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Electrochemically active Ir NPs on graphene for OER in acidic aqueous electrolyte investigated by in situ and ex situ spectroscopies
Abstract
An electrode for the oxygen evolution reaction based on a conductive bi-layered free standing graphene support functionalized with iridium nanoparticles was fabricated and characterized by means of potentiometric and advanced X-ray spectroscopic techniques. It was found that the electrocatalytic activity of iridium nanoparticles is associated to the formation of Ir 5d electron holes. Strong Ir 5d and O 2p hybridization, however, leads to a concomitant increase O 2p hole character, making oxygen electron deficient and susceptible to nucleophilic attack by water. Consequently, more efficient electrocatalysts can be synthesized by increasing the number of electron-holes shared between the metal d and oxygen 2p.
... X-ray spectroscopy is a tool well suited to study the role of Ir−O species on IOH surfaces, due to their surface sensitivity when using soft X-rays and distinct signals from different Ir−O species. 20,23−26 In an effort to utilize this spectroscopy under wet conditions, a variety of in situ approaches have been developed 27,28 and used on iridium dioxide, 29,30 anodized metallic iridium thin films 30,31 and nanoparticles, 25,32,33 amorphous IOHs with varying pretreatment, 20 and mesoporous IOH films. 34 By comparing to calculated spectroscopy of a rutile-type IrO 2 model system, these studies were able to identify electron-deficient oxygen species that are reactive. ...
... 25,30−32 This negative charge transfer behavior between Ir and O occurs when iridium is oxidized beyond Ir IV . 32,33,35 The dynamic behavior of the electrondeficient oxygen species under applied bias was further used to connect Ir−O species to electrochemical oxidation events 20,25 and their impact on the reaction barrier of the rate-determining step in the OER employing ab initio molecular dynamics. 36 The preliminary consensus on Ir−O speciation in the above studies is that surface oxygens bound to one iridium atom (μ 1 -O) are oxyls when stripped of all protons and are the most active species, oxygens bound to two iridium atom (μ 2 -O) contribute to a larger surface electron hole density and serve as proton acceptors in the rate-determining O−O coupling step, and the remaining μ 3 -O species contribute to stability through connectivity. ...
... Beyond +4, IOHs enter a negative charge transfer regime and electron holes increasingly reside on oxygen. 32,33,35 This hole character on oxygen is known to play a crucial role for the reactivity of IOHs in the OER. We measured this hole character via the Loẅdin charges on oxygen (tables within Figure 4) and the O K-edge WLI, which scales with the empty oxygen PDOS of the probed element. ...
The oxygen evolution reaction (OER) provides the protons for many electrocatalytic power-to-X processes, such as the production of green hydrogen from water or methanol from CO2. Iridium oxohydroxides (IOHs) are outstanding catalysts for this reaction because they strike a unique balance between activity and stability in acidic electrolytes. Within IOHs, this balance varies with the atomic structure. While amorphous IOHs perform best, they are least stable. The opposite is true for their crystalline counterparts. These rules-of-thumb are used to reduce the loading of scarce IOH catalysts and retain the performance. However, it is not fully understood how activity and stability are related at the atomic level, hampering rational design. Herein, we provide simple design rules (Figure 12) derived from the literature and various IOHs within this study. We chose crystalline IrOOH nanosheets as our lead material because they provide excellent catalyst utilization and a predictable structure. We found that IrOOH signals the chemical stability of crystalline IOHs while surpassing the activity of amorphous IOHs. Their dense bonding network of pyramidal trivalent oxygens (μ3Δ-O) provides structural integrity, while allowing reversible reduction to an electronically gapped state that diminishes the destructive effect of reductive potentials. The reactivity originates from coordinative unsaturated edge sites with radical character, i.e., μ1-O oxyls. By comparing to other IOHs and literature, we generalized our findings and synthesized a set of simple rules that allow prediction of stability and reactivity of IOHs from atomistic models. We hope that these rules will inspire atomic design strategies for future OER catalysts.
... X-ray spectroscopy is a tool well suited to study the role of Ir−O species on IOH surfaces, due to their surface sensitivity when using soft X-rays and distinct signals from different Ir−O species. 20,23−26 In an effort to utilize this spectroscopy under wet conditions, a variety of in situ approaches have been developed 27,28 and used on iridium dioxide, 29,30 anodized metallic iridium thin films 30,31 and nanoparticles, 25,32,33 amorphous IOHs with varying pretreatment, 20 and mesoporous IOH films. 34 By comparing to calculated spectroscopy of a rutile-type IrO 2 model system, these studies were able to identify electron-deficient oxygen species that are reactive. ...
... 25,30−32 This negative charge transfer behavior between Ir and O occurs when iridium is oxidized beyond Ir IV . 32,33,35 The dynamic behavior of the electrondeficient oxygen species under applied bias was further used to connect Ir−O species to electrochemical oxidation events 20,25 and their impact on the reaction barrier of the rate-determining step in the OER employing ab initio molecular dynamics. 36 The preliminary consensus on Ir−O speciation in the above studies is that surface oxygens bound to one iridium atom (μ 1 -O) are oxyls when stripped of all protons and are the most active species, oxygens bound to two iridium atom (μ 2 -O) contribute to a larger surface electron hole density and serve as proton acceptors in the rate-determining O−O coupling step, and the remaining μ 3 -O species contribute to stability through connectivity. ...
... Beyond +4, IOHs enter a negative charge transfer regime and electron holes increasingly reside on oxygen. 32,33,35 This hole character on oxygen is known to play a crucial role for the reactivity of IOHs in the OER. We measured this hole character via the Loẅdin charges on oxygen (tables within Figure 4) and the O K-edge WLI, which scales with the empty oxygen PDOS of the probed element. ...
The oxygen evolution reaction (OER) provides the protons for many electrocatalytic power-to-X processes, such as the production of green hydrogen from water or methanol from CO2. Iridium oxo-hydroxides (IOHs) are outstanding catalysts for this reaction because they strike a unique balance between activity and stability in acidic electrolytes. Within IOHs, this balance varies with atomic structure. While amorphous IOHs perform best, they are least stable. The opposite is true for their crystalline counterparts. These rules-of-thumb are used to reduce the loading of scarce IOH catalysts and retain performance. However, it is not fully understood how activity and stability are related on the atomic level, hampering rational design. Herein, we provide simple design-rules (Figure 12) derived from literature and various IOHs within this study. We chose crystalline IrOOH nanosheets as our lead material because they provide excellent catalyst utilization and a predictable structure. We found that nanosheets combine the chemical stability of crystalline IOHs with the activity amorphous IOHs. Their dense bonding network of pyramidal trivalent oxygens (μ3∆–O) provides structural integrity, while allowing reversible reduction to an electronically gapped state that diminishes the destructive effect of reductive potentials. The reactivity originates from coordinative unsaturated edge sites with radical character, i.e. μ1–O oxyls. By comparing to other IOHs and literature, we generalized our findings and synthesized a set of simple rules that allow prediction of stability and reactivity of IOHs from atomistic models. We hope that these rules will inspire atomic design strategies for future OER catalysts.
... Indeed, we ACS Catalysis pubs.acs.org/acscatalysis Research Article observe the asymmetric Ir 4f doublet (Ir 4f 7/2 at 61.8 eV) also reported in the literature for conducting Ir 4+ oxides 18,27,43,48,50 (compare to Figure 5d). Increasing the potential induces changes in the Ir 4f spectra. ...
... Recent work suggests that this would lead to oxygen-centered oxidation. 11,18 Hence, the changes in the Ir 4f spectrum may be caused by a combination of Ir-oxidation and modified Ir−O hybridization due to O-centered oxidation. This interpretation also holds for crystalline IrO 2 , which shows a small increase in the Ir 4f asymmetry as well due to the oxidation of the surface. ...
... As discussed earlier, the oxidation will be centered not only on the Ir atoms but also on the oxygen atoms. 11,18,27 Hence, highly oxidized Ir oxides containing a high density of μ 1 -O/μ 2 -O groups will have a strong electrophilic character in the active μ 1 -O sites, which ensures a low OER barrier 26 (as schematically depicted in Figure 8). Important to note is that any change in the OER barrier will have an exponential effect on the catalytic activity. ...
... In addition, some practical advantages demonstrated that the Ir with a high valence state is responsible for the high OER. For instance, using in situ and ex situ x-ray spectra, Juan-Jesús Velasco-Vélez et al. investigated the electrochemically active iridium nanoparticles for OER in acidic conditions and revealed that the catalytic activity is from the formation of shared electron-holes in the O 2p and Ir 5d, which leads to the generation of electron-deficient oxygen species 30 . Du and coworkers synthesized an Ag 1 /IrO x single-atom catalyst, uncovering the high-valence Ir x+ (x > 4) is responsible for the high catalytic OER performance 31 . ...
... Theoretical investigation for active sites using DFT. Inspired by the unique structure with the electron-deficient surface that benefits the OER efficiency as demonstrated using in situ and ex situ x-ray spectroscopies 30 , we further investigate the catalytic activity by means of DFT. Firstly, we check the favorite sites for HO* adsorption at various IMO sites since the HO* is the first intermediate formed in the OER (Fig. 6a). ...
... Thus, we adopt sites A, B, and C to further study mechanism of site activity. With its electron-deficient surface, IMO has a high OER catalytic efficiency and fast kinetic processes, resulting in the boosted OER performance 30,42 . The B site is more favored for the OER process compared to sites A and C because the configuration of HOO* cleaves as O* and HO* during the optimization process at sites of A and C sites (Fig. 6d-f). ...
The poor catalyst stability in acidic oxidation evolution reaction (OER) has been a long-time issue. Herein, we introduce electron-deficient metal on semiconducting metal oxides-consisting of Ir (Rh, Au, Ru)-MoO 3 embedded by graphitic carbon layers (IMO) using an electrospinning method. We systematically investigate IMO's structure, electron transfer behaviors, and OER catalytic performance by combining experimental and theoretical studies. Remarkably, IMO with an electron-deficient metal surface (Ir x+ ; x > 4) exhibit a low overpotential of only~156 mV at 10 mA cm −2 and excellent durability in acidic media due to the high oxidation state of metal on MoO 3. Furthermore, the proton dissociation pathway is suggested via surface oxygen serving as proton acceptors. This study suggests high stability with high catalytic performance in these materials by creating electron-deficient surfaces and provides a general, unique strategy for guiding the design of other metal-semiconductor nanocatalysts.
... XAS principles specify that both the E0 and WL position shifts toward higher excitation energies with iridium oxidation state. The WL intensity, also tied to the total Ir 5d-band hole count, should also monotonically increase with iridium oxidation state [44][45][46] . The WL position and intensity differences result from the fluorescence measurement configuration ( Figure S34) and changing sample nature (schematic in main text Figure 1a). ...
... eV after 1000 growth cycles when am-hydr-IrOx dominates the spectroscopic signal. The immediate WL intensity increase upon am-hydr-IrOx growth signifies a change in the average Ir 5d-band state of the material, indicating the presence of a higher oxidation species (i.e., am-hydr-IrOx) in the system [44][45][46] . Both, WL position and intensity changes thus illustrate that an oxidation process is taking place, converting Ir 0 into am-hydr-IrOx. ...
Understanding the oxygen evolution reaction (OER) and Ir dissolution mechanisms in amorphous, hydrous iridium oxides (am-hydr-IrO x ) is hindered by the reliance on crystalline iridium oxide theoretical models to interpret its...
... It has been shown that, as a result of a strong hybridization of the iridium and oxygen orbitals, the positive charge is shared between cations and anions. [33] Specifically, the formation of the reactive oxyl species depends on the oxidation state of the iridium. [34] As the catalyst is exposed to a high anodic potential, the accumulation of positive charge in electron-deficient oxygen species results in a decrease of the activation energy for the nucleophilic attack of water molecules and the formation of an OÀO bond, which is currently understood to be the ratedetermining step of the OER (Figure 1 e). ...
... e) OER scheme showing the formation of oxyl species, as a result of hybridization of Ir and O orbitals, which are prone to nucleophilic attack by water and the formation of an OÀO bond. [33] Copyright 2019, Elsevier. structure, the hydration and conductivity of the oxide play a significant role in the activity-stability properties (Figure 3 a). ...
The widespread utilization of proton exchange membrane (PEM) electrolyzers currently remains uncertain, as they rely on the use of highly scarce iridium as the only viable catalyst for the oxygen evolution reaction (OER), which is known to present the major energy losses of the process. Understanding the mechanistic origin of the different activities and stabilities of Ir‐based catalysts is, therefore, crucial for a scale‐up of green hydrogen production. It is known that structure influences the dissolution, which is the main degradation mechanism and shares common intermediates with the OER. In this Minireview, the state‐of‐the‐art understanding of dissolution and its relationship with the structure of different iridium catalysts is gathered and correlated to different mechanisms of the OER. A perspective on future directions of investigation is also given.
... The CVs in Figure 1C and 1D indicate that both the thin film and bilayer graphene coated with Ir NPs behave similarly. The control experiments with the plain graphene electrode were reported in previous work, 38 indicating clearly an enhanced catalytic activity for the electrode decorated with Ir NPs. Therefore, it is possible that similar active species are present during the electrocatalytic oxidation of water to dioxygen on both the thin-film and iridium NP electrodes. ...
... The Ir L 3 -edge probes the dipole allowed transitions from a core Ir 2p 3/2 electron to the partially occupied Ir 5d and Ir 6sp orbitals, which are hybridized with the O 2p orbitals. 38 Transitions to the 5d orbitals are lower in energy and well separated from transitions to the 6sp. These 2p to 5d transitions give rise to the so-called white line. ...
Iridium and ruthenium and their oxides/hydroxides are the best candidates for the oxygen evolution reaction under harsh acidic conditions owing to the low overpotentials observed for Ru-and Ir-based anodes and the high corrosion resistance of Ir-oxides. Herein, by means of cutting edge operando surface and bulk sensitive X-ray spectroscopy techniques, specifically designed electrode nano-fabrication and ab initio DFT calculations, we were able to reveal the electronic structure of the active IrO x centers (i.e. oxidation state) during electrocatalytic oxidation of water in the surface and bulk of high-performance Ir-based catalysts. We found the oxygen evolution reaction is controlled by the formation of empty Ir 5d states in the surface ascribed to the formation of formally Ir V species leading to the appearance of electron-deficient oxygen species bound to single iridium atoms (µ 1-O and µ 1-OH) that are responsible for water activation and oxidation. Oxygen bound to three iridium centers (µ 3-O) remains the dominant species in the bulk but do not participate directly in the electrocatalytic reaction, suggesting bulk oxidation is limited. In addition a high coverage of a µ 1-OO (peroxo) species during the OER is excluded. Moreover, we provide the first photoelectron spectroscopic evidence in bulk electrolyte that the higher surface to bulk ratio in thinner electrodes enhances the material usage involving the precipitation of a significant part of the electrode surface and near-surface active species.
... Note that PES is a surface sensitive technique due to the short inelastic mean free path (IMFP) of photoelectrons in solids or liquids 25,26 , making it an excellent complement to TFY-XAS. The Ir L 3 -edge probes the dipole allowed transitions from a core Ir 2p 3/2 electron to the partially occupied Ir 5d and Ir 6sp orbitals, which are hybridized with the O 2p orbitals 27 . Transitions to the 5d orbitals are lower in energy and well separated from transitions to the 6sp. ...
... While the large lifetime broadening (about 5 eV, see SI) does not allow the discrimination of ne structure in the white-line due to, for instance, transitions into t 2g -like and e g -like 5d orbitals, analysis of the whiteline intensity can still give insight into the electronic structure of iridium. In particular, a sum-rule relates the total number of 5d holes to the integral area of the white-line 11,27 ; that is, the white-line is linearly proportional to the iridium oxidation state 28 . Note that while the sum-rule is a property of the dipole operator and rigorously holds 29,30 for L 3 + L 2 , previous work shows no change in the L 3 :L 2 branching ratio for oxidized iridium compounds, making L 3 alone su cient for a white-line analysis 28 . ...
Iridium and its oxides/hydroxides are the best candidates for the oxygen evolution reaction under harsh acidic conditions owing to the low overpotential and the high corrosion resistance observed for Ir-based anodes. Herein, by means of cutting edge operando surface and bulk sensitive X-ray spectroscopy techniques, specifically designed electrode nano-fabrication and ab initio DFT calculations, we were able to reveal the electronic structure of the active IrO x centers (i.e. oxidation state) during electrocatalytic oxidation of water in the surface and bulk of high-performance Ir-based catalysts. We found the oxygen evolution reaction is controlled by the formation of empty Ir 5d states in the surface ascribed to the formation of formally Ir V species leading to the appearance of electron-deficient oxygen species bound to single iridium atoms (µ 1 -O and µ 1 -OH) that are responsible for water activation and oxidation, due to the bound oxygen’s transformation into an oxyl susceptible to nucleophilic attack water. Oxygen bound to three iridium centers (µ 3 -O) remains the dominant species in the bulk but do not participate directly in the electrocatalytic reaction, suggesting bulk oxidation is limited. In addition a high coverage of a µ 1 -OO (peroxo) species during the OER is excluded. Moreover, we provide the first photoelectron spectroscopic evidence in bulk electrolyte that the higher surface to bulk ratio in thinner electrodes enhances the material usage involving the precipitation of a significant part of the electrode surface and near-surface active species.
... In the example shown in figure 7(d), for instance, 80% of the signal comes from the top 0.43 nm of the ∼2.1 nm Ir nanoparticles (details in SI section S2.1). At open circuit potential (0.25 V RHE ), only the metallic state is detected within this probing depth, showing that the particles do not significantly oxidize without applied potential, in good agreement with previous results [2,66]. Raising the potential to 1.8 V RHE , where the oxygen evolution reaction occurs, oxidizes the particles. ...
... In our experience, only a double layer of graphene provides sufficient stability and the holes in the SiN x membrane should not exceed 1.3 µm diameter. Deposition of the material of interest onto the graphene window has been performed using electrodeposition [21] and physical vapor deposition [66]. Note that calcination of the samples is limited to about 200 • C in order to prevent damage to the graphene layer. ...
In situ X-ray spectroscopies offer a powerful way to understand the electronic structure of the electrode-electrolyte interface under operating conditions. However, most X-ray techniques require vacuum, making it necessary to design spectro-electrochemical cells with a delicate interface to the wet electrochemical environment. The design of the cell often dictates what measurements can be done and which electrochemical processes can be studied. Hence, it is important to pick the right spectro-electrochemical cell for the process of interest. To facilitate this choice, and to highlight the challenges in cell design, we critically review four recent, successful cell designs. Using several case studies, we investigate the opportunities and limitations that arise in practical experiments.
... The three X-ray absorption near edge structure (XANES) spectra at the Ir L 3 -edge (Fig. 1f) show similar characteristic features except for the peak positions and relative intensities. The prominent peak of Ir L 3 -edge, which is historically called the white line (WL), corresponds to the electron transition from the occupied 2p 3/2 orbital to the partially occupied Ir 5d orbitals and the magnitude of its integrated intensity is directly proportional to the density of unoccupied 5d orbitals (Supplementary Fig. 13) 32,33 . The inset in Fig. 1f shows the relationship of corresponding integrated intensities for the three samples indicated, it is obvious that the WL intensity of the Ir L 3 -edge for Ir-NSG is considerably higher than that for metallic Ir but lower than that for IrO 2 , revealing that the number of unoccupied states in the 5d band for Ir-NSG is between those of IrO 2 and metallic Ir, which has been utilized to correlate the catalytic activities of noble metal-based electrocatalysts to changes in their local electronic states 33,34 . ...
... The prominent peak of Ir L 3 -edge, which is historically called the white line (WL), corresponds to the electron transition from the occupied 2p 3/2 orbital to the partially occupied Ir 5d orbitals and the magnitude of its integrated intensity is directly proportional to the density of unoccupied 5d orbitals (Supplementary Fig. 13) 32,33 . The inset in Fig. 1f shows the relationship of corresponding integrated intensities for the three samples indicated, it is obvious that the WL intensity of the Ir L 3 -edge for Ir-NSG is considerably higher than that for metallic Ir but lower than that for IrO 2 , revealing that the number of unoccupied states in the 5d band for Ir-NSG is between those of IrO 2 and metallic Ir, which has been utilized to correlate the catalytic activities of noble metal-based electrocatalysts to changes in their local electronic states 33,34 . The Fourier transforms of the phase-uncorrected extended X-ray absorption fine structure (EXAFS) spectra are plotted in Supplementary Fig. 14 and Fig. 1g to probe the local environment of Ir. ...
Water electrolysis offers a promising energy conversion and storage technology for mitigating the global energy and environmental crisis, but there still lack highly efficient and pH-universal electrocatalysts to boost the sluggish kinetics for both cathodic hydrogen evolution reaction (HER) and anodic oxygen evolution reaction (OER). Herein, we report uniformly dispersed iridium nanoclusters embedded on nitrogen and sulfur co-doped graphene as an efficient and robust electrocatalyst for both HER and OER at all pH conditions, reaching a current density of 10 mA cm−2 with only 300, 190 and 220 mV overpotential for overall water splitting in neutral, acidic and alkaline electrolyte, respectively. Based on probing experiments, operando X-ray absorption spectroscopy and theoretical calculations, we attribute the high catalytic activities to the optimum bindings to hydrogen (for HER) and oxygenated intermediate species (for OER) derived from the tunable and favorable electronic state of the iridium sites coordinated with both nitrogen and sulfur. Water electrolysis offers a promising energy conversion technology, although there is still a need to understand the catalysis on the atomic-level. Here, the authors report Ir nanoclusters coordinated with both N and S as an efficient and pH-universal electrocatalyst for overall water splitting.
... The surface-sensitive spin-orbit split Ir 4 f X-ray photoelectron spectra for the as-prepared samples in Figure 5a were fitted using the theory-based model developed by Pfeifer et al. and summarised in Table S3. [35,36] A peak with a maximum at 61.7 eV and Doniach-Ŝunjić (DS) lineshape together with the associated shake-up satellites~1.0 eV higher in binding energy (BE) is consistent with the Ir4f 5/2 component of Ir IV in rutile IrO 2 . ...
Efficient water‐splitting is severely limited by the anodic oxygen evolution reaction (OER). Iridium oxides remain one of the only viable catalysts under acidic conditions due to their corrosion resistance. We have previously shown that heat‐treating high‐activity amorphous iridium oxyhydroxide in the presence of residual lithium carbonate leads to the formation of lithium‐layered iridium oxide, suppressing the formation of low‐activity crystalline rutile IrO2. We now report the synthesis of Na‐IrOx and K‐IrOx featuring similarly layered crystalline structures. Electrocatalytic tests confirm Li‐IrOx retains similar electrocatalytic activity to commercial amorphous IrO2 ⋅ 2H2O and with increasing size of the intercalated cation, the activity towards the OER decreases. However, the synthesised electrocatalysts that contain layers show greater stability than crystalline rutile IrO2 and amorphous IrO2 ⋅ 2H2O, suggesting these compounds could be viable alternatives for industrial PEM electrolysers where durability is a key performance measure.
... Shan et al., 2021 also reported that short-range regular Ir single atoms integrated into the cobalt oxide spinel structure exhibit much higher acidic OER activity and excellent stability [119]. Velasco-Vélez et al., 2019 prepared electrochemically active Ir nanoparticles on graphene and reported that it exhibited improved acidic OER activity and stability [120]. Forgie et al., 2010 compared pure ruthenium (Ru) with Ru-Co, Ru-Cu, and Ru-Ir for catalyst activity and reported a significant improvement in oxygen evolution reactions [121]. ...
This review covers the methods of hydrogen production through water electrolysis, encountered problems, used catalysts, methods of preparing these catalysts, and especially the method of electrode modification using the electrophoretic deposition technique. The methods of hydrogen production mechanism, electrode and electrocatalyst structures, and electrode modification techniques are classified based on types of the electrolyzers namely Alkaline Water Electrolysis (AEL), Anion Exchange Membrane (AEM) Water Electrolysis, Proton Exchange Membrane Electrolysis (PEMEL) and Solid Oxide Electrolysis (SOEL). The mechanisms of hydrogen evolution and oxygen evolution reactions, which are crucial in hydrogen production, have been provided in all types of electrolyzers. The parameters influencing these reactions on the anode and cathode electrodes have been discussed. This review summarizes and compares important parameters that show the catalyst's performance in detail. The fundamental principles of the electrophoretic deposition technique have been explained, and examples of electrocatalysts prepared using this technique have been provided.
... The presence of a large number of D-band holes increases the number of O 2p holes, and the consequential electrophilic nature facilitates nucleophilic acid-base-type O-O bond formation [39,46]. Hence, the greater the number of D-band holes, the higher the catalytic activity, as proposed previously. ...
Large-scale deployment of polymer electrolyte membrane water electrolysis, which is a promising technology for green hydrogen production, requires significant reduction of Ir in catalyst layer due to its high material cost. However, as the Ir amount in the catalyst layer decreases, number of electrically-isolated-inactive catalysts increases, resulting in poor performance. Here we show that the nanostructured textiles essentially avoids the formation of such inactive catalysts, enables unprecedented long-term hydrogen evolution with ultralow Ir mass loadings at high rates. We reveal that achieved high performance is derived from the low resistivity of prepared nanostructured textile which have a unique porous, three-dimensional structure. We also show that the nanostructured textiles facilitate simple catalyst-coated membrane production, which does not require liquid-phase processes or produce chemical waste. The method we developed can also be applied to a variety of zero-gap electrochemical conversion cells that require a high-rate catalytic reactions with small amounts of catalyst.
... Graphene, with decent electrical conductivity and chemical stability, high mechanical strength, and flexibility, has been used as a promising substrate material in diverse fields. Velasco-Vélez and co-workers [85] prepared electrochemically active Ir NPs on graphene and improved the acidic OER activity and stability. Shao and co-workers further adopted graphitic carbon nitride (g-C 3 N 4 ) and nitrogen-doped graphene (NG) as a substrate to anchor monodispersed Ir NPs (Ir/g-C 3 N 4 /NG). ...
Hydrogen, as a clean energy carrier, is of great potential to be an alternative fuel in the future. Proton exchange membrane (PEM) water electrolysis is hailed as the most desired technology for high purity hydrogen production and self-consistent with volatility of renewable energies, has ignited much attention in the past decades based on the high current density, greater energy efficiency, small mass-volume characteristic, easy handling and maintenance. To date, substantial efforts have been devoted to the development of advanced electrocatalysts to improve electrolytic efficiency and reduce the cost of PEM electrolyser. In this review, we firstly compare the alkaline water electrolysis (AWE), solid oxide electrolysis (SOE), and PEM water electrolysis and highlight the advantages of PEM water electrolysis. Furthermore, we summarize the recent progress in PEM water electrolysis including hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) electrocatalysts in the acidic electrolyte. We also introduce other PEM cell components (including membrane electrode assembly, current collector, and bipolar plate). Finally, the current challenges and an outlook for the future development of PEM water electrolysis technology for application in future hydrogen production are provided.
... The Ir 4f spectrum of the Ir-IrO x /C-20 catalyst shows an increase in intensity at higher binding energy at BEs of 62.4 and 65.4 eV, which can be attributed to more Ir 4f 5/2 and Ir 4f 7/2 components of iridium atoms that have undergone oxidation into the Ir IV under OER ( Figure S35), in agreement with reported results. 37,47 The XANES spectra show an increase in the white-line intensity at the Ir L III -edge compared with the fresh Ir-IrO x /C-20 catalyst, indicating the oxidation of the IrO x during the OER ( Figure S36). The LCF of Ir-IrO x /C-20 after the OER durability test ( Figure S37a) shows the Ir 0 and Ir IV components are 28.0% ...
... Details on the cell were given previously. 36,37,39 For the studies reported here, we used a cell configuration in which the CE (Pt wire) and the RE (Ag/AgCl (3M), DRIREF-2SH, World Precision Instruments) were immersed in a liquid electrolyte (0.05 M H 2 SO 4 ) that flowed continuously through the cell. The CE and the RE immersed into the electrolyte stream were separated from the evacuated XPS measurement chamber of the end station by a sandwiched membrane electrode assembly (MEA) based on a Nafion 117 PEM (Alfa Aeser), which previously was purified from organic contaminants and then activated as described in ref 36. ...
Surface-sensitive ambient pressure X-ray photo-electron spectroscopy and near-edge X-ray absorption fine structure spectroscopy combined with an electrocatalytic reactivity study, multilength-scale electron microscopy, and theoretical modeling provide insights into the gas-phase selective reduction of carbon dioxide to isopropanol on a nitrogen-doped carbon-supported iron oxyhydroxide electrocatalyst. Dissolved atomic carbon forms at relevant potentials for carbon dioxide reduction from the reduction of carbon monoxide chemisorbed on the surface of the ferrihydrite-like phase. Theoretical modeling reveals that the ferrihydrite structure allows vicinal chemisorbed carbon monoxide in the appropriate geometrical arrangement for coupling. Based on our observations, we suggest a mechanism of three-carbon-atom product formation, which involves the intermediate formation of atomic carbon that undergoes hydrogenation in the presence of hydrogen cations upon cathodic polarization. This mechanism is effective only in the case of thin ferrihydrite-like nanostructures coordinated at the edge planes of the graphitic support, where nitrogen edge sites stabilize these species and lower the overpotential for the reaction. Larger ferrihydrite-like nanoparticles are ineffective for electron transport.
... Measured distances between atoms were of 0.26, and 0.25 nm for GO and G. This distribution and distance are attributed to the presence of at least a bi-layer structure, as seen in literature [41], where the second layer does not lie directly under the layer above but is slightly displaced from it and thus these data are obtained. For graphene, the flakes presented a more damaged structure than graphene oxide. ...
Polyurethane based materials show great potential for many applications, and their reinforcement with different kinds of nano-entities can improve their properties or supply them with new ones, widening their fields of applications to new opportunities. In this work, nanocomposites composed of a biobased waterborne polyurethane and carbonaceous reinforcements were prepared and characterized. Parting from graphite, graphene, and graphene oxide were obtained through a mechanical and a chemical route, respectively, and graphene oxide was reduced into graphene through a thermal process. Successful exfoliation, oxidation, and reduction processes were proven when characterizing graphene, graphene oxide, and reduced graphene oxide. Nancomposites reinforced with graphene and graphene oxide showed improved mechanical and thermomechanical properties, whereas they did not show electrical conductivity. Coatings of the systems with graphene and reduced graphene oxide were studied, to grant electrical properties to the composites. Electrical conductor materials were obtained after coating the systems, as shown by Electrostatic Force Microscopy and electrical conductivity measurements.
... By increasing the potential from 1.25 to 1.65 V RHE , it is expected that the spectrum broadens toward high binding energies. 8,27 This is not the case for the measurement on the same spot, after 15 min of beam exposure, but is true for the fresh spot, which was only irradiated during the data acquisition of 150 s. Consequently, the change caused by the applied bias is slow or hindered when the area is damaged by the beam. ...
Electrochemistry is a promising building block for the global transition to a sustainable energy market. Particularly the electroreduction of CO2 and the electrolysis of water might be strategic elements for chemical energy conversion. The reactions of interest are inner-sphere reactions, which occur on the surface of the electrode, and the biased interface between the electrode surface and the electrolyte is of central importance to the reactivity of an electrode. However, a potential-dependent observation of this buried interface is challenging, which slows the development of catalyst materials. Here we describe a sample architecture using a graphene blanket that allows surface sensitive studies of biased electrochemical interfaces. At the examples of near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and environmental scanning electron microscopy (ESEM) we show that the combination of a graphene blanket and a permeable membrane leads to the formation of a liquid thin film between them. This liquid thin film is stable against a water partial pressure below 1 mbar. These properties of the sample assembly extend the study of solid-liquid interfaces to highly surface sensitive techniques, such as electron spectroscopy/microscopy. In fact, photoelectrons with an effective attenuation length of only 10 Å can be detected, which is close to the absolute minimum possible in aqueous solutions. The in situ cells and the sample preparation necessary to employ our method are comparatively simple. Transferring this approach to other surface sensitive measurement techniques should therefore be straightforward. We see our approach as a starting point for more studies on electrochemical interfaces and surface processes under applied potential. Such studies would be of high value for the rational design of electrocatalysts.
Understanding the structure of nanoparticles under (electro)catalytic operating conditions is crucial for uncovering structure–property relationships. By combining operando X-ray total scattering and pair distribution function analysis with operando small-angle X-ray scattering (SAXS), we obtained comprehensive structural information on ultrasmall (<3 nm) iridium nanoparticles and tracked their changes during oxygen evolution reaction (OER) in acid. When subjected to electrochemical conditions at reducing potentials, the metallic Ir nanoparticles are found to be decahedral. The iridium oxide formed in the electrochemical oxidation contains small rutile-like clusters composed of edge- and corner-connected [IrO6] octahedra of a very confined range. These rutile domains are smaller than 1 nm. Combined with complementary SAXS data analysis to extract the particle size, we find that the OER-active iridium oxide phase lacks crystalline order. Additionally, we observe an iridium oxide contraction under OER conditions, which is confirmed by operando X-ray absorption spectroscopy. Our results highlight the need for multitechnique operando studies for a complete understanding of the electrochemically formed Ir oxide active in OER.
A special membrane electrode assembly to measure operando X‐ray absorption spectra and resonant photoemission spectra of mesoporous templated iridium oxide films is used. These films are calcined to different temperatures to mediate the catalyst activity. By combining operando resonant photoemission measurements of different films with ab initio simulations these are able to unambiguously distinguish µ2‐O (bridging oxygen) and µ1‐O (terminal oxygen) in the near‐surface regions of the catalysts. The intrinsic activity of iridium oxide scales with the formation of µ1‐O (terminal oxygen) is found. Importantly, it is shown that the peroxo species do not accumulate under reaction conditions. Rather, the formation of µ1‐O species, which are active in O−O bond formation during the OER, is the most oxidized oxygen species observed, which is consistent with an O−O rate‐limiting step. Thus, the oxygen species taking part in the electrochemical oxidation of water on iridium electrodes are more involved and complex than previously stated. This result highlights the importance of employing theory together with true and complementary operando measurements capable of probing different aspects of catalysts surfaces during operation.
Water splitting is a promising technology in the path towards complete renewable energy within the hydrogen economy but overcoming the sluggishness of the oxygen evolution reaction (OER) is a major challenge. Iridium-based oxides remain the most attractive materials for the OER under acidic conditions since they offer the combination of activity and stability. Gaining knowledge about how these materials have such an ability is of great interest to develop improved electrocatalysts for the OER. Among the different iridium-based oxides the materials with high concentrations of electron deficient oxygen (O I−) have been shown to have higher OER activity, however, they also have high dissolution rates, seemingly due to the presence or formation of Ir III species. In contrast, rutile-type IrO 2 , which does not contain Ir III species, has high dissolution resistance but the OER activity remains comparatively low as only low coverages of O I− species are formed under OER. The apparent link between O I− and Ir III species that comes from these observations has yet to be proven. In this work, using ab initio thermodynamics and in situ X-ray photoelectron and absorption spectroscopy we show that the same electrophilic O I− species that appear on Ir-based oxides under OER can be formed on Ir IV+δ by mild thermal oxidation of rutile-type IrO 2 , without the presence Ir III species.
The lack of high efficiency and pH‐universal bifunctional electrocatalysts for water splitting to hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) hinders the large‐scale production of green hydrogen. Here, an IrPd electrocatalyst supported on ketjenblack that exhibits outstanding bifunctional performance for both HER and OER at wide pH conditions is presented. The optimized IrPd catalyst exhibits a specific activity of 4.46 and 3.98 A mgIr⁻¹ in the overpotential of 100 and 370 mV for HER and OER, respectively, in alkaline conditions. When applied to the anion exchange membrane electrolyzer, the Ir44Pd56/KB catalyst shows a stability of >20 h at a current of 250 mA cm⁻² for water decomposition, indicating promising prospects for practical applications. Beyond offering an advanced electrocatalyst, this work also guides the rational design of desirable bifunctional electrocatalysts for HER and OER by regulating the microenvironments and electronic structures of metal catalytic sites for diverse catalysis.
Revealing the electronic properties determining the characteristics and performance of different materials of interest under operation conditions has become paramount in many research areas specially in energy materials as batteries or catalysis. Operando spectroscopy is of prime interest even more in presence of complex environments as gases and liquids, for example in applications as batteries, capacitors, electro- and thermal-catalysis etc. X-ray spectroscopy is a non-destructive method that provides relevant information of a material of interest in an element specific way. Unfortunately this technique is hardly compatible with liquid and gases (specially in the soft X-ray regimen). In this article a detailed description of the different approaches used for investigate complex electrochemical systems under working conditions with X-ray absorption spectroscopy in the soft X-ray regimen is discussed.
Hydrogen fuel cell vehicles have always been regarded as the main direction for developing new energy vehicles in the future due to their advantages of zero emission, high cruising range, and strong environmental adaptability. Currently, although the related technologies have gradually matured, there are still many factors hindering its development. One of the main reasons is that the price of hydrogen fuel increases the cost of using vehicles, which puts it at a competitive disadvantage compared with traditional fuel vehicles and pure electric vehicles. Herein, we summarize the recent development status of hydrogen fuel cell vehicles at home and abroad, and analyze the cost and sustainability brought by the latest scientific research progress to the hydrogen production industry, which is derived from basic research on electrocatalysts used in industrial electrocatalytic water splitting with an alkaline electrolyte. Finally, the development of hydrogen fuel cell vehicles was analyzed and prospected, which is one of the main application fields of hydrogen in the future.
The combination of operando and computational X-ray spectroscopies has shown promise for building accurate models of active catalyst surfaces. Operando spectroscopy captures metastable active surfaces; computational spectroscopy uses this information to aid in building models for first principles reaction simulations. Herein, we review recent efforts and outline future opportunities to study the oxygen evolution reaction (OER) by combining operando spectroscopies and first principles modeling. We begin by showcasing how explicit simulation of operando-collected spectra has helped validate an OER mechanism over Ir-based catalysts involving electron-deficient oxygen, or OI-. We continue by reviewing efforts on 3d transition metal (TM) oxyhydroxides, where operando studies again suggest OI- is critical. For these materials, changes in OI- coverage have been argued to cause qualitative mechanistic differences, though, comparative operando and computational spectroscopic studies are lacking. We close by outlining how such comparative studies would aid in testing mechanistic claims on 3d TM oxyhydroxides.
Photoelectron spectroscopy offers detailed information about the electronic structure and chemical composition of surfaces, owing to the short distance that the photoelectrons can escape from a dense medium. Unfortunately, photoelectron based spectroscopies are not directly compatible with the liquids required to investigate electrochemical processes, especially in the soft X-ray regime. To overcome this issue, different approaches based on photoelectron spectroscopy have been developed in our group over the last few years. The performance and the degree of information provided by these approaches are compared with those of the well established bulk sensitive spectroscopic approach of total fluorescence yield detection, where the surface information gained from this approach is enhanced using samples with large surface to bulk ratios. The operation of these approaches is exemplified and compared using the oxygen evolution reaction on IrO x catalysts. We found that all the approaches, if properly applied, provide similar information about surface oxygen speciation. However, using resonant photoemission spectroscopy, we were able to prove that speciation is more involved and complex than previously thought during the oxygen evolution reaction on IrO x based electrocatalysts. We found that the electrified solid-liquid interface is composed of different oxygen species, where the terminal oxygen atoms on iridium are the active species, yielding the formation of peroxo species and, finally, dioxygen as the reaction product. Thus, the oxygen-oxygen bond formation is dominated by peroxo species formation along the reaction pathway. Furthermore, the methodologies discussed here open up opportunities to investigate electrified solid-liquid interfaces in a multitude of electrochemical processes with unprecedented speciation capabilities, which are not accessible by one-dimensional X-ray spectroscopies.
In this short review, a brief analysis of opportunities and challenges offered by in situ X-ray spectroscopic methods in electrocatalysis will be presented. Moreover, a selection of the most recent contributions of in situ spectroscopies on two reactions will be reported to highlight current understanding on the reaction mechanisms, namely the oxygen evolution reaction (OER) over Ir based electrocatalysts and the CO2 reduction reaction (CO2RR) over Cu.
Widespread utilization of proton exchange membrane (PEM) electrolyzers is currently still uncertain, as it inevitably relies on the use of highly scarce iridium as the only viable catalyst for oxygen evolution reaction (OER), which is known to present the major energy losses of the process. The understanding of the mechanistic origin of different activities and stabilities of Ir-based catalysts is therefore crucial for a scale-up of green hydrogen production. It is known that structure importantly influences the dissolution, which is the main degradation mechanism and shares common intermediates with the OER. In this mini-review, the state-of-the-art understanding of the dissolution phenomenon and its relationship with the structure of different iridium catalysts is gathered and correlated to different mechanisms of OER. In conclusion, the perspective on future investigation directions is given.
Understanding the atomic-scale mechanistic details of the oxygen evolution reaction (OER) remains an unresolved challenge in electrochemistry owing to the complexity of the OER. In this short review we discuss how, with the advent of new experimental and computational methodologies, the OER can be treated with increasingly sophisticated models to aid in our complete understanding. For the case of steady state catalyst surfaces, we define a six-rung ladder of complexity to frame how far this understanding reaches and in which aspects our understanding could still improve.
Hydrogen production from solar energy is currently considered the best alternative to fossil fuels. Thus, materials enabling efficient and sustainable energy conversion and storage need to be developed. Iridium is still the only material used in proton exchange membrane electrolyzers that efficiently catalyze hydrogen evolution counter-reaction, namely, the oxygen evolution reaction (OER) for electrochemical water splitting in acidic media. With no practical alternatives that can sustain the harsh reaction conditions, new approaches need to be developed to increase the utilization of this scarce metal. Hereby, a carbon–ceramic nanocomposite material is investigated, where Ir nanoparticles and nanoflakes of titanium oxynitride (TiONx) are deposited on the surface of reduced graphene oxide nanoribbons (rGONRs). OER performance is shown to be dependent on the mutual distribution of the Ir–TiONx–rGONR phases and in the best case leads up to 30 times higher activity relative to the commercial IrO2 benchmark. Adjusting the domains of different chemical nature within the same hybrid nanocomposite material through the formation of heterojunctions is shown to boost OER performance. This work demonstrates how fine-tuning of morphology, composition, and particle distribution of the carbon–ceramic catalytic material can introduce a strong synergistic effect on OER activity and stability of iridium.
This review features state-of-the-art in situ and operando electron microscopy (EM) studies of heterogeneous catalysts in gas and liquid environments during reaction. Heterogeneous catalysts are important materials for the efficient production of chemicals/fuels on an industrial scale and for energy conversion applications. They also play a central role in various emerging technologies that are needed to ensure a sustainable future for our society. Currently, the rational design of catalysts has largely been hampered by our lack of insight into the working structures that exist during reaction and their associated properties. However, elucidating the working state of catalysts is not trivial, because catalysts are metastable functional materials that adapt dynamically to a specific reaction condition. The structural or morphological alterations induced by chemical reactions can also vary locally. A complete description of their morphologies requires that the microscopic studies undertaken span several length scales. EMs, especially transmission electron microscopes, are powerful tools for studying the structure of catalysts at the nanoscale because of their high spatial resolution, relatively high temporal resolution, and complementary capabilities for chemical analysis. Furthermore, recent advances have enabled the direct observation of catalysts under realistic environmental conditions using specialized reaction cells. Here, we will critically discuss the importance of spatially-resolved operando measurements and available experimental setups that enable (1) correlated studies where EM observations are complemented by separate measurements of reaction kinetics or spectroscopic analysis of chemical species during reaction or (2) real-time studies where the dynamics of catalysts are followed with EM and the catalytic performance is extracted directly from the reaction cell that is within the EM column or chamber. Examples of current research in this field will be presented. Challenges in the experimental application of these techniques and our perspectives on the field's future directions will also be discussed.
Iridium-based catalysts are state-of-the-art catalysts for oxygen evolution reaction (OER) under highly corrosive acidic condition. However, it remains challenges to simultaneously achieve enhanced activity and stability from Ir based OER catalyst. Herein, the amine molecules directed assembly method is used to coat of ultrasmall Ir nanoparticles (∼1.3 nm) on carbon (Ir NP/C). The catalyst with ultralow loading of 13.5 ᶙgIr cm⁻² exhibits 70 mV lower overpotential to reach a current density of 10 mA cm⁻² and much longer lifetime than that of commercial IrO2 with high loading of 102 ᶙgIr cm⁻². Experimental results suggest that the presence of a higher proportion of Ir-N on the top atomic layer of ultrasmall Ir nanoparticles, as evident from the X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) spectra of Ir NP/C, facilitated the enhanced performance. The findings could promote the understanding and developing of active Ir-based OER nanocatalysts with longer lifetime.
The catalyst in the oxygen electrode is the core component of the aqueous metal-air battery, which plays a vital role in the determination of the open circuit potential, energy density, and cycle life of the battery. For rechargeable aqueous metal-air batteries, the catalyst should have both good oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalytic performance. Compared with precious metal catalysts, non-precious metal materials have more advantages in terms of abundant resource reserves and low prices. Over the past few years, great efforts have been made in the development of non-precious metal bifunctional catalysts. This review selectively evaluates the advantages, disadvantages and development status of recent advanced materials including pure carbon materials, carbon-based metal materials and carbon-free materials as bifunctional oxygen catalysts. Preliminary improvement strategies are formulated to make up for the deficiency of each material. The development prospects and challenges facing bifunctional catalysts in the future are also discussed.
Cobalt oxides have been identified as highly active catalysts for the electrochemical water splitting and oxygen evolution reaction. Using near-ambient pressure resonant photoelectron spectroscopy, we studied changes in the metal-oxygen coordination of size-selected core-shell CoO
x
nanoparticles induced by liquid water. In dry conditions, the nanoparticles exhibit an octahedrally coordinated Co2+ core and a tetrahedrally coordinated Co2+ shell. In the presence of liquid water, we observe a reversible phase change of the nanoparticle shell into octahedrally coordinated Co2+ as well as partially oxidized octahedrally coordinated Co3+. This is in contrast to previous findings, suggesting an irreversible phase change of tetrahedrally coordinated Co2+ after the oxygen evolution reaction conditioning. Our results demonstrate the appearance of water-induced structural changes different from voltage-induced changes and help us to understand the atomic scale interaction of CoO
x
nanoparticles with water in electrochemical processes.
Cette thèse porte sur l’étude et le développement d’électrocatalyseurs à base d’iridium pour la réaction de dégagement de dioxygène (OER) dans les électrolyseurs à membrane échangeuse de protons. En raison de la dégradation marquée des électrocatalyseurs en conditions OER, nous nous sommes particulièrement intéressés à la recherche d’un compromis optimal entre activité catalytique et stabilité. Différents électrocatalyseurs (supportés sur noir de carbone, supportés sur oxydes métalliques dopés et non-supportés) ont été synthétisés et caractérisés par des méthodes électrochimiques et physico-chimiques, notamment par spectroscopie photoélectronique X, microscopie électronique en transmission à localisation identique et spectrométrie de masse à plasma à couplage inductif. Les électrocatalyseurs supportés sont les moins stables en conditions OER, notamment du fait de l’agglomération, la coalescence, la dissolution et le détachement des nanoparticules d’oxyde d’iridium. Ces deux derniers mécanismes de dégradation sont exacerbés par la corrosion des supports carbonés et la dissolution des éléments composant les supports oxydes métalliques dopés. Les électrocatalyseurs non-supportés offrent ainsi le meilleur compromis entre activité et stabilité. Les degrés d’oxydation Ir(III) et Ir(V) ont été identifiés comme les plus actifs pour l’OER en électrolyte acide tandis que l’oxyde Ir(IV) est le plus stable, l’espèce la moins stable étant l’iridium métallique Ir(0). La dégradation des couches catalytiques en cellule d’électrolyse PEM ne semble impacter que très peu les performances globales d’électrolyse par rapport à la dégradation des collecteurs de courant.
Hydrogen is a clean and renewable energy carrier for powering future transportation and other applications. Water electrolysis is a promising option for hydrogen production from renewable resources such as wind and solar energy. To date, tremendous efforts have been devoted to the development of electrocatalysts and membranes for water electrolysis technology. In principle, water electrolysis in acidic media has several advantages over that in alkaline media, including favorable reaction kinetics, easy product separation, and low operating pressure. However, acidic water electrolysis poses higher requirements for the catalysts, especially the ones for the oxygen evolution reaction. It is a grand challenge to develop highly active, durable, and cost‐effective catalysts to replace precious metal catalysts for acidic water oxidation. In this article, an overview is presented of the latest developments in design and synthesis of electrocatalysts for acidic water oxidation, emphasizing new strategies for achieving high electrocatalytic activity while maintaining excellent durability at low cost. In particular, the reaction pathways and intermediates are discussed in detail to gain deeper insight into the oxygen evolution reaction mechanism, which is vital to rational design of more efficient electrocatalysts. Further, the remaining scientific challenges and possible strategies to overcome them are outlined, together with perspectives for future‐generation electrocatalysts that exploit nanoscale materials for water electrolysis.
Iridium oxide based catalysts are uniquely active and stable in the oxygen evolution reaction. Theoretical work attributes their activity to oxyl, or µ1-O, species. Verifying this intermediate experimentally has, however, been challenging. In the present study, these challenges were overcome by combining theory with new experimental strategies. Ab initio molecular dynamics of the solid liquid interface were used to predict spectroscopic features, while a sample architecture, developed for surface sensitive X-ray spectroscopy of electrocatalysts in confined liquid, was used to search for these species under realistic conditions. Through this approach, we have identified µ1-O species during oxygen evolution. Potentiodynamic X-ray absorption additionally shows that these µ1-O species are created by electrochemical oxidation currents in a deprotonation reaction.
We study the electrochemical interface between rutile IrO2 (110) and water to investigate how the inclusion of an explicit solvent influences the stabilities of adsorbed intermediates in the oxygen evolution reaction (OER). Solvent is modeled by explicit non-dissociated water molecules and their structure is determined by a global optimization method. We find that the inclusion of an explicit solvent can significantly affect the geometry of adsorbed intermediates, changing from an interaction with the surface to an interaction with the water bilayer. These water structures consist of stacked octagonal sheets in an ordered network. Solvent stabilization is pronounced for adsorbed *OH and *OOH, which are capable of donating hydrogen bonds. We find little to no change in adsorbate binding energy as the number of layers of solvent is increased from 1 to 3, suggesting a single water bilayer is sufficient to describe the system. With either *O or *OH co-adsorbates, the energetics of the reaction pathway are relatively unchanged with the inclusion of explicit solvent.
Water splitting performed in acidic media relies on the exceptional performance of iridium-based materials to catalyze the oxygen evolution reaction (OER). In the present work, we use in situ X-ray photoemission and absorption spectroscopy to resolve the long-standing debate about surface species present in iridium-based catalysts during the OER. We find that the surface of an initially metallic iridium model electrode converts into a mixed-valent, conductive iridium oxide matrix during the OER, which contains OII− and electrophilic OI− species. We observe a positive correlation between the OI− concentration and the evolved oxygen, suggesting that these electrophilic oxygen sites may be involved in catalyzing the OER. We can understand this observation by analogy with photosystem II; their electrophilicity renders the OI− species active in O-O bond formation, i.e. the likely potential- and rate-determining step of the OER. The ability of amorphous iridium oxyhydroxides to easily host such reactive, electrophilic species can explain their superior performance when compared to plain iridium metal or crystalline rutile-type IrO2.
Tremendous effort has been devoted towards elucidating the fundamental reasons for the higher activity of hydrated amorphous IrIII/IV oxyhydroxides (IrO
x
) in the oxygen evolution reaction (OER) in comparison with their crystalline counterpart, rutile-type IrO2, by focusing on the metal oxidation state. Here we demonstrate that, through an analogy to photosystem II, the nature of this reactive species is not solely a property of the metal but is intimately tied to the electronic structure of oxygen. We use a combination of synchrotron-based X-ray photoemission and absorption spectroscopies, ab initio calculations, and microcalorimetry to show that holes in the O 2p states in amorphous IrO
x
give rise to a weakly bound oxygen that is extremely susceptible to nucleophilic attack, reacting stoichiometrically with CO already at room temperature. As such, we expect this species to play the critical role of the electrophilic oxygen involved in O-O bond formation in the electrocatalytic OER on IrO
x
. We propose that the dynamic nature of the Ir framework in amorphous IrO
x
imparts the flexibility in Ir oxidation state required for the formation of this active electrophilic oxygen.
Electrochemically grown cobalt on graphene exhibits exceptional performance as a catalyst for the oxygen evolution reaction (OER) and provides the possibility of controlling the morphology and the chemical properties during deposition. However, the detailed atomic structure of this hybrid material is not well understood. To elucidate the Co/graphene electronic structure, we have developed a flow cell closed by a graphene membrane that provides electronic and chemical information on the active surfaces under atmospheric pressure and in the presence of liquids by means of X-ray photoelectron spectroscopy (XPS). We found that cobalt is anchored on graphene via carbonyl-like species, namely Co(CO)x , promoting the reduction of Co(3+) to Co(2+) , which is believed to be the active site of the catalyst.
Access to clean, affordable and reliable energy has been a cornerstone of the world's increasing prosperity and economic growth since the beginning of the industrial revolution. Our use of energy in the twenty-first century must also be sustainable. Solar and water-based energy generation, and engineering of microbes to produce biofuels are a few examples of the alternatives. This Perspective puts these opportunities into a larger context by relating them to a number of aspects in the transportation and electricity generation sectors. It also provides a snapshot of the current energy landscape and discusses several research and development opportunities and pathways that could lead to a prosperous, sustainable and secure energy future for the world.
Gotcha! Using electrochemical surface-enhanced Raman spectroscopy, surface-bound hydroxyperoxy (AuOOH) is pinpointed in situ as a reaction intermediate of oxygen evolution on gold catalyst (see picture).
We have performed x-ray absorption spectroscopy (XAS) measurements on a
series of Ir-based 5d transition metal compounds, including Ir, IrCl3, IrO2,
Na2IrO3, Sr2IrO4, and Y2Ir2O7. By comparing the intensity of the "white-line"
features observed at the Ir L2 and L3 absorption edges, it is possible to
extract valuable information about the strength of the spin-orbit coupling in
these systems. We observe remarkably large, non-statistical branching ratios in
all Ir compounds studied, with little or no dependence on chemical composition,
crystal structure, or electronic state. This result confirms the presence of
strong spin-orbit coupling effects in novel iridates such as Sr2IrO4, Na2IrO3,
and Y2Ir2O7, and suggests that even simple Ir-based compounds such as IrO2 and
IrCl3 may warrant further study. In contrast, XAS measurements on Re-based 5d
compounds, such as Re, ReO2, ReO3, and Ba2FeReO6, reveal statistical branching
ratios and negligible spin-orbit coupling effects.
The performance of new materials and devices often depends on processes taking place at the interface between an active solid element and the environment (such as air, water or other fluids). Understanding and controlling such interfacial processes require surface-specific spectroscopic information acquired under real-world operating conditions, which can be challenging because standard approaches such as X-ray photoelectron spectroscopy generally require high-vacuum conditions. The state-of-the-art approach to this problem relies on unique and expensive apparatus including electron analysers coupled with sophisticated differentially pumped lenses. Here, we develop a simple environmental cell with graphene oxide windows that are transparent to low-energy electrons (down to 400 eV), and demonstrate the feasibility of X-ray photoelectron spectroscopy measurements on model samples such as gold nanoparticles and aqueous salt solution placed on the back side of a window. These proof-of-principle results show the potential of using graphene oxide, graphene and other emerging ultrathin membrane windows for the fabrication of low-cost, single-use environmental cells compatible with commercial X-ray and Auger microprobes as well as scanning or transmission electron microscopes.
The large-scale practical application of fuel cells will be difficult to realize if the expensive platinum-based electrocatalysts for oxygen reduction reactions (ORRs) cannot be replaced by other efficient, low-cost, and stable electrodes. Here, we report that vertically aligned nitrogen-containing carbon nanotubes (VA-NCNTs) can act as a metal-free electrode with a much better electrocatalytic activity, long-term operation stability, and tolerance to crossover effect than platinum for oxygen reduction in alkaline fuel cells. In air-saturated 0.1 molar potassium hydroxide, we observed a steady-state output potential of -80 millivolts and a current density of 4.1 milliamps per square centimeter at -0.22 volts, compared with -85 millivolts and 1.1 milliamps per square centimeter at -0.20 volts for a platinum-carbon electrode. The incorporation of electron-accepting nitrogen atoms in the conjugated nanotube carbon plane appears to impart a relatively high positive charge density on adjacent carbon atoms. This effect, coupled with aligning the NCNTs, provides a four-electron pathway for the ORR on VA-NCNTs with a superb performance.
Over the past 100 years, the global average temperature has increased by approximately 0.6 degrees C and is projected to continue to rise at a rapid rate. Although species have responded to climatic changes throughout their evolutionary history, a primary concern for wild species and their ecosystems is this rapid rate of change. We gathered information on species and global warming from 143 studies for our meta-analyses. These analyses reveal a consistent temperature-related shift, or 'fingerprint', in species ranging from molluscs to mammals and from grasses to trees. Indeed, more than 80% of the species that show changes are shifting in the direction expected on the basis of known physiological constraints of species. Consequently, the balance of evidence from these studies strongly suggests that a significant impact of global warming is already discernible in animal and plant populations. The synergism of rapid temperature rise and other stresses, in particular habitat destruction, could easily disrupt the connectedness among species and lead to a reformulation of species communities, reflecting differential changes in species, and to numerous extirpations and possibly extinctions.
In order to address the economic and environmental consequences of our global energy system, we consider the availability
and consumption of energy resources. Problems arise from our dependence on combustible fuels, the environmental risks associated
with their extraction, and the environmental damage caused by their emissions. Yet no primary energy source, be it renewable
or nonrenewable, is free of environmental or economic limitations. As developed and developing economies continue to grow,
conversion to and adoption of environmentally benign energy technology will depend on political and economic realities.
Identifying and building a sustainable energy system are perhaps two of the most critical issues that today's society must
address. Replacing our current energy carrier mix with a sustainable fuel is one of the key pieces in that system. Hydrogen
as an energy carrier, primarily derived from water, can address issues of sustainability, environmental emissions, and energy
security. Issues relating to hydrogen production pathways are addressed here. Future energy systems require money and energy
to build. Given that the United States has a finite supply of both, hard decisions must be made about the path forward, and
this path must be followed with a sustained and focused effort.
This article reviews the basic theoretical aspects of graphene, a one atom thick allotrope of carbon, with unusual two-dimensional Dirac-like electronic excitations. The Dirac electrons can be controlled by application of external electric and magnetic fields, or by altering sample geometry and/or topology. We show that the Dirac electrons behave in unusual ways in tunneling, confinement, and integer quantum Hall effect. We discuss the electronic properties of graphene stacks and show that they vary with stacking order and number of layers. Edge (surface) states in graphene are strongly dependent on the edge termination (zigzag or armchair) and affect the physical properties of nanoribbons. We also discuss how different types of disorder modify the Dirac equation leading to unusual spectroscopic and transport properties. The effects of electron-electron and electron-phonon interactions in single layer and multilayer graphene are also presented.
Nature’s water splitting catalyst, an oxygen-bridged tetra-manganese calcium (Mn4O5Ca) complex, sequentially activates two substrate water molecules generating molecular O2. Its reaction cycle is composed of five intermediate (Si) states, where the index i indicates the number of oxidizing equivalents stored by the cofactor. After formation of the S4 state, the product dioxygen is released and the cofactor returns to its lowest oxidation state, S0. Membrane-inlet mass spectrometry measurements suggest that at least one substrate is bound throughout the catalytic cycle, as the rate of ¹⁸O-labeled water incorporation into the product O2 is slow, on a millisecond to second timescale depending on the S state. Here, we demonstrate that the Mn4O5Ca complex poised in the S0 state contains an exchangeable hydroxo bridge. Based on a combination of magnetic multiresonance (EPR) spectroscopies, comparison to biochemical models and theoretical calculations we assign this bridge to O5, the same bridge identified in the S2 state as an exchangeable fully deprotonated oxo bridge [Pérez Navarro et al. Proc. Natl. Acad. Sci. U.S.A. 2013 110, 15561]. This oxygen species is the most probable candidate for the slowly-exchanging substrate water in the S0 state. Additional measurements provide new information on the Mn ions that constitute the catalyst. A structural model for the S0 state is proposed that is consistent with available experimental data and explains the observed evolution of water exchange kinetics in the first three states of the catalytic cycle.
Significance
In this short paper, a detailed investigation is made of one of the leading suggestions for O–O bond formation in photosystem II. The mechanism studied is a nucleophilic attack by a water molecule on an oxyl group in the oxygen-evolving complex. The calculations show quite clearly that this type of mechanism is not possible in this case.
One of the main goals in catalysis is the characterization of solid/gas interfaces in a reaction environment. The electronic structure and chemical composition of surfaces become heavily influenced by the surrounding environment. However, the lack of surface sensitive techniques that are able to monitor these modifications under high pressure conditions hinders the understanding of such processes. This limitation is known throughout the community as the “pressure gap.” We have developed a novel experimental setup that provides chemical information on a molecular level under atmospheric pressure and in presence of reactive gases and at elevated temperatures. This approach is based on separating the vacuum environment from the high-pressure environment by a silicon nitride grid—that contains an array of micrometer-sized holes—coated with a bilayer of graphene. Using this configuration, we have investigated the local electronic structure of catalysts by means of photoelectron spectroscopy and in presence of gases at 1 atm. The reaction products were monitored online by mass spectrometry and gas chromatography. The successful operation of this setup was demonstrated with three different examples: the oxidation/reduction reaction of iridium (noble metal) and copper (transition metal) nanoparticles and with the hydrogenation of propyne on Pd black catalyst (powder).
Iridium-based materials are among the most active and stable electrocatalysts for the oxygen evolution reaction. Amorphous iridium oxide structures are found to be more active than their crystalline counterparts. Herein, we combine synchrotron-based X-ray photoemission and absorption spectroscopies with theoretical calculations to investigate the electronic structure of Ir metal, rutile-type IrO2, and an amorphous IrOx. Theory and experiment show that while the Ir 4f line shape of Ir metal is well described by a simple Doniach-Šunjić function, the peculiar line shape of rutile-type IrO2 requires the addition of a shake-up satellite 1eV above the main line. In the catalytically more active amorphous IrOx, we find that additional intensity appears in the Ir 4f spectrum at higher binding energy when compared with rutile-type IrO2 along with a pre-edge feature in the O K-edge. We identify these additional features as electronic defects in the anionic and cationic frameworks, namely, formally OI- and IrIII, which may explain the increased activity of amorphous IrOx electrocatalysts. We corroborate our findings by in situ X-ray diffraction as well as in situ X-ray photoemission and absorption spectroscopies.
Iridium oxide based electrodes are among the most promising candidates for electrocatalyzing the oxygen evolution reaction, making it imperative to understand their chemical/electronic structure. However, the complexity of iridium oxide's electronic structure makes it particularly difficult to experimentally determine the chemical state of the active surface species. To achieve an accurate understanding of the electronic structure of iridium oxide surfaces, we have combined synchrotron-based X-ray photoemission and absorption spectroscopies with ab initio calculations. Our investigation reveals a pre-edge feature in the O K-edge of highly catalytically active X-ray amorphous iridium oxides that we have identified as O 2p hole states forming in conjunction with Ir(III). These electronic defects in the near-surface region of the anionic and cationic framework are likely critical for the enhanced activity of amorphous iridium oxides relative to their crystalline counterparts.
In this paper we present a comprehensive study on low hydration Ir/IrO2 electrodes, made of an Ir core and a IrO2 shell, that are designed and synthetized with an innovative, green approach, in order to have a higher surface/bulk ratio of Ir-O active centers. Three materials with different hydration degrees have been deeply investigated in terms of structure and microstructure by means of Transmission Electron Microscopy (TEM) and Synchrotron Radiation techniques such as High Resolution (HR) and Pair Distribution Function (PDF) quality X-Ray Powder Diffraction (XRPD), X-Ray Absorption Spectroscopy (XAS), and for what concerns their electrochemical properties by means of cyclic voltammetry and steady state I/E curves. The activity of these materials is compared and discussed in the light of our most recent results on hydrous IrOx. The main conclusion of this study is that the Ir core is non-interacting with the IrOx shell, the latter being able to easily accommodate Ir in different oxidation states, as previously suggested for the hydrated form, thus explaining the activity as electrocatalysts. In addition, in-operando XAS experiments assessed that the catalytic cycle involves Ir(III) and (V), as previously established for the highly hydrated IrOx material.
The interaction of chemical vapor deposition (CVD)-grown graphene films with water was studied by means of in situ angle-dependent X-ray absorption spectroscopy (XAS). We found that when the graphene layer is in contact with water there is a reduction in the π* peak intensity in the carbon K-edge absorption spectra, accompanied by an extension of the σ* peak to lower energies, which are indicative of chemical modifications of the graphene and a reduction in the number of unsaturated carbon bonds due to the covalent attachment of contaminant species. In addition to the chemical changes a decrease in the dichroic ratio measured by polarized XAS measurements was observed, which indicates an increase on the nanometer scale corrugation. These changes can strongly influence the electronic properties, mechanical robustness, and resistance to sloughing, as well as graphene reactivity.
Active and highly stable oxide-supported IrNiOx core–shell catalysts for electrochemical water splitting are presented. IrNix@IrOx nanoparticles supported on high-surface-area mesoporous antimony-doped tin oxide (IrNiOx /Meso-ATO) were synthesized from bimetallic IrNix precursor alloys (PA-IrNix /Meso-ATO) using electrochemical Ni leaching and concomitant Ir oxidation. Special emphasis was placed on Ni/NiO surface segregation under thermal treatment of the PA-IrNix /Meso-ATO as well as on the surface chemical state of the particle/oxide support interface. Combining a wide array of characterization methods, we uncovered the detrimental effect of segregated NiO phases on the water splitting activity of core–shell particles. The core–shell IrNiOx /Meso-ATO catalyst displayed high water-splitting activity and unprecedented stability in acidic electrolyte providing substantial progress in the development of PEM electrolyzer anode catalysts with drastically reduced Ir loading and significantly enhanced durability.
We discuss the generation of a library of projector augmented-wave (PAW) and ultrasoft pseudopotentials (PPs) for all elements of the periodic table from H to Pu. The PPs are compared with two libraries: pslibrary.0.3.1 and the GBRV library (Garrity et al., 2013). The PPs are tested on the lattice constants of the fcc and bcc structures of the 63 elements of the GBRV library. The same parameters are used to generate fully relativistic PPs that are compared with the scalar relativistic PPs. The PPs of lanthanides and actinides are tested on all-electron data available in the literature.
A comparative investigation was performed to examine the intrinsic catalytic activity and durability of carbon supported Ru, Ir, and Pt nanoparticles and corresponding bulk materials for the electrocatalytic oxygen evolution reaction (OER). The electrochemical surface characteristics of nanoparticles and bulk materials were studied by surface-sensitive cyclic voltammetry. Although basically similar voltammetric features were observed for nanoparticles and bulk materials of each metal, some differences were uncovered highlighting the changes in oxidation chemistry. On the basis of the electrochemical results, we demonstrated that Ru nanoparticles show lower passivation potentials compared to bulk Ru material. Ir nanoparticles completely lost their voltammetric metallic features during the voltage cycling, in contrast to the corresponding bulk material. Finally, Pt nanoparticles show an increased oxophilic nature compared to bulk Pt. With regard to the OER performance, the most pronounced effects of nanoscaling were identified for Ru and Pt catalysts. In particular, the Ru nanoparticles suffered from strong corrosion at applied OER potentials and were therefore unable to sustain the OER. The Pt nanoparticles exhibited a lower OER activity from the beginning on and were completely deactivated during the applied OER stability protocol, in contrast to the corresponding bulk Pt catalyst. We highlight that the OER activity and durability were comparable for Ir nanoparticles and bulk materials. Thus, Ir nanoparticles provide a high potential as nanoscaled OER catalyst.
An iridium oxide nanoparticle electrocatalyst under oxygen evolution reaction conditions was probed in situ by ambient-pressure X-ray photoelectron spectroscopy. Under OER conditions, iridium undergoes a change in oxidation state from Ir(IV) to Ir(V) that takes place predominantly at the surface of the catalyst. The chemical change in iridium is coupled to a decrease in surface hydroxide, providing experimental evidence which strongly suggests that the oxygen evolution reaction on iridium oxide occurs through an OOH-mediated deprotonation mechanism.
Combustion emissions adversely impact air quality and human health. A
multiscale air quality model is applied to assess the health impacts of
major emissions sectors in United States. Emissions are classified
according to six different sources: electric power generation, industry,
commercial and residential sources, road transportation, marine
transportation and rail transportation. Epidemiological evidence is used
to relate long-term population exposure to sector-induced changes in the
concentrations of PM2.5 and ozone to incidences of premature
death. Total combustion emissions in the U.S. account for about 200,000
(90% CI: 90,000-362,000) premature deaths per year in the U.S. due
to changes in PM2.5 concentrations, and about 10,000 (90% CI:
-1000 to 21,000) deaths due to changes in ozone concentrations.
The largest contributors for both pollutant-related mortalities are road
transportation, causing ˜53,000 (90% CI: 24,000-95,000)
PM2.5-related deaths and ˜5000 (90% CI: -900 to
11,000) ozone-related early deaths per year, and power generation,
causing ˜52,000 (90% CI: 23,000-94,000)
PM2.5-related and ˜2000 (90% CI: -300 to 4000)
ozone-related premature mortalities per year. Industrial emissions
contribute to ˜41,000 (90% CI: 18,000-74,000) early deaths
from PM2.5 and ˜2000 (90% CI: 0-4000) early
deaths from ozone. The results are indicative of the extent to which
policy measures could be undertaken in order to mitigate the impact of
specific emissions from different sectors — in particular black
carbon emissions from road transportation and sulfur dioxide emissions
from power generation.
The electrocatalytic activity of mixed Ru‐Ir oxide electrodes fabricated by thermal decomposition are compared (using cyclic
voltammetry and potentiodynamic technique), for their ability to evolve hydrogen and oxygen in both
and
solutions. Cyclic voltammetry provides information about the redox transitions of surface oxyruthenium and oxyiridium groups,
and also generates an effective index, (voltammetric charge
), which can be used to determine the electrocatalytic activity of the electrode. In this study,
(obtained by numerical integration from CV), indicates that maximum activity results from a coating solution with 60 to 80
mol % Ir content. It is noted that, for acidic solutions, voltammetric charge in the region of hydrogen adsorption/desorption,
, exhibits the same trends as
. The potentiodynamic technique, on the other hand, is employed to yield Tafel plots providing
vs. E relations. It is found that the activity of
is worst for oxygen evolution in alkaline solutions, while its electrochemical behavior for hydrogen evolution is the same
for both acidic and basic solutions. In contrast, it is found that the electrochemical behavior of
for hydrogen evolution is significantly influenced by pH.
The activities of the oxygen evolution reaction (OER) on iridium-oxide- and ruthenium-oxide-based catalysts are among the highest known to date. However, the OER activities of thermodynamically stable rutile iridium oxide (r-IrO2) and rutile iridium oxide (r-RuO2), normalized to catalyst mass or true surface area are not well-defined. Here we report a synthesis of r-IrO2 and r-RuO2 nanoparticles (NPs) of 6 nm, and examine their OER activities in acid and alkaline solutions. Both r-IrO2 and r-RuO2 NPs were highly active for OER, with r-RuO2 exhibiting up to 10 A/goxide at 1.48 V versus reversible hydrogen electrode. When comparing the two, r-RuO2 NPs were found to have slightly higher intrinsic and mass OER activities than r-IrO2 in both acid and basic solutions. Interestingly, these oxide NPs showed higher stability under OER conditions than commercial Ru/C and Ir/C catalysts. Our study shows that these r-RuO2 and r-IrO2 NPs can serve as a benchmark in the development of active OER catalysts for electrolyzers, metal-air batteries, and photoelectrochemical water splitting applications.Keywords: iridium oxide; ruthenium oxide; oxygen evolution; water splitting; electrocatalysis; rutile; nanomaterials; acid; alkaline
Structural and electronic aspects of IrO2 films prepared by electrodeposition on Au substrates were investigated by in situ LIII-edge X-ray absorption and surface enhanced Raman spectroscopies in both acid and alkaline aqueous solutions. Linear correlations were found between the extent of oxidation of Ir3+ in the films determined from a statistical fit of the white line, which includes contributions from each of the sites differing by a single electron, and from coulometric analysis of the voltammetric curves. Analysis of the extended X-ray absorption fine structure (EXAFS) yielded Ir−O bond lengths decreasing in the sequence 2.02, 1.97, and 1.93 Å for Ir3+, Ir4+, and Ir5+ sites, respectively. Whereas SERS provided evidence for the presence of crystalline IrO2 in the highly hydrated films, the lack of intense shells in the Fourier transform of the EXAFS function beyond the nearest oxygen neighbors indicates that the films do not display long-range order.
To develop the proton exchange membrane water electrolyzer (PEMWE) with satisfactory performance and durability for application in a regenerative fuel cell (RFC) system, anode electrocatalysts of Ir, Ru and their oxides were prepared and incorporated into the catalyst coated membranes (CCM) of the PEMWE. Repetitive cyclic voltammetry of the Nafion bonded electrodes, steady state polarization curves and stability testing of the single PEMWE cells were performed to investigate the performance and stability of these electrocatalysts. The structure and specific surface areas of the electrocatalysts were characterized by XRD and BET. IrO2 exhibited a slightly lower performance but a markedly higher stability than the other electrocatalysts. By modifying the preparation process via a colloidal iridium hydroxide hydrate precursor, which permitted a lower heat treatment temperature, its performance was enhanced, corresponding to a terminal voltage of 1.63 V at 1 A/cm2.
Scitation is the online home of leading journals and conference proceedings from AIP Publishing and AIP Member Societies
Photosystem II is the site of photosynthetic water oxidation and contains 20 subunits with a total molecular mass of 350 kDa. The structure of photosystem II has been reported at resolutions from 3.8 to 2.9 Å. These resolutions have provided much information on the arrangement of protein subunits and cofactors but are insufficient to reveal the detailed structure of the catalytic centre of water splitting. Here we report the crystal structure of photosystem II at a resolution of 1.9 Å. From our electron density map, we located all of the metal atoms of the Mn(4)CaO(5) cluster, together with all of their ligands. We found that five oxygen atoms served as oxo bridges linking the five metal atoms, and that four water molecules were bound to the Mn(4)CaO(5) cluster; some of them may therefore serve as substrates for dioxygen formation. We identified more than 1,300 water molecules in each photosystem II monomer. Some of them formed extensive hydrogen-bonding networks that may serve as channels for protons, water or oxygen molecules. The determination of the high-resolution structure of photosystem II will allow us to analyse and understand its functions in great detail.
Water splitting cells with direct semiconductor/liquid contacts are attractive because they avoid significant fabrication and systems costs involved with the use of separate electrolyzers wired to p-n junction solar cells. Another attractive advantage of photoelectrochemical water splitting directly at the semiconductor surface is the ease with which an electric field can be created at a semiconductor/liquid junction. Water splitting cells require semiconductor materials that are able to support rapid charge transfer at a semiconductor/aqueous interface, that exhibit long-term stability, and that can efficiently harvest a large portion of the solar spectrum. In contrast to the use of a single band gap configuration (S2) to split water, the use of a dual band gap (D4) water splitting cell configuration, where the electric field is generated at a semiconductor liquid junction or through a buried junction, appears to be the most efficient and robust use of complementary light absorbing materials.
Iridium half-sandwich complexes of the types Cp*Ir(N-C)X, [Cp*Ir(N-N)X]X, and [CpIr(N-N)X]X are catalyst precursors for the homogeneous oxidation of water to dioxygen. Kinetic studies with cerium(IV) ammonium nitrate as primary oxidant show that oxygen evolution is rapid and continues over many hours. In addition, [Cp*Ir(H(2)O)(3)]SO(4) and [(Cp*Ir)(2)(μ-OH)(3)]OH can show even higher turnover frequencies (up to 20 min(-1) at pH 0.89). Aqueous electrochemical studies on the cationic complexes having chelate ligands show catalytic oxidation at pH > 7; conversely, at low pH, there are no oxidation waves up to 1.5 V vs NHE for the complexes. H(2)(18)O isotope incorporation studies demonstrate that water is the source of oxygen atoms during cerium(IV)-driven catalysis. DFT calculations and kinetic experiments, including kinetic-isotope-effect studies, suggest a mechanism for homogeneous iridium-catalyzed water oxidation and contribute to the determination of the rate-determining step. The kinetic experiments also help distinguish the active homogeneous catalyst from heterogeneous nanoparticulate iridium dioxide.
There is a growing interest in oxygen electrochemistry as conversions between O(2) and H(2)O play an important role in a variety of renewable energy technologies. The goal of this work is to develop active bifunctional catalyst materials for water oxidation and oxygen reduction. Drawing inspiration from a cubane-like CaMn(4)O(x), the biological catalyst found in the oxygen evolving center (OEC) in photosystem II, nanostructured manganese oxide surfaces were investigated for these reactions. Thin films of nanostructured manganese oxide were found to be active for both oxygen reduction and water oxidation, with similar overall oxygen electrode activity to the best known precious metal nanoparticle catalysts: platinum, ruthenium, and iridium. Physical and chemical characterization of the nanostructured Mn oxide bifunctional catalyst reveals an oxidation state of Mn(III), akin to one of the most commonly observed Mn oxidation states found in the OEC.
There is intense interest in graphene in fields such as physics, chemistry, and materials science, among others. Interest in graphene's exceptional physical properties, chemical tunability, and potential for applications has generated thousands of publications and an accelerating pace of research, making review of such research timely. Here is an overview of the synthesis, properties, and applications of graphene and related materials (primarily, graphite oxide and its colloidal suspensions and materials made from them), from a materials science perspective.
Chemistry with its key targets of providing materials and processes for conversion of matter is at the center stage of the energy challenge. Most energy conversion systems work on (bio)chemical energy carriers and require for their use suitable process and material solutions. The enormous scale of their application demands optimization beyond the incremental improvement of empirical discoveries. Knowledge-based systematic approaches are mandatory to arrive at scalable and sustainable solutions. Chemistry for energy, "ENERCHEM" contributes in many ways already today to the use of fossil energy carriers. Optimization of these processes exemplified by catalysis for fuels and chemicals production or by solid-state lightning can contribute in the near future substantially to the dual challenge of energy use and climate protection being in fact two sides of the same challenge. The paper focuses on the even greater role that ENERCHEM will have to play in the era of renewable energy systems where the storage of solar energy in chemical carries and batteries is a key requirement. A multidisciplinary and diversified approach is suggested to arrive at a stable and sustainable system of energy conversion processes. The timescales for transformation of the present energy scenario will be decades and the resources will be of global economic dimensions. ENERCHEM will have to provide the reliable basis for such technologies based on deep functional understanding.
Graphene is a wonder material with many superlatives to its name. It is the thinnest known material in the universe and the
strongest ever measured. Its charge carriers exhibit giant intrinsic mobility, have zero effective mass, and can travel for
micrometers without scattering at room temperature. Graphene can sustain current densities six orders of magnitude higher
than that of copper, shows record thermal conductivity and stiffness, is impermeable to gases, and reconciles such conflicting
qualities as brittleness and ductility. Electron transport in graphene is described by a Dirac-like equation, which allows
the investigation of relativistic quantum phenomena in a benchtop experiment. This review analyzes recent trends in graphene
research and applications, and attempts to identify future directions in which the field is likely to develop.
There is near unanimous scientific consensus that greenhouse gas emissions generated by human activity will change Earth's climate. The recent (globally averaged) warming by 0.5 degrees C is partly attributable to such anthropogenic emissions. Climate change will affect human health in many ways-mostly adversely. Here, we summarise the epidemiological evidence of how climate variations and trends affect various health outcomes. We assess the little evidence there is that recent global warming has already affected some health outcomes. We review the published estimates of future health effects of climate change over coming decades. Research so far has mostly focused on thermal stress, extreme weather events, and infectious diseases, with some attention to estimates of future regional food yields and hunger prevalence. An emerging broader approach addresses a wider spectrum of health risks due to the social, demographic, and economic disruptions of climate change. Evidence and anticipation of adverse health effects will strengthen the case for pre-emptive policies, and will also guide priorities for planned adaptive strategies.
Graphene is a rapidly rising star on the horizon of materials science and condensed-matter physics. This strictly two-dimensional material exhibits exceptionally high crystal and electronic quality, and, despite its short history, has already revealed a cornucopia of new physics and potential applications, which are briefly discussed here. Whereas one can be certain of the realness of applications only when commercial products appear, graphene no longer requires any further proof of its importance in terms of fundamental physics. Owing to its unusual electronic spectrum, graphene has led to the emergence of a new paradigm of 'relativistic' condensed-matter physics, where quantum relativistic phenomena, some of which are unobservable in high-energy physics, can now be mimicked and tested in table-top experiments. More generally, graphene represents a conceptually new class of materials that are only one atom thick, and, on this basis, offers new inroads into low-dimensional physics that has never ceased to surprise and continues to provide a fertile ground for applications.
Metal-oxos are critical intermediates for the management of oxygen and its activation. The reactivity of the metal-oxo is central to the formation of O-O bonds, which is the essential step for oxygen generation. Two basic strategies for the formation of O-O bonds at metal-oxo active sites are presented. The acid-base (AB) strategy involves the attack of a nucleophilic oxygen species (e.g., hydroxide) on an electrophilic metal-oxo. Here, active-site designs must incorporate the assembly of a hydroxide (or water) proximate to a high-valent metal-oxo of even d electron count. For the radical coupling (RC) strategy, two high-valent metal-oxos of an odd d electron count are needed to drive O-O coupling. This Forum Article focuses on the different electronic structures of terminal metal-oxos that support AB and RC strategies and the design of ligand scaffolds that engender these electronic structures.
Al(110) has been studied for temperatures up to 900 K via ensemble density-functional molecular dynamics. The strong anharmonicity displayed by this surface results in a negative coefficient of thermal expansion, where the first interlayer distance decreases with increasing temperature. Very shallow channels of oscillation for the second-layer atoms in the direction perpendicular to the surface support this anomalous contraction, and provide a novel mechanism for the formation of adatom-vacancy pairs, preliminary to the disordering and premelting transition. Such characteristic behavior originates in the free-electron-gas bonding at a loosely packed surface. Comment: 4 pages, two-column style with 5 PostScript figures embedded. Uses RevTeX and epsf macros. To appear in Phys. Rev. Lett. Also available at http://www.physics.rutgers.edu/~marzari/preprints/index.html#al110
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