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Operando XAS Study of the Surface Oxidation State on a Monolayer IrO x on RuO x and Ru Oxide Based Nanoparticles for Oxygen Evolution in Acidic Media
Abstract
Herein we present surface sensitive operando XAS L-edge measurements on IrOx/RuO2 and RuOx thin films as well as mass-selected RuOx and Ru nanoparticles. We observed shifts of the white line XAS peak towards higher energies with applied electrochemical potential. Apart from the case of the metallic Ru nanoparticles, the observed potential dependences were purely core-level shifts caused by a change in oxidation, which indicates no structural changes. These findings can be explained by different binding energies of oxygenated species on the surface of IrOx and RuOx. Simulated XAS spectra show that the average Ir/Ru oxidation state change is strongly affected by the coverage of atomic O. The observed shifts in oxidation suggests that the surface has a high coverage of O at potentials just below the potential where oxygen evolution is exorgenic in free energy. This observation is consistent with the notion that the metal-oxygen bond is stronger than ideal.
... The scarcity and relatively low OER activity of Ir are insufficient to meet industrial requirements 13,14 , while, Ru-based catalysts generally suffer from poor stability on account of the formation of soluble Ru oxides (such as RuO 4 ) during OER process 15,16 . Up to date, the OER catalysts of RuIr bimetallic oxides have been extended from their component-dependence 17-21 , optimization of bimetallic oxide nanostructures (one-dimensional 22 , threedimensional 23 and core-shell structures 24 , etc.) to modification of electronic properties [25][26][27] . Notably, the redox of Ru in RuIr bimetallic oxides could be affected by Ir species in RuIr bimetallic oxides system 13,19 , in which Ru exhibits a strong oxidation state [19][20][21]25,26,28 . ...
... The scarcity and relatively low OER activity of Ir are insufficient to meet industrial requirements 13,14 , while, Ru-based catalysts generally suffer from poor stability on account of the formation of soluble Ru oxides (such as RuO 4 ) during OER process 15,16 . Up to date, the OER catalysts of RuIr bimetallic oxides have been extended from their component-dependence [17][18][19][20][21] , optimization of bimetallic oxide nanostructures (one-dimensional 22 , threedimensional 23 and core-shell structures 24 , etc.) to modification of electronic properties [25][26][27] . Notably, the redox of Ru in RuIr bimetallic oxides could be affected by Ir species in RuIr bimetallic oxides system 13,19 , in which Ru exhibits a strong oxidation state [19][20][21]25,26,28 . ...
... Furthermore, the XANES analyses also reveal that the valence state of Ru increased slightly and the oxidation state of Ir decreased slightly after the stability test compared with before the test ( Supplementary Fig. 22). These may be due to the strong interaction between Ir-Ru and the electron transition behavior that limits the redox elasticity of Ru 13,19-21 , avoiding the generation of more soluble Ru high-valent complexes and also the origin of high stability of the Ru 0.5 Ir 0.5 O 2 [25][26][27] . The morphology and crystal structure of Ru 0.5 Ir 0.5 O 2 after acid OER stability test were further confirmed by morphology characterization. ...
The oxygen evolution reactions in acid play an important role in multiple energy storage devices. The practical promising Ru-Ir based catalysts need both the stable high oxidation state of the Ru centers and the high stability of these Ru species. Here, we report stable and oxidative charged Ru in two-dimensional ruthenium-iridium oxide enhances the activity. The Ru0.5Ir0.5O2 catalyst shows high activity in acid with a low overpotential of 151 mV at 10 mA cm⁻², a high turnover frequency of 6.84 s⁻¹ at 1.44 V versus reversible hydrogen electrode and good stability (618.3 h operation). Ru0.5Ir0.5O2 catalysts can form more Ru active sites with high oxidation states at lower applied voltages after Ir incorporation, which is confirmed by the pulse voltage induced current method. Also, The X-ray absorption spectroscopy data shows that the Ru-O-Ir local structure in two-dimensional Ru0.5Ir0.5O2 solid solution improved the stability of these Ru centers.
... 53,55 The in situ/ex situ X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), soft X-ray absorption spectroscopy (sXAS), 77 and XAS could help explore the oxidation state and the structural change of catalysts before/after operations. 78 Neutron imaging 79 and synchrotron radiography 80 are reported as powerful tools to monitor the bubbles' behavior in PEMWEs with different anode designs. 81 ...
... Kao et al. 86 performed sXAS to probe the molecular-scale structure of electrode interfaces by measuring total electron yield. Pedersen et al. 87 applied operando XAS to study the oxidation state changes of RuO x nanoparticles and IrO x /RuO 2 thin films at high electrochemical potentials. They found that IrO x showed a saturation of oxidation state at high electrochemical potentials and a decrease in oxidation state occurred for the RuO 2 nanoparticles. ...
Benefiting from the merits of short response time, high current density, and differential pressure, the proton exchange membrane water electrolyzer (PEMWE) is attracting increasing attention from both academic and industry researchers. A limiting factor that impedes the widespread application of the PEMWE is its reliance on the rarest elements, such as iridium and platinum. In order to optimize the device performance as well as to reduce the usage of rare elements, it is important but difficult to directly observe the reaction within the electrolyzer under working conditions. Thus, operando characterization methods are urgently needed to probe in real time the water electrolysis process during operation. In this perspective, we highlight the important role and summarize the recent advances of operando characterization methods in obtaining kinetic insights about PEMWEs. Based on the demands of kinetic optimization, an outlook of future characterization methods is given at the end.
... The OER catalysts have been developing for many years and various excellent catalysts have been proposed. Among these catalysts, Iridium (Ir)-based and Ruthenium (Ru)based catalysts have shown the best OER performance in acid [6,7]. For example, Paoli et al. synthesized mass-selected RuO 2 nanoparticles that show 0.6 A mg −1 at 0.25 V overpotential [8]. ...
The development of a new type of oxygen evolution reaction (OER) catalyst to reduce the energy loss in the process of water electrolysis is of great significance to the realization of the industrialization of hydrogen energy storage. Herein, we report the catalysts of NiFe double-layer hydroxide (NiFe-LDH) mixed with different equivalent terephthalic acid (TPA), synthesized by the hydrothermal method. The catalyst synthesized with the use of the precursor solution containing one equivalent of TPA shows the best performance with the current density of 2 mA cm−2 at an overpotential of 270 mV, the Tafel slope of 40 mV dec−1, and excellent stable electrocatalytic performance for OER. These catalysts were characterized in a variety of methods. X-ray diffraction (XRD), Fourier Transform Infrared Spectrometer (FTIR), and Raman spectrum proved the presence of TPA in the catalysts. The lamellar structure and the uniform distribution of Ni and Fe in the catalysts were observed by a scanning electron microscope (SEM) and a transmission electron microscope (TEM). In X-ray photoelectron spectroscopy (XPS) of NiFe-LDH with and without TPA, the changes in the peak positions of Ni and Fe spectra indicate strong electronic interactions between TPA and Ni and Fe atoms. These results suggest that a certain amount of TPA can boost catalytic activity.
... The glassy carbon sheet was synthesized by pyrolyzing 20 mm thick Kapton tape in a N 2 environment at 1000 1C for 1 h. 35 Extended X-ray absorption fine structure (EXAFS) spectra were acquired in transmission mode with the Co film and glassy carbon sheet samples at a 451 angle to the beam. XAS data was processed using the Demeter Software suite and standard data analysis protocols. ...
An illustration of solar-driven synthesis of ammonia using nitrates with >10% solar-to-fuel efficiencies that can potentially decarbonize and decentralize ammonia production.
Highly active and durable electrocatalysts for the oxygen evolution reaction (OER) are crucial for proton exchange membrane water electrolysis (PEMWE). While doped RuO2 catalysts demonstrate good activity and stability, the presence of dopants limits the number of exposed active sites and complicates Ru recovery. Here, we present a monometallic RuO2 (d-RuO2) with lattice hydroxyl in the periodic structure as a high-performance OER electrocatalyst. The obtained d-RuO2 catalyst exhibits a low overpotential of 150 mV and long-term operational stability of 500 h at 10 mA cm–2, outperforming many Ru/Ir-based oxides ever reported. A PEMWE device using d-RuO2 sustains operation for 348 h at 200 mA cm–2. In-situ characterization reveals that the incorporation of lattice hydroxyl increases the Ru–Ru distance, which facilitates the turnover of the Ru oxidation state and promotes the formation of stable edge-sharing [RuO6] octahedra during the OER, thereby accelerating the formation of O–O bonds and suppressing the overoxidation of Ru sites. Additionally, the small particle size of the catalyst decreases the three-phase contact line and promotes bubble release. This study will provide insights into the design and optimization of catalysts for various electrochemical reactions.
When it comes to the ability of the water splitting process to create hydrogen, the challenging dynamics of the water-oxidation reaction (WOR) provide a substantial impediment. So, in order to start both hydrogen and oxygen production during water electrolysis, efficient catalysts are needed. Herein, we report the development of NaBiO2@PPy nanocomposite capable of oxygen evolution reactions (OER) in alkaline medium. The fabricated NaBiO2@PPy nanocomposite and its individuals are characterized via various techniques to confirm the structural, elemental, textural, and morphological characteristics. This material produces a current density of 10 mA/cm2 for oxygen evolution comparatively at low overpotentials of 227 mV and maintains its stability for up to 50 h. Furthermore, the fabricated NaBiO2@PPy nanocomposite also shows less Tafel slope of 30.8 mV/dec. Therefore, this catalytic material would be a good option for creating a cost-effective, and environmentally friendly alkaline medium. In order to enhance the performance of water electrolysis, this work offers a revolutionary designed material and electrode fabrication method for future applications.
One challenge remaining in the development of Ir-based electrocatalyst is the activity-stability paradox during acidic oxygen evolution reaction (OER), especially for the surface reconstructed IrOx catalyst with high efficiency. To address this, a phase selective Ir-based electrocatalyst is constructed by forming bridged W-O moiety in NiIrOx electrocatalyst. Through an electrochemical dealloying process, an nano-porous structure with surface-hydroxylated rutile NiWIrOx electrocatalyst is engineered via Ni as a sacrificial element. Despite low Ir content, NiWIrOx demonstrates a minimal overpotential of 180 mV for the OER at 10 mA·cm⁻². It maintains a stable 300 mA·cm⁻² current density during an approximately 300 h OER at 1.8 VRHE and shows a stability number of 3.9 × 10⁵ noxygen · nIr⁻¹. The resulting W – O–Ir bridging motif proves pivotal for enhancing the efficacy of OER catalysis by facilitating deprotonation of OER intermediates and promoting a thermodynamically favorable dual-site adsorbent evolution mechanism. Besides, the phase selective insertion of W-O in NiIrOx enabling charge balance through the W-O-Ir bridging motif, effectively counteracting lattice oxygen loss by regulating Ir-O co-valency.
The reduction of greenhouse gas emissions and the shift away from petrochemical-derived materials are critical goals in modern industrial development and societal progress. Addressing these intertwined challenges demands innovative and sustainable solutions. Here, we present the first example of synthesizing poly[R-(–)-3-hydroxybutyrate] (PHB) from atmospheric CO2, utilizing a streamlined and integrated process that combines both chemo- and bio-catalytic conditions. Central to our approach is the development of an immobilized catalytic system that efficiently converts atmospheric CO2 into sodium formate, establishing a sustainable carbon source for formatotrophic organisms. Through Adaptive Laboratory Evolution (ALE), we enhanced the growth rate of the bacterium Cupriavidus necator H16, enabling it to utilize formic acid and formate as the sole carbon and energy sources. The evolved strain, C. necator ALE26, achieved a 1.8-fold increase in the maximum growth rate (μmax = 0.25 ± 0.02 h−1), attributed to the loss of the megaplasmid pHG1. Employing the adapted strain, we report the highest PHB production rate in continuous fermentation using C. necator for growth on formate. The development of the different stages (sorption and chemo- and bio-transformation) under compatible conditions that minimize the number of work-up stages demonstrates a major advancement in converting atmospheric CO2 into valuable biopolymers, thus simultaneously contributing to the reduction of greenhouse gases in the atmosphere and to a circular economy of biobased polymers that diminish fossil fuel dependence.
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.
Understanding the structure–property relationship in electrocatalysts under working conditions is crucial for the rational design of novel and improved catalytic materials. This paper presents the Aarhus University reactor for electrochemical studies using X-rays (AUREX) operando electrocatalytic flow cell, designed as an easy-to-use versatile setup with a minimal background contribution and a uniform flow field to limit concentration polarization and handle gas formation. The cell has been employed to measure operando total scattering, diffraction and absorption spectroscopy as well as simultaneous combinations thereof on a commercial silver electrocatalyst for proof of concept. This combination of operando techniques allows for monitoring of the short-, medium- and long-range structure under working conditions, including an applied potential, liquid electrolyte and local reaction environment. The structural transformations of the Ag electrocatalyst are monitored with non-negative matrix factorization, linear combination analysis, the Pearson correlation coefficient matrix, and refinements in both real and reciprocal space. Upon application of an oxidative potential in an Ar-saturated aqueous 0.1 M KHCO3/K2CO3 electrolyte, the face-centered cubic (f.c.c.) Ag gradually transforms first to a trigonal Ag2CO3 phase, followed by the formation of a monoclinic Ag2CO3 phase. A reducing potential immediately reverts the structure to the Ag (f.c.c.) phase. Following the electrochemical-reaction-induced phase transitions is of fundamental interest and necessary for understanding and improving the stability of electrocatalysts, and the operando cell proves a versatile setup for probing this. In addition, it is demonstrated that, when studying electrochemical reactions, a high energy or short exposure time is needed to circumvent beam-induced effects.
The integration of chirality, specifically through the chirality-induced spin selectivity (CISS) effect, into electrocatalytic processes represents a pioneering approach for enhancing the efficiency of energy conversion and storage systems.
Hierarchical fluoride-containing zeolite HY (Hie-HY(F)) is an interesting material because it tunes both a high mesoporosity and a moderate acid property, which could benefit the transformation of xylose to xylitol. Herein, we synthesized a new Hie-HY(F) by etching NH4Y with ammonium fluoride (NH4F) under ultrasonication at 60 °C for 20, 40, and 60 min. After etching with NH4F solution, the Hie-HY(F)s contained fluoride in the zeolite frameworks with high mesoporosity and moderate acid properties. The sonication time longer than 20 min caused the structure to collapse rather than the creation of mesoporosity. The Hie-HY(F)s were further used as supports for the Ru catalyst. When loaded with Ru precursor and calcined in air, RuO2 phase was formed in all catalysts. The RuO2 supported on Hie-HY(F)s had a lower reduction temperature than that on HY. Ru supported on Hie-HY(F)-20 gave the reactivity of xylose conversion to xylitol, the highest yield of 80.1 %, providing the reusability up to four cycles.
The slow kinetic process of oxygen evolution reaction (OER) and poor electrochemical stability in acidic environments of electrocatalysts seriously restrict the efficiency of hydrogen production from proton exchange membrane water...
The proton exchange membrane water electrolysis (PEMWE) powered by renewable electricity offers a facile route for clean hydrogen production. The oxygen evolution reaction (OER) at the anode plays a major role in affecting the overall device efficiency due to its sluggish OER kinetics. Thus, it remains a challenge to develop robust and active catalysts for OER in acid media for efficient PEMWE. Currently, iridium (Ir)‐based materials, such as mono‐ and multimetallic Ir, Ir‐based oxides, pyrochlore iridate oxides, and Ir‐based perovskites, are the most promising OER catalysts in acid media for PEMWEs. Extensive research has been conducted to enhance the specific activity of Ir species to make cost‐effective. The present review aims to provide the recent progress on addressing the long‐term durability issue of Ir‐based catalyst for OER in acidic conditions, aspiring to inspire the researchers to design highly efficient and stable Ir‐based catalysts. Moreover, the detailed OER mechanism along with the dissolution nature of Ir species is discussed and summarized. Finally, the status, challenges, and prospects for the development of Ir‐based OER catalysts are discussed.
In this study, we address the significant challenge of overcoming limitations in the catalytic efficiency for the oxygen evolution reaction (OER). The current linear scaling relationships hinder the optimization of the electrocatalytic performance. To tackle this issue, we investigate the potential of designing single-atom catalysts (SACs) on Mo2CO2 MXenes for electrochemical OER using first-principles modeling simulations. By employing the Electrochemical Step Symmetry Index (ESSI) method, we assess OER intermediates to fine-tune the activity and identify the optimal SAC for Mo2CO2 MXenes. Our findings reveal that both Ag and Cu exhibit effectiveness as single atoms for enhancing OER activity on Mo2CO2 MXenes. However, among the 21 chosen transition metals (TMs) in this study, Cu stands out as the best catalyst for tweaking the overpotential (ηOER). This is due to Cu's lowest overpotential compared to other TMs, which makes it more favorable for the OER performance. On the other hand, Ag is closely aligned with ESSI = ηOER, making the tuning of its overpotential more challenging. Furthermore, we employ symbolic regression analysis to identify the significant factors that exhibit a correlation with the OER overpotential. By utilizing this approach, we derive mathematical formulas for the overpotential and identify key descriptors that affect the catalytic efficiency in the electrochemical OER on Mo2CO2 MXenes. This comprehensive investigation not only sheds light on the potential of MXenes in advanced electrocatalytic processes but also highlights the prospect of improved activity and selectivity in OER applications.
Oxygen evolution reaction (OER) under acidic conditions becomes of significant importance for the practical use of a proton exchange membrane (PEM) water electrolyzer. In particular, maximizing the mass activity of iridium (Ir) is one of the maiden issues. Herein, the authors discover that the Ir-doped calcium copper titanate (CaCu₃Ti₄O₁₂, CCTO) perovskite exhibits ultrahigh mass activity up to 1000 A gIr -1 for the acidic OER, which is 66 times higher than that of the benchmark catalyst, IrO2 . By substituting Ti with Ir in CCTO, metal-oxygen (M-O) covalency can be significantly increased leading to the reduced energy barrier for charge transfer. Further, highly polarizable CCTO perovskite referred to as "colossal dielectric", possesses low defect formation energy for oxygen vacancy inducing a high number of oxygen vacancies in Ir-doped CCTO (Ir-CCTO). Electron transfer occurs from the oxygen vacancies and Ti to the substituted Ir consequentially resulting in the electron-rich Ir and -deficient Ti sites. Thus, favorable adsorptions of oxygen intermediates can take place at Ti sites while the Ir ensures efficient charge supplies during OER, taking a top position of the volcano plot. Simultaneously, the introduced Ir dopants form nanoclusters at the surface of Ir-CCTO, which can boost catalytic activity for the acidic OER.
Heteroatom-doping is a practical means to boost RuO2 for acidic oxygen evolution reaction (OER). However, a major drawback is conventional dopants have static electron redistribution. Here, we report that Re dopants in Re0.06Ru0.94O2 undergo a dynamic electron accepting-donating that adaptively boosts activity and stability, which is different from conventional dopants with static dopant electron redistribution. We show Re dopants during OER, (1) accept electrons at the on-site potential to activate Ru site, and (2) donate electrons back at large overpotential and prevent Ru dissolution. We confirm via in situ characterizations and first-principle computation that the dynamic electron-interaction between Re and Ru facilitates the adsorbate evolution mechanism and lowers adsorption energies for oxygen intermediates to boost activity and stability of Re0.06Ru0.94O2. We demonstrate a high mass activity of 500 A gcata.−1 (7811 A gRe-Ru−1) and a high stability number of S-number = 4.0 × 106 noxygen nRu−1 to outperform most electrocatalysts. We conclude that dynamic dopants can be used to boost activity and stability of active sites and therefore guide the design of adaptive electrocatalysts for clean energy conversions. RuO2 is a promising anode catalyst for proton exchange membrane water electrolyzers but suffers from poor catalytic stability. Here the authors present a rhenium-doped RuO2 with a unique dynamic electron accepting-donating that adaptively boosts activity and stability in acidic water oxidation.
The storage of renewable energy is a pressing challenge to overcome in the transition towards a power grid based on plentiful, yet intermittent energy supplies. The renewables-driven electrolysis ofwater to formhydrogen fuel is an attractive avenue, but requires better oxygen-evolution reaction (OER) catalysts to be feasible at scale. RuO2 is touted as one of the superior OER catalysts, but only under acidic conditions – RuO2 electrocatalysts suffer from poor stability under alkaline conditions. In this work, we evaluate three photodeposited RuO2 OER electrocatalysts, all prepared via a scalable photodeposition method. Based on electrochemical and spectroscopic studies (x-ray photoelectron spectroscopy and X-ray absorption spectroscopy) our main findings are that nanocrystalline RuO2 catalysts outperform their amorphous counterpart, and are stable under alkaline (0.1 M KOH) conditions. This works thus lifts a major hurdle towards the use of RuO2 for alkaline water electrolysis.
We design a hexagram-like 1D/2D van der Waals heterostructure composed of regularly crosslinked aligned 1D Bi2S3 nanowires on 2D Bi2Se3 nanoplates, termed Bi2S3/Bi2Se3 vdWHs, for used as anode materials for potassium ion batteries (PIBs) and hybrid capacitors (PIHCs). Thanks to the mixed dimensional topological heterostructures, the abundant network-contacted heterojunctions facilitate ordered ion/electron transport around the surface network and interior topological materials, and simultaneously promoting the K⁺ diffusion, electron transfer, and electrolyte infiltration. The Bi2S3/Bi2Se3 vdWHs deliver an attractive specific capacity over 600 mA h g⁻¹ at 50 mA g⁻¹, a high-rate capability up to 2500 mA g⁻¹, and excellent cycling stability. Theoretical models, in tandem with operando X-ray diffraction and HRTEM analysis reveal the behavior of heterogeneous interfacial reaction in terms of the trapping ability and diffusion kinetics, confirming the reversible conversion reaction of Bi2S3/Bi2Se3 vdWHs. Finally, the full cells of PIBs and PIHCs coupled with Bi2S3/Bi2Se3 vdWHs anodes exhibit excellent performances of 208 and 106 Wh kg⁻¹ over 850 and 3000 cycles, respectively, demonstrating their feasibility towards practical applications. Our study provides a new insight into architectural strategies for heterogeneous interfaces to realize intelligent kinetic control strategies of chalcogenide topological materials for advanced energy storage.
Water oxidation is the step limiting the efficiency of electrocatalytic hydrogen production from water. Spectroelectrochemical analyses are employed to make a direct comparison of water oxidation reaction kinetics between a molecular catalyst, the dimeric iridium catalyst [Ir2(pyalc)2(H2O)4-(μ-O)]2+ (IrMolecular, pyalc = 2-(2'pyridinyl)-2-propanolate) immobilized on a mesoporous indium tin oxide (ITO) substrate, with that of an heterogeneous electrocatalyst, an amorphous hydrous iridium (IrOx) film. For both systems, four analogous redox states were detected, with the formation of Ir(4+)-Ir(5+) being the potential-determining step in both cases. However, the two systems exhibit distinct water oxidation reaction kinetics, with potential-independent first-order kinetics for IrMolecular contrasting with potential-dependent kinetics for IrOx. This is attributed to water oxidation on the heterogeneous catalyst requiring co-operative effects between neighboring oxidized Ir centers. The ability of IrMolecular to drive water oxidation without such co-operative effects is explained by the specific coordination environment around its Ir centers. These distinctions between molecular and heterogeneous reaction kinetics are shown to explain the differences observed in their water oxidation electrocatalytic performance under different potential conditions.
Carefully assessing the energetics along the pathway of the oxygen evolution reaction (OER), our computational study reveals that the “classical” OER mechanism on the (110) surface of iridium dioxide (IrO2) must be reconsidered. We find that the OER follows a bi-nuclear mechanism with adjacent top surface oxygen atoms as fixed adsorption sites, whereas the iridium atoms underneath play an indirect role and maintain their saturated 6-fold oxygen coordination at all stages of the reaction. The oxygen molecule is formed, via an Ir-OOOO-Ir transition state, by association of the outer oxygen atoms of two adjacent Ir-OO surface entities, leaving two intact Ir-O entities at the surface behind. This is drastically different from the commonly considered mono-nuclear mechanism where the O2 molecule evolves by splitting of the Ir-O bond in an Ir-OO entity. We regard the rather weak reducibility of crystalline IrO2 as the reason for favoring the novel pathway, which allows the Ir-O bonds to remain stable and explains the outstanding stability of IrO2 under OER conditions. The establishment of surface oxygen atoms as fixed electrocatalytically active sites on a transition-metal oxide represents a paradigm shift for the understanding of water oxidation electrocatalysis, and it reconciles the theoretical understanding of the OER mechanism on iridium oxide with recently reported experimental results from operando X-ray spectroscopy. The novel mechanism provides an efficient OER pathway on a weakly reducible oxide, defining a new strategy towards the design of advanced OER catalysts with combined activity and stability.
The efficiency of the synthesis of renewable fuels and feedstocks from electrical sources is limited, at present, by the sluggish water oxidation reaction. Single-atom catalysts (SACs) with a controlla-ble coordination environment and exceptional atom utilization efficiency open new paradigms toward designing high-performance water oxidation catalysts. Here, using operando X-ray absorption spectroscopy measurements with calculations of spectra and elec-trochemical activity, we demonstrate that the origin of water oxidation activity of IrNiFe SACs is the presence of highly oxidized Ir single atom (Ir 5.3+) in the NiFe oxyhydroxide under operating conditions. We show that the optimal water oxidation catalyst could be achieved by systematically increasing the oxidation state and modulating the coordination environment of the Ir active sites anchored atop the NiFe oxyhydroxide layers. Based on the proposed mechanism, we have successfully anchored Ir single-atom sites on NiFe oxyhydroxides (Ir 0.1 /Ni 9 Fe SAC) via a unique in situ cryogenic-photochemical reduction method that delivers an over-potential of 183 mV at 10 mA · cm −2 and retains its performance following 100 h of operation in 1 M KOH electrolyte, outperform-ing the reported catalysts and the commercial IrO 2 catalysts. These findings open the avenue toward an atomic-level understanding of the oxygen evolution of catalytic centers under in operando conditions. highly oxidized Ir sites | water oxidation | operando X-ray absorption spectroscopy | DFT calculations E fficient and cost-effective electrocatalysts play critical roles in energy conversion and storage and the societal pursuit of sustainable energy (1-3). The water oxidation reaction, also known as oxygen evolution reaction (OER), in particular, is an enabling process for diverse clean energy technologies, including water splitting (4-6), solar fuels (2), CO 2 reduction (7), and rechargeable metal-air batteries (8). Unfortunately, the kinetics of the OER are sluggish, which limits the power conversion efficiency and the overall efficiency. Very recently, higher valence transition metal ions such as Co 4+ (9-11), Ni 4+ (12-14), and Fe 4+/5+ (15, 16) generated through a potential-dependent deprotonation reaction have been incorporated into metal oxides/hydroxides, resulting in enhanced water oxidation activity. Incorporating precious metals such as Ir, Ru, and Pt is much less explored, but they offer greater opportunities due to their tendency to form single-atom sites. Computational work has predicted either direct substitution of Ni 4+ and Fe 4+ by Ir 4+ and Ru 4+ or that the metal would be close to its most stable +4 oxidation state based on the high stability of the rutile phase (17-20). On the other hand, the high activity of Sr-leached SrIrO 3 /IrO x and Li-removed Li x IrO 3 catalysts (5, 21, 22) and a recent prediction of highly active and high-oxidation homogeneous water oxidation systems (23) indicate that increased oxidation may lead to improved activity if the active site can be stabilized under operating conditions. Single-atom catalysts (SACs) have offered an ideal system to precisely control the local coordination environments and oxidation states of the single-site centers (24-29). The single-atom nature of these active centers leads to well-defined coordination environments and enhanced metal-support interactions, which provide remarkable catalytic performance in a number of heterogeneous reactions, including in the water-gas shift reaction (24), heterogeneous reduction of CO 2 processes (27), CO oxidation (28), and oxygen reduction reaction (29). We adopted this strategy and sought to incorporate high-oxidation Ir metal sites into the support to enhance the water oxidation activity. Here, we developed an in situ cryogenic-photochemical reduction method for anchoring Ir single sites on the NiFe oxy-hydroxide support. Density functional theory (DFT) calculations predict unusually stable IrO 6 octahedral SAC anchored atop the Significance The efficiency with which renewable fuels and feedstocks are synthesized from electrical sources is largely limited by the sluggish water oxidation reaction. We show that the optimal water oxidation catalyst could be achieved by systematically modulating the coordination of the Ir active sites using an in situ cryogenic-photochemical reduction synthesis method. We achieved a highly oxidized Ir single site (Ir +5.3) in the best atom utilization by single-atom catalysts on electrochemically stable supports. The origin of water oxidation activity in an Ir single-atom catalyst is revealed experimentally and theoretically. The concept and strategy of this work are expected to pioneer novel approaches to engineer single-atom catalysts. ENGINEERING Downloaded from https://www.pnas.org by Stanford Libraries on April 22, 2022 from IP address 171.66.12.195.
Cobalt oxide decorated amorphous carbon/cobalt silicate (denoted as Co-rich C/CoSi) is developed and proved as a potential OER catalyst. The three-dimensional (3D) Co-rich C/CoSi composite is synthesized using the renewable bamboo leaves as raw materials and silicon suppliers undergoing the calcination, hydrothermal and adsorption processes. The decoration of Co can greatly improve the OER performance of carbon/cobalt silicate (C/CoSi). The overpotential of Co-rich C/CoSi is 349 mV at 10 mA·cm⁻² and Tafel slope is 77 mV·dec⁻¹, which are lower than those of C/CoSi (448 mV and 84 mV·dec⁻¹). The values are even superior to many reported transition metal-based electrocatalysts. Furthermore, the Co-rich C/CoSi shows large electroactive surface area and long stability (98.7% OER capacity retention after 15 hours). This work not only develops a strategy to synthesize 3D Co-rich C/CoSi composite, but also demonstrates the decoration of Co can significantly enhance the OER performance of C/CoSi.
Out-of-equilibrium processes are ubiquitous across living organisms and all structural hierarchies of life. At the molecular scale, out-of-equilibrium processes (for example, enzyme catalysis, gene regulation, and motor protein functions) cause biological macromolecules to sample an ensemble of conformations over a wide range of time scales. Quantifying and conceptualizing the structure–dynamics to function relationship is challenging because continuously evolving multidimensional energy landscapes are necessary to describe nonequilibrium biological processes in biological macromolecules. In this perspective, we explore the challenges associated with state-of-the-art experimental techniques to understanding biological macromolecular function. We argue that it is time to revisit how we probe and model functional out-of-equilibrium biomolecular dynamics. We suggest that developing integrated single-molecule multiparametric force–fluorescence instruments and using advanced molecular dynamics simulations to study out-of-equilibrium biomolecules will provide a path towards understanding the principles of and mechanisms behind the structure–dynamics to function paradigm in biological macromolecules.
Reducing the noble metal loading of oxygen evolution reaction (OER) electrocatalyst for polymer electrolyte membrane electrolyzers is a necessity for enabling their large‐scale deployment for hydrogen production. This work introduces a highly active and stable OER catalyst which is based on a core‐shell architecture by covering a monodispersed layer of iridium nanoparticles on the core of TiO2 using a one‐step polyol reduction method. The monolayer of iridium coating is substantiated by analyzing the accurate loading from inductively coupled plasma mass spectrometry (ICP‐MS) combined with morphology/size from transmission electron microscopy (TEM) and the theoretical model of the most‐dense arrangement of iridium nanoparticles on TiO2 surface. The conductive network constructed by interconnected iridium nanoparticles on the surface together with full participation of iridium atoms in the catalytic reaction benefitting from the less than 2 nm size and monolayer adsorption of iridium nanoparticles endow the catalyst with extremely high OER activity (486 A gIr⁻¹ at 1.5 V vs RHE) and stability (100 h at 10 mA cm⁻²). This work shows a new way to prepare iridium‐based electrocatalysts with high mass activity.
Significance
The efficiency with which renewable fuels and feedstocks are synthesized from electrical sources is largely limited by the sluggish water oxidation reaction. We show that the optimal water oxidation catalyst could be achieved by systematically modulating the coordination of the Ir active sites using an in situ cryogenic–photochemical reduction synthesis method. We achieved a highly oxidized Ir single site (Ir +5.3 ) in the best atom utilization by single-atom catalysts on electrochemically stable supports. The origin of water oxidation activity in an Ir single-atom catalyst is revealed experimentally and theoretically. The concept and strategy of this work are expected to pioneer novel approaches to engineer single-atom catalysts.
The voltage reversal of water electrolyzers and fuel cells induces a large positive potential on the hydrogen electrodes, followed by severe system degradation. Applying a reversible multifunctional electrocatalyst to the hydrogen electrode is a practical solution. Ir exhibits excellent catalytic activity for hydrogen evolution reactions (HER), and hydrogen oxidation reactions (HOR), yet irreversibly converts to amorphous IrO x at potentials > 0.8 V/RHE, which is an excellent catalyst for oxygen evolution reactions (OER), yet a poor HER and HOR catalyst. Harnessing the multifunctional catalytic characteristics of Ir, here we design a unique Ir-based electrocatalyst with high crystallinity for OER, HER, and HOR. Under OER operation, the crystalline nanoparticle generates an atomically-thin IrO x layer, which reversibly transforms into a metallic Ir at more cathodic potentials, restoring high activity for HER and HOR. Our analysis reveals that a metallic Ir subsurface under thin IrO x layer can act as a catalytic substrate for the reduction of Ir ions, creating reversibility. Our work not only uncovers fundamental, uniquely reversible catalytic properties of nanoparticle catalysts, but also offers insights into nanocatalyst design.
Water oxidation, or the oxygen evolution reaction (OER), which combines two oxygen atoms from two water molecules and releases one oxygen molecule, plays the key role by providing protons and electrons needed for the hydrogen generation, electrochemical carbon dioxide reduction, and nitrogen fixation. The multielectron transfer OER process involves multiple reaction intermediates, and a high overpotential is needed to overcome the sluggish kinetics. Among the different water splitting devices, proton exchange membrane (PEM) water electrolyzer offers greater advantages. However, current anode OER electrocatalysts in PEM electrolyzers are limited to precious iridium and ruthenium oxides. Developing highly active, stable, and precious‐metal‐free electrocatalysts for water oxidation in acidic media is attractive for the large‐scale application of PEM electrolyzers. In recent years, various types of precious‐metal‐free catalysts such as carbon‐based materials, earth‐abundant transition metal oxides, and multiple metal oxide mixtures have been investigated and some of them show promising activity and stability for acidic OER. In this review, the thermodynamics of water oxidation, Pourbaix diagram of metal elements in aqueous solution, and theoretical screening and prediction of precious‐metal‐free electrocatalysts for acidic OER are first elaborated. The catalytic performance, reaction kinetics, and mechanisms together with future research directions regarding acidic OER are summarized and discussed. Water oxidation also known as the oxygen evolution reaction (OER) plays the key role by providing protons and electrons needed for hydrogen generation and carbon dioxide reduction. This review comprehensively summarizes the thermodynamics, Pourbaix diagram, theoretical screening and prediction, catalytic performance, as well as reaction kinetics and mechanisms of precious‐metal‐free electrocatalysts for acidic OER.
Water splitting is regarded as the essential reaction for converting intermittent energy into hydrogen fuel. Sustainability of the oxygen-evolving electrode determines efficient and economic hydrogen production. Using Ir derivatives in an electrolyte, we report an in situ catalyst regenerative deposition which substantially prolongs the lifetime of an IrOx oxygen-evolving electrode. Taking pH-dependent and reversible conversion between hexahydroxyiridate (IV) ([Ir(OH)6]²⁻) and IrOx, an IrOx-regenerative deposition concurrent with oxygen evolution reaction (OER) is demonstrated. Time of electrolysis and overpotential of the IrOx electrode are studied as a function of pH of the Ir species contained in alkaline electrolyte. The regenerative deposition process is verified by ex situ cyclic voltammetry. The transmission electron microscope, zeta potential, and diffusivity studies suggest the prominent role of hydrous [Ir(OH)6]²⁻-adsorbed IrOx nanoparticles in determining the efficiency of catalyst redeposition. Per the behaviour, a sustainable oxygen-evolving electrode succeeds in acidic electrolyte to exhibit an approximately 1390-fold enhancement in durability compared with a system without Ir species. Under the presence of the Ir species, lower catalyst dissolution rate is observed. The design offers an approach for efficient utilization of IrOx catalysts towards practical hydrogen production.
The proton exchange membrane (PEM) water electrolysis is one of the most promising hydrogen production techniques. The oxygen evolution reaction (OER) occurring at the anode dominates the overall efficiency. Developing active and robust electrocatalysts for OER in acid is a longstanding challenge for PEM water electrolyzers. Most catalysts show unsatisfied stability under strong acidic and oxidative conditions. Such a stability challenge also leads to difficulties for a better understanding of mechanisms. This review aims to provide the current progress on understanding of OER mechanisms in acid, analyze the promising strategies to enhance both activity and stability, and summarize the state‐of‐the‐art catalysts for OER in acid. First, the prevailing OER mechanisms are reviewed to establish the physicochemical structure–activity relationships for guiding the design of highly efficient OER electrocatalysts in acid with stable performance. The reported approaches to improve the activity, from macroview to microview, are then discussed. To analyze the problem of instability, the key factors affecting catalyst stability are summarized and the surface reconstruction is discussed. Various noble‐metal‐based OER catalysts and the current progress of non‐noble‐metal‐based catalysts are reviewed. Finally, the challenges and perspectives for the development of active and robust OER catalysts in acid are discussed.
Proton exchange membrane (PEM) water electrolyzers hold great significance for renewable energy storage and conversion. The acidic oxygen evolution reaction (OER) is one of the main roadblocks that hinder the practical application of PEM water electrolyzers. Highly active, cost‐effective, and durable electrocatalysts are indispensable for lowering the high kinetic barrier of OER to achieve boosted reaction kinetics. To date, a wide spectrum of advanced electrocatalysts has been designed and synthesized for enhanced acidic OER performance, though Ir and Ru based nanostructures still represent the state‐of‐the‐art catalysts. In this Progress Report, recent research progress in advanced electrocatalysts for improved acidic OER performance is summarized. First, fundamental understanding about acidic OER including reaction mechanisms and atomic understanding to acidic OER for rational design of efficient electrocatalysts are discussed. Thereafter, an overview of the progress in the design and synthesis of advanced acidic OER electrocatalysts is provided in terms of catalyst category, i.e., metallic nanostructures (Ir and Ru based), precious metal oxides, nonprecious metal oxides, and carbon based nanomaterials. Finally, perspectives to the future development of acidic OER are provided from the aspects of reaction mechanism investigation and more efficient electrocatalyst design.
The oxygen evolution reaction catalyst optimization is hindered because in the desirable acidic conditions the sole active catalysts are RuO2 and IrO2. Thus, the understanding of the factors controlling the reactivity of these materials is mandatory. In this contribution, DFT (PBE-D2) periodic calculations are performed to analyze the catalytic activities of the main ((110), (011), (100) and (001)) IrO2 surfaces. Results show that the reaction only occurs if the Ir=O species on the surfaces exhibit an oxyl character. The water nucleophilic attack mechanism is the most favorable pathway on the (110), (100) and (001) surfaces. In contrast, for the (011) facet the oxo-coupling is preferred. The required overpotentials for the four IrO2 surfaces depend on the feasibility to oxidize the Ir-OH to Ir-O species and this is tuned by the coordination of the unsaturated iridium sites: the (100) and (001) surfaces appear to be more active than the (110) and (011).
Iridium‐based oxides, currently the state‐of‐the‐art oxygen evolution reaction (OER) electrocatalysts in acidic electrolytes, are cost‐intensive materials which undergo significant corrosion under long‐term OER operation. Thus, numerous researchers have devoted their efforts to mitigate iridium corrosion by decoration with corrosion‐resistant metal oxides and/or supports to maximize OER catalyst durability whilst retaining high activity. Herein a one‐step, facile electrochemical route to obtain improved IrOx thin film OER stability in acid by decorating with amorphous tungsten sulphide (WS3−x) upon electrochemical decomposition of a [WS4]²⁻ aqueous precursor is proposed. The rationale behind applying such WS3−x decoration stems from the generation of a tungsten oxide phase, a well‐known corrosion‐resistant photoactive OER catalyst. The study demonstrates the viability of the proposed WS3−x decoration, allowing the tailoring of experimental parameters responsible for WS3−x nanoparticle size and surface coverage. OER stability tests coupled by ex situ SEM and XPS corroborate the beneficial effect of WS3−x decoration, yielding improved OER specific activity metrics along with minimized Ir surface roughening, a characteristic of electrodissolution. Iridium decoration with electrodeposited, corrosion‐resistant oxides is consequently shown to be a promising route to maximize OER stabilities.
In this work, we employ electronic structure methods to investigate the structure and reactivity of IrOx nanoparticle models as catalysts for the oxygen evolution reaction (OER). Based on the explicit inclusion of the potential and pH in a constant potential framework, a computational approach is applied to investigate the thermodynamics of the proton and electron transfer process of IrOx cluster models. We address structural changes of the clusters under electrochemical conditions by constructing potential–pH diagrams from our computational results. Comparison of two IrOx cluster structures suggests that the charge transport to the clusters strongly depends on the pH. As a result, structures with a maximum number of on-top hydroxyl (OHμ1) species are stable at low potentials and deprotonation becomes favorable with increasing potential. An assessment of the Ir oxidation states in our models shows that mixed oxidation states, i.e., IrIV and IrV, occur around the OER onset potential and increase to higher oxidation states (IrVI) in the high potential regime. Furthermore, an investigation of the water adsorption mechanism is carried out at different potentials.The results suggest that the potential controls the energetics of intermediates as well as transition states during the OER.
Highly-efficient oxygen evolution reaction (OER) and reduction of carbon dioxide (CO2RR) represent the two biggest scientific challenges in artificial photosynthesis. Many efficient and cost-affordable electrocatalysts have been reported in the development of electrochemical OER and CO2RR; however, during the electro-derived oxidation or reduction processes, a critical fact that most catalysts are going to undergo a structural reconstruction and/or surface rearrangement has been widely observed, which greatly subverts the traditional conception of “catalysts”. In this respect, the research trends have gradually transferred from optimizing catalyst materials to elucidating the real active sites of the catalysts as well as understanding the underlying mechanisms behind these complex reactions. Most importantly, the in situ/operando characterization techniques are powerful tools to achieve this goal. Herein, recent advances in the in situ X-ray diffraction and absorption spectroscopy that have provided a unique opportunity to investigate the structural reconstruction and/or surface rearrangement of catalysts under realistic OER and CO2RR conditions are thoroughly reviewed. Finally, the challenges of the material design are discussed, and the future perspective for developing next-generation catalyst with imperative requirements of material nature is provided.
Understanding the reaction mechanism of various heterogeneous catalytic reactions is of fundamental importance in catalysis science. In the past, scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) have proved to be powerful surface-sensitive techniques to characterize surface reactions on model catalysts under UHV conditions. The recent development of high-pressure scanning tunneling microscopy (HP-STM) and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) has largely extended the application of these two excellent surface-sensitive imaging and electron spectroscopy techniques to a variety of catalytic systems under realistic conditions. In this mini review, we will review a series of catalytic systems studied by HP-STM and AP-XPS, including reactant coadsorption systems, coadsorption + reaction systems, and poisoned reaction systems. We will also illustrate one of the main difficulties in the practical execution of experiments where the initial surface cleanliness is easily compromised by the adsorption of adventitious contaminants. All of these examples will demonstrate that the combined use of HP-STM and AP-XPS can provide a deeper understanding of the structure and dynamics of reactant coadsorption on model catalysts, although great care has to been taken to maintain the cleanness of the in situ instrumentation.
The electrochemical reduction of CO2 has gained significant interest recently as it has the potential to trigger a sustainable solar-fuel-based economy. In this Perspective, we highlight several heterogeneous and molecular electrocatalysts for the reduction of CO2 and discuss the reaction pathways through which they form various products. Among those, copper is a unique catalyst as it yields hydrocarbon products, mostly methane, ethylene, and ethanol, with acceptable efficiencies. As a result, substantial effort has been invested to determine the special catalytic properties of copper and to elucidate the mechanism through which hydrocarbons are formed. These mechanistic insights, together with mechanistic insights of CO2 reduction on other metals and molecular complexes, can provide crucial guidelines for the design of future catalyst materials able to efficiently and selectively reduce CO2 to useful products.
Oxygen evolution was investigated on model, mass selected RuO2 nanoparticles in acid, prepared by magnetron sputtering. Our investigations include electrochemical measurements, electron microscopy, scanning tunneling microscopy and X-ray photoelectron spectroscopy. We show that the stability and activity of nanoparticulate RuO2 is highly sensitive to its surface pretreatment. At 0.25 V overpotential, the catalysts show a mass activity of up to 0.6 A/mg and a turnover frequency of 0.65 s-1, one order of magnitude higher than the current state-of-the-art.
The pyrochlore solid solution (Na0.33Ce0.67)2(Ir1−xRux)2O7 (0≤x≤1), containing B-site RuIV and IrIV is prepared by hydrothermal synthesis and used as a catalyst layer for electrochemical oxygen evolution from water at pH<7. The materials have atomically mixed Ru and Ir and their nanocrystalline form allows effective fabrication of electrode coatings with improved charge densities over a typical (Ru,Ir)O2 catalyst. An in situ study of the catalyst layers using XANES spectroscopy at the Ir LIII and Ru K edges shows that both Ru and Ir participate in redox chemistry at oxygen evolution conditions and that Ru is more active than Ir, being oxidized by almost one oxidation state at maximum applied potential, with no evidence for ruthenate or iridate in +6 or higher oxidation states.
Mass-selected nanoparticles can be conveniently produced using magnetron sputtering and aggregation techniques. However, numerous pitfalls can compromise the quality of the samples, e.g. double or triple mass production, dendritic structure formation or unpredicted particle composition. We stress the importance of transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS) for verifying the morphology, size distribution and chemical composition of the nanoparticles. Furthermore, we correlate the morphology and the composition of the PtxY nanoparticles with their catalytic properties for the oxygen reduction reaction. Finally, we propose a completely general diagnostic method, which allows us to minimize the occurrence of undesired masses.
Low-temperature fuel cells are limited by the oxygen reduction reaction, and their widespread implementation in automotive vehicles is hindered by the cost of platinum, currently the best-known catalyst for reducing oxygen in terms of both activity and stability. One solution is to decrease the amount of platinum required, for example by alloying, but without detrimentally affecting its properties. The alloy Pt x Y is known to be active and stable, but its synthesis in nanoparticulate form has proved challenging, which limits its further study. Herein we demonstrate the synthesis, characterization and catalyst testing of model Pt x Y nanoparticles prepared through the gas-aggregation technique. The catalysts reported here are highly active, with a mass activity of up to 3.05 A mg Pt −1 at 0.9 V versus a reversible hydrogen electrode. Using a variety of characterization techniques, we show that the enhanced activity of Pt x Y over elemental platinum results exclusively from a compressive strain exerted on the platinum surface atoms by the alloy core. P olymer electrolyte membrane fuel cells (PEMFCs) hold the potential to provide a zero-emission power source for future automotive applications. However, their widespread commer-cialization is hindered by the high loadings of Pt required to catalyse the oxygen reduction reaction (ORR) at the cathode 1–4 . An order of magnitude increase in ORR mass activity (that is, current density per unit mass Pt) over state-of-the art commercial pure Pt catalysts would bring the precious-metal loading in fuel cells to a similar level to that used for emission control in internal combustion engines 3,5 . Some alloys of PtX (X = Co, Ni, Cu) show higher ORR activity in comparison to pure Pt (refs 1–4), but typically their long-term performance is compromised by their poor stability against dealloying 6,7 . In recent years, progress has been made towards the stabilization of Pt-based catalysts 8–12
We present a multicrystal Johann-type hard x-ray spectrometer (∼5-18 keV) recently developed, installed, and operated at the Stanford Synchrotron Radiation Lightsource. The instrument is set at the wiggler beamline 6-2 equipped with two liquid nitrogen cooled monochromators - Si(111) and Si(311) - as well as collimating and focusing optics. The spectrometer consists of seven spherically bent crystal analyzers placed on intersecting vertical Rowland circles of 1 m of diameter. The spectrometer is scanned vertically capturing an extended backscattering Bragg angular range (88°-74°) while maintaining all crystals on the Rowland circle trace. The instrument operates in atmospheric pressure by means of a helium bag and when all the seven crystals are used (100 mm of projected diameter each), has a solid angle of about 0.45% of 4π sr. The typical resolving power is in the order of EΔE∼10 000. The spectrometer's high detection efficiency combined with the beamline 6-2 characteristics permits routine studies of x-ray emission, high energy resolution fluorescence detected x-ray absorption and resonant inelastic x-ray scattering of very diluted samples as well as implementation of demanding in situ environments.
The platinum 2p3/2, 2p1/2, and 2s x-ray absorption spectra, recorded by monitoring the fluorescence intensity with eV energy resolution, show a spectral broadening that is significantly less than the 2p and 2s core hole lifetime broadening. The background of such spectral sharpening is discussed. It is shown, experimentally as well as theoretically, that the lifetime broadening of the 2p and 2s core holes is replaced by a new lifetime broadening. It is demonstrated that from the combination of normal x-ray absorption, selective x-ray absorption, and x-ray emission, the individual lifetimes of all participating core states can be determined. The difference between the 5d5/2 and 5d3/2 densities of states can be obtained from a combination of the 2p1/2 and 2p3/2 x-ray absorption spectra. The present spectra are limited by the experimental resolution. With the prospect of an improved experimental resolution for x-ray excitation and decay, the Pt edge absorption spectra could be obtained with even better resolution, thus providing a high-resolution hard x-ray probe of the empty density of states with important advantages for in-situ and high-pressure studies.
We report on a source for producing size-selected nanoclusters based on the combination of radio frequency magnetron plasma sputtering and gas condensation. The use of plasma sputtering to vaporize a target is applicable to a large range of materials; Ag, Au, Cu, and Si have been attempted to date. The source, combined with a time-of-flight mass filter, can produce clusters in the size range from 2 up to at least 70 000 atoms, depending on the target material, with a constant mass (M) resolution (M/ΔM ∼ 25) at an intensity that produces atomic monolayer coverage in as little as a few minutes. The source is also attached to an ultrahigh vacuum analysis chamber, which allows in situ surface chemical and structural analysis. Examples of cluster deposition experiments with the source are also presented.
A new mass selection technique has been developed, which allows one to size-select charged particles from atoms to nanoparticles of almost unlimited size. It provides a mass resolution of m/Δm = 20–50 and a transmission of about 50% for the selected size, both independent of mass. The technique is based on the time-of-flight principle, but differs fundamentally from time-of-flight mass selection normally used. The basic idea is to use time-limited high voltage pulses to displace laterally a preaccelerated ion beam, without changing its direction or shape. As the movement of the ions perpendicular to their original beam direction is independent of their forward velocity, mass resolution and calibration does not depend on the ion beam energy. A mass selector of this type has been implemented successfully into a cluster deposition experiment and has proven to be reliable and simple to operate. © 1999 American Institute of Physics.
Hydrous ruthenium oxide (RuO2·xH2O or RuOxHy) is a mixed electron−proton conductor with a specific capacitance as high as 720 F/g/proton, making it a candidate material for energy storage. The correlation between the structure and properties of RuO2·xH2O materials is not well understood due to their amorphous nature and compositional variability. In this study, ruthenium oxides with the compositions RuO2·2.32H2O, RuO2·0.29H2O, and anhydrous RuO2 are characterized using thermogravimetric analysis (TGA), X-ray diffraction (XRD), and X-ray absorption near-edge structure (XANES) and extended X-ray fine structure (EXAFS) analyses. XANES cannot be used to distinguish between Ru(III) and Ru(IV) in the hydrous oxides, but the EXAFS analyses show large differences in the short-range structures of the materials. Whereas anhydrous RuO2 has the rutile structure comprising chains of RuO6 octahedra linked in three dimensions, the structure of RuO2·0.29H2O is rutile-like at the RuO6 core, but less connected and progressively disordered beyond the RuO6 core. The structure of RuO2·2.32H2O is composed of chains of disordered RuO6 octahedra that exhibit no chain-to-chain linking or three-dimensional order. Although the local structures of RuO2·0.29H2O and RuO2·2.32H2O markedly differ, their specific capacitances are large and essentially equivalent, so nonunique local structures can balance effective electron transport (along dioxo bridges) with the effective proton transport (through structural water) necessary for charge storage.
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.
Hydrogen produced by water splitting is a promising solution for a sustained economy from renewable energy sources. Proton exchange membrane (PEM) electrolysis is the utmost suitable technology for this purpose, although the quest for low cost, highly active and durable catalysts is persistent. Here we develop a nanostructured iridium catalyst after electrochemically leaching ruthenium from metallic iridium-ruthenium, Ir0.7Ru0.3Ox (EC), and compare its physical and electrochemical properties to the thermally treated counterpart: Ir0.7Ru0.3O2 (TT). Ir0.7Ru0.3Ox (EC) shows an unparalleled 13-fold higher oxygen evolution reaction (OER) activity compared to the Ir0.7Ru0.3O2 (TT). PEM electrolyzer tests at 1 A cm⁻² show no increase of cell voltage for almost 400 h, proving that Ir0.7Ru0.3Ox (EC) is one of the most efficient anodes so far developed.
A catalyst functions by stabilizing reaction intermediates, usually through surface adsorption. In the oxygen evolution reaction (OER), surface oxygen adsorptions play an indispensable role in the electrocatalysis. The relationship, however, between the adsorption energetics and OER kinetics has not yet been experimentally measured. Herein, we report an experimental relationship between the adsorption of surface oxygen and the kinetics of the OER on IrO2(110) epitaxially grown on a TiO2(110) single crystal. The high quality of the IrO2 film grown using molecular-beam epitaxy affords the ability to extract the surface oxygen adsorption and its impact on the OER. By examining a series of electrolytes, we further find that the adsorption energy changes linearly with pH, which we attribute to the electrified, interfacial water. We support this hypothesis by showing that an interfacial modification via the electrolyte salt can lead to the adsorption energy shift. The dependence of the adsorption energy on pH has implications on the OER kinetics, but it is not the only factor; the dependence of the OER electrocatalysis on pH stipulates two OER mechanisms, one operating in acid and another operating in alkaline. Our work points to the subtle, adsorption-kinetics relationship in the OER, and highlights the importance of the interfacial electrified interaction in electrocatalyst design.
The release of carbon dioxide (CO2) and other greenhouse gases (GHGs) due to human activity is increasing global av-erage surface air temperatures, disrupting weather patterns, and acidifying the ocean (1). Left unchecked, the continued growth of GHG emissions could cause global average tem-peratures to increase by another 4°C or more by 2100 and by 1.5 to 2 times as much in many midcontinent and far northern locations (1). Although our understanding of the impacts of climate change is increasingly and disturbingly clear, there is still debate about the proper course for U.S. policy—a debate that is very much on display during the current presidential transition. But putting near-term poli-tics aside, the mounting economic and scientific evidence leave me confident that trends toward a clean-energy econ-omy that have emerged during my presidency will continue and that the economic opportunity for our country to har-ness that trend will only grow. This Policy Forum will focus on the four reasons I believe the trend toward clean energy is irreversible.
How to efficiently oxidize H2O to O2 (Oxygen Evolution Reaction –OER) in photoelectrochemical cells (PEC) is a great challenge due to its complex charge transfer process, high overpotential, and corrosion. So far no OER mechanism has been fully explained atomistically with both thermodynamic and kinetics. IrO2 is the only known OER catalyst with both high catalytic activity and stability in acidic conditions. This is important because PEC experiments often operate at extreme pH conditions. In this work we performed first principles calculations integrated with implicit solvation at constant potentials to examine the detailed atomistic reaction mechanism of OER at the IrO2 (110) surface. We determined the surface phase diagram, explored the possible reaction pathways including kinetic barriers, and computed reaction rates based on the micro-kinetic models. This allowed us to resolve several long-standing puzzles about the atomistic OER mechanism.
The low efficiency of the electrocatalytic oxidation of water to O2 (oxygen evolution reaction-OER) is considered as one of the major roadblocks for the storage of electricity from renewable sources in form of molecular fuels like H2 or hydrocarbons. Especially in acidic environments, compatible with the powerful proton exchange membrane (PEM), an earth-abundant OER catalyst that combines high activity and high stability is still unknown. Current PEM-compatible OER catalysts still rely mostly on Ir and/or Ru as active components, which are both very scarce elements of the platinum group. Hence, the Ir and/or Ru amount in OER catalysts has to be strictly minimized. Unfortunately, the OER mechanism, which is the most powerful tool for OER catalyst optimization, still remains unclear. In this review, we first summarize the current state of our understanding of the OER mechanism on PEM-compatible heterogeneous electrocatalysts, before we compare and contrast that to the OER mechanism on homogenous catalysts. Thereafter, an overview over monometallic OER catalysts is provided to obtain insights into structure-function relations followed by a review of current material optimization concepts and support materials. Moreover, missing links required to complete the mechanistic picture as well as the most promising material optimization concepts are pointed out.
Oxygen electrochemistry plays a key role in renewable energy technologies such as fuel cells and electrolyzers, but the slow kinetics of the oxygen evolution reaction (OER) limit the performance and commercialization of such devices. Here we report an iridium oxide/strontium iridium oxide (IrOx/SrIrO3) catalyst formed during electrochemical testing by strontium leaching from surface layers of thin films of SrIrO3. This catalyst has demonstrated specific activity at 10 milliamps per square centimeter of oxide catalyst (OER current normalized to catalyst surface area), with only 270 to 290 millivolts of overpotential for 30 hours of continuous testing in acidic electrolyte. Density functional theory calculations suggest the formation of highly active surface layers during strontium leaching with IrO3 or anatase IrO2 motifs.The IrOx/SrIrO3 catalyst outperforms known IrOx and ruthenium oxide (RuOx) systems, the only other OER catalysts that have reasonable activity in acidic electrolyte.
A current challenge faced in water electrolysis is the development of structure–activity relationships for understanding and improving IrOx-based catalysts for the oxygen evolution reaction (OER). We report a simple and scalable modified Adams fusion method for preparing highly OER active, chlorine−free iridium oxide nanoparticles of various size and shape. The applied approach allows for the effects of particle size, morphology, and the nature of the surface species on the OER activity of IrO2 to be investigated. Iridium oxide synthesized at 350 °C from Ir(acac)3, consisting of 1.7 ± 0.4 nm particles with a specific surface area of 150 m² g⁻¹, shows the highest OER activity (E = 1.499 ± 0.003 V at 10 A gox⁻¹). Operando X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) studies indicate the presence of iridium hydroxo (Ir−OH) surface species, which are strongly linked to the OER activity. Preparation of larger IrO2 particles using higher temperatures results in a change of the particle morphology from spherical to rod−shaped particles. A decrease of the intrinsic OER activity was associated with the predominant termination of the rod-shape particles by highly ordered (110) facets in addition to limited diffusion within mesoporous features.
Water splitting is hindered by the sluggish kinetics of the oxygen evolution reaction (OER). The choice of materials for this reaction in acid is limited to the platinum group metals; high loading required of these scarce and expensive elements severely limit the scalability of such technology. Ruthenium oxide is among the best catalysts for OER, however the reported activity and stability can vary tremendously depending on the preparation conditions and pre-treatment. Herein, we investigate the effect of oxidation treatment on mass-selected Ru nanoparticles in the size range between 2 and 10 nm. The effect of two distinct oxidation pre-treatments on the activity and stability have been investigated: (1) thermal oxidation; and (2) oxidation with an oxygen plasma under vacuum. We report that activity and stability can be tuned by using different oxidation pre-treatments. Thermally oxidized particles exhibited the lowest activity, although over an order of magnitude higher than the state of the art, and the highest stability. Plasma-treated particles showed intermediate performance between as-deposited and thermally oxidized NPs.
Electrode/electrolyte interfaces play a vital role in various electrochemical systems, but in situ characterization of such buried interfaces remains a major challenge. Several efforts to develop techniques or to modify existing techniques to study such interfaces are showing great promise to overcome this challenge. Successful examples include electrochemical scanning tunneling microscopy (EC-STM), surface-sensitive vibrational spectroscopies, environmental transmission electron microscopy (E-TEM), and surface X-ray scattering. Other techniques such as X-ray core-level spectroscopies are element-specific and chemical-state-specific, and are being widely applied in materials science research. Herein we showcase four types of newly developed strategies to probe electrode/electrolyte interfaces in situ with X-ray core-level spectroscopies. These include the standing wave approach, the meniscus approach, and two liquid cell approaches based on X-ray photoelectron spectroscopy and soft X-ray absorption spectroscopy. These examples demonstrate that with proper modifications, many ultra-high-vacuum based techniques can be adapted to study buried electrode/electrolyte interfaces and provide interface-sensitive, element- and chemical-state-specific information, such as solute distribution, hydrogen-bonding network, and molecular reorientation. At present, each method has its own specific limitations, but all of them enable in situ and operando characterization of electrode/electrolyte interfaces that can provide important insights into a variety of electrochemical systems.
We discuss recent developments in nanostructured molybdenum sulfide catalysts for the electrochemical hydrogen evolution reaction. To develop a framework for performing consistent and meaningful comparisons between catalysts, we review standard experimental methodologies for measuring catalyst performance and define two metrics used in this perspective for comparing catalyst activity: the turnover frequency, an intrinsic activity metric, and the total electrode activity, a device-oriented activity metric. We discuss general strategies for synthesizing catalysts with improved activity, namely, increasing the number of electrically accessible active sites or increasing the turnover frequency of each site. Then we consider a number of state-of-the-art molybdenum sulfide catalysts, including crystalline MoS2, amorphous MoSx, and molecular cluster materials, to highlight these strategies in practice. Comparing these catalysts reveals that most of the molybdenum sulfide catalysts have similar active site turnover frequencies, so the total electrode activity is primarily determined by the number of accessible active sites per geometric electrode area. Emerging strategies to overcome current catalyst limitations and potential applications for molybdenum sulfide catalysts including photoelectrochemical water splitting devices and electrolyzers are also considered.Keywords: molybdenum sulfide; hydrogen evolution; electrocatalysis; water splitting; nanomaterials
Carbon-free energy generation is a necessity to meet rising global energy needs while minimizing environmental impact. Hydrogen has the potential to be a cost competitive and scalable solution to replace fossil fuels, and water electrolysis is an attractive concept for producing hydrogen with zero carbon footprint when integrated with a renewable energy source. Proton Energy Systems is a world leader in hydrogen generation from PEM electrolysis and has demonstrated the commercial viability of this technology in the industrial gas market, with pathways defined to reach targets in the energy markets. Recent catalyst research at Proton has demonstrated efficiency improvements while maintaining stability. Ongoing collaboration with 3M has also shown feasibility to reduce the catalyst loading by over an order of magnitude vs. current commercial loadings. This paper will discuss Proton's fueling efforts, particularly at high pressure, and advancements in efficiency which enable localized generation of hydrogen where it is needed.
Heterogeneous catalysis is of great importance for modern society. About 80% of the chemicals are produced by catalytic reactions. Green energy production and utilization as well as environmental protection also need efficient catalysts. Understanding the reaction mechanisms is crucial to improve the existing catalysts and develop new ones with better activity, selectivity, and stability. Three components are involved in one catalytic reaction: reactant, product, and catalyst. The catalytic reaction process consists of a series of elementary steps: adsorption, diffusion, reaction, and desorption. During reaction, the catalyst surface can change at the atomic level, with roughening, sintering, and segregation processes occurring dynamically in response to the reaction conditions. Therefore, it is imperative to obtain atomic-scale information for understanding catalytic reactions.
In the present study, we used a surface-science approach to establish a functional link between activity and stability of monometallic oxides during the OER in acidic media. We found that the most active oxides (Au ≪ Pt < Ir < Ru ≪ Os) are, in fact, the least stable (Au ≫ Pt > Ir > Ru ≫ Os) materials. We suggest that the relationships between stability and activity are controlled by both the nobility of oxides as well as by the density of surface defects. This functionality is governed by the nature of metal cations and the potential transformation of a stable metal cation with a valence state of n = +4 to unstable metal cation with n > +4. A practical consequence of such a close relationship between activity and stability is that the best materials for the OER should balance stability and activity in such a way that the dissolution rate is neither too fast nor too slow.Keywords: Electrochemistry; Oxygen evolution reaction; monometallic oxides
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.
The electrochemical production of hydrogen and hydrocarbons is considered to play a decisive role in the conversion and storage of excess amounts of renewable energy. The electrocatalysis of the oxygen evolution reaction (OER), however, faces significant challenges for practical implementation of electrolyzers. In this work, a comparative study on the activity and stability of oxidized polycrystalline noble metals during the OER is presented. All studied metals exhibit transient and steady-state dissolution. Transient dissolution takes place during oxide formation and reduction. Steady-state dissolution depends on the OER mechanism on each surface: On metals such as Ru and Au, for which oxygen from the oxide participates in the OER, the Tafel slope is low and the dissolution rate is high. In contrast, on metals for which oxygen evolves directly from adsorbed water, such as Pt and presumably Pd, the Tafel slopes are high and the dissolution rates are low. This should be considered in the design of optimal OER catalysts.
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 IrIV to IrV 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.
Sputter deposition of Ir/IrOx on p+-n-Si without interfacial corrosion protection layers yielded photoanodes capable of efficient water oxidation (OER) in acidic media (1 M H2SO4). Stability of at least 18 h was shown by chronoamperomety at 1.23 V versus RHE (reversible hydrogen electrode) under 38.6 mW/cm2 simulated sunlight irradiation (λ > 635 nm, AM 1.5G) and measurements with quartz crystal microbalances. Films exceeding a thickness of 4 nm were shown to be highly active though metastable due to an amorphous character. By contrast, 2 nm IrOx films were stable, enabling OER at a current density of 1 mA/cm2 at 1.05 V vs. RHE. Further improvement by heat treatment resulted in a cathodic shift of 40 mV and enabled a current density of 10 mA/cm2 (requirements for a 10% efficient tandem device) at 1.12 V vs. RHS under irradiation. Thus, the simple IrOx/Ir/p+-n-Si structures not only provide the necessary overpotential for OER at realistic device current, but also harvest 100 mV of free energy (voltage) which makes them among the best-performing Si-based photoanodes in low-pH media.
The activities of the oxygen evolution reaction (OER) on IrO2 and RuO2 catalysts are among the highest known to date. However, the intrinsic OER activities of surfaces with defined crystallographic orientations are not well-established experimentally. Here we report that the (100) surface of IrO2 and RuO2 is more active in alkaline environments (pH 13) than the most thermodynamically stable (110) surface. The OER activity was correlated with the density of coordinatively undersaturated metal sites of each crystallographic facet. The surface-orientation-dependent activities can guide the design of nanoscale catalysts with increased activity for electrolyzers, metal-air batteries, and photoelectrochemical water splitting applications.
Objective evaluation of the activity of electrocatalysts for water oxidation is of fundamental importance for the development of promising energy conversion technologies including integrated solar water-splitting devices, water electrolyzers, and Li-air batteries. However, current methods employed to evaluate oxygen-evolving catalysts are not standardized, making it difficult to compare the activity and stability of these materials. We report a protocol for evaluating the activity, stability, and Faradaic efficiency of electrodeposited oxygen-evolving electrocatalysts. In particular, we focus on methods for determining electrochemically active surface area and measuring electrocatalytic activity and stability under conditions relevant to an integrated solar water-splitting device. Our primary figure of merit is the overpotential required to achieve a current density of 10 mA cm(-2) per geometric area, approximately the current density expected for a 10% efficient solar-to-fuels conversion device. Utilizing the aforementioned surface area measurements, one can determine electrocatalyst turnover frequencies. The reported protocol was used to examine the oxygen-evolution activity of the following systems in acidic and alkaline solutions: CoOx, CoPi, CoFeOx, NiOx, NiCeOx, NiCoOx, NiCuOx, NiFeOx, and NiLaOx. The oxygen-evolving activity of an electrodeposited IrOx catalyst was also investigated for comparison. Two general observations are made from comparing the catalytic performance of the OER catalysts investigated: (1) in alkaline solution, every non-noble metal system achieved 10 mA cm(-2) current densities at similar operating overpotentials between 0.35 and 0.43 V, and (2) every system but IrOx was unstable under oxidative conditions in acidic solutions.
The energy infrastructure for fossil fuels is well-established, accounting for approximately 87% of the 16 TW of power consumed globally. For renewable and sustainable energy conversion technologies to play a relevant role at the terrestrial scale, they must be able to scale to the TW level of deployment. This would place a significant demand on the current and future supply of raw materials (chemical elements) used by those technologies. Oftentimes, the average crustal abundance of a chemical element is cited as a measure of its scalability, however another important metric for scalability is the existence (of lack thereof) of mineable ores with a high concentration of the targeted element. This paper aims to provide an overview of the availability of all elements. This is accomplished via a compilation of data for global primary production rates for each element, as a measure of availability at the present time. This work also addresses the potential future availability based on current and possible future primary sources.
The electrocatalytic behaviour of Ti-supported RuO2, prepared by thermal decomposition of RuCl3, as substrate for oxygen evolution was investigated by a variety of techniques. B.E.T. adsorption experiments showed that the RuO2 layers are highly porous with large surface area values which, however, decrease rapidly as the annealing temperature increases above 300°C. Both the charge involved in the cyclic voltammograms and the oxygen evolution rates are dependent on the true (rather than apparent) area of these electrode surfaces, and also on the pH of the solution. The results for oxygen evolution are discussed in terms of electrochemically generated unstable surface oxides whose decomposition is catalysed by protons in acid, and hydroxide ions in base. The lower reactivity of the oxide at intermediate pH value is attributed on the one hand to loss of protons by OH groups, resulting in oxygen bridging, and on the other to lack of enhanced coordination of surface ruthenium species by OH– ions, which in this pH region are present only at low activity. The charge associated with voltammetric sweeps is accounted for in terms of surface redox processes rather than bulk penetration of protons into the oxide. The need for surface area measurements as a guide to the interpretation of the electrochemical data in the case of these oxide systems is stressed.
Anodic evolution of oxygen on mixed oxides RuxIr1−xO2 has been investigated for x=0,0.3,0.5,0.8 and 1 using electrochemical as well as surface physical techniques. In terms of Tafel slope and corrosion rate the electrochemical behaviour of the mixed oxide is mainly determined by the iridium component for x <0.5. XPS results show that there is no change in surface composition during O2 evolution. Valence band spectra and cyclic voltammetry results suggest that band mixing occurs, giving rise to a shift of oxidation potentials. While the activity of the RuO2 component in the mixed oxide is lost the stability of the slightly activated IrO2 component is maintained. A model for the stabilization effect is proposed.
Semi‐empirical values of the natural widths of K, L1, L2, and L3 levels, Kα1 and Kα2 x‐ray lines, and KL1L1, KL1L2 and KL2L3 Auger lines for the elements 10?Z?110 are presented in tables and grapahs. Level width Γi (i=K, L1,L2, L3) is obtained from the relation Γi=ΓR,i/ωi, using the theoretical radiative rate ΓR,i from Scofield’s relativistic, relaxed Hartree‐Fock calculation and the fluorescence yield ωi from Krause’s evaluation. X‐ray and Auger lines widths are calculated as the sums of pertinent level widths. This tabulation of natural level and line widths is internally consistent, and is compatible with all relevant experimental and theoretical information. Present semi‐empirical widths, especially those of Kα1 and Kα2 x‐rays, are compared with measured widths. Uncertainties of semi‐empirical values are estimated.
The ‘legend’ of DSA® (Dimensionally Stable Anodes), one of the greatest technological breakthrough of the past 50 years of electrochemistry, is reviewed with the aim to emphasise the reasons for their success. In this respect, the industrial success, which came first, was due to factors differing from those responsible for the successive boom of fundamental research on the same materials. This article scrutinises these factors highlighting the aspects which make these materials so much intriguing.
Hydrogen is often considered the best means by which to store energy coming from renewable and intermittent power sources. With the growing capacity of localized renewable energy sources surpassing the gigawatt range, a storage system of equal magnitude is required. PEM electrolysis provides a sustainable solution for the production of hydrogen, and is well suited to couple with energy sources such as wind and solar. However, due to low demand in electrolytic hydrogen in the last century, little research has been done on PEM electrolysis with many challenges still unexplored. The ever increasing desire for green energy has rekindled the interest on PEM electrolysis, thus the compilation and recovery of past research and developments is important and necessary. In this review, PEM water electrolysis is comprehensively highlighted and discussed. The challenges new and old related to electrocatalysts, solid electrolyte, current collectors, separator plates and modeling efforts will also be addressed. The main message is to clearly set the state-of-the-art for the PEM electrolysis technology, be insightful of the research that is already done and the challenges that still exist. This information will provide several future research directions and a road map in order to aid scientists in establishing PEM electrolysis as a commercially viable hydrogen production solution.
Six transition metal dioxides,
,
,
,
,
, and
, have been examined as electrodes in
solution. The oxides
,
,
, and
have broad current‐potential profiles, indicating the formation of a surface layer which can exist over a range of compositions.
Steady‐state measurements of
reduction showed catalytic activities which were low compared to common
catalysts such as Pt, but of the same order as other oxide catalysts. Activities were lowest for
and
, which form resistive surface layers of a higher oxide. The only material studied which was sufficiently stable to allow
measurement of both
evolution and
reduction was
. The current‐potential profiles of
and
are characterized by distinct changes of oxidation state. These two oxides were not sufficiently stable to allow the measurement
of
reduction.
High-energy-resolution fluorescence-detected Xray absorption spectroscopy (HERFD-XAS) has been applied to study the chemical state of similar to 1.2 nm size-selected Pt nanoparticles (NPs) in an electrochemical environment under potential control. Spectral features due to chemisorbed hydrogen, chemisorbed O/OH, and platinum oxides can be distinguished with increasing potential. Pt electro-oxidation follows two competitive pathways involving both oxide formation and Pt dissolution.
Trends in electrocatalytic activity of the oxygen evolution reaction
(OER) are investigated on the basis of a large database of
HO* and HOO* adsorption energies on oxide surfaces. The theoretical
overpotential was calculated by applying standard
density functional theory in combination with the computational
standard hydrogen electrode (SHE) model. We showed
that by the discovery of a universal scaling relation between
the adsorption energies of HOO* vs HO*, it is possible to analyze
the reaction free energy diagrams of all the oxides in a
general way. This gave rise to an activity volcano that was the
same for a wide variety of oxide catalyst materials and a universal
descriptor for the oxygen evolution activity, which suggests
a fundamental limitation on the maximum oxygen evolution
activity of planar oxide catalysts.
The stoichiometric RuO2(110) surface is terminated by bridge-coordinated oxygen atoms (Oβ) and by coordinatively unsaturated Ru (Rucus) atoms. Exposure to gaseous O2 leads to the formation of two additional surface species: a molecularly chemisorbed state (Oδ) bridging two neighboring Rucus atoms and weakly held O atoms (Oγ) in terminal position above the Rucus atoms. Characterization of the energetics and kinetics as well as structural, vibrational, and electronic properties is achieved by combined application of experimental (low-energy electron diffraction, high-resolution electron loss spectroscopy, thermal desorption spectroscopy) and theoretical (density functional theory) methods. The interplay between the different oxygen species accounts for the high sticking coefficient for dissociative adsorption as well as for the continuous restoration of the surface structure in the course of catalytic oxidation reactions.
In this paper, density functional theory (DFT) calculations are performed to analyze the electrochemical water-splitting process producing molecular oxygen (O2) and hydrogen (H2). We investigate the trends in the electro-catalytic properties of (1 1 0) surfaces of three rutile-type oxides (RuO2, IrO2, and TiO2). The two first of these oxide anodes show lower O2-evolving over-potentials than metal anodes, due to weak O binding but strong hydroxyl (HO∗) binding on the surface. Furthermore, the binding energies of O, HO, and HOO on the (1 1 0) surfaces fulfill universal linear relations similar to those found on metal surfaces.
We have studied the effect of nanostructuring in Pt monolayer model electrocatalysts on a Rh(111) single-crystal substrate on the adsorption strength of chemisorbed species. In situ high energy resolution fluorescence detection X-ray absorption spectroscopy at the Pt L(3) edge reveals characteristic changes of the shape and intensity of the "white-line" due to chemisorption of atomic hydrogen (H(ad)) at low potentials and oxygen-containing species (O/OH(ad)) at high potentials. On a uniform, two-dimensional Pt monolayer grown by Pt evaporation in ultrahigh vacuum, we observe a significant destabilization of both H(ad) and O/OH(ad) due to strain and ligand effects induced by the underlying Rh(111) substrate. When Pt is deposited via a wet-chemical route, by contrast, three-dimensional Pt islands are formed. In this case, strain and Rh ligand effects are balanced with higher local thickness of the Pt islands as well as higher defect density, shifting H and OH adsorption energies back toward pure Pt. Using density functional theory, we calculate O adsorption energies and corresponding local ORR activities for fcc 3-fold hollow sites with various local geometries that are present in the three-dimensional Pt islands.
Requisites for electrode materials to be suitable for technological applications are outlined and discussed. Oxides with metallic or quasi-metallic conductivity meet these requirements best. Most of these electrodes are prepared by thermal procedures. It is shown that the temperature of preparation affects the catalytic activity through the surface area and the chemical composition (non-stoichiometry). The co-variation of these parameters is best followed in situ by voltammetric curves and point of zero charge measurements. Examples are given for pure RuO2, IrO2, Co3O4 and IrO2 + RuO2 mixtures. Kinetic and mechanistic details are discussed for O2 evolution on RuO2, IrO2 and Co3O4 and for Cl2 evolution on RuO2 and Co3O4. Finally, the electrocatalytic properties of different oxides are correlated with the energy change involved in the lower → higher valency state transition. Experimental data for both O2 and Cl2 evolution can thus be organized into a “volcano” curve enabling predictive interpolations to be made.
Density functional theory calculations are used as the basis for an analysis of the electrochemical process, where by water is split to form molecular oxygen and hydrogen. We develop a method for obtaining the thermochemistry of the electrochemical water splitting process as a function of the bias directly from the electronic structure calculations. We consider electrodes of Pt(1 1 1) and Au(1 1 1) in detail and then discuss trends for a series of different metals. We show that the difficult step in the water splitting process is the formation of superoxy-type (OOH) species on the surface by the splitting of a water molecule on top an adsorbed oxygen atom. One conclusion is that this is only possible on metal surfaces that are (partly) oxidized. We show that the binding energies of the different intermediates are linearly correlated for a number of metals. In a simple analysis, where the linear relations are assumed to be obeyed exactly, this leads to a universal relationship between the catalytic rate and the oxygen binding energy. Finally, we conclude that for systems obeying these relations, there is a limit to how good a water splitting catalyst an oxidized metal surface can become.
Currently, hydrogen is primarily used in the chemical industry, but in the near future it will become a significant fuel. There are many processes for hydrogen production. This paper reviews the technologies related to hydrogen production from both fossil and renewable biomass resources including reforming (steam, partial oxidation, autothermal, plasma, and aqueous phase) and pyrolysis. In addition, electrolysis and other methods for generating hydrogen from water, hydrogen storage related approaches, and hydrogen purification methods such as desulfurization and water-gas-shift are discussed.
In this work, IrO(2)-based powders are screened by cyclic voltammetry for the determination of the electrochemical active sites and for the qualitative evaluation of the iridium atoms speciation. All results are obtained using a cavity-microelectrode as powder holder, thus exploiting the features of this innovative tool, whose best potentialities have been recently introduced by our group. All the studied materials have been prepared by the sol-gel technique and differ in calcination temperature and method of mixing the metal oxide precursors. The electrochemical results are complemented with the information obtained by X-ray absorption spectroscopy (XAS), that give insights on the local structure of each selected sample, confirming the trends found by cyclic voltammetry and give new and unexpected insights on the powder structural features.