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Enhancing Oxygen Evolution Reaction via a Surface Reconstruction-Induced Lattice Oxygen Mechanism
Green hydrogen production can be achieved through electrolysis of fresh water or the use of renewable energy to electrolyze seawater. However, due to the low activity and poor stability of oxygen evolution reaction catalysts, direct electrolysis of alkaline seawater faces significant challenges. Herein, The catalyst F‐CoFe(OH)‐CO3/NF with three‐dimensional nanosphere structure was prepared, The introduction of CO3²⁻ into the intermediate layer of CoFe hydroxide improves the corrosion resistance of alkaline electrolyte and the doping of F⁻ is to design three‐dimensional layered nanostructures, increase the active site, and accelerate the diffusion of the electrolyte. By in situ Raman analysis, partial oxidation of CoFe hydroxide to CoFe (oxy)hydroxide as the active center can accelerating the adsorption of oxygen‐related intermediates. In 1 M KOH, it requires overpotentials of 210 mV and 251 mV to drive current densities of 10 and 100 mA cm⁻², respectively. And it remained stable at the current density of 100 mA cm⁻² for 120 h in 1 M KOH. F‐CoFe(OH)‐CO3/NF can also catalyzes the decomposition of electrolytic seawater. Compared with hydroxide, anion‐doped carbonate hydroxide is more efficient and stable in electrolyte solution, which is of great importance for the development of a new stable electrocatalyst for water decomposition.
Neither electrocatalytic activity nor structural stability is inconsequential in water electrolysis. Unfortunately, they have to be compromised in practice, especially in the anodic redox chemistry of lattice oxygen. Herein, the discovery of a La1−xCexFeO3 perovskite is presented which shows both good stability and high catalytic activity. Using advanced operando characterizations, it is identified that the self‐healing evolution of the La1−xCexFeO3 perovskite plays a key role during water oxidation in the lattice oxygen‐mediated mechanism (LOM) pathway. Unlike irreversible reconstruction, the formation of reconstructed active‐phase α‐FeOOH is reversed by re‐crystallization of surface La1−xCexFeO3 upon return to noncatalytic conditions. The self‐healing transformation of the α‐FeOOH termination layer on the stable La1−xCexFeO3 core imparts remarkable long‐term stability as well as excellent electrocatalytic performance. As a result, a composition La0.9Ce0.1FeO3@FeOOH is designed that exhibits ultralow overpotentials of 257 and 312 mV to achieve 10 and 100 mA cm⁻², respectively. The findings provide insight into self‐healing behavior toward engineering perovskite oxides for efficient and stable oxygen electrocatalysis.
The regulation of atomic and electronic structures of active sites plays an important role upon rational design of oxygen evolution reaction (OER) catalysts toward electrocatalytic hydrogen generation. However, the precise identification of the active sites for surface reconstruction behavior during OER remains elusive for water‐alkali electrolysis. Herein, we report irreversible reconstruction behavior accompanied by copper dynamic evolution for cobalt iron layered double hydroxide (CoFe LDH) precatalyst to form CoFeCuOOH active species with high‐valent Co species, identifying the origin of reconstructed active sites through operando UV‐Visible (UV‐Vis), in‐situ Raman and X‐ray absorption fine‐structure (XAFS) spectroscopies. Density functional theory analysis rationalizes this typical electronic structure evolution causes the transfer of intramolecular electron to form ligand holes, promoting reconstruction of active sites. Specifically, unambiguous identification of active sites for CoFeCuOOH is explored by in‐situ 18O isotope‐labeling differential electrochemical mass spectrometry (DEMS) and supported by theoretical calculation, confirming mechanism switch to oxygen‐vacancy‐site mechanism (OVSM) pathway on lattice oxygen. This work enables us to elucidate the vital role of dynamic active‐site generation and representative contribution of OVSM pathway for efficient OER performance. This article is protected by copyright. All rights reserved
Electron transfer process during oxygen evolution reaction often either proceed solely via a metal redox chemistry (adsorbate evolution mechanism (AEM), with metal bands around the Fermi level) or an oxygen redox chemistry (lattice oxygen oxidation mechanism (LOM), with oxygen bands around the Fermi level). Unlike AEM, LOM involves oxygen redox chemistry instead of metal redox, which leads to the formation of a direct oxygen‐oxygen (O‐O) bond. As a result, such process is able to bypass the rate‐determining step, i.e., O‐O bonding, in AEM, which highlights the critical advantage of LOM as compared to the conventional AEM. Thus, it has been well reported that LOM‐based catalysts were able to demonstrate higher OER activities as compared to AEM‐based catalysts. Here, a comprehensive understanding of the oxygen redox in LOM and all documented and possible characterization techniques that can be used to identify the oxygen redox are reviewed. This review will interpret the origins of oxygen redox in the reported LOM‐based electrocatalysts and the underlying science of LOM‐induced surface reconstruction in transition metal oxides. Finally, the perspectives on the future development of LOM electrocatalysts will also be provided. This article is protected by copyright. All rights reserved
Reversible anionic redox reactions represent a transformational change for creating advanced high-energy-density positive-electrode materials for lithium-ion batteries. The activation mechanism of these reactions is frequently linked to ligand-to-metal charge transfer (LMCT) processes, which have not been fully validated experimentally due to the lack of suitable model materials. Here we show that the activation of anionic redox in cation-disordered rock-salt Li1.17Ti0.58Ni0.25O2 involves a long-lived intermediate Ni3+/4+ species, which can fully evolve to Ni2+ during relaxation. Combining electrochemical analysis and spectroscopic techniques, we quantitatively identified that the reduction of this Ni3+/4+ species goes through a dynamic LMCT process (Ni3+/4+–O2− → Ni2+–On−). Our findings provide experimental validation of previous theoretical hypotheses and help to rationalize several peculiarities associated with anionic redox, such as cationic–anionic redox inversion and voltage hysteresis. This work also provides additional guidance for designing high-capacity electrodes by screening appropriate cationic species for mediating LMCT. Understanding reversible anionic redox reactions is key to designing high-energy-density cathodes for lithium-ion batteries. Anionic redox activation in cation-disordered rock-salt Li1.17Ti0.58Ni0.25O2 is shown to involve intermediate Ni3+/4+ species that can evolve to Ni2+ during relaxation.
Tuning the reactivity of lattice oxygen is of significance for lowering the energy barriers and accelerating the oxygen evolution reaction (OER). Herein, single‐atomic Mo sites were anchored on Ni‐Fe oxyhydroxides nanoarrays by a facile MOFs‐derived strategy, exhibiting superior performance towards OER in alkaline media. The in‐situ electrochemical spectroscopies and the isotope labeling experiments reveal the involvement of lattice oxygen during OER cycles. Combining the theoretical and experimental investigation of electronic configuration, it is comprehensively confirmed that the incorporation of single‐atomic Mo sites enables the higher oxidation state of metal and the strengthened metal‐oxygen hybridization, as well as the formation of oxidized ligand holes above Fermi level. In a word, the considerable acceleration of water oxidation is achieved via enhancing the reactivity of lattice oxygen and triggering the lattice oxygen activation. This work may provide new insights for designing ideal electrocatalysts via tuning the chemical state and activating the anions ligands. This article is protected by copyright. All rights reserved
The density functional theory calculation results reveal that the adjacent defect concentration and electronic spin state can effectively activate the CoIII sites in the atomically thin nanosheets, facilitating the thermodynamic transformation of *O to *OOH, thus offering ultrahigh charge transfer properties and efficiently stabilizing the phase. This undoubtedly evidences that, for metal sulfides, the atom‐scale cation/anion vacancy pair and surface electronic spin state can play a great role in enhancing the oxygen evolution reaction. Inspired by the theoretical prediction, interconnected selenium (Se) wired ultrathin Co3S4 (Sex‐Co3S4) nanosheets with Co/S (Se) dual‐vacancies (Se1.0‐Co3S4‐VS/Se‐VCo) pairs are constructed by a simple approach. As an efficient sulfur host material, in an ultralow‐concentration KOH solution (0.1 m), Se1.0‐Co3S4‐VS/Se‐VCo presents outstanding durability up to 165 h and a low overpotential of 289.5 mV at 10 mA cm–2, which outperform the commercial Co3S4 nanosheets (NSs) and RuO2. Moreover, the turnover frequency of Se1.0‐Co3S4‐VS/Se‐VCo is 0.00965 s–1 at an overpotential of 0.39 V, which is 5.7 times that of Co3S4 NSs, and 5.8 times that of commercial RuO2. The finding offers a rational design strategy to create the multi‐defect structure in catalysts toward high‐efficiency water electrolysis.
Water oxidation is a crucial reaction for renewable energy conversion and storage. Among the alkaline oxygen evolution reaction (OER) catalysts, NiFe based oxyhydroxides show the highest catalytic activity. However, the details of their OER mechanism are still unclear, due to the elusive nature of the OER intermediates. Here, using a novel differential electrochemical mass spectrometry (DEMS) cell interface, we performed isotope‐labelling experiments in ¹⁸O‐labelled aqueous alkaline electrolyte on Ni(OH)2 and NiFe layered double hydroxide nanocatalysts. Our experiments confirm the occurrence of Mars‐van‐Krevelen lattice oxygen evolution reaction mechanism in both catalysts to various degrees, which involves the coupling of oxygen atoms from the catalyst and the electrolyte. The quantitative charge analysis suggests that the participating lattice oxygen atoms belong exclusively to the catalyst surface, confirming DFT computational hypotheses. Also, DEMS data suggest a fundamental correlation between the magnitude of the lattice oxygen mechanism and the faradaic efficiency of oxygen controlled by pseudocapacitive oxidative metal redox charges.
Anodic oxygen evolution reaction (OER) is recognized as kinetic bottleneck in water electrolysis. Transition metal sites with high valence states can accelerate the reaction kinetics to offer highly intrinsic activity, but suffer from thermodynamic formation barrier. Here, we show subtle engineering of highly oxidized Ni4+ species in surface reconstructed (oxy)hydroxides on multicomponent FeCoCrNi alloy film through interatomically electronic interplay. Our spectroscopic investigations with theoretical studies uncover that Fe component enables the formation of Ni4+ species, which is energetically favored by the multistep evolution of Ni2+→Ni3+→Ni4+. The dynamically constructed Ni4+ species drives holes into oxygen ligands to facilitate intramolecular oxygen coupling, triggering lattice oxygen activation to form Fe-Ni dual-sites as ultimate catalytic center with highly intrinsic activity. As a result, the surface reconstructed FeCoCrNi OER catalyst delivers outstanding mass activity and turnover frequency of 3601 A gmetal−1 and 0.483 s−1 at an overpotential of 300 mV in alkaline electrolyte, respectively. Electrocatalytic water oxidation is facilitated by high valence states, but these are challenging to achieve at low applied potentials. Here, authors report a multicomponent FeCoCrNi alloy with dynamically formed Ni4+ species to offer high catalytic activity via lattice oxygen activation mechanism.
The development of oxygen evolution reaction (OER) electrocatalysts remains a major challenge that requires significant advances in both mechanistic understanding and material design. Recent studies show that oxygen from the perovskite oxide lattice could participate in the OER via a lattice oxygen-mediated mechanism, providing possibilities for the development of alternative electrocatalysts that could overcome the scaling relations-induced limitations found in conventional catalysts utilizing the adsorbate evolution mechanism. Here we distinguish the extent to which the participation of lattice oxygen can contribute to the OER through the rational design of a model system of silicon-incorporated strontium cobaltite perovskite electrocatalysts with similar surface transition metal properties yet different oxygen diffusion rates. The as-derived silicon-incorporated perovskite exhibits a 12.8-fold increase in oxygen diffusivity, which matches well with the 10-fold improvement of intrinsic OER activity, suggesting that the observed activity increase is dominantly a result of the enhanced lattice oxygen participation.
Cobalt pnictides show good catalytic activity and stability on oxygen evolution reaction (OER) behaviors in a strong alkaline solution. Identifying the intrinsic composition/structure‐property relationship of the oxide layer on the cobalt pnictides is critical to design better and cheaper electrocatalysts for the commercial viability of OER technologies. In this work, the restructured oxide layer on the cobalt pnictides and its effect on the activity and mechanism for OER is systematically analyzed. In‐situ electrochemical impedance spectroscopy (EIS) and near edge x‐ray absorption fine structure (NEXAFS) spectra indicate that a higher OER performance of cobalt pnictides than Co3O4 is attributed to the more structural disorder and oxygen defect sites in the cobalt oxide layer evolved from cobalt pnictides. Using angle resolved x‐ray photoelectron spectroscopy (AR‐XPS) further demonstrates that the oxygen defect sites mainly concentrate on the subsurface of cobalt oxide layer. The current study demonstrated promising opportunities for further enhancing the OER performance of cobalt‐based electrocatalysts by controlling the subsurface defects of the restructured active layer.
Herein, we report the synthesis and electrochemical oxygen evolution experiments for a graphene supported Ni3MnO4 catalyst. The changes that occur at the Ni active sites during the electrocatalyic oxygen evolution reaction (OER) were elucidated by a combination of operando Ni L-edge X-ray absorption spectroscopy (XAS) and Ni 2p3d resonant inelastic X-ray scattering (RIXS). These data are compared to reference measurements on NiO, β-Ni(OH)2, β-NiOOH, and γ-NiOOH. Through this comparative analysis, we are able to show that under alkaline conditions (0.1 M KOH) the oxides of the Ni3MnO4 catalyst are converted to hydroxides. At the onset of catalysis (1.47 V) the β-Ni(OH)2-like phase is oxidized and converted to a dominantly γ-NiOOH phase. The present study thus challenges the notion that the β-NiOOH phase is the active phase in OER and provides further evidence that the γ-NiOOH phase is catalytically active. The ability to use Ni L-edge XAS and 2p3d RIXS to provide a rational basis for structure-activity correlations is highlighted.
The development of efficient and low-cost electrocatalysts for the oxygen evolution reaction (OER) is critical for improving the efficiency of water electrolysis. Here, we report a strategy using Fe substitution to enable the inactive spinel CoAl2O4 to become highly active and superior to the benchmark IrO2. The Fe substitution is revealed to facilitate surface reconstruction into active Co oxyhydroxides under OER conditions. It also activates deprotonation on the reconstructed oxyhydroxide to induce negatively charged oxygen as an active site, thus significantly enhancing the OER activity of CoAl2O4. Furthermore, it promotes the pre-oxidation of Co and introduces great structural flexibility due to the uplift of the oxygen 2p levels. This results in the accumulation of surface oxygen vacancies along with lattice oxygen oxidation that terminates as Al³⁺ leaches, preventing further reconstruction. We showcase a promising way to achieve tunable electrochemical reconstruction by optimizing the electronic structure for low-cost and robust spinel oxide OER catalysts.
The oxygen evolution reaction (OER) is a key process in electrochemical energy conversion devices. Understanding the origins of the lattice oxygen oxidation mechanism is crucial because OER catalysts operating via this mechanism could bypass certain limitations associated with those operating by the conventional adsorbate evolution mechanism. Transition metal oxyhydroxides are often considered to be the real catalytic species in a variety of OER catalysts and their low-dimensional layered structures readily allow direct formation of the O–O bond. Here, we incorporate catalytically inactive Zn2+ into CoOOH and suggest that the OER mechanism is dependent on the amount of Zn2+ in the catalyst. The inclusion of the Zn2+ ions gives rise to oxygen non-bonding states with different local configurations that depend on the quantity of Zn2+. We propose that the OER proceeds via the lattice oxygen oxidation mechanism pathway on the metal oxyhydroxides only if two neighbouring oxidized oxygens can hybridize their oxygen holes without sacrificing metal–oxygen hybridization significantly, finding that Zn0.2Co0.8OOH has the optimum activity. Oxygen evolution is one half of the overall water splitting reaction to produce hydrogen. Although this reaction is well studied, there remains debate over the particulars of the catalytic mechanism. Here, the authors investigate Co–Zn oxyhydroxide electrocatalysts, and suggest that the mechanism depends on the amount of Zn2+ they contain.
The efficiency with which renewable fuels and feedstocks are synthesized from electrical sources is limited at present by the sluggish oxygen evolution reaction (OER) in pH-neutral media. We took the view that generating transition-metal sites with high valence at low applied bias should improve the activity of neutral OER catalysts. Here, using density functional theory, we find that the formation energy of desired Ni4+ sites is systematically modulated by incorporating judicious combinations of Co, Fe and non-metal P. We therefore synthesized NiCoFeP oxyhydroxides and probed their oxidation kinetics with in situ soft X-ray absorption spectroscopy (sXAS). In situ sXAS studies of neutral-pH OER catalysts indicate ready promotion of Ni4+ under low overpotential conditions. The NiCoFeP catalyst outperforms IrO2 and retains its performance following 100 h of operation. We showcase NiCoFeP in a membrane-free CO2 electroreduction system that achieves a 1.99 V cell voltage at 10 mA cm–2, reducing CO2 into CO and oxidizing H2O to O2 with a 64% electricity-to-chemical-fuel efficiency.
Owing from the transient nature of the intermediates involved into the oxygen evolution reaction (OER) as well as their interaction with water molecules, the exact role played by the proton on the activity and mechanism for OER catalysts remains elusive. To shed further light on this issue we herein propose the use of chemical probes to selectively interact with deprotonated oxygen in order to better study them. For that, we introduce tetraalkylammonium (TAA) cations, previously known for their surfactant properties, which block the oxygen active sites and modify the hydrogen bonds network on the surface of OER catalysts. Combining chemical probes with isotopic and pH dependent measurements, we further demonstrate that the introduction of iron for into Ni oxy-hydroxide amorphous films deeply modifies the proton diffusion properties, and therefore related to the activity and mechanism of the OER reaction pertaining to the NiOOH films.
Lithium-ion batteries are now reaching the energy density limits set by their electrode materials, requiring new paradigms for Li+ and electron hosting in solid-state electrodes. Reversible oxygen redox in the solid state in particular has the potential to enable high energy density as it can deliver excess capacity beyond the theoretical transition-metal redox-capacity at a high voltage. Nevertheless, the structural and chemical origin of the process is not understood, preventing the rational design of better cathode materials. Here, we demonstrate how very specific local Li-excess environments around oxygen atoms necessarily lead to labile oxygen electrons that can be more easily extracted and participate in the practical capacity of cathodes. The identification of the local structural components that create oxygen redox sets a new direction for the design of high-energy-density cathode materials.
Perovskite oxides are attractive candidates as catalysts for the electrolysis of water in alkaline energy storage and conversion systems. However, the rational design of active catalysts has been hampered by the lack of understanding of the mechanism of water electrolysis on perovskite surfaces. Key parameters that have been overlooked include the role of oxygen vacancies, B-O bond covalency, and redox activity of lattice oxygen species. Here we present a series of cobaltite perovskites where the covalency of the Co-O bond and the concentration of oxygen vacancies are controlled through Sr2+ substitution into La1-xSrxCoO3-δ. We attempt to rationalize the high activities of La1-xSrxCoO3-δ through the electronic structure and participation of lattice oxygen in the mechanism of water electrolysis as revealed through ab initio modelling. Using this approach, we report a material, SrCoO2.7, with a high, room temperature-specific activity and mass activity towards alkaline water electrolysis.
Microkinetic analyses of aqueous electrochemistry involving gaseous H2 or O2, i.e., hydrogen evolution reaction (HER), hydrogen oxidation reaction (HOR), oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), are revisited. The Tafel slopes used to evaluate the rate determining steps generally assume extreme coverage of the adsorbed species (θ ≈ 0 or ≈1), although, in practice, the slopes are coverage-dependent. We conducted detailed kinetic analyses describing the coverage-dependent Tafel slopes for the aforementioned reactions. Our careful analyses provide a general benchmark for experimentally observed Tafel slopes that can be assigned to specific rate determining steps. The Tafel analysis is a powerful tool for discussing the rate determining steps involved in electrocatalysis, but our study also demonstrated that overly simplified assumptions led to an inaccurate description of the surface electrocatalysis. Additionally, in many studies, Tafel analyses have been performed in conjunction with the Butler-Volmer equation, where its applicability regarding only electron transfer kinetics is often overlooked. Based on the derived kinetic description of the HER/HOR as an example, the limitation of Butler-Volmer expression in electrocatalysis is also discussed in this report.
In recent years, the oxygen evolution reaction (OER) has attracted increased research interest due to its crucial role in electrochemical energy conversion devices for renewable energy applications. The vast majority of OER catalyst materials investigated are metal oxides of various compositions. The experimental results obtained on such materials strongly suggest the existence of a fundamental and universal correlation between the oxygen evolution activity and the corrosion of metal oxides. This corrosion manifests itself in structural changes and/or dissolution of the material. We prove from basic thermodynamic considerations that any metal oxide must become unstable under oxygen evolution conditions irrespective of the pH value. The reason is the thermodynamic instability of the oxygen anion in the metal oxide lattice. Our findings explain many of the experimentally observed corrosion phenomena on different metal oxide OER catalysts.
The interaction between the transition metal 3d and the oxygen 2p states via hybridization underpins many of the phenomena in transition metal oxide materials. We report the empirical trend of this interaction using the pre-edge feature of the O K-edge X-ray absorption spectrum. Our assessment method is built on the dipole approximation and the configuration interaction between the transition metal 3d and the oxygen 2p states. We found that hybridization increases with the number of 3d electrons, consistent with the expected electronegativity trend. We support this analysis with density functional calculations, which reveal a systematic increase in the transition metal 3d and the oxygen 2p state mixing with increasing 3d-electron number. Oxidation of the transition metal was also found to increase hybridization, which we believe reflects the reduced transition metal 3d and oxygen 2p energy difference, causing increased covalency. We compare the analysis from the surface-sensitive electron-yield and the bulk-sensitive fluorescence-yield spectra, revealing that either method can be used to study the hybridization trend. We finally compare and discuss the influence of the lanthanide ions and the influence of the covalency on oxygen electrocatalysis. Our study describes an efficient and simple approach to understand the hybridization trend in transition metal oxides, which has considerable implications for electrochemical energy conversion processes.
The La0.85Sr0.15Cr0.95Ni0.02Co0.02O3 (LSC) interconnect materials for solid oxide fuel cells (SOFCs) were synthesized by EDTA–citrate complexing method. Thermal decomposition behavior of the gel, phase formation and morphology of LSC powders were characterized by thermogravimetry/differential thermal (DSC/TG) analysis, X-ray diffraction (XRD) and scanning electron microscope (SEM), respectively. It appeared that lower pH value (pH 4.9) of the precursor solution resulted in a transient liquid phase, SrCrO4, in the calcined LSC powder. The sintering characteristics, electrical conductivity and thermal expansion properties of sintered bars were investigated. La0.85Sr0.15Cr0.95Ni0.02Co0.02O3 prepared in the condition of pH 4.9 showed an electrical conductivity of 15.6Scm−1 at 800°C and a thermal expansion coefficient (TEC) of 10.8×10−6K−1 (20–900°C), which is suitable for use as interconnect materials for SOFCs.
In this paper we study through a multiplicity of experimental and theoretical techniques the electrochemical evolution of oxygen on gold, the metal on which water splitting was initially discovered more than two centuries ago. The evidence obtained with a combination of in situ surface-enhanced Raman spectroscopy, online electrochemical mass spectrometry and density functional theory calculations suggests the existence of several mechanisms for the evolution of O2 on Au electrodes, depending on the electrode potential. Significantly, at approximately 2.0 V vs. RHE the first O2 that is evolved consists of two oxygens from the surface oxide, suggesting an oxide decomposition or oxide disproportionation step. At somewhat higher potentials, O2 is formed by a combination of oxygen from the oxide lattice and oxygen provided by water. The oxide decomposition step implies a more three-dimensional mechanism for oxygen evolution than suggested in previous mechanisms, which involve only surface-adsorbed intermedi
We present a method for calculating the stability of reaction intermediates of electrochemical processes on the basis of electronic structure calculations. We used that method in combination with detailed density functional calculations to develop a detailed description of the free-energy landscape of the electrochemical oxygen reduction reaction over Pt(111) as a function of applied bias. This allowed us to identify the origin of the overpotential found for this reaction. Adsorbed oxygen and hydroxyl are found to be very stable intermediates at potentials close to equilibrium, and the calculated rate constant for the activated proton/ electron transfer to adsorbed oxygen or hydroxyl can account quantitatively for the observed kinetics. On the basis of a database of calculated oxygen and hydroxyl adsorption energies, the trends in the oxygen reduction rate for a large number of different transition and noble metals can be accounted for. Alternative reaction mechanisms involving proton/electron transfer to adsorbed molecular oxygen were also considered, and this peroxide mechanism was found to dominate for the most noble metals. The model suggests ways to improve the electrocatalytic properties of fuel-cell cathodes.
We present an efficient scheme for calculating the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set. In the first part the application of Pulay's DIIS method (direct inversion in the iterative subspace) to the iterative diagonalization of large matrices will be discussed. Our approach is stable, reliable, and minimizes the number of order N-atoms(3) operations. In the second part, we will discuss an efficient mixing scheme also based on Pulay's scheme. A special ''metric'' and a special ''preconditioning'' optimized for a plane-wave basis set will be introduced. Scaling of the method will be discussed in detail for non-self-consistent calculations. It will be shown that the number of iterations required to obtain a specific precision is almost independent of the system size. Altogether an order N-atoms(2) scaling is found for systems up to 100 electrons. If we take into account that the number of k points can be implemented these algorithms within a powerful package called VASP (Vienna ab initio simulation package). The program and the techniques have been used successfully for a large number of different systems (liquid and amorphous semiconductors, liquid simple and transition metals, metallic and semiconducting surfaces, phonons in simple metals, transition metals, and semiconductors) and turned out to be very reliable.
Based on the advancements in atomically dispersed multi-site catalysts for FZABs, this review discusses the design methodologies to regulate the performance of bifunctional oxygen electrocatalysts from the electronic and geometric structures.
Electricity-driven water splitting can facilitate the storage of electrical energy in the form of hydrogen gas. As a half-reaction of electricity-driven water splitting, the oxygen evolution reaction (OER) is the major bottleneck due to the sluggish kinetics of this four-electron transfer reaction. Developing low-cost and robust OER catalysts is critical to solving this efficiency problem in water splitting. The catalyst design has to be built based on the fundamental understanding of the OER mechanism and the origin of the reaction overpotential. In this article, we summarize the recent progress in understanding OER mechanisms, which include the conventional adsorbate evolution mechanism (AEM) and lattice-oxygen-mediated mechanism (LOM) from both theoretical and experimental aspects. We start with the discussion on the AEM and its linked scaling relations among various reaction intermediates. The strategies to reduce overpotential based on the AEM and its derived descriptors are then introduced. To further reduce the OER overpotential, it is necessary to break the scaling relation of HOO* and HO* intermediates in conventional AEM to go beyond the activity limitation of the volcano relationship. Strategies such as stabilization of HOO*, proton acceptor functionality, and switching the OER pathway to LOM are discussed. The remaining questions on the OER and related perspectives are also presented at the end.
Assessing the reaction pathway of multi-electron-transfer reactions is an essential yet difficult task for the rational design of electrocatalysts. In this work, we develop a heuristic approach that combines thermodynamic adsorption energetics calculated through density functional theory with microkinetic modeling using the steady state approximation to interpret the potential-dependent Tafel behavior of consecutive electrochemical reactions. In doing so, we introduce a kinetic framework for ab initio calculations that ensures self-consistent adsorption energetics based on kinetically limited adsorbate coverages. The approach is applied to experimental results on CoOx(OH)2-x single crystal electrocatalyst particles yielding coverage dependent mechanistic information and identification of the rate-limiting step with standard rate constants for the oxygen evolution reaction on the (11-20) surfaces of the β-Co(OH)2, β-CoOOH, and CoO2 bulk phases. This generalizable method enables catalyst benchmarking based on determining the active species involved and associated intrinsic reaction rate constants in consecutive multi-electron-transfer reactions.
In this study, the long-term chemothermal stability of chemically delithiated Li1Ni0.8Co0.15Al0.05O2 (NCA) was systematically investigated at relevant operating temperatures of polymer solid-state batteries using ex-situ synchrotron-based hard and soft X-ray absorption spectroscopy. The reduction of nickel on the surface, subsurface, and in the bulk of secondary NCA particles was studied and directly related to aging time, temperature, the presence of polymeric electrolyte (poly(ethylene oxide) or polycaprolactone), and lithium salt (lithium tetrafluoroborate or lithium bis(trifluoromethanesulfonyl)imide). Depending on the polymer and/or lithium salt accompanying the delithiated Li0.3NCA, reduction of nickel at the surface, subsurface, and bulk occurs to varying extents, starting at the surface and propagating into the bulk material. Our results indicate how degradation (reduction of nickel) is strongly correlated to temperature, time, and the presence of blended polymer and/or lithium salt in the cathode. The relative stability of the NCA material in cathodes having different polymer and lithium salt combinations identified in the ex-situ spectroscopy study is directly demonstrated in solid-state polymer batteries.
Multiple electrochemical processes are involved at the catalyst/electrolyte interface during the oxygen evolution reaction (OER). With the purpose of elucidating the complexity of surface dynamics upon OER, we systematically studied two Ni-based crystalline oxides (LaNiO3-δ and La2Li0.5Ni0.5O4) and compared them with the state-of-the-art Ni-Fe (oxy)hydroxide amorphous catalyst. Electrochemical measurements such as rotating ring disk electrode (RRDE) and electrochemical quartz microbalance microscopy (EQCM), coupled with a series of physical characterizations including transmission electron microscopy (TEM) and X-ray absorption spectroscopy (XAS) are conducted to unravel the exact pH effect on both the OER activity and the catalyst stability. We demonstrate that for Ni-based crystalline catalysts the rate for surface degradation depends on the pH and is greater than the rate for surface reconstruction. This behavior is unlike for amorphous Ni oxyhydroxide catalyst which is found more stable and pH independent.
Recent years have witnessed significant development of electrocatalysis for clean energy and related potential technologies. The precise identification toward active sites of catalysts and the monitoring of product information are highly desirable to understand how the materials catalyze a specific electrocatalytic reaction. For a long period, the identification of active sites and the cognition of corresponding catalytic mechanisms are generally based on various ex situ characterization methods which actually could not capture dynamic structure and intermediate information during electrocatalytic processes. With recent developments of in situ and operando characterization techniques, it has been extensively observed that most of the catalysts would undergo structural self-reconstruction as a result of electro-derived oxidation or reduction process of the catalysts at a given potential, often accompanied by the increase or decrease of catalytic activity as well as the change of catalytic selectivity. In fact, such structural self-change in the catalytic process does make it difficult to identify the true catalytically active sites efficiently, thus hindering the understanding of the real catalytic mechanism. Therefore, we believe that understanding the self-reconstruction by the combination of reliable characterization techniques and theoretical calculations holds the key to rational design of advanced catalysts. In this Account, we provide in-depth insights into recent progress regarding structural self-reconstruction of electrocatalysts in several typical electrochemical reactions with the emphasis on fundamental knowledge, structure-property relationships, structural evolution process, and modulation of self-reconstruction. To deliver a clear understanding, it has to be pointed out in advance that these catalysts with drastic structural and activity self-change in electrocatalytic processes are suggested to be called precatalysts under nonreaction conditions. The restructured active components in realistic reaction conditions are true catalysts. The structural self-reconstruction process bridges the precatalysts with true catalysts. To understand the self-reconstruction behavior, the following three critical aspects will be carefully disclosed and discussed in depth. First, fundamental origin of structural self-reconstruction of electrocatalysts is introduced. It is noteworthy that the atomic-level correlations between the self-reconstruction behavior and intrinsic structure of precatalysts are emphasized due to the fact that even if some precatalysts are congeneric, they often exhibit a diverse self-reconstruction phenomenon and catalytic performance. Second, the self-reconstruction process should be monitored by advanced characterization techniques, which is central to precisely unveil the self-reconstruction behavior. In situ or operando characterizations have been considered as judicious methods to track the self-reconstruction, capture dynamic structure and analyze real-time reaction products. Finally, based on the dynamic structure and product information together with comprehensive theory calculations, the enhancement or degradation mechanism of catalytic activities can be unambiguously clarified. With thoughtful studies toward the complete self-reconstruction process of electrocatalysts, some feasible methods to tune the self-reconstruction and improve the performance can be rationally proposed. Based on this progress, we hope to provide new insight into electrocatalysis, particularly the self-reconstruction and true active sites of electrocatalysts, and then to offer guidelines for rational design of advanced electrocatalysts.
The slow kinetics of the oxygen evolution (OER) and oxygen reduction (ORR) reactions hamper the development of renewable energy storage and conversion technologies. Transition-metal oxides (TMOs) are cost-effective replacements to conventional noble metal catalysts for driving these electrochemical systems. Strain is known to greatly affect the electronic structure of TMO surfaces, leading to significant changes in their electrocatalytic activities. In this study, we explore the influence of strain on the OER and ORR mechanisms on the LaNiO3(001) surface using density functional theory (DFT). Through a comparison of the overpotential and the largest change in Gibbs free energy (∆G) in the reaction pathway, we determined that OER activity on the LaNiO3 surface is directly related to the desorption of -H from the surface, which can be tuned as a function of strain. Moreover, tensile strain shuts off the reaction pathway to forming the –O2H intermediate state, due to the dissociation of –O2H into –O2 and –H. This is largely a consequence of the strong binding of H to the surface O, leading to a significant increase in the largest ∆G for the ORR on the tensile-strained surfaces by promoting an alternative reaction pathway. Overall, our results show that tensile strain on LaNiO3(001) leads to a decrease in both OER and ORR activities. Interestingly, in both cases, we find that the reaction is driven by the interactions with surface O ions, thus calling for a reinterpretation of the role that Ni eg orbital polarization plays in defining the OER and ORR catalytic activity on the TMO surfaces. Here, it is an indirect measure of changes in Ni-O hybridization, which controls the binding of -H species to the surface. As such, these results highlight the importance of surface O ions; particularly as it relates to defining molecule-surface interactions that ultimately tune and enhance the electrocatalytic efficiency of perovskite materials through the modulation of strains.
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This study demonstrates the importance of considering lattice oxygen participation in understanding trends in the oxygen evolution reaction (OER) on ABO3 (A = lanthanum or strontium, B = transition metal) perovskites. Using density functional theory, we show that the lattice oxygen mechanism (LOM) can lead to higher OER activity than the conventional adsorbate evolving mechanism (AEM) by minimizing the thermodynamically required overpotential. We also show that the OER activity volcano for AEM is universal for all perovskites, whereas that for LOM depends on the identity of the A cation in ABO3. This explains experimental observations that perovskites such as Pr0.5Ba0.5CoO3–δ and SrCoO3–δ show higher OER activities than the conventionally predicted optimum compounds such as LaNiO3 and SrCoO3. Furthermore, we show that LOM is preferred to AEM in achieving bifunctional catalysts capable of promoting both OER and ORR. Using our overall activity volcano, we finally suggest several candidate materials that are predicted to be highly active for OER via LOM.
There is still an ongoing effort to search for sustainable, clean and highly efficient energy generation to satisfy the energy needs of modern society. Among various advanced technologies, electrocatalysis for the oxygen evolution reaction (OER) plays a key role and numerous new electrocatalysts have been developed to improve the efficiency of gas evolution. Along the way, enormous effort has been devoted to finding high-performance electrocatalysts, which has also stimulated the invention of new techniques to investigate the properties of materials or the fundamental mechanism of the OER. This accumulated knowledge not only establishes the foundation of the mechanism of the OER, but also points out the important criteria for a good electrocatalyst based on a variety of studies. Even though it may be difficult to include all cases, the aim of this review is to inspect the current progress and offer a comprehensive insight toward the OER. This review begins with examining the theoretical principles of electrode kinetics and some measurement criteria for achieving a fair evaluation among the catalysts. The second part of this review acquaints some materials for performing OER activity, in which the metal oxide materials build the basis of OER mechanism while non-oxide materials exhibit greatly promising performance toward overall water-splitting. Attention of this review is also paid to in situ approaches to electrocatalytic behavior during OER, and this information is crucial and can provide efficient strategies to design perfect electrocatalysts for OER. Finally, the OER mechanism from the perspective of both recent experimental and theoretical investigations is discussed, as well as probable strategies for improving OER performance with regards to future developments.
Understanding how materials that catalyse the oxygen evolution reaction (OER) function is essential for the development of efficient energy-storage technologies. The traditional understanding of the OER mechanism on metal oxides involves four concerted proton–electron transfer steps on metal-ion centres at their surface and product oxygen molecules derived from water. Here, using in situ ¹⁸O isotope labelling mass spectrometry, we provide direct experimental evidence that the O2 generated during the OER on some highly active oxides can come from lattice oxygen. The oxides capable of lattice-oxygen oxidation also exhibit pH-dependent OER activity on the reversible hydrogen electrode scale, indicating non-concerted proton–electron transfers in the OER mechanism. Based on our experimental data and density functional theory calculations, we discuss mechanisms that are fundamentally different from the conventional scheme and show that increasing the covalency of metal–oxygen bonds is critical to trigger lattice-oxygen oxidation and enable non-concerted proton–electron transfers during OER.
Instabilities resulting from side reactions between the high-voltage cathode and the electrolyte are major barriers to meeting the calendar and cycle life requirements in lithium-ion batteries for vehicular applications. The present study reports a new approach for minimizing the effect of these reactions. LiMn1.5Ni0.5O4 (LMNO) with two distinct morphologies, octahedron with (111) and plate with (112) surface facets, were synthesized in a similar size and investigated for structural changes and electrochemical stability during long-term cycling and storage in the presence of a liquid carbonate electrolyte. Bulk and surface analyses using ICP, XRD, FTIR, soft and hard XAS revealed that in the charged state, the high-valent transition metals in Mn1.5Ni0.5O4 (MNO) oxidatively decompose the electrolyte which results in electron transfer from the electrolyte to the cathode. As a compensation mechanism, Li(+) ions are re-inserted into MNO and the cathode self-discharges. Surface facets where the local redox reactions occur heavily influence the reaction kinetics and selectivity which ultimately determine the nature of the products and rate of self-discharge. Significantly lower self-discharge was observed on octahedra with the (111) facets, benefiting from their ability for promoting sufficient passivation after the initial interaction with the electrolyte. The importance of particle engineering reported in this work has a broad implication in the development of next generation cathode materials with improved performance.
An algorithm is presented for carrying out decomposition of electronic charge density into atomic contributions. As suggested by Bader [R. Bader, Atoms in Molecules: A Quantum Theory, Oxford University Press, New York, 1990], space is divided up into atomic regions where the dividing surfaces are at a minimum in the charge density, i.e. the gradient of the charge density is zero along the surface normal. Instead of explicitly finding and representing the dividing surfaces, which is a challenging task, our algorithm assigns each point on a regular (x,y,z) grid to one of the regions by following a steepest ascent path on the grid. The computational work required to analyze a given charge density grid is approximately 50 arithmetic operations per grid point. The work scales linearly with the number of grid points and is essentially independent of the number of atoms in the system. The algorithm is robust and insensitive to the topology of molecular bonding. In addition to two test problems involving a water molecule and NaCl crystal, the algorithm has been used to estimate the electrical activity of a cluster of boron atoms in a silicon crystal. The highly stable three-atom boron cluster, B3I is found to have a charge of −1.5e, which suggests approximately 50% reduction in electrical activity as compared with three substitutional boron atoms.
A method is given for generating sets of special points in the Brillouin zone which provides an efficient means of integrating periodic functions of the wave vector. The integration can be over the entire Brillouin zone or over specified portions thereof. This method also has applications in spectral and density-of-state calculations. The relationships to the Chadi-Cohen and Gilat-Raubenheimer methods are indicated.
Nowadays, sol–gel procedures are well established in the synthesis of complex oxides as they allow to obtain phase pure products and to control precisely their stoichiometry. This quality makes them a tool of choice for the preparation of perovskite-type oxides. To optimize the functional properties of these materials, it is essential to set accurately their possible complex stoichiometries. However, details of the formation of the perovskite crystal remain obscure. Different stages of an ethylene-diamine-tetraacetic acid (EDTA)/citrate-gel based synthesis process for mixed conducting (Ba0.5Sr0.5)(Fe0.8Zn0.2)O3−δ of cubic perovskite structure are elucidated. The combination of analytical transmission electron microscopy with X-ray diffraction reveals that the perovskite-type oxide is formed already at moderate temperatures at around 700°C via nanoscale solid state reactions between finely-dispersed crystalline intermediates identified as a spinel and a carbonate. The reaction scheme, however, is intricate and includes stuffed tridymite structures as transient phases. The ultrafine intermixing of extremely small reactants makes EDTA/citrate-gel based procedures superior to classical solid state routes with respect to applications that demand phase purity and stoichiometry control.
Perovskites such as Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF82) can be highly active for the oxygen evolution reaction (OER) upon water oxidation in alkaline solution. Here we report that BSCF82 can quickly undergo amorphization of its surface at OER potentials, which is accompanied by reduced surface concentrations of Ba2+ and Sr2+ ions as well as increased pseudocapacitive and OER currents. Such quick amorphization during OER was also observed for perovskite catalysts with similar OER activities such as Ba0.5Sr0.5Co0.4Fe0.6O3−δ and SrCo0.8Fe0.2O3−δ. In contrast, perovskite oxides with lower OER activities than BSCF82 did not undergo this transformation when subjected to identical electrochemical conditions. These findings demonstrate that the active chemistry and structure of oxide catalysts during OER can significantly differ from those of the as-synthesized material and that understanding how the oxide surface may change and impact the OER activity is critical to the design of highly active and stable OER catalysts.
An approach for electronic structure calculations is described that generalizes both the pseudopotential method and the linear augmented-plane-wave (LAPW) method in a natural way. The method allows high-quality first-principles molecular-dynamics calculations to be performed using the original fictitious Lagrangian approach of Car and Parrinello. Like the LAPW method it can be used to treat first-row and transition-metal elements with affordable effort and provides access to the full wave function. The augmentation procedure is generalized in that partial-wave expansions are not determined by the value and the derivative of the envelope function at some muffin-tin radius, but rather by the overlap with localized projector functions. The pseudopotential approach based on generalized separable pseudopotentials can be regained by a simple approximation.
A simple formulation of a generalized gradient approximation for the exchange and correlation energy of electrons has been proposed by Perdew, Burke, and Ernzerhof (PBE) [Phys. Rev. Lett. 77, 3865 (1996)]. Subsequently Zhang and Yang [Phys. Rev. Lett. 80, 890 (1998)] have shown that a slight revision of the PBE functional systematically improves the atomization energies for a large database of small molecules. In the present work, we show that the Zhang and Yang functional (revPBE) also improves the chemisorption energetics of atoms and molecules on transition-metal surfaces. Our test systems comprise atomic and molecular adsorption of oxygen, CO, and NO on Ni(100), Ni(111), Rh(100), Pd(100), and Pd(111) surfaces. As the revPBE functional may locally violate the Lieb-Oxford criterion, we further develop an alternative revision of the PBE functional, RPBE, which gives the same improvement of the chemisorption energies as the revPBE functional at the same time as it fulfills the Lieb-Oxford criterion loc
Building upon recent study of cobalt-oxide electrocatalysts in fluoride-buffered electrolyte at pH 3.4, we have undertaken a mechanistic investigation of cobalt-catalyzed water oxidation in aqueous buffering electrolytes from pH 0-14. This work includes electrokinetic studies, cyclic voltammetric analysis, and electron paramagnetic resonance (EPR) spectroscopic studies. The results illuminate a set of interrelated mechanisms for electrochemical water oxidation in alkaline, neutral, and acidic media with electrodeposited Co-oxide catalyst films (CoO(x)(cf)s) as well as for a homogeneous Co-catalyzed electrochemical water oxidation reaction. Analysis of the pH dependence of quasi-reversible features in cyclic voltammograms of the CoO(x)(cf)s provides the basis for a Pourbaix diagram that closely resembles a Pourbaix diagram derived from thermodynamic free energies of formation for a family of Co-based layered materials. Below pH 3, a shift from heterogeneous catalysis producing O(2) to homogeneous catalysis yielding H(2)O(2) is observed. Collectively, the results reported here provide a foundation for understanding the structure, stability, and catalytic activity of aqueous cobalt electrocatalysts for water oxidation.
The occupation of d orbitals controls the magnitude and anisotropy of the inter-atomic electron transfer in transition-metal oxides and hence exerts a key influence on their chemical bonding and physical properties. Atomic-scale modulations of the orbital occupation at surfaces and interfaces are believed to be responsible for massive variations of the magnetic and transport properties, but could not thus far be probed in a quantitative manner. Here we show that it is possible to derive quantitative, spatially resolved orbital polarization profiles from soft-X-ray reflectivity data, without resorting to model calculations. We demonstrate that the method is sensitive enough to resolve differences of ~3% in the occupation of Ni e(g) orbitals in adjacent atomic layers of a LaNiO(3)-LaAlO(3) superlattice, in good agreement with ab initio electronic-structure calculations. The possibility to quantitatively correlate theory and experiment on the atomic scale opens up many new perspectives for orbital physics in transition-metal oxides.