Double perovskites as a family of highly active catalysts for oxygen evolution in alkaline solution

Tunable metal hydroxide-organic frameworks for catalysing oxygen evolution

The oxygen evolution reaction is central to making chemicals and energy carriers using electrons. Combining the great tunability of enzymatic systems with known oxide-based catalysts can create breakthrough opportunities to achieve both high activity and stability. Here we report a series of metal hydroxide-organic frameworks (MHOFs) synthesized by transforming layered hydroxides into two-dimensional sheets crosslinked using aromatic carboxylate linkers. MHOFs act as a tunable catalytic platform for the oxygen evolution reaction, where the π-π interactions between adjacent stacked linkers dictate stability, while the nature of transition metals in the hydroxides modulates catalytic activity. Substituting Ni-based MHOFs with acidic cations or electron-withdrawing linkers enhances oxygen evolution reaction activity by over three orders of magnitude per metal site, with Fe substitution achieving a mass activity of 80 A [Formula: see text] at 0.3 V overpotential for 20 h. Density functional theory calculations correlate the enhanced oxygen evolution reaction activity with the MHOF-based modulation of Ni redox and the optimized binding of oxygenated intermediates.

Oxygen Evolution Reaction in Energy Conversion and Storage: Design Strategies Under and Beyond the Energy Scaling Relationship

The oxygen evolution reaction (OER) is the essential module in energy conversion and storage devices such as electrolyzer, rechargeable metal-air batteries and regenerative fuel cells. The adsorption energy scaling relations between the reaction intermediates, however, impose a large intrinsic overpotential and sluggish reaction kinetics on OER catalysts. Developing advanced electrocatalysts with high activity and stability based on non-noble metal materials is still a grand challenge. Central to the rational design of novel and high-efficiency catalysts is the development and understanding of quantitative structure-activity relationships, which correlate the catalytic activities with structural and electronic descriptors. This paper comprehensively reviews the benchmark descriptors for OER electrolysis, aiming to give an in-depth understanding on the origins of the electrocatalytic activity of the OER and further contribute to building the theory of electrocatalysis. Meanwhile, the cutting-edge research frontiers for proposing new OER paradigms and crucial strategies to circumvent the scaling relationship are also summarized. Challenges, opportunities and perspectives are discussed, intending to shed some light on the rational design concepts and advance the development of more efficient catalysts for enhancing OER performance.

Activating lattice oxygen in NiFe-based (oxy)hydroxide for water electrolysis

Transition metal oxides or (oxy)hydroxides have been intensively investigated as promising electrocatalysts for energy and environmental applications. Oxygen in the lattice was reported recently to actively participate in surface reactions. Herein, we report a sacrificial template-directed approach to synthesize Mo-doped NiFe (oxy)hydroxide with modulated oxygen activity as an enhanced electrocatalyst towards oxygen evolution reaction (OER). The obtained MoNiFe (oxy)hydroxide displays a high mass activity of 1910 A/g_metal at the overpotential of 300 mV. The combination of density functional theory calculations and advanced spectroscopy techniques suggests that the Mo dopant upshifts the O 2p band and weakens the metal-oxygen bond of NiFe (oxy)hydroxide, facilitating oxygen vacancy formation and shifting the reaction pathway for OER. Our results provide critical insights into the role of lattice oxygen in determining the activity of (oxy)hydroxides and demonstrate tuning oxygen activity as a promising approach for constructing highly active electrocatalysts.

Anion-Exchange Membrane Water Electrolyzers

This Review provides an overview of the emerging concepts of catalysts, membranes, and membrane electrode assemblies (MEAs) for water electrolyzers with anion-exchange membranes (AEMs), also known as zero-gap alkaline water electrolyzers. Much of the recent progress is due to improvements in materials chemistry, MEA designs, and optimized operation conditions. Research on anion-exchange polymers (AEPs) has focused on the cationic head/backbone/side-chain structures and key properties such as ionic conductivity and alkaline stability. Several approaches, such as cross-linking, microphase, and organic/inorganic composites, have been proposed to improve the anion-exchange performance and the chemical and mechanical stability of AEMs. Numerous AEMs now exceed values of 0.1 S/cm (at 60–80 °C), although the stability specifically at temperatures exceeding 60 °C needs further enhancement. The oxygen evolution reaction (OER) is still a limiting factor. An analysis of thin-layer OER data suggests that NiFe-type catalysts have the highest activity. There is debate on the active-site mechanism of the NiFe catalysts, and their long-term stability needs to be understood. Addition of Co to NiFe increases the conductivity of these catalysts. The same analysis for the hydrogen evolution reaction (HER) shows carbon-supported Pt to be dominating, although PtNi alloys and clusters of Ni(OH)₂ on Pt show competitive activities. Recent advances in forming and embedding well-dispersed Ru nanoparticles on functionalized high-surface-area carbon supports show promising HER activities. However, the stability of these catalysts under actual AEMWE operating conditions needs to be proven. The field is advancing rapidly but could benefit through the adaptation of new in situ techniques, standardized evaluation protocols for AEMWE conditions, and innovative catalyst-structure designs. Nevertheless, single AEM water electrolyzer cells have been operated for several thousand hours at temperatures and current densities as high as 60 °C and 1 A/cm², respectively.

Engineering Sulfur Vacancies in Spinel-Phase Co3S4 for Effective Electrocatalysis of the Oxygen Evolution Reaction

Restricted by the sluggish kinetics of the oxygen evolution reaction (OER), efficient OER catalysis remains a challenge. Here, a facile strategy was proposed to prepare a hollow dodecahedron constructed by vacancy-rich spinel Co₃S₄ nanoparticles in a self-generated H₂S atmosphere of thiourea. The morphology, composition, and electronic structure, especially the sulfur vacancy, of the cobalt sulfides can be regulated by the dose of thiourea. Benefitting from the H₂S atmosphere, the anion exchange process and vacancy introduction can be accomplished simultaneously. The resulting catalyst exhibits excellent catalytic activity for the OER with a low overpotential of 270 mV to reach a current density of 10 mA cm^-2 and a small Tafel slope of 59 mV dec^-1. Combined with various characterizations and electrochemical tests, the as-proposed defect engineering method could delocalize cobalt neighboring electrons and expose more Co²⁺ sites in spinel Co₃S₄, which lowers the charge transfer resistance and facilitates the formation of Co³⁺ active sites during the preactivation process. This work paves a new way for the rational design of vacancy-enriched transition metal-based catalysts toward an efficient OER.

Effect of electron structure on the catalytic activity of LaCoO3 perovskite towards toluene oxidation

LaCoO₃ perovskites with different spin states of Co³⁺ were prepared by calcination at 600-1000 °C. LaCoO₃ with electron filling in the e_g orbital at 1 exhibited a moderate interaction between the surface oxygen, resulting in the best catalytic activity. This was verified by the O p-band center.

Tuning the Polarity of a Fibrous Poly(vinylidene fluoride-co-hexafluoropropylene)-Based Support for Efficient Water Electrolysis

Water electrolysis under alkaline conditions is of interest due to the applicability of non-precious metal-based materials for electrocatalysts. However, the successful design and synthesis of earth-abundant and efficient catalysts for the oxygen evolution reaction (OER) remain a significant challenge. This work presents cost-effective and straightforward ways to improve the OER activity under alkaline conditions by activating the catalyst-support and reactant-support interaction. Micro/nano-sized fibrous poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) was synthesized via simple and scalable electrospinning and subsequently coated with Cu by electroless deposition to obtain the electrocatalyst with a large specific surface area, enhanced mass transport, and high catalyst utilization. Scanning electron microscopy, infrared spectroscopy, and X-ray diffraction confirmed the successful synthesis of the series of Cu/PVdF-HFP fibrous catalysts with varied ferroelectric polarizability of the PVdF-HFP support in the order of stretch-anneal > anneal > stretch > without pre-treatment of the catalyst. The best OER activity was confirmed for the Cu/PVdF-HFP catalyst with stretch and annealed treatment among the catalysts tested, suggesting that both the reaction kinetics and energetics of stretch-annealed Cu/PVdF-HFP catalysts were optimal for the OER. The electron delocalization between Cu and PVdF-HFP substrates (electron transfer from Cu to the negatively charged (δ^- _eff) PVdF-HFP region at the Cu|PVdF-HFP interface) and the enhanced transport of reactive hydroxide species and/or the increase in the local pH by positively charged (δ⁺ _eff) PVdF-HFP region concertedly accelerate the OER activity. The overall activity for the prototype water electrolyzer increased 10-fold with stretch-anneal treatment compared to the one without pre-treatment, highlighting the effect of tuning the catalyst-support and reactant-support interaction on improving the efficiency of the water electrolysis.

Lattice B-doping evolved ferromagnetic perovskite-like catalyst for enhancing persulfate-based degradation of norfloxacin

Series of B-doped perovskite-like materials CeCu_0.5Co_0.5O₃ (B-C³O) were fabricated with unique ferromagnetic property due to partial substitution of non-magnetic 2p-impurities boron in the lattice. Then, B-C³O was used for activating peroxymonosulfate (PMS) for the degradation of norfloxacin (NOR), one kind of emerging pollutants with the concentration level up to mg/L in wastewaters. The results indicated that 5.0% B-C³O exhibited stable catalytic ability at pH 3.0-9.0 and high degradation efficiency in co-existing inorganic Cl^-, SO₄^2-, NO₃^-, H₂PO₄^- and organic humic acid. Non-radical ¹O₂, radicals •OH and SO₄^•-, as well as ClO^- were detected with synergy effect for NOR degradation. By quantifying free radicals, •OH with 0.52 µM and SO₄^•- with 10.91 µM were obtained at 180 min, verifying the leading role of SO₄^•-. The degradation process involved the defluorination and decarboxylation, as well as opening of quinolone and piperazinyl rings. Adopting alfalfa as the model plant, the toxicity effect before and after NOR degradation was finally evaluated with seed germination rate and chlorophyll content as the physiological indicators. In summary, non-metal B-doping not only provides a creative strategy for the development of ferromagnetic perovskite-like materials, but also affords excellent catalysts for aiding the advanced oxidation technology for removal of emerging pollutants in wastewaters.

Thermodynamic Stability and Intrinsic Activity of La1−xSrxMnO3 (LSM) as an Efficient Bifunctional OER/ORR Electrocatalysts: A Theoretical Study

Perovskite oxides, being transition metal oxides, show promise as bifunctional catalysts being able to catalyze both oxygen evolution reactions (OER) and oxygen reduction reactions (ORR). These two reactions play a crucial role in energy storage and energy conversion devices. An important feature of perovskite catalyst is their ability to be tuned, as tuning can positively affect both reactivity and stability. In this study, Density Functional Theory (DFT) has been utilized to calculate both the equilibrium phase stability and the overpotentials (reactivity performance indicator of the catalysts) of La1−xSrxMnO3 (LSM) structures with different stoichiometry by introducing Mn and O vacancies for both the OER/ORR reactions. The electronic structures reveal that combined Mn and O vacancies can lead to higher catalytic activity for both OER and ORR due to the optimum filling of antibonding orbital electrons. Moreover, both O p-band centers and equilibrium phase stability plots show that LSM structures can be stable at normal OER/ORR operating conditions in an alkali medium.

Highly active postspinel-structured catalysts for oxygen evolution reaction

The rational design principle of highly active catalysts for the oxygen evolution reaction (OER) is desired because of its versatility for energy-conversion applications. Postspinel-structured oxides, CaB₂O₄ (B = Cr³⁺, Mn³⁺, and Fe³⁺), have exhibited higher OER activities than nominally isoelectronic conventional counterparts of perovskite oxides LaBO₃ and spinel oxides ZnB₂O₄. Electrochemical impedance spectroscopy reveals that the higher OER activities for CaB₂O₄ series are attributed to the lower charge-transfer resistances. A density-functional-theory calculation proposes a novel mechanism associated with lattice oxygen pairing with adsorbed oxygen, demonstrating the lowest theoretical OER overpotential than other mechanisms examined in this study. This finding proposes a structure-driven design of electrocatalysts associated with a novel OER mechanism.

Postspinel-structured oxides, CaB₂O₄ (B = Cr³⁺, Mn³⁺, and Fe³⁺), have exhibited systematically higher catalytic activities in the oxygen evolution reaction (OER) than nominally conventional counterparts of perovskite LaBO₃ and spinel ZnB₂O₄.

Insights into Oxygen Migration in LaBaCo2O6-δ Perovskites from In Situ Neutron Powder Diffraction and Bond Valence Site Energy Calculations

Layered cobalt oxide perovskites are important mixed ionic and electronic conductors. Here, we investigate LaBaCo₂O_6-δ using in situ neutron powder diffraction. This composition is unique because it can be prepared in cubic, layered, and vacancy-ordered forms. Thermogravimetric analysis and diffraction reveal that layered and disordered samples have near-identical oxygen cycling capacities. Migration barriers for oxide ion conduction calculated using the bond valence site energy approach vary from E _b ∼ 2.8 eV for the cubic perovskite to E _b ∼ 1.5 eV for 2D transport in the layered system. Vacancy-ordered superstructures were observed at low temperatures, 350-400 °C for δ = 0.25 and δ = 0.5. The vacancy ordering at δ = 0.5 is different from the widely reported structure and involves oxygen sites in both CoO₂ and LaO planes. Vacancy ordering leads to the emergence of additional migration pathways with low-energy barriers, for example, 1D channels with E _b = 0.5 eV and 3D channels with E _b = 2.2 eV. The emergence of these channels is caused by the strong orthorhombic distortion of the crystal structure. These results demonstrate that there is potential scope to manipulate ionic transport in vacancy-ordered LnBaCo₂O_6-δ perovskites with reduced symmetry.

Electrocatalysis in Alkaline Media and Alkaline Membrane-Based Energy Technologies

Hydrogen energy-based electrochemical energy conversion technologies offer the promise of enabling a transition of the global energy landscape from fossil fuels to renewable energy. Here, we present a comprehensive review of the fundamentals of electrocatalysis in alkaline media and applications in alkaline-based energy technologies, particularly alkaline fuel cells and water electrolyzers. Anion exchange (alkaline) membrane fuel cells (AEMFCs) enable the use of nonprecious electrocatalysts for the sluggish oxygen reduction reaction (ORR), relative to proton exchange membrane fuel cells (PEMFCs), which require Pt-based electrocatalysts. However, the hydrogen oxidation reaction (HOR) kinetics is significantly slower in alkaline media than in acidic media. Understanding these phenomena requires applying theoretical and experimental methods to unravel molecular-level thermodynamics and kinetics of hydrogen and oxygen electrocatalysis and, particularly, the proton-coupled electron transfer (PCET) process that takes place in a proton-deficient alkaline media. Extensive electrochemical and spectroscopic studies, on single-crystal Pt and metal oxides, have contributed to the development of activity descriptors, as well as the identification of the nature of active sites, and the rate-determining steps of the HOR and ORR. Among these, the structure and reactivity of interfacial water serve as key potential and pH-dependent kinetic factors that are helping elucidate the origins of the HOR and ORR activity differences in acids and bases. Additionally, deliberately modulating and controlling catalyst-support interactions have provided valuable insights for enhancing catalyst accessibility and durability during operation. The design and synthesis of highly conductive and durable alkaline membranes/ionomers have enabled AEMFCs to reach initial performance metrics equal to or higher than those of PEMFCs. We emphasize the importance of using membrane electrode assemblies (MEAs) to integrate the often separately pursued/optimized electrocatalyst/support and membranes/ionomer components. Operando/in situ methods, at multiscales, and ab initio simulations provide a mechanistic understanding of electron, ion, and mass transport at catalyst/ionomer/membrane interfaces and the necessary guidance to achieve fuel cell operation in air over thousands of hours. We hope that this Review will serve as a roadmap for advancing the scientific understanding of the fundamental factors governing electrochemical energy conversion in alkaline media with the ultimate goal of achieving ultralow Pt or precious-metal-free high-performance and durable alkaline fuel cells and related technologies.

Heterogeneous Activation of Persulfate by LaMO3 (M=Co, Fe, Cu, Mn, Ni) Perovskite Catalysts for the Degradation of Organic Compounds

Sulfate radical-based advanced oxidation processes (SR-AOPs) are lately applied for the degradation of various pollutants through the formation of reactive oxidant species (ROS) from activation of oxidants, such as persulfate (PS) or peroxymonosulfate (PMS). In this study, LaMO3 (M=Co, Fe, Cu, Mn, Ni) perovskite catalysts were synthesized, characterized by several techniques, and tested for the activation of persulfate towards the degradation of phenolic pollutants. The effect of substitution of position B of La-based perovskites as well as calcination temperature was studied. Overall, the results showed that the decisive role in the catalytic activity was the presence of structures that enhance the transfer of electrons between perovskite and oxidant. LaNiO3 followed by LaCoO3 were found as the most active catalysts. Finally, the stability of the catalysts was studied, showing that B-metal leaching is significant for both catalysts, with LaCoO3 being the most stable one.

Electronic Structure-Based Descriptors for Oxide Properties and Functions

ConspectusThe transition from fossil fuels to renewable energy requires the development of efficient and cost-effective energy storage technologies. A promising way forward is to harness the energy of intermittent renewable sources, such as solar and wind, to perform (electro)catalytic reactions to generate fuels, thus storing energy in the form of chemical bonds. However, current catalysts rely on the use of expensive, rare, or geographically localized elements, such as platinum. Widespread adoption of new (electro)catalytic technologies hinges on the discovery and development of materials containing earth-abundant elements, which can efficiently catalyze an array of (electro)chemical reactions.In the context of catalysis, descriptors provide correlations between fundamental physical properties, such as the electronic structure, and the resulting catalytic activity. The use of easily accessible descriptors has proven to be a powerful method to advance and accelerate discovery and design of new catalyst materials. The position of the oxygen electronic 2p band center has been proposed to capture the basic physical properties of oxides, including oxygen vacancy formation energy, diffusion barrier of oxygen ions, and work function. Moreover, the adsorption strength of relevant reaction intermediates at the surface of oxides can be strongly correlated with the energy of the oxygen 2p states, which affects the catalytic activity of reactions, such as oxygen electrocatalysis, and oxidative dehydrogenation of organic molecules. Such descriptors for catalytic activity can be used to predict the activity of new catalysts and understand trends and behavior among different catalysts.In this Account, we discuss how the energy of the oxygen 2p states can be used as a descriptor for oxide bulk and surface chemical properties. We show how the oxide redox properties vary linearly with the position of the oxygen 2p band center with respect to the Fermi level, and we discuss how this descriptor can be expanded across different materials and structural families, including possible generalizations to compounds outside oxides. We highlight the power of the oxygen 2p band center to predict the catalytic activity of oxides. We conclude with an outlook examining under which conditions this descriptor can be applied to predict oxide properties and possible opportunities for further refining and accelerating property predictions of oxides by leveraging material databases and machine learning.

Regulating Na Occupation to Introduce Non-Fermi-Liquid States of NaxCoO2 for Enhanced Water Oxidation Activity

The couplings among fundamental quantum parameters provide versatile freedom of manipulations for useful electronic structures, based on which optimized oxygen evolution reaction (OER) performances can be achieved. In this work, we demonstrate the successful regulation of the electronic structure in layered Na_xCoO₂ oxides to introduce a non-Fermi-liquid (NFL) state by adjusting the Na content and Na occupation in the lattice. The presence of an NFL is facilitated by the weakened electron-electron correlation when the on-site Coulomb repulsion of Co⁴⁺ with Na⁺ and oxygen vacancy with Na⁺ is balanced. As a feature of NFL, the metallic states in the vicinity of the Fermi energy contribute to a fast electron transfer efficiency and eventually to an improved OER performance. These findings open up a new avenue to design highly efficient OER electrocatalysts in strong electron-correlated transition metal material systems by consideration of couplings among the fundamental quantum parameters.

Fundamental understanding of electrocatalysis over layered double hydroxides from the aspects of crystal and electronic structures

Layered double hydroxides (LDHs) composed of octahedral ligand units centered with various transition metal atoms display unique electronic structures and thus attract significant attention in the field of electrocatalytic oxygen evolution reactions (OER). Intensive experimental explorations have therefore been carried out to investigate the LDHs synthesis, amorphous control, intrinsic material modifications, interfacing with other phases, strain, etc. There is still the need for a fundamental understanding of the structure-property relations, which could hinder the design of the next generation of the LDHs catalysts. In this review, we firstly provide the crystal structure information accompanied by the corresponding electronic structures. Then, we discuss the conflicts of the active sites on the NiFe LDHs and propose the synergistic cooperation among the ligand units during OER to deliver a different angle for understanding the current structure-property relations beyond the single-site-based catalysis process. In the next section of the OER process, the linear relationship-induced theoretical limit of the overpotential is further discussed based on the fundamental aspects. To break up the linear relations, we have summarized the current strategies for optimizing the OER performance. Lastly, based on the understanding gained above, the perspective of the research challenges and opportunities are proposed.

Perovskite With Tunable Active-Sites Oxidation State by High-Valence W for Enhanced Oxygen Evolution Reaction

Perovskite oxides have been established as a promising kind of catalyst for alkaline oxygen evolution reactions (OER), because of their regulated non-precious metal components. However, the surface lattice is amorphous during the reaction, which gradually decreases the intrinsic activity and stability of catalysts. Herein, the precisely control tungsten atoms substituted perovskite oxides (Pr_0.5Ba_0.5Co_1-xW_xO₃-_δ) nanowires were developed by electrostatic spinning. The activity and Tafel slope were both dependent on the W content in a volcano-like fashion, and the optimized Pr_0.5Ba_0.5Co_0.8W_0.2O₃-_δ exhibits both excellent activity and superior stability compared with other reported perovskite oxides. Due to the outermost vacant orbitals of W⁶⁺, the electronic structure of cobalt sites could be efficiently optimized. Meanwhile, the stronger W-O bond could also significantly improve the stability of latticed oxide atoms to impede the generation of surface amorphous layers, which shows good application value in alkaline water splitting.

Highly active and stable surface structure for oxygen evolution reaction originating from balanced dissolution and strong connectivity in BaIrO3 solid solutions

Catalysts for the oxygen evolution reaction (OER) are receiving great interest since OER remains the bottleneck of water electrolyzers for hydrogen production. Especially, OER in acidic solutions is crucial since it produces high current densities and avoids precipitation of carbonates. However, even the acid stable iridates undergo severe dissolution during the OER. BaIrO₃ has the strongest IrO₆ connectivity and stable surface structure, yet it suffers from lattice collapse after OER cycling, making it difficult to improve the OER durability. In the present study, we have successfully developed an OER catalyst with both high intrinsic activity and stability under acidic conditions by preventing the lattice collapse after repeated OER cycling. Specifically, we find that the substitution of Ir-site with Mn for BaIrO₃ in combination with OER cycling leads to a remarkable activity enhancement by a factor of 28 and an overall improvement in stability. This dual enhancement of OER performance was accomplished by the novel strategy of slightly increasing the Ir-dissolution and balancing the elemental dissolution in BaIr_1−xMn_xO₃ to reconstruct a rigid surface with BaIrO₃-type structure. More importantly, the mass activity for BaIr_0.8Mn_0.2O₃ reached ∼73 times of that for IrO₂, making it a sustainable and promising OER catalyst for energy conversion technologies.

We have prevented lattice collapse and developed an OER catalyst with both high activity and stability by slightly increasing Ir-dissolution and balancing the elemental dissolution in BaIr_1−xMn_xO₃ for reconstructing the rigid catalytic surface.

Emerging Electrocatalysts for Water Oxidation under Near-Neutral CO2 Reduction Conditions

Electrocatalytic CO₂ reduction reaction (CO₂ RR), which produces valuable fuels and chemicals under near-neutral conditions, offers a renewable approach to alleviate the global energy crisis as well as the increasing concerns on climate change. However, to implement this strategy, one of the major challenges, the sluggish kinetics of the paired oxygen evolution reaction (OER) at anode, needs to be surmounted. It is therefore highly desirable to explore high-performance and cost-effective OER electrocatalysts suitable for CO₂ RR conditions, which is very different from those widely investigated under acidic or alkaline conditions. In this review, the ongoing development of OER electrocatalysts under near pH-neutral CO₂ -saturated (bi)carbonate solutions are highlighted and the future opportunities are discussed. It is started with a brief introduction on OER paired with CO₂ RR, the relevant theoretical tools such as density functional theory (DFT) and particularly machine learning (ML), and the operando characterization techniques. Then, there are some detailed discussions of recent progress on the rational design of OER electrocatalysts under CO₂ RR conditions ranging from noble-metal oxides to nonprecious metal phosphides, carbonates, (hydro)oxides, and so on. Finally, a perspective for developing OER electrocatalysts integrated with CO₂ electroreduction is proposed.

Hexagonal perovskite Sr6(Co0.8Fe0.2)5O15 as an efficient electrocatalyst towards the oxygen evolution reaction

The high overpotential required for the oxygen evolution reaction (OER)—due to the transfer of four protons and four electrons—has greatly hindered the commercial viability of water electrolysis. People have been...

Quenching as a Route to Defect-Rich Ru-Pyrochlore Electrocatalysts toward the Oxygen Evolution Reaction

Defects have a significant impact on the electrocatalysts performance. Introducing defect structures in metal oxides such as pyrochlores and perovskites has proved to be an effective strategy to enhance electrocatalytic activity. However, it is hard to build numerous defect sites in such high-temperature oxides due to the strong metal-oxygen bonds and the so-called self-purification effect, which becomes increasingly important as the particle size reduced to the nanoscale. Here, a facile strategy is demonstrated to fabricate defect-rich yttrium ruthenate oxides Y₂ Ru₂ O_{7- δ} with the pyrochlore structure (denoted D_rich -YRO) by the liquid nitrogen (<-196 °C) quenching. Owing to the almost instantaneous cooling in oxygen-deficient condition, a large number of defects-including oxygen vacancies, grain boundaries, pores and surficial disorder-are preserved in the room temperature material and act as electrocatalytic active sites for oxygen evolution. As a result, D_rich -YRO shows excellent catalytic activity and high electrochemical stability, along with a high performance in the operation of proton exchange membrane electrolyzer. The quenching strategy employed in this work provides a facile approach for constructing defect-rich structures in high-temperature oxides and should lead to new applications in energy conversion and storage devices for such materials.

Dual nature of high-temperature electronic transport in layered perovskite-like cobaltites: exhaustive consideration of experimental features observed

Complex oxides with the general formula Pr_1-xY_xBaCo_2-yNi_yO_6-δ (x = 0, 0.1, y = 0, 0.2) were successfully synthesized via combustion of organo-metallic precursors. The high-temperature dc conductivity of the obtained sintered materials was studied in a wide range of oxygen partial pressures and temperatures by means of the 4-probe method. The resulting dependencies were juxtaposed with the previously published data on oxygen non-stoichiometry for the oxides considered. The comprehensive analysis of these datasets in attempts to explain the observed trends has shown the large inadequacy of currently existing conduction models. Consequently, a new model approach was developed to account for the numerous experimental and theoretical peculiarities being characteristic for the cobaltites with a layered double perovskite structure. One of the key propositions made postulates mixed nature of the band structure for materials studied with spin states of Co ions acting as a spatial descriptor of a particular type of conductivity: semiconducting or a metallic one. The abovementioned hypothesis was validated by magnetic, thermodynamic and structural arguments obtained both theoretically and experimentally. The models suggested were shown to be adequate in describing large arrays of data collected. Additionally, the reasons behind doping and temperature/pressure influences on the respective conductivity changes in Pr_1-xY_xBaCo_2-yNi_yO_6-δ were uncovered. Transport and thermodynamic parameters determined were used to evaluate transference numbers and mobilities of different charge carriers which revealed the dominating role of metallic conductivity under oxidative conditions and the superiority of semiconducting charge transport in reducing environments. The obtained conclusions were further supported by utilizing the derived model equations for successful description of conductivity/non-stoichiometry data for other layered cobaltites. Also, interesting correlations between cation composition, thermodynamic and transport properties were found. Finally, general review of the formulated approach was made and further research directions were proposed.

Promoting the Oxygen Evolution Activity of Perovskite Nickelates through Phase Engineering

Perovskite oxides have emerged as promising candidates for the oxygen evolution reaction (OER) electrocatalyst due to their flexible lattice structure, tunable electronic structure, superior stability, and cost-effectiveness. Recent research studies have mostly focused on the traditional methods to tune the OER performance, such as cation/anion doping, A-/B-site ordering, epitaxial strain, oxygen vacancy, and so forth, leading to reasonable yet still limited activity enhancement. Here, we report a novel strategy for promoting the OER activity for perovskite LaNiO₃ by crystal phase engineering, which is realized by breaking long-range chemical bonding through amorphization. We provide the first and direct evidence that perovskite oxides with an amorphous structure can induce the self-adaptive process, which helps enhance the OER performance. This is evidenced by the fact that an amorphous LaNiO₃ film on glassy carbon shows a 9-fold increase in the current density compared to that of an epitaxial LaNiO₃ single crystalline film. The obtained current density of 1038 μΑ cm^-2 (@ 1.6 vs RHE) is the largest value among the literature reported values. Our work thus offers a new protocol to boost the OER performance for perovskite oxides for future clean energy applications.

Developing sustainable, high-performance perovskites in photocatalysis: design strategies and applications

Solar energy is attractive because it is free, renewable, abundant and sustainable. Photocatalysis is one of the feasible routes to utilize solar energy for the degradation of pollutants and the production of fuel. Perovskites and their derivatives have received substantial attention in both photocatalytic wastewater treatment and energy production because of their highly tailorable structural and physicochemical properties. This review illustrates the basic principles of photocatalytic reactions and the application of these principles to the design of robust and sustainable perovskite photocatalysts. It details the structures of the perovskites and the physics and chemistry behind photocatalytic reactions and describes the advantages and limitations of popular strategies for the design of photoactive perovskites. This is followed by examples of how these strategies are applied to enhance the photocatalytic efficiency of oxide, halide and oxyhalide perovskites, with a focus on materials with potential for practical application, that is, not containing scarce or toxic elements. It is expected that this overview of the development of photocatalysts and deeper understanding of photocatalytic principles will accelerate the exploitation of efficient perovskite photocatalysts and bring about effective solutions to the energy and environmental crisis.

Lattice site-dependent metal leaching in perovskites toward a honeycomb-like water oxidation catalyst

Metal leaching during water oxidation has been typically observed in conjunction with surface reconstruction on perovskite oxide catalysts, but the role of metal leaching at each geometric site has not been distinguished. Here, we manipulate the occurrence and process of surface reconstruction in two model ABO₃ perovskites, i.e., SrSc_0.5Ir_0.5O₃ and SrCo_0.5Ir_0.5O₃, which allow us to evaluate the structure and activity evolution step by step. The occurrence and order of leaching of Sr (A-site) and Sc/Co (B-site) were controlled by tailoring the thermodynamic stability of B-site. Sr leaching from A-site mainly generates more electrochemical surface area for the reaction, and additional leaching of Sc/Co from B-site triggers the formation of a honeycomb-like IrO_xH_y phase with a notable increase in intrinsic activity. A thorough surface reconstruction with dual-site metal leaching induces an activity improvement by approximately two orders of magnitude, which makes the reconstructed SrCo_0.5Ir_0.5O₃ among the best for water oxidation in acid.

5f Covalency Synergistically Boosting Oxygen Evolution of UCoO4 Catalyst

Electronic structure modulation among multiple metal sites is key to the design of efficient catalysts. Most studies have focused on regulating 3d transition-metal active ions through other d-block metals, while few have utilized f-block metals. Herein, we report a new class of catalyst, namely, UCoO₄ with alternative CoO₆ and 5f-related UO₆ octahedra, as a unique example of a 5f-covalent compound that exhibits enhanced electrocatalytic oxygen evolution reaction (OER) activity because of the presence of the U 5f–O 2p–Co 3d network. UCoO₄ exhibits a low overpotential of 250 mV at 10 mA cm^–2, surpassing other unitary cobalt-based catalysts ever reported. X-ray absorption spectroscopy revealed that the Co²⁺ ion in pristine UCoO₄ was converted to high-valence Co^3+/4+, while U⁶⁺ remained unchanged during the OER, indicating that only Co was the active site. Density functional theory calculations demonstrated that the OER activity of Co^3+/4+ was synergistically enhanced by the covalent bonding of U⁶⁺-5f in the U 5f–O 2p–Co 3d network. This study opens new avenues for the realization of electronic structure manipulation via unique 5f involvement.

Design principles of noble metal-free electrocatalysts for hydrogen production in alkaline media: combining theory and experiment

Water electrolysis is a promising solution to convert renewable energy sources to hydrogen as a high-energy-density energy carrier. Although alkaline conditions extend the scope of electrocatalysts beyond precious metal-based materials to earth-abundant materials, the sluggish kinetics of cathodic and anodic reactions (hydrogen and oxygen evolution reactions, respectively) impede the development of practical electrocatalysts that do not use precious metals. This review discusses the rational design of efficient electrocatalysts by exploiting the understanding of alkaline hydrogen evolution reaction and oxygen evolution reaction mechanisms and of the electron structure-activity relationship, as achieved by combining experimental and computational approaches. The enhancement of water splitting not only deals with intrinsic catalytic activity but also includes the aspect of electrical conductivity and stability. Future perspectives to increase the synergy between theory and experiment are also proposed.

Structural Variations of Metal Oxide-Based Electrocatalysts for Oxygen Evolution Reaction

Electrocatalytic oxygen evolution reaction (OER), an important electrode reaction in electrocatalytic and photoelectrochemical cells for a carbon-free energy cycle, has attracted considerable attention in the last few years. Metal oxides have been considered as good candidates for electrocatalytic OER because they can be easily synthesized and are relatively stable during the OER process. However, inevitable structural variations still occur to them due to the complex reaction steps and harsh working conditions of OER, thus impending the further insight into the catalytic mechanism and rational design of highly efficient electrocatalysts. The aim of this review is to disclose the current research progress toward the structural variations of metal oxide-based OER electrocatalysts. The origin of structural variations of metal oxides is discussed. Based on some typical oxides performing OER activity, the external and internal factors that influence the structural stability are summarized and then some general approaches to regulate the structural variation process are provided. Some operando methods are also concluded to monitor the structural variation processes and to identify the final active structure. Additionally, the unresolved problems and challenges are presented in an attempt to get further insight into the mechanism of structural variations and establish a rational structure-catalysis relationship.

Degradation of tetrabromobisphenol A through peroxymonaosulfate oxidation activated by La0.5Sr0.5CoxMn1-xO3-δ perovskite

La_0.5Sr_0.5Co_xMn_1-xO_3-δ (LSCM) perovskite composite oxides prepared by co-doping of Co and Mn in B site-activated peroxymonosulfate (PMS) to degrade tetrabromobisphenol A. The characterization results indicated that the LSCM with x=0.3-0.8 have hexagonal R-3c structure. The activation effect of LSCM on PMS decreased gradually with the increase of Mn doping, among which LSCM82 (x=0.8) had good oxygen desorption performance certificated by O₂-TPD and lower relative acidity (1.975). Moreover, the redox pairs of Co/Mn multi-valence ions were the main contributor to its catalytic activity. The electron spin resonance results suggested that SO₄^•- and •OH existed in the system and SO₄^•- is the main free radical. Therefore, LSCM82 perovskite catalyst has broad application prospects in aqueous solutions.

Recent Progress in Advanced Electrocatalyst Design for Acidic Oxygen Evolution Reaction

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.

Halide-Doping Effect of Strontium Cobalt Oxide Electrocatalyst and the Induced Activity for Oxygen Evolution in an Alkaline Solution

Perovskites of strontium cobalt oxyhalides having the chemical formulae Sr2CoO4-xHx (H = F, Cl, and Br; x = 0 and 1) were prepared using a solid-phase synthesis approach and comparatively evaluated as electrocatalysts for oxygen evolution in an alkaline solution. The perovskite electrocatalyst crystal phase, surface morphology, and composition were examined by X-ray diffraction, a scanning electron microscope, and energy-dispersive X-ray (EDX) mapping. The electrochemical investigations of the oxyhalides catalysts showed that the doping of F, Cl, or Br into the Sr2CoO4 parent oxide enhances the electrocatalytic activity for the oxygen evolution reaction (OER) with the onset potential as well as the potential required to achieve a current density of 10 mA/cm2 shifting to lower potential values in the order of Sr2CoO4 (1.64, 1.73) > Sr2CoO3Br (1.61, 1.65) > Sr2CoO3Cl (1.53, 1.60) > Sr2CoO3F (1.50, 1.56) V vs. HRE which indicates that Sr2CoO3F is the most active electrode among the studied catalysts under static and steady-state conditions. Moreover, Sr2CoO3F demonstrates long-term stability and remarkably less charge transfer resistance (Rct = 36.8 ohm) than the other oxyhalide counterparts during the OER. The doping of the perovskites with halide ions particularly the fluoride-ion enhances the surface oxygen vacancy density due to electron withdrawal away from the Co-atom which improves the ionic and electronic conductivity as well as the electrochemical activity of the oxygen evolution in alkaline solution.

Facilitating room-temperature oxygen ion migration via Co-O bond activation in cobaltite films

Oxygen ion migration in strongly correlated oxides can cause dramatic changes in the crystal structure, chemical and magnetoelectric properties, which holds promising for a wide variety of applications in catalysis, energy conversion, and electronics. However, the high strength and stability of metal-oxygen (M-O) bonds cause a large thermodynamic barrier for oxygen migration. Here, we designed Co-O bond activation in cobaltite (SrCoO_x) films by Au-nanodot-decoration. Charge transfer from Au to SrCoO_x effectively weakens the Co-O bond, meanwhile Co-O-Au synergistic bonding remarkably decreases the migration barrier of oxygen ions. Fast oxygen evolution occurs at the perimeter of the Au/SrCoO_x interface, and the chemical potential gradient of O^2- drives inner ion diffusion to the surface. Consequently, bias-free topotactic phase reduction from perovskite SrCoO_3-δ to brownmillerite SrCoO_2.5 has been achieved at room temperature. Our finding explores a new dimension to accelerate oxygen ion kinetics in transition-metal oxides from the aspect of interfacial bond activation, which is significant for developing oxide/noble-metal interfaces for high-efficiency ion migration and redox catalysis at low temperature.

Structural Transformation of Heterogeneous Materials for Electrocatalytic Oxygen Evolution Reaction

Electrochemical water splitting for hydrogen generation is a promising pathway for renewable energy conversion and storage. One of the most important issues for efficient water splitting is to develop cost-effective and highly efficient electrocatalysts to drive sluggish oxygen-evolution reaction (OER) at the anode side. Notably, structural transformation such as surface oxidation of metals or metal nonoxide compounds and surface amorphization of some metal oxides during OER have attracted growing attention in recent years. The investigation of structural transformation in OER will contribute to the in-depth understanding of accurate catalytic mechanisms and will finally benefit the rational design of catalytic materials with high activity. In this Review, we provide an overview of heterogeneous materials with obvious structural transformation during OER electrocatalysis. To gain insight into the essence of structural transformation, we summarize the driving forces and critical factors that affect the transformation process. In addition, advanced techniques that are used to probe chemical states and atomic structures of transformed surfaces are also introduced. We then discuss the structure of active species and the relationship between catalytic performance and structural properties of transformed materials. Finally, the challenges and prospects of heterogeneous OER electrocatalysis are presented.

Coherent Acoustic Interferometry during the Photodriven Oxygen Evolution Reaction Associates Strain Fields with the Reactive Oxygen Intermediate (Ti-OH*)

The oxygen evolution reaction (OER) from water requires the formation of metastable, reactive oxygen intermediates to enable oxygen-oxygen bond formation. Conversely, such reactive intermediates could also structurally modify the catalyst. A descriptor for the overall catalytic activity, the first electron and proton transfer OER intermediate from water, (M-OH*), has been associated with significant distortions of the metal-oxygen bonds upon charge-trapping. Time-resolved spectroscopy of in situ, photodriven OER on transition metal oxide surfaces has characterized M-OH* for the charge trapping and the symmetry of the lattice distortions by optical and vibrational transitions, respectively, but had yet to detect an interfacial strain field arising from a surface coverage M-OH*. Here, we utilize picosecond, coherent acoustic interferometry to detect the uniaxial strain normal to the SrTiO₃/aqueous interface directly caused by Ti-OH*. The spectral analysis applies a fairly general methodology for detecting a combination of the spatial extent, magnitude, and generation time of the interfacial strain through the coherent oscillations' phase. For lightly n-doped SrTiO₃, we identify the strain generation time (1.31 ps), which occurs simultaneously with Ti-OH* formation, and a tensile strain of 0.06% (upper limit 0.6%). In addition to fully characterizing this intermediate across visible, mid-infrared, and now GHz-THz probes on SrTiO₃, we show that strain fields occur with the creation of some M-OH*, which modifies design strategies for tuning catalytic activity and provides insight into photo-induced degradation so prevalent for OER. To that end, the work put forth here provides a unique methodology to characterize intermediate-induced interfacial strain across OER catalysts.