Operando X‐Ray Absorption Spectroscopy Shows Iron Oxidation Is Concurrent with Oxygen Evolution in Cobalt–Iron (Oxy)hydroxide Electrocatalysts

Stability of electrocatalytic OER: from principle to application

Hydrogen energy, derived from the electrolysis of water using renewable energy sources such as solar, wind, and hydroelectric power, is considered a promising form of energy to address the energy crisis. However, the anodic oxygen evolution reaction (OER) poses limitations due to sluggish kinetics. Apart from high catalytic activity, the long-term stability of electrocatalytic OER has garnered significant attention. To date, several research studies have been conducted to explore stable electrocatalysts for the OER. A comprehensive review is urgently warranted to provide a concise overview of the recent advancements in the electrocatalytic OER stability, encompassing both electrocatalyst and device developments. This review aims to succinctly summarize the primary factors influencing OER stability, including morphological/phase change and electrocatalyst dissolution, as well as mechanical detachment, alongside chemical, mechanical, and operational degradation observed in devices. Furthermore, an overview of contemporary approaches to enhance stability is provided, encompassing electrocatalyst design (structural regulation, protective layer coating, and stable substrate anchoring) and device optimization (bipolar plates, gas diffusion layers, and membranes). Hopefully, more attention will be paid to ensuring the stable operation of electrocatalytic OER and the future large-scale water electrolysis applications. This review presents design principles aimed at addressing challenges related to the stability of electrocatalytic OER.

Direct Identification of O─O Bond Formation Through Three-Step Oxidation During Water Splitting by Operando Soft X-ray Absorption Spectroscopy

Anionic redox allows the direct formation of O─O bonds from lattice oxygens and provides higher catalytic in the oxygen evolution reaction (OER) than does the conventional metal ion mechanism. While previous theories have predicted and experiments have suggested the possible O─O bond, it has not yet been directly observed in the OER process. In this study, operando soft X-ray absorption spectroscopy (sXAS) at the O K-edge and the operando Raman spectra is performed on layered double CoFe hydroxides (LDHs) after intercalation with [Cr(C2O4)3]3-, and revealed a three-step oxidation process, staring from Co2+ to Co3+, further to Co4+ (3d6L), and ultimately leading to the formation of O─O bonds and O2 evolution above a threshold voltage (1.4 V). In contrast, a gradual oxidation of Fe is observed in CoFe LDHs. The OER activity exhibits a significant enhancement, with the overpotential decreasing from 300 to 248 mV at 10 mA cm-2, following the intercalation of [Cr(C2O4)3]3- into CoFe LDHs, underscoring a crucial role of anionic redox in facilitating water splitting.

Active oxygen species mediate the iron-promoting electrocatalysis of oxygen evolution reaction on metal oxyhydroxides

Iron is an extraordinary promoter to impose nickel/cobalt (hydr)oxides as the most active oxygen evolution reaction catalysts, whereas the synergistic effect is actively debated. Here, we unveil that active oxygen species mediate a strong electrochemical interaction between iron oxides (FeOxHy) and the supporting metal oxyhydroxides. Our survey on the electrochemical behavior of nine supporting metal oxyhydroxides (M(O)OH) uncovers that FeOxHy synergistically promotes substrates that can produce active oxygen species exclusively. Tafel slopes correlate with the presence and kind of oxygen species. Moreover, the oxygen evolution reaction onset potentials of FeOxHy@M(O)OH coincide with the emerging potentials of active oxygen species, whereas large potential gaps are present for intact M(O)OH. Chemical probe experiments suggest that active oxygen species could act as proton acceptors and/or mediators for proton transfer and/or diffusion in cooperative catalysis. This discovery offers a new insight to understand the synergistic catalysis of Fe-based oxygen evolution reaction electrocatalysts.

Confirming High-Valent Iron as Highly Active Species of Water Oxidation on the Fe, V-Coupled Bimetallic Electrocatalyst: In Situ Analysis of X-ray Absorption and Mössbauer Spectroscopy

Iron (Fe)-based bimetallic oxides/hydroxides have been widely investigated for promising alkaline electrochemical oxygen evolution reactions (OERs), but it still remains argumentative whether Fe³⁺ or Fe⁴⁺ intermediates are highly active for efficient OER. Here, we rationally designed and prepared one Fe, V-based bimetallic composite nanosheet by employing the OER-inert V element as a promoter to completely avoid the argument of real active metals and using our recently developed one-dimensional conductive nickel phosphide (NP) as a support. The as-obtained hierarchical nanocomposite (denoted as FeVO_x/NP) was evaluated as a model catalyst to gain insight into the iron-based species as highly active OER sites by performing in situ X-ray absorption spectroscopy and ⁵⁷Fe Mössbauer spectroscopy measurements. It was found that the high-valent Fe⁴⁺ species can only be detected during the OER process of the FeVO_x/NP nanocomposite instead of the iron counterpart itself. Together with the fact that the OER activities of both the vanadium and iron counterparts are by far worse than that of the FeVO_x/NP composite, we can confirm that the high-valent Fe⁴⁺ formed are the highly active species for efficient OER. As demonstrated by density functional theory simulations, the composite of Fe and V metals is proposed to cause a decreased Gibbs free energy as well as theoretical overpotential of water oxidation with respect to its counterparts, as is responsible for its excellent OER performance with extremely low OER overpotential (290 mV at 500 mA cm^-2) and extraordinary stability (1000 h at 100 mA cm^-2).

Navigating surface reconstruction of spinel oxides for electrochemical water oxidation

Understanding and mastering the structural evolution of water oxidation electrocatalysts lays the foundation to finetune their catalytic activity. Herein, we demonstrate that surface reconstruction of spinel oxides originates from the metal-oxygen covalency polarity in the M_T-O-M_O backbone. A stronger M_O-O covalency relative to M_T-O covalency is found beneficial for a more thorough reconstruction towards oxyhydroxides. The structure-reconstruction relationship allows precise prediction of the reconstruction ability of spinel pre-catalysts, based on which the reconstruction degree towards the in situ generated oxyhydroxides can be controlled. The investigations of oxyhydroxides generated from spinel pre-catalysts with the same reconstruction ability provide guidelines to navigate the cation selection in spinel pre-catalysts design. This work reveals the fundamentals for manipulating the surface reconstruction of spinel pre-catalysts for water oxidation.

Deciphering the Structural and Chemical Transformations of Oxide Catalysts during Oxygen Evolution Reaction Using Quick X-ray Absorption Spectroscopy and Machine Learning

Bimetallic transition-metal oxides, such as spinel-like CoxFe3-xO4 materials, are known as attractive catalysts for the oxygen evolution reaction (OER) in alkaline electrolytes. Nonetheless, unveiling the real active species and active states in these catalysts remains a challenge. The coexistence of metal ions in different chemical states and in different chemical environments, including disordered X-ray amorphous phases that all evolve under reaction conditions, hinders the application of common operando techniques. Here, we address this issue by relying on operando quick X-ray absorption fine structure spectroscopy, coupled with unsupervised and supervised machine learning methods. We use principal component analysis to understand the subtle changes in the X-ray absorption near-edge structure spectra and develop an artificial neural network to decipher the extended X-ray absorption fine structure spectra. This allows us to separately track the evolution of tetrahedrally and octahedrally coordinated species and to disentangle the chemical changes and several phase transitions taking place in CoxFe3-xO4 catalysts and on their active surface, related to the conversion of disordered oxides into spinel-like structures, transformation of spinels into active oxyhydroxides, and changes in the degree of spinel inversion in the course of the activation treatment and under OER conditions. By correlating the revealed structural changes with the distinct catalytic activity for a series of CoxFe3-xO4 samples, we elucidate the active species and OER mechanism.

Mesoporous Single Crystals with Fe-Rich Skin for Ultralow Overpotential in Oxygen Evolution Catalysis

The oxygen evolution reaction (OER) is a key reaction in water splitting and metal-air batteries, and transition metal hydroxides have demonstrated the most electrocatalytic efficiency. Making the hydroxides thinner for more surface commonly fails to increase the active site number, because the real active sites are the edges. Up to now, the overpotentials of most state-of-the-art OER electrocatalysts at a current density of 10 mA cm^-2 (η₁₀ ) are still larger than 200 mV. Herein, a novel design of mesoporous single crystal (MSC) with an Fe-rich skin to boost the OER is shown. The edges around the mesopores provide lots of real active sites and the Fe modification on these sites further improves the intrinsic activity. As a result, an ultralow η₁₀ of 185 mV is achieved, and the turnover frequency based on Fe atoms is as high as 16.9 s^-1 at an overpotential of 350 mV. Moreover, the catalyst has an excellent catalytic stability, indicated by a negligible current drop after 10 000 cyclic voltammetry cycles. The catalyst enables Zn-air batteries to run stably over 270 h with a low charge voltage of 1.89 V. This work shows that MSC materials can provide new opportunities for the design of electrocatalysts.

High-Performance Bifunctional Ni-Fe-S Catalyst in situ Synthesized within Graphite Intergranular Nanopores for Overall Water Splitting

Low-cost and efficient bifunctional catalysts are urgently needed for overall water splitting used in large-scale energy storage. In this study, we develop a nickel and iron (di)sulfide (Ni-Fe-S) composite catalyst that is in situ synthesized and fixed within the intergranular nanopores inside high pure polycrystalline graphite. Two precursor solutions (reactants) may permeate the graphite intergranular pores to a depth of more than 3.5 mm. The nanoscale pores serve as an array of nanoreactors for the synthesis of the Ni-Fe-S nanoparticles under conditions much milder than usual. The prepared catalyst efficiently catalyzes both the hydrogen and oxygen evolution reactions (HER and OER) in 1.0 M KOH. It delivers a current density of 400 mA cm^-2 at a full cell voltage of around 2.3 V without considerable activity decay over 24 h electrolysis. The active species of the catalyst are different for the HER and OER and discussed accordingly. The synthesis strategy based on the nanopores in a monolithic conductive substrate proves to be a simple, efficient, and promising way to prepare electrocatalysts that are cheap, abundant, and industrially attractive.

Recent advances in activating surface reconstruction for the high-efficiency oxygen evolution reaction

A climax in the development of cost-effective and high-efficiency transition metal-based electrocatalysts has been witnessed recently for sustainable energy and related conversion technologies. In this regard, structure-activity relationships based on several descriptors have already been proposed to rationally design electrocatalysts. However, the dynamic reconstruction of the surface structures and compositions of catalysts during electrocatalytic water oxidation, especially during the anodic oxygen evolution reaction (OER), complicate the streamlined prediction of the catalytic activity. With the achievements in operando and in situ techniques, it has been found that electrocatalysts undergo surface reconstruction to form the actual active species in situ accompanied with an increase in their oxidation state during OER in alkaline solution. Accordingly, a thorough understanding of the surface reconstruction process plays a critical role in establishing unambiguous structure-composition-property relationships in pursuit of high-efficiency electrocatalysts. However, several issues still need to be explored before high electrocatalytic activities can be realized, as follows: (1) the identification of initiators and pathways for surface reconstruction, (2) establishing the relationships between structure, composition, and electrocatalytic activity, and (3) the rational manipulation of in situ catalyst surface reconstruction. In this review, the recent progress in the surface reconstruction of transition metal-based OER catalysts including oxides, non-oxides, hydroxides and alloys is summarized, emphasizing the fundamental understanding of reconstruction behavior from the original precatalysts to the actual catalysts based on operando analysis and theoretical calculations. The state-of-the-art strategies to tailor the surface reconstruction such as substituting/doping with metals, introducing anions, incorporating oxygen vacancies, tuning morphologies and exploiting plasmonic/thermal/photothermal effects are then introduced. Notably, comprehensive operando/in situ characterization together with computational calculations are responsible for unveiling the improvement mechanism for OER. By delivering the progress, strategies, insights, techniques, and perspectives, this review will provide a comprehensive understanding of the surface reconstruction in transition metal-based OER catalysts and future guidelines for their rational development.

In Situ/Operando Electrocatalyst Characterization by X-ray Absorption Spectroscopy

During the last decades, X-ray absorption spectroscopy (XAS) has become an indispensable method for probing the structure and composition of heterogeneous catalysts, revealing the nature of the active sites and establishing links between structural motifs in a catalyst, local electronic structure, and catalytic properties. Here we discuss the fundamental principles of the XAS method and describe the progress in the instrumentation and data analysis approaches undertaken for deciphering X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra. Recent usages of XAS in the field of heterogeneous catalysis, with emphasis on examples concerning electrocatalysis, will be presented. The latter is a rapidly developing field with immense industrial applications but also unique challenges in terms of the experimental characterization restrictions and advanced modeling approaches required. This review will highlight the new insight that can be gained with XAS on complex real-world electrocatalysts including their working mechanisms and the dynamic processes taking place in the course of a chemical reaction. More specifically, we will discuss applications of in situ and operando XAS to probe the catalyst's interactions with the environment (support, electrolyte, ligands, adsorbates, reaction products, and intermediates) and its structural, chemical, and electronic transformations as it adapts to the reaction conditions.

Highly Efficient Alkaline Water Splitting with Ru-Doped Co-V Layered Double Hydroxide Nanosheets as a Bifunctional Electrocatalyst

Active electrocatalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are decisive for achieving efficient energy conversion from electricity to hydrogen fuel through water electrolysis. In this study, tremella-like Ru-doped Co-V layered double hydroxide nanosheets on Ni Foam (Ru-CoV-LDH@NF) was fabricated by a one-pot solvothermal reaction. As-prepared Ru-CoV-LDH@NF, with a nominal Ru loading of around 51.6 μg cm^-2 exhibits excellent bifunctional catalytic activity towards HER and OER in alkaline media. To accomplish a current density of 10 mA cm^-2 , it demands 32 mV and 230 mV overpotentials for HER and OER, respectively. The alkali electrolyzer utilizing Ru-CoV-LDH/NF as bifunctional electrocatalyst affords 10 mA cm^-2 electrolytic current density at an extremely low cell voltage of 1.50 V, showing excellent performance compared to a Pt/C-RuO₂ -based electrolyzer and many other bifunctional electrocatalyst-based ones. The incorporation of Ru changes the morphology of the resultant nanosheets to offer high electrochemical surface areas for electrocatalysis; at the same time, it significantly boosts the intrinsic HER/OER electrocatalytic activity. For HER, the energy barrier of the Volmer step is efficiently reduced upon Ru doping, whereas the Ru dopants optimize the absorption strength of *O intermediates to facilitate the OER process. This work offers a feasible means to optimize the Co-based hydroxide materials for improved electrocatalysis in overall water splitting.

Dual Role of Silver Moieties Coupled with Ordered Mesoporous Cobalt Oxide towards Electrocatalytic Oxygen Evolution Reaction

Herein, we show that the performance of mesostructured cobalt oxide electrocatalyst for oxygen evolution reaction (OER) can be significantly enhanced by coupling of silver species. Various analysis techniques including pair distribution function and Rietveld refinement, X-ray absorption spectroscopy at synchrotron as well as advanced electron microscopy revealed that silver exists as metallic Ag particles and well-dispersed Ag2 O nanoclusters within the mesostructure. The benefits of this synergy are twofold for OER: highly conductive metallic Ag improves the charge transfer ability of the electrocatalysts while ultra-small Ag2 O clusters provide the centers that can uptake Fe impurities from KOH electrolyte and boost the catalytic efficiency of Co-Ag oxides. The current density of mesostructured Co3 O4 at 1.7 VRHE is increased from 102 to 211 mA cm-2 with incorporation of silver spices. This work presents the dual role of silver moieties and demonstrates a simple method to increase the OER activity of Co3 O4 .

Stoichiometry-Controlled Synthesis of Nanoparticulate Mixed-Metal Oxyhydroxide Oxygen Evolving Catalysts by Electrochemistry in Aqueous Nanodroplets

Mixed-metal oxyhydroxides-especially those of Ni and Fe-are one of the most active classes of materials known for catalyzing the oxygen evolution reaction (OER). Here, nanoparticulate mixed metal oxyhydroxides (of Ni, Fe, and Co) were prepared on an electrode surface by electrochemical reaction of a precursor solution encapsulated in aqueous nanodroplets (AnDs), with each of the droplets containing 10 s of attoliters of fluid. Electrode reactions and synthesis can be monitored in situ by electrochemistry as single AnD stochastically lands and interacts with the working electrode. Resultant metal oxyhydroxide nanoparticles can be size and composition controlled precisely by modulating the precursor solution stored in the AnD. Nanoparticulate metal oxyhydroxides were implemented as catalysts for the OER and exhibited superior catalysis compared to their thin-film counterparts, demonstrating a hundred-thousand-fold enhancement in atom efficiency at comparable turnover rates.

Modes of Fe Incorporation in Co-Fe (Oxy)hydroxide Oxygen Evolution Electrocatalysts

Ni-Fe (oxy)hydroxide and Co-Fe (oxy)hydroxide are among the most active oxygen evolution reaction (OER) catalysts in alkaline media. Fe is essential for the high activity, but the details of how Fe is incorporated into the Ni or Co (oxy)hydroxide structure and affects catalysis remain incompletely understood. This study concerns two different modes of Fe incorporation to form Co(Fe)O_x H_y , which both yield increased OER activity but result in Fe and Co species that differ in chemical reactivity and electrochemical response. Co(Fe)O_x H_y films that were cathodically deposited from mixed Co and Fe nitrate solution (co-deposited) result in Fe species that interact strongly with the Co species (as evidenced by an anodic shift in the Co redox wave) and are difficult to leach out under electrochemical conditions. Fe incorporated into a CoO_x H_y film by cycling in Fe-spiked KOH electrolyte similarly enhance activity, but do not strongly electronically interact with the majority of the Co in the film and are removed by cycling in Fe-free KOH. These results support the hypothesis that co-deposition of Co(Fe)O_x H_y leads to films where the Co and Fe are mixed within the nanosheet structure and cycling in Fe-spiked KOH incorporates Fe species largely at surface, edge, or defect sites, where they drive OER but do not otherwise significantly modulate the electrochemical response of the Co.