Role of Lattice Oxygen in the Oxygen Evolution Reaction on Co3O4: Isotope Exchange Determined Using a Small-Volume Differential Electrochemical Mass Spectrometry Cell Design

Enhancing the oxygen evolution reaction activity and stability of high-valent CoOOH by switching the catalytic pathway through doping low-valent Cu

The oxygen evolution reaction (OER) is a critical process in electrochemical energy storage and conversion systems. The adsorbate evolution mechanism (AEM) pathway possesses the characteristics of high stability but slow catalytic kinetics. We propose that combining AEM with the lattice oxidation mechanism (LOM) pathway can potentially enhance the OER catalytic activity and stability. However, the triggering of LOM is an important challenge due to the high thermodynamic activation barrier of lattice oxygen. To solve this problem, we performed theoretical calculations and experiments which suggest that the introduction of low-valent Cu in CoOOH (CuxCo1-xOOH) could directionally modulate the local coordination environment of CoO bonds. This approach can activate lattice oxygen and generate oxygen vacancies to enhance the nucleophilic attack of *OH and directly establish OO coupling, thereby facilitating the smoothly switching from AEM to LOM pathway by increasing voltage and thus activating lattice oxygen in CuxCo1-xOOH. The switching of AEM and LOM enables CuxCo1-xOOH showing an outstanding overpotential of only 252 mV (10 mA cm-2) and durability of only 2.80 % degradation after 280h. This work provides a new way for designing efficient and stable electrocatalysts with AEM and LOM pathway switching.

In Situ Synthesis of Ternary Ni-Fe-Mo Nanosheet Arrays for OER in Water Electrolysis

Water electrolysis is a promising path to the industrialization development of hydrogen energy. The exploitation of high-efficiency and inexpensive catalysts become important to the mass use of water decomposition. Ni-based nanomaterials have exhibited great potential for the catalysis of water splitting, which have attracted the attention of researchers around the world. Here, we prepared a novel Mo-doped NiFe-based layered double hydroxide (LDH) with a nanoarray microstructure on Ni foam. The doping amount of Mo can significantly change the microstructure of the electrocatalysis, which will further affect the oxygen evolution reaction (OER) performance of water splitting. This novel nanomaterial required only an overpotential of 227 mV for 10 mA cm-2 and a Tafel slope of 54.8 mV/dec in 1 M KOH. Meanwhile, there was no Mo, and the NiFe-LDH needed 233 mV to attain to 10 mA cm-2. Compared to the NiFe-LDH without Mo, the NiFeMo-LDH nanosheet arrays exhibited enhanced activities with 17.1 mV/dec less Tafel in OER. The good performance of the electrocatalyst is ascribed to the special nanosheet arrays and the heterostructure of the Ni-Fe-Mo system. These features help to increase the active surface, enhancing the efficient charge transfer and the reactive activity in OER.

Nickel-Molybdenum-Based Three-Dimensional Nanoarrays for Oxygen Evolution Reaction in Water Splitting

Water splitting is an important approach to hydrogen production. But the efficiency of the process is always controlled by the oxygen evolution reaction process. In this study, a three-dimensional nickel-molybdenum binary nanoarray microstructure electrocatalyst is successfully synthesized. It is grown uniformly on Ni foam using a hydrothermal method. Attributed to their unique nanostructure and controllable nature, the Ni-Mo-based nanoarray samples show superior reactivity and durability in oxygen evolution reactions. The series of Ni-Mo-based electrocatalysts presents a competitive overpotential of 296 mV at 10 mA·cm-2 for an OER in 1.0 M KOH, corresponding with a low Tafel slope of 121 mV dec-1. The three-dimensional nanostructure has a large double-layer capacitance and plenty of channels for ion transfer, which demonstrates more active sites and improved charge transmission. This study provides a valuable reference for the development of non-precious catalysts for water splitting.

Recent development and applications of differential electrochemical mass spectrometry in emerging energy conversion and storage solutions

Electrochemical energy conversion and storage are playing an increasingly important role in shaping the sustainable future. Differential electrochemical mass spectrometry (DEMS) offers an operando and cost-effective tool to monitor the evolution of gaseous/volatile intermediates and products during these processes. It can deliver potential-, time-, mass- and space-resolved signals which facilitate the understanding of reaction kinetics. In this review, we show the latest developments and applications of DEMS in various energy-related electrochemical reactions from three distinct perspectives. (I) What is DEMS addresses the working principles and key components of DEMS, highlighting the new and distinct instrumental configurations for different applications. (II) How to use DEMS tackles practical matters including the electrochemical test protocols, quantification of both potential and mass signals, and error analysis. (III) Where to apply DEMS is the focus of this review, dealing with concrete examples and unique values of DEMS studies in both energy conversion applications (CO2 reduction, water electrolysis, carbon corrosion, N-related catalysis, electrosynthesis, fuel cells, photo-electrocatalysis and beyond) and energy storage applications (Li-ion batteries and beyond, metal-air batteries, supercapacitors and flow batteries). The recent development of DEMS-hyphenated techniques and the outlook of the DEMS technique are discussed at the end. As DEMS celebrates its 40th anniversary in 2024, we hope this review can offer electrochemistry researchers a comprehensive understanding of the latest developments of DEMS and will inspire them to tackle emerging scientific questions using DEMS.

The mechanism of water oxidation using transition metal-based heterogeneous electrocatalysts

The water oxidation reaction, a crucial process for solar energy conversion, has garnered significant research attention. Achieving efficient energy conversion requires the development of cost-effective and durable water oxidation catalysts. To design effective catalysts, it is essential to have a fundamental understanding of the reaction mechanisms. This review presents a comprehensive overview of recent advancements in the understanding of the mechanisms of water oxidation using transition metal-based heterogeneous electrocatalysts, including Mn, Fe, Co, Ni, and Cu-based catalysts. It highlights the catalytic mechanisms of different transition metals and emphasizes the importance of monitoring of key intermediates to explore the reaction pathway. In addition, advanced techniques for physical characterization of water oxidation intermediates are also introduced, for the purpose of providing information for establishing reliable methodologies in water oxidation research. The study of transition metal-based water oxidation electrocatalysts is instrumental in providing novel insights into understanding both natural and artificial energy conversion processes.

Electrocatalytic water oxidation with manganese phosphates

As inspired by the Mn4CaO5 oxygen evolution center in nature, Mn-based electrocatalysts have received overwhelming attention for water oxidation. However, the understanding of the detailed reaction mechanism has been a long-standing problem. Herein, homologous KMnPO4 and KMnPO4•H2O with 4-coordinated and 6-coordinated Mn centers, respectively, are prepared. The two catalysts constitute an ideal platform to study the structure-performance correlation. The presence of Mn(III), Mn(IV), and Mn(V) intermediate species are identified during water oxidation. The Mn(V)=O species is demonstrated to be the substance for O-O bond formation. In KMnPO4•H2O, the Mn coordination structure did not change significantly during water oxidation. In KMnPO4, the Mn coordination structure changed from 4-coordinated [MnO4] to 5-coordinated [MnO5] motif, which displays a triangular biconical configuration. The structure flexibility of [MnO5] is thermodynamically favored in retaining Mn(III)-OH and generating Mn(V)=O. The Mn(V)=O species is at equilibrium with Mn(IV)=O, the concentration of which determines the intrinsic activity of water oxidation. This study provides a clear picture of water oxidation mechanism on Mn-based systems.

Methanol assisted water electrooxidation on noble metal free perovskite: RRDE insight into the catalyst's behaviour

In this work, we have hypothesized that noble metal-free perovskites are an essential class of oxygen evolution reaction (OER) catalysts in an alkaline medium and thus, they are a suitable candidate for the assisted water oxidation catalysts. Herein, we demonstrate that the origin of the methanol-assisted OER activity at near thermodynamic potential on perovskite electrode arises due to the involvement of additional hydroxyls as a result of dissociative chemisorption of methanol. When the perovskite electrode is screened for methanol electrooxidation reaction in 0.5 M KOH + 0.5 M methanol electrolyte, it delivers a two times higher current density. This imparts an 82 % increase in the evolution of oxygen gas moles with complete oxidation of methanol to carbon dioxide. Along with the electrochemical characterization to understand the electrocatalyst property, Rotating ring disk electrode (RRDE) technique is explored for the first time in literature to validate the catalyst's involvement during OER. RRDE is effective in understanding the lattice oxygen behaviour and methanol-assisted water electrooxidation during OER. Our results suggest new insights and ideas towards the oxygen evolution reaction process and the mechanistic insight into the elevated OER due to assisted methanol electrooxidation.

Cobalt-free layered perovskites RBaCuFeO5+δ (R = 4f lanthanide) as electrocatalysts for the oxygen evolution reaction

Co-based perovskite oxides are intensively studied as promising catalysts for electrochemical water splitting in an alkaline environment. However, the increasing Co demand by the battery industry is pushing the search for Co-free alternatives. Here we report a systematic study of the Co-free layered perovskite family RBaCuFeO5+δ (R = 4f lanthanide), where we uncover the existence of clear correlations between electrochemical properties and several physicochemical descriptors. Using a combination of advanced neutron and X-ray synchrotron techniques with ab initio DFT calculations we demonstrate and rationalize the positive impact of a large R ionic radius in their oxygen evolution reaction (OER) activity. We also reveal that, in these materials, Fe3+ is the transition metal cation the most prone to donate electrons. We also show that similar R3+/Ba2+ ionic radii favor the incorporation and mobility of oxygen in the layered perovskite structure and increase the number of available O diffusion paths, which have an additional, positive impact on both, the electric conductivity and the OER process. An unexpected result is the observation of a clear surface reconstruction exclusively in oxygen-rich samples (δ > 0), a fact that could be related to their superior OER activity. The encouraging intrinsic OER values obtained for the most active electrocatalyst (LaBaCuFeO5.49), together with the possibility of industrially producing this material in nanocrystalline form should inspire the design of other Co-free oxide catalysts with optimal properties for electrochemical water splitting.

Activating lattice oxygen in high-entropy LDH for robust and durable water oxidation

The oxygen evolution reaction is known to be a kinetic bottleneck for water splitting. Triggering the lattice oxygen oxidation mechanism (LOM) can break the theoretical limit of the conventional adsorbate evolution mechanism and enhance the oxygen evolution reaction kinetics, yet the unsatisfied stability remains a grand challenge. Here, we report a high-entropy MnFeCoNiCu layered double hydroxide decorated with Au single atoms and O vacancies (AuSA-MnFeCoNiCu LDH), which not only displays a low overpotential of 213 mV at 10 mA cm-2 and high mass activity of 732.925 A g-1 at 250 mV overpotential in 1.0 M KOH, but also delivers good stability with 700 h of continuous operation at ~100 mA cm-2. Combining the advanced spectroscopic techniques and density functional theory calculations, it is demonstrated that the synergistic interaction between the incorporated Au single atoms and O vacancies leads to an upshift in the O 2p band and weakens the metal-O bond, thus triggering the LOM, reducing the energy barrier, and boosting the intrinsic activity.

Single Co3O4 Nanocubes Electrocatalyzing the Oxygen Evolution Reaction: Nano-Impact Insights into Intrinsic Activity and Support Effects

Single-entity electrochemistry allows for assessing electrocatalytic activities of individual material entities such as nanoparticles (NPs). Thus, it becomes possible to consider intrinsic electrochemical properties of nanocatalysts when researching how activity relates to physical and structural material properties. Conversely, conventional electrochemical techniques provide a normalized sum current referring to a huge ensemble of NPs constituting, along with additives (e.g., binders), a complete catalyst-coated electrode. Accordingly, recording electrocatalytic responses of single NPs avoids interferences of ensemble effects and reduces the complexity of electrocatalytic processes, thus enabling detailed description and modelling. Herein, we present insights into the oxygen evolution catalysis at individual cubic Co₃O₄ NPs impacting microelectrodes of different support materials. Simulating diffusion at supported nanocubes, measured step current signals can be analyzed, providing edge lengths, corresponding size distributions, and interference-free turnover frequencies. The provided nano-impact investigation of (electro-)catalyst-support effects contradicts assumptions on a low number of highly active sites.

On the formation and diffusion of oxygen vacancies in non-stoichiometric mixed Co3-xMnxO4spinel structures from first-principles calculations

Using first-principles simulations, we focus on the study of Co₃O₄-Mn₃O₄mixed oxides, which have recently shown alluring features as thermochemical heat storage materials. We provide fundamental atomistic-level insight into the thermodynamics and kinetics of a series of non-stoichiometric Co_3-xMn_xO_4-y(0 ⩽x⩽ 3 andy= 0, 0.125, 0.250) bulk systems, by examining in detail the formation and diffusion processes of oxygen vacancies as a function of Mn content. We find a preference for the formation of vacancies atx= 1.5. And we predict a significant drop of diffusion barriers forx⩾ 1.5, when Mn atoms start to populate the spinel octahedral sites as Mn³⁺. Our results pave the way for better understanding the underlying mechanisms that govern oxygen vacancy dynamics in Co_3-xMn_xO₄in general, and, in particular, the reversible reduction and re-oxidation reactions of these promising mixed oxides for thermal energy storage. Nevertheless, some discrepancies are found between our calculations on bulk models and recent experimental insights from the literature, which suggests that surface and finite size effects might play an important role in controlling the observed macroscopic behavior of these materials during reversible reduction and re-oxidation cycles.

Boosting the charge transfer of FeOOH/Ni(OH)2 for excellent oxygen evolution reaction via Cr modification

For the electrocatalytic oxygen evolution reaction (OER) in alkaline media, there is an urgent need to optimize the adsorption strength of OH*. Here, a flower-like hybrid of Cr-doped FeOOH/Ni(OH)2 was used as an OER catalyst with a low overpotential of 291 mV at 50 mA cm-2. The results showed that faster charge transfer was achieved at the electrode/solution interface during the OER process after the FeOOH/Ni(OH)2 was modified by Cr, which facilitates the rate-determining step of the Volmer reaction. Furthermore, the results of faradaic efficiency and X-ray photoelectron spectroscopy (XPS) measurements confirmed that the synergistic effect between the ternary metal and oxygen vacancies led to excellent OER performance. This work provides a new strategy for the preparation of high-efficiency and low-cost oxygen evolution electrocatalysts.

Correlation between Oxygen Vacancies and Oxygen Evolution Reaction Activity for a Model Electrode: PrBaCo2 O5+δ

The role of the perovskite lattice oxygen in the oxygen evolution reaction (OER) is systematically studied in the PrBaCo₂ O_5+δ family. The reduced number of physical/chemical variables combined with in-depth characterizations such as neutron dif-fraction, O K-edge X-ray absorption spectroscopy (XAS), electron energy loss spectroscopy (EELS), magnetization and scanning transmission electron microscopy (STEM) studies, helps investigating the complex correlation between OER activity and a single perovskite property, such as the oxygen content. Larger amount of oxygen vacancies appears to facilitate the OER, possibly contributing to the mechanism involving the oxidation of lattice oxygen, i.e., the lattice oxygen evolution reaction (LOER). Furthermore, not only the number of vacancies but also their local arrangement in the perovskite lattice influences the OER activity, with a clear drop for the more stable, ordered stoichiometry.

Structural transformations of solid electrocatalysts and photocatalysts

Heterogeneous catalysts often undergo structural transformations when they operate under thermal reaction conditions. These transformations are reflected in their evolving catalytic activity, and a fundamental understanding of the changing nature of active sites is vital for the rational design of solid materials for applications. Beyond thermal catalysis, both photocatalysis and electrocatalysis are topical because they can harness renewable energy to drive uphill reactions that afford commodity chemicals and fuels. Although structural transformations of photocatalysts and electrocatalysts have been observed in operando, the resulting implications for catalytic behaviour are not fully understood. In this Review, we summarize and compare the structural evolution of solid thermal catalysts, electrocatalysts and photocatalysts. We suggest that well-established knowledge of thermal catalysis offers a good basis to understand emerging photocatalysis and electrocatalysis research.

Breaking the scaling relationship via dual metal doping in a cobalt spinel for the OER: a computational prediction

The scaling relationship in the OER is found to be broken on a reconstructed Co₃O₄(110) surface via dual metal doping.

A review on fundamentals for designing oxygen evolution electrocatalysts

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.