Oxygen Evolution Activity of Amorphous Cobalt Oxyhydroxides: Interconnecting Precatalyst Reconstruction, Long-Range Order, Buffer-Binding, Morphology, Mass Transport, and Operation Temperature

Oxidation of interfacial cobalt controls the pH dependence of the oxygen evolution reaction

Transition metal oxides often undergo dynamic surface reconstruction under oxygen evolution reaction conditions to form the active state, which differs in response to the electrolyte pH. The resulting pH dependency of catalytic activity is commonly observed but poorly understood. Herein we track Co oxidation state changes at different pH-directed (hydr)oxide/electrolyte interfaces using operando X-ray absorption spectroscopy characterizations. Combined with in situ electrochemical analyses, we establish correlations between Co redox dynamics, the flat band potential and Co oxidation state changes to explain the pH dependency of the oxygen evolution activity. Alkaline environments provide a low flat band potential that yields a low-potential Co redox transformation, which favours surface reconstruction. Neutral and acidic environments afford an anodic shift of the Co redox transformation that increases the catalytic overpotential. The larger overpotential in neutral environments is attributable to poor Co atom polarizability and slow Co oxidation state changes. These findings reveal that interfacial Co oxidation state changes directly determine the pH dependency of the oxygen evolution reaction activity.

Insight into the structural reconstruction of alkaline water oxidation electrocatalysts

The oxygen-evolution reaction (OER) is an indispensable component of various energy storage and conversion electrocatalytic systems. However, the slow reaction kinetics have forced the development of advanced, efficient, and inexpensive OER electrocatalysts to break through the bottleneck of its application. Recently, the structural reconstruction of precatalysts has provided a promising avenue to boost the catalytic activity of electrocatalysts. Structural reconstruction implies atomic rearrangement and composition change of the pristine catalytic materials, which is a very complex process. Therefore, it is very crucial to have a deep understanding of the reconstruction chemical process and then modulate the reconstruction by deliberate design of electrochemical conditions and precatalysts. However, a systematic review of the structural reconstruction process, research methods, influencing factors and structure-performance relationship remains elusive, significantly impeding the further developments of efficient electrocatalysts based on structural reconstruction chemistry. This critical review is dedicated to providing a deep insight into the structural reconstruction during alkaline water oxidation, comprehensively summarizing the basic research methods to understand the structural evolution process and various factors affecting the structural reconstruction process, and providing a reference and basis for regulating the dynamic reconstruction. Moreover, the impact of reconstruction on the structure and performance is also covered. Finally, challenges and perspectives for the future study on structural reconstruction are discussed. This review will offer future guidelines for the rational development of state-of-the-art OER electrocatalysts.

Structure-Selection Dynamics of Cobalt Nanoparticles from Solution Synthesis and Their Impact on the Oxygen Evolution Reaction

Resolving the three-dimensional structure of transition metal oxide nanoparticles (TMO-NPs), upon self-restructuring from solution, is crucial for tuning their structure-functionality. Yet, this remains challenging as this process entails complex structure fluctuations, which are difficult to track experimentally and, hence, hinder the knowledge-driven optimization of TMO-NPs. Herein, we combine high-energy synchrotron X-ray absorption and X-ray total scattering experiments with atomistic multiscale simulations to investigate the self-restructuring of self-assembled Co-NPs from solution under dark or photocatalytic water oxidation conditions at distinct reaction times and atomic length-scales. Using the atomic range order as a descriptor, we reveal that dissolution of a Co-salt in BO3 buffer leads to a self-optimization route forming disordered oxyborate Co3BOx-NPs unveiling a high oxygen yield due to the formation of surface oxo/hydroxo adsorbates. Those Co3BOx-NPs further self-restructure into distorted Co3O4-NPs and, lastly, into distorted CoOOH-NPs through a rate-limiting step integrating Co3+-states during the course of a representative photocatalytic assay. Self-restructuring does not proceed from amorphous-to-ordered states but through stochastic fluctuations of atomic nanoclusters of ≈10 Å domain size. Our key insight into the structure-selection dynamics of TMO-NPs from solution offers a route for tuning their structure-function relationships for wide-ranging emergent technologies.

A Knowledge-Based Molecular Single-Source Precursor Approach to Nickel Chalcogenide Precatalysts for Electrocatalytic Water, Alcohol, and Aldehyde Oxidations

The development and comprehensive understanding of nickel chalcogenides are critical since they constitute a class of efficient electro(pre)catalysts for the oxygen evolution reaction (OER) and value-added organic oxidations. This study introduces a knowledge-based facile approach to analogous NiE (E = S, Se, Te) phases, originating from molecular β-diketiminato [Ni2E2] complexes and their application for OER and organic oxidations. The recorded activity trends for both target reactions follow the order NiSe > NiS > NiTe. Notably, NiSe displayed efficient performance for both OER and the selective oxidation of benzyl alcohol and 5-hydroxymethylfurfural, exhibiting stability in OER for 11 days under industrially pertinent conditions. Comprehensive analysis, including quasi in situ X-ray absorption and Raman spectroscopy, in combination with several ex situ techniques, revealed a material reconstruction process under alkaline OER conditions, involving chalcogen leaching. While NiS and NiSe experienced full chalcogen leaching and reconstruction into NiIII/IV oxyhydroxide active phases with intercalated potassium ions, the transformation of NiTe is incomplete. This study highlights the structure-activity relationship of a whole series of analogous nickel chalcogenides, directly linking material activity to the availability of active sites for catalysis. Such findings hold great promise for the development of efficient electrocatalysts for a wide range of applications, impacting various industrial processes and sustainable energy solutions.

Dynamic evolution processes in electrocatalysis: structure evolution, characterization and regulation

Reactions on electrocatalytic interfaces often involve multiple processes, including the diffusion, adsorption, and conversion of reaction species and the interaction between reactants and electrocatalysts. Generally, these processes are constantly changing rather than being in a steady state. Recently, dynamic evolution processes on electrocatalytic interfaces have attracted increasing attention owing to their significant roles in catalytic reaction kinetics. In this review, we aim to provide insights into the dynamic evolution processes in electrocatalysis to emphasize the importance of unsteady-state processes in electrocatalysis. Specifically, the dynamic structure evolution of electrocatalysts, methods for the characterization of the dynamic evolution and the strategies for the regulation of the dynamic evolution for improving electrocatalytic performance are summarized. Finally, the conclusion and outlook on the research on dynamic evolution processes in electrocatalysis are presented. It is hoped that this review will provide a deeper understanding of dynamic evolution in electrocatalysis, and studies of electrocatalytic reaction processes and kinetics on the unsteady-state microscopic spatial and temporal scales will be given more attention.

Lattice-Disordered High-Entropy Alloy Engineered by Thermal Dezincification for Improved Catalytic Hydrogen Evolution Reaction

A disordered crystal structure is an asymmetrical atomic lattice resulting from the missing atoms (vacancies) or the lattice misarrangement in a solid-state material. It has been widely proven to improve the electrocatalytic hydrogen evolution reaction (HER) process. In the present work, due to the special physical properties (the low evaporation temperature of below 900 °C), Zn is utilized as a sacrificial component to create senary PtIrNiCoFeZn high-entropy alloy (HEA) with highly disordered lattices. The structure of the lattice-disordered PtIrNiCoFeZn HEA is characterized by the thermal diffusion scattering (TDS) in transmission electron microscope. Density functional theory calculations reveal that lattice disorder not only accelerates both the Volmer step and Tafel step during the HER process but also optimizes the intensity and distribution of projected density of states near the Fermi energy after the H2O and H adsorption. Anomalously high alkaline HER activity and stability are proven by experimental measurements. This work introduces a novel approach to preparing irregular lattices offering highly efficient HEA and a TDS characterization method to reveal the disordered lattice in materials. It provides a new route toward exploring and developing the catalytic activities of materials with asymmetrically disordered lattices.

Selective oxygen reduction reaction: mechanism understanding, catalyst design and practical application

The oxygen reduction reaction (ORR) is a key component for many clean energy technologies and other industrial processes. However, the low selectivity and the sluggish reaction kinetics of ORR catalysts have hampered the energy conversion efficiency and real application of these new technologies mentioned before. Recently, tremendous efforts have been made in mechanism understanding, electrocatalyst development and system design. Here, a comprehensive and critical review is provided to present the recent advances in the field of the electrocatalytic ORR. The two-electron and four-electron transfer catalytic mechanisms and key evaluation parameters of the ORR are discussed first. Then, the up-to-date synthetic strategies and in situ characterization techniques for ORR electrocatalysts are systematically summarized. Lastly, a brief overview of various renewable energy conversion devices and systems involving the ORR, including fuel cells, metal-air batteries, production of hydrogen peroxide and other chemical synthesis processes, along with some challenges and opportunities, is presented.

Optimizing the Synergistic Effect of Co and Fe for Efficient and Durable Oxygen Evolution under Alkaline Conditions

Developing robust oxygen evolution reaction (OER) electrocatalysts is crucial for advancing anion exchange membrane water electrolysis (AEMWE). In this study, we present a catalyst optimizing the synergistic effect of Co and Fe by creating a CoFe-based layer on a Fe-based electrode (Fe@CoFe). The Fe@CoFe exhibits an overpotential of 168 mV at 10 mA cm-2 under half-cell conditions and a current density of 10 A cm-2 at 2 V in the AEMWE system with 1 M KOH. Moreover, it showcases a degradation rate of 76 μV h-1 for 2000 h at 500 mA cm-2 in the single-cell system. This study demonstrates the feasibility of achieving efficient and durable water electrolysis using a transition metal-based catalyst exclusively fabricated via electrodeposition.

Intermetallic Cobalt Indium Nanoparticles as Oxygen Evolution Reaction Precatalyst: A Non-Leaching p-Block Element

Merely all transition-metal-based materials reconstruct into similar oxyhydroxides during the electrocatalytic oxygen evolution reaction (OER), severely limiting the options for a tailored OER catalyst design. In such reconstructions, initial constituent p-block elements take a sacrificial role and leach into the electrolyte as oxyanions, thereby losing the ability to tune the catalyst's properties systematically. From a thermodynamic point of view, indium is expected to behave differently and should remain in the solid phase under alkaline OER conditions. However, the structural behavior of transition metal indium phases during the OER remains unexplored. Herein, are synthesized intermetallic cobalt indium (CoIn3) nanoparticles and revealed by in situ X-ray absorption spectroscopy and scanning transmission microscopy that they undergo phase segregation to cobalt oxyhydroxide and indium hydroxide. The obtained cobalt oxyhydroxide outperforms a metallic-cobalt-derived one due to more accessible active sites. The observed phase segregation shows that indium behaves distinctively differently from most p-block elements and remains at the electrode surface, where it can form lasting interfaces with the active metal oxo phases.

Deciphering Photoinduced Catalytic Reaction Mechanisms in Natural and Artificial Photosynthetic Systems on Multiple Temporal and Spatial Scales Using X-ray Probes

Utilization of renewable energies for catalytically generating value-added chemicals is highly desirable in this era of rising energy demands and climate change impacts. Artificial photosynthetic systems or photocatalysts utilize light to convert abundant CO2, H2O, and O2 to fuels, such as carbohydrates and hydrogen, thus converting light energy to storable chemical resources. The emergence of intense X-ray pulses from synchrotrons, ultrafast X-ray pulses from X-ray free electron lasers, and table-top laser-driven sources over the past decades opens new frontiers in deciphering photoinduced catalytic reaction mechanisms on the multiple temporal and spatial scales. Operando X-ray spectroscopic methods offer a new set of electronic transitions in probing the oxidation states, coordinating geometry, and spin states of the metal catalytic center and photosensitizers with unprecedented energy and time resolution. Operando X-ray scattering methods enable previously elusive reaction steps to be characterized on different length scales and time scales. The methodological progress and their application examples collected in this review will offer a glimpse into the accomplishments and current state in deciphering reaction mechanisms for both natural and synthetic systems. Looking forward, there are still many challenges and opportunities at the frontier of catalytic research that will require further advancement of the characterization techniques.

In Situ Insertion of Silicon Nanowires into Hollow Porous Co/NC Polyhedra Enabling Large-Current-Density Hydrogen Evolution Electrocatalysis

N-doped carbon (NC)-encapsulated transition metal (TM) nanocomposites are considered as alternatives to Pt-based hydrogen evolution reaction (HER) electrocatalysts; however, their poor electron transfer and mass diffusion capability at high current densities hinder their practical application. Herein, an oriented coupling strategy for the in situ grafting of ultrafine Co nanoparticle-embedded hollow porous C polyhedra onto Si nanowires (Co/NC-HP@Si-NWs) is proposed to address this concern. Experimental investigations reveal that the intimate coupling between the Si-NW and Co/NC nanocage forms a multithreaded conductive network, lowering the energy barrier for internal electron transfer. When functionalized as an HER electrocatalyst in 0.5 m H2 SO4 , Co/NC-HP@Si-NWs deliver overpotentials as low as 57 and 440 mV at 10 and 500 mA cm-2 , respectively, which are much better than those of the pristine Co/NC-HP. Moreover, Co/NC-HP@Si-NWs show an outstanding cycle durability of 24 h at 10 and 500 mA cm-2 . The findings of this study are expected to inspire revolutionary work on the development of Si-mediated TM-based electrocatalysts for the HER.

Oxygen Evolution/Reduction Reaction Catalysts: From In Situ Monitoring and Reaction Mechanisms to Rational Design

The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are core steps of various energy conversion and storage systems. However, their sluggish reaction kinetics, i.e., the demanding multielectron transfer processes, still render OER/ORR catalysts less efficient for practical applications. Moreover, the complexity of the catalyst-electrolyte interface makes a comprehensive understanding of the intrinsic OER/ORR mechanisms challenging. Fortunately, recent advances of in situ/operando characterization techniques have facilitated the kinetic monitoring of catalysts under reaction conditions. Here we provide selected highlights of recent in situ/operando mechanistic studies of OER/ORR catalysts with the main emphasis placed on heterogeneous systems (primarily discussing first-row transition metals which operate under basic conditions), followed by a brief outlook on molecular catalysts. Key sections in this review are focused on determination of the true active species, identification of the active sites, and monitoring of the reactive intermediates. For in-depth insights into the above factors, a short overview of the metrics for accurate characterizations of OER/ORR catalysts is provided. A combination of the obtained time-resolved reaction information and reliable activity data will then guide the rational design of new catalysts. Strategies such as optimizing the restructuring process as well as overcoming the adsorption-energy scaling relations will be discussed. Finally, pending current challenges and prospects toward the understanding and development of efficient heterogeneous catalysts and selected homogeneous catalysts are presented.

In Situ Coupling of Carbon Dots with Co-ZIF Nanoarrays Enabling Highly Efficient Oxygen Evolution Electrocatalysis

Metal-organic frameworks (MOFs) are regarded as one promising class of precatalysts for electrocatalytic oxygen evolution reaction (OER), yet most of them suffer from poor conductivity and lack of coordinatively unsaturated metal sites, which hinders the fast electrochemical reconstruction and thus a poor OER activity. To address this issue, a unique heterocomposite has been constructed by in situ inserting carbon dots (CDs) into cobalt-based zeolitic imidazolate framework (Co-ZIF) nanosheet arrays (Co-ZIF/CDs/CC) in the presence of carbon cloth (CC) via one-pot coprecipitation for alkaline OER. Benefiting from the synergism between CDs and Co-ZIF subunits such as superior conductivity, strong charge interaction as well as abundant metal sites exposure, the Co-ZIF/CDs/CC exhibits an enhanced promotion effect for OER and contributes to the deep phase transformation from CDs-coupled Co-ZIF to CDs-coupled active CoOOH. As expected, the achieved Co-ZIF/CDs/CC only requires an overpotential of 226 mV to deliver 10 mA cm^-2 in 1.0 M KOH, which is lower than that of Co-ZIF/CC and superior to most previously reported CC-supported MOF precatalysts. Moreover, it can also maintain a large current density of 100 mA cm^-2 for 24 h with negligible activity decay.