Progress and Challenges Toward the Rational Design of Oxygen Electrocatalysts Based on a Descriptor Approach

Promoting Electrocatalytic Oxygen Reactions Using Advanced Heterostructures for Rechargeable Zinc-Air Battery Applications

In order to facilitate electrochemical oxygen reactions in electrically rechargeable zinc-air batteries (ZABs), there is a need to develop innovative approaches for efficient oxygen electrocatalysts. Due to their reliability, high energy density, material abundance, and ecofriendliness, rechargeable ZABs hold promise as next-generation energy storage and conversion devices. However, the large-scale application of ZABs is currently hindered by the slow kinetics of the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). However, the development of heterostructure-based electrocatalysts has the potential to surpass the limitations imposed by the intrinsic properties of a single material. This Account begins with an explanation of the configurations of ZABs and the fundamentals of the oxygen electrochemistry of the air electrode. Then, we summarize recent progress with respect to the variety of heterostructures that exploit bifunctional electrocatalytic reactions and overview their impact on ZAB performance. The range of heterointerfacial engineering strategies for improving the ORR/OER and ZAB performance includes tailoring the surface chemistry, dimensionality of catalysts, interfacial charge transfer, mass and charge transport, and morphology. We highlight the multicomponent design approaches that take these features into account to create advanced highly active bifunctional catalysts. Finally, we discuss the challenges and future perspectives on this important topic that aim to enhance the bifunctional activity and performance of zinc-air batteries.

Facilitating charge transfer via a Semi-Coherent Fe(PO3)2-Co2P2O7 heterointerface for highly efficient Zn-Air batteries

Developing reversible oxygen electrodes for both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is crucial for achieving high-performance rechargeable Zn-air batteries (ZABs). This study introduced an nitrogen-doped carbon confined with a semi-coherent Fe(PO3)2-Co2P2O7 heterojunction for bifunctional oxygen electrocatalysis. This nanocomposite yielded an ORR half-wave potential of 0.908 V and an OER overpotential of 291 mV at 10 mA/cm2. ZABs incorporating this catalyst yielded impressive performance, including a peak power density of 203 mW/cm2, a specific capacity of 737 mAh/gZn, and promoted stability. Both experimental and theoretical simulations demonstrated that the unique electric field between Fe(PO3)2 and Co2P2O7 promoted efficient charge transport across the heterointerface. This interaction likely modulated the d-band center of the heterojunction, expedite the desorption of oxygen intermediates, thus improving oxygen catalysis and, consequently, ZAB performance. This work illustrates a significant design principle for creating efficient bifunctional catalysts in energy conversion technologies.

Experimentally validating sabatier plot by molecular level microenvironment customization for oxygen electroreduction

Microenvironmental modifications on metal sites are crucial to tune oxygen reduction catalytic behavior and decrypt intrinsic mechanism, whereas the stochastic properties of traditional pyrolyzed single-atom catalysts induce vague recognition on structure-reactivity relations. Herein, we report a theoretical descriptor relying on binding energies of oxygen adsorbates and directly associating the derived Sabatier volcano plot with calculated overpotential to forecast catalytic efficiency of cobalt porphyrin. This Sabatier volcano plot instructs that electron-withdrawing substituents mitigate the over-strong *OH intermediate adsorption by virtue of the decreased proportion of electrons in bonding orbital. To experimentally validate this speculation, we implement a secondary sphere microenvironment customization strategy on cobalt porphyrin-based polymer nanocomposite analogs. Systematic X-ray spectroscopic and in situ electrochemical characterizations capture the pronounced accessible active site density and the fast interfacial/outward charge migration kinetics contributions for the optimal carboxyl group-substituted catalyst. This work offers ample strategies for designing single-atom catalysts with well-managed microenvironment under the guidance of Sabatier volcano map.

Advancing electrocatalytic reactions through mapping key intermediates to active sites via descriptors

Descriptors play a crucial role in electrocatalysis as they can provide valuable insights into the electrochemical performance of energy conversion and storage processes. They allow for the understanding of different catalytic activities and enable the prediction of better catalysts without relying on the time-consuming trial-and-error approaches. Hence, this comprehensive review focuses on highlighting the significant advancements in commonly used descriptors for critical electrocatalytic reactions. First, the fundamental reaction processes and key intermediates involved in several electrocatalytic reactions are summarized. Subsequently, three types of descriptors are classified and introduced based on different reactions and catalysts. These include d-band center descriptors, readily accessible intrinsic property descriptors, and spin-related descriptors, all of which contribute to a profound understanding of catalytic behavior. Furthermore, multi-type descriptors that collectively determine the catalytic performance are also summarized. Finally, we discuss the future of descriptors, envisioning their potential to integrate multiple factors, broaden application scopes, and synergize with artificial intelligence for more efficient catalyst design and discovery.

Perovskite CoSn(OH)6 nanocubes with tuned d-band states towards enhanced oxygen evolution reactions

The CoSn(OH)6 perovskite hydroxide is a structure stable and inexpensive electrocatalyst for the oxygen evolution reaction (OER). However, the OER activity of CoSn(OH)6 is still unfavorable due to its limited active sites. In this work, an Fe3+ doping strategy is used to optimize the d-band state of the CoSn(OH)6 perovskite hydroxide. The CoSn(OH)6 catalyst with slightly Fe3+ doped nanocubes is synthesized by a facile hydrothermal method. Structure characterization shows that Fe3+ ions are incorporated into the crystal structure of CoSn(OH)6. Owing to the regulation of the electronic structure, CoSn(OH)6-Fe1.8% exhibits an OER overpotential of 289 mV at a current density of 10 mA cm-2 in OER electrochemical tests. In situ Raman spectroscopy shows that no obvious re-construction occurred during the OER for both CoSn(OH)6 and CoSn(OH)6-Fe1.8%. DFT calculations show that the introduction of Fe3+ into CoSn(OH)6 can shift the d-band center to a relatively high position, thus promoting the OER intermediates' adsorption ability. Further DFT calculations suggest that incorporation of an appropriate amount of Fe3+ into CoSn(OH)6 significantly reduces the rate-determining Gibbs free energy during the OER. This work offers valuable insights into tuning the d-band center of perovskite hydroxide materials for efficient OER applications.

Bird's-Eye View of the Activity Distribution on a Catalyst Surface via a Machine Learning-Driven Adequate Sampling Algorithm

Rational design of catalysts relies on a deep understanding of the active centers. The structure and activity distribution of active centers on a surface are two of the central issues in catalysis and important targets of theoretical and experimental investigations. Herein, we report a machine learning-driven adequate sampling (MLAS) framework for obtaining a statistical understanding of the chemical environment near catalyst sites. Combined strategies were implemented to achieve highly efficient sampling, including the decomposition of degrees of freedom, stratified sampling, Gaussian process regression, and well-designed constraint optimization. The MLAS framework was applied to the rate-determining step in NH3 synthesis, namely the N2 activation process. We calculated the produced population function, PA, which provides a comprehensive and intuitive understanding of active centers. The MLAS framework can be broadly applied to other more complicated catalyst materials and reaction networks.

Efficient Machine Learning Model Focusing on Active Sites for the Discovery of Bifunctional Oxygen Electrocatalysts in Binary Alloys

The distinctive characteristics of alloy catalysts, encompassing composition, structure, and modifiable adsorption sites, present significant potential for the development of highly efficient electrocatalysts for oxygen evolution/reduction reactions [oxygen evolution reactions (OERs)/oxygen reduction reactions (ORRs)]. Machine learning (ML) methods can quickly establish the relationship between material features and catalytic activity, thus accelerating the development of alloy electrocatalysts. However, the current abundance of features presents a crucial challenge in selecting the most pertinent ones. In this study, we explored seven intrinsic features directly derived from the material's structure, with a specific focus on the chemical environment of active sites and their nearest neighbors. An accurate and efficient ML model to predict potential bifunctional oxygen electrocatalysts based on the intrinsic features of AB-type alloy active sites and intermediate free energies in the OERs/ORRs was established. These features possess clear physical and chemical meanings, closely linked to the electronic and geometric structures of active sites and neighboring atoms, thereby providing indispensable insights for the discovery of high-performance electrocatalysts. The ML model achieved R2 scores of 0.827, 0.913, and 0.711 for the predicted values of the three intermediate (OH, O, OOH) free energies, with corresponding mean absolute errors of 0.175, 0.242, and 0.200 eV, respectively. These results indicate that the ML model exhibits high accuracy in predicting the intermediate free energies. Furthermore, the ML model exhibited a prediction efficiency 150,000 times faster than traditional density functional theory calculations. This work will offer valuable insights and a framework for facilitating the rapid design of potential catalysts by ML methods.

Dipole Effect on Oxygen Evolution Reaction of 2D Janus Single-Atom Catalysts: A Case of Rh Anchored on the P6m2-NP Configurations

Catalytic performance of single-atom catalysts (SACs) relies fundamentally on the electronic nature and local coordination environment of the active site. Here, based on a machine-learning (ML)-aided density functional theory (DFT) method, we reveal that the intrinsic dipole in Janus materials has a significant impact on the catalytic activity of SACs, using 2D γ-phosphorus carbide (γ-PC) as a model system. Specifically, a local dipole around the active site is a key degree to tune the catalytic activity and can be used as an important descriptor with a high feature importance of 17.1% in predicting the difference of adsorption free energy (ΔGO* - ΔGOH*) to assess the activity of the oxygen evolution reaction. As a result, the catalytic performance of SACs can be tuned by an intrinsic dipole, in stark contrast to those external stimuli strategies previously used. These results suggest that dipole engineering and the revolutionary DFT-ML hybrid scheme are novel approaches for designing high-performance catalysts.

Significance of Engineering the MnO6 Octahedral Units to Promote the Oxygen Reduction Reaction of Perovskite Oxides

The electronic structure and geometric configuration of catalysts play a crucial role to design novel perovskite-type catalysts for oxygen reduction reaction (ORR). Nowadays, many studies are more concerned with the influence of electronic structure and ignore the geometric effect, which plays a nonnegligible role in enhancing catalytic performances. Herein, this work regulates the MnO6 octahedral tilting degree of LaMnO3 by modulating the concentration of Y3+ , excluding the electronic effect from the valence state of manganese. Plotting the MnO6 octahedral tilting degree as a function of concentration of Y3+ produces a volcano-shaped plot. The octahedral tilting can reduce the Mn-O covalency, generating more highly active Mn3+ and oxygen vacancies during ORR process. The specific activity has a positive correlation with octahedral tilting degree. Meanwhile, the octahedral tilting stabilizes Mn-O interactions during ORR process and promote stability. Based on experimental results and DFT calculations, octahedral tilting alters the rate-determining step (RDS) and decrease the energy barrier. Subsequent extended experiment confirms that octahedral tilting is the key factor to affect the catalytic performances.

Recent Advances in Hybrid Seawater Electrolysis for Hydrogen Production

Seawater electrolysis (SWE) is a promising and potentially cost-effective approach to hydrogen production, considering that seawater is vastly abundant and SWE is able to combine with offshore renewables producing green hydrogen. However, SWE has long been suffering from technical challenges including the high energy demand and interference of chlorine chemistry, leading electrolyzers to a low efficiency and short lifespan. In this context, hybrid SWE, operated by replacing the energy-demanding oxygen evolution reaction and interfering chlorine evolution reaction (CER) with a thermodynamically more favorable anodic oxidation reaction (AOR) or by designing innovative electrolyzer cells, has recently emerged as a better alternative, which not only allows SWE to occur in a safe and energy-saving manner without the notorious CER, but also enables co-production of value-added chemicals or elimination of environmental pollutants. This review provides a first account of recent advances in hybrid SWE for hydrogen production. The substitutional AOR of various small molecules or redox mediators, in couple with hydrogen evolution from seawater, is comprehensively summarized. Moreover, how the electrolyzer cell design helps in hybrid SWE is briefly discussed. Last, the current challenges and future outlook about the development of the hybrid SWE technology are outlined.

Intermetallic Nanocrystals for Fuel-Cells-Based Electrocatalysis

Electrocatalysis underpins the renewable electrochemical conversions for sustainability, which further replies on metallic nanocrystals as vital electrocatalysts. Intermetallic nanocrystals have been known to show distinct properties compared to their disordered counterparts, and been long explored for functional improvements. Tremendous progresses have been made in the past few years, with notable trend of more precise engineering down to an atomic level and the investigation transferring into more practical membrane electrode assembly (MEA), which motivates this timely review. After addressing the basic thermodynamic and kinetic fundamentals, we discuss classic and latest synthetic strategies that enable not only the formation of intermetallic phase but also the rational control of other catalysis-determinant structural parameters, such as size and morphology. We also demonstrate the emerging intermetallic nanomaterials for potentially further advancement in energy electrocatalysis. Then, we discuss the state-of-the-art characterizations and representative intermetallic electrocatalysts with emphasis on oxygen reduction reaction evaluated in a MEA setup. We summarize this review by laying out existing challenges and offering perspective on future research directions toward practicing intermetallic electrocatalysts for energy conversions.

Role of Transition Metals in Pt Alloy Catalysts for the Oxygen Reduction Reaction

In pursuit of higher activity and stability of electrocatalysts toward the oxygen reduction reaction, it has become standard practice to alloy platinum in various structural configurations. Transition metals have been extensively studied for their ability to tune catalyst functionality through strain, ligand, and ensemble effects. The origin of these effects and potential for synergistic application in practical materials have been the subject of many theoretical and experimental analyses in recent years. Here, a comprehensive overview of these phenomena is provided regarding the impact on reaction mechanisms and kinetics through combined experimental and theoretical approaches. Experimental approaches to electrocatalysis are discussed.

Progress in Manipulating Dynamic Surface Reconstruction via Anion Modulation for Electrocatalytic Water Oxidation

The development of efficient and economical electrocatalysts for oxygen evolution reaction (OER) is of paramount importance for the sustainable production of renewable fuels and energy storage systems; however, the sluggish OER kinetics involving multistep four proton-coupled electron transfer hampers progress in these systems. Fortunately, surface reconstruction offers promising potential to improve OER catalyst design. Anion modulation plays a crucial role in controlling the extent of surface reconstruction and positively persuading the reconstructed species' performances. This review starts by providing a general explanation of how various types of anions can trigger dynamic surface reconstruction and create different combinations with pre-catalysts. Next, the influences of anion modulation on manipulating the surface dynamic reconstruction process are discussed based on the in situ advanced characterization techniques. Furthermore, various effects of survived anionic groups in reconstructed species on water oxidation activity are further discussed. Finally, the challenges and prospects for the future development directions of anion modulation for redirecting dynamic surface reconstruction to construct highly efficient and practical catalysts for water oxidation are proposed.

Recent advances in developing multiscale descriptor approach for the design of oxygen redox electrocatalysts

Oxygen redox electrocatalysis is the crucial electrode reaction among new-era energy sources. The prerequisite to rationally design an ideal electrocatalyst is accurately identifying the structure-activity relationship based on the so-called descriptors which link the catalytic performance with structural properties. However, the quick discovery of those descriptors remains challenging. In recent, the high-throughput computing and machine learning methods were identified to present great prospects for accelerating the screening of descriptors. That new research paradigm improves cognition in the way of oxygen evolution reaction/oxygen reduction reaction activity descriptor and reinforces the understanding of intrinsic physical and chemical features in the electrocatalytic process from a multiscale perspective. This review summarizes those new research paradigms for screening multiscale descriptors, especially from atomic scale to cluster mesoscale and bulk macroscale. The development of descriptors from traditional intermediate to eigen feature parameters has been addressed which provides guidance for the intelligent design of new energy materials.

Ni-Fe Oxides/TiO2 Heterojunction Anodes for Reactive Chlorine Generation and Mediated Water Treatment

Reactive chlorine-mediated electrochemical water treatment necessitates selective chlorine evolution reaction (ClER) versus parallel oxygen evolution reaction (OER) in mild pH (7-10), with minimal deployments of precious electrocatalysts. This study reports Ni_0.33Fe_0.67O_y/TiO₂ heterojunction anode prepared by a straightforward sol-gel coating with thermal decomposition at 425 °C. The ClER current efficiency (CE, 70%) and energy efficiency (2.3 mmol W h^-1) were comparable to benchmarking Ir₇Ta₃O_y/TiO₂ at 30 mA cm^-2 in 50 mM NaCl solutions with near-neutral pH. Correlations of ClER CE of variable Ni_xFe_1-xO_y/TiO₂ (x: 0.33, 0.8-1) with the flat-band potential and p-band center, as experimental descriptors for surface charge density, nominated the outer TiO₂ to be the active ClER center. The underlying Ni_0.33Fe_0.67O_y, characterized as biphasic NiFe₂O₄ and NiO, effectively lowered the O binding energy of TiO₂ by electronic interaction across the junction. The OER activity of Ni_0.33Fe_0.67O_y superior to the other Fe-doped Ni oxides suggested that the conductive OER intermediates generated on Ni_0.33Fe_0.67O_y could also facilitate the ClER as an ohmic contact. Stability tests and NH₄⁺ degradation in synthetic and real wastewater confirmed the feasibility of Ni_0.33Fe_0.67O_y/TiO₂ heterojunction anode for mediated water treatments in mild pH.

Phase Evolution on the Hydrogen Adsorption Kinetics of NiFe-Based Heterogeneous Catalysts for Efficient Water Electrolysis

Transition metal layered double hydroxides, especially nickel-iron layered double hydroxide (NiFe-LDH) shows significant advancement as efficient oxygen evolution reaction (OER) electrocatalyst but also plays a momentous role as a precursor for NiFe-based hydrogen evolution reaction (HER) catalysts. Herein, a simple strategy for developing Ni-Fe-derivative electrocatalysts via phase evolution of NiFe-LDH under controllable annealing temperatures in an argon atmosphere is reported. The optimized catalyst annealed at 340 ^o C (denoted NiO/FeNi₃ ) exhibits superior HER properties with an ultralow overpotential of 16 mV@10 mA cm^-2 . Density functional theory simulation and in situ Raman analyses reveal that the excellent HER properties of the NiO/FeNi₃ can be attributed to the strong electronic interaction at the interface of the metallic FeNi₃ and semiconducting NiO, which optimizes the H₂ O and H adsorption energies for efficient HER and OER catalytic processes. This work will provide rational insights into the subsequent development of related HER electrocatalysts and other corresponding compounds via LDH-based precursors.

A corpus of CO2 electrocatalytic reduction process extracted from the scientific literature

The electrocatalytic CO₂ reduction process has gained enormous attention for both environmental protection and chemicals production. Thereinto, the design of new electrocatalysts with high activity and selectivity can draw inspiration from the abundant scientific literature. An annotated and verified corpus made from massive literature can assist the development of natural language processing (NLP) models, which can offer insight to help guide the understanding of these underlying mechanisms. To facilitate data mining in this direction, we present a benchmark corpus of 6,086 records manually extracted from 835 electrocatalytic publications, along with an extended corpus with 145,179 records in this article. In this corpus, nine types of knowledge such as material, regulation method, product, faradaic efficiency, cell setup, electrolyte, synthesis method, current density, and voltage are provided by either annotating or extracting. Machine learning algorithms can be applied to the corpus to help scientists find new and effective electrocatalysts. Furthermore, researchers familiar with NLP can use this corpus to design domain-specific named entity recognition (NER) models.

Metal-organic framework-derived advanced oxygen electrocatalysts as air-cathodes for Zn-air batteries: recent trends and future perspectives

Electrochemical energy storage devices with stable performance, high power output, and energy density are urgently needed to meet the global energy demand. Among the different electrochemical energy storage devices, batteries have become the most promising energy technologies and ranked as a highly investigated research subject. Recently, metal-air batteries especially Zn-air batteries (ZABs) have attracted enormous scientific interest in the electrochemical community due to their ease of operation, sustainability, environmental friendliness, and high efficiency. The oxygen electrocatalytic reactions [oxygen reduction reaction (ORR) and oxygen evolution reaction (OER)] are the two fundamental reactions for the development of ZABs. Noble metal-based electrocatalysts are widely considered as the benchmark for oxygen electrocatalysis, but their practical application in rechargeable ZAB is hindered due to several shortcomings. Thus, to replace noble metal-based catalysts, a wide range of transition-metal-based materials and heteroatom-doped metal-free carbon materials has been extensively investigated as oxygen electrocatalysts for ZABs. Recently, metal-organic frameworks (MOFs) with unique structural flexibility and uniformly dispersed active sites have become attractive precursors for the synthesis of a large variety of advanced functional materials. Herein, we summarize the recent progress of MOF-derived oxygen electrocatalysts (MOF-derived carbon nanomaterials, MOF-derived alloys/nanoparticles, and MOF-derived single-atom electrocatalysts) for ZABs. Specifically, we highlight MOF-derived single-atom electrocatalysts owing to the wide exploration of these emerging materials in electrocatalysis. The influence of the active sites, structural/compositional design, and porosity of MOF-derived advanced materials on the oxygen electrocatalytic performances is also discussed. Finally, the existing challenges and prospects of MOF-derived electrocatalysts in ZABs are briefly highlighted.

Fully-Conjugated Covalent Organic Frameworks with Two Metal Sites for Oxygen Electrocatalysis and Zn-Air Battery

Covalent organic frameworks (COFs) are a promising alternative toward catalysis, due to the unique framework structure and the excellent chemical stability. However, the scarcity of unsaturated metal sites and the low conductivity have constrained the advancement of these materials for catalysis of electrochemical reactions. Exploring next-generation conductive metal-covalent organic frameworks (M-COFs) with extra metal active sites is crucial for improving their catalytic activity. Herein, a novel fully-conjugated M-COFs (Co-PorBpy-Co) with two types of metal sites is proposed and achieved by solvothermal method in the presence of carbon nanotube (CNT). The electrocatalyst constructed by the Co-PorBpy-Co exhibits excellent oxygen reduction reaction (ORR) activity (E_1/2 = 0.84 V vs RHE, n = 3.86), superior to most COFs-based catalysts. Theoretical result shows the CoN₂ sites are extremely active for ORR, and Co-PorBpy-Co exhibits excellent conductivity for electron transfer. The Zn-air battery constructed by Co-PorBpy-Co/CNT manifests excellent power density (159.4 mW cm^-2 ) and great cycling stability, surpassing that of 20 wt% Pt/C catalyst. This work not only proposes a novel design concept for electrocatalysts, but establishes a mechanism platform for single-metal atom electrocatalysis and synergistic effect.

Hierarchical meso-micro porous FeNC derived from tripolycyanamide-based microporous polymer as efficient electrocatalyst for oxygen reduction reaction

Designing porous FeNC nanomaterials with highly efficient active sites is an effective strategy for constructing high-performance oxygen reduction reaction (ORR) electrocatalysts. N-containing porous organic polymers (POPs) have emerged as promising candidates for the preparation of porous FeNC catalysts. Here, N-rich tripolycyanamide-based microporous polymer (TCAMP)-coated SiO₂ nanospheres (SiO₂@TACMP) were prepared as the precursors of an Fe-N doped hierarchical meso-micro porous carbon (Fe-N-HMC) electrocatalyst for the ORR. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) characterizations demonstrated that the Fe-N-HMC catalyst possessed a higher content percentage of Fe-N_x active sites and a better distribution of Fe nanoparticles than its Fe-N doped microporous carbon (Fe-N-MC) counterpart. N₂ adsorption-desorption isotherm analysis showed that Fe-N-HMC catalyst exhibited a hierarchical meso-micro porous system, with a Brunauer-Emmett-Teller (BET) surface area (S_BET) of 733 m² g^-1 (∼2 times of Fe-N-MC's S_BET). As a result, Fe-N-HMC catalyst presented a highly efficient ORR performance with a half-wave potential of 0.856 mV, which is similar to the commercial grade 20 wt% Pt/C catalyst and superior to the Fe-N-MC catalyst. Moreover, the as-synthesized Fe-N-HMC catalyst displayed a better durability and methanol tolerance than the commercial Pt/C catalyst. Therefore, Fe-N-HMC shows great promise as an ORR catalyst for fuel batteries and metal-air cells due to its low-cost, high activity, and good stability.

Ruthenium Azobis(benzothiazole): Electronic Structure and Impact of Substituents on the Electrocatalytic Single-Site Water Oxidation Process

The present article deals with the structurally and spectroelectrochemically characterized newer class of ruthenium-azoheteroarenes [Ru^II(Ph-trpy)(Cl)(L)]ClO₄, [1]ClO₄-[3]ClO₄ (Ph-trpy: 4'-phenyl-2,2':6',2″-terpyridine; L1: 2,2'-azobis(benzothiazole) ([1]ClO₄); L2: 2,2'-azobis(6-methylbenzothiazole) ([2]ClO₄); L3: 2,2'-azobis(6-chlorobenzothiazole) ([3]ClO₄)). A collective consideration of experimental (i.e., structural and spectroelectrochemical) and theoretical (DFT calculations) results of [1]ClO₄-[3]ClO₄ established selective stabilization of (i) the unperturbed azo (N═N)⁰ function of L, (ii) the exclusive presence of the isomeric form involving the N(azo) donor of L trans to Cl, and (iii) the presence of extended, hydrogen-bonded trimeric units in the asymmetric unit of [2]ClO₄ (CH---O) via the involvement of ClO₄^- anions. The detailed electrochemical studies revealed metal-based oxidation of [Ru^II(Ph-trpy)(Cl)(L)]⁺ (1⁺-3⁺) to [Ru^III(Ph-trpy)(Cl)(L)]²⁺ (1²⁺-3²⁺); however, the electronic form of the first reduced state (1-3) could be better represented by its mixed Ru^II(Ph-trpy)(Cl)(L^•-)/Ru^III(Ph-trpy)(Cl)(L^2-) state. Both native (1⁺-3⁺) and reduced (1-3) states exhibited weak lower energy transitions within the range of 1000-1200 nm. Further, [1]ClO₄-[3]ClO₄ delivered an electrochemical OER (oxygen evolution reaction) process in alkaline medium on immobilizing them to a carbon cloth support, which divulged an amplified water oxidation feature for [2]ClO₄ due to the presence of electron-donating methyl groups in the L2 backbone. The faster OER kinetics and high catalytic stability of [2]ClO₄ could also be rationalized by its lowest Tafel slope (85 mV dec^-1) and choronoamperometric experiment (stable up to 12 h), respectively, along with high Faradic efficiency (∼97%). A comparison of [2]ClO₄ with the reported analogous ruthenium complexes furnished its excellent intrinsic water oxidation activity.

Ab initio study of changing the oxygen reduction activity of Co-Fe-based perovskites by tuning the B-site composition

Perovskite oxides are promising low-cost and stable alternative electrocatalysts for the oxygen reduction reaction (ORR), relative to the precious metal-based electrocatalysts. Despite the experimental research on substituting various transition metals into the B-site of perovskite catalysts to improve the ORR performance, the detailed ORR mechanism due to the substitution process is rarely studied. In this paper, the ORR activity of La_0.5Sr_0.5Co_xFe_1-xO₃ perovskites (x = 0, 0.25, 0.5, 0.75, and 1) is studied by density functional theory (DFT). The ORR mechanism in alkaline solution is theoretically examined as a function of the Co/Fe composition at different potentials. The substitution of Co for Fe at the B-site of the perovskites dramatically changes the theoretical overpotential and enhances the activity. The HOO* formation is the potential-determining step for all the Co/Fe compositions. In comparison with the other compositions, the Co_0.5/Fe_0.5 composition exhibits the lowest overpotential and bonding with the reaction intermediates moderately. Furthermore, the oxygen binding energy is correlated with the bulk oxygen p-band center relative to the Fermi level. Among all the Co/Fe compositions, the Co_0.5/Fe_0.5 composition shows neither too low nor too high oxygen p-band center value. These results provide deep insights into the ORR mechanism on B-site substituted perovskites and guidelines for the design of cost-effective and Pt-free electrocatalysts for oxygen reduction.

Tuning the d-Band States of Ni-Based Serpentine Materials via Fe3+ Doping for Efficient Oxygen Evolution Reaction

The serpentine germanate materials are promising oxygen evolution reaction (OER) electrocatalysts due to their unique layered crystal structure and electronic structure. However, the catalytic activities still need to be improved to satisfy the practical applications. Adjusting the d-band center of metal active site to balance the adsorption and desorption of intermediates is considered an effective approach to improve the OER activity. In this work, an element dopant strategy was proposed to optimize the d-band state of Ni₃Ge₂O₅(OH)₄ serpentine to improve the OER activity. The density functional theory calculations revealed that Fe³⁺ doping increased the d-band center of the Ni₃Ge₂O₅(OH)₄ serpentine, which optimized the adsorption strength of intermediates on surface Ni and Fe atoms so that the Fe³⁺ doped Ni₃Ge₂O₅(OH)₄ (Ni_2.25Fe_0.75Ge₂O₅(OH)₄) exhibited much reduced Gibbs free energy changes in the rate-determining step compared with pristine serpentine. Inspired by the theoretical calculations, the Ni_xFe_3-xGe₂O₅(OH)₄ nanosheets with different amounts of doped Fe³⁺ were designed and synthesized. The structural characterizations indicated that Fe³⁺ was successfully doped into Ni₃Ge₂O₅(OH)₄ and replaced the Ni²⁺. The Fe³⁺ doped Ni_xFe_3-xGe₂O₅(OH)₄ nanosheets showed greatly improved OER activity than Ni₃Ge₂O₅(OH)₄ and Fe₃Ge₂O₅(OH)₄. Further electrochemical analysis illustrated that Fe³⁺ doping reduced the adsorptive/formative resistance of intermediates and the charge transfer resistance and facilitated the kinetic process of OER. The in situ Raman spectra indicated that the Fe³⁺ doped Ni₃Ge₂O₅(OH)₄ possesses a more active Ni-O bond than pristine Ni₃Ge₂O₅(OH)₄. This work provides an effective strategy to tune the d-band center of serpentines for efficient electrocatalytic OER.

Fe Ions-Doped TiO2 Aerogels as Catalysts of Oxygen Reduction Reactions in Alkaline Solutions

Aerogels have interconnected networks and preeminent pore structures. When used as the catalysts for oxygen reduction reaction (ORR), they can facilitate the mass transfer and expose more active sites. Here, we synthesized the Fe-doped titanium oxide-based aerogels (TA/Fes) by the sol-gel method combined with thermal treatment. The specific surface areas of the TA/Fes ranged from 475 to 774 m²·g^-1, and the pore volumes varied from 0.96 to 1.72 cm³·g^-1. The doping effect of the Fe ions and the oxygen vacancies in anatase enhance the electrical conductivity, leading to the low Rct (313.3-828.2 Ω). All samples showed excellent stability (2.0-4.5 mV) and 4e^- pathway. The limiting current density of TA/Fe3 reached 5.34 mA·cm^-2, which was comparable to that of commercial Pt/C. The preparation method is inspiring and the as-prepared aerogel catalysts have potential in promoting the scale of fuel cells.

Mesokinetics as a Tool Bridging the Microscopic-to-Macroscopic Transition to Rationalize Catalyst Design

Heterogeneous catalysis is the workhorse of the chemical industry, and a heterogeneous catalyst possesses numerous active sites working together to drive the conversion of reactants to desirable products. Over the decades, much focus has been placed on identifying the factors affecting the active sites to gain deep insights into the structure-performance relationship, which in turn guides the design and preparation of more active, selective, and stable catalysts. However, the molecular-level interplay between active sites and catalytic function still remains qualitative or semiquantitative, ascribed to the difficulty and uncertainty in elucidating the nature of active sites for its controllable manipulation. Hence, bridging the microscopic properties of active sites and the macroscopic catalytic performance, that is, microscopic-to-macroscopic transition, to afford a quantitative description is intriguing yet challenging, and progress toward this promises to revolutionize catalyst design and preparation.In this Account, we propose mesokinetics modeling, for the first time enabling a quantitative description of active site characteristics and the related mechanistic information, as a versatile tool to guide rational catalyst design. Exemplified by a pseudo-zero-order reaction, the kinetics derivation from the Pt particle size-sensitive catalytic activity and size-insensitive activation energy suggests only one type of surface site as the dominant active site, in which the Pt(111) with almost unchanged turnover frequency (TOF₁₁₁) is further identified as the dominating active site. Such a method has been extended to identify and quantify the number (N_i) of active sites for various thermo-, electro-, and photocatalysts in chemical synthesis, hydrogen generation, environment application, etc. Then, the kinetics derivation from the kinetic compensation effects suggests a thermodynamic balance between the activation entropy and enthalpy, which exhibit linear dependences on Pt charge. Accordingly, the Pt charge can serve as a catalytic descriptor for its quantitative determination of TOF_i. This strategy has been further applied to Pt-catalyzed CO oxidation with nonzero-order reaction characteristic by taking the site coverages of surface species into consideration.Hence, substituting the above statistical correlations of N_i and TOF_i into the rate equation R = ∑N_i × TOF_i offers the mesokinetics model, which can precisely predict catalytic function and screen catalysts. Finally, based on the disentanglement of the factors underlying Pt electronic structures, a de novo strategy, from the interfacial charge distribution to reaction mechanism, kinetics, and thermodynamics parameters of the rate-determining step, and ultimately catalytic performance, is developed to map the unified mechanistic and kinetics picture of reaction. Overall, the mesokinetics not only demonstrates much potential to elucidate the quantitative interplay between active sites and catalytic activity but also provides a new research direction in kinetics analysis to rationalize catalyst design.

Rational Design of Atomic Site Catalysts for Electrocatalytic Nitrogen Reduction Reaction: One Step Closer to Optimum Activity and Selectivity

The electrocatalytic nitrogen reduction reaction (NRR) has been one of the most intriguing catalytic reactions in recent years, providing an energy-saving and environmentally friendly alternative to the conventional Haber–Bosch process for ammonia production. However, the activity and selectivity issues originating from the activation barrier of the NRR intermediates and the competing hydrogen evolution reaction result in the unsatisfactory NH3 yield rate and Faradaic efficiency of current NRR catalysts. Atomic site catalysts (ASCs), an emerging group of heterogeneous catalysts with a high atomic utilization rate, selectivity, and stability, may provide a solution. This article undertakes an exploration and systematic review of a highly significant research area: the principles of designing ASCs for the NRR. Both the theoretical and experimental progress and state-of-the-art techniques in the rational design of ASCs for the NRR are summarized, and the topic is extended to double-atom catalysts and boron-based metal-free ASCs. This review provides guidelines for the rational design of ASCs for the optimum activity and selectivity for the electrocatalytic NRR.

Graphical Abstract

Rational design of atomic site catalysts (ASCs) for nitrogen reduction reaction (NRR) has both scientific and industrial significance. In this review, the recent experimental and theoretical breakthroughs in the design principles of transition metal ASCs for NRR are comprehensively discussed, and the topic is also extended to double-atom catalysts and boron-based metal-free ASCs.

Machine Learning for Electrocatalyst and Photocatalyst Design and Discovery

Electrocatalysts and photocatalysts are key to a sustainable future, generating clean fuels, reducing the impact of global warming, and providing solutions to environmental pollution. Improved processes for catalyst design and a better understanding of electro/photocatalytic processes are essential for improving catalyst effectiveness. Recent advances in data science and artificial intelligence have great potential to accelerate electrocatalysis and photocatalysis research, particularly the rapid exploration of large materials chemistry spaces through machine learning. Here a comprehensive introduction to, and critical review of, machine learning techniques used in electrocatalysis and photocatalysis research are provided. Sources of electro/photocatalyst data and current approaches to representing these materials by mathematical features are described, the most commonly used machine learning methods summarized, and the quality and utility of electro/photocatalyst models evaluated. Illustrations of how machine learning models are applied to novel electro/photocatalyst discovery and used to elucidate electrocatalytic or photocatalytic reaction mechanisms are provided. The review offers a guide for materials scientists on the selection of machine learning methods for electrocatalysis and photocatalysis research. The application of machine learning to catalysis science represents a paradigm shift in the way advanced, next-generation catalysts will be designed and synthesized.

Electron spin modulation engineering in oxygen-involved electrocatalysis

Electrocatalytic oxygen reduction reaction (ORR) and oxygen evolution reduction (OER) are regarded as the key reactions via the sustainable system (fuel cell and water splitting), respectively. In OER, the transition from singlet oxygen species to triplet oxygen molecules is involved, meanwhile the ORR involves the transition from triplet oxygen molecules to singlet oxygen species. However, in these processes, the number of unpaired electrons is not conserved, which is not thermodynamically favorable and creates an additional energy barrier. Fortunately, regulating the electrocatalysis by spin-state modulation enables a unique effect on the catalytic performance, but the current understanding on spin-state engineering for electro-catalyzing ORR and OER is still insufficient. Herein, this review summarized the in-spin engineering for the state-of-the-art ORR and OER electrocatalysts. It began by introducing engineering of spin-state to e_gfilling for ORR and OER process, and then moved to spin polarization and spin-pinning effect for OER process. Various designed strategies focusing on how to regulate the spin-state of the active center have been summarized up. The connectivity of the structures of typical ORR (e.g. metal-nitrogen-carbon) and OER (e.g. design strategies oxides, metal organic frameworks) catalysts depending on the spin level is also discussed. Finally, we present the outlook from the aspects of template catalysts, characterization methods, regulation strategies, theoretical calculations, which will further expand the possibility of better electrocatalytic performance through spin-state modulation. This review concluded some open suggestions and prospects, which are worthy of the community's future work.

Improving the Accuracy of Atomistic Simulations of the Electrochemical Interface

Atomistic simulation of the electrochemical double layer is an ambitious undertaking, requiring quantum mechanical description of electrons, phase space sampling of liquid electrolytes, and equilibration of electrolytes over nanosecond time scales. All models of electrochemistry make different trade-offs in the approximation of electrons and atomic configurations, from the extremes of classical molecular dynamics of a complete interface with point-charge atoms to correlated electronic structure methods of a single electrode configuration with no dynamics or electrolyte. Here, we review the spectrum of simulation techniques suitable for electrochemistry, focusing on the key approximations and accuracy considerations for each technique. We discuss promising approaches, such as enhanced sampling techniques for atomic configurations and computationally efficient beyond density functional theory (DFT) electronic methods, that will push electrochemical simulations beyond the present frontier.

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.

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.

Strain engineering in single-atom catalysts: GaPS4 for bifunctional oxygen reduction and evolution

We report here a theoretical study on 34 transition metal doped two-dimensional GaPS4 catalysts denoted as transition metal transition metal@VS-GaPS4.

Functionality and design of Co-MOFs: unique opportunities in electrocatalysts for oxygen reduction reaction

The first report highlighting miraculous and intelligent electrocatalysts that can be tailored to form useful structures and morphologies with active sites for the oxygen reduction reaction.

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.

Machine Learning in Screening High Performance Electrocatalysts for CO2 Reduction

Converting CO₂ into carbon-based fuels is promising for relieving the greenhouse gas effect and the energy crisis. However, the selectivity and efficiency of current electrocatalysts for CO₂ reductions are still not satisfactory. In this paper, the development of machine learning methods in screening CO₂ reduction electrocatalysts over the recent years is reviewed. Through high-throughput calculation of some key descriptors such as adsorption energies, d-band center, and coordination number by well-constructed machine learning models, the catalytic activity, optimal composition, active sites, and CO₂ reduction reaction pathway over various possible materials can be predicted and understood. Machine learning is now realized as a fast and low-cost method to effectively explore high performance electrocatalysts for CO₂ reduction.

Tuning Reconstruction Level of Precatalysts to Design Advanced Oxygen Evolution Electrocatalysts

Surface reconstruction engineering is an effective strategy to promote the catalytic activities of electrocatalysts, especially for water oxidation. Taking advantage of the physicochemical properties of precatalysts by manipulating their structural self-reconstruction levels provide a promising methodology for achieving suitable catalysts. In this review, we focus on recent advances in research related to the rational control of the process and level of surface transformation ultimately to design advanced oxygen evolution electrocatalysts. We start by discussing the original contributions to surface changes during electrochemical reactions and related factors that can influence the electrocatalytic properties of materials. We then present an overview of current developments and a summary of recently proposed strategies to boost electrochemical performance outcomes by the controlling structural self-reconstruction process. By conveying these insights, processes, general trends, and challenges, this review will further our understanding of surface reconstruction processes and facilitate the development of high-performance electrocatalysts beyond water oxidation.

Advanced Transition Metal-Based OER Electrocatalysts: Current Status, Opportunities, and Challenges

Oxygen evolution reaction (OER) is an important half-reaction involved in many electrochemical applications, such as water splitting and rechargeable metal-air batteries. However, the sluggish kinetics of its four-electron transfer process becomes a bottleneck to the performance enhancement. Thus, rational design of electrocatalysts for OER based on thorough understanding of mechanisms and structure-activity relationship is of vital significance. This review begins with the introduction of OER mechanisms which include conventional adsorbate evolution mechanism and lattice-oxygen-mediated mechanism. The reaction pathways and related intermediates are discussed in detail, and several descriptors which greatly assist in catalyst screen and optimization are summarized. Some important parameters suggested as measurement criteria for OER are also mentioned and discussed. Then, recent developments and breakthroughs in experimental achievements on transition metal-based OER electrocatalysts are reviewed to reveal the novel design principles. Finally, some perspectives and future directions are proposed for further catalytic performance enhancement and deeper understanding of catalyst design. It is believed that iterative improvements based on the understanding of mechanisms and fundamental design principles are essential to realize the applications of efficient transition metal-based OER electrocatalysts for electrochemical energy storage and conversion technologies.

Revealing the Correlation of OER with Magnetism: A New Descriptor of Curie/Neel Temperature for Magnetic Electrocatalysts

Developing accurate descriptors for oxygen evolution reaction (OER) is of great significance yet challenging, which roots in and also boosts the understanding of its intrinsic mechanisms. Despite various descriptors are reported, it still has limitations in the facile prediction, given that complicated analytical techniques as well as time-consuming modeling and calculations are indispensable. In the present work, strong correlation of magnetic property with OER performance is revealed by in-depth investigations on the crystal and electronic structures. A facile descriptor of Curie/Neel temperature (T_C/N ) is developed for La_{2- x} Sr_x Co₂ O_{6- δ} perovskite oxides, based on the inference that both magnetism and OER are rooted in the electron exchange interaction. Specifically, both the T_C/N and OER activity are proportional to the degree of p-d orbital hybridization, which increases with enlarged bond angle of Co─O─Co and/or increased oxidation of Co. This finding reveals that T_C/N from magnetic characterizations is an effective descriptor in designing novel OER electrocatalysts, and interdisciplinary researches are advantageous for revealing the controversial mechanisms of OER process.

Hierarchical Hollow MOF-Derived Bamboo-like N-doped Carbon Nanotube-Encapsulated Co0.25Ni0.75 Alloy: An Efficient Bifunctional Oxygen Electrocatalyst for Zinc-Air Battery

The synthesis of nonprecious electrocatalysts for oxygen electrocatalysis is of considerable interest for the development of electrochemical energy devices. Herein, we demonstrate a facile approach for the synthesis of bamboo-like nitrogen-doped carbon nanotube-encapsulated Co_0.25Ni_0.75 alloy electrocatalyst (Co_0.25Ni_0.75@NCNT) and its bifunctional oxygen electrocatalytic performance toward oxygen reduction and oxygen evolution reactions. The Co_0.25Ni_0.75 alloy wrapped with NCNT is obtained by a one-step carbothermal reduction approach using dicyandiamide and NiCo-MOF precursors. Dicyandiamide acts as a nitrogen source, and the in situ generated Co_0.25Ni_0.75 alloy nanoparticles catalyze the growth of bamboo-like NCNTs. The hollow NiCo-MOF plays a sacrificial role in providing a suitable environment for the controlled growth of Co_0.25Ni_0.75 alloy and NCNT. Co_0.25Ni_0.75@NCNT efficiently catalyzes both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) at a favorable overpotential. It shows a low potential gap (ΔE) of ∼0.8 V between the two reactions, and it qualifies for the development of air cathode in metal-air batteries. The enhanced bifunctional activity and excellent durability stem from the chemical composition and the synergistic effect between Co_0.25Ni_0.75 alloy and encapsulating NCNT. The original phase and morphology of the catalyst is preserved after an extensive durability test. Aqueous rechargeable Zn-air battery (ZAB) is fabricated using a Co_0.25Ni_0.75@NCNT-based air cathode. The battery has high open-circuit voltage (1.53 V) and a maximum peak power density of 167 mW cm^-2 with only 1.6% loss in the voltaic efficiency after 36 h charge-discharge cycles. As a proof-of-concept demonstration, the as-fabricated ZAB is successfully used for the electrochemical water splitting in alkaline solution.