Research progress in improving the oxygen evolution reaction by adjusting the 3d electronic structure of transition metal catalysts

Phosphorus-doped nickel cobalt oxide (NiCo2O4) wrapped in 3D hierarchical hollow N-doped carbon nanoflowers as highly efficient bifunctional electrocatalysts for overall water splitting

Properly design and fabricate capable electrocatalysts with 3D hierarchical hollow framework to realize cost-effective and efficacious overall water splitting (OWS) are particularly meaningful for the large-scale arrangement of pivotal energy technology. In this study, P-doped NiCo2O4 nanoparticles encapsulated in N-doped carbon hierarchical hollow nanoflowers (P-NiCo2O4@NCHHNFs) were constructed using the hydrothermal-pyrolysis-phosphorization approach. This fascinating architecture can not merely serve as a conductive pathway for electron transfer, but at the same time effectively inhibited the aggregation and corrosion of the NiCo2O4 nanoparticles. Additionally, the P doping not only regulates electronic structure configuration to boost the intrinsic activity of the catalyst, but also enhance electrochemical surface areas to reveal more accessible active sites. Attributing to these characteristics, the as-prepared P-NiCo2O4@NCHHNFs exhibit preeminent electrocatalytic performance with low overpotentials of 283 mV and 162 mV for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) (at 10 mA cm-2), respectively. Specifically, by using the P-NiCo2O4@NCHHNFs as bifunctional catalysts, a low potential of 1.56 V (at 10 mA cm-2) is sufficient to drive overall water splitting with splendid durability. This study proposed an innovative strategy for the conceiving and fabricating high-performance catalysts via heteroatom-doping.

Revealing the Electro-oxidation Mechanism of 5-Aminotetrazole on Nickel-Based Oxides and Synthesizing 5,5'-Azotetrazolate Salts

With the gradual expansion of the application of organic electromechanical synthesis in the field of energetic materials, it is necessary to explore deeply the mechanisms behind the organic electromechanical oxidation of energetic materials in order to develop efficient electrocatalysts. Electrochemical synthesis of 5,5'-azotetrazolate (ZT) salts is not only environmentally friendly and efficient but also can replace oxygen evolution reaction (OER) combined with hydrogen production, significantly reducing the battery voltage of overall water splitting (OWS) and achieving low energy consumption hydrogen production. Here, we prepared the Co-modified nickel-based oxide electrodes (Ni3-xCoO4/carbon cloth (CC), x = 1, 2) as a medium to reveal the oxidative coupling mechanism of 5-aminotetrazole (5-AT). Experimental and theoretical calculations verified that Ni-catalyzed oxidative coupling of 5-AT is a proton-coupled electron transfer (PCET) process, including electron transfer of electrocatalytic intermediates (Ni2+-O + OH- = Ni3+-O(OH) + e-) and spontaneous dehydrogenation process (Ni3+-O(OH) + X-H = Ni2+-O + X•). The Ni3+-O(OH) is an extremely fast nonreducing electron transfer center that serves as a chemical oxidant to directly abstract hydrogen atoms from the 5-AT. Simultaneously, the synergistic effect of Co doping on the electric cloud around Ni causes the upshift of the d-band centers, which is conducive to the easier adsorption of OH*, forming the generation of active intermediate Ni3+-O(OH). Thus, Ni2CoO4/CC has higher Faraday efficiency (FE) and yield for the oxidation reaction of 5-AT, with a yield of approximately 72.3% after electrolysis at 1.7 V vs reversible hydrogen electrode (RHE).

Abrupt Change from Ionic to Covalent Bonding in Nickel Halides Accompanied by Ligand Field Inversion

The electronic configuration of transition metal centers and their ligands is crucial for redox reactions in metal catalysis and electrochemistry. We characterize the electronic structure of gas-phase nickel monohalide cations via nickel L2,3-edge X-ray absorption spectroscopy. Comparison with multiplet charge-transfer simulations and experimental spectra of selectively prepared nickel monocations in both ground- and excited-state configurations are used to facilitate our analysis. Only for [NiF]+ with an assigned ground state of 3Π can the bonding be described as predominantly ionic, while the heavier halides with assigned ground states of 3Π or 3Δ exhibit a predominantly covalent contribution. The increase in covalency is accompanied by a transition from a classical ligand field for [NiF]+ to an inverted ligand field for [NiCl]+, [NiBr]+, and [NiI]+, resulting in a leading 3d9 L̲ configuration with a ligand hole (L̲) and a 3d occupation indicative of nickel(I) compounds. Hence, the absence of a ligand hole in [NiF]+ precludes any ligand-based redox reactions. Additionally, we demonstrate that the shift in energy of the L3 resonance is reduced compared to that of isolated atoms upon the formation of covalent compounds.

An IrRuOx catalyst supported by oxygen-vacant Ta oxide for the oxygen evolution reaction and proton exchange membrane water electrolysis

The sustainable development of proton exchange membrane water electrolysis (PEMWE) requires a dramatic reduction in Ir while maintaining good catalytic activity and stability for the oxygen evolution reaction (OER). Herein, high-surface-area Ta2O5 with abundant oxygen vacancies is synthesized via a facile process, followed by anchoring IrRuOx onto a Ta2O5 support (IrRuOx/Ta2O5). IrRuOx and Ta2O5 work synergistically to afford excellent catalytic performance for the acidic OER. At 0.3 mgIr cm-2, IrRuOx/Ta2O5 only needed an overpotential of 235 mV to deliver 10 mA cm-2 in an acidic half cell and needed a cell potential of 1.91 V to deliver 2 A cm-2 in a PEM water electrolyzer. The characterization results show that doping Ir into RuOx significantly improves the stability and the electrochemically active surface area of RuOx. In IrRuOx/Ta2O5, IrRuOx interacts with Ta2O5 through more electron-rich Ir, indicating strong synergy between the catalyst and the support. The use of a metal oxide support improves the catalyst dispersion, optimizes electronic structures, facilitates mass transport, and stabilizes active sites. This work demonstrates that compositing Ir with less expensive Ru and anchoring catalyst nanoparticles on platinum-group metal (PGM)-free metal oxide supports represents one of the most promising strategies to reduce Ir loading and achieve an activity-stability trade-off. Such a strategy can benefit future catalyst design for other energy storage and conventional processes.

Realizing a high OER activity in new single-atom catalysts formed by introducing TMNx (x = 3 and 4) units into carbon nanotubes using high-throughput calculations

Exploring highly efficient electrocatalysts for the oxygen evolution reaction (OER) is of great significance for hydrogen production through water splitting. By means of high-throughput density functional theory (DFT) calculations, we investigated the OER catalytic activity of a series of one-dimensional carbon nanotube (CNT)-based systems containing TMN4 or TMN3 functional units. Through the screening of 3d/4d/5d transition metals (TMs) from Group IVB to Group VIII, eight newly obtained TMNx@CNT (x = 3 and 4) systems were found to exhibit excellent OER activity, with very low overpotentials in the range 0.29-0.51 V, where the Co, Rh, Ir, Ti, Fe, and Ru atoms could be used as active sites. It was found that under the framework of TMN3@CNTs, the pre-adsorption of some species from water dissociation on the relevant TM sites (TM = Ti, Fe, and Ru) could lead to a high OER catalytic activity, which was different from the general situation where OER reactions directly occur on the clean surfaces of the remaining systems with Co/Rh/Ir metal centers. Moreover, the catalytic mechanisms were analyzed in detail. This work can be conducive to obtaining low-cost and high-performance OER single-atom electrocatalysts based on excellent CNT nanomaterials.

Insights into Heterogeneous Catalysis on Surfaces with 3d Transition Metals: Spin-Dependent Chemisorption Models and Magnetic Field Effects

This Perspective provides an overview of recent developments in the field of 3d transition metal (TM) catalysts for different reactions, including oxygen-based reactions such as the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The spin moments of 3d TMs can be exploited to influence chemical reactions, and recent advances in this area, including the theory of chemisorption based on spin-dependent d-band centers and magnetic field effects, are discussed. The Perspective also explores the use of scaling relationships and surface magnetic moments in catalyst design as well as the effect of magnetism on chemisorption and vice versa. In addition, recent studies on the influence of a magnetic field on the ORR and the OER are presented, demonstrating the potential of ferromagnetic catalysts to enhance these reactions through spin polarization.

Research Progress of Hydrogen Production Technology and Related Catalysts by Electrolysis of Water

As a clean and renewable energy source for sustainable development, hydrogen energy has gained a lot of attention from the general public and researchers. Hydrogen production by electrolysis of water is the most important approach to producing hydrogen, and it is also the main way to realize carbon neutrality. In this paper, the main technologies of hydrogen production by electrolysis of water are discussed in detail; their characteristics, advantages, and disadvantages are analyzed; and the selection criteria and design criteria of catalysts are presented. The catalysts used in various hydrogen production technologies and their characteristics are emphatically expounded, aiming at optimizing the existing catalyst system and developing new high-performance, high-stability, and low-cost catalysts. Finally, the problems and solutions in the practical design of catalysts are discussed and explored.

Morphology-Engineering Construction of Anti-Aggregated Co/N-Doped Hollow Carbon from Metal-Organic Frameworks for Efficient Biomass Upgrading

The controlled pyrolysis of metal/carbon-containing precursors is commonly used for fabricating multifunctional metal/carbon-based catalysts, nevertheless, the inevitable agglomeration of these precursors in pyrolysis is extremely negative for efficient catalysis. This study reports the first example of suppressing the interfacial fusion and agglomeration of metal/carbon-based catalyst in its pyrolysis-involved fabrication process by developing a facile morphology-engineering strategy. Metal-organic framework precursors are chosen as a proof of concept and five Co/N-doped hollow carbons with different morphologies (rhombic dodecahedron, cube, plate, interpenetration twin, and rod) are synthesized via the pyrolysis of their corresponding core-shell ZIF-8@ZIF-67 precursors. It is demonstrated that the interpenetration twin precursor shows the minimum interfacial contact of interparticles due to its partly-concave morphology with abundant facets, which endows it with the best resistibility from interfacial fusion and thus aggregation of interparticles during pyrolysis. Benefiting from its unique anti-aggregated structure with high specific surface area, abundant fully-exposed active sites, and good dispersibility, the resultant 36-facet Co/N-doped hollow carbon exhibit remarkably improved catalytic property for biomass upgrading as compared with its aggregated counterparts. This study highlights the crucial role of engineering morphology to prevent metal/carbon-containing precursors from detrimental agglomeration during pyrolysis, demonstrating a new approach to constructing anti-aggregated metal/carbon-based catalysts.

Quantum dot-doped CeOx-NiB with modulated electron density as a highly efficient bifunctional electrocatalyst for water splitting

Development of economical, efficient and durable non-noble metal electrocatalysts for the hydrogen/oxygen evolution reaction (HER/OER) holds great promise, but still faces great challenges. Herein, a strategy of doping metal borides with rare earth metal oxides and introducing silicon carbide (SiC) quantum dots has been explored to develop efficient bifunctional electrocatalysts. A novel electrocatalyst consists of SiC quantum dot-decorated CeO_x-NiB supported on nickel foam via a one-step mild electroless plating reaction (denoted as CeO_x-NiB/SiC@NF). Notably, the modulated electron density of the CeO_x-NiB/SiC@NF electrode significantly boosts the electrochemically active surface area and electron transfer, and optimizes the hydrogen/water absorption free energy, which delivers current densities of 50 mA cm^-2 and 10 mA cm^-2 at overpotentials of only 131 mV and 234 mV for the HER and the OER, respectively. The target electrode requires only 1.43 V to provide 10 mA cm^-2 for overall water splitting in 1.0 M KOH. Moreover, the electrode also exhibits good stability and durability at the industrial-grade current density (0.5-1 A cm^-2). This work provides a new idea for the development of efficient and durable non-precious metal catalysts.

Transformation mechanism of high-valence metal sites for the optimization of Co- and Ni-based OER catalysts in an alkaline environment: recent progress and perspectives

As an important semi-reaction process in electrocatalysis, oxygen evolution reaction (OER) is closely associated with electrochemical hydrogen production, CO₂ electroreduction, electrochemical ammonia synthesis and other reactions, which provide electrons and protons for the related applications. Considering their fundamental mechanism, metastable high-valence metal sites have been identified as real, efficient OER catalytic sites from the recent observation by in situ characterization technology. Herein, we review the transformation mechanism of high-valence metal sites in the OER process, particularly transition metal materials (Co- and Ni-based). In particular, research progress in the transformation process and role of high-valence metal sites to optimize OER performance is summarized. The key challenges and prospects of the design of high-efficiency OER catalysts based on the above-mentioned mechanism and some new in situ characterizations are also discussed.

Heterogeneous catalysis mediated by light, electricity and enzyme via machine learning: Paradigms, applications and prospects

Energy crisis and environmental pollution have become the bottleneck of human sustainable development. Therefore, there is an urgent need to develop new catalysts for energy production and environmental remediation. Due to the high cost caused by blind screening and limited valuable computing resources, the traditional experimental methods and theoretical calculations are difficult to meet with the requirements. In the past decades, computer science has made great progress, especially in the field of machine learning (ML). As a new research paradigm, ML greatly accelerates the theoretical calculation methods represented by first principal calculation and molecular dynamics, and establish the physical picture of heterogeneous catalytic processes for energy and environment. This review firstly summarized the general research paradigms of ML in the discovery of catalysts. Then, the latest progresses of ML in light-, electricity- and enzyme-mediated heterogeneous catalysis were reviewed from the perspective of catalytic performance, operating conditions and reaction mechanism. The general guidelines of ML for heterogeneous catalysis were proposed. Finally, the existing problems and future development trend of ML in heterogeneous catalysis mediated by light, electricity and enzyme were summarized. We highly expect that this review will facilitate the interaction between ML and heterogeneous catalysis, and illuminate the development prospect of heterogeneous catalysis.

Structural engineering and electronic state tuning optimization of molybdenum-doped cobalt hydroxide nanosheet self-assembled hierarchical microtubules for efficient electrocatalytic oxygen evolution

Cobalt-based hydroxide are ideal candidates for the oxygen evolution reaction. Herein, we use molybdenum oxide nanorods as sacrificial templates to construct a self-supporting molybdenum-doped cobalt hydroxide nanosheet hierarchical microtubule structure based on a structural engineering strategy to improve the active area of the catalyst. X-ray-based spectroscopic tests revealed that Mo (VI) with tetrahedral coordination intercalated into the interlayer of cobalt hydroxide, promoting interlayer separation. At the same time, Mo is connected with Co through oxygen bonds, which promotes the transfer of Co charges to Mo and reduces the electron cloud density of Co ions. In 1 M KOH, optimized molybdenum-doped cobalt hydroxide nanosheet microtubules only needs an overpotential of 288 mV to drive a current density of 10 mA cm^-2, which is significantly better than that of pure Co(OH)₂ nanosheets and RuO₂. Structural engineering and electronic state regulation can effectively improve the oxygen evolution activity of cobalt-based hydroxide, which provides a design idea for the development of efficient oxygen evolution catalysts.

Interfacial engineering of a tri-phase CoFe/CoFeOx/Co–Fe3O4 electrocatalyst for promoting the oxygen evolution reaction

A tri-phase interfacial structure of CoFe alloy/CoFe oxide/cobalt-doped iron oxide as a highly efficient and cost-effective oxygen evolution reaction electrocatalyst was elaborately constructed by a one-step chemical bath deposition approach.

A new hexanuclear Fe(III) nanocluster: Synthesis, structure, magnetic properties, and efficient activity as a precatalyst in water oxidation

The oxo-bridged hexanuclear iron cluster formulated, [Fe6III(µ4-O)2(edteH)2(piv)4(SCN)4]∙2MeCN∙2H2O (1) (where, edteH = N,N,N′,N′-tetrakis(2-hydroxyethyl)ethylenediamine; piv = pivalic acid) is synthesized by the reaction of FeCl2∙4H2O with edteH4 and piv in the presence...