Coordination Shell Dependent Activity of CuCo Diatomic Catalysts for Oxygen Reduction,Oxygen Evolution,and Hydrogen Evolution Reaction

Challenges in rational designing dual-atom catalysts (DACs) give a strong motivation to construct coordination-activity correlations. Here, thorough coordination-activity correlations of DACs based on how the changes in coordination shells (CSs) of dual-atom Cu,Co centers influence their electrocatalytic activity in oxygen reduction reaction(ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) is constructed. First, Cu,Co DACs with different CSs modifications are fabricated by using a controlled “precursors-preselection” approach. Three DACs with unique coordination environments are characterized as secondary S atoms that directly bond to Cu,Co-N₆ in lower CSs, indirectly bond in neighboring CSs, and are doped in higher CSs, respectively. Then, experimentally and theoretically, a coordination correlation resembling a planet-satellite system, where satellite coordinated atoms (heteroatom N, S) surround Cu-Co dual-atom entity in various orbitals CSs. By evaluating electrocatalytic activity indicators, differences are identified in electronic structure and electrocatalytic performance of Cu and Co centers in ORR, OER, and HER. Interestingly, initial CSs modifications for DACs may not always be advantageous for electrocatalysis. This work offers valuable insight for designing DACs for diverse applications.     

In Situ Grown Pristine Cobalt Sulfide as Bifunctional Photocatalyst for Hydrogen and Oxygen Evolution

Herein, transition metal chalcogenides of pristine cobalt sulfides are rationally designed to act as robust bifunctional photocatalysts for visible‐light‐driven water splitting for the first time. Through moderate solvothermal route, cobalt sulfides are synthesized in situ growth and observed by scanning electron microscope image analysis. Noteworthily, 3D hierarchical cobalt sulfides acting as bifunctional photocatalysts are implemented to catalyze the visible‐light‐driven oxygen evolution reaction and hydrogen evolution reaction. This efficient, earth‐abundant, and nonnoble water splitting catalyst for artificial photosynthesis is thoroughly analyzed by various spectroscopic techniques with the aim of investigating its photocatalytic mechanism under visible‐light illumination. The main catalyst of CoS‐2 exhibits considerable H₂ evolution rate of 1196 µmol h^?1 g^?1 and O₂ yield of 63.5%. The efficient activity is attributed to the effective electron transfer between the photosensitizer and catalyst, which is verified by transient absorption experiments. The effective electron transfer between the photosensitizer and catalyst during water oxidation is verified by the dramatic decline of [Ru(bpy)₃]³⁺ concentration in the presence of the catalyst CoS‐2. At the same time, transient absorption experiments support a rapid electron transfers from ³EY* (excited photosensitizer eosin‐Y) to the catalyst CoS‐2 for efficient hydrogen evolution.     

Bifunctional Electrode Design Targeting Co-Enhanced Kinetics and Mass Transport for Hydrogen and Water Oxidation Reactions

Bifunctional catalysts based on noble metals have achieved practical-level performances in round-trip energy conversion systems. However, the required amount of noble metals should be substantially reduced via a new catalyst design that can pursue the synergy of the constituent materials’ intrinsic properties and architectural maneuver over reactant/product transport. In this study, cross-stacked Ir and Pt nanowires resolved the bottlenecks of two reactions essential for the hydrogen-based energy system: i) hydrogen spillover phenomenon between Pt and Ir nanowires to expedite the hydrogen oxidation reaction and ii) spacing Ir nanowires sufficiently to enhance the mass transport of the oxygen evolution reaction. Simultaneously accommodating the different strategies within the single catalyst layer, a new horizon to design a bifunctional electrode is proposed with the high performance of polymer electrolyte membrane unitized regenerative fuel cells: 47% of round-trip efficiency at 0.5 A cm⁻² with total noble metal loading < 0.3 mg cm⁻².     

In Situ Engineering of a Cobalt-free Perovskite Air Electrode Enabling Efficient Reversible Oxygen Reduction/Evolution Reactions

Reversible protonic ceramic electrochemical cells (R-PCECs) have received increasing focus for their good capability of converting and storing energy. However, the widely used cobalt-based air electrodes are less thermomechanically compatible with the electrolyte and lack stability, which largely limits the development of R-PCECs. Herein, a cobalt-free perovskite with a nominal composition of PrBa_0.8Ca_0.2Fe_1.8Ce_0.2O_6δ (PBCFC) is reported, which is in–situ engineered to a (Ba, Ce) deficient-PBCFC phase, a BaCeO₃, and a CeO₂ phase under typical operating conditions, delivering a low area–specific resistance of 0.10 Ωcm² at 700 ^oC. The generated BaCeO₃ and CeO₂ particles increase the conduction/transfer of protons and oxygen ions, thus providing extra active sites for the oxygen reactions. When utilized as an air electrode on a single cell, it achieves encouraging performance at 700 °C: a peak power density of 1.78 Wcm⁻² and a current density of 5.00 Acm⁻² at 1.3 V in the dual mode of the fuel cell (FC) and electrolysis (EL) mode with reasonable Faradaic efficiencies. In addition, the cells exhibit favorable operational durability of 65 h (FC mode), 95 h (EL mode), and promising cycling stability of 200 h.     

Hierarchical NiCo2S4 Nanowire Arrays Supported on Ni Foam: An Efficient and Durable Bifunctional Electrocatalyst for Oxygen and Hydrogen Evolution Reactions

A recent approach for solar‐to‐hydrogen generation has been water electrolysis using efficient, stable, and inexpensive bifunctional electrocatalysts within strong electrolytes. Herein, the direct growth of 1D NiCo₂S₄ nanowire (NW) arrays on a 3D Ni foam (NF) is described. This NiCo₂S₄ NW/NF array functions as an efficient bifunctional electrocatalyst for overall water splitting with excellent activity and stability. The 3D‐Ni foam facilitates the directional growth, exposing more active sites of the catalyst for electrochemical reactions at the electrode–electrolyte interface. The binder‐free, self‐made NiCo₂S₄ NW/NF electrode delivers a hydrogen production current density of 10 mA cm^–2 at an overpotential of 260 mV for the oxygen evolution reaction and at 210 mV (versus a reversible hydrogen electrode) for the hydrogen evolution reaction in 1 m KOH. This highly active and stable bifunctional electrocatalyst enables the preparation of an alkaline water electrolyzer that could deliver 10 mA cm^–2 under a cell voltage of 1.63 V. Because the nonprecious‐metal NiCo₂S₄ NW/NF foam‐based electrodes afford the vigorous and continuous evolution of both H₂ and O₂ at 1.68 V, generated using a solar panel, they appear to be promising water splitting devices for large‐scale solar‐to‐hydrogen generation.     

Proximity Electronic Effect of Ni/Co Diatomic Sites for Synergistic Promotion of Electrocatalytic Oxygen Reduction and Hydrogen Evolution

The modulation effect manifests an encouraging potential to enhance the performance of single-atom catalysts; however, the in-depth study about this effect for the isolated diatomic sites (DASs) remains a great challenge. Herein, a proximity electronic effect (PEE) of Ni/Co DASs is proposed that is anchored in N-doped carbon (N-C) substrate (NiCo DASs/N-C) for synergistic promoting electrocatalytic oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER). Benefiting from the PEE of adjacent Ni anchored by four nitrogen (Ni-N₄) moiety, NiCo DASs/N-C catalyst exhibits superior ORR and HER activity. In situ characterization results suggest Co anchored by four nitrogen (Co-N₄) as main active site for O₂ adsorption-activation process, which promotes the formation of key *OOH and the desorption of *OH intermediate to accelerate the multielectron reaction kinetics. Theoretical calculation reveals the adjacent Ni-N₄ site as a modulator can effectively adjust the electronic localization of proximity Co-N₄ site, promoting the *OH desorption and *H adsorption on Co-N₄ site, thereby boosting ORR and HER process significantly. This study opens a new opportunity for rationally regulating the electronic localization of catalytic active centers by proximity single-atom moiety, as well as provides guidance for designing high-efficiency bifunctional electrocatalysts for promising applications.     

Phosphorous‐Doped Graphite Layers with Outstanding Electrocatalytic Activities for the Oxygen and Hydrogen Evolution Reactions in Water Electrolysis

Advances demonstrate that the incorporation of phosphorous into the network of nitrogen, sulfur, or fluorine‐doped carbon materials can remarkably enhance their oxygen and hydrogen evolution activities. However, the electrocatalytic behaviors of pristine phosphorous single‐doped carbon catalysts toward the oxygen and hydrogen evolution reactions (OER and HER) are rarely investigated and their corresponding active species are not yet explored. To clearly ascertain the effects of phosphorous doping on the OER and HER and identify the active sites, herein, phosphorous unitary‐doped graphite layers with different phosphorous species distributions are prepared and the correlations between the oxygen or hydrogen evolution activity and different phosphorous species are investigated, respectively. Results indicate that phosphorous single‐doped graphite layers show a superior oxygen evolution activity to most of the reported OER catalysts and the commercial IrO₂ in alkaline medium, and comparable hydrogen evolution activity to most reported carbon catalysts in acidic medium. Moreover, the relevancies unveil that the C? O? P species are the main OER active species, and the defects derived from the decomposition of C₃? P = O species are the main active sites for HER, as evidenced by density functional theory calculations, showing a new perspective for the design of more effective phosphorous‐containing water‐splitting catalysts.     

Co3O4 Nanoparticles with Ultrasmall Size and Abundant Oxygen Vacancies for Boosting Oxygen Involved Reactions

Ultrasmall size and abundant defects are two crucial factors for improving the performance of catalysts. However, it is a big challenge to introduce defects into ultrafine catalysts because of the surface tension and self‐purification effect of the nanoparticles. In the present work, physical laser fragmentation with chemical oxidization reaction is combined to synthesize Co₃O₄ nanoparticles (L‐CO) with ultrasmall size (≈2.1 nm) as well as abundant oxygen vacancies, thus providing an effective solution to the long‐standing contradiction between the size reduction and defect generation. The ultrasmall particle size allows more catalytic sites to be exposed. The surficial oxygen vacancies enhance the intrinsic activity, while the internal oxygen vacancies improve the electron transfer, and all of these benefits make L‐CO an active and durable bifunctional catalyst for oxygen reduction/evolutions. As the air cathode of zinc–air battery, L‐CO displays excellent rechargeable performance with a power density of ≈337 mW cm^?2, outperforming the commercial noble metal couple (Pt/C+RuO₂).     

Shape Control of Mn3O4 Nanoparticles on Nitrogen‐Doped Graphene for Enhanced Oxygen Reduction Activity

Three kinds of Mn₃O₄ nanoparticles with different shapes (spheres, cubes, and ellipsoids) are selectively grown on nitrogen‐doped graphene sheets through a two‐step liquid‐phase procedure. These non‐precious hybrid materials display an excellent ORR activity and good durability. The mesoporous microstructure, nitrogen doping, and strong bonding between metal species and doped graphene are found to facilitate the ORR catalytic process. Among these three kinds of Mn₃O₄ particles, the ellipsoidal particles on nitrogen‐doped graphene exhibit the highest ORR activity with a more positive onset‐potential of –0.13 V (close to that of Pt/C, –0.09 V) and a higher kinetic limiting current density (J_K) of 11.69 mA cm^–2 at –0.60 V. It is found that the ORR performance of hybrid materials can be correlated to the shape of Mn₃O₄ nanocrystals, and specifically to the exposed crystalline facets associated with a given shape. The shape dependence of Mn₃O₄ nanoparticles integrated with nitrogen‐doped graphene on the ORR performance, reported here for the first time, may advance the development of fuel cells and metal‐air batteries.     

Epitaxial Heterogeneous Interfaces on N‐NiMoO4/NiS2 Nanowires/Nanosheets to Boost Hydrogen and Oxygen Production for Overall Water Splitting

Developing bifunctional efficient electrocatalysts for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is in high demand for the development of overall water‐splitting devices. In particular, the electrocatalytic performance can be largely improved by designing positive nanoscale‐heterojunction with well‐tuned interfaces. Herein, a novel top‐down strategy is reported to construct the oxide/sulfide heterostructures (N‐NiMoO₄/NiS₂ nanowires/nanosheets) as a multisite HER/OER catalyst. Starting with the NiMoO₄ nanowires, nitridation in a controlled manner enables activation of Ni sites in NiMoO₄ and then yields oxide/sulfide heterojunction by directly vulcanizing the highly composition‐segregated N‐NiMoO₄ nanowires. The abundant epitaxial heterogeneous interfaces at atomic‐level facilitate the electron transfer from N‐NiMoO₄ to NiS₂, which further cooperate synergistically toward both the hydrogen and oxygen generation in alkali solution. Furthermore, with N‐NiMoO₄/NiS₂ grown carbon fiber cloth as the engineering electrode, the assembled N‐NiMoO₄/NiS₂–N‐NiMoO₄/NiS₂ system can deliver a current density of 10 mA cm^?2 with the cell voltage of 1.60 V in the water‐splitting reaction. This current density is 3.39 times higher than that of the Pt–Ir set (2.95 mA cm^?2). The excellent catalytic performance offered of N‐NiMoO₄/NiS₂ nanowires/nanosheets presents a great example to demonstrate the significance of interface engineering in the field of electrocatalysis.     

Practical Classification of Catalysts for Oxygen Reduction Reactions: Optimization Strategies and Mechanistic Analysis

Oxygen reduction reaction (ORR) is the core reaction of fuel cell/metal–air battery at the cathode, and it is of great significance to develop high-performance ORR catalyst. Generally speaking, ORR catalysts are classified according to the class of materials they use, which often makes it difficult for different types of catalysts to have a clear connection with some unified optimization mechanism and creates difficulties in designing improvement strategies. Here, this review proposes three major strategies that can affect the activity of ORR catalysts: designing morphology, defect control, and heteroatom doping. The core content of this review is to analyze the principles of various optimization strategies that affect the performance of catalysts individually or synergistically at the micro level. In addition, the problems and challenges still faced in the design of ORR catalysts are sorted out, and possible solutions are proposed. This review makes the research direction of ORR catalysts clearer, and provides theoretical support and new ideas for the design of high-efficiency electrocatalysts.     

Pyrolysis of Self‐Assembled Iron Porphyrin on Carbon Black as Core/Shell Structured Electrocatalysts for Highly Efficient Oxygen Reduction in Both Alkaline and Acidic Medium

A simple carbonization of evaporation‐induced self‐assembled iron(III) porphyrin (FeP) layers uniformly coated on carbon black, leading to an unprecedented core/shell structured nonprecious metal electrocatalysts (NPMEs) composed of N‐doped graphene‐like layers uniformly coated on carbon is reported. The thickness of graphene‐like shell can be readily adjusted up to about 6.6 nm by varying the amount of FeP loaded on carbon. Interestingly, the obtained NPME exhibits one of the highest oxygen reduction reaction (ORR) activity in both alkaline (half‐wave potential of 0.87 V vs reversible hydrogen electrode‐RHE) and acidic (half‐wave potential of 0.75 V vs RHE) medium. In particular, the core/shell structured NPME demonstrates a remarkable durability in acidic conditions superior to that of commercial Pt/C, which likely comes from the exposure of inner active sites after the outermost layer is consumed. Furthermore, the core/shell NPME displays direct 4e and indirect 4e process toward ORR in alkaline and acidic medium, respectively. This study points out a new avenue for the design of high‐performance NPMEs in both alkaline and acidic media, which may have potential applications in polymer electrolyte membrane fuel cells (PEMFCs), metal‐air batteries, and electrolyzers.     

Hollow Multivoid Nanocuboids Derived from Ternary Ni–Co–Fe Prussian Blue Analog for Dual‐Electrocatalysis of Oxygen and Hydrogen Evolution Reactions

Hydrogen generation from electrochemical water‐splitting is an attractive technology for clean and efficient energy conversion and storage, but it requires efficient and robust non‐noble electrocatalysts for hydrogen and oxygen evolution reactions (HER and OER). Nonprecious transition metal–organic frameworks (MOFs) are one of the most promising precursors for developing advanced functional catalysts with high porosity and structural rigidity. Herein, a new transition metal‐based hollow multivoid nanocuboidal catalyst synthesized from a ternary Ni–Co–Fe (NCF)‐MOF precursor is rationally designed to produce dual‐functionality toward OER and HER. Differing ion exchanging rates of the ternary transition metals within the prussian blue analog MOF precursor are exploited to produce interconnected internal voids, heteroatom doping, and a favorably tuned electronic structure. This design strategy significantly increases active surface area and pathways for mass transport, resulting in excellent electroactivities toward OER and HER, which are competitive with recently reported single‐function nonprecious catalysts. Moreover, outstanding electrochemical durability is realized due to the unique rigid and interconnected porous structure which considerably retains initial rapid charge transfer and mass transport of active species. The MOF‐based material design strategy demonstrated here exemplifies a novel and versatile approach to developing non‐noble electrocatalysts with high activity and durability for advanced electrochemical water‐splitting systems.     

Generalized Synthetic Strategy for Amorphous Transition Metal Oxides-Based 2D Heterojunctions with Superb Photocatalytic Hydrogen and Oxygen Evolution

2D amorphous transition metal oxides (a-TMOs) heterojunctions that have the synergistic effects of interface (efficiently promoting the separation of electron−hole pairs) and amorphous nature (abundant defects and dangling bonds) have attracted substantial interest as compelling photocatalysts for solar energy conversion. Strategies to facilely construct a-TMOs-based 2D/2D heterojunctions is still a big challenge due to the difficulty of preparing individual amorphous counterparts. A generalized synthesis strategy based on supramolecular self-assembly for bottom–up growth of a-TMOs-based 2D heterojunctions is reported, by taking 2D/2D g-C₃N₄ (CN)/a-TMOs heterojunction as a proof-of-concept. This strategy primarily depends on controlling the cooperation of the growth of supramolecular precursor and the coordinated covalent bonds arising from the tendency of metal ions to attain the stable configuration of electrons, which is independent on the intrinsic character of individual metal ion, indicating it is universally applicable. As a demonstration, the structure, physical properties, and photocatalytic water-splitting performance of CN/a-ZnO heterojunction are systematically studied. The optimized 2D/2D CN/a-ZnO exhibits enhanced photocatalytic performance, the hydrogen (432.6 µmol h⁻¹ g⁻¹) and oxygen (532.4 µmol h⁻¹ g⁻¹) evolution rate are 15.5 and 12.2 times than bulk CN, respectively. This synthetic strategy is useful to construct 2D a-TMOs nanomaterials for applications in energy-related areas and beyond.     

Understanding the Synergistic Effects of Cobalt Single Atoms and Small Nanoparticles: Enhancing Oxygen Reduction Reaction Catalytic Activity and Stability for Zinc-Air Batteries

The development of earth-abundant oxygen reduction reaction (ORR) catalysts with high catalytic activity and good stability for practical metal-air batteries remains an enormous challenge. Herein, a highly efficient and durable ORR catalyst is reported, which consists of atomically dispersed Co single atoms (Co-SAs) in the form of Co-N4 moieties and small Co nanoparticles (Co-SNPs) co-anchored on nitrogen-doped porous carbon nanocage (Co-SAs/SNPs@NC). Benefiting from the synergistic effect of Co-SAs and Co-SNPs as well as the enhanced anticorrosion capability of the carbon matrix brought by its improved graphitization degree, the resultant Co-SAs/SNPs@NC catalyst exhibits outstanding ORR activity and remarkable stability in alkaline media, outperforming Co-SAs-based catalyst (Co-SAs@NC), and benchmark Pt/C catalyst. Density functional theory calculations reveal that the strong interaction between Co-SNPs and Co-N4 sites can increase the valence state of the active Co atoms in Co-SAs/SNPs@NC and moderate the adsorption free energy of ORR intermediates, thus facilitating the reduction of O₂. Moreover, the practical zinc-air battery assembled with Co-SAs/SNPs@NC catalyst demonstrates a maximum power density of 223.5 mW cm^–2, a high specific capacity of 742 W h kg^–1 at 50 mA cm^–2 and a superior cycling stability.     

MO‐Co@N‐Doped Carbon (M = Zn or Co): Vital Roles of Inactive Zn and Highly Efficient Activity toward Oxygen Reduction/Evolution Reactions for Rechargeable Zn–Air Battery

A highly efficient bifunctional oxygen catalyst is required for practical applications of fuel cells and metal–air batteries, as oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are their core electrode reactions. Here, the MO‐Co@N‐doped carbon (NC, M = Zn or Co) is developed as a highly active ORR/OER bifunctional catalyst via pyrolysis of a bimetal metal–organic framework containing Zn and Co, i.e., precursor (CoZn). The vital roles of inactive Zn in developing highly active bifunctional oxygen catalysts are unraveled. When the precursors include Zn, the surface contents of pyridinic N for ORR and the surface contents of Co–N_x and Co³⁺/Co²⁺ ratios for OER are enhanced, while the high specific surface areas, high porosity, and high electrochemical active surface areas are also achieved. Furthermore, the synergistic effects between Zn‐based and Co‐based species can promote the well growth of multiwalled carbon nanotubes (MWCNTs) at high pyrolysis temperatures (≥700 °C), which is favorable for charge transfer. The optimized CoZn‐NC‐700 shows the highly bifunctional ORR/OER activity and the excellent durability during the ORR/OER processes, even better than 20 wt% Pt/C (for ORR) and IrO₂ (for OER). CoZn‐NC‐700 also exhibits the prominent Zn–air battery performance and even outperforms the mixture of 20 wt% Pt/C and IrO₂.