High-Performance Bifunctional Electrocatalysts for Flexible and Rechargeable Zn-Air Batteries: Recent Advances.

Flexible rechargeable Zn-air batteries (FZABs) exhibit high energy density, ultra-thin, lightweight, green, and safe features, and are considered as one of the ideal power sources for flexible wearable electronics. However, the slow and high overpotential oxygen reaction at the air cathode has become one of the key factors restricting the development of FZABs. The improvement of activity and stability of bifunctional catalysts has become a top priority. At the same time, FZABs should maintain the battery performance under different bending and twisting conditions, and the design of the overall structure of FZABs is also important. Based on the understanding of the three typical configurations and working principles of FZABs, this work highlights two common strategies for applying bifunctional catalysts to FZABs: 1) powder-based flexible air cathode and 2) flexible self-supported air cathode. It summarizes the recent advances in bifunctional oxygen electrocatalysts and explores the various types of catalyst structures as well as the related mechanistic understanding. Based on the latest catalyst research advances, this paper introduces and discusses various structure modulation strategies and expects to guide the synthesis and preparation of efficient bifunctional catalysts. Finally, the current status and challenges of bifunctional catalyst research in FZABs are summarized.

A Superior Bifunctional Electrocatalyst in Which Directional Electron Transfer Occurs Between a Co/Ni Alloy and Fe─N─C Support.

Stable, efficient, and economical bifunctional electrocatalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) are needed for rechargeable Zn-air batteries. In this study, a directional electron transfer pathway is exploited in a spatial heterojunction of CoyNix@Fe─N─C heterogeneous catalyst for effective bifunctional electrolysis (OER/ORR). Thereinto, the Co/Ni alloy is strongly coupled to the Fe─N─C support through Co/Ni─N bonds. DFT calculations and experimental findings confirm that Co/Ni─N bonds play a bridging role in the directional electron transfer from Co/Ni alloy to the Fe─N─C support, increasing the content of pyridinic nitrogen in the ORR-active support. In addition, the discovered directional electron transfer mechanism enhances both the ORR/OER activity and the durability of the catalyst. The Co0.66Ni0.34@Fe─N─C with the optimal Ni/Co ratio exhibits satisfying bifunctional electrocatalytic performance, requiring an ORR half-wave potential of 0.90 V and an OER overpotential of 317 mV at 10 mA cm-2 in alkaline electrolytes. The assembled rechargeable zinc-air batteries (ZABs) incorporating Co0.66Ni0.34@Fe─N─C cathode exhibits a charge-discharge voltage gap comparable to the Pt/C||IrO2 assembly and high robustness for over 60 h at 20 mA cm-2.

Ternary Heteroatomic Doping Induced Microenvironment Engineering of Low Fe-N4-Loaded Carbon Nanofibers for Bifunctional Oxygen Electrocatalysis.

Fabricating highly efficient and long-life redox bifunctional electrocatalysts is vital for oxygen-related renewable energy devices. To boost the bifunctional catalytic activity of Fe-N-C single-atom catalysts, it is imperative to fine-tune the coordination microenvironment of the Fe sites to optimize the adsorption/desorption energies of intermediates during oxygen reduction/evolution reactions (ORR/OER) and simultaneously avoid the aggregation of atomically dispersed metal sites. Herein, a strategy is developed for fabricating a free-standing electrocatalyst with atomically dispersed Fe sites (≈0.89 wt.%) supported on N, F, and S ternary-doped hollow carbon nanofibers (FeN4 -NFS-CNF). Both experimental and theoretical findings suggest that the incorporation of ternary heteroatoms modifies the charge distribution of Fe active centers and enhances defect density, thereby optimizing the bifunctional catalytic activities. The efficient regulation isolated Fe centers come from the dual confinement of zeolitic imidazole framework-8 (ZIF-8) and polymerized ionic liquid (PIL), while the precise formation of distinct hierarchical three-dimensional porous structure maximizes the exposure of low-doping Fe active sites and enriched heteroatoms. FeN4 -NFS-CNF achieves remarkable electrocatalytic activity with a high ORR half-wave potential (0.90 V) and a low OER overpotential (270 mV) in alkaline electrolyte, revealing the benefit of optimizing the microenvironment of low-doping iron single atoms in directing bifunctional catalytic activity.

Plasma-Engineering of Oxygen Vacancies on NiCo2O4 Nanowires with Enhanced Bifunctional Electrocatalytic Performance for Rechargeable Zinc-air Battery.

Designing an efficient, durable, and inexpensive bifunctional electrocatalyst toward oxygen evolution reactions (OER) and oxygen reduction reactions (ORR) remains a significant challenge for the development of rechargeable zinc-air batteries (ZABs). The generation of oxygen vacancies plays a vital role in modifying the surface properties of transition-metal-oxides (TMOs) and thus optimizing their electrocatalytic performances. Herein, a H2/Ar plasma is employed to generate abundant oxygen vacancies at the surfaces of NiCo2O4 nanowires. Compared with the Ar plasma, the H2/Ar plasma generated more oxygen vacancies at the catalyst surface owing to the synergic effect of the Ar-related ions and H-radicals in the plasma. As a result, the NiCo2O4 catalyst treated for 7.5 min in H2/Ar plasma exhibited the best bifunctional electrocatalytic activities and its gap potential between Ej = 10 for OER and E1/2 for ORR is even smaller than that of the noble-metal-based catalyst. In situ electrochemical experiments are also conducted to reveal the proposed mechanisms for the enhanced electrocatalytic performance. The rechargeable ZABs, when equipped with cathodes utilizing the aforementioned catalyst, achieved an outstanding charge-discharge gap, as well as superior cycling stability, outperforming batteries employing noble-metal catalyst counterparts.

Rational Construction of 3D Self-Supported MOF-Derived Cobalt Phosphide-Based Hollow Nanowall Arrays for Efficient Overall Water Splitting At large Current Density.

Developing efficient nonprecious bifunctional electrocatalysts for hydrogen and oxygen evolution reactions (HER and OER) in the same electrolyte with a low overpotential and large current density presents an appealing yet challenging goal for large-scale water electrolysis. Herein, a unique 3D self-branched hierarchical nanostructure composed of ultra-small cobalt phosphide (CoP) nanoparticles embedded into N, P-codoped carbon nanotubes knitted hollow nanowall arrays (CoPʘNPCNTs HNWAs) on carbon textiles (CTs) through a carbonization-phosphatization process is presented. Benefiting from the uniform protrusion distributions of CoP nanoparticles, the optimum CoPʘNPCNTs HNWAs composites with high abundant porosity exhibit superior electrocatalytic activity and excellent stability for OER in alkaline conditions, as well as for HER in both acidic and alkaline electrolytes, even under large current densities. Furthermore, the assembled CoPʘNPCNTs/CTs||CoPʘNPCNTs/CTs electrolyzer demonstrates exceptional performance, requiring an ultralow cell voltage of 1.50 V to deliver the current density of 10 mA cm-2 for overall water splitting (OWS) with favorable stability, even achieving a large current density of 200 mA cm-2 at a low cell voltage of 1.78 V. Density functional theory (DFT) calculation further reveals that all the C atoms between N and P atoms in CoPʘNPCNTs/CTs act as the most efficient active sites, significantly enhancing the electrocatalytic properties. This strategy, utilizing 2D MOF arrays as a structural and compositional material to create multifunctional composites/hybrids, opens new avenues for the exploration of highly efficient and robust non-noble-metal catalysts for energy-conversion reactions.

CoFe Alloys Dispersed on Se, N Co-Doped Graphitic Carbon as Efficient Bifunctional Catalysts for Zn-Air Batteries.

The development of a stable and efficient non-noble metal catalyst with both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is paramount to achieving the widespread application of Zn-air batteries (ZABs) but remains a great challenge. Herein, a novel Co3 Fe7 alloy nanoparticle dispersed on Se, N co-doped graphitic carbon (denoted as CoFe/Se@CN) was prepared through a facile hydrothermal and pyrolysis process. The synthesized CoFe/Se@CN exhibits outstanding ORR and OER properties with an ultralow potential gap of 0.625 V, which is mainly attributed to the abundant porous structure, the rich structural defects formed by doping Se atoms, and the strong synergistic effects between the CoFe alloys and graphitic carbon nanosheet. Furthermore, the ZAB fabricated by CoFe/Se@CN shows a high peak power density of 160 mW cm-2 and a large specific capacity of 802 mA h g-1 with favorable cycling stability, outperforming that of Pt/C+RuO2 . Our study offers a plausible strategy to explore bifunctional carbon-based materials with efficient electrocatalytic properties for rechargeable ZABs.

Robust wrinkled MoS2/N-C bifunctional electrocatalysts interfaced with single Fe atoms for wearable zinc-air batteries.

The ability to create highly efficient and stable bifunctional electrocatalysts, capable of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in the same electrolyte, represents an important endeavor toward high-performance zinc-air batteries (ZABs). Herein, we report a facile strategy for crafting wrinkled MoS2/N-doped carbon core/shell nanospheres interfaced with single Fe atoms (denoted MoS2@Fe-N-C) as superior ORR/OER bifunctional electrocatalysts for robust wearable ZABs with a high capacity and outstanding cycling stability. Specifically, the highly crumpled MoS2 nanosphere core is wrapped with a layer of single-Fe-atom-impregnated, N-doped carbon shell (i.e., Fe-N-C shell with well-dispersed FeN4 sites). Intriguingly, MoS2@Fe-N-C nanospheres manifest an ORR half-wave potential of 0.84 V and an OER overpotential of 360 mV at 10 mA⋅cm-2 More importantly, density functional theory calculations reveal the lowered energy barriers for both ORR and OER, accounting for marked enhanced catalytic performance of MoS2@Fe-N-C nanospheres. Remarkably, wearable ZABs assembled by capitalizing on MoS2@Fe-N-C nanospheres as an air electrode with an ultralow area loading (i.e., 0.25 mg⋅cm-2) display excellent stability against deformation, high special capacity (i.e., 442 mAh⋅g-1Zn), excellent power density (i.e., 78 mW⋅cm-2) and attractive cycling stability (e.g., 50 cycles at current density of 5 mA⋅cm-2). This study provides a platform to rationally design single-atom-interfaced core/shell bifunctional electrocatalysts for efficient metal-air batteries.

Construction of Co2 P-Ni3 S2 /NF Heterogeneous Structural Hollow Nanowires as Bifunctional Electrocatalysts for Efficient Overall Water Splitting.

Designing efficient and stable transition metal-based catalysts for electrocatalytic water splitting is vital for the development of hydrogen production. Herein, a facile synthetic strategy is developed to fabricate transition metal-based heterogeneous structural Co2 P-Ni3 S2 hollow nanowires supported on nickel foam (Co2 P-Ni3 S2 /NF). Owing to the multiple active sites provided by transition metal compounds, large surface area of the unique hollow nanowire morphology, and the synergistic effect of Co2 P-Ni3 S2 heterostructure interfaces, Co2 P-Ni3 S2 /NF requires ultralow overpotentials of 110, 164 mV for HER and 331.7, 358.3 mV for OER at large current densities of 100, 500 mA cm-2 in alkaline medium, respectively. Importantly, the two-electrode electrolyzer assembled by Co2 P-Ni3 S2 /NF displays a cell voltage of 1.54 V at 10 mA cm-2 and operates stably over 24 h at 100 mA cm-2 , which performs better than reported transition metal-based bifunctional electrocatalysts. This work presents a successful fabrication of transition metal-based bifunctional HER/OER electrocatalysts at large-current density and brings new inspiration for developing applicable energy conversion materials.

Hierarchical Nanospheres with Polycrystalline Ir&Cu and Amorphous Cu2 O toward Energy-Efficient Nitrate Electrolysis to Ammonia.

Electrochemical reduction reaction of nitrate (NITRR) provides a sustainable route toward the green synthesis of ammonia. Nevertheless, it remains challenging to achieve high-performance electrocatalysts for NITRR especially at low overpotentials. In this work, hierarchical nanospheres consisting of polycrystalline Iridium&copper (Ir&Cu) and amorphous Cu2 O (Cux Iry Oz NS) have been fabricated. The optimal species Cu0.86 Ir0.14 Oz delivers excellent catalytic performance with a desirable NH3 yield rate (YR) up to 0.423 mmol h-1  cm-2 (or 4.8 mg h-1  mgcat -1 ) and a high NH3 Faradaic efficiency (FE) over 90% at a low overpotential of 0.69 V (or 0 VRHE ), where hydrogen evolution reaction (HER) is almost negligible. The electrolyzer toward NITRR and hydrazine oxidation (HzOR) is constructed for the first time with an electrode pair of Cu0.86 Ir0.14 Oz //Cu0.86 Ir0.14 Oz , yielding a high energy efficiency (EE) up to 87%. Density functional theory (DFT) calculations demonstrate that the dispersed Ir atom provides active site that not only promotes the NO3 - adsorption but also modulates the H adsorption/desorption to facilitate the proton supply for the hydrogenation of *N, hence boosting the NITRR. This work thus points to the importance of both morphological/structural and compositional engineering for achieving the highly efficient catalysts toward NITRR.

Developing Superior Hydrophobic 3D Hierarchical Electrocatalysts Embedding Abundant Catalytic Species for High Power Density Zn-Air Battery.

It is essential but still challenging to design and construct inexpensive, highly active bifunctional oxygen electrocatalysts for the development of high power density zinc-air batteries (ZABs). Herein, a CoFe-S@3D-S-NCNT electrocatalyst with a 3D hierarchical structure of carbon nanotubes growing on leaf-like carbon microplates is designed and prepared through chemical vapour deposition pyrolysis of CoFe-MOF and subsequent hydrothermal sulfurization. Its 3D hierarchical structure shows excellent hydrophobicity, which facilitates the diffusion of oxygen and thus accelerates the oxygen reduction reaction (ORR) kinetic process. Alloying and sulfurization strategies obviously enrich the catalytic species in the catalyst, including cobalt or cobalt ferroalloy sulfides, their heterojunction, core-shell structure, and S, N-doped carbon, which simultaneously improve the ORR/OER catalytic activity with a small potential gap (ΔE = 0.71 V). Benefiting from these characteristics, the corresponding liquid ZABs show high peak power density (223 mW cm-2 ), superior specific capacity (815 mA h gZn -1 ), and excellent stability at 5 mA cm-2 for ≈900 h. The quasi-solid-state ZABs also exhibit a very high peak power density of 490 mW cm-2 and an excellent voltage round-trip efficiency of more than 64%. This work highlights that simultaneous composition optimization and microstructure design of catalysts can effectively improve the performance of ZABs.

In Situ Porousized MoS2 Nano Islands Enhance HER/OER Bifunctional Electrocatalysis.

2D molybdenum disulfide (MoS2 ) is developed as a potential alternative non-precious metal electrocatalyst for energy conversion. It is well known that 2D MoS2 has three main phases 2H, 1T, and 1T'. However, the most stable 2H-phase shows poor electrocatalysis in its basal plane, compared with its edge sites. In this work, a facile one-step hydrothermal-driven in situ porousizing of MoS2 into self-supporting nano islands to maximally expose the edges of MoS2 grains for efficient utilization of the active stable sites at the edges of MoS2 is reported. The results show that such active, aggregation-free nano islands greatly enhance MoS2 's hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) bifunctional electrocatalytic activities. At a low overpotential of 248 and 300 mV, the porous MoS2 nano islands can generate a current density of 10 mA cm-2 in HER and OER, which is much better than typical nanosheet morphology. Surprisingly, the porous MoS2 nano islands even exhibit better performance than the current commercial RuO2 catalyst in OER. This discovery will be another effective strategy to promote robust 2H-phase, instead of 1T/1T'-phase, MoS2 to achieve efficient endurable bifunctional HER/OER, which is expected to further replace precious metal catalysts in industry.

Biomass-Derived Bifunctional Cathode Electrocatalyst and Multiadaptive Gel Electrolyte for High-Performance Flexible Zn-Air Batteries in Wide Temperature Range.

High-efficiency and low-cost bifunctional electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), as well as gel electrolytes with high thermal and mechanical adaptability are required for the development of flexible batteries. Herein, abundant Setaria Viridis (SV) biomass is selected as the precursor to prepare porous N-doped carbon tubes with high specific surface area and the 900 °C calcination product of SV (SV-900) shows the optimum ORR/OER activities with a small EOER -EORR of 0.734 V. Meanwhile, a new multifunctional gel electrolyte named C20E2G5 is prepared using cellulose extracted from another widely distributed biomass named flax as the skeleton, epichlorohydrin as the cross-linker and glycerol as the antifreezing agent. C20E2G5 possesses high ionic conductivity from -40 to + 60 °C, excellent tensile and compressive resistance, high adhesion, strong freezing and heat resistance. Moreover, the symmetrical cell assembled with C20E2G5 can significantly inhibit Zn dendrite growth. Finally, flexible solid-state Zn-air batteries assembled with SV-900 and C20E2G5 show high open circuit voltage, large energy density, and long-term operation stability between -40 and + 60 °C. This biomass-based approach is generic and can be used for the development of diverse next-generation electrochemical energy conversion and storage devices.

Bimetallic Phosphide Heterostructure Coupled with Ultrathin Carbon Layer Boosting Overall Alkaline Water and Seawater Splitting.

Seawater electrolysis is promising for green hydrogen production but hindered by the sluggish reaction kinetics of both cathode and anode, as well as the detrimental chlorine chemistry environment. Herein, a self-supported bimetallic phosphide heterostructure electrode strongly coupled with an ultrathin carbon layer on Fe foam (C@CoP-FeP/FF) is constructed. When used as an electrode for the hydrogen and oxygen evolution reactions (HER/OER) in simulated seawater, the C@CoP-FeP/FF electrode shows overpotentials of 192 mV and 297 mV at 100 mA cm-2 , respectively. Moreover, the C@CoP-FeP/FF electrode enables the overall simulated seawater splitting at the cell voltage of 1.73 V to achieve 100 mA cm-2 , and operate stably during 100 h. The superior overall water and seawater splitting properties can be ascribed to the integrated architecture of CoP-FeP heterostructure, strongly coupled carbon protective layer, and self-supported porous current collector. The unique composites can not only provide enriched active sites, ensure prominent intrinsic activity, but also accelerate the electron transfer and mass diffusion. This work confirms the feasibility of an integration strategy for the manufacturing of a promising bifunctional electrode for water and seawater splitting.

First-Row Transition Metals for Catalyzing Oxygen Redox.

Rechargeable zinc-air batteries are widely recognized as a highly promising technology for energy conversion and storage, offering a cost-effective and viable alternative to commercial lithium-ion batteries due to their unique advantages. However, the practical application and commercialization of zinc-air batteries are hindered by the sluggish kinetics of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Recently, extensive research has focused on the potential of first-row transition metals (Mn, Fe, Co, Ni, and Cu) as promising alternatives to noble metals in bifunctional ORR/OER electrocatalysts, leveraging their high-efficiency electrocatalytic activity and excellent durability. This review provides a comprehensive summary of the recent advancements in the mechanisms of ORR/OER, the performance of bifunctional electrocatalysts, and the preparation strategies employed for electrocatalysts based on first-row transition metals in alkaline media for zinc-air batteries. The paper concludes by proposing several challenges and highlighting emerging research trends for the future development of bifunctional electrocatalysts based on first-row transition metals.

Heteroatom Doped Amorphous/Crystalline Ruthenium Oxide Nanocages as a Remarkable Bifunctional Electrocatalyst for Overall Water Splitting.

Developing robust and highly active bifunctional electrocatalysts for overall water splitting is critical for efficient sustainable energy conversion. Herein, heteroatom-doped amorphous/crystalline ruthenium oxide-based hollow nanocages (M-ZnRuOx (MCo, Ni, Fe)) through delicate control of composition and structure is reported. Among as-synthesized M-ZnRuOx nanocages, Co-ZnRuOx nanocages deliver an ultralow overpotential of 17 mV at 10 mA cm-2 and a small Tafel slope of 21.61 mV dec-1 for hydrogen evolution reaction (HER), surpassing the commercial Pt/C catalyst, which benefits from the synergistic coupling effect between electron regulation induced by Co doping and amorphous/crystalline heterophase structure. Moreover, the incorporation of Co prevents Ru from over-oxidation under oxygen evolution reaction (OER) operation, realizing the leap from a monofunctional to multifunctional electrocatalyst and then Co-ZnRuOx nanocages exhibit remarkable OER catalytic activity as well as overall water splitting performance. Combining theory calculations with spectroscopy analysis reveal that Co is not only the optimal active site, increasing the number of exposed active sites while also boosting the long-term durability of catalyst by modulating the electronic structure of Ru atoms. This work opens a considerable avenue to design highly active and durable Ru-based electrocatalysts.

Turn "Waste" Into Wealth: MoO2 @coal Gangue Electrocatalyst with Amorphous/Crystalline Heterostructure for Efficient Li-O2 Batteries.

Unreasonable accumulation of coal gangue in mining area has become the major source of global pollution. Probing the high-valued utilization of coal gangue has become a key approach to address the problem. Herein, a promising catalyst of MoO2 @coal gangue with amorphous/crystalline heterostructure derived from mine solid waste, which acts as an efficient cathode for Li-O2 batteries is first reported. Impressively, the as-prepared catalyst exhibits a favorable initial discharge capacity of 9748 mAh g-1 and promising long-term cyclic stability over 2200 h. Experimental results coupled with density functional theory (DFT) analysis reveal that the synergistic interaction between high-activity MoO2 and stable SiO2 , unique amorphous/crystalline heterostructure and the modified interfacial adsorption of LiO2 intermediate are critical factors in promoting the electrochemical performance. This work provides a new insight to design marked electrocatalysts by mine solid waste for Li-O2 batteries.

Nonmetallic Nitrogen-Doped MnO2 as Highly Efficient Oxygen Electrocatalyst for Rechargeable Zinc-Air Batteries.

Zinc-air batteries (ZABs) have been considered as one of the most promising energy storage devices to solve the problem of energy crisis and environmental pollution. In this work, we reported the synthesis of nitrogen-doped MnO2 (N-MnO2 ) to replace the noble metal electrocatalysts for air cathode in ZABs. The doped N atoms here introduced more Mn3+ and oxygen vacancies for MnO2 , enhancing charge transfer property and accelerating surface intermediate product during the oxygen reduction reaction (ORR). Hence, the best N-MnO2 achieved remarkable electrocatalytic activities towards ORR (half-wave potential of 0.797 V vs. RHE), and reversible oxygen overpotential of around 0.842 V, which is better than or comparable to the Pt/C and Mn-based catalysts reported recently. Moreover, the homemade ZABs based on N-MnO2 showed the maximum power density of 132.8 mW cm-2 and excellent cyclic stability.

Zeolitic Imidazolate Frameworks Derived Co9 S8 /Nitrogen-Doped Hollow Porous Carbon Spheres as Efficient Bifunctional Electrocatalysts for Rechargeable Zinc-Air Batteries.

The development of nonprecious metal-based electrocatalysts with remarkable catalytic activity and long-cycling lifespan toward oxygen reduction reaction (ORR) and evolution reaction (OER) is especially important for rechargeable zinc-air batteries (ZABs). Herein, monodispersed Co9 S8 nanoparticles embedded in nitrogen-doped hierarchically porous hollow carbon spheres (Co9 S8 NPs/NHCS) are synthesized through a template-assisted strategy followed by a co-assembly, thermal annealing, and sulfurization process. Benefiting from larger specific surface area, hierarchically porous hollow structure, and carbon nanotubes self-growth, the obtained Co9 S8 NPs/NHCS-0.5 electrocatalyst exhibits decent performance for ORR (E1/2 =0.85 V) and OER (E10 =1.55 V). A rechargeable ZAB assembled using the Co9 S8 NPs/NHCS-0.5 as air cathode delivers a maximum power density of 116 mW cm-2 , high open circuit voltage of 1.47 V, and good durability (no obvious voltage decay after 1200 cycles (200 hours)). Such a hierarchically porous hollow structure of Co9 S8 NPs/NHCS-0.5 provides a confined space shell and an interconnected hollow core to achieve outstanding bifunctional catalytic activity and cycling stability, which surpass the benchmark Pt/C-RuO2 .

Yttrium- and Cerium-Codoped Ultrathin Metal-Organic Framework Nanosheet Arrays for High-Efficiency Electrocatalytic Overall Water Splitting.

The construction of low-cost, highly efficient, and stable electrocatalysts is a significant challenge for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Herein, we report a facile strategy to fabricate ultrathin metal-organic framework (MOF) nanosheet arrays doped with two rare-earth elements, Y and Ce, and self-supported on nickel foam (NF) to enhance the HER and OER performance by constructing abundant active sites and bimetallic synergistic effects. The NiYCe-MOF/NF features an ultrathin nanosheet array structure and is uniformly and richly codoped by Y and Ce. When it was explored as both the anode and cathode electrocatalysts for overall water splitting, it achieved 10 mA cm-2 at 136 and 245 mV for the HER and OER in an alkaline electrolyte, respectively. Notably, an extremely low cell voltage of 1.54 V was required to achieve 100 mA cm-2 in 1.0 M KOH solution, making it a promising substitute for noble-metal catalysts.

Ru-FeNi Alloy Heterojunctions on Lignin-derived Carbon as Bifunctional Electrocatalysts for Efficient Overall Water Splitting.

Rational design of efficient, stable, and inexpensive bifunctional electrocatalysts for oxygen evolution reactions (OER) and hydrogen evolution reactions (HER) is a key challenge to realize green hydrogen production via electrolytic water splitting. Herein, Ru nanoparticles and FeNi alloy heterojunction catalyst (Ru-FeNi@NLC) encapsulated via lignin-derived carbon was prepared by self-assembly precipitation and in situ pyrolysis. The designed catalyst displays excellent performance at 10 mA cm-2 with low overpotentials of 36 mV for HER and 198 mV for OER, and only needs 1.48 V for overall water splitting. Results and DFT calculations show the unique N-doped lignin-derived carbon layer and Ru-FeNi heterojunction contribute to optimized electronic structure for enhancing electron transfer, balanced free energy of reactants and intermediates in the sorption/desorption process, and significantly reduced reaction energy barrier for the HER and OER rate-determining steps, thus improved reaction kinetics. This work provides a new in situ pyrolysis doping strategy based on renewable biomass for the construction of highly active, stable and cost-effective catalysts.