Enhancing ORR/OER active sites through lattice distortion of Fe-enriched FeNi3 intermetallic nanoparticles doped N-doped carbon for high-performance rechargeable Zn-air battery

Ni-Doped SFM Double-Perovskite Electrocatalyst for High-Performance Symmetrical Direct-Ammonia-Fed Solid Oxide Fuel Cells

Ammonia has emerged as a promising fuel for solid oxide fuel cells (SOFCs) owing to its high energy density, high hydrogen content, and carbon-free nature. Herein, the electrocatalytic potential of a novel Ni-doped SFM double-perovskite (Sr1.9Fe0.4Ni0.1Mo0.5O6-δ) is studied, for the first time, as an alternative anode material for symmetrical direct-ammonia SOFCs. Scanning and transmission electron microscopy characterization has revealed the exsolution of Ni-Fe nanoparticles (NPs) from the parent Sr2Fe1.5Mo0.5O6 under anode conditions, and X-ray diffraction has identified the FeNi3 phase after exposure to ammonia at 800 °C. The active-exsolved NPs contribute to achieving a maximal ammonia conversion rate of 97.9% within the cell's operating temperatures (550-800 °C). Utilizing 3D-printed symmetrical cells with SFNM-GDC electrodes, the study demonstrates comparable polarization resistances and peak power densities of 430 and 416 mW cm-2 for H2 and NH3 fuels, respectively, with long-term stability and a negligible voltage loss of 0.48% per 100 h during ammonia-fed extended galvanostatic operation. Finally, the ammonia consumption mechanism is elucidated as a multistep process involving ammonia decomposition, followed by hydrogen oxidation. This study provides a promising avenue for improving the performance and stability of ammonia-based SOFCs for potential applications in clean energy conversion technologies.

Synergistic effect of Fe-Ru alloy and Fe-N-C sites on oxygen reduction reaction

In the pursuit of optimizing Fe-N-C catalysts for the oxygen reduction reaction (ORR), the incorporation of alloy nanoparticles has emerged as a prominent strategy. In this work, we effectively synthesized the FeRu-NC catalyst by anchoring Fe-Ru alloy nanoparticles and FeN4 single atom sites onto carbon nanotubes. The FeRu-NC catalyst exhibits significantly enhanced ORR activity and long-term stability, with a high half-wave potential of 0.89 V (vs. RHE) in alkaline conditions, and the half-wave potential remains nearly unchanged after 5000 cycles. The zinc-air battery (ZAB) assembled with FeRu-NC demonstrates a power density of 169.1 mW cm-2, surpassing that of commercial Pt/C. Density functional theory (DFT) calculations reveal that the synergistic interaction between the Fe-Ru alloy and FeN4 single atoms alters the electronic structure and facilitates charge transfer at the FeN4 sites, thereby modulating the adsorption and desorption of ORR intermediates. This enhancement in catalytic activity for the ORR process underscores the potential of this approach for refining M-N-C catalysts, providing novel insights into their optimization strategies.

Nickel-Induced charge transfer in semicoherent Co-Ni/Co6Mo6C Heterostructures for reversible oxygen electrocatalysis

To achieve high-performance Zn-air batteries (ZABs), the development of bifunctional air electrodes capable of efficiently mediating both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) is imperative. In this study, we present an N-doped carbon hollow nanorod encapsulating a semi-coherent Co-Ni/Co6Mo6C heterojunction, tailored for reversible oxygen catalysis. This nanohybrid demonstrated an ORR half-wave potential of 0.907 V alongside an OER overpotential of η10 = 352 mV. When incorporated into ZABs, this catalyst exhibited extraordinary performance metrics, including a high-power density of 343.7 mW/cm2, a specific capacity of 681 mAh/gZn, and enhanced durability. The distinctive electric field within the heterojunction facilitated electron transfer across the semi-coherent interface during reversible oxygen electrocatalysis, enhancing the adsorption and release of active intermediates. Thus, this heightened ORR-OER catalytic efficiency culminated in superior ZABs performance. Our findings afford a pivotal design paradigm for the advancement of productive bifunctional catalysts within the field of energy conversion technologies.

Future prospects of high-entropy alloys as next-generation industrial electrode materials

The rapid advancement of electrochemical processes in industrial applications has increased the demand for high-performance electrode materials. High-entropy alloys (HEAs), a class of multicomponent alloys with unique properties, have emerged as potential electrode materials owing to their enhanced catalytic activity, superior stability, and tunable electronic structures. This review explores contemporary developments in HEA-based electrode materials for industrial applications and identifies their advantages and challenges as compared to conventional commercial electrode materials in industrial aspects. The importance of tuning the composition, crystal structure, different phase formations, thermodynamic and kinetic parameters, and surface morphology of HEAs and their derivatives to achieve the predicted electrochemical performance is emphasized in this review. Synthetic procedures for producing potential HEA electrode materials are outlined, and theoretical discussions provide a roadmap for recognizing the ideal electrode materials for specific electrochemical processes in an industrial setting. A comprehensive discussion and analysis of various electrochemical processes (HER, OER, ORR, CO2RR, MOR, AOR, and NRR) and electrochemical applications (batteries, supercapacitors, etc.) is included to appraise the potential ability of HEAs as an electrode material in the near future. Overall, the design and development of HEAs offer a promising pathway for advancing industrial electrode materials with improved performance, selectivity, and stability, potentially paving the way for the next generation of electrochemical technology.

Iron Triad-Based Bimetallic M-N-C Nanomaterials as Highly Active Bifunctional Oxygen Electrocatalysts

The use of precious metal electrocatalysts in clean electrochemical energy conversion and storage applications is widespread, but the sustainability of these materials, in terms of their availability and cost, is constrained. In this research, iron triad-based bimetallic nitrogen-doped carbon (M-N-C) materials were investigated as potential bifunctional electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The synthesis of bimetallic FeCo-N-C, CoNi-N-C, and FeNi-N-C catalysts involved a precisely optimized carbonization process of their respective metal-organic precursors. Comprehensive structural analysis was undertaken to elucidate the morphology of the prepared M-N-C materials, while their electrocatalytic performance was assessed through cyclic voltammetry and rotating disk electrode measurements in a 0.1 M KOH solution. All bimetallic catalyst materials demonstrated impressive bifunctional electrocatalytic performance in both the ORR and the OER. However, the FeNi-N-C catalyst proved notably more stable, particularly in the OER conditions. Employed as a bifunctional catalyst for ORR/OER within a customized zinc-air battery, FeNi-N-C exhibited a remarkable discharge-charge voltage gap of only 0.86 V, alongside a peak power density of 60 mW cm-2. The outstanding stability of FeNi-N-C, operational for about 55 h at 2 mA cm-2, highlights its robustness for prolonged application.

Switching Electric Double Layer Potential by Phase Structure Control for Advanced Oxygen Reduction Reaction of Cobalt@Nitrogen Doped Carbon Core-Shell

The key to design an advanced oxygen reduction reaction (ORR) electrocatalyst is a well-balance between the adsorption and desorption of oxygen intermediates. This study systematically evaluated the ORR activity of HCP and FCC cobalt core-shell cobalt/N-doped carbon (Cobalt@NC) catalyst via theoretical and experimental studies. The electronic structure calculations using density functional theory (DFT) calculations revealed that the ORR activity of carbon layer can be improved by 1) switching the electrostatic potential in the electrical double layer due to the polarization induced at the carbon-cobalt interface and 2) modulating the electron population in the bonding orbital in the C-O bonds in an ORR. The results revealed that an O atom is bounded stronger to the outer NC shell with FCC Cobalt than HCP Cobalt, which hindered the desorption steps of OH*. Experimentally, plasma-engineered HCP Cobalt@NC also showed remarkably advanced performance toward ORR compared to that FCC Cobalt@NC. The kinetic current density of HCP Cobalt@NC at 0.85 V versus RHE is calculated as 6.24 mA cm-2, which is six folds higher than FCC Cobalt@NC and even outperform 20 wt.% Pt/C. In a practical Aluminium-air battery, HCP Cobalt@NC also exhibited slightly higher peak power density (110.57 mW cm-2) compared to 20 wt.% Pt/C.

Engineering core-shell hollow-sphere Fe3O4@FeP@nitrogen-doped-carbon as an advanced bi-functional electrocatalyst for highly-efficient water splitting

Given the rapidly increasing energy demand and environmental pollution, to achieve energy conservation and emission reduction, hydrogen production has emerged as a promising alternative to traditional fossil fuels because of its high gravimetric energy density, and renewable and environmentally friendly characteristics. Herein, a core-shell hollow-sphere Fe3O4@FeP@nitrogen-doped-carbon (labeled as H-Fe3O4@FeP@NC) with a dual-interface, novel morphology, and superior conductivity is prepared as an advanced bi-functional electrocatalyst for electrochemical overall water splitting using a collaborative strategy comprising of facile self-assembly and phosphating. The prepared catalyst exhibits superior electrocatalytic activity compared to H-Fe3O4@NC and H-Fe3O4 for oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Additionally, the overpotential of H-Fe3O4@FeP@NC for OER/HER (258/165 mV at 10 mA/cm2) is significantly lower than those of H-Fe3O4@NC (274/209 mV) and H-Fe3O4 (287/213 mV) at 10 mA/cm2. Meanwhile, the as-synthesized H-Fe3O4@FeP@NC, as an electrode pair, displays a low cell voltage of 1.69 V at 10 mA/cm2 and excellent stability after 100 h, indicating its practical application for overall water splitting. This work presents a practical and economical strategy toward the fabrication of catalyst for efficient water splitting and fuel cell.

Fe nanoparticles confined by multiple-heteroatom-doped carbon frameworks for aqueous Zn-air battery driving CO2 electrolysis

Metal-organic frameworks (MOF) derived carbon materials are considered to be excellent conductive mass transfer substrates, and the large specific surface area provides a favorable platform for loading metal nanoparticles. Tuning the coordination of metals through polyacid doping to change the MOF structure and specific surface area is an advanced strategy for designing catalysts. Modification of Fe-doped ZIF-8 pre-curing by pyrolysis of phosphomolybdic acid hydrate (PMo), Fe nanoparticles confined by Mo and N co-doped carbon frameworks (Fe-NP/MNCF) were fabricated, and the impact of PMo doping on the shape and functionality of the catalysts was investigated. The Zn-air battery (ZAB) driven CO2 electrolysis was realized by using Fe-NP/MNCF, which was used as bifunctional oxygen reduction reaction (ORR) and carbon dioxide reduction reaction (CO2RR) catalysts. The results show that the half-wave potential (E1/2) of Fe-NP/MNCF is 0.89 V, and the limiting diffused current density (jL) is 6.4 mA cm-2. The ZAB constructed by Fe-NP/MNCF shows a high specific capacity of 794.8 mAh gZn-1, a high open-circuit voltage (OCV) of 1.475 V, and a high power density of 111.6 mW cm-2. Fe-NP/MNCF exhibited efficient CO2RR performance with high CO Faraday efficiency (FECO) of 87.5 % and current density for the generation of carbon dioxide (jCO) of 10 mA cm-2 at -0.9 V vs RHE. ZAB-driven CO2RR had strong catalytic stability. These findings provide new methods and techniques for the preparation of advanced carbon-based catalysts from MOFs.

Confined covalent organic framework anchored Fe sites derived highly uniform electrocatalysts for rechargeable aqueous and solid-state Zn-air batteries

Exploiting clean, highly efficient energy storage and conversion device like Zn-air battery is of significance for alleviating the energy and environmental crises of this society. Metal organic coordination polymers/frameworks have been regarded as ideal templates to synthesize non-noble metal catalysts for a long time. However, the high density of metal nodes inevitably leads to the heavy aggregation of metal nanoparticles during thermolysis transformation process, which greatly hinders the maximizing of electrochemical performances. Herein, covalent organic framework (COF) has been employed to anchor the quantificational Fe ions (COF-Fe) and then confined into the macropores of g-C3N4 to improve the dispersion of metal active sites and avoid severe aggregation during high temperature pyrolysis. After calcination, the metal nanoparticles highly dispersed Fe-CFN catalysts can be obtained. The optimal Fe-CFN-800 catalysts exhibit excellent ORR and OER performances with the potential difference between ORR and OER of merely 0.723 V. Moreover, experimental way and DFT theoretical calculations are also employed to disclose the reaction mechanism. Finally, the all-solid-state and aqueous Zn-air batteries assembled with the optimized Fe-CFN-800 as cathode present excellent performances with high peak power density, flexible rate performance, strong discharge stability and long-term charge-discharge cycling performance.

Superhydrophilicity boron-doped cobalt phosphide nanosheets decorated carbon nanotube arrays self-supported electrode for overall water splitting

Transition metal borides (TMBs) or phosphides (TMPs) have attracted great attention to the design of bifunctional electrocatalysts for energy storage. The superaerophobicity and superhydrophilicity of the catalytic electrode surface are crucial factors to determine the reaction process of the gas electrode. Herein, we report a self-supported electrode of carbon nanotube (CNTs) array grown on carbon cloth (CC) modulated together by boron-doped cobalt phosphide (CoP-B/CNTs/CC). The electrode requires the overpotential of 73.8 mV and 189.5 mV at the current density of ±10 mA cm-2 for hydrogen and oxygen evolution reactions in an alkaline electrolyte (1.0 M KOH), respectively, meanwhile maintaining outstanding long-term durability for more than 300 h. The excellent activity of CoP-B/CNTs/CC is attributed to boron doping regulating its electronic structure and further enriching active sites. The attractive stability of CoP-B/CNTs/CC is due to the unique geometric structure of the self-supported electrode. Furthermore, the superaerophobicity and superhydrophilicity of the electrode surface also accelerate the reaction process of the gas electrode. Expectedly, water splitting cells assembled using CoP-B/CNTs/CC electrodes as cathode and anode, respectively, require a cell voltage of 1.54 V at 10 mA cm-2, which is lower than that of the Pt/C/CC||RuO2/CC couple (1.69 V at 10 mA cm-2). Importantly, CoP-B/CNTs/CC||CoP-B/CNTs/CC achieve stable cell voltage under the step current changes (10 mA cm-2, 50 mA cm-2, and 100 mA cm-2) over 300 h. This work highlights a new path to understanding the effects of the static and dynamic behavior of bubbles on the surface of self-supporting electrodes on catalytic performance.

Boosting oxygen reduction reaction kinetics through perturbating electronic structure of single-atom Fe-N3S1 catalyst with sub-nano FeS cluster

Single atomic Fe-N4 catalyst exhibits a great prospect for oxygen reduction reaction (ORR) and adjusting the intrinsic coordination structure and the carbon matrix structure effectively improves the catalytic activity. However, controlling the active site coordination structure and its surrounding environment at atomic level remains a challenge. In this paper, Fe-N3S1 and FeS sub-nano cluster were innovatively concatenated on S, N co-doped carbon matrix (SNC), denoted as FeS/FeSA@SNC catalysts, for modulating ORR catalysis performance. Both experimental measurements and theoretical calculations have confirmed that the local electron configuration of Fe center is modulated by this unique structure combination leading to optimized ORR kinetics. Based on this design, the synthesized FeS/FeSA@SNC delivers ORR activity with a half-wave potential of 0.9 V (vs. RHE), excelling that of commercial Pt/C (0.87 V) and the Zn-air battery (ZAB) with this cathode catalyst delivers a peak power density of 126 mW cm-2. This work presents a novel strategy for manipulating the single-atom active sites through control the local coordination structure and provides a reference for the development of novel efficient ORR electrocatalysts.

Duplex Interpenetrating-Phase FeNiZn and FeNi3 Heterostructure with Low-Gibbs Free Energy Interface Coupling for Highly Efficient Overall Water Splitting

The sluggish kinetics of both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) generate the large overpotential in water electrolysis and thus high-cost hydrogen production. Here, multidimensional nanoporous interpenetrating-phase FeNiZn alloy and FeNi3 intermetallic heterostructure is in situ constructed on NiFe foam (FeNiZn/FeNi3@NiFe) by dealloying protocol. Coupling with the eminent synergism among specific constituents and the highly efficient mass transport from integrated porous backbone, FeNiZn/FeNi3@NiFe depicts exceptional bifunctional activities for water splitting with extremely low overpotentials toward OER and HER (η1000 = 367/245 mV) as well as the robust durability during the 400 h testing in alkaline solution. The as-built water electrolyzer with FeNiZn/FeNi3@NiFe as both anode and cathode exhibits record-high performances for sustainable hydrogen output in terms of much lower cell voltage of 1.759 and 1.919 V to deliver the current density of 500 and 1000 mA cm-2 as well long working lives. Density functional theory calculations disclose that the interface interaction between FeNiZn alloy and FeNi3 intermetallic generates the modulated electron structure state and optimized intermediate chemisorption, thus diminishing the energy barriers for hydrogen production in water splitting. With the merits of fine performances, scalable fabrication, and low cost, FeNiZn/FeNi3@NiFe holds prospective application potential as the bifunctional electrocatalyst for water splitting.

Different Growth Behavior of MOF-on-MOF Heterostructures to Enhance Oxygen Evolution

The exploration of non-noble metal electrocatalysts with high catalytic activity and stability for oxygen evolution reaction (OER) has become particularly urgent. Here, FeNi-based Prussian blue analogues (PBAs) were obtained by adding different solvents, where PBA particles preferentially grew on the surface plane/edge of coordination polymer precursor (Ni-ABDC) with various polarities. This resulted in the formation of FeNi-PBA-plane/edge morphologies, respectively. Notably, on account of more exposed PBAs, FeNi nanoparticles were uniformly supported on the porous N-doped carbon nanomaterials. Among them, the calcined FeNi-NC-800 underwent an interesting pre-activation process and exhibited a low overpotential of 281 mV at 10 mA cm^-2 and a small Tafel slope of 82 mV dec^-1 in 1.0 m KOH. This bimetallic sample showed superior OER activity and stability in comparison with control materials, which could be attributed to its abundant FeNi nanoparticles, high nitrogen content, large specific surface area, and synergistic effects between Fe and Ni atoms. In addition, relevant theoretical calculation on the optimal catalyst, FeNi-NC-800, further demonstrated its efficient OER performance with effective Fe-doping in the Ni-based oxyhydroxides.

Cu-doped CaFeO3 perovskite oxide as oxygen reduction catalyst in air cathode microbial fuel cells

Cathode electrocatalyst is quite critical to realize the application of microbial fuel cells (MFCs). Perovskite oxides have been considered as potential MFCs cathode catalysts to replace Pt/C. Herein, Cu-doped perovskite oxide with a stable porous structure and excellent conductivity was successfully prepared through a sol-gel method. Due to the incorporation of Cu, CaFe_0.9Cu_0.1O₃ has more micropores and a larger surface area, which are more conducive to contact with oxygen. Doping Cu resulted in more Fe³⁺ in B-site and thus enhanced its binding capability to oxygen molecules. The data from electrochemical test demonstrated that the as-prepared catalyst has good conductivity, high stability, and excellent ORR properties. Compared with Pt/C catalyst, CaFe_0.9Cu_0.1O₃ exhibits a lower overpotential, which had an onset potential of 0.195 V and a half-wave potential of -0.224 V, respectively. CaFe_0.9Cu_0.1O₃ displays an outstanding four-electron pathway for ORR mechanism and demonstrates superiors corrosion resistance and stability. The MFC with CaFe_0.9Cu_0.1O₃ has a greater maximum power density (1090 mW m^-3) rather than that of Pt/C cathode (970 mW m^-3). This work demonstrated CaFe_0.9Cu_0.1O₃ is an economic and efficient cathodic catalyst for MFCs.

Monodispersed Eu2O3-Modified Fe3O4@NCG Composites as Highly Efficient and Ultra-stable Catalysts for Rechargeable Zn-Air Batteries

Constructing highly efficient cathode catalysts for Zn-air batteries (ZABs) is an attractive research topic in sustainable energy storage area. Herein, the rare-earth metal oxide modification strategy has been proposed to construct the highly efficient and ultra-stable catalysts for ZABs. Accordingly, a graphene oxide-doped carbon-supported Eu₂O₃-modified Fe₃O₄ (Fe₃O₄/Eu₂O₃@NCG) catalyst is developed with layered Fe-Eu-MOF/GO as a precursor. Detailed characterization reveals that Fe₃O₄/Eu₂O₃@NCG possesses unique structural properties, including carbon-metal-carbon configuration, plentiful oxygen vacancies, and variable metal-active sites, which endows the catalyst with strong conductivity, high activity, and ultra-long stability. The optimal Fe₃O₄/Eu₂O₃@NCG catalyst exhibits an outstanding electrochemical performance, and the potential difference (E_gap) between oxygen reduction reaction and oxygen evolution reaction is merely 0.68 V at 0.1 M KOH condition. Moreover, density functional theory calculations are employed to investigate the reaction mechanism and the synergetic effect between Fe and Eu atoms. Most importantly, the Fe₃O₄/Eu₂O₃@NCG-based aqueous ZAB delivers a high power density (218 mW/cm²), specific capacity (854 mA h/g@5 mA/cm²), and an impressive ultra-long cycle property with more than 1000 h (>6000 cycles) charge-discharge cycle life. In addition, the Fe₃O₄/Eu₂O₃@NCG-based all-solid-state ZAB also exhibits an outstanding performance, achieving >460 h cycle life (>2760 cycles) and strong practical application capability.

Metal-Organic Frameworks (MOFs) Derived Materials Used in Zn-Air Battery

It is necessary to develop new energy technologies because of serious environmental problems. As one of the most promising electrochemical energy conversion and storage devices, the Zn-air battery has attracted extensive research in recent years due to the advantages of abundant resources, low price, high energy density, and high reduction potential. However, the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) of Zn-air battery during discharge and charge have complicated multi-electron transfer processes with slow reaction kinetics. It is important to develop efficient and stable oxygen electrocatalysts. At present, single-function catalysts such as Pt/C, RuO₂, and IrO₂ are regarded as the benchmark catalysts for ORR and OER, respectively. However, the large-scale application of Zn-air battery is limited by the few sources of the precious metal catalysts, as well as their high costs, and poor long-term stability. Therefore, designing bifunctional electrocatalysts with excellent activity and stability using resource-rich non-noble metals is the key to improving ORR/OER reaction kinetics and promoting the commercial application of the Zn-air battery. Metal-organic framework (MOF) is a kind of porous crystal material composed of metal ions/clusters connected by organic ligands, which has the characteristics of adjustable porosity, highly ordered pore structure, low crystal density, and large specific surface area. MOFs and their derivatives show remarkable performance in promoting oxygen reaction, and are a promising candidate material for oxygen electrocatalysts. Herein, this review summarizes the latest progress in advanced MOF-derived materials such as oxygen electrocatalysts in a Zn-air battery. Firstly, the composition and working principle of the Zn-air battery are introduced. Then, the related reaction mechanism of ORR/OER is briefly described. After that, the latest developments in ORR/OER electrocatalysts for Zn-air batteries are introduced in detail from two aspects: (i) non-precious metal catalysts (NPMC) derived from MOF materials, including single transition metals and bimetallic catalysts with Co, Fe, Mn, Cu, etc.; (ii) metal-free catalysts derived from MOF materials, including heteroatom-doped MOF materials and MOF/graphene oxide (GO) composite materials. At the end of the paper, we also put forward the challenges and prospects of designing bifunctional oxygen electrocatalysts with high activity and stability derived from MOF materials for Zn-air battery.

S-Doping Promotes Pyridine Nitrogen Conversion and Lattice Defects of Carbon Nitride to Enhance the Performance of Zn-Air Batteries

The efficient operation of Zn-air batteries (ZABs) requires highly active and stable reversible air catalysts. Studies have shown that heteroatom-doped carbonaceous nanomaterials are effective metal-free electrocatalysts for the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR). Herein, we design a facile and scalable catalyst doping scheme to manufacture S-doped carbon nitride (S-C₃N₄). Surprisingly, this metal-free catalyst exhibits excellent OER and ORR electrocatalytic activities in alkaline electrolytes, being comparable to those of commercial Pt/C. For the first time, it is proved by experiments that S doping can not only effectively increase the lattice defects of C₃N₄ but also promote the conversion of pyrrolic nitrogen to pyridine nitrogen, thereby enhancing the bifunctional catalytic activity (OER and ORR). When the catalyst is used as an air electrode for rechargeable ZABs, its performance is obviously better than that provided by commercial Pt/C. Our findings and material design strategies are expected to provide new ideas for the synthesis of various high-performance carbon-based electrocatalysts.

Iron-nickel alloy nanoparticles encapsulated in nitrogen-doped carbon nanotubes as efficient bifunctional electrocatalyst for rechargeable zinc-air batteries

In this work, we present an efficient bifunctional electrocatalyst comprising iron-nickel (FeNi) alloy nanoparticles confined in nitrogen-doped carbon nanotubes (denoted FeNi-NCNT) for zinc-air batteries. The FeNi-NCNT electrocatalyst is fabricated by anion-exchange of nickel(II) ion/zinc(II) ion-2,4,6-tris-(di(pyridin-2-yl)amino)-1,3,5-triazine complex with ferricyanide anion followed by mixing with melamine and then carbonization. The resultant FeNi-NCNT electrocatalyst displays excellent bifunctional activity with a low reversible oxygen electrode index of 0.725 V. The rechargeable aqueous zinc-air battery fabricated with FeNi-NCNT air cathode manifests both high specific capacity (819 mAh g^-1_Zn at 5.0 mA cm^-2) and long cycle life (1500 cycles/1000 h at 10 mA cm^-2, 600 cycles/400 h at 25 mA cm^-2, and 165 cycles/110 h at 50 mA cm^-2). Moreover, flexible solid-state zinc-air battery assembled with FeNi-NCNT air cathode can deliver a specific capacity of 765 mAh g^-1_Zn at 5.0 mA cm^-2, a power density of 84.8 mW cm^-2, and a cycle life of 110 h (330 cycles) at 2.0 mA cm^-2.

Controllable growth of Fe-doped NiS2 on NiFe-carbon nanofibers for boosting oxygen evolution reaction

The construction of high-efficiency and low-cost electrocatalysts toward oxygen evolution reaction (OER) to improve the overall water decomposition performance is a fascinating route to deal with the clean energy application. Herein, Fe-doped NiS₂ crystals grown on the surface of carbon nanofibers (CNFs) encapsulated with NiFe alloy nanoparticles ((Ni,Fe)S₂/NiFe-CNFs) are fabricated through an electrospinning-calcination-vulcanization process, which has been used as a splendid electrocatalyst for OER. Benefitting from the abundant electrochemical active sites from the incorporation of Fe element in NiS₂ and the synergistic effect between NiFe-CNFs and surface sulfides, the obtained (Ni,Fe)S₂/NiFe-CNFs catalyst exhibits highly electrochemical activities and satisfactory durability toward OER in an alkaline medium with a low overpotential of only 287 mV at a high current density of 30 mA cm^-2, and with a little decline in the current retention after 48 h, suggesting its superior OER performance even compared with some noble metal-based electrocatalysts. Additionally, a two-electrode system conducted by using the (Ni,Fe)S₂/NiFe-CNFs and commercial Pt/C as electrodes, only needs a cell voltage of 1.54 V to afford 10 mA cm^-2 for overall water splitting, which is even much better than the RuO₂||Pt/C electrolyzer. This study offers a promising approach to prepare high-efficiency OER catalysts toward overall water splitting.

Random Occupation of Multimetal Sites in Transition Metal-Organic Frameworks for Boosting the Oxygen Evolution Reaction

Developing robust oxygen evolution reaction (OER) electrocatalysts with excellent performance is essential for the conversion of renewable electricity to clean fuel. Herein, we present a facile concept for the synthesis of efficient high-entropy metal-organic frameworks (HEMOFs) as electrocatalysts in a one-step solvothermal synthesis. This strategy allows control of the microstructure and corresponding lattice distortion by tuning the metal ion composition. As a result, the OER activity was improved by optimizing the coordination environment of the metal catalytic center. The optimized Co-rich HEMOFs exhibited a low overpotential of 310 mV at a current density of 10 mA cm^-2 , better than a RuO₂ catalyst tested under the same conditions. The finding of lattice distortion of the HEMOFs provides a new strategy for developing high-performance electrocatalysts for energy conversion.

Boron, nitrogen co-doped biomass-derived carbon aerogel embedded nickel-cobalt-iron nanoparticles as a promising electrocatalyst for oxygen evolution reaction

The electrocatalytic performance of oxygen evolution reaction (OER) electrocatalysts is highly reliant on the activity of its catalytic active site, which may be augmented by raising the number of active sites. In this study, nanoscaled nickel-cobalt-iron (NiCoFe) alloy was embedded on conductive boron(B), nitrogen(N) co-doped/biomass-derived carbon aerogel as an OER electrocatalyst. The synthesized electrocatalysts were calcined under different temperatures and with variable dopants. The optimal electrocatalyst (BN/CA-NiCoFe-600) demonstrated a low overpotential of 321 mV (at current density of 10 mA cm^-2) and a minute Tafel slope of 42 mV dec^-1, which was even smaller than that of IrO₂ and RuO₂. Its mass activity and specific activity were calculated to be 201.7 A g^-1, and 34.1 cm^-2_ECSA, respectively. Furthermore, the electrocatalyst showed excellent stability and durability. This work provides an easy and practical synthetic strategy for acquiring very active and durable electrocatalysts for OER.

Red Bean Pod Derived Heterostructure Carbon Decorated with Hollow Mixed Transition Metals as a Bifunctional Catalyst in Zn-Air Batteries

Design and synthesis of low-cost and efficient bifunctional catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in Zn-air batteries are essential and challenging. We report a facile method to synthesize heterostructure carbon consisting of graphitic and amorphous carbon derived from the agricultural waste of red bean pods. The heterostructure carbon possesses a large surface area of 625.5 m²  g^-1 , showing ORR onset potential of 0.89 V vs. RHE and OER overpotential of 470 mV at 5 mA cm^-2 . Introducing hollow FeCo nanoparticles and nitrogen dopant improves the bifunctional catalytic activity of the carbon, delivering ORR onset potential of 0.93 V vs. RHE and OER overpotential of 360 mV. Electron energy-loss spectroscopy (EELS) O K-edge map suggests the presence of localized oxygen on the FeCo nanoparticles, suggesting the oxidation of the nanoparticles. Zn-air battery with these carbon-based catalysts exhibits a peak power density as high as 116.2 mW cm^-2 and stable cycling performance over 210 discharge/charge cycles. This work contributes to the advancement of bifunctional oxygen electrocatalysts while converting agricultural waste into value-added material.

Perspective on intermetallics towards efficient electrocatalytic water-splitting

Intermetallic compounds exhibit attractive electronic, physical, and chemical properties, especially in terms of a high density of active sites and enhanced conductivity, making them an ideal class of materials for electrocatalytic applications. Nevertheless, widespread use of intermetallics for such applications is often limited by the complex energy-intensive processes yielding larger particles with decreased surface areas. In this regard, alternative synthetic strategies are now being explored to realize intermetallics with distinct crystal structures, morphology, and chemical composition to achieve high performance and as robust electrode materials. In this perspective, we focus on the recent advances and progress of intermetallics for the reaction of electrochemical water-splitting. We first introduce fundamental principles and the evaluation parameters of water-splitting. Then, we emphasize the various synthetic methodologies adapted for intermetallics and subsequently, discuss their catalytic activities for water-splitting. In particular, importance has been paid to the chemical stability and the structural transformation of the intermetallics as well as their active structure determination under operating water-splitting conditions. Finally, we describe the challenges and future opportunities to develop novel high-performance and stable intermetallic compounds that can hold the key to more green and sustainable economy and rise beyond the horizon of water-splitting application.

Silver decorated cobalt carbonate to enable high bifunctional activity for oxygen electrocatalysis and rechargeable Zn-air batteries

Rechargeable zinc-air batteries (ZABs) is primarily driven by the couple of oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Currently,it is still challenging to develop cost-effective, highly efficient, and robust bifunctional catalysts for ZABs. Herein, a novel silver decorated cobalt carbonate (Ag@CoCO₃) hybrid catalyst is proposed as the potential bifunctional catalyst to drive OER and ORR for ZABs. Engineering Ag nanoparticles onto the surface of CoCO₃ microsphere not only facilitates the charge transfer, but also modulates the electronic structure, which are beneficial to intrinsic bifunctional activity. As a result, this Ag@CoCO₃ catalyst yields a substantially enhanced bifunctionality compared to the pristine CoCO₃ catalyst. Moreover, the homemade Ag@CoCO₃ based ZABs provides a high peak power density of 146 mW cm^-2, superior to 107 mW cm^-2 for CoCO₃ based ZABs and 111 mW cm^-2 for commercial Pt/C-IrO₂ based ZABs.

In situ immobilization of Fe/Fe3C/Fe2O3 hollow hetero-nanoparticles onto nitrogen-doped carbon nanotubes towards high-efficiency electrocatalytic oxygen reduction

The rational design of affordable, efficient and robust electrocatalysts towards the oxygen reduction reaction (ORR) is of vital importance for the future advancement of various renewable-energy technologies. Herein, we develop a feasible and delicate synthesis of Fe/Fe3C/Fe2O3 hollow heterostructured nanoparticles in situ immobilized on highly graphitic nitrogen-doped carbon nanotubes (referred to as Fe/Fe3C/Fe2O3@N-CNTs hereafter) via a simple hydrogel-bridged pyrolysis strategy. The simultaneous consideration of interfacial manipulation and nanocarbon hybridization endows the formed Fe/Fe3C/Fe2O3@N-CNTs with sufficiently well-dispersed and firmly immobilized active components, regulated electronic configuration, enhanced electrical conductivity, multidimensional mass transport channels, and remarkable structural stability. Consequently, benefiting from the compositional synergy and architectural superiority, the as-obtained Fe/Fe3C/Fe2O3@N-CNTs exhibit excellent ORR catalytic activity, impressive durability and superior selectivity in an alkaline electrolyte, outperforming the commercial Pt/C catalyst and a majority of the previously reported Fe-based catalysts. Furthermore, the rechargeable Zn-air battery using Fe/Fe3C/Fe2O3@N-CNTs + RuO2 as an air-cathode exhibits a higher power density, larger specific capacity and better cycling stability as compared with the state-of-the-art Pt/C + RuO2 counterpart. The explored hydrogel-bridged pyrolysis strategy enabling the concurrent heterointerface construction, nanostructure engineering and nanocarbon hybridization may inspire the future design of high-efficiency electrocatalysts for diverse renewable energy applications.

Insight into the amorphous nickel-iron (oxy)hydroxide catalyst for efficient oxygen evolution reaction

The specific roles of Ni and Fe in nickel-iron (oxy)hydroxide catalyst (NiFeO_x(OH)_y) are extensively discussed during oxygen evolution reaction (OER). However, there still remains controversy about whether Ni or Fe species as the dominate active site. In this work, we reported the NiFeO_x(OH)_y catalysts with varied atomic ratio of nickel and iron for OER to explore the dominate active site during OER processes. From the electrochemical performances, the similar Tafel slopes of catalysts with Fe species can achieve at a level of 40 mV dec^-1, outperforming the Tafel slopes of catalysts without Fe species. Thus, it can be concluded that the present Fe site can serve as the dominant active site in NiFeO_x(OH)_y for OER. Meanwhile, the Ni species is proved as the OH^- adsorption site, which is beneficial to the Fe site to deliver a better OER performance. As a result, the catalyst with an optimal Ni/Fe interface (atomic ratio of 1 : 1.18) displays outstanding OER performances. It only requires a low overpotential of 250 mV to deliver current density of 10 mA cm^-2 and exhibits a small Tafel slope of 39 mV dec^-1. This catalyst also shows remarkable stability with negligible potential decay after 50 h at a current density of 50 mA cm^-2. This work offers a new sight into the specific roles of Ni and Fe in NiFeO_x(OH)_y for OER.

Ultrasonic Plasma Engineering Toward Facile Synthesis of Single-Atom M-N4/N-Doped Carbon (M = Fe, Co) as Superior Oxygen Electrocatalyst in Rechargeable Zinc-Air Batteries

As bifunctional oxygen evolution/reduction electrocatalysts, transition-metal-based single-atom-doped nitrogen-carbon (NC) matrices are promising successors of the corresponding noble-metal-based catalysts, offering the advantages of ultrahigh atom utilization efficiency and surface active energy. However, the fabrication of such matrices (e.g., well-dispersed single-atom-doped M-N₄/NCs) often requires numerous steps and tedious processes. Herein, ultrasonic plasma engineering allows direct carbonization in a precursor solution containing metal phthalocyanine and aniline. When combining with the dispersion effect of ultrasonic waves, we successfully fabricated uniform single-atom M-N₄ (M = Fe, Co) carbon catalysts with a production rate as high as 10 mg min^-1. The Co-N₄/NC presented a bifunctional potential drop of ΔE = 0.79 V, outperforming the benchmark Pt/C-Ru/C catalyst (ΔE = 0.88 V) at the same catalyst loading. Theoretical calculations revealed that Co-N₄ was the major active site with superior O₂ adsorption-desorption mechanisms. In a practical Zn-air battery test, the air electrode coated with Co-N₄/NC exhibited a specific capacity (762.8 mAh g^-1) and power density (101.62 mW cm^-2), exceeding those of Pt/C-Ru/C (700.8 mAh g^-1 and 89.16 mW cm^-2, respectively) at the same catalyst loading. Moreover, for Co-N₄/NC, the potential difference increased from 1.16 to 1.47 V after 100 charge-discharge cycles. The proposed innovative and scalable strategy was concluded to be well suited for the fabrication of single-atom-doped carbons as promising bifunctional oxygen evolution/reduction electrocatalysts for metal-air batteries.