Advanced non-noble materials in bifunctional catalysts for ORR and OER toward aqueous metal-air batteries

Versatile LaCo0.6Ni0.4O3-δ Nanofiber Membrane for High Performance Oxygen Electrocatalysis over a Wide Temperature Range

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are key processes in numerous oxygen-involved applications over a wide temperature range. Despite advances in nanofiber engineering to increase active site density and catalytic efficiency for ORR/OER, conventional electrode fabrication methods often compromise the integrity of nanofibrous structures. Herein, a robust strategy is presented for the fabrication of LaCo0.6Ni0.4O3-δ (LCN) nanofibrous membranes using optimized electrospinning techniques. This approach achieves high specific surface area, increased porosity, rapid mass transport, and precise control of morphology and thickness. The resulting LCN nanofibers exhibit exceptional ORR and OER catalytic activity at room temperature, rivaling commercial Pt/C and RuO₂ catalysts. Moreover, in solid oxide cells (SOCs) operating at elevated temperatures, LCN nanofibrous membranes deliver remarkable ORR and OER performance, with a peak power density of 0.802 W cm- 2 at 700 °C and excellent stability over 180 h. These results highlight the potential of nanofibrous perovskite catalysts for practical oxygen electrocatalytic applications and demonstrate that the LCN nanofibrous membrane, combined with a self-assembly approach, exploits on the advantages of high porosity and specific surface area. This work opens up new avenues for the use of nanofibrous electrodes in a wide temperature range.

Smart Compositional Design of B-Site Ordered Double Perovskite for Advanced Oxygen Catalysis at Ultra-High Current Densities

Perovskite oxides have been considered promising oxygen catalysts for oxygen reduction and evolution reactions (ORR and OER), owing to structural and compositional flexibility, and tailorable properties. Ingenious B-site ordered La1.5Sr0.5NiMn0.5Fe0.5O6 (LSNMF) double perovskite is strategically designed by simultaneously interposing Ni0.5Mn0.5 and Ni0.5Fe0.5 into B' and B″ sites. Controlling B-site cation systematically tailors the electronic structure of the B-site cation with a d-band center (Md) upshift close to the Fermi level, increasing the overlap of the Md center and O 2p center (OP). The strong interaction of Md and Op facilitates the adsorption of oxygen and activates the lattice oxygen to participate in the OER process, thereby enhancing the ORR and OER activity. For ORR, LSNMF exhibited an onset potential of 0.9 V along with a high limiting current of -8.05 mA cm-2. At the same time, for OER at 1 m KOH, LSNMF effectively reached a maximum current density of 3000 mA cm-2. Most importantly, the difference between EORR (at -1 mA cm-2) and EOER (at 10 mA cm-2), ΔE is 0.69 V, which stands among the best of recently reported perovskites. The as-designed LSNMF is stable, efficient, lucrative, and a promising candidate for practical application.

Three-dimensional porous structured cobalt- and nitrogen-doped carbon nanotube electrocatalyst derived from cobalt-based zeolitic imidazolate framework nanoleaves for high performance zinc-air battery

The development of efficient and cost-effective electrocatalysts to overcome the intrinsic sluggish kinetics of the oxygen reduction reaction (ORR) in zinc-air batteries is crucial. In this study, we introduce a strategy that integrates a template-assisted synthesis with subsequent thermal treatment to fabricate an active and stable cobalt-based nitrogen-doped carbon electrocatalyst, denoted as Co-N-CNT. The strategy adjusts the disordered architecture of the zeolitic imidazolate framework (ZIF) through the synergistic effect of bimetallic species, restricted the growth of zeolitic imidazolate framework nanoleaves (ZIF-L) using salt templates, and directed the transformation from a two-dimensional blade-like morphology to a three-dimensional multi-tiered composite structure. Notably, the Co-N-CNT-800 sample, synthesized at an optimized pyrolysis temperature of 800 °C, exhibits a half-wave potential of 0.89 V and demonstrates stability with sustained cycling over 21 h, which is comparable to the performance of commercial Pt/C electrocatalysts. Moreover, when employed as the cathode in zinc-air batteries, Co-N-CNT-800 not only surpasses Pt/C in terms of power density but also exhibits long-term charge/discharge stability. This findings offer a viable pathway for the design of active and cost-effective ORR electrocatalysts, holding promise for applications in the electrochemical energy storage and conversion systems.

A Molecular Catalyst-Driven Sustainable Zinc-Air Battery Assembly

Bidirectional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) electrocatalysts are key for molecular oxygen-centric renewable energy transduction via metal-air batteries. Here, a molecular cobalt complex is covalently tethered on a strategically functionalized silica surface that displayed both ORR and OER in alkaline media. The detailed X-ray absorbance spectroscopy (XAS) studies indicate that this catalyst retains its intrinsic molecular features while playing a central role during bidirectional electrocatalysis and demonstrating a relatively lower energy gap between O2/H2O interconversions. This robust molecular catalyst-silica composite (deposited on a porous carbon paper) is assembled along with a zinc foil and polymeric gel membrane to devise an active single-stack quasi-solid zinc-air battery (ZAB) setup. This quasi-solid ZAB assembly displayed impressive power density (60 mW cm-2@100 mA cm-2), specific capacity (818 mAh g-1@ 5mA cm-2), energy density (757 Whkg-1 @5mA cm-2), and elongated charging/discharging life (28 h). An appropriate assembly of these ZAB units is able to power practical electronic appliances, requiring ≈1.6-6.0V potential requirements.

Recent Advances in Rechargeable Zn-Air Batteries

Rechargeable Zn-air batteries are considered to be an effective energy storage device due to their high energy density, environmental friendliness, and long operating life. Further progress on rechargeable Zn-air batteries with high energy density/power density is greatly needed to satisfy the increasing energy conversion and storage demands. This review summarizes the strategies proposed so far to pursue high-efficiency Zn-air batteries, including the aspects of the electrocatalysts (from noble metals to non-noble metals), the electrode chemistry (from the oxygen evolution reaction to the organic oxidation reaction), electrode engineering (from powdery to free-standing), aqueous electrolytes (from alkaline to non-alkaline) and the battery configuration (from liquid to flexible). An essential evaluation of electrochemistry is highlighted to solve the challenges in boosting the efficiency of rechargeable metal-air batteries. In the end, the perspective on current challenges and future research directions to promote the industrial application of rechargeable Zn-air batteries is provided.

A sensitive electrochemical immunosensor based on high-efficiency catalytic cycle amplification strategy for detection of cardiac troponin I

An electrochemical immunosensor based on the novel high efficiency catalytic cycle amplification strategy for the sensitive detection of cardiac troponin I (cTnI). With its variable valence metal elements and spiny yolk structure, the Cu2O/CuO@CeO2 nanohybrid exhibits high speed charge mobility and exceptional electrochemical performance. Notably, fluorite-like cubic crystal CeO2 shell would undergo redox reaction with Cu2O core, which successfully ensures the continuous recycling occurrence of "fresh" Cu (II)/Cu (I) and Ce (Ⅳ)/Ce (Ⅲ) pairs at the electrode interface. The "fresh" active sites continue to emerge constantly, resulting in a significant increase in the current signal. In light of the electrochemical characterization, the electron transfer pathway and catalytic cycle mechanism among CeO2, Cu2O and CuO were further discussed. The developed electrochemical immunosensor detected cTnI from 100 fg/mL to 100 ng/mL with a LOD of 15.85 fg/mL under optimal conditions. The analysis results indicate that the immunosensor would hold promise for broad application prospects in the biological detection for other biomarkers.

Screening Transition-Metal-Incorporated β-AgVO3 for Augmented Oxygen Reduction Activity

Exploring cost-effective alternatives to Pt-based catalysts for the oxygen reduction reaction (ORR) in fuel cells is crucial for their large-scale deployment in green energy applications. Silver vanadate (AgVO3) is a well-studied material for photocatalytic applications. Here, we investigate the electrocatalytic ORR activity of the thermodynamically stable β phase of AgVO3 through computational modeling based on DFT. It is found that β-AgVO3 exhibits weak catalytic activity for the ORR, with vanadium being the preferable active site. Incorporating single atoms of transition metals at surface-level vacancies in β-AgVO3 significantly modifies the ORR activity. We study the scaling of free energy changes for the ORR intermediates *OOH, *OH, and *O for various transition metals incorporated, which leads to an optimal overpotential for the system. The optimal overpotential thus obtained is remarkably lower than that of pristine β-AgVO3. For the transition metal atoms we consider here, Co-incorporated β-AgVO3 exhibits the best ORR catalytic activity due to its optimal binding of ORR species to the vanadium site. It is also observed that some of the transition metals considered like Re, Rh, Os, or Mn show weak activity, either due to strong or weak binding. Analysis of the electronic structure of the adsorbate-catalyst interface shows a strong correlation between optimal activity and evolution of midgap states in β-AgVO3, due to transition metal incorporation. Our study concludes that the ORR activity of a stable mixed transition metal oxide like β-AgVO3 can be enhanced with a minimal loading of transition metals, which could help in developing a novel series of ORR catalysts.

Exploration of metal-free 2D electrocatalysts toward the oxygen electroreduction

The advancement of economical and readily available electrocatalysts for the oxygen reduction reaction (ORR) holds paramount importance in the advancement of fuel cells and metal-air batteries. Recently, 2D non-metallic materials have obtained substantial attention as viable alternatives for ORR catalysts due to their manifold advantages, encompassing low cost, ample availability, substantial surface-to-volume ratio, high conductivity, exceptional durability, and competitive activity. The augmented ORR performances observed in metal-free 2D materials typically arise from heteroatom doping, defects, or the formation of heterostructures. Here, the authors delve into the realm of electrocatalysts for the ORR, pivoting around metal-free 2D materials. Initially, the merits of metal-free 2D materials are explored and the reaction mechanism of the ORR is dissected. Subsequently, a comprehensive survey of diverse metal-free 2D materials is presented, tracing their evolutionary journey from fundamental concepts to pragmatic applications in the context of ORR. Substantial importance is given on the exploration of various strategies for enhancing metal-free 2D materials and assessing their impact on inherent material performance, including electronic properties. Finally, the challenges and future prospects that lie ahead for metal-free 2D materials are underscored, as they aspire to serve as efficient ORR electrocatalysts.

Triphasic Oxygen Storage in Wet Nanoparticulate Polymer of Intrinsic Microporosity (PIM-1) on Platinum: An Electrochemical Investigation

The triphasic interaction of gases with electrode surfaces immersed in aqueous electrolyte is crucial in electrochemical technologies (fuel cells, batteries, sensors). Some microporous materials modify this interaction locally via triphasic storage capacity for gases in aqueous environments linked to changes in apparent oxygen concentration and diffusivity (as well as activity and reactivity). Here, a nanoparticulate polymer of intrinsic microporosity (PIM-1) in aqueous electrolyte is shown to store oxygen gas and thereby enhance electrochemical signals for oxygen reduction in aqueous media. Oxygen reduction current transient data at platinum disk electrodes suggest that the reactivity of ambient oxygen in aqueous electrolyte (typically Doxygen = 2.8 × 10-9 m2 s-1; coxygen = 0.3 mM) is substantially modified (to approximately Dapp,oxygen = 1.6 (±0.3) × 10-12 m2 s-1; capp,oxygen = 50 (±5) mM) with important implications for triphasic electrode processes. The considerable apparent concentration of oxygen even for ambient oxygen levels is important. Potential applications in oxygen sensing, oxygen storage, oxygen catalysis, or applications associated with other types of gases are discussed.

Enhancing Electrocatalytic Kinetics via Synergy of Co Nanoparticles and Co/Ni-N4-C- and N-Doped Porous Carbon

Transition-metal species embedded in carbon have sparked intense interest in the fields of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). However, improvement of the electrocatalytic kinetics remains a challenge caused by the synergistic assembly. Here, we propose a biochemical strategy to fabricate the Co nanoparticles (NPs) and Co/Ni-N4-C co-embedded N-doped porous carbon (CoNPs&Co/Ni-N4-C@NC) catalysts via constructing the zeolitic imidazolate framework (ZIF)@yeast precursor. The rich amino groups provide the possibility for the anchorage of Co2+/Ni2+ ions as well as the construction of Co/Ni-ZIF@yeast through the yeast cell biomineralization effect. The functional design induces the formation of CoNPs and Co/Ni-N4-C sites in N-doped carbon as well as regulates the porosity for exposing such sites. Synergy of CoNPs, Co/Ni-N4-C, and porous N-doped carbon delivered excellent electrocatalytic kinetics (the ORR Tafel slope of 76.3 mV dec-1 and the OER Tafel slope of 80.4 mV dec-1) and a high voltage of 1.15 V at 10 mA cm-2 for the discharge process in zinc air batteries. It provides an effective strategy to fabricate high-performance catalysts.

Enhancing Interaction between Lanthanum Manganese Cobalt Oxide and Carbon Black through Different Approaches for Primary Zn-Air Batteries

Due to the need for decarbonization in energy generation, it is necessary to develop electrocatalysts for the oxygen reduction reaction (ORR), a key process in energy generation systems such as fuel cells and metal-air batteries. Perovskite-carbon material composites have emerged as active and stable electrocatalysts for the ORR, and the interaction between both components is a crucial aspect for electrocatalytic activity. This work explores different mixing methods for composite preparation, including mortar mixing, ball milling, and hydrothermal and thermal treatments. Hydrothermal treatment combined with ball milling resulted in the most favorable electrocatalytic performance, promoting intimate and extensive contact between the perovskite and carbon material and improving electrocatalytic activity. Employing X-ray photoelectron spectroscopy (XPS), an increase in the number of M-O-C species was observed, indicating enhanced interaction between the perovskite and the carbon material due to the adopted mixing methods. This finding was further corroborated by temperature-programmed reduction (TPR) and temperature-programmed desorption (TPD) techniques. Interestingly, the ball milling method results in similar performance to the hydrothermal method in the zinc-air battery and, thus, is preferable because of the ease and straightforward scalability of the preparation process.

Strategic Design and Insights into Lanthanum and Strontium Perovskite Oxides for Oxygen Reduction and Oxygen Evolution Reactions

Perovskite oxides exhibit bifunctional activity for both oxygen reduction (ORR) and oxygen evolution reactions (OER), making them prime candidates for energy conversion in applications like fuel cells and metal-air batteries. Their intrinsic catalytic prowess, combined with low-cost, abundance, and diversity, positions them as compelling alternatives to noble metal and metal oxides catalysts. This review encapsulates the nuances of perovskite oxide structures and synthesis techniques, providing insight into pivotal active sites that underscore their bifunctional behavior. The focus centers on the breakthroughs surrounding lanthanum (La) and strontium (Sr)-based perovskite oxides, specifically their roles in zinc-air batteries (ZABs). An introduction to the mechanisms of ORR and OER is provided. Moreover, the light is shed on strategies and determinants central to optimizing the bifunctional performance of La and Sr-based perovskite oxides.

Innovative Strategy for Developing PEDOT Composite Scaffold for Reversible Oxygen Reduction Reaction

Metal-air batteries are an emerging technology with great potential to satisfy the demand for energy in high-consumption applications. However, this technology is still in an early stage, facing significant challenges such as a low cycle life that currently limits its practical use. Poly(3,4-ethylenedioxythiophene) (PEDOT) conducting polymer has already demonstrated its efficiency as catalyst for oxygen reduction reaction (ORR) discharge as an alternative to traditional expensive and nonsustainable metal catalysts. Apart from that, in most electrochemical processes, three phenomena are needed: redox activity and electronic and ionic conduction. Material morphology is important to maximize the contact area and optimize the 3 mechanisms to obtain high-performance devices. In this work, porous scaffolds of PEDOT-organic ionic plastic crystal (OIPC) are prepared through vapor phase polymerization to be used as porous self-standing cathodes. The scaffolds, based on abundant elements, showed good thermal stability (200 °C), with potential ORR reversible electrocatalytic activity: 60% of Coulombic efficiency in aqueous medium after 200 cycles.

Isolated Binary Fe-Ni Metal-Nitrogen Sites Anchored on Porous Carbon Nanosheets for Efficient Oxygen Electrocatalysis through High-Temperature Gas-Migration Strategy

Atomically dispersed dual-site catalysts can regulate multiple reaction processes and provide synergistic functions based on diverse molecules and their interfaces. However, how to synthesize and stabilize dual-site single-atom catalysts (DACs) is confronted with challenges. Herein, we report a facile high-temperature gas-migration strategy to synthesize Fe-Ni DACs on nitrogen-doped carbon nanosheets (FeNiSAs/NC). FeNiSAs/NC exhibits a high half-wave potential (0.88 V) for the oxygen reduction reaction (ORR) and a low overpotential of 410 mV at 10 mA cm-2 for the oxygen evolution reaction (OER). As an air electrode for Zn-air batteries (ZABs), it shows better performances in aqueous ZABs and excellent stability and flexibility in solid-state ZABs. The high specific surface area (1687.32 m2/g) of FeNiSAs/NC is conducive to electron transport. Density functional theory (DFT) reveals that the Fe sites are the active center, and Ni sites can significantly optimize the free energy of the oxygen-containing intermediate state on Fe sites, contributing to the improvement of ORR and the corresponding OER activities. This work can provide guidance for the rational design of DACs and understand the structure-activity relationship of SACs with multiple active sites for electrocatalytic energy conversion.

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.

High Catalytic Activity of Co-centered 2D Metal Organic Frameworks toward Bifunctional Oxygen Evolution and Reduction Reactions: Rationalized by Spin Polarization Effect

CoX4 (X = NH, S, and O) motifs have demonstrated their high catalytic activity in the platforms of metal organic frameworks (MOFs), however, the underlying reason is still unrevealed. Herein, we propose monolayers constructed by linking TMNxO4-x motifs (TM = Fe, Co, Ni, Cu) with trioxotriangulenes (TOTs) as suitable models to clarify the structure-property-performance relationship of 2D MOFs for the oxygen evolution/reduction reaction (OER/ORR). The highly robust catalytic activity of CoNxO4-x for both the OER and the ORR has been confirmed, even surpassing that of most previously reported 2D MOFs and SACs. This activity is attributed to the moderate interaction between Co and the key intermediate species, which can be modulated by the coordinating atoms. We reveal spin momentum as a reliable activity descriptor in rationalizing the OER/ORR activity, which can be extended to many other 2D MOFs. The elucidated structure-activity relationship is significant for the development of effective bifunctional OER/ORR electrocatalysts.

Enhancing the electrocatalytic activities of metal organic frameworks for the oxygen evolution reaction with bimetallic groups

Controlling the ratio of metals in bimetallic organic frameworks (MOFs) can not only alter the structures but also tailor the properties of MOFs. Herein, we report a series of electrocatalytically active CoxNiy-based bimetallic MOFs that are synthesized with the 3,5-pyridinedicarboxylic acid (3,5-H2pdc) ligand (where x : y = 20 : 1, 15 : 1, 10 : 1, 5 : 1, 1 : 1, and 1 : 20) and a facile, scalable, low temperature synthetic route. The materials have one-dimensional (1D), rod-like microstructures with different aspect ratios. While they all electrocatalyze the oxygen evolution reaction (OER) in alkaline solution (1 M KOH), their electrocatalytic performances vary substantially depending on their compositions. The CoxNiy-MOF with an optimal ratio of x : y = 15 : 1 (Co15Ni1-MOF) electrocatalyzes the OER with the highest maximum current density (92.2 mA cm-2 at 1.75 V vs. RHE) and the smallest overpotential (384 mV vs. RHE at 10 mA cm-2) in a 1 M KOH solution. It is also stable under constant current application during the electrocatalytic OER. This work demonstrates the application of bimetallic MOFs that are synthesized following a simple, low temperature synthetic route for the OER and their tailorable electrocatalytic properties for the OER by varying the ratio of two metals and the synthetic conditions used to produce them.

A Comprehensive Evaluation of Battery Technologies for High-Energy Aqueous Batteries

Aqueous batteries have garnered significant attention in recent years as a viable alternative to lithium-ion batteries for energy storage, owing to their inherent safety, cost-effectiveness, and environmental sustainability. This study offers a comprehensive review of recent advancements, persistent challenges, and the prospects of aqueous batteries, with a primary focus on energy density compensation of various battery engineering technologies. Additionally, cutting-edge high-energy aqueous battery designs are emphasized as a reference for future endeavors in the pursuit of high-energy storage solutions. Finally, a dual-compatibility battery configuration perspective aimed at concurrently optimizing cycle stability, redox potential, capacity utilization for both anode and cathode materials, as well as the selection of potential electrode candidates, is proposed with the ultimate goal of achieving cell-level energy densities exceeding 400 Wh kg-1 .

Enhanced electrocatalytic oxygen redox reactions of iron oxide nanorod films by combining oxygen vacancy formation and cobalt doping

A synergistic effect of Co-doping and vacuum-annealing on electrochemical redox reactions of iron oxide films is demonstrated in the present work. In this research, a series of defect-rich iron oxy/hydroxide nanorod arrays: α-FeOOH, Fe2O3, and FeOx nanorod thin film catalysts were synthesized via a hydrothermal approach followed by thermal and vacuum treatments. Besides, a cobalt doping process was employed to prepare the thin film of Co-doped FeOx nanorods. The morphology, crystallinity, and electrochemical activities of Co-doped oxygen-deficient FeOx (Co-FeOx/FTO) show strong correlations with metal concentration and thermal treatments. The electrochemical measurements demonstrated that the as-deposited Co-doped FeOx NR catalyst could achieve a maximum OER current of 30 mA cm-2, which was six times greater than that recorded by as-deposited Co-doped FeOOH NR catalysts (5.7 mA cm-2) at 1.65 V vs. RHE, confirming the superior electrocatalytic OER activity at the as-deposited Co-doped FeOx NR catalyst after cobalt doping. It is believed that these results are attributed to two factors: the synergistic effect of Co doping and the defect-rich nature of FeOx nanorod catalysts that are used in sustainable energy systems.

Agarose-gel-based self-limiting synthesis of a bimetal (Fe and Co)-doped composite as a bifunctional catalyst for a zinc-air battery

Exploring efficient noble-metal-free electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is crucial for the development of rechargeable Zn-air batteries. Herein, a self-limiting method using an agarose gel was proposed to prepare bimetallic (iron and cobalt) nitrogen-doped carbon composites (FeCo-NC). The resulting FeCo-NC catalyst has a high surface area and a hierarchical porous structure. The optimized FeCo-NC electrocatalyst exhibits a small potential difference (ΔE) = 0.72 V between the ORR half-wave potential and the OER potential at a current density of 10 mA cm^-2 in alkaline media. Impressively, the FeCo-NC Zn-air battery exhibits a high open-circuit voltage, large power density, and outstanding charge-discharge cycling stability. This study provides an effective means of designing electrocatalysts and energy conversion systems.

Atomically Dispersed Fe/N4 and Ni/N4 Sites on Separate-Sides of Porous Carbon Nanosheets with Janus Structure for Selective Oxygen Electrocatalysis

Dual single atoms catalysts have promising application in bifunctional electrocatalysis due to their synergistic effect. However, how to balance the competition between rate-limiting steps (RDSs) of reversible oxygen reduction and oxygen evolution reaction (OER) and fully expose the active centers by reasonable structure design remain enormous challenges. Herein, Fe/N₄ and Ni/N₄ sites separated on different sides of the carbon nanosheets with Janus structure (FeNi_jns /NC) is synthesized by layer-by-layer assembly method. Experiments and calculations reveal that the side of Fe/N₄ is beneficial to oxygen reduction reaction (ORR) and the Ni/N₄ side is preferred to OER. Such Janus structure can take full advantage of two separate-sides of carbon nanosheets and balance the competition of RDSs during ORR and OER. FeNi_jns /NC possesses superior ORR and OER activity with ORR half-wave potential of 0.92 V and OER overpotential of 440 mV at J = 10 mA cm^-2 . Benefiting from the excellent bifunctional activities, FeNi_jns /NC assembled aqueous Zn-air battery (ZAB) demonstrates better maximum power density, and long-term stability (140 h) than Pt/C+RuO₂ catalyst. It also reveals superior flexibility and stability in solid-state ZAB. This work brings a novel perspective for rational design and understanding of the catalytic mechanisms of dual single atom catalysts.

Hydrogel-Derived Co3ZnC/Co Nanoparticles with Heterojunctions Supported on N-Doped Porous Carbon and Carbon Nanotubes for the Highly Efficient Oxygen Reduction Reaction in Zn-Air Batteries

It is crucial for metal-air batteries and fuel cells to design non-precious-metal catalysts instead of platinum-based materials to boost the sluggish oxygen reduction reaction (ORR). Herein, Co₃ZnC/Co nanoparticles with heterojunctions supported on N-doped porous carbon and carbon nanotubes (CNTs) are fabricated by pyrolyzing the hydrogel prepared from melamine and citric acid chelated with Co²⁺/Zn²⁺ ions. This hybrid shows strong ORR catalytic activity as its half-wave potential reaches 0.88 V (vs reversible hydrogen electrode (RHE)) in 0.1 M KOH and Zn-air batteries with the catalyst have higher discharge plateaus and capacity than those employing Pt/C. The hybrid mixed with RuO₂ can also be used as an efficient bifunctional catalyst for rechargeable Zn-air batteries. The excellent performance is primarily derived from the Co₃ZnC/Co heterojunctions, the electron transfer of which boosts the ORR catalysis. Moreover, the suitable ratio of Co/Zn in precursors results in the epitaxial growth of hollow CNTs and abundant mesopores, hence promoting the adsorption of oxygen and the transport of ORR-related species.

Recent Progress of Diatomic Catalysts: General Design Fundamentals and Diversified Catalytic Applications

In recent years, some experiments and theoretical work have pointed out that diatomic catalysts not only retain the advantages of monoatomic catalysts, but also introduce a variety of interactions, which exceed the theoretical limit of catalytic performance and can be applied to many catalytic fields. Here, the interaction between adjacent metal atoms in diatomic catalysts is elaborated: synergistic effect, spacing enhancement effect (geometric effect), and electronic effect. With regard to the classification and characterization of various new diatomic catalysts, diatomic catalysts are classified into four categories: heteronuclear/homonuclear, with/without carbon carriers, and their characterization measures are introduced and explained in detail. In the aspect of preparation of diatomic catalysts, the widely used atomic layer deposition method, metal-organic framework derivative method, and simple ball milling method are introduced, with emphasis on the formation mechanism of diatomic catalysts. Finally, the effective control strategies of four diatomic catalysts and the key applications of diatomic catalysts in electrocatalysis, photocatalysis, thermal catalysis, and other catalytic fields are given.

Boosting the activity and stability via synergistic catalysis of Co nanoparticles and MoC to construct a bifunctional electrocatalyst for high-performance and long-life rechargeable zinc-air batteries

The high overpotential of the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) leading to slow air cathode kinetics is still a major challenge for zinc-air batteries (ZABs), hindering the commercialization of ZABs. With the advantages of cost-effectiveness and feasibility of synthesis at room temperature, zeolite imidazole frameworks (ZIFs) are regarded as advanced precursors. But a majority of ZIF-derived catalysts show only one catalytic activity, which limits their performance in ZABs as well as the cycling stability. In addition, molybdenum carbide (MoC) is recognized as an excellent candidate for renewable energy conversion due to its good chemical resistance and thermal stability. Herein, we report a ZIF-67-derived Co/MoC-NC multiphase doped carbon bifunctional ORR/OER catalyst with multiple active sites for the cathode of ZABs. The synergistic catalysis of Co nanoparticles and MoC nanoparticles in Co/MoC-NC which are embedded in a thin layer of N-doped graphitic carbon and immobilized on N-doped graphitic carbon, respectively, demonstrates superior ORR catalytic performance and durability both under alkaline and acidic conditions (E_1/2 = 0.87 V in 1.0 M KOH and E_1/2 = 0.76 V in 0.5 M H₂SO₄). Simultaneously, Co/MoC-NC also exhibits favorable OER performance (10 mA cm^-2, η = 320 mV) in 1 M KOH. Furthermore, a remarkable peak-power density of 215.36 mW cm^-2 and great cycling stability could be achieved while applying Co/MoC-NC in the cathode of ZABs (over 300 h). This work will provide a viable design concept for designing and synthesizing multifunctional catalysts to construct rechargeable ZABs.

Constructing atomically-dispersed Mn on ZIF-derived nitrogen-doped carbon for boosting oxygen reduction

Exploring durable and highly-active non-noble-metal nanomaterials to supersede Pt-based nanomaterials is an effective way, which can reduce the cost and boost the catalytic efficiency of oxygen reduction reaction (ORR). Herein, we constructed atomically-dispersed Mn atoms on the ZIF-derived nitrogen-doped carbon frameworks (Mn-N_x/NC) by stepwise pyrolysis. The Mn-N_x/NC relative to pure nitrogen-doped carbon (NC) exhibited superior electrocatalytic activity with a higher half-wave potential (E_1/2 = 0.88 V) and a modest Tafel slope (90 mV dec⁻¹) toward ORR. The enhanced ORR performance of Mn-N_x/NC may be attributed to the existence of Mn-N_x active sites, which can more easily adsorb intermediates, promoting the efficiency of ORR. This work provides a facile route to synthesize single-atom catalysts for ORR.

A High-Energy-Density Magnesium-Air Battery with Nanostructured Polymeric Electrodes

The greenhouse emissions are biggest challenge of the present era. The renewable power sources are required to have characteristics of good charge capacity, energy density with proven charging discharging cycles for energy storage and applications. Mg-air batteries (MABs) are an alternative renewable power source due to their inexpensive cost. In particular, the previous reports presented the metal-air battery structure, with a specific energy overall output of 765 W h kg⁻¹. This paper is focused mainly on the MAB, which employed nanocomposite polymeric electrodes with a proven energy density of 545 W h kg⁻¹ and a charge capacity of 817 mA h g⁻¹ when electrolyzed at a cycling current density of 7 mA cm⁻².

Facile synthesis of multiphase cobalt-iron spinel with enriched oxygen vacancies as a bifunctional oxygen electrocatalyst

The multiphase cobalt-iron spinel was firstly synthesized via a facile cold plasma method and applied as a bifunctional electrocatalyst for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Compared with the single-phase obtained by the traditional calcination method, the CoFe₂O₄ and Co₃O₄ phase were obtained by the plasma method. The multivalence states of cobalt and iron facilitated electron transport in electrochemical reactions. The plasma sample had a small particle size (5 nm) due to the low operation temperature. Notably, electron impact produced more oxygen vacancies and a larger surface area on Co_xFe_yO₄, which increased the active sites and electronic conductivity. Electrochemical investigations indicated that the multiphase spinel obtained with a quasi-four-electron transfer process showed an onset potential of 0.76 V versus the RHE for the oxygen reduction reaction. In the oxygen evolution reaction, the potential of current density at 10 mA cm^-2 was 1.53 V versus RHE. As for the overall electrocatalytic activity, the multiphase spinel had a ΔE (the difference between E₁₀(OER) and E_1/2(ORR)) of 0.89 V, exhibiting greater bifunctional activity than the other prepared catalysts.

Defect-rich Fe-doped NiS/MoS2 heterostructured ultrathin nanosheets for efficient overall water splitting

With the demand for efficient hydrogen/oxygen evolution reaction (HER/OER) bifunctional electrocatalysts, defect-rich two-dimensional (2D) heterostructured materials attract increasing attention due to abundant active sites and facile mass/charge transfer. However, precise manipulation of lattice defects in a 2D heterostructured material is still a challenge. Herein, through pyrolytic sulfurization of a layered Fe-doped Ni/Mo MOF precursor, a series of defect-rich Fe-doped NiS/MoS₂ ultrathin nanosheets were obtained. For 0.1Fe-NiS/MoS₂, abundant lattice defects induced by Fe atoms provide more water adsorption sites, and intimate interface between NiS and MoS₂ can optimize the adsorption energy of a HER/OER intermediate. As a result, both HER and OER activities are significantly enhanced. The respective overpotential is 120 mV and 297 mV for the HER and OER. Small Tafel slopes of 69.0 mV dec^-1 and 54.7 mV dec^-1 indicate favorable electrochemical reaction kinetics. The catalytic performance of this material can be compared with those of 20% Pt/C and RuO₂ catalysts and top-rated MoS₂-based materials. For overall water splitting, only 1.66 V voltage is required to deliver 10 mA cm^-2. Long-term stability of 0.1Fe-NiS/MoS₂ presents a prospect for its practical application.

Heteroatom-Doped Carbon-Encapsulated FeP Nanostructure: A Multifunctional Electrocatalyst for Zinc-Air Battery and Water Electrolyzer

The rational design and synthesis of efficient multifunctional electrocatalysts for renewable energy technologies is of significant interest. Herein, we demonstrate a novel approach for the synthesis of a nitrogen and phosphorus dual-doped mesoporous carbon-encapsulated iron phosphide (FeP@NPC) nanostructure and its multifunctional electrocatalytic activity toward an oxygen reduction reaction, oxygen evolution reaction, and hydrogen evolution reaction for zinc-air battery (ZAB) and alkaline water-splitting applications. FeP@NPC is obtained by the carbothermal reduction of the precursor complex [Fe(bpy)₃](PF₆)₂ in the presence of melamine without any traditional phosphidating agent. The PF₆^- counteranion is used for the phosphidation of Fe. FeP@NPC obtained at 900 °C (FeP@NPC-900) exhibits excellent bifunctional oxygen electrocatalytic performance with a very low potential gap (ΔE = E_1/2^ORR - E_j10^OER) of 670 mV. The ZAB device delivers a peak power density of 190.15 mW cm^-2 (iR-corrected), specific capacity of 785 mA h g_Zn^-1, and energy density of 706.5 Wh kg_Zn^-1 at 50 mA cm^-2. The ZAB exhibits excellent charge-discharge cycling stability for over 35 h with negligible voltaic efficiency loss (0.9%). Three CR2032 coin-cell-based ZABs made of an FeP@NPC-900 air cathode connected in series power 81 LEDs for 15 min. FeP@NPC-900 also has promising electrocatalytic activity toward water splitting in acidic as well as in alkaline pH. The benchmark current density of 10 mA cm^-2 is achieved with a two-electrode alkaline water electrolyzer at a cell voltage of 1.65 V. ZAB-powered water electrolyzer is made by integrating two rechargeable ZABs connected in series with the two-electrode water electrolyzer. The ZAB powers the electrolyzer for 24 h without a significant loss in the open-circuit voltage. The catalyst retains its initial structural integrity even after continuous water electrolysis for 24 h.

Transition Metal and N Doping on AlP Monolayers for Bifunctional Oxygen Electrocatalysts: Density Functional Theory Study Assisted by Machine Learning Description

It is vital to search for highly efficient bifunctional oxygen evolution/reduction reaction (OER/ORR) electrocatalysts for sustainable and renewable clean energy. Herein, we propose a single transition-metal (TM)-based defective AlP system to validate bifunctional oxygen electrocatalysis by using the density functional theory (DFT) method. We found that the catalytic activity is enhanced by substituting two P atoms with two N atoms in the Al vacancy of the TM-anchored AlP monolayer. Specifically, the overpotential of OER(ORR) in Co- and Ni-based defective AlP systems is found to be 0.38 (0.25 V) and 0.23 V (0.39 V), respectively, showing excellent bifunctional catalytic performance. The results are further presented by establishing the volcano plots and contour maps according to the scaling relation of the Gibbs free-energy change of *OH, *O, and *OOH intermediates. The d-band center and the product of the number of d-orbital electrons and electronegativity of the TM atom are the ideal descriptors for this system. To investigate the activity origin of the OER/ORR process, we performed the machine learning (ML) algorithm. The result indicates that the number of TM-d electrons (N_e), the radius of TM atoms (r_d), and the charge transfer of TM atoms (Q_e) are the three primary descriptors characterizing the adsorption behavior. Our results can provide a theoretical guidance for designing highly efficient bifunctional electrocatalysts and pave a way for the DFT-ML hybrid method in catalysis research.

Metal-Free Pyrene-Based Conjugated Microporous Polymer Catalyst Bearing N- and S-Sites for Photoelectrochemical Oxygen Evolution Reaction

The development of an efficient, sustainable, and inexpensive metal-free catalyst for oxygen evolution reaction (OER) via photoelectrochemical water splitting is very demanding for energy conversion processes such as green fuel generators, fuel cells, and metal-air batteries. Herein, we have developed a metal-free pyrene-based nitrogen and sulfur containing conjugated microporous polymer having a high Brunauer-Emmett-Teller surface area (761 m² g⁻¹) and a low bandgap of 2.09 eV for oxygen evolution reaction (OER) in alkaline solution. The π-conjugated as-synthesized porous organic material (PBTDZ) has been characterized by Fourier transform infrared spectroscopy (FT-IR), solid-state ¹³C (cross-polarization magic angle spinning-nuclear magnetic resonance) CP-MAS NMR, N₂ adsorption/desorption analysis, field-emission scanning electron microscope (FESEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA) experiments. The material acts as an efficient catalyst for photoelectrochemical OER with a current density of 80 mA/cm² at 0.8 V vs. Ag/AgCl and delivered 104 µmol of oxygen in a 2 h run. The presence of low bandgap energy, π-conjugated conducting polymeric skeleton bearing donor heteroatoms (N and S), and higher specific surface area associated with inherent microporosity are responsible for this admirable photoelectrocatalytic activity of PBTDZ catalyst.

In-situ construction of 3D hetero-structured sulfur-doped nanoflower-like FeNi LDH decorated with NiCo Prussian blue analogue cubes as efficient electrocatalysts for boosting oxygen evolution reaction

At present, it is urgent for us to develop non-noble metal-based catalysts with abundant reserves and high efficiency towards oxygen evolution reaction (OER) in water electrolysis devices. Herein, cubic NiCo-Prussian blue analogue (PBA)/ flower-like FeNi layered double hydroxide (LDH) heterostructure was facilely in-situ formed on porous nickel foam (NF) via hydrothermal strategy coupled by subsequent sulfurizing treatment (named as S-FeNi LDH@PBA/NF), showing largely facilitated electron transfer over homogeneous counterpart. Also, we investigated the effects of different Fe/Ni feeding ratios on their catalytic properties in some detail. The as-prepared S-FeNi LDH@PBA/NF demonstrated the superior OER activity (e.g. only 243 mV of overpotential required for 50 mA cm^-2) and stability. Accordingly, using the catalyst as anode, the home-assembled S-FeNi LDH@PBA/NF//Pt/C/NF electrolyzer exhibited small Tafel slope (83.1 mV dec^-1) and ultra-stability, showing the potential feasibility in practical water electrolysis. This strategy provides a hopeful model to enhance the OER performance by effectively constructing advanced catalyst with promising heterostructure and optimal electronic structure.

Electronically modulated nickel boron by CeOx doping as a highly efficient electrocatalyst towards overall water splitting

Exploiting economic, efficient and durable non-noble metal electrocatalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is promising, but still faces enormous challenges. Herein, the strategy of doping a metal boride with a rare earth metal oxide has been explored to develop a highly efficient bifunctional electrocatalyst. The novel electrocatalyst CeO_x-NiB consists of CeO_x-doped NiB supported on nickel foam, and was fabricated by a one-step mild electroless plating reaction. Remarkably, the CeO_x-NiB@NF electrode delivers a current density of 10 mA cm^-2 at overpotentials of only 19 mV and 274 mV for the HER and OER, respectively. Two-electrode electrolyzers with the CeO_x-NiB@NF electrode require only 1.424 V to deliver 10 mA cm^-2 for overall water splitting in 1.0 M KOH, outperforming the Pt-C/NF∥IrO₂/NF electrolyzer. Meanwhile, the electrode also has good stability (can work for 100 hours at 10 mA cm^-2) and industrial-grade current density. This work provides a new idea for the development of efficient and durable non-precious metal catalysts.

Ultrasonic synthesis of Mn-Ni-Fe tri-metallic oxide anchored on polymer-grafted conductive carbon for rechargeable zinc-air battery

As a promising electrochemical energy device, a rechargeable zinc-air battery (RZAB) requires cost-effective cathode catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Some earth-abundant transition metal oxides have certain levels of bi-functional ORR/OER catalytic activities yet low electronic conductivities. The addition of high-electronic-conductivity material such as carbon black could result in another problem because there is low compatibility between metal oxide and carbon. In this work, polymer chains are ultrasonically prepared to act as binders to anchor metal-oxide active sites to porous domains of carbon black. The monomer N-isopropyl acrylamide is polymerized under ultrasonication instead of using conventional radical initiators which are dangerous and harmful. Reactive free radicals produced by ultrasonic irradiation can also help to form the Mn-Ni-Fe tri-metallic oxide. Thus, aided by the amide-type polymer as an adhesive, the tri-metallic oxide anchored on polymer-grafted carbon black prepared by ultrasonication possess a large number of metal-oxide active sites and hierarchical pores, contributing substantially to the enhanced ORR/OER electrocatalytic performance in the RZABs. Accordingly, this work provides interesting insight into the effective combination of inherently incompatible components for the fabrication of composite materials from an ultrasonic standpoint.

Atomically dispersed Pd catalysts promote the oxygen evolution reaction in acidic media

A Pd-doped Pt₃Sn-based single atom alloy catalyst (Pd-Pt₃Sn) was synthesized via a hydrothermal method. The overpotential of Pd-Pt₃Sn is lower than that of commercial Pd/C and IrO₂ catalysts at 10 mA cm^-2. This is due to the synergistic effect between Pt, Sn and Pd and the influence of electronic effects on their catalytic performance.

Pyrolysis-Free Synthesized Catalyst towards Acidic Oxygen Reduction by Deprotonation

Acidic oxygen reduction is vital for renewable energy devices such as fuel cells. However, many aspects of the catalytic process are still uncertain-especially the large difference in activity in acidic and alkaline media. Thus, the design and synthesis of model catalysts to determine the active centers and the inactivation mechanism are urgently needed. We report a pyrolysis-free synthesis route to fabricate a catalyst (CPF-Fe@NG) for oxygen reduction in acidic conditions. By introducing a deprotonation process, we extended the oxygen reduction reaction (ORR) activity from alkaline to acidic conditions. CPF-Fe@NG demonstrated outstanding performance with a half-wave potential of 853 mV (vs. RHE) and good stability after 10000 cycles in 1 M HClO₄ . The pyrolysis-free route could also be used to assemble fuel cells, with a maximum power density of 126 mW cm^-2 . Our findings offer new insights into the ORR process to optimize catalysts for both mechanistic studies and practical applications.

Electrochemical degradation of chloramphenicol using Ti-based SnO2-Sb-Ni electrode

Antibiotic residues may be very harmful in aquatic environments, because of limited treatment efficiency of traditional treatment methods. An electrochemical system with a Ti-based SnO₂-Sb-Ni anode was developed to degrade a typical antibiotic chloramphenicol (CAP) in water. The electrode was prepared using a sol-gel method. The performance of electrode materials, impact factors and dynamic characteristics were evaluated. The Ti-based SnO₂-Sb-Ni electrode was compact and uniform as shown by characterization using SEM and XRD. The electrocatalytic oxidation of CAP was carried out in a single-chamber reactor by using a Ti-based SnO₂-Sb-Ni electrode. For 100 mg L^-1 CAP, the CAP removal ratio of 100% and the TOC removal ratio of 60% were obtained at the current density of 20 mA cm^-2 and in a neutral electrolyte at 300 min. Kinetic investigation has shown that the electro-oxidation of CAP on a Ti-based SnO₂-Sb-Ni electrode displayed a pseudo-first-order kinetic model. Free radical quenching experiments presented that the oxidation of CAP on Ti-based SnO₂-Sb-Ni electrode resulted from the synergistic effect of direct oxidation and indirect oxidation (·OH and ·SO₄^-). Doping Ni on the Ti/SnO₂-Sb electrode for CAP degradation was presented in this paper, showing its great application potential in the area of antibiotic and halogenated organic pollutant degradation.

Hierarchically porous Fe,N-doped carbon nanorods derived from 1D Fe-doped MOFs as highly efficient oxygen reduction electrocatalysts in both alkaline and acidic media

Rationally designing low-cost yet highly efficient electrocatalysts for the oxygen reduction reaction (ORR) in both alkaline and acidic media remains highly challenging. Herein, we report the facile synthesis of Fe,N-doped carbon nanorods (denoted as Fe-N/C-NR) with abundant hierarchical pores and highly active sites by the pyrolysis of a one-dimensional (1D) Fe-doped zeolitic imidazolate framework (Fe-ZIF-8) as a self-sacrificing template. The unique 1D nanoarchitecture of the resultant Fe-N/C-NR can provide fast electron and electrolyte transport towards exposed active sites, and their hierarchically porous structures with large surface areas can efficiently facilitate mass diffusion and increase the density of exposed active sites. Furthermore, it is demonstrated that the coexistence of highly dispersed Fe-Nx sites and Fe3C/Fe nanoparticles (NPs) in these electrocatalysts can provide a large number of desired catalytic centers with highly intrinsic activity and structural stability. As a result, the optimized 5Fe-N/C-NR exhibits excellent catalytic activity for the ORR, with a high half-wave potential (E1/2) of 0.90 V vs. RHE in alkaline medium, superior to that of commercial Pt/C (0.86 V vs. RHE), and also a high E1/2 of 0.81 V vs. RHE in acidic medium, comparable to that of commercial Pt/C (0.81 V vs. RHE). Moreover, its robust ORR durability can far surpass that of commercial Pt/C in both acidic and alkaline media, further highlighting the merit of this MOF-templated strategy. Our findings might shed light on the rational design of cost-effective and highly efficient ORR electrocatalysts for practical applications.