MnS-BaS Heterostructures as Effective Catalysts for Oxygen Reduction Reaction

The inadequate electrical conductivity of metal sulfides, along with their tendency to agglomerate, has hindered their use in energy storage and catalysis. The construction of a heterojunction can ameliorate these deficiencies to some extent. In this paper, MnS-BaS heterojunction catalysts were prepared by a hydrothermal method, which is a simple and inexpensive process. The MnS-BaS heterojunction catalysts exhibited superior performance owing to the strong synergistic interaction between MnS and BaS. Density functional theory (DFT) calculations reveal strong interactions at the heterojunction interface and significant electron transfer between MnS and BaS, which further modulates the electronic structure of Mn. The elevation of the center of the d-band enhances the adsorption of oxygen and oxygen-containing intermediates on the catalyst, thus promoting the oxygen reduction reaction (ORR). The practical application of MnS-BaS catalysts was tested by assembling zinc-air batteries. This study provides a rational strategy for designing transition metal catalysts that are efficient and low cost.

Highly Active CoNi-CoN3 Composite Sites Synergistically Accelerate Oxygen Electrode Reactions in Rechargeable Zinc-Air Batteries

Reaching rapid reaction kinetics of oxygen reduction (ORR) and oxygen evolution reactions (OER) is critical for realizing efficient rechargeable zinc-air batteries (ZABs). Herein, a novel CoNi-CoN3 composite site containing CoNi alloyed nanoparticles and CoN3 moieties is first constructed in N-doped carbon nanosheet matrix (CoNi-CoN3 /C). Benefiting from the high electroactivity of CoNi-CoN3 composite sites and large surface area, CoNi-CoN3 /C shows a superior half-wave potential (0.88 V versus RHE) for ORR and a small overpotential (360 mV) for OER at 10 mA cm-2 . Theoretical calculations have demonstrated that the introduction of CoNi alloys has modulated the electronic distributions near the CoN3 moiety, inducing the d-band center of CoNi-CoN3 composite site to shift down, thus stabilizing the valence state of Co active sites and balancing the adsorption of OER/ORR intermediates. Accordingly, the reaction energy trends exhibit optimized overpotentials for OER/ORR, leading to superior battery performances. For aqueous and flexible quasi-solid-state rechargeable ZABs with CoNi-CoN3 /C as catalyst, a large power density (250 mW cm-2 ) and high specific capacity (804 mAh g-1 ) are achieved. The in-depth understanding of the electroactivity enhancement mechanism of interactive metal nanoparticles and metal coordinated with nitrogen (MNx ) moieties is crucial for designing novel high-performance metal/nitrogen-doped carbon (M─N─C) catalysts.

Extending the Cycle Lifetime of Solid-State Zinc-Air Batteries by Arranging Stable Zinc Species Channels

The solid-state zinc-air batteries have attracted extensive attention due to their high theoretical energy density, high safety, and the compact structure. In this work, a novel hydrogel solid-state electrolyte was developed that was equipped with an interpenetrating network of zinc polyacrylate (PAZn) and polyacrylamide (PAM). At the same time, a cyclodextrin derivative with sulfonate groups was introduced as an additive. From the design of anionic groups in the network, effective and stable channels for zinc species have been established. The unique structure of the additives regulates the uniform deposition of zinc. After using this solid-state electrolyte, the cycle lifetime of solid-state zinc-air batteries assembled have been significantly extended. The byproducts were greatly suppressed and generated the smooth zinc electrode surface after the charge-discharge cycling.

Toward a Metal Anode-Free Zinc-Air Battery for Next-Generation Energy Storage

Rechargeable aqueous zinc-air batteries (ZABs) promise high energy density and safety. However, the use of conventional zinc anodes affects the energy output from the battery, so that the theoretical energy density is not achievable under operation conditions. A large portion of the zinc is shielded by anode passivation during the discharge process and remains electrochemically unused, making the operation of rechargeable ZABs inefficient up to date. In a metal anode-free ZAB, there is no unnecessary excess zinc if the zinc reservoir can be precisely adjusted by electrodeposition of zinc from the electrolyte. In this respect, an anode-free battery uses the electrolyte offering a dual-mode functionality not only providing ionic conductivity but also being the source of zinc. In addition, it is shown that a defined porous anode architecture is crucial for high rechargeability in this new type of ZAB. 3D-spatially arranged carbon nanotubes as geometrically defined host structures allow a homogeneous zinc deposition from the electrolyte. Together with carbon nanohorns as an active 2e- catalyst on the cathode side, the rechargeability of this new concept reaches up to 92%.

External Field-Responsive Ternary Non-Noble Metal Oxygen Electrocatalyst for Rechargeable Zinc-Air Batteries

Despite the increasing effort in advancing oxygen electrocatalysts for zinc-air batteries (ZABs), the performance development gradually reaches a plateau via only ameliorating the electrocatalyst materials. Herein, a new class of external field-responsive electrocatalyst comprising Ni0.5 Mn0.5 Fe2 O4 stably dispersed on N-doped Ketjenblack (Ni0.5 Mn0.5 Fe2 O4 /N-KB) is developed via polymer-assisted strategy for practical ZABs. Briefly, the activity indicator ΔE is significantly decreased to 0.618 V upon photothermal assistance, far exceeding most reported electrocatalysts (generally >0.680 V). As a result, the photothermal electrocatalyst possesses comprehensive merits of excellent power density (319 mW cm-2 ), ultralong lifespan (5163 cycles at 25 mA cm-2 ), and outstanding rate performance (100 mA cm-2 ) for liquid ZABs, and superb temperature and deformation adaptability for flexible ZABs. Such improvement is attributed to the photothermal-heating-enabled synergy of promoted electrical conductivity, reactant-molecule motion, active area, and surface reconstruction, as revealed by operando Raman and simulation. The findings open vast possibilities toward more-energy-efficient energy applications.

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.

Mesoporous Carbon for High-Performance Near-Neutral Zinc-Air Batteries

Near-neutral zinc-air batteries (ZABs) have garnered significant research interest due to their high energy density, exceptional electrochemical reversibility, and adaptability to ambient air. However, these batteries suffer from substantial electrochemical polarization, low energy efficiency, and poor rate performance. In this study, a mesoporous carbon (meso-C) with a high specific surface area (1081 m2 g-1 ) and abundant porous structure for the cathode of near-neutral ZABs using a scalable synthesis method is prepared. The meso-C-based cathode is endowed with stable hydrophobicity and abundant electrochemical active sites, which considerably improve the energy efficiency, rate performance, and cycle life of the battery compare to commercial carbon black-based cathode when applied to near-neutral ZABs with 1 mol kg-1 (1 m) zinc acetate and 1 m zinc trifluoromethanesulfonate electrolytes. Additionally, the mesopores of meso-C facilitate the construction of better three-phase reaction interfaces and contribute to better electrochemical reversibility. The work presents a general and scalable approach for carbon materials in the cathode of near-neutral ZABs.

Flash Nitrogen-Doped Carbon Nanotubes for Energy Storage and Conversion

In recent years, nitrogen-doped carbons show great application potentials in the fields of electrochemical energy storage and conversion. Here, the ultrafast and green preparation of nitrogen-doped carbon nanotubes (N-CNTs) via an efficient flash Joule heating method is reported. The precursor of 1D core-shell structure of CNT@polyaniline is first synthesized using an in situ polymerization method and then rapidly conversed into N-CNTs at ≈1300 K within 1 s. Electrochemical tests reveal the desirable capacitive property and oxygen catalytic activity of the optimized N-CNT material. It delivers an improved area capacitance of 101.7 mF cm-2 at 5 mV s-1 in 1 m KOH electrolyte, and the assembled symmetrical supercapacitor shows an energy density of 1.03 µWh cm-2 and excellent cycle stability over 10 000 cycles. In addition, the flash N-CNTs exhibit impressive catalytic performance toward oxygen reduction reaction with a half-wave potential of 0.8 V in alkaline medium, comparable to the sample prepared by the conventional long-time pyrolysis method. The Zn-air battery presents superior charge-discharge ability and long-term durability relative to commercial Pt/C catalyst. These remarkable electrochemical performances validate the superiorities of the Joule heating method in preparing the heteroatom-doped carbon materials for wide applications.

Recent Advancements of Graphene-Based Materials for Zinc-Based Batteries: Beyond Lithium-Ion Batteries

Graphene-based materials (GBMs) possess a unique set of properties including tunable interlayer channels, high specific surface area, and good electrical conductivity characteristics, making it a promising material of choice for making electrode in rechargeable batteries. Lithium-ion batteries (LIBs) currently dominate the commercial rechargeable battery market, but their further development has been hampered by limited lithium resources, high lithium costs, and organic electrolyte safety concerns. From the performance, safety, and cost aspects, zinc-based rechargeable batteries have become a promising alternative of rechargeable batteries. This review highlights recent advancements and development of a variety of graphene derivative-based materials and its composites, with a focus on their potential applications in rechargeable batteries such as LIBs, zinc-air batteries (ZABs), zinc-ion batteries (ZIBs), and zinc-iodine batteries (Zn-I2 Bs). Finally, there is an outlook on the challenges and future directions of this great potential research field.

In situ Construction of Multistage Core-Shell Nanostructure as Bifunctional Catalyst for Ultrastable Zinc-Air Batteries

Many efforts have been devoted to obtaining excellent cathode catalysts for Zinc air batteries (ZABs), but the inevitable use of binder will damage the catalytic activity and weaken long-term stability, inefficient mass transfer of oxygen is also chargable for the limited activity. Herein, in situ grown hydrogen substituted graphdiyne (HGDY) on carbon paper has been prepared and used as cathode catalyst layer in ZABs. Multiple catalytic sites are firmly combined and end with the boosted bifunctional catalytic activity of oxygen reduction and oxygen evolution. Moreover, the specific surface area, sufficient active sites, multilevel pore nanostructure and robust conductivity are fully exposed to establish efficient catalytic interface and skeleton. Cu/Co nanoparticles are uniformly distributed and warped by HGDY network, which can stably exist during the catalytic process. As a result, a current density of 18.75 mA cm-2 and a Tafel slope of 61.06 mV dec-1 for oxygen reduction and a ultralong operation for more than 2300 h in aqueous ZAB have been achieved, which is beyond many reported bifunctional catalysts in ZAB system.

Hydrophobic and Homogeneous Conductive Carbon Matrix for High-Rate Non-Alkaline Zinc-Air Batteries

Non-alkaline zinc-air batteries (ZABs) that use reversible O2 /ZnO2 chemistry exhibit excellent stability and superior reversibility compared to conventional alkaline ZABs. Unlike alkaline ZABs, ZnO2 discharge products are generated on the surface of the air cathodes in non-alkaline ZABs, requiring more gas-liquid-solid three-phase reaction interfaces. However, the kinetics of reported ZABs based on carbon black (CB) is far from satisfactory due to the insufficient reaction areas. The rational structural design of the air cathode is an effective way to increase active surfaces to further enhance the performance of non-alkaline ZABs. In this study, multi-walled carbon nanotubes (MW-CNTs) with unique mesoporous structures and high pore volumes are selected to replace CB in the air cathode preparation. Due to the larger electrochemically active surface area, superior hydrophobicity, and uniform electroconductibility of MW-CNTs-based cathodes, primary ZABs exhibit high specific capacity (704 mAh gZn-1 ) with a Zn utilization ratio of 85.85% at 1.0 mA cm-2 , excellent discharge rate performance, and negligible self-discharge. Furthermore, rechargeable ZABs also demonstrate outstanding rate capability and excellent cycling stability at various current densities. This work provides a fundamental understanding of the criteria for the cathode design of non-alkaline ZABs, thus opening a new pathway for more sustainable ZABs.

A Versatile Extended Stöber Approach to Monodisperse Sub-40 nm Carbon Nanospheres for Stabilizing Atomically Dispersed Fe─N4 Sites Toward Efficient Oxygen Reduction Electrocatalysis

The development of atomically dispersed iron-nitrogen-carbon (Fe─N─C) catalysts as an alternative to precious platinum holds great potential for the substantial progress of a variety of oxygen reduction reaction (ORR)-associated energy conversion technologies. Nevertheless, the precise synthesis of Fe─N─C single atomic catalysts (SACs) with a high density of accessible active sites and pronounced electrocatalytic performance still remains an enormous challenge. Herein, an innovative extended Stöber method is designed for the controllable preparation of monodisperse small-sized N-doped carbon colloidal nanospheres (≈40 nm) anchoring atomically isolated Fe─N4 sites (abbreviated as Fe-SA@N-CNSs hereafter) with a narrow size distribution and high uniformity. Benefiting from the single Fe─N4 moieties and the unique spherical carbon substrate, the resultant Fe-SA@N-CNSs exhibit excellent ORR activity, outstanding long-term durability, and methanol tolerance in KOH electrolyte. More impressively, when further assembled into a flexible solid-state rechargeable zinc-air battery (ZAB), the Fe-SA@N-CNSs-driven ZAB delivers a higher open circuit voltage, a larger power density, and robust cycling/mechanical stability, outperforming the state-of-the-art Pt/C-based counterpart and further testifying the great potential of the as-prepared Fe-SA@N-CNSs in diverse ORR-related practical energy devices. The developed extended Stöber method provides an efficient and versatile avenue toward the preparation of a series of well-defined SACs for diverse electrocatalytic systems.

A 3d-4d-5d High Entropy Alloy as a Bifunctional Oxygen Catalyst for Robust Aqueous Zinc-Air Batteries

High entropy alloys (HEAs) are highly suitable candidate catalysts for oxygen evolution and reduction reactions (OER/ORR) as they offer numerous parameters for optimizing the electronic structure and catalytic sites. Herein, FeCoNiMoW HEA nanoparticles are synthesized using a solution-based low-temperature approach. Such FeCoNiMoW nanoparticles show high entropy properties, subtle lattice distortions, and modulated electronic structure, leading to superior OER performance with an overpotential of 233 mV at 10 mA cm-2 and 276 mV at 100 mA cm-2 . Density functional theory calculations reveal the electronic structures of the FeCoNiMoW active sites with an optimized d-band center position that enables suitable adsorption of OOH* intermediates and reduces the Gibbs free energy barrier in the OER process. Aqueous zinc-air batteries (ZABs) based on this HEA demonstrate a high open circuit potential of 1.59 V, a peak power density of 116.9 mW cm-2 , a specific capacity of 857 mAh gZn -1 , and excellent stability for over 660 h of continuous charge-discharge cycles. Flexible and solid ZABs are also assembled and tested, displaying excellent charge-discharge performance at different bending angles. This work shows the significance of 4d/5d metal-modulated electronic structure and optimized adsorption ability to improve the performance of OER/ORR, ZABs, and beyond.

Advanced Architectures of Air Electrodes in Zinc-Air Batteries and Hydrogen Fuel Cells

The air electrode is an essential component of air-demanding energy storage/conversion devices, such as zinc-air batteries (ZABs) and hydrogen fuel cells (HFCs), which determines the output power and stability of the devices. Despite atom-level modulation in catalyst design being recently achieved, the air electrodes have received much less attention, causing a stagnation in the development of air-demanding equipment. Herein, the evolution of air electrodes for ZABs and HFCs from the early stages to current requirements is reviewed. In addition, the operation mechanism and the corresponding electrocatalytic mechanisms of ZABs are summarized. In particular, by clarifying the air electrode interfaces of ZABs at different scales, several approaches to improve the air electrode in rechargeable ZABs are reviewed, including innovative electrode structures and bifunctional oxygen catalysts. Afterward, the operating mechanisms of proton-exchange-membrane fuel cells (PEMFCs) and anion-exchange-membrane fuel cells (AEMFCs) are explained. Subsequently, the strategies employed to enhance the efficiency of the membrane electrode assembly (MEA) in PEMFCs and AEMFCs, respectively, are highlighted and discussed in detail. Last, the prospects for air electrodes in ZABs and HFCs are considered by discussing the main challenges. The aim of this review is to facilitate the industrialization of ZABs and HFCs.

Highly-Exposed Co-CoO Derived from Nanosized ZIF-67 on N-Doped Porous Carbon Foam as Efficient Electrocatalyst for Zinc-Air Battery

Non-precious-metal based electrocatalysts with highly-exposed and well-dispersed active sites are crucially needed to achieve superior electrocatalytic performance for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) toward zinc-air battery (ZAB). Herein, Co-CoO heterostructures derived from nanosized ZIF-67 are densely-exposed and strongly-immobilized onto N-doped porous carbon foam (NPCF) through a self-sacrificial pyrolysis strategy. Benefited from the high exposure of Co-CoO heterostructures and the favorable mass and electron transfer ability of NPCF, the Co-CoO/NPCF electrocatalyst exhibits remarkable performance for both ORR (E1/2  = 0.843 V vs RHE) and OER (Ej = 10 mA cm-2  = 1.586 V vs RHE). Further application of Co-CoO/NPCF as the air-cathode in rechargeable ZAB achieves superior performance for liquid-state ZAB (214.1 mW cm-2 and 600 cycles) and flexible all-solid-state ZAB (93.1 mW cm-2 and 140 cycles). Results from DFT calculations demonstrate that the electronic metal-support interactions between Co-CoO and NPCF via abundant C-Nx sites is favorable for electronic structure modulation, accounting for the remarkable performance.

Spontaneous Internal Electric Field in Heterojunction Boosts Bifunctional Oxygen Electrocatalysts for Zinc-Air Batteries: Theory, Experiment, and Application

Heterojunctions are a promising class of materials for high-efficiency bifunctional oxygen electrocatalysts in both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). However, the conventional theories fail to explain why many catalysts behave differently in ORR and OER, despite a reversible path (* O2 ⇋* OOH⇋* O⇋* OH). This study proposes the electron-/hole-rich catalytic center theory (e/h-CCT) to supplement the existing theories, it suggests that the Fermi level of catalysts determines the direction of electron transfer, which affects the direction of the oxidation/reduction reaction, and the density of states (DOS) near the Fermi level determines the accessibility for injecting electrons and holes. Additionally, heterojunctions with different Fermi levels form electron-/hole-rich catalytic centers near the Fermi levels to promote ORR/OER, respectively. To verify the universality of the e/h-CCT theory, this study reveals the randomly synthesized heterostructural Fe3 N-FeN0.0324 (Fex N@PC with DFT calculations and electrochemical tests. The results show that the heterostructural F3 N-FeN0.0324 facilitates the catalytic activities for ORR and OER simultaneously by forming an internal electron-/hole-rich interface. The rechargeable ZABs with Fex N@PC cathode display a high open circuit potential of 1.504 V, high power density of 223.67 mW cm-2 , high specific capacity of 766.20 mAh g-1 at 5 mA cm-2 , and excellent stability for over 300 h.

Bifunctional Single Atom Catalysts for Rechargeable Zinc-Air Batteries: From Dynamic Mechanism to Rational Design

Ever-growing demands for rechargeable zinc-air batteries (ZABs) call for efficient bifunctional electrocatalysts. Among various electrocatalysts, single atom catalysts (SACs) have received increasing attention due to the merits of high atom utilization, structural tunability, and remarkable activity. Rational design of bifunctional SACs relies heavily on an in-depth understanding of reaction mechanisms, especially dynamic evolution under electrochemical conditions. This requires a systematic study in dynamic mechanisms to replace current trial and error modes. Herein, fundamental understanding of dynamic oxygen reduction reaction and oxygen evolution reaction mechanisms for SACs is first presented combining in situ and/or operando characterizations and theoretical calculations. By highlighting structure-performance relationships, rational regulation strategies are particularly proposed to facilitate the design of efficient bifunctional SACs. Furthermore, future perspectives and challenges are discussed. This review provides a thorough understanding of dynamic mechanisms and regulation strategies for bifunctional SACs, which are expected to pave the avenue for exploring optimum single atom bifunctional oxygen catalysts and effective ZABs.

Palladium Enhanced Iron Active Site - An Efficient Dual-Atom Catalyst for Oxygen Electroreduction

Metal-nitrogen-carbon (M-C/N) electrocatalysts have been shown to have satisfactory catalytic activity and long-term durability for the oxygen reduction reaction (ORR). Here, a strategy to prepare a new electrocatalyst (Fe&Pd-C/N) using a unique metal-containing ionic liquid (IL) is exploited, in which Fe & Pd ions are positively charged species atomically dispersed by coordination to the N of the N-doped C substrate, C/N. X-ray absorption fine structure, XPS and aberration-corrected transmission electron microscopy results verified a well-defined dual-atom configuration comprising Fe+2.x -N4 coupled Pd2+ -N4 sites and well-defined spatial distribution. Electronic control of a coupled Fe-Pd structure produces an electrocatalyst that exhibits superior performance with enhanced activity and durability for the ORR compared to that of commercial Pt/C (20%, Johnson Matthey) in both alkaline and acid media. Density functional theory calculations indicate that Pd atom can enhance the catalytic activity of the Fe active sites adjacent to Pd sites by changing the electronic orbital structure and Bader charge of the Fe centers. The excellent catalytic performance of the Fe&Pd-C/N electrocatalyst is demonstrated in zinc-air batteries and hydrogen-air fuel cells.

Tailoring Oxygen Reduction Reaction Kinetics on Perovskite Oxides via Oxygen Vacancies for Low-Temperature and Knittable Zinc-Air Batteries

High kinetics oxygen reduction reaction (ORR) electrocatalysts under low temperature are critical and highly desired for temperature-tolerant energy conversion and storage devices, but remain insufficiently investigated. Herein, oxygen vacancy-rich porous perovskite oxide (CaMnO3 ) nanofibers coated with reduced graphene oxide coating (V-CMO/rGO) are developed as the air electrode catalyst for low-temperature and knittable Zn-air batteries. V-CMO/rGO exhibits top-level ORR activity among perovskite oxides and shows impressive kinetics under low temperature. Experimental and theoretical calculation results reveal that the synergistic effect between metal atoms and oxygen vacancies, as well as the accelerated kinetics and enhanced electric conductivity and mass transfer over the rGO coated nanofiber 3D network contribute to the enhanced catalytic activity. The desorption of ORR intermediate is promoted by the regulated electron filling. The V-CMO/rGO drives knittable and flexible Zn-air batteries under a low temperature of -40 °C with high peak power density of 56 mW cm-2 and long cycle life of over 80 h. This study provides insight of kinetically active catalyst and facilitates the ZABs application in harsh environment.

Electrocatalysts for Zinc-Air Batteries Featuring Single Molybdenum Atoms in a Nitrogen-Doped Carbon Framework

Bifunctional catalysts can facilitate two different electrochemical reactions with conflicting characteristics. Here, a highly reversible bifunctional electrocatalyst for rechargeable zinc-air batteries (ZABs) is reported featuring a "core-shell structure" in which N-doped graphene sheets wrap around vanadium molybdenum oxynitride nanoparticles. Single Mo atoms are released from the particle core during synthesis and anchored to electronegative N-dopant species in the graphitic shell. The resultant Mo single-atom catalysts excel as active oxygen evolution reaction (OER) sites in pyrrolic-N and as active oxygen reduction reaction (ORR) sites in pyridinic-N environments. ZABs with such bifunctional and multicomponent single-atom catalysts deliver high power density (≈376.4 mW cm-2 ) and long cycle life of over 630 h, outperforming noble-metal-based benchmarks. Flexible ZABs that can tolerate a wide range of temperatures (-20 to 80 °C) under severe mechanical deformation are also demonstrated.

Universal Synthesis of Half-Metallic Diatomic Catalysts for Efficient Oxygen Reduction Electrocatalysis

Developing efficient and low-cost noble-free metal electrocatalysts is an urgent requirement. Herein, a one-step, solid-state template-assisted method for fabricating isolated half-metallic diatomic M, Zn─N─C (M═Fe, Co, and Ni) catalysts is reported. In particular, the fabricated Fe, Zn─N─C structure exhibits superior oxygen reduction reaction capabilities with a half-wave potential of 0.867 V versus RHE. The Mossbauer spectra reveal that the Fe, Zn─N─C half-metallic diatomic catalyst has a large proportion of the D2 site (ferrous iron with a medium spin state). Density functional theory (DFT) reveals that in Fe, Zn─N─C structures, the zinc sites play a unique role in accelerating the protonation process of O2 in ORR. In assembled zinc-air batteries, a maximum power density of 138 mW cm-2 and a capacity of 748 mAh g zn-1 can be obtained. This work fabricates a series of efficient M, Zn─N─C diatomic electrocatalysts, and the developed solid-state reaction method can hopefully apply in other energy conversion and storage fields.

MXenes for Zinc-Based Electrochemical Energy Storage Devices

As an economical and safer alternative to lithium, zinc (Zn) is promising for realizing new high-performance electrochemical energy storage devices, such as Zn-ion batteries, Zn-ion hybrid capacitors, and Zn-air batteries. Well-designed electrodes are needed to enable efficient Zn electrochemistry for energy storage. Two-dimensional transition metal carbides and nitrides (MXenes) are emerging materials with unique electrical, mechanical, and electrochemical properties and versatile surface chemistry. They are potential material candidates for constructing high-performance electrodes of Zn-based energy storage devices. This review first briefly introduces the working mechanisms of the three Zn-based energy storage devices. Then, the recent progress on the synthesis, chemical functionalization, and structural design of MXene-based electrodes is summarized. Their performance in Zn-based devices is analyzed to establish relations between material properties, electrode structures, and device performance. Last, several research topics are proposed to be addressed for developing practical MXene-based electrodes for Zn-based energy storage devices to enable their commercialization and broad adoption in the near future.

Strain-Regulated Pt-NiO@Ni Sub-Micron Particles Achieving Bifunctional Electrocatalysis for Zinc-Air Battery

Highly active bifunctional electrocatalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) have always been the key factors to affect the performance of zinc-air batteries. However, integrating the independent reaction sites of ORR and OER in a catalyst remains a major challenge. Herein, a collaborative strategy based on defect induction and doping is proposed to prepare the strain-regulated Pt-NiO@Ni sub-micron particles (Pt-NiO@Ni SP). Benefiting from the synergistic effect of tensile strain and Pt-doped, the metallic Ni-based sub-micron particles with tensile strain as the catalyst carriers can effectively optimize the electronic distribution of atomic structures in Pt and NiO on the surface of particles, leading to reduce the energy barrier of intermediates for ORR and OER. Consequently, the Pt-NiO@Ni SP exhibits outstanding bifunctional catalytic activity with the ΔE index of 0.65 V under a low Pt loading, outperforming that of the benchmark Pt/C+IrO2 catalysts (0.76 V). Impressively, the Pt-NiO@Ni SP-based liquid zinc-air battery develops a high open-circuit potential (1.47 V), excellent energy density (188.2 mW cm-2 ), and favorable cyclic charge-discharge cycling durability (200 h at 20 mA cm-2 ). This work provides an innovative avenue for the rational construction of highly active bifunctional electrocatalysts for practical applications.

Regulating the Coordination Geometry and Oxidation State of Single-Atom Fe Sites for Enhanced Oxygen Reduction Electrocatalysis

FeNC catalysts demonstrate remarkable activity and stability for the oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells and Zn-air batteries (ZABs). The local coordination of Fe single atoms in FeNC catalysts strongly impacts ORR activity. Herein, FeNC catalysts containing Fe single atoms sites with FeN₃ , FeN₄ , and FeN₅ coordinations are synthesized by carbonization of Fe-rich polypyrrole precursors. The FeN₅ sites possess a higher Fe oxidation state (+2.62) than the FeN₃ (+2.23) and FeN₄ (+2.47) sites, and higher ORR activity. Density functional theory calculations verify that the FeN₅ coordination optimizes the adsorption and desorption of ORR intermediates, dramatically lowering the energy barrier for OH^- desorption in the rate-limiting ORR step. A primary ZAB constructed using the FeNC catalyst with FeN₅ sites demonstrates state-of-the-art performance (an open circuit potential of 1.629 V, power density of 159 mW cm^-2 ). Results confirm an intimate structure-activity relationship between Fe coordination, Fe oxidation state, and ORR activity in FeNC catalysts.

Developing Outstanding Bifunctional Electrocatalysts for Rechargeable Zn-Air Batteries Using High-Purity Spinel-Type ZnCo2 Se4 Nanoparticles

Zinc-air batteries are gaining popularity as viable energy sources for green energy storage technologies. The cost and performance of Zn-air batteries are mostly determined by the air electrodes in combination with an oxygen electrocatalyst. This research aims at the particular innovations and challenges relating to air electrodes and related materials. Here, a nanocomposite of ZnCo₂ Se₄ @rGO that exhibits excellent electrocatalytic activity for the oxygen reduction reaction, ORR (E_1/2  = 0.802 V), and oxygen evolution reaction, OER (η₁₀  = 298 mV@10 mA cm^-2 ) is synthesized. In addition, a rechargeable zinc-air battery with ZnCo₂ Se₄ @rGO as the cathode showed a high open circuit voltage (OCV) of 1.38 V, a peak power density of 210.4 mW cm^-2 , and outstanding long-term cycling stability. The electronic structure and oxygen reduction/evolution reaction mechanism of the catalysts ZnCo₂ Se₄ and Co₃ Se₄ are further investigated using density functional theory calculations. Finally, a perspective for designing, preparing, and assembling air electrodes is suggested for the future developments of high-performance Zn-air batteries.

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

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

Pomegranate-Like FeNC with Optimized FeN4 Configuration as Bi-Functional Catalysts for Rechargeable Zinc-Air Batteries

Catalysts with FeNC moieties have demonstrated remarkable activity toward oxygen reduction reaction (ORR), but precise synthesis and configuration regulation of FeNC to achieve bi-functional catalytic sites for ORR and oxygen evolution reaction (OER) remain a great challenge. Herein, a pomegranate-like catalyst with optimized FeN₄ configuration is designed. The unique framework affords a large surface area for sufficient active site exposure and abundant macroporous channels for mass transport. By twisting chemical bonds, the electronic structure of FeN₄ is regulated, and the adsorption/desorption of oxygen species is facilitated. Compared to noble metal-based catalysts (Pt/C+IrO₂ ), the optimized FeNC exhibits impressive onset potential (0.96 V versus reversible hydrogen electrode), larger limiting current density (5.85 mA cm^-2 ), and better long-term life for ORR, as well as, lower OER overpotential. When integrated into Zn-air batteries, it demonstrates a respectable peak power density (71.6 mW cm^-2 ) and ideal cycling stability (30 h), exceeding that of commercial Pt/C+IrO₂ . The exploration offers a guideline for designing advanced bi-functional electrocatalysts.

Geometric and Electronic Engineering of Atomically Dispersed Copper-Cobalt Diatomic Sites for Synergistic Promotion of Bifunctional Oxygen Electrocatalysis in Zinc-Air Batteries

The development of rechargeable zinc-air batteries is heavily dependent on bifunctional oxygen electrocatalysts to offer exceptional oxygen reduction/evolution reaction (ORR/OER) activities. However, the design of such electrocatalysts with high activity and durability is challenging. Herein, a strategy is proposed to create an electrocatalyst comprised of copper-cobalt diatomic sites on a highly porous nitrogen-doped carbon matrix (Cu-Co/NC) with abundantly accessible metal sites and optimal geometric and electronic structures. Experimental findings and theoretical calculations demonstrate that the synergistic effect of Cu-Co dual-metal sites with metal-N₄ coordination induce asymmetric charge distributions with moderate adsorption/desorption behavior with oxygen intermediates. This electrocatalyst exhibits extraordinary bifunctional oxygen electrocatalytic activities in alkaline media, with a half-wave potential of 0.92 V for ORR and a low overpotential of 335 mV at 10 mA cm^-2 for OER. In addition, it demonstrates exceptional ORR activity in acidic (0.85 V) and neutral (0.74 V) media. When applied to a zinc-air battery, it achieves extraordinary operational performance and outstanding durability (510 h), ranking it as one of the most efficient bifunctional electrocatalysts reported to date. This work demonstrates the importance of geometric and electronic engineering of isolated dual-metal sites for boosting bifunctional electrocatalytic activity in electrochemical energy devices.

Synergistic Bimetallic CoCu-Codecorated Carbon Nanosheet Arrays as Integrated Bifunctional Cathodes for High-Performance Rechargeable/Flexible Zinc-Air Batteries

The unremitting exploration of well-architectured and high-efficiency oxygen electrocatalysts is promising to speed up the surface-mediated oxygen reduction/evolution reaction (ORR/OER) kinetics of rechargeable zinc-air batteries (ZABs). Herein, bimetallic CoCu-codecorated carbon nanosheet arrays (CoCu/N-CNS) are proposed as self-supported bifunctional oxygen catalysts. The integrated catalysts are in situ constructed via a simple sacrificial-templated strategy, imparting CoCu/N-CNS with 3D interconnected conductive pathways, abundant mesopores for electrolyte penetration and ion diffusion, as well as Cu-synergized Co-N_x /O reactive sites for improved catalytic activities. By incorporating a moderate amount of Cu into CoCu/N-CNS, the bifunctional activities can be further increased due to synergistic oxygen electrocatalysis. Consequently, the optimized CoCu/N-CNS realizes a low overall overpotential of 0.64 V for OER and ORR and leads to high-performance liquid ZABs with high gravimetric energy (879.7 Wh kg^-1 ), high peak power density (104.3 mW cm^-2 ), and remarkable cyclic stability upon 400 h/1000 cycles at 10 mA cm^-2 . More impressively, all-solid-state flexible ZABs assembled with the CoCu/N-CNS cathode exhibit superior rate performance and exceptional mechanical flexibility under arbitrary bending conditions. This CoCu/N-CNS monolith holds significant potential in advancing cation-modulated multimetallic electrocatalysts and multifunctional nanocatalysts.

A Fluorinated Covalent Organic Framework with Accelerated Oxygen Transfer Nanochannels for High-Performance Zinc-Air Batteries

The establishment of abundant three-phase interfaces with accelerated mass transfer in air cathodes is highly desirable for the development of high-rate and long-cycling rechargeable zinc-air batteries (ZABs). Covalent organic frameworks (COFs) exhibit tailored nanopore structures, facilitating the rational tuning of their specific properties. Here, by finely tuning the fluorinated nanopores of a COF, a novel air cathode for rechargeable ZABs is unprecedentedly designed and synthesized. COF nanosheets are decorated with fluorinated alkyl chains, which shows high affinity to oxygen (O₂ ), in its nanopores (fluorinated COF). The fluorinated COF nanosheets are stacked into well-defined O₂ -transport channels, which are then assembled into aerophilic "nano-islands" on the hydrophilic FeNi layered-double-hydroxide (FeNi LDH) electrocatalyst surface. Therefore, the mass-transport "highway" for O₂ and water is segregated on the nanoscale, which significantly enlarges the area of three-phase boundaries and greatly promotes the mass-transfer therein. ZABs based on the COF-modified air cathode deliver a small charge/discharge voltage gap (0.64 V at 5 mA cm^-2 ), a peak power density (118 mW cm^-2 ), and a stable cyclability. This work provides a feasible approach for the design of the air cathodes for high-performance ZABs, and will expand the new application of COFs.

Regulating Reversible Oxygen Electrocatalysis by Built-in Electric Field of Heterojunction Electrocatalyst with Modified d-Band

Developing bifunctional catalysts for oxygen electrochemical reactions is essential for high-performance electrochemical energy devices. Here, a Mott-Schottky heterojunction composed of porous cobalt-nitrogen-carbon (Co-N-C) polyhedra containing abundant metal-phosphides for reversible oxygen electrocatalysis is reported. As a demonstration, this catalyst shows excellent activity in the oxygen electrocatalysis and thus delivers outstanding performance in rechargeable zinc-air batteries (ZABs). The built-in electric field in the Mott-Schottky heterojunction can promote electron transfer in oxygen electrocatalysis. More importantly, an appropriate d-band center of the heterojunction catalyst also endows oxygen intermediates with a balanced adsorption/desorption capability, thus enhancing oxygen electrocatalysis and consequently improving the performance of ZABs. The work demonstrates an important design principle for preparing efficient multifunctional catalysts in energy conversion technologies.

Altering Ligand Microenvironment of Atomically Dispersed CrN4 by Axial Ligand Sulfur for Enhanced Oxygen Reduction Reaction in Alkaline and Acidic Medium

Because of the instability and Fenton reactivity of non-precious metal nitrogen-carbon based catalyst when processing the oxygen reduction reaction (ORR), seeking for electrocatalysts with highly efficient performance becomes very highly desired to speed up the commercialization of fuel cell. Herein, chromium (Cr)-N₄  electrocatalyst containing extraterrestrial S formed axial S₁ -Cr₁ N₄  bonds (S₁ Cr₁ N₄ C) is achieved via an assembly polymerization and confined pyrolysis strategy. Benefiting from the adjusting  coordination configuration and electronic structure of the metal center through axial coordination, S₁ Cr₁ N₄ C exhibits enhanced the intrinsic activity (half-wave potential (E_1/2 ) is 0.90 V versus reversable hydrogen electrode, RHE) compared with that of CrN₄ C and Pt/C catalysts. More notably, the catalyst is almost inert in catalyzing the Fenton reaction, and thus shows the high stability. Density functional theory (DFT) results further reveal that the existence of axial S atoms in S₁ Cr₁ N₄ C moiety has the better ORR activity than Cr₁ N₄ C moieties. The axial S ligand in S₁ Cr₁ N₄ C moiety can break the electron localization around the planar Cr₁ N₄  active center, which facilitated the rate-limiting reductive release of OH* and accelerated overall ORR process. The present work opens up a new avenue to modulate the axial ligand type of the single-atoms (SAs) active center to enhance intrinsic SAs performances.

High Durability of Fe-N-C Single-Atom Catalysts with Carbon Vacancies toward the Oxygen Reduction Reaction in Alkaline Media

Single-atom catalysts (SACs) have attracted extensive interest to catalyze the oxygen reduction reaction (ORR) in fuel cells and metal-air batteries. However, the development of SACs with high selectivity and long-term stability is a great challenge. In this work, carbon vacancy modified Fe-N-C SACs (Fe_H -N-C) are practically designed and synthesized through microenvironment modulation, achieving high-efficient utilization of active sites and optimization of electronic structures. The Fe_H -N-C catalyst exhibits a half-wave potential (E_1/2 ) of 0.91 V and sufficient durability of 100 000 voltage cycles with 29 mV E_1/2 loss. Density functional theory (DFT) calculations confirm that the vacancies around metal-N₄ sites can reduce the adsorption free energy of OH*, and hinder the dissolution of metal center, significantly enhancing the ORR kinetics and stability. Accordingly, Fe_H -N-C SACs presented a high-power density and long-term stability over 1200 h in rechargeable zinc-air batteries (ZABs). This work will not only guide for developing highly active and stable SACs through rational modulation of metal-N₄ sites, but also provide an insight into the optimization of the electronic structure to boost electrocatalytical performances.

High-Activity Fe3 C as pH-Universal Electrocatalyst for Boosting Oxygen Reduction Reaction and Zinc-Air Battery

Transition metal catalysts are regarded as one of promising alternatives to replace traditional Pt-based catalysts for oxygen reduction reaction (ORR). In this work, an efficient ORR catalyst is synthesized by confining Fe₃ C nanoparticles into N, S co-doped porous carbon nanosheets (Fe₃ C/N,S-CNS) via high-temperature pyrolysis, in which 5-sulfosalicylic acid (SSA) demonstrates as an ideal complexing agent for iron (ΙΙΙ) acetylacetonate while g-C₃ N₄ behaves as a nitrogen source. The influence of the pyrolysis temperature on the ORR performance is strictly examined in the controlled experiments. The obtained catalyst exhibits excellent ORR performance (E_1/2  = 0.86 V; E_onset  = 0.98 V) in alkaline electrolyte, coupled by exhibiting the superior catalytic activity and stability (E_1/2  = 0.83 V, E_onset  = 0.95 V) to Pt/C in acidic media. In parallel, its ORR mechanism is carefully illustrated by the density functional theory (DFT) calculations, especially the role of the incorporated Fe₃ C played in the catalytic process. The catalyst-assembled Zn-air battery also exhibits a much higher power density (163 mW cm^-2 ) and ultralong cyclic stability in the charge-discharge test for 750 h with a gap increase down to 20 mV. This study provides some constructive insights for preparation of advanced ORR catalysts in green energy conversion units correlated systems.

Waxberry-like MnS/Ni3S4 as High-efficiency Bi-functional Catalyst for Zn-air Batteries

In this paper, waxberry-liked MnS/Ni3S4 composite catalyst was designed and synthesized. In this core-shell structure, MnS is located inside and Ni3S4 is wrapped on the surface of MnS to form a spherical structure. This structure makes the material show excellent stability in the electrocatalytic process. The diffusion staggered region structure formed at the two-phase interface greatly enhances the synergistic interaction between MnS and Ni3S4. At the same time, the defects and vacancies formed by the diffusion mechanism at the interface of the two phases increase the active site and improve the interfacial electron transfer kinetics. Therefore, MnS/Ni3S4 composites showed excellent catalytic performance for ORR/OER. At 10 mA cm-2, the overpotential of it is only 320 mV, and the half-wave potential can reach 0.81 V. The catalyst also exhibited extraordinary cycle stability and small voltage gap when used as cathode of Zn-air batteries. When the current density is 3 mA cm-2, the cyclic discharge can be stable for 400h and the voltage difference between the front and back does not increase more than. When the current density increases to 5 mA cm-2, the cyclic charge and discharge can be stable for more than 300 h.

Optimizing d-Orbital Electronic Configuration via Metal-Metal Oxide Core-Shell Charge Donation for Boosting Reversible Oxygen Electrocatalysis

Tuning the d-orbital electronic configuration of active sites to achieve well-optimized adsorption strength of oxygen-containing intermediates toward reversible oxygen electrocatalysis is desirable for efficient rechargeable Zn-Air batteries but extremely challenging. Herein, this work proposes to construct a Co@Co₃ O₄ core-shell structure to regulate the d-orbital electronic configuration of Co₃ O₄ for the enhanced bifunctional oxygen electrocatalysis. Theoretical calculations first evidence that electron donation from Co core to Co₃ O₄ shell could downshift the d-band center and simultaneously weak spin state of Co₃ O₄ , result in the well-optimized adsorption strength of oxygen-containing intermediates on Co₃ O₄ , thus contributing a favor way for oxygen reduction/evolution reaction (ORR/OER) bifunctional catalysis. As a proof-of-concept, the Co@Co₃ O₄ embedded in Co, N co-doped porous carbon derived from thickness controlled 2D metal-organic-framework is designed to realize the structure of computational prediction and further improve the performance. The optimized 15Co@Co₃ O₄ /PNC catalyst exhibits the superior bifunctional oxygen electrocatalytic activity with a small potential gap of 0.69 V and a peak power density of 158.5 mW cm^-2 in ZABs. Moreover, DFT calculations shows that the more oxygen vacancies on Co₃ O₄ contribute too strong adsorption of oxygen intermediates which limit the bifunctional electrocatalysis, while electron donation in the core-shell structure can alleviate the negative effect and maintain superior bifunctional overpotential.

Engineering the Electronic Structure of Single-Atom Iron Sites with Boosted Oxygen Bifunctional Activity for Zinc-Air Batteries

Rechargeable zinc-air batteries typically require efficient, durable, and inexpensive bifunctional electrocatalysts to support oxygen reduction/evolution reactions (ORR/OER). However, sluggish kinetics and mass transportation challenges must be addressed if the performance of these catalysts is to be enhanced. Herein, a strategy to fabricate a catalyst comprising atomically dispersed iron atoms supported on a mesoporous nitrogen-doped carbon support (Fe SAs/NC) with accessible metal sites and optimized electronic metal-support interactions is developed. Both the experimental results and theoretical calculations reveal that the engineered electronic structures of the metal active sites can regulate the charge distribution of Fe centers to optimize the adsorption/desorption of oxygenated intermediates. The Fe SAs/NC containing Fe₁ N₄ O₁ sites achieves remarkable ORR activity over the entire pH range, with half-wave potentials of 0.93, 0.83, and 0.75 V (vs reversible hydrogen electrode) in alkaline, acidic, and neutral electrolytes, respectively. In addition, it demonstrates a promising low overpotential of 320 mV at 10 mA cm^-2 for OER in alkaline conditions. The zinc-air battery assembled with Fe SAs/NC exhibits superior performance than that of Pt/C+RuO₂ counterpart in terms of peak power density, specific capacity, and cycling stability. These findings demonstrate the importance of the electronic structure engineering of metal sites in directing catalytic activity.

Tailored Heterojunction Active Sites for Oxygen Electrocatalyst Promotion in Zinc-Air Batteries

Rechargeable zinc-air batteries (ZABs) are promising energy storage systems due to their low-cost and safety. However, the working principle of ZABs is based on oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), which display sluggish kinetic and low stability. Herein, this work proposes a novel method to design a heterogeneous CoP/CoO electrocatalyst on mesopore nanobox carbon/carbon nanotube (CoP/CoO@MNC-CNT) that enriched active sites and synergistic effect. Moreover, the well-defined heterointerfaces could lower the energy barrier for intermediate species adsorption and promote OER and ORR electrochemical performances. The CoP/CoO@MNC-CNT electrocatalyst presents a high half-wave potential of 0.838 V for ORR and a small overpotential of 270 mV for OER. The ZABs-based CoP/CoO@MNC-CNT air-cathode shows an open-circuit voltage of 1.409 V, the long-term cycle life of 500 h with a small voltage difference change of 7.7%. Additionally, the flexible ZABs exhibit highly mechanical stability, demonstrating their application potential in wearable electronic devices.

In situ Self-Catalyzed Growth of Manganese-Embedded 3D Flakes-Coated Carbon Rod as an Efficient Oxygen-Reduction Reaction Catalyst of Zinc-Air Batteries

The in situ self-catalyzed growth of manganese-embedded 3D flakes-coated carbon rods (GFC) as an efficient oxygen-reduction reaction (ORR) catalyst of Zinc-air batteries is described for the first time. By optimizing the amount of Mn in the precursor, a series of 3D graphene-like flakes-coated carbon rods were synthesized. GFC with a doping amount of Mn of 10 % (GFC-10) exhibits excellent ORR performance with an onset potential of 0.94 V (vs. reversible hydrogen electrode). The Zinc-air battery is constructed with GFC-10 as the cathode catalyst, and it exhibits a peak power density of 128.9 mW cm^-2 and a cycling stability of 75 h at a current density of 10 mA cm^-2 , which are superior to the commercial 20 wt% Pt/C-based Zinc-air battery. Interestingly, the introduction of Mn facilitates the self-catalyzed growth of carbon rods, and the change of Mn amount can effectively regulate the morphology of materials. The improved ORR performance of the catalyst is ascribed to the synergistic effect of unique hierarchical porous structure, high-charge transport capacity, abundant carbon defects/edges and Mn-N_x sites. This research provides a new avenue to fabricating highly active Mn-based electrocatalysts for renewable energy systems.

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

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

All-in-One Process for Mass Production of Membrane-Type Carbon Aerogel Electrodes for Solid-State Rechargeable Zinc-Air Batteries

This study presents a mass-production process for conductive carbon membrane-type sponge electrodes derived from recyclable cellulose biowaste. It includes an all-in-one hydrogel fabrication process for mass production, which significantly shortens the complex and expensive process for the conventional process of catalytic electrodes based on conductive supporting substrates such as the gas diffusion layer (GDL). The presence of pre-adsorbed melamine powder in the all-in-one hydrogel induces internal diffusion of the gaseous reactant for the uniform growth of carbon nanotubes (CNTs) onto the sponge-like porous carbon aerogel with a relatively thick and tortuous pore structure, thereby providing the electrochemical properties and mechanical strength simultaneously required for the air electrodes of rechargeable and quasi solid-state zinc-air batteries.

Dealloying-Derived Porous Spinel Oxide for Bifunctional Oxygen Electrocatalysis and Rechargeable Zinc-Air Batteries: Promotion of Activity Via Hereditary Al-Doping

The large-scale fabrication of highly efficient and low-cost bifunctional catalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is critical to the development of rechargeable zinc-air batteries (ZABs). Herein, a scalable dealloying strategy was proposed to obtain hierarchically porous spinel-type oxide with minor hereditary Al doping. Benefiting from the well-structured porosity and native dopant, O-np-Ni₅ Co₁₀ (Al), namely Al-NiCo₂ O₄ , exhibited excellent electrocatalytic ORR and OER activities, giving a small potential gap of 0.71 V. The rechargeable ZAB with O-np-Ni₅ Co₁₀ (Al) as cathode catalyst delivered a high specific capacity of 757 mAh g^-1 , a competitive peak power density of 142 mW cm^-2 , and a long-term discharge-charge cycling stability. Furthermore, density functional theory calculations evidenced that appropriate Al doping into NiCo₂ O₄ could significantly reduce the Gibbs free energy difference to 1.71 eV. This work is expected to inspire the design of performance-oriented bifunctional electrocatalysts for wider applications in renewable energy systems.

Modulation of Ligand Fields in a Single-Atom Site by the Molten Salt Strategy for Enhanced Oxygen Bifunctional Activity for Zinc-Air Batteries

Achieving full utilization of active sites and optimization of the electronic structure of metal centers is the key to improving the intrinsic activity of single-atom catalysts (SACs) but still remains a challenge to date. Herein, a versatile molten salt-assisted pyrolysis strategy was developed to construct ultrathin, porous carbon nanosheets supported Co SACs. Molten salts are capable of inducing the formation of a Co single-atom and porous graphene-like carbon, which facilitates full exposure of the active center and simultaneously endows the Co SACs with abundant defective Co-N₄ configurations. The reported Co SACs deliver an excellent bifunctional activity and good stability for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Moreover, metal-air batteries (MABs) assembled with the Co SACs as air electrode also deliver excellent performance with high power densities of 160 mW·cm^-2, large capacities of 760 mAh·g^-1, and superior long-term charge/discharge stability, outperforming those of commercial Pt/C+RuO₂. DFT theoretical calculation results show that the defects in the second coordination shell (CS) of Co SACs promote desorption of the OH* intermediate for the ORR and facilitate deprotonation of OH* for the OER, which can serve as the favorable active site for oxygen bifunctional catalysts. Our work provides an efficient strategy for the preparation of SACs with fully exposed active centers and optimized electronic structures.

Engineering Amorphous/Crystalline Structure of Manganese Oxide for Superior Oxygen Catalytic Performance in Rechargeable Zinc-Air Batteries

Although amorphous materials are popular in oxygen electrocatalysis, their performance requires further improvement to meet the need for rechargeable zinc-air batteries. In this work, an amorphous/crystalline layered manganese oxide (ACMO) was designed, and its unique amorphous/crystalline homogeneous structure activated its oxygen reduction activity with a positive half-wave potential of 0.81 V and oxygen evolution activity with a moderate overpotential of 407 mV at 10 mA cm^-2 . Moreover, the amorphous/crystalline structure endowed ACMO with excellent stability. While employed as the air-electrode material for rechargeable zinc-air batteries, ACMO overcame the poor cycling stability of manganese oxide and cycled stably for 1000 cycles (≈17 days) at 10 mA cm^-2 . Besides, it delivered a high power density of 159.7 mW cm^-2 and a narrow voltage gap of 0.66 V. This work gives an insight into designing oxide materials with amorphous/crystalline structure and feasible guidance for harmonizing electrochemical activity and stability.

Wood-Derived Monolithic Catalysts with the Ability of Activating Water Molecules for Oxygen Electrocatalysis

Oxygen reduction reaction (ORR) is the key reaction on cathode of rechargeable zinc-air batteries (ZABs). However, the lack of protons in alkaline conditions limits the rate of ORR. Herein, an activating water strategy is proposed to promote oxygen electrocatalytic activity by enhancing the proton production from water dissociation. FeP nanoparticles (NPs) are coupled on N-doped wood-derived catalytically active carbon (FeP-NWCC) to associate bifunctional active sites. In alkaline, FeP-NWCC possesses outstanding catalytic activities toward ORR (E_1/2  = 0.86 V) and Oxygen evolution reaction (OER) (overpotential is 310 mV at 10 mA cm^-2 ). The liquid ZABs assembled by FeP-NWCC deliver superior peak power density (144 mW cm^-2 ) and cycle stability (over 450 h). The quasi-solid-state ZABs based on FeP-NWCC also display excellent performances. Theoretical calculation illustrates that the superb bifunctional performance of FeP-NWCC results from the elevated dissociation efficiency of water via FeP NPs to assist the oxygen catalytic process. The strategy of activating water provides a new perspective for the design of ORR/OER bifunctional catalysts. This work is a model for the application of forest biomass.

Boosting the Electrocatalytic Oxygen Evolution of Perovskite LaCo1- x Fex O3 by the Construction of Yolk-Shell Nanostructures and Electronic Modulation

Realizing the rational design of perovskite oxides with controllable compositions and nanostructures remains a tremendous challenge for the development of efficient electrocatalysts. Herein, a ligand-assisted synthetic strategy to fabricate perovskite oxides LaCo_{1- x} Fe_x O₃ with yolk-shell nanostructures is developed. Benefiting from the unique structural and compositional merits, LaCo_0.75 Fe_0.25 O₃ exhibits an overpotential of 310 mV at a current density of 10 mA cm^-2 and long-term stability of 100 h for the oxygen evolution reaction. In situ Raman spectroscopy demonstrates that Fe substitution facilitates the pre-oxidation of Co sites and induces the surface reconstruction into active Co oxyhydroxides at a relatively lower applied potential, guaranteeing excellent catalytic performances. Density functional theory calculations unravel that the appropriate introduction of Fe into perovskite LaCoO₃ leads to the improved electroactivity and durability of the catalyst for the oxygen evolution reaction (OER). Fe-3d orbitals show a pinning effect on Co-3d orbitals to maintain the stable valence state of Co sites at the low overpotential of the OER. Furthermore, Zn-air batteries (ZABs) assembled with LaCo_0.75 Fe_0.25 O₃ display a high open circuit potential of 1.47 V, superior energy density of 905 Wh kg^-1  _Zn , and excellent stability in a large temperature range. This work supplies novel insights into the future developments of perovskite-based electrocatalysts.

Defect-Engineered Co3 O4 @Nitrogen-Deficient Graphitic Carbon Nitride as an Efficient Bifunctional Electrocatalyst for High-Performance Metal-Air Batteries

The ability to craft high-efficiency and non-precious bifunctional oxygen catalysts opens an enticing avenue for the real-world implementation of metal-air batteries (MABs). Herein, Co₃ O₄ encapsulated within nitrogen defect-rich g-C₃ N₄ (denoted Co₃ O₄ @ND-CN) as a bifunctional oxygen catalyst for MABs is prepared by graphitizing the zeolitic imidazolate framework (ZIF)-67@ND-CN. Co₃ O₄ @ND-CN possesses superb bifunctional catalytic performance, which facilitates the construction of high-performance MABs. Concretely, the rechargeable zinc-air battery based on Co₃ O₄ @ND-CN shows a superior round-trip efficiency of ≈60% with long-term durability (over 340 cycles), exceeding the battery with the state-of-the-art noble metals. The corresponding lithium-oxygen battery using Co₃ O₄ @ND-CN exhibits an excellent maximum discharge/charge capacity (9838.8/9657.6 mAh g^-1 ), an impressive discharge/charge overpotential (1.14 V/0.18 V), and outstanding cycling stability. Such compelling electrocatalytic processes and device performances of Co₃ O₄ @ND-CN originate from concurrent compositional (i.e., defect-engineering) and structural (i.e., wrinkled morphology with abundant porosity) elaboration as well as the well-defined synergy between Co₃ O₄ and ND-CN, which produce an advantageous surface electronic environment corroborated by theoretical modeling. By extension, a rich diversity of other metal oxides@ND-CN with adjustable defects, architecture, and enhanced activities may be rationally designed and crafted for both scientific research on catalytic properties and technological development in renewable energy conversion and storage systems.

Defective Fe3 O4- x Few-Atom Clusters Anchored on Nitrogen-Doped Carbon as Efficient Oxygen Reduction Electrocatalysts for High-Performance Zinc-Air Batteries

It remains a challenge to develop cost-effective, high-performance oxygen electrocatalysts for rechargeable metal-air batteries. Herein, zinc-mediated zeolitic imidazolate frameworks are exploited as the template and nitrogen and carbon sources, onto which is deposited a Fe₃ O₄ layer by plasma-enhanced atomic layer deposition. Controlled pyrolysis at 1000 °C leads to the formation of high density of Fe₃ O_{4- x} few-atom clusters with abundant oxygen vacancies deposited on an N-doped graphitic carbon framework. The resulting nanocomposite (Fe₃ O_{4- x} /NC-1000) exhibits a markedly enhanced electrocatalytic performance toward oxygen reduction reaction in alkaline media, with a remarkable half-wave potential of +0.930 V versus reversible hydrogen electrode, long-term stability, and strong tolerance against methanol poisoning, in comparison to samples prepared at other temperatures and even commercial Pt/C. Notably, with Fe₃ O_{4- x} /NC-1000 as the cathode catalyst, a zinc-air battery delivers a high power density of 158 mW cm^-2 and excellent durability at 5 mA cm^-2 with stable 2000 charge-discharge cycles over 600 h. This is ascribed to the ready accessibility of the Fe₃ O_{4- x} catalytic active sites, and enhanced electrical conductivity, oxygen adsorption, and electron-transfer kinetics by surface oxygen vacancies. Further contributions may arise from the highly conductive and stable N-doped graphitic carbon frameworks.

A Single-Atom Fe-N-C Catalyst with Ultrahigh Utilization of Active Sites for Efficient Oxygen Reduction

Fe-N-C single-atom catalysts (SACs) are emerging as a promising class of electrocatalysts for the oxygen reduction reaction (ORR) to replace Pt-based catalysts. However, due to the limited loading of Fe for SACs and the inaccessibility of internal active sites, only a small portion of the sites near the external surface are able to contribute to the ORR activity. Here, this work reports a metal-organic framework-derived Fe-N-C SAC with a hierarchically porous and concave nanoarchitecture prepared through a facile but effective strategy, which exhibits superior electrocatalytic ORR activity with a half-wave potential of 0.926 V (vs RHE) in alkaline media and 0.8 V (vs RHE) in acidic media while maintaining excellent stability. The superior ORR activity of the as-designed catalyst stems from the unique architecture, where the hierarchically porous architecture contains micropores as Fe SAC anchoring sites, meso-/macro-pores as accessible channels, and concave shell for increasing external surface area. The unique architecture has dramatically enhanced the utilization of previously blocked internal active sites, as confirmed by a high turnover frequency of 3.37 s^-1 and operando X-ray absorption spectroscopy analysis with a distinct shift of adsorption edge.

Recycling spent LiNi1-x-yMnxCoyO2 cathodes to bifunctional NiMnCo catalysts for zinc-air batteries

SignificanceIn recent years, lithium-ion batteries (LIBs) have been widely applied in electric vehicles as energy storage devices. However, it is a great challenge to deal with the large number of spent LIBs. In this work, we employ a rapid thermal radiation method to convert the spent LIBs into highly efficient bifunctional NiMnCo-activated carbon (NiMnCo-AC) catalysts for zinc-air batteries (ZABs). The obtained NiMnCo-AC catalyst shows excellent electrochemical performance in ZABs due to the unique core-shell structure, with face-centered cubic Ni in the core and spinel NiMnCoO₄ in the shell. This work provides an economical and environment-friendly approach to recycling the spent LIBs and converting them into novel energy storage devices.