Advanced Architectures and Relatives of Air Electrodes in Zn-Air Batteries

Will Iron Forge the Future of Metal-Air Batteries in Grid Scale Energy Storage?

The community is exploring sustainable alternatives for grid-scale energy storage. Besides lithium-ion batteries (LIBs), such technologies with a focus on sustainability aspects offer only a limited solution for grid-scale energy storage. Rechargeable metal-air batteries (MABs) based on affordable abundant multivalent metal anodes in aqueous medium provide promising theoretical metrics, such as volumetric capacity, but do not completely fulfill their potential when scaled from lab to commercial products. Both the metal anode and the air cathode need to be addressed: corrosion, hydrogen evolution reaction (HER) during charging, and passivation all diminish the anode's effective volumetric energy density and shelf life, while the air cathode's challenges include sluggish kinetics, low efficiency, and poor stability. Nevertheless, this Perspective highlights iron-air MABs as an appealing sustainable alternative for grid-scale energy storage, since iron is abundant and affordable, recyclable, has multielectron reversible redox activity, historically rich experience in production and processing, and is safe to handle. Given that further research will be directed to exploring the composition and design of electrolytes and electrodes, it may lead to advances in scaling and commercialization, as well as reducing the environmental impact of secondary batteries utilized for grid-scale energy storage in the next decades.

Selective Electrocatalytic Production of Formic Acid from Plastic Waste Using a Nickel Metal-Organic Framework Constructed from a Biomass-Derived Ligand

A novel nickel-based metal organic framework (MOF) [Ni(FDC)(CH3OH)1.5(H2O)0.5](H2O)0.35 (UOW-6) utilizing biomass-derived 2,5-furan dicarboxylate (FDC) as a ligand is reported as an electrocatalyst for anodic ethylene glycol (EG) oxidation with cathodic hydrogen evolution. The MOF structure was analyzed using single crystal X-ray-diffraction, thermogravimetric analysis (TGA) and thermodiffractometry, to establish its structure and verify phase purity. The material was dropcast on carbon fiber paper as a catalyst, and by using a three-electrode system, UOW-6 requires only 1.47 V to attain a current density of 50 mA cm-2. During oxidation of the EG, UOW-6 shows unprecedented selectivity towards formic acid with a Faradaic efficiency of 94 % and remarkable stability over 20 days. The combination of electrochemical measurements and in situ Raman confirmed in situ formed NiOOH at the surface of UOW-6 as the catalytically active sites for EG oxidation. This work not only presents a pioneering application of FDC-based MOFs for polyethylene terephthalate (PET) upcycling but also underscores the potential of electrocatalysis in advancing sustainable plastic valorization strategies.

Self-powered electrochemical energy systems to produce fuels

In the pursuit of efficient fuel production, the challenges posed by the requirement of an external power source have prompted the need for self-powered energy systems by obtaining energy from the environment. Until now, significant progress on developing self-powered energy systems has been made. However, a more basic and in-depth study on their configuration is required for industrial applications. In this review, we outline the latest advancements of self-powered electrochemical energy systems constructed with solar energy, rechargeable batteries/fuel cells and triboelectric nanogenerators. Critical evaluations of the electrochemistry are highlighted to address the issues in elevating the efficiency of fuel production. In addition, the existing challenges and future prospects are also discussed, aiming to develop highly-efficient self-powered energy systems for green fuel production in the future.

High-Density Accessible Iron Single-Atom Catalyst for Durable and Temperature-Adaptive Laminated Zinc-Air Batteries

Designing single-atom catalysts (SACs) with high density of accessible sites by improving metal loading and sites utilization is a promising strategy to boost the catalytic activity, but remains challenging. Herein, a high site density (SD) iron SAC (D-Fe-N/C) with 11.8 wt.% Fe-loading is reported. The in situ scanning electrochemical microscopy technique attests that the accessible active SD and site utilization of D-Fe-N/C reach as high as 1.01 × 1021 site g-1 and 79.8%, respectively. Therefore, D-Fe-N/C demonstrates superior oxygen reduction reaction (ORR) activity in terms of a half-wave potential of 0.918 V and turnover frequency of 0.41 e site-1 s-1. The excellent ORR property of D-Fe-N/C is also demonstrated in the liquid zinc-air batteries (ZABs), which exhibit a high peak power density of 306.1 mW cm-2 and an ultra-long cycling stability over 1200 h. Moreover, solid-state laminated ZABs prepared by presetting an air flow layer show a high specific capacity of 818.8 mA h g-1, an excellent cycling stability of 520 h, and a wide temperature-adaptive from -40 to 60 °C. This work not only offers possibilities by improving metal-loading and catalytic site utilization for exploring efficient SACs, but also provides strategies for device structure design toward advanced ZABs.

Tetramethylguanidine-Modified Graphene Oxide as a Gel Polymer Electrolyte Additive for Improving the Performance of Flexible Zinc-Air Batteries

Flexible zinc-air batteries (FZABs) present a promising solution for the next generation of power sources in wearable electronics, owing to their high energy density, cost-effectiveness, and safety. However, solid-state electrolytes for FZABs continue to face challenges related to rapid water loss and low ionic conductivity. In this study, a hydrophilic and stable tetramethylguanidine-modified graphene oxide as an additive, which is incorporated into sodium polyacrylate to develop a high-performance gel polymer electrolyte (GPE), is designed. The addition of additives makes GPE more hydrophilic, allowing for a wider hydrogen bonding network and more efficient ion transport channels. Due to its stable structure, abundant water channels and fast OH- conductivity, GPE also offers excellent mechanical properties, long-lasting water retention, and high ionic conductivity (173.9 mS cm-1). FZABs assembled with this GPE exhibit a high open-circuit voltage of 1.558 V, a cycle life of 230 h, a specific capacity of 810.3 mAh g-1, and a peak power density of 130.5 mW cm-2, coupled with impressive flexibility. These characteristics underscore their significant potential for applications in wearable electronics.

Formation of H2O2 in Near-Neutral Zn-air Batteries Enables Efficient Oxygen Evolution Reaction

Rechargeable Zn-air batteries (ZABs) with near-neutral electrolytes hold promise as cheap, safe and sustainable devices, but they suffer from slow charge kinetics and remain poorly studied. Here we reveal a charge storage mechanism of near-neutral Zn-air batteries that is mediated by formation of dissolved hydrogen peroxide upon cell discharge and its oxidation upon charge. This H2O2-mediated pathway facilitates oxygen evolution reaction (OER) at ~1.5 V vs. Zn2+/Zn, reducing charge overpotentials by ~0.2-0.5 V and mitigating carbon corrosion-a common issue in ZABs. The manifestation of this mechanism strongly depends on the electrolyte composition and positive electrode material, contributing up to ~60 % of the capacity with ZnSO4 solutions and carbon nanotubes. Enhancing the H2O2-mediated pathway offers a route to higher energy efficiency and durability in near-neutral ZABs, advancing practical, sustainable energy storage.

In situ visualization of interfacial processes at nanoscale in non-alkaline Zn-air batteries

Zn-air batteries (ZABs) present high energy density and high safety but suffer from low oxygen reaction reversibility and dendrite growth at Zn electrode in alkaline electrolytes. Non-alkaline electrolytes have been considered recently for improving the interfacial processes in ZABs. However, the dynamic evolution and reaction mechanisms regulated by electrolytes at both the positive and Zn negative electrodes remain elusive. Herein, using in situ atomic force microscopy, we disclose that thin ZnO2 nanosheets deposit in non-alkaline electrolyte during discharge, followed by the formation of low-modulus products encircled around them. During recharge, the nanosheets are completely decomposed, revealing the favorable reversibility of the O2/ZnO2 chemistry. The circular outlines with low-modulus, composed of C = C and ZnCO3, are left which play a key role in promoting the oxygen reduction reaction (ORR) during the subsequent cycles. In addition, in situ optical microscopy shows that Zn can be uniformly dissolved and deposited in non-alkaline electrolyte, with the formation of homogeneous solid electrolyte interphase. Our work provides straightforward evidence and in-depth understanding of the interfacial reactions at both electrode interfaces in non-alkaline electrolyte, which can inspire strategies of interfacial engineering and material design of advanced ZABs.

Ruthenium clusters decorated on lattice expanded hematite Fe2O3 for efficient electrocatalytic alkaline water splitting

Electrocatalytic water splitting in alkaline media plays an important role in hydrogen production technology. Normally, the catalytic activity of commonly used transition metal oxides usually suffers from unsatisfactory electron conductivity and unfavorable binding strength for transition intermediates. To boost the intrinsic catalytic activity, we propose a rational strategy to construct lattice distorted transition metal oxides decorated with noble-metal nanoclusters. This strategy is verified by loading ruthenium clusters onto lithium ion intercalated hematite Fe2O3, which leads to significant distortion of the FeO6 unit cells. A remarkable overpotential of 21 mV with a Tafel slope of 39.8 mV dec-1 is achieved at 10 mA cm-2 for the hydrogen evolution reaction in 1.0 M KOH aqueous electrolyte. The assembled alkaline electrolyzer can catalyse overall water splitting for as long as 165 h at a current density of 250 mA cm-2 with negligible performance degradation, indicating great potential in the field of sustainable hydrogen production.

Fe/Fe3C particles encapsulated in hollow carbon nanoboxes for high performance zinc-air batteries

Zinc-air batteries are recognized for their environmental friendliness and high energy density; however, the slow kinetics of the oxygen reduction reaction (ORR) at the air electrode hinder their commercial viability. The research focuses on synthesizing cubic hollow carbon structures derived from Metal-Organic Frameworks (MOFs), which enhance catalytic performance through improved conductivity and mass transfer. The resulting Fe/Fe3C/HCNB catalyst demonstrates a half-wave potential of 0.826 V for ORR and achieves a peak power density of 274 mW cm-2 in zinc-air batteries, surpassing commercial Pt/C catalysts. Electrochemical impedance spectroscopy reveals that the hollow structure enhances hydrophilicity and reduces solution resistance, facilitating greater active site engagement in electrochemical reactions. The study concludes that the unique structural features of Fe/Fe3C/HCNB significantly improve discharge performance and stability, positioning it as a promising alternative for zinc-air battery applications.

Metal-chelated polydopamine nanomaterials: Nanoarchitectonics and applications in biomedicine, catalysis, and energy storage

Polydopamine (PDA)-based materials inspired by the adhesive proteins of mussels have attracted increasing attention owing to the universal adhesiveness, antioxidant activity, fluorescence quenching ability, excellent biocompatibility, and especially photothermal conversion capability. The high binding ability of PDA to a variety of metal ions offers a paradigm for the exploration of metal-chelated polydopamine nanomaterials with fantastic properties and functions. This review systematically summarizes the latest progress of metal-chelated polydopamine nanomaterials for the applications in biomedicine, catalysis, and energy storage. Different fabrication strategies for metal-chelated polydopamine nanomaterials with various composition, structure, size, and surface chemistry, such as the pre-functionalization method, the one-pot co-assembly method, and the post-modification method, are summarized. Furthermore, emerging applications of metal-chelated polydopamine nanomaterials in the fields ranging from cancer therapy, theranostics, antibacterial, catalysis to energy storage are highlighted. Additionally, the critical remaining challenges and future directions of this area are discussed to promote the further development and practical applications of PDA-based materials.

Accelerating charge separation in p-n heterojunction photocathode for photoelectrochemical oxygen reduction and evolution in photo-enhanced zinc-air battery

For the first time, we constructed a band-matched ZnO/NiO staggered p-n heterojunction photoelectrochemical (PEC) catalyst with superior charge separation and transfer efficiency to optimize the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) kinetics demands of a photo-enhanced zinc-air battery (PZAB). The ingenious design of heterojunction architecture effectively stimulates an internal self-built electric field (BEF), thus accelerating charge separation and migration during PEC catalysis. Upon illumination, the photocathode not only endows a promoted PEC catalytic competence to drive ORR and OER, but also reduces the charge-discharge voltage gap of PZAB. Density functional theory (DFT) calculations further disentangled the PEC catalysis activity origins by substantiating the strongly decreased Gibbs free energy potently for generating the essential intermediates *OH and *OOH during ORR and OER, respectively, and visualized the charge densities in photocathode to illustrate the authentic separation of photogenerated carriers. Remarkably, our work experimentally and theoretically sheds lights on engineering innovative and efficient bifunctional PEC catalysts to drive PZABs with photo-enhancement.

Recent advances on COF-based single-atom and dual-atom sites for oxygen catalysis

Covalent organic frameworks (COFs) have emerged as promising platforms for the construction of single-atom and dual-atom catalysts (SACs and DACs), owing to their well-defined structures, tunable pore sizes, and abundant active sites. In recent years, the development of COF-based SACs and DACs as highly efficient catalysts has witnessed a remarkable surge. The synergistic interplay between the metal active sites and the COF has established the design and fabrication of COF-based SACs and DACs as a prominent research area in electrocatalysis. These catalytic materials exhibit promising prospects for applications in energy storage and conversion devices. This review summarizes recent advances in the design, synthesis, and applications of COF-based SACs and DACs for oxygen catalysis. The catalytic mechanisms of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are comprehensively explored, providing a comparative analysis to elucidate the correlation between the structure and performance, as well as their functional attributes in battery devices. This review highlights a promising approach for future research, emphasizing the necessity of rational design, breakthroughs, and in-situ characterization to further advance the development of high-performance COF-based SACs and DACs for sustainable energy applications.

High-Conductivity and Ultrastretchable Self-Healing Hydrogels for Flexible Zinc-Ion Batteries

Aqueous zinc-ion batteries are promising candidates for flexible energy storage devices due to their safety, economic efficiency, and environmental friendliness. However, the uncontrollable dendrite growth and side reactions at the zinc anode hinder their commercial application. Herein, we designed and synthesized a dual network self-healing hydrogel electrolyte with zwitterionic groups (PAM-PAAS-QCS), which can be used for the large deformations of flexible devices due to its excellent stretchability (ε = 5100%). The incorporation of zwitterionic groups into the PAM-PAAS-QCS hydrogel electrolyte endows it with high ionic conductivity (33.61 mS/cm), a wide electrochemical stability window, and the ability to suppress zinc dendrite formation and side reactions. Besides, the Zn//Zn symmetric cell with PAM-PAAS-QCS can stably plate and strip zinc for 1500 h at 0.5 mA/cm2, and the Zn//Polyaniline full cell retains 82.4% of its capacity after 1500 cycles at 1 A/g. Additionally, flexible batteries based on both the original and self-healed PAM-PAAS-QCS hydrogel electrolytes demonstrate good cycling stability and stable charge-discharge performance under various bending conditions. This self-healing hydrogel electrolyte with excellent stretchability and high ionic conductivity is expected to pave the way for the development of high-performance flexible energy storage and wearable devices.

General and Facile Synthesis of Co/CoO Nanoparticals Supported by Nitrogen-Doped Graphenic Networks as Efficient Oxygen Electrocatalyst for Zn-Air Batteries

Reasonable design of low-cost, high-efficiency and stable bifunctional oxygen electrocatalysts is of great significance to improve the reaction efficiency of Zn-air batteries, which is still a huge challenge. Here, we report a highly efficient bifunctional oxygen electrocatalyst with three-dimensional (3D) N-doped graphene network-supported cobalt and cobalt oxide nanoparticles (Co/CoO-NG), which can be in situ synthesized by inducing metal ions on metal plates via graphene oxide as an inducer. This 3D network structure and open active center show excellent bifunctional oxygen electrocatalytic activity under alkaline conditions, and can be used as an air electrode in rechargeable Zn-air batteries, with significantly better power density (244.28 mW cm-2) and stability (over 340 h) than commercial Pt/C+RuO2 mixtures. This work is conducive to advancing the practical application of graphene-based materials as air electrodes for rechargeable zinc-air batteries.

An Overview and Future Perspectives of Rechargeable Flexible Zn-Air Batteries

Environmental friendliness and low-cost zinc-air batteries for flexible rechargeable applications have great potential in the field of flexible electronics and smart wearables owing to high energy density and long service life. However, the current technology of flexible rechargeable zinc-air batteries to meet the commercialization needs still facing enormous challenges due to the poor adaptability of each flexible component of the zinc-air batteries. This review focused on the latest progress over the past 5 years in designing and fabricating flexible self-standing air electrodes, flexible electrolytes and zinc electrodes of flexible Zn-air batteries, meanwhile the basic working principle of each component of flexible rechargeable zinc-air batteries and battery structures optimization are also described. Finally, challenges and prospects for the future development of flexible rechargeable zinc-air batteries are discussed. This work is intended to provide insights and general guidance for future exploration of the design and fabrication on high-performance flexible rechargeable zinc-air batteries.

Promoting Electrocatalytic Oxygen Reactions Using Advanced Heterostructures for Rechargeable Zinc-Air Battery Applications

In order to facilitate electrochemical oxygen reactions in electrically rechargeable zinc-air batteries (ZABs), there is a need to develop innovative approaches for efficient oxygen electrocatalysts. Due to their reliability, high energy density, material abundance, and ecofriendliness, rechargeable ZABs hold promise as next-generation energy storage and conversion devices. However, the large-scale application of ZABs is currently hindered by the slow kinetics of the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). However, the development of heterostructure-based electrocatalysts has the potential to surpass the limitations imposed by the intrinsic properties of a single material. This Account begins with an explanation of the configurations of ZABs and the fundamentals of the oxygen electrochemistry of the air electrode. Then, we summarize recent progress with respect to the variety of heterostructures that exploit bifunctional electrocatalytic reactions and overview their impact on ZAB performance. The range of heterointerfacial engineering strategies for improving the ORR/OER and ZAB performance includes tailoring the surface chemistry, dimensionality of catalysts, interfacial charge transfer, mass and charge transport, and morphology. We highlight the multicomponent design approaches that take these features into account to create advanced highly active bifunctional catalysts. Finally, we discuss the challenges and future perspectives on this important topic that aim to enhance the bifunctional activity and performance of zinc-air batteries.

Hierarchical Porous Carbon Supported Co2P2O7 Nanoparticles for Oxygen Evolution and Oxygen Reduction in a Rechargeable Zn-Air Battery

The oxygen reduction/evolution reaction (ORR/OER) represents a pivotal process in metal-air batteries; however, it is constrained by the limitations of slow kinetics. Nevertheless, the creation of long-lasting and bifunctional catalysts represents a significant challenge. This study presents a series of hierarchical porous carbon-supported cobalt pyrophosphate (Co2P2O7-N/C-T) catalysts, prepared through the pyrolysis of porphyrin-based NTU-70 nanosheets with red phosphorus at varying temperatures. The Co2P2O7-N/C-800 not only demonstrates remarkable OER performance with an overpotential of only 290 mV at a current density of 10 mA cm-2 in 1 M KOH, but also exhibits an excellent ΔE of 0.74 V in 0.1 M KOH, which is lower than that of Pt/C + RuO2 (0.76 V). The utilization of Co2P2O7-N/C-800 as an air cathode in a rechargeable Zn-air battery (ZAB) results in a stable discharge voltage plateau of 1.405 V and a high gravimetric energy density of 801.2 mA h gZn-1. This work presents a promising strategy for the design of efficient bifunctional catalysts and demonstrates the critical importance of the interplay between the active center and the supported hierarchical porous carbon.

Aluminium, Nitrogen-Dual-Doped Reduced Graphene Oxide Co-Existing with Cobalt-Encapsulated Graphitic Carbon Nanotube as an Activity Modulated Electrocatalyst for Oxygen Electrocatalyst for Oxygen Electrochemistry Applications

There is a rising need to create high-performing, affordable electrocatalysts in the new field of oxygen electrochemistry. Here, a cost-effective, activity-modulated electrocatalyst with the capacity to trigger both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) in an alkaline environment is presented. The catalyst (Al, Co/N-rGCNT) is made up of aluminium, nitrogen-dual-doped reduced graphene oxide sheets co-existing with cobalt-encapsulated carbon nanotube units. Based on X-ray Absorption Spectroscopy (XAS) studies, it is established that the superior reaction kinetics in Al, Co/N-rGCNT over their bulk counterparts can be attributed to their electronic regulation. The Al, Co/N-rGCNT performs as a versatile bifunctional electrocatalyst for zinc-air battery (ZAB), delivering an open circuit potential ≈1.35 V and peak power density of 106.3 mW cm-2, which are comparable to the system based on Pt/C. The Al, Co/N-rGCNT-based system showed a specific capacity of 737 mAh gZn -1 compared to 696 mAh gZn -1 delivered by the system based on Pt/C. The DFT calculations indicate that the adsorption of Co in the presence of Al doping in NGr improves the electronic properties favoring ORR. Thus, the Al, Co/N-rGCNT-based rechargeable ZAB (RZAB) emerges as a highly viable and affordable option for the development of RZAB for practical applications.

Photo-Assisted Rechargeable Metal Batteries: Principles, Progress, and Perspectives

The utilization of diverse energy storage devices is imperative in the contemporary society. Taking advantage of solar power, a significant environmentally friendly and sustainable energy resource, holds great appeal for future storage of energy because it can solve the dilemma of fossil energy depletion and the resulting environmental problems once and for all. Recently, photo-assisted energy storage devices, especially photo-assisted rechargeable metal batteries, are rapidly developed owing to the ability to efficiently convert and store solar energy and the simple configuration, as well as the fact that conventional Li/Zn-ion batteries are widely commercialized. Considering many puzzles arising from the rapid development of photo-assisted rechargeable metal batteries, this review commences by introducing the fundamental concepts of batteries and photo-electrochemistry, followed by an exploration of the current advancements in photo-assisted rechargeable metal batteries. Specifically, it delves into the elucidation of device components, operating principles, types, and practical applications. Furthermore, this paper categorizes, specifies, and summarizes several detailed examples of photo-assisted energy storage devices. Lastly, it addresses the challenges and bottlenecks faced by these energy storage systems while providing future perspectives to facilitate their transition from laboratory research to industrial implementation.

Recent Advances in Catalyst Design and Performance Optimization of Nanostructured Cathode Materials in Zinc-Air Batteries

This review focuses on the advanced design and optimization of nanostructured zinc-air batteries (ZABs), with the aim of boosting their energy storage and conversion capabilities. The findings show that ZABs favor porous nanostructures owing to their large surface area, and this enhances the battery capacity, catalytic activity, and life cycle. In addition, the nanomaterials improve the electrical conductivity, ion transport, and overall battery stability, which crucially reduces dendrite growth on the zinc anodes and improves cycle life and energy efficiency. To obtain a superior performance, the importance of controlling the operational conditions and using custom nanostructural designs, optimal electrode materials, and carefully adjusted electrolytes is highlighted. In conclusion, porous nanostructures and nanoscale materials significantly boost the energy density, longevity, and efficiency of Zn-air batteries. It is suggested that future research should focus on the fundamental design principles of these materials to further enhance the battery performance and drive sustainable energy solutions.

Zn-based batteries for sustainable energy storage: strategies and mechanisms

Batteries play a pivotal role in various electrochemical energy storage systems, functioning as essential components to enhance energy utilization efficiency and expedite the realization of energy and environmental sustainability. Zn-based batteries have attracted increasing attention as a promising alternative to lithium-ion batteries owing to their cost effectiveness, enhanced intrinsic safety, and favorable electrochemical performance. In this context, substantial endeavors have been dedicated to crafting and advancing high-performance Zn-based batteries. However, some challenges, including limited discharging capacity, low operating voltage, low energy density, short cycle life, and complicated energy storage mechanism, need to be addressed in order to render large-scale practical applications. In this review, we comprehensively present recent advances in designing high-performance Zn-based batteries and in elucidating energy storage mechanisms. First, various redox mechanisms in Zn-based batteries are systematically summarized, including insertion-type, conversion-type, coordination-type, and catalysis-type mechanisms. Subsequently, the design strategies aiming at enhancing the electrochemical performance of Zn-based batteries are underscored, focusing on several aspects, including output voltage, capacity, energy density, and cycle life. Finally, challenges and future prospects of Zn-based batteries are discussed.

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.

Recent Advances in Carbon Nanotube Utilization in Perovskite Solar Cells: A Review

Due to their exceptional optoelectronic properties, halide perovskites have emerged as prominent materials for the light-absorbing layer in various optoelectronic devices. However, to increase device performance for wider adoption, it is essential to find innovative solutions. One promising solution is incorporating carbon nanotubes (CNTs), which have shown remarkable versatility and efficacy. In these devices, CNTs serve multiple functions, including providing conducting substrates and electrodes and improving charge extraction and transport. The next iteration of photovoltaic devices, metal halide perovskite solar cells (PSCs), holds immense promise. Despite significant progress, achieving optimal efficiency, stability, and affordability simultaneously remains a challenge, and overcoming these obstacles requires the development of novel materials known as CNTs, which, owing to their remarkable electrical, optical, and mechanical properties, have garnered considerable attention as potential materials for highly efficient PSCs. Incorporating CNTs into perovskite solar cells offers versatility, enabling improvements in device performance and longevity while catering to diverse applications. This article provides an in-depth exploration of recent advancements in carbon nanotube technology and its integration into perovskite solar cells, serving as transparent conductive electrodes, charge transporters, interlayers, hole-transporting materials, and back electrodes. Additionally, we highlighted key challenges and offered insights for future enhancements in perovskite solar cells leveraging CNTs.

In-Situ Nanoarchitectonics of Fe/Co LDH over Cobalt-Enriched N-Doped Carbon Cookies as Facile Oxygen Redox Electrocatalysts for High-Rate Rechargeable Zinc-Air Batteries

The reality of long-term rechargeable and high-performance zinc-air batteries relies majorly on cost-effective and eminent bifunctional electrocatalysts, which can perform both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). Herein, we demonstrate a new approach for the synthesis of in-situ-grown layered double hydroxide of iron and cobalt over a cobalt nanoparticle-enriched nitrogen-doped carbon frame (CoL 2:1) by a simple coprecipitation reaction with facile scale-up and explore its electrocatalytic ORR and OER activity for an electrically rechargeable zinc-air battery. Consequently, the developed composite displays excellent ORR and OER activity with an ORR half-wave potential of 0.84 V, a limiting current density of 5.85 mA/cm2, and an OER overpotential of 320 mV with exceptional stability. The outstanding bifunctionality index of the catalyst (ΔE = 0.72 V) inspired us to utilize it as a cathode catalyst in an in-house developed prototype zinc-air battery. The battery could easily supply a specific capacity of 804 mAh/g with a maximum peak power density of 161 mW/cm2. The battery exhibits an attractive charge-discharge profile with a lesser voltage gap of 0.76 V at 10 mA/cm2 with durability for a period of 200 h and a voltage efficiency of 97%, which surpassed the corresponding Pt/C + RuO2-based zinc-air battery. Further, a maximum load of 50 mA/cm2 could easily be sustained during cycling, revealing its outstanding stability. A series-connected two CoL 2:1-based zinc-air batteries effortlessly enlighten a pinwheel fan and LED panel simultaneously, revealing its practicality. The high electrical conductivity and greater specific surface area of Co/N-C and its robust attachment with Fe/Co LDH preserves both active sites, thereby resulting in exceptional performance. Our method is capable of being flexible enough to create various bifunctional Co/N-C-based composite electrodes, opening up a feasible pathway to rechargeable zinc-air batteries with maximum energy density.

Universal Sublimation Strategy to Stabilize Single-Metal Sites on Flexible Single-Wall Carbon-Nanotube Films with Strain-Enhanced Activities for Zinc-Air Batteries and Water Splitting

Single-metal-site catalysts have recently aroused extensive research in electrochemical energy fields such as zinc-air batteries and water splitting, but their preparation is still a huge challenge, especially in flexible catalyst films. Herein, we propose a sublimation strategy in which metal phthalocyanine molecules with defined isolated metal-N4 sites are gasified by sublimation and then deposited on flexible single-wall carbon nanotube (SWCNT) films by means of π-π coupling interactions. Specifically, iron phthalocyanine anchored on the SWCNT film prepared was directly used to boost the cathodic oxygen reduction reaction of the zinc-air battery, showing a high peak power density of 247 mW cm-2. Nickel phthalocyanine and cobalt phthalocyanine were, respectively, stabilized on SWCNT films as the anodic and cathodic electrocatalysts for water splitting, showing a low potential of 1.655 V at 10 mA cm-2. In situ Raman spectra and theoretical studies demonstrate that highly efficient activities originate from strain-induced metal phthalocyanine on SWCNTs. This work provides a universal preparation method for single-metal-site catalysts and innovative insights for electrocatalytic mechanisms.

Synergistic effect of surfactant and 1,10-decanedithiol as corrosion inhibitor for zinc anode in alkaline electrolyte of zinc-air batteries

The self-discharge by corrosion of zinc-air batteries (ZABs) will result in the reduced coulombic efficiency and lower energy efficiency. The additives in electrolyte should not only inhibit the occurrence of self-corrosion during battery dormancy, but also achieve a stable cycle of adsorption-desorption during battery operation, improving the durability of discharge cycles. But the former requires strong binding between additives and zinc to form a dense protective film, while the latter requires easy desorption of additives and zinc without affecting discharge power, which is contradictory to balance. In this study, a dynamic combination of additives and zinc, as well as a design of multi-channel strategy for the corresponding protective layer, have been proposed to solve the issues of self-corrosion and discharge cycle stability. Specifically, the surfactant (octylphenol polyoxyethylene ether phosphate (OP-10P)) and 1,10-decanedithiol (DD) have been selected as the combined anti-corrosion additives in ZABs with concentrated alkaline solution. The synergistic inhibition mechanism and the stabilization mechanism in zinc-air full cells have been studied systematically. The results indicated that the combined inhibitors inhibited the self-corrosion of Zn efficiently in the dormancy, and the inhibition efficiency reached 99.9 % at the optimized proportion. OP-10P achieve the preferential adsorption on the zinc surface, and then the chelates of DD with Zn2+ deposit on the outer layer to form the protective film with fine hydrophobic performance. The stability of ZABs in discharge and charging cycles has been improved owing to the multilayer adsorption film on zinc surface, which retains ion transport channels with the homogeneously pores to weaken the dendrites and side reactions during galvanostatic cycles. A probable model on zinc surface was established to discuss the actual working mechanism.

Recent Advances in Laser-Induced Graphene-Based Materials for Energy Storage and Conversion

Laser-induced graphene (LIG) is a porous carbon nanomaterial that can be produced by irradiation of CO2 laser directly on the polymer substrate under ambient conditions. LIG has many merits over conventional graphene, such as simple and fast synthesis, tunable structure and composition, high surface area and porosity, excellent electrical and thermal conductivity, and good flexibility and stability. These properties make LIG a promising material for energy applications, such as supercapacitors, batteries, fuel cells, and solar cells. In this review, we highlight the recent advances of LIG in energy materials, covering the fabrication methods, performance enhancement strategies, and device integration of LIG-based electrodes and devices in the area of hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, zinc-air batteries, and supercapacitors. This comprehensive review examines the potential of LIG for future sustainable and efficient energy material development, highlighting its versatility and multifunctionality in energy conversion.

A Review of Rechargeable Zinc-Air Batteries: Recent Progress and Future Perspectives

Zinc-air batteries (ZABs) are gaining attention as an ideal option for various applications requiring high-capacity batteries, such as portable electronics, electric vehicles, and renewable energy storage. ZABs offer advantages such as low environmental impact, enhanced safety compared to Li-ion batteries, and cost-effectiveness due to the abundance of zinc. However, early research faced challenges due to parasitic reactions at the zinc anode and slow oxygen redox kinetics. Recent advancements in restructuring the anode, utilizing alternative electrolytes, and developing bifunctional oxygen catalysts have significantly improved ZABs. Scientists have achieved battery reversibility over thousands of cycles, introduced new electrolytes, and achieved energy efficiency records surpassing 70%. Despite these achievements, there are challenges related to lower power density, shorter lifespan, and air electrode corrosion leading to performance degradation. This review paper discusses different battery configurations, and reaction mechanisms for electrically and mechanically rechargeable ZABs, and proposes remedies to enhance overall battery performance. The paper also explores recent advancements, applications, and the future prospects of electrically/mechanically rechargeable ZABs.

Bio-catalyzed oxidation self-charging zinc-polymer batteries

Oxidation self-charging batteries have emerged with the demand for powering electronic devices around the clock. The low efficiency of self-charging has been the key challenge at present. Here, a more efficient autoxidation self-charging mechanism is realized by introducing hemoglobin (Hb) as a positive electrode additive in the polyaniline (PANI)-zinc battery system. The heme acts as a catalyst that reduces the energy barrier of the autoxidation reaction by regulating the charge and spin state of O2. To realize self-charging, the adsorbed O2 molecules capture electrons of the reduced (discharged state) PANI, leading to the desorption of zinc ions and the oxidation of PANI to complete self-charging. The battery can discharge for 12 min (0.5 C) after 50 self-charging/discharge cycles, while there is nearly no discharge capacity in the absence of Hb. This biology-inspired electronic regulation strategy may inspire new ideas to boost the performance of self-charging batteries.

All-Solid-State Mg-Air Battery Enhanced with Free-Standing N-Doped 3D Nanoporous Graphene

Nitrogen (N) doping of graphene with a three-dimensional (3D) porous structure, high flexibility, and low cost exhibits potential for developing metal-air batteries to power electric/electronic devices. The optimization of N-doping into graphene and the design of interconnected and monolithic graphene-based 3D porous structures are crucial for mass/ion diffusion and the final oxygen reduction reaction (ORR)/battery performance. Aqueous-type and all-solid-state primary Mg-air batteries using N-doped nanoporous graphene as air cathodes are assembled. N-doped nanoporous graphene with 50-150 nm pores and ≈99% porosity is found to exhibit a Pt-comparable ORR performance, along with satisfactory durability in both neutral and alkaline media. Remarkably, the all-solid-state battery exhibits a peak power density of 72.1 mW cm-2 ; this value is higher than that of a battery using Pt/carbon cathodes (54.3 mW cm-2 ) owing to the enhanced catalytic activity induced by N-doping and rapid air breathing in the 3D porous structure. Additionally, the all-solid-state battery demonstrates better performances than the aqueous-type battery owing to slow corrosion of the Mg anode by solid electrolytes. This study sheds light on the design of free-standing and catalytically active 3D nanoporous graphene that enhances the performance of both Mg-air batteries and various carbon-neutral-technologies using neutral electrolytes.

Metal-Organic Frameworks-Derived FeS-Co9S8/NCA Porous Aerogel Electrocatalyst as a High-Performance Cathode for Zinc-Air Batteries

Herein, a novel strategy to establish a porous FeS-Co9S8/carbon aerogel (FeS-Co9S8/NCA) electrocatalyst for oxygen evolution reaction (OER) is fabricated via applying a green biomass carrageenan sulfuration method to CoFe-metal-organic frameworks (MOFs). The FeS-Co9S8/NCA exhibits optimized catalytic activity toward the OER with a lower overpotential of 322 mV, which is overmatched to the majority of transition metal sulfides (TMSs), as well as lifted long-term durability without evident variation in the LSV curves after 3000 cycles. Rechargeable liquid zinc-air battery (ZAB) assembled with FeS-Co9S8/NCA as the OER catalyst indicated a maximum power density of 176 mW cm-2 and superior cycling stability without raised polarization even after 48 h, outperforms commercial RuO2-based ZAB. Furthermore, the flexible solid-state ZAB built with FeS-Co9S8/NCA also demonstrated outdistance properties and bendability. The excellent performance stems from the hierarchical porous aerogel structure, which offers a multiscale mass/electron transport channel, together with the interfacial synergy effect between FeS and Co9S8, which serves as the active site of the OER reaction. Thus, this work instituted a novel strategy for obtaining both clean and efficient transition metal sulfide electrocatalysts for the OER reaction and an environmentally friendly biomass material-based sustainable electrocatalyst.

Recent progress of MOF-functionalized nanocomposites: From structure to properties

Metal-organic frameworks (MOFs) are novel crystalline porous materials assembled from metal ions and organic ligands. The adaptability of their design and the fine-tuning of the pore structures make them stand out in porous materials. Furthermore, by integrating MOF guest functional materials with other hosts, the novel composites have synergistic benefits in numerous fields such as batteries, supercapacitors, catalysis, gas storage and separation, sensors, and drug delivery. This article starts by examining the structural relationship between the host and guest materials, providing a comprehensive overview of the research advancements in various types of MOF-functionalized composites reported to date. The review focuses specifically on four types of spatial structures, including MOFs being (1) embedded in nanopores, (2) immobilized on surface, (3) coated as shells and (4) assembled into hybrids. In addition, specific design ideas for these four MOF-based composites are presented. Some of them involve in situ synthesis method, solvothermal method, etc. The specific properties and applications of these materials are also mentioned. Finally, a brief summary of the advantages of these four types of MOF composites is given. Hopefully, this article will help researchers in the design of MOF composite structures.

A Mechanistic Overview of the Current Status and Future Challenges in Air Cathode for Aluminum Air Batteries

Aluminum air batteries (AABs) are a desirable option for portable electronic devices and electric vehicles (EVs) due to their high theoretical energy density (8100 Wh K-1 ), low cost, and high safety compared to state-of-the-art lithium-ion batteries (LIBs). However, numerous unresolved technological and scientific issues are preventing AABs from expanding further. One of the key issues is the catalytic reaction kinetics of the air cathode as the fuel (oxygen) for AAB is reduced there. Additionally, the performance and price of an AAB are directly influenced by an air electrode integrated with an oxygen electrocatalyst, which is thought to be the most crucial element. In this study, we covered the oxygen chemistry of the air cathode as well as a brief discussion of the mechanistic insights of active catalysts and how they catalyze and enhance oxygen chemistry reactions. There is also extensive discussion of research into electrocatalytic materials that outperform Pt/C such as nonprecious metal catalysts, metal oxide, perovskites, metal-organic framework, carbonaceous materials, and their composites. Finally, we provide an overview of the present state, and possible future direction for air cathodes in AABs.

Rechargeable Zinc-Air Batteries: Advances, Challenges, and Prospects

Rechargeable zinc-air batteries (Re-ZABs) are one of the most promising next-generation batteries that can hold more energy while being cost-effective and safer than existing devices. Nevertheless, zinc dendrites, non-portability, and limited charge-discharge cycles have long been obstacles to the commercialization of Re-ZABs. Over the past 30 years, milestone breakthroughs have been made in technical indicators (safety, high energy density, and long battery life), battery components (air cathode, zinc anode, and gas diffusion layer), and battery configurations (flexibility and portability), however, a comprehensive review on advanced design strategies for Re-ZABs system from multiple angles is still lacking. This review underscores the progress and strategies proposed so far to pursuit the high-efficiency Re-ZABs system, including the aspects of rechargeability (from primary to rechargeable), air cathode (from unifunctional to bifunctional), zinc anode (from dendritic to stable), electrolytes (from aqueous to non-aqueous), battery configurations (from non-portable to portable), and industrialization progress (from laboratorial to practical). Critical appraisals of the advanced modification approaches (such as surface/interface modulation, nanoconfinement catalysis, defect electrochemistry, synergistic electrocatalysis, etc.) are highlighted for cost-effective flexible Re-ZABs with good sustainability and high energy density. Finally, insights are further rendered properly for the future research directions of advanced zinc-air batteries.

Surface Engineering Based on Conductive Agent Dispersion Uniformity: Strategies toward Performance Consistency of Lithium-Ion Batteries

The consistency of lithium-ion battery performance is the key factor affecting the safety and cycle life of battery packs. Surface engineering of electrodes in production processes plays an important role in improving the consistency of battery performance. In this study, the drying process in the electrode manufacturing process is studied as the effect on surface engineering of the electrode materials, with consideration on impacting the battery performance. Specifically, the solid content of the slurry and drying temperature are considered to be the two factors that affect conductive agent dispersion uniformity in the porous electrodes. To achieve surface engineering on the dispersion uniformity of the conductive agent, the optimal processing parameters can be obtained by adjusting the temperature and solid content of the slurry. The mechanism of dispersion uniformity of the conductive agent is mainly related to the polyvinylidene fluoride grid structure. In the manufacturing of lithium-ion batteries, the electrode coated with 66% solid slurry and dried at 90-100 °C presents stable energy storage performance, which is beneficial to maintain the stable performance of the battery pack in the application.

An 'active site anchoring' strategy for the preparation of PBO fiber derived carbon catalyst towards an efficient oxygen reduction reaction and zinc-air batteries

In order to promote the wide application of clean energy-fuel cells, it is urgent to develop transition metal-based high-efficiency oxygen reduction reaction (ORR) catalytic materials with a low cost and available rich raw material resources to replace the currently used precious metal platinum-based catalytic materials. Herein, a novel 'active-site-anchoring' strategy was developed to synthesize highly-activated carbon-based ORR catalysts. Firstly, poly(p-phenylene benzobisoxazole) (PBO) fiber with a stable chemical structure was selected as the main precursor, and iron was complexed on its surface, and then poly-dopamine (PDA) was coated on the surface of PBO-Fe to form a PBO-Fe-PDA composite structure. Therefore, carbon-based catalyst PBO-Fe-PDA-900 with abundant Fe2O3 active sites was prepared by anchoring iron sites by PDA after pyrolysis. As a result, the PBO-Fe-PDA-900 catalyst displayed a 30 mV higher half-wave potential (0.86 V) than that of a commercial Pt/C electrocatalyst. Finally, PBO-Fe-PDA-900 was used as a cathode material for zinc-air batteries, showing a high peak power density superior to Pt/C. This work offers new prospects for the design of efficient, non-precious metal-based materials in zinc-air batteries.

Single-atom Zn with nitrogen defects on biomimetic 3D carbon nanotubes for bifunctional oxygen electrocatalysis

Single atoms catalysts (SACs) have promising development in electrocatalytic energy conversion. Nevertheless, rational design SACs with reversible oxygen electrocatalysis still remain challenge. Herein, we synthesized atomically dispersed Zn with N defect on three-dimensional (3D) biomimetic carbon nanotubes by secondary pyrolysis (Zn-N-C-2), which possesses excellent oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) bifunctional catalytic activities. The biomimetic 3D structure and unique "leaf-branch" system are beneficial to fully expose the active sites. Density functional theory (DFT) calculations show that Zn-N3-D can optimize the charge distribution and facilitate electron transfer step of OH*→O*. Zn-N-C-2 exhibits higher ORR activity than commercial Pt/C with a half-wave potential (E1/2) of 0.85 V and OER overpotential of 450 mV at 10 mA cm-2. After being assembled into the air cathode of aqueous Zn-air battery (ZAB), it demonstrates superior performances with long-term charge and discharge for more than 200 h. This work not only clarifies the controlled synthesis of N-defects Zn SACs with excellent bifunctional electrocatalyst, but also provide in-depth understanding of structural-performance relationships by regulating local microenvironments.

P-doped binary Ni/Fe-N-C for enhanced oxygen electrocatalysis performance

Adjusting the micro-environment of highly dispersive metals on carbon supports has been proved to be effective for achieving enhanced electrocatalysis performance. Herein, we delicately design a phosphorus-doped binary NiFe-nitrogen-carbon material (denoted as P-NiFe-NC), taking advantage of the coupling reaction between phenylphosphonamide (P dopant) and formamide (the carbon and nitrogen sources). The XPS N 1s fine scan reveals the strong interplay of N and P by the positively shifted binding energy of pyridinic N species after P incorporation, and the chemical state of Fe species is influenced accordingly. In addition, the P doping can enlarge the specific surface area and increase the meso/macroporosity of NiFe-NC, thus contributing to the enhancement of mass transfer inside the pores. The P-NiFe-NC sample exhibits favorable bifunctional oxygen electrocatalysis performance, rendering an ORR half-wave potential of 0.85 V and an OER potential of 1.69 V@10.0 mA cm-2, superior to those of P-free NiFe-NC. Assembled into Zn-air batteries, P-NiFe-NC delivers a high specific power of 161.36 mW cm-2 and stable charge/discharge for over 100 h (corresponding to 300 cycles).

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.

Self-Decoupled Oxygen Electrocatalysis for Ultrastable Rechargeable Zn-Air Batteries with Mild-Acidic Electrolyte

Rechargeable zinc-air batteries (ZABs) have been considered promising as next-generation sustainable energy storage devices; however, their large-scale deployment is hampered by the unsatisfactory cyclic lifespan. Employing neutral and mild-acidic electrolytes is effective in extending the cyclability, but the rapid performance degradation of the bifunctional catalysts owing to different microenvironmental requirements of the alternative oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is still a serious limitation of their cyclic life. Herein, we propose a "self-decoupling" strategy to significantly improve the stability of the bifunctional catalysts by constructing a smart interface in the bifunctional air electrode. This smart interface, containing a resistance-switchable sulfonic acid doped polyaniline nanoarray interlayer, is nonconductive at high potential but conductive at low potential, which enables spontaneous electrochemical decoupling of the bifunctional catalyst for the ORR and OER, respectively, and thus protects it from degradation. The resulting self-decoupled mild-acidic ZAB delivers stable cyclic performances in terms of a negligible energy efficiency loss of 0.015% cycle-1 and 3 times longer cycle life (∼1400 h) compared with the conventional mild-acidic ZAB using a normal bifunctional air electrode and the same low-cost ZnCo phosphide/nitrogen-doped carbon bifunctional catalyst. This work provides an effective strategy for tolerating alternative oxidation-reduction reactions and emphasizes the importance of smart nanostructure design for more sustainable batteries.

Recent advances of bifunctional catalysts for zinc air batteries with stability considerations: from selecting materials to reconstruction

With the growing depletion of traditional fossil energy resources and ongoing enhanced awareness of environmental protection, research on electrochemical energy storage techniques like zinc-air batteries is receiving close attention. A significant amount of work on bifunctional catalysts is devoted to improving OER and ORR reaction performance to pave the way for the commercialization of new batteries. Although most traditional energy storage systems perform very well, their durability in practical applications is receiving less attention, with issues such as carbon corrosion, reconstruction during the OER process, and degradation, which can seriously impact long-term use. To be able to design bifunctional materials in a bottom-up approach, a summary of different kinds of carbon materials and transition metal-based materials will be of assistance in selecting a suitable and highly active catalyst from the extensive existing non-precious materials database. Also, the modulation of current carbon materials, aimed at increasing defects and vacancies in carbon and electron distribution in metal-N-C is introduced to attain improved ORR performance of porous materials with fast mass and air transfer. Finally, the reconstruction of catalysts is introduced. The review concludes with comprehensive recommendations for obtaining high-performance and highly-durable catalysts.

Photoelectrochemical Engineering for Light-Assisted Rechargeable Metal Batteries: Mechanism, Development, and Future

Rechargeable battery devices with high energy density are highly demanded by  our  modern society. The use of metal anodes is extremely attractive for future rechargeable battery devices. However, the notorious metal dendritic and instability of solid electrolyte interface issues pose a series of challenges for metal anodes. Recently, considering the indigestible dynamical behavior of metal anodes, photoelectrochemical engineering of light-assisted metal anodes have been rapidly developed since they efficiently utilize the integration and synergy of oriented crystal engineering and photocatalysis engineering, which provided a potential way to unlock the interface electrochemical mechanism and deposition reaction kinetics of metal anodes. This review starts with the fundamentals of photoelectrochemical engineering and follows with the state-of-art advance of photoelectrochemical engineering for light-assisted rechargeable metal batteries where photoelectrode materials, working principles, types, and practical applications are explained. The last section summarizes the major challenges and some invigorating perspectives for future research on light-assisted rechargeable metal batteries.

Biomass-Derived Carbon Aerogels for ORR/OER Bifunctional Oxygen Electrodes

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are crucial electrochemical reactions that play vital roles in energy conversion and storage technologies, such as fuel cells and metal-air batteries. Typically, noble-metal-based catalysts are required to enhance the sluggish kinetics of the ORR and OER, but their high costs restrict their practical commercial applications. Thus, highly active and strong non-noble metal catalysts are essential to address the cost and durability challenge. Based on previous research, carbon-based catalysts may present the best alternatives to these precious metals in the future owing to their affordability, very large surface areas, and superior mechanical and electrical qualities. In particular, carbon aerogels prepared using biomass as the precursors are referred to as biomass-derived carbon aerogels. They have sparked broad attention and demonstrated remarkable performance in the energy conversion and storage sectors as they are ecologically beneficial, affordable, and have an abundance of precursors. Therefore, this review focuses on various nanostructured materials based on biomass-derived carbon aerogels as ORR/OER catalysts, including metal atoms, metal compounds, and alloys.

Binary glycerol-based deep eutectic solvents containing zinc nitrate hexahydrate salt for rechargeable zinc air batteries applications with enhanced properties

Deep eutectic solvents (DESs) have attracted interest due to their unique and favorable electrochemical characteristics. This study reported a novel binary glycerol-zinc salt deep eutectic solvents were prepared with a combination of hydrogen bond donor (glycerol (Gly)) and hydrogen bond acceptor (Zinc nitrate hexahydrate (ZNH)) at different molar ratios of 1:2, 1:3, 1:4, 1:5, and 1:6. The various physicochemical properties including viscosity, refractivity index, conductivity, thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), cyclic voltammetry (CV) and electrochemical impedance (EIS) were measured. The results showed that among the various combinations tested, DES 1:2 resulted in a low viscosity value of 690, 500, 310, 220, and 160 mPa (mPa s) at shear rate (_S^-1) values of 20, 30, 60, 100, and 200 respectively. Moreover, DES 1:2 resulted in more electrochemically stable solvents with a lower refractive index value of 1.446, and a higher conductivity (σ) of 4.41 mS/cm. The findings found disclose the features, nature and of properties of prepared DESs as a potential solvents for different electrochemical storage applications.

Modulating the Oxygen Reduction Reaction Performance via Precisely Tuned Reactive Sites in Porphyrin-Based Covalent Organic Frameworks

Covalent organic frameworks (COFs) have emerged as promising electrocatalysts due to their controllable architectures, highly exposed molecular active sites, and ordered structures. In this study, a series of porphyrin-based COFs (TAPP-x-COF) with various transition metals (Co, Ni, Fe) were synthesized via a facile post-metallization strategy under solvothermal synthesis. The resulting porphyrin-based COFs showed oxygen reduction reaction (ORR) activity with a trend in Co > Fe > Ni. Among them, TAPP-Co-COF exhibited the best ORR activity (E_1/2 = 0.66 V and jL = 4.82 mA cm^-2) in alkaline media, which is comparable to those of Pt/C under the same conditions. Furthermore, TAPP-Co-COF was employed as a cathode in a Zn-air battery, demonstrating a high power density of 103.73 mW cm^-2 and robust cycling stability. This work presents a simple method for using COFs as a smart platform to fabricate efficient electrocatalysts.

FeCo alloy entrapped in N-doped graphitic carbon nanotubes-on-nanosheets prepared by coordination-induced pyrolysis for oxygen reduction reaction and rechargeable Zn-air battery

Oxygen reduction reaction (ORR) on cathode severely suffers from sluggish kinetics in zinc-air batteries. Therefore, substantial efforts have been made to prepare advanced electrocatalysts for facilitating the ORR. Herein, we synthesized FeCo alloyed nanocrystals entrapped in N-doped graphitic carbon nanotubes on nanosheets (FeCo-N-GCTSs) by 8-aminoquinoline coordination-induced pyrolysis, whose morphology, structures, and property were characterized in details. Impressively, the obtained FeCo-N-GCTSs catalyst showed a positive onset potential (E_onset = 1.06 V) and half-wave potential (E_1/2 = 0.88 V), revealing excellent ORR activity. Further, the FeCo-N-GCTSs assembled zinc-air battery displayed the maximum power density of 133 mW cm^-2 and negligible gap change in the discharge-charge voltage plot over 288 h (ca. 864 cycles) at 5 mA cm^-2, outperforming the Pt/C + RuO₂ based counterpart. This work provides a facile route for construction of high-efficiency, durable and low-cost nanocatalysts for the ORR in fuel cells and rechargeable Zn-air batteries.

Photovoltaic-driven electrocatalytic upcycling poly(ethylene terephthalate) plastic waste coupled with hydrogen generation

The electrochemical upconversion of plastic wastes has been demonstrated as an attractive alternative to the sluggish OER process to simultaneously produce valued chemicals and reduce the energy consumption. Herein, we report a photovoltaic-driven electrocatalytic strategy to upcycle poly(ethylene terephthalate) (PET) into value-added formic acid products and co-produce green hydrogen. The waste PET was dissolved by KOH and then directly pumped into an electrochemical flow reactor (EFR) including CuO nanowires (NWs) anode and Pt/C 20% cathode (PV-EFR) and driven by the commercial silicon photovoltaic (PV) panels. This PV-EFR system exhibits a solar-to-chemical (STC) efficiency of 32.6% under AM 1.5 G simulated sunlight (100 mW cm^-2), and high Faradaic efficiencies (FE, ∼ 67% for formic acid, and ∼90% for green hydrogen) with exceptional 120 h long-term stability in the STC mode. Such a photovoltaic-driven electrocatalytic strategy exhibits great potential for the rational utilization of renewable energy sources to produce high-value chemicals and fuels by upconversion of waste plastics.

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.

Cobalt Nanoparticles Anchored on N-Doped Porous Carbon Derived from Yeast for Enhanced Electrocatalytic Oxygen Reduction Reaction

Biomass-derived carbon materials have received extensive attention for use in high-performance electrocatalysts. In this study, a highly efficient electrocatalyst is developed with Co nanoparticles anchored on N-doped porous carbon material (CoNC) by using yeast as a biomass precursor through a facial activation and pyrolysis process. CoNC exhibits comparable catalytic activity with commercial 20 % Pt/C for the oxygen reduction reaction (ORR) with a half-wave potential of 0.854 V. A home-made primary Zn-air battery exhibited an open circuit potential of 1.45 V and a peak power density of 188 mW cm^-2 . Moreover, the discharge voltage of the primary battery maintained at a stable value up to 9 days. The enhanced performance of CoNC was probably ascribed to its high content of pyridinic-N and graphitic-N species, extra Co loading and porous structure, which provided sufficient active sites and channels to promote mass/electron transfer for ORR. This work provides a promising strategy to develop an efficient non-noble metal carbon-based electrocatalyst for fuel cells and metal-air batteries.

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.