Efforts at Enhancing Bifunctional Electrocatalysis and Related Events for Rechargeable Zinc‐Air Batteries

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

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

Scalable One-Pot Fabrication of Carbon-Nanofiber-Supported Noble-Metal-Free Nanocrystals for Synergetic-Dependent Green Hydrogen Production: Unraveling Electrolyte and Support Effects

Electrocatalytic hydrogen evolution reactions (HER) are envisaged as the most promising sustainable approach for green hydrogen production. However, the considerably high cost often associated with such reactions, particularly upon scale-up, poses a daunting challenge. Herein, a facile, effective, and environmentally benign one-pot scalable approach is developed to fabricate MnM (M═Co, Cu, Ni, and Fe) nanocrystals supported over in situ formed carbon nanofibers (MnM/C) as efficient noble-metal-free electrocatalysts for HER. The formation of carbon nanofibers entails impregnating cellulose in an aqueous solution of metal precursors, followed by annealing the mixture at 550 °C. During the impregnation process, cellulose acts as a reactor for inducing the in situ reductions of MnM salts with the assistance of ether and hydroxyl groups to drive the mass production (several grams) of ultralong (5 ± 1 μM) carbon nanofibers ornamented with MnM nanoparticles (10-14 nm in size) at an average loading of 2.87 wt %. For better electrocatalytic HER benchmarking, the fabricated catalysts were tested over different working electrodes, i.e., carbon paper, carbon foam, and glassy carbon, in the presence of different electrolytes. All the fabricated MnM/C catalysts have demonstrated an appealing synergetic-effect-dependent HER activity, with MnCo/C exhibiting the best performance over carbon foam, close to that of the state-of-the-art commercial Pt/C (10 wt % Pt), with an overpotential of 11 mV at 10 mA cm-2, a hydrogen production rate of 2448 mol g-1 h-1, and a prolonged stability of 2 weeks. The HER performance attained by MnCo/C nanofibers is among the highest reported for Pt-free electrocatalysts, thanks to the mutual alloying effect, higher synergism, large surface area, and active interfacial interactions over the nanofibers. The presented findings underline the potential of our approach for the large-scale production of cost-effective electrocatalysts for practical HER.

Engineering Ti3C2-MXene Surface Composition for Excellent Li+ Storage Performance

Exploiting novel materials with high specific capacities is crucial for the progress of advanced energy storage devices. Intentionally constructing functional heterostructures based on a variety of two-dimensional (2D) substances proves to be an extremely efficient method for capitalizing on the shared benefits of these materials. By elaborately designing the structure, a greatly escalated steadiness can be achieved throughout electrochemical cycles, along with boosted electron transfer kinetics. In this study, chemical vapor deposition (CVD) was utilized to alter the surface composition of multilayer Ti3C2Tx MXene, contributing to contriving various layered heterostructure materials through a precise adjustment of the reaction temperature. The optimal composite materials at a reaction temperature of 500 °C (defined as MX500), incorporating MXene as the conductive substrate, exhibited outstanding stability and high coulombic efficiency during electrochemical cycling. Meanwhile, the reactive sites are increased by using TiS2 and TiO2 at the heterogeneous interfaces, which sustains a specific capacity of 449 mAh g-1 after 200 cycles at a current density of 0.1 A g-1 and further demonstrates their exceptional electrochemical characteristics. Additionally, the noted pseudocapacitive properties, like MXene materials, further highlight the diverse capabilities of intuitive material design. This study illuminates the complex details of surface modification in multilayer MXene and offers a crucial understanding of the strategic creation of heterostructures, significantly impacting sophisticated electrochemical applications.

Sustainable zinc-air battery chemistry: advances, challenges and prospects

Sustainable zinc-air batteries (ZABs) are considered promising energy storage devices owing to their inherent safety, high energy density, wide operating temperature window, environmental friendliness, etc., showing great prospect for future large-scale applications. Thus, tremendous efforts have been devoted to addressing the critical challenges associated with sustainable ZABs, aiming to significantly improve their energy efficiency and prolong their operation lifespan. The growing interest in sustainable ZABs requires in-depth research on oxygen electrocatalysts, electrolytes, and Zn anodes, which have not been systematically reviewed to date. In this review, the fundamentals of ZABs, oxygen electrocatalysts for air cathodes, physicochemical properties of ZAB electrolytes, and issues and strategies for the stabilization of Zn anodes are systematically summarized from the perspective of fundamental characteristics and design principles. Meanwhile, significant advances in the in situ/operando characterization of ZABs are highlighted to provide insights into the reaction mechanism and dynamic evolution of the electrolyte|electrode interface. Finally, several critical thoughts and perspectives are provided regarding the challenges and opportunities for sustainable ZABs. Therefore, this review provides a thorough understanding of the advanced sustainable ZAB chemistry, hoping that this timely and comprehensive review can shed light on the upcoming research horizons of this prosperous area.

Cobalt Phthalocyanine-Doped Polymer-Based Electrocatalyst for Rechargeable Zinc-Air Batteries

Rechargeable zinc-air batteries (RZAB) have gained significant attention as potential energy storage devices due to their high energy density, cost-effectiveness, and to the fact that they are environmentally safe. However, the practical implementation of RZABs has been impeded by challenges such as sluggish oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), including poor cyclability. Herein, we report the preparation of cobalt- and nitrogen-doped porous carbon derived from phloroglucinol-formaldehyde polymer networks with 2-methyl imidazole and cobalt phthalocyanine as precursors for nitrogen and cobalt. The CoN-PC-2 catalyst prepared in this study exhibits commendable electrocatalytic activity for both ORR and OER, evidenced by a half-wave potential of 0.81 V and E_j=10 of 1.70 V. Moreover, the catalyst demonstrates outstanding performance in zinc-air batteries, achieving a peak power density of 158 mW cm^-2 and displaying excellent stability during charge-discharge cycles. The findings from this study aim to provide valuable insights and guidelines for further research and the development of hierarchical micro-mesoporous carbon materials from polymer networks, facilitating their potential commercialisation and widespread deployment in energy storage applications.

Iron, Cobalt, and Nickel Phthalocyanine Tri-Doped Electrospun Carbon Nanofibre-Based Catalyst for Rechargeable Zinc-Air Battery Air Electrode

The goal of achieving the large-scale production of zero-emission vehicles by 2035 will create high expectations for electric vehicle (EV) development and availability. Currently, a major problem is the lack of suitable batteries and battery materials in large quantities. The rechargeable zinc-air battery (RZAB) is a promising energy-storage technology for EVs due to the environmental friendliness and low production cost. Herein, iron, cobalt, and nickel phthalocyanine tri-doped electrospun carbon nanofibre-based (FeCoNi-CNF) catalyst material is presented as an affordable and promising alternative to Pt-group metal (PGM)-based catalyst. The FeCoNi-CNF-coated glassy carbon electrode showed an oxygen reduction reaction/oxygen evolution reaction reversibility of 0.89 V in 0.1 M KOH solution. In RZAB, the maximum discharge power density (P_max) of 120 mW cm^-2 was obtained with FeCoNi-CNF, which is 86% of the P_max measured with the PGM-based catalyst. Furthermore, during the RZAB charge-discharge cycling, the FeCoNi-CNF air electrode was found to be superior to the commercial PGM electrocatalyst in terms of operational durability and at least two times higher total life-time.

Porous spinel-type transition metal oxide nanostructures as emergent electrocatalysts for oxygen reduction reactions

Porous spinel-type transition metal oxide (PS-TMO) nanocatalysts comprising two kinds of metal (denoted as A_xB_3-xO₄, where A, B = Co, Ni, Zn, Mn, Fe, V, Sm, Li, and Zn) have emerged as promising electrocatalysts for oxygen reduction reactions (ORRs) in energy conversion and storage systems (ECSS). This is due to the unique catalytic merits of PS-TMOs (such as p-type conductivity, optical transparency, semiconductivity, multiple valence states of their oxides, and rich active sites) and porous morphologies with great surface area, low density, abundant transportation paths for intermediate species, maximized atom utilization and quick charge mobility. In addition, PS-TMOs nanocatalysts are easily prepared in high yield from Earth-abundant and inexpensive metal precursors that meet sustainability requirements and practical applications. Owing to the continued developments in the rational synthesis of PS-TMOs nanocatalysts for ORRs, it is utterly imperative to provide timely updates and highlight new advances in this research area. This review emphasizes recent research advances in engineering the morphologies and compositions of PS-TMOs nanocatalysts in addition to their mechanisms, to decipher their structure-activity relationships. Also, the ORR mechanisms and fundamentals are discussed, along with the current barriers and future outlook for developing the next generation of PS-TMOs nanocatalysts for large-scale ECSS.

Heteroatom-Doped Porous Carbon-Based Nanostructures for Electrochemical CO2 Reduction

The continual rise of the CO2 concentration in the Earth's atmosphere is the foremost reason for environmental concerns such as global warming, ocean acidification, rising sea levels, and the extinction of various species. The electrochemical CO2 reduction (CO2RR) is a promising green and efficient approach for converting CO2 to high-value-added products such as alcohols, acids, and chemicals. Developing efficient and low-cost electrocatalysts is the main barrier to scaling up CO2RR for large-scale applications. Heteroatom-doped porous carbon-based (HA-PCs) catalysts are deemed as green, efficient, low-cost, and durable electrocatalysts for the CO2RR due to their great physiochemical and catalytic merits (i.e., great surface area, electrical conductivity, rich electrical density, active sites, inferior H2 evolution activity, tailorable structures, and chemical-physical-thermal stability). They are also easily synthesized in a high yield from inexpensive and earth-abundant resources that meet sustainability and large-scale requirements. This review emphasizes the rational synthesis of HA-PCs for the CO2RR rooting from the engineering methods of HA-PCs to the effect of mono, binary, and ternary dopants (i.e., N, S, F, or B) on the CO2RR activity and durability. The effect of CO2 on the environment and human health, in addition to the recent advances in CO2RR fundamental pathways and mechanisms, are also discussed. Finally, the evolving challenges and future perspectives on the development of heteroatom-doped porous carbon-based nanocatalysts for the CO2RR are underlined.

Tailoring the defects of sub-100 nm multipodal titanium nitride/oxynitride nanotubes for efficient water splitting performance

Deciphering the photocatalytic-defect relationship of photoanodes can pave the way towards the rational design for high-performance solar energy conversion. Herein, we rationally designed uniform and aligned ultrathin sub-100 nm multipodal titanium nitride/oxynitride nanotubes (TiON _x NTs) (x = 2, 4, and 6 h) via the anodic oxidation of Ti-foil in a formamide-based electrolyte followed by annealing under ammonia gas for different durations. XPS, XPS imaging, Auger electron spectra, and positron annihilation spectroscopy disclosed that the high nitridation rate induced the generation of a mixture of Ti-nitride and oxynitride with various vacancy-type defects, including monovacancies, vacancy clusters, and a few voids inside TiO _x NTs. These defects decreased the bandgap energy to 2.4 eV, increased visible-light response, and enhanced the incident photon-to-current collection efficiency (IPCE) and the photocurrent density of TiON _x NTs by nearly 8 times compared with TiO₂NTs, besides a quick carrier diffusion at the nanotube/electrolyte interface. The water-splitting performance of sub-100 nm TiON₆NT multipodal nanotubes was superior to the long compacted TiON _x NTs with different lengths and TiO₂ nanoparticles. Thus, the optimization of the nitridation rate tailors the defect concentration, thereby achieving the highest solar conversion efficiency.