Metal-organic framework-derived advanced oxygen electrocatalysts as air-cathodes for Zn-air batteries: recent trends and future perspectives

Electrochemical energy storage devices with stable performance, high power output, and energy density are urgently needed to meet the global energy demand. Among the different electrochemical energy storage devices, batteries have become the most promising energy technologies and ranked as a highly investigated research subject. Recently, metal-air batteries especially Zn-air batteries (ZABs) have attracted enormous scientific interest in the electrochemical community due to their ease of operation, sustainability, environmental friendliness, and high efficiency. The oxygen electrocatalytic reactions [oxygen reduction reaction (ORR) and oxygen evolution reaction (OER)] are the two fundamental reactions for the development of ZABs. Noble metal-based electrocatalysts are widely considered as the benchmark for oxygen electrocatalysis, but their practical application in rechargeable ZAB is hindered due to several shortcomings. Thus, to replace noble metal-based catalysts, a wide range of transition-metal-based materials and heteroatom-doped metal-free carbon materials has been extensively investigated as oxygen electrocatalysts for ZABs. Recently, metal-organic frameworks (MOFs) with unique structural flexibility and uniformly dispersed active sites have become attractive precursors for the synthesis of a large variety of advanced functional materials. Herein, we summarize the recent progress of MOF-derived oxygen electrocatalysts (MOF-derived carbon nanomaterials, MOF-derived alloys/nanoparticles, and MOF-derived single-atom electrocatalysts) for ZABs. Specifically, we highlight MOF-derived single-atom electrocatalysts owing to the wide exploration of these emerging materials in electrocatalysis. The influence of the active sites, structural/compositional design, and porosity of MOF-derived advanced materials on the oxygen electrocatalytic performances is also discussed. Finally, the existing challenges and prospects of MOF-derived electrocatalysts in ZABs are briefly highlighted.

Influencing Factors of Performance Degradation of Zinc–Air Batteries Exposed to Air

Zinc–air batteries feature high energy density, but they usually suffer from their short storage life after they start working, restricting their commercial applications. In the past, scholars did not reach an agreement on the influencing factors of the performance degradation of zinc–air batteries when exposed to air. Here, a series of comparative experiments were conducted to confirm the changes of the battery during storage after being exposed to air. The morphology and composition of the components of the battery were characterized by scanning electron microscopy (SEM) and X-ray diffraction analyses. SEM images revealed that with the increase of storage days, the corrosion of the zinc anode gradually deepens, but the surface morphology of the air cathode does not change much. The electrolyte of the batteries stored for different periods was examined through inductively coupled plasma spectroscopy and titration. After 20 days of storage, the concentration of CO32− reached 2.694 mol L−1, which indicates that more than 80% of the OH− in the electrolyte was consumed. The results show that after being exposed to air, the carbonation of the electrolyte is the main cause of the battery capacity decay.

Recoverable peroxidase-like Fe3O4@MoS2-Ag nanozyme with enhanced antibacterial ability

Antibacterial agents with enzyme-like properties and bacteria-binding ability have provided an alternative method to efficiently disinfect drug-resistance microorganism. Herein, a Fe₃O₄@MoS₂-Ag nanozyme with defect-rich rough surface was constructed by a simple hydrothermal method and in-situ photodeposition of Ag nanoparticles. The nanozyme exhibited good antibacterial performance against E. coli (~69.4%) by the generated ROS and released Ag⁺, while the nanozyme could further achieve an excellent synergistic disinfection (~100%) by combining with the near-infrared photothermal property of Fe₃O₄@MoS₂-Ag. The antibacterial mechanism study showed that the antibacterial process was determined by the collaborative work of peroxidase-like activity, photothermal effect and leakage of Ag⁺. The defect-rich rough surface of MoS₂ layers facilitated the capture of bacteria, which enhanced the accurate and rapid attack of •OH and Ag⁺ to the membrane of E. coli with the assistance of local hyperthermia. This method showed broad-spectrum antibacterial performance against Gram-negative bacteria, Gram-positive bacteria, drug-resistant bacteria and fungal bacteria. Meanwhile, the magnetism of Fe₃O₄ was used to recycle the nanozyme. This work showed great potential of engineered nanozymes for efficient disinfection treatment.

An Electrically Rechargeable Zinc/Air Cell with an Aqueous Choline Acetate Electrolyte

Due to the feasibility of an electrically rechargeable zinc/air cell made of a zinc foil as active material, an aqueous choline acetate (ChAcO) mixture as an electrolyte and a spinel MnCo₂O₄ (MCO) and NiCo₂O₄ (NCO) as a bi-functional oxygen catalyst was investigated in this work. The 30 wt.% water-containing aqueous ChAcO solution showed high contact angles close to those of KOH favoring triple-phase boundary formation in the gas diffusion electrode. Conductivity and pH value of 30 wt.% H₂O/ChAcO amounted to 5.9 mS cm⁻¹ and 10.8, respectively. Best results in terms of reversible capacity and longevity of zinc/air cell were yielded during 100 h charge/discharge with the MnCo₂O₄ (MCO) air electrode polarization procedure at 100 µA cm⁻² (2.8 mA g⁻¹_zinc). The corresponding reversible capacity amounted to 25.4 mAh (28% depth of discharge (DOD)) and the energy efficiency ranged from 29–54% during the first and seventh cycle within a 1500 h polarization period. Maximum active material utilization of zinc foil at 100 µA cm⁻² was determined to 38.1 mAh (42% DOD) whereas a further charging step was not possible due to irreversible passivation of the zinc foil surface. A special side-by-side optical cell was used to identify reaction products of the zinc/air system during a single discharge step in aqueous ChAcO that were identified as Zn(OH)₂ and ZnO by Raman analysis while no carbonate was detected.

Influence of Al Alloying on the Electrochemical Behavior of Zn Electrodes for Zn-Air Batteries With Neutral Sodium Chloride Electrolyte

Zn alloy electrodes containing 10 wt. % Al were prepared to examine the applicability as anodes in primary Zn-air batteries with neutral 2M NaCl electrolyte. These electrodes were investigated by electrochemical measurements and microscopic techniques (SEM, LSM, AFM). Based on the cyclic voltammetry and intermediate term (24 h) discharge experiments, the only active element in the as-prepared alloy was found to be Zn. It was further confirmed by LSM that Zn rich areas dissolved while Al remained passive during discharge. The passive state of Al was also demonstrated by conductive AFM investigations on the as-cast alloy surfaces. The results on potentiodynamic polarization and weight loss measurements indicated that the alloy electrode was less prone to corrosion than pure Zn electrode. The electrochemical behavior of the electrodes was modified under certain cathodic polarization previous to measurements. Accordingly, originating from Al activation due to application of cathodic potentials, potentiodynamic polarization studies showed a clear shift on the corrosion potentials of the alloy toward more negative values. On the basis of these results, with the precondition of Al activation prior to discharge experiments, the effect of Al alloying on the Zn electrodes was revealed as temporarily enhanced potentials on the discharge profiles in comparison to pure Zn electrodes.

Electrically Rechargeable Zinc-Air Batteries: Progress, Challenges, and Perspectives

Zinc-air batteries have attracted much attention and received revived research efforts recently due to their high energy density, which makes them a promising candidate for emerging mobile and electronic applications. Besides their high energy density, they also demonstrate other desirable characteristics, such as abundant raw materials, environmental friendliness, safety, and low cost. Here, the reaction mechanism of electrically rechargeable zinc-air batteries is discussed, different battery configurations are compared, and an in depth discussion is offered of the major issues that affect individual cellular components, along with respective strategies to alleviate these issues to enhance battery performance. Additionally, a section dedicated to battery-testing techniques and corresponding recommendations for best practices are included. Finally, a general perspective on the current limitations, recent application-targeted developments, and recommended future research directions to prolong the lifespan of electrically rechargeable zinc-air batteries is provided.

Pt-free silver nanoalloy electrocatalysts for oxygen reduction reaction in alkaline media

Silver nanoalloy electrocatalysts with comparable activity and better stability than commercial Pt/C for oxygen reduction reaction (ORR) in advanced metal–air batteries and fuel cells.

Recent advances in zinc-air batteries

Zinc-air is a century-old battery technology but has attracted revived interest recently. With larger storage capacity at a fraction of the cost compared to lithium-ion, zinc-air batteries clearly represent one of the most viable future options to powering electric vehicles. However, some technical problems associated with them have yet to be resolved. In this review, we present the fundamentals, challenges and latest exciting advances related to zinc-air research. Detailed discussion will be organized around the individual components of the system - from zinc electrodes, electrolytes, and separators to air electrodes and oxygen electrocatalysts in sequential order for both primary and electrically/mechanically rechargeable types. The detrimental effect of CO2 on battery performance is also emphasized, and possible solutions summarized. Finally, other metal-air batteries are briefly overviewed and compared in favor of zinc-air.