Application Of MOFs-derived Tungsten-based Electrocatalysts In Zn-air Batteries

In order to solve the problem of fossil fuel consumption and consequent environmental pollution,in recent years,people have made great efforts to develop clean and sustainable energy.The Zn-air battery（ZAB）is environmentally friendly,resource-rich,low-cost,and highly safe,with a high theoretical energy density of 1086 Wh kg^-1,and is considered to be one of the best alternative technologies to meet future energy storage and conversion needs.The discharge and charge processes of ZAB correspond to the oxygen reduction reaction（ORR）and oxygen evolution reaction（OER）on the air cathode,respectively.Therefore,the charge-discharge performance of ZAB is limited by the slow reaction kinetics.Platinum carbon（Pt/C）and Ruthenium dioxide（Ru O₂）are the most advanced commercial ORR and OER electrocatalysts,respectively,but they have scarce reserves and high cost.Therefore,the development of efficient,durable and low-cost oxygen electrocatalysts is the key to the large-scale application of ZAB.Tungsten carbide（WC）has a good electrical conductivity,its d-band center can be adjusted,so it is easy to balance the adsorption and desorption levels of oxygen.WC has been proved to be stable in both acidic and alkaline environments.Therefore,tungsten-based catalytic materials have great potential as oxygen electrocatalysts.In this paper,tungsten-based electrocatalytic materials were successfully prepared by chemical precipitation method and high temperature carbonization method,and the materials were applied to Zn-air batteries.The main contents are as follows:（1）The development of ORR/OER bi-functional non-noble metal based electrocatalysts is very important for realizing rechargeable ZAB and reducing the preparation cost of ZAB.In this work,WC and Co₇Fe₃ embedded in N,P co-doped hierarchical carbon（WC/Co₇Fe₃-NPHC）was prepared using zeolite imidazole frameworks as precursor.Density functional theory calculation shows that WC,Co₇Fe₃and N,P co-doped carbon regulate each other at the three-phase heterojunction so that the catalyst has moderate adsorption strength,which greatly improves the conductivity and electron transfer rate of the catalyst.The results show that the overall oxygen redox potential difference of WC/Co₇Fe₃-NPHC is low to 0.72 V.The ZAB assembled by WC/Co₇Fe₃-NPHC as an air cathode has a peak power density of 270 m W cm^-2.When WC/Co₇Fe₃-NPHC is applied to flexible solid ZAB,it can maintain good charge and discharge stability under different bending angles.This work opens new avenues for the development of low-cost,efficient electrocatalysts that could potentially replace the widespread use of precious metals in ZAB energy systems.（2）The development of electrocatalyst for ORR in seawater electrolyte is the key to realizing utilization of marine resources and application of ZAB.Exploring the adsorption and desorption behavior of oxygen can guide the design of catalyst fundamentally and improve the catalytic performance.On this basis,WC and Co on nitrogen-doped carbon（WC-Co@NC）with interfacial built-in electric field（BEF）was designed as ORR catalyst.The BEF of W-C-Co connections can promote the charge separation and directional migration,thus realizing the oxygen adsorption equilibrium and speeding up the process of ORR.These give WC-Co@NC a high half-wave potential of 0.872 V,and the alkaline seawater-based ZAB with WC-Co@NC as the air cathode has a peak power density of 189 m W cm^-2 and excellent stability.This work will provide a basis for the rational design of the cathode of alkaline seawater-based ZAB,and will also give a new direction for the utilization of seawater resources.