Construction Of Ferrous Tungstate-based Oxygen Reduction Electrocatalyst And Its Application In Zn-air Batteries

Zn-air battery is considered as an ideal energy conversion device,which is environmentally friendly,safe and inexpensive,and provides a reasonable solution to solve the problem of energy shortage.However,the slow multi-electron transfer path of cathode oxygen reduction reaction（ORR）seriously affects the power density and energy conversion efficiency of Zn-air batteries.Therefore,it is extremely important to develop efficient ORR electrocatalysts to improve the reaction kinetics and practical performance of Zn-air batteries.Platinum-based catalysts are commonly used as ORR electrocatalysts,but high cost and low stability limit their commercial application in zinc-air batteries.Therefore,the development of non-noble metal ORR catalyst with low cost,excellent activity and good cycle stability is an important link to realize Zn-air batteries as the next generation of energy technology device.Therefore,Fe WO₄ with appropriate electronic structure and high electrochemical stability was selected as the basis in this paper.The electronic structure of Fe WO₄ was optimized to enrich the number of active sites,improve the adsorption behavior of oxygen on the catalyst surface,and enhance the catalytic activity of oxygen reduction of Fe WO₄ based catalyst through nonmetallic element doping and defect regulation.The main research contents of this paper are as follows:（1）Using graphite phase carbon nitride（g-C₃N₄）with high nitrogen content as nitrogen source,bacterial fiber gel as carbon source,ferric chloride and tungstic acid dissolved in ammonia water as metal salt,thiourea as sulfur source,porous sponge aerogel was obtained by freeze-drying method.Then,the sulfur-doped Fe WO₄supported multi-nitrogen doped carbon（S-Fe WO₄/NC）ORR electrocatalyst was obtained by high temperature pyrolysis.It was found that the introduction of S atom with larger atomic radius could optimize the electronic structure of Fe WO₄ and improve the intrinsic ORR activity of Fe WO₄.After S doping,the potential range of O₂adsorption and reduction reaction on the surface of S-Fe WO₄/NC catalyst was reduced,and the process of oxygen reduction was accelerated.The ORR half-wave potential of S-Fe WO₄/NC was 40 m V higher than that of Fe WO₄/NC,and the limiting current density(5.29 m A cm^-2)was larger.As an air cathode material,the peak power density of S-Fe WO₄/NC-based Zn-air battery is 150.3 m W cm^-2,and the specific capacity is790.7 m Ah g^-1.The voltage difference of the battery can keep stable within 220 h without obvious attenuation,which indicates that the battery has good cycle stability.（2）Inspired by the previous work,F（χ=4.0）with the largest electronegativity was selected as the doping atom,and ammonium fluoride was used as the fluorine source to construct the F-doped Fe WO₄ supported multi-nitrogen doped carbon（F-Fe WO₄/NC）catalyst.The entry of nonmetallic element fluorine into the main lattice of Fe WO₄ enhances the adsorption capacity of hydroxide anions and oxygen-containing intermediates to ORR,and induces the metal center to have a high oxidation number,resulting in the generation of oxygen vacancy active center.Moreover,the introduction of fluorine increases the polarity of the chemical bond in Fe WO₄,which increases the surface wettability of F-Fe WO₄/NC and decreases the interface resistance between electrode and electrolyte,thus promoting the adsorption of reactants and electrolyte diffusion.The thermal decomposition of ammonium fluoride not only provides in situ F doping,but also promotes the formation of template-free porous structure by gas precipitation,resulting in a greater degree of defects in the F-Fe WO₄/NC catalyst,which plays a crucial role in improving the catalytic activity.F-Fe WO₄/NC showed excellent ORR catalytic activity with a half-wave potential of 0.85 V.The Zn-air battery assembled with F-Fe WO₄/NC as the cathode active material showed a peak power density of 173.5 m W cm^-2,a specific capacity of 800.2 m Ah g^-1 and a long-term cycle stability of 170 h.（3）Using chitosan as carbon and nitrogen sources,phosphotungstic acid as phosphorus and tungsten sources,and ferric chloride as iron sources,the P-Fe WO₄/PNC porous carbon catalyst supported by phosphorus-doped ferrous tungstate was prepared by Na Cl template assisted pyrolysis.The doping of P atoms with abundant valence electrons can increase the electron density of the Fe site in P-Fe WO₄/PNC,which helps to optimize the electronic structure of Fe WO₄,thus inhibiting the formation of HO₂^-on the catalyst surface.Na Cl template assisted pyrolysis is beneficial to the construction of porous carbon structures with large specific surface area,which can synergistically enhance the oxygen reduction activity of P-Fe WO₄/PNC catalyst.P-Fe WO₄/PNC showed excellent ORR catalytic performance with a half-wave potential of 0.86 V.When P-Fe WO₄/PNC catalyst is used as the active material of air electrode in Zn-air battery,the peak power density of 172.2 m W cm^-2,the specific capacity of810.1 m Ah g^-1 and the charge-discharge cycle of 240 h are stable.