Autocatalytically Grown Carbon Nanotube-based Electrocatalysts And Their Applications In Zinc-air Batteries

Among the many electrochemical energy devices,zinc-air batteries have a broad application potential in the market due to their abundant raw material sources,high theoretical density and environmental friendliness.However,the practical application of this energy technology is constrained by the slow oxygen reduction reaction(ORR)and oxygen precipitation reaction(OER)kinetics at the electrode interface.Despite Pt,Ir and Ru-based precious metals have attracted much attention in the field of catalysis due to excellent ORR or OER activities,their commercial development is restricted by adverse factors such as high price and easy deactivation.Therefore,how to design a non-precious metal bifunctional catalyst with high activity,strong alkali resistance and low cost is a hot issue in current research.Transition metal oxide and sulfide nanoparticles with excellent intrinsic activity are of great interest in the field of electrocatalysis,however,its further development is hindered by its own poor electrical conductivity.Anchoring these metal oxide or sulfide nanoparticles on a carbon substrate with excellent electrical conductivity is a good strategy to solve this problem.In this paper,self-catalyzed induced growth of nitrogen-doped carbon nanotubes as a conductive carrier is used to prepare oxygen catalysts with both ORR and OER activities by coupling transition metal nanoparticles,and they are applied to self-assembled zinc-air batteries to achieve specific practical value.The details are as follows:(1)The NCNTs/Cowith high electrical conductivity and high ORR catalytic activity was selected as the conductive substrate,and by coupling with Mn₃O₄nanoparticles with better intrinsic activity,it not only provided a substrate for the dispersion of Mn₃O₄,but also rationally constructed an Oxide-NCNTs-Coelectron transfer channel with a non-homogeneous interface,which accelerated the charge transfer rate of the catalytic process and reduced the intrinsic internal resistance of Mn₃O₄,providing a feasible strategy to solve the problem of poor conductivity of the semiconductor oxide itself,and finally realized the bifunctional catalytic activity of the composite.The optimal catalyst Mn₃O₄/NCNTs/Co-2 has an ultimate current density j=-5.7 mA cm^-2 with a Tafel slope of only 56 mV dec^-1 for the ORR and?₁₀=390 mV with a Tafel slope of 104 mV dec^-1 for the OER.The optimal catalyst Mn₃O₄/NCNTs/Co-2 can reach a peak power density of 128 mW cm^-2 in self-assembled alkaline zinc-air batteries with a voltage gap of only 0.91 V after 160 h of continuous charging and discharging at 5 mA cm^-2.(2)NCNTs/Ni catalytically grown with nickel salts are used as conductive substrates,which are coordinately coupled with CoS_x nanoparticles with high conductivity and intrinsic activity to obtain efficient ORR/OER catalysts.It was analyzed that the formation of Co-N bonds in CoS_x/NCNTs/Ni composites could effectively enhance the interfacial charge transfer,resulting in a synergistic effect and enhanced oxygen electrochemical reaction.Using NCNTs/Ni as a conductive substrate not only provides a site for the growth and dispersion of CoS_xnanoparticles to increase the exposure of active sites,but also its ultrathin carbon layer structure promotes the activation of O₂ at the catalytic interface to further enhance the ORR kinetics.The bifunctional catalyst CoS_x/NCNTs/Ni-2 exhibited excellent ORR activity with an E_1/2 of 0.79 V and Tafel slope of 96 mV dec^-1 for OER,the?₁₀=310mV and Tafel slope of 88 mV dec^-1 for OER.Notably,alkaline zinc-air batteries assembled with CoS_x/NCNTs/Ni-2 catalysts could achieve a peak power density of131 mW cm^-2 and remained stable after 200 h of continuous charge/discharge tests.