Design Of MOF Derived Cobalt Phosphide/Porous Carbon-Based Composite Materials For Zinc-Air Batteries

The depletion of traditional fossil fuel and serious environmental problems has inspired intense interest in developing clean energy storage and conversion systems,such as metal-air batteries and fuel cells.Zinc-air batteries are regarded as one of the most promising devices owing to their high theoretical specific capacity,low cost,and environmentally friendly.However,the performance of zinc-air batteries is severely limited by the sluggish kinetics of the oxygen reduction reaction/oxygen evolution reaction（ORR/OER）on the air electrode.Therefore,it is important to explore efficient electrocatalysts to accelerate the reaction rate.Noble Pt/Ir/Ru-based catalysts with excellent catalytic performance are considered the most advanced catalysts,but the scarce reserves,high price,and single catalytic ability hinder their large-scale application.It is urgent to develop the low-cost and high activity of non-noble metal catalysts to replace noble catalysts in zinc-air batteries.Here,MOF-derived cobalt phosphide/porous carbon matrix composites are synthesized by carbonization and phosphating methods as efficient electrocatalysts for ORR and OER.The structure and active components of the electrocatalysts are analyzed and explored by various characterization techniques.The performance of the zinc-air battery composed of these materials is further performed.The main content and innovation points are as follows:（1）Transition metal phosphates with low cost and high OER catalytic activity have become advanced non-precious metal catalytic materials.However,their inherent low conductivity and poor ORR catalytic activity limit their further development.Herein,Co₂P nanoparticle-decorated N,P,and Fe co-doped porous hollow carbon spheres are successfully synthesized by utilizing bimetallic zeolite imidazole framework（Zn Co-ZIF）on the surface of polystyrene（PS）microspheres and introducing phytic acid and ferrocene.The ORR performance of Co₂P/Fe/NP-HCSs is effectively improved owing to the N,P doping and the formation of Fe-N-C active sites.The half-wave potential of Co₂P/Fe/NP-HCSs is 0.872 V in an alkaline medium.The OER performance of Co₂P/Fe/NP-HCSs is improved owing to the higher conductivity of Co₂P and the exposure of Co₂P active sites in the carbon substrate.The overpotential of Co₂P/Fe/NP-HCSs at 10 m A cm^-2is only 341m V.Liquid zinc-air batteries based on Co₂P/Fe/NP-HCSs achieved large specific capacity(780 m Ah g^-1),high power density(112 m W cm^-2),and long cycle stability（over 3500min）.Additionally,the flexible zinc-air battery driven by Co₂P/Fe/NP-HCSs exhibits a high open-circuit voltage（1.39 V）.（2）MOF-derived carbonized materials with abundant metal-N-C sites and N-doped graphite possess high catalytic activity for ORR.Transition metal phosphide exhibits outstanding intrinsic catalytic activity for OER.Herein,Co₂P and Co nanoparticles co-modified N-doped porous carbon（Co₂P/Co/N-C）is synthesized by calcining the mixture of Co-based zeolite imidazole framework（ZIF-67）and phytic acid.The characterization results show that Co₂P nanoparticles are successfully formed at high temperatures by cross-linked Co metal of phytic acid.Furthermore,the porous N-doped carbon substrate decorated with Co nanoparticles and Co-N-C sites is obtained by using the precursor of ZIF-67.Owing to the unique porous structure and abundant active sites,Co₂P/Co/N-C composites exhibit excellent bifunctional catalytic activity in alkaline solutions with a half-wave potential of 0.892 V for ORR and an overpotential of only 368 m V at 10 m A cm^-2for OER.The liquid zinc-air batteries based on Co₂P/Co/NP-C show excellent specific capacity(825 m Ah g^-1)and long cycle life（over 140 h）.In conclusion,the MOF-derived cobalt phosphating/porous carbon matrix composites show excellent bifunctional activity and cycle stability in electrochemical testing and zinc-air batteries,which provides a novel route for the development of low-cost and high-activity non-precious-metal catalysts.