Room-temperature Synthesis Of MnO₂/C Composite For Oxygen Reduction And Oxygen Evolution Reaction

MnO₂,owing to its inexpensive and environment-friendly features,has wide applications in electrochemical field.To improve the conductivity of MnO₂ materials and suppress the agglomeration of MnO₂ particles,MnO₂ is generally mixed with carbon for use.To a certain extent,the introduction of carbon suppresses the agglomeration of MnO₂ particles,and improves the electrochemical stability of MnO₂.Nevertheless,the electrochemical stability of this physical mixture is very limited because of the weak interaction between MnO₂ and carbon.Thus,to further enhance MnO₂ stability,direct growth of MnO₂ on carbon surface was developed,such as electrodeposition,microwave-assisted synthesis,and hydrothermal method.Among these methods,the hydrothermal method smartly uses the redox reaction between C and KMnO₄,which is the most promising technique for practical production.Despite that,the hydrothermal synthesis was generally carried out beyond 100 ^oC.Therefore,this thesis develops a room-temperature method for MnO₂ growth on the C surface by employing Co^II-catalytic effect on the redox reaction between KMnO₄ and C,achieving MnO₂/C composite,and investigates the electrocatalytic performance of as-prepared MnO₂/C.（1）At room temperature,α-MnO₂/C_ZIF-67 composite was prepared via the redox reaction between KMnO₄ and carbonized ZIF-67 metal-organic framework(C_ZIF-67).As-prepared MnO₂ has a sheet-like structure and covers the whole C_ZIF-67 particle,showing a large specific surface area（⁷6 m²/g）.Also,control experiment results indicate that the room-temperature formation of MnO₂ originates from the catalytic effect of Co^II embedded into C_ZIF skeleton,revealing the room-temperature reaction mechanism of KMnO₄ and C_ZIF.（2）The electrocatalytic performance ofα-MnO₂/C_ZIF-67 was further evaluated by employing oxygen reduction as a model reaction.The results indicate thatα-MnO₂/C_ZIF-67 composite shows a high current density of 5.92 mA·cm^-2,11.28%higher than commercial Pt/C;the catalytic onset potential is⁰.88 V,slightly less better than Pt/C,but superior to most of reported MnO₂ catalysts.Besides,α-MnO₂/C_ZIF-67IF-67 exhibits better stability and tolerance to methanol poisoning than Pt/C:after 11 h continuous catalysis,the current density remains 81%of the original whereas the current density of Pt/C remains only 68%.（3）Ni^II-doped MnO₂/C_ZIF-67 composites(Ni^II-MnO₂/C_ZIF-67)was synthesized by one-step strategy involving C_ZIF-67 as carbon source and Ni（NO₃）₂ as an additive in KMnO₄ aqueous solution.As-prepared Ni^II-MnO₂/C_ZIF-67 composite has a comparable catalytic performance to commercial RuO₂ towards oxygen evolution reaction,with an overpotential of⁰.36 V,superior to MnO₂/C_ZIF-67（0.54 V）.Moreover,Ni^II-MnO₂/C_ZIF-67 catalyst has better catalytic stability than RuO₂/C and nickel hydroxides:after 1h continuous catalysis,its overpotential did not show a noticeable change.