Transition Metal(Mn,Co) Supported On Nitrogen-doped Carbon:Synthesis And Application In Oxygen Reduction Electrocatalysis

Oxygen reduction reaction?ORR? is an important cathodic reaction in energy storage and conversion devices such as metal-air batteries and fuel cells.It usually requires a highly active catalyst to reduce the reaction energy barrier and increase the reaction rate.Noble metal platinum?Pt? based catalysts are the most active ORR catalysts to date.However,the scarcity,high cost,poor stability and susceptibility to poisoning of Pt limit its large-scale applications.Therefore,it is an urgent task to develop low-cost and highly efficient non-precious metal catalysts.In this regard,nitrogen-doped carbon carrier-supported transition metal materials?M-N-C,M refers to transition metals such as Fe,Co,Mn,etc.? are the most promising alternative ORR catalysts.However,there is no clear understanding on the catalytic active sites and structure-activity relationships in this type of M-N-C catalysts,which restrict the rational design and controllable synthesis of efficient catalysts.In this dissertation,two transition metal?Mn,Co?/PDA precursors were prepared by using the biomimetic polymer polydopamine?PDA? as a universal carrier to chelate metal ions.Different M-N-C catalysts were obtained by pyrolysis of these precursors,and their morphology and ORR catalytic properties were systematically studied.The main work of this paper is as follows:?1? Bimetallic MnO/Co nanoparticles supported on N-doped graphene?MnO/Co-N-G? were preparedand their ORR catalytic properties were studied.Using the coordination ability of PDA,the binary Mn and Co transition metal ions were introduced into PDA and the Mn/Co-PDA-GO precursor was formed.Pyrolysis of such Mn/Co-PDA-GO precursor at an inert atmorsphere produces bimetallic MnO/Co supported on nitrogen-doped graphene catalyst?MnO/Co-N-G?.The morphology,structure and composition of MnO/Co-N-G catalyst were systematically investigated.In alkaline solution,the MnO/Co-N-G catalyst exhibits ORR activity superior to commercial Pt/C,with higher limiting current density,better methanol resistance and excellent stability.?2? Mn₃O₄ nanoparticle supported on nitrogen-doped carbon?Mn-N-C? with abundant micropores was prepared and its ORR activity was studied.In order to increase the micropore content and the density of M-N₄ active site in the M-N-C catalyst,N-rich and evaporable cyanamide?CM? molecule was further introduced to PDA.The transition metal Mn without Fenton catalytic activity were used to construct Mn chelated cyanamide-PDA modified carbon black?Mn-CM-PDA-CB? precursor by one-pot wet chemistry method,followed by high temperature treatment,Mn-N-C consists of Mn₃O₄ nanoparticles supported on nitrogen-doped carbon black was synthesized.Characterizations show that there is a large amount of micropores in as-synthesized Mn-N-C catalyst,and the rich nitrogen content increases the density of the active site of M-N₄.In alkaline media,the optimal Mn-N-C catalyst proceeds primarily via a four-electron process.It exhibits higher catalytic activity,better stability and excellent methanol tolerance.This master's thesis develops a simple and environmentally friendly method to prepare M-N-C ORR catalysts.Two efficient catalysts were prepared.Further,the ORR catalytic mechanism of these catalysts was briefly explored and discussed.It may provide valuable information to the research on non-precious metal based ORR catalysts.