Synthesis And Investigation On The Electrocatalic Performance Of Manganese Oxide And Its Composites

Fuel cell is a kind of energy conversion device that combines the advantages of safety,environmental friendliness and high efficiency.However,the oxygen reduction reaction（ORR）with slow reaction kinetics hinders its wide application.Currently,exploring low-cost,long-life high-performance ORR catalysts is a research hotspot for improving the reaction rate of fuel cell cathodes.Manganese dioxides have abundant reserve in nature,more importantly,owing to their unique structure,they behave excellent electrochemical activity.It is regretful that the manganese dioxides have low conductivity,which does not beinifit for electron transmission,thereby decreasing the catalytic efficiency of the catalysts in the ORR process.Both heterogeneous element-doped carbon materials and iron oxides have strong capabilities of electron transport.Combining them with manganese dioxides can improve the electrical conductivity and electrocatalytic performance of the materials,and the purpose of enhanced electrocatalytic performance of the catalyst can be gained.In this paper,compounds containing transition metal manganese were combined with nitrogen and sulfur co-doped carbon materials and iron oxides to prepare ORR catalysts.Corresponding experiments and electrochemical tests were carried out.The details of our work are as follows:1.Compounds containing manganese element,camphorsulfonic acid and ammonium persulfate were used as precursors,sulfur source and oxidant,respectively.Polyaniline was grown on their surface to form nanocomposites,which led to the formation of nitrogen and sulfur co-doped carbon materials（N,S-C/MnO-₂）after annealing treatment.The product behaved good ORR activity.In aqueous solution of 0.1 M KOH,the initial and half-wave potential of N,S-C/MnO-₂ is 0.891 and 0.796 V vs.RHE,respectively.It is worth noting that the limiting current density of N,S-C/MnO-₂ attained 5.182 m A cm^-2 at the potential of 0.565 V,surpassing that of commercial Pt/C catalysts.In addition,N,S-C/MnO-₂ also showed better stability and resistance to methanol poisoning than Pt/C.After cycling for 20000 s,N,S-C/MnO-₂ only exhibited 9 % current loss.The good electrocatalytic activity of N,S-C/MnO-₂ can be attributed to the large amount of Mn3+ in MnO-₂ and the synergistic effect between MnO-₂ and the carbon materials doped by heteroatoms.2.MnO-₂/Fe₂O₃ nanocomposites were achieved via using KMn O4 and Fe Cl3 as starting materials in aqueous solution followed by hydrothermal reaction and high-temperature treatment processes.The H+ produced by hydrolysis of Fe Cl3 reacted with KMn O4 to produce MnO-₂ and this process was also beneficial for shortening the reaction time of Fe Cl3 hydrolysis.The as-synthesized MnO-₂/Fe₂O₃ nanocomposites presented a high oxygen reduction reaction activity with an onset and half-wave potential of 0.953 and 0.859 V vs.RHE,respectively,and a limited current density of 4.857 m A cm^-2 at 0.565 V,which were comparable to those of commercial Pt/C catalyst.The excellent catalytic performance was considered to originate from the high Mn3+ content and the related oxygen vacancies.Furthermore,the unsatisfactory stability of the MnO-₂/Fe₂O₃ nanocomposites can be solved by coating a layer of polyaniline-derived carbon.