Coal-derived Carbon-based Electrocatalysts For Bifunctional Oxygen Reduction Reaction And Oxygen Evolution Reaction

The commercialization of metal-air batteries requires efficient,low-cost and stable bifunctional electrocatalysts for reversible electrocatalysis of the oxygen reduction reaction(ORR)and the oxygen evolution reaction(OER).The modification of natural coal,with or without its prior purification,with heteroatom elements such as N and S,or metal oxide species,has been demonstrated to form very promising electrocatalysts for the ORR and OER.However,natural coal contains many inorganic impurities,notably,Fe,Si,Al,Ca,and K,among others.It is therefore prudent to understand how such impurity elements may impact the electrocatalytic properties of coal-derived catalysts.Herein,we used source-abundant coal as a raw material to synthesize electrocatalysts for reversible oxygen electrodes and probed the influence of the presence of various trace metals that are notable impurities in coal,including: Al,Si,Ca,K,Fe,Mg,Co,Mn,Ni and Cu on the electrochemical performance of the prepared catalysts.Among the tested samples,1 wt.% addition of Fe,Al,Ca,K and Co promoted the ORR,while the presence of 1 wt.% Fe and Al favored the OER,with the catalysts showing stable performance for both reactions,as well resistance to methanol cross-over in the case of the ORR.Importantly,Zn-air batteries constructed with the prepared catalysts as cathodes and Zn as anode were able to power green LED lights for a test period of more than 80 h,and charge-discharge polarization curves at a current density of 5 m A cm-2 exhibited excellent and durable rechargeability during 500 cycles(ca.84 h)of continuous cycling.The relationship between electrochemical performance and physical morphology of the catalyst was further researched with high-resolution scanning electrochemical microscopy(SECM).The promotional effect of the trace elements is believed to accrue from a combination of factors including electronic structure modification of the active sites through synergistic interactions,enhancement of the active site density through creation of pores and defects,and formation of a conductive 3-dimensional hierarchical network of carbon nanotubes,and increase in the graphitization degree of the coal.