In-situ Study Of Fe-based Catalyst Towards Oxygen Reduction Reaction In Fuel Cell

Direct borohydride fuel cell(DBFC)is an energy conversion device which directly converts chemical energy stored in alkaline sodium borohydride solution and oxidant into electrical energy through electrochemical reactions.DBFC is considered as one of the most promising fuel cells because it uses liquid fuel and available non-noble metal catalysts.Recently,transition metal-nitrogen-carbon catalysts(Me-N-C)have been becoming the research hotspot in the field of cathode catalyst materials due to their remarkable advantages,such as wide source of raw materials,low price and good electrocatalytic activity.However,there is no unified standpoint of the catalytic sites about Me-N-C catalysts.The study of valence and electronic structure information of transition metals during catalytic reaction would be conducive to reveal the active sites and catalytic mechanism of Me-N-C catalysts,which makes further efforts to develop more cheap and efficient materials.The aim of this paper is to study iron-based catalysts with different valence states supported on nitrogen and carbon.Firstly,these catalysts were prepared by wet chemical method and heat treatment.Secondly,the catalytic activities and power performances of DBFC with synthesized catalysts were investigated by a series of electrochemical and single-cell tests.Finally,in-situ X-ray absorption fine structure(XAFS)and in-situ X-ray diffraction(XRD)technologies were carried out to further discuss the catalytic mechanism towards oxygen reduction reaction.It was found that Fe2O3-PPy-BP(Fe3+)could be successfully prepared by hydrothermal method using FeSO4,pyrrole and BP 2000,and Fe-PPy-BP(Fe0+)would be gained by the direct pyrolysis of Fe3+sample in Hi atmosphere.The mixture consisting of FeC1b,(NH4)2S2O3,pyrrole and BP 2000 would be transformed into FeS-PPy-BP(Fe2+)after chemical reaction in low temperature and then heat treatment in high temperature.In alkaline medium,the onset reduction potentials of iron-based catalysts with different valence states followed the sequence:FeS-PPy-BP(0.89 V)>Fe2O3-PPy-BP(0.81 V)>Fe2O3-PPy-BP(0.76 V).Fe-PPy-BP and Fe2O3-PPy-BP both catalyzed oxygen reduction through a mixed route of 2 e and 4 e,while FeS-PPy-BP was a 4-e transfer pathway towards ORR.Among three kinds of catalysts,the maximum power density of DBFC using FeS-PPy-BP as cathode catalyst reached 140.5 mW·cm-2,much higher than those of Fe-PPy-BP and Fe2O3-PPy-BP(112 mW·cm-2 and 67 mW·cm-2,respectively).With the increase of discharge current density and current time,the K-side absorption edge of Fe atom both in Fe-PPy-BP and FeS-PPy-BP catalysts gradually shifted to higher energy direction.Meanwhile,that of Fe atom in Fe2O3-PPy-BP kept unchanged.We also observed that the Fe0+ in Fe-PPy-BP was firstly converted to Fe2+and then further converted to Fe3+,while Fe2+ in FeS-PPy-BP could be partly converted to Fe3+ and Fe3+ in Fe2O3-PPy-BP would not be oxidized further.In the discharge process,it was common that all Fe atoms of Fe-N-C catalysts underwent irreversible reactions resulting in the intermediate product FeOOH.In this paper,the catalytic performances and ORR mechanism of Fe-N-C catalysts for ORR in alkaline solution were discussed,which provided experimental foundation for the better development and application of high efficient Fe-N-C catalysts.