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

Fuel cells have attracted significant attention because of its high efficiency and non-pollution,which direct conversion chemical energy of fuel and oxidant into electricity by electrochemical reactions.Pt/C catalyst are deployed as cathode catalyst catalytizing the oxygen reduction reaction（ORR）in fuel cells.However,to become commercially viable,fuel cells have to reduce its high catalyst cost caused by the exclusive use of platinum.Carbon supported transition metal nitrogen compounds（Me/N/C,Me=Co,Fe,etc.）have been studied as one of the most promising candidates for cathode electrocatalysts for fuel cells,though the catalytic activity and stability of Me/N/C catalysts still far away from that of the Pt/C catalyst.Therefore,clarify the physical essence and mechanism of the catalytic sites in Me/N/C catalysts,are one of the key issues for solving this problem.In the present work,the mechanism of ORR on Co-based catalysts are studied by in-situ X-ray absorptive fine structure（XAFS）combine with X-ray diffraction（XRD）experiment.A modification of polypyrrole-modified carbon-supported cobalt hydroxide（Co（OH）₂-PPy-BP）has been carried out to obtain Co-based catalysts with different valence state of Co.A kind of in-situ XAFS and XRD experiment methods are established,based on BL15U1 in Shanghai synchrotron radiation facility（SSRF）,to obtain the in-situ measurements of Co-based catalysts with different valences in the fuel cell.The change of electronic state and local structures around Co ions during the ORR reaction are obtained.Combine with the results of the catalytic performance of half-cells,the catalytic active sites of Co-based catalysts with different valences state are discussed.Moreover,series of PtCo/C catalyst with different Pt and Co atom ratio are used to investigate the effect of binary metal complexation on the catalytic activity of Co-based cathode catalyst,and the transform of catalytic mechanism are discussed.Co-PPy-BP（Co0）and CoO-PPy-BP(Co²⁺)are prepared by the heat-treatment of Co（OH）₂-PPy-BP to obtain different valence state of Co.The onset reduction potential of CoO-PPy-BP is higher than that of Co-PPy-BP in alkaline solution towards ORR.Both Co-PPy-BP and CoO-PPy-BP are present a 4-electron transfer pathways towards ORR.The Co0 in Co-PPy-BP is converted to Co²⁺ initially,and then change same with behaviour of CoO-PPy-BP(Co²⁺),which Co²⁺ oxidized into Co³⁺ with the increase of discharge current until up to 150 mA,the valence state of Co ion keep +3 valence during in-situ oxygen reduction.This indicates that valence state of Co ions both Co0 and Co²⁺ have been changed to Co³⁺ during oxygen reduction in alkaline media.Those evidences confirm that the circular change of between Co²⁺ and Co³⁺ play important role in the catalytic process of oxygen reduction on both Co0 and Co²⁺ cathode catalyst.CoOOH-PPy-BP(Co³⁺)catalyst was prepared by the impregnate in alkaline solution from precursor Co（OH）₂-PPy-BP.The CoOOH-PPy-BP catalyst has exhibited competitive onset potential against to commercial Pt/C catalyst.The ORR electrons is 3.89,which confirmed that ORR occurred via 4-electron transfer pathways on the CoOOH-PPy-BP catalyst in alkaline solution.However,in our in-situ measurement of CoOOH-PPy-BPcatalyst,no further oxidized of Co ion to higher valence was observed.The average coordination number of the nearest Co-O coordinate in CoOOH-PPy-BP cathode changed from two to only one when the cathode was discharged,which implies the generation of oxygen vacancies emerge on the surface of Co ion.A series of PtCo/C catalyst with different Pt and Co atom ratio are prepared by chemical methods.The onset potential of ORR shifted to negative with the increase of content of Co atoms,ORR occurs simultaneously via 4-electron transfer pathways still.The valence variation of Pt and Co in each PtCo/C catalyst,used in fuel cell cathode,are studied by In-situ X-ray absorptive near edge structure（XANES）measurements at various discharging currents.There is no valence variation of Pt is observed except PtCo3/C catalyst,which with few content in Pt presents an oxidized valence of Pt.For Co K-edge XANES,the initial valence of Co0,among all PtCo/C catalyst,oxidized to +3 valence while at various discharging currents.Moreover,when the discharge is finished,Co keep valence state of Co²⁺ or Co³⁺,instead of return back to valence of Co0.In this work,the ORR mechanism on the Co-based catalyst for ORR reaction in alkaline solution was discussed,which proposed a new experimental and theoretical foundation for the further development and application of Co-based cathode catalyst.