Vacancy-assisted oxygen reduction reaction on cobalt-based catalysts in direct borohydride fuel cell revealed by in-situ XAFS and XRD

The oxygen reduction reaction mechanism is the key issue for designing novel non-Pt electrocatalysts of H-2-O-2 fuel cells. Although the Men+/Me(n+1)+ redox model has been widely accepted, the valence state of the Men+ was found to keep unchanged in this work. Polypyrrole-modified carbon-supported cobalt oxyhydroxide catalyst (CoOOH-PPy-BP) was prepared by impregnation-chemical method and used as cathode catalyst in direct borohydride fuel cells. The CoOOH-PPy-BP exhibited compatible electrochemical properties with Co(OH)(2)-PPy-BP and a near 4e transfer oxygen reduction reaction. The variation of the local structure around Co ions during discharging was analyzed by in-situ X-ray absorption fine structure (XAFS) and X-ray diffraction (XRD) measurements. No new phase was detected by in-situ XRD while oxygen vacancies were detected by in-situ XAFS. Oxygen vacancies at the surface of CoOOH provided favorable sites for the O-2 absorption, accelerating the activation of the O-2. The electron holes generated due to the oxygen vacancies in the CoOOH can capture electrons from the anode to form excited cationic states [Co3++ e]. Then the absorption oxygen molecule is reduced by capturing electrons from [Co3++ e]. A new oxygen reduction reaction mechanism based on the oxygen vacancy instead of the previous Con+/Co(n+1)+ model is proposed. This work provides lights for the design of novel catalysts with excellent performance by introducing defects of oxygen vacancies artificially. (C) 2017 Elsevier Ltd. All rights reserved.