Oxidation State as a Descriptor in Oxygen Reduction Electrocatalysis
Ning Qin, Sicen Yu, Zongwei Ji, Yanfang Wang, Yingzhi Li, Shuai Gu, Qingmeng Gan, Zhenyu Wang, Zhiqiang Li, Guangfu Luo, Kaili Zhang, Zhouguang Lu
Abstract
Electrocatalysts for the oxygen reduction reaction (ORR) are critically important in the development of fuel cells and metal–air batteries. Intensive research interests have been devoted to improving the electrocatalytic performance by tuning the morphology and defect-active sites. Herein, we demonstrate that the oxidation state can also serve as an effective strategy for designing ORR electrocatalysts. Valence models of silver with gradient chemical valence from zero valence to trivalence were successfully built. Their oxidation states were evaluated by cryo-X-ray photoelectron spectroscopy, X-ray absorption fine structure, and electron paramagnetic resonance spectroscopy. For the first time, our results demonstrated that the electrocatalytic activities of silver species can be improved by increasing their valence, conforming the order of Ag < Ag<sub>2</sub>O < Ag<sub>2</sub>O<sub>2</sub> < Ag<sub>3</sub>O<sub>4</sub> < Ag<sub>2</sub>O<sub>3</sub>. Computational studies reveal that higher valence Ag species possess a higher proportion of d band holes and more electrons closer to the Fermi level. Therefore, the oxygen adsorption and activation energy on the Ag sites can be regulated to a near-optimal level and the ORR catalytic efficiency increases. This work clearly presents that oxidation state is another degree of freedom in designing efficient ORR electrocatalysts.