Geometric Occupancy and Oxidation State Requirements of Cations in Cobalt Oxides for Oxygen Reduction Reaction

Jing Liu,^†,+ Hongliang Bao,^&,+ Bingsen Zhang,^‡ Qingfeng Hua,^† Mingfeng Shang,^&  Jianqiang Wang,*^,&; Luhua Jiang*^,†

^†Nanomaterial & Electrocatalysis Laboratory, College of Materials Science and Engineering, Qingdao University of Science and Technology, 53 Zhengzhou Road, 266042, Qingdao, China.

^&;Shanghai Institute of Applied Physics, Chinese Academy of Science, 239 Zhangheng Road, 201204, Shanghai, China

^‡Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, 110016, Shenyang, China

ABSTRACT：Cobalt oxides, including spinel Co₃O₄ and rock-salt CoO, have been widely reported as promising catalysts for oxygen reduction reaction (ORR). However, three types of cobalt ions, i.e., Co²⁺ in the tetrahedral site (Co²⁺_Td), Co³⁺ in the octahedral site (Co³⁺_Oh) and Co²⁺ in the octahedral site (Co²⁺_Oh), are included in these oxides, and the roles of cobalt geometric occupancy and valance states have remained elusive. Here, for the first time, we investigated the effects of cobalt geometric occupancy on the ORR activity by substituting Co²⁺_Td and Co³⁺_Oh of Co₃O₄ with inactive Zn²⁺ and Al³⁺, respectively. The ORR activity decreases in the order of Co₃O₄ (Co³⁺_Oh, Co²⁺_Th) < ZnCo₂O₄ (Co³⁺_Oh) << CoAl₂O₄ (Co²⁺_Th) in accordance of the ORR overpotentials at the current density of 0.1 mA cm^-2_Ox. Furthermore, by comparatively investigating the activity and stability of Co₃O₄ (Co³⁺_Oh) and CoO (Co²⁺_Oh) nanoparticles, by virtue of the electrochemical technique, the high-resolution TEM and the in-operando fuel cell - X-ray absorption spectroscopy techniques, it was revealed that Co²⁺_Oh in CoO is the main active sites, which under electrochemical conditions tends to transform into Co³⁺_Oh and forms Co₃O₄ with hollow structure due to the Kirkendall effect; nevertheless, remains decent ORR activity due to the formation of the unique hollow structure.

KEYWORDS: cobalt oxide; oxygen reduction reaction; geometric occupancy; valance state; octahedral coordination

DOI: 10.1021/acsami.9b00481