In-situ Identification of Geometrical-Site-Dependent Activity and µO-O Bond Formation of Spinel Co3O4 Electrocatalyst toward Water Oxidation
Molecular Physics seminar
Monday 27 February 2017
to 11:00 at
Hsin-Yi Wang (School of Chemical and Biomedical Engineering, Singapore)
The energy crisis and related environmental issue is an important issue at a global level. Water electrolysis has been proposed as a promising technology for the production of H2 that can be directly used as the clean fuel. However, the efficiency of the electrolyser is greatly restricted on the anode side, where the oxygen evolution reaction (OER) is a thermodynamic up-hill reaction and which usually requires a high overpotential to drive the reaction. Thus, efficient electrocatalyst for high-performance OER is essential for the development of sustainable energy conversion technology.
Spinel cobalt oxide (Co3O4) is an earth-abundant material which has been extensively studied as effective OER catalyst due to its competitive activity compared to the noble catalyst. Although it has been known the spinel Co3O4 was comprised by one Co2+ in the tetrahedral site (Co2+Td) and two Co3+ in the octahedral site (Co3+Oh), the roles of two geometrical cobalt ions toward the OER have remained elusive. To individually understand geometrical-site-dependent OER activity of Co3O4 catalyst, we separately examined the properties of Co3+Oh and Co2+Td by substituting Co2+Td and Co3+Oh with inactive Zn2+ (d10) and Al3+ (d0), respectively. Following a thorough in-situ analysis by electrochemical impedance spectroscopy and X-ray absorption spectroscopy, it was revealed that Co2+Td site should be responsible for the electrochemical activity of water oxidation.
Apart from the geometrical-site-dependent activity of spinel Co3O4, we were also surprised that the anodic peak prior to the rise of OER current could be vanished if Zn2+ was substituted into the tetrahedral site of Co3O4. Thus, the underlying properties of anodic peak attracted our attentions. Through a combination of well-designed independent in-situ measurements including X-ray absorption and X-ray diffraction under operando conditions, and following a potential-resolved in-situ Raman analysis with their particularly designed cells, we revealed that not only the oxidation process of cobalt ions, but the formation of peroxide moieties (e.g., Co-OO-Co or Co-OOH) on the surface of Co3O4 could associate with the anodic peak current, and Co2+Td ions in Co3O4 should be the key species for the bridging process of µ-OO bond, which could be the important elementary step for the water oxidation.