Composition-controlled synthesis of LixCo3−xO4 solid solution nanocrystals on carbon and their impact on electrocatalytic activity toward oxygen reduction reaction

Abstract

Through incorporation of other metal elements like Li, K and Cu into the Co₃O₄ lattice, tuning electronic structures of carbon-supported Co₃O₄ may be an effective way to improve electrochemical properties of the oxide as an efficient catalyst for the oxygen reduction reaction (ORR). In this paper, we have synthesized Li-doped Co₃O₄ (Li/Co = 0, 2.5%, 5%, 7.5%) solid solution nanocrystals through direct nucleation and growth of the lithium-cobalt oxide on a acid-treated carbon black. The carbon supports Li_xCo_3−xO₄ spinel nanocrystals with average particle size of approximately 4 nm, covered evenly on the surface of carbon in this hybrid. The electrocatalysis experiments in an alkaline solution reveal a close correlation between the ORR electrocatalytic activities and the doped Li contents of these Li_xCo_3−xO₄/C catalysts. Interestingly, the half-wave potential and the mass-specific activity of these catalysts show a typical volcano plot as a function of Li contents. Among all the synthesized Co_3−xO₄/C samples, the sample prepared at the Li/Co atomic ratio of 5% displays the most positive half-wave potential and the largest mass-specific activity respectively, which are 70 mV and 3.3 times higher than the undoped Co₃O₄/C. Compared with that of the undoped Co₃O₄/C, the remarkly increased content of covalent OC–O–Co^III–O bonds formed at the interfaces between Li-doped Co₃O₄ and carbon support is responsible for the improved electro-catalytic performances of the doped catalyst. Moreover, the covalent electron transfer from the Co^III species to the electron-withdrawing OC–O species through the OC–O–Co^III–O bonds can not only promote the oxidation of active sites Co^III into Co^IV but also facilitate the surface hydroxide displacement, which can significantly contribute to the ORR kinetics. Therefore, the exact understanding of the unique interfacial electronic structures of the Li_xCo_3−xO₄/C hybrids is very important to develop the lithium-cobalt oxides on carbon as next-generation catalysts for ORR.