Spinel-Type Metal Oxide Nanoparticles Supported on Platelet-Type Carbon Nanofibers for Oxygen Evolution Reaction and Oxygen Reduction Reaction

Abstract

Zinc-air secondary battery (ZAB) is a promising candidate for the development of new generation of energy conversion systems due to its relatively high theoretical energy density of 1370 Wh kg-1. The decrease of overpotential for oxygen-reduction-reaction (ORR) and oxygen-evolution-reaction (OER) in air electrode, together with providing of high durability, are essential for commercialization of zinc air battery. As precious-metal-free electrocatalysts, transition metal oxide nanoparticles supported on nanocarbon materials are promising for highly active ORR/OER catalysts. The carbon has an important role to provide enough electrical conductivity, but the carbon degradation at high OER potential is a key issue for practical application. Very recently, we reported that platelet-type carbon nanofibers (pCNFs) with high graphitization degree are very resistive in the OER environments in the highly concentrated alkaline media [1]. Surprisingly, carbon edge plane at the sidewall of the carbon nanofibers is exposed to electrolyte, but negligible oxidative dissolution occurs at the OER condition for 1 month. Thus, pCNFs have high potential as the durable carbon material for the air electrode of ZAB. In the present study, novel hybrid electrocatalyst consisting of pCNFs and spinel-type oxide nanoparticles are prepared by a solvothermal method as a durable and active ORR/OER electrocatalyst.The commercially available pCNFs (Sigma-Aldrich) were used in this study. Commercial carbon black (Denka black, DB) were also used for comparison. Each carbon support was treated in concentrated HNO3 aqueous solution at 110℃ for the surface activation. MnCo2O4 nanoparticles (MCO) were loaded on each carbon support by means of solvothermal synthesis; an ethanol solution containing manganese acetate and cobalt acetate was heated at 150℃ for 3 h. The electrochemical measurements were carried out with the carbon counter electrode and Hg/HgO/KOH aq. reference electrode in 4 and 8 mol dm-3 KOH aqueous solution, for the electrocatalytic activity evaluation and for the durability tests, respectively.HRTEM image of the MCO/pCNF hybrid (Fig. 1) reveals the uniform distribution of MCO nanoparticles, which are 5 nm in diameter on average, on the looped carbon planes of the pCNFs. It is likely that the MCO is strongly coupled with pCNF. The Mn/Co ratio obtained by TEM-EDS analysis of MCO nanoparticles loaded on pCNFs was 1/2 in agreement with the target composition. The electrochemical measurements demonstrated markedly high activity of MCO/carbon hybrid towards ORR with the onset potential as high as 0.90 V vs RHE. High catalytic performance probably results from the good interface formation between MCO and carbon due to the solvothermal synthesis. The durability test using gas-diffusion electrodes was carried out under repeated galvanostatic ORR/OER polarization at ±20 mA cm-2 for 1 h. Although the MCO/DB electrode shows the overpotential increase after 30 cycles, the overpotential for MCO/pCNF is stable even after 80 cycles.