Oxygen Evolution Reaction of NiO Nanowalls with a Three-Dimensional Framework

Abstract

It is important to optimize the mass transport of electrons, ions and gases to enhance the catalytic activity and efficiency for electrolysis. We previously reported that nanosheets possess higher electrocatalytic activity compared to nanoparticles.1 This is attributed to the large electrochemically active surface area of nanosheets. However, nanosheets tend to re-stack, leading to poor mass transport and low efficiency. In this study, we propose a new electrocatalyst with a three-dimensional architecture composed of a framework of NiO nanowalls toward oxygen evolution reaction (OER).Macro porous carbon (MPC) was purchased from Toyo Tanso (CNovel®, Japan) and used as template. MPC was dispersed in 2-methoxyethanol, and nickel acetylacetonate was dissolved in 2-methoxyethanol. The suspension was mixed for 3 days and collected by centrifugation. The obtained powder (MPC@NiO x ) was dispersed in 2-methoxyethanol. After solvothermal treatment, the solution was centrifuged, and the dried powder was heated in air to yield macro porous NiO (3D-NiO). NiO particles were prepared without the macro porous carbon template. OER activity of the electrocatalysts was investigated using a three-electrode system consisting of a Pt mesh counter electrode and a Ag/AgCl (sat. KCl) reference electrode. 1.0 M KOH (30°C) was used as the electrolyte.TEM images of 3D-NiO showed a 3D structure constructed by 2D NiO nanowalls with very thin and smooth surface. The structure of 3D-NiO was replicated from the MPC template; MPC was oxidatively removed by calcination at 400°C. The redox peaks at a half peak potential of E 1/2 = 1.49 V vs. RHE were observed in the cyclic voltammograms for 3D-NiO and NiO particles. These redox peaks were attributed to the transformation of b-Ni(OH)2 and b-NiOOH (Ni(OH)2 « NiOOH + H+ + e-).2 The OER current at 1.8 V vs. RHE of 3D-NiO was 15 times larger than that of NiO particles. The onset potential of the OER for 3D-NiO was similar to that for NiO particles, indicating that the OER mechanism was not sensitive to the morphology of NiO. The result strongly indicates that the 3D structure with 2D nanowall is a superior design for OER electrocatalyst. Considering the diffusion path of evolved O2 molecules, the enhanced activity could also be attributed to the meso pore nature of 3D-NiO.This work was supported by Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for Young Scientist (Grant Number 18K14054). References D. Takimoto, K. Fukuda, S. Miyasaka, T. Ishida, Y. Ayato, D. Mochizuki, W. Shimizu and W. Sugimoto, Electrocatalysis, 8, 144 (2017).M.-S. Kim, T.-S. Hwang and K.-B. Kim, J. Electrochem. Soc., 144, 1537 (1997).