Oxygen Evolution Reaction By Hierarchical Nanostructure Pt-Co3O4: The Use of Bicontinuous Microemulsion As a Soft Template
Authors: Adesuji Elijah T , Guardado Villegas Esther , Sanchez-Domínguez Margarita , Videa Marcelo 
Published: May 2020

Hierarchical or 3D nanostructures, which are made up of interconnected nanoscaled units, exhibit large surface area and high porosity which are of high interests because of their excellent electrocatalytic properties [1]. These nanostructures are easily achievable and convenient to prepare using template assisted assembling of building units. Bicontinuous microemulsions were used in this work as soft templates because of its facile preparation and unique disposition made up of continuous intertwined channels, leading to interconnected 3D nanostructures of Pt-Co3O4 in a simple chemical reduction process. The Pt-Co3O4 was then used in electrocatalytic oxygen evolution reaction (OER) in an alkaline media and compared with nanostructured platinum obtained through a similar procedure using bicontinuous microemulsions.
Pt-Co3O4 and Pt were characterized by XRD, XPS, SEM, EDX, STEM, and cyclic and linear sweep voltammetry. The crystalline nature of the synthesized Pt nanostructures was confirmed by the XRD patterns. The XPS spectra confirmed the presence of cobalt oxides and metallic Pt0. From the SEM images, both Pt-Co3O4 and Pt showed interconnected needle-like structure. Cyclic voltammetry (CV) measurements showed the characteristic features of polycrystalline Pt electrode, whereas the same features were not as pronounced in Pt-Co3O4 nanostructure. Electrochemical active surface area (ECSA) for Pt obtained from the hydrogen adsorption charge were 9.5 m2/g for Pt-Co3O4 while Pt value was 33.9 m2/g. The electrochemical measurements in 0.1 M KOH at a scan rate of 1 mV s-1 gave an onset potential of approximately 1.43 V vs RHE for Pt-Co3O4, 100 mV lower than Pt which has an onset potential of 1.53 V vs RHE. An early onset potential is an indication for a good OER catalytic activity [2]. The glassy carbon electrode (GCE) showed no catalytic activity. The overpotential at the current density of 10 mA cm–2 (η10) measured for Pt-Co3O4 nanostructure is 222.6 mV vs RHE. The Tafel plot showed that the Tafel slope of Pt-Co3O4 was 143.4 mV dec–1 while for Pt was 259.3 mV dec–1. The enhanced OER ability of Pt-Co3O4 is attributed to the presence of Co in the nanomaterial which increased the active sites available to participate in OER. The OER performance obtained in this work is comparable superior to Pt supported on Ti [3]. The suitability of bicontinuous microemulsion as 3D templates to direct the synthesis of hierarchical nanostructures, which have practical applications including but not limited to electrocatalysis has been presented in this novel study.
[1] Q. Qu, J.H. Zhang, J. Wang, Q.Y. Li, C.W. Xu, X. Lu, Three-dimensional ordered mesoporous Co3O4 enhanced by Pd for oxygen evolution reaction, Sci Rep 7 (2017) 41542.
[2] Z.-y. Li, K.-h. Ye, Q.-s. Zhong, C.-j. Zhang, S.-t. Shi, C.-w. Xu, Au-Co3O4/C as an Efficient Electrocatalyst for the Oxygen Evolution Reaction, ChemPlusChem 79(11) (2014) 1569-1572.
[3] J. Zheng, Binary platinum alloy electrodes for hydrogen and oxygen evolutions by seawater splitting, Applied Surface Science 413 (2017) 72-82.

Figure 1