Fabrication and characterisation of open-cell high entropy alloy electrocatalyst for oxygen evolution reaction

• Main Author: Lim, Teik Yi
• Other Authors: Huang Yizhong
• Format: Final Year Project
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Engineering::Materials::Metallic materials::Alloys; Engineering::Materials::Material testing and characterization
• Online Access: https://hdl.handle.net/10356/140162
• Institution: Nanyang Technological University

Water electrolysis is a green method for producing hydrogen, which is currently high in global demand due to it playing an important role in clean energy storage. However, due to sluggish kinetics of the oxygen evolution reaction (OER) and high cost of noble-metal electrodes, water electrolysis has yet to have any widespread use in the field of hydrogen production. In order to overcome these bottlenecks, the electrocatalytic performance of OER electrocatalysts has to be improved and these electrocatalysts should be designed using cheaper materials. Recently, a novel class of materials known as high entropy alloys (HEAs) are said to have superior electrocatalytic abilities and corrosion resistance, making them promising candidates as OER electrocatalysts. In this study, to evaluate the OER performance of porous HEA electrocatalysts, the template replication method was used to fabricate open-cell CoCrFeMnNi foam electrodes with pore densities of 35, 45, and 60 pores per inch (ppi). The powder and foam morphology were characterised using scanning electron microscopy (SEM) and optical microscopy. Foam porosity was calculated using the gravimetric method whereas its crystal structure was analysed using powder x-ray diffraction (XRD). For evaluation of the electrocatalyst performance, electrochemical methods such as cyclic voltammetry (CV), linear sweep voltammetry (LSV) and electronic impedance spectroscopy (EIS) were employed. Elemental composition analysis was carried out using energy dispersive x-ray spectroscopy (EDX). Results show that the CoCrFeMnNi electrodes fabricated were open-cell in nature (porosity >95%) and have an FCC crystal structure. Regardless of pore density, all CoCrFeMnNi electrocatalysts recorded low overpotentials and Tafel slopes which suggest excellent OER performance, especially the 35 ppi electrode which only required an overpotential of 18 mV to reach a current density of 10 mA/cm2 and had a Tafel slope of 44 mV/dec. When comparing the overpotentials required for the 35 and 60 ppi electrodes, the 35 ppi electrode required an overpotential that was 7-times lower than that of the 60 ppi electrode, even though their porosities only differed slightly. This suggests that the porosity of an electrocatalyst has a large effect on its electrochemical performance. The electrodes showed signs of instability and element dissolution as indicated by a distinct change in electrolyte appearance and precipitation during EIS. Nonetheless, the OER performance of the open-cell CoCrFeMnNi electrocatalysts are still competitive in comparison to state-of-the-art electrocatalysts, mainly due to their synergistic effects which is an inherent characteristic of HEAs, and the increased surface area which can be associated to their open-cell structure.