Synthesis, characterisation, and properties of powder metallurgy transition metal-based high entropy alloys for electrocatalytic application

Abstract

Water electrolysis is an eco-friendly route for hydrogen production when compared to other routes, such as steam reforming, coal gasification, biomass gasification, etc. However, only 4% of hydrogen is being produced through water electrolysis. Water electrolysis proceeds via two half-cell reactions, namely the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). HER is a two-electron transfer process, while OER is a four-electron/proton transfer process. Hence, OER is energy-intensive and affects the overall efficiency of water electrolysis. To improve the efficiency of water electrolysis, it is necessary to develop an efficient electrocatalyst for the OER process. Ir and Ru-based oxides are the state-of-the-art electrocatalysts for OER. However, their natural abundance is minuscule, making them expensive. Transition metal-based electrocatalysts are emerging materials for OER applications due to their high natural abundance and low cost. However, these materials exhibit poor conductivity and, consequently, poor catalytic activity. To overcome the shortcomings of conventional electrocatalysts, a new class of multi-component alloys termed high entropy alloys (HEAs) is becoming popular for electrocatalytic applications. However, most of the HEAs for OER are not self-supporting and are synthesized in powder form, often coated onto conductive substrates such as Ni foam and carbon fiber. Hence, these powder-based electrocatalysts are not suitable for industrial-scale hydrogen production. In addition, most synthesized HEAs consist of Co, which is an inherently expensive material. To overcome these current drawbacks in this field, this thesis considered three Co-free, self-supporting HEA combinations, namely NiMnFeCu, NiMnFeCrCu, and NiMnFeCr, which were successfully synthesized. Among the synthesized HEAs examined, NiMnFeCr was found to exhibit an overpotential of 300 mV after it was subjected to 100 cycles of cyclic voltammetry (CV) activation. Furthermore, the NiMnFeCr HEA synthesized exhibited a compressive yield stress of 306 MPa, manifesting the excellent combined properties of not only catalytic activity but also mechanical strength. The research showed that NiMnFeCr was a suitable candidate material for industrial-scale water electrolysis and hence can be considered as a potential replacement for the conventional, prominently used electrocatalysts, such as Raney nickel, Ir, and Ru oxide-based catalysts. The excellent catalytic activity demonstrated by the HEA NiMnFeCr could also be attributed to its work function of 3.02 eV, which is the lowest value of work function among other studied HEAs.