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

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 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 OER process. Ir and Ru based oxides are the state-of-the-art electrocatalysts for OER, however their natural abundance is minuscule, hence they are 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. Hence to overcome the shortcomings of conventional electrocatalysts a new class of multi component alloys termed as high entropy alloys (HEAs) are getting popular for electrocatalytic applications. However, most of the HEAs for OER are not self-supporting and being synthesised in powder form are often coated onto conductive substrates such as Ni foam and carbon fibre. Hence this powder based electrocatalysts are not suitable for industrial scale hydrogen production. In addition, most synthesised 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 synthesised 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 of other studied HEAs.
Type of thesis
The University of Waikato
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