Advanced high entropy oxides for seawater splitting

Abstract

The demand for renewable energy is increasing due to the consumption of primary energy as known as the fossil fuels. Water electrolysis to produce hydrogen is an ideal clean energy choice because of its high energy density and zero carbon emissions. However, fresh water, which accounts for less than 3 % of the worlds water supply is a high demand resource for people's livelihood. Seawater on the other hand, accounts for more than 70 % of the earth, making it much desirable as a source of hydrogen. For seawater to produce hydrogen, a more efficient, stable and longer-lasting catalyst would need to be developed. This would avoid chlorine evolution, a ‘competition’ reaction during oxygen evolution reaction (OER), which accelerates the corrosion process suppressing the lifespan of the catalyst. High entropy materials (HEM), a potential promising catalyst candidate, are able to reach much lower OER overpotentials and higher corrosion resistance due to their tunable electronic structures. In this work, we used a pulsed laser irradiation scanning on mixed salt solutions (PLMS) method to produce a six element, high entropy ceramic nanoparticles as an OER catalyst for efficient and stable work in seawater environments (Yang et al., 2021). The (AlCoCrFeMnNi)O high-entropy ceramic (HEC) we proposed demonstrate a great performance in the seawater environment. The onset voltage of oxygen-evolution can reach as low as to 1.47 V vs. reversible hydrogen electrode (RHE), and in terms of stability, our catalyst can operate for 1000 h under 100 mA/cm². Furthermore, to understand the mechanism behind our (AlCoCrFeMnNi)O HEC, the in-situ X-ray absorption spectroscopy (XAS) measurement and In-situ extended X-ray absorption fine structure (EXAFS) are carried out in this work with varying applied voltages for OER. Co, Mn, and Ni identified as the active sites of OER and these three elements work intimately together, each with different reaction routes, including the adsorbate evolution mechanism (AEM) and lattice oxygen-participated mechanism (LOM). While Al, Cr, and Fe are not directly linked to the catalytic activity, they do play important roles to stabilize the high entropy structure throughout the whole reaction. Finally, a computational model was created using a 2x1x2 supercell to study various atom distributions. This study utilizes DFT calculations, geometry optimization, and electron density mapping. To highlight the influence of different atoms on bond lengths, particularly the impact of Al on structural distortion and addressing bond lengths involving Mn and Fe.