Chiral Co- and Ni-based catalysts for the oxygen evolution reaction in alkaline water splitting: a combined in operando XAS and DFT simulations study

In the context of the transition towards a sustainable energy economy based on renewable energy sources, hydrogen production via electrochemical water splitting represents a promising route to mitigate their intermittency by converting the electrical energy they produce into chemical energy stored in hydrogen. However, the efficiency of water splitting remains limited by the sluggish kinetics of the oxygen evolution reaction (OER), as it is a multistep, four-electron process. Developing high-performance, earth-abundant OER catalysts is essential to increase the efficiency of water electrolysers and fuel cells and to enable an economically competitive hydrogen production compatible with large-scale adoption. Conventional OER catalysts are based on platinum-group metals, which are scarce and precious materials, thus not suitable for mass-scale adoption. In alkaline media, the OER can be catalysed with comparable efficiency by oxides of 3d transition metals, such as cobalt and nickel, which are abundant and less expensive. Moreover, the OER is affected by the formation of by-products whose generation depends on the spin configuration of the electrons involved in the process. Their formation can be suppressed, thereby enhancing oxygen production, by exploiting the chirality-induced spin selectivity (CISS) effect, which takes advantage of the chiral properties of the catalyst. In this thesis, we investigated the catalytic activity of novel chiral catalysts that exploit the CISS effect to enhance the OER in the water splitting process. The catalysts are based on cobalt oxide and nickel hydroxide, which are 3d transition-metal compounds showing a comparable OER efficiency to noble-metal-based catalysts in alkaline media. We analysed two types of samples: cobalt oxide nanoparticles and nickel-based metal-hydroxide-organic frameworks (MHOFs). The catalyst samples were studied by means of X-ray absorption spectroscopy (XAS) both ex situ and in operando conditions at the soft X-ray beamline BACH at the Elettra synchrotron and at the hard X-ray beamline SAMBA at the SOLEIL synchrotron. The cobalt oxide nanoparticles were investigated ex situ and in operando at the SAMBA beamline in two different electrochemical setups: a three-electrode cell in alkaline medium and a complete reversible alkaline electrolyser. The in operando experiments were carried out simultaneously with chronoamperometric measurements to examine the electrochemical behaviour of the catalyst during operation. The chiral properties of the nickel-based MHOFs were investigated ex situ by natural circular dichroism at the BACH beamline. In addition, cobalt- and nickel-bearing commercial reference compounds were analysed only ex situ at both beamlines during previous beamtimes. The experimental studies were integrated with density functional theory (DFT) simulations on nickel hydroxide Ni(OH)2, which is present in the structure of the nickel-based MHOFs and is a precursor of the catalytically active nickel oxyhydroxide NiOOH. The DFT simulations assessed the crystal structure, the electronic properties and enabled the calculations of the XAS spectra at the oxygen K-edge. The band gap energy was tested against experimental and theoretical values reported in the literature, while the calculated XAS spectra were compared with the experimental measurements.