Multicomponent Alloy Nanoparticles as Efficient Oxygen Evolving Electrocatalysts

Designing efficient and cost-effective catalysts for electrochemical water splitting will be pivotal in the global effort towards net zero [1]. The two half reactions of water splitting; the hydrogen evolution reaction (HER, 2H⁺ + 2e⁻ → H₂) and the oxygen evolution reaction (OER, 2H₂O → O₂ + 4H⁺+ 4e⁻), currently rely on Pt group metals such as Pt and Ru, and Ir and Ru oxides, when performed in acidic media. Overpotentials are still observed for these catalysts due to the sluggish kinetics of the reactions, with OER typically the rate limiting reaction [2]. It is therefore instructive to find alternative oxides to replace Ir and Ru. Nonprecious transition metal-based oxides may provide cheaper and more abundant OER catalysts. Unfortunately, these oxides are not stable in the acidic media, and therefore must be studied in alkaline media [3].<br/>The activity of transition metal-based catalysts relates to their electronic structure, specifically the energy difference between the valence d-band centre and the Fermi level (E_F) [4]. An increase in the energy difference usually results in a stronger interaction of the orbital and the adsorbate. The strength of this interaction in terms of catalytic activity is described by the Sabatier Principle. Therefore, tailoring the optimal energy of the d-band for the reaction of interest will allow for improved catalytic efficiency. Ni, Fe and Co based catalysts and their alloys have shown great promise as OER catalysts [5,6]. Multicomponent alloys (MCAs) exhibit improved corrosion resistance relative to the elemental constituents whilst still containing catalytically active elements such as Ni,Fe and Co. This synergistic relationship between the elements make MCAs promising as successful OER catalysts.<br/>Here, size-selected nanoparticles are deposited via a state-of-the-art gas-phase synthesis. Using a quadrupole mass spectrometer placed in-line with the source, the size of the nanoparticles are well defined and controlled. Initial depositions of NiFeCoMoAl and NiFeCoMoCr have been successful, and characterised on an atomic scale using transmission electron microscopy (TEM). Hard and soft X-ray photoelectron spectroscopy (HAXPES and SXPS, respectively), measured at the i09 beamline at Diamond Light Source, gives depth resolved compositional information showing the presence of all elements as well as their possible arrangement within the nanoparticles. Meanwhile atom probe tomography (APT) measurements, made possible by simultaneous nanoparticle deposition and conventional magnetron sputtering, allow us to marry the information gathered using TEM and SXPS/HAXPES. The structural and chemical information thereby obtained is compared with electrochemical measurements of the intrinsic activities of different MCAs, revealing trends in valence band measurements from SXPS and activity. Unifying these complimentary techniques gives insight into the relationship between the atomic and electronic structure with electrochemical activities of MCA nanoparticles which will feed into future electrocatalytic design and will be a stepping stone in the wider effort of achieving net-zero emissions.<br/><br/>[1] UNFCCC, The Paris Agreement–Publication, Paris Climate Change Conference-Nov 2015, 2018<br/>[2] J. Am. Chem. Soc., 140, 7748, 2018<br/>[3] Small, 13, 1701931, 2017<br/>[4] Surf. Sci., 343, 211, 1995<br/>[5] Int. J. Hydrogen energy, 47, 23483, 2022<br/>[6] Chem. Rev., 122,11830, 2022