Nikkel-bevattende bimetaal-nanodeeltjies as katalisatore vir elektrochemiese watersplyting

Abstract

The hydrogen evolution reaction (HER) can occur in an acidic or alkaline medium. The alkaline HER is very important for water-splitting electrolysers because contamination of H2 and corrosion of the electrolysers and electrodes are eliminated. However, the alkaline HER requires extra energy to break the covalent bonds in the water molecule. This requirement of the alkaline HER causes the reaction kinetics for the dissociation of water to slow down. Therefore, designing more efficient alkaline HER catalysts is critical to lower the energy barrier and improve the hydrogen adsorption for the H–H bond formation. Although several highly active non-noble metal compounds have been identified as catalysts in an alkaline medium for the oxygen evolution reaction (OER), there is a lack of non-noble metal equivalents for the hydrogen evolution reaction (HER). This shortcoming slows the development of cost-effective water-splitting devices in alkaline media. Nickel was identified almost a century ago as a catalyst for OER and HER in alkaline media. In an attempt to increase the electrocatalytic activity and stability of Ni for the above reactions, two possible approaches can be used, namely the addition of a secondary metal or the increase of the contact surface. In this study, both approaches were applied. The contact surface of the catalyst can be increased by using nanoparticles. Because nanoparticles are so small, their surface-to-volume ratio is larger than their equivalent material. In this study, nanoparticles with a magic number of 55, in other words, nanoparticles consisting of 55 atoms, were investigated. The influence of the addition of a secondary metal to Ni can be investigated by building different Ni-containing bimetallic structures, namely NiCo, NiCr, NiMn and NiZn, using the Site Occupation Disorder (SOD) program. The most stable unique configurations for each Ni-containing bimetallic structure were identified and optimised. Afterwards, a bimetallic nanoparticle with 55 atoms was cut from each bulk structure and optimised. Two core-shell nanoparticles were also built and optimised, as well as two nanoparticles whose core and corner atoms in one consist of Ni atoms and the other of the secondary metal. Therefore, ten nanoparticles were constructed and optimised for each bimetallic combination. For each of the nanoparticles, three structural properties (volume, surface area and surface-to-volume ratio) and three electronic properties (Fukui function, Mulliken atomic charge and the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied

molecular orbital (LUMO)) were investigated, as well as stability and activity for electrochemical water splitting. The structural properties showed that only the core-shell nanoparticles, whose core atoms consist of the secondary metal, have a contact surface larger than the Ni nanoparticle, but still smaller than the Pt nanoparticle. The Fukui function showed that the probability of poisoning by the OH- group is greater with the bimetallic nanoparticle than with the Ni nanoparticle, but still lower than with the Pt nanoparticle. The Mulliken atomic charge showed that the position to which the Ni atoms withdraw electrons from the core depends on the secondary metal, and finally, the HOMO-LUMO energy gap showed that all the bimetallic nanoparticles are more stable than the Ni nanoparticle. The approach in this study enables researchers to investigate a larger, repeatable spectrum of different nanoparticles with bimetallic structures for developing WVR catalysts.