Screening of Pt/Ni bimetallic electrocatalysts using DFT calculations

Abstract

Clean, renewable energy could be produced from the electrocatalytic splitting and recombination of water in electrolysers and fuel cells, called electrochemical devices. However, the development of these electrochemical devices is limited by the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). These reactions require an overpotential of 1.23 V, leading to slow reaction kinetics. The overpotential could be lowered and the reaction kinetics improved by using electrocatalysts and using acidic electrolytes. In the case of electrocatalysts, the electrode materials that could be used are limited. It is limited even further if an acidic media is used because acidic media destabilises electrode materials by dissolution/corrosion. In previous studies, precious metal electrodes, amongst others, have shown catalytic activity towards the OER and ORR. Of these precious metal electrodes, platinum (Pt) is the most used commercial electrocatalyst. However, Pt is only catalytically active for the ORR but not for the OER. Additionally, Pt is a rare, expensive resource that is not stable in acidic media. For these reasons, various studies have been conducted to identify an alternative cost-effective and stable electrocatalyst for the OER and ORR. One possibility that has shown potential is the development of bimetallic surfaces since the activity, selectivity and stability (also called lifetime) of a metal, such as Pt, could be fine-tuned through the addition of a secondary metal. In this study, nickel (Ni) was chosen as the secondary metal because Ni-based electrodes showed excellent performance towards the OER. However, before the Pt/Ni bimetallic surfaces could be investigated as potential electrocatalysts for the OER and ORR, a computational model had to be developed. Pure Pt and Ni unit cells were used to develop the computational model by adapting existing density functional theory (DFT) models for Pt and Ni. The adapted computational model was validated by comparing the structural and electronic properties of the pure Pt and Ni unit cells with literature values. Slab systems with (111) surfaces were created for Pt and Ni. Further validation of the adapted computational model was done by comparing the structural and electronic properties of Pt (111) and Ni (111) slabs to literature values. The next step in this study was to construct Pt/Ni bimetallic systems. Two techniques for the construction of Pt/Ni bimetallic systems, namely a random manual and an automated systematic technique, were employed. The random manual technique is a simplistic technique that involves the manual substitution of Pt atoms with Ni atoms in a pre-existing Pt (111) surface constructed from optimised pure Pt bulk structures. Using the manual substitution technique, the ensemble and the ligand effect were investigated separately. Moreover, hydrogen adsorption investigations were performed on these bimetallic surfaces. The results obtained for these investigations showed that the adsorption ability of a bimetallic surface could be fine-tuned by using either the ligand or the ensemble effects. However, this random manual is not sufficient to investigate all inequivalent configurations and is not scientific reproducible. In contrast, the automated systematic technique is a more scientific reproducible technique that uses a computer program, SOD, to create all possible inequivalent configurations for a selected size of the Pt/Ni bulk structures. In addition, SOD allowed the user to obtain thermodynamic data, which was used to calculate thermodynamic properties such as the full-disorder entropy, mixing enthalpy and a probability distribution. These thermodynamic properties were used to narrow down the possible inequivalent configurations, and to obtain the optimal Pt/Ni ratio and Pt/Ni atomic arrangement for a bimetallic bulk structure. A Pt/Ni bimetallic surface was constructed from the optimal Pt/Ni ratio and Pt/Ni atomic arrangement bulk structures obtained from the automated systematic technique. Finally, the reactivity of the Pt/Ni bimetallic surfaces, constructed with the random manual technique and the automated systematic technique, were investigated. The surface and electronic properties, namely the surface energy, work function, d-band centre, binding energy and the adsorption energy which was obtained by the adsorption of an elementary hydrogen on the Pt/Ni bimetallic slab surfaces. From the comparison of the results obtained for the Pt/Ni bimetallic bulk structures and surface systems, constructed by the two techniques, it was obvious that the automated systematic technique gave more consisted results. The reason being that the surface slabs created using the random manual technique resulted in an uncertainty of the configuration stability, which was observed from the varying surface reactivity results for each of the adsorption sites. However, the results obtained using the automated systematic technique concluded that a bimetallic bulk structures consisting of a Pt0.5Ni0.5 mixture was stable and exhibited favourable magnetic properties, which is a contributing factor towards the reactivity of the surface. In addition, the d-band centre of the Pt/Ni bimetallic surface, constructed from the Pt0.5Ni0.5 bulk structures, shifted towards the Fermi level. This shift indicated an increase in the hydrogen binding energy (HBE). The HBE exhibited by the Pt/Ni (111) bimetallic surface was higher than the HBE obtained for the Pt (111) surface, but weaker than the HBE for the Ni (111) surface. This HBE indicated that the surface reactivity of the Pt surface was successfully modified by the addition of Ni. Lastly, the adsorption of a single H2O molecule on the bimetallic surfaces constructed from the Pt0.5Ni05 bulk structures was investigated. From the H2O adsorption results it could be concluded that these bimetallic surface slabs could be potential electrocatalysts for either the OER or ORR, since cleavage of the water molecule occurs on the surface. Although experimental data is needed to support the results in this thesis, the conclusion is that Pt/Ni bimetallic surfaces are the future for the water splitting reactions.