Platinum single-site catalysts for the hydrogen evolution reaction
Keywords: single-site catalysis
water splitting
platinum
hydrogen evolution reaction
Operando characterisation

To meet the International Energy Agency (IEA) net zero emissions scenario, terawatt-scale green hydrogen production is needed from renewable electricity using proton exchange membrane (PEM) water electrolysis. However, PEM water electrolysis now requires using rare and expensive noble metal electrocatalysts, undermining its economic viability for large-scale deployment. Single-site catalysts (SSCs), have active structures of isolated metal sites supported on substrates and thus could alleviate this problem by maximising the noble metal utilisation without comprising activity. The synthesis, activity, and stability of SSCs are determined by the interaction between isolated metal sites and supporting substrates. Platinum single-site catalysts (Pt SSCs) supported on carbon substrates have been proposed as efficient and stable electrocatalysts for the hydrogen evolution reaction (HER). However, due to the complexity of the atomic structure and the lack of in-situ characterisation techniques, the Pt SSCs’ synthesis strategy, formation mechanism, atomic structure, and electrocatalytic activity for the HER are still highly debated. Therefore, the main focus of this project is to systematically explore the synthesis, characterisation, and catalytic activity of Pt SSCs that are stabilised differently on carbon substrates for the HER.
To achieve this target, three different types of Pt SSCs were initially synthesised, as defined by the interaction between the single site and the substrate. Firstly, Pt single-site complexes physically adsorbed onto graphene were synthesised by interacting with aniline molecules. After clarifying its atomic structure, I present a mechanistic study of this platinum SSC for the HER using operando, synchrotron-X-ray absorption spectroscopy. The results indicate that the physically adsorbed Pt SSC are decomposed, and platinum agglomerates as nanoparticles at operating potentials. Then, to provide a stronger interaction between Pt sites and the substrate, Pt single sites were chemically bonded to the surface of a polyaniline substrate. With the advantage of complementary characterisation techniques, the atomic structure of this chemisorbed Pt SSC was clarified, and its active structure for the HER was recorded. Finally, a Pt SSC trapped at defects of N-doped graphene, the most stable single-site structure, was prepared by pyrolysis. In-situ characterisation techniques were employed to thoroughly clarify its formation mechanism and atomic structure. Using this well-defined model Pt SSC, it is found that the stable Pt SSC displays a lower intrinsic activity for the HER than metallic Pt nanoparticles.
This thesis provides a comprehensive study of how the interaction between Pt single sites and substrates affects the catalytic activity of Pt SSCs for the HER through advanced synthesis strategies and detailed characterisation techniques. My study clarifies the desired structure of Pt SSC for the HER and will aid the design and synthesis of improved SSC for the HER and other electrochemical reactions.