Strategies to improve the performance of platinum-based electrocatalysis from the atomic scale

• Main Author: Guo, Shasha
• Other Authors: Liu Zheng
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Science::Chemistry::Physical chemistry::Catalysis; Engineering::Materials::Energy materials
• Online Access: https://hdl.handle.net/10356/161326
• Institution: Nanyang Technological University

The current energy and climate crises urgently demand the global energy system to transition away from fossil fuels. Green hydrogen fuel, produced by water-splitting, is considered one of the best sustainable alternatives to the crises. Notably, an external energy input, such as electricity, is needed to drive the energy-uphill hydrogen evolution reaction (HER). Platinum metal is known to be the most efficient catalyst for HER, and is the state-of-art material in sustainable commercial technologies, namely water electrolyzers. However, the scarcity, low annual production, and high cost of platinum metal significantly limit the industrial scalability of electrolysis to reach terawatt-scale energy production. As a consequence, this pressing challenge calls for the rational design of practical Pt catalysts with minimized Pt loading and uncompromised catalytic activity. To this end, this thesis aims at constructing novel architecture for Pt-based catalysts with atomic precision, and studying the dynamic effects of HER over Pt/C. First, a chemical vapor co-deposition method is developed to synthesize wafer-scale single platinum atom catalysts. Comprehensive characterizations show that the synthesis method can effectively stabilize single platinum atoms on the two-dimensional films, and the density of single dispersed Pt atoms reaches ~10 wt%. More notably, these single atoms are arranged into metallic atomic chain structures with an average length of up to ~17 nm, further forming an interconnected single-atom chain network in the atomic thin film. As expected, these single dispersed platinum atoms serve as active centers and remarkably enhance the reactivity of the atomic thin film. However, due to the relatively low coverage of platinum atoms, the geometric area-based activity of as-prepared material is inferior to that of pure Pt film. Second, to satisfy industrial needs, an amorphization strategy is developed to fabricate a freestanding single platinum layer—monolayer amorphous PtSex. Extensive structural characterizations and first-principle calculations suggest that this structure can be considered as a single platinum atom layer stabilized by selenium atoms. Furthermore, electrochemical measurements combined with theoretical calculations show that this amorphous platinum layer possesses a fully activated surface with a current density close to 100% relative to a pure Pt surface, and mass activity one to two orders of magnitude higher than commercial Pt/C catalysts for hydrogen production. Likely, this structure is the ultimate design for tailor-made Pt-based catalysts with the lowest possible Pt loading, high platinum atom utilization efficiency, and ultra-high activity. Third, although thermodynamic equilibrium models are usually employed to analyze catalytic processes, heterogeneous catalysis is indeed a highly dynamic process. In particular, the hydrogen spillover phenomenon is frequently encountered in various heterogeneous catalysis; however, direct evidence of this phenomenon in HER is lacking. To gain insight into this phenomenon in HER, an in-situ single-molecule total internal reflection microscopy/micro-electrochemical platform is developed. Comprehensive investigations are performed over a model Pt/C system, namely well-defined Pt/graphene. Surprisingly, the graphene support directly participates in the reaction instead of being a spectator. The in-situ experiments evidence that the hydrogen spillover can activate graphene support towards the hydrogen combination step of HER. Furthermore, in order to optimize the practical Pt/C catalysts, on-chip devices were fabricated to the effective spatial extent of activated carbon and the optimized area percentage of Pt.