Atomically dispersed iridium electrocatalysts for the oxygen evolution reaction

Abstract

Proton-exchange membrane (PEM) water electrolysers can efficiently integrate with renewable energy sources for green hydrogen production. With the increasing occurrence of surplus renewable electricity periods, there is a growing market for PEM water electrolysers. However, the scalability of these water electrolysers faces a bottleneck due to the use of costly Ir-based catalysts for the anodic oxygen evolution reaction (OER). To address this cost issue and to enhance catalytic efficiency, research efforts are shifting towards exploring atomically dispersed Ir electrocatalysts for the acidic OER. While in theory, the atomically dispersed Ir electrocatalysts exhibit enhanced mass-specific activity due to the maximum utilisation of the Ir, in practice, they often show poor stability during synthesis and operation. Their degradation can be traced back to the aggregation of the atomically dispersed Ir into larger particles, i.e., a reduction of the catalytically active surface area of Ir. Enhancing catalyst stability requires a deeper understanding of the mobility, agglomeration behaviour, and bonding environment of Ir during synthesis and usage. This thesis focuses on the synthesis and the characterisation of the atomically dispersed Ir on TiO2 anatase nanosheets for the acidic OER. In the first half of this work, a facile synthesis strategy for preparing atomically dispersed Ir on TiO2 anatase with minimised Ir agglomeration is developed. Utilising in-situ heating annular dark field scanning transmission electron microscopy (ADF-STEM) microscopy, combined with X-ray absorption fine structure (XAFS), the Ir agglomeration behaviour and mobility on TiO2 with increasing synthesis temperature is correlated with the evolution of the bonding environment of the Ir, offering critical insights into the synthesis mechanism. In the second half of this thesis, a comparative study of OER performance between atomically dispersed Ir on TiO2 and oxidised Ir clusters is presented. By employing high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) to establish a statistically significant representation of the Ir dispersion on TiO2, correlated with the XAFS analysis of the local environment of Ir, this work provides a fundamental understanding of the structure-activity-stability of the atomically dispersed Ir on TiO2 anatase for the acidic OER.