Computational Studies of the Water Splitting Reaction on Ceria-Based Catalysts

Abstract

Ceria-based catalysts have attracted considerable interest for the electrocatalytic water splitting reaction (WSR) in solid oxide electrolysis cells (SOECs), due to the high efficiency and high stability for sustainable hydrogen production. Traditional SOECs suffer from instabilities, owing to cracking and polarization at gas/catalyst/electrolyte triple-phase boundaries (TPBs). Ceria-based catalysts as hydrogen electrodes in SOECs can be accessible to both ionic and electronic carriers, and gas molecules, which enables a wider surface area for electrocatalysis in comparison with the TPBs, significantly enhancing the efficiency of hydrogen production in the SOECs. However, the fundamental understanding of the WSR on ceria-based catalysts remains unclear. By using density functional theory corrected for on-site Coulomb interactions (DFT+U), the reaction mechanism for the WSR on low index facets of ceria, as well as the effect of strain and doping are systemically investigated in this thesis, providing a platform for understanding of the electrocatalytic WSR on ceria-based catalysts. Initially the effect of Gd doping in ceria (GDC) on the WSR is explored because of the highly increased ionic conductivity of ceria by the incorporation of gadolinium. It is found that the rate-determining step (RDS) of the WSR on both ceria and GDC is hydroxyl decomposition into H2. At the RDS, H2 evolution is more likely to proceed via the Ce-H and Gd-H moiety on the hydroxylated pure ceria and GDC, respectively. Notably, the formation of such Gd-H is more facile compared to that of Ce-H, which gives rise to the improved electrocatalytic activity of the WSR on ceria by the incorporation of Gd. In addition, doping ceria or creating oxygen vacancies in ceria generally leads to lattice strain. Thereby, the effect of strain on the WSR on the pure CeO2(111) is investigated. It is found that the RDS of the WSR on CeO2(111) remains unchanged under different lattice strains. However, the formation of intermediates as well as the reaction efficiency of the WSR could be effectively tuned by tailoring the lattice strain. The WSR activity on CeO2(111) is predicted to be strongly enhanced when CeO2(111) is compressed by more than 3.0%. Finally, a comparison of the catalytic activity of the WSR on the (110), (100) and (111) facets of ceria is presented. The WSR for H2 production on the (111) and (110) facets of ceria is 10~100 times faster in  comparison with that on the (100) facet of ceria at temperature (T) < 950 K, which reveals that the WSR on ceria is strongly facetdependent at low and intermediate temperature. Interestingly, this facet-dependence is found to diminish at high temperature, suggesting the temperature sensitivity in the facet-dependence of the WSR on ceria.