Computational unravelling of interface processes in energy materials: from cathodic metal hydride formation to semiconductor photo-absorption and dual-phase battery charging

In response to the urgent need for a global transition towards efficient and renewable energy technologies, this thesis enhances understanding and contributes to the development of materials used for energy generation, conversion, and storage. The aim of this thesis is to computationally unravel the interface processes and structure–property–performance relationships of (i) palladium (Pd) hydride as an electrocatalyst for the hydrogen evolution reaction (HER); (ii) copper zinc tin selenide (CZTSe) tetrapod semiconductor as a photovoltaic (PV) absorber; and (iii) dual-phase bismuth-tin (Bi-Sn) as an anode for magnesium-ions batteries (MIBs).

Various PdHx/Pd interface model systems are first studied and compared in calculated surface Pourbaix diagrams, using first-principles calculations based on density functional theory (DFT) in Chapter 3. The results show that around the equilibrium electrode potential and pH for the HER, Pd starts to be covered by hydrogen adatoms. Lowering the electrode potential increases the hydrogen population on the Pd surface and in subsurface layers. These findings provide insights into the stability and formation of hydrogen-containing Pd surfaces, which creates highly active sites for hydrogen evolution. Subsequently, the structure–activity relations for the HER on Pd surfaces are elucidated in Chapter 4. DFT results reveal an activity trend following Pd(111) > Pd(110) > Pd(100) and that the formation of subsurface hydride layers causes morphological changes and strain, which affect the HER activity and the nature of active sites. To extensively investigate the structural changes of Pd induced by hydride formation, molecular dynamics (MD) simulations are performed to elucidate the atomic-scale mechanisms of restructuring processes on Pd surfaces in Chapter 5. MD results show that surface alterations are related to the creation and propagation of structural defects, as well as phase transformations that take place upon formation of hydrides. These findings provide novel insights into the H-induced Pd surface reconstruction under reaction conditions, and are thus important for the design of next-generation highly active and selective Pd-based electrocatalysts.

Next, the electronic and optical properties of the four identified domains within the tetrapod-shaped CZTSe nanocrystal (NC) are explored using DFT calculations in Chapter 6. The results reveal that multiple domains within a single NC have independent and distinct electronic properties. Interestingly, the 3D periodic structure of only two domains exhibits a band gap. Moreover, the computationally predicted type-II band alignment between these two semiconductor domains facilitates electron-hole pair separation and enhances solar power conversion efficiency. These insights into the structure–property relationships in CZTSe NCs will guide the design of next-generation CZTSe-based solar cells and optoelectronic arrays.

Finally, a dual-phase engineering strategy to develop a Bi-Sn dual anode for MIBs is presented in Chapter 7. DFT calculations are employed to explain the mechanisms behind the experimentally improved performance of dual-phase Bi-Sn electrodes. It is computationally shown that the formation of the Mg3Bi2//Sn interfaces is energetically more favourable compared to the most stable exposed surfaces of the individual Mg3Bi2 and Sn phases. Furthermore, Mg insertion into Sn is facilitated when Mg3Bi2 is present. The computational findings point towards easier magnesiation/demagnesiation for the dual-phase Bi-Sn electrodes over pure Bi or pure Sn. The use of dual-phase engineering is a major step forward in the development of anodes for MIBs