Essadek, A-A; (2017) Computational Study of Nickel Supported on Yttria-stabilized Zirconia. Doctoral thesis , UCL (University College London).

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Abstract

Solid oxide fuel cells (SOFCs) are an electrochemical device that converts chemical energy by oxidizing fuel into electrical energy. The device consists of three components: the cathode, the electrolyte, and the anode. A common material used for the anode is Ni supported on yttria (Y₂O₃) stabilized zirconia (ZrO₂) (Ni/YSZ). The oxidation of the fuel, e.g. molecular hydrogen, takes place at the anode, at the Ni/YSZ interface. The performance of the SOFC depends on physico-chemical phenomena, such as Ni sintering, and electrochemical reactions taking place at the triple phase boundary (TPB), where the anode, the electrolyte and the gas phase meet. It is therefore important to understand, at the atomic scale, the microstructure of Ni/YSZ and its interaction with the gaseous phase. In this thesis, we employ ab initio techniques, based on the density functional theory with long-range dispersion corrections (DFT-D2), to investigate the two aspects of the physical chemistry of the TPB: the interaction between Ni and YSZ and the interaction of Ni/YSZ with molecules relevant to SOFC reactions, such as CO₂, CO, H₂O and H₂. To study those two aspects, we first adsorb one Ni atom on top of both ZrO2 and YSZ surfaces to understand the influence of the Y atom on the Ni adsorption. This provides insight into the preferential adsorption site of the single atom, which allows us to construct Ni clusters supported on both ZrO₂ and YSZ surfaces. We proceed by adsorbing clusters of up to 10 Ni atoms, taking into account several configurations and we study the diffusion of Ni atoms towards neighbouring clusters. We adsorb three molecules (H₂O, CO₂, and CO) on bare ZrO₂ and YSZ surfaces and on the oxide surfaces decorated with one supported Ni atom supported. From this first model, we describe the influence of the dopant and metal atom on the adsorption of the molecule at the TPB. Finally, to determine a more realistic molecule-Ni/YSZ interaction model, we consider several Ni cluster sizes (up to 20) to study the interaction of the CO₂ molecule with Ni/YSZ and to determine the influence of the cluster size on the CO₂ adsorption. We also evaluate the CO₂-H₂ co-adsorption on Ni/YSZ to investigate the reverse water gas shift (RWGS) reaction (CO₂+H₂→H2O+CO), which is one of the reactions taking place at the TPB of the SOFC.

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