Modeling and assessment of high-temperature photo-electrochemical devices

In this thesis, the feasibility and performance of photo-driven electrochemical (PEC) devices working at elevated temperatures (T>700 K) is investigated and modeled. In such high-temperature PEC devices, a part of the solar spectrum is used for generating photocarriers in high-temperature solar cells while the non-absorbed solar irradiation is transmitted towards a planar solid oxide electrolyzer (SOE), where the elevated temperature reduces the equilibrium voltage of the reaction, improves the kinetics of catalysts, and increases the ion conductivity of the solid electrolyte. Although this approach has been proposed as an alternative to PEC devices working at low temperature for reducing the use of rare materials, the quantitative evaluation and formulation of design guidelines for full high-temperature PEC devices remain unexplored.

Firstly, a 0-D model of low-temperature devices (LTDs) and high-temperature devices (HTDs) was developed and used to compare their characteristics and performance, considering also auxiliary components (pumps, compressors, heat exchangers, separators) and the option to couple the device with a secondary power cycle. Besides the generation of H2, we also explored the possibility of producing CO, CH4, and C2H4. HTDs reached solar-to-fuel efficiencies of around 10% and 12%, while LTDs reached efficiencies of around 16% and 29% when using single and double junction solar cells, respectively. Even though the performance was lower for HTDs, these devices release a mixture of reactants and products at temperatures high enough to power a secondary power cycle, producing additional energy. Under such conditions the combined cycle would be able to match the efficiency of LTDs and produce larger net powers.

Subsequently, 1D and 2D device models, incorporating high-temperature solar cells (HTSCs) and the SOE, were developed. Different assemblies of semiconductors were assessed to work at temperatures between 600 to 800 K. Only assemblies such as GaAs/GaP and GaAs/GaInP-GaAs/GaP showed photovoltages sufficiently high to drive the electrochemical reactions with high current densities, e.g., 2.5 Acm-2 and 0.35 V (GaAs/GaP) and around 1 Acm-2 and 0.9 V (GaAs/GaInP-GaAs/GaP) at 800 K and a solar concentration of 100 suns. Parametric studies using these models not only helped to maximize the efficiency, but also to determine the right operating conditions for the production of syngas with adequate H2/CO ratios for its later use in a Fischer-Tropsch reactor. Finally, these models helped to evaluate different designs with various levels of integration between the HTSCs and the electrodes of the SOE.