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Performance Modelling of Photoelectrochemical Devices in Reduced Gravitational Environments

Supervisor: Katharina Brinkert (Chemistry) and Sophia Haussener (EPFL)

Photoelectrochemical (PEC) devices, combining the processes of light absorption, charge separation, charge transfer and catalysis, are currently realised for light-assisted water-splitting and simultaneous fuel production which makes them not only interesting for the green energy transition on Earth: the reduced mass and volume in comparison to electrochemical cells powered by PV represent as well key advantages for the application in life support systems for long-term space travels, where a resupply of oxygen, food and fuels from Earth is not possible. The governing interface phenomena in photoelectrochemical devices and the impact of reduced gravitation are however not understood. Particularly, the impact of reduced gravitation on substrate- and product- transfer to and from the electrode surface as well as light reflection from gas bubbles adhering to the electrode surface are only two phenomena which are highly important for the photoelectrochemical device performance, but which are difficult to assess in terrestrial and microgravity experiments. This project aims at creating a computational model of a complete PEC device where the different processes impacting the device performance can be simulated and the effect of reduced gravitation can be directly predicted and quantified. Experimental photocurrent-voltage as well as gas bubble evolution data obtained in drop tower campaigns within the project "Solar-assisted oxygen and fuel production for long-term space travels and Moon habitats" will feed into the computational simulations to validate and ensure the accurate and predictive character of the model in predicting the impact of reduced gravitation on photoelectrochemical devices. The established model is also of significant importance for the prediction of the device performance and output for terrestrial applications where the impact of gas bubble desorption and mass transfer (limitations) are barely understood as well.

The project is divided into four main sub-projects:

1) Development of the photoelectrochemical half-cell device model (photocathode), including formulation of governing conservation and transport equations with appropriate boundary conditions and the identification of key interfacial reactions and parameters determining the half-cell efficiency (for terrestrial conditions). Wherever possible, commercial and/or open finite volume solvers will be used (Ansys, Fluent, Comsol, OpenFoam, etc.). Volume of Fluid methods will be explored for the incorporation of multi-phase phenomena, and - if needed - even Lattice Boltzmann methods will be explored. Initially, 1D multi-physics models will be developed, which will be extended to 2D (and 3D if necessary), in order to account for heterogeneities in multiple directions. The half-cell model will be validated by experimental data from an earlier micro-gravity campaign, literature or theoretical predictions (for example thermodynamic limits).

2) Identification of the governing terms impacted by gravitation and subsequent simulation of the impact of microgravity, Moon gravitation, and Mars gravitation on photoelectrochemical hydrogen evolution. These investigations include parameter studies that sweep material properties (interface kinetics, absorptivity, etc.), operating conditions (irradiation intensity, solution concentration, etc.) and design (dimensions, orientation, etc.) of the half-cell, in order to identify the dependence of gravitations effects on these choices.

3) Combination of the hydrogen evolution half-cell with the oxygen production half-cell and creation of a complete device model for light-assisted hydrogen and oxygen evolution based on the results obtained in 1). The computational domain effectively will be extended and the new boundary conditions and material characteristics (interfacial reaction etc.) will be added. Again, the device model will be validated by experimental data, literature or theoretical predictions (for example thermodynamic limits).

4) Numerical investigation of the impact of reduced gravitation on performance and operation of device model in analogy to 2). Identification of the 'ideal' photoelectrochemical device in terms of the semiconductor choice, electrocatalyst nanostructure and electrolyte composition for efficient operation in reduced gravitation based on the identified parameters in 1) - 4). Simple (for example parameter variation) and advanced (for example machine learning) optimization methods will be explored.

5) Extension of the device model to photoelectrochemical carbon dioxide reduction. Validation with data from literature. Identification of fundamental similarities and differences between water and CO2 splitting devices.

This research project will be jointly supervised by Dr. Katharina Brinkert, University of Warwick and Dr. Sophia Haussener, EPFL/Switzerland and is co-funded by the European Space Agency, ESA.