Presentation - Multiphysics model for design optimization of a monolithic photoelectrochemical cell device

Abstract:

The production of low-emission fuels is increasingly important to support the energy demand while combatting climate change. In this regard, harnessing solar energy for hydrogen production is an appealing prospect. The FreeHydroCells European project (Grant Agreement No 101084261) aims to create a novel, tandem, wireless, monolithic photoelectrochemical (PEC) cell to split water unassisted and provide a cheap, efficient and modular hydrogen generating solution. However, many challenges 
remain around developing deployable devices. It is being recognized that the design of PEC systems plays an equally important role as the photoelectrodes during the upscaling process [1]. A model was thus developed in COMSOL Multiphysics to optimize the design of a monolithic PEC cell device.

The 2D model couples charge, mass and momentum transfer to predict local current densities, potential distribution, ionic concentrations, volumetric gas fractions, pressure and velocities profiles in the electrolyte. A laminar two-phase electrolyte flow is considered, and the mixture model of Schillings 
et al. [2] is used to solve the bubble plumes evolving from the electrodes. The transport of charged species by convection, diffusion and migration is described by the Nernst-Planck equation, while the potential distribution is derived from the stationary charge conservation. The model is applied to amonolithic cell immersed in a potassium carbonate buffer at pH 12, and surrounded by a membrane toseparate the anodic and cathodic compartments. The membrane allows the transfer of ions, but it is assumed impermeable to gas and water. The equilibrium reactions in the bulk electrolyte are considered, as well as the kinetics of the electrode reactions using a Butler-Volmer equation with mass transport effects [3]. The optimum PEC monolithic cell in the project is still under confidential development, so the photocurrent densities related to it are not incorporated for the time being. Still, bubble-induced optical losses are assessed in addition to the hydrogen flux as an important output parameter for PEC devices.

Cells of various dimensions are compared. The results show that, when the size of the cell increases, the gas flux is significantly reduced. It drops from 4.3 mL/h/cm2
for a 1 cm² cell to 0.9 mL/h/cm2 for a larger cell of confidential 𝑛x𝑛 cm² surface area (-78%). Indeed, in a monolithic configuration, gas is mainly produced at the edges of the electrodes. Coupling multiple cells of smaller dimensions would thus be a clever way of improving the performance of upscaled systems. According to model predictions, the hydrogen produced by 𝑛 rectangular cells of 1x𝑛 cm² amounts to 3.4 mL/h/cm2, that is nearly four times higher than for the 𝑛x𝑛 cm² cell. However, the use of multiple small cells increases the optical loss due to bubbles: 17% of the incident radiative power is lost in the 𝑛-cell configuration, versus 6% when using one 𝑛x𝑛 cm² cell. While the gas flux should still be higher in the former case, these results show that the consideration of optical loss is crucial for the design of PEC devices. Apart from the cell size, the model is also used to evaluate the influence of several geometrical and operating parameters, such as membrane dimensions, electrolyte volume or pH.

[1] Vilanova et al. (2024) https://doi.org/10.1039/D1CS01069G
[2] Schillings et al. (2015) https://doi.org/10.1016/j.ijheatmasstransfer.2015.01.121
[3] Lee et al. (2022) https://doi.org/10.1016/j.fuel.2022.123273

Keywords: monolithic photoelectrochemical cell, water splitting, multiphysics model, design optimization, hydrogen, bubbles, optical loss

This presentaiton was delivered by Elise Bérut at the 5th International Solar Fuels Conference in Sptember 2025.