Numerical modelling of a gas diffusion-based CO₂ electrolyser with flowing catholyte

Abstract

Electrochemical CO₂ reduction may present a solution to close the carbon cycle and to utilise CO₂ emissions. However, for this technology to have a significant impact, it has to be successfully implemented on an industrial scale. Numerical simulations can aid with the study of process parameters and reactor design.

The overall aim of this project is to develop and utilise a numerical model that can describe phenomena arising in the CO₂ electrolyser inside the flooded catalyst layer (CL). First, the general operation of the electrolyser is addressed, and this is extended for the effect of liquid flow rate, electrolyser length, and operating pressure. To investigate the performance and limitations arising at the large-scale, the model is scaled-up to describe a one meter long electrolyser. The study is concluded with two considerations that could improve the electrolyser performance. These points were addressed by developing a 2D numerical model of a gas diffusion-based CO₂ electrolyser in COMSOL Multiphysics. We assessed the performance of the electrolyser in terms of current density, reflecting rate of species formation, and of faradaic efficiency for CO (FE), reflecting selectivity towards the desired product.

Investigating the small-scale electrolyser we find that at high current density (200 mA cm^-2), the pH in the CL immediately increases by 3 units and further diagonally increases from pH 10.1the inlet to 12.2 around the outlet. When operating the electrolyser with excess of CO₂ supply, we find the CL to perform the best near the gas phase boundary (311 mA cm^-2, 95% FE), while the regions close to the electrolyte are underperforming (250 mA cm^-2, 89% FE). This shows that the performance in certain regions of the CL needs to be improved.

When scaling-up the electrolyser to a length of one meter we find that the performance does not change dramatically when operating at excess of gas supply. However, if a high CO₂ conversion should be achieved, the long electrolyser shows a 10% decrease in FE, and CO₂ conversion compared to a small-scale electrolyser, at the same level of current density (115 mA cm^-2).  Analysing the current density locally, we find that difference between the inlet and outlet can be as large as 100 mA cm^-2. Next, we find that FE can fall to almost 50% around the outlet. This shows that when adding extra length to the long electrolyser, this extra length only adds a fraction of its potential performance.

The uneven utilization of the catalyst can be improved by varying the catalyst loading along the electrolyser length. This improves the FE by around 5% while using 40% less catalyst. We also find that while the current density is slightly lower, the amount of product generated per mass of catalyst has significantly increased. This shows that carefully engineering the catalyst loading can save the amount of catalyst needed and could potentially improve the cost-effectiveness of the CO₂ electrolyser.

In all cases the performance over the CL is unevenly distributed. To achieve a higher performance, research needs to find ways how to enhance the performance also in the poorly utilised regions of the CL. Scaling-up the electrolyser just by extending its length proves inefficient and inevitably leads to a lower performance. The beneficial buffering effect provided by the electrolyte at a small-scale does not translate to a large-scale. At this moment, performance of large-scale CO₂ electrolysers seems satisfactory only when operating at very low CO₂ conversion. From the investigated parameters that address the performance issues, higher operating pressure and smart catalyst loading seem only promising options, however, other options should be found to speed up the development.