Is maximising current density always the optimum strategy in electrolyser design for electrochemical CO2 conversion to chemicals?

Electrochemical conversion of CO2 to chemicals and fuels can potentially play a role in reducing CO2 emissions from industrial processes and providing non-fossil fuel routes to important chemical feedstocks. Most of the recent research on electrocatalysts for CO2 reduction (CO2R) focuses on achieving maximum selectivity for desired products at the highest possible current density. This approach assumes that maximising current density leads to the lowest cost of CO2R (e.g. $·kg-1 CO2 converted) because it requires the lowest catalyst loadings and electrode area per kg of CO2 treated and thus minimising the electrolyser equipment cost. Using a techno-economic analysis (TEA) model with experimental data from a two-cell vapor fed electrolyser, we show this assumption is not valid for CO2 conversion to CO if the process model accounts for relationships between current density, selectivity, cell voltage, ohmic losses, and product separation costs. Instead, our model predicts the lowest CO production costs at current densities from 500 – 700 A·m-2. At current densities above 1000 A·m-2, growing ohmic losses in the electrolyser lead to increasing power costs that become much larger than any capital savings related to reduced electrode area at the higher current density. Further, we investigate different opportunities that could bring down the CO production cost, however, in all the cases, the lowest CO production cost was found at current densities between 600 – 1400 A·m-2. This work also provides insights that can help identify feasible design spaces for both catalysts and electrolysers to develop CO2 conversion technologies that could soon compete on a cost basis with the natural reforming technologies to produce CO (0.60 $·kg-1 market price).