Insight in the behavior of bipolar membrane equipped carbon dioxide electrolyzers at low electrolyte flowrates

Electrochemical CO2 reduction is a promising carbon capture and utilization technique, which is urgently required to prevent earth's global warming. Extensive research on catalyst improvement, reactor engineering and electrode development has been performed in order to develop an industrially scalable system. However, despite the great process that has been made, there is still a lack of understanding in the fundamental behavior of the electrolyzers. To reduce this gap, we have investigated the influence of product selectivity and current density on the pH of the catholyte in bipolar membrane electrolyzers. More specifically we have found that, by targeting formate production, consumption of bulk bicarbonate occurs, which can cause a pH drop of the catholyte. Especially at low catholyte flowrates and high partial current densities, it was shown that this phenomenon can negatively affect the cells performance due to a high concentration of protons in combination with a high residence time of the catholyte. By implementing these fundamental findings, we were able to operate the electrolyzer under acidic conditions, without affecting the electrolyzers performance, allowing us to produce formic acid rather than formate which has not yet been achieved in bipolar membrane based electrolyzers. Under optimal conditions, we achieved a product stream with a formic acid/formate ratio of 0.67 and a combined concentration of 34 g/l, a partial current density of 152 mA/cm2 and a cell voltage of 6 V.