Producing formic acid at low pH values by electrochemical CO₂ reduction

The electrochemical reduction of CO₂ has the potential to become a key technology in the transformation to a sustainable circular carbon economy. Formic acid could be an important platform chemical for the eco-friendly chemical industries of tomorrow. However, in most cases the reduction of CO₂ is performed in highly alkaline electrolyte systems yielding formates instead of formic acid. Furthermore, existing cell configurations do not allow an operation at acidic conditions below the pKa of formic acid over a long period of time. Both challenges make such a process unprofitable since they are associated with high costs for downstream processing. The main challenge, however, is the so-called “carbonate problem”, which is the non-faradaic formation of bicarbonate and carbonate from CO₂ and OH− that cuts the carbon selectivity to faradaic products to half, even at 100 % faradaic efficiency. In the present work we evaluate three different cell configurations, using gas diffusion electrodes (GDEs) with a tin-oxide catalyst, at an industrially relevant current density of 200 mA cm-2. The quantification of outlet CO₂ allows us to compare carbon selectivities, while the variation of current densities supported by numerical simulation results give insights into the local pH value inside the GDE. We demonstrate that an electrolyzer equipped with a single-layer GDE, a liquid electrolyte, and a zero-gap anode can achieve and sustain low pH values, especially below the pKa of formic acid. Altogether this paves the way for an industrial production of formic acid.