Co-Electrolysis Cell Configurations for CO2 Electrochemical Reduction
Authors: Durst Julien , Herranz Juan S. , Paratcha Yohan , Permyakova Anastasia A. , Schmidt Thomas J. 
Published: February 2020

Although the electrochemical reduction of CO2 has been extensively studied over the past decades, with a renewed interest in the past 5 years, this topic has been tackled so far only by using a very fundamental approach, i.e., mostly by trying to understand and improve faradaic efficiencies, kinetics and selectivities toward certain products in half cell configurations and liquid based electrolytes (such as KHCO3) (1). The main drawback of this approach is that, due to the low solubility of CO2 in water, the maximum current which could be drawn falls in the range of 10-20 mA/cm2 even under the most optimized convection conditions. In this work, CO2 is intended to be electrochemically reduced in a co-electrolysis system, i.e., a device similar to a water electrolyzer where the hydrogen evolving cathode is substituted by a CO2 reduction one. By making the analogy with state-of-the-art water alkaline electrolyzers, such co-electrolysis system starts to be interesting from an energy efficiency point of view only if large currents (>100 mA/cm2) can be drawn from the anodic and cathodic electrochemical reactions (2). Due to the additional difficulty to prepare heavy metal impurity free pure liquid carbonate based electrolytes (below the ppm level to avoid cathodic metal plating and facilitated hydrogen evolution), the motivation is the development of a membrane-based cell using liquid electrolyte free gas diffusion electrodes for the CO2 reduction reaction. This strategy is proven to enable operation at cell current densities > 1 A/cm2 in Proton Exchange Membrane (PEM)-electrolyzer and Polymer Electrolyte Fuel Cells.
Only very few reports have already investigated the use of gas diffusion electrodes for the CO2 reduction reaction (3, 4). In what we identified as the most promising co-electrolysis approach, unsupported gold or silver assembled in the CO2 reduction electrode were separated from the oxygen evolution electrode through an 800 µm thick buffer layer filled with a 0.5 M KHCO3 solution and a 50 µm thick cation exchange membrane (3, 4). These were the first experimental reports showing that CO2 reduction currents of ≈140 mA/cm2 for forming CO could be achieved at an overpotential of ≈0.5 V. Nevertheless, in this cell configuration, the CO2 reduction electrode is in contact with a buffer layer consisting of a concentrated KHCO3 solution, and therefore the question whether such configuration could sustain high CO2 reduction currents over time due to contamination issues is questionable. In this contribution, we will report alternative liquid electrolyte free cell configurations for co-electrolysis operation. The most promising configurations will be characterized using a small scale electrolysis cell (1cm2 active area), and the CO2 reaction products will be analyzed by using an on line mass spectrometer (MS) coupled to the gas outlet of the electrolysis cell. Since only gaseous products can be detect by MS, unsupported Cu and Au nanoparticles which produce mostly CH4/C2H4 and CO species respectively will be used as catalysts in the CO2 reduction electrode.
References:

1 D. T. Whipple and P. J. Kenis, The Journal of Physical Chemistry Letters, 2010, 1, 3451.

2 K. Zeng and D. Zhang, Prog. Energ. Comb. Sci., 2010, 36, 307.

3 C. Delacourt, P. L. Ridgway, J. B. Kerr and J. Newman, J. Electrochem. Soc., 2008, 155, B42 (2008).

4 C. Delacourt and J. Newman, J. Electrochem. Soc., 2010, 157, B1911 (2010).