Co-Electrolysis Cell Configurations for CO2 Electrochemical Reduction

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).