(685e) Electrode and Catalyst Design for Electrochemical Reduction of CO2

Carbon dioxide is believed to be a major contributor to climate change due to its accumulation in the atmosphere as a greenhouse gas.  To significantly reduce carbon emissions several strategies are being explored, including carbon capture and sequestration (CCS) and increasing the use of carbon-neutral energy sources such as wind, solar, and nuclear.¹  Electrochemical reduction of CO₂ to useful fuels or small organic molecules is more promising than sequestration as it produces value-added products and provides a means of storing power from intermittent  sources such as wind and solar in a convenient and high energy density form.  While the electrochemical conversion of CO₂ has great potential, significant technological advances with regard to electrode structure and catalyst design are still needed for this process to become economically viable.

Many early efforts have focused on exploring different catalysts to make various products such as formic acid and syngas.  More recently, several researchers have proposed various reactor designs to electrochemically convert CO₂ into formic acid or syngas.  Previously, we developed an electrochemical flow reactor for CO₂ reduction in which the anode and cathode are separated by a flowing liquid electrolyte.  Operating this reactor at acidic pH, we succeeded in converting CO₂ to formic acid with high Faradaic and energetic efficiency of 89 and 45%.²  Furthermore, we studied efficient conversion of CO₂ to CO, and found ionic liquid electrolyte, EMIM BF₄, significantly reduced the overpotential to <0.2 V as opposed to ~0.8 V in aqueous electrolytes.  The ionic liquid acts as co-catalyst, stabilizing the unstable intermediate CO₂^- radical.³ While these results are encouraging, to date few efforts have focused on the effect of electrolyte composition and electrode structure.  Better understanding of the relationship between electrode structure and performance is needed (i) to enhance catalyst utilization, (ii) to reduce catalyst loading, and (iii) to improve electrode durability.  To address these challenges, we demonstrate a systematic characterization of the structure-activity relationships of complex electrodes as a function of component materials choice and synthesis (e.g., catalyst design), of electrode fabrication and processing methods (e.g., catalyst deposition and the choice of the gas diffusion layer), and of electrochemical environment (e.g., electrolyte composition).

With respect to catalyst design, we observed CO₂ reduction is highly dependent on the morphology of the catalyst layer.  The particle size of the catalyst has a tremendous influence on performance.  Moreover, unsupported catalysts are most commonly used for CO₂ reduction; however, the catalyst utilization is very low (less than 10%) due to agglomeration.  To overcome this issue, we have developed supported catalyst via loading catalyst particles on highly electrically conductive supports in uniform fashion.    This approach provides an extensive three-phase boundary where effective transport of electrons, protons and CO₂ can take place, resulting in significant performance enhancement.

Regarding electrode fabrication and processing, we investigated how physical properties of the gas diffusion layer such as porosity and hydrophobicity influence electrode performance in terms of effective gas transport of both reactant (CO₂) and product (CO and H₂) gases.  Also, to improve electrode performance and durability we have developed a fully automated air-brushing deposition technique that significantly enhances performance up to 40%.

Regarding electrochemical environment, we investigated the influence of electrolyte composition on the electrochemical reduction of CO₂ to CO.  In specific, we studied the effect of cation and anion size on partial current densities of the desired CO and undesired H₂ products.  For example, we have observed that large cations enhance the CO₂ reduction reaction to CO while suppressing hydrogen evolution reaction, resulting in high Faradaic yield toward CO production.

1.   C. Oloman and H. Li, Chemsuschem, 1, 385 (2008).

2.   D. T. Whipple, E. C. Finke and P. J. A. Kenis, Electrochem Solid St, 13, D109 (2010).

3.   B. A. Rosen, A. Salehi-Khojin, M. R. Thorson, W. Zhu, D. T. Whipple, P. J. A. Kenis and R. I. Masel, Science, 334, 643 (2011).