Electrocatalytic Reduction of CO2 to CO

Over the past decades, catalysts for fuel cells have been studied in great detail, leading to tremendous improvements in catalyst performance with respect to activity, stability, and durability. Furthermore many approaches have been developed to lower the amount of precious metals needed, for example by the use of metal alloys, or by supporting catalyst on carbon and other materials.  A lot of this progress has only been made possible by many detailed mechanistic studies that helped unravel the different pathways and rates of oxygen reduction and fuel oxidation on different catalysts.  Over his career Andrzej Wieckowski certainly has made key contributions to improving our understanding of electrocatalytic processes, particularly those for fuel cells.

More recently, another electrocatalytic process has gained significant attention in the research community: the electroreduction of carbon dioxide (CO₂) to small molecules such as methanol, formic acid, carbon monoxide (CO), or short hydrocarbons (typically C1-C3).  The drivers for these investigations are multiple, including the desire to identify ways to slow down the increase in (or even reduce) the atmospheric CO₂ levels, with various undesired climate change effects already being attributed to the current CO₂ level of >400 ppm.  The amounts of CO₂ being produced are so large that multiple approaches need to be implemented, including switching from fossil fuel-burning power plants to renewable energy sources, increasing the energy efficiency of buildings, increasing the fuel efficiency of vehicles, and underground carbon capture and sequestration (CCS) to curb the increase in atmospheric CO₂ levels.^[1-3]  Electrochemical reduction of produced and captured CO₂ into useful chemicals including carbon monoxide (CO), formic acid, methane, and ethylene^{[5, 6]} is another approach to address this grand challenge.    Coupled to renewable energy sources such as wind and solar, this process can produce carbon-neutral fuels or commodity chemicals by using CO₂ as the starting material.  Furthermore, electrochemical reduction of CO₂may provide a storage medium for the otherwise wasted excess renewable energy from intermittent sources.

Only recently, less than the last decade, have studies involving the conversion of CO₂ into useful chemicals really picked up in intensity.  Prior, Hori has reported seminal work on screening different metal catalysts and the various products that can be formed using those metals.^[7]  In these early studies conversion (<20 mA/cm²) and the selectivity for the desired product is typically low.  For electrochemical conversion of CO₂ to become economically feasible, more active and stable catalysts in as well as better electrodes are necessary such that the CO₂ electrolyzer can be operated at sufficient conversion (current density >250 mA/cm²), reasonable energetic efficiency (>60%), and sufficient selectivity for the desired product (Faradaic efficiency >90%).^[6]

This paper will focus on our recent efforts to understand and improve catalysts and electrodes for conversion of CO₂ into CO (Figure 1 top).  The electrochemical conversion of CO₂ is really the inverse process of a fuel cell.  As such, many lessons learned over the past 100 years with respect to fuel cell electrocatalysis can be applied to the challenge of electroreduction of CO₂. Multiple catalyst systems were studied with various collaborators: (i) Ag nanoparticles (varying size); (ii) various Ag and Au-based supported catalysts (carbon black, TiO₂, and carbon nanotubes); and (iii) metal free catalysts.  These catalysts have been characterized in a3-electrode cell as well as in an electrolyzer.  Routinely current densities of 90-100 mA/cm² are obtained, with some systems producing more than 200 mA/cm², at energy efficiencies of 45 to 60%.  The electrodes in all these cases are prepared using a fully-automated airbrushing method^[4], which allowed loadings to be reduced to 0.75 mg/cm² for Ag and 0.17 mg/cm²for Au.  Furthermore, the metal-free catalysts that exhibit even better performance than the state-of-the-art silver nanoparticles (Figure 1 bottom), which is encouraging as the catalyst cost could be reduced significantly.

Acknowledgements

We gratefully acknowledge our collaborators at Illinois (Masel, Gewirth) and Kyushu University (Nakashima, Fujigaya, Lyth) as well as financial support from UIUC, DOE, and I²CNER, a World Premier Institute at the University of Illinois at Urbana Champaign in the USA and at Kyushu University in Fukuoka, Japan.

References

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[2]  M. I. Hoffert, Science  2010, 329, 1292.

[3]  S. J. Davis, K. Caldeira, H. D. Matthews, Science  2010, 329, 1330.

[4]  H. R. Jhong, F. R. Brushett, P. J. A. Kenis, Advanced Energy Materials  2013, 3, 589.

[5]  D. T. Whipple, P. J. A. Kenis, J Phys Chem Lett  2010, 1, 3451.

[6]  H. R. Jhong, S. Ma, P. J. A. Kenis, Current Opinion in Chemical Engineering  2013, 2, 191.

[7]  Y. Hori, H. Wakebe, T. Tsukamoto, O. Koga, Electrochim Acta  1994, 39, 1833.