The 6-electrons, 6-protons transformation of CO₂ to methanol requires a thermodynamic energy input of 690kJ/mole under standard conditions, about 5.5 times more energy than the free energy required to split water. But, for this “price” one obtains a liquid fuel that has an enthalpic combustion content, which is about 2.8 times larger than hydrogen. This combined with methanol’s density (compared to hydrogen) provides a volumetric energy density that makes this compound a potential liquid fuel. However, the energy landscape for this type of reactivity is complicated by the large activation barrier associated with the one-electron reduction of CO₂. At most metallic electrodes a ~1V overpotential is needed to achieve a pragmatically viable rate of conversion to products.¹ This constraint holds true even if the product formation is shifted to formic acid, although this species only involves a two-electron reduction. A volt of “overpotential” is an extreme energetic burden. In certain cases, this issue can be overcome either by the choice of electrode materials employed^2,3 or by introducing a protonated aromatic amine as an electrocatalyst.⁴ For example, addition of pyridinium to a palladium electrode based electrochemical cell reduces the overpotential for methanol to ~200mV.⁵ Similar advantage can be obtained with aromatic amine catalysts for other C₁ species. This electrocatalytic approach can provide commercially useful products, while removing CO₂ from the environment, as long as a nonfossil based fuel is utilized as the energy source.

To this end, we have developed two types of light driven electrochemical reactors. In the first case, heavy post-transition metal electrodes such as indium, tin, bismuth or lead that are intrinsically catalytic for CO₂ to formate are powered by a commercial photovoltaic system.⁶ This has the advantage that the PV system and the electrochemical reactor can be separately optimized, but the approach increases the necessary system engineering. Our second solar fuels approach is the use of a photoelectrochemical cell. In this system, one or both of the electrodes in the electrochemical reactor is composed of a semiconductor that absorbs visible light, generating a voltage in-situ, and a simplified balance of plant. In these systems the introduction of a dissolved aromatic amine catalyst is of utility. To date, we have found p-type semiconductor such as GaP ⁷ and certain metal oxides semiconductors having a delafossite structure such as CuFeO₂,⁸ to be active for the chemistry of interest.

References:

(1)      White, J. L.; Baruch, M. F.; III, J. E. P.; Hu, Y.; Fortmeyer, I. C.; Park, J. E.; Zhang, T.; Liao, K.; Gu, J.; Yan, Y.; Shaw, T. W.; Abelev, E.; Bocarsly, A. B.: Light-Driven Heterogeneous Reduction of Carbon Dioxide: Photocatalysts and Photoelectrodes. Chem. Rev. 2015, DOI: 10.1021/acs.chemrev.5b00370.

(2)      Detweiler, Z. M.; White, J. L.; Bernasek, S. L.; Bocarsly, A. B.: Anodized indium metal electrodes for enhanced carbon dioxide reduction in aqueous electrolyte. Langmuir 2014, 30, 7593-600.

(3)      Baruch, M. F.; Pander, J. E.; White, J. L.; Bocarsly, A. B.: Mechanistic Insights into the Reduction of CO₂ on Tin Electrodes using in Situ ATR-IR Spectroscopy. ACS Catalysis 2015, 5, 3148-3156.

(4)      Cole, E. E. B.; Baruch, M. F.; L'Esperance, R. P.; Kelly, M. T.; Lakkaraju, P. S.; Zeitler, E. L.; Bocarsly, A. B.: Substituent Effects in the Pyridinium Catalyzed Reduction of CO₂ to Methanol: Further Mechanistic Insights. Top Catal 2015, 58, 15-22.

(5)      Seshadri, G.; Lin, C.; Bocarsly, A. B.: A New Homogeneous Electrocatalyst for the Reduction of Carbon-Dioxide to Methanol at Low Overpotential. J Electroanal Chem 1994, 372, 145-150.

(6)      White, J. L.; Herb, J. T.; Kaczur, J. J.; Majsztrik, P. W.; Bocarsly, A. B.: Photons to formate: Efficient electrochemical solar energy conversion via reduction of carbon dioxide. Journal of CO₂ Utilization 2014, 7, 1-5.

(7)      Yan, Y.; Gu, J.; Zeitler, E. L.; Bocarsly, A. B.: Photoelectrocatalytic Reduction of Carbon Dioxide. In Carbon Dioxide Utilisation: Closing the Carbon Cycle; Strying, P., Quadreilli, E. A., Armstrong, K., Eds.; Elsevier: London, 2015; pp 211-233.

(8)      Gu, J.; Wuttig, A.; Krizan, J. W.; Hu, Y.; Detweiler, Z. M.; Cava, R. J.; Bocarsly, A. B.: Mg-Doped CuFeO₂ Photocathodes for Photoelectrochemical Reduction of Carbon Dioxide. The Journal of Physical Chemistry C 2013, 117, 12415-12422.