(Invited) Photoelectrochemical Reduction of CO2: Electrocatalytic Formation of Solar Fuels
Authors: Bocarsly Andrew B. , Cole Emily , Hu Yuan , Frick Jessica Jane , Zhang Tao , White James , Pander James , Baruch Maor , Park James 
Published: February 2020

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 employed2,3 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:
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