The electrochemical transformation of CO2 to methanol, a process involving 6-electrons and 6-protons 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 reasonable as a liquid fuel. If one can convert CO2 to larger alcohols or other organics containing carbon-carbon bonds, the energy landscape appears even more appealing.
However, the energy landscape for this type of reactivity is complicated by the large activation barrier associated with the one-electron reduction of CO2. In electrochemical terms, this means that to achieve a reasonable rate of conversion to products requires ~1V of potential beyond that required by thermodynamics. A volt of “overpotential” is an extreme energetic burden. Even processes that might employ sunlight as the energy source in place of commercial electricity cannot overcome this limitation, since the overpotential requires the use of UV light, a portion of the solar spectrum that is not readily available at the earth’s surface. This issues can be overcome by introducing a protonated aromatic amine as an electrocatalyst. For example, addition of pyridinium to a platinum electrode based electrochemical cell reduces the overpotential for methanol to ~200mV. Similar advantage can be obtained with aromatic amine catalysts for other C1 species. Even more intriguing, we find that certain electrode material/aromatic amine combinations promote the formation of carbon-carbon bonds, leading to the formation of true fuel and/or chemical feedstock compounds. To this end, we have developed systems that produce isopropanol in relatively high yield. We conclude that 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.
In collaboration with the startup company, Liquid Light Inc., we have developed and demonstrated two types of light driven electrochemical reactors. In the first case, standard metal electrodes are powered by a commercial photovoltaic system. 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 this presentation we will evaluate the chemical and electrochemical mechanisms that can lead to a practical reaction scheme for the conversion of CO2 to organic products.
Prof. Jake Soper (404-894-4022)