In this work, we use an atmospheric-pressure plasma in argon as a cathode to electrochemically reduce carbon dioxide in aqueous solution. Using optical absorption spectroscopy, we directly show that solvated electrons reduce CO 2(aq) to form the carboxyl radical anion CO 2 − (aq) , and the reaction obeys 3D bulk reaction kinetics similar to those measured in radiolysis experiments. We then use liquid ion chromatography to show that the CO 2 − (aq) intermediate ultimately reacts to produce oxalate and formate. The overall faradaic efficiency of the reaction is close to 10% for a CO 2(aq) concentration of 34 mM. However, given the known reaction kinetics of solvated electrons, this efficiency should approach 100% as the concentration of CO 2(aq) is increased. At a rate of over 30 billion tons per year, carbon dioxide (CO 2 ) gas represents one of the largest sources of human waste, with most of it coming from energy and cement production.1 Natural carbon sinks do not have the capacity to absorb such a massive amount of CO 2 , so much of it ends up in the atmosphere, where it acts as a greenhouse gas. While one strategy is to capture and store (or sequester) waste CO 2 2 , a perhaps more compelling alternative is to capture and transform waste CO 2 into other chemicals of economic value. Scientists have approached the challenge of CO 2 reformation with a variety of tools and techniques including catalytic reduction, 3 photoelectrochemical reduction, 4,5 and electrochemical reduction 6 to produce chemicals including carbon monoxide (CO), formic acid (HCOOH), oxalic acid (C 2 H 2 O 4 ), formaldehyde (CH 2 O), and methanol (CH 3 OH) among others. In catalysis, large molecules or rare earth metals have sites with low electron affinity 7 onto which the CO 2 adsorbs and is reduced. In photoelectrochemical reduction, a photon (usually in UV range) promotes an electron in the cathode to an excited state with enough kinetic energy to reduce the CO 2(ads) . In electrochemical reduction, an applied voltage drives electrons from the metal to the CO 2(ads) , thus reducing it. These three approaches are not mutually exclusive and usually overlap with one another, yielding hybrid methods such as electrocatalytic 8,9 and photoelectrocatalytic 10 reduction processes. All of these approaches require the adsorption of CO 2 onto some substrate, where it can be reduced to form reactive intermediates that ultimately yield the final products. Hence, the adsorption processes inherently limits the overall rate of reduction. To ameliorate this, significant attempts have been made to increase the surface-to-volume ratio of the cathode substrate. For example, metallic nanoparticles have been used as substrates for electrocatalytic 11 and photocatalytic 12 reduction. Recent work has also attempted to incorporate catalytic molecules into metal-organic frameworks, which have an extremely large surface-to-volume ratio.
13Radiolysis is also capable of reducing CO 2(aq) in the bulk and avoids the adsorption step all together. This approach uses en...
