A photoelectrochemical approach to splitting carbon dioxide for a manned mission to Mars

A photoelectrochemical system for splitting carbon dioxide to carbon monoxide and oxygen is discussed. The Martian atmosphere consists of 95% carbon dioxide. Splitting carbon dioxide would provide both oxygen to support life and carbon monoxide, which can be used as a substitute for hydrogen fuel. The photoelectrochemical system involves a cathodic compartment where reduction of carbon dioxide to carbon monoxide occurs; and an anodic compartment where ‘oxide’ equivalents from the carbon dioxide–carbonate equilibrium are oxidized to oxygen. The trinuclear nickel clusters [Ni3(μ2-dppm)3(μ3-L)(μ3-I)](PF6) (L=CNR, R=CH3 (1), i–C3H7 (2), C6H11 (3), CH2C6H5 (4), t–C4H9 (5), 2,6–Me2C6H3 (6); L=CO (7); [dppm=bis(diphenylphosphino)methane] are found to catalyze the cathodic process of carbon dioxide reduction to carbon monoxide. These cluster catalysts undergo single electron reduction over a relatively narrow range of E1/2(+/0) (−1.08–−1.18 V vs. SCE in acetonitrile) to form neutral radicals, [Ni3(μ2-dppm)3(μ3-L)(μ3-I)]•. Specular reflectance infrared spectroelectrochemical (SEC) measurements were used to characterize these species and their reactions with CO2. Studies in the absence of CO2 show that the capping isocyanide or carbonyl ligand remains triply bridging (μ3, η1) upon single electron reduction. Electrochemical kinetics studies indicate that the rates of reaction with CO2 depend to first order on (cluster) and (CO2). The rate constants for the rate limiting step in the reduction of CO2 by the clusters, kCO2 (M−1 s−1), are 1.6±0.3 (1), 1.4±0.3 (2), 0.5±0.1 (3), 0.2±0.05 (4), 0.0±0.05 (5), 0.0±0.05 (6), and 0.1±0.1 (7), respectively. Thus, the relative rates of reaction of the alkyl or aryl substituted isocyanide- or carbonyl-capped clusters with CO2 follow the order: CNCH3 (1) CN(i–C3H7) (2)>CNC6H11 (3)>CNCH2C6H5 (4)>CO (7)>CN(t-C4H9) (5) CN(2,6-Me2C6H3) (6). On the basis of these kinetic and spectroscopic studies, a mechanism for the catalytic reduction of CO2 involving CO2 activation on the isocyanide-capped face of the trinuclear nickel clusters is proposed.