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

Abstract

Abstract 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 [Ni 3 (μ 2 -dppm) 3 (μ 3 - L )(μ 3 - I )](PF 6 ) ( L =CNR, R=CH 3 (1), i –C 3 H 7 (2), C 6 H 11 (3), CH 2 C 6 H 5 (4), t –C 4 H 9 (5), 2,6–Me 2 C 6 H 3 (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 E 1/2 (+/0) (−1.08–−1.18 V vs. SCE in acetonitrile) to form neutral radicals, [Ni 3 (μ 2 -dppm) 3 (μ 3 - L )(μ 3 - I )] • . Specular reflectance infrared spectroelectrochemical (SEC) measurements were used to characterize these species and their reactions with CO 2 . Studies in the absence of CO 2 show that the capping isocyanide or carbonyl ligand remains triply bridging (μ 3 , η 1 ) upon single electron reduction. Electrochemical kinetics studies indicate that the rates of reaction with CO 2 depend to first order on (cluster) and (CO 2 ). The rate constants for the rate limiting step in the reduction of CO 2 by the clusters, k CO2 (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 CO 2 follow the order: CNCH 3 (1) CN( i –C 3 H 7 ) (2)>CNC 6 H 11 (3)>CNCH 2 C 6 H 5 (4)>CO (7)>CN( t -C 4 H 9 ) (5) CN(2,6-Me 2 C 6 H 3 ) (6). On the basis of these kinetic and spectroscopic studies, a mechanism for the catalytic reduction of CO 2 involving CO 2 activation on the isocyanide-capped face of the trinuclear nickel clusters is proposed.