Substitution of the four paraphenyl hydrogens of iron tetraphenylporphyrin by trimethylammonio groups provides a watersoluble molecule able to catalyze the electrochemical conversion of carbon dioxide into carbon monoxide. The reaction, performed in pH-neutral water, forms quasi-exclusively carbon monoxide with very little production of hydrogen, despite partial equilibration of CO 2 with carbonic acid-a low pK a acid. This selective molecular catalyst is endowed with a good stability and a high turnover frequency. On this basis, prescribed composition of CO-H 2 mixtures can be obtained by adjusting the pH of the solution, optionally adding an electroinactive buffer. The development of these strategies will be greatly facilitated by the fact that one operates in water. The same applies for the association of the cathode compartment with a proton-producing anode by means of a suitable separator. CO 2 -to-CO conversion | contemporary energy challenges | electrochemistry | catalysis | solar fuels O ne of the most important issues of contemporary energy and environmental challenges consists of reducing carbon dioxide into fuels by means of sunlight (1-3). One route toward this ultimate goal is to first convert solar energy into electricity, which will then be used to reduce CO 2 electrochemically. Direct electrochemical injection of an electron into the CO 2 molecule, forming the corresponding anion radical CO 2 .− requires a very high energy [the standard potential of the CO 2 / CO 2 .− couple is indeed −1.97 V vs. normal hydrogen electrode (NHE) in N,N′dimethylformamide (DMF)] (4, 5). Electrochemical conversion of CO 2 to any reaction product thus requires catalytic schemes that preferably avoid this intermediate. Carbon monoxide may be an interesting step en route to the desired fuels because it can be used as feedstock for the synthesis of alkanes through the classic Fischer-Tropsch process. A number of molecular catalysts for the homogeneous electrochemical CO 2 -to-CO conversion have been proposed. They mainly derive from transition metal complexes by electrochemical generation of an appropriately reduced state, which is restored by the catalytic reaction. So far, nonaqueous aprotic solvents (mostly DMF and acetonitrile) have been used for this purpose (5-16). Brönsted acids have been shown to boost catalysis. However, they should not be too strong, at the risk of leading to H 2 formation at the expense of the CO. Trifluoroethanol and water (possibly in large amounts) have typically played the role of a weak acid in the purpose of boosting catalysis while avoiding hydrogen evolution. One of the most thoroughly investigated families of transitionmetal complex catalysts of CO 2 -to-CO conversion is that of iron porphyrins brought electrochemically to the oxidation degree 0. The importance of coupling electron transfer and introduction of CO 2 into the coordination sphere of iron with proton transfers required by the formation of CO, CO 2 + 2e − , appeared from the very beginning of these studies. Sustained formation of CO was indeed only achieved upon addition of weak and Lewis acids (20, The results thus obtained in nonaqueous or partially aqueous media enabled the discovery of remarkably efficient and selective catalysts of the CO 2 -to-CO conversion. They were also the occasion of notable advances in terms of mechanisms and theory of concerted bond-breaking proton-electron transfer (29). It must, however, be recognized that, from the point of view of practical applications, the use of nonaqueous solvents is not the most exciting aspect of these results. One would rather like to use water as the solvent, which would render more viable the CO 2 -to-CO half-cell reaction as well as its association with a water-oxidation anode through a proton-exchange membrane. Significance CO 2 -to-CO electrochemical conversion is a key step in the production of liquid fuels through dihydrogen-reductive FischerTropsch chemistry. Among molecular catalysts, iron porphyrins reduced electrochemically to the Fe(0) state are particularly efficient and led to a deeper understanding of mechanisms involving coupled bond-breaking proton-electron transfer processes. The replacement of nonaqueous solvents by water should make the CO 2 -to-CO half-cell reaction much more attractive for applications, particularly because it would allow association with a water-oxidation anode through a protonexchange membrane. Here it is demonstrated that electrochemical CO production catalyzed by a water-soluble iron porphyrin can occur with high catalytic efficiency. Manipulation of pH and buffering then allows conversions from those involving complete CO selectivity to ones with prescribed CO-H 2 mixtures
