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 − + 2AH ↔ CO + H 2 O + 2A − , appeared from the very beginning of these ...
Low-cost, efficient CO 2 -to-CO+O 2 electrochemical splitting is a key step for liquid-fuel production for renewable energy storage and use of CO 2 as a feedstock for chemicals. Heterogeneous catalysts for cathodic CO 2 -to-CO associated with an O 2 -evolving anodic reaction in high-energy-efficiency cells are not yet available. An iron porphyrin immobilized into a conductive Nafion/carbon powder layer is a stable cathode producing CO in pH neutral water with 90% faradaic efficiency. It is coupled with a water oxidation phosphate cobalt oxide anode in a home-made electrolyzer by means of a Nafion membrane. Current densities of approximately 1 mA/cm 2 over 30-h electrolysis are achieved at a 2.5-V cell voltage, splitting CO 2 and H 2 O into CO and O 2 with a 50% energy efficiency. Remarkably, CO 2 reduction outweighs the concurrent water reduction. The setup does not prevent high-efficiency proton transport through the Nafion membrane separator: The ohmic drop loss is only 0.1 V and the pH remains stable. These results demonstrate the possibility to set up an efficient, low-voltage, electrochemical cell that converts CO 2 into CO and O 2 by associating a cathodic-supported molecular catalyst based on an abundant transition metal with a cheap, easyto-prepare anodic catalyst oxidizing water into O 2 .CO 2 -to-CO conversion | carbon dioxide electrolyzer | electrochemistry | molecular catalysis | solar fuels T he production of carbon-based fuels or chemicals using the most abundant carbon source (CO 2 ) requires designing efficient, cheap, selective, and sustainable processes able to convert CO 2 into useful products (1-7). Carbon monoxide production is an important step to fuels because it can be used as a feedstock in the Fischer-Tropsch process. Compared with water splitting, electrochemical reduction of CO 2 into CO is a greater challenge. This is particularly true when aiming to carry out this reaction selectively in friendly conditions, namely at neutral pHs, ambient temperature, and with abundant and cheap materials as catalysts as opposed to solid-state high-temperature electrolyzers (8).We recently discovered that substitution of the four paraphenyl hydrogens of iron tetraphenylporphyrin by trimethylammonio groups provides a water-soluble iron porphyrin (WSCAT) able to catalyze selectively the electrochemical conversion of CO 2 into CO in neutral water in homogeneous conditions (9). The next challenge was to efficiently immobilize this molecular catalyst onto the cathode and to set up an integrated electrochemical cell able to split CO 2 and H 2 O into CO and O 2 according to[potentials referred to the standard hydrogen electrode (SHE)]. Immobilization of the catalyst was achieved by preparation of a suspension containing Nafion, WSCAT, and carbon powder (Materials and Methods) (10). This solution was then sprayed onto a carbon support (glassy carbon electrode for cyclic voltammetry experiments and carbon felt or carbon Toray for electrolysis) and air-dried. Interactions between the positively charged cataly...
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