Efficient electrolyzer for CO ₂ splitting in neutral water using earth-abundant materials

Abstract

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...

Iron Porphyrin Allows Fast and Selective Electrocatalytic Conversion of CO2 to CO in a Flow Cell

“…In this context, Fe is an ideal candidate being the most abundant transition metal. As previously shown, tetraphenyl Fe porphyrin substituted with trimethyl ammonium groups at the para position of each of the four phenyl groups ( FeP , Scheme (left), is able to produce CO with a selectivity of 90 % at neutral pH and at an overpotential of 450 mV. However, maximum current density obtained in an H‐cell device was in the range of 1 mA cm −2 .…”

Investigation of Electrocatalysts for Selective Reduction of CO2 to CO: Monitoring the Reaction Products by on line Mass Spectrometry and Gas Chromatography

“…Blank measurements using Ar-saturated electrolyte with and without polarization, and CO 2 -saturated electrolyte, without polarization were also recorded. The experimental data was collected and translated to faradaic efficiencies considering standard conditions and calculated by equation 6: (6) where e output is the number of mols of electrons required for reducing CO 2 to CO (recorded amount of product in mol × number of electrons required to obtain one molecule of the product); e input is the total number of mols of electrons Vol. 28, No.…”

“…In this context, the electroreduction of CO 2 to fuels with high-energy density or to industrial chemicals, that can be further processed to produce useful fuels, such as CO, using photovoltaic panels, with the consecutive utilization as fuel in fuel cells, would define a sustainable or regenerative cycle. [1][2][3][4][5][6][7] In the case of performing the CO 2 electroreduction to CO in parallel with the water electroreduction (or the hydrogen evolution reaction (HER)), the mixture CO + H 2 (syngas) is produced. 8,9 In the chemical industry, CO/H 2 mixtures are reacted to form methanol or other liquid fuels, such as diesel, by using the Fischer-Tropsch process.…”

A Hybrid Co Quaterpyridine Complex/Carbon Nanotube Catalytic Material for CO2 Reduction in Water

“…[4,5] These catalysts are mainly active in aprotic solvent with reaction products being mostly CO and more rarely HCOOH. [9,10] In that case,C O production was achieved with over 90 %selectivity at neutral pH and am oderate 450 mV overpotential. Ni cyclam was first reported as as elective and efficient catalyst of the CO 2 -to-CO conversion in water at am ercury surface electrode,with favorable specific interactions of the catalytic adsorbed species with the surface, [6,7] while the use of acarbon electrode leads to much less efficiencyi nt erms of rate (maximal turnover frequency of 90 s À1 )a nd turnover (4 cycles).…”