Electrochemical Reduction of CO₂ to Methane on Platinum Catalysts without Overpotentials: Strategies for Improving Conversion Efficiency

Abstract

The electrochemical CO2 reduction to hydrocarbons and alcohols with sustainable energies is a promising technology for reducing atmospheric CO2 and storing electricity as chemical energy. However, the development of catalysts with high activities, high product selectivities, and low overpotentials to drive the reaction remains a major challenge for practical applications. On Pt, CO2 reduction to CO occurs easily, but further reduction to hydrocarbons and alcohols is difficult because of the strong adsorption of CO on the Pt surface. Consequently, Pt is considered an inactive catalyst and has been studied less than other transition metals such as Cu and Au. Herein, we show that adsorbed CO is selectively reduced to methane on carbon-supported Pt catalysts at low CO2 partial pressures and, crucially, at potentials close to the thermodynamic equilibrium potential of the reaction (0.16 V vs reversible hydrogen electrode), i.e., without overpotentials. Although the estimated apparent faradaic efficiency of 6.8% is not sufficiently high for commercial applications, this is the first demonstration of electrochemical methane generation without an overpotential. We suggest that the overpotential deposition (OPD) of hydrogen atoms at the atop sites of the Pt surface facilitates the reduction of adsorbed CO to methane and that the onset of hydrogen OPD coincidentally matches the equilibrium potential of methane generation. Mechanistic considerations also answer some questions raised by the present experiments, such as why low CO2 pressures are favorable for selective methane generation and why the faradaic efficiency is low. Overall, the results provide useful information for rationally designing effective CO2RR catalysts.

Engineering Sn‐based Catalytic Materials for Efficient Electrochemical CO2 Reduction to Formate

“…To reduce the thermodynamically stable CO 2 , catalysts are needed to increase the kinetics of the electrochemical reduction reaction and achieve an appreciable yield. Most of the catalytic materials studied to date can be divided into 1) metallic such as Au, [4–5] Ag, [6–7] Pd, [8–9] Pt, [10] Zn, [11–12] Cu, [13–15] Ni, [16–17] Fe, [18] Sn, [19–20] In, [21–22] Bi, [23] and alloys which include a combination of 2 or more of these metals, [24–33] 2) non‐metallics such as MoS 2 , [34] carbon compounds and its derivatives such as N‐doped carbon and carbon nanofibers [35–37] and 3) molecular catalysts [38–39] …”

H2-CO2 polymer electrolyte fuel cell that generates power while evolving CH4 at the Pt0.8Ru0.2/C cathode

“…Therefore, we demonstrated a H 2 -CO 2 fuel cell that generates electric power while e ciently reducing CO 2 to CH 4 . follows the L-H mechanism involving CO ads and H ads (ref 23 ). According to equation 1, this reaction theoretically proceeds at a CO ads -to-H ads molar ratio of 1:6; however, this reaction proceeded when the ratio was 1:11 or higher for a Pt/C electrocatalyst 24 , with the best ratio reported to be 1:18 (ref 24 ).…”

“…Cell fabrication. A polymer electrolyte fuel cell (PEFC) was fabricated by essentially following the same procedure reported previously [23][24][25] , with the exception that Pt 0.8 Ru 0.2 /C was used as the cathode instead of Pt/C. Brie y, a 6 × 6 cm Na on 117 membrane and 3 × 3 cm pieces of carbon paper pretreated with acetone were used as the proton-exchange membrane and gas diffusion layers, respectively.…”

“…The use of a protonexchange membrane and an ionomer was suggested to facilitate CO 2 reduction; however, the C1 yield was quite low. We recently demonstrated that CH 4 can be produced by the reduction of CO 2 in the absence of an overpotential and with a faradaic e ciency of 6.8% using the MEA 23 . This CH 4 -generation reaction proceeds by a Langmuir-Hinshelwood (L-H) mechanism associated with CO ads and H adsorbed on the metal (H ads ):…”

Reproducible Electrodeposition of Hydrogen Molybdenum Bronze Films and Electrochemical Reduction of Carbon Dioxide

“…Electrochemical reduction experiments resulted in a faradaic efficiency of 93% requiring an over potential of 0.560 V. Performance significantly dropped within the first two hours and retained 63% percent faradaic efficiency after 12 hours [14]. Platinum catalysts have been used to reduce the required overpotential for reducing CO2 to methane but the faradaic efficiency is only 6.8% [15]. There is a need to continue to develop carbon supported catalyst to reduce the required overpotential for reduction of CO2 and maintain reduction over long periods of time.…”