Electroreduction of CO 2 to CH 4 without overpotential using Pt‐black catalysts: Enhancement of faradaic efficiency

The use of metal electrocatalysts (Cu, Au, and Ag) is the mainstream in primary CO2 reduction systems, but their large overpotential induces a decrease in energy conversion efficiency. Recently, it has been discovered that carbon-supported Pt can make a game change to reduce CO2 to CH4 electrocatalytically without overpotential; however, its faradaic efficiency remains only 12.3% thus far. One approach is to investigate the role of carbon supports for improving the efficiency. In this study, improved CO2 reduction was carried out by employing a Pt-black electrocatalyst without a carbon support in a polymer electrolyte membrane electrolyzer cell. When supplying CO2 diluted by Ar gas at CO2 concentrations of 4-7 vol.%, the efficient CH4 generation was observed. The reason behind the improved production of CH4 occurs by a Langmuir-Hinshelwood mechanism associated with adsorbed CO (COads) and H (Hads) on the Pt electrocatalyst. Moreover, a high-density COads environment (surface) can be created at the Pt-black catalyst, achieving CH4 generation with a faradaic efficiency of 23.2% at 0.18 V vs RHE. Therefore, this study succeeded in improving the faradaic efficiency of CH4 production from CO2 reduction without overpotential using a Pt-black electrocatalyst.