Electrochemical CO₂ Reduction to Methane with Remarkably High Faradaic Efficiency in the Presence of a Proton Permeable Membrane

The combustion of fossil fuels is a major contributor to rising levels of anthropogenic carbon dioxide (CO 2 ) in the atmosphere. Due to the adverse effects of climate change caused in large part by CO 2 , new technologies must be developed that contribute to a global renewable energy supply. Electrochemical reduction of CO 2 offers a viable pathway to generating value-added products and synthetic fuels to meet our future energy demands while decreasing greenhouse gas emissions. By constructing electrocatalysts capable of modulating proton transfer, we are able to tune product selectivity. Previous studies have demonstrated that two-electron products (CO and HCOOH) can be produced with high selectivity, but multiple-electron transfer products (CH 4 , CH 3 OH, and C2+ products) typically are produced at much lower Faradaic efficiencies. In this work, we construct polymer-modified Cu electrodes that exhibit extraordinarily high CH 4 production (88% Faradaic efficiency). These electrodes afford a new strategy for increasing the selectivity of CO 2 reduction electrocatalysts, an attribute that is key in developing industrially-relevant CO 2 conversion devices. Nafion is a widely used fluoropolymer that is often mixed with electrocatalysts to facilitate proton transport. In contrast to these Nafion-catalyst composites, this work studies electrodes covered by Nafion overlayers for the CO 2 reduction reaction. By varying the thickness, substrates, and voltage, we perform a detailed study of the effect of Nafion overlayers on metal and carbon mesh electrodes for CO 2 reduction. Depending on the thickness of the Nafion membrane, CO 2 reduction occurs at either the polymer–electrolyte interface or electrode–polymer interface. A Nafion overlayer of 15 μm on a Cu electrode enables an extraordinarily high yield of CH 4 production (88% Faradaic efficiency) at a low overpotential (540 mV) via the stabilization of metal-bound CO intermediates. To the best of our knowledge, this yield is the highest for electrocatalytic CO 2 reduction to CH 4 production at room temperature reported.