Developing sustainable energy strategies based on CO 2 reduction is an increasingly important issue given the world's continued reliance on hydrocarbon fuels and the rise in CO 2 concentrations in the atmosphere. An important option is electrochemical or photoelectrochemical CO 2 reduction to carbon fuels. We describe here an electrodeposition strategy for preparing highly dispersed, ultrafine metal nanoparticle catalysts on an electroactive polymeric film including nanoalloys of Cu and Pd. Compared with nanoCu catalysts, which are state-of-the-art catalysts for CO 2 reduction to hydrocarbons, the bimetallic CuPd nanoalloy catalyst exhibits a greater than twofold enhancement in Faradaic efficiency for CO 2 reduction to methane. The origin of the enhancement is suggested to arise from a synergistic reactivity interplay between Pd-H sites and Cu-CO sites during electrochemical CO 2 reduction. The polymer substrate also appears to provide a basis for the local concentration of CO 2 resulting in the enhancement of catalytic current densities by threefold. The procedure for preparation of the nanoalloy catalyst is straightforward and appears to be generally applicable to the preparation of catalytic electrodes for incorporation into electrolysis devices. solar energy | carbon dioxide reduction | CuPd nanoalloy | electropolymerized film | hydrocarbon D eveloping sustainable energy resources and strategies to combat the hazards associated with the use of fossil fuels, which include global warming due to the increased concentration of greenhouse gases, is an important theme in the current energy and environmental research agenda (1-3). Electrochemical and photoelectrochemical CO 2 reduction to energy-dense hydrocarbon fuels could play a major role and become part of an integrated energy storage strategy, in combination with solar-or wind-generated electricity, as a way to store energy in the chemical bonds of carbon-based fuels (4-12). Metal-based catalysts for CO 2 reduction have been extensively studied over the last three decades. Metallic copper has proven to be the best available catalytic material for CO 2 reduction to hydrocarbons by electrochemical methods (13)(14)(15)(16)(17)(18)(19). Cu foils and single crystals have been extensively investigated but suffer from low surface areas, low catalytic current densities, and rapid deactivation during electrochemical CO 2 reduction (20). Recently, nanoparticle Cu catalysts (nanoCu) have been investigated as a way to increase catalytic current densities and stabilities. However, Faradaic efficiencies for electrocatalytic reduction to hydrocarbons decrease dramatically with particle size falling to ∼10% for particles less than 20 nm in diameter. The size effect has been attributed to an enhanced chemisorption strength for CO at small Cu nanoparticles compared with large particles or bulk Cu electrodes (13). An additional complication arises from the use of surfactants in the syntheses of nanoCu because they can lead to significant contamination issues requiring their rem...

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