Solar-powered synthesis of hydrocarbons from carbon dioxide and water

Abstract

The synthesis of hydrocarbons via electroreduction of CO 2 is an attractive approach to store energy generated from intermittent renewable sources of electricity (e.g., solar) through formation of the high-energy C-C and C-H bonds of reduced carbon compounds (1, 2). Establishment of such processes also represents a critical step toward the sustainable production of carbonbased commodity chemicals and energy-rich liquid fuels from nonpetroleum resources (3). Despite the promise that such strategies hold, facilitating the rapid, selective, and efficient electrosynthesis of multicarbon products from CO 2 is an inherently difficult proposition. Part of the challenge stems from the fact that CO 2 reduction half-reactions that generate value-added C 1 and multicarbon products take place within a narrow potential window that is less than 0.5 V wide ( Fig. 1).As a result of the reaction landscape illustrated in Fig. 1, it is virtually impossible to target a given CO 2 reduction product based purely on thermodynamic considerations. For instance, electrochemical reduction of CO 2 to ethylene occurs at 0.08 V vs. reversible hydrogen electrode (RHE). Accordingly, any electrochemical process that targets this product must be run at a potential at which production of other species such as ethane (E°= 0.14 V) and methane (E°= 0.17 V) is also thermodynamically feasible. Moreover, kinetics associated with CO 2 reduction reactions (CO 2 RRs) can often be sluggish. This is particularly true for CO 2 RR processes that generate reduced products with C-C bonds, which require application of modest overpotentials of at least 400 to 500 mV. In practice, electrochemical hydrocarbon evolution ultimately requires cathodic potentials that are more negative than -0.5 V vs. RHE, which ultimately brings all of the CO 2 couples of Fig. 1 into play. Matters are further complicated by the fact that each CO 2 reduction couple requires both eand H + equivalents. As a result, the kinetically facile reduction of protons to hydrogen gas (E°= 0.0 V) represents a competitive cathodic process that must be suppressed for efficient and selective hydrocarbon evolution to be realized.Each CO 2 reduction half-reaction requires multiple eand H + equivalents that are most logically provided by the oxidation of water (E°= 1.23 V). Accordingly, electrochemical cells (ECs) for sustainable hydrocarbon synthesis must juxtapose cathode and anode catalysts that can manage the demanding multielectron proton-coupled electron transfer reactions attendant to CO 2 reduction and H 2 O oxidation, respectively. Moreover, since production of hydrocarbons from Potential versus RHE (V) Fig. 1. Equilibrium potentials for reductive and oxidative half-reactions relevant to the synthesis of hydrocarbons from carbon dioxide, water, and sunlight. Electron and proton equivalents generated via water oxidation at the anode of an EC can be utilized for fuel-forming cathodic processes to generate hydrogen gas or a broad array of reduced carbon products. Promoting the efficient and selective...

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