Conversion of carbon dioxide (CO2) to carbon monoxide (CO) and other value-added carbon products is an important challenge for clean energy research. Here we report modular optimization of covalent organic frameworks (COFs), in which the building units are cobalt porphyrin catalysts linked by organic struts through imine bonds, to prepare a catalytic material for aqueous electrochemical reduction of CO2 to CO. The catalysts exhibit high Faradaic efficiency (90%) and turnover numbers (up to 290,000, with initial turnover frequency of 9400 hour(-1)) at pH 7 with an overpotential of -0.55 volts, equivalent to a 26-fold improvement in activity compared with the molecular cobalt complex, with no degradation over 24 hours. X-ray absorption data reveal the influence of the COF environment on the electronic structure of the catalytic cobalt centers.

The transformation of CO 2 into chemical feedstocks or fuels is an attractive goal, but catalysts capable of generating useful, multicarbon products have been challenging to design. Here, thin films of the intermetallic Ni 3 Al on glassy carbon are found to be electrocatalytic for aqueous CO 2 reduction. At−1.38 V vs Ag/AgCl, Ni 3 Al films produce a range of C1, C2, and C3 oxygenated organic species including 1-propanol and methanol at Faradaic efficiencies that are competitive with single-metal electrodes reported in the literature. To the best of our knowledge, Ni 3 Al on glassy carbon is the only noncopper-containing material shown to generate C3 products.

Electrochemical transformation of CO2 into commodity chemicals such as oxalate is a strategy for profitably remediating high atmospheric CO2 levels. Electrocatalysts for oxalate generation, however, have required prohibitively large applied potentials, forcing the use of nonaqueous electrolytes. Here, a thin film comprised of alloyed Cr and Ga oxides on glassy carbon is shown to electrocatalytically generate oxalate from aqueous CO2 with high Faradaic efficiencies at 690 mV overpotential. Oxalate is produced at a surface anion site via a CO-dependent pathway; the process is highly sensitive to the hydrogen-bonding environment and avoids the commonly invoked CO2 •– intermediate. Ultimately, this catalytic system accomplishes efficient CO2 to oxalate conversion in protic electrolyte.

Cofactor regeneration in enzymatic reductions is crucial for the application of enzymes to both biological and energy-related catalysis. Specifically, regenerating NADH from NAD + is of great interest, and using electrochemistry to achieve this end is considered a promising option. Here, we report the first example of photoelectrochemical NADH regeneration at the illuminated (l > 600 nm), metal-modified, p-type semiconductor electrode Pt/p-GaAs. Although bare p-GaAs electrodes produce only enzymatically inactive NAD 2 , NADH was produced at the illuminated Pt-modified p-GaAs surface. At low overpotential (À0.75 V vs. Ag/AgCl), Pt/p-GaAs exhibited a seven-fold greater faradaic efficiency for the formation of NADH than Pt alone, with reduced competition from the hydrogen evolution reaction. Improved faradaic efficiency and low overpotential suggest the possible utility of Pt/p-GaAs in energy-related NADHdependent enzymatic processes. Scheme 1. Structure of NAD + (nicotinamide) and two-electron redox conversion between NAD + and NADH (1,4-dihydronicotinamide), which occurs in enzymatic processes (ADPR = adenosine diphosphoribose).

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