Electrochemical sensors measure an amount or concentration of CO.sub.2, typically using catalysts that include at least one catalytically active element and one helper catalyst. The catalysts can be used to increase the rate, modify the selectivity or lower the overpotential of chemical reactions. These catalysts are useful for a variety of chemical reactions including electrochemical conversion of CO.sub.2. Chemical processes and devices employing the catalysts are also disclosed, including processes that produce CO, OH.sup.-, HCO.sup.-, H.sub.2CO, (HCO.sub.2).sup.-, H.sub.2CO.sub.2, CH.sub.3OH, CH.sub.4, C.sub.2H.sub.4, CH.sub.3CH.sub.2OH, CH.sub.3COO.sup.-, CH.sub.3COOH, C.sub.2H.sub.6, O.sub.2, H.sub.2, (COOH).sub.2, and (COO.sup.-).sub.2.
STATEMENT OF GOVERNMENT INTEREST
 This invention was made, at least in part, with U.S. government support under U.S. Department of Energy Grant DE-SC0004453. The government has certain rights in the invention.