Electrochemical devices comprising electrocatalyst mixtures include at least one Catalytically Active Element and, as a separate constituent, one Helper Catalyst. The electrocatalysts can be used to increase the rate, modify the selectivity or lower the overpotential of chemical reactions. These electrocatalysts are useful for a variety of chemical reactions including, in particular, the electrochemical conversion of CO2. Chemical processes employing these catalysts produce CO, HCO−, H2CO, (HCOO)−, HCOOH, CH3OH, CH4, C2H4, CH3CH2OH, CH3COO−, CH3COOH, C2H6, (COOH)2, or (COO−)2. Devices using the electrocatalysts include, for example, a CO2 sensor and a CO2 electrolyzer.