Proton donor and outer coordination sphere control of CO2 reduction selectivity by a cobalt electrocatalyst in water

We deployed two molecular cobalt complexes containing two 2,2'-bipyridine groups linked by -NH (complex 1) or -NCH3 (complex 2) groups for CO2 reduction in water.Electrocatalytic CO2 reduction to CO along with hydrogen (H2) formation was accomplished by 1 & 2 in the presence of weak acids with various pKa values.The applied potential and the pKa of the acids present play key roles in determining the activity and selectivity of both complexes, with a less cathodic potential and higher-pKa acid favoring CO over H2 production.We also observe an effect of catalyst structure on activity and selectivity: Under a CO2 atmosphere at -1.4 V vs. Ag/AgCl/KCl(1M), 1 shows higher activity and selectivity than 2 for CO generation under optimized conditions (pH 6.0, in the presence of 3-(cyclohexylamino)-1propanesulfonic acid, CAPS; pKa 10.4): 1TON,CO = 19900, 2TON,CO = 7920, 1FE,CO: H2 = 95:5, 2FE,CO: H2 = 80:20.We propose that the -NH group on complex 1 stabilizes a state with water and/or buffer molecules interacting with cobalt-coordinated CO2, assisting CO2 activation.At less cathodic potentials, activity and selectivity for CO formation is high under all conditions examined, up to a TON of 59,000 and 95% FE for 1.These results reveal that 1 and 2 exhibit outstanding activity and selectivity for CO2 reduction in water, with the impact of proton donors on selectivity providing insight into the mechanism.