Direct Electrochemical Reduction of Ammonium Carbamate on Transition Metal Surfaces: Finding Activity and Stability Descriptors beyond those for CO2 Reduction

Experiments and theory are combined to search for catalyst activity and stability descriptors for the direct reactive capture and conversion (RCC) of CO2 in ammonia capture solutions using Cu, Ag, Au, Sn and Ti electrodes. Two major phenomena emerge in RCC that are not predominant in the electrochemical CO2 reduction (CO2R) reaction, namely, the rapid corrosion and restructuring of the catalyst in the presence of the CO2-ammonia adducts, and the promotion of the competing hydrogen evolution reaction (HER). The prevalence of HER in RCC is correlated to the electrostatic attraction of the protonated amine and repulsion of the captured CO2 to the electrode, using the potential of zero charge (PZC). The stability of catalysts under RCC conditions is a function of the applied potential, and cannot be readily predicted using binding energy descriptors commonly used in the prediction of CO2R activity. Three different trends are experimentally observed under RCC testing: i) Cu, and Sn corrode under open circuit potential and produce predominantly hydrogen, ii) Au and Ag show activity for the reduction of dissolved CO2 and restructure under cathodic potentials, and iii) Ti does not corrode under open circuit conditions and only generated hydrogen as reduction product. This work shows that a direct correlation between calculated binding energies of CO2R intermediates, atomic oxygen, hydrogen, and ammonia, and the activity and stability of transition metal for RCC cannot be found, highlighting the need for further development of activity and stability descriptors beyond those known for CO2R.