Thermochemical adsorption at high overpotentials limits molecular CO-to-methanol electroreduction

Electrocatalysis modulates interfacial charge-transfer kinetics through potential control. However, it remains unclear how thermochemical steps, such as adsorption, dictate these overall kinetics. Here we determine CO-reduction limiting currents by measuring CO-oxidation limiting currents, allowing mass-transport effects to be deconvoluted from the CO-reduction kinetics. Using CO-to-methanol conversion on molecular catalysts as a model system, we show that while the reaction rate is limited by an electrochemical step at low overpotentials, the thermochemical CO adsorption step becomes rate limiting at high overpotentials. Cation-dependent variations in CO adsorption energies and activation energies align with the Brønsted-Evans-Polanyi principle, establishing CO adsorption energy as a robust kinetic descriptor. We further demonstrate that cobalt phthalocyanine supported on carbon nanotubes enables appreciable CO-to-methanol conversion due to its enhanced CO adsorption compared to other metal phthalocyanines. Ultimately, these findings illuminate an inherent thermochemical constraint on electrolysis and provide a kinetic framework to identify when such limitations become rate determining.