Bicarbonate Is Not a General Acid in Au-Catalyzed CO₂ Electroreduction

We show that bicarbonate is neither a general acid nor a reaction partner in the rate-limiting step of electrochemical CO₂ reduction catalysis mediated by planar polycrystalline Au surfaces. We formulate microkinetic models and propose diagnostic criteria to distinguish the role of bicarbonate. Comparing these models with the observed zero-order dependence in bicarbonate and simulated interfacial concentration gradients, we conclude that bicarbonate is not a general acid cocatalyst. Instead, it acts as a viable proton donor past the rate-limiting step and a sluggish buffer that maintains the bulk but not local pH in CO₂-saturated aqueous electrolytes.