Polarization-Driven Hydride Transfer Promotes Cobalt Tetrasulfophthalocyanine-Catalyzed Hydrogen Evolution by Five Orders of Magnitude

The hydricity of M–H intermediates is a key descriptor and optimization target for the rational design of molecular hydrogen evolution electrocatalysts. Adsorption to an electrode surface is known to augment catalytic rate, but it remains unclear how adsorption affects M-H hydricity. Herein, we quantitatively compare the catalytic activity and mechanism of the hydrogen evolution reaction (HER) from cobalt tetrasulfophthalocyanine (CoTSPc) in a dissolved aqueous form (CoTSPc (aq) ) and when adsorbed to the surface of a carbon electrode (CoTSPc/C). At the same electrode potential, CoTSPc/C catalyzes HER at a rate that is over five orders of magnitude faster than CoTSPc (aq) . Mechanistic studies indicate that both CoTSPc/C and CoTSPc (aq) catalyze HER via a rate-limiting hydride transfer to a proton donor in solution to form H 2 . Hydride transfer from Co-H in CoTSPc (aq) is a purely chemical step defined by a constant hydricity of 22.4 kcal mol −1 that is independent of applied potential. In contrast, hydride transfer from Co-H in CoTSPc/C is strongly polarization dependent, decreasing hydricity by ~23 kcal mol −1 V −1 to values as low as 1.2 kcal mol −1 at −1.1 V vs SHE, leading to the observed rate promotion. We attribute this mechanistic change to the positioning of the CoTSPc-H intermediate within the electrochemical double layer, such that the hydride traverses the interfacial electrostatic potential difference during the RLS. These results demonstrate the power of surface adsorption to generate highly reactive molecular hydrides and fundamentally reshape the free energy landscape of electrocatalysis.