Cobalt-based Metalloporphyrins As Efficient Electro-catalysts for Hydrogen Evolution From Acetic Acid and Water

Four molecular electrocatalysts based on cobalt complexes, CoT(X)PP (X = H (1), OH (2), CN (3), COOH (4)), were prepared from meso-tetra-p-X-phenylporphin (H₂T(X)PP, X = H, OH, CN, COOH) by reaction with cobalt acetate to be used for electrolytic proton or water reduction. The electrochemical properties and the corresponding catalytic activities of these four catalysts were investigated by cyclic voltammetry. Controlled potential electrolysis with gas chromatography analysis confirmed that the turn-over frequencies (TOF) per mol of catalyst per hour were 42.4, 38.6, 55.5, and 70.1 mol H₂ at an overpotential of 941.6 mV (in DMF) in the acetic acid solution containing catalyst. In neutral buffered aqueous solution (pH 7.0), these four molecular catalysts had TOF per mol of catalyst per hour of 352.53, 313.7, 473.4, and 714.6 mol H₂, respectively, with an overpotential of 837.6 mV, indicating that complex 4 had better activity than complexes 1, 2, and 3. The Faraday efficiencies of complexes 1-4 were 99.1, 99.6, 100.4, and 99.0% at 72 h of consecutive reduction on a glassy carbon electrode, respectively. These results indicate that the electronic properties of the ligands play a crucial role in determining the catalytic activity of the cobalt complex and are consistent with the phenomenon that the catalytic activity of the benzene porphyrins is significantly increased in the presence of electron-withdrawing groups, and the CoT(COOH)PP is the most active catalyst.

Graphical Abstract