Conjugated porphyrin polymer films with nickel single sites for the electrocatalytic oxygen evolution reaction

Directly fused nickel(ii) porphyrins are successfully investigated as heterogeneous single-site catalysts for the oxygen evolution reaction (OER). Conjugated polymer thin films from Ni(ii) 5,15-(di-4-methoxycarbonylphenyl)porphyrin (pNiDCOOMePP) and Ni(ii) 5,15-diphenylporphyrin (pNiDPP) showed an OER onset overpotential of 270 mV, and current densities of 1.6 mA cm(-2) and 1.2 mA cm(-2) at 1.6 V vs. RHE, respectively, representing almost a hundred times higher activity than those of monomeric thin films. The fused porphyrin thin films are more kinetically and thermodynamically active than their non-polymerized counterparts mainly due to the formation of conjugated structures enabling a dinuclear radical oxo-coupling (ROC) mechanism at low overpotential. More importantly, we have deciphered the role of the porphyrin substituent in the conformation and performance of porphyrin conjugated polymers as (1) to control the extension of the conjugated system during the oCVD reaction, allowing the retention of the valence band deep enough to provide a high thermodynamic water oxidation potential, (2) to provide a flexible molecular geometry to facilitate O-2 formation from the interaction between the Ni-O sites and to weaken the pi-bond of the *Ni-O sites for enhanced radical character, and (3) to optimize the water interaction with the central metal cation of the porphyrin for superior electrocatalytic properties. These findings open the scope for molecular engineering and further integration of directly fused porphyrin-based conjugated polymers as efficient heterogeneous catalysts.

Automated summary

Directly fused nickel(ii) porphyrins have been investigated as efficient heterogeneous catalysts for the oxygen evolution reaction (OER). The polymer thin films of these porphyrins show significantly higher activity than monomeric thin films due to the formation of conjugated structures enabling a dinuclear radical oxo-coupling mechanism at low overpotential. The role of the porphyrin substituent in the conformation and performance of porphyrin conjugated polymers has been deciphered, providing insights for molecular engineering and further integration of these materials as efficient catalysts.