Hydrogen-Bond Regulation of the Microenvironment of Ni(II)-Porphyrin Bifunctional Electrocatalysts for Efficient Overall Water Splitting.

Abstract

Accurately regulating the microenvironment around active sites is an important approach for boosting the overall water splitting performance of bifunctional electrocatalysts,whichcan drive both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) in the same electrolyte. Herein, pseudo-pyridine-substituted Ni(II)-porphyrins (o-NiTPyP, m-NiTPyP, and p-NiTPyP) with pseudo-pyridine N-atoms located at the ortho-, meta-, or para-position are prepared and used as model catalysts for alkaline water splitting. Experimental and theoretical results reveal that the pseudo-pyridine N-atom positions can regulate the microenvironment around the active sites and the adsorption free energy of H-donating substances by affecting the H-bonding interaction and the NNiN bond angles of active sites, and thus those pseudo-pyridine-substituted Ni(II)-porphyrins deliver better electrocatalytic activity than the Ni(II)-tetraphenylporphyrin (NiTPP) without pseudo-pyridine N-atoms. Among them, m-NiTPyP on carbon nanotubes delivers the lowest overpotentials of 267 and 138mV at 10mA cm-2 for the OER and HER, respectively. Specifically, m-NiTPyP as bifunctional electrocatalyst in an alkaline electrolyzer requires only 1.62V to drive efficient overall water splitting at 10mA cm-2 while remaining durable. This work proposes a new H-bond-regulating approach of the microenvironment of electrocatalysts for effectively boosting the overall water splitting activity and deeply understanding its related mechanism.