Porphyrin-catalyzed electrochemical hydrogen evolution reaction. Metal-centered and ligand-centered mechanisms

In the past few decades, several transition metal complexes have emerged as molecular catalysts for the electrochemically triggered hydrogen evolution reaction (HER). The catalytic activity of the metal complexes relies on their ability to participate in different proton-coupled electron transfer (PCET) processes to produce intermediates that can donate hydrides to the free protons in the media and release hydrogen. Most frequently, PCET processes occur at the central metal ion and yield a metal hydride as the key intermediate from which hydrogen is released. When the ligand of the metal complex is a redox-active species, such as a porphyrin macrocycle, it can also participate in the PCET steps of the catalytic cycle. In this case, porphyrin reduced species, such as phlorins or isobacteriochlorins, are suggested as key intermediates that provide the hydride to the free protons to produce hydrogen. When both the metal and ligand are redox-active entities, a large diversity of HER mechanisms is possible. This review aims to provide a molecular-level understanding of the different mechanistic pathways that have been proposed for the porphyrin-catalyzed HER. The influence of the reaction media and structure of the catalysts on the reaction mechanisms is systematically analyzed. We hope that this review becomes a useful guide for the optimization of the HER catalytic process and the rational design of high-activity porphyrin-based catalysts.