Phenothiazine redox mediators boost photocatalytic hydrogen evolution

Abstract

The photocatalytic production of green hydrogen constitutes an important step towards sustainable energy storage. An ambitious goal is to couple reductive and oxidative photocatalysis, i.e., the hydrogen-evolution reaction (HER) with water oxidation to achieve overall water splitting. Herein, we explore a pathway towards such coupled processes by using phenothiazines (PTs) as redox mediators (RMs) in HER. In this work, we explored the reductive half reaction utilizing a new hybrid SED-RM architecture with a photocatalytic system consisting of the thiomolybdate cluster (NH4)2[Mo3S13] as catalyst, ascorbic acid as sacrificial electron donor, and [Ru(bpy)₃](PF₆)₂ (bpy = 2,2′bipyridine) as photosensitiser (PS), we show that the catalytic performance of the system is improved by a factor of three (increase in turnover number (TON) from 6,760 to 20,660 and in turnover frequency (TOF) from 1,130 h⁻¹ to 3,440 h⁻¹ (for 6h)) with 10H-phenothiazine (PTH) as RM. Stern-Volmer experiments show that PTH and derivatives effectively quench the excited state of [Ru(bpy)₃](PF₆)₂ -independent of pH -enabling photocatalytic HER to be performed at the lower pH of 2.55, where the catalyst is more active due to a higher availability of protons. A near-linear correlation between the Stern-Volmer-quenching ability of the PT derivatives and the photocatalytic performance suggests the initial reductive quenching step to be to be an important kinetic factor in the catalytic cycle. Our study proposes RMs as a strategy to boost the performance of photocatalytic HER systems by mediating efficient electron transfers even at low pH values and paves the way towards coupled reductive and oxidative photocatalysis.