Spectroscopic Investigations Provide a Rationale for the Hydrogen-Evolving Activity of Dye-Sensitized Photocathodes Based on a Cobalt Tetraazamacrocyclic Catalyst

Abstract

Dye-sensitized photoelectrochemical cells (DSPECs) are a promising approach to produce solar fuels, e.g., by reduction of protons to molecular hydrogen. Here, we present functional NiO photocathodes sensitized with covalent organic dye-catalyst assemblies integrating a robust cobalt tetraazamacrocyclic complex. This catalyst proved to be decisive in improving the stability of these systems, with hydrogen being produced with a 26-fold increase in turnover numbers compared to similar photocathodes based on a cobaloxime catalyst, all other conditions being strictly identical otherwise. Transient absorption spectroelectrochemical (TA-SEC) measurements observed the catalytically competent CoI state in a functional dye-sensitized photocathode, with a lifetime of up to >1 ms, comparable to the timescale of catalysis. They also unveiled the lack of efficiency of the thermally activated electron transfer from the reduced dye to the catalyst, which first limits the photocurrent density for hydrogen production. A second consequence is the accumulation of photogenerated charges on the acceptor side of the dye, ultimately leading to its degradation, as observed in operando and post operando characterizations of the system. This study thus provides tracks to improve the performances of hydrogen-evolving dye-sensitized photocathodes toward their integration into functional DSPECs.