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 to improve the stability of these systems, 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 Co I 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 firstly 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 characterization of the system. This study thus provides tracks to improve the performances of hydrogen-evolving dye-sensitized photocathodes toward their integration into functional DSPECs.

Electrocatalytic reduction of protons to dihydrogen by the cobalt tetraazamacrocyclic complex [Co(N4H)Cl2]+: mechanism and benchmarking of performances

“…While Cat1 is becoming more and more popular as a molecular H 2 -evolving catalyst for the design of aqueous systems, it is increasingly important to advance the understanding of its H 2 evolution catalysis mechanism 13 and performance by providing insight into the catalytic steps involved. This is especially the case when considering structural modication 50 or molecular engineering 51 in order to either enhance catalytic activity or stability 16 or immobilise a catalytic centre for integration in photoelectrodes [7][8][9] or devices. In the societal context, both of these objectives are ultimately necessary to achieve industrial relevancy and technological maturity of hydrogen-producing electrolysers based on molecular catalysts made from earthabundant elements.…”

“…[1][2][3][4][5] Recently, the cobalt complex [Co(N 4 H)Cl 2 ] + (Cat1, Fig. 1) based on the tetraazamacrocyclic 2,12-dimethyl-3,7,11,17-tetraazabicyclo [11.3.1]heptadeca-1 (17),2,11,13,15-pentaene ligand, 6 described by Karn and Busch in 1966, has received increased interest [7][8][9] namely because this catalyst proves active and stable for the evolution of H 2 from fully aqueous solutions. [7][8][9][10][11][12][13][14][15][16][17] A study carried out under homogeneous conditions using chemical reductants or photochemical activation conrmed the superior activity of Cat1 in fully aqueous media, 18 and X-ray absorption spectroscopic monitoring of a homogeneous photocatalytic system for H 2 evolution based on Cat1 indicated an ECEC mechanism (E ¼ monoelectronic electrochemical reduction, C ¼ protonation step) starting from the bisaqua Co(II) complex.…”

Investigating Light-Induced Processes in Covalent Dye-Catalyst Assemblies for Hydrogen Production

“…Cat1 [20] and Cat2 [51] were prepared according to previously reported procedures. Detailed synthetic procedures for T2R, T2R-Cat1, and T2R-Cat2 are reported elsewhere [18]. For T2R-Cat2, the copper-catalyzed azide-alkyne cycloaddition (CuAAC) coupling we previously reported for related dyads based on push-pull dyes [12,13] was employed.…”

“…We recently addressed the photoinduced processes at work with our first photoelectrocatalytically active dyad, thanks to a combination of classical spectroscopic techniques and transient absorption spectroelectrochemistry [17]. In the present study, we investigate the ultrafast excited state dynamics of two novel dye-catalyst assemblies specifically designed for improved photoelectrocatalytic hydrogen production in dye-sensitized photocathodes [18]. They comprise a push-pull organic dye (T2R) covalently assembled with two different H2-evolving cobalt catalysts: either the cobalt diimine-dioxime complex Cat2 [19] or the cobalt tetraazamacrocyclic catalyst Cat1 [20][21][22] (Figure 1).…”

Hydrogen Evolution Mediated by Cobalt Diimine‐Dioxime Complexes: Insights into the Role of the Ligand Acid/Base Functionalities.

“…[1] Cobalt diiminedioxime complexes (Figure 1) with a tetradentate equatorial ligand similarly displayed good performances in terms of overpotential requirements, [2] are even more stable against hydrolysis [2b] and can be easily derivatized, [3] which allowed their recent incorporation into nanostructured cathode materials based on carbon nanotubes [3a,4] or dye-sensitized photocathode architectures. [5] While studies have been dedicated to understanding the tolerance to O 2 [6] or the stability of this series of catalysts during H 2 evolution, [7] little information is available to benchmark their catalytic performances or to understand their catalytic mechanisms. Both complexes [Co((DO) 2 BF 2 )pnBr 2 ] (1, Figure 1) and [Co(DO)(DOH)pnBr 2 ] (2) ((DOH)(DOH)pn = N 2, N 2 'propanediylbis(2,3-butandione 2-imine 3-oxime) were previously reported as stable and efficient molecular catalysts for hydrogen evolution.…”