Crossing the bridge from molecular catalysis to a heterogenous electrode in electrocatalytic water oxidation

Abstract

Significant progress has been made in designing single-site molecular Ru(II)-polypyridyl-aqua catalysts for homogenous catalytic water oxidation. Surface binding and transfer of the catalytic reactivity onto conductive substrates provides a basis for heterogeneous applications in electrolytic cells and dye-sensitized photoelectrosynthesis cells (DSPECs). Earlier efforts have focused on phosphonic acid (-PO3H2) or carboxylic acid (-CO2H) bindings on oxide surfaces. However, issues remain with limited surface stabilities, especially in aqueous solutions at higher pH under conditions that favor water oxidation by reducing the thermodynamic barrier and accelerating the catalytic rate using atom-proton transfer (APT) pathways. Here, we address the problem by combining silane surface functionalization and surface reductive electropolymerization on mesoporous, nanofilms of indium tin oxide (ITO) on fluorine-doped tin oxide (FTO) substrates (FTO|nanoITO). FTO|nanoITO electrodes were functionalized with vinyltrimethoxysilane (VTMS) to introduce vinyl groups on the electrode surfaces by silane attachment, followed by surface electropolymerization of the vinyl-derivatized complex, [RuII(Mebimpy)(dvbpy)(OH2)]2+ (12+; Mebimpy: 2,6-bis(1-methyl-1H-benzo[d]imidazol-2-yl)pyridine; dvbpy: 5,5′-divinyl-2,2′-bipyridine), in a mechanism dominated by a grafting-through method. The surface coverage of catalyst 12+ was controlled by the number of electropolymerization cycles. The combined silane attachment/cross-linked polymer network stabilized 12+ on the electrode surface under a variety of conditions especially at pH > ∼6. Surface-grafted poly12+ was stable toward redox cycling at pH ∼ 7.5 over an ∼4-h period. Sustained heterogeneous electrocatalytic water oxidation by the electrode gave steady-state currents for at least ∼6 h with a Faradaic efficiency of ∼68% for O2 production.

Molecular and heterogeneous water oxidation catalysts: recent progress and joint perspectives

“…376 We want to emphasize here that the stability and the charge transfer capability are two equally important considerations for the anchoring groups. 377 Future development and evaluation with both principles in mind will significantly facilitate the utilization of molecular WOCs on heterogeneous surfaces. 15 To facilitate the design and engineering of efficient hybrid devices, one important question has to be answered: what are the individual functions of the heterogeneous substrate and of the immobilized molecular species?…”

Supramolecular photocatalysts fixed on the inside of the polypyrrole layer in dye sensitized molecular photocathodes: application to photocatalytic CO2 reduction coupled with water oxidation

“…Methods of the second type rely on the introduction of a polymer layer, e.g., Meyers' 19 and our 12,27 groups employed the reductive electropolymerization of vinyl groups in the diimine ligands of metal complexes to afford a polymer layer that contained both the photosensitizer and catalyst units on the semiconductor electrode and, compared to the photoelectrode with only the anchoring groups described above, more strongly absorbed visible light owing to the accumulation of photosensitizer units. 19,[27][28][29][30][31][32] The reductive electropolymerization of molecular complexes on the dye-sensitized molecular photocathode also drastically suppressed the desorption of metal complexes from the electrode and enhanced the durability of photocatalysts for CO 2 reduction. 19,27 However, the dye-sensitized molecular photocathodes prepared via the reductive polymerization of vinyl groups required a relatively negative applied potential to achieve maximal photocurrent, and the photocurrent density remained low, probably because of the low conductivity of the saturated hydrocarbon chains formed by vinyl group polymerization.…”

An Iron(III) Complex with Pincer Ligand—Catalytic Water Oxidation through Controllable Ligand Exchange

“…Beyond the mechanistic aspects, in practical electrolysis and dye-sensitized photoelectrolysis cells further considerations have to be made. In such cells, instead of dissolved in the electrolyte, catalysts are better applied on conductive surfaces [41,42]. The conversion or degradation of homogeneous Fe catalysts due to the oxidation of ligands [43,44] as well as the question of homogeneous vs. heterogeneous reaction are crucial issues from the viewpoint of the application [26].…”

“…There have been successful attempts to graft the molecular reactivity onto conductive substrates through immobilization, thus providing advanced heterogeneous systems [42]. Shi et al demonstrated the convenient preparation of nanostructures by applying Fe II -phthalocyanine/carbon nitride nanosheet (FePc/CN) nanocomposites [46].…”