Dye-sensitized photo-electrochemical cells (DS-PECs) for water splitting hold promises for the large-scale storage of solar energy in the form of (solar) fuels, owing to the low cost and ease to process of their constitutive photoelectrode materials. The efficiency of such systems ultimately depends on our capacity to promote unidirectional light-driven electron transfer from the electrode substrate to a catalytic moiety. We report here on the first noble-metal free and covalent dyecatalyst assembly able to achieve photo-electrochemical visible light-driven H 2 evolution in mildly acidic aqueous conditions when grafted onto p-type NiO electrode substrate.Photosynthesis has inspired for many years the development of water splitting dye-sensitized photo-electrochemical cells (DS-PECs) for hydrogen production.1-3 A key step has been achieved very recently with the report of the first fully operative tandem DS-PEC.4 In such devices, limitation currently arises from the photocathode performances. Therefore different architectures based on the co-grafting4,5 of catalyst and dye onto nickel oxide (NiO), layerby-layer6 or supramolecular linkage of the catalyst to a grafted dye7 have been investigated. NiO is a p-type transparent conducting oxide specifically suitable for fast hole injection from the highest occupied molecular orbital (HOMO) of the excited dye.8 Then H 2 evolution requires that the photogenerated electron is efficiently and rapidly transferred to a catalyst. In that perspect, push-pull organic dyes appear as particularly attractive since they combine large absorptivity in the visible spectrum and spatial charge separation in the Europe PMC Funders Author ManuscriptsEurope PMC Funders Author Manuscripts excited state that limits the undesired recombination reaction from the reduced dye to the NiO electrode.9,10 The push-pull architecture is also instrumental to foster unilateral electron transfer through direct connection of the acceptor moiety, where the lowest unoccupied molecular orbital (LUMO) is centered and the photogenerated electron located, to the catalyst. We report here the first example of a covalent dye-catalyst molecular assembly integrated in an operative photoelectrode for H 2 evolution (Figure 1).We previously reported that, upon grafting on NiO, an easily affordable push-pull dye based on a triarylamine electron-donor part and an ethyl cyanoacetate electron-acceptor part separated by a thiophene unit generates large photocurrents in the presence of an irreversible electron acceptor in mildly acidic aqueous solution (pH 4-5).11 Cobalt diimine-dioxime complexes are proven catalysts for H 2 evolution at low overvoltage.12,13 When grafted onto electrode surfaces, they display sustained activity in pH = 4.5 aqueous solution14 and tolerance to oxygen.15 These features make them particularly attractive for incorporation into dye-sensitized H 2 -evolving photoelectrodes. To prepare a covalent dye-catalyst assembly, we first synthesized a terminal alkyne derivative (1, Figure 2) of the a...

The search for efficient noble metal-free hydrogen-evolving catalysts is the subject of intense research activity. A new family of molecular cobalt(II)-polypyridyl catalysts has recently emerged. These catalysts prove more robust under reductive conditions than other cobalt-based systems and display high activities under fully aqueous conditions. This review discusses the design, characterization, and evaluation of these catalysts for electrocatalytic and light-driven hydrogen production. Mechanistic considerations are addressed and structure-catalytic activity relationships identified in order to guide the future design of more efficient catalytic systems.