Hydrogen Evolution Catalyzed by Cobalt Diimine–Dioxime Complexes

Abstract

ConspectusMimicking photosynthesis and producing solar fuels is an appealing way to store the huge amount of renewable energy from the sun in a durable and sustainable way. Hydrogen production through water splitting has been set as a first-ranking target for artificial photosynthesis. Pursuing that goal requires the development of efficient and stable catalytic systems, only based on earth abundant elements, for the reduction of protons from water to molecular hydrogen. Cobalt complexes based on glyoxime ligands, called cobaloximes, emerged ten years ago as a first generation of such catalysts. They are now widely utilized for the construction of photocatalytic systems for hydrogen evolution.In this Account, we describe our contribution to the development of a second generation of catalysts, cobalt diimine-dioxime complexes. While displaying similar catalytic activities as cobaloximes, these catalysts prove more stable against hydrolysis under strongly acidic conditions thanks to the tetradentate nature of the diimine-dioxime ligand. Importantly, H 2 evolution proceeds via proton-coupled electron transfer steps involving the oxime bridge as a protonation site, reproducing the mechanism at play in the active sites of hydrogenase enzymes. This feature allows H 2 to be evolved at modest overpotentials, i.e. close to the thermodynamic equilibrium over a wide range of acid-base conditions in non-aqueous solutions.Derivatization of the diimine-dioxime ligand at the hydrocarbon chain linking the two imine functions enables the covalent grafting of the complex onto electrode surfaces in a more convenient manner than for the parent bis-bidentate cobaloximes. Accordingly we attached * to whom correspondence should be addressed. vincent.artero@cea.fr. Europe PMC Funders Group Europe PMC Funders Author ManuscriptsEurope PMC Funders Author Manuscripts diimine-dioxime cobalt catalysts onto carbon nanotubes and demonstrated the catalytic activity of the resulting molecular-based electrode for hydrogen evolution from aqueous acetate buffer. The stability of immobilized catalysts was found to be orders of magnitude higher than that of catalysts in the bulk. It led us to evidence that these cobalt complexes, as cobaloximes and other cobalt salts do, decompose under turnover conditions where they are free in solution. Of note this process generates in aqueous phosphate buffer a nanoparticulate film consisting of metallic cobalt coated with a cobalt-oxo/hydroxo-phosphate layer in contact with the electrolyte. This novel material, H 2 -CoCat, mediates H 2 evolution from neutral aqueous buffer at low overpotentials.Finally, the potential of diimine-dioxime cobalt complexes for light-driven H 2 generation has been attested both in water/acetonitrile mixtures and in fully aqueous solutions. All together, these studies hold promises for the construction of molecular-based photoelectrodes for H 2 evolution and further integration in dye-sensitized photo-electrochemical cells (DS-PECs) able to achieve overall water splitting. Introducti...

Wafer-sized multifunctional polyimine-based two-dimensional conjugated polymers with high mechanical stiffness

“…5d). These values are superior to those of the previously reported cobalt complex based molecular HER catalysts (Supplementary Table 1), including diimine-dioxime cobalt catalysts, cobalt microperoxidase-11and porphyrin-cobalt embedded in graphene oxide sheets353839.…”

Nickel phlorin intermediate formed by proton-coupled electron transfer in hydrogen evolution mechanism

“…Moreover, the coupling of ET and PT can be enhanced by incorporating proton relays into molecular catalysts, exploiting the proximal positioning of the proton donor and acceptor (12)(13)(14)(15)(16). Recognition and characterization of successful PCET motifs within molecular electrocatalysts provides insight into the design of efficient catalytic processes (17)(18)(19).…”

New Cobalt‐Bisterpyridyl Catalysts for Hydrogen Evolution Reaction

“…[1][2][3] The kinetic barrierf or such at ransformation is large, hence,c atalysts are required to lower it.Although metallic platinum remains the most effective catalytic materialf or the hydrogen evolution reaction( HER), its limited availability and high cost justifiest he quest for alternative catalysts based on cheaper and more abundant non-noble metals.C urrent research focuses primarily on two types of compounds:h eterogeneousm aterials and homogeneous metal complexes. However,t hese energy sourcess uffer from low energetic density and intermittency.T oo vercome such failings, ac ommon approachi nvolves storage of the generatede nergy by conversion into chemical energy through the formation of chemical bonds.…”