Homogeneous Catalytic Performance And Assemble Of Photocath- Ode Device Based On Cobalt Complexes For Hydrogen Production In Aqueous Solution

Sunlight is a kind of clean and renewable energy. It is important to utilize the solar energy to the production processes and daily life of human society by different techniques, among which, using solar energy to split water into hydrogen and oxygen is an ideal method for conversion of solar energy to applicable chemical energy. Because hydrogen has a high mass energy density, it is considered as a promising alternative to fossil fuels. During the past decades, the research on the multicomponent catalytic systems and photoelectrochemical cells for light-driven water splitting has achieved many progresses, but the efficiencies of the reported systems and devices are not high enough for application. Hence, it is still a big challenge to improve the efficiency of photocatalytic systems and to find high-efficient solar-to-hydrogen transformation path. On the basis of such background, the research of this thesis focused on the following two aspects:(1) to find new homogeneous photocatalytic systems based on cobalt polypyridine complexes; (2) to assemble photocathode functional devices based on cobaloxime molecular catalysts and CdSe quantum dots (QDs).A series of three-component homogeneous catalytic systems were studied. Each system contains a cobalt polypyridine complex as catalyst, a ruthenium complex [Ru(bpy)3]2+as photosensitizer, and ascorbic acid as sacrificial electron donor for visible-light-driven water reduction to hydrogen. The electrochemical studies on each component of the system indicated that the desired electron transfer pathway is thermodynamically feasible. The results of the photocatalytic studies of the systems showed that the TONs of cobalt catalystsl [Co(tpen)](BF4)2{tpen= N1,N1,N2,N2- tetrakis(pyridinyl-2-methyl)ethane-1,2-diamine},2 [Co(bztpen)](BF4)2{bztpen= N1-benzy1-N1,N2,N2-tris(pyridinyl-2-methyl)ethane-1,2-diami-ne} and 3 [Co(dbzbpen)](BF4)2{ dbzbpen= N1,N2-dibenzyl-N1,N2-bis(pyridinyl-2-methyl)-ethane-1,2-diamine} are about 639,2078, and 98, respectively, after 11 h illumination under optimized conditions. The quantum efficiencies of H2 generation are about 0.48% for the 1/[Ru(bpy)3]2+/H2A (ascorbic acid) and 1.51% for the 2/[Ru(bpy)3]2+/H2A system. It was found that during long-time illumination, the activity of the system was gradually decreased and the H2 evolution leveled off after 11 h illumination due to the decomposition of the cobalt catalyst and the [Ru(bpy)3]2+ photosensitizer. On the basis of the results of electrochemical studies and the related literatures, we proposed the mechanism with a preferably reductive quenching approach of the excited state of [Ru(bpy)3]2+ for the photoinduced H2 generation reaction catalyzed by these systems.We designed an efficient noble-metal-free photocathode, which was fabricated by chemically co-grafted CdSe QDs and a cobaloxime catalyst to the surface of open porous NiO film on a FTO glass substrate. CdSe QDs was grafted on the porous NiO by two methods, namely the successive ionic layer adsorption and reaction (SILAR) and the one-pot in situ adsorption and reaction (OIAR) procedures. Upon visible light illumination, this photocathode displayed a photocurrent density of 102±2 μA cm-2 at a bias potential of 0 V versus NHE in 0.1 M Na2SO4 solution at pH 7. The photocathodes prepared by SILAR and OIAR procedures exhibited similar photocurrents under identical conditions. The activity of this photocathode is 4-5 times higher than those reported to date for the dye-sensitized NiO cathodes with a cobaloxime catalyst chemically attached or physically adsorbed on the electrode surface under the identical conditions. The photocathode functions in a PEC cell for a long time with only 6% decay in photocurrent after 1.5 h illumination, indicating a good stability of the as-prepared photocathode.