Effect Of Ruthenium Catalyst Structure On The Water Oxidation Performance Of Photoanode In DS-PEC

Energy crisis and enviromental pollution are the problems to threaten the human suvival. So, exploitation and utilization of clean, sustainable and renewable energy such as solar energy and H2 energy is the concerned focus of humans. Dye-sensitized photoelectrochemical cell (DS-PEC) using solar energy to water splitting realizes the transformation of solar energy to clean chemical energy. Recently, The reseach subject has aroused widespread attention. The water oxidation on the photoanode in DS-PEC was mainly studied in this thesis. The effect of molecular structures of water oxidation catalysts on the performance of photoanode in DS-PEC was researched from the following several aspects.In order to investigate the effect of structure of flexible carbon chain in the axile ligand in the catalyst on the performance of DS-PEC, two photoanodes TiO2(PS1+C1) and TiO2(PS1+C2) were assembled by co-adsorption with the phosphoric derivative of Ru(bpy)3 (PS1) as the photosensitizer and the complex C1 containning shorter flexible chain and complex C2 containning longer flexible chain as the catalysts respectively. After 10 s photoelectrochemical measurement with 0.2 V (vs. NHE) bias and 300 mW/cm2 light intensity in 0.1 M Na2SO4 (pH= 6.4) aqueous solution, the stable photocurrent of the DS-PEC with the photoanode TiO2(PS1+C2) (1.4 mA/cm2) is found to be twice as that of the DS-PEC with the photoanode TiO2(PS1+C1) (0.7 mA/cm2). It is concluded that the complex C2 contaning longer flexible chain posseses the higher degree of motion freedom and more easily conducts intermolecular radical coupling reaction than the complex C1. In addition, the electrical conductivity of the long flexible chain is poor, so it could reduce the back electron transfer from TiO2 to the high oxidation state of complex C2. Therefore, catalyst C2 with long flexible carbon chain is beneficial to the improvement of DS-PEC.Secondly, in order to investigate the effect of catalyst assemble method on the performance of DS-PEC, a photoanode TiO2(PS1+C3) was assembled with complex C3 containning two-CH2CH2CH2-flexible chains in the two axile ligands as catalyst. After 10 s photoelectrochemical measurement in phosophate buffer solution (pH= 6.8), the stable photocurrent of the DS-PEC with the photoanode TiO2(PS1+C2) (1.50 mA/cm2) is almost fourfold as that of the DS-PEC with the photoanode TiO2(PS1+C3) (0.40 mA/cm2) under the same condition. It is concluded that the complex C3 fixing on the nano TiO2 by two phosphate claws posseses the lower degree of motion freedom which is disadvantage for intermolecular radical coupling reaction. On the other hand, the distance from the nano TiO2 to the active centre of catalyst C3 is closer that aggravates the back electron transfer. So, the corresponding performance of DS-PEC is seriously affected.In addition, two photoanodes TiO2(PS1+C4) and TiO2(PS1+C5) were assembled with the binuclear ruthenium complex C4 and a mononuclear ruthenium complex C5 as catalysts respectively by the same method above. After 10 s photoelectrochemical measurement in 0.1 M Na2SO4 (pH=6.4) aqueous solution, the stable photocurrent of the molecular device assembled with photoanode TiO2(PS1+C4) (1.1 mA/cm2) is found to be twice as that of the DS-PEC with photoanode TiO2(PS1+C5) (0.5 mA/cm2). It can be pointed out that the complex C4 conducts intramolecular radical coupling reaction on the surface of the photoanode and obtains superior performance than complex C5 conducting intermolecular radical coupling reaction.Finally, a PMMA overlayer was coated on the photoanode TiO2(PS1+C2) by a simple method to improve the stability of DS-PEC. This could efficiently prevent the desorption of the photosensitizer and catalyst from the semiconductor and enhance the long-term stability of the DS-PEC.Based on the bimolecular radical coupling mechanism of catalyst [Ru(bda)(pic)2] (H2bda = 2,2'-bipyridine-6,6'-dicarboxylic acid, pic= picoline) in the water oxidation process, a trinuclear ruthenium complex C6 contaning flexible bridge ligands and a binuclear ruthenium complex C7 for comparison were designed and synthesized. With (NH4)2Ce(NO3)6 as the oxidant, TON of catalyst C6 (86498) is twice as hight as catalyst C7 (44412) based on the number of molecules in CF3SO3H solution (pH 1.0). It is concluded that the intramolecular bimolecular radical coupling happened in the water oxidation process of trinuclear ruthenium complex C6 and the probability is much higher than the bimolecular catalyst C7 under the identical conditions.