Azarpira, Anahita: Synthesis and characterization of novel composite photoelectrodes based on chalcopyrite and silicon for the visible light-driven hydrogen and oxygen evolution. , Berlin, Technische Universität, Diss., 2016
https://depositonce.tu-berlin.de/handle/11303/5513
Open Accesn Version
Abstract:
In the presented work, efficient and stable multi-junction semiconductor electrodes are introduced for the two half-cell reactions of photo-assisted splitting of water, the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER). For efficient and stable HER in acidic electrolytes, novel composite photocathodes were developed which functionalize device-grade Cu(In,Ga)Se2 thin-film absorbers in conjunction with electrocatalytic Pt-implemented TiO2 layers. Varying Pt-concentrations were systematically investigated in order to optimize simultaneously (i) the conductivity of the Pt - TiO2 films, (ii) the electrocatalytic activity, and (iii) light-guidance toward the chalcopyrite absorber. It is shown that the gradual increase of the Pt-concentration passes through an efficiency- and stability-maximum of the device (at about 5 vol.% Pt of the precursor solution). At this maximum, optimized light-incoupling into the chalcopyrite light-absorber was achieved and 15 mAcm-2 at the thermodynamic potential for H2-evolution (0 V vs. RHE) were realized. Devices, fabricated according to this optimized parameter, operated over more than 24 hours with no sign of degradation. For the corresponding preparation of semiconductor-based photoanodes, electrophoretic deposition was used for formation of two different water oxidation catalysts, ZnO:Co and RuO2 on semiconductor supports. Firstly, for OER in alkaline solutions, an extensive analysis was carried out in order to determine optimized parameters for electrophoretic deposition of pre-synthesized ZnO:Co catalysts from varied organic solvents on fluorinated tin oxide. Evaluation of the electrochemical activity proved a clear solvent-dependence with highest activity upon deposition from acetonitrile and lowest activity upon deposition from ethanol. Detailed analysis of the respective films by various methods showed that the change in electrochemical activity is caused by a corresponding variation in the size of the active surface area. It is furthermore shown that less active but highly transparent ZnO:Co phases, prepared from ethanol-containing suspensions, can be successfully employed in a stacking configuration with low-cost triple-junction solar cells. Thereby, solar-to-hydrogen efficiencies of up to 5.0% were achieved. Secondly, for devising a silicon-based photoanode, applicable to OER in acidic media, a novel approach was developed: the capacity of the most efficient water oxidation catalyst in acidic electrolytes, RuO2, was exploited towards alcohol polymerization. Thereby, a stable organic protection layer could be formed which allows for the first time long-term operation of silicon-RuO2 junction as OER-photoanode. The interfacial layers are generated via iodine-mediated electro-reductive polymerization of alcohols, simultaneously forming during electrophoretic transport of RuO2. Reaction chemistry analyses suggest that the RuO2-induced catalysis introduces E2-elimination reactions which result in a carbon sp3-sp2 transformation within the film. For the two modes of photoelectrochemical operation, the photovoltaic and the photoelectrocatalytic mode, 20mAcm-2 and 15mAcm-2 photocurrent densities, respectively, were obtained with operational stability for 8 and 24 hrs. The interfacial organic-protection layer enables Si photovoltages of 500mV, demonstrating an extraordinary electronic interface quality.