Search

Photovoltaics (PVs) show high conversion efficiency in renewable energy. However, PV systems demand advanced preparations and relatively large amounts of staring materials. Photoelectrochemical (PEC) systems, with monolithic semiconductor (SC) electrode/Redox couple interface, emerged, but still demand special preparation conditions and relatively large amounts of starting materials. Polycrystalline film electrodes, are studied as replacement for monolithic SC electrodes, both theoretically and experimentally. Examples are dye-sensitized solar cells (DSSCs) and metal chalcogenides. With narrow-to-medium band gaps, metal chalcogenides are suitable for the abundant visible solar light. However, polycrystalline film electrodes suffer major shortcomings. Pristine films show low conversion efficiency. Being polycrystalline, they have low carrier mobility. They are also unstable to photo-corrosion, due to charge build-up in the space charge layer (SCL). Enhancement of polycrystalline film electrode PEC performance and stability was reported using different methods. Among those, charge transfer catalysis at the film electrode surface, is focused here. The modification involves attaching electroactive species to the electrode surface. Ability of electroactive species to behave as charge transfer catalysts at the solid/liquid interface is discussed. By such behavior, the catalysts speed up charge transfer and consequently increase photocurrent. Moreover, charge build-up in the SCL is prevented by quick charge transfer, which protects the electrode surface from photo-corrosion. Ability of charged electroactive species to shift flat band edge position, with its consequences, is also described. Recommended features of an effective catalyst species for PEC systems are described together with recommendations for more future research in the field.

Waste cadmium sulfide (CdS) film electrodes, originally deposited onto glass/fluorine doped tin oxide (glass/FTO) substrates, were used to prepare recycled CdS film electrodes. The waste glass/FTO/CdS were processed in acidic media to recover the glass/FTO substrates, the Cd2+ ions (in the acidic solutions) and the gaseous H2S (recaptured in basic media). All components of the waste electrodes were thus recovered. The recovered glass/FTO and the Cd2+ ions were then reused to produce new recycled glass/FTO/CdS electrodes by chemical bath deposition. The produced films were then characterized by X-ray diffractometry, scanning electron microscopy, electronic absorption spectroscopy and other techniques. The Cd2+ ions were recovered with efficiency higher than 90% from the waste films, as observed from atomic absorption spectrometry. The recycled films were assessed in photo-electrochemical conversion of light to electricity, and exhibited comparable efficiency to those freshly prepared from authentic starting materials and other literature values. Photoelectrochemical characteristics for the recovered films were further enhanced by avoiding stirring of the chemical deposition bath during preparation. The results manifest the feasibility of recycling CdS electrodes and enhancing their photo-electrochemical characteristics by simple low cost methods. Both environmental protection and economic goals can thus be potentially achieved.

Waste CdSe film electrodes can be recycled into useful films without sacrificing their photoelectrochemical performance. Cd2+ aqueous ions have been recovered from waste CdSe electrodes by soaking in minimal amount of HCl (10.0 M) at 80 °C for 60 min. The Se2− ions, converted into H2Se(g) by the acid, have been trapped in basic NaOH solution for further use. The recovered Cd2+ ions have been used to deposit new (recycled) CdSe film electrodes onto fluorine doped tin oxide films (FTO/Glass) using aqueous Na2SeSO3 solutions by two methods. Electrochemical deposition (ECD) has been performed using applied potential (−1.0 V vs Ag/AgCl). Chemical bath deposition (CBD) has been performed in a basic medium at 70 °C. Both ECD and CBD recycled films have been prepared using recovered Cd2+ solutions, whereas fresh films have been prepared using fresh solutions. In both cases, the recycled CdSe films do not show any inferior characteristics to the freshly prepared counterparts. Effects of annealing at 150 °C and cooling rate on physical and photoelectrochemical (PEC) characteristics have been studied for different CdSe film electrodes. In both recycled and fresh systems, the CBD film electrodes exhibit enhanced uniformity, compactness and photo-electrochemical (PEC) performance by annealing. Upon annealing, the recycled and fresh ECD films exhibit less film uniformity and compactness, and consequently lower PEC performance. In ECD and CBD films, fast cooling is advantageous over slow cooling. Conversion efficiency values of ~8% are observed for the CBD recycled pristine films once carefully annealed and quickly cooled. This is higher than literature values for pristine CdSe (~5%), TiO2/CdS/CdSe (~4%) film electrodes, and resembles that for Tl/CdSe0.65Te0.35 (~8) multi-layer systems. The results show the feasibility of recycling waste CdSe film electrodes.

Photoelectrochemical (PEC) characteristics of CdTe film electrodes, known to have low conversion efficiency when used in their pristine form, can be significantly enhanced by carefully controlling their annealing temperature and cooling rate. Pristine CdTe films were electrodeposited onto FTO/Glass substrates which were used as anodes. To reach films with optimal characteristics, different applied preparation potentials were intentionally examined, namely 1.0, 1.1 and 1.2 V, vs. Ag/AgCl reference electrode (or 1.2, 1.3 and 1.4 V NHE, respectively) where the 1.1 V applied potential showed best PEC characteristics, and was thus followed unless otherwise stated. To study effect of annealing temperature, three temperatures (150, 200 and 250 oC) were attempted to enhance PEC characteristics of the deposited films. Effect of cooling rate, on PEC performance of pre-annealed films, was also studied using quenching and slow cooling. Films quenched from annealing at all temperatures showed lower PEC performance compared to non-annealed electrode. Film electrodes slowly cooled from 150 or 200 oC show enhanced PEC performance compared to quenched or non-annealed films. Film slowly cooled from 250 oC exhibited lower PEC performance than the quenched counterpart. Annealing at 250 oC lowered PEC for both quenching and slow cooling. As a low band gap semiconductor film electrode, it is recommended to slowly cool CdTe from low annealing temperatures, and to quickly cool them from relatively higher annealing temperature. The annealing temperature and cooling rate effects on CdTe film PEC performance are attributed to their effects on other physical characteristics, namely crystallinity, morphology and chemical composition. The optimal conversion efficiency (6.9%) was observed for film deposited at 1.1 V applied potential when annealed at 200 oC and slowly cooled to room temperature.

Lithium (Li) coating of the inner wall of tokamaks has been considered a practical technique to reduce fuel recycling. Li-coating also improves the tokamak wall condition and reduces high-Z material release from the tokamak wall during discharges. In a recent campaign in the STOR-M tokamak (R/a = 0.46/0.12 m, B t = 0.7 T, I p = 25 kA), 100 mg of Li has been coated on the tokamak chamber using a physical vacuum evaporation applicator developed by General Fusion Inc. A reduction in the plasma impurity contents and increase in both the peak plasma current and duration have been observed after Li-coating. The line averaged electron density in the plasma reduced after Li-coating. An increase in hard x-ray radiation has also been observed, suggesting an enhanced production of suprathermal run-away electrons because of the reduced electron density. In addition the Li atom emission line has been used to measure the plasma flow velocity based on the Doppler shift of the emission.