Long-Term Thermal Stability of Liquid Dye Solar Cells

Laboratory-size dye solar cells (DSCs), based on industrially feasible materials and processes employing liquid electrolytes, have been developed. Cells based on two electrolyte solvents with different physical properties were subjected to thermal stress test at 80 °C for 2000 h in the dark to monitor their long-term thermal stability. The DSCs incorporating a methoxypropionitrile (MPN)-based electrolyte presented a severe efficiency loss at 1 sun AM 1.5G of more than 70% upon thermal aging, while the solar cells using tetraglyme (TG) as a high boiling point solvent attained a promising stability with only 20% loss of performance. To better understand the above behavior, systematic experiments, including optical microscopy, linear sweep voltammetry, UV–vis absorption, electrochemical impedance, and Raman spectroscopies were conducted. Virtually no dye degradation/desorption, electrolyte decomposition, semiconductor passivation, or loss of cathode activity could be identified. For the MPN-based cells, a sharp decrease in the short-circuit photocurrent was observed at high illumination intensities following thermal stress, attributed to charge-transfer limitations due to severe triiodide loss, verified by different experimental techniques. These degradation effects were efficiently mitigated by replacing MPN with the high-boiling-point solvent in the electrolyte

Mo-BiVO₄/Ca-BiVO₄ Homojunction Nanostructure-Based Inverse Opals for Photoelectrocatalytic Pharmaceutical Degradation under Visible Light

Homojunction engineering has emerged as a potent strategy to evade interfacial stability issues and improve the efficiency of nanostructured metal oxide photocatalysts, though rarely combined with the enhanced light capture ability of three-dimensional macroporous photonic crystal structures. Herein, the formation of nanoscale n-n+ homojunctions between different Mo- and Ca-doped BiVO4 nanocrystals in the skeleton of photonic band gap (PBG) engineered inverse opals is introduced as an advanced approach to simultaneously promote visible light harvesting, electron transport, and charge separation of BiVO4 nanomaterials for the photoelectrocatalytic degradation of pharmaceutical contaminants of emerging concern. Controlled deposition of BiVO4 inverse opal films with tailored PBGs was combined with compositional tuning by Mo- and Ca-doping for slow-photon-assisted visible-light-activated (VLA) photocatalysis. The introduction of shallow dopant states in the Mo-, Ca-doped BiVO4 nanoparticles with relatively weak structural distortions but significantly different donor concentrations resulted in a broad distribution of type-II homojunctions in the nanocrystalline inverse opal walls. Comparative photoelectrochemical evaluation showed that nanostructured homojunction Mo-BiVO4/Ca-BiVO4 photonic films largely outperformed their individual constituents in both photocurrent generation and the VLA photocatalytic degradation rate. Moreover, they exhibited markedly improved performance in the photoelectrocatalytic degradation of tetracycline and ciprofloxacin broad-spectrum antibiotics as well as salicylic acid under visible light, validating their application potential in VLA water remediation by pharmaceutical micropollutants

Influence of Fluorine Plasma Treatment of TiO₂ Films on the Behavior of Dye Solar Cells Employing the Co(II)/(III) Redox Couple

Fluorine plasma treatment was investigated as an appropriate means for the surface modification of TiO₂ thin film electrodes and the optimization of their performance as photoanodes in dye solar cells (DSCs) employing the Co­(II)/(III) redox shuttle and the organic D35 sensitizer. Detailed surface and structural characterization of the titania films by contact angle measurements, atomic force microscopy, profilometry, and Raman and UV–vis spectroscopy showed that high density SF₆ plasma provoked severe film densification and thus an increase of the nanoparticles packing density, leaving intact the crystallinity, particle size, and optical bandgap. Surface fluorination of the TiO₂ films was also identified by X-ray photoelectron spectroscopy. The combination of the above effects resulted in the enhancement of both photocurrent and power conversion efficiency of the corresponding DSCs at moderate plasma treatment durations, while the photovoltage decreased continuously as a function of the fluorine processing time. Electrochemical impedance spectroscopy analysis revealed a marked increase of the density and distribution of trap states due to fluorine induced surface states along with a systematic downward shift of the TiO₂ conduction band, probably attributed to the electrostatic coupling of intercalated Li⁺ cations with the polar Ti–F species at the TiO₂ surface, in agreement with the <i>V</i>_oc drop. In contrast, enhanced electron injection was inferred to underlie the observed <i>J</i>_sc and DSC performance improvements, as surface fluorination and the concomitant film densification slightly increased electron transport while hardly affecting dye loading capacity, light harvesting efficiency, and recombination kinetics, except for the case of prolonged plasma treatment. Effective control of the detrimental side effects of fluorine species can render this kind of plasma treatment a powerful method to tune the surface and electrical properties of TiO₂ films and optimize the behavior and performance of the resulting DSC devices

Photocatalytic Degradation of Microcystin-LR and Off-Odor Compounds in Water under UV‑A and Solar Light with a Nanostructured Photocatalyst Based on Reduced Graphene Oxide–TiO₂ Composite. Identification of Intermediate Products.

Microcystin-LR (MC-LR) is the most common and toxic variant of the group of microcystins (MCs) produced during the formation of harmful cyanobacterial blooms. Geosmin (GSM) and 2-methylisoborneol (MIB) may also be produced during cyanobacterial blooms and can taint water causing undesirable taste and odor. The photocatalytic degradation of MC-LR, GSM, and MIB in water under both UV-A and solar light in the presence of reduced graphene oxide–TiO₂ composite (GO–TiO₂) was studied. Two commercially available TiO₂ materials (Degussa P25 and Kronos) and a reference TiO₂ material prepared in the laboratory (ref-TiO₂) were used for comparison. Under UV-A irradiation, Degussa P25 was the most efficient photocatalyst for the degradation of all target analytes followed by GO–TiO₂, ref-TiO₂, and Kronos. Under solar light irradiation GO–TiO₂ presented similar photocatalytic activity to Degussa P25, followed by Kronos and ref-TiO₂ which were less efficient. Intermediate products formed during the photocatalytic process with GO–TiO₂ under solar light were identified and were found to be almost identical to those observed by Degussa P25/UV-A. Assessment of the residual toxicity of MC-LR during the course of treatment with GO–TiO₂ showed that toxicity is proportional only to the remaining MC-LR concentration. The photocatalytic performance of GO–TiO₂ was also evaluated under solar light illumination in real surface water samples, and GO–TiO₂ proved to be effective in the degradation of all target compounds

Enhanced CO₂ Capture in Binary Mixtures of 1‑Alkyl-3-methylimidazolium Tricyanomethanide Ionic Liquids with Water

Absorption of carbon dioxide and water in 1-butyl-3-methylimidazoliun tricyanomethanide ([C₄C₁im]­[TCM]) and 1-octyl-3-methylimidazolium tricyanomethanide ([C₈C₁im]­[TCM]) ionic liquids (ILs) was systematically investigated for the first time as a function of the H₂O content by means of a gravimetric system together with in-situ Raman spectroscopy, excess molar volume (<i>V</i>^E), and viscosity deviation measurements. Although CO₂ absorption was marginally affected by water at low H₂O molar fractions for both ILs, an increase of the H₂O content resulted in a marked enhancement of both the CO₂ solubility (ca. 4-fold) and diffusivity (ca. 10-fold) in the binary [C_<i>n</i>C₁im]­[TCM]/H₂O systems, in contrast to the weak and/or detrimental influence of water in most physically and chemically CO₂-absorbing ILs. In-situ Raman spectroscopy on the IL/CO₂ systems verified that CO₂ is physically absorbed in the dry ILs with no significant effect on their structural organization. A pronounced variation of distinct tricyanomethanide Raman modes was disclosed in the [C_<i>n</i>C₁im]­[TCM]/H₂O mixtures, attesting to the gradual disruption of the anion–cation coupling by the hydrogen-bonded water molecules to the [TCM]⁻ anions, in accordance with the positive excess molar volumes and negative viscosity deviations for the binary systems. Most importantly, CO₂ absorption in the ILs/H₂O mixtures at high water concentrations revealed that the [TCM]⁻ Raman modes tend to restore their original state for the heavily hydrated ILs, in qualitative agreement with the intriguing nonmonotonous transients of CO₂ absorption kinetics unveiled by the gravimetric measurements for the hybrid solvents. A molecular exchange mechanism between CO₂ in the gas phase and H₂O in the liquid phase was thereby proposed to explain the enhanced CO₂ absorption in the hybrid [C_<i>n</i>C₁im]­[TCM]//H₂O solvents based on the subtle competition between the TCM–H₂O and TCM–CO₂ interactions, which renders these ILs very promising for CO₂ separation applications

CO₂ Capture Efficiency, Corrosion Properties, and Ecotoxicity Evaluation of Amine Solutions Involving Newly Synthesized Ionic Liquids

The CO₂ capture efficiency of nine newly synthesized ionic liquids (ILs), both in their pure states as well as in binary and ternary systems with water and amines, was investigated. The study encompassed ILs with fluorinated and tricyanomethanide anions as well as ILs that interact chemically with CO₂ such as those with amino acid and acetate anions. Compared to amines, some of the novel ILs exhibited a majority of important advantages for CO₂ capture such as enhanced chemical and thermal stabilities and negligible vapor pressure; the previous features counterbalance the disadvantages of lower CO₂ absorption capacity and rate, making these ILs promising CO₂ absorbents that could partially or totally replace amines in industrial scale processes. In addition to their ability to capture CO₂, important issues including corrosivity and ecotoxicity were also examined. A thorough investigation of the capture efficiency and corrosion properties of several solvent formulations proved that some of the new ILs encourage future commercial-scale applications for appropriate conditions. ILs with a tricyanomethanide anion confirmed a beneficial effect of water addition on the CO₂ absorption rate (ca. 10-fold) and capacity (ca. 4-fold) and high efficiency for corrosion inhibition, in contrast with the negative effect of water on the CO₂ absorption capacity of ILs with the acetate anion. ILs with a fluorinated anion showed high corrosivity and an almost neutral effect of water on their efficiency as CO₂ absorbents. ILs having amino acid anions presented a reduced toxicity and high potential to completely replace amines in solutions with water but, in parallel, showed thermal instability and degradation during CO₂ capture. Tricyanomethanide anion-based ILs had a beneficial effect on the capture efficiency, toxicity, and corrosiveness of the standard amine solutions. As a consolidated output, we propose solvent formulations containing the tricyanomethanide anion-based ILs and less than 10 vol % of primary or secondary amines. These solvents exhibited the same CO₂ capture performance as the 20−25 vol % standard amine solutions. The synergetic mechanisms in the capture efficiency, induced by the presence of the examined ILs, were elucidated, and the results obtained can be used as guidance for the design and development of new ILs for more efficient CO₂ capture