Monolithic Quasi-Solid-State Dye Sensitized Solar Cells Prepared Entirely by Printing Processes

A complete printing process was developed to fabricate the quasi-solid-state dye-sensitized solar cells with monolithic structures (m-QS-DSSCs). First, a structure of m-DSSCs was constructed by sequentially printing TiO2 layers (main and scattering), a ZrO2 insulating layer, and a carbon counter electrode (CE) onto an FTO substrate (FTO/TiO2/ZrO2/carbon CE). Then, a quasi-solid-state printable electrolyte (QS-PE), prepared using polyethylene oxide/polymethyl methacrylate, was printed directly on top of the porous carbon counter electrode (CE), enabling the m-QS-DSSCs to be prepared entirely by printing processes. In this study, the porous structures and characteristics of the ZrO2 and carbon layers were optimized by controlling the film thicknesses and heat treatment conditions; furthermore, the Pt layer was coated to improve the catalytic activity of carbon CEs. The results revealed that an appropriate porous structure of carbon and ZrO2 films could be obtained by heating the films from 200 to 500 °C. Through these porous layers, the QS-PE can penetrate well into the photoelectrodes, increasing the charge transport in the cells and at the electrode/electrolyte interfaces; therefore, the m-QS-DSSCs can achieve an efficiency of 6.79% under 1 sun illumination. Furthermore, the structures can also be utilized to fabricate liquid cells for application in a dim light environment. The m-QS-DSSCs remained stable during a long-term stability test at room temperature

Performance Enhancement of Quantum-Dot-Sensitized Solar Cells by Potential-Induced Ionic Layer Adsorption and Reaction

Successive ionic layer adsorption and reaction (SILAR) technique has been commonly adopted to fabricate quantum-dot-sensitized solar cells (QDSSCs) in the literature. However, pore blocking and poor distribution of quantum dots (QDs) in TiO₂ matrices were always encountered. Herein, we report an efficient method, termed as potential-induced ionic layer adsorption and reaction (PILAR), for in situ synthesizing and assembling CdSe QDs into mesoporous TiO₂ films. In the ion adsorption stage of this process, a negative bias was applied on the TiO₂ film to induce the adsorption of precursor ions. The experimental results show that this bias greatly enhanced the ion adsorption, accumulating a large amount of cadmium ions on the film surface for the following reaction with selenide precursors. Furthermore, this bias also drove cations deep into the bottom region of a TiO₂ film. These effects not only resulted in a higher deposited amount of CdSe, but also a more uniform distribution of the QDs along the TiO₂ film. By using the PILAR process, as well as the SILAR process to replenish the incorporated CdSe, an energy conversion efficiency of 4.30% can be achieved by the CdSe-sensitized solar cell. This performance is much higher than that of a cell prepared by the traditional SILAR process

Poly(ethylene oxide)-co-Poly(propylene oxide)-Based Gel Electrolyte with High Ionic Conductivity and Mechanical Integrity for Lithium-Ion Batteries

Using gel polymer electrolytes (GPEs) for lithium-ion batteries usually encounters the drawback of poor mechanical integrity of the GPEs. This study demonstrates the outstanding performance of a GPE consisting of a commercial membrane (Celgard) incorporated with a poly­(ethylene oxide)-co-poly­(propylene oxide) copolymer (P­(EO-co-PO)) swelled by a liquid electrolyte (LE) of 1 M LiPF₆ in carbonate solvents. The proposed GPE stably holds LE with an amount that is three times that of the Celgard-P­(EO-co-PO) composite. This GPE has a higher ionic conductivity (2.8 × 10^–3 and 5.1 × 10^–4 S cm^–1 at 30 and −20 °C, respectively) and a wider electrochemical voltage range (5.1 V) than the LE-swelled Celgard because of the strong ion-solvation power of P­(EO-co-PO). The active ion-solvation role of P­(EO-co-PO) also suppresses the formation of the solid–electrolyte interphase layer. When assembling the GPE in a Li/LiFePO₄ battery, the P­(EO-co-PO) network hinders anionic transport, producing a high Li⁺ transference number of 0.5 and decreased the polarization overpotential. The Li/GPE/LiFePO₄ battery delivers a discharge capacity of 156–135 mAh g^–1 between 0.1 and 1 C-rates, which is approximately 5% higher than that of the Li/LE/LiFePO₄ battery. The IR drop of the Li/GPE/LiFePO₄ battery was 44% smaller than that of the Li/LE/LiFePO₄. The Li/GPE/LiFePO₄ battery is more stable, with only a 1.2% capacity decay for 150 galvanostatic charge–discharge cycles. The advantages of the proposed GPE are its high stability, conductivity, Li⁺ transference number, and mechanical integrity, which allow for the assembly of GPE-based batteries readily scalable to industrial levels

Graphite Oxide with Different Oxygenated Levels for Hydrogen and Oxygen Production from Water under Illumination: The Band Positions of Graphite Oxide

Graphite oxide (GO) photocatalysts derived from graphite oxidation can have varied electronic properties by varying the oxidation level. Absorption spectroscopy shows the increasing band gap of GO with the oxygen content. Electrochemical analysis along with the Mott–Schottky equation show that the conduction and valence band edge levels of GO from appropriate oxidation are suitable for both the reduction and the oxidation of water. The conduction band edge shows little variation with the oxidation level, and the valence band edge governs the bandgap width of GO. The photocatalytic activity of GO specimens with various oxygenated levels was measured in methanol and AgNO₃ solutions for evolution of H₂ and O₂, respectively. The H₂ evolution was strong and stable over time, whereas the O₂ evolution was negligibly small due to mutual photocatalytic reduction of the GO with upward shift of the valence band edge under illumination. The conduction band edge of GO showed a negligible change with the illumination. When NaIO₃ was used as a sacrificial reagent to suppress the mutual reduction mechanism under illumination, strong O₂ evolution was observed over the GO specimens. The present study demonstrates that chemical modification can easily modify the electronic properties of GO for specific photosynthetic applications

Tuning the Electronic Structure of Graphite Oxide through Ammonia Treatment for Photocatalytic Generation of H₂ and O₂ from Water Splitting

Graphite oxide (GO) synthesized from the oxidation of graphite powders exhibits p-type conductivity and is active in photocatalytic H₂ evolution from water decomposition. The p-type conductivity hinders hole transfer for water oxidation and suppresses O₂ evolution. Treating GO with NH₃ gas at room temperature tunes the electronic structure by introducing amino and amide groups to its surface. The ammonia-modified GO (NGO) exhibits n-type conductivity in photoelectrochemical analysis and has a narrower optical band gap than GO. Electrochemical analysis attributes the band gap reduction to a negative shift of the valence band. An NGO-film electrode exhibits a substantially higher incident photo-to-current efficiency in the visible light region than a GO electrode. Photoluminescence analyses demonstrate the above-edge emission characteristic of GO and NGO. NH₃ treatment enhances the emission by removing nonirradiative epoxy and carboxyl sites on the GO. In half-reaction tests of water decomposition, NGO effectively catalyzes O₂ evolution in an aqueous AgNO₃ solution under mercury-lamp irradiation, whereas GO is inactive. NGO also effectively catalyzes H₂ evolution in an aqueous methanol solution but shows less activity than GO. Under illumination with visible light (λ > 420 nm), NGO simultaneously catalyzes H₂ and O₂ evolutions, but with a H₂/O₂ molar ratio below 2. The n-type conductivity of NGO may hinder electron transfer and form peroxide species instead of H₂ molecules. This study demonstrates that the functionality engineering of GO is a promising technique to synthesize an industrially scalable photocatalyst for overall water splitting

Graphene Oxide Sponge as Nanofillers in Printable Electrolytes in High-Performance Quasi-Solid-State Dye-Sensitized Solar Cells

A graphene oxide sponge (GOS) is utilized for the first time as a nanofiller (NF) in printable electrolytes (PEs) based on poly­(ethylene oxide) and poly­(vinylidene fluoride) for quasi-solid-state dye-sensitized solar cells (QS-DSSCs). The effects of the various concentrations of GOS NFs on the ion diffusivity and conductivity of electrolytes and the performance of the QS-DSSCs are studied. The results show that the presence of GOS NFs significantly increases the diffusivity and conductivity of the PEs. The introduction of 1.5 wt % of GOS NFs decreases the charge-transfer resistance at the Pt-counter electrode/electrolyte interface (<i>R</i>_pt) and increases the recombination resistance at the photoelectrode/electrolyte interface (<i>R</i>_ct). QS-DSSC utilizing 1.5 wt % GOS NFs can achieve an energy conversion efficiency (8.78%) higher than that found for their liquid counterpart and other reported polymer gel electrolytes/GO NFs based DSSCs. The high energy conversion efficiency is a consequence of the increase in both the open-circuit potential (<i>V</i>_oc) and fill factor with a slight decrease in current density (<i>J</i>_sc). The cell efficiency can retain 86% of its initial value after a 500 h stability test at 60 °C under dark conditions. The long-term stability of the QS-DSSC with GOS NFs is higher than that without NFs. This result indicates that the GOS NFs do not cause dye-desorption from the photoanode in a long-term stability test, which infers a superior performance of GOS NFs as compared to TiO₂ NFs in terms of increasing the efficiency and long-term stability of QS-DSSCs

Photocatalytically Reduced Graphite Oxide Electrode for Electrochemical Capacitors

Graphene sheets are an ideal carbon material with the highest area available for electrolyte interaction and can be obtained by reducing graphite oxide (GO). This study presents the photocatalytic reduction of GO in water with mercury-lamp irradiation. The specific capacitance of the reduced GO in an H₂SO₄ aqueous solution reached levels as high as 220 F g^–1. This is because of the double layer formation and the reversible pseudocapacitive processes caused by oxygen functionalities at the sheet periphery. The rate capability for charge storage increases with irradiation time due to the continued reduction of oxygenated sites on the graphene basal plane. Alternating current impedance analysis shows that prolonged light irradiation promotes electronic percolation in the electrode, significantly reducing the capacitive relaxation time. With a potential widow of 1 V, the resulting symmetric cells can deliver an energy level of 5 Wh kg^–1 at a high power of 1000 W kg^–1. These cells show superior stability, with 92% retention of specific capacitance after 20 000 cycles of galvanostatic charge–discharge

Immobilization of Anions on Polymer Matrices for Gel Electrolytes with High Conductivity and Stability in Lithium Ion Batteries

This study reports on a high ionic-conductivity gel polymer electrolyte (GPE), which is supported by a TiO₂ nanoparticle-decorated polymer framework comprising poly­(acrylonitrile-<i>co</i>-vinyl acetate) blended with poly­(methyl methacrylate), i.e., PAVM:TiO₂. High conductivity GPE-PAVM:TiO₂ is achieved by causing the PAVM:TiO₂ polymer framework to swell in 1 M LiPF₆ in carbonate solvent. Raman analysis results demonstrate that the poly­(acrylonitrile) (PAN) segments and TiO₂ nanoparticles strongly adsorb PF₆^– anions, thereby generating 3D percolative space-charge pathways surrounding the polymer framework for Li⁺-ion transport. The ionic conductivity of GPE-PAVM:TiO₂ is nearly 1 order of magnitude higher than that of commercial separator-supported liquid electrolyte (SLE). GPE-PAVM:TiO₂ has a high Li⁺ transference number (0.7), indicating that most of the PF₆^– anions are stationary, which suppresses PF₆^– decomposition and substantially enlarges the voltage that can be applied to GPE-PAVM:TiO₂ (to 6.5 V vs Li/Li⁺). Immobilization of PF₆^– anions also leads to the formation of stable solid-electrolyte interface (SEI) layers in a full-cell graphite|electrolyte|LiFePO₄ battery, which exhibits low SEI and overall resistances. The graphite|electrolyte|LiFePO₄ battery delivers high capacity of 84 mAh g^–1 even at 20 C and presents 90% and 71% capacity retention after 100 and 1000 charge–discharge cycles, respectively. This study demonstrates a GPE architecture comprising 3D space charge pathways for Li⁺ ions and suppresses anion decomposition to improve the stability and lifespan of the resulting LIBs

Elucidating Quantum Confinement in Graphene Oxide Dots Based On Excitation-Wavelength-Independent Photoluminescence

Investigating quantum confinement in graphene under ambient conditions remains a challenge. In this study, we present graphene oxide quantum dots (GOQDs) that show excitation-wavelength-independent photoluminescence. The luminescence color varies from orange-red to blue as the GOQD size is reduced from 8 to 1 nm. The photoluminescence of each GOQD specimen is associated with electron transitions from the antibonding π (π*) to oxygen nonbonding (n-state) orbitals. The observed quantum confinement is ascribed to a size change in the sp² domains, which leads to a change in the π*−π gap; the n-state levels remain unaffected by the size change. The electronic properties and mechanisms involved in quantum-confined photoluminescence can serve as the foundation for the application of oxygenated graphene in electronics, photonics, and biology