Novel electrode and electrolyte membranes: Towards flexible dye-sensitized solar cell combining vertically aligned TiO 2 nanotube array and light-cured polymer network

Dye-sensitized solar cells strike back

Dye-sensitized solar cells (DSCs) are celebrating their 30th birthday and they are attracting a wealth of research efforts aimed at unleashing their full potential. In recent years, DSCs and dye-sensitized photoelectrochemical cells (DSPECs) have experienced a renaissance as the best technology for several niche applications that take advantage of DSCs' unique combination of properties: at low cost, they are composed of non-toxic materials, are colorful, transparent, and very efficient in low light conditions. This review summarizes the advancements in the field over the last decade, encompassing all aspects of the DSC technology: theoretical studies, characterization techniques, materials, applications as solar cells and as drivers for the synthesis of solar fuels, and commercialization efforts from various companies.

A single layer deposition of Li-doped mesoporous TiO2 beads for low-cost and efficient dye-sensitized solar cells

Cost-effective DSCs with superior electronic properties are gained by a reduction in electronic trap states and outstanding light scattering and harvesting.

Encapsulating MoO2 Nanocrystals into Flexible Carbon Nanofibers via Electrospinning for High-Performance Lithium Storage

Design and synthesis of flexible and self-supporting electrode materials in high-performance lithium storage is significant for applications in the field of smart wearable devices. Herein, flexible carbon nanofiber membranes with uniformly distributed molybdenum dioxide (MoO₂) nanocrystals are fabricated by a needlefree electrospinning method combined with the subsequent carbonization process, which exhibits outstanding structural stability under abrasion and deformation. The as-fabricated lithium-ion batteries (LIBs) exhibit a high discharge of 450 mAh g⁻¹ after 500 cycles at 2000 mA g⁻¹ by using the MoO₂/C nanofiber membrane as the self-supporting anode. Further, the nanofibers structure remains intact after 500 cycles, which reflects the excellent stability of the materials. This study provides a simple and effective method for the preparation of MoO₂/C nanofiber materials, which can not only maintain its excellent electrochemical and physical properties, but also easily realize large-scale production. It is undoubtedly beneficial for the development of flexible LIBs and smart wearable devices.

Mesoporous TiO2 anatase films for enhanced photocatalytic activity under UV and visible light

Mesoporous TiO2 films with enhanced photocatalytic activity in both UV and visible wavelength ranges were developed through a non-conventional atomic layer deposition (ALD) process at room temperature. Deposition at such a low temperature promotes the accumulation of by-products in the amorphous TiO2 films, caused by the incomplete hydrolysis of the TiCl4 precursor. The additional thermal annealing induces the fast recrystallisation of amorphous films, as well as an in situ acidic treatment of TiO2. The interplay between the deposition parameters, such as purge time, the amount of structural defects introduced and the enhancement of the photocatalytic properties from different mesoporous films clearly shows that our easily upscalable non-conventional ALD process is of great industrial interest for environmental remediation and other photocatalytic applications, such as hydrogen production.

N- and C-Modified TiO₂ Nanotube Arrays: Enhanced Photoelectrochemical Properties and Effect of Nanotubes Length on Photoconversion Efficiency

In this investigation, a new, facile, low cost and environmental-friendly method was introduced to fabricate N- and C-modified TiO₂ nanotube arrays by immersing the as-anodized TiO₂ nanotube arrays (TNTAs) in a urea aqueous solution with mechanical agitation for a short time and keeping the TNTAs immersed in the solution for 6 h at room temperature. Then, the TNTAs were annealed at different temperatures. The produced N-, C-modified TNTAs were characterized using FESEM, EDX, XRD, XPS, UV-Vis diffuse reflectance spectra. Modified optical properties with narrow band gap energy, E_g, of 2.65 eV was obtained after annealing the modified TNTAs at 550 °C. Modified TNTAs showed enhanced photoelectochemical performance. Photoconversion efficiency (PCE) was increased from 4.35% for pristine (unmodified) TNTAs to 5.18% for modified TNTAs, an increase of 19%. Effect of nanotubes length of modified TNTAs on photoelectrochemical performance was also studied. Photocurrent density and PCE were increased by increasing nanotube length with a maximum PCE of 6.38% for nanotube length of 55 µm. This high PCE value was attributed to: band gap reduction due to C- and N-modification of TNTAs surface, increased surface area of long TNTAs compared with short TNTAs, investigated in previous studies.

Evolution of nanomechanical properties and crystallinity of individual titanium dioxide nanotube resonators

Herein a complete characterization of single TiO₂ nanotube resonator was reported for the first time. The modal vibration response analysis allows a non-invasive indirect evaluation of the mechanical properties of the TiO₂ nanotube. The effect of post-grown thermal treatments on nanotube mechanical properties was investigated and carefully correlated to the chemico-physical parameters evolution. The Young's modulus of TiO₂ nanotube rises linearly from 57 GPa up to 105 GPa for annealing at 600 °C depending on the compositional and crystallographic evolution of the nanostructure. Considering the growing interest in single nanostructure devices, the reported findings allow a deeper understanding of the properties of individual titanium dioxide nanotubes extrapolated from their standard arrayed architecture.

Unveiling iodine-based electrolytes chemistry in aqueous dye-sensitized solar cells

Aqueous dye-sensitized solar cells (DSSCs) have recently emerged as promising systems, which can combine low cost and environmental compatibility with appreciable efficiency, long-term durability and enhanced safety. In the present study, we thoroughly investigate the chemistry behind the iodide/triiodide-based redox mediator, which presents - in a completely aqueous environment - several differences when compared to the behavior observed in the conventionally used organic solvents. The speciation of ions, the effect of the concentration of the redox mediator and the type of counter-ion are characterized from the electrochemical, spectroscopic, photovoltaic and analytical viewpoints. Furthermore, we demonstrate that aqueous DSSCs, often assumed as unstable, hold the potential to assure unparalleled stability after five months of aging without any addition of stabilizers or gelling agents, thus envisaging the construction of eco-friendly photovoltaic devices free of expensive, flammable and toxic solvents.

Photopolymer Electrolytes for Sustainable, Upscalable, Safe, and Ambient-Temperature Sodium-Ion Secondary Batteries

The first example of a photopolymerized electrolyte for a sodium-ion battery is proposed herein. By means of a preparation process free of solvents, catalysts, purification steps, and separation steps, it is possible to obtain a three-dimensional polymeric network capable of efficient sodium-ion transport. The thermal properties of the resulting solid electrolyte separator, characterized by means of thermogravimetric and calorimetric techniques, are excellent for use in sustainable energy systems conceived for safe large-scale grid storage. The photopolymerized electrolyte shows a wide electrochemical stability window up to 4.8 V versus Na/Na(+) along with the highest ionic conductivity (5.1 mS cm(-1) at 20 °C) obtained in the field of Na-ion polymer batteries so far and stable long-term constant-current charge/discharge cycling. Moreover, the polymeric networks are also demonstrated for the in situ fabrication of electrode/electrolyte composites with excellent interfacial properties, which are ideal for all-solid-state, safe, and easily upscalable device assembly.

Dispelling clichés at the nanoscale: the true effect of polymer electrolytes on the performance of dye-sensitized solar cells

In the field of dye-sensitized solar cells, polymer electrolytes are among the most studied materials due to their ability to ensure both high efficiency and stability, the latter being a critical point of these devices. Hundreds of polymeric matrices have been proposed over the years, and their functionalization with several groups, the variation of their molecular weight and the tuning of the crosslinking degree have been investigated. However, the true effect that polymeric matrices have on the cell parameters has often been addressed superficially, and hundreds of papers justify the obtained results with a simple bibliographic reference to other systems (sometimes completely different). This work proposes a system of nanoscale growth and crosslinking of a polymer electrolyte inside a nanostructured photoanode. Electrochemical and photovoltaic parameters are carefully monitored as a function of thickness and degree of penetration of the electrolyte. The results derived from this study refute many clichés generally accepted and taken for granted in many literature articles, and – for the first time – a compromise between the amount of polymer, cell efficiency and stability is achieved.

Electrodes/electrolyte interfaces in the presence of a surface-modified photopolymer electrolyte: application in dye-sensitized solar cells

Since hundreds of studies on photoanodes and cathodes show that the electrode/electrolyte interfaces represent a key aspect at the base of dye-sensitized solar cell (DSSC) performances, it is reported here that these interfaces can be managed by a smart design of the spatial composition of quasi-solid electrolytes. By means of a cheap, rapid, and green process of photoinduced polymerization, composition-tailored polymer electrolyte membranes (PEMs) with siloxane-enriched surfaces are prepared, and their properties are thoroughly described. When assembled in DSSCs, the interfacial action promoted by the composition-tailored PEMs enhances the photocurrent and fill factor values, thus increasing the global photovoltaic conversion efficiency with respect to the non-modified PEMs. Moreover, the presence of the siloxane-chain-enriched surface increases the hydrophobicity and reduces the water vapor permeation into the device, thus enhancing the cell's durability.

Aqueous dye-sensitized solar cells

This review highlights the efforts towards the realization of an artificial photosynthetic system able to convert sunlight into electricity by using a unique solvent, water, the solvent of life.