Thick titania films with hierarchical porosity assembled from ultrasmall titania nanoparticles as photoanodes for dye-sensitized solar cells

Thin mesoporous titania films prepared by surfactant templating feature some of the highest light conversion efficiencies per thickness ratios as anodes in dye-sensitized solar cells (DSCs). However, the fabrication of thicker films required for sufficient light absorption is very challenging using this approach, often resulting in cracking and delamination of the films. Here we present a simple and scalable method to prepare thick mesoporous titania photoanodes via a surfactant-directed assembly of crystalline ultrasmall TiO2 nanoparticles in combination with phase separation due to ethyl cellulose added to the coating solutions. Along with increasing film thickness, the ethyl cellulose introduces an interpenetrating macropore network into the films, leading to the formation of hierarchical porous films with bimodal porosity, with the smaller mesopores resulting from the structure-directing agent, Pluronic F127. In this way, films of up to 2 mm per layer without delamination can be produced, exhibiting a high surface area of 130 m(2) g(-1), about twice the value of films based on standard TiO2 nanoparticle paste. The preparation of multilayer films by a sequential spin-coating and calcination procedure enables the production of films with an overall thickness of up to 10 mm in only 5 steps, which showed high efficiencies of 7.7% in dye-sensitized solar cells

Tuning the crystallinity parameters in macroporous titania films

Although macroporous titania scaffolds are used for many different applications, not much is known about the importance of the synthesis strategy on the resulting materials' properties. We present a comparative study on the influence of different colloidal titania precursors for direct co-deposition with poly(methyl methacrylate) (PMMA) beads on the properties of the resulting macroporous scaffolds after calcination. The colloidal titania precursors for the film assembly differ in their size and initial crystallinity, ranging from amorphous sol-gel clusters to already crystalline pre-formed particles of 4 nm, 6 nm and 20 nm in size, as well as a combination of sol-gel and nanoparticle precursors in the so-called `Brick and Mortar' approach. The type of the precursor greatly influences the morphology, texture and the specific crystallinity parameters of the macroporous titania scaffolds after calcination such as the size of the crystalline domains, packing density of the crystallites in the macroporous walls and interconnectivity between the crystals. Moreover, the texture and the crystallinity of the films can be tuned by postsynthesis processing of the films such as calcination at different temperatures, which can be also preceded by a hydrothermal treatment. The ability to adjust the porosity, the total surface area and the crystallinity parameters of the crystalline macroporous films by selecting suitable precursors and by applying different post-synthetic treatments provides useful tools to optimize the film properties for different applications

All-inorganic core-shell silica-titania mesoporous colloidal nanoparticles showing orthogonal functionality

Colloidal mesoporous silica (CMS) nanoparticles with a thin titania-enriched outer shell showing a spatially resolved functionality were synthesized by a delayed co-condensation approach. The titaniashell can serve as a selective nucleation site for the growth of nanocrystalline anatase clusters. These fully inorganic pure silica-core titania-enriched shell mesoporous nanoparticles show orthogonal functionality, demonstrated through the selective adsorption of a carboxylate-containing ruthenium N3-dye. UV-Vis and fluorescence spectroscopy indicate the strong interaction of the N3-dye with the titania-phase at the outer shell of the CMS nanoparticles. In particular, this interaction and thus the selective functionalization are greatly enhanced when anatase nanocrystallites are nucleated at the titania-enriched shell surface

Oxide ceramic electrolytes for all-solid-state lithium batteries – cost-cutting cell design and environmental impact

All-solid-state batteries are a hot research topic due to the prospect of high energy density and higher intrinsic safety, compared to conventional lithium-ion batteries. Of the wide variety of solid-state electrolytes currently researched, oxide ceramic lithium-ion conductors are considered the most difficult to implement in industrial cells. Although their high lithium-ion conductivity combined with a high chemical and thermal stability make them a very attractive class of materials, cost-cutting synthesis and scalable processing into full batteries remain to be demonstrated. Additionally, they are Fluorine-free and can be processed in air but require one or more high temperature treatment steps during processing counteracting their ecological benefits. Thus, a viable cell design and corresponding assessment of its ecological impact is still missing. To close this gap, we define a target cell combining the advantages of the two most promising oxidic electrolytes, lithium lanthanum zirconium oxide (LLZO) and lithium aluminium titanium phosphate (LATP). Even though it has not been demonstrated so far, the individual components are feasible to produce with state-of-the-art industrial manufacturing processes. This model cell then allows us to assess the environmental impact of the ceramic electrolyte synthesis and cell component manufacturing not just on an abstract level (per kg of material) but also with respect to their contributions to the final cell. The in-depth life cycle assessment (LCA) analysis revealed surprising similarities between oxide-based all-solid-state batteries and conventional Li-ion batteries. The overall LCA inventory on the material level is still dominated by the cathode active material, while the fabrication through ceramic manufacturing processes is a major contributor to the energy uptake. A clear path that identifies relevant research and development directions in terms of economic benefits and environmental sustainability could thus be developed to promote the competitiveness of oxide based all-solid-state batteries in the market

Optimizing the Composite Cathode Microstructure in All-Solid-State Batteries by Structure-Resolved Simulations

All-solid-state batteries are considered as an enabler for applications requiring high energy and power density. However, they still fall short of their theoretical potential due to various limitations. One issue is poor charge transport kinetics resulting from both material inherit limitations and non-optimized design. Therefore, a better understanding of the relevant properties of the cathode microstructure is necessary to improve cell performance. In this article, we identify optimization potentials of the composite cathode by structure-resolved electrochemical 3D-simulations. In our simulation study, we investigate the influence of cathode active material fraction, density, particle size, and active material properties on cell performance. Special focus is set on the impact of grain boundaries on the cathode design. Based on our simulation results, we can predict target values for cell manufacturing and reveal promising optimization strategies for an improved cathode design

Highly soluble energy relay dyes for dye-sensitized solar cells

High solubility is a requirement for energy relay dyes (ERDs) to absorb a large portion of incident light and significantly improve the efficiency of dye-sensitized solar cells (DSSCs). Two benzonitrile-soluble ERDs, BL302 and BL315, were synthesized, characterized, and resulted in a 65% increase in the efficiency of TT1-sensitized DSSCs. The high solubility (180 mM) of these ERDs allows for absorption of over 95% of incident light at their peak wavelength. The overall power conversion efficiency of DSSCs with BL302 and BL315 was found to be limited by their energy transfer efficiency of approximately 70%. Losses due to large pore size, dynamic collisional quenching of the ERD, energy transfer to desorbed sensitizing dyes and static quenching by complex formation were investigated and it was found that a majority of the losses are caused by the formation of statically quenched ERDs in solution

Kinetics and Pore Formation of the Sodium Metal Anode on NASICON‐Type Na3.4_{3.4} Zr2_2Si2.4_{2.4}P0.6_{0.6}O12_{12} for Sodium Solid‐State Batteries

In recent years, many efforts have been made to introduce reversible alkali metal anodes using solid electrolytes in order to increase the energy density of next-generation batteries. In this respect, Na$_{3.4}$Zr$_2$Si$_{2.4}$P$_{0.6}$O$_{12}$ is a promising solid electrolyte for solid-state sodium batteries, due to its high ionic conductivity and apparent stability versus sodium metal. The formation of a kinetically stable interphase in contact with sodium metal is revealed by time-resolved impedance analysis, in situ X-ray photoelectron spectroscopy, and transmission electron microscopy. Based on pressure- and temperature-dependent impedance analyses, it is concluded that the Na|Na$_{3.4}$Zr$_2$Si$_{2.4}$P$_{0.6}$O$_{12}$ interface kinetics is dominated by current constriction rather than by charge transfer. Cross-sections of the interface after anodic dissolution at various mechanical loads visualize the formed pore structure due to the accumulation of vacancies near the interface. The temporal evolution of the pore morphology after anodic dissolution is monitored by time-resolved impedance analysis. Equilibration of the interface is observed even under extremely low external mechanical load, which is attributed to fast vacancy diffusion in sodium metal, while equilibration is faster and mainly caused by creep at increased external load. The presented information provides useful insights into a more profound evaluation of the sodium metal anode in solid-state batteries