Guided in Situ Polymerization of MEH-PPV in Mesoporous Titania Photoanodes

Incorporation of conjugated polymers into porous metal oxide networks is a challenging task, which is being pursued via many different approaches. We have developed the guided in situ polymerization of poly­(2-methoxy-5-(2′-ethylhexyloxy)-<i>p</i>-phenylenevinylene) (MEH-PPV) in porous titania films by means of surface functionalization. The controlled polymerization via the Gilch route was induced by an alkoxide base and by increasing the temperature. The selected and specially designed surface-functionalizing linker molecules mimic the monomer or its activated form, respectively. In this way, we drastically enhanced the amount of MEH-PPV incorporated into the porous titania phase compared to nonfunctionalized samples by a factor of 6. Additionally, photovoltaic measurements were performed. The devices show shunting or series resistance limitations, depending on the surface functionalization prior to in situ polymerization of MEH-PPV. We suggest that the reason for this behavior can be found in the orientation of the grown polymer chains with respect to the titania surface. Therefore, the geometry of the anchoring via the linker molecules is relevant for exploiting the full electronic potential of the conjugated polymer in the resulting hybrid composite. This observation will help to design future synthesis methods for new hybrid materials from conjugated polymers and n-type semiconductors to take full advantage of favorable electronic interactions between the two phases

Multilayered High Surface Area “Brick and Mortar” Mesoporous Titania Films as Efficient Anodes in Dye-Sensitized Solar Cells

The “brick and mortar” approach is employed to synthesize thick surfactant-templated mesoporous titanium dioxide films of up to 10 μm thickness using multilayer deposition. The films exhibit very high surface areas scaling linearly with the thickness, and roughness factors of up to 1600 cm²/cm² can be reached. For the first time, surfactant-derived mesoporous titanium dioxide films of such a large thickness and surface area can be prepared without serious cracking, delamination, or deterioration of the porous structure. The mesopores are rather large (12 nm), and stacking many layers does not affect their size or accessibility, which is shown by krypton and dye adsorption experiments. Applied in dye-sensitized solar cells, the films feature a high power conversion efficiency of over 7% already at thicknesses below 4 μm due to their high surface area and dye adsorption

Aqueous Processing of LiCoO₂–Li_6.6La₃Zr_1.6Ta_0.4O₁₂ Composite Cathode for High-Capacity Solid-State Lithium Batteries

To fabricate ceramic composite cathodes LiCoO2–Li6.6La3Zr1.6Ta0.4O12 (LCO-LLZTO) on an industrial scale, a water-based tape-casting process was developed, which is scalable and environmentally friendly. Additionally, the cosintering behavior of the two materials, often leading to poor electrochemical performance, was optimized via a Li2O-rich atmosphere. The resulting dense, free-standing, and phase-pure LCO-LLZTO mixed cathodes were assembled into full cells using a dual-layer solid polymer-ceramic separator and an In–Li anode. These cells show very high utilization rates for LCO of approximately 90% at a high areal capacity of over 3 mAh cm–2, demonstrating the potential of water-based tape-casting for a scalable and sustainable manufacturing of oxide-ceramic based solid-state Li batteries

Tailoring the Morphology of Mesoporous Titania Thin Films through Biotemplating with Nanocrystalline Cellulose

The tunable porosity of titania thin films is a key factor for successful applications in photovoltaics, sensing, and photocatalysis. Here, we report on nanocrystalline cellulose (NCC) as a novel shape-persistent templating agent enabling the straightforward synthesis of mesoporous titania thin films. The obtained structures are highly porous anatase morphologies having well-defined, narrow pore size distributions. By varying the titania-to-template ratio, it is possible to tune the surface area, pore size, pore anisotropy, and dimensions of titania crystallites in the films. Moreover, a post-treatment at high humidity and subsequent slow template removal can be used to achieve pore widening; this treatment is also beneficial for the multilayer deposition of thick films. The resulting homogeneous transparent films can be directly spin- or dip- coated on glass, silicon, and transparent conducting oxide (TCO) substrates. The mesoporous titania films show very high activity in the photocatalytic NO conversion and in the degradation of 4-chlorophenol. Furthermore, the films can be successfully applied as anodes in dye-sensitized solar cells

Nanocellulose-Assisted Formation of Porous Hematite Nanostructures

We report the formation of porous iron oxide (hematite) nanostructures via sol–gel transformations of molecular precursors in the confined space of self-organized nanocrystalline cellulose (NCC) used as a shape-persistent template. The obtained structures are highly porous α-Fe₂O₃ (hematite) morphologies with a well-defined anisotropic porosity. The character of the porous nanostructure depends on the iron salt used as the precursor and the heat treatment. Moreover, a postsynthetic hydrothermal treatment of the NCC/iron salt composites strongly affects the crystal growth as well as the porous nanomorphology of the obtained hematite scaffolds. We demonstrate that the hydrothermal treatment alters the crystallization mechanism of the molecular iron precursors, which proceeds via the formation of anisotropic iron oxyhydroxide species. The nanocellulose templating technique established here enables the straightforward fabrication of a variety of mesoporous crystalline iron oxide scaffolds with defined porous structure and is particularly attractive for the processing of porous hematite films on different substrates

Investigation of the pH-Dependent Impact of Sulfonated Polyaniline on Bioelectrocatalytic Activity of Xanthine Dehydrogenase

We report on the pH-dependent bioelectrocatalytic activity of the redox enzyme xanthine dehydrogenase (XDH) in the presence of sulfonated polyaniline PMSA1 (poly­(2-methoxyaniline-5-sulfonic acid)-<i>co</i>-aniline). Ultraviolet–visible (UV-vis) spectroscopic measurements with both components in solution reveal electron transfer from the hypoxanthine (HX)-reduced enzyme to the polymer. The enzyme shows bioelectrocatalytic activity on indium tin oxide (ITO) electrodes, when the polymer is present. Depending on solution pH, different processes can be identified. It can be demonstrated that not only product-based communication with the electrode but also efficient polymer-supported bioelectrocatalysis occur. Interestingly, substrate-dependent catalytic currents can be obtained in acidic and neutral solutions, although the highest activity of XDH with natural reaction partners is in the alkaline region. Furthermore, operation of the enzyme electrode without addition of the natural cofactor of XDH is feasible. Finally, macroporous ITO electrodes have been used as an immobilization platform for the fabrication of HX-sensitive electrodes. The study shows that the efficient polymer/enzyme interaction can be advantageously combined with the open structure of an electrode material of controlled pore size, resulting in good processability, stability, and defined signal transfer in the presence of a substrate

Nanocellulose-Templated Porous Titania Scaffolds Incorporating Presynthesized Titania Nanocrystals

Nanocrystalline cellulose (NCC) is an abundant biogenic nanomaterial with unique properties that enables the efficient synthesis of mesoporous crystalline titania. We significantly enhance the photocatalytic activity of titania thin films by introducing solvothermally synthesized preformed anatase nanoparticles into a sol–gel based biotemplated titania scaffold. The resulting dual source titania thin films containing different amounts of preformed crystalline species were investigated by time-resolved microwave conductivity (TRMC) measurements and tested in the photocatalytic conversion of 4-chlorophenol. The gradual addition of preformed nanoparticles leads to a consistent increase of the mean size of titania crystalline domains, whereas the porosity of the composite is well-preserved due to the shape-persistent nature of the NCC template. Microwave conductivity studies establish increased photoconductivity of the films containing preformed anatase nanoparticles in comparison to that of films made without the nanoparticles. The synergistic features of the dual source titania, namely the improved crystalline properties brought by the preformed nanocrystals in combination with the high surface area provided by the NCC-templated sol–gel titania, result in a very high photocatalytic activity of the films in the photocatalytic decomposition of 4-chlorophenol. In quantitative terms, the dual source titania films prepared with 75% nanoparticles exhibit a first order degradation rate constant of 0.53 h^–1 (1.47 × 10⁻⁴ sec⁻¹), which strongly outperforms the activity of commercial P90 nanopowder showing a rate constant of 0.17 h^–1 (0.47 × 10⁻⁴ sec⁻¹) under the same conditions

Nanostructured Ternary FeCrAl Oxide Photocathodes for Water Photoelectrolysis

A sol–gel method for the synthesis of semiconducting FeCrAl oxide photocathodes for solar-driven hydrogen production was developed and applied for the production of meso- and macroporous layers with the overall stoichiometry Fe_0.84Cr_1.0Al_0.16O₃. Using transmission electron microscopy and energy-dispersive X-ray spectroscopy, phase separation into Fe- and Cr-rich phases was observed for both morphologies. Compared to prior work and to the mesoporous layer, the macroporous FeCrAl oxide photocathode had a significantly enhanced photoelectrolysis performance, even at a very early onset potential of 1.1 V vs RHE. By optimizing the macroporous electrodes, the device reached current densities of up to 0.68 mA cm^–2 at 0.5 V vs RHE under AM 1.5 with an incident photon-to-current efficiency (IPCE) of 28% at 400 nm without the use of catalysts. Based on transient measurements, this performance increase could be attributed to an improved collection efficiency. At a potential of 0.75 V vs RHE, an electron transfer efficiency of 48.5% was determined

Impact of Ni–Mn–Co–Al-Based Cathode Material Composition on the Sintering with Garnet Solid Electrolytes for All-Solid-State Batteries

A systematic and comprehensive study of the thermal stability of the cathode active materials LiNi1/3Mn1/3Co1/3O2 (NMC111), LiNi0.6Mn0.2Co0.2O2 (NMC622), LiNi0.8Mn0.1Co0.1O2 (NMC811), and LiNi0.8Co0.15Al0.05O2 (NCA) in combination with the garnet solid electrolyte Li6.45La3Zr1.6Ta0.4Al0.05O12 was performed, and the respective thermal stability limits in air were assessed. Compared to prior studies on such material mixtures, additional Zr-containing secondary phases were detected, which had not been taken into consideration in a previously published work. Here, these phases were successfully identified for the first time by a combination of X-ray diffraction, Raman spectroscopy, and microstructural analysis

Electron Collection in Host–Guest Nanostructured Hematite Photoanodes for Water Splitting: The Influence of Scaffold Doping Density

Nanostructuring has proven to be a successful strategy in overcoming the trade-off between light absorption and hole transport to the solid/electrolyte interface in hematite photoanodes for water splitting. The suggestion that poor electron (majority carrier) collection hinders the performance of nanostructured hematite electrodes has led to the emergence of host–guest architectures in which the absorber layer is deposited onto a transparent high-surface-area electron collector. To date, however, state of the art nanostructured hematite electrodes still outperform their host–guest counterparts, and a quantitative evaluation of the benefits of the host–guest architecture is still lacking. In this paper, we examine the impact of host–guest architectures by comparing nanostructured tin-doped hematite electrodes with hematite nanoparticle layers coated onto two types of conducting macroporous SnO₂ scaffolds. Analysis of the external quantum efficiency spectra for substrate (SI) and electrolyte side (EI) illumination reveals that the electron diffusion length in the host–guest electrodes based on an undoped SnO₂ scaffold is increased substantially relative to the nanostructured hematite electrode without a supporting scaffold. Nevertheless, electron collection is still incomplete for EI illumination. By contrast, an electron collection efficiency of 100% is achieved by fabricating the scaffold using antimony-doped SnO₂, showing that the scaffold conductivity is crucial for the device performance