Composite short-side-chain PFSA electrolyte membranes containing selectively modified halloysite nanotubes (HNTs)

Aquivion membrane displays improved properties as compared to Nafion membrane, partly due to shorter side chains. However, some improvements are still necessary for proton exchange membrane fuel cell to operate at low relative humidity. To overcome this drawback, the addition of clay nanoparticle into the Aquivion matrix can be considered. In this study, different composite membranes have been prepared mixing short-side-chain PFSA (perfluorosulfonic acid) Aquivion and selectively modified halloysite nanotubes for PEMFC low relative humidity operation. Halloysites were grafted with fluorinated groups, sulfonated groups, or perfluoro-sulfonated groups on inner or outer surface of the tubes. The obtained composite membranes showed improved properties, especially higher water uptake associated with reduced swelling and better mechanical strength compared to pristine Aquivion membrane and commercially available Nafion HP used as reference. The best performance in this study was obtained with Aquivion loaded with 5 wt% of pretreated perfluoro-sulfonated halloysite. The composite membrane, referred to as Aq/pHNT-SF5, displayed the largest water uptake and proton conductivity among the panel of membranes tested. The chemical stability was not affected by the presence of halloysite in the Aquivion matrix.

Graphical abstract

     

Impact of three different TiO2 morphologies on hydrogen evolution by methanol assisted water splitting: Nanoparticles, nanotubes and aerogels

Increasing the activity of a photocatalyst goes through the improvement of both its absorption (light) and adsorption (reactant) properties. For a given semiconducting material, the charge carrier separation is also a very important step. Properly combining chosen phases is one option to improve this separation (example of the commercial P25) and depositing platinum on the surface of the catalyst, another one. In some cases, coupling both may nevertheless lead to a decrease of photoactivity or at least limit the potentiality of the catalyst. A third option, consisting in modifying the morphology of the photoactive phase, has shown very promising results.In this study, we have elaborated, characterized and evaluated the hydrogen evolution potentiality (through methanol assisted water splitting) of different TiO₂ morphologies: nanoparticles, nanotubes and aerogels. These materials have shown different behaviours depending on both their composition and morphology. Different types of separation processes have been claimed to account for the observed different photoactivities, with more or less pronounced synergetic effects, due to: the use of Pt as a co-catalyst, the mixture of different TiO₂ phases (anatase and TiO₂(B) or rutile) and the specific morphology of the samples (nanotubes or aerogels). Among all the tested samples, the TiO₂ aerogel supported Pt one exhibited very promising performances, three times as active as P25 supported Pt, which is already much more active than pure P25 in our testing conditions.     

Synthesis reaction of metatitanate BaTiO3

Thermogravimetric and X-ray diffraction analysis of the reaction mixture, during the synthesis of barium metatitanate BaTiO₃ from BaCO₃ and TiO₂ allowed clarification of the transformation time and the role of the reaction environment: in air, the occurrence of BaTiO₃ is in competition with that of orthotitanate Ba₂TiO₄. The latter forms between 25 and 95% of the transformation of the initial mixture by reaction between BaCO₃ and BaTiO₃. Under vacuum, metatitanate is not detected before 70% of transformation of the initial mixture. The orthotitanate occurs alone, oxide BaO produced by the decomposition of the carbonate and replaces it in the reaction syntheses. In CO₂, the synthesis of BaTiO₃ competes with the formation of a phase other than Ba₂TiO₄ but whose behaviour during calcination is comparable to that of the orthotitanate.     

Correlations between the catalytic layer composition,the relative humidity and the performance for PEMFC carbon aerogel based membrane electrode assemblies

The parameters influencing the water management or the gas diffusion have a large impact on the performances of the PEMFC. In this work, we focused on the influence of the composition of the cathode catalytic layer on the performance of the MEA in relation to the relative humidity. A carbon aerogel was synthesized as catalyst support with a texture minimizing diffusive losses. We studied the influence of the Nafion content by preparing various cathode catalytic layers whose hydric properties were modified by adding PTFE as a hydrophobic agent. We evaluated the impact of the cathodic relative humidity by testing MEAs on a single cell test bench. We showed that modifying the composition of the catalytic layers playing on the content of both Nafion and PTFE, enables to improve the performance due to a better water management. The best performance was obtained with a Nafion/Carbon mass ratio (N/C) of 0.5 and a PTFE/Carbon (PTFE/C) mass ratio of 0.35 allowing 40% more power at 0.4 V and 75%RH. A first positive evaluation of the impact of PTFE is also done with TEC10E40E.     

Role and behaviour of orthotitanate Ba2TiO4 during the processing of BaTiO3 based ferroelectric ceramics

Some anomalies observed in the dielectric properties of ceramics made from calcines of BaTiO₃ are due to the presence of Ba₂TiO₄. These effects are only negligible if the Ba₂TiO₄ concentration in the calcine is less than 1 or 2 wt %. For concentrations greater than this, Ba₂TiO₄ inhibits the growth of BaTiO₃ grains, shifts the Curie point towards low temperatures, decreasesε _r and causes large fluctuations in tanδ curves.     

Influence of the carbon texture of platinum/carbon aerogel electrocatalysts on their behavior in a proton exchange membrane fuel cell cathode

Seven carbon aerogels (CAs) with different pore size distributions were synthesized and used as a catalyst support in proton exchange membrane fuel cell (PEMFC) cathodes. The cathodes were tested in 50 cm² membrane electrode assemblies. The results demonstrate that the CA texture significantly influences performance by impacting gas diffusion and proton transport. Also, the Nafion^® loading must be adapted to the CA texture, since its easier penetration into large pores favors their obstruction and results in higher proton resistance and mass-transport voltage losses. Under fixed experimental conditions (notably Nafion^®/carbon ratio = 1), the best CA support displays high specific surface area and pore volume, a majority of mesopores with a pore size distribution peak around 25–30 nm and with some macropores. The work confirms that the carbon support structure must be controlled to reduce mass-transport voltage losses. Doing so would lead to reduce PEMFC overall cost per kW.     

Alkoxide-Hydroxide Route to Syntheltize BaTiO3-Based Powders

When preparing homogeneous, fine barium titanate powders, the major difficulty is to avoid the spontaneous self-condensation between the Ti-OH groups. In the usual way of preparing fine barium titanate powders, chelating agents (citrate, oxalate) or simply unidentate ligands (alkoxy or carboxyl groups) are used to complex titanium atoms. Another way is to mix barium and titanium precursors in a strongly basic medium. The condensation between the Ti(OH)^2-₆Ba²⁺ species directly gives the perovskite compound. Using an alkoxide-hydroxide route, a homogeneous Ba-Ti solution was prepared that completely advanced by condensation between the Ti(OH)^2-₆Ba²⁺ species and led to a controlled-stoichiometry powder. Concerning pure barium titanate, dried powders exhibited the cubic perovskite structure, and a direct sintering at 1150°C, without calcination, led to highly dense BaTiO₃ bodies with fine-grained uniform microstructure (1 μm) that exhibited a high permittivity value at room temperature ( K = 5400). The alkoxide-hydroxide method was also used to prepare dense alkaline-earth perovskite ceramics with complex compositions.