Structure and dynamics of hydrogen in nanocomposite solid acids for fuel cell applications

Abstract

The transition to sustainable energy sources is inevitable. A possible future scenario could be the hydrogen economy, where the fuel cell plays an important role in the conversion of hydrogen back to electricity. The technology behind the fuel cell however, still has significant room for improvement. A next step would be the realization of so called intermediate temperature fuel cells. Solid acids have been shown to be promising candidates for fuel cell electrolytes as they possess high proton conductivity in the intermediate temperature range, from ambient up to 250 °C. One major problem of the solid acids was the low proton conduction at temperatures below their superprotonic phase transition. This has been solved by nanostructuring the materials, where the solid acid is mixed with e.g. TiO2 or SiO2 nanoparticles of 40 nm in size and smaller. The conductivity increases with orders of magnitude. Here a multi-technique approach is used to study this matter. Using Neutron Diffraction, direct experimental proof of the occurrence of space charge effects in these nanocomposites was shown in the form of deuterium ion intercalation in TiO2 nanoparticles together with deuterium depletion in the solid acid CsHSO4 phase. Very high, particle size dependent proton densities, up to 10% in the 7 nm TiO2 particles were found. Quasi elastic neutron scattering experiments showed fractions of up to 25% of the hydrogen ions at temperatures below the superprotonic phase transition to possess mobilities similar to the protons above this transition. Nuclear magnetic resonance spectroscopy showed similar fractions as well as T1 relaxation times of roughly 2 orders of magnitude shorter compared to the bulk crystalline solid acids. These results lead to the conclusion that because of the space charge effect, vacancies are created in the solid acid at temperatures below the superprotonic phase transition temperature. These empty sites allow a large fraction of the hydrogen ions to move, to such an extent that they become almost as mobile as in the superprotonic phase. Furthermore a composite electrolyte of CsHSO4 and Nafion was synthesized. The composite electrolyte membrane showed good mechanical strength resulting from the Nafion polymer matrix. It exhibited similar proton conductivity to pure Nafion at low temperatures in a humid environment and showed high conductivity of 10-3 S/cm at intermediate temperatures around 140 °C, where Nafion filled with water is inoperable. Finally the investigation was extended to another solid acid: CsH2PO4. Using the multi-technique approach, these CsH2PO4 composites with nanoparticulate TiO2 or SiO2 were studied and compared to the CsHSO4 composites. Acidity was found to be indicative for the amount of space charge occurring. The results indicate a reduced space charge effect in CsH2PO4-TiO2 composites consistent with the reduced acidity of CsH2PO4 and lower proton accepting capacity of TiO2 compared to SiO2. Using the acidity combined with computer calculations might be a useful way to predict the extent of space charge in future research towards the optimal combination for electrolyte membrane material.