Proton conducting materials based on hydrofluorides

Abstract

Proton conducting materials have a great potential in applications for new energy sources and inorganic membrane reactors, which are highly efficient, energysaving and friendly to the environment. The utilization of hydrogen energy is an important subject for a sustainable development of the society in the 21st century, where proton conducting and hydrogen semipermeable membrane materials play a key role. Proton conductivity and materials are a very interesting subject in solid state ionics; every two years there is an international conference on solid state proton conductors. Proton conducting polymers have made it possible to develop a new generation of electrical vehicles powered by polymer electrolyte fuel cells (PEFCs) [1]. Production of hydrogen and generation of electric power using fossil fuels require a membrane reactor or fuel cell operating at high temperatures (>600 ◦C). It thus relies on the use of inorganic proton-conducting or hydrogen semipermeable membrane materials. These materials can be used in devices for hydrogen-energy related applications in two modes: membrane device and fuel cell. Proton-conducting solid state fuel cells (SSFCs) for converting the energy of hydrogen containing fuel have some advantages compared with oxygen-ion conducting solid oxide fuel cells (SOFCs). For stationary fuel cell plants operating at intermediate temperatures (say 400 to 800 ◦C), the use of inorganic proton conductors as electrolytes is very promising [2–4]. However, there is a need for even higher proton conductivity in order to improve the power output. In this work a high proton conductivity and good fuel cell performance are shown for a new type of materials based on MF-BaF2-CaH2 (M= Li, Na) hydrofluoride electrolytes although some reports claimed they are hydride ion, H−, conductors [5–7]. In order to enhance the mechanical properties, the MF-BaF2-CaH2 (M = Li, Na) hydrofluoride materials were prepared as a composite with Al2O3. The electrical properties of these hydrofluoride-composites were investigated as electrolytes in hydrogen-air fuel cells, a new method developed for investigating the systems with variability in the stoichiometry [8]. The fuel cells were constructed in the following way: (fuel gas, H2) Pt paste (Leitplatin 308A, Hanau, Germany)/ hydrofluoride-based electrolyte/Pt paste or Ag paste (Leitsilber 200, Hanau, Germany) (oxidant gas, 2% O2 in Ar). The hydrofluoride-based materials were prepared from LiF, NaF, BaF2, Al2O3 and CaH2. The LiF (or NaF), BaF2 and Al2O3 powders were first heated at 800 ◦C for 2 h, then Li (Na)F-BaF2-Al2O3 was mixed with CaH2 (about 20 mol% of the total sample amount) and heated again in a hydrogen atmosphere at 700 ◦C for 1 h to synthesize CaH2 with Li (Na)F-BaF2-Al2O3 as hydrofluoride type ceramics. In the fuel cells, the electrodes are non-blocking only for H+, H− or O2− mobile ionic charge carriers, since the electrodes are supplied with gas from external gas resources; for ions such as Li+, F− etc. the electrodes are blocking; these ions will thus not make a contribution to the stable fuel cell current output during fuel cell operation. The conductivity of the hydrofluoridebased electrolytes was obtained from measurements of the fuel cell I -V characteristics in subtraction of the influence from the electrodes. It should be mentioned that the conductivity obtained from the fuel cells correspond directly to the proton or oxygen ion conduction [8]. Fig. 1 shows typical I -V characteristics for a fuel cell with the electrolyte, LiF-BaF2-CaH2-Al2O3, see Fig. 1a, and the I -P characteristics at 740 ◦C, see Fig. 1b. In the I -V characteristics, the linear section in the central region reflects the IR loss mainly caused by the electrolyte, from which the resistance, and thus the conductivity of the electrolyte can be calculated, as shown in Fig. 1. Using ionic doping and forming composites with Al2O3 can significantly enhance the conductivity of the fluorides. For example, MF (M=Li, Na,) doped BaF2-Al2O3 has a conductivity of 10−3 to 10−2 S/cm for temperatures above 600 ◦C, while for