(Invited) Properties of the Electrode-Ammonium Polyphosphate-Composite Interface at Temperatures up to 250°C
Ammonium polyphosphate (APP) based composites as proton conducting electrolyte for the development of an intermediate temperature (200-400°C) fuel cell have drawn a lot of interest and attention in the past, as APP is non-toxic, commercially available and inexpensive. The intermediate temperature operation range of a fuel cell is in particular interest for mobile and stationary applications, as the thermal management of the cell will be similar to an internal combustion engine, without facing the high material requisitions of a solid oxygen fuel cell. In addition, the elevated operation temperature offers enhanced kinetics, an efficient conversion of some liquid fuels (e.g. small alcohols) in the cell, which have a relatively high volumetric energy density.
Because pure APP decomposes at intermediate temperatures, thermally stable composites with metal oxides (e.g. SiO2, TiO2) were developed during the last years (1-5). For the usability of the ceramic composites of APP as a flexible fuel cell membrane, the electrolyte was recently successfully imbedded in a polymer (6). Tests with commercial HT-PEM electrodes as MEA have shown, that this composite membrane can be used as hydrogen and alcohol fuel cell (7).
Here, we want to report investigations of the electrode-solid electrolyte interface. These were performed by obtaining capacity values of the solid-solid interface from impedance spectroscopy measurements. In addition, the kinetics of interfacial charge transfer reactions were considered.
The composite electrolyte, (NH4)3Si0.5Ti0.5P4O13 (ASi0.5Ti0.5PP) was prepared according to the procedure of Wang et al. (4). The membrane was manufactured by imbedding ASi0.5Ti0.5PP in a polymer (7). For the electrodes, a homogeneous powder mixture of unsupported Pt (black) and ASi0.5Ti0.5PP was prepared and pressed on a platinized titanium expanded metal, which served as current collector and was cleaned before use in a HF/HNO3/H2O bath. To fabricate a membrane electrode assembly (MEA), these electrodes were pressed on the membrane and were put into two layers of Klingersil-C4400, which served as sealing material. The geometric surface area of the electrodes, anode and cathode, were 2 cm² each. The experimental set-up was described earlier in detail (7).
From impedance measurements at different temperatures the interfacial capacity was evaluated. The values are comparable to other solid-solid electrolyte contacts and were of the order of µF/cm2. It is interesting to note that the capacity showed only a small temperature dependence. Further insight was obtained from evaluating the kinetics of charge transfer reactions at the solid-solid electrolyte interface.
The results will be discussed in terms of the charge distribution at the interface and what kind of potential distribution can be assumed. This will impact the understanding of charge transfer reactions taking place at the same interface.
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