Intermediate-temperature fuel cell (ITFC) systems have recently received significant interest for stationary distributed generation applications. As opposed to high-temperature (> 650 °C) solid-oxide fuel cells (SOFCs), intermediate-temperature systems (200-500 °C) may be capable of reaching cost targets by relaxing material re­quirements for interconnects and systems-level requirements for insulation, heat exchangers, and other balance-of-plant elements. ITFCs may operate on minimally-processed fossil fuels such as natural gas, rather than the ultra-purified hydrogen required for low-temperature (< 100 °C) polymer electrolyte membrane fuel cells (PEMFCs), a feature that could possibly accelerate adoption due to the existence of established distribution networks for these fuels.

Various proton conducting solid acids of the general formula MH_xXO₄ (M = K, Cs, Nd; x = 0-2; X = S, P) have attracted considerable interest as electrolytes for ITFCs. We will present an overview of distinguishing structural features of these solid acids, temperature induced phase transitions, and comment on the varying mechanisms of proton conduction using studies from NMR, XRD, FT-IR, Raman microscopy, and AC impedance spectroscopy.

The most promising candidate for solid acid fuel cells (SAFCs) is CsH₂PO₄ (CDP). The current state-of-the-art SAFC utilizes thin films of platinum nanoparticles on a porous framework of CDP electrolyte. Our methodology to improve Pt utilization, increase electrode surface area, and electrical interconnectivity was achieved by using carbon-based supports for the electrocatalyst and homogenizing framework with CDP. Using a combination of chemical functionalization of nanostructured carbon and infiltration of CDP, we fabricated nanocomposite electrodes that demonstrated vastly superior performance. For example, at 0.8V and 250 °C, a hydrogen-air SAFC using this new electrode architecture we found an improvement in power density by almost a factor of 2, with a decrease in Pt loading of a factor of 15. Furthermore, with Pt loadings of 0.18 mg_Pt cm^-2 we observed peak power densities of in excess of 300 mW cm^-2. We will present a detailed account of these effects, comment on improved stability and durability of these electrodes, and illustrate using a combination of SEM and TEM that nanoconfined CDP exists within the electrode. Finally, we will discuss future directions including advanced manufacturing enabled by the nanocomposite structure.

Acknowledgements

This work is supported by ARPA-E under Cooperative Agreement Number DE-AR0000499.