Various proton conducting solid acids of the general formula MHxXO4 (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 mgPt 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.
This work is supported by ARPA-E under Cooperative Agreement Number DE-AR0000499.