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The paper presented delivers the proof for one of the possible solutions to the so-called medium-temperature gap—the lack of electrolytic systems able to efficiently work in a temperature range spanning from 200 to 450 °C. Regardless of the progress made in this field, the commercially available systems are still operating either at close to ambient temperatures, where hydrogen purity requirements are a significant limit, or above ca. 600 °C, where they suffer from increased corrosion and excessive thermal stresses occurring during startup and shutdown. Alkali metal orthoborates (M3BO3 M = Li, Na, K, or the mixture of these), in contrast to commercially used tetra-(M2B4O7) and meta-(MBO2) borates of these metals, are compounds with relatively poorly understood structure and physicochemical properties. The possibility of their application as an electrolyte in a fuel cell is a relatively new idea and has been preliminary reported. Therefore, an extended phase-focused analysis of the materials applied was needed to re-optimize both the synthetic strategy and the application route. Results of PXRD and FT-IR investigations showed, on the one hand, a complicated multi-phase structure, including the main orthoborate phase, as well as the presence of additional borate-based phases, including boric oxoacid. On the other hand, DTA tests proved not only that their melting temperatures are lower than these characteristics for the tetra- and meta-counterparts, but also that cation mixing leads to a subsequent decrease in this important functional parameter of the materials studied. [ABSTRACT FROM AUTHOR]

The introduction of the hydrogen economy, despite its obvious technological problems, creates a need for a significant number of niche-focused solutions, such as small-sized (10–100 W) fuel cells able to run on hydrogen of lesser purity than what is considered a standard in the case of PEMFCs. One of the solutions can be derived from the fact that an increase in the operational temperature of a cell significantly decreases its susceptibility to catalyst poisoning. Electrolytes suitable for the so-called medium temperature operational range of 120–400 °C, hence developed, are neither commercialized nor standardized. Among them, phosphate silicate protonically conductive glasses were found not only to reveal interestingly high levels of operational parameters, but also, to exhibit superior chemical and electrochemical stability over their polymeric counterparts. On the other hand, their mechanical properties, including cracking fragility, still need elaboration. Initial studies of the composite phosphate silicate glasses with uranyl-based protonic conductors, presented here, proved their value both in terms of application in fuel cell systems, and in terms of understanding the mechanism governing the charge transport mechanism in these and similar systems. It was found that whereas systems containing 10–20 wt% of the crystalline additive suffer from significant instability, materials containing 45–80 wt% (with an optimum at 60%) should be examined more thoughtfully. Moreover, the uranyl hydrogen phosphate was found to surpass its arsenate counterpart as an interesting self-healing behavior of the phase structure of the derived composite was proved. [ABSTRACT FROM AUTHOR]