A review on synthesis and characterization of solid acid materials for fuel cell applications

• Solvent-induced precipitation is the fastest method to produce solid acids.
• Solvent-induced precipitation produces polycrystalline solid acid powder.
• Isothermal evaporation technique produces single crystals solid acid.
• Decomposition is observed in solid acids heated lower than 0.3 °C min−1.
• Humidification is required to maintain long term fuel cell performance.

Solid acids emerged as an electrolyte material for application in fuel cells due to their high protonic conductivity and stability at high temperatures between 100 °C and 250 °C. This paper gives an overview of the different solid acid materials and their properties, such as high protonic conductivity and thermal stability, in relation to phase transitions and mechanisms of proton transport. Various solid acid synthesis methods including aqueous and dry mixing, electrospinning, sol-gel, impregnation and thin-film casting will be discussed, and the impact of synthesis methods on the properties of solid acids will be highlighted. The properties of solid acids synthesized as either single crystals and or polycrystalline powders were identified via X-ray diffraction, nuclear magnetic resonance, thermal analyses, optical microscopy and infrared spectroscopy. A selection of electrolyte-electrode assembly methods and the performance of solid acid fuel cell prototypes are also reviewed.