Solid acid fuel cells (SAFCs) are a class of fuel cells characterized by the use of a solid acid material as the electrolyte. Similar to proton exchange membrane fuel cells and solid oxide fuel cells, they extract electricity from the electrochemical conversion of hydrogen- and oxygen-containing gases, leaving only water as a byproduct. Current SAFC systems use hydrogen gas obtained from a range of different fuels, such as industrial-grade propane and diesel. They operate at mid-range temperatures, from 200 to 300 °C.
Solid acid material In the context of SAFC's, solid acids are proton-conducting materials whose chemistry and properties lie between those of normal acids and normal salts—they are conductive due to the protons from the "acid" contribution, and they are brittle due to the "salt" contribution. These materials are based on oxyanion groups such as SO42-, PO43−, SeO42−, or AsO43− linked together by hydrogen bonds and charge-balanced by large cation species such as Cs+, Rb+, NH4+, or K+. The first example of a solid acid electrolyte material in a proof-of-concept SAFC was in 2000, using cesium hydrogen sulfate, CsHSO4. Since then, however, the best-performing SAFCs have been shown to use cesium dihydrogen phosphate, CsH2PO4. Various methods are available for synthesis of the solid acid materials and their composites, including slow isothermal evaporation of mixtures, solvent-induced precipitation, dry mixing, electrospinning, sol-gel, thin-film casting, and impregnation. The main parameters that must be tuned during synthesis are temperature, pressure, heating duration, and grinding/mixing because these are the factors that affect the resulting structure of the electrolyte. Currently, the preferred method in the literature is mixing with water followed by solvent-induced precipitation, due to the fact that this method is high-throughput and easy to reproduce. This method typically produces polycrystalline powders of the solid acids, whereas slow isothermal evaporation produces single crystals. The ideal method must be determined based on the desired function or application. Solid acids are ideal as electrolytes for fuel cell applications due to their "superprotonic" structures that occur at transition temperatures well-within the operating temperatures of the SAFC. CsH2PO4 demonstrates high proton conductivity values of 2.2×10−2 S cm−1 at 240°C, and CsHSO4 boasts a proton conductivity of 4 × 10−2 S cm−1 at 200°C. The solid acid "superprotonic" structure and property arises from a phase transition occurring between 100°C to 250°C for most solid acids, at which point the conductivity can increase by 2-3 orders of magnitude. CsH2PO4, in particular, experiences an increase in conductivity by four orders of magnitude through the superprotonic phase transition. This high conductivity allows for peak power densities as high as 415 mW/cm2 in CsH2PO4-based SAFCs and efficiencies of up to 50% on various fuels. An advantage of solid acids as an alternative to conventional polymer membrane electrolytes (PEM) in fuel cells is that they do not require hydration in order to function as an electrolyte. Polymer electrolyte membranes require constant hydration to maintain acceptable conductivity levels, but hydration can simultaneously degrade these membranes. This constrains the operating temperature of the PEM fuel cell to below 100°C. In contrast, the flexibility of the solid acid is such that the fuel cell can operate at mid-range temperatures, thereby eliminating the additional issues of electrocatalyst inefficiencies and intolerance to fuel impurities that arise from the lower operating temperatures of PEM fuel cells. The consequence of this higher tolerance is that SAFCs can run on hydrogen gas that has been extracted from biodiesels and other impure forms of hydrocarbons, paving the way for a more versatile and deployable fuel cell technology. An additional benefit of the higher operating temperatures of SAFCs is that non-platinum alloy and transition metal oxide electrocatalysts can be used, which are typically cheaper than the platinum catalysts found in PEM fuel cells. Despite the advantages of SAFCs over PEM fuel cells, there are also drawbacks. One major drawback of CsHSO4 is its solubility in water, and practical difficulties in fabricating sufficiently thin membranes. Additionally, reactions involving H2 gas, CsHSO4 electrolyte, and typical SAFC electrocatalysts can lead to the degradation of the anode material, eventually leading to SAFC performance loss after only modest usage. This combination of detrimental effects is the reason that recent research efforts have pivoted towards focusing more on developing CsH2PO4 electrolytes. Although CsH2PO4 does not require hydration to improve its conductivity, it does require some level of hydration to prevent dissociation into a salt and water vapor. Typically, this hydration is provided by humidifying the H2 and O2 supply to the fuel cell. Another study showed that the humidification of the supplies does not affect fuel cell performance, however, which suggests that further investigation of this behavior is needed. A third study indicated that humidification has little effects on the short-term SAFC performance tests but becomes necessary for long-term stability and performance. There is also controversy in the literature as to whether the higher operating temperatures of SAFCs could potentially also lead to the dehydration or decomposition of CsH2PO4. One approach to improve thermal stability has been to dope the solid acids with oxide materials such as silica or alumina, which can both increase stability and enhance proton transport for CsHSO4 but has yet to be explored for CsH2PO4.
Electrode reactions
Hydrogen gas is channeled to the anode, where it is split into protons and electrons. Protons travel through the solid acid electrolyte to reach the cathode, while electrons travel to the cathode through an external circuit, generating electricity. At the cathode, protons and electrons recombine along with oxygen to produce water that is then removed from the system. Anode: H2 → 2H+ + 2e− Cathode: 1/2O2 + 2H+ + 2e− → H2O Overall: H2 + 1/2O2 → H2O
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