Solid acids are chemical intermediates between salts and acids, such as CsHSO₄.^[3] Solid acids of interest for fuel cell applications are those whose chemistry is based on oxyanion groups (SO₄^2-, PO₄³⁻, SeO₄²⁻, AsO₄³⁻) linked together by hydrogen bonds and charge-balanced by large cation species (Cs⁺, Rb⁺, NH₄⁺, K⁺).^[1]

At low temperatures, solid acids have an ordered molecular structure like most salts. At warmer temperatures (between 140 and 150 degrees Celsius for CsHSO₄), some solid acids undergo a phase transition to become highly disordered "superprotonic" structures, which increases conductivity by several orders of magnitude.^[3] When used in fuel cells, this high conductivity allows for efficiencies of up to 50% on various fuels.^[4]

The first proof-of-concept SAFCs were developed in 2000 using cesium hydrogen sulfate (CsHSO₄).^[1] However, fuel cells using acid sulfates as an electrolyte result in byproducts that severely degrade the fuel cell anode, which leads to diminished power output after only modest usage.^[5]

Current SAFC systems use cesium dihydrogen phosphate (CsH₂PO₄) and have demonstrated lifetimes in the thousands of hours.^[6] When undergoing a superprotonic phase transition, CsH₂PO₄ experiences an increase in conductivity by four orders of magnitude.^[7][8][9] In 2005, it was shown that CsH₂PO₄ could stably undergo the superprotonic phase transition in a humid atmosphere at an "intermediate" temperature of 250 °C, making it an ideal solid acid electrolyte to use in a fuel cell.^[10] A humid environment in a fuel cell is necessary to prevent certain solid acids (such as CsH₂PO₄) from dehydration and dissociation into a salt and water vapor.^[11]

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: ½O₂ + 2H⁺ + 2e⁻ → H₂O

Overall: H₂ + ½O₂ → H₂O

The operation of SAFCs at mid-range temperatures allows them to utilize materials that would otherwise be damaged at high temperatures, such as standard metal components and flexible polymers. These temperatures also make SAFCs tolerant to impurities in their hydrogen source of fuel, such as carbon monoxide or sulfur components. For example, SAFCs can utilize hydrogen gas extracted from propane, natural gas, diesel, and other hydrocarbons.^[12][13][14]

Sossina Haile developed the first solid acid fuel cells in the 1990s.

In 2005, SAFCs were fabricated with thin electrolyte membranes of 25 micrometer thickness, resulting in an eightfold increase in peak power densities compared to earlier models. Thin electrolyte membranes are necessary to minimize the voltage lost due to internal resistance within the membrane.^[15]

According to Suryaprakash et al. 2014, the ideal solid acid fuel cell anode is a "porous electrolyte nanostructure uniformly covered with a platinum thin film." This group used a method called spray drying to fabricate SAFCs, depositing CsH₂PO₄ solid acid electrolyte nanoparticles and creating porous, 3-dimensional interconnected nanostructures of the solid acid fuel cell electrolyte material CsH₂PO₄.^[16]

SAFCs, like many other types of fuel cells, utilize electrochemical catalysts on the electrodes to increase cell efficiency. Platinum is the most common choice for SAFCs due to its high reaction activity and stability.^[17][18] Initially, platinum nanoparticles were deposited directly on the electrode surface, but these nanoparticles agglomerated throughout fuel cell operation.^[19][20] Recent studies have incorporated carbon-based supports (carbon nanotubes, graphene, etc.) to reduced agglomeration.^[21][22] Here platinum nanoparticles are deposited directly onto the carbon-based support via processes like atomic layer deposition^[23] or metal-organic chemical vapor deposition.^[24]

SAFCs have a high tolerance to catalyst poisoning due to the stability of CsH₂PO₄ at operating temperatures.^[25] However, one recent study has proposed local hotspots around the current collector fibers can cause catalyst poisoning.^[26] According to Wagner et al. 2021, local hotspots can form a liquid phase of CsH₂PO₄ that introduces phosphate groups to the platinum catalyst, degrading fuel cell operation. The introduction of a microporous current collector was found to improve the morphological stability of CsH₂PO₄ and, consequently, mitigate catalyst poisoning.

Compared to their high operating temperature counterparts such as high temperature protonic ceramic fuel cells or solid oxide fuel cells, solid acid fuel cells benefit from operating at low temperatures where plastic deformation and creep mechanisms are less likely to cause permanent damage to the cell materials. Permanent deformation occurs more readily at elevated temperatures because defects present within the material have sufficient energy to move and disrupt the original structure. Lower temperature operation also allows for the use of non-refractory materials which tends to decrease the cost of the SAFC.  

However, solid acid fuel cell electrolyte materials are still susceptible to mechanical degradation under normal operating conditions above their superprotonic phase transition temperatures due to the superplasticity enabled by this transition.^[6][27][28] For instance in the case of CsHSO₄, a study has shown that the material can undergo strain rates as high as ${\displaystyle 10^{-2}s^{-1}}$  for an applied compressive stress in the range of several MPa. Since fuel cells often require pressures in this range to properly seal the device and prevent leaks, creep is likely to degrade the cells by creating a short circuiting path. The same study showed that the strain rate, as modeled using the standard steady-state creep equation ${\displaystyle {\overset {\cdot }{\epsilon }}=A\sigma ^{n}e^{\frac {-Q}{kT}}}$ , has a stress exponent of  ${\displaystyle n=3.6}$  typically associated with a dislocation glide mechanism, and an activation energy of 1.02 eV.^[27] n is the stress exponent, Q is the creep activation energy, and A is a constant that depends on the creep mechanism.

Creep resistance can be obtained by precipitate strengthening using a composite electrolyte whereby ceramic particles are introduced to prevent dislocation motion. For example, the strain rate of CsH₂PO₄ was reduced by a factor of 5 by mixing in SiO₂ particles with a size of 2 microns, however resulting in a 20% decrease in protonic conductivity.^[6]

Other studies have looked at CsH₂PO₄/epoxy resin composites where micron size particles of CsH₂PO₄ are embedded in a cross-linked polymer matrix. A comparison between the flexural strength of an SiO₂ composite versus an epoxy composite demonstrated that while the strengths themselves are similar, the flexibility of the epoxy composite is superior, a property which is essential in preventing electrolyte fracture during operation. The epoxy composite also shows comparable but slightly lower conductivities than the SiO₂ composite when operating at temperatures below 200 °C.^[28]

Because of their moderate temperature requirements and compatibility with several types of fuel, SAFCs can be utilized in remote locations where other types of fuel cells would be impractical. In particular, SAFC systems for remote oil and gas applications have been deployed to electrify wellheads and eliminate the use of pneumatic components, which vent methane and other potent greenhouse gases straight into the atmosphere.^[4] A smaller, portable SAFC system is in development for military applications that will run on standard logistic fuels, like marine diesel and JP8.^[29]

In 2014, a toilet that chemically transforms waste into water and fertilizer was developed using a combination of solar power and SAFCs.^[30]