Solid acid fuel cell

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.^[3][4] These materials are based on oxyanion groups such as SO₄^2-, PO₄³⁻, SeO₄²⁻, or AsO₄³⁻ linked together by hydrogen bonds and charge-balanced by large cation species such as Cs⁺, Rb⁺, NH₄⁺, or K⁺.^[1] The first example of a solid acid electrolyte material in a proof-of-concept SAFC was in 2000, using cesium hydrogen sulfate, CsHSO₄.^[1][3] Since then, however, the best-performing SAFCs have been shown to use cesium dihydrogen phosphate, CsH₂PO₄.^[5][6]

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.^[4] 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.^[4] 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.^[4] 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.^[4][3] CsH₂PO₄, in particular, experiences an increase in conductivity by four orders of magnitude through the superprotonic phase transition.^[7][8][9] 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.^[10]

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.^[11][12][13] 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.^[14]

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.^[15] This combination of detrimental effects is the reason that recent research efforts have pivoted towards focusing more on developing CsH₂PO₄ electrolytes.^[14] 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.^[16] Typically, this hydration is provided by humidifying the H₂ and O₂ supply to the fuel cell.^[16] 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.^[3] 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.^[16] 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.^[17] 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.^[18]

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

In 2005, SAFCs were fabricated with thin CsH₂PO₄ electrolyte membranes of 25 micrometer thickness, resulting in an eightfold increase in peak power densities compared to earlier models.^[5] Thin electrolyte membranes are necessary to minimize the voltage lost due to internal resistance within the membrane.^[5] These thin electrolyte membranes were fabricated by slurry deposition, wherein toluene was used as the suspension medium for the solid acid. The layers of the cell were deposited as follows onto a gas diffusion stainless steel electrode: electrocatalyst, CsH₂PO₄ electrolyte, second electrocatalyst, and the final gas diffusion layer. As is typical for most fuel cell systems, the entire cell sandwich must be uniaxially pressed to enhance adhesion at the interfaces between layers.

The ideal solid acid fuel cell anode is a "porous electrolyte nanostructure uniformly covered with a platinum thin film". Such electrodes can be prepared by spray drying, e.g., depositing CsH₂PO₄ nanoparticles and creating porous, 3-dimensional interconnected nanostructures of the solid acid fuel cell electrolyte material CsH₂PO₄.^[19]

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.^[20][21] Initially, platinum nanoparticles were deposited directly on the electrode surface, but these nanoparticles agglomerated throughout fuel cell operation.^[22][23] Recent studies have incorporated carbon-based supports (carbon nanotubes, graphene, etc.) to reduced agglomeration.^[24][25] Here platinum nanoparticles are deposited directly onto the carbon-based support via processes like atomic layer deposition^[26] or metal-organic chemical vapor deposition.^[27]

SAFCs have a high tolerance to catalyst poisoning due to the stability of CsH₂PO₄ at operating temperatures.^[28] However, one recent study has proposed local hotspots around the current collector fibers can cause catalyst poisoning.^[29] 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.