Solid acid fuel cell

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 acids are chemical intermediates between salts and acids, such as CsHSO₄. 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⁺).

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. When used in fuel cells, this high conductivity allows for efficiencies of up to 50% on various fuels.

The first proof-of-concept SAFCs were developed in 2000 using cesium hydrogen sulfate (CsHSO₄). 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.

Current SAFC systems use cesium dihydrogen phosphate (CsH₂PO₄) and have demonstrated lifetimes in the thousands of hours. When undergoing a superprotonic phase transition, CsH₂PO₄ experiences an increase in conductivity by four orders of magnitude. 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. 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.

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.

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