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.
Contents
Design
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 (SO42-, 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.
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: 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.
Fabrication and Production
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.
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.
Applications
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. A smaller, portable SAFC system is in development for military applications that will run on standard logistic fuels, like marine diesel and JP8.
In 2014, a toilet that chemically transforms waste into water and fertilizer was developed using a combination of solar power and SAFCs.