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75 reactant stoichiometry and water removal. It also controls two purge valves on the side of the water management unit, which can rapidly discharge build-ups of contaminants in the reactants, as would be indicated by lower-than-expected current and voltage coming from the cells. There is a low-voltage cut-off function as well, which will again open the contactors to cut the circuit if high-power operation results in the cell dropping below 0.5 V. This prevents cell reversal, in which such low voltage levels can cause the cell to act in reverse, generating reactants on the wrong side of the cell’s membrane electrode assemblies (MEAs) and shortening its lifespan. Cell stack layers and materials With a few exceptions, such as the tubular solid oxide fuel cells featured in UST 29 (December 2019/January 2020), the vast majority of fuel cells generate power using bipolar plates (BPPs) sandwiching thin MEA sheets. These sheets consist of five key layers – a PEM (polymer electrolyte or proton exchange membrane), two catalyst layers and two gas diffusion layers (GDLs). As with most PEMFCs, the PEM layer is for transporting the protons, which in the EDR cell is made from DuPont’s Nafion, specifically the Nafion XL variant. It is 27.5 µm thick, weighs 55 g/m 2 , and is designed for higher mechanical strength than other variants. “Nafion XL has a web of fibre reinforcements supporting it structurally,” says Dr Valdez. “There are lots of membrane material variants to choose from, like Nafion 117 and Nafion 212. The latter is easier to handle than Nafion XL, partly because it’s twice as thick, while Nafion 117 is seven times thicker, making it really strong.” The strength and thickness of the membrane is important because pressure differentials between the hydrogen and oxygen inputs on either side can cause tears in the material. However, Nafion XL has one of the highest tensile strengths of any variant – 45 MPa in the machined direction (MD) and 40 MPa in the transverse direction (TD), compared with 43 MPa MD and 32 MPa TD for Nafion 117 for example – so tearing is not an issue. The catalyst hot-pressed onto either side of the Nafion XL is platinum black – pure platinum in its powdered form. Other materials, such as carbon black or iridium, can be used, and mixtures of carbon with nanometre quantities of platinum are popular for reducing expenditure on precious metals, Unmanned Systems Technology | December/January 2021 EDR fuel cell PEMFC Weight: 15.4 kg Dimensions: 566 180 x 110 mm Active area per cell: 75 cm 2 Max peak power: 8 kW Max continuous power: 6 kW Output supply: 80 V MTBF: 7000 hours Some key suppliers Carbon composite bipolar plates: Metro Mold & Design PEM layer: DuPont Hydrophilic material: DuPont Platinum black: Johnson Matthey Gas diffusion layers: Sigracet Washers: McMaster-Carr Tie rods: McMaster-Carr Solenoid valves: Gems Pressure regulators: Emerson Tubing: Swagelok Specifications The cell stack’s bipolar plates are constructed from carbon, to withstand the corrosive effects of pure oxygen The PEM layer is made from Nafion XL, which has fibre reinforcements that support it structurally and is designed to provide high mechanical strength