Uncrewed Systems Technology 047 l Aergility ATLIS l AI focus l Clevon 1 UGV l Geospatial insight l Intergeo 2022 report l AUSA 2022 report I Infinity fuel cell l BeeX A.IKANBILIS l Propellers focus I Phoenix Wings Orca

84 extremely important – imagine, say, a naval UUV working in mine countermeasures or a rocket being launched. “We know many companies use graphite BPPs, but we also know from experience with other companies’ fuel cells that do use graphite plates that if you drop them on the floor, you end up with a lot of shards of graphite, whereas if you drop one of ours on the floor, you’re more likely to dent the floor. Metallic BPPs are ideal for handling shock and vibration, and if they are carefully designed and manufactured they can still be reasonably lightweight.” Metallic BPPs cannot realistically be made solely through casting, and still feature the very fine and minute flow fields needed to allow precise delivery of hydrogen, oxygen, product water and unused reactant to their respective destinations. The flow fields are instead most often bonded onto the flat BPPs with adhesives such as epoxy resins, silicones or elastomers. BPPs made this way can exhibit good levels of flatness and strength, but having these distinctly separate (albeit bonded) layers means they are prone to degradation through delamination over time, and can December/January 2023 | Uncrewed Systems Technology Each active, current-producing cell in a PEMFC stack is otherwise known as a membrane electrode assembly (MEA). The number of layers in the MEA will differ according to the design, but generally include a single proton exchange membrane at the core, sandwiched by two catalyst layers and then two gas diffusion layers (GDLs) on the outermost sides. As water is produced at the site of the electrochemical reaction, it exudes outwards from the catalyst layer on the cathode side of the fuel cell (where the oxygen reactant is input), and then out from the gas diffusion layer. A sequence of components are key to the stages by which this waste water is passively removed and hence flooding of the stack is prevented. First, the GDL material is hydrophobic, so it repels the water, causing it to form as droplets on the GDL’s outer surface. Next, between each GDL and bipolar plate in the stack sits a screen material layer, a porous membrane and a water cavity. The membrane’s porous material can be metallic or a polymer, depending on factors such as weight, cost and availability. “The water then moves through the screen layer, which can be around 304.8 microns thick. The droplets build up through that screen material and touch the porous membrane on the other side,” says William Smith, Infinity’s president and founder. “Whether the porous membrane is a metal or a polymer, water will pass through its pores because they’re fine enough to conduct water easily, but they will resist intrusion of any gases owing to surface tension effects caused by van der Waals forces, up to differential pressures of 50 psi. “That resistance is maintained passively through the constant pressure level in the water cavity on the other side of that membrane, which is lower than that of the pressure in the MEA’s oxygen chamber. The differential pressure can be as low as 2-3 psid, but a higher differential pressure can simplify the overall control system design. “That results in a highly effective water-gas phase separator, and we incorporate this arrangement of screen material, porous membrane and water removal cavity in every layer of our fuel cell stack. Except for the pressure control, no other active control system is needed at the core of this water removal process.” The electrochemical reaction’s by-product water is therefore removed from the PEMFC stack’s cells without using any moving parts, and the hydrophobic GDL and pressure differential ensure that neither gravity nor its absence impact the process. The water cavities are joined using a manifold, through which the water can flow directly from the stack into an external water collection system. Notably, the passive water removal technology in the Gemini spacecraft (see main article) used a similar configuration, but with the porous material also installed outside the fuel cell stack. That stack was inside a pressurised oxygen chamber, making for a far more bulky and complicated system than Infinity’s turnkey approach. Advanced Product Water Removal The stages of the Advanced Passive Water Removal system. Water is pushed passively away from a hydrophobic layer and drawn into a hydrophilic layer, independent of gravity or machines