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

82 Dossier | Infinity APWR fuel cell This is a stack with 36 layers of the 50 cm 2 active area cells, roughly 25 cm long including the tie rods used to secure, tighten and pressurise the cells and BPPs; it is 10 cm in diameter and weighs just below 6 kg. As with all Infinity’s PEMFC solutions, it incorporates the APWR system as well as an inert concentrator and what are called active humidifier systems. “It’s nominally designed for generating 500 W to 1 kW, and ideally for operating for extremely long missions as part of a regenerative fuel cell system,” Smith explains. “If it’s installed in a lunar energy storage application, for example, combined with solar arrays, it can produce electricity and water at night when the arrays are not generating electricity, by running hydrogen and oxygen through the cell. This by-product [or just ‘product’] water can then be decomposed into hydrogen and oxygen during the lunar day in a separate electrolysis reaction using surplus electricity from the arrays.” Using a similar diffusion-bonded technology, Infinity has also developed a separate family of high-pressure electrolysers that can produce hydrogen and oxygen as an energy storage medium at 2000 psi and above for later energy needs. “That gives you an idea of how these solutions scale, particularly in terms of current density,” Smith says. “The number of cells sets the stack voltage, and of course the current multiplied by the voltage gives the power. With 36 cells and 0.8 V per cell, you get a PEMFC of about 28 V, and it could operate at a peak current of about 50 A at a slightly lower voltage per cell.” He notes however that this does not tell the whole story: the mission profile, application and objectives have consistently been the primary dictators of each fuel cell’s design. “Sometimes you’re after very high power levels that you want for short periods of time; other times you’re after maximum efficiency, and the latter would drive the design in a very different direction to the former. There’s a lot of optimisation that impacts size, weight, power, cost and so on,” he says. All-metal bonded cells The materials in the cells’ MEAs are optimised to work with pure oxygen. This is most noticeable in the catalyst layer, as Smith notes that typical carbon-supported catalyst materials can be degraded through contact with pure oxygen gas. “We therefore have a catalyst structure that’s compatible with pure oxygen, and is designed to be very hydrophobic so that it can remove the water effectively from the cathode side of the reaction,” he explains. “And within the MEA layers, we use a fairly conventional Nafion material as our PEM layer, with a very high catalyst loading. That’s because although mission use cases vary, most of the time we’re after high performance and long life. “In these applications, typically the amount of catalyst on the membrane does not have a major impact on cost as far as our users are concerned, so we ensure that it can not only withstand the corrosive pure oxygen environment but also the wetness of that environment, and still provide high, consistent power output over its lifetime.” He adds that NASA would like that lifetime to be in the 10,000-20,000 hour range at least, longer if possible. As mentioned, the BPPs are metallic, monolithic BPPs created through diffusion bonding. They serve to contain and pressurise the MEAs, as well as the various fluids flowing throughout the stack as reactants, coolants and waste products, and mount the pore chambers and phase separation membranes. “We want to make our BPPs as lightweight as possible, but there are trade-offs,” Smith says. “In many of our applications, ruggedness and the ability to withstand shocks and vibrations are December/January 2023 | Uncrewed Systems Technology In some space vehicles, Infinity’s technology might function as an electrolyser (pictured), powered by solar panels to turn water back into hydrogen and oxygen as a form of energy storage