UST035

76 but Teledyne’s approach is critical for operating with pure oxygen. “Oxygen attacks pure carbon, so our cathode absolutely has to be platinum black,” Dr Valdez explains. “We also use pure platinum for the anode, but since it’s just hydrogen there, which doesn’t harm carbon, we could actually use platinum-supported carbon material for the anode.” He adds that although the quantities of platinum typically used make the EDR cell more expensive than those from Ballard, Toyota and so on, fuel cells for use in space and underwater have to be manufactured for long-term survival given the use of pure oxygen. Also, the more platinum there is in the catalyst layer, the higher the energy efficiency and performance of the cell, so for space missions the EDR cell will probably always use pure platinum on the membrane. For some underwater applications, however, when the last 0.5% of power is not mission-critical, a platinum-carbon anode might be suitable. The GDL is a two-layer porous carbon material manufactured by German company Sigracet, and is a well-proven material widely used in the industry. The variant for the EDR cell has also been chosen for operating in a pure oxygen environment: at about 0.35 mm thick, it is around three times the thickness of most GDLs. As well as reducing the impact of oxygen corrosion, its thickness also provides further structural support for the Nafion XL membrane. Carbon composite BPPs sit between the MEAs – that is, between the anode of one cell and the cathode of the next – and are key to achieving the fuel cell’s required voltage, current, even distribution of hydrogen and oxygen, water management and structural integrity. While many companies in the unmanned and automotive sectors have switched to using metal BPPs, pure oxygen will rapidly corrode metallic plates, so carbon-based plates are Six tie rods are used and torqued as necessary to secure the cell stack with sufficient pressure to preventing expansion or leakages Metal plates are hydrophilic, so they need a coating or treatment to make them repel water so that it is wicked away December/January 2021 | Unmanned Systems Technology