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NASA engineers at the Glenn Research Center in Cleveland are developing a fuel cell that will let planetary rovers operate longer, especially in the cold and dark. Current rovers rely on batteries recharged with electricity from solar panels. But NASA wants to send rovers into canyons and valleys on Mars that aren’t in the sunlight because these shaded areas are more likely to have water (ice) near the surface than areas exposed to sunlight, and finding water would change NASA’s plans for exploring Mars. These areas are also colder than those exposed to sunlight, which reduces battery capacity. So today’s rovers have limited time — anywhere from a few hours to a few days — to explore shadowed areas. But a fuel cell could power a rover for weeks at time, despite the cold and darkness.
Fuel cells used in space exploration use hydrogen and pure oxygen, whereas those built for use on Earth rely on hydrogen and air, though only the oxygen from the air is really needed. Using pure oxygen eliminates the need to get rid of impurities found in air. This is one way NASA can boost fuel-cell efficiency.
In the pursuit of efficiency, the space agency also uses a nonflowthrough cell, which means capillary action wicks away water produced by the electrochemical reactions that generate electricity. Conventional flow-through cells need a pump to handle this task, which adds weight, takes up space, and is less reliable.
And although it seems contradictory, NASA’s nonflow-through cells are larger and heavier than conventional fuel cells. This lets NASA use higher pressures and temperatures inside — about 70°C and 45 psi for both the hydrogen and oxygen, compared with just a few pounds/square inch over ambient and room temperature for conventional cells. All these factors make the rover cell more efficient at converting hydrogen and water to electricity. So the larger size and weight of the NASA cell is offset by the additional electricity it can extract from the store of water carried onboard the rover or spacecraft.
NASA can also configure a rover to use solar panels to generate electricity for converting water to hydrogen and oxygen through electrolysis. The rover would use the hydrogen and oxygen to resupply the fuel cell’s reactant tanks and extend its power generation capability.