For Earth-bound explorers, a lack of water – particularly drinkable water – was often the greatest impediment. Those on long space journeys will not just need to drink water but use it to make oxygen. However, the techniques we use to split water on Earth run into problems outside strong gravitational fields. Fortunately, it looks like we now have an answer.
Earthly free oxygen is a product of plants and photosynthesizing bacteria. Future space voyages may take these forms of life along with us to ensure the astronauts can keep breathing, but the weight and space required may prove too much. The International Space Station gets its oxygen from electrolyzing water, running electricity through it so that oxygen appears at one electrode and hydrogen at the other, but there is room for improvement on the process.
Long space missions won't be able to stock up on water, and their power source will get weaker as they travel further from the Sun, so Caltech's Dr Katharina Brinkert is seeking lighter and more efficient options for turning it to oxygen.
On Earth, when we use electricity to split water to make hydrogen for fuel, the process works because the gasses bubble to the surface where they can be captured. In microgravity, gases don't rise in the same way, leading to gas bubble froth layers that increase resistance and slow gas production.
One option is to create artificial gravity, spinning the chamber where the water is being split like a centrifuge. Indeed, future voyages to the outer planets may do this to the entire spacecraft for the comfort of those on board. However, Brinkert wants to ensure alternatives are available.
In Nature Communications, she has announced a smaller scale solution.
Brinkert and colleagues used the Bremen Drop Tower to simulate near-zero gravity, albeit for just 9.3 seconds as their equipment underwent free fall. Water-splitting cells were packed together with cameras to record the development of bubbles of hydrogen gas at one electrode and oxygen at the other.
Brinkert trialed two electrolysis systems, both using indium phosphide and rhodium particles. These produced very similar results under Earth gravity, but one suffered a dramatic reduction in performance under gravity a million times lower. The other system, however, performed just as well in low gravity.
Unfortunately for Elon Musk (given his public contempt for nanotechnology), to use Brinkert's space-suitable version on his planned voyages to Mars will require self-assembling nanocrystalline rhodium particles in hexagonal cell patterns. The nanostructuring prevents the hydrogen bubbles from becoming large enough to induce current loss, which interferes with further water splitting.
Besides distant space missions, the work may also lead to better designs for water splitting on Earth, where the goal is hydrogen for clean energy storage.