Long-term space missions will require methods to produce fuels, such as hydrogen gas, on demand. Photoelectrochemical cells (PECs) can produce hydrogen using energy from sunlight, but the reduced gravity in space presents unique challenges to this chemistry.
An international team of researchers led by California Institute of Technology’s Hans-Joachim Lewerenz studied one of these low-gravity problems and demonstrated how a nanostructured catalyst surface could solve it (Nat. Commun. 2018, DOI: 10.1038/s41467-018-04844-y).
The team’s PEC consists of an electrode made of a light-absorbing semiconductor, p-type indium phosphide, coated with a layer of a rhodium catalyst. When exposed to light, the electrode reduces hydrogen cations from an acidic water solution, producing hydrogen gas.
To simulate the microgravity conditions of space, the researchers catapulted the device 120 m into the air and allowed it to fall in a specialized tower in Germany. During the tests, hydrogen bubbles accumulated on the flat surface of the electrodes, preventing efficient gas production.
To encourage the bubbles to detach from the electrodes, the researchers wanted to decrease the contact area between the bubbles and catalyst surface, so they shaped the rhodium layer into tiny peaks and holes. When the team tested these textured photoelectrodes in the drop tower, they found the cell’s efficiency matched that of a flat electrode in regular gravity.
Cornell University’s Mason A. Peck, an aerospace engineer and former chief technologist at NASA, said that demonstrating performance in microgravity is an important step for any potential space-faring technology. But ultimately, he notes, the practicality of such a system will depend on how efficient it is at converting solar energy to stored chemical energy. The ideal setup also would produce oxygen. Katharina Brinkert, the paper’s first author, says the team is working to develop a cell that will combine hydrogen and oxygen production.