NASA’s endeavor to further enable long-duration manned space exploration requires further closure of the oxygen loop of the life support system that is currently realized aboard the International Space Station. Currently, oxygen is recovered from crew-generated carbon dioxide via the use of a Sabatier carbon dioxide reduction system coupled with water electrolysis. Water is electrolyzed to form oxygen for crew consumption, as well as hydrogen. The hydrogen is reacted with carbon dioxide, forming water and waste methane gas. Since hydrogen is lost from the desired closed-loop system in the form of methane, there is insufficient hydrogen available to fully react all of the carbon dioxide, resulting in a net loss of oxygen from the loop. In order to further close the oxygen loop, NASA has been developing an advanced plasma pyrolysis technology that further reduces the waste methane to higher hydrocarbons in order to better utilize the hydrogen for oxygen recovery.
This hydrogen purification innovation allows the extraction and purification of nearly all of the hydrogen from a gas stream resulting from the plasma pyrolysis of methane. An electrochemical cell that uses a polymer matrix electrolyte is used to electrochemically separate hydrogen from a stream containing hydrogen, acetylene, and carbon monoxide, among other components. The efficiency of the hydrogen purifier is high, and the mass and volume of the device are small. It requires no reciprocating compressor, so it is largely maintenance-free.
In a typical electrochemical hydrogen separation application, a mixture containing hydrogen is fed to the anode chamber of a cell or a “stack” of cells. A voltage is applied across the cathode and anode such that the anode is held at positive potential with respect to the cathode. The voltage is high enough to oxidize hydrogen, but low enough so that other constituents of the chamber are not oxidized. Hydrogen is then electrooxidized, resulting in the formation of protons and electrons.
The cathode and anode of the cell are separated by a polymer that, being a solid super acid, is inherently conductive to protons, but relatively impermeable to hydrogen and other gases. At the cathode, the protons are reduced by the applied electrical potential, and gaseous hydrogen evolves.