Malfunction of an oxygen generator on bord of the ISS

The oxygen generation system (OGS) of the US sector of the International Space Station ISS experienced an unplanned shutdown. The system consisted in a water electrolyser stack and was operating at the 100% production rate. The function of the OGS was to provide oxygen to the cabin and was one of the orbital replacement units (ORU) of the ISS.
The fast shutdown was due to one electrolytic cell exceeding the safe high voltage limit of 3.0 volts. A neighbouring cell was very near that value as well, and all other cells exhibited an increase in voltage as well. While high voltage are in general a signal of ageing and end-of-life, the life of the OGS cells was expected to exceed the five-year life of the ORU, but failed well before it.

The unit was removed and replaced with an on-board spare unit to guarantee oxygen generation. The failed OGS was returned to earth in March 2011 for investigation and tests.

The INITIATING CAUSE was a cell stack high-voltage shutdown.

The failure was due to chemical degradation of the cell membranes during nominal electrolysis, resulting in the formation of degradation byproducts – specifically hydrofluoric acid and, to a much lesser extent, sulfuric acid. These acid constituents accumulated over time in the OGS recirculating water loop, lowering the loop pH to the point of corroding metallic hardware in both the cell stack and balance of plant hardware. The corrosion products accumulated in the recirculation loop fluid with impacted the performance and the safe operation of the electrolysers with a self-reinforcing degradation mechanism, bringing to increasing cell voltage, cell ohmic resistance, water conductivity and reduction of water flow, which eventually brought to too its shutdown.

Since the design of the water loop was improved after the event, a design shortcoming could be attributed as ROOT CAUSE.

electrolytic cell,
water loop,
Balance of plant

The OGS was built in 2003 launched in orbit 2006 and activated for the first time in 2007. Since then, it operated for approximately 240 days before shutdown on cell too high voltage in July 2010.
It was considered a critical function to maintaining six crew capability. Durin gits life, some failures interrupted or threatened to interrupt oxygen production. Filters in the recirculation loop clogged and had been replaced, hydrogen sensors fell out of specifications, a pump delta pressure sensor failed, a pump failed to start, and eventually the voltage on the cell stack increased out of tolerance.
[see Diderich et al., 41st ICES]

DESCRIPTION OF THE OXYGEN GENERATING SYSTEM
OGS = the NASA Oxygen Generating System was a unit made of a water PEM electrolyser stack, a power supply and balance of plant components. It used water from the Water Recovery System to produce oxygen delivered to the cabin atmosphere, and hydrogen originally vented (later, the hydrogen was used in a Sabatier system for producing again water). Before the adoption of the electrolysis-based system, the US orbital segment had used oxygen stored in a pressurised tank or generated by the Russian service module.

The OGS which failed was generating oxygen at a maximum rate of 5.4 kg/day when operated on day/night orbital cycles, with selectable rate between 2.3 and 9.2 kg/day when operated continuously.

ORBIT REPLACEMENT UNITS
The OGS was one of the several orbit replacement units of the ISS.
ORU’s = Orbital replacement units (or on-orbit replaceable units) are elements of the International Space Station that can be readily replaced at the end of their lifetime or when they fail. Examples of ORU’s are pumps, storage tanks, controller boxes, antennas, and battery units.
For a description of the various ORU’s, see Wikipedia: https://en.wikipedia.org/wiki/Orbital_replacement_unit]

As temporary measure, the failed ORU was removed and replaced with an on-bord spare oxygen generator ORU.

To reduce the degradation of the electrolyser membranes, a mixed resin deionisation bed was installed within on water recirculation loop of the ORU.
The generation of hydrofluoric acid (fluoride) within the cell stack cannot be avoided or mitigated because intrinsic to the design and operation of the membrane. Nevertheless, its accumulation in the recirculated water and its negative effects on the pH could be minimised by this corrective measure.
[See Diderich et al., 42nd ICES]

The lessons learnt from the on-bord operation of this first OGS supported an improved design of the unit, to reduce mass and crew maintenance time while increasing reliability. Th re-design involved the electrolysis cell stack, the hydrogen dome and the recirculation loop deionizing bed. The hydrogen sensors were replaced and the nitrogen purging and water flushing loop were added.
[See Takada et al., 52nd ICES]