Shrihari Sankarasubramanian, Pralay Gayen and Vijay K Ramani discuss work on methods to make fuel and oxygen on the red planet
MARS, the fourth planet from the Sun, has been an object of fascination for millennia. The Romans deified the red planet as the god of war while writers like HG Wells and Edgar Rice Burroughs imagined it as the home of alien civilisations. Scientists have long speculated that Mars may have hosted life at some point in its planetary history. This public and scientific interest has helped make Mars the most explored and studied of all the planets of the Solar system, with six orbiters and two rovers currently active on and around the planet. Going forward, the US National Aeronautics and Space Administration (NASA) plans to land humans on Mars by 2033. Eventually, Mars may become the staging ground for the exploration of the asteroid belt and the outer planets.
The so-called “tyranny of the rocket equation” – where increasing the payload mass of a rocket results in a multifold increase in the fuel required to launch it – means that typically 85% of a rocket’s mass is just fuel and oxidant with the payload being only ~5% of the rocket’s final weight. Thus, to extend mission durations and to cut the umbilical cord to the Earth, it is critical that we develop technologies to live off the land on Mars through in-situ resource utilisation (ISRU). ISRU technologies that can produce fuel and oxidant on Mars are particularly interesting, as they can eliminate the need to carry fuel and oxidant for the return trip during launch itself.
Despite Mars being called “Earth’s twin”, conditions are inhospitable for human landings. We see from the comparison in Table 1 that the temperature range on Mars is comparable to some of the coldest places on Earth, while the relatively sparse Martian atmosphere is predominantly CO2. Thus, any human landing mission should ensure life support O2 supply for the duration of the mission.
NASA is aiming to produce O2 on Mars to serve as an oxidiser for a return mission and to provide life support for astronauts during a possible 2033 landing mission. Its testbed, “MOXIE” (Mars oxygen in-situ resource utilisation Experiment), is designed to produce oxygen from the abundant CO2 (96%) in the Martian atmosphere via high temperature (~800oC) solid oxide electrolysis. MOXIE aims to produce CO and O2 and separate the gases into breathable, highly pure (>99.6%) O2 and a (highly toxic) waste CO stream. As an alternative, we have come up with a method to simultaneously produce H2 as fuel and O2 as oxidant through low temperature (-36oC) electrolysis of concentrated perchlorate brine solutions on Mars.
The search for water has been the holy grail of Mars exploration. From May 2008 to August 2008, NASA’s Phoenix lander (see Figure 1a) explored the polar regions of Mars. The aim was to understand polar climate and weather, atmospheric composition, and the historic role of water in shaping Martian geology and geography. The lander observed the sublimation of sub-surface ice over the course of 4 days in a shallow trench that it dug. The lander’s thermal and evolved gas analyser (TEGA) detected water vapour at 0oC, while the complementary microscopy, electrochemistry, and conductivity analyser (MECA) instrument detected bound water in the mildly alkaline (pH 7.7) regolith (soil).
The key to our proposed electrolyser is the discovery of perchlorate salts (predominantly magnesium perchlorate) in Mars’ soil at concentrations as high as 0.5 wt.%. While the high concentrations of oxidative perchlorate may limit lifeforms to highly-adapted extremophiles (organisms that can thrive in extreme environments), they also enable the presence of liquid water even at the sub-zero Martian terrestrial temperatures by depressing the freezing point of water below -60oC.
The Mars Express spacecraft shows that multiple sub-glacial water bodies presently exist underneath the Martian south pole deposits at Ultimi Scopuli. Thus, there is water on Mars, and quite a lot of it
Supplementing these observations from Phoenix, the Mars Reconnaissance Orbiter (MRO) has detected surface patterns on Mars possibly indicative of contemporary water flow (see Figure 1b). Finally, recently published data obtained by the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument onboard the Mars Express spacecraft shows that multiple sub-glacial water bodies presently exist underneath the Martian south pole deposits at Ultimi Scopuli. Thus, there is water on Mars, and quite a lot of it.
The freezing point of water is depressed to -68.6oC in 3.5 M Mg(ClO4)2 solutions(78 wt.%; maximum Mg(ClO4)2 solubility in water is 99.3 g/100mL). The low Martian atmospheric pressure of ~6.4 mb depresses the boiling point of pure water to -39.9oC. But in highly concentrated perchlorate brines the boiling point is elevated, following the relation ΔTb=KbbB.
In this equation ∆Tb is the elevation in boiling point compared to pure water, Kb is the ebullioscopic constant (0.512 for pure water), and bB is molality of the solution. This equation predicts a ∆Tb of 4.3oC for 2.8 M Mg (ClO4)2 solutions, resulting in a boiling point of -35.6oC. Therefore, perchlorate brines can exist in the liquid phase between temperatures of -68.6oC and -35.6oC under Martian atmospheric pressures. We have demonstrated the electrolytic splitting of this brine into ultra-pure H2 and O2 using our Martian regolithic brine electrolyser.
The Martian regolithic brine electrolyser schematic is shown in Figure 2a. The device produces H2 and O2 simultaneously during operation via the following reactions at its two electrodes:
Cathode : 2H2 + 4OH- ⇌ 4H2O + 4e- (E0 = 0.83V)
Anode: O2 + 2H2O + 4 e- ⇌ 4 OH- (E0 = 0.40V)
The device assembly is shown in Figure 2b. The standard plate-and-frame design consists of a membrane-electrode assembly, MEA (an anion exchange membrane, AEM, sandwiched between two electrodes), flow channels on either side of the MEA, with the assembly backed by steel endplates. The entire assembly is held together by bolts. The anode flow channels are machined into a corrosion-resistant titanium plate (2 × 2 mm2 single parallel flow channels) to enable operations at the high electrode potentials encountered at the anode, whereas the cathode side employs flow channels machined into a graphite plate (1 × 1 mm2 triple-serpentine flow channels). The anode consists of high-performance lead ruthenate pyrochlore oxygen evolution reaction (OER) electrocatalyst particles bound together with an anion exchange ionomer and deposited onto porous titanium foam. The cathode consists of platinum on carbon hydrogen evolution reaction (HER) electrocatalyst particles bound together with an anion exchange ionomer and deposited onto porous carbon paper. These electrodes sandwich a commercially-available 50 µm Fumasep FAA-3-50 AEM, forming the MEA. These components are shown in Figure 2c.
Typically, water electrolysers work with deionised water feeds so as to prevent any side reactions with other ions present in water. This is critically important, especially with feeds containing chloride ions, as chlorine gas can evolve at the oxygen electrode and deactivate the OER catalysts and also make the Cl2-contaminated O2 unusable for life support. Significant efforts are being made to develop catalysts that are OER selective in the presence of chloride ions, with lead ruthenate pyrochlores being an excellent candidate. These catalysts were also found to resist poisoning from perchlorate ions in solution. Thus, the versatility of these catalysts makes them suitable for terrestrial electrolysers with deionised or even salt water while also being suitable for our Martian electrolyser.
The assembled electrolyser was tested under Mars-like atmospheric and temperature conditions using the experimental setup shown in Figure 3. The low average Martian terrestrial temperatures (-36ºC) were achieved and maintained by immersing a vessel with 2.8 M Mg(ClO4)2 solution (simulated Martian regolithic brine, SMRB, in a dry-ice bath with an ethylene glycol and ethanol mixture. The SMRB feed was purged with pure CO2 to simulate the Martian atmosphere. It was then pumped into the electrolyser cell while the cell was surrounded by an insulating blanket containing chunks of dry ice. The electrolyser in turn was connected to a DC power supply. A flow diagram of the overall setup is shown in Figure 4a. During operations, the current response (proportional to the rate of electrochemical reaction) to a given applied potential was recorded. The production rates of H2 and O2 gas were obtained by collecting them using their negative displacement of water in suitably-arranged vessels (see Figure 4b). As seen in Figure 4c, we controlled the ratio of ethanol to ethylene glycol to control the temperature of the dry ice bath and all experiments were carried out at -36ºC. We were able to operate our electrolyser over 10 hours with no irreversible performance degradation. Furthermore, it exceeded the performance of typical terrestrial electrolysers that use deionised water feeds.
We believe that to explore Mars we need to apply all of the very best technologies available. Thus, we see our electrolyser as being complementary to the high-temperature approach taken with NASA’s MOXIE. MOXIE’s major advantage of being geographically unconstrained comes from its ability to exploit the widely-available atmospheric CO2 to produce O2.
Our system is limited to regions where regolithic brines are prevalent. But we also offer the advantage of producing both H2 fuel and O2. Further, we are able to run our system at near ambient temperatures on Mars while MOXIE expends a lot of its energy in maintaining an 800ºC reactor temperature. This is reflected in the performance comparison presented in Table 2.
We are able to match the O2 output of MOXIE on both a device weight and device volume normalised basis (while continuing to produce H2) using 12 W of electrical power as opposed to 300 W consumed by MOXIE (a 25-fold reduction in power consumption).
Given this dramatic advantage, we look forward to further testing and developing this system and perhaps one day deploying it on Mars!
2. Gayen, P, Sankarasubramanian, S, Ramani, V, Fuel and Oxygen Harvesting from Martian Regolithic Brine, PNAS, (2020).
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