Dec 2, 2020. By Kolemann Lutz
(Image credit: Nasa/JPL-Caltech) Depiction of Jezero Crater with a lake around 3.5 - 4 billion years ago where Mars 2020 Perseverance will land in mid-February, 2021.
December 2, 2020
A new perchlorate water brine electrolyzer system has a 25-fold higher oxygen production rate than NASA’s MOXIE using the same amount of power, or a power consumption rate 25 times less than MOXIE with the same oxygen production rate.
Removing toxic perchlorate (ClO4) salts from Martian regolith and brine water with machines around 225 million kilometers away on the arid surface is expensive and difficult. Compositional analysis from wet chemistry instrument on Phoenix lander indicated that Mg(ClO4)2 is a major component of Martian regolith and that magnesium perchlorate brine can remain in liquid phase at up to -70° C. Considering the annual average temperature on the surface is -63° C with an average daily variation of 100° C, liquid brine water could exist on the surface and subsurface.
US-based engineers at Washington University in Missouri developed and tested their new patented brine electrolysis system in a simulated Martian atmosphere. The researchers utilized a ruthenate pyrochlore (Pb2Ru2O7) electrocatalyst for the oxygen evolution reaction (OER) and a carbon doped platinum electrocatalyst for the hydrogen evolution reaction (HER). Having previously demonstrated high-performance alkaline water electrolyzers using the Pb2Ru2O7 for OER catalysts, electric current was passed through the lead ruthenate pyrochlore anode in ClO4 brine water in O2 and CO2 saturated environments over a range of temperatures, 21° C (average Earth temperature) to -36° C (Martian temperatures).
Considering that the ratio of overpotential to current density is lower for the ORR (94–152 mV/dec) at all of the temperatures compared to the OER (158–173 mV/dec) on Pb2Ru2O7, it was apparent that the oxygen electrode was the limiting electrode.
The anode electrolyzers, commercial anion-exchange membrane(AEM) that separates outcome gases, and carbon doped cathodes were operated with 200 mL per min in a CO2 saturated SMRB to mitigate H2O transport and membrane drying loss. As hydrogen was fed to the anode and oxygen gathered at the cathode within the fuel cell to convert the chemical energy, the electrolyzer showed excellent bifunctional ORR and OER electrocatalysts.
“The careful design and unique anode allow the system to function without the need for heating or purifying the water source”, mentioned by research team lead led by Vijay Ramani, distinguished professor in the Department of Energy, Environmental & Chemical Engineering at Washington University.
"Paradoxically, the dissolved perchlorate in the water, so-called impurities, actually help in an environment like that of Mars," said Shrihari Sankarasubramanian, a research scientist and co-author of the paper. Soluble Perchlorate (ClO4) salts reduce waters freezing point to -60°C in the current active water cycle on Mars.
Traditional water electrolyzers use highly purified, deionized H2O, which increases the cost of the system. Water-splitting electrolysis that is adapted to the local geochemical properties of celestial bodies, such as the salty brine on the Martian surface, can significantly improve the economic value proposition of hydrogen and oxygen production.
With oxygen representing a very small fraction (0.14 %) of the present Martian atmosphere, the oxygen reduction reaction (ORR) in fuel cells is critical for energy production due to the inactivity of the ORR.
A human requires around 550 Liters of pure gaseous oxygen per day. To satisfy those same oxygen requirements, the cell active area of the 2.2V brine electrolyzer is .375m and 1.2m^2 respectively.
Solid-state brine electrolyzers have greater efficiencies than alkaline water electrolyzers under simulated terrestrial and Martian conditions.
Brine water electrolysis could become essential life support to bring on one of the first uncrewed missions. "Our Martian brine electrolyzer radically changes the logistical calculus of missions to Mars and beyond" said Ramani. "This technology is equally useful on Earth where it opens up the oceans as a viable oxygen and fuel source".
Research Paper. P. Gayen, P. Sankarasubramanian, and V. Ramani. Fuel and oxygen harvesting from Martian regolithic brine. National Academy of Sciences of the United States of America. November 2, 2020