Material science
material such as graphite is the anode; the electrolytic cell is run at around 900°C. Oxygen is stripped off from the oxide cathode in its ionic form, and transported though the molten salt electrolyte. The negatively charged oxygen erodes the carbon anode and produces carbon dioxide. To generate oxygen by this process, not carbon dioxide, the Cambridge scientists needed an inert anode. For this, they used a mixture of calcium titanate and calcium ruthenate. Because this anode barely erodes, the reaction between the oxygen ions and anode produces oxygen. The cathode is regolith from Moon rocks.
‘NASA sent us some simulants and we
were able to generate oxygen through this process,’ says Fray. ‘At the start you get oxygen and iron, then the reduction of other oxides and more oxygen, so one advantage of the process is that you can get a mix of metals, an alloy.’ Fray envisages the power for the
process being supplied by solar panels, or a small nuclear power reactor. The relatively low temperature of 900°C, the simplicity of the process and possibility to use robotic technology are advantages, he says (Planetary Space Science, 2012, 74, 49).
Another plus, says Fray, is that the FFC
process has been refined and developed by the spin-out UK company Metalysis to provide an alternative to the Kroll process for extracting titanium metal and tantalum. Here the metal oxides serve as the cathode in the electrolytic reaction, with a carbon anode. The process should ultimately benefit any lunar process. One slight disadvantage of this
approach is that you have to bring the electrolyte calcium chloride to the Moon, Fray acknowledges, and you must also consolidate the Moon’s loose regolith before processing. But he envisages robots collecting, transporting and consolidating lunar material, something
Electrolysis of molten magma has the potential to produce oxygen on the Moon
NASA is keen on. Another competing process – high
temperature molten oxide electrolysis (MOE) – is emerging from the Massachusetts Institute of Technology (MIT) in the US. High temperature electrolysis is used to make liquid metal
and oxygen from a metal oxide feedstock but requires rare and consumable anode materials like iridium. It is being investigated as a way of producing iron without the colossal carbon dioxide emissions generated by traditional steel smelting. As well as more environmentally
friendly steel making (C&I, 2013, 6, 6), material scientist Donald Sadoway at MIT has also used this method to produce oxygen and metal from Moon rock simulants. This involves heating regolith up to 1600°C, turning it into a molten mass. When potential is applied across the electrodes, oxygen evolves at the anode and metal is deposited at the cathode. The successful deployment of MOE thus hinges upon finding an inert anode capable of sustained oxygen evolution. According to Sadoway, iridium is the ideal anode metal because its oxide cannot form at above 1200°C so it remains as a metal during electrolysis, rather than corroding away because of oxidation (Journal of the Electrochemical Society, doi:10.1149/1.3560477).
But it is expensive. ‘NASA could justify the cost of iridium because it costs $100,000 per kilo to move mass from Earth’s gravity to the Moon, so it matters little what the cost of the precious metal is. Making oxygen on the Moon has a very different price point [to steel production on Earth],’ says Sadoway. ‘Initially you would use iridium, though you might make some improvements in terms of precious metal alloys. You can wait on making metal on the Moon but you need six pounds of oxygen per day or you won’t be around tomorrow. You can’t take chances with that.’
Not to be thwarted, Sadoway went on
to discover a chromium alloy anode that is up to the task (Nature, doi: 10.1038/ nature12134). Fray acknowledges this discovery as having implications in the future for extraterrestrial oxygen and iron generation, and could be a boon to Solar System exploration (Nature, doi: 10.1038/ nature12102).
While some chemists see the high
temperatures in Sadoway’s approach as a deterrent, he disagrees. ‘What I found is that the higher temperatures give you higher throughput and the energetic cost is quite small,’ he explains. ‘When you get beyond a certain cell size, the current generates enough heat to keep the cell at operating temperatures. That is how aluminium electrolysis works. To run
34 Chemistry&Industry • November 2013
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NASA
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