A prediction in my SciFi novel “Red Gold”

Time to brag a bit! I know, bragging is BAD, but here I cannot help myself. Around science fiction, there are always these comments about things that SF predicted. You know, like the “flip-open” communicator in Star Trek that looks suspiciously like some mobile phones. Well, I am going to claim a partial success. There are two tricks with such predictions. The first is to find a need, which, of course, drives all successful inventions. The second is that nobody recalls the failures, so, predict away! However, in my case, unlike most others, the prediction is not what it is, but how it would work, and that is a lot harder. The reason I put science into my novels is not to predict or show off, but rather to try and show those interested some of the principles under which science works.

One problem in my novel Red Gold was, how would settlers on Mars power their transport? Since there is no air, any combustion motor would require carrying your own oxygen, so the obvious answer is, electricity. There are now two problems: how to get power, and how to get enough total energy. Electricity could come from either rechargeable batteries or fuel cells, and both use the same basic chemistry, although some chemistry for one is not suited to the other. For example, most fuel cells now run on hydrogen and air, and a rechargeable battery that generated gas on recharging would soon blow up. Similarly, a sodium – sulphur system that works in batteries might provide a challenge as to how to feed a fuel cell.

Basically, a fuel cell (or a battery) works by burning something in a controlled fashion such that instead of generating heat, the energy comes off as electric current. I decided that a fuel cell would be better than a rechargeable battery because the battery can only store so much charge, whereas a fuel cell can go indefinitely if you recharge the fuel and remove the waste. Further, I rejected the use of hydrogen and oxygen because both would have to be in gas bottles, and as we know from the use of compressed natural gas, compressed gas takes up too much volume for a given range. That suggested the use of metal. The metal I opted for was aluminium, which is desirable because each atom gives up three electrons, and it is a solid that is easily available. At first sight, this may seem strange because to get power the reaction must be fast. Thus iron rusts, but that is so slow and fuel cell might make snails win a race with the vehicle, yet iron rusts faster than aluminium corrodes.

Aluminium has been postulated for fuel cells for about 30 years, but no real progress has been made. There are two problems with it. First, the aluminium cation with three positive charges strongly attaches itself to solvent, which means it moves very slowly, which in turn means any possible fuel cell will have a very poor power output. The second is that aluminium reacts strongly with oxygen and forms an oxide coating on its surface that effectively protects it from all sorts of reagents, which is why aluminium corrodes so slowly. Aluminium was the metal of choice to contain white fuming nitric acid for the German rocket fighter in WW 2. (White fuming nitric acid was mixed with aniline, and the spontaneous combustion gave an impressive power output. Since the fuel tanks were just behind the pilot, he was effectively flying a bomb if something went wrong.)

To get around this, I opted for chlorine as the oxidizing agent, and there were three reasons for this. The first was that chlorine and chlorides totally disrupt that oxide layer, and hydrochloric acid reacts furiously with aluminium. The second was that chlorine would be a liquid at Martian temperatures, and hence apart from its corrosive nature, which can be got around with ceramics, it would be easy to handle. The fact that it is toxic is beside the point because everyone has to wear their own breathing system on Mars because it has no significant atmosphere. The third is that the reason aluminium is usually a problem is because when it burns, it forms a small cation with three positive charges on it, and these charges polarize the solvent and large amounts of solvent stick to each cation, they then do not move very quickly, and hence the power output is very low. However, if aluminium is burned in chlorine in a fuel cell, chloride anions bond to aluminium chloride to give (AlCl4)-, an anion with one negative charge, even though the three electrons have been given up. Of course, this was a bit detailed for a novel, so I just left it with the fuel cells, and left it to those with a bit of chemical knowledge to work out why I put it there. So, why the brag?

Last week, in Nature (vol 520, p 325 – 328) Lin et al. have developed an aluminium-chloride battery that has quite dramatic properties: charge/discharges over a minute with 3 kW/kg are claimed. If it works in a battery, it should work just as easily in a fuel cell. One of the key aspects is that the reaction is that (AlCl4)- reacts with Al to make (Al2Cl7)-, which makes the whole process so fast. Another important point is that the product of burning the aluminium, namely AlCl3, actually helps further reaction and does not impede the reaction, although of course, from a volume point of view it would have to gradually removed. There is a long way to go yet, and I doubt there would ever be such a fuel cell on Earth because chlorine is a rather dangerous gas, but it should work on Mars. Not, of course, that I shall live long enough to see. Nevertheless, the fact that I could predict some chemistry that would work when up to thirty years of work by others had not is very satisfying to a chemist.

If anyone is interested in Red Gold, it will be on a Kindle count-down special from May 1 for six days.

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