Non-Battery Powered Electric Vehicles

If vehicles always drive on a given route, power can be provided externally. Trams and trains have done this for a long time, and it is also possible to embed an electric power source into roads and power vehicles by induction. My personal view is commercial interests will make this latter option rather untenable. So while external power canreplace quite a bit of fossil fuel consumption, self-contained portable sources are required.

In the previous posts, I have argued that transport cannot be totally filled by battery powered electric vehicles because there is insufficient material available to make the batteries, and it will not be very economically viable to own a recharging site for long distance driving. The obvious alternative is the fuel cell. The battery works by supplying electricity that separates ions and converts them to a form that can recombine the ions later, and hence supply electricity. The alternative is to simply provide the materials that will generate the ions and make the electricity. This is the fuel cell, and effectively you burn something, but instead of making heat, you generate electric current. The simplest such fuel cells include the conversion of hydrogen with air to water. To run this sort of vehicle, you would refill your hydrogen tank in much the same way you refill a CNG powered car with methane. There are various arguments about how safe that is. If you have ever worked with hydrogen, you will know it leaks faster than any other gas, and it explodes with a wide range of air mixtures, but on the other hand it also diffuses away faster. Since the product is water (also a greenhouse gas, but one that is quickly cycled away, thanks to rain, etc) this seems to solve everything. Once again, the range would not be very large because cylinders can only hold so much gas. On the other hand, work has been going on to lock the hydrogen into another form. One such form is ammonia. You could actually run a spark ignition motor on ammonia (but not what you buy at a store, which is 2 – 5% ammonia in water), but it also has considerable potential for a fuel cell. However, someone would still have to develop the fuel cell. The problem here is that fuel cells need a lot more work before they are satisfactory, and while the fuel refilling could be like the current service station, there may be serious compatibility problems and big changes would be required to suppliers’ stations.

Another problem is the fuel still has to be made. Hydrogen can be made by electrolysing water, but you are back to the electricity requirements noted for batteries. The other way we get hydrogen is to steam reform oil (or natural gas) and we are back to the same problem of making CO2. There is, of course, no problem if we have nuclear energy, but otherwise the energy issues of the previous post apply, and we may need even more electricity because with an additional intermediate, we have to allow for inefficiencies.

As it happens, hydrogen will also run spark ignition engines. As a fuel, it has problems, including a rather high air to fuel ratio (a minimum of 34/1, although because it runs well lean, it can be as high as 180/1) and because hydrogen is a gas, it occupies more volume prior to ignition. High-pressure fuel injection can overcome this. However there is also the danger of pre-ignition or backfires if there are hot spots. Another problem might include hydrogen getting by the rings into the crankcase, where ignition, if it were to occur, could be a real problem. My personal view is, if you are going to use hydrogen you are better off using it for a fuel cell, mainly because it is over three times more efficient, and in theory could approach five times more efficient. You should aim to get the most work out of your hydrogen.

A range of other fuel cells are potentially available, most of them “burning” metal in air to make the electricity. This has a big advantage because air is available everywhere so you do not need to compress it. In my novel Red Gold, set on Mars, I suggested an aluminium chlorine fuel cell. The reason for this was: there is no significant free oxygen in the thin Martian atmosphere; the method I suggested for refining metals, etc. would make a lot of aluminium and chlorine anyway; chlorine happens to be a liquid at Martian temperatures so no pressure vessels would be required; aluminium/air would not work because aluminium forms an oxide surface that stops it from oxidising, but no such protection is present with chlorine; aluminium gives up three electrons (lithium only 1) so it is theoretically more energy dense; finally, aluminium ions move very sluggishly in oxygenated solutions, but not so if chlorine is the underpinning negative ion. That, of course, would not be appropriate for Earth as the last thing you want would be chlorine escaping.

This leaves us with a problem. In principle, fuel cells can ease the battery problem, especially for heavy equipment, but a lot of work has to be done to ensure it is potentially a solution. Then you have to decide on what sort of fuel cells, which in turn depends on how you are going to make the fuel. We have to balance convenience for the user with convenience for the supplier. We would like to make the fewest changes possible, but that may not be possible. One advantage of the fuel cell is that the materials limitations noted for batteries probably do not apply to fuel cells, but that may be simply because we have not developed the cells properly yet, so we have yet to find the limitations. The simplest approach is to embark on research and development programs to solve this problem. It was quite remarkable how quickly nuclear bombs were developed once we got started. We could solve the technical problems, given urgency to be accepted by the politicians. But they do not seem to want to solve this now. There is no easy answer here.

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.