Zeolites to Store Hydrogen.

A new potential strategy has been proposed to circumvent the vexed problem of storing hydrogen which must be solved if it is to be used on a large scale as a "fuel". Researchers in Spain (Journal of Materials Chemistry, 2006, Vol. 16, 2884) have found that a zeolite exchanged with magnesium cations (Mg2+) has an unprecedented high absorption enthalpy of - 17.5 kilojoules per mole (kJ/mol), which is close to the optimum value of -15 kJ/mol, and so the material shows promise for this purpose. There are a few points worth noting, however, before we go any further. For a start, hydrogen is not actually a primary "fuel" but an energy transfer (storage) medium. That is to say that hydrogen cannot just be dug-up out of the ground, but rather it must be "made" by some artificial means. Most of the hydrogen currently in use in the world (mainly for chemical purposes, such as the wholesale manufacture of fertilisers) is produced from natural gas by a process known as "reforming".

I have noted previously that this term is reminiscent of the reformation of the monasteries, and in practice too, since the methane is broken down and restructured into hydrogen and carbon monoxide, by reaction with steam at high temperatures:

CH4 + H2O --> CO + 3H2.

By an appropriate adjustment of the reaction conditions, an extra molecule of hydrogen can be squeezed out of the system similarly to the "water gas shift reaction", used to enhance the yield of hydrogen for use in the Fischer-Tropsch synthesis of artificial gasoline from coal, which I have discussed previously, hence:

CO + H2O --> CO2 + H2.

Oil, since it is made of hydrocarbons can similarly be converted to hydrogen; hence we have not broken our dependency on gas/oil by using hydrogen per se. Of course, hydrogen can also be produced by the electrolysis of water, but the electricity has to be made by some means, to do it with. Hence we are currently using gas, coal, oil and nuclear for this purpose. I have calculated in some of my earlier postings the huge amount of "renewable" infrastructure that would be needed to make enough hydrogen to substitute for the some 54 million tonnes of oil (equivalent) that we use each year, e.g. by wind-power. Not that I am knocking "renewables" at all since this is where we will need to go eventually, but I think that many who use that word, and ask, "why don't we use more renewables?" don't understand the sheer scale of energy density that we require to be so implemented by its means. There are serious analysts (notably) Ulf Bossel who have concluded that the putative "Hydrogen Economy" is a non-starter. And so it is, in terms of any kind of plastic replication of our status quo using hydrogen; the entire way of life and demand upon energy that we pursue has to change first, before such alternatives are even worth discussing.

So, why is storage of hydrogen such a big deal? There are a number of reasons, is the short answer. It may be feasible to produce hydrogen on a local scale, say to feed community "pods" as I have termed them, to supply essential transportation and to run machinery, especially if there is some local source of electricity e.g. hydroelectric or a wind farm of sufficient extent. However, it is not feasible to generate hydrogen in situ, and effectively burn it at source. The wind doesn't always blow, for instance, and so this secondary energy carrier must be compiled in some way, to be used as necessary. So that implies some system of central "tanks" to put it in. Also, if it is to be used as a "fuel" to run vehicles, it must be put into a fuel-tank of some design, just like gasoline is now. O.K. there are gas-powered cars and trucks, which run on compressed natural gas, and that might be one way of storing hydrogen. However, it takes energy to compress hydrogen, which detracts from the "black" side of the energy balance sheet, and since hydrogen is a very light gas (molecular weight of 2 compared to methane, at 16), the fuel/tank weight ratio (if I can phrase it so) is lower than is practicable. Ideally, that should be at least 6%, and it has been found that some zeolites have a maximum storage capacity of about 4.5% which brings these materials into the range of consideration for this purpose.
Hydrogen can also be liquefied, since a liquid "fuel" packs more of a punch weight for weight than a compressed gas (more molecules per unit volume or mass), but its liquefaction requires even more energy than its compression does. Liquefying hydrogen is difficult in any case, since it is one of (I believe) only three gases known that has a a negative Joule Thompson effect: i.e. the compressed gas warms on expansion, and unless it is first cooled to 193 K it cannot be liquefied by such compression-expansion cycles, as say air can - in order to produce liquid nitrogen as a coolant. On 10 May 1898, James Dewar used it to become the first to statically liquefy hydrogen. Using liquid nitrogen he pre-cooled gaseous hydrogen under 180 atmospheres, then expanded it through a valve in an insulated vessel, also cooled by liquid nitrogen. The process is, as you might imagine, costly in terms of energy. Hydrogen is a highly inflammable gas (think of the R101 Zeppelin going up), and so there are grave worries about the safety of hydrogen as a fuel.

Therefore, storing it in a solid matrix of some kind would be the best means. The Spanish study of zeolite MgY (magnesium exchanged zeolite Y) might therefore look promising. If hydrogen is to be stored in solid materials, a critical balance must be struck, i.e. for an optimum delivery cycle, the adsorption enthalpy should be neither too low (so that suficient storage will occur) nor too high (so that hydrogen can be released on demand. Previous measurements of this quantity for hydrogen adsorbed in zeolites are in the range of about -5 to -10 kJ/mol, all rather shy of the optimum -15 kJ/mol (so the absorption capacity and corresponding weight for weight ratio of hydrogen to the storage material is not ideal). At -17.5 kJ/mol the MgY zeolite seems to be the best yet, and I have no doubt that fine-tuning of this value is possible by varying the presence of other cations (it is not fully-exchanged), or by changing the type of zeolite framework.

Once again, I am left with concerns about scale. Let's just consider the case for the U.K. which uses 54 million tonnes of oil to supply its transportation = 54 x 10*6 x 42 x 10*9 = 2.268 x 10*18 Joules of energy.

Given that the heat of combustion of hydrogen = 285.83 kJ/mole, we would require 7.93 x 10*12 moles of H2 = 1.59 x 10*13 grams = 1.59 x 10*7 tonnes of H2. At an upper absorption capacity of 4.5%, we would need to put this lot into 100/4.5 x 1.59 x 10*7 = 3.53 x 10*8, or around 353 million tonnes of zeolite Y. Since zeolite Y is a synthetic zeolite, it needs therefore to be manufactured. Agreed, we wouldn't need to store an entire year's worth in one go, but say 10% of the total might be a conservative estimate, so we do need to manufacture at least 35 million tonnes of the zeolite. To place this into context, the entire world petrochemical industry only uses around 200,000 tonnes of zeolite Y annually, mainly for cracking oil fractions into gasoline and other fuels. Hence this new hydrogen-based fuel industry would require a considerable scale-up in zeolite production, on a world scale to around 100 times this! 3,500 million tonnes? Even if that quantity of zeolite could be made available, it would still be necessary to keep the material cool, certainly at liquid nitrogen temperature, in order for it to absorb and retain sufficient hydrogen to be any use as a "store" for it. Therefore the energy costs of liquefying sufficient nitrogen coolant should also be factored into the balance sheet.

Whatever course we chose, there is no avoiding the issue of gargantuan energy use. We are merely exposing this central point in different ways, with each new "energy solution" that is discovered, once a simple arithmetic scale-up is applied! Sorry to you guys in Spain; a lovely fundamental result in science, and which may have small scale applications in the future, but we are far from out of the woods yet.