The world is facing a challenge of energy needs—balancing supply and demand, costs, and environmental impact—and one potential solution to this problem is hydrogen.
Hydrogen is promising as a fuel source for combustion engines, fuel cells, and as an alternative to natural gas heating. The only remnant of its combustion is water and there are several ways to produce the gas without the threat of carbon emissions.
However, pure hydrogen gas is costly to produce sustainably and more costly still to store and transport compared to traditional fossil fuels. Energy researchers have been attempting to find the best ways to procure hydrogen as well as the most practical methods for its transport for it to be a true competitor with natural gas or petrochemicals. There are a few ways to approach this problem, find out more below.
For all its uses, pure diatomic hydrogen does have limitations preventing it from being practical on larger scales. The production of hydrogen fuel is not strictly free of greenhouse gases, and there are both sustainable and unsustainable methods of production which need to be taken into account. The most promising currently is the electrolytic splitting of water (using renewable energy), which produces hydrogen and oxygen gases as a result.
With the problem of production solved, the issue of efficiency materialises—at ambient pressure and temperature, there is simply not enough energy per unit volume of hydrogen gas to provide a comparable measure against fossil fuels. The energy density of hydrogen gas per kilogram is almost three times that of traditional fuels, however the realistic energy capability per litre is orders of magnitude smaller.
While the hydrogen gas can be compressed under high pressure, this requires specialised equipment as well as even more energy to do so, and can still only achieve about 5% hydrogen per unit weight (where the remaining 95% is the weight of the pressurised vessel). The same can be said for liquefied hydrogen which requires a temperature of –253°C or colder, necessitating cooling equipment and additional power.
The best solution for efficient usage and transport of hydrogen that scientists have found is not actually pure hydrogen at all. There are alternatives which have a lot of potential—namely chemical storage and physical storage.
Chemical storage is where the hydrogen atoms are stored within molecules through chemical bonds, only to be released after a chemical reaction takes place. There are many potential options for chemical carriers of hydrogen, such as metal hydrides or organic molecules (eg. alcohols, carbohydrates).
To be most effective, a material should have a hydrogen capacity of at least 7% by weight, and have a working temperature between 0 and 100°C. Many metal hydrides require a temperature of at least 200°C to release hydrogen. Organic hydrocarbons are in a similar position, with the added drawback of emitting CO2 as a reaction product.
Physical storage options allow hydrogen to be adsorbed onto the surface of a material in far greater amounts than leaving the gas contained by itself. The most common of these are highly porous sponge-like materials, such as activated carbon or metal-organic frameworks (MOFs). A MOF reported in 2020 was found to achieve an outstanding hydrogen capacity of 14% by weight. The limitation of many MOFs, however, is that they perform adsorption best at very low temperatures (many around –200°C) and lose efficacy as the temperature increases.
Ammonia has already made a name for itself as a vital component of fertilisers, with global annual production exceeding 200 million tonnes in 2021. It has also sparked inspiration as a method of chemical hydrogen storage.
The current ammonia production method is not a green one—the Haber process entails reacting nitrogen gas and hydrogen gas together at high temperatures and pressures, where the hydrogen in question is most often sourced from fossil fuels. However, energy scientists are making strides with alternative production methods, such as fuel cells and membrane reactors, which can give ammonia a greener footprint for fuels, fertilisers, and more.
Ammonia is an inorganic molecule, made up of one nitrogen atom and three hydrogen atoms. This hydrogen density makes it an attractive chemical carrier of hydrogen for energy purposes, as an alternative to transporting pure liquid hydrogen around. Rather than requiring temperatures sub –253°C, ammonia is a liquid at only –77°C at atmospheric pressure, or as high as –10°C under slightly higher pressures. Additionally, ammonia contains no carbon, so has great potential as a carbon-neutral fuel source. It can be split into hydrogen and nitrogen gases in a reverse fuel cell, where diatomic nitrogen can simply re-join the atmosphere with no detriment to the environment.
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