Focus on Climate Change
The greatest challenge of the 21st century will surely be the concerted attempt to supersede traditional carbon-based fuels with renewable and cleaner energy sources. In 2021 human activity contributed 36.3 billion tonnes of carbon dioxide into the environment according to the International Energy Agency ([1] IEA, 2022). Most notably carbon dioxide retains excess solar heat in the atmosphere which is driving rapid variations in temperatures and weather patterns across the planet. Other pollutants such as sulphur dioxide and nitrogen oxides, which can be emitted from car exhausts, contribute to worsening acid rain that weakens higher altitude forests and vegetation. There is now a significant inter-disciplinary effort between physicists and chemists to develop novel technologies that will enable cleaner energy production in the near future. These green sources must satisfy an ever-increasing global energy demand whilst simultaneously reducing any harmful environmental impacts to meet this challenge.
Figure 1: artist’s impression of terrestrial climate change. Credit: [2] NASA, 2015.
Hydrogen Fuel
Hydrogen is the prime candidate to be the green energy source of the future. It is the most common element in the cosmos making up 75% of the visible universe ([3] Scharf, 2011). It is an abundant form of renewable energy that only produces water when burned. Most impressive is its stored energy which is approximately three times higher than gasoline per gram. This means that a standard sized car would require 1kg of Hydrogen to drive 100km ([4] Crow, 2019). Hydrogen fuel sounds ideal but it presents difficulties that are inhibiting it from replacing gasoline in our automobiles any time soon. Hydrogen is known for being an explosive and flammable gas. This uncomfortable fact was etched into the minds of the public and regulators after the Hindenburg air disaster in 1937. It is a clear reminder of what can go wrong if hydrogen fuel is not used safely. One other difficulty that has been less publicised is the obstacles that the gaseous state of hydrogen under standard pressures and temperatures present to using it as an everyday fuel source.
Figure 2: Hindenburg disaster in New Jersey on May 6, 1937. Credit: [5] The Guardian, 2017.
The Problem with Gaseous Fuels
Gas molecules possess a great deal of kinetic energy in comparison to tighter held particles in solid and liquid states. It would be a nauseous experience if we could sit and ride along on a gas molecule. The molecule would be careening through space while colliding with its neighbours and ricocheting off the walls of the container. Thence it is unsurprising that gas molecules dislike being close together under standard conditions. In fact a fuel tank would need a volume of 11m3 to store just 1kg of hydrogen under atmospheric pressure at room temperature. As previously mentioned this would just be enough to travel only 100km (from Dublin to Tullamore). Hydrogen would need to be compressed at 700bar to squeeze in 5kg of the element at room temperature. ([4] Crow, 2019). This would allow a hydrogen powered car to travel the length of Ireland which is a reasonable range. However the pressurised fuel tank would have to be reinforced adding weight and hence decreasing the predicted mileage. A more fundamental problem is that regulators stipulate that fuel tanks used in consumer vehicles must be limited to pressures of at most 100bar ([6] Dumé, 2020). Luckily there is one other property of gases that may offer a promising low-pressure storage solution. The property is that gas molecules can be made to stick to surfaces and the solution comes in the form of nanoparticles called metal-organic frameworks.
Introducing MOFs
Metal-organic frameworks (MOFs) are porous crystalline materials that act like nano-sponges. They consist of metal ions that are connected via organic linker molecules and physically look like cages that can trap gas molecules inside them. MOFs are highly porous with an astonishing surface area commonly above 2000m2/g. NU-1501-M is an example of a promising MOF that was synthesised at Northwestern University in the United States. The M stands for metal which is either aluminium or iron. The nano-material can store 14% of its weight as hydrogen which sails past the 4.5% target set by the US Department of Energy in 2020 ([6] Dumé, 2020). NU-1501-M has enough internal surface area in just 1g to cover 1.3 American football fields. Researchers in the Snurr Group at Northwestern University are also designing low temperature tanks that can contain the same volume of hydrogen at 100bar as 700bar by using temperatures as low as -196ºC ([7] Bowser, 2016).
Figure 3: a cartoon of a metal-organic framework trapping a gas molecule inside its cage-like structure. Credit: [7] Bowser, Chemistry World, 2016.
The properties of these materials are impressive and the idea behind using them is rather rudimentary. The principle being employed here is that hydrogen gas molecules can be made to stick to these huge internal surface areas thus allowing larger volumes of gas to be stored without requiring highly pressurised containers. This provides an opportunity for the production of a hydrogen fuel tank that would satisfy current regulations.
MOF Design and Self-Assembly
The natural question arises as to how physicists can design and build these elegant nano-cages in the lab. The formation of a MOF takes place via a thermodynamic process called self-assembly. In physics, self-assembly occurs when ordered structures spontaneously emerge from disordered building blocks. It is an equilibrium process governed by the laws of thermodynamics. The second law of thermodynamics tells us that a quantity called the free energy (ΔG) must decrease for the process to happen spontaneously. Self-assembly is also responsible for the formation of other complex structures such as the DNA double-helix molecule.
Computational simulations are used to analyse metal-linker combinations. There are too many possible combinations to synthesise all of them and so virtual analysis is required. The Snurr Group at Northwestern has alone tested over 13,000 different MOFs for optimal low temperature storage of hydrogen ([7] Bowser, 2016). The amazing thing about MOFs is the customisability that they offer. It’s rather like a high-tech Lego set on the nanoscale. The pore size and polarisability (i.e. the stickiness) are two important characteristics that can be fine-tuned. The pores should be big enough to fit the particular gas molecule inside and sticky enough to just hold the molecule in place. In fact the design process can go one step further to create a single MOF with multiple pore sizes.
Figure 4: Simulation of a MOF designed to store hydrogen. The yellow points here indicate the binding sites. Credit: [4] Crow, Chemistry World, 2019.
Using MOFs as Emission Filters
Figure 5: a cartoon of the MOF Cu-BTC containing three cages for storing and hence separating three different kinds of gas molecules. Credit: [8] Grajciar et al, 2011.
A Promising Solution
The current development of metal-organic frameworks specific to low-pressure hydrogen storage looks promising. The area of research is still very much in its infancy but the demand for cleaner fuel sources is rising by the day. Countries such as Japan are leading the way and are aiming to have 800,000 hydrogen powered vehicles on their roads by 2030 ([4] Crow, 2019). Hydrogen still remains a highly combustible and explosive fuel source. However MOFs may be the key to taming this temperamental element. Its natural abundance, energy density and production of just water when burned in oxygen make it much too tempting to ignore.
References
[1] Press Statement by International Energy Agency I.E.A. (2022), Global CO2 emissions rebounded to their highest level in history in 2021, Available at: < https://www.iea.org/news/global-co2-emissions-rebounded-to-their-highest-level-in-history-in-2021 > [Accessed: 08/05/2022]
[2] Tenenbaum, LF. (2015). When global warming gets you down, come back stronger, Available at: < https://climate.nasa.gov/ask-nasa-climate/2352/when-global-warming-gets-you-down-come-back-stronger/ > [Accessed: 08/05/2022]
[3] Scharf, CA. (2011). The molecules that made the universe, SCIENTIFIC AMERICAN, Available at: < https://blogs.scientificamerican.com/life-unbounded/httpblogsscientificamericancomlife-unbounded20110802the-molecules-that-made-the-universe/ > [Accessed: 08/05/2022]
[4] Crow, JM. (2019). Hydrogen storage gets real, Chemistry World, Royal Society of Chemistry, Available at: < https://www.chemistryworld.com/features/hydrogen-storage-gets-real/3010794.article > [Accessed: 09/05/2022]
[5] Walters, J. (2017). The Hindenburg disaster, 80 years on: a ‘perfect storm of circumstances’, The Guardian, Available at: < https://www.theguardian.com/us-news/2017/may/07/hindenburg-disaster-80th-anniversary > [Accessed: 08/05/2022]
[6] Dumé, I. (2020). Ultraporous metal-organic frameworks could make clean energy carriers, physicsworld, Institute of Physics, Available at: <https://physicsworld.com/a/ultraporous-metal-organic-frameworks-could-make-clean-energy-carriers/ > [Accessed: 09/05/2022]
[7] Bowser, S. (2016). MOF menagerie fuels search for hydrogen storage solution, Chemistry World, Royal Society of Chemistry Available at: < https://www.chemistryworld.com/news/13000-strong-mof-menagerie-fuels-search-for-hydrogen-storage-solution/1017524.article > [Accessed: 09/05/2022]
[8] Grajciar, L.,Wiersum, AD., Llewellyn, PL., et al. (2011). Understanding CO2 Adsorption in CuBTC MOF: Comparing Combined DFT–ab Initio Calculations with Microcalorimetry Experiments, The Journal of Physical Chemistry 115 (36), 17925-17933, DOI: 10.1021/jp206002d
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