The consequences of the climate crisis are now being seen through highly unseasonal weather events. Among the most startling of these were wildfires that tore through large areas of Siberia, Alaska, Greenland and Canada in 2019, believed to be caused by consistently high temperatures around the arctic.
Developments like this are now forcing more urgent action from major industrialised nations. In April 2020, President Biden vowed to cut US emissions by at least 50% before 2030, while the UK recently reaffirmed its ambition to be ‘net zero’ by 2050. The achievability of these targets, however, will not only depend on limiting greenhouse gases through carbon capture but also a wholesale shift away from the use of fossil fuels.
This is what’s meant by the ‘energy transition’ – moving from current energy production and consumption practices towards a more efficient, low-carbon energy mix. It’s difficult, not least because it has to be managed alongside economic development, but most experts agree it’s feasible providing changes are made today and technological breakthroughs are fully exploited.
The discovery of metal-organic frameworks (MOFs) represents one of these breakthroughs. With the largest surface area known to science, these super adsorbent materials can hasten the replacement of oil, coal and gas by solving many of the challenges associated with today’s alternative fuels. This includes hydrogen, a clean energy source that many believe will be central to a more sustainable future.
What are MOFs?
MOFs are a new class of crystalline super-adsorbent. Composed of metal ions and organic linkers, they can be formed in one-, two- and three-dimensional structures. Their highly porous nature often sees them likened to sponges, though unlike that material MOFs can be designed to capture, store and release specific gases. This is done through a careful selection of metals and linkers during the first stages of synthesis.
It’s this bespoke quality along with a high storage capacity that gives MOFs so much promise. Other adsorbents, like activated carbon and silica gel, cannot hold anywhere near as much medium and are unable to be used in such a selective way. MOFs, on the other hand, offer record-breaking porosities (up to 10,000m2/g) and can be developed to target everything from ethylene and ammonia to carbon dioxide and methane from complex gas mixtures.
Hydrogen has long been seen as one of the best candidates for easing the world’s dependency on fossil fuels due to its natural abundance and clean burning properties. It features in many energy transition strategies, including the UK’s ‘ten point plan’ and European Union’s pathway to carbon neutrality. The latter, for example, seeks to use hydrogen wherever electrification is not suitable or cost-effective, such as in HGVs.
Other key areas of research include private transport. The phase out of diesel and petrol engines is already planned in many countries, with batteries and hydrogen fuel cells expected to take their place in the coming years. Major companies like Toyota have already introduced hydrogen-powered cars to the market but practical challenges, like onboard fuel storage and manufacturing costs, have slowed wider progress.
Domestic heating is another area where hydrogen is expected play an important role, particularly as it can be piped through existing infrastructure. This will lower the cost of transitioning building stock but more importantly limit the boiler’s environmental impact, which is responsible for a high percentage of the home’s carbon emissions. Early trials that have introduced a 20% blend of hydrogen into natural gas networks have been favourable but, much like the car industry, wider roll-out is dependent on access to pure sources of fuel. If this issue can be overcome, a UK-wide programme across all homes would equate to removing roughly 2.5 million cars from the road.
Unlocking the Hydrogen Economy
Hydrogen fuel may still face a number of challenges before it can become society’s standard green energy source but MOFs have proven versatile enough to offer solutions across the full chain of the hydrogen economy.
Take storage, for instance. The low volumetric density of hydrogen sees it take up more space in a container when compared to other gases, meaning cryo-pressurised and liquified storage are necessary to make it practical. However, these methods are not the most secure or efficient ways to handle hydrogen as it’s highly flammable. Solid adsorbent materials like MOFs offer one route out of this problem. High porosity and a large surface area mean they can store far greater volumes of gas in a confined space without the need to increase pressure.
One study at UC Berkeley saw record-breaking hydrogen storage capacities at near-ambient temperatures when researchers used an adsorbent-based product in a standard gas tank. These kinds of findings have also piqued the interest of the US Department of Energy. It found that MOFs enable storage systems to operate at a nominal pressure of less than 350 bar, offering “a range of advantages compared to existing systems” such as fast fill-up and discharge rates that would be essential for supporting the transport industry.
As the gas is held in a MOF’s structure by weak attractive forces, release is also a relatively straightforward process. An end user would only need to apply small amounts of heat or pressure to an adsorbent product to break its intermolecular chain. These unique properties have seen MOFs receive considerable interest from the automobile industry, which has historically struggled to store enough gas at the correct pressure without using expensive specialised tanks. While the application of MOFs in this area is still in development, results suggest they will be key for improving the driving distances possible from hydrogen-powered vehicles. Lower working tank pressures will also help to cut build and development costs, giving more people access to greener technologies that would otherwise be unaffordable.
Along with their storage potential, MOFs can also help realise the new levels of purity demanded by end-use applications. Fuel cells, for example, need an exceptionally pure supply of hydrogen to achieve optimum performance yet there are very few naturally occurring sources available. Today the cheapest way to produce useable hydrogen gas is by purifying hydrocarbons and other compounds, but these processes tend to leave behind impurities that need removing.
The removal of contaminants is a strong application area for MOFs because they can be tuned to target specific gases and remove unwanted molecules from gas mixtures. Their high stability also allows them to be recycled and reused multiple times, even in harsh industrial conditions where another materials falter. Early testing has proved favourable, with hydrogen now able to be purified from a range of sources, including in some of the most common post-production hydrogen gas mixtures containing methane, nitrogen, carbon dioxide and carbon monoxide. Again, this area of research is still developing but MOFs are slowly opening the door to a cheap, effective means of generating pure hydrogen that will be key for things like weaning home boilers off natural gas.
The ideas in this piece are just some of the ways MOFs are set to positively disrupt the energy transition. Indeed, this discussion has not touched on other key applications where these materials are set to transform how society gets its power. Among the most important of these is using MOFs as electrocatalysts to support water splitting – a process central to the large-scale production of green hydrogen. By now though it should be clear that these versatile products continue deliver on their promise, offering the type of progress that other so-called ‘wonder materials’ only hint at. MOFs cannot solve the energy transition as a standalone solution – nor can any other technology – but they will play a critical role both today and in the future.
For more information, please contact Dr Conor Hamill, Chief Operating Officer of MOF Technologies or visit: https://www.moftechnologies.com/