It’s going to take some pretty big innovations to address this climate crisis we’ve created for ourselves.

Among them, hydrogen could be the answer to a lot of the problems faced by the transportation, manufacturing, and electricity generation industries. Hydrogen has been held back by a few problems of its own, though.

If you want to learn about the challenges related to creating it, you can watch our video on the subject. But another big problem is storing the hydrogen, however we make it. For reasons I’ll explain, the gas is incredibly difficult to contain in large amounts.

But there may be an interesting solution. Because scientists have proposed making a giant hydrogen ‘battery’ using the very thing it’s intended to replace: Coal. [♪ INTRO] Molecular hydrogen, which is just two hydrogen atoms in a trench coat, is an enticing alternative to fossil fuels because when you burn it, water is the only waste product. Plain ol’ H2O.

Admittedly, to become a true alternative, scientists still have to find a way to make a bunch of hydrogen quickly that doesn’t also release carbon-based greenhouse gases into the atmosphere. But once they do, this tiny chemical could help us replace the fuels we currently rely on, like coal, oil, and gas. But even assuming we can make enough green hydrogen to suit our needs, there’s a problem with this plan: It’s really difficult to store.

Hydrogen is the lightest element, and molecular hydrogen is about as small and energetic as it gets. So while it might be a gas at room temperature and pressure, like many carbon-based fuels, it has an extremely low boiling point. If you wanted to store it as a liquid, you’d need to cool it to negative 253 degrees Celsius.

And then keep it there. If that’s too difficult, it helps to increase the pressure. But if you want to store a decent amount of hydrogen in a sensibly-sized tank, you’d need to maintain a pressure that’s about a hundred times higher than what you’d need for propane.

And even then, hydrogen molecules are so small that they will literally diffuse into the metal walls of the container, causing damage to its structural integrity. So if hydrogen is going to be the next big thing in decarbonised fuel, we need to solve the storage problem. One possible solution is to give up on the idea of pressurisation altogether, and find something that the molecules will stick to.

Something that can soak up a bunch of hydrogen gas like a proverbial sponge. And it turns out, one of those “somethings” seems to be coal. Researchers have proposed that hydrogen could be pumped down into natural coal beds to create hydrogen batteries.

It’s similar to the concept of storing excess carbon dioxide by pumping it into underground rocks, except we’d want to get the hydrogen back out again, eventually. And coalbeds could be an ideal host, because many are known to hold onto a lot of gas. Not hydrogen, though.

Methane. Methane gas is created as the coal forms and the organic matter inside it heats up. And instead of leaking up through the ground and escaping into the air, a lot of those small gas molecules adsorb, or get stuck to the surface of the rock.

But when I say surface, I don’t just mean the outside. Coal is super porous like a sponge, except the pores are much, much smaller than the ones in the thing sitting by your kitchen sink. These are like tens or hundreds of nanometers across.

On top of all this adsorption, a coal bed might be able to trap even more gas if there are layers of impermeable shale or mudstone sitting on top of it. Since methane gas is itself a decent, if very not green, fuel source, industries have already tapped into many of these coalbed reservoirs, and extracted as much as they can. And now, some scientists hope that we can use that existing infrastructure to fill depleted beds back up with hydrogen.

However, not all coals are created equal, and there are several factors that affect how efficient these coalbed hydrogen batteries could be. One is the diffusion potential, or how well the hydrogen can penetrate the coal. And another is the absorption adsorption potential, or how well the hydrogen sticks to the coal’s tiny pores.

Both of these factors depend on the type of coal you have, and its so-called rank… which is related to the percentage of carbon it contains and the amount of energy it releases when it burns. For example, a high rank coal like anthracite contains a lot of carbon, whereas lower rank coals like bituminous or sub-bituminous coals have proportionally less carbon within their structures. But there are other factors that change how effective a particular lump of coal will be as a storage medium, including the local temperature, the pore size, and the gas pressure.

So in 2023, scientists from Penn State released a study investigating how well different locations could serve as hydrogen batteries. They looked at samples from eight coalbeds across the US, including sub-bituminous coal in Montana, and anthracite in Pennsylvania. From their tests, they found that diffusivity was generally higher for hydrogen than it was for methane, which is promising for getting the gas into the coalbed in the first place.

Meanwhile, the adsorption of hydrogen varied between the different kinds of coal at the different sites. For coalbeds that were completely depleted of methane, they found that the higher ranking anthracite adsorbed hydrogen the best. But in many cases, some of the original methane remained in these old reservoirs.

And among these gassy coal beds, the researchers found that lower-rank, low-volatile sub-bituminous coal proved better at storing hydrogen. That’s because, in general, methane is better at sticking to coal than hydrogen is, so the high rank gassy coalbeds will prefer to hang onto their original methane, leaving less space for the hydrogen you try to pump down there. In contrast, the lower-rank coals hold on less tightly to methane, so hydrogen can more easily displace it.

Now, this is all very experimental work, dealing with small, crushed up samples in the lab to see whether a coalbed-based hydrogen battery is even possible. Let alone a legitimate future storage system. There’s still plenty of work to be done to see how it would work in the real world, how easy it would be to get the hydrogen back out of the coal bed, and how it would work with the existing infrastructure from the fossil fuel industry.

But these are exciting first steps towards addressing a sticky hydrogen problem, and paving the way for a versatile clean energy economy in years to come. Thanks for watching this episode of SciShow. We love exploring all the cool new ways that scientists are looking to bring humanity into a greener future, and we wouldn’t be able to do that without the support of our patrons.

If you’d like to help fuel the SciShow machine, head on over to Patreon.com/SciShow to find out all the cool perks you can get as an ever bigger thank you from the whole SciShow team. [♪ OUTRO]