Near the end of last year I published a video on solid hydrogen storage and it got a lot of questions, critiques, and push back from some of you. In some cases, rightfully so, and in other cases, not so much. In trying to simplify a pretty complex topic, I oversimplified some things, which created problems. So is solid hydrogen storage actually a thing? Is it something that’s currently being used? Let’s take another crack at solid hydrogen storage and try to address some of the shortcomings of my last video.
Many are still betting on green hydrogen as an essential piece for fueling the transition to clean mobility and energy storage. And that’s especially true for heavy-duty vehicles like buses, trucks, trains or ships, as well as long term energy storage. Yet, with the current available technologies, creating hydrogen and storing large amounts of it is still tricky and expensive. We need to take a closer look at the metal hydrides that make this storage possible.
What are metal hydrides?
A metal hydride is formed when hydrogen bonds with a metal.1 They’re sometimes referred to as solid-state hydrogen batteries. The very first metal hydrides date back to the 1930s.2 However, their energy applications didn’t start to solidify until the end of the last century. Since the early 1990s, nickel hydrides have been used in rechargeable batteries.3 A decade later, hydrogen was stored onto intermetallic compounds4 to work as a stationary backup power unit.5
So, how do these materials work? Basically, when a hydrogen gas molecule approaches the metal surface, it dissociates, or breaks down into two hydrogen atoms. These travel from the metal surface to its internal crystal structure. That’s where atomic hydrogen bonds with the metallic framework through a process called absorption.6 To reverse this mechanism and release the hydrogen, you need to heat up the compound, which requires extremely high temperatures that can go above 300 C (572 F).7 At those temperatures, it’s not exactly an energy efficient process to release the hydrogen.
Sometimes, this is referred to as “solid hydrogen”, even though it’s the metal structure that’s the solid. The hydrogen atoms are small enough to fit between the lattice of the metal structure, meaning they can sit between the metal lattice atoms. I won’t go into too much detail here, but they’re held in place by Van der Waals bonding, which occurs when the motion of electrons around a nucleus leaves an area locally charged. Other times, the hydrogen is captured into intermetallic compounds where the hydrogen is directly bonded to the metal atoms covalently. Regardless of the bonding mechanism in order to free the hydrogen from this bond, heat has to be added to give it enough energy to slip free and then diffuse through the lattice back out of the metal and into the gaseous form. That’s a lot to take in.
So what about at a practical level? How does solid hydrogen stack up against conventional storage techniques? Like compressing it or liquifying it? One of the main advantages of metal hydrides is that they can store hydrogen at much lower pressure compared to gas tanks, where hydrogen is kept to up to 10,000 psi (ca. 700 bar).8 The only way to withstand such a high pressure without having super heavy cylinders is to reinforce the vessels with something like carbon fiber. It’s a very safe system, but it is often cost prohibitive.9 Liquefaction is another way to store hydrogen that doesn’t require high pressure.10 The drawback of liquefaction is that you need to keep the liquid hydrogen in a cryogenic vessel to minimize evaporation at -252.8°C (-423°F)11. Just like the high heat for releasing hydrogen from some forms of metal hydrides, this is an energy-expensive process. It also doesn’t fully prevent hydrogen from boiling off. Instead, metal hydrides can work for a long time without losing any hydrogen. Despite a much higher capital cost, binding hydrogen with a metal framework may work out as a cheaper storage solution in the long run. That’s what the Department of Energy (DOE) concluded when comparing the operation and maintenance (O&M) costs of different hydrogen storage technologies.12 They pegged metal hydride storage at 0.02 $/kWh versus compressed gas and liquid hydrogen at 0.04 $/kWh and 0.06 $/kWh.
Energy density is another essential factor to look at when comparing storage options, especially when considering small-size vehicles. Small cars can’t carry bulky, heavy storage units. You need something that can store a lot of energy per unit of weight and volume to maximize driving range. This is why a technology with a high energy-to-weight/volume ratio13 like lithium-ion batteries are the preferred solution for light-duty, consumer electric vehicles (EVs).
With all of this in mind, here’s one of the things I should have explained better in my last video: the difference between gravimetric and volumetric energy density. Let’s take two identical plastic bottles. If you see the amount of water each of them can store as energy…which it is in a way…when you fill them up to the same level the two bottles have the same volumetric density. However, if one of the two bottles were made of metal, this would have a lower gravimetric energy density than the lighter plastic bottle. When it comes to storing hydrogen the universal rules still apply. From a gas to a liquid to a solid, you go from the least dense to the most dense. So a liquid form can save you a lot of space compared to a gaseous phase.14 Even when the gas is compressed at the highest practical pressure (700 bar), hydrogen gas has a volumetric density of only 40 Kg/m3.15 This falls short of liquid hydrogen’s 70 Kg/m3. However, when considering aluminium or magnesium hydrides, the volumetric hydrogen density balloons through the 80 Kg/m3 threshold … so, why’s that?
The reason is linked to how you create metal hydrides. As I mentioned earlier, before hydrogen bonds with a metal core, its molecule dissociates into two atoms. These are then tightly packed into the metallic structure, which results in a high hydrogen-to-metal ratio for most complexes.16 17 I had a chance to speak to Dr. Brandon Wood from Lawrence Livermore National Laboratory, who backed this up.
“The advantage of a metal hydride is that you form a direct chemical bond between hydrogen and something else. And that allows you literally to compact the hydrogen in much closer than you could even with a liquid” -Dr. Brandon Wood
But how close are we talking about?
“Angstroms apart, as opposed to what you have in a liquid or gas, which is much larger than that.” -Dr. Brandon Wood
And that’s true even when factoring in the space taken up by the metal hydride framework.
“This is not a matter of debate, it’s just a scientific fact. If you get something and it’s in a solid phase, it’s generally speaking more compact than it is in a liquid, even if you pay the dead weight for the matrix host.” -Dr. Brandon Wood
Yet, metal hydrides have a relatively low energy density by weight. For this reason, a tank filled up with metal hydrides is about four times heavier than a gasoline tank.18 Obviously, this is not a great solution for cars in comparison to other technologies available. However, they could be a cost-effective option as a stationary energy source where you don’t need to care about weight. Also, they’d be an advantage when storing hydrogen on heavy-duty EVs. For instance, industrial machines like forklifts need heavy counterweights to balance the hefty loads they carry. With no high pressure, the factor of safety required for designing new tanks is greatly reduced. All of this reduces costs and is another selling point for solid hydrogen.
Metal hydrides current applications
Some of the more vocal critics of my last video approached metal hydrides in general as a fantasy or fool’s errand. Let’s make sure we’re not in a fantasy land and look at the real world. Have these materials been used successfully for hydrogen storage? Yep.
Working as a storage unit for a fuel cell, metal hydrides have been propelling some German submarines since 2003.19 As you can imagine, extra weight is a bonus rather than a limitation in this case, as submarines need a lot of counterweight to stay underwater.20 With a price tag of $500 million, a fuel cell system costs as much as a diesel submarine.21 When equipped with a metal hydride storage unit, fuel cell submarines can run underwater for up to 14 days. That’s much longer compared to the max of 2 days for a battery hybrid submarine. Thyssen Krupp Marine Systems, who was the first to dive into this field released their 4th Generation Fuel Cell (FC4G) just two years ago.22
How about something a little closer to home? Hystorsys AS, a spin-off of the Norwegian Institute for Energy Technology (IFE)23, has a unit that can be mounted on a wall, like in your home. Besides stand-alone solutions, their technology, called HYMEHC, can also be a power backup for industrial sites. The storage unit of this thermally driven system is a vessel filled with a metal alloy powder. The powder has more surface area, speeding up the diffusion of hydrogen into the lattice. When absorbing the hydrogen onto it, they cool the unit down to 15 C (59 F)24, while heating it up to 95 C (203 F) to release the gas. By recycling waste heat, their device drives down its operational costs.25 HYMEHC is currently used in an off-grid house in combination with renewables and other energy storage solutions.26 27 This isn’t the only home solution. In Australia the company Lavo has a Tesla Powerwall-like device that uses metal hydrides too.
The global organization, GKN, is also working on a similar use case. Their engineers formulated a new metal alloy powder and compacted it into pellets. These allow for a more dense hydrogen storage solution compared to compressed gas tanks.28 The system’s working pressure when capturing hydrogen is 20x less than typical pressurized vessels. To withdraw hydrogen, you just heat the material to 65°C, significantly lower than the 300 C, mentioned earlier. With a storage capacity of 50 kWh per 100 kg of metal hydride, GKN pellets are on par with the lithium-ion battery used in a Tesla Model 3.29 You can use clean electricity to power an electrolyser, which generates green hydrogen, and stores it in their metal hydride. This way, they can store solid hydrogen for months if needed until feeding it to a fuel cell to generate electricity on the way back out. They started a pilot in 2019 using their technology as a winter power backup for an off-grid house in the Alps.30 By recovering the waste heat, GKN system achieves a round-trip efficiency of 90%, which is about the same as a typical lithium ion battery. They’ve also integrated the system into a hot water heater setup to get some extra use out of the thermal heat. Based on the company white paper31, their solid hydrogen storage technology becomes more cost-effective for capacities higher than 80 kWh. Just this October, GKN received $1.7M funding from the US DOE to test the scalability of their storage system.32 Two 9 MWh units will be connected to a megawatt-scale electrolyser and fuel cell facility.
Hydrogen in General
That leads me to the elephant in the hydrogen room, which has nothing to do with metal hydrides, and that’s cost and efficiency. When you consider the combination of power-to-hydrogen and hydrogen-to-energy processes, you achieve a max round-trip efficiency of only around 46%.33 This is still far from more mature technologies like pumped-storage hydropower, which delivers up to 85% of the electricity stored. Based on a Volkswagen study, power losses along the hydrogen fuel chain translates into an efficiency of 35% for hydrogen-powered electric cars in the best case scenario. Which is less than half of what you get for battery electric vehicles on average.34 35
Efficiency isn’t the only…roadblock. Cost is a major driving factor regardless of efficiency. Gasoline is still cheaper to make. Mostly because of the low cost of crude oil, accounting for 53% of its production fees.36 37 Adding up taxes, transportation and refinery costs, gasoline price at the pump is about $3.30/gallon on average.38 Instead, with the current technologies, you need nearly twice as much to produce 1 gallon of gasoline equivalent (gge) of hydrogen.39 40
It’s the one-two punch of efficiency and cost that make hydrogen a tough sell in the automotive industry. It’s not just the lower round trip efficiency on its own, which brings me back to metal hydrides. As hydrogen technologies like electrolyzers, fuel cells, and storage systems improve, the overall efficiency will increase and the costs drop. Metal hydrides could make much more sense than other technologies like batteries when it comes to longer term stationary storage applications. I’ll let Dr. Wood explain why…
“If you think about a battery, if you want to double its capacity, you basically need two identical cells.” -Dr. Brandon Wood
“Batteries tend to be very heavy. They’re never going to get super lightweight just because the chemistry that’s associated with the cathode in a battery requires heavier elements. So you double the capacity of the battery, you double its weight and you double its volume. That’s not true for hydrogen because the only thing you need to double is the storage tank. You don’t need to double the fuel cell stack.” -Dr. Brandon Wood
“So with a small fuel cell stack and a gigantic storage tank, you can get all the advantages of multiplying by 10 the capacity of a standard battery, without any of the weight limitations that you would get from that.” -Dr. Brandon Wood
Back in 2019, the National Renewable Energy Laboratory (NREL) estimated that hydrogen tanks coupled with fuel cells were more cost-effective than batteries when storing energy for more than 13 hours.41 Also, in a more recent study42, researchers compared the levelized cost of energy (LCOE) of the major long-duration energy storage technologies at different discharge times. Not surprisingly, lithium-ion batteries were one of the cheapest solutions for a 12-hour discharge. Yet, combining underground hydrogen storage with a heavy-duty fuel cell turned out to be more competitive for a 5-day duration.
The end result is that the viability of solid hydrogen depends on your use case. There’s a lot of research and predictions around the levelized cost of hydrogen and where it’s projected to go over the coming decades.43 44 45 46 47 48 49 Metal hydrides are already a … solid reality … for fueling heavy-duty mobility like submarines as well as ensuring the energy autonomy of remote households. However, storage is only one of the pieces in the hydrogen puzzle. We also need to achieve more cost-effective fuel production and power conversion.
As with all of my videos, I always include a link to the script with all citations and sources that you can check out for more information down in the description. Even though this update is twice the length of the previous video, and gave far more context around solid hydrogen, I’m still just scratching the surface. It’s part of the reason why it took me and my team a while to pull this one together. Just like all my videos, this isn’t meant to be the definitive, comprehensive source of knowledge, or calling a technology a success or a scam, but it’s a jumping off point. Things have to get boiled down to fit my usual 10-12 minute long video and be understandable for a large audience. It’s a challenge that all science and technology YouTube channels struggle with. In fact, Kurzgesagt put out a video not too long ago called, “We lied to you … and we’ll do it again.” It’s definitely worth watching.
I’m still learning this stuff and am always trying to improve. And for all of the people that gave me constructive criticism, thank you.
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