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Handheld Hydrogen: Metal Ammine Complexes in Tablet Form

Amminex Handheld Hydrogen (H2) tablets

Researchers at the Technical University of Denmark (DTU) have developed an ammonia-based solid-state hydrogen storage solution: a tablet that can be held in your hand.

The tablet is a metal ammine complex that stores 9.1% hydrogen by weight in the form of ammonia absorbed efficiently in magnesium chloride: Mg(NH3)6Cl2.  The storage is completely reversible, and by adding an ammonia decomposition catalyst, hydrogen can be delivered at temperatures below 347º C (656º F). The tablets can be recharged with additional ammonia.

Other metal ammines are possible as well, such as Ca(NH3)8Cl2, although the team spent most of its investigation with the magnesium chloride complex.

Jens Nørskov, Claus Hviid Christensen, Tue Johannessen, Ulrich Quaade and Rasmus Zink Sørensen are the five researchers behind the invention. Together with DTU and SeeD Capital Denmark, they have founded a company—Amminex A/S—which will focus on the further development and commercialization of the technology.

The technology is a step towards making the society independent of fossil fuels. We have a new solution to one of the major obstacles to the use of hydrogen as a fuel. And we need new energy technologies—oil and gas will not last, and without energy, there is no modern society.

—Professor Jens Nørskov, director of the Nanotechnology Center at DTU

Ammonia has been explored as a potential hydrogen storage medium since the 1970s. Due to the toxicity of ammonia in its liquid form, most recent work on ammonia as a hydrogen storage solution has focused on solids such as ammonia borane. Researchers at PNNL, for example, are investigating ammonia borane and polyammonia borane. This family of molecules demonstrates hydrogen capacities of > 12 wt%. (Earlier post.)

Metal ammine complexes have been known for more than a century. The DTU team is finding that the kinetics of ammonia adsorption and desorption with the metal ammine complexes are reversible and fast, and that the complex is simple to manufacture and easy to handle.

For use in a PEM fuel cell, the ammonia released from the tablet would then need to be decomposed to hydrogen, and the resulting gas cleansed of any remaining ammonia (probably by passing it over a small amount of unsaturated MgCl2).

Metal Ammine Complexes as Hydrogen Storage
Gravitmetric H2 density
(wt % H2)
Volumetric H2 density
(kg H2/l)
Energy density
US DOE Goal 2015 9.0 0.081 9.72
Mg(NH3)6Cl2 9.1 0.11 13.0
Ca(NH3)8Cl2 9.7 0.12 14.4

Should you drive a car 600 km using gaseous hydrogen at normal pressure, it would require a fuel tank with a size of nine cars. With our technology, the same amount of hydrogen can be stored in a normal gasoline tank.

—Professor Claus Hviid Christensen, Department of Chemistry at DTU

The DTU research team has just published its first paper on the storage method in the Journal of Materials Chemistry.

(A hat-tip to Distantbody!)




I don't get it. Why not just put liquid ammonia in your tank, and avoid the extra conversion step?


Safety and ease of handling...and regulatory issues related to those. Ammonia is on the EPA list of 366 extremely hazardous substances subject to community right-to-know provisions of the Superfund Act and emergency planning provisions of the Clean Air Act.

That&rsquo’s one reason researchers have concentrated on solid state these tablets, or the ammonia-borane complexes.

According to the DTU group, the tablets have the same volume density as liquid ammonia, but are more it’s easier to design a safe container for the metal ammine complex, and even if it is outside a container it is simple to handle at room temperature due to the slow kinetics.

The T-Raissi paper cited above has a good background on the past decades of looking at ammonia as a hydrogen storage compound. (The “Hydrogen Energy Economy” talk kicked off first in the early 1970s.)


OK, so ammonia is rather toxic, and storing it in some kind of solid will make it safer and easier to handle. However, is liquid ammonia worse (harder to handle, more dangerous if released accidentally) than gasoline, around which we have built our current automobile fueling infrastructure?

Just curious.


Well, neither is particularly good for humans...:-)

Ammonia seems to carry more of a potential immediate hazard from inhalation. In other words, breathing in even relatively low amounts of ammonia will irritate the lungs, and in higher concentrations, lead to pulmonary edema and possibly death. Inhaling gasoline fumes in small amounts will make you dizzy. (Very large amounts could put you into a coma. Gasoline is also a potential carcinogen.)

From the point of view of a widespread fueling infrastructure—in which there are inevitably going to be spills, leakages and the occasional customer getting a good whiff—liquid ammonia would be risky.


Not to mention VERY stinky. I wouldn't want a station near me.


also note that ammonia is a gas at room temp (boiling point = -33 C), so liquid ammonia I think is stored under pressure, which makes handling even more difficult. When released, it is detectable by smell at about 50 ppm, so it doesnt take much to stink a lot, and it's highly corrosive and highly toxic by inhalation. I think that adds up to a lot more dangerous than gasoline, which, with the exception of flammability, is relatively easy to work with because it's a liquid and isnt especially toxic for low exposures by skin contact or inhalation like you'd get when filling a tank.


These hydrogen systems seem to be getting more complicated, i.e.expensive, as time goes by. Thinc zinc.


Hydrogen - in any form - will never prove to be a practical fuel for small vehicles.

Think renewable biofuels...


I haven't read about how you get the ammonia out of the pellets, but I assume it is by heating them. That suggests that there will be a certain amount of ammonia being given off even at room temperature, which given our extreme sensitivity to this toxic gas, implies that there will be odor problems even with the pellets.

Also, in any refueling situation, it would be necessary to remove the old pellets from the gas tank and replace them with new, ammonia-charged pellets, so the whole process of fueling the car would be very different from what it is today. I'd imagine the pellets could still be delivered via some kind of hose, probably thicker than the hoses used for gasoline, but I don't know how you would get them out of the car. Maybe a button would open up the bottom of the car's pellet storage tank and they would fall out into an underground bin positioned beneath the car.


It is hard to imagine ammonia ever gaining consumer acceptance as a passenger vehicle fuel because of the smell alone. In addition, most ammonia is currently manufactured from natural gas with CO2 as a byproduct which eliminates much of the purpose of using it as fuel, unless additional terawatts of wind, solar and nuclear electrical plant capacity is built to produce ammonia through some other process involving the electrolysis of water.

Robert McLeod

What the article doesn't mention is that you have to heat up the pellets to 620 K to liberate all the ammonia from them, which is energy intensive. It's about 30 % of the heating value of the hydrogen to liberate it from the salt.

What this means is that without a fancy heat exchanger that can move waste heat from the stack to pre-heat the solid pellets -- really complicated conceptually -- the concept falls short of liquification. Over at theWatt we talked about this and basically came to the conclusion that this was unworkable for PEM fuel cells because the operating temperature is too low and there's not enough waste heat available.


This may not be the solution to providing the worlds future energy needs, but it gets people thinking in the right direction. Hydrogen can be sustainably produced from water and electrical power (generated from wind, tidal, or solar energy), but hydrogen is difficult to store and transport. There are huge wind resources in the Aleutian Islands and Patagonia, but no way to get the energy to where it is needed. 5 megawatt wind turbines on a 200 mile by 10 mile area of the Aleutians (with typical spacing between turbines) would generate enough energy to replace ALL of the petroleum consumed in the U.S. for transportation (~ 12 million barrels/day). By converting H2 to ammonia, using the Haber process and N2 from air, we have an easily transportable form of the energy. We are working on a different scheme whereby the ammonia is converted into guanidine (CH5N3) which is a stable solid. When combined with water at about 100 C, ammonia is released in two steps. One molecule of CO2 is also released, but that CO2 was required in the synthesis of guanidine so the process is sustainable and CO2 neutral. This process is similar to that mentioned in one post, the process by Amendola which uses urea, only we store about 1.5 x the amount of ammonia per unit weight.


P.S. To the above post

More info on guanidine for energy storage can be found at:

comments to:

[email protected]


Greetings folks--

I'm really happy to see this much interest in hydrogen storage by ammonia and ammonia "cousins".

I congratulate the DTU folks on their announcement of a way to store ammonia (therefore, hydrogen) in solid form. I just feel I need to provide some clarifying/factual information. This is good work by the Danes and deserves recognition. But, as far as ammonia as a hydrogen fuel, it's just a peek at what's possible...

I barely know where to start...(probably won't take you long to determine I'm an anhydrous ammonia fan)...

1. The DTU folks have issued a press release about a solid metal (magnesium) ammmine complex (MAC) with a hydrogen content of 9.1 percent. Anhydrous ammonia--NH3 ("ammonia") is 17.6 percent hydrogen. Also, ammonia is a pumpable liquid at room temperature with a mild pressure of only 125 psi, much like propane. In fact, you can store liquid ammonia quite readily in propane tanks.

2. The solid MAC reportedly liberates ammonia gas at 347C (620K, over 650F). Anhydrous ammonia is already a gas at room temperature.

3. The solid MAC is reported as "reversible", liberating ammonia (not hydrogen) gas at 347C. (Probably using the ammonia partial pressure (not hydrogen) for the reaction driver to establish/shift reversibility). So a "filling station" for this solid product would be an ammonia pressurization station, not a hydrogen station.

4. It is reported that the heat of formation for the magnesium ammine is 40-80 kJ/mol NH3. So, of course, the reversibility comes with a 10-20% energy burden.

5. Quoting loosely from T-Raissi report quoted in the DTU paper, "... anhydrous ammonia is a a wonderful fuel, except for its toxicity..." I'M NOT SURE WHERE THIS FOLKLORE COMES FROM, BUT AS A FARMERGUY, I KNOW THAT APPROXIMATELY 15 MILLION TONS OF NH3 (ANHYDROUS AMMONIA, PURE AND SIMPLE), ARE CONSUMED IN THE US MIDWEST AS FERTILIZER ON AN ANNUAL BASIS. Don't you think that the farmers (no farmer jokes...) have figured out the risks and have developed safe transport/ storage/ transfer/ handling procedures over the last 50 or more years? Check the accident stats in Iowa, Nebraska, Kansas, you name it. Not to mention, the current existence of 2000+ miles of ammonia pipeline in the US Midwest.


The folklore comes from the material safety data sheets (MSDS) required for every chemical. Just enter "ammonia MSDS" into Goggle and read the results. Here are quotes from one MSDS that I found:

"Exposure can cause coughing, chest pains, difficulty in breathing. Repeated significant overexposure can cause permanent lung function damage, edema and chemical pneumonitis. May cause serious damage to the eyes."

"Eyes:Eye irritant. May cause severe eye irritation with corneal injury and permanent vision impairment.

Skin: Skin irritant. Contact may cause severe skin irritation, chemical burns, and blistering. Contact with vaporizing liquid may cause frostbite due to rapid evaporative cooling. Cooling effect may mask the extent of corrosive injury received.

Inhalation: Irritating to entire respiratory tract. Excessive overexposure may cause severe irritation to the upper respiratory tract and potential lung damage."

Farmers and industrial workers use many dangerous chemicals routinely. I worked for a company that used phosphine gas. It is extremely toxic, yet no worker was ever injured to my knowledge. Farmers use organophospate pesticides routinely, yet I would not want some minimum wage person punping pesticide or phosphine gas near me. About a week ago I was at a gas station when a customer drove away while gasoline was still being pumped. The attendant told me that this happens frequently and sometimes the gasoline keeps flowing onto the ground. I would not want to think about that happening with anhydrous ammonia.

Despite what I say above, I believe that ammonia is the best sustainable fuel we have. We need to face the toxicity issue and work towards safe means of delivering ammonia to internal combustion engines or fuel cells. The Danish work is a good start, although probably not the answer. Ignoring the fact that anhydrous ammonia will never be accepted as a consumer fuel (although it could find acceptance in industrial applications such as locomotives and cargo ships) does not serve the long-term interests of ammonia advocates or of our country.


As another former farm guy, the father of one of my best childhood friends nearly died from an accidental blast to the face from anhydrous ammonia while refilling tanks. He temporarily lost his sight, it peeled several layers of skin from his face and was left with permanent scarring in his throat. Not to mention that to this day he has no sense of taste or smell. Of course, this is just one event, but the point is that this isn't the sort of stuff that a soccer mom is going to want to handle directly.

Speaking of smell, ammonia stinks (alot) and even the inevitable tiny leaks will be noticeable. Nobody will want to live or have a business within a block of an anydrous filling station.

Ammonia may eventually find acceptance in cargo ships, maybe locomotives, etc. but it is hard envisioning acceptance of the stuff in passenger vehicles.

Nikos Tsogkas

This is a very interesting announcement which is echoing acroos the news papers of the world. Best wishes in future progress and development. Your work's results will be anticipated by all people of the world. Our planet needs a breath of air. Good luck!


Science teacher
Oxford, UK

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