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PNNL team scales up ammonia borane slurry for on-board H2 storage for automotive applications

A team at Pacific Northwest National Laboratory (PNNL) is scaling up their work on the use of ammonia borane (AB) slurries optimized with an ultrasonic process for automotive hydrogen storage applications. Ammonia borane is an atttractive material for chemical hydrogen storage (CHS) due to its high hydrogen content of 14–16 wt % below 200 °C.

CHS materials release hydrogen at low temperatures but are not easily rehydrided as are metal hydrides, needing instead to use chemical regeneration. In work reported earlier this year (Choi et al.), the PNNL team optimized a model AB slurry in silicone oil, obtaining up to 40 wt % (ca. 6.5 wt % H2) loading. The US Department of Energy (DOE) gravimetric target for on-board hydrogen storage is near 10 wt %. In the current work, reported in the ACS journal Energy & Fuels, the team optimized the slurry production to prepare 50 wt % (ca. 8 wt % H2) AB slurries and proceeded toward making liter-size batches to show scalability.

To develop a new method to produce high solid loading AB slurries, we established a process to produce large volumes up to 1 L of slurry. … Two major challenges existed in producing both high loading and large volumes: (1) It is common knowledge that the high-energy tip-sonication process used in slurry production results in localized temperature gradients. Without proper cooling these localized gradients could produce a cascade thermal reaction resulting in a rapid exothermic reaction. (2) In addition to regulating the process temperature, providing adequate mixing is also as essential. It has been observed that AB readily absorbs silicone fluid up to 50% of its mass. As with many slurries, increasing the solids content increases the viscosity exponentially. While AB slurries are shear-thinning, the yield stress is quite high and common laboratory mixing methods proved unable to thoroughly mix slurries of high solid loading.

—Westman et al.

Image
Scale-up from 150 mL to produce <2 L AB slurries using an ultrasonic process. Credit: ACS, Westman et al. Click to enlarge.

Once the researchers determined how to design an appropriate reactor to handle the temperature issues, they addressed mixing issues. They found that the key to mixing high solids loading slurries was a high-speed/high-torque overhead mixer. With this method, they noted, it was not uncommon for slurries above 40 wt % to require mixing speeds in excess of 800 rpm and torques as high as 30 ft-lb.

A known issue with AB is foaming expansion during dehyrdogenation; AB can foam up to 10 times its original during hydrogen release.

One of the desirable aspects of a slurry AB is the possibility of controlling this foaming through the use of surfactants or other antifoaming agents (AFAs). Silicone fluids are often used as AFAs in many applications as they have very low surface tension resulting in small, fragile bubbles that rupture when they reach the surface. Unfortunately, the massive release of hydrogen during the first two equivalents, up to 19 wt %, exceeds the silicone fluids’ innate properties and the slurry can foam up to 10 times its original volume. In addition to the excessive volume change this foam tends to solidify if allowed to cool too rapidly. This poses a serious problem for automotive applications as the solidified AB foam is rigid and can easily plug tubing.

—Westman et al.

Testing identified a non-ionic octylphenol ethoxylate that was effective as an antifoaming agent.

Through the development of new and efficient processing techniques and the ability to adequately control the foaming, our results exhibited that stable homogeneous slurry of high solid loading is a viable hydrogen delivery source.

—Westman et al.

Resources

  • Matthew Westman, Jaehun Chun, Young Joon Choi and Ewa C. E. Rönnebro (2015) “Materials Engineering and Scale-up of Fluid Phase Chemical Hydrogen Storage for Automotive Applications” Energy & Fuels doi: 10.1021/acs.energyfuels.5b01975

  • Choi, Y. J.; Westman, M.; Karkamkar, A.; Chun, J.; Rönnebro, E. C. E. (2015) “Synthesis and Engineering Materials Properties of Fluid Phase Chemical Hydrogen Storage Materials for Automotive Applications,” Energy Fuels 29, 6695–6703 doi: 10.1021/acs.energyfuels.5b01307

Comments

Alain

I'm interested in the energy loss in the conversion process H2->slurry->H2.
If this is not extremely good, I wonder why not simply using carbohydrates (ethanol, gasoline, butanol, ...) + H2O as "hydrogen carrier" and simply release the CO2 in the atmosphere.
(carbohydrate + H2O --> H2 + CO2)
If the carbohydrates are produced from CO2, this is CO2-neutral. It obviates the use of potentially dangerous salts and slurries, and obviates the necessity to organise the whole recycling infrastructure of the slurries.
How many megajoules/kg of slurry ? (compared to 35-45MJ/kg of carbohydrate) This is important to stockpile energy reserves.

In my opinion, this slurry-strategy will never make it. It's interesting research, and could potentially be beneficial for niche-applications, but the drawbacks are huge compared to alternatives for automotive energy storage.
- huge amounts of toxic chemicals
- need for a large infrastructure to recycle the slurries
- very difficult to stockpile reserves of slurry (compared to stockpiling carbohydrates) and to transport it.

ai_vin

I think the "niche-application" they're looking at here is FCVs. As such they need something that not only stores a lot of hydrogen but also releases it without to much trouble. Carbohydrates require reforming.

Alain

I'm sure they aim for FCVs, but i guess a small reformer, combined with FC that doesn't get poisened by CO will be much easier to accomplish than the huge challenges of a slurry infrastructure.
(Or silmply a high-pressure H2 tank)

ai_vin

Like I said, they are looking for something that stores "a lot of hydrogen." Ammonia borane is an attractive material due to its high hydrogen content - it is actually more hydrogen-dense than liquid hydrogen while also able to exist at normal temperatures and pressures. It would give a FCV the range a high-pressure H2 tank can't without robbing the car of all its truck space.

ai_vin

For "truck space" read "trunk space" please.

sd

Why bother with Ammonia Borane when you can fuel vehicles with ammonia? Read http://www.nh3car.com/FAQ1.htm or
http://www.nationaldefensemagazine.org/archive/2008/August/Pages/AlternativeFuelsTakingASecondLookatAmmonia.aspx

Apparently, while ammonia is an inhalation hazard, it is considered safer than gasoline and combustion results in N2 and H2O. It is a gas at 15 psi but a liquid at about 150 psi so can be handled like propane.

ai_vin

Ammonia has a gravimetric hydrogen density of 17.8 mass%.
For ammonia borane it is 19.6%
These numbers are close enough that other factors will have to be the deciding ones.

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