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.
|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.
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