(25d) Hydrogen Generation From Thermolysis of Neat Ammonia Borane for On-Board Vehicle Applications

A safe and efficient on-board hydrogen storage system is a demanding challenge for PEM fuel cell vehicle applications. Current hydrogen storage approaches include compressed hydrogen gas, cryogenic and liquid hydrogen, adsorbents, metal hydrides, and chemical hydrides. Among the chemical hydrides, ammonia borane (H₃NBH₃, AB) has attracted considerable interest because of its high hydrogen content (19.6 wt%). Hydrogen can be released from AB through either hydrolysis or thermolysis. Hydrolysis provides low theoretical H₂ yield (~5.6 wt%) due to limited AB solubility in water. In addition, it generates B-O bonds which are not preferred from the spent fuel regeneration viewpoint, and NH₃ which must be removed for use in PEM fuel cells. On the other hand, thermolysis requires either relatively high temperature (~160 ^oC) to release two equivalents of hydrogen per AB, or additives (which constitute weight penalty) for lower temperature operation (≥ 80 ^oC) and shorter induction period. We have recently proposed and demonstrated a novel method involving noncatalytic hydrothermolysis of AB-water mixture [1, 2]. Using this approach, high 14.3 wt% hydrogen yield was obtained at 77 wt% AB and the set point temperature (T_SP) 85 ^oC. Nevertheless, formation of NH₃ and B-O bonds remain a challenge for this process. Thus, in the present work, thermolysis of neat AB was investigated under various operating conditions. It was shown that using this approach, high H₂ yield (~15 wt %; 2.3 H₂ molar equivalent) can be obtained at 14.7 psia and T_SP 90 ^oC with rapid kinetics. To our knowledge, this value is higher than by any other method using AB at near PEM fuel cell operating temperatures. It was also found that effective reaction heat management is required to obtain sharp heat evolution, which plays a critical role in providing high H₂ yield. The results indicate that the proposed method is promising for hydrogen storage, and could be used in PEM fuel cell based vehicles.

References:

[1] Diwan M, Diakov V, Shafirovich E, Varma A. Noncatalytic Hydrothermolysis of Ammonia Borane. International Journal of Hydrogen Energy. 2008;33:1135-1141.

[2] Diwan M., Hwang HT, Al-Kukhun A., Varma A. Hydrogen Generation from Noncatalytic Hydrothermolysis of Ammonia Borane for Vehicle Applications," AIChE Journal (in press, DOI 10.1002/aic.12240). 2010