|Hydrogen yield from the hydrothermolysis approach as a function of AB concentration, at different operating conditions. Source: Diwan et al. Click to enlarge.|
A team of chemical engineers at Purdue University have developed a noncatalytic hydrothermolysis approach to release hydrogen from ammonia borane (AB)—a promising hydrogen storage material containing 19.6 wt% hydrogen—in yields and under conditions promising for application in on-board hydrogen storage systems for fuel cell vehicles.
In a paper published online recently in AIChE Journal, the researchers report that the maximum hydrogen storage capacity from a 77 wt% solution of AB at 85 °C (185 °F)—i.e., near PEM fuel cell operating temperatures—was 11.6 and 14.3 wt% at 14.7 and 200 psia pressure, respectively. Research findings were also presented 15 June during the International Symposium on Chemical Reaction Engineering in Philadelphia.
To our knowledge, on a material basis, the AB hydrothermolysis process is the first one to provide such high hydrogen yield values at near PEM fuel cell operating temperatures without use of catalyst. Further, for the same temperature range, these yield values are higher than previously reported in the literature by either AB hydrolysis or thermolysis alone. It is worth noting that DOE system targets for H2 yield are 4.5 and 5.5 wt% for the years 2010 and 2015, respectively. The material-based H2 yield (~14.3 wt%) achieved in this work is sufficiently higher than the target values, suggesting that the noncatalytic AB hydrothermolysis method is promising for hydrogen storage in fuel cell based vehicle applications.
—Diwan et al.
The paper was written by former Purdue doctoral student Moiz Diwan, now a senior research engineer at Abbott Laboratories in Chicago; Purdue postdoctoral researcher Hyun Tae Hwang; doctoral student Ahmad Al-Kukhun; and Arvind Varma, R. Games Slayter Distinguished Professor of Chemical Engineering and head of the School of Chemical Engineering. Purdue has filed a patent application on the technology.There are two conventional approaches to releasing hydrogen from AB: thermolysis and catalytic hydrolysis. Due to limited AB solubility in water, catalytic hydrolysis provides low theoretical hydrogen yield (~5.6 wt%) and also requires expensive catalysts such as ruthenium, the authors note. Thermolysis, on the other hand, requires heating the material to more than 170 °C (338 °F) to release sufficient quantities of hydrogen. However, fuel cells that will be used in cars operate at about 85 °C.
The new process also promises to harness waste heat from fuel cells to operate the hydrogen generation reactor, Varma said.
In the AIChE paper, the researchers investigated their thermolysis approach over a wide range of AB concentrations, pressure, and temperature. In the experiments, they used deuterium (D2O) instead of H2O to clarify the reaction mechanism, enabling them to trace how much hydrogen is generated from the hydrolysis reaction and how much from the thermolysis reaction, details critical to understanding the process.
It was shown that with increasing AB concentration up to 77 wt%, the H2 molar equivalent increased, while the HD molar equivalent decreased. Thus the role of thermolysis (H2 yield), as compared to hydrolysis (HD yield), increases with AB concentration. The effect of pressure on hydrogen yield was found to be insignificant up to 60 wt% AB. An important finding is that for AB concentration > 43 wt %, Treactor ~85 °C is sufficient to release the same amount of hydrogen as at Treactor ~135 °C. This, however, does not hold for lower AB concentrations (≤ 32 wt%).
The maximum observed hydrogen storage capacity, obtained at 77 wt% AB concentration and Treactor ~85 °C, was 11.6 and 14.3 wt% at pressure 14.7 and 200 psia, respectively. Transient experiments showed that during the hydrothermolysis process, the sample temperature increased sharply due to heat evolution by reaction (both AB hydrolysis and the first step of AB thermolysis are exothermic). The H2 and HD evolution began simultaneously at Tsample ~105 °C, and reached maximum value within a few seconds, which suggests rapid reaction kinetics.
—Diwan et al.
Future work on hydrothermolysis will explore scaling up the reactor to the size required for a vehicle to drive 350 miles before refueling. Additional research also is needed to develop recycling technologies for turning waste residues produced in the process back into ammonia borane.
The technology may also be used to produce hydrogen for fuel cells to recharge batteries in portable electronics, such as notebook computers, cell phones, personal digital assistants, digital cameras, handheld medical diagnostic devices and defibrillators.
The recycling isn’t important for small-scale applications, such as portable electronics, but is needed before the process becomes practical for cars.
The research has been funded by the US Department of Energy by a grant through the Energy Center in Purdue’s Discovery Park.
Moiz Diwan, Hyun Tae Hwang, Ahmad Al-Kukhun, and Arvind Varma (2010) Hydrogen Generation from Noncatalytic Hydrothermolysis of Ammonia Borane for Vehicle Applications. AIChE Journal doi: 10.1002/aic.12240