ARTICLE pubs.acs.org/IECR
A Comparison of Ammonia Borane Dehydrogenation Methods for Proton-Exchange-Membrane Fuel Cell Vehicles: Hydrogen Yield and Ammonia Formation and Its Removal Ahmad Al-Kukhun, Hyun Tae Hwang, and Arvind Varma* School of Chemical Engineering, Purdue University, 480 Stadium Mall Drive West Lafayette, Indiana 47907, United States ABSTRACT: Current promising methods to release hydrogen from ammonia borane (NH3BH3, AB; 19.6 wt % H2) including neat thermolysis, thermolysis in ionic liquid bmimCl with or without proton sponge, thermolysis with nano-BN and hydrothermolysis, were investigated for hydrogen yield and ammonia formation. It was found that even trace moisture influences AB dehydrogenation significantly. The hydrothermolysis at 85 °C (13.5 wt % H2, 1 mol % NH3) and thermolysis in bmimCl with 3 wt % moisture at 110 °C (13 wt % H2, 0.2 mol % NH3) methods were found to be the most promising. Since the target for a proton exchange membrane (PEM) fuel cell is an ammonia concentration less than 0.1 ppm, different purification methods were evaluated. Using experiments and simulations, the proposed ammonia removal method, involving absorption in water followed by adsorption on carbon, was optimized and tested. This study demonstrates that, with high hydrogen yield and an effective method to remove ammonia and borazine, AB dehydrogenation is an attractive approach to generate hydrogen for PEM fuel cell vehicle applications.
1. INTRODUCTION Hydrogen is a potentially clean and environmentally friendly energy carrier because, together with oxygen in fuel cells to generate electricity, its only product is water. It has higher energy density on a mass basis than gasoline (120 MJ/kg for hydrogen vs 44 MJ/kg for gasoline) but far lower volumetric energy density (0.01 MJ/L for hydrogen at STP vs 32 MJ/L for gasoline).1 A major obstacle for the development of hydrogen powered vehicles is the lack of safe, lightweight, and energy efficient means for onboard hydrogen storage.2 Current approaches for hydrogen storage include compressed hydrogen gas, cryogenic and liquid hydrogen, metalorganic frameworks (MOFs), metal hydrides, and chemical hydrides.37 The latter are the most promising approaches for hydrogen storage because of the high gravimetric capacity and moderate storage/release temperature. Among chemical hydrides, ammonia borane (NH3BH3, AB), 19.6 wt % hydrogen content, has received extensive attention as a potential hydrogen storage medium.8,9 There are two distinct approaches for AB dehydrogenation: (1) hydrolysis using catalysts (eq 1), 1012 which generates borates and ammonia, and (2) thermolysis (eq 25),1316 which generates various products such as (poly)aminoborane, (poly)iminoborane, cyclotriborazane, borazine (N 3 B 3 H6 ), polyborazylene, etc. Hydrolysis provides low theoretical H 2 yield because of limited AB solubility in water and requires catalysts. Thermolysis, on the other hand, requires either a relatively high temperature (>150 °C) to release 2 or 2.5 equiv of hydrogen per AB, or additives (which constitute weight penalty) for lower temperature operation and shorter induction period.1519 Above 500 °C, complete dehydrogenation occurs forming boron nitride (BN).15,19 From a spent fuel regeneration viewpoint, however, BN is not preferred due to its high chemical and thermal stability.20 r 2011 American Chemical Society
Hydrolysis: NH3 BH3 þ 3H2 O f BðOHÞ3 þ NH3 þ 3H2
ð1Þ
Thermolysis: 1 NH3 BH3 f ðNH2 BH2 Þx þ H2 ; ð90 120°CÞ x 1 1 ðNH2 BH2 Þx f ðNHBHÞx þ H2 ; ð∼150°CÞ x x 1 1 ðNHBHÞx f ðNBHÞx þ 0:5H2 ; x x 1 1 ðNHBHÞx f ðNBÞx þ H2 ; x x
ð > 150°CÞ
ð > 500°CÞ
ð2Þ ð3Þ ð4Þ ð5Þ
For use in proton exchange membrane fuel cells (PEM FCs), ammonia and borazine (a cyclic volatile compound) must be removed from the H2 stream. It has been reported that as low as 13 ppm NH3 can decrease the FC performance, and that the degradation is irreversible for long-term exposure (15 h) to 30 ppm NH3.21 The U.S. Department of Energy (DOE) has set the target for ammonia concentration at
