This chapter reviews the progress of ammonia fuel cells – those that use ammonia directly or indirectly. Ammonia fuel cells have been previously reviewed [ 5 - 7 ]. Ref [ 5 ] was published in 2004 and provides a mini-review focusing only on decomposition catalysts. Ref [ 6 ] was published in 2008 and provides a good review up to that point, however it only addresses decomposition catalysts, ammonia fed SOFCs and SOFC modeling. Ref [ 7 ] was published more recently (2011) but gave a more general overview rather than integrate research findings in the different areas of research in ammonia fuel cells. The present work seeks to integrate the research findings and provide a wide picture of the research conducted in ammonia fuel cells, and to show the development of the field. It also highlights areas that warrant further investigation to fully develop the field. Section 2 discusses developments in hydrogen generation for fuel cells via thermal decomposition of ammonia, electro-oxidation of ammonia, and from ammonia products. Section 3 outlines the development of direct ammonia fuel cells, citing experimental studies and their results. Section 4 reviews the various works done on mathematical modeling and simulation of ammonia fuel cells.

Ammonia provides a source of hydrogen for fuel cells. It contains 17% hydrogen by weight, which can be extracted via thermal catalytic decomposition or electro-oxidation. Alternatively ammonia may be oxidized directly in fuel cells without the need for a separate reactor. Table 1 compares the storage capabilities of various fuels based on their higher heating value (HHV) [ 3 ]. Hydrogen has a very low energy density (per volume) because of its low density. Ammonia’s energy density is comparable to that of compressed natural gas (CNG) and methanol, but lower than gasoline and liquefied propane gas (LPG). Per unit volume, the cost of hydrogen energy is lower than that of ammonia energy, but hydrogen has less energy stored per volume than ammonia. Per unit energy, ammonia is the cheapest energy source listed in Table 1 – estimated at US$13.3/GJ. Note that these values are based on the HHV of the fuel and do not account for conversion of this energy to useful forms. The life cycle production cost of energy from ammonia is estimated at US$1.2/kWh compared to US$3.8/kWh for methanol and US$25.4/kWh for hydrogen [ 4 ]. Thus ammonia presents a very viable and cost effective fuel for fuel cells.

Recently the concept of an ammonia economy has gained eminence [ 1 ]. Like hydrogen, ammonia is carbon free and can be produced from any energy resource. However there are also some significant advantages in terms of storage and transport. Ammonia can be liquefied at room temperature at pressures of 8-10 bar and stored in a similar manner to propane, whereas hydrogen requires expensive cryogenic storage. In addition, ammonia allows for safer handling and distribution than hydrogen. Although it is toxic, its smell can be detected even at safe concentration levels (< 1 ppm). Ammonia has a narrower flammable range than hydrogen and is actually considered nonflammable when being transported, whereas hydrogen burns with an invisible flame. Ammonia is the second most widely produced commodity chemical in the world (second to sulfuric acid), with over 100 million tons per year being transported [ 2 ], and as such its worldwide distribution system is well established. Such is not the case for hydrogen. In fact, one major drawback with hydrogen technologies is the fact that the necessary hydrogen infrastructure does not presently exist. Essentially the ammonia economy can achieve the same benefits of a hydrogen economy, but using infrastructure that already exists.

The concept of a hydrogen economy was revived in the 1990s as interest in fuel cell technology surged. There was an explosion of research into fuel cells since then mainly because of its status as a hydrogen technology, and as such both concepts shared a symbiotic relationship. However there are a number of problems with the direct use of hydrogen in fuel cells. Firstly hydrogen does not exist naturally. Secondly it is not easy to store or transport because of its low volumetric energy density and its small molecular size.

2. Hydrogen generation from ammonia

Hydrogen can be produced from ammonia for use in fuel cells in various ways. Most of the literature is devoted to thermal decomposition or catalytic cracking of ammonia into nitrogen and hydrogen, with fewer articles addressing electrolysis or electro-oxidation. Some papers also address hydrolysis of ammonia products such as ammonia borane.

2.1. Catalytic decomposition of ammonia 2 N H 3 + 92.4 k J → N 2 + 3 H 2 E1 Ammonia is unstable at high temperatures and begins to decompose at 200 C [7]. The slightly endothermic decomposition reaction is shown in equation 1. Thermodynamically, 98-99% conversion of ammonia to hydrogen is possible at temperatures as low as 425 C. However in practice, the rate of conversion depends on temperature as well as catalysts. Thermal decomposition or catalytic cracking is the most common means of hydrogen generation from ammonia. Lipman and Shah [8] report that for large scale hydrogen generation (> 1000 m³/hour), reformation of natural gas remains the most cost effective process, however for small scale generation, (< 10 m³/hour), ammonia cracking becomes slightly more economical than natural gas reformation (see Table 2). This study is based on lifecycle cost analysis, taking into account investment and operation costs. Scale of H2 production (m³/hour) Cost of H2 production, US$ / (m³/hour) Water Electrolysis Natural Gas Reformation Methanol Reformation Ammonia Cracking 10 0.943 0.390 0.380 0.343 100 0.814 0.261 0.285 0.279 1000 0.739 0.186 0.226 0.241 Table 2. Life Cycle Cost of Hydrogen Production via Various Processes [8] The early studies done on ammonia decomposition focused more on ammonia synthesis, and as such considered iron based catalysts. Since then various metals, alloys, and compounds of noble metal characters have been tested for ammonia decomposition. These include Fe, Ni, Pt, Ru, Ir, Pd, Rh; alloys such as Ni/Pt, Ni/Ru, Pd/Pt/Ru/La; and alloys of Fe with other metal oxides including Ce, Al, Si, Sr, and Zr [5]. Various catalysts have been investigated for decomposing ammonia to produce hydrogen for alkaline fuel cells. These include WC, Ni/Al 2 O 3 , NiCeO 2 /Al 2 O 3 , Cr 2 O 3 , Ru/ZrO 2 , and Ru on carbon nano-fibres. Caesium-promoted ruthenium supported on graphite was also found to be very promising [7]. For these catalysts, a minimum temperature of 300 C is required for efficient release of ammonia for hydrogen production. The performance of the catalysts can be quantified using the rate of hydrogen production, conversion fraction of ammonia (fraction of ammonia that is converted to hydrogen), and activation energy. The rate of formation of hydrogen from ammonia decomposition has been measured experimentally, typically in units of millimoles of hydrogen produced per minute per gram of catalyst loaded (mmol/min/g). The performance of various catalysts for ammonia decomposition, reviewed in this section, is summarized in Table 3. Catalyst / Support Temp.

(°C) Rate of H2 Gen. (mmol/min/g) Conv. Eff.

(%) Ref. Nano-sized Ni/Santa Barbara Amorphous (SBA)-15 support 450

500

550

600

650 8.4

17.4

26.8

31.9

33.2 25.0

52.1

80.1

95.2

99.2 [11] Ni/SBA-15 550 12.7 37.8 [12] Ni/SiO 2 400

500

550

600

650 0.4

3.3

6.8

11.4

21.1 1.4

10.5

21.6

36.4

70.0 [10] Ni/SiO 2 550 11.6 34.6 [12] Ni/Al 2 O 3 550 12.7 37.8 [13] Ni/Al 2 O 3 500 24.1 71.9 [14] Ni/Al 2 O 3 coated cordierite monolith

Ni/Al 2 O 3 (unsupported particles < 200 μm) 550 16.5

13.2 50.0

40.0 [15] Ir/SiO 2 400

500

600

700 1.2

5.7

17.6

30.6 3.9

18.2

56.0

98.0 [10] Ru/SiO 2 400

500

600

650 4.5

20.0

30.3

30.9 14.3

64

97

99 [10] Ru/ZrO 2

Ru/Al 2 O 3 550 25.8

23.5 77.0

73.7 [16] Ru/CNT

Ru/K-CNT

Ru/K-ZrO 2 -BD

Ru/ZrO 2

Ru/Al 2 O 3

Ru/MgO

Ru/TiO 2 400 6.2

12.2

8.5

3.7

3.8

5.4

4.3 3.7

7.3

5.3

2.2

2.3

3.2

2.6 [16]a Ru/CNT

Ru/MgO–CNT 400 6.0

8.7 9.0

13.0 [17]b Ru/CNT treated with KNO 3

Ru/CNT treated with KOH

Ru/CNT treated with K 2 CO 3 400 33.3

31.6

31.3 49.7

47.2

46.7 [18]b Table 3. Summary of Ammonia Decomposition Catalysts Performance Reported Papapolymerou and Bontozoglou [9] studied the rate of decomposition at 225 – 925 C and 133 kPa ammonia partial pressure. They used the catalyst in the form of polycrystalline wires of foils, and ranked them in decreasing order of reaction rate: Ir > Rh > Pt > Pd. Choudhary et al [10] performed similar studies at 400 – 700 C with pure ammonia and ranked them: Ru > Ir > Ni. Comparing Ni, Ir and Ru supported in silica, Ru based catalysts have been reported to produce the highest decomposition rates as well as the highest conversion rate of ammonia. Yin et al [5] studied the effect of Ru loading within the silica support (in the range 0-35 wt.%) and found that the conversion rate of ammonia reached a peak at 15% weight loading of Ru. It increased with Ru loading from 0-15%, but above this, the sublayers of Ru were inaccessible thus rendering them redundant. Different supports have also been investigated. The purpose of the support is to enhance the dispersion and increase the effective area of the active catalyst. The support should be stable under reaction conditions and have a high specific surface area. For Ru catalyst, the various supports include silica, alumina, graphitized carbon, carbon nanotubes, and nitrogen doped carbon nanotubes [10,19-25]. Yin et al [16] ranked the supports for Ru in order of decreasing activity measured by ammonia conversion rate: Carbon nanotube (CNT) > MgO > TiO 2 > Al 2 O 3 > ZrO 2 > AC > ZrO 2 /BD. It was proposed that CNTs performed the best because they allowed the best dispersion of Ru and also because of their high purity. CNTs have the added advantage of high conductivity which aids in electron transfer thus facilitating the recombinative nitrogen desorption step (see section 2.2). They further showed that using a MgO-CNT support resulted in better performance of the Ru catalyst than using a MgO base or CNT base alone[17]. Temperature programmed hydrogenation results showed that MgO resulted in even greater stability for the CNT. Studies have shown that acidic conditions are not suitable for ammonia decomposition. Yin et al [16] prepared CNT with KOH and found that they resulted in better catalytic performance measured by reaction rate and conversion efficiency. N 2 -temperature programmed desorption (TPD) results showed that the stronger the basicity, the better the catalyst performance [16]. They later studied the effects of promoter cations and the amount of potassium on the morphological structure and catalysis of Ru/CNT [18]. Essentially they found that ammonia conversion increased as the electro-negativity of the promoter decreased. When Ru/CNT is treated with potassium nitrate, potassium hydroxide or potassium carbonate, the conversion rate of ammonia and the rate of hydrogen evolution are significantly improved (see Table 3). Yin et al [5] concluded that the best catalyst for ammonia decomposition is Ru supported on alkaline promoted CNT. The problem with Ru is that it is a noble metal which will significantly increase the cost of the fuel cell system. For mass production, it is preferable to use less expensive materials. Ru is widely accepted as the most active catalyst for ammonia decomposition, however the performance of Ni is very close [26]. It is possible to substitute the noble catalyst by other more economic active phases. Plana et al [15] studied the effect of having the catalyst in the form of a structured reactor with the hope of extracting greater activity from less expensive metals. The small scales of microstructured devices have inherent advantages, including high heat and mass transfer coefficients and high surface area to volume ratios. They considered cordierite monoliths, which are structured reactors with multiple channels of several hundreds of microns in diameter. Monoliths have uniform flow distribution and low pressure drop which is crucial for the energy-efficiency of the process. Furthermore, they are commercially available, and they can withstand high temperatures and their coating with catalyst layer is a mature technology [27]. They used coated cordierite monoliths with mesoporous calumina, on which they dispersed Ni by electrostatic adsorption. This catalytic structured reactor was thoroughly characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), N 2 phisisorption and temperature programmed reduction (TPR), and it was tested in NH 3 decomposition for in situ H 2 generation under realistic conditions such as pure NH 3 feed and high space velocity. The structured catalyst reactor consisted of Ni supported on alumina-coated monoliths. After prolonged reaction, Ni remained well dispersed with particle sizes of 6 nm and mesopores between 4-5 nm. Ni remained anchored within the alumina matrix and did not plug the pores. 100% conversion of ammonia was observed at 600 C. They found that at temperatures exceeding 500 C, the monolith reactors showed better performance than a packed bed catalyst – higher conversion of ammonia and more robustness. Table 3 shows that the best ammonia conversion and hydrogen generation rates via thermal decomposition are obtained using Ru/CNT catalysts treated with potassium based alkalis. Ni produces very good results as well but requires higher temperatures (500 – 600 C) to produce equivalent performance of Ru at 400 C. The advantage of Ni is that it is less expensive than Ru and can be loaded at high concentrations to achieve the desired results. An anode supported SOFC (with an anode thickness of 500 μm, 40% porosity and 50% Ni by volume) requires a Ni loading of 0.134 g/cm². If it is operated at a current density of 5000 mA/cm², it consumes hydrogen at the rate of 11.6 mmol/min/g of catalyst. If the cell operates at 600 C, then Ni can safely decompose ammonia at the required rate.

2.2. Reaction mechanism of ammonia decomposition Various studies have investigated the reaction mechanism of ammonia decomposition. The reaction steps include 1) adsorption of ammonia onto catalyst sites, 2) cleavage of N-H bond on adsorbed ammonia, 3) recombinative desorption of N 2 atoms [28]. These three steps are respectively illustrated in equations 2-4, where * refers to an active site and X* refers to species X adsorbed onto an active site. N H 3 + * → N H 3 * E2 N H 3 * + * → N H 2 * + H * E3 2 N * → N 2 + 2 * E4 Early studies observed that the rate of ammonia decomposition over Pt and Fe, shifted from zero order with respect to ammonia partial pressure at low temperatures (< 500 C) to first order at high temperatures [29]. Tsai and Weinberg [27] proposed that on Ru crystal catalysts, below approximately 400 C the recombinative desorption of nitrogen atoms (step 3) is rate limiting, whereas above 400 C the cleavage of the N–H bond of adsorbed NH 3 (step 2) is rate limiting. This was based on the observation that the apparent activation energy decreased from 180 kJ/mol at the low temperatures to 21 kJ/mol at high temperatures. It should be noted however, that these early studies did not consider the effects of hydrogen inhibition. Later studies observed that at low temperatures and low ammonia partial pressures, the released hydrogen acted as an inhibitor to the decomposition reaction. Bradford et al [19] sought to gain information on H 2 inhibition on NH 3 decomposition over Ru/C catalyst. NH 3 partial pressure was varied from 1.3-12.0 kPa with temperatures between 370-390 C, and a first order dependence of the reaction rate on NH 3 was observed. They proposed the following equation where α varies from 0.69 to 0.75, and β varies from -1.5 to -2, while the activation energy was 96.6 kJ/mol. r H 2 = k p N H 3 α p H 2 β E5 (5) (5) Egawa et al [30] used deuterated NH 3 on Ru single crystal surfaces and determined that the inhibition by H 2 was a consequence of an equilibrium established among adsorbed nitrogen atoms, gas-phase NH 3 , and gas-phase H 2 ; and that recombinative desorption of adsorbed nitrogen atoms was the rate determining step. Vitvitskii et al [31] came to a similar conclusion based on experimental results acquired with diluted NH 3 . Boudart et al [28] proposed that over W and Mo catalysts, N-H bond cleavage and recombinative desorption of surface nitrogen atoms are slow irreversible steps in NH 3 decomposition, NH 3 is activated via a direct dissociative adsorption step, and the adsorbed N atoms are the most abundant reactive intermediate. Skodra et al [32] found that at higher temperatures (350 – 650 C) and low ammonia partial pressures (0.5 – 2.0 kPa) over a Ru catalyst, hydrogen inhibition was no longer significant. They also observed a second order dependence of the rate of decomposition on ammonia partial pressure. This was explained by assuming step 3 above was the rate determining step. Shustorovich and Bell [33] suggested, based on the BOC Morse potential method, that the rate-determining step of ammonia decomposition is recombinative desorption of N 2 . Chellappa et al [34] investigated pure ammonia (high concentration) over Ni-Pt/Al 2 O 3 catalyst at 520 – 690 C, and H 2 inhibition was not observed. The reaction was first order with respect to NH 3 pressure, and the activation energy was 196.2 kJ/mol. Thus it appears that H 2 inhibition is only significant at low NH 3 concentrations and low temperatures [5]. Earlier studies reported a shift in reaction order from 0 to 1 with respect to ammonia partial pressure as temperature increases, however more recent studies report a shift in reaction order from 1 to 2 with temperature. β varies between -1.5 and -2 at low temperatures and low ammonia concentrations, but shifts to 0 as temperature and ammonia concentration increase. There also appears to be a consensus among researchers that the recombinative desorption of nitrogen atoms is the rate determining step in the decomposition reaction.

2.3. Electrolysis of Ammonia Electrolysis or electro-oxidation is another method of extracting hydrogen from ammonia. It has the advantage of scalability and versatility to interface with renewable energy sources including those whose electricity production varies with time [35]. Hydrogen can also be produced at moderate temperatures. It was first discussed by Vitse et al [35], who proposed the coupling of ammonia oxidation in an alkaline medium at the anode with the reduction of water at the cathode. 2 N H 3 + 6 O H − → N 2 + 6 H 2 O + 6 e − , E 0 = − 0.77 V / S H E E6 6 H 2 O + 6 e − → 3 H 2 + 6 O H − , E 0 = − 0.82 V / S H E E7 The thermodynamic potential for ammonia electrolysis in alkaline media is -0.77 V compared with -1.223 V for the electrolysis of water. The theoretical thermodynamic energy consumption is 1.55 Wh/g of H 2 from electrolysis of NH 3 compared to 33 Wh/g of H 2 from H 2 O [36]. This means that theoretically, ammonia electrolysis consumes 95% less energy to produce a quantity of hydrogen than water electrolysis. This however, does not account for kinetics of the reaction. The most widely accepted mechanism of ammonia oxidation is 1) the adsorption of ammonia on to Pt surfaces, 2) dehydrogenation of ammonia into various adsorbed intermediates (N, NH, NH 2 ), 3) reaction of the intermediates to form N 2 H 2,ad , N 2 H 3,ad and N 2 H 4,ad which then react with OH– to produce nitrogen [37]. The reaction of N 2 H 2,ad is considered the rate determining step. Vidal-Iglesias et al [38-39] conducted differential Electrochemical Mass Spectrometry (DEMS) studies on ammonia oxidation and suggested also the presence of an azide intermediate species ( N 3 - ) at certain potentials. Of the various adsorbed intermediates, NH and NH 2 are active, however, N remains adsorbed (N ad ) and acts as a poison. Although ammonia electrolysis is thermodynamically favorable, kinetics are slow. In practice, high overpotentials are required to drive the ammonia oxidation reaction, and deactivation of the Pt catalyst is observed at high current densities [40-41]. With Pt, N ad is only formed at very high potentials, thus making Pt the best choice of catalyst for electro-oxidation of ammonia. Alloys of Pt have been found effective as catalysts for ammonia oxidation, with the other metals in the alloy chosen for their ability to dehydrogenate ammonia. Endo et al [42] studied combined catalysts of Pt with other metals including Ir, Cu, Ni, and Ru. They concluded that only Ru and Ir can improve the catalytic properties of Pt. In another study, an alloy of bulk Pt with bulk Ir was tested, and the performance was found to be better than Pt alone, however oxidation current densities were still less than 1 mA/cm² [43]. De Vooys et al [40] studied ammonia oxidation and intermediates on various polycrystalline catalyst surfaces – Pt, Pd, Rh, Ru, Ir, Cu, Ag, and Au. They concluded that only Pt and Ir combine a good capability to dehydrogenate ammonia with a low affinity to produce N ad . In another study, a Pt-Ir powder mixture (50 wt.%) impregnated in Teflon and painted on a platinum screen was found to provide much lower overpotentials for the oxidation of ammonia than platinum black [44]. However, in these studies, a very high loading of precious metal catalysts was used (up to 51 mg/cm²) rendering them uneconomical for fuel cell use. Botte et al [35] studied the use of Pt-Ru alloys for ammonia oxidation catalysts. Individually, Pt and Ru resulted in fast dehydrogenation of ammonia at low potentials which resulted in fast deactivation of the catalyst. However, when combined, the Pt allowed for a significant rate of recombination of adsorbed nitrogen. A low loading of Ru prevented the fast ammonia dehydrogenation from prevailing over the nitrogen recombination step. They reported that catalyst preparation using co-electrodeposition allows for a low loading of noble metals (~2.5 mg/cm²). In another study, they evaluated the electrolysis of ammonia on a high surface area Raney Nickel substrate plated with Pt and Rh [45]. The electrodes were characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy. All tested electrodes demonstrated that Rh produced a synergistic effect when paired with Pt as a catalyst for ammonia electro-oxidation. Hydrogen was successfully produced from a 1M NH 3 /5M KOH solution at 14.54 Wh/g H 2 at a current density of 2.5 mA/cm² by an anode containing 1 mg/cm² Rh and 10 mg/cm² Pt at ambient temperature and pressure. When the Pt loading was reduced to 5 mg/cm², the required energy for electrolysis was 16.83 Wh/g of H 2 . They did not report results when Pt alone is used as catalyst. Their results indicated that rhodium can increase the kinetics of the electrolysis reaction while allowing a reduced loading of precious catalysts. Their XPS results indicated that 1 mg/cm² is the optimum loading of Rh, since it maximized the proportion of the noble metal coverage to exposed substrate metal. Nevertheless, the energy required to produce hydrogen is nearly 10 times higher than the theoretical thermodynamic value. In a follow up study, they considered the use of carbon fiber substrate electrodes instead of Raney Nickel [46]. An observed decrease in current density with Raney Nickel at polarization potentials indicated that blockage of active sites by OH– occurred. This blockage was possibly due to non-uniform coverage of the substrate. It resulted in a reduced surface area of the active catalyst. It was proposed that OH– competes with NH 3 for adsorption on to the Pt surface, thereby decreasing the number of available sites for electrolysis. Rh added to Pt has been shown to solve this problem by reducing the number of unused catalyst sites compared with Pt alone. It was also observed that the reactivity of the catalyst decreased over time, indicating that the Ni substrate was not stable. Better results were obtained using carbon fiber electrodes [46-47], which allowed for uniform surface coverage of the noble metal, prevented blockage of active sites, and were light weight compared with Ni. They also obtained promising results using Pt-Ir-Rh and Pt-Ir catalysts on the carbon fiber substrate, which resulted in 91-92% conversion of ammonia to hydrogen at room temperature and low ammonia concentrations, and with electrolysis occurring at current densities up to 25 mA/cm² and a precious metal loading of 5.5 mg/cm². This corresponds to an energy consumption of 18.15 Wh/g of H 2 which is higher than those reported by Cooper and Botte [45], and a hydrogen generation rate of 1.4 mmol/min/g of catalyst. The previous studies were based on bulk catalysts. Other studies have considered the effects of nano-sized Pt particles, but they found that the oxidation of ammonia was more sensitive to the structure of Pt particles rather than their size. Vidal-Iglesias et al [48-50] studied ammonia oxidation on stepped electrodes consisting of Pt (1 0 0) terraces and Pt (1 1 1) steps. They used voltammetry, chronoamperometry, and in situ infrared spectrometry to characterize the electrodes and concluded that electrocatalytic activity is increased by a factor of up to 7 when Pt (1 0 0) is used rather than Pt (1 1 1) or Pt (1 1 0) as the preferential orientation of nano-particles. They found that the oxidation was highly structure sensitive and that it took place exclusively on the Pt (1 0 0) sites. Vidal-Iglesias et al [51] further considered further the effect of adding nano-sized alloys to Pt. Ir, Pd, Rh and Ru were tested. Ru and Pd were found to decrease the oxidation current. In fact, as Ru content increased, the oxidation current decreased. They explained this result by proposing that Pd and Ru decreased the density as well as the dimensions of the Pt (1 0 0) sites. However, Ir and Rh were found to enhance the oxidation current at low potentials. They also studied the effect of particle size and found that 9 nm Pt particles produced better oxidation results than 4 nm particles. This is because the 9 nm particles had a larger number of Pt (1 0 0) sites. They thus concluded that oxidation of ammonia on nano-particles is highly structure sensitive. Much work has been done in developing catalysts to electrolyze ammonia. Calculations show that the rate of hydrogen generated via electrolysis is in the order of 0.1 to 1 mmol/min/g of catalyst, which is several orders of magnitude lower than what is reported for ammonia decomposition. Also the energy consumption required to produce hydrogen ranges from 14-18 Wh/g. This energy consumption needs to be reduced to 5.4 Wh/g in order to produce H 2 at a realistic cost of US$2/kg [45]. This means that oxidation overpotentials must be reduced to below 200 mV at much higher current densities than those reported in the literature.

2.4. Hydrogen production from ammonia borane Products of ammonia have also received some attention in the literature as sources of hydrogen, with most of the studies focusing on ammonia borane (NH 3 BH 3 ) or AB. Ammonia-borane complex has a high material hydrogen content (about 19.6 wt%) with a system-level H 2 energy storage density of about 2.74 kWh/L (versus 2.36 kWh/L for a liquid hydrogen). Hydrogen can be evolved via hydrolysis of AB. N H 3 B H 3 + 2 H 2 O → N H 4 + + B O 2 − + 3 H 2 E8 To employ H 2 as a direct fuel supply for PEMFCs a suitable catalyst is needed to accelerate the hydrolysis of AB. Various catalysts with excellent catalytic performance have been developed [52-63]. These include noble metal based catalysts such as Pt, Ru, Rh, Pd, Pt and Au supported on alumina; combinations of Pt with Ir, Ru, Co, Cu, Sn, Au and Ni supported on carbon, Rh(0) nano-clusters [52-57]; and also non-noble metal based catalysts such as Ni and Co on alumina, and Ni and Co nano-particles, Cu/Cu 2 O, Poly(N-vinyl-2-pyrrolidone) (PVP) stabilized Ni, Ni-SiO 2 and Fe–Ni alloys [58-63]. Unfortunately, most of the aforementioned catalysts, except for the magnetic Fe–Ni alloy catalyst, are difficult to use repeatedly in solution because they are in a powdery form or are supported weakly on a substrate. The development of catalysts with high durability is thus important for practical use. Mohajeri et al [64] studied the room temperature hydrolysis of ammonia borane using K 2 PtCl 6 and found the reaction rate to be third order (second order with respect to catalyst concentration and first order with respect to AB concentration) with an activation energy of 86.6 kJ/mol. Their average hydrogen generation rate was 590.3 mmol/min/g of catalyst, although this rate varied throughout the test. Good results were also obtained using non-precious metal catalysts. Eom et al [65] considered the effect of an electroless-deposited Co–P/Ni foam catalyst on H 2 generation kinetics in AB solution and investigated the cyclic behavior (durability) of the catalyst. The activation energy for the hydrolysis of AB using the Co–P/Ni foam catalyst was calculated to be 48 kJ/mol. Their hydrogen generation rates were an order of magnitude lower than ref [64] at room temperature, but increased with the temperature of the AB solution. After six cycles, the H 2 generation rate dropped to about 70% of the initial values. Xu et al [62-63,66-67] obtained excellent results for hydrolysis of an ammonia borane / sodium borohydride (NaBH 4 ) mixture in a 5:1 mass ratio. Their various works utilized different non-precious nano-catalysts including Ni/silica, Co/silica nano-spheres, unsupported Co nano-particles and Fe-Ni nano-particles. For unsupported Co and Fe-Ni, they reported extremely high hydrogen generation rates at room temperature. Some hydrogen generation rates they obtained are calculated using data provided in their references. 10 nm unsupported Co nano-particles evolved hydrogen at the rate of 775 mmol/min/g, while Fe 0.5 Ni 0.5 generated 178 mmol/min/g, both at room temperature. Although AB has 19.1 wt% hydrogen content, the gravimetric (mass) hydrogen storage capacity (GHSC) is relatively low. The GHSC of the AB – H 2 O system is only 9% when hydrolysis is intended in stoichiometric conditions and even lower with excess H 2 O. Practical studies showed that the effective GHSC is typically only 1% [58]. Demirci and Miele [68] considered the effect that storing AB in a solid form and regulating the supply of H 2 O would have on the effective GHSC. They used CoCl 2 as catalyst, and found that when water was supplied in stoichiometric quantities, an effective GHSC of 7.8% was achieved at 25 C. They reported the hydrogen generation rate to be 85.9 mmol/min/g of catalyst under these conditions. Results in this section show that ammonia borane is extremely promising as a hydrogen source. Hydrolysis of AB has yielded order of magnitude higher rates of hydrogen generation than ammonia decomposition, and it can be done at room temperatures using non-precious metal catalysts. These results are shown in Table 4, which shows significant variations in the performance of the hydrolysis catalysts. For example the hydrogen generation rate using unsupported Co nano-particles is 3 orders of magnitude higher than for Co nano-particles supported on silica. It should also be pointed out that in Ref [62], when the Ni loading increased, the rate of hydrogen generation increased but by an amount less than proportional to the increase in catalyst loading. In other words, as the Ni loading increased, the rate of hydrogen generation per gram of catalyst decreased. Thus there is a lot that is not yet fully understood. The effects of particle size, loading, support as well as other unknown factors need to be investigated in greater detail. Catalyst Conditions Hydrogen Generation Rate

(mmol/min/g) REF Co-P/ Cu sheet 30 °C 38.7 [65] Co-P/Ni foam 30 °C

40 °C

50 °C

60 °C 35.8

69.3

130.0

220.1 [65] K 2 PtCl 6 salt 25 °C 590.3 [64] 20-30 nm Ni/Si 2 O 3 25 °C 7.0 [62] 15-30 nm Co/ Si 2 O 3 nano-spheres 25 °C, NH 3 BH 3 / NaBH 4 mixture 0.7 [66] 10 nm unsupported Co particles 25 °C, NH 3 BH 3 / NaBH 4 mixture 775.0 [67] Fe 0.5 Ni 0.5 nano-particles 25 °C, NH 3 BH 3 / NaBH 4 mixture 178.1 [63] CoCl 2 25 °C, NH 3 BH 3 / NaBH 4 mixture 85.9 [68] Table 4. Performance of Various Catalysts for Ammonia Borane Hydrolysis