RsR4:oohs9 ARTICLE IN PRESS Ws (2008)xUK-E and Ca(OH).Faster aluminum consumption in NaOH solutior 人 gators [44].in whose study the optimum temper ature was Other than using aluminum or its alloys alone,combining defined as the temperature at which a high rate of hydroger sodium borohydride (NaBH)with aluminum (or aluminun producto in a and ys)in al alne so proved to be at ie the mass of NaOH and for the d h generation of hydrogen.However.besides temperature and the hydrolysis of NaBH and the catalytic effects of som lkah concentra n.ther e are many ot num alloys on th hyd rolysis of NaBH 47 However ng t of ions,metal pr reatments as well as mixing conditions in the on hydrogen production via the reaction of aluminum or its reactor [41.45].If alloys ar use 451.The follo e 4.2.Aluminum-water reaction in neutral condition (the ma as line ent on the state 2Al+6H-O2Al(OH)+3H (4 (ii)of aluminate ions would inhibit the Calculate the above equation the theoretical of the oxide laver on the metal surface.was able to shorter higher than that of other metals such as mo and Zn (33 wt the induction period,ie.the time required for the reaction 2.4 wt.%,respectively).If w ater produced from the driver achieve a steady-sta its supposed fully the abov (iv) reaction. nt effect on the 56L. aching the t of 6.0 wt for hydr storage systems set by the U.S.Department of Energy 4] Alloy 4 henoh in neu hydrogen generation rate in 16 tested alloy types vater is extremely low.Thus,improving the aluminum activity Martinez et al.[18]studied the influences of NaOH/Al mola can be a ct emperatur highe It was observed that with the same amount of aluminum the cutting,drilling rindins of aluminum and its allo higher the NaOH/Al molar ratio,the higher the initial hydroge water,by which the fresh metal surface was kept exposed in liberation rate.but the d300 [49].The highe lume ot hydrog nlin an aluminum can-based hydrogen production with stopped due to the rapid assivation of metal surface 1491 T proton exchang mem rane fuel cell (PEMFC).A protor faci tate continuo eneration of hydroger metal particle xchange (PE small which pecific expo It was concluded that aluminum cans have bette metal po wders is the high-eneray hall milling a process ir performance are fractured into small powder under the n addition NaOH.other hydr used as the of th size uti prod on 45 by tals 5.52 depen properly controlled since prolonged milling will caus and base concentration at th tme [31 dec in the surface and the oxidation of the co it th 31].A recent study [45]compared the hydro eratio [15.52-551.In addition to its effects on particle sizes.ball performances with three different hydroxides: NaOH.KOH milling induces pitting corrosion process by creating numerous Iction using aluminum and aluminum alloys.renew sustairconditions. An optimum temperature of 70–90 8C and optimum NaOH concentration of 5.75 M were given by early investi￾gators [44], in whose study the optimum temperature was defined as the temperature at which a high rate of hydrogen production was achieved in a controllable manner, and the optimum alkali concentration was thought as the concentration which minimizes the mass of NaOH and H2O required for the generation of hydrogen. However, besides temperature and alkali concentration, there are many other factors affecting the hydrogen production performance, including the morphology and initial amount of the metal, concentrations of aluminate ions, metal pretreatments as well as mixing conditions in the reactor [41,45]. If aluminum alloys are used, the alloy compositions will be the key factor to the hydrogen yield [45]. The following results were found from a comprehensive parametric study by Aleksandrov et al. [41]: (i) the maximum reaction rate was linearly dependent on the initial metal weight for metal powder, while the steady￾state reaction rate was a linear function of the surface area for metal foil; (ii) high concentrations of aluminate ions would inhibit the hydrogen liberation; (iii) polishing of the metal sample, which promotes the removal of the oxide layer on the metal surface, was able to shorten the induction period, i.e. the time required for the reaction to achieve a steady-state level; (iv) stirring rates, in contrast, had an insignificant effect on the reaction. Alloy composition effects were discussed in another work [45] and Al/Si alloy was found to show the highest initial hydrogen generation rate in 16 tested alloy types. Martınez et al. [18] studied the influences of NaOH/Al molar ratio on hydrogen production at a constant temperature (23 3 8C) directly using soft drink aluminum can wastes. It was observed that with the same amount of aluminum, the higher the NaOH/Al molar ratio, the higher the initial hydrogen liberation rate, but the total volumes of hydrogen produced at the molar ratios of 2.00 and 3.00 were found to be the same. Soon after, Martınez et al. [46] extended their work by coupling an aluminum can-based hydrogen production setup with a proton exchange membrane fuel cell (PEMFC). A proton exchange membrane electrolyzer (PEME) driven by solar energy was also installed in their experiment for comparisons. It was concluded that aluminum cans have a better performance. In addition to NaOH, other hydroxides were used as the reacting base for hydrogen production [31,45]. In potassium hydroxide (KOH) solution, a synergistic effect on the hydrogen liberation performance was found to be achieved by increasing temperature and base concentration at the same time [31]. Unfortunately, there was a consumption of KOH due to its reaction with CO2 in the air, which decreased the reaction rate [31]. A recent study [45] compared the hydrogen generation performances with three different hydroxides: NaOH, KOH, and Ca(OH)2. Faster aluminum consumption in NaOH solution was found. Other than using aluminum or its alloys alone, combining sodium borohydride (NaBH4) with aluminum (or aluminum alloys) in alkaline solutions was proved to be able to enhance both hydrogen production rate and conversion yield [47]. Such enhancement was attributed to both the pH increase caused by the hydrolysis of NaBH4 and the catalytic effects of some aluminum alloys on the hydrolysis of NaBH4 [47]. However, NaBH4, a complex hydride made from borax, is quite expensive for hydrogen production. The results of some selected studies on hydrogen production via the reaction of aluminum or its alloys with water in alkaline conditions are summarized in Table 1. 4.2. Aluminum–water reaction in neutral condition Aluminum can directly react with water without the help of alkalis: 2Al þ 6H2O ! 2AlðOHÞ3 þ 3H2 (4) Calculated from the above equation, the theoretical hydrogen yield of this reaction for the mixture of aluminum and water in stoichiometric ratios is only 3.7 wt.% but still higher than that of other metals, such as Mg and Zn (3.3 wt.% and 2.4 wt.%, respectively). If water produced from the driven fuel cell is supposed to be fully recovered for the above reaction, its theoretical hydrogen yield will increase to 5.6 wt.%, approaching the target of 6.0 wt.% for hydrogen storage systems set by the U.S. Department of Energy [48]. In comparison with those reactions assisted by alkalis, this method is much safer, but the surface passivation in neutral water occurs much more easily and the metal activity with water is extremely low. Thus, improving the aluminum activity in water can be an essential task for this scenario. Freshly exposed metal surface possesses a relatively higher chemical activity. The release of hydrogen gas was observed through cutting, drilling, or grinding of aluminum and its alloys in water, by which the fresh metal surface was kept exposed in water [49]. The highest volume of hydrogen generated per unit volume of metal removal was found in the case of grinding. However, the reaction stopped immediately after the machining stopped due to the rapid passivation of metal surface [49]. To facilitate continuous generation of hydrogen, metal particles with small sizes, which increase the specific exposed surface area of metals, are favorable [50,51]. One way to produce fine metal powders is the high-energy ball milling, a process in which materials are fractured into small powders under the action of the ball-powder collisions. The size reduction induced by ball milling strongly depends on the mechanical properties of metals [15,52]. Moreover, the milling time needs to be properly controlled since prolonged milling will cause decreases in the powder surface area and the oxidation of powders, both of which increase the corrosion resistance of metals and therefore inhibit the reaction of metal with water [15,52–55]. In addition to its effects on particle sizes, ball milling induces pitting corrosion process by creating numerous 4 H.Z. Wang et al. / Renewable and Sustainable Energy Reviews xxx (2008) xxx–xxx + Models RSER-547; No of Pages 9 Please cite this article in press as: Wang, H.Z., et al., A review on hydrogen production using aluminum and aluminum alloys, Renew Sustain Energy Rev (2008), doi:10.1016/j.rser.2008.02.009