4562r 2009 American Chemical Society pubs.acs.org/EF

Energy Fuels 2009, 23, 4562–4566 : DOI:10.1021/ef9004169Published on Web 07/20/2009

Feasibility Study ofHydrogenGeneration from theMilled Al-BasedMaterials forMicro

Fuel Cell Applications

Mei-qiang Fan,*,†,‡ Li-xian Sun,† and Fen Xu†

†Department of Material Science and Engineering, China JiLiang University, Hangzhou 310018, P R China, and‡Thermochemistry Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P R China

Received May 7, 2009. Revised Manuscript Received June 22, 2009

Hydrogen generation from hydrolysis of the milled Al materials in water at room temperature isinvestigated. The hydrolysis mechanism is based on the work of the microgalvanic cell, where AlOOH(Bohmite) and hydrogen are produced via consumption of Al and water. From the hydrolysis results, theAl-Bi-based materials have better hydrolysis properties than those of the Al-Sn-based materials.Approximately 1000 mL of H2 per gram of the Al-Bi-based materials can be generated, and the hydrogengeneration rate can be achieved at 160 mL/g/min in the first five minutes of hydrolysis. The high reactivityof the Al-Bi materials come from the additives (Zn, hydrides, and salts) and can be achieved in 5 h ofmilling. Meanwhile, the feasibility of the CO2-free hydrogen from Al alloys for fuel cell applications isconfirmed in our experiments, and the hydrogen generation can be controlled by feeding the water flowinto the Al alloys.

Introduction

With decreases of fossil fuel and appeals of environmentalprotection, a new sustainable energy economy will appear atthe 21st century. Hydrogen has many attractive advantagessuch as high energy density and nonpolluted reaction pro-ducts, so hydrogen is a perfect energy source for protonexchange membrane fuel cells in which chemical energyreleased from the reaction of hydrogen and oxygen to makewater is converted to electric energy. Fuel cells are alternativepower sources for providing clean energy for transportationand personal electronic applications where a safe, low cost,and light hydrogen gas generator is important. Hydrogenstorage on-board the vehicle is considered key to achievingmarket success for fuel cell vehicles.1 Hydrogen is usuallystored in tanks or in many materials such as metal hydrides,etc. However, their fatal shortcomings limit hydrogen energycommercialization. For example, the intermetallic hydridescontain transition or rare earth metal, so their hydrogenstorage capability in terms of specific energy density is toolow to meet the requirements of various applications. For thewide range of potential applications, it is important to select

appropriate materials (hydrocarbons,2,3 metal alloys,4-13

NaBH4,14-18 LiBH4

19) that exhibit high hydrogen yields.

2Alþ6H2Ow 2AlðOHÞ3þ3H2 ð1Þ

ΔH ¼-444:1 kJ=mol

According to eq 1, Al holds a promise as it has highhydrogen yield up to 11.1 wt % (mH2

/mAl) of theoretic value.Moreover, Al has lower cost (approximate 3 dollar/kg), andits hydrolysis residual (Al(OH)3) is environmentally friendly.The problemwith the use of Almetal as expendable hydrogensources is its high inertness to water. Several methods5-7 havebeendeveloped to accelerate theAl hydrolysis. Evgeny5 foundthat the mixture of nanoscale Al and NaBH4

5 had a highhydrogen yield at gentle temperature. Unfortunately, the highcost of the nanoscale Al and NaBH4 was a drawback of thismethod. Kunio Uehara6 found that the fresh surface of Al orAl alloys reacted with water and that hydrogen gas bubbleswere formed when these materials were cut or grounded inwater. The reaction occurred at room temperature, and itcontinued as long as cutting continued. It was known that thereaction ofAl orAl alloys in sodiumhydroxide7-10solution toproduce hydrogen had already been studied. The hydrogen

*Towhom correspondence should be addressed. Telephone and Fax:0086-571-86835740. E-mail: fanmeiqiang@126.com.(1) Steven, G. C.; James, F. M. J. Power Sources 2006, 159, 73–80.(2) Alexander, F.; Alexander, G. Energy Fuels 2006, 20 (3), 1242–

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2006, 20, 2612–2615.(4) Uan, J. Y.; Lin, M. C.; Cho, C. Y. Int. J. Hydrogen Energy 2009,

34, 1677–1687.(5) Evgeny, S.; Victor, D.; Arvind, V.Combust. Flame 2006, 144, 415–

418.(6) Kunio, U; Takeshita, H.; Hiromi, K. J. Mater. Process. Technol.

2002, 127, 174–177.(7) Lluis, S.; Jorge, M.; Maria, M.; Juan, C. J. Power Sources 2007,

169, 144–149.(8) Shkolnikov, E.; Vlaskin, M.; Iljukhin, A.; Zhuk, A.; Sheindlin, A.

J. Power Sources 2008, 185, 967–972.(9) Martinez, S. S.; Sanchez, L. A.; Alberto, A. Int. J. Hydrogen

Energy 2007, 32, 3159–3162.

(10) Wang, H. Z.; Leung, D. Y. C.; Leung, M. K. H. RenewableSustainable Energy Rev. 2009, 13, 845–853.

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K. B. J. Alloys Compd. 2005, 97, 58–62.(14) Murat, A.; Adnan, M.; Ibrahim, D. J. Power Sources 2007, 165,

844–853.(15) Pe~na-Alonso,R.; Sicurelli,A.; Callone,E.; Carturan,G. J. Power

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544–548.(19) Yoshitsugu, K.; Ken-ichirou, S.; Yasuaki, K. J. Power Sources

2006, 155, 325–328.

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generation rate could be easily controlled by NaOH solutionconcentration and working solution temperature. As a con-sequence, this process could be a feasible alternative forhydrogen production to supply fuel cells.

Furthermore, the high dispersed Al alloys11-13 possessedhigher activities and reacted very rapidly with water to evolvehydrogen, whenAl wasmelted with particular metals, such asGa, Sn, In, Hg, Pb, Bi, Mg, or Zn. Kravchenko13 developedthe Al-basedmetal composites doped withGa, In, Zn, and Snand found that the melted alloys produced 1060 mL/g ofhydrogen in 30min of hydrolysis in pure water at 82 �C.Here,water acted as anoxidizer forAl and simultaneously producedthe humid hydrogen. So far, the activation and hydrolysismechanism of the milled Al-based materials for hydrogenproduction have been seldom investigated.

Meanwhile, hydrogen production from hydrolysis of acti-vated aluminum alloys in water has not been used for fuelcells. Therefore, this workwants to carry out the above targetsand introduces a newmethod toproducehydrogen for the fuelcell with the cheap Al-based materials.

Experimental Section

Synthesis and Hydrolysis of Al-Based Materials. The startingmaterials were powders of pureAl, Bi, Sn, Zn,Ga,MgH2 (98%),CaH2 (98%), and some salts. The composites were mixed in anargon-filled glovebox. Then, milling was performed by a QM-1SP planetary 15 ball miller under a 0.2-0.3 MPa argon atmo-sphere. The ball-to-powder ratio corresponds to 30:1. Hydro-lysis reactions of the Al alloy (0.1-0.2 g) with 20 mL of distilledwater were carried out in a stainless steel chamber attached to agas buret graduated in 0.1 mL increments at room temperature.The water vapor in produced hydrogen flowed through acondenser and then was removed by a desiccator prior tomeasurement of the hydrogen volume. The produced hydrogenvolume was measured by monitoring water displaced from agraduated cylinder as the reaction proceeded, according to theexperiment by Dong et al.20 Hydrogen yield is defined as theproduced hydrogen volume over the theoretical hydrogen vol-ume that should be released, assuming that all material ishydrolyzed. Powder X-ray diffraction (XRD) studies werecarried out on PANalytical X-ray diffractometer (crystallinesilicon is the internal standard). Microstructure studies(scanning electron microscopy and energy dispersive X-rayanalysis) were performed on a CAMEBAX-microbeam elec-tron microprobe equipped with a KEVEX energy-dispersiveanalyzer.

Hydrogen for the Fuel Cell System.The hydrogen generator ismade up of an Al-H2O reactor attached to a water tank, apump, a micro flow meter, and a micro fuel cell. The hydrogenreactor is stainless steel, and its size is (3.14 � 362) mm2 � 173mmwith 700 mL internal volume. Figure 1 shows the schematicdiagram of the hydrogen generator and fuel cell system.

The amounts of produced hydrogen in the reactor weredetermined by the quantity of Al materials, and the hydrogengeneration rate was decided by water flow. The water flowwas 3mL/min in our experiments. When Al material contacted withwater, the reaction happened and resulted in a temperature rise.The temperatures of hydrolysis residual and hydrogen gas weremeasured near the bottom or the top of the reactor through twothermocouples, respectively. The hydrogen supplying rate wascontrolled by a hydrogen reductor (the reductor is an apparatuswhich can adjust the hydrogen pressure) and detected by amicroflow meter.

An amount of 11 g of Al-10 wt % Bi-10 wt % MgH2

material of milled 5 h was placed in the reactor. The water was

added into the reactor via the pump, and the hydrogen gas wasproduced when the Al material contacted with the water. Thehydrogen flow was controlled as 100 mL/min through thepressure reducing valve and the micro flow meter to meet therequirement of normal work of the fuel cell. The membraneelectrode assemblies andworking conditions of the fuel cell wereaccording to the experiments by Liang et al.21

Results and Discussion

Hydrolysis Properties of the Milled Al-Based Materials.

Figure 2 shows hydrogen generation from the hydrolysis ofthe milled Al materials in water at room temperature. Thedata on the kinetics of hydrogen generation distinctly reflectthat the Al materials have enough reactivity to react withwater and have high hydrogen yields at room temperature.From the hydrolysis results, the Al-Bi-based materials havebetter hydrolysis properties than those of the Al-Sn-basedmaterials. Approximatly 1000 mL of hydrogen per gram ofthe Al-Bi-based materials can be generated, and the hydro-gen generation rate can be achieved at 160 mL/g/min in thefirst five minutes of hydrolysis. The high reactivity of the Almaterials come from several factors. First are the othermetaladditions (Sn, Bi, Zn), which have been confirmed to im-prove Al reactivity by shifting the anode potential to a morenegative value.22 The Al-Bi alloy and Al-Sn alloy have -1.87 V and -1.54 V potential (reference HgO/Hg), respec-tively, evidently lower than the -1.29 V of H2O decomposi-tion potential. So hydrogenmolecules can be generated fromtheir surface as the creation of a microgalvanic cell betweenthe anode Al and the cathode Bi (or Sn). Second, alumina iseliminated from the Al surface in the milling process, and aquantity of defects and fresh surfaces are also produced,shown from the SEM image in Figure 3. Third, hydrolysis ofthe hydrides (or salts) produces a lot of heat and a number ofconductive free-moved ions which favor the work of themicrogalvanic cell.

After the hydrolysis reaction, the residuals were dark andbecame gray-white at 100 �C in the vacuumoven. TheXRDpatterns of the residual are shown inFigure 4.AlOOHpeaks,some small peaks of the additionmetals (Sn, Zn, andBi), andunreacted Al material dominate in the XRD patterns. Thehydrolysis of the milled Al materials in water is based on thework of the microgalvanic cell between Al (anode) andaddition metals (cathode) as the following formulas

3H2OðMÞþ3ew3

2H2ðMÞþ3OH- ð2Þ

M ¼ Bi, Sn, and Zn

Figure 1. Schematic diagrams of the hydrogen generator and fuelsystem.

(20) Dong,H.; Yang,H. Int. J.HydrogenEnergy 2003, 28, 1095–1100.

(21) Liang, Y.; Zhang, H.; Yi, B. Carbon 2005, 43, 3144–3152.(22) Shayeb,H. A.;Wahab, F.M.; Abedin, S.Z.Corros. Sci. 2003, 43,

655–669.

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Energy Fuels 2009, 23, 4562–4566 : DOI:10.1021/ef9004169 Fan et al.

Alþ3OH- -3ewAlOOH ðBohmiteÞþH2O ð3ÞAlþOH- -e-wAlOH ð4Þ

3AlOHw 2AlþAlOOHþH2O ð5Þ

The hydrolysis mechanism has been described earlier.23,24

According to eqs 4 and 5, the Al atom got an electron andcreated Al-OH intermediately, which quickly convertedto AlOOH (Bohmite) because the AlOH species had a

Figure 2. Hydrogen generation of the hydrolysis of Al-based materials in pure water. (a) Al-Sn-based materials; (b) Al-Bi-based materials.

Figure 3. SEM images of multicomponent Al-based materials. (a)The unmilled Al powders. (b) The fresh milled Al-16 wt % Bi alloy.

Figure 4. XRD spetrum of the hydrolysis residual of Al-basedmaterials. (a) Al-10 wt % Sn-10 wt % Zn alloy. (b) Al-10 wt %Bi-10 wt % MgH2 material. (c) Al-10 wt % Bi-10 wt % CaH2

material.

Figure 5. SEM images and EDX of multicomponent Al-basedmaterials. (a) Al-16 wt % Bi alloy instantly immerged in1 MNaClsolution. (b) The fresh milled Al-10 wt % Sn-5 wt % Zn-5 wt %MgH2. (c) The hydrolysis residual of the milled Al-10 wt % Sn-5wt%Zn-5wt%MgH2. Compositions in designed fields (mass%).

Table

materialscontents of elements of designed fields

(mass %)

hydrolysis residual of Al-Bialloys in 1 M NaCl solution

1. Al-56.18, Bi-0, O-29.32, Na-3.97,Cl-10.52

2. Al-49.88, Bi-9.17, O-33.67, Na-6.12, Cl-10.34

3. Al-45.41, Bi-9.17, O-30.66, Na-5.78, Cl-8.97

4. Al-51.19, Bi-0, O-37.32, Na-3.06,Cl-8.43

milled Al-10 wt % Sn-5 wt %Zn-5 wt % MgH2

1.Al-74.18, Sn-10.43, Zn-7.03, Mg-4.51, O-3.84

2. Al-75.27, Sn-10.15, Zn-6.62, Mg-4.42, O-3.54

3. Al-74.76, Sn-10.11, Zn-7.54, Mg-4.19, O-3.4

4.Al-74.54, Sn-10.2, Zn-7.1,Mg-4.55,O-3.62

hydrolysis residual of themilled Al-10 wt %Sn-5 wt%Zn-5 wt%MgH2

1. Al-37.09, Sn-4.99, Zn-2.35, Mg-1.84, O-53.73

2 Al-37.68, Sn-5.87, Zn-1.33, Mg-1.66, O-53.46

3. Al-37.40, Sn-5.59, Zn-2.24, Mg-1.78, O-52.99

4. Al-37.65, Sn-4.68, Zn-2.35, Mg-1.82, O-54.36

(23) Despic, A. R.; Radosevic, J.; Dabic, P.; Kliskic, M. Electrochim.Acta 1990, 35, 743–1746.(24) Kliskic, M.; Radoseic, J.; Gudic, S. Electrochim. Acta 2003, 48,

4167–4 174.

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significant oxidation potential. Meanwhile, the formation ofthe hydrogen molecule was stimulated by its affinity forcreation of an AlOH bond as one hydrogen molecule wasproduced for each electron introduced from the externalsource.

The creation of an Al-OH bond is often produced in thespot of Al (M) material. It can be confirmed from the SEMimage and EDXof Al-16 wt%Bi alloy after being instantlyimmersed in 1MNaCl solution in Figure 5a. There aremanypits where Bi is depleted completely in comparison to thelower Bi content in other places. In relation to the XRDpatterns in Figure 4 where Bi peaks still appear in thehydrolysis residuals of Al-Bi alloys, it is attributed to thatBi metal breaks away gradually from the Al surface in the Alhydrolysis process and it finally exists in the hydrolysisresidual. Further comparing to the SEM image and EDXof the milled Al-10 wt % Sn-5 wt % Zn-5 wt % MgH2

before and after hydrolysis reactions in Figure 5b and 5c, itsshape is converted from the flat to the solid body. It can beimaged that the AlOOH is produced and covered on thesurface of the Al-Sn-Zn alloy, while the water continues topenetrate the AlOOH layer and still reacts with Al. Finally,Al is depleted, and the loose solid body (including Sn and Znmetal) is produced. Therefore, the composition distributionsin the residual are still uniformly as those of the Al-10 wt%Sn-5 wt % Zn-5 wt % MgH2 before hydrolysis reactionfrom the EDX results.

Figure 6 shows the hydrogen yield of Al-10 wt %Bi-10 wt % MgH2 with different milling time. The 2 hmilled mixture has 651.8 mL/g, and the 5 h milled mixturepresents 953.3 mL/g in 11 min of hydrolysis. Prolonging themilling time can improve the Al reactivity and increase the

hydrogen yield, but it shows that themixtures do not increasehydrogen yields markedly as the milling time continues toincrease from 5 to 20 h. In Figure 7, except for the peak of thecomposition, no new peak appears and no peak displace-ment occurs with milling. It indicates that no reaction occursin the mixture. During prolonged milling (tg 3 h), consider-able energy is accumulated into the powder, and dislocationsoccur in all directions. Deformations are random, and thetexture disappears.25 An accentuation of the broadening of theAl diffraction peaks is observed as the milling time increases,reflecting the reduction of the crystallite size and the accu-mulation of microstrains. The Al crystallite size is reduced

Figure 7. XRD patterns of Al-10 wt %Bi-10 wt %MgH2 materialwith different milling time.

Figure 8. Temperature curve and pressure curve in the hydrogenreactor (water flow: 3 mL/g, MAl-10 wt % Bi-10 wt % MgH2 material:11 gand 298 K).

Figure 9. (a) I-V curve of the fuel cell. (b) Power generation of thefuel cell at 2 A.

Figure 6.Hydrogen generation from the hydrolysis of Al-10 wt %Bi-10 wt % MgH2 materials with different milling times.

(25) Wang, S.; Nishimura, H.; McCormick, P.G. Mater. Sci. Eng. A2001, 318, 22–33.

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from 42 (milled 2 h) to 37 nm (milled 5 h), and it continues todecrease from 35 (milled 10 h) to 32 nm (milled 20 h). It canalso be found that the peaks of Bi and MgH2 disappeargradually with increasing milling time. In the 10 h millingtime, the peaks of Bi andMgH2 still can be discovered in theXRD spectrum, but they are all not clearly discernible afterbeing milled for 20 h as Bi and MgH2 are homogeneouslydistributed in the composite. It explains that the milled Almaterial has improved hydrolysis properties with longermilling time from the XRD results.

Hydrogen Generator for a Fuel Cell. In the experiments,the pressure meter (0-6 MPa) rises slowly with the wateraddition of 3 mL/min and is up to the 1.9 MPa in 25 min inFigure 8 when no hydrogen overflows. Therefore, there isapproximately 11 000 mL of hydrogen stored in the hydro-gen reactor. The reactor has different temperatures of Al-water residual in the bottom and of hydrogen gas in the top.Shown in Figure 8, the temperature of Al-water residualquickly rises to 100 �C and increases slowly with the waterfurther adding. It should be found that a lot of heat isproduced, and the heat needs controlling via heat manage-ment or controlled water flow. However, the temperature ofthe produced hydrogen keeps steady at 35 �C.Obviously, thehydrogen gas does not contain much water vapor.

Figure 9 shows the I-V curve and power generation of thefuel cell. Thehydrogen supply rate canmeet thenormalworkoffuel cell, and 1.4 W is produced for 50 min. The open-circuit

voltage is 0.86 V, and the cell voltage can keep at 0.7 V whenthe hydrogen supply rate is 100 mL/g.

Summary

The milled Al-based material shows the good hydrolysisproperties at room temperature, especially that approximate1000mL ofH2 per gram of the Al-Bi-basedmaterials, can begenerated, and the hydrogen generation rate of 160mL/g/mincan be achieved in first five minutes of hydrolysis. The Alhydrolysis is stimulated by the work of the microgalvanic cellbetween the anode (Al) and the cathode (the additionalmetals(M)), which results in the generation of AlOOH (Bohmite)from the sacrificed anode (Al). Such results are confirmedfrom the XRD spectrum of the residual that the peaks ofadditional metals and AlOOH dominate absolutely. The Al-10 wt % Bi-10 wt % MgH2 mixture with different millingtime shows the reduction of the crystallite size and theaccumulation of microstrains favoring the creation of themicrogalvanic cell in water, and the 5 h milled mixture showshigh hydrogen yield as that of milled 10 h (or 20 h). The Almaterial in the generator reacts quickly with water and cansupply the CO2 free hydrogen for the normal work of the fuelcell.

Acknowledgment. This work was financially supported by theNational Natural Science Foundation of China (No. 20473091,20573112, and 50671098).