A self-regulating hydrogen generator for micro fuel cells

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A self-regulating hydrogen generator for micro fuel cells
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   Journal of Power Sources 185 (2008) 445–450 Contents lists available at ScienceDirect  Journal of Power Sources  journal homepage: www.elsevier.com/locate/jpowsour A self-regulating hydrogen generator for micro fuel cells Saeed Moghaddam a , ∗ , Eakkachai Pengwang a , Richard I. Masel b , Mark A. Shannon a a Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, 1206 West Green Street, Urbana, IL 61801, United States b Chemical and Biomolecular Engineering, University of Illinois at Urbana-Champaign, 213 Roger Adams Lab, 600 S. Mathews, Urbana, IL 61801, United States a r t i c l e i n f o  Article history: Received 29 May 2008Received in revised form 16 June 2008Accepted 17 June 2008Available online 3 July 2008 Keywords: Fuel cellHydrogen generationPassive controlMetal hydrideMicrovalvePower source for portable applications a b s t r a c t Theever-increasingpowerdemandsandminiaturizationofportableelectronics,micro-sensorsandactu-ators, and emerging technologies such as cognitive arthropods have created a significant interest indevelopmentofmicrofuelcells.Oneofthemajorchallengesindevelopmentofhydrogenmicrofuelcellsis the fabrication and integration of auxiliary systems for generating, regulating, and delivering hydrogengas to the membrane electrode assembly (MEA). In this paper, we report the development of a hydrogengas generator with a micro-scale control system that does not consume any power. The hydrogen gen-erator consists of a hydride reactor and a water reservoir, with a regulating valve separating them. Theregulating valve consists of a port from the water reservoir and a movable membrane with via holes thatpermitwatertoflowfromthereservoirtothehydridereactor.Waterflowstowardsthehydridereactor,butstopswithinthemembraneviaholesduetocapillaryforces.Watervaporthendiffusesfromtheviaholesintothehydridereactorresultingingenerationofhydrogengas.Whentherateofhydrogenconsumedbythe MEA is lower than the generation rate, gas pressure builds up inside the hydride reactor, deflectingthe membrane, closing the water regulator valve, until the pressure drops, whereby the valve reopens.We have integrated the self-regulating micro hydrogen generator to a MEA and successfully conductedfuel cell tests under varying load conditions.Published by Elsevier B.V. 1. Introduction The increasing demand for high energy density power sourcesdrivenbyadvancementsinportableelectronicsandMEMSdeviceshasgeneratedsignificantinterestindevelopmentofmicrofuelcellsand batteries [1,2]. In terms of energy density, metal hydrides (e.g. NaBH 4 ),methanol,andmosthydrocarbonfuelshaveanenergyden-sityuptoanorderofmagnitudehigherthanthecompetitivebatterytechnologies.Microfuelcells,however,canpotentiallyoutperformthe batteries only if their fuel to device volume ratio can be max-imized and the power consumption of their auxiliary systems toregulate fuel delivery and power output is significantly reduced.While fabrication of small-scale membrane electrode assembly(MEA) is widely reported in literature [3–8], shrinking the size of  the auxiliary systems (pump, valves, sensors, distribution compo-nents, and power and control electronics for these components)has remained a challenge. While this might be somewhat feasiblein centimeter-scale fuel cells, fitting all the auxiliary componentswithin a few cubic millimeters volume is quite a challenge. Devel-oping a new means of fuel delivery and control that can be scaled ∗ Corresponding author. Tel.: +1 217 244 5136; fax: +1 217 244 6534. E-mail addresses:  saeedmog@uiuc.edu (S. Moghaddam), epengwa2@uiuc.edu(E.Pengwang),r-masel@uiuc.edu(R.I.Masel),mshannon@uiuc.edu(M.A.Shannon). downed and consume little to no power opens an opportunity forfabricating millimeter-scale fuel cells and realizing new devicesthat are tied to the existence of such power sources.Despite the advancements in fuel cell components and fabrica-tion processing, there has been very little progress made on microfuel cell system integration. Integrated micro fuel cell architec-tures suggested in literature (e.g. in [9–12]) are scaled-downed versions of large-scale systems with numerous auxiliary compo-nents. These components can be much larger than the membraneelectrodeassembly,greatlyreducingtheoveralldeviceenergyden-sity. In addition, they consume power, which reduces availableoutput power from the micro fuel cell for a further reductionof the device energy density. Additionally, auxiliary componentsnormally require numerous microfabrication steps and have inte-gration difficulties that can result in higher production costs andadded complexity of micro fuel cells operation.Examples of fuel delivery and control systems can be found in[13–18]. Sarata et al. [13] proposed a pressure-based control sys- tem for a hydrogen generator comprising of a hydride reactor andwater for hydrolysis. The hydrogen generation rate is controlled bymonitoring the reactor pressure and then stopping the pumpingof water to the hydride chamber when the hydride chamber pres-sure increases above a reference value. The pressure sensor, pump,valve,andelectronicstoconductthiscontroloperationoccupysig-nificantspace,whichdirectlytranslatetolowerenergydensityand 0378-7753/$ – see front matter. Published by Elsevier B.V.doi:10.1016/j.jpowsour.2008.06.060  446  S. Moghaddam et al. / Journal of Power Sources 185 (2008) 445–450 highcostformicrofuelcells.Inanotherapproach[18],thepressure increase in a macro-scale hydride chamber was used to automat-ically push the water out of a conduit that connected the waterreservoir to the hydride reactor. This results in an increase in thediffusionlengthbetweenthewaterfrontandthehydrideandcon-sequently slows down the hydrogen generation rate. This passiveapproachmaybemoresuitableforminiaturization(e.g.fabricationof a microchannel between the water and hydride reservoirs andsoon).But,unfortunately,sincewaterdiffusionandtherebyhydro-gen generation is not completely stopped, pressure continues torisesuchthatfailurecanoccur.Furthermore,themovementsofthewaterfrontinsidethemicrochannel(i.e.dynamicsoftheadvancingand receding contact lines) and the pressure of the excess hydro-gen inside the device that pushes against the water front can becomplicated to predict and control.In this manuscript, we present the development of a microhydrogengeneratorwithaself-regulatingcontrolmechanism.Thecontrol scheme enables the hydrogen generator to automaticallystop generating hydrogen when it is not consumed by the microfuel cell. The volume of the control mechanism is less than 50nL (approximately 0.5% of the device volume) and requires no energyinput.Thistechnologyhasenabledfabricationofthefirstfullyinte-grated millimeter-scale fuel cell that operates much like a battery.Thistechnologycanalsobeimplementedincentimeter-scalemicrofuelcellstoenhancetheirenergydensityandreliabilityandreducetheir complexity and cost. Details of the control mechanism anddevelopmentofaself-regulatingmicrohydrogengeneratoranditsintegration with a MEA are discussed in this manuscript. 2. Operation principle The no-power, self-regulating hydrogen generator consists of ahydride reactor and a water reservoir, with a regulating valve sep-arating them, as shown in Fig. 1. The regulating valve consists of  a port and a membrane with via holes in it. Water flows throughthe port towards the hydride chamber, but stops within the mem-brane via holes due to capillary forces. Water vapor then diffusesinto the hydride chamber resulting in hydrogen generation (metalhydrides such as LiH, LiAlH 4 , and CaH 2  react with water vaporto produce H 2  [19]). When the rate of hydrogen consumption by the fuel cell is lower than generation rate, gas pressure builds upinside the hydride reactor and deflects the membrane towards thewater port, blocking the port and ceasing the water flow to thehydrideafterthewaterevaporates.Thisregulationaction,however,assumesthatthemembranedeflectsunderasmallerpressurethanneeded to break into the liquid meniscus formed inside the mem-brane via holes. Under such conditions, complete isolation of thehydride reactor from the water reservoir can occur. When hydro-gen consumption by the fuel cell is faster than the generation rate, Fig. 1.  Schematic cross-section of the self-regulating hydrogen generator and itsprincipleofoperation.(A)Membraneinreleasemode:waterexitsthereservoiranddiffuses into the hydride reactor through the membrane holes. (B) Membrane inclosed mode: small pressure buildup in the hydride reactor, when hydrogen is notused, bends the membrane and closes off the water port. the reverse happens, opening the membrane to allow water to dif-fuse into the hydride reactor, increasing the hydrogen generation.Essentially,thecontrolmechanismisapassivevalvethatautomat-ically regulates hydrogen production based on the hydride reactorpressure.Detailsofthevalveoperationandtestingarediscussedinthe following sections. 3. Valve fabrication and testing   3.1. Valve and water reservoir assembly The membrane separating the water reservoir and the hydridereactor was made of polyimide (PI) through spinning and curingPI 5878G (from HD Microsystems) on a 100mm diameter 500  mthick glass wafer. The final thickness of the PI layer was 5  m. Acircular 1.3mm diameter area at the center of the membrane wassputter coated with a 0.2  m thick Cr/Au layer to prevent waterdiffusionthroughthemembranewhenitisclosed.Acircularlydis-tributed array of 30  m diameter holes was etched through theCr/Au (wet etched) and PI (reactive ion etched) layers close to theperimeter of the Cr/Au coated area. Fig. 2 shows the front view of  the membrane. The membrane was then transfer-bonded (processis described in [20]) to the bottom of the water reservoir fabri- catedfrom  100   p -dopedsiliconthroughdeepreactiveionetching(DRIE) process (cf. Fig. 3). Note that the 3  m deep recess seen inthebottomviewofthereservoiristheseparatinggapbetweenthePI membrane and the bottom of the water reservoir, as depicted inthe schematic of  Fig. 1.  3.2. Membrane bulge test  A test piece was fabricated to determine the membrane deflec-tionwithpressure.Thetestpiecewasasilicondie(10mm × 10mm)with 2.4mm × 2.4mm opening at its middle, over which the PImembrane was bonded. The PI membrane was similar to that of the device in every aspect (i.e. size and microfabrication process)except that it did not have the 30  m holes. A bulge test setup wasusedtomeasurethemembranedeflectionatdifferentpressures(cf.Fig. 4). The test piece was installed on the pressure chamber of thesetup, as depicted in Fig. 4. The chamber pressure was increased using a piezoelectric actuator. As the results in Fig. 5 show, anappliedpressureofapproximately150Paissufficienttodeformthe Fig. 2.  Top view of the center of the polyimide membrane coated with Cr/Au toprevent water diffusion through the membrane. The holes are 30  m in diameter.  S. Moghaddam et al. / Journal of Power Sources 185 (2008) 445–450  447 Fig. 3.  Top and bottom views of the water reservoir. The inset figure in the bottomview shows a 3  m deep recess that separates the reservoir from the PI membrane.Thickness of the reservoir bottom wall is 20  m. Fig. 4.  Bulge test setup. Main system components: (A) laser sensor model 812330-SLS700/15fromLMISelcom,Inc.,(B)ajigforholdingthetestarticle,(C)piezoelectricactuator model P-239.60 HVPZT from Physik Instrumente GmbH and Co. KG, (D)water chamber, (E) micro positioning stage, (F) pressure transducer model PX 309-001GVS5VfromOMEGACo.,and(G)waterinletandoutletvalves.Thetestarticleisattachedtothejig(B).Themicropositioningstageisusedtoadjustthesamplerightbelow the optical sensor. Next, the water chamber (D) is filled with water using theinlet and outlet valves (G). The water level reaches the top of the water chamber,but does not come into contact with the membrane. Fig. 5.  Membrane deflection versus applied pressure. membrane3  m.Thispressureissignificantlylessthanthetypicalwater capillary pressure in the micro-scale via holes. For example,capillary pressure in a 30  m hole with a surface to liquid contactangle of     =50 degrees is approximately 6kPa (=2   cos   / r  ). Thissuggests that the membrane will deflect and seal the water portbefore hydrogen can break the capillary meniscus formed insidethe membrane holes.  3.3. Valve performance test  An experimental setup was fabricated to test the valve perfor-mance.Thesetupdeterminestheopenandclosestatesofthevalveas well as the water vapor released rate through the holes whenthe valve is open. Fig. 6 shows a schematic of the setup. The setup consists of two main chambers C-1 and C-2. Pressure inside eachchamber is adjusted by changing the liquid (Fomblin oil) level in Fig. 6.  Schematic of the test setup for measuring the valve performance. Schematicshowsthewaterchamberandmembraneassemblyheldbetweenthetop(C-1)andbottom (C-2) chambers of the setup. Two valves (V-1 and V-2) on the C-2 chamberareusedforpurgingitwithdrynitrogen.Twomanometers(M-1andM-2)areusedto measure and adjust the pressure in C-1 and C-2 chambers.  448  S. Moghaddam et al. / Journal of Power Sources 185 (2008) 445–450 Fig.7.  PerformanceofthevalvetestedinthetestsetupshowninFig.6.Resultsshowincrease in humidity in C-2 chamber due to water vapor release by the valve whenpressureinC-1andC-2chamberswasequal(theC-2chamberwaspurgedwithdrynitrogenfourtimes).Attheendofthefourcycles,resultsshowvalveclosureafterapressure of approximately 400Pa was applied on the membrane. The C-2 chamberwas purged for a few minutes after the fourth cycle. manometers M-1 and M-2. Two push-button valves V-1 and V-2allow purging of the C-2 chamber with dry nitrogen. A humiditysensor (Model SHT75, size 3.7mm × 2.2mm × 4.9mm, from Sen-sirion, Inc.) installed on the bottom of the C-2 chamber measuresthe relative humidity.The valve and water reservoir assembly was installed betweentheC-1andC-2chambers,asdepictedinFig.6.Waterwassupplied to the water reservoir (i.e. topside of the valve). The two cham-bers were kept at the same pressure. The C-2 chamber was purgedwith nitrogen until a humidity level of less than 1% was reached.Immediatelyafterpurgingthechamber(i.e.closingtheV-1andV- Fig.8.  Schematicassemblyoftheself-regulatinghydrogengeneratorandtheactualimages of the top and bottom of the hydrogen generator. 2 valves), the chamber humidity started to raise indicating watervapor release by the valve. The chamber was purged with nitrogenagainandtheincreaseintherelativehumiditywasmeasured.Thisprocess was repeated several times, as the results in Fig. 7 show. Finally, pressure inside the C-2 chamber was increased approxi-mately400PaabovethatoftheC-1chamberbyadjustingtheliquidlevel in manometer M-2. The chamber was purged with nitrogenfor several minutes. As can be seen in Fig. 7, the humidity did not rise at this point, showing that the valve was closed. 4. Hydrogen generator fabrication The hydride reactor (see its schematic cross-section in Fig. 1)wasfabricatedfrom  100   p -dopedsiliconusingKOHetchingpro-cess. The bottom wall of the hydride reactor was then anodized in25%HFelectrolytetoproduce10–20nmdiameterporesthatallowhydrogen to exit the reactor. A 3D schematic of the hydride reac-tor assembled with the water reservoir and membrane assemblyis shown in Fig. 8. A bottom image of the hydride reactor is also shown in Fig. 8, in which the porous bottom wall can be identified as the dark region in the middle of the mesa.Thehydridereactorwasfilled(about60–70%ofthe2.2  Linter-nalvolumeofthereactor)withLiAlH 4  (fromSigma–Aldrich,Inc.)inaglovebox.Thewaterreservoirandmembraneassemblywasthenepoxied (using Scotch-Weld 2216B/A Gray epoxy from 3M Co.) tothe hydride reactor. 5. Fabrication of MEA and integration with hydrogengenerator A hybrid silicon/Nafion ® MEA was fabricated on silicon-on-oxide (SOI) wafer with a 40  m thick device layer. The handlelayer of the wafer was patterned and etched in KOH solution untilreached the oxide layer. The 40  m thick device layer was thenetchedusing DRIEprocessto open100  m × 100  msquare open-ings that were 100  m apart (cf. Fig. 9) over a 1mm × 1mm area.Nafion ® solution, ∼ 28  Lof5wt.%Nafion ® ionomer1100EW(fromSolution Technology, Inc.), was then painted with a paintbrush on Fig. 9.  Self-regulating hydrogen generator assembled on a microfabricated hybridsilicon/Nafion MEA. The image was taken before epoxy (3M Scotch-Weld 2216B/AGray) was poured around the micro hydrogen generator to fix it on the MEA.  S. Moghaddam et al. / Journal of Power Sources 185 (2008) 445–450  449 Fig. 10.  Performance of the integrated device under different load conditions.Results show the device current output under varying voltage conditions. Voltagewas changed in square and saw-tooth forms. theperforatedsiliconmembranetofilltheopenings.Aleaktestwasperformedusinganin-housedevice.Catalystinkwasthenpreparedby dispersing platinum black HiSPEC 1000 (from Alfa Aesar Co.) inNafion ® solution, Millipore water, and isopropanol via sonication.Using the direct paint method, the catalyst ink was painted ontothe Nafion ® layer of the anode and cathode. The resulting catalystloading was approximately 20mgcm − 2 . In addition to the mem-brane area, a small amount of catalyst ink was painted onto thegold current collectors to provide electrical connection. The cur-rent collectors were made through sputter deposition of 0.1  mthick Cr/Au layers on anode and cathode sides.The micro hydrogen generator was then epoxied (using Scotch-Weld 2216B/A Gray epoxy from 3M Co.) onto the MEA to make anintegrated hydrogen generator-fuel cell assembly. 6. Integrated device performance test The integrated device was tested using a Solartron SI 1287potentiostat. The water reservoir was filled and tests were con-ducted.Inthefirsttest,thevoltagewasswitchedbetweentheopencircuit voltage ( ∼ 0.8V) and 0.3V several times (i.e. square waveform),ascanbeseeninFig.10.Theprimarygoalsofthisexperiment were the following.(1) Tofindoutifhydrogenbubblespassthroughthevalveandenterthe water reservoir.(2) Confirm valve closure through analysis of current transients aswellasphysicalevidence(failureofthedeviceduetofractureof its elements) suggesting continuous hydrogen generation andpressure build-up inside the device.Inthesecondtest,thedevicevoltagewaschangedinsaw-toothwave form between 0.3 and 0.7V to evaluate the response of thecontrol mechanism to gradual changes in load conditions. 7. Test results and discussions Results of both tests are provided in Fig. 10. During the course of the experiment, no bubbles were observed to enter the waterreservoir, indicating that the membrane deflection and capillaryforces did not allow hydrogen to pass through the valve. With-out the membrane, bubbles are observed to pass, even throughlong microchannels connecting the water reservoir to the hydridereactor.Also,thebottomwallofthewaterreservoirdidnotmeasur-ably bulge, suggesting that the hydrogen pressure did not increasemeasurably inside the device.Analysis of the device current output also provided interest-ing insight about hydrogen generation. As can be seen in Fig. 10,after 3min of device not consuming any hydrogen (i.e. open cir-cuitmode),nospikeincurrent(beyondthesteady-statevalue)wasobservedwhenthevoltagewasdroppedto0.3V.Thistestsuggeststhathydrogenwasnotproducedanddidnotaccumulateinsidethedevice when no current was drawn. Note that higher currents canbe generated with increased hydrogen pressure, considering thattheMEAiscapableofdeliveringanorderofmagnitudehighercur-rent(700mAcm − 2 attheoperatingvoltageof0.3V),thanwhatwasdelivered by the integrated device. The small spike ( ∼ 1mAcm − 2 versus8mAcm − 2 )seeninFig.10ismainlyanartifactoftheexper- iment. A similar negative spike can be seen when current goesto zero, which suggests the spikes are due to the measurementelectronics on abrupt voltage changes. Also, the near square-wavevariation of the current generated showed that the hydrogen gen-eration due to changes in the valve responded in less than asecond.The smooth variations of the output current in response to thegradual changes in voltage (i.e. saw-tooth wave form) suggestedthat the hydrogen generator provided sufficient hydrogen to theMEAwhenneededandreducedsupplywhentheconsumptionratewas low. 8. Conclusions Thedevelopmentofaself-regulatingmicrohydrogengeneratorfor micro fuel cells was reported. The device employs a regulatormicro valve for controlling the rate of hydrogen generation in ahydridereactor,eliminatingtheneedforcomplexauxiliarysystemscommonly suggested in hydrogen generators. The control mecha-nismtakesadvantageofcapillaryforcestomaintainwaterinsideaconfined volume connected to a water reservoir. It delivers watervapor to the hydride reactor when hydrogen pressure inside thehydridereactorislowandreliesondeflectionofamembranetosealoff the water reservoir from the hydride reactor when the reactorpressure increases due to excess generation of hydrogen over thatconsumedbythefuelcell.Severaluniquequalitiesofthehydrogengenerator regulator are:(1) It occupies a volume of less than 50nL (approximately 0.5% of thevolumeofthemillimeter-scaleintegrateddevicedevelopedin this study);(2) Unlikemostothercontrolmechanisms,itconsumesnoenergy;(3) It operates passively without a need for external electronics,allowing the fuel cells to operate similar to batteries; and(4) Enables fabrication of millimeter-scale fully integrated microfuel cells.  Acknowledgements This research is funded by the Defense Advanced ResearchProjects Agency (DARPA) under grant DST 2007-0299513- 000-
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