Kuan-Lun Chu, Scott Gold, Vaidyanathan (Ravi) Subramanian, Chang Lu, Mark A. Shannon, and Richard I. Masel
Abstract—Silicon-based fuel cells are under active developmentfor chip-scale electrical power supply. One of the greatest chal-lenges in micro-fuel-cell research is the development of a suitableproton conducting membrane material that is compatible withstandard silicon microfabrication technology. In this paper, theuse of nanoporous silicon as a novel proton conducting membranematerial in a microscale fuel cell membrane electrode assembly(MEA) is demonstrated. The devices were fabricated by firstcreating 100- m-thick silicon windows in a standard siliconwafer, anodizing to create pores in the windows, and then paintingcatalyst layers and insulators onto the porous structures. Using5 M formic acid and 0.5 M sulfuric acid as the fuel, the fuelcell peak power density reached about 30 mW/cm2 at currentdensity level of about 120 mA/cm2. These results represent thesuccessful integration of a new class of protonic conductor into amicrofabricated silicon fuel cell. [1455]
Index Terms—Micro fuel cells, porous silicon.
I. INTRODUCTION
THERE has recently been considerable interest in devel-oping silicon-based micro fuel cells for chip scale power
[1]–[5]. Fuel cells have several advantages over traditional bat-teries, including rapid recharging and much higher energy den-sities. Among the many types of fuel cell systems, proton ex-change membrane fuel cells (PEMFC) have been most widelyexamined as major future power sources for microscale devices.Significant efforts have focused on the development of CMOScompatible processes and materials to produce silicon-basedPEMFCs that can be integrated with microelectronic and mi-croelectromechanical systems (MEMS) devices.
Development of a suitable proton conducting membranematerial has proved to be one of the greatest challenges in themanufacture of micro fuel cells. Nafion or a similar polymeris most widely used as a proton conductor in silicon microfuel cells and in PEMFCs in general. However, Nafion is not
Manuscript received November 4, 2004; revised September 10, 2005. Thiswork was supported by the Defense Advanced Research Projects Agency underU.S. Air Force Grant F33615-01-C-2172. Subject Editor J. Judy.
K.-L. Chu, V. Subramanian, and R. I. Masel are with the Departmentof Chemical and Biomolecular Engineering, University of Illinois at Ur-bana-Champaign, Urbana, IL 61801 USA (e-mail: [email protected]).
S. Gold was with the Department of Chemical and Biomolecular Engineering,University of Illinois, Urbana, IL 61801 USA. He is now with the Chemical En-gineering Program and Institute for Micromanufacturing, Louisiana Tech Uni-versity, Ruston, LA 71270 USA.
C. Lu was with the Department of Chemical and Biomolecular Engineering,University of Illinois, Urbana, IL 61801 USA. He is now with the Agriculturaland Biological Engineering, Purdue University, West Lafayette, IN 47907-2093USA.
M. A. Shannon is with the Department of Mechanical and Industrial Engi-neering, University of Illinois, Urbana, IL 61801 USA.
Digital Object Identifier 10.1109/JMEMS.2006.872223
Fig. 1. An illustration of the fabrication scheme used to create the poroussilicon membranes.
Fig. 2. Nanoporous silicon formation setup for fabrication step in Fig. 1(f).
readily integrated with standard microfabrication techniquesused in making micro fuel cells, other microchemical systems,or MEMS-based devices, as it cannot be easily patterned usingphotolithography and bonding it to silicon and other commonlyused materials is extremely challenging under working fuelcell conditions due to its volumetric changes with changesin hydration level [6]. Countless efforts have been made todevelop a next-generation protonic conducting membrane ma-terial to replace Nafion. Solid-state protonic conductors includematerials such as solid acids, polymers, oxide ceramics, andintercalation compounds [7], [8]. One common approach has
672 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 15, NO. 3, JUNE 2006
(a) (b)
Fig. 3. SEM image of nanoporous silicon membrane (cross section view) at (a) low magnification and (b) high magnification.
Fig. 4. SEM image of porous silicon (top face view).
been to use inorganic–organic hybrid materials, often includingNafion as one of the components. Examples of such compositeproton conducting membrane materials reported in the litera-ture include Nafion-silica [9]–[11], Nafion-borosiloxane [12],and silica-polyethylene oxide [13] composites, among others.More recently, nanoporous silicon has been shown to havestrong potential as a proton conducting fuel cell membranematerial having proton conductivity and fuel crossover fluxcomparable to Nafion [14], [15]. Nanoporous silicon, whichis readily formed by anodic etching in hydrofluoric acid, iscompatible with conventional Si microfabrication technologies,presenting the possibility of fabricating different componentsof a fuel cell or even a fuel cell and its supported devices in amonolithic fashion. Nanoporous silicon should also be stable atelevated temperatures, unlike many polymeric materials.
The objective of this paper is to demonstrate the potentialof nanoporous silicon as a protonic conductor in a siliconmicro fuel cell. In the study reported in this paper, a membraneelectrode assembly (MEA), a crucial component of almost allfuel cell systems, was fabricated using nanoporous silicon.The performance of this MEA was evaluated by measuring itscurrent–voltage characteristics and power output.
Fig. 5. An illustration of the fuel cell as tested.
Fig. 6. SEM image of the nanoporous TiO layer on the anode side of thedevice.
II. EXPERIMENTAL METHODS
A. Nanoporous Silicon Membrane Fabrication
The procedure used to fabricate the nanoporous silicon mem-branes is illustrated in Fig. 1. Generally, the procedure was touse deep reactive ion etching (DRIE) to form 100 M mem-branes in a standard silicon wafer. The membranes were then an-odized, to convert them to porous silicon. Then catalyst layers,
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Fig. 7. Polarization and power density curves for two nanoporous silicon MEAs that were insulated with a TiO film. The measurements were done using fuela solution containing: : 5 M formic acid plus 0.5 M sulfuric acid; euro: 5 M formic acid with no sulfuric acid.
insulators, and current collectors were painted onto the wafersto create fuel cells.
In detail, the fabrication started with prime grade p-type(boron-doped) double side polished 100 silicon wafers with100 mm diameter and 0.01–0.02 -cm resistivity from WaferWorld Inc. Native oxide on the wafer surface was removedby immersion in a buffered oxide etch (BOE) solution priorto any other processing [Fig. 1(a)]. First, an approximately800-nm-thick silicon nitride film was deposited on both wafersides by low-pressure chemical vapor deposition [Fig. 1(b)].Photolithography was used to pattern four small circles onthe front side of each die with a total area (all four cells)of 0.0625 cm . Each die has a square shape with 2.25 cmarea. Reactive ion etching with freon plasma was then used toremove the silicon nitride from wafer back side and to exposethe four circles on the front side, as shown in Fig. 1(c). Siliconmembranes with thickness of 100 m were formed by DRIE[Fig. 1(d)]. For each die, the indented side etched down byDRIE is referred as the front side, and the opposite flat sideas the backside, in the rest of this paper. A 50 nm chrome andgold layer was then sputtered on the backside of the wafer forelectrical contact following removal of any native oxide witha BOE solution. AZ 4903 photoresist was spin-coated on topof the chrome layer to protect it from acid etching in the next
step [Fig. 1(e)]. A nanoporous silicon membrane was formedby immersing the die in an electrolyte solution composed ofethanol and 49 wt.% hydrofluoric acid (1:1 by volume) stirredby magnet rotor and passing a constant current of 40 mA/cmthrough the four circular membranes, while silicon die wasthe anode and a platinum wire coil acted as the cathode inthe electrolyte solution, as illustrated in Fig. 2. Nanoporeswere formed by electrochemical etching of silicon. The diewith porous silicon membrane was illustrated in Fig. 1(f). Thephotoresist and chrome on the die were removed by piranhasolution (1:3 volume ratio of 30 wt.% (aq): 98 wt.%
) and chrome etchant, respectively [Fig. 1(g)]. Scanningelectron microscopic (SEM) images of the nanoporous siliconmembrane surfaces and cross sections were obtained usinga Hitachi S4700 field emission gun SEM at the Center forMicroanalysis of Materials (CMM), University of Illinois atUrbana-Champaign, and are shown in Figs. 3 and 4.
B. Construction of the Fuel Cell
The final fuel cell is illustrated in Fig. 5. Generally, the fuelcell is constructed by painting a layer onto the membrane,adding anode and cathode catalyst layers onto the nanoporoussilicon membrane, and then insulating with a sol.
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Fig. 8. Polarization and power density curves for two different nanoporous silicon MEAs. Both were insulated with a TiO film. Both were fabricated withidentical procedures. The measurements were done using fuel a solution containing 5 M formic acid plus 0.5 M sulfuric acid.
In detail, the backside surfaces of four circular nanoporoussilicon membrane were painted by brush with nanopar-ticle colloid to form an insulating film on top of nanoporous sil-icon. When being painted, temperature of the silicon die wasmaintained between 40 and 50 C. This insulating film is toprevent short-circuiting between anode and cathode electrodesformed on the two opposite sides of nonporous silicon mem-brane in the following steps. The colloids were preparedby the hydrolysis of titanium isopropoxide (0.6 M) ( 99%) ob-tained from Aldrich Chemical Co. in acetic acid for 12–16 h.The sol thus prepared was autoclaved at 230 C for 12 h and al-lowed to cool to room temperature. An alternative to the abovemethod for insulating film was depositing film on thebackside surfaces of the four circular nanoporous silicon mem-branes (and also other area on the die) by PlasmaLab plasma-en-hanced chemical vapor deposition system. The thickness of this
film is around 6 nm. The catalysts for both anode andcathode were prepared by ink-painting method, which is com-monly used for macroscale fuel cell systems. The catalyst inkfor anode was prepared by mixing 10 mg of palladium black
powder (Sigma-Aldrich), 40 mg of 5% Nafion solution (Solu-tion Technology Inc.), and 100 mg Millipore , and then son-ication for 1 min. This catalyst ink for the anode was painted bybrush on top of the insulating film, which is or thinfilm on the back side of the porous silicon membrane. The inkfor cathode was prepared by mixing 10 mg of platinum blackpowder (Alfa-Aesar), 40 mg of 5% Nafion solution, and 100 mgMillipore , and sonication for 1 min. The catalyst ink forthe cathode was painted by brush on the front side surfaces ofthe porous silicon membranes. To make the anode catalyst layersurfaces hydrophilic so that fuel can be attracted to the catalystto facilitate anode reaction, an additional thin film waspainted by brush on top of the palladium film. The completenanoporous silicon membrane electrode assembly is illustratedin Fig. 5.
C. Micro Fuel Cell Performance Testing
Nanoporous silicon membrane electrode assembly was testedby dropping about 50 of fuel solution on the anode side of
CHU et al.: NANOPOROUS SILICON MEMBRANE ELECTRODE ASSEMBLY 675
Fig. 9. Polarization and power density curves for two different nanoporous silicon MEAs. Both were insulated with a SiO film. Both were fabricated withidentical procedures. The measurements were done using fuel a solution containing 5 M formic acid plus 0.5 M sulfuric acid.
silicon die, which was held horizontally. The fuel solution waseither 5 M formic acid or 5 M formic acid with 0.5 M sulfuricacid. The formic acid/sulfuric acid mixture is quickly wickedinto the and catalyst layers but does not flow throughthe porous silicon. The testing setup is illustrated with thenanoporous silicon MEA structure in Fig. 5. In all tests, thecathode was air-breathing.
III. RESULTS AND DISCUSSION
SEM images of the nanoporous silicon membranes are shownin Figs. 3 and 4. Generally, the films contain branched, inter-connected pores with an average diameter of less than 10 nm.The SEM images of the thin film (Fig. 6) and the catalystlayers show that they are also porous.
The polarization curves (voltage–current density curves) fornanoporous silicon MEA with insulating film were takenusing two kinds of fuel solutions, as illustrated in Fig. 7(a). Oneof the fuel solutions is 5 M formic acid and the other is 5 Mformic acid plus 0.5 M sulfuric acid, in which formic acid is
the fuel oxidized at the anode. Open cell potentials (the voltageoutput of MEA in the zero current density limit) shown by bothcurves in Fig. 7 were much lower than the theoretical electromo-tive force of formic acid fuel cell, which is 1.45 V, and also lowerthan Nafion membrane-based MEA, which is usually 1.0 V. Thisis attributed to higher fuel crossover through the membrane fromthe anode side to the cathode side in nanoporous silicon mem-brane than in Nafion membrane [14]. This is also the evidencethat fuel solution penetrated into the MEA from the topfilm, the anode catalyst layer, insulating film, and into thepores of the nanoporous silicon membranes. Fuel solution insidethe pores contributed to proton conductivity of the membrane. Itcan be seen from both Fig. 7(a) and (b) that nanoporous siliconMEA showed better performances when 0.5 M sulfuric acid wasadded in the fuel solution than when just using 5 M formic acidalone as fuel. This improvement in performances is attributed toincrease in proton conductivity (contributed by sulfuric acid) offuel solution inside the pores of nanoporous silicon membranes.Therefore in the following figures, all data were obtained byusing fuel solution of 5 M formic acid plus 0.5 M sulfuric acid.
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In order to explore the reproducability of the findings, Figs. 8and 9 show polarization curves and power density curves fornanoporous silicon MEAs made in two different batches. It canbe seen that there were differences in their performances thoughthey were prepared with the same procedure. The MEA withhigher open cell potential also has lower short-circuit currentdensity (the current density output when anode and cathode areshort-circuited). This is as expected if there were slight differ-ences in the proton conductivity in the porous silicon the two dif-ferent batches of porous silicon membranes. Physically, as theconductivity of the membrane decreases, the parasitic currentsdue to fuel transport through the membrane are reduced. Thatraises the open cell potential. Unfortunately, the proton conduc-tivity is reduced. That reduces the short circuit current.
At this point we are not completely clear why the batch-to-batch variations in the conductivity of the membranes exists. Inour pervious paper [15], we noted that the anodization of theporous silicon stops when the chrome on the backside of themembrane dissolves. That can result in incomplete anodizationof the membrane where some of the pores do not extend com-pletely through the membrane.
IV. CONCLUSION
In this paper, nanoporous silicon is demonstrated to be anovel proton conducting membrane material for membraneelectrode assemblies in a microscale silicon fuel cells. With 5M formic acid as the fuel and 0.5 M sulfuric acid added to thefuel to increase proton conductivity, the fuel cell peak powerdensity reached about 30 mW/cm at current density of about120 mA/cm .
ACKNOWLEDGMENT
The CMM Department of Energy National User Center forElectron Beam Microcharacterization, University of Illinois atUrbana-Champaign, provided access to the SEM used in thispaper.
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Kuan-Lun Chu received the B.S. degree in physicsfrom National Taiwan University, Taipei, Taiwan,in 1994, and the M. Eng. degree in electrical andcomputer engineering from Cornell University,Ithaca, NY, in 2001.
He is currently pursuing the Ph.D. degree in elec-trical and computer engineering at the University ofIllinois, Urbana-Champaign. His research area is mi-croelectromechanical systems for micropower gener-ation
Scott Gold received the B.S. degree in chemical engi-neering from the University of Kentucky, Lexington,the M.S. degree from the Georgia Institute of Tech-nology, Atlanta, and the Ph.D. degree from ArizonaState University, Tempe, in 2002.
He followed with a postdoctoral research positionat the Univesity of Illinois, Urbana-Champaign.He joined the faculty at Louisiana Tech University,Ruston, in 2004. He works in micromechincalsystems and micropower generation.
Vaidyanathan (Ravi) Subramanian received thePh.D. degree in chemical engineering from theUniversity of Notre Dame, South Bend, IN, in 2004.
He is currently a visiting research AssistantProfessor with Prof. Richard Masel at the Universityof Illinois, Urbana-Champaign, in the Chemicaland Biomolecular Engineering Department. Hisresearch area is synthesis and applications of semi-conductor-metal nanocomposites to alternate energysystems, sensors, and environmental systems.
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Chang Lu received the B.S. degree in chemistryfrom Peking University, Beijing, China, in 1998, andthe M.S. and Ph.D. degrees in chemical engineeringfrom the University of Illinois, Urbana-Champaign,in 2001 and 2002, respectively.
He followed with a post doctorate at theNanobiotechnology Center at Cornell University,Ithaca, NY. He is now an Assistant Professor atPurdue University, West Lafayette, IN. His researchgroup focuses on the use ofmicro/nano scale devicesand materials in the study of biological systems and
for harnessing biological energy. More specifically, part of the effort is directedtoward development of micro/nano devices for biological sample treatment andprocessing.
Mark A. Shannon received the B.S. and Ph.D. de-grees in mechanical engineering from the Universityof California, Berkeley, in 1989 and 1993, respec-tively.
He joined the faculty of the University of Illinois,Urbana-Champaign, in 1994. He is Willett Professorof Mechanical and Industrial Engineering, Electricaland Computer Engineering, Bioengineering, andDirector of the Center of Advanced Materials forthe Purification of Water with Systems. His researchis in microfabrication, micro electrical mechanical
systems, micro/nanofluidics, and BioMEMS. Recently, he and Richard Maselfounded Cbana Laboratories, a developer of microfluidic products.
Richard I. Masel received the M.S. degree in chem-ical engineering in 1973 from Drexel University,Philadelphia, PA, in 1973.
He joined the University of Illinois, Urbana-Cham-paign, in 1978. He is now Professor of Chemical andBiomolecular Engineering and Electrical and Com-puter Engineering. Masel has worked on kinetics andcatalysis, and more recently fuel cells, microchem-ical systems, and MEMS. He helped found Tekion,a developer of fuel cells for portable electronics, in2003. He served for three years as Chief Technical
Officer of Tekion before stepping down to the position of Chief Technical Ad-visor. Recently, he and Mark Shannon founded Cbana Laboratories, a developerof microfluidic products.
