Patent Application
20090092862
The present disclosure relates generally to the field of fuel cells. More specifically, the present disclosure relates to methanol fuel cells with a porous proton-exchange membrane.
Understanding that drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with specificity and detail through the use of the accompanying drawings as listed below.
It will be readily understood that the components of the embodiments as generally described and illustrated in the figures herein could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of various embodiments, as represented in the figures, is not intended to limit the scope of the invention, as claimed, but is merely representative of various embodiments. While the various aspects of the embodiments are presented in drawings, the drawings are not necessarily drawn to scale unless specifically indicated.
As those of skill in the art will appreciate, the principles of the invention may be applied to and used with a variety of fuel cell systems including an inorganic or organic fuel cell, direct methanol fuel cell (DMFC), reformed methanol fuel cell, direct ethanol fuel cell, proton-exchange membrane (PEM) fuel cell, microbial fuel cell, reversible fuel cell, formic acid fuel cell, and the like. Furthermore, the present invention may be used in a variety of applications and with fuel cells of various sizes and shapes. For purposes of example only, and not meant as a limitation, the present invention may be used for electronic battery replacement, mini and microelectronics, car engines, power plants, and an as an energy source in many other devices and applications. With reference now to the accompanying figures, particular embodiments will now be described in greater detail.
In general, fuel cell membranes, micro-fuel cells, and methods of fabrication thereof are disclosed. Furthermore, various devices, units, and assemblies that may include fuel cell membranes, micro-fuel cells, and methods of fabrication thereof as disclosed herein. Embodiments of the fuel cell membranes may be made of silicon dioxide and/or a doped silicon dioxide and relatively thin and have comparable area resistivities as thinker polymer membranes. The thinner the membrane, the easier it is for protons to move through it, thus increasing the amount of electrical current that can be generated. Meanwhile, the materials used to make the membranes are superior to currently used proton exchange membranes (PEMs) in preventing reactants from passing through the membrane, a common problem particularly in direct methanol fuel cells. In addition, the membranes can be fabricated using well-known micro-electronic fabrication techniques. In this regard, the membrane can be fabricated onto the micro-electronic structure to which the fuel cell is going to be used.
In an embodiment, the fuel cell membrane and the micro-fuel cell can be directly integrated into an electronic device. For example, the fuel cell membrane and the micro-fuel cell can be integrated to create a chip-scale fuel cell by placing the fuel cell membrane or the micro-fuel cell on the chip, integrating the fuel cell membrane or the micro-fuel cell in the substrate or printed circuit board, and interposing or attaching the fuel cell membrane or the micro-fuel cell to the chip as a separate part that is bonded to the chip. In general, the fuel cell membranes and micro-fuel cells can be used in technology areas such as, but not limited to, microelectronics (e.g., microprocessor chips, communication chips, and optoeletronic chips), micro-electromechanical systems (MEMS), microfluidics, sensors, and analytical devices (e.g., microchromatography), communication/positioning devices (e.g., beacons and GPS systems), recording devices, and the like.
The membrane can include materials such as, but not limited to, organic conducting materials and inorganic conducting materials. For example, the membrane can include material such as, but not limited to, silicon dioxide, doped silicon dioxide, silicon nitride, doped silicon nitride, silicon oxynitride, doped silicon oxynitride, metal oxides (e.g., titanium oxide, tungsten oxide), metal nitrides (e.g., titanium nitride), doped metal oxides, metal oxynitirdes (e.g., titanium oxynitride), doped metal oxynitrides, and combinations thereof. In general, the membranes can be doped with about 0.1 to 20% of dopant in the membrane and about 0.1 to 5% of dopant in the membrane. The doped silicon dioxide can include, but is not limited to, phosphorous doped silicon dioxide, boron doped silicon dioxide, aluminum doped silicon dioxide, arsenic doped silicon dioxide, and combinations thereof. In general, the doping causes atomic scale defects such as M-OH (M is a metal) and distort the lattice so that protons can be transported therethrough. The amount of doping can be from 0.1 to 20% by weight of dopant in membrane, 0.5 to 10% by weight of dopant in membrane, 10 and 2 to 5% by weight of dopant in membrane.
The membrane 12 has a thickness of less than about 10 micrometers (gm), about 0.01 to 10 gm, about 0.1 to 5 gm, about 0.1 to 2 gm, about 0.5 to 1.5 gm, and about 1 gm. The length of the membrane 12 can be from about 0.001 m to 100 m, and the width can be from about 1 gm to 1000 μm. It should be noted that the length and width are dependent on the application and can be adjusted accordingly. The membrane 12 has an area resistivity of about 0.1 to 1000 ohms cm2, about 0.1 to 100 ohms cm2, about 0.1 to 10 ohms cm2, about 1 to 100 ohms cm2, and about 1 to 10 ohms cm2. The area resistivity is defined as the resistivity across the area of the membrane exposed to the fuel (e.g., resistance times area or resistivity times thickness). The membranes 12 can be formed using methods such as, but not limited to, spin-coating, plasma enhanced chemical vapor deposition (PECVD), screen printing, doctor blading, spray coating, roller coating, meniscus coating, and combinations thereof.
The catalyst layer 14a and 14b can include a catalyst such as, but not limited to, platinum, platinum/ruthenium, nickel, palladium, alloys of each, and combinations thereof. In general, in one embodiment a platinum catalyst is used when the fuel is hydrogen and in another embodiment a platinum/ruthenium catalyst is used when the fuel is methanol. The catalyst layer 14a and 14b can include the same catalyst or a different catalyst. The catalyst layer 14a and 14b is typically a porous catalyst layer that allows protons to pass through the porous catalyst layer. In addition, there is an electrically conductive path between the catalyst layer and the anode current collector. The catalyst layer 14a and 14b can have a thickness of less than 1 g, about 0.01 to 100 gm, about 0.1 to 5 gm, and about 0.3 to 1 gm.
The catalyst layer 14a and 14b can include alternative layering of catalyst and the membrane material, which builds a thicker catalyst layer 14a and 14b (e.g., two or more layers). For example, two layers improve the oxidation rate of the fuel. This is advantageous because it can increase the anode catalyst loading and keep the catalyst layer porous. The high surface area will allow a high rate of oxidation of the fuel. A higher rate corresponds to higher electrical current and power. The membrane can be further processed by post-doping. The dopants can be diffused or implanted into the membrane to increase the ionic conductivity. The dopants can include, but are not limited to, boron and phosphorous. Each dopant can be individually diffused into the membrane from a liquid or from a solid source, or can be ion implanted using a high voltage ion accelerator. The conductivity of the membrane can be increased by diffusion of acidic compounds (e.g., carboxylic acids (in the form of acetic acid and trifluoracetic acid) and inorganic acids such as phosphoric acid and sulfuric acid) into the membrane.
Although the membrane layer 12 and polymer layer 16 are separate layers, they both operate as a fuel cell membrane. The combination of properties (e.g., ionic conductivity, fuel crossover resistance, mechanical strength, and the like) of the dual-layer membrane may be superior in some instances than either layer individually. For example, the polymer layer 16 may add additional mechanical support and stability to the membrane layer 12. In addition, in embodiments where the membrane layer 12 is silicon dioxide, this material is similar to the other insulators being used to fabricate the device, for example, when the membrane 12b is used with a semiconductor device. The polymer layer 16 can include polymers such as, but not limited to, Nafion (perfluorosulfonic acid/polytetrafluoroethylene copolymer), polyphenylene sulfonic acid, modified polyimide, and combinations thereof. For example, when Nafion is used as the polymer layer 16, the open circuit potential has been shown to increase without loss to current density, resulting in an increase in power density and efficiency.
The polymer layer 16 has a thickness of about 1 to 50 gm, 5 to 50 gm, and 10 to 50 gm. The length of the polymer layer 16 can be from about 0.01 m to 100 m, and the width can be from about 1 pm to 500 pm. It should be noted that the length and width are dependent on the application and can be adjusted accordingly. The polymer can be deposited using techniques such as, but not limited to, spin-coating, and therefore, the polymer can completely cover the substrate, and/or can be selectively deposited into a desired areas. The polymer layer 16 has an area resistivity of about 0.001 to 0.5 ohms cm2.
The first porous catalyst layer 14a is disposed on the bottom side of the membrane closed to the substrate 22. The second porous catalyst layer 14b is disposed on the top side of the membrane on the side opposite to the substrate 22. The micro-fuel cell 20a includes a first porous catalyst layer 14a and a second porous catalyst layer 14b, which form electrically conductive paths to the anode current collector 24 and the cathode current collector 26, respectively. The first porous catalyst layer 14a and the second porous catalyst layer 14b can include the same catalysts as those described previously, and also have the same thickness and characteristics as those described previously.
The anode current collector 24 collects electrons through the first porous catalyst layer 14a. The anode current collector 24 can include, but is not limited to, platinum, gold, silver, palladium, aluminum, nickel, carbon, alloys of each, and combinations thereof. The cathode current collector 26 emits electrons. The cathode current collector 26 can include, but is not limited to, platinum, gold, silver, palladium, 20 aluminum, nickel, carbon, alloys of each, and combinations thereof. The various anode current collectors 24 and the cathode current collector 26 can be electronically connected in series or parallel, depending on the configuration desired (e.g., the wiring could be from anode-to-cathode (in series) or anode-to-anode (in parallel)). In one embodiment, the individual micro-fuel cells can be connected electronically in series to form fuel cell stacks to increase the output voltage. In another embodiment, the connections can be made in parallel to increase the output current at the rated voltage.
The channels 32a, 32b, and 32c are substantially defined (e.g., bound on all sides in the cross-sectional view) by the membrane 28, the first porous catalyst layer 14a, and the substrate 22. A fuel (e.g., hydrogen and methanol) is flowed into the channels and interacts with the first porous catalyst layer 14a in a manner as described previously. The channels 32a, 32b, and 32c, can be in series, parallel, or some combination thereof. The anode current collector 24 is disposed adjacent the channels 32; 32b, and 32c, but is electrically connected to the porous catalyst layer 14a.
In yet another embodiment, the channels 32a, 32b, and 32c are formed by the removal (e.g. decomposition) of a sacrificial polymer layer from the area in which the channels 32a, 32b, and 32c are located. During the fabrication process of the structure 20a, a sacrificial polymer layer is deposited onto the substrate 12 and patterned. Then, the membrane 28 is deposited around the patterned sacrificial polymer layer. Subsequently, the sacrificial polymer layer is removed, forming the channels 32a, 32b, and 32c. The processes for depositing and removing the sacrificial polymer are discussed in more detail hereinafter.
Although a rectangular cross-section is illustrated for the channels 32a, 32b, and 32c, the three-dimensional boundaries of the channels can have cross-sectional areas such as, but not limited to, rectangular cross-sections, non-rectangular cross-sections, polygonal cross-sections, asymmetrical cross-sections, curved cross sections, arcuate cross sections, tapered cross sections, cross sections corresponding to an ellipse or segment thereof, cross sections corresponding to a parabola or segment thereof, cross sections corresponding to a hyperbola or segment thereof, and combinations thereof. For example, the three-dimensional structures of the channels can include, but are not limited to, rectangular structures, polygonal structures, non-rectangular structures, non-square structures, curved structures, tapered structures, structures corresponding to an ellipse or segment thereof, structures corresponding to a parabola or segment thereof, structures corresponding to a hyperbola or segment thereof, and combinations thereof. In addition, the channels can have cross-sectional areas having a spatially-varying height. Moreover, multiple air-regions can be interconnected to form microchannels and microchambers, for example.
The channels 32a, 32b, and 32c height can be from about 0.1 to 100 p.m, about 1 to 100 gm, 1 to 50 μm, and 10 to 20 μm. The channels 32a, 32b, and 32c width can be from about 0.01 to about 1000 μm, about 100 to about 1000 gm, about 100 to about 300 gm. The length of the channels 32a, 32b, and 32c can vary widely depending on the application and configuration in which they are used. The channels 32a, 32b, and 32c can be in series, parallel, serpentine, and other configurations that are appropriate for a particular application.
In another embodiment, the sacrificial polymer used to produce the sacrificial material layer can be a polymer that slowly decomposes and does not produce undue pressure build-up while forming the channels 32a, 32b, and 32c within the surrounding materials. In addition, the decomposition of the sacrificial polymer produces gas molecules small enough to permeate the membrane 28. Further, the sacrificial polymer has a decomposition temperature less than the decomposition or degradation temperature of the membrane 28.
The sacrificial polymer can include compounds such as, but not limited to, polynorbornenes, polycarbonates, polyethers, polyesters, functionalized compounds of each, and combinations thereof. The polynorbornene can include, but is not limited to, alkenyl-substituted norbornene (e.g., cyclo-acrylate norbomene). The polycarbonate can include, but is not limited to, norbomene carbonate, polypropylene carbonate, polyethylene carbonate, polycyclohexene carbonate, and combinations thereof. In addition, the sacrificial polymer can include additional components that alter the processability of the sacrificial polymer (e.g., increase or decrease the stability of the sacrificial polymer to thermal and/or light radiation). In this regard, the components can include, but are not limited to, photoinitiators and photoacid initiators.
The sacrificial polymer can be deposited onto the substrate using techniques' such as, for example, spin coating, doctor-blading, sputtering, lamination, screen or stencil-printing, melt dispensing, evaporation, CVD, MOCVD, and/or plasma-based deposition systems. The thermal decomposition of the sacrificial polymer can be performed by heating to the decomposition temperature of the sacrificial polymer and holding at that temperature for a certain time period (e.g., 1-2 hours). Thereafter, the decomposition products diffuse through the membrane 28 leaving a virtually residue-free hollow structure (channels 32a, 32b, and 32c).
The polymer layer 36 is similar that the polymer layer 16 described in
Now having described the structure 10 having micro-fuel cells 20a, 20b, 20c, and 20d in general, the following describes exemplar embodiments for fabricating the micro-fuel cell 20a, which could be extended to fabricate micro-fuel cells 20b, 20c, and 20d. It should be noted that for clarity, some portions of the fabrication process are not included in
As mentioned previously, a step can be added between the steps illustrated in
Microfabricated fuel cells have been designed and constructed on silicon integrated circuit wafers using many processes common in integrated circuit fabrication, including sputtering, polymer spin coating, reactive ion etching, and photolithography. Proton exchange membranes (PEM) have been made by low-temperature, plasma-enhanced chemical vapor deposition (PECVD) of silicon dioxide. Fuel delivery channels were made through the use of a patterned sacrificial polymer below the PEM and anode catalyst. Platinum-ruthenium catalyst was deposited by DC sputtering. The resistivity of the oxide films was higher than traditional polymer electrolyte membranes (e.g., Nafion™) but they were also much thinner.
The design and fabrication of the micro-fuel cells is based on a technique of using a sacrificial polymer to form the fuel delivery channels for the anode. This sacrificial polymer, Unity 2000P (Promerus LLC, Brecksville, Ohio), was patterned by ultraviolet exposure and thermal decomposition of the exposed areas. The membrane and electrodes coat the patterned features in a sequential buildup process. One of the last steps in the fabrication sequence is the thermal decomposition of the patterned Unity features, leaving encapsulated microchannels (e.g., similar to process shown in
Silicon dioxide was used as the encapsulating material and PEM. The deposition of SiO2 took place in a Plasma-Therm PECVD system (Plasma-Therm, St. Petersburg, Fla.) at temperatures of 60-200° C. The reactant gases were silane and nitrous oxide with a N2O:SiH4 ratio of 2.25 and operating pressure of 600 mTorr. Deposition times of 60-75 minutes produced film thicknesses, measured with an Alpha-Step surface profilometer (KLA-Tencor, San Jose, Calif.), between 2.4 and 3.4 μm.
The catalyst layers were sputter deposited using a CVC DC sputterer (CVC Products, Inc., Rochester, N.Y.). A 50:50 atomic ratio platinum/ruthenium target (Williams Thin-Film Products, Brewster, N.Y.) was used as the source target.
All electrochemical measurements, including impedance spectroscopy (IS) and linear voltamagrams, were performed with a Perkin Elmer PARSTAT 2263 (EG&G, Princeton, N.J.) electrochemical system. The scan rate for linear sweep voltametry was 1 mV/s. Ionic conductivity was measured with impedance spectroscopy through SiO2 films deposited onto aluminum-coated substrates and contacted with a mercury probe, as well as with actual cells. The frequency range for the impedance measurement was from 100 mHz to 1 MHz, with an AC signal amplitude of 10 mV. Half-cell devices were fabricated with the fuel delivery channels and sputtered catalyst under the SiO2 PEM. Instead of a cathode, epoxy was used to form a well on top of the devices and filled with a 1 M sulfuric acid solution. Measurements were made with a saturated calomel electrode (SCE) and a Pt wire as the reference and counter electrodes, respectively, placed in the sulfuric acid solution. A PHD 2000 Programmable Syringe Pump (Harvard Apparatus, Holliston, Mass.) delivered liquid fuels and controlled the flow rates. Hydrogen was supplied with a pressurized tank of ultra high purity grade gas that passed through a bubbler to humidify the feed.
Microfabricated fuel cells were successfully fabricated using many materials and processes common to integrated circuit fabrication. The performance of the micro-fuel cells with different fuels and temperatures was measured for cells with different features, including half-cells and full cells. The purpose was to investigate the individual fuel cells components (e.g.; anode, cathode, and PEM) as a function of processing conditions. In addition to catalytic activity, the key properties that were desired for the sputtered catalyst layers were porosity and electrical conductivity. The catalyst layer that contacts the membrane must be porous so that the protons generated during oxidation can come in contact with the PEM and pass to the cathode. The electrons generated at the anode catalyst need a path to the metal current collectors. Different amounts of Pt were sputtered onto substrates containing two solid electrodes patterned on opposite sides of an insulator. The sheet resistance of the Pt layers across the space between the electrodes was measured.
In various embodiments of the present invention, Pt/Ru layers with an average thickness of about 50-200 Å were used as porous, conducting layers on roughened Unity sacrificial polymer. A titanium adhesion layer was deposited on top of the Pt/Ru before SiO2 deposition. The amount of Ti needed for adhesion was minimized. About 45 Å (average thickness) of Ti was deposited between Pt/Ru and SiO2 in the sputtered electrodes.
Sputtering about 600 Å, or approximately 100 μg/cm2, of Pt/Ru prior to the deposition and patterning of the Unity sacrificial polymer produced a relatively solid (non-porous) layer on the substrate that increased the total amount of anode catalyst in the cell that could be utilized by a conducting analyte (e.g., acidic methanol). It also seemed to somewhat improve performance with hydrogen. Therefore, all results are discussed herein for cells fabricated with a solid layer of Pt/Ru on the bottom of the microchannels.
The requirements for the proton exchange membrane are different from the traditional PEM (e.g., Nafion) due to the mechanical properties and thickness required in microfabricated fuel cells. Here, SiO2 is shown to work as a stand-alone membrane. SiO2 films were deposited by PECVD and the ionic conductivity was measured with impedance spectroscopy at room temperature.
Half-cell devices were fabricated and tested to evaluate the anode performance with different fuels and provide a comparison for the full cell tests.
Microfabricated full-cells were fabricated and tested with linear voltametry at a scan rate of 1 mV/sec from the open-circuit potential. Table 1 compares the differences in process between five sets of cells that are presented here to demonstrate the key parameters (anode and cathode construction) that affect cell performance for these power devices.
A thick-film ink catalyst was coated onto the air-breathing cathode to improve its area and catalyst activity. When using the painted catalyst ink on top of the membrane, the full cell performance increased dramatically due to the increase in cathode catalyst loading. Because of the significant improvement to the oxygen reduction at the cathode, it was no longer the limiting electrode. The performance of cells with the thick-film cathode was a function of the anode composition.
The temperature dependence was such that greater power output could be achieved at elevated temperatures. Waste heat is produced in fuel cells, however, the size of these devices and the amount of power generated suggest that they would not be able to retain enough heat for operation at an elevated temperature. Integrated fuel cells could also use some heat released from the circuit (or other electronic devices) that they are built on.
Improvements in the activity and surface area of the anode can lead to higher currents and power densities. The anode performance was improved with a higher catalyst loading.
To improve the electrode performance, the catalyst surface area, particularly the catalyst that is in direct contact with the electrolyte, must be increased. A thin layer of SiO2 electrolyte could be deposited between two catalyst depositions because it was deposited through PECVD. Sample D had the same 34 μg/cm2 layer as C deposited on the patterned sacrificial polymer, followed by a deposition of 400 Å of SiO2, and then an additional 8.5 μg/cm2 of catalyst, before the thicker SiO2 PEM layer was deposited. The second layer of sputtered Pt/Ru was embedded in SiO2, increasing the catalyst/electrolyte contact area. With only 25% more Pt/Ru at the membrane, the peak power density of sample D was over four times greater than sample C at room temperature. This dramatic improvement in current and power density was due to the SiO2-encapsulated layer of Pt/Ru that allowed for more membrane/catalyst contact in addition to the increase in total catalyst weight. The two thin layers of Pt/Ru and the small amount of SiO2 between them most likely form a mixed matrix of catalyst and electrolyte that is conductive to both protons and electrons while increasing the overall catalyst surface area, particularly the area in contact with the electrolyte.
The performance of the hydrogen fuel cells was studied as a function of time to determine if the data collected through linear voltammetry matches steady-state values at constant potential.
The experiments have shown trends that are being used to further enhance the performance of microfabricated fuel cells. Adding catalyst to the bottom of microchannels is an effective technique for use with conductive fuels. However, increasing the catalyst at the membrane through the use of multiple SiO2 embedded layers to maintain porosity shows promise for increased current density. Additional areas of ongoing study include other improvements to the electrodes, such as increased anode area, and membrane properties, especially conductivity. Reducing the thickness of SiO2 would decrease the resistance of the PEM, but the mechanical strength must be maintained to avoid fuel cells breaking from the pressure of the fuel in the anode microchannels.
Micro-fuel cells utilizing sacrificial polymer-based microchannels and thin-film SiO2 membranes have been successfully fabricated and tested Low-temperature PECVD silicon dioxide shows promise for use in integrated thin-film devices. Lowering the deposition temperature dramatically increased the conductivity of the films to an acceptable level for the current densities achieved with the fabricated electrodes used in this study. Repeated alternate catalyst sputtering and SiO2 deposition steps to build up a catalyst matrix will provide an electrode with increased catalyst and membrane catalyst contact area. Additional catalyst that is not in contact with the membrane can be utilized when using a conductive analyte, such as acidic methanol.
Microfabricated fuel cells have been designed and constructed on silicon integrated circuit wafers using many processes common in integrated circuit fabrication, including sputtering, polymer spin coating, reactive ion etching, and photolithography. Phosphorous-doped silicon dioxide has been studied as a proton exchange membrane for use in these thin-film fuel cells. It is deposited through plasma-enhanced chemical vapor deposition (PECVD) and has ionic conductivities two orders of magnitude greater than low-temperature deposited SiO2 previously used in microfabricated cells. Films with a thickness of 6 and a resistivity of 100 kS)-cm have an area resistance of 60 )-cm2, which compares favorably to x.175 μm-thick film on Nafion 117. The use of phosphorous-doped SiO2 in the microfabricated fuel cells has improved the performance over previous cells that used an-doped silicon dioxide.
A schematic cross section of the microfabricated fuel cells is similar to that shown in
The deposition of SiO2 took place in a PECVD system at temperatures of about 75-250° C. The reactant gases were silane and nitrous oxide with an operating pressure of about 600 mTorr. Phosphorous-doped silicon dioxide (P—SiO2) was deposited by substituting a gas mixture of 0.3% phosphene and 5.0% silane in helium carrier gas for the standard silane gas (5.0% SiH4, in He). Typically, the ratio of theflow rates of N20 to PH3/SiH4 (or N20 to Silo) was 2.25 and the operating temperature 100° C. These values were varied one parameter at a time, while keeping other parameters the same. Film thicknesses were measured with an Alpha-Step surface profilometer (KLA-Tencor, San Jose, Calif.) after using a physical mask to prevent deposition on in a selected region on the substrate. Un-doped SiO2 PEM layers used in previous fuel cell devices (data from some of which are shown for comparison) were deposited using a Plasma-Therm PECVD system (Plasma-Therm, St. Petersburg, Fla.) at 100° C. and the other parameters the same.
Fourier transform infrared (FTIR) spectroscopy was performed using a Nicolet model 560 and Omnic software. All electrochemical measurements, including impedance spectroscopy (IS) and linear voltamagrams, were performed with a PerkinEkner PARSTAT 2263 (EG&G, Princeton, N.J.) electrochemical system. The scan rate for linear sweep voltametry was 1 mV/s. Hydrogen was supplied to the anode microchannels through fine tubing from a pressurized tank of ultra high purity grade gas that passed through a bubbler to humidify the feed. Ionic conductivity was measured with impedance spectroscopy through SiO2 films deposited onto aluminum-coated substrates and contacted with a mercury probe, as well as with actual cells. The frequency range for the impedance measurement was from 100 mHz to 1 MHz, with an AC signal amplitude of 10 mV.
SiO2 and phosphorous-doped SiO2 (P—SiO2) films were deposited onto aluminum-coated glass slides with thicknesses of 1.4-1.5 μm. P—SiO2 films on the aluminum-coated substrates were measured for ionic conductivity through the use of impedance spectroscopy (IS).
While a dramatic increase compared to un-doped SiO2, the conductivity of the P—SiO2 films remains lower than for other commonly used PEMs, such as Nafion, but they are also much thinner than other fuel cell membranes. Extruded Nafion membranes (equivalent weight of 1100) have area resistances of 0.1-0.35 Q-cm2 (15). The area resistance of a 3 μm thick P—SiO2 film deposited at 100° C. is 30 S2-cm2 at room temperature. The relatively high resistance leads to a decrease in cell voltage at higher current.
P—SiO2 films were used as PEMs in microfabricated fuel cells to compare to the un-doped SiO2. Again, the deposition temperature of the PECVD chamber was 100° C. Although, many different recipes were tested for ionic conductivity measurements, the mechanical strength of the films using some of the different gas ratios were not as good as the standard SiO2 recipe. For this reason, the initial fuel cell devices with phosphorous doping used the standard recipe with only the phosphene-silane gas substituted for silane. These films, however, were still not as strong as the previous SiO2 films and required a thicker deposition.
Microfabricated full-cells were fabricated using the processes previously described and tested with linear voltammetry at a scan rate of 1 mV/sec from the open-circuit potential. A 6-μm thick P-doped SiO2 was successfully used as a PEM in the devices.
The addition of phosphorous to SiO2 has been shown to increase the ionic conductivity of the films and improve the overall performance of microfabricated fuel cells when used as the PEM. The conductivity of P—SiO2 films deposited under the same process conditions as the previously used SiO2 membranes, except the addition of the phosphene gas, was approximately 50 times greater than the un-doped low temperature SiO2. Due to this increase in conductivity, thicker PEM layers could be deposited to improve the mechanical strength of the devices while still having a lower resistance to proton transport. The thicker films also improved the open-circuit potential, leading to better overall performance. The P—SiO2 sample outperformed all un-doped SiO2 samples, including ones with better anodes. P—SiO2 proved to be the preferred thin-film PEM material for these devices.
In yet another embodiment, microfabricated fuel cells, or other types of fuel cells may be used in a sensor network, such as a wireless mesh network, including an integrated self-contained sensor assembly 100 as shown in
The transceiver 110 may include one or more transmitters or receivers for use in wireless transmission and reception of data and information to and from the integrated self-contained sensor assembly 100. The transceiver 110 may also include an antenna. The transceiver 110 may be a low current drain device with both a transmitter and a receiver in the same integrated circuit. A number of chipsets are known by those of skill in the art over a wide range of frequencies to accommodate various transmit and receive power levels, duty cycles, and sleep mode capabilities. As used herein, the duty cycle is the amount of time the transceiver may be receiving or transmitting.
The host controller 120 may be a computer such as a microcomputer or a lap top computer or a customized controller targeted at specific mesh network applications. The host controller may have a number of hard wired interfaces to support other network functions including customer or off-the-shelf bus protocols, phone, and internet connections. The host controller 120 may be configured to receive and store data and information from the sensor and/or detectors 130. It is important to note that the integrated self-contained sensor assembly 100 may be configured with or without the host controller.
The sensors and/or detectors 130 may include devices, units, meters, switches, optical waveguides, and other instruments designed for specific applications. The applications may include, but should not be limited to, weather monitoring, soil and water management, pH monitoring, salinity monitoring, motion sensors, law enforcement and security, industrial controls, vibration and pressure monitors, home security and automation, and any other desired application. The sensors and/or detectors 130 may be connected to the integrated self-contained sensor assembly 100 over a wired connection or a wireless connection. In one embodiment, the sensors and/or detectors 130 may be configured to directly or indirectly communicate with the transceiver 110 thereby transmitting the information and data collected by the sensors and/or detectors 130 to the transceiver. In yet another embodiment, the sensors and/or detectors 130 may be configured to directly or indirectly communicate with the host controller 120 thereby transmitting and storing the information and data collected by the sensors and/or detectors 130 to the host controller 120.
The hybrid power module 200 may be designed to provide the energy storage and power generation needs of the integrated self-contained sensor assembly 100. Referring to
The fuel cell 210 included in the hybrid power module 200 is electrically connected with an electrical storage device 220 such as a rechargeable battery. The fuel cell 210 may also be combined with other electrical storage devices 220 such as a capacitor. The fuel cell 210 may be matched with a number of different storage devices 220 according to the needs of the application. The fuel cell 210 may also be combined with additional electrical power generation devices such as turbines, solar cells, geothermic power collectors, and thermoelectric devices. When connected with the electrical storage device 220 such as a rechargeable battery, the fuel cell 210 may trickle-charge the battery and keep it powered sufficiently to meet the needs of the sensor network and the integrated self-contained sensor assembly 100. Moreover, trickle-charging can dramatically extend the field life of the electrical storage device 220, thereby, reducing the frequency and cost of replacement.
In this way, the hybrid power module 200 may be configured to provide the desired electrical characteristics for the transceiver 110 and the sensors/detectors 130. Furthermore, hybrid power module 200 may include the best combination of fuel cell and energy storage device in order to achieve the performance and longevity desired for a specific sensor network.
In yet another embodiment, the hybrid power module 200 can be designed to meet electrical requirements including average and peak current, the duty cycle of the transceiver and/or sensors and storage capacity of the system. For example, the hybrid power module 200 may include a charge control circuit to optimize the desired voltage and current for a specific application. Furthermore, the hybrid power module 200 may include a storage device 210 that has be selected to meet specific storage needs while buffering the fuel cell 210 from peak current activities.
The hybrid module 200 may be configure to be stable in various environmental conditions such as temperature extremes and humidity while maintaining hermeticity and shock and vibration resistance. The hybrid power module 200 may also be configure with the desired input and output connections for integrated self-contained sensor assembly 100. Moreover, the hybrid module 200 may be sized, shaped, and packaged to meet the requirements of the integrated self-contained sensor assembly 100 and any associated sensor network.
It should be emphasized that the previously-described embodiments of this disclosure are merely possible examples of implementations, and are set forth for a clear understanding of the principles of this disclosure. Many variations and modifications may be made to the previously-described embodiments of this disclosure without departing substantially from the spirit and principles of this disclosure. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US06/47694 | 12/14/2006 | WO | 00 | 11/14/2008 |
| Number | Date | Country | |
|---|---|---|---|
| 60750151 | Dec 2005 | US |
