Patent Application
20070193885
The present invention relates to apparatus, methods and applications for electrochemical hydrogen manipulation, including the use of electrochemical cells to transfer, purify or compress hydrogen from a source gas that contains hydrogen.
Hydrogen based energy devices are of increasing interest, due in part to advantages provided in efficiency and environmental impact over traditional combustion based technologies.
A variety of electrochemical fuel cell technologies are known, wherein electrical power is produced by reacting a fuel such as hydrogen in an electrochemical cell to produce a flow of electrons across the cell, thus providing an electrical current. For example, in fuel cells utilizing proton exchange membrane technology, a gas containing hydrogen is reacted at an anode side of the fuel cell. Each hydrogen molecule that is reacted produces two protons which pass through a proton conductive membrane to a cathode side of the fuel cell. The protons at the cathode react with oxygen to form water, and the residual electrons at the anode travel through a conductive path around the proton conducting membrane from anode to cathode to produce an electrical current. The technology is closely analogous to conventional battery technology.
Electrochemical cells can also be used to selectively transfer (or “pump”) hydrogen from one side of the cell to another. For example, in a cell utilizing a proton exchange membrane, the membrane is sandwiched between a first electrode and a second electrode, a gas containing hydrogen is placed at the first electrode, and an electric potential is placed between the first and second electrodes, the potential at the first electrode with respect to ground (or “zero”) being greater than the potential at the second electrode with respect to ground. Each hydrogen molecule reacted at the first electrode produces two protons which pass through the membrane to the second electrode of the cell, where they are rejoined by two electrons to form a hydrogen molecule (sometimes referred to as “evolving hydrogen” at the electrode).
Electrochemical cells used in this manner are sometimes referred to as hydrogen pumps. In addition to providing controlled transfer of hydrogen across the cell, hydrogen pumps can also by used to separate hydrogen from a gas containing other components besides hydrogen. The hydrogen production from the cell can also be used to compress the hydrogen gas as it is evolved.
There is a continuing need for apparatus, methods and applications for electrochemical hydrogen manipulation, including the use of electrochemical cells to transfer, purify or compress hydrogen.
Apparatus, methods and applications are provided for electrochemical hydrogen manipulation. In one aspect, an electrochemical cell is provided utilizing an acid doped polybenzimidazole (PBI) membrane having a proton conductivity of at least 0.1 S/cm and comprising phosphoric acid (PA) in a ratio of at least 20 moles phosphoric acid to PBI repeating unit. As an example, the PBI membrane can be produced by a sol-gel process. In some embodiments, such systems can be operated utilizing hydrogen that is dry or otherwise un-humidified or less than saturated with water.
In another aspect, an electrochemical cell is provided that includes a polymeric layer that abuts an external surface of an acid doped PBI membrane. As examples, the polymeric layer can be a polymeric acid layer, e.g., polyvinyl phosphonic acid or a polyvinyl sulfonic acid. Other materials are also possible.
In another aspect, an electrochemical cell is provided that includes an acid doped PBI membrane associated with a porous support layer. As examples, the support layer can be encapsulated within the membrane, or can be provided along an external surface of the membrane. As an example, the support layer can be expanded polytetrafluoroethylene. Other materials are also possible.
In another aspect, apparatus and methods are provided wherein an electrochemical cell is used to provide hydrogen to an inlet of a mechanical compressor. In other embodiments, a mechanical compressor can be adapted to provide compressed hydrogen to an inlet of an electrochemical cell.
In another aspect, a method is provided for operating an electrochemical cell utilizing an acid doped PBI membrane and non-graphitic carbon based components such as flow field plates, etc. An electric potential is applied between first and second electrodes of the cell, and the potential is maintained below 0.8 volts.
In another aspect, a method is provided for utilizing an electrochemical cell to meter a flow of hydrogen. As an example, an electrical measurement can be taken from an electrochemical cell operating in a hydrogen pumping mode to correlate an amount of hydrogen transferred across the cell. The correlated hydrogen flow can be compared to a threshold value to allow the cell to be shut off when a desired amount of hydrogen has been transferred.
Other aspects and features of the invention will be apparent from the following Detailed Description and from the claims.
It will be appreciated that the apparatus, methods, and applications of the invention can include any of the features described herein, either alone or in combination.
One aspect of the invention is a method of operating an electrochemical cell, including at least the following steps: applying an electric potential between a first electrode and a second electrode of an electrochemical cell, wherein the first electrode has a higher electric potential with respect to zero than the second electrode, wherein the first and second electrodes have an acid doped PBI membrane between them, the membrane having a proton conductivity of at least 0.1 S/cm; and flowing a hydrogen gas across the first electrode and evolving hydrogen at the second electrode. As discussed above, each hydrogen molecule reacted at the first electrode produces two protons which pass through the membrane to the second electrode of the cell, where they are rejoined by two electrons to form a hydrogen molecule. In the present invention, the hydrogen gas can be pure hydrogen, or any gas containing any amount of hydrogen, for example containing various impurities. The hydrogen gas may also be referred to synonymously as a source gas, hydrogen source gas, hydrogen containing gas, etc.
The direction of hydrogen “pumping” across the membrane can be controlled according to the polarity of the electrical potential between the first and second electrodes. The hydrogen flows between the electrodes from higher to lower potential with respect to ground or zero. Thus, reversing the polarity across the cell can reverse the direction of hydrogen flow between the electrodes. Methods under the present invention may thus include the step of reversing a polarity of the electric potential between the first electrode and second electrode to reverse a direction of hydrogen flow through the cell. In this context, “reversing a direction” is taken to mean selectively evolving hydrogen at either electrode according to the polarity of the potential (in addition to actually reversing an active flow of hydrogen through the cell).
In another embodiment, instead of a potential being placed across the first and second electrodes, an electrical load can be placed across them, and as a result, hydrogen will be “pumped” from the side of the membrane having the higher partial pressure of hydrogen to the side having the lower partial pressure of hydrogen. Methods under the present invention may thus include the step of removing the electric potential between the first electrode and the second electrode, and connecting an electric load between the first electrode and the second electrode.
It will be appreciated that PBIs are a class of heterocyclic polymers. Various examples of PBI polymers are provided in the teachings of U.S. Pat. No. 4,814,399, which is hereby incorporated by reference. As discussed, for example, in the above referenced patent, PBI membranes used in electrochemical cells are normally imbibed with an ion conductive material such as phosphoric acid (PA). PBI membranes that are associated with PA through soaking, imbibing, through the sol-gel process discussed below, or by any other process are sometimes referred to as acid doped PBI membranes.
PBI membranes used with the present invention can be prepared by a sol-gel process, as described in the article, High-Temperature Polybenzimidazole Fuel Cell Membranes Via A Sol-Gel Process, Chem. Mater. Vol. 17, No. 21, 2005, which is incorporated by reference and excerpted below. It is noted that one inventor of the present case is an author of this article.
Under the sol-gel process, polymerization to produce PBI polymers can be carried out using polyphosphoric acid (PPA) as both the polycondensation agent and the polymerization solvent starting from tetraaminobiphenyl (TAB) and dicarboxylic acid. After polymerization, the PBI solution in PPA can be directly cast at approximately 200 to 220 C without isolation or redissolution of the polymers. Upon casting, hydrolysis of the PPA to PA induces a sol-gel transition that produces membranes with higher ratios of PA to PBI repeating unit than currently believed possible with other PBI membrane production techniques. For example, PBI membranes produced under the sol-gel process can have more than 20 moles of PA per PBI repeating unit (e.g., 20-40 moles of PA per PBI repeating unit). The main example discussed in the article referenced above had approximately 32 moles of PA per PBI repeating unit. It will be appreciated that over time, and particularly during operation of an electrochemical cell containing a PBI membrane, some PA may migrate from the membrane over time.
Without wishing to be bound by theory, it is believed that the higher PA loading in PBI membranes produced under the sol-gel process results in greater proton conductivity. As examples, such membranes generally have conductivities of at least 0.1 S/cm, or even at least 0.2 S/cm.
The following description of PBI membrane preparation under the sol-gel process is taken from the article referenced above.
Materials and PBI Synthesis. Isophthalic acid and terephthalic acid were purchased from Amoco (99+% pure) and dried prior to use. 3,3′,4,4′-Tetraaminobiphenyl (TAB, polymer grade) was donated by Celanese Ventures, GmbH and used as received. Polyphosphoric acid (115%) was used as supplied from Aldrich Chemical Co. and FMC Corporation. The general procedure for the synthesis of polybenzimidazoles (PBIs) is described as follows: Isophthalic acid (12.460 g, 75 mmol) and TAB (16.074 g, 75 mmol) were added to a three-neck resin reaction flask in a nitrogen atmosphere glovebox, followed by 200 to 600 g of polyphosphoric acid. The reaction mixture was stirred using a mechanical overhead stirrer and purged with a slow stream of nitrogen, and the reaction temperature was controlled by a programmable temperature controller with ramp and soak features. The typical polymerization temperatures were approximately 190-220 C for 16 to 24 h. During the polymerization, the reaction mixture became more viscous and developed a dark brown color. A small amount of the reaction mixture was poured into water and isolated as a brown mass. The mass was pulverized, neutralized with ammonium hydroxide, washed thoroughly with water, and dried in a vacuum oven for 24 h at 100 C to obtain the PBIs for further characterization.
Membrane Preparation. The membranes were prepared by casting the polymerization solution directly onto untreated glass substrates in air using a film applicator with a gate thickness ranging from 0.127 mm (5 mils) to 0.635 mm (25 mils) and allowed to cool from polymerization temperature (190 to 220 C) to room temperature in a few minutes. Hydrolysis was allowed to proceed under controlled conditions (for example, by exposing films for 24 h at 25 C and a relative humidity of 40±5%). Since both PBI polymer and polyphosphoric acid are extremely hygroscopic, moisture was absorbed from the atmosphere and hydrolyzed the polyphosphoric acid solvent to phosphoric acid. Some drain-off of water and phosphoric acid was then observed during the hydrolysis process which caused a shrinkage of membrane dimensions of 10 to 20%. The amount of water absorbed did not correlate directly with the membrane PA-doping level.
Characterization Methods. The phosphorus nuclear magnetic resonance spectra (31P NMR) were recorded on a Chemagnetics CMX-360 instrument operating at a frequency of 145.71 MHz using 85% PA as external reference. Polymer films were cast onto thin glass strips and assembled into an open-ended glass NMR tube with 7.0 mm diameter. The film strips were then hydrolyzed in an environmental chamber and taken out periodically for 31P NMR measurements. The membrane acid-doping levels were determined by titrating a preweighed piece of membrane sample with standardized sodium hydroxide solution with a Metrohm 716 DMS Titrino titrator. The samples were then washed with water and dried in a vacuum oven at 100 C for 4 h to obtain the dry weight of polymer. The acid-doping levels, X, expressed as moles of phosphoric acid per mole of PBI repeat unit (XH3PO4·PBI) were calculated from the equation: acid-doping level X=(VNaOH CNaOH)/(Wdry/Mw);
where VNaOH and CNaOH are the volume and the molar concentration of the sodium hydroxide titer, while Wdry is the dry polymer weight and Mw is the molecular weight of the polymer repeat unit, respectively.
Ionic conductivities were measured by a four-probe ac impedance method using a Zahner IM6e spectrometer over a frequency range from 1 Hz to 100 kHz. A rectangular piece of membrane (3.5 cm×7.0 cm) and four platinum wire current collectors were set in a glass cell. Two outer electrodes 6.0 cm apart supply current to the cell, while the two inner electrodes 2.0 cm apart on opposite sides of the membrane measure the potential drop. The four-probe technique offers many advantages over the two-probe techniques, including measuring the bulk property of the membrane instead of the surface property and minimizing the error stemming from contact resistance and electrode resistance. The cell was placed in a programmable oven to measure the temperature dependence of the proton conductivity. The membranes were dried by first heating from room temperature to 200 C and holding at 200 C for 1 h. The membrane samples were then cooled in a vacuum oven and taken out just before conductivity measurement in an effort to keep the samples dry. The conductivities of the membrane samples were measured from 20 to 160 C at intervals of 20 C. Before the measurements at each temperature set point, the samples were held at constant temperature for at least 10 min. Repeated conductivity measurements showed that reproducible results were obtained using this temperature profile and testing procedure. A two-component model with an ohmic resistance in parallel with a capacitor was employed to fit the experimental curve of the membrane resistance across the frequency range (the Nyquist plot). The conductivities of the membrane at different temperatures were calculated from the membrane resistance obtained from the ohmic resistance of the model simulation. Proton conductivity was then calculated from the following equation:
•=D/(LBR);
where D is the distance between the two current electrodes 2.0 cm apart, L and B are the thickness and width, respectively, and R is the resistance value measured.
One advantage of the electrochemical cells, systems and related methods provided under the invention is that they can generally be operated without having to humidify the gas from which hydrogen is removed. It is believed that electrochemical cells utilizing PBI membranes produced by traditional non-sol-gel processes, and other non-PBI fuel cells all require that the hydrogen source gas be humidified prior to hydrogen transfer. It is believed that utilization of subsaturated hydrogen source gas will result in an immediate and progressive performance degradation. The sol-gel PBI based systems under the invention do not exhibit such degradation. Without wishing to be bound by theory, it is believed that the higher ratio of PA to PBI repeating unit enables this aspect of performance. Thus, in some embodiments, the apparatus and methods provided include the distinction that the hydrogen source gas is unhumidified. In this context, unhumidified means that the gas is less than saturated with water, and no step has been taken to increase the saturation level of the gas. In some embodiments, the hydrogen source gas can be dry.
Some embodiments also provide the advantage that they can be used to transfer hydrogen from gasses containing carbon monoxide, including concentrated amounts of carbon monoxide that would be sufficient to interfere with the operation of other polymer electrolyte membrane electrochemical cells. Without wishing to be bound by theory, it is believed that the capability of PBI based membranes to be operated at relatively high temperatures enables this aspect of performance (e.g., operating temperatures from 100-200 C, over 140 C, etc.).
In another aspect of the invention, the electrochemical cell can include a polymeric film or layer abutting an external surface of the membrane. For example, the hydrogen source side of the membrane may have such a layer between the membrane and the electrode. Similarly, such a layer may also be placed between the hydrogen evolution side of the membrane and the electrode. Without wishing to be bound by theory, it is believed that such polymeric layers may assist long term retention of PA in the PBI membrane, particularly in the case of sol-gel PBI membranes having high ratios of PA to PBI repeating unit. As examples, the polymeric film can be a polymeric acid layer comprising polyvinyl phosphonic acids, polyvinyl sulfonic acids or other materials suitable for promoting proton transfer. Those of skill in the art will appreciate that other suitable materials can also be used. In some embodiments, the polymeric acid layer can be cross-linked onto the PBI membrane.
In another aspect of the invention, the electrochemical cell can include a porous support layer. For example, in some embodiments, the PBI membrane may have such a layer at its core. In other embodiments, the PBI membrane may have such a layer on the hydrogen source side, or the hydrogen evolution side, or both sides. In one example, the support layer can be a porous polymer film such as expanded polytetrafluoroethylene that is drawn through a sol-gel mixture of PA and PBI, such that the PBI is cast onto the support layer. In another example, the support layer can be a rigid layer such as a ceramic material. It will be appreciated that additional support layer compositions will be suitable. In addition to providing mechanical support, the support layer can also provides additional PA associated with the membrane (e.g., support layer with pores containing PA), thus improving performance and longevity. Where a support layer is placed across an external surface of the membrane, the support layer will generally need to be electrically conductive.
In another aspect, an electrochemical cell can be combined with a mechanical compressor adapted to receive an exhaust from the cell. In some mechanical compressors, the initial stages of compression can be less efficient, and therefore providing initial compression from an electrochemical cell can improve the efficiency of a combined compression system.
A related method of operating an electrochemical cell is provided, including at least the following steps: applying an electric potential between a first electrode and a second electrode of an electrochemical cell, wherein the first electrode has a higher electric potential with respect to zero than the second electrode, wherein the first and second electrodes have an acid doped polybenzimidazole membrane between them, the membrane having a proton conductivity of at least 0.1 S/cm; flowing a hydrogen gas across the first electrode and evolving hydrogen at the second electrode, wherein the hydrogen gas comprises hydrogen; and exhausting hydrogen from the second electrode to an inlet of a mechanical compressor.
Conversely, in some cases it may be desirable to minimize the pressure drop across an electrochemical cell used in a hydrogen pumping mode, or otherwise provide gas to the cell at elevated pressure. Therefore, a combined system is also provided wherein a mechanical compressor is adapted to supply a compressed source gas to an inlet of an electrochemical cell, wherein the source gas comprises hydrogen, and wherein the electrochemical cell is adapted to transfer hydrogen from the compressed source gas to an outlet of the electrochemical cell.
In another aspect, the invention provides a means for utilizing PBI based electrochemical cells with non-graphitic carbon based components. Operation of such systems has been problematic in the past because the relatively high cell voltages associated with traditional fuel cells (e.g., over 0.8 volt) have resulted in corrosion of the cell, requiring the use of expensive graphitic materials. However, under the present invention, in systems utilizing sol-gel based PBI membranes, it has been found that the cells can be operated at substantially lower voltages where corrosion will not occur (e.g., under 0.8 volts, under 0.6 volts, or even under 0.3 volts).
The invention therefore provides a related method of operating an electrochemical cell, including at least the following steps: applying an electric potential between a first electrode and a second electrode of an electrochemical cell, wherein the first electrode has a higher electric potential with respect to zero than the second electrode, wherein the first and second electrodes have an acid doped polybenzimidazole membrane between them, wherein the first and second electrodes each comprise non-graphitic carbon based components; flowing a gas comprising hydrogen across the first electrode; and maintaining the electric potential between the first and second electrodes below 0.8 volts.
In another aspect, the invention provides a method of metering a flow of a hydrogen, including at least the following steps: applying an electric potential between first and second electrodes of an electrochemical cell; providing gas comprising hydrogen to the first electrode; and removing the electric potential when a desired amount of hydrogen has been transferred to the second electrode.
This method may also include additional steps including the following: taking an electrical measurement from the electrochemical cell; correlating an amount of pumped hydrogen from the electrical measurement; comparing the correlated amount of pumped hydrogen to a threshold value; and generating a signal to remove the electric potential between the first and second electrodes when the correlated amount of pumped hydrogen is at least as high as the threshold value.
It is well known to those of ordinary skill in the art how electrical information from an electrochemical cell in a hydrogen pumping mode can be used to correlate the amount of hydrogen transferred across the cell.
In addition to the foregoing, such control methods may also be conducted according to non-electrical measurements, such as pressure measurements, etc.
As examples, such methods can be used to accurately control the flow of hydrogen gas into or out of a hydrogen storage vessel, or from one stream containing hydrogen to another, etc. Such methods can be used to meter hydrogen flow to propulsion systems, such as fuel cell and other hydrogen based automotive systems requiring metered hydrogen injection. As an example, rather than getting a bulk flow of hydrogen by opening a pressure valve on a tank of hydrogen, by utilizing electrochemical metering under the present invention, the amount of hydrogen released or injected from a source can be controlled with extreme precision, on essentially an atom-by-atom basis.
While most of the concepts described herein involve the use of PBI membranes produced under the sol-gel process, in claims where the specific nature of the membrane is not specified, any suitable membrane may be used, such as those based on non-sol-gel PBI, Nafion, PEEK, etc.
Discussion in the present case is generally made only with respect to the particular aspects of electrochemical cell technologies affected by the concepts described herein. Additional details for suitable designs and operating methods for electrochemical cells are well known in the art. As examples, the teachings of U.S. Pat. Nos. 4,620,914 and 6,280,865; and published U.S. patent application Ser. Nos. 10/213,798 and 10/478,852 are hereby incorporated by reference.
While the invention has been shown or described in only some of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes without departing from the scope of the invention.
This application claims priority under 35 USC 119 (e) from U.S. Provisional Application No. 60/763,457, filed Jan. 30, 2006, naming Benicewicz et al. as inventors, and titled APPARATUS AND METHODS FOR ELECTROCHEMICAL HYDROGEN MANIPULATION.” That application is incorporated herein by reference in its entirety and for all purposes.
| Number | Date | Country | |
|---|---|---|---|
| 60763457 | Jan 2006 | US |
