HK1069682B - Chemoelectric generating - Google Patents

Description

Chemical power generation

Technical Field

The present invention relates to fuel cells and more particularly to a novel system and method for providing reverse current charging to a fuel cell

Background

A fuel cell is an electrochemical device that generates usable electricity by converting chemical energy into electrical energy. A typical fuel cell includes a positive electrode and a negative electrode separated by an electrolyte, such as a Polymer Electrolyte Membrane (PEM). In a typical direct current methanol fuel cell (DMFC), a fuel, such as hydrogen or methanol, supplied to the negative electrode diffuses to the anode catalyst and is decomposed into protons and electrons. The protons pass through the PEM to the cathode, while the electrons travel through an external circuit to power a load.

Disclosure of Invention

According to an aspect of the present invention, there is provided a method of chemically generating electricity using a fuel cell having an anode, an electrolyte and a cathode, the method comprising: providing a fuel to the anode; providing an oxidant to the cathode; and performing reverse current charging by interrupting power generation of the fuel cell to supply a reverse current pulse having a specific amount of current level and duration to the fuel cell.

According to the present invention, the operation of the fuel cell is intermittently interrupted, and a reverse charging current is supplied to the cell during the interruption.

In another aspect, the flow rate of air at the cathode is increased.

In yet another aspect, the invention includes a power supply and energy storage device that provides reverse current charging to a fuel cell that is being powered during periods of fuel cell shutdown, and during normal operation, the fuel cell charges the energy storage element.

Other features, objects, and advantages of the invention will be apparent from the description which follows taken in conjunction with the accompanying drawings.

Drawings

FIG. 1 is a block diagram of a fuel cell operating system according to the present invention;

FIG. 2 shows a voltage versus time graph illustrating the effect of fuel cell preconditioning using reverse current charging in accordance with the present invention;

FIG. 3 shows a voltage versus time graph illustrating the improvement in long term degradation of fuel cell voltage using the reverse current charging of the present invention;

fig. 4 shows a voltage versus time graph illustrating the voltage recovery of a fuel cell after cell reversal using the reverse current charging of the present invention.

Like reference symbols in the various drawings indicate like elements.

Detailed Description

The method and system of the present invention will be described with reference to a Direct Methanol Fuel Cell (DMFC). The method and system are applicable to any type of fuel cell, however, including but not limited to fuel cells using carbon-based fuels such as methanol and ethanol; it is also applicable to a fuel cell using pure hydrogen or hydrogen containing carbon monoxide (CO) as an impurity as a fuel. FIG. 1 shows a block diagram of a system for operation of a DMFC 110 in which methanol is supplied to a negative electrode (anode) 120 and electrochemically oxidized to generate electrons (e)^-) And proton (H)⁺). The protons pass through the electrolyte 110 to the cathode 130. The electrolyte 100 may be in the form of a solid Polymer Electrolyte Membrane (PEM). The electrons travel through an external circuit (described below) to the positive electrode (cathode) 130 where they react with oxygen (or oxidant) and protons directed through the PEM to form water and heat. Oxygen may be provided to cathode 130 by various methods, such as flowing air or by means of a liquid carrier. The oxidant may be used for oxidation and/or to transport oxygen to the cathode by means of a liquid or gas. Many useful oxidizers, such as potassium chlorate (KClO)₃) And sodium chlorate (NaClO)₃) Which upon heating may decompose and release oxygen. Hydrogen peroxide (in liquid form) may also decompose and release oxygen when contacted with a catalyst or acid. While these oxidants may directly contact the cathode and react with the electrons to complete the reduction reaction, they may also decompose first, and then transport the liberated oxygen to the cathode.

Each side of the PEM contacts an electrode and the electrodes are typically in the form of a carbon paper coated with a catalyst such as platinum (Pt), a mixture of platinum and rubidium, or a platinum-rubidium alloy (Pt-Ru). The electrochemical reactions occurring at the anode and the cathode will be explained below.

Anodic (oxidation half reaction): CH (CH)₃OH+H₂O→CO₂+6H⁺+6e^-

Cathode (reduction half reaction): 3/2O₂+6H⁺+6e^-→3H₂O

And (3) net reaction: CH (CH)₃OH+3/2O₂→CO₂+2H₂O

The electrons generated at the anode pass through an external circuit that includes an electrical energy processing circuit and a load circuit (discussed below). The external circuit comprises an energy storage unit 150, which may comprise, for example, a battery and/or a capacitor. Energy from the fuel cell may be stored in the energy storage unit 150. The external circuitry may optionally include first intermediate power processing circuitry 140, if necessary, which conditions the power from the fuel cell for appropriate provision to the energy storage unit 150. The first intermediate power processing circuit may comprise, for example, a DC/DC converter. The energy stored in the energy storage unit 150 may be used to supply a load circuit 170 (e.g., a portable electronic device) via an optional second intermediate power processing circuit 160. The second power processing loop 160 may provide further power conditioning at the output of 150 as required by the load circuit 170 and may include a DC/DC or DC/AC converter. The first power processing circuit 140, the second power processing circuit 160, and the energy storage unit 150 cooperate to supply power to the load circuit 170.

The fuel cell is interrupted by the interaction of the power processing circuit 180, the second processing circuit 160, the energy storage unit 150 and the control box 190. The circuitry 180 and the control box 190 may include hardware modules, software modules, or a combination thereof. The reverse current is provided to the fuel cell through switch or relay 147 causing circuit 180 to draw power from the storage 150 unit. By drawing current, the circuit 180 provides a reverse current to the fuel cell, which is opposite to the normal fuel cell discharge current. Therefore, during the reverse current charging, the cathode potential is higher than that during normal operation, and the anode potential is lower than that during normal operation. For normal fuel cell operation, a switch or relay 147 is connected to terminal 145. During reverse current charging, a switch or relay 147 is connected to switch terminal 146 and provides energy from the energy stored in storage unit 150 to circuit 180. During the current reverse charging, the energy storage unit 150 continues to supply power to the load 170 by means of the second power processing circuit 160. Control box 190 draws power from energy storage unit 150 and controls how circuitry 180 provides reverse current pulses to the system. Reverse current charging is related to the number of reverse current pulses and the duration of each pulse and depends on fuel cell specifications, fuel cell operating conditions, fuel cell performance, and external circuit operating conditions. Depending on the operating state of the fuel cell (e.g., whether the fuel cell requires pre-processing, is in reverse, or has been operating for a period of time and has found a decay in performance), control box 190 may provide intermittent reverse current to the fuel cell to charge the fuel cell to improve the performance of the fuel cell. The control box 190 monitors various cell performance parameters such as fuel cell voltage, load current 175, power processing circuitry 160, and energy storage unit 150 by monitoring fuel supply status, elapsed run time, and long term performance degradation, and monitors fuel cell operating status, fuel cell reversal via status line 125.

Each monitored parameter of the reverse current charging pulse supplied to the fuel cell can be controlled by means of circuit 180 and switch or relay 147. For example, control box 190 may not activate power processing circuitry 140 during reverse current charging. When a drop in the output voltage of the fuel cell is observed, control box 190 may begin to provide a series of rapid reverse current pulses to the cell to increase the fuel cell power output level. The reverse current pulses may then be adjusted to a frequency less than that determined by the monitored cell performance, i.e., due to observed cell output increase and stability. Typically, the fuel cell is configured and arranged to provide a steady supply of power to the load circuit 170, and the additional energy stored in the power supply 150 may further be used as peak power required to meet the load.

Examples

Membrane Electrode Assemblies (MEAs) were fabricated or purchased commercially. In the area of 16cm²In-cell testing of active regionsA membrane electrode assembly. The experiment was performed using 1M methanol solution and compressed air. The reverse current is generally the same as the load current. The duration of reverse current charging ranges from a few seconds to a few minutes. During charging, the cell voltage is greater than the open circuit voltage, the cathode is in an oxidizing condition and the anode is in a reducing condition.

The MEA was prepared by the following method: Pt-Ru black (Johnson Matthey, London, UK) was mixed with a 5 wt% NAFION solution (Electrochem Inc, Woburn, Mass.) and water to form an ink. The anode was then prepared by applying a layer of the prepared ink to polytetrafluoroethylene (10 wt%) carbon paper (Toray, Torayca, Japan). A cathode was prepared using a similar method except Pt was used instead of Pt-Ru black (Johnson Matthey, London, UK). By contacting the anode and cathode with NAFIONR(Dupont, Wilmington, DE) membrane bonding the entire MEA was made. The membrane was assembled for testing in two heated graphite plates through which fuel and air were passed.

Example 1

This example is illustrative of the improvement in performance through the pretreatment of a fuel cell prepared by the present invention. As shown in fig. 2, after a brief application of reverse current to the MEA, the performance of the MEA after the pre-treatment (curve (a) in fig. 2) was significantly improved compared to the performance of the MEA before the brief reverse current charging pre-treatment (curve (b) in fig. 2).

MEA was fabricated in the usual shape having 4.5mg/cm²And Pt-Ru of 3mg/cm²And (3) Pt of (1). Using NAFION(Dupont, Wilmington, DE) as an electrolyte membrane. The performance of the freshly made MEA was tested with a 2A load before and after pretreatment at 70 ℃.

The pretreatment of the short reverse current charging is carried out according to the following method: reverse current charging on the MEA was performed for 180 minutes with 6 reverse current pulses periodically at 2A for 18 seconds. When reverse current charging is not performed, the battery output current is maintained at 2A. The power increases by 15% (as shown in fig. 2, under constant output current, a 15% increase in voltage translates to a 15% increase in power). It is worth noting that after reverse current charging, the battery provides electrical energy at a relatively high voltage.

Example 2

This example illustrates the effect of intermittent reverse current charging on mitigating long-term fuel cell performance decay. Fuel cells are typically operated at a constant load, such as constant current mode. In this mode, long-term operation causes a decay in the output voltage of the battery. In this embodiment, the fuel cell operation is interrupted and reverse current charging pulses are provided on an artificial basis. In operation, the system of FIG. 1 provides these functions, wherein switch 147 is intermittently switched between positions 145 and 146 by means of circuitry 180 and control box 190.

The MEA tested was loaded at 2.2mg/cm on the anode side²Pt-Ru (Johnson-Matthey) of (1), supported on the cathode side at 3.3mg/cm²Pt of (1) and the use of NAFIONFilm-formed. Teflon Toray carbon paper was used as the gas diffusion electrode. The cell was tested at 42 ℃ and an air flow rate of 550 cc/min. The fuel cell was disconnected from the load (0.78A) to interrupt the load current, thereby interrupting the fuel cell operation. During the interruption, a reverse current pulse is provided by the external power supply circuit.

The cells were tested for a first time period with one current charge/discharge cycle of reverse current charging, i.e., 0.81A/15min discharge followed by-0.81A/0.3 min charge. The cell was then tested for a second cycle, this time with only 0.78A constant current discharge. The graph in fig. 3 shows the output of the battery when tested over two time periods. The cell had only a 0.5mV/hr decay in performance during the period when the periodic interruption was experienced and reverse current charging occurred, and approximately a 3mV/hr decay during constant current operation.

It is worth noting that during the time that intermittent reverse current charging occurs, the current discharge is maintained at a level (0.81A) higher than that of the battery during constant current (0.78A). This ensures that sufficient energy is available during reverse current charging to meet the energy requirements of the load 170 and the reverse current 180 charging circuit 180.

Example 3

This example illustrates the recovery of fuel cell performance after cell reversal. During long-term operation of the fuel cell, the output voltage of a cell or cells in a large fuel cell stack may be reversed. When this occurs, the battery output voltage becomes a negative value. That is, during cell reversal, the anode becomes more efficient than the cathode. One common cause of reversal is reactant consumption. Although cell reversal can be caused by consumption of reactants in the anode or cathode, the most serious problem occurs when the anode fuel is limited. For example, carbon corrosion and anode catalyst damage from excessive oxidation can occur if the anode is empty of fuel. However, the battery can be reactivated using the current reversal method of the present invention.

Cell reversal can be mimicked by running the cell without fuel from time to time until the cell voltage becomes negative. It was found that by briefly supplying a reverse current to the battery, battery degradation can be reduced and most of the battery performance can be restored.

The MEA was first tested with a determined load (discharge current) as described below. After the battery voltage stabilizes, the fuel pump is turned off while the same amount of current is forced through the battery for a time sufficient to cause damage to the battery. If the cell voltage is lower than the voltage of the original cell under the same output current density condition after the fuel source is restored, it is determined that cell damage caused by cell reversal has occurred.

MEA purchased from Lynntech (College Station, TX) inThe membrane is pre-coated with a catalyst. The anode contained 4mg/cm²And the cathode contains 4mg/cm of Pt-Ru²And (3) Pt of (1). The membrane was tested using a teflon carbon paper as the anode gas diffusion electrode and a gold mesh with an air flow rate of 600cc/min as the cathode gas diffusion electrode. Figure 4 shows the performance curve (voltage versus time) of a fuel cell at a load of 70 c at 1A. After the test was run for a period of time (curve (a) in fig. 4), the fuel transfer pump was turned off while the cell was outputting the same amount of current. After a few minutes, the cell voltage reverses (curve (b) in fig. 4). With a cell voltage output of-1.7V, the anode is more efficient than the cathode. When the fuel pump is turned on and fuel delivery is resumed, the output voltage is significantly lower than before the cell reversal (curve (c) in fig. 5). After some short reverse current charging pulse was applied, most of the cell voltage was recovered (curve (d) in fig. 5).

Apparatus and techniques for improving fuel cell performance have been described herein. It will be apparent to those skilled in the art that many modifications and variations can be made to the specific embodiments described herein without departing from the inventive concepts. Accordingly, the invention comprises each and every feature and novel combination of features disclosed in or possessed by the apparatus and techniques herein disclosed and limited only by the spirit and scope of the appended claims.

Claims (23)

1. A method of chemically generating electricity using a fuel cell having an anode, an electrolyte, and a cathode, the method comprising:

providing a fuel to the anode;

providing an oxidant to the cathode; and

reverse current charging is performed by interrupting power generation of a fuel cell to provide a reverse current pulse having a specific amount of current level and duration to the fuel cell.

2. The method of claim 1, further comprising monitoring an operating condition of the fuel cell.

3. The method of claim 2, wherein monitoring operating conditions comprises monitoring performance of the fuel cell.

4. The method of claim 2 wherein said reverse current charging is performed when monitoring of the operating conditions of said fuel cell indicates a degradation in the performance of said fuel cell.

5. The method of claim 2, wherein monitoring the operating condition comprises monitoring a voltage of the fuel cell.

6. The method of claim 1, wherein the reverse current charge controls an amount of reverse current charge received by the fuel cell.

7. The method of claim 1, further comprising monitoring an operating condition of said fuel cell, and selecting said specified amount of reverse current pulses and a duration of each pulse in accordance with the monitored operating condition of the fuel cell.

8. The method of claim 1, wherein said reverse current charging increases the amount of charge received by said fuel cell when the monitored performance of the fuel cell decays.

9. The method of claim 1, wherein the reverse current charging reduces the amount of charge received by the fuel cell when the monitored performance of the fuel cell improves.

10. The method of claim 1, wherein said providing oxidant to said cathode is by air flow.

11. The method of claim 10, wherein the reverse current charging further comprises increasing the rate of air flow when an oxidant is provided to the cathode.

12. The method of claim 1, wherein said providing oxidant to said cathode is via a liquid.

13. The method of claim 1, wherein the oxidant is oxygen from air.

14. The process of claim 1 wherein the oxidant is oxygen from the decomposition of potassium chlorate.

15. The process of claim 1 wherein said oxidizing agent is oxygen from the decomposition of sodium chlorate.

16. The method of claim 1, wherein the oxidizing agent is oxygen from the decomposition of hydrogen peroxide.

17. A method according to any one of claims 1 to 16 for preconditioning a fuel cell prior to its first use.

18. The method of any one of claims 1 to 16, for recovering the performance of a fuel cell after its use.

19. The method of claim 2, further comprising:

applying the reverse current charge to the fuel cell via an external power supply circuit in accordance with the monitored operating condition.

20. The method of claim 19, wherein operating the fuel cell to provide power is supplying power to the external power supply circuit, which further supplies power to an external load circuit.

21. The method of claim 19, further comprising monitoring an operating condition of the external power supply circuit.

22. The method of claim 19, further comprising monitoring an operating condition of the external load circuit.

23. The method of claim 19, further comprising an external power supply circuit supplying power to the load circuit when selectively providing reverse current charging to the fuel cell.