Wicking strands for a polymer electrolyte membrane

Details Wicking strands for a polymer electrolyte membrane Wicking strands for a polymer electrolyte membrane

2000-02-08

2003-04-29

Ryan, Patrick (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S006000, C429S006000

Reexamination Certificate

active

06555262

ABSTRACT:

BACKGROUND OF THE INVENTION
This invention relates to fuel cells and, more particularly, to fuel cells incorporating a solid polymer electrolyte membrane to conduct protons between the electrodes of the fuel cell and including wicking strands to hydrate the membrane.
Work is commonly derived from fuel by a combustion process which uses the pressure of expanding gases to turn a turbine or move a reciprocating piston and, ultimately, provide torque to a driveshaft. This torque is then usually used for propulsion or to generate electrical power. In the latter case, the electrical power is oftimes reconverted into mechanical work.
The by-products of the combustion process are waste gases which contaminate the atmosphere or, if pollution is to be avoided or at least reduced, reacted with catalysts to produce benign compounds. The foregoing process is usually expensive and typically calls for operations and equipment that require extensive monitoring and maintenance to ensure that the emission of pollutants is kept below a prescribed maximum. Furthermore, there are energy losses inherent in the use of expanding gases to drive a turbine or piston engine due to the inefficiency of the combustion process and friction of moving parts.
One approach which avoids the foregoing disadvantages inherent to generating work by burning a fuel is the fuel cell, which produces electrical power directly from a chemical reaction which oxidizes a fuel with the aid of a catalyst. No intermediate steps, such as combustion, are needed, nor is the machinery to generate electrical power from the torque of a driveshaft. The chemical energy of the fuel is utilized much more efficiently. Since polluting waste gases are not emitted, the attendant processes and equipment required to neutralize these harmful by-products are unnecessary.
The simplest fuel cell consists of two electrodes separated by an electrolyte. The electrodes are electrically connected through an external circuit, with a resistive load lying in between them. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly, or “MEA,” consisting of a solid polymer electrolyte membrane, or “PEM,” also known as a proton exchange membrane, disposed between the two electrodes. The electrodes are formed from porous, electrically conductive sheet material, typically carbon fiber paper or cloth, that allows gas diffusion. The PEM readily permits the movement of protons between the electrodes, but is relatively impermeable to gas. It is also a poor electronic conductor, and thereby prevents internal shorting of the cell.
A fuel gas is supplied to one electrode, the anode, where it is oxidized to produce protons and free electrons. The production of free electrons creates an electrical potential, or voltage, at the anode. The protons migrate through the PEM to the other electrode, the positively charged cathode. A reducing agent is supplied to the cathode, where it reacts with the protons that have passed through the PEM and the free electrons that have flowed through the external circuit to form a reactant product. The MEA includes a catalyst, typically platinum-based, at each interface between the PEM and the respective electrodes to induce the desired electrochemical reaction.
In one common embodiment of the fuel cell, hydrogen gas is the fuel and oxygen is the oxidizing agent. The hydrogen is oxidized at the anode to form H
+
ions, or protons, and electrons, in accordance with the chemical equation:
H
2
=2H
+
+2
e
−
The H
+
ions traverse the PEM to the cathode, where they are reduced by oxygen and the free electrons from the external circuit, to form water. The foregoing reaction is expressed by the chemical equation:
½O
2
+2H
+
+2
e
−
=H
2
O
One class of fuel cells uses a solid PEM formed from an ion exchange polymer such as polyperfluorosulfonic acid, e.g., a Naflon® membrane produced by E. I. DuPont de Nemours. Ion transport is along pathways of ionic networks established by the anionic (sulfonic acid anion) groups that exist within the polymer. Water is required around the ionic sites in the polymer to form conductive pathways for ionic transport.
The ionic conductivity of such PEMs is thus a function of the water content in the polymer. More particularly, the conductivity will decrease as the water content drops below a minimum threshold level. As the conductivity drops, the efficiency of the fuel cell decreases until, if the polymer becomes excessively dry, the fuel cell becomes non-conductive.
There are several factors causing the removal of water from the anode surface of the PEM. Heat generated in the oxidation reaction, as well as by transport of the free electrons, i. e., IR type losses, causes evaporation. Water is also lost through electroosmotic transport by hydrogen water compounds, e.g., “hydronium ions,” (H
3
O)
+
. This is a process in which water molecules are “dragged” through the PEM by hydrogen protons migrating from the anode to the cathode. Each H
+
ion is believed to transport one or two water molecules along with it through the mechanism of electroosmotic “drag.”
Dehydration of the membrane is a problem endemic to PEM fuel cells. Abundant water collects on the cathode from being created by the reduction of the H
+
ions and from being transported through the PEM by electoosmosis, and some of this water will automatically migrate back through the PEM to the anode by virtue of the mechanism of diffusion. However, the rate of water migrating back to the anode by means of diffusion is not always sufficient to prevent excessive PEM drying under high current operating conditions, and thus diffusion alone cannot be relied upon to prevent drying under the range of operating conditions that a fuel cell might be expected to encounter.
One approach to maintaining adequate hydration of the PEM is to use a humidifier external to the fuel cell structure to introduce water as steam or a fine mist into the stream of hydrogen gas fuel flowing into the anode. Another method is to bubble the fuel gas through water kept at a temperature higher than the temperature at which the fuel cell is operated.
There are, however, limits on the effectiveness of humidification as a viable solution occasioned primarily by constraints inherent to the mechanism of condensation. More particularly, water condenses in the anode in an amount corresponding to the difference between the saturation vapor pressure of the humidified fuel gas at a humidification temperature and the saturation vapor pressure at a cell operation temperature. The difference between the humidification temperature and the cell operation temperature is typically too small to provide for condensation sufficient to avoid excessive dehydration of the PEM.
One solution has been to increase the temperature differential by increasing the humidification temperature. However, the increased humidification temperature causes an increase in the partial pressure of the water vapor which is greater than the attendant increase in the partial pressure of the fuel gas. This unequal increase in partial pressures causes a decrease in the quantity of fuel gas per unit of volume in the humidified gas mixture entering the fuel cell which, in turn, adversely affects the performance of the fuel cell.
Moreover, even with the gaseous mixture being saturated with water in an amount sufficient to prevent dehydration of the PEM, and assuming arguendo that the quantity of condensed water is similarly adequate, the application of the condensed water is not uniform over the surface of the anode. Rather, most of the water condenses on the part of the anode nearest incoming stream, leaving the more distant portions of the PEM subject to drying out.
The quantity of moisture carried by a saturated gas, and thus the amount of condensed water, can be increased by increasing the flow rate of the saturated gas, but this requires a recirculating gas system including recirculation pumps and some means of filtering impuritie

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