Patent Application
20100239921
1. Field of the Invention
This invention relates to alkaline fuel cells and alkaline batteries. In one aspect, this invention relates to alkaline fuel cells and alkaline batteries which are tolerant to CO2, a known “poison” to the alkaline electrolyte of alkaline fuel cells and alkaline batteries. In one aspect, this invention relates to alkaline fuel cells and alkaline batteries employing polymeric anion exchange alkaline electrolyte membranes. In one aspect, this invention relates to CO2 tolerant alkaline electrolyte membrane fuel cells and batteries. In one aspect, this invention relates to CO2 tolerant polymeric anion exchange alkaline electrolyte membranes.
2. Description of Related Art
Alkaline fuel cells produce power through a redox reaction between hydrogen and oxygen. At the anode electrode, hydrogen is oxidized according to the reaction
H2+2OH−→2H2O+2e−
producing water and releasing two electrons. The electrons flow through an external circuit to the cathode electrode at which oxygen is reduced according to the reaction
O2+2H2O+4e−→4OH−
producing hydroxide ions. The net reaction consumes one oxygen molecule and two hydrogen molecules in the production of two water molecules. Electricity and heat are formed as by-products of this reaction.
In a conventional alkaline fuel cell, the cathode and anode electrodes are gas diffusion electrodes comprising a catalyst layer, a catalyst support layer, and a gas diffusion layer, and the electrodes are separated by a porous matrix, e.g. a nylon sponge, saturated with an aqueous alkaline solution, such as potassium hydroxide (KOH). Aqueous alkaline solutions do not normally reject carbon dioxide (CO2) as a result of which the fuel cell is easily “poisoned” through the progressive carbonation of the solution in accordance with the reaction
CO2+2OH−→CO3−2+H2O
This reaction leads to a decrease in the overall performance of the alkaline fuel cell overtime. In addition, carbonateprecipitation in the cathode impairs the performance because of the three-phase boundary,
In the operation of a conventional alkaline fuel cell, air is provided to the gas diffusion layer of the cathode through which it is transmitted to the catalyst support layer and then the catalyst layer, which is saturated, for example, with KOH solution. If the air contains CO2 or if fuel oxidation produces CO2, the CO2 will react with the KOH to form K2CO3 in the catalyst layer, producing the following effects: a) reduction in the OH− concentration and fuel oxidation/oxygen reduction kinetics; b) increase in the electrolyte viscosity resulting in lower diffusion coefficients and lower limiting currents; c) the eventual precipitation of carbonate salts in the pores of the porous electrode; d) reduction in oxygen solubility; and e) reduction in electrolyte conductivity. A similar mechanism applies to alkaline batteries, especially air-metal batteries.
Alkaline membrane fuel cells, in which the electrolyte is an anion exchange membrane, offer significant advantages over conventional aqueous alkaline-solution based alkaline fuel cells in that membrane based systems avoid issues of electrolyte migration, mitigate corrosion concerns, can be operated with differential pressures, and offer design simplification. Most significantly, alkaline membrane fuel cells are able to operate without the noble metal catalysts employed in conventional alkaline fuel cells. However, low OH− conductivity, water swelling, and chemical stability are problematic with conventional alkaline exchange membranes. Low OH− conductivity of conventional anion exchange alkaline membrane fuel cells has been addressed by doping of the membrane with KOH. However, the use of KOH provides the opportunity for CO2 poisoning. For these reasons, conventional alkaline fuel cells typically operate on pure oxygen, or at least purified air.
The technology most widely used for CO2 removal is amine adsorption, but amine plants are complex and expensive. Membrane plants using CO2-selective cellulose acetate membranes have been in use since the 1980s and currently, the largest membrane facility for CO2 removal operates at 700 million scfd (standard cubic feet per day). Another known technology for CO2 removal is the morphysorb process which selectively removes acid gases, such as H2S, CO2, COS, and other components. Accordingly, there is a need for an anion exchange alkaline membrane fuel cell which is tolerant to CO2 so as to eliminate the need for CO2 removal.
It is, thus, one object of this invention to provide an alkaline fuel cell and alkaline battery which are both highly anion conductive and CO2 tolerant.
This and other objects of this invention are addressed by an electrochemical device comprising an alkaline electrolyte disposed between an anode electrode and a cathode electrode, and CO2 inhibitor means for reducing the effects of CO2 on the electrochemical device, said CO2 inhibitor means disposed in at least one of said electrolyte, said anode electrode, and said cathode electrode. In accordance with one embodiment of this invention, a CO2 inhibitor, which comprises a CO2-adsorption component bound with a polymer, is embedded within, and/or disposed on the surface of, the gas diffusion layer of at least one of the anode and cathode electrodes. In accordance with one embodiment, the electrolyte is a solid polymer anion exchange alkaline electrolyte membrane and the electrodes are gas diffusion electrodes comprising a catalyst layer adjacent the alkaline electrolyte membrane supported on a catalyst support layer and a gas diffusion layer disposed adjacent the catalyst layer. In accordance with another embodiment of this invention, the electrolyte is a CO2 tolerant solid polymer anion exchange alkaline electrolyte membrane.
This and other objects of this invention are also addressed by a method for inhibiting CO2 transmission in an alkaline electrochemical device having an alkaline electrolyte disposed between a gas diffusion cathode electrode and a gas diffusion anode electrode comprising the step of embedding a CO2 inhibitor in, or on the surface of, the gas diffusion layer of at least one of the gas diffusion anode electrode and the gas diffusion cathode electrode. In accordance with one embodiment, the electrochemical device is an anion exchange alkaline electrolyte membrane fuel cell having a solid polymer alkaline electrolyte membrane disposed between the electrodes. In accordance with another embodiment of this invention, the electrochemical device is an open-air alkaline battery.
These and other objects of this invention will be better understood from the following detailed description taken in conjunction with the drawings wherein:
As previously indicated, one of the advantages of an anion exchange alkaline electrolyte membrane fuel cell is the ability to operate without the use of expensive noble metal catalysts. Accordingly, in accordance with one embodiment of this invention, the anode catalyst layer comprises nickel and the cathode catalyst layer comprises silver.
To prevent poisoning of the alkaline electrolyte by the transmission of CO2 through the cathode electrode into the alkaline electrolyte, the porous cathode gas diffusion layer 107 comprises a CO2 inhibitor 114. Modification of the gas diffusion layer for inclusion of the CO2 inhibitor must take into account the stability of the inhibitor under fuel cell and battery operating conditions and the gas permeability of the inhibitor for fuel cells and batteries. The CO2 inhibitor for use in this invention is in the form of a polymer-bound CO2 adsorbent. While any CO2 adsorbent may be employed, in accordance with one preferred embodiment of this invention, the CO2 adsorbent is morpholine, which has been used to scavenge acid gases. However, morpholine by itself is easy to evaporate and not very stable in air.
In accordance with one embodiment of this invention, the anode electrode also comprises a CO2 inhibitor 113 disposed within the anode electrode gas diffusion layer, whereby CO2 which may be generated by the fuel oxidation is substantially prevented from being adsorbed by the alkaline electrolyte membrane. In particular, the catalyst layer of the anode electrode promotes CO2 desorption. In addition, any CO2 adsorbed on the membrane will be decomposed when the fuel cell temperature reaches more than 55° C.
Analysis of the reactant and product species at the anode of an alkaline membrane fuel cell shows that the only way to maintain OH− conductivity through the alkaline electrolyte membrane is to increase the decomposition rate of carbonate (CO32−), bicarbonate (HCO3−), and carbamate (N4NH2CO2). Thus, the percentage of nitrogen in the membrane must increase to provide a high N, —NH, and —NH2 concentration for OH− conductivity while maintaining the polymer's stable structure. With a high percentage of nitrogen, i.e. about 5% or greater, only carbamate is formed, which is not stable in aqueous solutions at elevated temperatures, i.e. greater than about 55° C.
High temperature proton exchange membranes based on poly(2,5-benzimidazole) (ABPBI) are known. The polybenzimidazole (PBI) backbone thereof has been shown to be stable under fuel cell operating conditions at temperatures as high as 160° C. I have found that animation of the ABPBI membrane for use in an alkaline electrolyte membrane fuel cell provides anion conductivity an order of magnitude better than commercially available anion exchange membranes, has good mechanical strength, has an operating temperature in the range of about −40° C. to about 160° C., and is CO2 tolerant. Aminated ABPBI for use in alkaline membrane fuel cells provides the following advantages: reduced swelling of the anion exchange membrane and increased membrane stability; production of water at the anode side which is consumed at the cathode side, thereby reducing the cathode water management problem and ultimately eliminating cathode humidification; use of the amine to cross-link to the ABPBI membrane, which amine is a strong base with a pH of 12.75 in a 60 wt % aqueous solution, providing a membrane having less fuel crossover than conventional linear membranes such as NAFION®; and no requirement of KOH in the fuel solution.
In this example, the alkaline electrolyte membrane in accordance with one embodiment of this invention was prepared by dissolving 3.5 g of poly(vinyl alcohol-co-ethylene) in 45 g of dimethyl sulfoxide)MSO). The solution was heated to 60° C. to facilitate the quick dissolve of the polymer. The solution was then cooled to room temperature, after which 1.5 g of tris(2-aminoethyl)amine was added into the polymer solution and stirred to be homogeneous. 1.6 g of formaldehyde (37% in water) solution was added drop by drop into the polymer solution while stirring. At less than two minutes of time, the solution was cast onto a flat TEFLON® substrate following which the clear solution turned cloudy. The cast membrane on the TEFLON substrate was placed overnight in an oven at 80° C. to dry and cure, Thereafter, the membrane was washed in deionized water and peeled off the substrate.
A single fuel cell was used to test the membrane conductivity. The membrane was assembled in a fuel cell block with a gas diffusion layer on both sides of the membrane serving as current collectors. The active area was 50 cm2. The single cell was placed into a fuel cell test station and humidified gas was passed through both sides of the cell. The carrying gas during the test was N2/N2 or N2/CO2. A Princeton Applied Research Potentiostat/Galvanostat model 263A and a Solartron SI1260 impedance gain/phase analyzer was used to measure resistance. All resistances were measured at 100 kHz. During a period of one minute, thirteen readings were taken at five second intervals. All of the readings were averaged to reach a resistance value.
The results of membrane resistance tests conducted at various temperatures with N2, CO2 purge gas are shown in
In this example, 5% poly(2,5)benzimidazole (ABPBI) was dissolved in 3% NaOH/EtOH solution at 70° C. Tris(2-aminoethyl)amine (TAEA) was added into the polymer solution to provide a ratio of ABPBI:TAEA of about 7:3 and stirred to be homogeneous. The solution was then cooled to room temperature and filtered to remove some undissolved polymer prior to casting. 1.6 g of formaldehyde (37% in water) solution was added drop by drop into 50 ml of the polymer solution while stirring. After less than two minutes, the solution was cast onto a flat TEFLON substrate.
In this example, a high yield aminated polymer was produced by dissolving 3.5 g Poly(ethylene-co-acrylic acid) (20% acrylic acid, Aldrich Chemical) in 45 g dimethyl sulfoxide (DMSO). The solution was then cooled to room temperature, after which 1.5 g of tris(2-aminoethyl)amine was added into the polymer solution and stirred to be homogeneous. The solution was then heated to 95° C. for the acrylic acid group to react with the amine group to remove water for 30 minutes. The solution was then cooled to room temperature and cast onto a flat TEFLON® substrate. The cast membrane was then placed in an oven at 95° C. to dry and cure. The structures of the reactants and end product are shown in
While in the foregoing specification this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for the purpose of illustration, it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic principles of this invention.
