COPRECIPITATED HYDROGELS IN PRESSURE TOLERANT GAS DIFFUSION ELECTRODES

Abstract: Gas diffusion electrodes (72) and gas generating or consuming electrochemical cells utilizing the same. The electrode (72) comprises an electronically conductive and electrochemically active porous body defining respective gas (82) and electrolyte (84) contacting surfaces, with a substantially gas impermeable layer (106) covering the electrolyte contacting surface (84). The layer comprises an electrolyte insoluble, ionomeric ionically conductive hydrophilic hydrogel (106) formed by coprecipitation between at least first and second precursor polymers.

Description: "COPRECIPITATED HYDROGELS IN PRESSURE TOLERANT GAS DIFFUSION ELECTRODES" BACKGROUND OF THE INVENTION Field of the Invention This invention relates generally to gas diffusion electrodes and, more particularly, this invention relates to gas diffusion electrodes adapted for use in electrochemical cells utilizing an aqueous alkaline electrolyte and consum¬ ing or generating a gas via the electrochemical process oc¬ curring within the gas diffusion electrode. Description of Related Art The use of gas diffusion electrodes in fuel cells and metal-air batteries is well known. Gas diffusion elec¬ trodes have also been used in the electrolysis, either oxi¬ dation or reduction, of gaseous reactants. It is also possi¬ ble to generate gases in such electrodes. In general, gas diffusion electrodes take the form of solid porous (gas and liquid permeable) bodies formed at least in part of an elec¬ tronically conductive, electrochemically active material, and may include a catalyst. Such electrodes generally de¬ fine an electrolyte contacting surface and a gas contacting surface. Electrochemical oxidation and reduction occur at the points in the electrode where the gas to be oxidized or reduced contacts both the electrolyte and the active mater¬ ial of the electrode. In the case of gas generation, elec¬ trolyte contacts the active material and gas is generated at this interface. Elecτrochemical cells utilizing such electrodes generally comprise the gas diffusion electrode, a spaced counter electrode, a liquid electrolyte (which is generally aqueous) which contacts both the counter electrode and the gas diffusion electrode, and a gas which contacts the gas diffusion electrode either (1) for reduction or oxidation of the gas or (2) produced via electrolytic generation. Cir¬ cuit connections are disposed between the counter and gas diffusion electrodes. Additionally, the counter electrode may also be a gas diffusion electrode. A well known example of such a design is the H2/O2 fuel cell. Electrochemical batteries, for example, the metal- air type, commonly utilize either an aqueous alkaline or neutral (e.g., saline) electrolyte, while fuel cells may commonly utilize either acidic electrolytes or alkaline electrolytes. Other types of electrolytes are also used, depending upon the specific gas which is consumed or gene¬ rated. The use in electrochemical batteries of an oxygen- containing gas such as air which is reduced at the gas dif¬ fusion electrode is well known. However, the gas need not be oxygen-containing nor need it be reduced at the gas dif¬ fusion electrode. For example, hydrogen gas is oxidized in some fuel cells. The present invention is generally appli¬ cable to all such types of gas diffusion electrodes and cells. The electronically conductive material in a gas diffusion electrode typically may be carbon. Additionally, a wide variety of catalysts such as platinum or transition metal organometallic catalysts (such as porphyrins) are available. In various applications, it is desirable that either or both the liquid electrolyte and the gaseous elec¬ trode reactant be flowed through the body of the ceil over the electrode surfaces. Flowing electrolyte and/or flowed gaseous reactant are of course accompanied by a pressure drop across the cell, especially on the electrolyte side. This can be lead to excess pressures either on the gas-side or the electrolyte-side of the electrode. Furthermore, it may be desirable in certain circumstances to operate at an elevated gas pressure with respect to the electrolyte pressure. One example of such a situation would be one in which the performance is increased by pressurizing the gaseous reactant. In battery and fuel cell applications, it is desirable to obtain as high a cell voltage as possible at any given current density. One means of accomplishing this ' is to utilize a relatively high gas pressure or flow rate. The use of a porous (e.g. typically 30-60% porosity) gas diffusion electrode, however, poses difficult flow management problems. When gas pressure exceeds liquid electrolyte pressure by a sufficient amount, "blow-through" of gas through the electrode into the liquid electrolyte results. In conventional gas diffusion electrodes, this so- called "blow-through pressure" is usually much lower than is desirable for tolerance of substantial differential pressures between the gas and liquid sides of the cell. For example, while it may be desirable to operate a cell at a gas vs. liquid differential pressure of up to 10 psi or more, typical air cathodes exhibit a gas blow-through pressure of less than about 0.25 psi. If the differential pressure exceeds the blow-through pressure, pumping of gas into the liquid electrolyte may result. (Typical blow- through pressures range from 0-1 psi, and are determined primarily by interfacial tension and pore size distribu¬ tion.) Conversely, if the liquid electrolyte pressure is higher than the gas pressure and the differential pressure exceeds the liquid bleed-through pressure, liquid may be pumped into the gas side of the cell, which may result in liquid in the gas manifold, with consequent pumping problems and a decrease in cell performance and useful cell life due to flooding of the active layer of the electrode. In gas-generating cells it is customary for the gas to be generated on the front face (electrolyte-side) of the electrode. The gas is thus generated as bubbles in the electrolyte, which can lead to removal of electrolyte from the cell and increased oh ic losses. Generation of gas in a gas diffusion electrode is more desirable, because the gas can exit the cell directly through the back of the elec¬ trode. Operation in this mode would require a certain " amount of pressure tolerance. Even higher pressure toler¬ ance would be required if the gas is generated in a pressur¬ ized state. If the differential pressure between the gas and liquid sides of an electrochemical cell using a porous gas diffusion electrode is to be maintained at a low level, im¬ practical pressure management problems result, especially in view of the fact that pressure levels vary from point to point on each side of the electrode. The problems described are not readily amenable to correction by the use of a gas barrier material between the - _3- gas and electrolyte sides of the electrode, since such barriers tend to block the flow of electrolytic ions through the electrode and also strongly contribute to voltage losses or do not allow operation at a sufficiently high current density for the desired application. It is desirable to maintain the potential across the electrode at as positive a level as possible while maintaining as high a current density as possible. For example, it may be desired to operate a cell at a current density of up to as high as 500 mA/cm 2 , typically at 100 mA/cm 2 , while minimizing the voltage loss across the electrode. A voltage loss of less than 0.05 volts is preferred, with voltage losses of up to 0.25 volts being generally acceptable. One approach to solving these problems is dis¬ closed in Juda and Ilan U.S. Patent No. 4,614,575 (September 30, 1986) , which involves the use of nonionic polymeric hy¬ drogel as a layer applied by painting onto the electrolyte side of the gas diffusion electrode. The maximum pressure tolerance disclosed by the Juda,et al patent is less than or equal to 40 inches of water (1.44 psi or 10.0 Pa) , which is significantly less than that possible with the present in¬ vention. SUT-MARY OF THE INVENTION It is an object of the present invention to over¬ come one or more of the problems described above. According to the present invention, an ionomeric, ionically conductive, substantially gas impermeable layer is disposed over substantially the entire electrolyte contact¬ ing surface of a gas diffusion electrode adapted for use in a gas generating or consuming electrochemical cell utilizing a liquid electrolyte. The layer comprises an electrolyte- insoluble, ionomeric ionically conductive hydrophilic hy¬ drogel, and is preferably formed in situ on the electrolyte contacting surface by a coprecipitation reaction between at least two precursor polymers. The invention also comprehends an electrochemical cell comprising the coated gas diffusion electrode spaced from a counter electrode and in contact with liquid electro¬ lyte. A gas to be oxidized, reduced or generated is in con¬ tact with the gas side of the electrode, and circuit connec¬ tions are disposed between the counter- and gas diffusion electrodes. The electrode and cell of the invention are capa¬ ble of operating at very high gas vs. electrolyte differen¬ tial pressures at high current densities without significant voltage loss. Other objects and advantages of the invention will be apparent to those skilled in the art from a review of the following detailed description taken in conjunction with the drawings and the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a transverse sectional view of one em¬ bodiment of an electrochemical cell in which the invention may be utilized; Fig. 2 is a schematic sectional view of a typical gas diffusion electrode with which the invention may be uti¬ lized; Fig. 3 is a sectional view of an electrode holder- useful in testing gas diffusion electrodes; Fig. 4 is a schematic exploded perspective view of an electrode assembly adapted for use with the electrode holder of Fig. 3 ; Fig. 5 is a schematic transverse sectional view of an electrode as used in Figs. 3 and 4; Fig. 6 is a series of polarization curves for oxy¬ gen reduction exhibited by an electrode made according to the invention at zero differential pressure; Fig. 7 is a series of polarization curves for oxy¬ gen reduction exhibited by an electrode made according to the invention at a differential pressure of 4 psi; Fig. 8 is a series of polarization curves for oxy¬ gen reduction carried out with another electrode made ac¬ cording to the invention; Fig. 9 is a series of polarization curves of an¬ other electrode made according to the invention using a fluorinated hydrogel layer; and, Fig. 10 is a plot of potential vs. time of another inventive electrode- DETAILΞD DESCRIPTION OF THE INVENTION Fig. 1 illustrates a typical embodiment of an electrochemical battery utilizing a gas diffusion electrode. This particular cell is an aqueous alkaline lithium-air cell. It is to be understood that the present invention is not limited to use in electrochemical batteries, nor to cells in which gas is consumed. Rather, the invention finds wide applicability in cells in which gas is either consumed or produced, via either reduction or oxidation, in which any of various electrolytes are used, etc. The cell of Fig. 1 is described in detail in U.S. Patent No. 4,528,249 (July 9, 1985) the disclosure of which is incorporated by reference. In Fig. 1, an electrochemical cell, generally de¬ signated 10, includes an anode 11, a gas consuming cathode 12, and a metal screen 13 interposed between the anode 11 and cathode 12 within an outer housing 14. In the embodi¬ ment of Fig. 1, the screen 13 is in electrical contact with the cathode 12, and is in mechanical (but not electrical) contact with the anode 11. In the exemplary embodiment, the anode 11 com¬ prises a lithium anoder which may comprise elemental lithium metal or lithium alloyed with alloying material such as small amounts of aluminum. The screen 13 is not in electrical contact, with the anode 11, due to the presence of an insulating, porous lithium hydroxide (LiOH) film which is formed on the anode surface by contact thereof with humid air, and is well known in the art. It is to be noted, however, that this particu- lar feature is peculiar to the aqueous lithium-air cell. in other types of metal-air batteries and fuel cells, either an electrically insulating porous separator layer or a simple electrolyte gap would be used. It should also be noted that the screen 13 is necessary to help restrain the gas diffu¬ sion electrode 12 against the gas pressure. The cathode 12 is in this case an air cathode through which atmospheric air flows. Those skilled in the art, however, will recognize that such a cathode may operate with any- oxygen-containing gas. One surface 15 of the cathode 12 is exposed to ambient atmosphere (or a source of another oxygen-containing gas) in a chamber 16 of the housing 14, and the opposite surface 17 of the cathode 12 is contacted by the liquid electrolyte 18 which is flowed through a second chamber 19 in the housing 14 as by a suitable pump 20. In the illus¬ trated embodiment, the electrolyte is provided from a reser¬ voir 21 for suitable delivery when needed. In Fig. 1, the anode 11 and cathode 12 each termi¬ nate in a respective terminal 26 or 28, and are connected to a load 30 through suitable circuit connections 32. Typically, the cathode 12 comprises a structure formed of a suitable porous hydrophobic material, such as polytetrafluoroethylene (PTFΞ) , mixed with carbon black, both pure and catalyst-containing. . A preferred form of the cathode 12 is described below in connection with Fig. 2. The screen 13 illustratively may comprise a woven metal wire screen formed of suitable non-corroding metal, which in the case cf alkaline electrolyte may be nickel or silver plated r.ickei. If desired, the screen 13 may serve as a current collector if connected to the terminal 28. In the embodiment of Fig. 1, liquid electrolyte, in this case an aqueous alkaline electrolyte such as aqueous lithium hydroxide, is flowed through the chamber 19 by means of the pump 20. As such, there is a pressure drop across the chamber 19 in the direction of flow. Further, air is flowed through the chamber 16 by means not shown, and there is a small pressure drop across the chamber 16 in the direction of flow by virtue thereof. However, those skilled in the art will recognize that the pressure drop across the gas chamber 16 is small in compari¬ son to that in the electrolyte chamber 19. As set forth above, Fig. 1 is intended to be exem¬ plary only, as the invention is applicable to anyof a variety of types of gaa diffusion electrodes and electrochemical cells. Fig. 2 is a schematic depiction of the structure of a preferred embodiment of the cathode 12. As shown in Fig. 2, the electrode 12 is formed essentially of a two or three component laminate defining the gas contacting surface 15 and the opposed electrolyte contacting -surface 17, An electronically conductive porous gas carrier layer 40 de¬ fines the gas contacting surface 15 and typically is a mix¬ ture of a hydrophobic material such as porous PTFE (e.g. Teflon brand PTFE) with, a carbon black such as Shawinigan black (Chevron Chemical Co. , Olefins and Derivatives Div. , Houston, TX) . A so-called "active 1 _yer" 42 comprises a layer 44 which comprises a mixture of carbon black, or catalyst sup¬ ported or. career, black, " and PTFE. An optional layer 46 of catalyst is disposed on the layer 44 at an interface 50. As shown _.n the schematic of Fig. 2, layers 44 and 46 appear to b discrete layers, but in practice ay define a single layer or two layers, since the catalyst is generally adsorbed onto the surface of the material of layer 44. In some cases, the materials of the three layers 40, 44 and 46 may be intermixed in a single layer. The entire structure of the electrode 12 of Fig. 2 is porous, generally exhibiting a porosity of 30-60.. A typical catalyst forming the layer 44 is heat- treated cobalt tetramethoxyphenyl porphyrin (CoTHPP) on a carbon black such as Vulcan XC-72 (Cabot Corp., Billerica, MA). The heat treatment is typically done at 400-1000'C In- inert gas. The structure of CoTMPP is shown below: This material is a currently preferred catalytic material. Other catalysts include platinum, Mnθ2 and tran¬ sition metal macrocycles other than CoTMPP. The function of the layer 40 is to allow ready transmission of gas to the active layer 44. Its hydropho- bicity also acts to repel liquid electrolyte which exists in the active layer 44 in order to avoid leakage of the liquid electrolyte into the gas side of the cell. It also provides electronic conductivity. The requisite consumption or generation of gas takes place in the active layer 44 where gas and liquid meet in the presence of the active material and optional cata¬ lyst, as is well known in the art. Fig. 3 illustrates an electrode holder useful in measuring characteristics of gas consuming or generating electrodes. The electrode holder, generally designated 60, comprises a solid body 62 of a nonconductive material defin¬ ing a gas inlet passage 64 communicating with a cell gas chamber 66 which in turn communicates with a gas outlet pas¬ sage 63. (A typical material of construction for the body 62 is 3M's Kel-F brand chloro fluorocarbon polymer.) An annu electrode seat 70 is defined in the body 62 in order to po¬ sition an electrode assembly (not shown in Fig. 3) which includes a gas diffusion electrode, generally designated 72, adjacent the cell chamber 66. A conductive (e.g. platinum) wire 74 contacts the seat 70 and extends therefrom through the outlet passage 68. A threaded plug 76 of the same ma¬ terial as the body 62 retains an electrode assembly 80 (shown in Fig. 4) in place. in the body 62. Fig. 4 illustrates the " electrode . assembly, gene¬ rally designated 80, which includes the gas diffusion elec¬ trode 72 of Fig. . The electrode 72 is shown in schematic form in Fig. 4 and formed as a cylindrical disk defining gas and electrolyte contacting surfaces 82 and 84 respectively. These surfaces are analogous to surfaces 15 and 17 of Fig. 1. An annular conductive metal (e.g. platinum) ring 86 ' is disposed on the gas surface 82 between the gas surface 82 and an annular rubber gasket 88. A similar rubber gasket 90 is disposed on the electrolyte side of the electrode 72 be¬ tween the electrolyte contacting surface 84 and an annular ring 92 of the same material as the body 62. When the assembly 80 is in place in the seat 70 of the electrode holder 60, the ring 86 is in electrical con¬ tact with the wire 74 and acts as a current collector. The electrode 72 as shown in Figs. 3 and 4 is schematic and these figures do not illustrate certain compo¬ nents such as the hydrophobic backing layer and associated screens. Fig. 5 illustrates an exploded sectional schematic view of a typical embodiment of the diffusion electrode 72. A silver plated nickel screen 100 is adjacent to and in contact with an electronically conductive hydrophobic backing layer 102, typically of Teflon brand PTFE on carbon black, which defines the surface 82. An active layer 104, which may in¬ clude a catalyst on carbon black, is adjacent to the layer 102 and defines the surface 84. A hydrophilic layer 106 is ap¬ plied to the surface 84 and is in contact with a steel rein¬ forcement screen 108. The layer 106 is described in detail below. When constructed, the screen 100 is not in physi¬ cal or electrical contact with the ring 86 and thus merely acts as a physical restraint. The gas inlet passage 64 and gas outlet passage 68 are connected with gas flow regulating means (not shown) which regulate the flow of gas through the passages 64 and 68 and the cell chamber 66, and thus the gas pressure in the chamber 66. When the electrode 72 is in place in the assembly 80 in the electrode holder 60, a central circular segment of each of the electrode surfaces 82 and 84 is exposed to gas and electrolyte sources, respectively. The electrode holder body 62 is positioned in a test cell such that the electrode surface 84 is exposed to a flowing or non-flowing (e.g. stirred) electrolyte. The remainder of the cell and asso¬ ciated temperature control means, etc. are omitted for clar¬ ity. For operation at elevated gas/electrolyte differ¬ ential pressures, the steel screen 108 acts as a reinforce¬ ment to prevent physical rupture of the electrode 72. Flow- through of gas from the cell chamber 66 through the elec¬ trode 72 into the electrolyte side of the cell is prevented by the layer 106 as described below. The layer 106 is formed in situ on the active lay¬ er surface 84 of the electrode. The layer is of an ionomer¬ ic hydrogel which is substantially impermeable to the gross passage of gas. The layer 106 is ionically conductiveand can conduct hydroxide (OH~) ions as well as water. It is also possib for bulk electrolyte to slowly diffuse through the polymer film. electrode 72 may be effectively wetted fay passage of liquid elect through the layer 106, while the layer 106 is virtually impermeab ?? r lnw. The hydrogel-forming coprecipitation reaction takes place between at least two precursor polymers which can react together to coprεcipitate to form a hydrophilic hydrogel which is insoluble or substantially insoluble in the electrolyte. Preferably, each precursor polymer is in¬ dividually soluble in the electrolyte as this facilitates removal of any excess reactant after hydrogel formation. At least one of the precursor polymers is an ionic polymer. Where the electrolyte is an alkaline electrolyte, it is preferred that a cationic polymer (e.g. containing ammonium or pyridinium groups, either pendant or part of the polymer chain) is reacted with an anionic or non-ionic polar polymer. In alkaline media, it is preferred that the ratio of number of equivalents of the cationic to the anionic polymer be greater than 1. In acidic media, this ratio is typically revers If each precursor polymer is an ionic polymer, the reaction product may be referred to as an "ion pair bonded polymer' 1 . The method of formation -of the hydrogel layer is very simple: the precursor polymers are individually ap¬ plied to the electrode surface, as by application of a solu¬ tion thereof. Preferably, the first precursor polymer solu¬ tion is applied to the surface, followed by application of ' the second precursor polymer solution. As stated above, the ratios of the number of equivalent of the respective precursor polymers will generally be greater th and typically in the range of 3-100:1, depending on the type of electrolyte to be used. In the case of an alkaline electrolyte, ratio of the number of equivalents of the cationic polymer applie compared to the number of equivalents applied anionic polymer wilJ generally be greater than one and may range as high as 100:1 or m In this case, the cationic polymer would typically be applied firs followed by application of the anionic polymer solution. This is conveniently carried out by varying the concentrations of the polymers in their respective solu¬ tions, with application of approximately equal volumes of polymer solutions. Application of the precursor polymer solution to the electrode layer results in independent absorption of the respective solutions into the body of the electrode, where the coprecipitation reaction takes place. Thus, while the exterior surface of the electrode is coated with the hydro¬ gel layer, it is to be understood that the layer will in most cases extend at least partially into the body of the electrode. Any solvent which does not damage the electrode structures and in which the polymers are soluble may be used, including aqueous solvents, alcohols, ethers or other organic solvents. It is preferred, however, to apply at least one and preferably both of the precursor polymers in low polarity organic solution whereby premature swelling of the polymers is avoided or minimized. It is important that the coprecipitated hydrogel be swellable upon contact with electrolyte,as this enhances the gas impermeability characteristics of the layer. Although a wide variety of precursor polymers are applicable, the currently preferred cationic polymer is poly (diallyl dimethyl ammonium chloride) , abbreviated pDMDAAC. The currently preferred anionic polymer is poly (styrene sulfonic acid) , abbreviated PSSA. Both materials are avail¬ able from Polysciences of Warrington, Pennsylvania. pDMDAAC is available as a 15 wt. percent solids solution in water, while PSSA is available.in a molecular weight of 70,000, in a wt. percent solids solution in water. The equivalent weights fo pDMDAAC and PSSA, respectively, are 161 and 185. Although each of these materials probably would provide a pressure tolerant gas diffusion electrode in that they do not microscopically pass gases, and both contain and pass ions, each of these polymers is independently soluble in the electrolyte and would promptly wash off of the elec¬ trode surface once placed in use. However, the combination of the two polymers with opposite charges on the molecular strands thereof results in a coprecipitated, insoluble hy¬ drophilic gel. This gel retains substantially all of the advanta¬ geous qualities of the original components, yet is insoluble in the electrolyte and should not wasn off of the electrode. After exposure to water, such a gel may contain up to 99.5 " wt. percent (or more) water. In a preferred form of the invention, the first precursor polymer is a cationic polymer and the second pre¬ cursor polymer is an anionic polymer which is perfluorinat- ed. A preferred perfluorinated polymer is a perfluorosul- fonic acid polymer sold by Du Pont under the trademark NAFION. (equivalent weight = 110) . Nafion solution in lower alco from Adrich s useful, and is currently available from Solution Technology, Mendenhall, PA. This is a chemically stable ion exchange resin. Reaction of the cationic polymer with the perfluorinat anionic polymer results m a partially fluorinated hydrogel. Th hydrogel is expected to have the particular advantage of high ox solubility and low rates of chemical decomposition. ' In general, electrochemical systems it is advantageous to utilize fluorinate components, especially perfluorinated materials, due to the extr resistance to chemical attack of such materials. Furtnermore, for systems using oxygen the relatively h solubility of oxygen m fluorocarbon materials is advantageou s . particular, the use of fluorinated species in gas diffusion elect provides twofold advantages. The high oxygen solubility results improved electrode (e.g. cathode) voltages, while enhanced chemical stability results in the delay of failure of the device due to chemical attack of the fluorinated species. ' EXAMPLES The following specific examples are given to il¬ lustrate the practice of the invention but are not to be considered as limiting in any way. The following generalized experimental procedure was used in each example. Cobalt tetramethoxyphenyl porphyrin (CoTMPP) was adsorbed on Vulcan XC-72 carbon (Cabot) by agitating a sus¬ pension of the latter in a solution of 10~4 y CoTMPP in acetone for at least 24 hours. The amount of the adsorbed macrocycle was calculated spectrophotometrically by determinin its loss from the filterpd solution. The solid c. talyst/carbon air-dried and then heat-treated to 450"C in a horizontal tube πrnace under continuous flow of purified argon. Porous gas-fed electrodes were fabricated as fol¬ lows: dilute (-2 mg/mL) Teflon T30 B aqueous suspension (Du Pont) was slowly added to an aqueous suspension of the catalyst/carbon while the latter was ultrasonically agitat¬ ed. The mixed suspension was then filtered with a lμm pore size polycarbonate filter membrane. The paste was worked with a spatula until slightly rubbery. The paste was shaped into a 1.75 cm diameter disk in a stainless steel die using hand pressure. This disk was then applied to another disk (-0.5 mm thick) of Teflon-carbon black hydrophobic porous sheet material (Eltech Systems Corp. , Fairport Harbor, O which contained a silver-plated Mi πesh. This dual .layer disk v pressed at 380 kg crn-^ P t room temperature and then heat-treate 290°C for 2 hours in flowing helium. The gas-fed electrode was placed in Teflon-Kel-F electrode holder as shown in Fig. 3. The gas (O2 or air) pressure was applied to the back-side (hyrophobic layer) of the electrode and was monitored at the outlet. A needle valve at the outlet was used to regulate the gas pressure. The O2 reduction measurements for the gas-fed electrodes were done galvanostatically in a concentrated alkaline electrolyte (0.5 M LiOH in 2:1 v/v 50% NaOH and 45% KOH) at 80-83°C with a research potentiostat (Stonehart Associates, Model BC1200). This potentiostat is equipped with positive feedback IR drop compensation" and correction circuits. The IR drop correction adjustment is made while monitoring the potential on an oscilloscope, with the current repetitively interrupted for 0.1 ms every 1.1 s. This procedure connects for any IR drop that is external to the electrode itself. Nickel foil was used as the counter electrode and a Hg/HgO, OH~ reference electrode was used. The polarization curves were recorded under steady-state conditions. Fig. 6 illustrates a series of oxygen reduction polarization curves for an electrode made as described above where in the electrolyte contacting side of the electrode was first coated with pDMDAAC then with PSSA, in a 10:1 PDMDAAC:PSSA) weight ratio (11.5:1 equivalent ratio). Runs were conducted using air and oxygen fed systems and measurements were taken after operation at 120 hours at a differential pressure of substantially zero. (Measurements in each case were taken at both increasing and decreasing current densities.) The results shown in Fig. 6 are excellent, with the potential at a current density of 100 mA/cm^ being approximately -0.085 to -0.090 V, which -is only 15-20 mV more negative than that for an electrode without the polymer film. Fig. 7 illustrates an oxygen reduction polarization curve for the same electrode run with oxygen at the relatively high differential pressure of 4 psi (28 kPa). The electrode was first broken in by operation at 144. hours at a differential pressure of zero. Measurements were taken first with increasing current density and then with decreasing current density. As shown in Fig. 7, results at 4 psi were excellent. Fig. 8 illustrates the results of runs with air and oxygen using a differential, pressure of 4 psi, but with a hydrogel layer formed first by application of a layer of pDMDAAC, followed by application of a PSSA solution at a 100:1 pDMDAAC:?SSA weight ratio (115:1 equivalent ratio). Both materials were applied from methanol solution. Fig. 9 depicts oxygen reduction polarization curves for an air cathode which had been treated with a combination of reactants which coprecipitated a fluorinated hydrogel. The inventive layer was placed on the electrode by first applying a. layer of pDMDAAC (0.3 mL 15 wt. percent pDMDAAC in 3 mL methanol) ■ followed by application of a methanol solution of Nafion polymer (0.1 L at 5 wt. percent) a 9:1 weight ratio (61.5:1 equivalent ration of oDMDAAC to Nafion. A differential pressure of 14.5 kPa was used with no gaseous blowthrough. This corresponds to a differential pressure of about 2.1 psi. Fig. 10 depicts the results using a similar gas diffusion electrode where, however, the two precursor polymers pDMDAAC and Nafion in methanol, 9:1 weight ratio or 61.5:1 equivalent ratio) were intimately mixed immediately prior to application to the electrode surface. (Tests were rune with oxygen.) This appears-to yield a higher resistance to blowthrough in that no gas flowed through the electrode at about 14.5 kPa. The extreme stability of the electrode ove.r the 16-17 hours of the run should be noted. Operation at high current densities (e.g. up to about 1 A/cm 2 or more) is possible according to the invention. Examination of the polarization curves presented above shows that according to the invention, very great increases in current (i.e. in current density) are available with only minor increases in the potential driving force over a wide range of current densities. (The Table/plats are relatively linear in Figs. 6 and 7 and exhibit about 0.039 V/decade current slopes.) The foregoing detailed description is given for clearness of understanding only and no unnecessary limitations are to be inferred therefrom, as modifications within the scope of the invention will be obvious to those skilled in the art.

Claims: CLAIMS We claim: 1. A gas diffusion electrode adapted for use in a gas generating or consuming electrochemical cell utilizing a liquid electrolyte, said electrode comprising an electronically conductive ' and electrochemically active porous body defining respective gas and electrolyte contacting surfaces, and a substantially gas impermeable layer covering substantially the entire said electrolyte contacting surface, said layer comprising an electrolyte-insoluble, ionomeric ionically conductive hydrophilic hydrogel formed on said electrolyte-contacting su b y coprecipitαtion between at least first and sec-one 1 precursor 2. The electrode of claim 1 wherein said hydro¬ gel layer is formed in situ on said electrolyte contacting surface by independent application thereto of respective solutions of -said precursor polymers in solvents in which the hydrogel is insoluble. 3. The electrode of claim 2 wherein said first precursor polymer contains cationic groups and said second precursor polymer contains anionic or non-ionic polar groups. 4. The electrode of claim 3 wherein said elec¬ trolyte is an alkaline aqueous electrolyte and the ratio of t number of equivalents of said first precursor polymer to said se precursor polymer is greater than 1. 5. The electrode of claim 4 wherein said first precursor polymer comprises poly (dimethyl diallyl ammonium chloride) and said second precursor polymer comprises poly (styrene sulfonic acid) . 6. The electrode of claim 5 wherein said ratio is in the range of about 3-100:1. 7. The electrode of claim 5 wherein substantial¬ ly equal volumes of said polymer solutions are applied to said electrolyte contacting surface. 8. The electrode of claim 3 wherein said elec¬ trolyte is an aqueous alkaline electrolyte and said first precursor polymer solution is applied to said electrolyte contacting surface prior to said second precursor polymer solution. 9. The electrode of claim 4 wherein said first precursor polymer is a cationic polymer and said second pre¬ cursor polymer is a perfluorinated anionic polymer. 10. The electrode of claim 9 wherein said second precursor polymer is a perfluorosulfonic acid polymer. 11. The electrode of claim 1 wherein said porous body comprises a laminate of a porous hydrophobic layer de¬ fining said gas contacting surface, and a porous active lay¬ er defining said electrolyte contacting surface, said active layer comprising an electrochemically active material. 12. The electrode of claim 11 wherein said elec¬ trochemically active material comprises carbon. 13. The electrode of claim 11 wherein a catalyst is adsorbed on said active material. 1 . The electrode of claim 13 wherein said cata¬ lyst is cobalt tetramethoxyphenyl porphyrin. 15. An electrochemical cell, comprising: (a) a gas diffusion electrode comprising an elec¬ tronically conductive and electrochemically active porous body defining respective gas and electrolyte contacting surfaces, and a substantially gas impermeable layer covering substantially the entire said electrolyte contacting surface, said layer comprising an electrolyte-insoluble, ionomeric ionically conductive hydrophilic hydrogel formed on said electrolyte-contacting surface py coρrecipiτ___ -.iuu L.t ee at least first and second precursor polymers; (b) a counter electrode spaced from said gas dif¬ fusion electrode; (c) a liquid electrolyte in contact with said counter electrode and with said layer on said electrolyte contacting surface of said gas diffusion electrode; (d) either a gas to be consumed via oxidation or reduction or gas formed via electrolytic generation in con¬ tact with said gas contacting surface of said gas diffusion electrode; and, (e) circuit connections between said gas dif¬ fusion electrode and said counter electrode. 16. The cell of claim 15 wherein said gas is an oxygen containing gas. 17. The cell of claim 16 wherein said gas is air. 18. The cell of claim 15 wherein at least one of said liquid electrolyte and said gas are flowed through said cell. 19. The cell of claim 15 wherein said hydrogel layer is formed in situ on said electrolyte contacting sur¬ face by independent application thereto of respective solu¬ tions of said precursor polymers in solvents in which the hydrogel is insoluble. 20. The cell of claim 19 wherein said first pre¬ cursor polymer contains cationic groups and said second pre¬ cursor polymer contains anionic or non-ionic polar groups. 21. The cell of claim 20 wherein said electrolyte is an alkaline aqueous electrolyte and the ratio of the number equivalents of said first precursor polymer to said second precu polymer is greater than 1. 22. The cell of claim 21 wherein said first pre¬ cursor polymer comprises poly (dimethyl diallyl ammonium chloride) and said second precursor polymer comprises poly (styrene sulfonic acid) . 23. The cell of claim 22 wherein said ratio is in the range of about 3-100:1. 24. The cell of claim 23 wherein substantially equal volumes of said polymer solutions are applied to said electrolyte contacting surface. 25. The cell of claim 20 wherein said electrolyte is an aqueous alkaline electrolyte and said first precursor polymer solution is applied to said electrolyte contacting surface prior to said second precursor polymer solution. 26. The cell of claim 21 wherein said first pre¬ cursor polymer is a cationic polymer and said second precur¬ sor polymer is a perfluorinated anionic polymer. 27. The cell of claim 26 wherein said second pre¬ cursor polymer is a perfluorosulfonic acid polymer. 28. The cell of claim 15 wherein said porous body comprises a laminate of a porous hydrophobic layer defining said gas contacting surface, and a porous active layer de¬ fining said electrolyte contacting surface, said active lay¬ er comprising an electrochemically active material. 29. The cell of claim 28 wherein said electro¬ chemically active material comprises carbon. 30. The cell of claim 28 wherein a catalyst is adsorbed on said active material. 31. The cell of claim 30 wherein said catalyst is cobalt tetramethoxyphenyl porphyrin.

Inventors: GORDON ARNOLD Z, HOSSAIN M SOHRAB, TRYK DONALD S, YEAGER ERNEST B, GORDON, ARNOLD, Z., HOSSAIN, M., SOHRAB, TRYK, DONALD, S., YEAGER, ERNEST, B.
Applicants: GOULD INC, GOULD INC.
Application year: 1988
Main classification: C25B9
IPC codes: C25B9, C25B11, H01M4, H01M12