Nonporous Hydrogen Depolarized Anode for Fuel Cells - Advances in

5 Nonporous Hydrogen Depolarized Anode for Fuel Cells HARRY G. OSWIN and STEWART M. CHODOSH

Downloaded by UNIV OF LEEDS on June 18, 2016 | http://pubs.acs.org Publication Date: January 1, 1969 | doi: 10.1021/ba-1965-0047.ch005

Leesona Moos Laboratories, Great Neck, Ν. Y.

This nonporous diffusion anode has been de veloped to the stoge where reproducibility and simplicity of manufacture have been amply demonstrated. Its extreme thinness and ability to handle high current densities make it suit able for very high power density systems. Further the ability of the alloy to handle im pure streams of hydrogen and its low suscep tibility to poisoning show that it is extremely suitable for use with reformer streams using cheap hydrocarbons, methanol, or ammonia as fuel. Its ability to operate in alkaline or acid electrolyte makes it a versatile anode. In addi tion, it can operate at negative, zero, or posi tive differential pressures.

solid nonporous diffusion-type anode has been developed which permits efficient utilization of impure hydrogen from reformed carbonaceous gas streams. This anode obviates some of the limitations of conventional porous structures, such as stringent quality control during anode manu facture, critical differential operating pressure, and contamination of the electrolyte by impurities in the hydrogen gas stream. Conventional Porous Anodes Conventional porous anodes require control of the three-phase an ode/fuel/electrolyte interface to stabilize the system and maintain an acceptable level of polarization. Three ways presently used to achieve such control are: 1. The biporous electrode structure developed by F . T. Bacon ( J ) essentially consists of two porous structures. One faces the electrolyte having a mean pore size smaller than that of the coarse-pore structure 61 Young and Linden; Fuel Cell Systems Advances in Chemistry; American Chemical Society: Washington, DC, 1969.


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facing the gas phase. Thus, b y careful control of the differential pressure applied across the electrode, a stable interface is maintained somewhere within the electrode. If a certain differential pressure is exceeded, bubbling w i l l commence at the largest pore i n the fine-pore layer. 2. The homoporous electrode structure, such as described b y Justi (4), is used i n a bubbling condition. Again, a certain differential pressure is required at which an acceptable rate of bubbling occurs through some of the larger pores, while a stable interface is maintained i n pores of smaller diameter. 3. Waterproofing the electrode structure consists of applying a waterproof layer to the surface of the porous structure. This apparently prevents gross flooding of the larger pores. Likewise, electrodes can be constructed b y incorporating waterproofing agents such as Teflon or K e l - F into a finely dispersed metal powder which is then fabricated into a porous layer. These electrodes have disadvantages such as the difficulty of controlling bubbling on waterproofed or homoporous structures with the consequent loss of fuel and its attendant dangers. In biporous structures, stringent quality control during manufacture is essential to achieve reproducibility. It was apparent some years ago that a nonporous hydrogen diffusion anode would eliminate many of these problems and have unique advantages of its own. T h e ability of palladium to diffuse hydrogen was recognized long ago, and an extensive literature is concerned with this phenomenon. There are references (8) i n the literature relating to the "electrochemical extraction" of hydrogen from palladium. Recently, it was shown that the rate of absorption of hydrogen by palladium could be increased b y covering the palladium surface with transition metal hydrides ( 9 ) . However, only a small amount of work (5-7) has been carried out on palladium alloy electrodes, which are more stable mechanically than pure palladium i n the presence of hydrogen ( 3 ) . The work described here, therefore, was carried out with the intent of developing a reliable, nonporous anode for fuel cell applications. It was considered desirable to develop an anode capable of extracting hydrogen at low partial pressures from impure (reformed hydrocarbons) gas streams. Physical and Electrochemical Mechanisms Involved The processes occurring i n the solid nonporous diffusion-type electrodes are shown i n Figure 1. The first stage involves chemisorption and dissociation of the hydrogen accompanied by the formation of metal hydrogen bonds at the surface. T h e hydrogen then diffuses as a proton interstitially through the bulk metal; the nature of the diffusion mechanism

Young and Linden; Fuel Cell Systems Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

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Hydrogen Depolarized Anode

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e ~ to Load Circuit


GAS

Figure 1.

Sequence of mechanisms involved at nonporous anode

at grain boundaries is not defined. O n reaching the electrolyte surface of the membrane, the protons emerge into specific bound surface states from which they are removed by the potential difference across the double layer. Thus, the total process involves three distinct activation energies—activation energy of dissociation; activation energy of the bulk diffusion process; and the activation energy for the transfer of protons at the electrolyte interface. There exists a concentration gradient of protons across the membrane which provides the driving force for the diffusion process. Part of the investigation has been to determine the role of these various processes and the rate-controlling mass-transfer mechanisms. Experimental Procedures The electrochemical data were obtained under controlled conditions of temperature and pressure using "half cells" i n most cases. The cathode of the cell consisted of a platinum gauze from which hydrogen was evolved.


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Certain of the early feasibility studies and the scale-up studies were conducted on cells containing oxygen depolarized cathodes. D u r i n g the feasibility stage, some of the electrodes investigated consisted of tubular structures of the type used for gaseous diffusion and separation of hydrogen. It had been hoped that these could be incorporated into concentric type cells giving high power density per unit volume, but the many problems associated with fabricating, sealing, and gasketing biporous tubular cathodes resulted i n the use of more suitable designs. It was also extremely difficult to calculate current-density distributions i n cells using circular cross-section electrodes (tubes) unless the surrounding cathode was exactly concentric. Polarization data are quoted on the E * scale—that is, using a reversible hydrogen electrode i n the same electrolyte as a reference. Electrode dimensions were in most cases 1-inch diameter flat membranes; i n scale-up studies, 3-inch, 5-inch, 6-inch square electrodes were used. Except where stated otherwise, the alloy used was 75% p a l l a d i u m - 2 5 % silver. The electrolytes consisted of U S P grades of potassium hydroxide, sulfuric acid, and phosphoric acid, from room temperature to 250° C . Luggin capillaries were used to measure the potential at the electrode surface. A l l of the data presented herein (except i n Figure 6) are steadystate polarizations and do not include any values derived from galvanostatic transients. Thus, all electrode IR drops—that is those caused by conductor resistances—are included i n the measurements. Feasibility Studies Initial feasibility studies were conducted on 2 5 % A g / 7 5 % P d alloy membranes at temperatures up to 250° C . Limiting currents up to several amps/sq. cm. were observed and polarizations of the order 150 mv. were obtained over the current density range 200 to 400 amps/sq. ft. H o w ever, at lower temperatures occasional irreproducibility and varying rates of surface poisoning prompted a study of surface treatment and preparation. Effects of Surface Pretreatment The effects of surface preparation—for example, the effect of treating gas and electrolyte surfaces, separately and i n combination—are shown i n Figure 2. In order to demonstrate the surface effects, pretreatment polarization values are shown for various membranes at 150° C . Pretreatment of the electrolyte surface has a significant effect on the activation polarization and has resulted in a more active surface, either b y lowering the activation energy of the process or by increasing the number of sites available.



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Current Density

Figure 2.

Effect of surface preparation on polarization of membrane anode

Limiting currents are not affected, however. Activation of the gas surface demonstrates a marked effect on the limiting current density obtainable from the membranes, confirming our suspicion that the surface absorption and dissociation processes constituted a rate-controlling mechanism. It is of interest to note also that pretreatment of the gas-side surface has an effect on the "activation-polarization" region as well as on limiting currents. This is accounted for by the increased diffusion of hydrogen to the electrolyte surface, resulting i n a greater concentration of hydrogen in the double layer. This increases the pre-exponential factor in the rate equation, resulting i n higher currents at a given polarization value. The combination of pretreating the gas and electrolyte sides results i n an electrode having both high limiting currents and low polarization. Diffusion studies, conducted concurrently, confirmed the results of the electrochemical investigation—namely, at lower temperatures the surface processes were rate controlling. However, limiting currents are always somewhat larger than those predicted from diffusion experiments. The probable explanation of this anomaly is that on polarizing the electrode, a very low partial pressure of hydrogen exists at the electrolyte interface. Taking a very simple view, the electrochemical process eliminates the recombination step: H + H H . The removal of hydrogen from the surface under conditions of polarization can be far more rapid than desorption of hydrogen into the gas phase. 2


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The investigation of surface treatment indicates clearly that the "electrolyte-surface" is potential-controlling while the "gas-surface" is rate controlling. F r o m a practical point of view, this phase of the investigation resulted i n methods of preparing the diffusion electrodes with extremely good reproducibility and with a minimum need for quality control procedures. Having easily reproducible electrodes available, a series of parametric investigations was conducted.


Temperature Dependence The effects of temperature on polarization and limiting currents are shown i n Figure 3 over the range from room temperature to 200° C . A l though at room temperature polarization is considerably increased, limiting currents of the order of 300 to 400 ma./sq. cm. are still obtainable, and the electrode can provide adequate starting power from ambient. Above 100° C . the measurement of limiting current becomes difficult because of the very high current values involved. F o r example, measurements at 200° C . have indicated limiting currents i n the region of 3500 to 4000 amps/sq. ft.; it is virtually impossible to measure such high limiting currents with accuracy.

Current Dtniity ( m A/cm ) 2

Figure 3.

Pohrization of paUadium-sHver/hydrogen anode as function of temperature


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Durability of Anodes Many anodes of various sizes have been investigated for periods up to 500 hours under varying load conditions. Corrosion appears to be the only factor controlling electrode durability. However, since the corrosion potential of the palladium/silver alloy is 800 mv. positive to hydrogen, only complete poisoning of the electrode surface can result i n such excessive polarization and subsequent corrosion. It has been our experience that b y using the electrolytes mentioned here and commercially pure gases, this situation does not arise, and no theoretical limit to the lifetime of the electrodes appears to exist.


Effect of Electrolyte Composition The effect of electrolyte concentration has been studied. There is no apparent effect on polarization or limiting current over the composition ranges of interest. Naturally, the specific resistivity of the electrolyte varies and this can affect the over-all polarization of the cell. This variability with electrolyte concentration indicates the minimum i m portance of concentration polarization at the surface due to electrolyte species. Polarization i n acidic or basic electrolytes is similar. Limiting Effects of Hydrogen Partial Pressure A study of the partial pressure effects of hydrogen (Figure 4) indicated that partial pressure has little or no effect on polarization until limiting current density regions are approached. Significant current densities can be sustained at partial pressures of hydrogen as low as 1 p.s.i.a. In connection with our original concept for a solid diffusion-anode capable of operating with impure hydrogen streams, this is an extremely i m portant finding since it determines the degree of utilization of the fuel. F o r example, if the limiting current density acceptable is 150 ma./sq.cm., then the partial pressure of hydrogen i n the purge gas—that is the gas being discarded from behind the electrode—would be approximately 1 p.s.i.a. If the in-going partial pressure of hydrogen is atmospheric (15 p.s.i.a.), this would represent about 9 3 % utilization of the hydrogen. A n in-going partial hydrogen pressure of 25 p.s.i.a. would result i n 96% fuel utilization. Effects of Membrane Thickness The effects of membrane thickness on the polarization and limiting current are shown on Figure 5. The effects on polarization at current densities below the limiting current density region are negligible. L i m i t ing current density, however, is an inverse function of thickness for a


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given partial pressure of hydrogen. This indicates that performance, as well as economics, can be improved by the use of extremely thin mem branes.

Eltctrodt: 7SXPd-25X Ag Electrolyte: Κ OH Ttmptrotuft: 200' C Fut I: Hydrogen


Counter Eltctrodt: Pt

Limiting Current Density (m A/cm ) 2

Figure 4.

Limiting current density of palladium-silver/hydrogen anode as function of hydrogen partial pressure


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Study of Poisoning Effects A comprehensive study of poisoning has been conducted. Poisoning of the electrode/electrolyte interface is of course affected ( as are all metal surfaces) b y excessive amounts of heavy metal ions and chloride ions; but i n a l l cases, using U S P grades of electrolyte and distilled water, no deterioration i n electrode performance has been observed under working conditions. A comprehensive study of poisoning of the gas surface has been made, including the effects of hydrocarbons, C H O H , H C H O , H C O O H , C O , C 0 , N H , N , and water i n the hydrogen-containing streams. B y control of the flow rates and conditions on the gas side of the membrane, none of these has been observed to cause significant polarization effects. Also, if a reformer stream is used for the fuel cell w e do not expect the membrane to be any more susceptible to poisoning than the reforming catalyst, which may, i n fact, selectively remove some of the undesirable materials, such as sulfur, and trace metals, such as vanadium. Carbon deposition on the gas surface has never been observed. Generally, we can expect sulfur poisoning to become more important at lower temperatures. Although sulfur is recognized as an important poisoning agent, the nature of its poisoning effect w i l l depend upon the form i n which it exists i n the gas stream; pertinent data w i l l soon be available. A n interesting effect, which is a form of poisoning, has been reported by Darling ( 2 ) . In the course of studies of hydrogen diffusion through palladium, Darling observed that even when using electrolytic hydrogen of high purity, a constant diffusion rate could only be maintained b y "purging" the high-pressure side of the membrane. W e observed the same phenomenon with "pure" hydrogen using the P d / A g anode. If the gas-vents were closed, a steady increase i n polarization ensued. A n experiment i n which the "pure" (electrolytic) hydrogen was first diffused through P d / A g anode resulted i n the disappearance of the phenomenon. It can only be concluded that an impurity is present i n small quantities. Since the "Darling effect" is observed over a temperature range from ambient to 600° C , it is likely that the impurity does not chemisorb or physically adsorb. Most probably it accumulates i n surface micropores, restricting hydrogen diffusion to active sites on the metal surface of such pores and cracks. 3


2

3

2

Electrochemical Characteristics of the Solid Diffusion Electrode Polarization values for the electrode at 200° C , derived b y transient techniques, are plotted semilog i n Figure 6. It would appear that the electrode exhibits Tafel behavior. Unfortunately, because of the large currents involved, it has not been possible to study the electrode


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polarization far into the linear Tafel region; IR contributions become excessively large. Essentially, at practical current densities the electrode polarization is i n the nonlinear Tafel region.

Current Dtnsity (m A/cm ) 2

Figure 7.

Anode polarization as function of membrane composition



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Figure 8.


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Scale-up effects on 1- and 5~inch diameter anodes

Effect of Membrane Composition The effects of various alloy compositions were studied, and results are shown i n Figure 7. The 2 5 % A g / 7 5 % P d alloy demonstrates superior polarization and higher limiting currents than all other compositions studied; there appears to be little economic advantage at this stage to increasing the amount of silver i n the alloy. Scale-up Studies Scale-up studies have been conducted on electrodes from 1-inch d i ameter to 6-inch square size. Results are shown i n Figure 8 for electrodes of 1- and 5-inch diameter. The differences i n polarization behavior are insignificant. Acknowledgment The authors acknowledge the technical contribution and experimental studies of M r . F . Malaspina and the results of the diffusion studies con ducted by M r . Ν. I. Palmer. Literature Cited (1) (2) (3) (4) (5) (6)

Bacon, F. T., Brit. Patent 667, 298 (1952). Darling, A. S., Fhtinum Metals Rev. 2, 16 (1958). Hunter, J. B., Ibid., 4, 130 (1960). Justi, E., U . S. Patent 2, 860, 175 (1958). Kussner, Α., Ζ. Elektrochemie66,675 (1963). Kussner, Α., Ζ. Physikalische Chemie,36,383 (1963).


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(7) Oswin, H. G., U. S. Patent 3,092,517 (1963). (8) Vetter, K . J., "Electrochemische Kinetik," p p . 492-96, Springer-Verlag, Berlin, 1961. (9) Wickes, E., Kussner, Α., Otto, K., Actes d u Deuxième Congres International de Catalyse, Paris, 1960, p. 1035. February

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