Fuel Cells Incorporating Ion Exchange Membranes - Advances in

14 Fuel Cells Incorporating Ion Exchange Membranes

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on October 16, 2015 | http://pubs.acs.org Publication Date: January 1, 1969 | doi: 10.1021/ba-1965-0047.ch014

Current State of Development CARL BERGER

Astropower Laboratory, Douglas Aircraft Co., Newport Beach, Calif.

The current state of development of ion exchange membrane fuel cells is reviewed with a discussion of approximate upper limits of performance and particular emphasis on the actual operation of such devices at the present time. The fuel cells analyzed are the single membrane fuel cell, the hydrogen-bromine fuel cell, and the dual membrane fuel cell. There are substantial differences between anticipated performances and those actually realized. Much of this is due to the problems inherent in removal of product water and represents definitive engineering limitations. Projections are made with respect to power and volume densities achievable in the foreseeable future. Suggestions are presented relative to fruitful avenues of research and development in the future.

J h e upsurge of interest i n the last several years i n fuel cell research is abundantly documented i n the literature found i n scientific, engineering, and business articles. The work reported here concerns applications of semipermeable membranes—in particular, ion-membrane fuel cells. Most representative of this group are the single membrane fuel cell (16), the dual membrane fuel cell (5), and a significant hybrid, the gas-liquid single membrane fuel cell (6). It may be of value to review briefly the advantages and disadvantages of an ion-membrane fuel cell i n comparison with fuel cells with porous electrodes i n contact with liquid electrolytes. Some of the advantages are: 1. Construction of electrode-catalyst configurations is simplified—the 188 In Fuel Cell Systems; Young, G., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1969.


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Ion Exchange Membranes

189

exact sizing of electrode pores, the criticality of catalyst deposition, and the requirements for waterproofing are a l l minimized. 2. There is no loss of gaseous reactants due to pore inexactitude. The gaseous reactants cannot be lost to the electrolyte but simply rebound back into the catalyst structure or the gas compartment if they do not react. 3. Compactness. 4. Light weight. The disadvantages of the ion-membrane fuel cell are: 1. Moderate current densities at practical voltage levels have been achievable although the compactness of configurations mitigates this problem to some extent. 2. Heat removal is more difficult than i n systems where an electrolyte can be circulated—e.g., approximately 30 to 5 0 % of the realizable power i n a fuel cell ends up as heat. The hydrogen-bromine fuel cell ( H B F C ) and the dual membrane fuel cell ( D M F C ) described here represent compromises instituted to overcome this problem. 3. The most highly developed ion-membrane fuel cells are organic and therefore sensitive to heat even when i n an aqueous environment. The hydrolysis of the ionic groups i n the organic matrix ( in the presence of catalyst ) is a possibility. The probability of such an occurrence would increase with elevation i n temperature. 4. Water removal from electrode-catalyst sites represents a variable difficult to control quantitatively and directly influences voltage output. The basic membrane used i n the three generalized configurations described here are of two physical species—a homogeneous fabric supported polymer (19) and a grafted polymeric type ( I ) . In both cases the polymers are sulfonated polystyrenes cross linked to a greater or lesser extent. The mechanism of operation of the membrane, however, differs appreciably i n the three types of fuel cells (Figure 1). A l l of the fuel cells shown i n Figure 1 have been amply described i n the literature (4-6,12,13, 16). The single membrane fuel cell (Figure 1 A ) uses hydrogen and oxygen as reactants. Hydrogen is converted to H + at the anode, electromigrates through the membrane, and unites with a reduced oxygen species at the cathode to form water, which must be removed. In the hydrogen-bromine fuel cell (Figure I B ) , the anode reaction is H

2

2H+ + 2e

and the cathode reaction B r + 2e 2

2Br~

The net result of the reaction is the formation of hydrogen bromide i n the aqueous catholyte.

In Fuel Cell Systems; Young, G., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

FUEL CELL SYSTEMS

190

In the dual membrane fuel cell (Figure 1 C ) , the anode and cathode reactions are identical to those i n the single membrane fuel cell. The différence i n these cells is that i n the former a layer of sulfuric acid is found between two membranes which serves to level out membrane water balance problems and functions importantly as a heat transfer fluid.

rA/Wi


rAA/Vi

Membrane

Membrane

Membrane

Ζ

Pt Catalyst

4

' Catalyst H,

H 0 ?

Anode Chamber

Catalyst

HBr Br.

Pt E l e c t r o d e j Cathode Anode ^1 Chambe r C hambe r

Single Membrane Fuel Cell (SMFC)

H,

Anode Chamber Cathode Chamber

Hydrogen-Bromine Fuel Cell (HBFC)

i Cathode: i Chamber

Dual Membrane Fuel Cell (DMFC)

Figure 1. Three representative ion-membrane fuel cells Single Membrane Fuel Cell The single membrane fuel cell ( S M F C ) is the system which has been most intensively investigated i n the last few years. The membrane used i n this case is a completely water leached ion-membrane where all of the electrical transport is due to the migration of H + ion formed at the anode from one sulfonic acid group to another until water is formed at the cathode. If the ion exchange membrane is considered a polymer network of a linear or branched variety crosslinked at various sites and swollen with solvent, an adequate physical network can be envisioned for the transport of solute. It is apparent i n envisioning this network as a "solid gel" that the velocity of H + ion in this network w i l l be altered and if, as the theory of aqueous electrolytes indicates, the velocities of ions i n "gel" structures is a function of the increased viscosity of the internal solvent phase (8), then it follows that a quasi-Stokes frictional resistance to flow ( F ) F

where

η = viscosity

= 6τψ

(1)

r = radius of migrating particle


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191


should be increased producing slower ionic migration whether the forces are purely those of diffusion or electromigration. In electromigration, this retardation w i l l be manifested by lower ionic mobilities. F o r example, the ionic mobility of H + ion i n an aqueous electrolyte is about 362 χ 1Q" cm./sec. i n contrast to a velocity of H + ion in sulfonated phenol formalde hyde resin of about 19 χ 1 0 cm./sec. ( 8 ). If one accepts as an operating basis that the S M F C is now utilizing the optimum catalysts obtainable for the hydrogen-oxygen system and that practical operating voltages much greater than 0.93 volt are not likely to be obtained (26) (a fact that the writer concurs i n as a result of his ex perience i n development of hydrogen-oxygen fuel cells), then theoretically the net power that can be obtained w i l l be a function of the ionic mo bility of the H + ion over a given transit thickness. Approximate calculations may be of some value i n guiding us with respect to the limiting current densities that can be achieved i n a leached H + transport system. Using the approach of Kortum and Bockris (21) and Spiegler and Coryell (28), the limiting current density of a leached membrane system may be defined as 5


-5

1 - - * - . ^ Z300 ma./cm.), and when used as a secondary battery, charge acceptance efficiency is high compared to the S M F C system. This is due to the considerable irreversibilities encountered on charging a leached hydrogen-oxygen ( S M F C ) system compared to the H B F C where overvoltage is a minor consideration. In practical systems this calls for a 20 to 3 0 % greater power requirement for recharging at a given current density ( 5 ) . T h e major factor which has held back the rapid development of this concept has been the lack of solid advances i n membrane technology. Recently (5), advances have been made which augur well for the development of this cell. It w i l l continue to suffer, however, from one basic limitation. T o prevent the migration of Br ~ into the membrane, the network of the ion exchange membrane must be made less porous—that is penetration of B r - must be decreased. The consequence of restricting the Br ~ penetration is a "tighter' internal structure which decreases ionic mobility. It therefore seems unlikely that effective operation of greater than 50 to 60 ma./sq. cm. at 0.62 to 0.57 volt w i l l be achieved i n multiple configurations of H B F C i n the next 36 months. The maximums could probably be improved b y 30 to 5 0 % if substantially more effort is devoted to this type of device than is presently contemplated. It is likely, however, that fuel cell optimization studies w i l l indicate that values of about 30 ma./sq. cm. and 0.72 volt are appropriate for design considerations at the present time. Since these values are satisfactory for secondary battery operation, practical applications i n such areas (weather satellites) may be envisioned. 3

3

3

D u a l Membrane F u e l C e l l . Various experimental considerations indicate the advantages of the D M F C . T h e membranes are continually in equilibrium with 6 N sulfuric acid, thereby eliminating problems related to water balance and drying of membranes (4). Moreover, the removal of generated heat can be efficiently performed b y circulation of the electrolyte. Finally, since water formed at the cathode migrates into the central electrolyte reservoir (6), we essentially eliminate the water transfer system required i n the S M F C , eliminate complexity, and increase reliability. Factors detrimental to the achievement of higher operating current densities i n the device are the probable low activity of equilibrated sulfuric acid i n the membrane, thereby lowering the conductivity substantially as compared to hydrobromic acid of the same concentration i n the membrane. Most importantly, the formation of water film on the oxygen



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199


electrode-membrane interface, suggests a limiting factor, the diffusion rate of the water from the interface into the membrane and the central reservoir. The water film appears to have a definitive means of leaving the area of the oxygen electrode by ordinary mass diffusional processes. If one assumes a diffusion constant of an order equivalent to that used i n calculating limiting currents i n membranes and circumvents the ambiguities of working with activities at membrane interfaces, then a rate of diffusion of sulfuric acid through the membrane to the oxygen electrode interface of about 8 to 16 ma./sq. cm. for a membrane 0.050 cm. thick or values of about 24 to 48 ma./sq. cm. for membranes 0.0165 cm. thick can be calculated. These values agree rather well with the data obtained during the course of a research program devoted to the D M F C ( 6 ) . It appears likely that using thinner membranes and with sufficient membrane development, current densities of 40 to 50 ma./sq. cm. at 0.67 to 0.63 volt can be achieved i n multiple configuration within the next few years. Improvement i n this system could be obtained b y providing a more open polymer network; however, laboratory efforts (29) have indicated that improved current density obtained this way causes increased "osmotic" leakage of electrolyte into the gas compartments. W h e n this occurs, steps must be taken to remove the fluid so that continuous effective performance of the fuel cell can be maintained. This, of course, would decrease the over-all efficiency and reliability of the D M F C . Because of the simplicity and ruggedness of this fuel cell, the D M F C units have been offered commercially to industry and government since 1962 ( I S ) . Conclusions The writer has taken operating parameters that he feels may be achieved within the next 12 months for multiple fuel cells of the three general classes of devices discussed here. One must bear i n mind that one of these ( H B F C ) , is fundamentally used as a secondary battery. O f particular interest are projections of approximate weight, volume, and power density based on estimates of reasonable voltages and current densities. There are shown i n Table I. Table I. Type SMFC DMFC HBFC

Volume and Weight Factors

Amps I Thickness, Pounds/ Voltage sq.ft. inches sq.ft. 0.72 0.72 0.72

75 30 30

0.205 0.194 0.165

1.37 1.97 1.67

Volume, cu. ft. 0.0171 0.0162 0.0138

Watts/ sq.ft.

Kw./ cu. ft.

Watts /lb

55.0 21.6 21.6

3.2 1.33 1.56

40.2 10.97 12.92

It is important to reiterate the basis on which the calculations were made: 1. The weights and dimensions refer to a unit cell with no instrumen-



200

FUEL CELL SYSTEMS

tation, electrolyte holdup, water removal, or any other system factors considered. F o r instance, it is clear that i n long missions requiring primary cells, the increased weight of fuel needed w i l l tend to improve markedly the watt-hours/lb. obtained from a given system. It is because of this variability of missions i n space, on land, or i n the sea that no attempt has been made to go beyond the unit cell structure i n analysis. Table I however, should be of value as a general starting point for systems analysis and is presented i n nonmetrical units for engineering convenience. 2. The S M F C and D M F C are primary cells and therefore not strictly comparable with H B F C . 3. The S M F C has been the object of a far greater investment of time and effort than either the D M F C or the H B F C . It is almost certain that the values of watts/lb. and kw./cu. ft. for the latter two would i n crease two to four times with an intensive development effort. Projections made i n this paper assume that the development of neither the D M F C nor H B F C w i l l be at as high a level i n the next 30 months as has been the case with the S M F C . It may be of value, to suggest possible research and development concepts that appear promising i n the improvement of ion membrane fuel cells. 1. First, because of processing advances i n producing thinner membranes (1) the membrane may be regarded as less of a structural electrolyte and more as a diffusion barrier up against an electrode. In this conceptual framework, we find that the membrane, for instance, can be regarded as a means for producing low cost porous electrodes since thin membrane barriers w i l l lessen the need for the elegant procedures used at present for preparation of metal and carbon electrodes. Moreover, such electrode membrane systems could be used i n various electrolytes, aqueous and nonaqueous. Finally, if very thin membranes are used (

Fuel Cells Incorporating Ion Exchange Membranes - Advances in

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