Current Density and Electrode Structure in Fuel Cells - Advances in

10 Current Density and Electrode Structure in Fuel Cells H. A. LIEBHAFSKY, E. J . CAIRNS, W. T. GRUBB, and L. W. NIEDRACH

General Electric Research Laboratory, Schenectady, Ν. Y.

Downloaded by UNIV OF ALABAMA on June 18, 2016 | http://pubs.acs.org Publication Date: January 1, 1969 | doi: 10.1021/ba-1965-0047.ch010

Continued progress on fuel cells requires a bet ter understanding of electrode structure.

A

survey of pertinent information has contributed to this end, but it has shown that the effect of electrode structure on cell performance will be difficult to isolate even after the badly needed extensive experimental work has been done. We believe that investigations of complete fuel cells will lead most directly to successful fuel batteries.

Therefore, data for complete fuel

cells are emphasized in this survey.

J h e effect of electrode structure on the current density obtainable at a given voltage in a fuel cell is second only to that of the electrocatalysts. The two effects are difficult to separate, and their relative importance changes with current density and voltage. As a permissible oversimplifica tion, one may say that the effect of electrode structure on current density is mainly physical. In the fuel cell of Figure 1, the electrode must serve functions that at first sight appear mutually self-exclusive: It must join yet separate the reacting gas and the electrolyte. It must join them that they may react; it must separate them to prevent massive transfer of gas into electrolyte or of electrolyte into gas. The physical burden imposed upon the electrode varies with the electrolyte and with the gases reacting. F o r a given electrolyte, the burden is least when access of reactant gas is unhindered b y the presence of other substances, as in the favorable case in which hydrogen or oxygen reacts, and water is easily rejected as liquid. The burden increases when the oxygen must be taken from air or when hydrazine is oxidized with the formation of nitrogen. Because intermediate compounds may form, the burden can be even greater when hydrocarbons are oxidized. The relationship between electrode structure and current density can be approached via simple models and via electrode kinetics. The models aid i n the understanding of transport processes, and the kinetic studies 116 Young and Linden; Fuel Cell Systems Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

10.

LIEBHAFSKY ET AL

Current Density and Electrode Structure

117

ΥΛ FUEL AQUEOUS

ANODE


OXIDATION

CATHODE

ELECTROLYTE

CHAMBER

CHAMBER

v..

PRODUCTS

f

L _ . ANODE ^A

VENT

S

CATHODE

ht

*

Figure 1. Schematic diagram of type of fuel cell considered Electrodes are envisaged as permeable to some degree by the gases and the free electrolyte. Electrodes relying on hydrogen diffusion through a solid—for example Pd-Ag alloys—are thus excluded

are important because the current density is the rate of the over-all elec trode reaction per unit area, expressed in electrical units; for a unidirec tional reaction C u r r e n t density =

k(A)(B), etc. e

R

t

(1)

where ( A ) ( B ) , etc. are related to the concentrations of species involved in the rate-determining step; Q = an energy of activation; c = a voltage term that increases with current density; and k! = a multiplicative constant analogous to NF in the thermodynamic relation A G = -NFE. Because fuel-cell electrodes are complex, and the processes occurring there are not well understood, models and mechanisms are mainly of qualitative use. To be satisfactory, a fuel cell must be acceptable i n at least these four respects: 1. 2. 3. 4.

Current density—voltage relationship Stoichiometry Reaction per pass Life

The electrodes in a satisfactory cell must have a structure that meets this severe set of requirements, and the electrodes in addition should have a fifth property—versatility—by which we mean, for example, that the cathode should operate satisfactorily on both oxygen and air and that

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

118

FUEL CELL SYSTEMS

the anode should operate satisfactorily on a number of fuels, some of which might be liquid. A satisfactory current density-voltage relationship means low overvoltage, and it is well known that the overvoltage, v = E

r

- E

(2)

a

increases with current density. (E = the reversible electromotive force; Ε = the actual cell voltage as measured at the terminals. ) Equation 1 is limited i n usefulness because it is difficult to tell what part of η belongs in this equation and because it is difficult to formulate the process that gives rise to c. Yet, it is advisable to use overvoltage i n the study of electrode structure. The usual procedure i n attempting to understand overvoltage is to sub divide it according to processes. W e propose for the complete fuel cell that η be subdivided initially according to location as follows: r


a

V =

VA +

VC +

(3)

VEl

where the subscripts, i n order, mean anode, cathode, and electrolyte. Once the overvoltage (η) at a given current density (i) is known, ap propriate measurements with Luggin capillaries give ηΑ and η thus establishing η ι b y difference. More than one measurement may be needed for an electrode because electrodes are not always uniform; also, the overvoltage and its components w i l l vary with current density. To make Equation 3 more useful i n the study of electrode structure, it is advisable to establish what fraction of each component of η is attrib utable to resistive losses. ( T h e most important of these resistive losses w i l l occur i n the electrolyte, but there w i l l be contributions from metallic conductors and from films on the electrodes. F o r simplicity's sake, the resistance of metallic conductors w i l l be assumed negligible and solid films w i l l be assumed absent. ) B y using interrupter techniques on the complete cell and on the Luggin capillary circuit for each electrode, one should be able to obtain ηΈι by difference from the equation 0>

Ε

η' =

V'A

+

V'c +

V'E
E . 6

c

d e

r

r

The results i n Table II deserve consideration except for the values of b, which are not significant because they are not necessarily minimum values. A l l the General Electric results ( N o . 2-5, inclusive) were ob tained with Niedrach-Alford electrodes (18) about 10 mils thick with platinum black as electrocatalyst; they should be comparable among themselves for the purpose of showing the effectiveness of this electrode structure for the different fuels. As here defined, d is the value of η at unit current density. O n Niedrach-Alford electrodes, hydrogen and methanol gave about the same value of d; that for propane was considerably higher. Bacon electrodes


134

FUEL CELL SYSTEMS l.2r

QJ

ι

I

•

200

I 400

ι

1 600

•

I 800

•

1

ι

1000


CURRENT DENSITY

I 1200

ι

I

ι

1400

I 1600

ι

I 1800

ι

I 2000

(ma/cm } 2

Figure 9. Current density-voltage curve showing high current density obtained in a Bacon hydrogen-oxygen cell give a notably small d for hydrogen under conditions that favor high re activity. Experiments on isolated anodes are needed to establish how much of d is attributable, i n each test to the anode reaction. The high value of a for methanol i n Table II may be related to its oxygen content. W i t h both propane and methanol, anodic oxidation was complete, and carbon dioxide and water were rejected as gases. Perhaps the difficulty of rejecting carbon dioxide at the anode is responsible for the lower values of i observed with carbonaceous fuels. The constant, c, together with the absolute value of i governs the sharpness of the downward break. A l l values of c i n Table II exceed the theoretical value 2.3 RT/NF. In N o . 2, which shows the highest value of c, the electrodes were known to be less permeable to gas than i n N o . 3 and 4; and this suggests that the difference between c and 2.3 RT/NF may i n general be due to restricted permeability. Table II should be regarded as a promising guide to further investi gations. Such investigations could well proceed along the lines laid out by Equations 3 and 4. If successful, they should reveal the effect of electrode structure on parameters a, b, c, and d. The Bacon cell (Table II, N o . 6 and Figure 9) gives outstanding per formance among those tested. As is well known, this cell does not use controlled wetting to keep gases and electrolyte apart but relies on the dual-layer structure in which the layer of smaller pores faces the electro lyte that fills them. The reactive zone is near the boundary of the two layers. The surface-active properties of the strongly alkaline electrolyte provide excellent opportunity for electrolyte film formation, and the low resistance of the electrolyte makes a large fraction of this film area effec tive for promoting electrode reactions. Rapid rates for these reactions are further favored by high pressure and high temperature. As Table II shows, the values of d and of b are the smallest given, and the value of a L

L


10.

LIEBHAF5KY ET AL


135

is intermediate. There is not even a hint of a limiting current density up to 2000 amp./sq. ft., and this very high current density is attained at a sacrifice of only about half of E . The difference i n operating conditions considered, it is clear that the Niedrach-Alford electrode is not far behind the Bacon electrode i n per formance on hydrogen and oxygen (Figure 10). To be sure, it uses plati num instead of nickel or nickel oxide as electrocatalyst, but it operates at a much lower pressure and temperature. L i k e the Bacon electrode, it shows no indication of a limiting current for oxygen over the currentdensity range investigated, which terminates just over 1000 ma./sq. cm. with Ε near 20% of E . The outstanding characteristic of the NiedrachAlford electrode is the excellent performance it gives on air, for which i has the high value of 620 ma./sq. cm. for one electrode and 255 for another, the second being of lower permeability (Table I I ) . If we as sume that there is no nitrogen diffusion barrier at i and that Equation 6 applies, i for operation on oxygen can be estimated as 620 (1/0.21) or about 3000 ma./sq. cm.; if, as is likely, some nitrogen barrier does exist, the estimated i is even higher. In addition to its other advantages, the Niedrach-Alford electrode is highly versatile as it shows good perform ance on a wide variety of fuels, gaseous and liquid hydrocarbons included. r

a

r


L

L

L

L

Satisfactory operation on air is, we believe, a hallmark of good elec trode structure, and we suggest that it be adopted as one criterion for judging electrode performance. •-2|—

'2

Figure 10. Comparative current density-voltage curves (IS) for operation on oxygen and on air in a cell with sulfuric acid as electrolyte

Note high current density achieved with air and absence of a limiting current during operation with oxygen


136

FUEL CELL SYSTEMS

Figure 10, compared with Figure 8, shows that the interpretation built around Reaction 13 is not valid over the entire current-density range, for Δ Ε increases continuously from about 0.02 volt at i = 0 to about 0.1 volt in the region (near 600 ma./sq. cm. ) where the E — i curve begins to turn down sharply. This behavior suggests that the interpreta tion designed for the Shell electrodes is valid here at i = 0 but that with the Niedrach-Alford electrode diffusion barriers begin to become impor tant even at low current densities and continue to grow i n importance until i is reached. Further work is required before these diffusion bar riers can be interpreted in terms of Equation 6 or 6a. The work of Clark ( 7 ) shows the seriousness of the nitrogen diffusion barrier for thick carbon electrodes. Clark seems to have made his elec trodes unusually thick to make prominent the diffusion barriers he wished to study. As his anodes were zinc, these diffusion barriers are associated with the cathodes. W e could not find thicknesses cited by Clark but we estimate 1 cm. as the thickness of the electrode in his Figure 2. Experiments b y Clark with oxygen have been mentioned. O f his nitrogen-oxygen experiments, we show the simpler set i n Figure 11. I n these experiments, Ρ was held constant at 0.205 atm. of oxygen, and P increased with the nitrogen content. The data of Figure 11 were shown by Clark (in his Figure 11) to have the inverse dependence of i on P called for by Equation 6a; i was taken as the value of i near 0.9 volt (dotted line i n Figure 11). Such experiments should be done on elec trodes such as Shell and Niedrach-Alford, for which the length of gaseous diffusion path is much shorter. α

a


L

T

L

L

P E R F O R M A N C E WITH AT

VARIABLE

—

0 "N 2

TOTAL

CLARK

2

7

Zn/KQH/0

2

S

1.0

MIXTURES

PRESSURE

PQ^.205 otm.

— 1? 62.9%\ λ 0*^Q \ 2 Λ

— AIR

a \

31.0% I 0

0 2 ,l—I

J

V Η

>i \79.2% \^40.Γ/.\ w

)p

\°

2

\

\0z

\

/TANK VOXYGEN \

ιI I I f h) ,ibQ 1I ιι I00 200 300 CURRENT DENSITY ( m a / c m 2 )

\

,ι

II 400

Figure 11. Limiting current densities (7) obtained with various oxygen-nitrogen mixtures at constant partial pressure of oxygen Note decrease of limiting current with increasing nitrogen content


T


10.

LIEBHAF5KY ET AL


137

Figure 12 shows the more complex results obtained by Clark (7) on oxygen-nitrogen mixtures at a fixed total pressure near 1 atm. The linear portion of the curve could be explained as was Figure 11, but the sharp upward rise could not. Figure 12 also shows that a small amount of nitro gen added to pure oxygen is more effective i n reducing limiting current density than the linear region would lead one to expect. T w o possible explanations come to mind: Nitrogen preferentially blocks the pores most electrochemically active, and nitrogen accumulates more rapidly at the active areas when the current density is high. A n y complete explanation must consider the rate at which nitrogen leaves the neighborhood of these areas, but much further work w i l l have to be done before a com plete, definitive explanation can be attempted. The lowering of i by the presence of nitrogen for carbon electrodes has been abundantly demonstrated by Clark (7). In Figure 11, i for air is about 100 ma./sq. cm. or about one third that for oxygen. Carbon electrodes like his are at a disadvantage i n air operation because the gas diffusion path is too long; thinner electrodes could of course be used in fuel batteries. The length of gaseous diffusion path seems a rational criterion for judging electrode structure. Again, everything else being equal, a short diffusion path is desirable. This holds true not only when nitrogen is present but even more when the partial pressure of water is appreciable. When the length of gaseous diffusion path and the thickness are propor tional (often they are not), the relation between thickness and limiting current density seems worth establishing (7). L

L

1200

1000

800

-

600

I

ο Ε

~

400

200 0

2

A

^

PARTIAL PRESSURE OF 0, (ATM

A

TO

)

Figure 12. Limiting current densities (7) obtained with oxygen-nitrogen mixtures at constant total and variable oxygen pressure


FUEL CELL SYSTEMS

138

After everything is said and done, we believe that thin electrodes are likely to be better than thick ones and that any good electrode structure must provide an adequate effective area of electrolyte film. Finally, Figure 13 shows current density-voltage curves for the two carbonaceous fuels of Table II. W i t h hydrogen as fuel, one may assume that the cathode behavior predominantly determines i . W i t h carbonaceous fuels, one may assume that anode behavior has a considerably greater influence. The data i n Figure 13 are presented i n a form to emphasize the difference i n the anodic behavior of methanol and propane.


L

0.2h

o«

I 10

I 20

I 30

I 40

I 50 i,

ma/cm

I 60

I 70

I 80

I 90

I I0

Current Density and Electrode Structure in Fuel Cells - Advances in

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