A New Fuel Cell Anode Catalyst

8 A New Fuel Cell Anode Catalyst RAYMOND J. JASINSKI

Research Division, Allis-Chalmers ManufacturingCo., Milwaukee, Wis. Downloaded by NORTH CAROLINA STATE UNIV on May 4, 2015 | http://pubs.acs.org Publication Date: January 1, 1969 | doi: 10.1021/ba-1965-0047.ch008

1

There are a number of possible commercial applications for hydrogen-air

fuel

cells—for

example, the οn-the-site fuel cell oxidation of by-product hydrogen.

One of the principal

items which affects the capital cost of the fuel cell power plant itself is the cost of the anode and cathode catalysts. hydrogen

anode

been developed.

A new nonnoble metal

catalyst—nickel

boride—has

This material possesses a

high surface area—20 sq. meters/gram—and is active in chemisorbing hydrogen.

A hydrogen

-oxygen fuel cell employing this catalyst oper ated continuously for 1200 hours at 32 ma./ sq. cm.

The initial cell voltage was 0.78 volt.

J h e operating cost of a fuel cell power plant is determined primarily b y the cost of the fuel. Although hydrogen is generally a more expensive fuel than methanol or propane, there are a number of situations i n which fuel cells operating on hydrogen could be economical. A n example of this is the on-the-site fuel cell oxidation of by-product hydrogen obtained from commercial processes, such as the electrolytic production of chlorine. One of the principal items which affects the capital cost of the fuel cell power plant itself is the cost of the anode and cathode catalysts. T h e majority of the ambient temperature, hydrogen-oxygen fuel cells described in the literature have employed noble metals such as platinum and palladium. These metals are expensive, and some question has been raised regarding an adequate natural abundance of the elements (12). There are two alternatives to developing inexpensive catalyst-anodes: Either reduce the amounts of the noble metal catalyst on the electrode to about 1 mg./sq. inch of electrode or employ less expensive nonnoble metal catalysts. 1

Present address:

Tyco Laboratories, Inc., Waltham 5 4 , Mass.

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

FUEL CELL SYSTEMS

96

The latter approach has been followed, for example, b y E . Justi ( 7 ) , who successfully constructed hydrogen anodes starting with the Raney alloy ( N i A l ) . It would appear, however, that the economic advantage of employing less expensive metals, such as nickel and aluminum, is more than offset b y the relatively involved, and hence costly, procedure of electrode preparation. Furthermore, the problem of adapting these procedures to the preparation of large electrodes for the construction of fuel cell power plants may limit the applications of this catalyst. This article discusses the use of a new material, nickel boride, as a hydrogen anode. The existence of catalytic activity is described as well as some of the general chemical properties of nickel boride. N o attempt is made, at this stage, to present a detailed electrochemical characterization of the material. The properties of nickel boride as an anode i n the hydrazine-oxygen cell and the potassium borohydride-oxygen cell have been discussed elsewhere (6). It was shown that, for these cells, the nickel boride catalyst-anodes were approximately 0.1 volt more effective than a palladium catalyst for the direct fuel cell oxidation of hydrazine and potassium borohydride. The same comparison holds true for platinum anodes. The chemical hydrogénation properties of this catalyst have been discussed only briefly i n the literature. Nickel boride has been reported to be superior i n catalytic activity to Raney nickel and more resistant to fatigue i n the hydrogénation of safrole, furfural, and benzonitrile ( 9 ) . Some of the properties of this material have also been described i n the Russian literature ( 5 ) . More recently, H . C . Brown (2) has discussed the hydrogénation of olefins b y nickel boride. Nickel boride can be formed b y heating nickel oxide to 700° to 1000° C . i n a stream of boron trichloride and hydrogen (3). Another method of preparation involves electrolysis of nickel oxide dissolved i n variable quantities of alkali tetraborates" ( 8 ) . T h e compound can also be formed directly from the elements b y diffusing powdered boron into the reduced nickel (4). However, the most convenient method has been that developed b y Schlesinger (11) i n which nickel boride is formed b y combining solutions of potassium borohydride and a nickel salt. This technique has also been employed by Paul, Buisson, and Joseph ( 9 ).

Downloaded by NORTH CAROLINA STATE UNIV on May 4, 2015 | http://pubs.acs.org Publication Date: January 1, 1969 | doi: 10.1021/ba-1965-0047.ch008

2

3

u

Chemisorption Properties Nickel boride, prepared b y the Schlesinger method, is formed i n an atmosphere of hydrogen, produced b y the decomposition of the excess potassium borohydride on the catalyst surface. As a result, there can be a considerable quantity of hydrogen chemisorbed on the material so that in some oases it may be pyrophoric. N o difficulty was experienced i n handling the electrodes. It is dersiable, however, that contact with air be held to a minimum.

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

8.

JASIN5KI

Anode Catalyst

97

The chemisorption of hydrogen on nickel boride was studied as a function of temperature. (The sample employed i n these experiments had a surface area of 15 sq. meters/gram, as determined from the adsorption of nitrogen at —196° C . ) Adsorbed hydrogen was removed b y evacuating the sample at 200° C . to a pressure of less than one micron. Heating at this temperature has only a minor effect on surface area. The adsorption of hydrogen at a pressure of 1 atmosphere, as a function of temperature, is shown in Table I.


Table I.

Adsorption of Hydrogen on Nickel Boride

Temperature, C

-196 0 25 150 275

Volume Adsorbed (STP), Cc./Gram 0.6 3.5 3.4 3 2.3 2

The variation of the quantity of hydrogen adsorbed with temperature, as shown i n Table I, is characteristic of activated adsorption. A t —196° C , the hydrogen has insufficient energy to overcome the energy barrier for adsorption. A t the higher temperatures (275° C ) , there is sufficient energy to break the chemical bonds holding the hydrogen to the surface of the solid and the quantity of gas adsorbed decreases. However, the drop i n hydrogen adsorption at 275° C . is complicated somewhat by the fact that the catalyst has probably undergone some sintering, thus lowering the over-all surface area. The shape of the hydrogen adsorption isotherm suggests that, at a pressure of 360 mm., a monolayer has been achieved. F r o m the volume of hydrogen adsorbed, it is possible to obtain an approximate value for the fraction of the total nickel boride surface available for hydrogen chemisorption. It was assumed in the calculations that the surface of the catalyst had the same atom ratio as the bulk compound—that is, 2 N i : l B . A n effective radius of the nickel atoms of 1.30 A . was chosen from the literature (14). The radius of the boron atoms was chosen as 0.89 A . (IS). It was also assumed that the hydrogen dissociated upon adsorption and that one hydrogen atom adsorbed per nickel atom. Finally, it was assumed that all the hydrogen adsorbing at room temperature chemisorbed on the catalyst surface and d i d not diffuse into the lattice. W i t h these assumptions, it can be shown that the nickel atoms adsorbing hydrogen account for 10 sq. meters/gram. Taking into account the boron on the surface raises this value to 12 sq. meters/gram. This surface area, computed from the hydrogen adsorption isotherm, is of the same order as that obtained from nitrogen adsorption, 15.2 sq.


FUEL CELL SYSTEMS

98

meters/gram. The lack of a closer correspondence i n area as computed by the two isotherms could be due to a number of factors. According to Beeck ( 1 ), the use of nitrogen adsorption to determine the area of nickel or iron catalysts generally leads to a value which is too high, because of a small amount of nitrogen chemisorption. The somewhat arbitrary choice of the nickel atom radius, representative of N i i n N i B , has an i n fluence on the surface area computed from hydrogen adsorption—that is, ± 0.1 A . is equivalent to db 1.5 sq. meters/gram. Nevertheless these data do indicate that a major portion of the nickel boride surface is active i n absorbing hydrogen, a necessary, but not suffi cient, property of a hydrogen anode catalyst. A preliminary study was also made of the physical structure of the catalyst as a function of sintering temperature. A second sample of nickel boride was heated under vacuum to successively higher temperatures. The surface area of the sample was determined between heat treatments from the nitrogen adsorption isotherm. The sample was first degassed at ambient temperature until a constant pressure of less than 1 micron was reached. A nitrogen adsorption isotherm, measured at —195° C , indicated a surface area of 22.5 sq. meters/gram. The boride was then heated under vacuum to 200° C . and held at a pressure of less than 1 micron for four hours. A second nitrogen isotherm indicated a surface area of 21.3 sq. meters/gram. The decrease is small, indicating little, if any, sintering of the nickel boride. Next, the sample was heated to 350° C . for Y7 / hours at a pressure of less than 1 micron. The surface area dropped to 13.3 sq. meters/gram. Finally the sample was heated to 475° C . for 2 / hours. The final surface area was 6.38 sq. meters/gram. These data are plotted in Figure 1. Very little chemisorption of hydrogen was noted on the nickel boride sample after sintering at 350° C . The sample was cooled to 25° C , and the hydrogen isotherm measured. After evacuating to a pressure of < 1μ at this temperature to remove only physically adsorbed hydrogen, another hydrogen isotherm was measured. The two isotherms were identical, indicating at most, weakly adsorbed hydrogen. This loss of a large number of active sites during sintering is confirmed by electrochemical data. F u e l cells containing nickel boride anodes which had been sintered at 500° C . did not attain a voltage greater than 0.4 volt, while the samples sintered at 400° C . attained a cell voltage of 1 volt but only very slowly. Apparently, the activity of the electrocatalyst was more sensitive to heat treatments than would be expected simply from changes i n surface area. A 3 X 3 inch anode, generating approximately 10 ma./sq. cm., must have at least 10 hydrogen atoms chemisorbing and reacting per second. The minimum current involved i n the voltmeter reading of 1 volt was approximately 10~ amps—10 hydrogen atoms reacting per second. A catalyst-anode sintered at 500° C . achieved an open-circuit voltage of


2

1

1

2

2

19

6

13


8.

JASINSKI

99

Anode Catalyst


2V

5

η I

0

ι

KX)

ι

ι

1

1

200

300

400

500

CALCINING

Figure 1.

T E M P E R A T U R E , ° C .

Variation in surface area with sintering temperature

only 0.4 volt. Assuming a direct proportionality between the rate of re action and the number of active sites, there must have been a decrease b y a factor of 10 in the number of active sites upon sintering at 500° C . T h e surface area, however, was decreased only b y a factor of four. This con clusion is also apparent from a comparison of the hydrogen chemisorption after sintering at 350° C . with the loss of surface area. The chemisorption of hydrogen was reduced to a negligible value, while the surface area had decreased by a factor of four. Therefore, the use of total surface area as an indicator of active catalyst site concentration in formulations of electrochemical reaction rates is a very tenuous procedure. A more valid approach would be the use of the hydrogen adsorption isotherm "surface area." This is not intended to imply an identity but rather a proportionality between the number of sites adsorbing hydrogen and the number of catalytically active sites. 6

Anodic Oxidation of Hydrogen Nickel boride was studied as a hydrogen anode catalyst in two f o r m s first as the powder and then supported on a porous nickel plaque. The powder was prepared by combining a basic solution of 5 % potassium boro-


FUEL CELL SYSTEMS


100

Figure 2.

Catalyst test electrode

hydride with a dilute aqueous solution ot a nickel salt—for example, nickel acetate. A voluminous black precipitate formed immediately and was accompanied by a rapid evolution of hydrogen gas. The details of the procedure are all well documented ( 9 ) and insoluble nickel boride is the sole solid reaction product. As a check on the procedure, a sample of this material was analyzed and shown to have a N i / B atom ratio of 2.05. The plaque-electrode was formed by depositing nickel boride i n the voids of a porous sintered nickel plaque. The substrate, commercially available, measured 3 X 3 X 0.03 inches. Since the nickel boride is an electrical conductor, it was possible to study the catalyst directly—that is, it was not necessary to first support the catalyst on a conducting substrate. The catalyst was formed into a plug and placed i n the catalyst test electrode shown i n Figure 2. The fuel gas was passed down the metal tube, through the catalyst plug, and into the "free" electrolyte. Studying the catalyst i n this form alleviated many of the problems involved i n operating hydrogen-oxygen fuel cells, such as maintaining the proper moisture balance i n the cell. The two electrodes were "driven" with the commutator (10) hence the voltages measured were free of internal resistance ( I R ) loss. In such a system,


8.

JASINSKI

Anode Catalyst

101

-I.0r

FUEL:

H

2

T E M P E R A T U R E : 80°C.

H-0.8 I

S -0.61 -J Ο >


u-0.4

Ο Ο Ζ

-0.6t ω

S< -0.4 -0.2h

°0

N I C K E L h

F U E L :

B O R I D E H

2

T E M P E R A T U R E : C O N S T A N T

100

200

" P L U G * A N O D E

eo°c

C U R R E N T

300

400 T I M E ,

D E N S I T Y

500

. 65

Β

600

ma/cm.

700

2

800

900

H O U R S

Figure 4. Extended operation of a nickel boride hydrogen electrode substrate. A voltage-current curve obtained for a fuel cell employing such an anode is shown i n Figure 6. The cell temperature i n this case was 78° C . It was possible to support a load of approximately 70 ma./sq. cm. at a cell voltage of 0.7 volt. Improvements i n cell and electrode design have increased this performance to 100 ma./sq. cm. at 0.7 volt. The plaque type of electrode structure employed i n these experiments is satisfactory for a demonstration of the existence of electrocatalytic activity. However, it is difficult to obtain more than a general estimate of the catalyst loading—that is, the weight of catalyst per unit geometric area. Obviously this information is necessary before an exact evaluation of catalyst cost is possible. Further experiments have confirmed that the use of nickel boride as a hydrogen anode does markedly reduce the anode catalyst cost. Extended Performance Tests The fuel cell data shown i n Figure 6 describe the initial perform ance of the catalyst electrodes—that is, the voltage-current characteristics obtained during the first day or two of operation. However, fuel cell electrodes generally show some loss i n activity with time, as shown i n Figure 4. This effect was also studied with a fuel cell of the type shown in Figure 5, employing an anode of nickel boride deposited within a porous nickel substrate. The initial performance was essentially that described by the voltage-current curve of Figure 6.



8.

JASINSKI

Anode Catalyst

103

ELECTRODE

H ELECTRODE

GAS CAVITY

GAS CAVITY

2

MEMBRANE ANODE H +2ÔH-^ 2 H 0 + 2e"

CATHODE '/ 0 + H 0 2e~—*-2ÔH 2

2

2

2

2

>H

INERTS-

2

OVERALL REACTION H + > 0 -*> H 0 2

Figure 5.

2

2

2

Allis-Chalmers hydrogen-oxygen brane fuel cell

capillary mem-

Before discussing the extended performance data, it is necessary to briefly mention the subject of fuel cell "controls". The principal problem, common to all hydrogen-oxygen fuel cells, centers on maintaining the proper water balance in the cell. It is desirable to withdraw from the operating fuel cell only that amount of water produced by the fuel cell reaction. If an excess is removed, the cell w i l l dry out; if insufficient water is removed, the electrode w i l l flood or the electrolyte w i l l be diluted. In either case, the extended performance of the hydrogen-oxygen fuel cell would be adversely affected. A high operating current density provides for a high rate of water production, and the problem of maintaining the proper amount of water in the cell becomes more severe. The life test on the nickel boride hydrogen anode was carried out at a relatively low current density—that is, 32 ma./sq. cm. A t this level, the rate of water production was determined to be sufficiently low so as not to require an involved water monitoring system. The moisture i n the cell was controlled empirically by adjusting the hydrogen flow rate to establish either a wetting or a desiccative condition as needed. The variation i n cell voltage with time is shown i n


FUEL CELL SYSTEMS

104

1.0

Hg/Qa FUEL C E L L N i B ANODE CELL TEMPERATURE*. 80°C« 2

0.8 UJ


Ο < 0.6 !J (X4

0.2 17

CURRENT DENSITY, ma/cm. 34 51 68 85

2

102 ι 6

3 4 CURRENT, amps

119 —J

Figure 6. Voltage-current characteristics of a hydrogen-oxygen fuel cell employ ing a nickel boride anode l.0r

5

H2/0 Ni2B

- j 0.41-

2

FUEL CELL ANODE

CELL TEMPERATURE : 80eC CONSTANT CURRENT DENSITY Γ 3 2

200

400

600 TIME,HOURS

Figure 7. Extended operation of a H /O anode 2

è

800

ma/cm.2

KXX)

1200

fuel cell employing a nickel boride

Figure 7 for a cell operated i n this manner for 1200 hours. There appears to be a small, gradual fall-off i n cell performance with time. Comparing these data with those of Figure 4 most of the fall-off after the first 200 ?


8.

JASINSKI

Anode Catalyst

105


hours must be ascribed to problems of cell control. Nevertheless, the data are sufficient to establish that it is possible to construct large nickel boride catalyst-electrodes which are capable of extended operation as hydrogen anodes. The data presented were taken with 3 X 3 inch electrodes. N o difficulty has been experienced i n adapting the preparative procedures to 6 X 4 inch nickel boride catalyst-anodes. The use of electrodes of this size i n the hydrazine-oxygen fuel cell has already been described ( 6 ) . Acknowledgment The author wishes to acknowledge the efforts of James Huff i n obtaining the gas adsorption data and the efforts of D a v i d Cary i n carry ing out the extended performance tests. The catalyst test electrode was developed with the assistance of P h i l Horwitz and Larry Swette. Literature Cited (1) Beeck, O . , " A d v a n c e i n Catalysis," V o l . 2., A c a d e m i c Press, N e w York, 1950. (2) Brown, C. Α . , Brown, H. C., J. A m . Chem. Soc. 85, 1003 (1963). (3) Deiss, W. J., B l u m , P., Compt. rend. 244, 464 (1957). (4) Fruchant, R., M i c h e l , Α., Bull. Soc. Chim. France 1959, 422. (5) Iverdovskii, I. P., Tupitsyn, I. F., Problemy Kinetiki i Kataliza, Akad. Nauk, S.S.S.R. Inst. Fiz. Khim., Soreshscanie, Moscow 9, 86 (1956). (6) Jasinski, R., presented at 123rd Meeting Electrochemical Society, Pitts burgh, Pa., A p r i l 15, 1963. (7) Justi, E., Pilkuhn, M., Schiebe, W., and W i n s e l , Α . , " H i g h D r a i n D i f f u s i o n Electrodes Operating at A m b i e n t Temperature and Low Pressure," Akad.

Wiss, Lit. ( M a i n z ) Abhand. Math. Nat. Klasse, N o . 8, K o m m Verlag Steiner,

Wiesbaden, 1959. (8) Marion, S., Bull. Soc. Chim., France, 1957, 522. (9) Paul, R., Buisson, P., Joseph, N., Ind. Eng. Chem. 44, 100 (1952). (10) Pollnow, G., K a y , R., J. Electrochem. Soc. 109, 648 ( 1 9 6 2 ) . (11) Schlesinger, H. I., U. S. Patent 2, 461, 611 (Jan. 9, 1945). (12) Slaughter, J., presented at 121st Meeting Electrochemical Society, L o s Angeles, Calif., M a y 1962. (13) Sneed, M. C., Brasted, R. C . , "Comprehensive Inorganic Chemistry," 7, D. V a n Nostrand C o . , N . J . (1958). (14) W e l l , A . F., "Structural Inorganic Chemistry," Clarendon Press, Oxford, 1962. R E C E I V E D February 17, 1964.


A New Fuel Cell Anode Catalyst

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