Redox Ion Exchanger Fuel Cell Jagdish Prasad Rawat' and Abdul Aziz Ansari Aligarh Muslim University, Aligarh-202002, India Our conventional sources of enerev. ... . such as fossil fuels. hydroelectricity,and nuclear energy,each has itsown limitations and asaociared hazards. This necessitates the develonment of sources of energy other than the conventional ones. The possibility of finding new sources of energy have been highlighted recently in our country ( I ) . Much emphasis has been aiven to research needs for advanced fuel cells (2). Its techn&ogy and applications have also been studied (3). In ceneral a hydrogen/oxygen fuel cell is used in which electrodes are made i f titanium coated u,ith platinum and the electrolyte is a cation-exchange resin 14). Oxygen is normallv the oxidant and hvdroaen is most width used t'uel. he knergy-producing reaction is the oxidation bf hydrogen bv oxveen. .Ion exchangers have been widely used as an e1ectrol)Te in full cells. Fuaita et al. (5) have made a fuel cell in which a combined body of cation-exchange membrane electrolyte between two electrodes had a porous collector of sintered Ti fibers pressed on electrodes. A sulfonated perfluorocarbon cation-exchange membrane was coated with P t to form a cathode on one side and with R h on the other side to form an anode. Mucoyama and Hirai (6) made fuel cells using electrolvtes that were meoared from p articles of stronelv acid catibn-exchange re& The resin particles were ma& into paste with addition of water and S i c powder and packed into the cavity between the fuel (MeOH) anode and an ion exchanee membrane coveriua the oxidant (air) cathode. A fibrousion-exchanger sheet gas been used as an electrolyte in a fuel cell prepared by Mitsubishi Rayon Co. Ltd. Japan (7).T h e fibrous cation exchanger bonded on a Ni mesh core to prepare an electrolyte was sandwiched between an anode and a cathode. Kummer and Oei (8) prepared a chemically regenerative redox fuel cell in which a suitable cation-exchange membrane was placed in a housing provided with openings on opposite sides of the membrane for the catholvte and anolvte resoectivelv so that necessarv exchanee " may take place to form a redox cell. The present work was undertaken t o study the usefulness of a redox reaction between Fe3+and Zn on an ion-exchanger bed for the preparation of a fuel cell. A comprehensive review of the literature reveals that the Zn2+/Znand Fe2+/Fe3+ system has not been used for the preparation of redox ionexchanger fuel cells so far. T h e system in question has been chosen because of a significant difference between the electrode potentials of Zn2+/Zn and Fe2+/Fe3+couples. Further, the system becomes important owing to its low cost, easy availability, and the ease with which the redox ion-exchanger can be prepared, that is, simply by incorporating Fe3+ ion in a n easily available cation exchanger (e.g., Dowex 50 W X 8). T h e redox iou-exchanger so formed can he regenerated after use. Most of the ion-exchanger fuel cells reported in literature (5-8) suffer from several shortcomings. Their assemblies are either too com~licated,hence the need for sophisticatedoperators,or theprocedures formaking thrmare cumbersome. Our redox ion-exchanaer fuel cell (REFCJ requires a very simple assembly, and the operation is very
-
-
easy, yet it furnishes a voltage of 0.69 f 0.05 V, which is comparable with the magnitude of voltage obtained from other fuel cells using ion exchangers.
Electrode Material for REFC In the present investigation a redox ion-exchanger has been used for the construction of the electrode. Redox ion-exchanger was prepared from Dowex 50 W X 8 cation exchanger by replacing the counter ion by Fe:'+ ion (9). Standard electrode potential of the couple Fe2+/Fe3+is little affected by incorporation of the couple into the ion exchanger (10). Dawex 50 W X 8 was kept in 1 M solution of ferricnitratefor 24h, then it was washed thoroughly with distilled water and was dried at 30% The resulting redox ionexchanger was packed in a glass tube narrow at one end and fitted with glass wool. Design and Working of REFC Redax ion-exchanger (5 g) packed in a glass tube worked as the cathode. while zinc metal was used as the anode. Bath the electrodes
to keep the cost low. A multirnetei was used for voltage end current measurements.A schematic diagram of the REFC is given in Figure 1. The concentration of zinc nitrate solution was kept constant at 1 M and that of Fe"+ ion in the redox exchanger at 1 M, to maintain standard conditions. Electrode potentials were calculated using the Nernst equation. Electrode potentials of the couples FeZ+/Fe"+and Zn"/Zn were found to be +0.771 V and -0.763 V, respectively. Since electrode potential of the couple Zn2+/Znis lower than that of the former is oxidized and furnishes electrons the couple Fe"+/FeZ+, to the latter. The energy producing reaction is oxidation of the substrate by the redox ion exchanger.
Zn t ZnZ++ 2e2Fe"
+ 22- + 2 F F
Author to whom correspondence should be addressed.
808
Journal of Chemical Education
(1)
(2)
so that net reaction is
+
Zn + 2Feat r Zn2+ 2Fe2+
(3)
Z in= nitrate S01ulion
R e d o r ion Exchongor Z i n c Piece
.
- - . . . . .. ..
l
at anode at cathode
Figure 1. A redox exchanger fuel cell.
Gloss woo1
Table 1.
Voltage and Current Data lor Slx Slmllar Redox Ion Exchanger Fuel Cells
Cell Voltage Cunem Number V mA
Standard deviation For For voltage current V mA
Relative standad deviation
For voltage For current %
u
%
Figure 2. Circuit diagram of the REFC showing internal resistance
voltage drop = 2.43 V
Table 2. Increase In Voltage and Current on Connecllng the Redox Ion Exchanger Fuel Cells In Serles and Parallel
Type of connection (a) Series (b) Parallel
Number Of cells
2 4 2 4
Theoretical value Voltage Current V mA
1.38 2.76 0.69 0.69
0.37 0.37 0.74 1.48
:.
Exoerimentai value Voltage Current V mA
1.22 2.43 0.68 0.66
0.36 0.37 0.68 1.30
Results and Discussion
From a single cell, a voltage of 0.69 f 0.05 V and 370 f 70 PA current was obtained. The voltage was taken as a steadystate voltage. In order to study the reproducibility of such fuel cells, six replicate measurements were made, and the results are summarized in Table 1. For six redox ion-exchanger fuel cells, voltage and current measurements were made individually, and i t was found that the value of the voltage varies from 0.64 to 0.72 V, whereas the value of the current lies within the 340-420-~Arange. Standard and relative standard deviations for voltage and current measurements were also computed and aregiven in Table 1. The magnitude of the voltage was increased additively by connecting more cells in series. For example, when four cells were connected in series, a voltage of 2.43 V was obtained, which is in eood aereement with a theoreticallv exnected value of 2.76% ( ~ a b i 2). e The theoretical value okolt'age for four cells in series is simnlv four times the voltage due to a single cell. A little discrepancy seen between the observed and theoretically predicted value may be attributed to experimental errors. The same arrangement of four cells in series furnishes a current of 370 PA, which is sufficient to display an electronic calculator. When the cells are connected in parallel, the current increases additively. Table 2 shows that on connecting four cells in parallel 1.30 mA current is obtained, which is approximately four times the current due to a single cell. The experimental value is in agreement with the theoretical value (1.48 mA). A fuel cell system consists of fuel cell stacks connected in narallel(l1). One difficulty that was observed with this redox ion-exchanger fuel cell was that an internal resistance is produced o n loading the cell (Fig. 2). As a result thcrc was a voltage drun in therrll,and amnewhat lesser vultage was available to the'load. However, internal resistance can be minimized by connecting more cells in parallel. The number of fuel cells required to obtain 10 mA current were calculated. The voltage obtained from four cells connected in series, without loading, was 2.43 V. On connecting a 1.5-V bulb (load), there was a voltage drop, and only 1.515 V was available to the load. Therefore,
- 1.515 V
current measured = 370 pA = 370 X 10-6A 0.915 V internal resistance = 370 X 10-6A
Since the current required is lOmA, the resistance should be minimized to a value
Now the number of fuel cell stacks connected in parallel
where each fuel cell stack consists of four cells connected in series. Thus 112 cells are required to obtain a 10 mA current. There are 28 such fuel cell stacks, and all these 28 fuel cell stacks should be connected in parallel. The resulting fuel cell system can furnish a voltage equal to 2.43 V and 10 mA current. Although these calculations are purely theoretical and have not been put to any experimental test, they may give an idea about the number of cells required to obtain 10 mA current. Redox ion-exchanger fuel cells have many advantages over others such as their low cost, pollution-free operation, regenerative nature, and easy reaction conditions. However, they require further improvements in current and voltage output, and need t o be more compact for their development.
-
Acknowledaernent
Theauthors are thankful toS. % 0srnan.Chairmanof I. the Ueoartrnent of Chemistrv at A. M. U. Alignrh, ior providing reskarch facilities. Literature Cited 1. Phys.Newr, Bull, Indian Phys. Assoc. I980.2(4). 2. Penner, S. S., Ed.: Pergamnn Pxess. New York. 1986: 229pp: C h e m Abslr. 1986,104. B ,""sn" ". Q 3. Pietrnerande. P. K i n d Techno1 Inr Co. Ilaiv. ICP 1986, 1 4 3 ) . 27-42. C h e m Abslr. 198fi. 10.5, R 229776 S . 4. Atkins.P. W. P h ~ s i c o Chemistry, l ELBS sndOxlord Univ.: 1 9 8 3 ; ~1061. 5 . Fuji*. J.: Fujita. Y., Japan Storage Batter." Co. Ltd. Jpn. Kokai Tokkyo Koho J P 615288. 15 Msrch 1986: C h e m Abstr. 1986.105. 1 4 2 7 3 ~ . 6. Mueuyama.Y.; Hirai.0.: Kobayashi, Y. HiLlchi Chemical Co. Ltd..Jpn. Kokai Tokkyu Kohod P6178067.21 April. 1986: Chrm.Abslr. 1986.106.191600a. 7. Mitsubiohi &yon Cu. Ltd, Jpn. Kokai Tokkyo Kohn 1 P 82128468. 10 Aug. 1982: Chrm. Ahatr. 1982,97.21%51 C. 8. Kummer.J.T.:Oei.D. G..Ford MuforCo..U.S. US4407902.4Oct. 1983;Chem.Abstr. ."*""7
Volume 67
Number 9
September 1990
809
