edited bv GEORGEL. GILBERT Denison University Granville. Ohio 43023
The Interconversionof Electrical and Chemical Energy: The Electrolysis of Water and the Hydrogen-Oxygen Fuel Cell Sus~lm BY ~ Serglo Roffla, Vittorlo Conclallni, and Carmen Paradlsi IMltuto Chlmlco "G. Clamlclan" UnlversHd dl Bologna Bdogna, Italy
CHECKED BY Jack Lambed Kansas State Unlverslty Manhatla", KS 66506
Several electrochemical demonstrations have been proposed to illustrate the interconversion between electrical and chemical energy (1-4). Some of them utilize the electrolysis of water in order to show the conversion of electrical energy to chemical energy while a t the same time they take advantaee of the ~ o t e n t i adifference l produced in the same cell as omsequence oi the electrolysis in order to s h m the reverse transiormation of chemical energy to clectrical energy ( 2 5 ) .Such electrical energy is often utilized for operating a small dc motor, an LED, etc. In the latter mode of operation it is said that the cell acts as an Hz-O2 fuel cell. Actually, there are two reasons why the electrochemical system obtained after electrolysis under the prevailing conditions is not a good representative of a fuel cell: the first lies in the fact that the potential difference one measures right after the electrolysis is fairly larger (typically a little higher than 2V) than the limiting thermodynamic value, 1.23 V, expected for an Hr02 fuel cell at 25 OC; the second is the lack of continuitv of the transformation, which, conversely, is a peculiar feature of a fuel cell. As far as the first question goes, the voltage read across the electrodes when interru~tinrelectrolvsis is hirher than 1.23 V because in a driven c&, i.;., in a ceil d r i v e d y an outside nower source. the cell voltage must be higher than that expected under equilibrium conditions d u e t o polarization Drocesses (Fin. 1) ( 6 ) .Thus, right after electrolysis, owing to the slow attainment of equilibrium conditions in the piesence of polarization phenomena, such as concentration pothe voltage maintains for a certain period a larization higher value than that corresponding to reversibility in aireement with what is found experimentally. In this connection it must be emphasized that only by virtue of'this overvoltaee is thecell. unlike an H - 0 , fuel cell. al~lctodrive electromeYchanical dekces, such aclock, that typically need emf's of -1.5 V. On the contrary, in a fuel cell, being a self-driven cell, i.e., an enerev the cell voltage in the presence of cur... ~roducer, . rent, always in consequence of the polarization phenomena, must he lower than the maximum reversible value (61 following a pattern schematized in Figure 2. From the above considerations it is evident that the cell, just after the electrolysis, is not a good model of an H r 0 2 fuel cell.
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Theapparatus described in this paper was set up in the occasion of the Exhibition on Energy "Energia: un concetto in mostra, gadget, esperimenti, film", held in Bologna, Italy. 9 February-25 April 1985. and organized by the city administration.
I cell current Figure 1. Schematic voltage-current
dependence in a driven cell.
L cell
current
Figure 2. Schematic voltage-current dependence in a selfdrivencell.
Coming to the second aspect, namely the problem of continuity of the transformation, it must be remarked that the experimental setup which utilizes the cell after electrolysis as a representative of a fuel cell in the chemical energyelectrical enerw interconversion, leads to sacrificing the continuity of the operation. In fact, once electroly& is stopped, owing to the decay of the potential difference with time, due mainly to polarization, a clock cannot be driven for more than a minute or so. On the contrary, a fuel cell is commonly intended to he a device that can continuously transform the chemical energy of a fuel and an oxidant in electrical energy by a processi&olving an essentially invariant electrode 1-electrolyte-electrode 2 system (8,9). A simnle anoaratus that allows one to ~ e r f o r mthe electrolysis of water and the back conversion of the products to H 2 0overcoming the drawbacks discussed above, is illustrated in Figure 3. A picture of the same is shown in Figure 4 and an enlareed reoroduction of one of the two fuel cells B and C is shownin ~ i g u r 5. e In the apparatus of Figure 3, hydrogen and oxygen are
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Figure 4. Picture of the apparatus schematized in Figure 3.
P c m i
Figure 3. Electrolytic cell (A) and HZ-O2 fuel cells (B and C ) for the study of electrical-chemical energy interconversion.
0
generated via electrolysis in the cell A. The gases are then led into the two fuel cells B and C connected in series. The overall potential difference that is generated across the leads Since the ufthe twocells is utilized todri~~eanelectricclock. voltage necessary 10 drive such a clock is -I..; V (usually supplied by a dry Leclanche cell), it is necessary to utilize a pair 01' R fuel cells, since either oithem nlone, under stationary state conditiuns, supplies -1 \'. In this setup the potential difference of a given fuel cell in fact never gets higher lhan the limiting thermodynamic value, 1.23 V, and the cuntinuitv is insured bv continuous feedine" so that the basic requirements of a fuel cell are satisfied. Materials and Methods The electrodes utilized were smooth platinum in the electrolysis cell and platinized platinum in fuel cells. Platinization was carried potentiostatically a t 0.05 V versus the reversible hydrogen electrode from a 0.04 M HzPtC16/lM HCI solution (10). 1 N H2SOa was utilized as cell electrolyte throughout. The two arms of the electrolysis cell are set with the two two-way stopcocks E and F on top so that the gas initiallv nresent. as well the eases electrolvticallv eenerated. can bediiven either outside& toward the fuel i e h . The experiment is performed filling first the fuel cells B and C with the 1N H2SOasolution so that the electrodes are com~letelvimmersed. After filline reservoir D of the electro1;sis cell with the same solution, the reservoir itself is moved while acting on the two-way stopcocks so as to drive all air out of the cell and fill it completely with the solution. The stopcocks are closed, the dc supply (-6 V) is connected to the electrolysis cell electrodes, whereupon H2 and 0 2 develop. While this occurs the levels of the liquid inside the cell are lowered. In particular, the arm containing hydrogen shows a decrease twice as large, since thevolume of hydrogen is twice that of oxveen. Gas Droduction continues until the electrode, where hydrogen is developed, has no solution around it. At this point the electrolysis is automatically suspended, and the arm where oxygen develops appears half filled, as required by reaction stoichiometry. If one wants electrolysis to continue further, one simply has to open the stopcock F on the hvdrogen containing arm, let a quantity of gaiout so that the solution touches again the eleckrode, and electrolysis will resume immediately, with further production of oxygen, and lowering of the solution in its arm. The procedure can be repeated until the volume of the two gases are practically equal in the respective arms. This concludes the conversion: electrical energy chemical energy. Let us now consider the reverse transformation. One first turns stopcocks E and F so that the electrolysis cell is connected with the tubes conveying the gases to the fuel cells.
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Figure 5. Enlarged reproduction ofOne of thetwo fuelcells B and C of Figure 3. Then, with the dc supply always connected, one maneuvers sto~cocksG. H. I. and L so as to let H? and 01throueh in the fuei cells while t h e level of the s o l u t i o ~in the cell ~ l r e m a i n s practically constant. In this way the potential difference across the electrodes of the two fuel cells increases and reaches. after a few minutes. a stationarv elobal value a little over 2 V.Once this condition is attained,the terminal leads of the cells will be connected to the clock, which will be driven into motion. Under the described conditions the svstem functions continuously, perfurming, at rhe same time, the transformation of the electrical enerxy into chemical energy in the elertrolgsis cell and the reverie transformation in the fuel cells. The experiment can be completed for demonstration purposes showing the textbook properties of hydrogen and oxygen, which can at any time be collected from stopcocks E and
F. Literature Cited 1. Dawron. R. E., Ed. Reuired N u f l d d C h o m i s f v Teachers' Guide 11;Longman: London, 1978: p 315. 2. Fein3tein.H. I.; Ga1e.V. J. Chem. Educ. 1977,54432. 3. Skinncr.J.F.J Cham.Edur. 1977,54, 619. 4. Gi1bert.G. L. J. C h e m Educ. 1980.57.216. 5. Ref I, p 324. 6. Bockria, J. O'M.; Reddy, A. K. N.Modern Elecfioch~miriry; Plenummosetta: New York, L973:Vol. 2, p 1129.
7. Bockris,J. O'M.:Conway,B.E.;Yeager,E.:White R.E.,Eds. Comprehansiu~Traotisr
oiEi~ctmchemisfry;Plenum: New York, 1981: Voi. 3. p 143. S. NEMA Standard, Publication No. CV-I, National Electrical Mfga, Asroe.: New York. 1968.
9. Cameron, D. CHEMTECH 1979.9.633. 10. Woods,R.EI~ctrochim.Aclo I968.13. 1967.
