Kamat, P. V.

Karkhanavala, M. D.

Moorthy, P. N.

English

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50INDIAN J. CHEM., VOL. lSA, APRIL 1977(JQI11\..•.EID- 30ox>~1010Fig. 2 - Thermc-osmotic permeability of solutions of sodiumchloride in water [(0) water, (e) 0'02SM, (4)) O'OSOM,(x) 0·075M and (f)) 0·100M sodium chlorideoo 0·150·05 0·'0(moleslt iter)Fig. 3 - Plots of thermo-osmotic coefficient against concen-tration of the electrolytespermeability measurement, density of the liquid onthe two sides of the membrane W2S measured aftersteady ~.tate was reached. It was found that densityremained practically constant, within experimentalerror. Density determinat icns were also madewhen the flow was allowed to take place underpressure grac:jent. Again the difference in densityon the two sides of the membrane were found to benegligible. Hence it may be concluded that tlCis negligible.From these experiments it may be concludedthat membrane does not possess semipermeability.This is due to the fact that membrane is not ofcopper feEocyanide but. of pyrex sinter impregnatedwith copper Ierrocyanide. .It is apparent from Fig. 3 that t~ermo-o~mo~lcpermeability coefficient decreases continuously withincrease in concentration of the electrolytes. Thermc-osmosis occurs when the mean free path is compar-able to pore diameter, i.e. the thermo-osmotic flow342would be more if the molecules move without under-going ~ny collision. In. the case of electrolytes,water ions are also carried away and they mustbe hydrated. When the concentraticn of the ionsis increased, the c~ances of molecular collision duringtransport would increase and hence thermo-osmoticvelocity would diminish with increase in ccncentra-tion of the electrolytes. The hydrodynamic perme-ability on the other hand would not depends~gnificantly: on. concentration of the electrolytessince the VISCOSItyof the aqueous solutions of theelectrolytes are not very much different.The author is highly thankful to Prof. R. P.Rastogi and Dr Kehar Singh of the University ofGorakhpur for their valuable suggestions and en-couragement.References1. VETO, F., Acta biochem. biophys. Acad. Hung., 1 (1966),203; 2 (1967), 95.2. VETO, F., Acta biochem., 2 (1960), 441.3. ALAXANDER, K. P. & WIRTZ, K., Z. phys. Chem., 195(1950), 165.4. HAASE, R. & STEINERT, C., Z. phys. Chem., 21 (1959),270.5. RASTOGI, R. P. & BLOKHRA, R. L., Trans. Faraday Soc.,60 (1964), 1386.6. RASTOGI, R. P. & SINGH, K, Trans. Faraday Soc., 62(1966), 1754.7. KEDEM, O. & DARLEL. M. S.- J. phys. Chern., 79 (1975),336.8. RASTOGI, R. P., SINGH, K & SHUKLA, P. C., IndianJ. Chem., 9 (1971), 1372.9. RASTOGI, R. P., SHUKLA, P. C. & YADAV, B., Biochim,biophys. A eta, 249 (1971), 454.10. CARR, C. W. & SOLLNER, K., J. electrochem, Soc., 109(1962), 616.11. KATCHALSKY, A. & CURRAN, P. F., Non-equilibriumthermodynamics in biophysics (Harvard UniversityPress, Cambridge, USA), 1967.12. RASTOGI, R. P. & SINGH, K, Trans. Faraday Soc., 63(1"967), 2917.Enhancement of the Power Output ofPhotogalvanic CellsP. V. KAMAT, M. D. KARKHANAV'ALA& P. N. MOORTHYChemistry Division, Bhabha Atomic Research CentreBombay 400085Received 6 July 1976; accepted 21 October 1976It is shown that the power output of photogafvantccells is limited by their high internal resistance whichcannot be reduced by reducing the ohmic resistancealone, e.g. by addition of inert electrolytes. To over-COInethis diff culty heterogeneous cells have been em-ployed wherein an internal bias potential is providedby an appropriate redox couple in the dark compart-ment. The net power output obtainable in such cellsis at least a factor of ten higher as compared with thatobtainable in homogeneous cells hitherto employed.A few other advantages of the heterogeneous cell overthe homogeneous one are also indicated.INVESTIGATIONS of phctcelectrc chemical pheno-menon have, in recent years, assumed greatprac~ica~ impcrt2nc.e in view of their possible~pphcatJOn .t? the direct C( nversic n of solar energyinto electricity, After the pioneering work ofBecquerrel! a big breakthrough was achieved whenRabinowitch" discovered that the photochemicalreduction of dyes of the thiazine class such as thio-nine was, under certain conditicns, fully reversibleand was accompanied by appreciable (up to 0·3 V)changes in the potential of an inert platinum elec-trode immersed in the scluticn. MOle recentlyGomer" has considered in detail the theoreticalaspects of these photogalvanic phencmena, whilePotter and Thaler" have evaluated experimentallythe power efficiencies of Ierrous-thionine photcgal-vanic cells of the type (A).In such homogeneous cells they have found thepower efficiency to be of the order of 3 X 10-4 %under the most optimum conditions. The usablepower output of such phc togalvanic cells is limitedby their high internal resistance (Ri). Petter andThaler+ have suggested that R, can be consideredto be the sum of the ohmic resistance (Ro) and acontribution (Rp) due to polarization at the phctc-electrode. The latter arises as a result of theslowness of discharge of the reduced dye speciesat the photoelectrr.de as compared to the heme-geneous reaction leading to its destructicn.We have undertaken the investigaticn of photc-galvanic phenomena in various dye redox systemswith the objective ( f improving their power outputs.The important findings are briefly reported here;full details will be published elsewhere.In homogeneous cells of type-A, Rs (as evaluatedby the procedure described by Hann et al:" has beenfound to be ,...,9 kohms, whereas the ohmic resistanceof the cell in darkness (as measured bv an a.c. ccn-ductivity bridge) was 1 kohm. It haS' been observ-ed that reduction of the latter alone by additir n ofinert electrolytes such as KK03, NaISO. or Keldid not improve the power output (Table 1), sincealthough Ro decreased in the presence of the inertelectrolytes as expected, R, and Rp showed anopposite trend, and hence a reduced power output.Pt/Fe+2, Fe+3, dye, H+(dark)H+ dye-, dye, Fe+2, Fe+3/ Pt(illuminated)(A)Pt/Fe+2, Fe+', H+(dark)Agar KCl I H+, Fe2+, Fe3+, Dye-/Ptbridge I (illuminated)(E)NOTESAn attempt was, therefore, made to overcomethe above problem by providing an internal biaspotential (dark potential). The contribution fromthis bias potential to the power cut put of the cellwas negligible. These experiments were carriedout in heterogeneous cells of the type (B). Theoptimum composition in the illuminated compart-ment was determined by separate experiment usingthe homogeneous cell (A) and was found to be:[Thionine] = 5 X 1O-5M, [Fe2+]= 5 X 1O-3MFeS04, [Fe3+]=1-5 X 1O-4M and pH = 2(H~SO.). The compositionof the dark compartment was varied as shownin Table 2.With this arrangement Rp and hence also R, werefound to decrease considerably, resulting in anincreased power output. The enhancement in thepower output was found to be dependent on theinternal bias potential, which in turn was governedby the difference in the [Fe3+J/[FeH] ratio in the twocompartments. In the above series of experiments,where the [Fe3+]/[Fe2+] ratio was maintained constantat .--0'04 in the illuminated compartment and in-creased in the dark ccrnpartment, the maximumpower output derivable was found to attain a limit-ing value of ,...,10 !lW at high [Fe3+]/[Fe2+] ratio.This limiting value was about ten times higher thanthe power output obtainable in the homogeneouscell under otherwise similar experimental cr.nditions.It thus appears that the hetercgenecus cell affordssignificantly higher power outputs; by suitablechoice of the photo active agent, the redox electrolyteTABLE 1 - CHARACTERISTICS OF HOMOGENEOUS FERROUS-THIONINE PHOTOGALVANIC CELLS[Incident light intensity (total above 400 nm) = 130 mW Iern": electrodes: bright Pt foil (each 4 cm-) : cell composition:[Thionine] = 5 X 10-'M, [Fe+2] = 5 x 10-3M, [Fe+S] =10-4M, pH = 2 (H2S04) (oxygen-free solution); open circuitcell voltage (Vo) in the dark ~ 2 mV]Inert Vo on Max. u, e;electrolyte and ilIumi- power (kohm)concn nation fLW(mV)17015016085RpNil0'25M KN03O·sM Na2S04O·sM Kel0·80·20·10·087·911·013·015·2to0·540·420·128·711·213·115·3TABLE 2 - CHARACTERISTICS OF HETEROGENEOUS FERROUS-THIONINE PHOTOGALVANIC CELLS~Inci~ent light intensity (total above 400 nm) = 130 mW Icm2; electrodes: bright Pt foil (each 4 cm'); composition ofrlfuminated compartment: [Thionine] = 5 x lO-'M, [Fe+2] = 5 x 10-3M, [Fe+3] = 10-4.11, pH = 2 (H2SO.) oxygen-freesolution)]~V(mV)Dark cornpt composition(PH 2; H2S04)Vodark(mV)Vo onill umination(mV)[Fe2+](1O-4M)50SOSOSO1002051[Fe3+](10-4M)11025010001000100010001000*Max.powerfLWRpRo(kohm)5 180 175 1·3 6·1 1-1 5·066 242 176 5·1 3·0 1·0 2·0108 286 178 6·7 2·6 0·7 1·9132 306 174 8·8 2·5 0·7 t·8112 290 178 7·6 2·6 0·5 H150 318 168 9·4 2·7 0·7 2·0182 342 160 9·5 2·7 0·7 2·0208 370 162 10·9 3·0 0·7 2·3*Corrected for contribution from dark potential.343INDIAN J. CHEM., VOL. 15A, APRIL 1977and the bridge separating the two compartmentsfurther improvements should be possible. Theother advantages of the heterogeneous cells are:(i) the dark compartment need only be formallydark; it need not be protected from light sincethere is no photoactive material therein; (ii) whereasthe homogeneous cell does not permit use of higher[Fe2+J/[Fe2+J ratios than the one chosen in the aboveexperiments (Table 1), as at higher Fe2+ concentra-tion the open circuit cell voltage itself decreases'',there is no limitation on the composition of thedark compartment; and (iii) with the heterogeneouscell it should also be possible to store the light energyin the form of chemical free energy and then releaseit by discharging through an external load.We are grateful to Dr K. N. Rao of this Divisionfor helpful discussion and comments.References1. BECQUEREL,E., C.r. htbd. Acad, Sci. Paris, 9 (1839), 561.2. RABINOWITCH,E., J. chem. Phys., 8 (1940), 551, 560.3. GOMER,R., Etectrochim. Acta, 20 (1975), 13.4. POTTER,A. E. & THALER,L. H., Solar Energy, 3 (1957), 1.5. HANN, R. A., READ, C., ROSSEINSKY,D. R. & WASSEL,P., Nature Lond., 244 (1973). 126.Effect of Temperature & Concentration onDissolution Potentials of Sodium Chloride,Bromide & IodideH. L. GIRDHAR& R. P. MATTADepartment of Chemistry, Kashmir University, Srinagar 6Received 17 April 1976; accepted 3 December 1976Dissolution potential and diffusion potential ofNaCI, NaBr and NaI have been measured at different.temperatures and at different concentrations at 25°.True dissolution potential decreases and diffusionpotential increases linearly with increasing tem-perature. The concentration dependence of diffusionpotential has been discussed on the basis of Planckequation and an empirical relation for the variation oftrue dissolution potential with concentration is given.DESPITE a good amount of experimental dataon. the. dissolution potential of electrolytes=s,there IS still no generalized mechanism for theorigin of such an effect because of the complexity ofthe phenomenon. Recentlt,5,6 the sign and magni-tude of dissolution potential have been explainedon the basis of the formation of electrical doublelayer at the solid/liquid interface. The dissolutionpotential has also been found":" to be influencedby the temperature of solvent and the concentrationof the dissolved electrolyte. The purpose of thepresent study is to explain quantitatively the tem-perature and concentratic n dependence of truedissclutic n potential and diffusicn potential ofsodium halides.Analytical reagent grade chemicals were used.All the measurements were made in deionized waterprepared by using Elgastat BIOS deionizer. Asreported earlier", the main contributions towardsthe observed dissolution potential are (i) true dis-solution potential and (ii) diffusion potential thatdevelops due to the formation of concentrationgradient when electrode loaded with wet crystalsis dipped into the cell. In order to find out thecontribution of the diffusion potential to the over-all observed dissolution potential it was alsomeasured separately. The details of the experi-mental technique for measuring the observed dis-solution potential and the diffusion potential havealready been describeds, The potentials were meas-ured within an accuracy of ± 2 mV with respectto the clean platinum electrode dipping in the cell.The desired temperature was controlled with anaccuracy of ± 0.10 The concentration dependencewas studied at 250 ± 0.10 by measuring the potentialin electrolytic solutions of different concentrations.The potentials measured at different concentra-tions are given in Table 1 and their variation withtemperature is shown in Figs. 1 and 2.For the same electrolyte solutions at two differentconcentrations (Cl and C2) the diffusion potentialis given by Planck equation".Diff. pot. = R'!.((U+-U_) In C1 ... (1)F u++u_ C2where U+ and u: are the ionic mobilities. At aparticular temperature, the concentration C1 ofthe saturated diffusing solution is constant whichis the condition of our experimental measurementof diffusion potential, Eq. (1) reduces toDiff. pot. = A-B In C2 ••• (2)TABLE1 - EFFECTOF CONCENTRATIONON DISSOLUTIONPOTENTIALAT 25°CSodium chloride Sodium bromide Sodium iodideObs. Diff. True Obs. Diff. True Obs. Diff. Truedissoln pot. dissoln dissoln pot. dissoln dissoln pot. dissolnpot. (mV) pot. pot. (mV) pot. pot. (mV) pot.(mV) (mV) (mV) (mV) (mV) (mV)-120 -40 -80 -238 -70 -168 -380 -112 -268-110 -38 -72 -230 -68 -162 -268 -100 -168-92 -36 -56 -214 -66 -148 -250 -90 -160-82 -34 -48 -202 -64 -138 -236 -86 -150-74 -30 -44 -194 -60 -134 -232 -72 -160-64 -22 -42 -192 -52 -140 -226 -60 -166-60 -20 -40 -180 -40 -140 -220 -54 -166-50 -14 -36 -170 -36 -134 -210 -44 -166Concentration(g eq/lit)0·00000·00010·0010·0050·010·050·10·2344

Enhancement of the Power Output of Photogalvanic Cells

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Enhancement of the Power Output of Photogalvanic Cells

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