A novel photogalvanic cell using anthraquinone-2-sulfonate

Int. J. Hydrogen Energy, Vol. 8, No. 2, pp. 91-96, 1983. Printed in Great Britain.

0360-3199/83/020091~36 1;03.00/0 Pergamon Press Ltd.

t~) 1983 International Association for Hydrogen Energy.

A NOVEL PHOTOGALVANIC CELL USING ANTHRAQUINONE-2- SULFONATE

A. RoY and S. ADITYA

Department of Applied Chemistry, University College of Science and Technology, 92 Acharya Prafulla Chandra Road, Calcutta 700009, India

(Received for publication 21 June 1982)

Abstract--A photoelectrochemical cell based on the photochemistry of anthraquinone-2-sulfonate (D) has been set up. In the presence of formate at pH 11.0, D on illumination produces D'- or D 2-. In the absence of oxygen, at the platinum electrode the anodic reaction is D' ~ D + e, or D 2- ~ D + 2e, and at the dark electrode the cathodic reaction is 02 + 2H20 + 4e ~ 4OH-. The open circuit potential of the cell is 500 mV. The short circuit current is 180 ~k. The cell has been recycled at least eight times. The efficiency increases with platinized platinum electrode in the dark chamber. The steady current under illumination is 65 ~tA with the same open circuit voltage of 500 mV. The short circuit current is 250 ~tA. With a CdS electrode in the illuminated chamber the efficiency is even better. The open circuit voltage is 560 mV. After charging by illumination for 8 h a steady current of 120 ~tA can be drawn from the cell, with illumination off, for 40 h. The short circuit current is 450 p.A. The maximum power output is 4.2 x 10 -6 W. The cell can be recycled at least four times without any loss in efficiency. Grey deposition on the CdS electrode possibly indicates electrode decomposition.

INTRODUCTION

Photoelectrochemical cells can be broadly classified into two types depending on whether the photoprocess starts at the interface or in the bulk of the solution leading to the photoelectrochemical process. In the first type, absorption of light initiates a charge separation at the interface. A typical example is TiO2: electrolyte: Pt, cell [1]. In tile second type, the active species is produced in the bulk of the solution and this is associated with the Faradic process at the electrode. Iron-thionine system presents one of the most well known examples [2-6]. Thionine is photoreduced by ferrous ion in the bulk of the solution on illumination.

The overall reaction in the illuminated solution (near the anode) is

½Th + Fe 2+ h v ½Leu.Th + Fe 3÷. dark

Since the rate of the reaction ½Leu. Th ~ ½Th + e is greater than that of the reaction Fe 3÷ + e ~ Fe 2+, the illuminated electrode becomes the anode. The dark electrode becomes the cathode where the reaction Fe 3÷ + e ~ Fe 2+ takes place. Leuco-dye is produced in the bulk of the solution. Diffusion to the electrode surface and back reactions are competitive. These facts pose a great disadvantage.

Archer and Albery [7] have considered the general reactions associated with photogalvanic cells which con- sist of plane electrodes. Light enters the cell through one of the electrodes or the solution near one of the electrodes is illuminated. An electron transfer occurs:

A + hv---~ A*

A* + X --~B + Y .

A, B and X, Y are redox couples and these may react

at the electrodes thus:

A + e---~ B

Y + e---~ X.

In the dark the equilibrium

A + x . k I ' B + y kb

lies to the left. On illumination, the species Y and B, which have higher free energies, are present in excess of the equilibrium concentration and these can be con- verted back to A and X with the production of useful work. In a photogalvanic cell part of the solution containing the redox couple is illuminated and the rest is kept in dark. The electromotive force developed depends on the difference between E(1) and E(d), where E(1) and E(d) are the potentials of the couple on illumination and in the dark, respectively. The authors have discussed the theoretical efficiency in light of the electrode kinetics and homogeneous reaction kinetics. Tributsch [8] has considered the role of an added dye (sensitizer) in such cells. Recently Mountz and Tien [9] reported about a PEC cell based on the combination of two photoactive electrodes. They set up the cell, Pt : iron-thionine : TPP-coated electrode, and recorded a potential of 382 mV against 168 mV for the Pt: iron-thionine : Pt cell. In the present work we report a photoelectrochemical cell based on the photochemistry of anthraquinone-2-Na-sulfonate (D) utilizing a process working on two redox systems in the presence of a scavenger (formate or alcohol), which may be considered as a fuel.

On illumination, anthraquinone-2-Na-sulfonate in aqueous solution, in the presence of formate, is reduced to its semi-quinone radical or quinol [10-12] by abstract-

91

92 A. ROY AND S. ADITYA

ing an electron from the formate. The rate of the reaction increases with increase in the concentration of formate, as well as that of D (observations made in this laboratory in connection with the studies on the photo-chemistry of anthraquinone-2-sulfonate). The photo-reactions may be represented as [12]:

D + hu---~ D* (1)

D * + D ~ D ' + + D ' - (2)

D *+ + HCOO---~ D + HCOO" (3) OH

D + HCOO" ~ D ' - + CO2 + H20 (4)

2D ' - ~ D + D 2-. (5)

o o- o-

0 Q O_

(D) (O'-) (D2-)

D ' - or D 2- are stable in the absence of oxygen and reactions (4) and (3) do not shift backwards in the dark. D ' - or D 2- will tend to pass on to D if they can get rid of one or two electrons according to the equations D ' - - -* D + e or D2---~ D + 2e, in an anodic process resulting in the passage of an electron to the electrode. If this electron is available at the electrode dipped in the portion of the solution in the dark, there would be a cathodic reaction. The possible cathodic reaction could be D + e ~ D ' - or 02 + 2H20 + 4e--* 4OH-. Thus a photoelectrochemical cell has been constructed utilizing the photochemistry of anthraquinone-2- sulfonate.

EXPERIMENTAL

The cell used for the investigation is shown in Fig. 1. The outer vessel was made of Corning glass (ca 50 ml). Through the rubber stopper was inserted an opaque tube with its lower end open and upper end closed with a rubber stopper. This provided an inner dark chamber and an outer chamber in which the solution could be illuminated. Electrodes were placed through the stoppers. An inlet and outlet for flushing were provided by hypodermic needles so that deoxy- genation of the solution with an inert atmosphere above or a solution saturated with air/oxygen with the cor- responding atmosphere above it was available if desired. The cell was also connected with a saturated calomel reference electrode so that the e.m.f, of each electrode could be measured with reference to the SCE. In the study reported, bright platinum as well as platinized platinum electrodes was used. To improve the performance of the cell, a CdS electrode was also used. Only polycrystalline CdS electrodes were used because of the high cost of single crystals.

A 125 W Philips HPL mercury vapour lamp was used for illumination. The lamp emits mainly in the 400--440,

~ C

. - q l . - - - - - - -

Fig. 1. PEC cell. a, Dark chamber; b, illuminated chamber; c, SCE; d, air inlet; A, ammeter; V, voltmeter.

530-540 and 570-580 nm regions. The electrode in the illuminated chamber was kept parallel to the light path.

Anthraquinone-2-Na-sulfonate (BDH) was crystal- lized five times from water before use. Formic acid (BDH, A R grade) and sodium hydroxide (S.M., G.R. grade) were used.

A platinized platinum electrode was prepared in the usual way by electro-depositing black platinum on bright platinum from chloroplatinic acid solution. A polycrystalline CdS electrode was prepared by electrolysing 0.1 N sodium sulfide solution using platinum as the cathode and a cadmium electrode as the anode on which yellow cadmium sulfide was deposited. The CdS electrode was then washed and polished before use in the photoelectrochemical cell.

An aqueous solution of anthraquinone-2-sulfonate and sodium formate having a pH between 10.5 and 11.0 was placed in the cell. For developing the e.m.f. (or charging of the cell), the solution in both chambers was deoxygenated by bubbling nitrogen. Illumination was started and a hypodermic needle was inserted through the stopper in the dark chamber so that there was a supply of oxygen in the dark chamber. The solution in the illuminated chamber (initially slightly yellow col- oured) slowly turned brown-red due to the formation of D ' - and D 2- and an e.m.f, developed across the two terminals. Flushing with nitrogen gas is not essential for the development of the potential. It only reduces the initial time of formation of the active species for the development of the potential. If the solution had not been deoxygenated earlier, D ' - or D 2- formed would react with oxygen consuming all the oxygen in the illuminated chamber, although the dark chamber is in contact with air as the diffusion of oxygen is slow through the solution. The concentration of the active species rose and the potential increased reaching a constant value. The formation of D ' - and D 2- on illumination in the presence of formate at pH 10.5-11.0 was confirmed by comparing the spectra of the solution with those of D ' - and D 2- formed by the reduction of D with

A NOVEL PHOTOGALVANIC CELL USING ANTHRAQUINONE-2-SULFONATE

sodium dithionite. D ' - shows absorption maxima at 400, 500 and 775, and D 2- at 430 and 530 nm. 6 0 0

RESULTS

With the formation of D ' - and D 2- in the illuminated chamber, the electrode dipped in the illuminated chamber became more and more negative with respect to that in the dark chamber and ultimately reached a steady value of 500 mV (measured with a Philips high impedence DC microvoltmeter, Model PP9004). The potential of the electrode in the illuminated chamber was -570 mV against the SCE and that of the dark electrode was - 7 0 mV. Even after illumination was turned off the potential of the cell remained steady for at least 72 h.

Fig. 2 shows the potential-t ime curves for the PEC cell. Curve a represents the growth of the potential with time when the outer chamber was illuminated. The solution in the outer chamber was deoxygenated. The dark chamber received a supply of air through the hypodermic needle or the air inlet provided. Curve b gives the potential- t ime curve when both solutions were deoxygenated. A small potential developed initially on illumination which dropped to the original value in the dark. If air is then allowed to enter the dark chamber as indicated by X in the curve, the potential starts to increase and reaches the same value as in curve a. If the solution in the dark chamber is deoxygenated again by bubbling nitrogen or argon the potential difference falls slowly to the initial value. By letting in air, the potential can be built up again. In order to discover the effect of oxygen/air at the dark electrode in the build- up of the e.m.f, of the PEC cell, the potential of each electrode was measured against the SCE. Fig. 3 shows the potential- t ime curve for both electrodes. Curves a and b represent the potentials of the illuminated and dark electrodes respectively, against SCE when the illuminated chamber was deoxygenated and the dark one was in contact with air while a and b show the same with both chambers deoxygenated. It is seen from the

4 5 0 03

300

o

I | I I 25 50 75 I00

Time, min

600

>E 450 ~ 0 - - - - 0 - - ~ ~

3 0 0 .2

# 150,

215 I I ~ I 50 75 I00 125 150 Time, min

Fig. 2. E.m.f. of the PEC cell. a, Illuminated chamber deoxygenated, dark chamber in contact with air; b, both chambers

deoxygenated. At X air is allowed in dark chamber.

93

Fig. 3. E.m.f. vs SCE. a, b: E.m.f. of illuminated and dark electrodes, respectively, with illuminated chamber deoxygenated, dark in contact with air. a', b': With both chambers

deoxygenated.

curves that the presence or absence of air/oxygen affects the potential of the dark electrode without affecting the potential of the electrode in the illuminated chamber.

Effect of the concentration of anthraquinone-2-sulfonate and formate

Anthraquinone-2-Na-sulfonate and sodium formate do not undergo any dark reaction, but on illumination with ultra-violet or visible light, anthraquinone-2-sulfonate undergoes reduction abstracting an electron from formate according to reactions (4) and (3). The quantum yield of the process increases with increasing concentration of the anthraquinone-2-sulfonate and formate. The solubility of the former is limited. Again the quantum yield of D ' - levels off with ",he increase in the concentration of formate.

Effect of pH of the solution

Above pH 5.0, the rate of photoreduction of D is constant provided the concentration of formate is not too low. A pH higher than 11 is not desirable because at higher pH photohydroxylation of the substrate occurs [121.

In view of the above considerations, we have used the following solution composition in the PEC cell: anthraquinone-2-sulfonate = 2.5 x 10 -3 M; Na formate = 5 x 10-' M; pH of the solution = 10.5-11.0. A slight variation in the concentration of D and formate does not alter the potential ol the PEC cell. As the pH of the solution is lowered the maximum of the potential decreases. At pH 6.5, the maximum potential of the cell is ca 380 mV.

Use of methanol in place of formate

Anthraquinone-2-Na-sulfonate undergoes similar photoreduction in the presence of an alcohol [13, 14]


as it does in the presence of formate, though the rate of the reaction is slower. We set up an anthraquinone PEC cell using methanol in place of formate. With a methanol concentration in the range 30--40% v/v, the ca F o cell workedas well as the one containing formate, the open circuit voltage being 500 mV.

From the above observations it is reasonable to assume that the electroactive species in the illuminated chamber is D ' - (not any species generated from the alcohol or formate) with the electrode reaction, D'----~ D + e, and at the electrode in the dark, the reaction is 02 + 2 H20 + 4e ~ 4 OH- . The value of E ° of D ' - / D has been reported by Hayon and Rao [15] as E ° = ( -250 - 59 pH) mV. This gives a value of c a

-900 mV for the anode. For the oxygen electrode, the potential is -400 mV. These values give an expected value of c a 500 mV as found in the present cell.

The efficiency of the cell was tested by discharging the cell through a 2000 f~ resistor. The current as well as the closed circuit voltage were measured. With bright platinum electrode (1 cm2), on illumination, the current reached a steady value of 43 ~tA. After the illumination was switched off, it fell to 22 ~tA in 1 h, then to 12 ~tA over a period of 4 h and slowly to 10 ~tA over a period of 8 h. The open circuit voltage, however, remained 500 mV. The charge-discharge process was repeated eight times without any change in the characteristics. After long use (illumination of cti 30 h) formate needs replenishment as it is consumed. The keeping quality of the cell is better if the contact of the solution in the dark chamber with air is made only when the cell is being discharged and closed at other times. Stirring the solution increased the current, signifying that the electroactive species had to reach the electrode for electron transfer.

To increase the efficiency of the cell, platinized platinum electrodes were tried. Replacing the bright platinum electrode in the illuminated chamber by a platinized platinum electrode did not produce any change in the current efficiency, but a platinized platinum electrode in the dark chamber increased the current efficiency. While with bright platinum electrodes the maximum current, under illumination with a load of 2000 f~, was 43 ktA, the current with platinized platinum elec- 4.0 trode was 65 ~A. The open circuit potential, however, remained 500 mV. The short circuit current was 250 lxA. ~,

Attempts were made to improve the performance of 2 3.c the cell using a semiconductor electrode. Only a polycrystalline CdS electrode was tried in the illuminated

. 2 4 3

chamber. The open circuit potential and current yielding ~, capacity improved which can be explained as follows. ~_ CdS has got a band gap of 2.4 eV and as such shows ,-o photo-effects in the visible range. Anthraquinone-2-sulfonate (D) and its reduced product D ' - have absorp- tions in the region below 550 nm. Thus the quantum efficiency of conversion is likely to increase as CdS absorbs light in the region 570-590 nm to excite an electron in the conduction band. The hole in the valence band may react with D ' - , produced on absorption of light by D in the presence of an electron donor, to give

h~

VB \

Cd$ solution

@

Platinum

Fig. 4. Schematic representation of the cell reaction. F, formate; Fox, oxidized product of formate.

back D. The reaction may be schematically represented as in Fig. 4. The PEC cell with the CdS electrode (area 1.5 cm x 1.0 cm) in the illuminated chamber and a 1 cm 2 platinized platinum electrode in the dark chamber had an open circuit potential of 560 mV and a short circuit current of 450 ~a,. It gave a steady current of 140 ~tA with a load of 2000 f~, under illumination. When the illumination was turned off, the current fell to 120 ~tA but remained steady at this value for several hours. After charging it for 8 h, the cell, with a load of 2000 f~, yielded a current of 120-110 ~tA for over 40 h. Fig. 5 shows the power characteristics of the cell.

The maximum power output was 4.2 x 10 -6 W. The cell was recycled at least four times without any loss of efficiency.

Examination of the electrode surface, however, showed some grey deposition, possibly formed by anodic dissolution of CdS.

Similar anodic dissolution of CdS electrode has been observed previously [16] and described according to the

i I I I , I

I 00 200 3 0 0 4 0 0

Current , ~ A

Fig. 5. Power characteristics of the PEC cell with the CdS electrode in the illuminated chamber and platinized Pt in the

dark.

A NOVEL PHOTOGALVANIC CELL USING ANTHRAQUINONE-2-SULFONATE

E vs. NHE

- - I .0 Conduction band

0

2.4eV

1.0

Valence band | I

- - nEd~s.(CdS+ ZH*+ 2e- -,,-Cd + H2S)

Eredox ( S 2 - -2e ~ S )

pFdi$. (CdS+ 2h + --~Cd 2+ + S )

EredOX C [FeCcN):- = rFeCcN)6] 4-+ h+]

Eredox(D'-+h+-- , - D )

Fig. 6. Relative positions of the band edges of CdS and Fermi levels of various redox systems.

95

reaction hu

CdS ' + hwt + e~ood

2 hv+al + CdS ) Sad + Cd 2+

where h+a~, e~-and and Sad denote a hole in the valence band, an electron in the conduction band and a sulfur atom adsorbed on the electrode surface. Fig. 6 shows the typical energy correlations between band edges and Fermi energies of electrode reactions for CdS in an aqueous solution, which indicates that anodic dissolution of CdS would occur at a low illumination intensity level. This dissolution would be prevented if the electrolyte contained a redox couple whose Fermi level was above that of the anodic dissolution reaction pEdis (as occurs with Sz-/S systems [17]). Kinetic parameters, e.g. rate of hole transfer to the redox couple, are also important in determining the electrode stabilization [18]. In the present system the Fermi level of the redox couple (D/D ' - ) is -0 .75V vs NHE, thus it lies between pEdis and the valence band. Although it can transfer an electron to the valence band it cannot effectively stop the anodic dissolution reaction. Similar dissolution has also been observed in the presence of [Fe(CN)6]3-/ [Fe(CN)6] 4- [16], whose Fermi level lies above D/D' -.

An idea about the performance of the cell reported in the present work (in comparison with other photogalvanic cells reported earlier) may be obtained from Table 1.

The advantage of this PEC cell over those photogalvanic cells reported previously is that here the pho- tochemical reaction producing the electroactive species does not show any back reaction, thereby imparting the storage capability of the cell. Further, the present cell is of the type which consumes cheap fuel like formate or alcohol in the overall process. It may be considered as a fuel cell, photochemically initiated and operating at room temperature.

AcknowledgementlWe would like to express our gratitude to the Council of Scientific and Industrial Research, India, for financial assistance during the course of this work.

REFERENCES

1. A. Fujishima, K. Kohayakawa and K. Honda, Bull. chem. Sac. Japan 48, 1041 (1975).

2. E. Rabinowitch, J. chem. Phys. 8, 551 (1940). 3. C. G. Hatchard and C. A. Parker, Trans. Faraday Sac.

57, 1093 (1961). 4. P. D. Wildes, D. R. Hobart, N. N. Lichtin, D. E. Hall

and J. A. Eckert, Sol. Energy 19,567 (1977).

Table 1. Open circuit potentials and short circuit currents of some photogalvanic cells

Photogalvanic cell Ref.

Open circuit Short circuit potential current

(mV) (/~A)

Pt/Fe-thionine/Pt [19] 135 1.9 SnOz/Fe-thionine/Pt [19] 170 2.8 TiOffFe-thionine/Pt [19] 155 1.5 TPP/Fe-thionine/Pt [9] 382 - - SnOffrhodamine B-HQ/Au [20] 80 8.0 Pt/Fe-poly(N-acryl- amidomethyl)thionine/Pt [21] 50 0.1


5. N. N. Lichtin, Solar Power and Fuels (T. R. Bolton, ed.). Academic Press, New York (1977).

6. T. Sakata, Y. Suda, J. Tanaka and H. Tsubomura, J. phys. Chem. 81,537 (1977).

7. M. D. Archer and W. J. Albery, Af-finidad 34, 247 (1977). 8. H. Tributsch, Photochem. Photobiol. 16, 261 (1972). 9. J. M. Mountz and H. T. Tien, Sol. Energy 21,291 (1978).

10. K. P. Clark and H. J. Stonehill, J. Chem. Soc., Faraday Tram. 1 68, 577, 1676 (1972).

11. B. E. Hulme, E. J. Land and G. O. Phillips, J. chem. Soc., Faraday Tram. 1 68, 1992 (1972).

12. A. Roy, D. Bhattacharyya and S. Aditya, J. Indian chem. Soc. 59, 585 (1982).

13. C. F. Wells, Tram. Faraday Soc. 57, 1703, 1719 (1961).

14. C. F. Wells, J. chem. Soc. 3100 (1962). 15. E. Hayon and P. S. Rao, J. Am. chem. Soc. 96, 1287

(1974). 16. H. Gerischer, J. electroanal. Chem. 58, 263 (1975). 17. A. B. Ellis, J. M. Bolts, S. W. Kaiser and M. S. Wrighton,

J. Am. chem. Soc. 99, 2848 (1977). 18. R. Memming, J. electrochem. Soc. 125, 117 (1978). 19. Y. Suda, Y. Shimoura, T. Sakata and H. Tsubomura, J.

phys. Chem. 82, 268 (1978). 20. J. Nasielski, A. K. Mesmaeker and P. Leempol. Nouv.

J. Chim. 2,497 (1978). 21. T. Tamilarasan and P. Natarajan, Indian J. Chem. Sect.

A 20, 213 (1978).

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A novel photogalvanic cell using anthraquinone-2-sulfonate