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Photogalvanic conversion of solar energy into electrical energy by using NaLS–xylose–methylene blue system
Electrical Power and Energy Systems 33 (2011) 155–158
Contents lists available at ScienceDirect
Electrical Power and Energy Systems journal homepage: www.elsevier.com/locate/ijepes
Photogalvanic conversion of solar energy into electrical energy by using NaLS–xylose–methylene blue system Krishna Kanwar Bhati ⇑, K.M. Gangotri JNV University, Jodhpur, Mohan B 619, BJS Colony, Jodhpur, India
a r t i c l e
i n f o
Article history: Received 13 April 2008 Received in revised form 29 June 2010 Accepted 2 August 2010
Keywords: NaLs Photopotential Photocurrent Storage capacity Conversion efficiency Fill factor
a b s t r a c t Photogalvanic cells having different surfactants, reductants and photosensitizers have been tried to get the better electrical output and storage capacity. Through literature survey shows that system having NaLs as a surfactant, xylose as a reductant and methylene blue as a photosensitizer has not been explored to get the required results and achievements so the efforts have been made by the system in photogalvanic cell to get better electrical output (i.e. photopotential 834 mV, photocurrent 90 lA, power and power at power point are 75.06 lW and 32.72 lW) and also good storage capacity i.e. 55 min in dark. The observed conversion efficiency and fill factor for this is 0.31%, 0.363%. The effect of different parameter like pH, diffusion length on electrical output of the cell was also studied and tentative mechanism for the generation of photocurrent was also purposed. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction The rapid usage of fossil fuels cannot continue indefinitely earth’s finite supply exhausts, we are therefore confirmed with the problem of planning our energy strategies and solar energy due to its abundance in availability, essentially non-polluting, hazard free nature it stand out as the brightest long range promises towards meeting the continually increasing demand of energy. Solar energy can be converted into many forms i.e. electrical energy, hydraulic potential energy, in the form of chemical bond etc. Solar cells convert solar energy directly into electrical energy. Photogalvanic solar cells are based on such photochemical reaction, which give rise to high energy products on excitation by a photon. These energy rich products loose energy electrochemically which lead to generation of electricity. The photogalvanic cells working on ‘‘photogalvanic effect” which was observed by Robinowitch [1]. Gangotri and Meena [2] have reported use of reductant and photo sensitizer in photogalvanic cell where as Gangotri et al. [3] has reported the effect of micelles on the performance and conversion efficiency of photogalvanic cells. Gangotri and Jagrti [4] has reported use of surfactant in photogalvanic cell for better conversion and good storage capacity, Gangotri and Pramila [5] reported uses of micelles in photogalvan-
⇑ Corresponding author. Tel.: +91 02912538578. E-mail address: [email protected] (K.K. Bhati). 0142-0615/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijepes.2010.08.001
ic cells whereas Genwa and Genwa [6] reported the comparative study of using different surfactants in photo galvanic cell.
2. Experimental 2.1. Structure of the compounds used (A) Methylene blue
(B) D(+) xylose
(C) Dodecyl sulfate, sodium salt (sodium lauryl sulfate)
CH3 ðCH2 Þ11 OSO3 Naþ
156
K.K. Bhati, K.M. Gangotri / Electrical Power and Energy Systems 33 (2011) 155–158
(D) Tween-20 (polyoxy ethylene sorbitan monolaurate)
C58 H114 O26 (E) CTAB (cetyl trimethyl ammonium bromide)
lamps of different wattage varies the Light intensity. A water filter was placed between the illuminated chamber and the light source to cutoff infrared radiations. Fig. 1 represent set-up of the photogalvanic cell.
CH3 ðCH2 Þ15 Nþ ðCH3 Þ3 Br 3. Result and discussion
(F) Oxalic acid
3.1. Effect of variation of anionic surfactant (NaLs) concentration Surfactant play vital role to solubilize the system by forming high molecular weight aggregates or micelles in dilute solution. Surfactant also enhanced the electrical output of the photogalvanic cell by making process of photo ejection of electron easier if the dye and surfactant are opposite in charge, the photo ejection of electron will be more pronounced and hence the efficiency of photo galvanic cell will increases. It was observed that electrical output of the cell was found to increase on increasing the concentration of anionic surfactant (NaLS) reaching a maximum value. On further increase in their concentration, a fall in photo potential, photocurrent and power of photogalvanic cell was obtained. The effect of variation of surfactant (NaLS) concentration on photo potential and photocurrent of xylose–methylene blue–NaLS system is reported in Table 1.
(G) Phenolphthalein
(H) Sodium hydroxide
NaOH 3.2. Effect of variation of dye (methylene blue) concentration 2.2. Experimental set-up of the photogalvanic cell A H-shaped glass tube was used which consist of known amount of the solutions of dye (photo sensitizer), reductant, surfactant and sodium hydroxide so as to keep the total volume of the mixture always 25.0 ml. Doubly distilled water was used to make up the solutions to desired volume. A platinum electrode (1.0 1.0 cm2) is dipped in one limb and a Saturated Calomel Electrode (SCE) was immersed in the another limb of the H-tube. The terminals of the electrode were connected to a digital pH meter (Systronics Model–335) and the whole cell was placed in the dark. The potential (mV) was measured in dark when the cell attains a stable potential. Then the limb containing platinum electrode was exposed to a 200 W tungsten lamp (Sylvania). Employing
It was observed that the photo potential and photocurrent were increased with the increase in concentration of the dye (methylene blue). A maxima was obtained for a particular value of methylene
Table 1 Effect of variation of surfactant (NaLS) concentration: (MB) = 3.60 105 M, light intensity = 10.4 mW/cm2, (xylose) = 2.00 103 M, temperature = 303 K, pH = 12.71. Xylose–methylene blue– NaLS system
(NaLS) 104 M 4.8
5.2
6.0
6.8
7.6
Photopotential (mV) Photocurrent (lA) Power (lW)
610.0 62.0 37.82
735.0 69.0 50.71
834.0 90.0 75.06
750.0 68.0 51.00
594.0 57.0 33.80
Fig. 1. Photogalvanic cell.
157
K.K. Bhati, K.M. Gangotri / Electrical Power and Energy Systems 33 (2011) 155–158
blue concentration, above which a decrease in the electrical output of the cell was obtained. The effect of variation of methylene blue concentration on photo potential and photocurrent of xylose–methylene blue–NaLS system is reported in Table 2. It was felt necessary to keep the concentration of methylene blue 105 M. At lower concentration of methylene blue, small number of dye molecules were available in the system for excitation and consecutive electron transfer whereas higher concentration of methylene blue also resulted in the lowering of photo potential, photocurrent because of the fact that the major portion of the light was absorbed by the dye molecules in the path causing a decrease in the intensity of light reaching the dye molecule near the electrodes. 3.3. Effect of variation of reductant (xylose) concentration With the increase in concentration of the reductant (xylose) the photo potential was found to be increase till it reaches maxima. On further increase in concentration of reductant (xylose) a decrease in the electrical output of the cell was observed. The effect of variation of the xylose concentration on the photo potential and photocurrent of xylose–methylene blue–NaLS system is reported in Table 3. It was observed that the fall in the concentration of reductant also resulted in fall in power output due to less number of molecules available for electron donation to dye molecules. On the other hand, large concentration of reducing agent again resulted into a decrease in electrical output, because the large number of reducing agent molecules hindered the dye molecules reaching the electrode in the desired time limit. 3.4. i–V Characteristics of the cell A digital pH meter (keeping the other circuit open) was used to measure the open circuit voltage (Voc) whereas short circuit current (isc) was measured by micrometer (keeping the other circuit closed). The electrical parameters in between these two extreme values (Voc and isc) were determined with the help of a carbon pot (log 470 K) connected in the circuit of micrometer, through which an external load was applied. It was observed in this system, i–V curve deviated from their expected regular rectangular shapes. The power point (A point on the
curve where the product of potential and current was maximum) in these i–V curves was determined and their fill factor were also calculated. These data are summarized in Table 4. 3.5. Effect of variation of pH Photogalvanic cell containing xylose–methylene blue–NaLS system was found to be quite sensitive to the pH of the solution. It was observed that there was an increase in the photo potential of this system with the increase in pH value (in the alkaline range). At pH = 12.71 a maxima was obtained. On further increase in pH, there was a decrease in photo potential. The effect of variation of pH on photo potential and photocurrent is reported in Table 5. It was quite interesting to observed that pH for optimum condition for reductant has a relation with its pKa value i.e. the desire pH should be slightly higher than their pKa values (pH > pKa).The reason may be the availability of reductant in its anionic form, which is better donor form. 3.6. Effect of diffusion length Effect of variation of diffusion length on the electrical output (imax, ieq) and initial rate of generation of current of the photogalvanic cells were observed by using the H-cell of different dimensions. The diffusion length (distance between the electrodes) greatly affects this system. It was observed that in the first few minutes of illumination there was a sharp increase in the photocurrent and then there was a gradual decrease to a stable value of photocurrent. This behavior of photocurrent indicates an initial rapid reaction followed by a slow rate determining step. The results were also discussed to know about electroactive species by considering various probable processes and Combination for electroactive species for the electrical output of the photogalvanic cells. These Processes and combinations are summarized in Table 6. If the oxidized form of the reductant is formed only in the illuminated chamber and it is considered to be the electroactive species in the dark chamber, then it must diffuse from the
Table 4 i–V characteristics of the cell. Systems
Voc (mV)
isc (lA)
Vpp (mV)
ipp (lA)
g
MB–xylose–NaLS
1001
90.0
595.0
45.0
0.363
Table 2 Effect of variation of dye (methylene blue) concentration: (NaLS) = 6.00 104 M, light intensity = 10.4 mW/cm2, (xylose) = 2.00 103 M, temperature = 303 K, pH = 12.71. Xylose–methylene blue–NaLS system
(Methylene blue) 105 M 3.44
3.52
3.60
3.76
3.84
Photopotential (mV) Photocurrent (lA) Power (lW)
695.0 57.0 39.61
780.0 75.0 58.50
834.0 90.0 75.06
720.0 71.0 51.12
615.0 60.0 36.90
Table 3 Effect of variation of reductant (xylose) concentration: (NaLS) = 6.00 104 M, light intensity = 10.4 mW/cm2, (MB) = 3.60 105 M, temperature = 303 K, pH = 12.71.
Table 5 Effect of variation of pH. (NaLS) = 6.00 104 M, light intensity = 10.4 mW/cm2, (MB) = 3.60 105 M, temperature = 303 K, (xylose) = 2.00 103 M. Xylose–methylene blue–NaLS system
pH 12.64
12.68
12.71
12.74
12.77
Photopotential (mV) Photocurrent (lA) Power (lW)
675.0 57.0 38.81
765.0 66.0 50.49
834.0 90.0 75.06
730.0 68.0 49.64
645.0 54.5 35.15
Table 6 Probable process and combinations for electroactive species.
Xylose–methylene blue–NaLS system
(Xylose) 103 M 1.92
1.96
2.0
2.08
2.12
In illuminated chamber
In dark chamber
Photopotential (mV) Photocurrent (lA) Power (lW)
660.0 55.0 36.30
770.0 68.0 52.36
834.0 90.0 75.06
680.0 65.0 44.20
605.0 53.0 30.25
Dye Semi or leuco form of the dye Semi or leuco form of the dye
Oxidized form of the reductant (R+) Oxidized form of the reductant (R+) Dye
158
K.K. Bhati, K.M. Gangotri / Electrical Power and Energy Systems 33 (2011) 155–158
illuminated chamber to the dark chamber to accept an electron from the electrode. As a consequence, the maximum photocurrent (imax) and rate of generation of photocurrent should decrease with an increase in diffusion length, but it is not observed experimentally. The value of (ieq) is also observed to be independent with respect to change in diffusion length (rather it decreases slightly). Therefore, it may be concluded that the main electroactive species are the leuco or semi leuco form of dye (photosensitizer) and the dye in the illuminated and the dark chamber, respectively. However, the reductant and its oxidized product act only as electron carriers in the path. 3.7. Performance of the cell The performance of the photogalvanic cell was measured in terms of t1/2 and was studied by applying the desired external load to have the potential and current corresponding to power point after removing the source of illumination. The time t1/2 was determined after removing the source of light. t1/2 is the time taken in reaching half the value of power point (32.72 lW) in dark. The performance of the cell in dark was found to be 55.0 min for this system. 3.8. Conversion efficiency of the cell With the help of current and potential values at Power point (pp) and the incident power of radiations, the conversion efficiency of the cell was determined as 0.31% in the xylose–methylene blue– NaLS system. 4. Mechanism The redox potential of the reductant is much higher than the photo sensitizer (dye) used in present work and hence these do not react in dark. On illumination at the platinum electrode, there was a rapid fall in potential and after some time, a constant value was obtained. On removing the source of light, the change in potential was reversed but it never reaches the initial value. It suggests that main reversible photochemical reaction is also accompanied by some irreversible side reactions. In this system having NaLS–xylose–MB photogalvanic systems, electroactive species are the leuco dye and dye, itself at the illuminated and dark electrodes, respectively. According to observed results, the most probable rate determining process for (ieq) should be the recycling reaction of oxidation product of the reducing agent and the semi or leuco dye (photosensitizer). On the basis of these observations, a mechanism is suggested for the generation of photocurrent in the photogalvanic cell as:
4.1. Illuminated chamber On irradiation, dye molecules get excited. hm
Dye ! Dye The excited dye molecules accept an electron from reductant and converted into semi or leuco form of dye, and the reductant into its excited form
Dye þ R ! Dye ðsemi or leucoÞ þ Rþ 4.2. At platinum electrode The semi or leuco form of dye loses an electron and converted into original dye molecule
Dye ! Dye þ e 4.3. Dark chamber At counter electrode: Dye molecules accept an electron from electrode and converted in semi or leuco form
Dye þ e ! Dye ðsemi or leucoÞ Finally leuco/semi form of dye and oxidized form of reductant combine to give original dye and reductant molecule and the cycle will go on
Dye þ Rþ ! Dye þ R where Dye, Dye*, Dye, R and R+ are the dye, excited form of dye, semi or leuco form of dye, reductant and oxidized form of the reductant, respectively. References [1] Robinowitch E. XLIII – the action of light on the ferrous ferric iodine iodide equilibrium. J Chem Soc Trans 1925;127:258–69. [2] Gangotri KM, Meena RC. Use of reductant and photosensitizer in photogalvanic cells for solar energy conversion and storage: oxalic acid–methylene blue system. J Photochem Photobiol A: Chem 2001;141(1):175. [3] Gangotri KM, Meena RC, Rajni Meena. Use of micelles in photogalvanic cells for solar energy conversion and storage: cetyl trimethyl ammonium bromide– glucose–toluidine blue system. J Photochem Photobiol A: Chem 1999;123:93–7. [4] Gangotri KM, Jagrati Meena. Role of surfactants in photo galvanic cells for solar energy conversion and storage. Energy Sources 2006;28:771–7. [5] Gangotri KM, Pramila S. Use of anionic micelles in photogalvanic cells for solar energy conversion and storage NaLS–mannitol–safranine-system. Energy Sources 2006;28:149–56. [6] Genwa KR, Genwa Mahaveer. Photogalvanic cell: a new approach for green and sustainable chemistry. Solar Energy Mater Solar Cells 2008;92(May):522–9.
Contents lists available at ScienceDirect
Electrical Power and Energy Systems journal homepage: www.elsevier.com/locate/ijepes
Photogalvanic conversion of solar energy into electrical energy by using NaLS–xylose–methylene blue system Krishna Kanwar Bhati ⇑, K.M. Gangotri JNV University, Jodhpur, Mohan B 619, BJS Colony, Jodhpur, India
a r t i c l e
i n f o
Article history: Received 13 April 2008 Received in revised form 29 June 2010 Accepted 2 August 2010
Keywords: NaLs Photopotential Photocurrent Storage capacity Conversion efficiency Fill factor
a b s t r a c t Photogalvanic cells having different surfactants, reductants and photosensitizers have been tried to get the better electrical output and storage capacity. Through literature survey shows that system having NaLs as a surfactant, xylose as a reductant and methylene blue as a photosensitizer has not been explored to get the required results and achievements so the efforts have been made by the system in photogalvanic cell to get better electrical output (i.e. photopotential 834 mV, photocurrent 90 lA, power and power at power point are 75.06 lW and 32.72 lW) and also good storage capacity i.e. 55 min in dark. The observed conversion efficiency and fill factor for this is 0.31%, 0.363%. The effect of different parameter like pH, diffusion length on electrical output of the cell was also studied and tentative mechanism for the generation of photocurrent was also purposed. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction The rapid usage of fossil fuels cannot continue indefinitely earth’s finite supply exhausts, we are therefore confirmed with the problem of planning our energy strategies and solar energy due to its abundance in availability, essentially non-polluting, hazard free nature it stand out as the brightest long range promises towards meeting the continually increasing demand of energy. Solar energy can be converted into many forms i.e. electrical energy, hydraulic potential energy, in the form of chemical bond etc. Solar cells convert solar energy directly into electrical energy. Photogalvanic solar cells are based on such photochemical reaction, which give rise to high energy products on excitation by a photon. These energy rich products loose energy electrochemically which lead to generation of electricity. The photogalvanic cells working on ‘‘photogalvanic effect” which was observed by Robinowitch [1]. Gangotri and Meena [2] have reported use of reductant and photo sensitizer in photogalvanic cell where as Gangotri et al. [3] has reported the effect of micelles on the performance and conversion efficiency of photogalvanic cells. Gangotri and Jagrti [4] has reported use of surfactant in photogalvanic cell for better conversion and good storage capacity, Gangotri and Pramila [5] reported uses of micelles in photogalvan-
⇑ Corresponding author. Tel.: +91 02912538578. E-mail address: [email protected] (K.K. Bhati). 0142-0615/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijepes.2010.08.001
ic cells whereas Genwa and Genwa [6] reported the comparative study of using different surfactants in photo galvanic cell.
2. Experimental 2.1. Structure of the compounds used (A) Methylene blue
(B) D(+) xylose
(C) Dodecyl sulfate, sodium salt (sodium lauryl sulfate)
CH3 ðCH2 Þ11 OSO3 Naþ
156
K.K. Bhati, K.M. Gangotri / Electrical Power and Energy Systems 33 (2011) 155–158
(D) Tween-20 (polyoxy ethylene sorbitan monolaurate)
C58 H114 O26 (E) CTAB (cetyl trimethyl ammonium bromide)
lamps of different wattage varies the Light intensity. A water filter was placed between the illuminated chamber and the light source to cutoff infrared radiations. Fig. 1 represent set-up of the photogalvanic cell.
CH3 ðCH2 Þ15 Nþ ðCH3 Þ3 Br 3. Result and discussion
(F) Oxalic acid
3.1. Effect of variation of anionic surfactant (NaLs) concentration Surfactant play vital role to solubilize the system by forming high molecular weight aggregates or micelles in dilute solution. Surfactant also enhanced the electrical output of the photogalvanic cell by making process of photo ejection of electron easier if the dye and surfactant are opposite in charge, the photo ejection of electron will be more pronounced and hence the efficiency of photo galvanic cell will increases. It was observed that electrical output of the cell was found to increase on increasing the concentration of anionic surfactant (NaLS) reaching a maximum value. On further increase in their concentration, a fall in photo potential, photocurrent and power of photogalvanic cell was obtained. The effect of variation of surfactant (NaLS) concentration on photo potential and photocurrent of xylose–methylene blue–NaLS system is reported in Table 1.
(G) Phenolphthalein
(H) Sodium hydroxide
NaOH 3.2. Effect of variation of dye (methylene blue) concentration 2.2. Experimental set-up of the photogalvanic cell A H-shaped glass tube was used which consist of known amount of the solutions of dye (photo sensitizer), reductant, surfactant and sodium hydroxide so as to keep the total volume of the mixture always 25.0 ml. Doubly distilled water was used to make up the solutions to desired volume. A platinum electrode (1.0 1.0 cm2) is dipped in one limb and a Saturated Calomel Electrode (SCE) was immersed in the another limb of the H-tube. The terminals of the electrode were connected to a digital pH meter (Systronics Model–335) and the whole cell was placed in the dark. The potential (mV) was measured in dark when the cell attains a stable potential. Then the limb containing platinum electrode was exposed to a 200 W tungsten lamp (Sylvania). Employing
It was observed that the photo potential and photocurrent were increased with the increase in concentration of the dye (methylene blue). A maxima was obtained for a particular value of methylene
Table 1 Effect of variation of surfactant (NaLS) concentration: (MB) = 3.60 105 M, light intensity = 10.4 mW/cm2, (xylose) = 2.00 103 M, temperature = 303 K, pH = 12.71. Xylose–methylene blue– NaLS system
(NaLS) 104 M 4.8
5.2
6.0
6.8
7.6
Photopotential (mV) Photocurrent (lA) Power (lW)
610.0 62.0 37.82
735.0 69.0 50.71
834.0 90.0 75.06
750.0 68.0 51.00
594.0 57.0 33.80
Fig. 1. Photogalvanic cell.
157
K.K. Bhati, K.M. Gangotri / Electrical Power and Energy Systems 33 (2011) 155–158
blue concentration, above which a decrease in the electrical output of the cell was obtained. The effect of variation of methylene blue concentration on photo potential and photocurrent of xylose–methylene blue–NaLS system is reported in Table 2. It was felt necessary to keep the concentration of methylene blue 105 M. At lower concentration of methylene blue, small number of dye molecules were available in the system for excitation and consecutive electron transfer whereas higher concentration of methylene blue also resulted in the lowering of photo potential, photocurrent because of the fact that the major portion of the light was absorbed by the dye molecules in the path causing a decrease in the intensity of light reaching the dye molecule near the electrodes. 3.3. Effect of variation of reductant (xylose) concentration With the increase in concentration of the reductant (xylose) the photo potential was found to be increase till it reaches maxima. On further increase in concentration of reductant (xylose) a decrease in the electrical output of the cell was observed. The effect of variation of the xylose concentration on the photo potential and photocurrent of xylose–methylene blue–NaLS system is reported in Table 3. It was observed that the fall in the concentration of reductant also resulted in fall in power output due to less number of molecules available for electron donation to dye molecules. On the other hand, large concentration of reducing agent again resulted into a decrease in electrical output, because the large number of reducing agent molecules hindered the dye molecules reaching the electrode in the desired time limit. 3.4. i–V Characteristics of the cell A digital pH meter (keeping the other circuit open) was used to measure the open circuit voltage (Voc) whereas short circuit current (isc) was measured by micrometer (keeping the other circuit closed). The electrical parameters in between these two extreme values (Voc and isc) were determined with the help of a carbon pot (log 470 K) connected in the circuit of micrometer, through which an external load was applied. It was observed in this system, i–V curve deviated from their expected regular rectangular shapes. The power point (A point on the
curve where the product of potential and current was maximum) in these i–V curves was determined and their fill factor were also calculated. These data are summarized in Table 4. 3.5. Effect of variation of pH Photogalvanic cell containing xylose–methylene blue–NaLS system was found to be quite sensitive to the pH of the solution. It was observed that there was an increase in the photo potential of this system with the increase in pH value (in the alkaline range). At pH = 12.71 a maxima was obtained. On further increase in pH, there was a decrease in photo potential. The effect of variation of pH on photo potential and photocurrent is reported in Table 5. It was quite interesting to observed that pH for optimum condition for reductant has a relation with its pKa value i.e. the desire pH should be slightly higher than their pKa values (pH > pKa).The reason may be the availability of reductant in its anionic form, which is better donor form. 3.6. Effect of diffusion length Effect of variation of diffusion length on the electrical output (imax, ieq) and initial rate of generation of current of the photogalvanic cells were observed by using the H-cell of different dimensions. The diffusion length (distance between the electrodes) greatly affects this system. It was observed that in the first few minutes of illumination there was a sharp increase in the photocurrent and then there was a gradual decrease to a stable value of photocurrent. This behavior of photocurrent indicates an initial rapid reaction followed by a slow rate determining step. The results were also discussed to know about electroactive species by considering various probable processes and Combination for electroactive species for the electrical output of the photogalvanic cells. These Processes and combinations are summarized in Table 6. If the oxidized form of the reductant is formed only in the illuminated chamber and it is considered to be the electroactive species in the dark chamber, then it must diffuse from the
Table 4 i–V characteristics of the cell. Systems
Voc (mV)
isc (lA)
Vpp (mV)
ipp (lA)
g
MB–xylose–NaLS
1001
90.0
595.0
45.0
0.363
Table 2 Effect of variation of dye (methylene blue) concentration: (NaLS) = 6.00 104 M, light intensity = 10.4 mW/cm2, (xylose) = 2.00 103 M, temperature = 303 K, pH = 12.71. Xylose–methylene blue–NaLS system
(Methylene blue) 105 M 3.44
3.52
3.60
3.76
3.84
Photopotential (mV) Photocurrent (lA) Power (lW)
695.0 57.0 39.61
780.0 75.0 58.50
834.0 90.0 75.06
720.0 71.0 51.12
615.0 60.0 36.90
Table 3 Effect of variation of reductant (xylose) concentration: (NaLS) = 6.00 104 M, light intensity = 10.4 mW/cm2, (MB) = 3.60 105 M, temperature = 303 K, pH = 12.71.
Table 5 Effect of variation of pH. (NaLS) = 6.00 104 M, light intensity = 10.4 mW/cm2, (MB) = 3.60 105 M, temperature = 303 K, (xylose) = 2.00 103 M. Xylose–methylene blue–NaLS system
pH 12.64
12.68
12.71
12.74
12.77
Photopotential (mV) Photocurrent (lA) Power (lW)
675.0 57.0 38.81
765.0 66.0 50.49
834.0 90.0 75.06
730.0 68.0 49.64
645.0 54.5 35.15
Table 6 Probable process and combinations for electroactive species.
Xylose–methylene blue–NaLS system
(Xylose) 103 M 1.92
1.96
2.0
2.08
2.12
In illuminated chamber
In dark chamber
Photopotential (mV) Photocurrent (lA) Power (lW)
660.0 55.0 36.30
770.0 68.0 52.36
834.0 90.0 75.06
680.0 65.0 44.20
605.0 53.0 30.25
Dye Semi or leuco form of the dye Semi or leuco form of the dye
Oxidized form of the reductant (R+) Oxidized form of the reductant (R+) Dye
158
K.K. Bhati, K.M. Gangotri / Electrical Power and Energy Systems 33 (2011) 155–158
illuminated chamber to the dark chamber to accept an electron from the electrode. As a consequence, the maximum photocurrent (imax) and rate of generation of photocurrent should decrease with an increase in diffusion length, but it is not observed experimentally. The value of (ieq) is also observed to be independent with respect to change in diffusion length (rather it decreases slightly). Therefore, it may be concluded that the main electroactive species are the leuco or semi leuco form of dye (photosensitizer) and the dye in the illuminated and the dark chamber, respectively. However, the reductant and its oxidized product act only as electron carriers in the path. 3.7. Performance of the cell The performance of the photogalvanic cell was measured in terms of t1/2 and was studied by applying the desired external load to have the potential and current corresponding to power point after removing the source of illumination. The time t1/2 was determined after removing the source of light. t1/2 is the time taken in reaching half the value of power point (32.72 lW) in dark. The performance of the cell in dark was found to be 55.0 min for this system. 3.8. Conversion efficiency of the cell With the help of current and potential values at Power point (pp) and the incident power of radiations, the conversion efficiency of the cell was determined as 0.31% in the xylose–methylene blue– NaLS system. 4. Mechanism The redox potential of the reductant is much higher than the photo sensitizer (dye) used in present work and hence these do not react in dark. On illumination at the platinum electrode, there was a rapid fall in potential and after some time, a constant value was obtained. On removing the source of light, the change in potential was reversed but it never reaches the initial value. It suggests that main reversible photochemical reaction is also accompanied by some irreversible side reactions. In this system having NaLS–xylose–MB photogalvanic systems, electroactive species are the leuco dye and dye, itself at the illuminated and dark electrodes, respectively. According to observed results, the most probable rate determining process for (ieq) should be the recycling reaction of oxidation product of the reducing agent and the semi or leuco dye (photosensitizer). On the basis of these observations, a mechanism is suggested for the generation of photocurrent in the photogalvanic cell as:
4.1. Illuminated chamber On irradiation, dye molecules get excited. hm
Dye ! Dye The excited dye molecules accept an electron from reductant and converted into semi or leuco form of dye, and the reductant into its excited form
Dye þ R ! Dye ðsemi or leucoÞ þ Rþ 4.2. At platinum electrode The semi or leuco form of dye loses an electron and converted into original dye molecule
Dye ! Dye þ e 4.3. Dark chamber At counter electrode: Dye molecules accept an electron from electrode and converted in semi or leuco form
Dye þ e ! Dye ðsemi or leucoÞ Finally leuco/semi form of dye and oxidized form of reductant combine to give original dye and reductant molecule and the cycle will go on
Dye þ Rþ ! Dye þ R where Dye, Dye*, Dye, R and R+ are the dye, excited form of dye, semi or leuco form of dye, reductant and oxidized form of the reductant, respectively. References [1] Robinowitch E. XLIII – the action of light on the ferrous ferric iodine iodide equilibrium. J Chem Soc Trans 1925;127:258–69. [2] Gangotri KM, Meena RC. Use of reductant and photosensitizer in photogalvanic cells for solar energy conversion and storage: oxalic acid–methylene blue system. J Photochem Photobiol A: Chem 2001;141(1):175. [3] Gangotri KM, Meena RC, Rajni Meena. Use of micelles in photogalvanic cells for solar energy conversion and storage: cetyl trimethyl ammonium bromide– glucose–toluidine blue system. J Photochem Photobiol A: Chem 1999;123:93–7. [4] Gangotri KM, Jagrati Meena. Role of surfactants in photo galvanic cells for solar energy conversion and storage. Energy Sources 2006;28:771–7. [5] Gangotri KM, Pramila S. Use of anionic micelles in photogalvanic cells for solar energy conversion and storage NaLS–mannitol–safranine-system. Energy Sources 2006;28:149–56. [6] Genwa KR, Genwa Mahaveer. Photogalvanic cell: a new approach for green and sustainable chemistry. Solar Energy Mater Solar Cells 2008;92(May):522–9.