Surfactant and natural sunlight enhanced Photogalvanic effect of Sudan I dye

1 Introduction

The photogalvanic cells are based on “Photogalvanic effect”. The term was, first time, used (Rabinowitch, 1940) to denote a special case of the so-called Becquerel effect, in which the influence of light on the electrode potential is due to a photochemical process in the body of the electrolyte (as distinct from photochemical or photoelectric processes in the surface layer of the electrode, which are the basis of the original Becquerel effect). Various PG cells using various dyes, reductants and micelles have been studied (Albery and Archer, 1977; Gangotri and Gangotri, 2009; Gangotri and Indora, 2010; Koli, 2014a; Pokhrel and Nagaraja, 2009; Potter and Thaller, 1959; Sharma et al., 2011) to increase their electrical performance. Despite recent advances (Koli et al., 2012; Pramila and Gangotri, 2007) in photogalvanics leading to tremendous improvement in their electrical output, some more is required to be done to make these cells practically significant in daily life. I have observed some fundamental drawbacks in the field of photogalvanics such as (i) photogalvanic solar energy conversion and storage is reported only in artificial and very low intensity light (i.e., 10.4 mW cm⁻² light intensity emitted from 200 W incandescent bulb), and (ii) the electrical output of these cells is still low from practical application point of view. Therefore, I am of the view that these cells have to show very high electrical output (that is practically significant in daily life) at natural sunlight intensity. Only then, these cells can be practically applicable in daily life. For this to happen, the electrical output of these cells has to be improved further to make it at least comparable to that for photovoltaics.

The use of small Pt electrode with Saturated Calomel Electrode (SCE) component of Combination Electrode and Fructose in alkaline medium (Koli et al., 2012), and anionic surfactants (Mukhopadhyay and Bhowmik, 1992) like Sodium Lauryl Sulfate (SLS) boost up efficiency of photogalvanic cells.

To my best knowledge, there is no report focusing on solar power and storage using Sudan I dye sensitizer, which is cheap and a low molecular weight molecule thought to be more effective in diffusion controlled (Gomer, 1975) PG cells. Therefore, the present study of use of Sudan-I dye as photosensitizer, Fructose as reductant, and SLS as surfactant in alkaline medium with small Pt electrode was undertaken at natural sunlight intensity (i) to further improve electrical performance of the photogalvanic cells, (ii) to study the solar power and storage at natural sunlight intensity, and (iii) to investigate for optimal conditions for optimum performance of the photogalvanic cell at natural sunlight intensity, and to observe whether photogalvanic cells at natural sunlight intensity follow same principles as for low intensity artificial light?

2 Material and methods

3 Results and discussion

The present study of use of Sudan-I dye as photosensitizer with Fructose as reductant and SLS as surfactant in alkaline medium with small Pt electrode at natural sunlight intensity has been done by fabricating 35 photogalvanic cells.

On illuminating each photogalvanic cell (filled with solution having Sudan-I dye sensitizer, Fructose reductant, SLS surfactant, and NaOH), the potential increases regularly and reaches a highest value (V_max), which then decreases and becomes quite constant (V_oc) after some time (Table 1, Fig. 1). The i–V characteristic of each cell (Table 2, Fig. 2) shows that highest power is extractable from a cell at a characteristic current and load resistance. Therefore, the performance of each cell has been studied at this characteristic current and resistance (different for different cells) in the dark. With time, the power decreases as a result of deactivation of Sudan I dye molecules (Table 3, Fig 3) in the dark. The cell continuously supplies power until its complete discharge. The cell does not have unlimited life time as life of excited molecules of Sudan I is not unlimited.

Close

Figure 1 Variation of potential with time during charging of cell.

Export to PPT

Close

Figure 2 Variation of potential and power with current (a) i–V characteristic of the cell, (b) power vs current.

Export to PPT

Close

Figure 3 Study of cell performance (a) power vs time, (b) potential vs time, (c) current vs time.

Export to PPT

The study of effect of variation of various variables such as concentrations (of Sudan-I dye, Fructose, SLS, NaOH), diffusion length, electrode area, etc. shows that the values of these variables affect the electrical performance of the cell at natural sunlight intensity. There is a characteristic value of each variable at which the cell shows highest performance. Therefore, the optimum performance of the cell can be obtained by carefully selecting the optimal values of these variables. Effect of temperature variation on the cell performance has not been done as it is avoidable because controlling temperature in natural sunlight conditions will complicate the technology and will enhance cost as well.

The optimum cell performance (in terms of power, current, and potential) at optimal values of cell variables can be explained on the basis of various principles (Koli et al., 2012; Pramila and Gangotri, 2007; Sharma et al., 2011) of physical chemistry such as particle nature of light and matter, diffusion, conductivity, etc.

The optimum cell performance is observed on 10.37 × 10⁻⁵ M concentration of the Sudan-I dye (Table 4). On the lower side of this concentration range, the electrical output is low as there will be lesser number of Sudan-I dye molecules to absorb light in path and to donate electrons to Pt electrode. The higher concentrations (>10.37 × 10⁻⁵ M) of Sudan-I dye will not permit the desired light intensity to reach the dye molecules near the electrodes and hence, there will be corresponding fall in the power of the cell.

The optimum cell performance is observed on 1.48 × 10⁻² M concentration of the SLS surfactant (Table 5). On the lower side of this concentration range, the electrical output is low as there will be lesser number of SLS molecules to facilitate electron transfer, solubility and stability of dye molecules. The higher concentrations (>1.48 × 10⁻² M) of SLS will hinder the motion of dye molecules toward the electrodes leading to corresponding fall in the power of the cell.

The optimum cell performance is observed on 2.37 × 10⁻³ M concentration of the Fructose reductant (Table 6). On the lower side of this concentration range, the electrical output is low as there will be lesser number of Fructose molecules to donate electrons to dye. The higher concentrations (>2.37 × 10⁻³ M) of Fructose will promote back electron transfer from dye molecule to reductant molecule, and will also hinder the motion of dye molecules toward the electrodes leading to corresponding fall in the power of the cell.

A general increase in cell parameters such as i_sc, P_pp, i_pp and CE was observed with increase in pH up to 13.73, and beyond this pH, the decrease in these parameters was found (Table 7). The performance of the cell is poor in acidic medium. It may be due to proton attachment to heteroatom and double bonds in dye and reductant leading to poor electron donating power of dye and reductant to Pt electrode. In alkaline medium, this effect is absent, and the anion formation of dye and reductant enhances their electron donation power. At very high pH, OH⁻ (from NaOH used in this system) may combine with cationic reductant (formed on electron donation from reductant to dye) inhibiting regeneration of reductant in original form, leading to poor performance of the cell.

Under the study, the pH has not been stabilized. The electrical parameters have been reported against the initial pH of the mixture of solutions of dye, reductant and NaOH.

Under the observed effect of electrode area, the i_max, i_sc, P_pp, CE and t_0.5 found highest for electrode area 0.40 × 0.20 cm². For electrodes of area larger than this, the cell parameters were found decreasing with increase in electrode area (Table Supplementary Data 1-S1). For the observed effect of electrode area, the better cell parameters were found for small electrodes owing to relatively less hindrance to diffusion of ions as the photogalvanic cells are based on ion diffusion mechanism.

Diffusion length significantly affects the performance of the photogalvanic cells as they are based on diffusion of ionic species. It was observed that with an increase in diffusion length, the photocurrent showed an increase and potential showed decrease (Table S2). As diffusion length increases, the current increases as conductivity of dye increases due to increase in the volume of solution between electrodes. The potential decreases with diffusion length. The reason may be that concentration gradient disturbs the dye (double layer) layer on Pt electrode. As diffusion length is small, concentration gradient factor is reduced and potential is increased.

The techniques of incident photon-to-current conversion efficiency (IPCE) and Electrochemical Impedance Spectroscopy (EIS) in favor of results and inferences could not be used as I am unable to do IPCE and EIS study for the want of the infrastructure, instruments and expertise in my laboratory. I understand that IPCE and EIS study of cell in this manuscript could have given some important insights to lead to further improvements in the cell performance. But, such experiments were not within the scope of the present manuscript. Because, the main aim of the present manuscript was to carry this work ahead for advancement over earlier reported work on photogalvanics by employing cell performance enhancer surfactant and natural sunlight for illumination.

However, it is very simple science that the current increases as the conductivity of dye increases due to increases in the volume of solution between electrodes. The increase of the volume of solution between electrodes means more number of leuco/semi-leuco dye molecules between electrodes. The more number of leuco/semi-leuco dye molecules between electrodes will have more conducting power, and hence current. It is based on the simple science of the photogalvanics that increased diffusion length provides for availability of more dye molecules between electrodes to reach Pt electrodes within the very short life of their excited states. Therefore, the availability of such dye molecules for electron donation to Pt will increase as the diffusion length will increase to an optimum value. Beyond this optimum diffusion length, the number of dye molecules may not be able to increase as they will not be able to reach Pt within their short life time due to increased distance between electrodes (Gomer, 1975).

Thus, the optimal values of the cell variables for Sudan I-Fructose-SLS-NaOH system at natural sunlight intensity (100 mW cm⁻²) may be summarized as 3.5 mL of M/500 Sudan-I dye (resultant concentration 10.37 × 10⁻⁵ M), 16.0 mL of M/100 Fructose (resultant concentration 2.37 × 10⁻³ M), 10.0 mL of M/10 SLS (resultant concentration 1.48 × 10⁻² M), 36.0 mL of 1 M NaOH (resultant pH 13.72), 6.3 cm diffusion length, and 0.4 × 0.2 cm² Pt electrode area. At these optimal values of the cell variables, the i–V characteristics of the cell (Table 2, Fig. 2) show that highest power (i.e. 1081.1 μW) is extractable from cell at current 1900 μA and characteristics load resistance 299.4 Ω.

The optimum cell performance at optimal values of the cell variable is summarized as dark potential (V_dark) 572 mV; maximum potential (V_max) 1060 mV; open-circuit potential (V_oc) 1048 mV; photopotential (ΔV) 488 mV; charging time (t) 25 min; maximum current (i_max) 5800 μA; equilibrium current (i_eq) or short-circuit current (i_sc) 4200 μA; power at power point (p_pp) 1081.1 μW; potential at power point (V_pp) 569 mV; current at power point (i_pp) 1900 μA; t_0.5 31 min; potential at t_0.5 (V_t0.5) 415 mV; current at t_0.5 (i_t0.5) 1300 μA; conversion efficiency (CE) 13.5%; fill factor (FF) 0.24; and rate of change of current over t_0.5 (Δi/Δt) 19.3 μA min⁻¹.

The CE is relatively high, but FF is not proportionately so high in present work. The reasons for relatively lower FF may be-One, the photo-instability of Sudan-I and limited life of excited state of Sudan-I coupled with requirement of diffusion of Sudan-I molecules through solution to reach and release electron to electrode may be the one reason.

The FF has been determined manually by knowing P_pp with the help of i–v characteristics. The manual P_pp determination takes some time. During this time, some excited molecules of photosensitizer Sudan-I are decayed leading to lower P_pp and in turn lower FF. Therefore, there is a time gap between determinations of i_sc and i_pp. During this time gap, some of the Sudan-I molecules might be undergoing photo-decay and deactivation leading to relatively fewer number of photosensitizer Sudan-I molecules capable of donating electrons to electrode resulting in relatively lower i_pp and V_pp (means P_pp). Otherwise this P_pp should have been higher had not been there such photo-decay and deactivation. The lowering of this P_pp is reinforced by requirement of diffusion of photosensitizer Sudan-I molecules to Pt electrode as some Sudan-I molecules might be getting photo-decayed and deactivated during this diffusion. Consequently, this relatively lower P_pp value results in relatively lower FF as it (FF) is directly proportional to P_pp. It is to be mentioned that photo-decay and deactivation follow non-zero order kinetics (Mosquera et al., 1994). So, initially the decay is fast, but later on it is relatively slow.

Two, the lower resistance (shunt resistance type-R_sh) at i_sc point and higher resistance (series resistance type-R_s) at V_oc point may also be the reason for relatively lower FF as the FF is directly affected by the values of R_sh and R_s. Increasing the R_sh and decreasing the R_s lead to a higher FF. A straightforward method of estimating the R_s for a cell is to find the resistance at the V_oc point, and an estimate for the value of the R_sh of a cell can be determined from the resistance near the i_sc point. In present study, the R_sh is low (of the order of 14.7 Ω) and R_s is very high (of the order of 4950 Ω or even higher).

The results in present work are very impressive and even higher than highest results reported so far in the field of photogalvanics (highest results are reported for Rhodamine B dye-Fructose system). This is explicit from the recently published work in the field of photogalvanic cells. CE 1.62%, i_sc 420 μA_, P_pp 168.75 μW, and t_0.5 130 min are reported for Tween 60-Amido Black 10B-Ascorbic acid system (Genwa and Sagar, 2013). CE 0.31%, i_sc 93 μA_, P_pp 29.8 μW, and t_0.5 65 min are reported for mixed dye (brilliant green and celestine blue) with EDTA reductant (Yadav and Lal, 2013). CE 0.14%, i_sc 45 μA_, P_pp 14.75 μW, FF 0.31, and t_0.5 40 min are reported for Azur B dye-EDTA reductant-Tergitol-7 surfactant system (Gangotri et al., 2013). CE 1.33%, i_sc 380 μA, P_pp 138.6 μW, and t_0.5 70 min are reported for Fast Green FCF dye with Fructose as reductant in NaOH as alkaline medium (Koli, 2014b). CE 7.58%, i_sc 972 μA_, P_pp 244.02 μW, and t_0.5 3.6 h are reported for Rhodamine B dye with Fructose as reductant in NaOH as alkaline medium (Koli et al., 2012).

Thus, the results in present work are consisting with the aim of further enhancement in the electrical output of the photogalvanic cell. The cell can be fabricated following same principle as followed for the construction of cells charged in artificial and low intensity light. The cell has been found to show the optimum performance at optimal values of the cell variable in natural sunlight as is reported for cells studied in the artificial and low intensity light.

Some of the reasons for higher results in present work may be the use of (i) anionic SLS surfactant, (ii) small Pt electrode, (iii) Sudan-I, and (iv) natural sunlight. The anionic surfactant enhances solubility and electron ejection capacity of the dye sensitizers. The photogalvanic cell is diffusion controlled. Therefore, any factor enhancing diffusion also enhances cell performance. The lower Pt electrode creates less hindrance to diffusion of molecules in solution inside the cell. As already reported, such high results for Sudan-I are artifacts of dye nature. The lower mass of Sudan-I coupled with alkaline medium facilitates more diffusion and transfer of electrons to Pt electrode and in turn more current and power. The anionic structure of Sudan-I in alkaline medium facilitates more transfer of electrons to Pt electrode and in turn more current and power. The use of the high intensity light (i.e., natural sunlight) for charging the cell in present work enables use of higher concentrations (greater number of molecules for electron exchange) of chemicals. As explained earlier the optimum performance of these cells can be explained by particle nature of the light and matter. In already reported Rhodamine B-Fructose system (Koli et al., 2012), the usable optimal resultant concentration of sensitizer was low (7.2 × 10⁻⁵ M Rhodamine B) as number of photons available for charging cell were lesser in number in artificial and low intensity light 10.4 mW cm⁻². Therefore, the number of dye sensitizer molecules capable of electron exchange with electrodes and reductant were lesser in number leading to low electrical output (i_sc 972 μA_, P_pp 244.02 μW).

In present work, the usable optimal resultant concentrations of sensitizer are high (10.37 × 10⁻⁵ M of Sudan-I) as number of photons available for charging cell are nearly ten times greater in number in natural sunlight (i.e., 100 mW cm⁻²) than that in artificial light (i.e., 10.4 mW cm⁻²). Therefore, the number of dye sensitizer molecules capable of electron exchange with electrodes, reductant, and surfactant are greater in number leading to very high electrical output (i_sc 4200 μA_, P_pp 1081.1 μW) in present work. It also indicates that geographical regions receiving high natural sunlight intensity will have high potential for having higher power generation from the photogalvanic cells as well.

Earlier published work on Rhodamine-B dye sensitization of photogalvanic cells at artificial and low intensity light emitted from 200 wattage incandescent bulb reported current 972 μA_, power 244.02 μW, and efficiency about 7.58% (Koli et al., 2012). The present manuscript carries this work ahead and reports tremendously greater advancement over earlier reported work by employing cell performance enhancer surfactant and natural sunlight for illumination.

Thus, the results in present work are highly impressive and far better than that for all dye based photogalvanic systems reported so far. Thus, the observed cell performance based on Sudan I dye-Fructose-SLS surfactant is encouraging enough for ultimate aim of development of applicable and affordable photogalvanic cells in the future.

This is the first time that the study has been done in natural sunlight for the class of photogalvanic cells reported in this manuscript. The photogalvanic cells reported in this work have ≈ 13.5% efficiency, and power density of the order of ≈ 135 Wm⁻² (w.r.t Pt electrode). The photovoltaic cells (PV) have practically 12–20% efficiency, and power density of the order of 140 Wm⁻².

The PV cells may be taken as a yardstick to judge the state of development of other similar techniques such as PG as it (PV) is presently only technique having commercial application in daily life world over. Although there are various efficient techniques such as dye-sensitized solar cells-DSSC (Guo et al., 2013; Yu et al., 2010; Zhang et al., 2013), etc. for direct conversion of solar light into power as do the photogalvanic cell described in this manuscript, overall peak power conversion efficiency for current DSSCs is about 11%, and commercial silicon panels operates between 14% and 17% (Gao et al., 2008).

I see that the photogalvanic cells have potential for use in natural sunlight conditions of daily life with some added advantages of inherent power storage capacity (Koli, 2013; Koli, 2014c), if their electrical output is improved further to match the level of performance of PV cells.

4 Conclusion

The use of surfactant improves electrical performance of the photogalvanic cells. The photogalvanic cells at natural sunlight intensity follow same principles as for low intensity artificial light as its optimum performance is obtainable in optimal conditions. Therefore, the efficient photogalvanic cells for use in natural sunlight can be fabricated by following the same principles as applicable for construction of cells in artificial and low intensity light. And, the Sudan-I dye-Fructose-SLS combination is a good alternative for fabrication of highly efficient photogalvanic cells usable in sunlight.