Simultaneous electrochemical solar power generation and storage using metanil yellow-formic acid as a new sensitizer-reductant couple in photogalvanic cells

With the rapid commercialization of solar and wind power as supplements and potential substitutes of fossil fuels, the need for power storage techniques to render renewable energy sources impervious to climatic variations has gained significant importance recently. In addition to this requirement of power storage, photo-galvanic (PG) cells hold special significance because these photo-electrochemical devices are capable of simultaneous solar power generation and storage. PG cells with performance as high as 649.6 μW power (Ppp), 2250 μA current (isc), 1048 mV potential (Voc), 8.12% conversion efficiency (CE), and 59 minutes power storage capacity (as half-time, t0.5) have been reported under artificial and low illumination intensities. To enable PG cells, a future source of solar energy conversion, with storage as well, their efficiency must be improved further to a level comparable to that of photovoltaic cells. The metanil yellow dye (photo-sensitizer)-formic acid (reductant) couple has not been exploited to date for this purpose. Therefore, in the present study, the metanil yellow dye as a photosensitizer and formic acid as a reductant have been used in the presence of sodium lauryl sulfate surfactant and sodium hydroxide alkaline medium to further increase the solar energy conversion efficiency and storage capacity of PG cells. The present study reports greatly enhanced electrical performance (compared to earlier results for similar cells) of Ppp 822 μW, isc 6000 μA, Voc 1110 mV, CE 20.41%, and t0.5 105 minutes. On the basis of the redox potential and reported data, a plausible mechanism has also been proposed for the photo-generation of current in metanil yellow-formic acid photogalvanics.

It belongs to a class of the dyes called phthalein dyes.

DESCRIPTION OF THE ELECTODES USED
(A) COMBINATION ELECTRODE: Combination electrode means a single cylinder containing both the reference electrode, and a glass membrane electrode.
(B) PLATINUM ELECTRODE: It is a metal electrode made of the Platinum (Pt) metal. It is a most inert electrode suitable for the efficient electron exchange between electrode electrolyte solutions. Under strong alkaline conditions, they actually function as oxidation electrodes. It provides a surface for oxidation and reduction reaction.

PREPARATION OF THE SOLUTIONS:
All these solutions have been prepared in single distilled water and kept in amber colour vessels to protect them from sunlight.

EXPERIMENTAL METHOD:
The experimental set up is as below (Figure S1) -  (Figure S1) to keep the total volume of reaction mixture always 68 ml. The cell will be H-shaped with two arms. Both arms shell be blackened from outside keeping a window in one arm (this arm called illuminated chamber with dipped Pt electrode). A platinum electrode is dipped into one window arm, and combination electrode is dipped in other arm. The terminals of the electrode are connected to a digital pH meter and the micro-ammeter [19][20]. The system is then exposed to the artificial sunlight (emitted from the incandescent lamp, i.e., tungsten lamp of different wattage). The study of the current voltage characteristics and determination of the power point was accomplished by making use of the resistance variation device called carbon (log 470 K).
The fill factor (FF) was calculated as (V pp  i pp )/(V oc  i sc ), and conversion efficiency (CE) was calculated as (V pp  i pp  FF × 100 %)/(A  I); where, 'A' is Pt electrode area, the 'I' is illuminating intensity in mWcm -2 . V pp  i pp is power at power point (P pp ) expressed in mW.
The size of open window (illuminated area of the cell) of illuminated arm has not been used for calculating the efficiency as-(i) the light diffraction, scattering, and edge effects effectively illuminate the larger solution area, (ii) the PG cell is diffusion controlled, therefore, only those dye molecules which are nearer to Pt anodic electrode shall be able to reach the electrode within their excited lifetime. It means that the number of such dye molecules shall be determinable with respect to Pt area but not the window size; (iii) the natural sunlight is available naturally and free of cost. So to illuminate the small or large area of the cell does not affect the cost and environment. But, the use of small or large Pt electrode affects the cost and environment. Therefore, the use of small Pt electrode area will be less costly and more eco-friendly as low demand for Pt will need less production of Pt leading to reduction in pollution; (iv) The use of Pt anode area for calculation of the efficiency is also supported by the published work of scientists Murthy et al. [34], who have used crosssectional area of the anode for the calculation of conversion efficiency of photogalvanic cell and have given a formula to calculate it, i.e., CF = (V pp x i pp x FF x 100 %)/(PA); and (v) the electrical performance is found independent of the area of illuminating window.
Here, it is to be kindly noted that the conversion efficiency and fill factors are two of the main parameters for judging the performance of the solar cells. Naturally, the higher efficiency should also accompany with the higher FF. But in case of the photogalvanic cells, the FF is relatively low due to certain reasons like photo decay of dye, etc. Because of this, the efficiency and FF does not seem coordinated but divergent to each other. For, the photogalvanic cells, the efficiency is high but the FF low. So to correlate them and to make efficiency linked to FF, the present formula as described in the manuscript has been used, and which is also substantiated by already published literature [19].

MEASUREMENT OF THE PHOTOPOTENTIAL: A digital pH meter (Systronics
Model-335) is used to measure the potential at different time intervals. The whole system was first placed in the dark till a stable potential is obtained. It is a dark potential. Then, the Pt electrode was illuminated with sunlight and the reaction mixture was allowed to reach equilibrium potential, again which was also measured. The difference between the dark potential and the equilibrium potential is termed as photo-potential (mV). The potential of the circuit in short-circuit conditions (i.e., current zero in circuit) is called open-circuit potential (V oc ).

MEASUREMENT OF THE PHOTOCURRENT:
Micro-ammeter was used to measure the current in the circuit.

MEASUREMENT OF THE pH:
The pH of the reaction mixture was measured by a digital pH (Systronics Modal: 335) and was also calculated by the used amount of sodium hydroxide solution by the following formula as follows-pH = 14-log [OH -].

CURRENT VOLTAGE (i-V) CHARACTERISTICS OF THE CELL:
The current-voltage (i-V) characteristics of the cell were studied using an external load (log 470 K) in the circuit.
The highest current obtained at the zero resistance of the circuit after the full charging of the cell is termed as maximum current (i max ), which after some time decreases to an equilibrium vale called short-circuit current (i sc ). In other words, the i sc is defined as the current of the circuit at zero potential. The potential at the short-circuit current is noted, then the resistance of the circuit is gradually decreased to note the potential value at each of the decreasing current value, and finally the potential is noted at zero current. The product of the current and its corresponding potential gives the power value of the cell. The highest product value of the power is called the power at power point of the cell (or the maximum power extractable at corresponding circuit resistance that is characteristics external load). The current and potential at power at power point are termed as current at power point and potential at power point, respectively. The graph between the current and its corresponding potential is termed as i-V curve of the cell. The curve is generally in 'U' shape. The maxima of the curve shows the power at power point and the current and potential values corresponding to the maxima are termed as the current at power point and potential at power point, respectively.

PERFORMANCE AND CONVERSION EFFICIENCY OF THE CELL:
The performance of the cell was determined at its power point from the rate of fall in the power of the cell after removing the source of light. An external load applied to have the current and the potential values equivalent to the value at the power point. Then, the time taken in fall of the power of the cell to its half value is recorded. It is denoted as half time (t 1/2 ) which has been taken as a measure for the performance of the cell in dark (storage capacity of the cell). The higher the t 1/2 means higher the power storage capacity of the cell.

THE EFFECT OF THE VARIATION OF THE NAOH (i.e. pH) CONCENTRATION:
The effect of the variation of the NaOH (i.e. pH) concentration on the metanil yellow-formic acid-SLS photogalvanic system has been studied by fabricating the four photogalvanic cells.

THE EFFECT OF VARIATION OF DISTANCE BETWEEN ILLUMINATING SOURCE AND PT ELECTRODE:
The effect of variation of distance between illuminating source and Pt electrode has been studied indirectly by using illuminating source of different wattage [i.e., incandescent lamps of 500 W, 200 W, 100 W, and 60 W; corresponding intensities (mWcm -2 ) respectively as 26, 10.4, 5.2, and 3.1]. The concentrations and values of other cell fabrication parameters were as explained earlier in para 10 -13.