Catalytic Behavior of Extended π-Conjugation in the Kinetics of Sensitizer-Mediator Interaction

Abstract

This chapter discusses the catalytic effect of extended π-conjugation on the electron transfer process between ferricyphen-ferrocyanide and ferricypyr-ferrocyanide in an aqueous medium. Ferricyphen and ferricypyr may be feasible options for the sensitizer in dye-sensitized solar cells due to their high reduction potential, stability, capability as an outer-sphere oxidant, and photosensitivity. Meanwhile, ferrocyanide could be used as a mediator in DSSCs instead of iodide to avoid iodate production and achieve a similar reduction potential and stability. This chapter compared the ability of competent putative sensitizers to oxidize the likely mediator in water. In contrast to the 2,2′-dipyridyl chelate, the extended π-conjugation in 1,10-phenanthroline accelerated the redox process by increasing the electron affinity of ferricyphen as compared to ferricypyr. The reactions had the same kinetics but different rate constants, indicating that the ferricyphen-ferrocyanide reaction was several times faster than the ferricypyr-ferrocyanide reaction, revealing and confirming the catalytic influence of extended π-conjugation on the redox process.

1. Introduction

A dye-sensitized solar cell (DSSC) relies on the interaction of the sensitizer and the flow of electrons from the mediator to the sensitizer to complete the circuit and convert photo energy to electrical energy. As a result, the sensitizer-mediator interaction in every DSSC is critical to its efficiency and stability. The reduced and oxidized forms of the sensitizer, as well as the reduced and oxidized forms of the mediator, must have adequate stability in the reaction medium to ensure a successful electron-transfer process. Meanwhile, the solubility of the oxidized and reduced sensitizers and mediators is an important metric to consider while building a DSSC. A photoanode, sensitizer, mediator, solvent, and the counter electrode are all common components of a DSSC, and the electrolyte is sometimes employed as well.

Scientists and engineers prefer DSSCs over other first and second-generation solar cells, such as thin-film and silicon-based solar cells, since they are less expensive, more stable, and environmentally friendly. Because DSSCs’ maximum efficiency is lower than that of first and second-generation solar cells, which is 12–14% under ideal conditions of materials and structure using Ru(II) dyes; ruthenium in the 2+ oxidation state, compared to 20–30% for the latter two types of cells, increasing their efficiency in a cost-effective and environmentally benign manner is still a hot topic. As a result, there is still a lot to learn about DSSCs and how to increase their efficiency, stability, and longevity by employing better conditions, materials, and structures.

Researchers have examined a number of materials for the photoanode and counter electrode to improve the efficacy of a DSSC, including nanomaterials, their composites, and nanocomposites of the same materials with varied morphologies [1, 2, 3, 4, 5]. In this investigation, heteroatom-doped graphene catalysts were used [6]. Heterogeneous FeNi₃/NiFe₂O₄ nanoparticles with modified graphene were also explored as electrocatalysts for dye-sensitized solar cells [7]. Studies on making ecologically acceptable and stable DSSCs using natural dyes and NHC-based iron as sensitizers [8, 9, 10], copper and other transition metal-based mediators [11, 12], and electrolytes based on gels and polymers [13, 14, 15, 16, 17] have been published. The steric, structural, and compositional effects of solvent on the efficiency of DSSC have also been investigated electrochemically [15, 18].

The use of potential Fe-based sensitizers and mediators has been revealed in this chapter based on their comparative kinetic study. The rate of the electron-transfer reactions is influenced by the structure of the sensitizer that has been discussed in this chapter. Iron-based sensitizers are less expensive and environmentally benign than ruthenium-based sensitizers, making them attractive to the socio-economic impact. The solubility of iron-based sensitizers in an aqueous medium, as well as the use of an aqueous medium rather than inflammable, volatile, poisonous, and expensive chemical solvents, are two further advantages. Iodate is produced by the electrolytic solution of iodide/triiodide, which is corrosive to stainless steel and a source of DSSC instability. Another approach to a stable DSSC is the use of an iron-based coordination complex such as ferrocyanide as a mediator that has one electron transfer chemistry and comparable redox potential to iodide/triiodide electrolyte. The one-electron transfer chemistry helps to reduce the recombination losses.

The effect of structure, such as extended π-conjugation, on the rate of the electrontransfer between the sensitizer-mediator in an aqueous medium, is the discussion in this chapter. Ferricyphen and ferricypyr are the names for dicyanobis(1,10-phenanthroline)iron(III) and dicyanobis(2,2′-dipyridyl)iron(III), respectively, where “ferri” stands for Fe(III) oxidation state, “cy” for cyanide, and “phen” or “pyr” for the chelate. Reduced variants are known as ferrocyphen and ferrocypyr. Both ferricyphen and ferricypyr are substitution inert outer sphere oxidants with octahedral geometry and similar Fe(III) transition metal coordination sites (Figure 1). Their reduction potentials are 0.80 V and 0.76 V, respectively, however, they were initially synthesized in the 1960s [19, 20, 21, 22]. They are potential sensitizers because of their photosensitive nature and their solubility in an aqueous medium in the oxidized form and comparatively low solubility in the reduced form which may be helpful for their adsorption on the photoanode. Each of the potential sensitizers easily oxidizes the selected potential mediator such as ferrocyanide in an aqueous medium without the need for any external triggering to initiate the reaction. Each of the redox reactions starts spontaneously after mixing the aqueous solutions of both reactants such as ferricyphen-ferrocyanide or ferricypyr-ferrocyanide. The reduction potential of ferrocyanide is comparable to the iodide electrolyte, hence displays its replacement over iodide. The oxidation of iodide by ferricyphen and ferricypyr has been studied in acetonitrile, aqueous tertiary butyl alcohol, and aqueous 1,4-dioxane [23, 24, 25, 26]. The comparative kinetic analysis shows the rapid kinetics of ferricyphen over ferricypyr in binary solvent media under optimized experimental conditions. The following section of the chapter will help to identify the role of pi-conjugation in reaction kinetics.

Figure 1.

(a) Dicyanobis(1,10-phenanthroline)iron(III). (b) Dicyanobis(2,2′-dipyridyl)iron(III).

2. Methodology and kinetics studies

Kinetics of the reduction of ferricyphen, and or, ferricypyr (oxidizing agent) was studied in an aqueous medium under the pseudo-first-order condition. The concentration of ferrocyanide (reducing agent) was always in excess of the oxidizing agents. The reactions were probed at room temperature i.e., 25 °C, and at constant ionic strength i.e., 0.06 M. The concentration ratio between the oxidizing agents and the reducing agent was always maintained at 1:2.5, 1:5, 1:7.5, and 1:10, respectively. The reactions were probed spectrophotometrically under ordinary experimental conditions. No specific or extraordinary experimental setup was required such as an inert atmosphere, dark room, and/or a catalyst. However, the fresh solutions of the reactants were prepared and wrapped in aluminum foil soon after preparation because ferricyphen and or ferricypyr get reduced when their aqueous solutions are exposed to light. The reactions were started upon mixing the reactants and a rise in the absorbance was monitored as a function of time (Figure 2A). The instrumental setup consisted of a home-built assembly as mentioned earlier [27]. The molar absorptivity of the reduced ferricyphen i.e., ferrocyphen, and reduced ferricypyr i.e., ferrocypyr are several folds higher than the oxidized ferrocyphen (ferricyphen) and ferrocypyr (ferricypyr). The spectra of the reactants and products are shown in Figure 2B and compared to the literature [21, 22, 23, 24, 25, 26, 27, 28, 29] that supports the electron transfer mechanism. The integration method was implemented to figure out the rate constant. Each experiment was repeated three to six times for accuracy and the rate constant is an average value. The reactions were kinetically examined, and it was determined that each one among them was completed in two phases.

Figure 2.

(A) Time course graphs at a varying concentration of ferrocyanide and fixed ferricyphen/ferricypyr. (B) UV-visible absorption spectral analysis of the sensitizer-mediator redox reactions.

The rate of both reactions was found independent of the concentration of the oxidizing agents i.e., sensitizers, and the reducing agent i.e., mediator during the first phase of the reaction. The rate of the second phase of both reactions, on the other hand, was shown to be first order and dependent on the concentrations of the sensitizers and mediator. The linear rate equations (integration method) of zero and first order were best fitted with the highest linear fit R² value on the time course data in the first and the second phases of the reactions. The slope of each plot yielded the observed zero order rate constant (k_obs) and the observed pseudo-first-order rate constant (k′_obs), respectively. The effect of variation in the concentration of each of the sensitizers and the mediator on each of the rate constant was studied. It is assumed that if the concentration of the reactant is low and is varied, the rate constant should not be varied rather the rate of the reaction is varied according to the Eqs. (1)–(5) considering the pseudo-first-order condition. However, the rate constant is varied when the concentration of the reactant that has been taken in excess is varied rather than the rate of the reaction in case of the first order dependence on the mediator. Similarly, if it is zero order, the rate constant will have no effect by variation in the concentration of the mediator. Figure 3 depicts the outcomes.

Figure 3.

Kinetic analysis of the sensitizer-mediator reactions.

$$\mathit{\text{Rate}}=k_{\mathit{obs}}{\left[\mathit{\text{Sensitizer}}\right]}^0\mathit{\text{first phase of the reaction}}$$E1

$$\mathit{\text{Rate}}=k_{\mathit{obs}}$$E2

Since sensitizer << mediator

$$\therefore k_{\mathit{obs}}=k_1{\left[\mathit{\text{Mediator}}\right]}^0\mathit{\text{first phase of the reaction}}$$E3

$$k_{\mathit{obs}}=k_1$$E4

$$\mathit{\text{Rate}}=k_{\mathit{obs}}^{\prime }\left[\mathit{\text{Sensitizer}}\right]\mathit{\text{second phase of the reaction}}$$E5

Since sensitizer << mediator

$$\therefore k_{\mathit{obs}}^{\prime }=k_2\left[\mathit{\text{Mediator}}\right]\mathit{\text{second phase ofthe reaction}}$$E6

Eqs. (4) and (6) reveal k₁ as the overall zero order rate constant of the first phase of the reactions and k₂ as the overall second order rate constant of the second phase of the reactions. A first order is observed corresponding to the concentration of the mediator in each case i.e., ferricyphen/ferrocyanide phase-II and ferricypyr/ferrocyanide phase-II (Figure 3). The plots have intercepts that interpret the initial zero order reaction phase corresponding to the concentration of the mediator. However, the rest of the plots (Figure 3) reveal the results according to the Eqs. (1)–(5). The rate constants either observed zero order rate constant (k_obs) and the observed pseudo-first order rate constant (k′_obs) were independent of the concentration of the sensitizers in both phases and the mediator in the first phase of the reaction. Because of the low concentration that was maintained to follow the pseudo-first order condition, the rate constants corresponding to the sensitizers were independent of the concentration terms of the sensitizers in both phases. As a result, the findings show that the pseudo-first order criterion was successfully implemented. It is worth noting that the first phase of both reactions was long enough to get the reactions to about 70% completion. In the first phase of both of the reactions, the zero order rate constant (k_obs) is the multiplication product with the molar absorptivity (ɛ) of either of ferrocyphen or ferrocypyr, respectively. The zero order integrated rate equation (linear-fit) was implemented on the absorbance data rather concentration data of the time course graphs. Therefore, the slope of the plot was the multiplication product of ɛ∙k_obs at the pathlength of the quartz cuvette equal to 1 cm. This multiplication of the constant value to the rate constant just adds a constant mathematical figure to the rate constant and does not affect the rate constant and overall findings of the data and the results.

For further rectification of the results, the effect of pH was monitored on the rate constants in each phase of the reactions under the pseudo-first order condition. The concentration of the mediator was always in excess over the sensitizers and the concentration of the nitric acid was always in excess over the mediator at room temperature and constant ionic strength 0.12 M. The results are revealed in Figure 4 by plotting the graphs between pH and the rate constants on x-y coordinates respectively for each of the sensitizer-mediator interaction. The first phase of each reaction was observed unaffected of pH that declares and confirms the zero order reaction in this phase of each reaction. However, the second phase of the reaction shows curvatures (Figure 4). The value of the pseudo-first order rate constant (k′_obs) decreased with decreasing pH and became constant at the low values of the pH as has been shown in the Figure 4. These results indicate the formation of the monoprotonated ferrocyanide upon increasing the acidity of the reaction medium via conversion of free ferrocyanide species to the rate-inhibiting monoprotonated ferrocyanide and though the value of the rate constant decreased in each case. The protonation of the sensitizers under the pH employed have not been mentioned in the literature [22]. However, ferrocyanide may form mono- to tetra-protonated species considering the charge on the free ferrocyanide and depending on the acidity of the reaction medium [30].

Figure 4.

Effect of pH on the rate constants of the first and second phases of each sensitizer-mediator reaction.

3. Catalytic effect of extended pi-conjugation

The comparative analysis of both of the sensitizer-mediator redox reactions revealed the catalytic effect of pi-conjugation on the rate of reaction. The second order rate constant (k₂) in case of ferricyphen-ferrocyanide has a greater value i.e., 1620 M⁻¹ s⁻¹ as compared to the ferricypyr-ferrocyanide that is 1050 M⁻¹ s⁻¹. The kinetic analysis of both of the reactions helped to identify the similar reaction mechanism of electron transfer between each sensitizer-mediator pair with a high rate in case of ferricyphen-ferrocyanide as compared to ferricypyr-ferrocyanide under similar experimental conditions. The sensitizer such as ferricyphen has extended π-conjugation in its structure that distributes the electron density over the entire chelate molecule while stabilizing it. As a result, in comparison to the 2,2′-dipyridyl chelate, the availability of the lone pair of electrons on the nitrogen atoms in the 1,10-phenanthroline chelate is reduced (Figure 5). The chelate such as 2,2′-dipyridyl does not carry an extended π-conjugation in its structure though its nitrogen atoms have high density of the lone pair of electrons that can be coordinated by the Fe(III) atom of ferricypyr relatively easily. However, in instance of 1,10-phennathroline the extended π-conjugation of the structure reduces the density of the lone electrons on the nitrogen atoms and therefore availability to Fe(III) in ferricyphen. As a result, in order to circumvent this electron deficiency, the electron affinity of Fe(III) in ferricyphen increases as compared to ferricypyr, which helps to enhance its electron accepting potential as compared to ferricypyr. Consequently, a catalytic effect of extended π-conjugation is observed in the reduction kinetics of the ferricyphen as compared to ferricypyr by ferrocyanide in aqueous medium.

Figure 5.

(1) 2,2’-Dipyridyl. (2) 1,10-Phenanthroline.

4. Conclusion

The kinetic study of the electron-transfer reaction between ferricyphen-ferrocyanide and ferricypyr-ferrocyanide revealed a complex mechanism. The reactions were completed into two phases. The initial phase of the reactions lasted long enough to complete the reaction at 70% efficiency and was independent of the sensitizer-mediator concentrations. This phase kinetics has been the most straightforward, with zero order corresponding to the sensitizer and mediator in the aqueous medium. The second phase of the reactions, on the other hand, was long enough to account for up to 30% of the total reaction time, with a first-order relationship of the rate of redox reactions on the concentration of sensitizers and mediator. In the second phase, the reactions followed an overall second order. The rate, that is second order rate constant, of the reaction was shown to be dependent on pH and the concentration terms involved, indicating that it was a rate-determining step involving interaction between ferricyphen or ferricypyr and ferrocyanide. However, as free ferrocyanide is consumed to generate monoprotonated ferrocyanide with increasing acidity, the rate of reaction in this phase slows down, indicating that this is the slow stage of the process. As a result, it has been discovered that monoprotonated ferrocyanide reduces sensitizer in the first phase of the reaction, which is a fast kinetic step in the electron transfer process. Meanwhile, the findings demonstrated that pi-conjugation in the sensitizer has a catalytic influence on the redox kinetics of the sensitizer-mediator interaction. When compared to less conjugated sensitizer that is ferricypyr, pi-conjugation increases the coordination compound’s or sensitizer’s (ferricyphen’s) electron affinity, allowing it to receive electrons and oxidize the mediator more quickly.