Open access peer-reviewed chapter

Solvent Catalysis in the Sensitizer-Mediator Redox Kinetics

Written By

Rozina Khattak

Submitted: 04 May 2022 Reviewed: 12 May 2022 Published: 20 June 2022

DOI: 10.5772/intechopen.105393

From the Edited Volume

Recent Advances in Chemical Kinetics

Edited by Muhammad Akhyar Farrukh

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Abstract

The sensitizer-mediator redox reaction is a vital component of the dye-sensitized solar cells (DSSCs). The efficiency and stability of dye-sensitized solar cells are aided by the kinetics of this redox process. Several reaction parameters influence the kinetics of a reaction, and if those parameters are controlled, the rate of the process and its results can be controlled. One of the most important aspects of the sensitizer-mediator interaction is the reaction medium. Aqueous DSSCs are unquestionably a good replacement when it comes to taking a green approach to avoiding toxic, flammable, and volatile organic solvents and their mixtures, which are commonly used in DSSCs and are known to harm the environment while also reducing the lifetime and stability of the DSSCs. The catalytic role of a small volume fraction of organic solvent in the aqueous electron transfer kinetics of a few putative sensitizer-mediator reactions is discussed in this chapter. In binary solvent media including dilute tertiary butyl alcohol (TBA)-water and dilute 1,4-dioxane-water, the reduction of dicyanobis(2,2′-dipyridyl)iron(III) and dicyanobis(1,10-phenanthroline)iron(III) was investigated. The reactions were carried out in a 10% TBA or dioxane to water media with a volume-volume fraction of both solvents using iodide as a reducing agent. The effect of several parameters on the rate constant was also calculated and analyzed.

Keywords

  • dye-sensitized solar cells
  • solvents
  • kinetics
  • redox reaction
  • catalysis

1. Introduction

In the kinetics of reactions, particularly redox reactions, the solvent has a significant effect. Redox reactions occur when two responding entities exchange electrons. The electron giver, or reducing agent, is the one who contributes the electron; the electron acceptor, or oxidizing agent, is the one who accepts the electron. The donation and reception of electrons alter the oxidation states of the reactants since electrons are such small charged particles. As a result, the solvent plays an important role in electron transfer reactions. A few of the most influential characteristics that govern redox reactions include solvation, viscosity, and hydrogen bonding [1]. The solvent organizes and reorganizes itself around the reactants and products before and after the electron transfer event. Similarly, the solvent organizes and reorganizes around the reactants during the production of the transition state [2]. According to the transition state theory of reactions in solution and the double sphere model, the rate constant is related to the dielectric constant of a medium using the following expression [3].

lnk=lnk0e2zAzB4πε0εrr#kBTE1

k and k0 are the rate constants for any dielectric constant and infinite dielectric constant, respectively, in Eq. (1). The symbols e, zA, zB, ɛ0, ɛr, r#, kB and T represent the constant value of electric charge (a constant in coulombs), charge on reactants A and B, permittivity constant, dielectric constant of medium, inter-nuclear distance between the reacting entities that form the transition state complex, Boltzmann constant, and temperature in Kelvin scale, respectively. The Eq. (1) correlates the rate constant with charges on the reactants and the dielectric constant of the medium in three ways.

  1. If the reactants have similar charges, i.e., positive and or negative, the rate constant will drop when the dielectric constant of the medium is decreased.

  2. If the reactants have different charges, the rate constant will rise as the dielectric constant of the medium falls.

  3. If one or both of the reactants are chargeless or neutral, lowering the dielectric constant of the medium has no effect on the rate constant’s value.

Other reaction parameters, such as the effect of ionic strength in a specific reaction, must be zero or close to zero in order to investigate the effect of the dielectric constant on the rate constant and, as a result, the rate of the reaction. Variation in ionic strength has a significant effect on the rate constant of any reaction, and we can find a theoretical value of the rate constant at zero ionic strength by extrapolating the graph to zero ionic strength, i.e., the intercept of the plot [4], using the transition state theory to formulate the primary salt effect. The theoretical value of the rate constant at zero ionic strength is known as the ideal value of the rate constant. The ideal rate constant for a reaction can be calculated in a variety of solvent systems with varied dielectric constants. The dielectric constant of reaction media can be changed by changing the proportion of one solvent to another. To determine the slope of the plot according to Eq. (1), it is advisable to plot the natural logarithm of the ideal rate constant (lnk) versus the reciprocal of the dielectric constant (1/ɛr) rather than the natural logarithm of the rate constant at any ionic strength. The value of the inter-nuclear distance between the reactants that constitute the transition state complex and are involved in the rate determining step can be calculated using the slope of the plot. The inter-nuclear distance between the reactants of various reactions can be compared and used to control the kinetics of the reactions under certain experimental conditions.

As a result, it is clear that changing the nature of the reaction media affects the entire electron transfer mechanism. When dealing with the kinetics of redox reactions, it is also worth noting that changing the solvents’ proportion in a reaction can change the viscosity and strength of hydrogen bonding, as well as the nature of hydrogen bonding, resulting in either the activation controlled mechanism or the diffusion controlled mechanism [5, 6, 7, 8, 9, 10, 11, 12]. It is critical to have precise information on the kinetics of any reaction in order to manage it and make use of it as needed in a process. When it comes to dye-sensitized solar cells (DSSCs), the sensitizer-mediator reaction is crucial to the electron transfer cycle as well as the cell’s stability, durability, and efficiency. The influence of variations in the sensitizer on the stability and/or efficiency of DSSCs has been explored by a number of researchers [13, 14, 15, 16, 17, 18, 19]. A variety of natural and synthetic dyes were utilized. Others, however, have looked into the effect of mediator variation on DSSC efficiency [20, 21, 22, 23, 24, 25, 26]. To avoid flammable, poisonous, and volatile organic solvents, combinations of such solvents with water were tried to increase the DSSCs’ stability and efficiency. Various organic solvents and their mixtures have been investigated for this purpose [15, 27, 28]. Aqueous-based dye sensitized solar cells are gaining popularity due to their environmentally beneficial and low-cost characteristics. In order to improve the DSSC’s efficiency and stability, aqueous-based sensitizers and electrolytes have recently been explored [29]. The effect of dilute binary solvent media consisting of dilute organic percentage and excess water on two potential photosensitizers to oxidize iodide is described in this chapter. Dicyanobis(2,2′-dipyridyl)iron(III) and dicyanobis(1,10-phenanthroline)iron(III) could be a potential replacement for ruthenium-based dyes due to their stability, solubility, and cost-effectiveness. Furthermore, unlike hazardous ruthenium-based chemicals, the most likely iron-based sensitizers are not environmentally detrimental. The rate constants for the reactions were calculated using 10 volume percent of tertiary butyl alcohol in water and an equal volume percent of 1,4-dioxane in water. When the rate constants were compared, it was discovered that such sensitizer-mediator interactions could have an impact on DSSC’s efficiency.

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2. Kinetics and solvent catalysis

When the proposed sensitizers, dicyanobis(2,2′-dipyridyl)iron(III) and dicyanobis(1,10-phenanthroline)iron(III), are introduced to the iodide solution in the binary solvent media, the oxidation of iodide is a spontaneous process that does not require any external triggering. When the reduction potential of the redox pair is taken into account, all of the reactions are electrochemically viable [30, 31, 32]. The visual color change of the solutions and the spectra of the products show the progress of the reactions and product(s) generation when compared to analogous iron(II) complexes (Figure 1) [33, 34]. Figure 1 shows the wavelength maxima corresponding to dicyanobis(2,2′-dipyridyl)iron(II) and dicyanobis(1,10-phenanthroline)iron(II), which allows time course graphs to be drawn as the absorbance increases as a function of time after the products are formed. The reactions were investigated in the visible region at the wavelength maximum of dicyanobis(2,2′-dipyridyl)iron(II) and dicyanobis(1,10-phenanthroline)iron(II) (Figure 2). Each reaction was investigated using a pseudo-order kinetic model with an excess and changing concentration of the mediator, iodide, in comparison to a fixed and low concentration of either dicyanobis(2,2′-dipyridyl)iron(III) or dicyanobis(1,10-phenanthroline)iron(III). The reactions were studied at room temperature in all the reaction media. The oxidant concentration was held constant at 0.08 mM and the iodide (reductant) concentration was adjusted between 0.08 mM and 4 mM at 1:1, 1:2.5, 1:5, 1:7.5, 1:10, 1:20, 1:30, 1:40, and 1:50 times to preserve the pseudo-first order kinetic model in all reaction media at 0.06 M ionic strength (μ). The integration method was implemented on the absorbance data, and zero order kinetics was observed corresponding to the sensitizers i.e., oxidizing agents. The absorbance was plotted against time that yielded a slope “ɛ∙b∙kobs” that is the multiplication product of the molar absorptivity of either of dicyanobis(2,2′-dipyridyl)iron(II) and or dicyanobis(1,10-phenanthroline)iron(II), the pathlength of the quartz cuvette (1 cm) and the observed zero order rate constant, respectively. The slope is a constant (ɛ∙b) times larger than the real value due to the inclusion of a constant mathematical number, which has no overall effect on the rate constant. Such a slope can be obtained by using absorbance without converting it to concentration (via implementing Beer-Lambert’s law). When a graph is drawn between the concentrations as a function of time, it yields a straight line with a slope equal to the zero order rate constant. Figure 2 shows representative kinetic traces at a 1:5 sensitizer:mediator ratio for comparative examination in various solvent media. In 10% (v/v) TBA-water, the reaction between dicyanobis(2,2′-dipyridyl)iron(III) and iodide is the slowest, but dicyanobis(1,10-phenanthroline)iron(III)-iodide is the fastest in identical media.

Figure 1.

Visible absorption spectra of the products of the redox reactions in three different reaction media.

Figure 2.

Representative time course graphs of reactions in different reaction media at 293 ± 1 K.

Controlling the reaction within the DSSC with respect to potential sensitizers such as dicyanobis(2,2′-dipyridyl)iron(III) or dicyanobis(1,10-phenanthroline)iron(III) is aided by zero order kinetics corresponding to oxidizing agents in all reaction media. The mediator, such as iodide, plays the main role in controlling the reaction kinetics in such sensitizer-mediator interactions. When the reaction mechanism is known in all of the selected media, it becomes much easier to exploit this sensitizer-mediator interaction to get the most out of the reaction in a DSSC where the rate of the reaction is solely dependent on the mediator. The zero order rate constant obtained for each reaction in all reaction media was displayed as a function of the iodide ion concentration (Figure 3) for this experiment [4, 35, 36]. The redox reaction between dicyanobis(2,2′-dipyridyl)iron(III)-iodide in either 10% TBA-water (bpy-TBA in Figure 3) or 10% dioxane-water (bpy-dioxane in Figure 3) underwent a first order with the zero order rate constant increasing linearly with increasing iodide concentration, yielding a straight line passing through the origin. The overall first-order rate constant of the reaction is determined by the slope of the plot. In the meantime, a third order kinetics was found in the reaction of dicyanobis(1,10-phenanthroline)iron(III)-iodide in 10% TBA-water (phen-TBA in Figure 3). As a result, it has been discovered that in the selected reaction media, dicyanobis(1,10-phenanthroline)iron(III)-iodide reacts much faster than dicyanobis(2,2′-dipyridyl)iron(III)-iodide, implying that in a DSSC, the recombination process may be faster in the “phen” system rather than the “bpy” system. To avoid repeating long names, the former potential sensitizer is referred to as “bpy” and the latter as “phen”. The main difference between the two sensitizers is in the chelate, where the phen system has more pi-conjugation than the bpy system. The rest of the coordination sites and geometry, on the other hand, are similar. Both are octahedral complexes that are substitution inert. Furthermore, in the instance of dicyanobis(2,2′-dipyridyl)iron(III)-iodide, the first order rate constant is 13 times bigger in 10% dioxane-water than in 10% TBA-water. Consequently, the results reveal that dioxane has a catalytic influence on the redox kinetics of the sensitizer-mediator relationship when compared to TBA. Similarly, dicyanobis(1,10-phenanthroline)iron(III)-iodide displays faster electron transfer than dicyanobis(2,2′-dipyridyl)iron(III)-iodide in the identical reaction medium (10% TBA-water).

Figure 3.

Kinetic study with respect to the reducing agent in different reaction media.

To evaluate the catalytic function of the solvent in the sensitizer-mediator interaction, it is obvious to calculate the ideal rate constant for each reaction in each solvent medium, such as the rate constant at zero ionic strength. Figure 4 depicts the plots of the primary salt effect on the rate constant according to the formulation (2). The optimal value of the rate constant was determined by the intercept of the plots. When the effect of the solvent is significant and there is no effect of ions on the rate constant, the ionic strength is assumed to be zero, and the ideal value of the rate constant is obtained. When the experimental data is plotted and extrapolated to zero ionic strength, the theoretical value of the rate constant, or ideal rate constant, is produced.

Figure 4.

Plots of primary salt effect in different solvent media.

logεbkobs=logεbkobsideal+2AzAzBμ1+μE2

Figure 4 shows the decelerating effect of increasing ionic strength on the observed zero order rate constant, indicating that opposite charges are involved in the rate determining step that leads to the formation of the transition state complex. bpy-5%/10% /15% dioxane, bpy-5%/10%/15% TBA, and phen-5%/10%/20% TBA were used to depict the effect of increasing ionic strength in different solvent media for “bpy” and “phen” systems. The term bpy-5% dioxane, on the other hand, refers to a sensitizer-mediator interaction involving dicyanobis(2,2′-dipyridyl)iron(III)-iodide and a 5% (v/v) 1,4-dioxane-water solvent system. Meanwhile, dicyanobis(1,10-phenanthroline)iron(III)-iodide in a 5% (v/v) TBA-water solvent solution is phen-5% TBA. The remainder of the terms has comparable connotations as well. The ideal value of the rate constant was obtained from the intercept of each plot and was used to build a graph. According to Eq. (1), the natural logarithm of the ideal rate constant was drawn on the y-axis and the reciprocal of the dielectric constant was drawn on the x-axis for each system, including bpy-dioxane, bpy-TBA, and phen-TBA. Figure 5 depicts the final results. The slope of the plots was used to calculate the inter-nuclear distance (r#) between the active species that constitute the transition state complex, and the results are presented in Table 1. Table 1 demonstrates that in the reaction of dicyanobis(2,2′-dipyridyl)iron(III) with iodide in 10% (v/v) TBA-water versus 10% (v/v) dioxane-water, the inter-nuclear distance is very long enough. This exhibits the catalytic impact of dioxane over TBA by displaying the quick electron transfer kinetics between the sensitizer-mediator in dioxane-water as compared to TBA-water. The inter-nuclear distance between the active reactants that form the transition state complex and lead to the rate-determining step of the reaction in dicyanobis(1,10-phenanthroline)iron(III)-iodide is smaller (53 pm) than dicyanobis(2,2′-dipyridyl)iron(III)-iodide, which is 130 pm in 10% (v/v) TBA-water. This confirms that in the former situation, electron transfer between the sensitizer and mediator is faster than in the latter case, and that the solvent has a catalytic impact in the “phen” system rather than the “bpy” system. However, by utilizing 1,4-dioxane instead of tertiary butyl alcohol, the reaction of the “bpy” system accelerated almost to the level of the “phen” system, where r# is 59 pm in the former instance and 53 pm in the latter case. As a result, in order to accelerate a sensitizer-mediator interaction for rapid recombination in DSSC, solvent can be used in an environmentally friendly and cost-effective manner to increase the stability and efficiency of the solar cell.

Figure 5.

Effect of solvent on the reaction kinetics of sensitizer-mediator interaction.

Sensitizer-MediatorSolvent System (Reaction Media)Inter-nuclear Distance (r#) in pm
[Fe(bpy)2(CN)2]+- I10% (v/v) TBA-water130
[Fe(bpy)2(CN)2]+- I10% (v/v) dioxane-water59
[Fe(phen)2(CN)2]+- I10% (v/v) TBA-water53

Table 1.

Catalytic effect of solvent in the sensitizer-mediator interaction.

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3. Conclusion

The redox reaction of dicyanobis(2,2′-dipyridyl)iron(III)-iodide and dicyanobis(1,10-phenanthroline)iron(III)-iodide could be used to improve the stability and efficiency of dye-sensitized solar cells. The proposed sensitizer-mediator interaction could be both cost-effective and ecologically advantageous due to the low cost of photosensitive iron complexes. To catalyze the indicated sensitizer-mediator processes in aqueous medium, a modest amount (10 volume percent) of organic solvent, such as tertiary butyl alcohol and 1,4-dioxane, can be utilized. Both solvents are relatively benign to the environment and inert to involvement in the redox process, so they do not cause parallel reactions or complex kinetics. In contrast to dicyanobis(2,2′-dipyridyl)iron(III)-iodide, which follows first order, tertiary butyl alcohol catalyzes the reaction of dicyanobis(1,10-phenanthroline)iron(III)-iodide to third order. Both reactions have rates that are independent of the concentration of oxidizing agents (potential sensitizers), emphasizing the need to use such oxidants in DSSCs to control a reaction that is solely dependent on the concentration of the reductant/reducing agent (mediator). Such sensitizer-mediator interactions are simple to manage, and the rate of reaction can be sped up or slowed down by adjusting the concentration of just one reactant. Furthermore, the rate of reaction was enhanced several times faster by using 1,4-dioxane instead of tertiary butyl alcohol, which may be useful in the recombination process in DSSCs. The inter-nuclear distances for the bpy-TBA and phen-TBA systems were 130 and 53 pm, respectively. However, with the bpy-dioxane and phen-TBA systems, it was 59 and 53 pm, respectively. These findings pertain to solvent catalysis by reducing inter-nuclear distances, resulting in a rapid electron transfer process. This research/study contributes to the development of a cost-effective and environmentally friendly strategy for improving the stability and efficiency of DSSCs.

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Written By

Rozina Khattak

Submitted: 04 May 2022 Reviewed: 12 May 2022 Published: 20 June 2022