Influence of Different Electrolytes on the Reaction Mechanism of a

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Influence of Different Electrolytes on the Reaction Mechanism of a Triiodide/Iodide Redox Couple on the Platinized FTO Glass Electrode in Dye-Sensitized Solar Cells Yingtong Tang, Xu Pan, Changneng Zhang, Songyuan Dai,* Fantai Kong, Linhua Hu, and Yifeng Sui Key Lab of NoVel Thin Film Solar Cells, Institute of Plasma Physics, Chinese Academy of Sciences, P.O. Box 1126, Hefei, Anhui 230031, People’s Republic of China ReceiVed: October 21, 2009; ReVised Manuscript ReceiVed: December 29, 2009

The reaction mechanisms of a triiodide/iodide redox couple at the platinized FTO glass electrode/electrolyte interface with different electrolytes were studied by cyclic voltammetry, steady polarization curve, and chronocoulometric methods. It was found that the reaction process of the triiodide/iodide redox couple in electrolytes without nitrogenous heterocyclic additives was controlled by adsorption, and different iodide concentrations could affect the reaction mechanism of the triiodide/iodide redox couple on the platinized FTO glass electrode by changing the adsorption characteristic of iodide (triiodide). After nitrogenous heterocyclic additives were added into the electrolytes, the reaction process was controlled by a diffusion process. The nitrogenous heterocyclic additives can absorb on the electrode and change the reaction mechanism of the triiodide/iodide redox couple by forming complex compounds with triiodide. The radius of the redox active species in the electrolyte with nitrogenous heterocyclic additives was bigger than that in electrolytes without nitrogenous heterocyclic additives. 1. Introduction The dye-sensitized solar cell (DSC) has been attracting considerable attention because of its high conversion efficiency, simple fabrication process, and low cost.1–4 DSCs generally consist of three main components: a dye-sensitized nanocrystalline titanium oxide (TiO2) layer on a transparent conductive glass substrate as a working electrode, a triiodide (I3-)/iodide (I-) redox couple in an organic solvent as an electrolyte, and a platinized conductive glass substrate as a counter electrode. The role of the counter electrode is to collect electrons from the external circuit and reduce I3- to I- in the electrolyte. To keep a low overvoltage and lessen energy loss in the DSC, the counter electrode should have low resistance and high electrocatalytical activity for the I3-/I- redox couple reaction.5,6 It is significant to research the reaction mechanism of the I3-/I- redox couple, which is of central importance for improving the efficiency and extending the service life of the DSC. Following a mechanism first proposed by Vetter,7 several authors8–10 have discussed the oxidation reaction of I- on platinum with a rate-limiting electron-transfer step, followed by dimerization and desorption of the adsorbed intermediate and reaction to I3-. Dane´ et al.11 and Macagno et al.12 presented a distinct mechanism with two consecutive electrochemical steps, and the rate-limiting step was the production of iodine (I2) between adsorbed I and I- in the electrolyte. However, all the above mechanisms do not include diffusion in electrolyte as an indispensable step of the electrode reaction process. Furthermore, the influence of different I- concentrations and nitrogenous heterocyclic additives on the reaction mechanism of the I3-/I- redox couple was not considered. As we know, nitrogenous heterocyclic additives, such as benzimidazole (BI), 1-methylbenzimidazole (MBI), and 4-t-butylpyridine (TBP), are usually added into electrolytes for enhancing open-circuit voltage, fill factor, and efficiency.2 These additives adsorb on the surface of the TiO2 electrodes and prevent injected * To whom correspondence should be addressed. E-mail: sydai@ ipp.ac.cn.

electrons from leaking to the electrolyte and I3- from approaching the surface of the TiO2 electrodes.13 However, the influence of nitrogenous heterocyclic additives on the reaction mechanism of the I3-/I- redox couple at the platinized counter electrode/electrolyte interface has not yet been sufficiently investigated. The reaction mechanism of the I3-/I- redox couple at the platinized FTO glass counter electrode is very intricate, both nitrogenous heterocyclic additives and different I- concentrations in the electrolytes will affect the reaction mechanism. Consequently, the aim of the present paper is not only to carry out a detailed study about the reaction mechanism of the I3-/I- redox couple at the platinized FTO glass electrode/electrolyte interface but also to investigate the influence of different I- concentrations on the reaction mechanism of the I3-/I- redox couple in electrolytes without additives. The influence of nitrogenous heterocyclic additives on the reaction mechanism of the I3-/I- redox couple is also taken into account. 2. Experimental Section 2.1. Chemicals and Materials. Anhydrous lithium iodide (LiI), iodine (I2), and methoxypropionitrile (MePN) were used as supporting electrolytes in the electrochemical studies. Benzimidazole (BI) (Alfa Chemicals) was used as a functional additive. There were four electrolytes used in this work: (1) 0.1 mol/L LiI and 0.1 mol/L I2 in MePN, (2) 0.2 mol/L LiI and 0.1 mol/L I2 in MePN, (3) 0.7 mol/L LiI and 0.1 mol/L I2 in MePN, and (4) 0.7 mol/L LiI, 0.1 mol/L I2, and 0.5 mol/L BI in MePN. These concentrations and additives are usual for DSCs; I2 was partially transformed into I3- following the equation12,14

I2 + I- a I3

(1)

2.2. Cell Assembly. The counter electrode, as one important component in the DSC, is usually made of thermally deposited platinum (Pt) on fluorine-deposed tin oxide (FTO) glass. The platinized FTO glass electrodes were obtained by printing

10.1021/jp910055c  2010 American Chemical Society Published on Web 02/11/2010

Reaction Mechanism of a Triiodide/Iodide Redox Couple

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Figure 1. Platinized FTO glass electrode/electrolyte/platinized FTO glass electrode cell model.

H2PtCl6 paste onto FTO glass and then were sintered in a belt furnace at 420 °C for 30 min. To simulate and investigate the reaction mechanism of the I3-/I- redox couple at the platinized FTO glass electrode/electrolyte interface, the cell used for all measurements was designed to be as similar as possible to the application of the DSC. The cell used in our experiment was designed in a symmetric platinized FTO glass electrode/ electrolyte/platinized FTO glass electrode cell used elsewhere,10 as shown in Figure 1. The two identical platinized FTO glass electrodes were sealed with a Surlyn film (DuPont), which served as a spacer. The electrolyte was filled from the holes made on one of the platinized FTO glass electrodes, and later, the holes were sealed by a cover glass and thermal adhesive film. The electrode area was approximately 4.0 cm2, and the distance between the two electrodes was about 20.0 µm. In the symmetric platinized FTO glass electrode/electrolyte/ platinized FTO glass electrode cell, the electric field on the surface of the two identical electrodes is homogeneous, and diffusion is the only transport mechanism in the electrolyte. Convection does not occur because the cell is very thin; migration is negligible because the electric field in the inner cell is shielded due to the high ionic concentration in the electrolyte.15 2.3. Instrumentation. The particle size of platinum and the electrode microstructure were studied using a field emission scanning electron microscope (SEM) (Sirion 200, FEI Corp.). The surface composition of the platinized FTO glass electrode was studied on an ESCALAB 250 XPS (Thermo Electron Corp., U.S.) with a monochromatized Al KR X-ray source, and the binding energies were referenced to the C 1s neutral carbon peak at 284.6 eV. Cyclic voltammetry scanning with various scan rates and the polarization curves in different electrolytes were obtained at a scan rate of 0.005 V/s. The chronocoulometric method was used to characterize the diffusion coefficient of I3- in different electrolytes. All the electrochemical tests were performed on a CHI 660 electrochemical workstation (U.S.A., CHI Instrument

Figure 3. XPS wide scan spectrum of the platinized FTO glass electrode. The thermal decomposition temperature is 420 °C. The inset is the enlargement of the Pt 4f peak.

Co.) in the symmetric platinized FTO glass electrode/electrolyte/ platinized FTO glass electrode cells at room temperature, and all the cells used in our experiments were newly prepared. 3. Results and Discussion 3.1. Characterization of the Platinized FTO Glass Electrode. The surface morphologies of the FTO glass substrate and the platinized FTO glass electrode in the SEM images are shown in Figure 2. The surface morphology of the FTO glass substrate exhibited a nearly spherical particular structure of the plated tin oxide on the glass surface having an average size of approximate 100 nm, and the Pt particles with a mean size of 15 nm were distributed homogeneously on the surface of the FTO glass electrode. The surface composition of the platinized FTO glass electrode was studied by XPS measurements. From the XPS wide scan spectrum of the platinized FTO glass electrode, as shown in Figure 3, the XPS peak of chloride, indicating the absence of chloride element on the surface of the platinized FTO glass electrode, could not be found, which implied that H2PtCI6 was decomposed completely during the thermal process at 420 °C. The existence of a O 1s peak was ascribed to tin oxide and the adsorption of oxygen in the air on the surface of the platinized

Figure 2. SEM images of the FTO glass substrate (a) and platinized FTO glass electrode (b). The thermal decomposition temperature is 420 °C.

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Figure 4. Electrode reaction process of the I3-/I- redox couple reaction in the symmetric platinized FTO glass electrode/electrolyte/platinized FTO glass electrode cell.

FTO glass electrode; the existence of a C 1s peak was ascribed to the adsorption of carbon on the surface of the platinized FTO glass electrode due to air exposure. The inset (Figure 3) provided the enlargement of the Pt 4f peak. Two peaks of Pt 4f7 and Pt 4f5 at the positions of 70.8 and 74.15 eV, respectively, correspond to the binding energy of Pt(0).16,17 XPS measurements revealed that pure Pt(0) existed on the FTO glass substrate after thermal decomposition at 420 °C, serving as the catalyst. 3.2. Theory of Heterogeneous Catalysis Reaction. In the symmetric platinized FTO glass electrode/electrolyte/platinized FTO glass electrode cell, the reaction of the I-/I3- redox couple was catalyzed by pure Pt(0) existing on the platinized FTO glass electrode, which was a heterocatalytic reaction. According to the mechanism of heterocatalysis,18 the typical reduction reaction process of the I3-/I- redox couple at the platinized FTO glass electrode/electrolyte interface follows three main stages: (1) The molecules of I3- diffuse from the bulk solution to the vicinal area in the platinized FTO glass electrode and then adsorb on the surface of Pt(0). (2) The molecules of I3- adsorbed on the surface of Pt(0) take part in a chemical reaction. (3) The molecules of I- leave the surface of Pt(0) because of desorption and then diffuse to the bulk solution. The above process is shown in Figure 4. The overall reaction rate of the I3-/I- redox couple depends on the rate of the slowest step, and the slowest step is the rate-determining step in a chemical reaction. To accelerate the overall reaction rate, the slowest reaction step should be accelerated. 3.3. Reaction Mechanism of the I3-/I- Redox Couple at the Platinized FTO Glass Electrode/Electrolyte Interface in Low I- Concentration Electrolyte without Additives. Figure 5 shows cyclic voltammograms of the I3-/I- redox couple on the platinized FTO glass electrode with various scan rates.19 It can be detected that the potentials of the reduction peaks (Epc) (around -0.4 V) shifted negatively, while the potentials of the oxidation peaks (Epa) (around +0.4 V) shifted positively with the increase of scan rates and the peakto-peak separation (∆Ep) increased. According to cyclic voltammetry theory,20 the ∆Ep can illustrate the reversibility of the electrode process: the peak potential (Ep) of reversible reactions is independent of scan rates, and ∆Ep is less than 30 mV; in an irreversible electrode process, Ep has a linear relationship with the logarithm of the scan rate and the ∆Ep is more than 30 mV. Therefore, the above experimental results indicated that the reaction of the I3-/I- redox couple

Tang et al.

Figure 5. Cyclic voltammograms of the symmetric platinized FTO glass electrode/electrolyte/platinized FTO glass electrode cells with different scan rates: 0.1 V/s (curve 1), 0.2 V/s (curve 2), 0.4 V/s (curve 3), 0.6 V/s (curve 4), 0.8 V/s (curve 5), and 1.0 V/s (curve 6). The electrolyte is 0.1 mol/L LiI and 0.1 mol/L I2 in MePN (init E ) -0.80 V, high E ) 0.80 V, low E ) -0.80 V).

at the platinized FTO glass electrode/electrolyte interface was a quasi-reversible electrode process. A pair of adsorption current peaks apart from the normal redox current peaks on the cyclic voltammograms also can be seen from Figure 5; therefore, the adsorption of the I3-/I- redox couple on the platinized FTO glass electrode was strong adsorption. The Ep and peak current (Ip) of the quasi-reversible electrode reaction with surface adsorption were determined by eqs 2 and 321 * Ep ) E0eq(O*ads/Rads )-

Ip )

RT ln(BO /BR) nF

n2F2 0 νΓ * RT Oads

(2)

(3)

0 where Eeq (O*ads/R*ads)is the standard equilibrium potential of the O*ads/R*ads system, BO is the adsorption equilibrium constant of the reaction reactant, BR is the adsorption equilibrium constant of the reaction product, n is the number of electrons involved in the electrode reaction, F is the Faraday constant, R is the universal gas constant, T is the absolute temperature, and Γ0O*ads is the adsorption quantity of the reactant. In the I3- reduction reaction process, the reaction reactant was I3- and the reaction product was I-. The adsorption current peak was in front of the redox current peak, as shown in Figure 5; hence, Ep was 0 (O*ads/R*ads). From eq 2, we can obtain the more positive than Eeq following equation:

BR〉〉BO

(4)

Therefore, the adsorption capability of reduction product Ion the platinized FTO glass electrode was stronger than the adsorption capability of reduction reactant I3-, i.e., BI- 〉〉 BI3-. In the oxidation reaction process of I-, the reaction reactant was I- and the reaction product was I3-. The adsorption current peak was in the back of the redox current peak, as shown in Figure 5. From eq 2, we can obtain the following equation:

BO〉〉BR

(5)



Figure 6. Relationship of oxidation (around 0.4 V) and reduction (around -0.4 V) peak current vs scan rate in Figure 5.

Therefore, the adsorption capability of oxidation reactant Ion the platinized FTO glass electrode was stronger than the adsorption capability of oxidation product I3-; this experiment result also testified that BI- 〉〉 BI3-. For all the above reasons, desorption of I- from Pt was quite difficult because of BI- 〉〉 BI3- in the low I- concentration electrolyte. The reduction peak current (Ipc) and the corresponding oxidation peak current (Ipa) from the cyclic voltammograms in Figure 5 versus the scan rates (ν) are plotted in Figure 6. According to eq 3, the good linear relationship between the peak currents and scan rates in Figure 6 indicated that the reaction process of the I3-/I- redox couple was adsorption (desorption)-controlled. In the low I- concentration electrolyte (0.1 mol/L LiI and 0.1 mol/L I2 in MePN solvent), the reaction process of the I3-/I- redox couple was controlled by desorption of I- from the platinized FTO glass electrode. Therefore, we suggested the following reaction mechanism:

I3-bulk a I3-surf (diffusion of I3 ) fast

(a)

I3-surf + Pt a I3 (Pt) (adsorption of I3 on Pt) fast

(b) I3 (Pt) + Pt a I2(Pt) + I (Pt) (production of I3 ) fast (c)

I2(Pt) + e- a I(Pt) + I-(Pt) (reduction of I2) fast

(d) I(Pt) + e- a I-(Pt) (reduction of I) fast

(e)

I-(Pt) a Isurf (desorption of I- from Pt) slow

(f)

Isurf a Ibulk (diffusion of I-) fast

(g)

Dane´ et al.11 (for H2SO4 as the solvent) and Macagno et al.12 (for ACN as the solvent) suggested a similar pathway for the I3-/I- redox couple reaction on a pure Pt electrode, but they did not consider diffusion in the electrolyte as an indispensable step of the electrode reaction process. According to them, instead of step (f), the rate-determining step was

I(Pt) + I- a I2 + Pt + e-

(6)

Figure 7. Polarization curves of the symmetric platinized FTO glass electrode/electrolyte/platinized FTO glass electrode cells with different electrolytes: electrolyte 1, 0.1 mol/L LiI and 0.1 mol/L I2 in MePNl; electrolyte 2, 0.2 mol/L LiI and 0.1 mol/L I2 in MePN; electrolyte 3, 0.7 mol/L LiI and 0.1 mol/L I2 in MePN (init E ) 0.80 V, final E ) -0.80 V, scan rate ) 0.005 V/s).

However, Vetter14 (for H2SO4 as the solvent) and Hauch10 (for ACN as the solvent) suggested a different reaction mechanism, and they suggested that the rate-determining step was

I-(Pt) a I(Pt) + e-

(7)

We will proceed to analyze the influence of different Iconcentrations and nitrogenous heterocyclic additives on the reaction mechanism of the I3-/I- redox couple in MePN solvent. 3.4. Influence of Different I- Concentrations on the Reaction Mechanism of the I3-/I- Redox Couple. Tafel analysis is used to study the interfacial charge-transfer properties of the I3-/I- redox couple in the symmetric platinized FTO glass electrode/electrolyte/platinized FTO glass electrode cells with different electrolytes; the experimental data of polarization curves are fit to the Bulter-Volmer equation. The Butler-Volmer equation is one of the most fundamental relationships in electrochemistry. It describes how the electrical current on an electrode depends on the electrode potential, considering that both cathodic and anodic reactions occur on the same electrode:

{

η -C [ RnF RT ]

I ) nFAks COsurf exp

surf R

[

exp -

]}

βnF η RT

(8)

Where I is the electrode current, A is the electrode active surface area, ks is the standard equilibrium constant, η is the overpotential, COsurf is the reaction reactant concentration near the electrode, CRsurf is the reaction product concentration near the electrode, R is the anodic transfer coefficient, and β is the cathodic transfer coefficient. The exchange current can be calculated from the intersection of the linear anodic and cathodic branches of the polarization curves. Figure 7 shows the polarization curves of the symmetric platinized FTO glass electrode/electrolyte /platinized FTO glass electrode cells with different electrolytes, and the polarization experiment results are listed in Table 1. The exchange current was directly proportional to I- concentration in MePN solvent, as shown in Table 1. Figure 8 shows the cyclic voltammograms with various scan rates in the high I- concentration electrolyte (0.7 mol/L LiI and 0.1 mol/L I2 in MePN solvent); the adsorption current peaks disappeared from the cyclic voltammograms compared to Figure 5, so the adsorption of I3- (I-) on the platinized FTO glass

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TABLE 1: Polarization Experiment Results in the Symmetric Cells with Different Electrolytes electrolyte

cathodic tafel

anodic tafel

electrolyte 1 electrolyte 2 electrolyte 3

4.002 5.290 4.777

4.030 5.256 4.732

exchange current/A 1.421 × 10-3 1.695 × 10-3 4.559 × 10-3

electrode was changed to weak adsorption. It could be deduced that, in the high I- concentration electrolyte, the vast majority of reaction activity points on the platinized FTO glass electrode were occupied by I- because of its very strong adsorption captivity and blocked the adsorption of I3- (free I- in the electrolyte) on the platinized FTO glass electrode. The ∆Ep increased with scan rates increasing, and the oxidation peak currents and the corresponding reduction peak currents from the cyclic voltammograms in Figure 8 were plotted in Figure 9 versus the scan rates and were as well as expected for an adsorption-controlled process. Therefore, in the high I- concentration electrolyte (0.7 mol/L LiI and 0.1 mol/L I2 in MePN solvent), owing to CI3- 〈〈 CI-, the electrode reaction process of the I3-/I- redox couple was controlled by the adsorption of I3on the platinized FTO glass electrode:

I3-surf + Pt a I3 (Pt)

(9)

The adsorption character of I3- (I-) can also be determined by steady-state polarization measurements. The cathodic polarization curves about the reduction reaction of I3- to I- in different electrolytes are shown in Figure 10.

Figure 10. Cathodic polarization curves of the symmetric platinized FTO glass electrode/electrolyte/platinized FTO glass electrode cells with different electrolytes: electrolyte 1, 0.1 mol/L LiI and 0.1 mol/L I2 in MePN; electrolyte 2, 0.7 mol/L LiI and 0.1 mol/L I2 in MePN (init E ) 0.80 V, final E ) -0.80 V, scan rate ) 0.005 V/s).

In the low I- concentration electrolyte (0.1 mol/L LiI and 0.1 mol/L I2 in MePN solvent), the current had an semilogarithmic relationship with the overpotential (line 1, Figure 10), which indicated that the electrochemical reaction process of the I3-/I- redox couple was controlled by succeeding desorption of reaction product, according to the following equation21

ηc ) Eeq - E ) -

RT RT ln I0back + ln I nF nF

(10)

where ηc is the cathodic overpotential, E is the electrode 0 potential, Eeq is the equilibrium potential, Iback is the exchange current of succeeding desorption, and I is the electrode current. Therefore, the electrochemical reaction process of I3- reduction in the low I- concentration electrolyte was controlled by the succeeding desorption of I-. While in the high I- concentration electrolyte (0.7 mol/L LiI and 0.1 mol/L I2 in MePN solvent), there was a limiting current (a horizontal line) on the cathodic polarization curve (line 2, Figure 10); the limiting current was caused by the limiting adsorption rate of the reactant, according to eq 1121 Figure 8. Cyclic voltammograms of the symmetric platinized FTO glass electrode/electrolyte/platinized FTO glass electrode cell with various scan rates: 0.1 V/s (curve 1), 0.2 V/s (curve 2), 0.4 V/s (curve 3), 0.6 V/s (curve 4), and 0.8 V/s (curve 5). The electrolyte is 0.7 mol/L LiI and 0.1 mol/L I2 in MePN solvent (init E ) -0.80 V, high E ) 0.80 V, low E ) -0.80 V).

Figure 9. Relationship of oxidation and reduction peak currents vs scan rates in Figure 8.

ηc )

( )

Id RT ln nF Id - I

(11)

where Id is the limiting adsorption current. This phenomenon indicated that the electrochemical reaction process of the I3reduction in the high I- concentration electrolyte was controlled by the prepositive adsorption of I3-. 3.5. Influence of Nitrogenous Heterocyclic Additives on the Reaction Mechanism of the I3-/I- Redox Couple. The nitrogenous heterocyclic additives, such as BI, have often been added into the electrolytes to improve the performance of DSCs.22,23 The conduction band shift behavior of the nanostructure TiO2 electrode with additives and without additives in the electrolytes was obtained from spectroelectrochemical techniques by measuring the absorbance at 780 nm as a function of applied potential.22 The TiO2 band was shifted negatively by nitrogenous heterocyclic adsorption on its surface and complex between Li+ and additives in the electrolyte, which improved


Figure 11. Cyclic voltammograms of the symmetric platinized FTO glass electrode/electrolyte/platinized FTO glass electrode cell with various scan rates: 0.2 V/s (curve 1), 0.4 V/s (curve 2), 0.6 V/s (curve 3), 0.8 V/s (curve 4), and 1.0 V/s (curve 5). The electrolyte is 0.7 mol/L LiI, 0.1 mol/L I2, and 0.5 mol/L BI in MePN solvent (init E ) -0.80 V, high E ) 0.80 V, low E ) -0.80 V).


Figure 13. Steady-state voltammograms of the symmetric platinized FTO glass electrode/electrolyte/platinized FTO glass electrode cells with different electrolytes: electrolyte 1, 0.7 mol/L LiI and 0.1 mol/L I2 in MePN; electrolyte 2, 0.7 mol/L LiI, 0.1 mol/L I2, and 0.1 mol/L BI in MePN (init E ) -0.80 V, final E ) 0.80 V, scan rate ) 0.001 V/s).

(0.7 mol/L LiI, 0.1 mol/L I2, and 0.5 mol/L BI in MePN solvent) was only controlled by the diffusion of I3-:

I3-bulk a I3-surf

Figure 12. Relationship of oxidation and reduction peak currents vs the square root of the scan rate in Figure 11.

the performance of DSCs.24 We will analyze the influence of nitrogenous heterocyclic additives on the reaction mechanism of the I3-/I- redox couple at the platinized FTO glass electrode/ electrolyte interface. Figure 11 shows the cyclic voltammograms under various scan rates in the high I- concentration electrolyte with 0.5 mol/L BI, and ∆Ep increased with scan rates increasing. The peak currents from the cyclic voltammograms of Figure 11 versus the square root of the scan rate is shown in Figure 12. The linear relationship between the peak current Ip and the square root of the scan rate (ν1/2) indicated that the reaction process of the I3-/I- redox couple was diffusion-controlled, according to the Randles-Sevcik equation20,25

To understand the influence of BI on the reaction mechanism of the I3-/I- redox couple on the platinized FTO glass electrode, we investigated the diffusion of I3- in different electrolytes by a steady-state voltammetry method. Figure 13 shows the steady-state voltammograms of the symmetric platinized FTO glass electrode/electrolyte/platinized FTO glass electrode cells with different electrolytes. The limiting current in BI electrolyte was smaller than the limiting current in electrolyte without BI. The diffusion coefficient of I3- (DI3-) and I- (DI-) in the two electrolytes can be calculated from the anodic and cathodic steady-state limiting currents (ILO), according to the following equation:28

IOL )

nFADO1/2CO π1/2t1/2

( )

1/2

DO1/2ν1/2

(14)

Therefore, the DI3- in the electrolyte without BI was bigger than that of the electrolyte with BI, and the diffusion of I3became slower in the BI electrolyte (0.7 mol/L LiI, 0.1 mol/L I2, and 0.5 mol/L BI in MePN solvent). The radius of the redox active specie (r) was related to the diffusion coefficient (D) by the well-known Stokes-Einstein equation29,30

DO ) nF Ip ) KnFACO RT

(13)

kBT 6πrφ

(15)

(12)

where K is the constant, CO is the bulk concentration of the electroactive species, and DO is the diffusion constant of the electroactive species. In LiI/I2 mixed solution, I2 preferred to exist as I3- and the absorption peak position of I2 was not possible to determine;26 therefore, I2 could be neglected in the electrolyte with 0.7 mol/L LiI and 0.1 mol/L I2 in MePN. Because of the large excess of I- in the electrolyte, only the diffusion of I3- limited the current.27 Therefore, the reaction process of the I3-/I- redox couple in the electrolyte with BI

where kB is the Boltzmann constant and φ is the viscosity of the electrolyte. We can draw a conclusion from eqs 14 and 15 that rI3-/rI- was inversely proportional to (IIL-/II3-L)2:

rI-3 rI-

)

() IIL-

IIL-3

2

(16)

The experimental rI3-/rI- was bigger in the electrolyte with BI than that without BI, as shown in Table 2.

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TABLE 2: Ratio of the Radius of Triiodide/Iodide in Different Electrolytes electrolyte electrolyte 1 electrolyte 2

cathodic IL 1.797 × 10

-1

1.448 × 10-1

anodic IL 1.796 × 10

-1

1.460 × 10-1

TABLE 3: Chronocoulometric Experiment Results in the Symmetric Cells with Different Electrolytes

0.9989 (9.989 × 10-1) 1.016 (10.16 × 10-1)

Due to the electron donor-acceptor interaction between BI and I3-, BI reacted with I3- in the electrolyte similar to MBI22 and TBP:31 BI + I3 a BII2 + I

(17)

+ 2BI + I3 a 2BII + 2I

(18)

We assumed that rI3- was constant, and then, rI3- was increased with addition of BI into the electrolyte. It was indicated that the reason for formation of large complexes was because of BI interaction with I3- in the electrolyte. As it was described above, the reduced current after adding additive was mainly due to a smaller diffusion coefficient of the bulk I3- complex in the electrolyte with BI compared with that without BI. However, the platinized FTO glass electrode was very active and organic compounds, such as nitrogenous heterocyclic additives, could adsorb on the platinized FTO glass electrode surface and reduce the active surface area. The chronocoulometric method is a convenient and reliable technique for investigating the adsorbed reactants on an electrode;32 the influence of BI on the diffusion coefficient of I3-/I- was also suitable to be investigated by chronocoulometric means. Figure 14 shows the chronocoulometric curves of the symmetric platinized FTO glass electrode/electrolyte/platinized FTO glass electrode cells in the electrolytes with BI and without BI; the chronocoulometric curves obeyed the following equation33

Q)

2nFADO1/2CO π1/2

t1/2 + Qdl + Qads

Qdl + Qads

Q vs t1/2

rI3-/rI-

(19)

where Q is the total coulomb, the intercept of the plot is the sum of Qdl and Qads, Qdl is the double-layer capacitive charge (integration of charging current), and Qads is the Faradaic component given by the adsorbed species. The linear slope of

electrolyte

forward slope -1

electrolyte 1 -5.908 × 10 electrolyte 2 -4.353 × 10-1

reverse slope 1.398 1.033

Qdl/C

Qads/C -1

8.795 × 10 1.234 5.946 × 10-1 1.013

Q versus t1/2 and the intercept of chronocoulometric curves are listed in Table 3. When BI was added into the electrolyte, the linear slope of Q versus t1/2 curves from the chronocoulometric experiment decreased, as shown in Table 3, and the DI3- can be calculated from the slope of Q versus t1/2 curves. The DI3- in the electrolyte without BI was bigger than that of the electrolyte with BI; therefore, the diffusion of I3- in the electrolyte with BI became slow. Meanwhile, both Qdl and Qads descended in the BI electrolyte, as shown in Table 3. Because Pt(0) had a mutual electrostatic effect34 with the pair of electrons on the nitrogenous heterocyclic of BI, the BI could adsorb on the electrode surface; hence, the adsorption capacity of I3-/Ion the electrode decreased. 4. Summary The reaction process of the I3-/I- redox couple was controlled by adsorption in electrolyte without nitrogenous heterocyclic additives. In the low I- concentration electrolyte, the reaction process of the I3-/I- redox couple was controlled by the desorption of I- from the platinized FTO glass electrode, whereas in the high I- concentration electrolyte, the reaction process of the I3-/I- redox couple was controlled by the adsorption of I3- on the platinized FTO glass electrode. The nitrogenous heterocyclic additives, such as BI, could change the reaction mechanism of the I3-/I- redox couple at the platinized FTO glass electrode/electrolyte interface. BI could not only form a complex between BI and I3- but also adsorb on the electrode surface; hence, the adsorption capacity of I3-/ I- on the electrode decreased. The electrode reaction process of the I3-/I- redox couple became diffusion-controlled in the LiI/I2 electrolytes with nitrogenous heterocyclic additives. Acknowledgment. This work was financially supported by the National Basic Research Program of China under Grant No. 2006CB202600, the National High Technology Research and Development Program of China under Grant No. 2009AA050603, the Funds of the Chinese Academy of Sciences for Key Topics in Innovation Engineering under Grant No. KGCX2-YW-326, and the Knowledge Innovation Program of the Chinese Academy of Sciences No. 075FCQ0125. References and Notes

Figure 14. Influence of BI on chronocoulometric curves of the symmetric platinized FTO glass electrode/electrolyte/platinized FTO glass electrode cells with different electrolytes: electrolyte 1, 0.7 mol/L LiI and 0.1 mol/L I2 in MePN; electrolyte 2, 0.7 mol/L LiI, 0.1 mol/L I2, and 0.1 mol/L BI in MePN.

(1) Regan, B. O.; Gra¨tzel, M. Nature 1991, 353, 737–740. (2) Nazzruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Mueller, E.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6382–6390. (3) Hagfeldt, A.; Gra¨tzel, M. Chem. ReV. 1995, 95, 49–68. (4) Gra¨tzel, M. Nature 2001, 414, 338–344. (5) Papageorgiou, N.; Maier, W. F.; Gra¨tzel, M. J. Electrochem. Soc. 1997, 144, 876–884. (6) Fnag, X.; Ma, T.; Yang, G.; Akiyama, M.; Kida, T.; Abe, E. J. Electroanal. Chem. 2004, 570, 257–262. (7) Vetter, K. J. Phys. Chem. 1952, 199, 285–299. (8) Newson, J. D.; Riddiford, A. C. J. Electrochem. Soc. 1961, 108, 69–76. (9) Thomas, A. B.; Brodd, R. J. J. Phys. Chem. 1964, 68, 3363–3367. (10) Hauch, A.; Georg, A. Electrochim. Acta 2001, 46, 3457–3466. (11) Dane´, L. M.; Janssen, L. J. J.; Hoogland, J. G. Electrochim. Acta 1968, 13, 507–518.

Reaction Mechanism of a Triiodide/Iodide Redox Couple (12) Macagno, V. A.; Giordano, M. C.; Arvia, A. J. Electrochim. Acta 1969, 14, 335–357. (13) Hagfeldt, A.; Gra¨tzel, M. Chem. ReV. 1995, 95, 49–68. (14) Vetter, K. J. Elektrochemische Kinetik; Springer: Berlin, 1961. (15) Greef, R.; Peat, R.; Peter, L. M.; Pletcher, D.; Robinson, J. Instrumental Methods in Electrochemistry; Ellis Horwood Ltd.: Chichester, England, 1985. (16) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-Ray Photoelectron Spectroscopy; Perkin-Elmer Physical Division: Eden Prairie, MN, 1992. (17) Bancroft, G. M.; Adams, I.; Coatsworth, L. L.; Bennewitz, C. D.; Brown, J. D.; Westwood, W. D. Anal. Chem. 1975, 47, 586–588. (18) Li, T.; Yang, Y.; Hou, J.; Chen, S.; Liu, R.; Wang, S. Forensic Sci. Int. 2006, 162, 113–120. (19) Randriamahazaka, H.; Sini, G.; Tran Van, F. J. Phys. Chem. C 2007, 111, 4553–4560. (20) Sevcik, A. Collect. Czech. Chem. Commun. 1948, 13, 349–377. (21) Zha, Q. X. Kinetics Instructional Theory of Electrode Processes; Science Press: Beijing, 2002. (22) Zhang, C. N.; Dai, J.; Huo, Z. P.; Pan, X.; Hu, L. H.; Kong, F. T.; Huang, Y.; Sui, Y. F.; Fang, X. Q.; Wang, K. J.; Dai, S. Y. Electrochim. Acta 2008, 53, 5503–5508. (23) Kusama, H.; Arakawa, H. Sol. Energy Mater. Sol. Cells 2006, 85, 333–338.

J. Phys. Chem. C, Vol. 114, No. 9, 2010 4167 (24) Nilsing, M.; Lunell, S.; Persson, P.; Ojama, L. Surf. Sci. 2005, 582, 49–60. (25) Randles, J. E. B. Trans. Faraday Soc. 1948, 44, 327–338. (26) Kebede, Z.; Lindquist, S. E. Sol. Energy Mater. Sol. Cells 1999, 57, 259–275. (27) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; John Wiley & Sons Inc.: NewYork, 1980. (28) Atkins, P. W. Physical Chemistry; Oxford University Press: Oxford, U.K., 1994. (29) Hayamizu, K.; Aihara, Y.; Nakagawa, H.; Nukuda, T.; Price, W. S. J. Phys. Chem. B 2004, 108, 19527–19532. (30) Saito, Y.; Hirai, K.; Matsumoto, K.; Hagiwara, R.; Minamizaki, Y. J. Phys. Chem. B 2005, 109, 2942–2948. (31) Boschloo, G.; Haggman, L.; Hagfeldt, A. J. Phys. Chem. B 2006, 110, 13144–13150. (32) Anson, F. C.; Osteryoung, R. A. J. Chem. Educ. 1983, 60, 293– 296. (33) Bard, A. J.; Faulkner, L. R. Electrochemical Methods Fundamentals and Applications, 2nd ed.; John Wiley & Sons Inc.: New York, 2001. (34) Seki, S.; Kobayashi, Y.; Miyashiro, H.; Ohno, Y.; Mita, Y.; Terada, N.; Charest, P.; Guerfi, A.; Zaghib, K. J. Phys. Chem. C 2008, 112, 16708–16713.

JP910055C

Influence of Different Electrolytes on the Reaction Mechanism of a

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