DOI: 10.1002/cssc.201403475
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Nanocomposite Semi-Solid Redox Ionic Liquid Electrolytes with Enhanced Charge-Transport Capabilities for DyeSensitized Solar Cells Iwona A. Rutkowska,[a] Magdalena Marszalek,[b] Justyna Orlowska,[a] Weronika Ozimek,[a] Shaik M. Zakeeruddin,[b, c] Pawel J. Kulesza,*[a] and Michael Grtzel*[b] The ability of Pt nanostructures to induce the splitting of the I¢I bond in iodine (triiodide) molecules is explored here to enhance electron transfer in the iodine/iodide redox couple. Following the dispersal of Pt nanoparticles at 2 % (weight) level, charge transport was accelerated in triiodide/iodide-containing 1,3-dialkylimidazolium room-temperature ionic liquid. If both Pt nanoparticles and multi-walled carbon nanotubes were introduced into the ionic-liquid-based system, a solidtype (nonfluid) electrolyte was obtained. By using solid-state voltammetric (both sandwich-type and microelectrode-based)
methodology, the apparent diffusion coefficients for charge transport increased to approximately 1 Õ 10¢6 cm2 s¢1 upon the incorporation of the carbon-nanotube-supported iodine-modified Pt nanostructures. A dye-sensitized solar cell comprising TiO2 covered with a heteroleptic RuII-type sensitizer (dye) and the semisolid triiodide/iodide ionic liquid electrolyte admixed with carbon-nanotube-supported Pt nanostructures yielded somewhat higher power conversion efficiencies (up to 7.9 % under standard reporting conditions) than those of the analogous Pt-free system.
Introduction The triiodide/iodide redox system has been the most commonly and most successfully used charge relay (mediator) in dyesensitized solar cells (DSCs).[1–8] The redox electrolyte plays a very important role in the DSC performance, and its usefulness largely depends on the dynamics of both interfacial electron transfer and bulk charge propagation within the system. Although photoelectric conversion efficiencies as high as 13 % have recently been obtained for porphyrin-sensitized solar cells utilizing organic liquid electrolytes containing cobalt(III/II) complexes,[9] triiodide/iodide-based systems have provided the most promising results in the area of practical DSCs. Nevertheless, problems such as leakage or evaporation of organic-solvent-based volatile liquids would require special attention such as careful sealing; otherwise, the cell performance would deteriorate during the long-term operation of the system.[10] To overcome these problems, several attempts, including the application of p-type semiconductors,[11] organic and inorganic hole conductors,[12, 13] and gel or polymer electrolytes,[14–24] have been proposed. There has also been growing interest in room[a] Dr. I. A. Rutkowska, J. Orlowska, W. Ozimek, Prof. P. J. Kulesza Faculty of Chemistry University of Warsaw Pasteura 1, 02-093 Warsaw (Poland) E-mail: [email protected] [b] Dr. M. Marszalek, Dr. S. M. Zakeeruddin, Prof. M. Grtzel Laboratory of Photonics and Interfaces Ecole Polytechnique F¦d¦rale de Lausanne (EPFL) Station 6, 1015 Lausanne (Switzerland) E-mail: [email protected] [c] Dr. S. M. Zakeeruddin Center of Excellence for Advanced Materials Research (CEAMR) King Abdulaziz University, Jeddah (Saudi Arabia)
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temperature ionic liquids, especially those with 1,3-dialkylimidazolium cations owing to their advantages such as negligible vapor pressure, high ionic conductivity and thermal stability, fairly wide electrochemical window, and ability to dissolve organic and inorganic solutes.[25, 26] The triiodide/iodide redox couple has been considered together with ionic liquids.[27, 28] The resulting redox-conducting electrolytes have several advantages such as high conductivity, low vapor pressure, high iodide concentration, and good electrochemical stability. Among their disadvantages is their high viscosity, which certainly contributes to the low mass-transport coefficient of the triiodide/iodide redox couple,[29] not only if the charge transport mechanism is predominantly physical at low concentrations but also if the Grotthus exchange mechanism is operative at high concentrations of the redox system. In general, both interfacial and bulk (self-exchange) electron transfers involving the triiodide/iodide redox system are somewhat complicated and appear slower than one would expect. Among the kinetic limitations there is a need to break the I¢I bond in the I3¢ or I2 molecule; it has also been well-established that Pt (e.g., deposited on the counter electrode) induces electron transfer within the iodine/iodide redox system. Strong interactions of Pt with iodide or iodine have been described.[30, 31] Notably, the iodine covalent radius (0.133 nm) and the Pt atomic radius (0.135 nm) are comparable. The formation of monolayer-type coverages of strongly adsorbed monoatomic iodine together with weakly bound electroactive iodine/iodide has also been postulated. In the present work, we explore the interfacial (electrocatalytic) phenomena of nanostructured Pt (namely, Pt nanoparticles three-dimensionally distributed in the electrolyte phase at 2 % weight level) and utilize them to
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Full Papers enhance triiodide/iodide electron transfers to develop more efficient charge relays for DSCs. Finally, to make the electrolyte more solid (nonfluid) and to improve the overall electron distribution within the redox-conducting electrolyte, we also introduce multi-walled carbon nanotubes (CNTs) into our nanocomposite system at 10 % weight level as supports for the dispersed iodine-modified Pt nanoparticles. The usefulness of admixing ionic liquid electrolytes with CNTs has also been demonstrated independently by others.[32–36] In particular, recent reports have emphasized the utility of platinized CNTs as the counter electrode material for DSCs with ionic liquids[37] as well as the catalytic activity of CNTs during the electrochemical reduction of I3¢[38] and their ability to facilitate electron transport through the ionic liquid electrolyte.[39] By using the microelectrode-based and sandwich-type electroanalytical methodologies of solid-state electrochemistry,[40, 41] we address here the charge-transport dynamics within the semisolid triiodide/ iodide ionic liquid electrolyte admixed with CNT-supported Pt nanostructures and comment on the reasonably high power conversion efficiencies of DSCs utilizing such electrolytes.
Results and Discussion Physicochemical identity of iodine-modified Pt nanoparticles To support the preparation procedures described in Experimental Section, we provide TEM images (Figure 1 A) of the iodine-modified Pt (Pt/I) nanoparticles. They have diameters of 4–6 nm, which are comparable to those determined previously for such Pt nanoparticles utilized in electrocatalysis.[42] Upon the dispersal of the nanoparticles in the aqueous liquid phase, we determined their electrokinetic zeta potential, which is approximately ¢32 mV (Pt/I) compared with ¢13 mV for bare Pt nanoparticles. First, this difference in zeta potential supports the view that iodine is present on the Pt. Second, it is generally accepted that zeta potentials with magnitudes exceeding 30 mV imply electrostatic repulsion and are a key indicator of the stability of colloidal suspensions.[43] Further, despite some statistical uncertainty (standard deviation) of 4–7 mV, a shift of the zeta potential toward more negative values upon the modification of the Pt nanostructures with iodine is indicative of a positive change in the net electrical charge at the interface formed by the nanoparticles decorated with iodine. The higher positive charge for Pt/I relative to that of bare Pt is consistent with the historic view for bulk Pt[40, 41] regarding monolayer-type coverages of monoatomic iodine (rather than anionic iodide) on Pt. The maximum coverage for irreversibly adsorbed monoatomic iodine was postulated to reflect a 1:1 atomic model in correspondence with the Pt surface.[40, 41] The formation of stable adsorbate iodine layers on Pt is also clear from conventional voltammetric diagnostic experiments performed in sulfuric acid (Figure 1 B), in which the responses characteristic of bare clean Pt, namely, hydrogen adsorption peaks (in the potential range 0–0.3 V) and Pt oxide formation peaks (at potentials higher than 0.7 V), are largely diminished after the iodine adsorption (compare the solid and dashed ChemSusChem 2015, 8, 2560 – 2568
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Figure 1. A) TEM micrograph of dried iodine-modified Pt nanoparticles; B) voltammetric probing of the interfacial behavior of the bare (solid line) and iodine-modified (dashed line) Pt nanoparticles deposited (100 mg cm¢2) on a glassy carbon disk electrode (electrolyte: deaerated 0.5 m H2SO4 ; scan rate: 10 mV s¢1). C) I-modified Pt nanoparticles supported on multi-walled carbon nanotubes.
lines in Figure 1 B). Notably, the iodine adsorbates on Pt are not electroactive. These observations agree with the literature data.[30, 31] It was postulated that up to 2 Õ 10¢9 mol cm¢2 of monoatomic iodine could be irreversibly adsorbed in the nonelectroactive state on the polycrystalline Pt surface. To obtain a solid-type (nonfluid) electrolyte, both Pt nanoparticles and multi-walled carbon nanotubes were introduced to the ionic-liquid-based system. A TEM image of the multiwalled-CNT-supported iodine-modified Pt (Pt/I) nanoparticles is shown in Figure 1 C. Here, it is apparent that the CNTs (thick-
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Full Papers nesses 10–12 nm; lengths in mm range) act as carriers for the Pt nanoclusters; although most of the Pt nanoparticles are dispersed, some of them form agglomerates (up to 20 nm). In the presence of such an extended network of CNT/Pt/I nanostructures, electron distribution should be facilitated in the redox electrolyte.[39]
Microelectrode-based diagnostic experiments To comment quantitatively on the enhancement of charge propagation through the incorporation of Pt-based nanostructures in triiodide/iodide-containing ionic liquids, the microelectrode-based methodology developed for solid-state voltammetry[40, 41] was considered (Figure 2 A). The usefulness of micro-
Figure 3. Microelectrode-based cyclic voltammetry of iodine/iodide redoxconducting electrolytes recorded at 50 mV s¢1 with a solid-state cell with a planar three-electrode configuration A) without Pt nanoparticles and upon the incorporation of B) Pt nanoparticles (2 % weight) and C) CNT-supported iodine-modified Pt (Pt/I) nanoparticles (2 % weight). Microdisk diameter 30 mm.
rameter [Eq. (1)] is expected to be approximately 1 (or more precisely 0.1 < t < 1; r is the electrode radius):[40, 41, 45–47] t ¼ 4 DAPP t=r2
Figure 2. Schematic diagrams of A) a microelectrode-based solid-state type cell with a planar three-electrode configuration and B) a two-electrode solidstate type cell in a sandwich configuration.
electrodes in studies of ionic-liquid-type systems has also been demonstrated by others.[44] The voltammetric responses recorded in the bulk triiodide/iodide redox-conducting electrolytes are illustrated in Figure 3 for an electrolyte without Pt nanoparticles and electrolytes with iodine-modified Pt nanoparticles and CNT-supported iodine-modified Pt (CNT/Pt/I) nanoparticles incorporated. The results are consistent with the view that all systems are well-behaved and the electron transfers are reversible. It should be remembered that the data refer to semi-infinite diffusion conditions. The voltammetric peak currents increase upon the addition of iodine-modified Pt, particularly for CNT/Pt/I; these results imply that the dynamics of charge transport become more facile in this case. From the shape of the voltammograms in Figure 3 and as the nature of mass transport depends on the time domain, it seems that a mixed (linear/radial) diffusional pattern is applicable here. Indeed, the voltammetric peaks of Figure 3 are rather flat, particularly those of the systems containing iodine-modified Pt. Under such conditions, the t dimensionless time paChemSusChem 2015, 8, 2560 – 2568
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ð1Þ
It should be remembered that for fast-scan-rate (short time) experiments with t ! 1, the diffusion field is small in comparison to the electrode radius, and the charge-transport mechanism follows the linear diffusional pattern. Consequently, a typical voltammetric peak with a diffusional term appears. On the other hand, for t @ 1, which corresponds to relatively slowscan-rate (long time) experiments, the hemispherical diffusion layer largely exceeds the size of the electrode, and the spherical (radial) diffusional pattern (leading to sigmoidal steadystate voltammograms) becomes predominant. The mixed diffusional pattern (t 1), which is applicable to cyclic voltammograms recorded at 50 mV s¢1 (Figure 2), permits us to estimate the charge transport (DAPP) parameter. As the experimental time (t) is roughly 2–3 s and r = 1.5 Õ 10¢3 cm, DAPP is expected to be approximately 10¢7 cm2 s¢1. Charge transfer in concentrated semisolid, semiliquid, or even liquid solutions of electrochemically well-behaved redox couples can occur concurrently by physical diffusion (DPHYS) and by electron hopping or self-exchange (kEX) between mixed-valent redox sites. The coupling of diffusion and electron rates was initially developed to describe electron self-exchange reactions in solutions of redox ions[48] and later expressed in the corrected form with DPHYS and kEX separated in the Dahms–Ruff relation [Eq. (2)]:[49, 50] DAPP ¼ DPHYS þDE ¼ DPHYS þkEX s2 C=6
ð2Þ
where DE, DAPP, and s are the electron diffusion coefficient, apparent (or effective) diffusion coefficient, and the average (equilibrium) distance between the redox sites in the investigated system, respectively. For typical liquid systems, DPHYS is usually a dominating kinetic factor, and any contribution from
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Full Papers electron hopping (DE) would only be important if the population (concentration) of redox centers (C) and kEX have high values. For semisolid (i.e., denser or more viscous than typical liquids) systems, DPHYS is low enough to play any important role in the overall charge-transport dynamics. To design systems capable of fast charge propagation under such conditions, care has to be exercised to make the DE contribution large enough not only by keeping C high but also by increasing electron hopping (kEX) rates to develop sizeable currents that are effectively diffusion-controlled.[51] For the present triiodide/iodide-containing ionic liquid, the truly physical diffusion of triiodides and iodides within the ionic liquid is rather slow, and the latter requirement for a high self-exchange rate constant translates to the necessity of fast electron transfer between iodine (I) and iodide (I¢). These elementary acts of electron transfer within the I/I¢ redox couples would require the existence of monoatomic I rather than I2 or I3¢ . In other words, to avoid complications with the slow chemical step,[45, 46] one has to has to develop a means of catalyzing the I¢I bond dissociation (151 kJ mol¢1). In other words, the ideal situation would appear if the I¢I bond-breaking process is fast enough to be neglected in kinetic terms. Under such conditions, the DE term in Equation (2) becomes sizeable enough to facilitate fast and effectively diffusional ( DAPP) charge transport (redox conduction).[40, 41, 51–53] The typical values for the diffusion coefficients (physical) of iodide, iodine, and triiodide in different nonaqueous solvents (e.g., DMF, DMSO) are 3 to 6 Õ 10¢6 cm2 s¢1.[54–56] The denser environments of our redox-conducting electrolytes, which are characterized by rather high viscosities of 40 cP at 25 8C or higher, should result in significantly reduced mobility (to which the physical diffusion rates should be inversely proportional) for the triiodide/iodide system. This comment applies particularly to electrolytes containing Pt nanostructures that are visually semisolid. Although the electrolytes are fairly viscous, their ionic conductivities (which are enhanced by the presence of large populations of I¢ ions) are sufficient to support microelectrode voltammetry. Clearly, the viscosities of our systems are not as high as those of redox melts (2–4 Õ 106 cP at 25 8C), which are characterized by DPHYS as low as 10¢13 to 10¢12 cm2 s¢1.[51, 57] In conclusion, as the DPHYS values for the iodine/iodide systems considered here are lower than those for liquids, the observed DAPP values cannot be controlled entirely by physical diffusion. If the concentration of redox centers is known, reliable charge propagation information (DAPP) can be obtained from long-time experiments with microelectrodes. Voltammetry at a slow-scan rate of 1 mV s¢1 (Figure 4) yields almost steadystate limiting currents (ILIM), which level out as radial diffusion conditions are attained. In this respect, the wave-shape response is typical; the appropriate radial diffusion equation[40, 41] is shown in Equation (3): ILIM ¼ 4 n Fr DAPP C
ð3Þ
where F is Faraday constant and r stands for radius of the microdisk electrode. More exact DAPP values can be obtained by ChemSusChem 2015, 8, 2560 – 2568
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Figure 4. Slow scan rate (1 mV s¢1) microelectrode-based voltammetry of iodine/iodide redox-conducting electrolytes A) without Pt nanoparticles and upon the incorporation of B) Pt nanoparticles and C) CNT-supported iodinemodified Pt (Pt/I) nanoparticles. The other conditions are the same as those for Figure 1.
Equation (3) because the charge-transport mechanism is clearly defined here (spherical) and the effects of uncompensated resistance (iRUNC) are less pronounced at slow scan rates. Under the conditions of the microelectrode-based voltammetric transient experiment in a three-electrode configuration, concentration gradients of mixed-valence redox sites can be generated during the appropriate potential scans.[48, 52, 53, 58, 59] The appearance of both negative and positive currents (Figure 4) reflects the presence of both oxidized (I2) and reduced (I¢) forms in the electrolytes. We start our analytical considerations from the oxidation steps (at 0.1 V in Figure 4) as the iodine species are first reduced at the microdisk electrode (this explains the origin of the negative currents below 0.6 V in Figure 4). The concentration gradients (effectively electron transfer between mixed-valence sites) are maximized at a 1:1 ratio of the oxidized to reduced forms.[40, 41, 60] No electron hopping is feasible between the fully oxidized or reduced species. As the total concentration of iodine (monoatomic) and iodide species in our electrolytes is 0.22+ +1.6 = 1.82 mol L¢1, the maximum population or concentration of mixed-valent I/I¢ sites to be generated would be half as much and equal to C = 9.1 Õ 10¢4 mol cm¢3. Under such conditions, the current should be maximized. Assuming that the monoatomic/anionic I/I¢ system can form ideal welldefined electron acceptor/electron donor sites, the following DAPP values (20 8C) have been determined by using Equation (3) with the data in Figure 3: 5.5 Õ 10¢7, 8.5 Õ 10¢7, and 9.5 Õ 10¢7 cm2 s¢1 for the triiodide/iodide redox-conducting electrolytes without Pt nanoparticles and upon the incorporation of iodine-modified Pt nanoparticles and CNT-supported iodinemodified Pt nanoparticles (CNT/Pt/I), respectively. To relate DAPP to kEX precisely, an estimate of DPHYS is required in Equation (2). The actual value of DPHYS is expected to be lower than DAPP because of the high viscosity of the system. With the assumption that the electron self-exchange term in Equation (2) is approximately equal to the measured DAPP at the highest concentration of mixed-valence sites (ca. 0.9 m), kEX becomes approximately 1010 m¢1 s¢1. Such high electron self-exchange rates are known for electron transfers between Mo(VI,V) or W(VI,V) in single crystals of heteropolyacids of mo-
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Full Papers lybdenum and tungsten.[53, 58, 59] In the present iodine/iodide system, it is reasonable to expect mutual compensation between physical and effective (electron self-exchange type) diffusion mechanisms. The increased currents (and values of diffusion coefficients, practically DE values) after the incorporation of Pt nanostructures reflect the existence of a Pt-induced catalytic enhancement effects toward the I¢I bond splitting in triiodide molecules. The current (and DE) increases are even more pronounced upon the incorporation of CNTs. Even if the CNTs are not catalytically active (we have no clear evidence for such an effect), the dispersed carbon nanostructures should provide percolation pathways and facilitate electron transport with the ionic-liquid-based charge relay. For electron transfers to be operative, the distances between the Pt nanoparticles and/or carbon nanotubes should be in the nanometer range. Although a direct estimation of the average distances between the Pt nanoparticles with use of the idealized cubic lattice model[40, 41, 48] (and by considering their loading and average sizes) is feasible, the actual distances for electron hops must be lower (< 10 nm) owing to the large population of CNT supports of different shapes. Further, the effective path for an electron transfer is expected to be even shorter because the Pt sites tend to be modified not only with irreversibly adsorbed monoatomic iodine but also with additional layers of electroactive iodine (~ 1 Õ 10¢9 mol cm¢2).[30, 31] Comparative measurements in sandwich configuration To preserve electroneutrality during charge transport in the iodine/iodide electrolytes, electron transfer to iodine must be accompanied by the unimpeded motion of iodide anions in the opposite direction.[40, 41, 48, 52] The extent of the displacement of the ions during individual electron hops is often unclear. It is commonly accepted that the motion of the ions may become rate-determining in the overall dynamics of charge transport (then termed macroscopic)[40, 52] during transient electrochemical experiments (e.g., voltammetric-type as in Figures 3 and 4). On the other hand, if the current–potential experiment is performed in a sandwich configuration (Figure 2 B) and the steady-state limiting current (ILIM) is observed, the concentration gradients C/d (d is a thickness for the investigated mixed-valence system that is defined by the distance between the sandwich-forming electrodes) are maximized, and the current flow would not require significant displacement of counterions, that is, only microscopic motion is needed.[40, 41, 48, 52] Under such conditions, the electron-hopping event is related primarily to the electron-transfer dynamics (electron diffusion, DE) rather than to the motion (diffusion) of the charge-compensating ionic species. A relevant relationship is described by Equation (4):[40, 41, 59] ILIM ¼ n F A C DE =d
ð4Þ
A is the cross-sectional area of the sandwich system, and C is the concentration of mixed-valence sites (here, the ratio of I to I¢ is fixed, and C = 0.22 mol L¢1). In general, the sandwich configuration seems to better resemble the situation occurring in ChemSusChem 2015, 8, 2560 – 2568
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DSCs in which the iodine/iodide electrolyte is “sandwiched” between the dye-modified TiO2 photoanode and the platinized optically transparent counter electrode. Thus, steady-state experiments are probably more suitable than transient experiments for the determination of electron-hopping rates (DE). The current responses versus the potential differences (DE) applied between the electrodes are illustrated in Figure 4 for the iodine/iodide redox-conducting electrolytes. In all cases, steady-state limiting currents were observed. From the data in Figure 5, DE values of 4.5 Õ 10¢7, 9.0 Õ 10¢7, and 1.1 Õ
Figure 5. Voltammetric (current vs. potential difference) responses recorded at 10 mV s¢1 with a solid-state cell in a sandwich configuration with iodine/ iodide redox-conducting electrolyte A) without Pt nanoparticles and upon the incorporation of B) iodine-modified Pt (Pt/I) nanoparticles and C) CNTsupported iodine-modified Pt (CNT/Pt/I) nanoparticles.
10¢6 cm2 s¢1 were estimated for the triiodide/iodide electrolytes without Pt nanoparticles and with iodine-modified Pt nanoparticles and CNT/Pt/I nanostructures introduced, respectively. As expected,[40, 41, 48, 52] the DE values obtained here are somewhat higher than those DAPP coefficients determined from transient microelectrode-based experiments (Figure 4). The use of Equation (4) generally requires low d values, that is, the application of thin films of investigated systems. For a given experimental time t, this thickness must not exceed the diffusion layer thickness (2 DEt)1/2.[40, 41, 48] Otherwise, instead of plateau currents, symmetrical peaks at approximately 0 V would be observed. Under the time scale of the voltammetric experiment with iodine/iodide redox-conducting salts (Figure 5), the diffusion layer thickness is in the range 90– 140 mm, that is, it is larger than d = 20 mm; thus, the use of Equation (4) is justified. On the whole, the results are consistent with the view that the introduction of Pt/I nanoparticles to the triiodide/iodide-containing electrolytes enhances the charge propagation rates (DAPP and DE) and related voltammetric currents regardless of the measurement approach applied. It is tempting to rationalize the effect in terms of the electrocatalytic activity of the nanostructured Pt on the triiodide/iodide electron transfers.
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Full Papers Probing the influence of Pt nanoparticles on iodine/iodide electrochemical behavior As mentioned, I2, I3¢ , and I¢ species adsorb strongly on Pt surfaces. An iodine fractional surface coverage of 0.5 was postulated for aqueous solutions;[30] a maximum of approximately 2 Õ 10¢9 mol cm¢2 of iodide or monoatomic iodine could be irreversibly adsorbed into a nonelectroactive state at a reduced Pt surface. It was estimated that the maximum coverage for irreversibly adsorbed iodine was approximately in 1:1 atomic correspondence with the Pt surface. An additional 1 Õ 10¢9 mol cm¢2 of iodine (but not anionic iodide) was adsorbed in an electroactive state at the surface bearing irreversibly adsorbed iodine. Consequently, the Pt surface was largely inert, that is, the formation of PtO and PtOH and the adsorption of hydrogen were largely suppressed.[30, 31] In the present work, we clarify that Pt nanoparticles of 4–6 nm can be also modified with both irreversibly adsorbed and additional layers of electroactive iodine in an analogous manner to polycrystalline bulk Pt. To investigate the behavior and possible adherence of the iodine/iodide system on Pt nanoparticles, we performed cyclic voltammetric diagnostic experiments (Figure 6) with a glassy
Figure 6. Diagnostic conventional cyclic voltammetric experiments: responses of the iodine/iodide system A) in solution (0.5 m KI and 0.5 m H2SO4) and B, C) the adsorbate upon medium transfer to the electrolyte (0.5 m H2SO4) only. Although (A) and (B) refer to responses recorded for the deposits of Pt nanoparticles, the response of CNT-supported Pt is presented in C). Electrode substrate: glassy carbon. Scan rate: 10 mV s¢1.
carbon electrode with Pt nanoparticles dispersed on the surface (loading, 100 mg cm¢2). The data in Figure 6 A are consistent with fairly distorted (electrochemically irreversible) behavior of the triiodide/iodide system in solution (separation of peaks ~ 200 mV). Upon transfer to the solution containing only supporting electrolyte, a set of symmetrical surface-type peaks were observed (Figure 6 B). The result is consistent with the view that, in addition to the irreversibly adsorbed monoatomic iodine, a large portion of the iodine/iodide system exists in the vicinity the electrode surface and adheres to nanostructured Pt/I sites (no significant adsorption on the bare glassy carbon occurs); therefore, reversible behavior consistent with fast electron transfer between iodine and iodide species is observed. On the basis of our data (Figure 6 B) and previous reports,[37, 38] ChemSusChem 2015, 8, 2560 – 2568
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it is also reasonable to expect that, regardless the mechanistic details, the adsorption of iodine on the surfaces of the Pt nanoparticles facilitates the dissociation of the I¢I bonds in I3¢ molecules. From the above observations, we can expect that the incorporation of iodine-modified Pt nanoparticles and their three-dimensional distribution within electrolyte layers produces a case between heterogeneous and homogenous electrocatalysis. The fact that the CNT-supported iodine-modified Pt (CNT/ Pt/I) produces the highest currents (Figures 3–5) can be interpreted in terms of better charge distribution within the redoxconducting electrolyte. We have found that iodine can also be adsorbed on CNT/Pt/I (Figure 6 B). The adsorption is as strong and durable on multi-walled CNTs as it is on Pt. Furthermore, the catalytic effect originating from the CNTs is not as pronounced as it is for Pt because the respective anodic and cathodic peaks originating from the iodine attached to the CNTs are much more drawn out and separated (Figure 6 B). Further research is needed in this area. Nevertheless, the addition of CNTs to electrolytes makes the electrolyte more solid (nonfluid) and facilitates better distribution of charge. Application of hybrid electrolytes in DSCs The promising iodine-modified Pt-based nanostructures have been tested as an additive to the redox-conducting electrolyte in dye-sensitized solar cells. As the electrolyte, we used a stateof-the-art ionic-liquid-based electrolyte[61] with slight modifications. It should be remembered that ionic-liquid-based electrolytes were successfully used in DSCs and provided efficient, stable devices, but their practical efficiency was typically limited by charge-transport issues related to the high viscosity of the medium. Therefore, the introduction of catalytic Pt-based centers to facilitate iodine–iodide electron transfers and the overall charge-transport dynamics seems to be justified. High concentrations of the redox (mediator) may permit the electron-self-exchange mechanism to be operative. In this study, we used the standard Z952 electrolyte [1,3-dimethylimidazolium iodide/1-ethyl-3-methylimidazolium iodide/1-ethyl-3-methylimidazolium tetracyanoborate/I2/N-butylbenzimidazole/guanidinium thiocyanate (DMII/EMII/EMITCB/I2/NBB/GuNCS) in the molar ratio 12:12:16:1.67:3.33:0.67] diluted with sulfolane (1:1), coded as Z988. The electrolyte is characterized by reduced viscosity, but the transport issues may be improved. The addition of nonvolatile sulfolane does not affect the stability of devices. For the photovoltaic study, we built a set of cells with doublelayered TiO2 (8+ +5 mm thick), C101 dye, and a standard Pt counter electrode. Herein, we want to focus on the comparison of the devices filled with the Z988 electrolyte and the systems containing the iodine-modified Pt nanoparticles and the CNTsupported iodine-modified Pt nanostructures (Figure 7). The freshly prepared cells have been considered, but the actual measurements required relaxation time to allow good infiltration of the viscous electrolyte into the mesoporous structure. A few hours of light soaking at an elevated temperature (60 8C) before the measurement was beneficial for the systems, mainly because of the temperature dependence of the viscosity of the
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Full Papers incorporating viscous ionic-liquid-based electrolytes, the mass limitations are not so severe, because the total amount of dye molecules to regenerate is smaller (the generated current densities are typically lower) and the percolation through the thinner layers of porous TiO2 is easier. Nevertheless, the current densities obtained for the cells with the modified electrolyte are higher by 0.7 mA cm¢2 at 1 Sun illumination. The detailed photovoltaic characterization is presented in Table 2.
Table 2. Photovoltaic parameters determined for the devices with C101 dye deposited on transparent TiO2 films.
Figure 7. Comparison of J–V curves recorded for DSCs utilizing Z988: A) pristine, that is, without Pt nanoparticles, and upon the incorporation of B) iodine-modified Pt (Pt/I) nanoparticles and C) CNT-supported iodine-modified Pt (CNT/Pt/I) nanoparticles.
Sun intensity
Jsc [mA cm¢2]
Voc [mV]
Fill factor
h [%]
Z988
1 0.1 1 0.1
13.56 1.47 14.38 1.57
688 633 719 652
0.71 0.80 0.66 0.78
7.03 7.80 7.10 7.90
Z988 with CNT-supported I-modified Pt nanoparticles
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Jsc [mA cm¢2]
Voc [mV]
Fill factor
h [%]
Z988
1 0.1 1 0.1
9.05 0.92 9.71 0.98
724 650 713 646
0.73 0.70 0.73 0.76
4.83 4.43 5.09 4.97
Conclusions
electrolytes. The photovoltaic parameters obtained with the above-described electrolytes under full sunlight (1 Sun) and low light (0.1 Sun) conditions are summarized in Table 1. The cell with regular Z988 electrolyte and C101 dye yielded reasonably high efficiency. The mass-transport issue cannot be avoided and manifested itself at high light intensity as a drop in the current dynamics plot. This current loss can be estimated to be approximately 1 mA cm¢2 by consideration of the current density obtained at a low light level (at which the masstransport limitation is not observed) and scaled up to the high light intensity conditions. Under low-light conditions, an excellent efficiency of 7.80 % is obtained, mainly owing to the high current density and fill factor. The incorporation of the optimum CNT-supported iodine-modified Pt nanoparticles into the electrolyte led to further improvements to the cell behavior. The current density increased under both conditions and reached approximately 14.5 mA cm¢2 for 1 Sun. Further, the addition of CNT/Pt/I increases Voc by approximately 30 mV. The cell with the modified electrolyte reached 7.10 % efficiency at 1 Sun illumination; however, under low light the efficiency was 7.90 %, and the short-circuit current density was up to approximnately1.6 mA cm¢2. Clearly, the addition of the Pt-based nanostructures has helped to obtain 1 mA cm¢2 higher shortcircuit current densities, which may be attributed to the shortened pathway of the redox species to the catalytically active centers dispersed in the electrolyte. A slight decrease of the fill factor upon the addition of platinized CNTs may originate from the nonideal distribution of the electrolyte components within the hybrid electrolyte upon their introduction to the DSC cell. The same effect has also observed for devices manufactured with a transparent semiconductor layer. Typically, for such cells ChemSusChem 2015, 8, 2560 – 2568
Sun intensity
Z988 with CNT-supported I-modified Pt nanoparticles
Table 1. Photovoltaic parameters determined for the devices with C101 dye (deposited on the double-layered TiO2).
Electrolyte
Electrolyte
The incorporation of dispersed carbon-nanotube-supported Pt nanostructures in iodine/iodide-containing ionic liquid electrolyte not only increases the charge-propagation dynamics under the conditions of the diagnostic microelectrode-based and sandwich-type voltammetric experiments but also produces a semisolid redox-conducting electrolyte of potential utility as a charge relay in dye-sensitized solar cells (DSCs). The results are consistent with the ability of nanostructured Pt to adsorb iodine as well as with the catalytic ability of Pt to break the I¢I bonds in the iodine or triiodide molecules. The fact that the most promising results were obtained with the iodine-modified Pt nanoparticles supported on a network of multiwalled carbon nanotubes reflects the improved electron distribution under such conditions, as was found previously for carbon nanotubes in electrode films of importance to electrocatalysis, bioelectrocatalysis, and charge storage (electrochemical capacitors, lithium-ion batteries). Consequently, the limitation originating from the chemical step (accompanying the electrode reaction) practically disappears, and the iodine/iodide redox couple behaves like a reversible electrochemical system. By applying the optimum semisolid redox electrolyte, we obtained 7.9 % power conversion efficiency in a DSC under standard recording conditions. This research parallels our recent efforts to fabricate composite electrolytes for high-performance and stable DSCs.[62, 63] Our results are also consistent with the view that ionic liquids could successfully serve as electrolytes not only in high-power charge-storage devices[64] but also in DSCs.
Experimental Section Materials and preparative procedures Sulfuric acid and potassium iodide were obtained from POCh (Gliwice, Poland). Platinum black was obtained from Alfa Aesar. 5 % Nafion-1100 solution and multi-walled carbon nanotubes (CNTs)
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Full Papers were purchased from Sigma–Aldrich. The ionic liquid components included 1-ethyl-3-methylimidazolium iodide (EMII), 1,3-dimethylimidazolium iodide (DMII), 1-ethyl-3-methylimidazolium tetracyanoborate (EMITCB), N-butylbenzimidazole (NBB), guanidinium thiocyanate (GuNCS), and sulfolane. The solvent-free Z988 electrolyte was composed of DMII, EMII, EMITCB, I2, NBB, and GuNCS in the molar ratio 1.6:1.6:2.13:0.22:0.44:0.08 and was further diluted with sulfolane (1:1 v/v). The RuII dye C101 [NaRu{4,4’-bis(5-hexylthiophene-2-yl)-2,2’-bipyridine}(4-carboxylic acid-4’-carboxylate-2,2’-bipyridine)(NCS)2] is used as a heteroleptic sensitizer with a high molar extinction coefficient (17 500 L mol¢1 cm¢1 at 547 nm wavelength). This dye is well-suited to coexist with the ionic-liquid-based electrolyte. To prepare the iodine-modified Pt nanoparticles, Pt black (18.0 mg) was introduced into a 10 mm solution of KI in deionized water (5.0 mL); the suspension was subjected to sonification for 2 h and then centrifuged. The supernatant solution was removed and replaced with fresh iodide solution. The centrifuging procedure was typically repeated two to three times. The iodide solution was decanted each time. The particles were then centrifuged and washed with water (5.0 mL) at least three to four times to ensure that all free ions were removed. Finally, the iodine-modified Pt nanoparticles were dried in air. To investigate their sizes, they were subsequently placed onto a carbon-coated nickel 400 mesh grid (Agar Scientific) and subjected to transmission electron microscopic (TEM) examination with a JEM 1400 instrument (JEOL Co., Japan, 2008). To produce a colloidal solution of multi-walled CNT-supported iodine-modified Pt nanoparticles, CNTs (Aldrich, 45.0 mg) and the iodine-modified Pt nanoparticles (prepared as mentioned above, 9.0 mg) were admixed and dispersed in water (5.0 mL). The suspension was subsequently stirred for 24 h. The CNT-supported iodine-modified Pt nanoparticles (CNT/Pt/I) were finally dried in air. The CNTs were purified as reported previously[65] with 12 m HCl solution for 1 h, followed by treatment with 3 m HNO3 for 8 h under reflux conditions. The samples were then washed with large amounts of water until the pH of the suspension became close to 7. To prepare the Z988 electrolyte containing iodine-modified Pt (Pt/ I), the respective nanoparticles were admixed with Z988 at 2 % weight level. The same statement applies to the electrolyte containing CNT/Pt/I, for which, in addition to the same loading of Pt nanoparticles, CNTs were present at 10 % weight level. To perform parallel voltammetric diagnostic experiments (in 0.5 m H2SO4) with Pt nanoparticles deposited on a glassy carbon electrode (for Figure 5), a drop (2 mL) of a colloidal suspension of the Pt nanoparticles was introduced onto the electrode surface and dried in air at room temperature (22 8C) for 30 min. The electrode was then covered with Nafion solution (1 mL, obtained by dissolving the 5 % commercial Nafion solution in ethanol at 1:10 v/v).
Electrochemical characterization The electrochemical measurements were performed by the voltammetric method with a CH Instruments Model 760D workstation (Austin, USA). The experiments were performed with the conventional and solid-state (three-electrode microelectrode-based and two-electrode sandwich) configurations described in the text. The conventional three-electrode cell consisted of a glassy carbon disk electrode (geometric area 0.071 cm2) as the working electrode, a carbon rod as the counter electrode, and a saturated (KCl) Ag/ AgCl reference electrode. The application of the above measurement systems in so-called solid-state voltammetry as well as their ChemSusChem 2015, 8, 2560 – 2568
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diagnostic utility have been demonstrated and reviewed previously.[40, 41]
Viscosity and zeta potential measurements The electrolyte (~ 1 g) from a sample stored in a dry-box was cast evenly on the plate of a Brookfield model DV-III rheometer and dried under vacuum at 60 8C for 48 h. The viscosities were determined under dry N2 at constant temperature with a Brookfield cone (model CP-52) and did not depend on the cone rotation rate. The zeta (electrokinetic) potentials of Pt and iodine-modified Pt nanoparticles (in aqueous colloidal dispersions at 25 8C) were determined indirectly from electrophoretic mobility by dynamic light scattering (laser, 532 nm, Zetaesizer Nano ZS system, Malvern Instruments).
Cell fabrication Screen-printed transparent and double layers of TiO2 particles were used as photoelectrodes in this study. For the transparent films, a 6 mm thick layer of 20 nm TiO2 particles was printed on the fluorine-doped SnO2 (FTO) conducting glass electrode. The double-layered films consisted of an 8 mm thick transparent layer of 20 nm TiO2 covered with a 5 mm thick scattering layer of 400 nm O2 particles. The porosities were evaluated as 67 % for the 20 nm TiO2 transparent layer and 42 % for the scattering layer, as determined from BET measurements. The TiO2 electrodes were sintered at 500 8C, cooled to 80 8C, dipped in the dye solution for 18 h (0.3 mm of the dye with 2 mm of chenodeoxycholic acid in chlorobenzene) for sensitization, and then assembled with a thermally platinized FTO conducting glass counter electrode. The working and counter electrodes were separated by a 25 mm thick hot-melt ring (Surlyn, DuPont) and sealed by heating. The internal space in the cell was filled with the electrolyte by using a vacuum pump.
Photovoltaic characterization A 450 W xenon light source (Oriel, USA) was used to characterize the solar cells. The spectral output of the lamp was matched in the wavelength region 350–750 nm with the aid of a Schott K113 Tempax sunlight filter (Przisions Glas&Optik GmbH, Germany) to reduce the mismatch between the simulated and true solar spectra to less than 2 %. The current–voltage characteristics of the cells under these conditions were obtained by applying an external potential bias to the cell and measuring the generated photocurrent with a Keithley (USA) Model 2400 digital source meter. A similar data acquisition system was used to control the incident photonto-current conversion efficiency (IPCE) measurement. Under computer control, the light from the 300 W xenon lamp (ILC Technology, USA) was focused through a Gemini-180 double monochromator (Jobin Yvon Ltd., UK) onto the investigated photovoltaic cell. The devices were masked to attain an illuminated active area of 0.159 cm2.
Acknowledgements This work was supported by the National Science Center (Poland) under project N N507 322040. J.O. was supported in part from the DSM project 120000-501/86-DSM-107500 (University of Warsaw). The technical help of D. Marks (University of Warsaw) is highly appreciated. M.G acknowledges financial support from
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Full Papers the Commission for Technology and Innovation CTI 17622.1 PFNM-NM, glass2energy sa (g2e), Villaz-St-Pierre, Switzerland. Keywords: iodine · nanotubes · nanoparticles · platinum · voltammetry [1] B. O’Regan, M. Grtzel, Nature 1991, 353, 737. [2] M. Grtzel, Acc. Chem. Res. 2009, 42, 1788. [3] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson, Chem. Rev. 2010, 110, 6595. [4] A. Hagfeldt, M. Grtzel, Chem. Rev. 1995, 95, 49. [5] P. Wang, S. M. Zakeeruddin, J. E. Moser, R. Humphry-Baker, M. Grtzel, J. Am. Chem. Soc. 2004, 126, 7164. [6] H. Nusbaumer, S. M. Zakeeruddin, J. E. Moser, M. Grtzel, Chem. Eur. J. 2003, 9, 3756. [7] H. Nusbaumer, J. E. Moser, S. M. Zakeeruddin, M. K. Nazeeruddin, M. Grtzel, J. Phys. Chem. B 2001, 105, 10461. [8] S. A. Sapp, M. Elliot, C. Contado, S. Caramori, C. A. Bignozzi, J. Am. Chem. Soc. 2002, 124, 11215. [9] A. Yella, H.-W. Lee, H. N. Tsao, C. Yi, A. K. Chandiran, M. K. Nazeeruddin, E. W.-G. Diau, C.-Y. Yeh, S. M. Zakeeruddin, M. Grtzel, Science 2011, 334, 629. [10] M. K. Nazeeruddin, P. Pechy, T. Renouard, S. M. Zakeeruddin, R. Humphry-Baker, P. Comte, P. Liska, L. Cevey, E. Costa, V. Shklover, L. Spiccia, G. B. Beacon, C. A. Bignozzi, M. Grtzel, J. Am. Chem. Soc. 2001, 123, 1613. [11] K. Tennakone, G. K. R. Senadeera, D. De Silva, I. R. M. Kottegoda, Appl. Phys. Lett. 2000, 77, 2367. [12] U. Bach, D. Lupo, P. Comte, J. E. Moser, F. Weissortel, J. Salbeck, H. Spreitzer, M. Grtzel, Nature 1998, 395, 583. [13] C. S. Karthikeyan, H. Wietasch, M. Thelakkat, Adv. Mater. 2007, 19, 1091 – 1095. [14] P. Wang, S. M. Zakeeruddin, J. E. Moser, M. K. Nazeeruddin, T. Sekiguchi, M. Grtzel, Nat. Mater. 2003, 2, 402. [15] D.-W. Kim, Y.-B. Jeong, S.-H. Kim, D.-Y. Lee, J.-S. Song, J. Power Sources 2005, 149, 112. [16] J. Wu, Z. Lan, D. Wang, S. Hao, J. Lin, Y. Huang, S. Yin, T. Sato, Electrochim. Acta 2006, 51, 4243. [17] M. Biancardo, K. West, F. C. Krebs, Sol. Energy Mater. Sol. Cells 2006, 90, 2575. [18] J. Xia, N. Masaki, M. Lira-Cantu, Y. Kim, K. Jiang, S. Yanagida, J. Am. Chem. Soc. 2008, 130, 1258. [19] Y. Yang, C.-h. Zhou, S. Xu, H. Hu, B.-l. Chen, J. Zhang, S.-j. Wu, W. Liu, X.z. Zhao, J. Power Sources 2008, 185, 1492. [20] J. Nei de Freitas, A. F. Nogueira, M.-A. De Paoli, J. Mater. Chem. 2009, 19, 5279. [21] B. Li, L. D. Wang, B. N. Kang, P. Wang, Y. Qiu, Sol. Energy Mater. Sol. Cells 2006, 90, 549. [22] T. Nguyen, X. Wang, J. Power Sources 2010, 195, 1024. [23] M. Wang, X. Pan, X. Fang, L. Guo, C. Zhang, Y. Huang, Z. Huo, S. Dai, J. Power Sources 2011, 196, 5784. [24] C.-L. Chen, H. Teng, Y.-L. Lee, J. Mater. Chem. 2011, 21, 628. [25] S. M. Zakeeruddin, M. Grtzel, Adv. Funct. Mater. 2009, 19, 2187. [26] Y. Bai, Y. Cao, J. Zhang, M. Wang, R. Li, P. Wang, S. M. Zakeeruddin, M. Grtzel, Nat. Mater. 2008, 7, 626. [27] H. Matsumoto, T. Matsuda, T. Tsuda, R. Hagiwara, Y. Ito, Y. Miyazaki, Chem. Lett. 2001, 26. [28] W. Kubo, T. Kitamura, K. Hanabusa, Y. Wada, S. Yanagida, Chem. Commun. 2002, 374. [29] H. Paulsson, L. Kloo, A. Hagfeldt, G. Boschloo, J. Electroanal. Chem. 2006, 586, 56. [30] R. F. Lane, A. T. Hubbard, J. Phys. Chem. 1975, 79, 808. [31] A. Wieckowski, B. C. Schardt, S. D. Rosasco, Surf. Sci. 1984, 146, 115.
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[32] J. Z. Chen, Y.-C. Yan, K.-J. Lin, J. Chin. Chem. Soc. 2010, 57, 1180. [33] C. S. N. O. A. Sreekala, J. Indiramma, K. B. S. P. Kumar, K. S. Sreelatha, M. Roy, J. Nanostructure Chem. 2013, 3, 19. [34] A. K. K. Kyaw, H. Tantang, T. Wu, L. Ke, C. Peh, Z. H. Huang, X. T. Zeng, H. V. Demir, Q. Zhang, X. W. Sun, Appl. Phys. Lett. 2011, 99, 021107. [35] H.-Y. Wang, F.-M. Wang, Y.-Y. Wang, C.-C. Wan, B.-J. Hwang, R. Santhanam, J. Rick, J. Phys. Chem. C 2011, 115, 8439. [36] C. Y. Neo, J. Ouyang, Electrochim. Acta 2012, 85, 1. [37] H.-Y. Chen, J.-Y. Liao, B.-X. Lei, D.-B. Kuang, Y. Fang, C.-Y. Su, Chem. Asian J. 2012, 7, 1795. [38] M. R. Karim, A. Islam, S. P. Singh, L. Han, Adv. OptoElectron. 2011, 357974. [39] J.-G. Chen, R. Vittal, M.-H. Yeh, C.-Y. Chen, C.-G. Wu, K.-C. Ho, Electrochim. Acta 2014, 130, 587. [40] P. J. Kulesza, M. A. Malik in Interfacial Electrochemistry: Theory, Experiment and Applications, Solid-State Voltammetry (Ed.: A. Wieckowski), Marcel Dekker, New York, 1999, pp. 673. [41] P. J. Kulesza, J. A. Cox, Electroanalysis 1998, 10, 73. [42] I. A. Rutkowska, P. J. Kulesza, Funct. Mater. Lett. 2014, 7, 1440005. [43] R. W. O’Brien, R. J. Hunter, Can. J. Chem. 1981, 59, 1878. [44] P. Wachter, M. Zistler, C. Schreiner, M. Berginc, U. O. Krasovec, D. Gerhard, P. Wasserscheid, A. Hinsch, H. J. Gores, J. Photochem. Photobiol. A 2008, 197, 25. [45] A. J. Bard, L.R Faulkner, Electrochemical Methods, VCH, New York, 1994. [46] Z. Galus, Fundamentals of Electrochemical Analysis, 2nd revised ed., Horwood, New York, 1994. [47] R. M. Wightman, D. O. Wipf in Electroanalytical Chemistry, Vol. 15, (Ed.: A. J. Bard), Marcel Dekker, New York, 1989. [48] N. A. Surridge, J. C. Jernigan, E. F. Dalton, R. P. Buck, M. Watanabe, H. Zhang, M. Pinkerton, T. T. Wooster, M. L. Longmire, J. S. Facci, R. W. Murray, Faraday Discuss. 1989, 88, 1. [49] G. Inzelt in Electroanalytical Chemistry, Vol. 18 (Ed.: A. J. Bard), Marcel Dekker, New York, 1994. [50] M. Majda in Techniques of Chemistry Vol. 22: Molecular Design of Electrode Surfaces, (Ed.: R. W. Murray), Wiley, New York, 1992. [51] P. J. Kulesza, E. Dickinson, M. E. Williams, S. M. Hendrickson, M. A. Malik, K. Miecznikowski, R. W. Murray, J. Phys. Chem. B 2001, 105, 5833. [52] E. F. Dalton, N. A. Surridge, J. C. Jernigan, K. O. Wilbourn, J. S. Facci, R. W. Murray, Chem. Phys. 1990, 141, 143. [53] R. W. Murray in Molecular Design of Electrode Surfaces (Ed.: R. W. Murray), Wiley, New York, 1992. [54] M. C. Giordano, J. C. Bazan, A. J. Arvia, Electrochim. Acta 1966, 11, 1553. [55] A. J. Arva, M. C. Giordano, J. J. Podesta, Electrochim. Acta 1969, 14, 389. [56] Ya. A. Yaraliev, Russ. Chem. Rev. 1982, 51, 566 [57] E. Dickinson, H. Masui, M. E. Williams, R. W. Murray, J. Phys. Chem. B 1999, 103, 11028. [58] P. J. Peerce, A. J. Bard, J. Electroanal. Chem. 1980, 114, 89. [59] M. G. Sullivan, R. W. Murray, J. Phys. Chem. 1994, 98, 4343. [60] P. Burgmayer, R. W. Murray, J. Electroanal. Chem. 1982, 135, 335. [61] V. Jovanovski, B. Orel, R. Jese, A. S. Vuk, G. Mali, S. B. Hocevar, J. Grdadolnik, E. Stathatos, P. Lianos, J. Phys. Chem B 2005, 109, 14387. [62] M. Marszalek, F. D. Arendse, J. D. Decoppet, S. S. Babkair, A. A. Ansari, S. S. Habib, M. Wang, S. M. Zakeeruddin, M. Grtzel, Adv. Energy Mater. 2014, 4, 1301235. [63] J. D. Decoppet, T. Moehl, S. S. Babkair, R. A. Alzubaydi, A. A. Ansari, S. S. Habib, S. M. Zakeeruddin, H. W. Schmidt, M. Grtzel, J. Mater. Chem. A 2014, 2, 15972. [64] T. Romann, O. Oll, P. Pikma, H. Tamme, E. Lust, Electrochim. Acta 2014, 125, 183. [65] M. Skunik, P. J. Kulesza, Anal. Chim. Acta 2009, 631, 153. Received: December 29, 2014 Revised: April 3, 2015 Published online on June 26, 2015
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Nanocomposite Semi-Solid Redox Ionic Liquid Electrolytes with Enhanced Charge-Transport Capabilities for DyeSensitized Solar Cells Iwona A. Rutkowska,[a] Magdalena Marszalek,[b] Justyna Orlowska,[a] Weronika Ozimek,[a] Shaik M. Zakeeruddin,[b, c] Pawel J. Kulesza,*[a] and Michael Grtzel*[b] The ability of Pt nanostructures to induce the splitting of the I¢I bond in iodine (triiodide) molecules is explored here to enhance electron transfer in the iodine/iodide redox couple. Following the dispersal of Pt nanoparticles at 2 % (weight) level, charge transport was accelerated in triiodide/iodide-containing 1,3-dialkylimidazolium room-temperature ionic liquid. If both Pt nanoparticles and multi-walled carbon nanotubes were introduced into the ionic-liquid-based system, a solidtype (nonfluid) electrolyte was obtained. By using solid-state voltammetric (both sandwich-type and microelectrode-based)
methodology, the apparent diffusion coefficients for charge transport increased to approximately 1 Õ 10¢6 cm2 s¢1 upon the incorporation of the carbon-nanotube-supported iodine-modified Pt nanostructures. A dye-sensitized solar cell comprising TiO2 covered with a heteroleptic RuII-type sensitizer (dye) and the semisolid triiodide/iodide ionic liquid electrolyte admixed with carbon-nanotube-supported Pt nanostructures yielded somewhat higher power conversion efficiencies (up to 7.9 % under standard reporting conditions) than those of the analogous Pt-free system.
Introduction The triiodide/iodide redox system has been the most commonly and most successfully used charge relay (mediator) in dyesensitized solar cells (DSCs).[1–8] The redox electrolyte plays a very important role in the DSC performance, and its usefulness largely depends on the dynamics of both interfacial electron transfer and bulk charge propagation within the system. Although photoelectric conversion efficiencies as high as 13 % have recently been obtained for porphyrin-sensitized solar cells utilizing organic liquid electrolytes containing cobalt(III/II) complexes,[9] triiodide/iodide-based systems have provided the most promising results in the area of practical DSCs. Nevertheless, problems such as leakage or evaporation of organic-solvent-based volatile liquids would require special attention such as careful sealing; otherwise, the cell performance would deteriorate during the long-term operation of the system.[10] To overcome these problems, several attempts, including the application of p-type semiconductors,[11] organic and inorganic hole conductors,[12, 13] and gel or polymer electrolytes,[14–24] have been proposed. There has also been growing interest in room[a] Dr. I. A. Rutkowska, J. Orlowska, W. Ozimek, Prof. P. J. Kulesza Faculty of Chemistry University of Warsaw Pasteura 1, 02-093 Warsaw (Poland) E-mail: [email protected] [b] Dr. M. Marszalek, Dr. S. M. Zakeeruddin, Prof. M. Grtzel Laboratory of Photonics and Interfaces Ecole Polytechnique F¦d¦rale de Lausanne (EPFL) Station 6, 1015 Lausanne (Switzerland) E-mail: [email protected] [c] Dr. S. M. Zakeeruddin Center of Excellence for Advanced Materials Research (CEAMR) King Abdulaziz University, Jeddah (Saudi Arabia)
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temperature ionic liquids, especially those with 1,3-dialkylimidazolium cations owing to their advantages such as negligible vapor pressure, high ionic conductivity and thermal stability, fairly wide electrochemical window, and ability to dissolve organic and inorganic solutes.[25, 26] The triiodide/iodide redox couple has been considered together with ionic liquids.[27, 28] The resulting redox-conducting electrolytes have several advantages such as high conductivity, low vapor pressure, high iodide concentration, and good electrochemical stability. Among their disadvantages is their high viscosity, which certainly contributes to the low mass-transport coefficient of the triiodide/iodide redox couple,[29] not only if the charge transport mechanism is predominantly physical at low concentrations but also if the Grotthus exchange mechanism is operative at high concentrations of the redox system. In general, both interfacial and bulk (self-exchange) electron transfers involving the triiodide/iodide redox system are somewhat complicated and appear slower than one would expect. Among the kinetic limitations there is a need to break the I¢I bond in the I3¢ or I2 molecule; it has also been well-established that Pt (e.g., deposited on the counter electrode) induces electron transfer within the iodine/iodide redox system. Strong interactions of Pt with iodide or iodine have been described.[30, 31] Notably, the iodine covalent radius (0.133 nm) and the Pt atomic radius (0.135 nm) are comparable. The formation of monolayer-type coverages of strongly adsorbed monoatomic iodine together with weakly bound electroactive iodine/iodide has also been postulated. In the present work, we explore the interfacial (electrocatalytic) phenomena of nanostructured Pt (namely, Pt nanoparticles three-dimensionally distributed in the electrolyte phase at 2 % weight level) and utilize them to
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Full Papers enhance triiodide/iodide electron transfers to develop more efficient charge relays for DSCs. Finally, to make the electrolyte more solid (nonfluid) and to improve the overall electron distribution within the redox-conducting electrolyte, we also introduce multi-walled carbon nanotubes (CNTs) into our nanocomposite system at 10 % weight level as supports for the dispersed iodine-modified Pt nanoparticles. The usefulness of admixing ionic liquid electrolytes with CNTs has also been demonstrated independently by others.[32–36] In particular, recent reports have emphasized the utility of platinized CNTs as the counter electrode material for DSCs with ionic liquids[37] as well as the catalytic activity of CNTs during the electrochemical reduction of I3¢[38] and their ability to facilitate electron transport through the ionic liquid electrolyte.[39] By using the microelectrode-based and sandwich-type electroanalytical methodologies of solid-state electrochemistry,[40, 41] we address here the charge-transport dynamics within the semisolid triiodide/ iodide ionic liquid electrolyte admixed with CNT-supported Pt nanostructures and comment on the reasonably high power conversion efficiencies of DSCs utilizing such electrolytes.
Results and Discussion Physicochemical identity of iodine-modified Pt nanoparticles To support the preparation procedures described in Experimental Section, we provide TEM images (Figure 1 A) of the iodine-modified Pt (Pt/I) nanoparticles. They have diameters of 4–6 nm, which are comparable to those determined previously for such Pt nanoparticles utilized in electrocatalysis.[42] Upon the dispersal of the nanoparticles in the aqueous liquid phase, we determined their electrokinetic zeta potential, which is approximately ¢32 mV (Pt/I) compared with ¢13 mV for bare Pt nanoparticles. First, this difference in zeta potential supports the view that iodine is present on the Pt. Second, it is generally accepted that zeta potentials with magnitudes exceeding 30 mV imply electrostatic repulsion and are a key indicator of the stability of colloidal suspensions.[43] Further, despite some statistical uncertainty (standard deviation) of 4–7 mV, a shift of the zeta potential toward more negative values upon the modification of the Pt nanostructures with iodine is indicative of a positive change in the net electrical charge at the interface formed by the nanoparticles decorated with iodine. The higher positive charge for Pt/I relative to that of bare Pt is consistent with the historic view for bulk Pt[40, 41] regarding monolayer-type coverages of monoatomic iodine (rather than anionic iodide) on Pt. The maximum coverage for irreversibly adsorbed monoatomic iodine was postulated to reflect a 1:1 atomic model in correspondence with the Pt surface.[40, 41] The formation of stable adsorbate iodine layers on Pt is also clear from conventional voltammetric diagnostic experiments performed in sulfuric acid (Figure 1 B), in which the responses characteristic of bare clean Pt, namely, hydrogen adsorption peaks (in the potential range 0–0.3 V) and Pt oxide formation peaks (at potentials higher than 0.7 V), are largely diminished after the iodine adsorption (compare the solid and dashed ChemSusChem 2015, 8, 2560 – 2568
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Figure 1. A) TEM micrograph of dried iodine-modified Pt nanoparticles; B) voltammetric probing of the interfacial behavior of the bare (solid line) and iodine-modified (dashed line) Pt nanoparticles deposited (100 mg cm¢2) on a glassy carbon disk electrode (electrolyte: deaerated 0.5 m H2SO4 ; scan rate: 10 mV s¢1). C) I-modified Pt nanoparticles supported on multi-walled carbon nanotubes.
lines in Figure 1 B). Notably, the iodine adsorbates on Pt are not electroactive. These observations agree with the literature data.[30, 31] It was postulated that up to 2 Õ 10¢9 mol cm¢2 of monoatomic iodine could be irreversibly adsorbed in the nonelectroactive state on the polycrystalline Pt surface. To obtain a solid-type (nonfluid) electrolyte, both Pt nanoparticles and multi-walled carbon nanotubes were introduced to the ionic-liquid-based system. A TEM image of the multiwalled-CNT-supported iodine-modified Pt (Pt/I) nanoparticles is shown in Figure 1 C. Here, it is apparent that the CNTs (thick-
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Full Papers nesses 10–12 nm; lengths in mm range) act as carriers for the Pt nanoclusters; although most of the Pt nanoparticles are dispersed, some of them form agglomerates (up to 20 nm). In the presence of such an extended network of CNT/Pt/I nanostructures, electron distribution should be facilitated in the redox electrolyte.[39]
Microelectrode-based diagnostic experiments To comment quantitatively on the enhancement of charge propagation through the incorporation of Pt-based nanostructures in triiodide/iodide-containing ionic liquids, the microelectrode-based methodology developed for solid-state voltammetry[40, 41] was considered (Figure 2 A). The usefulness of micro-
Figure 3. Microelectrode-based cyclic voltammetry of iodine/iodide redoxconducting electrolytes recorded at 50 mV s¢1 with a solid-state cell with a planar three-electrode configuration A) without Pt nanoparticles and upon the incorporation of B) Pt nanoparticles (2 % weight) and C) CNT-supported iodine-modified Pt (Pt/I) nanoparticles (2 % weight). Microdisk diameter 30 mm.
rameter [Eq. (1)] is expected to be approximately 1 (or more precisely 0.1 < t < 1; r is the electrode radius):[40, 41, 45–47] t ¼ 4 DAPP t=r2
Figure 2. Schematic diagrams of A) a microelectrode-based solid-state type cell with a planar three-electrode configuration and B) a two-electrode solidstate type cell in a sandwich configuration.
electrodes in studies of ionic-liquid-type systems has also been demonstrated by others.[44] The voltammetric responses recorded in the bulk triiodide/iodide redox-conducting electrolytes are illustrated in Figure 3 for an electrolyte without Pt nanoparticles and electrolytes with iodine-modified Pt nanoparticles and CNT-supported iodine-modified Pt (CNT/Pt/I) nanoparticles incorporated. The results are consistent with the view that all systems are well-behaved and the electron transfers are reversible. It should be remembered that the data refer to semi-infinite diffusion conditions. The voltammetric peak currents increase upon the addition of iodine-modified Pt, particularly for CNT/Pt/I; these results imply that the dynamics of charge transport become more facile in this case. From the shape of the voltammograms in Figure 3 and as the nature of mass transport depends on the time domain, it seems that a mixed (linear/radial) diffusional pattern is applicable here. Indeed, the voltammetric peaks of Figure 3 are rather flat, particularly those of the systems containing iodine-modified Pt. Under such conditions, the t dimensionless time paChemSusChem 2015, 8, 2560 – 2568
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ð1Þ
It should be remembered that for fast-scan-rate (short time) experiments with t ! 1, the diffusion field is small in comparison to the electrode radius, and the charge-transport mechanism follows the linear diffusional pattern. Consequently, a typical voltammetric peak with a diffusional term appears. On the other hand, for t @ 1, which corresponds to relatively slowscan-rate (long time) experiments, the hemispherical diffusion layer largely exceeds the size of the electrode, and the spherical (radial) diffusional pattern (leading to sigmoidal steadystate voltammograms) becomes predominant. The mixed diffusional pattern (t 1), which is applicable to cyclic voltammograms recorded at 50 mV s¢1 (Figure 2), permits us to estimate the charge transport (DAPP) parameter. As the experimental time (t) is roughly 2–3 s and r = 1.5 Õ 10¢3 cm, DAPP is expected to be approximately 10¢7 cm2 s¢1. Charge transfer in concentrated semisolid, semiliquid, or even liquid solutions of electrochemically well-behaved redox couples can occur concurrently by physical diffusion (DPHYS) and by electron hopping or self-exchange (kEX) between mixed-valent redox sites. The coupling of diffusion and electron rates was initially developed to describe electron self-exchange reactions in solutions of redox ions[48] and later expressed in the corrected form with DPHYS and kEX separated in the Dahms–Ruff relation [Eq. (2)]:[49, 50] DAPP ¼ DPHYS þDE ¼ DPHYS þkEX s2 C=6
ð2Þ
where DE, DAPP, and s are the electron diffusion coefficient, apparent (or effective) diffusion coefficient, and the average (equilibrium) distance between the redox sites in the investigated system, respectively. For typical liquid systems, DPHYS is usually a dominating kinetic factor, and any contribution from
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Full Papers electron hopping (DE) would only be important if the population (concentration) of redox centers (C) and kEX have high values. For semisolid (i.e., denser or more viscous than typical liquids) systems, DPHYS is low enough to play any important role in the overall charge-transport dynamics. To design systems capable of fast charge propagation under such conditions, care has to be exercised to make the DE contribution large enough not only by keeping C high but also by increasing electron hopping (kEX) rates to develop sizeable currents that are effectively diffusion-controlled.[51] For the present triiodide/iodide-containing ionic liquid, the truly physical diffusion of triiodides and iodides within the ionic liquid is rather slow, and the latter requirement for a high self-exchange rate constant translates to the necessity of fast electron transfer between iodine (I) and iodide (I¢). These elementary acts of electron transfer within the I/I¢ redox couples would require the existence of monoatomic I rather than I2 or I3¢ . In other words, to avoid complications with the slow chemical step,[45, 46] one has to has to develop a means of catalyzing the I¢I bond dissociation (151 kJ mol¢1). In other words, the ideal situation would appear if the I¢I bond-breaking process is fast enough to be neglected in kinetic terms. Under such conditions, the DE term in Equation (2) becomes sizeable enough to facilitate fast and effectively diffusional ( DAPP) charge transport (redox conduction).[40, 41, 51–53] The typical values for the diffusion coefficients (physical) of iodide, iodine, and triiodide in different nonaqueous solvents (e.g., DMF, DMSO) are 3 to 6 Õ 10¢6 cm2 s¢1.[54–56] The denser environments of our redox-conducting electrolytes, which are characterized by rather high viscosities of 40 cP at 25 8C or higher, should result in significantly reduced mobility (to which the physical diffusion rates should be inversely proportional) for the triiodide/iodide system. This comment applies particularly to electrolytes containing Pt nanostructures that are visually semisolid. Although the electrolytes are fairly viscous, their ionic conductivities (which are enhanced by the presence of large populations of I¢ ions) are sufficient to support microelectrode voltammetry. Clearly, the viscosities of our systems are not as high as those of redox melts (2–4 Õ 106 cP at 25 8C), which are characterized by DPHYS as low as 10¢13 to 10¢12 cm2 s¢1.[51, 57] In conclusion, as the DPHYS values for the iodine/iodide systems considered here are lower than those for liquids, the observed DAPP values cannot be controlled entirely by physical diffusion. If the concentration of redox centers is known, reliable charge propagation information (DAPP) can be obtained from long-time experiments with microelectrodes. Voltammetry at a slow-scan rate of 1 mV s¢1 (Figure 4) yields almost steadystate limiting currents (ILIM), which level out as radial diffusion conditions are attained. In this respect, the wave-shape response is typical; the appropriate radial diffusion equation[40, 41] is shown in Equation (3): ILIM ¼ 4 n Fr DAPP C
ð3Þ
where F is Faraday constant and r stands for radius of the microdisk electrode. More exact DAPP values can be obtained by ChemSusChem 2015, 8, 2560 – 2568
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Figure 4. Slow scan rate (1 mV s¢1) microelectrode-based voltammetry of iodine/iodide redox-conducting electrolytes A) without Pt nanoparticles and upon the incorporation of B) Pt nanoparticles and C) CNT-supported iodinemodified Pt (Pt/I) nanoparticles. The other conditions are the same as those for Figure 1.
Equation (3) because the charge-transport mechanism is clearly defined here (spherical) and the effects of uncompensated resistance (iRUNC) are less pronounced at slow scan rates. Under the conditions of the microelectrode-based voltammetric transient experiment in a three-electrode configuration, concentration gradients of mixed-valence redox sites can be generated during the appropriate potential scans.[48, 52, 53, 58, 59] The appearance of both negative and positive currents (Figure 4) reflects the presence of both oxidized (I2) and reduced (I¢) forms in the electrolytes. We start our analytical considerations from the oxidation steps (at 0.1 V in Figure 4) as the iodine species are first reduced at the microdisk electrode (this explains the origin of the negative currents below 0.6 V in Figure 4). The concentration gradients (effectively electron transfer between mixed-valence sites) are maximized at a 1:1 ratio of the oxidized to reduced forms.[40, 41, 60] No electron hopping is feasible between the fully oxidized or reduced species. As the total concentration of iodine (monoatomic) and iodide species in our electrolytes is 0.22+ +1.6 = 1.82 mol L¢1, the maximum population or concentration of mixed-valent I/I¢ sites to be generated would be half as much and equal to C = 9.1 Õ 10¢4 mol cm¢3. Under such conditions, the current should be maximized. Assuming that the monoatomic/anionic I/I¢ system can form ideal welldefined electron acceptor/electron donor sites, the following DAPP values (20 8C) have been determined by using Equation (3) with the data in Figure 3: 5.5 Õ 10¢7, 8.5 Õ 10¢7, and 9.5 Õ 10¢7 cm2 s¢1 for the triiodide/iodide redox-conducting electrolytes without Pt nanoparticles and upon the incorporation of iodine-modified Pt nanoparticles and CNT-supported iodinemodified Pt nanoparticles (CNT/Pt/I), respectively. To relate DAPP to kEX precisely, an estimate of DPHYS is required in Equation (2). The actual value of DPHYS is expected to be lower than DAPP because of the high viscosity of the system. With the assumption that the electron self-exchange term in Equation (2) is approximately equal to the measured DAPP at the highest concentration of mixed-valence sites (ca. 0.9 m), kEX becomes approximately 1010 m¢1 s¢1. Such high electron self-exchange rates are known for electron transfers between Mo(VI,V) or W(VI,V) in single crystals of heteropolyacids of mo-
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Full Papers lybdenum and tungsten.[53, 58, 59] In the present iodine/iodide system, it is reasonable to expect mutual compensation between physical and effective (electron self-exchange type) diffusion mechanisms. The increased currents (and values of diffusion coefficients, practically DE values) after the incorporation of Pt nanostructures reflect the existence of a Pt-induced catalytic enhancement effects toward the I¢I bond splitting in triiodide molecules. The current (and DE) increases are even more pronounced upon the incorporation of CNTs. Even if the CNTs are not catalytically active (we have no clear evidence for such an effect), the dispersed carbon nanostructures should provide percolation pathways and facilitate electron transport with the ionic-liquid-based charge relay. For electron transfers to be operative, the distances between the Pt nanoparticles and/or carbon nanotubes should be in the nanometer range. Although a direct estimation of the average distances between the Pt nanoparticles with use of the idealized cubic lattice model[40, 41, 48] (and by considering their loading and average sizes) is feasible, the actual distances for electron hops must be lower (< 10 nm) owing to the large population of CNT supports of different shapes. Further, the effective path for an electron transfer is expected to be even shorter because the Pt sites tend to be modified not only with irreversibly adsorbed monoatomic iodine but also with additional layers of electroactive iodine (~ 1 Õ 10¢9 mol cm¢2).[30, 31] Comparative measurements in sandwich configuration To preserve electroneutrality during charge transport in the iodine/iodide electrolytes, electron transfer to iodine must be accompanied by the unimpeded motion of iodide anions in the opposite direction.[40, 41, 48, 52] The extent of the displacement of the ions during individual electron hops is often unclear. It is commonly accepted that the motion of the ions may become rate-determining in the overall dynamics of charge transport (then termed macroscopic)[40, 52] during transient electrochemical experiments (e.g., voltammetric-type as in Figures 3 and 4). On the other hand, if the current–potential experiment is performed in a sandwich configuration (Figure 2 B) and the steady-state limiting current (ILIM) is observed, the concentration gradients C/d (d is a thickness for the investigated mixed-valence system that is defined by the distance between the sandwich-forming electrodes) are maximized, and the current flow would not require significant displacement of counterions, that is, only microscopic motion is needed.[40, 41, 48, 52] Under such conditions, the electron-hopping event is related primarily to the electron-transfer dynamics (electron diffusion, DE) rather than to the motion (diffusion) of the charge-compensating ionic species. A relevant relationship is described by Equation (4):[40, 41, 59] ILIM ¼ n F A C DE =d
ð4Þ
A is the cross-sectional area of the sandwich system, and C is the concentration of mixed-valence sites (here, the ratio of I to I¢ is fixed, and C = 0.22 mol L¢1). In general, the sandwich configuration seems to better resemble the situation occurring in ChemSusChem 2015, 8, 2560 – 2568
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DSCs in which the iodine/iodide electrolyte is “sandwiched” between the dye-modified TiO2 photoanode and the platinized optically transparent counter electrode. Thus, steady-state experiments are probably more suitable than transient experiments for the determination of electron-hopping rates (DE). The current responses versus the potential differences (DE) applied between the electrodes are illustrated in Figure 4 for the iodine/iodide redox-conducting electrolytes. In all cases, steady-state limiting currents were observed. From the data in Figure 5, DE values of 4.5 Õ 10¢7, 9.0 Õ 10¢7, and 1.1 Õ
Figure 5. Voltammetric (current vs. potential difference) responses recorded at 10 mV s¢1 with a solid-state cell in a sandwich configuration with iodine/ iodide redox-conducting electrolyte A) without Pt nanoparticles and upon the incorporation of B) iodine-modified Pt (Pt/I) nanoparticles and C) CNTsupported iodine-modified Pt (CNT/Pt/I) nanoparticles.
10¢6 cm2 s¢1 were estimated for the triiodide/iodide electrolytes without Pt nanoparticles and with iodine-modified Pt nanoparticles and CNT/Pt/I nanostructures introduced, respectively. As expected,[40, 41, 48, 52] the DE values obtained here are somewhat higher than those DAPP coefficients determined from transient microelectrode-based experiments (Figure 4). The use of Equation (4) generally requires low d values, that is, the application of thin films of investigated systems. For a given experimental time t, this thickness must not exceed the diffusion layer thickness (2 DEt)1/2.[40, 41, 48] Otherwise, instead of plateau currents, symmetrical peaks at approximately 0 V would be observed. Under the time scale of the voltammetric experiment with iodine/iodide redox-conducting salts (Figure 5), the diffusion layer thickness is in the range 90– 140 mm, that is, it is larger than d = 20 mm; thus, the use of Equation (4) is justified. On the whole, the results are consistent with the view that the introduction of Pt/I nanoparticles to the triiodide/iodide-containing electrolytes enhances the charge propagation rates (DAPP and DE) and related voltammetric currents regardless of the measurement approach applied. It is tempting to rationalize the effect in terms of the electrocatalytic activity of the nanostructured Pt on the triiodide/iodide electron transfers.
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Full Papers Probing the influence of Pt nanoparticles on iodine/iodide electrochemical behavior As mentioned, I2, I3¢ , and I¢ species adsorb strongly on Pt surfaces. An iodine fractional surface coverage of 0.5 was postulated for aqueous solutions;[30] a maximum of approximately 2 Õ 10¢9 mol cm¢2 of iodide or monoatomic iodine could be irreversibly adsorbed into a nonelectroactive state at a reduced Pt surface. It was estimated that the maximum coverage for irreversibly adsorbed iodine was approximately in 1:1 atomic correspondence with the Pt surface. An additional 1 Õ 10¢9 mol cm¢2 of iodine (but not anionic iodide) was adsorbed in an electroactive state at the surface bearing irreversibly adsorbed iodine. Consequently, the Pt surface was largely inert, that is, the formation of PtO and PtOH and the adsorption of hydrogen were largely suppressed.[30, 31] In the present work, we clarify that Pt nanoparticles of 4–6 nm can be also modified with both irreversibly adsorbed and additional layers of electroactive iodine in an analogous manner to polycrystalline bulk Pt. To investigate the behavior and possible adherence of the iodine/iodide system on Pt nanoparticles, we performed cyclic voltammetric diagnostic experiments (Figure 6) with a glassy
Figure 6. Diagnostic conventional cyclic voltammetric experiments: responses of the iodine/iodide system A) in solution (0.5 m KI and 0.5 m H2SO4) and B, C) the adsorbate upon medium transfer to the electrolyte (0.5 m H2SO4) only. Although (A) and (B) refer to responses recorded for the deposits of Pt nanoparticles, the response of CNT-supported Pt is presented in C). Electrode substrate: glassy carbon. Scan rate: 10 mV s¢1.
carbon electrode with Pt nanoparticles dispersed on the surface (loading, 100 mg cm¢2). The data in Figure 6 A are consistent with fairly distorted (electrochemically irreversible) behavior of the triiodide/iodide system in solution (separation of peaks ~ 200 mV). Upon transfer to the solution containing only supporting electrolyte, a set of symmetrical surface-type peaks were observed (Figure 6 B). The result is consistent with the view that, in addition to the irreversibly adsorbed monoatomic iodine, a large portion of the iodine/iodide system exists in the vicinity the electrode surface and adheres to nanostructured Pt/I sites (no significant adsorption on the bare glassy carbon occurs); therefore, reversible behavior consistent with fast electron transfer between iodine and iodide species is observed. On the basis of our data (Figure 6 B) and previous reports,[37, 38] ChemSusChem 2015, 8, 2560 – 2568
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it is also reasonable to expect that, regardless the mechanistic details, the adsorption of iodine on the surfaces of the Pt nanoparticles facilitates the dissociation of the I¢I bonds in I3¢ molecules. From the above observations, we can expect that the incorporation of iodine-modified Pt nanoparticles and their three-dimensional distribution within electrolyte layers produces a case between heterogeneous and homogenous electrocatalysis. The fact that the CNT-supported iodine-modified Pt (CNT/ Pt/I) produces the highest currents (Figures 3–5) can be interpreted in terms of better charge distribution within the redoxconducting electrolyte. We have found that iodine can also be adsorbed on CNT/Pt/I (Figure 6 B). The adsorption is as strong and durable on multi-walled CNTs as it is on Pt. Furthermore, the catalytic effect originating from the CNTs is not as pronounced as it is for Pt because the respective anodic and cathodic peaks originating from the iodine attached to the CNTs are much more drawn out and separated (Figure 6 B). Further research is needed in this area. Nevertheless, the addition of CNTs to electrolytes makes the electrolyte more solid (nonfluid) and facilitates better distribution of charge. Application of hybrid electrolytes in DSCs The promising iodine-modified Pt-based nanostructures have been tested as an additive to the redox-conducting electrolyte in dye-sensitized solar cells. As the electrolyte, we used a stateof-the-art ionic-liquid-based electrolyte[61] with slight modifications. It should be remembered that ionic-liquid-based electrolytes were successfully used in DSCs and provided efficient, stable devices, but their practical efficiency was typically limited by charge-transport issues related to the high viscosity of the medium. Therefore, the introduction of catalytic Pt-based centers to facilitate iodine–iodide electron transfers and the overall charge-transport dynamics seems to be justified. High concentrations of the redox (mediator) may permit the electron-self-exchange mechanism to be operative. In this study, we used the standard Z952 electrolyte [1,3-dimethylimidazolium iodide/1-ethyl-3-methylimidazolium iodide/1-ethyl-3-methylimidazolium tetracyanoborate/I2/N-butylbenzimidazole/guanidinium thiocyanate (DMII/EMII/EMITCB/I2/NBB/GuNCS) in the molar ratio 12:12:16:1.67:3.33:0.67] diluted with sulfolane (1:1), coded as Z988. The electrolyte is characterized by reduced viscosity, but the transport issues may be improved. The addition of nonvolatile sulfolane does not affect the stability of devices. For the photovoltaic study, we built a set of cells with doublelayered TiO2 (8+ +5 mm thick), C101 dye, and a standard Pt counter electrode. Herein, we want to focus on the comparison of the devices filled with the Z988 electrolyte and the systems containing the iodine-modified Pt nanoparticles and the CNTsupported iodine-modified Pt nanostructures (Figure 7). The freshly prepared cells have been considered, but the actual measurements required relaxation time to allow good infiltration of the viscous electrolyte into the mesoporous structure. A few hours of light soaking at an elevated temperature (60 8C) before the measurement was beneficial for the systems, mainly because of the temperature dependence of the viscosity of the
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Full Papers incorporating viscous ionic-liquid-based electrolytes, the mass limitations are not so severe, because the total amount of dye molecules to regenerate is smaller (the generated current densities are typically lower) and the percolation through the thinner layers of porous TiO2 is easier. Nevertheless, the current densities obtained for the cells with the modified electrolyte are higher by 0.7 mA cm¢2 at 1 Sun illumination. The detailed photovoltaic characterization is presented in Table 2.
Table 2. Photovoltaic parameters determined for the devices with C101 dye deposited on transparent TiO2 films.
Figure 7. Comparison of J–V curves recorded for DSCs utilizing Z988: A) pristine, that is, without Pt nanoparticles, and upon the incorporation of B) iodine-modified Pt (Pt/I) nanoparticles and C) CNT-supported iodine-modified Pt (CNT/Pt/I) nanoparticles.
Sun intensity
Jsc [mA cm¢2]
Voc [mV]
Fill factor
h [%]
Z988
1 0.1 1 0.1
13.56 1.47 14.38 1.57
688 633 719 652
0.71 0.80 0.66 0.78
7.03 7.80 7.10 7.90
Z988 with CNT-supported I-modified Pt nanoparticles
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Jsc [mA cm¢2]
Voc [mV]
Fill factor
h [%]
Z988
1 0.1 1 0.1
9.05 0.92 9.71 0.98
724 650 713 646
0.73 0.70 0.73 0.76
4.83 4.43 5.09 4.97
Conclusions
electrolytes. The photovoltaic parameters obtained with the above-described electrolytes under full sunlight (1 Sun) and low light (0.1 Sun) conditions are summarized in Table 1. The cell with regular Z988 electrolyte and C101 dye yielded reasonably high efficiency. The mass-transport issue cannot be avoided and manifested itself at high light intensity as a drop in the current dynamics plot. This current loss can be estimated to be approximately 1 mA cm¢2 by consideration of the current density obtained at a low light level (at which the masstransport limitation is not observed) and scaled up to the high light intensity conditions. Under low-light conditions, an excellent efficiency of 7.80 % is obtained, mainly owing to the high current density and fill factor. The incorporation of the optimum CNT-supported iodine-modified Pt nanoparticles into the electrolyte led to further improvements to the cell behavior. The current density increased under both conditions and reached approximately 14.5 mA cm¢2 for 1 Sun. Further, the addition of CNT/Pt/I increases Voc by approximately 30 mV. The cell with the modified electrolyte reached 7.10 % efficiency at 1 Sun illumination; however, under low light the efficiency was 7.90 %, and the short-circuit current density was up to approximnately1.6 mA cm¢2. Clearly, the addition of the Pt-based nanostructures has helped to obtain 1 mA cm¢2 higher shortcircuit current densities, which may be attributed to the shortened pathway of the redox species to the catalytically active centers dispersed in the electrolyte. A slight decrease of the fill factor upon the addition of platinized CNTs may originate from the nonideal distribution of the electrolyte components within the hybrid electrolyte upon their introduction to the DSC cell. The same effect has also observed for devices manufactured with a transparent semiconductor layer. Typically, for such cells ChemSusChem 2015, 8, 2560 – 2568
Sun intensity
Z988 with CNT-supported I-modified Pt nanoparticles
Table 1. Photovoltaic parameters determined for the devices with C101 dye (deposited on the double-layered TiO2).
Electrolyte
Electrolyte
The incorporation of dispersed carbon-nanotube-supported Pt nanostructures in iodine/iodide-containing ionic liquid electrolyte not only increases the charge-propagation dynamics under the conditions of the diagnostic microelectrode-based and sandwich-type voltammetric experiments but also produces a semisolid redox-conducting electrolyte of potential utility as a charge relay in dye-sensitized solar cells (DSCs). The results are consistent with the ability of nanostructured Pt to adsorb iodine as well as with the catalytic ability of Pt to break the I¢I bonds in the iodine or triiodide molecules. The fact that the most promising results were obtained with the iodine-modified Pt nanoparticles supported on a network of multiwalled carbon nanotubes reflects the improved electron distribution under such conditions, as was found previously for carbon nanotubes in electrode films of importance to electrocatalysis, bioelectrocatalysis, and charge storage (electrochemical capacitors, lithium-ion batteries). Consequently, the limitation originating from the chemical step (accompanying the electrode reaction) practically disappears, and the iodine/iodide redox couple behaves like a reversible electrochemical system. By applying the optimum semisolid redox electrolyte, we obtained 7.9 % power conversion efficiency in a DSC under standard recording conditions. This research parallels our recent efforts to fabricate composite electrolytes for high-performance and stable DSCs.[62, 63] Our results are also consistent with the view that ionic liquids could successfully serve as electrolytes not only in high-power charge-storage devices[64] but also in DSCs.
Experimental Section Materials and preparative procedures Sulfuric acid and potassium iodide were obtained from POCh (Gliwice, Poland). Platinum black was obtained from Alfa Aesar. 5 % Nafion-1100 solution and multi-walled carbon nanotubes (CNTs)
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Full Papers were purchased from Sigma–Aldrich. The ionic liquid components included 1-ethyl-3-methylimidazolium iodide (EMII), 1,3-dimethylimidazolium iodide (DMII), 1-ethyl-3-methylimidazolium tetracyanoborate (EMITCB), N-butylbenzimidazole (NBB), guanidinium thiocyanate (GuNCS), and sulfolane. The solvent-free Z988 electrolyte was composed of DMII, EMII, EMITCB, I2, NBB, and GuNCS in the molar ratio 1.6:1.6:2.13:0.22:0.44:0.08 and was further diluted with sulfolane (1:1 v/v). The RuII dye C101 [NaRu{4,4’-bis(5-hexylthiophene-2-yl)-2,2’-bipyridine}(4-carboxylic acid-4’-carboxylate-2,2’-bipyridine)(NCS)2] is used as a heteroleptic sensitizer with a high molar extinction coefficient (17 500 L mol¢1 cm¢1 at 547 nm wavelength). This dye is well-suited to coexist with the ionic-liquid-based electrolyte. To prepare the iodine-modified Pt nanoparticles, Pt black (18.0 mg) was introduced into a 10 mm solution of KI in deionized water (5.0 mL); the suspension was subjected to sonification for 2 h and then centrifuged. The supernatant solution was removed and replaced with fresh iodide solution. The centrifuging procedure was typically repeated two to three times. The iodide solution was decanted each time. The particles were then centrifuged and washed with water (5.0 mL) at least three to four times to ensure that all free ions were removed. Finally, the iodine-modified Pt nanoparticles were dried in air. To investigate their sizes, they were subsequently placed onto a carbon-coated nickel 400 mesh grid (Agar Scientific) and subjected to transmission electron microscopic (TEM) examination with a JEM 1400 instrument (JEOL Co., Japan, 2008). To produce a colloidal solution of multi-walled CNT-supported iodine-modified Pt nanoparticles, CNTs (Aldrich, 45.0 mg) and the iodine-modified Pt nanoparticles (prepared as mentioned above, 9.0 mg) were admixed and dispersed in water (5.0 mL). The suspension was subsequently stirred for 24 h. The CNT-supported iodine-modified Pt nanoparticles (CNT/Pt/I) were finally dried in air. The CNTs were purified as reported previously[65] with 12 m HCl solution for 1 h, followed by treatment with 3 m HNO3 for 8 h under reflux conditions. The samples were then washed with large amounts of water until the pH of the suspension became close to 7. To prepare the Z988 electrolyte containing iodine-modified Pt (Pt/ I), the respective nanoparticles were admixed with Z988 at 2 % weight level. The same statement applies to the electrolyte containing CNT/Pt/I, for which, in addition to the same loading of Pt nanoparticles, CNTs were present at 10 % weight level. To perform parallel voltammetric diagnostic experiments (in 0.5 m H2SO4) with Pt nanoparticles deposited on a glassy carbon electrode (for Figure 5), a drop (2 mL) of a colloidal suspension of the Pt nanoparticles was introduced onto the electrode surface and dried in air at room temperature (22 8C) for 30 min. The electrode was then covered with Nafion solution (1 mL, obtained by dissolving the 5 % commercial Nafion solution in ethanol at 1:10 v/v).
Electrochemical characterization The electrochemical measurements were performed by the voltammetric method with a CH Instruments Model 760D workstation (Austin, USA). The experiments were performed with the conventional and solid-state (three-electrode microelectrode-based and two-electrode sandwich) configurations described in the text. The conventional three-electrode cell consisted of a glassy carbon disk electrode (geometric area 0.071 cm2) as the working electrode, a carbon rod as the counter electrode, and a saturated (KCl) Ag/ AgCl reference electrode. The application of the above measurement systems in so-called solid-state voltammetry as well as their ChemSusChem 2015, 8, 2560 – 2568
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diagnostic utility have been demonstrated and reviewed previously.[40, 41]
Viscosity and zeta potential measurements The electrolyte (~ 1 g) from a sample stored in a dry-box was cast evenly on the plate of a Brookfield model DV-III rheometer and dried under vacuum at 60 8C for 48 h. The viscosities were determined under dry N2 at constant temperature with a Brookfield cone (model CP-52) and did not depend on the cone rotation rate. The zeta (electrokinetic) potentials of Pt and iodine-modified Pt nanoparticles (in aqueous colloidal dispersions at 25 8C) were determined indirectly from electrophoretic mobility by dynamic light scattering (laser, 532 nm, Zetaesizer Nano ZS system, Malvern Instruments).
Cell fabrication Screen-printed transparent and double layers of TiO2 particles were used as photoelectrodes in this study. For the transparent films, a 6 mm thick layer of 20 nm TiO2 particles was printed on the fluorine-doped SnO2 (FTO) conducting glass electrode. The double-layered films consisted of an 8 mm thick transparent layer of 20 nm TiO2 covered with a 5 mm thick scattering layer of 400 nm O2 particles. The porosities were evaluated as 67 % for the 20 nm TiO2 transparent layer and 42 % for the scattering layer, as determined from BET measurements. The TiO2 electrodes were sintered at 500 8C, cooled to 80 8C, dipped in the dye solution for 18 h (0.3 mm of the dye with 2 mm of chenodeoxycholic acid in chlorobenzene) for sensitization, and then assembled with a thermally platinized FTO conducting glass counter electrode. The working and counter electrodes were separated by a 25 mm thick hot-melt ring (Surlyn, DuPont) and sealed by heating. The internal space in the cell was filled with the electrolyte by using a vacuum pump.
Photovoltaic characterization A 450 W xenon light source (Oriel, USA) was used to characterize the solar cells. The spectral output of the lamp was matched in the wavelength region 350–750 nm with the aid of a Schott K113 Tempax sunlight filter (Przisions Glas&Optik GmbH, Germany) to reduce the mismatch between the simulated and true solar spectra to less than 2 %. The current–voltage characteristics of the cells under these conditions were obtained by applying an external potential bias to the cell and measuring the generated photocurrent with a Keithley (USA) Model 2400 digital source meter. A similar data acquisition system was used to control the incident photonto-current conversion efficiency (IPCE) measurement. Under computer control, the light from the 300 W xenon lamp (ILC Technology, USA) was focused through a Gemini-180 double monochromator (Jobin Yvon Ltd., UK) onto the investigated photovoltaic cell. The devices were masked to attain an illuminated active area of 0.159 cm2.
Acknowledgements This work was supported by the National Science Center (Poland) under project N N507 322040. J.O. was supported in part from the DSM project 120000-501/86-DSM-107500 (University of Warsaw). The technical help of D. Marks (University of Warsaw) is highly appreciated. M.G acknowledges financial support from
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