PT/NI COUNTER-ELECTRODES WITH IMPROVED STABILITY FOR DYE SENSITIZED SOLAR CELLS

PT/NI COUNTER-ELECTRODES WITH IMPROVED STABILITY FOR DYE SENSITIZED SOLAR CELLS G. Syrrokostas, G. Leftheriotis and P. Yianoulis Energy and Environment Lab, Physics Department, University of Patras, Rion, 26500, Greece Tel.: +02610997446 email: gesirrokos@upatras.gr ABSTRACT: In this work we investigate the stability of platinum (Pt) electrodes after storage in an electrolyte solution (0.5M KI, 0.05 M I 2 in 90%/10% PC/EG), having the standard synthesis used for dye sensitized solar cells. The electrodes were prepared by electrodeposition and thermal decomposition of hexachloroplatinic acid (H 2 PtCl 6 ) solutions. A decrease of up to 40% in the current density for triiodide reduction was caused by storage in the electrolyte, except in the case of constant voltage electrodeposition when the deposited Pt charge density was in the order of 60mC/cm 2. The use of electrodeposited Ni electrodes as substrates for Pt deposition was found to prevent the degradation of the electrodes, even after 50 days of storage in the electrolyte. Nickel has no catalytic effect in the reduction of triiodide ions and the amount of Ni can be kept to a minimum value, such as below 40mC/cm 2, without significant increment in the preparation cost. Keywords: Degradation, Dye sensitized, Electrodeposition, Stability 1 INTRODUCTION Dye sensitized solar cells (DSSC hereafter) can be used for the production of solar electricity [1] and can be integrated into many devices, such as photoelectrochromics [2,], energy storable solar cells [4] and many others, increasing the potential applications. However despite the considerable amount of work that has been done in the field, further improvement of their efficiency and stability is required. One of the main components of DSSCs is the platinized counter-electrode. Its role is to catalyze the reduction of triiodide ions and for that reason an ultra thin platinum film is deposited on a conductive substrate, such as SnO 2 :F/glass (FTO hereafter). We have found that such electrodes degrade during storage in an electrolyte with the same synthesis as the one used for DSSCs, due to Pt dissolution [5]. In this work we have studied the influence of the deposition method and of the Pt amount deposited, on the degradation of platinized electrodes during storage in an electrolyte. Also in order to improve the stability of electrodes prepared by electrodeposition, we have used nickel (Ni) films as substrates. A Ni film was electrodeposited on FTO first, and on top of this film, Pt nanoparticles were electrodeposited, thus creating composite dual or triple layered coatings. The morphology of the films was examined using scanning electron microscopy (SEM) and cyclic voltammetry was used in order to study the catalytic properties of the electrodes. 2 EXPERIMENTAL 2.1 Preparation of the platinum and the nickel films Platinum films were deposited on FTO (sheet resistance: 16.7Ω/sq). Before deposition the substrates were cleaned with a soft detergent. Ultrasound treatment followed, first in a solution containing a small amount of ethanol (5% v/v) and then in a solution containing acetone (5% v/v). They were then rinsed with deionized water. Calcination at 120 o C for 15 min completed the cleaning process. Platinum films were prepared by two different methods, by thermal decomposition and by electrodeposition of a hexachloroplatinic acid solution (H 2 PtCl 6 ). In order to prepare films by thermal decomposition one drop of an H 2 PtCl 6 solution in isopropanol (0.005 M) was spread on the substrate (FTO). The resulting films were not uniform and in order to increase the uniformity the substrates were heated at 70 o C before deposition. Then the films were annealed in 80 o C for 15 min, in order decomposition of H 2 PtCl 6 to take place [5]. Two different processes were used for the electrodeposition of platinum. Films were grown at a constant current of 0.5 ma/cm 2 or at a constant voltage of 400 mv for different time intervals. A potentiostat galvanostat (AMEL, model 205), a function generator (AMEL, model 586) and noise reducer (AMEL NR 2000) were used. An aqueous solution of 0.002M H 2 PtCl 6 and a three electrode configuration was used for both processes. A SnO 2 :F coated glass (FTO) was used as a working electrode, an Ag/AgCl electrode as a reference electrode and a Pt wire as a counter electrode. The same setup was used for the preparation of nickel films. An aqueous solution of 0.5M NiSO 4 x6h 2 O and 0.125M H BO was used and the electrodeposition process was carried out at a constant voltage of 1500 mv for different time intervals, on the same substrates. We have prepared dual or triple layered films with the form Pt/Ni/FTO/glass or Pt/Ni/Pt/FTO/glass by performing successive depositions. 2.2 Characterization of the films Scanning electron images (SEM) were taken in order to characterize the morphology of the films. The electrocatalytic activity of the prepared electrodes for triiodide reduction was evaluated using cyclic voltammetry. A solution containing 10 mm KI, 1 mm I 2 and 1 M LiClO 4 in propylene carbonate (PC) was used. The same electrode setup was used, as in the case of the film electrodeposition. The scan rate was 10mV/s. The Pt electrodes were used as counter electrodes for dye sensitized solar cells. The anode of the cells was a nanostructured TiO 2 film, sensitized with N dye (0. mm in EtOH). The space between the two electrodes was filled with a liquid electrolyte (0.5 M KI, 0.05M I2 in 90%/10% propylene carbonate/ethylene carbonate). 2. Soaking of the films in the electrolyte The platinum and the composite Ni/Pt or Pt/Ni/Pt films were immersed in a standard electrolyte consisting of 0.5M KI, 0.05 M I 2 in a mixture of 90%/10% propylene carbonate/ ethylene glycol and stored in the

dark. When the films were removed from the electrolyte for the cyclic voltammetry measurement they were rinsed gently with deionized water, to remove the excess electrolyte. RESULTS.1 Film morphology The morphology of the films is affected by the preparation method. When Pt is deposited on top of FTO/glass by electrodeposition with constant current density or by thermal decomposition, the films are composed by distinct nanoparticles covering only a part of the substrate, with sizes ranging from 100 to (Fig.1, 2). This results in the enhancement of electroactive surface area of the electrode [6]. More specifically Pt films prepared by thermal decomposition were composed by large Pt aggregates, with large gaps between them (Fig.1), whereas almost spherical nanoparticles were observed in the case of electrodeposition (Fig.2). Figure : Sem image of a film made by electrodeposition voltage: 400 mv, deposited charge density: 51.4 mc/cm 2 ). Figure 1: Sem image of a film made by thermal decomposition of H 2 PtCl 6 (0.005 M) solution in isopropyl alcohol. Figure 4: Sem image of a film made by electrodeposition voltage: 400 mv, deposited Pt charge density: 21. mc/cm 2 )..2 Electrochemical properties of the films In Fig. 5 a characteristic cyclic voltammogramm for triiodide reduction appears for two electrodes prepared by thermal decomposition and by electrodeposition on FTO/glass. 0.7 Peak I Peak II I I 2e 2I 2e 2 0.5 0. 0.1 Figure 2: Sem image of a film made by electrodeposition current density: 0.5 ma/cm 2, deposited charge density: 45 mc/cm 2 ). On the other hand uniform films result from the electrodeposition with constant voltage, when almost the same charge density of Pt was deposited as in the case of electrodeposition with constant current density (Fig. ). The morphology was not altered when Pt was deposited on top of Ni/FTO/glass substrate with constant voltage (Fig.4). -500-00 -100-0.1 100 00 500 700 900 1100-0.5-0.7 Peak I' I 2e I 2e 2I Peak II' 2 V (mv) vs Ag/AgCl Thermal decomp. Electrodeposition Figure 5: Cyclic voltammogramms for two Pt films prepared by thermal decomposition and electrodeposition on FTO/glass. The electrodes prepared by thermal decomposition showed larger catalytic activity for triiodide reduction, as shown by the larger current density for all voltammetry peaks and especially for peak I (Fig. 5), that corresponds to triiodide reduction, due to the larger electroactive area [7]. In Fig. 6 a characteristic cyclic voltammogramm for

η (%) triiodide reduction appears for two electrodes prepared by electrodeposition on FTO/glass and on a Ni/FTO/glass substrate. The current density for the electrodeposited electrodes on a bare FTO and on a previously Ni deposited FTO substrate was almost the same, as the underlying Ni film has no catalytic effect for triiodide reduction, as is shown from the absence of a reduction peak in the cyclic voltammogramm of a Ni/FTO/glass electrode (Fig. 6). 0.5 0.25 0.15 0.05-500 -00-100 100 00 500 700 900 1100-0.05-0.15 5 Peak I Peak II I I 2e 2I 2e 2 Peak I' I 2e I 2e 2I 2 Peak II' Pt/FTO/Glass V (mv) vs Ag/AgCl Pt/Ni/FTO/glass Ni/FTO/Glass Figure 6: Cyclic voltammogramms for two Pt films prepared by electrodeposition on FTO/glass and on Ni/FTO/glass substrate and for a Ni film prepared by electrodeposition on FTO/glass. Moreover in the case of electrodeposited Pt films with both processes (constant current and constant voltage) the variation of the current density for triiodide reduction with the deposited Pt charge density was studied and the results are shown in Fig. 7. 4 2 electrodeposited Pt films with constant voltage (400 mv) has a plateau above 50-60 mc/cm 2 of deposited charge density. The reason is that a uniform and compact film was already prepared when the deposited Pt charge was in the range of 50mC/cm 2 and the deposition of more charge had no influence in the catalytic active area of the electrode, since the excess charge is deposited on an already existing Pt film.. Storage of the electrodes in the electrolyte After evaluation of their catalytic activity, the electrodes were stored in an electrolyte solution, having the same composition with the electrolyte used in a DSSC, and were tested periodically. In Fig.8 appears the drop in current density during the first 50 days of storage in the electrolyte solution for Pt electrodes prepared by electrodeposition with constant current density. We observe that the drop is more significant during the first 10 days. Furthermore this drop is almost the same regardless the deposited Pt charge density. 2 1 y = 0.004ln(x) - 0.00 R² = 0.91 Pt charge density 57.6 mc/cm2 80.8 mc/cm2 11. mc/cm2 142.2 mc/cm2 y = 0.00ln(x) - 0.1 R² = 0.84 y = 0.004ln(x) - 0.11 R² = 0.971 y = 0.00ln(x) - 0.01 R² = 0.92 Figure 8: Variation of current density after electrolyte storage for electrodes prepared by electrodeposition with constant current density (0.5mA/cm 2 ). -0.24 Constant Current Constant Voltage The same trend has been observed for the efficiency of dye sensitized cells, prepared by the two kinds of Pt electrodes, although the drop in efficiency may be due to other factors, as it is more pronounced during the first 2- days (Fig. 9). -0.22 0 20 40 60 80 100 120 140 Q PT (mc/cm 2 ) Figure 7: Variation of current density for triiodide reduction with the deposited Pt charge density for electrodes prepared by electrodeposition with constant current density or constant voltage. We observe that the current density is almost linear with the deposited Pt charge density, when electrodeposition with constant current density is used (0.5 ma/cm 2 ). As shown from the SEM images above, only a part of the surface of the substrate is covered in the case of electrodeposition with constant current density. When more charge was deposited, a larger part of the substrate surface was covered, resulting in the enhancement of the electrocatalytic activity of the electrodes and the subsequent reduction in the charge transfer resistance. On the other hand the current density for -0.65-0.60-0.55-0.50-0.45 thermal decomp. electrodeposition 0.5 0 5 10 15 20-0.40 0 5 10 15 20 Figure 9: Variation for the current density corresponding to the reduction of triiodide (Peak I ) for two Pt electrodes prepared by thermal decomposition and electrodeposition stored in the electrolyte solution and variation for the efficiency of dye sensitized cells (inset). 2.5 2.0 1.5 1.0 6 5 4 2 1 0

When electrodeposition with constant voltage was used (Fig.10), no drop in the current density was observed below 60mC/cm 2 of the deposited charge. This value of charge density is the limit for the preparation of a uniform film, as discussed above. Thus, it can be argued that dissolution of Pt is more pronounced in the case of a uniform Pt film, for charge densities above 60mC/cm 2. 1 61.8 mc/cm2 87.8 mc/cm2 y = -2E-05x - 0.008 R² = 0.011 y = 0.00ln(x) - 0.02 R² = 0.798 y = 0.005ln(x) - 0.299 R² = 0.769 112 mc/cm2 Figure 10: Variation of current density with electrolyte storage for electrodes prepared by electrodeposition with constant voltage (400 mv) Films prepared by thermal decomposition are more catalytic active, as is shown from the larger current density for triiodide reduction, but degrade more compared to films prepared by electrodeposition (Fig. 11). Use of Ni/Pt/FTO/glass as substrate for the electrodeposition of Pt nanoparticles, seems to enhance stability of the electrodes preventing the current density reduction during the first 50 days of electrolyte storage, as can be seen in Fig.11. 4 2 1 Pt charge density Pt/FTO electrodep. Pt/Ni/Pt/FTO electrodep. Pt/FTO electrodep. Pt/FTO thermal decomp. - 21% - 5 % - 4 % + % -0.65 Figure 11: Variation of current density with electrolyte storage for electrodes prepared by electrodeposition and thermal decomposition on FTO/glass and on Ni/Pt/FTO/glass substrates. The deposition of Ni on Pt/FTO/glass enhances the uniformity of the nickel films, compared to the deposition of Ni on FTO/glass as a substrate. Finally different amounts of Ni where deposited and no change in the stability of Pt/Ni/FTO/ glass electrodes was observed -0.6-0.55-0.5-0.45-0.4 5 (Fig. 12). Thus, the amount of Ni can be kept to a minimum value, without affecting significant the overall cost of DSSC. These results are promising and we believe that tailoring the properties of the Ni/FTO substrate to better suit the Pt particles could lead to further improvements of the electrode long term stability. 2 1 Figure 12: Variation of current density with electrolyte storage for electrodes prepared by electrodeposition on Ni/FTO/glass substrates, with different amount of nickel and the same amount of Pt (45 mc/cm 2 ). 4 CONCLUSIONS The stability of the platinized counter electrode in a DSSC depends strongly on the preparation method. Electrodes prepared by thermal decomposition show enhanced catalytic activity towards triiodide reduction in the as prepared state, but degrade faster than electrodeposited Pt electrodes and this degradation has a direct impact on the efficiency of solar cells. As for the electrodeposited Pt electrodes, the preparation method (constant current density or constant voltage) leads to different film morphologies from distinct almost spherical nanoparticles to uniform films, for the same amount of deposited Pt charge. The current density for triiodide reduction increases linearly with the deposited Pt charge density, but only when the deposition is performed with constant current. Moreover the stability is affected from the amount of deposited charge in the case of constant voltage. Deposition of a Pt film on electrodeposited Ni/FTO/Glass substrates improves the stability, especially during the first days, and the amount of Ni needed can be kept to a low value (below 40 mc/cm 2 ). By altering the conditions of Pt deposition on Ni/FTO substrates (e.g deposition time), we believe that we can prepare electrodes with the same catalytic activity as the platinized electrodes (Pt/FTO). ACKNOWLEDGEMENTS y = -0.008ln(x) - 0.289 R² = 0.9678 y = -0.00ln(x) - 0.299 R² = 0.448 y = -0.002ln(x) - 0.284 R² = 0.016 Ni charge density 7.7 mc/cm2 72.2 mc/cm2 86.2 mc/cm2 This research has been co-financed by the European Union (European Regional Development Fund ERDF) and Greek national funds through the Operational Program "Regional Operational Programme" of the National Strategic Reference Framework (NSRF) - Research Funding Program: Support for research, technology and innovation actions in Region of Western

Greece Support of Research Services, Technology and Innovation of Patras University project, co-funded by the EU and the Greek Government. The authors acknowledge the financial support from the Karatheodori 2010-201 research grants of Patras University. REFERENCES [1] G. Syrrokostas, M. Giannouli, P. Yianoulis, Renewable Energy 4 (2009) 1759. [2] G. Leftheriotis, G. Syrrokostas, P. Yianoulis, Solid State Ionics 21 (201) 0. [] G. Leftheriotis, G. Syrrokostas, P. Yianoulis, Solar Energy Materials and Solar Cells 96 (2012) 86. [4] Y. Saito, S. Uchida, T. Kubo, H. Segawa, Thin Solid Films 518 (2010) 0. [5] G. Syrrokostas, A. Siokou, G. Leftheriotis, P. Yianoulis, Solar Energy Materials and Solar Cells 10 (2012) 119. [6] N. Papageorgiou, W. Maier, M. Grätzel, Journal of the Electrochemical Society 144 (1997) 876. [7] K. Hanson, C. Tobias, Journal of The Electrochemical Society 14 (1987) 2204.