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Bimetallic PtSe nanoparticles incorporating with reduced graphene oxide as efficient and durable electrode materials for liquid-junction photovoltaic devices
Materials Today Energy 16 (2020) 100384
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Bimetallic PtSe nanoparticles incorporating with reduced graphene oxide as efficient and durable electrode materials for liquid-junction photovoltaic devices Van-Duong Dao a, b a b
Faculty of Biotechnology, Chemistry and Environmental Engineering, Phenikaa University, Hanoi, 10000, Viet Nam Phenikaa Research and Technology Institute (PRATI), A&A Green Phoenix Group, 167 Hoang Ngan, Hanoi, 10000, Viet Nam
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 March 2019 Received in revised form 31 July 2019 Accepted 8 January 2020 Available online xxx
In this work, we introduce the synthesis of PtSe alloys with the different stoichiometric ratio of Pt and Se in PtSe alloy on reduced graphene oxide (RGO). And, then the developed nanohybrid materials are employed for the first time as counter electrodes (CEs) for efficient third-generation solar cells. As a result, PtSe nanoalloys is located on the surface of RGO without any agglomerations. Furthermore, the developed materials also provide a porous three-dimensional network structure, suggesting rapid electron transfer paths. Therefore, the highest efficiency of 6.26% was obtained for a cell fabricated with Pt0$74Se0.26/RGO electrode that is due to the lowest charge-transfer resistance of 0.89 U and diffusion impedance of 0.88 U. The optimized efficiency is also higher than those of 4.98% and 4.34% for devices assembled with Pt/RGO and Se/RGO CEs, respectively. This work presents a general strategy for designing and fabricating porous PtSe alloy/RGO CEs for energy conversion devices. © 2020 Elsevier Ltd. All rights reserved.
Keywords: PtSe/graphene Counter electrode Hybrid materials Dye-sensitized solar cells
1. Introduction Dye-sensitized solar cells (DSCs) have drawn wide attention for scientific research that is due to their incomparable advantages including low cost, easy to manufacture, environmental friendly and high theoretical power conversion efficiency (PCE) [1e3]. In general, dye-absorbed TiO2 as a working electrode, an electrolyte, and Ptloaded on SnO2:F as a counter electrode (CE) is structured a DSC [3,4]. It is well-known that the CE has an important component to make the proper connection to transport the electrons formed towards the outside of the cell as a media to transport electrons along the electric circuit. Furthermore, it also catalyzes the reduction of triiodide ions. The development of CE in DSCs has received a lot of attention [5e9]. Until now, the highest PCE of the DSCs with a Pt CE was recorded of 14.3% [10]. However, the Pt CE has some disadvantages such as high cost and very low storage capacity. In addition, Pt can react with the liquid electrolyte containing iodide to form PtI4 and PtI6 [11e13], which reduces the stability of CE and device. To solve this issue, PtM (M ¼ Fe, Pd, FeOx, TiO2, Mo, Cu, Ru, NiO, Ni, Co, and Cr) become alternative materials for DSCs [14e17]. This
E-mail address: [email protected]. https://doi.org/10.1016/j.mtener.2020.100384 2468-6069/© 2020 Elsevier Ltd. All rights reserved.
is due to a reduction of the Pt used in CE, improvement of catalytic activity for triiodide ions reduction and enhancement of long-term performance. However, numerous drawbacks in the synthesis methods exist such as the vacuum condition requirement, toxic chemical reagents, liquid environment, etc. To overcome these disadvantages, plasma technology has been developed [18]. Various PtM alloys/reduced graphene oxide (RGO) (M ¼ Au, Zn, Ni, Co, Fe, Pd, Cu, Sn, Ru, and Mo) have been synthesized and applied an efficient reduction of triiodide ion at CE of DSCs [19e21]. To reduce of Pt used in CEs of DSCs, we presented here the synthesis of PtSe alloy-incorporated with RGO by using plasma technology which is shown in Fig. 1a. Furthermore, to optimize catalyst material for the reduction of triiodide ions, we also carefully controlled the chemical state of Pt and Se in PtSe alloy on the RGO surface to obtain highest catalyst materials for regeneration iodide ions from triiodide ions at CEs of liquid-junction photovoltaic (PV) devices. As the results, the lowest charge-transfer resistance (Rct), and the lowest diffusion impedance (Zw) were obtained for Pt0$74Se0.26/RGO electrode. Moreover, the ratio of Rct/Zw for the developed Pt0$74Se0.26/RGO electrode become to ~1. Thus, the highest PCE of 6.26% was obtained for a cell fabricated with Pt0$74Se0.26/ RGO electrode which is also higher than those of 4.98% and 4.34% for DSCs with Pt NPs/RGO and Se NPs/RGO CEs, respectively.
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2. Experimental 2.1. Synthesis of the PtSe alloy/graphene Two precursor solutions of 10 mM chloroplatinic acid (H2PtCl6.xH2O (37.5% Pt basic) and 10 mM selenium tetrachloride (SeCl4) in DI water were prepared. In order to control the stoichiometric ratio of Pt and Se in PtSe alloy, a set of 10 mL each
solution was prepared by verifying the volume ratio of the precursors, as follows: 1:0, 0.9:0.1, 0.7:0.3, 0.5:0.5, 0.3:0.7, 0.1:0.9, and 0:1 and then added in a 200 mL pre-dispersed GO solution with a concentration of 1 g L1. Next, we conducted the ultrasonication for 30 min to ensure the dispersion of reagents. To synthesize PtSe alloy/graphene, we conducted an AC plasma system as Fig. 1a at 200 W for 1 h. Note that this process does not require any carrier gas. The developed materials were collected after removing
Fig. 1. (a) Schematic diagram and photos of the plasma system. (b) TEM image of Pt0$74Se0.26/RGO. (c) A cross-sectional compositional line profile of PtSe NPs on RGO. The inserted Fig. 1c shows the HAADF-STEM image of PtSe NPs/RGO. (d) TEM-EDS of Pt0$74Se0.26/RGO. e) XRD patterns of RGO. f) C1s core level XPS spectrum of Pt0$74Se0.26/RGO.
V.-D. Dao / Materials Today Energy 16 (2020) 100384
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Fig. 2. (a) Cyclic voltammograms of electrodes and b) CV of the Pt0$42Se0.58/RGO measured with 100 cycles.
Table 1 CE properties extracted from the CV and EIS spectra, Tafel, IPCE, and IeV measurements. Counter electrode
|Jred| (mA.cm2)
DE (mV)
Rct (U)
Jsc (mAcm2)
Jsc* (mAcm2)
Voc (mV)
FF (%)
PCE (%)
Pt0Se1/RGO Pt0$42Se0.58/RGO Pt0$65Se0.35/RGO Pt0$74Se0.26/RGO Pt0$78Se0.22/RGO Pt0$82Se0.18/RGO Pt1Se0/RGO
4.15 4.25 4.62 5.00 4.80 4.80 4.32
660 630 600 490 490 560 500
1.92 8.73 3.02 0.89 1.44 3.13 7.87
11.43 11.17 12.85 15.33 14.47 14.83 11.44
11.33 11.31 14.24 15.33 15.16 14.25 11.25
525 535 550 570 580 570 590
72.37 73.71 73.81 71.70 65.20 63.39 73.81
4.34 4.40 5.21 6.26 5.47 5.36 4.98
Jred: Peak current density of the reduction. € Peak-to-peak separation. AE: * The values are obtained from IPCE measurements. Rct: charge transfer resistance. Jsc: short-circuit current density. Voc: open-circuit voltage. FF: Fill Factor. PCE: Power conversion efficiency.
impurities from the solution several times by the centrifugation at 10,000 rpm for 5 min. It was further washed with DI water and subsequently isopropyl alcohol before dispersing in 6 mL DI water for further use. The XPS measurements indicate the change in the stoichiometric ratio of elements in PtSe alloy. We found that the change in volume ratio of Pt and Se precursors in mixture solution of 1:0, 0.9:0.1, 0.7:0.3, 0.5:0.5, 0.3:0.7, 0.1:0.9 results in the change in the stoichiometric ratio of elements of 1:0, 0.82:0.18, 0.78:0.22, 0.74:0.26, 0.65:0.35, 0.42:0.58 and 0:1 respectively. We denoted the CEs by Pt1Se0/RGO, Pt0$82Se0.18/RGO, Pt0$78Se0.22/RGO, Pt0$74Se0.26/RGO, Pt0$65Se0.35/RGO, and Pt0$42Se0.58/RGO and Pt0Se1/RGO. 2.2. Fabrication of electrodes, assembly, and measurements of DSCs Fabrication of electrodes, assembly, and measurements of DSCs are detailed in Supporting Information. 3. Results and discussion 3.1. Synthesis and characterization of the Pt0·74Se0.26/RGO materials To affirm the formation of PtSe alloy on the surface of graphene, we first synthesized the PtSe alloy on GO with the volume ratio of Pt and Se precursor of 1:1 and it was then denoted by Pt0$74Se0.26/
RGO as the stoichiometric ratio of Pt and Se in PtSe alloy. In order to affirm the PtSe alloy formation on the graphene surface, we conducted TEM measurement. As can be seen in Fig. 1b, the PtSe NPs are successfully synthesized and immobilized on RGO's surface without any aggregation. The size of PtSe NPs is in range of 3e8 nm and the average size is ~3 nm. We conducted the cross-sectional compositional line profile to analyze the elemental distribution in the PtSe alloy. HAADF-STEM-EDS of single NPs revealed the alloy structure and composition of NPs (Fig. 1c). TEM-EDS performance was conducted to determine the surface composition of the PtSe alloy on the surface of RGO. As shown in Fig. 1d, the chemical formula of the PtSe was Pt0$93Se0.07 which is a little different with XPS results (Pt0$74Se0.26). This is due to the limitation of the equipment used. To affirm the formation of RGO, we conducted XRD and XPS measurements. The obtained results are presented in Fig. 1e and f. It is easily found that the 2ɵ peak at 26.7 is obtained for RGO, suggesting the reduction of GO to RGO. The reduction of GO to RGO was further investigated through XPS. After fitting with Shirley þ Liner background subtraction, The C 1s spectrum of the Pt0$74Se0.26/RGO film was decomposed to five components: C]C in aromatic rings (284.4 eV) and CeC in aliphatic (283.8 eV). Note that the plasma has high kinetic energy that can cut out the bonds between C and O. therefore, we could not found the peak fitting of CeO, C]O groups in C 1s core-level XPS spectrum of Pt0$74Se0.26/RGO. The obtained result is in agreement with other efforts [22].
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Fig. 3. (a,b) Nyquist plots of symmetrical dummy cells with two identical CEs. The insert in Fig. 3a shows the equivalent circuit diagram used to fit the EIS spectra. (c) Bode phase plots of symmetrical dummy cells. (d) Tafel curves of different symmetrical dummy cells, similar to those used for the EIS measurements, are shown.
3.2. Design stoichiometric ratio of Pt and Se in PtSe alloy/RGO CEs for highly efficient DSCs In order to optimize both catalytic activity and electrical conductivity of the developed nanohybrid materials for the reduction of triiodide ions to iodide ions at CEs of DSCs, we controlled the stoichiometric ratio of Pt and Se in PtSe alloy to optimize an efficient PtSe/RGO CE. The change in a stoichiometric ratio of Pt and Se in PtSe alloy will effect on the morphology of CEs. Note that the CEs were fabricated by drop-casting as our previous work [23]. The obtained results are presented in Fig. S1 (Supporting Information). As can be seen in Fig. S1, there was a formation of a 3D network structure of RGO on the surface of the FTO glass substrate. This can support numerous channels for the triiodide ions to cross, and improve the reduction occur of triiodide ions. PtxSe1-x (0x 1) alloys are successfully synthesized and deposited on the RGO surfaces. We could not find any agglomerations. The PtSe NPs is not only well-localized on the surface of the RGO but it also localized at the grain boundary between the RGO sheets. It should be noted that the localized NPs at the grain boundary of graphene sheets can act as bridges to facilitate electron conductivity. On the other hand, this supports to enhance the electrical conductivity as well as the
electrochemical catalytic activity of the CEs. The change in NP sizes and density of NPs with the change in the stoichiometric ratio of Pt and Se in PtSe alloy are not easy obtained by SEM images. To evaluate the catalytic performance capabilities of the electrodes, we conducted a CV assessment as presented in Fig. 2a. This € of each CV as listed in Table 1 and was done using |Jred| and AE Table S1. Table 1 showed that |Jred| increases with decreasing the stoichiometric ratio of Pt and Se in PtSe alloy from 1:0 to 0.74:0.26 and that it decreases with further decreasing the stoichiometric ratio of Pt and Se in PtSe alloy. The Pt0$74Se0.26/RGO electrode exhibits the highest |Jred| value of 5.00 mA$cm2. This means the charge transfer rate across the Pt0$74Se0.26/RGO electrode is higher than those of the other electrodes tested. The stability of the Pt0$74Se0.26/RGO electrode was further affirmed by CV assessments with 100 cycles, as shown in Fig. 2b. There is no change in Jred and Joxd values over 100 cycles This affirms that there was no degradation of the Pt0$74Se0.26 on the RGO and this also affirms the strong adhesion of Pt0$74Se0.26/RGO on FTO glass. The obtained results indicate the high stability of Pt0$74Se0.26/RGO electrode under electrochemical reaction conditions. € is followed Table 1 and Table S1 indicate that the change in AE the sequence of Pt0Se1/RGO (660 mV) < Pt0$42Se0.58/RGO
V.-D. Dao / Materials Today Energy 16 (2020) 100384
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Fig. 4. (a) Characteristic current density-voltage curves of DSCs with different CEs measured under standard conditions. (b) Characteristic current density-voltage curves of DSCs with different CEs measured in the dark. (c) IPCE curves of different DSCs. (d) Start-stop switches of DSCs with different CEs. The on-off plots were achieved by alternately irradiating (1 kWm2) and darkening the DSC devices at 0 V. e) Time-resolved photocurrent responses during the long-term performance. (f) Relationship between the CEs, the efficiency, and the ratio of Rct/Zw.
(630 mV) < Pt0$65Se0.35/RGO (600 mV) < Pt0$74Se0.26/ RGO ¼ Pt0$78Se0.22/RGO (490 mV) < Pt1Se0/RGO € of 490 mV for (500 mV) < Pt0$82Se0.18/RGO (560 mV). AE Pt0$74Se0.26/RGO was the lowest value under study which indicates the highest rate of the redox reaction of I 3 /I [24,25].
The catalytic activities of the developed electrodes were further investigated by electrochemical impedance spectroscopy (EIS). Fig. 3a and b shows the Nyquist plots using different electrodes. The obtained results are fitted by Zview and listed in Table 1 and Table S2. It was found that the Rct value decreases with a decrease in
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the stoichiometric ratio of Pt and Se in PtSe alloy from 1:0 to 0.74:0.26. However, it increases with a further decrease in the stoichiometric ratio of Pt and Se in PtSe alloy. The lowest Rct value was found with the stoichiometric ratio of Pt and Se in PtSe alloy of 0.74:0.26. The change in the binding energy of PtSe alloys could be used to explain the change in catalytic activity due to the electron transfer from Se to Pt upon alloying [26,27]. For this purpose, we conducted XPS measurements. As the results, the change in the binding energy of Pt4f7/2 in Pt1Se0/RGO, Pt0$82Se0.18/RGO, Pt0$78Se0.22/RGO, Pt0$74Se0.26/RGO, Pt0$65Se0.35/RGO, and Pt0$42Se0.58/RGO were 70.71, 69.99, 70.22, 71.37, 72.77, and 72.96 eV, respectively. Tang et al. reported that the change in the binding energy of alloys results in the change in the work function of alloys [12]. It is expected that the work function of the PtSe alloy decrease compared to Pt. According to that, a decrease in work function results in decreasing binding energy of catalyst with electrons. On the other hand, a decrease in the work function of alloy results in increasing triiodide ions on the surface of the catalyst which helps enhance the rate of the reduction process. Now, we turn to consider the electron lifetime (t) data for the regeneration of iodide ions from triiodide ions (Fig. 3c) and the exchange current density (Fig. 3d). It was found that there is no significant difference in the trends of the fmax and J0 values which were a similar trend for the Rct values. Accordingly, a shorter t and a high J0 value mean a lower Rct [28] and a high short-circuit current density (Jsc) in DSCs [29]. We discuss the effect of CEs on the PV performance of cells. The PV parameters are presented in Fig. 4a and Table 1. As discussed above, the Pt0$74Se0.26/RGO CE showed excellent electrochemical catalysis for the triiodide reduction. Hereon, the DSC with a Pt0$74Se0.26/RGO electrode exhibited the highest efficiency of 6.26% under standard irradiations (Table 1). The Jsc, Voc, and FF values were found to be 15.33 mA$cm2, 570 mV, and 71.70%, respectively. In order to compare with the state-of-the-art device, we conducted an additional experiment with devices assembled with RGO and Pt electrodes. Note that the Pt electrode is prepared by thermal decomposition as our previous work [30]. The obtained results showed in Fig. S2 and Table S3. We found that the efficiencies of 2.53 and 4.35% were obtained for the cells with RGO and Pt electrodes, respectively. These efficiencies are lower than that of 6.26% which cell fabricated with Pt0$74Se0.26/RGO electrode. The obtained efficiency is slightly lower than that the 7.08% for a cell with nickel tungsten oxide/biobased carbon CE [31]. However, the enhancement of efficiency compared to the cell fabricated with Pt electrode in this work is 43.9% while the Yun et al. showed the enhancement of 9.6%. The positive shifts of I 3 /I redox energy level, which is described by DE value in CV measurements [25], was used to explain the change in Voc values. The change in dark currents, that is measured in Fig. 4b, are used to further affirm the change in Voc value. As can be seen in Fig. 4b, at a high forward bias, the change in the dark current has followed a sequence of Pt1Se0/ RGO > Pt0$82Se0.18/RGO > Pt0$78Se0.22/RGO > Pt0$74Se0.26/ RGO > Pt0$65Se0.35/RGO > Pt0$42Se0.58/RGO > Pt0Se1/RGO. It should be noted that the dark current relates the reaction between the CE and electrolyte [32e34]. Given that the dark current becomes a higher, the reaction between the CE and the electrolyte can be neglected. To affirm the change in Jsc values, we conducted IPCE measurements, as shown in Fig. 4c and Table 1. It was found that the change in the Jsc values is followed the sequence of Pt0$74Se0.26/RGO (15.33 mA$cm2) > Pt0$82Se0.18/RGO (14.83 mA$cm2) > Pt0$78Se0.22/RGO (14.47 mA$cm2) > Pt0$65Se0.35/RGO (12.85 mA$cm2) > Pt1Se0/RGO (11.44 mA$cm2) > Pt0Se1/RGO (11.43 mA$cm2) > Pt0$42Se0.58/RGO (11.17 mA$cm2). The obtained results are in good accordance with the change of Jsc in the PV
performances. The highest Jsc value of 15.33 mA$cm2 was found in the cell with the Pt0$74Se0.26/RGO electrode which is due to the high electrochemical catalytic activity of the CEs. The long-term stability of DSCs assembled with various CEs under 1 sun irradiation was further investigated by Jsc-time with a start-stop switch. As indicated in Fig. 4d, we could not find the change in the Jsc after several start/stop cycles. Fig. 4e shows the long-term performance of DSCs. The Jsc values do not change with the long-time process under 1 sun irradiation. The obtained result indicates the stability of CEs for the reduction of triiodide ions. We found that there is a similar trend in Jsc-time and the catalytic activities of the CEs. Fig. 4e presents the changes in the ratio of Rct/Zw and PCE with respect to the change in the stoichiometric ratio of Pt and Se in PtSe alloy. We found that the ratio of Rct/Zw is close to one results in a high PCE. The obtained results are in line with our previous observations [35]. 4. Conclusion In this study, by using plasma technology, well-dispersed PtxSe1(0x 1) alloy NPs were synthesized and immobilized on the surface of RGO. The morphologies of electrodes and the structure of PtSe alloy were investigated through SEM, TEM, HAADF-STEM-EDS performances. We have carefully controlled the stoichiometric ratio of Pt and Se in PtSe alloy to optimize the catalytic activity of the PtSe/RGO toward the reduction of triiodide ions to iodide ions, as reflected by the smallest Rct, Zw values and balance of Rct/Zw value. As a result, the PCE improved from 4.34 to 6.26% under standard conditions when the Pt0Se1/RGO electrode was substituted for a Pt0$74Se0.26/RGO electrode. Interestingly, the stability in the iodide electrolyte of the electrode showed a good result. The introduction of Pt1-xSex (1 x 0) alloys/RGO is a promising strategy for fabricating efficient and low-cost stable electrodes for liquidjunction PV devices.
x
Declaration of Competing Interest There is no conflict of interest. Acknowledgments This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.02e2018.27. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.mtener.2020.100384. References [1] S. Yun, Y. Qin, A.R. Uhl, N. Vlachopoulos, M. Yin, D. Li, X. Han, A. Hagfeldt, Newgeneration integrated devices based on dye-sensitized and perovskite solar cells, Energy Environ. Sci. 11 (2018) 476e526. [2] S. Yun, A. Hagfeldt, T. Ma, Pt-free counter electrode for dye-sensitized solar cells with high efficiency, Adv. Mater. 26 (2014) 6210. €tzel, A low-cost, high-efficiency solar cell based on dye[3] B. O'Regan, M. Gra sensitized colloidal TiO2 films, Nature 353 (1991) 737e740. [4] S. Yun, A. Hagfeldt, T. Ma, Superior catalytic activity of sub-5 mm-thick Pt/SiC films as counter electrodes for dye-sensitized solar cells, ChemCatChem 6 (2014) 1584e1588. [5] Q. Tang, J. Duan, Y. Duan, B. He, L. Yu, Recent advances in alloy counter electrodes for dye-sensitized solar cells. A critical review, Electrochim. Acta 178 (2015) 886e899. [6] T. Zhang, Y. Liu, S. Yun, Recent advances in counter electrodes for thiolatemediated dye-sensitized solar cells, Isr. J. Chem. 55 (2015) 943e954.
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Contents lists available at ScienceDirect
Materials Today Energy journal homepage: www.journals.elsevier.com/materials-today-energy/
Bimetallic PtSe nanoparticles incorporating with reduced graphene oxide as efficient and durable electrode materials for liquid-junction photovoltaic devices Van-Duong Dao a, b a b
Faculty of Biotechnology, Chemistry and Environmental Engineering, Phenikaa University, Hanoi, 10000, Viet Nam Phenikaa Research and Technology Institute (PRATI), A&A Green Phoenix Group, 167 Hoang Ngan, Hanoi, 10000, Viet Nam
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 March 2019 Received in revised form 31 July 2019 Accepted 8 January 2020 Available online xxx
In this work, we introduce the synthesis of PtSe alloys with the different stoichiometric ratio of Pt and Se in PtSe alloy on reduced graphene oxide (RGO). And, then the developed nanohybrid materials are employed for the first time as counter electrodes (CEs) for efficient third-generation solar cells. As a result, PtSe nanoalloys is located on the surface of RGO without any agglomerations. Furthermore, the developed materials also provide a porous three-dimensional network structure, suggesting rapid electron transfer paths. Therefore, the highest efficiency of 6.26% was obtained for a cell fabricated with Pt0$74Se0.26/RGO electrode that is due to the lowest charge-transfer resistance of 0.89 U and diffusion impedance of 0.88 U. The optimized efficiency is also higher than those of 4.98% and 4.34% for devices assembled with Pt/RGO and Se/RGO CEs, respectively. This work presents a general strategy for designing and fabricating porous PtSe alloy/RGO CEs for energy conversion devices. © 2020 Elsevier Ltd. All rights reserved.
Keywords: PtSe/graphene Counter electrode Hybrid materials Dye-sensitized solar cells
1. Introduction Dye-sensitized solar cells (DSCs) have drawn wide attention for scientific research that is due to their incomparable advantages including low cost, easy to manufacture, environmental friendly and high theoretical power conversion efficiency (PCE) [1e3]. In general, dye-absorbed TiO2 as a working electrode, an electrolyte, and Ptloaded on SnO2:F as a counter electrode (CE) is structured a DSC [3,4]. It is well-known that the CE has an important component to make the proper connection to transport the electrons formed towards the outside of the cell as a media to transport electrons along the electric circuit. Furthermore, it also catalyzes the reduction of triiodide ions. The development of CE in DSCs has received a lot of attention [5e9]. Until now, the highest PCE of the DSCs with a Pt CE was recorded of 14.3% [10]. However, the Pt CE has some disadvantages such as high cost and very low storage capacity. In addition, Pt can react with the liquid electrolyte containing iodide to form PtI4 and PtI6 [11e13], which reduces the stability of CE and device. To solve this issue, PtM (M ¼ Fe, Pd, FeOx, TiO2, Mo, Cu, Ru, NiO, Ni, Co, and Cr) become alternative materials for DSCs [14e17]. This
E-mail address: [email protected]. https://doi.org/10.1016/j.mtener.2020.100384 2468-6069/© 2020 Elsevier Ltd. All rights reserved.
is due to a reduction of the Pt used in CE, improvement of catalytic activity for triiodide ions reduction and enhancement of long-term performance. However, numerous drawbacks in the synthesis methods exist such as the vacuum condition requirement, toxic chemical reagents, liquid environment, etc. To overcome these disadvantages, plasma technology has been developed [18]. Various PtM alloys/reduced graphene oxide (RGO) (M ¼ Au, Zn, Ni, Co, Fe, Pd, Cu, Sn, Ru, and Mo) have been synthesized and applied an efficient reduction of triiodide ion at CE of DSCs [19e21]. To reduce of Pt used in CEs of DSCs, we presented here the synthesis of PtSe alloy-incorporated with RGO by using plasma technology which is shown in Fig. 1a. Furthermore, to optimize catalyst material for the reduction of triiodide ions, we also carefully controlled the chemical state of Pt and Se in PtSe alloy on the RGO surface to obtain highest catalyst materials for regeneration iodide ions from triiodide ions at CEs of liquid-junction photovoltaic (PV) devices. As the results, the lowest charge-transfer resistance (Rct), and the lowest diffusion impedance (Zw) were obtained for Pt0$74Se0.26/RGO electrode. Moreover, the ratio of Rct/Zw for the developed Pt0$74Se0.26/RGO electrode become to ~1. Thus, the highest PCE of 6.26% was obtained for a cell fabricated with Pt0$74Se0.26/ RGO electrode which is also higher than those of 4.98% and 4.34% for DSCs with Pt NPs/RGO and Se NPs/RGO CEs, respectively.
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2. Experimental 2.1. Synthesis of the PtSe alloy/graphene Two precursor solutions of 10 mM chloroplatinic acid (H2PtCl6.xH2O (37.5% Pt basic) and 10 mM selenium tetrachloride (SeCl4) in DI water were prepared. In order to control the stoichiometric ratio of Pt and Se in PtSe alloy, a set of 10 mL each
solution was prepared by verifying the volume ratio of the precursors, as follows: 1:0, 0.9:0.1, 0.7:0.3, 0.5:0.5, 0.3:0.7, 0.1:0.9, and 0:1 and then added in a 200 mL pre-dispersed GO solution with a concentration of 1 g L1. Next, we conducted the ultrasonication for 30 min to ensure the dispersion of reagents. To synthesize PtSe alloy/graphene, we conducted an AC plasma system as Fig. 1a at 200 W for 1 h. Note that this process does not require any carrier gas. The developed materials were collected after removing
Fig. 1. (a) Schematic diagram and photos of the plasma system. (b) TEM image of Pt0$74Se0.26/RGO. (c) A cross-sectional compositional line profile of PtSe NPs on RGO. The inserted Fig. 1c shows the HAADF-STEM image of PtSe NPs/RGO. (d) TEM-EDS of Pt0$74Se0.26/RGO. e) XRD patterns of RGO. f) C1s core level XPS spectrum of Pt0$74Se0.26/RGO.
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Fig. 2. (a) Cyclic voltammograms of electrodes and b) CV of the Pt0$42Se0.58/RGO measured with 100 cycles.
Table 1 CE properties extracted from the CV and EIS spectra, Tafel, IPCE, and IeV measurements. Counter electrode
|Jred| (mA.cm2)
DE (mV)
Rct (U)
Jsc (mAcm2)
Jsc* (mAcm2)
Voc (mV)
FF (%)
PCE (%)
Pt0Se1/RGO Pt0$42Se0.58/RGO Pt0$65Se0.35/RGO Pt0$74Se0.26/RGO Pt0$78Se0.22/RGO Pt0$82Se0.18/RGO Pt1Se0/RGO
4.15 4.25 4.62 5.00 4.80 4.80 4.32
660 630 600 490 490 560 500
1.92 8.73 3.02 0.89 1.44 3.13 7.87
11.43 11.17 12.85 15.33 14.47 14.83 11.44
11.33 11.31 14.24 15.33 15.16 14.25 11.25
525 535 550 570 580 570 590
72.37 73.71 73.81 71.70 65.20 63.39 73.81
4.34 4.40 5.21 6.26 5.47 5.36 4.98
Jred: Peak current density of the reduction. € Peak-to-peak separation. AE: * The values are obtained from IPCE measurements. Rct: charge transfer resistance. Jsc: short-circuit current density. Voc: open-circuit voltage. FF: Fill Factor. PCE: Power conversion efficiency.
impurities from the solution several times by the centrifugation at 10,000 rpm for 5 min. It was further washed with DI water and subsequently isopropyl alcohol before dispersing in 6 mL DI water for further use. The XPS measurements indicate the change in the stoichiometric ratio of elements in PtSe alloy. We found that the change in volume ratio of Pt and Se precursors in mixture solution of 1:0, 0.9:0.1, 0.7:0.3, 0.5:0.5, 0.3:0.7, 0.1:0.9 results in the change in the stoichiometric ratio of elements of 1:0, 0.82:0.18, 0.78:0.22, 0.74:0.26, 0.65:0.35, 0.42:0.58 and 0:1 respectively. We denoted the CEs by Pt1Se0/RGO, Pt0$82Se0.18/RGO, Pt0$78Se0.22/RGO, Pt0$74Se0.26/RGO, Pt0$65Se0.35/RGO, and Pt0$42Se0.58/RGO and Pt0Se1/RGO. 2.2. Fabrication of electrodes, assembly, and measurements of DSCs Fabrication of electrodes, assembly, and measurements of DSCs are detailed in Supporting Information. 3. Results and discussion 3.1. Synthesis and characterization of the Pt0·74Se0.26/RGO materials To affirm the formation of PtSe alloy on the surface of graphene, we first synthesized the PtSe alloy on GO with the volume ratio of Pt and Se precursor of 1:1 and it was then denoted by Pt0$74Se0.26/
RGO as the stoichiometric ratio of Pt and Se in PtSe alloy. In order to affirm the PtSe alloy formation on the graphene surface, we conducted TEM measurement. As can be seen in Fig. 1b, the PtSe NPs are successfully synthesized and immobilized on RGO's surface without any aggregation. The size of PtSe NPs is in range of 3e8 nm and the average size is ~3 nm. We conducted the cross-sectional compositional line profile to analyze the elemental distribution in the PtSe alloy. HAADF-STEM-EDS of single NPs revealed the alloy structure and composition of NPs (Fig. 1c). TEM-EDS performance was conducted to determine the surface composition of the PtSe alloy on the surface of RGO. As shown in Fig. 1d, the chemical formula of the PtSe was Pt0$93Se0.07 which is a little different with XPS results (Pt0$74Se0.26). This is due to the limitation of the equipment used. To affirm the formation of RGO, we conducted XRD and XPS measurements. The obtained results are presented in Fig. 1e and f. It is easily found that the 2ɵ peak at 26.7 is obtained for RGO, suggesting the reduction of GO to RGO. The reduction of GO to RGO was further investigated through XPS. After fitting with Shirley þ Liner background subtraction, The C 1s spectrum of the Pt0$74Se0.26/RGO film was decomposed to five components: C]C in aromatic rings (284.4 eV) and CeC in aliphatic (283.8 eV). Note that the plasma has high kinetic energy that can cut out the bonds between C and O. therefore, we could not found the peak fitting of CeO, C]O groups in C 1s core-level XPS spectrum of Pt0$74Se0.26/RGO. The obtained result is in agreement with other efforts [22].
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Fig. 3. (a,b) Nyquist plots of symmetrical dummy cells with two identical CEs. The insert in Fig. 3a shows the equivalent circuit diagram used to fit the EIS spectra. (c) Bode phase plots of symmetrical dummy cells. (d) Tafel curves of different symmetrical dummy cells, similar to those used for the EIS measurements, are shown.
3.2. Design stoichiometric ratio of Pt and Se in PtSe alloy/RGO CEs for highly efficient DSCs In order to optimize both catalytic activity and electrical conductivity of the developed nanohybrid materials for the reduction of triiodide ions to iodide ions at CEs of DSCs, we controlled the stoichiometric ratio of Pt and Se in PtSe alloy to optimize an efficient PtSe/RGO CE. The change in a stoichiometric ratio of Pt and Se in PtSe alloy will effect on the morphology of CEs. Note that the CEs were fabricated by drop-casting as our previous work [23]. The obtained results are presented in Fig. S1 (Supporting Information). As can be seen in Fig. S1, there was a formation of a 3D network structure of RGO on the surface of the FTO glass substrate. This can support numerous channels for the triiodide ions to cross, and improve the reduction occur of triiodide ions. PtxSe1-x (0x 1) alloys are successfully synthesized and deposited on the RGO surfaces. We could not find any agglomerations. The PtSe NPs is not only well-localized on the surface of the RGO but it also localized at the grain boundary between the RGO sheets. It should be noted that the localized NPs at the grain boundary of graphene sheets can act as bridges to facilitate electron conductivity. On the other hand, this supports to enhance the electrical conductivity as well as the
electrochemical catalytic activity of the CEs. The change in NP sizes and density of NPs with the change in the stoichiometric ratio of Pt and Se in PtSe alloy are not easy obtained by SEM images. To evaluate the catalytic performance capabilities of the electrodes, we conducted a CV assessment as presented in Fig. 2a. This € of each CV as listed in Table 1 and was done using |Jred| and AE Table S1. Table 1 showed that |Jred| increases with decreasing the stoichiometric ratio of Pt and Se in PtSe alloy from 1:0 to 0.74:0.26 and that it decreases with further decreasing the stoichiometric ratio of Pt and Se in PtSe alloy. The Pt0$74Se0.26/RGO electrode exhibits the highest |Jred| value of 5.00 mA$cm2. This means the charge transfer rate across the Pt0$74Se0.26/RGO electrode is higher than those of the other electrodes tested. The stability of the Pt0$74Se0.26/RGO electrode was further affirmed by CV assessments with 100 cycles, as shown in Fig. 2b. There is no change in Jred and Joxd values over 100 cycles This affirms that there was no degradation of the Pt0$74Se0.26 on the RGO and this also affirms the strong adhesion of Pt0$74Se0.26/RGO on FTO glass. The obtained results indicate the high stability of Pt0$74Se0.26/RGO electrode under electrochemical reaction conditions. € is followed Table 1 and Table S1 indicate that the change in AE the sequence of Pt0Se1/RGO (660 mV) < Pt0$42Se0.58/RGO
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Fig. 4. (a) Characteristic current density-voltage curves of DSCs with different CEs measured under standard conditions. (b) Characteristic current density-voltage curves of DSCs with different CEs measured in the dark. (c) IPCE curves of different DSCs. (d) Start-stop switches of DSCs with different CEs. The on-off plots were achieved by alternately irradiating (1 kWm2) and darkening the DSC devices at 0 V. e) Time-resolved photocurrent responses during the long-term performance. (f) Relationship between the CEs, the efficiency, and the ratio of Rct/Zw.
(630 mV) < Pt0$65Se0.35/RGO (600 mV) < Pt0$74Se0.26/ RGO ¼ Pt0$78Se0.22/RGO (490 mV) < Pt1Se0/RGO € of 490 mV for (500 mV) < Pt0$82Se0.18/RGO (560 mV). AE Pt0$74Se0.26/RGO was the lowest value under study which indicates the highest rate of the redox reaction of I 3 /I [24,25].
The catalytic activities of the developed electrodes were further investigated by electrochemical impedance spectroscopy (EIS). Fig. 3a and b shows the Nyquist plots using different electrodes. The obtained results are fitted by Zview and listed in Table 1 and Table S2. It was found that the Rct value decreases with a decrease in
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the stoichiometric ratio of Pt and Se in PtSe alloy from 1:0 to 0.74:0.26. However, it increases with a further decrease in the stoichiometric ratio of Pt and Se in PtSe alloy. The lowest Rct value was found with the stoichiometric ratio of Pt and Se in PtSe alloy of 0.74:0.26. The change in the binding energy of PtSe alloys could be used to explain the change in catalytic activity due to the electron transfer from Se to Pt upon alloying [26,27]. For this purpose, we conducted XPS measurements. As the results, the change in the binding energy of Pt4f7/2 in Pt1Se0/RGO, Pt0$82Se0.18/RGO, Pt0$78Se0.22/RGO, Pt0$74Se0.26/RGO, Pt0$65Se0.35/RGO, and Pt0$42Se0.58/RGO were 70.71, 69.99, 70.22, 71.37, 72.77, and 72.96 eV, respectively. Tang et al. reported that the change in the binding energy of alloys results in the change in the work function of alloys [12]. It is expected that the work function of the PtSe alloy decrease compared to Pt. According to that, a decrease in work function results in decreasing binding energy of catalyst with electrons. On the other hand, a decrease in the work function of alloy results in increasing triiodide ions on the surface of the catalyst which helps enhance the rate of the reduction process. Now, we turn to consider the electron lifetime (t) data for the regeneration of iodide ions from triiodide ions (Fig. 3c) and the exchange current density (Fig. 3d). It was found that there is no significant difference in the trends of the fmax and J0 values which were a similar trend for the Rct values. Accordingly, a shorter t and a high J0 value mean a lower Rct [28] and a high short-circuit current density (Jsc) in DSCs [29]. We discuss the effect of CEs on the PV performance of cells. The PV parameters are presented in Fig. 4a and Table 1. As discussed above, the Pt0$74Se0.26/RGO CE showed excellent electrochemical catalysis for the triiodide reduction. Hereon, the DSC with a Pt0$74Se0.26/RGO electrode exhibited the highest efficiency of 6.26% under standard irradiations (Table 1). The Jsc, Voc, and FF values were found to be 15.33 mA$cm2, 570 mV, and 71.70%, respectively. In order to compare with the state-of-the-art device, we conducted an additional experiment with devices assembled with RGO and Pt electrodes. Note that the Pt electrode is prepared by thermal decomposition as our previous work [30]. The obtained results showed in Fig. S2 and Table S3. We found that the efficiencies of 2.53 and 4.35% were obtained for the cells with RGO and Pt electrodes, respectively. These efficiencies are lower than that of 6.26% which cell fabricated with Pt0$74Se0.26/RGO electrode. The obtained efficiency is slightly lower than that the 7.08% for a cell with nickel tungsten oxide/biobased carbon CE [31]. However, the enhancement of efficiency compared to the cell fabricated with Pt electrode in this work is 43.9% while the Yun et al. showed the enhancement of 9.6%. The positive shifts of I 3 /I redox energy level, which is described by DE value in CV measurements [25], was used to explain the change in Voc values. The change in dark currents, that is measured in Fig. 4b, are used to further affirm the change in Voc value. As can be seen in Fig. 4b, at a high forward bias, the change in the dark current has followed a sequence of Pt1Se0/ RGO > Pt0$82Se0.18/RGO > Pt0$78Se0.22/RGO > Pt0$74Se0.26/ RGO > Pt0$65Se0.35/RGO > Pt0$42Se0.58/RGO > Pt0Se1/RGO. It should be noted that the dark current relates the reaction between the CE and electrolyte [32e34]. Given that the dark current becomes a higher, the reaction between the CE and the electrolyte can be neglected. To affirm the change in Jsc values, we conducted IPCE measurements, as shown in Fig. 4c and Table 1. It was found that the change in the Jsc values is followed the sequence of Pt0$74Se0.26/RGO (15.33 mA$cm2) > Pt0$82Se0.18/RGO (14.83 mA$cm2) > Pt0$78Se0.22/RGO (14.47 mA$cm2) > Pt0$65Se0.35/RGO (12.85 mA$cm2) > Pt1Se0/RGO (11.44 mA$cm2) > Pt0Se1/RGO (11.43 mA$cm2) > Pt0$42Se0.58/RGO (11.17 mA$cm2). The obtained results are in good accordance with the change of Jsc in the PV
performances. The highest Jsc value of 15.33 mA$cm2 was found in the cell with the Pt0$74Se0.26/RGO electrode which is due to the high electrochemical catalytic activity of the CEs. The long-term stability of DSCs assembled with various CEs under 1 sun irradiation was further investigated by Jsc-time with a start-stop switch. As indicated in Fig. 4d, we could not find the change in the Jsc after several start/stop cycles. Fig. 4e shows the long-term performance of DSCs. The Jsc values do not change with the long-time process under 1 sun irradiation. The obtained result indicates the stability of CEs for the reduction of triiodide ions. We found that there is a similar trend in Jsc-time and the catalytic activities of the CEs. Fig. 4e presents the changes in the ratio of Rct/Zw and PCE with respect to the change in the stoichiometric ratio of Pt and Se in PtSe alloy. We found that the ratio of Rct/Zw is close to one results in a high PCE. The obtained results are in line with our previous observations [35]. 4. Conclusion In this study, by using plasma technology, well-dispersed PtxSe1(0x 1) alloy NPs were synthesized and immobilized on the surface of RGO. The morphologies of electrodes and the structure of PtSe alloy were investigated through SEM, TEM, HAADF-STEM-EDS performances. We have carefully controlled the stoichiometric ratio of Pt and Se in PtSe alloy to optimize the catalytic activity of the PtSe/RGO toward the reduction of triiodide ions to iodide ions, as reflected by the smallest Rct, Zw values and balance of Rct/Zw value. As a result, the PCE improved from 4.34 to 6.26% under standard conditions when the Pt0Se1/RGO electrode was substituted for a Pt0$74Se0.26/RGO electrode. Interestingly, the stability in the iodide electrolyte of the electrode showed a good result. The introduction of Pt1-xSex (1 x 0) alloys/RGO is a promising strategy for fabricating efficient and low-cost stable electrodes for liquidjunction PV devices.
x
Declaration of Competing Interest There is no conflict of interest. Acknowledgments This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under grant number 103.02e2018.27. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.mtener.2020.100384. References [1] S. Yun, Y. Qin, A.R. Uhl, N. Vlachopoulos, M. Yin, D. Li, X. Han, A. Hagfeldt, Newgeneration integrated devices based on dye-sensitized and perovskite solar cells, Energy Environ. Sci. 11 (2018) 476e526. [2] S. Yun, A. Hagfeldt, T. Ma, Pt-free counter electrode for dye-sensitized solar cells with high efficiency, Adv. Mater. 26 (2014) 6210. €tzel, A low-cost, high-efficiency solar cell based on dye[3] B. O'Regan, M. Gra sensitized colloidal TiO2 films, Nature 353 (1991) 737e740. [4] S. Yun, A. Hagfeldt, T. Ma, Superior catalytic activity of sub-5 mm-thick Pt/SiC films as counter electrodes for dye-sensitized solar cells, ChemCatChem 6 (2014) 1584e1588. [5] Q. Tang, J. Duan, Y. Duan, B. He, L. Yu, Recent advances in alloy counter electrodes for dye-sensitized solar cells. A critical review, Electrochim. Acta 178 (2015) 886e899. [6] T. Zhang, Y. Liu, S. Yun, Recent advances in counter electrodes for thiolatemediated dye-sensitized solar cells, Isr. J. Chem. 55 (2015) 943e954.
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