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Synthesis and analysis of Mo2N as efficient counter electrodes for dye sensitized solar cells
Materials Today: Proceedings xxx (xxxx) xxx
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Synthesis and analysis of Mo2N as efficient counter electrodes for dye sensitized solar cells Priyada V. Rajeev, Subashini Gnanasekar, Raja Sellappan, Andrews Nirmala Grace ⇑ Vellore Institute of Technology, Vellore 632014, Tamil Nadu, India
a r t i c l e
i n f o
Article history: Received 6 March 2019 Received in revised form 20 May 2019 Accepted 21 May 2019 Available online xxxx Keywords: Transition metal nitrides DSSC Molybdenum nitride Nanosheet Counter electrode
a b s t r a c t The category of transition metal nitrides has been least explored for DSSC. Here, Mo2N was synthesized by a solgel followed by nitridation method and utilized as the counter electrode for DSSC. The structural studies were carried out with XRD, morphology with SEM, TEM, electrochemical studies with Cyclic Voltammetry, EIS and Tafel. Further I-V characteristic was studied using solar simulator Oriel Class AAA under standard test conditions. The Mo2N counter electrodes showed excellent electrochemical activity comparable to that of platinum. DSSC with Mo2N generated an average efficiency of 2.79% and hence can be a promising alternative to Platinum. Ó 2019 Elsevier Ltd. All rights reserved. Peer-review under responsibility of the scientific committee of the Exploring Nanostructures for Enhanced Power Conversion Efficiency of Solar Cells Conference.
1. Introduction The unceasing and growing need for energy with the rising population and development has activated the use of renewable energy resources. Solar energy, being one of the most potent renewable energy resources, has the proficiency for meeting the global energy needs. The enormous energy available from the sun will satisfy the present and future energy requirements [1]. The first generation of solar cells were developed using crystalline silicon wafers and provides a very high efficiency. Due to its high cost and difficulty in processing the silicon wafers, various alternatives have been introduced [2]. The Dye Sensitized Solar Cells (DSSC) has come into light in the recent years due to its added advantages against the conventional silicon solar cells. DSSC has attained consideration as it is less expensive, effective and simple in nature. The exploration of nanocrystalline materials has established numerous openings for the incorporation of these materials in improving the efficiency of existing solar cells. The efficiency of a DSSC depends on all factors including the dye, semiconductor oxide layer, electrolyte, and the counter electrode. The counter electrode plays a major role in catalyzing the reduction of triiodide ions in the electrolyte [3]. The rate of reduction reaction (I 3 to I ) plays a keen role in improving the activity of the DSSC. Major efforts on research have been ⇑ Corresponding author. E-mail address: [email protected] (A.N. Grace).
done to find alternatives for the standard platinum counter electrode. Due to its high cost and low resources, a better alternative to Platinum is required for commercial applications [4]. Immense research has been done on carbon based materials, polymers, oxides and composites [5]. Some of the counter electrodes with high efficiency reported till now include carbonaceous materials (CNT, carbon, graphene, N-doped carbon), polymers (PPy, PANI, PEDOT) [6–12] metal sulphides (NiCo2S4 [7], WS2 [13], MoS2 [13]), metal oxides (TaO [14], V2O3 [15], WO3 [18]) and transition metal carbides (VC [15], TiC [16], WC [17]). Nitrides show very high activity as compared to oxides and carbides due to its excellent electrical conductivity. The transition metal nitrides show unique physical and chemical properties due to their oxidation states. These interstitial nitrides form a predominant category for replacing platinum. Several nitrides have been tested as the counter electrodes for DSSC giving comparable performance as that of platinum such as CNx [18], VN [15], NbN [15], TiN [15], MoN [19], W2N [19] and VN peas [20]. All these nitrides have shown excellent catalytic activity and efficiency and hence represent an excellent alternative for platinum counter electrodes. In the present report, Mo2N has been chosen as an alternative counter electrode for DSSC as very less research has progressed in the material so far. Mo2N has been used widely in the field of fuel cells and supercapacitors providing improved specific capacitance. Hence this shows that Mo2N can act as a good catalyst and can be tested for enhancing the performance of DSSC. Mingxing Wu et al. coated Mo2N on Ti substrate and studied the activity as counter electrode and reported a Power Conver-
https://doi.org/10.1016/j.matpr.2019.05.438 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Peer-review under responsibility of the scientific committee of the Exploring Nanostructures for Enhanced Power Conversion Efficiency of Solar Cells Conference.
Please cite this article as: P. V. Rajeev, S. Gnanasekar, R. Sellappan et al., Synthesis and analysis of Mo2N as efficient counter electrodes for dye sensitized solar cells, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.05.438
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sion Efficiency (PCE) of 6.38% [19]. Here we synthesized Mo2N by a simple sol-gel followed by nitridation method and its structural, morphological and electrochemical studies have been done for analyzing the catalytic activity. The Mo2N was used as counter electrode for DSSC and tested its Power Conversion Efficiency. 2. Experimentation 2.1. Material synthesis The synthesis of transition metal nitrides is very challenging. A simple sol-gel followed by nitridation method is proceeded here for the synthesis of Molybdenum Nitride (Mo2N). 0.522 g of ammonium molybdate tetrahydrate was dissolved in 15 ml of distilled water (DI). After complete dissolution, 1.64 g of ethylenediamine tetraaceticacid (H4EDTA) was added to the above solution. Further 0.854 g of polyethyleneimine (PEI) was added and stirred properly till a clear pale yellow solution is obtained. A yellow gel like substance is formed at the bottom. This gel is collected in a ceramic boat and is preheated in an oven at 50 °C for 24 h. The boat with the gel was then subjected to high temperature treatment in a tubular furnace with a flow of nitrogen. The temperature was increased from room temperature to 700 °C at a ramp rate of 7 °C/min, held for 3 h and then cooled down to room temperature. Dark foam like structure of molybdenum nitride was formed which was ground and stored for further analysis. 2.2. Fabrication of electrode FTO glass slides were cleaned by sonicating initially in soap solution, then DI water, acetone and isopropanol. Mo2N counter electrode was prepared by making a thick slurry of Mo2N with DI water and Triton-X and coated on a cleaned FTO using the Doctor Blade Method for an area of 4 mm * 4mm. The FTO was further sintered at 250 °C and was allowed to cool down to room temperature. Platinum counter electrode was prepared by dispersing 10 mM Hexachloroplatinic acid in IPA and coated on a similar area of FTO, then sintered at 450 °C for 30 min and cooled down to room temperature. 2.3. Assembly of DSSC The photoanode of TiO2 was prepared from a thick paste of Titanium Dioxide (P25) nanoparticles, mixed with DI water, concentrated HNO3 and Triton-X. The paste was coated on a cleaned FTO using the Doctor Blade Method. The electrode was prepared for an area of 4 mm * 4mm. The coated FTO was sintered at 450 °C for the duration of 30 min for good adhesion and then cooled to room temperature. 0.3mM N719 dye solution was prepared in ethanol. The prepared TiO2 working electrode was immersed in the dye solution overnight for absorption of the dye. Excess dye was washed off with ethanol. The iodide/triiodide electrolyte was prepared in a 0.05 M I2, 0.5 M LiI and 0.1 M 4 tert-butyl pyridine in acetonitrile. The DSSC was assembled with dye absorbed TiO2 as working electrode and Mo2N as counter electrode. The assembly was held together and the electrolyte was injected in between the electrodes.
and stability of the counter electrode material. The electrochemical studies were carried out in a CHI660C Electrochemical workstation. The Cyclic Volatmmetry tests were carried out using I/I 3 electrolyte with Ag/AgCl as reference electrode, Platinum as counter electrode and Mo2N as working electrode. The Electrochemical Impedance Spectroscopy (EIS) analysis was done for analyzing the electrical properties of the Mo2N with a symmetrical cell and the electrolyte injected between electrodes. The Photovoltaic Conversion Efficiency for DSSC with Mo2N counter electrode was tested with Oriel Class AAA solar simulator under standard test conditions AM 1.5G. 3. Results and discussion 3.1. Structural and morphological analysis The crystallinity of Mo2N nanoparticles were confirmed with X-Ray Diffraction pattern as shown in Fig. 1. The peaks observed at 36.508°, 42.348°, 62.296°, 74.553° and 79.359° correspond to planes (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) respectively forming a cubic structure of Mo2N according to reference card JCPDS no: 025-1366. The crystallite size of the as prepared nanomaterial was calculated using The Debye-Scherer equation as shown in equation (1).
D¼
Kk bcosh
ð1Þ
where D is the crystallite size, K is a constant of value 0.9, k is the wavelength of value 1.5418 Å, b is the full width half maximum of the peak and h is the diffraction angle in radians. The DebyeScherer equation gives the crystallite size of the particle as 0.63 nm. The broad peak indicates very small crystallite size. All the 5 peaks observed are matching with Mo2N and no other impurities are observed hence showing formation of primal Mo2N. The Scanning Electron Microscope (SEM) images of the sample were observed at various magnifications as shown in Fig. 2(a–c). Crumbled nanosheets like structures were observed from the images. Irregular sheets with non-uniform dimensions are noticed. The Energy Dispersive X-Ray Analysis (EDAX) spectrum of Mo2N as given in Fig. 2d confirms the presence of Mo and N. 3.2. Electrochemical analysis The cyclic voltammetry analysis for various scan rates is plotted as shown in Fig. 3(a–c). The oxidation and reduction peaks
2.4. Characterization The X-Ray diffraction analysis was carried out in a Bruker D8 Advance X-Ray diffractometer using Cu(Ka) radiation (k = 1.5418 Å). The morphological analysis was done with Zeiss Scanning Electron Microscope (SEM) with a power of 8 kV. Electrochemical analysis was carried out for studying the catalytic activity
Fig. 1. X-ray diffraction pattern of molybdenum nitride.
Please cite this article as: P. V. Rajeev, S. Gnanasekar, R. Sellappan et al., Synthesis and analysis of Mo2N as efficient counter electrodes for dye sensitized solar cells, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.05.438
P.V. Rajeev et al. / Materials Today: Proceedings xxx (xxxx) xxx
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Fig. 2. SEM images of Mo2N at various magnifications (a) 20 mm (b) 2mm (c) 1mm and (d) EDAX of Mo2N.
Fig. 3. (a) Cyclic coltammetry of Mo2N and Pt for a scan rate of 20 mV (b) cyclic voltammetry of Mo2N at varying scan rates (c) cyclic voltammetry of Mo2N at 20 mV for 25 cycles.
observed confirms the I/I 3 redox reactions. The anodic peak represents redox reaction of I2/I 3 as represented in Eq. (2). The catho dic peaks represent the conversion of I at the counter 3 to I electrode as in equation (3).
3I2 þ 2e ! 2I3
ð2Þ
I3 þ 2e ! 3I
ð3Þ
The activity of Mo2N is compared with that of Pt as shown in Fig. 3(a). The peak current density provided by both Mo2N and Pt is almost same and shows similar catalytic activity as that of Pt. The observed Epp values for Mo2N and Pt are 0.48 V and 0.5 V respectively. The rate constant is determined by the peak to peak voltage (Epp). Hence the slightly lower Epp value for Mo2N confirms a faster reduction at the cathode as compared to Pt. The peak current density rises with increasing scan rates as depicted in Fig. 3(b) conveys increasing catalytic activity of Mo2N counter electrodes at higher scan rates. The distortionless uniform pattern represents
good reversibility of the reaction. The stability of Mo2N counter electrodes is analyzed for a scan rate of 20 mV for 25 cycles. The uniform shape of the curve delivers a stable redox activity at the counter electrode and is least affected by the electrolyte. Electrochemical Impedance Spectroscopy (EIS) tests were carried out using CE/electrolyte/CE for analyzing the catalytic activity. The Nyquist plots for Mo2N counter electrodes were shown in Fig. 4(a). The series resistance (Rs) gives an insight into the contacts and substrate used and is measured from the high impedance region. The charge transfer resistance (Rct) shows the resistance to diffusion of ions at the cathode. Nyquist plot was fitted with the equivalent circuit using EC lab software. From the Nyquist plot, the Mo2N and Pt counter electrode provides Rs of 1.46 O and 1.32 O respectively. The Rct values for Mo2N and Pt counter electrodes are observed as 38.2 O and 20.1 O respectively. The results indicate that Mo2N shows good catalytic activity comparable to Pt and hence can be a suitable alternative for counter electrodes in DSSC.
Please cite this article as: P. V. Rajeev, S. Gnanasekar, R. Sellappan et al., Synthesis and analysis of Mo2N as efficient counter electrodes for dye sensitized solar cells, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.05.438
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Fig. 4. (a) Electrochemical impedance spectroscopy of Mo2N and Pt counter electrode (b) Tafel plots of Mo2N and Pt counter electrode (c) I-V characteristics of DSSC with Mo2N and Pt counter electrode.
Table 1 Parameters obtained from J-V characteristics of DSSC with Mo2N and Pt counter electrode. CE
Voc (V)
Jsc (mA/cm2)
Fill factor
g (%)
Mo2N Pt
0.834 0.650
8.67 14.33
0.44 0.82
2.79 6.4
The Tafel plots were studied with two identical counter electrodes for analyzing the catalytic activity. The lower current density obtained for Mo2N counter electrode as compared to Pt might be a result of the higher series resistance and charge transfer resistance as analyzed from the EIS results. The higher charge transfer resistance limits the current density in Mo2N counter electrode.
3.3. I-V characteristics Fig. 4(c) shows I-V characteristics of Mo2N and Pt counter electrode. The Mo2N counter electrodes provides an open circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF) and efficiency (g) of 834.75 mV, 8.76 mA/cm2, 0.44 and 2.79% as compared to Pt with the parameters 650 mV, 14.33 mA/cm2, 0.82 and 6.4% respectively. The lower fill factor and efficiency observed for Mo2N may be due to the slower electron transfer and higher Rct as compared to Pt. Hence the results in Table 1 shows activity of Mo2N as an effective counter electrode and the performance can be further improved by either preparation of composites or by varying the electrolyte.
4. Conclusion A simple sol-gel technique followed by nitridation was used to synthesis Mo2N. The counter electrodes were prepared and structural and morphological analysis was done with XRD and SEM, electrochemical studies with CV and EIS. Finally the photovoltaic characteristics was tested under AM 1.5G illumination standard test conditions and achieved a power conversion efficiency of 2.79%. The performance can be further improved by preparation of various composites and also by changing the electrolyte. The results initialized the further work to make Mo2N an efficient counter electrode comparable to Pt.
Acknowledgements The authors gratefully acknowledge the financial support given by DST- SERB (EMR/2016/003681), Government of India and thanks Vellore Institute of Technology, Vellore for its support to work on this research. References [1] Muhammad Zahir Iqbal, Sana Khan, Sol. Energy 160 (2018) 130–152. [2] Romain Cariou, Jan Benick, Frank Feldmann, Oliver Höhn, Hubert Hauser, Paul Beutel, Nasser Razek, Markus Wimplinger, Benedikt Bläsi, David Lackner, Martin Hermle, Gerald Siefer, Stefan W. Glunz, Andreas W. Bett, Frank Dimroth, Nat. Energy volume 3 (2018) 326–333. [3] Jeongmin Lim, Sang Yeoul Ryu, Jeonghun Kim, Yongseok Jun, Lim, et al., Nanoscale Res. Lett. 8 (2013) 227. [4] Wasan Maiaugree, Seksan Lowpa, Madsakorn sTowannang, Phikun Rutphonsan, Apishok Tangtrakarn, Samuk Pimanpang, Prapen Maiaugree, Nattawat Ratchapolthavisin, Wichien Sang-aroon, Wirat Jarernboon, Vittaya Amornkitbamrung, Sci. Rep. 5 (2015) 15230. [5] Jayaraman Theerthagiri, Arumugam Raja Senthil, Jagannathan Madhavan, Thandavarayan Maiyalagan, ChemElectroChem 2 (2015) 928–945. [6] Hyunkook Kim, Hyonkwang Choi, Sookhyun Hwang, Youngjoo Kim, Minhyon Jeon, Nanoscale Res. Lett. 7 (1) (2012) 53. [7] E. Ramasamy, W.J. Lee, D.Y. Lee, J.S. Song, Appl. Phys. Lett. 90 (17) (2008) 173103. [8] Shaocong Hou, Xin Cai, Wu Hongwei, Yu Xiao, Ming Peng, Kai Yan, Dechun Zou, Energy Environ.Sci. 6 (2013) 3356. [9] Q. Li, J. Wu, Q. Tang, Z. Lan, P. Li, J. Lin, L. Fan, Electrochem. Commun. 10 (2008) 1299–1302. [10] Xiong Yin, Fan Wu, Nianqing Fu, Jing Han, Dongliang Chen, Peng Xu, Meng He, Yuan Lin, ACS Appl Mater. Interfaces 17 (5) (2013) 8423–8429. [11] Mingxing Wu, Yudi Wang, Xiao Lin, Naisen Yu, Liang Wang, Linlin Wang, Anders Hagfeldtc, Tingli Ma, Chem. Phys 13 (2011) 19298–19301. [12] Sining Yun, Liang Wang, Wei Guo, Tingli Ma, Electrochem. Commun. 24 (2012) 69–73. [13] Eric Singh, Ki Seok Kim, Geun Young Yeom, Hari Singh Nalwa, RSC Adv. 7 (2017) 28234. [14] Ling Cheng, Yu Hou, Bo Zhang, Shuang Yang, Jian Wei Guo, Long Wu, Hua Gui Yang, Chem. Commun. 49 (2013) 5945–5947. [15] Xiaoying Zhang, Xiao Chen, Kejun Zhang, Shuping Pang, Xinhong Zhou, Hongxia Xu, Shanmu Dong, Pengxian Han, Zhongyi Zhang, Chuanjian Zhang, Guanglei Cui, J. Mater. Chem. A 1 (2013) 3340–3346. [16] Jum Suk Jang, Dong Jin Ham, Easwaramoorthi Ramasamy, Jinwoo Lee, Jae Sung Lee, Chem. Commun. 46 (2010) 8600–8602. [17] Chaoyu Wu, Guoran Li, Xueqin Cao, Bao Lei, Xueping Gao, Green Energy Environ. (2017) 302e309.. [18] M. Wu, X. Lin, Y. Wang, L. Wang, W. Guo, D. Qi, X. Peng, A. Hagfeldt, M. Grätzel, T. Ma, J Am Chem Soc. 134 (7) (2012) 3419–3428. [19] Mingxing Wu, Qingyu Zhang, Jinqiu Xiao, Chunyu Ma, Xiao Lin, Chunyu Miao, Youjing He, Yurong Gao, Anders Hagfeldt, Tingli Ma, J. Mater. Chem. 21 (2011) 10761–10766. [20] Mingxing Wu, Hongyue Guo, Ya-nan Lin, Kezhong Wu, Tingli Ma, Anders Hagfeldt, J. Phys. Chem. C 118 (24) (2014) 12625–12631.
Please cite this article as: P. V. Rajeev, S. Gnanasekar, R. Sellappan et al., Synthesis and analysis of Mo2N as efficient counter electrodes for dye sensitized solar cells, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.05.438
Contents lists available at ScienceDirect
Materials Today: Proceedings journal homepage: www.elsevier.com/locate/matpr
Synthesis and analysis of Mo2N as efficient counter electrodes for dye sensitized solar cells Priyada V. Rajeev, Subashini Gnanasekar, Raja Sellappan, Andrews Nirmala Grace ⇑ Vellore Institute of Technology, Vellore 632014, Tamil Nadu, India
a r t i c l e
i n f o
Article history: Received 6 March 2019 Received in revised form 20 May 2019 Accepted 21 May 2019 Available online xxxx Keywords: Transition metal nitrides DSSC Molybdenum nitride Nanosheet Counter electrode
a b s t r a c t The category of transition metal nitrides has been least explored for DSSC. Here, Mo2N was synthesized by a solgel followed by nitridation method and utilized as the counter electrode for DSSC. The structural studies were carried out with XRD, morphology with SEM, TEM, electrochemical studies with Cyclic Voltammetry, EIS and Tafel. Further I-V characteristic was studied using solar simulator Oriel Class AAA under standard test conditions. The Mo2N counter electrodes showed excellent electrochemical activity comparable to that of platinum. DSSC with Mo2N generated an average efficiency of 2.79% and hence can be a promising alternative to Platinum. Ó 2019 Elsevier Ltd. All rights reserved. Peer-review under responsibility of the scientific committee of the Exploring Nanostructures for Enhanced Power Conversion Efficiency of Solar Cells Conference.
1. Introduction The unceasing and growing need for energy with the rising population and development has activated the use of renewable energy resources. Solar energy, being one of the most potent renewable energy resources, has the proficiency for meeting the global energy needs. The enormous energy available from the sun will satisfy the present and future energy requirements [1]. The first generation of solar cells were developed using crystalline silicon wafers and provides a very high efficiency. Due to its high cost and difficulty in processing the silicon wafers, various alternatives have been introduced [2]. The Dye Sensitized Solar Cells (DSSC) has come into light in the recent years due to its added advantages against the conventional silicon solar cells. DSSC has attained consideration as it is less expensive, effective and simple in nature. The exploration of nanocrystalline materials has established numerous openings for the incorporation of these materials in improving the efficiency of existing solar cells. The efficiency of a DSSC depends on all factors including the dye, semiconductor oxide layer, electrolyte, and the counter electrode. The counter electrode plays a major role in catalyzing the reduction of triiodide ions in the electrolyte [3]. The rate of reduction reaction (I 3 to I ) plays a keen role in improving the activity of the DSSC. Major efforts on research have been ⇑ Corresponding author. E-mail address: [email protected] (A.N. Grace).
done to find alternatives for the standard platinum counter electrode. Due to its high cost and low resources, a better alternative to Platinum is required for commercial applications [4]. Immense research has been done on carbon based materials, polymers, oxides and composites [5]. Some of the counter electrodes with high efficiency reported till now include carbonaceous materials (CNT, carbon, graphene, N-doped carbon), polymers (PPy, PANI, PEDOT) [6–12] metal sulphides (NiCo2S4 [7], WS2 [13], MoS2 [13]), metal oxides (TaO [14], V2O3 [15], WO3 [18]) and transition metal carbides (VC [15], TiC [16], WC [17]). Nitrides show very high activity as compared to oxides and carbides due to its excellent electrical conductivity. The transition metal nitrides show unique physical and chemical properties due to their oxidation states. These interstitial nitrides form a predominant category for replacing platinum. Several nitrides have been tested as the counter electrodes for DSSC giving comparable performance as that of platinum such as CNx [18], VN [15], NbN [15], TiN [15], MoN [19], W2N [19] and VN peas [20]. All these nitrides have shown excellent catalytic activity and efficiency and hence represent an excellent alternative for platinum counter electrodes. In the present report, Mo2N has been chosen as an alternative counter electrode for DSSC as very less research has progressed in the material so far. Mo2N has been used widely in the field of fuel cells and supercapacitors providing improved specific capacitance. Hence this shows that Mo2N can act as a good catalyst and can be tested for enhancing the performance of DSSC. Mingxing Wu et al. coated Mo2N on Ti substrate and studied the activity as counter electrode and reported a Power Conver-
https://doi.org/10.1016/j.matpr.2019.05.438 2214-7853/Ó 2019 Elsevier Ltd. All rights reserved. Peer-review under responsibility of the scientific committee of the Exploring Nanostructures for Enhanced Power Conversion Efficiency of Solar Cells Conference.
Please cite this article as: P. V. Rajeev, S. Gnanasekar, R. Sellappan et al., Synthesis and analysis of Mo2N as efficient counter electrodes for dye sensitized solar cells, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.05.438
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sion Efficiency (PCE) of 6.38% [19]. Here we synthesized Mo2N by a simple sol-gel followed by nitridation method and its structural, morphological and electrochemical studies have been done for analyzing the catalytic activity. The Mo2N was used as counter electrode for DSSC and tested its Power Conversion Efficiency. 2. Experimentation 2.1. Material synthesis The synthesis of transition metal nitrides is very challenging. A simple sol-gel followed by nitridation method is proceeded here for the synthesis of Molybdenum Nitride (Mo2N). 0.522 g of ammonium molybdate tetrahydrate was dissolved in 15 ml of distilled water (DI). After complete dissolution, 1.64 g of ethylenediamine tetraaceticacid (H4EDTA) was added to the above solution. Further 0.854 g of polyethyleneimine (PEI) was added and stirred properly till a clear pale yellow solution is obtained. A yellow gel like substance is formed at the bottom. This gel is collected in a ceramic boat and is preheated in an oven at 50 °C for 24 h. The boat with the gel was then subjected to high temperature treatment in a tubular furnace with a flow of nitrogen. The temperature was increased from room temperature to 700 °C at a ramp rate of 7 °C/min, held for 3 h and then cooled down to room temperature. Dark foam like structure of molybdenum nitride was formed which was ground and stored for further analysis. 2.2. Fabrication of electrode FTO glass slides were cleaned by sonicating initially in soap solution, then DI water, acetone and isopropanol. Mo2N counter electrode was prepared by making a thick slurry of Mo2N with DI water and Triton-X and coated on a cleaned FTO using the Doctor Blade Method for an area of 4 mm * 4mm. The FTO was further sintered at 250 °C and was allowed to cool down to room temperature. Platinum counter electrode was prepared by dispersing 10 mM Hexachloroplatinic acid in IPA and coated on a similar area of FTO, then sintered at 450 °C for 30 min and cooled down to room temperature. 2.3. Assembly of DSSC The photoanode of TiO2 was prepared from a thick paste of Titanium Dioxide (P25) nanoparticles, mixed with DI water, concentrated HNO3 and Triton-X. The paste was coated on a cleaned FTO using the Doctor Blade Method. The electrode was prepared for an area of 4 mm * 4mm. The coated FTO was sintered at 450 °C for the duration of 30 min for good adhesion and then cooled to room temperature. 0.3mM N719 dye solution was prepared in ethanol. The prepared TiO2 working electrode was immersed in the dye solution overnight for absorption of the dye. Excess dye was washed off with ethanol. The iodide/triiodide electrolyte was prepared in a 0.05 M I2, 0.5 M LiI and 0.1 M 4 tert-butyl pyridine in acetonitrile. The DSSC was assembled with dye absorbed TiO2 as working electrode and Mo2N as counter electrode. The assembly was held together and the electrolyte was injected in between the electrodes.
and stability of the counter electrode material. The electrochemical studies were carried out in a CHI660C Electrochemical workstation. The Cyclic Volatmmetry tests were carried out using I/I 3 electrolyte with Ag/AgCl as reference electrode, Platinum as counter electrode and Mo2N as working electrode. The Electrochemical Impedance Spectroscopy (EIS) analysis was done for analyzing the electrical properties of the Mo2N with a symmetrical cell and the electrolyte injected between electrodes. The Photovoltaic Conversion Efficiency for DSSC with Mo2N counter electrode was tested with Oriel Class AAA solar simulator under standard test conditions AM 1.5G. 3. Results and discussion 3.1. Structural and morphological analysis The crystallinity of Mo2N nanoparticles were confirmed with X-Ray Diffraction pattern as shown in Fig. 1. The peaks observed at 36.508°, 42.348°, 62.296°, 74.553° and 79.359° correspond to planes (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) respectively forming a cubic structure of Mo2N according to reference card JCPDS no: 025-1366. The crystallite size of the as prepared nanomaterial was calculated using The Debye-Scherer equation as shown in equation (1).
D¼
Kk bcosh
ð1Þ
where D is the crystallite size, K is a constant of value 0.9, k is the wavelength of value 1.5418 Å, b is the full width half maximum of the peak and h is the diffraction angle in radians. The DebyeScherer equation gives the crystallite size of the particle as 0.63 nm. The broad peak indicates very small crystallite size. All the 5 peaks observed are matching with Mo2N and no other impurities are observed hence showing formation of primal Mo2N. The Scanning Electron Microscope (SEM) images of the sample were observed at various magnifications as shown in Fig. 2(a–c). Crumbled nanosheets like structures were observed from the images. Irregular sheets with non-uniform dimensions are noticed. The Energy Dispersive X-Ray Analysis (EDAX) spectrum of Mo2N as given in Fig. 2d confirms the presence of Mo and N. 3.2. Electrochemical analysis The cyclic voltammetry analysis for various scan rates is plotted as shown in Fig. 3(a–c). The oxidation and reduction peaks
2.4. Characterization The X-Ray diffraction analysis was carried out in a Bruker D8 Advance X-Ray diffractometer using Cu(Ka) radiation (k = 1.5418 Å). The morphological analysis was done with Zeiss Scanning Electron Microscope (SEM) with a power of 8 kV. Electrochemical analysis was carried out for studying the catalytic activity
Fig. 1. X-ray diffraction pattern of molybdenum nitride.
Please cite this article as: P. V. Rajeev, S. Gnanasekar, R. Sellappan et al., Synthesis and analysis of Mo2N as efficient counter electrodes for dye sensitized solar cells, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.05.438
P.V. Rajeev et al. / Materials Today: Proceedings xxx (xxxx) xxx
3
Fig. 2. SEM images of Mo2N at various magnifications (a) 20 mm (b) 2mm (c) 1mm and (d) EDAX of Mo2N.
Fig. 3. (a) Cyclic coltammetry of Mo2N and Pt for a scan rate of 20 mV (b) cyclic voltammetry of Mo2N at varying scan rates (c) cyclic voltammetry of Mo2N at 20 mV for 25 cycles.
observed confirms the I/I 3 redox reactions. The anodic peak represents redox reaction of I2/I 3 as represented in Eq. (2). The catho dic peaks represent the conversion of I at the counter 3 to I electrode as in equation (3).
3I2 þ 2e ! 2I3
ð2Þ
I3 þ 2e ! 3I
ð3Þ
The activity of Mo2N is compared with that of Pt as shown in Fig. 3(a). The peak current density provided by both Mo2N and Pt is almost same and shows similar catalytic activity as that of Pt. The observed Epp values for Mo2N and Pt are 0.48 V and 0.5 V respectively. The rate constant is determined by the peak to peak voltage (Epp). Hence the slightly lower Epp value for Mo2N confirms a faster reduction at the cathode as compared to Pt. The peak current density rises with increasing scan rates as depicted in Fig. 3(b) conveys increasing catalytic activity of Mo2N counter electrodes at higher scan rates. The distortionless uniform pattern represents
good reversibility of the reaction. The stability of Mo2N counter electrodes is analyzed for a scan rate of 20 mV for 25 cycles. The uniform shape of the curve delivers a stable redox activity at the counter electrode and is least affected by the electrolyte. Electrochemical Impedance Spectroscopy (EIS) tests were carried out using CE/electrolyte/CE for analyzing the catalytic activity. The Nyquist plots for Mo2N counter electrodes were shown in Fig. 4(a). The series resistance (Rs) gives an insight into the contacts and substrate used and is measured from the high impedance region. The charge transfer resistance (Rct) shows the resistance to diffusion of ions at the cathode. Nyquist plot was fitted with the equivalent circuit using EC lab software. From the Nyquist plot, the Mo2N and Pt counter electrode provides Rs of 1.46 O and 1.32 O respectively. The Rct values for Mo2N and Pt counter electrodes are observed as 38.2 O and 20.1 O respectively. The results indicate that Mo2N shows good catalytic activity comparable to Pt and hence can be a suitable alternative for counter electrodes in DSSC.
Please cite this article as: P. V. Rajeev, S. Gnanasekar, R. Sellappan et al., Synthesis and analysis of Mo2N as efficient counter electrodes for dye sensitized solar cells, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.05.438
4
P.V. Rajeev et al. / Materials Today: Proceedings xxx (xxxx) xxx
Fig. 4. (a) Electrochemical impedance spectroscopy of Mo2N and Pt counter electrode (b) Tafel plots of Mo2N and Pt counter electrode (c) I-V characteristics of DSSC with Mo2N and Pt counter electrode.
Table 1 Parameters obtained from J-V characteristics of DSSC with Mo2N and Pt counter electrode. CE
Voc (V)
Jsc (mA/cm2)
Fill factor
g (%)
Mo2N Pt
0.834 0.650
8.67 14.33
0.44 0.82
2.79 6.4
The Tafel plots were studied with two identical counter electrodes for analyzing the catalytic activity. The lower current density obtained for Mo2N counter electrode as compared to Pt might be a result of the higher series resistance and charge transfer resistance as analyzed from the EIS results. The higher charge transfer resistance limits the current density in Mo2N counter electrode.
3.3. I-V characteristics Fig. 4(c) shows I-V characteristics of Mo2N and Pt counter electrode. The Mo2N counter electrodes provides an open circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF) and efficiency (g) of 834.75 mV, 8.76 mA/cm2, 0.44 and 2.79% as compared to Pt with the parameters 650 mV, 14.33 mA/cm2, 0.82 and 6.4% respectively. The lower fill factor and efficiency observed for Mo2N may be due to the slower electron transfer and higher Rct as compared to Pt. Hence the results in Table 1 shows activity of Mo2N as an effective counter electrode and the performance can be further improved by either preparation of composites or by varying the electrolyte.
4. Conclusion A simple sol-gel technique followed by nitridation was used to synthesis Mo2N. The counter electrodes were prepared and structural and morphological analysis was done with XRD and SEM, electrochemical studies with CV and EIS. Finally the photovoltaic characteristics was tested under AM 1.5G illumination standard test conditions and achieved a power conversion efficiency of 2.79%. The performance can be further improved by preparation of various composites and also by changing the electrolyte. The results initialized the further work to make Mo2N an efficient counter electrode comparable to Pt.
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Please cite this article as: P. V. Rajeev, S. Gnanasekar, R. Sellappan et al., Synthesis and analysis of Mo2N as efficient counter electrodes for dye sensitized solar cells, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2019.05.438