Nitrogen-doped carbon microspheres counter electrodes for dye-sensitized solar cells by microwave assisted method

Abstract

Nitrogen-doped carbon microspheres (NCSs) were fabricated via a microwave-assisted method followed by thermal treatment in an ammonia atmosphere and used as counter electrodes for dye-sensitized solar cells. The results show that the cell with NCSs treated at 900 °C exhibits a maximum conversion efficiency of 6.28% at one sun (AM 1.5G, 100 mW cm⁻²), which is comparable to the one for the cell with conventional Pt counter electrode.

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Introduction

Dye sensitized solar cells (DSSCs) have gained much attention over recent decades due to their low cost, easy fabrication and high power conversion efficiency.^1–7 In a typical DSSC, electrons from excited states of the dye are injected into the conduction band of the semiconductor, followed by the reduction of the oxidized dye with a charge mediator.^8–11 During this process, the counter electrode plays a crucial role in collecting electrons from external circuit and reducing I₃⁻ to I⁻ in electrolyte.^12–14 Although Pt is an effective counter electrode material, the high price of noble metal Pt restrains the large-scale applications of DSSCs.^15–18 Therefore, it is necessary to explore low cost and effective Pt-free counter electrodes.

Recently, nitrogen-doped carbon materials, such as carbon nanotubes and graphene, have been reported as effective candidate counter electrodes to replace Pt for DSSCs.^19,20 Nitrogen-doping is attractive because it can improve the electrochemical activity of the electrodes and alter their chemical reactivity, which helps to reduce the total internal resistance and thus improves the photovoltaic performance.^21,22 Moreover, nitrogen-doping can induce a large number of defects in the carbon structure and offer more active sites for ion insertion.^23,24 Wang et al.²⁵ prepared nitrogen doped diamond like carbon thin films as counter electrode and the DSSCs exhibited a conversion efficiency of 3.35%. Yen et al.²⁶ incorporated nitrogen-doped graphene as counter electrode of DSSCs and a conversion efficiency of 4.75% was achieved. However, further exploration on the nitrogen-doped carbon counter electrode for DSSCs is necessary because its current performance is still insufficient for practical applications.

As an important member in carbon family, carbon microspheres (CSs) have been proposed to be an attractive material for energy storage applications in lithium ion battery and supercapacitor, since their unique spherical structure favours a high accessible surface area, good catalytic efficiency and excellent chemical stability.^27,28 Yang et al.²⁹ prepared nitrogen (N)-doped hollow core–mesoporous shell carbon counter electrodes by a three-step processes for DSSCs, and a conversion efficiency of 8.86% was achieved. In previous work, CSs were used in DSSCs as counter electrodes and a conversion efficiency of 5.5% was achieved.³⁰ Thus it can be expected that this CSs with nitrogen-doping should be promising counter electrode materials for DSSCs. However, up to now, such an exploration has seldom reported.

In this work, a simple and fast microwave-assisted method was employed to synthesize CSs using sucrose dissolved in a mixing solvent of water and ethylene glycol. This microwave assisted method can offer an easy control over all experimental parameters without the requirement of repetitive immersing operation, organic linker or high temperature heating. After thermal treatment in an ammonia atmosphere, nitrogen-doped carbon microspheres (NCSs) were synthesized and used as counter electrodes of DSSCs. The N-doping into CSs can improve the electrocatalytic performance and enhance the interfacial charge-transfer properties. The results show that a maximum conversion efficiency of 6.28% can be achieved under one sun illumination for the as-synthesized cells.

Experimental

NCSs were prepared using the method reported in our previous work.³¹ In a typical process, 6.85 g sucrose and 2 g concentrated sulfuric acid were dissolved in 100 ml mixing solvent of water and ethylene glycol (6 : 4 v/v). Subsequently, 20 ml of the solution was heated at 160 °C with a microwave irradiation power of 100 W for 10 min using a microwave system (Explorer-48, CEM Co.). After being centrifuged and washed with deionized water, the resultant precipitate was dried in a vacuum oven at 80 °C for 24 h and finally thermally treated at different temperatures for 2 h in an ammonia atmosphere. In this work, the temperatures of thermal treatment were set at 500, 700 and 900 °C, and the corresponding obtained NCSs are named as NCS500, NCS700 and NCS900, respectively. The as-synthesized NCSs were coated onto the cleaned F-doped SnO₂ (FTO) (resistivity: 14 Ω □⁻¹, Nippon Sheet Glass, Japan) glass using the screen printing method. The NCSs were annealed at 400 °C for 30 min and then used as counter electrodes. The conventional Pt film was also used as counter electrode for comparison.

The TiO₂ electrodes were prepared by screen printing of TiO₂ paste on FTO glass followed by sintering at 500 °C for 30 min. After cooled to 80 °C, sintered electrodes were immersed into 0.5 mM N719 dye solution and kept for 24 h at room temperature. The dye-treated electrodes were rinsed with ethanol and dried under nitrogen flow. The redox electrolyte was composed of 0.6 M 1-butyl-3-methylimidazolium iodide (BMII), 30 mM I₂, 0.5 M tert-butylpyridine, and 0.1 M guanidinium thiocyanate (GuNCS) in a solvent mixture of 85% acetonitrile with 15% valeronitrile by volume. The TiO₂ electrodes and counter electrodes were sealed together with a 60 μm spacer (Surlyn). The active area of the cell is 0.2 cm².

The surface morphology and structure of the as-synthesized NCSs were examined by field emission scanning electron microscopy (FESEM, JEOL JSM-LV5610) and X-ray diffraction (XRD, Holland Panalytical PRO PW3040/60) with Cu Kα radiation (V = 30 kV, I = 25 mA). Nitrogen adsorption–desorption isotherms were measured at 77 K with an ASAP 2020 Accelerated Surface Area and Porosimetry System (Micrometitics, Norcross, GA). The X-ray photoelectron spectroscopy (XPS) measurement was performed on an imaging photoelectron spectrometer (Axis Ultra, Kratos Analytical Ltd) with a monochromatic Al Kα X-ray source. The cyclic voltammetry (CV) experiment was carried out by using an electrochemical workstation (Autolab PGSTAT 302N) in a three electrode mode, with a standard calomel electrode as reference electrode and a Pt foil as counter electrode at a potential scan rate of 50 mV s⁻¹. Electrochemical impedance spectroscopy (EIS) measurements were carried out using same electrochemical workstation under 100 mW cm⁻² illumination at an applied bias of V_oc, applying a 10 mV AC sinusoidal signal over the constant applied bias with the frequency ranging between 100 kHz and 0.1 Hz. J–V measurement was performed with a Keithley model 2440 Source Meter and a Newport solar simulator system (equipped with a 1 kW xenon arc lamp, Oriel) at one sun (AM 1.5G, 100 mW cm⁻²). In the process of the test, as-prepared cells were measured without an aperture. Incident photon to current conversion efficiency (IPCE) was measured as a function of wavelength from 300 to 800 nm using an Oriel 300 W xenon arc lamp and a lock-in amplifier M 70104 (Oriel) under monochromator illumination.

Results and discussion

The FESEM image of NCS900 is shown in Fig. 1. The morphologies of NCS500 and NCS700 (not shown here) are similar to that of NCS900. It can be observed that a spherical structure is obtained via the microwave-assisted reaction and the NCSs have an average diameter of around 2 μm. The porous structure of NCSs favors the diffusion of I₃⁻ onto the active site to be reduced.^12,32 The specific surface areas of NCSs were measured by the Brunauer–Emmett–Teller method and the values of NCS500, NCS700 and NCS900 are 96, 447 and 1630 m² g⁻¹, respectively. When the thermal treatment temperature increases from 500 to 900 °C, an increase in specific surface area of NCSs is observed, therefore offering a large electrode/electrolyte interface that can contribute to an efficient catalysis on the redox reaction of the electrolyte.^18,33

Fig. 1 FESEM image of NCS900.

Fig. 2a shows the XRD patterns of the as-synthesized NCSs. Two broad peaks at ∼24° and ∼43.5° correspond to (002) and (100) diffraction modes of graphitic structure, which is characteristic of disordered carbon material.^31,34 The (100) diffraction peak gradually becomes intense when the thermal treatment temperature increases from 500 to 900 °C, indicating that the carbonization degree increases with the increase of temperature.¹⁵

Fig. 2 (a) XRD patterns of NCS500, NCS700 and NCS900; (b) XPS N 1s of NCS900.

As an efficient tool to detect the nitrogen doping in the NCSs, an XPS analysis was performed. The carbon and nitrogen contents of NCSs calculated from XPS are listed in Table 1. It can be observed that when the thermal treatment temperature increases from 500 to 900 °C, the carbon content increases from 86.4% to 91.5%, which further confirms that the carbonization degree increases with the increase of temperature. NCS900 has a highest carbon content of 91.5%, which may favor a high electrical conductivity.³⁵ Similarly, the content of nitrogen increases when the thermal treatment temperature increases from 500 to 700 °C.²² However, it should be noted that NCS700 has a maximum nitrogen content of 7.8%, higher than that of NCS900, because heteroatoms are chemically unstable at higher temperature.^22,36 The high-resolution N 1s XPS spectrum (Fig. 2b) of the NCS900 is broad and asymmetric, indicating that there are several types of binding configurations related to the involved nitrogen atoms. Curve deconvolution shows that three components locate around 398.3, 400.2 and 402.3 eV, which are assigned to pyridinic, pyrrolic and quaternary (graphitic) nitrogen doped in the carbon, respectively.^37,38 The XPS analysis strongly indicates that nitrogen was successfully incorporated into CSs.²¹

Table 1 The carbon and nitrogen contents of various NCSs

Sample	C (%)	N (%)
NCS500	86.4	7.2
NCS700	87.9	7.8
NCS900	91.5	6.3

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CV measurements were carried out to analyze the electrochemical catalytic activity of the electrodes. Fig. 3 shows the CV curves of the NCSs and Pt electrodes under the potential from −0.5 to 1.2 V. Two pairs of oxidation and reduction peaks are observed, and the left pair is attributed to the redox reaction (1), while the right one results from the redox reaction (2).^39–42

3I⁻ ↔ I₃⁻ + 2e⁻ (1)

2I₃⁻ ↔ 3I₂ + 2e⁻ (2)

Fig. 3 CV curves of various NCSs and Pt counter electrodes.

In the DSSCs, electrons are injected into photo-oxidized dye from I⁻ ions in the electrolyte, and the produced I₃⁻ ions are reduced on the counter electrode. From Fig. 3, it can be observed that NCS900 shows similar cathodic and anodic peaks to those of Pt, indicating that NCSs are effective in catalyzing the reduction of triiodide to iodide. NCS900 electrode displays higher current density than NCS500 and NCS700 electrodes due to the larger specific surface area and higher reactivity in NCS900 electrode for catalysis.^43,44

EIS was used to examine the charge transfer process and the catalytic activity of counter electrodes. Fig. 4 shows the Nyquist plots of the cells with various NCSs and Pt counter electrodes, and the inset is corresponding equivalent circuit. Two semicircles, the left one at high frequency and the right one at low frequency, are observed in the Nyquist plots of EIS spectra. The high-frequency semicircle is related to the charge-transfer resistance (R_ct) and interfacial capacitance (CPE1) at the interface of the electrolyte/counter electrode, and the low-frequency one is related to the charge-transfer resistance (R_w) and interfacial capacitance (CPE2) at the TiO₂/dye/electrolyte interface.⁴³ Compared with NCS500 and NCS700, the fitted R_ct value of the cell with NCS900 is most close to that of the cell with Pt electrode, reflecting its comparatively high electrocatalytic performance. This result indicates a high catalytic activity and electrical conductivity of NCS900, leading to the excellent performance for the cell with NCS900 counter electrode.¹³

Fig. 4 Nyquist plots of the cells with various NCSs and Pt counter electrodes. Inset displays the corresponding equivalent circuit.

The J–V curves of DSSCs with NCSs and Pt counter electrodes are shown in Fig. 5a. The open circuit potential (V_oc), short circuit current density (J_sc), fill factor (FF), and conversion efficiency (η) of all the cells are listed in Table 2. The η of cell with NCS500 counter electrode is only 3.84%, due to low value of J_sc (13.9 mA cm⁻²) and FF (38.4). It is observed that for the cells with NCSs counter electrodes, the values of FF and η obviously increase with the increase of thermal treatment temperature and reach maximum values of 60.8% and 6.28% for the cell with NCS900 counter electrode, which should be related to the high electrocatalytic activity of NCS900. The large specific surface area of NCS900 provides sufficient electrode/electrolyte interface to reduce I₃⁻ ion and facilitates the rapid charge-transfer reaction. Beyond that, nitrogen-doping offers more active sites for I₃⁻ ion insertion and may contribute to the electrical conductivity.²² Therefore, a large improvement in cell efficiency is achieved here compared with our previous work that employed CSs counter electrode (5.5%).³⁰ The incident photon to current conversion efficiency (IPCE) spectra of DSSCs with NCSs and Pt counter electrodes, as shown in Fig. 5b, exhibits a similar trend as J–V curves. Moreover, the calculated short circuit current density values from the IPCE spectra of as-prepared DSSCs with various counter electrodes are also shown in Table 2. The results are basically consistent to the measured J_sc values under one sun illumination. It should be noted that the maximum η of 6.28% for the cell with NCSs counter electrodes is lower than that for the cell with Pt counter electrode (6.95%) due to lower FF and J_sc caused by the worse electrocatalytic ability of NCSs counter electrodes and the re-absorption of unabsorbed incident light reflected back to the photoanode for semi-transparent Pt counter electrode.^35,45–48 However, considering the low cost and superior chemical stability of carbon materials, the NCSs counter electrode is still a competitive alternative to conventional Pt counter electrode.

Fig. 5 (a) J–V curves and (b) IPCE spectra of as-prepared DSSCs with various counter electrodes.

Table 2 Photovoltaic parameters of as-prepared DSSCs with various counter electrodes

Sample	Jsc (mA cm^−2)	(mA cm^−2)	JVoc (V)	FF (%)	η (%)
NCS500	13.90	12.43	0.72	38.4	3.84
NCS700	14.38	13.12	0.72	53.8	5.60
NCS900	14.40	13.34	0.71	60.8	6.28
CS	13.30	12.93	0.74	55.6	5.52
Pt	14.69	13.66	0.73	65.0	6.95

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Conclusion

In summary, NCSs were synthesized via a microwave-assisted reaction followed by thermal treatment in an ammonia atmosphere and investigated as counter electrodes of DSSCs. The results of electrochemical experiments reveal that the NCSs counter electrodes exhibit excellent electrochemical catalytic activity for catalyzing the reduction of triiodide. A maximum photovoltaic conversion efficiency of 6.28% under one sun illumination is achieved for NCSs thermally treated at 900 °C, which is comparable to that for conventional Pt counter electrode. The NCSs should be a very promising candidate for counter electrode of DSSCs.

Acknowledgements

This work is supported by the National Science Foundation of China (NSFC) (21271136), the Provincial Natural Science Foundation of Anhui (1508085ME104 and 1408085QA20), the Important Project of Anhui Provincial Education Department (KJ2015A250 and KJ2015A210), the Key Program of Outstanding Youth Talents in Higher Education Institution (GXYQZD2016343), the Program of Innovative Research Team of Anhui Provincial Education Department (Photoelectric information materials and new energy devices), and the Scientific Research Foundation for the Anhui Provincial Returned Overseas Chinese Scholars (Electrospun low dimensional TiO₂ nano-structures and application in photovoltaic devices).

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