Synthesis of lignite-based Ni/C composite with low-medium temperature pyrolysis method as an e�cient Pt-free counter electrode for dye-sensitized solar cells

In order to obtain inexpensive Pt-free counter electrode materials for dye sensitized solar cells and expand the application �elds of low-rank coal, lignite-based Ni/C composite counter electrode materials were prepared by low-medium temperature pyrolysis method using Huolinhe Lignite as raw material. The structure and chemical components of as-synthesized lignite-based Ni/C composite counter electrode materials were characterized by XRD, FT-IR, TG, Raman, SEM, TEM and XPS, and the electrocatalytic activity of lignite-based Ni/C composite counter electrode were investigated by cyclic voltammetric curve, electrochemical impedance spectrum, Tafel polarization curve. It is found that the electrocatalytic activity of the lignite was improved after low-medium temperature pyrolysis and composited with Ni species to form lignite-based Ni/C composites, which helps to catalyze the electrolyte reduction and thus improve the photoelectric conversion e�ciency of the cell. The photoelectric conversion e�ciency ( η ) of the lignite-based Ni/C composite counter electrode was 3.42% (J sc = 11.49 mA cm -2 , V oc = 0.75 V, FF = 0.40) signi�cantly higher than that of the lignite counter electrode ( η = 0.20%, J sc = 3.16 mA cm -2 , V oc = 0.72 V, FF = 0.09). This indicates that low-medium temperature pyrolysis and composition with Ni is an effective method to improve the photovoltaic performance of coal-based counter electrode materials.

excellent properties make it promising for applications in the eld of optoelectronic materials and for the preparation of catalysts [23]. However, lignite and its derivatives have been neglected for the application in DSSCs as counter electrode materials because of their insu cient conductivity and electrocatalytic activity. Therefore, how to enhance the conductivity and electrocatalytic activity of lignite and its derivatives is a key point for its application in DSSCs [24].
In recent years, many strategies were put forward to control the composition and structure of lignite for its applications in different elds. For example, Zhu et al. successfully prepared supercapacitor electrode materials by pyrolytic activation using Shengli lignite as raw material, which led to a signi cant improvement in its electrochemical performance and measured a speci c capacitance of 449 F g − 1 at 0.2 A g − 1 [25]. Zhao et al. prepared coal-based carbon bers from lignite by electrostatic spinning and maintained them at 800°C under Ar atmosphere for 1 h to improve their conductivity, and nally applied them to supercapacitor electrode materials and measured the speci c capacitance of 230 F g − 1 at a current density of 1 A g − 1 [26]. These literatures indicate that pyrolysis of coal can change the coal molecular structure and thus increase its conductivity. Besides, it is also found that transition metal Ni modi cation can improve the electrocatalytic activity of carbon counter electrode materials for DSSCs application [27][28][29].
Therefore, in this study, lignite-based Ni/C composite counter electrode materials were prepared by lowmedium temperature pyrolysis method using Huolinhe Lignite as the raw material. The structure and chemical components of as-synthesized lignite-based Ni/C composite counter electrode materials were (99%), and iodine (99.8%) were purchased from Aladdin Reagent Co., China; dye N719 was available from Solaronix Co, Switzerland. All reagents were used directly without further puri cation. The lignite used in this experiment is provided by Huolinhe Coal Mine in Inner Mongolia province of China (average particle size < 200 mesh) and its proximate and ultimate analysis are shown in Table 1.

Counter electrode preparation
The Ni/C counter electrode was prepared as follows: 0.5 mg of lignite-based Ni/C composite was weighed, added into 1.0 mL of anhydrous ethanol, and ground into a paste, then 4 drops of 5% Na on solution were added and continuously ground to mixed thoroughly. Then drip 10 µL of this mixture onto FTO conductive glass with a microinjector to form a 1⋅1 cm 2 lm and dried in an oven at 80°C for 30 min to obtain lignite-based Ni/C composite counter electrode. As a comparison, the UC and HC counter electrodes were also prepared by the same method, while Pt counter electrode was prepared by screen printing method using H 2 PtCl 6 paste.

Fabrication of DSSCs
The TiO 2 photoanodes with an effective area of 0.16 cm 2 were prepared by screen printing method. The preparation process was as follows: rstly, 0.18 g TiO 2 nanoparticles, 0.09 g ethyl cellulose and 0.73 g terpineol was weighed into a 10 mL vial, and an appropriate amount of ethanol was added and stirring for about one week to form a paste suitable for screen printing. The screen printing process were repeated 6 times to form 8-12 µm lms. Thereafter, it was calcined in a mu e furnace at 500°C for 30 min. Then, the obtained TiO 2 photoanode lms were placed in 3 mol L − 1 dye N719 solution overnight to obtain dye-sensitized TiO 2 photoanodes. A mixture of 0.5 M LiI, 0.1 M 4-tert-butylpyridine, 0.05 M I 2 and acetonitrile-propenyl carbonate was used as the electrolyte, and the electrolyte was dropped to ful ll the space on the septum between the dye-sensitized TiO 2 photoanode and the counter electrode, all of which were assembled into a cell with a sandwich-like structure by holding with a clamp.

Materials characterization
An X-ray powder diffractometer (XRD, D8-Advance, Bruker) with a radiation source of Cu-Kα, an operating voltage of 40 kV and a current of 40 mA was used to carry out phase analysis. The surface morphology and structure of the Ni/C composites were observed by scanning electron microscopy (SEM, Hitachi, S-4800) and high-resolution transmission electron microscope (HR-TEM, Tecnai G2 F30 S-TWIN, FEI, America). X-ray photoelectron spectroscopy (XPS, Thermo VG, ESCALAB250) with a radiation of Al-Kα was used to carried out elemental composition and chemical states. Fourier transform infrared spectroscopy (FT-IR, Nicolet iS5) was used to determine the functional groups contained in Ni/C composite. Thermogravimetric analyzer (TG, LINSEIS STA PT 1600) was used to determine the thermal stability of Ni/C materials.

Electrochemical measurements
The electrochemical workstation system (CHI 760E, Chenhua, Shanghai) was used to carry out the electrochemical and photovoltaic properties. Cyclic voltammetry (CV) was performed in a three-electrode system with an Ag/AgCl electrode as the reference electrode, a Pt sheet as the auxiliary electrode, the 3 Results And Discussion

Morphology and structure
The XRD patterns of the lignite-based Ni/C composites are shown in Fig. 1a. For comparison, the XRD patterns of UC as well as HC are also plotted in the Fig. 1a. From the XRD patterns of UC, it can be seen that a broaden peak corresponds to the (002) crystal plane of graphite type carbon materials appears at 2θ angle of 25.9°, and diffraction peaks of SiO 2 species, which originate from the components contained in the lignite itself, are found at 2θ angles of 20.8°, 26.6°, 50.6°, 59.9° and 68.2°, respectively. After pyrolysis to form HC, diffraction peaks were found at the same positions as UC, which indicates that pyrolysis did not change the phase components of UC, which also contains SiO 2 species and carbon materials frameworks. As for Ni/C composites, besides the above peaks, some stronger diffraction peaks were also observed at 2θ angles of 44.5°, 51.8° and 76.4°, which is consistent with standard card PDF#87-0712 and correspond to the (111), (200) and (220) crystallographic planes of Ni, respectively. This indicates that Ni rather than NiO is present in the Ni/C materials. Although NiO should be formed by the direct decomposition of Ni (NO 3 ) 2 ·6H 2 O, the decomposition-generated NiO species is further reduced by elemental C to form Ni monomers. Therefore, there is no diffraction peaks of NiO species was obviously found and the Ni species is mainly existed as Ni monomers in Ni/C composites.
In order to investigate the changes of functional groups in the material, FT-IR was carried out. As seen in  [31]. The broadening of the sulfur-containing absorption peaks of HC and Ni/C indicates that the sulfur-containing functional groups are lost as sulfur dioxide during the low-medium temperature pyrolysis. The functional group located at 1087 cm − 1 is a C-H deformation vibration on the aromatic ring [32]. The functional groups located in the 1168-1612 cm − 1 region are associated with C = O and C-O stretching vibrations [33]. The sharp decrease in HC and Ni/C oxygenated functional groups in this region is mainly attributed to the loss of carbon dioxide and water during the low-medium temperature pyrolysis and also indicates an increase in graphitization. The functional groups at 2856 and 2924 cm − 1 are associated with asymmetric -CH 2 -and -CH 3 stretching vibrations in the coal [34]. In HC and Ni/C, the peak at this position almost disappears, indicating that -CH 2 -and -CH 3 were broken during the process of pyrolysis. The functional group at the peak of 3423 cm − 1 is associated with the stretching vibration of -OH in the carboxyl group. It can be seen from the Fig. that the absorption peak of UC at 3423 cm − 1 is the largest, indicating that the UC has the most -COOH. The HC has a broader and less intense -COOH peak compared to UC, indicating that the medium and low-temperature pyrolysis can cause some of the -COOH to be lost as carbon dioxide and water. Compared with HC, the -COOH peak of Ni/C is also wider and weaker, which is mainly due to the chemical reaction with -COOH after the addition of nickel nitrate hexahydrate, as well as the effect of low-medium temperature pyrolysis. The functional groups located at 3623 and 3969 cm − 1 can be attributed to the free -OH vibrations in UC [35], which disappeared for HC and Ni/C. When the temperature increased to 800°C, the total weight of Ni/C decreased by about 43%, which is slightly higher than that of UC due to the addition of Ni(NO 3 ) 2 ·6H 2 O, and the reduction of NiO to Ni by the C species in the materials. In order to observe the microscopic morphology of UC, HC and Ni/C, SEM for them were carried out and shown in Fig. 2. It can be seen from Fig. 2a that the UC is loosely dispersed, which is not compact enough to ensure the conductivity of the material, indicating worse electrocatalytic activity. However, as shown in Fig. 2b, the HC shows a much compact structure due to some lignite particles coalesces together during the pyrolysis process, and there are still a lot of voids between the compact structures, which are su cient to ensure electrolyte penetration and e cient electron transfer and resulted in increased electrocatalytic activity. The morphology of the Ni/C composites formed after Ni doping is shown in Fig. 2c ~ 2e, respectively. It is found that some small Ni particles are decorated on the surface of lignite, indicating the successfully modi ed with Ni particles during the pyrolysis process. Therefore, the increased active sites due to Ni decoration and the good conductivity after low-medium temperature pyrolysis ensures the charge transfer and the occurrence of the triiodide/iodide redox reaction, which indicates the high electrocatalytic activity of lignite-based Ni/C materials. To better prove the successful synthesis of Ni/C composites, EDX tests were performed on Fig. 2e, and the results are shown in Fig. 2f (carbon), 2g (oxygen) and 2h (nickel). It can be seen from Fig. 2f and 2g that the carbon and oxygen elements are evenly distributed. Although the distribution of nickel elements is not uniform due to the non-uniform distribution of oxygen-containing functional groups in coal, it again proves that the Ni/C composites were successfully synthesized. The EDS results show that the atomic concentrations of carbon, oxygen and nickel in the Ni/C material are 91.4, 7.72 and 0.14%, respectively. Although the atomic concentration ratio of oxygen to nickel is above 1:1, it still does not indicate that the Ni species existed as NiO, but metallic Ni monomers in Ni/C composites. The higher oxygen atomic concentration was partly caused by the formation of some hetero-metal oxide together with very small amount of nickel oxides and some residuals oxygen-containing group or adsorbed oxygen, which is con rmed by the previous XRD test and the same result was obtained in the subsequent XPS test.
Furthermore, the microscopic morphology and structure of Ni/C were observed by transmission electron microscope (TEM). The TEM image of Ni/C were shown in Fig. 3a ~ 3c. From Fig. 3a ~ 3c, it is observed that the Ni monomer is well attached to the lignite surface, which makes the lignite-based Ni/C composite material have better electrocatalytic activity. Furthermore, the high-resolution transmission electron microscope (HR-TEM) was performed on the selected local area in Fig. 3c. As shown in the HR-TEM image, a set of lattice fringes was observed. The calculated lattice spacing of the fringes was 2.035 nm, which corresponds to the (111) crystal plane of Ni monomer, and consists with the XRD results, further con rmed that the Ni species is mainly existed as Ni monomers in Ni/C composites.
To further demonstrate the successful synthesis of lignite-based Ni/C composites and investigate the elemental composition and their chemical state, XPS tests were performed, and the results are shown in Fig. 4. From the results in Fig. 4a, it can be seen that C1s and O1s peaks are present near the binding energy of 284.50 eV and 532.26 eV for both UC, HC and Ni/C composites. In addition, the Ni2p peak was found at 854.44 eV for the Ni/C composites. To further investigate the effect of low-medium temperature pyrolysis and modi cation of Ni on the carbon structure of UC, the high resolution C1s spectra of UC is given in Fig. 4b, and for comparison, the high resolution C1s spectra of HC and Ni/C composites are also plotted in Fig. 4b. The results show that three carbon structures exist in all materials, namely C-C/C = C located around 284 eV, C-O located around 285 eV and C = O located around 288 eV[36]. The peak positions were almost unchanged after low-medium temperature pyrolysis and Ni modi cation, indicating that the oxygen-containing group was not completely destroyed after low-medium temperature pyrolysis and Ni modi cation, which is consistent with the FT-IR and EDS analysis. Figure 4c shows the high-  Fig. 4d. Figure 4d shows the high-resolution t of Ni2p in the Ni/C composite. It can be seen from the Fig. that

Electrochemical impedance spectroscopy
In order to evaluate the charge transfer dynamics of the prepared materials, electrochemical impedance spectroscopy (EIS) was performed using a symmetric cell assembled by two same counter electrodes with a counter electrode-electrolyte-counter electrode structure, and the Nyquist plots and equivalent circuit of the prepared symmetric cells are shown in Fig. 5a and Fig. 5b, respectively. It can be seen from Fig. 5a that the Nyquist diagram of each material consists of a semicircle in the high-frequency region.
The intersection of the semicircle in the high-frequency region with the X-axis is the series ohmic resistance (R s ) between the counter electrode lm material and the FTO glass substrate, and the magnitude of R s has a great in uence on the conductivity of the counter electrode [38]. From the enlarged view of the boxed area in Fig. 5a (the inset of Fig. 5a), it is found that R s (Pt) < R s (Ni/C) < R s (UC) < R s (HC). During the pyrolysis process, the oxygen element in the material of UC reacts with carbon element leading to the reduction of oxygen element content, which makes the adhesion of HC to the surface of FTO glass substrate become poor. However, the R s of Ni/C composites prepared after Ni modi cation is greatly reduced, indicating that the doping of Ni can improve the conductivity. The radius of the semicircle in the high frequency area represents the charge transfer resistance (R ct ) of the material, which directly re ects the electrocatalytic activity of the material for the triiodide/iodide redox reaction [39]. It can be seen from Fig. 5a that R ct (Pt) < R ct (Ni/C) < R ct (HC) < R ct (UC). Furthermore, the R s and R ct values for the electrode materials were tted using the equivalent circuit shown in Fig. 5b, and the results are shown in Table 2. From Ω and 3.45 Ω, respectively. The lower R ct of Ni/C composites proved that the materials had better electrocatalytic activity. The above results indicate the conductivity of UC was signi cantly improved after low-medium temperature pyrolysis and modi cation by Ni, which is bene t for enhancing the photoelectric properties of DSSCs.

Tafel polarization curves
To investigate the electrocatalytic activity of the counter electrodes, Tafel polarization curve of symmetric cells were tested. In general, a Tafel polarization curve contains three regions: diffusion region, Tafel region and polarization region, and two parameters: exchange current density (J 0 ) and limit diffusion current density (J lim ) to evaluate the electrocatalytic activity of the counter electrode [40]. J 0 is the Y axis reading of the intersection of tangent line to the cathode branch in the Tafel zone and the perpendicular line to the zero potential [41], and J lim is the intersection of the cathode's branch and the Y-axis [42]. In addition, J 0 and J lim can also be calculated from the equations (1) and (2).
In Equations (1), n, F, T and R denote the number of electrons transferred by the reaction, Faraday's constant, absolute temperature, and gas constant, respectively. In Eq. (2), n, e, D, C, N A and l denote the number of electrons transferred by the reaction, basic charge, diffusion coe cient of triiodide, concentration of I 3 − , Avogadro's constant, and thickness of the spacer layer, respectively. As can be seen in Fig. 5c, UC has the smallest J 0 and J lim , indicating that it has the worst electrocatalytic activity. After low-medium temperature pyrolysis, its electrocatalytic activity was enhanced due to the formation of compact internal structure and the existence of certain interstices and edges in the HC materials, which helped the occurrence of the triiodide/iodide redox reaction, and thus the electrocatalytic ability was enhanced. After the modi cation with Ni, the electrocatalytic performance of Ni/C is further improved and more closed to Pt counter electrode. These con rm that the electrocatalytic performance of UC is enhanced after low-medium temperature pyrolysis and Ni modi cation.

Cyclic voltammetric curves
To further investigate the electrocatalytic ability of the counter electrode for the reduction of I 3 − to I − , cyclic voltammetry tests on UC, HC, Ni/C and Pt counter electrodes were performed at a scan rate of 10 mV s − 1 with a three-electrode system. Figure 5d shows the CV curves of different counter electrode, and it can be clearly observed that each curve has two pairs of redox peaks labeled as Ox1: 3I − -2e − ↔I 3 − /Red1: I 3 − +2e − ↔3I − and Ox2: I 3 − -2e − ↔3I 2 /Red2: 3I 2 + 2e − ↔2I 3 − , where the electrocatalytic activity is mainly in uenced by the rst pairs of redox peak [43]. From the rst pair of redox peak, the cathode peak current density (J pc ) and the peak spacing between Ox1/Red1 (E pp ) can be extracted to evaluate the electrocatalytic activity of counter electrode. The smaller the E pp and the larger the J pc , the better the electrocatalytic activity for reduction of triiodide to iodide [44]. As shown in Fig. 5d, the UC counter electrode has the largest E pp and the smallest J pc , indicating that the UC counter electrode has the worst electrocatalytic ability for reduction of triiodide to iodide. After low-medium temperature pyrolysis to form HC, E pp decreases and J pc increases, indicating that the electrocatalytic ability of UC has been enhanced after pyrolysis. The Ni/C counter electrode material formed after low-medium temperature pyrolysis and Ni modi cation has the largest J pc and smaller E pp , indicating that the redox reaction rate is faster on the Ni/C counter electrode, which is more favorable to the occurrence of triiodide/iodide redox reaction, and thus improve the photoelectric conversion e ciency of DSSCs.

J-V Characteristic curves
To investigate the practical effect of the lignite-based Ni/C counter electrode, the photovoltaic performance of the cells was tested under simulated solar irradiation (AM 1.5 G, 100 mW cm − 2 ). Table 2 and Fig. 6a show the photovoltaic parameters of different counter electrodes. It can be seen that the Ni/C composite counter electrode exhibited a photovoltaic conversion e ciency (η = 3.42%, V oc = 0.75 V, J sc = 11.49 mA cm − 2 , FF = 0.40), which was signi cantly better than the UC counter electrode (η = 0.20%, J sc = 3.16 mA cm − 2 , V oc = 0.72 V, FF = 0.09) and HC counter electrode (η = 2.14%, J sc = 9.06 mA cm − 2 , V oc = 0.72 V, FF = 0.33). In view of the above electrocatalytic and electrochemical characterization, it is suggested that the improvement of the photoelectric properties of the lignite-based Ni/C composite counter electrodes is due to the increase in conductivity and electrocatalytic activity by low-medium temperature pyrolysis and Ni modi cation. Although the conductivity (R ct ) and adhesion (R s ) of Ni/C composites to the FTO substrate are not as good as those of Pt materials, making the photovoltaic performance of the lignite-based Ni/C composites inferior to that of the Pt counter electrode (η = 5.10%, J sc = 12.56 mA cm − 2 , V oc = 0.73 V, FF = 0.56), the synthesis of Ni/C composites by a simple low-medium temperature pyrolysis and Ni modi cation method substantially improved the electrochemical properties of lignite-based counter electrode, thus greatly improving the photovoltaic performance of DSSCs and becoming a promising counter electrode material to replace Pt.
In order to further investigate the effects of synthesis condition (pyrolysis temperature and time) and modi cation amount of Ni on the photoelectric properties of lignite-based Ni/C counter electrode, the photoelectric properties of DSSCs with different Ni/C counter electrodes were tested and the results are shown in Fig. 6b ~ 6d From Fig. 6b, it can be found that the photoelectric properties of lignite-based Ni/C counter electrode gradually increased with the increase of pyrolysis temperature. When the pyrolysis temperature is 400 ℃, 500 ℃ and 600 ℃, the photoelectric conversion e ciency is low, which is mainly because the temperature is too low and the internal structure of lignite is not broken enough to form regular molecular structure, resulting in poor conductivity. When the temperature rises to 700 ℃, the internal structure of lignite tends to be regular, so the photoelectric conversion e ciency reaches the best. When the temperature rises to 800°C, the photoelectric conversion e ciency does not change much, which indicates that the structure of Ni/C is similar when the pyrolysis temperature is 700°C and 800°C, and the totally pyrolysis is occurred when the temperature reach to 700°C, which is consistent with TG results. As shown in Fig. 6c, with the increase of pyrolysis time, the photoelectric conversion e ciency of Ni/C counter electrode increased rst and then tended to be stable when the pyrolysis time was 1, 2, 3 and 4 h, which means that totally pyrolysis is occurred and Ni has formed a better crystal type during this time range, and there is no obvious difference on the role of redox reaction. Finally, the effect results of Ni mass percentage content on the photoelectric conversion e ciency are shown in Fig. 6d. It can be seen from Fig. 6d that the photoelectric conversion e ciency gradually increases with the increase of Ni mass percentage content, and reaches to the best when the Ni content is 6%. Although it is said that the modi cation of Ni can improve the catalytic activity of the material, it will sacri ce the conductivity of the material, so the photoelectric conversion e ciency of the lignite-based Ni/C composite decreases rapidly when the mass percentage content of Ni is more than 6%.

Conclusion
In conclusion, lignite-based Ni/C composite counter electrode materials were prepared by low-medium temperature pyrolysis and Ni modi cation. The low-medium temperature pyrolysis helps to improve the conductivity and electrocatalytic activity of lignite materials, which are further enhanced by Ni modi cation to form lignite-based Ni/C counter electrode materials. The photoelectric conversion e ciency of DSSCs based on Ni/C counter electrode was 3.42%, which was higher than that of DSSCs based on UC counter electrode (0.20%) and HC counter electrode (2.14%). What's more, the Ni/C composites have the advantages of simple preparation, low cost, and good catalytic activity for I 3 − reduction, which have great potential for replace Pt counter electrode in DSSCs. The exploration of lignitebased Ni/C composite counter electrodes is of signi cance to broaden the application of lignite and realize its e cient and clean utilization.
Declarations Figure 2 SEM images of (a) UC (b) HC (c) (d) and (e) Ni/C; Elemental mappings of (f) carbon (g) oxygen (h) nickel for Ni/C