CoSe2@N-Doped Graphene Nanocomposite High-Efficiency Counter Electrode for Dye-Sensitized Solar Cells

In present study, CoSe2, and CoSe2@N-doped graphene nanocomposite has been prepared in an inert atmosphere and used as a DSSC counter electrode. The fabricated nanocomposite was characterized using analytical techniques including FTIR, TGA, XRD, Raman, XPS, and BET. The assembled DSSC obtains a photoelectric conversion efficiency (PCE) of 7.65%, which is higher than the PCE (7.19%) of the Pt electrode assembly cell under the same conditions. The promising performance of the fabricated counter electrodes may be due to the excellent surface area of the nanocomposites, the doping of heteroatoms which provide the active sites to boost the catalytic activities towards I3− reduction. These results illustrate the rational design of CoSe2@N-doped graphene nanocomposite and has great potential for replacing noble Pt for the reduction of I3– in DSSCs.


Introduction
The sun has long been the most important renewable energy source on earth and the researchers are trying to utilize this vital source of energy for the benefit of humans. Even though, the world's biggest oil exporter, Saudi Arabia, has also pledged to cut its carbon emissions to net zero by 2060 and the plans and objective of Vision 2030. To meet the growing demand for energy, Saudi Arabia has focused on producing 50% of its electricity from renewable sources by 2030. Solar radiation can be converted directly into electricity by solar cells (photovoltaic cells) [1][2][3]. Among these solar cells, dye-sensitized solar cells (DSSC) have attracted widespread attention due to their low cost, easy preparation, and high solar energy conversion efficiency [4,5]. Generally, a typical DSSCs composed of a dye-adsorbed TiO 2 anode, a redox shuttle (I − /I 3 − ), and a counter electrode (CE). The function of CE is to collect the electron from the external circuit and catalyse the reduction of I 3 − into I − [6][7][8]. Pt is widely used as a counter electrode (CE) to fabricate DSSC due to its high conductivity and excellent catalytic activity towards I 3 − reduction, however, the limited resources and high price of Pt limits its industrial application [9,10]. Therefore, current studies are focused on finding alternatives to Pt. Some non-Pt materials such as porous carbon, cobalt based nanomaterials have been used as for water splitting, energy storage and counter electrode in DSSCs due to their large surface area, high conductivity, and low cost [11,12]. Moreover, several transition metal-based compounds specially, transition metal selenides, such as CoSe 2 , NbSe 2 , NiSe 2 , and ZnSe 2 have excellent catalytic activity towards I 3 − reduction [13][14][15]. Among these, cobalt selenide (CoSe 2 ) has attracted much attention because of its high catalytic activity, low cost and excellent long-term stability [16][17][18]. Moreover, previously it was noticed that Co-based selenide (sulfide)/N-doped carbon material hybrid has a great potential to reduce I 3 − in DSSCs [19][20][21][22][23]. However, the electron conductivity of CoSe 2 is not meet to the required standard due to its semiconducting behaviour and poor conductivity [24][25][26]. On the other hand, the carbon-based materials such as graphite carbon, carbon nanotubes, graphene and graphene oxide not only show the excellent electrical conductivity but also the high surface area to enhance the catalytic activity. Recently, it was noticed that the doping of the heteroatoms such as N and S into carbon provide the additional active sites, which enhance the charge transfer behaviour as well as enhance the catalytic activity of the nanocomposites. Therefore, the heterostructure nanocomposites based counter electrode have wide attention because they combine the advantages of each component and help improve the catalytic activity and the stability of the electrode [27][28][29][30]. Being inspired by the above investigations, here we have design low-cost, effective and environmentally friendly counter electrode for DSSC using CoSe 2 @N-doped graphene nanocomposite. The electrode materials were characterized using FTIR, XRD, Raman, BET, XPS, SEM and TEM. The electrochemical performance of CoSe 2 @N-doped graphene was analysed by electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and Tafel polarization curve, and the effects of N-doped graphene (NGN) loading with CoSe 2 were discussed in details. The DSSC fabricated using the CoSe 2 /NGN-15 electrode exhibits excellent PCE value of 7.65% higher than that of other fabricated electrode even higher than that of the Pt electrode (7.19%) under the same experimental conditions. This may be due to the synergetic effect between the CoSe 2 nanoparticles and graphene moreover the doping of the nitrogen into the graphene matrix also improve the DSSC performance and have good corrosion resistance to a corrosive redox electrolyte as CE materials. We hope that, the present work offers to fabricate Pt free counter electrode for DSSC and provides an opportunity to develop a highly efficient and low cost DSSC as the plans and objectives of Vision 2030.

Preparation of CoSe 2 @NGN
The counter electrode materials were prepared as following; 20 mg of the NGN was dispersed into 50 mL ethanol and sonicated for 30 min, Then 0.40 mmol of Co(NO 3 ) 2 ·6H 2 O and then 26 mmol (CH 3 COO)Na were added to this mixture and stirred continuously. After that 0.80 mmol of SeO 2 was added to this solution and stirred again for 3 h, under N 2 gas, and then 4.92 mmol NaBH 4 dissolved in 20 mL ethanol was dropped to the above mixture and stirred overnight. The obtained precipitate was filtered, washed dried and then annealed at 400 °C for 4 h in N 2 atmosphere to remove amorphous selenium. Resulting product CoSe 2 @NGN-20, was washed and saved for further uses. Other compositions were prepared using similar procedure after changing the amount of NGN as 0, 5, 10 and 15 as pure CoSe 2 , CoSe 2 @NGN-5, CoSe 2 @NGN-10 and CoSe 2 @NGN-15 respectively.

Cell Fabrication and Measurement
The detailed of counter electrode (CE) and DSSC fabrication processes, material characterization, and electrochemical measurement could be found in the Supporting Information.

Results and Discussion
The CoSe 2 @NGN nanocomposite was prepared using reduction and post calcination method as shown in Fig. 1.
The FTIR spectra of the nanocomposites illustrate in Fig. 2a and the peaks at 1567 cm −1 and 1647 cm −1 assigned to the C=C and C=N bonds. Other peaks at 1334, 1402, and 1054 cm −1 peak are assigned to the C-O, C-N, and C-O-C stretching, respectively [31,32]. While the peaks in the region of 3170-3382 cm −1 are corresponding to N-H, -NH 2 , and O-H bonds, respectively. Moreover, the FTIR peaks between 587 and 600 cm −1 support the bond Co-Se in the case of pure CoSe 2 and other CoSe 2 @NGN nanocomposites. The thermal stability of the nanocomposites was determine using TGA analysis and the results were illustrated in the Fig. 2b. The TGA outcomes revealed that as the amount of the NGN was increased from 5 to 20% the thermal stability of the nanocomposites was decreased and the reduced weight was found to be 86.74%, 49.02%, 41.64%, 38.43% and 34.88% corresponding to CoSe 2 and CoSe 2 @ NGN-5, CoSe 2 @NGN-10, CoSe 2 @NGN-15 and CoSe 2 @ NGN-20 respectively at 800 °C [33,34]. It was noticed that the with increasing the NGN contents the reduce weight was decreased due to the un-stability of the NGN under the flow of oxygen and convert into volatile CO 2 and H 2 O. The purity and the crystalline nature of the nanocomposites were also determine using XRD, as shown in Fig 131) and (122) planes are assigned to the orthorhombic CoSe 2 (PDF-53-0449) in the case of pure CoSe 2 and other nanocomposites. No peaks related to graphite was observed in the case of nanocomposites this may be due the lower contents of NGN in nanocomposites. The doping of the nitrogen atoms into to the graphene and growing the CoSe 2 nanoparticles into the NGN matrix was further determine using Raman spectra. As shown in Fig. 2d, two main Raman peaks were observed at 1354 cm −1 and 1592 cm −1 known as D and G bands, respectively. The G band assigned to the sp 2 hybridized C=C bonds, whereas the D band assigned to the sp 3 hybridized C-C bond [35][36][37]. The intensity of these two peaks increased with increasing the contents of NGN in to the nanocomposites. Other peaks at 163.46, 469.23 and 670.21 cm −1 assigned to the CoSe 2 nanoparticles and observed in all the nanocomposites. The defect density of carbon is proportional to the value of I D /I G and found to be 1.24, 1.23, 1.24 and 1.22 in the case of CoSe 2 @NGN-5, CoSe 2 @NGN-10, CoSe 2 @NGN-15 and CoSe 2 @NGN-20. However, these peaks are not noticed in the case of pure CoSe 2 .
The results indicated that the doping of N atoms and the existence of sp 3 C-C and C-N bonds enhanced the disordered of graphene lattice. Which lead the electron transfer and catalytic efficiency for I 3 − reduction in DSSC. The microstructure and the morphology of the fabricated electrode materials were determine using SEM and TEM techniques. As shown in Fig. 3a, the SEM image of the CoSe 2 @NGN-15 nanocomposite show the porous structure and grown the nanoparticles in the NGN matrix. The SEM image shown the spherical shaped CoSe 2 nanoparticles with the dimeter range of 20-50 nm are well dispersed into the NGN matrix. These results support by TEM image as shown in Fig. 3b, that the fabricated electrode material contains heterostructure which being conducive to the exposure the active sites at the surface of the catalyst to reduce the I 3 − . To deep understand the microstructural of CoSe 2 @NGN-15, TEM analysis was used. As shown in Fig. 3c, CoSe 2 nanoparticles with an average size of ~ 38.9 nm is uniformly embedded in the NGN matrix.
The HRTEM image as shown in Fig. 3d, displayed interplanar spacing of 0.29 nm, and 0.19 nm in the high-resolution TEM (HRETM) images are indexed to the (101), and (211) oriented facets of CoSe 2 nanoparticles. The inserted figure of selected area electron diffraction shows the polycrystalline nature of the CoSe 2 nanoparticles. The nitrogen adsorption-desorption isotherm was used to determine the porosity of the nanocomposites and the isotherm curves are illustrated in Fig. 4a.
It was noticed that all the nanocomposites show type-IV hysteresis loops and demonstrating the mesoporous structure [38][39][40]. In the case of the CoSe 2 @NGN-15, the hysteresis loops were observed from 0.40 to 0.99 and the specific surface area was found to be 421.0 m 2 /g, while for pure CoSe 2 , CoSe 2 @NGN-5, CoSe 2 @NGN-10, CoSe 2 @ NGN-20 the surface area was found to be 464.21, 440.2, 436.4 and 414.12 m 2 /g respectively. These results demonstrate that the surface area of the nanocomposites was decreased with increasing the contents of NGN into the electrode materials. In the case the CoSe 2 @NGN-5 the pore size distribution is found between 25 and 45 nm due to the lower amount of NGN. Figure 4b show the pore size dimeter was observed about in the range of 20-67 nm in the case of all the nanocomposites. The large surface area and nanoscale pore size of the CoSe 2 @NGN based electrode materials not only support the additional catalytic sites but also enhance the electron transfer during the I 3 − reduction. The elemental composition and the valance state of the elements present in the nanocomposite as monitored by XPS analysis, as shown in Fig. 5a, the XPS spectra of CoSe 2 @NGNs show the presence of C, N, Co, Se and O elements into the nanocomposite, however, in the case of CoSe 2 the peaks belong to C and N is absent. As shown in Fig. 5b, the C 1s peak was deconvoluted into four peaks and centred at 284.68, 285.71, 286.90 and 287.78 eV and assigned to the C-C, C-N, C=N, and O=C-O function groups presence into the NGN matrix. The N1s spectra of the nanocomposites was split into three peaks and show the binding energy at into 398.24, 400.36, and 401.43 eV, corresponding to pyridinic, pyrrolic/pyridonic, and graphitic nitrogen functional groups respectively as shown in Fig. 5c [41,42]. The XPS peak of Co 2p was deconvoluted into four characteristic peaks as shown in Fig. 5d, two of them are the main peaks and the binding energy cantered at 778.71 and 793.89 eV corresponding to the Co 2p 3/2 and Co 2p 1/2 , respectively, and other two peaks were observed at binding energy 780.23 and 796.91 eV are assigned to the satellite peaks of Co 2p 3/2 and Co 2p 1/2 respectively. Figure 5e shows the XPS spectrum of Se 3d and the main peak was split into two peaks and cantered at 54.74 and 55.83 eV and assigned to Se 3d 3/2 and Se 3d 5/2 respectively [43]. The XPS results shows that the O atoms also present into the matrix and the O 1s spectrum was deconvoluted into binding energies of 530.44, and 531.88 eV and assigned to the presence of C-O and Se-O bonds respectively.

Photovoltaic Performance
The DSSC was fabricated as sandwich structure using the N719 dye-loaded TiO 2 as a photo-anode, an iodine solution (I 3 − /I − ) as an electrolyte, and CoSe 2 @NGN (or Pt) as a counter electrode. The current density voltage curve (J-V) of the fabricated DSSCs using CoSe 2 @NGN-5, CoSe 2 @ NGN-10, CoSe 2 @NGN-15, and CoSe 2 @NGN-20 and Pt under 100 mW/cm 2 AM 1.5 G are illustrated in Fig. 6a. The values of photovoltaic factors including short circuit current density (J sc ), open circuit voltage (V oc ), filling factor (FF) and maximum power (P max ) are summarized in Table 1 [19][20][21][22][23]. Figure 6b show the incident photon-current conversion efficiency IPCE and in the case of CoSe 2 @NGN-15 nanocomposite the value of IPCE is higher than that of other nanocomposites with broadening in the region of  (350-800 nm) causing increasing the light harvesting ability. Therefore, CoSe 2 @NGN-15 based counter electrode has proper distribution of CoSe 2 into the NGN matrix and has promising surface area, resulting show higher the catalytic activity, and the better the photovoltaic performance of DSSC. On the other hand, the nanocomposites CoSe 2 @ NGN-5 and CoSe 2 @NGN-10 show the DSSC performance lower than that of Pt. Based on the above results the proposed photovoltaic mechanism of the CoSe 2 @NGN-5 CE is illustrated in Fig. 7.

Electrochemical Performance of CE Materials
The relationship between the photoelectric conversion efficiency of the fabricated DSSC with the counter electrode was further carried out using CV, EIS and Tafel plots. As shown in Fig. 8a, the cyclic voltammetry curve of the DSSC fabricated using CoSe 2 @NGN-5, CoSe 2 @NGN-10, CoSe 2 @NGN-15, and CoSe 2 @NGN-20 and Pt show the peaks due to the redox reaction of I 3 − as: I 3 − + 2e − → 3I − (i) and 3I 2 + 2e − → 2I 3 − (ii). It was noticed that the current density (J) and the potential (ΔE pp ) difference between the two peaks are important factors for comparing the catalytic activity of different CE materials. It was observed that the CE based on CoSe 2 @NGN-15 nanocomposite show higher current density and lower peaks difference than Pt based counter electrode, indicating that CoSe 2 @NGN-15 has superior catalytic ability than Pt for I 3 − reduction [46,47]. The stability experiment was carried out using CV cycles as shown in Fig. 8b, and the outcomes revealed that after 50 cycles on 50 mV/s scan rate, the current density, peak positions and E PP value is substantially unchanged, indicating CoSe 2 @NGN-15 having excellent electrochemical stability.
The electrocatalytic activity of the CoSe 2 @NGN-5, CoSe 2 @NGN-10, CoSe 2 @NGN-15, and CoSe 2 @NGN-20 and Pt electrode were further determine using the Tafel polarization curves asillustrated in Fig. 8c. All the Tafel polarization plots were divided into three zones, first the polarization zone occurs due to the electrochemical reaction and noticed at low over potential (|V| < 120 mV), in the second zone is sharp slope is and it is Tafel zone, in which, the current density (J o ) is changed and noticed at moderate over-potentials (120 mV < |V| < 400 mV), third is the limiting diffusion zone, which depend on the transport of ions and occurs at high over-potential (|V| > 400 mV), and used to determine the limiting diffusion current density (J lim ). The value of of J o and J lim related to the catalytic activity of the counter electrode. As shown in Fig. 8c, it has been noticed that CoSe 2 @NGN-15 has a higher value of J o as compared to Pt based CE, resulting shown higher catalytic activity towards I 3 − reduction, and the higher J lim value of CoSe 2 @ NGN-15, indicate that CoSe 2 @NGN-15 based CE has a faster ion diffusion rate than standard Pt CE [48]. The electrochemical impedance spectroscopy (EIS) measurements of the DSSC fabricated using CoSe 2 @NGN-5, CoSe 2 @NGN-10, CoSe 2 @NGN-15, and CoSe 2 @NGN-20 and Pt based CE is illustrated in Fig. 8d. The EIS results used to determine various resistances, charge-transfer between the interface of electrode/electrolyte and the catalytic efficiency of the counter electrode.
The EIS Nyquist plots, show three semicircles, the R s is represent to resistance between the FTO and CE materials, the R ct-1 is representing to the charge transfer resistance between the electrolyte and CEs and the R ct-2 represents the electron transfer resistance between the TiO 2 /electrolyte and the dye. It was noticed that in all the Nyquist plots the value of R ct-2 is unchanged and almost similar, these results may have been due to the use of similar type photoanaode materials. While the value of R s and R ct-1 was changed. The results reveal that CoSe 2 @NGN-15 show almost similar value of R s to the Pt and support that these is a good connection between the FTO glass and the electrode materials. While the R ct-1 value of CoSe 2 @NGN-15 was found to be lower, than that of other electrodes even lower than that of Pt based CE. On the other hand, it was noticed that the CoSe 2 @NGN-15 nanocomposite show lower value of Nernst diffusion impedance (Z w ) than the Pt base CE for diffusion of I − /I 3 − redox couple within the electrolyte [46,47]. This may be due to

Conclusion
Sustainability has been the focus of researcher vision; therefore, we have fabricated novel counter electrode materials for DSSC using CoSe 2 nanoparticles embedded into NGN. The experimental results show that the CoSe 2 @NGN electrode has excellent catalytic activity. The excellent performance of CoSe 2 @NGN-15 is attributed due to the synergetic effect of CoSe 2 nanoparticles and N-doped graphene matrix, because the N-doped graphene exhibit excellent conductivity and uniformly dispersed CoSe 2 which boost the catalytic activity of the nanocomposites. The preparation of this new type of non-Pt material provides a new method for DSSCs counter electrode research and promotes its industrialization process to utilised the sustainable solar energy for the benefit of humans.

Acknowledgements
The author thanks to Researchers Supporting Project number (RSP-2021/6), King Saud University, Riyadh, Saudi Arabia.
Funding This study was supported by King Saud University.

Conflict of interest
The author have not disclosed any conflict of interest.