Counter Electrode For Dye-sensitized Solar Cells Research Articles

Pt-free counter electrode based on FeNi alloy/reduced graphene oxide in liquid junction photovoltaic devices

This study presents a strategy to optimize FeNi nanoparticles (NPs) deposited onto reduced graphene oxide (RGO) in order to fabricate an efficient Pt-free counter electrode (CE) for use in a dye-sensitized solar cell (DSC). Nanohybrids exhibit a 3D network structure of RGO with high FeNi NP loading on the RGO surface. Well-dispersed Fe1-xNix (0 ≤ x ≤ 1) NPs ranging in size from 2 to 4 nm are stabilized with RGO after co-reduction of the metal precursor ions and graphene oxide. The developed catalysts are then applied as CEs in DSCs. As a result, the Fe0.7Ni0.3/RGO nanohybrid exhibited the highest electrocatalytic activity, corresponding to the lowest charge transfer resistance of 0.17 Ω, among the electrodes tested. The DSC employing Fe0.7Ni0.3/RGO CEs exhibits increases in efficiency of 87.68% and 75.65% relative to those of Fe0Ni1/RGO and Fe1Ni0/RGO devices, respectively, due to the optimization of the charge-transfer resistance and the reduced diffusion impedance values of the developed materials. This strategy is simple and efficient for fabricating cost-effective CE materials utilized in DSCs and fabricating efficient catalysts to facilitate methanol oxidation and oxygen reduction reactions.

Fabrication of Cost-Effective Dye-Sensitized Solar Cells Using Sheet-Like CoS2 Films and Phthaloylchitosan-Based Gel-Polymer Electrolyte

Platinum-free counter electrodes (CE) were developed for use in efficient and cost-effective energy conversion devices, such as dye-sensitized solar cells (DSSCs). Electrochemical deposition of CoS2 on fluorine-doped tin oxide (FTO) formed a hierarchical sheet-like structured CoS2 thin film. This film was engaged as a cost-effective platinum-free and high-efficiency CE for DSSCs. High stability was achieved using a phthaloychitosan-based gel-polymer electrolyte as the redox electrolyte. The electrocatalytic performance of the sheet-like CoS2 film was analyzed by electrochemical impedance spectroscopy and cyclic voltammetry. The film displayed improved electrocatalytic behavior that can be credited to a low charge-transfer resistance at the CE/electrolyte boundary and improved exchange between triiodide and iodide ions. The fabricated DSSCs with a phthaloychitosan-based gel-polymer electrolyte and sheet-like CoS2 CE had a power conversion efficiency (PCE, η) of 7.29% with a fill factor (FF) of 0.64, Jsc of 17.51 mA/cm2, and a Voc of 0.65 V, which was analogous to that of Pt CE (η = 7.82%). The high PCE of the sheet-like CoS2 CE arises from the enhanced FF and Jsc, which can be attributed to the abundant active electrocatalytic sites and enhanced interfacial charge-transfer by the well-organized surface structure.

Large area growth of MoTe2 films as high performance counter electrodes for dye-sensitized solar cells

A cost effective and efficient alternative counter electrode (CE) to replace commercially existing platinum (Pt)-based CEs for dye-sensitized solar cells (DSSCs) is necessary to make DSSCs competitive. Herein, we report the large-area growth of molybdenum telluride (MoTe2) thin films by sputtering-chemical vapor deposition (CVD) on conductive glass substrates for Pt-free CEs of DSSCs. Cyclic voltammetry (CV), Tafel curve analysis, and electrochemical impedance spectroscopy (EIS) results showed that the as-synthesized MoTe2 exhibited good electrocatalytic properties and a low charge transfer resistance at the electrolyte-electrode interface. The optimized MoTe2 CE revealed a high power conversion efficiency of 7.25% under a simulated solar illumination of 100 mW cm−2 (AM 1.5), which was comparable to the 8.15% observed for a DSSC with a Pt CE. The low cost and good electrocatalytic properties of MoTe2 thin films make them as an alternative CE for DSSCs.

CuO and CuO/Graphene Nanostructured Thin Films as Counter Electrodes for Pt-Free Dye-Sensitized Solar Cells

Copper oxide (CuO) and CuO/graphene nanostructured thin films were used as counter electrodes (CEs) for dye-sensitized solar cells (DSSCs). CuO and CuO/graphene pastes were prepared and coated on fluorine-doped tin oxide (FTO) glass substrates using a doctor-blade coating method. The substrates were then sintered at 350 °C for 30 min to form CuO and CuO/graphene nanostructures. The material properties of the CuO and CuO/graphene CEs were analyzed using a scanning electron microscope, transmission electron microscope, energy-dispersive spectrometer, thermogravimetric analysis instrument, X-ray diffractometer, Raman spectroscopy, X-ray photoelectron spectrometer, ultraviolet-visible spectrophotometer, and cyclic voltammetry instrument. The CuO and CuO/graphene CEs were used to fabricate DSSCs, and the device characteristics were analyzed using current density–voltage, incident photo-to-current conversion efficiency, and electrochemical impedance spectroscopy measurements. The results showed that when CuO and CuO/graphene were used as the CEs, the device conversion efficiencies were 2.73% and 3.40%, respectively. CuO is a favorable replacement for expensive platinum (Pt) because it features a simple fabrication process and is inexpensive and abundant. Furthermore, graphene, which exhibits high carrier mobility, may be added to enhance the electrical and catalytic abilities of CuO/graphene CEs. This is the first study to examine the use of CuO and CuO/graphene for developing Pt-free CEs in DSSCs.

Titania Nanobelts as a Scattering Layer with Cu2ZnSnS4 as a Counter Electrode for DSSC with Improved Efficiency

Various nanostructures of Titania (TiO2) play a significant role in the performance of dye sensitized solar cells (DSSCs) and affect the overall light harvesting efficiency of the cells. In this article we present the one dimensional nanostructure of TiO2(nanobelts) can increase the light scattering effect, light harvesting effect and electron transport in the DSSC to improve its performance. Pure anatase TiO2 nanobelts were synthesized by a hydrothermal method using commercial material (P25) and characterized by XRD and SEM. Cu2ZnSnS4 (CZTS) nanoparticles have been prepared by thermal decomposition method used as counter electrode material instead of platinum. DSSC have been fabricated by using a new type of double layered photoanode, which was prepared and optimized by using TiO2 nanoparticles as the main layer and TiO2 nanobelts as the over-layer (scattering layer). The photoelectric conversion efficiency (PCE) of the prepared cells was measured by solar simulator under 1 sun illumination (100 mW/cm2, AM 1.5). The DSSCs with CZTS nanoparticles based counter electrodes without scattering layer relatively low PCE of 5.76 %. However, the PCE was clearly improved with scattering layer and a maximum PCE of 7.85% was achieved.

Self-Assembled Cauliflower-Like CuS/ZnS@rGO Electrocatalyst with Cuprous Oxide as a Sacrificial Template for Dye-Sensitized Solar Cells with Excellent Photoelectric Properties and Electrochemical Stability

A cauliflower-like CuS/ZnS@rGO nanocomposite was selectively synthesized via a facile hydrothermal reduction method using Cu2O nanocrystals as the sacrificial template. Using this nanocomposite as the counter electrode (CE) in a dye-sensitized solar cell (DSSC) yielded excellent performance with a high-power conversion efficiency (PCE) of 8.45% and outstanding electrochemical stability. Experimental measurements confirmed that the CuS/ZnS heterojunction was uniformly inlaid on the reduced graphene oxide (rGO). This arrangement helped enhance the synergetic effect of the heterojunction, thereby avoiding the frequent recombination of electron-hole pairs to increase the efficient use of electrons, as confirmed by the behavior of the semiconductor. Furthermore, rGO dispersed the active sites and facilitated the transfer of electrons. As the CE of a DSSC, compared with CuS and ZnS CEs, the rGO@CZS composite dramatically enhanced performance and provided outstanding electrochemical stability. Hence, the results indicate that our material could be utilized as the all-important CE of DSSCs.

A 3D architecture composite of porous vanadium nitride nanoribbons and reduced graphene oxide as a high-efficiency counter electrode for dye-sensitized solar cells.

A three-dimensional (3D) porous architecture combining porous vanadium nitride nanoribbons with reduced graphene oxide was prepared through a hydrothermal process and subsequent thermal annealing in an ammonia/argon mixed atmosphere. Then, the obtained 3D porous vanadium nitride nanoribbon/reduced graphene oxide (PVNN/RGO) composite was explored as the counter electrode of dye-sensitized solar cells (DSCs). As evidenced by the electrochemical measurements, the 3D PVNN/RGO composite demonstrates excellent electrocatalytic performance, which is comparable to that of Pt. This can be attributed to the fact that the 3D architecture composite of porous vanadium nitride and reduced graphene oxide can simultaneously provide a favorable electrolyte diffusion channel, a fast electron-transport network, and an abundance of efficient electrocatalytic active sites. By employing such PVNN/RGO composite as the counter electrode, the fabricated DSC can achieve a conversion efficiency of 7.43%, which is comparable to that of the conventional Pt counter electrode (7.74%). Therefore, the 3D PVNN/RGO composite is a promising low-cost alternative to the expensive Pt as a counter electrode in DSCs.

Economic synthesis of Co3S4 ultrathin nanosheet/reduced graphene oxide composites and their application as an efficient counter electrode for dye-sensitized solar cells

Despite cobalt sulfide nanosheets gain great attention for their excellent electrochemical performance, the lack of economic and scalable synthesis method limits their applications. Herein, a simple and cost-effective noninjection hydrothermal process is proposed to synthesize Co3S4 ultrathin nanosheets on the surface of reduced graphene oxide (rGO) by controlling the polarity of the reaction solution and introducing NH3. Experimental and theory results reveal that the obtained Co3S4 nanosheets have higher electrochemical properties for the reduction of I3− than bulk due to their larger density of electron states in the nearby of Fermi level facilitating fast electron-transfer and providing highly active sites for catalytic reaction. With the synergistic effect of rGO, electrocatalytic performances of Co3S4 nanosheets can be further optimized. The dye-sensitized solar cell (DSSC) device with Co3S4 nanosheet/rGO as counter electrode (CE) material exhibits a power conversion efficiency (PCE) of up to 8.08%, which is superior to that of Co3S4 nanosheet (7.13%) and Pt (7.62%) at the same measurement condition. Moreover, Co3S4 nanosheet/rGO composites show enhanced cycling stability, indicating the great possibility of them for replacing Pt as an advanced CE in DSSCs.

Atmospheric-pressure plasma jet processed Pt/ZnO composites and its application as counter-electrodes for dye-sensitized solar cells

Nitrogen dc-pulse atmospheric pressure plasma jet (APPJ) is used to fabricate Pt/ZnO composites as the counter electrodes (CEs) of dye-sensitized solar cells (DSSCs). Due to the synergetic effect of the reactive plasma species and heat in nitrogen APPJ, the spin-coated precursors including chloroplatinic acid and zinc acetate can be reduced on fluorine-doped tin oxide (FTO) glass substrates in a few seconds. X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) analyses indicate that the precursors are reduced to Pt/ZnO under APPJ calcination. Electrochemical impedance spectroscopy (EIS) and Tafel measurement show the promising catalytic activities of Pt/ZnO CEs with low charge transfer resistance and high exchange current density. The efficiency of a DSSC with a 30-s APPJ-calcined Pt/ZnO CE is similar to that with a conventional furnace-annealed Pt CE for 15 min. The results indicate that nitrogen dc-pulse APPJ treatment is an efficient tool for rapidly fabricating Pt/ZnO composite CEs of DSSCs.

In Situ Self-Assembly-Generated 3D Hierarchical Co3O4 Micro/Nanomaterial Series: Selective Synthesis, Morphological Control, and Energy Applications

A simple in situ self-assembly selective synthetic strategy for one-step controllable formation of various three-dimensional (3D) hierarchical Co3O4 micro/nanomaterials with peculiar morphologies, uniform size, and high quality is successfully developed. The morphological control and related impact factors are investigated and clarified in detail. The results further clarify the corresponding mechanisms on the reaction process, product generation, and calcining process as well as the formation of specific morphologies. Furthermore, the superior catalytic properties of these materials are confirmed by two typical Co-based energy applications on the decomposition of an important solid rocket propellant, ammonium perchlorate (AP), and dye-sensitized solar cells (DSSCs). The addition of Co3O4 materials to AP obviously decreases the decomposition temperatures by about 118-140 °C and increases the exothermic heat to a great extent. As the substituted counter electrodes of DSSCs, the 3D hierarchical Co3O4 materials exhibit attractive photovoltaic performances. These findings provide a facile and effective way for designing new types of 3D hierarchical materials toward high catalytic activity for energy devices.

Prickly polyaniline nano/microstructures as the efficient counter electrode materials for dye-sensitized solar cells

Two kinds of prickly polyaniline samples, prickly polyaniline nanorods and microgranules, are prepared through the chemically oxidative polymerization method by regulating the concentration of aniline. Scanning electron microscopy images indicate that the diameter of prickly polyaniline nanorods (PPNRs) is about 80 nm and the size of prickly polyaniline microgranules (PPMGs) is about 400 nm. The as-prepared prickly polyaniline samples are subsequently explored as the Pt-free counter electrode materials for dye-sensitized solar cells (DSCs). Electrochemical impedance spectroscopy and cyclic voltammetry measurements demonstrate that PPNR electrode displays superior electrocatalytic activity for the I3 − reduction reaction to PPMG electrode, which can be attributed to the unique prickly nanorod structure that provides abundant electrocatalytic active sites and the fast charge transport pathway simultaneously. As a consequence, the DSC fabricated with PPNR counter electrode achieves a high conversion efficiency of 6.86% under illumination of 100 mW cm−2, which is close to the efficiency of a Pt electrode-based device. This work presents a promising way to develop Pt-free and high-efficiency counter electrode in DSCs.

Electrochemical Exfoliation of Graphene Flake Embedded in SiNWs as Counter Electrode for Dye-Sensitized Solar Cells

In this paper, a three-dimensional graphene wrapped silicon nanowire (SiNWs) architecture was synthesized by electrochemical exfoliation method and proposed to be the counter electrode for dye-sensitized solar cells (DSSCs). The results show that the few high-quality layers of graphene sheets have been obtained by electrochemical exfoliation method. The size of graphene is distributed from 550[Formula: see text]nm to 650[Formula: see text]nm, appropriately dispersed onto the SiNW supporter. The graphene/SiNW nanostructure still vertically aligned to the substrate finely. The channel between each wire is clear. The fabricated graphene/SiNWs electrode is a promising structure for future applications. The performance of the graphene/SiNWs as the counter electrode is found to be dependent on its dispersion in the whole backbone, with better dispersion offering more surface areas for the catalytic reduction reaction. Electrochemical tests reveal that the DSSC with graphene/SiNWs exhibits higher electro-catalytic activity and lower charge transfer resistance, suggesting that the 3D structure presents a potential way to fabricate low-cost, integrated and metal-free counter electrode for high-performance DSSCs. The DSSC based on graphene/SiNWs has an open-circuit voltage ([Formula: see text]) of 738.11[Formula: see text]mV, short-circuit current density ([Formula: see text]) of 15.48[Formula: see text]mA cm[Formula: see text], fill factor (FF) of 0.67 and conversion efficiency ([Formula: see text]) of 7.66%.

Influence of polyaniline in polyaniline-tin oxide nanocomposite as counter electrode for dye sensitized solar cells

Polyaniline (PANI) was synthesized in its emeraldine form by the oxidation of aniline with ammonia persulphate (APS). The coprecipitation technique was employed for the synthesis of PANI-SnO2 nanocomposite using prepared PANI, tin chloride dihydrate and ammonia solution precursors. The nanocomposite samples consisting of different weight percentage of PANI were prepared. The morphological features of the nanocomposites were analyzed from its Scanning Electron Microscopy (SEM) images. The crystalline size and structural properties were deciphered from the X-ray Diffraction (XRD) results of the samples. The Energy Dispersive X-ray Analyses (EDAX) of the samples provided the information on elemental composition. Furthermore, UV–vis spectroscopy was used to analyze the optical properties while the Electrochemical Impedance Spectroscopy (EIS) was used for the study of electrochemical properties of the samples, which have revealed that PANI-SnO2 nanocomposite can act as a suitable counter electrode in dye sensitized solar cells (DSSCs). Thus, this nanocomposite can be a cheaper replacement to the more expensive and commercialized platinum counter electrodes in DSSCs.

Selenization of Cu2ZnSnS4 Enhanced the Performance of Dye-Sensitized Solar Cells: Improved Zinc-Site Catalytic Activity for I3.

Cu2ZnSnS4 (CZTS) and Cu2ZnSn(S,Se)4 (CZTSSe) as promising photovoltaic materials have drawn much attention because they are environmentally benign and earth-abundant elements. In this work, the monodispersed, low-cost Cu2ZnSnS4 nanocrystals with small size have been controllably synthesized via a wet chemical routine. And CZTSSe could be easily prepared after selenization of CZTS. When they are employed as counter electrodes (CEs) for dye-sensitized solar cells (DSSCs), the power conversion efficiency (PCE) has been improved from 3.54% to 7.13% as CZTS is converted to CZTSSe, which is also compared to that of Pt (7.62%). The exact reason for the enhanced catalytic activity of I3- is discussed with the work function and density functional theory (DFT) when CZTSSe converted from CZTS. The results of a Kelvin probe suggest that the work function of CZTSSe (5.61 eV) is closer to that of Pt (5.65 eV) and higher than that of CZTS, which matched the redox shuttle potential better. According to the theory calculation, all the atomic and bond populations changed significantly when Se replaced partly the S on the CZTS system, especially in the Zn site. During the catalytic process as CEs, the adsorption energy obviously increased compared to those at other sites when I3- adsorbed on the Zn site in CZTSSe. So, Zn plays an important role for the reduction of I3- after CZTS is converted to CZTSSe. Based on above analysis, the reason for enhanced performance of DSSCs when CZTS converted to CZTSSe is mainly due to the enhancement of Zn-site activity. This work is beneficial for understanding the catalytic reaction mechanism of CZTS(Se) as CEs of DSSCs.

Hydrothermal synthesis of the composited WS2–W5O14–MWCNTs for high performance dye-sensitized solar cell counter electrodes

WS2 and W18O49/W5O14 (WS 2–W 18O 49/W 5O 14) powders were synthesized using a hydrothermal reaction of microsized WO3 particles in a HCl + Na2S·9·H2O solution. The resulting WS 2–W 18O 49/W 5O 14 powders were used as a catalyst in a counter electrode of a dye-sensitized solar cell with an efficiency of 3.37%. To enhance the DSSC’s performance, multi-walled carbon nanotubes (MWCNTs) was added to the aforementioned hydrothermal reaction producing WS 2–W 5O 14–MWCNTs composite, which can enhance an efficiency of up to 7.44%, comparable to that of Pt DSSCs (7.53%). This outstanding performance of the WS 2–W 5O 14–MWCNTs based DSSCs is attributed to high electrocatalytic activity and low charge-transfer resistance at the counter electrode and electrolyte interface at the WS 2–W 5O 14–MWCNTs electrode. This facile and cost-effective fabrication of the composited WS 2–W 5O 14–MWCNTs CE could lead to applications in commercial DSSC devices.

Effect of Reaction Period on Stoichiometry, Phase Purity, and Morphology of Hydrothermally Synthesized Cu2NiSnS4 Nanopowder

The effect of reaction period on the phase purity, morphology, and stoichiometry of Cu2NiSnS4 (CNTS) nanopowder prepared by hydrothermal method has been investigated. Polyvinylpyrrolidone (PVP) and thioglycolic acid were used as capping agent and sulfur source, respectively. The presence of cubic stannite crystal structure and its phase purity were confirmed by powder x-ray diffraction analysis and Raman spectroscopy. Furthermore, the morphological, crystallographic, and optical features of the prepared CNTS nanopowder were characterized by field-emission scanning electron microscopy, transmission electron microscopy, and ultraviolet–visible (UV–Vis) spectrophotometry. The elemental ratios of Cu/(Ni + Sn) and Ni/Sn showed that the stoichiometry of CNTS was maintained for the compounds synthesized at 230°C with reaction period of 24 h. The occurrence of Cu+, Ni2+, Sn4+, and S2− was evaluated by x-ray photoelectron spectroscopy. The prepared material was used as counter electrode in a dye-sensitized solar cell (DSSC) as an alternative to platinum (Pt), resulting in conversion efficiency of 0.92%. These results indicate that CNTS is a prospective material to replace conventional Pt-based counter electrodes in DSSCs.

Graphene oxide induced fabrication of pillared and double-faced polyaniline arrays with enhanced triiodide reduction capability

Counter electrode (CE) materials, featuring low cost, high electrical conductivity and electrocatalytic activity, are one of the key points to achieve practical application of dye-sensitized solar cells (DSSCs). To tackle with this issue, the pillared and double-faced polyaniline (PANI) nanocone arrays induced by graphene oxide (GO) (PANI/GO) are configured through in-situ polymerization, and the electrochemical behavior as the low-cost CE for DSSCs is revealed. The GO sheets adopted as a substrate and structure-directing agent could effectively induce the uniform and vertical growth of PANI arrays, thus producing a free and open networks. This unique structure is further capable of shortening the ionic diffusion path and providing the large contactable area to the electrolyte. And, a high power conversion efficiency of 8.19±0.08% can be achieved, being superior to those of Pt and PANI references. Moreover, the as-made PANI/GO CE also delivers an outstanding electrochemical stability. The present strategy provides a simple yet efficient method to engineer the high-efficiency and fast mass-transport catalyst for DSSCs and electrode materials for other energy storage/conversion.

Electrochemical deposition of Pt-Ni on reduced graphene oxide as counter electrode material for dye-sensitized solar cell

Abstract In dye-sensitized solar cell (DSSC) alloys of Pt with other metals or replacement of it with Pt-free alloys can be used to reduce the fabrication cost. Herein, we present the formation of Pt-Ni alloy/reduced graphene oxide (Pt-Ni/RGO) counter electrode (CE) by a simple and environmental-friendly electrochemical deposition method for fabrication of DSSC. First, RGO/FTO CE was prepared through electrophoretic deposition (EPD) method and then reduced electrochemically. Then, Pt-Ni alloy was loaded on CE by cyclic voltammetry (CV) strategy. The morphology of the Pt-Ni/RGO film was characterized by field emission scanning electron microscopy (FESEM) analysis. Pt-Ni nanoparticles were uniformly dispersed on the surface of RGO after electrodeposition. The Pt-Ni/RGO CE showed remarkably enhanced electrocatalytic activities toward triiodide (I3−) reduction reaction in comparable to RGO CE. CV and electrochemical impedance spectroscopy (EIS) confirmed that the Pt-Ni/RGO CE has superior electrocatalytic activity than that of RGO and Pt-Ni CEs. The enhanced electrocatalytic activities can be in association with the synergistic effect of transition metals. As a result, an efficiency of 6.14% with Jsc = 14.08 mA cm−2, Voc = 749 mV, and FF = 0.58 was achieved for DSSC assembled with the Pt-Ni/RGO CE under AM 1.5 illumination of 100 mW cm−2. Thus, the fabricated Pt-Ni/RGO CE could be used as a promising material for a low-cost CE for DSSC.

Photovoltaic Analysis of Platinum Counter Electrode Modified by Graphene Oxide and Magnetic Beads for Dye-Sensitized Solar Cell

Platinum (Pt) is widely used for counter electrode of dye-sensitized solar cell (DSSC). In order to improve the electro-catalytic ability of Pt counter electrode for DSSC, the graphene oxide (GO) and magnetic beads (MBs) were added into titanium dioxide (TiO2) colloid to modifying Pt-counter electrode. GO with high specific surface area could be used as the electrocatalyst for triiodide reduction in DSSC. Because of its high charge carrier mobility and photocatalytic activity, MBs could enhance the photovoltaic properties of DSSC. Moreover, the effects of Pt counter electrode modified by GO and MBs for DSSC were investigated by X-ray diffraction, solar simulator, and electrochemical impedance spectroscopy. Compared with the photovoltaic conversion efficiency (PCE) of 2.31% of the conventional DSSC using Pt counter electrode, the catalyst properties of Pt counter electrode with GO and MBs had been improved and the PCE achieved 3.62%.

Prussian Blue‐Derived Synthesis of Hollow Porous Iron Pyrite Nanoparticles as Platinum‐Free Counter Electrodes for Highly Efficient Dye‐Sensitized Solar Cells

Iron pyrite has long been an attractive material for environmental and energy applications, but is hampered by a lack of control over morphology and purity. Hollow porous iron pyrite nanoparticles were synthesized by a direct sulfurization of iron oxide derived from Prussian blue. The high efficiencies of these hollow porous iron pyrite nanoparticles as effective dye-sensitized solar cell counter electrodes were demonstrated, with an efficiency of 7.31 %.