Next Article in Journal
Magnetism and Thermal Transport of Exchange-Spring-Coupled La2/3Sr1/3MnO3/La2MnCoO6 Superlattices with Perpendicular Magnetic Anisotropy
Previous Article in Journal
Design and Fabrication of High Performance InGaAs near Infrared Photodetector
Previous Article in Special Issue
Solvothermally Synthesized Hierarchical Aggregates of Anatase TiO2 Nanoribbons/Nanosheets and Their Photocatalytic–Photocurrent Activities
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Nanomaterials
Volume 13
Issue 21
10.3390/nano13212896
nanomaterials-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Nanograss-Assembled NiCo2S4 as an Efficient Platinum-Free Counter Electrode for Dye-Sensitized Solar Cell

by
Shada A. Alsharif
University College of Umlij, University of Tabuk, Tabuk 71491, Saudi Arabia
Nanomaterials 2023, 13(21), 2896; https://doi.org/10.3390/nano13212896
Submission received: 2 October 2023 / Revised: 26 October 2023 / Accepted: 30 October 2023 / Published: 2 November 2023
(This article belongs to the Special Issue Nanomaterials for Energy Applications: Materials Synthesis, Device Fabrication, and Characterization)
Browse Figures
Versions Notes

Abstract

:
Dye-sensitized solar cells (DSSCs) are often viewed as the potential future of photovoltaic systems and have garnered significant attention in solar energy research. In this groundbreaking research, we introduced a novel solvothermal method to fabricate a unique “grass-like” pattern on fluorine-doped tin oxide glass (FTO), specifically designed for use as a counter electrode in dye-sensitized solar cell (DSSC) assemblies. Through rigorous structural and morphological evaluations, we ascertained the successful deposition of nickel cobalt sulfide (NCS) on the FTO surface, exhibiting the desired grass-like morphology. Electrocatalytic performance assessment of the developed NCS-1 showed results that intriguingly rivaled those of the acclaimed platinum catalyst, especially during the conversion of I3 to I− as observed through cyclic voltammetry. Remarkably, when integrated into a solar cell assembly, both NCS-1 and NCS-2 electrodes exhibited encouraging power conversion efficiencies of 6.60% and 6.29%, respectively. These results become particularly noteworthy when compared to the 7.19% efficiency of a conventional Pt-based electrode under similar testing conditions. Central to the performance of the NCS-1 and NCS-2 electrodes is their unique thin and sharp grass-like morphology. This structure, vividly showcased through scanning electron microscopy, provides a vast surface area and an abundance of catalytic sites, pivotal for the catalytic reactions involving the electrolytes in DSSCs. In summation, given their innovative synthesis approach, affordability, and remarkable electrocatalytic attributes, the newly developed NCS counter electrodes stand out as potent contenders in future dye-sensitized solar cell applications.

1. Introduction

The rising consumption of fossil fuel-based energy has recently prompted considerable research into sustainable alternatives for future energy requirements [1,2,3,4,5]. Consequently, scientists have been delving into harnessing solar power by employing diverse photovoltaic devices which are capable of covering maximum solar spectrum and converting solar light to real electricity at low cost [4,5,6,7]. The dye-sensitized solar cells (DSSCs) have gained prominence owing to their cost-effectiveness, straight forward assembly method, and flexibility in tweaking their components. DSSCs represent a third-generation photovoltaic technology that offers advantages such as flexibility, potential cost-effectiveness, and simple fabrication processes compared to traditional silicon-based solar cells. The photoanode is essentially the heart of a DSSC [7,8,9,10]. It is responsible for absorbing sunlight and converting it into an electrical current. Here is how: The dye molecules are attached to the semiconductor material, typically titanium oxide or zinc oxide. When these dye molecules are hit by photons from sunlight, they become excited and inject electrons into the semiconductor material [2,10,11,12]. TiO2 is more commonly used due to its excellent stability, high refractive index, and strong bonding with dye molecules. However, ZnO has a faster electron mobility, which can potentially lead to better efficiency under certain conditions. The counter electrode, typically made of platinum, plays a critical role in the operation of the DSSC: Electron Collection. Once the electrons have travelled through the external circuit, generating electricity, they re-enter the cell at the counter electrode. It is the job of this electrode to collect these electrons and send them back to the electrolyte, completing the circuit. Platinum facilitates the reduction of the tri-iodide ions (I3−) back to iodide ions (I−) in the electrolyte [5,6,7,8,9]. This process is essential for the regeneration of the dye and the continuation of the electrical current. Platinum is chosen for this role primarily due to its exceptional conductivity, vast number of active sites for the redox reaction, and ease of deposition on conductive substrates. Its chemical stability also ensures a longer lifespan for the DSSC [12,13,14,15]. This is essentially the electrolyte solution in a DSSC. Here is its role: Once the dye has been excited by sunlight and has injected an electron into the TiO2 or ZnO, it is left in an oxidized state. To get back to its ground state and be ready to absorb more photons, it needs an electron. This electron is provided by the iodide ion (I−), which then becomes oxidized [1,2,3,4,10,11,12]. The oxidized iodide ions (usually in the form of I3−) then travel to the counter electrode, where they are reduced back to their original form, with the help of electrons that have come through the external circuit. The synchronized operation of the photoanode, platinum counter electrode, and redox mediator ensures efficient photon-to-electron conversion in a DSSC, offering a promising alternative to conventional silicon-based solar cells [15,16,17,18,19,20]. Nevertheless, despite platinum’s myriad benefits, challenges/difficulties like its susceptibility to corrosion during the redox process, high cost, and limited availability hinder its expensive use in DSSCs. This has sparked initiatives to unearth alternatives to Pt for counter electrode in DSSCs, including metal oxide, bimetallic oxides, sulfide, bimetallic sulfide, and metal nitrides, metal–polymer nanohybrids, etc. Notably, bimetallic sulfide, especially nickel- and cobalt-based bimetallic sulfide, has generated interest in DSSCs counter electrode application due to its easy preparation, low cost, uncomplicated synthesis, high conductivity, high surface area, large number of catalytic sites, and pronounced catalytic activity for the redox reaction during the electricity conversion process.
Pt has long been recognized as a benchmark catalyst due to its facile synthesis, ample catalytic sites, and robust conductivity [18,19,20,21,22,23]. However, despite its numerous merits over other metal catalysts, challenges such as its significant operational cost, rapid corrosion during redox reaction, and scarcity hinder its expensive deployment in solar cell application. In recent years, there has been a proliferation of research aimed at identifying suitable replacements for Pt, with a focus on both their cost-effectiveness and efficiency. Consequently, the quest for viable alternatives to expensive Pt-based counter electrodes has driven research towards diverse materials like metal and its binary structure of nitride, sulfide, oxide sulfide, metal oxides, and different types of nanohybrids for use in DSSCs [23,24,25]. Particularly, ternary metal sulfides, especially NiCoS4, have emerged as frontrunners for counter electrode applications. Their appeal stems from attributes like exceptional electrical conductivity surpassing both similar types of the metal sulfide and metal oxide merged in a ternary framework with commendable balance of the mechanical and thermal stabilities and superior properties including electrochemical and catalytic performances, a credit towards the presence of the two cations of the metal contribution in the cobalt and nickel framework [20,21,22,23,24,25,26,27]. The increased attention on NiCoS4 has seen several studies exploring different fabrication techniques and their efficiencies. In various works, DSSCs equipped with NiCoS4 counter electrodes have displayed efficiencies ranging from 4.5% to 7%, highlighting its potential as a viable alternative to Pt [28,29,30]. A notable method that has been previously employed is the hydrothermal method, which, while effective, has its own set of challenges and benefits [29,30]. The promise of NiCo2S4 has catalyzed the development of varied fabrication techniques for counter electrodes on FTO glass substrates, each bearing its unique pros and cons concerning cost, fabrication complexity, and environmental impact: for instance, the NiCoS4-based electrode prepared by the sulfide–argon gas blend to deposit the NiCoS4 for DSSCs application [15,16,17,18,19]. As well as the Pt–NiCoS4-based counter electrode prepared by the sputtering and pyrolysis method and used as a counter electrode in DSSCs. Given various backdrops, streamlining counter electrode fabrication by utilizing simple reactants and a solitary process is vital for large scale production [26,27,28,29,30,31]. Hence, an approach that simplifies synthesis, eschewing complex chemicals and multistep procedures could offer notable benefits for directly growing NiCoS4 on FTO glass substrate, minimizing potential film detachment issues. This research, therefore, zeroes in on the direct cultivation of NiCoS4 on fluorine-doped coated glass using a simplified, singular solvothermal process, augmented by the addition of surfactant guiding agent. Post fabrication, the prepared electrode underwent rigorous characterization through various spectroscopic and microscopic methodologies, followed by integration and testing within a DSSC setup. Preliminary findings indicate that the fabricated counter electrode boasts a power conversion efficiency of 6.60%, a figure that commendably parallels the 7.19% efficiency of its Pt counterpart in a similar DSSC configuration.

2. Materials and Methods

2.1. Materials

Hexa-methylene tetramine, 1-propyl-3-methylimidazolium iodide, iodine, 4-tert-butylpyridine, lithium iodide, cis-bis (isothiocyanato)-bis(2,20-bipyridyl-4,40-dicarboxylato)-ruthenium (II) bis-trabutylammonium, and acetonitrile were sourced from Sigma-Aldrich (St. Louis, MO, USA). The cobalt nitrate, thiourea, detergent, and nickel nitrate were acquired from Sigma-Aldrich. The TiO2 paste was procured from Dyesol Industries Pty. Ltd., Sydney, Australia. The fluorine-doped tin oxide (FTO) was sourced from NSG Japan (Tokyo, Japan).

2.2. Methods

To investigate the structure and surface design of the NCS-1 and NCS-2 layered over the FTO, field emission scanning electron microscopy (JEOL-JSM-6700, JEOL, Akishima, Japan) and X-ray diffraction (Bruker, Mannheim, Germany) were performed. The chemical stated and its composition examined through the Thermo Scientific X-ray photoelectron spectroscopy (Thermo Scientific, Waltham, MA, USA). Artificial solar illumination was generated using a calibrated solar simulator (Oriel Sol 3A, Newport, Irvine, CA, USA) equipped with a Xenon lamp. The source meter (Keithley-2400, Keithley, Cleveland, OH, USA) integrated with specialized software was utilized to check the current density versus voltage under dark and illumination condition using Newport sola simulator of the developed DSSC cell. A mixture of the 0.6 M 1-propyl-3-methylimidazolium iodide, 0.1 M lithium iodide, 0.03 M iodine, and 0.05 M 4-tert-butylpyridine was prepared in acetonitrile as a solvent. The counter electrodes we prepared, specifically NCS-1 and NCS-2, were subjected to rigorous cyclic voltammetry testing. For these tests, the scan rate was set at 50 mV/s. Within the experimental setup, NCS-1 and NCS-2 were utilized as the working electrodes, while a platinum (Pt) wire served as the counter electrode. It is essential to note that the choice of Pt wire as the counter electrode offers a reliable and consistent standard for comparison, given its widespread acceptance in electrochemical analysis. All electrochemical measurements, including potential sweeps and current recordings, were conducted using a Metrohm Autolab electrochemical workstation (Utrecht, The Netherlands). This workstation, equipped with a potentiostat, ensured precision and accuracy in capturing the electrochemical behavior of the prepared counter electrodes.

2.3. Material Synthesis

2.3.1. Counter Electrode Fabrication on FTO-Coated Glass

The FTO underwent an ultrasonic cleaning process involving a sequence of detergent, ethanol, and water. Following this, precise quantities of the nickel nitrate (0.1 M) and cobalt nitrate (0.2 M) were dissolved in DI water and ethanol. The above reaction solution was further stirred for 20 min until all the precursor mixed well. The directing agent of hexa-methylene tetramine and thiourea (SC(NH2)2) was later added to the solution. This mixture was then placed into a Teflon-lined autoclave, with the FTO glass positioned strategically inside (in such way that the conductive surface faced downward). Following which was heat treatment at 170 °C in an electric oven for 12 h. The reaction vessel was cooled down to room temperature naturally and after that the sample was collected, rinsed with water and ethanol to remove the unreacted precursor part, and dried at 60 °C in oven overnight. The NiCo2S4 synthesized over FTO abbreviated as NSC-1 and NCS-2 depends on the concentration of the hexa-methylene tetramine (0.05 and 0.1 M) directing agent. The suggested formation mechanism could be written as follows.
Ni(NO3)2 + Co(NO3)2 + EtOH/H2O → Ni2+ + Co2+ + OH−
2Co(NO3)2 + Ni(NO3)2 + 4S → NiCo2S4 + 6NO2 + 2O2
SC(NH2)2 → NH2CN + H2S
Ni2+ + Co2+ + OH− → Ni-Co2OOH + H2O
Ni-Co2OOH + H2S → NiCo2S4 + H2O

2.3.2. Preparation of Photoanode and DSSC Assembly

Commercial FTO-coated glass was meticulously cleaned using an ultrasonication process. Subsequently, this glass was layered with TiO2 paste via sharp blades. To form a TiO2 compact layer, the glass was dipped into a TiF4 solution and treated at 70 °C for 30 min. This is followed by heating the glass at 450 °C for an hour to enhance the crystallinity of the TiO2 layer. Before applying the TiO2 paste, unwanted areas on the glass are shielded using 3 M magic tape. The coated FTO glass with TiO2 was then dipped in a dye solution for 12 h at 40 °C. The dye-soaked electrode was then paired with a counter electrode, sealed, and finally filled with the formulated electrolytes. The film undergoes a post-treatment in a TiF4 solution, identical to the initial pretreatment step. The treated electrodes are then immersed in an N719 dye solution in absolute ethanol. This is left at room temperature overnight in a dark environment. Visualize a DSSC as a composite system comprising two electrically conductive electrodes, sandwiching an electrolyte with a redox duo, specifically iodide/triiodide ions. These electrodes rest on glass layers coated with a transparent conductive oxide. The operational electrode features a TiO2 layer that is sensitized using specific dyes.

3. Results and Discussion

3.1. Formation Mechanism

The specific pathway of formation for these materials remains somewhat elusive, yet several factors, such as the nature of the precursor, the temperature, and the formation of complexes, play undeniable roles in determining the size, shape, and phase of the resulting materials. At the onset, under solvothermal conditions, cobalt and nickel ions engage in complex formation with thiourea, leading to the generation of rudimentary NiCo2S4 particles, as illustrated in Figure 1. These primary particles coalesce, adopting a grass-like nanostructure. It is theorized that as the process progresses, nanoparticles tend to gravitate around gas bubbles, giving rise to the initial stages of nanocluster formation. These emerging clusters, influenced by the inherent magnetic interactions among particles, begin to elongate, forming string-like aggregates. These elongated aggregates subsequently serve as platforms or ‘nucleation sites’ where further accretion occurs, eventually leading to the development of nanosheets. Due to their intrinsically elevated surface energy, these nanosheets tend to attract each other, culminating in intricate, reminiscent structures. On a related note, upon increasing the temperature, thiourea is subject to hydrolysis. This chemical breakdown results in the liberation of CO2 and NH3 gases. Intriguingly, the evolving gas bubbles appear to act as templates or molds, playing a pivotal role in dictating the final morphology of the synthesized materials. This templating effect of gas bubbles offers a potential avenue to control the shape and structure of materials in future synthetic endeavors.

3.2. Structural and Morphological Studies

The crystallinity and structural integrity of NCS-1 and NCS-2 when deposited on the FTO were rigorously evaluated using X-ray diffraction (XRD) techniques. The results are visualized in Figure 2. Delving into this figure allows for a juxtaposition between the XRD patterns of the pristine FTO and those of FTO when embellished with NCS-1 and NCS-2. Distinctive diffraction peaks emerge at 2θ values of 26.81, 31.57, 38.0, 32.0, and 55.30°. These peaks are not arbitrary; they correspond with crystallographic planes denoted as (111), (220), (311), (400), (422), (511), (440), (533), and (444). A deeper dive into the literature references, particularly those indexed by JCPDS (Card No: 20-0782), aligns these peaks and planes with the inherent cubic structure characteristic of both NCS-1 and NCS-2. In essence, these congruencies between observed diffraction peaks and the known crystalline planes for this compound type serve as solid indicators. They reinforce the notion that the NCS-1 and NCS-2 were indeed successfully anchored onto the FTO substrate. Furthermore, the consistency of the patterns suggests a homogenous and well-ordered crystalline deposition, which is crucial for effective material performance.
The intricate details of the surface morphology of the synthesized NCS-1 electrode were examined using field-emission scanning electron microscopy (FESEM), and the compelling visuals can be seen in Figure 3a–d. The morphology, which intricately influences the catalytic action and surface properties of materials, carries profound significance for the overall performance of the electrode. Upon examining Figure 3, one is immediately drawn to the unique, slender grass-like structures of NCS-1. These structures are not isolated; they appear to intertwine and link at various junctions across the surface. A closer, more discerning look emphasizes that these grass-like protrusions are not only interconnected but also maintain a level of structural cohesion. This interlinked framework augments the electrode’s conductive behavior. Essentially, the intertwined nature ensures a continuous path for electron transport, reducing resistances and enhancing conductivity, a factor that is pivotal for efficient electrode performance. Furthermore, the dense and connected nature might aid in providing a larger surface area, promoting greater interaction with the surrounding medium, which can be crucial in various electrochemical applications. The scanning electron microscopy images of NCS-2 show that the structure resembling grass is not distinct (Figure 3c,d). This means that instead of individual “grass-like” structures, the segments appear to merge together. This amalgamation can be attributed to the insufficient quantity of the directing agent used during the preparation. When the directing agent is not present in adequate amounts, it cannot guide the assembly of the structures effectively. As a result, the electrode performance diminishes because clear pathways, which are crucial for efficient conductivity and electron transfer, are obstructed. In simpler terms, think of it like building a pathway in a garden. If you do not have enough guidance or material to create distinct paths, the plants might grow everywhere and block the way, making it difficult to walk through. Similarly, the electrode’s performance is compromised due to the lack of clear structural pathways.
The XPS was utilized to delve into the chemical constituents of the NCS-1 over the FTO, with the results visualized in Figure 4. Transitioning to Figure 4a, the Ni 2p spectra brings forth two distinct peaks at BE value of 852.35 eV and 870.05 eV. These can be ascribed to the Ni 2p1/2 and Ni 2p3/2 electronic state, respectively [18,19,29,31]. This 18 eV difference in the BE is consistent with the spin-orbit splitting of the Ni 2p electron level. A closer look at Figure 4b shows the peak at 777.72 eV, which can be attributed to the Co 2p3/2 core level of cobalt. Typically, a binding energy around this range suggests the presence of cobalt in an oxidized state, potentially Co(II) or Co(III), depending on the chemical environment. The peak at 793.09 eV corresponds to the Co 2p1/2 core level. The approximately 15 eV difference between the peak is consistent with the spin-orbit splitting observed for cobalt in many of its compounds [20,22,25]. Moreover, a comprehensive examination of the S 2p spectra, illustrated in Figure 4c, reveals both a principal peak and associated satellite peak at BE values of 161.76 eV, 163.05 eV, and 160.58 eV. The observed BEs in the S 2p region for the NiCo2S4 sample suggest a complex sulfur environment, with predominant sulfide character along with potential variations in the local coordination environment or surface-oxidized species. The occurrence of these peaks is indicative of both metal-sulfur and cobalt sulfur bonds within the structure. Notably, these findings align well with data presented in prior research [20,21,22,23,24,25,26,27], adding further credibility to our observation and discussion.
The efficiency and attributes of the DSSC are significantly determined by the choice of materials used in its various components, as illustrated in Figure 5a. Delving into the cyclic voltammetry results for both the Pt and NCS electrodes offers a comprehensive understanding of their electrocatalytic behaviors. In particular, these results shed light on the redox properties of the catalysts in relation to the sensitizing dyes, as seen in Figure 5b [25,26,27,28,29,30,31]. The positive potential in the cyclic voltammetry curve is indicative of the redox reaction involving I2/2I3−. Conversely, the negative potential mirrors the redox dynamics of I−/I3−. These observed phenomena can be better understood through the ensuing equation. Furthermore, the relative positions and magnitudes of these potentials provide insights into the efficiency of electron transfer and the potential for recombination. An electrode material with rapid redox reactions ensures swift electron transfer, reducing energy losses and thereby improving overall device performance [23,25,29].
2I3− = 3I2 + 2e−
3I− = I3− + 2e−
The ability of the NCS-1 at the surface of the FTO substrate to reduce the I3− to I− was analyzed using cyclic voltammetry. The graph of the Pt and NCS electrodes revealed two evident peaks representing the oxidation and reduction process that occurred during the electrocatalytic reactions. Notably, the negative pair represents the redox conversion of I−/I3− while the positive pair indicates the I2/I3− transition during electrocatalytic reaction (Figure 5b). The graph also displays that the peak current in the case of the NCS is almost the same compared to the Pt electrode which suggests that the fast reduction rate is similar for the NCS-1 electrode [12,13,14,15,16,17]. The peak-to-peak separation and related current density (Epp) can be correlated with the faster electron transfer kinetics. The NCS-1 electrode has a slightly lower Epp than the Pt electrode which suggests that enhanced kinetics led to better electrocatalytic activity towards the lithium iodide electrolytes [28,29,30,31]. The above results showed that the NCS-1 counter electrode displayed similar catalytic activity to the Pt counter electrode for the reduction of the I3− to I−. A pivotal feature is the unique grass-like pattern over the FTO substrate which provides multiple benefits such as high surface area, short pathway towards the charge transfer kinetic process, and numerous catalytic sites which lead to swift the electron transfer [30,31]. The stability of NiCo2S4 electrodes is crucial for their electrochemical applications [32,33,34,35,36,37,38]. NiCo2S4’s unique structure offers resistance to mechanical and electrochemical stresses, ensuring longevity. Its reduced volume changes during cycling enhance stability compared to binary sulfides. Additionally, when paired with the right electrolyte, it can form a stable solid–electrolyte interphase, further improving its performance. The literature also indicates that nanostructured NiCo2S4 exhibits superior durability and efficiency. Overall, our prepared NiCo2S4 electrodes show promising stability, aligning with the existing literature. The stability of the NCS-1 (Figure S1) counter electrode was thoroughly evaluated. The results revealed that the electrode remained responsive after 20 cycles, indicating its durability and consistent performance. This means that even after being used multiple times (20 cycles in this context), the electrode did not degrade or lose its effectiveness. Furthermore, this behavior of maintaining its characteristics was observed even after prolonged usage, suggesting that the electrode is not only stable but also has a longer operational lifespan. In simpler terms, just like a rechargeable battery that holds its charge even after being used many times, the NCS-1 counter electrode continues to perform well over repeated cycles, showcasing its reliability and potential for extended applications.
Switching focus to the photovoltaic attributes of all the counter electrodes (Pt@FTO, NCS-1@FTO, and NCS-2@FTO) within the dye-sensitized solar cell, a DSSC device assembly fabricated using the abovementioned counter electrode and corresponding photo current density voltage curve are represented in Figure 5c. The comparative analysis of the photo current density voltage curve infers that the NCS-1 and NCS-2 counter electrodes’ performance is comparable to the well-known Pt-based CE of the DSSC. The important parameters of the photovoltaic such as open circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF), and photoelectric conversion efficiency (PCE) are listed in Table 1.
In the assembly of DSSCs using NCS-1 and NCS-2, the observed values for Voc were 0.75 V and 0.74 V, respectively, with fill factors (FF) being 66.56 and 63.21, and current densities (Jsc) at 13.05 and 13.05 mA·cm−2. When compared to the performance of the platinum (Pt) counter electrode, which exhibited a Voc of 0.75 V, an FF of 67.68, and a Jsc of 13.98 mA·cm−2, the metrics are strikingly similar. The comparable values between NCS-1 and NCS-2 can be attributed to their analogous electrocatalytic behaviors. Specifically, both counter electrodes exhibited consistent catalytic activity in facilitating the reduction of I3− ions, leading to increased production of I− ions. This, in turn, promotes efficient regeneration of the dye, thereby enhancing the device’s ability to capture light and effectively inject electrons, optimizing the performance of the DSSC. Furthermore, these findings are corroborated by the results from cyclic voltammetry (CV) tests, reinforcing the notion of comparable catalytic activities between the two materials. The DSSCs utilizing NCS-1 and NCS-2 as counter electrodes achieved photoelectric conversion efficiencies of 6.60% and 6.29%, respectively. These values closely approach the efficiency of 7.19% achieved with the benchmark Pt counter electrode. Such performance metrics underscore the potential of NCS materials as viable alternatives to traditional Pt counter electrodes in DSSC applications, paving the way for more cost-effective and sustainable photovoltaic solutions. The table presents a comprehensive comparison of the performance metrics of various dye-sensitized solar cell electrodes (Table S1). The most frequently assessed material, NiCo2S4, displays variable performance across different studies as tabulated in Table S1. The introduction of reduced graphene oxide into the NiCo2S4 matrix seems to offer an enhanced power conversion efficiency, signifying the synergistic effect of combining materials to optimize electron transport properties. Other materials like CoNi2S4 and CuInS2 also exhibit competitive performance, underscoring the rich landscape of potential electrode materials for DSSCs. Of particular note is the achievement in the present work with NiCo2S4, which approaches the highest efficiency levels, suggesting advancements in electrode fabrication or treatment that maximize the potential of this material.

4. Conclusions

In this study, we developed a simplified solvothermal technique to grow a distinct grass-like pattern on fluorine-doped tin oxide, which was designed to act as a counter electrode in dye-sensitized solar cell assembly. The examination of the morphology and structure confirmed the successful deposition of NCS on the FTO substrate surface with grass-like morphology. The electrocatalytic results displayed that the NCS-1 showed similar catalytic behavior towards the I3− and I− conversion as compared to the Pt counter electrode. The assembled DSSC assembly with NCS-1 and NCS-2 counter electrode produced power conversion efficiency of 6.60% and 6.29% which are close to the Pt counter electrode efficiency (7.19%). The comparative efficiency of the NCS-1, NCS-2, and Pt counter electrode could be due to the thin, sharp grass-like structure. This provides a large number of catalytic sites and high surface area, as identified through scanning electron microscopy, which are essential for the electrocatalytic reactions. Given its easy fabrication, cost effectiveness, and electrocatalytic activity performance, NCS emerges as a promising counter electrode material for dye-sensitized solar cell application.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/nano13212896/s1, Figure S1: The cyclic voltammetry (CV) was used to evaluate the NCS-1 counter electrode over 20 cycles; Table S1: Comparative Analysis of DSSC Electrode Materials: NCS vs. Alternatives. Refs. [15,16,28,35,36,37,38] are cited in the supplementary materials.

Funding

This research received no external funding.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Acknowledgments

The author is thankful to University of Tabuk, Tabuk, Saudi Arabia for providing all the research facilities.

Conflicts of Interest

The author declares no conflict of interest.

References

  1. McGehee, M.D. Paradigm Shifts in Dye-Sensitized Solar Cells. Science 2011, 334, 607–608. [Google Scholar] [CrossRef] [PubMed]
  2. Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. Dye-sensitized solar cells. Chem. Rev. 2010, 110, 6595–6663. [Google Scholar] [CrossRef] [PubMed]
  3. Muñoz-García, A.B.; Benesperi, I.; Boschloo, G.; Concepcion, J.J.; Delcamp, J.H.; Gibson, E.A.; Meyer, G.J.; Pavone, M.; Pettersson, H.; Hagfeldt, A.; et al. Dye-sensitized solar cells strike back. Chem. Soc. Rev. 2021, 50, 12450–12550. [Google Scholar] [CrossRef] [PubMed]
  4. Shang, Y.; Hao, S.; Yang, C.; Chen, G. Enhancing Solar Cell Efficiency Using Photon Upconversion Materials. Nanomaterials 2015, 5, 1782–1809. [Google Scholar] [CrossRef]
  5. Halim, M.A. Harnessing Sun’s Energy with Quantum Dots Based Next Generation Solar Cell. Nanomaterials 2012, 3, 22–47. [Google Scholar] [CrossRef]
  6. Kulkarni, P.; Nataraj, S.K.; Balakrishna, R.G.; Nagaraju, D.H.; Reddy, M.V. Nanostructured binary and ternary metal sulfides: Synthesis methods and their application in energy conversion and storage devices. J. Mater. Chem. A 2017, 5, 22040–22094. [Google Scholar] [CrossRef]
  7. Iftikhar, H.; Sonai, G.G.; Hashmi, S.G.; Nogueira, A.F.; Lund, P.D. Progress on Electrolytes Development in Dye-Sensitized Solar Cells. Materials 2019, 12, 1998. [Google Scholar] [CrossRef]
  8. Kim, H.-J.; Chebrolu, V.T.-V. Chemical bath deposition of NiCo2S4 nanostructures supported on a conductive substrate for efficient quantum-dot-sensitized solar cells and methanol oxidation. New J. Chem. 2018, 42, 18824–18836. [Google Scholar] [CrossRef]
  9. Biswas, S.; Kim, H. Solar Cells for Indoor Applications: Progress and Development. Polymers 2020, 12, 1338. [Google Scholar] [CrossRef]
  10. Ma, X.; Fu, J.; Gao, L.; Zhang, J.; Tao, S.; Guo, W.; Liu, X.; Yang, B.; Lu, J. Dual-duty NiCo2S4 nanosheet-based solar rechargeable batteries toward multi-scene solar energy conversion and storage. Nanoscale 2023, 15, 10584–10592. [Google Scholar] [CrossRef]
  11. Khoo, S.Y.; Miao, J.; Yang, H.B.; He, Z.; Leong, K.C.; Liu, B.; Tan, T.T.Y. One-Step Hydrothermal Tailoring of NiCo2S4 Nanostructures on Conducting Oxide Substrates as an Efficient Counter Electrode in Dye-Sensitized Solar Cells. Adv. Mater. Interfaces 2015, 2, 1500384. [Google Scholar] [CrossRef]
  12. Huang, N.; Zhang, S.; Huang, H.; Liu, J.; Sun, Y.; Sun, P.; Bao, C.; Zheng, L.; Sun, X.; Zhao, X. Pt-sputtering-like NiCo2S4 counter electrode for efficient dye-sensitized solar cells. Electrochim. Acta 2016, 192, 521–528. [Google Scholar] [CrossRef]
  13. Xue, G.; Bai, T.; Wang, W.; Wang, S.; Ye, M. Recent advances in various applications of nickel cobalt sulfide-based materials. J. Mater. Chem. A 2022, 10, 8087–8106. [Google Scholar] [CrossRef]
  14. Qiu, J.; He, D.; Wang, H.; Li, W.; Sun, B.; Ma, Y.; Lu, X.; Wang, C. Morphology-controlled fabrication of NiCo2S4 nanostructures decorating carbon nanofibers as low-cost counter electrode for efficient dye-sensitized solar cells. Electrochim. Acta 2020, 367, 137451. [Google Scholar] [CrossRef]
  15. Li, L.; Zhang, X.; Liu, S.; Liang, B.; Zhang, Y.; Zhang, W. One-step hydrothermal synthesis of NiCo2S4 loaded on electrospun carbon nanofibers as an efficient counter electrode for dye-sensitized solar cells. Sol. Energy 2020, 202, 358–364. [Google Scholar] [CrossRef]
  16. Nan, H.; Han, J.; Luo, Q.; Yin, X.; Zhou, Y.; Yao, Z.; Zhao, X.; Li, X.; Lin, H. Economically synthesized NiCo2S4/reduced graphene oxide composite as efficient counter electrode in dye-sensitized solar cell. Appl. Surf. Sci. 2018, 437, 227–232. [Google Scholar] [CrossRef]
  17. Xia, C.; Li, P.; Gandi, A.N.; Schwingenschlögl, U.; Alshareef, H.N. Is NiCo2S4 Really a Semiconductor? Chem. Mater. 2015, 27, 6482–6485. [Google Scholar] [CrossRef]
  18. Lin, J.-Y.; Chou, S.-W. Highly transparent NiCo2S4 thin film as an effective catalyst toward triiodide reduction in dye-sensitized solar cells. Electrochem. Commun. 2013, 37, 11–14. [Google Scholar] [CrossRef]
  19. Rao, S.S.; Durga, I.K.; Kundakarla, N.; Punnoose, D.; Gopi, C.V.V.M.; Reddy, A.E.; Jagadeesh, M.; Kim, H.-J. A hydrothermal reaction combined with a post anion-exchange reaction of hierarchically nanostructured NiCo2S4 for high-performance QDSSCs and supercapacitors. New J. Chem. 2017, 41, 10037–10047. [Google Scholar] [CrossRef]
  20. Chen, X.; Ding, J.; Chen, X.; Liu, X.; Zhuang, G.; Zhang, Z. Porous tremella-like NiCo2S4 networks electrodes for high-performance dye-sensitized solar cells and supercapacitors. Sol. Energy 2018, 176, 762–770. [Google Scholar] [CrossRef]
  21. Anuratha, K.S.; Ramaprakash, M.; Panda, S.K.; Mohan, S. Studies on synergetic effect of rGO-NiCo2S4 nanocomposite as an effective counter electrode material for DSSC. Ceram. Int. 2017, 43, 10174–10182. [Google Scholar] [CrossRef]
  22. Su, P.; Jiao, Q.; Li, H.; Li, Y.; Liu, X.; Wu, Q.; Shi, D.; Zhao, Y.; Wang, T.; Wang, W. Rational Design of NiCo2S4 Quantum Dot-Modified Nitrogen-Doped Carbon Nanotube Composites as Robust Pt-Free Electrocatalysts for Dye-Sensitized Solar Cells. ACS Appl. Energy Mater. 2021, 4, 4344–4354. [Google Scholar] [CrossRef]
  23. Deng, J.; Wang, M.; Song, X.; Yang, Z.; Yuan, Z. Ti Porous Film-Supported NiCo2S4 Nanotubes Counter Electrode for Quantum-Dot-Sensitized Solar Cells. Nanomaterials 2018, 8, 251. [Google Scholar] [CrossRef] [PubMed]
  24. Yin, J.; Wang, Y.; Meng, W.; Zhou, T.; Li, B.; Wei, T.; Sun, Y. Honeycomb-like NiCo2S4nanosheets prepared by rapid electrodeposition as a counter electrode for dye-sensitized solar cells. Nanotechnology 2017, 28, 345403. [Google Scholar] [CrossRef]
  25. Gayathri, V.; Peter, I.J.; Ramachandran, K.; Karazhanov, S.; Mohan, C.R. Graphene Quantum Dots Embedded in NiCo2S4/MWCNT Nanocomposite as a Promising Candidate for Supercapacitors and I3– Reduction in Dye-Sensitized Solar Cells. Energy Fuels 2021, 35, 13360–13369. [Google Scholar] [CrossRef]
  26. Li, L.; Zhang, X.; Fu, Y.; Han, X.; Wang, Y.; Zhang, Y.; Zhang, W.; Li, L. NiCo2S4 nanotube arrays grown in situ on Ni foam as a low-cost counter electrode for dye-sensitized solar cells. Sol. Energy 2021, 225, 297–304. [Google Scholar] [CrossRef]
  27. Liu, J.; Li, C.; Zhao, Y.; Wei, A.; Liu, Z. Synthesis of NiCo2S4 nanowire arrays through ion exchange reaction and their application in Pt-free counter-electrode. Mater. Lett. 2016, 166, 154–157. [Google Scholar] [CrossRef]
  28. Liu, K.; Wei, A.; Liu, J.; Liu, Z.; Xiao, Z.; Zhao, Y. NiCo2S4 nanosheet thin film counter electrodes prepared by a two-step approach for dye-sensitized solar cells. Mater. Lett. 2018, 217, 185–188. [Google Scholar] [CrossRef]
  29. Dostagir, N.H.M.D.; Rattanawan, R.; Gao, M.; Ota, J.; Hasegawa, J.-Y.; Asakura, K.; Fukouka, A.; Shrotri, A. Co Single Atoms in ZrO2 with Inherent Oxygen Vacancies for Selective Hydrogenation of CO2 to CO. ACS Catal. 2021, 11, 9450–9461. [Google Scholar] [CrossRef]
  30. Su, A.-L.; Lu, M.-N.; Chang, C.-Y.; Wei, T.-C.; Lin, J.-Y. Scalable Fabrication of Efficient NiCo2S4 Counter Electrodes for Dye-sensitized Solar Cells Using a Facile Solution Approach. Electrochim. Acta 2016, 222, 1410–1416. [Google Scholar] [CrossRef]
  31. Sivanantham, A.; Ganesan, P.; Shanmugam, S. Hierarchical NiCo2S4 Nanowire Arrays Supported on Ni Foam: An Efficient and Durable Bifunctional Electrocatalyst for Oxygen and Hydrogen Evolution Reactions. Adv. Funct. Mater. 2016, 26, 4661–4672. [Google Scholar] [CrossRef]
  32. Koolen, C.D.; Luo, W.; Züttel, A. From Single Crystal to Single Atom Catalysts: Structural Factors Influencing the Performance of Metal Catalysts for CO2 Electroreduction. ACS Catal. 2022, 13, 948–973. [Google Scholar] [CrossRef]
  33. Li, M.; Hua, B.; Wang, L.-C.; Sugar, J.D.; Wu, W.; Ding, Y.; Li, J.; Ding, D. Switching of metal–oxygen hybridization for selective CO2 electrohydrogenation under mild temperature and pressure. Nat. Catal. 2021, 4, 274–283. [Google Scholar] [CrossRef]
  34. Zhang, J.; Pham, T.H.M.; Ko, Y.; Li, M.; Yang, S.; Koolen, C.D.; Zhong, L.; Luo, W.; Züttel, A. Tandem effect of Ag@C@Cu catalysts enhances ethanol selectivity for electrochemical CO2 reduction in flow reactors. Cell Rep. Phys. Sci. 2022, 3, 100949. [Google Scholar] [CrossRef]
  35. Shi, Z.; Deng, K.; Li, L. Pt-free and efficient counter electrode with nanostructured CoNi2S4 for dye-sensitized solar cells. Sci. Rep. 2015, 5, 9317. [Google Scholar] [CrossRef]
  36. Wang, L.; He, J.; Zhou, M.; Zhao, S.; Wang, Q.; Ding, B. Copper indium disulphide nanocrystals supported on carbonized chicken eggshell membranes as efficient counter electrodes for dye-sensitized solar cells. J. Power Sources 2016, 315, 79–85. [Google Scholar] [CrossRef]
  37. Xiao, J.; Zeng, X.; Chen, W.; Xiao, F.; Wang, S. High electrocatalytic activity of self-standing hollow NiCo2S4 single crystalline nanorod arrays towards sulfide redox shuttles in quantum dot-sensitized solar cells. Chem. Commun. 2013, 49, 11734–11736. [Google Scholar] [CrossRef] [PubMed]
  38. Zhang, C.; Deng, L.; Zhang, P.; Ren, X.; Li, Y. Electrospun NiCo2S4 with extraordinary electrocatalytic activity as counter electrodes for dye-sensitized solar cells. J. Solid State Electrochem. 2017, 21, 3579–3588. [Google Scholar] [CrossRef]
Nanomaterials 13 02896 g001
Figure 1. Suggested schematic of the growth of the NCS-1 over the FTO substrate.
Figure 1. Suggested schematic of the growth of the NCS-1 over the FTO substrate.
Nanomaterials 13 02896 g001
Nanomaterials 13 02896 g002
Figure 2. XRD pattern of the NCS-1, NCS-2, and FTO substrate.
Figure 2. XRD pattern of the NCS-1, NCS-2, and FTO substrate.
Nanomaterials 13 02896 g002
Nanomaterials 13 02896 g003
Figure 3. (a,b) SEM images of the NCS-1 at different magnification. (c,d) SEM images of the NCS-2 at different magnification.
Figure 3. (a,b) SEM images of the NCS-1 at different magnification. (c,d) SEM images of the NCS-2 at different magnification.
Nanomaterials 13 02896 g003
Nanomaterials 13 02896 g004
Figure 4. XPS high resolution spectra (a) Ni 2p, (b) Co 2p, and (c) S 2p of the NCS-1 electrode.
Figure 4. XPS high resolution spectra (a) Ni 2p, (b) Co 2p, and (c) S 2p of the NCS-1 electrode.
Nanomaterials 13 02896 g004
Nanomaterials 13 02896 g005
Figure 5. (a) Schematic image of the assembled DSSC, (b) cyclic voltammograms of the Pt, NCS-1, and NCS-2, and (c) current density–voltage graph of the Pt, NCS-1, and NCS-2.
Figure 5. (a) Schematic image of the assembled DSSC, (b) cyclic voltammograms of the Pt, NCS-1, and NCS-2, and (c) current density–voltage graph of the Pt, NCS-1, and NCS-2.
Nanomaterials 13 02896 g005
Table 1. Photovoltaic performance summary of the Pt, NCS-1, NCS-2 assembled DSSC devices.
Table 1. Photovoltaic performance summary of the Pt, NCS-1, NCS-2 assembled DSSC devices.
ElectrodesVoc (V)Jsc (mA·cm−2)FFPCE (%)
Pt0.7513.9867.687.19
NCS-10.7513.0566.566.60
NCS-20.7413.0463.216.29
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Alsharif, S.A. Nanograss-Assembled NiCo2S4 as an Efficient Platinum-Free Counter Electrode for Dye-Sensitized Solar Cell. Nanomaterials 2023, 13, 2896. https://doi.org/10.3390/nano13212896

AMA Style

Alsharif SA. Nanograss-Assembled NiCo2S4 as an Efficient Platinum-Free Counter Electrode for Dye-Sensitized Solar Cell. Nanomaterials. 2023; 13(21):2896. https://doi.org/10.3390/nano13212896

Chicago/Turabian Style

Alsharif, Shada A. 2023. "Nanograss-Assembled NiCo2S4 as an Efficient Platinum-Free Counter Electrode for Dye-Sensitized Solar Cell" Nanomaterials 13, no. 21: 2896. https://doi.org/10.3390/nano13212896

APA Style

Alsharif, S. A. (2023). Nanograss-Assembled NiCo2S4 as an Efficient Platinum-Free Counter Electrode for Dye-Sensitized Solar Cell. Nanomaterials, 13(21), 2896. https://doi.org/10.3390/nano13212896

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop