Nickel cobalt sulfide nanoneedle array as an effective alternative to Pt as a counter electrode in dye sensitized solar cells

Abstract

Self supported nickel cobalt sulfide (NCS) nanoneedles are directly formed on FTO glass substrates by sulphurization of nickel cobalt oxide nanoneedles (grown by a hydrothermal method) in the presence of a hydrogen sulfide and argon gas mixture. These NCS nanoneedles when used as a counter electrode for dye sensitized solar cells (DSSCs) show efficient catalytic activity towards the I⁻/I₃⁻ redox couple, and lead to an impressive efficiency of 6.9%, compared with 7.7% obtained with a Pt electrode in similarly constructed devices.

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Introduction

Since the discovery of dye sensitized solar cells (DSSCs) in 1991, enormous efforts have been devoted to the enhancement of their performance. Various new and novel alternatives have been tested for all three key components of DSSCs, namely the working electrode, made up of semiconductor nanoparticles loaded with sensitizer dye, the redox electrolyte, and the counter electrode, in order to make these cells more efficient and cost effective. Over the years and from various research results, TiO₂ has emerged as the ideal material for the working electrode due to its fairly good electron mobility and stability towards the acidic end-groups of dyes.¹ Moreover, it is also an earth-abundant and non-toxic material. Efforts continue to synthesize TiO₂ with various morphology types, with specific surface facets and a high surface area to enhance the DSSC conversion efficiency.^2–5 Ruthenium sensitizers are considered as the most successful, and have yielded an efficiency of 11% with TiO₂ as the working electrode.^6,7 In 2012 Grätzel and co-workers used a cocktail of two organic dyes and a cobalt redox shuttle to achieve an efficiency of 12%.⁸ Efforts are continuing to replace the liquid redox electrolyte with solid p-type conductors. More recently, Chung et al. used fluorine-doped CsSnI₃ perovskite as a p-type material in DSSCs to achieve an efficiency of 10%.⁹ Another major breakthrough was achieved by Snaith and Grätzel, who used organic-based perovskites as p-type materials which gave efficiencies of 11% and 9.7% using Al₂O₃ and TiO₂ respectively as the working electrodes.^10,11 In a remarkable recent development, Grätzel and co-workers have achieved an impressive efficiency of about 15% using a sequential deposition technique for perovskite-based solar cells.¹² This is considered to be a game changer in the field.

The counter electrode also plays a very important role in determining the efficiency of this family of excitonic solar cells. Usually, a Pt nanoparticle-coated FTO realized by thermal decomposition,¹³ sputtering¹⁴ or chemical reduction¹⁵ is used as the counter electrode. The Pt counter electrode is very efficient for I⁻/I₃⁻ redox regeneration (the conversion of I₃⁻ to I⁻ occurs on the surface of Pt), which in turn helps in the regeneration of the oxidized dye. Thus, platinum acts as a catalyst for the charge transfer reaction occurring between iodide and tri-iodide.¹⁶ However, in view of the high cost and low natural abundance of Pt, in recent years significant efforts have been directed towards the replacement of this Pt catalyst with other inexpensive and earth abundant materials.¹⁶ The prerequisites for an efficient catalyst for DSSCs are that it should be easily available, low cost, stable in the ambient cell architecture, and should certainly have very good catalytic activity. Carbon is one of the leading candidates in this respect due to its merits like low cost, high catalytic activity and excellent corrosion resistance.¹⁷ To date various carbon forms like CNTs,¹⁸ functionalized graphene,¹⁹ mesoporous carbon,²⁰ carbon fibers,²¹ and laser synthesized carbon²² have been successfully used as counter electrodes in DSSCs with efficiencies comparable to or even exceeding that of DSSCs using platinum. But the main problems with carbon counter electrodes are the adhesion of these carbon materials to the substrate surface and its opaque nature.¹⁷ Various organic polymers like PEDOT nanotube arrays²³ and PProDOT-Et₂²⁴ have also been used as counter electrode materials in DSSCs. The advantages of these polymers are their high conductivity and good transparency, but a potential disadvantage is their stability. Unfortunately no detailed stability studies have been reported to date for these polymers.

Inorganic materials like sulfides, carbides, nitrides and some organic–inorganic composites have also been used as counter electrode materials.¹⁷ TiN and W₂N have shown good performance as counter electrodes in DSSCs.²⁵ Various composites like MoS₂–carbon,²⁶ carbon–PEDOT–PSS,²⁷ carbon–TiO₂,²⁸ etc. have also been used as counter electrodes in DSSCs. Copper-zinc-tin sulfide (CZTS) has shown good performance as a counter electrode, with an efficiency of 7.3%.²⁹ A CoS counter electrode has shown a conversion efficiency of 7.6%, equal to that of Pt.³⁰ NiS is another candidate which has yielded a DSSC conversion efficiency of 6.8%.³¹ Metal sulfides thus appear to show great promise as counter electrodes in DSSCs.³² To date there are only a few reports on the use of ternary systems (oxides/sulfides) as catalysts, even though it has been proved that these, in several cases, show higher conductivity than their binary counterparts. Ternary sulfides have also not been well investigated, except for a sole report stating the use of NiCo₂S₄, showing excellent electrocatalytic activity towards oxygen evolution reactions.³³ In a very recent work by Xiao et al., NiCo₂S₄ hollow nanorods were used as counter electrodes for quantum dot sensitized solar cells to give an impressive efficiency of 4.2%, in comparison with 3.05% for the cell using Pt as the counter electrode.³⁴ In the present work we have synthesized nickel cobalt sulfide (NCS) nanoneedles directly on FTO glass substrates. These are synthesized from nickel cobalt oxide nanoneedles grown by a hydrothermal method, followed by their sulphurization in the presence of hydrogen sulfide and argon gas. These NCS nanoneedles when used as a counter electrode for DSSCs have shown a highly efficient catalytic activity towards the I⁻/I₃⁻ redox couple and led to an impressive efficiency of 6.9% compared to 7.7% with a Pt electrode in a similar cell construction.

Chemicals

Cobalt chloride hexahydrate (CoCl₂·6H₂O) and nickel chloride hexahydrate (NiCl₂·6H₂O) were purchased from Merck. Urea was purchased from Sigma Aldrich. All chemicals were used without further purification.

Synthesis

FTO glass substrates were cleaned thoroughly with soap, water, ethanol and were finally dried in an oven before use. Synthesis of NiCo₂O₄ (NCO) nanoneedles was done by a simple hydrothermal method. 4 mmol of nickel chloride hexahydrate, 8 mmol of cobalt chloride hexahydrate and 15 mmol urea were dissolved in 75 ml of water and stirred for 15 min to get a clear pink solution. The whole solution was then transferred into a 100 ml Teflon autoclave, and the FTO substrates were dipped into it. The autoclave was sealed and kept at 120 °C for 6 h in an electric oven. After naturally cooling to room temperature, the FTO substrates were removed from the solution. A pink coloured mass was observed to have deposited on the FTO substrates, which were washed thoroughly several times with deionized water and ethanol, and kept overnight for drying at around 75 °C. The films were then annealed at 350 °C for 2 h at a heating rate of 2 °C min⁻¹. The films turned black in colour, which confirmed the formation of NiCo₂O₄ (NCO). Subsequently the NCO films were subjected to sulphurization to obtain NiCo₂S₄ (NCS), by annealing at 350 °C for 8 h by passing a H₂S and argon gas mixture through a split tube furnace.

Fabrication of the DSSCs

Fabrication of the DSSCs was done by a doctor blading method using commercially available TiO₂ nanopowder (Sigma Aldrich, 20 nm), with ethyl cellulose and α-terpineol as the binder and surfactant, respectively. After coating, the films were annealed at 450 °C for 60 min. The thickness was kept at around ∼12 μm. The films were subjected to TiCl₄ treatment at 70 °C for 30 min, followed by annealing at 450 °C for 30 min. Then they were dipped in ruthenium N719 dye solution (1 : 1 v/v acetonitrile + t-butyl alcohol) for 24 h at room temperature. The samples were then rinsed with ethanol to remove the excess dye on the surface. This was followed by redox electrolyte addition and top contact of Pt-coated FTO, NCO or NCS counter electrodes. The Pt counter electrode was prepared by drop casting 0.6 mM H₂PtCl₆ ethanolic solution on clean FTO. It was allowed to dry at room temperature then heated at 450 °C for 15 min. The electrolyte used was 0.6 M 1-propyl-2,3-dimethyl-imidazolium iodide, 0.05 M LiI, 0.05 M I₂, and 0.5 M 4-tert-butylpyridine in acetonitrile–valeronitrile solution (v/v 1 : 1).

Characterization

Various characterization techniques such as X-ray diffraction (XRD, Philips X'Pert PRO), high-resolution transmission electron microscopy (HR-TEM, FEI Tecnai 300), and scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDX) (FEI Quanta 200 3D) were used for the determination of various properties of the materials used. J–V characteristics were measured under irradiation at 100 mW cm⁻² (150 W xenon lamp, Oriel Instruments), 1 sun AM 1.5 simulated sunlight (solar simulator). Impedance measurements were recorded using an Autolab potentiostat.

Results and discussion

The synthesis of oriented NiCo₂O₄ nanoneedles followed by their conversion to NiCo₂S₄ nanoneedles by sulphurization is a three step process, as shown in Scheme 1. Initially, the reaction of nickel chloride, cobalt chloride and urea in aqueous medium at 120 °C leads to the formation of the precursor nickel cobalt hydroxide carbonate with the chemical composition NiCo₃(OH)₂·2H₂O and Co(CO₃)_0.5(OH)_0.11·H₂O on the surface of the FTO glass substrate, which is step one. The colour of the initial nickel cobalt hydroxide carbonate is pink.³³

Scheme 1 Formation of NCS nanoneedles from the precursors. Step I is the formation of nickel cobalt oxide hydroxide carbonate nanoneedles by a hydrothermal method. Step II is the formation of nickel cobalt oxide (NCO) nanoneedles by annealing in air for 2 hours. Step III is the sulphurization process for the formation of nickel cobalt sulfide (NCS) in H₂S/Ar gas at 350 °C for 8 hours.

Step two involves the transformation of this nickel cobalt hydroxide carbonate precursor to NiCo₂O₄ nanoneedles by further annealing the precursor at 350 °C in air for 2 hours. The formation of NiCo₂O₄ is confirmed by the change in colour from pink to jet black. The sulphurization of NCO is the third step, which is done in an inert atmosphere of argon and H₂S gas mixture in a split tube furnace at 350 °C for 8 hours, where the exchange of lattice oxygen with sulphur takes place leading to the conversion of the NiCo₂O₄ (NCO) nanoneedles into NiCo₂S₄ (NCS) nanoneedles. The as-synthesized nickel cobalt oxide (NCO) and nickel cobalt sulfide (NCS) samples were characterized by scanning electron microscopy (SEM) to study their morphology. SEM images of the as-grown nickel cobalt oxide and sulfide are shown in Fig. 1. From Fig. 1(a) we can easily see the uniformly arranged 1D NiCo₂O₄ nanoneedle array. The surface of the nanoneedles is quite smooth. The length of the nanoneedles is around 1 micron, and the average thickness is 200 nm (excluding the tips of the nanoneedles). Fig. 1(d) shows the SEM images of NiCo₂S₄. It is clear from the image that, even after sulphurization, there is no major change in the shape of the nanoneedles. The SEM images of the metal sulfide nanoneedles indicate that the process does not destroy the basic morphology of the 1D structure. It is also clear from the images that the films have more areas that are easily accessible to the electrolyte, which will be of help for their application as a counter electrode in DSSCs, as shown in Fig. 4.

Fig. 1 (a–c) FESEM images of the as-synthesized nickel cobalt oxide (NCO) and (d–f) nickel cobalt sulphide (NCS), showing maintained nanoneedle morphology.

The X-ray diffraction (XRD) results confirm the formation of the metal oxides and sulfides, as seen in Fig. 2. According to the PCPDFWIN 73-1702, the XRD pattern of Fig. 2(a) matches with that for the pure phase of NiCo₂O₄ spinel structure. The XRD pattern of NiCo₂S₄, shown in Fig. 2(b) matches well with the PCPDFWIN 43-1477 data. Most of the peaks are retained after sulfurization of NiCo₂O₄ because of the same basic unit cell structure of the two materials.

Fig. 2 XRD data for (a) nickel cobalt oxide (NCO) and (b) nickel cobalt sulfide (NCS).

Fig. 3 shows the transmission electron microscopy (TEM) images of the NCO and NCS nanoneedle samples. From Fig. 3a–c it is observed that the porous nanoneedle structures are composed of small NiCo₂O₄ nanoparticles.

Fig. 3 TEM images of (a–c) NCO nanoneedles and (d–f) NCS nanoneedles.

The size range of the nanoparticles is between 5 and 10 nm. These small nanoparticles are interconnected to form the 1D nanoneedle-like structures. Fig. 3d–f show the TEM images of the NCS nanoneedles, which are also composed of small nanoparticles.

Further, HRTEM also revealed the lattice spacing and the crystallinity of both the oxide and sulfide nanoparticles. No amorphous layer is observed in HRTEM, which indicates the high crystallinity of NiCo₂O₄ as well as NiCo₂S₄. The lattice spacing of the NiCo₂O₄ nanoneedles is 2.25 Å, which corresponds to the (311) planes, and that of sulfide nanoneedles is 3.37 Å, corresponding to (220) planes. The TEM images match well with the XRD data in Fig. 2. These NCO and NCS nanoneedles were tested as counter electrodes in DSSCs and compared with the usual Pt counter electrode. Iodide/tri-iodide electrolyte was used for the DSSCs. Cyclic voltammetry measurements were also performed to measure and compare the catalytic activity of these NCO and NCS counter electrodes.

Cyclic voltammetry was carried out in a three electrode system, with Ag/AgCl as the reference electrode, Pt foil as the counter electrode and drop casted Pt/NCS/NCO as the working electrode, with a scan rate of 50 mV s⁻¹. The electrolyte used for this study was 10 mM LiI, 1 mM I₂ and 0.1 M LiClO₄ in acetonitrile. Fig. 4 shows the cyclic voltammetry analysis of the Pt, NCS and NCO counter electrodes. In general two pairs of redox peaks are seen for the Pt counter electrode. The more negative pair is assigned to the oxidation and reduction of I⁻/I₃⁻ and the positive pair is assigned to oxidation and reduction of I₂/I₃⁻.¹⁸ Since the counter electrode used for a DSSC is responsible for the reduction of I₃⁻ to I⁻, the more negative pair of peaks is our main focus. From the data it is seen that, like Pt, NCS and to some extent NCO also are catalytically active for the reaction to regenerate the redox couple. The current density obtained for the NCS and NCO electrodes are higher than that of Pt. The higher current density may be due to the high porosity of the NCS and NCO films compared to Pt, as seen in the SEM images in Fig. 1. Due to the higher thickness, good porosity and elongated structures, there is intimate contact between the electrolyte and the catalyst which leads to a capacitive behaviour in these two cases, giving rise to a higher current. The peak currents and peak to peak separation are important parameters for determining the catalytic activity of counter electrodes. The rate constant of a redox reaction is inversely proportional to its peak separation (E_pp).³⁵ E_pp is calculated using the formula:

E_pp = |E_p (anodic) − E_p (cathodic)|

Fig. 4 Cyclic voltammetry analysis of Pt, NCS and NCO counter electrodes.

In a DSSC the peak of interest is the peak towards the more negative side. Therefore this peak is used for E_pp calculations. The E_pp for the Pt counter electrode is ∼681 mV, but that of NCS is ∼671 mV and for NCO it cannot be well defined. Thus it is clear that both the NCS and Pt drop casted counter electrodes are catalytically more active towards the reduction of I₃⁻ to I⁻ than NCO.

Fig. 5 and Table 1 show the J–V characteristics for cells with Pt, NCS and NCO counter electrodes. The Pt counter electrode shows an efficiency of 7.7%, with a current density of 14.2 mA cm⁻², V_oc of 0.8 V and fill factor (FF) of 63.4%. Interestingly, the NCS counter electrode also shows an efficiency of 6.9%, very close to that of the cell with a Pt counter electrode. The current density in the NCS counter electrode case is 13.38 mA cm⁻² with V_oc of 0.76 V and FF of 63.2%. The slight decrease in current density may be due to the increase in R_s and R_ct of the NCS electrode, as reflected in impedance measurements discussed in the next section. The NCO (oxide) nanoneedle electrode showed a low efficiency of 1.5% with overall low cell parameters. This can be attributed to its low catalytic activity as seen in the CV results, and also high resistance as observed in the impedance measurements.

Fig. 5 I–V data of Pt, NCS and NCO counter electrodes.

Table 1 I–V characteristics of cells

Name	Jsc (mA cm^−2)	Voc (V)	FF (%)	η (%)
Pt	14.20	0.8	63.4	7.7 ± 0.2
NCS	13.38	0.76	63.2	6.9 ± 0.3
NCO	8.2	0.67	26.7	1.5 ± 0.2

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Impedance measurements were performed using a symmetric cell assembly on all the three counter electrodes in the dark under zero bias. Fig. 6 shows the Nyquist plots for the Pt, NCS and NCO counter electrodes. The R_ct, i.e. the charge transfer resistance related to the catalytic activity, is 1.35 Ω for the Pt electrode while it is higher for the NCS electrode (7.71 Ω). The R_ct value for the NCO electrode is in kilo-ohms, which correlates with its poor catalytic properties and low efficiency.

Fig. 6 Impedance data of Pt, NCS and NCO counter electrodes and the equivalent circuit used to fit the impedance data.

The series resistance for the NCS electrode is 22.4 Ω, while that of Pt is 15.1 Ω. This resistance depicts the ‘electronic’ adhesion of the material on the FTO along with its sheet resistance. Higher R_ct and higher R_s appears to be the reason for the slightly lower efficiency in the case of the NCS electrode. Below is the equivalent circuit used to fit the impedance data. Thus it is clear that NCS is superior as a counter electrode for DSSCs to NCO. Tafel polarization (Fig. 7) was carried out for the Pt and NCS counter electrodes to determine the most efficient counter electrode between the two.

Fig. 7 Tafel polarization curves for Pt and NCS counter electrodes.

Tafel polarization measurement was also carried out for the Pt drop casted counter electrode and the NCS counter electrode. This measurement was carried out in a symmetric cell assembly with usual DSSC electrolyte mentioned above.

The scan rate for this measurement was 10 mV s⁻¹. A Tafel curve is a plot of the log of the current density vs. the applied voltage. It is usually divided into three parts: first at lower potential is the polarization zone, the curve at the middle with a steep slope is the Tafel zone which determines the catalytic properties of the electrode, and the third region is the diffusion region at higher voltage.³⁶ In the Tafel zone the intersection of the cathodic branch with the equilibrium potential value is J_o, which is the exchange current density.

From the graph it can be seen that Pt has a higher J_o value than NCS, which indicates that it is more effective in catalyzing the reduction of I₃⁻. J_o is inversely proportional to R_ct, from the equation:

J_o = RT/nFR_ct,

where R = gas constant, T = temperature, n = number of electrons involved in the reaction, and R_ct = charge transfer resistance.

Another important parameter that can be extracted from this Tafel graph is J_lim, which the limiting diffusion current density. J_lim for NCS is slightly higher than that of Pt. This may be due to the porous nature of the NCS counter electrode. Thus NCS can be a potentially useful candidate to replace Pt as the counter electrode in DSSCs.

Conclusions

Nickel cobalt sulfide (NCS) nanoneedles were synthesized by the sulphurization of nickel cobalt oxide (NCO) nanoneedles. These were tested as the counter electrode in a DSSC device. These NCS nanoneedles show excellent catalytic activity towards the I⁻/I₃⁻ redox reaction, which is a requirement for high efficiency DSSCs. An impressive conversion efficiency of 6.9%, comparable to that of a Pt-based DSSC (7.7%), was obtained with this NCS counter electrode in similarly made devices.

Acknowledgements

AB and SA would like to thank CSIR and TAPSUN for funding support. SBO would like to acknowledge MNRE and NISE (TAPSUN) for support. Thanks are also due to Mr Upendra Singh for discussion on synthetic strategy. K. Upadhyay and S. Bhatnagar acknowledge the support of the Director, IISER, Pune, for allowing them to undertake work at CSIR-NCL. Special thanks to Ms Dhanya, Ms Shravani, Mr Rounak and Mr Sambhaji for help in taking electron microscope images.

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Footnote

† Electronic supplementary information (ESI) available: Stability data for the NCS counter electrode is provided. NCS nanoneedles were also tested as a counter electrode in quantum dot sensitized solar cells. The I–V data and fabrication details are available. See DOI: 10.1039/c3ra45981k

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