Direct growth of NiCo₂S_x nanostructures on stainless steel with enhanced electrocatalytic activity for methanol oxidation

Abstract

In this report, NiCo₂S_x nanostructures are directly synthesized on stainless steel (SS) substrates by a facile electrodeposition method in addition to a post-vulcanization process. The binder free NiCo₂S_x/SS electrode shows high electrocatalytic activity for methanol oxidation compared with NiS_x and CoS_x. The prepared NiCo₂S_x is a promising non-platinum electrocatalyst for direct methanol fuel cells.

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Ever-worsening energy and global warming issues have triggered significant research efforts in the design and development of novel electrode materials for advanced energy devices.¹ In recent years, direct methanol fuel cells (DMFCs) have attracted a great deal of attention due to their high energy-conversion efficiencies and power densities, eco-friendliness, simple handling and processing.² It is accepted that the performance of a DMFC is governed by the electrocatalytic activity of its electrode materials and the ion/electron transport rate in the electrode and at the electrode–electrolyte interface.³

An efficient way to improve the performance of DMFCs is designing and synthetizing electrocatalyst with high activity and stability.⁴ In recent years, Pt based electrocatalysts have been widely investigated due to the high electrocatalytic activity.^5,6 But the low utilization, high cost and decaying activity of Pt based materials limit their commercial application. In order to solve these problems, non-platinum electrocatalysts have been proposed as an alternative such as NiO,^7,8 Co₃O₄,⁹ NiCo₂O₄^10,11 and MnO_x.¹² Particularly NiO, Co₃O₄ are the most widely investigated non-platinum catalysts because of their relative low cost and high activity. But the low conductivity of these metal oxides hampers their application. Therefore, the exploration of novel catalysts exhibiting both high conductivity and high catalytic activity is a key challenge in this field.

Recently, transitional metal sulphide such as NiS, CoS and CuS have been widely investigated as a new type electrode material owing to the potential application for optics, catalysis and energy storage devices.^13–16 Although many transition metal sulphides have been investigated, reports on binary transition metal sulphides as electrode materials are still limited.¹⁷ Similar to spinel NiCo₂O₄, binary NiCo₂S₄ can offer rich redox reactions owing to the contributions from both nickel and cobalt ions with different valence states. Particularly, according to the previous reports, NiCo₂S₄ exhibit much higher electric conductivity, at least 100 times higher than that of NiCo₂O₄, although NiCo₂O₄ is higher than nickel oxides and cobalt oxides by at least another 2 orders of magnitude.¹⁸ Due to the high electric conductivity and synergistic effect of Co and Ni, NiCo₂S₄ have been used as electrode materials for capacitor,¹⁹ but as far as we know, their application in methanol oxidation is blank. In this report, NiCo₂S_x is proposed as a potential high performance catalyst for methanol oxidation.

Another fascinating strategy to improve the performance of the DMFCs is accelerating the ion/electron transport rate and increasing the utilization efficiency of the electroactive species by direct synthesis of electroactive species on the electrode.⁹ In traditional electrode preparation processes, binders are necessary in order to avoid the loss of the electroactive species. But the presence of the binder not only hinder the kinetics of ion and electron transport in the electrode and at the electrode–electrolyte interface but also decrease the utilization efficiency of electroactive materials. Up to now, although there are several reports about the synthesis of the NiCo₂S₄ powder by hydrothermal method,^18,20 the reports about the direct synthesis of NiCo₂S₄ nanostructure on conductive substrates are rare.^17,21,22 Bearing this in mind, according to our previous report,¹⁰ electrodeposition and a subsequent vulcanization process were used for the directly synthesis of NiCo₂S_x on a SS electrode. The binder free NiCo₂S_x/SS electrodes were directly used for methanol oxidation, which displaying high electrocatalytic activity compared with the monometallic sulphides NiS_x and CoS_x. Our proposed NiCo₂S_x/SS binder free electrodes have several apparent advantages. On one hand, every electroactive nanostructures are directly attached to the conductive substrates, the electrolyte can easily penetrate into the electrode material. As a result, the electronic resistance and ion diffusion resistance are not a big concern. On the other hand, in the absence of binder, almost all electroactive surfaces are exposed to the electrolyte, the utilization efficiency of electroactive materials can be remarkably improved.

In this report, NiCo₂S_x nanostructures were directly grow on SS substrates by a facile two step method including electrodeposition and subsequent vulcanization process. The experimental details were shown in (ESI†). In order to identifying the composition and crystal structure of the prepared Co–Ni layered double hydroxides (Co–Ni LDHs) and NiCo₂S_x, XRD measurements were performed. As shown in Fig. 1, for the black curve, the (003) peak at 2θ of 11.3 is the signature peak of a parasitic LDHs phase.²³ The diffraction peaks marked with black square were index to Ni(OH)₂ (JCPDS: 01-74-2075) and Co(OH)₂ (00-030-0443). It is difficult to differentiate between the two phases, since they have similar structures and their diffraction peaks are very close.²⁴ After vulcanization, the green Co–Ni LDHs transferred into black NiCo₂S_x. The XRD pattern of NiCo₂S_x without calcination is shown in Fig. S1.† The crystallinity of the as proposed NiCo₂S_x is too weak to use for XRD characterization. In order to improve the crystal properties, the NiCo₂S_x used for XRD characterization was calcined in N₂. According to our experiment results, the electrocatalytic activity of calcined NiCo₂S_x is weak than that without calcination, so the NiCo₂S_x without calcination were used for the following characterization and electrochemical experiments. The XRD pattern of the calcined NiCo₂S_x in Fig. 1 (red curve) is different from Co–Ni LDHs (black curve). Four weak diffraction peaks can be indexed as spinel NiCo₂S₄, which is in accord with the standard file of JCPDS card 20-0782 and the previous report.²⁵ No impurities such as NiCo₂O₄ are observed. The XRD patterns of NiS_x and CoS_x were shown in Fig. S2.† From the XRD results we can find that, the crystallinity of these catalysts is too low to confirm the structure. In order to confirm that the Co–Ni LDHs were fully transferred into NiCo₂S_x after vulcanization, the X-ray fluorescence (XRF) and X-ray photoelectron spectroscopy (XPS) were carried out.

Fig. 1 XRD patterns of Co–Ni LDHs (black) and NiCo₂S_x (red).

The XRF spectrum of the proposed NiCo₂S_x is shown in Fig. 2A. The existence of S element in the sample signifies the successful conversion of the precursor into sulphide. The element of Na may come from the reaction reagent. The element ratio of Ni, Co and S is 1 : 2.3 : 3.0. In contrast, the XRF measurements of NiS_x and CoS_x were carried out and the responding patterns were shown in Fig. S3 and S4,† the element ratio of Ni and S, Co and S are 1 : 1.01 and 1 : 0.93 respectively. To further evaluate the elemental composition and chemical state of the product, the XPS measurements were carried out and the results are shown in Fig. 2B–D. The atomic ratio of Ni, Co and S is 1 : 2.3 : 3.4, which is in accord with the XRF results. By using Gaussian fitting method, the Co and Ni spectra can be fitted with two spin–obit doublets and two shake-up satellites (identified as Sat.). In Ni 2p spectra (Fig. 2B), two kinds of nickel species containing Ni²⁺ and Ni³⁺ can be observed. The binding energies at 854.9 eV and 871.8 eV are indexed to Ni²⁺ and the binding energies at 856.1 eV and 873.9 eV are indexed to Ni³⁺. As depicted in Fig. 2C, the binding energies at 781.03 eV and 796.74 eV are ascribed to Co²⁺. Another two fitting peaks at 779.10 eV and 794.22 eV are ascribed to Co³⁺, but these two peaks are weaker than the peaks of Co²⁺. These results indicate that the main oxidation state of cobalt species in NiCo₂S_x is Co²⁺ although the Co³⁺ is existence. These results match well with the reported data of Co 2p and Ni 2p spectra in NiCo₂S₄¹⁸ and NiCo₂O₄.²⁶ The S 2p spectrum can be fitted with three main peaks. The fitting peak at 163.7 eV is a typical peak of metal–sulphur bond,²⁷ while the fitting peak at 162.3 eV can be attributed to sulphur ion in low coordination at the surface.¹⁸ The binding energy at 168.8 eV can be attributed to the sulphur ion with higher oxide state of S₄O₆²⁻ at the surface,^28,29 which can be ascribed to partly oxidation of NiCo₂S_x. These XPS results demonstrate that the surface of our prepared NiCo₂S_x has a mixed composition containing Co²⁺, Co³⁺, Ni²⁺, Ni³⁺, S²⁻ and S^2.5+. Combining element ratio results from XRF and XPS measurements we can conclude that, after vulcanization, the Co–Ni LDHs were transferred into NiCo₂S_x. Our proposed NiCo₂S_x is a non-stoichiometric compound, the range of x is between 3 and 4. The solid redox couple of Ni²⁺/Ni³⁺and Co²⁺/Co³⁺ can afford enough active sites for methanol oxidation, which may be one of the important factors contributing to the high electrocatalytic performance of NiCo₂S_x.

Fig. 2 XRF (A), Ni 2p (B), Co 2p (C) and S 2p (D) XPS spectra of NiCo₂S_x on SS substrate.

The scanning electron microscopy (SEM) images of Co–Ni LDHs and NiCo₂S_x are shown in Fig. 3. As shown in Fig. 3A, the dense nanoflake arrays are observed in the bottom layers, but the flower like Co–Ni LDHs are observed in the surface layers which are constructed from many crinkly nanosheets (Fig. 3A inset image). From the cross sectional SEM image (Fig. 3B) we can clearly found that, the Co–Ni LDHs are directly grown on ITO substrate (the methods of synthetized Co–Ni LDHs on SS and ITO substrates are same, since the glass is fragile, in the process of obtaining the cross section, the morphology of the Co–Ni LDHs can be fully preserved, so the Co–Ni LDHs synthetized on ITO was used for cross sectional characterization). As shown in Fig. 3B, the bottom layer is vertically aligned, but the upper layers lose its uniformity and were loose packed. This may be due to the layered brucite crystal structure of Co–Ni LDHs, which shows weak interaction between layers and strong binding within layered planes. Neighbouring layers are bound together by weak van der waals forces.³⁰ After vulcanization in Na₂S, the morphology preserved NiCo₂S_x nanostructures (Fig. 3C) are obtained. It is expected that this unique structures might possess a large surface area. The criss-crossed nanostructure also facilitates charge transfer at the electrode–electrolyte interface and electrolyte penetration between NiCo₂S_x nanosheets. This may be one of the important factors for the high electrochemical activity. The Energy-dispersive X-ray spectroscopy (EDS) of NiCo₂S_x is shown in Fig. 3D. Peaks from S, Co, and Ni elements are clearly observed, which indicate that the NiCo₂S_x sample is basically made up of Ni, Co, and S elements. The element ratio of Ni, Co and S is 1 : 2.1 : 3.2, which is in accord with the XRF and XPS results. The O element may come from the absorbed CO₂ and H₂O or the partly oxidation of NiCo₂S_x. The Cr and Fe elements may be come from the SS substrates. The C element may come from the SS substrate and the absorbed CO₂.

Fig. 3 SEM images of Co–Ni LDHs (A) and NiCo₂S_x (C) on SS, (B) is the cross sectional SEM image of Co–Ni LDHs on ITO, (D) is the EDS spectra of NiCo₂S_x on SS.

In order to define the crystallographic properties of the proposed NiCo₂S_x, transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) were performed. A typical TEM image in Fig. 4A indicates the crinkly nanosheets structure of NiCo₂S_x, which is in agreement with the SEM results. The HRTEM image in Fig. 4B clearly shows that the lattice phase has random orientation. These results demonstrate the polycrystalline nature of the sample, the lattice spacing of 0.235 nm and 0.286 nm corresponding to the (400) and (311) planes of NiCo₂S₄ respectively.^18,22 The SAED pattern (Fig. 4C) also indicates the polycrystalline nature of NiCo₂S_x. The faint diffraction rings can be readily indexed to the (220), (311), (400), (511) and (440) planes of the NiCo₂S₄. These results are consistent with the HRTEM results and the previous reports.²⁵ The HRTEM and SAED results indicate that, although the proposed NiCo₂S_x is a non-stoichiometric compound, their crystallographic property is similar to the spinel NiCo₂S₄. Such crinkly and polycrystalline nanostructures are expected to possess excellent electrochemical performance.³¹

Fig. 4 TEM (A), HRTEM (B) and SAED (C) images of NiCo₂S_x on SS.

The specific surface area (SSA), average pore size and mesoporous volume are important factors affecting the electrocatalytic performance. To further describe the porous structure of the proposed NiCo₂S_x nanomaterials, N₂ adsorption and desorption measurements were performed. As shown in Fig. 5, a distinct hysteresis loop is observed in the large range of 0.7–1.0P/P₀. This result indicates that the as proposed NiCo₂S_x nanomaterials have a typical mesoporous structure.²⁶ The SSA is 64 m² g⁻¹, which is larger than the previously reported results.²⁰ The Barrett–Joyner–Halenda (BJH) pore size distribution (PSD) data (Fig. 5 inset) further defines the mesoporous structure of NiCo₂S_x. The pore distribution is relatively narrow and mainly centred in the range of 2–13 nm, which is the optimal pore size for the diffusion of active species in electrode materials.³² The average pore diameter and pore volume are 6.9 nm and 0.22 cm³ g⁻¹. The mesoporous structure and large SSA not only greatly improves the electrode–electrolyte contact area but also provides relative short pathways for redox species diffusion, which is another important factor for the high electrocatalytic activity of our proposed NiCo₂S_x nanostructures. Accordingly, our proposed NiCo₂S_x electrodes have huge potential for the electrocatalytic oxidation of methanol.

Fig. 5 N₂ adsorption–desorption isotherm loop and PSD data (inset image) of the NiCo₂S_x on SS.

The above structure and composition characterizations credibly demonstrate that, through a simple electrodeposition and vulcanization process, the NiCo₂S_x nanostructures with uniform morphology were directly synthetized on SS substrate. The binder free NiCo₂S_x/SS electrodes have huge potential for the electrocatalytic oxidation of methanol.

The electrocatalytic oxidation properties of methanol on NiCo₂S_x/SS electrode were elucidated by cyclic voltammetry (CV) and chronoamperometry tests. CoS_x/SS and NiS_x/SS electrodes synthetized by the same methods were used for comparison. As shown in Fig. 6A, the CV curves of CoS_x/SS electrode consist of two pairs of redox peaks, corresponding to the reaction of Co²⁺/Co³⁺ and Co³⁺/Co⁴⁺.^33,34 The redox peaks of NiS_x/SS electrode can be ascribed to the reaction of Ni²⁺/Ni³⁺.³⁵ The NiCo₂S_x/SS electrode shows one distinct redox couple. According to the previous reports,^18,22,36 the redox couple is an integrated redox couple of Ni²⁺/Ni³⁺, Co²⁺/Co³⁺ and Co³⁺/Co⁴⁺ based the following reaction:

NiCo₂S_x + 3OH⁻ = NiS_x−2yOH + 2CoS_yOH + 3e⁻

CoS_yOH + OH⁻ = CoS_yO + H₂O + e⁻

Fig. 6 CV curves of NiCo₂S_x/SS, CoS_x/SS and NiS_x/SS electrode in 1 M KOH without (A) and with (C) 0.5 M methanol at scan rate of 10 mV s⁻¹; (B) is CV curves of NiCo₂S_x/SS electrode in 1 M KOH with and without 0.5 M methanol; (D) is chronoamperometry curves of NiCo₂S_x/SS, CoS_x/SS and NiS_x/SS electrodes in 1 M KOH with 0.5 M methanol at 0 V (0–100 s) and 0.6 V (101–1100 s).

The CV curves of NiCo₂S_x/SS show much higher peak current and larger enclosed area compared with single NiS_x/SS and CoS_x/SS electrode. This may be due to the fact that NiCo₂S_x is a mix valence compound, the solid state redox couples of Co²⁺/Co³⁺ and Ni²⁺/Ni³⁺ are present in the structure, which can produce different active centre for redox reactions. These results indicate that our proposed NiCo₂S_x has much better electrochemical performance.

The methanol electrocatalytic oxidation property on NiCo₂S_x is shown in Fig. 6B. Comparing the CV curves in 1 M KOH electrolyte with and without methanol, we can found that, the methanol electrocatalytic oxidation on the NiCo₂S_x/SS electrode is clearly observed. A sharp increase in anodic current for methanol oxidation is noticed. As shown in Fig. S5,† the methanol electrocatalytic oxidation activity of Co–Ni LDHs is much lower than that of NiCo₂S_x. The activity of blank SS for methanol oxidation can be ignored as shown in Fig. S6.†

The degree of current density increase is an important factor for methanol electrocatalytic oxidation. For our proposed NiCo₂S_x nanomaterials, the current density at 0.6 V in KOH electrolyte with methanol is 5 times higher than that without methanol, which is comparable to the mesoporous NiCo₂O₄ nanoparticles (about 5 times at 0.6 V)³⁷ but higher than the mesoporous NiCo₂O₄ nanoparticles (2 times at 0.6 V)³⁸ and porous Co₃O₄/NiO core/shell nanowire array (3 times at 0.75 V).⁹ As shown in Fig. 6C, the NiCo₂S_x shows high anodic current density at 0.6 V with almost 3 times higher than that of single NiS_x and CoS_x. Similar to NiCo₂O₄, the enhanced electrocatalytic activity can be explained by the following reasoning. (a) The synergistic contributions from both nickel and cobalt ions are expected to of ferricher redox reactions than the two corresponding single component sulphides;¹⁷ (b) the presence of cobalt increased the charge-acceptance of Ni in the form of the Ni²⁺/Ni³⁺;³⁹ (c) cobalt allows the nickel to reach a higher oxidation state during the oxidation process and promotes the electron transfer process during the methanol oxidation.^40,41

The onset potential is another important factor for methanol electrocatalytic oxidation. For our proposed NiCo₂S_x, the onset potential is about 0.47 V, which is comparable to the previously reported Co₃O₄/NiO⁹ and NiCo₂O₄,¹⁰ but much higher than the well-known noble metal based catalysts.⁴² This is a common phenomenon exit in all metal oxide/hydroxide catalysts for alkaline DMFCs,^8,9 which is the main obstacle for the commercialization of metal oxide catalyst. Another obstacle for the metal oxide catalyst is the methanol oxidation mechanism. After consult the relevant references we found that, the methanol oxidation mechanism concerned two respects, the oxidation process and the oxidation product. The exact mechanism is complicated and presently not well understood. To date, there are many reports about the methanol oxidation mechanism on nickel based materials.^43–45 Fleischmann et al.⁴³ suggested that methanol oxidation takes place by the reduction of NiOOH to Ni(OH)₂; Taraszewska et al.⁴⁴ suggested that methanol molecules penetrate into a nickel hydroxide film and were oxidized by the OH⁻ trapped in the film. The identity of products formed from the methanol oxidation is typically unspecified. Singh et al.⁴⁶ reported that the product was CO₃²⁻ in a hydroxide solution after methanol oxidation over a Ni electrocatalyst; Golikand⁴⁵ and Fleischmann⁴³ mentioned that formate anions (HCOO⁻) or formic acid (HCOOH) were the possible theoretical products following a 4-electron pathway. In this communication, there were no sufficient data to discuss the mechanism of methanol oxidation on NiCo₂S_x nanomaterials, we just conjecture that the mechanism may similar with the nickel oxide or hydroxide because O and S are in the same family and have similar properties. The exact mechanism is the next step for our proposed NiCo₂S_x. Although there are many unsolved problems, transition metal based catalyst still attracted more and more attention due to the low cost and relative abundance. So, there is large scope for improving the methanol electrocatalytic performance and investigating the methanol oxidation mechanism on transition metal based catalyst.

The electrocatalytic activity and stability were also investigated by chronoamperometry method. The chronoamperometry curve of Co–Ni LDHs and blank SS substrate are shown in Fig. S7,† the current density of Co–Ni LDHs is much lower than that of NiCo₂S_x. The activity of blank SS for methanol oxidation can be ignored. As shown in Fig. 6D, the current density of NiCo₂S_x at 0.6 V is more than three times higher than NiS_x and CoS_x. But a slight current decay is observed in the first five minutes. The current density decrease also can be found in the cyclic CV test. As shown in Fig. 7, the current density only retained 50% after 1000 cycles. But the current density can be returned to 80% by replacing the electrolyte. These results indicated that the current decay is partly depended on the consumption of methanol. In order to further investigating the reason of current decay, the morphology characterization and charge transfer resistance measurement after 1000 cycles were carried out. Form the inset SEM image in Fig. 7 we can found that, the morphology of NiCo₂S_x is well preserved after 1000 cycles. The structure stability of catalyst is important for its electrocatalytical stability. The charge transfer resistance is another important factor affecting the electrocatalytic activity and stability of NiCo₂S_x. As shown in Fig. 8, the charge transfer resistance in 1 M KOH electrolyte with methanol is larger than that without methanol. After 1000 cycles, the charge transfer resistance further increased, this phenomenon is similar with the NiCo₂S₄ based supercapacitor electrode materials, which may be due to the adsorption of the oxidation intermediate.²¹ The adsorption may result in the current decay. The above results indicate that the current density decay is mainly due to the consumption of methanol and the poison of oxidation intermediate. Improving the long term stability is a clear next step for our proposed non-platinum NiCo₂S_x electrocatalysts.

Fig. 7 CV curves of NiCo₂S_x/SS electrode in 1 M KOH with 0.5 M methanol measured at different scans at a scan rate of 10 mV s⁻¹, the inset image is the SEM image of NiCo₂S_x on SS after 1000 cycles.

Fig. 8 The EIS plots of NiCo₂S_x in 1 M KOH electrolyte without (black) and with 0.5 M MeOH (red and blue) before (red) and after (blue) 1000 cycles.

Conclusions

In this report, NiCo₂S_x nanostructures were directly synthesized on the SS substrate by electrodeposition and a subsequent vulcanization process. The NiCo₂S_x/SS electrode can be directly used for binder free methanol oxidation electrode. The proposed NiCo₂S_x show high methanol electrocatalytic activity although the long term stability needed improving. The high electrocatalytic activity can be explained by the following reasons. (a) NiCo₂S_x is a mixed valence compound, thus the redox couples of Co²⁺/Co³⁺ and Ni²⁺/Ni³⁺ can offer rich redox reactions. (b) The mesoporous structure of NiCo₂S_x (with pore sizes of 2–13 nm) could act as channels for the rapid materials exchange in the reaction process. (c) In the absence of binder, almost all available surface area is exposed and able to participate in methanol oxidation, thus maximizing the utilization of electroactive species. For this promising methanol oxidation electrocatalyst, the relative high onset potential, poor long-term stability and the controversial mechanism are the main obstacle for the commercialization, this is the clear next step for our proposed NiCo₂S_x electrocatalyst.

Acknowledgements

The authors gratefully acknowledge the research start financial support from Chengdu University of Technology.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: The experimental details of synthetizing NiCo₂S_x and the catalytic activity of Co–Ni LDHs and blank SS. See DOI: 10.1039/c4ra11008k

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