A “One-Pot” Route for the Synthesis of Snowflake-like Dendritic CoNi Alloy-Reduced Graphene Oxide-Based Multifunctional Nanocomposites: An Efficient Magnetically Separable Versatile Catalyst and Electrode Material for High-Performance Supercapacitors

Abstract

In this paper, a simple “one pot” methodology to synthesize snowflake-like dendritic CoNi alloy-reduced graphene oxide (RGO) nanocomposites has been reported. First-principles quantum mechanical calculations based on density functional theory (DFT) have been conducted to understand the electronic structures and properties of the interface between Co, Ni, and graphene. Detailed investigations have been conducted to evaluate the performance of CoNi alloy and CoNi-RGO nanocomposites for two different types of applications: (i) as the catalyst for the reduction reaction of 4-nitrophenol and Knoevenagel condensation reaction and (ii) as the active electrode material in the supercapacitor applications. Here, the influence of microstructures of CoNi alloy particles (spherical vs snowflake-like dendritic) and the effect of immobilization of CoNi alloy on the surface of RGO on the performance of CoNi-RGO nanocomposites have been demonstrated. CoNi alloy having a snowflake-like dendritic microstructure exhibited better performance than that of spherical CoNi alloy, and CoNi-RGO nanocomposites showed improved properties compared to CoNi alloy. The k_app value of the (CoNi_D)₆₀RGO₄₀-catalyzed reduction reaction of 4-nitrophenol is 20.55 × 10^–3 s^–1, which is comparable and, in some cases, superior to many RGO-based catalysts. The (CoNi_D)₆₀RGO₄₀-catalyzed Knoevenagel condensation reaction showed the % yield of the products in the range of 80–93%. (CoNi_D)₆₀RGO₄₀ showed a specific capacitance of 501 F g^–1 (at 6 A g^–1), 21.08 Wh kg^–1 energy density at a power density of 1650 W kg^–1, and a retention of ∼85% of capacitance after 4000 cycles. These results indicate that (CoNi_D)₆₀RGO₄₀ could be considered as a promising electrode material for high-performance supercapacitors. The synergistic effect, derived from the hierarchical structure of CoNi_D-RGO nanocomposites, is the origin for its superior performance. The easy synthetic methodology, high catalytic efficiency, and excellent supercapacitance performance make (CoNi_D)₆₀RGO₄₀ an appealing multifunctional material.

1. Introduction

Bimetallic nanoparticles have gained immense attraction from the researchers because of their plethora of applications in diverse fields such as catalysis, optical, magnetic, biomedical, magnetic devices, etc.¹ In many cases, the physicochemical properties of the bimetallic alloys, particularly in their nanostructure form, are superior to those of the constituent metals. The alteration of the electronic structures of the metals, which occurs due to their alloy formation, is responsible for the improvement of the properties of the alloys.¹

Though compared to bulk materials, nanostructured materials exhibit exotic properties and superior performance in many applications (e.g., catalysis, magnetic, electrical, optical, etc.), but the agglomeration of nanoparticles due to their high surface energy quite often poses a major challenge. To overcome this limitation, the immobilization of the nanoparticles on a suitable support appeared as an attractive strategy. For the past couple of years, graphene or reduced graphene oxide (RGO) has attracted the attention of the scientists as an interesting support material for immobilization of a variety of nanoparticles for different applications.^2,3 The unique properties of graphene, such as large specific surface area (∼2600 m² g^–1),^4,5 chemical stability, mechanical flexibility with Young’s modulus (∼1 TP),⁶ high adsorption capacity of different ions on its surface, etc., make it a very promising support material and broaden the horizons of applications of graphene-based nanocomposites. The additional advantage that graphene offers as a support matrix is that it possesses high electrical conductivity and its π electrons affect the electronic structure of the nanoparticles, which are immobilized on its surface and in turn influence their properties.

Our current work deals with the catalytic and electrochemical properties of CoNi alloy and CoNi-reduced graphene oxide nanocomposites. CoNi alloy is an important transition metal alloy system that has been widely used for catalysis, magnetic devices, microwave absorption, etc.⁷ It has already been reported that CoNi alloy exhibit a superior catalytic and electrochemical property to the monometallic counterpart.⁸ The CoNi alloy system exhibits richer faradic redox reactions, higher electrical conductivity, stability, and enhanced electrochemical performance due to the synergistic effect arising from the intimate coexistence of Ni and Co in the alloy.^8,9 The electrochemical performances of various types of CoNi-based systems (such as CoNi oxides, CoNi hydroxides, NiCo chalcogenides, CoNi phosphides, etc.) have been reported by several researchers where the improved electrochemical properties of the bimetallic systems have been demonstrated.⁹⁻¹³

Several synthetic methodologies, such as electrodeposition, hydro/solvothermal, sol–gel, spray pyrolysis, polyol process, microemulsion, mechanical alloying, etc., have been reported by several researchers for the preparation of CoNi alloy with different microstructures (e.g., Ni₄₉Co₅₁ sphere, NiCo dumbbells, Ni₇₀Co₃₀ nanorings, NiCo nanochains, NiCo nanowire, handkerchief-like Ni₈₂Co₁₈, CoNi nanoneedle, Ni₁₉Co₈₁ nanotube array, etc.).¹⁴ Wu et al.¹⁴ have reported a solution-phase reduction method to prepare NiCo alloy having a dendritic structure. They have shown that the presence of sodium dodecylbenzenesulfonate resulted in the formation of flower-like microstructures, whereas cetyltrimethylammonium bromide produced dendritic Ni_33.8Co_66.2 alloy.¹⁴ Rashid et al. have synthesized chain-like CoNi nanostructures by employing a polymer-assisted methodology, where poly(vinyl methyl ether) was used.¹

Here, we report the synthesis of CoNi alloy particles with a snowflake-like dendritic structure and the nanocomposites (CoNi-RGO), where the surface of the nanometer-thin RGO sheets are decorated with these CoNi alloy nanoparticles. We have evaluated the performance of these materials as the catalyst for two important reactions: (i) reduction of 4-nitrophenol to 4-aminophenol in the presence of NaBH₄ and (ii) Knoevenagel condensation reaction between an aromatic aldehyde and ethyl cyanoacetate. We have also studied the electrochemical properties of these materials to explore the possibility of the use of these materials as an electrode material for high-performance supercapacitor applications. We have chosen to investigate the abovementioned applications of the synthesized CoNi-RGO nanocomposites for the following reasons:

• (i)Reduction of 4-nitrophenol reaction: The reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in the presence of an excess of NaBH₄ in an aqueous medium was chosen as a model reaction because this is simple and trustworthy, can easily monitor the reaction, and yields a single product. This reaction has been extensively used by the researchers across the globe to evaluate the catalytic property of the nanostructured catalysts developed by them.^15,16 Moreover, nitrophenols and their derivatives are used/produced during the commercial production of a variety of chemicals (e.g., synthetic dyes, herbicides, insecticides, pesticides, etc.), and 4-NP is used in numerous industries such as pharmaceutical, textile, leather, steel manufacturing, refinery, etc. The effluents from these industries contain a considerable amount of 4-NP, which causes water pollution due to its toxicity. It has detrimental effects on the central nervous system, kidney, liver, and blood for both humans and animals.¹⁷ On the other hand, 4-AP, which is produced from the reduction of 4-NP, is a useful chemical for polymer, pharmaceutical, herbicide, etc., production industries.^15,16,18 Several types of nanocatalysts, including noble metals, coinage metals, and catalysts where various metal nanoparticles are supported on solid supports (e.g., silica, alumina, polymer, dendrimer, etc.), graphene/RGO-supported catalysts, magnetic nanoparticle (such as ferrites)-based nanocomposites, etc., have been reported for the reduction reaction of 4-NP in the presence of excess of NaBH₄.^19,20 In Table S1, several nanocatalysts (including Co, Ni, and CoNi alloy catalysts), which were reported for the reduction of 4-NP in an aqueous medium in the presence of excess NaBH₄, and the associated rate constants for the catalysis reaction are listed.
• (ii)Knoevenagel condensation reaction: Knoevenagel condensation reaction is one of the important and widely used carbon–carbon bond-forming reactions. This reaction involves the condensation between a carbonyl compound and a compound having an active methylene group containing one or two electron-withdrawing groups (such as nitrile, acyl, and nitro). This reaction is extensively used for the commercial production of many cosmetics, perfumes, coumarin derivatives, fine chemicals, pharmaceutical chemicals, etc.²¹ Traditionally, this reaction is performed in the presence of different bases (e.g., ammonia, ethylenediamine, piperidine, etc.) and their salts.²² However, quite often, the use of homogeneous catalysts causes the problem in the purification of the product and demands tedious workup procedures.²³ Therefore, some heterogeneous catalysts like Al₂O₃,²⁴ zeolite,²⁵ ionic liquids,²⁶ metal–organic frameworks, metal ion-based coordination polymers, mesoporous silica-based catalysts, etc., have been developed for the Knoevenagel condensation reaction.²⁷ However, some of the disadvantages often experienced during the use of these catalysts are the harsh reaction conditions, low yield of the product, need of toxic/expensive organic solvents, low reusability of the catalysts, etc. Examples of some of the metal nanoparticle-based catalysts, which have been reported for the Knoevenagel reaction are Fe powder, Cu powder, Co powder, C/Co, C/Fe, Ag powder, Au powder, Ni nanoparticles, etc.^22,23 As nanosized catalysts are difficult to separate from the reaction mixture, development of magnetically separable catalysts has been adopted by several researchers as a promising strategy. NiFe₂O₄²¹ and CoFe₂O₄²⁸ nanoparticles have been reported as magnetically separable catalysts for the Knoevenagel reaction.
• (iii)Supercapacitor: Supercapacitors are of immense interest due to their applications in varieties of energy storage and conversion systems, such as electric vehicles, uninterruptible power supplies, laptop, solid-state devices, mobile phones, load leveling, intermittent wind, solar energy systems, etc.²⁹ The use of supercapacitors in the emergency doors of an Airbus A380 is one of many of its commercial applications. In the supercapacitors, the charging or discharging process generally takes place in seconds. Though supercapacitors possess a lower energy density (∼5 Wh kg^–1) than batteries, they can provide higher power density (∼10 kW kg^–1) for shorter times.³⁰ Electrical double layer (EDL) electrodes and pseudocapacitor electrodes are the two major classes of supercapacitors based on their different charge storage mechanisms. In EDL electrodes, which are mostly carbon-based materials, energy storage occurs via fast ion adsorption on the electrode surface. As this process is highly dependent on the available surface area of the active electrode materials, porous carbon and graphene-based electrodes exhibit high power density and long shelf life but low energy density.³¹ Fast redox reactions at the electrode/electrolyte interface are the origin of the high capacitance of pseudocapacitor electrodes. Several metals oxides, conducting polymers, etc., have been reported as electrode materials to construct pseudocapacitor electrodes.^32,33 However, one of the limitations associated with pseudocapacitor electrodes is their poor cycling performance, which might be due to the structural instability of electrode materials in the reaction process.³⁴ To achieve a supercapacitor with high capacitor performance, design of composites, composed of carbon-based materials and redox-active materials, is an attractive strategy, where the advantages of EDLCs and pseudocapacitor electrodes can be exploited at the same time.^29,30 Several scientists have explored a variety of graphene-based nanocomposites, transition metal (e.g, Fe, Co, and Ni)-based metal–organic framework materials, and silica-based composites for energy storage applications (such as supercapacitors, an electrode for Li-ion battery, etc.).³⁵⁻⁴⁰

In this paper, we report a “one pot” method to prepare nanocomposites, which are composed of varying amounts of snowflake-like dendritic CoNi alloy particles and RGO. To understand the electronic structures of CoNi alloy and Co-Ni-graphene superlattice, first-principles quantum mechanical calculations based on density functional theory (DFT) have been performed. We have evaluated the performance of CoNi alloy and CoNi-RGO nanocomposites for two different types of applications: (i) as the catalyst for the reduction reaction of 4-nitrophenol and Knoevenagel condensation reaction and (ii) as the active electrode material in the supercapacitor applications. The influence of microstructures of CoNi alloy particles (spherical vs snowflake-like dendritic) and the effect of immobilization of CoNi alloy on the surface of RGO on the catalytic and supercapacitive properties have been investigated in detail. To the best of our knowledge, this is the first time that the demonstration of the multifunctional nature of CoNi-RGO nanocomposites by showing its application as a catalyst for two different types of reactions and as an active electrode material for supercapacitors and the determination of its electronic structures and electronic properties by a DFT study have been reported.

2. Result and Discussion

2.1. Formation of the Materials and Their Characterizations

Two wet chemical methods were used to prepare CoNi alloy particles. In both of these synthesis methodologies (Method-I and Method-II), CoNi alloy particles were formed due to the reduction of Co²⁺ and Ni²⁺ in alkaline medium. In Method-I, ethylene glycol and N₂H₄ acted as the reducing agents. The reduction of Co²⁺ and Ni²⁺ by ethylene glycol and N₂H₄^41,42 can be presented by the reactions given below (reaction 1–4)

1

2

3

4

where M²⁺ = Co²⁺, Ni²⁺ .

Pure Co and Ni nanoparticles were also synthesized by employing Method-I and Method-II. Field emission scanning electron microscopy (FESEM) micrographs of the synthesized materials are illustrated in Figure 1. Figure 1a shows the nanometer-thin RGO sheets, and the X-ray diffraction (XRD) of pure RGO is shown in Figure S1.⁴³Figure 1b reveals that the pure Ni particles (Ni_S), which were obtained from Method-I, are spherical in nature and the diameter of the spheres lies in the range of 80–100 nm. Figure S1 shows the XRD pattern of Ni_S. Figure 1c shows that the pure Co particles, which were synthesized by Method-II, possess a dendritic microstructure (Co_D). The XRD pattern of Co_D is presented in Figure S1. The FESEM micrograph (Figure 1d) of the CoNi_S particles (prepared by Method-I) shows their spherical nature, and the diameter of the spheres is in the range of 230–500 nm. EDS analysis (Figure S2a) of these spheres confirms the presence of both Co and Ni in each sphere in the desired weight ratio, indicating the formation of CoNi alloy due to the simultaneous reduction of Ni²⁺ and Co²⁺. Elemental color mapping of the spheres also supports this result (Figure S2b). XRD analysis of CoNi_S (Figure 1i) shows a diffraction peak at 2θ = 44.37°, which corresponds to (111) and (002) planes of Ni and Co, respectively.¹ However, the weak intensity of this peak and the absence of other diffraction peaks of Co and Ni indicate the poor crystallinity of CoNi_S. In Method-II, the CoNi_D particles were formed from the reduction of Co²⁺ and Ni²⁺ by N₂H₄ (reaction 4). FESEM micrographs illustrate that CoNi_D particles possess a snowflake-like dendritic microstructure (Figure 1e,f).

Figure 1.

FESEM micrographs of the synthesized materials (a) RGO, (b) Ni_S (Method I), (c) Co_D (Method II), (d) CoNi_S (Method I), (e) CoNi_D (Method II), (f) enlarged micrograph of CoNi_D, (g) (CoNi_D)₆₀RGO₄₀, (h) enlarged micrograph of (CoNi_D)₆₀RGO₄₀ showing the CoNi_D particle on the surface of RGO, and (i) XRD patterns of CoNi_S, CoNi_D, and (CoNi_D)₆₀RGO₄₀.

EDS analysis of CoNi_D particles shows that the dendritic structure is composed of both Co and Ni (Figure S3a), and the elemental color mapping of these particles also supports this observation (Figure S3b). The XRD pattern of CoNi_D (Figure 1i) shows the diffraction peaks at 2θ = 41.71°, 44.71°, 47.54°, and 76.19°, which can be assigned to (100), (002), (101), and (110) planes of CoNi alloy with the hexagonal close-packed (hcp) phase of Co.¹⁴ In this synthesis method, CTAB acts as a microstructure directing agent. According to Wu et al., CTAB controls the growth rate kinetics of (100), (110), and (010) planes of CoNi alloy and interacts with the faces via the adsorption–desorption process and facilitates the growth of the dendritic structure.¹⁴

Investigations on the catalysis property and electrochemical properties were performed for both the spherical CoNi_S and dendritic CoNi_D. It was observed that the CoNi_D exhibited superior properties to CoNi_S. For further improvement of these properties, we have synthesized CoNi_D-RGO nanocomposites by using a “one pot” method where dendritic CoNi_D particles were immobilized on the surface of RGO. In this method, the formation of dendritic structured CoNi alloy particles (CoNi_D) and transformation of GO to RGO occur simultaneously. FESEM micrographs of CoNi_D-RGO nanocomposites (Figure 1g,h) reveal the presence of snowflake-like dendritic CoNi_D particles on the surface of nanometer-thin RGO sheets. EDS analysis and elemental color mapping (Figure S4a,b) also confirm the immobilization of CoNi_D on the RGO surface in CoNi_D-RGO nanocomposites. The CoNi_D particles in CoNi_D-RGO nanocomposites are to some extent smaller in size in comparison with pure CoNi_D particles. In the composite, the presence of RGO might have restricted the growth of the CoNi_D particles. In the XRD pattern of CoNi_D-RGO nanocomposites (Figure 1i), the presence of diffraction peaks at 2θ = 41.71°, 44.71°, 47.54°, and 76.19° indicates the presence of CoNi_D. In this XRD pattern, the absence of peaks at 2θ = 9.76° and 42.14°, which are the characteristic peaks of (001) and (101) planes of GO (Figure S5), is noticed.¹⁸ This could be due to the transformation of GO to RGO during the formation of CoNi_D-RGO nanocomposites. In this “one pot” methodology, the conversion of Co²⁺ and Ni²⁺ to CoNi_D alloy and reduction of GO to RGO occurred simultaneously. Raman spectra of the CoNi_D-RGO nanocomposite and pure GO are shown in Figure S6. The Raman spectra of GO shows that the peaks corresponding to D and G bands appear at 1342 and 1582 cm^–1, respectively, whereas, for CoNi_D-RGO, these bands red-shifted to 1336 and 1585 cm^–1. This shifting of D and G bands in the nanocomposite can be attributed to the conversion of GO to RGO. The I_D/I_G ratio of the nanocomposite (I_D/I_G = 1.10) is greater than that of GO (I_D/I_G = 0.96), and this increase in I_D/I_G might be due to the decrease in the average size of sp² domains upon the reduction of GO during the formation of the CoNi_D-RGO nanocomposite.¹⁸

2.2. First-Principles Calculations of Electronic Structure

We have performed the first-principles quantum mechanical calculations based on DFT to understand the interfacial interactions between Co, Ni, and graphene in the CoNi alloy and Co-Ni-graphene superlattices. Geometrical optimization was performed for each system before the calculation of DOS (density of states), TDOS (total density of states), and band structure. The superlattice structures of Co, Ni, graphene, Co–Ni interface, and Co-Ni-graphene superlattice before and after full relaxation are shown in Figure S7a,b. The obtained lattice parameters for the hcp unit cell of Co are a = b = 2.50 Å and c = 4.06 Å ,and those for the fcc Ni unit cell are a = b = c = 3.52 Å. The lattice parameter values thus obtained match well with the previously reported values.^44,45

In the Co-Ni-graphene superlattice, the lattice distortion is observed after relaxation with a ∼2.3% expansion in the z direction of the Co–Ni interface. In this superlattice, the binding energy between the Co–Ni interface and graphene is −3.49 eV. These results indicate the existence of strong interactions between the Co–Ni interface and graphene. The equilibrium interlayer distance between them is 2.96 Å.

The DOS and TDOS analysis for both Co and Ni unit cells show (Figures S8 and S9) that, in both cases, the valence bands and conduction bands originated mainly from the contribution of their 3d and 3p orbitals. Figure S10 shows the contributions from Ni 3d and Co 3d in the valence band region near the Fermi level in the TDOS of the Co–Ni interface. The presence of Ni 3d and Co 3d states across the Fermi level indicates its metallic nature. In the case of Co-Ni-graphene, the TDOS (Figure S11) shows the contribution from C 2p, Ni 3d, and Co 3d orbitals across the Fermi level and the asymmetric nature of the spin up and spin down phase. These facts clearly indicate the metallic nature and the ferromagnetic nature of Co-Ni-graphene.^46,47 The band structure of graphene (Figure S12) clearly shows its zero band gap nature.⁴⁸ The band structures of Co, Ni, Co-Ni, and Co-Ni-graphene also indicates their metallic character. The band structure of the hcp Co unit cell (Figure S8) and fcc Ni unit cell (Figure S9) match well with the previously reported band structures.^45,49,50 In the band structure of the Co–Ni interface (Figure S10), the appearance of new bands near the Fermi level in comparison with the band structures of individual Co and Ni unit cells indicates the occurrence of hybridization between 3d states of Co and Ni. In the band structure of Co-Ni-graphene (Figure S11), the appearance of new bands at both valence and conduction band regions occurs due to the hybridization between C 2p states of graphene, Ni 3d, and Co 3d states. The charge density plot and difference charge density interface and the electron plot of the Co–Ni interface and Co-Ni-graphene are shown in Figure 2a,b. In the Co–Ni interface, the density of some of the Co centers becomes relatively less in comparison with pure Co. This could be due to the orbital level interactions between Co and Ni in the interface and higher electronegativity of Ni (1.91) than that of Co (1.88). The difference charge density plot of the Co–Ni interface also shows this depletion of the electron density of Co. Figure 2c,d shows the interaction between Co–Ni, C–Ni, and C–Co in the interface in Co-Ni-graphene. These DFT calculations clearly demonstrate the existence of strong orbital level hybridization between Co, Ni, and graphene in Co-Ni-graphene. From this hybridization point of view, we can predict that the conductivity of Co-Ni-RGO nanocomposite will be higher than that of Co-Ni alloy. EIS measurements of the synthesized materials support this prediction by showing the lower R_ct (charge transfer resistance) value of (CoNi_D)₆₀RGO₄₀ (0.53 Ω) compared to CoNi_D (1.61 Ω). This fact indicates that the incorporation of RGO causes the enhancement of the conductivity of the nanocomposites. The detailed discussion on the results obtained from EIS measurements is provided in Section 2.4.

Figure 2.

Charge density plots of the (a) Co–Ni interface and (b) Co–Ni graphene interface and difference charge density plot of the (c) Co–Ni interface and (d) Co–Ni graphene interface (where the red color represents charge accumulation and the blue color represents charge depletion in the difference charge density plot).

2.3. Catalysis Reactions

2.3.1. Reduction of 4-NP to 4-AP

We have performed the room temperature reduction of 4-NP in an aqueous medium in the presence of aqueous NaBH₄ using the synthesized materials as catalysts. We have also performed this reaction without using any catalyst and observed that no reaction has occurred. This reaction was monitored by using UV–Vis spectroscopy. 4-NP in aqueous medium shows the maximum absorption (λ_max) at 317 nm, and the addition of NaBH₄ solution to 4-NP solution causes a red shift of this λ_max to 400 nm due to the formation of dark yellow-colored phenolate ions.^16,51,52 With the progress of the reaction, the color of 4-nitrophenolate starts to fade. With the increasing reaction time, the intensity of λ_max(4-NP) at 400 nm gradually decreases. Simultaneously, a new peak at λ_max = 300 nm grows, which indicates the formation of 4-aminophenol due to the reduction of 4-nitrophenol. This reduction reaction is a pseudo-first-order reaction. As a representative, time-dependent UV–Vis spectra of CoNi_S-, CoNi_D-, and (CoNi_D)₆₀RGO₄₀-catalyzed reaction are presented in Figure 3.

Figure 3.

Time-dependent UV–Vis spectra of the reduction reaction of 4-nitrophenol catalyzed by (a) CoNi_S, (b) CoNi_D, and (c) (CoNi_D)₆₀RGO₄₀ and (d) pseudo-first-order kinetic plot of this reaction.

To understand the role of individual Co, Ni, and RGO in the CoNi-RGO catalyst, we have performed the reaction with pure RGO, Ni_S, Co_S (spherical), and Co_D (dendritic structure) (Figure S13). The induction time, reaction completion time, and the value of k_app, which were observed for the reactions catalyzed by different synthesized materials, are listed in Table S3. In the present reaction condition, pure RGO does not show any catalytic activity. The values of k_app for pure Ni_S- and Co_S-catalyzed reactions are 2.93 × 10^–3 and 2.23 × 10^–3 s^–1, respectively. The use of Co_D as the catalyst results in a decrease in both the induction time (from 6 to 3 min) and the reaction completion time (32 to 24 min) compared to the Co_S nanoparticle. k_app increases significantly due to the use of spherical CoNi_S (k_app = 4.33 × 10^–3 s^–1) as the catalyst instead of pure Co or Ni nanoparticle. This fact clearly indicates the beneficiary effect of the use of CoNi alloy as a catalyst instead of pure Co or Ni. The influence of the microstructure of the catalyst on the reaction kinetics was investigated by performing the reaction using spherical CoNi_S and snowflake-like dendritic CoNi_D as the catalyst. For the CoNi_D catalyst, the value of k_app (6.11 × 10^–3 s^–1) is higher than that of CoNi_S (k_app = 4.33 × 10^–3 s^–1), and the reaction completion time reduces from 21 to 17 min. These results indicate that CoNi_D acts as a better catalyst than CoNi_S. To develop a catalyst having enhanced efficiency, we have synthesized nanocomposites by immobilizing different amounts of CoNi_D on the surface of RGO. Figure S13 shows the higher catalytic activity of CoNi_D-RGO nanocomposites than that of CoNi_D. The catalyst containing 60 wt % CoNi_D and 40 wt % RGO,(CoNi_D)₆₀RGO₄₀ exhibits the highest catalytic activity with almost no initiation time and the minimum reaction completion time of 4 min with the highest k_app of 20.55 × 10^–3 s^–1 (Figure 3).

The reduction of 4-NP to 4-AP is a six-electron transfer reaction.⁵³ The metal nanoparticle-catalyzed reduction reaction of 4-NP to 4-AP proceeds via an electron transfer (ET)-induced hydrogenation mechanism. In the initial stage, BH₄^– ions are adsorbed on the surface of metal nanoparticles. Electron transfer from BH₄^– to metal nanoparticles resulted in the production of active hydrogen atoms, and these hydrogen atoms reduce 4-NP to 4-AP. Here, the metal nanoparticles act as storage of electrons after ET from BH₄^–.^16,54 Ballauff et al. have proposed the Langmuir–Hinshelwood mechanism for the reduction of 4-NP to 4-AP on Au nanoparticles⁵⁵ where both the reactants were adsorbed on the surface of the catalyst before the reaction. The reconstruction of the surface of the metal nanoparticles occurs when BH₄^– reacts with it and forms metal-H bonds.^15,56 The reaction between metal-H and adsorbed 4-NP results in the formation of 4-AP. This reaction proceeds as 4-nitrophenol → 4-nitrosophenol → 4-hydroxyaminophenol → 4-aminophenol.⁵⁷ The possible mechanism of the (CoNi_D)₆₀RGO₄₀-catalyzed reaction of 4-nitrophenol can be schematically presented as Scheme 1.

Scheme 1. Plausible mechanism for the (CoNi_D)₆₀RGO₄₀-Catalyzed Reduction Reaction of 4-Nitrophenol in the Presence of Excess NaBH₄ in an Aqueous Medium.

In the present investigation, the following important points were noted: (i) Co-Ni alloy exhibits better catalytic activity than that of individual pure Ni and pure Co. DFT calculation shows the generation of electron deficiency in Co in the Co–Ni system. These electron-deficient Co sites might act as better electron-storing sites during the electron transfer process from BH₄^– than pure metal nanoparticles. (ii) CoNi_D with a snowflake-like dendritic structure exhibits better catalytic activity than that of spherical CoNi_S. This could be due to the fact that, in CoNi_D, the electron transfer occurs more efficiently via a fast ballistic charge transfer mechanism due to its well-defined sharp-edged dendritic microstructure, whereas in spherical CoNi_S, the electron transfer proceeds via a relatively slow diffusion-controlled pathway. (iii) Immobilization of CoNi_D on the surface of RGO enhances the catalytic activity of the catalyst. (CoNi_D)₆₀RGO₄₀ demonstrates better catalytic efficiency than that of CoNi_D. As the reduction of 4-NP to 4-AP is a six-electron transfer reaction, the electrical conductivity of the catalyst plays a crucial role in this reaction. The EIS measurements of the catalysts clearly show that the conductivity of (CoNi_D)₆₀RGO₄₀ is higher than that of CoNi_D. DFT calculation also shows the existence of orbital level hybridization in the interface between Co, Ni, and C in the Co–Ni graphene system. Due to this orbital level hybridization between Co–Ni, Co–C, and Ni–C, the conductivity of the (CoNi_D)₆₀RGO₄₀ nanocomposite becomes higher than that of CoNi_D. Due to this enhanced conductivity, the electron transfer process occurs more efficiently in the RGO-containing catalysts, and they show greater catalytic activity. Moreover, (CoNi_D)₆₀RGO₄₀ possesses a high adsorption capacity of 4-NP.¹⁸ According to Rout et al., the strong adsorption of 4-NP on the surface of RGO causes stretching of the N=O bond of the −NO₂ group of 4-NP (from 1.23 to 1.28 Å), and this stretching of bond activates −NO₂ for the reduction reaction.⁵⁸ These factors enhance the catalytic efficiency of (CoNi_D)₆₀RGO₄₀ toward the reduction of 4-NP to 4-AP.

2.3.2. Knoevenagel Reaction

As the (CoNi_D)₆₀RGO₄₀ nanocomposite exhibits the highest catalytic activity amongst the synthesized materials toward the reduction of 4-nitrophenol to 4-aminophenol, we have explored the utilization of this nanocomposite as a catalyst for the Knoevenagel condensation reaction. Here, we have performed the condensation of an aromatic aldehyde with ethyl cyanoacetate in ethanol medium in the presence of (CoNi_D)₆₀RGO₄₀. To validate the (CoNi_D)₆₀RGO₄₀ nanocomposite as a catalyst for this reaction, we have used several aromatic aldehydes with different substitutions in the −Ph ring and obtained ∼80–93% product yield depending upon the aldehyde. In Table 1, the percentages of the yield of the obtained products are listed. The purity and chemical structures of the obtained products were verified by using FTIR, DSC (for melting point determination), LC–MS, and ¹H NMR analysis (Figures S14–S17).^21,28

Table 1. (CoNi_D)₆₀RGO₄₀-Catalyzed Knoevenagel Condensation Reaction of an Aromatic Aldehyde and Ethyl Cyanoacetate^a.

^aReaction condition: 4-nitrobenzaldehyde (1 mmol), ethyl cyanoacetate (1.2 mmol), ethanol (4 mL), catalyst dose (25 mg), reaction temperature: 50 °C, reaction time: 12 h.

A plausible mechanism for the Knoevenagel condensation reaction is presented in Scheme 2 on the basis of reported mechanisms.^21,23,59 The CoNi alloy nanoparticles act as the catalytically active centers in the catalyst. A DFT study shows that the interfacial interactions between Co, Ni, and RGO in the Co-Ni-RGO interface generate electron-deficient Co centers. The interaction between the carbonyl group of the aromatic aldehyde and Co causes the polarization of this group and the electrophilic character of the carbonyl carbon atom increases. This polarization helps in the attack of methylene carbon of ethyl cyanoacetate to the carbonyl carbon of the aromatic aldehyde. The cyano group of ethyl cyanoacetate also interacts with Co, which enhances the acidity of methylene carbon of ethyl cyanoacetate and helps in the formation of nucleophilic species via deprotonation of methylene carbon of ethyl cyanoacetate. The attack of this nucleophilic carbon to the carbonyl carbon of aromatic aldehyde results in the formation of a new C–C bond. In the protic solvent, this intermediate dissociates from the catalyst. Dehydration of this intermediate generates the final product, α,β-unsaturated carbonyl compound. In the present catalyst (CoNi_D)₆₀RGO₄₀, the presence of RGO offers the following advantages: (i) nanometer-thin RGO sheets provide a strong but mechanically flexible support to host the catalytically active centers (i.e., CoNi_D), (ii) immobilization of CoNi_D nanoparticles on the surface of RGO helps to prevent their agglomeration, therefore the reactant molecules get better accessibility to the catalytically active sites, (iii) the high surface area of RGO helps to absorb the reactant molecules on the surface of the (CoNi_D)₆₀RGO₄₀ catalyst via π–π stacking interaction, and (iv) the conducting nature of the RGO support enhances the electron transfer process of the catalysis reactions.

Scheme 2. Plausible Mechanism for the (CoNi_D)₆₀RGO₄₀-Catalyzed Knoevenagel Condensation Reaction of an Aromatic Aldehyde and Ethyl Cyanoacetate in Ethanol Medium.

2.3.3. Magnetic Separation and Reusability Test of the Catalyst

A VSM was used to determine the magnetic property of (CoNi_D)₆₀RGO₄₀, which shows its saturation magnetization (M_s) and coercivity (H_c) values are 65 emu g^–1 and 300 Oe (Figure S18), respectively. By exploiting the magnetic character of this catalyst, its separation from the reaction mixture was conducted by using a permanent magnet externally (Figure 4a,c). To test the reusability of the catalyst, we have recycled the catalyst up to six reaction cycles. Figure 4b,d demonstrates that the recovered catalyst retains almost the same efficiency even up to the six cycles. The examination of the recovered catalyst by XRD and FESEM shows no significant change in its crystal structure and microstructure (Figure S19) and indicates the structural robustness of the (CoNi_D)₆₀RGO₄₀ catalyst.

Figure 4.

Magnetic separation of (CoNi_D)₆₀RGO₄₀ catalyst from the reaction mixture and performance of recycles catalyst for (a, b) reduction reaction of 4-nitrophenol and (c, d) Knoevenagel condensation reaction.

2.4. Electrochemical Properties

As (CoNi_D)₆₀RGO₄₀ demonstrated its efficient performance as a catalyst, we have now explored the capability of this nanocomposite as an active electrode material for the supercapacitor application. To evaluate the electrochemical properties of the synthesized nanocomposites, CV and GCD measurements were performed. CV measurements were conducted to understand the electrochemical reactions, which are occurring on the surface of the electrodes in the presence of aqueous 3 M KOH solution as the electrolyte. To investigate the effect of microstructure of the active electrode materials on their electrochemical properties, working electrodes were prepared with CoNi_S and CoNi_D. The electrochemical reactions of Co and Ni in the alkaline medium have been investigated by several researchers and several reactions have been proposed, and reactions can be presented as follows:⁶⁰⁻⁶³

5

6

7

8

9

According to Behl and Toni, during the electro-oxidation reaction of Co in the KOH medium, a thin film of Co(OH)₂ forms on the surface of Co, which is subsequently oxidized to Co₃O₄ and CoOOH.⁶¹ In the case of Ni, the formation of Ni(OH)₂ and NiOOH occurs during the electro-oxidation process.^62,63

In the present case, the CV curves of CoNi_S and CoNi_D show a distinct pair of redox peaks within the potential window of 0–0.55 V versus Hg/HgO, and the anodic peak appears at ∼0.38 V and the cathodic peak at ∼ 0.30 V (Figure 5). The redox peaks around this potential range can be attributed to the conversion of Ni²⁺/Co²⁺ to Ni³⁺/Co³⁺, and the corresponding electrochemical reactions can be presented as reactions 7 and 9.⁶⁴ The identical nature of anodic and cathodic peaks indicates the reversibility of the electrochemical process.⁶⁵ The presence of distinct redox peaks in CoNi_S and CoNi_D electrodes within the potential window of 0–0.55 V versus Hg/HgO indicates the faradic behavior (i.e., pseudocapacitive) of these electrodes in the electrochemical process. Comparison of the CV profiles of pure Co and pure Ni (Figure S20) with that of CoNi alloy shows that the redox peaks for the pure Co and pure Ni electrodes appear at ∼0.38/0.30 V and 0.50/0.32 V, respectively, and in the case of CoNi alloy, overlapping of these redox peaks causes the broadening of the oxidation and reduction peak.⁶²Figure 5 shows that the area under the CV curve for CoNi_D is larger than that of CoNi_S and indicates that the specific capacitance of CoNi_D is greater than that of CoNi_S. This could be due to the difference in the microstructure between CoNi_D and CoNi_S. The sharp-edged dendritic structure of CoNi_D offers more active sites for the electrochemical reactions and better electron transfer ability to the electron collector.⁶⁶Figure S21 presents the CV profile of pure RGO, and the quasi-rectangular shape of this CV curve indicates the EDLC feature of RGO.⁶⁷Figure 5a also illustrates that the peak area under the CV curve of the (CoNi_D)₆₀RGO₄₀ nanocomposite is even larger than that of CoNi_D, demonstrating the higher specific capacitance of (CoNi_D)₆₀ RGO₄₀ than that of CoNi_D. The redox peaks of (CoNi_D)₆₀RGO₄₀ appear almost in the same positions, where they appear for CoNi_D. This fact indicates that the electrochemical reactions, which are happening for the CoNi_D electrode, are also occurring for (CoNi_D)₆₀RGO₄₀. The CV curves of the electrodes, which were measured at different scan rates (10–100 mV s^–1), are presented in Figures S22–S24, which shows the effect of the increase in scan rate on the peak potential and the peak current. With the increase in scan rate, the shifting of the anodic and cathodic peaks toward anodic and cathodic directions, respectively, can be observed in these CV profiles. This could be due to the development of an overpotential, which limited the faradic reactions occurring on the surface of the electrodes. The increase in peak current with increasing scan rate indicates the excellent rate capability of electrode materials.^30,68 The Randles–Sevcik plots of the electrodes are shown in Figure 5b, which shows a linear increase in peak current with the progressive increase in scan rate. This could be due to the influence of scan rate on the migration and diffusion of electrolytic ions into the electrode.

Figure 5.

(a) CV curves of the synthesized materials at 10 mV s^–1 in 3 M KOH and (b) Randles–Sevcik plots.

When the scan rate is relatively low, a thick diffusion layer of ions forms on the surface of the electrode, which restricts the migration of electrolyte ions into the electrode and resulted in the lowering of peak current. On the other hand, at a relatively high scan rate, this diffusion layer cannot grow on the electrode surface. Therefore, the migration of ion increases into the electrodes and ultimately enhances the peak current.⁶⁹ The specific capacitance (C_s) values of the electrodes were calculated from the GCD curves of CoNi_S (Figure S25), CoNi_D (Figure 6a), and (CoNi_D)₆₀RGO₄₀ (Figure 6b) at different current densities from 1 to 10 A g^–1 under an applied potential between 0 and 0.55 V. In 3 M KOH, CoNi_S, CoNi_D, and (CoNi_D)₆₀RGO₄₀ shows the C_s values of 89, 130, and 278 F g^–1, respectively, at a current density of 1 A g^–1. The dendritic CoNi_D possesses higher C_s than that of spherical CoNi_S, and the C_s value has further increased when CoNi_D nanoparticles are immobilized on RGO in (CoNi_D)₆₀RGO₄₀. The synergistic effect arising from the combination of the EDLC nature of RGO and the pseudocapacitance feature of CoNi_D are responsible for the outstanding performance of the (CoNi_D)₆₀RGO₄₀ nanocomposite. The energy density (E) and power density (P) of the electrodes were determined from the GCD. CoNi_S, CoNi_D, and (CoNi_D)₆₀RGO₄₀ electrodes exhibit the energy densities of 3.74, 5.5, and 11.68 Wh kg^–1, respectively, at a power density of 275 W kg^–1 in 3 M KOH, and a decreasing trend of energy density value with increasing power density. Figure S26a (Ragone Plot) illustrates this decrease in energy density with increasing power density. The cyclic behavior of the (CoNi_D)₆₀RGO₄₀ electrode was evaluated at a constant current density of 7 A g^–1 within a potential window of 0–0.55 V in 3 M KOH and presented as Figure S26b, which shows the retention of ∼85% capacitance up to 4000 cycles. The XRD pattern and FESEM micrograph of (CoNi_D)₆₀RGO₄₀ after charge–discharge cycles exhibit the almost retention of its crystalline phase and microstructure (Figure S27), indicating its structural stability.

Figure 6.

Galvanostatic charge–discharge curves of (a) CoNi_D and (b) (CoNi_D)₆₀RGO₄₀ electrodes at different current densities (1–10 A g^–1) in 3 M KOH in a three-electrode system.

The addition of a complementary redox couple with matching potential (e.g., [Fe(CN)₆]^4–/[Fe(CN)₆]^3– (0.20/0.37 V)) in the electrolyte system has been adopted by the researchers to enhance the specific capacitance of the electrode.⁷⁰ We have added 0.1 M K₄[Fe(CN)₆] aqueous solution to the 3 M KOH solution. In this condition, along with the faradic reactions of Co and Ni, an additional redox reaction, which is offered by [Fe(CN)₆]^4–/[Fe(CN)₆]^3–, occurs. Figure 7a shows the CV curves of the (CoNi_D)₆₀RGO₄₀ electrode when measurements were conducted with 3 M KOH and 3 M KOH + 0.1 M K₄[Fe(CN)₆] mixture. The CV curves clearly show that the addition 0.1 M K₄[Fe(CN)₆] in 3 M KOH electrolyte solution causes the increase in the area under the CV curve and indicates the enhancement of C_s. The [Fe(CN)₆]^4–/[Fe(CN)₆]^3– redox couple acts as an electron buffer source in the redox reactions at the electrode/electrolyte interface, which enhances the specific capacitance. The addition of 0.1 M K₄[Fe(CN)₆] to 3 M KOH electrolyte solution results in the increase in C_s of (CoNi_D)₆₀RGO₄₀ to 501 F g^–1 at a current density of 6 A g^–1 (Figure 7b). This value of C_s decreases to 334 F g^–1 when the current density increases to 20 A g^–1 showing ∼66.7% retention of C_s and indicates its good rate capability. In this electrolyte system, the (CoNi_D)₆₀RGO₄₀ nanocomposite exhibits an energy density of 21.08 Wh kg^–1 at a power density of 1650 W kg^–1. Figure S28a presents the Ragone plot, which shows the decrease in energy density up to 14.05 Wh kg^–1 when the power density increases to 5500 W kg^–1. Figure S28b demonstrates the cyclic behavior of the (CoNi_D)₆₀RGO₄₀ and shows the retention of ∼85% capacitance after 4000 cycles when the testing was performed at a current density of 7 A g^–1.

Figure 7.

(a) CV curves of (CoNi_D)₆₀RGO₄₀ at 10 mV s^–1 in 3 M KOH electrolyte and 3 M KOH + 0.1 M K₄[Fe(CN)₆] electrolyte. (b) GCD curves of (CoNi_D)₆₀RGO₄₀ electrode in 3 M KOH + 0.1 M K₄[Fe(CN)₆] electrolyte.

To mimic a supercapacitor device, a two-electrode symmetric cell was devised using (CoNi_D)₆₀RGO₄₀ as an electrode material though it is a well-known fact that two-electrode systems show a lower C_s value than that of a three-electrode system.⁷¹ The reason for the lower capacitance value in the case of the two-electrode system as compared to the three-electrode system is due to the difference in the packaging and configuration of the cells. In a three-electrode system, only one working electrode contains the active material being analyzed, whereas in the case of a two-electrode system, two symmetric working electrodes where both the electrodes contain the active material are used, which leads to a difference in the applied voltage and charge transfer across a single electrode in comparison to two electrodes.⁷² Also, in a symmetrical two-electrode cell, the potential differences applied to each electrode are equal to each other and are one-half of the values of the applied voltage potential.⁷² CV and GCD measurements of the (CoNi_D)₆₀RGO₄₀ electrode in a two-electrode symmetric system to choose the appropriate operating potential window measurements were conducted using different potential windows at a scan rate of 10 mV s^–1, and it was observed that a stable voltammogram was obtained when the operating window was 0–0.8 V.⁷³Figure 8 shows the CV profile and GCD plot of the (CoNi_D)₆₀RGO₄₀ electrode when measurements were conducted in the two-electrode system using 3 M KOH, and 3 M KOH + 0.1 M K₄[Fe(CN)₆] electrolyte solutions in a potential window of 0–0.8 V. The C_s values thus obtained are 125 F g^–1 at a current density of 1 A g^–1 in 3 M KOH and 405 F g^–1 at a current density of 3 A g^–1 in 3 M KOH + 0.1 M K₄[Fe(CN)₆]. Figure S29 presents the specific capacitance and Coulombic efficiency of the (CoNi_D)₆₀RGO₄₀ electrode as a function of current density in 3 M KOH + 0.1 M K₄[Fe(CN)₆] electrolyte solution. It was observed that, though the specific capacitance of the electrode is decreased with increasing current density, the Coulombic efficiency is found to be increased from 50 to 90% when the current density is increased from 3 to 10 A g^–1.⁷⁴⁻⁷⁸ (CoNi_D)₆₀RGO₄₀ also exhibits the value of energy density of 9 Wh kg^–1 when the power density is 1200 W kg^–1in 3 M KOH + 0.1 M K₄[Fe(CN)₆] and energy density decreases with increasing power density. The energy density becomes 4 Wh kg^–1 when the power density is 8000 W kg^–1.

Figure 8.

(a, b) GCD curves of (CoNi_D)₆₀RGO₄₀ electrode in (a) 3 M KOH electrolyte and (b) 3 M KOH + 0.1 M K₄[Fe(CN)₆] electrolyte. (c) CV curves of (CoNi_D)₆₀RGO₄₀ electrode at 10 mV s^–1 in 3 M KOH and 3 M KOH + 0.1 M K₄[Fe(CN)₆] electrolyte in a two-electrode symmetric cell system.

The electrochemical impedance spectroscopy (EIS) measurements were performed for the synthesized materials to determine their charge transfer resistance (R_ct) and internal resistance (R_s). Nyquist plots were generated, and an equivalent circuit was constructed to fit the impedance data. In the Nyquist plot, the intercept of Z′ (real axis) in the high-frequency region provides the value of R_s. The combination of the following effects is the origin of R_s: (i) the contact resistance between the electrode material and the current collector, (ii) the intrinsic resistance of the electrode materials, and (iii) the ionic resistance of the electrolyte.⁷⁹ In the Nyquist plot, the diameter of the semicircle at the intermediate frequency range indicates the value of R_ct, and R_ct indicates the faradic pseudocapacitance of the electrode materials. The Nyquist plots of the synthesized materials are shown in Figure 9a, and R_ct and R_s values are listed in Table S3. EIS analysis revealed that (i) R_ct and R_s values of CoNi_D are lower than those of spherical CoNi, (ii) immobilization of CoNi_D on the surface of RGO causes a further decrease in R_ct and R_s values, and (iii) R_ct and R_s values of the (CoNi_D)₆₀RGO₄₀ nanocomposite are lower than those of pure RGO. This could be due to the fact that the charge transfer occurs more efficiently in CoNi_D because of its well-defined sharp-edged dendritic structure. In this case, the charge transfer occurs via a ballistic charge transport mechanism, which is faster than that of a diffusive transport mechanism that occurs in spherical CoNi_S, (iv) the (CoNi_D)₆₀RGO₄₀ nanocomposite possesses higher conductivity than that of pure RGO and CoNi_D. In the nanocomposite, CoNi_D nanoparticles are in close contact with RGO. Strong interfacial interaction between CoNi_D and electron-rich RGO provides a shorter transport path to the electrons, which helps to enhance the conductivity of the CoNi_D-RGO nanocomposite.

Figure 9.

(a) Electrochemical impedance spectra of the synthesized materials, and the insets show the high-frequency region and equivalent circuit used for the fitting of Nyquist plot. (b) |Z| versus frequency plot, (c) Phase angle versus frequency plots.

To understand the capacitive behavior and determine the “knee frequency”, Bode plots were generated by plotting |Z| versus frequency (Figure 9b) and phase angle versus frequency (Figure 9c). In the |Z| versus frequency plots, the materials with a higher conductivity show relatively lower |Z| value at the low frequency. Figure 9b shows a lower |Z| value of CoNi_D (|Z| = 32.48 Ω) than that of CoNi_S (|Z| = 38.91 Ω), indicating the higher conductivity of CoNi_D. (CoNi_D)₆₀RGO₄₀ (|Z| = 24.56 Ω) also shows a lower |Z| value than that of pure RGO (|Z|= 31.14 Ω) and CoNi_D, which also indicates the enhanced conducting nature of the (CoNi_D)₆₀RGO₄₀ nanocomposite compared to CoNi_D and RGO.

In the phase angle versus frequency plot, the phase angle of 90° in the low frequency suggests the ideal capacitive behavior of the electrode, and the phase angle of 0° indicated the pure resistance behavior.^80,81 The knee frequency (f_o) is the frequency at the phase angle of 45°, and in this frequency, the capacitive and resistive impedance are equal. In Figure 9c, the higher f_o value of (CoNi_D)₆₀RGO₄₀ compared to CoNi_S and CoNi_D indicates the high rate capability of this nanocomposite. The higher relaxation time (τ = f_o^–1) of (CoNi_D)₆₀RGO₄₀ (4.1 × 10^–3 s) than those of CoNi_S (7.3 × 10^–3 s) and CoNi_D (5.7 × 10^–3 s) indicates that, in the nanocomposite, the immobilization of dendritic CoNi_D particles on the surface of RGO creates shorter ion transport paths, which helps to increase its electrical conductivity by decreasing the internal resistance. The enhanced conductivity of (CoNi_D)₆₀RGO₄₀ is one of the important factors that are responsible for its superior supercapacitive nature than CoNi_S or CoNi_D.

Investigation of the electrochemical properties of the synthesized materials revealed that the C_s value of CoNi_D is greater than that of CoNi_S and this value further increases when (CoNi_D)₆₀RGO₄₀ is used as an electrode material. (CoNi_D)₆₀RGO₄₀ shows a C_s of 501 F g^–1 at a current density of 6 A g^–1 along with the good retention of the capacitance and rate capability property. It also possesses an energy density of 21.08 Wh kg^–1 at a power density of 1650 W kg^–1 in a three-electrode system and an energy density of 9 Wh kg^–1 when the power density is 1200 W kg^–1 in a two-electrode symmetric system, which are comparable to the commercial supercapacitors (3–9 Wh kg^–1 at 3000–10,000 W kg^–1).^30,82Figure 10 shows the illumination of seven red LED bulbs when three pair of electrodes were connected in series to construct the two-electrode system. The illumination of seven red LED bulbs for over 3 min demonstrates the potential of (CoNi_D)₆₀RGO₄₀ to be used as an active electrode material for the construction of a supercapacitor.

Figure 10.

Demonstration of the lighting of a series of red light-emitting diode (LED; 3 V each) powered by three (CoNi_D)₆₀RGO₄₀ electrodes connected in series. (a–d) Glowing LEDs at different time intervals.

3. Conclusions

We have reported a simple “one-pot” methodology to synthesize snowflake-like dendritic CoNi alloy-RGO nanocomposites. The effect of microstructure on the catalytic property and the electrochemical property of CoNi alloy has been demonstrated here. Compared to spherical CoNi_S, snowflake-like dendritic structure CoNi_D has exhibited better performance as a catalyst for the reduction reaction of 4-nitrophenol and Knoevenagel reaction and as an active electrode material for supercapacitor applications. The nanocomposite that was prepared by immobilizing (60 wt %) CoNi_D on the surface of RGO (40 wt %) showed superior performance in the aforesaid applications than CoNi_D and CoNi_S. The synergistic effect, which arises from the coexistence of CoNi_D and RGO in the hierarchical structure of the CoNi_D-RGO nanocomposite, is responsible for its superior performance than the individual components. First-principles calculations based on DFT provided insight into the interfacial interactions between Co, Ni, and graphene in the Co-Ni-graphene superlattice. The obtained electronic structures of the Co–Ni interface and Co-Ni-graphene superlattice were helpful to understand why CoNi alloy showed better performance than that of pure Co and Ni and why the CoNi-RGO nanocomposite did exhibit improved properties compared to CoNi alloy. (CoNi_D)₆₀RGO₄₀ demonstrated its multifunctional character by exhibiting its high efficiency as a magnetically separable catalyst for two important reactions and as an excellent electrode material for high-performance supercapacitors. The k_app value for the (CoNi_D)₆₀RGO₄₀-catalyzed reduction reaction of 4-nitrophenol is 20.55 × 10^–3 s^–1, which is comparable and, in some cases, superior to many RGO-based catalysts. The % yield of the products obtained for the (CoNi_D)₆₀RGO₄₀-catalyzed Knoevenagel condensation reaction between several aromatic aldehydes and ethyl cyanoacetate lies in the range of 80–93%. In addition to showing this versatile catalytic activity, (CoNi_D)₆₀RGO₄₀ also offers an advantage of its easy magnetic separation by the use of an external magnet. This overcomes the separation-related limitations that many nanostructured catalysts encounter. The recycled catalysts also showed satisfactory performance. The specific capacitance value of (CoNi_D)₆₀RGO₄₀ was 501 F g^–1 at a current density of 6 A g^–1. Along with good rate capability and ∼85% retention of capacitance, it also exhibited 21.08 Wh kg^–1 energy density at a power density of 1650 W kg^–1. These results indicate that (CoNi_D)₆₀RGO₄₀ could be considered as a favorable electrode material for the construction of high-performance supercapacitors. The easy synthetic methodology, high catalytic efficiency, and excellent supercapacitance performance make (CoNi_D)₆₀RGO₄₀ an appealing multifunctional material.

4. Experimental Section

4.1. Synthesis of Materials

Two wet chemical methods have been used to prepare CoNi alloy particles possessing two distinct types of morphology. These two methods were also used to prepare pure Co and pure Ni nanoparticles. A simple but novel “one-pot” method was employed to prepare CoNi-RGO nanocomposites.

4.1.1. Method-I: Synthesis of Spherical CoNi Alloy Particles (CoNi_S)

In this method, a calculated amount of CoCl₂·6H₂O and NiCl₂·6H₂O was dissolved in a mixture of ethylene glycol and polyvinylpyrrolidone (molar ratio 2:1). To this mixture, NaOH pellets were dissolved till the pH of the solution became ∼10 and then N₂H₄ was added to it (metal ion/N₂H₄ molar ratio 1:40). This reaction mixture was refluxed at 85 °C for 15 min. After cooling it at room temperature, the precipitate thus formed was separated, washed, and dried. As the alloy particles, which were formed in this method were spherical in shape, now onward, we will refer them as CoNi_S.

4.1.2. Method-II: Synthesis of Dendritic CoNi Alloy Particles (CoNi_D)

In this methodology, a coprecipitation-reduction technique was used in the presence of cetyltrimethylammonium bromide (CTAB).¹⁴ CoNi alloy particles were synthesized by using CoCl₂·6H₂O and NiCl₂·6H₂O (molar ratio 2:1) as starting materials. The metal chloride salts were mixed with a solution of CTAB (0.1 mmol) in ethanol. After stirring this mixture thoroughly, it was heated at 55 °C, and then a solution containing NaOH and N₂H₄ was added to it. This reaction mixture was refluxed for 30 min at 55 °C. The black precipitate thus formed was separated, washed with distilled water, and dried. As the FESEM micrograph of these particles revealed their snowflake-like dendritic structure, now onward, these CoNi alloy particles will be referred to as CoNi_D.

4.1.3. Synthesis of CoNi_D-RGO Nanocomposites

A “one-pot” synthetic methodology was employed to prepare the CoNi_D-RGO nanocomposites having a different amount of CoNi_D and RGO. First, graphene oxide (GO) was prepared by using a modified Hummers method.⁸³ Then, in a round-bottom flask, a calculated amount of GO was dispersed in a mixture of ethanol and CTAB. In this mixture, an appropriate amount of CoCl₂·6H₂O and NiCl₂·6H₂O (molar ratio 2:1) was added. This mixture was heated at 55 °C with continuous stirring. A solution containing NaOH and N₂H₄ was added to it dropwise, and then it was refluxed at 55 °C for 30 min. After reflux, the reaction mixture was allowed to cool down at room temperature. The precipitate thus formed was separated, washed, and finally dried at 60 °C. The flow chart for the synthesis of CoNi_D-RGO nanocomposite is presented as Scheme 3.

Scheme 3. Schematic Presentation of the Synthesis of (CoNi_D)-RGO.

4.2. Catalytic Activity Tests

4.2.1. Reduction of 4-Nitrophenol to 4-Aminophenol

For evaluating the catalytic property of the as-prepared catalysts, the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) was conducted in the presence of excess NaBH₄ in an aqueous medium. In a typical run, 4.5 mL of 9 × 10^–2 mM aqueous solution of 4-NP was mixed with 0.5 mL of H₂O and 1 mL of 0.2 M NaBH₄ solution followed by the addition of a 2 mL aqueous suspension of the catalyst (0.1 g L^–1). Four milliliters of this mixture was transferred immediately in a quartz cuvette, and the absorbance spectra of the solution were recorded using a UV–Vis spectrophotometer at an interval of 1 min. The progress of the reduction reaction of the 4-NP was monitored by following the decrease in the intensity of the λ_max peak at 400 nm of the 4-NP with time. In the reaction, the concentration of NaBH₄ was in excess, and it remained almost constant throughout the reaction. It is a well-proven fact that, in the aqueous medium, the nanoparticle-catalyzed reduction reaction of 4-nitrophenol in the presence of excess NaBH₄ follows the pseudo-first-order kinetics.^16,53 As the absorbance of 4-NP at λ_max is proportional to its concentration, the ratio of the absorbance of 4-NP A_t (measured at time t) to A₀ (at t = 0) is equal to C_t/C₀ (where C₀ is the initial concentration and C_t is the concentration at time t of 4-NP). The apparent rate constant (k_app) was determined using the following equations:

10

11

The value of k_app was calculated from the slope of the ln(A_t/A₀) versus time plot.

4.2.2. Knoevenagel Condensation Reaction

Different aromatic aldehydes and ethyl cyanoacetate were used as starting materials. In a typical reaction, an aldehyde and ethyl cyanoacetate (1:1 molar ratio) were dissolved in 4 mL of ethanol in a round-bottom flask. To this reaction mixture, 25 mg of catalyst was added and refluxed at 50 °C for 12 h. The progress of the reaction was monitored by using thin layer chromatography (TLC).

After completion of the reaction, the catalyst was separated with the help of an external magnet, and ethanol was evaporated from the reaction mixture. Ten milliliters of ethyl acetate was added to it, and the product was obtained by crystallization. The product was further purified by column chromatography using a mixture of ethyl acetate and petroleum ether as the eluent.

4.2.3. Recycling of the Catalyst

After each reaction cycle, the catalyst was recovered from the reaction mixture by applying the magnetic separation technique using a permanent magnet (N35 grade NdFeB magnet having an energy product BH_max = 33–36 MGOe) externally. After separation, the recovered catalyst was washed with distilled water and ethanol. Then, the catalyst was dried and used for the next reaction cycle.

4.3. Electrochemical Testing

To determine the electrochemical properties of the synthesized materials, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge–discharge (GCD) measurements were performed by using a three-electrode system. The synthesized materials (e.g., CoNi_S, CoNi_D, pure RGO, and CoNi_D-RGO nanocomposite) were used as active electrode materials for the fabrication of working electrodes. To prepare the working electrodes, a viscous paste was prepared by mixing 10 wt % poly(vinylidene fluoride), 10 wt % acetylene black, and 80 wt % active material in N-methyl-2-pyrrolidinone. This paste was cast on (1.5 cm × 1.5 cm) nickel foam (thickness, ∼0.2 mm). After casting the electrode material, the residual solvent was removed from the electrode by drying the electrode at 60 °C for 24 h in a vacuum oven. In the working electrode, the weight of the active materials was ∼3 mg. A double junction Hg/HgO electrode was used as the reference electrode and a gold electrode as the counter electrode. The electrochemical measurements were performed in two electrolytes: (i) 3 M KOH aqueous solution and (ii) a mixture of 3 M KOH and 0.1 M K₄[Fe(CN)₆]. All the CV measurements were recorded using a potential window of 0–0.55 V at different scan rates ranging from 10 to 100 mV s^–1. Galvanostatic charge–discharge (GCD) measurements were performed at different current densities ranging from 1 to 20 A g^–1. Electrochemical impedance spectroscopy (EIS) measurements were carried out in the frequency range of 0.01–10,000 Hz at an open circuit potential with an alternating current amplitude of 0.01 V.

Specific capacitance (C_s), energy density (E), and power density (P) of the synthesized materials were determined from the GCD measurements by using the following equations

12

13

14

where i (A) represents the charge or discharge current, ΔE (V) is the applied potential window, m (g) is the mass of the active electrode material, Δt (s) is the discharge time, E is the energy density (Wh kg^–1), C_s (F g^–1) is the specific capacitance based on the mass of the electroactive material, V is the potential window of discharge (V), P is the power density (W kg^–1), and Δt is the discharge time (s).

Coulombic efficiency of the synthesized material was calculated by using the following equation

15

where η is the Coulombic efficiency, t_D is the discharging time (s), and t_C is the charging time (s).

The synthesized materials were characterized by using an X-ray diffractometer, field emission scanning electron microscope, Raman spectrometer, and vibrating sample Magnetometer (VSM). UV–Vis spectroscopy, Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC), liquid chromatography–mass spectrometry (LC–MS), and ¹H NMR were used to monitor the catalysis reactions and characterize the products obtained from these reactions. Cyclic voltammetry (CV) was used to study the electrochemical properties of the synthesized materials. Details of the chemicals and the instruments used for this purpose are provided in the Supporting Information.

4.4. First-Principles Calculations

To perceive the electronic structures of Co unit cell, Ni unit cell, Co–Ni interface, and Co-Ni-graphene superlattice, we have conducted the first-principles quantum mechanical calculations based on DFT. Quantum ESPRESSO computational package^51,84 was used to calculate the ground-state structures, binding energy, density of states (DOS), projected density of states (PDOS), and total density of state (TDOS) using a plane-wave set and pseudopotentials.^51,84 DFT was used with generalized gradient approximation (GGA) and parameterized by Perdew, Burke, and Ernzerhof (PBE).⁸⁵ Kohn–Sham orbitals were expanded in a plane-wave basis set up to the kinetic energy cutoff 30 Ry (408.17 eV). The convergence criterion for the self-consistent calculation was 10^–7 Ry per Bohr (0.0257 eV Å^–1). For calculating the final electronic properties of the structures, an empirical dispersion-corrected density functional theory (DFT-D2) approach, which was proposed by Grimme, was employed.^53,86 Grimme-D2 corrections were used to account for the intermolecular interactions and van der Waals (vdW) interactions.⁸⁶

In the present study, five systems were investigated: (i) pure Co having a hexagonal close packing structure (space group P6₃/mmc), (ii) pure Ni having a face-centered cubic structure (space group Fm3̅m), (iii) graphene superlattice, (iv) Co–Ni interface, and (v) Co-Ni-graphene superlattice. Ultrasoft pseudopotentials for these systems were constructed by using 15, 16, and 4 electrons for Co (3p⁶3d⁷4s²), Ni (3p⁶3d⁸4s²), and C (2s²2p²), respectively. From the Quantum ESPRESSO website, the pseudopotentials of Co, Ni, and C were chosen.⁸⁷ To select the k-point mesh, the Monkhorst–Pack approach⁸⁸ was used, and the details of k points for each system are provided in the Computational details section in the Supporting Information. The Brillouin zone integration of these systems was performed with Methfessel–Paxton smearing technique⁸⁹ for Co and Ni unit cells. The Marzari–Vanderbilt⁹⁰ smearing technique was used for graphene, Co–Ni interface, and Co-Ni-graphene superlattice. Here, the smearing parameter was 0.005 Ry. The binding energy of the Co-Ni-graphene superlattice was calculated from the difference between the total energy of the Co-Ni-graphene superlattice and the sum of each system alone (Co-Ni interface and graphene). The sizes of the unit cells of the simulated systems are listed in Table S4. The details of the sample input files for the geometric optimization of Co, Ni, graphene, Co–Ni, and Co-Ni-graphene superlattice are provided in the Computational details section in the Supporting Information.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.9b02861.

• (Table S1) Comparison of the catalytic efficiency of different reported catalysts for the reduction reaction of 4-NP in the presence of NaBH₄; (Figure S1) XRD pattern of RGO, pure Co_D, and Ni_S; (Figure S2a) EDS spectra of synthesized CoNi_S nanocomposites; (Figure S2b) FESEM micrographs and elemental color mapping of synthesized CoNi_S nanocomposites; (Figure S3a) EDS spectra of synthesized CoNi_D nanocomposites; (Figure S3b) FESEM micrographs and elemental color mapping of synthesized CoNi_D nanocomposites; (Figure S4a) EDS spectra of synthesized (CoNi_D)₆₀RGO₄₀ nanocomposites; (Figure S4b) FESEM micrographs and elemental color mapping of synthesized (CoNi_D)₆₀RGO₄₀ nanocomposites; (Figure S5) room temperature wide-angle powder XRD pattern of GO; (Figure S6) Raman spectra of GO and the (CoNi_D)₆₀RGO₄₀ nanocomposite; (Figure S7a) the initial structure of Co unit cell, Ni unit cell, Co–Ni interface, Co-Ni-graphene superlattice, and graphene; (Figure S7b) the optimized structure of Co unit cell, Ni unit cell, Co–Ni interface, Co-Ni-graphene superlattice, and (e) graphene; (Figure S8) the band structure and density of states of Co unit cell; (Figure S9) the band structure and density of states of Ni unit cell; (Figure S10) the band structure and density of states of Co–Ni interface; (Figure S11) the band structure and density of states of Co-Ni-graphene superlattice; (Figure S12) the band structure and density of states of graphene; (Figure S13) time-dependent UV–Vis spectral changes of the reaction mixture of 4-NP catalyzed by Ni_S, Co_S, Co_D, (CoNi_D)₉₀RGO₁₀, (CoNi_D)₈₀RGO₂₀, (CoNi_D)₇₀RGO₃₀, (CoNi_D)₅₀RGO₅₀, and (CoNi_D)₄₀RGO₆₀, pseudo-first-order kinetic plot of 4-NP reduction reaction with pure Ni_S, Co_S, Co_D, (CoNi_D)₉₀RGO₁₀, (CoNi_D)₈₀RGO₂₀, (CoNi_D)₇₀RGO₃₀, (CoNi_D)₅₀RGO₅₀ and (CoNi_D)₄₀RGO₆₀; (Table S2) details of completion time and rate constant of the synthesized catalyst for reduction of 4-nitrophenol in the presence of NaBH₄; product of Knoevenagel condensation reaction (Table 1); (Figure S14) FTIR spectra of (E)-ethyl 2-cyano-3-phenyl acrylate, (E)-ethyl 3-(4-chlorophenyl)-2-cyanoacrylate, (E)-ethyl 2-cyano-3-(4-nitrophenyl)acrylate, (E)-ethyl 2-cyano-3-(2-methoxyphenyl)acrylate, (E)-ethyl 2-cyano-3-(p-tolyl)acrylate, (E)-ethyl 2-cyano-3-(4-methoxyphenyl)acrylate, and (E)-ethyl 3-(4-bromophenyl)-2-cyanoacrylate; (Figure S15) DSC of (E)-ethyl 2-cyano-3-phenylacrylate, (E)-ethyl 3-(4-chlorophenyl)-2-cyanoacrylate, (E)-ethyl 2-cyano-3-(4-nitrophenyl)acrylate, (E)-ethyl 2-cyano-3-(2-methoxyphenyl)acrylate, (E)-ethyl 2-cyano-3-(p-tolyl)acrylate, (E)-ethyl 2-cyano-3-(4-methoxyphenyl)acrylate, and (E)-ethyl 3-(4-bromophenyl)-2-cyanoacrylate; (Figure S16a) LC–MS full scan of (E)-ethyl 2-cyano-3-phenylacrylate; (Figure S16b) LC–MS full scan of (E)-ethyl 3-(4-chlorophenyl)-2-cyanoacrylate; (Figure S16c) LC–MS full scan of (E)-ethyl 2-cyano-3-(4-nitrophenyl)acrylate; (Figure S16d) LC–MS full scan of (E)-ethyl 2-cyano-3-(2-methoxyphenyl)acrylate; (Figure S16e) LC–MS full scan of (E)-ethyl 2-cyano-3-(p-tolyl)acrylate; (Figure S16f) LC–MS full scan of (E)-ethyl 2-cyano-3-(4-methoxyphenyl)acrylate; (Figure S16g) LC–MS full scan of (E)-ethyl 3-(4-bromophenyl)-2-cyanoacrylate; (Figure S17a) ¹H NMR spectrum of (E)-ethyl 2-cyano-3-phenylacrylate; (Figure S17b) ¹H NMR spectrum of (E)-ethyl 3-(4-chlorophenyl)-2-cyanoacrylate; (Figure S17c) ¹H NMR spectrum of (E)-ethyl 2-cyano-3-(4-nitrophenyl)acrylate; (Figure S17d) ¹H NMR spectrum of (E)-ethyl 2-cyano-3-(p-tolyl)acrylate; (Figure S17e) ¹H NMR spectrum of (E)-ethyl 2-cyano-3-(2-methoxyphenyl)acrylate; (Figure S17f) ¹H NMR spectrum of (E)-ethyl 2-cyano-3-(4-methoxyphenyl)acrylate; (Figure S17g) ¹H NMR spectrum of (E)-ethyl 3-(4-bromophenyl)-2-cyanoacrylate; (Figure S18) room temperature magnetic hysteresis loop of (CoNi_D)₆₀RGO₄₀; (Figure S19) FESEM images and XRD of the recycled (CoNi_D)₆₀RGO₄₀ catalyst; (Figure S20) cyclic voltammetry curves of pure Co and pure Ni electrodes at different scan rates in 3 M KOH electrolyte; (Figure S21) cyclic voltammetry curve of RGO in 3 M KOH in the three-electrode system; (Figure S22) cyclic voltammetry curves of CoNi_S in 3 M KOH at different scanning rates (10–100 mV s^–1); (Figure S23) cyclic voltammetry curves of CoNi_D in 3 M KOH at different scanning rates (10–100 mV s^–1); (Figure S24) cyclic voltammetry curves of CoNi_D)₆₀RGO₄₀ in 3 M KOH at different scanning rates (10–100 mV s^–1); (Figure S25) Galvanostatic charge–discharge curves of CoNi_S electrodes at different current densities (1–10 A g^–1) in 3 M KOH in a three-electrode system; (Figure S26) Ragone plots of CoNi_S, CoNi_D, and (CoNi_D)₆₀RGO₄₀ electrodes and cyclic stability of the (CoNi_D)₆₀RGO₄₀ electrode in 3 M KOH showing the capacitance retention after 4000 cycles using a charge–discharge at a constant current density of 7 A g^–1; (Figure S27) XRD pattern and SEM micrograph of the (CoNi_D)₆₀RGO₄₀ electrode after charge–discharge cycles; (Figure S28) Ragone plots of (CoNi_D)₆₀RGO₄₀ electrodes in 3 M KOH + 0.1 M K₄[Fe(CN)₆] and cyclic stability of the (CoNi_D)₆₀RGO₄₀ electrode in 3 M KOH + 0.1 M K₄[Fe(CN)₆] showing the capacitance retention after 4000 cycles using a charge–discharge at a constant current density of 7 A g^–1; (Figure S29) a plot of Coulombic efficiency and specific capacitance of (CoNi_D)₆₀RGO₄₀ as a function of current density; (Table S3) fitting results of the EIS data of all the synthesized nanocomposites; Materials, Characterization and Instrumentation, and Computational details sections; (Table S4) the sizes of the unit cells of simulated systems; and details of the input files for geometric optimization of the Co unit cell, Ni unit cell, graphene superlattice, Co–Ni interface, and Co-Ni-graphene superlattice (PDF)

The authors declare no competing financial interest.