Nickel nanoparticles coated on the exfoliated graphene layer as an efficient and stable catalyst for oxygen reduction and hydrogen evolution in alkaline media

In this work, we report simultaneous electrochemical exfoliation of graphite powder using SDS, anionic surfactant salts, and cyclic potential to prepare graphene on carbon paper. Then, Nickel is electro-reduced into graphene nanosheets on carbon paper and also on the bare carbon paper to use in alkaline media for hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR). Afterward, graphene and Ni-graphene are characterized using scanning electron microscopy, atomic force microscopy (AFM) and electrochemical technique. SEM images show the Cauliflower structure of Ni in the absence of graphene and nanoparticle shapeless in the presence of smooth graphene. The electrochemical results show an excellent catalytic activity of Ni-graphene/ carbon paper with an over potential of 90 mV (Versus Ag/AgCl), which is lower than the literature value for Ni in alkaline electrolyte for HER (120 mV dec−1). The effect of graphene support on the electrochemical impedance spectroscopy response, activation energy and HER activity of the samples are investigated carefully. Finally, we prepare a novel gas diffusion electrode by using Ni pasted on carbon paper for the ORR in fuel cells and compared it with standard Pt/C catalysts using linear sweep voltammetry.


Introduction
Due to urgent demand for the development of energy storage system, the production of clean and renewable fuel attracts great attentions [1,2]. Several of these sustainable technologies, such as fuel cells, metal-air batteries and electrolyzers, strongly depended on kinetics path way of oxygen reduction reaction (ORR) [3] and hydrogen evolution reaction (HER) [4,5]. In spite of extraordinary efforts, developing catalysts for ORR and HER with high activity at low costs remains a grand challenge.
Molecular hydrogen (H 2 ), which has the highest gravimetric energy density and non-polluting product, is highly regarded as the promising candidate for the future energy storage [6][7][8]. There are three main methods for hydrogen production namely coal gasification (chemical), steam reformation of hydrocarbons and electrochemical water electrolysis [9]. Nowadays, hydrogen production from coal gasification and methane reforming leads to CO 2 emissions. But, water electrolysis only produces H 2 and O 2 that are clean products. Although tremendous progress has been achieved, highly efficient electro-catalysts or low overpotential still set hurdles for real applications [7,[10][11][12][13][14][15]. Besides, the current bottleneck of cathode side resides in the sluggish ORR on fuel cells [3,16]. Hence, study of stable and efficient electro-catalysts for H 2 evolution from water and oxygen reduction for fuel cells still remain a great challenge.
Up to now, noble metal electrocatalysts such as Pt, Au,Ru and Pd still provide the best performance in water splitting for electrolyzers as well as ORR for cathode side of fuel cells, but extensive utilization are seriously limited by the high cost [11,[17][18][19][20][21]. Thus, developing highly active catalysts for HER and ORR based on large surface area, stability of performance, availability in abundant and low cost metals is important to affordable the hydrogen production and fuel cell industries. In regard to this, great progresses have been achieved under the Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. development of high-performance HER and ORR, excellent durability , cheaper and abundant electro-catalysts such as metal free electrocatalysts [22,23], transition-metals based hybrid [24], phosphide [25], oxide [26], sulfide [27][28][29] and carbon-based compounds [30][31][32][33][34].
Graphene, 2D form of carbon, has received great scientific attention in recent years due to its high electronic and thermal conductivity, extraordinary of charge mobility, high conductivity, large surface area, etc. [22,42,43].
3d transition metals encapsulation into graphene support has been suggested by several researchers as a strategy for HER and ORR in alkaline media [29,[44][45][46][47]. Toward this end, Li et al [30] synthesized the transition metal (Co and Ni) on a graphene protected Ni substrate by a simple electro-deposition method. They found that the Ni/G substrate is stable in alkaline electrolytes and thus can be used as HER catalyst. Moreover, Chang et al [40], found that rGO hybrids with NiCoP could exhibit a comparable HER performance in the alkaline and acidic environment. Recently, interesting finding on the effect of embedded graphene in Fe-Ni electrodes was used for hydrogen generation in alkaline solution [48]. The rGO introduction to porous 3D projections of nanosized Fe-Ni alloy successfully increases the electrochemically active surface area, resulting further water splitting. McKone et al [49], synthesized unsupported Ni-Mo nanopowders that exhibit high catalytic activity for the HER. They evaluated the composition, morphology, catalytic activity, and stability of Ni-Mo nanopowders during hydrogen evolution in aqueous acidic and alkaline conditions in detail. In regard to ORR several groups of researchers have conducted detailed case study of graphene or graphene decorated metal particles. For example, porous nitrogen-doped graphene prepared through pyrolysis of ammonium acetate as an efficient electro-catalyst for ORR. [50]. Jiang et al [51] prepared a 3D porous cellular NiCoO 2 /graphene using a sodium dodecyl sulfate via purification assembly of NiCoO 2 precursor and rGO for ORR. They reported that the fabricated catalyst shows excellent potentials toward several applications such as metal-air batteries, fuel cells, and water splitting.
But, to the best of our knowledge after extensive literature survey, Ni electrodeposited on graphene electrosynthesis has been unexplored either in synthesis method or for HER and ORR as dual applications. Therefore, this Ni-graphene material is expected to have remarkable potential as electrode material for water splitting and ORR. Other word, the graphene is readily fabricated onto substrate carbon paper by CV in graphite powder and SDS solution and Ni nanoparticles onto graphene substrate. The morphology, catalytic activity, and stability during hydrogen evolution of such catalyst under aqueous alkaline condition have been evaluated in detail.

Experimental section 2.1. Graphene synthesis
The graphene nanosheets are synthesized from sodium dodecyl sulfate (SDS) (14 mM), (Fluka), graphite powder (5 mg ml −1 ), and water with magnetic stirring on carbon paper by in situ cyclic voltammetry (CV) method [52,53]. The CV was carried out at 50 mV s −1 and 150 cycles in a potential range from 0.22 to −1.7 V at 50°C and saturated with N 2 .

2.2.
Ni-graphene and Ni-carbon paper electrodes fabrication and Pt/C catalyst Ni particles were synthesized by electro deposition method on the surface of Carbon paper (b) or graphene coated on carbon paper (c) electrodes from 6 mM NiSO 4 and SDS (32 mM) solution with chronoamperometry at 0.25 V relative to Ag/AgCl for 600 s. The Ni loadings on Ni-graphene and Ni-Carbon paper were estimated from the charge consumed during the sweep provided (assuming a 100% current efficiency). In order to compare the fabricated catalysts with standard catalyst we made a mixture containing a homogeneous suspension of commercially available Pt catalyst powder (10 wt%) on carbon black (Vulcan XC-72) E-TEC Inc. as a reference eletrocatalyst. Water, Isopropanol (Merck), 10 wt% Nafion solution and Pt based carbon were sonicated for 20 min, then pasted onto carbon paper (TGPH-0120T Toray). The obtained composite was dried in air at 90°C for 2 h and named Pt/C electrode.

Electrochemical characterization and measurements
The electrochemical measurements were carried out by a Biologic potentiostat-galvanostat VSP300. A threeelectrode cell was applied in all experiments with reference electrode( Ag/AgCl, KCl, 3 M) and Pt as a counter electrode, and carbon paper with or without graphene decorated Ni nanostructure as working electrode in 1M KOH solution for HER and also in 0.1M KOH and saturated O 2 for ORR.

Results and discussion
The production of graphene was carried out by cyclic voltammetry (CV) technique. Figure 1 shows CV profile of graphene synthesized on carbon paper at potential range from 0 to −1.4 V in colloidal dispersion of 0.10 g graphite with 20 ml SDS (14 mM) in different cycles. This figure 1 shows a broad reduction peak due to the surface oxygen functional groups and reduction of water to hydrogen at −1.25 V [52]. As mentioned in previous works by magnetic stirring of the solution driving graphite onto the electrode surface has occurred [53]. Figure 2 shows the potentiostatic curve, the plot of current versus time, of Ni based carbon paper (b) and Ni deposited on graphene-carbon paper (c). From this figure 2, it can be demonstrated that in the duration time, the currents value decreased dramatically. Moreover, the charge consuming for (b) electrode is much higher than that for (c) electrode, due to the graphene moderately smooth surface than only porous carbon paper as detected in SEM image. Figure 3 shows the SEM images of graphene on carbon paper. FE-SEM analysis of figures 3(a), (b), and (c) revealed the large, transparent sheets with small layered of graphene planes, growing on a surface of carbon paper. The surface of the aggregated sheets is properly smooth, transparent, and displayed in 3 enlarge with high resolution magnification (1 μm, 500nm and 200nm) to make sure graphene nanosheets is fabricated (i, ii and iii).
In order to directly confirm and identify the thickness and roughness of graphene layers AFM has been applied. The definitive show that the thin flakes in the sample gained by our system a few layers thick is obtained with the AFM.      Figure 4(ii) reveals the height profile obtained along the figure 4(i). The measured thickness of the graphene layers is 1.2-1.5 nm, which is larger than the interlayer spacing of graphite (0.34 nm), reflecting 4−5 nm for graphene layers. Figures 5(a), (b) reveals a typical FE-SEM micrograph of a cleaned Ni/ carbon paper electrode; the bright shape in figures 5(a), (b) is Ni. As shown in this figure, Cauliflower structure is detected on carbon paper. The average Ni cluster diameter is approximately 100 nm, which is significantly larger than preferred. Figures 5(c), (d) shows FE-SEM micrograph of amorphous Ni nanoparticles on the 2dimentional graphene sheets linked carbon paper. By contrast, deposition of several Ni particles on the graphene sheets in the condition like as Ni/carbon paper indicates a much rougher surface and spread amount of Ni nanoparticles. TEM image for Ni nanoparticles decorated on graphene sheets is presented in figure 5(E). This figure displays the spherical shape of the nickel nanoparticles on graphene layers. The average particle size of the nickel nanoparticles was calculated at around 17 nm. Figure 6 compares the CV curve of the Ni-carbon paper (b) and Ni-graphene carbon paper (c) electrodes in 1 M KOH solution. Polarization was started by scanning potential of 50 mV s −1 from 0.6 V to −0.2 V versus Ag/ AgCl. This figure displays the redox peaks for Ni-carbon paper(b) in the positive potential side (272 and 342 mV), this pair of peaks is related to the oxidation of Ni(OH)2 to NiOOH [54]. Also figure 6 shows the CV profile of Ni-graphene carbon paper (c) which Ni nanoparticle is deposited on the graphene based carbon paper. The potential peak of (c) electrode in forward and backward scan of CV curve has been shifted to the positive voltage which can be attributed to the influence of the graphene on the Ni deposition. Moreover, the higher Ni content in (b) electrode in comparison to (c) electrode will lead to increase the intensities of the redox activation peaks in (b).
The electrocatalytic HER performance of the electrocatalyst nanostructures were investigated in basic aqueous solution (1 M KOH) by linear sweep voltammetry (LSV). The LSV curves of the bare carbon paper (a), Ni-carbon paper (b), Ni-graphene carbon paper (c) and commercial Pt/C catalyst at a scan rate of 1 mV s −1 are seen in figure 7. As predictable, the Pt/C (d) electro-catalyst, selected as reference, shows the highest HER catalytic activity with 35 mV overpotential. However, the bare carbon paper (a) exhibits very poor HER activity with approximately 350 mV overpotential (figures 7(i) and (ii). In addition, the (c) electrode exhibits lower overpotential than (b) electrode. On the other hand, an interesting trend for HER performance could be observed as following: (d>c>b>a). Furthermore, the difference in performance between (b) and (c) was probably resulted from the difference in presence of graphene as a support for Ni nanoparticle that is Cauliflower structure or irregular solid shapes with nanosize, reflecting the significance of graphene sheets.
Moreover, the Tafel plots of different electrodes are shown in figure 7(iii). The Tafel slop of Pt/C catalyst (d) as a control electrode is about 70 mV dec −1 , lower than all other catalysts (c) 95 mV dec −1 and (b) 130 mV dec −1 . The reported Tafel slope in literature for Ni in alkaline electrolyte is nearly 120 mV dec −1 . Thus, reducing the Tafel slope to around 95 mV dec −1 for Ni-graphene carbon paper is the remarkable electrocatalytic HER property of graphene which might be resulted from faster electron transfer. LSV Polarization for Ni -graphene/ carbon paper before and after continuous LSV scanning of 1000 cycles is shown in figure 7(iv).
We further studied the effect of electrolyte temperature on the catalytic activity at the designed electrocatalyst (c electrode) for HER. Figure 7(v) shows the LSV polarization curves of the Ni deposited on graphenecarbon paper (c) at different temperatures. With increasing the temperature from 298 to 323 K, the HER overpotential decreased from value about 90 mV to 60 mV and the current density enhanced with the rise of temperature. The obtained results from LSV curves indicate that the current density can be significantly enhanced by increasing the electrolytic temperature, demonstrating the higher electrocatalytic activity and lower ohmic resistance of the cell. Figure 7(vi) shows the Arrhenius curve, the plot of the logarithm of the current density ln(i 0 ) versus the inverse temperature (1/T) [55], for the water splitting of the synthesis Ni -graphene/ carbon paper(c) as a best electrode in this work. The exchange current densities (i 0 ) were obtained by applying the extrapolation method to the Tafel plots ( figure 7(v)). The calculated value of Ea was 56.66 kJ mol −1 for HER, suggesting the higher HER activity.
The measured activation energy for the (c) catalyst was lower than that of the Ni 2 P/RGO hybrid catalyst, which yields an activation energy of 63 kJ mol −1 [56].
The catalytic durability was another key important parameter for HER performance. To obtain their long term stability, the catalysts were tested at high overpotential (η=260 mv) to evaluate the I-t curves. As shown in figure 8(i) the (c) electrode with graphene demonstrated excellent stability. Remarkably, there was minor current loss at the beginning I-t curve. The sample without graphene (b) exhibited poor stability. As a control, the current density of Pt/C decreased slowly (d).   The a.c. impedance investigation of the HER on the electrodes for the samples was achieved at selected overpotentials (η=200 mV) in 1 M KOH to better understand the underlying origin. The obtained plots are presented in figure 8(ii). It should be noticed that there is no any Warburg impedance for all samples, which illustrates the fast ionic transport and kinetic control of the electrolysis reaction on the surface of the electrodes predominant [57]. As shown from figure 8(ii) the Nyquist diagram of the electrodes shows only one semicircle at the complex plane. It could be seen that the value of charge transfer resistance for Pt/C(d) was smaller than that for (c) and (c) electrode is much smaller than those for (b) and( a) at the same overpotential, in well agreement with their different HER activities.
To evaluate the ORR electrocatalytic performance of the Ni-graphene catalyst, rotating disk electrode (RDE) measurements were employed in alkaline electrolyte (0.1 M KOH) at several rotation speeds. Ideal ORR catalysts should electrocatalyze ORR via a four-electron pathway rather than in an indirect two-electron process. Figure 9(i) and iii shows linear LSVs of Ni-graphene and Pt/C catalysts on GC with RDE in O 2 -saturated KOH electrolyte at several rpm. As shown in this figure the diffusion-limiting current density of Ni-graphene is higher than that of the commercial Pt/C, demonstrating its distinct electrocatalytic activity for ORR.
Moreover, the limiting current densities observed on modified GC electrodes increases gradually as the rotation speed increases . This tendency proposes that the diffusion of O 2 increase at the surface of the Nigraphene and Pt/C electrodes. Figures 9(ii) and (iv) displays the Koutecky-Levich (K-L) plots of Ni-graphene and Pt/C catalysts respectively. This figure represents the relationship between the inverse current density (j −1 ) and the inverse of the square root of the rotation rate (ω −1/2 ) at various voltages. Furthermore, the near parallelism of the fitting plots suggests a similar electron transfer number (n) for both electrodes (n=3.91 for Ni-graphene and n=3.98 for Pt/C). Figure 9(v) shows the LSV measurements at the different gas diffusion electrodes (geometric area of 1 cm 2 ) for the Ni/carbon paper, Ni-graphene/carbon paper and Pt/C on carbon paper in O 2 -saturated 0.1 M KOH at a scan rate of 5 mV s −1 . The obtain kinetic parameters of the electrodes in low current density reign were calculated to be 85, 71 and 53 mV dec −1 for Ni/carbon paper, Pt/C on carbon paper and Ni-graphene/carbon paper, respectively.

Conclusions
In this work, graphene nanosheets are prepared using graphite powder in SDS surfactant by CV, as green and ecofriendly, method. Graphene is successfully characterized by employing FE-SEM and AFM techniques. FE-SEM results confirm the graphene films on the surfaces of the carbon paper are quite thin and smooth. We also employed CV to deposit Ni on graphene surface to use as water splitting. SEM micrographs for Ni nanoparticle on carbon paper and graphene surfaces show Cauliflower structure for Ni on carbon paper and shapeless with higher dispersion on graphene between 60 and 40 nm diameters. The prepared electrodes were characterized using different electrochemical techniques for use as an active and stable catalyst for the HER and ORR in alkaline solution. It was found that the Ni-graphene based carbon paper has a superior activity in the HER than that Ni based only carbon paper. For example, low Tafel slope, low overpotential, values and high stability of Ni/carbon paper confirmed the low consumption of energy during the electrolysis of water. Moreover, K-L plot illustrates that 4-electron participates in the alkaline solution for ORR. The presence of graphene as a support hinders the nickel Cauliflower structure growth, leading to decreasing Ni nanoparticle size, which can increase the catalytic activity.