Strong and Hierarchical Ni(OH) 2 /Ni/rGO Composites as Multifunctional Catalysts for Excellent Water Splitting

: The lack of efficient and non-precious metal catalysts poses a challenge for electrochemical water splitting in hydrogen and oxygen evolution reactions. Here, we report on the preparation of growing Ni(OH) 2 nanosheets in situ on a Ni and graphene hybrid using supergravity electrodepo-sition and the hydrothermal method. The obtained catalyst displays outstanding performance with small overpotentials of 161.7 and 41 mV to acquire current densities of 100 and 10 mA cm −2 on hydrogen evolution reaction, overpotentials of 407 and 331 mV to afford 100 and 50 mA cm −2 on oxygen evolution reaction, and 10 mA·cm −2 at a cell voltage of 1.43 V for water splitting in 1 M KOH. The electrochemical activity of the catalyst is higher than most of the earth-abundant materials re-ported to date, which is mainly due to its special hierarchical structure, large surface area, and good electrical conductivity. This study provides new tactics for enhancing the catalytic performance of water electrolysis.


Introduction
With fossil fuel consumption and the increasing demand and supply of energy, it has become a dominant trend to discover novel technologies used for clean and scalable energy substitutes [1][2][3][4].Recently, electrochemical water splitting for hydrogen and oxygen fuel has drawn much attention to reduce overpotential and enhance the reaction rate [5][6][7].The Pt-based system is the most advanced catalyst for hydrogen evolution reaction (HER), and RuO2 and IrO2 catalysts present the supreme performance for oxygen evolution reaction (OER) [8][9][10].Nonetheless, these kinds of precious metal catalysts are limited in their applications because of their high price, scarcity, and low bifunctional activities [11,12].Therefore, designing an eco-friendly and energy-saving catalyst for water reduction and oxidation in a cathode and anode is very urgent [13][14][15][16].
remarkably active and stable OER catalyst in alkaline media [21].Li et al. successfully synthesized NiSe@NiOOH core-shell hyacinth-like nanostructures supported on nickel foam by a facile solvothermal selenization and subsequent in situ electrochemical oxidation.The electrochemical performance of NiSe@NiOOH/NF for OER was investigated [22].Li et al. synthesized a delicate NiCo(OH)x-CoyW catalyst with a bush-like heterostructure via gas-template-assisted electrodeposition and an electrochemical etchinggrowth process, which ensured a high active area and fast gas release kinetics for HER [23].Zhang et al. synthesized Ni@Ni(OH)2 core-shell nanomaterials through molten salt polymerization.Ni(OH)2 was coated on the Ni core to make the catalyst monomer adsorb H* and OH* simultaneously, forming good hydrogen chemical kinetics and improving the catalytic stability of the catalyst [24].
The documents mentioned above describe the application of Ni-based materials and Ni hydroxides on HER or OER.Furthermore, many significant studies are concerned with bifunctional catalysts.Jia et al. reported a heterostructured catalyst by coupling an exfoliated Ni-Fe layered double-hydroxide nanosheet and defective graphene.The catalyst has exhibited extremely high electrocatalytic activity for HER and OER in an alkaline solution [25].Liu et al. have successfully synthesized nickel oxysulfide fullerene-like hollow nanospheres grown in situ on 3D nickel foams as a kind of efficient bifunctional electrocatalyst in strongly alkaline electrolytes, which could be attributed to the synergetic effect of the proper ratios of sulfur and oxygen [26].Rao et al. synthesized hydrotalcite-like Ni(OH)2 nanosheets grown on Ni foam via a facile one-pot hydrothermal method.The resultant Ni(OH)2/NF electrode shows superior electrocatalytic activity and durability for HER and OER, as well as overall water splitting [27].Yang et al. prepared NiP2/NiSe2 porous precursor on carbon fiber cloth by simple hydrothermal reaction using nickel nitrate and obtained the basic product with good morphology.It is well applied to HER, OER, and overall water splitting [28].Wu et al. reported the modulation of metallic Mo2S3 and in situ epitaxial growth of bifunctional Ni-based catalyst (Mo2S3@NiMo3S4) to construct a metallic heterostructure which can facilitate the charge transfer for water splitting [29].
According to the analysis of the above examples, it can be seen that the composite materials of Ni hydroxides and Ni-based metals exhibit good performance in water splitting.This is caused by the fact that Ni hydroxides are inclined to cleave the HO-H bond for dissociation of water, but are inefficient for converting the produced hydrogen intermediates (Hads) into H2.On the contrary, most Ni-group metals benefit from the adsorption and recombination of the Hads, yet they are commonly incompetent in cleaving the HO-H bond.Therefore, according to their respective electrochemical characteristics, we can combine the catalytic advantages of Ni hydroxides and Ni-based metals to design an excellent composite electrocatalyst on both HER and OER.
In this paper, we prepared a hierarchical Ni(OH)2/Ni/rGO composite electrode, the main steps of which are as follows: First, a foamed nickel substrate subjected to ultrasonication and acidification was used as an electrodeposited cathode, and a Ni/rGO composite electrode was prepared by supergravity electrodeposition, which has been reported in our previous paper [30].Then, the Ni nanoparticles on the surface of the Ni/rGO electrode were reacted with urea via the hydrothermal method, and a layer of Ni(OH)2 nanoplates was vertically grown in situ on the surface of the Ni particles.The resulting Ni(OH)2/Ni/rGO electrodes have a unique tertiary structure.The primary structure is a Ni foam substrate.The Ni foam has a three-dimensional and porous architecture, increasing the surface area of the composite electrode and ensuring that the electrolyte or gas can pass through unimpededly.The secondary structure is the Ni/rGO part, and the rGO layers are interconnected to form a 3D-networked frame on which plenty of Ni nanoparticles are uniformly supported.In addition, the tertiary structure is Ni(OH)2 nanosheets.Based on the results, the aim of this study is to combine the different catalytic characteristics of Ni and Ni(OH)2 to create a new bifunctional metal-metal hydroxide system with excellent and stable HER and OER performances.Furthermore, the impact of urea concentration on the morphology, microstructure, and electrocatalytic properties of the composite electrodes was elaborately researched.We foretell that this study can provide a fresh perspective on non-precious metals as bifunctional electrocatalysts, which is attributed to their unique hierarchical structure, large specific surface area, good electrical conductivity, and synergistic effect between Ni and Ni(OH)2.

Morphology Analysis
The flow chart for a synthetic process of the hierarchical Ni(OH)2/Ni/rGO composite catalyst is obtained by two experimental steps (Scheme 1).The Ni/rGO composite electrode was fabricated via supergravity electrodeposition [30], which converted Ni ions in a plating solution into Ni nanoparticles grown in situ on rGO layers.The interconnected rGO layers are key in refining Ni particles and conducting electricity.In another hydrothermal process, Ni nanoparticles directly provide a Ni source that reacts with urea and forms Ni(OH)2 nanosheets on the surface of Ni particles.The obtained Ni(OH)2/Ni/rGO catalyst has a hierarchical architecture.To our knowledge, Ni foam possesses a special porous structure, making it more conducive to transferring gases and electrolytes and improving their electrocatalytic properties.Hence, the Ni foam substrate acts as the first structure.In addition, the secondary structure is the Ni/rGO hybrid, and the tertiary structure is the Ni(OH)2 nanosheets grown in situ vertically on Ni particles.S7b and S8b, the O 1s spectrum of both the Ni(OH)2/Ni/rGO-1 and Ni(OH)2/Ni/rGO-3 catalysts contributes two forms of oxygen, which are represented as O1 and O2.The signal O1 located at ~531.4 eV is attributed to a higher number of hydroxide species and adsorbed oxygen at the surface of the hybrid catalyst.The signal O2 located at ~530.8 eV generally ascribes oxygen atoms at the surface of hydroxyl groups.In addition, the intensity of the O 1s signal is sufficiently strong, which reflects the formation of Ni(OH)2 or is apparently oxidized at the surface of the material.1, it is apparent that the Ni(OH)2/Ni/rGO-3 cathode presents better catalytic activity among these catalysts, which demands overpotentials of 161.7 and 41.00 mV to acquire current densities of 100 and 10 mA cm −2 , respectively.Moreover, the Ni/rGO, Ni(OH)2/Ni/rGO-1, Ni(OH)2/Ni/rGO-2, and Ni(OH)2/Ni/rGO-4, Ni(OH)2/Ni/rGO-5 catalysts demand overpotentials of 196.0 and 62.01 mV, 183.5 and 64.69 mV, 176.8 and 68.67 mV, 171.6 and 51.00 mV, and 200.5 and 91.38 mV to acquire current densities of 100 and 10 mA cm −2 , respectively.And the obtained Pt/C catalyst demands overpotentials of 97 and 27 mV to acquire current densities of 100 and 10 mA cm −2 .The HER performance of the Ni(OH)2/Ni/rGO-3 composite catalyst is closest to the Pt/C cathode.Remarkably, compared to other similar Ni-based catalysts, the Ni(OH)2/Ni/rGO-3 hybrid synthesized in this paper exhibits much better catalytic activity on HER than other catalysts of reported papers, as displayed in Table S1.Ni(OH)2/Ni/rGO-3 shows lower overpotential than those of NiFe LDH-NS [25], FNHNs/NF [26], Ni(OH)2/NF [27], NiP2/NiSe2 [28], NiCo2S4 NA/CC [33], NiS/Ni foam [34], Ni2P [35], and NiMo HNRs/TiM [36].The j0 is also identified as significant data for the electrochemical catalysis mechanism, and this reflects the difficulty of the electron transport rate of the composite catalyst on HER.According to the known formula η = blog(j/j0) [37], where j is the current density, j0 is the exchange current density, and b is the Tafel slope, we can get j0 values.As revealed in Table 1, the obtained j0 value of the Ni(OH)2/Ni/rGO-3 catalyst is 3.018 mA cm −2 , which is much higher than those of the Ni/rGO (2.605 mA cm −2 ), Ni(OH)2/Ni/rGO-1 (2.039 mA cm −2 ), Ni(OH)2/Ni/rGO-2 (2.703 mA cm −2 ), Ni(OH)2/Ni/rGO-4 (2.149 mA cm −2 ), and Ni(OH)2/Ni/rGO-5 (2.149 mA cm −2 ) composite catalysts, suggesting that the Ni(OH)2/Ni/rGO-3 cathode demands a lower driving power to enhance the hydrogen evolution activity.

Electrochemical Activity and Stability
EIS testing was then operated to gain more information on the electrocatalytic property of composite catalysts in the case of a cathode overpotential of 125 mV on HER.The corresponding Nyquist curves are revealed in Figure 4c, and the curves were characterized by a model with the single-adsorbate mechanism [38], as shown in the inset of Figure 4c.As presented in Table 1, the Rct of the Ni/rGO, Ni(OH)2/Ni/rGO-1, Ni(OH)2/Ni/rGO-2, Ni(OH)2/Ni/rGO-3, Ni(OH)2/Ni/rGO-4, and Ni(OH)2/Ni/rGO-5 catalysts are 4.064, 3.799, 2.993, 2.106, 2.570, and 5.632 Ω cm −2 , respectively.The above results are consistent with the LSV and Tafel analysis assessment, and it further indicates that the Ni(OH)2/Ni/rGO-3 catalyst shows higher electrocatalytic activity and good electrical conductivity.
Chronopotentiometry measurement is currently aimed at assessing the durability and stability of electrocatalysts on HER.It is worth noting that the Ni(OH)2/Ni/rGO-3 cathode was measured at the current density of 100 mA cm −1 for 10 h.As presented in Figure 4d, the chronopotentiometric curve states that the Ni(OH)2/Ni/rGO-3 cathode almost keeps a constant potential around 1.10-1.15V.In addition, the SEM image after chronopotentiometry testing of Ni(OH)2/Ni/rGO-3 cathode is also displayed in Figure 4d.Obviously, after 10 h of chronopotentiometric testing, its morphology did not change visibly as shown in the SEM image.Thus, the result of chronopotentiometry analysis illustrates the good durability and stability of the Ni(OH)2/Ni/rGO-3 catalyst.Also, the LSV curves of the Ni(OH)2/Ni/rGO-3 sample before and after stability analysis (Figure S9) display negligible change.
Electrocatalytic properties of different electrocatalysts for OER were studied in a classical three-electrode configuration (4OH − → O2 + 2H2O + 4e − , OER in alkaline solution).As displayed in Figure 5a, the Ni(OH)2/Ni/rGO-1 sample presents outstanding performance for OER with a low overpotential.Nevertheless, RuO2 shows poor performance for OER.In this process, the overpotentials needed to reach 50 and 100 mA cm −2 for Ni/rGO, Ni(OH)2/Ni/rGO-1, Ni(OH)2/Ni/rGO-3, and Ni(OH)2/Ni/rGO-5 are 397 and 472 mV, 331 and 407 mV, 389 and 440 mV, 403 and 465 mV, respectively.Similarly, Ni(OH)2/Ni/rGO-1 presents superior OER activity than other reported papers [25,26,28,[34][35][36], as shown in Table S1.In addition, the long-term stability of this efficient oxygen evolution anode was measured.A chronopotentiometry test of the Ni(OH)2/Ni/rGO-1 catalyst at the current density of 100 mA cm −2 for 10 h was applied without obvious decay (Figure 5b) and the appearance of the Ni(OH)2/Ni/rGO-1 catalyst did not change (inset in Figure 5b).The good electrochemical property of the Ni(OH)2/Ni/rGO-1 catalyst on OER is mainly because Ni(OH)2 nanosheets help accelerate the cracking of water molecules.The LSV curves of the Ni(OH)2/Ni/rGO-1 sample before and after stability analysis (Figure S10) display a tiny change.The overpotentials needed to reach 50 and 100 mA cm −2 for Ni(OH)2/Ni/rGO-1 after the chronopotentiometry test are 366 and 420 mV.Finally, the Ni(OH)2/Ni/rGO-3 and Ni(OH)2/Ni/rGO-1 catalysts were used as HER and OER catalysts for water spitting in 1 M KOH. Figure 5c presents the LSV curve of overall water splitting, and it reaches 1.43 and 1.48 V at 10 and 50 mA cm −2 , respectively, which is much lower than some reported Ni-based catalysts [26][27][28][33][34][35][36], as shown in Table S1. Figure 5d shows a real overall water-splitting system in 1 M KOH solution.There produced many bubbles on the cathode and anode surfaces in succession.All of the experiments described above reveal that the Ni(OH)2/Ni/rGO catalyst is a highly effective bifunctional catalyst in strong alkaline electrolytes.
Based on a series of experimental results, this outstanding catalytic performance of the Ni(OH)2/Ni/rGO catalyst for HER, OER, and water splitting can be attributed to several features: (1) the Ni(OH)2/Ni/rGO catalyst has unique bifunctional metal-metal hydroxide system, which facilitates different parts of the overall multistep process in an alkaline environment.Ni(OH)2 nanosheets offer the active sites for splitting water, and metal Ni accelerate adsorption of the Hads and its following association to form H2 from these intermediates; (2) the rGO layer has very good electrical conductivity; meanwhile, the rGO sheets have positive effects on increasing the surface roughness of electrode and refining the Ni particles, thereby promoting the specific surface area of the composite catalyst and enhancing its electrochemical activity; the large surface area of Ni(OH)2/Ni/rGO catalyst gives it countless active sites; and (3) the superior property of Ni(OH)2/Ni/rGO catalyst comes from its unique tertiary structure.Taking advantage of the catalytic characteristics of rGO, Ni, and Ni hydroxide is to design a new-style and environment-friendly catalyst.These advantages might encourage the design and development of new low-cost and high-efficiency electrodes for water electrolysis.

Preparation of Ni/rGO Composite Catalyst
The growth method and characterization analysis of Ni/rGO composite catalyst were introduced in the supporting information and our reported paper [30].

Characterization
The microstructures of the obtained catalysts were studied by X-ray photoelectron spectroscopy (XPS, Manchester, England) with a Kratos XSAM-800 spectrometer and Xray diffraction (XRD, RigakuD/MAX 2500/PC, Japan) with Cu Kα radiation (λ = 1.5418Å).The morphologies of the obtained catalysts were studied by transmission electron microscopy (TEM, JEM 2100F, Japan Electronics Co., LTD, Japan) executed at 100 kV, atomic force microscopy (AFM, Bruker Multimode 8, Germany), and scanning electron microscopy (SEM, Carl Zeiss Super55, Carl Zeiss NTS GmbH, Oberkochen, Germany) with an accelerating voltage of 20 kV.The chemical composition analysis of the hybrid catalyst was conducted by applying an energy dispersive spectrometer (EDS) linked to the SEM instrument.

Electrochemical Measurements
All the tests were measured in a three-electrode electrochemical cell at 25 °C, and all the electrocatalytic performance measurements were performed using a CHI 660E electrochemical apparatus (CH Instruments, Inc., Shanghai, China).A Pt plate of 1 cm × 1 cm × 0.1 mm was used as the counter electrode, an Hg/HgO electrode in 1 M KOH was used as the reference electrode and the obtained Ni(OH)2/Ni/rGO composite coating of 1 cm × 1 cm was used as the working electrode.Generally, 30 mg 20 wt% Pt/C and RuO2 were dispersed in a 10 mL mixed solution of 5 mL distilled water and 5 mL ethanol with 100 μL Nafion solution, respectively, followed by 2 h ultrasonic treatment to get the uniform ink.The commercial Pt/C cathode and RuO2 anode were fabricated by dripping the above hybrid ink onto a Ni foam sheet with 1 cm × 1 cm and, afterward, drying at 25 °C for 10 h.The linear sweep voltammetry (LSV) curves were conducted at a scan rate of 5 mV s −1 .The Tafel curves were derived from the transformation of LSV tests.Furthermore, the electrochemical impedance spectroscopy (EIS) was carried out at a potential of −1.05 V vs. Hg/HgO (η = 125 mV), at a frequency from 100 kHz to 0.01 Hz with an AC amplitude of 5 mV.The stability measurements were performed at 100 mA cm −2 for 10 h each.

Conclusions
In summary, we report a successful method of a series of Ni(OH)2/Ni/rGO hybrid catalysts on Ni foam in an economical and simple way.We combine the catalytic characteristics of Ni and Ni(OH)2 by creating a new bifunctional metal-metal hydroxide system.

Figure
Figure S1 presents the pristine Ni foam substrate with a 3D macroporous morphology and smooth surface.Figure S2 presents a high-magnification SEM image of a prepared Ni/rGO catalyst.It shows that rGO layers form an interconnected texture on which lots of Ni nanoparticles are uniformly loaded.The low-magnification SEM image of the Ni(OH)2/Ni/rGO-1 cathode reveals that the surface of the Ni foam is entirely covered with the Ni(OH)2/Ni/rGO hybrid, as shown in Figure S3.As displayed in Figure 1a,b, the highmagnification SEM image of Ni(OH)2/Ni/rGO-1 further displays that the Ni(OH)2 nanosheets grow vertically in situ on the surface of the Ni particles and form a staggered structure.In this process, Ni nanoparticles act as a source of nickel and do not require adding a new nickel source.These Ni(OH)2 nanosheets present an average size of about ~10 nm with the thickness displayed in Figure S4.And the surface size of Ni(OH)2 nanosheets is about 5-500 nm.Furthermore, the elemental mappings (Figure S5a-c) demonstrate the existence of C, O, and Ni elements, which is also in good accordance with the EDS analysis (Figure S5d) of the Ni(OH)2/Ni/rGO-1 hybrid catalyst.The atom ratios of C, O, and Ni in the Ni(OH)2/Ni/rGO-1 coating are 58.81at%, 14.85 at% and 26.34 at%, respectively.As revealed in Figure 1c,d, the SEM image of the Ni(OH)2/Ni/rGO-2 catalyst

A
series of characterizations, including XRD and XPS analysis, were performed to identify the component of the Ni(OH)2/Ni/rGO hybrid.The XRD patterns of Ni/rGO, Ni(OH)2/Ni/rGO-1, Ni(OH)2/Ni/rGO-3, and Ni(OH)2/Ni/rGO-5 catalysts are displayed in Figure 3a.The 4 peaks at 44.92, 52.26, 76.72, and 93.24° arise from Ni (JCPDS No. 65-2865), which conform to the (111), (200), (220), and (311) lattice planes, respectively.It is observed that the preferential orientation of the Ni/rGO, Ni(OH)2/Ni/rGO-1, Ni(OH)2/Ni/rGO-3, and Ni(OH)2/Ni/rGO-5 composite catalysts are the (111) crystal plane.According to the illustration of reported papers [31,32], the Ni(111) plane has a more advantageous effect on enhancing the electrocatalytic performance of HER than other planes.In addition, Ni/rGO and Ni(OH)2/Ni/rGO-5 catalysts show a stronger peak intensity of Ni than those of Ni(OH)2/Ni/rGO-1 and Ni(OH)2/Ni/rGO-3 catalysts, which is due to a layer of Ni(OH)2 nanosheets covering the surface of Ni particles of Ni(OH)2/Ni/rGO-1 and Ni(OH)2/Ni/rGO-3 catalysts, so that the peak strength of Ni is weakened.It is worth noting that the diffraction peaks located at 33.62, 38.98, and 52.26° correspond to the (100), (101), and (102) planes of β-Ni(OH)2 (JCPDS No. 14-0117), which are only reflected on the Ni(OH)2/Ni/rGO-1 pattern.As displayed in Figure 3b, there are no characteristic diffraction peaks of Ni(OH)2 in the samples of Ni(OH)2/Ni/rGO-3 and Ni(OH)2/Ni/rGO-5 catalysts.This is because the Ni(OH)2 nanosheets gradually disappear and dissolve in the solution as the concentration of urea increases.XRD analysis indicates that Ni(OH)2 successfully forms on the Ni surface and falls off as the urea concentration increases.Such observations are in keeping with the analysis results of SEM and TEM.Meanwhile, we did not find the characteristics of rGO diffraction peaks in the XRD pattern of Ni(OH)2/Ni/rGO coatings.This is due to the diffraction intensity of Ni being too strong, thus weakening the diffraction intensity of rGO.

Figure 3 .
Figure 3. XRD patterns of (a) Ni/rGO, Ni(OH)2/Ni/rGO-1, Ni(OH)2/Ni/rGO-3, and Ni(OH)2/Ni/rGO-5 composite catalysts; (b) is the enlarged rectangle region of (a).XPS spectra of (c) Ni 2p and (d) C 1s of Ni(OH)2/Ni/rGO-1 catalyst.XPS spectra of (e) Ni 2p and (f) C 1s of Ni(OH)2/Ni/rGO-3 catalyst.XPS testing was further carried out to research the surface chemical composition of the Ni(OH)2/Ni/rGO hybrid catalysts.The survey XPS spectra in Figures S7a and S8a verify the coexistence of Ni, C, and O elements on the material surface of Ni(OH)2/Ni/rGO-1 and Ni(OH)2/Ni/rGO-3 catalysts.The high-resolution XPS spectrum of the Ni(OH)2/Ni/rGO-1 material for the Ni region, as shown in Figure 3c, indicates that the peaks at 855.5 and 861.1 eV accord with the Ni 2p3/2 levels, and signals at 873.0 and 879.2 eV conform to Ni 2p1/2 levels.The C element of the Ni(OH)2/Ni/rGO-1 sample is presented in Figure 3d.The main peak at 284.6 eV is from the C−C coordination in the rGO.It is different from the Ni(OH)2/Ni/rGO-1 sample.The high-resolution Ni 2p spectrum of Ni(OH)2/Ni/rGO-3 sample reflects an extra peak at 850.2 eV (Figure 3e), which arises from Ni 0 from the Ni/rGO catalyst.Once again, this implies that Ni(OH)2 nanosheets gradually disappear as the concentration of urea increases.The high-resolution C 1s spectrum of the

Table 1 .
Electrochemical parameters on HER recorded in 1 M KOH solution at 298 K.