Facile Synthesis of Ni(OH) 2 through Low-Temperature N-Doping for E ﬃ cient Hydrogen Evolution

: Nickel hydroxide is a potentially cheap non-precious metal catalytic material for alkaline hydrogen evolution reactions (HERs). Herein, a nickel form (NF)-based nitrogen-modi ﬁ ed nickel hydroxide (N-Ni(OH) 2 /NF) with interlaced two-dimensional (2D) nanosheet structures was synthesized by a simple one-step ammonia vapor-phase hydrothermal method for e ﬃ cient electrocatalytic HERs. The e ﬀ ect of the reaction temperature of the catalyst preparation on the HERs’ performance was studied in detail. The HER activity of N-Ni(OH) 2 /NF is enhanced by the large speci ﬁ c surface area, mass transfer and electron conductivity provided by a unique and suitable 2D nanostructure and nitrogen doping. The obtained N-Ni(OH) 2 /NF not only shows a superior HERs performance, but also exhibits good stability during long-term electrolysis.


Introduction
Hydrogen fuel is one of the most promising candidates for clean and sustainable energy supply systems [1,2].Alkaline water electrolysis is the most effective way to achieve hydrogen production.The development of highly efficient and low-cost nonprecious metal catalytic materials to replace Pt-based noble metal catalysts has been extremely crucial for improving the efficiency and economy of hydrogen evolution reactions (HERs) through water electrolysis in the past twenty years [3][4][5].As a common non-precious metal catalyst, the use of nickel-based materials in alkaline water electrolysis has also been studied for a long time [6,7].For alkaline HERs kinetics, the adsorption and activation of H2O molecules constitutes the energy barrier of water dissociation in the first Volmer step, which needs low Gibbs free energy for H2O adsorption (ΔGH2O) to save electricity consumption [8].The non-noble metal Ni is considered to be one of the most promising candidates for this, as it contains effective water dissociation centers to overcome the energy barrier in the Volmer step.However, the Ni catalyst in the metal state itself shows poor catalytic activity for HERs in alkaline media.According to recent reports, Ni(OH)2 can promote water dissociation and reduce the H* adsorption energy barrier by interacting with the hydroxyl group [6,9].In addition, an improvement in the catalytic properties can be achieved through the modification of heteroatoms (S, Se, P, N, etc.) [10][11][12].Previously reported metal sulfides, phosphides and selenides show high catalytic activities, such as the Ni/NixSy heterostructure [13], Ni/NixP hybrids [14] and NiSe2/Ni3Se4 dual-phase nanosheets [15].However, it is worth noting that there is not much work related to nitrogen-modified nickel hydroxides due to the difficulty and complexity of the experimental conditions.Recently, Li and co-workers reported a simple synthesis method for nitrogen-modified nickel nanoparticles, which motivated us to proceed to carry out a related study [16].
From another angle, improvements in electrocatalytic performance can also be achieved through the construction of nanostructures and the optimization of conductivity [17,18].One effective strategy is to grow unique nanostructures in situ on a self-supported conductive substrate such as nickel foam (NF), which contains more pores and open spaces to utilize active species efficiently and improve the dispersion of active sites [19,20].Close contact of the conductive substrate as electron gathering devices with the active sites also facilitates prompt charge transport, which is better than powder materials that rely on polymeric binders [21].Furthermore, as the HER proceeds for a prolonged period of time, the reconstruction of nanostructures results in an increase in surface roughness, which exposes more catalytic sites.In a word, the NF-supported in situ construction of interlaced nanosheet structures with open pores can further roughen the surface and increase the number of catalytic sites; therefore, it is helpful to construct the catalyst with higher HER activity [22,23].
Here, we designed a simple one-step method for preparing a 2D porous nanostructure of nitrogen-modified nickel hydroxide on nickel foam (N-Ni(OH)2/NF), as shown in Figure 1.In brief, N-modified nickel hydroxide was grown in situ on NF through a simple one-step ammonia vapor-phase hydrothermal process.As the framework and nickel source, NF partially converted to nickel hydroxide by reacting with water vapor, and the N-modified material was simultaneously obtained due to the presence of ammonia in the water vapor.The constructed 2D nanostructures with porous and open spaces provide a large specific surface area that facilitates the dispersion and exposure of the active sites.Thus, the contact and mass transfer between the electrolyte and the catalyst is also improved.The prepared N-Ni(OH)2/NF shows superior HER performance with a low overpotential at 351 mV to achieve 100 mA cm −2 , as well as good stability during the long-term HER process.

Results and Discussion
To explore the effect of reaction temperature on HER performance and optimize the catalyst with the best performance, the N-Ni(OH)2/NF samples were synthesized at three different temperatures: 90 °C, 130 °C and 180 °C, respectively (Figure 1).
The crystal structures of the as-prepared samples were characterized by XRD, and the corresponding XRD patterns are shown in Figure 2.For NF-supported samples, the characteristic peaks of metallic Ni (PDF no.00-003-1051) can be detected at 44.8°, 52.2° and 76.8° [24].Except for the peaks of metallic Ni and test substrate, the detection results for the other peaks are very weak in the N90-Ni(OH)2/NF sample, indicating that Ni(OH)2 formed at 90 °C has a low crystallinity.However, all peaks detected for Ni(OH)2 can be well-indexed to pristine β-Ni(OH)2 (PDF no.00-003-0177) in the samples of N130-Ni(OH)2/NF and N180-Ni(OH)2/NF [25,26], which is due to the surface chemical reaction between Ni and water vapor at high temperatures.To further confirm the composition of the obtained catalysts, the detailed chemical structures and elemental oxidation states of as-prepared N130-Ni(OH)2/NF were characterized by XPS (Figure 3).The XPS survey (Figure 3a) demonstrated the existence of Ni, O and N elements in N130-Ni(OH)2/NF.The XPS spectrum of N 1s (Figure 3b) deconvolved into two distinct peaks at 398.5 eV and 400.0 eV, respectively.This indicates strong and weak interactions between Ni and N in the sample, respectively, thus revealing the successful doping of the N atoms in the samples.In the Ni 2p region (Figure 3c), Ni 2p 1/2 (873.2eV) and Ni 2p 3/2 (855.7 eV) are assigned to Ni(OH)2, which proves that the metallic Ni species transform into metal hydroxides after ammonia vapor-phase hydrothermal treatment [27,28].Combined with the XRD spectrum, it was confirmed that the nickel hydroxide on the surface of the catalyst was mainly β-Ni(OH)2 crystal, which proved the successful synthesis of the N-Ni(OH)2/NF catalyst.Furthermore, the energy-dispersive Xray (EDX) spectra and element mapping results of the N130-Ni(OH)2/NF catalyst also confirmed the successful introduction of N atoms into Ni(OH)2, and all the elements were dispersed uniformly on the material surface (Figure S1).All the above results confirm the incorporation of N species into the nanosheet structure of Ni(OH)2.In Figure 4, the morphologies of the as-prepared samples at different temperatures are presented, respectively.Unlike the nanoparticles and the nano-disks structure of unsupported Ni(OH)2 reported in the other related literature, the NF-supported Nmodified Ni(OH)2 nanosheets display 2D interlaced nanostructures with many pores and gaps, which sharply contrasts with the smooth surface of the nickel foam (Figure S1a,b).Therefore, the influence of substrate dispersion and orientation on the crystal growth of nanomaterials may be considered as the reason for the distinct morphologies that were observed in 2D NF-supported N-modified Ni(OH)2.After the vapor-phase hydrothermal treatment at 90 °C, the N-modified Ni(OH)2 nanosheets had already successfully grown on the surface of the NF (Figure 4a).Furthermore, at a higher magnification, it can be observed that the N-modified Ni(OH)2 is composed of nanosheets with an average diameter of only 50 nm (Figure 4b) as a result of the low crystallinity that is consistent with the characterization of XRD.The corresponding morphology of N130-Ni(OH)2/NF and N180-Ni(OH)2/NF is shown in Figure 4c and Figure 4e, respectively.Figure S2 shows the morphology and distribution of N130-Ni(OH)2/NF at a larger size.For the N130-Ni(OH)2/NF and N180-Ni(OH)2/NF samples, they have the same morphology as the N90-Ni(OH)2/NF sample, indicating that the effect of hydrothermal treatment temperature on the morphology of the Ni(OH)2 nanosheets is not significant.The 2D interlaced nanosheets are still identifiable and similar to the sample observed in Figure 4a, which has structures containing many pores and gaps.However, a closer look at Figure 4d shows that the enlarged average thickness of the Ni(OH)2 nanosheets reaches ~80 nm, which is raised to ~180 nm in Figure 4f.In order to explain this problem more clearly, the thickness distribution of the N90-Ni(OH)2/NF, N130-Ni(OH)2/NF and N180-Ni(OH)2/NF nanosheets was statistically calculated using the distribution diagram (Figure S3).The rich pores and gaps between the 2D interleaved nanosheets can provide a large surface area to expose more active sites, simultaneously resulting in sufficient contact and rapid mass transfer at the electrolyte/catalyst interface.In fact, the size of the nanosheets increases with increasing treatment temperature, which should provide a larger surface area and further favor the rise in catalytic activity.According to the difference in catalyst loading on the substrate before and after the treatment reaction, it can be found that the difference between the three samples is not significant, indicating that the effect of treatment temperatures on catalyst loading is very small.This can be attributed to the fact that when the thickness of Ni(OH)2 nanosheets reaches a certain value, the diffusion of Ni in the deep layer of NF substrate to the surface and the further formation of Ni(OH)2 are hindered, but it does not prevent the diffusion of Ni(OH)2 species or the growth of Ni(OH)2 nanosheets on the surface.This is consistent with the characterization results from the XRD, and the calculation of the electrochemical active area also follows this pattern.Unexpectedly, the results of the electrochemical properties test did not meet this conjecture, i.e., the HER activity of the Ni(OH)2/NF catalysts did not increase with the increase in the Ni(OH)2 nanosheets' size.This means that the appropriate morphology, structure and size of Ni(OH)2 nanosheets are critical to improving the HER activity of Ni(OH)2/NF catalysts.The transmission electron microscope (TEM) images show that N130-Ni(OH)2/NF presented with thin layers of nanosheets (Figure 5a), which is consistent with the SEM images above.In the high-resolution TEM (HRTEM) image of N130-Ni(OH)2/NF, we observed lattice fringes attributed to Ni(OH)2 0 0 1 (0.453 nm) and 1 0 0 (0.268 nm) crystal faces, respectively (Figure 5b).The above results proved that Ni(OH)2 was successfully prepared by the rapid low-temperature ammonification process.As shown in Figure 5c, the TEM mapping image shows that there are uniformly distributed N elements in the structure of N130-Ni(OH)2/NF, which indicates that we have successfully implemented the N-doping process.The EDS mapping image shows that the content of N element in N130-Ni(OH)2/NF is 3.1% (atomic fraction).With the increase in synthesis temperature, the amount of N-doping increases gradually.At 130 ℃, we achieved the most suitable Ndoping amount and the most ideal morphology construction.The doped N element had a significant effect on the catalytic activity.The doping of N element is too small to cause obvious lattice distortion and electronic structure disturbance.If the N element is doped too much, the H adsorption capacity of the Ni active site will be weakened, and the activity will be reduced.Therefore, the amount of N doping needs to be regulated in a suitable range.The decomposition of ammonia and the growth rate of Ni(OH)2 can be affected by temperature control, so the amount of the N doping can be easily controlled.As shown in Figure 6, the HER electrocatalytic performances of all obtained samples were tested in 1.0 M KOH using bare NF as a reference.The current density measured at the applied potential is one of the key parameters for electrocatalysts, which reflects the catalytic activity of corresponding electrodes.In the polarization curves in Figure 5a, the results of the LSV indicate that the performance of bare NF as a HER catalyst is not ideal.The overpotential of bare NF achieved at 10 mA cm −2 is 270 mV, suggesting its low activity towards HERs in this potential region.For N-modified Ni(OH)2/NF, the improved HER performances are shown with a lower overpotential to deliver 10 mA cm −2 .For N130-Ni(OH)2/NF, the HER activity is further improved with low overpotentials at 239 mV and 347 mV, respectively, to achieve 10 and 100 mA cm −2 , which is comparable to current common high-current-density catalytic materials (Table S1).The improved HER performance of N-modified Ni(OH)2/NF may be attributed to the strong synergistic effect between 2D N-modified Ni(OH)2 nanosheets and NF, which not only enhances conductivity for promoting fast charge transfer but also improves the dispersion of electrocatalytic active sites.Moreover, the formation of an unusual 2D interlaced Ni(OH)2 nanosheets structure affected by NF in ammonia vapor-phase hydrothermal crystal growth may be conducive to exposing abundant active sites.
In Figure 6b, the Tafel plots that were plotted based on the LSV curves are displayed to estimate the HER kinetics of electrocatalysts.The Tafel slopes for N90-Ni(OH)2/NF, N130-Ni(OH)2/NF and N180-Ni(OH)2/NF are defined as being 126 mV dec −1 , 109 mV dec −1 and 118 mV dec −1 , respectively.According to the previous literature, HER can be divided into two steps in alkaline media, including the Volmer step (H2O + e − → Hads + OH − ) as the first step, and taking the Heyrovsky step (Hads + H2O + e − → H2 + OH − ) or the Tafel step (Hads + Hads → H2) as the second step [29,30].The lower Tafel slope of N130-Ni(OH)2/NF suggests improved HER reaction kinetics and a combined Volmer-Heyrovsky mechanism for HER in alkaline media.The electrochemical impedances of alkaline HER kinetics were evaluated using the Nyquist plots in Figure 5c.The fitting curves and R² (R-squared) corresponding to different Nyquist plots of samples are also shown in the figure, and the test data were fitted using a simple equivalent circuit schematic attached to the figure, indicating a good correlation.For this equivalent circuit, Rs is the solution resistance that represents the electrical transport properties, and CPE is related to the constant phase element, which acts as a different component depending on the alpha index.Rct, which is the charge transfer resistance, represents the electrocatalytic kinetics between the catalyst interface and the electrolyte interface.In Table S2, the calculated values of Rs and Rct are summarized.As can be seen in Figure 6c, N130-Ni(OH)2/NF has the lowest Rct value of 5.26 Ω and the smallest semicircle, which means that the electron transport between the 2D interleaved Ni(OH)2 nanosheets and the NF conductive substrate occurs fast and directly during the HER process.It can be surmised that the surface of the N130-Ni(OH)2/NF electrode is more uniform and shows a larger surface area than the other two samples.Therefore, there is more contact between the catalyst and the electrolyte, which promotes charge transfer in the reaction.In addition, a smaller Rct value indicates desirable catalytic kinetics and better conductivity, which is beneficial to the improvement of HER catalytic activity.
Moreover, the test of the electrochemical active surface area (ECSA) is used to evaluate the HER activity of the as-prepared N-Ni(OH)2/NF samples.All electrochemical tests were conducted in 1.0 M KOH aqueous solution.The Cdl value of N130-Ni(OH)2/NF in Figure 6d is calculated as 3.40 mF cm −2 , according to the CV curves in the non-Faradic region (Figure S4), which is significantly larger than the other two samples.The high electrochemical capacitance of N130-Ni(OH)2/NF indicates that its unique 2D interleaved nanosheets structure has the largest ECSA (Table S3).The results show that N130-Ni(OH)2/NF obtained using the simple vapor-phase hydrothermal method not only has abundant active sites, but also shows greater improved catalytic activity than the other two samples.Therefore, the superior HER performance of N130-Ni(OH)2/NF may be attributed to the following three aspects: (i) the unique and appropriate 2D interleaved nanosheets structure provides a large surface area and exposes rich active sites; (ii) compared with metallic nickel or nickel oxides, nickel hydroxides can produce more active sites with superior intrinsic electrocatalytic activity; (iii) the large open space provided by conductive and porous NF with a 3D framework can build up the interleaved 2D nanostructures with porous, ensuring substantial exposure of active sites and quick mass as well as electron transfer in the HER process.
Meanwhile, the amount of hydrogen evolved in the electrolytic cells was collected using the water drainage method; the testing process was conducted at a current density of 100 mA cm −2 .As shown in Figure 7, the amount of H2 is consistent with the theoretical value, implying a nearly 100% HER Faradaic yield for N130-Ni(OH)2/NF.This result also proves that the N130-Ni(OH)2/NF catalyst has an excellent alkaline water electrolysis HER performance.Another observation is that the current density of N180-Ni(OH)2/NF is similar to N90-Ni(OH)2/NF under the same potential, which indicates that an increase in the synthesis temperature may result in a magnification of the growth rate and crystal size, eventually causing a decrease in its HER activity compared with N130-Ni(OH)2/NF.The possible reason may be inferred as the poor conductivity of Ni(OH)2, as the excessive crystal particles and layer thickness reduce the conductivity of the catalyst, which leads to a decrease in HER activity.Catalytic stability is one of the key parameters determining the performance of N130-Ni(OH)2/NF electrodes in practical applications.The long-term electrolysis test carried out in 1.0 M KOH was used to evaluate the catalytic stability of N130-Ni(OH)2/NF.
In Figure 8a, the LSV curve results show that the potential drop at 100 mA cm −2 does not exceed 5 mV after 5000 cycles.To further examine the stability of the electrode during the electrocatalysis process, a chronoamperometry (CA) test was continuously performed for 12 h at −1.3 V (vs.SCE); the corresponding potential-time curve is shown in Figure 8b.The results show that the current density of N130-Ni(OH)2/NF exhibits only faint degradation even after an operation of 12 h.Detailed electrochemical tests and physical characterization were performed on the samples after 12 h stability tests.After 12 h, N130-Ni(OH)2/NF showed no significant decrease in HER activity (Figure S5a).At the same time, SEM images show that the nanosheet morphology is well-maintained (Figure S5b).An obvious uniform distribution of N elements can still be observed in the TEM mapping images (Figure S5c), proving that doped N elements can be stable in the structure.The above results provide conclusive evidence that N130-Ni(OH)2/NF is exceptionally stable during long-term electrolysis.

Experimental Section
Analytically pure ammonia solution was purchased from Xilong Scientific Co., Ltd.(Shantou, Guangdong, China).Nickel foam (NF, thickness of 1.2 mm) was provided by Shenzhen Poxon Machinery Technology Co., Ltd.(Shenzhen, Guangdong, China).All commercially available chemical reagents are of analytical grade and used in experiments without further purification.

Synthesis of N-Ni(OH)2/NF
Prior to synthesis, the NF was cut into small pieces of 1 × 2 cm 2 , followed by consecutive sonication in 1.0 M hydrochloric acid, acetone and ethanol for 30 min, respectively.Then, the clean NF pieces were dried at 60 °C for at least 8 h under vacuum.After that, the N-modified N-Ni(OH)2 samples were synthesized via an additional ammonia vapor-phase hydrothermal method in a closed autoclave (100 mL) at different temperatures (90 °C, 130 °C and 180 °C) for 12 h, respectively.The supply of the ammonia source was achieved by placing 5 mL of ammonia solution at the bottom of the autoclave.A custom-made Teflon holder was employed to vertically suspend the NF about 5 cm above the solution.During the heating process, the ammonia water breaks down into ammonia gas and water, and fills the entire environment acting on the NF.After the ammonia and water vapor treatment, the N-modified samples were rinsed with DI water and ethanol, respectively, followed by drying in a vacuum oven.According to the corresponding treatment temperature, the obtained samples were named N90-Ni(OH)2/NF, N130-Ni(OH)2/NF and N180-Ni(OH)2/NF, respectively.

Characterization
All the samples were tested using an X'Pert PRO MPD diffractometer (Malvern Panalytical, Malvern, UK) (Cu Kα), and X-ray diffraction (XRD) patterns (Rigaku, Tokyo, Japan) were recorded with a 2θ range of 10° to 85°.The morphology of the prepared samples was observed with scanning electron microscopy (SEM) (FEI, Hillsboro, OR, USA), and the instrument model was Hitachi S-4800.X-ray photoelectron spectra (XPS) were tested using a ThermoFisher Scientific II spectrometer (Waltham, MA, USA) with Al as the photo source.

Electrochemical Measurements
The electrocatalytic properties of the prepared samples were all measured using a three-electrode system connected to a computer and an electrochemical workstation (Gamry Reference 1010 Instruments, USA, Warminster, PA, USA).In this experiment, all prepared samples were used as working electrodes, while the counter and reference electrodes were a carbon rod and a saturated calomel electrode, respectively.All experimentally obtained electrochemical data were recorded using Gamry Framework Data Acquisition Software 6.11 and corrected for iR (current time resistance).Prior to testing, the electrolyte (1.0 M KOH) was degassed with N2 for 1 h and continued to remain above the solution during the HER measurement.The LSV test was performed at a scan rate of 2 mV s −1 .Electrochemical impedance spectroscopy (EIS) was performed at −1.25 V (vs.SCE) from 10 5 to 0.1 Hz with an AC voltage of 5 mV.The electrochemical double layer capacitance Cdl of the materials was obtained through a series of cyclic voltammetry (CV) measurements at various scan rates (40, 60, 80, 100 and 120 mV s −1 ) in the non-Faradaic potential region from −0.3 to −0.1 V vs. RHE.The derived linear slope was calculated as Cdl.The stability tests were undertaken by CV from 0 to −0.4 V (vs.SCE), or by chronoamperometry at −0.3 V (vs.SCE).The potential conversion between SCE and the reversible hydrogen electrode (RHE) is based on the equation as follows: E (vs.RHE) = E (vs.SCE) + 0.2415 V + 0.059 pH The values of ECSA can be calculated using the following equation: Cdl: Double layer capacitance of the catalyst measured in 1.0 M KOH (mF); geometric surface area of the working electrode is 2 cm −2 ; Cs: specific capacitance; the value of Cs is 0.04 mF cm −2 in 1.0 M KOH.
The Faradic efficiency (FE) was the ratio of the amount of experimentally evolved hydrogen to that of the theoretically expected hydrogen.The hydrogen was collected using the water drainage method.Then, the amount of H2 was calculated using the gas laws.The theoretically expected amount of H2 was then calculated based on the Faraday law.The amount of charge (Q) was calculated from the current-potential curve.Assuming two electrons are required to make one H2 molecule from two protons, the Faradaic yield for H2 = Q/2nF.

Conclusions
In summary, we have developed a simple low-temperature vapor-phase hydrothermal method to construct N-modified Ni(OH)2 on NF (N-Ni(OH)2/NF) with enhanced electrocatalytic activity during an alkaline HER process.By designing NF as a nickel source and framework in one simple step, it is possible to directly construct nickelbased hydroxide with a fine interleaved 2D nanosheets structure, which simplifies the synthesis process.The large specific surface area and abundant active sites from the interleaved 2D nanosheets structure promote superior HER activity of N-Ni(OH)2/NF and maintain its good stability during long-term electrolysis.The direct preparation method of this N-modified transition metal hydroxide may be worthy of further application to other non-precious metal electrolysis catalytic materials, which is beneficial for increasing activity and efficiency in alkaline water electrolysis.