Electrodeposited Ni-Sn-P electrodes for hydrogen evolution electrocatalysis

Developing high-activity, Nobel metal-free electrodes is of utmost significance in the context of the hydrogen evolution reaction (HER). In this study, we employed an electrodeposited process to fabricate Ni-Sn-P films on a copper substrate. The microstructure and electrocatalytic properties of these Ni-Sn-P films were thoroughly characterized and subsequently compared to those of the reference Ni-P films.Remarkably, the Ni-Sn-P electrode exhibited an impressively low overpotential of −84 mV at a current density of −10 mA cm−2 in a 1 M NaOH solution. This value was found to be 172 mV lower than that of the Ni-P electrode, thereby establishing the superior electrocatalytic activity of the Ni-Sn-P electrode. Moreover, the excellent electrocatalytic stability of the Ni-Sn-P electrode was demonstrated over a time span of at least 25 h.The exceptional attributes of the Ni-Sn-P electrode can be ascribed to its larger active surface area, lower Tafel slope, and reduced charge transfer resistance. The utilization of Ni-Sn-P films as electrode materials holds great potential in advancing the field of HER due to their enhanced performance compared to conventional Ni-P films.


Introduction
Energy issues have received worldwide concern due to the increasing depletion of traditional resources such as coal and natural gas [1,2].Hydrogen is environmentally friendly and can be stored on a large scale, which is considered as a renewable energy source and can effectively solve energy problems [3][4][5].Water electrolysis is one of the most attractive ways to produce hydrogen.Unfortunately, the sluggish kinetics of the HER results in excessive energy consumption [6].Hence, it is urgent to develop high-efficiency electrocatalysts that enhance current density at low overpotential during hydrogen evolution reaction [7].At present, the most efficient hydrogen evolution electrocatalysts are platinum and palladium [8], and both of them are expensive and scarce, thus restricting their applications.So, the development of low-cost and high-efficient HER catalysts is of great significance in the future.
For decades, Ni-P-based based electrocatalysts have been widely used to improve HER catalytic activity [9,10], while the activity is still lower.Lian et al reported that both metallic element doping and the active surface area increase of the electrode enhance the HER performance [11].Sn, Mo, Al, and Fe elements are added to Nibased alloy, and those alloys show high electrochemical activity in HER [12].Among those alloys, Sn doping alloy possesses an accelerated reaction rate for HER.V D Jouic et al reported that due to the addition of slightly higher than normal amounts of Sn to all phases, the cell size of almost all phases was somewhat larger than expected [13].Thus, we can conclude that the Sn doping to Ni-P electrocatalysts may enhance electrochemical activity in HER.But, regrettably, the studies about Ni-Sn-P electrocatalysts are scarce.
In this study, we prepared Ni-Sn-P films with a focus on enhancing the catalytic performance for the HER.We also investigated the microstructure and intrinsic properties of Ni-P films with and without Sn doping.Our analysis solely concentrated on the catalytic activity under identical working electrode sizes, while the mass activity of the samples was not taken into account.For information on mass activity, we suggest referring to other literature sources [14].

Material fabrication and sample preparation
Here, we adopted a high-density graphite sheet and copper plate as anode and cathode, respectively.The copper plate was washed with 0.3 M HCl, absolute ethanol, and deionized water in turn, and then the Ni-Sn-P film was prepared on copper plates in a two-electrode cell using direct-current electrodeposition.Accompanied by magnetic stirring, the deposition process was performed at 45 °C for 30 min, with a current density of 40 mA cm −2 and a deposition time of 30 min.To obtain excellent properties, the pH value of the electrolyte was set to 5.0.The pH value was adjusted by adding H 2 SO4, H 2 O, and NH 3 •H 2 O.In addition, the Ni-P electrode was also prepared using the same preparation process without the addition of SnSO 4 in the electrolyte as a comparison group.The composition of the electrolyte is shown in table 1.

Characterizations
The x-ray diffraction (XRD) patterns of the processed samples were obtained using an X' Pert PRO PANalytical diffractometer with a Cu-Kα radiation source operating at 40 kV and 100 mA.The surface morphologies and microstructures of the samples were examined by scanning electron microscopy (SEM, FEI Apreo S), transmission electron microscopy (TEM, Tecnai F30), and atomic force microscopy (AFM).X-ray photoelectron spectroscopy (XPS) measurements were performed by using a Kratos Axis Ultra x-ray photoelectron spectrometer.

Electrochemical measurements
The electrochemical tests were carried out in an electrochemical workstation (CS 310, Corrtest Instruments, Wuhan, China).The tests were conducted in 100 ml of 1 M NaOH electrolyte at 25 °C under atmospheric pressure.A platinum foil and a saturated calomel electrode (SCE) were used as the counter electrode and reference electrode, respectively.Ni-Sn-P film or Ni-P film served as the working electrode.To evaluate the electrocatalytic activities of Ni-Sn-P and Ni-P films, linear sweep voltammetry (LSV) was measured with a scan rate of 2 mV s −1 .The measured potentials were converted to the RHE reference scale using the equation:

RHE SCE
The Tafel slope b is derived from LSV conversion , where h is overpotential, and j is the current density.Cyclic voltammetry (CV) curves were tested at scan rates of 10, 20, 30, 40, 50, 60, 70 and 80 mV s −1 .The electrochemical impedance spectroscopy (EIS) experiments were performed at −0.65 V (versus RHE) with the frequency range of 0.1 Hz to 100 kHz in 1 M NaOH.The stability of the Ni-Sn-P electrode was tested at a constant voltage of −150 mV (versus RHE).

Results and discussion
Figures 1(a) and (b) show the surface morphology of the Ni-P and Ni-Sn-P films, respectively.The surface morphology of Ni-P film is dense and smooth, while that for Ni-Sn-P film is rough.During the deposition process, the relative velocities of the ions reaching the cathode surface are different [15], thus resulting in the local concentration differentiation and the formation of intermediate species of different sizes in different regions.The difference in surface morphology results from the different sizes of intermediate products.
According to the EDX result (figure 1(e)), the atomic concentration of Ni, P, and Sn elements are 73%, 15%, and 12% in the intermediate species, respectively.The particle size ranges from 200 to 500 nm for Ni-Sn-P film, which is larger than that for Ni-P film.
Figures 1(c) and (d) present the 3D surface topography of Ni-P and Ni-Sn-P films with an area of 20 × 20 μm.The root means square (RMS) for Ni-Sn-P film is about 180.25 nm, which is larger than that for Ni-P film (33.22 nm).It means that Ni-Sn-P film has a rougher surface, which is consistent with SEM results.
The XRD patterns of both Ni-P and Ni-Sn-P specimens are shown in figure 1(f).Note that the diffraction peaks appeared at 43.4°, 50.0°, and 74.0°.Combined with XRD patterns and PDF cards, it is concluded that the diffraction peaks correspond to the (111), (200), and (220) reflections of the Cu substrate.In addition, a broad diffraction halo appears at around 45°, which implies the formation of an amorphous phase.Compared with crystal film, the amorphous film can supply plentiful unsaturated coordination sites as active centers for HER [16,17].These sites are used for the adsorption of the reactants and hence improve the electrocatalytic performance of the catalyst.The microstructures of both Ni-P and Ni-Sn-P films were further investigated by TEM.According to the HRTEM images, there are no distinct crystallographic features in the Ni-P film (figure 2  indicates that the existence of Ni species in the sample has a very small positive charge [19].The formation of oxidized Ni species was also certified through two peaks at 855.7 eV and 873.4 eV.In figure 3(c), three peaks observed around 484.1, 485.3, and 486.6 eV belong to Sn 3d 5/2 level, and the other three peaks appeared at 492.5, 493.6, and 495.9 eV belonging to Sn 3d 3/2 level.The first two peaks of Sn 3d 3/2 and Sn 3d 5/2 level corresponds to Sn δ+ in Ni-Sn-P film and oxidized Sn species, respectively.While the third peaks of Sn 3d 3/2 and Sn 3d 5/2 levels are derived from the corresponding satellite.Due to the binding energy of the Sn 3d level is very close to that of the Sn atom, we can infer that Sn atoms in Ni-Sn-P film have a very small positive charge.In figure 3(d), the signals at 128.8 eV and 129.6 eV are assigned to P 2p 3/2 and P 2p 1/2 energy levels, and these values are very close to those of the P (0) species.We can infer that P in the Ni-Sn-P specimen has a very small negative charge (P δ− ).The peak at 133.1 eV is attributed to the oxidized phosphate species arising from the partial after-air exposure.According to the above results, Ni, Sn, and P are not simply physical mixing.The existence of Sn can balance the adsorption and desorption of hydrogen atoms.More importantly, the presence of P is beneficial for maintaining the amorphous feature of the Ni-Sn-P electrode [12], which guaranteed enough reaction sites as active centers for HER.Therefore, the synergetic effect of Ni, Sn, and P promotes the formation of hydrogen.
The LSV curves are shown in figure 4(a).When the current density is 10 mA cm −2 , the overpotential of Ni-Sn-P film is −84 mV, and the overpotentials of Pt/C, Ni-P, and Cu substrate are −13 mV, −256 mV, and −557 mV, respectively.The result indicates that the Ni-Sn-P film has better electrocatalytic activity than that of the Ni-P film.The mechanism of hydrogen evolution reaction in alkaline solution is as follows [20]: where M is the active site of the electrode surface and M-H is the M occupied by hydrogen atoms.According to the LSV conversion in figure 4(a), the Tafel slopes of the Ni-Sn-P specimen and Ni-P specimen are 62 mV/dec and 134.8 mV/dec, respectively.Thus, the HER mechanism of the Ni-Sn-P electrode corresponds to the Heyrovsky reaction, while that for the Ni-P specimen corresponds to the Volmer reaction.The results indicate that the Ni-Sn-P electrode surface has a more active site (M) of hydrogen adsorption than that for the Ni-P electrode, which is consistent with the results shown in figures 1 and 2(a)-(c).
In general, the intrinsic activity of the catalyst was investigated by estimating the electrocatalytic active surface area (ECSA).The ECSA is proportional to C dl and can be calculated by / = ECSA C C dl S [21].The C dl for the Ni-P specimen and Ni-Sn-P specimen derived from the CV plots are 0.15 and 1.16 mF cm −2 , respectively (figures 4(c)-(e)).In an alkaline environment, C s is typically 0.04 mF cm −2 [22].The C dl can be calculated based on the equation: where J anodic and J cathodic are the positive current density and negative current density of the intermediate potential, respectively.Now we calculate the ECSA of the Ni-P and Ni-Sn-P specimens.The value for the Ni-Sn-P specimen is 28.75 cm 2 , which is larger than that for the Ni-P specimen (3.75 cm 2 ).The difference between the two specimens  comes from the rough surface and smaller particle size of the Ni-Sn-P specimen (figure 1(b)).The larger active surface area of the Ni-Sn-P specimen is favorable for enhancing the HER activity.
Electrochemical impedance spectroscopy (EIS) is another important method to deeply understand the HER process for these two electrodes.Both EIS results and equivalent circuit diagram are depicted in figure 4(f), in which R , s R , ct and Q represent the solution resistance, charge transfer resistance, and the constant phase element, respectively.Of special noting is that the Ni-Sn-P specimen has a smaller semicircular diameter than that of the Ni-P specimen.The R ct value for the Ni-Sn-P specimen is 2.61 Ω, which is smaller than that for the Ni-P specimen (8.17 Ω).Therefore, the Ni-Sn-P specimen has a smaller impedance and faster charge-transfer kinetics on the surface.In addition, the Ni-Sn-P specimen shows long-term stability for at least 25 h, as shown in figure 4(g).

Conclusion
Highly active electrodes play a crucial role in promoting the hydrogen evolution reaction (HER).In this study, we employed a cost-effective electrodeposition method to fabricate Ni-Sn-P electrodes without the use of noble metals on a copper substrate.The key findings are summarized as follows: 1.The Ni-Sn-P electrode demonstrates superior catalytic activity and exceptional electrocatalytic stability compared to the Ni-P electrode.At a current density of 10 mA cm −2 , the Ni-Sn-P electrode exhibits an impressively low overpotential of −84 mV.Furthermore, the Ni-Sn-P electrode demonstrates excellent electrocatalytic stability, maintaining an overpotential of 150 mV for at least 25 h.
2. The microstructure of the Ni-Sn-P film displays a higher degree of disorder and amorphization, resulting in a larger active surface area for the HER.Additionally, the Ni-Sn-P electrode shows a reduced Tafel slope of 62 mV/dec and a lower charge transfer resistance of 2.61 Ω.
3. The outstanding electrocatalytic properties of the Ni-Sn-P electrode can be attributed to its larger active surface area, lower Tafel slope, and reduced charge transfer resistance.
Through the utilization of Ni-Sn-P films as electrode materials, the field of HER can benefit from their enhanced performance in comparison to conventional Ni-P electrodes.
(a)) and Ni-Sn-P film (figure 2(c)).The selected area electron diffraction (SAED) patterns inserted in figures 2(a) and (c) further prove that these films are amorphous phases.Of special noting is that a small amount of lattice fringe[18] appears in a high magnification image of Ni-P film (figure 2(b)).Therefore, we can conclude that the Ni-Sn-P film is more disorderly and amorphization.Moreover, the energy dispersive spectrometer (EDS) elemental mapping images manifest that Ni, Sn, and P elements are more evenly distributed in the Ni-Sn-P film (figures 2(d1)-(d4)).The chemical states of the Ni-Sn-P film were analyzed by XPS and the results are shown in figure 3(a).Of special noting is that Ni, Sn, P, C, and O coexist in that film.The reasons for O and C come from the oxidation of the sample surface and the carbon used for calibration, respectively.The high-resolution XPS spectra of Ni 2p, Sn 3d, and P 2p are shown in figures 3(b)-(d).Two peaks appear at 851.9 eV and 869.1 eV (figure 3(b)), which correspond to the Ni 2p 3/2 and Ni 2p 1/2 energy levels, respectively, and here we regarded as Ni δ+ in the Ni-Sn-P film.In addition, the Ni 2p binding energy in the Ni-Sn-P film is in close proximity to that of Nickel metal, which

Figure 3 .
Figure 3. XPS spectra of the Ni-Sn-P specimen.(a) The wide spectrum; (b-d) the high resolution of Ni 2p, Sn 3d, and P 2p, respectively.

Table 1 .
Composition of the electrolyte.