Characteristics of low-pressure-plasma-processed NiRu-MOFs/nickel foam for Hydrogen evolution reaction

NiRu bimetallic metal–organic frameworks (NiRu-MOFs) are grown by a simple hydrothermal method on nickel foam (NF) as an the electrocatalyst for the hydrogen evolution reaction. Low-pressure plasmas of pure Ar, 95%Ar + 5%H2, and 95% Ar + 5%O2 are used for the post-treatment of the NiRu-MOFs. NiRu-MOFs-AO with post-plasma treatment with 95%Ar + 5%O2 show the lowest overpotential of 123.3 mV at a current density of 10 mA cm−2 and a Tafel slope of 72.0 mV dec−1 in 1 M KOH electrolyte. Electrochemical impedance spectroscopy results indicate that post-plasma treatment can further reduce the charge transfer resistance. Moreover, the electric double-layer capacitance (Cdl) is calculated based on the cyclic voltammetry results, and the electrocatalyst subjected to 95%Ar + 5%O2 post-plasma treatment shows a 2Cdl value of 3.69 mF/cm2, suggesting a larger electrochemically active surface area after oxygen-containing-plasma treatment.


Introduction
Rapid developments in technology and industry have resulted in a massive increase in energy demand, and consequently, issues such as energy crisis, carbon dioxide emissions, and pollution are emerging [1][2][3][4][5][6].In this light, sustainable green energy sources need to be found urgently.Accordingly, hydrogen power has attracted attention as a green energy source [7].Hydrogen can be produced through fossil fuel reforming, biofuel reforming, coal gasification, and the hydrogen evolution reaction (HER) via water electrolysis [8,9].Of these, the last approach is considered promising because of its low pollution and need for a relatively simple equipment configuration [9,10].Further, hydrogen production via electrolysis powered by wind or solar energy is a green technology.
Nevertheless, some challenges in water electrolysis need to be overcome.One challenge is the high cost of catalysts.The efficiency of the HER is usually increased with platinum (Pt) as a catalyst to realize large-scale production and commercialization; however, Pt is expensive [4,5,[11][12][13][14].Therefore, studies are actively investigating non-Pt-based electrocatalysts having a lower cost.Ruthenium (Ru) is considered one of the alternatives to Pt because it is a less expensive non-Pt-based electrocatalyst.The price of Ru can be five times cheaper than Pt [15][16][17][18].Among 4d metals, Ru has similar properties to Pt, great activity for water dissociation, and proper Ru-H bond strength, enhancing hydrogen desorption, promising for catalyzing the HER [19,20].However, Ru belongs to the platinum group metal (PGM) [16,21].The earth-abundance of Ru is about 0.001 mg kg −1 in the crust [22,23] and the Ru production is estimated at 25t per year [24].Further, nickel (Ni), is a transition metal with a large earth-abundant of about 84 mg kg −1 [22,23].Ni has the advantages of low cost and great mechanical properties, and also with unpaired electrons in the d-orbital which promote hydrogen absorption can catalyze the HER [15,19,20].In this regard, to further lower the cost of the electrocatalyst, the noble-transition bimetallic material could be one of the strategies to reduce the usage amount of the precious metal [17,25,26].Moreover, the combination of different metals in an alloy may change the charge distribution in the material leading to the improvement of the electrocatalytic properties [17,25,27].In this light, the present study investigates a NiRu bimetal electrocatalyst.
Some studies have demonstrated that the modification of the crystal structure and surface properties of non-Pt-based electrocatalysts can further improve their HER performance [8,28].With regard to the crystal structure, metal-organic frameworks (MOFs) are novel materials that combine an organic ligand and a metal ion.MOFs have porous structure that provides a large surface area and more active sites for the catalyzing reaction, thus increasing the HER efficiency [2,9,11,[28][29][30][31][32][33][34][35][36][37][38].Studies have extensively investigated surface modification through plasma treatment to create more defects and thereby produce more active sites.Plasma treatment can also form dopants or gas-incorporated structures that modify the electronic properties of the catalyst [39][40][41][42][43][44][45][46].In this study, a bimetallic NiRu(BDC) 2 TED metal organic frameworks material was synthesized by a feasible and simple solvothermal method, and then, the effect of post-plasma treatment on the NiRu-MOFs with different types of gases was investigated [11,40,41].

Experimental
2.1.Pre-treatment of nickel foam substrates Nickel foam with a thickness of 1.7 mm was purchased from HOMYTECH.Hereafter, untreated nickel foam is referred as NF.The NF was cut into a 2 × 4 cm 2 rectangular shape.Then, it was successively immersed in 3 M HCl, ethanol, and de-ionized water with ultrasonication for 20 min for removing the native oxides on pristine NF.The nickel foam obtained after this cleaning process is referred as NF * .

Synthesis of NiRu-MOFs electrocatalysts
In this study, the solvothermal method was used to grow the NiRu-MOFs on NF * .The solution mixture contains 0.3 g of Ni(NO 3 ) 2 •6H 2 O, 0.175 g of 1,4-benzene dicarboxylic acid (BDC), 0.055 g of triethylenediamine (TED), and 29 mg of RuCl 3 dissolved in 30 ml of ethanol [11].After stirring the mixture for 15 min, NF * was sealed with the mixture in an autoclave and then heated at 130 °C for 24 h in an oven [11].After the solvothermal process, the nickel foam with the NiRu(BDC) 2 TED electrocatalyst were ultrasonicated in ethanol for 15 min and dried in the oven at 60 °C.The resultant sample is referred as NiRu-MOFs.

Low-pressure plasma post-treatment
Low-pressure-plasma post-treatment was then applied to the NiRu-MOFs sample.The plasma treatment was performed using pure Ar, 95%Ar + 5%H 2 , and 95%Ar + 5%O 2 to obtain NiRu-MOFs-A, NiRu-MOFs-AH, and NiRu-MOFs-AO samples, respectively [40,41].A low-pressure plasma machine (Harrick, Plasma cleaner PDC-32G) was used for performing the plasma post-treatment with a pressure of 0.6 torr, flow rate of 8 sccm, the power of 11 W, and the treatment time is 30 s in each sample.The sample fabrication procedure is illustrated in figure 1.

Electrochemical measurements
All electrochemical measurements were performed using an electrochemical workstation (Autolab PGSTAT204, Metrohm, Utrecht, The Netherlands) at room temperature.A conventional three-electrode configuration with Ag/AgCl, Pt and NF-based composite material as the reference electrode, counter electrode, and working electrodes, respectively, was used in the 1 M KOH electrolyte.The mass loading of the NiRu-MOFs active material on the Ni foam is ∼1.2 mg/cm 2 .According to the Nernst equation (E RHE = E Ag/AgCl + 0.059 × pH + 0.197), the measured potential against the Ag/AgCl electrode (E Ag/AgCl ) was converted to the relative potential corresponding to the reverse hydrogen electrode (E RHE ) [40,41,[47][48][49]. Linear sweep voltammetry (LSV) was performed with the scan rate of 5 mV s −1 .Electrochemical impedance spectroscopy (EIS) measurements were performed in a frequency range of 10 kHz to 0.1 Hz.Cyclic voltammetry (CV) with a potential scan speed of 20-300 mV s −1 , was performed in a potential range of 0.25 V to 0.05 V relative to the Ag/ AgCl electrode.

Water contact angle
Hydrophilicity is one of the important properties of electrocatalysts.The HCl pre-treatment of nickel foam removed native oxides and particles on its surface, thereby improving its hydrophilicity and reducing the electron transfer resistance [40,41].Figure 2 The results of the water contact angle show the sample after the acid cleaning process, solvothermal process, and the plasma post-treatment still remains hydrophilicity, which is beneficial to the water splitting.Better hydrophilicity of the material provides not only better interfacial contact between the electrode and the electrolyte but also a higher detach rate of gas bubbles, resulting in increased hydrogen gas production in the water splitting process [11,41,50,51].

SEM
The morphology of the pristine NF, NF * , NiRu-MOFs, NiRu-MOFs-A, NiRu-MOFs-AH, and NiRu-MOFs-AO is shown in the magnified SEM images.The pristine NF and NF * show a relatively smooth surface and porous backbone (figures 4(a) and (b), respectively).Furthermore, NiRu-MOFs nanosheets and clusters formed by the aggregation of nanosheets can be seen in figures 4(c)-(f).Compared to the case of NiRu-MOFs, a few more defects are seen in SEM images of the post-plasma-treated electrocatalysts; these lead to a larger active area and improve the HER efficiency [40,41,43].

XPS
The chemical composition is characterized by XPS analysis.Figures 5(a  with a satellite peak of 879.57eV corresponds to Ni 2+ 2p 1/2 [11,[55][56][57].Additionally, the fitted Ru 3p spectrum figures 7(a)-(d) shows two peaks at 462.86 eV and 484.98 eV that correspond to Ru 2+ 3p 3/2 and Ru 2+ 3p 1/2 , respectively [11,58].The XPS result indicated the successful synthesis of the NiRu-MOFs electrocatalyst on nickel foam substrates.In the equivalent circuit model, R s , R ct , and CPE stand for the series resistance, charge transfer resistance, and constant phase element, respectively [40,41].The R ct values of the samples are listed in table 1. Pristine NF shows the largest R ct of 11.68 Ω, and the R ct of NF * reduces to 7.89 Ω, possibly because of the removal of native oxides and particles [40,41].Compared to the R ct of NF and NF * , that of NiRu-MOFs decreases to 7.52 Ω owing to the growth of the catalyst material.Further, post-plasma-treated samples showed a lower R ct .The NiRu-MOFs-AO sample treated by Ar and O 2 gas plasma had the lowest charge transfer resistance of 1.78 Ω, implying that oxygen-containing plasma treatment best improves the charge transfer ability and decreases the interfacial impedance among the three samples [40,41].The electric double-layer capacitance (C dl ) could be used to estimate the electrochemically active surface area (ECSA), according to the equation of ECSA = C dl / C s , where the C s is the specific capacitance of the sample material [59,60].By the equation, the higher C dl value indicates the higher ECSA and more active sites for hydrogen evolution [11,[61][62][63][64]. Figure 8(d) shows the results of CV measurements with the scan rates of 20-300 mV s −1 .2C dl can be calculated based on the linear slope of the current density and scan rates [11,55].The corresponding 2C dl values of samples are shown in table 1.The sample treated with the oxygen-containing plasma showed a 2C dl value of 3.69 mF cm −2 , implying a larger ECSA (figure 8(d)).The mass activity (A/g) in figure 8(f) is calculated by the current density (j, mA/cm 2 ), measured at overpotential of 100 mV for HER, divided by the mass loading of the electrocatalyst (m, mg/cm 2 ) [65].Table 1 also lists the calculated mass activity.The NiRu-MOFs-AO sample has the highest mass activity among all of the samples at the overpotential of 100 mV for HER.

Stability test
The stability test is performed further to investigate this work's most promising catalyst material, NiRu-MOFs-AO.The NiRu-MOFs-AO sample was tested by the current density of 10 mA/cm 2 for 12 h.Figure 9 shows the result of the stability test.The initial overpotential of NiRu-MOFs-AO is 157.1 mV (v.s.RHE).After testing under the constant current density for 12 h, the overpotential increased from 157.1 mV to 265.1 mV (v.s RHE).According to the result of the long-term measurement, the electrocatalyst still has plenty of room for improvement in its stability.

Conclusion
Plasma-treated NiRu-MOFs catalyst on nickel foam are investigated.The NiRu(BDC) 2 TED catalyst treated by a plasma containing 95%Ar+5%O 2 gas achieves the lowest overpotential of 123.3 mV at a current density of 10 mA cm −2 and the lowest Tafel slope of 77 mV dec −1 .Moreover, the EIS analysis shows that post-plasma treatment reduces the charge transfer resistance, in keeping with the LSV results.Furthermore, the 2C dl value is estimated by CV analysis.The sample treated with 95%Ar + 5%O 2 plasma shows the largest 2C dl value, indicating the larger ECSA.Oxygen-containing plasma treatment results in the highest HER efficiency and mass activity; this may be due to the incorporation of oxygen and the defects on the material surface.
)-(f) show full survey spectra of NF, NF * , NiRu-MOFs, NiRu-MOFs-A, NiRu-MOFs-AH, and NiRu-MOFs-AO, respectively.The main constituent elements of the electrocatalysts are C, O, Ni, and Ru.Furthermore, the signal of N appears in the XPS spectra after the solvothermal process because of the TED (triethylenediamine) with N atoms in the organic ligands.High-resolution XPS spectra of Ni 2p are shown in figures 6(a)-(d).The binding energy position at 855.54 eV with a satellite peak of 861.42 eV corresponds to the Ni 2+ 2p 3/2 , and the binding energy position of 873.30 eV

3. 5 .
Electrochemical measurementThe electrochemical properties of bimetallic NiRu-MOFs electrocatalysts were measured in 1 M KOH electrolyte.The results of LSV curves and the HER overpotential at 10 mA cm −2 (η 10 ) of HER are shown in figure8(a) and table1.The overpotentials of pristine NF and NF * were 287.1 mV and 282.2 mV, respectively.

Table 1 .
Parameters corresponding to each electrode.