Performance of Low-Pressure-Plasma-Processed RuCo Electrocatalysts for Hydrogen Evolution Reaction

RuCo/acid-treated nickel foam (ANF) has been reported to be an excellent electrocatalyst for the hydrogen evolution reaction (HER). In this study, we perform plasma treatment with Ar, Ar/H2 (95:5), and Ar/O2 (95:5) as working gases for surface modification to explore the effect on HER performance. The developed electrocatalysts are tested in an alkaline solution (1 M KOH); the results show that Ar/H2 (95:5) plasma treatment significantly improves the electrocatalytic activity of RuCo/ANF, achieving an overpotential of 98 mV at a current density of 10 mA cm−2. Electrochemical impedance spectroscopy and cyclic voltammetry analyses shSow a large reduction in the charge transfer impedance and a significant increase in the electric double-layer capacitance. This study provides a facile strategy to activate RuCo to improve HER performance.

The consumption of fossil fuels is causing increasingly serious problems such as environmental pollution and energy scarcity.Therefore, it is imperative to develop clean alternatives to fossil fuels in energy research.2][3][4] The catalytic material selection for the working electrode is one of the most important challenges in electrochemical processes. 5,6Platinum (Pt) can be used as the catalyst for this reaction; however, it is expensive.For large-scale applications, efficient and low-cost catalysts are required to increase the reaction performance. 7Therefore, many studies have been conducted to develop alternative catalysts to Pt. 8,9 The 3d transition metal catalysts afford advantages such as low cost, high efficiency, 10 and abundant reserves, and therefore, they show promise for reducing the cost and increasing the activity of the electrocatalytic HER. 2 Recently, encouraging progress has been achieved by alloying transition metals with other noble metals. 11any studies have investigated alloying a 3d transition metal with Ru.The Ru catalyst, priced at just 1/25 of Pt metal, exhibits a 2.5fold higher hydrogen evolution turnover frequency (TOF) under alkaline conditions compared to the state-of-the-art Pt/C catalyst. 12he resulting synergistic effect between the two metals favors the adsorption and desorption of H atoms. 3,13 The low electronegativity of transition metals can regulate the electronic structure of Ru to optimize the HER activity.14 For example, Su et al. prepared RuCo alloy electrocatalysts in N-doped graphene and demonstrated that they were superior to commercial Pt/C.15 Lin et al. prepared RuCo aerogels with the use of excess reducing agent to maximize the use of active centers through synergistic effects.16 Pei et al. adjusted the Ru:Co ratio to achieve a low overpotential of 21 mV at 10 mA cm −2 .17 Alloying Ru with transition metals was found to significantly promote the electrocatalytic activity for the HER. 18 Te electrocatalytic performance could be further optimized by performing some modifications such as surface modification, 19 vacancy formation, 20 and morphological control.Inspired by the above reports, we alloy Ru with Co and perform surface modification to adjust the metal structure and increase the number of active sites.
Plasma treatment is a simple and convenient method for surface modification.Plasma surface treatment involves the interaction of energetic particles and reactive species in plasma with the surface of a material, resulting in the generation of specific functional groups on the material's surface and altering its surface structure, thereby achieving modification of the material. 21,22It is very suitable for solid materials such as nickel foam (NF). 23,24urfaces subjected to plasma treatment undergo activation or other chemical reactions caused by the reactive species. 25,26In fact, the incorporation of hydrogen radicals can reduce oxide. 22The generation of oxygen vacancies can modify the surface to make it more hydrophilic. 27The physical or chemical reactions involved in the reaction process allow the removal of residual contaminants on the surface to adjust the surface properties and number of active sites. 28,29In this study, the effect of different plasma species (Ar, Ar/H 2 , and Ar/O 2 ) on the performance of RuCo catalysts prepared by the hydrothermal method is investigated.

Experimental
Synthesis.-Generally, an NF piece with dimensions of 4.0 cm × 3.0 cm × 0.17 cm was ultrasonically cleaned in 0.1 M H 2 SO 4 , deionized (DI) water, ethanol (EtOH), and acetone for 20 min in sequence to remove organic residues and impurities on the surface 30 the processed NF is called acid-treated nickel foam (ANF).Then, RuCo/ANF was prepared by a one-step hydrothermal process.First, 1 mmol Co(NO 3 ) 2 •6H 2 O (0.291 g), 0.5 mmol RuCl 3 •xH 2 O (0.103 g) and 1 mmol terephthalic acid (0.166 g) were dispersed in 35 ml of EtOH and 0.5 ml of acetic acid solution. 31After 30 min of continuous stirring using a magnetic stirrer, the mixed solution was transferred to a 50 ml Teflon-lined stainless-steel autoclave, and ANF was immersed in the solution.The autoclave was subsequently placed in an oven at 130 °C for 5 h to allow the in situ growth of RuCo alloy on ANF.After natural cooling at room temperature, the electrode was removed and then washed repeatedly with DI water and EtOH.Then, the electrode was dried at 60 °C for 10 min; the obtained electrode was referred as RuCo/ANF.Finally, post-treatments were performed using low-pressure plasmas with Ar, Ar/H 2 (95:5), and Ar/O 2 (95:5) gases to obtain RuCo/ANF-Ar, RuCo/ANF-Ar/H 2 , and RuCo/ANF-Ar/O 2 , respectively.
Material characterization.-Themorphology and surface structure of the samples were examined by field-emission scanning electron microscopy (FE-SEM; JSM-7800F Prime, JEOL, Tokyo, Japan) with energy-dispersive spectroscopy (EDS).X-ray photoelectron spectroscopy (XPS; 5000 Versa Probe, ULVAC PHI, Kanagawa, Japan) was used to characterize the chemical compositions of the samples.Crystal phase analysis was performed by X-ray diffraction (XRD; Bruker D2 PHASER) using Cu-Kα radiation (λ = 1.54060Å) for wide-angle measurements in the 2θ range of 5°-60°and grazing incidence XRD (GIXRD; Bruker D8 Discover) for low-sweeping angular diffraction with low 2θ range of 5 to 20°.The water contact angle of the samples was measured using a goniometer (Sindatek, Model 100SB).The low-pressure plasma treatment was applied using a plasma cleaner (PDC-32G) with a pressure of 0.6 torr, a flow rate of 10 sccm, and power of 11 W.
Electrochemical measurement.-Theelectrochemical measurement were conducted using an electrochemistry workstation (Autolab PGSTAT204) in a three-electrode configuration by using Ag/AgCl, Pt, and ANF-based material as the reference, counter, and working electrodes, respectively.As the electrolyte, 1 M KOH was used.The potential E was converted to the corresponding potentials relative to RHE by the Nernst equation as follows: E RHE = E Ag/AgCl + 0.059 × pH + 0.197.Linear sweep voltammogram (LSV) curves were performed from −0.8 to −1.8 V (vs Ag/AgCl) at a scanning rate of 5 mV s −1 .Moreover, Tafel slopes were re-plotted as overpotential vs log(current density) from the LSV curves for evaluating the HER kinetics.Electrochemical impedance spectroscopy (EIS) analysis of the electrocatalysts was performed in the frequency range of 10 kHz to 0.1 Hz at an overpotential of 0.15 V (vs RHE).Cyclic voltammetry (CV) was performed in the scan range of −0.25 V to −0.05 V (vs Ag/ AgCl) with a potential scan speed of 20-300 mV s −1 .The electrochemical stability of RuCo/ANF-Ar/H 2 was tested in 1 M KOH with continuous reductive potential cycling over the range of −0.8 V to −1.8 V (vs Ag/AgCl) for 12 h.

Results and Discussion
SEM and SEM-EDS mapping.-Theelectrocatalytic activity of a material could be influenced by its morphological characteristics. 32o study the morphology and element distribution of the electrode surface, SEM and EDS mapping analysis were conducted.Figure 1(1-a, 2-a) shows that the NF skeleton has a smooth surface.After treatment with sulfuric acid, DI water, EtOH and acetone, the ANF sample exhibited a rough surface.Moreover, more cracks and pores were seen under high magnification (10000×), as shown in Fig. 1(1-b, 2-b). 33Figure 1c shows the spherical structure in which RuCo alloy grows in situ on the ANF skeleton and aggregates into tiny clusters.A low-magnification (100×) SEM image of RuCo/ ANF (Fig. 2a) and multi-element EDS mapping (Fig. 2b) showed that Ru and Co were uniformly distributed on the ANF framework.The element distribution diagrams of Ni, Co, and Ru are shown in Figs.2c-2e.From the EDS data in Fig. 2f, the Ru:Co atomic ratio in RuCo/ANF was approximately 1:1.25.Further, the proportion of Co was lower than the molar ratio Ru:Co = 1:2 in the hydrothermal precursor solution, indicating that the growth environment of the hydrothermal method was more favorable for Ru formation.Figures 1e-1f shows that the plasma treatments with different gases physically modified the surface morphology, helping in improving wettability.
Water contact angle.-During the HER, a large number of bubbles are generated.For the electrolyte to effectively penetrate the electrode surface, increasing the hydrophilicity of the electrode is critical.Figure 3a shows that the water contact angle of NF is 106°.No obvious penetration phenomenon was observed after the droplet made contact with the surface, indicating the hydrophobicity of NF. Figure 3b shows that the droplet will penetrate the ANF surface after 10 s owing the removal of native oxides. 34The rougher surface caused by acid treatment made the surface more hydrophilic. 35After the growth of the RuCo alloy, the time required for the droplet to penetrate a surface increased from 10 s to 23 s (Fig. 3c); this was attributed to the residual Cl and other organic pollutants in the precursor.Toward this end, we chose plasma with Ar, Ar/H 2 , and Ar/O 2 gases for surface treatment.Figures 3d-3f shows that the droplets penetrate the surface completely in an instant, indicating that plasma treatment significantly improved the wettability of the RuCo/ANF-Ar, RuCo/ANF-Ar/H 2 , and RuCo/ANF-Ar/O 2 samples.In addition to the successful removal of surface organic pollutants, the increase in the surface area of the plasma-treated electrode produced more vacancies, making the electrolyte solution penetrate the electrode more easily. 36,37[40] XPS and XRD.-XRD analysis was performed to identify the crystal structures of the prepared samples.Figure 4a shows the XRD patterns of NF, RuCo/ANF, RuCo/ANF-Ar, RuCo/ANF-Ar/H 2 , and RuCo/ANF-Ar/O 2 .Two strong diffraction peaks were seen at 2θ of 42.5°and 52.5°, corresponding to the face-centered cubic structure of the NF planes ( 111) and ( 200), respectively. 41The signal of the thin film was difficult to observe owing to the substrate signal being too strong.Thus, GIXRD was used for the thin film sample, as shown in Fig. 4b.3][44][45][46] After plasma treatment, the (200) diffraction peak shifted from a lower diffractionangle to a higher one, indicating that the interplanar spacing changed owing to lattice distortion caused by the shrinkage of the material structure. 47PS measurements were performed to understand the elemental composition of the prepared samples.9][50] The peaks became more obvious after plasma treatment was conducted to remove surface pollutants, thus confirming the existence of Co.With regard to Ru, Ru3d 5/2 was used to investigate the electronic property because Ru 3d 3/2 overlaps with C1s. 51,52Figure 4d shows that the RuCo/ANF peak at 281.8 eV is attributed to Ru 3d 5/2 , and this peak shifted slightly to a lower binding energy region after plasma treatment. 53Furthermore, the binding energies of 280.5 eV, 281.5 eV and 282.4 eV were assigned to Ru 4+ , Ru 3+ , and Ru 6+ , respectively. 51Interestingly, RuCo/ANF-Ar/O 2 has more Ru 4+ owing to the oxidation of Ru 2 O 3 .In summary, the XPS results indicated that Ru and Co were successfully grown in situ on ANF substrates via hydrothermal synthesis.
Electrochemical characterization of HER.-The prepared samples were placed in 1.0 M KOH solution to measure the electrocatalytic activity for HER.Figures 5a-5b shows the LSV curves and corresponding overpotential values at a current density of −10 mA cm -2 .NF exhibited an overpotential of 216 mV.After introducing RuCo alloy through the hydrothermal method, the overpotential was reduced greatly to 131 mV.Plasma treatment with Ar, Ar/H 2 (95:5), and Ar/O 2 (95:5) gases resulted in overpotentials of 120 mV, 98 mV, and 125 mV, respectively.Treatment with Ar/H 2 reducing gas optimizes the catalytic performance 24,54 and reduces the overpotential by 33 mV, resulting in the best HER activity.The Tafel slope provided additional insights into the HER mechanisms.6][57] The corresponding Tafel slope was also calculated (Fig. 5c).The Tafel slope of NF was 280 mV dec −1 .The slope decreased to 202 mV dec −1 after introducing RuCo alloy.Then, plasma treatment further reduced the slope.][60] Figure 5d shows the EIS result of each sample at an overpotential of 100 mV.RuCo/ANF-Ar/H 2 had the lowest electron transfer resistance of 3.0 Ω; this was lower than those of NF (6.8 Ω), RuCo/ANF (4.4 Ω), RuCo/ANF-Ar (4.0 Ω), and RuCo/ANF-Ar/O 2 (3.9 Ω), indicating that H doping significantly reduced the charge transfer resistance and increased the interfacial charge transfer rate.
To evaluate the electrochemical active surface area (ECSA), CV was performed in the potential range of −0.25 V to −0.05 V (vs Ag/ AgCl) without an apparent Faradaic process, 61 with the scan rate being varied from 20 mV s −1 to 300 mV s −1 for seven scans (Fig. 5e).The slope of the curve obtained from the current density difference (Δj) corresponding to each scan rate at −0.15 V (vs Ag/AgCl) was two times the double-layer capacitance (2C dl ), and the obtained C dl was proportional to the ECSA. 62Figure 5f shows that the 2C dl value of RuCo/ANF-Ar/H 2 is 33.3 mF cm −2 ; this is higher than those of NF (2.6 mF cm −2 ), RuCo/ANF (17.9 mF cm −2 ), RuCo/ANF-Ar (24.1 mF cm −2 ), and RuCo/ANF-Ar/O 2 (26.8 mF cm −2 ).C dl of RuCo/ANF-Ar/H 2 was approximately 13 times larger than that of NF and two times larger than that of RuCo/ANF without plasma treatment.This result revealed that the use of reducing gas in plasma treatment can significantly improve the C dl value and ECSA owing to the generation of more active sites during ion bombardment.
In addition to the catalytic activity for the HER, the stability is a critical factor for evaluating the electrocatalyst performance.To evaluate the long-term durability of the prepared samples, a 12-h continuous test was performed.As shown in Fig. 5g, After 12 h, at a current density of 10 mA cm −2 , there was a decrease in overpotential for all cases, indicating an improvement in catalyst performance after long time operation.The Nyquist plots in Fig. 5h also show a decrease in charge transfer resistance, which reduced significantly compared to that before the 12 h operation.Among them, RuCo/ANF-Ar/H 2 decreased from 3 Ω to 0.931 Ω.Table I indicates that each catalytic electrode experiences a reduction in material loss during long-term cycling, but it does not impact the catalytic performance.In addition, the Tafel slope changed significantly (Fig. 5i), indicating that the HER kinetics changed after a long cycle.The rate-determining process changed from the Volmer reaction to the Volmer-Heyrovsky mechanism, thereby effectively increasing the amount of converted hydrogen. 63,64

Conclusions
The HER performance of RuCo was evaluated by modifying its surface with Ar, Ar/H 2 (95:5), and Ar/O 2 (95:5) plasmas.The incorporation of H into the RuCo structure by Ar/H 2 plasma treatment was found to result in the best HER performance, with an overpotential of 98 mV at a current density of 10 mA cm −2 .This was evidenced by the increased hydrophilicity, enlarged ECSA, lower Tafel slope, and lower charge transfer resistance.In addition, owing to the H retained in the lattice, RuCo/ANF-Ar/H 2 exhibited better catalytic performance after a 12-h cycle test compared with those of other catalysts.In summary, this study provides a facile strategy to activate RuCo through plasma treatment, thereby providing additional active sites to greatly promote the HER activity.

Figure 2 .
Figure 2. (a) SEM image with 100× magnification of RuCo/ANF.(b) Multi element EDS mapping of Ni, Co, and Ru.(c) EDS maps of elemental distribution: Ni in red, (d) Co in green, and (e) Ru in red.(f) Corresponding EDS results of RuCo/ANF.