The Promotional Effect of Na on Ru for pH-Universal Hydrogen Evolution Reactions

Abstract

Alkali metals, as ideal electron donors, can effectively regulate the valence state distribution of the host metals. Nevertheless, no studies have reported the application of alkali metal promoters in the hydrogen evolution reaction (HER). Here, we designed an efficient and wide pH-universal hydrogen evolution catalyst that utilizes alkali metal to control the valence, size, and dispersion of Ru NPs. The experimental results reveal that the alkali metal additives contribute to the dispersion and stabilization of metallic Ru. More importantly, the interaction between Na and Ru regulates the distribution of Ru valence states and helps to form more active components of Ru⁰. Additionally, NaCl functioned as an in situ template to assist the construction of a porous carbon skeleton promotes mass transfer and exposes more active sites, further promoting the synergistic effect of Ru and Na. As a result, the optimal Ru_0.3/C−800 delivers high efficiency for HER with an overpotential as low as 29 mV in 1.0 M KOH and 83 mV in 0.5 M H₂SO₄ under 10 mA cm⁻². Particularly, the catalytic performance of Ru_0.3/C−800 even outbalanced that of commercial Pt/C in an alkaline medium. This rational construction strategy opens up new avenues for obtaining superior pH-universal electrocatalysts.

1. Introduction

As a sustainable secondary energy, hydrogen (H₂) energy has the advantages of wide raw material sources and zero carbon emission, and it has demonstrated an excellent application foreground in industries such as automotive, chemical, and aviation [1,2]. Compared with traditional steam reforming technology, producing hydrogen by water electrolysis fundamentally eliminates the dependence on fossil fuels and achieves zero carbon emissions [3,4]. Nowadays, the electrolysis of water has become the most competitive hydrogen production technology [5,6]. Alkaline electrolyzer, proton exchange membrane electrolyzer, and other types of electrolyzers are available on the market. The various operating environments put forward higher requirements for the design of catalysts. It is highly desirable to create economical and pH-universal catalysts. Moreover, the practical deployment of water electrolysis technology is heavily restricted by the sluggish kinetics of the cathode catalysts, which results in a high overpotential to drive H₂ production [7]. Currently, Pt-based electrocatalysts, as the benchmark for HER, exhibit excellent catalytic performance [8,9,10,11]. Unfortunately, the scarce reserves and exorbitant price largely impede their commercialization. Developing cost-effective electrocatalysts to substitute Pt/C is a central issue in the water industry [12]. As a Pt-group metal, Ru possesses a moderate metal–hydrogen bond strength similar to Pt–H, but the cost is only 1/30 that of Pt [13,14,15,16]. In general, the HER efficiency is determined by the strength of the metal–hydrogen bond [17]. Ru serving as a catalyst’s active center instead of expensive Pt metal is expected to keep the catalytic activity unchanged while reducing the catalyst cost [18,19]. However, the current Ru-based catalysts suffer from complex preparation methods, the high toxicity of reagents, and high metal loading [20]. Therefore, it is urgent and challenging to develop facile, economical, and green synthetic strategies for fabricating efficient Ru-based catalysts.

Alkali metal, serving as a structural or electronic regulator, plays vital role in thermocatalytic reactions [21]. Since the first report on the promotional effect of alkali metal in 1845, the function of alkali metal has been systematically studied in hydrogenation reactions, such as ammonia production [22], Fischer–Tropsch process [23], aromatics [24], hydrogenation of unsaturated hydrocarbons [25,26,27,28], and CO₂ [29]. In the meantime, it is generally believed that alkali metals act as electron donors to modulate the electronic structures of the active metals, thereby changing the adsorption of reacting intermediates [30,31,32]. For example, Zhang introduced Na dopant and oxygen vacancy, which made the d-band center of RuO₂ far away from the Fermi level, resulting in the weakening of the chemical bond between the oxygen intermediate and the surface of RuO₂, thus reducing the activation barrier of the oxygen evolution reaction (OER) to optimize the performance [33]. By regulating the interplay of Na and Co, Wu’s group obtained stable Na-Co₂C active sites, formed Na-CO bonds to strengthen the interaction between Na and Co₂C, dispersed the Co₂C, and reduced the particle size [34]. Despite alkali metal promoters being widely used in thermocatalysis, OER, etc., the application of alkali metal promoters in HER has not been reported, let alone the influencing mechanism of alkali metal on the HER performance.

Herein, “starch strips” derived from potatoes, commercially used in the preparation of puffed food were chosen as the carbon source. The “starch strips” were economical, widely sourced, and rich in NaCl. Utilizing abundant NaCl in “starch strips” as the pore-forming agent, the porous carbon-supported ultra-dispersed Ru-based composite (Ru/C) was synthesized by a simple impregnation–pyrolysis strategy; meanwhile, alkali metal Na promoters were introduced in situ. It was found that the introduction of Na greatly modulated the valence state distribution of Ru species, inducing the formation of more Ru⁰ states which, in turn, boosted the electrocatalytic performance. Benefitting from the interplay between Na and Ru, as well as the developed texture structure, Ru_0.3/C−800 functioned as a super and long-lasting HER catalyst over a broad pH range. In 1.0 M KOH and 0.5 M H₂SO₄, the overpotentials of Ru_0.3/C−800 were only 29 mV and 83 mV to achieve the current density of 10 mA cm⁻². Notably, the mass loading of Ru in Ru_0.3/C−800 was as low as 0.81 wt%, greatly reducing the synthetic cost compared with that of commercial Pt/C (20 wt%). Furthermore, the HER performance for Ru_0.3/C−800 outbalanced Pt/C in alkaline solution. This work provides a new method for alkali metal promoters to tune the electrocatalytic performance, which could lead to the design of novel catalysts for efficient hydrogen evolution.

2. Results and Discussion

2.1. Catalyst Synthesis and Characterization

The Ru/C was synthesized using the method of impregnation adsorption and one-step pyrolysis starting from RuCl₃ and “starch strips” derived from potatoes as the feedstock, which was ecofriendly, commercially available, and inexpensive (Figure 1). Benefitting from the confinement effect of the carbon skeleton, Ru species were converted into monodispersed nanoparticles (NPs) [35]. In this study, the as-prepared catalysts were named Ru_x/C−T, where x refers to the additional amounts of Ru (0, 0.2, 0.3, and 0.4 mmol), and T refers to pyrolysis temperature (700, 800, 900, and 1000 ℃). Transmission electron microscopy (TEM) was performed to observe their morphologies and microstructures. For Ru_0.3/C−800, a distinct pore structure was exhibited on the carbon substrate (Figure S1), which was attributed to the pore-forming effect of NaCl. It was not difficult to find that, the Ru NPs of Ru_0.3/C−800 were uniformly distributed on the carbon frame with a mean particle size of ~1.90 nm (Figure 2a). When the dose of Ru³⁺ was lower than 0.4 mmol, the Ru particles of Ru_0.2/C−800 had a uniform size. However, upon increasing the addition of Ru³⁺ to 0.4 mmol, the Ru particles of Ru_0.4/C−800 were significantly agglomerated (Figure S2). In addition, the pyrolysis temperature also played a vital part in the distribution of Ru particles. As displayed in Figure S3a, Ru_0.3/C−700 displayed the homogeneous distribution of Ru NPs due to the fact of insufficient nucleation under low pyrolysis temperature. However, with the increase in the pyrolysis temperature, the agglomeration of the Ru NPs occurred obviously (Figure S3b,c). Furthermore, to investigate the role of NaCl on the microstructure and metal phase of the catalysts, the “starch strips” were pretreated to wash off NaCl as much as possible, and the obtained catalyst was denoted as Ru_0.3/C−800−WF. Compared with Ru_0.3/C−800, the pore structure of Ru_0.3/C−800−WF was not obvious at the same scale, indicating the pore-forming effect of NaCl. In addition, Ru_0.3/C−800−WF only contained a small number of Ru particles, which demonstrates that NaCl also contributed to dispersing and anchoring the metallic Ru (Figure S4). The mass loading of Ru and Na was, furthermore, measured by an inductively coupled plasma-optical emission spectrometer (ICP-OES). As elevating the pyrolysis temperature from 800 ℃ to 1000 ℃, the Na content gradually reduced from 0.36 wt% to 0.004 wt% (Table S1). Accordingly, the content of Ru showed the same variation trend as that of Na content with the decreased content from 0.81 wt% to 0.11 wt%. Similarly, the Ru content of Ru_0.3/C−800−WF was reduced as the Na content decreased compared with Ru_0.3/C−800. Such results revealed that the stabilizing effect of Na on Ru was due to the interaction between them. Detected by high-resolution transmission electron microscopy (HRTEM), the interplanar spacings of Ru_0.3/C−800 were measured to be 0.205 nm and 0.231 nm (Figure 2b), coincident with the (101) and (100) planes of Ru, respectively. In addition, the uniform distribution of metallic Ru in the carbon matrix was further identified under a high-angle annular dark-field scanning transmission electron microscope (HADDF-STEM) (Figure 2c) [36]. The elemental mapping of Ru_0.3/C−800 (Figure 2d – i) demonstrated the homogeneous dispersion of C, O, N, Ru, Na, and Cl. Notably, it could be found that the element Ru was distributed around Na, as shown in the Figure S5, indicating the interaction between them, to a certain extent.

More structural details of the catalysts were then confirmed by X-ray diffractometer (XRD). From Figure 3a, a broad peak belonging to amorphous carbon was observed in all samples. Apart from that, the diffraction peaks coincided well with the (100), (002), and (101) planes of Ru (PDF#06-0663) located at ≈38.4°, 42.2°, and 44.0°, which appeared in Ru_0.3/C−800 and Ru_0.3/C−900. This result was associated with the good crystalline character of Ru at 800 °C and 900 °C (Figure S6). In addition, the typical diffraction peaks of NaCl (PDF#05-0628) at ≈31.7° and 45.5° were presented as well. It was worth noting that the intensity of the NaCl peaks weakened obviously along with the elevated pyrolysis temperature, which was due to the volatilization of NaCl as the temperature increased [37]. As for Ru_0.3/C−800−WF, there were only amorphous carbon peaks existing from the XRD pattern, revealing that NaCl was completely removed (Figure S7b). Moreover, as exhibited in Figure S7a, the NaCl peak intensity gradually weakened as the content of in Ru content increased.

X-ray photoelectron spectroscopy (XPS) was further employed to reveal the chemical valence and elemental compositions of the samples [17]. Here, the C 1s peak of 284.5 eV served as the standard for all of the peaks’ calibration [38]. Figure 3b visually showed the Na 1s peaks of Ru_0.3/C−800 and C−800. In contrast to C−800, the Na 1s peak of Ru_0.3/C−800 was shifted towards higher binding energy owing to the electron transfer from Na to Ru. From the Ru 3p spectra in Figure 3c, the peaks at 462.0 eV, 484.2 eV, 464.6 eV, and 486.5 eV were corresponding to Ru⁰ 3p_3/2, Ru⁰ 3p_1/2, Ruⁿ⁺ 3p_3/2, and Ruⁿ⁺ 3p_1/2, respectively [17,39]. The binding energy of Ru 3p of Ru_0.3/C−800 was negatively biased compared with Ru_0.3/C−800−WF. Moreover, the peak positions of Na 1s and Ru 3p varied with the pyrolysis temperature. As shown in Figure S8a, with the increase in the temperature, Na 1s shifted to the low binding energy. In Figure S8b, when the content of Na is high, the shift in the Ru 3p peak position was more obvious, because more Na transferred electrons to Ru. It was calculated that the ratio of Ru⁰/Ruⁿ⁺ for Ru_0.3/C−800 was 1.68, which was much higher than that of other control groups (Table S2). According to reports, Ru⁰ was the active center of catalytic hydrogen production, and the high content of Ru⁰ was helpful for improving catalytic hydrogen production [40]. In this system, Na, as an electronic structure regulator, could effectively adjust the valence distribution of Ru, promoting the formation of more Ru⁰, and hopefully improved catalytic activity.

Na not only served as an assistant to regulate the valence distribution of Ru and increase the content of Ru⁰ but also acted as a pore-forming agent that could affect the pore structure of the catalysts [41]. The pore structure was studied using the Brunauer–Emmett–Teller (BET) method, as displayed in Figure 3d. The N₂ adsorption/desorption plots exhibited that the specific surface area of Ru_0.3/C−800 was 592.0 m² g⁻¹, significantly larger than other catalysts (Table S3). The decreased specific surface areas of Ru_0.3/C−900 and Ru_0.3/C−1000 were possibly caused by the collapse of the mesoporous structure when the material calcined at high temperatures (especially over 800 °C). Furthermore, the pore size distribution of Ru_0.3/C−800 (inset in Figure 3d) manifested a large number of mesopores. The existence of a large number of mesopores was beneficial for the solution to enter during the reaction, which increased the contact area, exposed more active sites, and accelerated mass transfer [42]. Additionally, the defective nature of the catalysts was investigated by Raman spectroscopy [43]. A typical D band and G band of carbon was manifested in Ru_0.3/C−800, and the intensity ratio I_D/I_G was 0.856. Moreover, the value of I_D/I_G increased with the increase in the pyrolysis temperatures (Figure S9), possibly because too high pyrolysis temperature would lead to the collapse of the pore structure. It displayed that only an appropriate pyrolysis temperature can optimize the conductivity, as well as the catalytic abilities of the composites.

2.2. Electrochemical Characterization

The Ru powder, commercial Pt/C (20 wt%), Pt electrode, and Ru_x/C−T were tested for HER in 1.0 M KOH, 0.5 M H₂SO₄, and 1.0 M PBS, respectively. As is well known, the HER activity was evaluated by the overpotential (η₁₀) at the current density of 10 mA cm⁻² [43,44,45,46]. From Figure 4a, the blank carbon (C−800) displayed almost no activity for hydrogen generation within the applied potential window. However, upon introducing trace Ru in the system, the catalytic activity was significantly improved. With the increase in the Ru concentration, the η₁₀ distinctly declined from 73 mV (Ru_0.2/C−800) to 29 mV (Ru_0.3/C−800). Whereas, the η₁₀ further increased to 35 mV (Ru_0.4/C−800) when the Ru dosage was increased to 0.4 mmol. These results indicate that the appropriate Ru content was able to enhance the catalytic activity. It is noteworthy that the HER of Ru_0.3/C−800 exhibited an even better performance than Pt/C (20 wt%), Ru powder, and Pt electrode in 1.0 M KOH, as shown in Figure S10 and Figure 4a. Additionally, an effective catalyst for HER was able to initiate proton reduction with minimum overpotential and fast kinetics [46]. Normally, the Tafel slope is a vital indicator to investigate the reaction kinetics for HER [47]. According to the polarization curves of the samples, the corresponding Tafel slopes were obtained (Figure 4b). It was not hard to find that the order of Tafel slopes was Ru_0.3/C−800 (34.43 mV dec⁻¹) < Ru_0.4/C−800 (36.89 mV dec⁻¹) < Pt/C (59.44 mV dec⁻¹) < Ru_0.2/C−800 (78.74 mV dec⁻¹), which was in line with the trend of η₁₀. The smaller Tafel slope value of Ru_0.3/C−800 suggested a higher reaction rate, and the HER catalyzed by Ru_0.3/C−800 followed a Volmer–Heyrovsky mechanism [12]. Then, the charge transfer kinetics involved in HER was detected through EIS. The subsequent Nyquist diagram (Figure 4c) demonstrated that the curve of the sample was virtually semicircular, with the diameter of the curve standing for the associated charge transfer resistance (R_ct). In addition, the lower R_ct indicated a rapid electrode reaction rate. The Ru_0.3/C−800 had the smallest R_ct, indicating that the reaction rate was the fastest, which was consistent with the results of the voltammetry measurements. Furthermore, the electrochemical double-layer capacitance (C_dl), which was measured in the non-Faraday region of the CV curves, was used to evaluate the ECSA. The C_dl value was produced by linearly graphing the capacitance current density response vs. the scanning rate [17]. Figure S11 depicts the ECSA curves with various Ru contents. The C_dl of Ru_0.3/C−800 was 135.66 mF cm⁻² (Figure 4d), which was significantly higher than the C_dl of Ru_0.4/C−800 (113.54 mF cm⁻²), Ru_0.2/C−800 (65.31 mF cm⁻²), and C−800 (4.60 mF cm⁻²). The increase in the C_dl value means that Ru_0.3/C-800 possesses a large active surface area, which contributes to the exposure of active sites and the charge transfer [48].

On this basis, the role of the pyrolysis temperature on the catalytic activity was subsequently explored [18]. As shown in Figure 5a, Ru_0.3/C−700 presented a high overpotential of 214 mV in alkaline solution to achieve 10 mA cm⁻². Upon increasing the pyrolysis temperature, the overpotential dropped sharply, and the overpotential of Ru_0.3/C−800 only required 29 mV to achieve the same current density. However, further elevating the pyrolysis temperature, the overpotential increased to 52 mV and 90 mV for Ru_0.3/C−900 and Ru_0.3/C−1000, respectively. Additionally, as exhibited in Figure S12, the C_dl value rose first and then declined with the increase of the pyrolysis temperature. This result was in line with the activity trend. It was further confirmed that the high C_dl was the reason for the improved activity. However, the variation in the C_dl mainly came from the specific surface area, which was ultimately ascribed to the NaCl template for the forming of pores. Furthermore, the catalytic activity was closely correlated with the electrochemical reaction rate and the capability of charge transfer, which was verified by the Tafel (Figure 5b) and EIS (Figure 5c) results. In addition, Figure S13 illustrates the EIS fitting circuit for Ru_0.3/C−800, Ru_0.3/C−900, and Ru_0.3/C−1000 in 1.0 M KOH. It can be seen from Table S4 that the smaller R_ct value of Ru_0.3/C−800 corresponded to the higher reaction rates. More importantly, the overpotential of Ru_0.3/C-800-WF increased to 72 mV when the amount of NaCl in the raw material was washed off (Figure 5d), and Figure S14 shows that the corresponding Tafel, C_dl, and EIS results displayed a worsening trend. This was due to the pore-forming effect of NaCl, which regulated the specific surface area of the catalysts and, thus, affected their activity. Meanwhile, it can be seen from Figure 5e that the decrease in the NaCl content in the raw materials was accompanied by the decrease in the Ru⁰ component and the deterioration of activity. Moreover, the same results can be obtained from the catalysts calcined under varied temperatures. When the pyrolysis temperature rose, the overpotential for Ru_0.3/C−800, Ru_0.3/C−900, and Ru_0.3/C−1000 at 10 mA cm⁻² was inversely proportional to the Ru⁰ content. To put it simply, the higher the Ru⁰ content, the smaller the overpotential of the catalyst, and the better the electrochemical performance. These findings again demonstrate the interaction between Na and Ru; that is, Na not only adjusted the valence state of the Ru phase but also stabilized Ru. Simultaneously, an ideal catalyst not only has excellent catalytic activity but also high catalytic stability. Therefore, stability was another vital indicator to assess the performance of the catalyst [49,50,51]. On the one hand, the durability of Ru_0.3/C−800 was measured by continuous CV sweeps. Figure 5f reveals that the overpotential only fell by 2 mV after 1000 cycles, suggesting the excellent stability of Ru_0.3/C−800. On the other hand, under the corresponding potential, a further measurement was performed for 10 h by chronoamperometry. No significant reduction in the current density could be observed (inset in Figure 5f).

Furthermore, we also studied the electrocatalytic activity of the catalyst in acidic and neutral media, verifying the universality of the designed catalyst in a wide pH [52]. In acidic medium, a current density of 10 mA cm⁻² could be reached by the low overpotential of 83 mV for Ru_0.3/C−800, which was significantly lower than that of Ru_0.3/C−700 (basically inactivated), Ru_0.3/C−900 (96 mV), and Ru_0.3/C−1000 (131 mV). Meanwhile, the corresponding Tafel slope and EIS plots (Figure 5h,5i) showed the same trend as the activity curves. Additionally, the activity of Ru_0.3/C−800 was better than Ru powder in acidic medium (Figure S15a), and Ru_0.3/C−800 exhibited good stability (Figure S15b). Moreover, we conducted activity tests in a. neutral environment (Figure S16a). The performance was not ideal, but Ru_0.3/C−800 still had advantages compared with the Ru powder. Surprisingly, the performance of Ru_0.3/C−800 was excellent in terms of stability (Figure S16b). In summary, the above results strongly support the hypothesis that Ru_0.3/C−800 has good activity and stability for HER in a wide pH range.

3. Experimental

3.1. Chemicals

Potassium hydroxides (KOH), ruthenium (III) chloride (RuCl₃, Ru content: 45 wt%~55 wt%), sulfuric acid (H₂SO₄), Ru powder, and ethanol were bought from Aladdin Industrial Inc. The commercial Pt/C with 20 wt% Pt loading was purchased from Johnson Matthey Company (Sigma, Shanghai, China). The main ingredient in the “starch strips” was starch, which was bought from supermarkets and is usually used in the production of puffed foods. Deionized water (DIW) was used to prepare all solutions. The DIW used in the experiment was obtained using a water purifier Hitech-DW, Wenzhou, Zhejiang Province, China).

3.2. Synthesis of Electrocatalysts

3.2.1. Synthesis of Ru_0.3/C−800

First, 400 mg of RuCl₃ was weighed and dissolved in 20 mL of DIW. Another clean glass beaker was used and 17 mL of DIW was mixed with 3 mL of the prepared RuCl₃ solution. After, ~4 g of the “starch strips” were soaked in the above RuCl₃ solution at room temperature for 1 h. Furthermore, the treated “starch strips” were transferred to a watchglass and dried at 60 °C to obtain the precursor. Afterward, the precursor was calcined at 800 °C for 2 h in a tube furnace under an Ar environment. Finally, the product was ultrasonically washed several times with DIW. The final product, Ru_0.3/C−800, was obtained after drying.

3.2.2. Synthesis of C−800, Ru_0.2/C−800, and Ru_0.4/C−800

For the comparison of the samples of C−800, Ru_0.2/C−800, and Ru_0.4/C−800, the total volume of the reaction mixture remained at 20 mL. However, the addition of the RuCl₃ solution was changed to 0/2/4 mL, respectively. The other conditions remained unchanged.

3.2.3. Synthesis of Ru_0.3/C−700, Ru_0.3/C−900, and Ru_0.3/C−1000

For the comparison of the samples of Ru_0.3/C−700, Ru_0.3/C−900, and Ru_0.3/C−1000, the preparation procedure followed that of Ru_0.3/C−800, except that the calcination temperature varied to 700 °C, 900 °C, and 1000 °C.

3.2.4. Synthesis of Ru_0.3/C−800−WF (Wash First)

In addition, to demonstrate the influence of Na on the valence state of Ru, a control sample was set. In the early stage, the NaCl in the “starch strips” were washed as much as possible, and then the washed “starch strips” were used as raw materials for the synthesis of Ru_0.3/C−800−WF. The preparation process followed that of Ru_0.3/C−800.

3.3. Material Characterization

The HRTEM was carried out on a Talos F200S S-Twin (Thermo Fisher, Czech Republic), and the accelerating voltage was 200 kV. A D/tex Ultima TV wide-angle X-ray diffractometer (NIC, Japan) was employed to measure the metal phase of the samples. The diffractometer was equipped with Cu Kα radiation (1.54 Å), and the scanning rate was 5 °/min. The Raman spectra were collected on a Raman spectrometer (Labram HR800-LS55, HORIBA, France). The specific surface area and PSD curves were calculated by traditional BET and BJH methods, respectively. The X-ray photoelectron spectra (XPS) for the bonding states were obtained using a Thermo ESCALAB 250xi spectrometer (Thermo Fisher, Czech Republic). In addition, the samples were first dissolved with aqua regia. Inductively coupled plasma-optical emission spectrometry (ICP-OES) (PerkinElmer, Optima 5300 DV) was used to measure the contents of Ru and Na. The Ru content of Ru_0.3/C-800 was as low as 0.81 wt% as measured by ICP-OES.

3.4. Electrochemical Characterization

To assess the HER performance of the catalysts, all experiments were performed on a conventional three-electrode system with CHI 760E as the electrochemical workstation, and the temperature during the test was kept at room temperature. A saturated calomel electrode (SCE) and a carbon rod was employed as the reference and the counter electrode, respectively. A glassy carbon electrode 5 mm in diameter acted as the working electrode. A 3 mg catalyst was dispersed in the mixture of Nafion (20 μL) and ethanol (300 μL). Whereafter, the prepared ink was ultrasonic treated for 30 min to form a uniform suspension. Then, 30 μL ink was coated on the surface of the glassy carbon electrode and dried at room temperature. The electrochemical tests were operated in 0.5 M H₂SO₄, 1.0 M KOH, and 1.0 M PBS solution. Linear sweep voltammogram (LSV) curves were acquired at a scan rate of 5 mV s⁻¹ over a potential window of −0.4 to 0.1 V (relative to a reversible hydrogen electrode (RHE)). All polarization curves were IR-corrected. The Tafel equation (η = a + b log(j)) was employed to fit the slope of Tafel. In order to obtain the electrochemical active surface area (ECSA) of the prepared catalysts, we measured a series of cyclic voltammetry (CV) plots at different scanning rates from 10 to 50 mV s⁻¹ in the potential window of 0.1–0.2 V relative to RHE. Additionally, electrochemical impedance spectroscopy (EIS) was collected from the frequency range of 0.01 Hz to 100 kHz, and the test voltage was −0.023 V (vs. RHE).

4. Conclusions

In this work, pH-universal Ru/C electrocatalysts for HER were constructed using a two-step adsorption–pyrolysis strategy. In 1.0 M KOH and 0.5 M H₂SO₄, the overpotentials of Ru_0.3/C−800 were as low as 29 mV and 83 mV to obtain a current density of 10 mA cm⁻². This work has several advantages over previously reported results: (1) The “starch strips” containing natural alkali metal additives (NaCl) were used as the raw material, which were green, ecofriendly, and economical. (2) The alkali metal auxiliaries were able to stabilize Ru and regulate the distribution of Ru valence, which contributed to the formation of more active components Ru⁰. (3) NaCl as a salt template could induce a porous structure, promoting mass transfer and exposing more active sites. The synthetic strategy was not only general and scalable but also emphasized the critical role of alkali metal in promoting HER activity, which provides a new idea for developing efficient non-Pt HER catalysts.