Designing of Surface Engineering of Mesoporous Ru/RuO₂ Heterostructure for Effective Water‐Splitting Reaction in Proton Exchange Membrane Water Electrolyzer

Abstract

The development of effective proton exchange membrane water electrolyzer (PEMWE) electrocatalysts is critical for hydrogen production. Here, we synthesized a mesoporous Ru/RuO₂ heterostructure as a bifunctional electrocatalyst prepared by a surfactant‐assisted approach, followed by annealing at three different temperatures. Among them, prepared catalyst treated at 600°C has the highest activity toward the bifunctional activity of overall water splitting in an acidic medium with a turnover frequency of 13.59 and 4.2 s⁻¹, and an overpotential of 230 and 88 mV, consistently operating at 10 mA cm⁻² for 280 and 120 h for OER and HER, which is considerably superior to commercial RuO₂. It is worth noting that the zero‐gap PEMWE was fabricated using m‐Ru/RuO₂‐600 with fed water, delivering the overall current density of 0.6 A cm⁻² at 25°C and 1.0 A cm⁻² at 60°C. The integrated PEMWE was demonstrated for 300 h at 100 mA cm⁻². The enhanced performance of m‐Ru/RuO₂‐600 catalysts is due to the presence of both metallic Ru and RuO₂, which produce a synergistic effect, leading to a Ru⁺⁴–O–Ru⁰ interfacial structure that effectively alters the oxidation state of ruthenium, boosting kinetics, as confirmed by pre‐ and post‐OER and HER characterizations. This study offers a unique opportunity for low‐cost PEMWE.

We show off newly synthesized mesoporous Ru/RuO₂ heterostructures as potential OER nanocatalysts via a solvothermal approach with surfactant assistance for slow hydrolysis, followed by the results of RuO₂ catalyst treated at various temperatures for an effective proton exchange membrane water electrolyzer (PEMWE).

1. Introduction

Continuous consumption of fossil fuels accelerates the rate of CO₂ in the atmosphere. Due to significant climate change and the depletion of fossil fuels, researchers are seeking alternative energy sources that have no negative impact on the environment. Although solar, wind, and hydro energy are promising candidates for clean and sustainable energy sources [1, 2], they may not be available throughout the year in the whole world. Hence, energy storage molecular fuel is one of the best solution. For example, water electrolysis, fuel cells, metal‐air batteries, and CO₂‐to‐fuel conversion technologies are produced using renewable energy sources and represent sustainable energy technologies [3, 4, 5, 6]. Among all the energy carriers, hydrogen possesses unique characteristics, such as an excellent energy density (142 MJ/kg), being a green fuel with zero greenhouse gas emissions, faster refueling times compared to batteries, and significantly reducing dependence on fossil fuels [7]. PEMWE stands out as a particularly effective and promising approach for producing high‐purity hydrogen using renewable energy sources. They also offer numerous benefits compared to traditional alkaline water electrolyzers, including user‐friendliness, compact design, high efficiency, and higher current densities [8]. Theoretically, water oxidation (oxygen evolution reaction (OER)) occurs at 1.23 V vs. reversible hydrogen electrode (RHE) and hydrogen evolution reaction (HER) occurs at 0 V vs. RHE. Thus, compared with the HER, OER is kinetically sluggish because it involves a multistep process and requires additional overpotential, which is identified as the bottleneck of water electrolysis. Additionally, the anode contributes to the overall low efficiency of the process [9]. Ru oxides are among the best noble electrocatalysts for the OER, especially in acidic electrolytes. They possess a distinctive crystal structure and can be easily synthesized using methods such as air thermal oxidation, high‐temperature annealing, hydrothermal synthesis, and molten salt procedures [10, 11]. During the OER, Ru‐based electrocatalysts undergo electrochemical oxidation and surface reconstruction, leading to the transient formation of multiple oxidation states of RuO₂ with a short lifespan [12, 13]. As a result, ruthenium oxide, which is associated with its susceptibility to lattice oxygen release and high‐potential oxidation, consistently hinders the catalyst's stability performance [14]. Several Ru‐based catalysts have been synthesized to address this difficulty, including amorphous and crystalline ruthenium oxide [15, 16, 17, 18, 19], Ru‐based high entropy oxide [20, 21, 22], heteroatom‐doped RuO₂ [23, 24, 25, 26], perovskites [27, 28], pyrochlores [29, 30, 31], and mesoporous material [11, 32, 33, 34]. Although ruthenium‐based perovskites and mesoporous, amorphous ruthenium oxide exhibit encouraging bifunctional activity, their durability is still a major obstacle, casting doubt on their long‐term viability. Furthermore, the stability of RuO₂‐based catalysts composed solely of ruthenium and oxygen is insufficient; therefore, heterometals are typically incorporated to modulate the local electronic structure [34, 35, 36, 37, 38, 39, 40, 41, 42, 43]. Nevertheless, during the OER and HER, these heterometallic components may undergo chemical leaching, which can contaminate the PEM and increase resistance polarization [34, 38, 39]. Therefore, developing RuO₂‐based catalysts without incorporating other metal elements is a practical necessity, although it remains a challenging task.

In this study, surfactant‐assisted m‐Ru/RuO₂ was prepared by slow hydrolysis using Pluronic F‐123, followed by annealing at various temperatures (400°C, 600°C, and 800°C). The synthesized m‐Ru/RuO₂ electrocatalyst was systematically characterized using spectroscopic and electrochemical techniques to confirm the formation of a mesoporous Ru/RuO₂ heterostructure. Its bifunctional (OER and HER) activity and stability were compared with those of commercial RuO₂. Finally, an assembled PEM‐based water electrolyzer using mesoporous Ru/RuO₂‐600 (m‐Ru/RuO₂‐600) showed an excellent performance of water electrolysis and produced green hydrogen for 300 h at 100 mA cm⁻².

2. Results and Discussion

2.1. The Evaluation of Structures and Morphology

The schematic route to create the mesoporous Ru/RuO₂ heterostructure, using Pluronic P‐123 surfactant, shown in Figure 1a, illustrates a straightforward, slow hydrolysis, surfactant‐assisted synthesis method used to create three different porous materials. These materials were subsequently annealed at 400°C, 600°C, and 800°C in an air atmosphere and designated as m‐Ru/RuO₂‐400, m‐Ru/RuO₂‐600, and m‐Ru/RuO₂‐800, respectively. Figure 1b depicts the XRD pattern of m‐Ru/RuO₂ that has been calcined at 400°C, 600°C, and 800°C. The observed major peaks at 2θ angles are 27.8°, 35.12°, and 54.6°, and 69.87 for RuO₂ corresponding planes (110), (011), (121), and (211) respectively confirm the formation of rutile phase of RuO₂ in synthesized m‐Ru/RuO₂ by comparing the ruthenium oxide XRD data with the ICCD file data (PDF # 98‐006‐2889). Also, confirmed the peaks corresponding to 2θ angles of 38.12°, 41.66°, and 43.65° for planes (010), (002), and (011), which are represented by the metallic Ru peaks, respectively, by correlating the metallic‐Ru of XRD data with the ICCD file, is verified (PDF # 98‐008‐3627). When the calcination temperature is increased from 400°C to 600°C, the crystallinity and crystal size increase. However, at 800°C, these properties decrease due to aggregation. The particle sizes of the samples are determined using the Scherrer equation based on the (110) diffraction peak, and m‐Ru/RuO₂‐400, m‐Ru/RuO₂‐600, and m‐Ru/RuO₂‐800 have particle sizes of 39, 8.8, and 43.69 nm, respectively. Then, the prepared m‐Ru/RuO₂ catalyst underwent scanning electron microscopy (SEM) analysis, and the morphological results of these analyses are shown in Figure 1c–e. It depicts a tremella nanosheet with a porous structure, indicating a two‐dimensional material with a high aspect ratio. The nanosheet structure consists of interconnected particles that form a network with varying pore sizes depending on the annealing temperature. In addition, the EDS study (Figure S1, Table S1) clearly confirmed the complete removal of the surfactant (PEO‐PPO‐PEO) during annealing, which plays a critical role in controlling the size of heterostructures during particle growth and catalyst synthesis.

FIGURE 1.

(a) Schematic route of preparation of mesoporous Ru/RuO₂ heterostructure, (b) XRD pattern, (c–e) SEM images of synthesized, and (f,g) BET‐isotherm and pore‐distribution plot of m‐Ru/RuO₂‐400, m‐Ru/RuO₂‐600, and m‐Ru/RuO₂‐800.

The surface areas of all the m‐Ru/RuO₂ catalysts (m‐Ru/RuO₂‐400, m‐Ru/RuO₂‐600, and m‐Ru/RuO₂‐800) were analyzed using Brunauer's method. Figure 1f depicts Brunauer– Emmett–Teller (BET) analysis using N₂ adsorption–desorption isotherms. These studies found that the specific surface area of m‐Ru/RuO₂‐600 is 24.579 m² g⁻¹, with a type IV isotherm, indicating the mesoporous nature of the catalysts. The other m‐Ru/RuO₂‐400 shows the same kind of isotherm, with a specific surface area areas of 23.579 m² g⁻¹. Indisputably, m‐Ru‐/RuO₂‐800 shows a type‐I microporous nature with specific surface area of 55.942 m² g⁻¹. The presence of a hysteresis loop in the isotherms indicates that the materials possess a complex pore structure, comprising both micropores and mesopores. The results of utilizing the Barrett–Joyner–Halenda (BJH) algorithm to obtain pore radius and volume from the adsorption branch (Figure 1g) pore distribution plot show that m‐Ru/RuO₂‐600 has a greater pore size of 10.843 than other catalyst, which may contribute to catalytic activity. While m‐Ru/RuO₂‐800 has a large surface area and small pore size, as shown in Table S2, this may lead to mass transport limitations, thereby reducing catalytic performance. Consequently, it was clear from these findings that m‐Ru/RuO₂‐600 offers more active sites for converting water molecules into gas products in acidic water‐splitting reactions.

Figure 2a–c display high‐resolution TEM images of m‐Ru/RuO₂ synthesized at different temperatures. The m‐Ru/RuO₂‐600°C sample shows monodispersed small nanosheets, whereas catalysts treated at 400°C and 800°C reveal aggregated nanoparticles. All calcined catalysts possess a well‐developed crystal structure, with RuO₂ lattice fringes of 0.327, 0.25, and 0.23 nm for the Ru (011), RuO₂ (110), and (011) planes, respectively (Figure 2d–f). For m‐Ru/RuO₂‐600, the crystal lattice with a d‐spacing of 0.32 nm corresponds to RuO₂ (110). Also, Figure S2 illustrates the histograms of particle sizes measuring 32.98, 8.3, and 43.68 nm for m‐Ru/RuO₂‐400, m‐Ru/RuO₂‐600, and m‐Ru/RuO₂‐800, respectively. These results are consistent with the XRD data. The crystal lattices exhibit d‐spacings of 0.3 and 0.25 nm, perpendicular to each other, corresponding to the (110) and (011) planes. The selected area electron diffraction (SAED) patterns are shown in Figure 2g–i and display distinct dotted rings characteristic of polycrystalline materials for all the catalysts, confirming the presence of the m‐Ru/RuO₂ heterostructure crystalline planes. This value corresponds to the (011) plane of the Ru metallic phase in m‐Ru/RuO₂‐600 and is also in good agreement with the PXRD results indicating the presence of metallic Ru in the material. Figure 2j–l depicts the high‐angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) picture of all the prepared material. All of the predicted constituents, such as mixed elements, Ru and O, respectively, are present and distributed uniformly over the nanosheet structure, which agrees with SEM image results.

FIGURE 2.

(a–c) HR‐TEM image, (d–f) lattice fringes, (g–i) SAED pattern, and (j–l) HAADF area chosen for color mapping and color mapping of synthesized m‐Ru/RuO₂‐400, m‐Ru/RuO₂‐600, and m‐Ru/RuO₂‐800.

2.2. The Study of Surface and Electronic Structure

To determine the oxidation state of synthesized mesoporous materials at various temperatures using photoelectron spectroscopy (XPS), the XPS spectra of the Ru 3d orbital were analyzed and are displayed in Figure 3a–c. The energy separation between the observed peaks is approximately 4.4 eV, with a theoretical intensity ratio of 3:2. The peaks, corresponding to 3d_5/2 and 3d_3/2, have binding energies of 281.05 and 286.43 eV, indicating a +4 oxidation state of m‐Ru/RuO₂. A zero‐oxidation state reveals additional peaks at 280.75 and 284.92 eV, confirming the heterostructure of m‐Ru/RuO₂. A shake‐off‐satellite peak was noted at 282.57 eV, reflecting multielectron excitation and providing insights into the Ru–O bond's covalent and ionic characters. Furthermore, similar peaks for m‐Ru/RuO₂‐400 and m‐Ru/RuO₂‐800 indicate +4 and 0 oxidation states of Ru, respectively. The peak at 284.86 eV (C—C) and 286.62 eV (C—O) suggests C‐1s overlap with Ru‐3d due to adventitious carbon on the surface.

FIGURE 3.

Deconvoluted XPS profile of (a–c) Ru‐3d, (d–f) Ru‐3p, and (g–i) O‐1s of synthesized m‐Ru/RuO₂‐400, m‐Ru/RuO₂‐600, and m‐Ru/RuO₂‐800.

Figure 3d–f display deconvoluted XPS spectra of Ru‐3p orbitals of all the prepared materials, revealing four peaks with binding energies of 462.27, 484.83, 464.138, and 487.69 eV, corresponding to the 0, +4 position states of oxidation for the m‐Ru/RuO₂‐600 3p_3/2 and 3p_1/2 orbital. Ru 3p peaks are often broader and less intense than their comparable Ru 3d peaks. At common X‐ray energies, this is mainly because the deeper 3p orbital has a lower photoionization cross‐section and a shorter core‐hole lifespan. Similarly, m‐Ru/RuO₂‐400 and m‐Ru/RuO₂‐800 have similar peaks in Figure 3d–f. Additionally, three distinct peaks can be seen in the XPS spectra of the O‐1s orbital at high resolution in Figure 3g–i. These peaks can provide information about oxygen species present, distinguishing between lattice oxygen (O²⁻), surface hydroxyl group (Ru–O), and surface‐adsorbed water molecules, which are typically around 529, 530.24, and 532.4 eV. All the prepared materials having a metallic Ru and RuO₂ can lead to a synergistic effect, enhancing the catalyst performance. Due to different calcination temperatures, it can influence the optimal Ru/RuO₂, which can be quantified using Ru‐3d, and it can influence the number of active sites available for catalytic reaction, as shown in Table S3 and Figure S3. It can lead to a balance between activity and selectivity, stability for water‐splitting reactions. Finally, the thermal stability of the catalyst was studied using TGA, as shown in Figure S4.

2.3. Electrocatalytic Performance Toward OER in Acidic Medium

OER investigations were undertaken in a 0.5 M H₂SO₄ solution after utilizing various kinds of characterization methods to learn more about the as‐prepared catalyst. All the catalysts were made in a slurry form, and they were coated on the carbon paper (detailed procedure given in the Supporting Information). Carbon paper‐modified m‐Ru/RuO₂ heterostructure catalyst behavior test in a 0.5 M H₂SO₄ using cyclic voltammetry (CV) is shown in Figure 4a, clearly indicating the redox behavior, which can be attributed to transition between different oxidation states of Ru, such as Ru(III) to Ru(VI), which is involved in the OER mechanism. The CV curve confirms the m‐Ru/RuO₂ form of material through its characteristic redox behavior and OER activity. To explore the m‐Ru/RuO₂ catalytic activity, the linear sweep voltammetry (LSV) was recorded at a scan rate of 5 mV s⁻¹, as shown in Figure 4b. The LSV polarization studies indicate ‐ that m‐Ru/RuO₂‐600 has significantly greater OER activity with lower overpotential of 230 mV, resulting in a current density of 10 mA cm⁻². The availability of a more tailored active surface could be a contributor to this increased OER, as evidenced by BET analysis and TEM analysis. m‐Ru/RuO₂‐600 showed a smaller particle size (8.8 nm), a larger surface area to volume ratio, optimal Ru/RuO₂, and optimal pore size (10.85 nm), which influence more active sites for OER reaction to occur, whereas m‐Ru/RuO₂‐400 and m‐Ru/RuO₂‐800 show overpotentials of 310 and 440 mV, respectively, to achieve the 10 mA cm⁻² current density. Tafel slopes were analyzed from the iR drop‐free LSV polarization results to evaluate electron transport kinetics at the electrode–electrolyte interface, as shown in Figure 4c. The m‐Ru/RuO₂‐600 has a reduced Tafel slope value of 66 mV dec⁻¹ when comparing to m‐Ru/RuO₂‐400 and m‐Ru/RuO₂‐800, which have slope values of 68 and 112 mV dec⁻¹, respectively. These results indicate that compared to the other catalysts, m‐Ru/RuO₂‐600 shows superior performance.

FIGURE 4.

(a) Cyclic voltammetry response recorded at 50 mV s⁻¹. (b) Linear sweep polarization response recorded at 5 mV s⁻¹. (c) Tafel plot derived from LSV of figure from (b) m‐Ru/RuO₂ at different temperatures in 0.5 M H₂SO₄. (d) Schematic representation of OER using m‐Ru/RuO₂ transfer kinetics at its interface with the electrode working toward the OER reaction in acidic medium.

Recently, Yin Wang et al. reported the discovery and detailed mechanistic understanding of how a Ru⁴⁺–O–Ru⁰ interfacial structure, created by integrating metallic Ru with RuO₂, can enhance both the activity and stability of RuO₂ catalysts for acidic OER [44]. A Similar mechanistic pathway we observed in m‐Ru/RuO₂‐600 shows excellent performance compared to the Commercial RuO₂ due to the Ru/RuO₂ interface exhibiting a synergistic effect. In the case of Ru/ RuO₂, metallic Ru donates the electrons to RuO₂, which modulates the oxidation state and enhances the catalytic activity. That gave a more suitable mechanistic approach for mesoporous Ru/ RuO₂, as shown in Figure 4d (detailed mechanism explained in Figure S5). An electrochemical impedence spectroscopy (EIS) analysis was conducted to examine the electrode–electrolyte interface's charge transfer resistance (R _ct). Figure 5a highlights that m‐Ru‐600 has a lower R _ct value of 5.54 Ω, while m‐Ru/RuO₂‐400 and m‐Ru/RuO₂‐800 have higher R _ct values of 11.32 and 17.75 Ω, respectively. EIS analysis was performed within a potential range of 1.41–1.61 V vs. RHE, with Bode plots illustrating reactant adsorption and desorption regions on the electrode (Figure S6a–c). The high‐frequency area indicated catalyst oxidation and revealed phase angle reductions, suggesting electrocatalyst oxidation as voltage surpasses 1.41 V. In the low‐frequency domain, the phase angle decrease indicated the onset of the OER at potentials over 1.45 V. The m‐Ru/RuO₂‐600 catalyst demonstrated superior performance for OER, promoting activity at 1.56 V with lower resistance (5.54 Ω) and better charge–transfer kinetics compared to other catalysts, as detailed in Table S4.

FIGURE 5.

(a) EIS of m‐Ru/RuO₂ treated at various temperatures at 1.56 V. (b) TOF vs. m‐Ru/RuO₂ treated at various temperatures. (c) Plot of current density vs. scan rate of m‐Ru/RuO₂ treated at various temperatures. (d) Chronopotentiometry curve of response of m‐Ru/RuO₂. (e) Stability test for m‐Ru/RuO₂‐600 in 0.5 M H₂SO₄. (f) Comparison plot of overpotential vs. electrocatalyst response of synthesized m‐Ru/RuO₂‐600 with other reported electrocatalyst.

The mass‐dependent activities of all catalysts were analyzed by mass‐normalizing them with geometrically normalized ones based on the catalyst mass loading (0.1 mg cm⁻²). The TOF were determined at 1.56 V to demonstrate the intrinsic activity of all catalysts compared to RHE. The m‐Ru/RuO₂‐600 has a TOF value of 13.59 s⁻¹, while m‐Ru/RuO₂‐400 and m‐Ru/RuO₂‐800 have values of 1.94 and 0.91 s⁻¹, respectively, shown in Figure 5b. The TOF results show that m‐Ru/RuO₂‐600 has greater catalytic activity for OER in 0.5 M H₂SO₄. The double‐layer capacitance (Cdl) of all of the catalysts was analyzed from CV curves collected at varied scan rates between a potential range of 0.12–0.22 V (versus Hg/HgSO₄), as shown in Figure S7a–c. The CV curves revealed Cdl values of 13.58, 19.32, and 9.96 µF cm⁻² for m‐Ru/RuO₂‐400, m‐Ru/RuO₂‐600, and m‐Ru/RuO₂‐800 respectively, as shown in Figure 5c. The m‐Ru/RuO₂‐600 has the high Cdl value creating more  active sites for OER process. To confirm the long‐term stability of the m‐Ru/RuO₂ catalyst, chronopotentiometry analysis was performed for 1 h at a constant current density of 10 mA cm⁻², with the findings shown in Figure 5d. The excellent stability of m‐Ru/RuO₂‐600 for 280 h is demonstrated (Figure 5e). Finally, the m‐Ru/RuO₂‐600 comparison with previous literature is shown in Figure 5f.

2.4. Electrocatalytic Performance Toward HER in Acidic Medium

The prepared catalysts’ HER activity was assessed in a 0.5 M H₂SO₄ electrolyte that was N₂‐saturated. The catalytic performance of the synthesized mesoporous catalyst was evaluated, via a three‐electrode systemand compared with commercial benchmark catalyst (Pt/C) as shown in Figure 6a, while m‐Ru/RuO₂‐600 exhibited superior HER activity, with an overpotential of just 88 mV to reach 10 mA cm⁻². This significantly outperformed other catalysts which are m‐Ru/RuO₂‐400 (122 mV) and m‐Ru/RuO₂‐800 (192 mV), indicating that the synergistic proximity of Ru and RuO₂ nanoparticles is key to enhancing catalytic efficiency. The Tafel slope was analyzed to understand reaction dynamics. The Volmer step, which is the rate‐determining step (RDS) of water dissociation and M–H bond formation, is indicated by a Tafel slope of about 120 mV dec⁻¹. Analysis of the LSV curves revealed in Figure 6b that m‐Ru/RuO₂‐600 possesses a Tafel slope of 80.6 mV/dec, significantly lower than those of other m‐Ru/RuO₂‐400 and m‐Ru/RuO₂‐800 which consist of 172 and 209 mV/dec indicating that the RDS remains stable while kinetics accelerate the following heterostructure formation. This reduced Tafel slope indicates superior hydrogen evolution kinetics, driven by the synergistic effect of adjacent metallic Ru and RuO₂ nanoparticles. Xueqing Qiu et. al recently identified that the formation of a Ru⁴⁺–O–Ru²⁺ interfacial structure achieved by merging a metallic Ru/RuO₂ heterostructure is the key to improving acidic HER performance [45]. We observed similar mechanistic pathway suitable for a prepared heterostructure mesoporous catalyst, which was discussed in the Supporting Information. The unique amorphous‐crystalline configuration of mesoporous Ru/RuO₂ heterostructure enhances the charge transfer efficiency, leading to faster reaction rates. This is supported by EIS measurements, which reveal m‐Ru/RuO₂‐600 has 3.27 Ω lower resistance levels for this catalyst compared to other catalysts which show 9.43 and 9.82 Ω for m‐Ru/RuO₂‐400 and m‐Ru/RuO₂‐800, respectively, as shown in Figure 6c. EIS analysis was carried out at various potential −0.1 to −0.2 V vs. RHE. Moreover, the Bode plots for all catalysts are illustrated in Figure S8a–c. These figures indicate the areas where reactant adsorption and desorption occur on an electrode's surfaces. As illustrated in the Bode plots (Figure S8a–c), a slight shift in impedance occurs at high frequencies while increasing the potential, contrasted by a decline in the low‐frequency range. This trend indicates that higher frequency vibrations enhance charge transport at the electorde‐ electrolyte interface, likely due to cavitation‐driven surface reactivation and more efficient bubble detachment. The results show that m‐Ru/RuO₂‐600 promotes oxygen evolution at a lower applied potential of 1.56 V, resulting in lower resistance (3.27 Ω) compared to other catalysts. As a result, it is shown that m‐Ru/RuO₂‐600 has better charge–transfer kinetics at the interface than the other catalysts given in Table S5. The TOF were determined at −0.2 V to demonstrate the intrinsic HER activity of all catalysts compared to RHE. The m‐Ru/RuO₂‐600 has a TOF value of 4.37 s⁻¹, while m‐Ru/RuO₂‐400 and m‐Ru/RuO₂‐800 have values of 3.12 and 1.34 s⁻¹, respectively, as shown in Figure 6d. The TOF results show that m‐Ru/RuO_2‐600 has greater catalytic activity for HER in 0.5 M H₂SO₄. Furthermore, chronopotentiometry confirmed the material's robust stability in acidic media, showing no significant current loss over 10 h of continuous operation at −10 mA cm⁻²; among all the catalysts, m‐Ru/RuO₂‐600 show superior performance with 99 mV low overpotential, as shown in Figure 6e. The excellent stability of m‐Ru/RuO₂‐600 for 120 h is demonstrated by these data (Figure 6f). At current densities of 10 mA cm^–2, the catalyst remarkably maintained a constant potential for more than 120 h with no voltage decay (ΔE = 0.001 V). The strong heterointerface between metallic Ru and RuO₂, which demonstrates chemical robustness and cooperative electronic interactions, was responsible for this exceptional stability. These results, detailed in Figure 6g, underscore the catalyst's exceptional performance in acidic media compared to previously reported HER catalysts.

FIGURE 6.

(a) Linear sweep polarization response recorded at 5 mV s⁻¹. (b) Tafel plot derived from LSV of figure from prepared m‐Ru/RuO₂ at in 0.5 M H₂SO₄. (c) EIS at −0.2 V. (d) Plot of TOF vs. m‐Ru/RuO₂ treated at various temperatures, (e) chronopotentiometry curve of response of m‐Ru/RuO₂ treated at various temperatures, (f) stability test for m‐Ru/RuO₂‐600 in 0.5 M H₂SO₄, and (g) comparison plot of overpotential vs. electrocatalyst response of synthesized m‐Ru/RuO₂‐600 with other HER reported electrocatalyst.

2.5. PEMWE Performance in Zero‐Gap Electrolysis Cell

In order to see the real electrolyzer cell performance, the best HER and OER electrocatalysts at the moment are believed to be Pt and IrO₂, because they exhibit high catalytic activity at a relatively modest overpotential at 10 mA cm ⁻² current density. To maximize their activity, stability, and overall cell performance, more studies on Ru‐based bi‐functional catalysts for PEMWEs are required. The synthesized m‐Ru/RuO₂‐600 catalyst exhibits high efficacy for both the OER and HER, achieving a current density of 10 mA cm⁻² with overpotentials of 230 mV for OER and 88 mV for HER in acidic solution. This catalyst has been chosen for further testing in PEMWE at elevated current densities. A membrane electrode assembly (MEA) was made in a zero‐gap electrolysis cell configuration schematic representation shown in Figure 7a. MEA preparation is detailed in the Supporting Information, where m‐Ru/RuO₂‐600 serves as the electrocatalyst for OER and HER . The current–voltage curve in Figure 7b shows that the Pt/C||m‐Ru/RuO₂‐600‐based electrolyzer operated at 2.0 V with a current density of 1.4 A cm⁻² compared with the commercial catalyst. Similarly, m‐Ru/RuO₂‐600||m‐Ru/RuO₂‐600 shows excellent bifunctional activity, showing higher current density of 1.2 A cm⁻² compared with commercial catalyst. When increasing the temperature of the water fed, it replicates the cell performance as shown in Figure 7c. Figure 7d displays the superior performance of m‐Ru/RuO₂‐600 as a cathode, showing higher current density compared than Com.Pt/C. We conducted stepwise chronopotentiometry and EIS, which were used to evaluate the performance of various MEAs at room temperature and 60°C. Stepwise chronopotentiometry was tested at various current densities, from 100 to 600 mA, at room temperature as shown in Figure 7e–g. The results indicate that catalyst shows superior performance. Increasing the temperature improve the cell performance, reflecting in the performance metrics shown in Figure S9a–d. MEAs tested at variuous voltages from 1.5 to 2.0 V, as shown in Figure S10a–d, indicating that the m‐Ru/RuO₂‐600||m‐Ru/RuO₂‐600 configuration achieved a current density of 0.56 A cm⁻² at 2 V, significantly outperforming the Com. IrO₂||Com. Pt/C cell. The m‐Ru/RuO₂‐600||Com. Pt/C electrolyser delivered a current density of 0.62 A cm⁻² at 2.0 V (Figure 7h). Superior hydrogen production rates and energy efficiency calculated using different applied cell voltage exceed 80%, as detailed in Figure 7i. The long‐term stability was tested for all the MEAs, as shown in Figure 7j. Among all the MEAs, m‐Ru/RuO₂‐600||Com. Pt/C emphasizes a constant cell potential of 1.54 V with current density of 100 mA cm⁻² at 320 h. In the same way, we checked the cell performance at 200 and 500 mA cm⁻² (Figure S11a,b). The produced gas was analyzed using gas chromatography (GC). As shown in Figure S12a,b, the representative gas chromatogram confirms the generation of O₂ and H₂ gases with high‐purity during the water‐splitting reaction. This outstanding consistency demonstrates the electrolyzer's extreme stability and long‐term performance retention to realise green hydrogen economy for clean environment.

FIGURE 7.

(a) Schematic representation of zero‐gap electrolysis cell for PEMWE. (b) LSV OER response of m‐Ru/RuO₂‐600‐based MEA. (c) LSV HER response of m‐Ru/RuO₂‐600‐based MEA. (d) LSV OER response of m‐Ru/RuO₂‐600‐based MEA at 60°C. (e–g) Stepwise chronopotentiometry curve of m‐Ru/RuO₂‐600 MEAs compared with Com.IrO₂||Pt/C. (h) I vs. V plot. (i) Hydrogen production rate. (j) stability test for m‐Ru/RuO₂‐600 using chronopotentiometry technique at 100 mA/cm².

3. Conclusion

The study successfully addresses the critical problem of developing long‐lasting and highly effective electrocatalysts for OER in acidic environments, with the goal of replacing expensive iridium dioxide (IrO₂) while avoiding the drawbacks of ruthenium dioxide (RuO₂), such as dissolution and over‐oxidation. The synthesized superior performance of the m‐Ru/RuO₂ catalyst was achieved using a surfactant‐assisted method followed by annealing at different temperatures. The impact of the Ru/RuO₂ ratio and the significance of mesoporosity in enhancing the catalytic performance were examined. The outstanding OER shown by m‐Ru/RuO₂‐600 with reduced overpotentials of 230 mV and a Tafel slope of 66 mV/dec in 0.5 M H₂SO₄ at a current density of 10 mA cm⁻². Also, excellent HER is performed with less overpotential 88 mV, 80.3 mV/dec; it's stable for 120 h for HER and 280 h for OER. Furthermore, the zero‐gap electrolyzer cell was assembled with m‐Ru/RuO₂‐600; both the anode and cathode in the PEMWE cell also offer a remarkable overall water‐splitting current density of 0.55 A cm⁻² at room temperature and stable for 40 h. Current Density of 0.8 A cm⁻² was achived at 60°C. The excellent m‐Ru/RuO₂‐600, anode catalyst, achieved 1 A cm⁻² at 60°C. The integrated PEMWE proved to have good endurance at 100 mA cm⁻² for 300 h. This work provides an approach for developing high‐performance acidic bifunctional mesoporous catalysts, which are essential for green hydrogen production to realise hydrogen economy.

Supporting Information

Additional supporting information can be found online in the Supporting Information section. Supporting Information provides detailed information regarding the synthesis of active electrodes, physicochemical characterization, and electrochemical assessment of after OER and HER and PEMWE applications. Supporting Fig. S1: SEM images EDX (a–c) of m‐Ru/RuO₂‐400, m‐Ru/RuO₂‐600, m‐Ru/RuO₂‐800. Supporting Fig. S2: Derivative plot of particle size histogram using HR‐TEM images (a–c) particle size of m‐Ru/RuO₂‐400, m‐Ru/RuO₂‐600, m‐Ru/RuO₂‐800. Supporting Fig. S3: Optimal m‐Ru/RuO₂ ratio and atomic ratio of oxygen of m‐Ru/RuO₂‐400, m‐Ru/RuO₂‐600, m‐Ru/RuO₂‐800. Supporting Fig. S4: Bode‐plot of m‐Ru/RuO₂‐400, m‐Ru/RuO₂‐600, m‐Ru/RuO₂‐800 for OER. Supporting Fig. S5: CV curve of m‐Ru/RuO₂‐400, m‐Ru/RuO₂‐600, m‐Ru/RuO₂‐800 and Com. RuO₂ at different scan‐rate. Supporting Fig. S6: Bode‐plot of m‐Ru/RuO₂‐400, m‐Ru/RuO₂‐600, m‐Ru/RuO₂‐800 for HER. Supporting Fig. S7: Step‐wise chronopotentiometry curve for MEA at 60°C (a) m‐Ru/RuO₂‐600||Com.IrO₂ (b) Com.Pt/C,|m‐Ru/RuO₂‐600 (c) m‐Ru/RuO₂‐600|| m‐Ru/RuO₂‐600 (d) Com.Pt/C,|Com.IrO_{2 .} Supporting Fig. S8: (a–d) Plot of Current density vs time for PEMWE tested at different cell voltage for different MEA (e) EIS measurement for MEA at 2.0 V cell (f) power consumption plot for prepared MEAs. Supporting Fig. S9: (a–b) Plot of Cell voltage vs time for PEMWE tested at 200, 500 mA for different MEA. Supporting Fig. S10: Gas chromatogram of produced hydrogen and oxygen gas. Supporting Fig. S11: XRD pattern of before and after stability test for m‐Ru/RuO₂‐600. Supporting Fig. S12: (a) TEM image of after stability test for m‐Ru/RuO₂‐600. (b) lattice fringes. (c) SAED pattern. (d) HAADF area chosen for color mapping and (e–i) color mapping results of after stability test m‐Ru/RuO₂‐600. Supporting Fig. S13: XPS profile of after OER m‐Ru/RuO₂‐600: (a) Ru‐3d, (b) Ru‐3p, (c) O‐1s. Supporting Table S1: Atomic weight % of m‐Ru/RuO₂‐400, m‐Ru/RuO₂‐600, m‐Ru/RuO₂‐800. Supporting Table S2: BET‐isotherm surface area and pore distribution m‐Ru‐400, m‐Ru‐600, m‐Ru‐800. Supporting Table S3: Metallic Ru and RuO₂ distribution m‐Ru/RuO₂‐400, m‐Ru/RuO₂‐600, m‐Ru/RuO₂‐800. Supporting Table S4: Charge Transfer of m‐Ru/RuO₂‐400, m‐Ru/RuO₂‐800 at 1.56 V. Supporting Table S5: Charge transfer of m‐Ru/RuO₂‐400, m‐Ru/RuO₂‐800 at −0.2 V. Supporting Table S6: Comparisons of OER catalyst in acidic medium. Supporting Table 7: Comparisons of HER catalyst in acidic medium. Supporting V ideo S1: PEMWE in operation mode.

Supporting information

Supplementary Material