As a countermeasure to mitigate global warming caused by the indiscriminate use of fossil fuels, many researches have been conducted to develop the energy conversion systems connected with renewable energies such as wind and solar power [1,2]. Among the systems, water electrolysis has been recognized as one of eco-friendly energy-conversion process that produce only hydrogen and oxygen without any pollutants [3]. In addition, when water electrolysis is driven by electricity generated from renewable energy sources, and accordingly, an eco-friendly hydrogen production system can be established [4,5]. As compared to traditional water electrolysis system, membrane electrode assembly (MEA)-based configuration recently has attention owing to its compact structure, low ohmic resistance, and high hydrogen production efficiency at high operating current densities [6,7]. Among MEA-based systems, proton exchange membrane water electrolysis (PEMWE) generally exhibits higher hydrogen production rates with higher purity than anion exchange membrane water electrolysis [8–11].

Meanwhile, considering the acidic operating environment of PEMWE, a significant challenge arises from the reliance on platinum group metals as electrode materials for water electrolysis [12,13]. Specifically, Pt is recognized as the most effective electrocatalyst for the hydrogen evolution reaction (HER), a crucial half-reaction in water electrolysis [14]. However, its scarcity and high cost increase the MEA fabrication expenses, ultimately hindering the scale-up and industrial commercialization of PEMWE. Therefore, the development of non-Pt-based electrocatalysts and electrodes is essential for achieving a low-cost water electrolysis [15]. Among the alternatives to Pt, Ru has recently gained attention as an electrocatalyst due to its ~2.4 times lower price compared to Pt (Pt: $973.9/oz, Ru: $400.0/oz) [16]. Additionally, Ru exhibits a similar hydrogen bond strength (65 kcal/mol) to Pt (62 kcal/mol), potentially leading to comparable catalytic behavior [17,18]. However, pure Ru (η₁₀: 99 mV) has a lower intrinsic HER performance than Pt (η₁₀: 25 mV) [19], prompting numerous studies aimed at enhancing its performance and durability through alloying with elements such as Co [20,21], Fe [22], Ni [23,24], Cu [25], Mo [26], and Au [16,27–29]. Among these, the Ru-Au bimetallic catalyst is considered a promising Ru alloy catalyst for the HER because inert Au in an acidic environment can improve its activity and durability (η₁₀: 43 mV) [29]. However, the hydrogen binding characteristics of Ru-Au are not optimal for HER, resulting in still lower HER activity than that of Pt [28]. Z. Zhang et al. [26] reported that the addition of Mo to Ru allows for fine-tuning of the hydrogen binding energy and this research findings provides inspiration for studying the Ru-Au-Mo ternary alloy.

Generally, the conventional electrode for PEMWE has been fabricated by drop-casting or spraying the catalyst powder via the following processes: (1) synthesizing nanoparticle catalysts, (2) preparing the catalyst slurry by blending with an ionomer solution, and (3) loading the slurry onto the membrane or porous transport layer (PTL) [30–32]. This method involves multiple steps and requires a relatively long fabrication time. In contrast, thin-film deposition methods, such as sputtering [33–35], chemical vapor deposition [36–38], and electrodeposition [39–41], can directly fabricate the catalyst layer with high coverage on the substrate and thin-film structures are effective in improving cell performance by increasing catalyst utilization [42]. Among these methods, sputtering allows for the formation of thin-films with high quality and reproducibility in a short processing time, which simplifies the electrode preparation step [43]. This one-pot process also provides significant advantages in the fabrication of porous transport electrodes (PTE) for PEMWE. Unlike typical PTE fabrication methods, sputtering does not require polymer binders, which enhance the electrical conductivity during water electrolysis [44]. In addition, the thin-film structure contributes to minimizing ohmic resistance and promoting the mass transport of reactants and products [45]. Moreover, the sputtering method can easily scale up the electrode area, making it suitable for water electrolysis to produce large amounts of hydrogen. Despite these advantages, the sputtering method faces significant challenges in electrode fabrication [33]; Its deposition rate is still low (< 50 nm/min), which leads to a long sputtering time for the high loading of the catalyst layer. Accordingly, successful utilization of the sputtering method for the fabrication of the catalyst eventually requires the enhancements of the activity of the catalyst even under the low loading condition, by the development of catalyst alloying as aforementioned.

In this concerns, sputtering method can easily prepare atomically uniform alloy-based catalysts when using the co-sputtering of multiple targets, where the atomic concentration of the alloy is controllable by adjusting the sputtering power condition [33]. In this study, we present a novel approach to fabricating RuAuMo thin films using a co-sputtering method onto a conductive support, with the films serving as efficient electrocatalysts for the HER in an acidic medium. By precisely controlling the sputtering power (P_metal) of each target element, we were able to fine-tune the atomic concentrations of Ru, Au, and Mo in the thin films. The HER performance of the RuAuMo films was investigated as a function of atomic composition through electrochemical analysis, demonstrating significant enhancement in catalytic activity with optimized atomic ratios. Notably, we directly fabricated RuAuMo thin films onto a PTL for application as cathodes in PEMWE. The RuAuMo cathode exhibited superior cell performance compared to Ru and RuAu electrodes, emphasizing the synergetic effects of trimetallic incorporation in improving HER activity.

Each of the elements Ru, Au, and Mo was deposited via sputtering onto a Ti/TiN/Si wafer with a fixed sputtering time of 1800 s. Fig. 1 shows the thickness of each elemental deposit as a function of P_metal. The thicknesses of Ru, Au, and Mo increased monotonically with P_metal, indicating that the deposition rate increases with P_metal and varies with the type of metal target. In the cross-sectional SEM images, the Ru films fabricated at a P_Ru of 30 W had a thickness of 97 nm, which gradually increased to 250 nm at 90 W (Fig. 1b–d).

We fabricated bimetallic RuAu-based films via co-sputtering of Ru and Au targets. The P_Ru was fixed at 70 W, while that of P_Au varied from 10 to 50 W. Fig. 2a shows the atomic composition of RuAu-based bimetallic films analyzed by EDS, where the Au ratio gradually increased with higher P_Au. The HER performance of the as-prepared RuAu-bimetallic electrocatalysts was evaluated in a 1.0 M H₂SO₄ solution. The RuAu-bimetallic electrocatalysts were denoted as RuAu#, where # represents the P_Au in watts. Fig. 2b presents the polarization curves for RuAu electrocatalysts, with the inset displaying the η₁₀ of HER plotted against P_Au. The h₁₀ for pure Ru was 84.5 mV. At a P_Au of 10 W, a decreased HER h₁₀ was measured. However, as the P_Au was further increased, the HER h₁₀ also increased, reaching 85.0 mV for RuAu50. Although Au has good electrical conductivity, its weak hydrogen adsorption properties can deteriorate the HER performance of RuAu-bimetallic electrocatalysts [27,28]. Therefore, the HER h₁₀ of RuAu against P_Au, or Au concentration, exhibited a volcano-like tendency, with RuAu10 showing the maximum HER performance compared to bare Ru and other RuAu# samples. The Tafel plots derived from the HER polarization curves confirmed that the Tafel slopes for Ru and RuAu electrocatalysts ranged from 60 to 70 mV/dec, indicating that the HER proceeded via the Volmer–Heyrovsky mechanism (Fig. 2c) [14]. Additionally, RuAu10 exhibited a Tafel slope of 61.9 mV/dec, indicating faster reaction kinetics for HER among the electrocatalysts (RuAu30: 63.0 mV/dec and RuAu50: 67.5 mV/dec). Considering Randles circuit model, the Nyquist plot in Fig. 2d also exhibited the same tendency of HER polarization curve and Tafel plot for RuAu-bimetallic electrocatalysts. Based on these results, RuAu10, fabricated with P_Ru, P_Au of 70 W for Ru and 10 W for Au, was selected as the optimal bimetallic electrocatalyst.

To further enhance the electrochemical properties of RuAu-based films, Mo was introduced as a third metal for sputtering, with varying P_Mo from 0 to 110 W. The RuAuMo trimetallic films were denoted as RuAu10Mo#, where # represents the P_Mo in watts. The top-view SEM images in Fig. 3a–d revealed slightly rough surfaces for the RuAu10Mo# films. Additionally, all RuAuMo films exhibited similar morphologies regardless of P_Mo. The atomic concentrations of RuAu10Mo# films were investigated by EDS, as shown in Fig. 3e. As the P_Mo increased, the Mo atomic ratio increased, while the Ru ratio decreased. Conversely, the Au atomic composition in RuAu10Mo# films remained in the range of 6–12%, showing minimal influence from Mo sputtering.

Fig. 4 shows the XRD patterns of Ru and RuAuMo thin films fabricated by sputtering. For the Ru film, Ru (100), Ru (002), and Ru (101) peaks were detected at 38.3°, 42.1°, and 44.1°, respectively. The broaden peaks for Ru deposit was detected in XRD pattern, and it seems to be that the weak P_Ru generated the low-crystalline phase of Ru deposits with random growth [46,47]. As the Mo content in RuAuMo deposits increased, the intensities of these Ru peaks gradually decreased, and a new peak emerged at 41.3°, which was slightly shifted to 40.4°. This peak shift in the XRD pattern may have originated from the formation of the alloy structure of the trimetallic RuAuMo thin film. In previous literature, the similar XRD peak shift was reported that the behavior of RuMo alloy was owing to Ru lattice expansion [48]. In the RuAu10Mo110 sample with a Mo ratio of 44.8%, the intensity of Ru peaks was decreased while the influence of Mo crystallinity became dominant.

Meanwhile, XPS analyses of RuAu10 and RuAu10Mo# deposits were conducted in order to investigate the electronic structure of trimetallic RuAuMo thin film (Fig. 5). As shown in Fig. 5a, the Ru 3p_1/2 and 3p_3/2 spectra for RuAu10 and RuAu10Mo# film were deconvoluted into three peaks, which corresponded to the Ru⁰ peak and two Ru satellite peaks. Compared to metallic Ru (461.2 eV), the Ru⁰ peak of RuAu10 was shifted to a higher binding energy, indicating the presence of electronic interactions between Ru and Au. Previous literature suggests that the peak shift is due to electron transfer from Ru to Au along with the formation of the Ru-Au alloy [16,27,28]. Furthermore, when Mo species were added to RuAu via co-sputtering, the Ru⁰ peak in RuAu10Mo# was slightly shifted in the negative direction relative to that of RuAu10. Conversely, Fig. 5b shows that both RuAu10 and RuAu10Mo# exhibited an Au 4f peak shift compared to metallic Au (84.0 eV), confirming that the co-sputtering of Ru, Au, and Mo forms an alloy structure. Interestingly, a positive peak shift for Au 4f was observed in all samples, despite the higher electronegativity of Au (EN: 2.54) compared to Ru (EN: 2.20) and Mo (EN: 2.16). This shift may be attributed to the small fraction of Au analyzed by EDS, which resulted in a decrease in electron density of Au in RuAu10Mo# deposits [29]. The XRD and XPS analyses revealed that the sputtering method is suitable for fabricating the alloy structure of Ru, Au, and Mo.

Fig. 6 presents the HER performance of trimetallic RuAuMo# electrocatalysts measured in a 1.0 M H₂SO₄ solution. According to the polarization curves in Fig. 6a, the bimetallic RuAu and trimetallic RuAuMo electrocatalysts demonstrate better HER activity compared to monometallic Ru in an acidic medium, indicating that alloy structures with multiple elements contribute to enhanced HER performance. Fig. 6b shows the HER h₁₀ derived from the polarization curves. At a P_Mo of 30 W, the h₁₀ was minimized to 50.4 mV. However, a further increase in P_Mo deteriorated the activity (η₁₀ = 81.1 mV for RuAu10Mo110). In previous literatures, it has generally reported that the HER performance of the electrocatalyst is depending on various factors; morphology, catalyst structure, crystallinity, atomic composition, electronic structure, and support [49,50]. In this study, the RuAuMo electrocatalysts with varying P_Mo showed similar surface morphologies, as shown in Fig. 3. This suggests that other factors, such as surface composition, crystal structure, and electronic structure, could play a more prominent role in influencing the variations in HER performance.

Based on the results from the half-cell system, the RuAu10Mo30 electrocatalyst exhibited the best HER performance with facile charge transfer in an acidic medium and was selected as a cathode catalyst for PEMWE. In order to apply the PTE into PEMWE, the RuAu10Mo30 was deposited via sputtering onto porous Ti PTL as a support. Optimized Ru and RuAu on Ti PTL were also fabricated for comparison. The ICP-MS analysis revealed that the total amount of Ru, Au and Mo loaded on Ti PTL for RuAu10Mo30 was 30.96 μg/cm². Fig. 7a presents the i-V curves of the PEMWE single cell with Ru, RuAu, and RuAuMo electrodes. At a cell voltage of 1.8 V_cell, the RuAu10Mo30/Ti PTL cathode achieved a current density of 1.39 A/cm², demonstrating better PEMWE performance than Ru/Ti PTL (0.93 A/cm²) and RuAu10/Ti PTL (1.20 A/cm²). Furthermore, compared to values reported in previous literatures employing Ru-based cathodes, the performance of the trimetallic RuAuMo is competitive, when considering its low catalyst loading (Fig. 7b) [21,26,52–54].

In this study, trimetallic RuAuMo-based thin-film electrocatalysts for the HER were fabricated via a facile sputtering method, and their performance in a PEMWE cell was investigated. For the sputtering method, it was easy to control the atomic compositions of RuAuMo-based films by modulating the PMetal of each target element. With varying the P_Mo, a trade-off relationship between the Ru and Mo ratios was identified. In addition, XRD and XPS analysis confirmed that the RuAuMo films fabricated via sputtering formed an alloy structure. The trimetallic RuAuMo electrocatalyst exhibited enhanced HER activity compared to the monometallic Ru and bimetallic RuAu electrocatalysts, as the optimized RuAu10Mo30 electrocatalyst exhibited an HER h₁₀ of 50.4 mV in a half-cell system. RuAuMo was directly fabricated onto Ti PTL for use as a cathode for PEMWE. The as-prepared RuAu10Mo30/Ti PTL cathode exhibited a current density of 1.39 A/cm² at 1.8 V_cell. The mass activity of RuAu10Mo30/Ti PTL for PEMWE was relatively high compared to reported values in previous literature. Consequently, this study demonstrates the RuAuMo alloy thin-film electrocatalysts fabricated via sputtering is a promising cathode for PEMWE systems.