In situ capturing site-to-site reactive species in CO₂-laser-patterned high-entropy alloy nanoflowers for robust alkaline seawater electrolysis

Abstract

Designing and synthesizing highly competent and stable electrocatalysts for the hydrogen evolution reaction (HER) in both alkaline and natural seawater media remain major obstacles. Herein, we present an ultrafast synthetic approach for producing AuRuIrPdPt high-entropy alloys (HEAs) through continuous-wave CO₂-laser irradiation for 90 s. HEAs synthesized using different CO₂-laser powers (30%, 60%, and 90% of the total 25 W laser power) demonstrated distinctive regularly ordered structures with numerous active sites for the HER. The optimized HEA-60 revealed outstanding HER activity with low overpotentials of 37, 34, and 45 mV at 10 mA cm⁻² in alkaline, simulated seawater, and natural seawater, respectively, outperforming a commercial Pt/C catalyst. In situ/operando electrochemical Raman analysis revealed the involvement of metals (M = Pt, Pd, and Ru) in the HER process, with M–H and M–O observed as intermediates rather than M–OH. Moreover, an overall water-splitting assembly using the IrO₂(+)‖HEAs-60(−) configuration achieved an exceptionally low cell voltage of 1.62 V to reach 10 mA cm⁻² in a natural seawater electrolyte, demonstrating excellent stability. This study emphasizes the use of an ultrafast CO₂-laser-irradiation method for synthesizing extremely stable and active HEAs for hydrogen production via seawater electrolysis.

---

1. Introduction

Hydrogen is a promising substitute for fossil fuels, characterized by its status as a sustainable energy carrier with a high gravimetric energy density of 142.351 MJ kg⁻¹ and zero CO₂ emission.^1,2 Electrocatalytic overall water-splitting (OWS) systems provide a viable approach for clean hydrogen production owing to their low energy consumption, high-purity hydrogen production, and pollution-free process.^3–5 Over the past few decades, numerous efficient electrocatalysts have been developed to produce hydrogen from water splitting, with some surpassing conventional Pt-based catalysts in performance.^6–8 Generally, OWS systems utilize freshwater as the electrolyte, either in acidic or alkaline conditions. However, the wide-spread utilization of freshwater electrolytes is restricted owing to the global freshwater scarcity. An interesting alternative is to directly split seawater into hydrogen considering its abundant availability, which accounts for 96.5% of the total water resources worldwide.^9–12 Yet, seawater splitting faces major obstacles in achieving efficient catalytic performance for the HER owing to its high chloride ion content, high corrosiveness, low conductivity, and susceptibility to ionic poisoning.^13,14 Despite considerable research efforts aimed at improving HER catalytic performance, reported catalysts have thus far fallen short of meeting the requisite criteria for practical implementation in seawater environments. Alternatively, platinum (Pt), considered the benchmark HER catalyst owing to its near-optimal hydrogen adsorption energy, has received considerable attention as an outstanding catalyst. However, its high cost, scarcity, and insufficient stability hinder its practical application in large-scale hydrogen production.^15,16 Therefore, substantial obstacles persist in the pursuit of identifying innovative HER catalysts with exceptionally high efficiency.

Alloys represent a prevalent catalyst type, and they have found widespread utilization in numerous fields.^17–20 In contrast to bimetallic and trimetallic alloys, high-entropy alloys (HEAs) have recently attracted enormous interest in the area of OWS owing to their ability to modify the electronic structures of catalysts and provide a plethora of various active sites.^21–24 HEAs are characterized as complex solid states containing five or more metallic elements within a single phase, and they exhibit outstanding physiochemical properties, such as corrosion resistance, tunable composition, high strength, excellent hardness, and robust stability.^25–27 Notably, a key feature of HEAs is their ability to operate across a wide range of operational potential energies. This is attributed to the formation of a strong lattice when catalytically active components with varying atomic sizes are combined, thereby reducing the energy barrier for catalytic processes.²⁸ In addition, the flexible arrangements of different metal components in HEAs can synergistically induce strain and ligand effects, influencing their distinctive characteristics and potentially modifying the d-band and electronic structure of active sites.^29,30 These combined effects facilitate the development of advanced functional materials that are ideal for catalytic reactions. Consequently, a few HEA catalysts have been investigated regarding their performance in seawater splitting. For example, Wang et al. described the effectiveness of TiNbTaCrMo HEA as an excellent active catalyst for the HER in natural seawater electrolytes. This outstanding performance stemmed from its incredible corrosion resistance and strong adsorption capabilities with different elements, attributed to an upshifted d-band center closely aligned with the Fermi level.³¹ Similarly, Li et al. reported FeCoNiMnMo HEA as an effective and exceptionally stable catalyst for the OER in simulated seawater, achieving a stability of 100 mA cm⁻² over 200 h. This stability was attributed to the establishment of a protective oxide layer on the active catalyst surface containing K₂MoO₄, which was crucial for inhibiting Cl⁻ corrosion.³² Based on the above, it is evident that the exceptional properties of HEAs make them excellent candidates for electrocatalytic H₂ generation in seawater.

However, the existing HEA catalysts face several challenges, particularly in terms of their preparation methods. Various conventional routes for preparing HEAs, such as high energy ball milling, solvothermal or hydrothermal synthesis, microwave, high-temperature liquid shock, and fast-moving pyrolysis, have been employed in recent times.^33–37 However, the preparation of HEAs utilizing these approaches leads to an irregular elemental compositional allocation among the particles in the HEAs, as well as disparities in their size and morphology. Furthermore, several strategies need complex synthetic processes, yielding uncontrollable outcomes and consuming considerable time. Thus, the preparation process of HEA catalysts is very important in optimizing their structural characteristics. Recently, facile and rapid approaches, such as pulsed laser techniques, have been used for preparing alloys and HEA-based catalysts.^38–43 Compared with conventional methods, these approaches offer numerous advantages, including time efficiency, cost-effectiveness, single-step processing, absence of harmful byproducts, and elimination of the need for surfactant agents.⁴⁴ Notably, our research group has primarily focused on the pulsed laser approach for synthesizing various alloy-based materials with diverse sizes and morphologies applicable to energy and environmental purposes,^17,19,45 further supporting the innovative aspects of our current work. By exploring the continuous-wave (CW) CO₂-laser pyrolysis approach for HEA electrocatalyst synthesis, we aim to contribute new insights and techniques to the field, addressing the limitations of more complex synthetic processes and providing a simpler, quicker, and more efficient alternative. To the best of our knowledge, there are no documented instances of preparing HEAs using the CO₂-laser pyrolysis method.

Herein, we report an ultrafast approach that utilizes a CW CO₂-laser pyrolysis process at varying power levels (30%, 60%, and 90% of the total 25 W laser power) to produce AuRuIrPdPt HEA catalysts. The catalytic performance of the HEAs was evaluated for a HER and OWS electrolyzer system with three different electrolytes: alkaline 1.0 M KOH, alkaline simulated seawater using 0.5 M NaCl, and alkaline natural seawater electrolytes. The optimal HEA-60 catalyst exhibited outstanding electrocatalytic HER performance with all electrolytes, outperforming the commercial Pt/C catalyst. This study presents a highly active catalyst for H₂ generation via seawater electrolysis, along with an ultrafast synthetic approach for designing HEA electrocatalysts featuring exceptional catalytic performance.

2. Experimental section

2.1 Synthesis of the AuRuIrPdPt HEAs via a CO₂ laser

HEAs were synthesized using a CW CO₂ laser (Synrad; Model-48-2SAL, WA) with a 3.5 mm beam diameter. Each stock solution of Au, Ru, Ir, Pd, and Pt metal salts at a concentration of 10 mM was prepared separately. Then, 100 μL of each metal salt solution was taken from the stock solution and combined in a 10 mL glass vial, then diluted with 5 mL of deionized (DI) water. Subsequently, 600 μL of the metal solution mixture was poured into a ceramic crucible and irradiated with 30%, 60%, and 90% of the maximum laser output power (25 W) for 90 s. After CO₂-laser irradiation, the resulting colloidal solutions of HEAs were washed three times with DI water, centrifuged at 14 000 rpm for 5 min to collect the HEA products, and dried overnight at 60 °C. Herein, the three different HEAs prepared with different CO₂ laser powers of 30%, 60%, and 90% were named as HEA-30, HEA-60, and HEA-90, respectively.

Further details regarding the materials and their characterization are provided in the ESI.†

2.2 Electrochemical experiments

All the electrochemical studies were performed on a conventional half-cell reaction setup on a CHI708E workstation at ambient temperature unless otherwise specified. A graphite rod, a Hg/HgO, and the catalysts loaded on carbon cloth (CC) were utilized as the counter, reference, and working electrodes, correspondingly. The working electrode (WE) was assembled as follows: 0.2 mg of the synthesized HEA catalyst and 0.8 mg of activated carbon black were added in a solution comprising water (95 μL), ethanol (95 μL), and 5 wt% Nafion (10 μL) under sonication for 30 min to form a catalyst ink. Thereafter, the ink was uniformly deposited onto the CC (1 cm × 1 cm) utilizing the drop-casting process and dried at 60 °C. The attained potentials were calibrated converted to the RHE based on a previous report.⁴⁶ For the HER and OWS measurements, three different electrolyte solutions were employed: 1.0 M KOH, 1.0 KOH + 0.5 M NaCl, and 1.0 M KOH + natural seawater. LSV polarization profiles were obtained at 5 mV s⁻¹. Before performing the LSV studies, the WE was treated using the CV technique at 50 mV s⁻¹ for 100 cycles. EIS studies were performed at an overpotential of 50 mV vs. RHE in the 0.1–10⁵ Hz frequency range. The C_dl of the catalyst was determined from the non-faradaic region of the CV graphs at diverse scan rates of 10–100 mV s⁻¹ at 0.2–0.3 V vs. RHE. To estimate the active sites in the fabricated catalyst, CV graphs were generated in 1.0 M KOH solution at 50 mV s⁻¹ at −0.1 to 0.6 V vs. RHE.

The turnover frequency (TOF) value was considered by the following relation: TOF (s⁻¹) = j × N_A/n × S_A × F, where j represents the current density collected from the LSV curves (HER) at different potentials, N_A represents Avogadro's number (6.02214076 × 10²³), n is the two-electron reaction for the HER (n = 2), S_A is the active sites on the as-synthesized WE, and F is the faradaic constant (96 485.3C mol⁻¹). Similarly, the mass activity (MA) of the fabricated catalyst was accounted for using the following equation: MA = j/m, where m represents the mass loading of the working electrode in grams. The stability of the catalyst was assessed by chronopotentiometry at a fixed 10 mA cm⁻² for 10 h. OWS studies were performed in different electrolytes using a two-electrode assembly (IrO₂(+)‖HEAs-60(−)), with IrO₂ serving as the anode and HEA-60 as the cathode.

3. Results and discussion

3.1 Structural analyses and formation of the HEAs via CO₂ laser

AuRuIrPdPt HEAs were synthesized using CW CO₂-laser irradiation (Fig. 1a). Initially, a mixture of salt solutions was placed in a ceramic boat, then irradiated by a CW CO₂ laser in solution for heating at different laser powers (30–90% of the maximum laser output power of 25 W) for 90 s. The incident laser was primarily absorbed by the salts, resulting in photothermal-induced melting and the release of gaseous byproducts, followed by crystallization into the AuRuIrPdPt HEAs. Any remaining unreacted precursors were removed by washing in deionized water. Unlike the Nd:YAG laser, the CW CO₂ laser operates through heat transfer based on heat diffusion, resulting in a substantial heat effect on a material. Therefore, a similar effect to thermal synthesis can be expected. However, the CW CO₂-laser method offers the advantage of considerably reducing both the time and cost of the manufacturing process compared to thermal synthesis, owing to the instantaneous high energy of the CW CO₂ laser (Fig. 1a).^47–49 In our system, we optimized the CW CO₂-laser-irradiation time to a short period (90 s). As depicted in Fig. 1b–d, flower-shaped materials with a ∼250 nm (referred to as “flowers”) radius comprising randomly overlapping plates with a thickness of ∼50 nm were observed, alongside hairball-shaped particles (referred to as “nonflowers”) with a slightly larger size (∼1.2 μm) that were synthesized simultaneously with the flowers. Fig. 1e–g confirm that the laser power altered the particle distribution and size in the prepared materials. Herein, the synthesized HEAs were named as HEA-30, HEA-60, and HEA-90, corresponding to the irradiation at various powers, 30% (∼7 W), 60% (∼18 W), and 90% (∼25 W) of the total laser power, respectively. The particle size observed in HEA-60 was larger compared to that of HEA-30 and HEA-90 due to the specific heating and melting dynamics of the alloy at various power levels using the CO₂ laser. At 30% power, insufficient energy may result in smaller particles due to incomplete melting. At 90% power, excess energy can cause rapid cooling and solidification, resulting in smaller particles. However, 60% power provided sufficient energy for proper melting without causing rapid cooling, promoting the formation of larger particles. To investigate the effect of the CW CO₂-laser irradiation, HEA synthesis was attempted under the same conditions using a conventional solvothermal synthesis method. The XRD patterns of these samples indicated the absence of HEA formation (Fig. S1a†). The FE-SEM images depicted in Fig. S1b† reveal spherical particles with smooth surfaces, and significantly, the findings from ICP-OES indicated notably low levels of Ir and Ru metals (Fig. S1c†). This is because while lasers cause a reduction and cohesion of metal ions through providing instantaneous high energy, solvothermal methods require more time owing to heat diffusion.

Fig. 1 (a) Schematic representation of HEA synthesis via CW CO₂-laser irradiation, (b–d) FE-SEM images of HEA-30, HEA-60, and HEA-90 corresponding to different laser powers, and (e–g) distribution histograms of the flower- and nonflower-shaped materials.

The STEM-HAADF method, along with powder XRD patterns, were utilized to ascertain whether the synthesized materials meet the definition of high-entropy materials. Elemental mapping via HAADF-STEM revealed an even distribution of each element (Au, Ru, Ir, Pd, and Pt) within both the flower and nonflower particles (Fig. 2a and b, and S2†). Moreover, we confirmed that the elements were evenly distributed in both the flower and nonflower structures through FE-SEM-EDS mapping analysis, as shown in Fig. S3.† The crystal structure and phase characteristics of the as-synthesized HEA catalysts were evaluated by XRD analysis. As shown in Fig. 2c, the XRD pattern of the HEA catalysts synthesized with different CO₂-laser power irradiation exhibited peaks that fitted well with the face-centered cubic (fcc) crystal structure. Three distinct diffraction peaks at 38.1°, 44.5°, and 64.6° were observed corresponding to the (111), (200), and (220) planes of HEA, respectively,^50,51 with no additional peaks from individual components observed. This confirmed that the materials synthesized in our system aligned with high-entropy materials. Consistent conclusions could be drawn from the results of the ICP-OES analysis (Table S1†). Interestingly, the amount of Pt present in the HEAs was low compared to other metals, such as Au, Ru, Ir, and Pd, as shown in Table S1.† This indicates that the composition of the elements in HEAs was not solely determined by their reduction potentials. The lower content of Pt could be attributed to a combination of factors, such as complex alloying effects, kinetic factors, and intermetallic interactions that contribute to the final composition of the HEA. These assertions are supported by the ICP-OES results presented in Table S1.†

Fig. 2 (a and b) HAADF-STEM element mapping images of the (a) nonflowers and (b) flowers in HEA-60 synthesized via the CW CO₂ laser. (c) XRD patterns of HEA-30, HEA-60, and HEA-90 with the single-phase fcc structure.

XPS studies were performed to inspect the surface composition of the HEAs, as depicted in Fig. 3 and S3.† The XPS survey scan spectrum of HEA-60 indicated the elemental presence of Au, Ru, Ir, Pd, and Pt (Fig. 3a). For Au, three types of Au peaks were observed, namely for Au⁰, Au⁺, and Au³⁺. The binding energies (BEs) around 83.21 and 86.87 eV corresponded to the spin–orbital doublet Au⁰ 4f_7/2 and Au⁰ 4f_5/2 (Fig. 3b), correspondingly. The Ru 3p spectrum displayed the occurrence of metallic Ru (Fig. 3c), with peaks observed at 462.91 and 485.33 eV consistent with Ru⁰ 3p_3/2 and Ru⁰ 3p_1/2, respectively. In the Ir spectrum (Fig. 3d), two peaks at BEs of 61.83 and 64.76 eV were observed, representing Ir⁰ 4f_7/2 and Ir⁰ 4f_5/2, correspondingly. Similar to Au, Pd exhibited three different valence states. The Pd 3d spectrum of HEA-60 showed spin–orbit doublets corresponding to Pd⁰ 3d_5/2 (334.87 eV), Pd²⁺ 3d_5/2 (336.83 eV), Pd⁴⁺ 3d_5/2 (337.86 eV), Pd⁰ 3d_3/2 (340.44 eV), Pd²⁺ 3d_3/2 (341.74 eV), and Pd⁴⁺ 3d_3/2 (343.32 eV), as depicted in Fig. 3e. The Pt 4f spectrum exhibited two peaks at 72.18 and 75.54 eV, corresponding to Pt 4f_7/2 and Pt 4f_5/2, respectively (Fig. 3f). Fig. S4† shows a similar trend for the other samples, HEA-30 and HEA-90. The presence of oxidized states of Au and Pd while the other metals remained in metallic states could be attributed to a combination of factors, such as the lower electronegativities, surface reactivity, and the segregation tendencies of Au and Pd. This leads to in vacuo oxidation when the surface is exposed to air.

Fig. 3 XPS spectra of HEA-60: (a) XPS survey, (b) core-spectra Au 4f, (c) Ru 3p, (d) Ir 4f, (e) Pd 4f, and (f) Pt 4f.

3.2 Electrocatalytic activity for the HER in alkaline electrolyte

The electrochemical process for the HER of the synthesized AuPdPtRuIr HEA catalysts was first assessed in 1.0 M KOH electrolyte. HER curves of the synthesized HEA catalysts via CO₂-laser irradiation under various power conditions, i.e., 30% (∼7 W), 60% (∼18 W), and 90% (∼25 W), along with commercial Pt/C were attained at 5 mV s⁻¹ (Fig. 4a). The polarization curves indicate that HEA-60 exhibited superior HER performance, comparable to HEA-30 and HEA-90, and outperformed the commercial Pt/C catalyst. Specifically, HEA-60 exhibited an overpotential (η) of only 37 mV to achieve 10 mA cm⁻², surpassing HEA-30, HEA-90, and conventional Pt/C catalysts, which required η values of 43, and 40 mV, correspondingly, as illustrated in Fig. 4b. Even under a high current density situation of 50 mA cm⁻², HEA-60 sustained its exceptional catalytic action compared to the other catalysts, which could likely be attributed to its optimal elemental composition ratio and surface structure. Despite the HEA-60 catalyst exhibiting excellent catalytic performance compared to the benchmark Pt/C, varying the CO₂-laser-power condition altered the efficiency of the HEA catalysts toward the HER. Thus, the HEA comprising Au, Pd, Pt, Ru, and Ir exhibited enhanced electrocatalytic HER performance owing to synergistic effects among its constituent metals, a tunable composition for optimized catalytic properties, its structural stability under harsh conditions, and the efficient mass transport of reactants and products. These combined advantages position the AuPdPtRuIr HEA as a promising candidate for achieving superior HER performance. The Tafel slope is crucial for elucidating the HER reaction kinetics and was thus determined for the synthesized HEA catalysts from their LSV polarization curves, as depicted in Fig. 4c. HEA-60 showed the smallest Tafel slope value of 59.26 mV dec⁻¹ compared to HEA-30 (66.85 mV dec⁻¹), HEA-90 (64.08 mV dec⁻¹), and commercial Pt/C (67.31 mV dec⁻¹), respectively. The lower Tafel slope value of 59.26 mV dec⁻¹ in HEA-60 suggested its superior catalytic performance and faster HER kinetics compared to the other three catalysts. It is well established that the mechanism of the HER in an alkaline electrolyte involves three steps: the Volmer, Heyrovsky, and Tafel reactions. Generally, the HER reaction mechanism in an alkaline electrolyte comprises the following three reactions (equ (1)–(3)):⁵¹

H₂O + e⁻ → H_ads + OH⁻ (Volmer or H₂O dissociation, 120 mV dec⁻¹) (1)

H_ads + H₂O + e⁻ → H₂ + OH⁻ (Heyrovsky or electrochemical process, 40 mV dec⁻¹) (2)

H_ads + H_ads → H₂ (Tafel or chemical desorption, 30 mV dec⁻¹) (3)

Fig. 4 HER performances of HEA-30, HEA-60, HEA-90, and Pt/C in 1.0 M KOH. (a) LSV plots, (b) η at 10 and 50 mAcm⁻², (c) Tafel slopes, (d) Nyquist impedance plots, (e) ΔJ/2 vs. scan rate plots, (f) TOF, (g) MAs, (h) long-term HER stability of HEA-60 at −37 mV vs. RHE for 10 h, and (i) HER pathway mechanism.

According to the Tafel slope value of the HEA-60 catalyst, the reaction mechanism pathway for the HER followed a combination of the Vomer–Heyrovsky mechanism.^52,53

EIS studies were conducted to obtain a comprehensive understanding of the HER kinetic performance at the electrolyte/HEAs electrode interface. As depicted in the Nyquist plots in Fig. 4d, HEA-60 showed the smallest R_ct value of 2.75 Ω compared to HEA-30, HEA-90, and Pt/C, which exhibited R_ct values of 3.09, 2.87, and 4.02 Ω, respectively. The lowest R_ct value demonstrates the substantially greater charge transfer in HEA-60 for the HER in comparison with the other three catalysts. Furthermore, to investigate the specific activity responses of the catalysts, electrochemical C_dl and ECSA experiments were conducted. The C_dl values were calculated from the CV polarization curves (Fig. S5†) of the synthesized catalysts within the non-faradaic portion of 0.2–0.3 V vs. RHE, using diverse scan rates from 10–100 mV s⁻¹. The resulting C_dl values are presented in Fig. 4e, indicating that the HEA-60 catalyst displayed a higher C_dl value of 27.04 mF cm⁻² compared to HEA-30 (17.96 mF cm⁻²), HEA-90 (23.03 mF cm⁻²), and Pt/C (12.27 mF cm⁻²). The ECSA was determined by the following formula: ECSA = C_dl/C_s, where C_s signifies the specific capacitance of the CC (88 mF cm⁻²).⁴⁰ The calculated ECSA value of HEA-60 (0.307 mF cm⁻²) was greater than that of HEA-30 (0.170 mF cm⁻²), HEA-90 (0.261 mF cm⁻²), and Pt/C (0.139 mF cm⁻²). These observations indicate that the HEA-60 catalyst exhibited a higher C_dl and ECSA, potentially indicating it had additional catalytically active sites exposed, which would consequently lead to its enhanced HER performance.

To further support the enhanced HER performance of HEA-60, the TOF and MA values in the intrinsic HER activity were estimated for all the electrocatalysts. TOF assessment included determining the accessible active sites (SAs) on the surface of the working electrode by analyzing their complete CV curves (Fig. S6†). The results indicate that HEA-60 exhibited a higher S_A value of 6.5 × 10⁻⁴ mol cm⁻² compared to HEA-30 (5.8 × 10⁻⁴ mol cm⁻²), HEA-90 (6.2 × 10⁻⁴ mol cm⁻²), and Pt/C (5.5 × 10⁻⁴ mol cm⁻²), signifying the higher S_A value, which would support the enhanced HER activity of HEA-60. As shown in Fig. 4f, HEA-60 had a higher TOF value of 0.054 s⁻¹ at an η of 0.05 V compared to HEA-30 (0.052 s⁻¹), HEA-90 (0.053 s⁻¹) and Pt/C (0.044 s⁻¹). Similarly, at each HER potential, the HEA-60 catalyst exhibited the highest mass activity compared to the other catalysts, demonstrating its superior HER catalytic performance (Fig. 4g). Additionally, besides the electrocatalytic performance, the long-term working sturdiness of the HEA-60 catalyst is a decisive constraint for estimating its practical utility in real applications. Durability was thus evaluated under a HER potential of −37 mV vs. RHE for 10 h in 1.0 M KOH, as shown in Fig. 4h. After 10 h of measurement, the HEA-60 catalyst displayed good durability with a minor reduction in the current density. This decrease in current density might be attributed to the porous nature of the CC substrate, which can entrap gas bubbles, impeding efficient mass transport between the HEA electrode and the electrolyte. Based on the HER results, the underlying mechanism is schematically depicted in Fig. 4i.

3.3 Electrocatalytic activity for the HER in simulated seawater

To evaluate the efficiency of the synthesized HEA catalysts in simulated seawater toward hydrogen production, their HER performance was studied in a 1.0 M KOH + 0.5 M NaCl (simulated seawater) electrolyte solution. The simulated seawater activity of the synthesized HEA catalysts mirrored their performance in alkaline conditions (Fig. 5). As shown in Fig. 5a, HEA-60 required only a low η of 34 and 148 mV to reach 10 and 50 mA cm⁻², respectively, which were smaller than those of HEA-30 (45 and 197 mV) and HEA-90 (37 and 176 mV). Under equivalent current densities, the HEA-60 catalyst demonstrated a notably superior performance compared to the benchmark Pt/C, as illustrated in Fig. 5b. Tafel slopes are well-known indicators of catalyst reaction kinetics. As indicated in Fig. 5c, the Tafel slope of the HEA-60 catalyst was 58.89 mV dec⁻¹, which was lower than that of HEA-30 (63.82 mV dec⁻¹), HEA-90 (62.03 mV dec⁻¹), and Pt/C (65.95 mV dec⁻¹). These Tafel slope values confirmed the faster reaction kinetics and higher electrocatalytic performance of HEA-60 toward the HER in simulated seawater compared to the other three catalysts. In addition, the HER pathways of HEA-60 followed the Vomer–Heyrovsky mechanism.

Fig. 5 HER performances of HEA-30, HEA-60, HEA-90, and Pt/C in 1.0 M KOH + 0.5 M NaCl; (a) LSV plots, (b) η at 10 and 50 mA cm⁻², (c) Tafel slopes, (d) Nyquist impedance plots, (e) ΔJ/2 vs. scan rate plots, (f) TOF, (g) MAs, (h) HER stability of HEA-60 at −34 mV vs. RHE for 10 h, and (i) HER pathway mechanism.

Furthermore, Fig. 5d demonstrates that HEA-60 (1.24 Ω) exhibited a lower R_ct compared to the other synthesized HEA catalysts and Pt/C, indicating the faster reaction rate for HEA-60 toward the HER. Meanwhile, the ECSA was estimated from the C_dl values of the fabricated HEA electrodes, as shown in Fig. 5e and S7.† HEA-60 showed the highest C_dl and ECSA (24.71 and 0.280 mF cm⁻²), demonstrating its advantage for accelerating mass/ion transport while providing additional active sites. Impressively, the calculated S_A, TOF, and MA values for the HER performance in simulated seawater also indicated the high intrinsic activity of HEA-60 (Fig. S8† and 5f–g). The good durability of the HER performance in simulated seawater is crucial for assessing the practical utility of the HEA-60 catalyst. The HEA-60 catalyst exhibited stable performance over 10 h with only a trivial decrease in current density when subjected to a persistent HER potential of −34 mV vs. RHE, indicating its superior durability (Fig. 5h). These results demonstrate that HEA-60 exhibits excellent performance in hydrogen generation, surpassing the majority of HEA-based catalysts, in both alkaline and simulated seawater environments. Based on the HER results in simulated seawater, the mechanism underlying the reaction is schematically described in Fig. 5i.

3.4 Electrocatalytic activity for the HER in natural seawater and overall water splitting

After observing the remarkable HER performance of the HEA catalyst in alkaline conditions and simulated seawater, we next proceeded to assess its HER activity in a solution of alkaline natural seawater (1.0 M KOH in natural seawater, pH ∼13.7). The natural seawater sample was attained from Namildae Beach in Suncheon, South Korea (Fig. S9†). In the natural seawater electrolyte (Fig. 6a and b), the HEA-60 catalyst continued to exhibit excellent catalytic performance for the HER, necessitating η values of 45 and 217 mV to attain 10 and 50 mA cm⁻², correspondingly. As revealed in Fig. 6c, compared with the LSV polarization curves in different electrolyte solutions, the optimal HEA-60 catalyst exhibited only a minor deterioration in natural seawater, unlike in the other two electrolyte solutions. The slight decrease in activity could be ascribed to the existence of insoluble precipitates, like Ca(OH)₂ and Mg(OH)₂, in the seawater electrolyte. These substances can cover the electrode surface, hindering access to the active sites of the electrode (Fig. S10†). From Fig. 6d, the Tafel slope value of HEA-60 was evaluated to be 82.42 mV dec⁻¹, which was lower than that of HEA-30 (113.3 dec⁻¹), HEA-90 (101.68 dec⁻¹), and Pt/C (83.59 dec⁻¹), respectively. This observation also showed a slight variation from the values observed in both alkaline and simulated seawater (Fig. 4 and 5). To instigate the origin of the catalytic performances of the HEA catalysts, EIS, C_dl, ECSA, S_A, TOF, and MA investigations were conducted (Fig. S11–S13†, 6e and f). As demonstrated in Fig. 6e, HEA-60 exhibited a smaller R_ct of 2.6 Ω at the electrolyte/electrode interface compared with the other catalysts, indicating a faster electron transference during the catalytic HER process. The intrinsic function of the prepared HEA catalysts was assessed by evaluating the C_dl of HEA-30, HEA-60, HEA-90, and Pt/C using their CV graphs in non-faradaic regions at different scan rates (Fig. S11†). The C_dl values for HEA-30, HEA-60, HEA-90, and Pt/C were determined to be 18.3, 22.2, 21.4, and 22.1 mF cm⁻², correspondingly, as shown in Fig. 6f. HEA-60 offered the greatest C_dl, suggesting a higher ECSA in alkaline natural seawater. As exposed in Fig. S13a and b†, the TOF and MA values for the HEA-60 catalyst at an overpotential of 0.08 V were 0.066 s⁻¹ and 17 A g⁻¹, correspondingly, which were larger than those for HEA-30 (0.045 s⁻¹ and 9.7 A g⁻¹), HEA-90 (0.062 s⁻¹ and 13.4 A g⁻¹), and Pt/C (0.064 s⁻¹ and 15.5 A g⁻¹). Remarkably, the results obtained from the EIS, ECSA, MA, and TOF investigations showed excellent agreement with the low η and small Tafel slope value of the HEA-60 catalyst, indicating its exceptional intrinsic behavior for the HER in alkaline natural seawater. Furthermore, the long-term stability of HEA-60 was evaluated in an alkaline seawater solution (Fig. S13c†). Remarkably, the HEA-60 catalyst maintained consistent performance in alkaline natural seawater, with only minor changes observed even after continuous operation for 10 h, indicating its excellent catalytic stability performance even in challenging natural seawater conditions. Consequently, the HEA-60 catalyst not only showed efficiency for both alkaline and simulated seawater HER performance but also exhibited activity in alkaline seawater conditions.

Fig. 6 HER performances of HEA-30, HEA-60, HEA-90, and Pt/C in 1.0 M KOH + natural seawater; (a) LSV plots, (b) η at 10 and 50 mAcm⁻², (c) LSV plots compared with different electrolytes, (d) Tafel slopes obtained from the LSV curves measured in alkaline seawater electrolyte, (e) Nyquist impedance plots, (f) ΔJ/2 vs. scan rate plot, (g) schematic illustration of the fabricated OWS electrolyzer using IrO₂(+)‖HEA-60(−), (h) OWS polarization curves over IrO₂(+)‖HEA-60(−) in different electrolytes, and (i) long-term OWS stability of IrO₂(+)‖HEA-60(−) at a fixed voltage for 10 h.

Considering the excellent electrocatalytic performance of the HEA-60 catalyst in alkaline, simulated, and natural seawater electrolytes, we constructed an OWS electrolyzer with IrO₂ as the anode and optimized HEA-60 as the cathode (Fig. 6g). Interestingly, the fabricated electrolyzer displayed a superior OWS performance in all the electrolyte solutions (Fig. 6h). As demonstrated in Fig. 6h, the fabricated IrO₂(+)‖HEA-60(−) electrolyzer achieved a current density of 10 mA cm⁻² at a cell voltage of 1.60 V in 1.0 M KOH. Moreover, in the alkaline simulated and natural seawater electrolytes, the electrolyzer achieved a current density of 10 mA cm⁻² at cell voltages of 1.59 and 1.62 V, respectively (Fig. 6h). Moreover, the IrO₂(+)‖HEA-60(−) electrolyzer demonstrated durability, acting as a stable OER‖HER catalyst for over 10 h at a fixed cell voltage (Fig. 6i). We also calculated the yield of H₂ for the fabricated IrO₂(+)‖HEA-60(−) alkaline seawater electrolyzer, with the results depicted in Fig. S14.† It is clear from Fig. S14† that the yield of H₂ gradually increased with the electrolysis reaction time. For instance, 447 μmol cm⁻² of H₂ was produced in 15 min, reaching a maximum of 792 μmol cm⁻² in 60 min. Furthermore, the faradaic efficiency (FE) of the system remained at 80% even after 60 min, indicating that the fabricated electrolyzer with HEA-60 possessed exceptional HER selectivity and stability.

3.5 Post-HER characterization

The LSV polarization curves investigated before and after the HER stability tests were identical for the HEA-60 catalyst in different electrolyte solutions, as displayed in Fig. S15,† indicating only small increases in the overpotential under the original conditions. The catalyst's composition and structure help maintain its performance over time, even after the stability test, resulting in minimal changes in the overpotential required for the HER. Nevertheless, in the alkaline seawater medium, there was a more significant increase in overpotential compared to in the alkaline and simulated seawater, owing to the formation of Ca(OH)₂ and Mg(OH)₂ precipitates within the electrolyte. The structural, morphological, and compositional features of the HEA-60 catalyst after the long-term HER stability measurements in various electrolyte media were further assessed via XRD, FE-SEM with EDS, HRTEM, and XPS analyses. As depicted in Fig. S16,† the XRD patterns indicate there were no significant changes in the crystalline structure of the HEA-60 catalyst, verifying the retention of its crystallinity after the HER stability tests in the various electrolytes. Furthermore, the FE-SEM images demonstrate that both the flower and nonflower morphological structures of the HEA-60 catalyst remained unchanged after the HER stability tests in alkaline and simulated seawater electrolyte (Fig. S17a and b†). However, the FE-SEM image of the HEA-60 catalyst after the HER stability test in the alkaline seawater medium showed an agglomeration of the flower structure caused by mineral deposition from the seawater on the electrode surface (Fig. S17c†). Besides, the EDS mapping investigation results showed there was a uniform distribution of Au, Pd, Pt, Ru, and Ir elements within the HEA-60 structure. However, it also highlighted that in the simulated seawater, Na and Cl were deposited on the surface after testing. During the HER stability test, minerals found in the alkaline seawater caused Na, Cl, Mg, K, and Ca to deposit on the surface of the HEA-60 catalyst. These species have the potential to interact with the catalyst surface, which may result in the formation of surface complexes or the adsorption of ions, potentially blocking the active sites. This led to the noticeable reduction in potential seen after the stability tests in the simulated and alkaline seawater. Moreover, XPS analysis was performed to examine the surface chemical composition of the HEA-60 catalyst following the durability tests in various electrolytes, as shown in Fig. S18.† The Au 4f, Ru 3p, Ir 4f, Pd 3d, and Pt 4f peaks all exhibited slight decreases in their positions following the stability tests in the various electrolytes, indicating alterations in the electronic structure and chemical surroundings of all the metals within the HEA-60 catalyst. XPS analysis confirmed the presence of Ca(OH)₂ in the alkaline seawater environments, as shown in Fig. S19,† which was attributed to minerals in the seawater.

To verify the formation and structural retention features of the optimized HEA-60 catalyst before and after the long-term HER stability tests, HRTEM analysis was conducted (Fig. S20†). As shown in Fig. S20a,† the HEA-60 catalyst exhibited a nonflower-like shape before the stability test. The HRTEM investigation (Fig. S20b†) allowed estimating the inner interplanar distance to be 0.24 nm, corresponding to the (111) planes in the fcc crystal structure of the HEA-60 catalyst. Furthermore, the line scan EDS analysis of HEA-60 confirmed the spatial uniformity of their compositions within individual particles (Fig. S20c†). Similarly, HRTEM and EDS line scan analyses were performed after the HER stability tests of the HEA-60 catalyst in various electrolytes (Fig. S20d–f†). The HRTEM images after the stability tests in both alkaline and simulated seawater were similar to those before the tests. However, in alkaline seawater, some agglomeration could be observed due to the minerals present. The HRTEM images showed there were no significant changes in the inner interplanar distance after the stability tests in various electrolytes. Additionally, the EDS line scan confirmed that the HEA-60 catalyst maintained spatial uniformity in its composition throughout the particles after the stability tests. Notably, these results highlight the impressive durability and robust structure of the HEA-60 catalyst, positioning it as a desirable choice for long-lasting HER in the industrial production of hydrogen from seawater.

3.6 In situ Raman spectroscopy monitoring of the reactive species, and mechanism pathways

To explore the active sites and surface reaction mechanisms of the HEA-60 catalyst during the electrochemical HER process, in situ Raman spectral analysis was performed, and the findings are displayed in Fig. 7. Fig. 7a presents a schematic illustration of the in situ Raman working setup. As shown in Fig. 7b, the Raman spectrum of the HEA-60 catalyst revealed no distinctive peaks in the absence of applied voltage. However, two peaks could be observed at 1325 and 1586 cm⁻¹, corresponding to the D and G bands of the carbon cloth substrate. At an applied potential of 60 mV, new Raman bands were detected at 434.78 cm⁻¹, corresponding to the Pt–O bond, and a peak at 604.65 cm⁻¹ was identified as the Pd–O bond.^54,55 As the potential increased, the intensity of these Pt–O and Pd–O peaks steadily rose, indicating the improved performance in the HER. The appearance of Pt–O and Pd–O bonds implied the creation of Pt–OH* and Pd–OH* intermediates, derived from the splitting of H₂O molecules. These findings demonstrate that Pt and Pd directly participate in the Volmer step, serving as active sites for water dissociation. Remarkably, a Raman band appeared at 2074.25 cm⁻¹, indicating the presence of Ru–H bonds and strongly suggesting the formation of Ru–H* intermediates,⁵⁶ which demonstrate that Ru served as the primary active site for H* adsorption/desorption. Consequently, it could be inferred that the HER rate-determining step (Volmer step) was activated due to the corroborating effect of the Ru, Pd, and Pt surfaces (Fig. 7c). The Raman findings directly reveal that Pt, Pd, and Ru serve as active sites in the HEA-60 catalyst, facilitating various intermediates, whereas Au and Ir improve the electrical conductivity of the HEA-60 catalyst. In the conventional HEA-60 catalyst, the Pt and Pd sites facilitate the dissociation of H₂O, while the Ru sites simultaneously accelerate the combination of H* to H₂. This experimental verification confirmed the presence of multiple intermediates on various active sites within the HEA-60 catalyst.

Fig. 7 (a) Pictorial representation of the in situ Raman spectroscopy setup, (b) in situ Raman spectroscopy monitoring results of HEA-60 during the HER process under applied potentials in 1.0 M KOH, and the (c) HER mechanism pathway with the HEA-60 catalyst.

4. Conclusion

Herein, we demonstrated the design and establishment of a unique HEA electrocatalyst for efficient HER performance in alkaline seawater electrolytes. HEAs were synthesized in a single step using a simple and rapid CW CO₂-laser irradiation approach, employing various CO₂ laser powers: 30%, 60%, and 90% of the total 25 W laser power. The optimal HEA-60 catalyst demonstrated outstanding electrocatalytic HER activity with an ultralow η of 37, 34, and 45 mV at 10 mA cm⁻² in alkaline, simulated seawater, and natural seawater electrolytes, correspondingly, outperforming the commercial Pt/C catalyst. Thus, the exceptional HER performance of the HEAs was attributed to the rich active metal sites, high electrical conductivity, and fast charge-transfer reactions facilitated by the synergistic effects of each metal component in the HEAs, as well as the distinctive structure generated through CO₂-laser irradiation. As an efficient electrocatalyst, the assembled two-electrode system using HEA-60 as the cathode and IrO₂ as the anode required only a cell voltage of 1.62 V to achieve a current density of 10 mA cm⁻² in natural seawater electrolyte while maintaining its stability. This research offers a rapid method for developing highly effective electrocatalysts for hydrogen production through seawater electrolysis.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

No conflicts to declare.

Acknowledgements

This research was supported by Korea Basic Science Institute (National Research Facilities and Equipment Center) grant funded by the Ministry of Education. (No. 2019R1A6C1010042). The authors acknowledge the financial support from National Research Foundation of Korea (NRF), (2022R1A2C2010686, 2022R1A4A3033528, RS-2024-00405324).

References

1. S. Park, L. Liu, Ç. Demirkır, O. van der Heijden, D. Lohse, D. Krug and M. T. M. Koper, Solutal Marangoni effect determines bubble dynamics during electrocatalytic hydrogen evolution, Nat. Chem., 2023, 15, 1532–1540.
2. J. Theerthagiri, S. J. Lee, A. P. Murthy, J. Madhavan and M. Y. Choi, Fundamental aspects and recent advances in transition metal nitrides as electrocatalysts for hydrogen evolution reaction: A review, Curr. Opin. Solid State Mater. Sci., 2020, 24, 100805.
3. B. H. R. Suryanto, Y. Wang, R. K. Hocking, W. Adamson and C. Zhao, Overall electrochemical splitting of water at the heterogeneous interface of nickel and iron oxide, Nat. Commun., 2019, 10, 5599.
4. T. Begildayeva, J. Theerthagiri, S. J. Lee, Y. Yu and M. Y. Choi, Unraveling the Synergy of Anion Modulation on Co Electrocatalysts by Pulsed Laser for Water Splitting: Intermediate Capturing by In Situ/Operando Raman Studies, Small, 2022, 2204309.
5. X. Wang, M. Geng, S. Sun, Q. Xiang, S. Dong, K. Dong, Y. Yao, Y. Wang, Y. Yang, Y. Luo, D. Zheng, Q. Liu, J. Hu, Q. Wu, X. Sun and B. Tang, Recent advances of bifunctional electrocatalysts and electrolyzers for overall seawater splitting, J. Mater. Chem. A, 2024, 12, 634–656.
6. H. Sun, X. Xu, H. Kim, Z. Shao and W. Jung, Advanced electrocatalysts with unusual active sites for electrochemical water splitting, InfoMat, 2024, 6, e12494.
7. Q. Qian, Y. Zhu, N. Ahmad, Y. Feng, H. Zhang, M. Cheng, H. Liu, C. Xiao, G. Zhang and Y. Xie, Recent Advancements in Electrochemical Hydrogen Production via Hybrid Water Splitting, Adv. Mater., 2024, 36, 2306108.
8. C. E. Park, G. H. Jeong, J. Theerthagiri, H. Lee and M. Y. Choi, Moving beyond Ti₂C₃T_x MXene to Pt-Decorated TiO₂@TiC Core–Shell via Pulsed Laser in Reshaping Modification for Accelerating Hydrogen Evolution Kinetics, ACS Nano, 2023, 17, 7539–7549.
9. W. Chen, W. Wei, F. Li, Y. Wang, M. Liu, S. Dong, J. Cui, Y. Zhang, R. Wang, K. Ostrikov and S.-Q. Zang, Tunable Built-In Electric Field in Ru Nanoclusters-Based Electrocatalyst Boosts Water Splitting and Simulated Seawater Electrolysis, Adv. Funct. Mater., 2024, 34, 2310690.
10. Z. Peng, Q. Zhang, G. Qi, H. Zhang, Q. Liu, G. Hu, J. Luo and X. Liu, Nanostructured Pt@RuO_x catalyst for boosting overall acidic seawater splitting, Chin. J. Struct. Chem., 2024, 43, 100191.
11. K. Liu, X. Zhang, J. Li, Y. Liu, M. Wang and H. Cui, A hierarchical bimetallic nitride hybrid electrode with strong electron interaction for enhanced hydrogen production in seawater, Green Chem., 2024, 26, 4677–4683.
12. J. Sun, S. Qin, Z. Zhao, Z. Zhang and X. Meng, Rapid carbothermal shocking fabrication of iron-incorporated molybdenum oxide with heterogeneous spin states for enhanced overall water/seawater splitting, Mater. Horiz., 2024, 11, 1199–1211.
13. F. Sun, J. Qin, Z. Wang, M. Yu, X. Wu, X. Sun and J. Qiu, Energy-saving hydrogen production by chlorine-free hybrid seawater splitting coupling hydrazine degradation, Nat. Commun., 2021, 12, 4182.
14. M. Jadwiszczak, K. Jakubow-Piotrowska, P. Kedzierzawski, K. Bienkowski and J. Augustynski, Highly Efficient Sunlight-Driven Seawater Splitting in a Photoelectrochemical Cell with Chlorine Evolved at Nanostructured WO₃ Photoanode and Hydrogen Stored as Hydride within Metallic Cathode, Adv. Energy Mater., 2020, 10, 1903213.
15. Z.-Y. Yu, Y. Duan, X.-Y. Feng, X. Yu, M.-R. Gao and S.-H. Yu, Clean and Affordable Hydrogen Fuel from Alkaline Water Splitting: Past, Recent Progress, and Future Prospects, Adv. Mater., 2021, 33, 2007100.
16. H. Shi, X.-Y. Sun, S.-P. Zeng, Y. Liu, G.-F. Han, T.-H. Wang, Z. Wen, Q.-R. Fang, X.-Y. Lang and Q. Jiang, Nanoporous Nonprecious High-Entropy Alloys as Multisite Electrocatalysts for Ampere-Level Current-Density Hydrogen Evolution, Small Struct., 2023, 4, 2300042.
17. Y. Jeong, S. Shankar Naik, Y. Yu, J. Theerthagiri, S. J. Lee, P. L. Show, H. C. Choi and M. Y. Choi, Ligand-free monophasic CuPd alloys endow boosted reaction kinetics toward energy-efficient hydrogen fuel production paired with hydrazine oxidation, J. Mater. Sci. Technol., 2023, 143, 20–29.
18. Y. Jeong, S. S. Naik, J. Theerthagiri, C. J. Moon, A. Min, M. L. Aruna Kumari and M. Y. Choi, Manifolding surface sites of compositional CoPd alloys via pulsed laser for hydrazine oxidation-assisted energy-saving electrolyzer: Activity origin and mechanism discovery, Chem. Eng. J., 2023, 470, 144034.
19. Y. Yu, S. J. Lee, J. Theerthagiri, Y. Lee and M. Y. Choi, Architecting the AuPt alloys for hydrazine oxidation as an anolyte in fuel cell: Comparative analysis of hydrazine splitting and water splitting for energy-saving H₂ generation, Appl. Catal., B, 2022, 121603.
20. J. Theerthagiri, R. A. Senthil, M. H. Buraidah, J. Madhavan, A. K. Arof and M. Ashokkumar, One-step electrochemical deposition of Ni1−xMoxS ternary sulfides as an efficient counter electrode for dye-sensitized solar cells, J. Mater. Chem. A, 2016, 4, 16119–16127.
21. Z. Jin, J. Lv, H. Jia, W. Liu, H. Li, Z. Chen, X. Lin, G. Xie, X. Liu, S. Sun and H.-J. Qiu, Nanoporous Al-Ni-Co-Ir-Mo High-Entropy Alloy for Record-High Water Splitting Activity in Acidic Environments, Small, 2019, 15, 1904180.
22. S.-Q. Chang, C.-C. Cheng, P.-Y. Cheng, C.-L. Huang and S.-Y. Lu, Pulse electrodeposited FeCoNiMnW high entropy alloys as efficient and stable bifunctional electrocatalysts for acidic water splitting, Chem. Eng. J., 2022, 446, 137452.
23. A. Asghari Alamdari, H. Jahangiri, M. B. Yagci, K. Igarashi, H. Matsumoto, A. Motallebzadeh and U. Unal, Exploring the Role of Mo and Mn in Improving the OER and HER Performance of CoCuFeNi-Based High-Entropy Alloys, ACS Appl. Energy Mater., 2024, 7, 2423–2435.
24. Y. Wang, M. Ni, W. Yan, C. Zhu, D. Jiang, Y. Yuan and H. Du, Supported High-Entropy Alloys for Electrooxidation of Benzyl Alcohol Assisted Water Electrolysis, Adv. Funct. Mater., 2024, 34, 2311611.
25. A. Ahmad, A. Nairan, Z. Feng, R. Zheng, Y. Bai, U. Khan and J. Gao, Unlocking the Potential of High Entropy Alloys in Electrochemical Water Splitting: A Review, Small, 2024, 2311929.
26. H.-M. Zhang, S.-F. Zhang, L.-H. Zuo, J.-K. Li, J.-X. Guo, P. Wang, J.-F. Sun and L. Dai, Recent advances of high-entropy electrocatalysts for water electrolysis by electrodeposition technology: a short review, Rare Met., 2024, 43, 2371–2390.
27. S. Schweidler, M. Botros, F. Strauss, Q. Wang, Y. Ma, L. Velasco, G. Cadilha Marques, A. Sarkar, C. Kübel, H. Hahn, J. Aghassi-Hagmann, T. Brezesinski and B. Breitung, High-entropy materials for energy and electronic applications, Nat. Rev. Mater., 2024, 9, 266–281.
28. Z. Shi, L. Wang, Y. Huang, X. Y. Kong and L. Ye, High-entropy catalysts: new opportunities toward excellent catalytic activities, Mater. Chem. Front., 2024, 8, 179–191.
29. J.-T. Ren, L. Chen, H.-Y. Wang and Z.-Y. Yuan, Non-precious metal high-entropy alloys with d–d electron interactions for efficient and robust hydrogen oxidation reactions in alkaline media, Inorg. Chem. Front., 2024, 11, 2029–2038.
30. L. Zhang, S. Xia, X. Zhang, Y. Yao, Y. Zhang, S. Chen, Y. Chen and J. Yan, Low-Temperature Synthesis of Mesoporous Half-Metallic High-Entropy Spinel Oxide Nanofibers for Photocatalytic CO₂ Reduction, ACS Nano, 2024, 18, 5322–5334.
31. X. Wang, Q. Peng, X. Zhang, X. Lv, X. Wang and Y. Fu, Carbonaceous-assisted confinement synthesis of refractory high-entropy alloy nanocomposites and their application for seawater electrolysis, J. Colloid Interface Sci., 2022, 607, 1580–1588.
32. P. Li, Y. Yao, W. Ouyang, Z. Liu, H. Yin and D. Wang, A stable oxygen evolution splitting electrocatalysts high entropy alloy FeCoNiMnMo in simulated seawater, J. Mater. Sci. Technol., 2023, 138, 29–35.
33. M. Bondesgaard, N. L. N. Broge, A. Mamakhel, M. Bremholm and B. B. Iversen, General Solvothermal Synthesis Method for Complete Solubility Range Bimetallic and High-Entropy Alloy Nanocatalysts, Adv. Funct. Mater., 2019, 29, 1905933.
34. A. S. Rogachev, N. A. Kochetov, A. V. Panteleeva, K. V. Kuskov, D. Y. Kovalev, A. S. Shchukin, S. G. Vadchenko and Y. B. Scheck, High-Energy Ball Milling and Spark Plasma Sintering of the CoCrFeNiAl High-Entropy Alloy, Metals, 2020, 10, 1489.
35. J. B. Khan, P. K. Panda, P.-C. Yang, C.-T. Hsieh, Y. A. Gandomi, W.-R. Liu and J.-K. Chang, Microwave synthesis of high-entropy alloy catalysts on graphene oxide sheets for oxygen reduction and evolution reactions, Int. J. Hydrogen Energy, 2024, 53, 999–1008.
36. X. Cui, Y. Liu, X. Wang, X. Tian, Y. Wang, G. Zhang, T. Liu, J. Ding, W. Hu and Y. Chen, Rapid High-Temperature Liquid Shock Synthesis of High-Entropy Alloys for Hydrogen Evolution Reaction, ACS Nano, 2024, 18, 2948–2957.
37. S. Jiang, Y. Yu, H. He, Z. Wang, R. Zheng, H. Sun, Y. Liu and D. Wang, General Synthesis of Composition-Tunable High-Entropy Amorphous Oxides Toward High Efficiency Oxygen Evolution Reaction, Small, 2024, 2310786.
38. J. Theerthagiri, K. Karuppasamy, A. Min, D. Govindarajan, M. L. A. Kumari, G. Muthusamy, S. Kheawhom, H.-S. Kim and M. Y. Choi, Unraveling the fundamentals of pulsed laser-assisted synthesis of nanomaterials in liquids: Applications in energy and the environment, Applied Physics Reviews, 2022, 9, 041314.
39. S. J. Lee, J. Theerthagiri and M. Y. Choi, Time-Resolved Dynamics of Laser-Induced Cavitation Bubbles During Production of Ni Nanoparticles via Pulsed Laser Ablation in Different Solvents and Their Electrocatalytic Activity for Determination of Toxic Nitroaromatics, Chem. Eng. J., 2021, 130970.
40. Y. Oh, J. Theerthagiri, A. Min, C. J. Moon, Y. Yu and M. Y. Choi, Pulsed laser interference patterning of transition-metal carbides for stable alkaline water electrolysis kinetics, Carbon Energy, 2024, e448.
41. J. Kim, T. Begildayeva, J. Theerthagiri, C. J. Moon, A. Min, S. J. Lee, G.-A. Kim and M. Y. Choi, Manifolding active sites and in situ/operando electrochemical-Raman spectroscopic studies of single-metal nanoparticle-decorated CuO nanorods in furfural biomass valorization to H₂ and 2-furoic acid, J. Energy Chem., 2023, 84, 50–61.
42. R. V. Blasques, J. R. Camargo, W. B. Veloso, G. N. Meloni, F. A. Fernandes, B. F. Germinare, L. R. Guterres e Silva, A. de Siervo, T. R. L. C. Paixão and B. C. Janegitz, Green Fabrication and Analytical Application of Disposable Carbon Electrodes Made from Fallen Tree Leaves Using a CO₂ Laser, ACS Sustain. Chem. Eng., 2024, 12, 3061–3072.
43. H. Wang, Y. Sun, X. Liu and T. Li, CO₂ laser-induced formation of a nitrogen-doped graphene aerophobic layer on the nickel foam surface for enhanced electro-oxidation of urea, Surface. Interfac., 2024, 46, 104074.
44. J. Theerthagiri, K. Karuppasamy, S. J. Lee, R. Shwetharani, H.-S. Kim, S. K. K. Pasha, M. Ashokkumar and M. Y. Choi, Fundamentals and comprehensive insights on pulsed laser synthesis of advanced materials for diverse photo- and electrocatalytic applications, Light: Sci. Appl., 2022, 11, 250.
45. Y. Yu, S. Jun Lee, J. Theerthagiri, S. Fonseca, L. M. C. Pinto, G. Maia and M. Yong Choi, Reconciling of Experimental and Theoretical Insights on the Electroactive Behavior of C/Ni Nanoparticles with AuPt Alloys for Hydrogen Evolution Efficiency and Non-enzymatic Sensor, Chem. Eng. J., 2022, 134790.
46. S. Hong, W. Lee, Y. J. Hwang, S. Song, S. Choi, H. Rhu, J. H. Shim and A. Kim, Role of defect density in the TiO_x protective layer of the n-Si photoanode for efficient photoelectrochemical water splitting, J. Mater. Chem. A, 2023, 11, 3987–3999.
47. E. Borsella, S. Botti, M. Cremona, S. Martelli, R. M. Montereali and A. Nesterenko, Photoluminescence from oxidised Si nanoparticles produced by CW CO₂ laser synthesis in a continuous-flow reactor, J. Mater. Sci. Lett., 1997, 16, 221–223.
48. A. I. Kostyukov, V. N. Snytnikov, M. I. Rakhmanova, N. Y. Kostyukova and V. N. Snytnikov, Luminescent properties of Al₂O₃:Tb³⁺ nanoparticles obtained by cw CO₂ laser vaporization, Opt. Mater., 2020, 110, 110508.
49. M. Fan, J. Xu, Y. Wang, Q. Yuan, Y. Zhao, Z. Wang and J. Jiang, CO₂ Laser-Induced Graphene with an Appropriate Oxygen Species as an Efficient Electrocatalyst for Hydrogen Peroxide Synthesis, Chem.–Eur. J., 2022, 28, e202201996.
50. Y. Men, D. Wu, Y. Hu, L. Li, P. Li, S. Jia, J. Wang, G. Cheng, S. Chen and W. Luo, Understanding Alkaline Hydrogen Oxidation Reaction on PdNiRuIrRh High-Entropy-Alloy by Machine Learning Potential, Angew. Chem., Int. Ed., 2023, 62, e202217976.
51. G. Lee, N.-A. Nguyen, V.-T. Nguyen, L. L. Larina, E. Chuluunbat, E. Park, J. Kim, H.-S. Choi and M. Keidar, High entropy alloy electrocatalyst synthesized using plasma ionic liquid reduction, J. Solid State Chem., 2022, 314, 123388.
52. N. Rahamathulla, N. Vadivel, J. Theerthagiri, R. S. Raj, C. J. Moon, A. P. Murthy, S. Kheawhom and M. Y. Choi, Underlying aspects of surface amendment strategies adopted in electrocatalysts for overall water splitting under alkaline conditions, Curr. Opin. Electrochem., 2024, 43, 101428.
53. S. J. Lee, J. Theerthagiri, P. Nithyadharseni, P. Arunachalam, D. Balaji, A. Madan Kumar, J. Madhavan, V. Mittal and M. Y. Choi, Heteroatom-doped graphene-based materials for sustainable energy applications: A review, Renew. Sustain. Energy Rev., 2021, 143, 110849.
54. M. Wei, Y. Sun, J. Zhang, F. Ai, S. Xi and J. Wang, High-entropy alloy nanocrystal assembled by nanosheets with d–d electron interaction for hydrogen evolution reaction, Energy Environ. Sci., 2023, 16, 4009–4019.
55. S. Lin, H. Ze, X.-G. Zhang, Y.-J. Zhang, J. Song, H. Zhang, H.-L. Zhong, Z.-L. Yang, C. Yang, J.-F. Li and Z. Zhu, Direct and Simultaneous Identification of Multiple Mitochondrial Reactive Oxygen Species in Living Cells Using a SERS Borrowing Strategy, Angew. Chem., Int. Ed., 2022, 61, e202203511.
56. J. Hao, Z. Zhuang, K. Cao, G. Gao, C. Wang, F. Lai, S. Lu, P. Ma, W. Dong, T. Liu, M. Du and H. Zhu, Unraveling the electronegativity-dominated intermediate adsorption on high-entropy alloy electrocatalysts, Nat. Commun., 2022, 13, 2662.

---

Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta02120g

‡ These authors contributed equally to this work.

---