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Article

A Quinary-Metallic High-Entropy Electrocatalyst with Driving of Cocktail Effect for Enhanced Oxygen Evolution Reaction

by
Jing-Yi Lv
1,
Zhi-Jie Zhang
1,
Hao Zhang
1,
Jun Nan
2,
Zan Chen
2,
Xin Liu
3,
Fei Han
3,
Yong-Ming Chai
1,* and
Bin Dong
1,*
1
State Key Laboratory of Heavy Oil Processing, College of Chemistry and Chemical Engineering, China University of Petroleum (East China), Qingdao 266580, China
2
CNOOC Tianjin Chemical Research and Design Institute Co., Ltd., Tianjin 300131, China
3
CNOOC Shanxi Precious Metals Co., Ltd., Jinzhong 030621, China
*
Authors to whom correspondence should be addressed.
Catalysts 2025, 15(8), 744; https://doi.org/10.3390/catal15080744
Submission received: 2 July 2025 / Revised: 23 July 2025 / Accepted: 26 July 2025 / Published: 5 August 2025
(This article belongs to the Special Issue Non-Novel Metal Electrocatalytic Materials for Clean Energy)
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Abstract

The complex system of high-entropy materials makes it challenging to reveal the specific function of each site for oxygen evolution reaction (OER). Here, with nickel foam (NF) as the substrate, FeCoNiCrMo/NF is designed to be prepared by metal–organic frameworks (MOF) as a precursor under an argon atmosphere. XRD analysis confirms that it retains a partial MOF crystal structure (characteristic peak at 2θ = 11.8°) with amorphous carbon (peaks at 22° and 48°). SEM-EDS mapping and XPS demonstrate uniform distribution of Fe, Co, Ni, Cr, and Mo with a molar ratio of 27:24:30:11:9. Electrochemical test results show that FeCoNiCrMo/NF has excellent OER characteristics compared with other reference prepared samples. FeCoNiCrMo/NF has an overpotential of 285 mV at 100 mA cm−2 and performs continuously for 100 h without significant decline. The OER mechanism of FeCoNiCrMo/NF further reveal that Co and Ni are true active sites, and the dissolution of Cr and Mo promote the conversion of active sites into MOOH following the lattice oxygen mechanism (LOM). The precipitation–dissolution equilibrium of Fe also plays an important role in the OER process. The study of different reaction sites in complex systems points the way to designing efficient and robust catalysts.

Graphical Abstract

1. Introduction

The water splitting for hydrogen production offers significant potential in mitigating environmental pollution and energy scarcity issues, comprising two half-reactions [1,2]. The oxygen evolution reaction (OER) is a crucial half-reaction in the water splitting [2,3,4]. Nonetheless, its sluggish dynamics seriously hampered the execution of commercial applications [5,6,7]. The emphasis on resolving this challenge is centered on developing efficient catalysts. Recent research underscores the importance of non-precious metal OER catalysts in equilibrating numerous influencing factors. Binary and multi-metal catalysts are particularly noteworthy among diverse catalysts due to their ability for electron modulation across constituent metals [8,9,10,11,12]. The aim of the research is to create OER catalysts utilizing cost-effective materials and suitable intrinsically active transition metals, including Fe, Co, and Ni [8,13,14,15]. As the research continues to deepen, it is interesting to note that high-priced metals, such as Mo and W, have strong electron attraction ability, hence diminishing the energy barrier associated with metal oxidation transitions. These metals enhance the electronic environment of the active metal, decreasing electron affinity at Fe, Co, and Ni sites, thus augmenting OER activity [16,17,18,19,20]. Moreover, the active site in the OER catalyst is generated through chemical reconfiguration during a distinct operation [21,22]. The precise function of different components in the catalyst is ambiguous, limiting substantial enhancement in the electrocatalytic properties of OER. Consequently, comprehending the mechanism of the action of catalysts enhances the development of more efficient and appealing catalysts.
In contrast to traditional materials, high-entropy materials (HEM) incorporate various elements to design efficient OER active sites [23,24,25,26]. High-entropy materials exhibit four primary properties, including an intriguing and distinctive cocktail effect, which enables the material to demonstrate characteristics that surpass conventional expectations [27]. As research progresses, the potential applications of high-entropy catalysts in the OER field are becoming increasingly expansive. These materials are anticipated to play a crucial role in advancing renewable energy technologies, including hydrogen production through the electrolysis of water [28,29,30,31]. When different components are combined in specific ratios, a comprehensive effect similar to a “cocktail” will be produced. The synergistic effect between the elements can optimize the adsorption and desorption processes of the reaction intermediates, reduce the activation energy barrier of the reaction, and consequently lead to a notable improvement in catalytic performance. Moreover, they have the capability to create several active sites on the catalyst surface. The active sites exhibit varying electronic structures and chemical properties, enabling them to effectively absorb and convert different reaction intermediates. However, the cocktail effect complicates the design of HEM significantly [32]. Simultaneously, examining the reaction mechanism of multi-component systems presents significant challenges [33,34]. Due to the complexity of high-entropy materials, it is challenging to reveal the specific functions of different active sites. Consequently, investigating the active sites and mechanisms of HEM catalysts is essential for the development of efficient and durable catalysts.
According to the above discussion, this work develops a quinary high-entropy catalyst FeCoNiCrMo/NF via MOF-derived calcination, aiming to clarify the role of each component in OER. The electrochemical test results demonstrate that FeCoNiCrMo/NF exhibited impressive OER performance, achieving an overpotential of 285 mV at 100 mA cm−2. Additionally, the Tafel slope and electrochemical impedance spectrum suggested the faster charge transfer rate for the catalyst. The chronopotentiometry revealed that FeCoNiCrMo/NF catalyst was capable of continuous operation for 100 h without significant degradation. The OER mechanism of FeCoNiCrMo/NF had been investigated, revealing that Co and Ni served as actual active sites. Additionally, the leakage of Cr and Mo promoted the conversion of active sites into MOOH in accordance with the LOM mechanism. The precipitation–dissolution equilibrium of Fe also played an important role in the OER reaction. This study not only reveals the synergistic effect in complex high-entropy systems, but also provides a paradigm for designing multi-component catalysts by rationally regulating each element’s function. It thereby establishes a foundation for future exploration of high-entropy materials in renewable energy technologies.

2. Results and Discussion

The synthesis of the FeCoNiCrMo electrocatalyst-based MOF precursor is illustrated in Figure 1a. A piece of Ni foam (Figure S1) (NF, 2.5 × 4 cm−2) is treated with 1M HCl and acetone, serving as a substrate for growing MOF and as the Ni source. Additionally, porous Ni foam provides a suitable substrate for the deposition of MOFs. The compound 2,5-dihydroxyterephthalic acid, characterized by its abundance of -OH and -COOH groups, facilitates a consistent distribution of five metal ions. During a hydrothermal process, it coordinates with five metal elements, leading to the formation of MOF-74, which appears as a spherical agglomerate (Figure S2) [35]. The SEM images of the precursor clearly depict spherical particles that uniformly and thoroughly cover the NF surface. The main morphology of the calcined catalyst closely resembles that of its parent MOF. Nonetheless, it is noteworthy that numerous cracks are present in the central area of NF, potentially resulting from the carbonization of the organic linker in the MOF (Figure 1b). Additionally, magnified SEM images reveal that the carbonized area is linked to the spherical particles, suggesting that a secondary fine structure emerges on the primary structures (Figure 1c,d). The structure of the catalyst is well preserved in the thermal conversion process. Consequently, only partial carbonization of the carbon in an organic ligand occurs at 300 °C. The TEM and electron diffraction images of the catalyst reveal its low crystalline (Figure 1e–g). The distribution of elements is analyzed by SEM EDS elemental mapping, indicating that the catalyst comprises Fe, Co, Ni, Cr, Mo, O, and C elements, all of which are uniformly distributed (Figure 1h,i). Additionally, the carbon content in FeCoNiCrMo/NF is 59.34%, which is uniformly distributed in the catalyst. The multiple functions of carbon derived from catalysts as structural support, electron transport channel, and active site provider jointly promote the improvement of catalytic performance. The molar ratio of the principal elements Fe:Co:Ni:Cr:Mo is 27:24:30:11:9, aligning with the definition of HEM [24], where the molar percentage of each element ranging from 5% to 35% ensures the formation of a homogeneous solid solution and the occurrence of the “cocktail effect”.
To avoid the interference of strong NF peaks, the synthesized product is scraped off the substrate for XRD characterization. The XRD test is performed to investigate the crystal structures of Per-FeCoNiCrMo and FeCoNiCrMo (Figure S3 and Figure 2a). The XRD patterns of Per-FeCoNiCrMo exhibit similarities to those of MOF-74 at approximately 11.8°, 14.6°, 18.0°, 20.0°, 21.4°, 23.3°, 24.4°, and 25.7°, as reported the literature [36]. Following calcination, the XRD peak at about 11.8° for FeCoNiCrMo is retained, aligning with the (002) crystal plane of the theoretically simulated XRD patterns of the MOF-74 powders, which suggest that the synthesized electrocatalyst shares the same crystal structure as MOF-74. However, the observed peaks deviate slightly from the simulated MOF-74 powder, indicating lattice distortions caused by the coordination of five metal elements with the organic ligands. The prominent diffraction peaks observed at 44.5°, 51.8°, and 76.1° correspond to the NF (PDF#04-0850) [37], which is attributed the inevitable residue of nickel foam during the scraping process. Furthermore, certain metal–organic frameworks will undergo calcination into amorphous carbon at 300 °C, as evidenced by XRD, with peaks observed at 22° and 48° corresponding to amorphous carbon. To further confirm the chemical bonds and functional groups in the material, the FT-IR spectra of FeCoNiCrMo are analyzed (Figure 2b). Acid derivatives at 3400–2500 cm−1 typically produce hydroxyl stretching vibrations. Dicarboxylic acids produce two strong bands at 1675–1750 cm−1, which are attributed to the stretching vibration of the carboxyl group C=O. Another strong band appears at 1230–1140 cm−1 due to the stretching of the C-O bond [8,38]. The prominent peak at 1410 cm−1 is attributed to the benzene ring within the ligand. Peaks in the FT-IR spectrum at 1300–1000 cm−1 are identified, which is attributed to plane ring bending vibrations of C–H, and in the region 950–780 cm−1, the C–H out-of-plane bending vibrations are detected. Significantly, distinct bands are observed around 500 cm−1 and associated with metal–O or metal–O–H-related vibrations [28,39]. The results obtained demonstrate the successful preparation of partially carbon-derived MOF-74.
To elucidate the surface chemical states of FeCoNiCrMo, X-ray photoelectron spectroscopy (XPS) was employed (Figure S4). The detailed binding energies and corresponding oxidation states of all elements, along with relevant literature references for comparison, are summarized in Table S3. All binding energies are referenced to the C 1s peak of adventitious carbon at 284.8 eV. The XPS survey spectrum for the FeCoNiCrMo electrocatalyst clearly shows prominent peaks for the elements Fe, Co, Ni, Cr, Mo, O, and C, aligning with the findings from the SEM EDS elemental mapping results (Figure S5 and Figure 1h,i). Similarly, for the FeCoNi electrocatalyst, Fe, Co, Ni, O, and C elements are found (Figure S6a). The XPS signals of Fe for the FeCoNiCrMo electrocatalyst are attributed to six distinct peaks. The Fe 2p spectrum has been deconvoluted into two pairs of 2p3/2/2p1/2 peaks at 710.2/723.5 eV and 712.8/725.5 eV corresponding to Fe2+ and Fe3+ species [40]. The peaks observed at 717.1 eV and 731.2 eV correspond to the satellite peaks of Fe 2p3/2/2p1/2 [17,40]. Two pairs of typically fitted peaks of Co 2p are located at 780.9 and 796.6 eV, with binding energies at 779.7 eV and 794.8 eV corresponding to Co3+ 2p3/2 and Co3+ 2p1/2, while those at 780.9 eV and 796.9 eV are attributed to Co2+ 2p3/2 and Co2+ 2p1/2, respectively (Figure 2d) [41,42,43,44,45,46]. As can be seen from Ni 2p analysis of XPS, Ni 2p3/2 and Ni 2p1/2 are positioned at 855.9 and 873.5 eV. The fitting data reveal the coexistence of Ni2+ and Ni3+. The peaks at 855.9 eV and 873.3 eV are associated with Ni2+ 2p3/2 and Ni2+ 2p1/2, while the peaks at 857.5 eV and 874.9 eV are attributed to Ni3+ 2p3/2 and Ni3+ 2p1/2, respectively (Figure 2e) [17,47]. For the Cr 2p XPS, two obvious peaks at the binding energies of 576.97 and 586.4 eV are well matched to the Cr 2p3/2 and Cr 2p1/2 of Cr3+ for FeCoNiCrMo (Figure 2f) [17]. Moreover, the fitted peaks located at the binding energies of 231.9 and 235.1 eV can be assigned to the Mo 3d5/2 and Mo 3d3/2 of Mo6+ (Figure 2g) [48]. The XPS analysis of the FeCoNiMo catalyst is investigated to probe the influence of Mo and Cr. The Fe 2p XPS spectrum reveals signal peaks at 712.1 and 724.4 eV, which are assigned to Fe 2p3/2 and 2p1/2. The binding energies at 711.1 eV and 724.0 eV correspond to Fe2+ 2p3/2 and Fe2+ 2p1/2, while the peaks at 712.7 eV and 727.0 eV are attributed to Fe3+ 2p1/2 and Fe3+ 2p3/2, respectively (Figure S5a). For the Co XPS spectrum, the peaks located at 781.0eV and 796.6 eV correspond to Co3+ 2p3/2 and Co3+ 2p1/2, and those at 783.6 eV and 799.3 eV are attributed to Co2+ 2p3/2 and Co2+ 2p1/2, respectively (Figure S5b) [49]. Two pairs of typically fitted peaks of Ni 2p are observed at 855.8 and 873.3 eV. The fitting data reveal the presence of both Ni2+ and Ni3+. The peaks at 855.5 eV and 873.0 eV are associated with Ni2+ 2p3/2 and Ni2+ 2p1/2, while the peaks at 857.0 eV and 875.5 eV are attributed to Ni3+ 2p3/2 and Ni3+ 2p1/2, respectively (Figure S5c). The Mo 3d spectra reveals two peaks, with Mo 3d5/2 and Mo 3d3/2 appearing at 232.1 and 235.2 eV, respectively. The XPS spectra of Mo are shifted towards higher binding energies compared to the XPS of FeCoNiCrMo, suggesting that Mo favors the modulation of the electronic structure (Figure S5d). Furthermore, there are two pairs of fitted peaks for Fe 2p observed at 712.9 and 724.3 eV in the FeCoNi electrocatalyst (Figure S6b). As can be seen from the Co analysis of XPS, the peaks at 781.1 eV and 796.5 eV are attributed to Co 2p3/2 and Co 2p1/2 for the FeCoNi electrocatalyst (Figure S6c). The peaks at 855.6 and 873.2 eV in the Ni 2p spectrum represent Ni 2p3/2 and Ni 2p1/2 (Figure S6d). Additionally, when it comes to O 1s spectra of FeCoNiCrMo (Figure 2i), Metal-O (529.1 eV), and O vacancy (at 531.1 eV), a member of O-C=O or adsorptive H2O (533.3 eV) are all present [7,50]. Electron paramagnetic resonance (EPR) is additionally utilized to assess the existence of oxygen vacancies in the catalysts. The EPR spectrum of FeCoNiCrMo shows a characteristic signal at g = 2.003, indicating the presentation of significant oxygen vacancies, which is in agreement with the XPS results (Figure S7). The presence of oxygen vacancies has the potential to influence the surface reconfiguration of the catalyst, transforming it into metal oxyhydrogen for OER [51]. Oxygen vacancies can provide adsorption sites for reaction intermediates. In addition, the increase in oxygen vacancy concentration with low forming energy reduces charge transfer resistance and speeds up reaction kinetics, thus enabling improved OER performance, which is consistent with previous studies [28,42]. The deconvoluted C1s peaks of the FeCoNiCrMo electrocatalyst could be matched to C―C bond (284.8 eV), C―O bond (285.7 eV), C=O bond (288.0 eV), and O-C=O bond (289.1 eV) (Figure 2h) [52]. The decomposed peaks for the O and C1s XPS peaks of ternary materials exhibit similarities to those of as-prepared FeCoNiCrMo (Figure S6e,f). Taking the above analysis into account, Cr and Mo can promote electron transfer among elements. The interplay among components enhances the OER characteristics of the catalyst.
To illustrate the significant impact of the interaction among the five elements on enhancing the OER activity of the catalyst, electrochemical measurements are conducted on the as-prepared samples at room temperature using a standard three-electrode system in 1.0 M KOH. The linear sweep voltage (LSV) curves clearly demonstrate that the FeCoNiCrMo electrocatalyst exhibits an optimal OER electrochemical property, requiring an overpotential of 285 mV to reach 100 mA cm−2, which is better than those of FeCoNiMo (η100 = 314 mV), FeCoNiCr (η100 = 300 mV), CoNiCrMo (η100 = 385 mV), FeCoNi (η100 = 353 mV), and Ni foam (η100 = 443 mV) (Figure 3a). The above properties indicate a synergistic effect among the metals of the partially carbon-derived catalyst. The Tafel slope of the FeCoNiCrMo electrocatalyst is 68.45 mV dec−1, which is lower than that of FeCoNiMo (70.16 mV dec−1), FeCoNiCr (82.96 mV dec−1), CoNiCrMo (93.43 mV dec−1), FeCoNi (114.2 mV dec−1), and Ni foam (150.6 mV dec−1) (Figure 3b), implying favorable OER electrocatalytic kinetics achieved by modifying the electronic structure of the FeCoNiCrMo sample. The electrochemical parameters of the oxygen evolution reaction (OER) for FeCoNiCrMo surpass those of other synthetic materials. Moreover, FeCoNiCrMo demonstrates a discernible advantage relative to commercial RuO2 and other documented high-performance electrocatalysts (Figure 3c,d and Figure S8 and Table S4) [53,54,55,56,57,58,59]. The conductivity of the electrode towards OER is investigated through electrochemical impedance spectroscopy (EIS) to facilitate a more comprehensive analysis of catalyst charge transfer. We used an equivalent circuit (Figure 3e) to fit the Nyquist plot, which includes a solution resistance (R1), a charge transfer resistance (R2), and a constant phase element (CPE) to describe the non-ideal double-layer capacitor. The fitting results are summarized in Table S5. In all samples, the R2 value of the FeCoNiCrMo catalyst (0.48317Ω·cm2) is the smallest, indicating that the electron transfer rate at the catalyst–electrolyte interface is the fastest during the OER process. This is consistent with its lowest Tafel slope (68.45 mV dec−1) and the best LSV performance, confirming the acceleration of the reaction kinetics. The CPE parameter n of FeCoNiCrMo is 0.61022, which is different from 1, indicating that the double-layer capacitor is non-ideal. This is attributed to the porous structure of the catalyst (Figure 1b–d), where the surface roughness leads to an uneven charge distribution. This confirms that the oxygen vacancies (Figure S7) further enhance the charge transfer by acting as electron donors, promoting the formation of MOOH. Additionally, the electrochemical double layer capacitance (Cdl) obtained by the CV scanning test can be used to evaluate the electrochemical surface area (ECSA) of the catalyst (Figure S9). Its spherical aggregates (Figure 1b–d) expand the electrochemical active surface area, thereby exposing more Co/Ni active sites. This explains the lower overpotential compared to similar materials with aggregated structures. XPS shows mixed valence states (Figure 2d,e), which provide a structural basis for the lattice oxygen mechanism (LOM). The high-valent Co3+/Ni3+ helps lattice oxygen participate in the formation of Ni (Co)OOH, thereby accelerating the reaction kinetics, which is reflected in the smaller Tafel slope.
The electrochemical stability of catalysts is a criterion used to evaluate commercial applications, especially at elevated current densities. The FeCoNiCrMo catalyst is performed to 500 CV cycles, and the LSV exhibits no changes significantly, indicating that the properties of the material remained stable (Figure 3f). Furthermore, the durability of the catalyst is studied by chronopotentiometry at a current density of 100 mA cm−2. It is obvious that the properties of the FeCoNiCrMo catalyst are performed to 100 h without apparent performance degradation (Figure 3g). Compared with the FeCoNiMo electrocatalysts material, the performance is decreased, indicating that the high-entropy environment of the five-element material enhances stability.
To further unravel the stability of the catalyst, the catalyst is collected and characterized post-durability testing. The FeCoNiCrMo electrode effectively preserves scattered nanoclusters, demonstrating commendable stability for commercial water electrolysis applications (Figure 4a,b). The crystal structure of the catalyst is studied by XRD, revealing a prominent peak for NF without the presence of other crystal structures (Figure S10). One of the reasons is low load. In addition, the diffraction peak of the MOOH phase is absent in the XRD pattern, as the diffraction signal from the low-abundance surface layer MOOH is weaker than that of the foam nickel substrate. The supported catalyst has difficulty in detaching from the nickel foam, signifying a robust connection between the stabilized material and the substrate.
The chemical states of elements on the surface of the FeCoNiCrMo electrocatalysts post-stability are analyzed by XPS. Remarkably, it is found that there are remarkable differences in the surface elements of the catalyst before and after the durability test (Figure S11). After OER, the reduced concentrations of Cr and Mo indicate that these elements are solubilized during the oxygen evolution process (Figure 4c) [60,61]. Conversely, other components exhibited minimal dissolution. It is important to maintain the valence states of the five elements, which once again proves the excellent stability of the material. In particular, the XPS fine spectra of Co and Ni reveal an increase in both Co3+/Co2+ and Ni3+/Ni2+ ratios from 0.52 to 1.81 and from 0.31 to 0.47, respectively, indicating that the reconfiguration of Co and Ni into higher oxidation states facilitated the reaction to adhere to the LOM mechanism (Figure 4d,e) [17,62,63], consistent with Trotochaud et al. [21], who reported that Ni3+ in (Ni, Fe)OOH is critical for LOM by facilitating lattice oxygen participation. The XPS fine spectra of Cr and Mo are analyzed, and the chemical valence of small amounts of Cr and Mo is Cr3+ and Mo6+, which is consistent with the chemical valence states before stability (Figure 4f,g). Additionally, the O 1s spectra of FeCoNiCrMo after the stability test exhibit the presence of Metal-O (529.8 eV), O vacancy (at 531.2 eV), and species associated with O-C=O or adsorbed H2O (531.8 eV) (Figure 4h). Additionally, the XPS spectrum of C 1s (Figure 4i) shows no significant changes in peak positions or intensities: the relative proportion of C—C bonds (284.8 eV) remains dominant, and there is no obvious increase in oxidized carbon species (e.g., C=O or O—C=O). This indicates that the carbon skeleton derived from MOF maintains good stability under prolonged electrolysis conditions (Figure 4i). The above results show that the catalyst has advantageous catalytic activity and excellent stability. The unique MOF-74 with amorphous carbon facilitates exposure of the active sites. The high-entropy environment of five elements promotes the interaction between electrons and improves stability.
Theoretical investigations are conducted to provide a more profound understanding of the catalytic mechanism of the OER (Figure 5). Transition metal-based materials, especially those comprising Fe, Co, and Ni, attracted considerable interest in OER catalyst research owing to their economic viability and appropriate catalytic performance. The low overpotential and fast kinetics (Figure 3a,b) are sustained over 100 h (Figure 3g) because the spherical structure (Figure 1b–d) tolerates volume changes during OER, while the uniform element distribution (Figure 1h,i) prevents localized degradation. Additionally, the dynamic Fe precipitation–dissolution equilibrium (0.0006 mg/L, Table S2) and Cr/Mo dissolution-induced reconstruction (Figure 4c) replenish active sites, avoiding performance decay, which is consistent with the minimal shift in LSV curves after 500 cycles (Figure 3f). However, a significant restriction of stable iron-based catalysts is the loss of iron, which hinders their extensive utilization [8]. It is well established that the establishment of a dynamic catalytic interface is crucial for maintaining stable OER activity [64,65]. This dynamic interface relies on a balanced ionic environment. The significance of iron in improving OER catalytic performance, known as the “iron effect,” is unequivocal [66]. To further elucidate the function of iron in the FeCoNiCrMo catalyst, the OER behavior of the CoNiCrMo catalyst is investigated. The comparative CV curve analysis of the CoNiCrMo catalyst in a 1M KOH solution with trace Fe3+ revealed improved catalytic performance compared to electrolytes lacking iron (Figure S12a). Additionally, in a chronopotentiometry experiment using the CoNiCrMo catalyst (Figure S12b), the potential increased after 50 h at a current density of 100 mA cm−2. However, with gradual Fe3+ addition to the electrolyte, the potential of the material decreases significantly, indicating the existence of a precipitation–dissolution equilibrium for Fe in the electrolyte. After stable operation at an optimized potential for 15 h, the potential increases, possibly due to an excess of Fe disrupting the catalytic interface. Furthermore, XPS analysis of the stabilized FeCoNiCrMo material shows an increase in Co and Ni content with a concurrent decrease in Fe content (Figure 4c). This suggests that the observed decrease in Fe resulted in the precipitation–dissolution equilibrium. Anantharaj et al. demonstrated that Fe3+ in the electrolyte can be in situ deposited onto the catalyst surface, forming a highly active Fe-doped NiOOH species, while excessive Fe leaching disrupts the catalytic interface, which is a mechanism consistent with our observations [66]. Therefore, the continuous site update achieved through iron balance and Cr/Mo reconstitution results in a stable constant potential electrolysis curve (Figure 3g). Comparison of XPS spectra prior to and following the stabilization of FeCoNiCrMo reveals an augmentation in the levels of Co and Ni in elevated oxidation states, signifying a remodeling event (Figure 4d,e). Previous research demonstrated that surface reconstruction products serve as the genuine active sites for the OER in relation to the LOM mechanism [17,62,63,67]. Furthermore, during the OER process, Cr and Mo are released from the catalyst, leading to prompt reconstruction of MOOH by Cr and Ni [53,60]. ICP-MS analysis of the electrolyte after the 100 h stability test detected a low but measurable Fe concentration (0.0006 mg/L, Table S1), indicating continuous Fe dissolution from the catalyst and reprecipitation onto the catalyst surface, forming a dynamic equilibrium (Table S2). Additionally, after 100 h of the reaction, the electrolyte contains Cr and Mo at concentrations of 0.1880mg/L and 0.3226mg/L, respectively. Liu et al. [60] reported that the leaching of Cr3+ in the high-entropy alloy leads to the surface reconstruction into MOOH, which is a key step in LOM, and this is consistent with our experimental results. Furthermore, the FeCoNiMo and FeCoNiCr perform a CV scan in 10 mM Cr3+ and MoO42− spiked 1 M KOH (Figure S13). The negative shift of the Ni2+/3+ redox peak as the scan proceeds can be attributed to the Ni reconstruction process Ni2+ → Ni3+ → Ni2+. The high-valence Mo6+ exhibits a significant electron-withdrawing capacity, facilitating the electron transfer of Ni/Co atoms, making it easy for Ni/Co atoms to form metal-oxyhydrogen in situ. This result is similar to the work of Mei et al. [48], who demonstrated that the high-valent Mo6+ can enhance the electron transfer at the Ni/Co sites. In addition, the area of the closed curves gradually increases, showing a gradual increase in the degree of catalyst reconstruction, which indicates that the dissolution of Mo and Cr can improve the reconstruction of the catalyst. The above experimental results reveal the role of various elements in the catalyst. The improvement of OER performance depends on any one of these elements.

3. Experimental Section

All chemicals were used directly and not purified.

3.1. Electrocatalyst Synthesis

The FeCoNiCrMo electrocatalyst was synthesized by a hydrothermal method and was then annealed. First, the nickel foam (NF, 4 × 2.5 cm2) was sonicated in 1 M HCl and acetone for 20 min to remove the oil from the surface, respectively. Then, the nickel foam was rinsed with water and anhydrous ethanol and dried in a vacuum drying oven at 60 °C for 12 h. FeCl2·6H2O (1 mmol), Co (NO3)2·6H2O (1 mmol), C10H14MoO6 (0.4 mmol), Cr (NO3)2·9H2O (0.4 mmol), and 2,5-dihydroxy-1,4benzenedicarboxylic acid (DHTA, 1.5 mmol), and a piece of cleaned NF were dissolved in the mixed solution (VDMF:VH2O:Vethanol = 14:1:1) and transferred into a Teflon-lined stainless steel autoclave (100 mL) for 48 h at 150 °C to produce pre-FeCoNiCrMo. The pre-FeCoNiCrMo was rinsed several times with deionized water and anhydrous ethanol, then dried at 60 °C in an air oven. Finally, dried pre-FeCoNiCrMo was placed at the tube furnace with Ar atmosphere at 300 °C for 2 h to acquire the FeCoNiCrMo electrocatalyst. For comparison purposes, other electrocatalysts had been synthesized in the same way, only with the addition of different metal precursors.

3.2. Materials Characterization

The morphology and structure of the prepared samples were observed using Zeiss Sigma 500 scanning electron microscopy (SEM, Carl Zeiss AG, Oberkochen, Baden-Württemberg, Germany) and JEM2100plus transmission electron microscopy (TEM, JEOL Ltd., Tokyo, Japan). The crystal structure of the catalysts was characterized using X-ray diffraction (XRD, Bruker AXS GmbH, Karlsruhe, Germany) on a D8 Advance instrument with Cu Kα radiation (λ = 1.54 Å). The valence states and surface chemistry of the samples were analyzed using ESCALAB 250Xi X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, Waltham, MA, USA). All binding energies were referenced to the C 1s peak of adventitious carbon at 284.8 eV.

3.3. Electrochemical Measurements

The electrochemical tests covered in this article were performed on an electrochemical instrument (Gamry Interface 1010E, Gamry Instruments, Inc., Philadelphia, PA, USA), simultaneous constructing of the three-electrode system, of which Pt foil, saturated calomel electrode, and prepared substrate catalysts were used, respectively, as the counter electrode, the reference electrode, and the working electrode. Except when otherwise noted, 1 M KOH aqueous solution was used as the electrolyte in the electrochemical tests. Before testing, CV curves were performed for 100 cycles at the scan rate of 20 mV s−1 in a potential range of 1.07–1.67 mV (vs. reversible hydrogen electrode (RHE)). The linear sweep voltammetry (LSV) curves were plotted from 1.07 to 1.77 V vs. RHE with a scanning speed of 5 mV s−1 in 1 M KOH solution with iR compensation. The recorded potential on the Gamry Reference 1010 instrument was converted to the RHE potential as follows: ERHE = ESCE + 0.243 V + 0.059 pH. The Tafel slope was determined from the LSV curve using the following equation: E = a + b × log|j|, where E represents the potential, b represents the Tafel slope, and j represents the current density. The cyclic voltammetry (CV) measurement at different scan rates ranging from 2 to 12 mV s−1 in a non-Faradaic region from of 0.25 to 0.35 V vs. SCE to calculate the electrical double-layer capacitances (Cdl). Electrochemical active surface area (ECSA) was calculated as follows: ECSA = Cdl/Cs, where Cs represents specific capacitance. A commonly utilized specific capacitance value of 0.040 mF cm−2 in 1 M KOH was utilized for this calculation. Electrochemical impedance spectroscopy (EIS) analysis was monitored with a frequency ranging from 10 to 105 Hz at 1.57 V vs. RHE. The electrochemical stability was obtained by using the CV cycle for 1000 cycles and chronopotentiometry tests at 10 mA cm−2.

4. Conclusions

Herein, the synthesized FeCoNiCrMo/NF catalyst used MOF as the precursor and further calcined with Ar atmosphere. The structure of the catalyst was confirmed by SEM, XPS, and other characterization techniques, which is the MOF product with partial amorphous carbon. The electrochemical test suggest that FeCoNiCrMo/NF has attractive OER performance with an overpotential of 285 mV at 100 mA cm−2, and the Tafel slope is 68.45 mV dec−1, indicating the faster charge transfer rate. The chronopotentiometry test reveals that the FeCoNiCrMo/NF catalyst could perform continuously for 100 h at 100 mA cm−2 without obvious decline. The OER reaction of FeCoNiCrMo/NF follows the LOM mechanism. The result shows that Co and Ni were the actual active sites, and the double leakage of Cr and Mo promoted the conversion of active sites to MOOH. The precipitation–dissolution equilibrium of Fe also plays an important role in efficient and stable OER catalysts. Each of the five elements has its own role and interacts with each other. The research of different reaction sites for complex systems points out the prospect of designing efficient and robust catalysts.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal15080744/s1, Figure S1,S2: The different resolution SEM images Ni foam and pre-FeCoNiCrMo; Figure S3,S4: The XRD patterns and XPS survey spectrum of pre-FeCoNiCrMo electrocatalyst; Figure S5,S6: XPS fine spectra for the FeCoNiMo and FeCoNi electrocatalyst; Figure S7: The EPR spectra of FeCoNiCrMo electrocatalyst; Figure S8: The LSV curve comparison of FeCoNiCrMo and RuO2; Figure S9: The CV curves at different scan rates in a non-Faradaic region from of 0.25 to 0.35 V vs. SCE and double-layer capacitance (Cdl) plots of different catalysts were calculated based on CV data; Figure S10,S11: XRD patterns and full spectrum of the FeCoNiCrMo electrocatalyst after the stability test; Figure S12: The comparison of polarization curves and Chronopotentiometry test result; Figure S13: The CV curves of FeCoNiMo and FeCoNiCr; Table S1: The elemental composition of the catalyst is measured by EDS;. Table S2: Content of Fe, Cr, and Mo elements in 1M KOH solution after stability test by ICP-MS; Table S3: XPS binding energies and corresponding oxidation states of elements in FeCoNiCrMo electrocatalyst [17,21,48,60,61,65,66]; Table S4: Comparison of OER performance of FeCoNiCrMo electrocatalyst with other reported electrocatalysts [19,28,40,41,49,51,53,54,55,56,57,58,59]; Table S5: EIS fitting parameters for catalysts; Table S6: The final elemental composition of the catalyst is measured by ICP-MS.

Author Contributions

Conceptualization, J.-Y.L. and Z.-J.Z.; methodology, J.-Y.L.; software, Z.-J.Z.; validation, H.Z.; formal analysis, J.-Y.L.; investigation, Z.-J.Z.; resources, J.N.; data curation, Z.-J.Z.; writing—original draft preparation, J.-Y.L.; writing—review and editing, Z.-J.Z.; visualization, X.L. and F.H.; supervision, Z.C.; project administration, B.D. and Y.-M.C.; funding acquisition, Y.-M.C. All authors have read and agreed to the published version of the manuscript.

Funding

This work is financially supported by National Natural Science Foundation of China (22479161, 52274308 and U22B20144) and the Fundamental Research Funds for the Central Universities (24CX03012A).

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Conflicts of Interest

Authors J.N. and Z.C. were employed by the company CNOOC Tianjin Chemical Research and Design Institute Co., Ltd. Authors X.L. and F.H. were employed by the company CNOOC Shanxi Precious Metals Co., Ltd. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Catalysts 15 00744 g001
Figure 1. (a) Preparation process of the FeCoNiCrMo electrocatalyst. (b–d) the different resolution SEM images, (e,f) TEM images, and (g) electron diffraction image of the FeCoNiCrMo electrocatalyst. (h,i) SEM EDS elemental mapping of C, O, Ni, Fe, Co, Cr, and Mo for the FeCoNiCrMo electrocatalyst.
Figure 1. (a) Preparation process of the FeCoNiCrMo electrocatalyst. (b–d) the different resolution SEM images, (e,f) TEM images, and (g) electron diffraction image of the FeCoNiCrMo electrocatalyst. (h,i) SEM EDS elemental mapping of C, O, Ni, Fe, Co, Cr, and Mo for the FeCoNiCrMo electrocatalyst.
Catalysts 15 00744 g001
Catalysts 15 00744 g002
Figure 2. (a) XRD patterns with standard cards, (b) FT-IR spectrum of the prepared electrocatalyst. (c–i) XPS spectra of Fe 2p, Co 2p, Ni 2p, Cr 2p, Mo 3d, C 1s, and O 1s for the FeCoNiCrMo electrocatalyst.
Figure 2. (a) XRD patterns with standard cards, (b) FT-IR spectrum of the prepared electrocatalyst. (c–i) XPS spectra of Fe 2p, Co 2p, Ni 2p, Cr 2p, Mo 3d, C 1s, and O 1s for the FeCoNiCrMo electrocatalyst.
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Catalysts 15 00744 g003
Figure 3. (a) The polarization curves in 1 M KOH solution, (b) Tafel plots of the different electrocatalysts of the as-fabricated catalysts. (c) A comparison of overpotential at 100 mA cm−2 and Tafel slope for six samples. (d) A comparison of overpotential and time of durability between FeCoNiCrMo and other reported electrocatalysts. (e) Nyquist plots for the as-prepared materials. (f) The comparison of polarization curves before and after the 500th cycle. (g) Chronopotentiometry test at 100 mA cm−2 of the FeCoNiCrMo and FeCoNiMo electrocatalysts; internal illustrations show LSV curves of the FeCoNiCrMo catalysts before and after stability tests.
Figure 3. (a) The polarization curves in 1 M KOH solution, (b) Tafel plots of the different electrocatalysts of the as-fabricated catalysts. (c) A comparison of overpotential at 100 mA cm−2 and Tafel slope for six samples. (d) A comparison of overpotential and time of durability between FeCoNiCrMo and other reported electrocatalysts. (e) Nyquist plots for the as-prepared materials. (f) The comparison of polarization curves before and after the 500th cycle. (g) Chronopotentiometry test at 100 mA cm−2 of the FeCoNiCrMo and FeCoNiMo electrocatalysts; internal illustrations show LSV curves of the FeCoNiCrMo catalysts before and after stability tests.
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Figure 4. (a,b) SEM images of FeCoNiCrMo after the stability test. (c) Variation in surface element content before and after the stability test. (d–i) XPS spectra of Co 2p, Ni 2p, Cr 2p, Mo 3d, O 1s, and C 1s for FeCoNiCrMo after the stability test.
Figure 4. (a,b) SEM images of FeCoNiCrMo after the stability test. (c) Variation in surface element content before and after the stability test. (d–i) XPS spectra of Co 2p, Ni 2p, Cr 2p, Mo 3d, O 1s, and C 1s for FeCoNiCrMo after the stability test.
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Figure 5. The schematic reaction mechanism of the FeCoNiCrMo catalyst.
Figure 5. The schematic reaction mechanism of the FeCoNiCrMo catalyst.
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MDPI and ACS Style

Lv, J.-Y.; Zhang, Z.-J.; Zhang, H.; Nan, J.; Chen, Z.; Liu, X.; Han, F.; Chai, Y.-M.; Dong, B. A Quinary-Metallic High-Entropy Electrocatalyst with Driving of Cocktail Effect for Enhanced Oxygen Evolution Reaction. Catalysts 2025, 15, 744. https://doi.org/10.3390/catal15080744

AMA Style

Lv J-Y, Zhang Z-J, Zhang H, Nan J, Chen Z, Liu X, Han F, Chai Y-M, Dong B. A Quinary-Metallic High-Entropy Electrocatalyst with Driving of Cocktail Effect for Enhanced Oxygen Evolution Reaction. Catalysts. 2025; 15(8):744. https://doi.org/10.3390/catal15080744

Chicago/Turabian Style

Lv, Jing-Yi, Zhi-Jie Zhang, Hao Zhang, Jun Nan, Zan Chen, Xin Liu, Fei Han, Yong-Ming Chai, and Bin Dong. 2025. "A Quinary-Metallic High-Entropy Electrocatalyst with Driving of Cocktail Effect for Enhanced Oxygen Evolution Reaction" Catalysts 15, no. 8: 744. https://doi.org/10.3390/catal15080744

APA Style

Lv, J.-Y., Zhang, Z.-J., Zhang, H., Nan, J., Chen, Z., Liu, X., Han, F., Chai, Y.-M., & Dong, B. (2025). A Quinary-Metallic High-Entropy Electrocatalyst with Driving of Cocktail Effect for Enhanced Oxygen Evolution Reaction. Catalysts, 15(8), 744. https://doi.org/10.3390/catal15080744

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