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Article

Unveiling the Stacking Faults in Fe2B Induces a High-Performance Oxygen Evolution Reaction

by
Haoyu Li
1,
Xin Liu
1,
Xiaoyan Liu
1,2,
Jian Cao
1,2,
Lili Yang
1,2,
Huilian Liu
1,2,
Pinwen Zhu
3,
Qiang Zhou
3,
Xingbin Zhao
4,
Yanli Chen
1,2,*,
Maobin Wei
1,2,* and
Qiang Tao
3,*
1
Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University, Changchun 130103, China
2
National Demonstration Center for Experimental Physics Education, Jilin Normal University, Siping 136000, China
3
Synergetic Extreme Condition High-Pressure Science Center, State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, China
4
Institute of High Pressure Physics, School of Physical Science and Technology, Ningbo University, Ningbo 315211, China
*
Authors to whom correspondence should be addressed.
Catalysts 2025, 15(1), 89; https://doi.org/10.3390/catal15010089
Submission received: 26 December 2024 / Revised: 10 January 2025 / Accepted: 13 January 2025 / Published: 18 January 2025
(This article belongs to the Section Electrocatalysis)
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Versions Notes

Abstract

:
Fe2B is a potentially promising electrocatalyst for the oxygen evolution reaction (OER) due to its excellent electronic conductivity, which is superior to that of traditional oxide catalysts. However, the activity of Fe2B is still not satisfactory. In this study, meta-stable microstructure stacking faults (SFs) were incorporated into Fe2B through a one-step high-pressure and high-temperature (HPHT) method. Pressure suppressed atomic diffusion but formed SFs when the grain grew. Fe2B with SFs exhibited remarkable OER activity, with low overpotential values of only 269 and 344 mV required to reach current densities of 10 and 100 mA cm−2, respectively; because of the presence of SFs, the overpotential for the OER was reduced to only 67.7% of that of Fe2B without SFs at 10 mA cm−2. Theoretical and experimental investigations confirmed that these SFs regulate the d-band center of Fe2B toward the Fermi level, optimizing the catalytic site activity. Furthermore, SFs reduced the charge transfer between Fe atoms and boron (B) atoms, increasing the number of free electrons in the structure and thereby increasing conductivity. Finally, this study suggests a strategy to construct microstructures in crystals, providing new insights into designing excellent catalysts via microstructure engineering.

1. Introduction

Energy demand is on the rise with the rapid development of society, particularly the demand for green renewable energy [1,2,3,4,5]. However, most forms of green renewable energy are intermittent, such as solar energy and wind energy. The storage of intermittent energy is crucial to addressing the energy crisis [6,7]. Electrochemical water splitting is an effective route to transform intermittent energy into chemical energy; for instance, solar energy is converted into electricity, and the produced electricity is used to decompose water into hydrogen and oxygen [8,9]. Hydrogen, as a green renewable energy source with the highest energy density, is more convenient to store than electricity. Electrochemical water splitting involves the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), with the four-electron transfer process during the OER being the main factor affecting the kinetics of water splitting [10,11,12,13]. An appropriate OER electrocatalyst is necessary to enhance the efficiency of water splitting. In general, noble-metal-based catalysts, such as IrO2 and RuO2, can effectively improve the rate of the OER; however, their high prices and limited reserves restrict their commercial application [14,15]. Hence, low-cost and abundant catalysts are necessary as alternatives to noble-metal catalysts, following which large-scale applications of water splitting can be achieved.
Transition-metal-based catalysts exhibit remarkable water-splitting properties [16,17,18,19,20,21,22]. Iron (Fe) is the most abundant transition metal element in the earth, with a higher content than that of promising catalytic elements such as nickel (Ni) and cobalt (Co). Its remarkable electrical properties, non-toxicity, magnetic properties, thermal properties, and ductility result in its wide use in smelting, manufacturing, magnetic applications, and catalysis [23,24,25,26,27]. Although elemental Fe exhibits poor OER activity, its compounds have variable chemical valence states, making them prone to redox reactions during the OER [28]. Therefore, numerous Fe-based electrocatalysts have been reported. For example, Fe2O3, a common oxide of Fe, can facilitate O–O coupling in the OER process, while showing satisfactory activity [29]. Li et al. confirmed that the Fe in FeOOH can increase its chemical valence state to +3.5 to achieve self-activation during the *OOH step, and the reaction energy barrier can be effectively reduced when Fe is the active center [30].
However, the electrical conductivity of Fe oxides is not ideal and limits further improvement in their catalytic activity [31,32,33,34,35]. Among transition metal compounds, borides have received extensive attention because of their remarkable electrical conductivity and chemical stability [36,37,38,39,40]. Many reports, as well as our previous work, have demonstrated that transition metal borides have metallic properties and can provide high-speed channels for electron transfer during catalysis, making them one of the most promising potential electrocatalysts. In fact, the intermetallic compound iron boride (Fe2B) merits particular attention. For example, the B atom in reduced graphene oxide (rGO)-supported iron boride (Fe2B/rGO) can regulate the electronic structure and enhance the oxygen reduction reaction (ORR) performance [41]. Fe-B-O@Fe2B, synthesized using surface-activated amorphous Fe2B, exhibits remarkable activity for the OER; its overpotential is 273 mV at a current density of 10 mA cm−2 [42]. In addition, the highest abundance of Fe in the earth portends that iron borides should be lower-cost boride catalysts, even more so than the promising cobalt boride and nickel boride catalysts. Moreover, the weak electronegativity of Fe (weakest among Fe, Co, and Ni) allows Fe to offer more electrons to compounds with B; thus, iron borides are easier to synthesize than both cobalt borides and nickel borides [43,44,45]. However, most studies have reported the structure of Fe2B as amorphous or composited with other crystalline phases, which is not conducive to the analysis of its intrinsic activity. In addition, further regulation of the electronic structure of Fe2B is still critical to activate catalytic sites. Thus, the need for new strategies to obtain a single phase of Fe2B, with both remarkable conductivity and activity, is crucial.
Additionally, the introduction of microstructures (atomic vacancy, local distortion, dislocations, stacking faults [SFs; the crystal plane slips that result in different stacking orders], etc.) into catalysts can modulate their electronic structures, optimize active sites, and facilitate reaction dynamics, which has been confirmed [46,47,48]. However, the introduction of microstructures into transition metal boride (TMB) catalysts used for water splitting has been rarely reported. This is mainly because TMBs have high strength with strong chemical bonds, and they need to cross a high-energy barrier to induce microstructures within the structure. Moreover, the microstructures can always be eliminated by high synthesizing temperatures. Therefore, usually complicated treatments are necessary, and a one-step method to introduce microstructures into Fe2B is difficult.
The high-pressure and high-temperature (HPHT) method is an efficient route to synthesizing TMBs. In addition, it is an effective modification method that can capture meta-stable microstructures (such as dislocations and SFs) through rapid quenching. This process has been verified for classical MoS2 catalysts, in which HPHT introduced a considerable quantity of meta-stable SFs and enhanced not only the layers’ interaction to optimize conductivity but also the active sites in MoS2 [49]. Hence, the synthesis of a single phase of Fe2B, the construction of microstructures in Fe2B, and revealing the basic mechanism of Fe2B catalysts with/without microstructures are crucial for further optimizing the OER performance of Fe2B.
Thus, this study suggests a strategy for enhancing OER dynamics via the fabrication of SFs. Fe2B with and without SFs was synthesized via the HPHT method and high-temperature sintering, respectively. Pressure changed the growth dynamics of Fe2B with different morphologies. SFs adjusted the states of both Fe and B atoms, and hence, Fe2B with SFs delivered remarkable OER performance. The reasons for the superior performance of Fe2B with SFs were analyzed via theoretical calculations. Finally, this study offers a new insight into the fabrication of remarkable catalysts via microstructure engineering of TMBs.

2. Results and Discussion

2.1. Subsection Electrocatalyst Characterization

Fe2B was prepared via the HPHT method according to the flowchart shown in Scheme 1. Metallic Fe powder and amorphous boron powder were fully mixed at a stoichiometric ratio of 2:1 and then placed in an HPHT assembly. Fe2B–HPHT was synthesized at a pressure of 5 GPa and a temperature of 1300 °C in 20 min. As a comparative experiment, the same mixture was sintered in a high-temperature furnace at 1200 °C for 1 h to obtain Fe2B–HS. A detailed synthesis process is provided in the Section 3. We studied the structure, morphology, and electronic structure of Fe2B prepared with both methods, which was helpful in analyzing the origin of Fe2B’s catalytic activity.
Figure 1a,b show that the crystal structure of Fe2B is tetragonal and belongs to the space group I4/mcm. From the top view (Figure 1a), it can be seen that Fe exists in the form of metal bonds connected to each other, similar to a bird’s nest. In addition, B exists in the form of isolated atoms in the middle of the bird’s nest, resembling individual “eggs”. The metal bonds extend in all directions throughout the entire crystal structure (Figure 1b), ensuring that Fe2B has remarkable electrical conductivity. As shown in Figure 1c, X-ray diffraction (XRD) results confirmed that Fe2B was successfully synthesized via both methods. The XRD spectroscopy results of Fe2B–HPHT and Fe2B–HS matched with those of tetragonal Fe2B (JCPDS No. 89-1993). XRD spectroscopy results showed that Fe2B–HPHT has better crystallinity than Fe2B–HS. This may be because compared with conventional conditions, high pressure facilitates reaction dynamics to more easily cross the energy barrier. In addition, the atomic spacing can be shortened in a high-pressure environment, enhancing the overlap of electron clouds and thus facilitating the formation of Fe–B chemical bonds, which is conducive to the synthesis of TMBs [50].
The intrinsic morphology of bulk samples should be observed from the cross section (fracture surface). Accordingly, Fe2B–HPHT’s cross section was observed using scanning electron microscopy (SEM). Figure 2a–c show that the bulk of Fe2B–HPHT has a dense morphology with almost no pores (Figure 2a). From the magnified view of the cross-sectional morphology (Figure 2b,c), it can be clearly seen that the dense surface comprises closely connected micron-size grains with particle morphology. There were two kinds of grain sizes in the samples; one was 0.5–1 μm and another was about 5 μm. Therefore, from the grain growth mechanism, we inferred that big grains grow from the integration of small grains under HPHT. Simultaneously, under the influence of pressure, a good growth interface may be formed between the grains, and the gas or moisture in the material is squeezed to the sample surface, obtaining a good dense bulk material. SEM images, as shown in Figure 2d–f, indicated that the cross section of Fe2B–HS has a certain number of cracks or pores. In addition, the grain size of Fe2B–HS was about 5 μm, and the grain growth in Fe2B–HS followed the step-growth mechanism. Furthermore, during the growth process, the gas or moisture in the pre-pressed bulk sample was released from the material, because of which many pores or cracks appeared in Fe2B–HS. The difference in morphology between Fe2B samples prepared via the two methods is mainly attributed to the following: During the sintering process at atmospheric pressure and high temperature, the grain growth could relax to the most stable state, as a result of which the grains in Fe2B–HS have anisotropic growth rates in different directions and are generated via the step-growth mechanism. However, the pressure suppresses atomic diffusion and reduces the growth rate of the crystal planes, which have a higher growth rate in Fe2B–HS; thus, relatively isotropic growth rates in Fe2B–HPHT result in particle morphology.
The micromorphology of Fe2B was further characterized using transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). Figure 3a shows TEM images of Fe2B–HPHT. From the enlarged view of the green-framed area in the image (Figure 3b), it can be seen that the interplanar distances are 0.253 and 0.359 nm, corresponding well with the (200)/(020) and (110) facets of Fe2B, and the angle between the crystal faces is self-consistent, showing that the zone axis is [001]. The fast Fourier transformation (FFT) pattern of the pink-framed area shows that the exposed surface in Fe2B–HPHT is (001). The aforementioned results indicated that the crystal structure of Fe2B–HPHT is in agreement with the XRD results (Figure 1c). Meanwhile, SFs appeared at the exposed surface of Fe2B–HPHT (Figure 3b), with the FFT pattern confirming SFs (Figure 3b, insert pattern). To further confirm the presence of SFs, inverse FFT (IFFT) was performed on the pink-framed area (Figure 3c). The following observations were made from the pattern: The crystal surface slides along the [110] direction; the lattice points of the crystal surface return to the initial position after the four sliding lattice points and regular SFs periodically appear. Because the change in the microscopic region does not affect the overall structure of the Fe2B, the change cannot be observed in the crystal surface information and XRD results (Figure 1c and Figure 3b). The slip of the crystal faces along the [110] direction may be attributed to the largest spacing between the crystal faces of (110) in the structure, which can easily slip under the influence of an external force. For comparison, TEM and HRTEM images of Fe2B–HS were also examined (Figure 3d,e). The images show that Fe2B–HS has good crystallinity, with the detection of (002), (112), and (110) crystal faces. From HRTEM and FFT images (Figure 3e, insert pattern), it can be seen that SFs are not present in Fe2B–HS.
SFs were present in Fe2B–HPHT but not in Fe2B–HS; the possible mechanism of SF generation is shown in Figure 3f. Actually, the mechanism of SF formation arises from the different growth mechanisms in the two methods. In the synthesis of Fe2B–HS, Fe2B was grown in a stable environment, and the grains could achieve free growth with the step-growth mechanism. This process could form regular lattices via high-temperature-induced atoms to reach the most stable state. However, in the case of Fe2B–HPHT, the growth of grains was dominated by grain integration. If it was a martensite transformation, this process needed to rotate the grains with the same orientation; if it was a diffusion process, the process first needed to form defects and then eliminate the defects by other ways, such as high-temperature annealing. However, high pressure always suppresses atomic diffusion, which can impede both grain rotation and elimination of defects. Thereby, the integration of grains with different orientations by high temperature can generate a high inner stress and then actuate the crystal plane slip to form SFs, and these SFs can be maintained by pressure that suppresses atomic diffusion [49]. These meta-stable SFs may have generated a local strain and altered the local electronic structure, facilitating the electrocatalytic reaction.
X-ray photo-electron spectroscopy (XPS) was performed to investigate the surface chemical states of Fe2B prepared via the two methods. Fe 2p spectra are shown in Figure 4a. Binding energies of 706.89 and 719.85 eV belonged to 2 p3/2 and 2 p1/2 of Fe–B in Fe2B–HPHT, respectively, and binding energies of 707.51 and 720.46 eV belonged to 2 p3/2 and 2 p1/2 of Fe–B in Fe2B–HS, respectively [41,51]. However, binding energies of 709.01 and 721.57 eV in Fe2B–HPHT and those of 709.61 and 721.83 eV in Fe2B–HS corresponded to the presence of Fe3+ at the surface of the samples, which may have been due to the oxygen absorbed during the test environment or surface partial oxidation [52]. The detection of the Fe–B peak confirmed that Fe2B was successfully synthesized by both methods, and the result was in agreement with XRD and HRTEM analyses (Figure 1c and Figure 3b). Meanwhile, an Fe–B peak (187.75 eV) and a B–O peak (192.11 eV) in Fe2B–HPHT were also detected via B 1s spectroscopy (Figure 4b) [37,53]. Interestingly, the binding energy position of Fe–B in Fe2B–HPHT was lower than that in Fe2B–HS, about 0.62 eV lower in Fe 2 p3/2 and 0.66 eV lower in B1s. This phenomenon may be attributed to the presence of SFs in the structure of Fe2B–HPHT, which makes it possible to exist only via the transfer of a small number of electrons between Fe and B.

2.2. OER Performance

Electrocatalytic OER activities of Fe2B–HPHT and Fe2B–HS were tested in 1 M KOH solution using a standard three-electrode system. Figure 5a shows OER polarization curves of the samples. As a comparison sample, commercially purchased Fe foil was used to test for OER activity. The overpotentials of Fe foil, Fe2B–HS, and Fe2B–HPHT at a 10 mA cm−2 current density were 364, 290, and 269 mV, respectively. At a contrast current density of 100 mA cm−2, the overpotential of Fe2B–HPHT was only 344 mV, which is only ~67.7% of that of Fe2B–HS at 100 mA cm−2. The overpotential of Fe2B–HPHT at 100 mA cm−2 was even lower than that of Fe2B–HS at 50 mA cm−2 (387 mV). These results indicated that SFs play an important role in reducing overpotential at high current density. Fe2B–HPHT exhibited remarkable OER activity in 1 M KOH solution (Figure 5b). The slope of the Tafel curve represents the degree to which the rate of the electrode reaction responds to a potential change [54]. Figure 5c shows that Fe2B–HPHT had the lowest Tafel slope, indicating that the catalyst kinetics of Fe2B–HPHT were faster than those of Fe foil and Fe2B–HS. SFs in the structure of Fe2B–HPHT could optimize the adsorption/desorption ability for the hydroxyl group on the surface during the OER, due to reconstructing of the local electronic structure, and eventually accelerate the catalytic reaction kinetics. Moreover, electrochemical impedance and the electrochemically active surface area (ECSA) are also key factors in evaluating the performance of electrocatalysts. Figure 5d shows the electrochemical impedance spectroscopy results for Fe2B–HPHT and Fe2B–HS. Through fitting the equivalent circuit, it could be determined that the semicircle of Fe2B–HPHT was obviously smaller than that of Fe2B–HS, meaning that the former’s charge transfer resistance (Rct) was also relatively lower. This is because Fe2B–HPHT is a dense bulk material with low grain boundary resistance, and there exists a three-dimensional metal framework in its structure (Figure 1a,b), affording it good electrical conductivity. Hence, Fe2B–HPHT can provide a high-speed channel for charge transfer during catalytic reaction processes, particularly at high current density, and ultimately expedite the catalytic reaction. However, the ECSA result was not satisfactory. Figure 5e shows electrochemical double-layer capacitances (Cdl) of Fe2B–HPHT and Fe2B–HS; it can be seen that Cdl is directly proportional to the ECSA. The Cdl values of Fe2B–HPHT and Fe2B–HS were 0.103 and 0.126 mF cm−2, respectively, showing that the ECSA of Fe2B–HS is slightly larger than that of Fe2B–HPHT, which is consistent with SEM results (Figure 2). Electrocatalyst durability in the reaction process is also a standard to evaluate catalyst quality. The current density of Fe2B–HPHT showed negligible degradation in 1 M KOH solution for 20 h (Figure 5f). This result confirmed that Fe2B–HPHT has good durability. All the aforementioned results confirmed that Fe2B–HPHT exhibits remarkable OER activity. The performance comparison with the reported catalysts is shown in Table S1.
Theoretical calculations were performed to further confirm the effect of SFs on the catalytic performance of Fe2B–HPHT. In this section, the model with SFs is noted as Fe2B–SF and that without SFs as Fe2B. The theoretical models of Fe2B with and without SFs are shown in Figure S1; the Fe2B–SF model was constructed on the basis of HRTEM results (Figure 3c). Figure S2 shows the charge change of Fe2B with and without SFs. The results demonstrated that (a) the average charge of Fe in Fe2B is +0.25, (b) the average charge of B in Fe2B is −0.50, and (c) the charge of the Fe atom in Fe2B–SF is reduced (in Fe2B–SF, the average charge of Fe was +0.19 and that of B was −0.38). These results indicated that SFs prefer to stabilize when there exists low electron transfer from Fe to B. The results also confirmed that the lower Fe-to-B electron transfer in Fe2B–HPHT than that in Fe2B-HS is attributable to the presence of SFs in Fe2B–HPHT, as observed from the XPS results. Hence, it can be said that SFs change not only the states of Fe and B atoms but also the electronic structures, both of which modulate the OER performance.
As is known, the crystal surface slides along the [110] direction, so the edge of the slip plane is mainly exposed to the (001) crystal plane. We used the (001) crystal plane as the plane to study the OER catalytic mechanisms of Fe2B–SF, which helped us fully observe the effects of the slip plane on catalytic activity. The OER was performed in an alkaline environment, and two types of catalytic reaction processes were considered with the surface exposed to the terminal. Accordingly, the four electron transfer states of OH adsorption on the model surface are shown in Figure 6a, Figures S3 and S4. First, OH− attached to the surface of the catalyst is oxidized to obtain *OH, while losing an electron. Next, *OH undergoes proton-coupled electron transfer with another OH− to form *O. Subsequently, the *O traps another OH− and leaves the electron to form *OOH. Finally, proton-coupled electron transfer results in the release of O2 [55]. We compared the reaction energy barrier of difference termination of Fe2B and Fe2B–SF during the OER process (Figure 6b,c); we observed that the third reaction step (O* to OOH*) is the rate-determining step (RDS) for Fe2B and Fe2B–SF. Fe sites during the B termination of Fe2B and Fe2B–SF exerted a moderate adsorption effect on OH*, O*, etc., and thus exhibited higher OER activity. However, the activity of Fe sites during Fe termination was excessively high, leading to overall weak activity. The largest energy barrier of the RDS in Fe2B–SF’s B termination was 1.86 eV, corresponding to the theoretical overpotential of 0.63 eV. Fe2B had a higher energy change of the RDS, i.e., 2.20 eV, than that of Fe2B–SF. The partial density of states (PDOS) of Fe2B and Fe2B–SF was calculated to further investigate the effect of SFs on their catalytic activity (Figure 6d). Electron densities of Fe2B and Fe2B–SF at the Fermi level were non-zero, which revealed that they have metallic properties, and the electrons at the Fermi level were mainly Fe 3d orbital electrons. After the introduction of SFs into the structure, the d-band center of Fe2B moved toward the Fermi level (from −2.04 to −1.83 eV), increasing the activity of Fe sites and facilitating the catalytic reaction. The electron location function (ELF) was used to describe the electronic states in Fe2B and Fe2B–SF (Figure 6e,f, Figures S5 and S6). The results demonstrated that Fe2B–SF has more free electrons, which may be because the presence of SFs reduces the charge transfer between Fe and B (Figure 4 and Figure S2); in addition, the remaining electrons existed in the state of an electron gas in the structure, enhancing the conductivity of Fe2B–SF and increasing its catalytic activity in the catalytic reaction.

3. Experimental and Theoretical Calculations

3.1. Synthesis of Fe2B–HPHT

Fe2B–HPHT was synthesized via the HPHT method. The simple synthesis process was as follows. First, metallic iron powder (Aladdin Biochemical Technology Co., Ltd., Shanghai, China) and amorphous boron powder (Aladdin Biochemical Technology Co., Ltd., Shanghai, China) were mixed in a stoichiometric ratio of 2:1 and were fully ground in an agate mortar for more than 3 h. Next, the mixed powder was cold-pressed into a tablet with a 4 mm diameter and a 2.5 mm thickness. Finally, the tablet was placed in the HPHT assembly, in a 6 × 14400 KN cubic anvil apparatus, at a pressure of 5 GPa and a temperature of 1300 °C for 20 min so as to obtain Fe2B–HPHT. A square tungsten carbide anvil with an area of 23.5 mm × 23.5 mm was used to create the high-pressure environment (pressure limit was 6 GPa). The heater was a graphite tube, and the temperature was calibrated in advance with a W5%Re-W26%Re thermo-couple (temperature limit was 2300 °C). The calibration of pressure was completed according to the change in resistance from the pressure-induced phase transition of Bi (2.55 GPa, 2.69 GPa) and Ba (5.5 GPa). A pyrophyllite composite block with a volume of 32.5 mm × 32.5 mm × 32.5 mm was used for pressure transmitting. MgO was used as the insulation material. The method is consistent with that in a previous work [45].

3.2. Synthesis of High-Temperature-Sintered Fe2B (Fe2B–HS)

High-temperature-sintered Fe2B (Fe2B–HS) was synthesized via high-temperature sintering in a tube furnace. Metallic iron powder (Aladdin Biochemical Technology Co., Ltd., Shanghai, China) and amorphous boron powder (Aladdin Biochemical Technology Co., Ltd., Shanghai, China) were mixed in a stoichiometric ratio of 2:1 and were fully ground in an agate mortar for more than 3 h. The mixed powder was then cold-pressed into a tablet with a 4 mm diameter and a 2.5 mm thickness. Finally, the tablet was placed in a high-temperature tube furnace and sintered at 1200 °C for 1 h to obtain Fe2B–HS.

3.3. Characterization

Powder X-ray diffraction (XRD) was performed with a Rigaku D/Max 2550 X-ray diffractometer operating with Cu Kα X-ray radiation (λ = 1.5418 Å) (Rigaku, Tokyo, Japan). Scanning electron microscopy (SEM) images were obtained with an FEI Magellan 400L microscope (Hillsboro, OR, USA) operating at 18 KV. High-resolution transmission electron microscopy (HRTEM) images were obtained with a JEM-2200FS TEM system (JEOL, Tokyo, Japan) operating at 200 KV. X-ray photoelectron spectroscopy (XPS) spectra were recorded on an ESCALAB 250 analyzer (Thermo Scientific, Waltham, MA, USA) with Al Kα radiation. All the peaks were corrected by the C 1 s line at 284.8 eV as a standard, and curve fitting and background subtraction were accomplished.

3.4. Electrochemical Measurements

Electrochemical measurements were performed using a standard three-electrode system (CHI 760E (Chenhua Instrument Co., Ltd., Shanghai, China)) in 1 M KOH solution. Fe2B–HPHT/Fe2B–HS, an Hg/HgO electrode, and a carbon rod were used as the working electrode, reference electrode, and counter electrode, respectively. The overpotential was expressed as the value vs. the reversible hydrogen electrode (RHE), work, according to Evs.RHE = Evs.Hg/HgO + 0.059 × pH + 0.098. The process of working electrode fabrication was consistent with processes in previous reports [56,57]. The cross section of Fe2B–HPHT/Fe2B–HS tablets served as the exposed surface involved in the electrocatalytic reaction. The sample was fixed on an L-type copper electrode with conductive silver paste, followed by sealing of the electrode with a modified acrylate adhesive. The exposed surface area of the samples was measured using a Leica M125 C system [56]. For LSV measurements, the scan rate was set to 5 mV/s. A comparison of the catalytic activities of different materials was expressed mainly based on the overpotential (the difference between the actual oxygen precipitation potential and the oxygen equilibrium potential) obtained for the reaction at constant current density. In order to probe the electrochemical double-layer capacitance of the material, cyclic voltammetry (CV) was used at non-Faradaic overpotentials as the means for estimating effective electrochemical surface areas. With this procedure, a series of CV measurements were performed at various scan rates (20, 40, 60, 80, and 100 mV s−1) from 1.15 to 1.25 V vs. the RHE region, in which the sweep segments were set to 40 to ensure consistency. EIS was performed on the different materials at a potential of 1.5 V vs. the RHE. A sinusoidal voltage with an amplitude of 5 mV and a scanning frequency ranging from 100 kHz to 1 Hz were applied during the measurements.

3.5. Theoretical Calculations

The Vienna Ab Initio Simulation Package (VASP 5.4.4) was used for all spin-polarized density functional theory (DFT) calculations within the generalized gradient approximation (GGA) using the Perdew–Burke–Ernzerhof (PBE) functional formulation [58,59,60]. Projected augmented wave (PAW) pseudopotentials were used to describe the interactions between ionic cores and valence electrons. The valence electronic states were expanded in plane-wave basis sets with a cutoff energy of 450 eV. The DFT-D3(BJ) method was used to describe dispersion effects in the system. Partial occupancies of electronic bands were allowed using the Gaussian smearing method, with a smearing width of 0.05 eV. The electronic energy was considered self-consistent when the energy change was smaller than 10−6 eV, while a force change smaller than 0.02 eV/Å was used to determine convergence of the geometry optimization. The Fe2B (001) surface was modeled using p( 2 × 2 ) slabs separated by a ~15 Å vacuum spacing along the Z direction. Each periodic slab comprised five layers of the Fe–B plane. A 3 × 3 × 1 k-point mesh was used for surface slab calculations. All geometry structures were optimized using a force-based conjugate gradient algorithm until the forces on all the relaxed atoms were below 0.02 eV/Å. During the geometry optimization, the bottom three Fe2B layers were fixed, while the top two layers and adsorbates were fully relaxed. The d-band electronic structure of a transition metal is crucial for the adsorption energy of the adsorbate. The simplest model to describe the d-band is the d-band center. It can be obtained by
ε d = ∫ − ∞ ∞ n d ( ε ) ε d ε ∫ − ∞ ∞ n d ( ε ) d ε
where ε is the energy referring to E-Fermi and n d ( ε ) is the DOS projected into d-states.

4. Conclusions

Fe2B with SFs was captured via rapid annealing and pressure relief. SFs in Fe2B could move the d-band center to the Fermi level and induce more free electrons, resulting in high-speed electron channels in Fe2B–SF, which could expedite electron transfer during the redox reaction. In addition, SFs could reduce the energy barrier of the RDS by ~18.2%, which enhanced the reaction dynamics of the OER. Hence, it can be said that SFs in Fe2B optimized the catalytic sites in the structure and effectively increased the density of active sites. Finally, Fe2B with SFs delivered remarkable OER performance with lower overpotentials of 269 and 344 mV to reach current densities of 10 and 100 mA cm−2, respectively. The presence of SFs could effectively reduce the overpotential, particularly at high current density (100 mA cm−2), which was only ~67.7% of that of Fe2B without SFs. This study revealed the inner mechanisms of the formation of SFs in Fe2B and their effect on its catalytic performance, and it also suggested a new strategy to obtain remarkable catalysts via microstructure engineering of TMBs.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/catal15010089/s1: Table S1: Comparison of overpotentials to achieve 10 mA cm−2 for the OER of various kinds of iron-based electrocatalysts in 1 M KOH solution; Figure S1: Top view of the crystal structure of (a) Fe2B and (b) Fe2B–SF; Figure S2: (a) Charge of Fe and B in Fe2B and (b) charge of Fe and B in Fe2B–SF; Figure S3: OER process of the Fe2B structural model with B termination; Figure S4: OER process of the Fe2B–SF structural model with B termination; Figure S5: Plots of the electron location function for Fe2B: (001) crystal plane slice cut along the (a) Fe–Fe bond and (b) B–B bond and (c) (100) crystal plane slice; Figure S6: Plots of the electron location function for Fe2B–SF: (001) crystal plane slice cut along the (a) Fe–Fe bond and (b) B–B bond and (c) (100) crystal plane slice. References [35,61,62,63,64,65,66,67,68,69,70,71] are cited in the Supplementary Materials.

Author Contributions

Conceptualization: H.L. (Haoyu Li) and Y.C.; methodology, X.Z., P.Z., Q.Z. and H.L. (Haoyu Li); validation, L.Y.; investigation, H.L. (Haoyu Li) and J.C.; data curation, H.L. (Haoyu Li), X.L. (Xiaoyan Liu) and L.Y.; visualization: X.L. (Xin Liu) and H.L. (Huilian Liu); formal analysis: P.Z. and Q.Z.; writing—original draft, H.L. (Haoyu Li) and Y.C.; writing—review and editing: Y.C. and Q.T.; funding acquisition: Y.C., H.L. (Huilian Liu), P.Z., M.W. and Q.T. All authors have read and agreed to the published version of the manuscript.

Funding

The authors acknowledge funding support from the Science and Technology Development Project of Jilin Province (YDZJ202201ZYTS308, 20240211002GX, SKL202402004, 20220402031GH), the National Major Science Facility Synergetic Extreme Condition User Facility Achievement Transformation Platform Construction (2021FGWCXNLJSKJ01), the National Key R&D Program of China (2018YFA0703400), Industrial Technology and Development Project of Jilin Provincial Development and Reform Commission (2023C44-4), and the Open Research Fund of Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education (Jilin Normal University, 202405).

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

The authors acknowledge the “Solid Environment High Pressure and High Temperature (SEHPHT) Station” for all high-pressure experiments. The authors acknowledge Xinyi Yang and Yixuan Wang for help with testing.

Conflicts of Interest

The authors declare no conflicts of interest.

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Catalysts 15 00089 sch001
Scheme 1. Preparation process and oxygen evolution reaction (OER) of Fe2B via the high-pressure and high-temperature (HPHT) method and high-temperature sintering (HS).
Scheme 1. Preparation process and oxygen evolution reaction (OER) of Fe2B via the high-pressure and high-temperature (HPHT) method and high-temperature sintering (HS).
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Catalysts 15 00089 g001
Figure 1. (a) Top view of the Fe2B crystal structure, (b) side view of the Fe2B crystal structure, and (c) XRD results of Fe2B–HPHT and Fe2B–HS.
Figure 1. (a) Top view of the Fe2B crystal structure, (b) side view of the Fe2B crystal structure, and (c) XRD results of Fe2B–HPHT and Fe2B–HS.
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Figure 2. (a–c) SEM images of Fe2B–HPHT and (d–f) SEM images of Fe2B–HS.
Figure 2. (a–c) SEM images of Fe2B–HPHT and (d–f) SEM images of Fe2B–HS.
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Figure 3. (a) TEM images of Fe2B–HPHT; (b) HRTEM images of Fe2B-HPHT, with the insert showing a fast Fourier transformation (FFT) pattern of the pink-framed area; (c) the inverse fast Fourier transformation (IFFT) pattern of the pink-framed area in (b); (d) TEM images of Fe2B–HS; (e) HRTEM images of red-framed area in (d), with the insert showing an FFT pattern of the light-blue-framed area; and (f) a diagram of SF formation in Fe2B–HPHT.
Figure 3. (a) TEM images of Fe2B–HPHT; (b) HRTEM images of Fe2B-HPHT, with the insert showing a fast Fourier transformation (FFT) pattern of the pink-framed area; (c) the inverse fast Fourier transformation (IFFT) pattern of the pink-framed area in (b); (d) TEM images of Fe2B–HS; (e) HRTEM images of red-framed area in (d), with the insert showing an FFT pattern of the light-blue-framed area; and (f) a diagram of SF formation in Fe2B–HPHT.
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Figure 4. High-resolution XPS spectra of (a) Fe 2p and (b) B 1s.
Figure 4. High-resolution XPS spectra of (a) Fe 2p and (b) B 1s.
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Figure 5. (a) Polarization curves of Fe2B–HPHT, Fe2B–HS, and Fe foil in 1 M KOH electrolyte for the OER; (b) comparison of overpotentials of Fe2B–HPHT, Fe2B–HS, and Fe foil at 10, 50, and 100 mA cm−2 in 1 M KOH; (c) Tafel slope; (d) Nyquist plots; (e) double-layer capacitances; and (f) curve for current density vs. time (I–t) with Fe2B–HPHT for the OER.
Figure 5. (a) Polarization curves of Fe2B–HPHT, Fe2B–HS, and Fe foil in 1 M KOH electrolyte for the OER; (b) comparison of overpotentials of Fe2B–HPHT, Fe2B–HS, and Fe foil at 10, 50, and 100 mA cm−2 in 1 M KOH; (c) Tafel slope; (d) Nyquist plots; (e) double-layer capacitances; and (f) curve for current density vs. time (I–t) with Fe2B–HPHT for the OER.
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Catalysts 15 00089 g006
Figure 6. (a) OER process of the Fe2B–SF structural model with B termination; (b) the Gibbs free–energy (ΔG) diagram of Fe2B and Fe2B–SF on the (001) crystal plane with B termination; (c) the Gibbs free-energy (ΔG) diagram of Fe2B and Fe2B–SF on the (001) crystal plane with Fe termination; (d) partial density of states (PDOS) of Fe2B and Fe2B–SF; (e) plots of the electron location function for the Fe2B (100) crystal plane slice; and (f) plots of the electron location function for the Fe2B–SF (100) crystal plane slice. The * in figure represents the “active site”.
Figure 6. (a) OER process of the Fe2B–SF structural model with B termination; (b) the Gibbs free–energy (ΔG) diagram of Fe2B and Fe2B–SF on the (001) crystal plane with B termination; (c) the Gibbs free-energy (ΔG) diagram of Fe2B and Fe2B–SF on the (001) crystal plane with Fe termination; (d) partial density of states (PDOS) of Fe2B and Fe2B–SF; (e) plots of the electron location function for the Fe2B (100) crystal plane slice; and (f) plots of the electron location function for the Fe2B–SF (100) crystal plane slice. The * in figure represents the “active site”.
Catalysts 15 00089 g006
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MDPI and ACS Style

Li, H.; Liu, X.; Liu, X.; Cao, J.; Yang, L.; Liu, H.; Zhu, P.; Zhou, Q.; Zhao, X.; Chen, Y.; et al. Unveiling the Stacking Faults in Fe2B Induces a High-Performance Oxygen Evolution Reaction. Catalysts 2025, 15, 89. https://doi.org/10.3390/catal15010089

AMA Style

Li H, Liu X, Liu X, Cao J, Yang L, Liu H, Zhu P, Zhou Q, Zhao X, Chen Y, et al. Unveiling the Stacking Faults in Fe2B Induces a High-Performance Oxygen Evolution Reaction. Catalysts. 2025; 15(1):89. https://doi.org/10.3390/catal15010089

Chicago/Turabian Style

Li, Haoyu, Xin Liu, Xiaoyan Liu, Jian Cao, Lili Yang, Huilian Liu, Pinwen Zhu, Qiang Zhou, Xingbin Zhao, Yanli Chen, and et al. 2025. "Unveiling the Stacking Faults in Fe2B Induces a High-Performance Oxygen Evolution Reaction" Catalysts 15, no. 1: 89. https://doi.org/10.3390/catal15010089

APA Style

Li, H., Liu, X., Liu, X., Cao, J., Yang, L., Liu, H., Zhu, P., Zhou, Q., Zhao, X., Chen, Y., Wei, M., & Tao, Q. (2025). Unveiling the Stacking Faults in Fe2B Induces a High-Performance Oxygen Evolution Reaction. Catalysts, 15(1), 89. https://doi.org/10.3390/catal15010089

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