Magnetic-Field-Induced Strain Enhances Electrocatalysis of FeCo Alloys on Anode Catalysts for Water Splitting

Abstract

In water splitting, the oxygen evolution reaction (OER) performance of transition metal alloy catalysts needs to be further improved. To solve this problem, the method of an external magnetic field was used to improve the OER catalytic performance of the alloy catalyst. In this paper, FeCo alloys with different composition ratios were prepared by an arc melting method, and OER catalysts with different compositions were obtained by annealing treatment. Under the action of a magnetic field, all three groups of catalysts showed a better catalytic performance than those without a magnetic field. The overpotentials of Fe₃₅Co₆₅, Fe₂₂Co₇₈ and Fe₁₅Co₈₅ at a current density of 20 mA cm⁻² were reduced by 12 mV, 6 mV and 2 mV, respectively. It is found that, due to the magnetostrictive properties of FeCo alloys, the catalyst itself will generate strain under the action of a magnetic field, and the existence of strain may be the main reason for the enhanced OER performance of the magnetic field. Therefore, this work provides a new idea for the development of magnetic material catalysts and a magnetic field to improve the performance of catalysts.

1. Introduction

In recent years, with the development of industrialization, the burning of fossil fuels has caused serious environmental pollution and climate change, leading to an increasing demand for clean energy for global sustainable development. Due to its high energy density and carbon neutrality, hydrogen is a clean energy carrier and is considered a promising alternative to fossil fuels [1]. Among various hydrogen production methods, electrocatalytic water splitting is the most attractive one [2]. Compared with traditional steam reforming or pyrolysis routes that rely on fossil fuel consumption, electrocatalytic water splitting is theoretically clean, and zero carbon emissions can be achieved by using intermittent renewable electricity [3]. The electrolysis of water includes a cathode hydrogen evolution reaction (HER) and an anode oxygen evolution reaction (OER) [4,5]. The slow multi-step electron and proton transfer process of OER leads to high overpotential, and the corresponding high energy consumption is the bottleneck for improving electrochemical energy conversion efficiency [4]. Hence, researchers have made considerable efforts to explore highly active OER catalysts [6]. At present, Ru/Ir-based catalysts are the benchmarks of OER. However, their large-scale application is severely limited by their scarcity and high cost [7]. In recent years, transition metals, such as Fe, Co and Ni, with their abundant element reserves, environmentally friendly characteristic and low-cost, have received widespread attention due to their outstanding OER performance, and are widely regarded as outstanding alternatives to expensive ruthenium/iridium-based catalysts [5,8,9,10,11]. Compared with single-metal-based catalysts, metal alloys show better OER activity, which benefits from strain effects and synergistic effects, which have an impact on crystal structure changes and electronic modulation [12]. Binary alloys such as NiFe [10,13] and FeCo [14,15,16] have been reported as effective OER catalysts, but the performance improvement of these catalysts has reached a bottleneck. It is still a great challenge to obtain more efficient and stable catalysts.

In addition to the development of efficient and stable catalysts, the proposal of an external field has also aroused the interest of researchers, such as a gravity field [17], light field [18], ultrasonic field [19] and electric field [20], which can effectively improve the mass transfer on the electrode surface and change the reaction kinetics. Interestingly, the latest research confirms that coupling the magnetic field and electrochemistry is a promising new strategy to enhance the electrochemical reaction [21,22,23]. For example, Elias et al. prepared a Ni-W alloy as the HER electrode material, and, under the action of a magnetic field of 0.1–0.4 mT, the hydrogen production efficiency was significantly improved. Magnetohydrodynamic driving (MHD)-induced convection and the rapid detachment of H2 bubbles lead to a reduced polarization resistance and exposure of active sites, which, in turn, accelerate the catalytic reaction [24]. Westsson et al. confirmed the action of the magnetic field on the hydrogen adsorption of Pt-based catalysts with a core–shell structure [25]. Galán-Mascarós et al. showed that highly magnetic electrocatalysts, such as mixed Ni-Fe-Zn-based oxides, exhibit higher oxygen evolution reaction activity when a magnetic field is applied to the anode. After the introduction of a magnetic field, the ferromagnetic domain of oxygen atoms is increased to suppress the antiferromagnetic domain, which is beneficial to the formation of active sites in parallel configuration, and the catalytic reaction activity is improved by the evolution of singlet spin to triplet spin [21]. Li et al. put the electrolytic cell between permanent magnets with a medium magnetic field and proved the influence of the magnetic field on the Co₃O₄ catalyst. Under the action of the magnetic field, the OER performance was significantly improved [26]. Wei et al. further studied the possible improvement of the catalytic activity of the same material for the oxygen reduction reaction (ORR). By applying an external magnetic field, a small increase in the selectivity of the 4e-pathway was achieved. The magnetic field induces the degeneracy of the unpaired electron spins in Co₃O₄, resulting in enhanced electronic energy states that contribute to the activation energy of the electron transfer reaction, thereby promoting the redox reaction rate. This work minimizes the generation of H₂O₂ and improves energy utilization [27]. These recent studies have shown the influence of magnetic fields on the catalytic performance of transition metals and their oxides.

It is well known that the FeCo alloy is not only a catalyst with an excellent performance that plays an important role in the field of catalysis, but also a magnetostrictive material with a wide range of applications. Recently, it has been shown that magnetostriction enhancement occurs at the junction of the FeCo alloy bcc/(fcc + bcc) phases, and the enhancement is due to the precipitation of nanoprecipitated phases in the bcc matrix. Under the action of a magnetic field, greater strains can be generated [28,29]. The engineered strain effect in solid-solution alloys enhances catalytic activity by shifting the d-band center and changing the intermediate adsorption energy [30,31], which may facilitate a reduction in the reaction potential of the OER. Magnetic fields promote electrocatalysis in many ways, but, for now, the effect on metal alloy catalysts has rarely been reported.

Herein, an attempt was made to explore the changes in the OER electrocatalytic properties of FeCo alloys under magnetic fields. We prepared FeCo alloys as OER catalysts by a mechanosynthesis method, and the alloys were properly annealed. The electrochemical properties were tested in the presence and absence of a magnetic field and compared. The results indicate that, in the presence of a magnetic field, strain is generated within the FeCo alloy, resulting in a significant reduction in the OER overpotential and exhibiting a better OER performance than in the absence of a magnetic field. Therefore, this experiment provides a new idea to study the performance enhancement of magnetostrictive materials and catalysts, such as magnetic shape memory alloys with a magnetic field.

2. Experimental Process

2.1. Materials Preparations

The alloy compositions selected in the experiment were Fe₃₅Co₆₅, Fe₂₂Co₇₈ and Fe₁₅Co₈₅, respectively. The raw materials for preparing the samples were all Fe and Co elements with a purity higher than 99.95% (mass fraction), purchased from Zhongnuo New Materials Technology Co., Ltd. (Beijing, China) The test samples were first smelted in a vacuum arc furnace under the protection of an argon atmosphere. In order to ensure the uniform composition, the alloys were repeatedly turned and remelted four times, and, after cooling, a button-shaped ingot was formed. The samples were annealed in an argon atmosphere to make the alloy composition uniform and the atoms arranged in a highly orderly manner. The annealing temperature was 700 °C, kept for 24 h and quenched in salt water to room temperature. The cooled block alloys were polished into particles with 800-mesh sandpaper, separated with permanent magnets and washed with ethanol several times.

2.2. Material Characterizations

The phase structure of the sample was analyzed by X-ray diffraction (XRD) using the German Bruker D8 Discover Cu-Kα radiation X-ray diffractometer (Bruker, Rheinstetten, Germany). The morphological characteristics of the prepared samples were characterized by field emission environment scanning electron microscope (SEM, Quanta 450 FEG, Thermo Fisher Scientific, Waltham, MA, USA). An energy spectrometer (EDS) was used to determine the content of chemical elements in the product. Using Al-Kα excitation source, X-ray photoelectron spectroscopy (XPS, Escalab 250Xi, Thermo Fisher Scientific, Waltham, MA, USA) was used to study the surface states of samples with different Co contents. At room temperature, a vibrating sample magnetometer (VSM, Lakeshore 7307, Lake Shore Cryotronics, Inc., Westerville, OH, USA) was used to measure the magnetostriction of the sample under a maximum magnetic field of 900 mT.

2.3. Electrochemical Measurements

The electrochemical performances of the catalysts were studied in a standard three-electrode system using CHI750E electrochemical workstation. In particular, 10 mg catalyst and 5 mg carbon black (acid-treated XC-72) were placed into a 4mL centrifuge tube, and 600 μL deionized water and 400 μL isopropanol were added into the centrifuge tube, respectively. Finally, 30 μL Nafion concentration of 5% was added, and ultrasound was performed for 40 min to prepare electrode ink. The ink was dropped on the foamed copper (1 × 1 cm²) substrate as the working electrode, and the carbon rod and the saturated calomel (saturated potassium chloride as the filling liquid) electrode were used as the counter electrode and the reference electrode, respectively. The tests were all carried out in an alkaline environment and at ambient temperature. The electrochemical test was performed in a 1 mol/L KOH solution saturated with oxygen. Before the measurement, the working electrode was activated by cyclic voltammetry (CV) at a scan rate of 10 mV/s, and then linear sweep voltammetry (LSV) was performed at a scan rate of 5 mV/s to describe the OER performance. All of the potentials versus Hg/HgO electrode were calibrated to the reversible hydrogen electrode (RHE) according to the Nernst equation $\mathrm{E}\left(\mathrm{RHE}\right)=\mathrm{E}\left(\mathrm{SCE}\right)+0.0591\times \mathrm{pH}+0.24$ V.

3. Results and Discussion

A brief schematic diagram of the experimental setup is shown in Figure S1. The three-electrode system was placed in a magnetic field as shown. The magnetic field is provided by the VSM system, and the strength and direction of the magnetic field can be easily adjusted. It is important that the magnetic field strength remains constant during each step of the test. The magnetic fields applied in the experiments are all provided by the VSM system. The XRD patterns of the three samples Fe₃₅Co₆₅, Fe₂₂Co₇₈ and Fe₁₅Co₈₅ are shown in Figure 1. It can be clearly found that the three samples have different phase structures due to the different percentages of Co. When the Co composition is 65 at.%, the three typical peaks at 2θ are 45.2°, 65.7° and 83.3°, corresponding to (110), (200) and (211) crystal planes, respectively, and are the bcc-FeCo phase structure (PDF#50-0795). When the Co content is 78 at.%, in addition to the three peaks mentioned above, 2θ peaks of 44.2°, 51.5° and 75.8° appear in the spectrum, corresponding to (111), (200) and (211) crystals, respectively. This is a typical fcc-Co (PDF#15-0806) phase structure, which shows that the sample Fe₂₂Co₇₈ has a two-phase structure of bcc + fcc. When the Co content is 85 at.%, a single-phase structure of fcc is obtained, which is consistent with the results reported in the literature [30]. It is well known that the bcc-FeCo phase behaves as a replacement solid solution. Since the atomic radius of the Co element (0.126 nm) is smaller than that of the Fe element (0.127 nm), the substitution of Fe by more Co in the FeCo phase may cause the lattice shrinkage of FeCo products [32,33,34].

The microscopic morphology and particle size of the samples were analyzed by SEM. The SE IMAGES and EDS spectra of Fe₃₅Co₆₅, Fe₂₂Co₇₈ and Fe₁₅Co₈₅ are shown in Figure 2. The SEM in Figure 2a–c shows that the alloys powder obtained by sandpaper grinding are irregular micro-particles with fine dispersion and a particle size of 1–10 μm; the smaller particles will affect the magnetic properties of ferromagnetic materials [35]. The EDS in Figure 2d–f confirms that the composition ratios of Fe₃₅Co₆₅, Fe₂₂Co₇₈ and Fe₁₅Co₈₅ are in line with the stoichiometric ratio of the initial ingredients, which also shows that the surface of the alloy particles is slightly oxidized.

XPS was used to measure the electronic state of the elements of Fe and Co atoms in samples with different Co contents. Figure 3 shows the XPS narrow scan spectra of Fe 2p and Co 2p of the Fe₃₅Co₆₅ sample. The Co 2p spectrum (Figure 3a) clearly shows two spin-orbit double peaks at 780.2 eV and 796.2 eV, corresponding to Co²⁺ 2p_3/2 and Co²⁺ 2p_1/2, respectively, as oxidation peaks. The two oscillating satellites are at 786.2 eV and 802.3 eV (denoted as “Sat”). In addition, the zero-valent Co peaks are concentrated at 777.8 eV and 793 eV, indicating the presence of cobalt derived from FeCo alloys [36]. Similarly, the Fe 2p spectrum (Figure 3b) shows two spin-orbit double peaks for Fe²⁺ 2p_3/2 (711.8 eV) and Fe²⁺ 2p_1/2 (724.6 eV) and an oscillating peak (719.3 eV). In addition, the peak at 707.1 eV corresponds to zero-valent iron [37,38], which originates from the FeCo alloy.

Figure 4a–c shows that the FeCo samples exhibited a typical hysteresis loop at room temperature, indicating that the samples had ferromagnetic characteristics at room temperature, and their saturation magnetization was above 180 emu g⁻¹. The hysteresis loop in Figure 4d shows the anti-magnetic properties of the copper foam substrate. Additionally, it was shown that the catalysts were nearly saturated and magnetized under the action of a permanent magnetic field (~700 mT), which suggests that the impact of a permanent magnetic field on the magnetic catalyst was effective.

The magnetostriction of products with different initial Co contents was studied at room temperature using a VSM with a maximum magnetic field of 900 mT at room temperature. The magnetostriction coefficient test method is as follows: bond the strain resistance sheet to the material to be tested. Under the action of the magnetic field, the size of the magnet will change, and the stress will be applied to the strain resistance, thereby changing the resistance value of the strain resistance. The proportional relationship is transformed into the strain value of the specimen and displayed, which is the magnetostriction coefficient λ. The magnitude of magnetostriction is described by the magnetostriction coefficient λs (λs = λ_∥ − λ_⊥), as shown in Figure 5. In the Fe_100−xCo_x alloy system, the magnetostriction first gradually becomes larger with the increase in Co content, and the maximum magnetostriction occurs at the interface (x~65) where the bcc phase and the (bcc + fcc) phase intersect. The reason is that the precipitation of nano-precipitates in the bcc matrix will cause the tetragonal distortion of the bcc matrix, thereby increasing the large magnetostriction. However, with the continuous increase in Co content and the growth of nano-precipitated phases, large-sized fcc-Co phases gradually appear, which leads to a sharp decrease in magnetostriction [28,29].

In order to visualize the influence of the magnetic field on the OER performance, LSVs were carried out in a KOH electrolyte as shown in Figure 6a–c. It can be clearly seen that the OER performance of all three catalysts is enhanced under the action of the magnetic field. Interestingly, under the same magnetic field strength, the change in the sample Fe₃₅Co₆₅ is the most obvious, where the LSV curve is shifted to the left more and the overpotential decreases the most. For Fe₂₂Co₇₈, the LSV curve also shows a more pronounced leftward shift, which indicates that its OER properties also improve to some extent under the magnetic field, while the change is smaller compared to Fe₃₅Co₆₅. For Fe₁₅Co₈₅, on the other hand, the LSV curve shows only a small deviation in the presence or absence of a magnetic field. Intriguingly, all three samples appeared to perform better than before under the action of a magnetic field. The amount of variation in the OER performance of these catalysts under a magnetic field shows a clear agreement with the strain generated under the magnetic field. Therefore, a preliminary conclusion can be drawn that the applied magnetic field changes the OER performance of the catalyst by affecting the strain of the material. In addition, the variation in the OER properties of blank copper foam in the presence or absence of a magnetic field was also tested. Copper is the anti-magnetic substance from Figure 4 that exhibits weak magnetic properties, so the response to the magnetic field is relatively small. As shown in Figure S2, the LSV curve does not change significantly, but showed a slight deviation similar to Fe₁₅Co₈₅, which could be the influence of the magnetic field on the mass transfer of the solution [24,39,40]. Figure 6d shows the corresponding relationship between the magnitude of magnetostriction and the change in overpotential. It can be clearly seen that the greater the strain caused by the magnetic field, the more the overpotential decreases, which means the more obvious the effect of the magnetic field on the catalyst. Meanwhile, Figure S3 shows the impedance change with and without the presence of a magnetic field. From the figure, we can see that, under the action of a magnetic field, the catalysts all exhibit smaller impedance, which is consistent with the corresponding electrocatalytic OER activity.

In addition, the capacitance (C_dl) of the electrochemical double layer (EDL), which is related to the electrochemically active surface area (ECSA) of the catalysts [41,42], was determined using the CV curves in the potential ranges of 0.1−0.15 V. As shown in Figure S4, it can be seen clearly that the C_dl values of the three samples under a magnetic field were greater than those under no magnetic field. This further confirms that the external magnetic field increases the active area and improves the catalytic performance of the material.

To further verify the effect of magnetic-field-induced strain on the OER performance, the OER performance of sample Fe₃₅Co₆₅ was tested at different magnetic field strengths. The LSV curves measured under the gradient of a magnetic field strength of 0, 300 and 700 mT are shown in Figure 7 and Table S1. An interesting phenomenon can be seen: as the magnetic field strength increases, the LSV curve without the magnetic field (black line) starts to gradually shift to the left; when the magnetic field strength reached 300 mT (red line), a significant shift started to appear, and as the magnetic field strength increased to 700 mT (green line), the curve continued to shift toward a lower voltage and the current density became larger. It can be concluded that there is a correlation between the magnitude of the magnetic field strength and the degree of improvement in the OER performance. In fact, the magnetostriction of the FeCo catalyst has reached saturation at approximately 500 mT, and the overpotential change in the sample at this magnetic field strength has also reached the maximum. A further increase in the magnetic field will not further improve the performance of the catalyst (Figure S5), but the maximum magnetic field of 700 mT was finally chosen to maintain the consistency of the experiment. Table S2 summarizes examples of magnetic fields affecting electrochemical catalytic reactions in recent years [21,24,25,27,43,44,45,46,47,48].

Based on the above results, it can be explained in conjunction with the magnetostriction curve of Figure 5. As the magnetic field strength increases, the strain generated within the alloy increases and the improvement in OER properties becomes greater and greater (Figure 8). However, when the magnetic field increases to a certain strength, the performance no longer changes (Figure 8b). The reason is that the magnetostriction of the FeCo alloy reaches saturation and the strain no longer occurs as the magnetic field continues to increase, so the OER performance also does not change anymore. Under the action of a magnetic field, magnetostriction occurs, and the strain caused by magnetostriction causes the metal atoms to have less d-orbital overlap, which reduces the bandwidth and increases the d-band center energy, thereby driving more antibonding states above the Fermi level and allowing the adsorbates to interact with them, increasing the binding energy of the adsorbates, which, in turn, enhances the OER performance [30,31,49,50].

4. Conclusions

Fe_100−xCo_x alloys with different composition ratios were prepared by a mechanical alloying method, and three OER catalysts with different phase structures were obtained after annealing. After applying an external magnetic field, the three catalysts showed different enhancements in performance due to the different strains generated, with Fe₃₅Co₆₅ increasing the most significantly and Fe₂₂Co₇₈ also showing more pronounced changes, whereas Fe₁₅Co₈₅ showed minimal changes, which corresponded well to their magnetic strains. The relationship between the strain magnitude and enhanced catalytic performance is also well illustrated by comparing the performance of different magnetic field gradients. Therefore, in this study, the magnetic field induced the strain of the catalyst, and the strain effect may be the main reason for the enhanced OER performance.