Magnetocatalysis: The Interplay between the Magnetic Field and Electrocatalysis

Abstract

Open AccessCCS ChemistryCOMMUNICATION1 Oct 2021Magnetocatalysis: The Interplay between the Magnetic Field and Electrocatalysis Guowei Li†, Qun Yang†, Kaustuv Manna†, Qingge Mu, Chenguang Fu, Yan Sun and Claudia Felser Guowei Li† *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Max Planck Institute for Chemical Physics of Solids, Dresden 01187 †G. Li, Q. Yang, and K. Manna contributed equally to this work.Google Scholar More articles by this author , Qun Yang† Max Planck Institute for Chemical Physics of Solids, Dresden 01187 †G. Li, Q. Yang, and K. Manna contributed equally to this work.Google Scholar More articles by this author , Kaustuv Manna† Department of Physics, Indian Institute of Technology, Delhi, New Delhi 110016 †G. Li, Q. Yang, and K. Manna contributed equally to this work.Google Scholar More articles by this author , Qingge Mu Max Planck Institute for Chemical Physics of Solids, Dresden 01187 Google Scholar More articles by this author , Chenguang Fu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027 Google Scholar More articles by this author , Yan Sun *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Max Planck Institute for Chemical Physics of Solids, Dresden 01187 Google Scholar More articles by this author and Claudia Felser *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Max Planck Institute for Chemical Physics of Solids, Dresden 01187 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202100991 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Magnetic fields are known as clean, economic, and effective tools to modify band and magnetic structures of materials. When coupled with catalytic processes such as the hydrogen evolution reaction (HER), they have the potential to control catalytic efficiency. Herein, we studied the magnetic response of a series of materials as HER catalysts, specifically ferromagnetic Co2VGa, Co2MnGa, and Ni, ferrimagnetic Mn2CoGa, and paramagnetic Pt. We found that all the Heusler compounds exhibit impressively high intrinsic activity. Most importantly, the presence of a magnetic field can change the HER efficiency significantly, regardless of the magnetic state of the catalyst. We observed boosting HER kinetics for ferromagnetic Ni and paramagnetic Pt, and a decrease of efficiency for ferro/ferrimagnetic Heusler alloys. Our calculations suggest that the binding energy between the reaction intermediate and the catalyst can be tailored effectively by controlling the number of transferred electrons. These findings highlight the vital role of magnetic fields in tailoring heterogeneous catalytic reactions that involve adsorption of reactants. Download figure Download PowerPoint Introduction As one of the most innovative and noninvasive tools, the magnetic field is widely used for controlled crystal synthesis, regulating physical properties, and promoting device performance.1–4 Both bulk band structures and surface properties can be reshaped in the presence of a magnetic field, thus inducing phenomena such as the quantum Hall effect and topological surface states and resulting in increased carrier mobility and fast electron–hole separation.5,6 These strategies are commonly used by chemists to design and tailor various chemical reactions. Magnetic fields have been used occasionally to influence the chemical synthesis, heavy metal adsorption, and photocatalysis processes.7–10 In contrast to tools such as high pressure and high temperature that require critical experimental conditions, a commercial permanent magnet is sufficient to influence many chemical reactions.11,12 For example, an extremely low magnetic field of 55 mT (600 times the magnetic field of earth) can accelerate catalytic adenosine 5′-triphosphate (ATP) synthesis by 50%.13 Recently, there has been growing research interest regarding the effects of magnetic interactions on the rate of hydrogen-energy-related reactions such as in water splitting and fuel cells (Figure 1a). With a moderate magnetic field, water-splitting kinetics can be promoted significantly by using a ferromagnetic electrode such as Ni, Co, and their alloys.14,15 The magnetism-independent reinforcement of catalytic efficiency can be explained by the magnetohydrodynamics effect induced by the Lorentz force, which acts either on ionic currents or oxygen/hydrogen bubble formation and release.16 Galán-Mascarós et al.17 found that the enhanced oxygen evolution reaction (OER) efficiency is closely related to the bulk magnetization of the catalysts and the pH value of the electrolyte. This highlights the vital role of the radical pair mechanism, which claims that the interconversion of the electronic singlet and triplet states of a radical pair (H•, OH•) determines the catalytic efficiency.18,19 However, subsequent studies found that both paramagnetic material (Pt) and diamagnetic topological Weyl semimetals (NbP, TaP…) have an equivalent effect in accelerating hydrogen/oxygen reduction reactions.20,21 The studies on these topological semimetals have highlighted the vital role of room-temperature magnetoresistance and changes in carrier mobility as a result of the magnetic field.21 Apparently, the mechanism of the interaction of the magnetic field with the catalytic process remains unclear. Figure 1 | Crystal and magnetic structures of Heusler alloys. (a) Schematic illustration of the role of magnetic field on the binding between hydrogen and metal d band. (b) Crystal structure of full-Heusler structure distinguishing the octahedral and tetrahedral positions. (c) FT EXAFS spectrum χ(R) of Co2MnGa bulk crystal at Co edge. (d) The partial DOS of Co and Mn atoms in Co2MnGa crystal. Download figure Download PowerPoint High-quality bulk single crystals are paramount for clarifying the contribution from the magnetic field as they have determined magnetic and crystal structures. Among them, the large family of Heusler compounds has attracted tremendous attention due to the high tunability of its crystal structures and physical properties via numerous element substitutions, predictable electrical and magnetic structures, promising applications, and so on.22–26 In particular, in magnetic Heusler alloys, the electron and surface properties are strongly coupled with the magnetic state, which results in a unique electron transfer behavior, tailored band structures, and even the emergence of robust topological surface states.27,28 The tunable composition, band structures, and magnetic structures of Heusler family compounds provide an ideal platform to investigate the relationship between spin, magnetic field, and surface reactions.29 Using ligand and ensemble effects of elemental substitution, it was found that the Co-based Heusler alloys have great selectivity for hydrogenation reactions. More interestingly, the reaction rate can be boosted significantly by element substitution without changing selectivity.30,31 Heusler compounds have recently been confirmed to be potential catalysts for OERs. The catalytic activities are closely related to the eg orbital filling of the active sites, suggesting the importance of the magnetic properties of the catalysts.29 In this work, we studied the catalytic behavior of several Co-based Heusler bulk crystals. Our results suggest that Heusler compounds are potential HER catalysts with high intrinsic activity. More importantly, we demonstrate theoretically and experimentally how the magnetic field interacts and controls the catalytic reaction efficiency via suitable manipulations of the band structures of the bulk crystals. With this, we can control the adsorption behavior of the reaction intermediates and thus accelerate or slow down the chemical reactions. Results and Discussion Synthesis and structure of Heusler single crystals Three full-Heusler alloys in the cubic space group with the chemical formula X2YZ (Co2MnGa, Co2VGa, and Mn2CoGa) were investigated in this work, and their structures are displayed in Figure 1b. These compounds crystallize either in a regular Heusler structure with a space group of F m 3 ¯ m (Co2MnGa, Co2VGa) or in an inverse Heusler structure with a space group of F 4 ¯ 3 m (Mn2CoGa). The structure can be viewed as a zinc blende-type sublattice constructed equally by X and Z elements, with the extra X element occupying the tetrahedral sites and Y sitting at the octahedral sites. In contrast to disordered substitutional alloys, Heusler alloys have well-defined crystal structures and a predictable bonding environment. There is a magnetic interaction between the two X atoms, and they are coupled ferromagnetically or antiferromagnetically with the Y atom.32 They form a covalent interaction through hybridization with the s and p states of the main-group element Z. The bonding states are well confirmed by the similarly extended X-ray absorption fine structure (EXAFS) spectroscopy between the Co2MnGa bulk single crystal and Co foil ( Supporting Information Figure S1). The Fourier-transform (FT) k3-weighted spectrum of Co K-edge shows the difference in detail (Figure 1c), with the well-defined main peak at 2.2 Å, representing the superposition of Co-Co contributions in the Co2MnGa structure. The small peak at 3.3 Å can be attributed to Co-Ga coordination, while the next more pronounced peak at 4.2 Å is a result of the Co-Mn and Co-Ga coordination shells with multiple scattering paths.33,34 Such unique bonding states in Heusler alloys lead to interesting properties in comparison with those of simple alloys. In addition to the appearance of covalent bonding interactions, another advantage of the full-Heusler alloys is the predictable band and magnetic structures, which are ideal for investigating the interaction with the magnetic field. A typical band structure of Co2MnGa is displayed in Supporting Information Figure S2, which suggests a half-metallic nature. The band structures are almost the same for the investigated Heusler alloys, except at the Fermi level, which is a result of the difference in valence electrons.35 The metallic density of electronic states at the Fermi level originates only from the spin-up channel, while the states for the spin-down channel form a gap (Figure 1d). Besides, partial density of states shows two peaks that are mostly derived from the Co and Mn d states. High-quality bulk single crystals of Co2MnGa, Mn2CoGa, and Co2VGa were synthesized using the Bridgman–Stockbarger method (details can be seen in the Supporting Information). The high quality of the as-grown crystals was confirmed by the well-defined Laue diffraction spots together with the cubic symmetry pattern (Figure 2a and Supporting Information Figure S3). Scanning electron microscopy (SEM) images and element analysis further confirmed high-quality crystal surfaces and composition ( Supporting Information Figures S4 and S5), with a near perfect stoichiometric ratio to 2∶1∶1 for all the crystals (X2YZ). Figure 2 | HER catalytic assessment of Heusler bulk single-crystal catalysts. (a) Laue diffraction pattern of Co2MnGa bulk crystal and the photograph of the investigated bulk crystals. (b) LSV curves and (c) the corresponding Tafel analysis of Co2MnGa, Co2VGa, and Mn2CoGa single-crystal catalysts. (d) A comparison of the TOF between Heusler compounds and reported state-of-the-art nanostructured catalysts. (e) LSV curves of the Co2MnGa and Co2VGa nanostructures, and the corresponding Tafel slopes (f). Download figure Download PowerPoint Electromechanical HER assessment of Heusler single-crystal catalysts To understand the intrinsic catalytic activities of the Heusler alloys, the grown single crystals were cut into cuboids along the crystallographic <001> direction to expose their (100) surfaces (inset Figure 2a). The electrodes used for the performance evaluation of the HER were prepared by attaching single crystals to Ti wires. All the electrochemical measurements were carried out using a conventional three-electrode cell containing 1 M KOH solution at a slow scan rate of 1 mV s−1 to minimize the capacitive current. The iR-corrected ( Supporting Information Figures S6–S8) linear sweep voltammetry (LSV) curves show that these electrodes achieved geometric current densities of 10 mA cm−2 at low overpotentials of 178, 220, and 256 mV for the bulk single crystals of Co2VGa, Mn2CoGa, and Co2MnGa, respectively (Figure 2b). In the meantime, the contribution of Ti wire and Ag paint to the HER activity can be neglected. From the extrapolation of the linear region of the LSVs (overpotential vs log j) (Figure 2c), Tafel slopes of ∼100 mV dec−1 for all the single-crystal electrodes were obtained. The Tafel slope values suggest a two-electron transfer process following a Volmer–Heyrovsky mechanism.36 As the figure of merit of overpotential is highly dependent on the specific surface areas and number of active sites, the turnover frequency (TOF) is used to reveal the intrinsic electrocatalytic activity of the Heusler bulk crystals.37 For this purpose, the electrochemical active surface areas (ECSAs) were determined from double-layer capacitance measurements (calculation details in the Supporting Information Figures S9–S14).38,39 TOF is then defined as the number of electrons consumed per active site per second at a given overpotential. The ECSAs were determined to be 11.75, 51.3, and 47.8 cm2, for Co2MnGa, Mn2CoGa, and Co2VGa, respectively, suggesting that these bulk crystals have much smaller active surfaces than those of nanostructured catalysts.40 TOFs were calculated at an overpotential of 200 mV by assuming that the transition metals in the (100) surfaces are active sites. As shown in Figure 2d, the bulk single crystals of Mn2CoGa, Co2MnGa, and Co2VGa exhibit high TOFs of 0.594, 1.2, and 1.475 s−1, respectively, which are comparable with those of the most reported high-performance transition-metal chalcogenide, phosphide, and nitride electrocatalysts.41–46 This highlights the high intrinsic catalytic activities of Heusler family alloys as HER catalysts. In addition to their high catalytic activity is the excellent surface stability of these bulk single-crystal catalysts. SEM images after HER measurements ( Supporting Information Figure S15) suggest the high degree of surface composition homogeneity. Although slight surface oxidation is observed from the corresponding energy-dispersive system (EDS) analysis ( Supporting Information Figure S16), the concentration of oxidized phases such as Co(OH)2 and CoOOH can be ignored judging from the Raman spectra ( Supporting Information Figure S17). The HER activity can be further boosted by decreasing the size of the Heusler alloys to the micrometer scale with a solution method. It only needs an overpotential of 129 (Co2VGa) and 159 mV (Co2MnGa) to deliver a current density of 10 mA cm−2, which is accompanied by a decreased Tafel slope of 78 and 83 mV dec−1, respectively (Figures 2e and 2f). The multiple-step Chrono potentiometric measurements at a current density of 15–400 mA cm−2 show a fast response of current to applied potential, suggesting good mass-transport properties and mechanical robustness toward hydrogen evolution ( Supporting Information Figure S18).47 HER behaviors of Heusler single-crystal catalysts under magnetic fields After the catalytic behaviors were established for the bulk single crystals, we explored the effect of an external magnetic field on the HER process. These crystals are ideal candidates for the following reasons: (1) The Currie temperature of the investigated Heusler crystals ( Supporting Information Figure S19) is significantly higher than room temperature (328 K for Co2VGa, 687 K for Co2MnGa, and 714 K for Mn2CoGa). (2) All the investigated crystals are soft magnetic materials with low coercivity that easily respond to the external magnetic field (Figure 3a). (3) The magnetoresistance of these bulk single crystals is extremely low at room temperature, which is important for us to exclude the contribution from changed resistivities with an external magnetic field ( Supporting Information Figure S20). (4) Most importantly, they have well-determined crystal surfaces, which are necessary to understand the change in local catalytic active sites. The magnetic field strength is determined to be 0.26 T in the magnetocatalysis experiments from the distance–field strength relation ( Supporting Information Figure S21). To understand the changes that occur on the active sites, ECSAs with and without a magnetic field were obtained by measuring the double-layer capacitance ( Supporting Information Figures S12–S14). As displayed in Figure 3b, there are no significant changes in the ECSAs in the presence of a magnetic field, indicating that the number of catalytic active sites remained unchanged. TOF values were calculated to reveal the changes in intrinsic activities (Figure 3c). In particular, the TOF for Co2VGa decreased greatly, by 43% to only 0.81 s−1 in the presence of the magnetic field. Similarly, Co2MnGa and Mn2CoGa also exhibited a sharp decline of 33% and 12%, respectively. Apparently, the activities of the local active sites are significantly reduced when an external magnetic field is applied. LSVs indicate a significant decrease in HER activity when a magnetic field is applied, and the activities are scaled with ECSAs (Figure 3d for Mn2CoGa crystal, Supporting Information Figure S22 for Co2MnGa, and Co2VGa single crystals). The decreased HER activity can be further confirmed by the corresponding current versus time measurements ( Supporting Information Figure S23), from which we can see the lower current densities in the presence of a magnetic field of 0.26 T. Figure 3 | HER catalytic behaviors under the external magnetic field. (a) The hysteresis loops for the three Heusler bulk crystals at room temperature (300 K). (b) The comparison of the ECSAs without and with the magnetic field. (c) The changes of TOF values for the investigated single-crystal catalysts without and with the magnetic field. (d) LSV curves of Mn2CoGa bulk crystal in the presence of the magnetic field. (e) The change in of ΔGH values for different adsorption sites in Mn2CoGa (e) and Co2MnGa (f) with and without magnetic fields. The insets show the changes of catalytic active sites. Download figure Download PowerPoint Now, we try to understand the role of the magnetic field theoretically from the viewpoint of thermodynamics. Before the discussion, it is important to exclude the “side effects” of the magnetic field in the catalysis reactions. The contribution from the changed electron transfer kinetics is excluded because of the extremely low magnetoresistance at room temperature. In addition, it is reasonable to exclude the magnetic-field-induced hydrodynamic effect because it will generally boost catalytic efficiency due to improved convection. For the HER process, the volcano plot based on Sabatier’s principle predicted the catalytic activities precisely, which states that the free energy of adsorption of hydrogen (ΔGH) should be close to zero for a good catalyst.48 As Co2MnGa and Mn2CoGa single crystals share the same element combination, we calculated the hydrogen adsorption behaviors on the (100) crystal surfaces. Without considering the spin polarization, the Mn sites are the active sites with a ΔGH value of 0.26 and 0.19 eV for Mn2CoGa and Co2MnGa catalysts, respectively (Figures 3e and 3f). These values are comparable with those of noble metal-based alloys such as Pt-Ni nanoparticles and PtSn4 single crystals,45,49 which explain the high TOFs and catalytic activities. Next, we considered the spin polarization at both Co and Mn sites to understand the effect of the magnetic field because all the investigated crystals are polarized under such a strong field ( Supporting Information Tables S1 and S2). As summarized in Figures 3e and 3f, the inclusion of spin polarization weakened the hydrogen adsorption ability at all sites for Mn2CoGa and Co2MnGa catalysts, including Mn, Co, and hollow positions. This explains well the decreased HER activities in the presence of a magnetic field. For Mn2CoGa crystals, although the originally active Mn sites were also significantly depressed, the Co sites were activated, which compensates for the loss of activity to a certain degree. These results also explain why the HER activity is reduced for Co2MnGa compared to Mn2CoGa. Boosting HER efficiency with magnetic fields It is disappointing that the magnetic field did not promote HER activity. However, we realized that the magnetic field could indeed “interact” with the catalytic active sites directly. With this in mind, we investigated other commonly used HER catalysts, including Ni and Pt. By depositing 100 nm Ni nanoparticles and 3 nm Pt nanoparticles on carbon paper (see Supporting Information for details), we found an increase in HER activity from the corresponding LSV curves (Figure 4a). Using a similar strategy, we analyzed the hydrogen adsorption on Ni and Pt nanoparticles. The ΔGH values for Ni and Pt were −0.21 and −0.13 eV for spin-unpolarized calculations, and decreased to only 0.1 and 0.05 eV, respectively, after the consideration of spin-polarization (Figure 4b), which is consistent with the improved HER activities. To understand the reason for the weakened hydrogen adsorption, Bader charge analysis on the charge density of Ni was carried out (Figure 4c). The results show that more electrons were transferred from Ni to H atoms when spin-polarization is considered (0.254 vs 0.239 electrons). Generally, more electron transfer suggests stronger bonding strength between the adsorbate and the substrate, which is contradictory to the results of our experiments. To answer this question, the interaction mechanism between the H intermediate and the Ni surface was investigated by the local DOS (LDOS). Orbital-decomposed DOS of the d orbital ( d x y , d x z , d y z , d z 2 , and d x 2 − y 2 ) projected onto the individual Ni atoms before and after H intermediate adsorption is shown in Figure 4d left and Supporting Information Figure S24, and indicates that the Ni 3d orbitals with z component ( d x z , d y z , and d z 2 ) are actively participating in H bonding and produce antibonding state orbitals at around 0.24 eV above the Fermi level (Figure 4d, left). According to the d-band center model of Hammer and Nørskov, the position of antibonding states determines the hydrogen adsorption energy and the corresponding HER activity of a catalyst. However, the antibonding states were located below the Fermi level once the Ni spins were polarized (Figure 4d, right). This means that the antibonding states are filled by the transferred electrons, thus weakening the hydrogen adsorption energy. The existence of the magnetic field results in the spin polarization of the metal surface, leading to the existence of two sets of d-band centers.50 Therefore, the minority and majority spin d-bands respond differently to the magnetic field and subsequently tailor the adsorption strength. With this in mind, we did the same analysis on the Heusler Co2MnGa crystal to understand the decreased HER activity with a magnetic field ( Supporting Information Figures S25 and S26). The interaction between Mn and H was calculated because Mn sites are active sites, as shown in Figure 3f. It was found that the Mn 3d-H 1s antibonding state orbitals are located at around 2 eV and 0.8 eV for spin-unpolarized and spin-polarized calculations, respectively, indicating weaker binding energy in the presence of a magnetic field, which is in good agreement with the Gibbs free energies as shown in Figure 3f. Unfortunately, the extremely weak H binding energy would result in a low reaction intermediate coverage, which explained the decreased catalytic efficiencies of the Heusler compounds. Figure 4 | Mechanism for the enhanced HER efficiency (a) The LSVs of Ni and Pt nanoparticles in the presence of a magnetic field. (b) ΔGH changes for Ni and Pt nanoparticles in the presence of the magnetic field. (c) Bader charge transfer between Ni atom and H atom for Ni catalyst without (above) and with a magnetic field (below). (d) The antibonding states between Ni and H without (left) and with a magnetic field (right). Download figure Download PowerPoint Conclusion We observed high intrinsic HER activity of Heusler bulk single crystals, which makes them a cheaper alternative to Pt-based electrocatalysts for the HER. The high tunability of crystal symmetry along with flexible adjustment of physical properties and magnetic moment via suitable selected site elemental substitution, make the Heusler family of compounds a rich platform for future HER-based practical applications. Most importantly, our results demonstrate that an external magnetic field can interact with the hydrogen adsorbate directly, resulting in adjustable binding energies, and thus slow down or accelerate the catalysis kinetics of HER catalysts. We believe that the results can help understand the role of the magnetic field in catalysis reactions and provide a new way for the development of high-performance HER catalysts. Supporting Information Supporting Information is available and includes Figures S1–S26. Conflict of Interest There is no conflict of interest to report. Funding Information This work was financially supported by the European Research Council (ERC Advanced grant no. 742068 ‘TOPMAT’). We also acknowledge funding by the DFG through SFB 1143 (project ID. 247310070), the Würzburg-Dresden Cluster of Excellence on Complexity and Topology in Quantum Matter ct.qmat (EXC2147, project ID 39085490), and DFG project HE 3543/35–1. Acknowledgments The authors thank Dr. Haojie Cao for performing the EXAFS measurement.