Electrospinning Machine | Effects of Steady Magnetic Fields on NiRuO2 Nanofibers for Electrocatalytic Hydrogen Evolution Reaction and Oxygen Evolution Reaction

As the energy crisis becomes increasingly severe, developing efficient and stable hydrogen/oxygen evolution catalysts is a key path to promote energy transition. However, the widely studied Ru-based electrocatalysts have inherent drawbacks of slow kinetics in the oxygen evolution reaction (OER) and face high water dissociation free energy barriers in the hydrogen evolution reaction (HER), severely restricting overall catalytic efficiency. Furthermore, the performance improvement of Ru-based catalysts is approaching the technical limit, urgently requiring exploration of new catalyst design strategies to break through the existing bottleneck.

Recently, Professor Long Yunze's team from Qingdao University (the first authors of the paper are Master's student Li Lingyun and Ph.D. student Liu Jinhua, and the corresponding authors are Yang Wenhua, Li Kai, Zhang Jun, and Long Yunze) published their latest research results "Effects of Steady Magnetic Fields on NiRuO₂ Nanofibers for Electrocatalytic Hydrogen Evolution Reaction and Oxygen Evolution Reaction" in the journal Journal of Materials Chemistry A. The researchers prepared nickel-doped ruthenium dioxide nanofiber catalysts via electrospinning combined with calcination. These catalysts exhibited excellent electrocatalytic performance under an applied steady magnetic field (800 mT), with the OER overpotential dropping to 280 mV and the HER overpotential being only 1 mV at a current density of 10 mA cm⁻². 

The study found that the magnetic field can drive a spin state transition from high spin to low spin, enhancing the interaction between catalytic active sites and reaction intermediates; it also enhances the hybridization of M-3d and O-2p orbitals, optimizing electron coupling effects and electrocatalytic performance. This work not only reveals the profound impact of spin state on the catalytic pathway but also opens new avenues for developing efficient water electrolysis catalysts and promoting the practical application of clean energy technologies.

 Figure 1: Morphological characterization of NiRuO₂ nanofibers (NFs).

As shown in Figure 1, various morphological characterizations proved the successful preparation of NiRuO₂ nanofibers (NFs). SEM (Fig. 1a) and TEM (Figs. 1b, c) show the rough fiber surface, which is conducive to exposing more active sites and promoting efficient electrocatalytic reactions. HRTEM (Fig. 1d) indicates that the rutile structure of RuO₂ remains intact; the selected area electron diffraction pattern (SAED, Fig. 1e) shows clear polycrystalline diffraction rings, confirming its polycrystalline nature; elemental mapping (Fig. 1f) confirms the coexistence and uniform distribution of Ni, Ru, and O elements within the fibers.

Figure 2: Structural and magnetic characterization of NiRuO₂ NFs.

As shown in Figure 2, characterization results from XRD (Fig. 2a), Raman (Fig. 2b), and XPS (Figs. 2c, d) confirm the successful doping of Ni into the RuO₂ lattice system, altering its electronic structure. The hysteresis loop (Fig. 2e) shows that NiRuO₂ NFs exhibit weak residual magnetization and moderate saturation magnetization at room temperature, with the introduction of Ni leading to a stronger magnetic response. M-T (Fig. 2f) shows obvious antiferromagnetic behavior, proving the main structure is RuO₂.

Figure 3: HER and OER electrocatalytic performance of NiRuO₂ NFs.

The NiRuO₂ NFs catalyst exhibits highly efficient and stable HER and OER performance (Figs. 3a, c), far exceeding that of pure RuO₂ NFs catalyst. Performance was further enhanced under a steady magnetic field: when the magnetic field strength was 800 mT, the OER overpotential was 280 mV and the HER overpotential was 1 mV at a current density of 10 mA cm⁻². It also showed good reaction kinetics (reduced Tafel slope in Fig. 3e) and very low charge transfer resistance (Fig. 3f). Additionally, NiRuO₂ NFs demonstrated excellent long-term stability both with and without the magnetic field (Fig. 3h).

Figure 4: Theoretical calculations and mechanism analysis.

The magnetic susceptibility curves obtained from quasi-in-situ M-T tests (Figs. 4a, b) show that the applied magnetic field increases the low-spin occupancy, inducing a spin state transition from high spin to low spin. Density functional theory calculations further reveal: a downshift of the d-band center (Fig. 4c), reducing the adsorption strength between active sites and oxygen-containing intermediates. Meanwhile, calculations of the key intermediate bond orders under high/low spin states (Fig. 4d) show that the low spin state can reduce the bond order of active radical groups, weakening intermediate adsorption and promoting reaction kinetics. Furthermore, the magnetic field significantly reduces the reaction energy barrier of the oxide path mechanism (OPM, Fig. 4e), effectively improving water electrolysis efficiency. In summary, this work successfully prepared nickel-doped RuO₂ nanofiber catalysts. Through the regulation of an external magnetic field, precise optimization of the electronic spin state (high-spin to low-spin transition) was achieved, significantly enhancing the HER and OER activity of the catalyst. This strategy provides new theoretical guidance and technical insights for designing efficient, magnetically tunable electrocatalysts.

Paper link: http://doi.org/10.1039/D5TA04061B