Porous Heterogeneous Sulfide Nickel/Nickel Iron Alloy Catalysts for Oxygen Evolution Reaction of Alkaline Water Electrolysis at High Current Density

Abstract

Alkaline water electrolysis is the important pathway for the green hydrogen production, where oxygen evolution reaction (OER) is the rate-limiting step due to the sluggish reaction kinetics. Transition metal heterogeneous catalyst is the kind of important OER catalyst for alkaline water electrolysis due to its good performance, low price and environmental friendliness. In this work, the porous sulfide nickel@nickel iron alloy catalyst (i.e. NM/NS@Ni₃Fe) is prepared by the designed high-temperature vulcanization and multi-step electrodeposition method. The NM/NS@Ni₃Fe catalyst exhibits an outstanding OER performance in an alkaline environment, with a low potential of 1.53 V at high current density of 1000 mA cm⁻² and a low Tafel slope of 89 mV dec⁻¹. The excellent OER performance is attributed to the unique electronic structure of Ni₃S₂/Ni₃Fe heterogeneous interface and the catalyst layer with porous structure. The results indicate that Ni₃S₂ provides good electronic conductivity and the low electronegativity S atoms increase the formation of oxygen vacancies, which effectively improves the OER performance. In addition, the hydrophilic and porous structure of the electrode facilitates bubbles release and electrolyte flow at high current density. It provides the guidance for the design of porous heterogeneous OER catalysts with good-performance.

1 Introduction

Green hydrogen, a “zero carbon emission” clean fuel, has been identified as having a crucial role in future energy innovation [1]. The eco-friendly energy gas can be obtained by many paths, such as water electrolysis to produce hydrogen, photo-splitting of water to produce hydrogen, biomass to produce hydrogen, and nuclear power to produce hydrogen [2]. Among them, the coupling of renewable energy power generation (such as wind energy, and solar energy) and water electrolysis hydrogen production technology is considered to be the main technical path to obtaining green hydrogen in the future [3, 4]. However, water electrolysis is limited by the slow oxygen evolution reaction (OER) [5]. Therefore, the high-performance OER catalysts that can speed up the electrochemical reaction and promote the decomposition of H₂O molecules are crucial for the electrolysis of water, especially for high current density conditions (>500 mA cm⁻²) [6, 7].

In order to accelerate the decomposition of water molecules and obtain superior electrochemical performance at high current densities, the fabrication of heterojunction catalysts has become a research hotspot [8]. For NiS_x/Ni₃Fe, low reaction activation energies and unique coordination structures, are considered potential catalysts for water electrolysis [9, 10]. In this work, porous Ni₃S₂/Ni₃Fe heterojunction catalyst electrodes are successfully prepared to rely on vulcanization and multi-step electrodeposition techniques. The prepared NM/NS@Ni₃Fe catalyst exhibited an impressive OER performance in alkaline environment, showing a low high current potential of 1.53 V@1000 ma cm⁻² and a low Tafel slope of 89 mV dec⁻¹. This heterogeneous catalyst shows good practical applications, especially for OER reactions at high currents.

2 Materials and Methods

2.1 Chemicals

All chemicals used in this work are analytical grade. NiCl₂·6H₂O, NH₄Cl, FeCl₂·4H₂O, H₃BO₃, sulfur powder and HCl are purchased from Sinopharm Reagent Co. Commercial RuO₂ is purchased from Aladdin Reagent Co., and NM (wire diameter of 100 μm, 80 mesh) is purchased from China Kangwei Wire Mesh Co.

2.2 Preparation of NM/NS

The nickel mesh (1 × 2 cm²) is submerged in 3 M HCl for 20 min to remove the surface oxides, then ultrasonic cleaning in deionized water three times to remove HCl residue. Next, the cleaned nickel mesh is taken as the cathode and the high-purity nickel rod (99.5 wt%) as the anode. The electrodeposition solution is a mixture of 0.15 M NiCl₂·6H₂O and 2 M NH₄Cl. Electrodeposition is operated at room temperature for 10 min at a current density of 1000 mA cm⁻² [11]. After drying at 60 ℃ for 4 h, it is placed in a crucible and vulcanized in a tube furnace containing 60 mg of S powder at 300 ℃ for 60 min. Then annealed in the air for 60 min to room temperature and it is denoted as NM/NS.

2.3 Preparation of NM/NS@Ni₃Fe

Continuously, 0.21 M FeCl₂·4H₂O and 0.6 M H₃BO₃ are added into the solution and the solution is heated to 80 ℃. Next, NM/NS is maintained for 3 min at an electro-deposition current density of 10 mA cm⁻². The target electrode is denoted as NM/NS@Ni₃Fe.

2.4 Materials characterization

X-ray diffraction (XRD) data are acquired by X-ray scanning using a device (Bruker D8 Advanced) with a Rigaku 2550 light source (Cu Kα, λ = 1.5418 Å). The scanning range is 10–90° with a scanning rate of 0.2°/s. The surface morphologies are investigated by scanning electron microscopy (ZEISS GeminiSEM 300) at a scanning voltage of 3 kV.

2.5 Electrochemical Characterization

Electrochemical performances are estimated by a three-electrode system connected to an electrochemical workstation (Reference 3000) in 30 wt% KOH solution (pH = 14.6). A high-purity graphite rod is used as the counter electrode accompanied by a HgO/Hg reference electrode. Linear scanning voltammetric (LSV) curves are performed with the scan range of 1.0–1.9 V versus the reversible hydrogen potential (vs. RHE) and the scan rate is 5 mVs⁻¹. The potential (vs. RHE) is converted by the equation: E_(RHE) = E_(HgO/Hg) + 0.0591 × pH + 0.098. All LSV curves are corrected by ir-compensation in the three-electrode system. Tafel slopes are obtained using a logarithmic transformation base on LSV curves. EISs are tested at a constant potential (0.49 V vs. HgO/Hg) scanning from 0.01 to 100,000 Hz.

3 Results and Discussion

Commercial NM is used as a substrate because of its lower price, resistance to alkaline corrosion, and two-dimensional planes conferring fast bubble release. The preparation process is shown in Fig. 1a. Initially, the high-current (1 A cm⁻²) is used to electro-deposited porous nickel (PN), its porous property can confer a large loading area for catalysis. Then, the PN is vulcanized, the sulfur vapor reacts with the metallic Ni to produce Ni₃S₂ at high temperatures. Finally, the electrode NM/NS@Ni₃Fe is prepared by electro-depositing the nickel-iron alloy again in the nickel-iron solution. Figure 1b and c show NM/PN electrode that reflects a distinct porous property, which provides a large area for the active sites. Figure 1c shows the SEM image of NM/NS@Ni₃Fe, showing filamentous nickel sulfide on the surface relative to PN. The filamentous morphology can increase the roughness of the catalyst surface, which can improve the catalytic performance through its super-hydrophilicity.

As shown in Fig. 2, the composition of the prepared electrodes is determined by XRD testing. NM and PN exhibit strong characteristic response peaks at 44.50 (111) 51.85 (113) and 76.38 (220), respectively, indicating that Ni metal is the predominant component. After vulcanization, PN is vulcanized to generate Ni₃S₂, showing a strong response at the characteristic angles 21.75° (101), 31.10° (110), 37.78° (003), 49.93° (113), 55.16° (122), respectively. When the NiFe alloy is deposited on the surface, the characteristic peaks of Ni₃Fe do not emerge. The signal peak is blocked because of its low content and its close response to the Ni metal.

Fig. 1

a Schematic diagram of the preparation process of NM/NS@Ni₃Fe; b and c SEM images of NM/PN; d and e SEM images of NM/NS@Ni₃Fe.

Fig. 2.

XRD patterns of various electrodes (NM/PN; NM/NS; NM/NS@Ni₃Fe).

The alkaline OER performance of the prepared electrodes is examined in a 30 wt% KOH solution, illustrated in Fig. 3. NM/NS@Ni₃Fe exhibits a lower potential of 1.53 V at a high current density of 1000 mA cm⁻², which is much lower than the NM/NS (1.71 V) and commercial RuO₂ (1.60 V). At high current density (100–1000 mA cm⁻²), NM/NS@Ni₃Fe possesses a small Tafel slope of 89 mV dec⁻¹, which is smaller than 201 for NM/NS and 110 for RuO₂, indicating that NM/NS@Ni₃Fe has a faster electrochemical reaction kinetics. The excellent OER performance is attributed to the Ni₃S₂ and Ni₃Fe heterostructures, whose unique electron transport properties confer a low reaction energy barrier to the active site. Moreover, NM/NS@Ni₃Fe exhibits a smaller electrochemical impedance value of 0.62 Ω, which is smaller than 3.8 Ω for NM/NS and 1.2 Ω for RuO₂, demonstrating the faster electron transfer ability of NM/NS@Ni₃Fe. The correlation between phase shift and frequency further confirms that NM/NS@Ni₃Fe possesses better electron transfer capability with phase peaks shifted to higher frequencies (Fig. 3d).

Fig. 3.

Electrochemical properties of various electrodes (NM/NS@Ni₃Fe, NM/NS, RuO₂ and NM): a LSV curves, b Tafel slope, c EIS spectrum and d Relationship between frequency and phase shift.

4 Conclusion

Porous heterogeneous Ni₃S₂@Ni₃Fe electrodes are successfully prepared using a continuous hydrogen template method, high-temperature vulcanization and electrodeposition methods. NM/NS@Ni₃Fe exhibits a potential of only 1.53 V for a current density of 1000 mA cm⁻² and a lower Tafel slope of 89 mV dec⁻¹, superior to commercial RuO₂. Its good performance is attributed to the unique electronic structure of the Ni₃S₂ and Ni₃Fe and the porous properties of the electrodes. Such porous heterogeneous NM/NS@Ni₃Fe electrode not only exhibits good catalytic properties but also has the potential to be applied due to its facile and inexpensive fabrication.