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Preparation of Nanoscale (VNbTaZrHf)C High-Entropy Carbides via Molten Salt Electro-deoxidation and Its Catalytic Performance of Hydrogen Evolution Reaction  PDF

  • Yang Yu 1 ✉
  • Chen Geng 2
  • Li Hui 2
  • Liang Jinglong 2
  • Hu Meilong 3
  • Hu Mengjun 4 ✉
1. Comprehensive Testing and Analyzing Center, North China University of Science and Technology, Tangshan 063009, China； 2. College of Metallurgy and Energy, North China University of Science and Technology, Tangshan 063210, China； 3. College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China； 4. Key Laboratory of Material Processing and Mold Technology, School of Mechanical Engineering, Chongqing Industry Polytechnic College, Chongqing 401120, China

Updated：2024-06-20

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Abstract

The nanoscale (VNbTaZrHf)C high-entropy carbide (HEC) powders with face-centered cubic structure were prepared by electro-deoxidation of metal oxides and graphite in CaCl2 at 1173 K. Appropriate temperature conditions are favorable for suppressing the in-situ sintering growth of HEC particles. Electrochemical performance tests were conducted in 1 mol/L KOH solution to investigate the catalytic performance of (VNbTaZrHf)C HEC. The catalytic performance of (VNbTaZrHf)C HEC for hydrogen evolution reaction (HER) was evaluated through polarization curves, Tafel slope, electrochemical impedance spectroscopy, and double-layer capacitance value cyclic voltammetry tests. Results show that the double-layer capacitance value of (VZrHfNbTa)C HEC is 40.6 mF/cm2. The larger the double-layer capacitance value, the larger the electrochemically active surface area. Due to the high-entropy effect and nanoscale structure of (VNbTaZrHf)C HEC, it exhibits superior catalytic performance to HER. This research provides a novel method for the preparation of HECs via molten salt electro-deoxidation.

Yeh[

1] and Cantor[2] et al synthesized multi-component alloys with single-phase crystal structure and proposed the concept of entropy stabilization in 2004. As a result, the idea of entropy stabilization attracts much attention. Various factors, such as chemistry, crystallography, and the design of different component composition, offer endless possibilities for the development of new alloys with excellent mechanical, oxidation-resistant, thermal, and corrosion-resistant properties. The concept of entropy stabilization materials has recently expanded into high-entropy carbides (HECs), borides, nitrides, and oxides in high-entropy ceramics.

HECs gain widespread attention due to their exceptional mechanical, oxidation-resistant, and corrosion-resistant properties. Han et al[

3] synthesized VNbMoTaWC5 and TiVNbTaWC5 HECs by low-pressure carbothermal shock method and prepared SiC with excellent thermal stability and antioxidant capacity. Li et al[4] achieved uniform composition and low oxygen content in single-phase (ZrTiTaNbMo)C powders by magnesium thermal reduction at 1623 K. Kovalev et al[5] prepared equimolar (TiZrHfNbTa)C HEC powders through high-energy ball milling, and the HEC powders exhibited excellent thermal stability. Xia et al[6] prepared (NbMoTaW)C and (NbMoTaWHf)C HECs with good mechanical properties using NbC, MoC, TaC, WC, and HfC powders as raw materials through the arc melting method. Although various methods, such as carbothermal reduction, magnesiothermic reduction, high-energy ball milling, and arc melting, have been used to synthesize HECs, the obtained materials are generally at the micrometer level and cannot be effectively applied in the electrocatalytic hydrogen evolution reaction (HER).

In electrochemical hydrolysis, the reaction at the cathode is considered as HER. HER is a promising green approach to produce hydrogen from renewable energy sources, and its efficiency depends on the electrocatalyst[

7–15]. Noble metals, such as Pt[16–18], Ir[19–21], and Pd[22–23], possess excellent electrocatalytic performance, but the high cost restricts their industrial production. Transitional metal phosphides[24–26], sulfides[27–29], carbides[30–32], and nitrides[33–35] have emerged as new materials for HER electrocatalysts due to their low preparation cost, diverse types, and environmentally friendly characteristic. However, the catalytic activity and stability of these materials are inferior. The high-entropy material exhibits better electrocatalytic water decomposition performance due to the continuous distribution of self-adsorption, easy control of electronic structure, and excellent structural stability. High-entropy alloys (HEAs), nitrides, sulfides, and phosphides are considered as the new generation electrocatalysts, relying on four core effects[36-37]. The ZrHfNbTa series ceramic HEA is the new research direction of HEAs. Adding V element to the system can effectively improve the mechanical properties of materials, and five metal ions can be combined with carbon to form covalent and ionic bonding in (VZrHfNbTa)C HEC materials. These materials have high adjustability in component and structure, showing great potential in the electrocatalysis field. However, the investigation of HECs as electrocatalyst is still at the infancy stage because the synthesis of nano-scale HEC materials is still challenging and costly.

In this research, VNbTaZrHf HEA and (VZrHfNbTa)C HEC powders were prepared through the one-step preparation method, which involved the direct electro-deoxidation of the stoichiometric metal oxides and graphite in the molten CaCl2 at 1173 K. A three-terminal electrochemical cell was used to investigate the catalytic performance of these HEC powders for hydrogen precipitation.

1 Experiment

The raw materials used in this research were mixed powders of Ta2O5, ZrO2, Nb2O5, HfO2, VO2, and graphite. VNbTaZrHf HEA and (VNbTaZrHf)C HEC were separately prepared according to the molar ratio of oxides and graphite with the total mass of 2 g. A binder was prepared by dissolving 0.04 g polyethylene glycol in isopropanol, then it was added to the raw material, and finally the mixed material was uniformly ground. The mixed material was dried at room temperature and pressed into samples at the pressure of 10 MPa using cylindrical grinding tool with diameter of 25 mm. Finally, the pressed samples were sintered at 523 K for 3 h.

The water-free CaCl2 (500 g) was placed in a corundum crucible and vacuum-dried at 673 K for 12 h. Then, the crucible was heated to 1173 K in a reactor with nickel flakes and graphite rods as the cathode and anode, respectively. The molten salt was pre-electrolyzed at 2.8 V to remove impurities and water. The cathode was made of mixed oxide precursors, and the anode was made of a graphite rod with diameter of 1 cm and height of 10 cm. The molten salt was subjected to electro-deoxidation at 3.1 V for 10 h. Finally, the products were cleaned by ultrasound, dried, and fully ground at 373 K.

The electrolytic voltage was provided by direct current power supply (DP310, MESTEK, Shenzhen, China). The phase of precursors and electro-deoxidation products was examined by X-ray diffractometer (XRD, D/max 2500PC, Rigaku, Japan). The microstructure and element distribution of precursors and electro-deoxidation products were characterized by scanning electron microscope (SEM, TESCAN VEGA II with Oxford INCA Energy 350) at backscattered electron (BSE) mode, energy dispersive spectrometer (EDS), field emission SEM (FESEM, ThermoFisher Scientific Quattro S), scanning transmission electron microscope (STEM, ThermoFisher Scientific Talos F200S), and high resolution transmission electron microscope (HRTEM). X-ray photoelectron spectroscope (XPS, ThermoFisher Scientific ESCALAB250Xi) was used to determine the elements of products and their oxidation states.

The electrochemical performance of HEC and HEA was evaluated by Chenhua CHI 660 instrument (Shanghai Chenhua Instrument Co. Ltd., China). The working electrodes were made by 80wt% VNbTaZrHf or (VNbTaZrHf)C powder+10wt% binder (polyvinylidene fluoride)+10wt% conductive agent (carbon black)+solvent (N-methyl pyrrolidone). The reference electrode was Ag/AgCl, and the counter electrode was platinum sheet.

The electrolyte was 1 mol/L KOH solution, and the area of electrode sheet immersed in the test solution was 1 cm2. The conversion equation from Ag/AgCl reference electrode to reversible hydrogen electrode (RHE) is shown in Eq.(1), as follows:

E(RHE)=E(Ag/AgCl)+0.0591×pH value+0.197 (1)

where E is the potential.

According to Eq.(1), the measured potential difference was 0.2224 V. The potentials measured in subsequent tests were converted to the ones concerning RHE. The catalytic proper-ties of the prepared materials were studied through linear sweep voltammetry (LSV), cyclic voltammetry (CV) method, AC impedance spectroscopy (EIS) method, and Tafel curves.

2 Results and Discussion

2.1 Electro-deoxidation of mixed raw material powders

Fig.1 displays XRD patterns of the VNbTaZrHf HEA and (VNbTaZrHf)C HEC precursors. Notably, the characteristic diffraction peaks of the oxides can be observed and no new phase can be detected. This result unequivocally demonstrates that the oxide mixture precursor remains inert to the chemical reactions, even after sintering treatment at 573 K for 3 h. During the sintering process, the oxide mixture precursor combines with binder to form a cohesive substance.

Fig.1  XRD patterns of VNbTaZrHf HEA and (VNbTaZrHf)C HEC precursors

Fig.2 shows SEM-BSE images and EDS element distributions of VNbTaZrHf HEA and (VNbTaZrHf)C HEC precursors. As shown in Fig.2a, the constituents of VNbTaZrHf HEA precursor exhibit a homogeneous dispersion: no apparent agglomeration or sintering traces can be observed between the particles. Distinct powder particles are discernible, and the sintering marks can hardly be observed. The graphite phase is represented by the black particles, as shown in Fig.2b. This further confirms that no chemical reaction occurs in the precursor during sintering at 573 K for 3 h. Furthermore, EDS analysis reveals the variation in the uniformity of particle sizes among different oxide phases. Specifically, Nb2O5, VO2, and ZrO2 exhibit relatively larger particle sizes, resulting in their aggregation, and the distribution of other phases is more uniform.

Fig.2  SEM-BSE images and EDS element distributions of VNbTaZrHf HEA (a) and (VNbTaZrHf)C HEC (b) precursors

2.2 Structure and morphology characterization

Fig.3 shows XRD patterns of the reduced products of VNbTaZrHf HEA and (VNbTaZrHf)C HEC samples after electro-deoxidation process. It can be observed that the products consist of body-centered cubic (bcc) HEAs and face-centered cubic (fcc) HECs. The electrolysis process obeys the deoxidation mechanism to form elemental metals and then promote the alloying process. Additionally, it is commonly reported that the electro-deoxidation process of HECs involves the release of metals from their oxides and an in-situ reaction with surrounding graphite atoms[

38].

Fig.3  XRD patterns of reduced products of VNbTaZrHf HEA and (VNbTaZrHf) HEC samples

Fig.4 shows SEM-BSE images and EDS element distributions of VNbTaZrHf HEA and (VNbTaZrHf)C HEC after electro-deoxidation process. As shown in Fig.4a, the powder particles are nodular and the nodular particle size is approximately 1 μm. As shown in Fig.4b, the reduced product has aggregates with sizes smaller than 500 nm. It can be seen that the particle size of HECs is significantly smaller than that of HEAs due to the inhibition of particle growth during the moderate temperature electro-deoxidation process. The deoxidation temperature has no significant effect on the in-situ sintering of the (VNbTaZrHf)C HEC particles with high melting point, resulting in the smaller particle sizes. EDS analysis reveals that the elements are uniformly distributed within the particles. According to the quantitative analysis data in Table 1, the atomic ratio of the two products is close to the designed atomic ratio of the alloy.

Fig.4  SEM-BSE images and EDS element distributions of VNbTaZrHf HEA (a) and (VNbTaZrHf)C HEC (b) after electro-deoxidation process

Table 1  EDS quantitative analysis results of VNbTaZrHf HEA and (VNbTaZrHf)C HEC after electro-deoxidation process (at%)
PowderCVZrHfNbTaO
VNbTaZrHf HEA 0.94 18.67 21.47 21.06 19.35 17.44 1.07
(VNbTaZrHf)C HEC 61.1 5.27 9.31 9.26 7.34 6.66 1.11

Fig.5 shows STEM images, EDS surface scanning results, HRTEM images, and corresponding fast-Fourier transform (FFT) patterns of the VNbTaZrHf HEA and (VNbTaZrHf)C HEC after electro-deoxidation process. According to STEM images, the VNbTaZrHf HEA powder has a dense block-like structure, and the (VNbTaZrHf)C particles have smaller sizes of less than 100 nm and exhibit cluster-like morphology. The STEM-EDS results demonstrate the uniform distribution of elements. HRTEM images reveal the polycrystalline nature of the product particles of VNbTaZrHf HEA and (VNbTaZrHf)C HEC. FFT patterns display discontinuous diffraction rings, indicating that VNbTaZrHf HEA has bcc structure, whereas (VNbTaZrHf)C HEC has fcc structure.

Fig.5  STEM images, EDS surface scanning results, HRTEM images, and corresponding FFT patterns of VNbTaZrHf HEA (a) and (VNbTaZrHf)C HEC (b) after electro-deoxidation process

Fig.6 shows XPS full-spectra analysis of the product powders of VNbTaZrHf HEA and (VNbTaZrHf)C HEC samples. The result indicates that both product powders contain the corresponding elements and a small amount of residual oxygen from the alloy. The VNbTaZrHf HEA powder has a small amount of cathodic carbon deposition, suggesting that the electro-deoxidation products are not contaminated by the impurities from the molten salt. High-resolution XPS results demonstrate the existence of strong interactions among the elements in both products. However, a small amount of Zr in the VNbTaZrHf HEA can be observed in the form of oxides, and Nb is susceptible to oxidation, thereby forming NbO after air exposure.

Fig.6  XPS spectra and high-resolution XPS spectra of VNbTaZrHf HEA (a–h) and (VNbTaZrHf)C HEC (i–p) after electro-deoxidation process: (a, i) overall spectra; (b, j) V element; (c, k) Zr element; (d, l) Hf element; (e, m) Nb element; (f, n) Ta element; (g, o) C element; (h, p) O element

2.3 Catalytic hydrogen precipitation performance of (VNbTaZrHf)C HEC powder

The electrochemical HER performance of the VNbTaZrHf HEA and (VNbTaZrHf)C HEC catalysts was tested in 1 mol/L KOH electrolyte. Fig.7a displays HER polarization curves of VNbTaZrHf HEA and (VNbTaZrHf)C HEC samples. It can be observed that (VNbTaZrHf)C HEC has an overpotential of 258.8 mV at the current density of 10 mA/cm2, which is much lower than that of VNbTaZrHf HEA (398.2 mV). This result indicates that the catalytic activity of (VNbTaZrHf)C HEC is higher than that of VNbTaZrHf HEA, which is possibly due to the strong electronic interactions resulting from the addition of carbon element. Fig.7b shows the fitted Tafel plots of VNbTaZrHf HEA and (VNbZrHf)C HEC samples, presenting the reaction kinetics in the electrochemical process. The Tafel slope results in Fig.7b and Table 2 show that the Tafel slope of (VNbTaZrHf)C HEC (69.0 mV·dec-1) is much smaller than that of Pt/C (99 mV·dec-1) and Ir/C (122 mV·dec-1). It is also much smaller than the Tafel slope of VNbTaZrHf HEA (146.3 mV·dec-1). Smaller Tafel slope indicates better reaction kinetics. There-fore, the reaction kinetics of (VNbTaZrHf)C HEC is much better than that of VNbTaZrHf HEA. Fig.7c shows EIS results of (VNbTaZrHf)C HEC and VNbZrHfTa HEA samples. The obtained EIS curve has a typical semicircular arc shape. Generally, the smaller the radius of arc curvature, the lower the resistance against the charge transport. Through comparison, it is found that the radius of curvature of VNbTaZrHf HEA is larger, which means that the charge transfer resistance is higher. Increasing the electrical

Fig.7  Polarization curves (a), Tafel plots (b), and EIS results (c) of (VNbTaZrHf)C HEC and VNbTaZrHf HEA samples

Table 2  Comparison of Tafel slopes of (VNbTaZrHf)C HEC, VNbTaZrHf HEA, and commonly used HER electro-catalysts
Electro-catalystPtIrHECHEA
Tafel slope/mV·dec-1 99 122 69.0 146.3

conductivity of the products can reduce the resistance against charge transport, and the carbon content in the product is closely related to the electrical conductivity. The addition of carbon is beneficial to improve the conductivity of the products, therefore reducing the resistance against charge transfer and ultimately improving the kinetics of catalytic reactions.

Fig.8a–8b display CV test results of VNbTaZrHf HEA and (VNbTaZrHf)C HEC samples to determine their double-layer capacitance values. Fig.8c shows the fitted capacitance curve with the current density as the vertical coordinate and the corresponding scanning rate as the horizontal coordinate. The slope of the fitted line in Fig.8c can reflect the magnitude of the double- layer capacitance value of corresponding material. Since the double-layer capacitance value of material is propor- tional to the electrochemically active area, the electro-chemically active area is used to compare the double-layer capacitance values in this research. The double-layer capacitance of (VNbTaZrHf)C HEC is 40.6 mF/cm2, which is larger than that of VNbTaZrHf HEA (26.8 mF/cm2). This result indicates that the addition of C element increases the number of active sites and has a significant effect on the activity of active sites.

Fig.8  CV curves of VNbTaZrHf HEA (a) and (VNbTaZrHf)C HEC (b); calculated double-layer capacitance of VNbTaZrHf HEA and (VNbTaZrHf)C HEC (c)

3 Conclusions

1) The overpotential of (VNbTaZrHf)C HEC and VNbTaZrHf HEA is 258.8 and 398.2 mV at the current density of 10 mA/cm2, respectively. The Tafel slope of (VNbTaZrHf)C HEC and VNbTaZrHf HEA is 69 and 146.3 mV/dec, respectively. EIS curves of both (VNbTaZrHf)C HEC and VNbTaZrHf HEA have typical semi-circular shapes, and the curvature radius of (VNbTaZrHf)C HEC is smaller.

2) The addition of carbon enhances the electrical conductivity of HEC and reduces the charge transfer resistance, improving the catalytic reaction kinetics.

3) The double-layer capacitance value of (VNbTaZrHf)C HEC is 40.6 mF/cm2, whereas that of VNbTaZrHf HEA is 26.8 mF/cm2, indicating that the catalytic performance of nanoscale (VNbTaZrHf)C HEC powders is significantly better than that of VNbTaZrHf HEA. This research provides new idea for the preparation of HECs via molten salt electro-deoxidation and expands the types of catalytic materials.

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