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Article

Vanadium Nitride Supported on N-Doped Carbon as High-Performance ORR Catalysts for Zn–Air Batteries

by
Yidan Fu
1,2,
Lina Han
1,2,*,
Pengfei Zheng
1,2,
Xianhui Peng
1,2,
Xianglan Xian
1,2,
Jinglin Liu
1,2,
Xiaoyuan Zeng
2,
Peng Dong
2,
Jie Xiao
2 and
Yingjie Zhang
1,2,*
1
School of Materials Science and Engineering, Kunming University of Science and Technology, Kunming 650000, China
2
National and Local Joint Engineering Laboratory for Lithium-Ion Batteries and Materials Preparation Technology, Kunming University of Science and Technology, Kunming 650000, China
*
Authors to whom correspondence should be addressed.
Catalysts 2022, 12(8), 877; https://doi.org/10.3390/catal12080877
Submission received: 17 July 2022 / Revised: 4 August 2022 / Accepted: 6 August 2022 / Published: 9 August 2022
(This article belongs to the Special Issue Advanced Catalysts for Achieving Hydrogen Economy from Liquids)
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Abstract

:
It is desirable to prepare low-cost non-noble metal catalysts using a simple and efficient method. Herein, we display for the first time that nitrogen-doped hierarchical porous carbon-supported vanadium nitride (VN/NC/C-x) catalysts can be regulated by dicyandiamide (DCDA). The introduction of DCDA not only effectively controls the pore structure, but also plays an important role in adjusting oxygen vacancies and d-electrons. In addition, DCDA is not only a significant raw material for the N-doped carbon, but also a nitrogen source for the preparation of vanadium nitride. The VN/NC/C-3 catalyst was prepared after optimization of the preparation parameters, and the macro/micro structure demonstrates a superior ORR performance in alkaline media with a positive onset potential of 0.85 V and a half-wave potential of 0.75 V, the limiting current density is as high as 4.52 mA·cm−2, and the Tafel slope is only 75.54 mV·dec−1. The VN/NC/C-3-based Zn–air battery exhibits a highest peak power density (161.82 mW∙cm−2) and an excellent energy density (702.28 mAh·kgZn−1 and 861.51 Wh·kgZn−1). This work provides a valuable synthetic approach for the preparation of other transition metal nitride catalysts with a relative economic value and high performance.

Graphical Abstract

1. Introduction

In recent years, Zn–air batteries have been regarded as the most promising clean energy by scientists for alleviating the energy crisis because of their high theoretical energy density, good safety, and environmental friendliness. However, the biggest technical challenge in practical applications is the slow kinetics of the air cathode, thus requiring the development of efficient catalysts [1,2,3,4]. Platinum is currently generally considered to be one of the most effective ORR catalysts; unfortunately its high cost and poor durability make it impractical for large-scale use [5,6,7]. To address this issue, non-noble metal catalysts with a high activity and durability need to be synthesized.
Transition metal nitrides (TMNs) are interstitial compounds formed by the insertion of nitrogen atoms into the crystal lattice of the parent metal [8,9]. TMNs are widely used as catalysts because of their outstanding electrical conductivity and superior catalytic activity. Generally speaking, TMNs have metallic, ionic, and covalent bonds, among which M–N bonds lead to the expansion of the transition metal lattice and the contraction of the metal d-band [10,11,12]. The d-band produces a noble metal-like behavior in catalysis, because of d-band defects created by the introduction of nitride atoms and a higher density of states (DOS) near the Fermi level [13,14,15]. Therefore, the adsorption between the reactants and the surface of TMNs is optimized, enabling TMNs to become the first choice for electrode materials and catalysts in the future. Among TMNs, vanadium nitride (VN) has received extensive attention because of their corrosion resistance properties and the availability of d electrons for efficient catalysis for various reactions [16,17,18]. However, the ORR performance of VN is too poor to meet the practical applications.
To address the above problems, the catalytic performance of VN can be improved by rational strategies. (1) The conductivity and electron transfer rate of VN-based catalysts can be improved by loading nitrogen-doped carbon materials with a large specific surface area. Furthermore, nitrogen-doped carbon supports can suppress the agglomeration of nanoparticles, and the synergistic effect between VN and NC can further enhance the catalytic activity. Zhao et al. [19] prepared Co/VN@NC through the hard template removal method with a good ORR performance (E1/2 = 0.83 V), and DFT calculations indicated that NCs play an important role in enhancing the conductivity of the catalysts. (2) Introducing a second metal element or oxygen vacancies to increase active sites. The introduction of the second metal element can redistribute the charge, resulting in a lower reaction barrier and better catalytic activity and stability of the catalyst. Xu et al. [20] obtained CoNx/Zn-NC by introducing Zn on nitrogen co-doped porous carbon and loading CoNx, forming Co and Zn dual active sites, and exhibiting an excellent ORR electrocatalytic performance (E1/2 = 0.85 V). Luo et al. [21] prepared the V0.95Co0.05N catalyst through ammonia complexation and then nitridation in an ammonia atmosphere, and the results showed that it had a comparable performance to Johnmn Matthey (JM) 20 wt% Pt/C in 0.1 M KOH solution. Zhang et al. [22] reported that VCoN materials exhibited an excellent catalytic activity for ORR, which shows an ORR performance compared with commercial 20% Pt/C.
On the other hand, oxygen vacancy defects are also an important factor to increase the active sites. It has been reported that the introduction of oxygen vacancy defects into the catalyst surface can tune the electronic structure and coordination environment of the catalyst, so that transition metals near the oxygen vacancies can reduce the activation energy and thus significantly improve the electrocatalytic activity. For NiO/CoN [23], CoFe@Fe3N [24], and Ti0.95Co0.05N@CNx [25], etc., it has been shown that ORR activity can be regulated by changing the degree of oxidation. However, the current commonly used method is destructive to the structure, and the introduction of oxygen vacancies is unstable. Therefore, it is of great significance to explore mild methods to introduce oxygen vacancies.
Studies on transition metal nitrides have been reported, and a common synthetic method to obtain such catalysts is to prepare vanadium pentoxide, followed by nitridation in an ammonia atmosphere. Tang et al. [26] employed a facile solvothermal method to fabricate vanadium nitride (VN), which were then converted to nitride in NH3 at 500 °C. Zhao et al. [27] prepared bulk VN by calcining NH4VO3 in air and then in an NH3 atmosphere. However, this method has various disadvantages. In the first place, dealing with ammonia gas is difficult because it is harmful [28]. In the second place, complex mechanical devices hinder large-scale applications [27,29]. Therefore, our way forward should be to rationally design a simple, ammonia-free synthetic route.
In this work, a facile, safe, and inexpensive synthetic method was employed to prepare nitrogen-doped hierarchically porous carbon-supported vanadium nitride (VN/NC/C-x). Dicyandiamide (DCDA) is not only a significant raw material for N-doped carbon, but also a nitrogen source for the preparation of vanadium nitride. By adjusting the amount of DCDA, the surface chemical state and surface area of VN/NC/C-x can be well tuned. XPS confirmed that varying the amount of DCDA can tune the ratio of V with a low valence in the catalyst, thereby enriching d-electrons, promoting O2 dissociation, and achieving an optimal catalytic performance. Furthermore, the content of V-N and pyridinic-N in the VN/NC/C-3 catalyst was significantly higher than for the other catalysts. As expected, VN/NC/C-3 exhibited an excellent ORR catalytic activity in 0.1 M KOH with a higher onset potential, maximum limiting current density, and minimum Tafel slope. At the same time, the VN/NC/C-3-based Zn–air battery exhibited a larger specific capacity and higher peak power density.

2. Results and Discussion

2.1. Material Characterization of VN/NC/C-x

Field emission scanning electron microscope (FESEM) tests were used to study the effect of the amount of DCDA on the morphology of VN/NC/C-x catalysts. Figure 1a and Figure S1 demonstrate that the prepared VN/NC/C-x catalysts exhibited interconnected spheroid-like structures with diameters ranging from 40 to 60 nm. The spherical particles agglomerated seriously in the VN/NC/C-2 catalysts, and the spherical particles were distributed more uniformly when the amount of DCDA was increased to 3 g in the preparing process. A small number of lumpy particles formed in the VN/NC/C-4 and VN/NC/C-5 catalysts. Therefore, the best value for the preparation of interconnected spherical structures for DCDG was 3 g. The average particle size of VN/NC/C-3 was approximately 50.17 nm, which was larger than those of the other prepared catalysts. This may be due to the different carbon contents of the catalysts. It was shown that the appropriate amount of DCDA not only provided a nitrogen and carbon source for VN/NC/C-x, but also inhibited the decomposition of P123. To further study the microstructure of the VN/NC/C-x catalysts, transmission electron microscopy (TEM) was performed. The TEM image exhibited in Figure 1b and Figure S2a–c depicts that the carbon layers were interconnected to form a connected spherical structure, the VN sub-nanoparticles were uniformly distributed on the spherical N-doped porous carbon, and the average particle size of VN in VN/NC/C-3 was observed to be about 4.50 nm. Correspondingly, Figure 1c and Figure S2d–f show HRTEM images of the VN/NC/C-x catalysts. The analysis showed that the lattice constant of VN/NC/C-3 was 0.210 nm, which corresponded to the (200) plane of the face-centered cubic phase VN. However, the lattice constant of VN/NC/C-3 was larger than that of VN/NC/C-2 (0.207 nm), VN/NC/C-4 (0.208 nm), and VN/NC/C-5 (0.206 nm). The reason for this may be that the amount of DCDA affects the formation of V-N (V3+), V-N-O (V4+), and V-O (V5+), while VN/NC/C-3 has more V3+ content (the radii of V3+, V4+, and V5+ were 0.64 Å, 0.58 Å, and 0.54 Å, respectively) [30]. In addition, EDS mapping analysis was performed on the VN/NC/C-3 catalyst (Figure 1d). EDS mapping images further confirmed that C, N, V, and O were homogeneously located in the region, while the distributions of the nitrogen atom were similar as that of carbon, proving the existence of C−N bonds. Some oxygen elements distributed in the region of V may have been due to the oxidation of vanadium nitride in air. The above results demonstrate the successful synthesis of VN/NC/C-x catalysts.
The crystalline phase of the synthesized VN/NC/C-x was confirmed by X-ray diffraction (XRD) measurements (Figure 2a). For all of the synthesized VN/NC/C-x, there were five diffraction peaks centered at 37.61°, 43.69°, 63.52°, 76.22°, and 80.28°, corresponding to (111), (200), (220), (311), and (222) planes of VN (JCPDS no. 35-0768), respectively, indicating that the crystalline phase of VN/NC/C-x was consistent with VN [25]. Furthermore, the broad diffraction peaks around 24.70° corresponded to the characteristic structure of graphitized carbon [31]. The four characteristic peaks observed in Figure 2b are located around 139.60 cm−1, 283.18 cm−1, 406.66 cm−1, and 693.28 cm−1, respectively, which are consistent with the classical vibrational modes Eg, B1g, A1g, and Eg of VN. The Eg, B1g, A1g, and Eg peaks correspond to the bending vibration of O-V-O, the symmetric stretching vibration of V=O, the antisymmetric stretching vibration of V-O-V, and the bending vibration of O=V=O, respectively [21,32]. The positions of the Raman peaks varied with particle size and pore size [33,34]. Compared with VN/NC/C-2 (140.26 cm−1), the Eg peak positions of VN/NC/C-3, VN/NC/C-4, and VN/NC/C-5 were shifted to 138.70 cm−1, 139.05 cm−1, and 139.24 cm−1, respectively (Figure S3). It has been proven that the VN nanoparticles existing in VN/NC/C-3 had a larger particle size and pore size, which was consistent with the TEM and SEM results. In addition, the Raman spectra of the VN/NC/C-x catalysts all displayed two typical peaks of carbon—the D band at 1344 cm−1 is assigned to disordered carbon, while the G band at 1586 cm−1 corresponds to sp2 bonding of graphitic carbon. It is well known that the area ratio (SD/SG) of the D-band to the G-band has been used to evaluate the degree of graphitization of carbon. As shown in Figure 2c, the SD/SG ratio of the VN/NC/C-3 catalyst is about 1.431, which is lower than that of the other catalysts. It indicates that the VN/NC/C-3 catalyst has the highest degree of graphitization. This result confirms that adding an appropriate amount of DCDA during the preparation process can affect the graphitization degree of carbon [35,36].
As shown in Figure 2d, the N2 adsorption−desorption isotherms of all of the catalysts reveal the characteristics of the type IV curves with H3-type hysteresis loops, indicating the existence of micropores and mesoporous. The specific surface area of VN/NC/C-3 was as high as 133.95 m²·g−1, the pore volume was up to 0.43 cm3·g−1 (Table S1), and the pore size distributions calculated from the N2 adsorption data were centered at 0.6 nm, 1.2 nm, 3 nm, 9 nm, and 26 nm (Figure 2e), corresponding to the microporous wall and mesoporous channel. Furthermore, a certain amount of macropores (with a pore size greater than 50 nm) also existed in the VN/NC/C-3 catalyst. Therefore, the surface area and pore volume of VN/NC/C-x could be tuned by changing the amount of DCDA to obtain micro/meso/macropores materials with high surface areas. Such a hierarchical porous structure with a high surface area is highly desirable for electrolyte/oxygen transportation and reaction sites, and is beneficial for improving the catalytic performance of the catalyst.
The surface electronic states and composition of VN/NC/C-x can be understood by XPS measurement of the spectra (Figure 3 and Figure S4). C, N, O, and V peaks were observed in the survey spectra (Figure 3a). It can be seen from Table S2 that the composition of the surface elements of N, C, O, and V of VN/NC/C-x changed significantly with the change in the amount of DCDA added. This result shows that the amount of DCDA had a significant effect on the surface structure of the sample, which further affected the adsorption and dissociation of oxygen molecules on the catalyst surface. An analysis of Table S2 shows that the surface of VN/NC/C-x had a greater oxygen content, which could improve its catalytic activity. From Table S3, the semi-quantitative analysis of the catalyst by XPS shows that the specific weight fractions (weight%) of VN were found to be 20.27%, 33.26%, 15.53%, and 18.98% for VN/NC/C-2, VN/NC/C-3, VN.NC/C-4, and VN/NC/C-5, respectively. The C1s spectra shown in Figure S4 were suitable for two valence states: C-C (284.75 eV) and C-N (286.26 eV). The above C-N bonds indicate that nitrogen elements were successfully doped into the carbon skeleton, which proves that the prepared catalysts consisted of nitrogen-doped carbon materials [37]. Numerous studies have demonstrated that C-N acts as the active site of catalysts. Table S4 shows that the VN/NC/C-3 catalyst had the largest proportion of C-N bonds (23.50%), which could act as the active site in the ORR process. The N1s spectrum (Figure 3b) can be divided into five peaks at 402.12, 401.12, 399.69, 398.12, and 396.87 eV, corresponding to oxidized-N, graphitic-N, pyrrolic-N, pyridinic-N, and V-N, respectively. This result further indicated that N was doped into the carbon structure and VN/NC/C-x was successfully synthesized. Compared with the other catalysts, VN/NC/C-3 had the highest V-N content (21.34%), as well as pyridine-N (15.22%) (Table S5 and Figure 3c). Pyridinic-N has the ability to accept electrons and is one of the active substances of the ORR [38]. At the same time, V-N has the most efficient metal active sites in catalysis, which can decrease the overpotential to improve the ORR activity. Therefore, changing the amount of DCDA can improve the formation of pyridinic-N and V-N species.
The high-resolution O 1s (Figure 3d) spectrum of the VN/NC/C-x catalysts were fitted into three peaks at 530.37 eV, 530.37 eV, and 530.37 eV, corresponding to OOH, N-V-O, and C-OH, respectively. It is well known that oxygen vacancies in materials can tune the coordination environment and electronic structure, thereby reducing activation energy and significantly improving the catalytic performance. An analysis of Tables S2 and S6 shows that compared with other catalysts, the O content in VN/NC/C-3 was high (13.37%) and there were more oxygen vacancies (52.99%). Therefore, VN/NC/C-3 with oxygen-rich vacancies exhibited a remarkable catalytic activity. In conclusion, adjusting the amount of DCDA could increase the number of coordinating oxygen defects on the surface.
The V 2p2/3 spectrum in Figure 3e could be fitted into three peaks at 514.23 eV, 515.87 eV, and 517.29 eV generated by V-N (V3+), V-N-O (V4+), and V-O (V5+), respectively [39,40]. The presence of V-N represents the successful synthesis of VN. V-N-O and V-O are generated because of the oxidation of vanadium nitride, which can be used as a protective layer to avoid further oxidation of the material and to improve the ORR activity [41]. Depending on the oxidation state, there are more than two, one, and zero d electrons in V3+, V4+, and V5+, respectively [21], demonstrating that the valence state is inversely proportional to the number of d electrons. In Table S7 and Figure 3f, calculated using the following formula for the average valence state = 3 × V-N% + 4 × V-N-O% + 5 × V-O%, the average valence state of V in the VN/NC/C-x catalyst was VN/NC/C-2 (4.67), VN/NC/C-3 (4.44), VN/NC/C-4 (4.59), and VN/NC/C-5 (4.66). After comprehensive comparison, it can be determined that the average valence state of V in VN/NC/C-x was VN/NC/C-3 < VN/NC/C-4 < VN/NC/C-5 < VN/NC/C-2. Therefore, the d-electron content of VN/NC/C-3 was higher than that of the other catalysts. In conclusion, it was demonstrated that the addition of DCDA has a significant effect on the electronic structure of VN/NC/C-x, increasing the d-electron content to obtain electrons to form more active sites, which further improved the electrocatalytic performance.

2.2. ORR Activity and Durability

The cyclic voltammetry (CV) curves of VN/NC/C-x (Figure S5) showed a distinct oxygen reduction peak in O2-saturated solution compared with that in the N2-saturated alkaline solution, indicating a superb oxygen reduction activity. Moreover, the oxygen reduction peak of VN/NC/C-3 was more positive, indicating that VN/NC/C-3 showed a better ORR performance. Therefore, the amount of DCDA was adjusted to effectively improve the catalytic activity. As shown in Figure S6, in order to further investigate the electrocatalytic performance of the VN/NC/C-x catalyst for ORR, a linear sweep voltammetry (LSV) test was performed. The onset potential (Eonset) and half-wave potential (E1/2) of the VN/NC/C-3 were 0.85 V and 0.75 V, respectively, which were higher than the other prepared catalysts. Furthermore, the limiting current density of the VN/NC/C-3 (4.52 mA∙cm−2) catalyst was significantly larger than those of the other VN/NC/C-2 (3.49 mA∙cm−2), VN/NC/C-4 (4.05 mA∙cm−2), and VN/NC/C-5 (3.75 mA∙cm−2) catalysts. All of the above results indicate that the VN/NC/C-3 catalyst had a better ORR catalytic activity than the VN/NC/C-2, VN/NC/C-4, and VN/NC/C-5 catalysts. The superior catalytic performance benefitted from the high degree of graphitization, the large specific surface area, the high d-electron, and the abundant oxygen defects and nitrogen species, which could be optimized by adjusting the amount of DCDA during the preparation. In addition, the synergistic effect between NC and VN could also enhance the overall catalytic activity.
The Tafel slope played a key role in judging the kinetic performance of ORR (Figure 4a). Impressively, VN/NC/C-3 possessed a smaller Tafel slope of 75.54 mV/dec, which was superior to VN/NC/C-2 (97.24 mV/dec), VN/NC/C-4 (87.08 mV/dec), and VN/NC/C-5 (92.67 mV/dec). This result suggests that VN/NC/C-3 had a superior reaction kinetics rate, and an appropriate amount of DCDA was more favorable for the reaction kinetics of the VN/NC/C-x catalysts. Furthermore, the excellent reaction kinetic rate of VN/NC/C-3 could be attributed to (1) the high intrinsic activity of VN and the excellent electrical conductivity provided by NC. (2) The synergistic effect of NC and VN lowering the energy barrier for O2 adsorption. (3) The abundant oxygen vacancies and porous structures provided abundant mass transport paths, accelerating the initial electron transfer step in ORR. In summary, an appropriate amount of DCDA was beneficial for oxygen reduction catalysis.
In Figure S7, the K-L plots of all catalysts showed a good linear relationship, representing the first-order reaction kinetics of ORR and the concentration of dissolved oxygen in the electrolyte [42,43]. At 0.30 V, 0.35 V, 0.40 V, and 0.45 V, the electron transfer number of VN/NC/C-3 was determined by the slope of each line to be 3.97, 3.85, 3.78, 3.73, 3.67, and 0.50 V, respectively. Thus, the average for all those values was 3.80, which demonstrated that the ORR on the VN/NC/C-3 catalyst followed the effective four-electron transfer pathway, suggesting that O2 was directly reduced to OH−. In the same way, the average values of the electron transfer number for VN/NC/C-2, VN/NC/C-4, and VN/NC/C-5 were 3.33, 3.67, and 3.49, which were smaller than that of VN/NC/C-3. Thus, it was demonstrated that the ORR pathways of VN/NC/C-2, VN/NC/C-4, and VN/NC/C-5 were the joint action of the 2e− and 4e− transfer mechanisms. The RRDE test was used to further analyze the yield of the peroxide intermediate (H2O2) and to study the electron transfer pathway of the VN/NC/C-3 catalyst. As shown in Figure 4b,c, the H2O2 yield detected on VN/NC/C-3 was less than 10%, indicating the high catalytic selectivity of VN/NC/C-3. The corresponding n for VN/NC/C-2, VN/NC/C-3, VN/NC/C-4, and VN/NC/C-5 were calculated to be 3.37, 3.82, 3.66, and 3.45, respectively, which are similar to the rotating disk electrode (RDE) data described above. These results further suggest a nearly 4e− ORR pathway for VN/NC/C-3 in an alkaline solution.
In practical applications, the anti-interference ability of catalysts is of great significance. An analysis of the curve in Figure 4d shows that there was a loss of current when VN/NC/C-x was exposed to methanol. Among them, the fluctuation of VN/NC/C-3 was weaker, which indicated that VN/NC/C-3 had a good methanol resistance, stronger anti-cross effect, and good selectivity for ORR. To characterize the durability of VN/NC/C-x, it was further evaluated by current–time (i-t) chronoamperometry. In Figure 4e, the VN/NC/C-3 catalyst maintained 87.80% of the initial current after 10 h, which was larger than that of VN/NC/C-2 (61.60%), VN/NC/C-4 (75.70%), and VN/NC/C-5 (65.60%). It directly demonstrates that VN/NC/C-3 exhibited a stronger stability to ORR. It has been reported that SCN− can coordinate with transition metal ions to seal metal active sites and hinder their catalytic performance [44]. The effect of the active center of VN/NC/C-x catalysts on the ORR activity were investigated by adding 50 mM KSCN to the electrolyte. As shown in Figure 4f, we found that in the presence of KSCN, the limiting current density and half-wave potential of the VN/NC/C-3 catalyst decreased by 0.21 mA·cm−2 and 6 mV, respectively, which was larger than those of VN/NC/C-2 (0.13 mA·cm−2 and 0 mV), VN/NC/C-4 (0.19 mA·cm−2 and 3 mV), and VN/NC/C-5 (0.15 mA·cm−2 and 2 mV). The reason is that SCN− can poison the vanadium-containing active sites, thereby reducing the catalytic performance [45].
Figure 4g and Figure S8 depict the CV curves of VN/NC/C-x at different scan rates from 20 to 120 mV·s−1. It can be clearly seen that the electrochemical area increased as the scan speed increased. The double-layer capacitance (Cdl) of VN/NC/C-x was tested to calculate the electrochemically active surface area (ECSA). According to Figure 4h, the Cdl values of VN/NC/C-2, VN/NC/C-3, VN/NC/C-4, and VN/NC/C-5 were 0.75, 11.89, 3.23, and 10.39 mF·cm−2, respectively. The electrochemical surface area of VN/NC/C-x was calculated by ECSA = Cdl/Cs × A (A = 0.196 cm2, Cs = 0.04 mF/cm2) [46]. The results show that the ESCA (73.57 cm2) of VN/NC/C-3 was larger than that of VN/NC/C-2 (4.64 cm2), VN/NC/C-4 (19.99 cm2), and VN/NC/C-5 (64.29 cm2), indicating that VN/NC/C-3 had more reactive sites exposed in the ORR reaction. In Figure 4i, the Nyquist curve of the VN/NC/C-x catalyst is presented, and the lower right inset shows the equivalent circuit. An analysis of the equivalent circuit diagram showed that the Rct value of VN/NC/C-x in the high frequency region was VN/NC/C-3 (11.85 Ω) < VN/NC/C-4 (14.47 Ω) < VN/NC/C- 5 (37.46 Ω) < VN/NC/C-2 (83.71 Ω). This means that VN/NC/C-3 had a higher conductivity and faster electron transport. It was obvious from the comparison that the diffusion resistance of the VN/NC/C-3 catalyst in the low frequency region was smaller than that of VN/NC/C-2, VN/NC/C-4, and VN/NC/C-5. This was because VN/NC/C-3 had a larger specific surface area, high electrical conductivity, and higher pore volume, which were beneficial for electrolyte and oxygen transport.

2.3. Zn–Air Battery Performance

To further evaluate the practical application of VN/NC/C-x catalysts, Zn–air batteries were constructed with VN/NC/C-x as the air cathode catalysts. In Figure 5a, the Zn–air battery of VN/NC/C-3 could run stably for more than 8 h at an open circuit voltage of about 1.30 V, which was better than VN/NC/C-2 (1.25 V), VN/NC/C-4 (1.27 V), and VN/NC/C-5 (1.26 V). The power density of the VN/NC/C-x-based cells (Figure 5b) was determined from the j–V polarization curve. Obviously, the VN/NC/C-3 battery achieved a peak power density of 161.82 mW·cm−2 at 310 mA·cm−2, which was superior to VN/NC/C-2 (100.70 mW·cm−2), VN/NC/C-4 (129.80 mW·cm−2), and VN/NC/C-5 (117.41 mW·cm−2). The energy density and specific capacity in Figure 5c show that ZABs based on VN/NC/C-3 provided a specific capacity of 702.28 mAh·gZn−1 and an energy density of up to 861.51 Wh·kgZn−1 when fully discharged. Both were higher than the specific capacities and energy densities of VN/NC/C-2 (552.96 mAh·gZn−1, 646.59 Wh·kgZn−1), VN/NC/C-4 (665.92 mAh·gZn−1, 778.67 Wh·kgZn−1), and VN/NC/C-5 (619.95 mAh·gZn−1, 724.92 Wh·kgZn−1). Finally, the performance of VN/NC/C-3-based Zn–air batteries was compared with recent literature on metal nitride/C catalysts, as shown in Table S9. The performance of the VN/NC/C-3-based Zn–air batteries was comparable to those catalysts, once again proving that VN/NC/C-3-based Zn–air batteries show great potential in practical applications.
Furthermore, the rate capability of the Zn–air battery was tested using discharge curves (Figure 5d). It can be seen that the discharge voltage continued decreasing while increasing the current density. The discharge voltage of VN/NC/C-3 was 1.26 V at an initial current density of 0.5 mA·cm−2, while the discharge voltage of VN/NC/C-3 was as high as 1.15 V, even at a higher current density of 10 mA·cm−2. When the current density returned to the initial value of 0.5 mA·cm−2, the discharge voltage returned back to the initial state. The rate performance shows that the Zn–air battery also had an excellent reversibility and high-rate discharge performance, which is attributed to the porous structure and good ORR catalytic kinetics of VN/NC/C-3.

3. Materials and Methods

3.1. Chemicals

Ammonium metavanadate (NH4VO3, AR) and potassium thiocyanate (KSCN, AR) were procured from Aladdin. (Shanghai, China). Dicyandiamide (DCDA, 99.5%) was supplied by Acros Science Co., Ltd. (Beijing, China). Poly (ethylene glycol)-block-poly (propylene glycol)-block-poly (ethylene glycol) (P123, Mw = 5800) and Nafion (5.0 wt%) were purchased from Sigma-Aldrich. (Shanghai, China). Ethanol was bought from Tianjin Fengchuan Chemical Reagent Co., Ltd. (Tianjin, China).

3.2. Preparation of VN/NC/C-x

A series of VN/NC/C-x catalysts (x represents the amount of DCDA) were prepared by a one-step pyrolysis method. P123, NH4VO3, and 50 mL of deionized water were sealed and stirred well at 50 °C for 14 h. The final molar ratio of NH4VO3/P123/H2O in the mixture was 1: 0.0605: 325.1988. After that, the precursor was obtained after adding x g (x = 2, 3, 4, and 5) DCDA and drying at 60 °C. Afterwards, the precursor was heated at 800 °C for 90 min at 5 °C·min−1 under a nitrogen atmosphere to obtain VN/NC-x (x = amount of DCDA). Finally, VN/NC-x and conductive carbon black were ball-milled in a ball mill at a mass ratio of 1:1 for 40 min. The synthesized catalyst was named VN/NC/C-x (x = amount of DCDA) and was further characterized.

3.3. Electrochemical Measurements

The electrochemical performance of the catalysts was carried out on an electrochemical workstation CHI760E using a three-electrode system. The three-electrode system consisted of a rotating disk working electrode (RDE, 5 mm diameter, 0.196 cm2), a Pt counter electrode, and an Ag/AgCl (saturated 3 M NaCl) reference electrode for alkaline media. Prepare the working electrode as follows: 5 mg of catalyst was poured into a mixed solution containing alcohol (0.4 mL), deionized water (0.6 mL), and Nafion (50 μL), and the mixture was sonicated to obtain a homogeneous mixed solution. Then, 15 µL or 19 µL of catalyst ink was evenly dropped onto the glassy carbon electrode and allowed to dry naturally. The voltage obtained from the test results needs to be converted to the reversible hydrogen voltage using the equation ERHE = EAg/AgCl + 0.059 × pH + 0.197.

3.4. Zn–Air Battery

All Zn–air battery measurements were performed on an electrochemical workstation (CHI760E) and a LAND-CT2001A battery test system at room temperature. The catalyst ink was prepared by sonicating a mixture of 5 mg of catalyst, 400 µL of ethanol, 1600 µL of deionized water, and 50 µL of Nafion for 2 h. The prepared catalyst was coated on carbon paper as the air cathode with a catalyst loading of 0.5 mg∙cm−2, zinc foil (16.0 cm2) as the anode, and the electrolyte was 6 M KOH solution.

3.5. Characterization

X-ray diffraction (XRD) was obtained using a Rigaku miniFlex 600 diffractometer (40 kV Cu, λ = 1.5406 Å). (Rigaku Corporation, Tokyo, Japan). Raman analysis was performed by Renishaw’s inVia Raman microscope spectrometer at an excitation wavelength of 532 nm. (Renishaw, Wotton-under-Edge, UK). Thermogravimetric-differential thermal Analysis (TG-DSC) was tested in a high-purity argon atmosphere using a STA 449F3 type of equipment. The morphology of the synthesized catalysts was measured using a field emission scanning electron microscope (FESEM, FEI Quanta 200, 20 kV). (FEI, Lausanne, Switzerland). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were run using a Tecnai G2 TF30 S-Twin at an operating voltage of 300 kV. (FEI, Lausanne, Switzerland). Energy Dispersive Spectroscopy (EDS) analysis was performed on Tecnai TEM with an EDS detector in scanning element electron microscopy (STEM) mode. N adsorption and desorption isotherms were measured at 77 K using a Micromeritics ASAP 2460 instrument. (Micromeritics, Norcross, GA, USA). The Brunauer−Emmett−Teller (BET) method was applied to analyze the specific surface area of the catalysts, and the pore size distribution (PSD) was measured using nonlocal density function theory (NLDFT). To study the chemical compositions and surface element valence states of the elements, X-ray photoelectron spectroscopy (XPS) was performed on Thermo Fisher Scientific K-Alpha+ (Al Kα, 1486.6 eV) XPS. Corrected for surface contamination C 1s (284.8 eV), photoelectron beams were processed using Avantage software (version 5.9918). (ThermoFisher Scientific, Waltham, MA, USA).

4. Conclusions

In conclusion, nitrogen-doped hierarchical porous carbon-supported vanadium nitride (VN/NC/C-x) catalysts were prepared using a facile synthetic strategy. The enhancement of the electrocatalytic performance of the as-prepared VN/NC/C-x catalysts by adjusting the amount of DCDA should be attributed to the following points: (1) VN nanoparticles well-dispersed on the N-doped carbon and well optimized particle size; (2) the large specific surface area, micro/meso/macropore structure materials are beneficial for electrolyte/oxygen transportation and reaction sites; (3) the high degree of graphitization can accelerate electron transport during the ORR process; and (4) oxygen vacancies, abundant pyridine-N, and d-electron have a significant effect on the electronic structure and endow more active sites. As expected, VN/NC/C-3 is rich in V-N and pyridine-N, and exhibits an excellent ORR performance in alkaline media. In addition, VN/NC/C-3-based Zn–air batteries have the highest peak power density and an excellent energy density. We hope that the results presented here may provide a valuable synthetic approach for the preparation of other transition metal nitride catalysts with a relative economic value and high performance.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal12080877/s1. FESEM images of (a, d) VN/NC/C-2, (b, f) VN/NC/C-4, (c, g) VN/NC/C-5, and (e) VN/NC/C-3. Figure S2: TEM images of (a) VN/NC/C-2, (b) VN/NC/C-4, and (c) VN/NC/C-5; HRTEM images of (d) VN/NC/C-2, (e) VN/NC/C-4, and (f) VN/NC/C-5. Figure S3: The Raman spectra of the VN/NC/C-x in the range of 100–200 cm−1. Figure S4: High resolution XPS spectra of C 1s for VN/NC/C-x. Figure S5: Cyclic voltammetry (CV) curves of VN/NC/C-x in N2- and O2-saturated 0.1 M KOH solution. Figure S6: LSV profiles of VN/NC/C-x in O2-saturated 0.1 M KOH solution at 1600 rpm under an electrode rotating speed of 1600 rpm. Figure S7: LSV polarization curves and corresponding K-L plots of (a, b) VN/NC/C-2, (c, d) VN/NC/C-3, (e, f) VN/NC/C-4, (g, h) VN/NC/C-5 catalysts measured in O2-saturated 0.1 M KOH at various rotation speeds from 400 rpm to 2025 rpm at scan rate of 10 mV s−1. Figure S8: CV curves of (a) VN/NC/C-2, (b) VN/NC/C-4 and (c) VN/NC/C-5 at various rates of 20–120 mV s−1. Table S2: The atomic contents of C, N, O, and V in VN/NC/C-x. Table S3: The specific weight fractions (weight%) of C, N, V, and VN in VN/NC/C-x. Table S4: Carbon species in VN/NC/C-x catalysts determined XPS using C 1s. Table S5: Nitrogen species in the VN/NC/C-x catalysts determined XPS using N 1s. Table S6: Oxygen species in the VN/NC/C-x catalysts determined by XPS using O 1s. Table S7: Vanadium species in the VN/NC/C-x catalysts determined by XPS using V 2p. Table S8: Comparison of ORR performance of VN/NC/C-x catalysts. Table S9: Comparison of Zn–air battery performance of VN/NC/C-3 with other state-of-the-art electrode materials. [4,19,20,34,35,37,47,48,49,50,51,52]

Author Contributions

Conceptualization, Y.F. and L.H.; methodology, Y.F. and L.H.; software, Y.F.; investigation, Y.F., P.Z. and X.P.; data curation, Y.F. and X.X.; writing—original draft preparation, Y.F.; writing—review and editing, Y.F., L.H., J.L., X.Z. and J.X.; funding acquisition, L.H., Y.Z. and P.D. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (51802134 and 51864024).

Data Availability Statement

Not applicable.

Acknowledgments

The authors thank the National Natural Science Foundation of China, the follow-up project of the Thousand Young Talents Program of Yunnan Province and Yunnan Major Scientific and Technological Projects (grant no. 202202AG050003) for their support.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) FESEM, (b) TEM, and (c) HRTEM images of VN/NC/C-3. (d,d1–d4) EDS elemental mapping of C, N, V, and O.
Figure 1. (a) FESEM, (b) TEM, and (c) HRTEM images of VN/NC/C-3. (d,d1–d4) EDS elemental mapping of C, N, V, and O.
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Figure 2. (a) XRD patterns of VN/NC/C-x. The Raman spectra of VN/NC/C-x in the range of (b) 50–800 cm−1 and (c) 1000–2000 cm−1. (d) N2 adsorption−desorption isotherms and (e) the pore-size distribution of VN/NC/C-x.
Figure 2. (a) XRD patterns of VN/NC/C-x. The Raman spectra of VN/NC/C-x in the range of (b) 50–800 cm−1 and (c) 1000–2000 cm−1. (d) N2 adsorption−desorption isotherms and (e) the pore-size distribution of VN/NC/C-x.
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Figure 3. (a) XPS survey spectra of the VN/NC/C-x; (b) high resolution N1s of VN/NC/C-x; (c) different proportions of N 1s in VN/NC/C-x; (d) high resolution O 1s of VN/NC/C-x; (e) high resolution V 2p of VN/NC/C-x; (f) different proportions of V 2p3/2 in VN/NC/C-x.
Figure 3. (a) XPS survey spectra of the VN/NC/C-x; (b) high resolution N1s of VN/NC/C-x; (c) different proportions of N 1s in VN/NC/C-x; (d) high resolution O 1s of VN/NC/C-x; (e) high resolution V 2p of VN/NC/C-x; (f) different proportions of V 2p3/2 in VN/NC/C-x.
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Figure 4. (a) Tafel plots of the various catalysts. (b) The RRDE data of the VN/NC/C-x. (c) The hydrogen peroxide yield and the electron transfer number. (d) ORR chronoamperometric responses after adding methanol of the VN/NC/C-x. (e) Current–time (i-t) curves of the VN/NC/C-x. (f) The effect of the KSCN interaction on the catalytic activity of VN/NC/C-x. (g) The voltammograms of VN/NC/C-3 at different scan rates. (h) Δj fit of VN/NC/C-x (Δj = ja−jc) versus scan rate (vs RHE) at the set potential. (i) Nyquist plots of VN/NC/C-x, the inset shows the equivalent circuit.
Figure 4. (a) Tafel plots of the various catalysts. (b) The RRDE data of the VN/NC/C-x. (c) The hydrogen peroxide yield and the electron transfer number. (d) ORR chronoamperometric responses after adding methanol of the VN/NC/C-x. (e) Current–time (i-t) curves of the VN/NC/C-x. (f) The effect of the KSCN interaction on the catalytic activity of VN/NC/C-x. (g) The voltammograms of VN/NC/C-3 at different scan rates. (h) Δj fit of VN/NC/C-x (Δj = ja−jc) versus scan rate (vs RHE) at the set potential. (i) Nyquist plots of VN/NC/C-x, the inset shows the equivalent circuit.
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Catalysts 12 00877 g005 550
Figure 5. (a) Open circuit voltage, (b) Polarization and corresponding power density curves, (c) discharge curves of the VN/NC/C-x based Zn–air batteries. (d) Galvanostatic discharge curves of the battery at different current densities.
Figure 5. (a) Open circuit voltage, (b) Polarization and corresponding power density curves, (c) discharge curves of the VN/NC/C-x based Zn–air batteries. (d) Galvanostatic discharge curves of the battery at different current densities.
Catalysts 12 00877 g005
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Fu, Y.; Han, L.; Zheng, P.; Peng, X.; Xian, X.; Liu, J.; Zeng, X.; Dong, P.; Xiao, J.; Zhang, Y. Vanadium Nitride Supported on N-Doped Carbon as High-Performance ORR Catalysts for Zn–Air Batteries. Catalysts 2022, 12, 877. https://doi.org/10.3390/catal12080877

AMA Style

Fu Y, Han L, Zheng P, Peng X, Xian X, Liu J, Zeng X, Dong P, Xiao J, Zhang Y. Vanadium Nitride Supported on N-Doped Carbon as High-Performance ORR Catalysts for Zn–Air Batteries. Catalysts. 2022; 12(8):877. https://doi.org/10.3390/catal12080877

Chicago/Turabian Style

Fu, Yidan, Lina Han, Pengfei Zheng, Xianhui Peng, Xianglan Xian, Jinglin Liu, Xiaoyuan Zeng, Peng Dong, Jie Xiao, and Yingjie Zhang. 2022. "Vanadium Nitride Supported on N-Doped Carbon as High-Performance ORR Catalysts for Zn–Air Batteries" Catalysts 12, no. 8: 877. https://doi.org/10.3390/catal12080877

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