Trans. Nonferrous Met. Soc. China 31(2021) 1427-1438

Preparation and properties of transition metal nitrides caged in N-doped hollow porous carbon sphere for oxygen reduction reaction

Yan-juan LI¹, Min WANG¹, Sa LIU¹, Jing-xia GAO¹, Shun YANG¹, Zi-hao LIU¹, Xiao-yong LAI², Xiao YAN¹

1. Jiangsu Key Laboratory of Green Synthetic Chemistry for Functional Materials, School of Chemistry and Materials Science, Jiangsu Normal University, Xuzhou 221116, China;

2. State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering, College of Chemistry and Chemical Engineering, Ningxia University, Yinchuan 750021, China

Received 1 June 2020; accepted 28 December 2020

Abstract: A series of transition metal nitrides (M_xN_y, M=Fe, Co, Ni) nanoparticle (NP) composites caged in N-doped hollow porous carbon sphere (NHPCS) were prepared by impregnation and heat treatment methods. These composites combine the high catalytic activity of nitrides and the high-efficiency mass transfer characteristics of NHPCS. The oxygen reduction reaction results indicate that Fe₂N/NHPCS has the synergistic catalytic performance of higher onset potential (0.96 V), higher electron transfer number (~4) and higher limited current density (1.4 times as high as that of commercial Pt/C). In addition, this material is implemented as the air catalyst for zinc-air battery that exhibits considerable specific capacity (795.1 mA·h/g) comparable to that of Pt/C, higher durability and maximum power density (173.1 mW/cm²).

Key words: N-doped porous carbon; transition metal nitrides; Zn-air battery; oxygen reduction reaction; fuel cell

1 Introduction

Cathode oxygen reduction reaction (ORR) is a critical reaction for metal-air batteries and fuel cells that are considered as the promising electrical devices with high energy efficiency and environmental friendliness [1-6]. However, the practical application of these devices in the industry was severely hampered by the sluggish kinetics of ORR [7-9]. Pt is recognized as the most effective electrocatalyst; however, the high price and limited reserves, especially vulnerable to the toxicity of CO and methanol, make it not suitable for commercialization [10,11]. Therefore, it is of fundamental and practical importance to develop efficient electrocatalysts with inexpensive cost, excellent durability and environmental friendliness. Numerous catalysts with high performance were explored, including metal oxides [3,12,13], metal sulfides [2,14,15], metal carbides [11,16], metal- free carbon [1,17], transition metals and nitrogen co-doped carbon as host (M—N_x—C, M=Fe, Co, Ni) [18-20]. Notably, a series of M—N_x—C electrocatalysts are proved, in which both M and N can be as the active sites, to exhibit higher activity, four-electron selectivity and excellent stability.

Transition metal nitrides (M_xN_y, M=Fe, Co, Ni, etc) have aroused intense attention due to their unique structural characteristics. The existence of mixture bonds (including ionic bonds, metal bonds and covalent bonds) formed between N atom and metal endows nitrides with conductivity similar to metal, which makes it have similar performance with noble metal electrode [21-26]. Furthermore, the theoretical calculations predicted that M_xN_y is more conducive to the transportation of charge- carrier and has higher electrical conductivity than the corresponding oxides [27,28]. However, the catalytic activity and stability of nitrides for ORR are not ideal due to the lack of catalytic centers and small specific surface area. In order to overcome the above problems, it is a good strategy to combine nitrides and carbon-based material with numerous channel structures [29-32].

Nitrogen-doped hollow porous carbon sphere (NHPCS) with large specific surface area, abundant pores, and high electron conductivity can provide abundantly accessible active sites near the external surface of the catalysts to participate in the ORR, while the active sites buried in the dense carbon matrix remain inactive [29,33,34]. However, there are few studies on the application of metal nitrides (M_xN_y, M= Fe, Co, Ni) incorporated into N-doped hollow porous carbon sphere as highly efficient electrocatalyst towards ORR both in fuel cells and zinc-air batteries.

Herein, a series of M_xN_y (M=Fe, Co, Ni) nanoparticles (NPs) assembled into NHPCS (M_xN_y/NHPCS) were constructed. These materials have several merits: Firstly, metal element will preferentially combine with nitrogen in the carbon matrix and then react with NH₃ to form nitrides with M—N/C bond, which is more stable than the original M—N_x species and conducive to good dispersion of nitrides. Secondly, the introduction of nitride into the pore-rich structure does not destroy the large specific surface area of the carbon material carrier, which leads to the exposure of more active sites and provides an open channel for oxygen. Thirdly, the NHPCS interacting with M_xN_y NPs is beneficial to electron transfer due to its high conductivity. Such a structure gives the material excellent properties. The prepared Fe₂N/NHPCS manifests outstanding catalytic activity for ORR with excellent stability and better methanol tolerance. Additionally, the prepared catalyst was implemented as the air catalyst for zinc-air battery that exhibits a comparable specific capacity of 795.1 mA·h/g and maximum power density of 173.1 mW/cm².

2 Experimental

2.1 Synthesis of M_xN_y/NHPCS

The NHPCS was obtained according to previously reported method [35]. 0.1 g of NHPCS was added into 30 mL of the aqueous solution containing 125 mg of Co(NO₃)₂·6H₂O (or 125 mg of Ni(NO₃)₃·6H₂O or 90 mg of Fe(NO₃)₃·6H₂O) and stirred for 2 h. Then, the solvent was evaporated at 45 °C, and dried at 200 °C to get M_xO_y/NHPCS, and subsequently, Co₂N/NHPCS was annealed at 550 °C in NH₃ for 2 h. For Ni₃N/NHPCS, the annealed temperature in NH₃ is 600 °C. While for Fe₂N/NHPCS, the Fe₃O₄/ NHPCS was heated at 750 °C for 3 h.

2.2 Characterization of materials

The structural features and morphologies were characterized by powder X-ray diffractometer (XRD, D8 ADVANCE X-ray diffractometer), scanning electron microscope (SEM/EDS, S-8010) and transmission electron microscope (TEM, Tacnai G2 F20). The X-ray photoelectron spectroscopy (XPS) spectrum was harvested on a VG ESCALAB LKII instrument with Mg KR-ADES (hν=1253.6 eV). The nitrogen adsorption-desorption isotherms were recorded on an Autosorb-IQ2-VP analyzer at 77 K.

2.3 Electrochemical measurement

Electrochemical properties were tested using electrochemical workstation (CHI 760E). The test system adopted three-electrodes rotating disc electrode or rotating ring disc electrode (RDE or RRDE), where Pt foil (1 cm²) and saturated calomel electrode (SCE) were selected as counter electrode and reference electrode, respectively. The ink of all catalysts was made as follows: 5 mg of product was added into a mixture solution containing 0.75 mL of isopropanol and 50 μL of nafion solution and then ultra-sonicated. The cleaned glassy carbon electrode (RDE, area: 0.19625 cm²) was used as work electrode and the surface was covered by certain amount of the above ink. The loading of the prepared catalysts was 637 μg/cm². Simultaneously, commercial Pt/C was selected as a reference with Pt loading of 25.4 μg/cm². The test condition was in 0.1 mol/L KOH solution with saturated N₂ or O₂ before all tests. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were harvested in an electrochemistry window of 0-1.2 V (vs RHE). Current density-time (J-t) curves were recorded at 0.6 V (vs RHE) in 0.1 mol/L KOH solution with 1.0 mol/L methanol.

2.4 Performance of zinc-air batteries (ZABs)

A cleaned Zn plate was selected as an anode. 6 mol/L KOH was used as the electrolyte for primary ZAB. The air electrode of primary ZAB was fabricated as follows: a gas diffusion layer (GDL) attached with carbon cloth (the active area of 1 cm²) was covered by 65 μL of catalyst ink, in which a catalyst loading of 0.5 mg/cm² was used. Afterward, the air electrode was dried at room temperature. The commercial 20%Pt/C was selected as a reference. The discharge polarization curves were obtained by LSV at a scanning rate of 1 mV/s.

3 Results and discussion

3.1 Structural features and morphologies

M_xN_y/NHPCS was synthesized through an anion conversion from M_xO_y under ammonia atmosphere. The crystal structure of M_xN_y/NHPCS was examined by XRD (Fig. 1). Compared with the standard card, the characteristic peaks in the three products are assigned to the crystalline planes of Fe₂N (PDF No. 2-1206), Co₂N (PDF No. 6-647), and Ni₃N (PDF No. 10-280), respectively, showing that the M_xN_y/NHPCS materials are successfully harvested.

Fig. 1 XRD patterns of M_xN_y/NHPCS

The typical morphologies of all catalysts are shown in Fig. 2, where well-defined spheres with diameters of 80-100 nm can be seen. The NHPCS spheres with smooth surface are dispersed well. From the SEM image of Fe₂N/NHPCS, no obvious change observed indicates that the small Fe₂N particles are homogenously distributed (Fig. 2(b)). While numerous small size particles are anchored on the surface of Co₂N/NHPCS (Fig. 2(c)) and Ni3N/NHPCS (Fig. 2(d)), which is detrimental to the catalytic performance.

Fig. 2 SEM images of NHPCS (a), Fe₂N/NHPCS (b), Co₂N/NHPCS (c) and Ni₃N/NHPCS (d)

Taking Fe₂N/NHPCS as an instance, more detailed microstructures of the nanocomposite were detected by TEM, as shown in Fig. 3. Figure 3(a) shows that the NHPCS spheres have a hollow structure with a diameter of ~100 nm, in which the size of shell is ~60 nm. The original morphology can still be maintained (Fig. 3(b)) after loading nitrides Figs. 3(b, c). In addition, the Fe₂N nanoparticles with diameters of 10-20 nm are uniformly anchored in the hollow carbon, which plays three primary roles. Firstly, the carbon cladding limits the expansion and dissolution of Fe₂N NPs, which significantly improves the stability of catalysts. Secondly, the hollow and mesopores structures provide enough space for mass transfer of O₂ and intermediate species. Thirdly, the carbon shells further enhance the electrode conductivity. Furthermore, HRTEM observations show that the Fe₂N particles with a darker periphery display intelligible distance of 0.21 and 0.22 nm, which are in accordance with the (101) and (002) crystal planes of Fe₂N, respectively (Fig. 3(d)).

N₂ adsorption-desorption was employed to investigate the textural structure of samples. The results are shown in Fig. 4. All isotherms present Type IV, indicating that there are a large number of mesopores in all catalysts [36,37], which contribute to the high surface area and provide more active adsorption sites. The NHPCS has a high specific surface area of 1024.25 m²/g, large pore volume of 0.78 cm³/g, and pore size of 3.94 nm. For M_xN_y/ NHPCS, the specific surface area and pore volume decrease, indicating that the M_xN_y is successfully loaded. For Fe₂N/NHPCS, two new pores centered at 6.0 and 13.1 nm appear due to non-compact packed Fe₂N particles, which lead to larger pore volume (0.84 cm³/g) (Table 1). Furthermore, the curves sharply increase in the relative pressure range of 0.9-1.0, confirming the existence of stacked macropores, which not only offer a propagation path for the ORR centers to capture active species and electrons but also enhance the diffusion rate of O₂. In conclusion, the synergistic effect of the above results will accelerate the catalytic reaction.

X-ray photoemission spectroscopy (XPS) was used to further study the surface chemistry of all samples. N element can be detected in NHPCS (Fig. 5(a)), suggesting successfully doping of heteroatoms into carbon matrix. The nitrogen deconvolution shows three major forms of N doping in the NHPCS at 397.9, 399.8 and 401.6 eV [38,39] with contents of 33.76% pyridinic N, 43.97% pyrrolic N and 22.27% graphitic N, respectively. Each spectrum can be divided into four peaks for all M_xN_y/NHPCS catalysts. According to previous reports [40,41], the doped N in the form of pyridinic N and graphitic N, especially the former, is considered to be more active for catalysis in comparison to pyrrolic N. In particular, the total content of pyridinic N and graphitic N in Fe₂N/NHPCS and Ni₃N/NHPCS catalysts is up to 63%, respectively (Table 2), which enhances the catalytic performance. Furthermore, the addition peaks situated at 399.6, 397.6 and 397.5 eV in Figs. 5(b-d) are contributed to Fe—N, Co—N and Ni—N bonds, respectively, suggesting that the metal nitrides are successfully harvested. Generally, the N atoms in carbon matrix can enhance conductivity and capture metal atoms, and nitrides provide catalytic active sites. The synergistic effect of the two is conducive to improve catalytic performance.

Fig. 3 TEM/HRTEM images of NHPCS (a) and Fe₂N/NHPCS (b-d)

Fig. 4 N₂ adsorption-desorption isotherms and pore size distribution of NHPCS (a), Fe₂N/NHPCS (b), Co₂N/NHPCS (c) and Ni₃N/NHPCS (d) (V and d in inserts are volume and diameter of pores, respectively)

Table 1 Texture parameters of all composites obtained from adsorption-desorption isotherm

3.2 ORR performance

Cyclic voltammograms (CVs) were first carried out to detect the catalytic activities in 0.1 mol/L KOH solution with saturated O₂ and N₂ (Fig. 6(a)). The non-characteristic is observed from the CV curve under N₂ condition, while all the prepared catalysts exhibit a certain redox peak under O₂-saturated condition. The difference of current density and potential between oxygen and nitrogen can indicate the catalytic performance for ORR and the values order from high to low as follows: Fe₂N/NHPCS > Ni₃N/NHPCS > Co₂N/ NHPCS > NHPCS, indicating that Fe₂N/NHPCS processes the best catalytic activity.

Fig. 5 N 1s XPS spectra of NHPCS (a), Fe₂N/NHPCS (b), Co₂N/NHPCS (c) and Ni₃N/NHPCS (d)

Fig. 6 CV curves of catalysts in O₂ (red lines) and N₂ (black lines) saturated 0.1 mol/L KOH solution at 50 mV/s (a) and LSV curves of catalysts at scan rate of 10 mV/s and rotating speed of 1600 r/min (b)

Table 2 Contents of C, O, M and different nitrogen species in all electrocatalysts (wt.%)

To further estimate the electrocatalytic properties of all samples, LSV curves on RDE are presented in Fig. 6(b). Compared with other prepared samples, Fe₂N/NHPCS exhibits the highest electrocatalytic activity towards ORR with an onset potential of 0.96 V (vs RHE) and a half-wave potential (φ_1/2) of 0.81 V (vs RHE), which are slightly lower than those of Pt/C (0.98 and 0.86 V (vs RHE), respectively). However, the limited current density of Fe₂N/NHPCS is 7.68 mA/cm² and 1.4 times as high as that of Pt/C, exceeding the theoretical diffusion limited current density (5.7 mA/cm²) calculated by Levich’s equation, which is ascribed to the unique porous architectural structure characterized by TEM observation and N₂ adsorption-desorption isotherm. The unique porous structure facilitates the rapid mass transfer by shortening the diffusion pathways, in which the macroporous frameworks can serve as reactant buffering reservoirs and mesoporous frameworks can ensure a large specific surface area that is favorable to the exposure of active sites, and multi-electron transport channels [34,42,43]. According to the TEM image (Fig. 3(c)) and N₂ adsorption-desorption isotherm (Fig. 4(b)), the Fe₂N is not solid particle and new pores appear at 6.0 and 13.1 nm, which might facilitate the mass transport, resulting in abnormally high limited current density.

In order to investigate the effect of catalyst load on properties of the electrode, the LSV curves with different loads from 159 to 796 μg/cm² are displayed in Fig. 7(a). The one with a load of 637 μg/cm² manifests the highest activity and sequentially all the electrodes discussed are with the same loading.

RDE measurements with different electrode rotation speeds and catalyst loads were performed to further understand the ORR kinetics and the catalytic pathway of the resulted catalysts. The results are shown in Fig. 7(b). Generally, the direct four-electron reduction of O₂ which can deliver higher energy efficiency is desirable. All LSV curves exhibit well limiting current density with the catalyst loading of 637 μg/cm² and constant onset potential, indicating that the catalytic behaviour is diffusion-controlled. Koutecky-Levich (K-L) plots display good linearity and near parallelism, indicating that the electron transfer numbers of all samples are similar.

Fig. 7 LSV curves of Fe₂N/NHPCS with different loads at 1600 r/min (a), LSV curves of Fe₂N/NHPCS at various speeds and insert showing K-L plots (b), electron transfer number of Fe₂N/NHPCS by RRDE (c), yield of H₂O₂ (d) and Tafel plots (e) of Fe₂N/NHPCS and Pt/C catalysts obtained from LSV curves

According to the K-L equation (Eq. (1)), the electron transfer number (n) is in the range of 3.80-4.10, suggesting the conversion reaction from O₂ to H₂O is a quasi-four electron process. According to K-L equation, n can also be acquired by RRDE as shown in Fig. 7(c), and the results are well consistent with that obtained from K-L equation. Furthermore, the amount of the ORR intermediate, peroxide (H₂O₂) is also quantified (Fig. 7(d)) and the average H₂O₂ yield is ~5.8% at 0.45 V (vs RHE) according to Eq. (2). The Tafel plots of ORR on Fe₂N/NHPCS and Pt/C catalysts are shown in Fig. 7(e). The Tafel slope of Fe₂N/NHPCS is apparently smaller than that of Pt/C, which indicates the high intrinsic catalytic activity of Fe₂N/NHPCS.

(1)

where J represents the measured current density, J_k is the kinetic current density at a constant potential, J_d represents the diffusion-limited current density, ω is angular velocity of the disk, n represents total number of electrons transferred during the ORR test, k is the electron transfer rate constant, F is the Faraday constant (96485 C/mol), D₀ represents the diffusion coefficient of O₂ in 0.1 mol/L KOH solution (1.9×10^-5 cm²/s), υ is kinematic viscosity of the electrolyte (0.01 cm²/s), and C₀ is saturation concentration of O₂ in 0.1 mol/L KOH solution (1.2×10^-6 mol/cm³) [44,45].

(2)

where η is the yield of H₂O₂, I_R and I_D represent the diffusion-limited current and disk current, respectively, and the value of N is 0.424 (the collection efficiency of ring electrode) [46].

Fig. 8 Chronoamperometric responses of Fe₂N/NHPCS and Pt/C with 1 mol/L methanol added at around 50 s and potential of 0.6 V (vs RHE) (a), LSV curves of Fe₂N/NHPCS (b) and Pt/C (c) before and after 5000 cycles and TEM/HRTEM images of Fe₂N/NHPCS after 5000 cycles (d)

The permeation of methanol from anodic to cathodic and the failure of catalyst during long-time utilization are also the key indicators to evaluate the performance of fuel cells. The methanol tolerance of Fe₂N/NHPCS and Pt/C (Fig. 8(a)) was estimated by chronoamperometry measurement. There is no obvious response observed in J-t curve for Fe₂N/NHPCS electrode after adding 1 mol/L methanol at about 50 s, which suggests that Fe₂N/NHPCS has good anti-crossover effect ability. While for Pt/C, the current density increases significantly. Durability is another indicator to measure the performance, and the LSV curves of catalysts before and after 5000 cycles are represented in Figs. 8(b, c). For Fe₂N/NHPCS, almost no change can be observed at the onset potential and half-wave potential (φ_1/2) except for a very slow attenuation in the limited current density. But for Pt/C, onset potential and φ_1/2 both negatively shift more significantly (34 and 39 mV, respectively), and the current density drops sharply, which is just 87.8% of the original one. The excellent performance of Fe₂N/NHPCS may be due to the maintenance of the structure (Fig. 8(d)). The above results confirm that Fe₂N/NHPCS has potential application in fuel cell.

3.3 ZABs performance

The home-made primary zinc-air batteries (ZABs) were assembled (Fig. 9(a)), using zinc plate as anode and Fe₂N/NHPCS as the air electrode catalyst, which were inspired by high ORR activities of Fe₂N/NHPCS and Pt/C. A string of red- LEDs can be lighted by two Fe₂N/NHPCS-based ZABs in series. As represented in Fig. 9(b), the open-circuit voltage (OCV) of Fe₂N/NHPCS-based ZAB is 1.30 V, which is slightly higher than 1.19 V of Pt/C-based ZAB. The typical discharge curves for Fe₂N/NHPCS and Pt/C at a current density of 20 mA/cm² display a similar voltage plateaus. However, the Fe₂N/NHPCS manifests better durability. The discharge polarization curves are shown in Fig. 9(c). The maximum power density of Fe₂N/NHPCS-based ZAB is 173.1 mW/cm², about 1.14 times that of the Pt/C-based ZAB (152.3 mW/cm²), which illustrates that the prepared Fe₂N/NHPCS possesses the highly electrochemical performance.

Fig. 9 Photograph of home-made solid-state device of Zn-air batteries (a), open-circle voltage curves (b), specific capacities of zinc-air batteries (normalized to mass of consumed Zn) (c) and current density-voltage and current density-power density curves of zinc-air batteries using Fe₂N/NHPCS and Pt/C catalysts (d)

4 Conclusions

(1) A series of M_xN_y (M=Fe, Co, Ni) composite caged into N-doped hollow porous carbon sphere (NHPCS) for the oxygen reduction reaction were successfully harvested.

(2) For ORR, the prepared Fe₂N/NHPCS catalyst shows comparable performances in terms of onset potential, half-wave potential and high limited current density. Additionally, Fe₂N/NHPCS also has outstanding methanol resistance and superior durability compared with Pt/C catalyst.

(3) Fe₂N/NHPCS as the air catalyst for zinc-air battery displays high durability, specific capacity (795.1 mA·h/g), and maximum power density of 173.1 mW/cm².

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (Nos. 51702137, 51802128), the Natural Science Foundation of Jiangsu Province, China (No. BK20181013), the Priority Academic Program Development of Jiangsu Higher Education Institutions, China (No. 18KJB430013), and the Foundation of State Key Laboratory of High- Efficiency Utilization of Coal and Green Chemical Engineering, China (No. 2020-KF-20).

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用于氧还原反应的氮掺杂空心多孔碳球封装的过渡金属氮化物的制备和性能

李彦娟¹，王 敏¹，刘 飒¹，高京夏¹，杨 舜¹，刘子豪¹，赖小勇²，闫 霄¹

1. 江苏师范大学 化学与材料科学学院 江苏省功能材料绿色合成化学重点实验室，徐州 221116；

2. 宁夏大学 化学化工学院 煤炭高效利用与绿色化工国家重点实验室，银川 750021

摘 要：通过浸渍和热处理方法制备一系列氮掺杂空心多孔碳球(NHPCS)封装的过渡金属氮化物(M_xN_y, M=Fe, Co, Ni)纳米颗粒复合材料。此纳米复合材料兼具氮化物的高催化活性和氮掺杂多孔碳球的高效传质特性。氧气还原反应结果表明，Fe₂N/NHPCS表现出优异的催化性能，其具有较高的起始电位(0.96 V)、电子转移数(~4)和极限电流密度(为工业Pt/C的1.4倍)。此外，该材料作为锌-空气电池的空气催化剂，表现出与Pt/C相媲美的比容量(795.1 mA·h/g)、更优的耐久性和最大的功率密度(173.1 mW/cm²)。

关键词：氮掺杂多孔碳；过渡金属氮化物；锌-空气电池；氧还原反应；燃料电池

(Edited by Wei-ping CHEN)

Corresponding author: Xiao YAN, Tel: +86-516-83403166, E-mail: yanxiao@jsnu.edu.cn

DOI: 10.1016/S1003-6326(21)65587-5

1003-6326/ 2021 The Nonferrous Metals Society of China. Published by Elsevier Ltd & Science Press

Yan-juan LI and Min WANG contributed equally to this work