Muhammad Irfan†
,
Izhar Ullah Khan†,
Jiao Wang,
Yang Li and
Xianhua Liu*
Tianjin Key Lab. of Indoor Air Environmental Quality Control, School of Environmental Science and Engineering, Tianjin University, Tianjin, 300354, PR China. E-mail: lxh@tju.edu.cn
First published on 11th February 2020
Metal nitrides are broadly applicable in the field of electrochemistry due to their excellent electrical properties. In this study, a 3D nanostructured Ni3N–Co3N catalyst was prepared by using a versatile urea glass method, and was tested as an anode catalyst for a glucose fuel cell. The synthesized Ni3N–Co3N exhibits uniform particle dispersion in structure, morphology, and composition, and has a interpenetrating three-dimensional network structure. Notably, the Ni3N–Co3N significantly improved the catalytic activity of glucose oxidation compared to Ni3N, Co3N, and conventional activated carbon electrodes. The superior electrochemical performance could be attributed to its porous structure and unique properties, which provided a fast transport network for charge and mass transfer as well as good synergetic effect. The glucose fuel cell equipped with a Ni3N–Co3N anode achieved 30.89 W m−2 power and 97.66 A m−2 current densities at room temperature. This investigation provides potential directions for the design of cost-effective bimetallic catalysts for a wide range of glucose fuel cell applications.
Despite the significant interest from scientists in developing glucose fuel cell technology, the direct utilization of glucose in fuel cells is largely underdeveloped due to the sluggish glucose oxidation reaction.7–9 Thus, a significant effort is required in developing low-cost and highly efficient anode catalysts which can facilitate glucose oxidation. Platinum-based metals have been commonly employed as anode catalysts in fuel cells due to their low over-potentials and fast kinetics in the redox reactions.10,11 However, high cost, resource limitation, and easily poisoned sensitivity hamper their wide applications.12 Scientists are working to find substitutes of noble metals by improving the properties of transition metals such as (Ni, Co, Fe, Cu, and Zn). Various types of active substances, such as transition metals and their oxides/hydroxides, are frequently used in fuel cells,13 supercapacitors,14 batteries,15 glucose sensors16,17 etc. as electrode materials owing to their unique layered structure and chemical stability. Among these materials, metallic nickel, nickel alloys and nickel compounds are preferred because of their low cost, good electrochemical stability, resistance to poisoning and high catalytic activity in alkaline environment.18 They are also widely used as bimetallic catalysts for water splitting,19 oxygen reduction reaction (ORR),20,21 ethanol,22,23 hydrazine,24 methanol25 and glucose oxidation.13 Nickel and Cobalt oxides are considered propitious transition metal oxide combinations used for batteries,26 fuel cell13 and supercapacitors.15,27,28 Gao et al.,13 used Ni–Co composite catalyst in a direct glucose fuel cell and obtained 23.97 W m−2 peak power density. Similarly, Fen et al.,29 and Jing et al.,30 used bimetallic nickel–cobalt catalyst for urea hydrogen peroxide and methanol fuel cell. These studies demonstrated interesting electrochemical properties of nickel and cobalt, indicating their potentials as electrode materials. The cost-effective association of these two metals has higher electrochemical activity in binary metal oxide attributed to the multiple redox mechanism and the synergistic effects in fuel cells.13,31
Transition metal nitrides are another type of material that exhibits excellent conductance, good stability and speeds up charge transport when employed as electrode.32–34 On the other hand, the presence of nitrogen strongly influences the electronic properties of the metal by increasing the density of electrons on the surface of the metal. Therefore, the metal nitride has a higher electro-catalytic activity in the reduction reaction than the corresponding pure metal.
The urea-glass route is a carbothermal reduction method for the synthesis of various metal carbides and nitrides in the presence of an N/C source.35 It has the advantages of being simple, scalable, and versatile. A key feature of the “urea glass technology” is the formation of a gel-like starting material consisting of a polymer composite between a metal precursor and urea, and environmental treatment of the corresponding carbides and nitrides.22 This is an easy way to minimize the use of toxic solvents and does not require purification. In fact, this is a simple and safe way to use urea as a nitrating agent instead of high-pressure ammonia to produce nitrides.
In this study, a low-cost and efficient nickel–cobalt nitride (Ni3N–Co3N) composite with 3D-porous nanostructure was prepared by using a facile urea glass method. That prepared composite was applied for direct glucose oxidation in the simple and non-toxic method. The performance results demonstrated that Ni3N–Co3N nanoparticles are an effective platform for the electrooxidation of glucose in an alkaline medium. The formation of Ni3N–Co3N was confirmed by XRD, SEM and XPS techniques. Our results showed that nickel–cobalt nitride composite has good electrocatalytic activity and is a promising fuel cell catalyst.
Fig. 1 (A) X-ray diffraction (XRD) patterns of Ni3N, Co3N, and Ni3N–Co3N; (B–D) SEM images of Ni3N–Co3N at 10 μm, 1 μm, and 200 nm resolution. |
The morphology of Ni3N–Co3N nanoparticles can be observed by SEM images which is illustrate in Fig. 1B–D, that showed the agglomeration of particles, with particle size on the nanometer scale. At 1 μm-magnification SEM image (Fig. 1C), the composite exhibits a similar network structure and abundant interconnected channels, which significantly contributes to the diffusion and transfer of ions from the bulk solution to the inner surface of the porous material. Fig. 1D reveals that roughly spherical Ni3N–Co3N nanoparticles having a diameter of about 20 nm are agglomerated with smaller nanoparticles. Thus, it can be inferred that the particle size of the Ni3N–Co3N catalyst depends on the degree of agglomeration between the smaller particles. The small dots on agglomerated particles can bring a beneficial effect on substrate contact through increasing the surface area of the composite.
To further, study the electronic states of Ni3N–Co3N, X-ray photoelectron spectroscopy (XPS) measurements were performed. The survey spectrum (Fig. 2A) indicates the existence of C 1s, O 1s, N 1s, Ni and Co. The O 1s peaks slighter shift towards higher binding energy due to the Ni and Co attachment. Narrow range Ni 2p spectra is provided in Fig. 2B, two peaks observed at a binding energy of 856.01 eV and 855.45 eV which indicates the presence of Ni3+ and Ni2+ (ref. 40 and 41) respectively. In Ni 2p spectrum, the peak of nickel nitride shifts to higher binding energy, which is probably due to the nickel atoms are surrounded by Co atoms and having fewer electrons. Therefore, the decrease in the electron shielding effect causes the positive shift of the Ni 2p peak, which confirmed the peak shift in XRD results due to alloying of nickel and cobalt. Fig. 2C, indicates that cobalt is present in the oxidation states of Co3+ (780.68 eV) and Co2+ (782.06 eV), similar to the literature results for N atom coordinated Co3+ and Co2+.41 The N 1s spectrum has three peaks: pyridine-N (398.4 ± 0.2 eV), pyrrolic N (400.2 eV), and oxidized N (402.3 eV) (Fig. 2D).42 Recent studies have shown that pyridine-N and pyrrolic N act as effective chemically active sites for the dyadic response. In addition, graphite N may contribute to electron transport, while oxidized N improves surface wettability and facilitates the transport of ions from electrolyte solutions to the interface.43 In the fitting of O 1s XPS spectrum (Fig. 3A), the composite sample showed three peaks at 530.6, 531.5, 533.2 and 537.7 eV, respectively. The peak value of 530.6 eV corresponds to quinone. The peak at 531.5 eV is due to CO in the carboxyl (COOH) and/or carbonyl (CO) groups. The fitted peak at binding energy = 533.2 eV represents single bond oxygen (–O–). Moreover, these oxygen-containing groups not only enhance the wettability, but also reduce the internal resistance, and can also reversibly react with hydrogen ions under alkaline conditions, and the electrochemical performance and catalytic ability can be greatly improved. According to the above analysis, the functional groups of the heteroatoms have been successfully bonded to the basal or graphite lattice edges, and due to good wettability, can contribute to glucose oxidation, and enhanced conductivity as well as low ion diffusion/transport. Further confirmation of the XPX spectrum, energy-dispersive X-ray spectroscopy (EDX) was performed (Fig. 3B). The EDX spectra represent the occurrence of Ni, Co, and N which is consistent with Co 2p, Ni 2p, and N 1s respectively.
Fig. 2 XPS survey spectrum of Ni3N–Co3N (A); high resolution of Ni 2p (B), Co 2p (C) and N 1s spectra (D). |
Ni2+ + OH− ↔ Ni3+ + e− | (1) |
Co2+ + OH− ↔ Co3+ + e− | (2) |
Ni3+ + glucose → Ni2+ + glucolactone | (3) |
Co3+ + glucose → Co2+ + glucolactone | (4) |
Ni3Co3 + glucose → Ni2Co2 + glucolactone | (5) |
Fig. 5 Schematic representation of Ni3N–Co3N modified electrode and proposed acting mechanism of Ni3N–Co3N catalyst in a glucose fuel cell. |
Two redox couples (Ni3+/Ni2+ and Co3+/Co2+) may play important roles in this process. In an alkaline condition, both Ni2+ and Co2+ can be transformed into Ni3+ and Co3+, respectively, and give electrons to the current collector. The formed Ni3+ and Co3+ can transform glucose into oxidation products and regenerate Ni2+ and Co2+. There may exist a synergic effect between the two redox couples. It has been proposed that nickel can effectively catalyze the glucose oxidation and cobalt can facilitate the transportation of reaction products produced from glucose oxidation.13 Furthermore, the 3D porous nanostructure of Ni3N–Co3N can benefit the uniform dispersion of catalytically active sites on the composite surface.
In order to determine the substantial responses of Ni3N, Co3N and Ni3N–Co3N nanoparticles, LSV was performed which can be ascribed to glucose oxidation by these metal nitride catalysts. LSV curves slopes (Fig. 6A) were found ascending in the following manner: Co3N < Ni3N < Ni3N–Co3N. A peak current density of 28.3 mA cm−2 was obtained at −0.4 V vs. Hg/HgO in this work which is almost 1.39 times higher than that of Co3N (11.81 mA cm−2) and 0.45 times higher than of Ni3N (19.42 mA cm−2). The obtained current density of bimetallic catalyst Ni3N–Co3N is higher as compared to mono metal nitride catalysts because the electronic environment of the metal surface is changed by the formation of a heteroatom bond (Fig. 5). That modification improved its electronic structure through the ligand effect, and the geometry of the bimetallic structure also transformed from that of parent metals, resulting in a strain effect that modifies the electronic structure by variation in the orbital overlap.46 Secondly, the nitride doping improved its electrical conductivity and catalytic ability. Owing these reasons, the current density of Ni3N–Co3N is much higher than that of a previous work done on Ni4–Co2/AC composite catalyst 21.03 mA cm−2 (ref. 13) at −0.4 V vs. Hg/HgO. Cao et al.47 suggested that the presence of nitrogen can considerably improve the electronic properties of metals by increasing electrons density on its surface. Wang et al.48 also declared that the addition of an appropriate amount of nitride could remarkably enhance catalyst performance.
EIS is a powerful diagnostic tool for fuel cells to characterise various limitations and improve fuel cell performance. There are three basic sources of fuel cell voltage loss: charge transfer activation or “kinetic energy” loss, ion and electron transfer or “ohmic” losses, and concentration or “mass transfer” losses. Among other factors, the environmental impact spectrum is an experimental technique that can be used to separate and quantify these polarization sources. Fig. 6B shows the Nyquist plots of all the anodes fitted by using the equivalent electrical circuit method.49 The total resistance (Rt) in the equivalent circuit is mainly comprised of three resistances: ohmic resistance (Rs), diffusion resistance (Rd) and charge transfer resistance (Rct).38,50 The fitted EIS data are listed in Table 1. Rt values declined in the following order: Co3N > Ni3N > Ni3N–Co3N. The Ni3N–Co3N shows the lowest Rt values compared to Ni3N and Co3N which are 3.9356 Ω, 4.5438 Ω, 6.682 Ω, respectively.
Anode | Rs (Ω) | Rct (Ω) | Rd (Ω) | Rt (Ω) |
---|---|---|---|---|
Ni3N–Co3N | 0.6985 | 0.2671 | 2.97 | 3.9356 |
Ni3N | 0.9758 | 1.056 | 2.008 | 4.5438 |
Co3N | 0.772 | 1.093 | 4.817 | 6.682 |
To further demonstrate the performance of these electrocatalysts in actual operating fuel cells, a full glucose fuel cell was assembled. Different anodes containing 5% catalysts and a same Cu2O–Cu air–cathode were employed to investigate the power density and polarization curves. The performance of full glucose fuel was examined by varying resistance from 9000 to 10 Ω. Fig. 6C depicts the current density and power density curves of these anodes in glucose fuel cells. The fuel cell equipped with the Ni3N–Co3N anode exhibits a maximum power density of 30.89 W m−2 which is almost 2.6 times greater than that of bare AC anode (11.94 W m−2), 1.7 times greater than Co3N anode (17.47 W m−2), and 1.3 times higher than Ni3N anode (23.62 W m−2). While, the current density of 97.66 A m−2, 86.30 A m−2, 73.81 A m−2, and 58.42 A m−2 were obtained in fuel cells equipped with Ni3N–Co3N, Ni3N, Co3N, and bare AC, respectively.
The obtained power density of Ni3N–Co3N is higher than that of reported noble and transitional metals like, Ni4–Co2/AC (23.97 W m−2),13 Ag (20.3 W m−2),51 Pd–Pt/GO/Ni (12.5 W m−2)52 and Au MnO2/C (11 W m−2),53 correspondingly. However, the potentials of cathodes remained unchanged as shown in Fig. 6D, which reveals that variation in power density is mainly ascribed to the modification in anodes. The fuel cell equipped with the low-cost Ni3N–Co3N catalyst noticeably has higher power density. It can be supposed that the interactive property of Ni and Co and the presence of nitrogen in the composite boost up the electro-catalytic activity. It has been reported that a covalent bond between metals and nitrogen in metal nitrides and the number of unpaired d-electron offered for intra-bond polarization makes the metals be easily reduced.54
Footnote |
† Both authors contributed equally to this work. |
This journal is © The Royal Society of Chemistry 2020 |