Qian
Huang‡
a,
Shuxian
Zhuang‡
b,
Xin
You
b,
Jinpeng
Zhang
a,
Ao
Xie
b,
Yu
Chen
c,
Yang
Tang
*a,
Yongmei
Chen
b,
Mingfei
Shao
d,
Xiao Jin
Yang
b and
Pingyu
Wan
*b
aInstitute of Applied Electrochemistry, Beijing University of Chemical Technology, Beijing 100029, China. E-mail: tangyang@mail.buct.edu.cn
bNational Fundamental Research Laboratory of New Hazardous Chemicals Assessment & Accident Analysis, Beijing University of Chemical Technology, Beijing 100029, China. E-mail: pywan@mail.buct.edu.cn
cInstitute of Future Technology, Ningbo Ronbay New Energy Technology Co., Ltd., Ningbo 315000, China
dState Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China
First published on 15th November 2021
The aqueous zinc-based batteries such as the Zn–air battery and the Zn-ion battery, featured with low-cost, high safety, high specific capacity and environmental-friendliness, have attracted intensive interest for their energy storage capability. For high performance zinc-based batteries, electrocatalysts with good conductivity, convenient mass transfer path and adequate accessible active sites are required. Herein, the fabrication of transition-metal/nitrogen co-doped honeycomb-like carbon materials with a hierarchical porous structure using a freeze-drying and one-step pyrolysis method is described. The as-prepared FeZnN–C is used as the cathode material of the zinc–air battery, which displays a high peak power density of 257 mW cm−2, a superior discharging voltage platform of about 1.36 V and a large specific capacity of 785 mA h gZn−1. The prepared Zn/ZnN–C was used as the anode of a Zn-ion battery which exhibits great cycle stability and a high charging/discharge rate. This work provides a novel and simple method for the construction of highly efficient electrocatalysts with hierarchical macro–meso–micro-porous structure and multi-active sites, which could be used for potential applications involving aqueous zinc-based batteries.
Currently, the transition metals (TM = Co, Cu, Fe, Ni, and Zn) and heteroatoms (X = B, F, N, P, S) of the co-doped carbon material (TMX-C) featuring low-cost, comparable catalytic activity and stability have been regarded as one of the most promising alternatives to Pt-based materials for ORR, especially in an alkaline medium.18–20 Among numerous TMX-Cs reported in the past decades, the FeNC-based electrocatalysts have attracted the most intensive study because of their good ORR performance.21,22 The high temperature pyrolysis of the TM, nitrogen and carbon precursors is the most common method to prepare TMN-Cs. However, the Fe atoms and carbon-based frameworks always aggregate during the high-temperature process, which decreases the active sites and hinders the porous structure formation, respectively.23 To improve the actual performance of the FeNC-based materials, it is essential to adopt two strategies: (1) fabricating of a hierarchical micro–meso–macro-porous structure with a large specific surface area and good conductivity to improve the apparent activity by increasing accessible active sites, optimizing electron conduction and providing a convenient path for electrolyte and oxygen diffusion,24,25 and (2) introducing multiple metal components to further adjust the electron state and O2 adsorption model, improve intrinsic activity and selectivity.26 It is disclosed that the coordination of multiple metals [(Cu, Fe)–N–CNT,27 (Fe, Co)/N–C,28 Ni–Co–Zn–N–PC29 and Fe–Zn–SA/NC30] and nitrogen atoms is conducive to easily breaking the O–O bonds.
In order to construct and maintain the highly porous structure, conventional templates (SiO2,31 MgO,32 F127 (ref. 33) and P123 (ref. 34)) are commonly used, which inevitably involve a high cost, use of toxic regents or tedious template-removal steps.35 In contrast, using NaCl as a low-cost, highly water soluble and chemically stable inorganic salt is convenient to operate, and it is easy to remove and recycle high purity NaCl by simple recrystallisation.36 Therefore, using crystalline NaCl as a template is a simple and environmental-friendly way to construct a porous structure. However, according to previous research,37 the porous structure carbon prepared by a method of rapid freezing of NaCl mainly consists of macro-pores, with only a few micro- and meso-pores. Wang et al.38 mixed poly(vinylpyrrolidone) and NaCl followed by rapid freezing, and then ground the freeze-dried mixtures with zinc chloride (ZnCl2). The mixture was pyrolyzed at 900 °C and then annealed again at 900 °C after washing the pyrolysis product with H2SO4. In addition to the macro-pores created by NaCl, as expected, the ZnCl2 enters the carbon framework during the pyrolysis and sublimation process because of capillary action, simultaneously forming a meso–micro-pore structure. However, this process is rather complicated not only because of the additional step to introduce Zn but also because of the second high temperature treatment.
In this work, honeycomb-like hierarchically porous FeZnN–C electrocatalysts were fabricated by freeze-drying precursors of carbon, nitrogen and metals, followed by a one-step pyrolysis process, in which the ZnCl2 and NaCl act as a co-template. The NaCl provides a framework for the precursor molecules and creates a macro-porous structure, whereas the ZnCl2 acts as a chemical activator to introduce abundant micro- and meso-pores and as a metal precursor to fabricate a Zn modified FeN–C material. The prepared FeZnN–C shows an excellent ORR performance with a high half-wave potential (E1/2) of 0.901 V (vs. RHE) and a diffusion-limiting current density (JL) of −6.04 mA cm−2, an extremely low average peroxide yield of only 1.25% and a high electron-transfer number (n) up to 3.97, which were superior to those of a commercial Pt/C catalyst (E1/2 of 0.851 V, JL of −5.27 mA cm−2, a H2O2 yield of 2.85% and a n of 3.94). The zinc–air battery using FeZnN–C as an electrocatalyst for cathode displays a high peak power density of 257 mW cm−2, high discharging potential of 1.36 V at 10 mA cm−2 and a large specific capacity of 785 mA h gZn−1. Moreover, the ZnN–C prepared by a similar method is further used to fabricate Zn/ZnN–C as a dendrite-free Zn containing anode for the advanced zinc-ion battery, which exhibits a high cycling stability and a high charging/discharging rate.
The electron transfer number (n) and hydrogen peroxide yield (H2O2%) at various potentials were calculated as follows:
n = 4Id/(Id + Ir/N) |
H2O2% = (200Ir/N)/(Id + Ir/N) |
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Scheme 1 Schematic diagram of the synthetic route for honeycomb-like hierarchically porous carbon materials and their application in a zinc–air battery and a zinc-ion battery. |
The micro-crystal structure of carbon was further investigated by XRD and Raman spectroscopy. As shown in Fig. 2a, all these carbon materials exhibit similar and excessively broad diffraction peaks at 25° and 43°, belonging to the (0 0 2) and (1 0 0) planes of graphitic carbon, respectively, which was consistent with the unperfect carbon lattice with abundant defects in the HRTEM result. Moreover, FeZnN–C and ZnN–C exhibit relatively higher intensities at low angles below 10°, which can be attributed to the diffraction of micro-pores in the carbon matrix and indicate more micro-pores than those of FeN–C and N–C.39 The Raman spectra (Fig. 2b) of the four previously described materials show two peaks at about 1353 cm−1 and 1583 cm−1, which were attributed to the D band and the G band, respectively. The ID/IG values of FeZnN–C, ZnN–C, FeN–C and N–C were 0.93, 0.94, 0.96 and 0.98, respectively. The lower ID/IG values of FeZnN–C and ZnN–C proved that the presence of the Zn component improved the degree of graphitisation of the carbon structure, which would optimize electronic conductivity, and thus facilitate the ORR process.
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Fig. 2 (a) The XRD patterns, (b) the Raman spectra, (c) the nitrogen adsorption–desorption isotherms, and (d) the corresponding pore size distribution of N–C, FeN–C, ZnN–C and FeZnN–C catalysts. |
The enriched porous structure benefitted from ZnCl2 was further confirmed by the N2 adsorption–desorption isotherms of the four catalysts (Fig. 2c). All these electrocatalysts exhibit type IV isotherm curves with a high N2 uptake in the medium and high relative pressure areas, which indicated the existence of meso- and macro-pores. In the comparison with the FeN–C and N–C, FeZnN–C and ZnN–C showed a much sharper uptake in the low relative pressure (P/P0 < 0.05) area, which suggested there were plentiful micro-pores inside the materials prepared with the addition of ZnCl2. The BET specific surface areas calculated from the isotherms of FeZnN–C and ZnN–C were as large as 1455.2 m2 g−1 and 1134.4 m2 g−1, respectively, which were significantly increased from the 304.4 m2 g−1 and 237.5 m2 g−1 for FeN–C and N–C, respectively. It was calculated that the specific surface areas of the catalysts with the addition of ZnCl2 (FeZnN–C, ZnN–C) were approximately 4.7 times higher than the results for the catalysts without ZnCl2 (FeN–C, N–C), respectively. Fig. 2d shows the corresponding pore size distribution of the four materials. The FeZnN–C and ZnN–C possessed an obviously intensive pore size distribution in the range of 1–1.5 nm (micro-pores) and 2–4 nm (small meso-pores), suggesting that the addition of ZnCl2 could increase the number of smaller meso-pores and give extra micro-pores. It was inferred that the FeZnN–C and ZnN–C possessed simultaneously, a hierarchical porous structure with abundant micro-, meso- and macro-pores, in which the micro-pores provided a larger specific surface area to facilitate the exposure of active sites whereas the meso-/micro-pores promoted the transmission of electrolytes during the ORR process. It was expected that the unique architecture with pores of various sizes and a large specific surface area would contribute to the high electrocatalytic performance, especially the high diffusion-limiting current densities for FeZnN–C or ZnN–C.
The chemical states and composition of FeZnN–C, ZnN–C, FeN–C and N–C were investigated by XPS measurement. The XPS survey spectra for these samples are shown in Fig. 3a, and they verify the existence of C, N, O and Zn. The presence of a low content of Fe in FeZnN–C and FeN–C could be observed from magnification of the XPS spectra (Fig. S3a, see ESI†). The Zn mainly existed in the chemical state of Zn2+ as shown in Fig. S3b (see ESI†). Table S1 (see ESI†) and Fig. 3b show the atomic percentage of each element for the four catalysts. The FeZnN–C had a relatively high N content of 9.85% with 0.21% Fe and 0.60% Zn. The contents and chemical state of N and the metals have an important effect on the determination of the catalytic activity for the ORR. As shown in Fig. 3c, the N 1s spectrum of FeZnN–C was deconvoluted into five peaks, which were attributed to pyridinic-N (398.3 eV), metal-N (399.1 eV), pyrrolic-N (399.8 eV), graphitic-N (401 eV) and oxidized N (403 eV). According to the results given previous reports,40,41 pyridinic-N, graphitic-N and metal-N are three effective N species that have high ORR activity, whereas pyrrolic-N and oxidized N are ineffective N species. Thus, Fig. 3d and Table S1 (see ESI†) further compare the contents of different N species and the chemical states of these materials. Out of all the catalysts, the percentage of effective N species of FeZnN–C was the highest, not only with respect to all the nitrogen species (85.7%) but also with respect to all the elements' composition (8.44%). It was expected that among these materials, FeZnN–C would exhibit the highest ORR activity. In the comparison of these catalysts, it was observed that the introduction of ZnCl2 can not only enrich the porous structure, but also increase the content ratio of the effective nitrogen species, thereby simultaneously increasing the apparent and intrinsic catalytic activity.
To clarify the ORR kinetics, the LSV curves of FeZnN–C and 20% Pt/C at different rotating speeds were measured and the results are shown in Fig. S4a and b (see ESI†). Fig. S4c (see ESI†) shows the corresponding Koutecký–Levich curves obtained at 0.4 V and 0.6 V, from which it was derived that FeZnN–C undergoes a four-electron ORR process, similar to 20% Pt/C. As shown in Fig. 4d, both FeZnN–C and ZnN–C exhibited a low yield of H2O2 produced by a two-electron process, and their electron transfer numbers (n) were very close to 4. In particular, the H2O2 yield on FeZnN–C was as low as 1.25% and the average n was 3.97 in the potential range of 0.2–0.9 V. It was observed that the ORR performance of FeZnN–C surpassed that of 20% Pt/C (H2O2 yield of 2.85%, n of 3.94) and most of the non-noble metal electrocatalysts reported previously (Table S2, see ESI†).
As shown in Fig. 4e, the Tafel slopes of FeZnN–C, ZnN–C, FeN–C, N–C and 20% Pt/C were 65.6 mV dec−1, 77.8 mV dec−1, 74.5 mV dec−1, 78.5 mV dec−1 and 68.6 mV dec−1, respectively. The smallest Tafel slope of FeZnN–C indicated that the rate-determining step of the ORR process was the second electron transfer step and this demonstrated the optimal intrinsic catalytic activity.42Fig. 4f shows the electrochemical impedance spectroscopy (EIS) of the five catalysts at 0.811 V. In the high frequency area, the same initial value represents the solution resistance (Rs) of 42 Ω for all the materials in the same test system. In the low frequency area, the diameter of the semicircle represents the charge transfer resistance (Rct). The lowest Rct of FeZnN–C indicated that it had the best electronic conductivity of ORR among all the materials. Both the smallest Tafel slope and lowest Rct show the improved ORR kinetics obtained with FeZnN–C.
On the other hand, the double-layer capacitance (Cdl) was also measured by CV in the non-Faraday response region at different scan rates (Fig. S5a–d, see ESI†) to evaluate the electrochemical surface area (ECSA) of these catalysts. The Cdl for FeZnN–C, ZnN–C, FeN–C and N–C were 20.60 mF cm−2, 15.97 mF cm−2, 12.52 mF cm−2 and 10.80 mF cm−2 (Fig. S5e, see ESI†), and the corresponding ECSA estimated from the Cdl were 515 cm2, 399 cm2, 313 cm2 and 270 cm2, respectively. The largest Cdl and ECSA values of FeZnN–C also revealed the largest charge storage capacity and specific surface area, which would provide the most sites of activation, thus presenting the highest apparent catalytic activity towards the ORR. The ECSA normalised LSV curves of N–C, FeN–C, ZnN–C and FeZnN–C in the electrochemical polarization-controlled region are shown in Fig. S6 (see ESI†), which verified that FeZnN–C showed the best intrinsic ORR activity. Furthermore, the methanol tolerance tests and thiocyanate (SCN−) poison tests were also performed, and the results are shown in Fig. 4g and h, respectively. The relative ORR current at 0.6 V on a 20% Pt/C electrode decreased sharply and immediately when methanol was added to the electrolyte. In contrast, there was almost no decay for the relative ORR current on FeZnN–C, which showed its higher resistance to methanol poisoning than that of 20% Pt/C (Fig. 4g), and this means that it is a promising ORR catalyst for use in fuel cells. SCN− is always used to evaluate the active sites of TMN-C because most non-noble metals are susceptible to SCN−. The results of the SCN− poison test showed that the E1/2 of FeZnN–C, ZnN–C and FeN–C negatively shifted by 21 mV, 38 mV and 44 mV, respectively, indicating that both Fe- and Zn-containing sites in FeZnN–C were simultaneously effective for ORR.
A zinc–air battery was assembled using the prepared FeZnN–C as an electrocatalyst for an oxygen diffusion cathode (2 × 2 cm2) with a commercial Zn plate (2 × 2 cm2) as anode and 6 M KOH as electrolyte. A similar zinc–air battery with commercial Pt/C as a cathodic electrocatalyst was also assembled for comparison. The polarization and power density curves of these two zinc–air batteries are shown in Fig. 5a. It was observed that the FeZnN–C battery achieved a peak power density of 257 mW cm−2 at a current density of 310 mA cm−2, which was superior to that of the Pt/C battery (191 mW cm−2 at 248 mA cm−2). Table S3 (see ESI†) also shows that FeZnN–C was much better than most of the previously reported ORR catalysts. Fig. 5b shows the discharge voltages of these two batteries at different constant current densities of 1 mA cm−2, 10 mA cm−2, 50 mA cm−2 and 100 mA cm−2. The cell voltage of the FeZnN–C battery was 60–70 mV higher than that of Pt/C at same current densities (Table S4, see ESI†). Typically, at 10 mA cm−2, the discharge voltage of the FeZnN–C battery could achieve 1.36 V with a specific energy density of 785 mA h gZn−1, which was higher than that of the Pt/C battery (1.30 V with 698 mA h g−1) as shown in Fig. 5c. In addition, the FeZnN–C battery only had a slight decline in discharge voltage at 10 mA cm−2 during each 30 h cycle with a total of six cycles in the durability test during which the zinc plate was replaced regularly, indicating the great durability of FeZnN–C. In contrast, the discharge voltage of the Pt/C battery was not only lower than that of the FeZnN–C battery, but also obviously fluctuated during the durability test with six cycles at the current density of 10 mA cm−2, which indicated that the FeZnN–C battery had better durability than the Pt/C one. These zinc–air battery tests strongly verified that FeZnN–C was a promising electrocatalyst for improving discharge voltage, power density and stability.
Therefore, the ZnN–C prepared at 650 °C, featuring a honeycomb-like hierarchically porous structure, a low voltage gap, a high CE and great Zn affinity, was used as the substrate for zinc deposition. The Zn/ZnN–C catalyst was obtained with an area capacity of 10 mA h cm−2, and the SEM images and schematic diagram are shown in Fig. 6a–c. It can be seen that the surface of the ZnN–C becomes uniform and flat, and notably no dendritic Zn was observed on the Zn/ZnN–C surface even for the high area capacity electrode. Two symmetrical coin cells, Zn/ZnN–C‖Zn/ZnN–C and Zn plate‖Zn plate, were assembled and tested in 2 M ZnSO4 to examine the zinc plating/stripping reversibility of Zn/ZnN–C and commercial Zn plate. The zinc-ion battery performances using Zn/ZnN–C and commercial Zn plate as zinc anodes were firstly tested at the same current density of 0.5 mA cm−2 (Fig. 6d). The polarisation voltage of Zn plate‖Zn plate was 45.3 mV and 92.7 mV in the first and 50th cycle, respectively (Fig. 6e and f). Comparatively, that of Zn/ZnN–C‖Zn/ZnN–C was as low as 15.5 mV in the first cycle (Fig. 6e), and only slightly increased to 18 mV after 50 cycles (Fig. 6f) and to 35 mV after 300 cycles (Fig. 6g). Moreover, the durability of the Zn/ZnN–C‖Zn/ZnN–C was much longer than that of Zn plate‖Zn plate. Short circuiting and large fluctuations of voltage did not happen to the former cell. The large specific surface area with the abundant active sites of ZnN–C containing Zn–N would provide sufficient sites for zinc deposition,43 which was conducive to the formation of a uniform and flat Zn layer, which further improving the cell performance. At the same time, the abundant porous structure of ZnN–C facilitated mass transfer and avoided the formation of Zn dendrites during the plating/stripping process, thereby ensuring the stability of the symmetrical coin cells.
The rate performances of the two batteries described previously are shown in Fig. S8 (see ESI†). Compared with commercial Zn plate‖Zn plate, Zn/ZnN–C‖Zn/ZnN–C exhibited a better rate performance at all the current densities of 0.5, 1, 2, 5 and 10 mA cm−2, which was reflected in the low polarization voltage. Moreover, the Zn/ZnN–C‖Zn/ZnN–C showed a much more stable charging/discharging voltage than commercial Zn plate‖Zn plate. Fig. S9 (see ESI†) further showed that Zn/ZnN–C‖Zn/ZnN–C was superior to Zn plate‖Zn plate in the long-term charging/discharging process at different current densities – area capacities of 1 mA cm−2 to 1 mA h cm−2, 2 mA cm−2 to 2 mA h cm−2 and 5 mA cm−2 to 2 mA h cm−2. The Zn plate‖Zn plate showed a large vibration of voltage after dozens of hours of testing, suggesting that it suffers from short-circuiting, whereas the prepared Zn/ZnN–C‖Zn/ZnN–C displayed a more stable charging and discharging voltage in addition to the lower polarization voltage. Moreover, the Zn/ZnN–C still maintained a uniform and flat surface after repeated Zn stripping/deposition processes, because of the porous structure which is beneficial to mass transfer (Fig. S10a and b, see ESI†). On the other hand, there are dense and sharp Zn dendrites on the surface of the commercial Zn plate (Fig. S10c and d, see ESI†) after a long-term test with 50 cycles at a current density – area capacity of 1 mA cm−2 to 1 mA h cm−2.
Furthermore, the zinc-ion battery employing Zn/ZnN–C as the anode and MnO2/C as the cathode was assembled. The battery performance was evaluated in 2 M ZnSO4 to 0.2 M MnSO4. As shown in Fig. 6h, the Zn/ZnN–C‖MnO2/C battery exhibited typical charging and discharging curves with the platform voltage at 1.33–1.39 V and 1.50–1.60 V, respectively. The Zn/ZnN–C‖MnO2/C battery delivered discharge capacities of 397.7, 362.8, 330.2, 279.1 and 279.5 mA h g−1 at different current densities (0.1, 0.2, 0.5, 1 and 2 A g−1), respectively, displaying its good rate capability. The specific capacity of this battery hardly declined during each test at various current densities as shown in Fig. 6i. These results demonstrate that the Zn/ZnN–C is a highly efficient, dendrite-free anode.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1se01427g |
‡ These co-first authors contributed equally to this work. |
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