Xinlei Luo‡
a,
Ziheng Zheng‡a,
Bingxue Houb,
Xianpan Xie*a and
Cheng Cheng Wang*a
aSchool of Entrepreneurship and Innovation, Shenzhen Polytechnic, Shenzhen, 518055, China. E-mail: xxp@szpt.edu.cn; wangchengcheng@szpt.edu.cn
bAviation Engineering Institute, Civil Aviation Flight University of China, Guanghan, 618037, China
First published on 21st June 2023
A novel catalyst obtained from the pyrolysis of a Co/Fe/Zn zeolitic imidazolite framework was prepared as an oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) electrocatalyst. The Co–N–C-900 catalyst displays promising ORR and OER activity with E1/2 = 0.854 V and Ej=10 = 1.780 V. The rechargeable Zn–air battery equipped with a Co–N–C-900 cathode electrocatalyst illustrates a high peak power density of 275 mW cm−2, which is much superior than that of commercial 20% Pt/C. Significantly, the designed Zn–air battery with the Co–N–C-900 catalyst presents good cycling stability for 180 h in the rechargeable Zn–air battery.
NPMC electrocatalysts M–N–C (M = Fe or Co)1–13 have been widely studied as efficient oxygen electrocatalysts. ORR activity can be largely improved by introducing Co into nitrogen doped carbon due to more active sites.14,15 Therefore, carefully designing Co–N–C materials is desirable for obtaining electrocatalysts with promising electrochemical performance. MOFs are excellent candidates for metal carbon composites due to a great variety of compositions including metal ions and organic ligands. Zeolitic imidazolate frameworks (ZIFs) are reckoned as precursors due to C, N, and transition metals. Recent years, some researchers are paying more attention to apply Co–N–C electrocatalysts prepared by ZIFs structure in the fields of Zn–air battery. Rui Hao studied21 trimetallic zeolitic imidazolite frameworks-derived Co nanoparticles@CoFe–nitrogen-doped porous carbon as bi-functional electrocatalysts for Zn–air battery, and they found that Co@CoFe–N–C catalysts displayed desirable ORR and OER activity. Zn–air battery equipped with Co@CoFe–N–C catalyst could exhibit high power density of 174.1 mW cm−2 and outstanding stability of 100 h. J. Tan22 also did some further study on the Co, ZIF-8 derived Co–N–C with high ORR performance and it was used as a support to disperse OER active Ni3Fe nanoparticles. At current densities of 10 mA cm−2 for OER, potentials were 1.54 V and potentials were 0.79 V at current densities of 3 mA cm−2 for ORR, respectively. Rechargeable Zn–air battery assembled with Ni3Fe/Co–N–C exhibited superior efficiency and excellent durability. Although extensive efforts were made due to preparation of Co–N–C electrocatalysts in the aspects of Zn–air battery, some organic solvents and surfactants were still used in the study, which could lead to waste and environmental pollution.
In this study, ZIFs structure were used because of its high nitrogen/carbon contents, high microporous surface area, tunable pore texture, and highly dispersed metal sites after thermal decomposition. It is widely known that Co–N–C was extensively studied to identify ORR and OER electrocatalytic performance. In the present study, in order to make the preparation process easier and scalable, water was innovatively only used as solvents in facile preparation of Co–N–C electrocatalysts in this study. Results showed that Co–N–C catalyst fired at 900 °C in N2 atmosphere exhibited high ORR activity with E1/2 of 0.854 V under O2-saturated 0.1 M KOH, which was comparable with commercial Pt/C from JM (Johnson Matthey) under the same testing condition. Moreover, regarding to OER activity, it also showed comparable current density and onset potential. Assembled initial Zn–air battery by Co–N–C exhibited relatively stable charging and discharging cycle stability for more than 180 h. This work opens light on a promising technique to prepare Co–N–C electrocatalysts and enhance its application of rechargeable zinc–air battery.
For ORR tests, linear scan voltammetry (LSV) was tested from 0 to −1 V (vs. RHE) at 10 mV s−1 in oxygen saturated 0.1 M KOH from 400 rpm to 2500 rpm with the same loading. For OER tests, LSV was conducted from 0 to 1 V (vs. RHE) at 10 mV s−1 at 1600 rpm in oxygen saturated 0.1 M KOH. IR correction and corrected curves including LSV were already done, which were shown in the previous paper.17 Pt/C and IrO2 from Johnson Matthey (Pt/C-JM) were studied for ORR and OER under the same condition. Potentials were given with RHE reference electrode (ERHE = Evs. SCE + ESCE + 0.059 × pH, ESCE = 0.247 V at 20 °C).
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Fig. 1 XRD patterns of as-prepared Co–N–C precursor (a), Co–N–C-800 (b), Co–N–C-900 (c), and Co–N–C-900–acid (d) catalysts. |
SEM was used to identify Co–N–C precursor's morphology and Co–N–C catalysts' morphology. According to Fig. 2a, it can be shown that the size of rhombic dodecahedron shaped ZIF-8 was 300 nm. After firing at 800 °C, the surface of the particles became quite rough, and there were some tiny particles covering. Moreover, the surface of Co–N–C-900 particles (Fig. 2e and f) looked differently from that of Fig. 2c and d. It is also worth mentioning that there was no huge difference of Co–N–C-900–acid catalysts compared to that Co–N–C-900 particles (Fig. 2g and h). It can be seen from our results, the shape of the nanoparticles was rhombus, however, in this recent published paper, it illustrated that a novel nanocapsules with cobalt core and nitrogen-doped carbon shells (Co@NC) supported on MOFs-derived hierarchical porous carbon framework.16 By rationally controlling the size and mesoporous ratio, can they obtain promising electrochemical performance. It is worthwhile for us to do further study regarding to our experiment.
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Fig. 2 SEM image of as-prepared Co–N–C precursor (a and b), Co–N–C-800 (c and d), Co–N–C-900 (e and f), and Co–N–C-900–acid catalysts (g and h). |
Fig. 3 showed TEM images of Co–N–C-900 catalysts. Only rhombic structure can be observed from high magnification pictures. Elemental mapping in TEM clearly shows that C, N and Co elements were distributed homogeneously in the structure. Fig. S2† also showed TEM images of Co–N–C-900–acid catalysts. Elemental mapping in TEM shows that Co, C, N elements were distributed in the structure. Cobalt existed in Co–N–C-900 catalysts in a highly dispersed state. The composition of Co–N–C-900 and Co–N–C-900–acid catalysts were further investigated by XPS. Fig. 4 shows selected survey scan of catalysts after heat-treated at 900 °C, indicating the presence of Co, O, N and C elements of Co–N–C-900 and Co–N–C-900–acid catalysts. Obviously, there were no differences compared to these two catalysts, in which might illustrate that acid wash did not show obvious effect on the composition of catalysts in this study. In order to identify the assumption, the deconvolution of Co 2p and N 1s in Co–N–C-900 and Co–N–C-900–acid catalysts was shown in Fig. 5b–e. Co0, Co2+ and Co3+ were deconvoluted in this study and Table 1.
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Fig. 4 XPS of Co–N–C-900 and Co–N–C-900–acid catalysts (a), high resolution of Co 2p (b and c), N 1s (d and e) of Co–N–C-900 and Co–N–C-900–acid catalysts. |
Total atomic N% | Pyridinic N | Pyrrole N | Graphitic N | Oxidized N | |
---|---|---|---|---|---|
Co–N–C-900 | 8.21 | 23.26% | 24.97% | 38.24% | 13.54% |
Co–N–C-900–acid wash | 12.17 | 18.91% | 17.26% | 50.95% | 12.88% |
Table 1 lists the calculated results of N after deconvolution. 8.21% and 12.17% of N could account for the whole electrocatalysts. It can also be seen that pyridinic N, pyrrole N, graphitic N and oxidized N were all deconvoluted for two electrocatalysts. According to previous results, the relatively higher content of graphitic N of Co–N–C–acid wash electrocatalyst was desirable due to conveying higher positive charge density near carbon atoms and improving current density by benefiting oxygen adsorption and weakening of oxygen to oxygen bonds. Furthermore, pyridinic and pyrrole nitrogen could cooperate with cobalt to form Co–Nx. It was more active than nitrogen doped carbon materials for ORR. Table 2 lists the calculated results of Co0, Co2+ and Co3+ after deconvolution. It can be shown that 1.88% and 1.46% of Co can account for the whole electrocatalysts. Moreover, for Co–N–C-900 catalysts, the atomic ratio of Co0, Co2+ and Co3+ was nearly the same. While, for Co–N–C-900–acid wash catalysts, the atomic ratio of Co0 was more than that of Co2+ and Co3+.
Total atomic Co% | Co0 | Co2+ | Co3+ | |
---|---|---|---|---|
Co–N–C-900 | 1.88 | 33.84% | 32.49% | 33.66% |
Co–N–C-900–acid wash | 1.46 | 40.14% | 33.77% | 26.09% |
According to the XPS deconvolution results of Co and elemental mapping results of Co–N–C-900, we might make an assumption that heterointerfaces existed in the electrocatalyst, and it is also widely known that heterointerfaces can significantly improve not only the electrocatalytic activity and stability but also the ionic conductivity of heterostructure electrolytes and increasing the durability of heterostructure electrodes with faster oxygen exchange kinetics at the heterointerfaces. Such improvements can be credited to many factors, containing lattice strain, dislocation, cation segregation, changes in electronic structure, and expansion of triple phase boundaries. It has been reported that the atomic structure of the heterointerfaces such as local strain and dislocations can extremely influence the ionic conductivity and/or the electrocatalytic activity of the heterostructure.17
ORR kinetics of Co–N–C-800, Co–N–C-900, Co–N–C-900–acid and Pt/C electrocatalysts were investigated by K–L curves (Fig. S3†). Results indicated that all samples exhibited low peroxide yield and approximately electron transfer number of 4. 4e− reduction was dominant in the ORR activity of Co–N–C-800, Co–N–C-900, Co–N–C-900–acid and Pt/C. In order to compare electrochemical performance of ORR activity of these electrocatalysts, Fig. S4† lists the comparison of onset potential and current density of Co–N–C-800, Co–N–C-900, Co–N–C-900–acid and Pt/C, in which illustrate that Co–N–C-900 exhibited relatively higher ORR performance in total.
OER activity of Co–N–C-800, Co–N–C-900, Co–N–C-900–acid and IrO2 was also evaluated under O2-saturated 0.1 M KOH at 10 mV s−1 at 1600 rpm (Fig. 6). Onset potential of Co–N–C-900 was 1.58 V, which was 60 mV higher than IrO2 (1.52 V). Maximum current density measured at 1.9 V was 22.5 mA cm−2, which was higher than IrO2 (9 mA cm−2). Remarkably, Co–N–C-900 afforded current density (10 mA cm−2) at 1.81 V, while Co–N–C-800 afforded current density (10 mA cm−2) at 1.85 V, which was higher than that Co–N–C-800. IrO2 afforded current density (10 mA cm−2) at 1.98 V. Fig. 7 lists the comparison of ORR and OER activity of these Co–N–C-800, Co–N–C-900, Co–N–C-900–acid electrocatalysts under the same condition for the sake of clearly identifying the electrochemical performance, and it can clearly identify that higher sintering temperature (900 °C) could lead to better ORR and OER performance of Co–N–C electrocatalysts. One of the main reasons that could lead to relatively higher ORR and OER performance was probably due to the electron transfer to adsorb oxygen catalysis species was improved, the oxygen exchange rate was accelerated, and are the real active sites of the electrocatalytic OER and ORR processes.18–20
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Fig. 7 ORR and OER potential difference curves of Co–N–C-800 (a), Co–N–C-900 (b) and Co–N–C-900–acid (c) catalysts in O2-saturated 0.1 M KOH with 1600 rpm and scan rate of 5 mV s−1. |
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Fig. 8 (a) OCV for three Co–N–C-900 assembled Zn–air batteries. (b and c) Power supplying for LED. (d) I–V–P curves of curve of Co–N–C-900 assembled Zn–air battery. |
The stability of rechargeable Co–N–C-900-based Zn–air battery was tested by charging and discharging for 10 min individually with repeated cycles at 5 mA cm−2, as shown in Fig. 9. Its voltage gap (Δη) was 1.1 V initially, and the voltage gap changed to 0.8 V after operating for 50 h. After 180 hours, voltage gap decreased to 0.8 V and the voltage gap was quite stable, respectively. This results can be comparable with what other researchers have published recently.23 Such phenomenon might be due to irreversible Zn plating–stripping process. Co–N–C-900-based Zn–air battery exhibits good recharge ability, and its performance is comparable to other results tested in our study. Moreover, Co–N–C-900 electrocatalyst is cheaper than IrO2 and Pt/C, in which shows its high economic potential.
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Fig. 9 Galvanostatic charge–discharge profiles of Zn–air batteries at 5 mA cm−2. (a) Pt/C–IrO2 assembled, Co–N–C-800 assembled (b), Co–N–C-900 assembled (c), Co–N–C-900–acid assembled (d). |
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ra02191b |
‡ The authors are making the same contribution to this paper. |
This journal is © The Royal Society of Chemistry 2023 |