Efficient oxygen electrocatalysts for zinc-air batteries: MOF-derived nitrogen-doped carbon networks loaded with cobalt and iron nanoparticles

Efficient oxygen electrocatalysts for zinc-air batteries: MOF-derived nitrogen-doped carbon networks loaded with cobalt and iron nanoparticles
Due to the increasing depletion of non-renewable energy sources and rising carbon dioxide emissions, new energy storage and conversion technologies are urgently needed to realize the “dual-carbon” goal. Rechargeable zinc-air batteries have attracted much attention due to their high theoretical energy density, good safety and low cost. However, the slow reaction kinetics at the air electrode side hinders the practicalization of rechargeable zinc-air batteries. Precious metal-based catalysts, such as Pt and IrO₂, can effectively promote the oxygen reduction or oxygen precipitation reaction at the air electrode side, respectively, and thus enhance the overall performance of

rechargeable zinc-air batteries. However, precious metals are scarce, expensive, unstable and single-functional, so it is important to find cheap, efficient and stable bifunctional oxygen electrocatalysts. Based on this, the team of Associate Professor Zhaojun Xie from Nankai University and Professor Zhen Zhou from Zhengzhou University prepared cobalt nanoparticles and highly dispersed iron species loaded on nitrogen-doped porous carbon substrates by controlled doping with the help of metal-organic frameworks as precursors. Based on the synergistic effect between the highly dispersed iron species and cobalt nanoparticles, as well as the large specific surface area and hierarchical porous structure of the catalysts, the target catalysts exhibited good oxygen reduction and oxygen precipitation performance. Rechargeable zinc-air batteries using it as an anode catalyst exhibited high peak power density and good cycling stability. The present work provides guidance for the rational design of advanced oxygen electrocatalysts and offers catalyst solutions for the practicalization of rechargeable zinc-air batteries.

1. Metal-organic frameworks were employed as precursors with the help of two different doping means, thereby successfully preparing nitrogen-doped carbon substrate-loaded cobalt nanoparticles and highly dispersed iron species (CoNP@FeNC-0.05).
2. The designed catalysts exhibited oxygen reduction as well as oxygen precipitation properties comparable to those of commercial catalysts
3. Electrochemical tests and structural analyses showed that the synergistic interaction between cobalt nanoparticles and highly dispersed iron species, and the large specific surface and hierarchical porous structure of the metal-organic frameworks as precursors play an important role in the enhancement of oxygen electrocatalytic performance.
Description
Finding inexpensive, efficient and stable bifunctional oxygen electrocatalysts is crucial to enhance the practical performance of rechargeable zinc-air batteries. Here, a team of Associate Professor Zhaojun Xie from Nankai University and Professor Zhen Zhou from Zhengzhou University prepared a series of carbon-based catalysts loaded with cobalt nanoparticles and highly dispersed iron species by using metal-organic frameworks (MOFs) as precursors and controlling the doping mode. Using the zeolite imidazolium skeleton ZIF-8 as the precursor, the iron and cobalt sources were introduced by in-situ doping and bis-solvent method, respectively, which led to the simultaneous anchoring of highly dispersed iron species and domain-limited growth of cobalt nanoparticles. The optimized catalyst (CoNP@FeNC-0.05) exhibited high oxygen reduction half-wave potential and small oxygen precipitation overpotential in alkaline electrolyte. Further, compared to the commercial catalyst (Pt/C+RuO₂), 以CoNP@FeNC-0.05, especially in terms of power density and cycling stability. Combining the results of electrochemical tests and structural characterization, the highly dispersed Fe-N sites have high catalytic activity for the oxygen reduction reaction (ORR), while the cobalt nanoparticles can improve the oxygen precipitation reaction (OER) process, and the synergistic effect exhibited by combining the two can further enhance the ORR and OER performance of the catalysts. In addition, the large specific surface area and porous structure of ZIF-8 as precursor can promote the electron transfer process and electrocatalytic mass transfer process, thus improving the ORR and OER performance.
I Characterization of morphology and structure
CoNP@FeNC-0.05Firstly Fe³⁺ substitutes part of Zn²⁺ to combine with 2-methylimidazole to form iron-doped ZIF-8, which facilitates the homogeneous dispersion of iron elements. The iron-doped ZIF-8 was then dispersed in hexane, and a methanol solution containing cobalt nitrate was added dropwise to the above dispersion. Utilizing this dual-solvent method enabled the introduced Co²⁺ to be mainly adsorbed within the cavities of the iron-doped ZIF-8 rather than on the surface of the iron-doped ZIF-8 polyhedra, thus facilitating the obtaining of small-sized, homogeneously dispersed cobalt nanoparticles.

II Oxygen Reduction, Oxygen Precipitation Performance Tests
CoNP@FeNC-0.05superior to commercial Pt/C catalysts. In addition to this, it is also shown that the introduction of transition metal elements on nitrogen-heterogeneous carbon substrates can improve the ORR performance of the catalysts. And it can be seen that compared with Pt/C and CoNP@NC catalysts, FeNC catalysts also have good ORR activity, indicating that the highly dispersed Fe-N sites are good ORR active sites, and the introduction of cobalt nanoparticles can further improve the ORR performance of FeNC catalysts CoNP@FeNC-0.05Tafel curves of the catalysts were obtained using the i-t method, CoNP@FeNC-0.05Tafeland CoNP@FeNC-0.05ORR. The electron transfer number and hydrogen peroxide yield of the catalyst were tested by rotating the ring-disk electrode. CoNP@FeNC-0.054 The corresponding hydrogen peroxide yield was less than 5%, indicating that the catalyst mainly reduces oxygen to hydroxide via the 4e-path. Next, the OER performance of the catalysts was tested by first comparing the overpotentials corresponding to different catalysts at 10 mA cm-², and it was found that cobalt nanoparticles possessed higher OER activity compared to Fe-N species, and the synergistic interaction between cobalt nanoparticles and Fe-N species could further improve the OER performance of the catalysts, thus the CoNP@FeNC- 0.05 catalyst exhibited good OER performance. CoNP@FeNC-0.05良好的OER The FeNC catalysts have higher bilayer capacitance values, indicating that the FeNC catalysts have a larger electrochemically active area, CoNP@FeNC-0.05 suggesting that due to the synergistic interaction between cobalt nanoparticles and Fe-N species the CoNP@FeNC- 0.05 catalyst has higher ORR, OER intrinsic activity.
(a) LSV profile measured in 0.1 M KOH with a sweep rate of 5 mV s-¹ at 1600 rpm; (b) ORR Tafel profile of the catalyst; (c) CoNP@FeNC-0.05(d) LSV profile of the catalyst in 1 M KOH with a sweep rate of 5 mV s -¹ at 1600 rpm; (e) OER Tafel curve of the catalyst; (f) Curve of current density versus sweep rate in 0.1 M KOH, with the slope representing the Cdl value of the catalyst.
III Performance test of rechargeable zinc-air battery
CoNP@FeNC-0.05 were tested by assembling rechargeable zinc-air batteries with polished zinc flakes as negative electrodes. Compared to the zinc-air battery with Pt/C+RuO₂ as the positive catalyst, CoNP@FeNC-0.05 Further, the charge-discharge voltage difference atCoNP@FeNC-0.0575 mA cm-² was 1.36 V, whereas the charge-discharge voltage difference of the Pt/C+RuO₂ catalyst at 63 mA cm-² was already 1.53 V, indicating that the CoNP@FeNC- 0.05 catalyst has better ORR and OER activities compared to Pt/C+RuO₂ catalyst. The power density profile of the catalysts can be obtained from the discharge polarization curves, and the CoNP@FeNC-0.05于Pt/C+RuO₂ catalyst exhibits a higher peak power density. Cycling tests were conducted at a current density of 5 mA cm-² for CoNP@FeNC-0.05500 h, while the Pt/C+RuO₂ catalyst was only able to operate stably for 237 h. CoNP@FeNC-0.05以Pt/C+RuO₂ as the The zinc-air battery with cathode catalyst showed better charge/discharge reversibility and cycle stability.