Zeolitic Imidazolate Framework Film Derived CoFe‐N‐C Nanofoams for Reliable Zn‐air Batteries

Abstract

reduce or completely remove the usage of precious metals. [2] Zeolitic imidazolate framework (ZIF), constructed by coordination characteristics of metal elements surrounded by N species, [3] is regarded as a promising precursor for synthesizing metal-nitrogen-carbon (M-N-C) with highly dispersed active metal nanoparticles, even single-atom. [4] Nevertheless, for M-N-C application, the catalyst powders are commonly mixed with ion conductive polymer binder, which is assembled on the membrane or gas diffusion layer to form the membrane electrode assembly (MEA) or gas diffusion electrode (GDE). The structure of the resulting electrode/catalyst layer is difficult to control, probably reducing electroconductivity, hindering the active site exposure, [5] and influencing mass transfer. [6] Thus, the control of the cathode microstructure is becoming highly attractive for ZABs.One promising way is the direct growth of ZIFs precursor onto a carbon substrate, forming a self-standing electrode without the polymer binder. [7] From this point, assorted binderfree air electrodes have been developed with the aim of further improving the ZAB performance. [8] As a new kind of carbon substrate, carbon nanotube film (CNTF) is considered to be a promising binder-free scaffold for air batteries, attributed to its high specific surface area, [9] high conductivity, [10] and good mass transfer properties. [11] In our previous work, [11b] through fast current-driven synthesis (FCDS) and high-temperature pyrolysis method, the Co-N-C supported on the CNTF electrode was prepared and exhibited a power density of 241 mW cm −2 , much higher than that on the other substrates (e. g. carbon fiber paper), demonstrating the great potential application of CNTF for ZABs. In this work, to further improve the power density of ZAB, bimetal (Co, Fe) doped ZIF is introduced to synthesize a ZIF film supported on a CNTF. After carbonization, a binder-free electrode with a unique CoFe-N-C nanofoam was successfully obtained. Beneficial from the hierarchically porous structure and bimetal active site, the liquid-state ZAB with the CoFe-based CNTF cathode shows a power density of 294 mW cm −2 and good cycling stability. Results ZIF Film Synthesis and CarbonizationThe ZIF film was synthesized by the fast current-driven synthesis (FCDS) method in the double electrode system. The design of the efficient, flexible air electrode for enhancing the Zn-air batteries (ZABs) performance is vital in developing a new portable rechargeable power supply. Emerging research indicates that non-precious metal materials have promising activity for oxygen reduction/evolution in alkaline conditions while constructing a high-performance and reliable electrode is still a big challenge at the present stage. In this work, a bimetal doped zeolitic imidazolate framework (ZIF) film is synthesized by fast current-driven synthesis (FCDS) on a thin carbon nanotube film (CNTF). After carbonization at 950 °C, CoFe-N-C nanofoam formed on the CNTF, resulting in binder-free, flexible electrode...