Enhancing bioelectrochemical performance of two-dimensional material attached by covalent/metal organic frameworks as cathode catalyst for microbial fuel cells

Highlights

• COF-300/ZIF-8@Ti₃AlC₂ was prepared by solvothermal method.

• COF-300/ZIF-8@Ti₃AlC₂ was multi-level, high conductivity, stability and durability.

• Maximum power density of COF-300/ZIF-8@Ti₃AlC₂-MFC was 587.01 mW/m².

• 2-D layered Ti₃AlC₂, 3-D COF-300 and ZIF-8 jointly improved MFC performance.

Abstract

In this study, covalent/metal organic framework and two-dimensional material (COF-300/ZIF-8@Ti₃AlC₂) were composited by a three-step distributed feed method for cathode catalyst in microbial fuel cells (MFCs). The growth of irregular cube-like COF-300/ZIF-8 on the Ti₃AlC₂ substrate was observed. Al, Zn, Ti, N, C and O elements were observed uniformly and more active sites were offered through it. The external output voltage of COF-300/ZIF-8@Ti₃AlC₂-MFC was 576 mV and this could be almost unchanged for 9 days. The external output power density was 587.01 mW/m², and that was 1.25 times of COF-300@Ti₃AlC₂-MFC (469.30 mW/m²) and 1.67 times of COF-300/ZIF-8-MFC (352.09 mW/m²). Ti₃AlC₂ enhanced the electrical conductivity of the composite by its rich surface functional groups and much more surface active sites. COF-300/ZIF-8 mixture improved the instability and disorder of the monomer. This study would supply technical support for the expanded request of composite materials in microbial electrochemistry.

Introduction

With the increasingly serious water pollution, wastewater treatment was a key challenge (Chen et al., 2022a, Chen et al., 2022b). Biological treatment has become a key approach with microbial fuel cells (MFCs) coming into view for its ability to degrade wastewater while producing electricity (Zhao et al., 2022, Zhu et al., 2022). The MFC was a device that combined the function of degrading wastewater with electricity generation (Chen et al., 2022a, Yang et al., 2022a). It was clean, pollution free and environmentally friendly, but its low power output was a key limiting factor for its large-scale application (Chen et al., 2021a, Chen et al., 2021b, Chen et al., 2021c). The type of anode microorganism, electrolyte solution and electrode material were the main factors that restricted its power output (Li et al., 2020, López-Hincapié et al., 2020). The cathode catalyst played a decisive role in the output of the cathode, so it was necessary to look for some affable cathode materials to increase the electrical performance of MFCs (He et al., 2020, Tang et al., 2019, Taşkan, 2020, Wang et al., 2021).

In recent years, many novel materials appeared, such as layered double hydroxide (LDH), two-dimensional material (MXene), covalent organic frameworks (COFs) and metal organic framework (MOFs) (Chen et al., 2022b, Chen et al., 2022c, Chen et al., 2022d). These have been used as cathode catalysts to enhance the redox activity of MFCs (Kumar et al., 2017, Li et al., 2018, Tajdid Khajeh et al., 2020, Yang et al., 2022c). On one hand, MXene was a new kind of two-dimensional materials, among which Ti₃AlC₂ had the advantages of rich surface functional groups, high young’s modulus, large layer spacing, conductivity of metal, large specific surface area, etc. It has been certificated to be an excellent electrode material for batteries (Bhardwaj et al., 2022, Wang et al., 2022). Previous studies have displayed that MXene composites could greatly improve the performance as cathode materials for lithium ion batteries (Abdurehman Tariq et al., 2022). MXene loaded composites could improve the performance of asymmetric supercapacitors (Liu et al., 2022). Ti₃AlC₂ was a class of MXene, it was possessed the advantages of high level and lively chemical properties. However, synthesizing large-area integrated MXene nanosheets with precise pores was still a huge challenge. In practical application, in order to expand the application range and improve the electrochemical and flexible properties of Ti₃AlC₂ as electrode, Ti₃AlC₂ could be compounded with many materials to prepare high-efficient composite electrode materials (Tahir et al., 2021, Yang et al., 2022b).

On the other hand, MOFs were kinds of materials which were composed of organic framework and metal. MOFs were possessed of wide range of applications for great surface area, multi-layer porosity and three-dimensional structure (Ashouri et al., 2022, Jing et al., 2022, Tao et al., 2022). ZIF-67 was a class of MOFs, which was the material with three-dimensional structure and possessed of large specific surface area. However, MOFs were sensitive to air, water vapor and acid-base because of the weak interaction between MOFs and metal ions. Then, COFs were appeared due to the demand for ordered porous materials with both stability and structural diversity. COFs were kinds of ordered porous materials which only had organic ligands linked by reversible covalent bonds, and it had various and controllable physical and chemical properties (Cao et al., 2021, Zhang et al., 2021, Zhang et al., 2022). However, the chemical stability and sequence of COFs were still poor, most of the reported COFs were highly hydrophobic (Yan et al., 2022). Therefore, it was suggested that the COF-300/ZIF-8 mixture formed by the combination of MOFs and COFs that might improve the instability of the monomer material to some extent and play a positive effect on its lasting power generation. By combining the complex COF-300/ZIF-8 with two-dimensional sheet nano-material Ti₃AlC₂, the three-dimensional structure could be constructed, which could reduce the possibility of agglomeration of two-dimensional sheet nano-material Ti₃AlC₂, and obtained more effective active sites and improved the electrochemical performance. At the same time, Ti₃AlC₂ electrode could store more electric charge and output more electric energy by surface functional group modification.

In this study, the mixture material COF-300/ZIF-8@Ti₃AlC₂ was perfectly prepared by distributed feeding method. Firstly, the nanopatch substrate Ti₃AlC₂ was prepared. Then, the three-dimensional material MOFs (ZIF-8) and COFs (COF-300) were combined by mother liquor mixing. Finally, COF-300/ZIF-8 was successfully grown on the substrate Ti₃AlC₂ and COF-300/ZIF-8@Ti₃AlC₂ was acquired. The oxidation reduction properties were tested using stainless steel mesh in MFCs as cathode catalyst. The crystal structure of the material surface was tested by X-ray diffraction (XRD), functional group composition was tested by fourier transform infrared spectroscopy (FT-IR), surface morphology of the material was observed by scanning electron microscope (SEM), the composition of elements were obtained by energy dispersive spectroscopy (EDS), and the states of the elements were demonstrated by X-ray photoelectron spectroscopy (XPS) (Zhou et al., 2021, Zhou et al., 2018, Zhou et al., 2019a, Zhou et al., 2019b). The redox activity test of the material was obtained by plotting linear sweep voltammetry (LSV) and cyclic voltammetry (CV) curves in an electrochemical system. Finally, the composites were loaded into the cathodic region of the MFC to undergo catalytic reactions, and the cathodic-anode voltage, current and power density were tested.