Study On The Electricity Generation And Wastewater Decontamination Performance Of Microbial Fuel Cell With MnO₂@graphene As Cathode Catalyst

Microbial fuel cell?MFC?is a new bio-electrochemical systems?BES?which has been paid great attention due to its bifunction of simultaneous electricity generation and organic wastewater or organic/inorganic matters degradation in the soil.Because of its characteristics of resource and harmlessness,MFC is one of the research hotspots in wastewater treatment in recent years.In the process of energy conversion of MFC,the electron transport rate restricts the efficiency of water treatment and electricity generation.In light of this situation,we will construct an air-cathode MFC and enhance the electron tansport rate by increasing the oxygen reduction rate?ORR?in the cathode in order to improve the performance of wastewater treatment and simultaneous electricity generation.Firstly,MnO₂@graphene composites were prepared by a chemical co-precipitation method using KMnO4,Mn C2O4·2H2O and graphene as raw materials.The microstructure,morphology of the prepared composites were analyzed using X-ray diffraction?XRD?,scanning electron microscopy?SEM?,Fourier transform infrared?FT-IR?spectrum,Raman spectroscopy,X-ray photoelectron spectroscopy?XPS?and specific area measurements.The electrochemical performances of MnO₂@graphene composites as catalysts for cathodic oxygen reduction reaction in microbial fuel cell were analyzed using cyclic voltammetry,electrochemical impedance spectrometry?EIS?.Results show that spherical MnO₂ with uniform particle size of 400 nm is tightly formed on the surface of paper-like graphene by electrostatic interaction.Moreover,the peak potential of Mn O₂@graphene electrode?-0.440 V vs Ag/AgCl?is very close to that for Pt/C electrode?-0.434 V vs Ag/AgCl?,though the initial peak potential of MnO₂@graphene electrode is0.048 V negative than that for Pt/C electrode.With the increase of cycling times,the initial peak potential of MnO₂@graphene electrode decreased,while there is only a small decline for the peak current density of MnO₂@graphene electrode,indicating the MnO₂@graphene composites have better catalytic activity and cycling stability for cathodic oxygen reduction reaction in microbial fuel cell than that for Pt/C catalysts.EIS results show that the electron-transfer resistance of Mn O₂@graphene is only 12.6?,which is smaller than that for Pt/C catalysts and MnO₂ catalysts,suggesting MnO₂@graphene catalysts promote the cathodic oxygen reduction reaction by decareasing the electron-transfer resistance and accelerating the charge transfer due the introduction of the excellent conductive graphene.Secondly,an air-cathode MFC was constructed using self selected F026 as electrogenic bacterium in anode of MFC and MnO₂@graphene as cathodic catalyst.The effect of initial COD concentration of substrate,pH and cathodic catalyst on the electricity generation?stable output voltage and maximum power density?of MFC with glucose as anode substrate were investigated in detail.Resuls show that the three factors have certian influence on the stable output voltage and maximum power density.In contrast,the cathodic catalyst and the pH has the greatest and minimal impact on the electricity generation of MFC,respectivly.The optimal conditions of electricity generation of MFC were obtained by single factor experiment method as follows:MnO₂@graphene as cathodic catalyst,initial COD concentration of substrate of 200 mg/L and pH 7.5.MFC delivers a stable output voltage of 0.73 V and a maximum power density of2130.3mW/m2.Finally,taking into account the optimal survival conditions of F026 and the cost factors inthe actual domestic wastewater,the effects of catalysts on the removal of COD and NH₄⁺-N by MFC under optimal pH of 7.5 with F026 as electrogenic bacterium and actual domestic wastewater as anode substrate were studied.The catalytic action mechanism of three kinds of catalysts?MnO₂@graphene,Pt and Mn O₂?_was discussed.Experimental results show that the MFC with MnO₂@graphene as cathodic catalyst has the best COD and NH₄⁺-N removal rate.The COD removal rate of MFC is more than 90% within 60 d and the maximum removal rate is 99.5%.While the NH₄⁺-N removal rate of MFC is more than 90.5% within 90 d and the maximum removal rate is 92.2%.Three kinds of catalysts follow a cathode catalytic-electricity generation mechanism to enhance the wastewater treatment effect.In the cathode of MFC,the ORR is accelerated by cathodic catalyst,and rate of the electron consumption by oxygen is improved,which accelerates the decomposition of organic matters by electrogenic bacterium in anode and produce more electrons,resulting in better performance of simultaneous electricity generation and wastewater treatment.