Enzymatic Oxygen Micro-Probe for Analysis of Microbial Fuel Cells
Authors: Grattieri Matteo , Babanova Sofia , Santoro Carlo , Guerrini Edoardo , Cristiani Pierangela , Trasatti Stefano P. , Atanassov Plamen 
Published: February 2020

Microbial fuel cell (MFC) is an innovative electrochemical biotechnology capable to produce electricity directly from wet marginal biomass and various wastes. Several interests related to this area of science are progressively increasing worldwide [1, 2]. The Single Chamber Microbial Fuel Cell (SCMFC), or rather a membraneless air-cathode MFC configuration, is the most studied MFC system due to simple design and the higher performance. The membrane removal leads to positive effects such as costs reduction, decrease of the MFCs ohmic resistance and as a result enhance the MFC output. Unfortunately, the membrane removal allows the oxygen transport and diffusion into the solution (electrolyte) and it thus lower the anodic performance, which are optimized for strictly anaerobic conditions. In fact, the presence of oxygen in the SCMFC solution affects the biofilm formation and development and disturb severely the biofilm settlement on the electrode.
Generally, biofilm action is not limited in a monolayer of bacteria directly in contact with the electrodes, but it needs to be considered as a 3D structure next to the surface, of hundred micrometer thickness. The biofilm is involved (completely or partially) in the electrochemical and chemical processes carring out an effective electron transfer (EET) to/from the electrodes in this three dimensional space [3]. Consequently, the determination of oxygen profiles close to the anode and cathode electrode becomes of fundamental importance. Until now, to the best of our knowledge, no exhaustive studies have been focused on the determination of oxygen profile into the electrolyte of a SCMFC and how the presence of oxygen affects the MFC output.
This work aimed to measure the oxygen presence/absence in the different zones inside the SCMFC solution and particularly close to the electrodes surfaces. Accordingly, a “home made” enzymatic oxygen micro-probe based on Bilirubin Oxidase (BOx) enzyme has been designed. The main advantage of utilizing an enzyme is its selectivity towards a specific reaction and in this case BOx is able to catalyze oxygen reduction reaction to water [4]. The advantage of utilizing a micro-probe is the possibility of obtain a precise oxygen measurment inside the SCMFC solution that would permit the optimization of this technology, allowing further cell configuration development for future on-field application. The probe was prepared by immobilization of BOx on modified carbon fiber embaded in plastic body and sealed with epoxy resin (Fig. 2A). The diameter of the sensor, exposed to the electrolyte, was approximatelly 100 μm. This micro-probe was calibrated via Cyclic Voltammetry (CV) using phosphate buffer 50 mM with additional indifferent electrolyte (KCl 100 mM) in different aeration conditions using a typical three electrode electrochemical cell. The reference electrode was an Ag|AgCl electrode and a Pt wire was used as counter electrode. The calibration curve was created by plotting the current at -0.4 V vs Ag|AgCl as a function of the oxygen concentration (Figure 1).
The oxygen concentration in the electrolyte during the voltammetry measurements was monitored via DO probe and used for the calibration plot.
In order to precisely measure the oxygen content inside the MFCs` electrolyte for localized analysis, a computer controlled stage (Newark System Inc.) was used. The analysis setup is shown at Figure 2B. A Luggin capillary was used during the analysis in order to reduce the ohmic drop. The oxygen concentration was studied both for the anode and cathode biofilm.
[1] C. Santoro, Y. Lei, Li, P. Cristiani, Biochemical Engineering J. 2012; 62:8–16
[2] W. Li, G. Sheng, X. Liu, H. Yu. Bioresource Technology. 2011; 102: 244–252
[3] L. Huang, J.M. Regan, X. Quan, Bioresource Technology, (2011), 102: 316-323
[4] S. Brocato, C. Lau, P. Atanassov, Electrochimica Acta. 2012; 61: 44-49