Membraneless Hybrid Biofuel Cells: Integrating Microbial Anode and Enzymatic Cathode
Authors: Santoro Carlo , Babanova Sofia , Li Baikun , Cristiani Pierangela , Ieropolous Ioannis , Atanassov Plamen 
Published: February 2020

MFC is a promising bio-electrochemical device capable of converting organic compounds (e.g. organic wastes) into useful electricity. In single chamber microbial fuel cells (SCMFCs), the anode is immersed in a solution containing organics that are oxidized and converted by the anodophillic bacteria attached at the anode, and the cathode is exposed to air for oxygen reduction reaction (ORR). As in nature, oxygen is the most preferable final electron acceptor due to its high reduction potential (~0.62 V vs Ag/AgCl at pH=7). The most common cathodes used in SCMFCs need noble catalyst (e.g. platinum) to achieve low overpotentials and subsequently high Open Circuit Potentials (OCP) of 0.3-0.4 V vs Ag/AgCl 1. But even those cathodes often suffer severe potential losses especially at low current densities2. On the other hand, it has been found and routinely proved that air-breathing enzyme-based cathodes exploring birilubin oxidase (BOx) could achieve a cathode OCP higher than 0.5 V vs Ag/AgCl due to the low overpotential and high kinetics of the enzymatic catalyzed reactions3. The concept of a Hybrid MFCs had been previously proved and studied in two-chamber MFC4. For the first time, hybrid membraneless SCMFCs with microbial-based anode and enzyme-based cathode (BOx) were developed in this study and explored for high power generation. Air-breathing gas-diffusional cathode were prepared using 30%wt PTFE treated carbon cloth (Fuel Cell Earth) as current collector and cathode support. Teflonized carbon black (Vulcan XC72R with 35%wt PTFE was utilized as a gas-diffusional layer. The inner side of the teflonized carbon black (XC35) was additionally pretreated to achieve a gradient from hydrophobic to hydrophilic properties across the layer. The carbon cloth, the XC35 layer and a multi-walled nanotube paper (MWNTP) were fused together by hydraulic pressure. The MWNTP was used to support the catalytic layer on which BOx dissolved in phosphate buffer solution (PBS) was applied and then kept at 4o C for 16 hours for enzyme immobilization.
Membraneless and membrane-based (Nafion 117) SCMFC configuration was studied and compared in this experimentation. SCMFCs (volume of 130 ml) fed with PBS and NaOAc were run in a batch mode for a week. The batch mode SCMFC tests showed that the cathode OCP was higher than 0.5 V vs Ag/AgCl in all SCMFCs tested and stabilizes for an hour. The initial cathode polarization curves showed low overpotentials and ohmic resistances at low current generation (Figure 1a). All cathodes had significantly higher performances in comparison to the Pt-based cathode till 0.2 V vs. Ag/AgCl. Greater ohmic and mass-transfer losses during the cathode polarization were observed when a polymeric membrane (Nafion 117) was used. Markedly high power densities (referred to geometric cathode area) up to 200 μW/cm2 (2 W/m2) were reached in membraneless SCMFCs with PBS and sodium acetate (Figure 1b). When a polymeric membrane (Nafion 117) was applied in SCMFCs, the power generation dropped to 24-26 μW/cm2 (0.24-0.26 W/m2), which is most likely due to increased ohmic and diffusional losses given by the presence of a solid barrier.
This study showed the possibility of using enzymatic cathode (bilirubin oxidase) in membraneless SCMFCs with high power generation. The highest power density (2 W/m2) ever reported in a single bottle MFC fed with acetate in phosphate buffer was achieved in this study. The main problem related with the exploration of enzymatic electrodes in wastewater MFCs is the enzymes short lifetime and deactivation cased from contaminants in the wastewater.
Further studies should be conducted in order to enhance the lifetime and the long-term operation efficiency of the developed enzymatic MFCs.

References:

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