Effects of Polymer Matrices and Carbon Nanotubes on the Generation of Electric Energy in a Microbial Fuel Cell

Abstract

The anode of a microbial fuel cell (MFC) was formed on a graphite electrode and immobilized Gluconobacter oxydans VKM-1280 bacterial cells. Immobilization was performed in chitosan, poly(vinyl alcohol) or N-vinylpyrrolidone-modified poly(vinyl alcohol). Ethanol was used as substrate. The anode was modified using multiwalled carbon nanotubes. The aim of the modification was to create a conductive network between cell lipid membranes, containing exposed pyrroloquinoline quinone (PQQ)-dependent alcoholdehydrogenases, and the electrode to facilitate electron transfer in the system. The bioelectrochemical characteristics of modified anodes at various cell/polymer ratios were assessed via current density, power density, polarization curves and impedance spectres. Microbial fuel cells based on chitosan at a matrix/cell volume ratio of 5:1 produced maximal power characteristics of the system (8.3 μW/cm²) at a minimal resistance (1111 Ohm cm²). Modification of the anode by multiwalled carbon nanotubes (MWCNT) led to a slight decrease of internal resistance (down to 1078 Ohm cm²) and to an increase of generated power density up to 10.6 μW/cm². We explored the possibility of accumulating electric energy from an MFC on a 6800-μF capacitor via a boost converter. Generated voltage was increased from 0.3 V up to 3.2 V. Accumulated energy was used to power a Clark-type biosensor and a Bluetooth transmitter with three sensors, a miniature electric motor and a light-emitting diode.

Keywords: boost converter accumulation; immobilization of bacterial cells; interaction of cell membranes with carbon nanotubes; microbial fuel cell; polymer matrix.

Figure 1

Scheme for the MFC and its external connections (a); block diagram for converter accumulation of energy (b).

Figure 2

Effect of the applied potential on the electrode signals at the introduction of ethanol (1) and on the electrode charge transfer resistance measured by electrochemical impedance spectroscopy after addition of ethanol (2). The electrode was modified using bacterial cells and chitosan (2:1, v/v).

Figure 3

Micrographs of Gluconobacter oxydans cells in polymer gels: (a) chitosan; (b) PVA; (c) mPVA.

Figure 4

Effect of the ratio of cells and polymer on the anode surface on MFC power (1) and internal resistance (2) (the number of cells on the electrode surface is constant, 0.12 mg/mm²): 1, MFC power; 2, R_in; (a) chitosan; (b) mPVA.

Figure 5

Typical polarization curves obtained from cyclic voltammograms and, calculated based on them, power characteristics of polymer/G. oxydans electrodes: 1, chitosan; 2, mPVA; 3, PVA. Inset: typical signals of polymer/G. oxydans electrodes: 1, chitosan; 2, mPVA; 3, PVA.

Figure 6

Nyquist plots of the impedance spectra of MWCNT-modified polymer/G. oxydans electrodes: 1, chitosan; 2, mPVA; 3, PVA.

Figure 7

Charging a 6800-μF capacitor from series-connected MWCNT-modified MFCs using a converter unit: 1, charge voltage; 2, voltage across the accumulating capacitance; 3, input voltage coming to the converter. (A), low-efficiency slow charge phase; (B), high-efficiency fast charge phase. Registration of the capacitor total charge after a 48-h continuous operation of the MFCs is shown.

Figure 8

Dependence of the charging of a 6800-μF capacitor on MFC operating conditions: 1, nonmodified MFCs; 2, MWCNT-modified MFCs; 3, MWCNT-modified MFCs after a 24-h operation; 4, MWCNT-modified MFCs after a 48-h operation.