Understanding the Biofilm Anode in Microbial Electrochemical Cells

Bruce Rittman
Arizona State University

In a microbial fuel cell (MFC) or microbial electrolysis cell (MEC), bacteria present at the anode catalyze the oxidation of organic fuel sources, including domestic wastewater, animal manures, plant residues, and photosynthetic microorganisms.  The MFC converts the energy value stored in the organic fuel to electrical energy, while the MEC converts it to hydrogen gas (H2).  The most unique feature of an MFC or an MEC is the biofilm that lives on the anode and oxidizes the fuel.  This oxidation is illustrated by the simple oxidation half reaction for acetic acid:  CH3COOH + 2H2O à 2CO2 + 8H+ + 8e-.  The biofilm includes anode-respiring bacteria (ARB) that have the ability to transfer the electrons (e-) to the anode, a conductive solid.  The biofilm matrix that the ARB produce is conductive, which allows the electrons to move efficiently to the anode, even when the ARB are not immediately next to the anode.  Being conductive, the biofilm matrix becomes part of the anode itself; thus, we call it the biofilm anode.  The commonality of the biofilm anode in MFCs and MECs gives rise to our acronyn MXC.

This talk presents recent results that characterize the key phenomena occurring in the biofilm anode. 

  1. The unique properties of ARB. 
  2. The Nernst-Monod expression to quantify the relationship between the rate of electron conduction and the electrical potential in the biofilm anode.
  3. How the rate of electron transport can be controlled by slow H+ transport more than slow electron conduction for high-performance biofilm anodes.
  4. The interactions of ARB with other microorganisms in the biofilm anode.

Sponsored by the Department of Civil and Environmental Engineering and Earth Sciences