Ionic and Electron Transport in Microbial Anode Respiration: The Limitations of Performing Electrochemistry in Neutral-pH Water

School for Engineering of Matter, Transport and Energy, ASU

Anode-respiring bacteria (ARB) catalyze the complete oxidation of organic compounds (e.g. acetate, glucose) into electrical current and carbon dioxide.  ARB naturally produce a biofilm at the electrode surface of up to 100 micrometers, where even cells on the outer part of the biofilm are participating in current production.  The specific mechanism by which the extracellular electron transport (EET) occurs throughout the biofilm is not well understood.  Using a variety of electrochemical techniques, our group has evidence that supports the hypothesis that ARB carries EET using a network of redox proteins (e.g. cytochromes) to carry out electron transport through the biofilm.

While the topic of electron transport is the focus of most ARB research, there are no indications that this is the rate-limiting step in current production or power production in microbial electrochemical cells (MXCs).  Ionic transport becomes an important topic in determining rate-limiting and potential loss processes.  ARB require near-neutral pH in the medium to grow, differing from chemical fuel cells commonly employed, which run under acidic or alkaline conditions.  This pH requirement results in a major transport limitation, as H+ ions (now in mM range) should be transported from anode to cathode to achieve electron neutrality.  In an MXC anode, H+ ions accumulate in the ARB biofilm, creating an acidification that limits current generation.  At the cathode, local gradients leading to pH > 12 is typical in MXC operation; as a consequence, the pH gradient results in Nernstian concentration overpotential of > 300 mV.  Thus, understanding and controlling ionic transport in MXCs is essential to ensure an efficient operation.  I will discuss our current efforts to characterize and overcome ionic transport limitations in order to develop efficient MXC designs.