Elsevier

Chemical Engineering Journal

Volume 230, 15 August 2013, Pages 532-536
Chemical Engineering Journal

Short communication
Catalytic response of microbial biofilms grown under fixed anode potentials depends on electrochemical cell configuration

https://doi.org/10.1016/j.cej.2013.06.044Get rights and content

Highlights

  • Microbial biofilms show different catalytic response at different anode potentials.

  • Membrane-less bioelectrochemical cell poised at +0.2 V vs. Ag/AgCl showed maximum current density of >5000 mA/m2.

  • Membrane separated bioelectrochemical cell poised at −0.3 V vs. Ag/AgCl showed current density of 4000 mA/m2.

  • Preliminary microbial analysis shows differences in both configurations.

Abstract

In microbial electrochemical cells the anode potential can vary over a wide range, which alters the thermodynamic energy available for bacterial-electrode electron exchange (termed electroactive bacteria). We investigated how anode potential affected the microbial catalytic response of the electroactive biofilm. Microbial biofilms induced to grow on graphite electrodes by application of a fixed applied anode potential in membrane-separated and membrane-less electrochemical cells show differences in electrocatalytic response. Maximum current density is obtained using +0.2 V vs. Ag/AgCl to induce biofilm growth in membrane-less cells, in contrast to a maximum achieved at lower applied potentials in a membrane-separated electrochemical cell configuration. This insight into differences in optimal applied potentials based on cell configuration can play an important role in selection of parameters required for microbial fuel cells and bio-electrochemical systems.

Introduction

In microbial electrogenesis bacteria oxidize substrates to generate electricity by transferring electrons to a solid electrode, leading to potential for electricity generation in microbial fuel cells (MFC) or to offset the potential required for hydrogen, and other value-added chemical, production at cathodes in bio-electrochemical systems (BES) [1], [2]. BES operating conditions affect growth kinetics and metabolism of the microbial community (Fig. 1). In microbial fuel cells, with current flow between an anode and a cathode across a fixed resistance load, the potential imposed on the anode is difficult to control, because of variations in anode potential as a function of mass transport to, catalytic activity at, and current flow between, anode and cathode. Growth of microbial biofilms can however be induced by precise control of anode potential vs. a reference electrode using a potentiostat. The electron transfer capabilities of microbial films induced to grow by imposition of specific anode potentials can be thus explored in fundamental studies of electroactive biofilms on electrodes. However, results comparing applied anode potentials show a range of response in terms of effect of potential on fuel cell power and/or current density [3], [4], [5], [6], [7], [8], as reviewed recently by Kumar et al. [4].

There are few studies comparing current densities achieved using a fixed resistance load MFC to induce microbial biofilm growth on anodes to those using fixed anode potentials. Wang et al. [9] compare biofilm growth under an applied anode potential of +0.4 V vs. Ag/AgCl to that across a fixed 1000 Ω resistance load between anode and a ferricyanide-reducing cathode, separated by an Ultrex cation exchange membrane. The application of a fixed anode potential produced a reproducible current in fewer cycles, and shorter time, compared to that using the fixed resistance load cell. Other studies on BESs using either individual applied anode potentials (different values), or a single applied potential, show variable results in terms of current or power generation, as reviewed recently [4]. There are however no reports to our knowledge comparing electrocatalytic properties of anode potential-induced biofilms as a function of the electrochemical cell configuration (i.e., in membrane-separated vs. membrane-less electrochemical cells). When MFCs and BESs are operated, anode potentials measured during peak power generation show typical values around −0.40 to −0.48 V vs. Ag/AgCl for mixed cultures oxidizing acetate [4]. Applied anode potentials more positive than this therefore provide a driving force for microbially catalyzed oxidation of acetate in biofilms on anodes. We probe the effect of −0.3 V, −0.2 V and +0.2 V vs. Ag/AgCl applied anode potentials on biofilm performance, as a function of electrochemical cell configuration, using anaerobic sludge as a mixed culture inoculum and acetate as feed.

Section snippets

Reactor configuration and operation

Experiments were conducted in either membrane-less or membrane-separated electrochemical cell configuration. The membrane-less configuration was a single borosilicate glass cell containing a Ag/AgCl (3 M NaCl) reference electrode, and both anode and cathode electrodes. For the membrane-separated configuration, the anodic and cathodic borosilicate glass half-cells were separated by a 12 cm2 Nafion® 117 proton exchange membrane (Sigma, Ireland), with the reference electrode in the anode half-cell.

Results and discussion

Slow scan cyclic voltammetry (CV) of the biofilms, induced to grow on graphite electrodes, Fig. 2, display sigmoidal curves typical of bioelectrocatalytic oxidation of substrate [7], [12], [13], [14], [15], [16] indicative of an electrochemical redox transition coupled to a catalytic oxidation reaction. The observed CV shape and mid-point potential (∼−0.41 V) are the same as those observed for biofilms, oxidizing acetate, induced to grow on anodes from single cultures of Geobacter sulfurreducens

Conclusions, outlook and implications

Linking anode potential to evolution of microbial biofilm current and/or power generation and microbial diversity information has been an issue of debate over the past decade. Our preliminary results indicate that consideration of the electrochemical cell configuration, with or without separating membrane, is required in any attempt to select an applied anode potential or to alter microbial activity in such systems. Among membrane-less bioelectrochemical cells, the electrochemical cell poised

Acknowledgments

Authors acknowledge funding from FP 7 People Programme of European Commission, Marie Curie Intra European Fellowship for Career Development (Grant A/6342 – PIEF-GA-2009-237181) and Science Foundation Ireland (Charles Parsons Energy Research Award06/CP/E006).

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