Elsevier

Bioelectrochemistry

Volume 78, Issue 1, April 2010, Pages 39-43
Bioelectrochemistry

Microbial electrolysis cell with a microbial biocathode

https://doi.org/10.1016/j.bioelechem.2009.05.005Get rights and content

Abstract

This study demonstrates, for the first time, the proof-of-principle of an MEC in which both the anodic and cathodic reaction are catalyzed by microorganisms. No expensive chemical catalysts, such as platinum, are needed. Two of these MECs were simultaneously operated and reached a maximum of 1.4 A/m2 at an applied cell voltage of 0.5 V. At a cathode potential of − 0.7 V, the biocathode in the MECs had a higher current density (MEC 1: 1.9 A/m2, MEC 2: 3.3 A/m2) than a control cathode (0.3 A/m2, graphite felt without biofilm) in an electrochemical half cell. This indicates that hydrogen production is catalyzed at the biocathode, likely by electrochemically active microorganisms. The cathodic hydrogen recovery was 17% for MEC 1 and 21% for MEC 2. Hydrogen losses were ascribed to diffusion through membrane and tubing, and methane formation. After 1600 h of operation, the current density of the MECs had decreased to 0.6 A/m2, probably caused by precipitation of calcium phosphate on the biocathode. The slow deteriorating effect of calcium phosphate, and the production of methane show the importance of studying the combination of bioanode and biocathode in one electrochemical cell, and of studying long term performance of such an MEC.

Introduction

Microbial electrolysis is a process for the production of green hydrogen from organic matter [1], [2], [3]. In a microbial electrolysis cell (MEC), organic material is converted by electrochemically active microorganisms into CO2, H+, and electrons. These electrons are transferred to the anode and flow from the anode, via an electrical circuit containing a power supply, to the cathode. At the cathode, hydrogen is produced via reduction of protons or water. The flow of negative charge outside the cell is compensated by cation transport from anode to cathode inside the cell. To drive hydrogen production under biological conditions, theoretically 0.14 V has to be applied [3]. In practice however, more than 0.14 V has to be applied, partly due to the cathode overpotential.

So far in microbial electrolysis, mainly cathodes have been used that apply platinum as catalyst. The choice for platinum originates from its excellent performance in water electrolyzers and fuel cells. Platinum however, is an expensive catalyst. Besides, platinum performance can be negatively affected by components often present in waste streams [4]. Therefore, there is need for an alternative catalyst at the cathode.

In 2008, Rozendal et al. developed a microbial biocathode as a potential alternative for platinum [5]. This biocathode was obtained by enriching a biofilm of hydrogen oxidizing, electrochemically active microorganisms on a graphite felt anode (bioanode). Next, the polarity of this bioanode was reversed, turning it into a hydrogen producing biocathode. At a controlled potential of − 0.7 V versus normal hydrogen electrode (NHE), the biocathode had a current density of 1.1 A/m2, whereas the control cathode (graphite felt without biofilm) had a current density of only 0.3 A/m2. Subsequently, effluent of the biocathode was used to inoculate the control cathode, which then also turned into a biocathode that reached a current density of 1.1 A/m2. This biocathode did not require a mediator for hydrogen production, which is an advantage compared to other reported hydrogen producing biocathodes [6], [7], [8]. The biocathode however, was only tested in an electrochemical half cell (Fig. 1A), in which ferrocyanide was oxidized at the anode.

The objective of this study was to give a proof-of-principle of an MEC in which both the anodic and cathodic reaction are catalyzed by microorganisms, i.e. a full biological MEC (Fig. 1B). Such a study is needed because the performance of a full biological MEC can be different from the performance predicted from half cell studies, because the bioanode and biocathode in a full biological MEC influence each other via transport of materials through the membrane. For this purpose, anode and cathode chamber of an MEC were inoculated with electrochemically active microorganisms, and the MEC was operated at an applied cell voltage of 0.5 V.

Section snippets

Experimental set-up

Experiments were performed with 2 identical electrochemical cells (MEC 1 and MEC 2) with graphite felt electrodes in the same experimental set-up that was used by Rozendal et al. [5]. Anode and cathode chamber (each with a volume of 0.25 L) were continuously fed with medium (1.3 mL/min) from separate supply tanks (volume 25 L). Both anode and cathode medium consisted of 0.74 g/L KCl, 0.58 g/L NaCl, 0.68 g/L KH2PO4, 0.87 g/L K2HPO4, 0.28 g/L NH4Cl, 0.1 g/L MgSO4·7H2O, 0.1 g/L CaCl2·2H2O and

Start-up and operation at an applied cell voltage of 0.5 V

Two MECs with graphite felt electrodes were started up by applying a cell voltage of 0.5 V and inoculating the anode chamber with electrochemically active microorganisms. After start-up, the current density of both MECs increased (Fig. 2). The current density of the MECs showed more fluctuations than that of the biocathode in the electrochemical half cell [5]. After 300 h for MEC 1, and after 200 and 400 h for MEC 2, a drop and subsequent increase in current density was observed. As will be

Conclusions and implications

This study demonstrated for the first time that it is possible to operate a full biological MEC, i.e. no expensive chemical catalysts such as platinum are needed. Start-up of the full biological MEC was slow compared to an MEC with platinum cathode. However, it can be expected that further enrichment of microorganisms on the biocathode will decrease the start-up time of the full biological MEC, and increase the current densities up to bioanode level. Current densities in this study were already

Acknowledgements

This work was performed in the TTIW-cooperation framework of Wetsus, centre of excellence for sustainable water technology (www.wetsus.nl). Wetsus is funded by the Dutch Ministry of Economic Affairs, the European Union Regional Development Fund, the Province of Fryslân, the City of Leeuwarden and the EZ/Kompas program of the “Samenwerkingsverband Noord-Nederland”. The authors like to thank Tom Sleutels for critically reading the manuscript, and Shell, Paques bv and Magneto Special Anodes bv of

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