Journal of Power Sources 196 (2011) 164–168 Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour Short communication Improved microbial electrocatalysis with neutral red immobilized electrode Kaipeng Wang, Yuwen Liu, Shengli Chen ∗ Hubei Electrochemical Power Sources Key Laboratory, Key Laboratory of Analytical Chemistry for Biology and Medicine (Ministry of Education), Department of Chemistry, Wuhan University, Bayi Road, Wuhan, Hubei 430072, China article info Article history: Received 14 May 2010 Received in revised form 16 June 2010 Accepted 16 June 2010 Available online 23 June 2010 Keywords: Microbial fuel cell Covalent immobilization Neutral red Extracellular electron transfer abstract Efficient electron transfer (ET) between microbes and electrodes is a key factor for electricity genera- tion in microbial fuel cell (MFC). The utilization of reversible redox electron-mediator can enhance such extracellular ET but could result in environmental contamination and low cost-effectiveness. These lim- itations may be overcome by immobilizing electron-mediator molecules on electrode surface. In this paper, we present a stepwise amidation procedure to covalently immobilize neutral red (NR), which has been proved to be an appropriate mediator to harvest microbial metabolic electrons due to its excellent electrochemical reversibility and compatible redox potential to the major metabolic electron carriers (e.g., of NADH/NAD + ), on carbon electrodes. In this procedure, immobilization of NR is realized by acylchlo- rination of the carboxylated carbon surface with thionyl chloride followed by amidation reaction with NR. It is shown that such a stepwise amidation procedure can significantly increase the amounts of NR molecules immobilized on carbon surface without altering their redox properties. In addition, the use of NR-immobilized carbon electrodes as MFC anode can significantly increase the power output and the utilization of carbon sources (organic fuel). © 2010 Elsevier B.V. All rights reserved. 1. Introduction Microbial fuel cell (MFC) is a device that directly converts the chemical energy stored in organic carbon sources into elec- tricity through extracellular electron transfer from microbes to solid electrodes [1,2]. Besides the abundance in nature, microbes mostly have versatile metabolic abilities to oxidize organic com- pounds from natural hydrocarbons to domestic and industrial wastes, which make MFC a potentially competitive solution of the worldwide crisis on energy shortage and environmental dam- age. Currently, the power density of MFC remain too low to be competitive to the conventional chemical fuel cell, mainly due to the very low rate of direct electron transfer from microbial cells to solid electrode [3]. Therefore, mechanism of electron trans- fer from microbial cells to extracellular solid electrode in MFC and methods to accelerate such transfer have been the subject of extensive studies. It has been previously shown that rapid electron transfer from microbes to electrodes might be real- ized by introducing proper electron-mediators such as thionin, azine, and 2-hydroxy-1,4-naphthoquinone (HNQ) in culture solution [4–6]. Ideally, a redox-mediator to deliver metabolic electrons to fuel cell anode should have good electrochemical reversibility so that ∗ Corresponding author. Tel.: +86 27 68754693, fax: +86 27 68754067. E-mail address: slchen@whu.edu.cn (S. Chen). its own oxidation at electrode requires negligible activation barrier. It should also have a redox potential close to the redox potential of NAD + /NADH couple, which is the major electron carrier in micro- bial metabolism and locates on the top of the electron transfer chain in microbial respiration. As has been shown by Park et al. [7,8], neutral red (NR) was one of the redox molecules with these novel properties. The comparable redox potential of NR to NAD + /NADH will render metabolic electrons to be harvested at the top of res- piration chain, therefore resulting in a relatively negative anode potential (high energy efficiency) in MFC. Although employing soluble electron-mediators can signifi- cantly improve the performance of MFC, it could cause secondary environmental contamination as mediators flow out with the effluent. In addition, the continuous reinforcement of electron- mediators also brings about cost problem [9]. Such embarrassments might be overcome using immobilized electron-mediator. Previous studies have shown that it was difficult to physically immobilize a large amounts of mediator directly on electrode surface, and that mediator physically adsorbed on electrode showed little effi- ciency because it was easily removed from the electrode surface and flowed out with the effluent [10]. Adachi et al. [11] have intro- duced that the power output of MFC could be increased using a mediator-polymer-modified anode, in which a bioactive redox compound 9,10-anthraquinone-2,6-disulfate (AQDS) was immobi- lized on the anode surface with a thin layer of a functional polymer. But for the redox potential of AQDS (-0.184 V vs. NHE) was not com- patible to NAD + /NADH couple, a little improved MFC performance 0378-7753/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jpowsour.2010.06.056