Elsevier

Journal of Cleaner Production

Volume 115, 1 March 2016, Pages 332-336
Journal of Cleaner Production

Chemically activated graphite enhanced oxygen reduction and power output in catalyst-free microbial fuel cells

https://doi.org/10.1016/j.jclepro.2015.12.067Get rights and content

Highlights

  • Graphite activated by H3PO4 or HNO3 has improved oxygen reduction potential.

  • Power density of a cathode activated by H3PO4 or HNO3 was higher than the control.

  • Chemically activated graphite was used as the catalyst-free cathodes.

  • Chemical activation is a cost-effective in-situ technology.

Abstract

In search of a cost effective cathode material for microbial fuel cells (MFCs), graphite was chemically treated with H3PO4, HNO3, ZnCl2, urea or melamine, and the effect of chemical activations on the oxygen reduction reaction (ORR) was examined. The performance of MFCs with activated graphite as the catalyst-free cathodes was then compared to those with untreated graphite. Results suggested that H3PO4 and HNO3 activations could improved ORR, showing the highest ORR activity in graphite treated with 14.62 M H3PO4 for 12 h at 30–50 °C. MFCs with H3PO4 and HNO3 activated graphite cathodes generated maximum power densities (7.9 W/m3 and 6.5 W/m3, respectively) 2.4 and 1.8 times higher than that of the untreated control. The chemical activation process involves just a simple immersion step, and it does not require heating, electrochemical process or expensive chemicals. Therefore, it is a highly cost-effective approach to improve the performance of MFCs. We recommend an in-situ modification of graphite cathodes in scale-up MFCs with either H3PO4 or HNO3 to optimize MFCs' various industrial applications.

Introduction

Microbial fuel cells (MFCs) have attracted significant attention recently due to their promising potential to harvest bioelectrochemical energy from wastewater and simultaneously remove organic pollutants using microorganisms (Koók et al., 2016, Liew et al., 2014). Oxygen reduction reaction (ORR) is a fundamental process occurring on the cathode of an MFC, therefore making the cathode a key factor for the overall process efficiency (Huang et al., 2015). Platinum (Pt) has been widely accepted as the cathodic catalyst for ORR because of its high catalytic activity that improves MFC performance (Oh et al., 2004, Deng et al., 2010a, Deng et al., 2010b). Its high cost, however, hinders the commercial application of the MFC technology. Therefore, developing a novel, low cost and platinum-free cathode material is highly desired.

Recently, non-precious metal catalysts such as cobalt (cobalt tetramethylphenylporphyrin, CoTMPP) and iron (iron phthalocyanine, FePc) compounds have been demonstrated as potential platinum catalyst replacements in MFC cathodes based on their oxygen reduction capabilities (Cheng et al., 2006, Zhao et al., 2005, Lu et al., 2015). Lu et al. (2009) tested the natural rutile (a semiconductor mineral) as a novel cathode catalyst for MFCs, and proved it a cost-effective alternative. Other catalysts have also been reported to enhance cathode performance and reduce cathode costs. These include MnO2 (Li et al., 2010), cobalt naphthalocyanine (CoNPc) (Kim et al., 2011), PbO2 (Morris et al., 2007) and Co/Fe/N/CNT (carbon nanotube CNT) (Deng et al., 2010a, Deng et al., 2010b). The commercial use of these potential alternatives, however, requires further investigations: platinum-free cathode materials tend to have lower power density and inferior catalytic capability; the production processes of these alternatives are complicated; and their cost is still unendurable high.

Chemical activation has been proven a low-cost and effective method to improve the adsorption capability and electrochemical performance of MFCs through the alteration of the surface area of cathodes, the microbial species living there and their population size. Chemical activators that modify carbon materials can be classified to three categories based on their activation mechanisms: oxidants (e.g. HNO3, H3PO4, O3 and H2O2), reductants (e.g. ammonia, urea and melamine formaldehyde), and neutral chemicals (e.g. steam and ZnCl2) (Tamon and Okazaki, 1996, Xu and Liu, 2009). A previous study (Qiao et al., 2015) reported that melamine and urea activated graphite anode improved the voltage output and power density of MFCs, while HNO3 and H3PO4 activated graphite inhibited the anodic performance. In this study, the effects of chemically activated graphite by H3PO4, HNO3, ZnCl2, urea and melamine on ORR were investigated through cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The widely available and low cost graphite materials were then used as catalyst-free and chemically activated cathodes to evaluate their potential enhancement to MFC power generation.

Section snippets

Chemical activation

Graphite rods (6 mm diameter, Xinxia Mechanical and Electrical Materials Inc., Shanghai, China) were used as the electrodes for ORR electrochemical tests. They were polished with sandpapers (10 μm), washed with deionized water and absolute ethyl alcohol, and air-dried for 1 h (Qiao et al., 2015) before chemical activation. To activate the graphite, the pre-washed rods were immersed in 6.80 M H3PO4 (P), 5.10 M HNO3 (O), 0.59 M ZnCl2 (Zn), 1.67 M urea (U) or 0.63 M melamine formaldehyde (M)

Enhancement of ORR by chemically activated graphite

As shown in Fig. 1(A), electrodes activated by H3PO4 (P) and HNO3 (O) exhibited a sharp increase in the reduction current when compared with the untreated graphite electrode (B), while the opposite was observed with electrodes treated with urea (U) and melamine (M). The catalytic currents derived from the untreated graphite electrode (B) and the graphite electrodes chemically activated by H3PO4 (P), HNO3 (O), ZnCl2 (Zn), urea (U) and melamine (M) were 0.604, 1.97, 1.08, 0658, 0.311 and

Conclusions

In search of a more cost-effective material for large-scale MFCs, chemically activated graphite was used to build the catalyst-free cathodes, and the performances of the resulting MFCs were examined through ORR and power generation analyses. MFCs with H3PO4 and HNO3 activated graphite cathodes produced maximum power densities of 7.9 W/m3 and 6.5 W/m3 respectively, 2.4 and 1.8 times higher than that of the control MFC with an untreated graphite cathode. The chemical activation technique is a

Acknowledgements

This work was supported by the National Natural Science Foundation of China (NSFC) (20906026), Shanghai Pujiang Program (09PJ1402900) and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (B200-C-0904).

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