Chemically activated graphite enhanced oxygen reduction and power output in catalyst-free microbial fuel cells
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).
References (24)
- et al.
Methanol resistant ruthenium electrocatalysts for oxygen reduction synthesized by pyrolysis of Ru3(CO)12 in different atmospheres
Int. J. Hydrog. Energy
(2009) - et al.
Power generation using an activated carbon fiber felt cathode in an upflow microbial fuel cell
J. Power Sources
(2010) - et al.
Development of high performance of Co/Fe/N/CNT nanocatalyst for oxygen reduction in microbial fuel cells
Talanta
(2010) - et al.
A new clean approach for production of cobalt dihydroxide from aqueous Co(II) using oxygen-reducing biocathode microbial fuel cells
J. Clean. Prod.
(2015) - et al.
Application of Co-naphthalocyanine (CoNPc) as alternative cathode catalyst and support structure for microbial fuel cells
Bioresour. Technol.
(2011) - et al.
Bioelectrochemical treatment of municipal waste liquor in microbial fuel cells for energy valorization
J. Clean. Prod.
(2016) - et al.
Manganese dioxide as a new cathode catalyst in microbial fuel cells
J. Power Source
(2010) - et al.
Review of evolution, technology and sustainability assessments of biofuel production
J. Clean. Prod.
(2014) - et al.
Behavior of metal ions in bioelectrochemical systems: a review
J. Power Source
(2015) - et al.
The use of electrochemical impedance spectroscopy (EIS) in the evaluation of the electrochemical properties of a microbial fuel cell
Bioelectrochemistry
(2008)