Iron–nitrogen-functionalized carbon as efficient oxygen reduction reaction electrocatalyst in microbial fuel cells

doi:10.1016/j.ijhydene.2016.04.154

International Journal of Hydrogen Energy

Volume 41, Issue 43, 16 November 2016, Pages 19637-19644

https://doi.org/10.1016/j.ijhydene.2016.04.154 Get rights and content

Highlights

•
Electrocatalysts based on iron, supported on nanostructured carbon.
•
Oxygen and nitrogen functionalization of carbon.
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Application as cathodes of microbial fuel cells (MFCs).
•
High power and voltage generation.
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Cost reduction with respect to Pt-based MFCs.

Abstract

Iron phthalocyanine (FePc) was supported on carbon nanotubes (CNT) and black pearls (BP). The carbon supports were modified by a two-step treatment with nitric acid and ammonia gas to facilitate catalyst dispersion and obtain effective ORR active sites. The treatment allowed introducing oxygen and nitrogen functionalities on the carbon surface while maintaining an extensively developed porous structure, as demonstrated by elemental analysis and BET measurements.

Electrochemical activity of the electrocatalysts was assessed by cyclic voltammetry and rotating disk voltammetry experiments. The catalyst obtained by supporting Fe on CNT modified with ammonia gas (Fe-CNT(NH₃)) displayed the highest catalytic activity towards ORR at neutral pH as a results of the highest density of pyridinic nitrogen on the sample surface, as indicated by Xray photoelectron spectroscopy (XPS).

The applicability of Fe-based electrocatalysts as ORR cathodes of microbial fuel cells (MFCs) was demonstrated by assembling single chamber air-cathodes MFCs and comparing the performance with that a MFC equipped with a reference Pt/C cathode.

Introduction

Microbial fuel cell (MFC) technology has been recently proposed to contribute to wastewater treatment while producing energy through biological oxidization of organic matter at the anode and chemical reduction of oxygen at the cathode [1], [2], [3], [4]. Different oxidants have been investigated as the electron acceptors at the cathode of MFCs, oxygen being the most sustainable electron acceptor due to its low cost, availability, and high redox potential [5], [6], [7].

Platinum (Pt) has extensively been considered the best performing catalyst for oxygen reduction reaction (ORR) at the cathode of MFCs, though its high cost and limited reserves in nature poses serious limitations to the widespread commercialization of MFCs [8], [9], [10]. Moreover, the use of Pt as cathodic catalyst is also compromised by its weakness to poisoning and deactivation in the presence of typical MFC metabolites [11], [12].

Increasing efforts have thus been made to explore cost-effective and stable Pt-free materials and recent breakthroughs in the synthesis of non platinum group metal catalysts and activated carbon nanostructures for efficient ORR at the cathode of MFCs make replacement of Pt a realistic possibility [13], [14], [15], [16], [17], [18]. It has been demonstrated that transition metal phthalocyanines supported on carbon nanotubes (CNTs) displayed an enhanced electrocatalytic performance towards ORR [19], [20], [21], [22], [23], [24] as a result of π-stacking interactions between planar aromatic structure of the macrocycle with the sidewalls of CNTs [25].

It has been proposed that the active sites for ORR arise from the coordination of carbon, nitrogen and metal atoms [3], [20], [26], [27], [28], [29], [30], surface chemistry and textural features of carbon supports playing a crucial role on ORR catalysis [31], [32], [33]. Nitrogen doping of nanostructured carbon is a promising strategy to modulate the electronic properties of carbon nanomaterials, to create highly active metal/nitrogen/carbon active sites and improving catalyst durability by enhancing π-bonding [34], [35], [36], [37], [38], [39].

However, composition and structure of ORR active site are still under debate in the current literature and further investigation on iron-based electrocatalysts may lead to the development of materials to completely eliminate the need for platinum in MFCs.

We have previously reported on the preparation and characterization of electrocatalysts derived from iron phthalocyanine (FePc) supported on carbon nanotubes (CNT). FePc/CNT displayed catalytic activity towards ORR at neutral pH, leading to higher power density than reference Pt/C when assembled as cathode of a MFC [24].

We have now extended our investigations, studying the effect of carbon functionalization on catalytic performance of FePc supported on CNT and black pearls (BP). Composition and activity of Fe/N/C sites were analyzed by rotating disk voltammetry, elemental analysis, BET surface area measurements, and x ray photoelectron spectroscopy. The applicability of these materials was evaluated by power and voltage generation of air-cathode MFC assembled with FePc-based cathodes.

Section snippets

Support treatments and catalyst preparation

Multi-walled carbon nanotubes (CNTs, > 95% carbon content, diameter × length 6–9 nm × 1 μm) were purchased from Aldrich and Black pearls 2000 (BP) were purchased by Cabot corporation (MA, US). Carbon supports were modified by a two-step treatment with nitric acid and ammonia gas. The HNO₃ treatment consisted in refluxing in concentrated HNO₃ 65 wt.% at 90 °C for 16 h. Then, the materials were filtered and washed with distilled water until neutral pH was obtained. The carbon pastes were then

Carbon support characterization

Surface area and chemical composition of the carbon supports were analyzed by BET measurements and EA, respectively, before and after HNO₃ and NH₃ treatments. Table 1 shows the corresponding results in terms of total surface area, oxygen and nitrogen content.

Surface area of CNTs increased after HNO₃ and NH₃ treatments, due to amorphous carbon removal and external exposition of inner cavities, as previously reported [40]. On the contrary, due to partial collapse of the porous structure,

Conclusions

This work demonstrated Fe-CNT(NH₃) as an effective oxygen reduction catalyst at MFC cathodes. The treatment of CNT with ammonia gas significantly improved catalytic activity, leading to kinetic current density values much higher than the standard Pt/C at neutral pH.

The performance of single-chamber air-cathode MFC was larger for Fe-CNT(NH₃) cathode, thereby generating higher power and current density as compared to Pt/C.

The significant improvement of cathode performance in MFC was ascribed to

Acknowledgments

The authors gratefully acknowledge financial support given by Spanish MINECO (ENE2014-52158-C2-1-R) and FEDER. Moreover, thanks are due to Ms C. D'Ottavi (Dept. Chemical Sciences and Technology, University of Rome “Tor Vergata”) for her valuable technical support.

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Citation Excerpt :
It is worth noting that the air-cathode catalysts are one of the major influences affecting the performance of MFCs because of the high overpotential and poor kinetics of the oxygen reduction reaction (ORR) (Tang et al., 2016a; Ucar et al., 2017). In general, Pt group metal (PGM) electrocatalysts are the best performing ORR catalysts, however, their high expense, low abundance, and deactivation in the presence of microbial fuel cell metabolites greatly limited the industrial application (Halakoo et al., 2015; Iannaci et al., 2016). Recently, people are trying to reduce the potential loss of ORR as well as develop cost-effective nanomaterials to substitute the expensive PGM catalysts.
Microbial fuel cells (MFCs) are devices that treat sewage and generate electricity. Recent researches have demonstrated that the characteristics of carbon precursors can tremendously influence the performance of the MFC cathode. Carbon nanomaterials with good crystallinity as well as high specific surface area (e.x., graphene and carbon nanotube) can not only accelerate charge transport but also afford a good dispersion of catalytic active components, leading to high MFC performance. On these bases, the preparation of highly-active Fe-N/C catalysts using different carbon substrates are mainly discussed in this review. It is pointed out that increasing the surface area and conductivity as well as elevating the density of active sites to reduce the oxygen reduction overpotential is still the emphasis of the current works. At present, although the researchers have made some progress, the output power density is far from meeting the actual application needs.
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Iron–nitrogen-functionalized carbon as efficient oxygen reduction reaction electrocatalyst in microbial fuel cells

Highlights

Abstract

Introduction

Section snippets

Support treatments and catalyst preparation

Carbon support characterization

Conclusions

Acknowledgments

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