Cobalt oxides nanoparticles supported on nitrogen-doped carbon nanotubes as high-efficiency cathode catalysts for microbial fuel cells

Highlights

• Facile synthesis of Co/N-CNT by pyrolysis of carbon nitride and cobalt acetate

• High ORR activity of Co/N-CNT in alkaline media

• Excellent poison tolerance of Co/N-CNT in sulfide-containing electrolyte

• Maximum power density of MFC with Co/N-CNT air cathode 16.6% higher than with Pt/C

Abstract

Microbial fuel cell is a unique energy technology where both wastewater treatment and electricity generation take place concurrently. However, the performance is typically rather limited due to the sluggish electron-transfer kinetics of oxygen reduction reaction at the cathode. Thus, development of high-performance cathode catalysts is of fundamental significance for the wide-spread application of microbial fuel cell. In this study, nanocomposites based on cobalt oxide nanoparticles supported on nitrogen-doped carbon nanotubes (Co/N-CNT) were synthesized by controlled pyrolysis of graphitic carbon nitride and cobalt acetate. Electrochemical tests indicated that the Co/N-CNT nanocomposites exhibited a high ORR electrocatalytic activity with a half-wave potential of +0.82 V and onset potential of +0.91 V vs. RHE, mostly via a four-electron reduction pathway. This was ascribed to the formation of high-efficiency CoN active sites that facilitated the ORR kinetics. A microbial fuel cell using the as-prepared Co/N-CNT as the cathode catalyst achieved a maximum power density of 1260 mW m⁻², which was 16.6% higher than that based on state-of-art Pt/C catalyst (1080 mW m⁻²). The results suggest that Co/N-CNT nanocomposites may serve as viable cathode catalysts in microbial fuel cell application.

Introduction

Microbial fuel cell (MFC) has been attracting extensive attention lately because it can degrade organic pollutants in wastewater and concurrently produce electricity using anode bacteria [[1], [2], [3], [4]]. Of these, MFC with an air cathode is a promising design for practical utilization, due to the direct use of freely available oxygen in the air as electron acceptors [[5], [6], [7]]. Nevertheless, despite substantial progresses in recent years, the power density of MFC has remained relatively low, primarily due to the high overpotential and sluggish electron-transfer kinetics of oxygen reduction reaction (ORR) at the cathode [8,9]. This is a major limitation hindering the practical application of MFC. Platinum (Pt)-based nanoparticles have been used extensively as the catalysts of choice for ORR; yet the high cost, low earth abundance, and low poison tolerance in the presence of contaminants in wastewater have been detrimental to the device performance and applications [10,11]. Within this context, a range of non-precious metal/carbon nanocomposites have been prepared by using select organometallic complexes as precursors, such as iron(II) phthalocyanine and cobalt tetramethoxyphenylporphyrin, and exhibited rather remarkable electrocatalytic performance as the air cathode catalysts of MFC [12,13]. In fact, carbon-supported non-precious metal-based catalysts have been proposed as viable alternatives as cathode catalysts for MFC, because of their low costs, high natural abundance, apparent catalytic activity, and good mechanical strength [9,[14], [15], [16], [17], [18], [19], [20], [21], [22], [23]]. For nitrogen-doped carbon, metal‑nitrogen coordination (MN_x) moieties are typically formed within the carbon matrix (M-N-C) and serve as the ORR active sites [10,24,25]. For instance, an MFC using Fe,N-codoped carbon as the cathode catalyst has been found to deliver a maximum power density (P_max) of 3118.9 mW m⁻², which is markedly higher than that obtained with a Pt/C cathode (2017.6 mW m⁻²) under the same operating conditions [14].

In general, the preparation of M-N-C hybrids entails two steps, synthesis of the carbon substrates, such as graphene and carbon nanotubes, followed by controlled pyrolysis after the addition of select nitrogen-containing precursors to facilitate N doping (e.g., HNO₃, NH₃, and urea) and metal salts to incorporate metal dopants. Graphitic carbon nitride (C₃N₄) represents a unique precursor. It is a conjugated organic semiconductor consisting of sp² hybridized nitrogen and carbon atoms [26], where the abundant pyridinic nitrogen moieties can be exploited for the coordination of transition metal centers [27,28].

Herein, we demonstrate that Co,N-codoped carbon nanotubules can be synthesized pyrolytically by using cobalt acetate as the cobalt precursor and C₃N₄ as the carbon and nitrogen sources. The structures of the resulting Co/N-CNT nanocomposites were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Raman measurements. Electrochemical tests showed that the Co/N-CNT nanocomposites exhibited a high ORR electrocatalytic activity with a half-wave potential of +0.82 V and onset potential of +0.91 V vs. RHE, mostly via a four-electron reduction pathway. This was ascribed to the high efficiency of CoN active sites that facilitated the ORR kinetics. Furthermore, the performance of the resulting Co/N-CNT nanocomposites as MFC air cathode catalysts was examined in a home-made MFC, which achieved a maximum power density of 1260 mW m⁻², 16.6% higher than that based on a state-of-art Pt/C catalyst (1080 mW m⁻²). These results suggest that Co/N-CNT nanocomposites may serve as viable ORR catalysts in MFC applications.