Iron-Based Electrocatalysts Supported on Nanostructured Carbon to Enhance Oxygen Reduction in Microbial Fuel Cells
Authors: Mecheri Barbara , Iannaci Alessandro , D'Epifanio Alessandra , Nieto-Monge Maria Jesus , Lázaro Maria Jesus , Licoccia Silvia 
Published: February 2020

Sustainable waste management and energy supply can find a concrete answer in the technology of microbial fuel cells (MFCs). One of the major barriers to the widespread diffusion of this technology is the high cost of materials, among which the platinum-based cathode accounts for about 50 % of the total cell cost. Although platinum is a highly effective oxygen reducing catalyst, its activity is decreased by a variety of poisoning substances present in wastewater [1]. Latest development on nanostructured carbon for energy storage and conversion has brought to light carbon nanofibers (CNF), carbon nanotubes (CNT) and activated carbon (AC) as good candidates to replace costly Pt. These materials have high surface area and conductivity, and nitrogen doping has been demonstrated to be an effective approach to modulate their electronic properties [2]. Moreover, recent advances on non platinum group metal (NPGM) catalysts demonstrated the possibility to replace Pt with earth-abundant elements, such as Mn, Fe, Co, and Cu [3]. Among all, iron based catalysts supported on carbon have being used at cathodes of MFCs, active sites arising from Fe/N/C coordination.
Aim of this study is to evaluate and compare catalytic activity towards oxygen reduction reaction (ORR) of catalysts based on iron phthalocyanine (FePc) supported over different carbon substrates with physical and morphological differences.
Commercial carbon black, carbon nanotubes, and black pearls were used as catalyst support. Furthermore, the use of different advanced nanostructured carbon materials, such as carbon nanofibers and carbon xerogels was also investigated, owing to their porous structure and surface properties which can be tailored according to catalytic requirements [4, 5]. To facilitate catalyst dispersion and obtain effective ORR active sites, the supports were modified by a two-step treatment with nitric acid and ammonia gas. The treatment allowed introducing oxygen and nitrogen functionalities on the carbon surface while maintaining an extensively developed porous structure, as demonstrated by elemental analysis, BET, and X-ray photoelectron spectroscopy (XPS). Electrocatalysts derived from iron phthalocyanine (FePc) and the different carbon supports were prepared and their electrochemical activity towards ORR was assessed by cyclic voltammetry and rotating disk voltammetry experiments. The electrochemical results demonstrated that the prepared electrocatalysts are all active towards ORR at neutral pH; the higher the density of pyridinic nitrogen on the sample surface, the greater is the performance, as indicated by XPS.
The applicability of FePc-based materials as ORR cathodes of MFCs was evaluated by assemblying FePc cathodes in single chamber air-catodes MFCs operating with sodium acetate in phosphate buffer solution, which coulombic efficiency, polarization and power density curves, and voltage generation cycles over time were acquired. The body of results demonstrated the potential ability of FePc-electrocatalysts to substitute platinum for ORR in MFCs.
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