Fe3O4@Pt Core-Shell Nanocatalyst Supported on N-Doped Functionalized Graphene As Novel Cathode Catalysts for Microbial Fuel Cells
Authors: Garza-Duran Eréndira , Vargas-Gutiérrez Gregorio , Escobar-Morales Beatriz , Alonso-Lemus Ivonne Liliana , Rodríguez-Varela F.J. 
Published: February 2020

Microbial Fuel Cells (MFCs) are bio-electrochemical systems that use bacteria to generate bioelectricity via the oxidation of organic matter contained in a substrate (such as wastewater). However, this new method of renewable energy recovery has several technical challenges. At the cathode, for example, the Oxygen Reduction Reaction (ORR) takes place. This reaction is kinetically slow, remaining one of the main drawbacks of MFCs. Even though Pt/C nanocatalysts are widely used to promote the ORR, their performance in MFC is low due to the complex operating conditions. Therefore, alternative cathode nanocatalysts must be developed. Regarding to this issue, core-shell nanocatalysts are a promising alternative for MFCs cathodes.

In this study, Fe3O4@Pt core-shell nanoparticles have been supported on N-doped and functionalized graphene (N-Gf) to obtain the Fe3O4@Pt/N-Gf nanocatalyst. First, nitrogen-doped graphene (N-G) has been synthesized by a one-step ball milling process using graphite as carbon source and melamine as both exfoliating agent and nitrogen source. Then, N-G has been functionalized by a mild acid treatment and labeled as N-Gf. Separately, the Fe3O4 core has been obtained by co-precipitation reaction of Fe2+/Fe3+, using citric acid as surfactant. The Pt shell has been deposited on the core by the Bromide Anion Exchange (BAE) method, resulting in a Fe3O4@Pt nanostructure having a 1:1 Fe3O4:Pt molar ratio. The 20 wt.% Fe3O4@Pt/N-Gf core-shell nanocatalyst has been obtained also by the BAE method.

Regarding N-Gf, its X-ray diffraction (XRD) pattern shows well-defined peaks at 2θ=26.5°, 44.39 and 54.4°, characteristic of graphitic structures. Moreover, its ID/IG intensity ratio calculated from Raman spectra is 1.40, which indicates a high degree of disorder in the carbon lattice, most likely due to the incorporation of N species into the structure. Additionally, an N content of 1.40 at. % has been determined by energy-dispersive X-ray spectroscopy (EDS). The XRD pattern of the magnetite phase shows reflections at 2θ= 30.44, 35.53, 43.46, 54.01, 57.79, 63.06 and 74.75°, attributed to the (220), (311), (400), (422), (511) (440) and (531) planes. The crystallite size of Fe3O4 has been calculated as 9.2 nm. In addition, its Raman spectrum shows signals corresponding to the A1g, Eg and T2g vibrational modes, characteristic of magnetite.

Meanwhile, the catalytic activity of the Fe3O4@Pt/N-Gf nanocatalyst for the ORR has been evaluated by the Rotating Ring-Disk Electrode (RRDE) technique in 0.5 M H2SO4 and in H2SO4 having a pH=6.1. The latter is because of the pH of the pharmaceutical residual water used as substrate in the MFC. Electrochemical parameters such as hydrogen peroxide percentage, number of electrons transferred, and onset and half wave potentials has been determined. The results show that Fe3O4@Pt/N-Gf has a catalytic activity for the ORR in low and almost neutral pH comparable to that of a commercial Pt/C nanocatalyst. Therefore, these results suggest that Fe3O4@Pt/N-Gf is a promising cathode nanocatalyst for MFC applications. To the best of the authors knowledge, this is the first time that such nanostructures have been evaluated in MFCs.