Sub-Micron Surface Structured Stainless Steel As an Anode for Microbial Fuel Cell

Abstract

Fossil foil depletion and their environmental impact make it inevitable to find alternative energy resources. One of the promising energy sources is to generate electricity through degrading organic compounds, using biofuel cells. Microbial fuel cells (MFCs) are biofuel cells that produce electricity while treating wastewater, allowing for more sustainable wastewater treatment and energy production1. For MFC to be a real-life application, the material of its components should be efficient, cost-effective, and commercially available. MFCs anodes are the interface where the bio-electrochemical reaction takes place through electron transfer from the bacteria to the electrode to produce an electrical current2. Herein we test a 3D nanostructured 316L stainless steel (SS) anode to provide a high specific surface area for the bacteria to adhere to the surface, and in turn, enhances the bacterial catalytic behavior. Furthermore, the SS nanostructured samples were annealed in various gaseous atmospheres to identify the effect of different surface oxidation states on MFC anodes performance. The surface of the bare SS and the nanostructured anodes were imaged using the field emission scanning electron microscopy (FESEM), before and after using them in the MFCs. The as anodized SS had a 3D nanostructured surface of interconnected nanorods that kept its morphology upon annealing. X-ray diffraction (XRD) was performed at a glancing angle (θ <5°), which interpreted that 316 L as received SS had a face-centered cubic structure that has gamma phase iron with the γ(111), γ (200), γ (220), γ (311), and γ (222) facets 3, while the film formed on the as anodized SS had amorphous regions that were transferred to crystalline film upon annealing for one hour in 450 C temperature. X-ray photoelectron spectroscopy (XPS) was used to characterize the composition of the fabricated SS anodes that interpreted that the film formed on SS was iron-chromium oxyhydroxide film4 and upon annealing the ratio of metal oxides especially Fe2O3 and Fe3O4 increased which increased the metal surface conductivity. The surface conductivity of annealed anodes was tested using cyclic voltammetry (CVs) in the ferricyanide solution. The annealed samples had enhanced to electron transfer kinetics on its surface relative to both the as-received and as anodized samples that showed nearly no redox activity. The fabricated SS samples were tested using a dual-chamber MFCs inoculated with sludge, using acetate as a substrate and potassium ferricyanide as the oxidant in the cathode. The power output of the MFCs with different anodes was observed using a data acquisition system for 20 days. CVs were done on the anodes on the 10th day. The results emphasized that the nanostructured SS samples highly increased cell voltage (≈ 80 times). This enhancement in the MFCs power performance is explained by the enhanced biofilm growth offered by the high specific area for the nanostructured SS. The FESEM images taken after the end of the experiment showed an obvious biomass growth on the surface of the as anodized SS relative to the biofilm on the as-received one. In addition, the annealed samples showed an enhanced activity after annealing. Annealing, especially in O2, increased the surface content of the more biocompatible Fe2O3 component, as interpreted by the XPS, which in turn increased the voltage production (≈ 120 times) relative to the as-received sample. The enhancement of the power generation was further supported using the CVs, with the anodized anode annealed in O2 showing an enhancement anodic current in comparison to smooth SS samples.