DEVELOPMENT OF TRANSITION METAL-NITROGEN-CARBON NANO-COMPOSITES AS ELECTROCATALYSTS FOR APPLICATION IN SINGLE CHAMBER MICROBIAL FUEL CELLS

Abstract

The uncertainty facing the global energy system is a critical challenge to meeting the sustainable development goals of developing countries. The alarming rise in energy demand led to the exploitation of fossil fuels, raising severe environmental concerns. Concerns about clean and renewable energy sources have triggered the research for green energy conversion and storage devices, where microbial fuel cells (MFCs) are a promising energy generation technology. MFCs are sustainable alternatives for clean and carbon-free energy generation. However, the technology’s efficiency depends on factors such as electrode cost, reaction kinetics at the electrodes, cell design, substrate, etc. The slow oxygen reduction reaction (ORR) kinetics at the cathode and biofilm enrichment at the anode hamper its propensity. The high capital cost of platinum (Pt) metal catalysts, limited availability, and its poisoning behavior during various electrochemical applications are major concerns for commercializing the MFCs. Studies have reported the development of electrocatalysts with high ORR activity, good durability, biocompatibility, and low cost as the key to optimizing the power performance of MFC systems. Carbonaceous materials could be efficient alternatives to Pt catalysts for ORR. Carbon materials doped with transition metal/heteroatom offer abundant M − Nx sites offering good activity and power output due to high site exposure and mass transfer rate. Biomass-derived carbons further reduce the catalyst cost. The work reported in the thesis summarizes the history behind the evolution of MFCs and progress in carbonaceous materials as the catalyst for ORR and their utilization for power generation in MFCs. Carbonaceous materials derived from polymers and biomass-based precursors have been explored for ORR catalysis. Incorporating transition metals and heteroatoms in the carbon could further enhance the catalytic activity towards ORR. Nitrogen (N)-doped carbons are popular research hot spots offering abundant catalytic sites improving the conductivity, stability, and ORR activity. This study covers the development of different transition metal-nitrogen-carbon nano-composites (M-N-C) primarily for power generation in MFCs using activated sludge and soil as inoculum/substrate. The performance of copper and nitrogen co-doped carbons catalysts (Cu/NC) for ORR is investigated to examine the effect of pyrolysis temperature, nitrogen functionalities, and Cu−Nx sites. Cu/NC-700 modified MFC exhibited a maximum power density of 489.2 mWm−2, higher than NC-700 (107.3 mWm−2), which could result from synergistic interaction between copper and nitrogen atoms, high density of Cu−Nx sites, and high pyridinic-N content. Fe-containing ureaformaldehyde resins-derived catalyst interweaved with polyaniline (PANI) pyrolyzed at 800◦C highlights the role of metal sites and PANI in enhancing the conductivity, charge transfer, and power generation (637.53 mWm−2) in MFCs. PANI@Fe/NC exhibits nanofiber aggregation, which on pyrolysis results in unique coralline-like short rods attributing to the large surface area of PANI@Fe/NC. In the next work, an environmentally benign approach is followed to synthesize a composite of magnesium cobaltite (MgCo2O4) embedded in N-doped carbon to optimize the performance of MFCs. The aim was to reduce the toxicity of cobalt that prohibits its application in aqueous MFC systems to some extent. Partially replacing cobalt with magnesium in the spinal oxide provides an alternate analog that could improve the bandgap/bandwidth of Co-based catalysts. The regulatory effect of Mg improves the active sites, electronic structure, and conductivity and suppresses the metal leaching, attributed to the formation of a highly stable MgCo2O4 spinel structure over Co3O4. Tri-metallic carbon composites as cathodes are rarely discussed for microbial fuel cells. Inspired by these observations, for the first time, a 3D hierarchically porous, trimetallic composite of FeCoNi alloy embedded in polyacrylamide-derived N-doped carbon has been explored for power generation in MFCs. FeCoNi@NC catalyst performed better than Pt/C, exhibiting superior electrocatalytic activity with a high oxygen reduction peak potential of 0.394 V (vs. RHE) and a corresponding peak current density of -0.159 mA. FeCoNi@NC records a high power density of 963.5 mWm−2, current density of 2483.2 mA m−2, 3.287 and 1.136 times higher than NC and 10 wt% Pt/C, respectively. The effect of sulfur (S) doping on the performance of M-N-C catalysts towards ORR has been examined. Heteroatoms alter the electronic distribution of the elements responsible for defect density in carbons and facilitate the formation of active intermediates in ORR. Doping sulfur could enhance the electrochemical properties due to its high degree of sulfur group dissociation. In polymer-derived carbons, the sulfo group tends to preserve the polymer’s nitrogen atoms and charging-discharging degree, making it suitable for use as an electrode material. Transition metal oxides and chalcogenides carbons were synthesized. NiCo2S4/NiCo2O4@NSC showed an oxygen reduction peak at 0.150 V, limiting current of -0.093 mA, close to -0.107 mA for Pt/C and generates maximum power density of 831.74 mWm−2, comparable to that of Pt/C (857.92 mWm−2). In another work, we developed a nickel (Ni) chelate-bearing sulfur (S) and nitrogen (N) groups from thiourea-formaldehyde resins (Ni-TF) derived catalyst for ORR. To improve the electrical conductivity and electron transfer and systematically investigate the influence of N/S content, Ni-TF particles were interweaved with polyaniline (PAN) and copolymer aniline-2- sulfonic acid and aniline (SPAN). Catalyst (SPAN-PAN/Ni-SNC) exhibits high oxygen reduction potential, superior stability, and low charge transfer resistance (Rct) of 95.1 Ω, favoring ORR activity. Furthermore, single chamber MFC based on SPAN-PAN/Ni-SNC exhibits the highest power density of 2045.1 mWm−2 at 8343.8 mA m−2 compared to 873.67 mWm−2 and 5453.6 mA m−2 for Ni-SNC. Finally, a cobalt oxide-modified graphite felt (GF) electrode is used as an anode in soil microbial fuel cells (SMFCs) to promote the development of a high-performing electroactive biofilm and, therefore, boost the electrocatalytic processes. The interweaving of PANI onto the Co3O4- GF electrode led to a porous structure that, while favoring microbial attachment, provides stability to the electrode over prolonged periods of operation. PANI-Co3O4-GF anode led to a highperforming SMFC system generating a peak power density of 70 mWm−2, corresponding to a current density of 143 mA m−2. This power density value was nearly three times greater than the power generated by the same SMFC system but with a plain GF anode. These studies provide exciting perspectives on developing composite carbon-based electrode materials for highly performing microbial fuel cells, thus inspiring future trends in the field.