الفهرس 
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Abstract
This thesis focused on the modification of the structure of the air cathode surface and improving the power generation performance of MFC in wastewater treatment.
This work contains three chapters:
Chapter I: Introduction and Literature Review
Water pollution by organic pollutants is a worldwide problem. The majority of organic pollutants originate from several industrial sources. However, these are not the sole sources of pollutants. Consumers’ use of fuels for transportation and heating, as well as the use of pesticides, fertilizers, and detergents, among other things, all contribute to the release of pollutants directly into the environment. Recent research has focused on the microbial fuel cell (MFC) as a sustainable bio-electrochemical energy conversion device due to its ability to directly harvest electrical energy during the wastewater treatment process; however, commercialization and manufacturing on an industrial scale are hampered by the sluggish kinetics of the oxygen reduction reaction (ORR) occurring on the cathode side at neutral pH conditions, as well as the scarcity and high cost of platinum (Pt). The introduction includes the fuel cell classification, among which is the microbial fuel cell; the factors affecting its performance and commercialization; and a literature survey on the application of metal ferrites as cathode catalysts in microbial fuel cell.
Chapter II: Materials and Experiments
The experimental part included;
1. Description of the materials used.
2. Catalysts preparation;
- Chemical treatment of Carbon Vulcan XC-72R and multi-walled carbon nanotubes (MWCNT).
- Preparation of Spinel-structure Activated Carbon supported MFe2O4 (M= Mn, Cu, and Ni) (MFe2O4/AC) Composites.
- Preparation of CuFe2O4 Composite.
- Preparation of Activated Carbon nanotubes supported CuFe2O4 (CuFe2O4/CNTa) Composite.
3. Physical characterizations of the electrocatalysts. The catalysts were characterized by using the following techniques;
- X-ray Diffraction (XRD).
- Fourier Transform Infrared (FT-IR) and Raman Spectroscopy.
- Nitrogen Adsorption-Desorption Isotherms.
- Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray Spectroscopy (EDX).
- High-Resolution Transmission Electron Microscopy (HR-TEM).
- Dynamic Light Scattering (DLS).
- Electron Spin Resonance (ESR).
4. Electrochemical Measurements including; cyclic voltammetry (CV) and linear sweep voltammetry (LSV).
Chapter III: Results and Discussion
This chapter included the following parts;
1. Electrocatalysts Physical characterization
- X-ray diffraction (XRD) analysis was employed to characterize the crystallinity of the catalysts and the diffractograms were recorded in 2θ range of 10-90o. The XRD identified the metal ferrites formation and exhibited the location of the corresponding peaks. The metal ferrites recognized were; MnFe2O4/AC, CuFe2O4/AC, NiFe2O4/AC, and CuFe2O4/CNTa, and its uniform dispersion over the activated carbon surface (AC, CNTa) were confirmed by disappearance of the carbon diffraction peaks. Also, CuFe2O4 formation was confirmed by its corresponding XRD peaks.
- Raman Spectroscopy showed bands for only the AC and NiFe2O4/AC, while the other prepared composite catalysts are Raman silent.
- FT-IR spectra in the range from 400 - 4000 cm-1 for the prepared samples showed strong bands in the lower mid-infrared 400-700 cm-1 range that originate from the stretching vibrations of the metal-oxygen bond (M–O; M= Mn, Ni, Cu, and Fe). All the catalysts (AC, CuFe2O4/AC, MnFe2O4/AC, NiFe2O4/AC, CuFe2O4, and CuFe2O4/CNTa) show a strong and broad peak around 3450 cm-1 due to the presence of OH-free asymmetric stretching. The absorption peak around 3650 cm-1 (one peak) is assigned to (N–H) stretching frequency of amide II.
- N2 adsorption-desorption isotherms results showed that all the synthesized composite catalysts exhibited classical type IV isotherm and a well-defined hysteresis loop close to high relative pressure. The pore size distribution curves pointed out that all the samples have mesopore sizes structured.
- FE-SEM & EDX Microscopy showed the morphology of the prepared catalysts and confirmed the uniform distribution of the transition metals over the surface of the carbon support.
- HR-TEM Microscopy images of all composites show low agglomeration, high dispersion, and homogeneous distribution of the composites nanoparticles on the carbon surface.
- DLS Analysis shows that the average particle size is 712 nm for CuFe2O4/AC, (295-995 nm) for NiFe2O4/AC, and (220-1106 nm) for MnFe2O4.
- ESR Spectroscopy qualitatively, verified the presence of oxygen vacancies and demonstrated the assimilation of the oxygen defects in MnFe2O4/AC, CuFe2O4/AC, and NiFe2O4/AC.
- The ESR is in good agreement with the electrochemical analysis due to the highest ∆Hpp value for MnFe2O4/AC compared to CuFe2O4/AC and NiFe2O4/AC.
2. Electrochemical Performance of the Electrocatalysts towards ORR
- The high dispersion of the ferrites on the carbon surface is considered to be responsible for the high electrochemical activity.
- Additionally, our results collected from LSV carried at 800 rpm revealed that MnFe2O4/AC had higher Eonset (-0.223 V vs. Ag/AgCl), kinetic current density (jK) (-5 mA cm-2), and lower Tafel slope (-330 mV dec-1) than NiFe2O4/AC (-0.270 V vs. Ag/AgCl, -2.67 mA cm-2, and -414 mV dec-1), and CuFe2O4/AC (-0.280 V vs. Ag/AgCl, -3.05 mA cm-2, and -577 mV dec-1), respectively.
- On the other hand; CuFe2O4 had higher Eonset (-0.260 V vs. Ag/AgCl) and jK (-3.29 mA cm-2), and lower Tafel slope (-286 mV dec-1) than CuFe2O4/CNTa (-0.330 V vs. Ag/AgCl, -2.64 mA cm-2, and -318 mV dec-1). Between the both catalysts, CuFe2O4 showed higher ORR catalytic activity owing to the good combination of morphology and crystal structure.
- Meanwhile, the K-L plot calculations for ORR catalyzed by MnFe2O4/AC and CuFe2O4 in neutral phosphate buffer solution (pH=7) and at different rpm confirmed that it is a 4e- pathway mechanism.
- MnFe2O4 displayed the highest ORR performance, and this can be pointed to (i) mesoporosity that provides more accessibility to active sites, (ii) highly-dispersed Mn and Fe species that work as the main active sites, and (iii) the large ferromagnetic particles.
3. Application in Microbial Fuel Cell (MFC)
- The maximum voltage output, power density, current density, chemical oxygen demand (COD) removal, and columbic efficiency of MnFe2O4/AC-based ACSCMFC fed-batch operation in UV-Visible light were 236 mV, 29 mW m-3, 300 mA m-3, 78.75±4.7 %, and 61.20 %, respectively. These results were lower than that in dark conditions operation; 171 mW m-3, 1277 mA m-3, 78.24±12.9 %, and 84.95 %, respectively. Thus, MnFe2O4/AC is suggested as a promising alternative to Pt- electrocatalyst cathode for MFCs at neutral conditions.
- On the other side; CuFe2O4-based ACSCMFC fed batch operation in UV-Visible light showed maximum voltage output (245 mV), power density (170.5 mW m-3), current density (1545.5 mA m-3), COD removal % (75.32±18.8), and columbic efficiency % (72.3), which is different than operation by CuFe2O4/CNTa; 270 mV, 209.95 mW m-3, 957.5 mA m-3, 81.2±9.37 %, and 58.16 %, respectively.
4. SEM Analysis of Electroactive Anodic Biofilm
- The SEM images determined the microbial attachment on the anode electrode surface and revealed that the formed anodic biofilm covered the surface and internal pores of carbon felt anode. Moreover, the electron micrographs revealed the bacteria appearance as rod-shaped individual cells in actual physical contact with the electrode.
- The formed biofilm (bacteria) is responsible for the electron transfer and current generation in MFC.
5. Microbial Community Analysis of Anodic biofilm
- The 16S rDNA gene clone libraries for anodic biofilm at the end of the MFC-fed batch operation are identified phylogenetically and grouped by phylum or class, using ABI3730xl DNA sequencer.
- The dominant strains are; Stenotrophomonas rhizophila, Brevibacillus parabrevis, Gelidibacter japonicas, and Bacillus flexus.
Conclusion
- MnFe2O4/AC and CuFe2O4 are suggested to be model alternatives for the commercial Pt/C catalyst. Finally, this research work has contributed to a simple, efficient, and low-cost synthesis technique for the development of naturally-abundant and efficient ORR composite electrocatalysts with improved electrocatalytic performance.
- MnFe2O4/AC and CuFe2O4 are considered low-cost Pt-alternative ORR cathode catalysts for MFC applications at pH=7. We suggest taking into consideration our research results, since it shed light on the design and synthesis of spinel-structure ferrite oxides and expand their applications in energy conversion technologies, including fuel cells, particularly, MFCs operated in neutral conditions at pH=7.