Mechanism Elucudation, System Manipulation And Functionization Of Iron-fed Air-cathode Fuel Cell

To deeply understand the iron-related systems，it is essential to elucidate the kineticsinvolved in the electro-oxidation of Fe(II). Therefore, in this thesis we examine theFe(II) oxidation process in an electrochemical system. The speciation of Fe(II) isincorporated into the model, and contributions of individual Fe(II) species to the overallFe(II) oxidation rate are quantitatively evaluated. The results show that the kineticmodel can accurately predict the electro-oxidation rate of Fe(II) in air-cathode fuel cells.FeCO3, Fe(OH)2, and Fe(CO23)2are the most important species determining theelectro-oxidation kinetics of Fe(II). The Fe(II) oxidation rate is primarily controlled bythe oxidation of FeCO3species at low pH,whereas at high pH Fe(OH)2and Fe(CO3)22are the dominant species. Solution pH, carbonate concentration, and solution salinity areable to influence the electro-oxidation kinetics of Fe(II) through changing bothdistribution and kinetic activity of Fe(II) species.The iron-fed fuel cell suffers from the problem of performance degradation whichsignificantly reduces its power output during long-term operation. In this work, theperformance degradation of iron-fed fuel cell is comprehensively evaluated with theobjective to elucidate the mechanisms involved in such a phenomenon. The ironcontaminant is present in the form of α-FeO(OH). Both the electrode and membrane aredeteriorated by iron contamination. The α-FeO(OH) contaminant not only forms foulinglayers on the surfaces of carbon electrode and membrane, but also migrates into themembrane to damage the membrane structure. To solve the performance problems， aseries of Fe(II)-fed fuel cells are operated with various chelating anions, includingphosphate and borate ligands. The average power densities of these fuel cells variedover a wide range from0.08±0.5to107.85±1.50mW·m2. Carbonate-amended fuelcells operated at pH6.0~8.0exhibited greater charge-recovery efficiencies than others,which ranged from93.5%to96.1%. The redox potential of an anodic solution andredox activity of Fe(II) were two important factors affecting the electro-oxidation ofFe(II) in fuel cells.This thesis aims to develop a novel fuel-cell-assisted chelated-iron process whichemploys an air-cathode fuel cell for the catalyst regeneration. By using such a process,sulfur and electricity are effectively recovered from H2S and the problem of chelatedegradation is well controlled. Experiment on a synthetic sulfide solution shows thefuel-cell-assisted chelated-iron process could maintain high sulfur recovery efficiencies generally above90.0%. The EDTA is preferable to NTA as the chelatingagent forelectricity generation, given the Coulombic efficiencies (CEs) of17.8±0.5%to75.1±0.5%for the EDTA-chelated process versus9.6±0.8%to51.1±2.7%for theNTA-chelated process in the pH rangeof5.0~9.0. The Fe(III)/S2ratio exhibits notableinfluence on the electricity generation, with the CEs improved by more than25%as theFe (III)/S2molar ratio increased from2.5:1to3.5:1. Application of this novel process intreating a H2S-containing biogas stream achieves99%of H2S removal efficiency,78%of sulfur recovery efficiency, and78.6%of energy recovery efficiency, suggesting thefuel-cell-assisted chelated-iron process is effective to remove the H2S from gas streamswith favorable sulfur and energy recovery efficiencies.