Reaction Mechanism And Catalyst Applications Of Direct Carbon Solid Oxide Fuel Cells

As an oxygen concentration cell, solid oxide fuel cell(SOFC) is a novel all-solid-state electric generation device which converts chemical energy of fuel directly into electric energy through conducting highly reactive oxygen ions with high energy efficiency and wide fuel feasibility. Direct carbon solid oxide fuel cell(DC-SOFC) is defined as a SOFC that directly operated on solid carbon as fuel, without any feeding gases. DC-SOFC not only includes the advantages of solid carbon, such as high energy density, rich sources and low cost, but also exhibits the superiorities of all-solid-state configuration of SOFC, which has no need for any high temperature liquid medium. This thesis focuses on the anode reaction mechanism and relative practical application of DC-SOFC, committing to providing technical support and theoretical basis for the practical application of DC-SOFC. Firstly, a preliminary study on the kinetics of DC-SOFC is carried out in order to provide the theoretical basis for the feasibility and self-sustainability of a “fixed bed” DC-SOFC. On the basis of this study, effects of types, loading amount and loading technique of Boudouard reaction catalyst on the performance and stability of DC-SOFC were explored. A biochar derived from orchid tree leaves is utilized as the fuel of a DC-SOFC, which has biologically accumulated calcium acting as catalyst for the Boudouard reaction, resulting in high cell performance. A SOFC system operating on waste plastics as fuel is developed, which reveals an ideal output performance, expanding the fuel feasibility range of SOFC.The kinetics of the DC-SOFCs is carried out through analyzing the correlations of the cell reaction rates to the emitting rates of CO and CO2. It turns out that higher operating current corresponds to higher rates of consuming and producing CO, through electrochemical oxidation at the anode and the Boudouard reaction at the carbon fuel, respectively. The rate of consuming CO can be maintained constant by controlling the operating current while the rate of producing CO decreases with time because of carbon consumption. When the CO producing rate becomes smaller than the CO consuming rate, the operation will be terminated. Compared to the rates of the chemical reactions, the diffusion rates of CO and CO2 are so fast that their impeding effect on the cell performance can be neglected.A wet agglomeration process is developed for preparing Fe-loaded carbon fuel for direct carbon solid oxide fuel cells(DC-SOFCs). This technique involves a simple mechanical mixing of carbon and Fe2O3 powder, along with a proper amount of polyvinylbutyral(PVB) ethanol solution as binder. Experimental results show that its output performance is comparable to that of the cell with carbon fuel prepared by the impregnation technique and is better than those of cells, respectively with pure carbon and carbon mechanically mixed with Fe oxide as the fuels. Ca-loaded carbon has the advantages of more low-cost, higher output performance and fuel utilization, as carbon fuel for DC-SOFCs.A Ca-loaded activated carbon is developed as fuel for direct carbon solid oxide fuel cells. It turns out that 5 wt % Ca-loaded activated carbon is the most favor carbon fuel for DC-SOFC, as the cell powered by 5 wt. % Ca-loaded activated carbon gives a higher performance than the best reported 5 wt. % Fe-loaded activated carbon. However, when the amount of Ca increases from 5 wt.% to 7 wt.%, the discharging time and fuel utilization of the cell reduce to 73%, with limited superior output performance, which might due to the agglomeration and caking of CaO. It is a promising catalyst to load on carbon fuels for DC-SOFC in good quality, with low cost and environmental friendliness.Biochar derived from orchid tree leaves is utilised as the fuel of a direct carbon solid oxide fuel cell(DC-SOFC). The performance of the DC-SOFC operated on the leaf char is higher than that operated on the best reported carbon fuel for DC-SOFCs, Fe-loaded activated carbon. XRD, Raman spectroscopy, SEM and EDX are applied to characterize the leaf char. It turns out that the leaf char is with porous structure and there is much Ca along with some K and Mg uniformly distributing in the leaf char. The present work suggests that plants, which can biologically accumulate some chemical elements(such as alkaline or alkaline earth metals, as well as transition metals) acting as active Boudouard reaction catalysts, may provide fuels for DC-SOFCs in large quantity and good quality, with low cost and environmental friendliness.A solid oxide fuel cell(SOFC) system fuelled by waste plastic is reported. Benefit from the widely alternative for fuel of SOFC, waste plastic is found to be an excellent fuel source for SOFC and exhibits a high output performance of 196 mW cm-2 at 800 oC. The performance of the SOFC operated with waste plastics and pyrolysis catalyst was compared with that free of pyrolysis catalyst and that using a hydrogen fuel. The microstructures of pyrolysis catalyst and pyrolysis product of waste plastic, as well as the impedance spectra and discharging performance of the corresponding SOFCs have been measured and analyzed in detail.