Investigation Of Nickel-based Anode Solid Oxide Fuel Cells

Solid oxide fuel cell (SOFC) is an energy conversion device with high conversionefficiency, low pollution emission and practical fuel flexibility, which make it the mostoutstanding in the fuel cell family, so it is significant to accelerate its commercial application.Mass production of SOFCs is one important way to achieve the goal of SOFCcommercialization. The conventional Ni-based anode is the most commonly used anodematerial of SOFCs, which shows good performance. However, the problem that Ni-basedanode becomes deactivated in the use of fossil fuel due to carbon deposition has been alwayshindering the development of SOFCs in practical use. To solve this problem, a clearknowledge on the conditions under which carbon deposition occurs and the mechanisms ofthe deactivation is necessary, so that proper method can be applied to improve the Ni-basedanode for SOFCs operating on carbon-containing fuels with high performance, which is ofgreat theoretical significance in application of SOFCs. This thesis focuses on the topic ofconventional Ni-based anode (Ni-YSZ) and the researches are carried out for the keyquestions about the fabrication technique of SOFC industrialization and the mechanismanalysis of deactivation of Ni-based anode in SOFCs operating on carbon-containing fuels,aiming to accelerate the SOFCs commercialization.To achieve the goal of SOFC industrialization, a low pressure injection molding (LPIM)technique is successfully developed to fabricate porous NiO-YSZ anode substrates forcone-shaped tubular anode-supported SOFCs. It is fast, low-cost, and especially suitable formass production of small and complex-shaped ceramic components with high accuracy in sizeand low deformation in shape. The critical technological parameters of LPIM technique areinvestigated in detail and several single cells and a two-cell-stack are assembled and theirelectrochemical performances and microstructures are characterized. The results show thatwith15wt.%paraffin as plasticizer, porous anode substrates can be obtained, and largerporosity can be obtained and controlled by adding proper amount of graphite as pore formerin preparation. At800oC, with moist hydrogen as fuel, the cell with the anode substratefabricated with5wt.%pore former shows a maximum power density of531mW cm-2, whilethe cell without any pore former,491mW cm-2. A two-cell-stack, fabricated without graphite,gives an open circuit voltage of1.75V and a maximum output of5.32W with a powerdensity of412mW cm-2, at800oC.For the purpose of simple fabrication technique for fundamental research of SOFCs,tape-casting technique is developed to fabricate Ni-based anode supported SOFCs by two different sintering techniques (pre-calcining and co-sintering), and their electrochemicalperformances are characterized using moist hydrogen and methane as fuels. The results revealthat with hydrogen as fuel, the cell after pre-calcining shows maximum power densities of569mW cm-2and786mW cm-2at750oC and800oC, respectively and gives an open circuitvoltage of1.05V while the cell after co-sintering gives an OCV of1.02V which is a littlelower that that of pre-calcining and shows MPD of324mW cm-2and497mW cm-2at750oCand800oC, respectively, which is lower than that of pre-calcining, indicating co-sinteringtechnique needs to be improved. With methane as fuel, the cell after pre-calcining showsMPD of333mW cm-2,559mW cm-2and827mW cm-2at700oC,750oC and800oC,respectively, which is a bit lower that the performances in hydrogen, and I-V curve showssome activation polarization in low current density region. The cell degrades after open circuittesting for2.5h, the impedance spectra show that the ohmic resistance remains the same afterlifetime testing, the main reason for the cell degradation is the increase of polarizationresistance of electrode, which is cause by carbon deposition on the anode.On the basis of above work, we have studied the effect of carbon fiber growth on thedeactivation of nickel-based anode for SOFCs operated on methane. The results show thatlarge amount of fibers with diameter of30-60nm have grown on Ni-YSZ anode from moistCH4fuel at700oC, the EDX measurement confirms that these fibers are carbon fibers.Carbon fiber growth will cause inner stress to the Ni-YSZ anode, then destroy the anodestructures and lead to fracture of the whole anode supported SOFC. Meanwhile, those cellsare also found broken even if there is no obvious carbon fiber. The possible reason is that athigh temperatures, the carbon dissolves and diffuses fast in the Ni crystal (face-centered cubicstructure), then the carbon atoms occupy the interval sites of the FCC crystal cells of themetal, casusing the swelling of the crystal or bulk volume, which causes inner stress of theanode so severe as to result in the crack of the cell. So some methods could be proposed toinhibit and avoid the Ni-based anode degradation due to the carbon fiber growth. Oneapproach is to add some oxide into the Ni-YSZ anode, like MgO or Al2O3, and anotherapproach is to alloy some metal of poor carbon dissolvent with nickel, lower the solubilityand diffusion of carbon in the Ni crystal.Under the inspiration of the above work, carbon deposition of CH4and CO on Ni-YSZ issystematically studied at SOFC operating conditions. The results are compared and analyzedbased on thermodynamic equilibrium calculation, chemical kinetics and experimentalmeasurements. The results show that at lower temperatures, the rate of deposition reaction iscontrolled by kinetics and at high temperatures it is limited by thermodynamics. There are strict correlations between each other of the carbon deposition rate, deformation of thesintered anode, and the microstructure morphology, i.e., larger amount a carbon causes moreserious deformation and more disintegrated microstructure. The principle cause ofdeactivation or damage of Ni-based anode exposed to carbon-containing fuels is the highsolubility of C in Ni and strong C-Ni interaction, which allow for carbon diffusing through thebulk Ni and depositing on the Ni surface in the form of graphite. Carbon formation on theNi-YSZ anode can be avoided by oxygen ion flux through the electrolyte but the mechanismis still not clear, so much research work still needs to continue to be carried out.Finally, to improve the electrical conversion efficiency of direct carbon solid oxide fuelcells (DC-SOFC), a two-step electrochemical oxidation method has been proposed anddeveloped. The results show that with the increase of discharging current, lifetime of the celldecreases and the fuel conversion efficiency reduces while the flow rate of outlet gasincreases. However, there’s still some difference between the experiment value measured andthe theoretical value, which is can be enhanced by improving the sealing property of the outletgas device. With the operation of the cell discharging, the flow rate of outlet gas decreases.The composition analysis of outlet gas indicates the ratio of CO reduces fast with time, whichis caused by insufficient carbon fuel feed. How to optimize the energy conversion efficiencyand the outlet gas production rate of the first cell and the power efficiency of the second cellto make total efficiency the highest, is the key to the future research.