Fuel cells are electrochemical energy conversion devices which can continuously convert the chemically energy of a fuel and an oxidant into electrical energy and heat, without involving direct combustion as an intermediate step. Since fuel cells convert chemical energy directly into electricity, they offer many advantages over traditional power sources such as improved efficiency, greater fuel diversity, high scalability, and have a lower impact on the environment. Theoretically, fuel cells can produce electricity as long as fuel (hydrogen, ethanol, methanol, or gaseous fossils fuels like natural gas) and oxidant (oxygen or air) are fed, but its degradation on service limits the practical life of the devices.
Compared to other types of fuel cells, a solid oxide fuel cell (SOFC) is an all-solid-state fuel cell based on a solid oxide electrolyte, which potentially offers the highest energy efficiency with minimum emissions and hold promise for direct utilization of a wide variety of fuels, from hydrogen to natural gas, coal gas, reformed gasoline or diesel, and gasified carbonaceous solids (e.g. municipal solid waste and biomass). SOFCs are simple, reliable, environmentally benign and highly efficient (up to *85% energy efficiency when combined with gas turbine) compared to engines and modern thermal power plants (*30%).
Solid oxide fuel cells (SOFC) have been through a century of research that now become a promising power generation system with fairly high efficiency, less pollution, and fuel flexibility as compared with the traditional thermal power plants. The design of cell architecture and electrode microstructure may greatly influence the performance and reliability of SOFC systems. Recently, anode hollow fibre has received considerable attention for achieving a high cell performance. In particular, micro-tubular SOFCs have been under an active research in the past few decades due to their excellent performance.
The electrical property and/or electrochemical property of SOFC anode are normally governed not only by the electrical conductivity of the metalâ€" ceramic composite itself but also by the appropriate connection of the gas diffusion path, in order to reduce the ohmic and diffusional-polarization loss of the cell performance, respectively. It is well known that both electrical conductivity and gas permeability of SOFC anode are strongly related to the microstructural parameters such as particle size, composition and spatial distribution of each constituent phase. Thus, characterization of electrical and/or electrochemical properties of anode substrate and their dependence on the microstructural parameters is very important for the performance optimization of the fuel cells.
The performance and durability of SOFCs depend strongly on the microstructure and morphology of cell components. The performance of solid electrodes for solid oxide fuel cells is related to available surface area for reaction and the ability of reactants to reach the reaction area. The rate of the chemical reaction of the fuel and oxygen in the cell can be limited by many effects that show up as polarization of the cell. Polarization causes voltage drops within the cell that limit the output voltage of the fuel cell and consequently reduce the area-specific output power at a given current density. Source of polarization can include electrical resistance to current flow, resistance to ionic transport through the electrolyte, kinetics of the reaction at the interfaces, charge-transfer resistance, and polarization because of resistance to diffusion of the gases to the active three-phase boundary reaction regions.
The concentration polarization originated from the resistance to the diffusion of fuels and by products may turn to be the rate determining step in total cell reaction due to the long diffusion pathways or poor porosity. During operation of the SOFC, concentration polarization can be avoided by the fast diffusion of the fuel and the reaction products through the whole anode substrate. The electrical conductivity of the anodes should also be sufficiently high to transport the electrons produced by the oxidation reaction to the external current-collector to prevent electrical polarization. Moreover, high electrochemical activity and good compatibility with the other fuel cell components, especially the matching of its sintering shrinkage with that of the electrolyte, are necessary to produce high-quality anodes for SOFCs.
The most frequently used anode materials are Ni-YSZ composites, which display excellent catalytic properties for fuel oxidation and good current collection. Porous Ni/YSZ cermet is currently the most common anode material for SOFC applications because of its low cost. It is also chemically stable in reducing atmospheres at high temperatures and its thermal expansion coefficient is close to that of YSZ-electrolyte. Nickel serves as an excellent reforming catalyst for electrochemical oxidation of hydrogen. It also provides predominant electronic conductivity for anode. The YSZ constitutes a framework for the dispersion of Ni particles and acts as inhibitor for the coarsening of Ni powders during both consolidation and operation. Additionally, it offers a significant part of ionic contribution to the overall conductivity.
Many methods have been studied to optimize the electrochemical performance of SOFC anode. Adding a pore forming agent (pore former) is an effective method for forming porous structures with desired porosity and pore size distribution for gas diffusion through the anode. The pore former is a combustible additive, and burned away during sintering, thus leaving voids in the resulting ceramic body. In particular, different types of pore formers such as flour, rice, corn starch, wheat, graphite, carbon black, and spherical synthetic polymers (such as PMMA) have been used in the fabrication of the SOFC anode.
The microstructure of a sintered ceramic body is significantly influenced by the particle size, shape, and the quantity of the pore former. In particular, the presence of high porosity may degrade mechanical strength, elastic modulus, as well as ionic, electronic, and thermal-conductivity of ceramic bodies, due to the reduction of the contact area between the particles. In addition, while very large pore sizes would lead to reduce total length of TPBs available for the reaction, very fine pore sizes decrease the counter-current diffusion of fuel and water vapour, consequently limiting the overall reaction rate; both cases are evidenced by an increase in cell polarization that limits the output voltage and the cell power density at a given current density. For these reasons, achieving a balanced amount of the anode porosity, pore size distribution, and sintering shrinkage are required to successfully fabricate an anode supported SOFC with good cell performance.
In fact, not only the porosity but also the sintering shrinkage and coefficient thermal expansion (CTE) are of importance to the overall performance of anode-supported SOFCs. The sintering shrinkage and CTE of the anode should be close to that of the electrolyte film to decrease the stress between them. The immoderate stress caused by shrinking and CTE mismatch may produce cracks in the electrolyte film, distort the fuel cell and in turn diminish the cell performance
Objective of Study
This study will evaluate the microstructural characteristics and properties including porosity, pore size distribution, sintering shrinkage, mechanical strength, and electrical conductivity of the SOFC anode using various types of pore-formers in the fabrication of Ni/YSZ ceramic anode. The objective of this study is to achieve the optimum characteristic of anode hollow fibre that allows it to give highest efficiency in SOFC operation.
Scope of Study
A few parameters are considered to meet the objective of this study. Attention is paid on the effects of porosity when the porosity is uniformly distributed in the anode hollow fibre.
1.3.1 To select at least two type of material powders that potentially to be used as pore-former for anode hollow fibre
1.3.2 To prepare anode dope suspensions that contain different loading of pore-former
1.3.3 To fabricate anode hollow fibre via phase inversion-based extrusion process/sintering
1.3.4 To characterize the morphology, porosity and properties of anode hollow fibre with various structures.
