Development of low temperature solid oxide fuel cell utilizing glycerol as a fuel

Abstract

The solid oxide fuel cell (SOFC) is a high efficiency energy conversion device with high efficiency and low pollution emissions. Among the various types of fuel cells, the SOFC combines the benefits of having all solid components and fuel flexibility. However, there are many problems to solve for commercialization. The major problem is related to the conventional yttria stabilized zirconia that should be used at high temperature of more than $800^\circ C$. High operating temperature results in serious problems such as long-term performance stability and material selection. In chapter 3, novel electrolyte which can operate at low temperature was investigated. $La_2Mo_2O_9$(LAMOX) and Pr, Ho and Dy substituted $La_2Mo_2O_9$ SOFC electrolyte materials were prepared by solid state processing method, and investigated using X-ray diffraction, DSC analysis, and electrochemical methods. XRD results confirmed that a single phase solid solution was formed for only Pr doped electrolyte materials. Among the substituted $La_2Mo_2O_9$ of 20 mol% Pr, Ho, Dy, the specimen containing Pr exhibit $\alpha - \beta$ phase transformation and high conductivity. The conductivity was evaluated using AC-impedance spectroscopy in temperature range between 623 and 973 K. The oxide ion conductivity was found to increase with increasing Pr concentration , and at $700^\circ C$, $La_{2-x}Pr_xMo_2O_{9-\delta }$ compositions with x values of 0.2 and 0.4 exhibited oxide-ion conductivities of 0.036 and 0.043 S/cm, respectively. Therefore Pr substituted $La_2Mo_2O_9$ would be a promising candidate as an electrolyte of intermediate-temperature solid oxide fuel cells. The solid oxide fuel cell is the ideal device for production of electrical power from natural gas and other hydrocarbons, as it operates at temperatures where methane and other hydrocarbons react readily with oxygen and steam. Carbon dioxide produced during reforming cause global warming. Bioderived glycerol will not contribute to the greenhouse effect. Carbon dioxide is produced by using glycerol as a fuel and is re-moved by the carbon biosphere circle. In chapter 4, in order to reduce the emission of $CO_2$ and air pollution, the technology to increase the efficiency of energy conversion and to develop the bio-derived fuel has been intensively investigated. In this study, this was demonstrated by using glycerol obtained as a by-product during biodiesel production, as a fuel in solid oxide fuel cell (SOFC). The performance of tubular SOFC Ni-Yttria stabilized zircornia (YSZ)/YSZ/-LSM cell was investigated at $650 - 800^\circ C$ by feeding glycerol to a Ni-8YSZ anode. The steam to glycerol mole ratio was varied from 3 to 5. It was found that there was no carbon depositions which was confirmed by Energy Dispersive X-ray spectroscopy(EDX) and the performance of the tubular SOFC with glycerol was close to 79 % that of SOFC with hydrogen. The maximum power density of $265 mW/cm^2$ was obtained with glycerol feed rate of 0.139 cc/min. In chapter 5, a 5 cm x 5 cm anode supported solid oxide fuel cell (SOFC) with a samarium doped ceria (SDC) electrolyte is fabricated for low temperature operation. Its electrochemical performance is extensively improved when a sufficient pore structure is sufficiently obtained in the anode. 30 vol.% poly methyl methacrylate (PMMA) is added as pore former in order to realize optimum anode micro-structure. A cermet of SDC and lanthanum strontium cobalt ferrite (LSCF) is used as the cathode layer. The thicknesses of electrolyte and cathode layer are $10 and 13 \mu m$, respectively. A single cell with $10-\mu m$ thick SDC electrolyte is tested between $500 and 600 ^\circ C$. The open circuit voltage (OCV) is 0.89 V at $500^\circ C$ with humidified hydrogen (3 % $H_2O$) and decreases to 0.83 V at $600^\circ C$ due to thermodynamic effects. A stability test is performed for 50 h. Voltage is maintained for the duration of the test. In chapter 6, glycerol obtained as a by-product during biodiesel production was used as a fuel in $5 x 5 cm^2$ solid oxide fuel cell (SOFC) based samarium doped ceria (SDC) operated at low temperature. The performance of $5 x 5 cm^2$ SOFC $Ni-SDC/SDC/La_{0.6}Sr_{0.4}Co_{0.2}Fe_{0.8}-SDC$ cell is investigated at $500 - 600^\circ C$ by feeding glycerol to a Ni-SDC anode. The steam to glycerol mole ratio is fixed at 2. It is found that the performance of the $5 x 5 cm^2$ SOFC with glycerol feed is close to 88 % that of SOFC with hydrogen with the current density of $450 mA cm^{-2}$. The maximum power density of $222 mW cm^{-2}$ is obtained with glycerol feed rate of $0.3 cc min^{-1}$ at $600^\circ C$