Efficient Electrochemical Energy Conversion of Carbon from Coal and Waste

Abstract

Direct carbon fuel cell directly converts the chemical energy stored in the fuel (carbon) to electricity. It is a high-temperature fuel cell having practical efficiency of 80%, normally operates at or above 700 oC with significantly low CO2 emission compared to coal burning power plants which release large amount of notorious gases NO2, SO2 and CO2. In this PhD research work main objectives are to synthesize combination of efficient electrolytes and electrodes materials not only operational compatible with carbon fuel, but also are electrochemical stable, having high conductivity and should provide excellent performance. Further, this thesis is divided into three parts; Electrolytes, Electrodes and theoretical calculation. Therefore, commonly used coprecipitation technique has been employed to synthesize various electrolytes, calcium doped ceria, single carbonate- doped ceria, binary carbonate-doped ceria, and ternary carbonate-doped ceria, barium co-doped ceria, calcium co-doped ceria, magnesium co-doped ceria and strontium co-doped ceria. However, in addition to electrolytes mainly two types of electrodes known as oxides LiNiCuZnO (LNCZO), LiNiCuZnFeO (LNCZFO) and perovskite LaSrNiTiO3- (LSNT, LaSrFeTiO3- (LSFT, LaSrCoTiO3- (LSCT and LaSrZnTiO3- (LSZT have been prepared using sol-gel technique. The prepared materials are characterized using various structural techniques; X-ray diffraction (XRD), Scanning electron microscopy, Thermal analysis, UV-Visible spectroscopy, Raman spectroscopy, Fourier transforms infrared spectroscopy, DC/AC conductivity and electrochemical performance. On the top of all characterization the XRD results reveal the prominent cubic structure of all the electrolytes and perovskite electrodes, whereas composite structure of LNCZO and LNCZFO is confirmed. Moreover, two types of carbon fuel categorized as coal based (lignite, bituminous, sub-bituminous) and waste biochar (walnut shells, almond shells) have been used to evaluate the overall electrochemical performance of direct carbon fuel cell (DCFC). Amongst all the discussed electrolytes (Li,Na)2CO3–doped ceria(LN-SDC) has shown the highest ionic conductivity of 0.31 Scm-1 with maximum performance of 617 mWcm-2 in combination of LNCZO electrode at 600 oC for hydrogen as fuel and air as oxidant. Secondly the combination of co-doped ceria electrolyte calcium co-doped ceria (CSDC) and LNCZFO electrode had depicted the performance of 630mWcm-2 at 650 oC with hydrogen fuel, where as co-doped ceria electrolyte (CSDC) has shown highest ionic conductivity of 0.124 Scm-1. Nevertheless in comparison above mentioned electrolytes LN-SDC with LNCZFO electrodes exhibited a performance of 58mWcm-2 for sub-bituminous fuel. Instead of obtained power densities of the cell comprised of cathode-electrolyteanode (LSCF|LN-SDC|LSFT) are 78,73,57,29 and 26 mWcm-2 at 700 oC with fuel as sub-bituminous, walnut shells, almond shells, bituminous and lignite respectively. The prepared LSFT and LSCT also have been tested as cathode which shows good performance with carbon fuel. Further to elaborate, theoretical calculations using Density Functional Theory (DFT) technique are performed to co-relate the effect of structure, dopant radius, lattice constant of doped system, density of states and band gap with the experimental results and at some point both DFT simulation and experimental results have shown the best match in terms of increase in lattice constant by decreasing band gap.