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.