| dc.description.abstract | lectrochemical double layer capacitors (EDLCs) can store and deliver electrical
energy at high power density with long cycle life, and can absorb peak
charge/discharge loads. EDLCs can be used with batteries to insulate the latter
from the adverse e ects of shock loads. Other high technology applications also
harness this feature of EDLCs. EDLCs store energy in the form of oppositely
charged double layers on two electrodes. The electrodes are typically made of
porous carbon which o ers high speci c surface area ( 1500 m2/g) and high
capacitance at low cost. The high surface area is contributed by pores ranging
from micropores (< 2 nm) to macropores (> 50 nm). The charging of an EDLC
moves electrons from one electrode to other. This leads to buildup of double
layers on electrode-electrolyte interfaces in pores. Connecting a load across an
EDLC leads to
ow of electrons through the load, discharge of double layers,
movement of ions in pores and electrolyte, and decrease in capacitor's potential.
There are two main issues with EDLCs that need to be addressed to take
them to the next level in energy storage applications|low energy e ciency at
moderate to high currents and high self-discharge. The objective of the work
reported here is twofold: a) to obtain detailed and careful electrode speci c
experimental measurements of dynamic characteristics of inhouse EDLCs with
known structural details and physical properties, instead of the ones available
commercially, and b) use comprehensive transport model based investigations to
quantitatively understand factors that limit performance of EDLCs.
In the rst part of the work, we present a detailed analysis of charge-redistribution
(CR) in EDLCs using a macro-homogeneous transport model. CR manifests as
rapid decrease in voltage beyond IR drop after charging is stopped, and recovery
of voltage beyond IR recovery after discharging is stopped. The model considers
local electrolyte concentration dependent ionic conductivity, nite electronic
conductivity of porous solid matrix, constant capacitance of double layer, ionic
ux driven by potential and concentration gradients, di erent sizes of ions, and
Bruggeman correction for ionic conductivity in pores. The model predicts that
the depletion of ions in pores requires signi cant potential drop to move ions to
ii
build double layers. Its magnitude, predicted by the earlier models to reach an
asymptotic value with time, is found in the present model to increase with time
due to the depletion e ects.
The model predicts CR to occur at two time scales|rapid changes in cell
potential over a couple of seconds, driven purely by potential gradients across
pores, and slow changes over time scale of hundreds of seconds, driven by electrolyte
concentration gradients inside and outside the porous electrode. The
large di erences in CR at two electrodes on account of involvement of di erent
ions on them are highlighted. The model predicted two time scale relaxation
of cell potential, one after the other, follows linear scaling with log t, but with
di erent slopes. The magnitude of both the slopes is a ected by the extent
of retardation of movement of ions in pores. The model shows that holding a
rapidly charged EDLC for short intervals of potentiostatic charging alleviates
CR induced loss of charging capacity substantially.
An extensive experimental study is undertaken next to obtain discharge characteristics
of carbon-H2SO4 EDLCs at di erent electrolyte concentrations. The
low current measurements, in the absence of any transport limitations, show that
charging and discharging cannot be explained using a single set of potential dependent
capacitance values. With the focus of the thesis on discharge of EDLCs,
the discharge current is varied over a large range. The experiments show close to
100% charge utilization at low discharge currents, which decrease to as much as
50% at high currents. The highly non-linear decrease observed in cell potential
at high currents persists at the lowest currents used as well. These EDLCs are
subjected to cyclic-voltammetry and impedance spectroscopy for their complete
electrochemical characterization.
In order to unambiguously delineate the dynamic e ects during discharge, the
variation of potential di erence across each electrode at low currents is used to
recover voltage and electrolyte concentration dependent electrode capacitance.
The model predictions for time variation of cell and individual electrode potentials,
obtained using i) measured capacitance, (i) measured electrolyte conductivity
in bulk, and iii) Bruggeman correction for conductivity in pores, are in
perfect agreement with the measurements at low discharge currents. At high
discharge currents, the model substantially underpredicts the loss of charge utilization.
The measured cell potential drops to zero far more rapidly and the cell
iii
potential recovery in open circuit mode is far too strong. Since there is no tted
parameter in the model whose value can be altered, the model unambiguously
points to much lower ionic conductivity in charged pores than what is given by
the Bruggeman correction.
With incorporation of additional retardation of movement of ions through
correction factors for electrolyte conductivity in pores, the model quantitatively
explains a large body of experimental data. The model also predicts the measured
cyclic-voltammetry pro les well; the agreement between the two is excellent
at high scan rates. The study thus highlights the need to understand the
cause of unusually low ionic conductivities in charged porous electrodes.
Self-discharge (SD) is the loss of stored energy when a device is not in use. In
the second part of the thesis, we carried out an experimental and modelling study
to understand SD better. The experimental study is focused on investigation
of role played by SD in galvanostatic, potentiostatic, and open-circuit modes.
The open circuit potential of the capacitor used in the present work decreased
from 1.0 to 0.4 V in 31 h. The electrode speci c measurements highlight quite
di erent rates of self-discharge at the two electrodes. The measurements show
that a capacitor starting with the same open circuit potential, but with di erent
histories such as maintaining it under a potentiostat versus letting it reach the
same voltage as it self-discharges from a higher cell potential, lead to di erent
self-discharge rates. When a capacitor is maintained at a xed cell potential by
supplying it with the needed current, named
oat current, our measurements
show that potential di erences for the individual electrodes undergo slow internal
adjustment. The measurements carried out at low currents to charge and
discharge EDLC show that passing of a charging current through EDLC aggravates
SD, while passing of an external current to discharge EDLC diminishes
SD.
The measurements performed on electrodes with di erent amount of active
material on them and with di erent distance between the electrodes establish
that self-discharge neither occurs uniformly everywhere in the electrodes nor
does it occur by a mechanism controlled by projected area of the electrodes.
The widely used Conway's diagnotic tools also do not lead to identi cation of
any of the three widely held mechanisms for self-discharge.
The model developed to predict charge-redistribution in EDLCs at short
iv
time scales is extended to predict slow self-discharge using the measured values
of electrode potential dependent
oat currents|the rate at which charge must
be supplied to maintain cell potential constant|after four hours of maintaining
EDLCs at desired potential. The model on the other hand requires substantially
smaller values of
oat currents to match experimental measurements on
self-discharge during OCR. The additional measurements carried out over 50 h
of potentiostatic holding show that the magnitude of the current required to
keep cell potential constant keeps decreasing, while the individual electrode potentials
stop readjusting just after about 6 h. The model quantitatively explains
experimental observations on self-discharge under open circuit and galvanostatic
discharge conditions. It successfully predicts the experimentally observed internal
readjustment of electrode potentials on potentiostatic holding of our EDLC.
The model validates the need to address the e ect of history on self-discharge
rates for electrodes. | en_US |
