Application of ionic conductors
In 1973, the oil price quadrupled and the world had a sudden awareness of
its petroleum dependence. Resources of fossil fuels are decreasing. Complexity
of power distribution is increasing. Mobile power system dependences are
increasing. Environmental pollution is increasing. Medical assistance is
increasing. All of the problems might be (at least partially) solved by using
Solid State Ionics (SSI) technologies. Their applications range from small
batteries for pacemaker implants, to high power batteries for energy storage
systems , electrochromic windows for energy conservation, sensors for
chemical pollutant detection , etc ...
Solid-state ionic devices can be used to harness chemical energy to produce
electricity in a fuel cell, convert one chemical species to another, separate
one chemical species from another, or detect chemical species by producing an
electric signal.
Gas sensors
Two potentiometric designs have evolved: surface-modified solid
electrolyte gas sensors and mixed potential gas sensors. In the former, the
surface of a solid electrolyte is coated with an auxiliary phase which will
react electrochemically and reversibly with the analyte and generate an
interfacial potential. Sensitivity and selectivity to the analyte are provided
by the auxiliary phase, e.g., the Na2CO3 /NASICON system can be used for
CO2 sensing This approach allows the use of several conventional ceramic
solid electrolytes, including YSZ, β-alumina, or NASICON to construct
sensors for many gases especially the environmental gaseous pollutants such
as CO2, CO, NOx , SOx , H2, Cl2, and NH3, etc. An important advantage of
this approach is the development of detection methods that survive harsh
conditions where typical liquid electrochemical sensors would be
inappropriate.
In a mixed potential sensor design more than one electrochemical reaction
takes place at the electrodes so that a mixed potential is established by
competing reactions. The catalytic activity of the electrode material is
particularly important, e.g., the Pt/YSZ/Au sensor can measure CO and
hydrocarbons due to the difference in catalytic activities between the Pt and
Au electrodes.
Oxygen Sensor
PO' 2
emf =
"
RT PO 2
ln '
4F PO
2
PO" 2
yttria stabilized zirconia (YSZ)
The cell operates at temperatures 500~1000 ℃, can be used to measure oxygen partial
pressure as low as 10-16 atm
Typical response of a commercial ZrO2oxygen
sensor to change on Air/Fuel of an engine
The main application of oxygen sensors in the
gasoline run automobiles is to control the air-to-fuel
ratio (λ). The oxygen partial pressure changes
abruptly in the vicinity of the stoichiometric
mixture of the air and the fuel (λ = 1: air = 14.5 kg;
fuel = 1 kg). The figure shows the characteristic
sensor output, the λ-curve, for a range of air-fuel
mixture in the combustion engine. The emissions of
toxic gases such as CO, NOx , and hydrocarbons
(HC) depend on the λ point at which the engine
functions. In the fuel rich region, the emission of
CO and HC dominates and in the lean region, NOx
emission is larger. These exhaust gases are
converted to non-toxic gases CO2, N2, and H2O by
a three way catalytic converter located in the
exhaust system. The O2 sensor output is fed back to
the engine control so that the engine operates
between the rich and lean fuel conditions centered
around the stoichiometric ratio.
HCs Sensor
Pt/YSZ/Nb2O5 sensor, using Nb2O5 and Pt electrodes on tape-cast YSZ. The EMF
response of this sensor to different concentrations of propylene (HCs)in air, in the
temperature range 500-700°C.
Sensor SO2
Pt mesh
T=790oC
Ag
V1
emf (mV)
V1
Pt catalyst
air+SO2
Ag
V2
Pt
3.63%Y2(SO4)3 in pure Ag2SO4
Ag
Al2O3
K2SO4 K2SO4+1%Ag2SO4
PSO2=1000ppm
Graphite embedding
Ag+Ag2SO4 reference electrode
V2
PSO2=100ppm
V1
V1
PSO2=10ppm
days
Schematic diagram of a K2SO4-electrolyte sensor and its
observed emf variation in time
Variation of emf with time for swapping of SO2
concentration from 20to 1000pp
oxygen generators
Ceramic oxygen generators (COG) are
receiving the greatest attention for medical
and aerospace applications because they
can readily produce a pure oxygen gas
stream from ambient air. Compact COGs
are being developed to provide a
continuous supply of oxygen-enriched air
for people with breathing disorders.
Similarly, these devices can enrich the
breathing oxygen concentration for high
altitude aircrafts. More recently this
technology has sparked the interest of NASA
for space exploration. Because of the
distance involved, if we are to travel to
other planets in the future, we need to
utilize available planetary resources. The
technology envisioned to make this possible
is based on a COG converting CO2 to O2 and
CO. Experimental results demonstrating the
efficacy of this technology are shown by the
Faradaic oxygen production from CO2/CO
gas mixtures .
Faradaic production of O2 from CO/CO2 gas mixture in a ceramic
oxygen generator.
Catalyst
Fuel cell- proton conductor
Proton exchange
membrane (PEM) fuel cell
The proton exchange
membrane is one of the
most advanced fuel cell
designs. Hydrogen gas
under pressure is forced
through a catalyst,
typically made of platinum,
on the anode (negative)
side of the fuel cell. At this
catalyst, electrons are
stripped from the
hydrogen atoms and
carried by an external
electric circuit to the
cathode (positive) side.
The positively charged hydrogen ions (protons) then pass through the proton exchange
membrane to the catalyst on the cathode side, where they react with oxygen and the electrons
from the electric circuit to form water vapour (H2O) and heat. The electric circuit is used to do
work, such as power a motor.
Fuel cell- oxygen conductor
A solid oxide fuel cell is made up of four
layers, three of which are ceramics (hence the
name). A single cell consisting of these four
layers stacked together is typically only a few
millimeters thick. Hundreds of these cells are
then connected in series to form what most
people refer to as an "SOFC stack". The
ceramics used in SOFCs do not become
electrically and ionically active until they reach
very high temperature and as a consequence the
stacks have to run at temperatures ranging from
500 to 1,000 °C. Reduction of oxygen into
oxygen ions occurs at the cathode. These ions
can then diffuse through the solid oxide
electrolyte to the anode where they can
electrochemically oxidize the fuel. In this
reaction, a water byproduct is given off as well
as two electrons. These electrons then flow
through an external circuit where they can do
work. The cycle then repeats as those electrons
enter the cathode material again.
Electrochromic
smart car window
Electrochromic Properties of Sol-gel Coating
of Nb2O5 and Nb2O5:Li+
It can be observed that, close to -1.0 V,
there is an increase in the cathodic
current associated with Nb2O5
reduction with simultaneous Li+ cation
insertions. During this process it is
observed a change in the optical
properties of the films, from
transparent to gray color.
Variable resistor devices
As an active material, inserted tungsten trioxide, MxWO3, is often used in
switching applications between very high and very low resistance: low
inserted tungsten trioxide is a dielectric while high inserted is a metallic
conductor . A more linear dependence between resistivity and insertion was
observed in mixed conductors based on some silver salts. They have been used
in the design of controllable resistors. Figure shown above gives an example
of such an electrochemical device.
Medical Applications of Solid State Ionics
Biomedical applications of solid state power sources; biofuel cells; and iontophoretic*
and related devices used for controlled transdermal** drug delivery and monitoring of
physiological parameters.
•Iontophoresis is a technique using a small electric charge to deliver a medicine or other chemical through the skin.
**Transdermal is a route of administration wherein active ingredients are delivered across the skin for systemic distribution.
Examples include Transdermal patches used for medicine delivery, and Transdermal implants used for medical or aesthetic purposes.
Biofuel cell
Penny Sized Biofuel Cell