Polymer electrolytes and fuel cells

POLYMER ELECTROLYTES AND FUEL
CELLS
APPLIED ASPECTS OF BIOTECHNOLOGY
SHARAVANAKKUMAR SK
III B.Sc. BIOTECHNOLOGY
PSG COLLEGE OF ARTS AND SCIENCE

Electrolytic Cell
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An electrolytic cell is an electrochemical cell that drives a non-
spontaneous redox reaction through the application of electrical
energy. They are often used to decompose chemical compounds, in a
process called electrolysis.
An electrolytic cell has three components,
Electrolyte
Cathode(Positive electrode)
Anode(Negative electrode)
Figure: A typical electrolytic cell

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Electrolyte:
An electrolyte is a substance that produces an electrically
conducting solution when dissolved in a polar solvent, such
as water. The dissolved electrolyte separates into cations and
anions, which disperse uniformly through the solvent.
Electrically, such a solution is neutral.
Electrodes:
An electrode is a solid electric conductor that carries electric
current into non-metallic solids, or liquids, or gases, or
plasmas, or vacuums. Electrodes are typically good electric
conductors, but they need not be metals.
Types:
1.Classical liquid electrolytes
2.Gel electrolytes
3.Dry polymer electrolytes
4.Dry single-ion-conducting polymer electrolytes
5.Solvated single-ion-conducting polymer electrolytes

Polymer Electrolyte
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Polymer electrolytes are very similar to the non-aqueous
electrolytes in general chemical makeup; however, instead of
using a liquid the lithium salts are dissolved into a polymer
gel matrix.
A polymer electrolyte is also referred to as a solid solvent that
possesses ion transport properties similar to that of the
common liquid ionic solution.
It usually comprises a polymer matrix and electrolyte,
wherein the electrolyte such as a lithium salt dissolves in a
polymer matrix.
polymer electrolytes are mainly used in electrochemical
devices such as batteries, electro-chromic devices, solar cells,
and super capacitors.

Properties
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Polymer electrolytes are potential materials for solving the
never-ending demand for high energy density in energy devices.
Polymer electrolytes are defined as linear macromolecular
chains bearing a large number of charged or chargeable groups
when dissolved in a suitable solvent.
Polymer molecules that have one or a few ionic groups, in most
cases terminal and anionic, are called macroions.
These are primarily living polymers wherein polymer molecules
that are present in a polymerizing reaction system grow as long
as monomers (e.g., esters or nitriles of metha-crylic acid) are
continuously supplied.
The ionic charge of the macroions gets transferred to the next
monomer added, and this process continues by keeping the
macroions charged for the addition of further monomers.

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Polymers having a considerable number of ionic groups and a
relatively nonpolar backbone are known as ionomers.
Those polymers with a number of ionic groups and that can
dissolve in water are known as polyelectrolytes.
Polymers with a much higher number of ionic groups get cross-
linked or undergo three-dimensional polymerization, which
constitutes the technically important group of ion exchangers.
To act as a successful polymer host, a polymer or the active part of
a copolymer should generally have a minimum of three essential
characteristics:
1.Atoms or groups of atoms with sufficient electron-donating power
to form coordinate bonds with cations.
2.Low barriers to bond rotation so that the segmental motion of the
polymer chain can take place readily.
3.A suitable distance between coordinating centers because the
formation of multiple intra-polymer ion bonds appears to be
important.

Bio-Polymer Electrolytes
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The development of new materials that can be applied as solid
electrolytes has led to the creation of modern systems of energy
generation and storage.
Among different poly (ethylene oxide)-based electrolytes,
natural polymers, such as hydroxyethylcellulose,
hydroxypropylcellulose or carboxymethylcellulose
(polysaccharides), starch, polyvinyl alcohol, polyethylene glycol,
and chitosan or proteins such as gelatin are considered polymer
electrolytes.
Those polymers can undergo biodegradation as well as show the
ionic conductivity improvement by inorganic dopants, acids,
and gelation; these polymer electrolytes are called biopolymer
electrolytes.
Some natural biopolymer electrolytes that are widely used in
research are chitosan, starch, and cellulose materials.
The existence of polar groups in biopolymers is necessary to
dissolve salts and form stable ion-biopolymer complexes.

Types
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Classification based on the sources and origins:
1.Synthetic biopolymer electrolytes based on bio-derived
monomers
e.g. Poly-(lactic acid)
2.Natural biopolymer electrolytes
e.g. Polysaccharides, Proteins.
Biopolymers from microbial fermentation
e.g. Polyhydroxybutyate, Hyaluronic acid, poly-γ-glutamic acid
Classification based on their physical state and composition:
1. Gel biopolymer electrolytes
2. Hydrogel biopolymer electrolytes
3. Solid biopolymer electrolytes
4. Blend biopolymer electrolytes
5. Composite polymer electrolytes

Dopants
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Dopants are materials added with the biopolymers to increase
the conductivity of electrons in the electrolyte.
Lithium Salts as Dopants in Biopolymer:
Lithium salts have to completely dissolve in the applied
solvent at the optimized concentration and ions should be
able to transfer through the biopolymer matrix.
Lithium ions should not undergo oxidative decomposition at
the cathode.
Lithium ions must be inert to the electrolyte solvent/
biopolymer.
Both anion and cation should be inert toward other cell
components.
The anion must be nontoxic and remain thermally stable at
the energy device’s working conditions.
e.g. LiClO4, LiPF6, LiBF4, LiAsF6, LiCF3SO3, LiN(CF3O2)2
LiC(CF3SO2)2.

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Acids as Dopants in Biopolymer Electrolytes:
The advantage of proton-conducting biopolymer gel systems
consisting of H2SO4, o-H3PO4, and HCl in comparison with
lithium salt-doped polymer electrolytes arises from their
potentially higher conductivity and smaller ionic radius.
Furthermore, with biopolymer complexes with low inorganic
salt concentrations, ion pairing is possible, while at high
concentration the formation of large ionic aggregation may
occur.
Thus, lithium salts containing polymer electrolytes lack
higher power density and mainly depend upon electron-rich
functional groups of polymers for higher conductivity.
e.g. H3PO4 is a proton conductor in poly(ethylene amine),
poly(vinyl alcohol), poly(silamine)chitosan or iota-
carrageenan.

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Alkaline Dopants in Polymer Electrolytes:
Alkaline dopants are generally the salts containing OH–ions, the
involvement of OH ions in the conduction via hyper-coordinated
or per-solvated complexes.
e.g. KOH, Tetra-alkyl-ammonium hydroxide (TAAOH)
Plasticizing Salts or Ionic Liquids:
Ionic liquids are low-temperature molten salts. They consist of a
large cation and a charge-delocalized anion. These ionic liquids
are similar to organic electrolytes and can have vapor pressure up
to 250–450°C. The presence of organic ions provides unlimited
structural variations.12Biopolymer Electrolytes
Plasticizers are used to reduce the crystalline phase of polymer
and increase the ambient temperature ionic conductivity in
polymer-salt complexes.
e.g. Imidazolium, Pyrrolidinium, Quaternary ammonium salts,
Bis(trifluoromethanesulphonyl)imide, Bis(fluoro-sulphonyl)imide,
and Hexafluorophosphate.

SOLID BIOPOLYMER ELECTROLYTES (SBPE)
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Solid biopolymer electrolytes are a liquid-free, high molecular
weight, polar polymer host having an ionically conducting
phase formed by dissolving salts.
Many polymer electrolyte materials will exhibit to a greater or
lesser extent the following properties:
Adequate conductivity for practical purposes
Good mechanical properties
Chemical, electrochemical, and photochemical stability
No possibility of leakage
Ease of processing
Shape flexibility
Lowering the cell weight-nonvolatile, all-solid-state cells do
not need a heavy steel casing

BLEND BIOPOLYMER ELECTROLYTES (BBPE)
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BBPEs are a physical mixture of two or more polymer chains
forming a homogeneous (liquid) solvent-free system.
Sometimes, though the various phases are chemically bonded
together wherein the ionically conducting phase is formed by
dissolving inorganic salts.
Hydrogen bonding, charge transfer interactions, and dipole-
dipole forces form the basis for the miscibility of polymer blends.
The manifestation of properties of polymer blends depends upon
the miscibility of the components and structure.

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 Based on the fact that the polymers are compatible or miscible
they are classified:
1.Compatible blend: A mixture of polymer in which the overall
property is enhanced when compared with its non-blend
components. Herein, there are only physical forces of attraction
that keep the polymers in single phase.
2.Incompatible blend: A mixture of polymers in which the overall
property is less when compared with its non-blend components.
3.Polymer alloys: Compatibilizers are used to improve the
property balance these blends are commercial polymer blends
having interface.
4.Miscible blend: A mixture of polymers in which a homogenous
phase exists at a microscopic level due to physical forces and
hydrogen bonding.
Preparation of Polymer Blends:
1.Solution mixing-laboratory or paint industry, but the problem
is removing a solvent.
2.Interpenetrating networks-cross linked materials.
3.Melt mixing (physical, reactive)-most important method in

Gel Biopolymer electrolytes (GBPE)
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GBPE is often known as a plasticized polymer electrolyte that
is neither liquid nor solid, or conversely both liquid and solid.
Gel contains a solid skeleton of polymers or long-chain
molecules cross-linked intra-molecularly or inter-molecularly,
entrapping an uninterrupted liquid phase.
The chemical composition and other factors such as hydrogen
bonding vary the chemistry of gels from viscous fluid to
moderately rigid solids.
Nevertheless, they are very soft and stretchy or “jelly like.”
Due to a large number of liquids filled in microspores most
gel materials exhibit liquid like characteristics microscopically,
but macroscopically have solid character.
The presence of the ultra-porous structure in the gel system
is likely to provide channels for ion migration.

Polymer electrolyte membrane fuel cells
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Polymer electrolyte fuel cells are electrochemical devices,
converting the chemical energy of fuel directly into electrical
energy.
The working principle of PEFCs is based on the anode-
oxidation of hydrogen (fuel) to protons:
H2 → 2H+ + 2e−
The reduction of oxygen to water at the cathode:
4H+ + O2 + 4e− → 2H2O
Based on the thermodynamic data of the reactions, the
theoretical cell voltage is calculated via:
U= −ΔGnF
With the Gibbs free energy of the electrochemical reactions
Δ G, the number of the electrons n and the Faraday constant
F. At 25°C, the theoretical hydrogen/oxygen fuel cell voltage
is 1.23 V.

Components
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PEMFCs are built out of membrane electrode assemblies
(MEA) which include the electrodes, electrolyte, catalyst, and
gas diffusion layers.
An ink of catalyst, carbon, and electrode are sprayed or
painted onto the solid electrolyte and carbon paper is hot
pressed on either side to protect the inside of the cell and
also act as electrodes.
The pivotal part of the cell is the triple phase boundary (TPB)
where the electrolyte, catalyst, and reactants mix and thus
where the cell reactions actually occur.
Importantly, the membrane must not be electrically
conductive so the half reactions do not mix.
Operating temperatures above 100 °C are desired so the
water byproduct becomes steam and water management
becomes less critical in cell design.

Electrolyte Membrane
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To function, the membrane must conduct hydrogen ions
(protons) but not electrons as this would in effect "short circuit"
the fuel cell.
The membrane must also not allow either gas to pass to the
other side of the cell, a problem known as gas crossover.
Finally, the membrane must be resistant to the reducing
environment at the cathode as well as the harsh oxidative
environment at the anode.
Splitting of the hydrogen molecule is relatively easy by using a
platinum catalyst. However, splitting the oxygen molecule is
more difficult, and this causes significant electric losses.
An appropriate catalyst material for this process has not been
discovered, and platinum is the best option.
A cheaper alternative to platinum is Cerium(IV) oxide catalyst
used by the research group of professor Vladimir Matolín in the
development of PEMFC.

Figure: Components of PEMFCs Figure: SEM micrograph of
PEMFCs

Electrodes
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An electrode typically consists of carbon support, Pt particles,
Nafion ionomer, and/or Teflon binder.
The carbon support functions as an electrical conductor; the
Pt particles are reaction sites; the ionomer provides paths for
proton conduction.
Teflon binder increases the hydrophobicity of the electrode
to minimize potential flooding.
Gas diffusion layer
The GDL electrically connects the catalyst and current
collector. It must be porous, electrically conductive, and thin.
The reactants must be able to reach the catalyst, but
conductivity and porosity can act as opposing forces.
Optimally, the GDL should be composed of about one third
Nafion or 15% PTFE.
The carbon particles used in the GDL can be larger than
those employed in the catalyst because surface area is not
the most important variable in this layer.
GDL should be around 15–35 µm thick to balance needed
porosity with mechanical strength.

Efficiency
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The maximal theoretical efficiency applying the Gibbs free
energy equation ΔG = −237.13 kJ/mol and using the heating
value of Hydrogen (ΔH = −285.84 kJ/mol) is 83% at 298 K.
η = Δ G /Δ H = 1 − [T Δ S /Δ H]
The practical efficiency of a PEMs is in the range of 50–60% .
Main factors that create losses are:
Activation losses
Ohmic losses
Mass transport losses

Applications
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PEM fuel cells focuses on transportation primarily because of
their potential impact on the environment
e.g. the control of emission of the green house gases (GHG)
Distributed or stationary and portable power generation.
Most major motor companies work solely on PEM fuel cells
due to their high power density and excellent dynamic
characteristics as compared with other types of fuel cells.
Light weight and PEMFCs for buses, which use compressed
hydrogen for fuel, can operate at up to 40% efficiency.
PEM fuel cells were used in the NASA Gemini series of
spacecraft, but they were replaced by Alkaline fuel cells in the
Apollo program and in the Space shuttle.
Toyota Mirai, a car model which uses the PEMFCs are
produced and yet to be launched in the market.

References
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Tokiwa Y, Suzuki T, Tokiwa Y, Suzuki T. Hydrolysis of polyesters by lipase.
Nature1977;270:76–8.
Gross RA, Kumar KB. Polymer synthesis by in vitro enzyme catalysis. Chem-
Rev2001;101:2097–124.
Mao H, Reamers JN, Jhong Q, von Sacken U. Proceedings of the
symposium on rechargeable lithium and lithium ion batteries, 94, 245, the
electrochemical society proceeding series, Pennington, NJ; 1995.
Uma T, Mahalingam T, Stimming U. Conductivity studies on poly(methyl
methacrylate)-Li2SO4polymer electrolyte systems. Mater Chem-Phys 2005;
90:245–9.
Stephan AM, Saito Y, Manuel Stephan A, Saito Y. Ionic conductivity and
diffusion coefficient studies of PVdF–HFP polymer electrolytes prepared
using phase inversion technique. Solid-State Ionics 2002;148:475–81.
Loyselle, Patricia; Prokopius, Kevin. "Teledyne Energy Systems, Inc., Proton
Exchange Member (PEM) Fuel Cell Engineering Model Power-plant. Test
Report: Initial Benchmark Tests in the Original Orientation". NASA. Glenn
Research Center. hdl:2060/20110014968.

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