7. Electroactive and Electro Optical Polymers (Chapter 23)

Introduction Most classical organic polymers are electrical insulators. Polymers are useful because of their ease of fabrication, flexibility or strength, lightness of weight, and chemical inertness. Three different types of electrically conducting polymers are known – ion polymer solid electrolyte systems, composites of electronic conducting materials in nonconducting polymers, and polymers that conduct electricity by electronic transport.

1. Ionic Conduction in Solid Polymers 1.1 The Phenomenon A salt (such as sodium chloride) conducts electricity by the migration of Na+ and Cl- ions to opposite electrodes under an electrical potential. Solvation of water molecules In general, the greater the concentration of salt in the water, the higher is the electrical conductivity of the solution Some polymer can function as solid solvents for salts. Host polymer must be sufficiently flexible and provide enough “free volume” to allow ionic migration.

1.2 Uses for Solid Polymeric Electrolytes Various applications: electrochromic displays, solid-state photoelectrochromic cells, sensors, electrochemical transistors, fuel cells, batteries, and supercapacitors

1.3 Polymer Electrolyte Membranes Polymer electrolyte membrane (PEM) or proton exchange membrane A semipermeable membrane generally made from ionomers and designed to conduct protons while being impermeable to gases such as oxygen or hydrogen PEMs are primarily characterized by proton conductivity, permeability, and thermal stability.

1.3 Polymer Electrolyte Membranes Fuorinated polymer membranes - Perfluorinated membranes (ex, Nafion Aciplex, Dow membrane) - Perfluorinated composite membranes (Gore-Select, Nafion/SiO2) - Partially fluorinated membranes (BAM3G) Hydrocarbon polymer membranes - Sulfonated aromatic polymer membranes - Hydrocarbon composite membranes (ionomer/matrix) - Strong acid-doped polymer complexes - Sulfonated polymer/base complexes (sulfonated PBI/NaOH) - Acid-base polymer blends (Sulfonated PEEK/PBI)

1.3.1 Fuel cell components Proton exchange membrane fuel cell stack Membrane electrode assembly (MEA)

1.3.2 Nafion perfluorosulfonic acid PEMs Nafion is a sulfonated tetrafluorethylene copolymer Discovered by Walther Grot of DuPont in the late 1960s by modifying Teflon PEMs are primarily characterized by proton conductivity, permeability, and thermal stability. Nafion was the first synthetic polymer ever developed with ionic properties and it started an entirely new class of polymers called ionomers.

Polymer electrolyte membrane: Nafion Nanfion membrane Ionic channel Key issue is control of ionic channel for high proton conductivity and low methanol permeability -High ionic conductivity (0.1 S/cm) -Good physicochemical properties -Good mechanical properties -Electrochemical stability Polymer electrolyte membrane and electrochemical binder

Ionic Channels

1.3.3 Sulfonated poly(arylene ether)s Excellent thermal and mechanical properties Resistance to oxidation and acid catalyzed hydrolysis High proton conductivity (0.15 S/cm) High water uptake (> 70%)

1.3.4 PEMs for lithium ion battery

2. Electronically Conducting Polymers Electronic conduction pathway in solid state Migration of electrons to jump from chain to chain Poly(sulfur nitride), polyacetylene, poly(phenylene vinylene), polypyrrol, polythiophene, polyaniline Presence of arrays of “conjugated” delocalized double bonds

2.1 Band Theory Energy levels of two electrons before and after the formation of the bond Bonding level, occupied by the two electrons, an unoccupied antibonding levels Valence band is the highest range of electron energies in which electrons can move freely within the atomic lattice of the material. Conduction band is the range of electron energies enough to free an electron from binding with its atom to move freely within the atomic lattice of the material as a “delocalized electron”.

Insulators are materials that have band gaps that are so wide that electron jumps of this magnitude are virtually prohibited. Semiconductors have a narrower band gap. An electron can jump from the highest energy levels of the valence band to the lowest levels of the conduction band. Metals are materials with an unfilled conduction band and a partly filled valence band.

2.2 Conductivity and doping A continuous molecular orbital Separation into a sequence of alternating single and double bonds Reduction (doping) of polyacetylene to give a polaron, a bipolaron, and a solition pair.

2.3 Polyaniline Electrochromic Display

2.4 Conducting Polymer-based Energy Storage Flexible energy devices Conducting polymer-coating electrodes

2.5 Conducting Polymer-based Solar Cells Poly(3,4-ethylenedioxythiophene) Polystyrene sulfonate