Electrochemistry and polymers
What is electrochemistry?
Electrochemistry is the study of electron movement, which forms the basis for electricity. While in organic synthesis, we use proton acceptors and donors to aid in the movement of electrons to form new bonds, electrochemistry utilises electric charge to move electrons.
Setting up an electrochemical cell
Electrochemical cells consist of a working, counter, and reference electrode and can be divided into two-, three- and four- electrode systems. Three-electrode system configurations are the most reported for conducting polymer synthesis and characterisation. The three-electrode system sets up the working and counter, facing each other to generate an electric field. The reference electrode should be situated close to the working electrode but not interfere with the electric field generated by the counter electrode.
The type of reference electrode used will depend on the type of system employed. In organic systems, a Ag/Ag+ from Ag/AgNO3 is used, while in aqueous systems, Ag/AgCl is used. All electrodes are immersed in an electrolyte solution such as NaCl (aqueous), tetrabutylammonium perchlorate or hexafluorophosphate (organics) to facilitate ion transfer between the working and counter electrodes.
In this reaction, molecules will collide with an electrode or conductive substrate. The result is either oxidation or reduction of the material at the electrode/electrolyte interface in the electrochemical cell.
how do we study CP electrochemistry?
In conducting polymer science, we use electrochemistry to study the oxidation and reduction processes that occur, as well as the means for charge storage. In a typical experiment, a film is prepared on a conductive surface. For example, the conducting polymer in solution is cast across an indium-tin oxide (ITO) coated glass surface. This film now serves as the working electrode.
Once a potential is applied to the film, many different processes occur. First, there is a transfer of charge from the electrode to the film. Second, there is a flux of ions that migrate towards the positive/negatively charged polymer. As the polymer is usually a porous structure, ions will also diffuse through the film. The flux and diffusion of ions into the CP matrix produces capacitance.
The most common form of electrochemistry that is used to assess CP behaviour is cyclic voltammetry. In this experiment, we will scan from low potentials to high potentials and record the electrical output in form of a current.
An important value to assess the performance of carbon-based electronics is the specific or volumetric capacitance. This value will indicate the amount of charge that can be stored. This can be determined from several CV experiments. To achieve this, several CV measurements at different scanning rates i.e. 10, 50 and 100 mV/s are required. Next, the difference between the anodic peak current and cathodic peak current is determined (Ipa – Ipc in mA). This value is then divided by the scan rate (in mV/s): Ipa – Ipc (mA)/scan rate (mV/s)
This calculation yields units of A⋅s/V = C/V = F
Where A is amps, s is seconds, C is coulombs, V is volts and F is farads.
If this experiment is conducted at 10, 50 and 100 mV/s, then there will be 3 values from the calculations. These values will be plotted against the scan rate to generate a linear plot. The slope of the curve is then extrapolated and is indicative of the total capacitance.
Following, the volumetric capacitance or specific capacitance (C*) is calculated by dividing the capacitance by the volume or mass, respectively.
Using EIS to study the very fast electrochemical processes?
Electrical impedance spectroscopy (EIS) scans from high to low frequencies and records the output of the electrochemical cell. In this experiment, we apply a small amount of AC current to ensure that the system obeys ohms law or a pseudolinear system, which is important to enable the extraction of information.
Frequency is represented in Hertz, named after a German scientist Heinrich Hertz, who proved that EMR existed in waves. Hertz is representative of the number of waves per second: Frequency=# of cycles/time (s)
Therefore, a larger frequency indicates a higher number of cycles or waves per second. This in turn relates to a higher energy. As high frequency waves of AC current are applied to the electrochemical cell, processes that occur in the nanosecond to millisecond time domain can be obtained. These include the electrolyte or bulk cell resistance, charge transfer resistance and the migration of ions towards the electrochemical interface. Analysis of the EIS can also allow the extrapolation of the electrical double layer capacitance.
As the frequency decreases, the time of each sinosoidal wave increases. At these frequencies, the electrochemical behaviour of the polymer film can be understood, including the diffusive behaviour. The volumetric or specific capacitance can also be calculated from the EIS plots at the lowest frequency, usually 0.1 Hz. We extrapolate the capacitance at 0.1 Hz because at this time point, the electrical double layer, and the diffusion of ions through the film will have been achieved.