1.0.1 Electrochemistry
1.0.1.1 Electrochemical Systems
Electrochemistry is a branch of chemistry that studies the movement of electrons as a result chemical changes. Electrochemical cells facilitate chemical reactions, and harness the flow of electrons as electricity. Batteries are the most common electrochemical devices allowing us to effectively store electricity for our phones, cars, and even the electricity grid. Electrochemistry will play a critical role in buffering variable renewable energy sources such as wind and solar and de-carbonizing fossil fuel reliant reliant industries such as passenger vehicles.
1.0.1.2 The Cell
A cell is a unit that enables the chemical reactions that deliver electricity. The cell is composed of two dissimilar chemical materials referred to as the cathode and the anode, these electrodes are contained in a liquid electrolyte. Cells are classified as either Galvanic or Electrolytic based on whether the redox reaction is spontaneous or non-spontaneous, respectively. As illustrated in diagram 1, when a galvanic cell is connected to a load, the anode oxidizes spontaneously causing electrons and positive ions to move towards the cathode to reduce. An example of a galvanic cell would be a discharging battery or a fuel cell in operation.
The driving force of this reaction is dictated by the composition of the electrodes and resulting cell potential. Cell potential is the difference in potential energy of the electrodes. Electrons want to move from high to low potential, so if the anode has a higher potential than the cathode a spontaneous reaction will occur. Cell potential can be calculated using the Nernst equation which factors in the standard potential difference between anode and cathode (E0), along with the influence of non-standard conditions such as concentration (Q), temperature (T), and number of electrons transferred during the reaction (n).
Electrolytic cells do not undergo spontaneous reaction and thus require an external energy source to drive diffusion. Examples of electrolytic cells are, charging a battery by oxidizing the cathode, or an electrolyzer splitting Hydrogen and oxygen molecules from water. This is fundamental to electrochemical energy storage.
1.0.1.3 The Electrode Interface
Positive ions travel back and forth through the electrolyte during charging and discharging to bond with electrons at the interface of the electrodes. The Helmholtz or electrical double layer is a region of oppositely charged ions that are held close to an electrode to keep a neutral charge balance, much like two plates in a capacitor. Beyond the double layer is the diffusion layer, this is a concentration gradient of ions, and molecules that builds as ions are either trying to reduce or oxidize at the electrode. At he boundary of the diffusion layer we have the electrolyte solution at its standard concentration. As the distance increases from the electrode the potential decreases.
Polarization refers to the changing of the electrodes potential as it oxidizes or reduces due to charge transfer, ohmic losses, and the reaction activation. These phenomena facilitate charge transfer in the electrodes during cell operation and the polarization is the resulting in the cell’s potential energy. The plot below is an example of a polarization curve for a redox flow battery, this showcases the potential dropping as charge is transferred during anode oxidization.
1.0.1.4 Summary and Examples
We have discussed what makes up an electrochemical cell and how it operates, however, the most defining feature of an electrochemical system are its active materials. Active materials are substances that participate in the chemical reactions taking place within the cell, and are commonly found at the electrodes. For example a Lithium-ion battery may have a graphite anode, a Nickel-Manganese-Cobalt cathode, and Lithium ions that traverse back and forth. Table 1 is provided below to break down the active materials in common electrochemical cells, along with some key traits.
1.0.2 Modelling the cell
Accurately modelling an electrochemical cell is useful for simulating behaviour, tracking cell performance, and managing operation, however, they can be very complex. The equivalent circuit model is commonly created to simplify the cell by using circuit elements such as resistors, capacitors, and inductors to represent cell components and processes. This bridges the gap between the chemical/physical understanding of the electrochemical phenomena and electrical engineering. The diagram below highlights how various components of a battery cell are translated to electrical circuit elements.
Resistors represent instantaneous losses in the cell in the form of voltage drops, this can be helpful in tracking cell degradation to components such as the electrolyte impeding charge transfer, the solid electrolyte interfacial layer, or contact resistances between the electrodes and current collectors. Capacitors model components that store energy and vary with time and state of charge, this includes charge store at in the double-layer, at the electrode-electrolyte interface, and bulk capacitance in the electrodes. Components such as the current collectors and wiring have inductive properties by storing charge in a magnetic field and can be modelled with an inductor. Using different combinations and structures of these basic circuit components various battery chemistries, fuel cells, and other electrochemical systems can be modelled.
1.0.2.1 Randles Circuit
The foundation of of equivalent circuit modelling is the Randles Circuit. The Randles Circuit is made up of a resistor in series with a parallel pair containing a capacitor and another resistor. This is often a strong starting point for creating a more advanced equivalent circuit model. As shown in the diagram below the circuit consists of the solution resistance to ion flow in the electrolyte (Rs), the double-layer capacitance representing the storage of charge in the electrode (Cdl), and charge transfer or polarization resistance dicatating charge transfer due to electrode potentials and concentrations (Rct/Rp).
Using the building blocks of the Randles circuit and resistor-capacitor pairs an accurate model of an electrochemical cell can be created. Electrochemical impedance spectroscopy is one of the best methods for determining the correct configuration and circuit component parameters.
1.0.3 What is Electrochemical Impedance Spectroscopy?
EIS is testing method to gain insight into the state of electrochemical systems. EIS operates by sending small electrical signals and measuring the system’s response. The signals are small alternating currents or sinusoidal voltage that are delivered at a range of frequencies. By using Ohm’s law we can determine the impedance within the system based on the measured differences in voltage signal in and current response out. The diagram below highlights how the input signal is changed vertically based on real impedance or resistance and horizontally lagged due to imaginary impedance, based on the condition of the system.
The measured impedance provides insights into system degradation, charge status, performance, future health, or safety. By sweeping the input signal at various frequencies the state of different processes and components can be measured, painting a detailed picture of the system. EIS impedance data is most commonly visualized using a nyquist plot. This allows us to see the real and imaginary impedance at each measured frequency. The diagram below is an example of a Nyquist plot with impedance measurements at frequencies between 0.1 and 10k Hertz.
Want to learn more? Speak with us today!
