Squidstat

	Admiral Instruments Squidstat Plus [SOP]
Voltage Scan Range	±10 V
Measured Voltage Accuracy	0.1% of measured, 1 mV min
Measured Voltage Resolution	0.003% of range, 15 μV min
Current Range	± 1 A
Measured Current Accuracy	0.1% of range, 100 pA min
Measured Current Resolution	0.003% of range, 3 pA min
EIS Frequency Range	10 µHz - 2 MHz
Time Resolution	10 ns

The Squidstat Plus is a single channel combined potentiostat galvanostat with FRA/EIS capabilities. It is capable of applying ±10V of potential to a sample or ±1A of current. EIS measurements can be made within the 10 µHz - 2 MHz frequency range. It has 5 alligator clip cables: 2 are lead used to apply potential and current, one attaches to the working electrode and the other to counter electrode; 2 are sense leads used for measuring potential and current at the working and counter electrodes; the fifth is a reference electrode, used to determine what potential will be assigned to 0 V.

Galvanostat/Potentiostat

Diagram of Potentiostat/Galvanostat

A galvanostat applies a current to an electrical circuit or cell and measures a response in the potential difference between electrodes, whereas a potentiostat applies a potential to an electrical cell or circuit and measures the resulting response in current. They both operate in a similar manner.

A working electrode (WE) and counter electrode (CE) must be attached to the sample to be tested, as well as a reference electrode (RE), and a sense (S) connection. The Squidstat employs two sense connections and uses the reference electrode only to set the zero point. The measurements are made between the counter and working senses, which are often stacked on top of the working and counter electrodes. The reference electrode is by convention often placed on the counter electrode to set it as the zero point.

A Galvanostat uses a feedback loop to control the current between the working and counter electrode, while recording the potential difference between the two. The current amplifier (CA) forces a current to run through the electrical cell into the working electrode, where it is then measured by either a Current Follower (LowCF) or a Shunt (HighCR). This provides feedback so that the instrument can maintain a specific current while monitoring the voltage.

A Potentiostat will carefully control the potential difference between the counter and working electrode. This while elicit a current response from the cell under study, which is recorded.

Experiment Set Up

Set Up for a 2 Electrode Test with a Capacitor

The most common set up for the Squidstat is a 2 electrode set up. In this configuration the different leads are stacked together so that only one clamp must be attached to each electrode or to each end of the electrical circuit. The two counter electrode cables (the black ones), are stacked together, and then stacked into the reference lead (the green one). The two working electrodes (the red ones) are also stacked. The working electrode side would then be attached to the cathode, and the counter electrode side to the anode.

This configuration is sufficient for most simple experiments. Another popular set up is the 3 electrode set up. In this set up the reference lead is disconnected from the counter leads and attached to the reference electrode. This allows for very precise control of the potential at the working electrode. The reference lead can also be ignored if the experiment is run with respect to the open circuit potential, as this replaces the reference from the reference lead. Additionally, the counter electrode sense lead is not strictly necessary when testing electrical cells with very low impedance.

Frequency Response Analysis (FRA)

Frequency response analysis (FRA) is a measurement technique used in a variety of applications, including electrochemistry. This technique allows signals to be analyzed based on frequency instead of time. FRA relies on Fourier Transforms to process data. This assumes that all periodic functions can be represented as a sum of sinusoidal functions. Used this analysis the frequency of as signal can be obtained.

FRA Procedure in a Potentiostat/Galvanostat

A signal, X(t), is imputed to the measurement device. This then records the resulting signal, and outputs it back to the analyzer. In the case of electrochemistry the output signal consists of two parts, a current wave, and a potential wave. Using Fourier transforms these are both represented sinusoidal, and can then be analyzed more effectively by the user.

Electrochemical Impedance Spectrophotometry (EIS)

Electrochemical impedance spectrophotometry (EIS) is the application of a sinusoidal potential or current wave to an electrochemical cell in order to determine the impedance of the circuit. It is a branch of frequency response analysis, which is used by the Squidstat. The application of a potential wave causes a corresponding current wave in the cell, and vis versa with the application of a current wave. The response wave is always separated from the applied wave by some phase shift φ. Measuring this phase shift allows the impedance of the cell to be characterized.

Impedance is essentially the generalized form of resistance. Ohms Law, ${\text{R}}={\frac {\text{V}}{\text{I}}}$. Ohms Law is accurate only when applied to an ideal resistor, which simplifies the relationship. In any other circuit or electrical device, Ohms Law becomes less precise, necessitating the use of impedance. Impedance, like resistance, measures the resistance of a circuit to electrical flow. It differs from resistance in that it is dependent on frequency, and the current and potential waves may not necessarily be in phase with each other. When Ohms law is used to calculate resistance, independence of frequency and in phase waves are assumed.

Impedance has 2 components, the phase shift ($\phi$) in the wave caused by the applied signal, and the amplitude ($Z_{0}$) of the resulting wave. Impedance is represented by the letter Z and consists of a real and imaginary part.

Nyquist Plots

Nyquist plots are often used to represent impedance. These are generated through EIS testing across a range of frequencies. The negative imaginary component of impedance is plotted on the y axis and the real component on the x axis. A parametric curve, from the frequencies tested, is plotted. Nyquist plots are popular because they allow the components of the system to be easily identified, and it is easy to see key data points. Different types of circuits produce very distinct shapes. To identify these shapes, Nyquist plots generally follow the convention of being orthonormal. However, neither the phase shift, nor the frequency is not shown on Nyquist plots. Generally the lower left portion is a higher frequency, and the right side is corresponds to the lower frequencies.

In a Nyquist plot, an ideal resistor is represented by a dot on the x axis at the resistance of that resistor. An ideal capacitor appears as a vertical line. However, most real capacitors appear as a slanted line, which starts at some x intercept, and then moves away from the origin. When both a capacitor and a ristorante are connected in parallel, the result is a half circle. This is because at low frequency the impedance of the capacitor increases, and most of the current flows through the resistor. But at high frequency the current through the capacitor dominates, and impedance of the circuit is determined primarily by the capacitor.

Example Nyquist Plots

Bode Plots

Bode plots are another common method of graphing EIS data. It plots frequency on the x axis, and uses two y axis. The primary axis shows the magnitude of the impedance, and the secondary axis shows the phase shift. Bode plots are useful because they allow the frequency and phase shift to be read easily. The phase shift also allows some determination of the type of circuit. An ideal resistor has a phase shift of 0º, and an ideal capacitor has a phase shift of -90º, and +90º corresponds to an ideal inductor.

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Example Bode Plots