THE INSTRUMENT

One workstation, three ways to poke a device

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Sweep the voltage

A potentiostat controls the voltage and measures the current. This is cyclic voltammetry (CV)

➡️

Hold the current

A galvanostat controls the current and measures the voltage. This is galvanostatic charge–discharge (GCD)

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Ping with AC

A tiny AC signal sweeps across frequencies and the response is recorded. This is impedance spectroscopy (EIS)

Most electrochemical workstations do all three, so one device can be tested every way.

THREE-ELECTRODE CELL

Testing a material on its own

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Working electrode

The material under test. Its potential is what you control and study

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Reference electrode

Holds a steady, known potential, so the working electrode can be measured against it

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Counter electrode

Completes the circuit and carries the current, usually with a larger area than the working electrode

A three-electrode cell tells you about a material. It doesn't tell you what a finished device will do.

TWO-ELECTRODE CELL

Testing the real device

(+) | separator | (−)

A two-electrode cell is the device itself: the full cell voltage is applied across both electrodes with no reference. It captures the separator, the electrolyte, the contacts, and both electrodes working together, so device claims should come from this setup.

Materials: three-electrode. Devices: two-electrode.

THE FACTOR OF FOUR

Why one carbon can give two numbers

C_cell = ½ C_electrode

In a symmetric cell the two electrodes act as capacitors in series, so the cell holds half of one electrode's capacitance, and it carries the mass of both electrodes. Dividing by the total mass gives one quarter of the single-electrode value: the same carbon might be quoted at 200 F/g or 50 F/g, depending on the setup.

Always check whether a specific capacitance is per electrode or per device.

UNIT 25 STUDY COMPLETE

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Ready for the Fab Challenge?

You've covered potentiostats, three- vs. two-electrode cells, and the factor of four.