CYCLE LIFE

How long does it last?

retention = Cₙ / C₁

Cycle the device thousands of times at a fixed current and track its capacitance. Capacitance retention is the fraction left after n cycles, reported with coulombic efficiency. A common end-of-life definition is a 20% loss of capacitance or a doubling of ESR.

Long-life claims need thousands of cycles, not a few hundred.

SELF-DISCHARGE

Holding a charge at rest

charge → rest → watch V

Charge the device, disconnect it, and watch the open-circuit voltage decay. Real cells sag over hours from leakage currents and unwanted reactions. Self-discharge decides whether a supercapacitor can power a sensor that wakes up once a day.

Fast delivery is only half the story: holding the charge matters too.

WHY THE VOLTAGE DECAYS

Three usual suspects

Ohmic leakage A small current through the separator or around the cell

Faradaic leakage Impurities or dissolved redox species react and drain charge

Charge redistribution Charge relaxes into deeper pores, so the terminal voltage sags at first

Redox-active electrolytes and very high voltages tend to speed up self-discharge.

SPOTTING SHAKY NUMBERS

Red flags in a supercapacitor paper

Material data sold as device data A symmetric full cell may deliver about a quarter of it

Only active-material mass counted A real device weighs far more once collectors and packaging are included

Only the slowest scan rate Ask for the rate capability across scan rates or currents

Plateaus called capacitance Redox plateaus are better quoted as capacity, in mAh/g or C/g

A few hundred cycles as proof Look for thousands of cycles, with coulombic efficiency

A vague voltage window Energy scales with V², so how the window was chosen matters

A bold number with no method behind it deserves a second look.

A TESTING CHECKLIST

Match the claim to the test

A device-level claim Two-electrode cell

Capacitance GCD or CV, with the current or scan rate and the normalization stated

Resistance IR drop from GCD, plus the Nyquist intercept

Energy and power GCD at several currents, summarized on a Ragone plot

Lifetime Thousands of cycles: retention and coulombic efficiency

Storage Self-discharge at open circuit

Key idea Good results show the method, the rate, and the denominator.

UNIT 30 STUDY COMPLETE

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Ready for the Fab Challenge?

You've covered cycle life, leakage, and a checklist for spotting shaky claims.