IONS HAVE SIZE

Ion size versus pore size

ion ≈ pore?

In water, an ion carries a shell of water molecules, so a small ion can act large. Pores have to be big enough for ions to enter, or the surface inside is wasted. In very small pores (under about a nanometer), ions can shed part of their shell and pack tightly, and capacitance per area can actually rise.

Choose the pores for the ions, or choose the ions for the pores.

TEMPERATURE

Hot, cold, and everything between

Aqueous Freezes near 0 °C (salt lowers it) and dries out when hot

Organic Typically about −40 °C to 65 °C, limited by volatility and flammability

Ionic liquid Stable when hot, but viscous and slow in the cold

Gel Hydrogels can dry out or freeze, so packaging must hold water in

Match the electrolyte to where the device will actually live.

REDOX-ACTIVE ELECTROLYTES

When the electrolyte stores charge too

I⁻ ⇄ I₃⁻

Some electrolytes carry a dissolved redox couple, such as iodide and triiodide, hydroquinone, or vanadium salts. The couple reacts quickly and reversibly at the electrode surface, adding pseudocapacitance from the liquid itself. It's the same redox idea behind the current amplification in Part 2, used here for storage.

The catch: dissolved redox species can wander to the other electrode and discharge it, so self-discharge rises.

A DECISION GUIDE

Pick the electrolyte for the job

Cheap, safe, moderate energy Neutral or acidic aqueous

Pseudocapacitive oxides An aqueous electrolyte matched to the oxide, such as alkaline or neutral salts

Highest energy in a big cell Organic, around 2.7 V

Extreme heat or strict safety Ionic liquid

Flexible, thin, or wearable Gel electrolyte

Thinnest, safest, slowest Solid electrolyte

Key idea There is no best electrolyte. Choose the one whose window, speed, and packaging fit the device.

UNIT 24 STUDY COMPLETE

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Ready for the Fab Challenge?

You've covered ion and pore size, temperature, redox-active electrolytes, and a decision guide.