The Seawater Capacitor

What happens when you connect a small solar panel to two vessels of seawater - separated and not electrically linked - each containing a graphite electrode? You can build up a voltage. Not a lot, and not for long, but enough to suggest something interesting is going on.

This isn’t a battery. There’s no sustained redox chemistry between the two chambers, no Faradaic current flowing. Instead, the system behaves like a capacitor: charge accumulates on each graphite electrode in response to the solar panel’s output, creating a measurable potential difference across the isolated cells.

The Setup

Two Erlenmeyer jars of seawater, not connected by any salt bridge or ion-permeable membrane
Graphite electrodes (reclaimed from pencil leads), submerged in each vessel
Copper foil and wire connect each graphite rod to the terminals of a small solar panel. I made it gas-tight just in case of any chlorine evolution
The solar panel provides DC output, but no current can flow between the jars

Observations

The system developed a voltage difference of ~2.3 V between the jars
Disconnecting the panel and probing the terminals confirmed stored charge, similar to a Leyden jar or modern electrochemical double-layer capacitor

Why It Works

Even though no ionic current flows between the jars, the solar panel drives charge separation at each graphite–seawater interface. Ions in the seawater respond locally, forming a Helmholtz double layer. Each half-cell builds an electrochemical potential, opposing the panel voltage and eventually reaching equilibrium, just like a capacitor charging.

Limitations

Very low capacitance - this stores microcoulombs at best
No discharge circuit included, so effective energy output wasn’t tested
Without a salt bridge, the system is closed: no redox loop can form

Next Steps, if any:

Could this scale up? Probably not in any practical sense of energy storage. But it’s an elegant demonstration of solar-powered ionic charge separation, and a reminder that the boundary between “battery” and “capacitor” is more fluid than textbooks suggest.

I may revisit this with a more advanced setup - perhaps a semi-permeable membrane, salt bridge, or proper discharge circuit. But for now, it’s just seawater, sunshine, and a simple question: how much electrode surface area and seawater do you need to do something useful?