A small electrolytic cell made out if acrylic that works with an aqueous solution of Urea.
A LM317T with a potentiometer is used to control the voltage.

The top of the cell is sealed with an O-Ring and the contacts for the electrodes are sealed with epoxy.
There is a hole (sealed with Blu-Tack) as outlet for the produced gases to which one can attach some plumbing or a pneumatic actuator.

This is an old project and you can some blue crystals forming on the carbon electrodes which I assume is some
form of copper(II) originating from the wiring.

An industrial application using this principle which inspired this build can be seen here:

BOLTR: Disposable Automatic Industrial Bearing Lubricator.