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A 'half cell' is a structure that contains a conductive electrode and a surrounding conductive electrolyte separated by a naturally-occurring Helmholtz double layer. Chemical reactions within this layer momentarily pump electric charges between the electrode and the electrolyte, resulting in a potential difference between the electrode and the electrolyte. The typical reaction involves a metal atom in the electrode being dissolved and transported as a positive ion across the double layer, causing the electrolyte to acquire a net positive charge while the electrode acquires a net negative charge. The growing potential difference creates an intense electric field within the double layer, and the potential rises in value until the field halts the net charge-pumping reactions.
A standard half cell, used in electrochemistry, consists of a metal electrode in a 1 molar aqueous solution of the metal's salt, at 298 Kelvin (25^o Celsius). The electrochemical series, which consists of standard electrode potentials and is closely related to the reactivity series, was generated by measuring the between the metal half cell in a circuit with a standard hydrogen half cell, connected by a salt bridge.
The half cell of a Daniell cell:
Zn + Cu+² -> Zn+² + Cu :Original equation
Zn -> Zn+² + 2e- :Half Cell of Zn
Cu+² + 2e- -> Cu :Half Cell of Cu