Voltaic pile

 ( Battery Types ) 

Many scientific instruments that belonged to Alessandro Volta are preserved in the University History Museum of the University of Pavia, where Volta taught from 1778 to 1819; the piles on display, unfortunately, are not original, as the ones preserved in Pavia were lent on the occasion of the centenary of the invention and subsequently lost in a fire.

Humphry Davy and Andrew Crosse were among the first to develop large voltaic piles.Encyclopædia Britannica, 1911 edition, Volume V09, Page 185 Davy used a 2000-pair pile made for the Royal Institution in 1808 to demonstrate carbon arc discharge Tracking Down the Origin of Arc Plasma Science. II. Early Continuous Discharges and isolate five new elements: barium, calcium, boron, strontium and magnesium.

The modern, atomistic understanding of a cell with zinc and copper electrodes separated by an electrolyte is the following. When the cell is providing an electrical current through an external circuit, the metallic zinc at the surface of the zinc anode is oxidized and dissolves into the electrolyte as electrically charged ions (Zn²⁺), leaving two negatively charged () behind in the metal:

:anode (oxidation): ZnZn²⁺ + 2

This reaction is called redox. While zinc is entering the electrolyte, two positively charged hydrogen ions (H⁺) from the electrolyte accept two electrons at the copper cathode surface, become reduced and form an uncharged hydrogen molecule (H₂):

:cathode (reduction): 2 H⁺ + 2 H₂

This reaction is called redox. The electrons used from the copper to form the molecules of hydrogen are made up by an external wire or circuit that connects it to the zinc. The hydrogen molecules formed on the surface of the copper by the reduction reaction ultimately bubble away as hydrogen gas.

One will observe that the global electro-chemical reaction does not immediately involve the electrochemical couple Cu²⁺/Cu (Ox/Red) corresponding to the copper cathode. The copper metal disk thus only serves here as a "chemically inert" noble metallic conductor for the transport of electrons in the circuit and does not chemically participate in the reaction in the aqueous phase. Copper does act as a catalyst for the hydrogen-evolution reaction, which otherwise could occur equally well (though at a slower rate in the absence of the catalyst) directly at the zinc electrode without current flow through the external circuit. The copper electrode could be replaced in the system by any sufficiently noble/inert and catalytically active metallic conductor (Ag, Pt, stainless steel, graphite, ...). The global reaction can be written as follows:

:Zn + 2H⁺Zn²⁺ + H₂

This is usefully stylized by means of the electro-chemical chain notation:

:(anode: oxidation) Zn | Zn²⁺ || 2H⁺ | H₂ | Cu (cathode: reduction)

in which a vertical bar each time represents an interface. The double vertical bar represents the interfaces corresponding to the electrolyte impregnating the porous cardboard disk.

When no current is drawn from the pile, each cell, consisting of zinc/electrolyte/copper, generates 0.76 V with a brine electrolyte. The voltages from the cells in the pile add, so the six cells in the diagram above generate 4.56 V of electromotive force.

The first to publish the discovery of a dry pile that produced a current was Johann Wilhelm Ritter in 1802, albeit in an obscure journal; over the next decade, it was announced repeatedly as a new discovery. One form of dry pile is the Zamboni pile. Francis Ronalds in 1814 was one of the first to realize that dry piles also worked through chemical reaction rather than metal-to-metal contact, even though corrosion was not visible due to the very small currents generated.