The thin electrolyte layer and thick ceramic support layer preferably define a sealing perimeter surrounding the porous electrode layer.

The membrane comprises a thin electrolyte layer supported on a porous electrode layer which in turn is supported on a thick ceramic support layer, preferably a ceramic electrolyte support.

A solution of volatile lithium, aluminium and phosphorous (10) is spraced onto a heated substrate (14) containing a thin film current collector provides the mixed oxide, Li2O-xAl2O3-yP2O5. electrolyte thin film.

An electrode structure which has a pair of electrode catalyst layers (1) and (1) and a polymer electrolyte film(2) sandwiched between the electrode catalyst layers (1) and (1).