NOVELTY - An electrochemical cell comprises: (a) continuous sheet (302) of substrate material having two faces, the substrate material being folded to form undulating sheet having first troughs (305A) on the first face and second troughs (305B) on the second face, the first troughs alternating with second troughs along undulating sheet so that each first trough has a first wall; and (b) ion-conducting region (404) on each of the first walls.    USE - Used as e.g. fuel cell, hydrogen-fueled fuel cell or electrolysis cell (claimed) for gaseous and liquid reactants.    ADVANTAGE - The electrochemical cell is reliable and cost effective to manufacture.    DETAILED DESCRIPTION - An electrochemical cell comprises continuous sheet of substrate material having two faces, the substrate material being folded to form undulating sheet having first troughs on the first face and second troughs on the second face, the first troughs alternating with second troughs along undulating sheet so that each first trough has a first wall in common with a corresponding adjacent second troughs; and ion-conducting region on each of the first walls and allowing ions to pass through the substrate material between the corresponding troughs. Each of the troughs comprises respective electrode in ionic communication with ion-conducting region.    An INDEPENDENT CLAIM is also included for a method for making an electrochemical cell layer, comprising:    (a) laying out unit cells on a substrate by forming a first set of pores in a sheet of substrate material;    (b) filling the first of pores to create an electron-conducting region;    (c) forming a second set of pores on the sheet of substrate material;    (d) filling the second set of pores to create an ion-conducting region; and    (e) folding the substrate to provide an undulating structure.    DESCRIPTION OF DRAWING(S) - The figure shows a pleated layer electrochemical cell.    Substrate sheet (302)    Troughs (305A-B, 306)    Ion-conducting region (404)