NOVELTY - A device (1) for generating fine bubbles, comprises a substrate (3) having holes (5) through it. Each hole comprises a gas inlet and a gas outlet. The width of the gas outlet is greater than the width of the gas inlet.    USE - The device is used for generating fine bubbles. The device is used in a catholyte regeneration system of a redox fuel cell. The device is used in treating sewage water with small oxygen bubbles. The device is used in growing and reproducing yeast bacteria with small oxygen bubbles. The device is used in carbonization of beverages with small carbon dioxide bubbles. The device is used in removing oil from oil-bearing reserves such that oil is carried at the interface between the gas and liquid of each small bubble as it bubbles through it (all claimed).    DETAILED DESCRIPTION - A device (1) for generating fine bubbles, comprises a substrate (3) having holes (5) through it. Each hole comprises a gas inlet and a gas outlet. The width of the gas outlet is greater than the width of the gas inlet. The average gas inlet width is 2-10 microns. The average gas outlet is 5-100 microns. The inlet diameter is 1/10th to 1/5th of the outlet diameter. INDEPENDENT CLAIMS are:    (1) a method of manufacturing a device for generating fine bubbles, by providing a substrate; and perforating the substrate at predetermined locations with holes of predetermined widths;    (2) a method of generating fine bubbles; and    (3) a catholyte regeneration system for a redox fuel cell, comprising: a chamber; a first inlet port for receiving into the chamber reduced redox mediator couple from the cathode region of the cell; a first outlet port for supplying oxidized redox mediator couple to the cathode region of the cell; a second inlet port for receiving a supply of oxidant; and a second outlet port for venting gas, water vapor and/or heat from the chamber, a catholyte channel in fluid communication with the first inlet port, a device above having an active surface, and the catholyte channel being arranged to direct a flow of catholyte adjacent to or towards the active surface.    DESCRIPTION OF DRAWING(S) - The drawing shows a plan view of a device.    Device (1)    Substrate (3)    Hole (5)