Most recently, a novel class of 'bubble-free' electrolysis cell, known as a capillary-fed cell, has been developed. In such cells, the gases are produced in bulk form directly from the liquid electrolyte that lightly, but completely, coats the electrodes. A thin, wicking separator between the electrodes draws the alkaline liquid electrolyte up to the electrodes from a reservoir at the base of the cell. The electrodes, which are sandwiched tightly against opposite sides of the separator in a zero-gap configuration, draw small amounts of liquid electrolyte laterally out of the separator causing them to be completely covered with only a thin layer of the liquid electrolyte. The gas formed on the electrode surface can then migrate through the thin layer of liquid electrolyte into an adjoining gas chamber, without forming gas bubbles. The outcome of this arrangement is that the energy efficiency of the cell is significantly improved. Indeed, a capillary-fed water electrolysis cell has been reported that requires only 40.4 kWh to produce 1 kg of hydrogen under the standard operating conditions employed in many commercial alkaline electrolyzers. This supersedes the efficiency target of <42 kWh kg -1 set by the International Renewable Energy Agency (IRENA) for the year 2050. While the above capillary-fed cell is essentially bubble-free at low current densities, some gas bubbles are produced at higher current densities. This is likely because one or both electrodes become overhydrated; that is the layer of liquid electrolyte on the surface of the electrodes becomes too thick. For example, water is produced at the oxygen-generating anode in alkaline electrolysis cells. At higher current densities the quantity of water produced in the capillary-fed alkaline cell may conceivably thicken the liquid layer on the anode, slowing the migration of the oxygen gas product into its gas chamber and leading to the formation of some bubbles. Understanding the processes that occur at each electrode and their relationship with the current density is critical to further improvements in energy efficiency.