Electrochemical cells provide electrical power via a chemical reaction. A typical electrochemical cell includes a pair of electrodes called an anode and a cathode separated by an electrolyte composition. The anode, cathode, and electrolyte are contained in a case and when the anode and cathode are electrically connected to a load, a chemical reaction between the anode, cathode, and electrolyte releases electrons and delivers electrical energy to the load.
Metal-air electrochemical cells utilize oxygen from ambient air as a reactant in an electrochemical reaction to provide a relatively lightweight power supply and include an air permeable cathode and a metallic anode separated by an aqueous electrolyte. Metal-air cells have a relatively high energy density because the cathode utilizes oxygen from ambient air as a reactant in the electrochemical reaction rather than a heavier material, such as a metal oxide or another depolarizable metallic composition. For example, during operation of a zinc-air cell, oxygen from the ambient air is converted at the cathode to hydroxide ions, zinc is oxidized at the anode, reacts with hydroxide ions, and water and electrons are released to provide electrical energy.
Cells that are useful for only a single discharge cycle are called primary cells, and cells that are rechargeable and useful for multiple discharge cycles are called secondary cells. An electrically rechargeable metal-air cell is recharged by applying voltage between an anode and the cathode of the cell and reversing the electrochemical reaction. During recharging, the cell discharges oxygen to the atmosphere through the air permeable cathode and the anode is electrolytically reformed by reducing to the base metal the metal oxides formed during discharge.
Hydrogen gas may be produced at the anode as a by-product during recharging of a metal-air cell and other rechargeable electrochemical cells. Hydrogen production normally occurs when the cell is nearly fully recharged and during overcharge of the cell. Hydrogen production increases significantly during overcharging. Typically, vents in the cell case release the hydrogen produced at the anode to the atmosphere. The hydrogen is released to prevent the hydrogen from reacting with other components of the cell and causing cell failure. In addition, it is desirable to prevent buildup of hydrogen in large quantities in the cell.
One drawback to the production of hydrogen and its release from the cell is that water is lost from the cell as a direct result. Water is also lost from a metal-air cell by evaporation through the gas-permeable cathode, but it is estimated that, when the atmosphere surrounding the cell is at about 50% relative humidity, 30-40% of the hydrogen loss from the cell is through electrolytic reaction and production of hydrogen gas. Loss of water through electrolytic reaction can eventually cause a rechargeable metal-air cell to fail due to drying out. This is particularly a problem when metal-air cells are excessively overcharged repeatedly.
Therefore, there is a need for a rechargeable electrochemical cell, and particularly, metal-air cell, in which loss of hydrogen, and thus water, through electrolytic reaction is controlled.