Aqueous decomposition voltage window

Broader decomposition voltage window (up to > 4 V) and therefore deposition of metals which is impossible in aqueous solution No reaction with substrate No hydrogen evolution Large number of complex ions possible Poisonous effect Easy inflammability. Low conductivity. Relatively high cost Limited solubility of many salts Disintegration of change at the electrodes Moisture and oxygen sensitivity... 

Generally, the available cell voltage of electrochemical cells depends on the thermodynamics of the two electrode reactions in the prevailing electrolyte, hence the difference in the electrode potentials, and is confined, according to the Electrochemical Series of Standard Potentials, to a few volts. Cells with an aqueous electrolyte exhibit a limitation given by the stability window of water, namely 1.23 V at standard conditions. As stated above, the H2/O2 fuel cell allows practical open circuit voltages of around 1.0 V. At cell voltages above 1.23 V, typically around 1.5 V, decomposition of water into H2 and O2 occurs. 

---