The electrochemical oxidation and reduction reactions take place at the anode and cathode surface, respectively. At the anode, the oxygen evolution reaction (OER: 4OH -→ 2H 2 O + O 2 + 4e -) produces oxygen gas, O 2 , while at the cathode, hydrogen gas is produced via reduction of water in the hydrogen evolution reaction (HER: 4H 2 O + 4e -→ 2H 2 + 4OH -) . The standard equilibrium potential of the overall water splitting reaction is 1.23 V. However, the practical operating cell voltage for hydrogen evolution lies between 1.5-2.0 V. The additional voltage, also referred to as overvoltage or overpotential, must be applied to overcome the energy barriers of the OER. The overall system efficiency thus remains between 59-70 % for commercially available electrolysers .