Water electrolysis electrode potentials with pH
Electrolysis electrode potentials with pH Generally, the water next to the electrodes c will change pH due to the ions produced or consumed. If the electrode compartments are separated by a suitable porous membrane then the concentration of H3O+ in the anolyte and OH- in the catholyte (and hence the increase in the respective conductivities) are both expected to increase more than if there is free mixing between the electrodes, when most of these ions will neutralize each other. Small but expected differences in the solutions’ pHs next to the anode (anolyte) and cathode (catholyte) cause only a slight change to the overall potential difference required (1.229 V). Increasing the acid content next to the anode due to the H3O+ produced will increase its electrode potential (for example: pH 4 E = +0.992 V) and increasing the alkaline content next to the cathode due to the OH- produced will make its electrode potential more negative (for example: pH 10 E = -0.592 V). If the anode reaction is forced to run at pH 14 and the cathode reaction is run at pH 0.0, then the electrode potentials are +0.401 V and 0 V respectively (see above right). d
(a) Anode pH 0 2 H2O -> O2 + 4H+ + 4e- E° = +1.229 V
(b) Anode pH 14 4 OH- -> O2 +H2O + 4e- E° = +0.401 V
(c) Cathode pH 0 4 H+ + 4e- -> 2H2 E° = 0.0 V
(d) Cathode pH 14 4 H2O + 4e- -> 2H2 +4OH- E° = -0.828 V
This does not mean that because the electrolysis can be achieved with a (minimum) voltage of +0.403 V (see equations c and b, above) [2515], it breaks the thermodynamic requirement of 1.229 V as there is a further input of energy required in keeping the electrode compartments at the required pHs and solute concentration.