The given voltage of -15 V cannot be by calculation, its to high. Or do you have some cells in series?
Values from table book
2H2O + 2e- -> H2 + 2OH- +0,828V
2H2O + O2 + 4e- -> 4OH- -0,401 V
2H2O -> O2 + 4H+ + 4e- +1,229 V
4OH- -> 2H2O + O2 + 4e- +0,401 V