Question

Why we chose 2H2O+2e--->H2+2OH- at cathode instead we chose O2+4H+ +4e--->2H2O at cathode?

Answer 1

2H₂O + 2e^- --> H₂+2OH^-
In water, solution concentration of water molecules is higher then concentration of dissolwed oxygen. So, process of reduction for hydrogen in water molecule is more preferred.