I have already answered some your questions posted earlier.

1. oxidation of water

2. measure the Coulombic efficiency (CE), also called faradaic efficiency or current efficiency of product formation. As a minimum, measure the H2-yield.

3. I don't understand this "the formulation of the cell half reactions?"

4. Why do you need the water reduction electrodes, if you want to reduce CO?

The photocatalytic CO2 reduction process is based on a serious of Redox (Reduction & Oxidation) reactions in which the electrons (e-) reduce the CO2 while the holes (h+) oxidize the water.