Question

How to differentiate two elwctrode and three elwctrode system?

Answer 1

1. In two-electrode system, you in fact use the counter electrode as reference electrode. unlike the reference electrode, the potential of counter electrode changes during your measurement. that is the reason you got different results. 

2. It's always better to use two electrode system to reflect the efficiency of your system(working+counter electrodes) because there is no reference electrode in real PEC devices. However, if you want to characterize only your working electrode and assume that the counter electrode reaction is not the rate-limiting reaction (like using big Pt foil as counter electrode) then you can use three-electrode system. but remember to convert the potential of your reference electrode into the potential of your reaction (like HER, HOR...) when you calculate the efficiency