1. How do you find onset potential in two-electrode system?

2. The on-set potential measured at 10 mAcm-2 current density. Why?

1-We can measure the onset potential of two electrode system at 10 mA/cm2.

2- We can measure the onset potential at 10,20 and 100 mA/cm2 but 10 is significant performance index as it is the value for a 10% efficient solar-to-fuel conversion device. The other reason is that most of the reported literature measured it at 10, so we can compared it with literature easily. 

There is a difference in onset potential and potential at 10 mAcm-2 current density. It is correct that you can easily compare your data at a particular current density but not the onset potential and comparisons the data between two-electrode and three-electrode systems  are not normally suggested.

According to the definition of "so-called onset potentials, i.e. the lowest (for the anodic reactions) or the highest (for the cathodic reactions) potentials at which a reaction product is formed at a given electrode and at defined conditions, is very important for the evaluation of the catalytic activity and even more for the comparison of different catalysts."

Thank You man, i know that onset potential, over potential and potential ate any specific current density are different things.

Your "onset" potential is likely to depend on membrane resistance and on stirring rate. 

Dear Muhammed,

I agree with Sujoy and would not talk about an onset potential at a given current density. It is more correct to talk about an operating potential at 10 mA/cm².  Although you should be aware that 10mA/cm² does not correspond to 10% STH efficiency (this is 8.13 mA/cm²).

The answer to your initial question is: NO.

For any two-electrode water splitting measurement the total operating potential will be:

E = E° + ƞ(cathode) + ƞ(anode) + ƞ(CP) + ƞ(ohmic) + ƞ(other)

E° = 1.229 V

ƞCP stands for concentration polarisation and account for shift in the Nernst equation due changes in concentration from products (O2 and H2) or intermediates (H+, OH-). Even in KOH CP will occur but i will be small. Therefore it is best, if this is possible, to flush the cathode side with H2 and the anode side with O2 during a two-electrode measurement.

ƞOhmic losses are predominantly due to losses in the electrolyte, which follows Ohms law. You can minimize ohmic losses by placing electrodes close to each other and by using a concentrated electrolyte. Be aware that if you are using a supporting electrolyte such as NaCl, CP will increase.

ƞOther stands for other losses such as bubble resistance, conctact resistance etc...

So to summarize. Your operating potential will be higher than 1.64V, depending on the concentration of your electrolyte and the space between both electrodes.

Hope this helps,

Tom