Reading the paper, my guess is that the overpotential they mention is the overpotential versus the standard hydrogen electrode. Overpotential is typically calculated as applied potential minus standard potential. In this paper, they take 0V as the standard potential of the SHE, where the reduction of H+ to H2 is the reaction taking place.

When the authors say that the overpotential is for example, 0.28 V, they mean that the reduction of H+ to H2 at the modified electrode begin to occur at 0.28 V in comparison to 0V in the SHE (taken as reference). If you're using a Ag/AgCl reference electrode, you could give your potential vs this electrode or calculate the potential vs the SHE. If you want to compare with the paper, maybe it's a better idea to calculate the potential vs the SHE.

The issue of the two scales in the Figures 5A and 5B, it is because the authors have presented the real value of the potential versus the SHE (in the first figure), while in the second one, they have presented the absolute value of the overpotential (taking 0V as the standard potential and the absolute value of the applied potential).

Hope that helps.

Dear Paul,

Overpotential is the applied potential to liberate a particular gas at particular electrode. Accordingly there are two types of overvoltage. Hydrogen Overvoltage is the  voltage required to be appled on cathode to start evolution of hydrogen at it.

Oxygen overvoltage- Voltage to be applied on Anode  to start evolution oxygen at it..

Three electrode Cell is to be used.

Working electrode- Glassy Carbon modified with MoS2

Auxillary electrode- Platinum Electrode

Reference Electrode- Ag/AgCl electrode

Electrolyte- 0.5 N H2SO4 

Applied voltage is to be varied with scan rate of 2 mV/S and current should be recorded.

In this way you can notice the evolution of Hydrogen at Glassy Carbon electrode at some applied potential and it is the  hydrogen overpotential w.r.t.Reference electrode. You can calculate Hydrogen overpotential w.r.t.SHE  considering  half cell potential of Ag/AgCl electode.

      I hope you will be satisfied with the answer.

Thank you Dr. Dipak for your valuable answer. I have acquired the theoretical aspect of HER and I performed it with M0S2 modified GCE by applying 0.2V to -1.2V by LSV vs Ag/AgCl. The potential applied is added with +0.197V to calculate over potential vs SHE. Now is this potential is HER over-potential ? Although as per Bard's encyclopedia of electrochemistry : HER overpotential= applied potential-0.059*pH, which is also an obvious fact. The paper I have uploaded has 2 different scales of potential in 2 plots.On e is potential applied wrt SHE and another is overpotential.

Reading the paper, my guess is that the overpotential they mention is the overpotential versus the standard hydrogen electrode. Overpotential is typically calculated as applied potential minus standard potential. In this paper, they take 0V as the standard potential of the SHE, where the reduction of H+ to H2 is the reaction taking place.

When the authors say that the overpotential is for example, 0.28 V, they mean that the reduction of H+ to H2 at the modified electrode begin to occur at 0.28 V in comparison to 0V in the SHE (taken as reference). If you're using a Ag/AgCl reference electrode, you could give your potential vs this electrode or calculate the potential vs the SHE. If you want to compare with the paper, maybe it's a better idea to calculate the potential vs the SHE.

The issue of the two scales in the Figures 5A and 5B, it is because the authors have presented the real value of the potential versus the SHE (in the first figure), while in the second one, they have presented the absolute value of the overpotential (taking 0V as the standard potential and the absolute value of the applied potential).

Hope that helps.