Plz, be professional. Your question is confusing, You are asking about RDE (Rotating Disk Electrode), but show CV (Cyclic Voltammogram). "The scan range is -0.1 -0.7 V vs SCE." What is on your graph?

I'm agree with Yurii V Geletii . Take care in your questions text and the information and for finding your answer read the supplementary information of the papers about OERs that calculated the electrochemical activities of catalysts. It's just a simple search.

In general, the electrocatalytic activity can be quantified based on the current density (at a specific voltage）per mass (A/g).

I apologize Dr. Yurii V Geletii for my confusing question. I appreciate your advice so I can revise my question. I would like to know the equation to calculate the ECA for OER. Thank you.

Thank you Dr. Sina Safavi for your advice, and I apologize for my confusing question. I found that ECA can be calculated by integrating the area of total charge correspond to the desorption peak of hydrogen in the CV curve, then divide by mass of catalyst, coulombic charge, and scan rate. However, I am not sure is correct or not. I would appreciate if you can share the paper that you refer to in your comment. Thank you.

Thank you Dr. Lauren B. Resnick for your answer. However, current density per mass is a mass activity (A/g). Please kindly correct me if I am wrong. Thank you.

Dear Anafi Nuraini

First of all I didn't get my PhD yet so I'm not Dr. please pay attention to these little things in conversations. Actually it's not obvious that are you looking for Electrocatalytic activity or Electrochemical active surface area or Electrochemical activation? But I guess you look for Electrocatalytic activity but you are using wrong phrase for it. Prof. Bin Zhao gave you the best and the most correct answer. When you do experiments and determine the activity of a catalyst the number you reached is for whole of the catalyst that you used so you must divide it to catalyst mass. Also I hope the attached papers could help you.

Chapter PEM Fuel Cell Electrocatalysts and Catalyst Layers

Article Experimental Methods for Quantifying the Activity of Platinu...

https://www.nature.com/articles/s41929-018-0076-7

https://iopscience.iop.org/article/10.1149/2.0151611jes/meta

Yours "I found that ECA can be calculated by integrating the area of total charge correspond to the desorption peak of hydrogen in the CV curve, then divide by mass of catalyst, coulombic charge, and scan rate" Very new for electrochemistry.

Electrochemistry is not about using correct formulas. You need to understand what is behind each technique. Start from textbooks on electrochemistry