I read several papers about the electrochemical hydrogen charging but found out that they all chose unit hydrogen fugacity in order to calculate the equilibrium potential for hydrogen in the charging solution by using Nernest equation. I want to know if there is any physical meaning behind for chosing hydrogen fugacity as 1 atm.

Another way of asking this question: Why when the potential of the hydrogen ebolution reaction at working electrode surface in the charing solution is in equilibrium, the hydrogen fugacity in the solution is equal to 1 atm?

Reference paper:Article Potential-pH diagrams

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