where Φ is the electrostatic potential within the electrolyte and membrane phases and 𝑅, 𝑇, and 𝐹 are the ideal gas constant, the temperature, and Faraday's constant, respectively. In the above expression, the first term is the reference chemical potential of species i, the second term accounts for changes in activity of i, the third term accounts for electrostatic potential and only applies for charged ionic species (i.e., all species except CO2). The activity (𝑎 . ) of a given species is defined by