The Poisson–Boltzmann equation describes the electrochemical potential of ions in the diffuse layer. The three-dimensional potential distribution can be described by the Poisson equation ∇ ∇ 2 ψ ψ = ∂ ∂ 2 ψ ψ ∂ ∂ x 2 + ∂ ∂ 2 ψ ψ ∂ ∂ y 2 + ∂ ∂ 2 ψ ψ ∂ ∂ z 2 = − − ρ ρ e ε ε r ε ε 0 , where