Ab Initio Atomistic Thermodynamics for Fuel Cell Catalysis - Fuel Cell Catalysis: A Surface Science Approach - page 136

Figure 5.4 Atomistic model of the electrochemical half-cell, showing the electrode/electro-
lyte interface (x 1 , x , x 2 ), which is connected to the bulk electrode and electrolyte (reser-
voirs). The lower panel indicates the electrostatic potential within the electrode and the bulk
electrolyte (solid lines), and possible shapes for the potential drop between them (dashed lines).
also show linear behavior as function of distance to the electrode x.Att . 0, the
system tries to reach thermodynamic equilibrium by redistributing the ions within
the electrolyte such that counter-ions in the electrolyte are accumulated at or near
the electrode. Some ions might even lose parts of their solvation shell and adsorb at
the electrode surface (specific adsorption) or just weakly interact as solvated ions
(nonspecific adsorption). When thermodynamic equilibrium is reached, the electro-
chemical potentials of anions and cations are constant throughout the electrolyte.
Furthermore, the electrolyte ions will exactly compensate the electrode excess
charges, leading to a potential drop within the interfacial region.
Figure 5.4 shows schematically the electrochemical half-cell and the electrostatic
potential after the system has reached equilibrium. It combines a single electrode/
electrolyte interface in contact with two reservoirs: bulk electrode (x , x 1 ) and bulk
electrolyte (x . x 2 ). Since, by definition, the excess charges on the electrode are
fully shielded within the interface, far from the electrode, the electrostatic potential
assumes the constant value f S with respect to the reference potential f ref .
Regarding the dimensions, the width of such an interface might range from a few to
several hundreds of ˚ ngstr ¨ms, which is rather small compared with the size of the
bulk electrolyte region in realistic systems. Therefore, the latter region can be
considered as an electrolyte reservoir.