Electric Potential Distribution at Interfaces in PEFC by First Principle Simulation

The electrochemical potential dictates the charge transfer across the interfaces in electrochemical systems including PEFCs. In the present work we use ab initio molecular dynamics (AIMD) methods to evaluate the electric potential distribution across the interfaces and the so called electrochemical potential. In addition, we try to correlate the evolution of the potential distribution with the charge transfer at the interfaces.  Four types of interfaces are considered: 1) 2 M H₂SO₄/Pt, 2) 2M H₂SO₄+O₂/Pt, 3) 2M H₂SO₄+H₂/Pt, and 4) Nafion cluster/Pt. Fig. 1 illustrates the first layer of 2M H₂SO₄ on top of Pt crystal. No bond formation and breaking was observed in a run of 400 steps. However in the cases for 2M H₂SO₄+O₂/Pt and H₂SO₄+H₂/Pt and, it is observed that either oxygen desorbs from Pt and enters electrolyte in the form of water (Fig. 2) or hydrogen desorbs from Pt and enters electrolyte in the form of proton (Fig. 3). In the case of Nafion on Pt (Fig. 4) no bond formation/breaking between electrolyte species and Pt is observed.  Accordingly, as shown in Fig. 5, the variation ranges for the H₂SO₄+O₂/Pt and H₂SO₄+H₂/Pt interfaces are 10 times greater than that for the H₂SO₄/Pt interface. The variation ranges for the H₂SO₄+O₂/Pt and H₂SO₄+H₂/Pt interfaces are approximately 0.8 e. As shown in Figs. 6, 7, and 8, the interface potential difference for H₂SO₄+O₂/Pt and H₂SO₄+H₂/Pt interfaces is much greater than that of the H₂SO₄/Pt interface. The first Pt layer locates in high electric field (10⁷ V m^-1) zone for the H₂SO₄+O₂/Pt and H₂SO₄+H₂/Pt interfaces while at the first layer Pt for the H₂SO₄/Pt interface the electric field is close to zero. These findings are believed to be closely related to the slow electron exchange rate rate for the 2 M H₂SO₄/Pt case and fast electron exchange rate rates for 2M H₂SO₄+O₂/Pt and 2M H₂SO₄+H₂/Pt cases.