First Principle Based Simulation of Cyclic Voltammogram: Bromide Adsorption on Pt(111) Surface

Introduction:

Adsorption of halide anions on the electrode surface is fundamental phenomena and important in the various applications, such as the synthesis of the shape-controlled nano-particles. Cyclic voltammetry is the most important experiment for understanding the surface reactions. However it is sometimes difficult to assign the origin of peak positions and/or shapes of the voltammogram.  Comparison of the experimental results with the simulated voltammograms might be helpful for assigning and clarifying of the atomistic behavior.  In this study, we focus on the bromide adsorption on the Pt(111) surface, which is complicated process because Br^- adsorbs simultaneously with the desorption of H⁺. The transition of Br^-adsorption structure during voltammetry has been discussed, but is still unclear. We simulated voltammogram based on the density functional theory (DFT) calculations and discussed the peak positions, shape, and adsorption/desorption processes.

Method:

In this study, following two redox reactions were considered,

H⁺_(aq) + * + e^-<--> H*           (1),

Br ^-_(aq) + * <--> Br* + e^-         (2),

where * indicates the Pt(111) surface. The adsorption free energies (ΔG_ads) for these reactions were calculated by DFT. (√3x√3)-Br, (3x3)-Br, and (√3x√3)-H adosprtion structures were considered by using the slab models. The surface coverage (θ) dependence of the ΔG_ads are also calculated. Methodology of the voltammogram simulation has been disscussed by some research groups [1-3]. We adopted a slimilar approach to their works. The θ_Br and θ_H were calculated at each potential steps from the ΔG_ads(θ) under the steady-state approximation. The currents were evaluated by the variation of the θ_Br and θ_Hduring potential sweep.

All DFT calculations were performed by the “interface” program package [4]. The RPBE exchange-correlation functional, the norm-conserving pseudopotential, and numerical basis sets were used.

Results and discussion:

 Figure 1 shows the calculated θ_H and θ_Br dependence of the ΔG_ads. This result describes the repulsive interaction between adsorbed atoms, for example, the ΔG_ads increases with the increase of θ_Br in the case of θ_H=0.33. The simulated voltammograms by using these ΔG_ads values are shown in Fig. 2. Although the peak shape is broad, the peak positions are in good agreements with the experimental one [5]. Figure 3 shows the calculated surface coverage θ_H and θ_Br. The θ_Br gradually increases from 0.1V, which corresponds the (√3x√3)-Br^- adsorption. The θ_Br exceeds 0.33 around 0.3V, which means that the adsorption structure of Br^-gradually changes from (√3x√3) to (3x3). Therefore, it is indicated that the shoulder peak around 0.3V in Fig. 2 is originated from the transition to (3x3)-Br adsorption structure.

References:

[1] I. S. P. Savizi and M. J. Janik, Electrochim. Acta, 56 (2011) 3996.

[2] V. Viswanathan, H. A. Hansen, J. Rossmeisl, T. F. Jaramillo, H. Pitsch, and J. K. Nørskov, J. Phys. Chem. C, 116 (2012) 4698.

[3] R. Jinnouchi, K. Kodama, T. Hatanaka, and Y. Morimoto, Phys. Chem. Chem. Phys., 13 (2011) 21070.

[4] R. Jinnouchi and A. B. Anderson, J. Phys. Chem. C, 112 (2008) 8747.

[5] H. A. Gasteiger, N. M. Markovic, and P. N. Ross, Jr., Langmuir, 12 (1996) 1414.