First Principle Based Simulation of Cyclic Voltammogram: Bromide Adsorption on Pt(111) Surface
Authors: Kawamura Yoshiumi , Baba Takeshi 
Published: February 2020

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 (ΔGads) 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 ΔGads 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 θBr and θH were calculated at each potential steps from the ΔGads(θ) under the steady-state approximation. The currents were evaluated by the variation of the θBr and θHduring 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 θH and θBr dependence of the ΔGads. This result describes the repulsive interaction between adsorbed atoms, for example, the ΔGads increases with the increase of θBr in the case of θH=0.33. The simulated voltammograms by using these ΔGads 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 θH and θBr. The θBr gradually increases from 0.1V, which corresponds the (√3x√3)-Br- adsorption. The θBr 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:

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Acta, 56 (2011) 3996.
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[5] H. A. Gasteiger, N. M. Markovic, and P. N. Ross, Jr., Langmuir, 12 (1996) 1414.