One-Step Fabrication of Whisker Catalyst By Galvanic Displacement Reaction and Its Application As the Electrocatalyst
Authors: Baek Seungyeon , Kim Myung Jun , Kim Kwang Hwan , Kim Jae Jeong 
Published: February 2020

The rational design of noble metal catalysts such as Pd, Pt, and Au is of considerable interest because of their superior properties in a number of applications, including catalysts, electronics, sensing, photonics, imaging, and biomedicine.1 Although noble metals are promising catalysts, their high cost represents an obstacle to commercialization.2 Therefore, extensive studies have introduced a template-mediated method of coating only a few layers of noble metal on non-expensive template through galvanic displacement reaction.
Galvanic displacement is mediated by the difference in standard reduction potentials; the metal template is spontaneously oxidized upon coming into contact with an ionized metal source with a more positive reduction potential. The electrons generated during oxidation spontaneously reduce the metal cation and it proceeds continuously until a metal film fully covered the template. Due to its uniform-deposition characteristic, galvanic displacement has been accompanied with a two-step method using a morphology-modified template with large surface area-to-volume ratio such as nanoparticles, nanowires, hollow or porous structures.3-5 This two-step process, however, is too complicate to easily control and obtain reproducible result.
We developed a one-step synthetic approach to preparation of a whisker catalyst on non-morphologically modified template by galvanic displacement. Different morphology of deposits were obtained by control of galvanic displacement condition. Whisker was deposited on Cu template during galvanic displacement reaction in PdCl2 solution (Figure 1 (a)), but film type of deposit was formed in PdSO4 solution (Figure 1 (b)). Not only the counter ions included in Pd precursor but convection which can be applied by rotating a template also determined morphology of deposit (Figure 1 (c)). Electrochemically active surface area (ECSA) of flat Pd catalyst and whisker catalyst was measured from the area of Pd oxide reduction peak at –0.3 V in cyclic voltammogram as shown in Figure 2. It showed that whisker catalyst which was fabricated by the galvanic displacement method exhibited considerably enhanced active sites than the flat Pd catalyst. The validity of the suggested galvanic displacement method for formation of whisker-type metal catalysts was confirmed using Pt and Au (Figure 3).
In this presentation, we will focus on the mechanistic analyses of whisker formation by taking a significant consideration on kinetics of coupled-redox reactions of galvanic displacement. Finally, we proposed a universal method of controlling the morphology of noble metal catalysts.

References

1. A. Mohanty, N. Garg, and R. Jin, Angew. Chem. Int. Ed., 49, 29 (2010).
2. M. Shao, K. Sasaki, N. S. Marinkovic, L. Zhang, R. R. Adzic, Electrochem. Commun., 9, 12 (2007).
3. C. Koenigsmann, A. C. Santulli, K. Gong, M. B. Vukmirovic, W.-p. Zhou, E. Sutter, S. S. Wong, R. R. Adzic, J. Am. Chem. Soc., 133, 25 (2011).
4. Y. G. Guo, J. S. Hu, H. M. Zhang, H. P. Liang, L. J. Wan, C. L. Bai, Adv. Mater., 17, 6 (2005).
5. K. H. Kim, T. Lim, M. J. Kim, S. Choe, S. Baek, J. J. Kim, Electrochem. Commun., 62, 13 (2016).

Figure Captions

Figrue 1. Surface and cross-sectional FESEM images of Cu template after galvanic displacement reaction (a, c) in the PdCl2 solution and (b) in the PdSO4 solution. The template remained (a, b) stationary or (c) rotating at 300 rpm during the reaction.
Figure 2. Cyclic voltammograms of flat Pd catalyst and whisker catalyst in 0.5 M KOH solution with a scan rate of 50 mV s-1.
Figure 3. Surface and cross-sectional FESEM images of Cu template after galvanic displacement in the K2PtCl4 solution and in the AuCl3 solution.

Figure 1