(280a) Stability of Platinum in the Electrochemical Environment: Reconstruction, Roughening, and the Third Peak

Platinum is a widely used catalyst in aqueous and electrochemical environments. It is used in fuel cells and electrolyzers, for example, to catalyze hydrogen oxidation, hydrogen evolution, and the oxygen reduction reactions [1,2]. The activity and selectivity of a catalyst is dictated by its surface structure, which can change in the reaction environment. In our prior work, we have used density functional theory to examine the surface energy (thermodynamic stability) of the low index Pt(111), Pt(100), and Pt(110) surfaces in the presence of adsorbed hydrogen, hydroxide, water, oxygen, surface oxide, and alkali metal cations, spanning the complete electrochemical window of water [3]. We found that the relative stability of these surfaces was altered in the electrochemical environment. While Pt(111) is the most stable surface in vacuum, the strong binding of hydrogen to Pt(100) and of hydroxide and water to Pt(110) make these surfaces more stable at low potentials and high potentials, respectively. We used these results to better understand the reconstruction of platinum electrodes. For example, cycling the potential of a Pt(111) electrode above 1.3 V_RHE results in the growth of islands expressing, primarily, (110) step sites [4, 5]; the Pt(110) surface is most stable at potentials where the surface oxide formed at high potentials (> 1.3 V_RHE) is reduced [3]. Similarly, the formation of (100) terrace sites upon square wave cycling of spherical nanoparticles to low potentials (below 0 V_RHE) [6] may be driven by the greater stability of the Pt(100) surface at low potentials due to its strong binding of hydrogen [3]. In this talk we will show how we have recently extended this work to include the surface energy of the Pt(553) and Pt(533) stepped surfaces. We have additionally considered the thermodynamics of subsurface hydrogen formation and hydrogen induced step-edge roughening. Using density functional theory, we find this step-edge roughening to be both thermodynamically and kinetically favorable. We compare closely to experiment, where it may be responsible for the appearance of the anomalous anodic “third peak” which can be seen in cyclic voltammograms on cycling a stepped or polycrystalline platinum electrode to low potentials (below -0.2 V_RHE) [7, 8].

1) H. A. Gasteiger, J. E. Panels, and S. G. Yan, "Dependence of PEM fuel cell performance on catalyst loading," Journal of Power Sources, vol. 127, pp. 162-171, 2004.

2) H. A. Gasteiger, S. S. Kocha, B. Sompalli, and F. T. Wagner, "Activity benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen reduction catalysts for PEMFCs," Applied Catalysis B: Environmental, vol. 56, pp. 9-35, 2005.

3) I. T. McCrum, M. A. Hickner, and M. J. Janik, “First-principles calculation of Pt surface energies in an electrochemical environment: thermodynamic driving forces for surface faceting and nanoparticle reconstruction,” Langmuir, vol 33, pp. 7043-7052, 2017.

4) M. Wakisaka, S. Asizawa, H. Uchida, M. Watanabe. “In Situ STM Observation of Morphological Changes of the Pt(111) Electrode Surface During Potential Cycling in 10 Mm HF Solution,” Phys. Chem. Chem. Phys, vol. 12, pp. 4184– 4190, 2010.

5) L. Jacobse, Y.-F. Huang, M. T.M. Koper, M. J. Rost, “Correlation of surface site formation to nanoisland growth in the electrochemical roughening of Pt(111)”, Nature Materials, vol. 17, pp. 277-282, 2018.

6) N. Tian, Z.-Y. Zhou, S.-G. Sun, Y. Ding, Z. L. Wang, “Synthesis of Tetrahexahedral Platinum Nanocrystals with High-Index Facets and High Electro-Oxidation Activity” Science, vol. 316, pp. 732– 735, 2007.

7) K. Kinoshita, J. Lundquist, P. Stonehart, “Hydrogen adsorption on high surface area platinum crystallites”, Journal of Catalysis, vol. 31, pp. 325-334, 1973.

8) O. Diaz-Morales, T. J.P. Hersbach, C. Badan, A. Garcia, M. T.M. Koper, “Hydrogen adsorption on nano-structured platinum”, Faraday Discussions, Accepted Manuscript, 2018, DOI: 10.1039/C8FD00062J.