Deeply penetrating X-rays are well suited to investigate the electrochemical systems and interfaces under in-situ / operando environments. In particular, a recently developed X-ray Bragg Coherent Diffractive Imaging (BCDI) is a powerful tool for 3D imaging of internal structures of nano-size particles, [1,2] and the internal compositions of alloy nanoparticles. [3-5] Further, the well-annealed alloy nanoparticles can be electrochemical treated to increase the surface strain and to enhance the certain electrocatalytic activities such as oxygen reduction reaction (ORR). [6] After introduction of the technique and examples, we will focus on how strain in a single Pt nano-grain evolves in response to applied potential. The strain responses then will be discussed within the context of surface oxidation and oxygen-evolution reaction (OER).

For Pt nanograins of a film grown on a substrate, the strain response is tightly related to the place-exchange surface oxidation and electrochemical double layer to the applied potential range relevant to OER. During polarization, marked changes in surface strain arise from the Coulombic attraction between the electrode surface and electrolyte ions, while the strain in the bulk of the crystal remained unchanged. The concurrent surface redox reactions have a strong influence on the magnitude and nature of the strain dynamics under polarization.

In careful BCDI analyses, we found that only the top 10 nm of Pt at the grain/electrolyte interface responded sensitively to applied potential, while a much more muted response was observed for the remainder of the bulk Pt grain. The potential-dependent strain responses were not present at grain boundaries locked to the neighbor grains and at the bottom interface in contact to the substrate, neither of which were exposed to electrolyte. Our measurements further suggested that even highly stable electrodes such as Pt are subject to irreversible structural changes during polarization, highlighting the importance of in-situ/operando techniques to draw faithful structure-activity relationships in electrochemistry. These observations were possible due to the high strain sensitivity of BCDI combined with the near-zero X-ray scattering background of the submicron scale nanopipette electrochemical cell.

[1] Y. Liu et al., Nano Letters 17, 1595 (2017).

[2] W. Cha, Y. Liu, H. You, G. B. Stephenson, and A. Ulvestad, Advanced Functional Materials 27, 1700331 (2017).

[3] T. Kawaguchi et al., Physical Review Letters 123, 246001 (2019).

[4] T. Kawaguchi, W. Cha, V. Latyshev, S. Vorobiov, V. Komanicky, and H. You, Journal of the Korean Physical Society 75, 528 (2019).

[5] S. Choi et al., Nano Letters 20, 8541 (2020).

[6] T. Kawaguchi, V. Komanicky, V. Latyshev, W. Cha, E. R. Maxey, R. Harder, T. Ichitsubo, and H. You, Nano Letters 21, 5945 (2021).