Influence of Growth Mechanism and Potential Cycling on the Active Surface Area of Electrodeposited Highly Porous Pt Nanoparticles
Authors: Hubin Annick , Ustarroz Jon , Geboes Bart , Bals Sara , Breugelmans Tom 
Published: February 2020

It is widely known that nanostructured materials are one of the key elements in the recent advances of various technological fields. More precisely, energy conversion devices (fuel cells, batteries, solar cells, etc.) have seen their efficiency boosted thanks to the engineering of materials in the nanoscale. The efficiency of these processes depends on many factors. Among them, increasing the surface to volume ratio (SVR) of the active materials is sought after in most energy conversion technologies. Moreover, large active surface areas are also targeted for highly sensitive (bio)sensors. Hence, optimizing the surface properties of nanostructured materials is of great interest.
In this work, we explored different electrochemical deposition procedures and experimental conditions to obtain distributions of Pt nanostructures with large surface areas. Furthermore, to evaluate the available Pt surface, in addition to conventional hydrogen underpotential deposition (H UPD), we used high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and quantitative 3D electron tomography [1, 2].
Pt dendritic nanoparticles (NPs) with large surface area were successfully electrodeposited using a potentiostatic double-pulse procedure under forced convection. Large nucleation overpotential led to a large surface coverage of roughened spheroids, providing a large roughness factor (Rf) but low mass-specific electrochemically active surface area (EASA). Lowering the nucleation overpotential led to highly porous Pt NPs with pores stretching to the center of the structure. At the expense of smaller Rf, the obtained EASA values of these structures were in the range of those of large surface area supported fuel cell catalysts. The active surface area of the Pt dendritic NPs was measured by electron tomography, and it was found that the potential cycling in the H adsorption/desorption and Pt oxidation/reduction region, which is generally performed to determine the EASA, led to a significant reduction of that surface area due to a partial collapse of their dendritic and porous morphology. Interestingly, the extrapolation of the microscopic tomography results in macroscopic electrochemical parameters indicated that the surface properties measured by H UPD were comparable to the values measured on individual NPs by electron tomography after the degradation caused by the H UPD measurement. These results highlighted that the combination of electrochemical and quantitative 3D surface analysis techniques is essential to provide insights into the surface properties, the electrochemical stability, and, hence, the applicability of these materials. Moreover, it indicated that care must be taken with widely used electrochemical methods of surface area determination, especially in the case of large surface area and possibly unstable nanostructures, since the measured surface can be strongly affected by the measurement itself.
[1] B. Geboes, J. Ustarroz, K. Sentosun, H. Vanrompay, A. Hubin, S. Bals, T. Breugelmans, ACS Catalysis, 6 (9) (2016) 5856-5864.
[2] J. Ustarroz, B. Geboes, H. Vanrompay, K. Sentosun, S. Bals, T. Breugelmans, A. Hubin, ACS Applied Materials & Interfaces, 9 (19) (2017) 16168-16177.