Self-Terminated Electrodeposition of Ultrathin Pt Films on Ru: A Promising Way for Preparing Core-Shell Ru-Pt Catalysts
Authors: Liu Yihua , Moffat Thomas P. , You Hoydoo 
Published: February 2020

The high cost and limited supply of Pt have been a major impediment to large-scale commercialization of fuel cells technologies, where satisfied device performance usually requires a large quantity of Pt catalysts to generate electricity from chemical reactions.  In pursuit of lower-cost catalysts, core-shell design has gained strong attraction, given that Pt usage is limited only to an ultrathin shell.  Despite extensive investigation, development in core-shell catalysts continues facing formidable challenges.  In recent works [1-2], we have reported an inexpensive and simple electrodeposition process for preparing core-shell Pt catalysts.  A unique feature of this method is that Pt growth becomes self-terminated upon depositing a monolayer film.  So far, the reports have been limited to deposition on Au and Ni.  To exploit the full potential of this new process, we need to extend its application to a wider range of substrate materials.  

This talk will highlight results from Pt electrodeposition on Ru substrate electrodes.  The growth was controlled by a pulsed potential method using an aqueous electrolyte consisting of 0.5 M NaCl + 3 mM K2PtCl4.  After the deposition, Ru electrodes were characterized by an array of surface-sensitive techniques.  Fig. 1 shows XPS-derived Pt film thickness as a function of deposition time for growth at - 0.8 VSCE.  Following a steady rise in the first 30 s, Pt films essentially stop growing upon reaching a monolayer thickness.  Characterized by hydrogen underpotential deposition (H upd), a Ru electrode covered with a monolayer-thick Pt film exhibits Pt-like voltammetric features (Fig. 2).  In agreement with H upd results, low energy ion scattering spectroscopy (LEISS) reveals that a monolayer-thick Pt deposit covers about 75% of a Ru surface (Fig. 3).   Deviation from an ideal self-terminating growth has been observed and will be discussed. 

A deposition procedure that enables a layer-by-layer Pt growth will also be presented.  Precise controls over Pt coverage and overlayer thickness will be highlighted.  The last part of the talk will be focused on the catalysis of Pt thin films toward oxygen reduction reaction (ORR) and small organic molecule oxidations. 

References:

[1] Y. Liu, D. Gokcen, U. Bertocci, T. P. Moffat, Science, 338, 1327 (2012).

[2] Y. Liu, C. M. Hangarter, D. Garcia, T. P. Moffat, Surf. Sci. 631, 141 (2015)

Figure 1. Thickness of Pt overlayers grown at -0.8 VSCEon Ru electrodes as a function of deposition time.  The film thickness was derived from the analysis of Pt 4f / Ru 3d intensity ratios as given by x-ray photoelectron spectroscopy (XPS). The inset shows typical Pt 4f and Ru 4d XPS spectra from Ru electrode with a monolayer-thick Pt deposit.   

Figure 2. Cyclic voltammetric study of electrodes in a deoxygenated 0.1 M HClO4 at 50 mV/s, showing a Pt-like surface for Ru electrode that has a monolayer-thick Pt deposit.  

Figure 3.  Coverage of Pt on Ru electrodes as a function of deposition time for growth at -0.8 VSCE.  The coverage was determined based on the analysis of Pt and Ru ion scattering intensities.  Typical LEISS spectra are shown in the inset.

Figure 1