(Invited) Growth and Catalytic Performance of Pd and Pd-Pt Alloy Ultrathin Films on Au By Surface Limited Redox Replacement of H UPD Layers in One-Cell Configuration
Authors: Dimitrov Nikolay , Achari Innocent , Ambrozik Stephen 
Published: February 2020

In the ongoing effort of solving the energy problem of humanity the design, fabrication and testing of catalysts has been playing a central role for the last couple of decades. Besides the known application of Pd hydrogen gas storage a growing interest has been seen in the use of this metal and its alloys in catalytic reactions with relevance to fuel cells. While Pt is still considered best catalyst for fuel cell applications its high cost and limited supply calls more and more for intensifying the effort of replacing Pt with Pd. Means of growing Pd on conductive materials include a variety of expensive vacuum deposition approaches that need sophisticated equipment and high temperature. An alternative to those is electrodeposition that besides the standard bulk deposition, offers a variety of ultra-thin film growth approaches that could facilitate the deposition of uniform and flat continuous Pd films on polycrystalline Au. Among those, the electrochemical atomic layer deposition of Pd under optimum conditions was first developed for automated flow cell.

Our group has been working recently on developing fine-tuned approaches for the growth of epitaxial Pd on Au using always readily available one-cell configuration instead, of sophisticated automated flow cells. The growth uses surface limited redox replacement (SLRR) reaction of underpotentially deposited (UPD) Cu or H sacrificial layers. After a set of iteratively applied deposition cycles, the as-deposited Pd or Pt-Pd films are tested for surface roughness using characteristic HUPD and CuUPD cyclic voltammetry (CV) curves and the efficiency of the deposition technique is measured by stripping charge determination. Also, XPS characterization was used for determining both, the alloy composition in the case of Pt-Pd deposition and the presence / absence of impurities incorporating in the deposit in the case of pure Pd growth. The quasi-2D growth resulting in smooth and uniform Pd-film morphology has also been confirmed up to 20 SLRR cycles by in-situ STM. Consequently, the accordingly deposited films at minimum thickness are tested for catalytic activity and durability in formic acid oxidation reactions.

In this presentation results of this work will be presented in detail and critically discussed. The emphasis is on the illustration of deposition approach that has been developed to take place in one cell and in the specifically in the case of SLRR of HUPD results in a theoretically contamination free deposits. It is also demonstrated that this approach provides for maximum deposition efficiency engaging also adsorbed H in the replacement scenario. The CV results indicate the deposition of smooth Pd films taking place for up to 30 SLRR cycles for HUPD and CuUPD sacrificial layers, respectively. This is followed by a rapid transition to dendritic growth at higher thickness. The underlying kinetic reasons for such abrupt transition are also considered and critically discussed. Finally, the applicability of the proposed facile SLRR approach has been demonstrated through testing for catalytic activity and durability in formic acid oxidation of ultrathin films of Pd, Pt and Pt-Pd alloys with different composition. The critical analysis of the testing outcome suggested that a 1:2 Pt-Pd alloy exhibits activity and durability that are comparable with those of pure Pt and substantially better to those of pure Pd.