The drive to adopt alternative energy technologies for a more sustainable energy future has been inhibited by insufficient activity, poor durability and high cost of fuel cell catalysts. To overcome these drawbacks there has been significant interest in the exploration of core-shell nanoparticles, related overlayer thin films and alloy architectures. These bimetallic systems offer significant tunability based on ligand effects associated with nearest neighbor bonding ¹, strain effects associated with lattice mismatch between the constituent metals , and geometric effects dictated by the local atomic distribution of elements. In order to realize such effects significant effort is underway to develop inexpensive, easy fabrication methods for forming these alloy and over-layer architectures ^2-7.

Recently, self-terminated electrodeposition reactions have been discovered and adopted for the fabrication of ultra-thin metal films with monolayer level control ^8-11. For the electrodeposition of Pt and/or Ir on Au and Ni¹² the deposition of growing metal at high overpotentials is quenched at the onset of hydronium reduction by hydrogen adsorption. In this work, the electrodeposition of Pt-Rh alloys at potentials coincident with hydronium reduction will be discussed. In contrast to Pt, the electrodeposition of Rh is not terminated by hydrogen adsorption. The introduction of minimal quantities of [RhCl₆]^3- into the NaCl-[PtCl₄]^2- electrolyte disrupt the quenching of the Pt partial deposition reaction. The impact of Rh concentration on deposition rate, film morphology and composition was examined with voltammetry, SEM, XPS, and ISS. Finally, the tunability of as deposited films of varied thickness and composition toward the ethanol oxidation reaction will be discussed.

(1) Bligaard, T.; Norskov, J. K. Electrochimica Acta 2007, 52, 5512.

(2) Nutariya, J.; Fayette, M.; Dimitrov, N.; Vasiljevic, N. Electrochimica Acta 2013.

(3) Viyannalage, L. T.; Vasilic, R.; Dimitrov, N. Journal of Physical Chemistry C 2007, 111, 4036.

(4) Brankovic, S. R.; Dimitrov, N.; Sieradzki, K. Electrochemical and Solid- State Letters 1999, 2, 443.

(5) Brankovic, S. R.; Wang, J. X.; Adzic, R. R. Surface Science 2001, 474, L173.

(6) Ambrozik, S.; Dimitrov, N. Electrochimica Acta 2015, 169, 248.

(7) Adzic, R. R.; Zhang, J.; Sasaki, K.; Vukmirovic, M. B.; Shao, M.; Wang, J. X.; Nilekar, A. U.; Mavrikakis, M.; Valerio, J. A.; Uribe, F. Top Catal 2007, 46, 249.

(8) Ahn, S. H.; Tan, H.; Haensch, M.; Liu, Y.; Bendersky, L. A.; Moffat, T. P. Energy Environ. Sci. 2015, 8, 3557.

(9) Liu, Y.; Gokcen, D.; Bertocci, U.; Moffat, T. P. Science 2012, 338, 1327.

(10) Ritzert, N. L.; Moffat, T. P. J. Phys. Chem. C 2016, 120, 27478.

(11) Wang, R.; Bertocci, U.; Tan, H.; Bendersky, L. A.; Moffat, T. P. J. Phys. Chem. C 2016, 120, 16228

(12) Liu, Y.; Hangarter, C. M.; Garcia, D.; Moffat, T. P. Surface Science 2015, 631, 141.