Self-terminating Electrodeposition of Pt on WC Electrocatalysts
Thomas P. Moffat
Self-terminating electrochemical deposition is used to grow Pt nanoparticles on tungsten monocarbide (WC) from a pH 4 electrolyte containing 3 mmol/L K2PtCl4 - 0.5 mol/L NaCl. An unconventional potentiodynamic deposition program is used where nucleation is promoted at large overpotentials followed by growth termination at still larger overpotentials to yield a high coverage of Pt nanoparticles on WC with minimal substrate oxidation. Following three Pt deposition cycles between -0.8 VSCE and -0.45 VSCE, corresponding to a monolayer equivalent charge, the surface is covered by ≈ 3 x 1011 particles/cm2 that are ≈ 6.7 ± 1.1 nm in diameter. The number and size of nanoparticles increase monotonically during the first five deposition cycles. With further cycling a growth instability leads to the development of a sub-population of (100) faceted platelets. Area-normalized kinetic parameters for hydrogen evolution (HER) and oxidation (HOR) on Pt-WC were determined in 0.5 mol/L H2SO4. For the lowest surface coverage of Pt nanoparticles on WC, ≈ 0.01, an exchange current density of ≈ 100 mA/cm2 is achieved similar to the highest reported values for Pt nanoparticles and ultramicroelectrodes. The area normalized exchange current density decreases with increasing Pt coverage as the relative contribution of point versus planar diffusion decreases. This work demonstrates that electrodeposition of Pt during potentiodynamic cycling at large overpotentials provides an attractive approach to achieving ultra-low loadings of well-dispersed Pt nanoparticles on a non-precious metal support like WC.