Oxygen Reduction Reaction on Electrodeposited Pt_{100–<i>x</i>}Ni_<i>x</i>: Influence of Alloy Composition and Dealloying

The electrocatalytic activity of electrodeposited Pt_{100–<i>x</i>}Ni_<i>x</i> thin films toward the oxygen reduction reaction (ORR) in perchloric acid was studied for <i>x</i> ranging between 2 and 95. The alloy composition was controlled by the potential applied during deposition. XRD and EDS were used to examine the structure and composition of the films before and after ORR measurements. Significant dealloying was evident for films with <i>x</i> > 45, and substantial shrinkage of the film thickness accompanied dealloying for films with <i>x</i> > 55. The onset of significant shrinkage occurs near the parting limit reported for bulk dealloying of fcc solid solutions. A maximum ORR specific activity of 2.8 mA/cm² at 0.900 V RHE was observed for alloys between Pt₄₅Ni₅₅ and Pt₅₅Ni₄₅. This represents an enhancement factor of 4.7 compared to electrodeposited Pt, thereby matching the best published results reported for Pt–Ni nanoparticles and thin films. A peak ORR mass activity of 0.78 A/mg_Pt at 0.900 V RHE was observed for alloy film compositions between Pt₃₈Ni₆₂ and Pt₄₅Ni₅₅. In comparison to electrodeposited Pt, these films exhibit a 10-fold improvement in mass activity

Self-Terminated Electrodeposition of Ni, Co, and Fe Ultrathin Films

Self-terminated Fe-group metal (Ni, Co, Fe) electrodeposition occurs at potentials negative of the onset of water reduction where OH^– generation leads to the formation of a blocking hydroxide monolayer. Quenching of metal deposition is accompanied by an increase in dissipative energy loss in microbalance experiments attributed to increased hydrogen bonding to the adjacent double layer. Pulse deposition at −1.5 V_SSCE in 5 mmol/L (NiCl₂, CoCl₂, FeSO₄) – 0.1 mol/L NaCl pH 3.0 electrolytes yields fully coalesced ultrathin films of Ni, Co, Fe, or alloys thereof, on Au. The film thickness is controlled by the nucleation, growth, and termination dynamics constrained by the electrochemical cell time constant. Precipitation of bulk Ni­(OH)₂ and related phases is minimized by using short deposition times and dilute metal cation concentrations to limit supersaturation. The rapid deposition of smooth, compact ultrathin Fe, Co, and Ni films should facilitate mechanistic and durability studies of Fe-group metal catalysis and the fabrication of emerging microdevices