Resumen de Electrodeposició d'electrocatalitzadors nanoestructurats de base níquel-platí per a sistemes d'energia d'hidrogen
Electrodeposition is a powerful and facile, yet sophisticated method for the synthesis of nanostructured electrocatalysts. The electrodeposition of nickel-platinum (Ni-Pt) thin films from aqueous media leads to the formation of nanocrystalline, single-phase alloy films with homogeneous composition. Remarkably, the composition is tuneable by simple variation of the potential used in potentiostatic deposition, in a range between 99 at% and 58 at% Ni, i.e. 1-42 at% Pt. With the addition of the amphiphilic block copolymer Pluronic P-123 in the electrolyte, micelle-assisted electrodeposition yields homogeneously mesoporous thin films with a uniform pore size of approx. 10 nm. With respect to dense films synthesised using the same parameters, the mesoporous thin films show identical composition and crystallite size. All electrodeposited Ni-Pt thin films exhibit stable performance during 200 cycles of hydrogen evolution reaction (HER) in 0.5 M H2SO4, where Ni84Pt16 showed the highest activity with an overpotential of -90 mV at -10 mA/cm2.
24 h galvanostatic long-term measurements show that this overpotential initially decreases over time before it stabilises. Furthermore, the application of anodic potentials in 0.5 M H2SO4 provokes failure of the Ni-rich thin films due to metal dissolution. Thus, reliable long-term application in acidic media is only possible at cathodic potentials. In turn, the Ni-Pt alloy thin films are stable over a wide potential range in 1 M NaOH, and keep their catalytic activity below an irreversible passivation around 1.0 V vs Ag|AgCl. In addition, the reversible redox reaction Ni(OH)2<->NiOOH is observed at lower anodic potentials. Notably, this reaction, which is of use in electrochemical supercapacitors, is enhanced by increasing the platinum content, showing that the Pt in the alloy catalyses this redox reaction.
Another potential application of single-phase Ni-Pt alloy thin films are magnetic nano- or microelectromechanical systems, where magnetic and mechanical properties are important. Due to the single-phase nature of the alloy films, saturation magnetisation and Curie temperature scale directly with the Ni content. Interestingly, the same is true for the coercivity of the mesoporous thin films, allowing thus to tune all of these parameters by the electrodeposition potential. A Curie temperature near room temperature is found for Ni58Pt42.
For application at oxygen reduction reaction (ORR), Pt-Ni and Pt-Ni-Mo nanoparticles are electrodeposited onto a gas diffusion layer for proton exchange membrane fuel cells (PEMFC), unifying synthesis and application of the electrocatalyst onto the electrode in a single step. Particle sizes ranging from 40 to 80 nm are obtained for particles with different compositions. The pulse electrodeposition process accomplishes the deposition of nanoparticles onto the most active sites of the substrate leading to very high mass activities at ORR, the highest of which were observed for Pt67Ni33 and Pt66Ni32Mo2. These compositions also showed the best durability in PEMFC.
