Porous PtPd alloy nanotubes: towards high performance electrocatalysts with low Pt-loading†
The electrochemical properties of Pt-based 1D electrocatalysts reported so far prove to be superior to those of known 0D nanomaterials. Particularly, the combination of Pt and Pd either as nanoalloys or Pt-coated Pd nanotubes (NTs) demonstrably boosts the catalyst performance. This provided the main impetus for the present work. The porous PtPd-NTs are processed into the nanopores of anodized aluminum oxide (AAO) template films using electrodeposition from an electrolyte containing appropriate concentrations of Pt and Pd ions and a small amount of a non-ionic surfactant. The range of PtPd alloys investigated is from 95 to 0 at% Pd. The deposition conditions allow diffusion-kinetics controlled growth along the AAO nanopore walls. The NTs with a wall thickness of approximately 15 nm exhibit closely-knit nanoparticles that are separated by mesopores. The salient result of this work is the strong dependence of the electrochemical properties on the Pt content with a maximum of 10 at% Pt. Below and beyond this value, both the electrochemical active surface area (ECSA) and the specific electrocatalytic activity towards methanol electrooxidation substantially decrease. The best ECSA values obtained amount to 138 m2 gmetal−1. Even for the 5 at% Pt alloy, the ECSA value obtained is higher than that of Pt-black and pure Pt-NTs. These results are unsurpassed in comparison to similar nanostructures. The catalytic performance for methanol electrooxidation also behaves in the same way with a maximum of 932 A gPt−1 for the 10 at% Pt alloy NTs. Our results show a practical way to boost the catalyst performance while keeping the Pt content low. They are discussed in terms of structure and morphology effects, and rationalized using DFT calculations that show that in a Pd50 : Pt50 mixture, CO preferentially adsorbs on Pd top-sites, due to charge transfer from Pd to Pt, thus keeping the active Pt sites free from intermediate adsorbates.