Metalizing carbon nanotubes with Pd–Pt core–shell nanowires enhances electrocatalytic activity and stability in the oxygen reduction reaction

We describe an electrostatically induced self-assembly method to prepare ultrathin Pd nanowires (NWs) surrounding individual multiwalled carbon nanotubes, i.e., PdNW/MWNTs, that are noticeable for improving performance in the oxygen reduction reaction (ORR) of their supported PtML electrocatalyst. The carbonaceous by-products in MWNTs, rather than the nanotubes themselves, are modified with the oxygenated terminals to allow the negatively charged and hydrophilic surface while retaining the intrinsic nature of the MWNTs. Encompassing the nanotubes' length are 2-nm-thick Pd NWs that are closely packed and homogeneously dispersed due to the unique processes for preparing PdNW/MWNTs and its components. Although the crystal lattice of the Pd NWs expands somewhat, which should cause an unfavorable interaction with supported PtML, this adverse effect is counterweighed by the shape-determined features of Pd NWs, including their high specific surface area, excellent contiguousness, and low-energy atomic configuration. Consequently, these distinct chemical and physical properties substantially expedite the desorption of the intermediates to refresh the active centers during the reduction of oxygen with the PtML electrocatalyst while ensuring a desirable electron transfer rate, so improving the overall ORR kinetics. Indeed, PtML/PdNW/MWNTs exhibits the Pt mass and specific activities of 1.45 A/mgPt and 0.65 mA/cm2 Pt, respectively, each of which are several times those of the Pt/C and even higher than those of the PtML supported on Pd nanoparticles. These high activities remained over a long-term stability test using the latest US Department of Energy-established protocol.