Creation of a Highly Active Pt/Pd/C Core–Shell-Structured Catalyst by Synergistic Combination of Intrinsically High Activity and Surface Decoration with Melamine or Tetra-(tert-butyl)-tetraazaporphyrin

Abstract

A Pt/Pd/C core−shell-structured catalyst with an extraordinarily high activity of 3625 A g −1 -Pt at 0.9 V versus a reversible hydrogen electrode (RHE) for the oxygen reduction reaction (ORR) was created by a synergistic combination of intrinsically high activity and surface decoration with melamine or tetra-(tert-butyl)-tetraazaporphyrin (tBuTAP). The intrinsically highly active Pt/Pd/C catalyst with an ORR mass activity of ca. 1000 A g −1 -Pt at 0.9 V versus RHE was synthesized by a direct displacement reaction method, and the activity was activated to ca. 1500 A g −1 -Pt via a high activation protocol (HAP) (a rectangular potential cycling of 0.05−1.0 V vs an RHE conducted in 0.1 mol dm −3 HClO 4 under an Ar atmosphere at 80 °C). The ORR activities of the Pt/Pd/C catalysts were further increased by surface decoration with melamine or tBuTAP, and the mass activity of ORR of the catalyst activated by an HAP was enhanced to 3625 A g −1 -Pt at 0.9 V versus RHE via the melamine decoration, which is about 11-fold that of a reference Pt/C catalyst (320 A g −1 -Pt). Cyclic voltammograms, linear sweep voltammograms, and CO stripping voltammograms indicated that the surface decoration further destabilized the OH species (OH ad ) adsorbed on the Pt shell and suppressed the formation of oxide on the Pt shell, which could explain the extraordinary enhancement of ORR activity. We present guidelines for creating Pt-based catalysts with extraordinarily high activity for the ORR by the synergistic combination of the intrinsically active Pt-based catalyst [improvement from the inside of Pt nanoparticles (NPs)] and surface decoration (improvement from the outside of Pt NPs).

Catalyst overcoating engineering towards high-performance electrocatalysis

“…Depending on the molecular size of reactants and/or spectator species, the coating overlayer can be rationally constructed to control their transport on/to the metal surface. As such, extremely high catalytic selectivity toward the HOR vs. ORR (under shut-down/start-up conditions in H 2 -O 2 fuel cells) 63,64,260,261,265 and the ORR vs. MOR (the case associated with methanol crossover in DMFCs) 51,54,56,75,[219][220][221] has been achieved. ORR and HOR catalytic activity can be additionally enhanced because of the abundant active sites free from blocking by poisoning species.…”

“…2c, d and f). [58][59][60][61][62][63][64] As such, chemically adsorbed or assembled organic molecules with tunable coverage on the surface of the metal are generated. Similar to the direct colloidal reduction synthesis, this strategy relies on the fact that organic molecules are prone to chemically coordinate with metal entities on the surface.…”

“…2e). 64 2.1.3 Overcoating of metal nanoparticles with polymer layers. Polymeric coatings with high ionic conductivity such as polyaniline (PANI) and polypyrrole (Ppy) could be fabricated onto the surface of metal particles as homogeneous thin films (Fig.…”

“…The strong adsorption of OH ad on the metal surface causes the sluggish ORR kinetics. 64,223 Metal intrinsic structural properties with respect to their electronic structures strongly determine the adsorption of OH ad . Electron delocalization on Pt is thus reasonable for changing metal affinity with O ad and OH ad species and enabling the regulations on ORR catalytic activity.…”

“…As earlier reported by Markovic ´'s group, patterning Pt with a self-assembled monolayer of calix [4]arene molecules featuring a wide rim structure endowed the catalyst with a high selectivity for the HOR. 63,64 High occupation of Pt by the calix layer could not change HOR activity since it has fast kinetics in an acidic electrolyte. In contrast, the ORR was inhibited because of the insufficient sites for O 2 adsorption and O-O bond cleavage on the Pt surface following the organic patterning.…”