Configuration-Dependent Oxygen Reduction Reaction Performance of Iridium-Decorated Ni@Pd Nanocatalysts

Abstract

The widespread market introduction of fuel cells is severely restricted by the sluggish oxygen reduction reaction (ORR) kinetics at the cathode side, which demands highly efficient and durable catalysts for generating adequate rates. By keeping this in view, herein, we report a systematic study to unveil the effect of structure configuration on the ORR activity of tri-metallic nanocatalysts (NC)s comprising atomic iridium (Ir) cluster (0.5 wt % of Ir)-anchored Pd nanoparticles (NP)s on the tetrahedral symmetric Ni-oxide support (denoted as NPI), prepared via a temperature-controlled wet chemical reduction method. For the optimum condition when the impregnation temperature for Ni-crystal growth is 40 °C, the Ir-cluster-decorated Ni@Pd nanoislands are formed (hereafter denoted as NPI-40), outperforming the commercial Johnson Matthey-Pt/C (J.M.-Pt/C; 20 wt % Pt) catalyst by 181-fold with an unprecedented high mass activity of 12,163 mAmgIr–1 at 0.85 V vs RHE in alkaline ORR (0.1 M KOH). More importantly, NPI-40 NC retained 100% ORR performance with no degradation up to 20,000 accelerated degradation test (ADT) cycles, while only a 5 mV loss in half-wave potential (E1/2) is observed after 30 k ADT cycles. Besides, Ir-cluster-decorated Nicore@unconformable Pdshell and Ni-to-Pd epitaxial structures are formed at 25 °C (NPI-RT) and 70 °C (NPI-70) impregnation temperatures, respectively, showing the significantly decreased mass activities of 997 and 5146 mAmgIr–1 at 0.85 V vs RHE. The results of physical structure inspections and electrochemical analysis suggest that such an exceptional ORR performance of NPI-40 NC originates from the potential synergism between the high density of the occupied Ir-d orbital and the adjacent compressively strained Ni–Pd interface, where Ir sites offer optimal adsorption energy for O2 splitting and the Ni–Pd interface facilitates the subsequent desorption of hydroxide ions (OH–). We believe that the obtained results will open new avenues for the facile design of Pt-free NCs for fuel cell cathodes and will benefit fuel-cell technology via both ecologically and economically friendly means.