Developing efficient catalysts for the oxygen reduction reaction (ORR) to reduce cathode Pt loading without sacrificing the performance has been under intensive research.

To achieve renewable energy technologies, low-cost electrocatalysts for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are required to replace Pt and IrO2/RuO2 catalysts.

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Pt/PdAl, which comprises a PdAl core protected by a Pt monolayer, may experience Al dealloying because of the strong affinity of Al toward O. To circumvent this issue, the Pt/Os/PdAl catalyst has been designed to suppress the migration of Al by inserting an Os monolayer at the interface between the PdAl core and two Pt monolayers. On the basis of segregation energies, Al leaching from the core to the 1st layer is determined to be endothermic even under O coverage, indicating an energetic preference for Al to reside in the core structure. The Pt/Os/PdAl catalyst benefits from the energetic disadvantage of the inward movement of Os and the presence of the 2 ML Pt layer. As an ORR electrocatalyst, the relatively weak adsorption ability of Pt/Os/PdAl suggests improved ORR activity. Finally, a representative OOH association mechanism with low reaction barriers of 0.46, 0.31, 0.38 and 0.41 eV for the OOH formation, OOH dissociation, OH formation and HO formation steps suggests that the catalyst can effectively activate the O-O bond and eliminate OH, which can act as a catalytic poison. These findings suggest the design of stable sandwich catalysts as potential candidates for ORR electrocatalysis.

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Polymer electrolyte membrane fuel cells (PEMFCs) are one of the most prominent clean energy technologies designed to achieve hydrogen utilization and solve problems such as low efficiency and high pollution associated with fossil fuel combustion. In order to bring about PEMFC commercialization, especially for automobile applications, developing high-activity and -selectivity catalysts for the oxygen reduction reaction (ORR) is of critical importance. Based on the density functional theory, the catalytic activity of the conductive, two-dimensional metal–organic frameworks TM3(HITP)2 monolayer (where HITP = hexaiminotriphenylene; TM = Ni, Co, Fe, Pd, Rh, Ru, Pt, Ir, and Os) for ORR has been investigated systematically. Furthermore, the classical volcano curves of the ORR activity, as a function of the OH binding, are found where the Ni, Pd, and Pt located at the weak binding side suffer from the sluggish *OOH formation and prefer the inefficient 2e− mechanism, while for other elements belonging to the strong binding side, the reactions are hindered by the poison due to ORR intermediates. Based on the free energy profiles, the corresponding overpotentials μORR exhibit the inverted volcano curve as a function of the atomic number of the 3d/4d/5d TM active center in the same period. Based on the μORR data, ORR activity decreases in the order of Ir > Co ≈ Rh > Ni ≈ Pd > Pt ≈ Fe > Ru > Os. Herein, the Co, Rh, and Ir central atoms exhibit enhanced catalytic activity in combination with the desirable selectivity of the O2 reduction to H2O. This systematic work may open new avenues for the development of high-performance non-PGM catalysts for practical applications of ORR.

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