The Activity Improvement of the TM₃(hexaiminotriphenylene)₂ Monolayer for Oxygen Reduction Electrocatalysis: A...

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 μ_ORR 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 μ_ORR 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.