Cascade Anchoring Strategy for Fabricating High-Loading Pt Single Atoms as Bifunctional Catalysts for Electrocatalytic Hydrogen Evolution and Oxygen Reduction Reactions

Carbon supports containing single-atomically dispersed metal-N_x (denoted as M_SAC-N_xC_y, x, y: coordination number) have attracted increasing attention due to their superb performance in heterogeneous catalysis. However, large-scale controllable preparation of single-atom catalysts (SACs) with high concentration of supported metal-N_x is still a big challenge because of the metal atom agglomeration during synthesis at high density and temperatures. Herein, we report a stepwise anchoring strategy from a 1,10-o-phenanthroline Pt chelate to an N_x-doped carbon (N_xC_y) with isolated Pt single-atom catalysts (Pt_SAC-N_xC_y) containing Pt loadings up to 5.31 wt % measured via energy-dispersive X-ray spectroscopy (EDS). The results show that 1,10-o-phenanthroline Pt chelate predominantly contributes to the formation of chelate single metal sites that bind tightly to platinum ions to prevent metal atoms from aggregating, resulting in high metal loading. The high-loading Pt_SAC-N_xC_y exhibits a low hydrogen evolution (HER) overpotential of 24 mV at 0.010 A cm^-2 current density with a relatively small Tafel gradient of 60.25 mV dec^-1 and excellent stable performance. In addition, the Pt_SAC-N_xC_y catalyst shows excellent oxygen reduction reaction (ORR) catalytic activity with good stability, represented by the fast ORR kinetics under high-potential conditions. Theoretical calculations show that Pt_SAC-NC₃ (x = 1, y = 3) offers a lower H₂O activation energy barrier than Pt nanoparticles. The adsorption free energy of a H atom on a Pt single-atom site is lower than that on a Pt cluster, which is easier for H₂ desorption. This study provides a potentially powerful cascade anchoring strategy in the design of other stable M_SAC-N_xC_y catalysts with high-density metal-N_x sites for the HER and ORR.