Electrocatalytic Oxygen Reduction Reaction over the Au22(L8)6 Nanocluster with Promising Activity: A DFT Study

The sub-nanocluster metal catalysts exhibit outstanding activities for a variety of catalytic reactions. Here, from density functional theory (DFT) simulations, we systematically explored the potential of an experimentally well-defined Au-22(L-8)(6) nanocluster (where L-8 = 1,8-bis(diphenylphosphino)) as an electrocatalyst to accelerate the sluggish kinetics of the oxygen reduction reaction (ORR) in fuel cells. Especially, the Au-22(L-8)(6) nanocluster with the exposure of in situ coordination unsaturated (cus) Au is able to avoid the block effect of the ligands and exhibits excellent activity in electrocatalysis. The DFT results showed that the cus Au atoms are the active sites for O-2 activation, and all of the ORR intermediates can favorably bind with the cus Au. Notably, the 2e(-) reduction on Au-22(L-8)(6) is unfavorable since the generated H2O2 species would decompose into two OH species spontaneously. Instead, ORR prefers to proceed via the 4e(-) pathway to form H2O and the predicted overpotential is 0.49 V, which is comparable to that of the Pt-based catalysts (0.40 V). The extended gold surfaces, e.g., Au(111), Au(100), Au(110), and Au(211), are also investigated for comparison. Their ORR activities are found to be much lower than that for Au-22(L-8)(6). The activity trend, Au-22(L-8)(6) (0.49 V) > Au(211) (0.70 V) > Au(110) (0.82 V) > Au(100) (0.92 V) > Au(111) (1.22 V), was justified by the geometry descriptor of the generalized coordination number of Au. Our results show the promising activity of Au-22(L-8)(6) for ORR and would stimulate the design of novel and highly efficient sub-nanocluster electrocatalysts.