Substitution of Fe in hydroxyapatite as an efficient single-atom catalyst for oxygen reduction reaction in biofuel cells: A first-principles study

Highlights: • The detailed behaviors of the ORR on Fe@HAP were first time presented. • ORR prefers to four-electron pathway on Fe@HAP • The magnetism of the single Fe atom plays an important role for ORR. • Fe@HAP is an efficient single-atom biological catalyst for ORR in biofuel cells. Single-atom biological catalysts have attracted enormous attentions for low cost, high stability and catalytic activity in therapeutics and biomedical devices. The properties of Fe atom substituted Ca_II in HAP surface (Fe@HAP) and Fe adsorbed over HAP surface (Fe/HAP), and the detailed kinetic and thermodynamic behaviors of the oxygen reduction reaction (ORR) processes have been investigated by using the first-principles study. It is found that Fe dopant by substituted Ca_II ion into HAP surface could be more stably anchored at HAP surface than Fe adsorbate, and the introduced Fe dopant can enormously increase the endogenous ORR catalytic activity of magnetic HAP catalysts. Furthermore, the ORR process on Fe@HAP prefers to the 4e^- pathway with the small reaction barrier (0.73 eV) for the rate-limiting step, and the two OH species formed from the hydrogenation of the adsorbed O₂ prefer to further hydrogenate into two H₂O molecules and their self-dehydrogenation is not preferable according to the transition states simulation, which is also confirmed by the Gibbs free energy calculation. Consequently, our results revealed that the Fe@HAP could be an efficient catalyst as cathode material in the applications of the implantable biofuel cells and shed light on the design of biomaterials for various applications.