Double-Heterostructured Nickel Phosphate-Phosphides as High-Activity Electrocatalyst for Ammonia Borane Electrooxidation

The direct electrooxidation reaction of ammonia borane (ABOR) as the anodic reaction of direct ammonia borane fuel cells (DABFCs) is greatly dependent on the properties of electrocatalysts. Both the active sites and charge/mass transfer characteristics are the key to promoting the processes of kinetics and thermodynamics, which can further improve the electrocatalytic activity. Hence, the catalyst double-heterostructured Ni₂P/Ni₂P₂O₇/Ni₁₂P₅ (d-NPO/NP) with the optimistic redistribution of electrons and active sites is prepared for the first time. The d-NPO/NP-750 catalyst obtained after pyrolysis at 750 °C shows the outstanding electrocatalytic activity toward ABOR with an onset potential of −0.329 V vs RHE which is better than all the published catalysts. The density functional theory (DFT) computations illustrate that the Ni₂P₂O₇/Ni₂P acts as the activity enhancement heterostructure with a high d-band center (−1.60 eV) and the low activation energy barrier, while the Ni₂P₂O₇/Ni₁₂P₅ acts as the conductivity enhancement heterostructure with the highest density of valence electrons.