Furthermore, it is essential to understand the possible mechanism for the electrochemical oxidation of nitrite at the Ni-PDCA/SPCE. Referring to the structural evidence confirmed by XRD and FT-IR, the Ni-PDCA structure does not contain open metal sites due to two water molecules coordinated to the Ni center. Therefore, Ni centers might notactively participate in the electrochemical oxidation of nitrite through direct charge transfer from nitrite to Ni2+. Another possible way for the Ni-PDCA structure to interact with nitrite is direct charge transfer between the nitrite and the ligand. The ligand H3PDCA has carboxylate and pyrazole functionalities.