However, the carboxylate groups have built MOF structures by forming hydrogen bonds with the Ni centers.Thus, it is assumed that nitrite electrooxidation takes place at the pyrazole ring. Considering structural and experimental findings, a possible mechanism, as presented in Fig. 4, is proposed for nitrite oxidation at NiPDCA/SPCE. At first, a proton interacts with the protonated nitrogen (N2) of the pyrazole ring, followed by the interaction of nitrite at thepositively charged N2 site under the given electrochemical conditions. Next, one H2O molecule interacts with the nitrite attached to the pyrazole ring, and then nitrite is electrochemically oxidized to nitrate Eqs.1-(3). The Ni-PDCA is denoted as [A] in the following equations