A comparison of a tetradentate schiff base ligand and its Ni(II) complex in terms of their electrocatalytic activity for reduction of CO2

Electrochemical activation of carbon dioxide, CO2, mediated by homogeneous catalysis has been a major issue during recent years [1]. This is due to the high overpotential of CO2 reduction at the surface of ordinary electrodes [2]. As a matter of fact, studying the electrocatalytic activity of new electron transfer mediators is useful, not only to decrease the overpotential of CO2 reduction but also to increase the catalytic current [3].In this regard, the present paper reports the electrocatalytic activity of a Schiff base, N,N-bis(2-hydroxy-1-naphthaldehyde)-m-phenylenediamine, NMPD, and its Ni(II) complex for CO2 reduction. The overpotential of CO2 reduction was reduced for about 700 mV and 920 mV in the presence of NMPD and its Ni(II) complex respectively. However, in the presence of NMPD, a higher increase was observed in the catalytic current of CO2 reduction with respect to the Ni(II) complex. By chronoamperometry, the homogeneous catalytic rate constant, k, for the electron transfer between the NMPD ligand and CO2 as well as the Ni(II) complex and CO2 was found to be 24.90 ± 0.98 M-1 s-1 and 0.71 ± 0.05 M-1 s-1 respectively. Furthermore, the diffusion coefficients, D, of the NMPD ligand and the Ni(II) complex in the DMF solvent were also determined as 9.8 ×10-6 cm2 s-1 and 8.9 ×10-6 cm2 s-1 respectively. The results show that the electrocatalytic reduction of CO2 is both ligand-based and metal-based. Also, the electrocatalytic behavior of the free ligand toward CO2 reduction is better than that observed for the central metal of the Ni(II) complex.