Oxygen Reduction Reaction of Tetraaza[14]Annulene Iron Complexes

Abstract

Introduction Alkaline fuel cells (AFC) have attracted much attention because non-Pt-based catalysts can be potentially applied. Metallocomplex-based catalysts are the one of the most prospective candidates of non-Pt-based oxygen reduction reaction (ORR) catalysts. These catalysts have several advantages over Pt catalysts. First, since active sites consist of isolated metal atoms in these catalysts, metal resource can be used economically. Second, the catalytic activity can be improved by modifying structure of the ligand. Among these metallocomplexes, iron complexes-based catalysts are intensively studied because of low cost and relatively high activity. We have been particularly interested in tetraazaannulene (TAA) iron complexes. The TAA complex is one of the N4-type macrocyclic complexes, as shown in Fig. 1. The main advantage of TAA ligands is that the molecular size is smaller than those of other conventional N4macrocyclic ligands such as porphyrins and phthalocyanines. The density of small molecules on an electrode can be increased. Furthermore, the substituents would have strong electronic effects on the active sites due to the small molecular size. Especially, the introduction of a substituent into α- or β- carbon located at diketiminate motif (Fig. 1) is believed to give large electronic effects on the active center. This indicates that the ORR activity of TAA complexes might be enhanced by the introduction of suitable substituents. Experimental studies on ORR activities of iron complexes of TAA have not been conducted to date, while computational studies reported that iron complexes of TAA and its derivatives have good ORR activity1. In this work, we investigate the redox property and ORR of the iron complexes ligated by TAA (Fig. 1 (a)) and TAA-NO2 (an electron withdrawing derivative, Fig. 1 (b)) in alkaline conditions, and discuss the effect of the ligand structure on the electrochemical behavior. Experiments The ligands of TAA2 and TAA-NO2 3 were synthesized according to the reported procedures. The iron complexes abbreviated as Fe-TAA4 and Fe-TAA-NO2 were synthesized by refluxing the ligand with FeCl2･4H2O in an organic solvent in an inert atmosphere. The iron complex-modified carbon materials were prepared as follows. Ketjenblack (KB) was added to the solution of the iron complexes. The mixture was evaporated to dryness. The hydrodynamic voltammetry using a rotating ring(Pt)-disk(GC) electrode was performed in 0.1 M NaOH under 25 °C. A Pt coil and a reversible hydrogen electrode (RHE) were used as a counter electrode and a reference electrode, respectively. The iron complex-modified KB (Fe-TAA/KB or Fe-TAA-NO2/KB) was immobilized on a glassy carbon (GC) electrode. Results and discussion The ORR activities of the Fe-TAA/KB and Fe-TAA-NO2 were evaluated in the alkaline aqueous solution. Strong activities were observed; the onset potential, at which the value of the catalytic current is 0.2 mA cm- 2, reached 0.94 V (Fe-TAA) and 0.97 V (Fe-TAA-NO2) vs. RHE. The onset potential was increased by the introduction of the -NO2 substituents. In the both of the cases, few H2O2was detected at ring electrode. This indicates that the numbers of electron transfer number of ORR by these catalysts are close to 4. The correlation between the onset potentials and redox potentials will also be discussed. Acknowledgement The authors are grateful to Tokuyama Corporation for providing the anion exchange resin (AS-4). References 1) A.L. Pereira Silva, L.F. de Almeida, A.L. Brandes Marques, H.R. Costa, A.A. Tanaka, A.B. Ferreira da Silva, and J.d.J. Gomes Varela Junior, Journal of Molecular Modeling, 20, 2131-2140 (2014). 2) Y.G. Yatluk, and A.L. Suvorov, Chemistry of Heterocyclic Compounds, 23, 316-320 (1987). 3) N. Nishiwaki, T. Ogihara, T. Takami, M. Tamura, and M. Ariga, J. Org. Chem., 69, 8382-8386 (2004) 4) Sustmann, R., Korth, H. G., Kobus, D., Baute, J., Seiffert, K. H., Verheggen, E., Bill, E., Kirsch, M., de Groot, H. Inorg. Chem., 46, 11416-11430 (2007) Figure 1