Study of M(iii)-cyclam (M = Rh, Ru; cyclam = 1,4,8,11-tetraazacyclotetradecane) complexes as novel methanol resistant electrocatalysts for the oxygen reduction reaction

Abstract

Transition metal macrocyclic complexes have acquired relevance as electrocatalysts to perform oxygen electroreduction in acid media as an alternative to platinum. This work presents two macrocyclic complexes using cyclam as ligand, which has a much simpler molecular structure (smaller size, no π-electrons, etc.) than the well studied porphyrins and phthalocyanines as transition metal complexes. Such compounds are usually subjected to thermal treatments at relatively high temperatures (800-900 °C) which result in ligand decomposition, leaving the so-called MN _x active sites. In contrast, the complexes reported in this work are efficient electrocatalysts for the oxygen reduction reaction (ORR) in their original molecular structure, with no thermal treatments of any kind applied. The electrocatalytic activity of the Rh(iii)-cyclam and Ru(iii)-cyclam complexes during the ORR in the absence and presence of methanol (2 mol L^-1) was evaluated by voltammetry techniques. The kinetic parameters of the novel materials for the reaction were determined. The exchange current density (j ₀) values, directly related to the charge transfer velocity, are of the same order as or higher than those of platinum/Vulcan® nanoparticles. In addition, they are practically unaffected by methanol, therefore, becoming interesting candidates to be evaluated as cathodes in polymer electrolyte membrane and direct methanol fuel cells.

Fig. 1. FTIR spectra of the metal–cyclam complexes and the cyclam ligand.

Fig. 2. Representation of the cyclam ligand structure.

Fig. 3. Scanning electron micrographs of the macrocyclic complexes synthesized.

Fig. 4. Cyclic voltammograms of the macrocyclic complexes in the absence and presence of 2.0 mol L⁻¹ methanol. Electrolyte: H₂SO₄ 0.5 mol L⁻¹. Sweep rate: 20 mV s⁻¹.

Fig. 5. ORR current–potential curves of the: (a) Rh-cyclam and (b) Ru-cyclam complexes, as well of (c) Pt/Vulcan® (30%), in the absence and presence of methanol (2.0 mol L⁻¹). Electrolyte: H₂SO₄ 0.5 mol L⁻¹. Sweep rate: 20 mV s⁻¹.

Fig. 6. Koutecky–Levich plots of the macrocyclic complexes for oxygen reduction in the absence and presence of 2.0 mol L⁻¹ methanol. Electrolyte: 0.5 mol L⁻¹H₂SO₄.

Fig. 7. Tafel plots of the macrocyclic complexes for oxygen reduction in the absence and presence of 2.0 mol L⁻¹ methanol. Electrolyte: 0.5 mol L⁻¹ H₂SO₄.