Uranyl N2O2-Schiff base complex as co-catalyst in ethanol electro-oxidation: synthesis, crystallographic, spectroscopic, electrochemical, and DFT characterization, and catalytic investigation

Abstract

Direct Ethanol Fuel Cells (DEFCs) are important clean energy conversion systems which can reach high energy densities using inexpensive and non-toxic fuels. One of the main obstacles to using DEFC systems is the high cost of platinum or platinum-alloy electrodes traditionally used in these systems. However, other less expensive co-catalysts, e.g., transition metal complexes, can be used to partially replace platinum or platinum-alloys in Pt-based electro-catalysts. The aim of this study is to describe and analyze the use of a new uranyl salen-type complex as co-catalyst in ethanol electro-oxidation. To this end, the structural and spectroscopic properties of the co-catalyst in question was investigated by single-crystal X-ray diffraction, DFT, elemental analysis (CHN), FTIR, UV-Vis, and ¹H and ¹³C NMR. Cyclic voltammetry indicated a quasi-reversible redox pair at 0.97/0.69 V (E_1/2 = 0.83 V) along with an anodic process at 1.14 V, both associated to the phenolate/phenoxyl radical couple. Six PtSn-based catalysts were produced by varying the PtSn:uranyl complex molar ratio. The ethanol oxidation reaction was investigated in acidic media via cyclic voltammetry and chronoamperometry. Direct scanning of the samples indicated that the peak-current density for the 6 : 1 PtSn/C : [UO₂(3-OMe-c-salcn)H₂O] catalyst was higher than that for other catalyst ratios. Moreover, as compared to the pure PtSn catalyst, 6 : 1 PtSn/C : [UO₂(3-OMe-c-salcn)H₂O] exhibited better catalytic activity in ethanol electro-oxidation reaction (EOR); it decreased the onset potential during ethanol oxidation. In addition, this catalyst exhibited peak-current densities about 2.3 times that of PtSn/C. pH affected the catalytic system performance, which decreased as pH increased (maximum efficiency at pH 0.3). Ethanol oxidation catalyzed by 6 : 1 PtSn/C : [UO₂(3-OMe-c-salcn)H₂O] was also investigated using cyclic voltammetry and chronoamperometry at different ethanol concentrations, indicating that the EOR peak increased as ethanol concentration increased.