3d-metal based anodic pincer electro-catalysts dispersed in solution for the electro-catalytic oxidation of (m)ethanol†

Abstract

Using banana-leaf extract as the reducing agent, a green protocol has been formulated for the synthesis of Ni/Co/Cr based metal oxide/hydroxide nanoparticles starting either from their metal salt precursors or from their bis(iminopyridine) pincer complexes. Thorough characterization has been performed using powder-XRD, FESEM, FETEM, XPS, MALDI and FT-IR analysis to infer the phase, morphology, size and chemical composition of these heterogeneous particles. The nano-particles (NPs) derived from (^iPr2NNN)CoCl₂ containing CoO₂ and Co₃O₄ mixed phase oxides possessing a spherical morphology turned out to be superior among the 12 nano-catalysts that were screened. While a current density of 24.90 ± 10.74 mA cm⁻² and 260.06 ± 3.06 mA cm⁻² was obtained for methanol electro-oxidation (MOR), a current density of 33.39 ± 1.28 mA cm⁻² and 235.03 ± 4.03 mA cm⁻² was observed during ethanol electro-oxidation (EOR) at room temperature in NaOH as the supporting electrolyte using a carbon based electrode assembly. At 60 °C, the current densities in the (^iPr2NNN)CoCl₂ NP catalyzed MOR increased up to 222.23 mA cm⁻² and 451.32 mA cm⁻² at a lower potential of 0.69 V and 1.22 V vs. Ag/AgCl. The electro-oxidation is majorly complete with zero evolution of CO₂ as it gets trapped by the supporting electrolyte as a value-added chemical Na₂CO₃. For the MOR and EOR, up to 98% and 82% respectively of Na₂CO₃ were obtained after 12 hours of controlled potential electrolysis. The (^iPr2NNN)CoCl₂ NPs were also found to be highly stable and fairly active with a current retention of 78.60% and 64.0% for MOR and EOR respectively. The catalytic activity and the yield of Na₂CO₃ obtained with the top-three best of the considered catalysts had a linear correlation with the electrochemical active surface area (ECSA), CA stability and current retention along with the diffusion coefficient while following the trend (^iPr2NNN)CoCl₂ NPs > (^Ph2NNN)NiCl₂(CH₃CN) NPs > (^Ph2NNN)CrCl₃ NPs. In this work, a unique approach of suspending the electro-catalysts in solution rather than using them on bulk electrodes has been executed to probe their electro-catalytic activity towards (m)ethanol which not only leads to high current densities but also generates industrially valuable Na₂CO₃.