Fe3O4 nano-octahedra and SnO2 nanorods modifying low-Pd amount electrocatalysts for alkaline direct ethanol fuel cells

Abstract

This work describes the ethanol oxidation reaction (EOR) in alkaline medium using low-palladium nanoparticle electrocatalysts modified by Fe₃O₄ nano-octahedra and SnO₂ nanorods. Operation studies on an alkaline direct ethanol fuel cell (ADEFC) were conducted using the developed electrocatalysts, and stability studies were performed using the advanced scanning flow cell (SFC) technique coupled to inductively coupled plasma mass spectrometry (online SFC-ICP-MS). The EOR was catalyzed by single (Pd/C and commercial Pd/C Alfa Aesar) and by synthesized binary/ternary electrocatalysts, in which Fe₃O₄ and SnO₂ nanostructures partially replaced the high-cost noble metal. The PdFe₃O₄/C was identified as the most promising synthesized material in the electrochemical studies, exhibiting the highest mass activity (1426 mA mg⁻¹ Pd) by cyclic voltammetry (CV), followed by the binary PdSnO₂/C (1135 mA mg⁻¹ Pd), and by the ternary (1074 mA mg⁻¹ Pd). This enhancement was attributed to the bifunctional mechanism enabled by Fe₃O₄ and SnO₂, therefore reducing poisoning and improving the EOR. Moreover, the operating results revealed that PdFe₃O₄/C showed the highest power density among the synthesized materials (31 mW cm⁻² at 70°C), even with a ∼45% reduction in Pd content compared to the commercial. XPS results showed that the Pd 3d₅/₂ and 3d₃/₂ peaks for PdFe₃O₄/C, PdSnO₂/C, and PdFe₃O₄SnO₂/C were shifted by ∼0.5 eV to higher binding energies compared to Pd/C, indicating a loss of electron density in Pd due to strong metal–oxide interactions. These interactions led to a downward shift in the d-band center of Pd, weakening the Pd-adsorbed bonds, facilitating the desorption of intermediates, and improving the catalyst tolerance to toxic species. Furthermore, the higher proportion of Pd oxides in the binary and ternary materials appeared to contribute to the supply of oxygenated species required for the oxidation of EOR intermediates. Thus, the observed enhancement resulted from synergistic bifunctional and electronic effects. Finally, online SFC-ICP-MS studies showed that Fe₃O₄ nano-octahedra contribute to the enhanced stability of the electrocatalyst, as PdFe₃O₄/C exhibits reduced Pd dissolution and no Fe dissolution.