Effects of SnO2 layer coated on carbon nanofiber for the methanol oxidation reaction

Carbon nanofibers (CNFs) are used as active materials for electrodes in various energy devices, such as lithium ion secondary batteries, supercapacitors, and fuel cells. Recent studies have shown that nanoscale coatings on carbon nanotubes increase the output and lifespan of these devices owing to the improvement of their mechanical and chemical properties. Among various coating methods, atomic layer deposition (ALD) can adjust the thickness of the coating layer conformally without any directional growth. Therefore, ALD can coat particles with high aspect ratios, such as CNFs, even at nanometer levels of thickness.In this work, we grew two different morphologies of a SnO₂ layer on CNF. We used two types of ALD equipment: flow-type ALD (static ALD), and fluidized bed reactor-type ALD (dynamic ALD). Static ALD could form a discontinuous SnO₂, while a uniform SnO₂ layer was formed by pre-inserting a layer of Al₂O₃. On the other hand, dynamic ALD formed a uniform SnO₂ layer without pre-insertion of an Al₂O₃ layer. X-ray photoelectron spectroscopy analysis revealed that both Sn⁴⁺ and Sn²⁺ were present in SnO₂ on the CNF deposited by static ALD, probably due to the formation of an interfacial layer between the SnO₂ and CNF. When the dynamic ALD method was used, only Sn⁴⁺ was present in the SnO₂ on CNF. Cyclic voltammetry analysis was performed to characterize the electrochemical properties of the SnO₂-coated CNF as an electrode on a direct methanol fuel cell. It was revealed that the discontinuous SnO₂ on CNF deposited by static ALD showed the highest current efficiency as well as enhanced electrocatalytic stability.