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

Abstract

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.