Developing a novel method for making metal oxide nanocoatings for electrocatalysis applications
Authors:   , Jasim Ahmed Mohammed 
Published: December 2023

In this dissertation, a new and novel technique to synthesize conformal films of metal oxides on top of nanoscale substrates in different geometries such as carbon nanotubes, carbon black particles, and iron oxide nanoparticles was developed. The coating technique is based on principles of condensing water molecules from an organic solvent, heptane, onto nanoscale substrates after oversaturating the heptane above its water saturation limit. When metal oxide precursors are introduced into the heptane dispersed with the nanoscale substrates, they diffuse to the surface of the substrates and react with the water film on their surface to form metal oxide nanocoatings. Due to the high surface tension between the water film and the heptane, the condensed water film takes a conformal shape on the substrates with minimum roughness, which also leads to producing uniform metal oxide coatings. This new coating technique is termed condensed layer deposition, or CLD. Various metal oxides were successfully made, such as TiO2, Nb2O5, and Al2O3 on various substrates, such as carbon black (CB), carbon nanotubes (CNTs), carbon nanofibers (CNFs), iron oxide particles (IOPs), and TiO2. By removing the carbon supports (CB, CNTs, CNFs), Nb2O5 nanoshells, TiO2 nanotubes and Al2O3 porous nanotubes have been obtained. It was found that the CLD technique can produce fluffy coatings with a high specific surface area of [approximately] 310 m2/g, when a large molecule size of organic precursor such as titanium terta-isopropoxide is used. The surface hydrophilicity was observed to increase by increasing the water ratio. Subsequently, a fluffy TiO2 coating on CNTs was made with tin oxide and used as a support for platinum nanoparticles for ethanol electrooxidation and for oxygen reduction reaction (ORR). The catalyst has shown a much higher ethanol mass activity peak of 560 mA/mg(Pt) than Pt/C (Pt on CB) which showed 296 mA/mg(Pt). In addition, the catalyst has a current density by over 2.2 times of that in Pt/C at 600 mV after 0.5 hours in durability test. In ORR, a remarkable improvement in ORR onset potential was achieved at 930 mV vs. RHE in 0.5 M sulfuric acid electrolyte. The mass activity was 2.66 times that of Pt/C at 900 mV. The same fluffy TiO2/CNTs was used to support platinum and iridium for methanol electrooxidation. Pt-Ir/C-TiO2/CNTs has shown a significant improvement in the anodic to the cathodic current ratio with a 5.6-fold increase. The i-t curve showed a high stability at 0.5 V after 3600 seconds by 12.5 times. On the other hand, Nb2O5 coating of 5 nm on CB was used for the oxygen reduction reaction. Electrochemical tests showed only a 1.7 percent activity loss after 5000 cycles, demonstrating an excellent durability of the electrocatalyst. Compared to the electrocatalyst without niobium oxide coating, it shows a 25 mV improvement in half-wave potentials, indicative of a better kinetics. The 5 nm Nb2O5 on CB was coupled with tin oxide supporting platinum toward methanol electrooxidation. Pt-TNb/C (tin and niobium oxide support) showed 2.7 A/mg(Pt) in mass activity at 0.8 V vs. Ag/AgCl which is better than most studies reported in the literature. Moreover, the Pt-TNb/C showed 0.0 percent loss after 1000 cycles in electrolyte containing 1.0 M CH3OH, whereas, Pt-Nb/C (niobium oxide support) and Pt/C suffered 21 percent and 30 percent loss, respectively. The chronoamperometry results showed the catalyst has a high CO tolerance after testing for 2 hours with a remarkable stability.