Niobium oxide coatings on nanostructured platinum electrocatalysts: benefits and limitations†

Abstract

Development of durable nanoscale electrocatalysts is an important step towards improving the affordability and sustainability of fuel cell technology. Nanostructured platinum catalysts are used to facilitate the two half reactions for hydrogen fuel cells. The sluggish kinetics of the cathodic oxygen reduction reaction and the less than optimal stability of cathode catalysts provide motivation for additional efforts to improve the catalytic performance of platinum. Metal oxide coatings on electrocatalysts have been found to increase durability of nanostructured catalysts and to impart additional properties such as increased activity and resistance to poisoning by contaminants. Niobium oxides have been studied as supporting materials for platinum fuel cell catalysts and shown to have a relatively high stability. It has also been suggested that niobium oxides can impart an increased activity due to strong metal support interactions. However, the lack of electrical conductivity of niobium pentoxide limits its viability as a support. Herein, coatings of niobium oxide were applied to nanotextured platinum catalysts prepared by electrodeposition against self-assembled templates to explore the impact of the coatings on the durability and electrocatalytic activity of the catalyst both during and after an accelerated stress test. The catalysts were characterized via scanning electron microscopy, X-ray photoelectron spectroscopy, conductive atomic force microscopy, and electrochemical techniques. Increasing the thickness of the coating from ∼0.5 nm to ∼4.5 nm was found to preserve the initial nanostructured morphology of the electrodeposited platinum catalyst. The thicker coatings did, however, result in larger charge transfer resistances towards the oxygen reduction reaction. These studies provide further evidence of the utility of ultrathin coatings to improve the properties of nanostructured electrocatalysts.