Oxidation of ammonia using PtRh/C electrocatalysts: Fuel cell and electrochemical evaluation

Abstract

This study reports on the use of PtRh/C electrocatalysts prepared by the borohydride reduction method with different Pt:Rh atomic ratios: (90:10, 70:30 and 50:50) which was investigated toward the ammonia electro-oxidation considering electrochemical and also direct ammonia fuel cell (DAFC) experiments. The DAFC experiments were conducted using different proportions of NH4OH and KOH as fuels. X-ray diffraction showed the formation of PtRh alloy while transmission electron micrographs showed the particles sizes between 4.1 and 4.5nm. Among the different NH4OH and KOH concentrations the combination of 3molL-1 NH4OH and 3molL-1 KOH was the most favorable due to the higher KOH concentration, which increased the electrolyte conductivity, thus, improving the ammonia oxidation. Moreover, among the PtRh/C electrocatalysts the Pt:Rh ratio of 90:10 showed to be the best suited one since it showed a power density almost 60% higher than Pt. X-ray photoelectron spectroscopy results revealed for this catalyst that the nanoparticles contain a high proportion of metallic Pt and Rh phases, supporting the alloy formation between Pt and Rh. The improved fuel cell efficiency can be related to the combination of different effects: the alloy formation between Pt and Rh (electronic effect), suppressing the adsorption strength of poisonous intermediates, and a synergic effect between Pt and Rh at this composition.