Exploring the effect of Ni/Cr contents on the sheet-like NiCr-oxide-decorated CNT composites as highly active and stable catalysts for urea electrooxidation

Abstract

The developing high-efficiency urea fuel cells have an irreplaceable role in solving the increasingly severe environmental crisis and energy shortages. The sluggish six-electron dynamic anodic oxidation reaction is the bottleneck of the rapid progress of urea fuel-cell technology. To tackle this challenge, we select the NiCr bimetallic system due to the unique synergic effect between the Ni and the Cr. Moreover, better conductivity is assured using carbon nanotubes (CNTs) as the support. Most importantly, we use a simple hydrothermal method in catalyst preparation for easy scale-up at a low cost. The results show that the hybrid catalysts of NiCrx-oxide-CNTs with different Ni/Cr ratios show much better catalytic performance in terms of active surface area and current density as compared to that of Ni-hydro-CNTs. The optimized NiCr2-oxide-CNTs catalyst exhibits not only the largest electrochemically active surface area (ESA, 50.7 m2 g-1) and the highest urea electrocatalytic current density (115.6 mA cm-2), but also outstanding long-term stability. The prominent performance of the NiCr2-oxide-CNTs catalyst is due to the combined effect of the improved charge transfer between Ni and Cr species, the large ESA, along with an elegant balance between the oxygen-defect sites and hydrophilicity. Moreover, we have proposed a synergistically enhanced urea catalytic mechanism.