New electrocatalyst developed for ORR

A team of researchers from Beihang University in Beijing, China has demonstrated a new type of low-cost nonprecious metal-based catalysts as an alternative to Pt-based catalysts for oxygen reduction reaction. In their work, a new type of vanadium nitride quantum dots anchored homogeneously onto nitrogen-doped graphene (denoted as VNQD-NG) is fabricated by a simple hydrothermal method and subsequent ammonia annealing process. The VNQD-NG exhibits high electrocatalytic activity, long durability and high selectivity for ORR, superior to commercially available Pt-C. Their report appears in the forthcoming issue of the journal NANO.

Developing suitable electrocatalysts for ORR is significant to the practical applications of fuel cells and metal-air batteries due to the kinetic sluggishness of ORR with a complex four-electron transfer process. Despite the high efficiency of platinum (Pt) and Pt-based alloys for ORR, their high cost along with the rare reserves in nature largely impedes the large-scale practical application. Therefore, developing low-cost nonprecious metal-based catalysts as an alternative to Pt-based catalysts are crucial. Vanadium nitrite (VN) not only has the similar structure and high density of states to those of above transition-metal nitrides, but also shows good corrosion resistance and high electrical conductivity, holding great promise as an active electrocatalyst for ORR.

The as-prepared VNQD-NG possesses numerous zero-dimensional (0D) VN quantum dots (QDs) with the sizes of 3?6 nm anchored homogeneously onto the surface of nitrogen-doped graphene, affording considerable structural edges and defects as active sites for ORR. Moreover, the VNQD-NG nanosheets can simultaneously construct a three-dimensional (3D) porous architecture to maximize the exposed active sites and provide sufficient electron transport pathways during ORR. Researchers executed cyclic voltammogram (CV) measurements in an aqueous 0.1 M KOH solution to investigate the ORR characteristics of VNQD-NG samples, reflecting a prominent electrocatalytic activity of the VNQD-NG for oxygen reduction. Chronoamperometric measurements show that a strong current response occurs at Pt-C catalyst when 3 M methanol was injected in oxygen-saturated solution, whereas the VNQD-NG maintains a stable current without any distinct response. Apparently, the VNQD-NG hybrid exhibits a high selectivity to methanol in the alkaline electrolyte, tolerance to crossover effects caused by fuel molecules permeating through the polymer membrane. Researchers also conducted the rotating ring-disk electrode (RRDE) measurements to verify the ORR electrochemical kinetics of VNQD-NG, displaying a comparable ORR onset potential to that of Pt-C catalyst, and higher electron transfer number (n) and kinetic-limiting current density (Jk) than that of commercial Pt-C and other non-Pt catalysts. The current-time (i-t) choronoamperometric response for VNQD-NG reveals a quite low attenuation of 77 % after 30,000 s, which provides further evidence that the stability of VNQD-NG is superior to that of commercial Pt-C (60 %).

VNQD-NG exhibits high electrocatalytic activity, high selectivity and long durability for ORR, better than commercially available Pt-C. These achievements could provide an extension of developing various other 3D porous metal nitride quantum dots onto graphene for broad applications in sensors, catalysis, and other electronic devices.