Oxygen reduction reaction carbon-nitride based nano-electrocatalysts for proton-exchange membrane fuel cells

The oxygen reduction reaction (ORR) is one of the fundamental electrochemical processes involved in the operation of fuel cells (FCs). The ORR is a very sluggish process and bottlenecks the performance of FCs working at T < 250°C, e.g. proton-exchange membrane fuel cells (PEMFCs). The latter systems are of a high scientific and technological relevance for their exceptional energy conversion efficiency, remarkable power density and environment-friendly operation. To achieve a performance level compatible with the applications, PEMFCs must mount suitable ORR electrocatalyts (ECs). Typical state-of-the-art ORR ECs for application in PEMFCs comprise Pt nanocrystals supported on active carbons with a large surface area. The performance and durability of these ECs is still insufficient; furthermore, the high loading of Pt raises the costs to unacceptable levels. One approach to address the above issues consists in the development of carbon nitride (CN)-based ECs. These systems are characterized by a porous conductive CN support, coordinating the metal alloy nanoparticles bearing the ORR active sites in “nitrogen coordination nests”. This contribution overviews the development of this family of ECs. Early unsupported materials were studied to identify the best preparation route and the chemical composition yielding the best ORR performance. A major breakthrough was achieved by devising “core-shell” CN-based ECs; in these systems, the CN “shell” embedding the ORR active sites is supported on a conductive carbonaceous “core”. It was shown that the morphology of the “core” plays a crucial role to modulate the performance of the CN-based EC. The performance in the ORR of the best “core-shell” CN-based ECs is outstanding. With respect to the reference ECs, the intrinsic ORR activity is no less than twice larger (>300 uA/cm2Pt at 0.9 V vs. ca. 150 uA/cm2Pt of the reference), while the Pt loading to achieve 1 kW or power is as little as ca. 1/3 (0.3 gPt/kW vs. 1.1 gPt/kW of the reference).