Quaternary FeCoNiMn-Based Nanocarbon Electrocatalysts for Bifunctional Oxygen Reduction and Evolution: Promotional Role of Mn Doping in Stabilizing Carbon

Template-Free Synthesis of Two-Dimensional Fe/N Codoped Carbon Networks as Efficient Oxygen Reduction Reaction Electrocatalysts

A direct pyrolysis and template-free synthesis strategy is demonstrated to synthesize the two-dimensional (2-D) Fe/N codoped carbon networks by virtue of 2-D graphitic-carbon nitride (g-C₃N₄) intermediates derived from melamine. Because of the stabilization and steric hindrance of additional N ligands with bisnitrogen-containing groups (phenanthroline, phthalonitrile, and phenylenediamine), the thin graphitic-layered Fe/N codoped carbon materials have successfully inherited the 2-D morphology from the g-C₃N₄ intermediate after direct carbonization treatment. After the easy removal of inactive Fe particles, the resultant sample exhibits numerous well-dispersed Fe atoms embedded in the carbon layers with a hierarchical (meso- and micro-) porous structure. Owing to the high active site density and open porous structure, the thin graphitic-layered Fe/N codoped carbon electrocatalysts exhibit superior oxygen reduction reaction performance (a half-wave potential of 0.88 V and a kinetics current density of 3.8 mA cm^-2), even better than the commercial Pt/C catalysts (0.85 V and 1.6 mA cm^-2, respectively). The facile and effective synthesis strategy without template to build the graphene-like nanoarchitectures inherited from the 2-D intermediates will lead to a great development of 2-D carbon materials in various electrochemical applications.

First-principles design of bifunctional oxygen reduction and evolution catalysts through bimetallic centers in metal–organic frameworks

Bi-metallic Fe_xCo_3−x(THT)₂ nanosheets exhibit bifunctional catalytic activity for both the ORR and OER. The ORR occurs on the Co atom, while the active site for the OER is the Fe atom.

Morphology Control of Carbon-Free Spinel NiCo2O4 Catalysts for Enhanced Bifunctional Oxygen Reduction and Evolution in Alkaline Media

Spinel NiCo₂O₄ is considered a promising precious metal-free catalyst that is also carbon-free for oxygen electrocatalysis. Current efforts mainly focus on optimal chemical doping and substituent to tune its electronic structures for enhanced activity. Here, we study its morphology control and elucidate the morphology-dependent catalyst performance for bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Three types of NiCo₂O₄ catalysts with significantly distinct morphologies were prepared using temple-free, Pluronic-123 (P-123) soft, and SiO₂ hard templates, respectively, via hydrothermal methods followed by calcination. Whereas the hard-template yields spherelike dense structures, soft-template assists the formation of a unique nanoneedle cluster assembly containing abundant meso- and macropores. Furthermore, the effect of morphology of NiCo₂O₄ on their corresponding bifunctional catalytic performance was systematically investigated. The flowerlike nanoneedle assembly NiCo₂O₄ catalyst via the soft-template method exhibited the highest catalytic activity and stability for both ORR and OER. In particular, it exhibited an onset and half-wave potentials of 0.94 and 0.82 V versus reversible hydrogen electrode, respectively, for the ORR in alkaline media. Although it is still inferior to Pt, the NiCo₂O₄ represents one of the best ORR catalyst compared to other reported carbon-free oxides. Meanwhile, remarkable OER activity and stability were achieved with an onset potential of 1.48 V and a current density of 15 mA/cm² at 1.6 V, showing no activity loss after 20 000 potential cycles (0-1.9 V). The demonstrated stability is even superior to Ir for the OER. The morphology-controlled approach provides an effective solution to create a robust three-dimensional architecture with increased surface areas and enhanced mass transfer. Importantly, the soft template can yield a high degree of spinel crystallinity with ideal stoichiometric ratios between Ni and Co, thus promoting structural integrity with enhanced electrical conductivity and catalytic properties.