Edge-sited Fe-N-4 atomic species improve oxygen reduction activity via boosting O-2 dissociation

The development of low-cost, efficient, and stable electrocatalysts toward the oxygen reduction reaction (ORR) is urgently demanded for scalable applications in fuel cells or zinc-air batteries (ZABs), but still remains a challenge. Herein, carbon materials with edge-sited Fe-N-4 atomic species (E-FeNC) were synthesized from pyrolysis of abundant Fe-containing biomass using silica spheres as hard template. The E-FeNC delivers remarkable ORB. performance with a half-wave potential of 0.875 V (vs. reversible hydrogen electrode (RHE)), much better than Pt/C (0.859 V), attributed to atomically dispersed Fe-N-4 moieties nearby graphitic edges. The density functional calculations reveal that O-2 molecule adsorbs on Fe-N-4 sites with an energetically favorable side-on configuration with elongated O=O bond rather than end-on form, boosting the subsequent dissociation pathway with a direct 4e reaction route. Using E-FeNC as cathode catalyst, the primary ZAB exhibits high specific capacity of 710 mA h g(-1) and power density of 151.6 mW cm(-2) . The rechargeable ZAB by coupling E-FeNC and NiFe layered double hydroxide (LDH) demonstrates long-term capacity retention over 200 h, superior to that using noble Pt/C and RuO2. This unique carbon material with atomically dispersed metal sites opens up an avenue for the design and engineering of electrocatalysts for energy conversion systems

Highly Localized C-N2 Sites for Efficient Oxygen Reduction

The search for oxygen reduction reaction (ORR) catalysts outperforming Pt, the state-of-the-art material, continues. Doped carbon-based materials offer a viable means for replacing Pt, but their activity improvement still remains a great challenge. Here, configurations of N-doped carbons are first analyzed using ab initio simulations toward ORR. The results show that a certain short-range ordered structure labeled as C-N2, which comprises of two nitrogen atoms flanking carbon, is the optimal choice. The predicted configuration of C-N2 is experimentally realized by triazine-doped carbon (triNC). The triNC with C-N2 sites demonstrates high ORR activity (onset potential 0.98 V, halfwave potential 0.89 V) comparable to commercial 20% Pt/C. The highly localized and positive-charged carbon atom in the C-N2 structure facilitates the dissociation of O-2 to increase the ORR kinetics, proved by theoretical calculation. A Zn-air cathode is fabricated using the triNC ORR electrocatalyst and outperforms the cathode using Pt/C in terms of specific capacity, energy density and long-term durability. The atomic-scale approach reported here provides a good strategy to achieve active carbon-based electrocatalysts for potential and scalable use in energy conversion and storage

A simple method to tune graphene growth between monolayer and bilayer

Selective growth of either monolayer or bilayer graphene is of great importance. We developed a method to readily tune large area graphene growth from complete monolayer to complete bilayer. In an ambient pressure chemical vapor deposition process, we used the sample temperature at which to start the H2 flow as the control parameter and realized the change from monolayer to bilayer growth of graphene on Cu foil. When the H2 starting temperature was above 700°C, continuous monolayer graphene films were obtained. When the H2 starting temperature was below 350°C, continuous bilayer films were obtained. Detailed characterization of the samples treated under various conditions revealed that heating without the H2 flow caused Cu oxidation. The more the Cu substrate oxidized, the less graphene bilayer could form

Dual-Metal Interbonding as the Chemical Facilitator for Single-Atom Dispersions

Atomically dispersed catalysts, with maximized atom utilization of expensive metal components and relatively stable ligand structures, offer high reactivity and selectivity. However, the formation of atomic-scale metals without aggregation remains a formidable challenge due to thermodynamic stabilization driving forces. Here, a top-down process is presented that starts from iron nanoparticles, using dual-metal interbonds (Rh-Fe bonding) as a chemical facilitator to spontaneously convert Fe nanoparticles to single atoms at low temperatures. The presence of Rh-Fe bonding between adjacent Fe and Rh single atoms contributes to the thermodynamic stability, which facilitates the stripping of a single Fe atom from the Fe nanoparticles, leading to the stabilized single atom. The dual single-atom Rh-Fe catalyst renders excellent electrocatalytic performance for the hydrogen evolution reaction in an acidic electrolyte. This discovery of dual-metal interbonding as a chemical facilitator paves a novel route for atomic dispersion of chemical metals and the design of efficient catalysts at the atomic scale