Nitrogen coordinated metal single atoms in carbon have aroused extensive interest recently and have been growing as the active research frontier in a wide range of key renewable energy reactions and devices. However, single-atom catalysts with isolated metallic active components cannot satisfy the demand of electrocatalysis with the requirement in selectivity. Herein, we develop a step-by-step self-assembly strategy to allocate nickel (Ni) and iron (Fe) single atoms respectively on the inner and outer walls of graphene hollow nanospheres (GHSs), for the first time realizing the separate-sided different single-atom functionalization of hollow graphene. The Ni or Fe single atom is demonstrated to be coordinated with four N atoms via the formation of a Ni-N 4 or Fe-N 4 planar experimental observations. As a proof-of-concept demonstration for realistic application, the Ni-N 4 /GHSs/Fe-N 4 endows the rechargeable Zn-air battery with excellent energy efficiency and cycling stability as an air-cathode, outperforming the performance of benchmark Pt/C+RuO 2 air-cathode. The current work paves a new avenue for precise control of single-atom sites on carbon surface for the high-performance and selective electrocatalysts.
The oxygen evolution reaction (OER) is pivotal for renewable energy conversion and storage devices, such as water electrolyzers and rechargeable metal–air batteries. However, the rational design of electrocatalysts with suitably high efficiencies and stabilities in strongly acidic electrolytes remains a significant challenge. Here, we show the demonstration of sub-10 nm, composition-tunable Rh–Ir alloy nanoparticles (NPs) prepared using a scalable microwave-assisted method as superior acidic OER catalysts. The OER activities showed a volcano-shaped dependence on Ir composition, with Ir-rich NPs (Ir ≥ 51%) achieving better OER performance than pure Ir NPs, as reflected by lower overpotentials and higher mass activities. Most significantly, Rh22Ir78 NPs achieved a maximum mass activity of 1.17 A mg–1 Ir at a 300 mV overpotential in 0.5 M H2SO4, which corresponds to a 3-fold enhancement relative to pure Ir NPs, making it one of the most active reported OER catalysts under acidic conditions. Density functional theory calculations reveal that owing to the synergy of ensemble and electronic effects by alloying a small amount of Rh with Ir, the binding energy difference of the O and OOH intermediates is reduced, leading to faster kinetics and enhanced OER activity. Furthermore, Rh–Ir alloy NPs demonstrated excellent durability in strongly acidic electrolyte. This work not only provides fundamental understandings relating to composition–electrochemical performance relationships but also represents the rational design of highly efficient OER electrocatalysts for applications in acidic media.
Nitrate (NO 3 − ) is a ubiquitous contaminant in groundwater that causes serious public health issues around the world. Though various strategies are able to reduce NO 3 − to nitrite (NO 2 − ), a rational catalyst design strategy for NO 2 − removal has not been found, in part because of the complicated reaction network of nitrate chemistry. In this study, we show, through catalytic modeling with density functional theory (DFT) calculations, that the performance of mono-and bimetallic surfaces for nitrite reduction can be rapidly screened using N, N 2 , and NH 3 binding energies as reactivity descriptors.With a number of active surface atomic ensembles identified for nitrite reduction, we have designed a series of "metal-on-metal" bimetallics with optimized surface reactivity and a maximum number of active sites. Choosing Pd-on-Au nanoparticles (NPs) as candidate catalysts, both theory and experiment find that a thin monolayer of Pd-on-Au NPs (size: ∼4 nm) leads to high nitrite reduction performance, outperforming pure Pd NPs and the other Pd surface compositions considered. Experiments show that this thin layer of Pd-on-Au has a relatively high selectivity for N 2 formation, compared to pure Pd NPs. More importantly, our study shows that a simple model, based upon DFTcalculated thermodynamic energies, can facilitate catalysts design relevant to environmental issues.
Alloying elements with strong and weak adsorption properties can produce a catalyst with optimally tuned adsorbate binding. A full understanding of this alloying effect, however, is not well-established. Here, we use density functional theory to study the ensemble, ligand, and strain effects of closepacked surfaces alloyed by transition metals with a combination of strong and weak adsorption of H and O. Specifically, we consider PdAu, RhAu, and PtAu bimetallics as ordered and randomly alloyed (111) surfaces, as well as randomly alloyed 140-atom clusters. In these alloys, Au is the weak-binding component and Pd, Rh, and Pt are characteristic strong-binding metals. In order to separate the different effects of alloying on binding, we calculate the tunability of Hand O-binding energies as a function of lattice constant (strain effect), number of alloy-substituted sublayers (ligand effect), and randomly alloyed geometries (ensemble effect). We find that on these alloyed surfaces, the ensemble effect more significantly tunes the adsorbate binding as compared to the ligand and strain effects, with the binding energies predominantly determined by the local adsorption environment provided by the specific triatomic ensemble on the (111) surface. However, we also find that tuning of adsorbate binding from the ligand and strain effects cannot be neglected in a quantitative description. Extending our studies to other bimetallics (PdAg, RhAg, PtAg, PdCu, RhCu, and PtCu), we find similar conclusions that the tunability of adsorbate binding on random alloys is predominately described by the ensemble effect.
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