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Well-defined metallic nanocrystals (NCs) have been explored as effective electrocatalysts for energy conversion and storage technologies (e.g., fuel cell or water splitting). It is commonly known that electrocatalytic performance can be enhanced by controlling composition, size, and surface morphology. In addition, precisely controlling the atomic arrangement inside NCs can improve performance, with their electronic structures being optimized via interfacial coupling. In this review, we summarize recent advances in atomic arrangement engineering approaches of metallic NCs. First, we introduce thermodynamic and kinetic principles to provide a basic understanding on atomic structureproperty correlations. Then, several representative cases of atomic ordering and planar stacking engineering are highlighted for different electrocatalytic processes. Finally, perspectives on the roles of calculations, characterization, and practical applications are outlined.

A facile H O oxidation treatment to tune the properties of metal disulfides for oxygen evolution reaction (OER) activity enhancement is introduced. With this method, the degree of oxidation can be readily controlled and the effect of surface S residues in the resulted metal (oxy)hydroxides for the OER is revealed for the first time. The developed NiFe (oxy)hydroxide catalyst with residual S demonstrates an extraordinarily low OER overpotential of 190 mV at the current density of 10 mA cm after coupling with carbon nanotubes, and outstanding performance in Zn-air battery tests. Theoretical calculation suggests that the surface S residues can significantly reduce the adsorption free energy difference between O* and OH* intermediates on the Fe sites, which should account for the high OER activity of NiFe (oxy)hydroxide catalysts. This work provides significant insight regarding the effect of surface heteroatom residues in OER electrocatalysis and offers a new strategy to design high-performance and cost-efficient OER catalysts.

The commercialization of proton exchange membrane fuel cells (PEMFCs) relies on highly active and stable electrocatalysts for oxygen reduction reaction (ORR) in acid media. The most successful catalysts for this reaction are nanostructured Pt-alloyw ith aP t-skin. The synthesis of ultrasmall and ordered L1 0 -PtCo nanoparticle ORR catalysts further doped with af ew percent of metals (W,G a, Zn) is reported. Compared to commercial Pt/C catalyst, the L1 0 -W-PtCo/C catalyst shows significant improvement in both initial activity and high-temperature stability.T he L1 0 -W-PtCo/C catalyst achieves high activity and stability in the PEMFC after 50 000 voltage cycles at 80 8 8C, which is superior to the DOE 2020 targets.E XAFS analysis and density functional theory calculations reveal that Wd oping not only stabilizes the ordered intermetallic structure,but also tunes the Pt-Pt distances in such away to optimize the binding energy between Pt and O intermediates on the surface.

Engineering the crystal structure of Pt-M (M = transition metal) nanoalloys to chemically ordered ones have drawn increasing attention in oxygen reduction reaction (ORR) electrocatalysis due to their high resistance against M etching in acid. Although Pt-Ni alloy nanoparticles (NPs) have demonstrated respectable initial ORR activity in acid, their stability remains a big challenge due to the fast etching of Ni. In this work, sub-6 nm monodisperse chemically ordered L1 0 -Pt-Ni-Co NPs are synthesized for the first time by employing a bifunctional core/shell Pt/NiCoO x precursor, which could provide abundant O-vacancies for facilitated Pt/Ni/Co atom diffusion and prevent NP sintering during thermal annealing. Further, Co doping is found to remarkably enhance the ferromagnetism (room temperature coercivity reaching 2.1 kOe) and the consequent chemical ordering of L1 0 -Pt-Ni NPs. As a result, the best-performing carbon supported L1 0 -PtNi 0.8 Co 0.2 catalyst reveals a half-wave potential (E 1/2 ) of 0.951 V vs. RHE in 0.1 M HClO 4 with 23-times enhancement in mass activity over the commercial Pt/C catalyst along with much improved stability (no performance degradation and Ni/Co loss in 5000 potential cycles). Density functional theory (DFT) calculations suggest that the L1 0 -PtNi 0.8 Co 0.2 core could tune the surface strain of Pt shells towards optimized Pt-O binding energy and facilitated reaction rate, thereby improving the oxygen reduction electrocatalysis.Received: ((will be filled in by the editorial staff))Revised: ((will be filled in by the editorial staff))

the-state-of-the-art ORR catalysts but their uses are largely restricted by the prohibitive cost and limited activity/stability. [3][4][5][6][7] In this regard, the development of non-Pt group metal (non-PGM) catalysts derived from earth-abundant elements for ORR is the fundamental solution for the widespread applications of PEMFCs. [8][9][10] Among various non-PGM ORR catalysts developed in last decade, transition metalnitrogen-carbon (M-N-C) catalysts with M-N x coordination active sites embedded in the basal planes of carbon matrixes were the most promising ones due to their decent activity in both acidic and alkaline media and ease of scale-up production. [11][12][13][14][15] The ORR performance of M-N-C electrocatalysts in alkaline electrolyte has been well demonstrated notwithstanding, [9,[16][17][18][19][20] their performance in acidic environment is still deficient and often degrades rapidly due to the etching of metal species and/or decomposition of active sites. [21,22] Generally, the M-N-C catalysts are prepared via high-temperature (T > 800 °C) pyrolysis process of transition metal (e.g., Fe, Co, Ni), nitrogen, and carbon precursors, during which the metal atoms are very easy to agglomerate into large particles. The aggregated metal and metal oxide/carbide particles will hinder the accessibility of M-N x /C active sites and lower the utilization of M atoms seriously, thus compromising their ORR activity. Furthermore, metal aggregates will be easily etched away in acid, leading to The development of high-performance oxygen reduction reaction (ORR) catalysts derived from non-Pt group metals (non-PGMs) is urgent for the wide applications of proton exchange membrane fuel cells (PEMFCs). In this work, a facile and cost-efficient supramolecular route is developed for making non-PGM ORR catalyst with atomically dispersed Fe-N x /C sites through pyrolyzing the metal-organic polymer coordinative hydrogel formed between Fe 3+ and α-L-guluronate blocks of sodium alginate (SA). High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and X-ray absorption spectroscopy (XAS) verify that Fe atoms achieve atomic-level dispersion on the obtained SA-Fe-N nanosheets and a possible fourfold coordination with N atoms. The best-performing SA-Fe-N catalyst exhibits excellentORR activity with half-wave potential (E 1/2 ) of 0.812 and 0.910 V versus the reversible hydrogen electrode (RHE) in 0.5 m H 2 SO 4 and 0.1 m KOH, respectively, along with respectable durability. Such performance surpasses that of most reported non-PGM ORR catalysts. Density functional theory calculations suggest that the relieved passivation effect of OH* on Fe-N 4 /C structure leads to its superior ORR activity to Pt/C in alkaline solution. The work demonstrates a novel strategy for developing high-performance non-PGM ORR electrocatalysts with atomically dispersed and stable M-N x coordination sites in both acidic and alkaline media.

PtM alloy catalysts (e.g., PtFe, PtCo), especially in an intermetallic L1 0 structure, have attracted considerable interests due to their respectable activity and stability for oxygen reduction This article is protected by copyright. All rights reserved.3 reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). However, metal-catalyzed formation of •OH from H 2 O 2 (i.e., Fenton reaction) by Fe-or Co-containing catalysts causes severe degradation of PEM/catalyst layers, hindering the prospects in commercial applications. Zinc (Zn) is known as an antioxidant in Fenton reaction, but rarely alloyed with Pt owing to its relatively negative redox potential. Here, we synthesized sub-4 nm intermetallic L1 0 -PtZn nanoparticles (NPs) as high-performance PEMFC cathode catalysts. In PEMFC tests, the L1 0 -PtZn cathode achieves outstanding activity (0.52 A mg Pt -1 at 0.9 V iR-free , and peak power density of 2.00 W cm -2 ) and stabilityonly 16.6 % loss in mass activity after 30,000 voltage cycles), exceeding the U.S. DOE 2020 targets and most of the reported ORR catalysts. Density function theory (DFT) calculations reveal that biaxial strains developed upon the disorder-order (A1-L1 0 ) transition of PtZn NPs would modulate the surface Pt-Pt distances and optimize Pt-O binding for ORR activity enhancement, while the increased vacancy formation energy of Zn atoms in ordered structure accounts for the improved stability. PtZn slabs. (f) Correlations between the formation energy and vacancy formation energy of M in various L1 0 -PtM systems. TOC Structurally ordered L1 0 -PtZn nanoparticles are developed as catalysts for oxygen reduction in proton exchange membrane fuel cells (PEMFCs). The L1 0 -PtZn catalyst with a "Pt-skin" achieves outstanding activity, power density, and stability in a PEMFC. The extraordinary fuel cell performance of L1 0 -PtZn/Pt is ascribed to the optimized biaxial strains, the Fenton reaction resistance, and the increased vacancy formation energy of Zn.

Highly efficient OER electrocatalyst based on metal-rich amorphous Co–Fe phosphide was fabricatedviaa non-toxic solvothermal method.