Sub‐6 nm Fully Ordered L10‐Pt–Ni–Co Nanoparticles Enhance Oxygen Reduction via Co Doping Induced Ferromagnetism Enhancement and Optimized Surface Strain

Engineering the crystal structure of Pt–M (M = transition metal) nanoalloys to chemically ordered ones has 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 L1₀‐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 L1₀‐Pt–Ni NPs. As a result, the best‐performing carbon supported L1₀‐PtNi_0.8Co_0.2 catalyst reveals a half‐wave potential (E_1/2) of 0.951 V versus reversible hydrogen electrode in 0.1 m HClO₄ with 23‐times enhancement in mass activity over the commercial Pt/C catalyst along with much improved stability. Density functional theory (DFT) calculations suggest that the L1₀‐PtNi_0.8Co_0.2 core could tune the surface strain of the Pt shell toward optimized Pt–O binding energy and facilitated reaction rate, thereby improving the ORR electrocatalysis.     

Fe‐N‐C Oxygen Reduction Fuel Cell Catalyst Derived from Carbendazim: Synthesis,Structure, and Reactivity

New non‐PGM catalysts from the family of Fe‐N‐C pyrolyzed materials are reported. They are synthesized using a templating silica powder with iron nitrate and carbendazim (CBDZ) precursors (sacrificial support method). The synthesis involves high temperature pyrolysis, followed by etching of the sacrificial support (silica) and obtaining a “self‐supported” open frame morphology catalyst. Both the temperature of heat treatment and Fe to CBDZ ratio play a crucial role in the final catalytic activity in oxygen reduction reaction (ORR). Prepared materials have extremely high durability in RDE tests, ending up with more than 94% of initial activity (by E_1/2 value) after 10 000 cycles in an oxygen atmosphere, which is the result we report for the first time. Evaluation of these new M‐N‐C catalysts in a single membrane electrode assembly (MEA) has shown an exceptionally high open circuit voltage (OCV) of 1 V and the world's second best performance with no IR correction. MEA tests have shown high current density of 700 mA cm^‐2 at 0.6 V and 120 mA cm^‐2 at 0.8 V. In‐depth structure‐to‐property correlation presents an evidence that Fe‐N_x centers are the active sites playing a key role in oxygen reduction reaction.     

Sequential Synthesis and Active‐Site Coordination Principle of Precious Metal Single‐Atom Catalysts for Oxygen Reduction Reaction and PEM Fuel Cells

Carbon‐supported precious metal single‐atom catalysts (PM SACs) have shown promising application in proton exchange membrane fuel cells (PEMFCs). However, the coordination principle of the active site, consisting of one PM atom and several coordinating anions, is still unclear for PM SACs. Here, a sequential coordination method is developed to dope a large amount of PM atoms (Ir, Rh, Pt and Pd) into a zeolite imidazolate framework (ZIF), which are further pyrolyzed into nitrogen‐coordinated PM SACs. The PM loadings are as high as 1.2–4.5 wt%, achieving the highest PM loadings in ZIF‐derived SACs to date. In the acidic half‐cell, Ir₁‐N/C and Rh₁‐N/C exhibit much higher oxygen reduction reaction (ORR) activities than nanoparticle catalysts Ir/C and Rh/C. In the contrast, the activities of Pd₁‐N/C and Pt₁‐N/C are considerably lower than Pd/C and Pt/C. Density function theory (DFT) calculations reveal that the ORR activity of PM SAC depends on the match between the OH* adsorption on PM and the electronegativity of coordinating anions, and the stronger OH* adsorption is, the higher electronegativity is needed for the coordinating anions. PEMFC tests confirm the active‐site coordination principle and show the extremely high atomic efficiency of Ir₁‐N/C. The revealed principle provides guidance for designing future PM SACs for PEMFCs.     

A New Approach to Fuel Cell Electrodes: Lanthanum Aluminate Yielding Fine Pt Nanoparticle Exsolution for Oxygen Reduction Reaction

Designing an electrocatalyst with low Pt content is an immediate need for essential reactions in low temperature fuel cell systems. In the present work, La_0.9925Ba_0.0075Al_0.995Pt_0.005O₃ is aimed at using with low (only 0.5%) Pt doping as an electrocatalyst for oxygen reduction reaction (ORR). The low doping level renders exsolution of 1–2 nm nanoparticles with uniform dispersion upon reduction in H₂/N₂ at low temperatures. Pt exsolved perovskite oxides deliver significantly enhanced catalytic activity for ORR and improved stability in alkaline media. This study demonstrates that LaAlO₃ with low noble metal content holds immense potential as an electrocatalyst in real fuel cell systems.     

Atomically Dispersed Fe‐Nx/C Electrocatalyst Boosts Oxygen Catalysis via a New Metal‐Organic Polymer Supramolecule Strategy

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 Fe³⁺ 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 excellent ORR 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 H₂SO₄ 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‐N₄/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.     

Anion Doping: A New Strategy for Developing High‐Performance Perovskite‐Type Cathode Materials of Solid Oxide Fuel Cells

Overcoming the sluggish activity of cathode materials is critical to realizing the wide‐spread application of intermediate‐temperature solid oxide fuel cells. Herein, a new way is reported to tune the performance of perovskite‐type materials as oxygen reduction electrodes by embedding anions (F^?) in oxygen sites. The obtained perovskite oxyfluorides SrFeO_{3?σ ?δ} F_σ and SrFe_0.9Ti_0.1O_{3?σ ?δ} F_σ (σ = 0.05 and 0.10) show improved electrocatalytic activity compared to their parent oxides, achieving area specific resistance values of 0.875, 0.393, and 0.491 Ω cm² for SrFeO_3?δ , SrFeO_2.95?δ F_0.05, and SrFeO_2.90?δ F_0.10, respectively, at 600 °C in air. Such improved performance is a result of the improved bulk diffusion and surface exchange properties due to anion doping. Moreover, favorable stability in performance is also demonstrated for the F^? anion‐doped perovskites as oxygen reduction electrodes at 650 °C for a test period of ≈200 h. A combination of anion doping and cation doping may provide a highly attractive strategy for the future development of cathode materials.     

Highly Efficient Oxygen Reduction Reaction Activity of Graphitic Tube Encapsulating Nitrided CoxFey Alloy

Nonprecious metals are promising catalysts to avoid the sluggish oxygen reduction reaction (ORR) in next‐generation regenerative fuel cells or metal–air batteries. Therefore, development of nonprecious metal catalysts for ORR is highly desirable. Herein, precise tuning of the atomic ratio of Fe and Co encapsulated in melamine‐derived nitrogen‐rich graphitic tube (NGT) is reported. The Co_1.08Fe_3.34 hybrid with metal? nitrogen bonds ( 1 : Co_1.08Fe_3.34@NGT) shows remarkable ORR catalytic activities (80 mV higher in onset potential and 50 mV higher in half‐wave potential than those of state‐of‐the‐art commercial Pt/C catalysts), high current density, and stability. In acidic solution, 1 also shows compatible performance to commercial Pt/C in terms of ORR activity, current density, stability, and methanol tolerance. The high ORR activity is ascribed to the co‐existence of Fe? N, Co? N, and sufficient metallic FeCo alloys which favor faster electron movement and better adsorption of oxygen molecules on the catalyst surface. In the alkaline anion exchange membrane fuel cell setup, this cell delivers the power density of 117 mW cm^?2, demonstrating its potential use for energy conversion and storage applications.     

Engineering Multimetallic Aerogels for pH‐Universal HER and ORR Electrocatalysis

The advent of noble metal aerogels (NMAs), that feature the high catalytic activity of noble metals and unique structural attributes of aerogels, has stimulated research on a new class of outstanding electrocatalysts. However, limited by the available compositions, the explored electrocatalytic reactions on NMAs are highly restricted and certain important electrochemical processes have not been investigated. Here, an effective gelation approach is demonstrated by using a strong salting‐out agent (i.e., NH₄F), thereby expanding the composition of NMAs to various multimetallic systems and providing a platform to investigate composition‐dependent electrocatalytic performance of NMAs. Combining structural features of aerogels and optimized chemical compositions, the Au–Pt and Au–Rh aerogel catalysts manifest remarkable pH‐universal (pH = 0–14) performance surpassing commercial Pt/C and many other nanoparticle (NP)‐based catalysts in the electrocatalytic oxygen reduction reaction, hydrogen evolution reaction, and water splitting, displaying enormous potential for the electrochemical hydrogen production, fuel cells, etc.     

Metal‐Organic Framework‐Derived Non‐Precious Metal Nanocatalysts for Oxygen Reduction Reaction

By virtue of diverse structures and tunable properties, metal‐organic frameworks (MOFs) have presented extensive applications including gas capture, energy storage, and catalysis. Recently, synthesis of MOFs and their derived nanomaterials provide an opportunity to obtain competent oxygen reduction reaction (ORR) electrocatalysts due to their large surface area, controllable composition and pore structure. This review starts with the introduction of MOFs and current challenges of ORR, followed by the discussion of MOF‐based non‐precious metal nanocatalysts (metal‐free and metal/metal oxide‐based carbonaceous materials) and their application in ORR electrocatalysis. Current issues in MOF‐derived ORR catalysts and some corresponding strategies in terms of composition and morphology to enhance their electrocatalytic performance are highlighted. In the last section, a perspective for future development of MOFs and their derivatives as catalysts for ORR is discussed.     

Defective Carbon–CoP Nanoparticles Hybrids with Interfacial Charges Polarization for Efficient Bifunctional Oxygen Electrocatalysis

The development of efficient catalysts for both oxygen reduction and evolution reactions (oxygen reduction reaction (ORR) and oxygen evolution reaction (OER)) is central to regenerative fuel cells and rechargeable metal–air batteries. It is highly desirable to achieve the efficient integration of dual active components into the catalysts and to understand the interaction between the dual components. Here, a facile approach is demonstrated to construct defective carbon–CoP nanoparticle hybrids as bifunctional oxygen electrocatalysts, and further probe the interfacial charge distribution behavior. By combining multiple synchrotron‐based X‐ray spectroscopic characterizations with density functional theory calculations, the interfacial charge polarization with the electrons gathering at the defective carbon surface and the holes gathering at the CoP surface due to strong interfacial coupling is revealed, which simultaneously facilitates the ORR and OER with remarkable bifunctional oxygen electrode activities. This work not only offers a bifunctional oxygen catalyst with outstanding performance, but also unravels the promoting factor of the hybrids from the view of interfacial charge distribution.     

Molecular Design of Single‐Atom Catalysts for Oxygen Reduction Reaction

Fuel cells are highly attractive for direct chemical‐to‐electrical energy conversion and represent the ultimate mobile power supply solution. However, presently, fuel cells are limited by the sluggish kinetics of the cathodic oxygen reduction reaction (ORR), which requires the use of Pt as a catalyst, thus significantly increasing the overall cost of the cells. Recently, nonprecious metal single‐atom catalysts (SACs) with high ORR activity under both acidic and alkaline conditions have been recognized as promising cost‐effective alternatives to replace Pt in fuel cells. Considerable efforts have been devoted to further improving the ORR activity of SACs, including tailoring the coordination structure of the metal centers, enriching the concentration of the metal centers, and engineering the electronic structure and porosity of the substrate. Herein, a brief introduction to fuel cells and fundamentals of the ORR parameters of SACs and the origin of their high activity is provided, followed by a detailed review of the recently developed strategies used to optimize the ORR activity of SACs in both rotating disk electrode and membrane electrode assembly tests. Remarks and perspectives on the remaining challenges and future directions of SACs for the development of commercial fuel cells are also presented.     

Gadolinium‐Induced Valence Structure Engineering for Enhanced Oxygen Electrocatalysis

Rare earth doped materials with unique electronic ground state configurations are considered emerging alternatives to conventional Pt/C for the oxygen reduction reaction (ORR). Herein, gadolinium (Gd)‐induced valence structure engineering is, for the first, time investigated for enhanced oxygen electrocatalysis. The Gd₂O₃–Co heterostructure loaded on N‐doped graphene (Gd₂O₃–Co/NG) is constructed as the target catalyst via a facile sol–gel assisted strategy. This synthetic strategy allows Gd₂O₃–Co nanoparticles to distribute uniformly on an N‐graphene surface and form intimate Gd₂O₃/Co interface sites. Upon the introduction of Gd₂O₃, the ORR activity of Gd₂O₃–Co/NG is significantly increased compared with Co/NG, where the half‐wave potential (E_1/2) of Gd₂O₃–Co/NG is 100 mV more positive than that of Co/NG and even close to commercial Pt/C. The density functional theory calculation and spectroscopic analysis demonstrate that, owing to intrinsic charge redistribution at the engineered interface of Gd₂O₃/Co, the coupled Gd₂O₃–Co can break the OOH*–OH* scaling relation and result in a good balance of OOH* and OH* binding on Gd₂O₃–Co surface. For practical application, a rechargeable Zn–air battery employing Gd₂O₃–Co/NG as an air–cathode achieves a large power density and excellent charge–discharge cycle stability.     

Nanocarbon Electrocatalysts for Oxygen Reduction in Alkaline Media for Advanced Energy Conversion and Storage

Alkaline oxygen electrocatalysis, targeting anion exchange membrane fuel cells, Zn‐air batteries, and alkaline‐based Li‐air batteries, has become a subject of intensive investigation because of its advantages compared to its acidic counterparts in reaction kinetics and materials stability. However, significant breakthroughs in the design and synthesis of efficient oxygen reduction catalysts from earth‐abundant elements instead of precious metals in alkaline media remain in high demand. Carbon composite materials have been recognized as the most promising because of their reasonable balance between catalytic activity, durability, and cost. In particular, heteroatom (e.g., N, S, B, or P) doping can tune the electronic and geometric properties of carbon, providing more active sites and enhancing the interaction between carbon structure and active sites. Importantly, involvement of transition metals appears to be necessary for achieving high catalytic activity and improved durability by catalyzing carbonization of nitrogen/carbon precursors to form highly graphitized carbon nanostructures with more favorable nitrogen doping. Recently, a synergetic effect was found between the active species in nanocarbon and the loaded oxides/sulfides, resulting in much improved activity. This report focuses on these carbon composite catalysts. Guidance for rational design and synthesis of advanced alkaline ORR catalysts with improved activity and performance durability is also presented.