Mesostructured Intermetallic Compounds of Platinum and Non‐Transition Metals for Enhanced Electrocatalysis of Oxygen Reduction Reaction

Alloying techniques show genuine potential to develop more effective catalysts than Pt for oxygen reduction reaction (ORR), which is the key challenge in many important electrochemical energy conversion and storage devices, such as fuel cells and metal‐air batteries. Tremendous efforts have been made to improve ORR activity by designing bimetallic nanocatalysts, which have been limited to only alloys of platinum and transition metals (TMs). The Pt‐TM alloys suffer from critical durability in acid‐media fuel cells. Here a new class of mesostructured Pt–Al catalysts is reported, consisting of atomic‐layer‐thick Pt skin and Pt₃Al or Pt₅Al intermetallic compound skeletons for the enhanced ORR performance. As a result of strong Pt–Al bonds that inhibit the evolution of Pt skin and produce ligand and compressive strain effects, the Pt₃Al and Pt₅Al mesoporous catalysts are exceptionally durable and ≈6.3‐ and ≈5.0‐fold more active than the state‐of‐the‐art Pt/C catalyst at 0.90 V, respectively. The high performance makes them promising candidates as cathode nanocatalysts in next‐generation fuel cells.     

Zn Single Atom Catalyst for Highly Efficient Oxygen Reduction Reaction

The authors report first a new type of nitrogen‐triggered Zn single atom catalyst, demonstrating high catalytic activity and remarkable durability for the oxygen reduction reaction process. Both X‐ray absorption fine structure spectra and theoretical calculations suggest that the atomically dispersed Zn‐N₄ site is the main, as well as the most active, component with O adsorption as the rate‐limiting step at a low overpotential of 1.70 V. This work opens a new field for the exploration of high‐performance Pt‐free electrochemical oxygen reduction catalysts for fuel cells.     

Relay Catalysis of Multi-Sites Promotes Oxygen Reduction Reaction

The two-electron pathway to form hydrogen peroxide (H₂O₂) is undesirable for the oxygen reduction reaction (ORR) in iron and nitrogen doped carbon (Fe–N–C) material as it not only lowers the catalytic efficiency but also impairs the catalyst durability. In this study, a relay catalysis pathway is designed to minimize the two-electron selectivity of Fe–N–C catalyst. Such a design is achieved by introducing two other sites, that is, MnN₄ site and α-Fe(110) face. A combination of transmission electron microscopy image and X-ray absorption spectra verify the three site formation. Electrochemical test coupled with post-treatment confirm the improvement of MnN₄ site and α-Fe(110) face on catalyst performance. Theoretical calculation proposes a relay catalysis pathway of three sites, that is, H₂O₂ released from the FeN₄ site migrates to the MnN₄ site or α-Fe(110) face, on which the captive H₂O₂ is further reduced to H₂O. The relay catalysis pathway positioned the as-prepared catalyst among the best ORR catalysts in both aqueous electrode and alkaline direct methanol fuel cell test. This study examples an interesting relay catalysis pathway of multi-sites for the ORR, which offers insights into the design of efficient electrocatalysts for fuel cells or beyond.     

Platinum Porous Nanosheets with High Surface Distortion and Pt Utilization for Enhanced Oxygen Reduction Catalysis

Unlike the well‐established shape/composition control, surface distortion is a newly emerged yet largely unexplored nanosurface engineering for boosting electrocatalysis. Tapping into the novel electrocatalysts for taking full use of the distortion effect is therefore of importance but remains a formidable challenge. Here, an approach to designing highly distorted porous Pt nanosheets (NSs) by electrochemical erosion of ultrathin PtTe₂ NSs is reported. The inherent ultrathin feature and massive leaching of Te have conspired to produce a highly distorted structure. As a result, the generated Pt NSs exhibit a much‐enhanced oxygen reduction reaction (ORR) mass and specific activity of 2.07 A mg_Pt^?1 and 3.1 mA cm^?2 at 0.90 V versus reversible hydrogen electrode, 9.8 and 10.7 times higher than those of commercial Pt/C. The highly distorted Pt NSs can endure 30 000 cycles with negligible activity decay and structure variation. Density functional theory calculations reveal that the electrochemical corrosion induced nanopores, boundaries, and vacancies consist of Pt sites with substantially low coordination numbers deviating from the one of pristine Pt (111) surface. These Pt sites actively act as electron‐depleting centers for highly efficient electron transfer toward the adsorbing O‐species. This study opens a new design for fully using the distortion effect to promote ORR performance and beyond.     

Porphyrin Aerogel Catalysts for Oxygen Reduction Reaction in Anion-Exchange Membrane Fuel Cells

Platinum group metal (PGM)-free catalysts for oxygen reduction reaction have shown high oxygen reduction reaction activity in alkaline media. In order to further increase the power density of anion-exchange membrane fuel cells (AEMFCs), PGM-free catalysts need to have a high site density to reach high current densities. Herein, synthesis, characterization, and utilization of heat-treated iron porphyrin aerogels are reported as cathode catalysts in AEMFCs. The heat treatment effect is thoroughly studied and characterized using several techniques, and the best performing aerogel is studied in AEMFC, showing excellent performance, reaching a peak power density of 580 mW cm⁻² and a limiting current density of as high as 2.0 A cm⁻², which can be considered the state-of-the-art for PGM-free based AEMFCs.     

Single-Atom Catalysts for H2O2 Electrosynthesis via Two-Electron Oxygen Reduction Reaction

Oxygen reduction reaction via the two-electron route (2e⁻ ORR) provides a green method for the direct production of hydrogen peroxide (H₂O₂) along with in situ utilization. The effective catalysts with high ORR activity, 2e⁻ selectivity, and stability are essential for the application of this technology. Single-atom catalysts (SACs) have attracted intensively attention for H₂O₂ electrosynthesis owing to the unique geometric and electronic configurations. In this review, the mechanism and theoretical predictions for 2e⁻ ORR over SACs are first introduced. Then, the recent advances of various SACs for the electrosynthesis of H₂O₂ are documented. And the correlation between the central atom, coordination atoms, and coordination environment of SACs and the corresponding electrocatalytic ORR performance including activity, selectivity, and stability are emphatically analyzed and summarized. Finally, the major challenges and opportunities regarding the future design of SACs for the H₂O₂ production are pointed out.     

Tunable Decoration of Reduced Graphene Oxide with Au Nanoparticles for the Oxygen Reduction Reaction

Reduced graphene oxide (rGO) films are decorated with non‐overlapping Au nanoparticles using diblock copolymer micelles that provide controllability over the number density as well as the diameter of the nanoparticles. This synthetic process produces a pure Au surface without extra layers. Further­more, the rGO film enables the transferability of the Au nanoparticles without deterioration of their arrays. Thus, the controllability of the Au nanoparticles and their transferability with rGO films allow the effective modification of electrochemical electrodes. With a glassy carbon electrode modified with an rGO film with Au nanoparticles, high electrochemical activity is observed in the oxygen reduction reaction (ORR). Furthermore, it is possible to identify a size‐dependent ORR mechanism, showing that Au nanoparticles with an average diameter of 8.6 nm exhibit a 4‐electron direct reduction of O₂ to H₂O.     

Coordination Shell Dependent Activity of CuCo Diatomic Catalysts for Oxygen Reduction,Oxygen Evolution,and Hydrogen Evolution Reaction

Challenges in rational designing dual-atom catalysts (DACs) give a strong motivation to construct coordination-activity correlations. Here, thorough coordination-activity correlations of DACs based on how the changes in coordination shells (CSs) of dual-atom Cu,Co centers influence their electrocatalytic activity in oxygen reduction reaction(ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) is constructed. First, Cu,Co DACs with different CSs modifications are fabricated by using a controlled “precursors-preselection” approach. Three DACs with unique coordination environments are characterized as secondary S atoms that directly bond to Cu,Co-N₆ in lower CSs, indirectly bond in neighboring CSs, and are doped in higher CSs, respectively. Then, experimentally and theoretically, a coordination correlation resembling a planet-satellite system, where satellite coordinated atoms (heteroatom N, S) surround Cu-Co dual-atom entity in various orbitals CSs. By evaluating electrocatalytic activity indicators, differences are identified in electronic structure and electrocatalytic performance of Cu and Co centers in ORR, OER, and HER. Interestingly, initial CSs modifications for DACs may not always be advantageous for electrocatalysis. This work offers valuable insight for designing DACs for diverse applications.     

Effects of Reaction Conditions on the Properties of Spherical Silver Powders Synthesized by Reduction of an Organometallic Compound

Silver powders were synthesized by reducing a silver organometallic compound, silver 2-ethylhexanoate, with di-n-octylamine. The effects of preparation conditions on the characteristics of the powders were investigated. Silver powders prepared from silver 2-ethylhexanoate and di-n-octylamine in the ratio 2:1 (MA21) at 150°C for 3 h had the best characteristics (average particle size 277 nm, narrow particle-size distribution, high tap density of 4.0 g/cm³), and were also obtained in high yield (98%). Use of an excessive amount of di-n-octylamine resulted in intense thermolysis and a low yield of silver powders of irregular morphology with a wide particle-size distribution. As the proportion of silver 2-ethylhexanoate was increased, the silver powders obtained had a bimodal particle-size distribution and a relatively low tap density. Silver films seemed to have high resistivity when the temperature used for synthesis of the silver powders was too low or reaction time was insufficient. The electrical resistivities of silver films prepared from MA21 powders and sintered at 300°C and 500°C for 30 min were 3.8 × 10^?6 Ω cm and 2.3 × 10^?6 Ω cm, respectively, close to that of bulk silver.     

Anchoring Single Copper Atoms to Microporous Carbon Spheres as High-Performance Electrocatalyst for Oxygen Reduction Reaction

Although the carbon-supported single-atom (SA) electrocatalysts (SAECs) have emerged as a new form of highly efficient oxygen reduction reaction (ORR) electrocatalysts, the preferable sites of carbon support for anchoring SAs are somewhat elusive. Here, a KOH activation approach is reported to create abundant defects/vacancies on the porous graphitic carbon nanosphere (CNS) with selective adsorption capability toward transition-metal (TM) ions and innovatively utilize the created defects/vacancies to controllably anchor TM–SAs on the activated CNS via TM N_x coordination bonds. The synthesized TM-based SAECs (TM-SAs@N-CNS, TM: Cu, Fe, Co, and Ni) possess superior ORR electrocatalytic activities. The Cu-SAs@N-CNS demonstrates excellent ORR and oxygen evolution reaction (OER) bifunctional electrocatalytic activities and is successfully applied as a highly efficient air cathode material for the Zn–air battery. Importantly, it is proposed and validated that the N-terminated vacancies on graphitic carbons are the preferable sites to anchor Cu-SAs via a Cu (N C₂)₃(N C) coordination configuration with an excellent promotional effect toward ORR. This synthetic approach exemplifies the expediency of suitable defects/vacancies creation for the fabrication of high-performance TM-based SAECs, which can be implemented for the synthesis of other carbon-supported SAECs.     

Evolution of Nanoporous Pt–Fe Alloy Nanowires by Dealloying and their Catalytic Property for Oxygen Reduction Reaction

The short life and high cost of carbon‐supported Pt nanoparticle catalysts (Pt/C) are two main problems with proton exchange membrane fuel cells. Porous Pt alloy nanowires have more durability and catalytic activity than Pt/C. Dealloying is a facile way to make nanoporous Pt. However, the process of porosity formation is difficult to control. In this paper, electrospinning and chemical dealloying techniques are used to make long, thin and yet nanoporous Pt–Fe alloy nanowires. The evolution of nanoporosity is observed and studied. It is found that non‐uniform composition in the precursor PtFe₅ alloy nanowires helps the formation of nanoporous structure. The overall wire diameter is about 10–20 nm and the ligament diameter only 2–3 nm. These porous long nanowires interweave to form a self‐supporting network with a high specific activity, 2.3 times that of conventional Pt/C catalysts, and also have better durability.     

Bipyridine Modified Conjugated Carbon Aerogels as a Platform for the Electrocatalysis of Oxygen Reduction Reaction

Aerogels offer a great platform for heterogeneous electrocatalysis owing to their high surface area and porosity. Atomically dispersed transition metal ions can be imbedded in these platforms at ultra-high site density to make them catalytically active for various reactions. Herein, the synthesis of a new class of conjugated microporous organic aerogels that are used as covalent 3D frameworks for the electrocatalysis of oxygen reduction reaction (ORR) is reported. Modified aerogels functionalized with bipyridine ligands enable copper ion complexation in a single-step synthesis. The aerogels’ structures are fully characterized using a wide array of spectroscopic and microscopic methods, and heat-treated in order to make them electronically conductive. After heat treatment at 600 °C, the aerogels maintained their macrostructure and became active ORR catalysts in alkaline environment, showing high mass activity and ultra-high site density.     

Polyformamidine‐Derived Non‐Noble Metal Electrocatalysts for Efficient Oxygen Reduction Reaction

A facile approach for the template‐free synthesis of highly active non‐noble metal based oxygen reduction reaction (ORR) electrocatalysts is presented. Porous Fe?N?C/Fe/Fe₃C composite materials are obtained by pyrolysis of defined precursor mixtures of polyformamidine (PFA) and FeCl₃ as nitrogen‐rich carbon and iron sources, respectively. Selection of pyrolysis temperature (700–1100 °C) and FeCl₃ loading (5–30 wt%) yields materials with differing surface areas, porosity, graphitization degree, nitrogen and iron content, as well as ORR activity. While the ORR activity of Fe‐free materials is limited (i.e., synthesized from pure PFA), a huge increase in activity is observed for catalysts containing Fe, revealing the participation of the metal dopant in the construction of active electrocatalytic sites. Further activity improvement is achieved via acid‐leaching and repeated pyrolysis, a result which is attributed to the creation of new active sites located at the surface of the porous nitrogen‐doped carbon by dissolution of the Fe and Fe₃C nanophases. The best performing catalyst, which was synthesized with a low Fe loading (i.e., 5 wt%) and at a pyrolysis temperature of 900 °C, exhibits high activity, excellent H₂O selectivity, extended stability, in both basic and acidic media as well as a remarkable tolerance toward methanol.     

Biological Redox Mediation in Electron Transport Chain of Bacteria for Oxygen Reduction Reaction Catalysts in Lithium–Oxygen Batteries

Governing the fundamental reaction in lithium–oxygen batteries is vital to realizing their potentially high energy density. Here, novel oxygen reduction reaction (ORR) catalysts capable of mediating the lithium and oxygen reaction within a solution‐driven discharge, which promotes the solution‐phase formation of lithium peroxide (Li₂O₂), are reported, thus enhancing the discharge capacity. The new catalysts are derived from mimicking the biological redox mediation in the electron transport chain in Escherichia coli, where vitamin K2 mediates the oxidation of flavin mononucleotide and the reduction of cytochrome b in the cell membrane. The redox potential of vitamin K2 is demonstrated to coincide with the suitable ORR potential range of lithium–oxygen batteries in aprotic solvent, thereby enabling its successful functioning as a redox mediator (RM) triggering the solution‐based discharge. The use of vitamin K2 prevents the growth of film‐like Li₂O₂ even in an ether‐based electrolyte, which has been reported to induce surface‐driven discharge and early passivation of the electrode, thus boosting the discharge capacity by ≈30 times. The similarity of the redox mediation in the biological cell and lithium–oxygen “cell” inspires the exploration of redox active bio‐organic compounds for potential high‐performance RMs toward achieving high specific energies for lithium–oxygen batteries.     

Bimetallic-Based Electrocatalysts for Oxygen Evolution Reaction

The electrochemical oxygen evolution reaction (OER) is a core electrode reaction for the renewable production of high-purity hydrogen, carbon-based fuel, synthetic ammonia, etc. However, the sluggish kinetics of the OER result in a high overpotential and limit the widespread application of OER-based technologies. Recent studies have shown that bimetallic-based materials with the synergism of different metal components to regulate the adsorption and dissociation energy of intermediates are promising OER electrocatalyst candidates with a lower cost and energy consumption. In the past two decades, tremendous efforts have been devoted to developing OER applications of bimetallic-based materials with a focus on compositions, phase, structure, etc., to highlight the synergism of different metal components. However, there is a lack of critical thinking and organized analysis of OER applications with bimetallic-based materials. This review critically discusses the challenges of developing bimetallic-based OER materials, summarizes the current optimization strategies to enhance both activity and stability, and highlights the state-of-the-art electrocatalysts for OER. The relationship between the componential/structural features of bimetallic-based materials and their electrocatalytic properties is presented to form comprehensive electronic and geometric modifications based on thorough analysis of the reported works and discuss future efforts to realize sustainable bimetallic-based OER applications.     

BiFeO_3粉体的固相合成及其微观结构研究

采用固相反应法制备了BiFeO3陶瓷粉体,研究了BiFeO3粉体在不同煅烧条件下的相结构与微观形貌的演变。结果表明,过低的煅烧温度和过短的煅烧时间得到的产物以杂相Bi25FeO40为主,提高煅烧温度和延长煅烧时间使杂相逐渐减少,有效地促进BiFeO3相的合成,但过高的煅烧温度和过长的煅烧时间则生成大量杂相Bi2Fe4O9并出现颗粒异常团聚及长大现象,800℃、1h为最佳的煅烧条件。另外,利用稀硝酸可有效清除产物中的杂相,获得较均匀、纯净的BiFeO3粉体,有利于进一步研究和应用。     

Improving the Activity for Oxygen Evolution Reaction by Tailoring Oxygen Defects in Double Perovskite Oxides

Developing low‐cost, high‐performance electro‐catalysts is essential for large‐scale application of electrochemical energy devices. In this article, reported are the findings in understanding and controlling oxygen defects in PrBa_0.5Sr_0.5Co_1.5Fe_0.5O_{5+ δ} (PBSCF) for significantly enhancing the rate of oxygen evolution reaction (OER) are reported. Utilizing surface‐sensitive characterization techniques and first‐principle calculations, it is found that excessive oxygen vacancies promote OH^? affiliation and lower the theoretical energy for the formation of O* on the surface, thus greatly facilitating the OER kinetics. On the other hand, however, oxygen vacancies also increase the energy band gap and lower the O 2p band center of PBSCF, which may hinder OER kinetics. Still, careful tuning of these competing effects has resulted in enhanced OER activity for PBSCF with oxygen defects. This work also demonstrates that oxygen defects generated by different techniques have very different characteristics, resulting in different impacts on the activity of electrodes. In particular, PBSCF nanotubes after electrochemical reduction exhibit outstanding OER activity compared with the recently reported perovskite‐based catalysts.     

Vertex‐Reinforced PtCuCo Ternary Nanoframes as Efficient and Stable Electrocatalysts for the Oxygen Reduction Reaction and the Methanol Oxidation Reaction

Noble metal binary alloy nanoframes have emerged as a new class of fuel cell electrocatalysts because of their intrinsic high catalytic surface area and accompanied high catalytic activity. However, their inferior structural and compositional stability during catalysis pose as formidable huddles to their practical applications. Herein, it is reported that introduction of an additional component to the binary catalytic system may serve as a simple and effective means of enhancing the structural and compositional stability of nanoframe‐based electrocatalysts. It is demonstrated that in situ doping of Co to the PtCu alloy nanoframe yields a ternary PtCuCo rhombic dodecahedral nanoframe (Co‐PtCu RNF) with a reinforced vertex structure. Co‐PtCu RNF exhibits superior electrocatalytic activity and durability for the oxygen reduction reaction to those of PtCu rhombic dodecahedral nanoframe (PtCu RNF) and Pt/C catalysts, due to its ternary composition and vertex‐strengthened frame structure. Furthermore, Co‐PtCu RNF shows enhanced activity for the methanol oxidation reaction as compared to PtCu RNF and Pt/C.     

Iron,Nitrogen Co-Doped Carbon Spheres as Low Cost,Scalable Electrocatalysts for the Oxygen Reduction Reaction

Atomically dispersed transition metal-nitrogen-carbon catalysts are emerging as low-cost electrocatalysts for the oxygen reduction reaction in fuel cells. However, a cost-effective and scalable synthesis strategy for these catalysts is still required, as well as a greater understanding of their mechanisms. Herein, iron, nitrogen co-doped carbon spheres (Fe@NCS) have been prepared via hydrothermal carbonization and high-temperature post carbonization. It is determined that FeN₄ is the main form of iron existing in the obtained Fe@NCS. Two different precursors containing Fe²⁺ and Fe³⁺ are compared. Both chemical and structural differences have been observed in catalysts starting from Fe²⁺ and Fe³⁺ precursors. Fe²⁺@NCS-A (starting with Fe²⁺ precursor) shows better catalytic activity for the oxygen reduction reaction. This catalyst is studied in an anion exchange membrane fuel cell. The high open-circuit voltage demonstrates the potential approach for developing high-performance, low-cost fuel cell catalysts.     

Understanding of Neighboring Fe-N4-C and Co-N4-C Dual Active Centers for Oxygen Reduction Reaction

Single atomic dispersed M-N-C (M = Fe, Co, Ni, Cu, etc.) composites display excellent performance for catalytic reactions. However, the analysis and understanding of neighboring M-N-C centers at the atomic level are still insufficient. Here, FeCo-N-doped hollow carbon nanocages (FeCo-N-HCN) with neighboring Fe-N₄-C and Co-N₄-C dual active centers as efficient catalysts are reported. Spherical aberration-corrected high angle annular dark-field scanning transmission electron microscopy, small area (1 nm²) electron energy loss spectroscopy, and X-ray absorption spectroscopy data analysis and fitting prove the neighboring Fe-N₄-C and Co-N₄-C dual active structure in FeCo-N-HCN. Experimental tests and density functional theory calculation results reveal that the FeCo-N-HCN catalyst displays better catalytic activity than Fe single-metal catalyst for oxygen reduction reaction (ORR), which is attributed to the synergistic effect of Fe-N₄-C and Co-N₄-C dual active centers reducing the reaction energy barriers for ORR. Although the catalytic performance of the FeCo-N-HCN catalyst is not comparable to the-state-of-art catalysts reported due to the low metal contents (Fe: 1.96 wt% and Co: 1.31 wt%), these results can refresh the understanding of neighboring M-N-C centers at the atomic level and provide guidance for the design of catalysts in the future.