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Pt‐based electrocatalysts are considered as one of the most promising choices to facilitate the oxygen reduction reaction (ORR), and the key factor enabling their success is to reduce the required amount of platinum. Herein, we focus on illuminating both the theoretical mechanisms which enable enhanced and sustained ORR activity and the practical methods to achieve them in catalysts. The various multi‐step pathways of ORR are firstly reviewed and the rate‐determining steps based on the reaction intermediates and their binding energies are analyzed. We then explain the critical aspects of Pt‐based electrocatalysts to tune oxygen reduction properties from the viewpoints of active sites exposure and altering the surface electronic structure, and further summarize representative research progress towards practically achieving these activity enhancements with a focus on platinum size reduction, shape control and core Pt elimination methods. We finally outline the remaining challenges and provide our perspectives with regard to further enhancing their activities. [ABSTRACT FROM AUTHOR]

Combining the advantages of homogeneous and heterogeneous catalysts, single‐atom catalysts (SACs) are bringing new opportunities to revolutionize ORR catalysis in terms of cost, activity and durability. However, the lack of high‐performance SACs as well as the fundamental understanding of their unique catalytic mechanisms call for serious advances in this field. Herein, for the first time, we develop an Ir‐N‐C single‐atom catalyst (Ir‐SAC) which mimics homogeneous iridium porphyrins for high‐efficiency ORR catalysis. In accordance with theoretical predictions, the as‐developed Ir‐SAC exhibits orders of magnitude higher ORR activity than iridium nanoparticles with a record‐high turnover frequency (TOF) of 24.3 e− site−1 s−1 at 0.85 V vs. RHE) and an impressive mass activity of 12.2 A mg−1Ir, which far outperforms the previously reported SACs and commercial Pt/C. Atomic structural characterizations and density functional theory calculations reveal that the high activity of Ir‐SAC is attributed to the moderate adsorption energy of reaction intermediates on the mononuclear iridium ion coordinated with four nitrogen atom sites. [ABSTRACT FROM AUTHOR]

Highly efficient transition-metal electrocatalysts hold great promise for overcoming the sluggish kinetics of the oxygen reduction reaction (ORR), while the dense stacking of active sites within bulk materials constrains electrocatalytic behaviors. Therefore, nano-structure engineering to obtain hierarchal morphology is crucial to enrich the active sites and facilitate the corresponding mass transfer. Here, the three-dimensional interconnected and ordered mesoporous (3DOM) Fe 2 N x decorated on TiO y (Fe 2 N x @TiO y) is constructed. By introducing nitrogen vacancies, the increased surface area, and active sites boost ORR kinetics, including a high half-wave potential (0.88 V vs reversible hydrogen electrode) and high current density (71 mA cm−2 at 0.8 V) have been reached. The zinc-air battery assembled with Fe 2 N x @TiO y catalysts presents a high specific capacity of 809 mAh g−1. Density functional theory analysis and X-ray absorption spectroscopy further confirm the promoter effects of nitrogen vacancies on modulating electronic structure of Fe, through regulating intermediates adsorption/desorption. The shift of its d -band center is also found toward the Fermi energy level, strengthening the adsorbate-substrate interaction. This allows oxygen species to be favorably stabilized onto active sites of Fe 2 N x @TiO y. [Display omitted] • The three-dimensional interconnected and ordered mesoporous Fe 2 N x embedded on TiO y (Fe 2 N x @TiO y) is fabricated. • The coordination environment of Fe sites is regulated through introducing nitrogen vacancies. • Nitrogen vacancies upshift the d band level of Fe active sites to the Fermi level, result in binding energy increases of the adsorbents. • Fe 2 N x @TiO y shows outstanding performance to power oxygen reduction reactions and zinc-air batteries. [ABSTRACT FROM AUTHOR]

We report a simple and efficient surfactant-free method to prepare 3D porous PtCu networks with ultrathin jagged nanowires and controllable composition. The morphological evolution and the influential effects of the important experimental parameters on the PtCu networks have been systematically studied. Relative to commercial Pt/C and Pt black catalysts, these porous PtCu networks exhibit much better activity and remarkably improved durability towards the oxygen reduction reaction (ORR). The excellent ORR performance could be attributed to their structural features, including the core-shell nanostructures with a Pt-skin, the 3D porous networks with high surface area, and the ultrathin (3.6 nm) jagged nanowires with plentiful edge/corner atoms. Notably, this method can be facilely extended to obtain PtCuAu trimetallic nanowire networks with high porosity, which exhibits its robustness for preparing novel 3D porous nanostructures with great potential in various catalytic applications. [ABSTRACT FROM AUTHOR]

Nanoscale architecturing of platinum group metal-free (PGM-free) electrocatalysts is expected to dramatically improve the overall catalytic performance for oxygen reduction reaction (ORR). Desired structures and morphologies for boosting active site density and enhancing mass and charge transfer are essential for developing next-generation PGM-free electrocatalysts. Herein, we report the design of a M-N-C type catalyst consisting of 3-dimensional graphitic meso-porous carbon spheres wrapped with 2-dimensional graphenized sheets. This heterostructure comprises resultant large electroactive surface area, abundant pore channels, and tuned chemical structures, which provide improved electrocatalytic performance. Meanwhile, these pore structures can be regarded as nano-channel reactors to catalyze ORR with easily accessible active sites, effective mass transfer, and smooth charge transfer. The obtained catalyst delivers a high maximum power density of 0.83 W cm −2 in a single H 2 –O 2 fuel cell measurement, ranking it as one of the most promising PGM-free catalysts in proton exchange membrane fuel cells (PEMFCs). Moreover, reasonable fuel cell stability was also observed through accelerated degradation testing. This work provides a new avenue for PGM-free catalysts design that can be a step towards practical commercial of PEMFCs. [ABSTRACT FROM AUTHOR]

A facile polymer-silicon-dual-template strategy is developed to construct mesoporous carbon spheres with densely and accessible atomic Fe-Nx sites. This result in an efficient air electrode with 3D hierarchically porous frameworks, which leads to enhanced mass transfer and consequently superior aluminum-air flow battery performance. [Display omitted] • Dens Fe-Nx sites coupled mesoporous carbon spheres are constructed by a facile dual-template strategy. • Single atomic Fe-Nx moieties could serve as highly active and stable centers for catalyzing ORR. • The 3D porous air electrode accelerates oxygen transfer and enhances accessibility of Fe-Nx sites. • The assembled aluminum-air flow battery delivers high gravimetric energy density and superior discharging stability. Single atomic iron modified nitrogen-doped carbons (SA Fe-N-C) are promising alternatives to Pt-based counterparts for facilitating the sluggish oxygen reduction reaction (ORR). However, both the low density of Fe-Nx active sites and their less utilization render these catalysts inefficient. To address these issues, a synergistic polymer-silicon dual templates strategy is reported to fabricate dense SA Fe-Nx sites on mesoporous carbon spheres (SA-Fe-Nx-MPCS). Thanks to the dense and accessible SA Fe-Nx sites, the SA-Fe-Nx-MPCS exhibits better ORR catalytic activity and stability than the commercial Pt/C catalyst. Moreover, the SA-Fe-Nx-MPCS result in a 3D porous framework with abundant channels in air cathode, facilitating mass transfer and enhancing accessibility of Fe-Nx moieties. The assembled aluminum-air flow battery exhibits a large peak power density of 130 mW cm−2 and a high gravimetric energy density of ca. 2905 Wh kg Al −1, as well as a superior stable discharging voltage over 300 h. [ABSTRACT FROM AUTHOR]

Metal-nitrogen-carbon (M-N-C) materials show great advantages for catalyzing the oxygen reduction reaction (ORR) in proton-exchange membrane fuel cells (PEMFCs). However, both the low density of single atomic (SA) MNx active sites and restricted mass transfer render these M-N-C based air electrodes inferior in cell performance. In this study, a new ZIF8-derived Fe-N-C catalyst/electrode design combining local chemistry tuning and primary morphology tailoring to address the above two critical issues is shown. The introduction of nitrogen-carbon defects in ZIF8 host enables a controlled atomic-scale dispersion of FeNx moieties, increasing their content in support materials. Also, the simultaneous structural arrangement of individual ZIF8 nano-grains endows the catalyst with a unique porous micro-spheric morphology. This result in an advanced 3D air electrode featuring dense SA FeNx sites and ample, multiscale macro-sized pore channels, which can significantly increase the intrinsic catalytic activity, facilitate bulk mass transport, and generate more effective triple-phase interfaces for ORR. The present catalyst/electrode design exhibits a record large peak power density of ca. 0.60 W cm−2 under practical air conditions. This approach provides a feasible way for boosting the air cathode interfacial ORR and further enlightens electrode designs for energy devices involving multiphase electrochemical reactions. Macro-scale hierarchically porous electrode coupled with enriched atomic FeNx sites is designed to boost the triple-phase interfacial oxygen electrocatalysis. Benefiting from the enhanced intrinsic catalytic activity and improved mass transfer, this air electrode delivers superior high-power density in hydrogen-air PEMFC. [Display omitted] • Local chemistry turning and morphology tailoring of the catalyst are simultaneously achieved by a simple strategy. • The introduction of N-C defects promotes the atomic-scale dispersion of FeNx sits and their contents in catalyst. • The structural arrangement of nanosized carbon supports leads to multiscale hierarchically porous 3D air electrode. • The dense single atomic FeNx sites and enhanced mass transfer boost the interfacial ORR at cathode. • The H 2 -air PEMFC with this air cathode delivers a record high power density of ca. 0.60 W cm−2. [ABSTRACT FROM AUTHOR]

In article number 1803737 Dong Su, Zhengyu Bai, Zhongwei Chen and co‐workers report an edge engineering strategy for boosting oxygen reduction performance of Fe–N–C catalysts. By generating abundant N‐enriched edges, the number of FeN4 sites as well as their intrinsic activity and utilization are substantially increased, which is vital for the development of next generation H2‐air fuel cells. [ABSTRACT FROM AUTHOR]

Pt-based nanomaterials are regarded as the most efficient electrocatalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). However, widespread adoption of PEMFCs requires solutions to major challenges encountered with ORR catalysts, namely high cost, sluggish kinetics, and low durability. Herein, a new efficient method utilizing Co-based metal-organic frameworks is developed to produce PtCo bimetallic nanoparticles embedded in unique nitrogen-doped hollow porous carbon capsules. The obtained catalyst demonstrates an outstanding ORR performance, with a mass activity that is 5.5 and 13.5 times greater than that of commercial Pt/C and Pt black, respectively. Most importantly, the product exhibits dramatically improved durability in terms of both electrochemically active surface area (ECAS) and mass activity compared to commercial Pt/C and Pt black catalysts. The remarkable ORR performance demonstrated here can be attributed to the structural features of the catalyst (its alloy structure, high dispersion and fine particle size) and the carbon support (its nitrogen dopant, large surface area and hollow porous structure). [ABSTRACT FROM AUTHOR]

Replacing high cost platinum (Pt) catalysts with alternative non-precious materials for the oxygen reduction reaction (ORR) is highly desirable to reduce the cost of polymer electrolyte membrane fuel cell (PEMFC) systems. Transition metal-nitrogen-carbon complexes (M-N-C) prepared by high temperature heat treatment techniques are the most promising class of non-precious materials with respect to ORR activity and stability. In this work, one-dimensional nanofibers are prepared by electrospinning an iron–polyaniline/polyacrylonitrile (Fe–PANI-PAN) metal-polymer blend, followed by subsequent heat treatment. This represents the first time a blend of polymers (PANI-PAN) has been used to prepare M-N-C nanofiber catalysts via electrospinning. The addition of 10 wt. % PANI to the electrospinning mixture provides 100 and 70 mV improvements to the ORR onset potential and half-wave potential, respectively, rendering the most active non-precious ORR nanofiber catalysts prepared by electrospinning to date. The high activity is attributed to the porous structure of the nanofibers, combined with increased nitrogen contents provided by the PANI incorporation. [ABSTRACT FROM AUTHOR]