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An oxygen reduction reaction (ORR) is the key half reaction of proton exchange membrane fuel cells (PEMFCs), and is highly dependent on Pt-based nanocrystals as core electrocatalysts. Despite the exceptional ORR activity from adjusting the electronic structures of surface or near-surface atoms, several serious issues, including the corrosion of carbon supports, the preferential leaching of active metal elements, the instability of surface low-coordinated atoms and the sintering/agglomeration of nanocrystals, still exist, challenging the ORR durability of developed Pt-based ORR catalysts. From the point of view of the catalyst structure design, in this review, we summarized the state-of-the-art structural regulation strategies for improving the ORR durability of Pt-based catalysts. The current limitation of Pt-based binary catalysts for ORR electrocatalysis is firstly discussed, and the detailed strategies are further classified into the optimization of supports, metal-doped alloys, core/shell structures, intermetallics and high-entropy alloys, etc. The structure–performance relationship is detailedly explained, especially emphasizing the elimination of the above restrictions. Finally, the existing challenges and future research direction are further presented, aiming at practicing the PEMFC devices of the ultradurable Pt-based catalysts. [ABSTRACT FROM AUTHOR]

Proton exchange membrane (PEM) fuel cells using Pt-based materials as electrocatalysts have achieved a decent performance, represented by the launched Toyota Mirai vehicle. The ideal PEM fuel cells consume stored pure hydrogen and air. However, SO2, as a primary air contaminant, may be fed along with air at the cathode, leading to Pt site deactivation. Therefore, it is important to improve the SO2 tolerance of catalysts for the stability of the oxygen reduction reaction (ORR). In this work, we develop the Pt/C-TiO2 catalyst against SO2 poisoning during ORR. Impressively, the hybrid Pt/C-TiO2 catalyst with 20 mass % TiO2 shows the best ORR and anti-toxic performance: the kinetic current density of ORR is 20.5% higher and the degradation rate after poisoning is 50% lower than Pt/C. The interaction between Pt and TiO2 as well as the abundant hydroxyl groups on the surface of TiO2 are both revealed to account for the accelerated removal of poisonous SO2 on Pt surfaces. [ABSTRACT FROM AUTHOR]

Opening up electrocatalysts for oxygen reduction reaction (ORR) is essential for practical application in fuel cells and metal-air batteries; however, how to make the catalysts with both good performance and low cost is difficult. Recently, research on the ORR of molybdenum disulfide-based catalysts in alkaline electrolytes has been on the rise. However, the development of MoS2 catalyst for acidic ORR is still in its infancy. Herein, without using reductant and morphology control reagent, we firstly obtained flowerball-like MoS2/Vulcan XC-72R (VC) nanocomposites via hydrothermal method. The designed composite exhibits a nearly 4e− ORR process with 0.78 and 0.92 V onset potentials in 0.1 M KOH and HClO4, respectively. Furthermore, the flowerball-like composite shows utmost electrochemical stability judging by 87 and 80% current retention for about 5.5 h either in alkaline or acid media, long term durability for continuous 10,000 cycles, and stronger resistance to methanol than the commercial Pt/C catalyst. The abundant Mo edges as catalytic active centers of flowerball-like structure, high electron conductivity, and enhanced mass transport in either alkaline or acidic electrolyte are favorable for catalytic performance. The prepared catalyst provides great potential for the substitution of noble metal based catalysts in fuel cells and metal-air batteries. [ABSTRACT FROM AUTHOR]

In the current work, heteroatom-doped graphene materials containing different atomic ratios of nitrogen and sulphur were employed as electrocatalysts for the oxygen reduction reaction (ORR) in acidic and alkaline media. To this end, the hydrothermal route and different chemical reducing agents were employed to synthesize the catalytic materials. The physicochemical characterization of the catalysts was performed by several techniques, such as X-ray diffraction, Raman spectroscopy and elemental analysis; meanwhile, the electrochemical performance of the materials toward the ORR was analyzed by linear sweep voltammetry (LSV), rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) techniques. The main results indicate that the ORR using heteroatom-doped graphene is a direct four-electron pathway, for which the catalytic activity is higher in alkaline than in acidic media. Indeed, a change of the reaction mechanism was observed with the insertion of N into the graphenic network, by the rate determining step changes from the first electrochemical step (formation of adsorbed OOH) on glassy carbon to the removal of adsorbed O (Oad) from the N-graphene surface. Moreover, the addition of sulphur atoms into the N-graphene structure increases the catalytic activity toward the ORR, as the desorption of Oad is accelerated. [ABSTRACT FROM AUTHOR]

Nitrogen-doped ordered mesoporous carbon was synthesized by using polyaniline as the carbon source and SBA-15 as the template. The microstructure, composition and electrochemical behavior were extensively investigated by the nitrogen sorption isotherm, X-ray photoelectron spectroscopy, cyclic voltammetry and rotating ring-disk electrode. It is found that the pyrolysis temperature yielded a considerable effect on the pore structure, elemental composition and chemical configuration. The pyrolysis temperature from 800 to 1100 °C yielded a volcano-shape relationship with both the specific surface area and the content of the nitrogen-activated carbon. Electrochemical tests showed that the electrocatalytic activity followed a similar volcano-shape relationship, and the carbon catalyst synthesized at 1000 °C yielded the best performance. The post-treatment in NH3 was found to further increase the specific surface area and to enhance the nitrogen doping, especially the edge-type nitrogen, which favored the oxygen reduction reaction in both acid and alkaline media. The above findings shed light on electrocatalysis and offer more strategies for the controllable synthesis of the doped carbon catalyst. [ABSTRACT FROM AUTHOR]

To explore high-performing alternatives to platinum-based catalysts is highly desirable for lowering costs and thus promoting fuel cell commercialization. Herein, self-supported Fe-N-C materials were prepared by the pyrolysis of dual precursors including EDTA ferric sodium (EDTAFeNa) and melamine (MA), followed by acid-leaching and final annealing. Towards an oxygen reduction reaction (ORR) in 0.1 M KOH, the as-prepared MA/EDTAFeNa-HT2 delivered onset (Eonset) and half-wave (E1/2) potentials of 0.97 and 0.84 V vs. RHE, respectively, identical with that of a state-of-the-art Pt/C catalyst, accompanied with predominantly a four-electron pathway. The introduction of MA and extension of acid-leaching promoted a positive shift of 50 mV for E1/2 relative to that of only the EDTAFeNa-derived counterpart. It was revealed that the enhancement of ORR activity is attributed to a decrease in magnetic Fe species and increase in pyridinic/quanternary nitrogen content whilst nearly excluding effects of the graphitization degree, variety of crystalline iron species, and mesoscopic structure. The usage of dual precursors exhibited great potential for the large-scale production of inexpensive and efficient Fe-N-C materials. [ABSTRACT FROM AUTHOR]

Structurally ordered intermetallic structure is an efficient catalyst design strategy to significantly improve the catalytic performance of Pt alloy electrocatalysts for oxygen reduction reaction in fuel cells. However, a high structural ordering degree generally relies on high-temperature annealing, which results in detrimental catalyst particle sintering. Herein, we reveal that the incompatibility between high ordering degree and minimum particle sintering during thermal annealing can be resolved through rational development of structurally ordered Pt ternary alloys. Ordering transformation mediated by high-temperature annealing of three representative Pt ternary alloys (Pt–Fe–Co, Pt–Ni–Co and Pt–Fe–Ni) at a similar Pt composition was systematically studied. It was found that Fe can significantly promote the structural ordering due to a faster atomic diffusion, whereas Co can effectively inhibit the particle sintering. As a result of the synergy between Co and Fe, the ordered PtCoFe catalyst exhibited the highest ordering degree after thermal annealing at 600 °C with the minimum nanoparticle growth, leading to the highest catalytic activity (0.65 A/mgPt at 0.9 V, 4 times that of pure Pt catalyst) and best stability (16% drop after 10,000 potential cycles). This study provides important clues for the rational design of high-performance structurally ordered ternary Pt alloys. [ABSTRACT FROM AUTHOR]

Electrocatalysis for the oxygen reduction reaction (ORR) at the cathode plays a critical role in fuel cells and metal-air batteries. However, the high-cost and sluggish kinetics of the catalytic reaction have hindered its development. Therefore, developing efficient catalysts to address these issues is of vital significance. In this work, we summarized the recent progress of nitrogen (N)-doped metal-free catalysts for the ORR, owing to their high catalytic activity (comparable to Pt/C) and cost-effectiveness. The synthetic strategy and the morphology structure to catalytic performance are mainly discussed. Furthermore, the design of N-doped nanomaterials with other heteroatoms in aiming to further enhance the ORR performance is also reviewed. At the end of the review, we provide a brief summary of the N-doped carbon-based catalysts in enhancing the ORR performance and give future perspectives for their further development. [ABSTRACT FROM AUTHOR]