Oxygen Reduction Reaction Durability Research Articles

Advanced transition metal/nitrogen/carbon-based electrocatalysts for fuel cell applications

The development of advanced transition metal/nitrogen/carbon-based (M/N/C) catalysts with high activity and extended durability for oxygen reduction reaction (ORR) is critical for platinum-group-metal (PGM) free fuel cells but still remains great challenging. In this review, we summarize the recent progress in two typical M/N/C catalysts (atomically dispersed metal-nitrogen-carbon (M-N-C) catalysts and carbon-supported metal nanoparticles with N-doped carbon shells (M@NC)) with an emphasis on their potential applications in fuel cells. Starting with understanding the active sites in these two types of catalysts, the representative innovative strategies for enhancing their intrinsic activity and increasing the density of these sites are systematically introduced. The synergistic effects of M-N-C and M@NC are subsequently discussed for those M/N/C catalysts combining both of them. To translate the material-level catalyst performance into high-performance devices, we also include the recent progress in engineering the porous structure and durability of M/N/C catalysts towards efficient performance in fuel cell devices. From the viewpoint of industrial applications, the scale-up cost-effective synthesis of M/N/C catalysts has been lastly briefed. With this knowledge, the challenges and perspectives in designing advanced M/N/C catalysts for potential PGM-free fuel cells are proposed.

Alkaline earth metal based single atom catalyst for the highly durable oxygen reduction reaction

Developing nonprecious group single-atom catalyst (SACs) based on alkaline earth metal has been rarely explored. Mg metal hampered the catalytic activity towards ORR due to the strong bonding of active centers with oxygenated group intermediate, whereas N-bonded Mg atoms have optimal bonding strength with intermediate oxygen species by adjusting the p-band center position. The co-ordination environment of Mg-Nx plays very crucial role in exhibiting the excellent catalytic activity towards ORR. We provide very simple MOF free methodology without any post acid treatment or any subsidiary sacrificial metal like Zn. The Mg-N-C catalyst exhibited a half-wave potential of 0.80 V versus the reversible hydrogen electrode, approaching the recently reported Fe-N-C catalyst. Electrochemical calculation further support the Mg-Nx sites as the origin of ORR via efficient 4-electron transfer pathway in basic medium. Importantly, current density is found to decrease less than 2% in diffusion limited current and loss of activity by only 13% at 0.9 V after 10,000 cycles in alkaline medium which far superior to the durability limit set by the US department of energy and overpasses the state-of-the-art Pt/C catalyst. The charge transfer resistance is the crucial parameter influencing on the ORR activity along with (pyridinic N+ pyrrolic N) content and degree of graphitization. This methodology can be applied to design a variety of other alkaline earth metal based M-N-C electrocatalysts and studied for various applications.

Bimetallic Ag–Cu nanosheets assembled flower-like structure for oxygen reduction reaction

Fabricating low-cost and Pt-free fuel cell catalysts towards highly active and durable oxygen-reduction reaction catalysis remains a great challenge. Herein, we synthesize a bimetallic Ag–Cu nanosheets assembled flower-like structure by our developed cyclic scanning electrodeposition (CSE) method. The formation mechanism of this structure is investigated systematically, where Cu2+ can affect oxidative etching during the formation process. The prepared bimetallic nanostructure is studied for kinetics towards oxygen reduction reaction (ORR) in alkaline media. The electrochemical test results indicate that the ORR catalytic ability of bimetallic Ag–Cu nanostructure is competitive with Pt/C-20%, better than Pt disk, and far superior to the pure Ag nanostructure. The electrons transfer number (n) is close to four, which is further proved by scanning electrochemical microscope (SECM) experiments. Density functional theory (DFT) calculations reveal that the high ORR activity is attributable to the widely distributed Ag–Cu steps, which is formed by decorating Cu atoms on the Ag nanosheets.

Synergistic Bimetallic Metallic Organic Framework-Derived Pt-Co Oxygen Reduction Electrocatalysts.

The rational design of Pt-based electrocatalysts is of paramount importance for the commercialization of proton exchange membrane fuel cells (PEMFCs). Pt-Co alloys and nitrogen-doped carbons have been shown to be effective in enhancing the kinetics of the oxygen reduction reaction (ORR). Herein, we reported on two kinds of Pt-Co electrocatalysts, PtCo ordered intermetallic and PtCo2 disordered alloys, supported on bimetallic MOF-derived N-doped carbon. The synergistic interaction between Pt-Co nanoparticles and Co-N-C enhanced the overall ORR activity and maintained the integrity of both structures and their electrochemical properties during long-term stability testing. The optimal activity for both PtCo and PtCo2 occurred after 20 000 potential cycles. The enhanced performance of PtCo was ascribed to the formation of a two-atomic-layer Pt-rich shell and the lattice strain caused by the core-shell PtCo@Pt structure. The increased activity of PtCo2 was ascribed to the formation of large, spongy, and small solid nanoparticles during electrochemical dealloying and thus the exposure of more Pt sites on the surface. The strategy described herein advances our understanding of the structure-activity relationship in electrocatalysis and sheds light on the future development of more active and durable ORR electrocatalysts.

Intermetallic PtCu Nanoframes as Efficient Oxygen Reduction Electrocatalysts.

Nanoframe alloy structures represent a class of high-performance catalysts for the oxygen reduction reaction (ORR), owing to their high active surface area, efficient molecular accessibility, and nanoconfinement effect. However, structural and chemical instabilities of nanoframes remain an important challenge. Here, we report the synthesis of PtCu nanoframes constructed with an atomically ordered intermetallic structure (O-PtCuNF/C) showing high ORR activity, durability, and chemical stability. We rationally designed the O-PtCuNF/C catalyst by combining theoretical composition predictions with a silica-coating-mediated synthesis. The O-PtCuNF/C combines intensified strain and ligand effects from the intermetallic PtCu L11 structure and advantages of the nanoframes, resulting in superior ORR activity to disordered alloy PtCu nanoframes (D-PtCuNF/C) and commercial Pt/C catalysts. Importantly, the O-PtCuNF/C showed the highest ORR mass activity among PtCu-based catalysts. Furthermore, the O-PtCuNF/C exhibited higher ORR durability and far less etching of constituent atoms than D-PtCuNF/C and Pt/C, attesting to the chemically stable nature of the intermetallic structure.

Effect of lanthanide (Ln=La, Nd, and Pr) doping on electrochemical performance of Ln2NiO4+δ−YSZ composite cathodes for solid oxide fuel cells

In this study, we prepared three different Ruddlesden−Popper (RP)-phase nickelate catalysts, i.e., La2NiO4+δ (LNO), Nd2NiO4+δ (NNO), and Pr2NiO4+δ (PNO) via solid-state route. These materials were used to fabricate composite cathodes through their combination with yttria stabilized zirconia (YSZ), which is the most widely used electrolyte material for solid oxide fuel cells. X-ray diffraction (XRD) analysis revealed that after the initial sintering all three RP-based materials had good chemical compatibility with YSZ without any reactive impurities. The electrochemical impedance spectroscopy study showed that, among the three samples, the PNO−YSZ cathode exhibited the best oxygen reduction reaction (ORR) activity, with an area specific resistance (ASR) of 0.64 Ω cm2 at 800 °C, which was 16% and 41% lower compared to that of NNO-YSZ (0.76 Ω cm2), and LNO-YSZ (1.09 Ω cm2), respectively. During the initial long-term operation for 60 h at 800 °C, the measured ASR degradation rate of LNO−YSZ was 3.56 %h−1, while NNO−YSZ and PNO−YSZ showed better stability in comparison (2.23 and 1.87 %h−1, respectively). The post XRD analysis revealed the presence of a pyrochlore secondary phases (Ln2Zr2O7) in all three samples, which was considered a major degradation mechanism of Ln2NiO4+δ-YSZ composite cathodes. However, the amount of the impurity phase exhibited the same tendency as the degradation rate of different cathodes (LNO > NNO > PNO), demonstrating that Pr doping at the A-site of the RP-phase effectively suppressed the formation of a the pyrochlore impurity phase against YSZ. Thus, our results suggest that the ORR activity and durability of Ln2NiO4+δ−YSZ composite cathodes strongly depends on the A-site lanthanides.

The enhanced activity of Pt\u2013Ce nanoalloy for oxygen electroreduction

The widespread use of low-temperature polymer electrolyte membrane fuel cells for clean energy source require significant reductions in the amount of expensive electrocatalyst Pt for the oxygen reduction reaction (ORR). Pt based binary alloys are promising materials for more active and stable electrocatalysts. In this paper, we studied Pt–Ce nanoalloy, which was prepared by hydrogen reduction techniques as ORR electrocatalysts. Among all PtCe alloy catalysts, the PtCe/C-800 ℃ shows superior ORR activity, stability and durability compared to commercial Pt/C. The results presented in this paper will provide the future perspectives to research based on Pt-RE (RE = Ce, Dy, Gd, Er, Sm, and La) alloy as an novel electrocatalyst for various electrocatalytic reactions.

Superior Fe x N electrocatalyst derived from 1,1′-diacetylferrocene for oxygen reduction reaction in alkaline and acidic media

Abstract Although Fe–N/C catalysts have received increasing attention in recent years for oxygen reduction reaction (ORR), it is still challenging to precisely control the active sites during the preparation. Herein, we report Fe x N@RGO catalysts with the size of 2–6 nm derived from the pyrolysis of graphene oxide and 1,1′-diacetylferrocene as C and Fe precursors under the NH3/Ar atmosphere as N source. The 1,1′-diacetylferrocene transforms to Fe3O4 at 600°C and transforms to Fe3N and Fe2N at 700°C and 800°C, respectively. The as-prepared Fe x N@RGO catalysts exhibited superior electrocatalytic activities in acidic and alkaline media compared with the commercial 10% Pt/C, in terms of electrochemical surface area, onset potential, half-wave potential, number of electrons transferred, kinetic current density, and exchange current density. In addition, the stability of FGN-8 also outperformed commercial 10% Pt/C after 10000 cycles, which demonstrates the as-prepared Fe x N@RGO as durable and active ORR catalysts in acidic media.

The marriage of crystalline/amorphous Co/Co3O4 heterostructures with N-doped hollow carbon spheres: efficient and durable catalysts for oxygen reduction

The design of non-precious metal-N-C (M-N-C) materials with efficient oxygen reduction activity are extremely necessary for renewable energy technologies. These M-N-C electrocatalysts are often composed of metal nanoparticles and M-Nx sites simultaneously due to their structural heterogeneity induced by the high-temperature strategy. However, these metal nanoparticles usually suffer from a partial electrochemical/chemical oxidation or dissolution in strong alkaline electrolytes, which may decrease the number of active sites for electrocatalytic oxygen reduction reaction (ORR). Herein, we demonstrated a facile strategy to get this obstacle that fabricate the core-shell crystalline/amorphous Co/Co3O4 heterostructures incorporated in N-doped hollow carbon spheres (Co/Co3O4@NHCS). Specifically, the amorphous Co3O4 shell could effectively prevent the electrochemical oxidation/dissolution of metal Co core in strong alkaline solution. It could also create abundant interface that provide sufficient electron supply and reaction places, and thus make it promising the high efficiency of electron transfer. Furthermore, the N-doped defective graphitic carbon could optimize and facilitate the charge/mass transport that could maximize the catalytic activity. Deliberately coupling the crystalline/amorphous Co/Co3O4 heterostructures with Co-Nx sites are advantageous to achieve the remarkable catalytic activity and superior durability for ORR in alkaline media. As expected, the resulted Co/Co3O4@NHCS catalysts possess the superior ORR activity with a half-wave potential of 0.82V (vs. RHE), similar to that of the commercial Pt/C. Most importantly, it shows excellent durability with almost negligible loss in catalytic performance after 3500 cycles. This work opens an avenue for rational design the efficient ORR electrocatalysts toward practical applications in fuel cells.

Interconnected porous nitrogen-doped carbon framework: Recoverable template fabrication and excellent electrocatalytic performance for oxygen reduction reaction

The development of efficient metal-free catalysts for oxygen reduction reaction (ORR) is of great importance for accelerating the commercial application of fuel cell. Here, a 3D interconnected N-doped porous carbon skeleton (PC) was prepared by one-step pyrolysis strategy with the help of recyclable NaCl. Combining the advantages of template and pyrolysis, the obtained N-PC-900 product showed a large specific surface area (470.9 m2 g−1), porous structure and high N contents (pyridinic N 30.2% and graphitic N 40.2%). The high ID1/IG value of 1.76 suggested that N-PC-900 had more defects which could be produced at 900 °C. In alkaline condition, the prepared N-PC-900 exhibited positive onset potential (0.90 V), half-wave potential (0.75 V) and high current density (5.38 mA/cm2 at 0.2 V), which was close to the 20% Pt/C. The number of transferred electrons was 3.87–3.98 at a potential of 0.055–0.9 V, meaning a 4e− pathway towards the ORR. Meanwhile, the N-PC-900 displayed better durability and methanol tolerance for ORR in both acid and alkaline conditions than 20%Pt/C. This study provided a convenient and low-cost method to prepare high-performance metal-free ORR catalysts, and the product may have great potentials in ORR catalysis.

A stable PdCu@Pd core-shell nanobranches with enhanced activity and methanol-tolerant for oxygen reduction reaction

Palladium-based catalyst represents one of the most promising candidates among all non-platinum oxygen reduction reaction (ORR) catalysts. However, it is still challenging to precisely engineering their morphology and composition. Herein, highly branched core-shell PdCu@Pd nanocrystals have been fabricated via a simple one-pot synthesis that co-directed by hydrogen gas and halide ions. Such unique PdCu@Pd nanocrystals exhibit prominent ORR activity, durability and methanol-tolerance in alkaline media owing to their core-shell composition, highly-branched structure and defect-rich surface. This work elucidates a facile strategy to design highly branched core-shell nanomaterials, shedding light on the exploration and optimization of novel non-platinum ORR catalysts.

Gradient-Concentration Design of Stable Core-Shell Nanostructure for Acidic Oxygen Reduction Electrocatalysis.

Manipulating the surface structure of electrocatalysts at the atomic level is of primary importance to simultaneously achieve the activity and stability dual-criteria in oxygen reduction reaction (ORR) for proton exchange membrane fuel cells. Here, a durable acidic ORR electrocatalyst with the "defective-armored" structure of Pt shell and Pt-Ni core nanoparticle decorated on graphene (Pt-Ni@PtD /G) using a facile and controllable galvanic replacement reaction to generate gradient distribution of Pt-Ni composition from surface to interior, followed by a partial dealloying approach, leaching the minor nickel atoms on the surface to generate defective Pt skeleton shell, is reported. The Pt-Ni@PtD /G catalyst shows impressive performance for ORR in acidic (0.1 m HClO4 ) electrolyte, with a high mass activity of threefold higher than that of Pt/C catalyst owing to the tuned electronic structure of locally concave Pt surface sites through synergetic contributions of Pt-Ni core and defective Pt shell. More importantly, the electrochemically active surface areas still retain 96% after 20 000 potential cycles, attributing to the Pt atomic shell acting as the protective "armor" to prevent interior Ni atoms from further dissolution during the long-term operation.

Highly active N-doped carbon encapsulated Pd-Fe intermetallic nanoparticles for the oxygen reduction reaction

Developing highly efficient non-Pt catalysts for fuel cells and metal-air batteries is highly desirable but still challenging due to the sluggish oxygen reduction reaction (ORR). Herein, a facile and efficient strategy is demonstrated to prepare N-doped carbon encapsulated ordered Pd-Fe intermetallic (O-Pd-Fe@NC/C) nanoparticles via a one-step thermal annealing method. The obtained O-Pd-Fe@NC/C nanoparticles show enhanced ORR activity, durability and anti-poisoning capacity in both acid and alkaline medium. When O-Pd-Fe@NC/C serving as cathode catalyst for Zn-air battery, it exhibits higher voltage platform and superior cycling performance with respect to the Zn-air battery based on the mixture of Pt/C and Ir/C catalysts. The enhanced electrocatalytic performance can be ascribed to the formation of face-centered tetragonal (fct) Pd-Fe nanoparticles, the protective action of the N-doped carbon layer and the interface confinement effect between them. The in situ formed N-doped carbon shell not only restrains the Pd-Fe ordered intermetallics from aggregating effectively during the thermal annealing process, but also provides a strong anchoring effect to avoid the detachment of Pd-Fe nanoparticles from the carbon support during the potential cycling. This facile carbon encapsulation strategy may also be extended to the preparation of a wide variety of N-doped carbon encapsulated intermetallic compounds for fuel cell application.

Rational design of perfect interface coupling to boost electrocatalytical oxygen reduction

Abstract The rational design and preparation of efficient Pt-based electrocatalysts towards the oxygen reduction reaction (ORR) is a key issue for the widespread application of hydrogen fuel cells. Despite many Pt/support hybrid materials that have been reported as promising electrocatalysts for the ORR, precisely controlling the contact facet between the Pt and the support has never been demonstrated to mediate the ORR. Herein, based on theoretical calculations, we constructed an interesting Pt/support electrocatalyst by chemically coupling Pt nanoparticles (NPs) with single crystal (Sc) LiTiO2 nano-octahedra (Pt NPs/Sc-LiTiO2) for the first time. Specifically, the Pt {111} atomic crystal planes are in contact with the highly lattice-matched Sc-LiTiO2 {111} crystal planes to form specific crystal-plane coupling heterostructures, leading to strong cooperative effects between the Sc-LiTiO2 and the Pt as well as to the epitaxial growth and favorable exposed facets of Pt on the surface of Sc-LiTiO2. These key features endow Pt NPs/Sc-LiTiO2 with a mass activity of 1.44 A mg−1Pt and specific activity of 1.78 mA cm− 2 at 0.9 V, which are 8.0 and 7.1-fold higher than those of the state-of-the-art Pt/C, respectively. Meanwhile, it can undergo 20000 sweep cycles with negligible activity decay and no obvious changes in morphology or composition, showing excellent ORR durability. The resulting ORR performance is also comparable to or even better than those of most first-class Pt-based electrocatalysts. Our synthetic strategy could be easily extended to the design and fabrication of other robust metal/support electrocatalysts.

Pt-on-Pd Dendritic Nanosheets with Enhanced Bifunctional Fuel Cell Catalytic Performance.

Pd-Pt bimetallic nanocrystals have become appealing in the electrocatalytic field by virtue of their synergy effects derived from the electronic coupling between two metals. Herein, a facile seed-mediated growth approach is reported for synthesis of Pt-on-Pd dendritic nanosheets (DNSs) through the growth of Pt branches on ultrathin Pd nanosheets (NSs). The as-obtained Pt-on-Pd DNSs exhibit superior catalytic activity toward both oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR), with mass activities (MAs) 2.2 times higher for ORR and 3.4 times higher for MOR than commercial Pt/C catalysts. Moreover, these spatially separated Pt branches supported on 2D NSs also endow the Pt-on-Pd DNSs with impressive durability for ORR with only 18.9% loss in MA, whereas the Pt/C catalyst loses 50.0% after 10,000-cycle accelerated durability tests. This 2D DNS architecture can be extended to other 2D metallic NS substrates for constructing Pt-based electrocatalysts with excellent electrocatalytic performance.

CTAB-capped copper nanoparticles coated on N doped carbon layer and encapsulated in ZIF-67: a highly-efficient ORR catalyst

The development of cathode materials with high cost-effectiveness, reliable performance, and good oxygen reduction activity are important to develop the next generation rechargeable metal-air battery system. Herein, we have developed a novel technique for synthesizing a catalyst with superior activity and durability for oxygen reduction reaction (ORR). Porous carbon with doped Cu nanoparticles was prepared by pyrolyzing Cu-containing ZIF-67 (Cu/ZIF-67). The findings show that Cu0.5%/Co-NC-900 exhibits superior ORR performance with an onset potential (Eo) of 0.98 V and a half-wave potential (E1/2) of 0.85 V, superior to all other prepared as well commercial Pt/C (20 wt.%, Eo 0.96 V and E1/2 0.82 V) catalysts in an alkaline solution. The excellent ORR performance of Cu0.5%/Co-NC-900 is due to the synergistic contribution to gradual mass transfer of Cu, Co and N doping structure, high pyridine content, and porous multilayer structure. This novel and rational metal doping strategy will dramatically enhance electrocatalyst operation and provide a template for new metal-doped porous carbon or catalyst designs.

A magnetic field strategy to porous Pt-Ni nanoparticles with predominant (111) facets for enhanced electrocatalytic oxygen reduction

For the first time, a magnetic field strategy was introduced in obtaining graphene supported porous Pt-Ni alloying nanoparticles with predominant (111) facets as efficient catalysts for enhancing electrocatalytic oxygen reduction. For the first time, we developed porous Pt-Ni alloying nanoparticles with predominant (111) facets under intense magnetic fields. Electrochemical analysis revealed that the Pt-Ni alloying nanoparticles obtained at 2 Tesla exhibited a superior catalytic activity and durability for oxygen reduction reaction. This work demonstrated that the imposition of intense magnetic field could be considered as a new approach for developing efficient alloying electrocatalysts with preferential facets.

Integrating Multiple Advantages into 1 Nm Pt3Ni Bimetallic Alloy Nanowires for Oxygen Reduction Reaction

Exploiting highly active and durable oxygen reduction reaction (ORR) electrocatalyst is still imperative for clean and efficient energy conversion device, such as fuel cells and metal-air batteries. For this purpose and maximize the utilization of noble Pt, we present here a facile, scalable strategy for the high-precise synthesis of 1 nm thick Pt3Ni bimetallic alloy nanowires (Pt3Ni BANWs). The seed-mediated growth mechanism of Pt3Ni BANWs is identified by examining the morphology and composition of the intermediate products collect at different reaction stages. As expected, the Pt3Ni BANWs delivered enhanced mass activity (0.546 A mg-1 Pt, exceeding the DOE 2020 target) in comparison to Pt nanowires assembly (Pt NWA, 0.098 A mg-1 Pt) and Pt/C (Pt, 0.135 A mg-1 Pt) due to the rational integration of multiple compositional and structural advantages, such as alloy feature, ultrathin 1D nanostructure and high-index facets. Moreover, the Pt3Ni BANWs displayed enhanced durability (37% MA retention) than Pt NWA and Pt after 50,000 potential cycles. All these results indicate that the ultrathin Pt3Ni BANWs are potential candidates for catalyzing ORR with acceptable activity and durability. The present work could not provide a facile strategy but also a general guidance for the design of superb performance Pt-based nanowire catalysts for ORR. Figure 1. Progressive formation mechanism of the ultrathin Pt3Ni BANWs. Keywords: Oxygen Reduction Reaction, Pt-Ni alloy, Nanowires, Seed-mediated growth

Co/N-doped carbon nanotube arrays grown on 2D MOFs-derived matrix for boosting the oxygen reduction reaction in alkaline and acidic media

The development of high-performance and cost-effective electrocatalysts towards oxygen reduction reaction (ORR) is of significant importance, but still challenging for the practical applications in related energy systems. ORR process typically suffers from sluggish kinetics, the exploration of ORR electrocatalyst thus requires elaborate design. Herein, an effective strategy is developed for growing Co/N-doped carbon nanotube arrays on 2D MOFs-derived matrix via the pyrolysis of Co/Zn metal-organic-framework (MOF) nanosheets. The Co/Zn-MOF nanosheets serve as both the self-template for the 2D carbonized framework morphology and C/N source for the in-situ growth of 1D N-doped carbon nanotubes. The constructed hierarchical architecture effectively integrates the 0D/1D Co nanoparticle/N-doped carbon nanotube interface and 1D (nanotubes)/2D (nanosheets) junction into frameworks with highly exposed active surface, enhanced mass-transport kinetics and electrical conductivity. As a result, the designed composite exhibits superior ORR activity and durability in alkaline media as compared to commercial Pt/C. Particularly, it shows promising ORR performance with a half-wave potential of 0.78 V versus reversible hydrogen electrode and negligible activity attenuation after 5000 potential cycles in acidic electrolyte. The designed strategy can be extended to construct other MOFs-derived carbon matrixes with diverse hierarchical structures and provide an efficient avenue for searching high-performance electrocatalysts.

Facile synthesis of synergistic Pt/(Co-N)@C composites as alternative oxygen-reduction electrode of PEMFCs with attractive activity and durability

The development of cost-effective electrocatalyst with high activity and durability for oxygen reduction reaction (ORR) in acidic condition is a key step towards the wide-spread application of proton exchange membrane fuel cells (PEMFCs), while the high cost of benchmark Pt/C is one of the main inhibiting factors. Here, we report a new composite catalyst by adopting electrocatalytically active Co,N-modified carbon prepared from a cost-effective method with mass production capability. The as-prepared Pt/(Co-N)@C catalyst shows a favourable mass activity of 0.227 A mgPt−1 as well as superior durability with only 15.4% of performance loss after 30,000 cycles, making it a highly promising ORR electrocatalyst for practical PEMFCs.