Oxygen Reduction Reaction Specific Activity Research Articles

Spin-Polarized PdCu-Fe3O4 In-Plane Heterostructures with Tandem Catalytic Mechanism for Oxygen Reduction Catalysis.

Alloying has significantly upgraded the oxygen reduction reaction (ORR) of Pd-based catalysts through regulating the thermodynamics of oxygenated intermediates. However, the unsatisfactory activation ability of Pd-based alloys toward O2 molecules limits further improvement of ORR kinetics. Herein, the precise synthesis of nanosheet assemblies of spin-polarized PdCu-Fe3O4 in-plane heterostructures for drastically activating O2 molecules and boosting ORR kinetics is reported. It is demonstrated that the deliberate-engineered in-plane heterostructures not only tailor the d-band center of Pd sites with weakened adsorption of oxygenated intermediates but also endow electrophilic Fe sites with strong ability to activate O2 molecules, which make PdCu-Fe3O4 in-plane heterostructures exhibit the highest ORR specific activity among the state-of-art Pd-based catalysts so far. In situ electrochemical spectroscopy and theoretical investigations reveal a tandem catalytic mechanism on PdCu-Fe3O4─Fe sites that initially activate molecular O2 and generate oxygenated intermediates being transferred to Pd sites to finish the subsequent proton-coupled electron transfer steps.

Concave Grain Boundaries Stabilized by Boron Segregation for Efficient and Durable Oxygen Reduction.

The oxygen reduction reaction (ORR) is a critical process that limits the efficiency of fuel cells and metal-air batteries due to its slow kinetics, even when catalyzed by platinum (Pt). To reduce Pt usage, enhancing both the specific activity and electrochemically active surface area (ECSA) of Pt catalysts is essential. Here, ultrafine, grain boundary (GB)-rich Pt nanoparticle assemblies are proposed as efficient ORR catalysts. These nanowires offer a large ECSA and a high density of concave GB sites, which improve specific activity. Atoms at these GB sites exhibit increased coordination and lattice distortion, leading to a favorable reduction in oxygen binding energy and enhanced ORR performance. Furthermore, boron segregation stabilizes these GBs, preserving active sites during catalysis. The resulting boron-stabilized Pt nanoassemblies demonstrate ORR specific and mass activities of 9.18 mA cm-2 and 6.40 A mg-1 Pt (at 0.9 V vs. RHE), surpassing commercial Pt/C catalysts by over 35-fold, with minimal degradation after 60000 potential cycles. This approach offers a versatile platform for optimizing the catalytic performance of a wide range of nanoparticle systems.

Activity and Durability of Pt/Ir Catalysts for Oxygen Reduction as a Function of Different Platinum Shells

We performed a systematic experimental and theoretical analysis of the oxygen reduction reaction (ORR) activity and durability of Pt/Ir catalysts featuring different platinum shells. Four Pt/Ir catalysts with different platinum monolayers (ML) on an iridium core were synthesized. The nanostructure and composition of the catalysts were studied using scanning transmission electron microscopy with energy–dispersive X–ray spectroscopy, X–ray photoelectron spectroscopy, and scanning electron microscopy with energy–dispersive X–ray spectroscopy. ORR activity and catalyst durability were studied using cyclic voltammetry with rotating disk electrode. Density functional theory calculations were performed to estimate the ORR activity of Pt(111) and nML Pt/Ir(111) surfaces (n = 0, 1, 2). Although the specific ORR activities of the synthesized Pt/Ir catalysts were lower or comparable to those of 50%Pt/C, the mass activities were higher due to the enhanced utilization of platinum. Accelerated stress tests (ASTs) revealed that the durability of 1 ML Pt/Ir surpassed that of the other studied catalysts. The factors influencing the trends in specific and mass activities, durability, and the feasibility of implementing a Pt/Ir system in practical proton exchange membrane fuel cells (PEMFCs) are discussed.

Enhanced oxygen reduction reaction on caffeine-modified platinum single-crystal electrodes

Enhancing the activity of the oxygen reduction reaction (ORR) is crucial for fuel cell development, and hydrophobic species are known to increase the ORR activity. This paper reports that caffeine enhanced the specific ORR activity of Pt(111) 11-fold compared to that without caffeine in a 0.1 M HClO4 aqueous solution. Moreover, caffeine increased the ORR activity of Pt(110) 2.5-fold; however, the activity of Pt(100) was unaffected. The infrared (IR) band of PtOH (blocking species of the ORR) decreased for all the surfaces. Caffeine was adsorbed with its molecular plane perpendicular to the Pt(111) and Pt(110) surfaces and tilted relative to the Pt(100) surface. Thus, the effects of caffeine on the ORR activity can be rationalized by a decrease in PtOH coverage and the difference in adsorption geometry of caffeine.

Boron-intercalation-triggered crystalline transition of Pd nanosheet assemblies for an enhanced oxygen reduction reaction

The development of effective and stable cathode electrocatalysts is highly desired for fuel cells. Controlling the composition and morphology of Pd-based materials can provide a great opportunity to improve their oxygen reduction reaction (ORR) performance. Here, we report the synthesis of hexagonal close-packed (hcp) Pd2B nanosheet assemblies (Pd2B NAs) via the boronation reaction between as-synthesized Pd NAs and N,N-dimethylformamide. The hcp Pd2B NAs with uniform pore distribution can provide sufficient active sites for ORRs. The insertion of B atoms can induce the phase transition from face-centered cubic structure to hcp structure, as the most thermodynamically stable phase in the Pd-B alloy, which is beneficial for enhancing the ORR stability and toxicity resistance. Therefore, the hcp Pd2B NAs exhibit superior mass activity, specific activity and excellent stability for ORR. The present strategy of boron-intercalation-triggered crystalline transition of Pd-based nanomaterials is valuable for the design of metal–nonmetal catalysts with enhanced performance.

Structural Effects on ORR Performances of Carbon-Free Connected Platinum–Cobalt Catalysts with Chemically Ordered Superlattice Structures

Polymer electrolyte fuel cells (PEFCs) have been considered as the promising next-generation energy-conversion technology, owing to their high energy efficiency and clean emissions. However, a sluggish oxygen reduction reaction (ORR) rate and the low durability of conventional catalysts in PEFCs limit the wider commercialization. To address these issues, our group has developed a carbon-free, connected Pt1–Fe1 catalyst with a porous hollow capsule structure. This catalyst comprises the nano-sized network formed by the connection of Pt1–Fe1 nanoparticles and exhibits an enhanced ORR specific activity (9 times higher than a commercial Pt/C) as well as excellent durability against start-stop operations due to the carbon-free structure.[1,2] However, during the load cycle operation, iron is dissolved from the catalyst, leading to the loss of ORR activity and the degradation of polymer electrolytes caused by the Fenton reaction.In this work, carbon-free and iron-free connected Ptx-Co1 catalysts with chemically ordered superlattice structures have been developed for the first time, as shown in Fig. 1(a). In addition, the metal compositions and chemically ordered (superlattice) degrees in the catalysts were controlled to investigate the structural effects on ORR activity and durability of the connected Ptx-Co1 catalysts.Herein, we employed the silica-coating method for the catalyst synthesis, which involves the combination of silica coating and high-temperature annealing. As shown in Fig. 1(b), the surface of the Ptx-Co1 nanoparticles/SiO2 sample was coated with thin silica layers to prevent the detachment of Ptx–Co1 nanoparticles from the SiO2 template as well as the large catalyst agglomeration and coalescence during the annealing process. We controlled the chemically ordered degrees of the connected Pt1–Co1 and Pt3–Co1 catalysts by using different annealing temperatures. The XRD patterns of the catalysts showed the peaks corresponding to L10 type and L12 type chemically ordered structures for the connected Pt1–Co1 and Pt3–Co1 catalysts annealed at 600°C, respectively. These two catalysts possessed ordered degrees (S) of ca. 50–60%. The TEM images of the connected Ptx–Co1 catalysts showed the formation of connected Ptx–Co1 nanonetwork and porous hollow capsule structures.The electrochemical measurements of ORR activity and durability were conducted in a 0.1 M HClO4 electrolyte solution. The connected Pt1–Co1 and Pt3–Co1 catalysts achieved 6- and 10-times higher ORR specific activities than that of Pt/C, respectively. Subsequently, the ORR activity and load cycle durability (0.6 V for 3 s Û 1.0 V for 3 s at 60 °C) of the connected Pt3–Co1 catalyst with different ordered degrees (S = 0%, 28%, and 58%) were evaluated. As the ordered degrees in the connected Pt3–Co1 catalysts were higher, the ORR specific activity was increased. Also, the retention of the specific activity of the ordered catalyst (S = 58%) was higher than that of the disordered catalyst (S = 0%) after 10,000 load cycles. The STEM-EDX line mapping results showed that 83% of the alloyed Co retained in the catalyst with a higher ordered degree even after 10000 load cycles, whereas the disordered and lower ordered catalysts retained lower Co ratios after the test, indicating the suppression effect of an ordered structure on metal dissolution. In addition, the L12 ordered Pt3–Co1 structure remained in the connected catalyst (S = 58%) after load cycle test, confirmed by the TEM diffraction analysis.In summary, we successfully developed the connected Ptx-Co1 catalysts with chemically ordered superlattice structures. In addition, we clarified the structural effects of metal composition and ordered degree in connected Ptx-Co1 catalysts on ORR activity and load cycle durability. These results provide useful guidelines for the design of advanced ORR catalysts towards high-performance PEFCs. Acknowledgement The part of this presentation is based on results obtained from a project, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

Carbon-Free, Connected Core-Shell Nanoparticle Catalysts with Advanced Oxygen-Reduction Performances for PEFCs

As the world faces diminishing non-renewable energy sources and a surge in global pollution, establishing a hydrogen and low-carbon economy has become crucial. Polymer electrolyte fuel cells (PEFCs) hold great promise as a sustainable energy source with the potential for high efficiency, and clean power generation in both stationary and mobile applications. However, the conventional platinum nanoparticle catalysts on carbon support (Pt/C) used for the oxygen reduction reaction (ORR) at the cathode shows sluggish ORR activity, leading to high cost and inferior PEFC performance. Moreover, the carbon-supports used for the Pt catalysts are susceptible to degradation caused by carbon-corrosion during start-stop operations, which can severely compromise the overall performance of the PEFCs. Hence, in order to significantly enhance the commercial viability of PEFCs, considerable progress must be made in terms of cost, performance, and durability.Previously our group has reported carbon-free, connected Pt-alloy catalysts to address the above issues. This catalyst is composed of connected Pt-alloy nanonetworks with high electronic conductivity and exhibits enhanced ORR activity, high start-stop durability, and thin catalyst layers. Specifically, this catalyst exhibited 9 times higher specific activity than that of the commercial Pt/C, making it an excellent candidate for cathode catalyst applications.[1,2] However, high-temperature annealing process is required to develop connected networks between the Pt-alloy nanoparticles, resulting in larger crystallite sizes and reduced surface areas. This limitation has prevented connected Pt-alloy catalysts from achieving significant improvement in mass activity.In this work, we have proposed a novel synthesis method that enables the development of connected core-shell nanoparticle catalysts without any high-temperature treatment. This method eliminates the particle growth and exhibit a high surface area, making it highly effective for ORR applications. Here, a carbon-free catalyst with a core-shell nanonetwork structure having a non-Pt (Pd) core and Pt shell was developed. Additionally, the relationship between the thickness of the Pt-shell layer and ORR activity was investigated.Connected core-shell catalysts with Pd nanoparticles as core and Pt as atomic shell (Pd@Pt) were synthesized in a one-pot polyol process that does not require high-temperature annealing as shown in Fig. 1. The formation of connected Pd@Pt core-shell nanonetworks (< 10 nm) was confirmed by TEM and STEM-EDX analysis. Also, the structure of Pt shell on core metals was successfully tuned by controlling the reaction temperature and time, precursor ratio, etc. Here, the number of the Pt layers on the surface of the connected Pd@Pt core-shell catalysts were varied from 1–6 atomic layers and the results show that the ORR mass activity was notably enhanced in the catalyst with a moderate shell thickness ranging from 2.5 to 3.5 Pt atomic layers. This suggests that connected Pd@Pt catalyst with a moderate Pt layer achieves both high surface area and ORR specific activity. The high surface area of the developed catalysts is attributed to a new synthesis method that does not use high temperature annealing and a low Pt core-shell structure. The improvement in ORR specific activity can be attributed to the more suitable d-band center position of the Pt shell, which is influenced by strain and ligand effects resulting from the Pd core. The ORR performance of the carbon-free connected Pd@Pt catalyst showed 2–4 times enhancement in electrochemical surface area (ECSA) and 1.5–3 times improvement in ORR mass activity compared to the connected Pt-alloy catalysts prepared using high-temperature annealing. Further, high durability of the connected core-shell catalyst against load cycles (0.6 ↔1.0 V) at 60 °C in acidic electrolyte solution was also demonstrated, indicating that a high stability nanonetwork connected by Pt atomic shell.In summary, we have successfully demonstrated a new synthesis method for the formation of connected core-shell nanoparticles by Pt atomic shell without high-temperature annealing and prepared a carbon-free, connected Pd@Pt core-shell catalyst with advanced ORR performance, highlighting its great potential as a cathode catalyst. Acknowledgement The part of this presentation is based on results obtained from a project, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

Regeneration Strategies for Ruthenium-Poisoned ORR Catalysts in Reformate PEM Fuel Cells

Reformate polymer electrolyte membrane (PEM) fuel cells have great potential to transform the global maritime transportation in a more environmentally friendly and CO2-neutral manner. Platinum (Pt)-ruthenium (Ru) alloy nanoparticles (NPs) supported on carbon (PtRu/C) are the most commonly used electrocatalyst material to accelerate the hydrogen oxidation reaction (HOR) of the reformate gas (H2/CO2/CO) at the anode [1]. The high CO tolerance of PtRu/C catalysts is based on the ensemble effect [2]. However, these materials suffer extremely from the Ru dissolution and Ruz+ crossover to the cathode, which are the main degradation processes in reformate PEM fuel cells. At the cathode, the Ruz+ species immediately redeposit on the Pt/C catalyst, resulting in a drastic decay of the entire performance of the reformate PEM fuel cell. Even if the Ru coverage on the Pt/C catalyst is less than 20 at. %, there is an 8-fold decrease in kinetics of the oxygen reduction reaction (ORR) [3]. Therefore, it is necessary to develop a regeneration protocols to mitigate the Ru poisoning at the cathode.In this study, we developed and tested different regeneration protocols to recover the Ru-poisoned ORR catalysts using a rotating disc electrode (RDE) set-up. To simulate the worst situation for poisoned ORR catalysts, commercially available Pt-Ru alloy NPs supported on carbon were used and characterized by TEM, XRD, XAS, and XPS. In general, our regeneration protocols can be classified into dynamic and steady-state conditions. All protocols included a pre-activation step by holding the potential at 1.4 VRHE for 5 minutes in 0.1 M HClO4. During the dynamic regeneration process, the potential was switched between 1.4 VRHE and 1.6 VRHE with a duration time of 10 s by 10 times using a pulse voltammetry technique. In contrast, the steady-state regeneration protocol consisted of only one pulse at 1.6 VRHE for 100 s. For the recovery procedure of the poisoned ORR catalysts, the experimental parameters were correlated with the changes in ORR performance and electrochemically active surface area (ECSA) via CO stripping as well as the loss of Ru content and particle structure established from μ-XRF, ICP-OES, and TEM.After the dynamic regeneration process, the ORR mass activity (MA) and specific activity (SA) of PtRu2/Vulcan increase initially from 0.07 ± 0.01 to 0.21 ± 0.02 A mgPt+Ru -1 and 62 ± 9 to 190 ± 16 μA cmPt+Ru -2 at 0.85 VRHE, iR-free, respectively. Hence, the dynamic regeneration protocol leads to an improvement of the MA by a factor of 2 and of the SA by 2.1 for PtRu2/Vulcan. In comparison, the steady-state regeneration experiment shows an enhancement by 2.2 (0.26 ± 0.03 A mgPt+Ru -1) for MA and by 3.5 (276 ± 16 μA cmPt+Ru -2) for SA, respectively. Therefore, the steady-state regeneration process is more effective compared to the dynamic regeneration protocol. Based on the µ-XRF data, we observed that the Ru content of the PtRu2/Vulcan decreases from 62 ± 2 at. % to either 5 ± 1 at. % or 8 ± 1 at. % after the steady-state or dynamic regeneration process. The loss of Ru from the particle surface can also be monitored by CO stripping, where the CO oxidation peak is shifted positively from 0.58 VRHE to 0.67 VRHE and 0.63 VRHE after steady-state and dynamic regeneration protocol, respectively. As the particle size of the PtRu2 NPs is around ~ 5.3 ± 1.2 nm, the ECSA is mainly unchanged after both regeneration protocols.Based on our results, the steady-state regeneration process with 1.6 VRHE for 100 s is more effective than the dynamic one with pulses from 1.4 to 1.6 VRHE each with 10 s by 10 times to recover the ORR activity of the PtRu2 alloy NPs. The proposed regeneration protocols can improve the performance and life-time of the reformate PEM fuel cell.

Development of Highly-Active Catalysts toward Oxygen Reduction by Controlling the Shape and Composition of Pt-Ni Nanocrystals.

Electrocatalysts comprised of Pt-Ni alloy nanocrystals have garnered substantial attention due to their outstanding performance in catalyzing the oxygen reduction reaction (ORR). Herein, we present the synthesis of Pt-Ni nanocrystals with a variety of controlled shapes and compositions in an effort to investigate the impact of the Ni content on the formation of {111} facets and thereby the ORR activity. By completely excluding O2 from the reaction system, we could prevent the generation of Ni(OH)2 on the surface of the nanocrystals and thereby achieve a linear relationship between the atomic ratio of Pt to Ni in the nanocrystals and the feeding ratio of the precursors. The atomic ratio of Pt to Ni in the Pt-Ni nanocrystals was tunable within the range of 1.2-7.2, while their average sizes were kept around 9 nm in terms of edge length. In addition, a quantitative correlation between the area ratio of {111} to {100} facets and the feeding ratio of Pt(II) to Ni(II) was obtained by adjusting the mole fraction of the Ni(II) precursor in the reaction mixture. For the catalysts comprising octahedral nanocrystals, their specific ORR activities exhibited a positive correlation with the Pt/Ni atomic ratio. After the accelerated durability test, both specific and mass activity displayed a volcano-type trend with a peak value at a Pt/Ni atomic ratio of 1.6.

Promoting Oxygen Reduction Reaction by Excitation of Localized Surface Plasmon of Shape‐ and Facet‐Controlled Octahedral Au@Pt Core‐Shell Nanocrystals

AbstractMuch attention has been paid to the development of high‐performance electrocatalytic reactions by exploiting the plasmonic properties of noble metal nanoparticles. In this study, ultrathin Pt layers with different coverage ratios were prepared on plasmonic octahedral Au nanoparticles (NPs) by underpotential deposition (UPD) of Cu followed by redox replacement with Pt. The effect of the Cu UPD cycle on the deposited Cu and redox‐substituted Pt layers on octahedral Au NPs was investigated to evaluate the electrocatalytic activity of Pt‐decorated octahedral NPs for the oxygen reduction reaction (ORR). By varying the number of cycles of Cu UPD and Pt replacement, the coverage by the Pt layer on the Au NPs could be easily controlled. The resulting Pt‐coated octahedral Au NPs exhibited three times higher specific activity for ORR than commercial Pt/C nanocatalysts due to the deposited Pt layer reflecting the {111} plane of the core. The ORR activity of Pt‐coated octahedral Au NPs could be enhanced by photoexcitation with monochromatic LED irradiation, and the increase in enhancement factor was even greater when irradiated with a wavelength that could efficiently excite the localized surface plasmon resonance (LSPR) of the NPs. Analysis shows that this improvement is due to improved diffusion of oxygen to the electrode.

Cation-mediated Pt site-selective trimetallic nanostructures for high-performance electrocatalysis

Precisely structure-controlled Pt-based multimetallic nanocrystals represent one of most promising candidates in the realm of electrocatalysis, but their rational design and engineering have been restricted. Herein, a cation-mediated strategy is reported for exquisitely manipulating Pt sites on Pd hexagonal nanosheets by regulating the reduction kinetics of the Pt precursor. The key is to introduce metal cations (Co3+, Ni2+, Fe3+), which not only coordinate the relationship between the deposition rate and diffusion rate of Pt atoms but also act as dopants to optimize the electronic structure of nanomaterials. As a result, a series of PdPtM (M = Co, Ni, Fe) nanostructures, including alloy nanorings (PdPtCo NRs), core–shell nanosheets with Pt-rich shells (PdPtNi CS-NSs), and Pt-rich nanoparticle-decorated nanosheets (PdPtFe NP-D NSs), are successfully constructed. Impressively, these Pt site-selective nanocrystals remarkably boost the oxygen reduction reaction (ORR), ethylene glycol oxidation reaction, and glycerol oxidation reaction performance. In particular, the optimal PdPtCo NRs deliver the highest ORR mass and specific activities of 2.18 A mg−1Pd+Pt and 3.23 mA cm−2 at 0.9 V vs. RHE, enhanced by 21.8 and 20.2 times, respectively, compared to those of commercial Pt/C. The excellent ORR stability of this catalyst is corroborated by the negligible changes after 15,000 cycles. This cation-mediated strategy may pave a new avenue to develop high-efficiency catalysts for energy-related applications.

Ternary PtZrNi nanorods for efficient multifunctional electrocatalysis towards oxygen reduction and alcohol oxidation

Pt-based alloys with precise structure and composition design have been considered to be effective and robust novel electrocatalysts for fuel cells. Whereas, the sluggish kinetics of oxygen reduction reaction (ORR) and low intrinsic activity of Pt limited their real application on a large scale. Herein, a novel ternary PtZrNi nanorods (PtZrNi NRs) was synthesized via a facile wet-chemical method to achieve high electrocatalytic performance for both ORR and alcohol oxidation reaction owing to the synergism of chosen three elements and prominent one-dimensional morphology. Specifically, the PtZrNi NRs show enhanced mass and specific activities of 0.755 A mgPt-1 and of 0.97 mA/cm2 at 0.9 VRHE towards ORR in acidic media, which are 4.7 and 4.4 times of those of commercial Pt/C, respectively. Additionally, in alkaline media, the PtZrNi NRs also exhibit superior ORR mass and specific activities of 3.216 A mgPt-1and 4.13 mA/cm2, enhanced by 34.6 and 31.3 times compared with those of commercial Pt/C, respectively. The PtZrNi NRs retain the nanorod shape well without agglomeration after an accelerated durability test (20000 cycles). This work may offer a new perspective for engineering high-performance Pt-based electrocatalysts for commercial fuel cells.

Tensile-Strained Platinum-Cobalt Alloy Surface on Palladium Octahedra as a Highly Durable Oxygen Reduction Catalyst.

Designing shape-controlled Pt-based core-shell nanocrystals is a prospective strategy to maximize the utilization of Pt while maintaining high activity for oxygen reduction reaction (ORR). However, the core-shell structures with ultrathin Pt shell exhibit limited electrochemical durability. Therefore, a thicker shell is proposed to successfully improve the durability of the core-shell structures by preventing the core from dissolution. Nevertheless, the deposition of Pt tends to switch to the Stranski-Krastanov (S-K) growth mode with the increase of the number of layer, resulting in the absence of a conformal morphology. Herein, we realize the deposition of three-to-five-layer epitaxial Pt-Co layers on Pd octahedral seeds by introducing tensile strain in the epitaxial layer to impede the S-K growth. The as-obtained Pd@Pt-Co octahedra with four layers exhibit enhanced mass activity (0.69 A/mgPt) and specific activity (1.00 mA/cm2) for ORR, which are 4.93 and 5 times that of the commercial Pt/C, respectively. Furthermore, it shows only 17% decay for specific activity after a 30,000-cycle durability test. This work is expected to enlighten the design and synthesis of related core-shell nanocrystals with facetted multicomponent shells, offering a promising strategy for designing cost-effective and efficient catalysts.

Oxygen Reduction Reaction on Pt-Y/C Catalysts: Activity and Long-Term Stability Study

Pt-Y/C catalysts were prepared by a modified formic acid method and their structural characteristics and activity for oxygen reduction reaction (ORR) were evaluated. X-ray diffraction analysis showed that no alloy was formed between the metals and X-ray photoelectron spectroscopy (XPS) experiments showed that yttrium is presented as Y2O3, Y(OH)3 and Y-O-Pt species. CO stripping voltammetry and in situ X-ray absorption spectroscopy confirmed the interaction between Pt nanoparticles and yttrium species. The Pt-Y/C 7:3 material showed the higher specific activity (103 μA cmPt -2) for ORR. Following repetitive potential cycling, the activity for the ORR of the Pt-Y/C 7:3 catalyst declined in a similar proportion to the Pt/C material. Transmission electron microscopy analysis after stability for both catalysts showed that Pt/C particle size slightly increased and that for Pt-Y/C 7:3 remained the same. XPS made in ultrathin catalyst layer, agreeing with energy-dispersive X-ray spectroscopy analysis, revealed that the intensities of the Y 3d peaks were suppressed relative to the initial one and that yttrium in the form of Y2O3 and Y(OH)3 were dissolved, while the species Y-O-Pt still remain in the catalyst, maintaining the higher activity.

Two-dimensional template-directed synthesis of one-dimensional kink-rich Pd3Pb nanowires for efficient oxygen reduction

Developing facile synthetic strategies toward ultrafine one-dimensional (1D) nanowires (NWs) with rich catalytic hot spots is pivotal for exploring effective heterogeneous catalysts. Herein, we demonstrate a two-dimensional (2D) template-directed strategy for synthesizing 1D kink-rich Pd3Pb NWs with abundant grain boundaries to serve as high-efficiency electrocatalysts toward oxygen reduction reaction (ORR). In this one-pot synthesis, ultrathin Pd nanosheets were initially generated, which then served as self-sacrificial 2D nano-templates. A dynamic equilibrium growth was subsequently established on the 2D Pd nanosheets through the center-selected etching of Pd atoms and edge-preferred co-deposition of Pd/Pb atoms. This was followed by the oriented attachment of the generated Pd/Pb alloy nanograins and fragments. Thus, kink-rich Pd3Pb NWs with rich grain boundary defects were obtained in high yield, and these NWs were used as electrocatalytic active catalysts. The surface electronic interaction between Pd and Pb atoms effectively decreased the surface d-band center to weaken the binding of oxygen-containing intermediates toward improved ORR kinetics. Specifically, the kink-rich Pd3Pb NWs/C catalyst delivered outstanding ORR mass activity and specific activity (2.26 A⋅mgPd−1 and 2.59 mA⋅cm−2, respectively) in an alkaline solution. These values were respectively 13.3 and 10.8 times those of state-of-the-art commercial Pt/C catalyst. This study provides an innovative strategy for fabricating defect-rich low-dimensional nanocatalysts for efficient energy conversion catalysis.

Continuous Production of Carbon-Supported and Surfactant-Free Pt-M (M=Fe, Co, Ni, and Cu) Nanocrystals for Catalyzing Oxygen Reduction

Scalable production of carbon-supported Pt-M (M=Co, Ni, and Fe) alloy nanocrystals is of great importance for their practical application as catalysts towards the oxygen reduction reaction (ORR), a process key to the operation of proton-exchange membrane fuel cells. Here we report the use of a fluidic device for the in situ nucleation and growth of Pt-M nanocrystals on a commercial carbon support in a continuous and scalable fashion. The use of dimethylformamide not only enables well dispersion of the carbon powders for the creation of a homogeneous reaction mixture but also helps reduce metal precursors for the heterogeneous nucleation and growth of nanocrystals on the carbon surface. The size, shape, and composition of the nanocrystals can all be tuned by changing the metal precursors added into the reaction mixture, resulting in Pt-M nanocrystals uniformly distributed across the surface of the carbon support. Among the nanocrystals, the carbon-supported Pt-Co nanocrystals show the highest ORR specific and mass activities at 0.9 V, demonstrating 11.4- and 8.8-fold enhancements over the state-of-the-art commercial Pt/C catalyst.

Pt–Fe–Cu Ordered Intermetallics Encapsulated with N-Doped Carbon as High-Performance Catalysts for Oxygen Reduction Reaction

Ternary platinum (Pt)-based ordered intermetallics represent a group of promising electrocatalysts in energy-conversion applications, because of their multielemental coupling that can potentially boost the activity and durability of the oxygen reduction reaction (ORR). Yet, the achievable catalysis performance is still susceptible to the inevitable transition metal leaching that can hardly be eliminated in an acidic environment. Herein, we report a nitrogen (N)-modified carbon (shell) encapsulated Pt–Fe–Cu ordered intermetallic nanoparticles (core) electrocatalyst for acidic ORR, where the Pt–Fe–Cu core presents a face-centered tetragonal (fct) phase. It is demonstrated that N-doped carbon shells can not only protect Pt–Fe–Cu cores from dissolution, agglomeration, coalescence, and Ostwald ripening but also enable the electronic structure regulation of the central Pt sites through the strong Fe–N coordination. The optimized Pt–Fe–Cu intermetallic with N-doped carbon shells delivers superior ORR activity and is more chemically stable over disordered Pt–Fe–Cu alloy, Pt–Fe–Cu intermetallics without a N-doped carbon shell, and commercial Pt/C, where the achievable ORR mass and specific activities are nearly 5-fold and 4-fold higher than those of commercial Pt/C in the acidic media, respectively.

Robust Oxygen Reduction Electrocatalysis Enabled by Platinum Rooted on Molybdenum Nitride Microrods.

Robust oxygen reduction electrocatalysis is central to renewable fuel cells and metal-air batteries. Herein, Pt nanoparticles (NPs) rooted on porous molybdenum nitride microrods (Pt/Mo2N MRs) are rationally constructed toward the oxygen reduction reaction (ORR). Owing to the desired composition with strong electronic metal-support interactions (EMSIs) and a porous one-dimensional structure supporting ultrafine NPs, the developed Pt/Mo2N MRs possess much higher ORR mass and specific activities than commercial Pt/C. In situ Raman and density functional theory calculations reveal that the EMSI weakens the adsorption of intermediates over Pt/Mo2N MRs via an associative mechanism. Moreover, the porous Mo2N support stabilizes these high activities. Impressively, a homemade zinc-air battery driven by Pt/Mo2N MRs delivers excellent performance including a peak power density of 167 mW cm-2 and a high rate capability that ranged from 5 to 50 mA cm-2. This work highlights the role of EMSI in promoting robust ORR electrocatalysis, thus providing a promising approach for efficient and robust cathode materials for advanced metal-air batteries.

Fe2 Dimers for Non-Polar Diatomic O2 Electroreduction.

Non-polar diatomic molecule activation is of great significance for catalysis. Despite the high atomic efficiency, the catalytic performance of single-atom catalysts is limited by insufficient receiving sites for diatomic molecule adsorption. Here, Fe2 dimers were successfully synthesized through precisely regulating the metal loading on metal-organic frameworks. The unique role of metal dimers in activating diatomic O2 molecules was explored. In alkaline electrolytes, the specific oxygen reduction reaction activity of Fe2 dimers was 7 times higher than that of Fe1 counterparts. The hydrogen atom transfer probes indicated a different activation mode for O2 on Fe1 and Fe2 dimers, respectively. Theoretical calculation results revealed that Fe2 dimers opened up a new reaction pathway by promoting the direct breaking of O=O bonds, thus avoiding the usual formation of *OOH intermediates, which helped explain the lower H2 O2 yield and higher specific activity.

Oxygen reduction reaction on PdM/C (M = Pb, Sn, Bi) alloy nanocatalysts

In this work three PdM/C bimetallic catalysts were prepared with a second metal being either Bi, Pb or Sn. The synthesised Pd-alloy nanoparticles were supported on Vulcan carbon XC-72R. These catalysts were studied using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HR-TEM) and electrochemically using the rotating disk electrode (RDE) technique. The prepared Pd-based electrocatalysts were compared to pure Pd/C catalyst synthesised using the citrate method. The oxygen reduction reaction (ORR) kinetics on Pd-alloy catalysts in alkaline solution was systematically investigated. Pd content was determined by microwave plasma atomic emission spectroscopy, which was used to calculate the mass activities (MA) for ORR. Mass activities revealed that BiPd had two-fold higher and PbPd showed 1.6-times increase in MA as compared to Pd/C, however SnPd showed lower MA than Pd/C. BiPd/C catalyst showed around 45% increase in specific activity (SA) for ORR, PdSn catalyst showed around 25% increase and PbPd no increase in SA in comparison to the Pd/C catalyst. Methanol tolerance was also studied for the PdM/C alloy catalysts and a higher tolerance was observed for BiPd/C losing 17 mV in the half-wave potential value and even better methanol tolerance was in evidence with the other two catalysts that lost only ∼10 mV in 0.1 M KOH + 0.5 M MeOH.