Oxygen Reduction Reaction Mass Activity Research Articles

Carbon-anchoring synthesis of Pt1Ni1@Pt/C core-shell catalysts for stable oxygen reduction reaction

Proton-exchange-membrane fuel cells demand highly efficient catalysts for the oxygen reduction reaction, and core-shell structures are known for maximizing precious metal utilization. Here, we reported a controllable “carbon defect anchoring” strategy to prepare Pt1Ni1@Pt/C core-shell nanoparticles with an average size of ~2.6 nm on an in-situ transformed defective carbon support. The strong Pt–C interaction effectively inhibits nanoparticle migration or aggregation, even after undergoing stability tests over 70,000 potential cycles, resulting in only 1.6% degradation. The stable Pt1Ni1@Pt/C catalysts have high oxygen reduction reaction mass activity and specific activity that reach 1.424 ± 0.019 A/mgPt and 1.554 ± 0.027 mA/cmPt2 at 0.9 V, respectively, attributed to the optimal compressive strain. The experimental results are generally consistent with the theoretical predictions made by our comprehensive microkinetic model which incorporates essential kinetics and thermodynamics of oxygen reduction reaction. The consistent results obtained in our study provide compelling evidence for the high accuracy and reliability of our model. This work highlights the synergy between theory-guided catalyst design and appropriate synthetic methodologies to translate the theory into practice, offering valuable insights for future catalyst development.

Modulating the Bader Charge Transfer in Single p‐Block Atoms Doped Pd Metallene for Enhanced Oxygen Reduction Electrocatalysis

AbstractMetallene is considered as an emerging family of electrocatalysts due to its atomically layered structure and unique surface stress. Here we propose a strategy to modulate the Bader charge transfer (BCT) between Pd surface and oxygenated intermediates via p‐d electronic interaction by introducing single‐atom p‐block metal (M=In, Sn, Pb, Bi) into Pd metallene nanosheets towards efficient oxygen reduction reaction (ORR). X‐ray absorption and photoelectron spectroscopy suggests that doping p‐block metals could facilitate electron transfer to Pd sites and thus downshift the d‐band center of Pd and weaken the adsorption energy of O intermediates. Among them, the developed Bi−Pd metallene shows extraordinarily high ORR mass activity of 11.34 A mgPd−1 and 0.86 A mgPd−1 at 0.9 V and 0.95 V in alkaline solution, respectively, representing the best Pd‐based ORR electrocatalysts ever reported. In the cathode of a Zinc‐air battery, Bi−Pd metallene could achieve an open‐circuit voltage of 1.546 V and keep stable for 760 h at 10 mA cm−2. Theoretical calculations suggest that the BCT between Pd surface and *OO intermediates greatly affects the bond length between them (dPd‐*OO) and Bi doping could appropriately reduce the amount of BCT and stretch the dPd‐*OO, thus enhancing the ORR activity.

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.

Modulating the Bader Charge Transfer in Single p-Block Atoms Doped Pd Metallene for Enhanced Oxygen Reduction Electrocatalysis.

Metallene is considered as an emerging family of electrocatalysts due to its atomically layered structure and unique surface stress. Here we propose a strategy to modulate the Bader charge transfer (BCT) between Pd surface and oxygenated intermediates via p-d electronic interaction by introducing single-atom p-block metal (M=In, Sn, Pb, Bi) into Pd metallene nanosheets towards efficient oxygen reduction reaction (ORR). X-ray absorption and photoelectron spectroscopy suggests that doping p-block metals could facilitate electron transfer to Pd sites and thus downshift the d-band center of Pd and weaken the adsorption energy of O intermediates. Among them, the developed Bi-Pd metallene shows extraordinarily high ORR mass activity of 11.34 A mgPd -1 and 0.86 A mgPd -1 at 0.9 V and 0.95 V in alkaline solution, respectively, representing the best Pd-based ORR electrocatalysts ever reported. In the cathode of a Zinc-air battery, Bi-Pd metallene could achieve an open-circuit voltage of 1.546 V and keep stable for 760 h at 10 mA cm-2. Theoretical calculations suggest that the BCT between Pd surface and *OO intermediates greatly affects the bond length between them (dPd-*OO) and Bi doping could appropriately reduce the amount of BCT and stretch the dPd-*OO, thus enhancing the ORR activity.

Enhancing Intrinsic Electrocatalytic Activity of Pt/C Nanoparticles for Oxygen Reduction Reaction in Acidic Media by Microwave-Assisted Synthesis

Abstract: This study is focused on the enhancement of the intrinsic electrocatalytic activity of Pt nano-particles supported on C (Pt/C NPs) towards Oxygen Reduction Reaction (ORR) in acidic media. The goal was to investigate the effect of microwave-assisted synthesis on the electrocatalytic performance of Pt/C NPs towards ORR. Thus, Pt/C NPs were synthesized using a microwave-assisted method and by a conventional heating method; structural and morphological characteristics were analyzed by X-ray dif-fraction (XRD) and transmission electron microscopy (TEM). Electrochemical studies were performed using the rotating disk electrode technique to evaluate the ORR performance. Microwave-assisted syn-thesis produced Pt/C NPs with a smaller particle size (6.3 ± 0.2 nm) than conventionally synthesized nanoparticles (8.6 ± 0.3 nm). Electrochemical analysis showed that the microwave-synthesized Pt/C NPs exhibited higher mass activity (4.6 ± 0.8 mA･g-1Pt) for ORR compared to conventionally synthesized nanoparticles (1.9 ± 0.4 mA･mA･g-1Pt). These results demonstrate that microwave-assisted synthesis en-hances the intrinsic electrocatalytic activity of Pt/C NPs for ORR in acidic media. These findings have important implications for the development of efficient electrocatalysts for fuel cell applications. Background: The synthesis and characterization of platinum nanoparticles on C are crucial for advancing electrocatalysis, particularly in the context of potential applications in fuel cells. This study builds on previous research, focusing on two distinct synthesis methods to enhance our understanding of their im-pact on nanoparticle properties and electrocatalytic performance. Objective: To investigate the synthesis efficiency, structural characteristics, and electrocatalytic activities of platinum nanoparticles on C using microwave-assisted heating and conventional synthesis reactor heating. The objective is to discern any significant differences in particle size, structure, and electrocata-lytic performance between the two synthesis methods. Methods: The synthesis involved a comparative analysis of platinum nanoparticles using microwave-assisted and conventional heating methods. Chemical composition analysis verified the synthesis efficiency, and structural and morphological characterizations were performed using X-ray Diffraction and Transmission Electron Microscopy. Electrochemical studies employed the rotating disk electrode technique, with activation and evaluation conducted through cyclic voltammetry, and the oxygen reduction reaction studied via linear sweep voltammetry in an acidic media (0.5 mol⋅L-1 H2SO4). Results: Well-supported platinum nanoparticles with a face-centered cubic structure were obtained on C using both synthesis methods. However, microwave-synthesized particles (6.3 ± 0.2 nm) exhibited a smaller size compared to conventionally synthesized particles (8.6 ± 0.3 nm). Electrochemical assess-ment revealed superior mass activity for microwave-synthesized material (4.6 ± 0.8 mA･g-1Pt), outper-forming commercial Pt nanoparticles (3.0 ± 0.3 mA･g-1Pt) and conventionally synthesized material (1.9 ± 0.4 mA･mA･g-1Pt). Conclusion: This study concludes that microwave-assisted synthesis yields platinum nanoparticles on C with enhanced electrocatalytic performance, as evidenced by the smaller particle size and superior mass activity compared to conventionally synthesized material and commercial Pt nanoparticles. These findings highlight the potential of microwave-synthesized Pt nanoparticles for applications in fuel cells.

Decoupling the Synergy Between PGM and PGM-Free Moieties toward Oxygen Reduction Reaction.

Recently, coupling the conventional low Pt-group-metal (low-PGM, LP) and emerging PGM-free (PF) moiety to form a composite LP/PF catalyst is proposed to be an advanced strategy to improve the intrinsic activity and stability of oxygen reduction reaction (ORR) catalysts. Milestones in terms of ORR mass activity are created by this type of catalyst. However, the specific synergy between LP and PF moieties has not been well elucidated. Herein, two model catalysts are synthesized, i.e., atomically dispersed Co/N/C supporting Pt single atoms (Co/N/C@Pt-SAs) and PtCo nanoparticles (Co/N/C@PtCo-NPs). Interestingly, the Co/N/C@PtCo-NPs catalyst presents higher ORR mass activity prior to Co/N/C@Pt-SAs. This is theoretically due to the dual "built-in electric field" in Co/N/C@PtCo-NPs: one electric field with a direction from Pt to Co in NPs and another from Pt to Co/N/C; that is, Pt gains higher electron density in Co/N/C@PtCo-NPs than that in Co/N/C@Pt-SAs, thus forming an asymmetric electron cloud, and regulating the adsorption and activation of oxygen-containing species. In addition, the existence of Co significantly decreases the average valence state of PtCo NPs, indicating a stronger affinity between PtCo NPs and Co/N/C substrate, to account for the enhanced stability.

Fe[sbnd]N doped carbon materials from oily sludge as electrocatalysts for alkaline oxygen reduction reaction

Alkaline oxygen reduction reaction (ORR) presents an important role for energy conversion technologies and requires the development of an efficient electrocatalyst. Pt-based catalysts provide suitable activity; however, Pt production accessibility and high costs create hurdles to their commercial implementation. Fe coordinated within N-doped carbon materials (FeNC) are a promising alternative due to their high ORR catalytic activity, although the currently commercially available FeNC materials rely on harsh synthetic protocols which can lead to increased environmental impacts. In this work we target this issue by taking advantage of an oily sludge waste currently generated in refineries to synthesize FeNC materials, thus, avoiding the environmental impact caused by the management of this waste. The solid particles within oily sludge, which present a high concentration of C and Fe, were combined by different nitrogen sources and pyrolyzed at high temperatures.The prepared materials present a hierarchical pore structure with surface areas up to 547 m2 g−1. X-ray photoelectron spectroscopy analysis found that the impregnation of N using phenanthroline promotes the formation of pyridinic-N structures, which enhances the ORR performance compared to melamine doping. Additional doping of Fe with phenanthroline results in an ORR mass activity of 1.23 ± 0.04 A gFeNC−1 at 0.9 VRHE, iR-free in a rotating disc electrode (0.1 M KOH). This catalyst also shows a lower relative loss in activity at 0.9 VRHE after 8000 cycles in O2-saturated conditions compared to a commercial FeNC catalyst, PMF D14401, (−63.5 vs −69 %, respectively), demonstrating promise as a cheap and simple route to FeNC catalysts for alkaline ORR.

Engineering ORR Electrocatalysts from Co8Pt4 Carbonyl Clusters via ZIF‐8 Templating

AbstractTo reduce the costs of proton exchange membrane fuel cells, the amount of Pt necessary to drive efficient oxygen reduction reaction (ORR) should be minimized. Particle nanostructuring, (nano‐)alloying, and metal‐doping can yield higher activities per Pt mass through tailoring catalysts owning a high number of active sites and precise electronic properties. In this work, the atom‐precise [NBnMe3]2[Co8Pt4C2(CO)24] (Co8Pt4) cluster is encapsulated and activated in a zeolitic imidazolate framework (ZIF)‐8, which unlocks the access to defined, bare Pt−Co nanoclusters, Co8±xPt4±yNC@ZIF‐8, for the fabrication of highly active ORR catalysts. Upon controlled C‐interfacing and ZIF‐8‐digestion, Co‐doped Pt NPs (Pt27Co1) with a homogenous and narrow size distribution of (1.1±0.4) nm are produced on Vulcan® carbon. Restructuring of the Pt27Co1/C catalyst throughout the ORR measurement was monitored via high‐angle annular dark field‐scanning transmission electron microscopy and X‐ray photoelectron spectroscopy. The measured ORR mass activity of (0.42±0.07) A mgPt−1 and the specific activity of (0.67±0.06) mA cmECSA−2 compare favourably with the catalyst obtained by direct C‐interfacing the pristine Co8Pt4 cluster and with state‐of‐the‐art Pt/C reference catalysts. Our results demonstrate the potential of ZIF‐8‐mediated Pt−Co NP synthesis toward devising ORR catalysts with high Pt‐mass activity.

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.

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.

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).

(Invited) Durable and Highly Active Fe-N-C Catalysts for PEM Fuel Cells

Atomically dispersed and nitrogen-coordinated single iron sites (FeN4) embedded in carbon (Fe-N-C) catalysts are the most promising platinum group metal (PGM)-free catalysts. However, they have yet to match their Pt counterparts for oxygen reduction reaction (ORR) activity and stability in proton exchange membrane fuel cells (PEMFCs). Here, we developed viable Fe-N-C catalysts, which, for the first time, demonstrated competitive activity to that of Pt/C catalysts and dramatically enhanced stability and durability under practical PEMFC operating conditions. The most active Fe-N-C catalyst achieved a record half-wave potential (E1/2 = 0.915 V vs. RHE at 0.6 mgcatcm-2) and an ORR mass activity of 10.5 mA mgcat at 0.9 V in (RDE) tests, exceeding a Pt/C baseline catalyst (60 µgPt cm-2) by 40 mV in acidic electrolytes. This compelling activity of the Fe-N-C catalyst in aqueous acids on rotating disk electrode (RDE) was successfully transferred to fuel cell membrane electrode assembly (MEAs), generating an initial current density of 44.2 mA cm-2 exceeding the U.S. DOE 2025 target (i.e., 44 mA cm-2) at 0.9 VIR-free under O2. Under practical hydrogen-air conditions, record 151 mA cm-2 at 0.8 V and peak power density of 601 mW cm-2 were achieved. Importantly, we discovered that depositing nitrogen-carbon species on the catalyst surface via chemical vapor deposition (CVD) dramatically enhanced catalyst stability, evidenced by performance durability after accelerated stress tests (30 000 square-wave voltage cycles under H2/air) and long-term steady-state life tests (> 300 hours at 0.67 V). Innovative identical location-scanning transmission electron microscopy (IL-STEM) experiments confirmed that the CVD process leads to deposition of nitrogen-doped carbon onto the catalyst surfaces. Along with theoretical modeling, a reconstruction of the carbon structure adjacent to FeN4 sites leads to increased robustness against demetallation and carbon oxidation. This work opens new avenues for developing earth-abundant iron-based catalysts with extraordinary activity and stability, thus competing with Pt and addressing the cost barrier of current PEMFCs.

Oxygen Reduction Reaction Activity of Platinum Nanosheets Derived from Layered Platinic Acid

Platinum nanoparticles are mainly utilized as the cathode catalyst for polymer electrolyte fuel cells. However, the utilization of platinum for the oxygen reduction reaction (ORR) is insufficient and one must tackle the trade-off between high surface area and high stability. We have previously shown that metallic nanosheets such as ruthenium nanosheets exhibit high electrocatalytic activity and stability owing to the two-dimensional structure with an atomic scale thickness.1, 2 Platinum nanosheets, if available, should also have the potential to increase the utilization of the atoms and durability. While a few studies have reported the synthesis of platinum nanosheets,3 the thickness of the nanosheets is more than 1 nm, and the utilization of platinum has so far been lower than that of the Pt nanoparticles.In this study, we report double-layered platinum nanosheets derived from layered platinic acid.4 Layered platinic acid was synthesized through a solid-state-reaction and subsequent acid treatment. From the layered platinic acid, platinum oxide nanosheets were obtained as a colloid via chemical exfoliation. The thickness of the platinum oxide nanosheet was ~0.9 nm, consistent with the thickness of a single PtO6 octahedron. The platinum oxide nanosheets on silicon wafer were topotactically reduced to platinum nanosheets by mild thermal treatment in hydrogen. The thickness of the platinum nanosheet was ~0.5 nm, corresponding to a two atomic layer platinum nanosheet.Carbon supported platinum nanosheets were prepared by the reduction of platinum oxide nanosheets supported on carbon. The electrochemically active surface area (ECSA) of the carbon supported platinum nanosheets was 100 to 120 m2 (g-Pt)-1, which was considerably larger than that of 3 nm platinum nanoparticle (80 m2 (g-Pt)-1). In addition, the platinum nanosheet showed 1.7 times higher mass activity towards the oxygen reduction reaction compared to that of 3 nm platinum nanoparticles.Various methods for the topotactic reduction of platinum oxide nanosheets on carbon was studied. In addition to the mild thermal treatment in hydrogen gas, electrochemical reduction was also conducted. The ECSA of the platinum nanosheet prepared by electrochemical reduction at constant potential was 100 to 120 m2 (g-Pt)-1, which was almost same as the platinum nanosheet reduced by the mild thermal treatment in hydrogen. The specific activity was 1.2 to 1.3 times higher than that of the platinum nanosheet reduced by the mild thermal treatment in hydrogen. Therefore, the platinum nanosheet reduced by electrochemical reduction showed high mass activity for ORR.AcknowledgementThis work was supported as part of the “Polymer Electrolyte Fuel Cell Program” and the FC-Platform projects from the New Energy and Industrial Technology Development Organization (NEDO) of Japan (20001201-0, 22101136-0).

Constructing Nitrogen-Doped Carbon Hierarchy Structure Derived from Metal-Organic Framework as High-Performance ORR Cathode Material for Zn-Air Battery.

The development of efficient and low-cost catalysts for cathodic oxygen reduction reaction (ORR) in Zn-air battery (ZAB) is a key factor in reducing costs and achieving industrialization. Here, a novel segregated CoNiPt alloy embedded in N-doped porous carbon with a nanoflowers (NFs)-like hierarchy structure is synthesized through pyrolyzing Hofmann-type metal-organic frameworks (MOFs). The unique hierarchical NFs structure exposes more active sites and facilitates the transportation of reaction intermediates, thus accelerating the reaction kinetics. Impressively, the resulting 15% CoNiPt@C NFs catalyst exhibits outstanding alkaline ORR activity with a half-wave potential of 0.93V, and its mass activity is 7.5 times higher than that of commercial Pt/C catalyst, surpassing state-of-the-art noble metal-based catalysts. Furthermore, the assembled CoNiPt@C+RuO2 ZAB demonstrates a maximum power density of 172mW cm-2 , which is superior to that of commercial Pt/C+RuO2 ZAB. Experimental results reveal that the intrinsic ORR mass activity is attributed to the synergistic interaction between oxygen defects and pyrrolic/graphitic N species, which optimizes the adsorption energy of the intermediate species in the ORR process and greatly enhances catalytic activity. This work provides a practical and feasible strategy for synthesizing cost-effective alkaline ORR catalysts by optimizing the electronic structure of MOF-derived catalysts.

Spontaneous ORR Activity Drop of Fe-N-C Catalysts upon Atmospheric Storage

Iron-Nitrogen-Carbon (Fe-N-C) catalysts are promising materials to replace the scarce and expensive platinum-group materials in proton exchange membrane fuel cells (PEMFC) cathodes. Fe-N-C catalysts can be prepared by multiple ways [1-5], and each synthesis method can be fine-tuned to produce single Fe atoms coordinated to nitrogen atoms (FeNx), identified as the most active sites toward the oxygen reduction reaction (ORR). However, despite these notable efforts, a rapid drop in ORR activity is noticed within the first hours of operation in thin-film rotating disk electrode (RDE) (liquid electrolyte) or in PEMFC systems (solid electrolyte) [6]. Unravelling the degradation mechanisms at stake and developing strategies to improve the durability of Fe-N-C catalysts has thus become an important objective for researchers worldwide [7].In this work, we provide evidences for spontaneous ageing of Fe-N-C catalysts synthesized using different synthetic approaches, and thus featuring distinct chemical, textural and structural properties. For all materials, a pronounced drop of ORR activity is measured, indicating that the “spontaneous ageing” phenomenon is universal. The drop of ORR mass activity over time may reach as high as 90 % of the initial value, and relates to a drop of the active site density and/or the turnover frequency. These adverse effects can be mitigated using storage under dry and oxygen-free atmosphere.

Insights into the Structure - Composition - Activity Relationship of PtCo Alloy Nanoparticles towards Oxygen Reduction Reaction (ORR)

PtCo alloy nanoparticles (NPs) are widely used as highly active oxygen reduction reaction (ORR) catalysts for polymer electrolyte membrane fuel cells (PEMFCs). Despite large efforts, the critical relationships between structure, composition and ORR performance of catalyst materials are not fully understood to date. In this study, we prepared two PtCo alloy NP catalysts with an atomic ratio of 1:1 using wet-impregnation route by varying the annealing parameters under reductive conditions. The as-prepared PtCo alloy catalysts were structurally characterized using ex-situ HR-TEM, EDX, XRD, and EXAFS. We show that the annealing temperature and holding time affect the particle size, composition and homogeneity of the PtCo NPs. With higher annealing temperature and longer holding time, the particle size grows from 3.1 ± 0.7 nm (400 °C, 4 h) to 4.4 ± 0.6 nm (800 °C, 6 h) and simultaneously, the alloy formation within the NPs improves. After electrochemical activation in 0.1 M HClO4, the electrochemically active Pt surface area (ECSA) for activated PtCo T400 (65 ± 8 m2 gPt -1) is slightly lower than that for pure Pt/C (70 ± 11 m2 gPt -1), but significantly higher than that for the activated PtCo T800 (50 ± 4 m2 gPt -1). However, the activated PtCo T800 shows the highest ORR mass activity (0.56 ± 0.14 A mgPt -1 at 0.9 VRHE, iR-free) than the activated PtCo T400 (0.43 ± 0.03 A mgPt -1) and Pt/C (0.24 ± 0.04 A mgPt -1). Altogether, we provide deeper understanding of the structure - composition - ORR activity relationships for two differently annealed PtCo alloy catalyst materials.

Insights into the Structure - Composition - Activity Relationship of PtCo Alloy Nanoparticles towards Oxygen Reduction Reaction (ORR)

PtCo alloy nanoparticles (NPs) are widely used as highly active oxygen reduction reaction (ORR) catalysts for PEMFCs. Despite large efforts, the critical relationships between structure, composition and ORR performance of catalyst materials are not fully understood to date. In this study, we prepared two PtCo alloy NP catalysts with an atomic ratio of 1:1 using wet-impregnation route by varying the annealing parameters under reductive conditions. The as-prepared PtCo alloy catalysts were structurally characterized using ex-situ HR-TEM, EDX, XRD, and EXAFS. We show that the annealing temperature and holding time affect the particle size, composition and homogeneity of the PtCo NPs. With higher annealing temperature and longer holding time, the particle size grows from 3.1 ± 0.7 nm (400 °C, 4 h) to 4.4 ± 0.6 nm (800 °C, 6 h) and simultaneously, the alloy formation within the NPs improves. After electrochemical activation in 0.1 M HClO4, the electrochemically active Pt surface area (ECSA) for activated PtCo T400 (65 ± 8 m2 gPt -1) is slightly lower than that for pure Pt/C (70 ± 11 m2 gPt -1), but significantly higher than that for the activated PtCo T800 (50 ± 4 m2 gPt -1). However, the activated PtCo T800 shows the highest ORR mass activity (0.56 ± 0.14 A mgPt -1 at 0.9 VRHE, iR-free) than the activated PtCo T400 (0.43 ± 0.03 A mgPt -1) and Pt/C (0.24 ± 0.04 A mgPt -1). Altogether, we provide deeper understanding of the structure - composition - ORR activity relationships for two differently annealed PtCo alloy catalyst materials.

Subnanoscale Dual-Site Pd-Pt Layers Make PdPtCu Nanocrystals CO-Tolerant Bipolar Effective Electrocatalysts for Alcohol Fuel Cell Devices.

Finding a high-performance low-Pt bipolar electrocatalyst in actual direct alcohol fuel cells (DAFCs) remains challenging and desirable. Here, we developed a crystalline PdPtCu@amorphous subnanometer Pd-Pt "dual site" layer core-shell structure for the oxygen reduction reaction (ORR) and alcohol (methanol, ethylene glycol, glycerol, and their mixtures) oxidation reaction (AOR) in an alkaline electrolyte (denoted D-PdPtCu). The prepared D-PdPtCu/C achieved a direct 4-electron ORR pathway, a full oxidation pathway for AOR, and high CO tolerance. The ORR mass activity (MA) of D-PdPtCu/C delivered a 52.8- or 59.3-fold increase over commercial Pt/C or Pd/C, respectively, and no activity loss after 20000 cycles. The D-PdPtCu/C also exhibited much higher AOR MA and stability than Pt/C or Pd/C. Density functional theory revealed the intrinsic nature of a subnanometer Pd-Pt "dual site" surface for ORR and AOR activity enhancement. The D-PdPtCu/C as an effective bipolar electrocatalyst yielded higher peak power densities than commercial Pt/C in actual DAFCs.

Boost the Utilization of Dense FeN4 Sites for High‐Performance Proton Exchange Membrane Fuel Cells

Iron‐nitrogen‐carbon (Fe‐N‐C) catalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs) have seriously been hindered by their poor ORR performance of Fe‐N‐C due to the low active site density (SD) and site utilization. Herein, we reported a melamine‐assisted vapor deposition approach to overcome these hindrances. The melamine not only compensates for the loss of nitrogen caused by high‐temperature pyrolysis but also effectively etches the carbon substrate, increasing the external surface area and mesoporous porosity of the carbon substrate. These can provide more useful area for subsequent vapor deposition on active sites. The prepared 0.20Mela‐FeNC catalyst shows a fourfold higher SD value and site utilization than the FeNC without the treatment of melamine. As a result, 0.20Mela‐FeNC catalyst exhibits a high ORR activity with a half‐wave potential (E1/2) of 0.861 V and 12‐fold higher ORR mass activity than the FeNC in acidic media. As the cathode in a H2‐O2 PEMFCs, 0.20Mela‐FeNC catalyst demonstrates a high peak power density of 1.30 W cm−2, outstripping most of the reported Fe‐N‐C catalysts. The developed melamine‐assisted vapor deposition approach for boosting the SD and utilization of Fe‐N‐C catalysts offers a new insight into high‐performance ORR electrocatalysts.

Unique (100) Surface Configuration Enables Promising Oxygen Reduction Performance for Pt3Co Nanodendrite Catalysts.

Selective exposure of active surfaces of Pt-based electrocatalysts has been demonstrated as an effective strategy to improve Pt utilization and promote oxygen reduction reaction (ORR) activity in fuel cell application. However, challenges remain in stabilizing those active surface structures, which often suffer undesirable degradation and poor durability along with surface passivation, metal dissolution, and agglomeration of Pt-based electrocatalysts. To overcome the aforementioned obstacles, we here demonstrate the unique (100) surface configuration enabling active and stable ORR performance for bimetallic Pt3Co nanodendrite structures. Using elaborate microscopy and spectroscopy characterization, it is revealed that the Co atoms are preferentially segregated and oxidized at the Pt3Co(100) surface. In situ X-ray absorption spectroscopy (XAS) shows that such (100) surface configuration prevents the oxygen chemisorption and oxide formation on active Pt during the ORR process. Thus, the Pt3Co nanodendrite catalyst shows not only a high ORR mass activity of 730 mA/mg at 0.9 V vs RHE, which is 6.6-fold higher than that of the Pt/C, but also impressively high stability with 98% current retention after the acceleration degradation test in acid media for 5000 cycles, far exceeding the Pt or Pt3Co nanoparticles. Density functional theory (DFT) calculation also confirms the lateral and structural effects from the segregated Co and oxides on the Pt3Co(100) surface in reducing the catalyst oxophilicity and the free energy for the formation of an OH intermediate in the ORR.