Octahedral PtNi Research Articles

A Review of Strategies to Improve the Stability of Carbon-supported PtNi Octahedral for Cathode Electrocatalysts in Polymer Electrolyte Membrane Fuel Cells

<p>Polymer electrolyte membrane fuel cells (PEMFCs) are green energy conversion devices, for which commercial markets have been established, owing to their application in fuel cell vehicles (FCVs). Development of cathode electrocatalysts, replacing commercial Pt/C, plays a crucial role in factors such as cost reduction, high performance, and durability in FCVs. PtNi octahedral catalysts are promising for oxygen reduction reactions owing to their significantly higher mass activity (10–15 times) than that of Pt/C; however, their application in membrane electrode assemblies (MEAs) is challenged by their low stability. To overcome this durability issue, various approaches, such as third-metal doping, composition control, halide treatment, formation of a Pt layer, annealing treatment, and size control, have been explored and have shown promising improvements in stability in rotating disk electrode (RDE) testing. In this review, we aimed to compare the features of each strategy in terms of enhancing stability by introducing a stability improvement factor for a direct and reasonable comparison. The limitations of each strategy for enhancing stability of PtNi octahedral are also described. This review can serve as a valuable guide for the development of strategies to enhance the durability of octahedral PtNi.</p>

Enhancing the Hydrogen-Air Fuel Cell Performance of Octahedral Ptni Nanoalloys with Rational Design of Dopants, Layering and Support in the PEM Fuel Cells

This study focuses on the scale-up synthesis of carbon-supported octahedral PtNiX nanoparticles to improve their oxygen reduction reaction electrocatalytic activity, stability, and high current density for fuel cell applications. Octahedral PtNiX nanoparticles doped with different elements such as Rh or Ir and with high Pt loading on carbon supports were synthesized, characterized by physio-chemical methods and tested in single MEA-based fuel cells after rotating disk electrode (RDE) pre-screening measurements. Different carbon supports were compared, including N-doped carbon. We investigated the sensitivity of the I/C ratio for the ink on the performance of the nanoparticles in RDE, and show that it is an important parameter to consider during synthesis. Our study shows that the sensitivity of the I/C ratio is more pronounced for PtNi alloy octahedra than for pure Pt catalyst. Moreover, we also use in-situ measurement techniques, such as X-ray Absorption Spectroscopy (XAS) in a liquid cell, to study the impact of the dopants during the operation. In the MEA, we achieve a state-of-the-art current density of around 1500 mA/cm2 at 0.6V for octahedral PtNiX nanoparticles in fuel cell applications. This high current density is attributed to the superior electrocatalytic activity and stability of the doped nanoparticles, as well as their high loading of Pt. Overall, our results demonstrate the potential of octahedral PtNiX nanoparticles as highly efficient electrocatalysts for fuel cell applications. The study highlights the importance of considering the I/C ratio during catalyst ink preparation, and the use of in-situ measurement techniques to improve the fundamental understanding of catalyst structure-activity and -stability for the design of future electrocatalysts.

The doping and co-impregnation process for the synthesis of the third metal in Pt3Ni electrocatalysts

Octahedral Pt3Ni on carbon support (Pt3Ni/C) is well-known to be excellent electrocatalysts for the oxygen reduction reaction (ORR) due to its exceptional activity. However, their long-term stability is poor, since the octahedral shape is entirely shattered and Ni is simply leached out during operation. To improve activity and stability, third metals M (i.e., Ga, Ag, Cu, Rh, Co, Fe, and Mo) were added to Pt3Ni electrocatalysts by gas-phase synthesis using two distinct approaches referred to as Co-impregnation (Pt3Ni-M/C) and Metal-doping (M-Pt3Ni/C). The size distributions of octahedron and truncated octahedron were almost identical, with an average particle size of around 8–9 nm for both approaches. The Fe–Pt3Ni/C exhibited the highest pristine ORR activity in terms of both mass activity (1.64 A mgPt−1) and specific activity (65.7 A mgPt−1), which were 2.8 times greater than Pt3Ni/C and 27.3 times more than commercial Pt/C catalyst. For an accelerated durability test (ADT) of 16,000 cycles, the Cu–Pt3Ni/C exhibited the most stable catalysts with an activity retention of 69.9%, followed by Ga-Pt3Ni (61.7%) and Rh–Pt3Ni (52.2%). The Cu–Pt3Ni/C catalyst was 5.8 times more stable than the Pt3Ni/C catalyst. When the same metal was used, it was discovered that the Metal-doping (M-Pt3Ni/C) approach provided superior ORR activity and stability than the Co-impregnation (Pt3Ni-M/C) technique.

Post-Synthesis Heat Treatment of Doped PtNi-Alloy Fuel-Cell Catalyst Nanoparticles Studied by In-Situ Electron Microscopy

Octahedral-shaped PtNi-alloy nanoparticles are highly active oxygen reduction reaction catalysts for the cathode in proton exchange membrane fuel cells. However, one major drawback in their application is their limited long-term morphological and compositional stability. Here, we present a detailed in situ electron microscopy characterization of thermal annealing on octahedral-shaped PtNi catalysts as well as on doped octahedral PtNi(Mo) and PtNi(MoRh) catalysts. The evolution of their morphology and composition was quantified during both ex situ and in situ experiments using energy dispersive X-ray spectroscopy in a scanning transmission electron microscope under a hydrogen atmosphere and in vacuum. Morphological changes upon heating, i.e., a gradual loss of the octahedral shape and a continuous rounding of the particles, were observed, as well as evidence for increased alloying. Furthermore, the evolution of the shape of the PtNi(Mo) nanoparticles was quantified using in situ experiments under hydrogen atmosphere in a transmission electron microscope. The shape change of the particles was quantified using segmentation maps created by a neural network. It has been demonstrated that morphological changes crucially depend on the composition and surface doping: doping with Mo or Mo/Rh significantly stabilizes the structure, allowing for persistence of a truncated octahedral shape during heat treatments.

The effect of supporting carbons on the gas phase synthesis of octahedral Pt3Ni electrocatalysts with various H2:CO ratios

The gas phase synthesis of octahedral Pt3Ni/C electrocatalysts using several carbon substrates (Ketjen black, Graphene, and Vulcan XC-72R) was investigated. Different carbon substrates altered the morphology and alloy of Pt3Ni nanoparticles, with octahedral morphology and alloy metal preferentially developing on Ketjen black and Graphene, while spherical shape and bimetallic metal preferentially developing on Vulcan. Furthermore, the shape was shown to be regulated throughout the reduction process, with the H2:CO ratio playing a crucial role in controlling octahedral morphology and carrying out the ORR activity. At a 1:3 H2:CO ratio, the Pt3Ni/Ketjen black exhibited the highest ORR activity for both mass activity (1.02 A mgPt−1) and specific activity (5.09 mA cm−2) that were 16.5 and 66.1 times larger than commercial Pt/C catalysts, respectively (0.062 A mgPt−1 and 0.077 mA cm−2). The best ORR activity of Pt3Ni onto Graphene and Vulcan XC-72R was exhibited with a 1:1 H2:CO mixture. The catalysts were tested using a 4000-voltage-cycle accelerated durability test, and the Pt3Ni/Ketjen catalyst fared the best in terms of ORR stability and durability.

Excess dopant effect in platinum‐based alloys toward the oxygen electroreduction reaction

AbstractDoping the PtNi octahedra with trace amount of transition metals has recently been shown to significantly improve the activity and durability of the oxygen reduction reaction (ORR). However, the role of excessive amounts of doped elements on ORR performance is unclear. In this work, PtNi octahedra were synthesized with and without excess Mo dopant. Also, spherical PtNi nanoparticles containing excess Co dopant were synthesized. The ORR performance revealed that PtNiMo octahedra and PtNiCo nanoparticles showed inferior ORR activities compared to bimetallic PtNi octahedra. Based on the electrochemical analysis, we found that unfavorable oxygen‐binding strengths of PtNiMo and PtNiCo catalysts decrease the ORR performances.

Highly Active and Stable Large Mo-Doped Pt-Ni Octahedral Catalysts for ORR: Synthesis, Post-treatments, and Electrochemical Performance and Stability.

Over the past decade, advances in the colloidal syntheses of octahedral-shaped Pt-Ni alloy nanocatalysts for use in fuel cell cathodes have raised our atomic-scale control of particle morphology and surface composition, which, in turn, helped raise their catalytic activity far above that of benchmark Pt catalysts. Future fuel cell deployment in heavy-duty vehicles caused the scientific priorities to shift from alloy particle activity to stability. Larger particles generally offer enhanced thermodynamic stability, yet synthetic approaches toward larger octahedral Pt-Ni alloy nanoparticles have remained elusive. In this study, we show how a simple manipulation of solvothermal synthesis reaction kinetics involving depressurization of the gas phase at different stages of the reaction allows tuning the size of the resulting octahedral nanocatalysts to previously unachieved scales. We then link the underlying mechanism of our approach to the classical "LaMer" model of nucleation and growth. We focus on large, annealed Mo-doped Pt-Ni octahedra and investigate their synthesis, post-synthesis treatments, and elemental distribution using advanced electron microscopy. We evaluate the electrocatalytic ORR performance and stability and succeed to obtain a deeper understanding of the enhanced stability of a new class of relatively large, active, and long-lived Mo-doped Pt-Ni octahedral catalysts for the cathode of PEMFCs.

The effect of supporting carbons on the solid-state chemistry of octahedral Pt3Ni electrocatalysts with various H2:CO ratios

The effect of supporting carbons on the solid-state chemistry of octahedral Pt3Ni electrocatalysts with various H2:CO ratios

On the electrocatalytical oxygen reduction reaction activity and stability of quaternary RhMo-doped PtNi/C octahedral nanocrystals.

Recently proposed bimetallic octahedral Pt–Ni electrocatalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cell (PEMFC) cathodes suffer from particle instabilities in the form of Ni corrosion and shape degradation. Advanced trimetallic Pt-based electrocatalysts have contributed to their catalytic performance and stability. In this work, we propose and analyse a novel quaternary octahedral (oh-)Pt nanoalloy concept with two distinct metals serving as stabilizing surface dopants. An efficient solvothermal one-pot strategy was developed for the preparation of shape-controlled oh-PtNi catalysts doped with Rh and Mo in its surface. The as-prepared quaternary octahedral PtNi(RhMo) catalysts showed exceptionally high ORR performance accompanied by improved activity and shape integrity after stability tests compared to previously reported bi- and tri-metallic systems. Synthesis, performance characteristics and degradation behaviour are investigated targeting deeper understanding for catalyst system improvement strategies. A number of different operando and on-line analysis techniques were employed to monitor the structural and elemental evolution, including identical location scanning transmission electron microscopy and energy dispersive X-ray analysis (IL-STEM-EDX), operando wide angle X-ray spectroscopy (WAXS), and on-line scanning flow cell inductively coupled plasma mass spectrometry (SFC-ICP-MS). Our studies show that doping PtNi octahedral catalysts with small amounts of Rh and Mo suppresses detrimental Pt diffusion and thus offers an attractive new family of shaped Pt alloy catalysts for deployment in PEMFC cathode layers.

Boost reactivity of tri-iodide reduction electrode by highly faceted octahedral PtNi nanocrystals

PtNi octahedra were utilized as the counter electrodes (CEs) of N719-dye based DSSCs and well characterized via high-resolution transmission electron microscopy, powder x-ray pattern scan, and on-axis transmission Kikuchi Diffraction technique, rendering the crystallographic orientation of individual particle. The photovoltaic results show that the power conversion efficiency (PCE) of devices with different CEs is in the following sequence: Pt thin film < Pt7Ni3 octahedra < Pt5Ni5 octahedra < Pt4Ni6 octahedra. The device of Pt4Ni6 octahedra reveal the PCE over 10%. The electrochemical measurements show the tri-iodide reduction activity is in a sequence of Pt4Ni6 octahedra > Pt5Ni5 octahedra > Pt7Ni3 octahedra > Pt thin film. This study provides fundamental understanding of the exposed surface of PtNi octahedra, for which the coverage of {111} facets plays a key role in the tri-iodide reduction and thereby leads to the increase of device PCE with significant decrease of the Pt component in CEs.

Effects of the Composition and the Particle Size of Octahedral Pt-Ni Nanoparticles on Their Durability

Carbon-supported octahedral Pt-Ni nanoparticles (oct-Pt-Ni/C) have been expected to be one of the most promising cathode catalysts for polymer electrolyte membrane fuel cells because of their extremely high catalytic activities for oxygen reduction reaction. While the effects of the Pt-Ni composition and the particle size on the activity have been widely studied to achieve the highest activity, their effects on the durability have been hardly examined. Here, we synthesized oct-Pt-Ni/C with different Pt-Ni compositions and sizes by changing concentrations of the precursors and the protecting agents, respectively. Then, their degradation behaviors were systematically examined by using an RDE-based accelerated durability test (ADT). Although the oct-Pt-Ni/C exhibited extremely high activities (630–3400 A g−1-Pt) with a volcano-like dependency against the Ni atomic ratio, all these ORR activities rapidly decreased in the beginning to approach that of Pt/C (400 A g−1-Pt). After the ADT, all the oct-Pt-Ni/C lost their octahedral shape through Ni leaching. There was no significant dependency of the degradation rate on the Pt-Ni composition and the particle size for the oct-Pt-Ni/C with the sizes smaller than 7 nm, which are needed to exhibit at least decent performances in a fuel cell. Therefore, further efforts are needed to develop highly durable oct-Pt-Ni/C.

Thermal annealing synthesis of double-shell truncated octahedral Pt-Ni alloys for oxygen reduction reaction of polymer electrolyte membrane fuel cells

Shape-controlled Pt-Ni alloys usually offer an exceptional electrocatalytic activity toward the oxygen reduction reaction (ORR) of polymer electrolyte membrane fuel cells (PEMFCs), whose tricks lie in well-designed structures and surface morphologies. In this paper, a novel synthesis of truncated octahedral PtNi35 alloy catalysts that consist of homogeneous Pt-Ni alloy cores enclosed by NiO-Pt double shells through thermally annealing defective heterogeneous PtNi3.5 alloys is reported. By tracking the evolution of both compositions and morphologies, the outward segregation of both PtOx and NiO are first observed in Pt-Ni alloys. It is speculated that the diffusion of low-coordination atoms results in the formation of an energetically favorable truncated octahedron while the outward segregation of oxides leads to the formation of NiO-Pt double shells. It is very attractive that after gently removing the NiO outer shell, the dealloyed truncated octahedral core-shell structure demonstrates a greatly enhanced ORR activity. The as-obtained truncated octahedral Pt21Ni core-shell alloy presents a 3.4-folds mass-specific activity of that for unannealed sample, and its activity preserves 45.4% after 30000 potential cycles of accelerated degradation test (ADT). The peak power density of the dealloyed truncated octahedral Pt2.1Ni core-shell alloy catalyst based membrane electrolyte assembly (MEA) reaches 679.8 mW/cm2, increased by 138.4 mW/cm2 relative to that based on commercial Pt/C.

Advanced Nanostructures for Proton Exchange Membrane Fuel Cells: From Liquid Electrolyte to Membrane Electrode Assembly

During the last decade, the landscape of Pt-based electrocatalysts for the oxygen reduction reaction, cathodic reaction of the proton exchange membrane fuel cell, dramatically changed. Huge enhancements in specific and mass activity, versus the standard, mono-metallic platinum nanoparticles supported on carbon, have been achieved. This was made possible by following two different approaches, namely (i) the order-based and (ii) the disorder-based approaches. The order-based consists in modifying the platinum lattice parameter and coordination number, thus impacting its electronic structure, in an identical fashion throughout the nanostructure. To undergo this path, the Pt can be alloyed with 3d-transition metals or rare-earth elements, or the nanostructures can be shaped to exhibit a preferential orientation, such as (111) facets, with a specific Pt atoms packing. By opposition, the disorder-based approaches imply that the electrocatalytic activity is carried, not by the entire nanostructure, but by some specific active sites exhibiting an ideal generalized coordination number and an ideal distance with their nearest neighbour 1,2. Such approaches resulted in notable nanostructures, such as the PtNi octahedra 3, the PtNi nanoframes 4 or others that presented notable activity enhancements in liquid electrolyte vs. standard Pt/C. The highest activity improvement reported was for the PtNi nanowires 5, namely 35-fold higher specific activity than standard Pt/C. However, there is only little reports on their activity in membrane electrode assemblies. From these reports, it mainly appears that the liquid electrolyte activity is often not reproduced, and that the improvements factors in solid electrolyte environment range from 1 to 5 vs. standard Pt/C 6,7. However, an extensive comparison of the liquid vs. solid performance for a wide range of electrocatalysts is currently missing. In this work, a group of electrocatalysts was synthesized from using microwave-assisted syntheses, including standard Pt/C, PtM alloys, ordered (nano cubes, octahedra) and disordered nanostructures. These electrocatalysts were characterized in liquid electrolyte and solid electrolyte environment, and the changes in specific surface, mass and specific activity were assessed.(1) Calle-Vallejo, F.; Tymoczko, J.; Colic, V.; Vu, Q. H.; Pohl, M. D.; Morgenstern, K.; Loffreda, D.; Sautet, P.; Schuhmann, W.; Bandarenka, A. S. Science. 2015, 350, 185–189.(2) Le Bacq, O.; Pasturel, A.; Chattot, R.; Previdello, B.; Nelayah, J.; Asset, T.; Dubau, L.; Maillard, F. ChemCatChem 2017, 9, 2324–2338.(3) Cui, C.; Gan, L.; Li, H.-H.; Yu, S.-H.; Heggen, M.; Strasser, P. Nano Lett. 2012, 12, 5885–5889.(4) Chen, C.; Kang, Y.; Huo, Z.; Zhu, Z.; Huang, W.; Xin, H. L.; Snyder, J. D.; Li, D.; Herron, J. A.; Mavrikakis, M.; Chi, M.; More, K. L.; Li, Y.; Markovic, N. M.; Somorjai, G. A.; Yang, P.; Stamenkovic, V. R. Science. 2014, 343, 1339–1343.(5) Li, M.; Zhao, Z.; Cheng, T.; Fortunelli, A.; Chen, C.-Y.; Yu, R.; Zhang, Q.; Gu, L.; Merinov, B. V.; Lin, Z.; Zhu, E.; Yu, T.; Jia, Q.; Guo, J.; Zhang, L.; Goddard, W. A.; Huang, Y.; Duan, X. Science. 2016, 354, 1414–1419.(6) Li, B.; Wang, J.; Gao, X.; Qin, C.; Yang, D.; Lv, H.; Xiao, Q.; Zhang, C. Nano Res. 2019, 12, 281–287.(7) Mauger, S. A.; Neyerlin, K. C.; Alia, S. M.; Ngo, C.; Babu, S. K.; Hurst, K. E.; Pylypenko, S.; Litster, S.; Pivovar, B. S. J. Electrochem. Soc. 2018, 165, 238–245. Figure 1

Oxygen Reduction Reaction Activity and Durability of Pt-Ni Catalysts Synthesized By a Continuous Flow Reactor

In recent years, polymer electrolyte membrane fuel cell (PEMFC) technology has attracted much attention and enabled the successful commercialization of fuel cell electric vehicles (FCEVs). However, the continuation of FCEV deployment still faces various hurdles, of which one major challenge is related to the high cost of platinum-based catalysts.[1] Substantial efforts have been dedicated for improving catalyst structure and composition to enhance their performance and thus lower the loadings for cost reduction. For example, shape-controlled catalysts such as octahedral PtNi and PtCo alloy nanoparticles showed much higher activity for oxygen reduction reaction (ORR) than the conventional Pt/C catalyst. [2-5] However, those advanced catalysts were typically prepared by batch synthesis with quantities often as little as a few milligrams, making it difficult for validation and further development of membrane electrode assembly (MEA) for commercial application. Attempt in increasing the catalyst amount by multiple batches can be time-consuming and sometimes challenging to maintain the consistency.We recently designed a flow reactor (FR) to explore the continuous synthesis of Pt alloy nanoparticles (Fig. 1.) under elevated temperature and pressure (up to 350 °C and 1000 psi). It has several advantages including fast heat-exchange, fast mixing, uniform concentration and uniform temperature as compared to batch synthesis. These features made it possible to accurately control the reaction parameters to produce monodisperse Pt alloy catalysts up to 10 grams per day with high reproducibility. In this study, systematic investigation was conducted on the FR synthetic conditions for highly active catalyst. We were able to control the size and composition of resulting PtNi nanoparticles to achieve much higher ORR activity than commercial catalysts by optimizing synthesis conditions. As shown in Fig. 2 and Fig. 3, octahedral PtNi alloy catalysts synthesized by the FR showed much higher mass activity and specific activity compared to commercial benchmark. Attempts were also made to use machine learning tools to assist experimental design and data analysis for process optimization. It is expected that the combination of the latest development of data science and the inherent features of such flow reactors can substantially accelerate future FC catalyst R&D. Figure 1

Differential Surface Elemental Distribution Leads to Significantly Enhanced Stability of PtNi-Based ORR Catalysts

Summary PtNi-based nanomaterials represent an emerging class of highly active catalysts for the oxygen reduction reaction (ORR) in fuel cells. However, they suffer from poor stability in operating conditions, which is the key challenge in maintaining their activity advantage over Pt in practice. We report significantly enhanced stability and activity of octahedral PtNi nanoparticles by tuning their surface elemental distribution through the introduction of a third element (Cu) during synthesis. To uncover the mechanism behind this observation, we performed kinetic Monte Carlo (KMC) simulations initialized using growth-tracking experiments and demonstrated that PtNiCu has improved Ni and Cu retention compared with PtNi, in agreement with experiments. The tracked movement of individual atoms in KMC reveals that the enhanced stability can be attributed to increased surface Pt composition in as-synthesized catalysts, which reduces the generation of surface vacancies and suppresses the surface migration and subsequent dissolution of subsurface Cu and Ni atoms.

Controlling Near-Surface Ni Composition in Octahedral PtNi(Mo) Nanoparticles by Mo Doping for a Highly Active Oxygen Reduction Reaction Catalyst.

We report and study the translation of exceptionally high catalytic oxygen electroreduction activities of molybdenum-doped octahedrally shaped PtNi(Mo) nanoparticles from conventional thin-film rotating disk electrode screenings (3.43 ± 0.35 A mgPt-1 at 0.9 VRHE) to membrane electrode assembly (MEA)-based single fuel cell tests with sustained Pt mass activities of 0.45 A mgPt-1 at 0.9 Vcell, one of the highest ever reported performances for advanced shaped Pt alloys in real devices. Scanning transmission electron microscopy with energy dispersive X-ray analysis (STEM-EDX) reveals that Mo preferentially occupies the Pt-rich edges and vertices of the element-anisotropic octahedral PtNi particles. Furthermore, by combining in situ wide-angle X-ray spectroscopy, X-ray fluorescence, and STEM-EDX elemental mapping with electrochemical measurements, we finally succeeded to realize high Ni retention in activated PtNiMo nanoparticles even after prolonged potential-cycling stability tests. Stability losses at the anodic potential limits were mainly attributed to the loss of the octahedral particle shape. Extending the anodic potential limits of the tests to the Pt oxidation region induced detectable Ni losses and structural changes. Our study shows on an atomic level how Mo adatoms on the surface impact the Ni surface composition, which, in turn, gives rise to the exceptionally high experimental catalytic ORR reactivity and calls for strategies on how to preserve this particular surface composition to arrive at performance stabilities comparable with state-of-the-art spherical dealloyed Pt core-shell catalysts.

Octahedral Pt-Ni nanoparticles prepared by pulse-like hydrothermal method for oxygen reduction reaction

Herein, the pulse temperature is provided during the nucleation period of the nanoparticles to shorten the nucleation time for Pt-based nanoparticle growth. Herein, 3-nm-sized Pt-Ni alloy octahedral catalysts were prepared by pulse-like hydrothermal method. The influence of the pulse on nucleation is demonstrated by comparing the morphology obtained from the conditions of constant temperature and pulse temperature. It has been found that pulse temperature could decrease the nucleation time, and the final morphology of the nanoparticle is different due to the different growth temperatures and times. When used in oxygen reduction reaction, the octahedron Pt-Ni exhibited a 50 mV positive shift of the half-wave potential and the mass-specific activity and area-specific activity are 5 and 7 times higher than that of commercial 40% Pt/C at 0.9 V. Through this way, kinds of multiple alloy compounds could be composed with less time in the future.

(Invited) Nano-Structured ORR Catalysts for PEM Fuel Cells

Polymer electrolyte membrane fuel cells (PEMFCs) are the most attractive power source for zero-emission vehicles owing to their environmental friendliness, high energy efficiency, and high power density. However, their high cost and low durability have prevented PEMFCs from being fully implemented and commercialized, mainly due to the use of platinum as a catalyst to overcome the sluggish kinetics of the oxygen reduction reaction (ORR) in the cathode. To reduce the use of Pt, extensive studies have been conducted and a large variety of ORR catalysts have been developed, such as Pt-M alloys as well as shape-controlled, dealloyed, and structurally ordered catalysts. Although those catalysts exhibited excellent intrinsic activity in aqueous environment, they have been hardly implemented in membrane-electrode assemblies (MEAs) for practical application. In this talk, we report Pt-M intermetallic catalysts and M-doped PtNi octahedral nanoparticles on a carbon support (M-PtNi/C) that exhibit superior catalytic activity and stability in half-cell and MEA tests as an ORR catalyst for PEMFCs. To prevent agglomeration of Pt-M nanoparticles during heat treatment in which to transform the Pt-M alloy into Pt-M intermetallic structure, a facile synthetic process was conducted at low temperatures. Octahedral PtNi nanoparticles are known as one of the best ORR catalyst with its superior activity toward ORR. However, dissolution of Ni from the surface and collapse of the octahedral shape degrade the activity rapidly under the PEMFC operation conditions. To stabilize Ni and the octahedral shape, small amount of dopant was incorporated into the surface of octahedral PtNi. Electrochemical, chemical, and structural stability was thoroughly investigated through a harsh durability test. The effects of intermetallic structure and doping elements on the surface properties of Pt-based nanoparticles were examined by X-ray absorption fine structure (XAFS) analysis and density functional theory (DFT) calculation.

Concave curvature facets benefit oxygen electroreduction catalysis on octahedral shaped PtNi nanocatalysts

PtNi alloy nano-octahedra after thermal reductive annealing: imperfect PtNi octahedra with concave Pt curvature catalytically outperform well-alloyed cuboctahedral nanoparticles with {111} and {100} nanofacets in electrochemical ORR.

Deconvolution of octahedral Pt3Ni nanoparticle growth pathway from in situ characterizations

Understanding the growth pathway of faceted alloy nanoparticles at the atomic level is crucial to morphology control and property tuning. Yet, it remains a challenge due to complexity of the growth process and technical limits of modern characterization tools. We report a combinational use of multiple cutting-edge in situ techniques to study the growth process of octahedral Pt3Ni nanoparticles, which reveal the particle growth and facet formation mechanisms. Our studies confirm the formation of octahedral Pt3Ni initiates from Pt nuclei generation, which is followed by continuous Pt reduction that simultaneously catalyzes Ni reduction, resulting in mixed alloy formation with moderate elemental segregation. Carbon monoxide molecules serve as a facet formation modulator and induce Ni segregation to the surface, which inhibits the (111) facet growth and causes the particle shape to evolve from a spherical cluster to an octahedron as the (001) facet continues to grow.