Degradation Of Pt Research Articles

Electrochemical Degradation of Pt3Co Nanoparticles Investigated by Off-Lattice Kinetic Monte Carlo Simulations with Machine-Learned Potentials

Electrochemical Degradation of Pt₃Co Nanoparticles Investigated by Off-Lattice Kinetic Monte Carlo Simulations with Machine-Learned Potentials

Tailoring the Metallic and Oxide Sites of Pd/C and Ni/C Catalysts for Optimum Hydrogen Oxidation Reaction (HOR) and Durability in Alkaline Media

In recent years, Pd- and Ni-based, or Pd-Ni alloys catalysts for HOR in alkaline electrolytes obtained increasing attention due to their higher durability than Pt-based catalysts. [1-3] However, to prevent passivation in high pH media and maintain active metal/oxide sites, it is necessary to protect the surface of Pd and Ni. It is well documented that different types of carbon cover layers surrounding Ni particles, such as graphite or carbon nanotube, can be beneficial for several applications, e.g. hydrogenation, dehydrogenation, methanol oxidation [4-6], and HOR. Gao et al. reported different degree of graphite coverage-Ni particles of controlled metal/oxide ratio that can enhance the HOR activity of Ni in alkaline electrolytes [7]. In this work, we explore different carbon layer properties, morphology and thickness that cover Pd and Ni particles. Thicker cover layers lead to low HOR activity initially because of the un-balanced metal/oxide sites (the amount of metal sites is higher originally), but can be improved (activated) by voltage pulsing (0.1 - 1.23 V vs. RHE, 3 s each potential, 300 cycles) in 0.1 M KOH, 25°C, Ar gas, without agglomerating the particles (as shown in Figure 1). After full activation, protected Pd and protected Ni were compared with bare Pd and bare Ni (no protecting layers) before and after accelerated stress test (AST) in the potential range of [0.1, 1.23] V vs. RHE. IL-TEM, XRD and XPS techniques show that for protected Pd and Ni, the passivation and degradation happen much slower than for bare Pd and bare Ni (which are immediately deactivated upon AST); also, the proportion of metal or oxide sites can be tailored by different activation procedure, which renders these protected catalysts suitable for long-term alkaline HOR. Figure 1 IL-TEM of Pd C1 (Pd-capped 1) at its initial state vs. post-activation (300 cycles-Ar). Top: 200 k magnified micrographs. Bottom: 500 k magnified micrographs.[1] Lafforgue, C.; Zadick, A.; Dubau, L.; Maillard, F.; Chatenet, M. Selected Review of the Degradation of Pt and Pd-Based Carbon-Supported Electrocatalysts for Alkaline Fuel Cells: Towards Mechanisms of Degradation. Fuel Cells 2018, No. 0, 1–10.[2] Sankar, S.; Anilkumar, G. M.; Tamaki, T.; Yamaguchi, T. Binary Pd−Ni Nanoalloy Particles over Carbon Support with Superior Alkaline Formate Fuel Electrooxidation Performance. ChemCatChem 2019, 11 (19), 4731–4737.[3] Zhao, Y.; Wang, G.; Xiao, L.; Lu, J.; Zhuang, L. Hydrogen Oxidation Reaction on Pd-Ni(OH)2 Composite Electrocatalysts in an Alkaline Electrolyte. ChemistrySelect 2020, 5 (26), 7803–7807. )[4] Ortega-Domínguez, R. A.; Vargas-Villagrán, H.; Peñaloza-Orta, C.; Saavedra-Rubio, K.; Bokhimi, X.; Klimova, T. E. A Facile Method to Increase Metal Dispersion and Hydrogenation Activity of Ni/SBA-15 Catalysts. Fuel 2017, 198, 110–122.[5] Jiao, C.; Sun, L.; Xu, F.; Liu, S. S.; Zhang, J.; Jiang, X.; Yang, L. NiCo Nanoalloy Encapsulated in Graphene Layers for Improving Hydrogen Storage Properties of LiAlH4. Sci. Rep. 2016, 6 (January), 4–11.[6] Tong, X.; Qin, Y.; Guo, X.; Moutanabbir, O.; Ao, X.; Pippel, E.; Zhang, L.; Knez, M. Enhanced Catalytic Activity for Methanol Electro-Oxidation of Uniformly Dispersed Nickel Oxide Nanoparticles-Carbon Nanotube Hybrid Materials. Small 2012, 8 (22), 3390–3395.[7] Yang, Y.; Sun, X.; Han, G.; Liu, X.; Zhang, X.; Sun, Y.; Zhang, M.; Cao, Z.; Sun, Y. Enhanced Electrocatalytic Hydrogen Oxidation on Ni/NiO/C Derived from a Nickel-Based Metal–Organic Framework. Angew. Chemie - Int. Ed. 2019, 58 (31), 10644–10649.[8] Gao, Y.; Peng, H.; Wang, Y.; Wang, G.; Xiao, L.; Lu, J.; Zhuang, L. Improving the Antioxidation Capability of the Ni Catalyst by Carbon Shell Coating for Alkaline Hydrogen Oxidation Reaction. ACS Appl. Mater. Interfaces 2020, 12 (28), 31575–31581. Figure 1

In Operando Spectroscopic Study for Durable Ammonia Electrolysis on Platinum

Ammonia has recently been considered as a promising energy carrier and a carbon-free fuel. Combining with renewable energy sources, ammonia oxidation reaction (AOR) is crucial reaction for onsite hydrogen-fuel production and electricity generation in future energy scenario. In spite of promising beginning-of-life AOR activity of platinum, its implementation is hampered by vulnerable AOR catalysis. Hence, improved durability is critically needed but challenged by insufficient understanding of its degradation mechanism during NH3 electrolysis. In this work, we study the degradation mechanisms of Pt during AOR catalysis with operando and in/ex situ spectroscopies. Passivation of the Pt surface and complexation of the dissolved Pt ions by NH3 (or NH3 derivatives during AOR) are identified. Such chemical modifications accelerate Pt dissolution by forming stable complex while decelerate Pt redeposition by passivation of Pt surface, which lead to the instable nature of Pt electrocatalysis toward AOR. Based on the potential-resolved investigations, a stable operando potential window and strategy for operating AOR are suggested in this study. The preliminary AOR protocol demonstrates prolonged NH3 electrolysis without significant degradation of Pt, which implicates the possible usage of NH3 as an energy carrier when the proposed operation strategy is further developed.

Electrochemical Degradation of Pt–Ni Nanocatalysts: An Identical Location Aberration-Corrected Scanning Transmission Electron Microscopy Study

The evolution of Pt-Ni nanoparticles supported on amorphous carbon is investigated before and after electrochemical potential cycling (0.6-1.1 V), using aberration-corrected scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDS). During voltage cycling and due to the dissolution of nanoparticles, single ions/atoms and ionic/atomic clusters emerge and diffuse across the carbon support toward larger nanoparticles, where they redeposit. We observe that the preferred locations for the dissolution are the steps and corners of the nanoparticles. On the other hand, the redeposition process happens often on {111} type planes. In addition, contrary to the conventional view, where larger particles grow isotropically from smaller ones, this research work shows that anisotropic growth of smaller particles occurs during potential cycling. The reason for this behavior seems to be related to the fact that smaller particles with thicker Pt-rich shells trigger the nucleation and deposition of Pt.

Oxide scale formation and microstructural degradation of conventional, Pt- and Pt/Ir-modified NiAl diffusion coatings during thermocyclic exposure at 1100 °C

The microstructural degradation of Pt- and Pt/Ir-modified NiAl coatings during thermocyclic exposure at 1100 °C was investigated. Detailed microstructural investigation revealed the different beneficial effects of Pt and Ir. Platinum was found to be active in the healing mechanism of the oxide scale, whereas Ir incorporation provided an obstruction to interdiffusion. Oxidized surface morphologies exhibited the formation of a dense alumina scale on top of the PtAl coating, whereas Al-depletion in the NiAl coating was accelerated due to scale spallation and interfacial void formation. Additionally, Pt inward- and Ni outward diffusion was substantially reduced in the Ir-containing coatings.

Observation of Degradation of Pt and Carbon Support in Polymer Electrolyte Fuel Cell Using Combined Nano-X-ray Absorption Fine Structure and Transmission Electron Microscopy Techniques

It is hard to directly visualize spectroscopic and atomic-nanoscopic information on the degraded Pt/C cathode layer inside polymer electrolyte fuel cell (PEFC). However, it is mandatory to understand the preferential area, sequence, and relationship of the degradations of Pt nanoparticles and carbon support in the Pt/C cathode layer by directly observing the Pt/C cathode catalyst for the development of next-generation PEFC cathode catalysts. Here, the spectroscopic, chemical, and morphological visualization of the degradation of Pt/C cathode electrocatalysts in PEFC was performed successfully by a same-view combination technique of nano-X-ray absorption fine structure (XAFS) and transmission electron microscopy (TEM)/scanning TEM-energy-dispersive spectrometry (EDS) under a humid N2 atmosphere. The same-view nano-XAFS and TEM/STEM-EDS imaging of the Pt/C cathode of PEFC after triangular-wave 1.0-1.5 VRHE (startup/shutdown) accelerated durability test (tri-ADT) cycles elucidated the site-selective area, sequence, and relationship of the degradations of Pt nanoparticles and carbon support in the Pt/C cathode layer. The 10 tri-ADT cycles caused a carbon corrosion to reduce the carbon size preferentially in the boundary regions of the cathode layer with both electrolyte and holes/cracks, accompanied with detachment of Pt nanoparticles from the degraded carbon. After the decrease in the carbon size to less than 8 nm by the 20 tri-ADT cycles, Pt nanoparticles around the extremely corroded carbon areas were found to transform and dissolve into oxidized Pt2+-O4 species.

Load Cycle Durability of Pt/Nb-SnO2 in Polymer Electrolyte Fuel Cells Under Potentiostatic/Galvanostatic Operation

Polymer electrolyte fuel cells (PEFCs) used in fuel cell vehicles (FCVs) are still faced with the challenge of increasing the stack durability. It is essential to resolve the degradation of the cathode catalyst layer due to carbon corrosion, dissolution and particle growth of Pt, and decomposition of membranes under various types of FCV operating conditions such as startup/shutdown, load cycling, and idling. In particular, it is difficult to suppress the degradation of Pt, because the speed of the FCV is controlled by the driver, leading to load cycling, which is known to decrease the durability of carbon black-supported Pt cathode catalysts. In particular, the degradation of Pt is accelerated at high relative humidity and long holding times at open circuit voltage (OCV) due to Pt oxide growth [1]. The dissolved Pt2+ ions are likely to diffuse into the ionomer and membrane during load cycling due to the limited ability of the carbon black support to trap these ions. In contrast, Pt on an Nb-SnO2 support might be expected to be slower to diffuse into the ionomer and membrane due to strong metal-substrate interactions [2]. In fact, we reported that the durability of a Pt/Nb-SnO2 cathode catalyst was superior to that of a Pt/GCB cathode catalyst during potentiostatic simulated load cycling [3]. In this research, we evaluated the load cycle durability for membrane electrode assemblies using a Pt/Nb-SnO2 cathode catalyst in PEFCs under galvanostatic and potentiostatic cycling operation. The Pt loading amount for both the Pt/Nb-SnO2 cathode catalyst and the Pt/CB anode catalyst was 0.1 mg cm-2. The cell performances were evaluated under H2/O2 at 80 °C and 100% RH before and after the durability evaluation. Figure 1a shows the protocol for the durability evaluation involving OCV and load holding times at a low current density of 0.33 A cm-2 (sample A1), at a high current density of 0.55 A cm-2 (sample A2), and, in the case of Fig. 1b, the load cycles between the upper cell voltage at 0.94 V and the lower cell voltage at 0.6 V were operated by a potentiostat under H2/N2 (sample A3). A1, A2 and A3 conditions were performed with H2; utilizations of the reactant gases were H2 (70%)/air (40%), H2 (70%)/O2 (20%) and H2 (100 mL min-1)/N2 (100 mL min-1) at 80 °C and 80% RH in Fig. 2. These durability evaluations can be characterized as follows: A1, A2, with the OCV/load holding times of 60 s/3 s; and A3, with upper/lower cell voltage holding times of 60 s/3 s; the sweep rate of 165 mV s-1 from the lower to upper cell voltage, simulates the durability test of A1. Figures 3 and 4 show the cell performances and mass activities under H2/O2 at 80 °C and 100% RH before and after the durability evaluation. After the durability evaluation, the cell performances for both A1 (H2/air condition) and A2 (H2/O2 condition) were nearly the same as their initial performances, but that for A3 (H2/N2 condition) decreased. The mass activity change decreased with the higher O2 partial pressure in the cathode after the durability evaluation. The mass activity of A2 (H2/O2 condition) was the highest of all conditions after the durability evaluation, in spite of the upper cell voltage being the highest. The behavior was in contrast to the case of a carbon-supported Pt catalyst (not shown here). These results suggest that the durability of the Pt/Nb-SnO2 cathode catalyst could correlate with the thickness of the electron depletion layer between the surface of the Nb-SnO2 and Pt. We reported that the electron depletion layer was induced by adsorption of oxygen species on the surface of SnO2 [4]. Therefore, we consider that the degradation from Pt dissolution in the electrochemical oxidation reaction (Pt → Pt2+ + 2e-) could be suppressed by increasing the thickness of the electron depletion layer due to an increase in the amount of adsorbed oxygen species as a function of O2 partial pressure. We will discuss in detail the degradation mechanisms of the Pt/Nb-SnO2 cathode catalyst in comparison with Pt/GCB during the load cycle durability evaluation. C. Takei, K. Kakinuma, K. Kawashima, K. Tashiro, M. Watanabe, and M. Uchida, J. Power Sources, 324, 729-737 (2016).Y. Chino, K. Taniguchi, Y. Senoo, K. Kakinuma, M. Hara, M. Watanabe, and M. Uchida, J. Electrochem. Soc., 162, F736-F743 (2015).R. Kobayashi, K. Kakinuma, A. Iiyama, and M. Uchida, 232nd ECS Meeting, 1388 (2017).Y. Senoo, K. Kakinuma, M. Uchida, H. Uchida, S. Deki, and M. Watanabe, RSC Adv., 4, 32180-32188 (2014). Figure 1

Selected Review of the Degradation of Pt and Pd‐based Carbon‐supported Electrocatalysts for Alkaline Fuel Cells: Towards Mechanisms of Degradation

AbstractIt is usually believed that carbon‐supported electrocatalysts are stable in alkaline environment, owing to the better thermodynamics stability of many metals and oxides at high pH. By focusing on a selected literature review concerning Pt/C and Pd/C nanoparticles, and in particular from identical‐location transmission electron microscopy (ILTEM), it is demonstrated that this “common knowledge” is erroneous in aqueous alkaline electrolytes: both Pt/C and Pd/C suffer pronounced loss of electrochemical surface area (ECSA), and the latter is linked to the detachment of the metal nanoparticles from the carbon support. Raman and X‐ray photoelectron spectroscopy show that these severe degradations are neither linked to massive corrosion of the carbon support nor to an overall change in carbon chemistry, but instead to a very localized corrosion of the carbon in the vicinity of the metal nanoparticles, leading to nucleation and growth of solid carbonate (when the electrolyte contains alkali metal cations), which expels the metal nanoparticles from their support. The mechanisms and extent of degradation depend on the nature of the metal nanoparticles, but also on their texture and on the nature of the support onto which they are immobilized.

Influence of Electrolyte Composition and pH on Platinum Electrochemical and/or Chemical Dissolution in Aqueous Acidic Media

Comprehension of the impact of electrolyte nature and concentration on Pt degradation is essential for the improvement of durability of catalyst layers (CLs), which are the heart of polymer electrolyte membrane fuel cells (PEMFCs). Electrochemical and chemical dissolution of polycrystalline Pt in aqueous CF3SO3H, H2SO4, and HClO4 solutions of different concentrations (c = 0.1 and 0.5 M) upon potential switching and holding in the 0.60–1.20 V versus RHE range is analyzed using inductively coupled plasma mass spectroscopy. This potential range mimics the conditions encountered in operating PEMFCs. Trifluoromethanesulfonic acid (CF3SO3H) is employed because it is the smallest fluorinated sulfonic acid and can serve as a model molecule. Degradation of Pt in H2SO4 and HClO4 solutions is examined for comparative analysis. The results reveal that the electrolyte concentration has a significant impact on Pt electrochemical and chemical dissolution. The amount of dissolved Pt in 0.1 M solutions of CF3SO3H, H2SO4, ...

In Situ Liquid Cell TEM Study of Morphological Evolution and Degradation of Pt–Fe Nanocatalysts During Potential Cycling

Nanocatalyst degradation is a serious limiting factor for the commercialization of proton exchange membrane fuel cells. Although the degradation has been extensively studied in the past through various ex situ electrochemical methods, employing an in situ technique can greatly improve our understanding of the mechanisms involved during the electrochemical cycling. In this work, we have employed an in situ liquid cell inside a TEM for a simultaneous investigation of the structural evolution and electrochemical response of Pt–Fe nanocatalysts. We demonstrate that the coarsening processes of these nanocatalyst particles, including the nucleation and growth, are not uniform, both in space and in time scale. The growth rate is found to be both site- and potential-dependent. Furthermore, these particles were found to exhibit considerably different behaviors when attached to an electrode as opposed to when isolated in the electrolyte. With Pt–Fe nanoalloy system as a candidate material, this work demonstrates th...

Durability of Low-Temperature Fuel Cell Electrocatalysts

Durability of Low-Temperature Fuel Cell Electrocatalysts

Synthesis and Characteristics of Pt/graphene by Co-Reduction Method for Oxygen Reduction Reactions

40%(w)Pt/graphene composites were prepared by sodium borohydride chemical coreduction,and were subsequently used as an electrocatalyst for oxygen reduction reactions.The electrocatalytic activity and stability was evaluated by cyclic voltammetry.The results indicated that the initial activity of Pt/graphene was lower than that of Pt/C due to the oxygen diffusion inhibition;however,the Pt/graphene showed superior durability characteristics.Degradation tests showed a 50%degradation of Pt/ graphene,which was substantially less than that of Pt/C(79%).X-ray diffraction and transmission electron microscope results showed that the composite formed strong interactions between the platinum nanoparticles and the graphene supports.The graphene supports may also prevent the graphene sheets from folding or re-stacking,which would hinder platinum nanoparticles?aggregation.The performance of a single cell was also tested,confirming an improvement in durability.

Co oxidation accompanied by degradation of Pt–Co alloy cathode catalysts in polymer electrolyte fuel cells

The electronic structure of Co ions in Pt(3)Co alloy cathode catalysts for polymer electrolyte fuel cells (PEFCs) under different operation conditions has been investigated using X-ray absorption spectroscopy (XAS). The line shapes of the Co 2p XAS spectra indicate that the Co atoms in the cathode layer are predominantly metallic in all cells, and the spectrum of the first-operated cell is the most metallic among them. The Co 2p XAS spectra slightly change depending on the operation conditions, and the differences in the spectra can be explained by the presence of cobalt oxides. Decomposition analysis of the Co 2p XAS spectra reveals that the operation of Pt-Co PEFCs yields trivalently oxidised cobalt in the cathode layer.

Synthesis and Solid-State Structure of a Pt-Ru-P Ternary Metal Phosphide (PtRuP2) as a Carbon Nanocomposite

Reactive thermal degradation of Pt(PPh3)(Cl)(mu2-Cl)2Ru(Cl)(eta3:eta3-C10H16)/Vulcan carbon powder composites gives a nanocomposite powder containing nanocrystals of the expected PtRu alloy phase along with nanocrystals of an unknown substance. Yields of the unknown phase increase when PPh3 is added to the composite prior to thermal treatment. The new substance has been identified as a ternary metal phosphide, PtRuP2. Full-profile Rietveld analysis of the XRD pattern of this phase is consistent with a primitive Pm3m cubic unit cell (CsCl-type) having a cell constant of 2.78 A. Fully disordered Pt and Ru atoms occupy the (0, 0, 0) atomic positions with P atoms occupying interstitial sites at the (1/2, 1/2, 1/2) atomic positions.

Degradation of polymer electrolyte fuel cells under the existence of anion species

The degradation of Pt supported on carbon catalysts (Pt/C) due to four anion species (Cl−, F−, SO42−, NO3−) was investigated by operation tests using single cells of a polymer electrolyte fuel cell (PEFC). A loss of cell voltage and of electrochemical surface area (ECA) of the cathode catalyst, and an increase in oxygen gain were shown only when a solution containing Cl− was supplied. Scanning electron microscope (SEM) and transmission electron microscope (TEM) analysis after 50h tests showed partial Pt dissolution and deposition in the membrane (Pt band) only in the inlet-side of the membrane electrode assembly (MEA). The partial loss of Pt led to the concentration of current density and degradation of cell performances.

High temperature degradation of Pt/Ti electrodes in micro-hotplate gas sensors

This paper examines the high temperature degradation of Pt–Ti thin films which configure the electrodes of a micro-hotplate designed in agreement with those used for gas sensor applications. FEM simulations of the micro-hotplate have been combined with atomic force microscopy (AFM) and Auger electron spectroscopy (EAS) measurements. Experimentally, the main mechanical failure mechanisms that have been identified are hillocks and delaminations detected in the Pt–Ti micro-electrodes produced by the high thermo-mechanical stresses induced during the operation of the micro-hotplate. Additionally, concurrent with the annealing-induced Pt microstructure changes, Ti from the adhesion layer and N from the Si3N4, migrated into the Pt film. It was shown with AES that, for high temperature operations, the titanium and nitrogen reach the Pt surface. Mechanical failures affect the contact between electrodes and the gas sensing material, and the Ti and N diffusion modifies the electrodes electrical resistivity. It is expected that both kinds of effects alter the electrical connection between the sensing layer and the electrodes, thus influencing the operation of the whole gas sensor.