High temperature stability study of carbon supported high surface area catalysts—Expanding the boundaries of ex-situ diagnostics

Corrosion Dynamics of Carbon-Supported Platinum Electrocatalysts with Metal-Carbon Interactions Revealed by In Situ Liquid Transmission Electron Microscopy

Carbon support is essential for electrocatalysis, but limitations remain, as carbon corrosion can lead to electrocatalyst degradation and affect the long-term durability of electrocatalysts. Here, we studied the corrosion dynamics of carbon nanotubes (CNTs) and Vulcan carbon (VC) together with platinum (Pt) nanoparticles in real time by liquid cell (LC) transmission electron microscopy (TEM). The results showed that CNTs with a high degree of graphitization exhibited higher corrosion resistance compared to VC. Furthermore, we observed that the main degradation path of Pt nanoparticles in Pt/CNTs was ripening, while in Pt/VC, it was aggregation and coalescence, which was dominated by the interactions between Pt nanoparticles and different hybridization of carbon supports. Finally, we performed an ex situ CV stability test to confirm the conclusions obtained from in situ experiments. This work provides deep insights into the corrosion mechanism of carbon-supported electrocatalysts to optimize the design of electrocatalysts with a higher durability.

Towards a realistic prediction of catalyst durability from liquid half-cell tests

Liquid half-cell measurements provide a convenient laboratory method for determining relevant parameters of electro-catalysts applied in e.g. polymer electrolyte membrane fuel cells. While these measurements may be effective in certain contexts, their applicability to real-world systems, such as single-cells in a membrane electrode assembly (MEA) configuration, is not always clear. This is particularly true when assessing the stability of these systems through accelerated stress tests (ASTs). Due to different electrode compositions and operating conditions, nanoscale degradation proceeds differently. Nevertheless, given the high demands of MEA measurements in terms of time, testing equipment complexity, and amount of catalyst material, application-relevant predictions of catalyst durability from liquid half-cell tests are highly desirable. This study combines electrochemical and nanoparticle analysis based on transmission electron microscopy to conduct a typical voltage cycling AST for rotating disc electrode (RDE) measurements, showing that the loss of the electrochemically active surface area (ECSA) of the used Pt/Vulcan catalyst is strongly enhanced at 80 °C compared to room temperature, which goes along with increased nanoparticle coarsening. Additionally, a high ionomer/carbon mass ratio (I/C = 0.7) accelerates the ECSA loss, and further investigations of its influence suggest a combination of several factors, including the high local proton concentration and the presence of adsorbing anions. At the same temperature (80 °C) and I/C ratio (0.7), the ECSA loss vs. AST cycle number of the Pt/Vulcan catalyst is essentially identical for a voltage cycling AST conducted in either an RDE half-cell or an MEA configuration, suggesting that liquid electrolyte half-cell based ASTs can provide application-relevant results. Thus, our study points out a way for predicting the stability of electro-catalysts in MEAs based on RDE experiments that require less specialized equipment and only μg-quantities of catalysts.

Stability challenges of carbon-supported Pt-nanoalloys as fuel cell oxygen reduction reaction electrocatalysts

Carbon-supported Pt-based nanoalloys (CSPtNs) as the oxygen reduction reaction (ORR) electrocatalysts are considered state-of-the-art electrocatalysts for use in proton exchange membrane fuel cells (PEMFCs). Although their ORR activity performance is already adequate to allow lowering of the Pt loading and thus commercialisation of the fuel cell technology, their stability remains an open challenge. In this Feature Article, the recent achievements and acquired knowledge on the degradation behaviour of these electrocatalysts are overviewed and discussed.

Understanding the Crucial Significance of the Temperature and Potential Window on the Stability of Carbon Supported Pt-Alloy Nanoparticles as Oxygen Reduction Reaction Electrocatalysts

The present research provides a study of carbon-supported intermetallic Pt-alloy electrocatalysts and assesses their stability against metal dissolution in relation to the operating temperature and the potential window using two advanced electrochemical methodologies: (i) the in-house designed high-temperature disk electrode (HT-DE) methodology as well as (ii) a modification of the electrochemical flow cell coupled to an inductively coupled plasma mass spectrometer (EFC-ICP-MS) methodology, allowing for highly sensitive time- and potential-resolved measurements of metal dissolution. While the rate of carbon corrosion follows the Arrhenius law and increases exponentially with temperature, the findings of the present study contradict the generally accepted hypothesis that the kinetics of Pt and subsequently the less noble metal dissolution are supposed to be for the most part unaffected by temperature. On the contrary, clear evidence is presented that in addition to the importance of the voltage/potential window, the temperature is one of the most critical parameters governing the stability of Pt and thus, in the case of Pt-alloy electrocatalysts, also the ability of the nanoparticles (NPs) to retain the less noble metal. Lastly, but also very importantly, results indicate that the rate of Pt redeposition significantly increases with temperature, which has been the main reason why mechanistic interpretation of the temperature-dependent kinetics related to the stability of Pt remained highly speculative until now.

Platinum Dissolution in Realistic Fuel Cell Catalyst Layers

Pt dissolution has already been intensively studied in aqueous model systems and many mechanistic insights have been gained. Nevertheless, transfer of new knowledge to real‐world fuel cell systems is still a significant challenge. To close this gap, we present a novel in situ method combining a gas diffusion electrode (GDE) half‐cell with inductively coupled plasma mass spectrometry (ICP‐MS). With this setup, Pt dissolution in realistic catalyst layers and the transport of dissolved Pt species through Nafion membranes were evaluated directly. We observed that 1) specific Pt dissolution increased significantly with decreasing Pt loading, 2) in comparison to experiments on aqueous model systems with flow cells, the measured dissolution in GDE experiments was considerably lower, and 3) by adding a membrane onto the catalyst layer, Pt dissolution was reduced even further. All these phenomena are attributed to the varying mass transport conditions of dissolved Pt species, influencing re‐deposition and equilibrium potential.

Low-PGM and PGM-Free Catalysts for Proton Exchange Membrane Fuel Cells: Stability Challenges and Material Solutions

Fuel cells as an attractive clean energy technology have recently regained popularity in academia, government, and industry. In a mainstream proton exchange membrane (PEM) fuel cell, platinum-group-metal (PGM)-based catalysts account for ≈50% of the projected total cost for large-scale production. To lower the cost, two materials-based strategies have been pursued: 1) to decrease PGM catalyst usage (so-called low-PGM catalysts), and 2) to develop alternative PGM-free catalysts. Grand stability challenges exist when PGM catalyst loading is decreased in a membrane electrode assembly (MEA)-the power generation unit of a PEM fuel cell-or when PGM-free catalysts are integrated into an MEA. More importantly, there is a significant knowledge gap between materials innovation and device integration. For example, high-performance electrocatalysts usually demonstrate undesired quick degradation in MEAs. This issue significantly limits the development of PEM fuel cells. Herein, recent progress in understanding the degradation of low-PGM and PGM-free catalysts in fuel cell MEAs and materials-based solutions to address these issues are reviewed. The key factors that degrade the MEA performance are highlighted. Innovative, emerging material concepts and development of low-PGM and PGM-free catalysts are discussed.

Particle Size Effect on Platinum Dissolution: Practical Considerations for Fuel Cells

The high costs of polymer membrane electrolyte fuel cells (PEMFCs) remain a roadblock for a competitive market with combustion engine vehicles. The PEMFC costs can be reduced by decreasing the size of Pt nanoparticles in the catalyst layer, thereby increasing the Pt dispersion and utilization. Furthermore, high-power performance loss due to O₂ transport resistance is alleviated by decreasing the particle size and increasing dispersion. However, firm conclusions on how Pt particle size impacts durability remain elusive due to synthetic difficulties in exclusively varying single parameters (e.g., particle size and loading). Therefore, here the particle size of Pt nanoparticles was varied from 2.0 to 2.8 and 3.7 nm while keeping the loading constant (30 wt %) on a Vulcan support using the two-step surfactant-free toolbox method. By studying the electrochemical dissolution in situ using online inductively coupled plasma mass spectrometry (online ICP-MS), mass-specific dissolution trends are revealed and are attributed to particle-size-dependent changes in electrochemically active surface area. Such degradation trends are critical for the start/stop of PEMFCs and currently require the implementation of potential control systems in consumer vehicles. Additionally, shifts in the onset of anodic dissolution and also oxidation to more negative potentials with decreasing particle size were observed. These results indicate a similar mechanism of anodic dissolution related to place-exchange when moving from extended polycrystalline Pt to nanoparticle scales. The negative shifts in the onset as the particle size decreases highlight a practical limitation for PEMFCs during load/idle conditions: without further material improvements, which inhibit Pt dissolution, reduction in costs and improvement in high-power performance via increased Pt utilization and dispersion will not be possible by decreasing particle sizes further.

Dissolution of Platinum Single Crystals in Acidic Medium

Platinum single crystal basal planes consisting of Pt(111), Pt(100), Pt(110) and reference polycrystalline platinum Pt(poly) were subjected to various potentiodynamic and potentiostatic electrochemical treatments in 0.1 M HClO4 . Using the scanning flow cell coupled to an inductively coupled plasma mass spectrometer (SFC-ICP-MS) the transient dissolution was detected on-line. Clear trends in dissolution onset potentials and quantities emerged which can be related to the differences in the crystal plane surface structure energies and coordination. Pt(111) is observed to have a higher dissolution onset potential while the generalized trend in dissolution rates and quantities was found to be Pt(110)>P(100)≈Pt(poly)>Pt(111).

Electrochemical On-line ICP-MS in Electrocatalysis Research

Electrocatalyst degradation due to dissolution is one of the major challenges in electrochemical energy conversion technologies such as fuel cells and electrolysers. While tendencies towards dissolution can be grasped considering available thermodynamic data, the kinetics of material's stability in real conditions is still difficult to predict and have to be measured experimentally, ideally in-situ and/or on-line. On-line inductively coupled plasma mass spectrometry (ICP-MS) is a technique developed recently to address exactly this issue. It allows time- and potential-resolved analysis of dissolution products in the electrolyte during the reaction under dynamic conditions. In this work, applications of on-line ICP-MS techniques in studies embracing dissolution of catalysts for oxygen reduction (ORR) and evolution (OER) as well as hydrogen oxidation (HOR) and evolution (HER) reactions are reviewed.

Importance of non-intrinsic platinum dissolution in Pt/C composite fuel cell catalysts

The dissolution of different platinum-based nanoparticles deposited on a commercial high-surface area carbon (HSAC) support in thin catalyst films is investigated using a highly sensitive electrochemical flow cell (EFC) coupled to an inductively coupled plasma mass spectrometer (ICP-MS). The previously reported particle-size-dependent dissolution of Pt is confirmed on selected industrial samples with a mean Pt particle size ranging from 1 to 4.8 nm. This trend is significantly altered when a catalyst is diluted by the addition of HSAC. This indicates that the intrinsic dissolution properties are masked by local oversaturation phenomena, the so-called confinement effect. Furthermore, by replacing the standard HSAC support with a support having an order of magnitude higher specific surface area (a micro- and mesoporous nitrogen-doped high surface area carbon, HSANDC), Pt dissolution is reduced even further. This is due to the so-called non-intrinsic confinement and entrapment effects of the (large amount of) micropores and small mesopores doped with N atoms. The observed more effective Pt re-deposition is presumably induced by local Pt oversaturation and the presence of nitrogen nucleation sites. Overall, our study demonstrates the high importance and beneficial effects of porosity, loading and N doping of the carbon support on the Pt stability in the catalyst layer.