Catalyst Degradation Mechanisms Research Articles

Humidity-Induced Degradation Mapping of Pt3Co ORR Catalyst for PEFC by In-Operando Electrochemistry and Ex Situ SAXS.

Understanding the degradation mechanisms of Pt-alloy catalysts is crucial for enhancing their durability. This study investigates the impact of relative humidity on Pt and Pt3Co catalysts using potential-cycling-based accelerated stress tests. Two conditions are investigated: 100% relative humidity on both sides, and a gradient with 30% at the anode and over 100% at the cathode. Pt3Co demonstrates sensitivity, with 77% performance loss and reductions in electrocatalyst surface area. Results demonstrate a 30% decrease in potential loss for Pt catalysts and a 77% increase for Pt3Co catalysts, indicating significant performance degradation in high humidity conditions, with Pt3Co exhibiting greater sensitivity. Measurements of electrochemically active surface area reinforce these findings. Resistance analysis using electrochemical impedance spectroscopy using equivalent circuit modeling reveals a threefold increase in Pt3Co MEAs' cathode charge transfer resistance and mass transport resistance during accelerated stress tests. Local current distribution analysis highlights differences between Pt catalyst and Pt3Co, with the latter displaying dealloying effects. Small-angle X-ray scattering reveals changes in particle and cluster sizes, indicating structural changes. Scanning electron microscopy highlights catalyst and membrane thickness variations, suggesting heterogeneity in Pt3Co. Under humidity gradients, Ostwald ripening plays a significant role in altering the catalyst's Pt3Co structure and subsequently impacting its performance.

Degradation and Protection Mechanisms of Platinum-Based Catalysts in Hydrogen-Bromine Flow Batteries

Cost-effective and large-scale electrical energy storage is pivotal in facilitating the assimilation of renewable energy sources like wind and solar, as well as enhancing the resilience of the future electricity grid. Hydrogen-Bromine flow battery (HBFB) technology is a candidate for a low cost and flexible system. Bromine is abundantly available on Earth. In addition, the reaction between bromine and bromide is characterized by rapid kinetic properties, thus no platinum group metals (PGMs) are required on the bromine side of the cell as a catalyst to sustain a high current density of >500 mA/cm2. Moreover, hydrogen-based flow batteries can be coupled with hydrogen pipelines & electrolyzers to boost the integration of new energy systems.However, the selection of bromine brings technical challenges that can limit the commercial deployment of such HBFB systems. We have proven a lifetime of more than 8,000 hours of continuous cycling at the lab scale. During this proof-of-concept validation, we found that bromine species (liquid) crosses through a dense proton exchange membrane to the hydrogen side of the cell. On this side, a Platinum-based catalyst layer is present to promote hydrogen oxidation-evolution reactions (HOR/HER). The crossover liquid interacts first with the catalyst layer before it crosses through the gas diffusion layer and exits the membrane electrode assembly (MEA). Chemical interactions between the crossover liquid and the catalyst can lead to catalyst migration, Ostwald ripening, or even permanent catalyst dissolution. This is a concern and must be addressed. The root of this concern is that the Platinum-based catalyst is highly reactive (high surface-active area respective to the loading, m2/g), and also the crossover bromine species are likely corrosive (highly acidic, pH <0, and contain polybromides). Therefore, chemical interaction cannot be fully avoided. Nevertheless, the cathodic protection reduces this interaction, which prolongs the lifetime of the electrochemical system. The cathodic protection consists of an applied positive potential and the presence of hydrogen gas on the catalyst nanoparticles. The catalyst degradation together with cathodic protection needs to be better understood and quantified to gain the confidence that the HBFB system fundamentally can last for more than 10 years of continuous operation. This confidence is essential to deliver a minimum viable product (MVP) of HBFB, e.g. on a 100 kW – 1 MW scale, to the market.To address the aforementioned scientific challenges, this work studies the liquid crossover phenomena and the interaction between the crossover liquid and platinum-based catalyst in depth. Liquid crossover rate, concentration, and bromine speciation from working electrochemical cells are quantified at different states of charge (SoCs) and different modes (charge or discharge). This information is used to set up further ex-situ experiments to understand and determine the chemical interaction between crossover liquid and catalyst. Electrochemical ex-situ tests for Pt dissolution measurement such as determination of the electrochemically active surface area (ECSA), are performed before and after soaking hydrogen electrodes in different concentrations of hydrogen bromide solutions that mimic the crossover liquid. These results are then combined with in-situ tests, where the catalyst dissolution rate is quantified in actual crossover liquid and electrolyte by the inductively coupled plasma (ICP) method. Finally, post-mortem analyses on the catalyst layer based on scanning electron microscopy (SEM), transmission electron microscopy (TEM), etc. will give quantitative and qualitative insights into the impact of cycling and liquid crossover on the catalyst performance, morphology, crystal structure, and chemical state of the catalyst.The goal of this work is to quantify the extrapolated lifetime of HBFB systems within a high confidence level. Then conclude if this technology is fundamentally sound and can be scaled up to an MVP to be deployed to the market. As a low cost energy storage solution, the HBFB’s lifetime should be at least 5 years and preferably more than 10 years to attract commercial attention. Moreover, the collective knowledge gained in this work about the catalyst degradation mechanisms will spark further ideas and guide the next generation of the system and its operating conditions to reduce the degradation rate. These presented ideas may also shine a light on catalyst degradation and protection mechanisms in other electrochemical applications that operate in corrosive conditions.Figure 1 Membrane electrode assembly architecture, incl. electrochemical reactions and species transport phenomena in the charge and discharge modes. Figure 1

Balancing Activity and Stability through Compositional Engineering of ORR Catalysts

Widespread adoption of advanced energy technologies, such as proton-exchange membrane fuel cells (PEMFCs), relies on the development of efficient and durable electrocatalysts for the oxygen reduction reaction (ORR). Given its unique properties, platinum has been the exclusive option for meeting activity demands, whether in its monometallic form or various bimetallic alloys with other metals1. Nevertheless, the challenge persists with durability due to catalyst degradation under the hostile electrochemical environment of the PEMFC cathode, leading to an irreversible detrimental effect on fuel cell performance. Studies of the degradation mechanisms of ORR catalysts and their underlying principles have highlighted that, among various degradation mechanisms, inhibiting the dissolution of Pt is crucial for suppressing their deterioration2.Owing to its high oxidation potential, gold has attracted increasing attention as an element that can be utilized to stabilize Pt and Pt-based ORR catalysts3. However, as Au is inactive towards ORR, achieving a balance between catalyst activity and stability requires precise compositional engineering.Targeting that aim, varying amounts of Au were added to model Pt and PtNi catalysts prepared by magnetron co-sputtering. A systematic study, employing multiple complementary characterization techniques such as Synchrotron Radiation Photoelectron Spectroscopy (SRPES), X-ray Diffraction (XRD), Rotation Disc Electrode (RDE), Scanning Flow Cell coupled to an Inductively Coupled Mass Spectrometer (SFC-ICP-MS), enabled us to gain a comprehensive understanding of the composition–activity–stability relationship to find optimal Pt–Au and PtNi-Au alloying for maintaining high ORR activity and improving catalyst stability4.

Investigating Corrosion Degradation of Porous Transport Layers for PEM Water Electrolyzers Using on-Line ICP-MS

The degradation of polymer electrolyte membrane water electrolyzer (PEMWE) is the net result of degradation phenomena across each component, which includes structural changes to the catalyst layer, deactivation of the electrolyte, loss of performance due to impurities in the feed water, and the corrosion and passivation of titanium-based bipolar plates (BPPs) and porous transport layers (PTLs).1, 2 This presentation will focus on PTL degradation, which has received relatively little attention compared to loss of performance related to catalyst and electrode degradation mechanisms. Generally, PTL degradation can be categorized into chemical/corrosion degradation and mechanical degradation. 3 In this work, we focus on corrosion degradation of the anode PTL. Titanium (Ti) is the state-of-the-art material for PTLs due to its stability and intrinsic electrical conductivity. Ti can however, develop an oxide layer in the harsh oxidizing conditions of the anode, thus reducing the effective electrical conductivity and cell performance. Coating the Ti parts with platinum group metals (Pt or Ir) is often used to protect against oxidation.4 An electrochemical flow cell system connected to an inductively-coupled plasma-mass spectrometer (ICP-MS) capable of detecting trace concentrations (<ppb) of dissolved elements in solution has been used to investigate the corrosion rates of the Ti-PTLs with and without coatings toward understanding factors influencing degradation mechanisms. The influence of various parameters such as potential, potentiodynamic profile parameters (e.g., scan rate, upper and lower potential limits), electrolyte type, and pH on the corrosion processes has been investigated. References Feng, X. Yuan, G. Liu, B. Wei, Z. Zhang, H. Li, H. Wang, J. Power Sources 2017, 366, 33.Babic, M. Suermann, F. N. Büchi, L. Gubler, T. J. Schmidt, J. Electrochem. Soc. 2017, 164, F387.Park, H. Oh, T. Ha, Y.I. Lee, K. Min, Appl. Energy 155 (2015) 866-880.Rakousky, U. Reimer, K. Wippermann, M. Carmo, W. Lueke, D. Stolten, J. Power Sources 326 (2016) 120–128. Acknowledgements This research is supported by the U.S. Department of Energy, Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office under the auspices of the H2NEW consortium. Argonne National Laboratory is managed for the U.S Department of Energy by the University of Chicago Argonne, LLC under contract DE-AC-02-06CH11357.

(Invited) PGM Catalysts on Atomic-Metal-Rich Carbon for Heavy-Duty Fuel Cell Catalysts: Performance and Durability Enhancement

Proton exchange membrane fuel cells (PEMFCs) are a promising zero-emission power source for heavy-duty vehicles (HDVs). However, long-term durability of up to 25,000 h is challenging because current carbon support, catalyst, membrane, and ionomer developed for traditional light-duty vehicles cannot meet the stringent requirement. Therefore, understanding catalyst degradation mechanisms under the HDV condition is crucial for rationally designing highly active and durable platinum group metal (PGM) catalysts for high-performance membrane electrode assemblies (MEAs). Herein, we report a PGM catalyst consisting of platinum nanoparticles with a high content (40 wt %) on atomic-metal-site (e.g., MnN4)-rich carbon support. MEAs with the Pt (40 wt %)/Mn–N–C cathode catalyst achieved significantly enhanced performance and durability, generating 1.41 A cm–2 at 0.7 V under HDV conditions (0.25 mgPt cm–2 and 250 kPaabs pressure) and retaining 1.20 A cm–2 after an extended and accelerated stress test up to 150,000 voltage cycles. Electron microscopy studies indicate that most fine Pt nanoparticles are retained on or/and in the carbon support covered with the ionomer throughout the catalyst layer at the end of life. During the long-term stability test, the observed electrochemical active surface area reduction and performance loss primarily result from Pt depletion in the catalyst layer due to Pt dissolution and redeposition at the interface of the cathode and membrane. The first-principle density functional theory calculations further reveal a support entrapment effect of the Mn–N–C, in which the MnN4 site can specifically adsorb the Pt atom and further retard the Pt dissolution and migration, therefore enhancing long-term MEA durability.

Simultaneous 2D Projection and 3D Topographic Imaging of Gas-Dependent Dynamics of Catalytic Nanoparticles.

Catalyst deactivation through pathways such as sintering of nanoparticles and degradation of the support is a critical factor when designing high-performance catalysts. Here, structural changes of supported nanoparticle catalysts are investigated in controlled gas environments (O2, H2O, and H2) at different temperatures by imaging simultaneously the nanoparticle structures in 2D projection and the 3D surface-sensitive topography. Platinum nanoparticles on carbon support as a model system are imaged in an environmental transmission electron microscope (ETEM), with concurrent acquisition of high-angle annular dark field scanning TEM (HAADF-STEM) and secondary electron (SE) images. Particle migration and coalescence occurs and shows gas-dependent kinetics, with nanoparticles moving across and through the support during and after coalescence. The temperature required for motion is lower in O2 than in H2O and H2, explained through the nature of the gas/nanoparticle interactions. In O2 and H2, the carbon support degrades by trench formation along migration pathways, and the particles move continuously, indicating a chemical reaction between gas and support. In H2O gas, motion is more discontinuous and oriented particle attachment occurs, as expected from theoretical predictions. These results suggest that multimodal imaging in ETEM that combines HAADF-STEM and SE data provides comprehensive information regarding catalyst dynamics and degradation mechanisms.

Benchmarking Stability of Iridium Oxide in Acidic Media under Oxygen Evolution Conditions: A Review: Part II

Part I () introduced state-of-the-art proton exchange membrane (PEM) electrolysers with iridium-based catalysts for oxygen evolution at the anode in green hydrogen applications. Aqueous model systems and full cell testing were discussed along with proton exchange membrane water electrolyser (PEMWE) catalyst degradation mechanisms, types of iridium oxide, mechanisms of iridium dissolution and stability studies. In Part II, we highlight considerations and best practices for the investigation of activity and stability of oxygen evolution catalystsviashort term testing.

Critical Roles of Chalcogenide Anion on Strengthening Stability of Ni2Mo6Te8 for Almost Exclusive Electrocatalysts Nitrate to Ammonia Conversion

AbstractElectrochemical hydrogenation of nitrate to ammonia using renewable electricity is a promising route for sustainability but lacks catalysts that can deliver balanced selectivity, activity, and durability. Here, a new family of noble metal‐free and high‐performing Chevrel phase Ni2Mo6T8 (T = S, Se, and Te) catalysts that have similar structural and textural properties and differ presumably only in chalcogenide anion is systematically studied. The side‐by‐side comparisons allow the uncovering of the critical roles of chalcogenide anions in impacting kinetic activities and long‐term durability. The incorporation of anions with larger size and smaller electronegativity from sulfide to selenide and telluride invokes stronger inhibition of the otherwise competing hydrogen evolution reaction (HER) and steers the hydrogenation toward the selective formation of ammonia, thus improving both Faradic selectivity and the turnover frequency to high levels of 99.4% and 21.5 s−1, respectively, on the Ni2Mo6Te8 catalyst. More significantly, the bulkier anion in the Ni2Mo6T8 catalyst kinetically inhibited the intercalation of electrolyte cations, a major degradation mechanism in the catalyst family examined here and delivered several times improved durability. Therefore, this study introduces novel active motifs for selective nitrate reduction and provides insights into the catalyst degradation mechanism and practical ways to improve durability.

Revealing the Synthesis and Degradation Mechanisms of Pt-Rare Earth Metal Alloy Catalysts by Multiple in-Situ Techniques

In comparison to conventional Pt alloys with late transition metals such as nickel and cobalt, Pt-rare earth metal alloys (Pt-RE) such as Pt-Ce, Pt-La, and Pt-Nd could have much-improved stability for the oxygen reduction reaction (ORR) in acidic media. However, this promise has only been demonstrated with polycrystalline electrodes rather than the more industrially relevant form of nanoparticles. A major challenge lies in the synthesis of Pt-RE alloy particles with well-defined shapes, sizes, and crystalline structures. We recently invented a solid-state chemical process for the synthesis of Pt-Re alloy nanoparticles. Here, we report our recent findings on the understanding of synthesis mechanisms and structural changes of Pt-RE alloy particles under operating ORR conditions, which are based on the results obtained from multiple in-situ techniques including in-situ transmission electron microscope, X-ray diffraction, and thermal-gravimetric mass spectrometry, etc. We aim to answer two questions through the studies, that is, 1) how do the alloy crystals come into formation during the synthesis? and 2) how do the structures of the alloy particles change during operating ORR conditions?

Catalyst Degradation Mechanism Study of Lithium-Sulfur Batteries Based on Electrodeposition

Lithium-sulfur batteries have emerged as a promising option for next-generation energy storage devices due to their high energy density and low cost. However, the two main obstacles for practical Li-S batteries are the notorious lithium polysulfide intermediate shuttle and the sluggish kinetics of sulfur cathodes, especially under lean electrolyte conditions. In this work, we propose a detailed plan to gain insight into the catalyst degradation mechanism based on electrodeposition by controlling the nucleation and deposition orientation of lithium sulfide. Firstly, we will explain why catalysts can be easily prepared by electrodeposition as cathode hosts, and others do not. Next, the key factors affecting catalysts' deactivation and cell degradation will be disclosed. Then, different types of metal catalysts with suitable particle sizes will be developed that are highly active and selective for Li-S cells. Ultimately, extensive data analysis will be performed to visualize the data and discover generalized rules for structure-activity correlation.

Stabilization of Platinum Catalyst Surface-Treated by Atomic Layer Deposition of Cobalt for Polymer Electrolyte Membrane Fuel Cells

Polymer electrolyte membrane fuel cells (PEMFCs) are attracting attention as next-generation energy-conversion devices because they are environmentally friendly and have high energy-conversion efficiencies. A membrane electrode assembly (MEA), composed of an anode, membrane, and cathode, is the most important part of a PEMFC where electrochemical reactions occur. A Pt/C catalyst, wherein Pt is supported on a carbon, is used as the catalyst layer for the anode and cathode. However, as Pt is a precious metal, current research is focused on reducing its loading and improving durability of PEMFCs. This study focused on improving the durability of catalysts by suppressing agglomeration, one of the degradation mechanisms of Pt catalysts. A protective layer of Co metal was deposited using the atomic layer deposition (ALD) process to improve the durability of catalysts. The ALD process is a type of chemical vapor deposition (CVD) process. This process is suitable for the PT/C catalyst as it allows for the deposition of a uniform thin film even on the porous catalyst layer. If the protective layer is thick, it may block the active site of the catalyst, resulting in decreased activity and reduced performance. However, ALD can form a thin protective layer because it can be controlled at the atomic level by adjusting the number of cycles. After fabricating a single cell using ALD Co-Pt/C as a catalyst for the cathode, The U.S. Department of Energy's electrocatalyst durability protocol was used for an accelerated degradation test (ADT). After Co-ALD was used as a cathode for 10 and 20 cycles, the active area of the Pt catalyst particles was reduced owing to the metal Co protective layer. The initial performance and electrochemical surface area (ECSA) of Co-ALD 10, 20 cycles decreased compared to the case of using Co-ALD 0 cycle. However, after ADT, the performance and ECSA reduction rate rather decreased, confirming that the co-protective layer through the plasma-enhanced atomic layer deposition (PEALD) process can improve the durability of the Pt/C catalyst. To identify the cause of the durability improvement, the cathode catalyst particles before and after ADT were compared through high resolution transmission electron microscopy (HR-TEM) analysis. As a result, in the HR-TEM images after ADT, the Pt catalyst particles were relatively uniformly distributed on the carbon support in the Co-Pt/C catalyst compared to Co-ALD 0 cycle. In addition, it was confirmed that the size of the Pt catalyst particles of Co-Pt/C was smaller than that of bare Pt/C (Co-ALD 0 cycle). It was concluded that when the Co protective layer was deposited on the Pt/C cathode, the protective layer suppressed the agglomeration of Pt catalyst particles, therefore, the increase in Pt particle size improved compared to the Co-ALD 0 cycle, and it was revealed that durability improved by suppressing catalyst degradation.

Durability Study of Anode Catalysts from Iridium-Based Titanium Supports in Polymer Electrolyte Membrane Water Electrolyzers

Electrochemical water splitting has been recognized as one of the most promising methods for hydrogen production. Electrocatalysts are critical materials in water electrolysis. The sluggish reaction kinetics for the anode reaction, oxygen evolution reaction (OER), hinders the development of water electrolysis technology. Iridium-based material is an leading catalyst for OERs, but the high scarcity and high cost limit its application in polymer electrolyte membrane water electrolyzers (PEMWEs). Moreover, Ir dissolution during the OER operation is another limitation. Robust and effective support-based catalysts should be developed to reduce the catalyst loading and provide high durability in PEMWEs. Herein, we synthesized an OER catalyst of Ir deposited on Ti supports using a polyol reduction method. The Ir/Ti catalysts with different Ir loading were prepared and applied in the PEMWEs for performance evaluation. The cell performance (Figure 1A) with the optimized catalyst (Ir/Ti-50%Ir) exhibited better performance than the commercial IrO2 in the same low loading (0.1 mgIr cm-2) and very close current density (<10 mV difference) achieved at 4 A cm-2 as to the electrode with a high catalyst loading (0.4 mgIr cm-2). Accelerated stress tests (ASTs) performed between 2.0 V and 1.45 V were used to investigate the catalyst durability in the PEMWEs. Figure 1B shows a volcano shape for the correlation between degradation behavior and Ir percent in the catalyst. Our results, combined with the ex-situ characterization, provide a synthesis strategy for robust OER catalysts, and elucidate the catalyst degradation mechanism in the PEMWEs. Figure 1

Thin Films Fabricated by Pulsed Laser Deposition for Electrocatalysis

Thin Films Fabricated by Pulsed Laser Deposition for Electrocatalysis

Transmission Electron Microscopy Observation of the Fuel Cell Catalyst Degradation during the Oxygen Reduction Reaction

Increasing durability of catalysts used in fuel cells is a necessary condition for their widespread commercialization. Fulfilling this condition requires understanding the catalyst degradation mechanism to propose how to reduce it. Transmission electron microscopy can help solve this problem thanks to the fact that it enables direct observation and thus unambiguous interpretation of the processes taking place. For this purpose, Identical Location Transmission Electron Microscopy (IL-TEM) was applied for observations of a commercial catalyst (platinum nanoparticles with a diameter of about 2 nm deposited on Vulcan carbon black) before and after durability tests. Obtained results may contribute to the development of better models of phenomena occurring during cell operation.

Preparing Co-Pi/BiVO4 for effective degradation of sulfur-containing organics in wastewater

To effectively degrade sulfur-containing organics in wastewater, a stable and excellent composite catalyst CoO x /BiVO 4 (Co-Pi/BiVO 4 ) was prepared by electrodeposition in phosphate neutral buffer solution. With the optimized Co(NO 3 ) 2 :BiVO 4 of 1:3, the COD removal and desulfurization of the wastewater under visible light reached 80% and >78%, respectively. These values were higher than those achieved with CoO x /BiVO 4 and BiVO 4 in visible light. At the same time, the morphology, crystal form, chemical state, oxygen evolution, and electrical impedance of Co-Pi/BiVO 4 are tested and compared with related substances. It is concluded that Co-Pi/BiVO 4 has a strong oxidation and stability. Finally, the kinetic and degradation mechanisms of related catalysts are deeply analyzed, and the protonation pathways of hydroxyl radicals are constructed. • Co-Pi/BiVO 4 was prepared by electrodeposition in a phosphate buffered solution. • The degradation of Co-Pi/BiVO 4 was higher than that of CoO x /BiVO 4 in visible light. • The degradation rate of Co-Pi/BiVO 4 on high concentration of ethanethiol exceeds 78%. • Co-Pi/BiVO 4 may reduce the recombination of electron-hole and produce more ·OH.

(Invited) Elucidating the Electrochemically Active Site Density of PGM-Free ORR Catalysts in Situ Fuel Cells Using Fourier Transform Alternating Current Voltammetry

The rising interest in polymer electrolyte fuel cell (PEFC) technology, part of the global shift in energy production to clean sources, is accompanied by efforts to drive down the cost of this technology, which focus primarily on the cathode catalyst, the most expensive PEFC component. While platinum-group metals (PGMs) continues to be the materials of choice for oxygen reduction reaction (ORR) catalysts, use of these materials in PEFCs must be significantly reduced or eliminated without a penalty in the overall cell performance for PEFC technology to become fully viable.The most promising class ORR catalysts that do not utilize PGMs (i.e., PGM-free catalysts), involve first-row transition metals, such as iron and cobalt incorporated in a nitrogen-doped carbon (M-N-C catalysts). While advancements in M-N-C activity have been impressive, the much sought-after improvement in durability has been impeded by limited information on changes in the PGM-free catalyst active site density, activity and its degradation rate during fuel cell testing. Currently, degradation of PGM-free catalysts during fuel cell operation is often quantified using the low-current region of polarization curves. While this approach is well established, it neglects complications from such factors as catalyst pore structure, membrane conductivity, ionomer content, nature of the support, and the inhomogeneity of active sites. Hence, there exists a critical need for a method with high specificity towards catalytic activity.In this presentation we will report for the first time on the use of Fourier-transform alternating current voltammetry (FTacV) as an electrochemical method for accurately quantifying the electrochemically active site density of PGM-free ORR catalysts and following their degradation in situ during operation of polymer electrolyte fuel cells. Using this method, we were able to detect changes in performance of electrochemically active species (electrocatalytic centers in this case), allowing us to calculate the electrochemical active site density (EASD) for the first time, which is necessary to elucidate the degradation mechanisms of PGM-free ORR catalysts that occur in situ fuel cells. large-amplitude FTacV, a well-established electrochemical method with distinct advantages over dc methods, was utilized to quantify the electrochemically active site density of PGM-free FeNC catalysts in situ in PEFC. First, we will demonstrate that an accurate measurement of the EASD can be made using this method. To further emphasize the strength of the technique, we will present our findings during degradation of commercial FeNC catalysts in operating PEFC. The peak currents from higher harmonics produced by this method are correlated to the fuel cell performance, and decrease after durability tests in a manner that indicates EASD loss may not be the only catalyst degradation mechanism, thus inviting further studies of yet-unknown degradation pathway(s).

Quantifying the electrochemical active site density of precious metal-free catalysts in situ in fuel cells

Advances in the development of precious-group metal-free (PGM-free) catalysts for the oxygen reduction reaction (ORR) in fuel cell cathodes have produced active catalysts that reduce the performance gap to the incumbent Pt-based materials. However, utilization of state-of-the-art PGM-free catalysts for commercial applications is currently impeded by their relatively low durability. Methods designed to study catalyst degradation in the operation of fuel cells are therefore critical for understanding durability issues and, ultimately, their solutions. Here we report the use of Fourier-transform alternating current voltammetry as an electrochemical method for accurate quantification of the electrochemically active site density of PGM-free cathode catalysts, and to follow their degradation in situ during the operation of polymer electrolyte fuel cells. Using this method, we were able to quantify the electrochemical active site density, which will enable the elucidation of degradation mechanisms of PGM-free ORR catalysts in situ in fuel cells.

Synergistic oxygen vacancy-rich CuO/visible light activation of peroxymonosulfate for degradation of rhodamine B: fast catalyst synthesis and degradation mechanism.

This work outlines the synthesis of copper oxide nanoparticles (CuO-SC) loaded with a number of oxygen vacancies by a fast sodium citrate assisted precipitation method with no need of calcination. X-ray diffraction, scanning electron microscopy, UV-Vis diffuse reflectance spectroscopy, time-resolved fluorescence lifetime and electrochemical impedance spectra were used to characterize the as-synthesized nanocomposites. The results indicated that the CuO-SC nanoparticles had regular fusiform shape with high surface area, wide light harvesting window, fast charge transport and high carrier concentration. As a result, the catalytic activity of the CuO-SC/peroxymonosulfate (PMS)/visible light (Vis) system for the degradation of rhodamine B (RhB) was much higher than that of as-prepared CuO nano powder in the absence of sodium citrate. Almost 98.0% of the initial RhB dyes was decomposed in 20 min with 0.12 g L−1 PMS and 0.3 g L−1 catalyst. Meantime, it exhibited high catalytic stability with little deactivation after four runs and a wide application range of pH. Moreover, RhB can be readily degraded with backgrounds of Cl−, NO3−, SO42−, HCO3− and low concentration of humic acid in a CuO-SC/PMS/Vis system. Combined with the results of electron spin resonance paramagnetic spectroscopy, X-ray photoelectron spectroscopy and radical quenching experiments, holes, superoxide radicals and a small amount of sulfate radicals, hydroxyl radicals and singlet oxygen were involved in the CuO-SC/PMS/Vis system. Furthermore, a possible degradation mechanism based on the synergistic effect of radical reaction and non-radical reaction was proposed based on the above results.

Identical Location Scanning Transmission Electron Microscopy Study of Fuel Cell Catalyst Degradation

Fuel cell electric vehicles (FCEVs) powered by proton exchange membrane fuel cells (PEMFCs) provide a sustainable, low-carbon alternative to the world’s current energy structure based on fossil fuels. At the heart of this solution, the PEMFC technology is met with two main barriers for wide commercialization: the cost and degradation of electrocatalysts for the oxygen reduction reaction (ORR) [1]. The main thrusts for ORR electrocatalyst development are currently Pt-transition metal (TM) alloy nanoparticles [1] and atomically-dispersed platinum group metal-free (PGM-free) catalysts [2] that reduce the Pt loading or replace Pt completely.The degradation of Pt-TM alloy nanoparticles, such as PtCo, involves the combined processes of Pt and Co dissolution, Pt oxide formation, and Pt redeposition. The interplay between different properties of catalysts, such as particle size distribution (PSD), Co composition, Pt-skin surface strain, and carbon support pore structure complicates attempts to link degradation mechanisms with loss in ORR activity and overall fuel cell performance [1]. Further, inhomogeneities of the catalyst layer limit the ability for electron microscopy to analyze the degradation process on the nanoscale. Similarly, the study of PGM-free catalyst degradation is also met with inhomogeneities which stem from the complex morphology of graphitic carbon particles, wide range of defects, and plurality of Fe active sites [2, 3].Identical-location scanning transmission electron microscopy (IL-STEM) allows the same catalytic material to be studied before and after ex situ cycling, minimizing the influence of inhomogeneities and therefore providing clearer insight into degradation mechanisms at the nanoscale [4]. Since the specimen is identical to a conventional STEM specimen, imaging and analytical methods like energy-dispersive X-ray spectroscopy and electron energy-loss spectroscopy can be performed at high spatial resolution. IL-STEM has previously been used to elucidate Pt catalyst degradation mechanisms such as migration, coalescence, and detachment from the carbon support [5-7]. Ostwald ripening has not been observed in most IL-STEM studies [5-7], however, in stark contrast to membrane electrode assembly (MEA) tests where it is identified as a primary degradation mechanism [8, 9]. Therefore, reproducing degradation processes that occur in MEAs, which is critical for accurately identifying mechanisms that can inform mitigation strategies, is still a challenge for IL-STEM.In this work, we seek to identify IL-STEM experimental conditions and protocols that produce end of life (EOL) catalyst morphology and composition that mimic those found in MEAs. A range of parameters are explored, such as specimen preparation method, electrolyte concentration, gaseous environment, upper and lower cycling potentials, and potential wave shape to reproduce the catalyst degradation observed in both Pt-based and PGM-free MEAs. The contribution of different degradation mechanisms revealed by IL-STEM in each system will be presented, including particle coalescence, Ostwald ripening, TM leaching, and carbon corrosion. The resulting insights into fundamental fuel cell catalyst degradation mechanisms will be used to guide development of catalysts with enhanced performance and durability. [10]References[1] RL Borup et al., Curr. Opin. Electrochem. 21 (2020), p. 192.[2] L Osmieri et al., Curr. Opin. Electrochem. 25 (2021), p. 100627.[3] T Asset et al., Joule 24 (2020), p.33.[4] N Hodnik et al., Curr. Opin. Electrochem. 15 (2019), p. 73.[5] K Schlogl et al., J. Electroanal. Chem. 662 (2011), p. 355.[6] Y Yu et al., Nano Lett. 12 (2012), p. 4417.[7] L Dubau et al., Electrochim. Acta 110 (2013), p. 273.[8] H Yu et al., Electrochim. Acta 247 (2017), p. 1169.[9] TE O’Brien et al., ECS Trans. 98 (2020), p. 505.[10] This work was supported by the U.S. Department of Energy (DOE), Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office under the M2FCT and ElectroCat 2.0 consortia. Electron microscopy was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

Stabilization of Pt on Carbon Supports Doped with Atomically-Dispersed Metal/Metal-Oxide Additives

The shift in focus of Proton Exchange Membrane Fuel Cell (PEMFC) development for use in heavy-duty truck applications has accelerated the need for highly durable and efficient Membrane Electrode Assemblies (MEA) in order to meet the long lifetime requirements (~30,000 hours of runtime or 1 million mile driving distance) and high efficiency to minimize the total cost of ownership dominated by the H2 fuel cost.1 These requirements for heavy-duty truck are far stringent compared to the light-duty vehicle applications (~8,000 hours of run time). This brings into focus the need to develop highly-durable catalyst and membrane materials to satisfy the longer lifetime demanded by heavy-duty trucks.The primary cause of cell voltage loss in an operating fuel cell largely arises from the electrochemical surface area (ECSA) loss of the Pt-based cathode catalyst layer. Two mechanisms, namely Pt migration/coalescence and dissolution/redeposition are responsible for the coarsening of Pt nanoparticles leading to surface area loss.2,3 In this presentation, we will explore the concept of using metal/metal-oxide (M/MOx) based dopants in the carbon support as additives to mitigate the Pt coarsening behavior. Specifically, we evaluate dopants that are in the form of M/MOx nanoclusters and/or atomically-dispersed M in the graphitic carbon lattice (M-Cy). We will discuss fuel cell MEA results by investigating the stability when subjected to accelerated voltage cycling tests along with a suite of ex situ characterizations to understand the chemical nature of the catalyst materials with dopants. Acknowledgements: This work is partially supported by U.S Department of Energy, Office of Energy Efficiency and Renewable Energy under grant DE-EE0008821. References Marcinkoski, J. et al. Hydrogen Class 8 Long Haul Truck Targets. Subsection of the Electrified Powertrain Roadmap. Technical Targets for Hydrogen-Fueled Long-Haul Tractor-Trailer Trucks. DOE Advanced Truck Technologies 1–31 (2019). Available at: https://www.hydrogen.energy.gov/pdfs/19006_hydrogen_class8_long_haul_truck_targets.pdf. Cherevko, S., Kulyk, N. & Mayrhofer, K. J. J. Durability of platinum-based fuel cell electrocatalysts: Dissolution of bulk and nanoscale platinum. Nano Energy 29, 275–298 (2016). Yu, K. et al. Degradation mechanisms of platinum nanoparticle catalysts in proton exchange membrane fuel cells: The role of particle size. Chem. Mater. 26, 5540–5548 (2014). Figure 1