Nanoscale analysis of structural and chemical changes in aged hybrid Pt/NbO x /C fuel cell catalysts

For the use of polymer electrolyte membrane fuel cells (PEMFCs) in heavy duty applications, on-site generation of hydrogen rich gas by reforming hydrocarbons or methanol are a suitable alternative to pure H2 due to higher availability and higher energy density of hydrocarbon fuels [1]. The so-called reformate contains in addition of H2 also CO, which is a catalyst poison. PtRu alloys are the most promising catalysts for PEM fuel cells operated with reformate due to their high CO-tolerance [2]. It is known that the less noble Ru is unstable in acidic PEMFC conditions and is lowering the durability of the membrane electrode assembly (MEA) by dissolution and crossing over through the membrane to the cathode [3]. Ru dissolution and crossover result in a decreased CO-tolerance on the anode and a lower activity of the cathode towards oxygen reduction due to Ru blocking the Pt surface of the cathode catalyst [4]. To prolong the lifetime and CO-tolerance of reformate PEMFCs, the stability of the anode catalysts must be improved. One approach is nano-structuring the catalysts by encapsulating the less noble Ru-core with a Pt-shell. In doing so, the corrosion resistant Pt-shell supposes to protect the Ru-core from dissolution and thus mitigates the Ru crossover phenomenon.In this work, the influence of the Pt-shell thickness on the stability and performance of the Ru@Pt catalysts is investigated. Two catalysts with varying Pt-shell thicknesses were synthesized on Vulcan XC72R carbon via a two-step polyol. The catalyst with the thinner Pt-shell is called Ru@1Pt and with the thicker shell Ru@2Pt. The as-synthesized catalysts were physically characterized by XRD, ICP-OES, TEM and EDS. A representative EDS map of Ru@1Pt is shown in fig. 1a. The physical characterization of the obtained catalysts unveils a Ru@Pt core-shell structure with crystalline hcp Ru and fcc Pt. To further investigate the catalysts electrochemically, MEAs were manufactured using Ru@Pt catalysts as anodes and commercial membranes and cathode catalysts. An accelerated stress test (AST) was developed to target Ru dissolution by potential cycling and was applied to investigate and compare the stability of the catalysts. The MEAs were electrochemically characterized by cyclic voltammetry, CO-stripping and U-I curves before, during and after the AST. Post-mortem cross-section STEM analysis of the stressed cells were performed to evaluate the elemental composition and morphology of the anode and cathode after the stress test.During the AST, CO-stripping on the Ru@1Pt anode showed a peak shift to higher potentials, indicating a decrease in CO-tolerance due to Ru loss. By contrast, on the Ru@2Pt anode, the peak shifted to lower potentials implying a stronger promoting influence of Ru. On the cathode side, CV measurements before and after the AST revealed a double layer increase and a decrease in the HUPD. This change implies the poisoning of Pt with Ru and were more pronounced for Ru@1Pt compared to Ru@2Pt. As seen by the CO-stripping and CV results, the Ru dissolution and crossover was more severe for Ru@1Pt than for Ru@2Pt. Hence, a higher fuel cell performance loss was observed for the cell with Ru@1Pt in H2, as can be seen in fig. 1b. In addition, further U-I curves were measured under varying reformate conditions to identify the influence of CO on the cell performance. Cross-section EDS maps of the stressed MEAs were used to evaluate the Ru dissolution and its distribution in the different layers of the MEA. A higher Ru:Pt ratio was found on the cathode for the cell with Ru@1Pt compared to a cell with Ru@2Pt, which further verifies a higher degree of Ru crossover for the catalyst with the thinner Pt-shell. The above findings could evidence that the shell thickness plays a significant role in the corrosion resistance of core-shell reformate anode fuel cell catalysts.

Degradation issues remain among the main factors impeding the development of commercially viable fuel cell (FC) systems for transportation and stationary power applications. The necessity to keep costs and performances at reasonable levels led to the widespread use of carbon supported platinum (Pt) for electrodes and thin PFSA membranes as electrolyte. Both are quite fragile materials but currently without any convincing alternative. To better understand, and eventually limit degradation mechanisms, several kinds of accelerated stress tests (AST) have been developed targeting each material independently: namely open circuit voltage (OCV) for PFSA membranes, potentiostatic regimes for carbon, and potential cycling for Pt [1, 2]. Other accelerated stress tests corresponding to common fuel cell operation sequences were also proposed like constant current, start/stop, hydrogen starvation/dead-ended anode protocols [3,4], temperature, relative humidity or potential/load cycling, or even freeze/thaw protocols [5]. In this work, we focus on load cycling events, which are very close to the potential cycling performed during AST to study Pt and carbon degradation mechanisms. For obvious reasons, load cycling in actual fuel cell systems and potential cycling in AST are usually performed in overfed conditions [6]: i.e. the fuel cell is supplied with the gas flow rates corresponding to the highest current before the intensity increases. And in most of the cases, the gas flow rates are even permanently kept at the highest values during AST. To avoid supplying fuel and air in excess, most of fuel cell stacks are hybridized either with batteries or supercapacitors (SC) to improve the system dynamics (which in return increases its size and cost). Thanks to hybridization, the FC voltage can drop by a few hundreds of mV per cell during the time needed by the gas supply lines to adapt the flow rates, while batteries or SC keep the system power at the required value. However, the impact of such short reactant starvation events on FC performance and durability is not very well known. Therefore, we performed repeated current Heaviside steps as AST on PEM single cells; typical values were from 0.25 to 0.95 A/cm² (with corresponding FC voltages between 0.65 and 0.8 V). The gas flow rates were set either to the maximum current intensity (with air and H2 stoichiometries of 3 and 1.5 in open-anode or 1 in dead-end mode, respectively) or we let the gas lines adjust the flow rates when the current input changed, which took about half a second (during this time, the FC could fell to about 0.1 V). As shown on the Figure, the performance and electrochemical surface area repeatedly aged significantly slower in the second case, i.e. with short gas starvation events. To better understand this difference, similar experiments were performed in a segmented cell [7] with measurements of local currents, anode and cathode potentials. We observed that local cathode potentials were higher in overfed conditions while they dropped for a short time in the other case due to (most probably) oxygen depletion in the cathode compartment. Local negative currents were also observed sometimes when the load decreased, but since they were not associated to high local potentials, they did not worsen degradation. In the same way, since the FC dynamics were limited most probably by air supply, the anode (and thus cathode) local potentials remained low. These results suggest that FC voltage drops during transients due to short lacks of reactants can improve the durability of the catalyst layer, as well as optimize the system efficiency in terms of hydrogen consumption and air compressor power demand which can be a useful observation for the design of optimized hybridization strategies.

Metal semiconductors are important materials for photocatalytic technology, and their morphology and size have the great influence on photocatalytic performance. Therefore, the detailed exploration of the size effect is significant for the photocatalytic reaction. Herein, CdS nanoparticles (NPs) with different particle sizes were prepared for photodegradation of methylene blue, and the effects of particle size on photodegradation were studied using CdS NPs as a catalyst. The rate-determining step of photodegradation is determined theoretically by deriving the kinetic order of the photodegradation reaction, and the relationships of size and performance are explored. The results show that the particle size of the CdS NP catalyst has an obvious influence on the photodegradation rate and the rate constant, and the size effects show a "volcano" trend, implying that the catalyst activity is best when the particle size of the catalyst is moderate. The size influences on the adsorption/desorption, light utilization, and carrier efficiency of the catalyst originate from the influence of size on the surface properties and energy band. This study provides a new understanding on the effect of catalyst size on its properties, and this finding of optimum sizes of catalyst possessing better activity is valuable, which has a significant guidance for developing high-performance catalytic materials.

Power source research at USC: Development of advanced electrocatalysts for polymer electrolyte membrane fuel cells

The long-term durability of the catalyst layers of a low-working temperature fuel cell such as a polymer electrolyte membrane fuel cell (PEMFC) is of significant scientific interest because of their operation criteria and high initial cost. Identification of degradation mechanisms quantitatively during an accelerated stress test (AST) is essential for assessing and improving the durability of such catalyst layers. In this study, we present a quantitative analysis of the degradation mechanisms such as (i) electronic connectivity loss due to carbon support corrosion, (ii) proton connectivity loss due to ionomer/catalyst interface loss, (iii) catalyst loss due to dissolution or detachment, and (iv) physical surface area loss due to particle growth that is responsible for the electrochemical surface area (ECSA) loss in Pt-based catalyst layers for PEMFCs during an AST performed through potential cycling (linear sweep cyclic voltammetry) between 0.4 and 1.6 V for 7000 cycles in Ar-saturated 1 M H2SO4. Using a half-membrane electrode assembly (half-MEA), where a gas diffusion electrode with genuine three-phase boundaries is used as a working electrode through a solid electrolyte, we have observed the ECSA loss due to ionomer/catalyst interface loss and identified a catalyst heterogeneous degradation pattern during an AST. Results suggest a significant ECSA loss due to catalyst isolation (∼64% of ECSA loss) from loss of electron and proton connectivities by catalyst support corrosion (∼45%) and ionomer/catalyst interface loss (∼19%), followed by particle growth (∼30%) and dissolution/detachment (6%). Such knowledge and methodology can effectively contribute to catalyst material screening and electrode structure development to advance the PEMFC technology.

To compete with well-established fossil fuel-based power sources and to maximize their potential as environmental friendly alternative, proton exchange membrane (PEM) fuel cells have to meet several technical criteria, as described by the US Department of Energy’s Multi Year Research, Development and Demonstration Plan (MYRD&D) [1]. Those criteria include long-term durability under normal operation conditions and stability to external disturbances, both of which are strongly related to the aging processes of fuel cell electrocatalysts. Proposed degradation mechanisms for the most commonly used platinum group metal nanoparticles supported on a carbon support material include [2]: Ostwald ripening, coalescence via particle migration, particle detachment from the carbon support, and Pt dissolution into the electrolyte. These catalyst aging processes significantly depend on the robustness of the metal nanoparticles and support material as well as the stabilizing particle – carbon interactions [3, 4]. As all of the degradation processes result in a loss of electrochemically active surface area and often appear simultaneously, differentiating between these aging processes is a highly complicated task which we track in our comprehensive project. In our work, we provide a deeper understanding about the degradation mechanisms occurring on carbon-supported platinum nanoparticles (Pt/C) as electrocatalyst for PEM fuel cells. Thin film electrodes made from Pt/C were electrochemically analyzed by exposing them to accelerated stress test (AST) protocols using a rotating disc electrode (RDE) setup. Our protocols include (i) potential cycling between 0.5 – 1.0 V vs. RHE (10,000 cycles) to resemble normal operation conditions in a fuel cell, (ii) 0.5 – 1.5 V vs. RHE (2,000 cycles) to simulate more extreme conditions e.g. during start-up and shutdown of the cell, and finally (iii) 1.0 – 1.5 V vs. RHE (2,000 cycles) to analyze corrosion effects on the carbon support material. As indicators for the catalyst degradation we monitored the electrochemically active surface area (ECSA), as well as the specific and mass activities at 0.9V vs. RHE of the Pt nanoparticles over the course of the AST experiments. Furthermore, the catalysts were characterized using various techniques such as transmission electron microscopy (TEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The losses of ECSA and ORR activity were then correlated to the structural changes such as particle size and carbon structure as well as to the experimental parameters such as number of cycles and potential range of the AST protocols. We observed a strong decrease in ECSA by up to 70% during simulated “start/stop” experiments, while “lifetime” and “carbon corrosion” protocols showed relatively small losses of ~30% and ~10%, respectively. These observations were accompanied by a strong growth in particle size during “start/stop” and less particle growth during “lifetime” and “carbon corrosion” protocols. Since the degradation rates of the Pt/C altered during the runs of these AST protocols in different manners, we performed intermediate stops to monitor the changes in structure and morphology of the Pt nanoparticles as well as of the carbon. Based on our results, we developed a model to clarify the degradation mechanisms during different types of accelerated stress tests and to identify the critical parameters for maintaining the catalytic performance of the Pt/C catalysts under operating fuel cell conditions.

For proton exchange membrane fuel cells (PEMFC), cathode catalyst durability is presently a key limiting factor to commercialization for automotive applications. The oxygen reduction reaction takes place on the cathode – transforming O2 + 4H+ + 4e- to 2H2O. Degradation occurs during normal operational load profiles for idle-to-peak power transients (rapid stop-go drive cycles), as the cathode voltage quickly switches from 0.9-1.2V (idle) to ~0.6V (peak). Cathode catalyst durability is dependent upon multiple operational factors, including; potential limits, temperature, and pH. As a result of the transient potentials, the electrochemical surface area (ECSA) of the carbon supported platinum catalyst (Pt/C) is reduced, due to a combination of Pt dissolution, Pt particle growth (migration and/or redeposition), and carbon corrosion (Pt detachment). Typically, accelerated stress tests (AST) are performed on thin films to evaluate catalyst durability and decouple contributions from ancillary fuel cell materials and structures. The thin films consists of catalyst deposited on a conductive rotating disk electrode and analyzed in liquid electrolyte (HClO4 or H2SO4) vs. a separate counter and reference electrode – ex-situ to the actual PEMFC environment. This removes the additional complexity of the catalyst layers 3-deminsional structure (containing both Pt/C and proton conducting ionomer), diffusion media and membrane – in-situ. The AST used in this study includes 25,000 potential cycles from idle to peak potential limits using either square or triangular waveforms (SW or TW, respectively) at 10 sec intervals, for both Pt/C thin films and catalyst layers. ECSA was measured periodically during the cycles, either by integration of the hydrogen desorption area or by CO stripping analysis. The following operational limits were used with a lower potential limit of 0.6V – several upper potential limits (UPL) from 0.9 to 1.2V, multiple temperature ranges between 25°C and 70°C, and different electrolyte pH ranges 0 to 1. Ex-situ AST results in 0.1 M HClO4 at 70°C show that the loss in ECSA was affected by both the UPL and the potential wave profile. For the 0.9V UPL dominated by Pt dissolution, the SW was more aggressive than the TW, resulting in a 45.6% and 36.8% ECSA loss, respectively. While the higher UPL of 1.2V had nearly identical ECSA losses for both potential wave profiles of 89%, due to the dominance of carbon corrosion. Figure 1A shows the effects of pH on AST induced ECSA losses at room temperature for the UPL of 1.2V using a TW. The pH values correspond to different concentrations of HClO4 (0.1 M HClO4 = pH of 1). As the pH is lowered there is an increase in ECSA loss. In Figure 1B the TW was found to cause more catalyst degradation at lower pH values. This potentially could be related to in-situ relative humidity dependent degradation – to be further studied in-situ on a catalyst layer. Figure 1

Electrolysis is near parity as an energy storage process when compared to the cost of competing technologies, including batteries, pumped hydro, and compressed air.[1] When coupled with renewable energy sources, such as wind and solar, electrochemical water splitting can also produce hydrogen using a nearly carbon free pathway.[2] Iridium nanoparticles are commonly used as electrocatalysts in acidic electrolyzers due to moderate stability and activity in the oxygen evolution reaction. The activity and durability of iridium nanoparticles has been studied in rotating disk electrode half-cells to develop methods for establishing electrochemical baselines. Mercury underpotential deposition has been used to determine the surface areas and specific activities of iridium catalysts.[3] This evaluation method has proved beneficial to those used previously since similar surface areas are obtained prior to and following exposure to high potential and the irreversible oxidation of surface iridium. Through mercury adsorption, the intrinsic activities of oxygen evolution catalysts can be evaluated and used to direct future electrocatalyst development. The activities of iridium nanoparticle have been evaluated over a wide range of potentials in the kinetic region and transport limitations generally become a factor at potentials greater than 1.5 V vs a reversible hydrogen electrode. Initial optimization of electrode coating processes has further provided a robust benchmark in oxygen evolution. Accelerated stress tests have been applied in rotating disk electrode half-cells using potential cycling and potential holds. At moderate potential, activity losses are generally attributed to losses in surface area by nanoparticle agglomeration. Although bubble formation may destabilize the catalyst layer, similar experiments in hydrogen peroxide and formic acid oxidation have suggested that this loss is unique to the higher potentials in oxygen evolution. At elevated potential, heavy activity losses are observed and are increasingly due to iridium dissolution into the electrolyte. This study establishes rotating disk electrode testing protocols for iridium in oxygen evolution. The observed losses in accelerated stress tests have significant implications on acidic electrolyzers, particularly at low iridium loadings. [1] K. Harrison, M. Peters, in: U.S. Department of Energy (Ed.), http://www.hydrogen.energy.gov/pdfs/review13/pd031_harrison_2013_o.pdf, 2013. [2] K. Harrison, M. Peters, C. Ainscough, in: U.S. Department of Energy (Ed.), http://www.hydrogen.energy.gov/pdfs/progress13/ii_a_2_harrison_2013.pdf, 2013. [3] S.P. Kounaves, J. Buffle, Journal of The Electrochemical Society, 133 (1986) 2495-2498.

Proton Exchange Membrane Fuel Cells (PEMFC) are high-efficiency energy converters that can be deployed for mobile, transport and stationary applications in an eco-friendly manner. Nevertheless, their large-scale development requires further improvements, specifically regarding their lifetime in operation1. In abnormal PEMFC operating conditions, e.g. start-up/shutdown events2 or localized fuel starvation3, the positive electrode may reach potentials of 1.5 V vs the reversible hydrogen electrode (RHE), which triggers extensive carbon support corrosion at the cathode side.To tackle this problem, one of the solution studied in the literature is to use (more robust) carbon-free catalyst supports based on metal oxides4-6. To represent viable alternatives to conventional carbon blacks, metal-oxide substrates must fulfill three criteria: be electron-conducting, resist corrosion and possess an opened porous structure compatible with facile ionomer insertion and efficient mass-transport properties.This study focuses on aerogel structures obtained using a sol-gel route, based on antimony-doped tin dioxide (ATO) with different doping levels (0, 5, 10 and 15 at.%). A Pt nanoparticles colloidal solution, obtained via a modified polyol route, was deposited on the ATO (Pt/ATO) featuring the best properties (10 at.% Sb-doped SnO2), and, for comparison, on Vulcan XC72 carbon (Pt/C) and on an undoped tin dioxide aerogel (Pt/SnO2). This strategy allowed the straightforward comparison of the Pt/ATO, Pt/C and Pt/SnO2 electrocatalysts in terms of catalytic activity for the oxygen reduction reaction (ORR) and durability. A 2-fold enhancement in specific activity for the ORR was measured on Pt/ATO over the reference Pt/C, suggesting strong metal support interactions between the Pt nanoparticles and the ATO support. An accelerated stress test (AST) protocol (5,000 or 10,000 potential cycles between 1.0 and 1.5 V vs RHE) enabled to determine the robustness of the Pt/ATO and the Pt/C electrocatalysts during model start-up/shut-down events. Figure 1 illustrates that Pt/C did not withstand operation in these conditions, as observed from the massive detachment of Pt nanoparticles from the carbon support following its corrosion. On the contrary, the Pt nanoparticles did not detach from the ATO support during ageing. However, evidences were provided that a core@shell structure with a Sb-poor surface covering a core with a Sb content close to the nominal forms during the AST. This core@shell structure restricts the capacity of the Pt nanoparticles to exchange electrons, as evidenced by the attenuated Pt surface oxide formation/reduction features, and the decreased catalytic activity for the ORR.

Degradation of phosphoric acid doped polybenzimidazole membrane based fuel cells under accelerated potential cycling conditions is investigated in current study. Three unit cells with identical high temperature membrane electrode assembly were assembled with three different cathode flow field designs. The fuel cell is cycled between 0.5 V and 0.9 V with 3 min dwelling time for each voltage set point. Performance degradation mechanisms associated with differences in cathode flow design are identified. The fuel cell with multiple serpentine design is operated for a maximum of 4,821 potential cycles with only 38.6% of initial performance remaining at the end‐of‐test (EoT), whereas straight and parallel design operated for only 3,188 cycles. The electrochemical characterization studies reveal the cause of observed performance losses from polarization curves. Irrespective of the design type used there are very high activation losses observed which accelerated with accelerated stress tests (AST) testing. The impedance studies reveal high charge transfer resistance related to increased platinum crystal growth on cathode and reduced electrochemical surface area (ECSA) of catalyst. Overall the AST results in irreversible performance loss and severe degradation of the cathode catalyst support and catalyst itself.

Proton Exchange Membrane (PEM) failure is one of the key criteria in deciding the durability of the PEM fuel cell system. Physical properties such as ionic conductivity and mechanical properties depend on the microstructure. The microstructure is itself strongly dependent on the water content of the membrane (1, 2). It is therefore important to fully understand the effect of degradation stressors on the morphological changes and water-uptake characteristics in order to improve membrane durability.In this work, standard perfluorosulfonic acid (PFSA) ionomer membranes are subjected to a combined chemical and mechanical accelerated stress test (AST) used for rapid benchmarking of in-situ membrane stability. The degradation stressors in the combined chemical/mechanical AST was previously shown by our group to change the molecular structure and lead to membrane material loss (3). The chemical phase of the AST generates hydroxyl radicals that attack both the side chain and main chain of the polymer, while the mechanically generated stresses due to humidity cycling accelerate mechanical failures. The observed structural changes are anticipated to alter the water-uptake properties of the membrane, as shown previously for ex-situ testing (4). The degradation also alters the morphology, ion exchange capacity, mechanical properties, and proton conductivity of the membrane (5). Water-uptake measurements of partially AST degraded catalyst coated membrane (CCM) samples are carried out at room temperature using a dynamic vapor sorption system. In spite of severe fluoride release and membrane thinning, the water-uptake per unit mass of the AST degraded CCMs is found to be relatively constant. Since the test specimens are CCMs which contain a significant amount of catalyst layer material, an attempt is made to measure and separate the mass of the ionomer by thermogravimetric analysis (TGA). The ionomer material loss throughout the AST process is significant due to fluoride release and ionomer degradation, up to roughly 50% at end-of-life (3). The water-uptake per unit mass of ionomer in the CCM is determined by extracting the ionomer weight obtained by heating the CCM in the TGA chamber. In contrast to the previous results normalized by specimen dry weight, the water-uptake increases significantly per unit mass of the ionomer in the CCM, as shown in Fig. 1. Although the increased water-uptake may indicate enlarged solvated hydrophilic domains in the membrane, which is good for enhanced proton mobility, the proton conductivity is found to decrease (Fig. 1). This may be attributed to a decay in proton concentration due to sulfonic acid functional group loss during side chain degradation. The overall effects of the combined chemical/mechanical AST on the membrane morphology and properties are presented and compared to provide additional insight into the complex degradation mechanism.ACKNOWLEDGMENTSResearch funding provided by Automotive Partnership Canada (APC), Natural Sciences and Engineering Research Council of Canada (NSERC) and Ballard Power Systems (BPS) is gratefully acknowledged. BPS is also acknowledged for providing access to experimental facilities, material samples and technical support. REFERENCES1. J.Benziger, A. Bocarsly, M. J. Cheah, P. Majsztrik, B. Satterfield, and Q. Zhao, in Fuel Cells and Hydrogen Storage 141, Bocarly and Mingos, Editors, p. 85 Springer, Berlin Heidelberg 141 (2011).2. Y. Yang, A. Siu, T. J. Peckham, S. Holdcroft, in Advances in Polymer Sciences 215: Fuel cells 1, G. G. Scherer, Editor p. 55 Springer, Berlin Heidelberg 215 (2008).3. C. Lim, L. Ghassemzadeh, F. Van Hove, M. Lauritzen, J. Kolodziej, G.G. Wang, S. Holdcroft, E. Kjeang, J. Power Sources 257 102 (2014).4. L. Ghassemzadeh and S. Holdcroft, J.Am.Chem.Soc. 135 8181 (2013).5. S. V. Venkatesan and E. Kjeang, 11th International Fuel Cell Science, Engineering and Technology Conference, ASME, Minneapolis, MN (2013).

Nanoporous Pt nanoparticles (NPs) are promising fuel cell catalysts due to their large surface area and increased electrocatalytic activity toward the oxygen reduction reaction (ORR). Herein, we report on the influence of the growth mechanisms on the surface properties of electrodeposited Pt dendritic NPs with large surface areas. The electrochemically active surface was studied by hydrogen underpotential deposition (H UPD) and compared for the first time to high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) quantitative 3D electron tomography of individual nanoparticles. Large nucleation overpotential leads to a large surface coverage of roughened spheroids, which provide a large roughness factor (Rf) but low mass-specific electrochemically active surface area (EASA). Lowering the nucleation overpotential leads to highly porous Pt NPs with pores stretching to the center of the structure. At the expense of smaller Rf, the obtained EASA values of these structures are in the range of those of large surface area supported fuel cell catalysts. The active surface area of the Pt dendritic NPs was measured by electron tomography, and it was found that the potential cycling in the H adsorption/desorption and Pt oxidation/reduction region, which is generally performed to determine the EASA, leads to a significant reduction of that surface area due to a partial collapse of their dendritic and porous morphology. Interestingly, the extrapolation of the microscopic tomography results in macroscopic electrochemical parameters indicates that the surface properties measured by H UPD are comparable to the values measured on individual NPs by electron tomography after the degradation caused by the H UPD measurement. These results highlight that the combination of electrochemical and quantitative 3D surface analysis techniques is essential to provide insights into the surface properties, the electrochemical stability, and, hence, the applicability of these materials. Moreover, it indicates that care must be taken with widely used electrochemical methods of surface area determination, especially in the case of large surface area and possibly unstable nanostructures, since the measured surface can be strongly affected by the measurement itself.

The design of catalysts for Polymer Electrolyte Membrane Fuel Cells is guided by two important principles, improving specific catalytic activity and increasing lifetime [1]. Over the past 3 decades, there was a tremendous effort to understand at the atomic and molecular levels the key parameters that control the catalytic activity of the interface for the oxygen reduction reaction (ORR), the limiting reaction for high performance devices. Understanding the functional links between structure-activity relationships from Pt single crystalline surfaces to PtNi alloy single crystals established the importance of controlling structure, composition in both bulk and particularly at the surface, leading to real catalytic reactivity enhancement of almost 2 orders of magnitude [2]. These studies paved the way to synthesis of advanced nanostructures with well-defined parameters such as particle size, distribution, and alloy composition, culminating with PtNi nanoframes, displaying one of the highest specific activities measured for the ORR [3]. While there are room for improvement with increasing precision in nanostructure synthesis, the corresponding understanding of durability of Pt-based is far less developed. One of the limitations is that measuring durability has always been linked to a particular experimental protocol, leading to the development of accelerated stress tests (AST). These protocols measure the changes in surface area, as well as catalytic activity as a function of cycle number, but provide very little information about the mechanism of degradation. Transmission Electron Microscopy helped shed light on particle size evolution as a function of cycle number, where small particles (ca. 3nm) often coalesce and grow to large sizes, ranging from 5 to 7nm [4]. Carbon corrosion can also contribute to Pt degradation especially under start-stop conditions. However, only recently after the development of an in situ that is sensitive enough to measure rates of Pt dissolution as a function of electrode potential we were able to begin establishing structure-stability relationships that goes beyond simple changes to surface area. In this talk, we will present how the stability trends observed from well-defined single crystal Pt surfaces can help understand durability of nanoparticle systems. First, we reveal how the mechanism of Pt-oxide induced dissolution is highly sensitive on surface structure, where dissolution from (110) surface is almost 10 times higher than on (111) surfaces [5]. The effect anions and other species present at the interface is also relevant for stability of Pt surface atoms. Beyond Pt dissolution, we discuss the effects of redeposition processes that can occur after fast changes in electrode potential, leading to a clear pathway to understand the kinetics of nanoparticle evolution. Further comparison between the rates of Pt dissolution measured from nanoparticles to extended surfaces (single crystal and thin films), helped elucidate the importance of controlling particle size, distribution and support loading. Overall, establishing the functional link between structure-stability can provide a deeper understanding on how to control the durability of fuel cell catalysts, together with advances in catalytic activity that can bring fuel cell technology one step closer to a cost–effective widespread commercialization. [1] Stamekovic, V.; Strmcnik, D.; Lopes, P. P.; Markovic, N. M., Nature Materials, 16, 57-69, 2017 [2] Stamenkovic, V. et al., Science 315, 493-497, 2007 [3] Chen, C. et al., Science 343, 1339-1343, 2014 [4] Borup, R. et al., Chemical Reviews, 107, 3904-3951, 2007 [5] Lopes, P. P. et al., ACS Catalysis, 6, 2536-2544, 2016

A hybrid Pt/NbO/CNTs catalyst with high activity and durability for oxygen reduction reaction in PEMFC

Heavy-duty trucks play a dominant role in U.S. freight movement, carrying 72% of freight by value and nearly two-thirds by tonnage as of 2021. This freight movement demands significant use of energy, as trucks consumed 5.55 quads of energy in 2022, which is 20% of highway transportation energy use1. As a result, polymer electrolyte fuel cells (PEFCs) are emerging as a crucial technology for heavy-duty transportation due to their high efficiency, long range, quick refueling, and zero emissions. Catalyst durability remains a critical challenge for the widespread commercial adoption of PEFCs.The performance and durability of electrocatalysts in PEFCs are strongly influenced by the locality of Pt nanoparticles, specifically whether they are positioned inside or outside the pores of the carbon support. Pt inside the mesopores is not accessed by ionomer and local pH is different compared to Pt covered by ionomer on the surface of the carbon. Pt nanoparticle placement is support-dependent, with Vulcan support having very little number of meso-pores favoring exterior Pt placement, while Ketjenblack (high surface area carbon) with mesopores in the range of 6-10 nm tends to have Pt within its pores2. Pt degradation rates within the pores vs. on the surface of the carbon is still a debated topic. Scanning transmission electron tomography has revealed that there is an increase in fraction of Pt nanoparticles within the interior of the carbon support and an increase in the average carbon mesopore size after the accelerated stress tests (ASTs)3. Particles located on the exterior carbon surface had a slightly greater increase in size distribution and loss of specific surface area. This suggested that during AST carbon corrosion occurred and pores opened allowing ionomer to access more Pt inside the pores. However, the effects of Pt locality on durability cannot be clearly decoupled when studied on different carbon supports, and there are still limitations in tomography analysis.To address these challenges, a model membrane electrode assembly (MEA) system has been developed using catalysts in which Pt nanoparticles are selectively placed inside or outside the mesopores, while maintaining consistent nanoparticle size, crystallinity, and loading. Electrocatalyst ASTs conducted on these MEAs indicate that Pt locality influences degradation mechanisms, suggesting distinct degradation behaviors depending on whether Pt is positioned inside or outside the carbon pores.(1) Kusoglu, A. Heavy-Duty Transportation in 10 Charts. Electrochem Soc Interface 2024, 33 (1), 26–29. https://doi.org/10.1149/2.F04241IF.(2) Padgett, E.; Andrejevic, N.; Liu, Z.; Kongkanand, A.; Gu, W.; Moriyama, K.; Jiang, Y.; Kumaraguru, S.; Moylan, T. E.; Kukreja, R.; Muller, D. A. Editors’ Choice—Connecting Fuel Cell Catalyst Nanostructure and Accessibility Using Quantitative Cryo-STEM Tomography. J Electrochem Soc 2018, 165 (3), F173–F180. https://doi.org/10.1149/2.0541803jes.(3) Amichi, L.; Yu, H.; Ziabari, A.; Rahman, O.; Arregui-Mena, D.; Hu, L.; Neyerlin, K. C.; Cullen, D. A. A 3D Nanoscale View of Electrocatalyst Degradation in Hydrogen Fuel Cells. Adv Energy Mater 2024. https://doi.org/10.1002/aenm.202402310.