Mathematical modeling of performance and durability plays a central role in developing electrochemical devices, where complex trade-offs between functionality, longevity, and cost are frequently done. To shorten learning cycles when developing a proton exchange membrane fuel cell (PEMFC) to last tens of thousands of hours, accelerated stress tests (ASTs) are commonly used. However, this can mischaracterize degradation mechanisms leading to unpreferred outcomes. In this work, we investigate platinum (Pt) surface area loss and carbon support degradation under non-accelerated, realistic operating conditions. Leveraging a unique dataset spanning thousands of hours of durability testing, we developed semi-empirical models to project the operational lifespans of both Pt and carbon components. The models were then applied to evaluate the viability of PEMFCs across a diverse range of potential fuel cell applications.

In any electrochemical device, the separator or membrane allows specific ions to transport but blocks electrons and other chemical species, enabling the electrochemical energy to be harvested. However, small amounts of undesired species are known to permeate through the membrane, reducing overall system efficiency and lifetime. An emerging concept called the "Selective Transport Layer" preferentially allows only protons to pass through but reduces the permeance of other species by a meaningful degree. In this study, we demonstrate that a 60 nm thick graphene oxide composite layer can be very effective in reducing gas and ion permeation, even for a gas as small as H2, while not noticeably increasing proton transport resistance. In electrode and membrane accelerated stability tests, we show that both electrode and membrane durability are improved by a factor of two. Microscopy and mathematic simulations confirm that the graphene oxide composite is effective in blocking transport of dissolved Pt2+. The improved durability and reduced H2 fuel crossover are expected to substantially reduce initial and operating costs of the fuel cell system. How this technology may affect other membrane-based electrochemical devices is also discussed.

We report the layer-by-layer growth of poly(ethylenimine) (PEI) that has been modified with sulfamate functionalities (S-PEI) using Zr4+-complexation interlayer linking chemistry. We have also deposited adlayers of graphene oxide (GO) that have been modified to possess sulfate functionalities (S-GO), onto the S-PEI layers. The multilayer assemblies are formed with sulfamate/sulfate and sulfate/sulfate (S-PEI + S-GO) interlayer linking chemistry. In all cases the adlayer thickness is consistent with predictions based on van der Waals volume and/or molecular mechanics calculations. X-ray photoelectron spectroscopy (XPS) is used to characterize the Zr/S stoichiometry in the multilayer assembly. The utility of these hybrid multilayer structures is demonstrated in a Proton Exchange Membrane (PEM) fuel cell, where they are shown to reduce H2 gas crossover by 15% with only a 12 nm thick layer.

Proton selective, atomically thin two-dimensional (2D) materials interfaced with state-of-the-art proton exchange membranes (PEMs) enable overcoming the inherent trade-off between proton conductance and gas crossover. Monolayer graphene-integrated PEMs show significantly reduced gas crossover with negligible impact on proton conductance. However, the influence of fabrication methods on membrane-electrode assemblies (MEAs) using PEMs interfaced with monolayer graphene (synthesized via chemical vapor deposition (CVD)) via an ultra-thin (∼700 nm) ionomer carrier layer remains elusive. Here, we systematically investigate three MEA fabrication processes: gas diffusion electrode (GDE), directly sprayed catalyst-coated membrane (DS-CCM), and decal transfer catalyst-coated membrane (DT-CCM) using monolayer CVD graphene-integrated with perfluorosulfonic acid (PFSA) PEMs (∼12-25 μm thick). Although the GDE process minimizes processing or impact on PEM properties, the rough surface of the GDE could damage CVD graphene coated with the ∼700 nm ionomer carrier layer, limiting the suppression of H2 crossover. The DS-CCM approach exposes the PEM to solvents, resulting in degradation and diminished performance. DT-CCM emerges as the most effective route, with minimal graphene damage, yielding a pronounced reduction (∼25-44%) in H2 crossover without impacting proton conductance. Notably, the decal approach remains effective even for thinner (∼12 μm thick) PEMs with inherently higher proton conductance, where reduced crossover can enable enhanced membrane durability and fuel cell efficiency. These findings establish fabrication-sensitive design rules for integrating 2D materials into MEAs and highlight the advantages of the decal-transfer approach for next-generation PEMs.

Catalyst requirements for proton exchange membrane (PEM) fuel cells differ by applications. Commercial heavy-duty vehicle (HDV) applications consume more H 2 fuel and demand higher durability than many others and the total cost of ownership (TCO) of the vehicle is largely related to the performance and durability of catalysts. This article is written to bridge the gap between the industrial requirements and academic activity for advanced cathode catalysts with an emphasis on durability. From a materials perspective, the underlying nature of the carbon support, Pt-alloy crystal structure, stability of the alloying element, cathode ionomer volume fraction, and catalyst-ionomer interface play a critical role in improving performance and durability. We provide our perspective on four major approaches, namely, mesoporous carbon supports, ordered PtCo intermetallic alloys, thrifting ionomer volume fraction, and shell-protection strategies that are currently being pursued. While each approach has its merits and demerits, their key developmental needs for future are highlighted.

Proton Exchange Membrane (PEM) fuel cells are a suitable electrochemical power source for heavy duty vehicle (HDV) applications due to their high performance, efficiency, and durability. The cathode of the fuel cell uses a higher geometric loading of platinum (~0.2 to 0.4 mgPt/cm2) for the electrocatalysis of the kinetically sluggish Oxygen Reduction Reaction (ORR), which requires higher weight percent loading of the metal (~50%) on the carbon support to decrease the catalyst layer thickness and hence, the reactant transport losses. Conventional supports for platinum catalysts, such as the KetjenBlackTM type high surface area carbon (HSC), feature limited mesopore area for the dispersion of Pt nanoparticles leading to increased aggregation and poor durability. Here, we show a new class of carbon materials developed by Pajarito Powder known as the Engineered Catalyst Support (ECS) with higher mesopore fraction for the dispersion of higher weight percentage of Pt nanoparticles. ECS materials can disperse up to 50% Pt by weight of the catalyst, thereby enabling lower catalyst layer thickness with higher performance retained after durability test. ECS materials are tunable in terms of their mesopore structure, graphitization levels and the use of metal-based additives as dopants on the carbon support to anchor catalyst nanoparticles. A comprehensive set of physico-chemical and electrochemical studies in membrane electrode assembly (MEA) are reported to understand the performance and durability of Pt/ECS catalysts. Acknowledgements: This work is partially supported by U.S Department of Energy, Hydrogen and Fuel Cell Technologies Office under grant DE-EE0008821 and DE-EE0010749.

A pore-scale model of a PEMFC cathode catalyst layer was developed using the pore network approach and used to predict polarization behavior. A volumetric image of a PEMFC catalyst layer was obtained using FIB-SEM with 4 nm resolution in all 3 directions. The original image only differentiated between solid and void, so a simple but effective algorithm was developed to insert tightly packed, but non-overlapping carbon spheres into the solid phase, which were then decorated with catalyst sites. The resultant image was a 4-phase image containing void, ionomer, carbon, and catalyst, each in proportion to the known Pt loading, carbon-to-ionomer ratio, and porosity. A multiphase pore network model was extracted from this image, and multiphysics simulations were conducted to predict the polarization behavior of an operating cell. It was shown that not only can beginning of life polarization performance be predicted with minimal fitting parameters, but degraded performance 30 k cycles was also well captured with no additional fitting. This latter result was accomplished by deleting catalyst sites from the network in proportion to the experimentally observed distribution of electrochemical surface area loss, obtained from TEM image of catalyst loading. The model included partitioning of oxygen into the ionomer phase, explicitly incorporating the oxygen transport resistance which dominates cell performance at higher current density. Although Knudsen diffusion is present at the scales present (<100nm), it represented a negligible fraction of the total transport resistance, which was dominated by the low solubility and slow diffusivity in the ionomer phase. This work showed that the performance of a typical PEMFC is highly dependent on the structural details of the catalyst layer, to the extent that polarization curves can be well predicted by direct inspection of an image of the catalyst layer. This work paves the way for a deeper understanding of the structure-performance relationship in these complex materials and the search for optimized catalyst layer designs.

Improving the activity and durability of carbon-supported platinum catalysts for the oxygen-reduction reaction (ORR) in acidic electrolytes is crucial to reducing the high overpotentials and power loss over time in proton-exchange membrane fuel cells (PEMFCs). We found that platinum nanoparticle catalyst deposited on an engineered carbon support in the presence of zirconium enabled higher ORR activity and 25% better retention in electrochemically active surface area (ECSA), thereby improving durability. The use of zirconium precursor in the carbon synthesis process led to the formation of atomically dispersed Zr and ZrO2 nanoparticles on the support. Upon Pt deposition and subsequent heat treatment, the ZrO2 particles preferentially rearranged on and around the platinum nanoparticles in a chemically reduced form as zirconium suboxide (ZrO2-x) surface-decorated nanoclusters, which mitigated Pt nanoparticle coarsening. Analysis of the here-to-fore unknown catalyst structure as well as its performance and durability in a PEM fuel cell membrane electrode assembly (MEA) is discussed.

As proton exchange membrane fuel cells mature into commercial devices capable of powering a wide range of stationary and automotive applications, they need materials with tunable properties to improve their performance and durability. Carbon supports used for platinum (Pt) nanoparticle dispersion is typically based on a furnace black-type material with a random structure, thereby hindering progress in catalyst development. To address this challenge, engineered carbons with a tunable mesoporous structure offer an opportunity to maximize catalyst performance and durability. In this article, we report on the development of a graphitized, mesoporous carbon support with a high degree of ordering, labeled here as ECS4005, for the dispersion of Pt nanoparticles. Pt/ECS4005 shows significantly improved kinetic activity due to its mesoporous structure that mitigates Pt poisoning by sulfonate functional groups in the ionomer while simultaneously enabling favorable accessibility to reactants.

Proton Exchange Membrane (PEM) fuel cells are a suitable electrochemical power source for heavy duty vehicle (HDV) applications due to their high efficiency and durability. The cathode of the fuel cell uses a higher geometric loading of platinum (∼0.2 to 0.4 mg Pt /cm 2 ) for the electrocatalysis of the kinetically sluggish Oxygen Reduction Reaction (ORR) which requires higher weight percent loading of the metal (∼50%) on the carbon support to decrease the catalyst layer thickness and hence, the reactant transport losses. The conventionally used supports for platinum catalyst, such as the KetjenBlack TM type high surface area carbon (HSC) features limited mesopore area for the dispersion of Pt nanoparticles leading to increased aggregation and poor durability. Here, we show a new class of carbon materials known as the Engineered Catalyst Support (ECS) developed by Pajarito Powder with higher mesopore fraction for the dispersion of higher weight percentage of Pt nanoparticles. ECS materials can disperse up to 50% Pt by weight of the catalyst thereby enabling lower catalyst layer thickness with higher performance retained after durability test. A comprehensive set of physico-chemical and electrochemical studies in membrane electrode assembly (MEA) are reported to understand the performance and durability of Pt/ECS catalysts.

Pt surface oxide coverage (POC) and Oxygen Reduction Reaction (ORR) were measured under a range of operating conditions in a Proton Exchange Membrane Fuel Cell (PEMFC). Sensitivity analysis suggests that adsorption of sulfonate groups of the ionomer on the Pt surface plays an important role in POC and ORR kinetics. Although both Pt oxide growth and ORR activity decay follow a logarithm of time behavior, it is found that ORR kinetics are affected by at least two different types of Pt oxide. A semi-empirical ORR kinetic model is proposed taking into account the effect of ionomer and Pt oxide types. The model is capable of providing a quantitative prediction of POC and ORR activity over a range of potential, temperature, relative humidity, and time that is relevant to normal PEMFC operation.

Interest is growing in applying fuel cells to heavy duty transportation applications, which requires the fuel cell design to prioritize efficiency and durability to provide a low lifetime cost of ownership. This motivates further development of catalysts for the oxygen reduction reaction (ORR), as ORR kinetics are a key factor in fuel cell efficiency and catalyst degradation is a major factor limiting lifetime. Pt-alloy catalysts have seen success in light-duty fuel cell applications, where they provide enhanced activity believed to originate (at least in part) from strain of the active Pt surface by the underlying Pt-alloy core. Concerns remain regarding the viability of PtCo catalysts for heavy duty applications, as they may not sustain their activity advantage over extended lifetimes and leaching of the base metal can lead to additional performance degradation. These concerns motivate investigation into the fundamental mechanisms of strain-induced activity enhancements to guide further catalyst development and understand the ultimate limits for the performance and stability of PtCo catalysts. However, practical Pt-alloy fuel cell catalysts are complex, and understanding of real strain effects in these systems remains elusive. Here we will present an investigation into strain effects in Pt-Co ORR catalysts for fuel cells, combining scanning nanobeam electron diffraction (NBED), continuum elastic strain modelling, and full-cell electrochemistry measurements. We leverage recent developments in NBED to make high-throughput strain maps in heterogeneous, carbon-supported PtCo catalysts and identify different mechanisms of strain relaxation, including dislocations and geometric effects. Supported by continuum elastic modelling, we correlate shell thickness, strain state, and activity of PtCo catalysts. Finally, we will comment on how these results provide rational guidance to optimize stable, efficient Pt-alloy catalysts.

Surface strain plays a key role in enhancing the activity of Pt-alloy nanoparticle oxygen reduction catalysts. However, the details of strain effects in real fuel cell catalysts are not well-understood, in part due to a lack of strain characterization techniques that are suitable for complex supported nanoparticle catalysts. This work investigates these effects using strain mapping with nanobeam electron diffraction and a continuum elastic model of strain in simple core-shell particles. We find that surface strain is relaxed both by lattice defects at the core-shell interface and by relaxation across particle shells caused by Poisson expansion in the spherical geometry. The continuum elastic model finds that in the absence of lattice dislocations, geometric relaxation results in a surface strain that scales with the average composition of the particle, regardless of the shell thickness. We investigate the impact of these strain effects on catalytic activity for a series of Pt-Co catalysts treated to vary their shell thickness and core-shell lattice mismatch. For catalysts with the thinnest shells, the activity is consistent with an Arrhenius dependence on the surface strain expected for coherent strain in dislocation-free particles, while catalysts with thicker shells showed greater activity losses indicating strain relaxation caused by dislocations as well.

PtM alloy electrocatalysts (M = Fe, Co, Ni) have been the subject of many investigations aimed at increasing their attractive properties, in particular their oxygen reduction reaction (ORR) activity, while reducing total platinum-group-metal content and improving durability. Despite some success, these catalysts still have relatively high Pt content and lack the necessary durability, as M metals leach out from the alloys during potential cycling. Previously, we synthesized nitrogen (N)-doped PtMN/C catalysts consisting of thin Pt shells on M nitride cores by a two-step method, which showed higher ORR activity and stability than their PtM counterparts. In the present study, we developed a facile onestep synthesis method, which comprises a single thermal annealing process of the N-doped PtNiN/C alloy. The ORR performance of the one-step-synthesized PtNiN/C catalyst is much higher than that of the two-step-synthesized PtNiN/C, as revealed by rotating disk electrode measurements. Membrane electrode assembly fuel cell testing demonstrated superb durability and high activity. Formation of Pt monolayer shells on the nitrided (PtxNi1-x)(4)N cores was confirmed by in situ X-ray absorption spectroscopy. The origins of the enhanced activity and stability of the one-step-synthesized PtNiN/C catalyst are elucidated based on density functional theory calculations together with the experimental results.

Achieving high power performance and durability with low Pt loadings are critical challenges for proton exchange membrane fuel cells. PtCo catalysts developed on new carbon black supports show promise by simultaneously providing good oxygen reduction kinetics and local oxygen transport. We investigate the role of nanoscale morphology in the performance of these catalysts supported on accessible (HSC-e and HSC-f) and conventional (Ketjen Black) porous carbons using 3D electron tomography, nitrogen sorption, and electrochemical performance measurements. We find that the accessible porous carbons have hollow interiors with mesopores that are larger and more numerous than conventional porous carbons. However, mesopore-sized openings (>2nm width) are too rare to account for significant oxygen transport. Instead we propose the primary oxygen transport pathway into the interior is through 1-2nm microporous channels permeating the carbon. The increased mesoporosity in the accessible porous carbons results in a shorter diffusion pathlength through constrictive, tortuous micropores in the support shell leading to lower local oxygen transport resistance. In durability testing, the accessible porous carbons show faster rates of electrochemical surface area loss, likely from fewer constrictive pores that would mitigate coarsening, but maintain superior high current density performance at end of life from the improved local oxygen transport.

Improvements in Pt-based catalysts and support durability are important to enable mass-market penetration of proton exchange membrane fuel cells (PEMFCs) in the automotive sector. This work focuses on a membrane electrode assembly (MEA) scale analysis of cathode catalyst layer (CCL) degradation processes occurring during catalyst accelerated stability testing (AST). We examined the degradation of the carbon support, decrease in electrochemically active surface area (ECSA), and formation of a platinum (Pt) band. We utilized a combined electrochemical and ex-situ nanoscale X-ray computed tomography (nano-CT) imaging technique to characterize these important degradation processes in state-of-the-art MEAs and examined the effects of AST cycling on two carbon supports, Vulcan (Vu) and a KetjenBlack high surface area carbon (HSC), and two Pt-based catalysts, pure Pt and PtCo. We found that neither support nor catalyst type completely suppressed the formation of a Pt band, but we did find that the PtCo catalyst and HSC support combination was able to limit the ECSA loss and Pt band formation better than the pure Pt and Vu combination. Our results also showed some loss of macroporosity in the HSC support after catalyst AST cycling, helping to explain the reduced performance seen with HSC-supported catalysts at high current densities.

Oxides on the surface of an electrocatalyst inhibit oxygen reduction reaction (ORR) activity at high potentials. Although this process has been described for Pt/Vulcan electrocatalysts by an ORR kinetic model1, there is an opportunity to improve the existing model to accurately predict performance loss under non-standard temperature, relative humidity and transient operating conditions. Also, the established kinetic model does not accurately predict the performance for Pt-alloy electrocatalysts. Here, we conducted time-resolved measurements of ORR kinetics for both Pt and Pt-alloy electrocatalysts under various temperature and relative humidity conditions in a PEM fuel cell. The oxide coverage was measured in a H2-N2 environment using cyclic voltammetry. ORR polarization measurements were conducted in H2-O2 environment. The experimental data was used to develop a kinetic model that depends on the oxide coverage, which showed improved prediction capability based on dynamic polarization measurements of Pt and Pt-alloy catalysts. This work was partially supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy under grant DE-EE0007271. Reference 1. P. Subramanian, T. A. Greszler, J. Zhang, W. Gu, and R. Makharia, Journal of The Electrochemical Society, 159 (5), B531-B540 (2012)