To increase the commercialization of fuel cell electric vehicles, it is imperative to improve the activity and performance of electrocatalysts through combined efforts focused on material characterization and device-level integration. Obtaining fundamental insights into the ongoing structural and compositional changes of electrocatalysts is crucial for not only transitioning an electrode from its as-prepared to functional state, also known as "conditioning", but also for establishing intrinsic electrochemical performances. Here, we investigated several oxygen reduction reaction (ORR) electrocatalysts via in situ and ex situ characterization techniques to provide fundamental insights into the interfacial phenomena that enable peak ORR mass activity and high current density performance. A mechanistic understanding of a fuel cell conditioning procedure is described, which encompasses voltage cycling and subsequent voltage recovery (VR) steps at low potential. In particular, ex situ membrane electrode assembly characterization using transmission electron microscopy and ultra-small angle X-ray scattering were performed to determine changes in carbon and Pt particle size and morphology, while in situ electrochemical diagnostics were performed either during or after specific points in the testing protocol to determine the electrochemical and interfacial changes occurring on the catalyst surface responsible for oxygen transport resistances through ionomer films. The results demonstrate that the voltage cycling (break-in) step aids in the removal of sulfate and fluoride and concomitantly reduces non-Fickian oxygen transport resistances, especially for catalysts where Pt is located within the pores of the carbon support. Subsequent low voltage holds at low temperature and oversaturated conditions, i.e., VR cycles, serve to improve mass activities by a factor of two to three, through a combined removal of contaminants, surface-blocking species (e.g., oxides), and rearrangement of the catalyst atomic structure (e.g., Pt−Pt and Pt−Co coordination).

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Neutron reflectometry analysis methods for under-determined, multi-layered structures are developed and used to determine the composition depth profile in cases where the structure is not known a priori. These methods, including statistical methods, sophisticated fitting routines, and coupling multiple data sets, are applied to hydrated and dehydrated Nafion nano-scaled films with thicknesses comparable to those found coating electrode particles in fuel cell catalyst layers. These results confirm the lamellar structure previously observed on hydrophilic substrates, and demonstrate that for hydrated films they can accurately be described as layers rich in both water and sulfonate groups alternating with water-poor layers containing an excess of fluorocarbon groups. The thickness of these layers increases slightly and the amplitude of the water volume fraction oscillation exponentially decreases away from the hydrophilic interface. For dehydrated films, the composition oscillations die out more rapidly. The Nafion-SiO2 substrate interface contains a partial monolayer of sulfonate groups bonded to the substrate and a large excess of water compared to that expected by the water-to-sulfonate ratio, λ, observed throughout the rest of the film. Films that were made thin enough to truncate this lamellar region showed a depth profile nearly identical to thicker films, indicating that there are no confinement or surface effects altering the structure. Comparing the SLD profile measured for films dried at 60 °C to modeled composition profiles derived by removing water from the hydrated lamellae suggests incomplete re-mixing of the polymer groups upon dehydration, indicated limited polymer mobility in these Nafion thin films.

Cerium is a radical scavenger which improves polymer electrolyte membrane (PEM) fuel cell durability. During operation, however, cerium rapidly migrates in the PEM and into the catalyst layers (CLs). In this work, membrane electrode assemblies (MEAs) were subjected to accelerated stress tests (ASTs) under different humidity conditions. Cerium migration was characterized in the MEAs after ASTs using X-ray fluorescence. During fully humidified operation, water flux from cell inlet to outlet generated in-plane cerium gradients. Conversely, cerium profiles were flat during low humidity operation, where in-plane water flux was negligible, however, migration from the PEM into the CLs was enhanced. Humidity cycling resulted in both in-plane cerium gradients due to water flux during the hydration component of the cycle, and significant migration into the CLs. Fluoride and cerium emissions into effluent cell waters were measured during ASTs and correlated, which signifies that ionomer degradation products serve as possible counter-ions for cerium emissions. Fluoride emission rates were also correlated to final PEM cerium contents, which indicates that PEM degradation and cerium migration are coupled. It is proposed that cerium migrates from the PEM due to humidification conditions and degradation, and is subsequently stabilized in the CLs by carbon catalyst supports. Widespread adoption of polymer electrolyte membrane (PEM) fuel cell technology is currently hindered by insufficient component durability and high cost.1 During operation, reactive radical species generated by electrochemical fuel cell processes attack vulnerable functional groups in the ion-conducting, or ionomer, molecules which constitute the PEM and are present in the catalyst layers (CLs).2 These attacks reduce PEM thickness and generate local pinholes, which release hydrofluoric acid (HF), sulfuric acid (H 2 SO 4 ), and fluorinated polymer fragments into effluent cell waters; increase crossover of reactant gases through the PEM; and lead to cell failure.1,2 Since the PEM is constrained by cell hardware, hygrothermal cycling generates mechanical stresses which cause physical damage to the PEM in the form of cracks, tears, and pinholes.1,2 Furthermore, during typical operation, cells experience both chemical and mechanical stresses simultaneously, which results in synergy between the degradation modes. Localized mechanical stresses increase PEM susceptibility to radical attack by reducing the activation energy necessary for such attacks to proceed 3,4 and chemical degradation of the ionomer diminishes the bulk mechanical properties of the PEM, such as ultimate tensile strength, strain-to-failure, and fracture toughness, which further increases its susceptibility to physical failure. 5-11Owing to its rapid and regenerative redox with radical species and stability in acidic media, 7 cerium dramatically improves PEM durability by neutralizing radicals before they attack the ionomer. Cerium ions may be directly exchanged with protons in the ionomer 12,13 or ...

A composite membrane consisting of a Nafion proton exchange ionomer and ceria-coated multiwall carbon nanotubes (MWCNTs) was prepared by a solutioncasting method. Reinforcement due to the presence of MWCNTs provides increased mechanical strength to the membrane, and the addition of ceria improves the membrane's chemical durability by scavenging free radicals. The ceria coating also insulates the MWCNTs, which helps to preserve the membrane's low electrical conductivity in the through-thickness direction. The morphology and loading of the CeO 2 /MWCNT precursor were verified using transmission electron microscopy and thermogravimetric analysis. The mechanical and chemical durability of the synthesized composite [CeO 2 / MWCNT]/Nafion membranes was compared with that of pure Nafion membranes. Composite membranes demonstrated improved tensile strength and dimensional stability during hydration, without significantly affecting electronic or ionic conductivity, and maintain equivalent polarization performance in a fuel cell system. They also showed increased durability in an open-circuit-voltage-hold fuel cell test. Such material property enhancements can extend the membrane lifetime, which increasing the economic viability of fuel cell technology.

A combined chemical/mechanical accelerated stress test (AST) was developed for proton exchange membrane (PEM) fuel cells based on relative humidity cycling (RHC) between dry and saturated gases at open circuit voltage (OCV). Membrane degradation and failure were investigated using scanning electron microscopy and small-and wide-angle X-ray scattering. Changes to membrane thickness, hydrophilic domain spacing, and crystallinity were observed to be most similar between field-operated cells and OCV RHC ASTs, where local thinning and divot-type defects are the primary failure modes. While RHC in air also reproduces these failure modes, it is not aggressive enough to differentiate between different membrane types in >1,333 hours (55 days) of testing. Conversely, steady-state OCV tests result in significant ionomer morphology changes and global thinning, which do not replicate field degradation and failure modes. It is inferred that during the OCV RHC AST, the decay of the membrane's mechanical properties is accelerated such that materials can be evaluated in hundreds, instead of thousands, of hours, while replicating the degradation and failure modes of field operation; associated AST protocols are recommended as OCV RHC at 90 • C for 500 hours with wet/dry cycle durations of 30s/45s and 2m/2m for automotive and bus operation, respectively.

PtCo-alloy cathode electrocatalysts release Co cations under operation, and the presence of these cations in the membrane electrode assembly (MEA) can result in large performance losses. It is unlikely that these cations are static, but change positions depending on operating conditions. A thorough accounting of these Co cation positions and concentrations has been impossible to obtain owing to the inability to monitor these processes in operando. Indeed, the environment (water and ion content, potential, and temperature) within a fuel cell varies widely from inlet to outlet, from anode to cathode, and from active to inactive area. Synchrotron micro-X-ray fluorescence (μ-XRF) was leveraged to directly monitor Co 2+ transport in an operating H 2 /air MEA for the first time. A Nafion membrane was exchanged to a known Co cation capacity, and standard Pt/C electrocatalysts were utilized for both electrodes. Co Kα 1 XRF maps revealed through-plane transient Co transport responses driven by cell potential and current density. Because of the cell design and imaging geometry, the distributions were strongly impacted by the MEA edge configuration. These findings will drive future imaging cell designs to allow for quantitative mapping of cation through-plane distributions during operation. In pursuit of vehicle electrification, considerable efforts have been devoted to elucidating the degradation mechanisms of proton exchange membrane (PEM) fuel cells. [1][2][3][4][5][6][7] While the majority of these studies are based around post-mortem analyses of fuel cell materials (e.g. changes in nanoparticle sizes and shape distributions), a full accounting of the losses that contribute to membrane-electrode assembly (MEA) performance degradation requires looking beyond wellresearched catalyst nanoparticle degradation processes.2,8 Indeed, the performance of a PEM fuel cell is affected by many internal and external factors, such as fuel cell design and assembly, material degradation, operational conditions, and impurities or contaminants.9 The above-mentioned factors interact, and they are closely related to the cation activities in the fuel cell. In the current PEM fuel cell system, in addition to protons, various other cations are introduced from a variety of sources. For example, cerium ions are intentionally introduced to the MEA to improve membrane durability by neutralizing radical species before they attack the ionomer. 10,11 In contrast, when Pt-alloy catalysts are used, cobalt, nickel, or other 3d transition metal cations can leach out during fuel cell operation.1,3,5 Finally, cell component corrosion or impurities in the reactant/fuel flows may serve as additional cation sources. 12,13 In general, these cations exhibit greater affinities for the sulfonic acid groups in the ionomers than protons. Proton flux to the cathode is reduced not only due to proton site occupancy but also decreased proton mobility caused by cationic interaction.14-16 Strong interactions between the cations and sulfonic acid sites may also induce io...

In PEM fuel cells, cerium migration is influenced by proton flux,as well as gradients in electric potential, ion concentration, andwater content. These factors were investigated in ex situexperiments and in operating fuel cells. Potential-inducedmigration was measured ex situ in hydrated window cells. CeriumcontainingMEAs were also fabricated and tested under ASTs.MEA disassembly and subsequent XRF analysis were used toobserve rapid cerium migration within the MEA. During MEA hotpressing, humidification, and low RH operation at OCV, ionicdiffusion causes uniform migration from the membrane into thecatalyst layers. During high RH operation at OCV, in-plane ceriumgradients arise due to variations in water content. These gradientsmay diminish the scavenging efficacy of cerium by reducing itsproximity to generated radicals.