Reducing Pt in proton exchange membrane fuel cells is the subject of intense research and development. Recently, researchers have observed significant performance loss due to a transport limitation at the Pt surface. This is investigated here with loading studies that fix electrode thickness and bulk properties. Within these layers, the impact of Pt dispersion is probed by varying the wt% of Pt/C while holding Pt loading and electrode thickness constant by diluting with carbon, effectively varying the average distance between Pt particles while maintaining gas phase loss in the catalyst layer. Results elucidate how the electrode structure impacts local transport loss. It is demonstrated that local transport loss is not fully captured with a normalized Pt area. Additional geometric considerations that account for ionomer surface area relative to the Pt particles are required to resolve performance loss at low Pt loading as electrode structure varies. Furthermore, within this ionomer layer an interfacial resistance at both the gas and Pt interfaces are necessary to account for performance trends observed. These results demonstrate that residual performance loss associated with low cathode Pt loading can be mitigated by electrode design, where oxygen flux through the gas/ionomer interface to the Pt surface is minimized.

Cerium and manganese ions are very effective reversible scavengers of •OH in an operating PFSA-based PEM fuel cell. The use of these ions in very small quantities can reduce the fluoride release rate by up to three orders of magnitude relative to an unmitigated sample and thereby afford extremely durable membranes. A chemically rational mechanism that accounts for the remarkable effectiveness of these chemical mitigants is presented.

Understanding how water produced in the cathode catalyst layer is removed during proton exchange membrane fuel cell (PEMFC) operation is critical for optimization of materials and model development. The present work combines in situ and ex situ experiments designed to elucidate the dominant water discharge mechanism when considering capillary and vapor transport at normal PEMFC operating conditions. The flux of water vapor driven by the thermal gradient in the cathode diffusion layer can alone be sufficient to remove product water at high current densities even with saturated gas in the delivery channels. The role of an intermediate microporous layer and its impact in vapor vs liquid transport is also considered. We propose that the primary role of the microporous layer is to prevent condensed water from accumulating on and blocking oxygen access to the cathode catalyst layer.

The total accumulated charge collected from test points b-i can be viewed as Supplemental Figure 1. As such, the charge per unit area in test point b represents only the charge collected at b; each successive test point contains the sum of charge collected at all previous test points in addition to the current test point.Supporting Figure 1: Charge Accumulation as a function of test point. Ex Situ Characterization of the SEI Layer: Sample PreparationUpon completion of in situ testing and associated radiation screening (approximately 60 days after NR experiments), cells were disassembled in a glovebox with atmospheric specifications as stated in the methods section. The working electrodes with SEI were rinsed with diethyl carbonate, DEC, and dried.

A strategy to exploit substituted M x Cu 6−x Sn 5 ͑M = Fe, Co, Ni, Zn͒ electrodes with NiAs-type structures that transform on electrochemical lithiation in lithium cells to intermediate face-centered-cubic Li 2 Cu 1−y M y Sn structures before Li z Sn alloys ͑1 Ͻ z Ͻ 4.4͒ are produced is discussed. Zn substitution for Cu is possible only to x Ϸ 1 before the onset of secondary phases, whereas Co and Ni substitution appears to maintain the NiAs-type structure for the whole range of x ͑0 Յ x Յ 5͒. Substituted electrodes provide higher rechargeable capacities than Cu 6 Sn 5 . The results of the study have implications for tailoring and optimizing ͑i͒ the design of multicomponent intermetallic electrode structures containing elements that are either active or inactive towards lithium, ͑ii͒ electrode compositions, and ͑iii͒ the reversibility of lithium insertion/metal displacement reactions in intermetallic electrode systems.

Ultrathin electrodes, such as the 3M Nanostructured Thin Film (NSTF) electrode, provide a plausible pathway to reduce platinum cost in low temperature fuel cells. However, several operational shortcomings, involving relatively poor electrode proton conduction and tendencies to collect water in the cathode, were observed in our fuel cell tests. This can be greatly mitigated when a few-micron thick dispersed-catalyst layer is placed adjacent to the NSTF layer, forming a dispersed-catalyst/NSTF hybrid electrode. This dispersedcatalyst layer is also called the "interlayer" because it is located between the NSTF layer and the microporous layer of the cathode diffusion medium. In this study, development of the hybrid electrode was pursued. Emphasis on developing lab-scale fabrication methods that can easily translate to roll-to-roll manufacturing process was a key element of the hybrid electrode development. The fuel cell performance of the electrode showed high sensitivity to fabrication methods. When the dispersed-catalyst layer was coated directly on the NSTF electrode, voltage at high current density dropped significantly. The voltage loss was surmised to be caused by ionomer seepage into the NSTF layer during the coating process. This voltage loss could be eliminated by placing the dispersedcatalyst layer on the gas-diffusion layer and then located adjacent to the NSTF cathode. Interaction between the dispersed-catalyst and NSTF layers and how it affects the fuel cell performance is discussed.In a proton exchange membrane fuel cell (PEMFC), Pt or Pt alloy nanoparticles supported on high surface area carbon particles are generally used in the cathode. The highly dispersed nature of the catalyst allows for good utilization of the precious metal. However, the electrochemical stability of the carbon support as well as the dissolution of Pt nanoparticles raise concerns about the durability of the catalyst. 1-4 3M's Nanostructured Thin Film (NSTF) is an extended surface catalyst which consists of a thin film of Pt alloy coated on individual lath-shaped single crystalline whiskers of an organic compound, perylene red (PR149). 5-7 The ∼0.5-1 μm long support whiskers are electrically non-conductive, thereby greatly reducing their susceptibility to electro-oxidative corrosion. The continuous layer of Pt serves as the electrical conductor for the electrode. Due to the low curvature (smaller number of low-coordination-number Pt atoms) and bulk-like characteristics of Pt in this form, NSTF electrodes exhibit higher chemical and electrochemical stability and 5-10 times higher Pt-areaspecific ORR activity than dispersed carbon supported catalysts. 7-10 In addition, owing to its unique structure of an extended surface of Pt covering a support, one expects that the Pt or Pt-alloy layer thickness (and therefore the Pt loading) could be reduced while maintaining the electrochemical properties of the catalyst. 11 Therefore, NSTF catalyst technology provides a plausible pathway for achieving higher ORR activity using small amounts of ...

The corrosion of carbon in microporous layers (MPL) has been identified and characterized through the use of potential hold at 1.2V vs RHE and accelerated startup/shutdown (SUSD) testing. Carbon oxidation in the MPL at the MPL/electrode interface leads to increased mass transport limitation of the gas diffusion layer (GDL), as shown by subsequent fuel cell testing. Mitigation of such corrosion in the MPL is achieved through employing graphitized carbon material. A 25% improvement in startup/shutdown degradation rate at 1.2A/cm2 has been observed with implementation of a graphitized carbon in the MPL.

Understanding how water produced in the cathode catalyst layer is removed during PEMFC operation can be critical for materials optimization and representative models. The present work combines in-situ and ex-situ experiments to determine the dominant water discharge mechanism when considering capillarity and vapor transport. Water flux of vapor driven by the thermal gradient in the cathode diffusion layer is shown to be sufficient to remove product water at high current densities with saturated gas in the gas delivery channels. The role of an intermediate microporous layer and its impact in vapor versus liquid transport is also considered. Through novel experimental techniques and capturing all key physical interactions it is concluded that vapor diffusion is the fundamental mechanism by which water is removed from the cathode catalyst layer.