Substantial progress has been made in reducing proton-exchange membrane fuel cell (PEMFC) cathode platinum loadings from 0.4-0.8 mgPt/cm(2) to about 0.1 mgPt/cm(2). However, at this level of cathode Pt loading, large performance loss is observed at high-current density (>1 A/cm(2)), preventing a reduction in the overall stack cost. This next developmental step is being limited by the presence of a resistance term exhibited at these lower Pt loadings and apparently due to a phenomenon at or near the catalyst surface. This issue can be addressed through the design of catalysts with high and stable Pt dispersion as well as through development and implementation of ionomers designed to interact with Pt in a way that does not constrain oxygen reduction reaction rates. Extrapolating from progress made in past decades, we are optimistic that the concerted efforts of materials and electrode designers can resolve this issue, thus enabling a large step toward fuel cell vehicles that are affordable for the mass market.

Electrification of the automobile provides a means of sustaining personal mobility in the face of petroleum resource limitations and environmental imperatives. Lithium ion (Li ion) batteries and hydrogen fuel cells provide pure-electrification solutions for different mass and usage segments of automotive application. Battery electric vehicles based on current and targeted Li ion battery technology will be limited to small-vehicle low-mileage-per-day applications; this is due to relatively low specific energy (kWh/kg) and long recharge time constraints. We briefly discuss new generations of Li ion positive and negative electrode intercalation compounds that are needed and under development to achieve energy storage density, durability, and cost targets. Lithium−air batteries give promise of extending the range, but scientists and engineers must surmount a plethora of challenges if growing research investments in this area are to prove effective. Hydrogen fuel cell vehicles have demonstrated the required ∼300 mile range and the ability to operate in all climates, but the cost of Pt-based catalysts, a low efficiency of utilization of presently cost-effective renewable sources of primary energy (e.g., electricity from wind), and the development of hydrogen infrastructure present significant challenges. Dramatic decreases in the amount of Pt used are required and are being brought to fruition along several lines of development that are described in some detail.

In this paper, electrochemical impedance spectroscopy ͑EIS͒ is used to resolve various sources of polarization loss in a pure hydrogen-fueled polymer electrolyte fuel cell ͑PEFC͒. EIS data are fitted to a fuel cell model in which the catalyst layer physics are accurately represented by a transmission line model. Extracted parameters include cell ohmic resistance, catalyst layer electrolyte resistance, and double-layer capacitance. The results showed that the catalyst layer electrolyte resistance for a stateof-the-art electrode ͑47 wt % Pt on Vulcan XC-72 carbon, 0.8 Nafion ͑1100EW͒-to-carbon weight ratio, 13 µm thick͒ at 80°C and fully humidified conditions was approximately 100 m⍀-cm 2 ; this translates to a dc voltage loss of about 33 mV at a current density of 1 A/cm 2 . Similar results were obtained for two experimental methods, one using H 2 ͑anode͒ and O 2 ͑cathode gas feed͒ and another with H 2 and N 2 supplies, and for two cell active areas, 5 and 50 cm 2 . The measured catalyst layer electrolyte resistance increased with decreasing ionomer concentration in the electrode, as expected. We also observed that the real impedance measured at 1 kHz, often interpreted as the ohmic resistance in the cell, can include contributions from the electrolyte in the catalyst layer.

Researchers worldwide view the high theoretical specific energy of the lithium-air or lithium-oxygen battery as a promising path to a transformational energy-storage system for electric vehicles. Here, we present a self-consistent material-to-system analysis of the best-case mass, volume, and cost values for the nonaqueous lithium-oxygen battery and compare them with current and advanced lithium-based batteries using metal-oxide positive electrodes. Surprisingly, despite their high theoretical specific energy, lithium-oxygen systems were projected to achieve parity with other candidate chemistries as a result of the requirement to deliver and purify or to enclose the gaseous oxygen reactant. The theoretical specific energy, which leads to predictions of an order of magnitude improvement over a traditional lithium-ion battery, is shown to be an inadequate predictor of systems-level cost, volume, and mass. This analysis reveals the importance of system-level considerations and identifies the reversible lithium-metal negative electrode as a common, critical high-risk technology needed for batteries to reach long-term automotive objectives. Additionally, advanced lithium-ion technology was found to be a moderate risk pathway to achieve the majority of volume and cost reductions. Broader contextThe commercialization of battery electric vehicles has provided a glimpse of one potential future paradigm of the transportation sector. Moving to an electricitybased transportation system could enable a domestically produced, potentially near-zero emission energy source if coupled to clean, domestic sources of electricity production. However, the batteries used in electric vehicles in 2013 are too expensive, large, and heavy for mass market adoption; signicant progress is needed. The lithium-air or lithium-oxygen battery is a high visibility archetype for the "best-case" possible electrochemical energy-storage system for electric vehicles. We present a material-to-systems analysis of the lithium-oxygen chemistry with comparison to current and future lithium-based chemistries to identify scientic challenges and technological possibilities. Through translation of materials-level science to the systems-level engineering, we show that a lithiumoxygen battery system for automotive applications has comparable cost, volume, and mass to other advanced chemistries that are in more mature states of development and have less technical risk. This result demonstrates that system-level analysis is necessary and may contradict trends predicted from active materials based specic energy and energy density calculations that are the basis for many research investment decisions.

Gas‐diffusion media (also known as gas diffusers and gas‐diffusion backings) are required in most polymer electrolyte fuel cell (PEFC) designs. Their function is to provide uniform reactant (H 2 , O 2 , and electrons) access to and product (H 2 O) removal from the electrodes, efficient heat removal from the membrane electrode assembly (MEA), and mechanical support to the MEA. The vast majority of gas‐diffusion media are based on carbon‐fiber materials; a variety of forms are used, with carbon‐fiber paper and carbon cloth receiving widest application. This chapter describes the production and properties of currently available and emerging materials. Commonly employed treatments and coatings used to tailor the wicking and hydrophobic properties of diffusion media for efficient water removal are discussed. Finally, ex‐situ and in‐situ methods for characterizing diffusion media are described.