Experimental section, discussion on high current density and heat rejection limit, and physical and electrochemical properties of different carbon supports (PDF)

The oxygen reduction reaction (ORR) kinetics of a high-surface-area carbon-supported platinum catalyst (Pt∕C) were measured in an operating proton exchange membrane fuel cell (PEMFC). The ORR kinetics of Pt∕C can be described over a wide range of temperature, pressure, and current density using four catalyst-specific parameters: transfer coefficient, exchange current density, reaction order with respect to oxygen partial pressure, and activation energy. These parameters were extracted using a combined kinetic and thermodynamic model, either referenced to the reversible cell potential (i.e., using exchange current density as activity parameter) or referenced to a constant ohmic-resistance-corrected (i.e., iR-free) cell voltage. The latter has the advantage of using an activity parameter (activity at 0.9V iR-free cell voltage) which can be measured explicitly without extrapolation, in contrast to the exchange current density required in the former model. It was found that much of the variation in the published values for these catalyst-specific kinetic parameters derives from applying the same parameter name (e.g., activation energy) without specifying which of its many possible definitions is being used. The obviously significant numerical differences both for “oxygen reaction order” and for “activation energy” due to different definitions (often tacitly assumed and rarely explicitly stated in the literature) are illustrated by the kinetic ORR parameters which we determined for Pt∕C : (i) at zero overpotential, where reaction order and activation energy are ∼0.5 and 67kJ∕mol , respectively, and (ii) at 0.9V iR-free cell voltage, where reaction order and activation energy are ∼0.75 and 10kJ∕mol , respectively.

The exchange current density for the hydrogen oxidation/evolution reactions was determined in a proton exchange membrane fuel cell. Ultralow Pt-loaded electrodes ͑0.003 mg Pt /cm 2 ͒ were used to obtain measurable kinetic overpotential signals ͑50 mV at 2 A/cm 2 ͒. Using a simple Butler-Volmer equation, the exchange current density and transfer coefficient were determined to lie within the range of 235-600 mA/cm Pt 2 and 0.5-1, respectively. Due to the fast kinetics, no measurable voltage losses are predicted for pure-H 2 /air proton exchange membrane fuel cell applications when lowering the anode Pt loadings from its current value of 0.4 mg Pt /cm 2 to the automotive target of 0.05 mg Pt /cm 2 .

Polymer-electrolyte fuel cells are a promising energy-conversion technology. Over the last several decades significant progress has been made in increasing their performance and durability, of which continuum-level modeling of the transport processes has played an integral part. In this review, we examine the state-of-the-art modeling approaches, with a goal of elucidating the knowledge gaps and needs going forward in the field. In particular, the focus is on multiphase flow, especially in terms of understanding interactions at interfaces, and catalyst layers with a focus on the impacts of ionomer thin-films and multiscale phenomena. Overall, we highlight where there is consensus in terms of modeling approaches as well as opportunities for further improvement and clarification, including identification of several critical areas for future research. Fuel cells may become the energy-delivery devices of the 21 st century. Although there are many types of fuel cells, polymer-electrolyte fuel cells (PEFCs) are receiving the most attention for automotive and small stationary applications. In a PEFC, fuel and oxygen are combined electrochemically. If hydrogen is used as the fuel, it oxidizes at the anode releasing proton and electrons according toThe generated protons are transported across the membrane and the electrons across the external circuit. At the cathode catalyst layer, protons and electrons recombine with oxygen to generate waterAlthough the above electrode reactions are written in single step, multiple elementary reaction pathways are possible at each electrode. During the operation of a PEFC, many interrelated and complex phenomena occur. These processes include mass and heat transfer, electrochemical reactions, and ionic and electronic transport. * Electrochemical Society Active Member. z E-mail: azweber@lbl.govOver the last several decades significant progress has been made in increasing PEFC performance and durability. Such progress has been enabled by experiments and computation at multiple scales, with the bulk of the focus being on optimizing and discovering new materials for the membrane-electrode-assembly (MEA), composed of the proton-exchange membrane (PEM), catalyst layers, and diffusionmedia (DM) backing layers. In particular, continuum modeling has been invaluable in providing understanding and insight into processes and phenomena that cannot be resolved or uncoupled through experiments. While modeling of the transport and related phenomena has progressed greatly, there are still some critical areas that need attention. These areas include modeling the catalyst layer and multiphase phenomena in the PEFC porous media.While there have been various reviews over the years of PEFC modeling 1-7 and issues, [8][9][10][11][12][13][14] as well as numerous books and book chapters, there is a need to examine critically the field in terms of what has been done and what needs to be done. This review serves that purpose with a focus on transport modeling of PEFCs. This is not meant to be an exhaustive review...

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.

A general form of the thermal energy equation for a battery system is derived based on first principles using the volume-averaging technique. A thermal-electrochemical coupled modeling approach is presented to simultaneously predict battery electrochemical and thermal behaviors. This approach couples the thermal energy equation with the previous multiphase micro-macroscopic electrochemical model via the heat generation and temperature-dependent physicochemical properties. The thermal-electrochemical model is multidimensional and capable of predicting the average cell temperature as well as the temperature distribution inside a cell. Numerical simulations are performed on a Ni-MH battery to demonstrate the significance of thermal-electrochemical coupling and to investigate the effects of thermal environment on battery electrochemical and thermal behaviors under various charging conditions.

A micro-macroscopic coupled model, aimed at incorporating solid-state physics of electrode materials and interface morphology and chemistry, has been developed for advanced batteries and fuel cells. Electrochemical cells considered consist of three phases: a solid matrix (electrode material or separator), an electrolyte (liquid or solid), and a gas phase. Macroscopic conservation equations are derived separately for each phase using the volume averaging technique and are shown to contain interfacial terms which allow for the incorporation of microscopic physical phenomena such as solidstate diffusion and ohmic drop, as well as interfacial phenomena such as phase transformation, precipitation, and passivation. Constitutive relations for these interfacial terms are developed and linked to the macroscopic conservation equations for species and charge transfer. A number of nonequilibrium effects encountered in high-energy-density and high-power-density power sources are assessed. Finally, conditions for interfacial chemical and electrical equilibrium are explored and their practical implications are discussed. Simplifications of the present model to previous macrohomogeneous models are examined. In a companion paper, illustrative calculations for nickel-cadmium and nickel-metal hydride batteries are carried out. The micro-macroscopic model can be used to explore material and interfacial properties for desired cell performance. InfroductionElectrochemical power sources such as lead-acid, nickelcadmium (Ni-Cd), nickel-metal hydride (Ni-MIT), and lithiurn batteries, as well as various fuel cells, are widely used in consumer applications and electric vehicles. These and future applications place an ever-increasing demand for developing more advanced power sources with higher energy density, higher power density, and longer cycle life. Mathematical modeling is indispensable in this development process, because a cell model, once validated experimental-

The following model/experiment comparisons aid in predicting the maximum performance of a normalH2∕oxygen proton exchange membrane fuel cell (PEMFC) for a cathode catalyst with known oxygen reduction reaction (ORR) kinetics in the absence of gas transport resistances. Specific focus was on modeling the voltage loss within the cathode catalyst layer, which results from a balance between slow ORR kinetics and resistance to proton transport. A unique plot of proton resistance correction vs the ratio of ohmic (expressed by the proton conduction sheet resistance, Rsheet ) to charge transfer resistance (expressed by the ratio of the Tafel slope of the ORR over the current density, b∕i ) in a PEMFC cathode (iRsheet∕b) was developed, based on an analytical solution to effective proton resistance in the porous cathode electrode of a proton exchange membrane fuel cell. Additionally, a plot of catalyst utilization (u) vs iRsheet∕b was developed to serve as a guideline of experimental design parameters such that catalyst utilization is kept above 90%, which is a prerequisite for measuring the ORR kinetics. The model is applicable as long as the ORR follow simple Tafel kinetics.