Energy storage is increasingly seen as a valuable asset for electricity grids composed of high fractions of intermittent sources, such as wind power or, in developing economies, unreliable generation and transmission services. However, the potential of batteries to meet the stringent cost and durability requirements for grid applications is largely unquantified. We investigate electrochemical systems capable of economically storing energy for hours and present an analysis of the relationships among technological performance characteristics, component cost factors, and system price for established and conceptual aqueous and nonaqueous batteries. We identified potential advantages of nonaqueous flow batteries over those based on aqueous electrolytes; however, new challenging constraints burden the nonaqueous approach, including the solubility of the active material in the electrolyte. Requirements in harmony with economically effective energy storage are derived for aqueous and nonaqueous systems. The attributes of flow batteries are compared to those of aqueous and nonaqueous enclosed and hybrid (semi-flow) batteries. Flow batteries are a promising technology for reaching these challenging energy storage targets owing to their independent power and energy scaling, reliance on facile and reversible reactants, and potentially simpler manufacture as compared to established enclosed batteries such as lead-acid or lithium-ion. Broader context Cost-effective electrochemical energy storage has the potential to dramatically change how society generates and delivers electricity. A few key market opportunities include supporting high fractions of intermittent renewable energy sources and deferring upgrades of existing electricity grid infrastructure. Unfortunately, present state-of-the-art technologies are too expensive for broad deployment. Reductions in manufacturing costs and associated overheads are identied as the single largest cost-savings opportunity for today's battery-based storage options. In addition, increasing production volume and market competition will lead to lower material costs. Both aqueous and nonaqueous ow batteries are promising technology platforms capable of achieving the low costs required for widespread implementation. Non-aqueous systems enable higher cell voltages than their aqueous counterparts but also require higher active material solubility to offset higher electrolyte costs. For both battery types, a key enabling development will be the discovery of tailored molecules that are long lived, provide large cell voltages, and have costs similar to existing commodity chemicals.

This paper presents a mathematical model of oxidation and dissolution of supported platinum catalysts in polymer electrolyte membrane fuel cells ͑PEMFCs͒. Kinetic expressions for the oxidation and dissolution reactions are developed and compared to available experimental data. The model is used to investigate the influences of electrode potential and particle size on catalyst stability.

A mathematical model is developed to describe the impedance response of a porous electrode composed of spherical intercalation particles. The model considers a porous electrode without solution-phase diffusion limitations. The model is developed by first deriving the impedance response of a single intercalation particle, obtained by solving a set of governing equations which describe charge-transfer and double-layer charging at the surface, solid-phase diffusion inside the particle, and an open-circuit potential which varies as a function of intercalant concentration. The model also considers the effect of an insulating film surrounding the particle. The governing equations are linearized to take advantage of the small amplitude of the perturbing current in impedance analysis. Once the impedance of a single particle is determined, this result is incorporated into a model which describes a porous electrode limited by ohmic drop in the solution and solid phases, and by the impedance of the particles of which the porous electrode is composed. The model can be used to examine the effect of physical properties and particle-size distributions in the porous electrode, and the usefulness of impedance analysis to measure solid-phase diffusion coefficients is scrutinized.

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...

This paper presents a mathematical model of the corrosion of carbon catalyst supports in polymer electrolyte membrane (PEM) fuel cells. The model describes how a maldistribution of hydrogen across the fuel electrode can induce both oxygen evolution and carbon corrosion on the positive electrode of the fuel cell in the fuel-starved region. Implications of this reverse-current mechanism are explored by simulating a cell with a nonuniform distribution of hydrogen along the fuel channel in both steady-state and transient operation.