Cathode Thickness Research Articles

Cost Reduction Strategies for Lithium Ion Electrode Processing and Pouch Cell Formation Steps

For lithium ion batteries (LIBs) to take their place in widespread commercialization of hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and full electric vehicles (EVs), system cost must still be reduced by 2-3× to about $200 kWh. [1] Two important ways to achieve significant system cost reduction are to: 1) lower the electrode processing cost (elimination of costly organic solvents, increasing the cathode thickness, and reduction of primary solvent drying time); and 2) reduce the formation time associated with the anode solid electrolyte interface (SEI) layer.Purchasing and handling the N-methylpyrrolidone (NMP) solvent used in much of the electrode formulation and coating steps add unnecessary manufacturing cost to the LIB pack. In addition, more processing energy (heated air flow) is required to remove NMP during drying of the electrode coatings than other solvents (such as water and lower alcohols) with much lower boiling points and higher vapor pressures. NMP recovery involves significant capital expense since multiple condensers or distillation towers are needed, and its use adds to the cost of the coating line equipment in making it explosion proof. Cathode thicknesses, which currently limit the cell specific capacity (mAh/g on a total-unit-cell weight basis) and energy density and indirectly add to cell cost, should also be significantly increased, so much fewer inactive components (current collectors and separators) are required per cell.Electrode wetting and formation cycling also add significant process energy cost per kWh of usable energy, which is often neglected in LIB pack cost calculations. Furthermore, the wetting and formation steps are a huge process time bottleneck and add substantial capital cost to a LIB production plant. In a large-scale LIB manufacturing plant, the footprint associated with the wetting and formation steps can be as large as 20-25% of the entire layout.There have been two useful LIB cost studies presented recently by Argonne National Laboratory (ANL) [2] and TIAX, LLC, [3] but these studies consider the entire 18650 cell production process without much granularity on individual processing and fabrication steps. These cost models are also heavy on contributions from material, labor, and capital equipment without detailed consideration of process energy requirements. This paper will give a more in depth review of the process energy consumption associated with electrode processing and formation cycling, two particularly costly elements of lithium-ion cell production, and how manufacturing cost savings could be realized.The cell fabrication steps mentioned above contribute significantly to the current overall pack cost of $400-600/kWh and will be the focus of the calculations in this presentation. Formulation, coating, and drying aspects will be considered with respect to electrode processing cost, and wetting time and low-rate cycling will be addressed with respect to total cell formation time. This cost assessment approach will show the critical link between electrode processing aides, process energy consumption, and LIB pack cost. Initial calculations show that about 20% of the total battery pack cost can be saved by switching to aqueous electrode processing and doubling the electrode thicknesses. Acknowledgment This research at Oak Ridge National Laboratory (ORNL), managed by UT Battelle, LLC, for the U.S. Department of Energy under contract DE-AC05-00OR22725, was sponsored by the Office of Energy Efficiency and Renewable Energy (EERE) Vehicle Technologies Office (VTO) Applied Battery Research (ABR) subprogram (Program Managers: Peter Faguy and David Howell).

Numerical Simulation of the Primary, Secondary and Tertiary Potential Distribution on Porous Cathode in a Flow-through Packed Bed Reactor. Influence of Thickness, Concentration and Hydrodynamics in Porous Media

Packed bed electrodes can be used for electrochemical recovery of heavy metals from a variety of industrial and laboratory model solutions. The packed bed electrode forms a porous flow-through (Figure 1) configuration providing large surface area usually depleting the concentration of several metal ions below 0.1 ppm. For design porpoises of these electrodes, it is necessary to study the influence of geometry (cathode thickness), flow rate and concentration on potential distribution.Numerical simulations of primary, secondary and tertiary current distributions on the porous cathode of a packed bed electrochemical reactor (Figure 1) were calculated. For primary and secondary current distribution, Laplace equations with its respective boundary conditions (constant potential for primary current distribution and charge transfer kinetics described by a Tafel approximation in secondary current distribution) were evaluated using the finite element method. For tertiary current distribution, the influence of hydrodynamics was taken in account, considering Navier-Stokes model with Brinkman extension for porous media. Once Brinkman equations was solved and local velocity distributions were obtained, local mass transfer coefficients were evaluated by means of following relationship: Kma=cvb where Km is local mass transport coefficient, a is the specific area, v is local velocity, c and b are empirical constants taken from literature. The effects of flow rate, ion concentration, matrix conductivity and thicknes on the distribution of current on porous cathode are analyzed. Results of current distribution obtained by Nava et. al 2008 [1] enable comparisons with the simulations.

Invited: Li-Air Flow Batteries

Although Li-air batteries have an extremely large theoretical energy density they suffer from several drawbacks: (1) The Li2O2/Li2O discharge product deposits on the air side of the electrode reducing the pore size and limiting access of the O2 in the cathode. The discharge products deposit mostly near the air side of the electrode because the O2 concentration is higher on this side. This inhomogenous deposition of reaction products severely limits the usage of cathode volume, which limits the maximum capacity and energy density of the battery; (2) the cyclability and energy efficiency of Li-air batteries are poor due to the lack of effective catalysts to convert solid Li2O2/Li2O discharge products into Li ions; and (3) the current and power densities of Li-air batteries are much lower compared to conventional batteries due to an extremely low oxygen diffusion coefficient in liquid solution.We have demonstrated a novel rechargeable Li-air flow battery (Fig. 1&2), which consists of a lithium-ion conducting glass-ceramic membrane sandwiched by a Li-metal anode in organic electrolyte and a carbon nanofoam cathode through which oxygen-saturated aqueous electrolyte (0.85 M CH3COOH in deionized water) flows. It features a flow cell design in which aqueous electrolyte is bubbled with compressed air, and is continuously circulated between the cell and a storage reservoir to supply sufficient oxygen for high power output.The physical structure of the air cathode is crucial for electrochemical performance. Most conventional air-cathode structures are prepared via a traditional “brick-and-mortar” fabrication approach based on mixing and pressing powders of carbon, catalyst particles, and a polymeric binder into a composite electrode that exhibits an ad-hoc porous structure. Carbon paper supported carbon nanofoam electrode exhibits the following desirable properties: (i) electrical conductivity (10-200 S cm−1); (ii) mechanical integrity; (iii) high specific surface areas (300-500 m2 g−1); and (iv) through-connected porosity in three dimensions with macroporous free volume, which is critical in maintaining facile water/air transport throughout the volume of the air cathode.Fig. 3(a) displays the charge-discharge curves at various current densities. The discharge and charge voltages keep at 3.2 V and 3.9 V, respectively, at a current density of 1 mA cm-2. Even at the current density of 5 mA cm-2, the discharge and charge voltages still keep at 1.5 V and 5.2 V, respectively. With the growth of applied current density, the discharge voltage linearly decreases, while its power density sharply increases as shown in Fig. 3(b). At the current density of 4 mA cm-2, a Li-air flow battery reaches its maximal power density of 7.64 mW cm-2.In Li-air flow batteries, a Li-metal foil was used as the anode electrode. The safety of the Li metal is always an important consideration. Table 1 shows theoretical energy densities of Li-air flow batteries if different anode materials such as Li metal, silicon, and graphite carbon are used. The theoretical specific energy was calculated based on EC reaction as1,2: 4Li+O2+4CH3COOH « 4CH3COOLi+2H2O (1) The biggest difference between the new Li-air flow batteries and conventional Li-air batteries1,3 is that oxygen is supplied from the aqueous electrolyte instead of diffusing from the window in the cathode side. Hence, the thickness of cathode utilized is not limited by the oxygen diffusion length in the electrolyte any more. Instead, by using a thick cathode the reaction rate which is proportional to the volume of cathode can be increased, so the cell’s power performance can be improved4.

Optical properties of CdO films prepared by DC planar magnetron sputtering

CdO films have been produced on glass substrates by DC planar magnetron sputtering technique in Ar and O2 gases 1:4 and distance 6cm between cathode and anode and thickness 174 nm. It is observed from optical properties that the films possess transmittance 80% in visible and near infrared region of spectrum and direct band gap values in the rang 2.31-2.4 e V.

Analysis of a Mathematical Model of Lithium-Sulfur Cells Part III: Electrochemical Reaction Kinetics, Transport Properties and Charging

Sensitivity analysis of a mathematical model of a lithium-sulfur (Li-S) battery was performed by investigating the response of the model to variation of the exchange current densities, diffusion coefficients, and cathode thickness over a wide range; the results of the analysis were used to explain the some aspects of the behavior of the system which may be seen in experiments. In particular, among all the exchange current densities, the exchange current density of the elemental sulfur reduction has the most significant effect on the discharge capacity of the cell. The variation of the diffusion coefficients was also analyzed, providing information on the non-uniformity of precipitants in the cell after discharge. An optimum cathode thickness was presented to gain the highest capacity of the cell. Finally, the simulation of charging was studied, showing that the model needs a large solubility product of di-lithium sulfide to be able to simulate the charge process of a cell.

All-Solid Lithium Metal Polymer Batteries; Effect of LiFePO4 Cathode Density and Thickness on Electrochemical Performance

All-solid lithium metal polymer batteries (LMPBs) consisting of lithium transition metal cathode, polymer electrolyte and Li metal anode have been investigated for achieving much higher energy density than lithium-ion batteries (LIBs) or providing special design properties such as flexibility[1-2]. Unfortunately, commercially available LMPBs failed to go to market, because solid polymer electrolytes (SPEs) show low ionic conductivity and poor interfacial stability towards electrodes comparing to liquid electrolytes. However, most of researches on SPEs have been focused for electrolytes themselves without broad cell performance data such as cycle life and rate capability. Moreover, even though the cathode should be developed or optimized for LMPB systems, few researches on cathodes (for example, cathode thickness, density, or composition) unlike LIBs have been done systematically for LMPBs up to now [3]. So, in this work, we investigate the room temperature electrochemical properties of all-solid LMPBs while changing cathode thicknesses and densities. The LMPBs consist of LiFePO4/SPE/Li metal, in which SPEs for electrolyte and cathode binder are manufactured by in-situ thermal polymerization after cathode/high porous separator/Li cell assembly. The optimum thickness and density of LiFePO4 cathodes could be found by evaluating cycle performances at room temperatures and rate capability. At the same time, morphological changes within the cathode and electrochemical changes before and after polymerization are also compared precisely. Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government(MEST)(2010-0025738).

Carbon corrosion of proton exchange membrane fuel cell catalyst layers studied by scanning transmission X-ray microscopy

Scanning Transmission X-ray Microscopy (STXM) at the C 1s, F 1s and S 2p edges has been used to investigate degradation of proton exchange membrane fuel cell (PEM-FC) membrane electrode assemblies (MEA) subjected to accelerated testing protocols. Quantitative chemical maps of the catalyst, carbon support and ionomer in the cathode layer are reported for beginning-of-test (BOT), and end-of-test (EOT) samples for two types of carbon support, low surface area carbon (LSAC) and medium surface area carbon (MSAC), that were exposed to accelerated stress testing with upper potentials (UPL) of 1.0, 1.2, and 1.3 V. The results are compared in order to characterize catalyst layer degradation in terms of the amounts and spatial distributions of these species. Pt agglomeration, Pt migration and corrosion of the carbon support are all visualized, and contribute to differing degrees in these samples. It is found that there is formation of a distinct Pt-in-membrane (PTIM) band for all EOT samples. The cathode thickness shrinks due to loss of the carbon support for all MSAC samples that were exposed to the different upper potentials, but only for the most aggressive testing protocol for the LSAC support. The amount of ionomer per unit volume significantly increases indicating it is being concentrated in the cathode as the carbon corrosion takes place. S 2p spectra and mapping of the cathode catalyst layer indicates there are still sulfonate groups present, even in the most damaged material.

Deposition and optimization of thin lead layers for superconducting accelerator photocathodes

A combination of a ultra high vacuum arc deposition system and a recrystallization method was used to optimize the smoothness and thickness of thin-layer lead cathodes for superconducting niobium electron injectors. A non-filtered arc system was chosen to deposit Pb films on niobium. The films then underwent melting and recrystallization by treating them with pulsed argon ion beams in a rod plasma injector.

Modeling of a membraneless single-chamber microbial fuel cell with molasses as an energy source

Microbial fuel cell (MFC) is a novel bio-electrochemical system that can use various organic substances as energy source. Computational models of MFC are needed for prediction and optimization of the MFC performance. A comprehensive computational modeling of a membraneless single-chamber MFC, in which bacteria consumed molasses as a substrate, is reported here. The simulated cathode had a layer of polytetrafluoroethylene, which allowed oxygen molecules to diffuse through to take part in the reduction reaction. The substrate molecules diffused through the biofilm, which deposited on the anode surface, and were oxidized by the bacteria localized within the film. The simulation program accepted inputs such as the initial amount of molasses, thickness of the biofilm layer, and dimensions of the MFC chamber. Some outputs of the program include concentration profiles of molasses and oxygen as functions of time and location, and the open-circuit voltage of the MFC as a function of time. As the cathode thickness decreased or the biofilm increased, the voltage increased. To obtain a higher voltage, increasing the biofilm thickness was more effective than decreasing the cathode thickness when the initial COD levels were >5,000 mg/L.

Investigation of the Active Thickness of Solid Oxide Fuel Cell Cathode

In solid oxide fuel cell (SOFC) electrode, electrochemical reaction only occurs in a narrow region near the interface of electrode and electrolyte, which is regarded as the “active thickness”. However, as a key parameter for SOFC electrode microstructure design and optimization, the active thickness of SOFC cathode reported by different research groups vary significantly (from 13 μm [1] to 253 μm [2]) due to different measuring conditions or simulation assumptions.In this study, a detailed SOFC model is developed, in which the dusty gas model (DGM), Ohm’s law and the concentration related Butler-Volmer equation are used to describe the gas diffusion in pores, ion and electron conduction in electrolyte and electrodes, and electrochemical reaction kinetics, respectively. The effective ionic conductivity, electronic conductivity and the TPB length are calculated with the percolation theory and coordinate number theory [3]. Current density distribution in cathode is investigated under various operating conditions and 99% total current density is used as the indicator of the active thickness. This work could provide useful information for SOFC cathode microstructure optimization and for performance enhancement. Keywords: Solid oxide fuel cell; Cathode thickness; Triple phase boundary length[1] Lu, Z., Hardy, J. S., Templeton, J. W., & Stevenson, J. W. (2012).Extended reaction zone of La0.6Sr0. 4Co0.2Fe0. 8O3 cathode for solid oxide fuel cell. Journal of Power Sources, 198: 90-94.[2] Jiang, Z. Y., Xia, C. R., & Chen, F. L. (2010). Efficient thickness of solid oxide fuel cell composite electrode. Chinese Journal of Chemical Physics, 23(2), 217.[3] Chen, D., Bi, W., Kong, W., & Lin, Z. (2010). Combined micro-scale and macro-scale modeling of the composite electrode of a solid oxide fuel cell. Journal of Power Sources, 195(19), 6598-6610.

Effect of Pressure on the Charge Transfer Processes in Stable Free-Radical Organic Polymer Cathode Materials

Organic radical batteries (ORBs) comprise a relatively new technology that uses cathodes based on stable polymeric organic radical-based polymers. These materials show great promise as cathodes in batteries because the neutral, radical species (at left, Figure 1) is remarkably stable and the one-electron oxidation depicted is fully reversible. The rate at which charge can be pushed into or extracted from an electrode is determined by the timescales on which charges are able to move within an electrode and across an electrode-electrolyte, electrode-current collector interface. This work focused on various test-cell pressures to determine how the charge/discharge processes based on the radical polymer’s redox interfacial and intramolecular mass-transfer processes would be influenced by the external pressure on the organic radical polymer electrode matrix. For the Redox process, reversible oxidation/reduction process for a stablized nitroxide radical PTMA, poly(4-methacryloyloxy-2,2,6,6-tetramethylpiperidine-N-oxyl, the expansion-contraction of the polymer by the inclusion-exclusion of the solvated anions during charge-discharge can have a direct affect on the mass transfer mechanism with the electrode matrix. Therefore, we have observed that the rate of the interfacial charge transfer may be affected by; nitroxide radical concentration, cathode capacity, pendant group chain length, cathode thickness and the current collector surface.AC impedance measurement of the composite electrodes at pressure from 25 to 85 inch-lbs revealed a correlation between the overall electron transfer resistance of the composite electrode and the composition of the current collector surface; capacity of nitroxide polymer; and the charge-discharge rate limits. These data suggest that the ionic conduction pathways both across the solvent-electrode interface and within the electrode matrix can be altered by pressure.Specifically, we have observed a gradual decrease in charge transfer resistance with an increase in pressure to 65 in-lbs. It is postulated that due to compression of the porous polymer structure variations possible in mass transfer process, i.e., anionic interfacial migration/diffusion, and intra-electrode diffusion pathways. Then as we continue to increase pressure, that the inability for the polymer matrix to swell, causes an increase in charge transfer resistance, possible due minimal pathways for anion mass-transfer.Overall, our results indicate that the interfacial electron transfer processes both within the electrode matrix, and across the electrolyte-electrode interface can dominate the charge/discharge performance of an organic radical battery system. Therefore, the performance of organic cathode materials could be improved by suitably designing the interfacial structure for specific charge-discharge rate requirements to maximize interfacial charge transfer through distinct composition for maximal pressure, correct polymer expansion and maximized redox sites.The detailed electrochemical investigation including cyclic voltammetry, AC impedance and spectroelectrochemical studies will be presented.Acknowledgements: This work is supported by a grant from the US Department of Energy Basic Energy Sciences Materials Division. Alliance for Sustainable Energy, LLC, managing and operating contractor for the National Renewable Energy Laboratory under U.S. Department of Energy M&O Contract No., DE-AC36-08GO28308

Optimization of the Cathode Microstructure in Lithium Air Batteries through Multiscale Physical Modeling

The performance of lithium air battery depends not only on the proper choice of cathode and electrolyte materials [1], but also on the geometric and microstructural optimization of the cathode. Physical modeling and numerical simulation afford straightforward ways to investigate possible schemes for performance improvement [2]. Here we report the galvanostatic discharge simulation results of several techniques: (i) a bi-layer cathode consisting of a high specific surface area porous material (e.g. Ketjen black carbon) and a large average pore radius porous material (e.g. Super P carbon); (ii) gradient of porosity across the cathode thickness, where the porosity varies linearly from the separator to the air inlet. In case (i), either the high specific surface material or the large pore radius material is arranged near the air inlet. The performance of the cell mainly depends on the cathode material near the air inlet. In case (ii), allocating larger porosity near the air inlet leads to both a higher cell voltage and a larger discharge capacity. Finally, we propose to insert a porous polymer "oxygen diffusion channel" into the active cathode materials. This channel is free of pore choking, since it is inactive in oxygen reduction reaction due to the absence of conduction electrons. Hence, a compromise may be reached between high surface area and smooth oxygen flow. The possible benefits and drawbacks of this scheme are discussed.[1] M. M. O. Thotiyl, S. A. Freunberger, Z. Peng, Y. Chen, Z. Liu, and P. G. Bruce, Nat. Mater. 12(11), 1050 (2013).[2] A. A. Franco and K.-H. Xue, ECS J. Solid State Sci. Technol. 2(10), M3084 (2013).

An OLED Surface-Plasmon-Polariton Mode Energy Extraction Method Using a Dielectric Extraction Layer

A new surface-plasmon-polariton mode energy extraction method using a deielectric extraction layer (DEL) was suggested. A numerical analysis was conducted to varify the feasibility of the extraction method and to find out the optimal values of the design pararmeters. Refractive index of the DEL should be larger than that of oragnic layers of organic light-emitting diodes. In addition, the thickness of the semi-transparent cathode had an optimal range.

Nano-Scaled Mixed Conductors for High Performance SOFCs at ≤ 600°C

IntroductionMixed ionic-electronic conductors as LSCF (La0.58Sr0.4Co0.2Fe0.8O3- δ) and cermets like Ni/8YSZ (8 mol% Y2O3 stabilized ZrO2) are well-established cathode and anode materials for solid oxide fuel cells. At operating temperatures of 600 °C or below, electrode polarization losses as well as ohmic losses in the solid electrolyte rise steeply. Thereby, anode-supported cells (ASC), proving highest performance and excellent durability at temperatures > 700 °C, are unsuitable in their actual “standard-type” design.In this contribution, the solid-gas electrochemistry at both electrode/electrolyte interfaces of a standard ASC was significantly improved by modifications using nanotechnology:A thin-film 8YSZ electrolyte of ~1.5 µm, made by a wet-chemical method, in combination with a dense Ce0.8Gd0.2O1.9 (CGO) thin-film buffer layer efficiently reduced the ohmic loss contribution by 80% [1].A nano-scaled La0.6Sr0.4CoO3-δ (LSC) thin-film cathode of 200 nm, deposited by metal organic deposition (MOD), cut the area specific resistance of the two-layer cathode of 30 µm down by two orders of magnitude, compared to µm-scaled cathode structures [2].A nano-scaled Ni/8YSZ anode layer of 10 to 100nm, grown in operando by a reverse current treatment (RCT) [3], lowered the area specific resistance of a standard µm scaled anode by up to 40% [4]. ExperimentalAnode-supported half cells were manufactured by Forschungszentrum Jülich. They consist of a regular anode support (thickness 0.5 mm, NiO/8YSZ), with a µm-scaled anode functional layer (AFL, thickness 7 µm, NiO/8YSZ) and an ultra-thin-film electrolyte (thickness 1.5 µm, 8YSZ). Onto this electrolyte, a dense CGO layer (thickness 850 nm) was deposited by physical vapor deposition (PVD) to prevent secondary phase formation. At IWE, the nano- scaled LSC thin-film cathode (thickness 200 nm) was deposited by spin-coating of a metal-organic solution on top. This required a modified fabrication route, including anode reduction at 800 °C prior to cathode deposition and two consecutive heat treatments (170 °C and 650 °C) of the LSC thin film. Subsequently, the cathode was completed by screen printing a µm-scaled LSC current collector on top. The nano-scaled anode/electrolyte interface was evolved from a reverse current treatment during cell operation at 600 °C (~1% H2O in H2). Results and DiscussionThe nanostructures and phase compositions of both electrode/electrolyte interfaces were studied by high-resolution and scanning electron microscopy methods, among others. MOD derived LSC thin-film cathodes typically consist of a heterointerface made of the perowskite La0.6Sr0.4CoO3-δ and the Ruddlesden-Popper type phase (La,Sr)2CoO4 ± δ , which, in combination with the nanoscaled microstructure, leads to an extremely enhanced oxygen surface exchange [2]. The Ni/YSZ anode had undergone a complete rearrangement in the close vicinity to the electrolyte, resulting in an enormous increase of triple phase boundaries on the nanoscale [3]. A reaction model explaining the solid-solid transformations during the reverse current treatment, which was set up in model experiments, is presented. Analysis by electrochemical impedance spectroscopy (EIS) and current/voltage (C/V) curves revealed an area specific resistance (ASR) of 359 mΩ∙cm² at 600 °C and 60% H2O in H2and a power density of 900 mW/cm² at 700 mV in dry hydrogen atmosphere. This performance is by 400% better than measured for a standard-type ASC with a LSCF cathode, which underlines the potential of introducing nanotechnology for mixed conducting electrodes in SOFC.ConclusionsThis study has created a better understanding, how custom-tailored nanostructures potentially support the performance of mixed-conducting electrodes for SOFC at ≤ 600 °C. Despite alternative approaches reported in literature [5, 6], quite simple nanotechnology strategies were applied, which are also applicable on the larger scale, so that we see great potential in this “nano” ASC design.

A High-Rate Rechargeable Li-Air Flow Battery

The Li-air batteries have been attracted much attention because of the possibility of extremely high theoretical energy density. However, the practical energy density of Li-air batteries is much less than the theoretical predication. In addition to the energy density, Li-air batteries also suffered from other major deficiencies including extremely low power (or current) density and poor cyclability, and sensitivity to the moisture. In particular, the current and power densities of Li-air batteries are much lower compared to conventional batteries due to an extremely low oxygen diffusion coefficient in liquid solution1. A solid Li-ion conducting glass ceramic (LIC-GC) membrane has not only a good conductivity for Li-ions but also good chemical stability in both non-aqueous and aqueous solutions, as well as the ability to isolate the two electrolytes. This membrane is used in the Li-air battery which consists of dual electrolytes - a non-aqueous electrolyte and an aqueous electrolyte in the anode and cathode electrodes, respectively. This dual electrolyte configuration can avoid the solid discharge product in the organic electrolyte configuration resulting in improved rechargeability. In this study, a new structure of Li-air flow battery system for possible large scale applications is proposed. It consists of a LIC-GC membrane sandwiched by a Li-metal anode in organic electrolyte and a carbon nanofoam cathode through which oxygen-saturated aqueous electrolyte flows. It features a flow cell design in which aqueous electrolyte is bubbled with compressed air, and is continuously circulated between the cell and a storage reservoir to supply sufficient oxygen for high power output.The experimental Li-air flow battery was shown in Fig. 1 which consists of two units: the electrochemical (EC) reaction unit; and a combined electrolyte storage/oxygen exchange unit. The EC reaction unit is similar to conventional Li-air batteries using hybrid electrolytes, which converts chemical energy into electrical energy and vice versa. However, a major difference is that the cathode electrode does not open directly to the atmosphere to receive oxygen; instead it circulates the electrolyte continuously between the EC reaction unit and electrolyte storage unit. Hence, the thickness of cathode utilized is not limited by the slow oxygen diffusion in the electrolyte any more. Instead, by using a thick cathode the reaction rate which is proportional to the volume of cathode can be increased, so the cell’s power performance can be improved2. Furthermore, the energy and power capabilities can be totally separated according to the load requirements. The maximum output power of the system is given by the maximum current density and the electrode size of the EC reaction unit; the electrolyte storage unit determines the maximum energy storage and delivery capacity; and the oxygen exchange unit regenerates (i.e. refreshes) the electrolyte to become EC reactive.Fig. 2 displays the charge-discharge curves at various current densities. With the growth of applied current density, the discharge and charge voltage difference linearly increases. According to the electrochemical impedance spectra (EIS) analysis, this voltage difference is mainly attributed to the resistance of LIC-GC membrane and causes hydrogen evolution at high current density during discharge. At the current density of 4 mA/cm2, a Li-air flow battery reaches its maximal power density of 7.64 mW/cm2. The theoretical cell specific energy is calculated as 477 Wh/kg based on the weight of Li-metal, acid and H2O3.In conclusion, a rechargeable Li-air flow battery is demonstrated to be cycled at a rate up to 5 mA/cm2, and the power performance can be further improved by reducing the resistance of LIC-GC membrane.ReferencesP. Andrei, J. P. Zheng, M. Hendrickson, and E. J. Plichta, J. Electrochem. Soc., 157, A1287 (2010).P. Andrei, J. P. Zheng, M. Hendrickson, and E. J. Plichta, J. Electrochem. Soc., 159(6), A770 (2012).J.P. Zheng, P. Andrei, M. Hendrickson, and E.J. Plichta, J. Electrochem. Soc. 158, A43 (2011).

Enhanced Electrolyte Utilizing Fluorocarbon Additives

Perfluorocarbons, the main component of artificial blood, have been investigated as additives for lithium oxygen battery due to their ability to absorb significant volume of oxygen. They are expected to increase the amount of oxygen dissolved in the electrolyte and therefore decrease the polarization during discharge. In this work, fluorocarbon compound (PFC) is incorporated in the ether electrolytes and its performance is tested in lithium oxygen battery. Our results demonstrate that addition of PFC improves the discharge voltage significantly up to 200mV for 20% PFC addition in 0.1 M LiClO4:TEGDME electrolyte. We also discover that the effect of PFC additives is more pronounce at thicker electrode, signifying that PFC additives might enhance the concentration of oxygen and or diffusion of oxygen throughout the electrode thickness. To further investigate the effect of altering oxygen solubility, the pressure of the system was varied. The impedance spectroscopy demonstrates that the charge transfer resistance value decreases with addition of PFC, decrease of cathode thickness and increase of pressure. Further result from rotating ring disk in order to quantify the concentration of oxygen soluble in the electrolyte and its difussion coefficient will be presented. This novel approach of utilizing the phenomenal characteristics of perfluorocarbon reveals an unconventional approach towards enhancing the lithium O2 battery performance.

Performance of MIEC Cathodes in SOFC Stacks Evaluated by Means of FEM Modeling

Solid oxide fuel cells (SOFC) are connected in series to stacks by metal interconnectors (MIC). Hereby, the potentially high performance of anode-supported cells (ASC) is significantly reduced. [1,2]. Conclusively, gas diffusion polarisation at the mixed conducting cathode contributes close to the sum of all ohmic losses to the overall polarization, both controlling stack performance.In order to investigate the matter further, a 2D FEM repeat unit model was developed, which accounts to ohmic and polarisation loss on a stack level [3]. In the model the physical processes i) gas diffusion in the porous electrodes, ii) electric /ionic conduction in the electrodes and electrolyte as well as iii) the electrochemical electrode reactions are incorporated. Electrode microstructure is implemented via 3D reconstruction of FIB-tomography images [5,6]. Electrode kinetic parameters are determined via EIS evaluations [4]. Performance predicted by the model was validated with the help of current/voltage-characteristics measured on high performance anode supported cells.The model calculations demonstrate the interdependence of cathode material parameters (thickness, porosity, conductivity, electrode kinetics) and MIC-design, and widen the understanding on their influence on stack performance. The results show i) that an optimal MIC-design depends on the cathode material parameters and ii) a well-chosen cathode thickness increases the overall power output.[1] L. Blum, W. A. Meulenberg, H. Nabielek, R. Steinberger-Wilckens, Worldwide SOFC Technology Overview and Benchmark, International Journal of Applied Ceramic Technology2 pp. 482-492 (2005).[2] M. Kornely, A. Leonide, A. Weber, E. Ivers-Tiffée, Journal of Power Sources 196 (17), pp. 7209-7216 (2011).[3] H. Geisler, M. Kornely, A. Weber, E. Ivers-Tiffée, ECS Trans. 57, pp. 2871-2881 (2013).[4] A. Leonide, Y. Apel, E. Ivers-Tiffée, ECS Trans. 19, pp. 81-109 (2009)[5] J. Joos, T. Carraro, A. Weber, E. Ivers-Tiffée, J. Power Sources 196, pp. 7302-7307 (2011).[6] J. Joos, M. Ender, I. Rotscholl, N. H. Menzler, E. Ivers-Tiffée, J. Power Sources 246, pp.819-830 (2014)

Effects of Pore Size Distribution on the Discharge Characteristics of Li-Air Batteries with Organic Electrolyte

In this presentation we investigate the effects of the pore size distribution in the cathode on the discharge characteristics of Li-air batteries with organic electrolyte. First, we present a new physics-based model for the discharge of these batteries, which takes into consideration the probability distribution function (PDF) of the pore sizes in the cathode material. Then, we present numerical results obtained by solving the model equations on a finite element grid and compare the numerical results with experimental data already published in the literature for a Lithium-air battery with a cathode thickness of 0.2 mm, in which the PDF of the pore sizes was determined using liquid porosimetry. The transport model that we introduce here is a generalization of our previous model presented in [1], in which all the pores were assumed to have the same size. Unlike in [1], the new model considers the exact form of the pore size distribution function, f(r), where r is the radius of the pore which is assumed to be cylindrical with nonuniform radius. (For instance, with these notations, f(r)*dr is the number of pores per unit area of the cathode with radius between r and r+dr). The model is based on the theory of concentrated solutions developed by Newman [2] and takes into consideration the transport of lithium-ions and oxygen diffusion inside the cell, the electron conductivity of the carbon cathode, and the formation and deposition of lithium peroxide at the cathode. The model accurately describes the porosity change inside the cathode by carefully adjusting the PDF of the pores inside the cathode during the discharge of the battery. The reaction rates at the anode and cathode are described by Butler-Volmer equations. The transport equations consist of a system of four partial differential equations (two for the Li-ions, one for the oxygen diffusion and one for the electron transport), which are coupled with differential equations for the local change in porosity, the specific surface area (which is computed as an integral involving the pore size distribution function), and the resistance of the deposit layer (Li2O2 or Li2O). Since the electron transport mechanism in the deposit layer is still under investigation in the literature we consider a combination of two different conduction processes: (1) the deposit layer is treated as an oxide with relatively high resistivity and (2) the transport through the conduction layer is governed by quantum tunneling. In this later case, we derive approximate expressions for the voltage drop across the deposit layer and solve it self-consistently with the full systems of transport equations.The transport equations are solved throughout the whole volume of the battery, including cathode, anode protective layer, and separator. We also assume organic binary monovalent electrolyte and no convection in the battery. The final system of equations is discretized on a finite element mesh and solved numerically in RandFlux [3]. The effect of the pore size distribution function on the discharge curves, specific capacity, and state-of-charge will be discussed at the conference and in the full article.

Performance of MIEC Cathodes in SOFC Stacks Evaluated by Means of FEM Modeling

In planar solid oxide fuel cell (SOFC) stacks, anode supported cells are connected in series and in parallel by metal interconnectors (MIC), which always goes to the cost of performance.Further insight was gained with the help of a 2D-FEM repeat unit model, accounting for ohmic and polarisation losses on stack level. Implemented physical processes are i) gas diffusion in the porous electrodes, ii) electric/ionic conduction in the electrodes/electrolyte iii) and electrochemical electrode reactions. Material and kinetic parameters have been determined experimentally and the model is validated against measured data over a broad range of operating conditions.The 2D-FEM repeat unit model calculates performance in dependence on cathode dimensions, microstructure and chemical composition versus MIC-flowfield design. The calculations show, that performance is mainly limited by the oxidant gas transport underneath the contact rib for a given flowfield design. Thereby a well-chosen cathode thickness increases the overall power output of a planar SOFC stack most efficiently.

Influence of the Interaction Between Phosphoric Acid and Catalyst Layers on the Properties of HT‐PEFCs

AbstractThe performance of high‐temperature polymer electrolyte fuel cells is influenced by the components in the cell. In order to determine the effects of the platinum loading and the thickness of the anode and the cathode as well as of the phosphoric acid content on the cell performance, a three‐level factorial plan encompassing 15 experimental points was designed and carried out. The experimental results were analyzed statistically and expressed as graphic cubes in the range studied. It was found that each factor and their interactions have strong effects on the cell performance. The interactions are attributed to the distribution of phosphoric acid and gases in the membrane electrode assembly. The water vapor transport influences the concentration of phosphoric acid, which modifies not only the proton conductivity but also the cathode kinetics. The optimized cell performance was obtained with an acid amount of 20 mg cm–2, a cathode thickness of 125 μm (platinum loading of 1.0 mg cm–2), and an anode thickness of 75 μm (platinum loading of 0.6 mg cm–2).