Porous Air Electrodes Research Articles

Study of a novel microstructured air electrode/electrolyte interface for solid oxide cells

Solid Oxide Cells (SOCs) are promising high temperature electrochemical devices to obtain clean energies from renewable sources. Their high operating temperatures (800–1000 °C) contribute to the degradation of the cell components. Intermediate Temperature SOCs (IT-SOCs) appear as an alternative to decrease the operating temperatures (600–800 °C) and avoid cell degradation, nevertheless, the electrochemical performance is affected by energy dissipation, principally by the air electrode overpotentials. This work presents the surface modification of Ce0.80Sm0.20O2-δ (SDC) electrolyte by Femtosecond Laser Micromachining (FLM) to increase the surface/area ratio and therefore improve the electrochemical performance. A pattern with an equally spaced pillar shape microstructure was obtained and characterized. (La0.60Sr0.40)0.95Co0.20Fe0.80O3-δ (LSCF) powder was used as porous air electrode to determine the electrochemical benefits of the pattern. Polarization resistance (Rp) of air electrode in patterned sample was about five times lower than in flat one at 600 °C and after 45 h, which suggested an improvement in the electrical and chemical features over time. These enhancements could be explained by the synergistic effect among surface/area ratio, nano-microcrystalline domains and superficial Ce3+ concentration in the patterned electrolyte. Rp values are higher than those reported for best air electrodes, however, FLM has proven its benefits in electrochemical performance.

A Quantitative Understanding of Electron and Mass Transport Coupling in Lithium–Oxygen Batteries

AbstractThe lithium–oxygen battery has the highest theoretical specific energy among all battery systems, while the actual value falls significantly short. The hindered oxygen and/or electron transport result(s) in limited utilization of the porous air electrode, while achieving a quantitative understanding of the electrochemistry and mass transport coupling is challenging. Herein, a porous electrode with highly consistent and controllable channel units is pioneered that excludes the randomness of disordered pores and consequently enables the investigation of control mechanisms. A three‐dimensional dynamic heterogeneous model is developed, providing the first spatio‐temporal distribution of LiO2 and revealing its reversed diffusion trajectories at limited electron transport. The synergistic combination of experiments and models identifies the crucial role of channel sizes on mechanisms that are divided into mass, hybridization, and electron transport control. For macropores, improving Li2O2 conductivity and mitigating solid‐liquid interface damage are urgent compared to enhancing oxygen diffusion. The unit model offers a promising approach to quantitatively understand the reaction and transport mechanisms in other battery systems with porous electrodes. This work represents a break through in knowledge of control mechanisms and guides the design of disordered electrodes for high‐performance lithium–oxygen batteries.

A Facile and Scalable Strategy for Fabrication of Superior Bifunctional Freestanding Air Electrodes for Flexible Zinc–Air Batteries

AbstractThe large‐scale fabrication of efficient and inexpensive bifunctional catalysts is highly desirable but very challenging for oxygen reduction reaction and oxygen evolution reaction (ORR–OER) in metal–air batteries. Here, a facile and scalable approach for the fabrication of hierarchically porous air electrode consisting of cobalt nanoparticles embedded in bamboo‐like nitrogen‐rich carbon nanotubes (Co/N@CNTs), which are in situ grown onto the surface of carbon nanotube macrofilm (CNMF) through a catalytic growth of crosslinked carbon nanotubes is reported. The resulting hybrid macrofilm (Co/N@CNTs@CNMF) can be directly used as a freestanding air electrode without adding any binder or addivities. More importantly, when incorporated in a zinc–air battery (ZAB), the Co/N@CNTs@CNMF electrode demonstrates drastically enhanced ORR and OER activity while maintaining excellent durability during cycling. Further, when it is used to assemble an all‐solid‐state ZAB, the cell also displays excellent mechanical flexibility, implying promising perspectives as power sources for wearable electronics.

Highly Efficient Li−Air Battery Using Linear Porosity Air Electrodes

An advanced transient state model was developed based on the dynamic behavior of the porous air electrode of non-aqueous Li-air battery, which was determined by a numerical solution of the combined continuity, transport, and kinetics equations. The effects of linear porosity in air electrode on the detail performance such as the distribution of the oxygen concentration, Li2O2 volume fraction, porosity, and oxygen diffusion coefficient of non-aqueous Li-air battery during the discharge were investigated. The results revealed that the employing linear porosity air electrode leaded to the higher specific capacity, the uniform porosity and the preferable oxygen diffusion coefficient of Li-air battery caused by the high-efficiency utilization of porous air electrode and sufficient oxygen transfer. The discharge current density had significant effects on the property of Li-air battery based on linear porosity air electrode due to the great increasing of ohm polarization and serious air electrode passivation. The porosity became uniformly with the reaction, which indicated utilization rate of air electrode near membrane side was significantly improved due to the large initial oxygen concentration difference. The detailed results provided a deeper understanding of producing more efficient Li-air batteries as potential power sources to expand the range of electric vehicles.

A self-supported electrode as a high-performance binder- and carbon-free cathode for rechargeable hybrid zinc batteries

Developing highly efficient and flexible air-breathing electrodes is of significant importance for various energy conversion and storage technologies. In this work, we report the synthesis of the self-supported sandwich-structured electrode with Co3-xNixO4/Co3O4 nanowire arrays (NS@Co3-xNixO4/Co3O4). This novel nanostructure demonstrates very high reversible Faradaic redox reaction of CoNi−O ↔ CoNi−O−OH and excellent bifunctional activity toward both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Moreover, the air cathode, prepared by pressing the stainless steel (SS)@Co3O4 sandwiched between two Ni foam (NF)@Co3-xNixO4, possesses a dense and interconnected structure with a high loading of active catalyst. The Ni foam in this work not only acts as a substrate, but also serves as a nickel precursor for the formation of the Co3-xNixO4 catalyst. Meanwhile, combining the 3D porous structure of NF and intrinsic high OER activity of SS, the assembled porous air electrode can take full advantage of the sandwich structure to promote the electrocatalytic reaction kinetics of NS@Co3xNixO4/Co3O4. Rechargeable battery system assembled from this NS@Co3-xNixO4/Co3O4 electrode based on both Zn−Co3-xNixO4 and Zn−air electrochemical reactions exhibits much higher energy efficiency and durability than those of commercial Pt/C and RuO2 electrocatalysts. This protocol opens a new avenue for the rational design of highly efficient and stable self-supported air electrode for metal-air batteries.

Three-Dimensional Array of TiN@Pt3Cu Nanowires as an Efficient Porous Electrode for the Lithium-Oxygen Battery.

The nonaqueous lithium-oxygen battery is a promising candidate as a next-generation energy storage system because of its potentially high energy density (up to 2-3 kW kg-1), exceeding that of any other existing energy storage system for storing sustainable and clean energy to reduce greenhouse gas emissions and the consumption of nonrenewable fossil fuels. To achieve high round-trip efficiency and satisfactory cycling stability, the air electrode structure and the electrocatalysts play important roles. Here, a 3D array composed of one-dimensional TiN@Pt3Cu nanowires was synthesized and employed as a whole porous air electrode in a lithium-oxygen battery. The TiN nanowire was primarily used as an air electrode frame and catalyst support to provide a high electronic conductivity network because of the high-orientation one-dimensional crystalline structure. Meanwhile, deposited icosahedral Pt3Cu nanocrystals exhibit highly efficient catalytic activity owing to the abundant {111} active lattice facets and multiple twin boundaries. This porous air electrode comprises a one-dimensional TiN@Pt3Cu nanowire array that demonstrates excellent energy conversion efficiency and rate performance in full discharge and charge modes. The discharge capacity is up to 4600 mAh g-1 along with an 84% conversion efficiency at a current density of 0.2 mA cm-2, and when the current density increased to 0.8 mA cm-2, the discharge capacity is still greater than 3500 mAh g-1 together with a nearly 70% efficiency. This designed array is a promising bifunctional porous air electrode for lithium-oxygen batteries, forming a continuous conductive and high catalytic activity network to facilitate rapid gas and electrolyte diffusion and catalytic reaction throughout the whole energy conversion process.

A Porous Air Electrode for Li-O2 Batteries Based on a Green Method

Since Li-oxygen (Li-O2) battery was first roported in 1996 by Abraham and Jiang 1, it had been attracting increasingly attention due to its remarkably high theoretical specific energy of 11400 Wh/kg, which is very close to that of gasoline (13000 Wh/kg) and is regarded as the most promising alternative to fossil fuel based energy2. However, the practical capacity of Li-O2 battery is always far below the theoretical value. The prime reason is that, up discharging, the insoluble discharge product, namely Li2O2, will block the paths for both oxygen and the electrolyte and result in an early termination of the discharge process. So the specific capacity of Li-O2 electrode strongly depends on the porosity of the electrode. And it is very crucial to build porous air electrodes with plenty of voids to accommodate Li2O2 and to facilitate the transportation of oxygen and electrolyte. Up to now, various mesoporous materials or nano-structured materials have been explored to make porous electrodes for Li-O2batteries and have promoted the electrochemical performance significantly. However, present electrode manufacturing prefer to use polyvinyliden-di-fluoride (PVdF) as the binder and use volatile organic solvents such as N-methyl-pyrrolidone (NMP). Most of these solvents cost a lot and lead to the harmful environment pollution during the electrode manufacturing process. In addition, PVdF is also a relatively costly polymer and is necessary to be substitued by an economic binder. Herein, we proposed a green method to build a porous air electrode for Li-O2 batteries by a freeze-dry process with carboxymethyl cellulose (CMC) as the binder. The electrode is characterized with abundant voids for discharge product, which also provide plenty of effective passages for both oxygen and electrolyte. This electrode demonstrate not only a high specific capacity but also an excellent rechargeable ability by combination with high conductive MWCNT. As shown in Figure 1a, at a current density of 100 mA g-1, the porous air cathode demonstrates an initial discharge capacity of 6,702 mAh g-1 with an average discharge potential of about 2.70 V. The increased discharge capacity can be ascribed to the fact that the plenty of room in the porous electrode accommodate more discharge product. Meanwhile, the abundant space facilitates the transportation of oxygen and electrolyte, thus extending the discharge process. The high conductivity of MWCNT also promotes the electrochemical performance of Li-O2batteries. By all the virtues above, the porous air cathode shows an enhanced cycling life as high as 53cycles (Figure 1b). Acknowledgement This work was supported by Science Foundation of Ministry of Education of China (grant 413064), the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning. Reference 1. K. M. Abraham, J. Electrochem. Soc. 1996, 143, 1-5. 2. Z. Q. Peng, S. A. Freunberger, Y. H. Chen, P. G. Bruce, Science, 2012, 337, 563 – 566. Figure 1

The Effects of Electrolyte Variations on the Oxygen Efficiency in Lithium Air Batteries

While the existing lithium-ion batteries have been extensively used in electric vehicles (EVs), their limited specific energy restricts the driving range. In this context, rechargeable lithium air (or Li-O2) batteries have attracted considerable interest in recent years as a potential power source for EVs, owing to their high theoretical specific energy. A typical non-aqueous lithium air battery consists of a porous air electrode, a lithium metal anode, and electrolyte containing lithium salt. The electrolyte is generally regarded as a critical component as it is directly related to the stability and the performance of a lithium air battery. While physicochemical properties of potential electrolytes are often discussed, the ability of electrolyte to utilize the oxygen during reversible charge/discharge cycle is rarely explored. In fact, previously reported gas analysis indicated low current efficiency for O2 evolution during charging (Li2O2 → 2Li+ + O2 + e-), accompanied the evolution of byproduct gases such as H2 and CO2.1-2 Furthermore, the O2 revolution (therefore, rechargeability) seemed highly dependable on the choice of electrolyte. In this work, we investigated the oxygen efficiency during discharge/charge cycle in lithium-air using differential electrochemical mass spectrometry (DEMS). We explored various electrolytes that have the potential to prompt reversible electrochemistry in Li-O2 battery. In addition to this, the effect of a catalyst and lithium salt variation on the O2 evolution was investigated. 1) B. D. McCloskey, D. S. Bethune, R. M. Shelby, G. Girishkumar, and A. C. Luntz, J. Phys. Chem. Lett., 2, 1161 (2011) 2) C. J. Barile and A. A. Gewirth, J. Electrochem. Soc., 160, A549 (2013)

Recent Progress in 1D Air Electrode Nanomaterials for Enhancing the Performance of Nonaqueous Lithium–Oxygen Batteries

AbstractLithium–oxygen (Li–O2) batteries are considered as the most promising candidates owing to their higher theoretical energy density than other energy storage devices. However, the unfavorable structure of the air cathode has become a major cause of low performance for Li–O2 batteries, limiting their use in practical applications. To deal with this issue, the rational construction of porous air electrodes, where crucial reactions take place, is in high demand. Among the various dimensional materials, 1D nanomaterials have been frequently applied for air electrodes in several different forms, owing to their fascinating morphologies and unique characteristics. We present a focus review on the recent research relating to 1D nanomaterials adopted as air electrodes in Li–O2 cells. This paper gives the current issues surrounding Li–O2 cells, which are analyzed in terms of the efforts to apply the advantages of 1D nanomaterial electrodes for enhancing the electrochemical performance of the given cells.

Nitrogen-Doped Holey Graphene for High-Performance Rechargeable Li–O2 Batteries

Li–air batteries represent cutting edge electrochemical energy storage devices, but their practical applications have been precluded by the high cathode cost, the low discharge/charge efficiency, and/or the short battery lifetime. Here, we developed a low-cost, but very efficient, air electrode from porous nitrogen-doped holey graphene for rechargeable nonaqueous Li–O2 cells. The resultant Li–O2 cell can deliver a high round-trip efficiency (85%) and a long cycling life (>100 cycles) under controlled discharge/charge depths or a high capacity of 17 000 mAh/g under the full discharge/charge condition, superior to most other carbonaceous air cathodes. The observed superb performance for the air electrode based on the nitrogen-doped holey graphene can be attributed to its efficient metal-free catalytic activity and three-dimensional mass transport pathway. Therefore, this work represents a new approach to low-cost, efficient, metal-free, binder-free, and hierarchically porous air electrodes useful for energy...

Binder-Free and Carbon-Free 3D Porous Air Electrode for Li-O2 Batteries with High Efficiency, High Capacity, and Long Life.

Pt-Gd alloy polycrystalline thin film is deposited on 3D nickel foam by pulsed laser deposition method serving as a whole binder/carbon-free air electrode, showing great catalytic activity enhancement as an efficient bifunctional catalyst for the oxygen reduction and evolution reactions in lithium oxygen batteries. The porous structure can facilitate rapid O2 and electrolyte diffusion, as well as forming a continuous conductive network throughout the whole energy conversion process. It shows a favorable cycle performance in the full discharge/charge model, owing to the high catalytic activity of the Pt-Gd alloy composite and 3D porous nickel foam structure. Specially, excellent cycling performance under capacity limited mode is also demonstrated, in which the terminal discharge voltage is higher than 2.5 V and the terminal charge voltage is lower than 3.7 V after 100 cycles at a current density of 0.1 mA cm(-2) . Therefore, this electrocatalyst is a promising bifunctional electrocatalyst for lithium oxygen batteries and this depositing high-efficient electrocatalyst on porous substrate with polycrystalline thin film by pulsed laser deposition is also a promising technique in the future lithium oxygen batteries research.

Three-dimensional transient modeling of a non-aqueous electrolyte lithium-air battery

A three-dimensional transient model is developed for a non-aqueous-electrolyte lithium-air battery to investigate numerically key phenomena during discharging. The proposed model rigorously assesses lithium peroxide (Li2O2) formation and growth in an air electrode, and its complex interactions with electrochemical reactions and species/charge transport. We assume that the porous air electrode mainly consists of sphere-like carbon particles, and hence a spherical film growth model is adopted to simulate the precipitation of Li2O2 on the spherical carbon surfaces. The model is first validated against voltage evolution data measured at different discharging current densities. Good agreement between the simulation results and experimental data is achieved, demonstrating that the model accurately captures voltage decline behaviors due to accumulated Li2O2 in the air electrode. Additional simulations are carried out with different air electrode designs in order to establish the optimization strategy for the air electrode. Detailed simulation results, including multidimensional contours, clearly indicate that the electrode thickness and degree of electrolyte filling are key factors for improving the discharging performance of lithium-air batteries.

In-situ Three-dimensional Visualization of Precipitation Behavior in a Porous Air Electrode for Aqueous Lithium-air Battery

In-situ Three-dimensional Visualization of Precipitation Behavior in a Porous Air Electrode for Aqueous Lithium-air Battery

Impedance modelling of air electrodes in alkaline media

A mathematical model of the impedance response of porous air electrodes in alkaline solutions, based on flooded agglomerate theory, was developed. The model results provides insight into characteristic features of the impedance spectra according to the relative rate of the various reactions and mass transfer processes occurring. The modelled spectra where dissolution, transport and reaction of oxygen dominates the impedance are close to ideal semicircles. With increasing contribution from ionic transport in the solution inside the pores, two distinct features occur, namely the appearance of a 45° branch at high frequencies, and an inductive loop at low frequenices. Examples from the literature where inductive loops occurs are given.

Development of Zirconia Electrolyte Films on Porous Doped Lanthanum Manganite Cathodes by Electrophoretic Deposition

ABSTRACTElectrophoretic deposition (EPD) was explored as an inexpensive route for fabricating the 8mol% yttria stabilized zirconia electrolyte in solid oxide fuel cells (SOFCs). Normally, deposition of particulate ceramic powders onto a sintered porous surface yields a non uniform coating which, after sintering, results in porosity, surface roughness and cracking in the coating. To overcome this problem, the present study used a fugitive graphite interlayer between the porous air electrode supported (AES) cathode tube (doped-LaMnO3) and the deposited zirconia film. By this approach, a fairly dense green coating (˜ 60%) was obtained, which yielded a smooth surface and pore-free microstructure after sintering. Preliminary results on the effect of a fugitive interlayer on the unfired (green) and fired zirconia coatings are discussed.

Electrochemical vapor deposition of solid oxide films

In the electrochemical vapor deposition (EVD) process, halides of the cations are volatilized and passed over one side of a porous substrate, and oxygen is passed on the other side of the substrate. The gases diffuse and react within the porous substrate and the pores get filled with the desired solid oxide. Subsequent solid-state electrochemical transport through the solid oxide in the pores continues the reaction, and the film grows over the porous substrate. In the process of fabricating tubular solid oxide fuel cells, the EVD process is used to deposit lanthanum chromite interconnection films and yttria-stabilized zirconia electrolyte films over porous air electrodes. The EVD growth of these solid oxide films is found to be parabolic in nature, with indicates that the solid-state transport (diffusion) of ions through the film controls the growth. This solid-state transport can be analyzed using Wagner's treatment of the electrochemical transport. Conductivity measurements and the information on the defect structure of the oxide can be combined with the EDV growth rate measurements to calculate the solids-state transport properties (transport number and partial conductivities) of the rare-controlling minority species in the oxide. Using this approach, the EVD growth of other electronic, mixed conducting and ionic solid oxide films can also be analyzed.

A mathematical model and optimization of the structure for porous air electrodes

A mathematical model for porous air electrodes is developed to optimize the structure of the electrode. The electrode consists of two layers; a gas-supplying layer and a reaction layer. The reaction layer is assumed to consist of porous catalytical agglomerates surrounded by a hydrophobic gas-supplying zone, which is made from the same material as the gas-supplying layer. It is assumed in the model that these agglomerates have a cylindrical shape. The model takes into account the diffusion of oxygen in the gas-supplying layer, the diffusion of dissolved oxygen, the electrochemical reactions taking place, ionic ohmic drop in the cylinder and also electronic ohmic drop due to the finite conductivity of solid material. To calculate a polarization curve altogether 13 parameters must be known; four geometrical parameters, six parameters which are characteristic of the electrode and three parameters which determine the experimental conditions. The performance of the air electrode is calculated for different geometries and the optimum geometry is determined. In order to simulate a real air electrode the characteristic parameters are measured. Comparisons have also been made between calculated and measured polarization curves.

Oxygen transport at porous air electrodes: Experimental

Porous carbon air electrodes have been operated in fully developed and laminar channel flow and their limiting current density measured with variations in the air velocity, the electrode length and the air channel thickness. The results are placed on a theoretical basis, using a theoretical model previously developed for the two extreme conditions of a thin mass transfer boundary layer in the air channel and complete oxygen exhaustion. The implications of the results on cell design are discussed.