A.M. Kannan

Pt-M (M = Fe, Co, Ni and Cu) electrocatalysts synthesized by an aqueous route for proton exchange membrane fuel cells

Abstract Nanostructured Pt–M (M=Fe, Co, Ni, and Cu) alloy catalysts synthesized by a low temperature (70 °C) reduction procedure with sodium formate in aqueous medium have been investigated for oxygen reduction in sulfuric acid and as cathodes in single proton exchange membrane fuel cells (PEMFC). The Pt–M alloy catalysts show improved catalytic activity towards oxygen reduction compared to pure platinum. Among the various alloy catalysts investigated, the Pt–Co catalyst shows the best performance with the maximum catalytic activity and minimum polarization occurring at a Pt:Co atomic ratio of around 1:7. While mild heat treatments at moderate temperatures (200 °C) improve the catalytic activity due to a cleaning of the surface oxides, annealing at elevated temperatures (900 °C) degrade the activity due to an increase in particle size.

Surface/Chemically Modified LiMn2 O 4 Cathodes for Lithium-Ion Batteries

LiMn 2 O 4 spinel oxide has been surface/chemically modified with Li x CoO 2 , LiNi 0 . 5 Co 0 . 5 O 2 , Al 2 O 3 , and MgO using a chemical processing procedure followed by heat-treatment at 300-800°C. The surface/chemically modified samples show much better capacity retention at both 25 and 60°C than does the unmodified LiMn 2 O 4 (41% fade in 100 cycles at 60°C). Among the various compositions investigated, the LiNi 0 . 5 Co 0 . 5 O 2 -modified sample shows superior capacity retention with only 2.8% fade in 100 cycles at 60°C with around 110 mAh/g. The Al 2 O 3 -modified sample shows a higher capacity of 130 mAh/g, but with a faster fade (16% in 100 cycles at 60°C). The Li 0 . 7 5 CoO 2 -modified sample shows the best combination of capacity (124 mAh/g) and retention (8% fade in 100 cycles at 60°C). The modified samples also exhibit better capacity retention after aging at 55°C at 60-70% depth of discharge.

High Capacity Surface-Modified LiCoO2 Cathodes for Lithium-Ion Batteries

The surface of LiCoO 2 cathodes was modified with Al 2 O 3 , TiO 2 , and ZrO 2 by a chemical processing procedure followed by heat treatment at 300°C in air for 4 h. The surface-modified LiCoO 2 samples show much better capacity retention at both 25 and 60°C than the unmodified LiCoO 2 cathode to higher cutoff charge voltages of as high as 4.7 V vs. lithium. For example, Al 2 O 3 -modified LiCoO 2 shows approximately 180 mAh/g at 4.5 to 3.2 V with a capacity fade of only 8% in 100 cycles, compared to 32% for the unmodified LiCoO 2 . Transmission electron microscopic studies reveal that the guest materials are present as loose oxides (Al 2 O 3 and ZrO 2 ) or as monolayers (TiO 2 ) on the surface of LiCoO 2 particles. The improved capacity retention and the higher reversible capacity (180 mAh/g) of the surface-modified LiCoO 2 compared to the unmodified LiCoO 2 (140 mAh/g) could be due to a suppression of the chemical and structural instabilities of the charged Li 1 - x CoO 2 cathodes and/or reduction of interparticle stresses and strains.

Comparison of the chemical stability of the high energy density cathodes of lithium-ion batteries

Abstract With an objective to assess the chemical stabilities and their consequences in cell performance, the variations of oxygen content with lithium content (1− x ) in chemically delithiated Li 1− x CoO 2 , Li 1− x Ni 0.85 Co 0.15 O 2 , and Li 1− x Mn 2 O 4 cathodes have been monitored with redox titrations. The Li 1− x CoO 2 system tends to lose oxygen from the lattice at deep lithium extraction, while the Li 1− x Ni 0.85 Co 0.15 O 2 system does not lose oxygen at least for (1− x )>0.3. The chemical instability with a tendency to lose oxygen at deep lithium extraction could be the reason for the limited practical capacity of the Li 1− x CoO 2 system (140 mA h/g) compared to that realized with the Li 1− x Ni 0.85 Co 0.15 O 2 system (180 mA h/g). The Li 1− x Mn 2 O 4 spinel maintains an oxygen content of 4.0 without losing any oxygen for 0.15⩽(1− x )⩽1.

Synthesis and electrochemical evaluation of high capacity nanostructured VO2 cathodes

A metastable form of vanadium dioxide—designated as VO2(B)—has been synthesized by an ambient temperature reduction of aqueous vanadate ions with potassium borohydride and sodium dithionite followed by heating in vacuum at 230 °C. The samples have been characterized by X-ray diffraction, thermogravimetric analysis (TGA), infrared spectroscopy, scanning electron microscopy (SEM), and electrochemical discharge–charge cycling in lithium cells. These oxides exhibit a high reversible capacity of >300 mAh/g at 3.5–1 V and a moderate reversible capacity of close to 200 mAh/g at 3.5–2 V with good cyclability in a wide temperature range of −30 to 60 °C. Also, the VO2(B) cathodes exhibit excellent safety characteristics as indicated by the absence of exothermic peaks below 350 °C in differential scanning calorimetry (DSC).

Hybrid Microgrid Model Based on Solar Photovoltaic Battery Fuel Cell System for Intermittent Load Applications

Microgrids are a subset of the modern power structure using distributed generation to supply power to communities rather than vast regions. The relatively smaller scale mitigates transmission loss with better control, greater security, increased reliability, and design flexibility. This study explores the modeled performance and cost viability of a hybrid grid-tied microgrid that utilizes the combination of solar photovoltaic (PV), batteries, and fuel cell (FC) systems. The proposed concept highlights that each community home is equipped with more solar PV than is required for normal operation. As the homes are part of a microgrid, excess or unused energy from one home is collected for use elsewhere within the microgrid footprint. The surplus power that would have been discarded becomes a community asset and is used to run intermittent services. The modeled community does not have parking adjacent to each home allowing for the installment of a privately owned slower Level 2 charger. This makes electric vehicle (EV) ownership untenable. Based on this study, an optimum configuration is recommended to provide a Level 3 dc quick charger for an intermittent service. The addition of batteries and FCs is meant to increase load leveling, improved reliability, and instill limited island capability.

Nanostructured gas diffusion and catalyst layers for proton exchange membrane fuel cells

Nanostructured components are introduced in membrane electrodes assembly MEA in proton exchange membrane fuel cell as asolution to improve the performance. Single-walled carbon nanotubes and multiwalled carbon nanotubes supported platinum areused to fabricate the gas diffusion layer GDL and the catalyst layers in the MEAs, respectively. The physicochemical andelectrochemical characterizations of these nanotube-based components demonstrate excellent GDL surface morphology and uni-form distribution of the platinum catalyst over the carbon nanotube support. The fuel cell testing using these nanostructuredcomponents exhibits promising fuel cell performance using hydrogen-air and hydrogen-oxygen at ambient pressure.© 2006 The Electrochemical Society. DOI: 10.1149/1.2422751 All rights reserved.Manuscript submitted September 19, 2006; revised manuscript received October 16, 2006.Available electronically December 26, 2006.

Characterization of the Bismuth-Modified Manganese Dioxide Cathodes in Rechargeable Alkaline Cells

Bismuth-modified manganese dioxide (BMD) cathodes are shown to exhibit good cycling characteristics with a theoretical two-electron capacity in rechargeable alkaline cells. With an aim to understand the discharge-charge mechanisms, the BMD cathodes are characterized by X-ray diffraction, scanning electron microscopy, and wet-chemical analysis at various levels of discharge and charge during the first two cycles and after various numbers of cycles. It is found that a well-ordered, crystalline birnessite MnO 2 is formed at the end of first charge, irrespective of the initial form of the manganese oxide. The discharge-charge mechanism involves a reversible conversion of birnessite MnO 2 to MnOOH to Mn(OH) 2 in the subsequent cycles. Wet-chemical analyses demonstrates for the first time that the discharge/charge process in rechargeable alkaline cells involves a reversible dissolution/incorporation of K + ions from/into the cathode lattice into/from the electrolyte. The incorporation of the K + ions into the lattice appears to stabilize a well-ordered birnessite structure during charge.

Oxide-based bifunctional oxygen electrode for rechargeable metal/air batteries

Statistical methods for optimizing the morphology of oxide-based, bifunctional oxygen electrodes for use in rechargeable metal/air batteries are examined with regard to binder composition, compaction time, and compaction load. Results show that LaNiO3 with PTFE binder in a nickel mesh envelope provides a satisfactory electrode.

Nano-electrocatalyst materials for low temperature fuel cells: A review

Abstract Low temperature fuel cells are an attractive technology for transportation and residential applications due to their quick start up and shut down capabilities. This review analyzed the current status of nanocatalysts for proton exchange membrane fuel cells and alkaline membrane fuel cells. The preparation process influences the performance of the nanocatalyst. Several synthesis methods are covered for noble and non-noble metal catalysts on various catalyst supports including carbon nanotubes, carbon nanofibers, nanowires, and graphenes. Ex situ and in situ characterization methods like scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy and fuel cell testing of the nanocatalysts on various supports for both proton exchange and alkaline membrane fuel cells are discussed. The accelerated durability estimate of the nanocatalysts, predicted by measuring changes in the electrochemically active surface area using a voltage cycling method, is considered one of the most reliable and valuable method for establishing durability.