29 research outputs found
Explaining the Atypical Reaction Profiles of Heme Enzymes with a Novel Mechanistic Hypothesis and Kinetic Treatment
Many heme enzymes show remarkable versatility and atypical kinetics. The fungal extracellular enzyme chloroperoxidase (CPO) characterizes a variety of one and two electron redox reactions in the presence of hydroperoxides. A structural counterpart, found in mammalian microsomal cytochrome P450 (CYP), uses molecular oxygen plus NADPH for the oxidative metabolism (predominantly hydroxylation) of substrate in conjunction with a redox partner enzyme, cytochrome P450 reductase. In this study, we employ the two above-mentioned heme-thiolate proteins to probe the reaction kinetics and mechanism of heme enzymes. Hitherto, a substrate inhibition model based upon non-productive binding of substrate (two-site model) was used to account for the inhibition of reaction at higher substrate concentrations for the CYP reaction systems. Herein, the observation of substrate inhibition is shown for both peroxide and final substrate in CPO catalyzed peroxidations. Further, analogy is drawn in the âsteady state kineticsâ of CPO and CYP reaction systems. New experimental observations and analyses indicate that a scheme of competing reactions (involving primary product with enzyme or other reaction components/intermediates) is relevant in such complex reaction mixtures. The presence of non-selective reactive intermediate(s) affords alternate reaction routes at various substrate/product concentrations, thereby leading to a lowered detectable concentration of âthe product of interestâ in the reaction milieu. Occam's razor favors the new hypothesis. With the new hypothesis as foundation, a new biphasic treatment to analyze the kinetics is put forth. We also introduce a key concept of âsubstrate concentration at maximum observed rateâ. The new treatment affords a more acceptable fit for observable experimental kinetic data of heme redox enzymes
Electrochemical performance behavior of combustion-synthesized LiNi0.5Mn0.5O2 lithium-intercalation cathodes
LiNiO2, partially substituted with manganese in the form of a LiNi0.5Mn0.5O2 compound, has been synthesized by a gelatin assisted combustion
method [GAC] method. Highly crystalline LiNi0.5Mn0.5O2 powders with R3m symmetry have been obtained at an optimum temperature of 850 âŚC,
as confirmed by PXRD studies. The presence of cathodic and anodic CV peaks exhibited by the LiNi0.5Mn0.5O2 cathode at 4.4 and 4.3V revealed
the existence of Ni and Mn in their 2+ and 4+ oxidation states, respectively. The synthesized LiNi0.5Mn0.5O2 cathode has been subjected to
systematic electrochemical performance evaluation, via capacity tapping at different cut-off voltage limits (3.0â4.2, 3.0â4.4 and 3.0â4.6 V) and the
possible extraction of deliverable capacity under different current drains (0.1C, 0.5C, 0.75C and 1C rates). The LiNi0.5Mn0.5O2 cathode exhibited
a maximum discharge capacity of 174 mAh gâ1 at the 0.1C rate between 3.0 and 4.6V. However, a slightly decreased capacity of 138 mAh gâ1
has been obtained in the 3.0â4.4V range, when discharged at the 1C rate. On the other hand, extended cycling at the 0.1C rate encountered an
acceptable capacity fade in the 3.0â4.4V range (<10%) for up to 50 cycles
High Voltage and High Capacity Characteristics of LiNi1/3Co1/3Mn1/3O2 Cathode for Lithium Battery Applications
Possibility of synthesizing LiNi1/3Co1/3Mn1/3O2 cathode via., soft chemistry based Gelatin Assisted
Combustion [GAC] approach has been examined through the present study. GAC method with a
calcination temperature as high as 750°C for a period of 24h. was found to be essential to prepare
LiNi1/3Co1/3Mn1/3O2 powders with good hexagonal ordering and better cycling performance. The
intensity ratio of (003) and (104) bragg peaks is greater than unity, which is an indication for the
absence of cation mixing. The observed CV peaks confirm the presence of Ni, Co and Mn ions in their
+3 oxidation state. A maximum discharge capacity of ~180mAh/g has been exhibited by the
synthesized LiNi1/3Co1/3Mn1/3O2 cathode, when charged up to 4.6V. Hence, it is demonstrated that the
LiNi1/3Co1/3Mn1/3O2 cathode synthesized through the present study could be exploited both as a high
voltage and high capacity cathode material for use in rechargeable lithium battery applications
A preliminary investigation into the new class of lithium intercalating LiNiSiO4 cathode material
A unique attempt to exploit silicate chemistry for a possible enhancement of the
electrochemical properties of a lithium ion system via exploration of the novel category lithium
intercalating LiNiSiO4 cathode has been made through the present study. A novel citric acid
assisted modified solâgel method (CAM solâgel) has been adopted to synthesize the title
compound with a formation temperature positioned well below 500 âŚC, as derived from thermal
studies. A powder x-ray diffraction (PXRD) pattern evidenced the absence of undesirable peaks
and confirmed the formation of a hexagonal lattice structure with enhanced crystallinity and
phase purity, and the presence of uniformly distributed particles of âź200 nm size with well
defined grain boundaries is obvious from the scanning electron microscopy (SEM) image of
LiNiSiO4 material. Further, magic angle spinning (MAS) 7Li nuclear magnetic resonance
(NMR) results from LiNiSiO4 confirmed the presence of a layered type of crystal arrangement.
A cyclic voltammetry (CV) study performed on a LiNiSiO4 cathode revealed an excellent
reversibility without any change in the peak position upon extended cycling, thus substantiating
the structural stability upon progressive cycling
Synthesis and Characterization of LiMXFe1-XPO4 (M = Cu, Sn; X = 0.02) Cathodes - A study on the Effect of Cation Substitution in LiFePO4 Material
An attempt has been made for the possible augmentation and exploration of partially substituted
LiFePO4 material as a positive electrode for lithium battery applications. In this regard, cationic
substitution of Cu and Sn (2%) to the native LiFePO4/C electro active material has been carried out
via. ball milling, with a view to understand the effect of respective transition and non-transition metals
upon LiFePO4 individually. Uniformly distributed particles (SEM) of LiMXFe1-XPO4/C (M= Cu, Sn)
with phase pure nature (XRD) and finer crystallite size (<1mm) were obtained. Further, it is interesting
to note that irrespective of the nature of the dopant metal, the simple route of ball milled LiMXFe1-
XPO4/C [M= Cu, Sn] cathodes endowed with improved conductivity and stable reversible capacity
values (chare-discharge). In other words, the LiCu0.02Fe0.98PO4/C cathode delivered a reversible
capacity of ~105 mAh/g with an excellent capacity retention characteristic. On the other hand
LiSn0.02Fe0.98PO4/C cathodes exhibited an average specific capacity of ~100mAh/g with progressively
enhanced efficiency values. Results of Fourier Transform Infra Red (FTIR) spectroscopy and Cyclic
Voltammetric studies of LiMXFe1-XPO4/C (M= Cu, Sn) composites are also appended and correlated
suitably
CAM solâgel synthesized LiMPO4 (M=Co, Ni) cathodes for rechargeable lithium batteries
The emerging category cathode candidates
such as LiCoPO4 and LiNiPO4 were synthesized at 800 �C
using Citric acid assisted modified solâgel (CAM solâgel)
method and examined for possible lithium intercalation
behavior. Compound formation temperature is confirmed
from thermogravimetry and differential thermal analysis
(TG/DTA). Powder X-ray diffraction (PXRD) pattern
evidenced the absence of undesirable peaks and confirmed
the formation of phase pure LiMPO4 (M=Co, Ni) compounds
with an orthorhombic structure and finer crystallite
size. Presence of nanosized particles as observed from
TEM image of LiCoPO4 and the presence of preferred
local cation environment as understood from FTâIR studies
are the added advantages of CAM solâgel synthesis. Further,
Cyclic voltametry (CV) and Impedance spectroscopy
(EIS) studies performed on the synthesized LiCoPO4 and
LiNiPO4 cathodes revealed excellent reversibility and
structural stability of CAM solâgel synthesized cathodes,
especially upon storage as well as during cycling
LiMgy1Cry2Mn2-y1-y2O4 (0.0 < 0.30; Y2=0.30-Y1) as a cathode active material for lithium batteries
LiMn2O4 is an attractive 4 V positive material in lithium rechargeable batteries owing to its favourbale electrochemical characteristics besides its economic and environmental advantages. However, problems of limited cyclability, especiallyat elevated temperatures, have limited the uitlity and commercialization of this cathdoe material. Stabilization of the LiMn2O4 spinel structure has been sought to be ralzied by doping the spinel with suitable cations. In this paper, the results of an exploratory research on the cpacity and cyclability of LiMn2o4 cathodes simultaneously doped with Cr3+ and Mg2+ arereported
LiMgy1Cry2Mn2-y1-y2O4 (0.0 < 0.30; Y2=0.30-Y1) as a cathode active material for lithium batteries
LiMn2O4 is an attractive 4 V positive material in lithium rechargeable batteries owing to its favourbale electrochemical characteristics besides its economic and environmental advantages. However, problems of limited cyclability, especiallyat elevated temperatures, have limited the uitlity and commercialization of this cathdoe material. Stabilization of the LiMn2O4 spinel structure has been sought to be ralzied by doping the spinel with suitable cations. In this paper, the results of an exploratory research on the cpacity and cyclability of LiMn2o4 cathodes simultaneously doped with Cr3+ and Mg2+ arereported
Electrochemical behaviour of LiMyMn2âyO4 (M = Cu, Cr; 0 <= y <= 0.4)
Spinel lithium manganese oxide, LiMn2O4, is beset with problems of capacity fade upon repeated
cycling. The loss in capacity upon cycling is attributable to JahnâTeller distortion and manganese dissolution in
the electrolyte in the charged state. One way to circumvent this capacity fade is to introduce other 3d transition
metal ions in the LiMn2O4 lattice. In this paper, we report on the effect of partial substitution of manganese in
the LiMn2O4 phase with copper (II) and chromium (III) ions. It has been shown that the higher octahedral
stabilization energy of trivalent chromium imparts greater structural stability to chromium-doped LiMn2O4
spinels. Both copper and chromium reduce the capacity of the spinel in the 4 V region. In terms of its good
reversible capacity and ability to sustain cycling with minimal capacity fade, LiCr0Ă1Mn1Ă9O4 may be considered
as a potential cathode material for lithium rechargeable cells
Iron doped lithium cobalt oxides as lithium intercalating cathode materials
Layered transition metal oxides of the formula LiMO 2 have good lithium insertion
properties for which reason LiCoO 2 and LiNiO2 have been exploited in practical lithium rocking
chair batteries. Another member of the LiMO2 series, LiFeO2, should be an attractive cathode
material considering the cheapness and environment-friendliness of iron compounds. Its rock-salt
structure, however, does not allow significant amounts of lithium to be reversibly intercalated in
its structure. Synthesis of layered LiFeO2 and study of its lithium intercalating properties have
been of limited success. Therefore, an attempt has been made here to study LiCol_~FeyO 2 solid
solutions (0 < y -< 0.4) as prospective cathode materials. XRD, FTIR, Atomic absorption
spectroscopy, Particle size and Surface area analysis were carried out in this regard towards the
physical characterization of the entire series of LiCo~_yFeyO 2 compounds. The electrochemical
discharge capacity of these materials is explained as a function of the iron content