4 research outputs found
Incorporation of Lithium by MgH<sub>2</sub>: An Ab Initio Study
The incorporation of lithium by MgH<sub>2</sub> through an electrochemical
conversion reaction is a valuable alternative to Li intercalation
into graphite for next-generation Li-ion cells. The incorporation
of lithium occurs by the reduction of magnesium hydride to magnesium
metal nanoparticles surrounded by an amorphous matrix of lithium hydride.
In this study we present a computational investigation of the conversion
reaction of MgH<sub>2</sub> to give Mg and LiH by first-principles
methods. Density functional theory calculations have been performed
using plane waves and projector augmented wave (PAW) pseudopotentials
within the generalized-gradient approximation by the VASP code. The
existence of intermediate phases has been checked by finite temperature
ab initio thermodynamic calculations. Also, the occurrence of solid
solutions in the first stages of lithium incorporation has been studied
by the supercell approach by predicting their thermodynamic stability
at 0 K. Five different solid solutions have been mimed by forming
0D defects in the MgH<sub>2</sub> lattice: (a) vacancies in the hydride
sites; (b) interstitial lithium insertion; (c) substitution of lithium
in hydride sites; (d) substitution of lithium in magnesium sites;
(e) substitution of lithium in magnesium sites with parallel formation
of vacancies in the hydride sites. Preliminary results about the thermodynamics
of the conversion reaction in nanometric MgH<sub>2</sub> clusters
are also discussed
Origin of the Voltage Hysteresis of MgH<sub>2</sub> Electrodes in Lithium Batteries
Magnesium
hydride has been proposed as innovative anode material
for Li ion cells due to its large theoretical capacity and high-energy
efficiency compared to other conversion materials. In this work, we
report a combined experimental-theoretical study about the origin
of voltage hysteresis in the conversion reaction of MgH<sub>2</sub> in lithium cells. Experimentally, the extent of the thermodynamic
voltage hysteresis in the first galvanostatic dischargeācharge
cycle has been determined by the GITT technique and decoupled from
the kinetic overpotentials. Theoretically, the origin of the thermodynamic
voltage hysteresis has been evaluated and studied by means density
functional theory calculations within the supercell approach. Different
elementary reactions have been modeled upon reduction and oxidation
on the surfaces of the active phases (i.e., MgH<sub>2</sub>, LiH,
and Mg), and the associated theoretical voltages have been predicted.
Experimental and theoretical results have been compared and discussed
to draw a comprehensive description of the elementary surface reactions
of the MgH<sub>2</sub> conversion in lithium cells
Origin of the Voltage Hysteresis of MgH<sub>2</sub> Electrodes in Lithium Batteries
Magnesium
hydride has been proposed as innovative anode material
for Li ion cells due to its large theoretical capacity and high-energy
efficiency compared to other conversion materials. In this work, we
report a combined experimental-theoretical study about the origin
of voltage hysteresis in the conversion reaction of MgH<sub>2</sub> in lithium cells. Experimentally, the extent of the thermodynamic
voltage hysteresis in the first galvanostatic dischargeācharge
cycle has been determined by the GITT technique and decoupled from
the kinetic overpotentials. Theoretically, the origin of the thermodynamic
voltage hysteresis has been evaluated and studied by means density
functional theory calculations within the supercell approach. Different
elementary reactions have been modeled upon reduction and oxidation
on the surfaces of the active phases (i.e., MgH<sub>2</sub>, LiH,
and Mg), and the associated theoretical voltages have been predicted.
Experimental and theoretical results have been compared and discussed
to draw a comprehensive description of the elementary surface reactions
of the MgH<sub>2</sub> conversion in lithium cells
Sulfonated Graphene Oxide Platelets in Nafion Nanocomposite Membrane: Advantages for Application in Direct Methanol Fuel Cells
Graphene oxide (GO) is well known
as an excellent amphiphilic material due to its oxygen-containing
functional groups and its chemical tunability. By intercalation chemistry,
organo-modified GO containing sulfonilic terminal groups were prepared
and used as nanoadditive in Nafion polymer for the creation of hybrid
exfoliated composites. The incorporation of hydrophilic 2D platelike
layers in the Nafion membranes is expected to induce advantages in
terms of thermal stability and mechanical and barrier properties (limitation
of the methanol crossover by increased tortuosity and obstruction
effect), although it may negatively affect the proton conductivity.
In this work, we show how different preparation methods of the nanocomposites
influence morphology, transport properties, and barrier effect to
methanol. The hybrid membranes are characterized by powder X-ray diffraction
and microscopies (SEM, TEM, and AFM). Water and methanol transport
properties inside the nanocomposites are investigated by NMR spectroscopy
(diffusivity and relaxation times), unveiling a reduction of the methanol
diffusion and, nevertheless, an increase in the proton mobility and
water retention at high temperatures. Finally, the electrochemical
properties are investigated by direct methanol fuel cell (DMFC) tests,
showing a significant reduction of the ohmic losses at high temperatures,
extending in this way the operating range of a DMFC