In the present study,
an in-depth investigation on the structural
transformation in a mesoporous γ-MnO2 cathode during
electrochemical reaction in a zinc-ion battery (ZIB) has been undertaken.
A combination of in situ Synchrotron XANES and XRD studies reveal
that the tunnel-type parent γ-MnO2 undergoes a structural
transformation to spinel-type Mn(III) phase (ZnMn2O4) and two new intermediary Mn(II) phases, namely, tunnel-type
γ-Zn
x
MnO2 and layered-type
L-Zn
y
MnO2, and that these phases
with multioxidation states coexist after complete electrochemical
Zn-insertion. On successive Zn-deinsertion/extraction, a majority
of these phases with multioxidation states is observed to revert back
to the parent γ-MnO2 phase. The mesoporous γ-MnO2 cathode, prepared by a simple ambient temperature strategy
followed by low-temperature annealing at 200 °C, delivers an
initial discharge capacity of 285 mAh g–1 at 0.05
mA cm–2 with a defined plateau at around 1.25 V
vs Zn/Zn2+. Ex situ HR-TEM studies of the discharged electrode
aided to identify the lattice fringe widths corresponding to the Mn(III)
and Mn(II) phases, and the stoichiometric composition estimated by
ICP analysis appears to be concordant with the in situ findings. Ex
situ XRD studies also confirmed that the same electrochemical reaction
occurred on repeated discharge/charge cycling. Moreover, the present
synthetic strategy offers solutions for developing cost-effective
and environmentally safe nanostructured porous electrodes for cheap
and eco-friendly batteries.
Highly active and stable electrocatalysts for hydrogen evolution have been developed on the basis of molybdenum compounds (Mo2C, Mo2N, and MoS2) on carbon nanotube (CNT)-graphene hybrid support via a modified urea-glass route. By a simple modification of synthetic variables, the final phases are easily controlled from carbide, nitride to sulfide with homogeneous dispersion of nanocrystals on the CNT-graphene support. Among the prepared catalysts, Mo2C/CNT-graphene shows the highest activity for hydrogen evolution reaction with a small onset overpotential of 62 mV and Tafel slope of 58 mV/dec as well as an excellent stability in acid media. Such enhanced catalytic activity may originate from its low hydrogen binding energy and high conductivity. Moreover, the CNT-graphene hybrid support plays crucial roles to enhance the activity of molybdenum compounds by alleviating aggregation of the nanocrystals, providing a large area to contact with electrolyte, and facilitating the electron transfer.
Large-scale carbon fixation requires high-volume chemicals production from carbon dioxide. Dry reforming of methane could provide an economically feasible route if coke- and sintering-resistant catalysts were developed. Here, we report a molybdenum-doped nickel nanocatalyst that is stabilized at the edges of a single-crystalline magnesium oxide (MgO) support and show quantitative production of synthesis gas from dry reforming of methane. The catalyst runs more than 850 hours of continuous operation under 60 liters per unit mass of catalyst per hour reactive gas flow with no detectable coking. Synchrotron studies also show no sintering and reveal that during activation, 2.9 nanometers as synthesized crystallites move to combine into stable 17-nanometer grains at the edges of MgO crystals above the Tammann temperature. Our findings enable an industrially and economically viable path for carbon reclamation, and the “Nanocatalysts On Single Crystal Edges” technique could lead to stable catalyst designs for many challenging reactions.
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