3D porous nanostructures built from 2D δ-MnO2 nanosheets are an environmentally friendly and industrially scalable class of supercapacitor electrode material. While both the electrochemistry and defects of this material have been studied, the role of defects in improving the energy storage density of these materials has not been addressed. In this work, δ-MnO2 nanosheet assemblies with 150 m2 g−1 specific surface area are prepared by exfoliation of crystalline KxMnO2 and subsequent reassembly. Equilibration at different pH introduces intentional Mn vacancies into the nanosheets, increasing pseudocapacitance to over 300 F g−1, reducing charge transfer resistance as low as 3 Ω, and providing a 50% improvement in cycling stability. X-ray absorption spectroscopy and high-energy X-ray scattering demonstrate a correlation between the defect content and the improved electrochemical performance. The results show that Mn vacancies provide ion intercalation sites which concurrently improve specific capacitance, charge transfer resistance and cycling stability.
Identifying
the catalytically active site(s) in the oxygen reduction
reaction (ORR), under real-time electrochemical conditions, is critical
to the development of fuel cells and other technologies. We have employed in situ synchrotron-based X-ray absorption spectroscopy
(XAS) to investigate the synergistic interaction of a Co–Mn
oxide catalyst which exhibits impressive ORR activity in alkaline
fuel cells. X-ray absorption near edge structure (XANES) was used
to track the dynamic structural changes of Co and Mn under both steady
state (constant applied potential) and nonsteady state (potentiodynamic
cyclic voltammetry, CV). Under steady state conditions, both Mn and
Co valences decreased at lower potentials, indicating the conversion
from Mn(III,IV) and Co(III) to Mn(II,III) and Co(II), respectively.
Rapid X-ray data acquisition, combined with a slow sweep rate in CV,
enabled a 3 mV resolution in the applied potential, approaching a
nonsteady (potentiodynamic) state. Changes in the Co and Mn valence
states were simultaneous and exhibited periodic patterns that tracked
the cyclic potential sweeps. To the best of our knowledge, this represents
the first study, using in situ XAS, to resolve the
synergistic catalytic mechanism of a bimetallic oxide. Strategies
developed/described herein can provide a promising approach to unveil
the reaction mechanism for other multimetallic electrocatalysts.
For Li–S batteries, operando X-ray diffraction and X-ray microscopy are combined to visualize the evolution of both the morphology and crystal structure of the materials during the cycling.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.