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.

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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.