Pseudocapacitance is commonly associated with surface or near-surface reversible redox reactions, as observed with RuO2·xH2O in an acidic electrolyte. However, we recently demonstrated that a pseudocapacitive mechanism occurs when lithium ions are inserted into mesoporous and nanocrystal films of orthorhombic Nb2O5 (T-Nb2O5; refs 1,2). Here, we quantify the kinetics of charge storage in T-Nb2O5: currents that vary inversely with time, charge-storage capacity that is mostly independent of rate, and redox peaks that exhibit small voltage offsets even at high rates. We also define the structural characteristics necessary for this process, termed intercalation pseudocapacitance, which are a crystalline network that offers two-dimensional transport pathways and little structural change on intercalation. The principal benefit realized from intercalation pseudocapacitance is that high levels of charge storage are achieved within short periods of time because there are no limitations from solid-state diffusion. Thick electrodes (up to 40 μm thick) prepared with T-Nb2O5 offer the promise of exploiting intercalation pseudocapacitance to obtain high-rate charge-storage devices.

A sulfur/carbon composite has been prepared to serve as a cathode for lithium/sulfur batteries. The effects of seven different liquid electrolytes on the electrochemical performance were investigated using galvanostatic discharge–charge tests on coin cells. The electrolytes included ether, sulfone, and carbonate solvents with common lithium salts. It was found that the solvent plays a key role on the electrochemical performance of the lithium/sulfur battery cathode while the lithium salt has no significant effects. Additional characterization, using in situ sulfur K-edge X-ray absorption spectroscopy (XAS), provided insights into the soluble sulfur species in the discharged and charged batteries. We find that the use of low-viscosity ethereal solvents results in a more complete reduction of soluble polysulfides, while soluble polysulfides remained more oxidized in viscous ethereal solvents. Moreover, XAS revealed that reduced sulfur species chemically react with carbonate-based solvents, making this class of solvents inappropriate for elemental sulfur cathodes of lithium batteries.

Mn3O4 has been investigated as a high-capacity anode material for rechargeable lithium ion batteries. Spongelike nanosized Mn3O4 was synthesized by a simple precipitation method and characterized by powder X-ray diffraction, Raman scattering and scanning electron microscopy. Its electrochemical performance, as an anode material, was evaluated by galvanostatic discharge–charge tests. The results indicate that this novel type of nanosized Mn3O4 exhibits a high initial reversible capacity (869 mA h/g) and significantly enhanced first Coulomb efficiency with a stabilized reversible capacity of around 800 mA h/g after over 40 charge/discharge cycles.

We report on the fabrication and measurement of devices designed to study the electrochemical behavior of individual monolayer graphene sheets as electrodes. We have examined both mechanically exfoliated and chemical vapor deposited (CVD) graphene. The effective device areas, determined from cyclic voltammetric measurements, show good agreement with the geometric area of the graphene sheets, indicating that the redox reactions occur on clean graphene surfaces. The electron transfer rates of ferrocenemethanol at both types of graphene electrodes were found to be more than 10-fold faster than at the basal plane of bulk graphite, which we ascribe to corrugations in the graphene sheets. We further describe an electrochemical investigation of adsorptive phenomena on graphene surfaces. Our results show that electrochemistry can provide a powerful means of investigating the interactions between molecules and the surfaces of graphene sheets as electrodes.

The pursuit of energy storage systems with high energy density has revealed several exciting possibilities, including the unexpectedly reversible conversion reactions between metal oxides and lithium for lithium ion battery anodes. The mechanistic complexity of the drastic chemical and structural changes as well as the sensitivity of the reaction intermediates and products to ambient conditions mean that the reaction mechanism is best studied by non-destructive techniques in the native battery environment (in operando). This work applies synchrotron X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) to directly observe the conversion reaction of a Mn 3 O 4 anode previously shown to have promising electrochemical performance. The results enable the assignment of electrochemical features to specific reactions, including the formation of LiMn 3 O 4 , MnO, metallic Mn, and non-metal-centered reactions, and elucidate the difference between the first and subsequent lithiation reactions. In operando XAS clearly shows that a significant fraction of the charge is stored in non-Mn-centered reactions, a result with serious implications for Mn 3 O 4 , in particular, and other metal oxide conversion anodes, in general.This study emphasizes the importance of in situ/in operando studies on next-generation electrode materials to confirm that the observed charge transfer is due to the desired electrochemical reactions.

The lithium-sulfur battery is an extremely attractive system for electrical energy storage because of its exceptional theoretical capacity and energy density. However, the practical values typically obtained are much lower and inherently determined by the complex chemistry of reduced sulfur species. The lack of methods to probe sulfur species under realistic battery conditions has frustrated chemical understanding and control. We have employed in situ X-ray diffraction (XRD) and sulfur K-edge X-ray absorption near edge spectroscopy (XANES) to probe the sulfur intermediates and products formed in battery electrodes during operation of prototype lithium-sulfur batteries. Correlations between the X-ray and electrochemical data show that the reduction of sulfur to lithium sulfide is mediated through dissociation and disproportionation reactions of a few dominant sulfur species. Deliberate control of these chemical equilibria is essential to approach the theoretical capacity of the lithium-sulfur system.

Replacing platinum by a less precious metal such as palladium, is highly desirable for lowering the cost of fuel-cell electrocatalysts. However, the instability of palladium in the harsh environment of fuel-cell cathodes renders its commercial future bleak. Here we show that by incorporating trace amounts of gold in palladium-based ternary (Pd6CoCu) nanocatalysts, the durability of the catalysts improves markedly. Using aberration-corrected analytical transmission electron microscopy in conjunction with synchrotron X-ray absorption spectroscopy, we show that gold not only galvanically replaces cobalt and copper on the surface, but also penetrates through the Pd–Co–Cu lattice and distributes uniformly within the particles. The uniform incorporation of Au provides a stability boost to the entire host particle, from the surface to the interior. The spontaneous replacement method we have developed is scalable and commercially viable. This work may provide new insight for the large-scale production of non-platinum electrocatalysts for fuel-cell applications.

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