Pseudocapacitive contributions to charge storage in highly ordered mesoporous group V transition metal oxides with iso-oriented layered nanocrystalline domains

In situ carbonization of a soft-template to directly synthesize crystalline mesoporous metal oxides with high surface areas

High-surface-area crystalline mesoporous tantalum oxide has been successfully synthesized using a pluronic tri-block polymer as the template.

Facile control of long range orientation in mesoporous carbon films with thermal zone annealing velocity

Ordered mesoporous carbons exhibit appealing properties for many applications, but their function and performance can depend critically on their structure. The in-plane orientation of 2D cylinders from the cooperative assembly of Pluronic P123 and resol has been controlled by application of cold zone annealing (CZA). By varying the moving rate, the preferential in-plane orientation of the self-assembled cylinders can be tuned through the entire 180° range possible from ϕ = 50° to ϕ = -130° (relative to the moving direction). At a moving rate of 2 μm s(-1), this simple and easy CZA process leads to cylinders that are well aligned parallel to the moving direction with a high orientational factor of S = 0.98. Moreover, the in-plane oriented cylinders can be nearly perfectly aligned transverse to the moving direction (S = 0.95) by simply decreasing the moving velocity to 0.5 μm s(-1). We attribute the parallel alignment to the flow that develops from the motion of the thermal gradients, while the transverse alignment is related to flow cessation (inertial effect). The preferential orientation is retained through the carbonization process, but there is some degradation in orientation due to insufficient crosslinking of the resol during CZA; this effect is most prominent for the higher moving rates (less time for crosslinking), but can be overcome by post-CZA annealing at uniform elevated temperatures to further crosslink the resol. CZA is a simple and powerful method for fabricating well-aligned and self-assembled mesoporous carbon films over large areas.

High-rate electrochemical energy storage through Li+ intercalation pseudocapacitance

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.

Enhancing pseudocapacitive charge storage in polymer templated mesoporous materials

Growing global energy demands coupled with environmental concerns have increased the need for renewable energy sources. For intermittent renewable sources like solar and wind to become available on demand will require the use of energy storage devices. Batteries and supercapacitors, also known as electrochemical capacitors (ECs), represent the most widely used energy storage devices. Supercapacitors are frequently overlooked as an energy storage technology, however, despite the fact that these devices provide greater power, much faster response times, and longer cycle life than batteries. Their limitation is that the energy density of ECs is significantly lower than that of batteries, and this has limited their potential applications. This Account reviews our recent work on improving pseudocapacitive energy storage performance by tailoring the electrode architecture. We report our studies of mesoporous transition metal oxide architectures that store charge through surface or near-surface redox reactions, a phenomenon termed pseudocapacitance. The faradaic nature of pseudocapacitance leads to significant increases in energy density and thus represents an exciting future direction for ECs. We show that both the choice of material and electrode architecture is important for producing the ideal pseudocapacitor device. Here we first briefly review the current state of electrode architectures for pseudocapacitors, from slurry electrodes to carbon/metal oxide composites. We then describe the synthesis of mesoporous films made with amphiphilic diblock copolymer templating agents, specifically those optimized for pseudocapacitive charge storage. These include films synthesized from nanoparticle building blocks and films made from traditional battery materials. In the case of more traditional battery materials, we focus on using flexible architectures to minimize the strain associated with lithium intercalation, that is, the accumulation of lithium ions or atoms between the layers of cathode or anode materials that occurs as batteries charge and discharge. Electrochemical analysis of these mesoporous films allows for a detailed understanding of the origin of charge storage by separating capacitive contributions from traditional diffusion-controlled intercalation processes. We also discuss methods to separate the two contributions to capacitance: double-layer capacitance and pseudocapacitance. Understanding these contributions should allow the selection of materials with an optimized architecture that maximize the contribution from pseudocapacitance. From our studies, we show that nanocrystal-based nanoporous materials offer an architecture optimized for high levels of redox or surface pseudocapacitance. Interestingly, in some cases, materials engineered to minimize the strain associated with lithium insertion can also show intercalation pseudocapacitance, which is a process where insertion processes become so kinetically facile that they appear capacitive. Finally, we conclude with a summary of simple design rules that should result in high-power, high-energy-density electrode architectures. These design rules include assembling small, nanosized building blocks to maximize electrode surface area; maintaining an interconnected, open mesoporosity to facilitate solvent diffusion; seeking flexibility in electrode structure to facilitate volume expansion during lithium insertion; optimizing crystalline domain size and orientation; and creating effective electron transport pathways.

General method for the synthesis of hierarchical nanocrystal-based mesoporous materials

Block copolymer templating of inorganic materials is a robust method for the production of nanoporous materials. The method is limited, however, by the fact that the molecular inorganic precursors commonly used generally form amorphous porous materials that often cannot be crystallized with retention of porosity. To overcome this issue, here we present a general method for the production of templated mesoporous materials from preformed nanocrystal building blocks. The work takes advantage of recent synthetic advances that allow organic ligands to be stripped off of the surface of nanocrystals to produce soluble, charge-stabilized colloids. Nanocrystals then undergo evaporation-induced co-assembly with amphiphilic diblock copolymers to form a nanostructured inorganic/organic composite. Thermal degradation of the polymer template results in nanocrystal-based mesoporous materials. Here, we show that this method can be applied to nanocrystals with a broad range of compositions and sizes, and that assembly of nanocrystals can be carried out using a broad family of polymer templates. The resultant materials show disordered but homogeneous mesoporosity that can be tuned through the choice of template. The materials also show significant microporosity, formed by the agglomerated nanocrystals, and this porosity can be tuned by the nanocrystal size. We demonstrate through careful selection of the synthetic components that specifically designed nanostructured materials can be constructed. Because of the combination of open and interconnected porosity, high surface area, and compositional tunability, these materials are likely to find uses in a broad range of applications. For example, enhanced charge storage kinetics in nanoporous Mn(3)O(4) is demonstrated here.

Ordered mesoporous metal oxides: synthesis and applications

Great progress has been made in the preparation and application of ordered mesoporous metal oxides during the past decade. However, the applications of these novel and interesting materials have not been reviewed comprehensively in the literature. In the current review we first describe different methods for the preparation of ordered mesoporous metal oxides; we then review their applications in energy conversion and storage, catalysis, sensing, adsorption and separation. The correlations between the textural properties of ordered mesoporous metal oxides and their specific performance are highlighted in different examples, including the rate of Li intercalation, sensing, and the magnetic properties. These results demonstrate that the mesoporosity has a direct impact on the properties and potential applications of such materials. Although the scope of the current review is limited to ordered mesoporous metal oxides, we believe that the information may be useful for those working in a number of fields.

Mesoporous manganese oxide nanowires for high-capacity, high-rate, hybrid electrical energy storage

Arrays of mesoporous manganese dioxide, mp-MnO(2), nanowires were electrodeposited on glass and silicon surfaces using the lithographically patterned nanowire electrodeposition (LPNE) method. The electrodeposition procedure involved the application, in a Mn(ClO(4))(2)-containing aqueous electrolyte, of a sequence of 0.60 V (vs MSE) voltage pulses delineated by 25 s rest intervals. This "multipulse" deposition program produced mp-MnO(2) nanowires with a total porosity of 43-56%. Transmission electron microscopy revealed the presence within these nanowires of a network of 3-5 nm diameter fibrils that were X-ray and electron amorphous, consistent with the measured porosity values. mp-MnO(2) nanowires were rectangular in cross-section with adjustable height, ranging from 21 to 63 nm, and adjustable width ranging from 200 to 600 nm. Arrays of 20 nm × 400 nm mp-MnO(2) nanowires were characterized by a specific capacitance, C(sp), of 923 ± 24 F/g at 5 mV/s and 484 ± 15 F/g at 100 mV/s. These C(sp) values reflected true hybrid electrical energy storage with significant contributions from double-layer capacitance and noninsertion pseudocapacitance (38% for 20 nm × 400 nm nanowires at 5 mV/s) coupled with a Faradaic insertion capacity (62%). These two contributions to the total C(sp) were deconvoluted as a function of the potential scan rate.

Ordered mesoporous α-Fe2O3 (hematite) thin-film electrodes for application in high rate rechargeable lithium batteries

Herein is reported the synthesis of ordered mesoporous α-Fe(2)O(3) thin films produced through coassembly strategies using a poly(ethylene-co-butylene)-block-poly(ethylene oxide) diblock copolymer as the structure-directing agent and hydrated ferric nitrate as the molecular precursor. The sol-gel derived α-Fe(2)O(3) materials are highly crystalline after removal of the organic template and the nanoscale porosity can be retained up to annealing temperatures of 600 °C. While this paper focuses on the characterization of these materials using various state-of-the-art techniques, including grazing-incidence small-angle X-ray scattering, time-of-flight secondary ion mass spectrometry, X-ray photoelectron spectroscopy, and UV-vis and Raman spectroscopy, the electrochemical properties are also examined and it is demonstrated that mesoporous α-Fe(2)O(3) thin-film electrodes not only exhibit enhanced lithium-ion storage capabilities compared to bulk materials but also show excellent cycling stabilities by suppressing the irreversible phase transformations that are observed in microcrystalline α-Fe(2)O(3).