Energy Materials

A recently introduced, novel electrical energy storage concept, the electrochemical flow capacitor (EFC), holds much promise for grid-scale energy storage applications. The EFC combines the principles behind the operation of flow batteries and supercapacitors, and enables rapid charging/discharging and decoupled energy/power ratings. Electrical charge is stored in a flowable carbon slurry composed of low-cost and abundantly available carbon particles in pH-neutral, aqueous electrolyte. Charge storage and transfer is analogous to solid carbon electrodes in conventional supercapacitors. Here, the effects of carbon particle solid fraction, shape, and size on the electrochemical and rheological properties of slurry electrodes are investigated. A static cell configuration is utilized for studying the electrochemical properties of the flowable electrodes. The electrochemical properties of the slurry electrodes tested in a static cell are found to be similar to that of solid electrodes in conventional supercapacitors for both, large spherical and anisometric activated carbons. Flow properties of the slurry electrodes are obtained for shear rates corresponding to pumping shear rates by rheometry. Results indicate that electrochemical and rheological properties of slurries depend on their concentration, shape and size of the carbon particles used in the slurries. For a range of concentrations, slurries based on spherical carbon particles show lower viscosities compared to anisometric activated carbon based slurries while performing similar electrochemically.

Owing to recent power- and energy-density advances, higher efficiencies, and almost unlimited lifetimes, electrical double-layer capacitors (EDLCs, also known as supercapacitors) are now used in a wide range of energy harvesting and storage systems, which include portable power and grid applications. Despite offering key performance advantages, many device components pose significant environmental hazards once disposed. They often contain fluorine, sulfur, and cyanide groups, which are harmful if discarded by using conventional landfill or incineration methods, and they are constructed by using multiple metallic parts, which contribute to a high ash content. We explore designs for a fully operational supercapacitor that incorporates materials completely safe to dispose of and easy to incinerate. The components, which include material alternatives for the current collector, electrolyte, separator, particle binder, and packaging, are all mutually compatible, and most of them exhibit better performance than commonly used materials. We selected a graphite foil as current collector, sodium acetate as electrolyte, an ester as porous membrane based on acetate cellulose, and polymers based on polyvinyl alcohol as environmentally benign solutions for device components. The presented materials all originate from simple and inexpensive source compounds, which decreases the environmental impact of their manufacture and renders them more viable for integration into commercial devices for large-scale stationary and transportation energy storage applications.

Molecular dynamics (MD) simulations of supercapacitors with single-walled carbon nanotube (SWCNT) electrodes in room-temperature ionic liquids were performed to investigate the influences of the applied electrical potential, the radius/curvature of SWCNTs, and temperature on their capacitive behavior. It is found that (1) SWCNTs-based supercapacitors exhibit a near-flat capacitance-potential curve, (2) the capacitance increases as the tube radius decreases, and (3) the capacitance depends little on the temperature. We report the first MD study showing the influence of the electrode curvature on the capacitance-potential curve and negligible dependence of temperature on capacitance of tubular electrode. The latter is in good agreement with recent experimental findings and is attributed to the similarity of the electrical double layer (EDL) microstructure with temperature varying from 260 to 400 K. The electrode curvature effect is explained by the dominance of charge overscreening and increased ion density per unit area of electrode surface.

The performance of supercapacitors is determined by the electrical double layers (EDLs) formed at electrolyte/electrode interfaces. To understand the energy storage mechanism underlying supercapacitors, molecular dynamics (MD) simulations were used to study the capacitive behavior of carbon-based supercapacitors with room-temperature ionic liquid (RTIL) electrolytes. The performance of porous supercapacitors was found to be correlated with the ion/pore size and applied voltage. Supercapacitors composed of RTILs on the outer, positively curved surfaces of onion-like carbons (OLCs) or carbon nanotubes (CNTs) exhibited significant effects on capacitance and the distinctive feature that differential capacitance varies only weakly with voltage. Investigations of temperature influence revealed a positive temperature dependence of capacitance for OLC-based supercapacitors and a weak dependence of capacitance on temperature for CNT-based supercapacitors, in line with experimental observations. Molecular insights into RTIL-based supercapacitors, reviewed in this Perspective, could facilitate the design and development of a new generation of energy storage devices.

A detailed understanding of ion adsorption within porous carbon is key to the design and improvement of electric double-layer capacitors, more commonly known as supercapacitors. In this work nuclear magnetic resonance (NMR) spectroscopy is used to study ion adsorption in porous carbide-derived carbons. These predominantly microporous materials have a tuneable pore size which enables a systematic study of the effect of pore size on ion adsorption. Multinuclear NMR experiments performed on the electrolyte anions and cations reveal two main environments inside the carbon. In-pore ions (observed at low frequencies) are adsorbed inside the pores, whilst ex-pore ions (observed at higher frequencies) are not adsorbed and are in large reservoirs of electrolyte between carbon particles. All our experiments were carried out in the absence of an applied electrical potential in order to assess the mechanisms related to ion adsorption without the contribution of electrosorption. Our results indicate similar adsorption behaviour for anions and cations. Furthermore, we probe the effect of sample orientation, which is shown to have a marked effect on the NMR spectra. Finally, we show that a 13C -> 1H cross polarisation experiment enables magnetisation transfer from the carbon architecture to the adsorbed species, allowing selective observation of the adsorbed ions and confirming our spectral assignments.

In situ electrochemical dilatometry was used to study, for the first time, the expansion behavior of a porous carbon electrode in a pure ionic liquid, 1-ethyl-3-methyl-imidazolium-tetrafluoroborate. For a single electrode, an applied potential of −2 V and +2 V against the potential of zero charge resulted in maximum strains of 1.8% and 0.5%, respectively. During cyclic voltammetry, the characteristic expansion behavior strongly depends on the scan rate, with increased scan rates leading to a decrease of the expansion. Chronoamperometry was used to determine the equilibrium specific capacitance and expansion. The obtained strain versus accumulated charge relationship can be fitted with a simple quadratic function. Cathodic and anodic expansion data collapse on one parabola when normalizing the surface charge by the ratio of ion volume and average pore size. There is also a transient spike in the height change when polarity is switched from positive to negative that is not observed when changing the potential from negative to positive indicating the size and the shape of the ion is influencing the expansion behavior.

The adsorption of molecules in a confined environment (pores and narrow channels) differs from adsorption on flat surfaces. While the immobilization of proteins on porous carbon and the transport of protein molecules through carbon nanotube channels are of great practical importance, the interaction of proteins with the carbon surface in confinement is poorly understood. In this study the adsorption of bovine serum albumin (BSA) and tumor necrosis factor-α (TNF-α) was studied in carbon nanotubes grown by chemical vapor deposition in cylindrical pores of anodic alumina membranes. BSA adsorption depends on the channel diameter of the carbon nanotubes, the termination of nanotube surfaces (surface chemistry), and the pH of medium. Amination of the carbon surface leads to increased adsorption of the proteins at neutral pH, while oxidation decreases the sorption capacity. The differences have been explained by favorable or unfavorable electrostatic interactions between protein molecules and the carbon surface.

LiFePO4 is one of most promising cathode materials for lithium-ion batteries (LIB) due to its superior rate handling ability, reduced cost, low environmental hazards, and safe long-term cyclability. Application of electrochemical quartz crystal admittance (EQCA) method to LIB electrodes provides direct access to potential-driven shifts of frequency (∆fexp) and width (∆Γ) of the resonance peaks simultaneously with the charge due to Li-ions insertion/extraction. In addition to conventional monitoring of mass changes in the electrode coating, the parameters ∆fexp and ∆Γ reflect via hydrodynamic solid-liquid interactions, in-situ mechano-structural changes in the composite electrodes occurring during the operation of a LIB. Applying the model that takes into account such interactions, potential-induced changes of the effective thickness and permeability of the composite electrode have been determined which are evident of non-uniform deformation of the electrode coating caused by ions insertion/extraction process. Using EQCA as a unique mechanical probe of the insertion-type electrodes, the dynamic effect of the local host environment on the foreign Na+-ions insertion/extraction has been studied in mixed solution of Li and Na salts. As a highly reliable and quantitative tool, EQCA methodology may provide surprisingly wide scope for further investigations resulting in a broader understanding of coupled electrochemical and mechanical events in LIB during their long-term operation. This includes information about the distortion/deformation of the electrode intercalation particles and the entire composite electrode under polarization, and is able to clarify the role of polymeric binder in the composite electrodes as the factor stabilizing long-term cyclability of Li-ions batteries.

Microporous carbon materials are widely used in gas storage, sorbents, supercapacitor electrodes, water desalination, and catalyst supports. While these microporous carbons usually have a particle size in the 1-100 μm range, here the synthesis of porous carbide-derived carbon (CDC) with particle diameters around 30 nm by extraction of titanium from nanometer-sized titanium carbide (TiC) powder at temperatures of 200 °C and above is reported. Nanometer-sized CDCs prepared at 200-400 °C show a disordered structure and the presence of CN sp1 bonds. Above 400 °C, the CN bond disappears with the structure transition to disordered carbon similar to that observed after synthesis from carbide micropowders. Compared to CDCs produced from micrometer-sized TiC, nano-CDC has a broader pore size distribution due to interparticle porosity and a large contribution from the surface layers. The material shows excellent electrochemical performance due to its easily accessible pores and a large specific surface area.