Energie-Materialien

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.

Desalination by capacitive deionization (CDI) is an emerging technology for the energy- and cost-efficient removal of ions from water by electrosorption in charged porous carbon electrodes. A variety of carbon materials, including activated carbons, templated carbons, carbon aerogels, and carbon nanotubes, have been studied as electrode materials for CDI. Using carbide-derived carbons (CDCs) with precisely tailored pore size distributions (PSD) of micro- and mesopores, we studied experimentally and theoretically the effect of pore architecture on salt electrosorption capacity and salt removal rate. Of the reported CDC-materials, ordered mesoporous silicon carbide-derived carbon (OM SiC-CDC), with a bimodal distribution of pore sizes at 1 and 4 nm, shows the highest salt electrosorption capacity per unit mass, namely 15.0 mg of NaCl per 1 g of porous carbon in both electrodes at a cell voltage of 1.2 V (12.8 mg per 1 g of total electrode mass). We present a method to quantify the influence of each pore size increment on desalination performance in CDI by correlating the PSD with desalination performance. We obtain a high correlation when assuming the ion adsorption capacity to increase sharply for pore sizes below one nanometer, in line with previous observations for CDI and for electrical double layer capacitors, but in contrast to the commonly held view about CDI that mesopores are required to avoid electrical double layer overlap. To quantify the dynamics of CDI, we develop a two-dimensional porous electrode modified Donnan model. For two of the tested materials, both containing a fair degree of mesopores (while the total electrode porosity is [similar]95 vol%), the model describes data for the accumulation rate of charge (current) and salt accumulation very well, and also accurately reproduces the effect of an increase in electrode thickness. However, for TiC-CDC with hardly any mesopores, and with a lower total porosity, the current is underestimated. Calculation results show that a material with higher electrode porosity is not necessarily responding faster, as more porosity also implies longer transport pathways across the electrode. Our work highlights that a direct prediction of CDI performance both for equilibrium and dynamics can be achieved based on the PSD and knowledge of the geometrical structure of the electrodes.

Porous carbon electrodes have significant potential for energy-efficient water desalination using a promising technology called Capacitive Deionization (CDI). In CDI, salt ions are removed from brackish water upon applying an electrical voltage difference between two porous electrodes, in which the ions will be temporarily immobilized. These electrodes are made of porous carbons optimized for salt storage capacity and ion and electron transport. We review the science and technology of CDI and describe the range of possible electrode materials and the various approaches to the testing of materials and devices. We summarize the range of options for CDI-designs and possible operational modes, and describe the various theoretical–conceptual approaches to understand the phenomenon of CDI.

In a recent study, Wimalasiri and Zou [1] have reported the use and performance of composite electrodes of carbon nanotubes (CNT) and graphene for application as porous electrodes in capacitive deionization (CDI). While CDI is emerging as an attractive technology for water desalination, and novel electrode materials and composites are important contributions to the advancement of the field, there are several issues in this study that we must comment on.

A novel approach to tracking intercalation-induced phase transitions in Li-ion battery materials demonstrated herein consists of simultaneous analysis of intercalation charge and the accompanying mechanical (geometric) changes in a microarray electrode composed of LixFePO4 intercalation particles probed by the electrochemical quartz-crystal admittance (EQCA) method. A recently elaborated approach to population dynamics of active (phase-transforming) nanoparticles has been used here for modeling current transients applying small potential steps to LixFePO4 electrodes. The number fraction of (phase) transformed particles thus calculated was directly compared with the changes in the effective thickness and permeability length of the electrode coating derived by EQCA. Geometric changes of thin active mass originating from different molar volumes of the parent and transformed phase result in nonuniform deformations of intercalation particles. This study confirms the collective behavior of LixFePO4 intercalation particles during electrochemically induced phase transition. The use of EQCA as a highly precise and sensitive probe of mass and geometric changes in the electrode layer of intercalation particles paves the way for dynamic in situ studies of nonuniform intercalation particles deformations which can hardly be assessed by other available techniques.