Prof. Dr. Volker Presser

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.

High power electrochemical double layer capacitors (also called supercapacitors) rely on highly conductive electrode materials with a large specific surface area, which is easily accessible for ions. Exohedral materials with a large ion-accessible outer surface and little or no porosity within the particles are particularly attractive for supercapacitor electrodes because they decrease mass transport limitations and enable very high charge/discharge rates. This study focuses on the investigation of charge induced expansion effects of spherical exohedral carbons, that is, onion-like carbons (OLC, diameter: 5–7 nm) and carbon black (diameter: ≈40 nm). Employing electrochemical in-situ dilatometry we studied the expansion behavior within ±1 V potential window versus carbon in an organic electrolyte (tetraethylammonium-tetrafluoroborate in acetonitrile). The expansion had a very small amplitude (<0.2% at ±0.08 C·m-2 accumulated charge; i.e., approximately ±1 V versus carbon) and was fully reversible. This was explained by ion adsorption on the exohedral carbons. Molecular dynamics (MD) simulations were employed to calculate the solvation shell of the respective cation and anion and the results were used to further evaluate the measured expansion. In summary, the experiments and simulations revealed that ion intercalation or ion sieving, which are commonly found in microporous (endohedral) carbons, were absent. Finally, low sweep rates resulted in a slight electrode compaction on a cycle-by-cycle basis, which can be explained by charge-induced restructuring of the nanoparticle network.

Sierpinski carbon: Macroporous carbide-derived carbon monoliths (DUT-38) were synthesized starting from SiC-PolyHIPEs, resulting in a hierarchical micro-, meso-, and macroporous structure. The high specific surface area and high macropore volume renders PolyHIPE-CDC an excellent adsorbent combining high storage capacity with excellent adsorption rates in gas storage and air filtration.

The nanoscale interactions of room temperature ionic liquids (RTILs) at uncharged (graphene) and charged (muscovite mica) solid surfaces were evaluated with high resolution X-ray interface scattering and fully atomistic molecular dynamics simulations. At uncharged graphene surfaces, the imidazolium-based RTIL ([bmim+][Tf2N-]) exhibits a mixed cation/anion layering with a strong interfacial densification of the first RTIL layer. The first layer density observed via experiment is larger than that predicted by simulation and the apparent discrepancy can be understood with the inclusion of, dominantly, image charge and π-stacking interactions between the RTIL and the graphene sheet. In contrast, the RTIL structure adjacent to the charged mica surface exhibits an alternating cation-anion layering extending 3.5 nm into the bulk fluid. The associated charge density profile demonstrates a pronounced charge overscreening (i.e., excess first-layer counterions with respect to the adjacent surface charge), highlighting the critical role of charge-induced nanoscale correlations of the RTIL. These observations confirm key aspects of a predicted electric double layer structure from an analytical Landau-Ginzburg-type continuum theory incorporating ion correlation effects, and provide a new baseline for understanding the fundamental nanoscale response of RTILs at charged interfaces.

This Letter is in response to a recent paper by Ma et al. (CrystEngComm, 2010, 12, 750-754) which arguably studied vanadium carbide nanostructures whereas all available evidence indicates the study of vanadium oxide. We feel that it is important to communicate to the community several inconsistencies so that the interesting material reported can be seen in the right light, especially with several groups nowadays having reported similar structures from vanadium oxide synthesis.