... and the field of Capacitive Deionization keeps on growing at an increasing speed ! While over 60 scientific publications are now written and published annually, citations to the CDI literature have grown from a number less than 100 per year before 2010, to about 2000 per year at the end of 2015, and this number continues to rise. Analysing citation data for the past 10 years, an exponential growth in citation rate is clearly observed, with a doubling of the citation rate every 18 months !
Currently CDI devices
consume significantly more energy than the theoretical thermodynamic minimum, and this is partly due to resistive power
dissipation. To enhance the performance of CDI, identification of resistances in the CDI cell is important. Recently, two articles have been published on characterizing resistances in CDI and membrane-CDI (MCDI).
Researchers from Stanford University (US) published an article in Environmental Science and Technology. They characterize electrical resistances in a CDI system, present an equivalent circuit model and propose measurable figures of merit to describe cell resistance. They also found that contact resistance between current collectors and porous electrodes is the major contributor to cell resistance in nearly all published CDI cells. Contact resistance can be reduced by either introducing contact pressure between current collectors and electrodes or using pore-filling adhesive to create a point contact configuration. They emphasize here that energy consumption of the CDI process is the unrecoverable dissipated energy during an operation cycle, which should not include stored capacitive energy.
Researchers from Wageningen University and Wetsus (The Netherlands) published an article in Water Research. They outline a method to identify electronic and ionic resistances. They illustrate their method by calculating the resistances in an MCDI cell, for which they derived a full-scale model. This model is validated against experimental data and used to calculate the ionic resistances across the MCDI cell. Furthermore, they present a way to measure ionic and electronic resistances in a CDI cell, as well as establish the spacer channel thickness and porosity after assembly of the MCDI cell. Based on their findings, they show that, for MCDI, the carbon electrode thickness can be increased without significantly increasing energy consumption, which has the advantage that desalination time can be lengthened significantly.
* Y. Qu, T.F. Baumann, J.G. Santiago, M. Stadermann, Characterization of Resistances of a Capacitive Deionization System, Environ Sci Technol, 49 (2015) 9699-9706.
* J.E. Dykstra, R. Zhao, P.M. Biesheuvel, A. van der Wal, Resistance identification and rational process design in Capacitive Deionization, Water Research, 88 (2016) 358-370.
October 25-29, 2015, the International Conference on CDI&Electrosorption was organized by the Institute of New Materials (INM) in Saarbrücken, Germany. In a beautiful location on the pittoresque campus of Saarland University, 120 attendees from all around the globe participated in lectures, a CDI-tutorial, and a poster session, with lively discussions during coffee breaks and lunches. An inspiring and diverse program was organized by the conference chair prof. Volker Presser (INM) and co-chair prof. Matthew Suss (Technion, Israel). Many new contacts were made during the opening mixer, the memorable excursion to an old steel and coal factory, and during the delicious gala dinner. We look back at a wonderful and very successful CDI&Electrosorption conference.
In a collaboration involving scientists from five different countries in two continents, members of the CDI&E working group used the past year to come with a Perspective-paper on capacitive ionization and electrosorption. Published on invitation in the high-impact journal Energy&Environmental Science (IF=25) as a prestigious Perspective-contribution, the paper is expected to generate attention inside and outside the CDI-field. To help quick dissemination of its content, the authors have chosen for the OPEN ACCESS-format.
As corresponding authors Prof.Dr. Mathew Suss and Prof.Dr. Volker Presser explain: "The idea for this perspective was conceived of during the last CDI-conference in Leeuwarden, the Netherlands, and our aim is that it serves the growing CDI-community in outlining current trends in CDI developments, in standardizing metrics, and to help by identifying 'white areas' in CDI, both experimentally and theoretically. It was a most exhilarating task to work together with so many different authors on different continents to see this paper growing over the year. Many discussions helped us to focus on the most important elements and to converge on the key trends and best metrics for CDI performance. We have done our very best to put together a paper that helps to catalyze scientific and industrial developments in the CDI&E-field."
As reported during the 8th International Conference "Interfaces against Pollution," May 2014, the energy consumption of CDI operation can be significantly reduced by tuning the discharge voltage, which is the cell voltage applied during cell discharge, when the adsorbed salt is released and a concentrated brine stream is produced. Commonly in CDI, the charging voltage is tuned to an optimum value, where salt adsorption is high but leakage currents are still low. The discharge voltage is by default set to zero Volt. Following an earlier study from Bar-Ilan University, Israel, a cooperation of Seoul University (South Korea), Wageningen University and Wetsus (the Netherlands) found and reported on the positive influence of tuning the discharge voltage to values higher than zero. In contrast to the earlier work, it was found that salt adsorption per cycle did not markedly decrease, while the charge efficiency went up to values approaching the theoretical limit of one (unity). This meant that the energy consumption significantly decreased (being inversely proportional to charge efficiency), even without considering energy recovery, something that is possible with positive discharge voltages.
In the same study it was also found that with a non-zero discharge voltage, it becomes easier in CDI to achieve a stable effluent concentration by using constant-current operation; something that before this study was thought to be possible only for membrane-assisted CDI. As senior author prof.dr. J. Yoon remarks: "This was a very insightful study that clearly showed the potential of tuning the operational conditions of CDI to enhance the performance of a CDI cell. It was remarkable how accurately the porous electrode transport theory, using the Donnan concept to describe salt adsorption, could describe the data. For design purposes, such a model is indispensable."
link to journal
link to pdf of paper
For our upcoming CDI&E conference, organized by Prof. Dr. Volker Presser (INM) and Prof. Dr. Matthew Suss (Technion), to be held in Saarbrücken, 26-29 October 2015, the flyer is available now !
A direct link to the conference website is www.cdi2015.de where you can submit your abstract for an oral or poster presentation. The conference registration has opened as well - note that the number of participants is limited, and thus make sure you register in time !
The abstract submission deadline is set to June 19, 2015.
In an American-Israel-Dutch-Polish cooperation, the existing "normal" modified-Donnan model to describe the electrical double layer structure in (micro-)porous carbons, was significantly improved (ergo: the i-mD model) without jeopardizing the model's mathematical simplicity, allowing it to be incorporated in transport theory, and allowing it to be solved using simple spreadsheet-software such as Excel. This i-mD model was published in a special issue of the Journal of Solid State Electrochemistry, commemorating the late prof. V.S. Bagotsky. The i-mD model has the same advantage as the classical mD-model (developed in 2011 for CDI) namely that it is mathematically simple and can be used for transport modeling in porous carbons (note that the Gouy-Chapman-Stern theory will fail for sufficiently small pores, as the inherent pore overlap is not included in GCS theory), but in addition describes salt adsorption at high salinity much better. This was a weak point of the "normal" mD-model. The improvement consists of a slightly different formulation, not introducing more mathematical "fit"-parameters, and actually, the new model has a strong physical background, based on attractive ion-ion correlation forces. Prof. Martin Bazant (MIT), the senior author of the paper, comments "It is quite remarkable how such a simple model fits data so beautifully. I was very surprised myself. As far as I know, the simple form of the ion correlation force expression that we present, is new. Possibly in the future we will find out if we need a more accurate expression, but for the moment this simple model works like a charm, and it has a physical basis which has the advantage that further extensions such as considering the case of ionic mixtures becomes possible."
link to the journal
PDF via ResearchGate
The PhD defense of Taeyoung Kim (PhD student in SNU under supervision of Prof. Jeyong Yoon) is scheduled for 28 October 2014. The title of thesis is ‘Desalination performance and mechanism of carbon electrodes with respect to physicochemical and electrochemical properties in capacitive deionization’. After graduation, Taeyoung is going to keep working on research in various fields, thus looking for a postdoctoral position worldwide.
This year, Yoon’s group in Seoul National University, Republic of Korea, has published three papers on CDI.
Hierarchically porous carbon was synthesized and used as a CDI electrode in collaboration with Prof. Park’s group (Carbon Nanomaterials Design Lab., SNU) (Yang et al., Carbon 71 (2014) 294-302). In this paper, a novel carbon material derived from metal organic framework (MOF) was prepared and its pore structure was characterized. Unique pore structure of MOF-derived carbon (MDC) consisting of micro-, meso-, and macropores allowed for a rapid and considerable amount of desalination compared to activated carbon (microporous carbon) and carbon aerogel (meso- and macroporous carbon). Thus, this paper has successfully elucidated the role of each pore size in CDI.
In the next paper, a new method to evaluate rate capability of CDI electrodes was proposed using a potential sweep method (Kim et al., Electrochimica Acta 139 (2014) 374-380). Potential sweep method can enable sweeping of potential in various rates, thus resulting desalination performance can be an indicator for rate capability. As a result, electrode thickness, flow rate, and salt concentration were found to parameters affecting rate capability, which is in good agreement with previous CDI papers.
Recently, constant voltage and constant current operations were compared focusing on salt adsorption capacity and energy consumption (Kang et al., Desalination (2014) accepted). Results said that constant current operation is advantageous in terms of energy consumption because of its low voltage profile.
Korean CDI research network has been established (Prof. Jeyong Yoon as chairman). Various research groups with different disciplines joined the network including Hyundai Heavy Industries (Yong-Shin So, in charge of general affairs), Korean Institute of Energy Research (KIER), and SionTech.