Volker Presser
Volker Presser

Volker Presser of the INM - Leibniz Institute for New Materials and Saarland University, in Germany, will be giving a lecture entitled "Carbon and carbon hybrid materials for electrochemical desalination" at the 28th International Conference on Diamond and Carbon Materials (DCM), 3-7 September 2017, in Gothenberg, Sweden as part of the Materials Today "Materials in Society" lecture series.

The conference, hosted by Elsevier and Materials Today will of course feature as always high-level research on a wide range of carbon-based materials covering all the well-known materials diamond, carbon nanotubes, and graphene, as well as novel materials such as carbon nanodots and carbon nitrides that are beginning to play an increasingly prominent role in composites as well as pure material form. Traditionally, the meeting has spanned the spectrum from preparation through fundamental physical and chemical concepts, to applications and will continue to do so.

As such, one of the most exciting and pressing areas in which materials science could solve a critical global problem is in the area of desalination. As climate change and other factors begin to impinge even more on fresh water supplies and environmental pressures marginalize vulnerable communities particularly in remote parts of the world and in the developing world, desalination is becoming increasingly important. At the forefront of research into new materials science approaches to solving the problem of desalination is Volker Presser.

Presser obtained his PhD in Applied Mineralogy in 2006 from the Eberhard Karls University in Tübingen, Germany. He then spent several years as a Humboldt Research Fellow and Research Assistant Professor working specifically between 2010 and 2012 at the A.J. Drexel Nanotechnology Institute in the team of Yury Gogotsi at Drexel University, Philadelphia, USA. He has been a full professor in the Department of Materials Science and Engineering at Saarland University and Program Division Leader at the INM - Leibniz Institute for New Materials in Saarbrücken, Germany, since 2015. His work focuses on nanocarbon and hybrid nanomaterials for electrochemical energy storage, harvesting, and, of course, water desalination.

I asked him about the history and the state of the art in water desalination technology. "Water desalination is a large and fast developing field - ranging from large scale production of drinking water to remediation of industrial water and limitation of pollution regarding agriculture and mining," he explains. "The field of electrochemical desalination by use of carbon materials was started several decades ago, but has not seen much development since the advent of nano-engineered carbons in the late 1990s and early 2000s. Until a few years ago, energy efficient water desalination via ion electrosorption was a niche field just for brackish water - but has now diversified to different concentration regimes and even continuous operation, especially by the use of suspension electrodes."

The obvious question then is why do we need new carbon materials to update the technology? "Capacitive deionization accomplishes desalination by ion immobilization during charging via the formation of the electrical double-layer," Volker explains. "The latter critically requires a large interface area and large pore volume to accomplish a large ion removal capacity (i.e., removed ions per gram of electrode material). Also, small micropores (especially below 1 nanometer) have a higher relative desalination capacity compared to mesopores - therefore, the design of the carbon pore size distribution is very important." He suggests that, "Ultimately, we also need to hybridize carbon with other materials to make the electrode more resistant towards oxidation during electrochemical operation and to enable energy efficient desalination of saline media with high ion concentration."

Once this side of the technology matures, we will reap the benefits of our ability to make carbon materials from abundant and renewable resources to help us design cost-efficient and sustainable technologies. "Carbon is also tremendously versatile," Volker adds. "We can tune the pore structure on a sub-nanometer level, modify the electrical conductivity, and obtain everything between ultrasmall nanoparticles, like 5 nm carbon onions to monoliths or large-scale carbon textiles."

I asked Volker about how he came to the area of water desalination from capacitive deionization from the field of electrochemical energy storage with carbon supercapacitors. "While different in device architecture and application," he told me, "these technologies are both based on the same mechanism, namely, ion electrosorption." He adds that, "It was fascinating to me to see how my knowledge of designing carbon nanomaterials greatly benefitted the advancement of electrodes for desalination. Even more to my surprise: this works both ways! Energy storage mostly considers just net charges, while desalination needs to consider permselectivity and the in-pore ion population of cations and anions. This unique understanding of the structure of the electrical double layer allows us to refine our understanding of ion electrosorption and to create better supercapacitors as well."

Volker suggests that effort in this area in recent years has led to a consolidation of the field. "It was important to us to publish authoritative review articles in collaboration with other leading experts," he says. These reviews now stand as highly cited contributions for our community. In the meantime, the researchers have constantly tried to advance their electrode materials. For example, dissolved oxygen is a real problem for carbon electrodes and peroxide formation leads to progressive degradation and ultimate device failure. The removal of dissolved oxygen or the use of additional ion exchange membranes should be avoided if possible because of additional energy costs or increasing device costs. One recent discovery has overcome several problems in this regard, Volker explains: "It was very exciting to see that nanoscale titania decorated on carbon surfaces makes such hybrid electrodes robust towards dissolved oxygen and enables stable system performance."

He adds that desalination beyond brackish water is of growing importance. "My team has shown that the Faradaic ion intercalation materials known from the battery community can be used for advanced desalination even when using high molar concentrations, as with sea water," he explains. "For conventional capacitive deionization, energy efficient desalination is only possible for low saline concentrations, the brackish water of the Baltic Sea, for example. In such Faradaic materials, carbons remain key components and we see a great potential for nanohybrid electrodes."

Looking to the future, Volker points out that research on carbons and carbon hybrid materials for water desalination is moving fast as new concepts, materials, and mechanisms are discovered. "It is important to not only provide the proof-of-concept for new materials or new cell designs, but to also advance the maturity of the technology. For that, we need to establish rigorous stability testing procedures and to make our electrodes and cells more robust towards real world application problems, such as biofouling and scale formation." He also confesses that he is very susceptible to the excitement surrounding the work, "It piques our scientific curiosity," he says.

Registration is open for the  the 28th International Conference on Diamond and Carbon Materials (DCM), 3-7 September 2017, in Gothenberg.

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".