In this work, ion transport at charge selective interfaces is investigated. ‘This is important in a multitude of technologies,’ says Anne Benneker. ‘Electrochemical cells, such as fuel cells, and electrodialysis both serve as good examples.’
At elevated potentials, the ion flux (current) is not linearly dependent on the driving force (potential), and the efficiency of the electrodialysis process drops. ‘However, electrodialysis processes also show an overlimiting current behaviour, in which the ion flux is increases again as a function of increasing potential,’ Anne says. ‘In this work, the effect of external properties, such as temperature and membrane geometry, on charge transport in electrodialysis systems in different regimes, is investigated.’
Temperature effects were first studied using a numerical model. ‘From there we concluded that the effect of temperature gradients was big enough to build experiments,’ she says. ‘The effects of temperature on charge transport were investigated in a macro-scale electrodialysis stack. For the effect of geometry on charge transport, microfluidic experiments were performed using two different types of microfluidic devices in which ion concentration and electro-kinetic flows were visualised.’
In this thesis different charge selective materials were investigated; (i) nanochannels (charge selective as a result of their overlapping Debye layers), (ii) commercially available polymeric charged membranes and (iii) patterned, charged hydrogels.
Experiments yielded knowledge on the development of ion depletion zones and the influence of geometry and temperatures, on the onset of hydrodynamic instabilities near membrane interfaces and on charge transport through the membrane.
‘There is a difference between numerical analysis and the actual experiments,’ Anne says. ‘The temperature gradient is more prone to change in experiments. We performed experiments on various scales, therefore the title of my thesis says: from small to big.’
‘For the development of experiments, I came into contact with other groups at MESA+, in this case the BIOS Lab-on-a-Chip Group. It turned out one particular, flexible chip design, developed together with two colleagues, fitted my research on charge transport very well. Especially in studying the influence of geometry on charge transport this chip design was very useful. We were able to design and fabricate three different geometries using this platform.’
Anne: ‘My first memorable moment was when I could unambiguously perceive, through my microscope, electroosmotic flows adjacent to charge selective nanochannels. Experiments can be quite challenging, so I was relieved to actually see what I expected, and really hoped, to see.’
‘Within my research during the past years I learnt that scientific research flourishes when true collaborations and open-minded exchange of ideas occur. In the beginning I learnt a lot from colleagues and technicians. As the project proceeded, I noticed that my expertise could be of use for other colleagues as well. One of the fortes of Mesa+ is the truly open research atmosphere, benefitting from each other’s expertise and jointly generating novel ideas. Some colleagues from abroad were really surprised by this, as they were more used to a somewhat competitive attitude and approach.’
During the research Anne found that temperature gradients can be of potential use in enhancing the efficiency of desalination by electrodialysis. ‘However, it should be noted that this comes at a price, since heat is not free,’ Anne says. Stacks of commercially available ion exchange membranes were used for the investigation of the influence of temperature and temperature gradients on macro-scale systems.
Anne: ‘My research topic is feasible for application at various fields in the middle and more far away future. However, my research was quite academic, aimed towards better understanding the actual phenomena and processes taking place.’
After her PhD Defence Anne is interested in a post-doc position. ‘From childhood on I was already interested in transport phenomena. For example I always wondered how to describe the flow through rivers,’ she says. ‘A post-doc position on computational fluid dynamics, in combination with the work I did during my PhD, would be great as it fits within this interest field and it will add to my skills and knowledge.’
Anne: ‘Regarding my future job, I would like to continue my work as an academic teacher which I am now practising on process technology. Being a teacher in academic or chemical engineering or applied physics education, would suit me well. A nice combination with research sounds like an interesting option to me. I really enjoy when students show their interest in the field and when they are able to show their growing understanding.’