Studying titanium dioxide via electrochemical impedance spectroscopy

Titanium dioxide is a photocatalyst which under illumination can degrade organic compounds via advanced oxidation processes. Upon illumination of the titanium dioxide, electron-hole pairs are formed, which can recombine or react further to oxidize the organic compounds and degrade them to minerals. This process could be interesting for water treatment however, the photon efficiency (degradation rate over the photon flux) is typically very low, below 10% [1]. The low photon efficiency is due to the fast recombination of the electron-hole pairs, therefor there is no efficient use of the photons going into the photocatalyst compared to how much organic compounds are degraded. 

Electrochemical impedance spectroscopy (EIS) is a sensitive measuring technique, which places an alternating current over the system and measures the resulting potential [2]. With the illumination of titanium dioxide, ions and radicals are produced near the wall which will change the solution resistance in the channel. This could be measured accurately with EIS under different conditions. The results are analyzed via equivalent circuits, which represent the electrical elements in the system, by combining them with the physical elements in the system. In this project, various parameters could be studied, like microchannel design, electrolyte concentration, current amplitude and range, organic contaminant degradation, and light intensity. 

Please do not hesitate to contact Nicole Timmerhuis for more information! (n.a.b.timmerhuis@utwente.nl)

[1] O. Tokode, et.al., J. Photochem. Photobiol., A,319-320:96-106, mar 2016

[2] A. Sacco, Renew. Sust. Energ. Rev., 79:814-829, nov 2017 

Diffusioosmotic flow for enhancing conversion

Many heterogeneous systems have mass transfer limitations near the solid-liquid interface. These limitations in the boundary layer are due to fast reaction at the wall, creating a concentration drop towards the solid. Or it could be from the velocity which is standing still near the wall due to a laminar, parabolic flow pattern in the microchannel. The main focus of this project is to overcome this mass transfer limitation by creating an osmotic flow near the solid-liquid interface in the boundary layer.


An osmotic flow is created by a pressure gradient in the liquid, which is directly proportional to the concentration gradient for non-electrolytes and to the natural logarithm of the concentration gradient for electrolytes [1]. The concentration gradient is created by the interaction between the liquid particles and solid interface, which can be repulsive or attractive. If different solid interfaces are used, alternating each other, a continuous osmotic flow can be created. This is schematically shown in the figure below.

This project can be approached both numerically and experimentally. Optimal conditions need to be found for maximal diffusioosmotic flow in the boundary layer in order to enhance the conversion as much as possible. Design parameter would be for example flow velocity, organic compound to degrade, and length of the titanium dioxide patch. 

Please do not hesitate to contact Nicole Timmerhuis for more information! (n.a.b.timmerhuis@utwente.nl)

[1] D.C. Prieve, J.L. Anderson, J.P. Ebel, M.E. Lowell, J. Fluid. Mech., 148:247-269, 1984

Ion transport through graphene

Graphene is increasingly studied as a potential material for membrane applications. It does not have any separation capability in its pristine state, however a perforated monolayer graphene membrane can selectively separate material due to steric or electrostatic exclusion [1, 2].  In our work, we use ion-bombarded perforated monolayer graphene membrane supported on a polymer foil to study diffusional ion transport properties through graphene [3]. This membrane allows positive ions to pass through and blocks negative ions, acting as a cation-exchange membrane (CEM). We have studied the selectivity of the membrane with varying concentrations from range where Donnan potential dominates to concentrations where the diffusion potential dominates and the membrane loses all its selectivity. We have also analyzed our experimental results with theory, based on Donnan equilibrium and the Teorell, Mayer and Sievers (TMS) theory. 

 

So far, we have done our measurements without applying any electric field across the membrane and the observations are based on diffusional ion transport. Now, we need to explore how the ions transport through the membrane when an electric field is applied across the membrane. The aim of the current student’s project is to study the ion transport behaviour of the graphene membrane with the application of electric field. Also, in the later part of the project the ion transport will be studied with varying number of pores in the membrane. Visualization of the ion transport with dyes will be possible by FLIM (Fluorescence Lifetime Imaging Microscopy).  Finally, the experimental result will be validated with theoretical understanding in terms of Nernst Planck framework. 

For more information, feel free to contact Madakranta (Dona) Ghosh:
m.ghosh@utwente.nl

[1] O’Hern et al. “Selective Ionic Transport through Tunable Subnanometer Pores in Single-Layer Graphene Membranes.” Nano Lett. 2014, 1234-1241

[2] Rollings et al. “Ion selectivity of graphene nanopores” Nature Comm., 2016, 7, 11408

[3] Madauß et al. “Fabrication of nanoporous graphene/polymer composite membranes” Nanoscale, 2017, 9, 10487

Catalytically Induced Convective Flow

The electrochemical decomposition of hydrogen peroxide  has been reported to generate interfacial fluid flow by micro-pumps, and interdigitated microelectrodes[1], as well as the motion of bimetallic platinum/gold nano-motors [2]. In principle, the immersion of platinum/gold nanorods in hydrogen peroxide, triggers a series of oxidation and reduction reactions, where  is oxidized at the platinum anode into protons, electrons, and oxygen molecules, while reduction takes place at the gold cathode. The resulting ionic flux generates an electric field that is coupled with the charge density, thereby inducing an electric body force that drives interfacial fluid motion.The heterogeneous rate constant which largely influences ionic flux by way of proton generation and consumption is an important parameter that needs to be understood under steady state and transient conditions.

The kochi and Kolinger’s approach [3] will be used in determining the reaction rate constant. Several electrochemical experimental techniques such as chronoamperometry, chronopotentiometry and cyclic voltammetry, will be used in determining important parameters such as, diffusion coefficient, charge transfer, number of electrons, and peak potentials needed for evaluating the rate constant. The experimental heterogenous rate constant will be implemented in existing numerical models by way of the Damköhler’s number to ascertain its influence on the reaction flux, proton distribution, induced potential and convective flow.

For more information, contact:

Abimbola ASHAJU (a.a.ashaju@utwente.nl), Soft Matter, Fluidics and Interfaces, Meander 317)

[1]          T. R. Kline, W. F. Paxton, Y. Wang, D. Velegol, T. E. Mallouk, and A. Sen, “Catalytic micropumps: Microscopic convective fluid flow and pattern formation,” J. Am. Chem. Soc., vol. 127, no. 49, pp. 17150–17151, 2005.

[2]          T. R. Kline, W. F. Paxton, T. E. Mallouk, and A. Sen, “Catalytic Nanomotors: Remote-Controlled Autonomous Movement of Striped Metallic Nanorods**,” pp. 744–746, 2005.

[3]          R. J. Klingler and J. K. Kochi, “Electron-transfer kinetics from cyclic voltammetry. Quantitative description of electrochemical reversibility,” J. Phys. Chem., vol. 85, no. 12, pp. 1731–1741, 1981.

Slippery liquid infused membranes (SLIMs)

The rapid industrial growth in the field of oil and gas and petrochemical has lead to the production of large amount of oily waste water. Due to the environmental issues, the necessity to treat oily waste water is an inevitable challenge. Polymeric membranes have shown to be a good candidate for oily waste water treatment mainly due to their facile fabrication procedures. However, the main disadvantage of application of polymeric membranes is fouling [1]. To improve the anti-fouling properties of membranes, different strategies have been examined as follows [2,3]:
-       Physical/chemical surface modification
-       Blending with hydrophilic additives
-       Superhydrophobic/superolephilic membranes

In this project, application of a new type of membrane, called, slippery liquid-infused membrane (SLIM) for treating oily waste water will be investigated. The concept of SLIM is inspired by slippery liquid-infused porous surfaces (SLIPS) which have shown anti-biofouling properties and multi-phase transport capability without clogging [4]. The liquid-filled pore will be opened in response to the system pressure. Therefore, a liquid-lined path is made for passage of the transport fluid. Once the pressurizing is stopped, the infusion liquid (fluorinated oil) can be re-infused back and closed status of the pore can be recovered. This is called gating mechanism which is schematically shown in figure 1 [5].


Figure 1. Schematic of gating mechanism in SLIM and comparison of a dry pore (a) to liquid-filled pore (b).

The presence of the thin liquid layer on the pore wall is essential for anti-fouling properties of SLIMs. So far, the retention of the infusion liquid has been tested by pushing pure water as well as different types of surfactant solutions (anionic, cationic, non-ionic and zwitterionic) through SLIM. As schematically shown in figure 2, displacement of the infusion liquid by water (immiscible displacement) has lead to the preferential flow path ways for water transport through the membrane. The displacement mechanism corresponds to capillary fingering invasion regime which was confirmed by microfluidic experiments using a chip mimicking porous membrane (figure3) [6].


Figure 2. Immiscible displacement in SLIM showing preferential flow path ways for water transport.


Figure 3. Liquid-liquid displacement in the microfluidic chip. The chip is infused with oil (yellow phase) and water is pushed at flow rate of 0.2 ml/s from the right side (blue phase).

In order to better investigate the effect of surfactants on retention of the infusion liquid on the pore wall, new type of microfluidic experiments have been planned [7]. Particle image velocimetry (PIV) will be used to investigate the velocity profiles and better understand Maragoni type stresses on the liquid-lined wall.

For more information, please contact Hanieh Bazyar: h.bazyar@utwente.nl

[1] M. Padaki, R. Surya Murali, M. S. Abdullah, M. Misdan, A. Moslehyani, M. A. Kassim, N. Hilal, A. F. Ismail, Desalination, 2015, 357, 197.
[2] A. Mansourizadeh, A. J. Azad, Journal of Polymer Research, 2014, 21, 1.
[3] L. Feng, Z. Zhang, Z. Mai, Y. Ma, B. Liu, L. Jiang, D. Zhu, Angewandte Chemie International Edition, 2004, 43, 2012.
[4] X. Hou, Y. Hu, A. Grinthal, M. Khan, J. Aizenberg, Nature, 2015, 519, 70.
[5] H. Bazyar, S. Javadpour and R. G. H. Lammertink, Adv. Mater. Interfaces, 2016, 3, 1600025.[6] H. Bazyar, P. Lv, J. A. Wood, S. Porada, D. Lohse, R. G. H. Lammertink, Soft Matter, 2017, submitted.
[7] M. A. Sikkink. Superhydrophobic Non-Newtonian Slip Flow, 2017, graduation thesis.

Multicomponent nanofiltration modeling

Nanofiltration membranes are commonly employed in industrial practice for separations of molecular species while working at relative low operation pressures. The unique property of these membranes is that they allow passage of certain solutes, while they can retain others. This makes nanofiltration a cost-effective step in the separation or purification of several streams. This can for instance be drinking water production, or resource recovery from waste streams. For an effective process, it is essential to understand the transport of the different solutes to and through the membrane. 

One observed non-ideality is the emergence of non-intuitive phenomena, such as negative retention of solutes (the permeate of a process is more concentrated than the retentate). This is, in particular, more common in mixture systems due to the fascinating non-ideal thermodynamics involved and has important consequences in the design and operation of membrane processes for separations. Building a model framework that can capture these effects and understand the role of mixture interactions in a simple manner is therefore of crucial importance.

In this assignment, you will work to build and compare multicomponent mass transfer models and compare the role of choosing an effective mixture thermo-dynamics rule + model framework in describing experimental data for hollow fiber filtration. The aim is to be able to describe industrially relevant hollow fiber filtration data obtained from actual membrane filtration experiments. The assignment will be performed at the UT in close collaboration with an industrial partner (NX Filtration).


For more information please contact Jeff Wood (j.a.wood@utwente.nl) or Joris de Grooth (j.degrooth@utwente.nl)