Ionic Diode Mechanisms for Desalination and Electroosmosis

: (Alternative Format Thesis)

Abstract

Ionic diodes refer to systems that produce rectification effects for ionic current, where an alternating current (AC) input is rectified into a direct current (DC) output. First suggested in 1959, a diverse range of designs for ionic diodes have been developed, including biomimetic, nanofluidic, and microfluidic ionic diodes. In the past decade, microhole ionic diodes have emerged as a subject of interest in the area, in particular for their potential in practical devices. Apart from applications such as electrochemical sensing and gating, which have dominated the discussion of ionic diode applications in literature, microhole ionic diodes have also been demonstrated to provide unidirectional ‘pumping’ effects for both ionic and neutral species, at a reasonable and applicable scale.

The system of a microhole ionic diode is constructed by asymmetrically depositing a semi permeable thin membrane on a microhole. The microhole is drilled into a thin film that separates two chambers of an electrochemical cell. When external bias is applied, concentration polarisation of ions in the microhole creates rectification effects. As a result of the rectified ionic current, both ionic and neutral species are transported from one chamber to the other. When coupling multiple microhole ionic diodes into an ionic circuit, the undesired side processes can be minimised (e.g. driver electrode reactions caused by unbalanced charge when a diode is operating), and energy efficiency can be improved. This allows AC-electricity driven devices based on coupled ionic diodes to be developed, for purposes such as water purification/capture.

In this thesis, the main objectives are (i) to study and understand the mechanism of microhole ionic diode at a fundamental level, in particular about diode performance, as it is crucial for applicational systems; (ii) to investigate the ‘pumping’ effects of microhole ionic diodes, and the role of semi-permeable materials (e.g. the performance under different conditions and the electroosmotic drag effects for different ionomers); and (iii) to construct ionic circuits based on coupled microhole ionic diodes, and to demonstrate their applications in desalination and water pumping systems.

First, a microhole ionic diode based on permeability-tuneable ionomer PIM-EA-TB was constructed. PIM-EA-TB is a member of the polymers of intrinsic microporosity (PIMs) family, which derives from an ethano-anthracene (EA) structural motif formed in a Tröger base (TB) reaction (see Section 1.1.6). The single diode system is shown to produce diode effects in ethanolic electrolyte, when protonation is introduced into the polymer by H+ or Cu2+ cations. Higher resistivity is observed in the system, comparing to aqueous environment, which is attributed to better size/ion exclusion in ethanol. Electrochemical gating of the diode is demonstrated by adding a second working electrode to induce/remove protonation of PIM-EA-TB.

Next, a COMSOL finite element model for an cation-conducting Aquivion-based ionic diode is developed. Transient phenomena in the microhole system are simulated, then tuned and compared to experimental data from electrochemical measurements. The mechanism of concentration polarisation in the microhole was revealed with respect to the diode geometry.

To exploit the ‘pumping’ effect of ions, two microhole ionic diodes (based on cation conducting Nafion and anion-conducting Sustainion) are constructed. The diodes are shown to produce good rectification effects in aqueous NaCl, and are coupled into an ionic circuit. With an AC-voltage applied, the system enables the transport of Na+ cations and Cl- anions from one chamber to another, resulting in desalination/salination. Effects of 4- electrode and 2-electrode configurations are investigated, with respect to the energy efficiency of the desalination device.

Then, potential of microhole ionic diodes in pumping neutral molecules is explored. This is first demonstrated with PIM-EA-TB diodes transporting caffeic acid. Caffeic acid is reported to accumulate/immobilise into the microporous PIM-EA-TB in acidic and neutral environments, via hydrogen bonding. When the polymer is employed for a microhole ionic diode, immobilisation of caffeic acid is shown to regulate the ionic current. This is attributed to both steric hindrance and polymer-charge removal as a result of bonding caffeic acid. The pumping of caffeic acid through the PIM-EA-TB diode is investigated under different pH conditions, and monitored by LC-MS.

The electroosmotic pumping of water molecules in a PIM-EA-TB diode is studied. The system is shown to allow significant flux of water to pass through. Lower pH (i.e. higher protonation levels of PIM-EA-TB) is shown to reduce the electroosmotic drag coefficient (i.e. number of water molecules electroosmotically transported with one ion). But the overall absolute rate of water transport is observed to be pH-independent. This is rationalised with a ‘piston mechanism’ in which water molecules are pushed through the rigid polymer by ions.

Furthermore, methylation of PIM-EA-TB is introduced as an alternative method to induce fixed-charge and produce a semi-permeable ionomer. With different levels of methylation creating controllable charge-doping of PIM-EA-TB, the electroosmotic drag coefficient of the methyl-PIM-EA-TB microhole ionic diode is shown to be tuneable. By coupling two PIM-EA-TB diodes with different levels of methylation in an AC-voltage driven ionic circuit, a prototype of water pumping device was developed. Net-zero ion transport is achieved as the diodes pump anions in the opposite directions. And one of the diodes (lower methylation level) allow higher water flux to pass through than the other one (higher methylation level), resulting in water accumulation in one chamber of the ionic circuit.

Finally, a microhole ionic diode is developed with butylated PIM-EA-TB. Similar to protonation and methylation, butylation of PIM-EA-TB induces fixed positive charges into the polymer, but produces a lower ionic current. However, outstanding rectification ratios are observed in butyl-PIM-EA-TB diodes. This is attributed to a new ionic diode mechanism, where charge carriers are removed from a surface layer in the ionomer. Overall, in this thesis, the concept of microhole ionic diode is investigated and discussed at a fundamental level, and the applicational perspective of coupling ionic diodes into ionic circuits for water treatment is highlighted. Novel prototype devices for water desalination and water pumping are developed and demonstrated, with their advantages and drawbacks discussed in terms of optimising the performance of microhole ionic diodes.

Date of Award	11 Sept 2024
Original language	English
Awarding Institution	University of Bath
Supervisor	Frank Marken (Supervisor) & Chris Bowen (Supervisor)