Interfacial structure and double layer capacitance of ionic liquids

Central to this thesis are ionic liquids: organic salts that are in liquid phase at room temperature.

‘There are many unique properties which make these liquids promising for future applications,’ says Monchai Jitvisate. ‘It may be used in the future as a new material in green energy applications, for example in next generation battery systems. Here, the liquid’s behaviour differs fundamentally from traditional electrolytes.’

Though some first product components using ionic liquids are operational already, for example in solar cells, fundamental understanding falls behind. Other unique properties of phosphor ionic liquids include: high ionic strength, extremely low vapor pressure, a wide electrochemical window, and high thermal and chemical stabilities.

Apart from electrochemical energy storage devices: electrically tuneable lubrication, high-temperature/vacuum material synthesis, and novel self-assembly media, are feasible application areas, thus Monchai. ‘In this extremely challenging academic research field, we as a Mesa+ Nanoionics Group, focused on the interfacial behaviour of the ionic liquids, both in pure and dilute forms, aiming to reveal their true interfacial nature.’

Because ionic liquids do not obey well-established classical electrical double layer models for electrolytes, experimental studies were performed in this thesis.

Atomic force microscopy (AFM) was used to directly probe the double layer force. Electrochemical techniques were used to measure the response to the electric field of the liquids, in terms of a double layer capacitance.


‘The observations from these two experimental techniques led to a molecular scale picture of the double layer and its charging mechanism,’ Monchai states. ‘On the AFM experiments, I collaborated with the Physics of Complex Fluids Group. It is great that at Mesa+ all expertise and up-to-date equipment is available.’


Monchai’s first publication was in Journal of Physical Chemistry C, in his second PhD year. ‘It made me feel more confident, knowing I was on the right research track,’ he says. ‘In this highly competitive research field, soon other researchers found new things.’

‘My most noteworthy publication was in Journal of Physical Chemistry Letters. We showed a straightforward experimental approach – not needing new assumptions – to check, and affirm, theoretical theories describing the way energy is stored at liquids/electrode interfaces. In this way the true nature of the material was brought one step closer.

‘I am very happy to have contributed to this progress, and that colleagues in the field adopted the approach. Designers in industry can use the outcomes as well, leading to new or better products.’  

Double layer capacitance was measured using electrochemical techniques. Capacitance curves well fitted recently proposed theoretical models for ionic liquids, leading to an understanding of the charging process of the ions. ‘The results explained in this thesis, are relevant for many applications, ranging from tunable lubrication to electrochemical energy storage devices,’ Monchai says.


In the last chapter of his thesis, Monchai describes some future challenges.

‘Apart from learning new skills in new fields within physics, I am now confident to have reached a fair level of expertise,’ Monchai says. ‘Interfacial physical properties and theories were totally new for me, as was working on the nanoscale.’


Monchai: ‘After the PhD Defence I will return to my home country Thailand, as I received a scholarship to join Mesa+ for this project. I hope to find an ambitious research group at one of Thailand’s Universities. Depending on the financial possibilities, performing research on ionic liquids would be a good topic for me, to contribute and take a leading role in. Otherwise, I will contribute to another research area, benefiting from the skills, knowledge and experience I gained here in Twente.’