Why ionic liquids make efficient “glue” for electrochemical electrodes
In our recent Communications Chemistry paper (DOI: 10.1038/s42004-025-01877-5), we focus on carbon paste electrodes (CPEs). These electrodes are made by mixing graphite particles with a binder to form a conductive paste. CPEs are attractive because they are simple, inexpensive, and highly customizable. Traditionally, the binder has been treated as an inert “glue” whose only role is mechanical. However, growing evidence suggests that binders can strongly influence electrochemical behavior.
Why graphite surfaces matter
Graphite is not a uniform material. It exposes two main types of surfaces: (1) the basal planes, which are flat and relatively unreactive, and (2) the edge planes, which are chemically more active and much better at exchanging electrons with molecules in solution.
Fast electrochemical reactions depend critically on access to these edge planes. Unfortunately, conventional binders such as paraffin oils tend to coat both basal and edge planes indiscriminately, blocking the most active sites and slowing electron transfer.
Ionic liquids as active binders
Ionic liquids (ILs)—salts that are liquid near room temperature—offer a promising alternative. They consist entirely of ions and combine high ionic conductivity, chemical stability, and negligible vapor pressure. These properties have made ILs popular as electrolytes, but they are increasingly explored as binders for CPEs.
Our study poses a simple yet important question: How do the various ionic liquids interact differently with graphite at the molecular level, and what impact does this have on electrochemical performance?
To answer this, we combined molecular dynamics (MD) simulations with electrochemical experiments, mainly cyclic voltammetry (CV). This allowed us to directly connect microscopic interfacial behavior with macroscopic electrochemical signals.
A focus on quaternary ammonium ionic liquids
We concentrated on a class of aprotic quaternary ammonium (QA) ionic liquids paired with the hydrophobic and electrochemically stable anion [NTf₂]⁻. These ILs have flexible, aliphatic cations, in contrast to more rigid, aromatic cations found in common imidazolium- or pyridinium-based ionic liquids.
What simulations revealed is that the MD simulations provided a clear picture of how QA-based ionic liquids behave at graphite surfaces. Unlike conventional oil binders, QA-ILs do not coat graphite uniformly. Instead, they preferentially wet the basal planes while largely leaving the edge planes exposed. This selective wetting is crucial because it preserves access to the most electrochemically active sites.
The simulations also unraveled that the aliphatic QA cations move very smoothly along the graphite surface. Their motion resembles low-friction “gliding” rather than the hopping behavior typical of rigid aromatic cations. This smooth motion indicates weak friction at the interface and enables rapid rearrangement of ions during electrochemical processes.
The [NTf₂]⁻ anion further enhances this behavior. Its charge is delocalized, making it flexible and mobile. Together, the QA cation and [NTf₂]⁻ anion form a highly dynamic interfacial layer that supports fast charge transport and efficient electrical double-layer (EDL) formation. So, the gliding cations and flexible anion enhance EDL capacitance and stability.
Cyclic voltammetry experiments strongly supported the simulation results. Carbon paste electrodes prepared with QA-based ionic liquid binders showed:
- Faster electron transfer kinetics than electrodes using traditional oil binders.
- A dominant non-Faradaic (capacitive) response, indicating efficient charge storage at the electrode surface.
- Only weak Faradaic (redox) currents, especially compared with electrodes based on aromatic ionic liquid binders.
This behavior is particularly desirable for applications such as supercapacitors, where rapid charging and discharging rely on capacitive processes rather than slower chemical reactions.
We also found subtle but meaningful trends within the QA family. For example, changing the length of the alkyl chains on the cation slightly altered how strongly the ions interacted with graphite. These differences translated into measurable changes in the balance between capacitive and Faradaic currents. Importantly, these experimental observations were fully consistent with both MD simulations and supporting quantum chemical calculations.
Well-known aromatic ionic liquids, such as those based on imidazolium or pyridinium cations, behave quite differently. Their rigid planar structures interact more strongly with the surface. This often leads to stronger adsorption, reduced ion mobility, and CV responses dominated by Faradaic processes. While such behavior may be useful in some contexts, it is less suitable for high-power energy storage.
Our results highlight a dual role of QA-based ionic liquid binders. (1) Structural role: by selectively wetting basal planes and leaving edge planes exposed, they enhance electron transfer. (2) Dynamic role: their flexible ions organize and move efficiently at the graphite interface, supporting rapid charge accumulation.
The choice of an anion was found to be equally important. The hydrophobic and redox-stable [NTf₂]⁻ anion proved well suited for aqueous electrochemical systems, whereas more hydrophilic and hydrolysis-prone anions such as [BF₄]⁻ are less reliable under similar conditions.
Looking ahead
This work confirms that binders should no longer be viewed as passive components in carbon paste electrodes. Instead, their molecular structure can be deliberately tuned to control surface exposure, ion dynamics, and ultimately electrochemical performance.
By linking atomistic simulations with experimental electrochemistry, we provide a clear design to offer fast screening principles for next-generation carbon-based electrodes, particularly for supercapacitors and related energy-storage devices. Sometimes, improving electrochemical performance does not require changing the electrode material itself—but choosing a smarter kind of “glue” to hold it together.