Quantification of ion confinement and desolvation in nanoporous carbon supercapacitors with modelling and in situ X-ray scattering

Storage and release of electric energy on a wide range of timescales play a crucial role for a sustainable energy management when implementing green technologies. This applies in particular to electric cars, microelectronics, or new forms of energy conversion. Today electric buses, aircraft doors or systems to recover breaking energy from vehicles already utilize a ultrafast energy storage technology called electrical double-layer capacitors or supercapacitors. These systems reveal higher power densities and much longer cycle lifetimes (>1 Million) than batteries. Although the principle design is similar to a conventional electrochemical cell, the charge storage mechanism in supercapacitors is purely physical: If the supercapacitor is charged electrons (or holes) attract cations (or anions) at the electrode-electrolyte interface forming an electric double layer and thereby provide the capacitive behavior. In order to store a large amount of energy the electrodes are highly porous with typical specific surface area of several thousand square meters per Gramm of the material. The tiny pores in such nanoporous carbon electrodes are not much larger than the (hydrated) ions themselves. Within the cross-linked network of pores, the ions have to share space with water molecules and ions of opposite charge. In this confined space large amounts of energy can be stored, yet ion transport could be hindered due to mutual blocking of ions with opposite charge, comparable to ion traffic jams. High energy densities therefore come along with low power densities. This subtle trade-off between power and energy needs to be understood on an atomistic level in order to improve the overall performance of supercapacitors.
An interdisciplinary team of scientists from the Institute of Physics of the Montanuniversitaet Leoben, the Graz University of Technology, the INM – Leibniz Institute of New Materials in Saarbrücken and the University of Vienna have recently reported a novel experimental and data analysis tool to increase the fundamental understanding of such phenomena. Combining in situ X-ray scattering and atomistic modeling they were able to make ion electrosorption visible on a sub-nanometer scale and claim to use this knowledge towards optimized electrode materials.
In situ scattering experiments were carried out at the Austrian SAXS beamline at ELETTRA using a custom-built in situ supercapacitor cell. Different nanoporous carbon materials were used as electrode material and high molar aqueous CsCl solutions as electrolyte. Installing a potentiostat at the beamline the SAXS intensities of the electolyte-filled working electrode were recorded during charging and discharging. The amount of information in the time-dependent SAXS data is huge; however, the complexity of the system makes their interpretation difficult. Therefore a novel data analysis approach, as visualized in Figure 1, was developed. First, a 3D pore model is generated from a simple ex situ SAXS measurement of the carbon electrode in air (A+B). The pore model is then populated with a specific number of cations and anions associated to each voltage step and obtained from the in situ experiment (C). Using a Monte Carlo simulation, the equilibrium configurations of ions are determined and a subsequent Fourier Transformation provides a simulated scattering intensities for each cell voltage. These simulated patterns can be compared with real in-situ measurements (D+E).
Using this analytical tool the ion positions can be tracked within the real space pore structure as a function of the applied voltage. Interestingly, ions do not just change their concentration within the electrode upon charging, but also they also change their preferred positions within the nanopores. By defining a parameter called “Degree of confinement” (DoC) the local ion rearrangement was investigated quantitatively. As a voltage is applied counter-ions preferably move into sites with high degree of confinement. This rearrangement is accompanied with a partial loss of the hydration shell each ion is carrying. As a major conclusion, ion charge was found to be stored in pore systems enabling the largest change of the ions’ DoC. By this way, the repulsive interaction between ions of the same charge are most effectively screened and ions can be packed most densely. In situ SAXS, therefore, allows a direct prediction of the capacitive performance of  nanoporous carbon electrodes. The developed method and insights are of great relevance also for other, related technologies dealing with ion electrosorption, like for instance capacitive seawater desalination.

Figure 1. The Scheme of the in-situ SAXS experiments and the data evaluation strategy. Reproduced with permission from [1]. © Nature Publishing Group.

This research was conducted by the following research team:

Christian Prehal¹, Christian Koczwara¹, Oskar Paris¹, Nicolas Jäckel², Anna Schreiber², Volker Presser²,Max Burian³, Heinz Amenitsch³, Markus A. Hartmann⁴

¹ Institute of Physics, Montanuniversitaet Leoben, Leoben, Austria
² INM – Leibniz Institute for New Materials, Saarbrücken, Germany
³ Austrian SAXS Beamline, Outstation of the Institute of Inorganic Chemistry, Graz University of Technology, c/o Elettra - Sincrotrone Trieste, Italy
⁴ Faculty of Physics, University of Vienna, Vienna, Austria

Oskar Paris, email: