Combining modeling and in situ x-ray scattering to quantify confinement and desolvation in nanoporous carbon supercapacitors


A fundamental understanding of the mechanisms controlling ion transport and arrangement in carbon nanopores is essential to improve the performance of supercapacitors or devices for capacitive desalination. In situ small angle x-ray scattering represents an excellent tool to study global ion fluxes and local ion rearrangements in nanopores of carbon electrodes during charging and discharging. Using a novel data analysis strategy, we find that charge is most effectively stored in sites of the carbon structure with highest possible geometrical confinement, accompanied with partial desolvation.

Figure 1.  Scheme of the in situ SAXS experiments and the data evaluation strategy. Reproduced and adapted with permission from Prehal, C. et al., Nat. Energy 2, 16215 (2017). © Nature Publishing Group..

Storage and release of electric energy on a wide range of timescales are crucial 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 that recover breaking energy from vehicles already utilize an 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 design is like 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 as many ions as possible, the electrodes are highly porous with typical specific surface area of several thousand square meters per Gramm of the material. The 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.

In the present work we report a novel experimental and data analysis tool to increase the fundamental understanding of such phenomena. Combining in situ x-ray scattering and atomistic modeling we visualized ion electrosorption on a sub-nanometer scale and to benefit the further development of 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 concentrated aqueous CsCl solutions as electrolyte. Installing a potentiostat at the beamline, 2D small-angle x-ray scattering (SAXS) patterns of the electrolyte-filled working electrode were recorded during charging and discharging. There are considerable changes of the time-dependent SAXS data, however, the complexity of the system makes their interpretation difficult. Therefore a novel data analysis approach, as visualized in Fig. 1, was introduced. First, a 3D pore model was generated from a simple ex situ SAXS measurement of the carbon electrode in air using the concept of Gaussian Random Fields (Fig. 1a and b). The pore model was then populated with a specific number of cations and anions associated to each voltage step and obtained from the in situ experiment (Fig. 1c). Using a Monte Carlo simulation, the equilibrium configurations of ions were determined and a subsequent Fast Fourier Transformation provided simulated scattering intensities for each cell voltage. These simulated patterns could be compared with real in situ measurements (Fig. 1d and 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 they also change their preferred positions within the nanopores. 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 is 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.

Retrieve article

Quantification of ion confinement and desolvation in nanoporous carbon supercapacitors with modelling and in situ X-ray scattering;
C. Prehal, C. Koczwara, N. Jäckel, A. Schreiber, M. Burian, H. Amenitsch, M. A. Hartmann, V. Presser, O. Paris;
Nat. Energy 2, 16215 (2017). doi: 10.1038/nenergy.2016.215

Last Updated on Tuesday, 14 May 2019 17:05