Electrode/electrolyte interfaces are critical to all electrical energy storage technologies, yet there remains limited understanding of how physiochemical properties of these devices are altered by the interfacial electric field generated during charging. The structural and dynamical responses of the electrolyte to an applied potential have been extensively studied, whereas the analogous responses of the electrode material during operation remain largely unexplored, even for widely used materials such as graphitic electrodes. Herein, we demonstrate that graphene-based supercapacitor electrodes undergo complex, electric field induced, changes in electronic structure during operation that have their origin in modifications to the electrode surface chemistry and morphology.

… Our results bolster a nascent model in which inter-facial capacitance and charge storage are not solely determined by the isolated properties of the electrode and electrolyte, but can be strongly influenced by polarization-induced and electrolyte-mediated modifications to the electrode itself.

—Bagge-Hansen et al.

The complex and dynamic behavior of polarized electrode–electrolyte interfaces strongly influence the core functionality of all electrochemical energy storage systems—and especially for electric double layer (EDL) capacitors, or supercapacitors. Supercapacitors store electrical energy solely by polarization of the electrode–electrolyte interface. Accordingly, the LLNL team noted, the desire for significantly increased capacity and efficiency of EDL capacitors and other electrochemical energy storage systems has motivated extensive research focused on the electrolyte—i.e., the transport, proximity, and arrangement of ions approaching the electrode surface.

On the other hand, very little is known about the equally important dynamic physiochemical response of the electrode to charge and discharge, including any electric-field induced changes to the electronic structure during operation. Instead, the electrode is conventionally considered to be static, with charge accumulation or depletion as the only response to polarization of the interface. This lack of understanding of the dynamic physio-chemical changes of the electrode is largely due to the paucity of experimental and theoretical methods for characterization of the electrode electronic structure under operating conditions.

—Bagge-Hansen et al.

Graphitic supercapacitors are ideal model systems to probe interfacial phenomena because they are chemically relatively stable, extensively characterized experimentally and theoretically and are interesting technologically. The team used its recently developed 3D nanographene (3D-NG) bulk electrode material as a model graphitic material.

3D-NG (graphene aerogel, earlier post) is composed of interconnected single-layer graphene sheets, is binder- and substrate-free, and has a well-characterized hierarchical pore structure.

Our newly developed X-ray adsorption spectroscopy capability allowed us to detect the complex, electric-field induced changes in electronic structure that graphene-based supercapacitor electrodes undergo during operation. Analysis of these changes provided information on how the structure and bonding of the electrodes evolve during charging and discharging. The integration of unique modeling capabilities for studying the charged electrode-electrolyte interface played a crucial role in our interpretation of the experimental data.

—Jonathan Lee, corresponding author

Discovering that the electronic structure of graphitic carbon supercapacitor electrodes can be tailored by charge-induced electrode-electrolyte interactions opens a new window toward more efficient electrochemical energy storage systems. In addition, the experimental and modeling techniques developed during the research are readily applicable to other energy storage materials and technologies.

Other Livermore researchers include Michael Bagge-Hansen, Brandon Wood, Tadashi Ogitsu, Trevor Willey, Ich Tran, Arne Wittstock, Monika Biener, Matthew Merrill, Marcus Worsley, Theodore Baumann, Tony van Buuren and Jürgen Biener. The research was conducted in collaboration with scientists at additional institutions, including Minoru Otani from the Nanosystem Research Institute in Japan; David Prendergast from the Molecular Foundry; and Jinghua Guo and Cheng-Hao Chuang of the Advanced Light Source Division at Lawrence Berkeley National Laboratory. A substantial portion of the research was supported by LLNL’s Laboratory Directed Research and Development (LDRD) program.

Resources

• Bagge-Hansen, M., Wood, B. C., Ogitsu, T., Willey, T. M., Tran, I. C., Wittstock, A., Biener, M. M., Merrill, M. D., Worsley, M. A., Otani, M., Chuang, C.-H., Prendergast, D., Guo, J., Baumann, T. F., van Buuren, T., Biener, J. and Lee, J. R. I. (2015) “Potential-Induced Electronic Structure Changes in Supercapacitor Electrodes Observed by In Operando Soft X-Ray Spectroscopy.” Adv. Mater., 27: 1512–1518. doi: 10.1002/adma.201403680