Exactly matched pore size for the intercalation of electrolyte ions determined using the tunable swelling of graphite oxide in supercapacitor electrodes

Abstract

The intercalation of solvent molecules and ions into sub-nanometer-sized pores is one of the most disputed subjects in the electrochemical energy storage applications of porous materials. Here, we demonstrate that the temperature- and concentration-dependent swelling of graphite oxide (GO) can be used to determine the smallest pore size required for the intercalation of electrolyte ions into hydrophilic pores. The structure of Brodie graphite oxide (BGO) in acetonitrile can be temperature-switched between the ambient one-layer solvate with an interlayer distance of ∼8.9 Å and the two-layer solvate (∼12.5 Å) at low temperature, thus providing slit pores of approximately 2.5 and 6 Å. Using in situ synchrotron radiation X-ray diffraction (XRD) and the temperature dependence of capacitance in supercapacitor devices, we found that solvated tetraethylammonium tetrafluoroborate (TEA-BF₄) ions do not penetrate into both the 2.5 and 6 Å slit pores formed by BGO interlayers. However, increasing the electrolyte concentration results in the formation of a new phase at low temperature. This phase shows a distinct interlayer distance of ∼15–16.6 Å, which corresponds to the insertion of partly desolvated TEA-BF₄ ions. Therefore, the remarkable ability of the GO structure to adopt variable interlayer distances allows for the determination of pore sizes that are optimal for solvated TEA-BF₄ ions (about 9–10 Å). The intercalation of TEA-BF₄ ions into the BGO structure is also detected as an anomaly in the temperature dependence of supercapacitor performance. The BGO structure remains to be expanded, even after the removal of acetonitrile, adopting an interlayer distance of ∼10 Å.