Stratigraphic analysis of intercalated graphite electrodes in aqueous inorganic acid solutions

Stefania De Rosa,Lamberto Duò,Paolo Branchini,Valentina Spampinato,Gianlorenzo Bussetti,Rossella Yivlialin,Luca Tortora,Alexis Franquet

doi:10.1007/s12274-021-3614-6

Abstract

A detailed stratigraphic investigation of the intercalation mechanism when graphite electrodes are immersed inside diluted perchloric (HClO4) and sulfuric (H2SO4) electrolytes is obtained by comparing results when graphite crystals are simply immersed in the same acid solutions. By combining time-of-flight secondary ion mass spectrometry (ToF-SIMS) and in-situ atomic force microscopy (AFM), we provide a picture of the chemical species involved in the intercalation reaction. The depth intensity profile of the ion signals along the electrode crystal clearly shows a more complex mechanism for the intercalation process, where the local morphology of the basal plane plays a crucial role. Solvated anions are mostly located within the first tens of nanometers of graphite, but electrolytes also diffuse inside the buried layers for hundreds of nanometers, the latter process is also aided by the presence of mesoscopic crystal defects. Residual material from the electrolyte solution was found localized in well-defined circular spots, which represent preferential interaction areas. Interestingly, blister-like micro-structures similar to those observed on the highly oriented pyrolytic graphite (HOPG) surface were found in the buried layers, confirming the equivalence of the chemical condition on the graphite surface and in the underneath layers.

Highlights

Ion batteries development and, more recently, graphene industrial production are two important examples of technological areas where anion intercalation into stratified crystal structures plays a crucial role [1,2,3]
By combining time-of-flight secondary ion mass spectrometry (ToF-SIMS) and in-situ atomic force microscopy (AFM), we provide a picture of the chemical species involved in the intercalation reaction
When the fourth intercalation stage is reached during a cyclic-voltammetry, the graphite electrode becomes a so-called graphite intercalated compound (GIC)

Summary

Introduction

Ion batteries development (by exploiting the change of the transport properties) and, more recently, graphene industrial production (by quenching the layer-layer interaction) are two important examples of technological areas where anion intercalation into stratified crystal structures plays a crucial role [1,2,3]. In a very recent work, we employed time-of-flight secondary ion mass spectrometry (ToF-SIMS) technique proving that a more sensitive analysis of the chemical species present on the HOPG electrode surface after the first intercalation stage is possible despite only ex-situ measures are possible [30]. By using this technique, the correlation between chemical and morphological changes can be highlighted by combining mass spectra and chemical maps [31, 32]. Molecular species coming from the electrolyte (ClO4– and SO4H–), as well as oxygenbased molecular fragment ions generated as a consequence of the EC treatment (CO2H– and O2–), were revealed and located within the graphitic planes

Sample preparation

Results and discussion

Conclusions