Electrochemical Analysis of Alkaline Metal Cation Adsorption on a Graphene
Interface by Electrochemical Surface X-ray Diffraction

S. Yasuda1), K. Tamura2), M. Kato3), H. Asaoka1) and I. Yagi3)
1) : Nanoscale Gr., JAEA 2): MSRC, JAEA 3): Hokkaido Univ.

Understanding electrochemical behavior of the alkaline metal
cation–graphene interface in electrolyte is essential forunderstanding the fundamental electrochemical interface anddevelopment of graphene-based technologies [1]. However,most studies have been mainly conducted based on typicalelectrochemical analysis and computational evaluation, and thefundamental properties of cations on graphene at the electrified
interface by the experimental approach still remain poorlyunderstood. Here, we report analysis of the electrochemical
behavior of both alkaline metal cations and graphene using
electrochemical surface X-ray diffraction (EC-SXRD) in which
the interfacial structure of cations can be elucidated [2]. As anmodel interface, monolayer graphene synthesized on an
atomically flat Au(111) electrode (graphene/Au(111)) wasemployed. Crystal truncation rod (CTR) surface diffraction
measurements of EC-SXRD were conducted on thegraphene/Au(111) electrodes at BL14B1, SPring-8 using a κdiffractometer.
Figure 1a shows the (0, 0, L) CTR profile of thegraphene/Au(111) electrode in 0.1 M KOH at 0 V vs Ag/AgCl,and one intense Bragg peak is clearly observed at L = 3. Whenthe potential is set from 0 to −1.15 V where the adsorption of K+
ions occurs, dips in CTR profile around L = 2 and 4.5 are clearlychanged (Figure 1b). These changes in the profile only appear ata more negative potential where the adsorption of K+ ionsoccurs and is eliminated when the potential is subsequently set
to more positive potential where the desorption of K+ ionsoccurs. Each profile is fitted using a three-layer structural model(first layer, adsorbates, such as ions and water; second layer,graphene; third layer, reconstructed Au(111) surface) tounderstand the potential-induced profile change. The model
analysis revealed that the water structural layer is formed ongraphene at 0 V, while the electrochemical dehydration of
hydrated K+ is involved at -1.15 V, and resultant cationicadsorbed layer structure is formed on the graphene surface. Inaddition, electrochemical analysis (date not shown here) foundthat the charge density for the K+ adsorption can be estimated tobe ∼17 μC cm–2, which corresponds to ∼0.1 × 1015 K+ cm–2, andis equivalent to ∼1 nm2 per one potassium ion. This value isclose to the estimated value from the EC-SXRD diffraction data(∼0.14 × 1015 atoms cm–2) and seems very reasonable for thediameter of hydrated K+, indicating that the adsorption anddesorption behavior is based on one potassium ion with onepositivecharge.
Interestingly,
electrochemical-Raman
spectroscopy also revealed that under K+ adsorption potential,
graphene becomes a highly electron doped state with lattice
expansion, which is a similar feature of the potassium–graphite
intercalation compound (data not shown here).
From these results, we can consider the mechanism of thecation adsorption and desorption on graphene and the associatedelectron high doping in graphene. Figure 2 shows a schematic
illustration of electrochemically driven adsorption anddesorption of cations on graphene proposed by our analyses.
When the potential is set around a more positive potential than apotential of zero charge (PZC) in which the value is located at 0.6V for our electrochemical system, the positively charged

Fig. 1 CTR profiles of the graphene/Au(111) electrode in 0.1 M KOH
at (a) 0 V and (b) −1.15 V vs Ag/AgCl. Circles and solid lines denoteexperimental data and the fitted curve, respectively.

Fig. 2 Schematic illustration of electrochemically driven adsorption
and desorption of cations involved in dehydration and hydration
process.

graphene would prevent the approach of hydrated cations to thesurface, and thus the water layer structure is mainly formed on
the graphene interface. The hydrated cations attract negatively
charged graphene at a more negative potential than the PZC andwould approach the graphene surface at the more negative
potential. When the potential reaches a certain value where it
attracts sufficient energy to overcome the hydration energy,
partial dehydration of cations electrochemically occurs, and thedehydrated cations are adsorbed on the graphene surface. The
partially dehydrated cation adsorption is also considered totrigger the high electron doping in graphene with lattice
expansion. Because the dehydrated cations are adsorbed on thegraphene surface, electrons would be simultaneously doped in
the graphene from the Au electrode to compensate for thepositive charge by the cation adsorption, reaching a high doping
level. When the potential is swept positively back to the PZC,
the adsorbed cations would be hydrated again, desorbing fromthe graphene surface because the attractive energy becomes
lower than the hydration energy. These findings have not beenreported so far and provide a fundamental understanding of theelectrochemical electrolyte–graphene interface and developmentof novel catalytic and sensing graphene-based carbon electrodeapplications.
References[1] Z. Cheng et al., Nat. Commun. 10, 4858 (2019).
[2] S. Yasuda et al., J. Phys. Chem. C 125, 22154 (2021).