[Paper Review] Electrochemistry, Ion Adsorption and Dynamics in the Double Layer: A Study of NaCl(aq) on Graphite

Aaron R. Finney, Ian J. McPherson+2 more|arXiv (Cornell University)|Apr 23, 2021

Electrostatics and Colloid Interactions90 references70 citations

TL;DR

This study uses constant chemical potential molecular dynamics (CµMD) simulations to investigate the NaCl(aq)–graphite double layer across a wide range of bulk concentrations, revealing that specific Na+ adsorption induces interfacial charging even without applied potential. Beyond ~0.6 M, ion crowding and over-screening lead to alternating compact charge layers, anomalous negative shifts in the potential of zero charge, and reduced ion diffusion (up to fivefold slower at >5 M), with solution-side contributions dominating electrochemical capacitance—challenging the long-held assumption that capacitance reflects only the electrode's density of states.

ABSTRACT

Graphite is a ubiquitous electrode material with particular promise for use in e.g., energy storage and desalination devices, but very little is known about the properties of the graphite-electrolyte double layer at technologically relevant concentrations. Here, the (electrified) graphite-NaCl(aq) interface was examined using constant chemical potential molecular dynamics simulations; this approach avoids ion depletion (due to surface adsorption) and maintains a constant concentration of ions beyond the surface. Specific Na+ adsorption at the graphite basal surface causes charging of the interface in the absence of an applied potential. At moderate bulk concentrations, this leads to accumulation of counter-ions in a diffuse layer to balance the effective surface charge, consistent with established models of the electrical double layer (DL). Beyond 0.6 M, however, a combination of over-screening and ion crowding in the DL results in alternating compact layers of ion density perpendicular to the interface. The transition to this regime is marked by an increasing DL size and anomalous negative shifts to the potential of zero charge with incremental changes to the bulk concentration. Our observations are supported by changes to the position of the differential capacitance minimum measured by electrochemical impedance spectroscopy. Furthermore, a striking level of agreement between the differential capacitance from simulations and experiments allows us to critically assess the accepted norm that electrochemical capacitance measurements report simply on the density of states of the graphite material. Finally, ion crowding at the highest concentrations (beyond 5 M) leads to the formation of liquid-like NaCl clusters confined to highly non-ideal regions of the double layer, where ion diffusion is up to five times slower than in the bulk.

Motivation & Objective

• To understand the structure and dynamics of the NaCl(aq)–graphite electrical double layer at technologically relevant concentrations.
• To resolve discrepancies between classical Gouy-Chapman-Stern models and experimental observations at high electrolyte concentrations.
• To assess the contribution of the solution side of the double layer to electrochemical capacitance, challenging the assumption that capacitance reflects only the electrode's electronic density of states.
• To investigate ion speciation, coordination, and diffusion in confined interfacial regions under varying bulk concentrations and surface charge.

Proposed method

• Constant chemical potential molecular dynamics (CµMD) simulations were employed to maintain electroneutrality and constant bulk concentration, avoiding ion depletion due to surface adsorption.
• Simulations used classical force fields parameterized from DFT, with explicit water and NaCl ions, on a graphite slab (8 graphene layers) symmetrically positioned in the simulation cell.
• Surface charge was applied uniformly to the graphite, and the system was coupled with a reservoir to maintain fixed bulk ion concentrations (0.23–9.2 M).
• Ion density profiles, coordination numbers, and diffusion coefficients were calculated from 100 ns trajectories using 50 × 1 ns windows for statistical averaging.
• Differential capacitance was evaluated from simulation data and compared with experimental electrochemical impedance spectroscopy (EIS) measurements.
• The electrode screening factor and electric field/potential profiles were computed to analyze charge screening and double layer structure.

Experimental results

Research questions

• RQ1 How does the structure of the NaCl(aq)–graphite double layer evolve with increasing bulk concentration, particularly beyond 0.6 M?
• RQ2 What causes the anomalous negative shifts in the potential of zero charge with incremental concentration increases, and how is this related to ion adsorption and screening?
• RQ3 To what extent does the solution side of the double layer contribute to the measured electrochemical capacitance, and how does this challenge the conventional interpretation of capacitance as a probe of the electrode's density of states?
• RQ4 How do ion dynamics, speciation, and coordination change in the interfacial region, especially at high concentrations (>5 M), and what are the implications for ion transport?

Key findings

• Specific Na+ adsorption at the graphite basal plane induces interfacial charging even without an applied potential, establishing a surface charge density of up to 1 e nm⁻².
• At bulk concentrations above ~0.6 M, ion crowding and over-screening lead to alternating compact layers of charge density perpendicular to the interface, deviating from the diffuse layer model.
• The double layer size increases with concentration, and the potential of zero charge shifts anomalously negative with incremental concentration increases, consistent with experimental EIS measurements.
• Ion diffusion in the double layer slows by up to fivefold at concentrations above 5 M, due to confinement and formation of liquid-like NaCl clusters in highly non-ideal regions.
• The solution side of the double layer provides the dominant contribution to electrochemical capacitance, contradicting the long-standing assumption that capacitance reflects only the electrode’s electronic density of states.
• A striking agreement between simulated and experimental differential capacitance values validates the CµMD approach and reveals that ion speciation and solvation dynamics are critical to interfacial electrochemistry.