Constant Potential Simulations Research Articles

Electrolytes in conducting nanopores: Revisiting constant charge and constant potential simulations.

Simulating electrolyte-electrode systems poses challenges due to the need to account for the electrode's response to ion movements in order to maintain a constant electrode potential, which slows down the simulations. To circumvent this, computationally more efficient constant charge (CC) simulations are sometimes employed. However, the accuracy of CC simulations in capturing the behavior of electrolyte-electrode systems remains unclear, especially for microporous electrodes. Herein, we consider electrolyte-filled slit nanopores and systematically analyze the in-pore ion structure and diffusivity using CC and constant potential simulations. Our results indicate that CC simulations provide comparable pore occupancies at high bulk ion densities and for highly charged pores, but they fail to accurately describe the ion structure and dynamics, particularly in quasi-2D (single-layer) pores and at low ion densities. We attribute these results to the superionic state emerging in conducting nanoconfinement and its interplay with excluded volume interactions.

Mechanistic understanding of the electrocatalytic conversion of CO into C2+ products by double-atom catalysts

Electrocatalytic CO reduction reaction (CORR) has gained increasing attentions to generate valuable multi-carbon (C2+) products, and designing high-performance electrocatalysts for CORR presents a great challenge for realizing its practical application. With the unique dual-atom site, double-atom catalyst (DAC) can provide enough geometric space to bind two CO molecules simultaneously for the key C–C coupling reaction. Herein, taking the typical 3d transition metal (TM) dimer anchored N-doped graphene (denoted as M1M2@NG) as representative, using first-principles calculations we theoretically investigate the catalytic mechanism and activity origin of CORR on DACs. It is found that the metal sites with higher d-band centers can effectively bind CO molecule, and further the d-band centers can be used as descriptors to predict the coupling free energy and coupling energy barrier for the key C–C coupling step of CO dimers. Intriguingly, among 21 candidates, CrCo and MnCo@NG DACs exhibit excellent performance for selectively generating ethanol (CH3CH2OH), with small C–C coupling energy barriers of 0.52 and 0.57 eV and ultralow limiting potentials of only −0.20 and −0.34 V, respectively, which has been further confirmed by the constant-potential simulations. Moreover, both heteronuclear CrCo and MnCo@NG exhibit significantly enhanced CORR performance compared with their homonuclear counterparts, which is ascribed to the unique d orbital synergy effect of the two different TM atoms. Our work provides rational guidelines for the design of efficient DACs for CORR, and brings useful insights into the boosted CORR performance enabled by the electronic synergy effect of the heteronuclear dual-atom site.

Constant potential simulations on a mesh.

Molecular dynamics simulations in a constant potential ensemble are an increasingly important tool to investigate charging mechanisms in next-generation energy storage devices. We present a highly efficient approach to compute electrostatic interactions in simulations employing a constant potential method (CPM) by introducing a particle-particle particle-mesh solver specifically designed for treating long-range interactions in a CPM. Moreover, we present evidence that a dipole correction term-commonly used in simulations with a slab-like geometry-must be used with caution if it is also to be used within a CPM. It is demonstrated that artifacts arising from the usage of the dipole correction term can be circumvented by enforcing a charge neutrality condition in the evaluation of the electrode charges at a given external potential.

Investigation of the Ionic Liquid Graphene Electric Double Layer in Supercapacitors Using Constant Potential Simulations

In this work, we investigate the effect of the cation structure on the structure and dynamics of the electrode–electrolyte interface using molecular dynamics simulations. A constant potential method is used to capture the behaviour of 1-ethyl-3-methylimidazolium bis (trifluoromethane)sulfonimide ([Cmim][NTf]) and butyltrimethylammonium bis(trifluoromethane) sulfonimide ([N][NTf]) ionic liquids at varying potential differences applied across the supercapacitor. We find that the details of the structure in the electric double layer and the dynamics differ significantly, yet the charge profile and capacitance do not vary greatly. For the systems considered, charging results in the rearrangement and reorientation of ions within ∼1 nm of the electrode rather than the diffusion of ions to/from the bulk region. This occurs on timescales of (10 ns) for the ionic liquids considered, and depends on the viscosity of the fluid.

Molecular mechanism of anion size regulating the nanostructure and charging process at ionic liquid–electrode interfaces

The effects of anion on the interfacial structure and charging process of the ionic liquid-based supercapacitor were explored via constant potential simulations.

Molecular Simulations of the Electric Double Layer Structure, Differential Capacitance, and Charging Kinetics for N-Methyl-N-propylpyrrolidinium Bis(fluorosulfonyl)imide at Graphite Electrodes

Molecular dynamics simulations were performed on N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide (pyr(13)FSI) room temperature ionic liquid (RTIL) confined between graphite electrodes as a function of applied potential at 393 and 453 K using an accurate force field developed in this work. The electric double layer (EDL) structure and differential capacitance (DC) of pyr(13)FSI was compared with the results of the previous study of a similar RTIL pyr(13)bis(trifluoromethanesulfonyl)imide (pyr(13)TFSI) with a significantly larger anion [ Vatamanu, J.; Borodin, O.; Smith, G. D. J. Am. Chem. Soc. 2010, 132, 14825]. Intriguingly, the smaller size of the FSI anion compared to TFSI did not result in a significant increase of the DC on the positive electrode. Instead, a 30% higher DC was observed on the negative electrode for pyr(13)FSI compared to pyr(13)TFSI. The larger DC observed on the negative electrode for pyr(13)FSI compared to pyr(13)TFSI was associated with two structural features of the EDL: (a) a closer approach of FSI compared to TFSI to the electrode surface and (b) a faster rate (vs potential decrease) of anion desorption from the electrode surface for FSI compared to TFSI. Additionally, the limiting behavior of DC at large applied potentials was investigated. Finally, we show that constant potential simulations indicate time scales of hundreds of picoseconds required for electrode charge/discharge and EDL formation.