Strain-Based In Situ Study of Anion and Cation Insertion into Porous Carbon Electrodes with Different Pore Sizes
Jennifer M. Black1,Guang Feng2,*,Pasquale F. Fulvio3,Patrick C. Hillesheim3,Sheng Dai3,4,Yury Gogotsi5,Peter T. Cummings2,Sergei V. Kalinin1,Nina Balke1,*
1 Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN, USA
2 Chemical & Biomolecular Engineering, Vanderbilt University, Nashville, TN, USA
3 Chemical Sciences Division Oak Ridge National Laboratory, Oak Ridge, TN, USA
4 Department of Chemistry University of Tennessee, Knoxville, TN, USA
5 Department of Materials Science and Engineering and A. J. Drexel Nanotechnology Institute Drexel University, Philadelphia, PA, USA
Article first published online: 8 OCT 2013, Adv. Energy Mater.,DOI: 10.1002/aenm.201300683
Keywords: electrochemical capacitors; atomic force microscopy; molecular dynamics; ionic liquids
Atomic force microscopy is used to monitor the expansion of porous carbon electrodes, which results from insertion/adsorption of ions in carbon pores during charging. The strain data collected at various potential scan rates are used to obtain information on anion and cation kinetics. Molecular dynamics simulations are performed to determine the molecular origins of charge-induced expansion in porous carbons.
The expansion of porous carbon electrodes in a room temperature ionic liquid (RTIL) is studied using in situ atomic force microscopy (AFM). The effect of carbon surface area and pore size/pore size distribution on the observed strain profile and ion kinetics is examined. Additionally, the influence of the potential scan rate on the strain response is investigated. By analyzing the strain data at various potential scan rates, information on ion kinetics in the different carbon materials is obtained. Molecular dynamics (MD) simulations are performed to compare with and provide molecular insights into the experimental results; this is the first MD work investigating the pressure exerted on porous electrodes under applied potential in a RTIL electrolyte. Using MD, the pressure exerted on the pore wall is calculated as a function of potential/charge for both a micropore (1.2 nm) and a mesopore (7.0 nm). The shape of the calculated pressure profile matches closely with the strain profiles observed experimentally.
N2 adsorption measurements were performed to determine the surface area and pore size distribution s of the carbon membranes used in this study. Figure S1 a and b show the N2 adsorption isotherms as well as the calculated pore size distributions (PSDs) for the MC, MC - A, and MC - G membranes. These are type IV isotherms with H1 hysteresis loops characteristic of materials with large mesopores. 
The steepness of the capillary condensation step results from the uniform diameter of the main mesopores and consequently narrow pore size distribution, which i s usually reported for ordered and disordered soft - templated carbon materials. The surface area of the MC, MC - A, and MC - G carbons w ere determined to be 579, 798, and 282 m 2 g - 1 , respectively. Mechanical indentation experiments were also performed to dete rmine the hardness of the carbon membranes, and results are shown in Figure S1c. From the indentation experiments the Young’s modulus for the MC, MC - A, and MC - G w ere determined to be 6.783, 4.586, and 9.655 GPa, respectively.
To compare the strain behavior of a non-porous carbon with the porous carbons used in this study, the charge induced expansion of a non-porous glassy carbon electrode was also measured using in-situ AFM.
Figure S2 shows the relative height change of MC membrane and glassy carbon over three cyclic voltammogram cycles performed at 1 mV s-1. The MC membrane experiences an expansion of ca. 0.15% with the maximum occurring at the most positive and most negative applied potentials. For the glassy carbon electrode the maximum strain observed was very small (<0.001 %), and was unrelated to the potential applied to the electrode. The absence of strain observed in the non-porous glassy carbon provides further support that the expansion experienced in porous carbon materials is related to the ion insertion/adsorption in the carbon pores.
References:  X. Q. Wang, Q. Zhu, S. M. Mahurin, C. D. Liang, S. Dai, Carbon, 2010, 48, 557