Relationship Between Pore Size and Reversible and Irreversible
Immobilization of Ionic Liquid Electrolytes in Porous Carbon Under Applied
Electric Potential
Shannon M. Mahurin,1 Eugene Mamontov,2 Matthew W. Thompson,3 Pengfei Zhang,1 C. Heath
Turner,4 Peter T. Cummings,3 Sheng Dai1,5
1
Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831
Chemical and Engineering Materials Division, Oak Ridge National Laboratory, Oak Ridge, TN
37831
3
Dept. of Chemical and Biomolecular Engineering, Vanderbilt University, Nashville, TN 37235
4
Dept. of Chemical and Biological Engineering, University of Alabama, Tuscaloosa, AL 35487
5
Dept. of Chemistry, University of Tennessee, Knoxville, TN 37996
2
I. Synthesis
The soft-templated carbon was synthesized using a previously described method. Briefly,
approximately 6.6 g of phloroglucinol and 12.6g F127 were initially dissolved in an
HCl/ethanol/H2O solution. After mixing, 13 g of formaldehyde solution (37%) was added and
the resulting solution was stirred for approximately 3 min until phase separation was observed.
After stirring for an additional 12 min, the polymer-rich gel phase and the liquid phase were
separated by centrifugation at 7000 RPM for 5 min. The gel was re-dispersed in hexamine with
stirring. Any bubbles present in the solution were removed by centrifugation and the solution
was then cast onto a hydrophilic Mylar sheet using a doctor blade. The film was allowed to dry
at ambient conditions for 4 h following by heating to 80 °C overnight (12 h). The free-standing
polymer film was then cut into disks using a punch with 25 mm diameter. Carbonization of the
polymer disks was performed flowing argon (100 mL/min) at 450 °C for 2 h and subsequent 850
°C for an additional 2 h with a heating rate of 5 °C/min. The polymer disks were loaded between
two pieces of carbon paper to maintain the flat-sheet morphology.
Microporous carbon was derived from Matrimid 5218 films, which was purchased from
Huntsman Advanced Materials. The polymer was dissolved in tetrahydrofuran as a 10 weight %
solution and cast with a doctor blade (blade slit size ranging from 0.254 mm to 0. 635 mm) onto
a glass surface, while quickly covering the solution to promote slow, even evaporation and left to
dry overnight. The free-standing Matrimid films were then removed from the glass plates and cut
or punched to obtain the final desired shape. To obtain flat membranes after carbonization, the
films were placed between carbon paper and sandwiched between quartz plates. Carbonization
was conducted using a tube furnace under N2 at 100 mL/min at a heating rate of 3 °C/min up to
600 °C and held at this temperature for 2 h.
The RTIL was loaded into the carbon electrodes using an impregnation method in which the
electrode was first immersed in an ionic liquid/ethanol mixture. After solvent evaporation, the
electrode was placed in a vacuum oven at 60 °C overnight to remove residual solvent and
promote RTIL diffusion into the pores. Excess ionic liquid was removed from the outer surface
by gently swabbing with a Kimwipe. This was a critical step because any bulk RTIL on the
surface would mask the signal from the RTIL confined in the pores. The electrode was then
dried in a vacuum oven at 60 °C.
The electrochemical cell consisted of two aluminum plates with the RTIL-loaded carbon
membrane mounted between them. The counter electrode was a high surface area carbon
immersed in RTIL in the bottom half of the cell and surrounded by a highly porous polymer
sheet to prevent a direct connection to the cell. Only the upper half of the cell was exposed to
the neutron beam during the measurement to eliminate scattering from bulk RTIL and from the
counter electrode and connecting wires. The neutron beam is 3 cm by 3 cm so the upper half of
the cell was designed such that the size allowed for full interception of the neutron beam with a
width of 3 cm and height of 3.5 cm.
II. Nitrogen adsorption isotherms
450
1.6
400
350
1.2
dV(log r)
Amount Absorbed (cm3 STP/g)
Nitrogen adsorption isotherms were measured at −196 °C using a TriStar 3000 volumetric
adsorption analyzer manufactured by Micromeritics Instrument Corp. (Norcross, GA). Before
adsorption measurements the carbon electrodes were degassed in flowing nitrogen from 1 to 2 h
at 200 °C. The specific surface area of the samples was calculated using the Brunauer–Emmett–
Teller (BET) method within the relative pressure range of 0.05–0.20.
300
250
200
0.8
0.4
150
100
0.0
50
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P0)
Fig. S1. SAXS and XRD data for the two carbon samples
0
50
100
150
Pore Size (A)
200
250
III. X-ray Scattering of Carbon Samples
Fig. S2. SAXS and XRD data for the two carbon samples.
X-ray scattering measurements were acquired with a Panalytical Empyrean diffractometer with
Ni-filtered CuK ( = 1.54 Å) radiation operating at 45 kV and 40 mA. Wide angle scattering
was collected from 5-90° on the as-synthesized monolithic electrodes. Small angle scattering
was collected on both the mesoporous and microporous electrodes by stacking multiple
electrodes to the SAXS holder. The background was essentially the empty holder with no
electrodes present.
The XRD data show similar broad curves for both the mesoporous and the Matrimid carbons.
The mesoporous carbon has two peaks at 24.2° and 43.2° while the Matrimid carbon has two
similar peaks at 23.5° and 43.2°. The 23.5º peak of the Matrimid carbon is shifted slightly to
lower angle and is slightly broader which indicates that the pores of the Matrimid carbon are less
uniform and more turbostratic in nature. The SAXS curve for the Matrimid carbon shows very
little scattering indicating only microporosity in agreement with the BET measurements while
the mesoporous carbon shows a broad peak confirming the presence of mesopores.