(562j) Preparation of Flexible Denim-Derived Activated Carbon Cloths for Environmental and Electrochemical Applications

Cordero-Lanzac, T. - Presenter, Universidad de Málaga
García-Mateos, F. J., Universidad de Málaga
Rosas, J. M., Universidad de Málaga
Rodríguez-Mirasol, J., Universidad de Málaga
Cordero, T., Universidad de Málaga

150%">The particular fibrous morphology of textile waste among
the rest of biomass sources makes attractive the possibility of their
valorization as precursor of carbon materials for selected applications.1 Activated carbon cloths are
highly conductive materials with excellent mechanical flexibility, low weight,
and chemical stability.2 However, modifications on its
properties are usually required in order to obtain carbons with comparable
features to those materials currently used. Within the commonly reported
modifications, the incorporation of heteroatoms and the customization of the
porous texture stand out.3 In this work, a one-step
strategy for preparing activated carbons with P and N heteroatoms is presented.
Activated carbons with different porous texture and surface chemistry are
obtained and tested as adsorbent and binderless electrode for pollutant removal
and energy storage, respectively.

activated carbon cloths were prepared by chemical activation of denim cloth
waste (DCW) with H3PO4. The composition of the DCW,
analyzed by means of ultimate analysis, was 43.9 wt% C, 6.3 wt% H, 0.4 wt% N
and 49.4 wt% O (by difference). This high C content makes DCW suitable as a
potential activated carbon precursor. Different impregnation ratios, in terms
of wt. H3PO4/wt. DCW, and activation temperatures were
used in order to investigate the effect of the preparation conditions on the
properties of the final material. Then, DCW was impregnated with H3PO4
using 0.5 and 1 impregnation ratios, and the mixture was heat-treated at
60 ºC for 12 h. Subsequently, samples were carbonized at 500, 600, 700,
800 or 900 ºC in a tubular furnace under a continuous flow of N2
(150 cm3 min-1), reaching the final temperature
at a heating rate of 10 ºC min-1. After an isothermal stage of
2 h, samples were cooled down and the resulting activated carbons were washed
with distilled water at 60 ºC until negative phosphate was detected in the
eluent. Activated carbons were named with the used impregnation ratio and
carbonization temperature (e.g. AC0.5-900).

150%">The morphology of the carbon materials were
characterized by scanning electron microscopy (SEM), their surface chemistry
was analyzed through X-ray photoelectron spectroscopy (XPS) analysis and their porous
structure by means of N2 adsorption-desorption at -196 ºC and CO2
adsorption at 0 ºC.

150%">On the one hand, the potential application of these
materials as adsorbent was analyzed using phenol as a model molecule. For the
adsorption runs, samples of 30 mg (dry basis) were placed in a spinning basket
batch reactor at laboratory scale. Experiments were carried out at 25 ºC
using 100 mL of a phenol solution with different concentrations (from 1 to
130 mg L-1). Phenol kinetic adsorption experiments were also carried
out at 25 ºC and using 40 mg of activated carbon cloths and 150 mL of phenol
solution with an initial phenol concentration of 100 mg L-1. In this
application, samples activated with the same impregnation ratio (1 g H3PO4/g
DCW) and different carbonization temperatures (500-800 ºC) were compared.
On the other hand, the feasibility of these materials as binderless carbon electrodes
were tested in a three-electrode cell, using a Pt wire as counter electrode, a
reference Ag/AgCl/KCl 3M electrode and an aqueous solution of H2SO4
(1 M) as acid electrolyte. Activated carbon samples of 0.5 cm2
were directly used as working electrode, with the requirement of neither binder
nor conductivity promoter. Samples were placed inside a folded stainless steel
sheet which acts as the current collector. Cyclic voltammetry (CV) experiments
were carried out between the potential limits of 0.0 and 0.8 V at a scan
rate of 1 mV s-1, whereas galvanostatic charge/discharge
(GCD) measurements were performed at 250 mA g-1. Gravimetric
capacitances were calculated from the discharge brand of the GCD profiles.

150%">As observed in the SEM micrographs of Figure 1a,
activated carbon shows the fibrous morphology attributed to the DCW precursor.
In fact, woven carbon fibers are observed for samples activated at both
impregnation ratios and any carbonization temperature (AC1-700 in Figure 1b).
Fibers of ca.7.4 ± 0.7 μm were yielded
after the chemical activation. Interestingly, flexible and easy-to-handle
fibers are obtained after the thermal treatment (see inset of Figure 1),
which encourages a versatile usage of these materials in several experimental
configurations. Regarding the surface chemistry of activated carbon cloths,
three heteroatoms are mainly identified by XPS analyses: oxygen, phosphorus and
nitrogen. P-groups are anchored during the activation treatment, whereas N is
present in the initial composition of the precursor and forms different
structures during the thermal treatment. In this case, carbonization
temperature plays a key role in the heteroatom functionalities. Thereby, the
formation of condensed structures, such as -C3P=O or pyridinic and
quaternary N, are favored at high temperatures. Otherwise, more oxidized
-C-(O)-PO3 and pyrrolic N are the main structures observed when low
carbonization temperatures are used. The impregnation ratio is nonetheless
crucial in the development of the porosity of the activated carbon, thus achieving
a more mesoporous structure upon increasing the impregnation ratio. Selecting
combinations of impregnation ratios and carbonization temperature allows for
obtaining activated carbon with the desired porosity for each application (external
surface area values from t-plot method: AC0.5-500, 56 m2 g-1;
AC1-500, 85 m2 g-1; AC0.5-900, 51m2 g-1
and; AC1-900, 528 m2 g-1).


line-height:150%">Figure 1. SEM
micrographs of the selected samples (a) AC0.5-500 and (b)


150%">Phenol adsorption experiments were carried out with four
selected carbon samples activated with the same impregnation ratio and
different temperatures (from 500 to 800 ºC). Figure 2 shows the phenol adsorption
isotherms obtained at 25 ºC. As observed, all experimental data can be fitted
to Langmuir-type adsorption isotherms in order to estimate the maximum adsorption
capacity of each activated carbon. A strong influence of the carbonization
temperature on the adsorption isotherms is observed, thus registering an
increase in the total amount of adsorbed phenol with temperature. AC1-800
exhibits the highest adsorption capacity (qL) value
(114 mg g-1), whereas AC1-500 shows the lowest one
(57 mg g-1). Langmuir constants (KL) are
predicted similar in all cases with maximum and minimum values of 0.091 and
0.045 L mg-1.


line-height:150%">Figure 2.
Phenol adsorption isotherms at 25 ºC obtained for different activated carbon cloths


150%">For electrochemical applications, the main drawback of
these materials as binderless electrodes is the low conductivity of the working
electrodes due to: (i) a low order of carbon structures or (ii) an inefficient contact
between carbon samples and current collector. In order to overcome these
operational hurdles, flexible activated carbons prepared at the highest
carbonization temperature (900 ºC) ensure an appropriate conductivity of
the carbon-based working electrode without using any conductivity promoter. In
this regard, AC0.5-900 and AC1-900 were characterized by means of cyclic
voltammetry and galvanostatic charge/discharge (Figure 3a and Figure
, respectively). Both of them show a rectangular voltammogram shape,
associated with the development of the electric double layer (EDL). This
accumulation of charges on the porous structure of activated carbon is the most
common mechanism of energy storage in porous materials. However, a little hump
at 0.33 V is also observed. This anodic current is attributed to the
pseudocapacitive interaction between the electrolyte and the oxygenated surface
groups. In this particular case, it is associated with the fast redox reactions
between the protons of the acid electrolyte and the oxygen of C=O and P=O
surface groups.4 The presence of these faradic
reactions is also suggested by the non-linear GCD profiles observed in both
samples (Figure 3b). The electrodes show a relatively low IR drop, and
the triangle shape indicates their suitable capacitive behavior. From the GCD profiles,
gravimetric capacitances of 237 ± 11 and 189 ± 15 F g-1
are calculated for AC0.5-900 and AC1-900, respectively. This significant
difference could be ascribed to the marked difference in the specific area of
each sample (form BET equation: AC0.5-900, 2032 m2 g-1
and AC1-900, 1530 m2 g-1).


line-height:150%">Figure 3. (a)
Cyclic voltammograms and (b) GCD profiles for AC0.5-900 and AC1-900



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