3.1.4. Thermal Analysis

The thermogravimetric curves of representative PAni-ACC composites obtained at selected electropolymerization times are shown in Figure 8a and compared to the thermal evolution of the untreated ACC. During the first thermogravimetric run (25–120 ◦C, inset to Figure 8a), the carbon support and the PAni-loaded samples all exhibit a weight loss below 100 ◦C. This loss is associated with the endothermic release of moisture or weakly adsorbed water solvent molecules, and perhaps with the evaporation of some residual monomers [50–52]. The amount of water lost by the carbon support is lower than the water released from any polymer-modified fabric, probably because PAni imparts a significant hydrophilic character. However, no clear pattern correlating the amount of deposited

polymer and the water content was found. During the second thermogravimetric run from 25–1000 ◦C only negligible residual water is lost from the samples (Figure 8a).

**Figure 8.** (**a**) TGA curves of unmodified ACC (black line) and hybrid PAni-ACC composites synthesized after 40 min (red line) and 120 min (blue line) of galvanostatic electropolymerization; the inset shows the thermal pattern during the first thermogravimetric run up to 120 ◦C; the main panel shows the thermal behavior of the resulting dried samples; (**b**) Evolution of the total weight loss at 1000 ◦C for heat-dried PAni-ACC composites obtained at different electropolymerization times.

The dry carbon cloth undergoes a first smooth mass loss in the range 200–400 ◦C, followed by a continuous decomposition and a more pronounced loss between 900–1000 ◦C (Figure 8a, black line). These features are well connected with the thermal decomposition of oxygen surface complexes in carbonaceous materials, which are known to evolve as CO2 and CO [30]. The thermogravimetric curves of hybrid PAni-ACC composites show a prominent loss between 150–350 ◦C and a subsequent steady weight decrease up to 1000 ◦C. The observed thermal pattern is a characteristic feature of PAni-like powders or films in their emeraldine salt (i.e., doped) state [51–53]. The sharp loss in the temperature range 150–350 ◦C has been attributed to either chain scission or cross-linking processes (e.g., leading to phenazine-like segments) with dopant removal [50–52], while the loss above 400 ◦C has been related to structural degradation of the polymer backbone leading to its complete carbonization at the highest temperature [52–54].

Our results show that both the sharp feature within 150–350 ◦C and the total weight loss are closely related to the amount of CP deposited on the carbon fabric (Figure 8a, red and blue lines). In samples with a moderate PAni loading (red line in Figure 8a), the loss corresponding to the decomposition processes of the underlying carbon cloth (800–1000 ◦C) is still discerned. However, it is missed in heavily-loaded PAni-ACC composites (Figure 8a, blue line). The total weight loss of dry samples as a function of the electropolymerization time (Figure 8b) reflects the evolution of the polymer amount in the hybrid material. Figure 8b shows an abrupt increase at the early stages of the electropolymerization process, a stabilization region and a second increase at the longest treatment time. Thus, the total weight loss vs. time plot mimics the evolution of the photoelectron N/C vs. time plot (Figure 4).

Thermogravimetric data can be used to estimate the PAni loading of the different hybrid composites. The estimate was done as follows: First, the total weight loss of dry hybrid PAni-ACC at 1000 ◦C was corrected by the loss corresponding to the carbon support at the same temperature (7.57 wt.%); then, the amount of polymeric material in the composite was estimated after considering that about 45 wt.% residue was left after carbonization at 1000 ◦C [53,54]. PAni-loadings are expressed either as the mass of polymer (mg) per unit geometric area (cm2) of carbon fabric or as wt.%. The results are summarized in Table 2 for composites electrosynthesized at 10–120 min time. The data confirm that a massive

deposit of PAni occurs at 120 min of electropolymerization that accounts for ca. 50% of the composite material mass.


**Table 2.** PAni loadings of dried hybrid PAni-ACC composites synthesized after different electropolymerization times.

1 After subtraction of the total loss from the carbon fabric support (see text). 2 After considering that a 45 wt.% polymer residue remains on the heat-treated sample. 3 Calculated on the basis of cloth aerial density.

*3.2. Enhanced Electrical and Adsorptive Properties.*

3.2.1. Capacitance and Surface Sheet Resistance.

The electrical properties of the untreated ACC and PAni-coated hybrid composites were obtained from CV and four strip probe conductivity measurements (Figure 9). CVs became stabilized within two cycles and remained unchanged thereafter. No loss of material was seen during CV recording.

**Figure 9.** (**a**) Stabilized cyclic voltammograms of untreated ACC and hybrid PANi-ACC composites synthesized at different electropolymerization times. Scan rate: 1 mV·s<sup>−</sup>1, supporting electrolyte 1.0 M H2SO4; (**b**) Evolution of the surface sheet resistance upon the time of electropolymerization.

The CV of the untreated ACC (Figure 9a), dashed line) shows a capacitive response with a distorted rectangular shape that indicates a deviation from the ideal double-layer capacitor behavior [11,15]. This is a symptom of a slow charging/discharging response caused by a potential difference across micropores [18,55]. In addition to the main double layer charge/discharge contribution, a redox couple is discerned (E1/2 = 0.25 V). This feature is associated with a faradic process involving redox transitions of surface carbon–oxygen groups, thus behaving as a pseudocapacitance [11,30]. At low or intermediate electropolymerization time (0–60 min), a moderate increase in the voltammetric current occurs as a result of PAni film formation. This increase in current has a pseudocapacitive character and is mainly due to reversible redox transitions corresponding to the doping/dedoping processes in PAni. The broad redox couple located within 0.4–0.2 V has been assigned to the reversible transition between leucoemeraldine and emeraldine salt oxidation states of the polymer in several PAni-coated 3D porous carbon materials, like foams [15] and activated carbon fibers or clothes [11,22,55]. The pair of sharp and small peaks at ~0.0 V may be related to the presence of some phenazine- or phenoxazine-like moieties in the PAni backbone [41,56,57]. In accordance with the modest increase in the CV response, the specific capacitance of the PAni-ACC composites rises by up to a 12% (Table 3). Most of this increase occurs within the first 10 min of electropolymerization and then it tends to level off during the first hour of electropolymerization. The CV corresponding to thick PAni coatings formed at the longest treatment (Figure 9a, dotted line) appears noticeably tilted. Furthermore, the pseudocapacitive features of PAni are barely distinguishable and the total specific capacitance falls even below the capacitance of the unmodified ACC (Table 3). The abrupt decrease in the specific capacitance parallels the loss of available BET surface area measured by N2 adsorption isotherm. The evolution of surface sheet resistance with time (Figure 9b) follows up the reported changes in the CV response and the specific capacitance. Within the first 60 min of electropolymerization, the electrical resistance of the hybrid PAni-ACC composite diminishes, but it rises again at prolonged treatment, i.e., when a thick nanofibrous PAni coating develops.

**Table 3.** Gravimetric specific capacitance of hybrid PAni-ACC composites as a function of the electropolymerization time.


### 3.2.2. Adsorption of Acid Red 27

The experimental equilibrium data for the uptake of Acid Red 27 on unmodified ACC (Figure 10a) at 25 ◦C were fitted with the Langmuir and Freundlich models (see Appendix A), [58,59]. The isotherm parameters can be obtained from the slope and y-intercept of the *Ce*/*qe* vs. *Ce* and ln *qe* vs. ln *Ce* plots. The correlation coefficients for the linear regression fittings of Langmuir and Freundlich models to the experimental data were 0.9997 and 0.9397, respectively (Figure S1). Therefore, the equilibrium adsorption of Acid Red 27 on ACC is best described by the Langmuir isotherm, with a maximum adsorption capacity of 146.6 mg g<sup>−</sup><sup>1</sup> and *b*=0.82 L mg<sup>−</sup>1. The simulated adsorption equilibrium curve based on Langmuir parameters is also shown in Figure 10a (solid line).

The isothermal kinetic curves for the adsorption of Acid Red 27 on ACC are shown in Figure 10b for concentrations ranging 25–150 mg L−1. The dye uptake is faster at the initial stages of adsorption and then it decreases as the process approaches equilibrium, owing to the increasing occupancy of surface sites and slow diffusion into the internal porous structure [14]. The adsorption rate increases with the increasing initial concentration. The equilibrium uptake shows the same dependence, although it levels off at high initial concentrations because maximum adsorption capacity is approached.

Kinetic data in Figure 10b were modelled according to the well-known pseudo-first order (PFO or Lagergren equation) and Ho's pseudo-second order (PSO) models (Equations (A5) and (A6), Appendix A) [60]. The relevant kinetic parameters can be inferred from the slope and y-intercept of ln(*qe* − *q*) vs. *t* (PFO model) and *t*/*q* vs. *t* (PSO model) plots (see Figures S2 and S3, respectively). The calculated rate constants, the theoretically predicted *qe*, and the corresponding *R*<sup>2</sup> are listed in Table 4 for different initial Acid Red 27 concentrations. Also tabulated are the calculated PSO initial adsorption rates, *r*0. Both models provide a good fitting of the experimental data over the whole time span according to the square correlation coefficient. Then, the goodness of fit was further evaluated with the so-called average relative error (ARE, Equation (A10), Appendix A). The ARE in Table 4 is lower for PSO-based fittings than for PFO kinetics over the whole range of initial concentrations. Further, the equilibrium uptakes predicted by the PSO model are generally closer to the experimental ones. Then, the PSO model correlates the experimental kinetic data better than the PFO model does. The second order rate constant shows a decreasing tendency as the initial concentration increases, whereas the initial adsorption rate follows a growing trend. The simulated adsorption curves derived from PSO kinetics are drawn in Figure 10b as solid lines.

**Figure 10.** (**a**) Experimental (solid symbols) and Langmuir-based simulated adsorption isotherm (solid line) for Acid Red 27 on untreated ACC at 25 ◦C; (**b**) Experimental (symbols) and PSO modelled (solid lines) kinetic curves for the adsorption of Acid Red 27 on untreated ACC at different initial dye concentrations; (**c**) Experimental (symbols) and PSO modelled (solid lines) kinetic adsorption curves of 50 mg L−<sup>1</sup> Acid Red 27 solution onto hybrid PAni-ACC composites formed at different electropolymerization times.


**Table 4.** Pseudo-first-order and pseudo-second-order kinetic parameters for the adsorption of Acid Red 27 on ACC (*C*0: mg·L−1; *qe*: mg·g<sup>−</sup>1; *k*1: min−1; *k*2: <sup>g</sup>·mg<sup>−</sup>1·min−1; *r*0: mg·g<sup>−</sup>1·min−1).

The PSO model is known to show widespread good fit to the adsorption kinetics of most dyes and organic pollutants on activated carbon and other synthetic or natural adsorbents [9,60]. However, it has been claimed [60] that the rate constant, *k*2, should not be regarded as being truly representative of intrinsic kinetics, but as an empirical parameter that lumps different controlling mechanisms in a complex manner. In fact, our data invariably show a noticeable deviation from linearity at the initial stage of adsorption (Figure S3). This typical downward curvature was previously realized by Azizian et al. [61], who ascribed it to a mixed rate control by both di ffusion and surface reaction. In order to check the significance of mass transfer on the Acid Red 27 adsorption kinetics onto ACC, we used the Vermeulen model (Equation (A9), Appendix A) [60]. According to this model, the *Bt* vs. *t* plot (Boyd plot) should be a straight line passing through the origin for a pure intraparticle di ffusion mechanism. If the line has a non-zero intercept, the adsorption is also controlled by external film di ffusion [31,60]. In our case, the plots are linear for the whole concentration range (see Figure S4), but they do not pass through the origin (Table S3), thus pointing out that film di ffusion is also involved in governing the adsorption rate.

The evolution of Acid Red 27 adsorption uptake onto di fferent PAni-ACC hybrid composites is shown in Figure 10c for a single dye initial concentration of 50 mg L−1. The kinetic curve for the bare ACC is also depicted for the sake of comparison. PAni layers deposited at short time (thin and compact films) remarkably enhance the rate of adsorption, but the total amount of dye adsorbed at equilibrium remains basically unchanged. However, higher PAni loading achieved at prolonged electropolymerization (thick and porous nanofibrous coating) leads to both a decrease in the adsorption rate and a considerable loss of the adsorption capacity at equilibrium. As above, PFO and PSO modelling of the experimental data were attempted and the kinetic parameters of interest, as well as proofs for testing the goodness of fit, are summarized in Table 5. Again, the PSO model provides an overall better description of the adsorption kinetics. Solid lines in Figure 10c were then traced by using the calculated kinetic parameters of the PSO fitting. These parameters corroborate that hybrid PAni-ACC composites of low polymer loading and smooth film morphology promote a faster adsorption of the dye, while the equilibrium uptake remains una ffected. On the contrary, the nanofibrous thick PAni coating is detrimental to both the adsorption rate and the dye loading into the adsorbent composite material.


**Table 5.** Pseudo-first-order and pseudo-second-order kinetic parameters for the adsorption of Acid Red 27 (50 mg <sup>L</sup>−1) on hybrid PAni-ACC composites synthesized at di fferent electropolymerization times. (*t*: min; *qe*: mg·g<sup>−</sup>1; .*k*1: min−1; *k*2: <sup>g</sup>·mg<sup>−</sup>1·min−1; *r*0: mg·g<sup>−</sup>1·min−1).

Finally, the involvement of mass transfer on the adsorptive capability of the hybrid composites was evaluated with the aid of *Bt* vs. *t* plots (Figure S5 and Table S4). In this instance, only the composites formed at 10 to 60 min of electropolymerization show good linear plots with a non-zero intercept, which again suggests that both external and intraparticle di ffusion may play a role in the adsorption process. Instead, the materials obtained at longer treatment time do not show a linear behavior and therefore the involvement of mass transfer is unclear.
