**5. Conclusions**

Flexible PAni-ACC composites of relatively large size can be easily fabricated by simple galvanostatic electropolymerization of dissolved aniline in a filter-press electrochemical cell, whose modular and stackable design is particularly well suited for easy scaling-up to pilot plant or even technical scales. The morphology of the deposited PAni is strongly dependent on the polymerization time (i.e., the amount of charge passed), ranging from dense and thin films to porous and thick coatings made of interconnected nanofibrils or nanowires. With the exception of some scattered spots, thin films appear featureless in SEM, and their occurrence is indirectly deduced from the XPS N/C and S/C ratios. Composites with a polymer thin film morphology show mixed XPS and valence band patterns from the carbon fabric and the conducting polymer, whereas those for composites with PAni nanofibrous morphology are typical of pure PAni, in a state close to that of (bi) sulfate-doped emeraldine salt.

PAni loadings range from 5 to 18 mg cm<sup>−</sup><sup>2</sup> (~25–50 wt.%), but they increase non-linearly with the electropolymerization time, as shown by combined XPS and TGA data. We propose that aniline polymerization occurs initially inside the carbon pores, but the polymerization rate levels o ff once aniline is depleted and supplied by slow internal pore di ffusion. This mechanism would also facilitate the self-assembly of aniline oligomers into thin films. The analysis of the N2 and CO2 isotherm data indicates that thin film PAni deposition occurs on micro- and mesopores, while the ultramicropore volume remains una ffected. Accordingly, the BET surface area decreases by 20%–25%. Thick nanofibrous coatings formed in highly-loaded PAni-ACC composites cause a strong blockage of pore entrance and a dramatic loss of the specific surface area to values typical of pure PAni.

The conductivity, electrochemical and liquid-phase adsorptive properties of the hybrid PAni-ACC composites are strongly conditioned by the microstructure of deposited PAni. The cyclic voltammograms of composites with thin film morphology show pseudocapacitive features related to reversible leucoemeraldine-emeraldine transition and electron-transfer in phenazine/phenoxazine-like segments. Accordingly, the specific capacitance is enhanced and the sheet resistance falls to a minimum. In addition, the pseudo-second order kinetics for the adsorption of Acid Red 27 are remarkably promoted. We believe that the extra pseudocapacitance, the improved conductivity and the attractive electrostatic interactions between dye molecules and PAni counterbalance the loss in a specific surface area. On the contrary, in composites with a thick nanofibrous morphology, the pronounced decrease in surface area, and perhaps higher PAni-PAni intraparticle and/or PAni–carbon contact resistances, can be at the origin of the decreased capacitance, conductivity, and adsorption rate and capacity.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1996-1944/12/16/2516/s1, Figure S1: Isotherm linearized fittings for Acid Red 27 adsorbed on ACC, Figure S2: Pseudo-first order ln(*q*e − *q*) vs. *t* plots for the adsorption of Acid Red 27 on ACC, Figure S3: Pseudo-second order *t*/*q* vs. *t* plots for the adsorption of Acid Red 27 on ACC., Figure S4: Boyd plots for the adsorption of Acid Red 27 on ACC, Figure S5: Boyd plots for the adsorption of Acid Red 27 on hybrid PAni-ACC composites, Table S1: Elemental surface composition (in at.%) of ACC and hybrid PAni-ACC composites, Table S2: C1s and N1s peak positions and relative abundance, Table S3: Boyd plot parameters for the adsorption of Acid Red 27 on ACC, Table S4: Boyd plot parameters for the adsorption of Acid Red 27 on hybrid PAni-ACC composites.

**Author Contributions:** Conceptualization, C.Q.; Methodology, C.Q. and R.B.; Validation, L.L. and E.B.; Formal Analysis, C.Q. and R.B.; Investigation, L.L. and E.B.; Visualization, L.L., E.B. and C.Q.; Data Curation, L.L., R.B. and C.Q.; Writing-Original Draft Preparation, C.Q.; Writing-Review & Editing, all authors.; Supervision, C.Q.; Funding Acquisition, C.Q. and R.B.

**Funding:** This research was funded by Spanish Ministerio de Economía y Competitividad and FEDER funds, (grants MAT2016-76595-R and RYC-2017-23618) and Generalitat Valenciana (grant PROMETEO/2018/087).

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.
