Conductive carbon fabric generation from single-step upcycling of textile waste

Abstract

Environmental impacts from the fashion industry are at the top of global pollution. Fiber production, fabric preparation and distribution, and disposal of textiles combined with the excessive consumerism of clothing result in the wastage of thousands of million cubic meters of fresh water, the release of gigatons of CO₂ equivalent, and tens of millions of metric tons of textile waste generation every year. This situation shows that there is an urgent and mandatory need to change the fashion paradigm, but, even if accomplished, the current textile waste spread worldwide still needs to be managed in an environmentally conscious way. Upcycling textile waste by pyrolysis is gaining interest as an alternative management option. The goal is to endow waste with new functionalities for its repurposing into new applications. This study focuses on applying pyrolysis to convert discarded clothing into a conductive carbon textile while avoiding treatments with hazardous chemicals. Envisioned to be applied for current collection in all-organic primary power sources, the ultimate goal is to replace synthetic polymers in commercial carbon current collectors. Actual textile waste has been successfully pyrolyzed without the need for pre-treatments or activation. The structural composition of the samples was studied by SEM, X-ray diffraction, Raman spectroscopy, ATR-FTIR spectroscopy, EDS and BET surface area. Electrical and electrochemical characterization showed their suitability as current collectors, which was demonstrated by building an aqueous metal-free organic primary battery. A system of innocuous quinone-based redox chemistry coupled with the revalorized collectors delivered 11.17 mA cm⁻² and 1.4 mW cm⁻² of power density, proving the feasibility of the proposed application.

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1. Introduction

The global fashion industry is one of the main actors in the world's pollution scene. Estimations attributed to this industry are 10% of global greenhouse gas (GHG) emissions, 35% of primary microplastics in the ocean, and 20% of global clean water pollution.¹ This is not only due to the materials and techniques used in the textile industry but also to the sheer volume of cloth production and demand. In 2021, 113 million metric tons of textile fabric were produced worldwide.² From those, less than 1% (of all total garments produced globally) were properly recycled into new products at the end of their lifespan.³

The reason behind this problem lies in the overly excessive consumption of fashion and new clothing designs, encompassed under the term ‘fast fashion’.^4–7 It is called this because it involves rapid design, production, distribution, and marketing of clothing. This means nowadays it takes only a few days for a garment to go from the design stage to being sold in stores.^8,9 The combination of these two factors – short timings and huge quantities – is pointed out as the main cause of such a detrimental impact on the natural ecosystems.

The apparel life cycle is mainly composed of fiber production, yarn preparation, fabric preparation, dyeing and finishing, assembly, distribution, and disposal. Within this cycle, the 4th step (dyeing and finishing), followed by the 1st and 2nd (fiber production and yarn preparation), are the most detrimental stages, in all impact indicators.¹⁰ Regarding the disposal, the massive amount of textile waste that ends up in landfills is remarkable. Specifically, it is estimated that 92 million tons of textile waste are created annually by the fashion industry. From that, 73% end up straight in landfills or incinerated, which accounts for 67 million tons per year.¹¹ In particular, deep in the Atacama Desert of north Chile, lays the biggest clothes dump in the world: each year, 39 000 tons of textile waste are thrown there, some of them even with the product label still attached.¹²

This scenario evidences the pressing need for a new fashion industry paradigm, one strongly committed to reduction, both in production and user consumerism of apparel. However, besides tackling this paradigm shift, there is still an unsolved practical issue: several million tons of textile waste which are already produced and dispersed all around the globe. Current solutions for textile waste management involve reuse‡ (second-hand selling, fashion sharing, donations, etc.),¹⁵ recycling (clothes, toys, tents, general textiles, etc.),¹⁶ and upcycling (mechanical treatment, depolymerization, composites fabrication, etc.).¹⁷

Another remarkable option regarding upcycling solutions is pyrolysis, a well-established process in energy recovery, biomass revalorization,^18,19 and char production.^20–22 Electrically conductive carbon textiles from solid residue pyrolysis have been widely studied, and their proposed applications range from electromagnetic shielding to current collection.^23–25 The final properties of the pyrolyzed materials (namely, flexibility, elasticity, electrical conductivity, and tensile strength) have been linked both to the process conditions and the parent material's nature: primarily, composition and morphology.^26–30 Thus, the weaving pattern from the textile parent material has been highlighted as one key parameter due to its maintenance after pyrolysis, leading to high porosity, high surface area, and ensuring optimal mechanical properties. There are more novel ways to obtain conductive carbon from biomass resources, such as Laser-Induced Graphene (LIG) or Flash Joule Heating (FJH), but these require complex equipment and still face challenges to obtain the desired woven porous carbon structure from the precursor.^31,32 Regarding pyrolysis, either the precursor or the pyrolyzed textile is usually treated or activated with hazardous chemicals.^26–30 Similarly, carbon-based commercial current collectors also have high porosity and surface area, and their morphology can range from flexible carbon felt to stiff carbon papers. Carbon felt is widely applied as a current collector in Redox Flow Batteries (RFBs) due to the mentioned properties.^33,34 However, 90% of carbon-based current collectors are manufactured by pyrolyzing fabrics made of poly-acrylonitrile (PAN), a synthetic polymer obtained from fossil fuel refineries, where hazardous chemicals are required throughout the process to treat and activate the materials' surfaces.³⁵

Herein, carbon fabrics from cotton textile waste upcycling by pyrolysis have been developed and applied as current collectors in metal-free aqueous organic redox batteries. These battery chemistries have been explored as a promising alternative to substitute scarce metals such as lithium, cobalt, or platinum, whose extraction and recycling have lately raised concerns both socially and environmentally, altogether with the toxic compounds used in their fabrication. The novelty of the work relies on the obtention of ready-to-use conductive carbon felt in one single step, without pretreatment or activation (neither chemical nor physical), to avoid hazardous chemicals and reduce energy requirements. The applicability has been demonstrated by using the carbonized samples as current collectors in an aqueous metal-free organic primary battery, based on a quinone redox couple, innocuous and already studied before,^36–38 to showcase the suitability of the obtained conductive textiles with organic chemistries and to keep the environmental impact at its minimum. This needs to be addressed since each organic species has particular molecular properties and will interact distinctly with each current collector material. Thus, this redox couple has been selected to demonstrate the capability of the obtained carbon fabrics to be effectively used as current collectors with an organic redox probe. As one indispensable element of this work revolved around using actual textile waste, t-shirt, towel, and cloth samples were obtained from a clothing recycling plant, managed by a social help organization named Koopera (Bizkaia, Spain). Also, since the resulting electrical conductivity of pyrolyzed samples increases with the pyrolysis temperature, the methodology has been adjusted to leverage the electrical conductivity performance with the energy consumption, according to literature protocols.^24,25 The final properties obtained for the pyrolyzed textiles have been found to be sufficient to electrochemically characterize the aforementioned battery.

This way, revalorizing cotton textile waste by pyrolysis allows us to tackle different factors at once: reduction of the current detrimental impact of textile waste disposed of in the environment, avoidance of fossil fuel-based materials (including the compounds used during their synthesis), avoidance of weaving steps when preparing the parent materials, and avoidance of hazardous chemicals when treating the surface of both the precursor and the pyrolyzed material.

2. Results and discussion

2.1 Materials pyrolysis

Fig. 1a shows a close-up view of the three selected textiles, made of 100% cotton. Cotton fibers are composed of 88–97% cellulose.³⁹ Hence, Whatman Grade 1 (W1) paper was used as a control for the pyrolysis process due to its certified composition of 99.9% alpha-cellulose purity. The samples have no electrical conductivity or rigidity. Qualitatively assessed, elasticity ranges from very elastic t-shirt samples to almost non-elastic cloth samples; towel samples fall in between. Each sample type is then cut with a CO₂ laser into rectangles and pyrolyzed, following the methodology described in Section 3.2, without any pre-treatment. Fig. 1 portrays and visually summarizes the results: (b) the different samples before and (c) after pyrolysis, (d) alongside the self-standing properties of each type of textile. All samples were properly pyrolyzed. It can be observed how their rigidity changes after the pyrolysis, control Whatman 1 being the stiffest and cloth the most flexible. This might be related to the properties of the original sample before the pyrolysis: Whatman 1 is a stiff piece of paper that was self-standing before the pyrolysis, while the rest of the textile samples were not self-standing before the thermal process. However, t-shirt textile was the most elastic, followed by towel textiles and cloth textiles being the least elastic. These original properties, which mean inherent internal tensions during the pyrolysis process, seem to revert to the later mechanical ones. Although the composition of all samples is mostly the same (cellulose), it is remarkable how the difference between weaving patterns (i.e., fibers' arrangement) on the original textiles is a crucial factor for the final morphology of the pyrolyzed products.

Fig. 1 Photographic record of all the distinct stages: (a) each type of sample, as received, (b) each type of sample in the crucible right before the pyrolysis, (c) all the pyrolysis repetitions' outcomes, (d) self-standing properties of each type of sample after the pyrolysis. Scale bars: 2 cm, each one corresponding to the set of photographs above it, across all sample types.

2.2 Electrical characterization

Fig. 2 shows the results for mass and area yield, as well as conductivity values, for each type of textile in contrast to the control. Fig. S1† shows the resistivity values for each pyrolyzed sample. The first thing observed is a direct correlation between mass and area loss, which also correlates with conductivity and thickness. This could be related to the formation of a higher number of bonds when the mass loss is smaller. Thus, control W1 samples, whose mass yield is (4.2 ± 0.2)% of the initial one, have the smallest conductivity value, (0.10 ± 0.04) S m⁻¹; and towel samples, whose mass yield is (10.6 ± 0.4)% of the initial one, have the highest conductivity value, (23.5 ± 2.6) S m⁻¹. Both t-shirt and cloth samples perform similarly, with (8.5 ± 0.8)% and (8.6 ± 0.5)% of mass yield (respectively); (40.8 ± 1.3)% and (41.1 ± 1.7)% for area yield (respectively); and (2.5 ± 0.3) S m⁻¹ and (2.4 ± 0.2) S m⁻¹ of conductivity, respectively. Linear fitting operations performed to obtain the conductivity values can be seen in Fig. S2,† and the related values have been gathered in Table S1.†

Fig. 2 Left – mass (solid circles) and area (open circles) yield, final value in percentage of the initial one, N = 3. Right – electrical conductivity (σ – solid rhombus) values for each sample type after pyrolysis, N = 3.

2.3 Morphological and chemical characterization

SEM images obtained both before and after the pyrolysis process for all samples are shown in Fig. 3. In control W1 images, disordered fibers can be clearly seen, amalgamated by the paper fabrication procedures, plus a decrease in fibers' diameter after the pyrolysis is observed. Textile samples maintain the woven structure after the pyrolysis, which was one of the main purposes of using these woven residues. Furthermore, they also show a decrease in the fibers' diameter. The porous structure of the pyrolyzed materials can also be observed, which is one of the pursued features for high electrochemical output.

Fig. 3 SEM images for all samples both before and after the pyrolysis. Zoom increases from left to right. Textiles' structural component (fibers) evolution by the thermal process can be seen; it is remarkable to say that the weaving pattern is retained in all textile cases, though more degraded. Scale bars (placed both on top and bottom) apply to each column of pictures.

Next, the structural composition of the different samples was studied. X-ray diffraction (XRD) (Fig. 4a) and Raman spectroscopy (Fig. 4b) were performed on all the samples. For both studies, the top graphs show the results for the cellulosic samples (before pyrolysis) and the bottom graphs show the results for the carbonized samples (after pyrolysis). Starting with X-ray diffractograms of cellulosic samples (Fig. 4a top graph), cellulose peaks can be seen at 2θ = 15° (1–10), 16.5° (110), 20.5° (102), 22.5° (200) and 34.5° (004).⁴⁰ Considering the difference seen between the peak's intensity at 34.5° (004) against the one from the peak at 22.5° (200), it can be seen that control W1 crystallinity in the zenithal plane is the lowest, while t-shirt and cloth samples lay in between, with towel ones having the highest. It follows the same pattern as the electrical conductivity, as seen in Fig. 2, so it could be that the crystallinity of the fabrics regarding the 004 crystallites positively influences the production of more electrically conductive carbon fibers. Other than that, the only slight difference is on control W1, which does not show the 2θ = 20.5° (102) peak. This could be due to the source of the material: control W1 is a chromatography paper, in which cellulose undergoes many different processing steps, compared to textile cellulose, before the product takes its final form. Fig. 4a bottom graph shows the results for the carbonized samples, after pyrolysis, where graphite peaks can be seen at 2θ = 22.5° (002) and 44.5° (100). The origin of the broad (002) reflection is interpreted as due to a small number of well-stacked layers with a uniform interlayer distance. The (100) reflections are attributed to in-plane order. It is known that the inter-graphene distance depends on the carbon source, the method of preparation, and chemical treatment.⁴¹

Fig. 4 Characterization results for all the samples before (top) and after (bottom) the pyrolysis process for each technique. (a) XRD diffractograms. Crystallite planes' orientations are shown on the top of each peak, which also applies to all the ones below. (b) Raman spectra. Vertical lines show the different peaks referring to cellulose and carbon resonance bands. (Inset): vertical zoom of the carbon textile curves (dashed zone) and their deconvolution (green), showing three peaks. (c) ATR-FTIR spectra. The relevant bands across all curves are indicated with vertical lines, while relevant bands of a specific spectrum are indicated with a vertical line and a circle. (d) EDS results, with the percentages of carbon (C) and oxygen (O) content indicated. Regarding O values after pyrolysis for the carbonized samples: control W1 (10.7 ± 0.4)%, t-shirt (11.6 ± 1.4)%, cloth (12.5 ± 0.7)%, and towel (10.1 ± 1.5)%.

Following the analysis, Raman spectroscopy performed before the pyrolysis process (Fig. 4b top graph) clearly showcases carbohydrates' peak at 2894 cm⁻¹ for all samples, alongside peaks at 1097 cm⁻¹, 1120 cm⁻¹ and 1378 cm⁻¹, all of them characteristics of cellulose.⁴² However, a significant amount of noise can be seen, probably caused by samples' fluorescence: specifically, towel samples show the largest amount of it, possibly due to their greater thickness, thus increasing the effect of this phenomenon. Regarding the t-shirt's 1599 cm⁻¹ peak, this could be caused by the presence of lignin in the sample, supported by the 2933 cm⁻¹ peak.⁴³ Finally, the 3281 cm⁻¹ peak could be assigned to the H₂O stretching vibration.^44,45 The bottom graph shows the spectra after the pyrolysis process: D (1350 cm⁻¹) and G (1590 cm⁻¹) bands, characteristic of carbon, can be clearly observed. The D-band is induced by any kind of disorder; it is an in-plane breathing vibration of the aromatic ring structures (A_1g symmetry). The G-band is assigned to the in-plane stretching vibration of sp² carbon (E_2g symmetry). Peaks at 2700 cm⁻¹ and 2900 cm⁻¹ are the 2D band (overtone of the D band) and D + G band (a combination of both bands), respectively.⁴⁶ The peak at 3180 cm⁻¹ could be assigned to NH₃ vibration, one possible by-product of the pyrolysis process.⁴⁷

Fig. 4c showcases the different fabrics' infrared spectra analyzed by attenuated total reflectance (ATR-FTIR) before (top) and after (bottom) the pyrolysis process, alongside the relevant bands. All fabrics before the pyrolysis present the main bands associated with cellulose: 3331 cm⁻¹ and 3278 cm⁻¹ bands are assigned to O–H stretching; 2896 cm⁻¹ and 2855 cm⁻¹ bands to C–H symmetric stretching; 1637 cm⁻¹ band to O–H bending; 1428 cm⁻¹ and 1363 cm⁻¹ bands to C–H symmetric bending; 1314 cm⁻¹ band to O–H in-plane bending; 1160 cm⁻¹, 1104 cm⁻¹, 1055 cm⁻¹, 1025 cm⁻¹, 993 cm⁻¹, and 899 cm⁻¹ bands are all assigned to C–O stretching in C–O–C and C–O–H; and 686 cm⁻¹, 661 cm⁻¹, 600 cm⁻¹, 552 cm⁻¹, and 517 cm⁻¹ bands to O–H out-of-plane bending. Some of the fabrics present individual and very weak signals: t-shirt samples show a band at 1738 cm⁻¹, associated with C=O stretching; and towel samples show two bands at 1576 cm⁻¹ and 1537 cm⁻¹, associated with N–H bending. Overall, the spectra match those from cellulose reported in the literature.⁴⁸ Regarding the carbonized samples, none of the ATR-FTIR spectra show any representative band, characteristic of pyrolyzed, but not activated, carbon.

Fig. 4d shows the energy dispersive X-ray spectroscopy (EDS) analyses, where it can be seen how the proportion of carbon (C) and oxygen (O) varies from 47% C–53% O to 90% C–10% O. The most outlying value is 51% C–49% O for the t-shirt fabric, which might be due to the aforementioned slight presence of lignin within the sample. Otherwise, the values are the expected ones for both cellulosic compounds before the pyrolysis and carbon fibers after the pyrolysis. Overall, the formation of carbon structures has been demonstrated to occur due to the pyrolysis process, formerly absent, thus confirming the effectiveness and viability of the upcycling technique.

In parallel, the contact angle with water was tested to check the compatibility with aqueous solutions (usual solvent of primary bio-batteries), shown in Fig. S3.† T-shirt and towel samples were found to be hydrophilic, while control W1 and cloth samples were hydrophobic.

Lastly, the porosity and surface area of the studied fabrics were assessed, and the results are summarized in Fig. 5. The surface area, obtained by the Brunauer–Emmett–Teller (BET) method, is observed to increase after the pyrolysis for all the fabrics. Likewise, the pore volume also increases after the pyrolysis across all the fabrics, though the change for control W1 is smaller than for the other fabrics. Regarding the pores' mean diameter after the pyrolysis, for control W1 and t-shirt samples a decrease is observed; oppositely, for cloth and towel fabrics an increase is observed. Due to the pyrolysis process, the fabrics experience a certain shrinkage, as seen in Fig. 2; however, the fibers' diameter also shrinks, as seen in Fig. 3, so the pore volume increases. Overall, the carbonized samples show a surface area of the same order as commercial carbon felt, usually between 10¹–10³ m² g⁻¹.

Fig. 5 Porosity and surface area results for all the studied fabrics, before (top) and after (bottom) the pyrolysis. On the left side, the results are portrayed visually, while the detailed values are shown on the right. An overall increase in the surface area and pore volume is observed for all fabrics.

2.4 Electrochemical characterization

Next, charge transfer capabilities were studied, via cyclic voltammetry (CV). p-Benzoquinone (p-BQ) was used as the organic redox species due to its reversible nature at neutral pH and its previously reported suitability as a cathode in organic metal-free power sources.^36–38 For this study, pyrolyzed control W1 was not found to be suitable as a control, since it is not a common standard in electrochemical measurements. Also, it could not be measured since it would break under the clamp connection due to its brittleness; thus, it was not characterized electrochemically. Therefore, a commercial carbon felt (control C100) was defined as the control for the electrochemical study, since it is the material envisioned to be substituted, commonly used as a current collector in RFB and working electrode in porous configurations. Specifically, the surface area of the commercial carbon felt C100 was also measured, obtaining a BET surface area of 211.0 m² g⁻¹, pore volume of 5.04 cm³ g⁻¹, and mean diameter of 47.8 nm. The surface area is lower than that of all 3 pyrolyzed fabrics, though the pore volume and mean diameter are larger. Previous to the CV, open circuit potential (OCP, V vs. Ag/AgCl) was measured, finding a higher value for the control C100 sample than for pyrolyzed textile samples (0.3 V of difference). This could be attributed to the catalysis of hydrogen reduction by the pyrolyzed fabrics in this specific system. The observed partial potential is −0.197 V vs. Ag/AgCl, which equals 0.0 V vs. SHE, the reduction potential of hydrogen. Fig. 6 shows the voltammograms obtained for all the different samples acting as working electrode in a 3-electrode cell, and the oxidation and reduction current peaks analysis. Regarding the CV comparison, it can be observed that the control exhibits a steeper slope in the region governed by faradaic currents (i.e., before the current peak) in comparison with the pyrolyzed textiles. This can be attributed to the higher electrical resistance of the pyrolyzed materials since commercial carbon felt C100 was measured to have an electrical conductivity of (1084 ± 15) S cm⁻¹, which is between two and three orders of magnitude greater than the pyrolyzed textiles' conductivity. Fig. S4† shows the cyclic voltammogram comparison with the blank solution (only water with the electrolyte, without active redox species), where the higher resistivity of the pyrolyzed samples over the commercial carbon felt when transferring the electrical current within itself can be seen.

Fig. 6 Cyclic voltammograms of 0.01 M p-benzoquinone solution in 1 M KCl using the pyrolyzed samples as the working electrode in a three-electrode electrochemical cell. S_rate = 10 mV s⁻¹. IUPAC convention used: top-right, high potentials and oxidation reactions; bottom-left, low potentials and reduction reactions.

This assessment is a qualitative test to see if the collectors do, indeed, exchange charge with the diluted species in an aqueous solution. It is rather difficult to assess quantitatively, for example, porous carbon electrodes' electrochemical surface area (ECSA), since they usually have porosities with length scales below the diffusion length of the redox probes (which typically is of the order of microns). This is problematic because standard electrochemical methods rely on the diffusion of redox probes (or ions) from the bulk to the surface electrode. Given those porosity scales, probes' diffusion from the bulk solution to the surface of the electrode gets highly convoluted, thus leaving obsolete the standard mathematical model used to compute these parameters.^49,50Fig. 6 shows the baselines and corrected peak current values, obtained using the electrochemical analysis software. No significant differences were found in the electrochemical behavior as current collectors of the samples, but towel samples seem to have more symmetrical shape and peak current values than the other two, important characteristics towards reproducible electrochemical performance.

2.5 Proof-of-concept

Charge transfer properties of the samples have been demonstrated, thus leading to the measurement of a full primary galvanic cell. Fig. 7a shows a scheme of the experimental set-up to assemble and measure the cell, as well as an actual photograph built with the pyrolyzed towel samples acting as current collectors. Fig. 7b shows the polarization curves of a full cell build-up with pyrolyzed samples of towel textile against the commercial carbon felt used before, control C100. Commercial carbon felt delivers a max power density of (4.16 ± 0.52) mW cm⁻², an open circuit voltage (OCV) of 0.605 V, and a max current density of (26.7 ± 1.2) mA cm⁻². However, pyrolyzed towel delivers a max power density of (1.40 ± 0.12) mW cm⁻², an open circuit voltage (OCV) of 0.495 V, and a max current density of (11.17 ± 0.07) mA cm⁻². Remarkably, pyrolyzed towel samples show a much greater reproducibility, and an electrochemical performance in the same order of magnitude, than the commercial carbon felt, even though conductivity values of pyrolyzed towel samples are two orders of magnitude smaller than those of commercial carbon felt. The standard deviation of the maximum power density value for control C100 is 4.33 times the one of pyrolyzed towel. Regarding cell stability, the herein-used quinone redox couple has been optimized for up to 4 days in previous studies. In this work, the objective was not to develop an optimized prototype; rather, to demonstrate the feasibility of the carbonized fabrics to properly function as current collectors in organic-based batteries and the comparison to their commercial equivalent. Regarding current and power outputs of pyrolyzed towel samples, those would be sufficient to enable this configuration as a power source for low energy demanding applications, such as precision agriculture sensors, validating the potentiality of this material. Those devices require at least 3 V to operate in a current range of 50 μA–5 mA.³⁸ Since aqueous-based chemistries are limited to the electrochemical stability window of water (1.23 V), different strategies can be applied to palliate such a gap; namely, cell stacking in series or the usage of a DC–DC converter to boost the voltage, both of them already established in the literature. Finally, Fig. S5† shows a close-up picture of the set-up. This way, the viability of the revalorization process and the applicability of these revalorized materials as current collectors in an aqueous metal-free organic primary battery have been demonstrated as a possible and feasible way to give a second life to huge amounts of textile residues spread around the globe.

Fig. 7 (a) Exploited view of the galvanic cell set-up. On the left a picture of the final assembly is shown (scale bar = 0.5 cm) and, on the right, a visual scheme. (b) Polarization curves of the batteries set up with pyrolyzed towel samples (purple, solid circles) and control C100 commercial carbon felt (grey, open squares). Arrows indicate Y-axis correspondence (both linked to X-axis, current density). Shaded areas show the standard deviation of N = 3 measurements. S_rate = 20 mV s⁻¹.

3. Materials and methods

3.1 Materials

3.1.1 Cellulosic samples. Cellulose chromatography paper Whatman Grade 1 was used as a reference (Cytiva, USA). Samples were carbonized in an alumina crucible; thus, determining their dimensions: 1.5 cm × 8 cm. All samples were cut with a CO₂ laser (Fusion Edge 12 30W, Epilog Laser, USA) to ensure dimension accuracy. Textile waste samples were obtained from Koopera, a social help organization with a clothing recycling plant located in Mungia, Bizkaia, Spain. Three different pieces were obtained, all made from 100% cotton textile: one t-shirt, one towel, and one cloth. All samples were used as received.

3.1.2 Chemicals. To perform the electrochemical assays, aqueous solutions of 0.01 M p-benzoquinone (p-BQ), as the main redox species, in 1 M KCl, as the supporting electrolyte, were prepared to study the charge transfer properties within a three-electrode electrochemical half-cell. To measure the full galvanic primary cell, an aqueous solution of 0.1 M p-BQ in 1 M KCl and 0.1 M oxalic acid (to extreme Nernst's potential) was prepared as the cathode; and 0.1 M H₂BQS in 1 M KCl and 0.1 M potassium hydroxide (KOH) was prepared as the anode. All species were used as received, obtained from Sigma Aldrich.

3.1.3 Others. Commercial carbon felt AvCarb C100 (AvCarb Material Solutions, Fuel Cell Earth, USA) was used (as received) as a reference control material when performing the electrochemical assays.

3.2 Methodology

3.2.1 Pyrolysis. A combustion boat of alumina, 99.7% (Nanoker Research, SL, Oviedo, Spain) was used to hold one sample during the pyrolysis. A balance between the oven capabilities (Rotary Tube Furnace O1200-50IT, ZYLAB Instruments) and different reference protocols from the literature were used to define the starting protocol:^24,25 carbonization at 1000 °C in an inert atmosphere of N₂ (atmospheric pressure), heating rate of 500 °C h⁻¹ and 1 h of dwelling time. Then, the oven was left to cool down until reaching room temperature. Finally, the samples were taken out and characterized without further treatment.

3.2.2 Characterization. Raman spectroscopy, X-ray diffraction (XRD), ATR-FTIR spectroscopy, EDS and BET techniques were used to assess the carbonization degree of the pyrolyzed samples. All techniques were performed by SGIKER services in the UPV/EHU campus in Leioa, Bizkaia, Spain.

To determine the samples' hydrophobicity (contact angle) a goniometer with a camera was used. Images were analyzed with an open-source program, ImageJ.

Electrical conductivity was assessed by the Transmission Line Method (TLM): a self-made set-up was marked at certain distances with a CO₂ laser (Mini Epilog 30, Epilog Laser) and later used to measure the resistance with a multimeter (15XP-B, Amprobe).

To study the electrochemical properties of the pyrolyzed samples, Cyclic Voltammetry (CV) and Linear Sweep Voltammetry (LSV) techniques were used, alongside a potentiostat/galvanostat (PalmSens BV EmStat4s HR, Netherlands).

4. Conclusions

The global fashion industry and its consumerism are known to be at the top of most pollutant agents in the world, both during the fabrication processes and at the disposal stage. Regarding the latter, tens of millions of metric tons of textile waste end up in landfills, without upcycling or reusing, contributing intensively to the natural environment degradation. Herein, we propose an alternative usage, where cellulosic textile waste has been successfully pyrolyzed into conductive carbon textiles, in order to substitute commercial carbon felt, which is currently fabricated with synthetic polymers obtained by fossil fuel refining. Other natural textile materials could be upcycled in the same way, such as hemp or linen, but their presence in the current textile market is much smaller than that of cotton; synthetic polymers such as polyesters cannot undergo the same process and keep the woven structure, due to their thermoplastic properties, and already existing methods allow for easy separation of these materials. Relatively low temperatures (1000 °C) have been enough to obtain proper electrical conductivities for the proposed application, without pretreatment or activation, neither chemical nor physical, to avoid hazardous chemicals and reduce the energy requirements. Further methods to reduce the energy consumption or the process complexity need to be studied, such as hydrothermal carbonization or usage of lower temperatures.^18,19 Electrochemical studies have shown the potentiality of the obtained materials to be able to work with organic redox chemistries. To tighten up, a full organic aqueous galvanic cell has been successfully assembled and measured, delivering 11.17 mA cm⁻² of current density and 1.4 mW cm⁻² of power density values, both in the same order of magnitude as the commercial carbon felt control. This way, the feasibility of upcycling cotton textile waste by pyrolysis and converting it into electrically conductive fabrics to be applied as current collectors in aqueous organic primary batteries has been demonstrated.

Author contributions

Carles Tortosa: conceptualization, data curation, formal analysis, investigation, methodology, project administration, validation, visualization, writing – original draft, writing – review & editing. Marina Navarro: conceptualization, supervision, validation, visualization, writing – review & editing. Pedro Guerrero: supervision, validation, writing – review & editing. Koro de la Caba: supervision, validation, writing – review & editing. Juan Pablo Esquivel: conceptualization, funding acquisition, project administration, supervision, validation, writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors would like to thank Koopera for providing the samples that have made this work possible. The authors acknowledge financial support from project BIDEKO (PLEC2021-007801), funded by MCIN/AEI/10.13039/501100011033 and European Union ‘‘NextGenerationEU/PRTR’’.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Electrical characterization raw data, contact angle measurement, blank cyclic voltammograms and close-up photographs of the proof-of-concept. See DOI: https://doi.org/10.1039/d3se01722b

‡ Other types of waste are also undergoing direct reuse studies, such as spent lithium-ion batteries or some of their components. For the interested reader, it is recommended to take a look at the herein-cited articles.^13,14

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