Chicken feather-derived carbon electrodes for capacitive deionization using poly(vinyl alcohol)-glutaraldehyde as the binder

Freshwater scarcity has emerged as a great concern globally due to the world's booming population and rising water demands (Elimelech & Phillip 2011). To overcome this situation, there are multiple options for water purification, the feasible and effective way to acquire freshwater from saline or brackish water has become the demand of the hour due to their abundant reserves on the earth, thermal-based (Ng et al. 2015), chemically activated systems (Youssef et al. 2014), membrane-based and electrical processes (Demirer et al. 2013) are used. These conventional techniques, for example, reverse osmosis (Kaschemekat et al. 1983) and multi-stage flash distillation consume high energy, have relatively poor efficiency, are environmentally damaging and take up massive infrastructure. As a result, an alternative is immediately needed. In this regard, capacitive deionization (CDI) has been introduced as an emerging technique for excellent desalination of low or moderate salt concentrations where salt ions are removed electrostatically from water. An electric field of low direct current voltage (usually 1.2 V) is given to the porous carbon electrodes which attract the counter-ions toward their respective electrodes (Welgemoed & Schutte 2005; Xu et al. 2008, 2014; Li et al. 2009). CDI has a facile operation, high energy efficiency and is eco-friendly.

Electrosorption reaction takes place at the electrodes surface, hence it is significant to enhance the ions adsorption capacity of electrodes for better desalination. Various carbon materials, such as carbon fiber, carbon cloth, carbon aerogels and carbon nanotubes have widely been used as electrodes for CDI applications to improve the capacitance of carbon electrodes (Oren 2008). Also, electrodes should be mechanically stable and low cost which are crucial for commercially achievable CDI process. Instead of having fine electrical and physical properties these carbon materials have disadvantages due to their complicated synthesis procedures and are expensive for CDI applications. Whereas activated carbons are inexpensive and are excellent materials with high surface area. Therefore, for the CDI process AC is extensively used as carbon electrode materials. Also, for the synthesis of carbon electrodes polymer binders are used which exhibit thermal and chemical stabilities. However, polymer binders enhance internal resistance and may block some carbon electrode pores (Kinoshita 1988).

Therefore, with achieving applications, the feasibility of production is required in the industries and we need the prior factors in which the environment should be friendly and cost-effective. The fabrication of electrodes takes place by brushing or printing the electrode paste or applying a slurry on a substrate on a metal element such as nickel foam, a mixture of active materials, binders and conductive agents when stirred ultrasonically, the electrode slurry or paste is formed. To avoid the falling active materials during the performance of the electrode, binders play a vital role which attaches the conductive agent and active material to the substrate and it strengthens the electrode and the pore size of it. Nevertheless, the surface area and pores on the material are covered by the binders or cohesive agents. So, the properties of binders and their components direct influence the electrochemical performances of supercapacitors (Zhu et al. 2016).

In the CDI process, to attain better desalination performance, it is important to increase the adsorption capacity (capacitance) of carbon electrodes. The carbon electrodes capacitance depends on different factors such as oxygenated functional groups, pore size distribution, surface area and pore structure. Similarly, the wettability of the electrode surface area is crucial for better adsorption capacity and if there is no contact between the surface area and the aqueous solution that is not usable for salt removal. Hence, for high CDI performance, it is essential to enhance the wetted surface area of carbon electrodes, surface structure and functionalities that have a critical role in electrode wettability. Carbon electrodes prepared by using hydrophobic polymer binders such as polyvinylidene fluoride or polytetrafluoroethylene have low wettability of electrodes, which accounts for the decrease in adsorption capacity.

Poly(vinyl alcohol) (PVA) and poly(vinyl acetate) are water-soluble polymers, these can be used for enhancing the wetted surface area of polymer-bonded carbon electrodes. Because of innate hydrophilicity, thermal and chemical stability, good flexibility (Gao & Yang 2017) and high antifouling potential, PVA membranes possess wide applicability and are also utilized as a water treatment application. Though, PVA exhibits a large number of hydroxyl functionalities, but possesses poor water repellence and a high swelling degree in water, which cause limitation of PVA. The water repellence can be enhanced by acetalized groups compared to unmodified PVA which can destroy the film's reliability and reduce its performance. However, the water resistance of membranes can be improved through numerous cross-linking, methods for instance, heat treatment, freezing, irradiation and chemical cross-linking. Herein, the chemical cross-linking procedure exhibits the advantages to decrease the energy expenditure and time utilization. The most widely used chemical cross-linking agents of PVA include carboxylic acid, acid anhydride, acid chlorides, boric acid, epichlorohydrin, glycidyl methacrylate, and aldehydes. Aldehyde cross-linking is the most common route utilized because of the high rate of cross-linking. The formation of a three-dimensional network is achieved through the acetylation reaction of aldehyde compounds with PVA's hydroxyl groups (Zhang et al. 2010). Normally, acetalized PVA membranes synthesis is facile and settled: an aldehyde such as formaldehyde (Liao et al. 2018) or glycidyl acrylate mixed with hydrochloric acid, is combined with PVA aqueous solution and later pasted into membranes (Holland & Hay 2002) and the prepared membranes are thermosetting.

In this study, poultry chicken feathers waste was carbonized to produce carbon materials, which were then chemically activated at high temperatures by KOH. The slurry of carbon material and PVA solution was prepared and GA was added as a cross-linking agent and stirred to achieve a homogenous slurry. Scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR), and Brunauer-Emmett-Teller (BET) and cyclic voltammetry (CV) were used to evaluate the physical and chemical properties of the AC materials. The electrosorption performance of the synthesized CF-AC-PVA-GA electrodes was also investigated.

Poultry chicken feathers obtained from a farmers’ market were washed with distilled water to remove impurities. PVA was purchased from Daejung (Korea), glutaraldehyde and potassium hydroxide were supplied from Sigma Aldrich (Germany).

CF-AC electrodes were fabricated by mixing CFs and polymer (PVA and GA). The mass ratios of AC, PVA and GA were taken 90:10, where the binders (PVA-GA) ratio was 1:1. Carbon slurry was prepared through mixing CF-AC with PVA solution and GA was added in the slurry dropwise as a cross-linking agent. PVA solution was dissolved in water (dissolved in DI water at 90 °C for 4 h), and later was crosslinked with GA because of its hydrophilic nature. The mixture was magnetically stirred overnight at room temperature (Park et al. 2011). The slurry was pasted on the copper sheet (2 cm × 4 cm) and dried at 70 °C for 1 h and affixed by heating in a thermostat oven at 120 °C for 3 h and pressed under 5 tons for 10 min to get a modified CF-AC-PVA-GA electrode as shown in Figure 5. Electrodes were washed with distilled water to remove loosely bound carbon and the amount of carbon was measured on electrodes by subtracting the weight of the same size copper sheet.

XRD pattern of CF-AC was recorded on D8 Advance Bruker powder diffractometer with a Cu-Kα radiation (Beijing Scientific Technology Co., Ltd). The Scanning electron microscopy (SEM) using Jeol LV-6490 was used to study the physical structure of carbons at an accelerating power 15 kV. Furthermore, the FTIR spectra were recorded using Thermo Scientific Nicolet TM iS10, samples were mixed with KBr and ground with mortar and pestle, and pressed into a transparent disk for analysis. Whereas, surface information and pore structure of CF-AC were characterized by nitrogen adsorption at 77 K using an Autosorb-iQ-MP. Samples were degassed at 300 °C for 6 h before adsorption experiments. CF-AC pores distribution was determined by employing Barrett–Joyner–Halenda (BJH) model for pores and Brunauer–Emmett–Teller (BET) for surface area.

Here, C shows the specific capacitance (F/g), i is current density (A), v represents scan rate (V/s) and m is the activated carbon electrode mass (g).

Later, the salt adsorption capacity (SAC) was calculated using Equation (2). Here, C° and C are initial and final concentrations (mg/L), V is the volume of salt water (mL) and m is the mass (g) of the electrode.

As can be seen in the SEM image in Figure 3(b), the SEM analysis of the CF-AC material revealed that it had flakes with a rough shape. The high rough surface of CF was visible after chemical activation. FTIR analysis revealed that the surface structure of the synthesized CF-AC materials included a variety of oxygenated functional groups (Figure 3(c)). The several distinctive peaks of the synthesized AC materials between 3,380 and 3,490 cm⁻¹ are related to -OH stretching vibration. The asymmetric stretching of the oxygenated functional group in the O–N molecule and the C = O molecule, respectively, may be responsible for the bands at 1,640 and 1,435 cm⁻¹ (Sugumaran et al. 2012; Xu et al. 2014).

Additionally, as shown in Figure 3(c), distinctive bands of the carbonized feather were identified as C = C stretching vibrations or C = O (stretching) around 1,700–1,600 cm⁻¹ and approximately 1,050 cm⁻¹ (Alfredy et al. 2019). While C–N stretching is shown by the band at 1,240 cm⁻¹ (Tuna et al. 2015). This shows that the corresponding amide peak was reduced by becoming a broad band between 1,240 and 1,200 cm⁻¹. Another amide peak that can be seen at 1,570 cm⁻¹ also seemed to be lessened by converting into a wider band at 1,600 cm⁻¹, indicating that the N–H bond was probably broken (Brebu & Spiridon 2011; Tuna et al. 2015; Partuti et al. 2019; Chen et al. 2021).

The produced activated carbon electrode must be reliable in order to be used in desalination processes. Carbon electrodes should specifically be able to withstand the shear stresses seen in a water stream (Zhang et al. 2012). To explore their stability, the AC electrodes were made with various binder ratios, as shown in Figure 5. Table 1 lists the various ratios of activated carbon with binder ratios of PVA and GA (PVA/GA: 1:1), such as 85:15, 90:10, and 95:5, along with their electrosorption capacities. No active material was lost when rubbing a carbon electrode with an 85:15 ratio because of its relatively smooth and uniform surface. In comparison to the 95:5 fabricated electrode, which had significant surface cracks, the prepared electrode with a 90:10 ratio revealed only tiny cracks and lost a modest amount of active material after rubbing (Fang et al. 2016). Thus, the prepared electrode with 90:10 ratio was utilized for further studies.

To improve the wettability of the CDI electrode, the activated carbon synthesized from chicken feathers was chemically treated with KOH and subsequently functionalized using PVA binder. The fabricated activated carbon was used as an electrode material for the desalinization process. However, as PVA is a water-soluble polymer binder, in order to make it insoluble, PVA was crosslinked with glutaraldehyde through an acetalization reaction. The PVA-GA carbon electrodes were prepared at different binder's ratios and heated at 120 °C overnight in order to make it a homogenous slurry. Through nitrogen adsorption–desorption isotherms, it was observed that most of the synthesized possessed mesopores which contribute to higher electrosorption performance. CV exhibited good conductivity properties of the functionalized activated carbon (CF-AC-PVA-GA) and the value was found to be 81.08 F/g. The functionalized CF-PVA-GA-activated carbon electrodes were used for the removal of salt where the SAC was found to be 3.89 mg g⁻¹ at 1.2 V. Furthermore, CF-AC-PVA-GA electrodes exhibited good stability in the shear conditions and, it was observed that the adsorption/desorption cycles of the fabricated CDI electrodes were more reproducible and stable more than 16 cycles with minimum loss of the active material. In conclusion, PVA binder fabricated carbon electrodes can be efficiently utilized for CDI performances with considerable certainty of better results.

A.H. and N.J.U. acknowledge the Ministry of Science and Technology, the People's Republic of Bangladesh, for financial support to partially carry out this work under the project ‘Monitoring and source identification of persistent toxic substances in atmospheric and biotic environment in Bangladesh’.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.