1. Introduction

The growing global population, the depletion of natural resources, and the negative impact of industrial manufacturing processes are driving factors for the increasing demand for more sustainable manufacturing processes and products for the energy storage industry. As a consequence, scientists have explored solutions for sustainable materials and products for greener storage devices through a tenfold increase in scientific publications in 2010–2020 [1,2,3,4,5].

Due to the diversity of the required small and large energy systems, the development of electrochemical energy storage devices, such as supercapacitors (SCs) and batteries, has been extensively reported in the literature; batteries have higher energy densities at the cost of low power densities, while SCs are characterized by high power densities and low energy densities, with the advantages of having a high power capacity and being endowed with fast charging/discharging cycles [6], light weight [5], and both economic and environmental advantages such as the use of aqueous electrolytes [2]. SCs have a huge relevance for applications such as electrical automobiles [7] and telecommunications [8] by integrating strategies based on the incorporation of solar electricity or the harvesting of mechanical movement converted into electricity [5,9,10].

The optimal configuration of supercapacitors has been explored with the combination of porous electrodes and electrolytes to improve the charge separation at the Helmholtz double layer and the electrode–electrolyte interface [4]. Supercapacitors are composed of electrodes that depend on features such as high specific surface area, surface chemistry, and electrical conductivity [11]. To date, several materials (e.g., graphite [12], metal oxides/hydroxides [13], and conducting polymers [14]) have been employed as high-performance electrodes for SCs. On the other hand, two-dimensional nanostructures (graphene, MXene, metal dichalcogenides) present outstanding properties such as high package density, high surface area, transparency, and chemical/mechanical stability that enable their use in supercapacitors [15]. Despite these advantages, the typical stacking/aggregation of 2D structures reduces the electrochemical performance of the devices. To circumvent these drawbacks, different strategies have focused on the exfoliation of black phosphorous [16] and on the surface modification of structures with nitrogen, sulfur, and phosphorous-based groups (heteroatom doping strategies) that reinforce the pseudocapacitance of the resulting material [17]. The incorporation of materials with characteristic redox reactions is another important strategy that is conducted given the fast electrochemical kinetics in systems such as vanadium redox flow batteries [18] and with the incorporation of carbonaceous materials to avoid corrosive and degradative processes in Zn anodes [19].

However, these electrode materials have the drawbacks of high production costs and/or non-eco-friendly fabrication methods. Thus, it is crucial to develop electrode materials with the properties of sustainability, eco-friendly behavior, low cost, and efficient response. In this direction, carbon electrodes from biomass precursors have attracted huge attention due to their worldwide availability and abundance, non-toxicity, and high surface area with hierarchical porous structure materials [20,21] to improve the mechanisms of efficient charge separation.

Pyrolysis is a standard method to convert biomass into porous materials whose properties are highly dependent on the pyrolysis conditions and the chemicals used for the activation or doping processes [22,23,24]. For instance, nitrogen doping methods in carbon preparation for SCs have been shown to boost its conductivities and hydrophilicity, which improve its surface wettability, resulting in an increased pseudocapacitance effect, thus delivering improved energy/power densities.

One of the ways to improve the electrochemical conductivity of carbon-based electrodes is coating with conducting polymers [25,26,27], such as polypyrrole (PPy), which is one of the most promising support materials due to its excellent electrical conductivity, environmental and thermal stability, and easy procedure of preparation [28,29,30,31,32]. Thus, it enables a promising possibility for the development of low-cost and commercially viable SCs.

The general mechanisms that are combined in the improvement of the electrochemical efficiency of the electrodes are based on two important processes: the electrical double layer capacitance (EDLC) acquired from the adsorption of electrolyte ions on conductive electrodes [33] and pseudocapacitive effects that allow good performance in energy storage through reversible redox on electrodes [34]. The adequate combination of EDLC and the pseudocapacitive effect in SCs improves the overall energy density of the devices by the combination of available sites for charge accumulation of EDLC and the rapid ionic transport of pseudocapacitive prototypes provided by the PPy-based biochar doping [35,36,37].

Herein, we aimed to explore the use of birch wood to produce porous biochar to be used as carbon electrodes for high-performance supercapacitors. Moreover, the prepared porous biochar was subjected to a polypyrrole, and its effect on both physicochemical and electrochemical properties was fully investigated. The obtained results suggest an improved efficiency of the polypyrrole-doped biochar due to the pseudo-capacitive effect and the feasibility of the proposed approach for the fabrication of sustainable SCs based on biomass wastes.

2. Materials and Methods

2.1. Materials

Polyvinylidene fluoride (PVDF), cetyl trimethyl ammonium bromide (CTAB), dimethylformamide (DMF), and pyrrole (PPy) were obtained from Sigma Aldrich (St. Louis, MO, USA). Ammonium persulfate (APS) was obtained from Química Moderna (Barueri, SP, Brazil). Carbon black was purchased from Micromeritics (Norcross, GA, USA). All materials were used as received, except for the pyrrole, which was distilled before use.

2.2. Biochar Preparation

The biochar was prepared using birch wood wastes as a precursor. First, 20.0 g of the dried biomass was mixed with H3PO4 (50%) at a ratio of 1:4 (weight) and mixed until forming a homogeneous paste [38,39]. Then, the paste was kept at room temperature for 2 h and dried at 105 °C overnight. The dried paste was pyrolyzed at 700 °C for 2 h under an N2 atmosphere, with an initial heating rate of 10 °C per min. The pyrolyzed material was ground and washed several times with boiling water until the pH value of the filtrate water was similar to the ultra-pure water.

2.3. Preparation of Birch-PPy Powder

The birch-PPy powder was prepared according to the procedure described as follows: birch powder was dispersed into 5 mL of Milli-Q water by sonication. Meanwhile, another solution containing polypyrrole and CTAB was prepared as follows: 35 uL of pyrrole was added into 5 mL of Milli-Q water and also incorporated with 9.6 mg of CTAB. This solution was stirred until the complete dispersion of the CTAB and mixed into the solution containing birch biochar to prepare solution A. After this, solution B was prepared by adding 114.6 mg of APS into 5 mL of milli-Q water. Then, solution A was stirred in an ice bath and received solution B (dropwise) in a process that initiated the polymerization. After 2 h of reaction, the dark solution was centrifuged at 5000× g for 5 min to obtain the precipitated particles. The black powder was filtered and washed several times with Milli-Q water to remove any residue from the polymerization process and was dried in an oven at 40 °C for 1 h. Finally, the birch-PPy at ambient temperature was stored for use. The loaded mass of polypyrrole in the composite was 11 mg.

2.4. Fabrication of the Biochar SCs Electrodes

Graphite paper (1 × 1 cm) was used as a support for the coating with the biochar-based material slurries, prepared with a mass ratio of 8:1:1 (biochar:PVDF:carbon black) as follows: 10 mg of PVDF was added to 500 μL of DMF and then heated to 60 °C and stirred until the PVDF was completely dispersed. After this step, 80 mg of birch biochar and 10 mg of carbon black were added to the mixture, which was kept under continuous stirring at 60 °C to provide a homogeneous dispersion of the carbonaceous derivative. The preparation of the electrode, after this step, was conducted as follows: First, 20 μL of the as-prepared slurry was dropped and spread on the substrate. Then, the coated graphite paper was heated at 50 °C to eliminate the residues of the solvent. The biochar-PPy-based electrodes were prepared using the same method, although instead of using the pure birch biochar, the composite biochar-PPy was used for the preparation of the slurry.

2.5. Electrolyte Preparation and Supercapacitor Assembly

Polyvinyl alcohol (PVA) was explored as a solid-state film for the preparation of the electrolyte. The standard procedure for preparation was conducted as follows: First, 1 g of PVA was added into 10 mL of Milli-Q water and heated to 70 °C under stirring for 1 h. After this, the temperature was raised to 100 °C, and the solution was stirred for an additional 1 h. Then, a transparent aspect of the solution was observed, and the solution was cooled to room temperature. As a following step, the solution was stirred for 15 min and received 1 mL of H3PO4. To avoid bubbles, the solution was sonicated for 2 min. Then, the solution was placed in a mold, and a thin film of PVA was obtained after 48 h. The PVA film was cut into pieces with 1 cm² of area and used as a solid-state electrolyte. The SCs were assembled in a sandwich configuration in which two electrodes were placed parallel to each other, separated by the PVA solid electrolyte film.

2.6. Characterization

The morphology evaluation of the biochars was carried out using a scanning electron microscope Vega 3XM (Tescan) with an electron acceleration of 10 kV, with the collected images collected with magnifications of 500 x, 1 kx, 3 kx, and 5 kx. The chemical composition of the materials was evaluated from Fourier transform infrared spectrum (FTIR) using the KBr method in an IR Prestige-21 Fourier transform infrared spectrometer (Shimadzu).

The specific surface area and pore volume of the biochar materials were measured through N2 adsorption-desorption via the BET (Brunauer, Emmett and Teller) and BJH (Barrett-JoynerHalenda) method, respectively, on a Surface Area Analyzer (ASAP 2020, Micromeritics).

Electrochemical characterization of the supercapacitors explored the two-electrode configuration with measurements provided by a potentiostat Autolab PGSTAT 302 N (Methrom) for the acquisition of voltammetry curves at different scan rates (10 mV s−1 to 200 mV s−1), with a potential window range of 0 to 0.8 V. Galvanometric curves was performed with a current density ranging from 1 mA to 5 mA, and the impedance spectrum was evaluated in a frequency range of 1 Hz to 1 MHz with data fitted by modified Randles circuit using the software Zview 2 version 3.4.


Ravi Moreno

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