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].

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.