Valorisation of Poultry Litter for Energy Storage: A Characterisation Study for Supercapacitor Applications

1. Introduction

The global shift to renewable energy has accelerated research into effective energy storage systems [1]. Supercapacitors have emerged as attractive electrochemical energy storage devices due to their high power density, fast charge-to-discharge rates, and outstanding cycling stability [2,3]. Supercapacitors, unlike batteries, store energy by reversible redox processes at electrode-electrolyte interfaces [4,5]. The performance of supercapacitors is significantly dependent on electrode material properties, particularly specific surface area, pore architecture and electrical conductivity [6,7]. They were examined because of their tuneable porosity and chemical stability [8,9].

However, typical carbon precursors obtained from fossil fuels present environmental problems and have high manufacturing costs [6].

Biomass waste has gained popularity as an alternative carbon source for electrode production [10]. Agricultural leftovers, forestry byproducts, and animal waste provide renewable, abundant, and cost-effective feedstocks for the synthesis of porous carbons [10]. The conversion of waste biomass into value-added carbon materials adheres to circular economy concepts, addressing waste management concerns while also developing sustainable energy storage options [11].

2. Background

The Poultry litter, which includes manure, bedding materials (sawdust, sunflower husks) and feed residues, is a major agricultural waste source, with global production surpassing millions of tonnes per year [9,12]. Improper management of this waste, particularly land application, often leads to excessive nutrient runoff, especially nitrogen and phosphorus, into water bodies [13,14]. This nutrient overload can trigger rapid eutrophication, resulting in harmful algal blooms that deplete oxygen, block sunlight, and cause fish kills. While such blooms are typically viewed as an environmental threat, some studies demonstrate that microalgae like “Dunaliella salina” can be cultivated under optimised nutrient conditions to produce high lipid yields for biodiesel [14]. This highlights a paradigm shift: instead of merely mitigating nutrient pollution, waste-derived nutrients could potentially support algal biofuel production[14]. Current disposal methods for poultry litter, including land application and direct combustion, are largely unsustainable due to their contributions to greenhouse gas emissions, air pollution, and nutrient runoff [13,14]. Therefore, converting chicken litter into carbonaceous materials for supercapacitor electrodes offers an environmentally benign alternative that aligns with circular economy principles.

Several studies have established the viability of employing animal waste-derived carbons for energy storage applications [15,16]. However, the physicochemical characteristics of poultry litter that govern its suitability for biochar production and subsequent activation into supercapacitor electrodes require systematic investigation. The study objects to comprehensively characterise poultry litter biomass by evaluating its proximate composition, elemental constituents, inorganic content and thermal properties to assess its potential as a precursor for porous carbon materials intended for supercapacitor applications [17].

Figure 1. PL Biomass to Supercapacitor Conversion Pathway.

Figure 1. PL Biomass to Supercapacitor Conversion Pathway.

3. Materials and Methods

3.1. Sample Collection and Preparation Procedure

Poultry litter samples consisting of sunflower husks, sawdust, feathers and manure were collected from Imbizo farm in Bulawayo, Zimbabwe. The feedstock was dried in an oven (Systronix Scientific) at 110°C for 24 hours to eliminate moisture [9]. The dried samples were ground using a ball mill (Pulveisette 6) for 15 minutes to achieve a uniform particle size [6,9].

Demineralisation was performed by washing the ground poultry litter with deionised water at a biomass-to-solvent ratio of 1:10 (w/v) at 30°C for 2 hours under continuous magnetic stirring [9,18]. This process removes water-soluble inorganic constituents that may interfere with subsequent carbonisation and activation processes. The samples were rinsed repeatedly with deionised water until pH 7 was achieved (Envtek, ENV49) and dried at 110°C for 24 hours, and stored in a desiccator prior to analysis [19,20].

3.2. Proximate Analysis

Proximate analysis was conducted using a thermogravimetric analyser (Leco TGA 701) according to ASTM D7582 MVA standard procedures [9]. Moisture content was determined by weight loss upon heating to 118°C. Volatile matter was measured by further heating to 970°C under an inert atmosphere. Ash content was determined by combustion in air at 600°C. Fixed carbon content was calculated by difference using Equation (1):

$$\mathrm{Fixed}\mathrm{carbon}=100-\left(\mathrm{Moisture}+\mathrm{VM}+\mathrm{Ash}\right)$$

(1)

3.3. Ultimate Analysis

Elemental composition (carbon, hydrogen, nitrogen, sulfur) was determined using an elemental analyser (Thermo Scientific Flash 2000 CHNS/O) following ASTM D5291-96 standards [6]. Oxygen content was calculated by difference using Equation (2) [26]:

$$\mathrm{O}=100-(\mathrm{C}+\mathrm{H}+\mathrm{N}+\mathrm{S}+\mathrm{Ash})\mathrm{\%}$$

(2)

3.4. Calorific Value Determination

Higher heating value (HHV) was measured using an oxygen bomb calorimeter (IKA C6000; Supercal2) according to ASTM D5865-12 standards [21]. Lower heating value (LHV) was calculated considering the latent heat of vaporisation of water formed during combustion.

3.5. Inorganic Element Analysis

The inorganic elemental composition of demineralised poultry litter was analysed using a handheld X-ray fluorescence spectrometer (Olympus Delta-50 Premium) under the Geochem method [22]. Elements detected were quantified as weight percentages.

4. Results and Discussion

4.1. Proximate Composition

The proximate analysis results for demineralised poultry litter are presented in Table 1. The moisture content of 4.89% falls within the acceptable range for thermochemical conversion processes, as excessive moisture (>10%) reduces process efficiency and increases energy requirements [29]. The volatile matter content of 70.26% indicates substantial organic components that decompose during pyrolysis to generate porous carbon structures [23].

The ash content of 6.34% is moderate compared to other biomass feedstocks. Agricultural residues typically exhibit ash contents ranging from 2-20%, with higher values associated with soil contamination and inorganic nutrient accumulation [24]. The fixed carbon content of 18.51% represents the fraction available for conversion into carbonaceous structures during pyrolysis. This value is comparable to other lignocellulosic materials such as switchgrass (15.02%) and red cedar (15.62%)[6,25].

4.2. Elemental Composition

Ultimate analysis results (Table 2) reveal carbon content of 43.06%, which serves as the primary building block for porous carbon networks. This value is lower than woody biomass (typically 45-50%) but comparable to herbaceous agricultural residues [26][6]. The hydrogen content of 6.13% and oxygen content of 40.68% indicate substantial oxygen-containing functional groups that may contribute to pseudocapacitance through faradaic reactions when incorporated into electrode materials [27].

The nitrogen content of 2.11% is noteworthy, as nitrogen doping in carbon materials enhances electrochemical performance by introducing electron-donor properties and improving wettability [9,28]. Heteroatom-doped carbons derived from nitrogen-rich precursors exhibit improved specific capacitance through additional pseudocapacitive contributions [29]. The sulfur content of 1.68% may similarly contribute to enhanced electrochemical activity [30][6,9].

The atomic ratios of H/C and O/C provide insights into the carbonisation behaviour and aromaticity development during pyrolysis [9]. The calculated H/C ratio of 1.71 (atomic basis) and O/C ratio of 0.71 suggest moderate aromatic condensation potential, though these values will evolve significantly during thermal treatment[31,32].

4.3. Calorific Value

The higher heating value of 16.86 MJ kg⁻¹ (Table 3) is lower than conventional woody biomass (18-22 MJ kg⁻¹) but consistent with values reported for animal manures and agricultural residues [33]. The energy content reflects the combined contributions of organic components and influences the thermal behaviour during carbonisation [34,35].

4.4. Inorganic Element Composition

X-ray fluorescence analysis revealed the presence of multiple inorganic elements in the demineralised poultry litter (Table 4). Calcium (4.12%) and potassium (3.42%) were the most abundant, reflecting the mineral composition of poultry feed and bedding materials [36]. Silicon (0.89%), aluminium (1.06%), and iron (1.23%) originate from soil contamination and bedding materials.

The existence of alkali (water soluble basis) and alkaline (pH neutralising) earth metals (K, Ca, Mg) significantly influences pyrolysis behaviour and subsequent activation processes [37]. During thermal treatment, these elements can catalyse carbon gasification reactions, affecting pore development and surface area evolution [38]. Potassium, in particular, is known to enhance the reactivity of carbonaceous materials during activation, promoting the formation of microporous structures [39].

Transition metals such as iron, copper, and zinc may contribute to graphitisation during high-temperature treatment and thus potentially improving electrical conductivity of the resulting carbon materials [40]. However, excessive inorganic content present may necessitate additional washing steps to prevent pore blockage and ensure consistent electrochemical performance [41].

4.5. Implications for Supercapacitor Electrode Fabrication

The characterisation results provide several insights regarding the suitability of poultry litter for supercapacitor electrode production (Table 5).

4.6. Comparative Assessment with Other Biomass Feedstocks

Table 6 compares the characteristics of poultry litter to other biomass sources studied for supercapacitor device applications. The carbon content is significantly lower than wood-derived biomass (red cedar: 47.51%), but equivalent to herbaceous materials (switchgrass: 43.19%, sorghum: 40.68%) [43]. The ash content is higher than woody biomass, but lower than some agricultural leftovers like rice husk (15-20%) [44].

Poultry litter has a higher nitrogen content (2.11%) than other feedstocks (<0.6%), providing a distinct advantage in producing heteroatom-enriched carbons. Studies have showed that nitrogen-containing functional groups enhance specific capacitance through faradaic reactions and improve electrolyte wettability [45,46].

Similarly, the elevated sulphur content may contribute to enhanced electrochemical performance, as sulphur-doping has been shown to increase pseudocapacitance and rate capability [47,48].

Figure 2. Circular Economy Framework of PL-Derived Supercapacitor.

Figure 2. Circular Economy Framework of PL-Derived Supercapacitor.

4.7. Optimisation Considerations for Biochar Production

Based on the characterisation results, several parameters require optimisation for producing biochar suitable for supercapacitor applications (Table 7).

5. Conclusions

The study characterised poultry litter biomass to evaluate its suitability for biochar production intended for supercapacitor electrode applications. The substance contains 43.06% carbon, 70.26% volatile matter and 18.51% fixed carbon, making it an ideal precursor for porous carbon synthesis. The intrinsic nitrogen (2.11%) and sulphur (1.68%) concentration allows for self-doping during carbonisation, which can improve electrochemical performance via pseudocapacitive contributions. The presence of potassium (3.42%) and other inorganic elements may aid in the creation of pores during chemical activation, thus reducing the need for activating agents. The moderate ash concentration (6.34%) demands adequate demineralisation procedures to avoid pore obstruction and provide constant electrode performance.

Poultry litter has specific advantages over typical biomass feedstocks, such as heteroatom enrichment and waste valorisation potential. The conversion of poultry litter into porous carbon materials adheres to circular economy concepts, addressing agricultural waste management issues while also developing sustainable energy storage solutions. Further research should focus on optimising pyrolysis and activation parameters to maximise surface area while preserving beneficial heteroatom functionalities, and evaluating the electrochemical performance of resulting carbon materials in supercapacitor configurations.