# Reuse of Spent Coffee Grounds: Alternative Applications, Challenges, and Prospects--A Review
## 5 SCGs in Bioelectricity  
SCGs can also be used as an alternative material to expensive metal-based catalysts used as electrodes in fuel cells. The method was used to synthesize phosphorus- and nitrogen-doped carbon from SCGs and ammonium polyphosphate (Figure 2). The carbon matrix produces spherical particles with diameters ranging from 100 nm to several hundred nm. BET results showed a high porosity of the structure, a predominance of mesopores over other pores and an area of 506.91 m\({}^{2}\)/g. Studies by [PERSON] et al. (2015) also indicate the presence of an intense oxygen reduction peak in 0.1 M potassium hydroxide solution at 0.379 V and high current density. The researchers confirm the possibility of using SCGs as electrode replacements in fuel cells [35]. The use of SCGs as an alternative to catalysts in fuel cells is an innovative solution. The synthesis of carbon doped with phosphorus and nitrogen provides high porosity and good electrochemical properties, but the process requires advanced equipment, leading to high costs. Future research should focus on optimizing the process and assessing its feasibility on an industrial scale. SCGs subjected to various water extraction methods, combined with organisms from coastal bottom sediments, can be used to generate bioelectricity using a sediment-based fuel cell (SMFC) system. [PERSON] et al. (2024) evaluated various water extraction methods for SCGs, such as rinsing and drying SCGs (SMFC-CRD), immersion, rinsing and drying (SMFC-CRID), drying alone (SMFC-CD), and untre  
Figure 2: Schematic of electrode formation in SCG-based fuel cells (own elaboration, based on [PERSON] et al. 2015 [35]). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Materials** & **Process Conditions** & **Results** & **References** \\ \hline SCGs lipids extracted with pure hexane and isopropanol—methanol addition Lipids extracted from dry SCGs with pure hexane & Reaction time: 3 h, temperature: 60 \({}^{\circ}\)C, 80 rpm &
\begin{tabular}{c}odine value of about 70.0 g \(\mathrm{\SIUnitSymbolMicro}\)/100 g \\ biodiesel, an acid value of about 1.8 mg \\ KOH/g biodiesel and ester content of \\ about 86\% \\ \end{tabular} & [28] \\ \hline SCGs lipids, methanol and ethanol & - & Yield of biodiesel was 97.5 \(\pm\) 0.5\% & [29] \\ \hline SCGs treatment using H\({}_{2}\)SO\({}_{4}\) & Reaction time: 12 h, temperature: 70 \({}^{\circ}\)C & Yield of biodiesel was 17.08 \(\pm\) 0.70\% & [31] \\ \hline SCGs and NaOH as a catalyst & Reaction time: 3 h, temperature: 50 \({}^{\circ}\)C & 77\% biodiesel yield & [32] \\ \hline SCGs, KOH as a catalyst and methanol & Reaction time: 2 h, temperature: 60 \({}^{\circ}\)C & 86\% biodiesel yield & [33] \\ \hline SCGs, NaOH as a catalyst and methanol & Reaction time: 90 min, temperature: 60 \({}^{\circ}\)C & 90\% biodiesel yield & [34] \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Methods for obtaining biodiesel from SCGs. concentration of caffeine contained in coffee waste was reduced by pretreatment, and the SMFC-CRID method yielded the lowest concentration of 0.021 \(\pm\) 0.001 mg/g. Therefore, it can be concluded that the SMFC system using SCGs with water extraction performs better than without extraction. This is due to the fact that the toxicity of caffeine contained in SCGs after water extraction--which is toxic to the metabolic activity of microorganisms in the anode area--is reduced. The lowest caffeine content in the system shows the highest maximum power density by reducing the caffeine concentration using water extraction with rinsing and immersion. The highest maximum power density of 60.6 mW/m\({}^{2}\) was achieved for SMFC-CRD, which corresponds to the closed-circuit current density and low caffeine concentration. The study may be a proposal for reusing SCGs to generate bioelectricity. Pretreatment of SCGs with aqueous extraction helps improve the performance of the SMFC system while allowing for the reuse of waste. Further research is needed to determine the practical, commercial use of this system [36].