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Article

Valorization of Spent Bio-Adsorbents into High-Performance Eco-Friendly Anodes for Direct Urea Fuel Cells

by
Samar M. Mahgoub
1,
Ahmed A. Allam
2,
Hala Mohamed
1,
Hassan A. Rudayni
2,
Rehab Mahmoud
3,*,
Kholoud Khaled Mohammed
4 and
Amal Zaher
5
1
Materials Science and Nanotechnology Department, Faculty of Postgraduate Studies for Advanced Sciences, Beni-Suef University, Beni-Suef 62528, Egypt
2
Department of Biology, College of Science, Imam Mohammad Ibn Saud Islamic University (IMSIU), Riyadh 11623, Saudi Arabia
3
Department of Chemistry, Faculty of Science, Beni-Suef University, Beni-Suef 62528, Egypt
4
Department of Biotechnology and Life Sciences, Faculty of Postgraduate Studies for Advanced Sciences (PSAS), Beni-Suef University, Beni-Suef 62511, Egypt
5
Environmental Science and Industrial Development Department, Faculty of Postgraduate Studies for Advanced Sciences, Beni-Suef University, Beni-Suef 62511, Egypt
*
Author to whom correspondence should be addressed.
Catalysts 2025, 15(12), 1113; https://doi.org/10.3390/catal15121113
Submission received: 31 October 2025 / Revised: 19 November 2025 / Accepted: 26 November 2025 / Published: 29 November 2025
(This article belongs to the Section Electrocatalysis)
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Versions Notes

Abstract

The commercialization of direct urea fuel cells (DUFCs) is hampered by the scarcity of low-cost, high-performance electrocatalysts for the urea oxidation reaction (UOR), while water treatment processes generate spent adsorbents as a secondary waste. This study presents a circular economy solution by transforming a waste product—spent progesterone-loaded Reishi mushroom biosorbents—into high-performance anodes for DUFCs. We demonstrate that the thermal conversion of Ganoderma lucidum into biochar (Biochar/RM), followed by its “activation” through progesterone (PG) adsorption, creates a superior electrocatalytic composite (Biochar/RM/PG). Electrochemical evaluation revealed that this spent adsorbent delivers an exceptional UOR activity, achieving a peak current density of 225.52 mA cm−2, which is 79% higher than its pristine counterpart. This enhancement is driven by a unique synergy: the biochar provides a conductive, porous framework, while the thermally transformed PG acts as an in situ dopant, creating nitrogen-rich active sites and optimizing the surface architecture for urea electro-oxidation. The catalyst further demonstrated remarkable operational stability over 3600 s. This work establishes a pioneering “waste-to-wealth” strategy, simultaneously addressing the challenges of pharmaceutical wastewater management and the need for sustainable energy materials.

1. Introduction

The escalating levels of urea in water bodies, originating from industrial effluent, agricultural runoff, and municipal wastewater, pose a severe threat to aquatic ecosystems [1,2]. Paradoxically, urea is also a hydrogen-rich molecule, positioning urea-polluted wastewater as a potential resource for clean energy via the electrochemical urea oxidation reaction (UOR: CO(NH2)2 + H2O → N2 + 3H2 + CO2) [3,4,5]. The commercialization of this technology, however, is hindered by the need for active, stable, and low-cost anodic catalysts to replace expensive precious metals [6,7,8,9,10]. While research has focused on non-precious transition metal-based catalysts [6,7], a sustainable and circular approach to catalyst fabrication remains a critical challenge.
Concurrently, the removal of endocrine-disrupting pharmaceuticals like progesterone (PG) via adsorption generates spent adsorbents, creating a secondary waste stream [8,9,10]. The concept of a circular economy advocates for the valorization of such waste into valuable resources. Biochar, a tunable carbon material, shows promise for both adsorption and electrocatalysis [11,12] and Reishi mushroom (Ganoderma lucidum) has demonstrated a high capacity for PG adsorption [13].
Herein, we introduce a novel strategy that bridges these two environmental challenges. Unlike conventional biochar-based UOR catalysts derived directly from pristine biomass, this work establishes a unique valorization pathway by using spent progesterone-loaded biosorbents as precursor materials. We hypothesize that the thermal transformation of Reishi mushroom (RM) into biochar, followed by its “activation” through PG adsorption, creates a unique composite with superior electrocatalytic properties [14,15].This approach converts a waste product into a high-value catalyst, effectively closing the loop between water treatment and energy conversion.
The novelty of our work. Unlike conventional biochar-based UOR catalysts, which are typically derived directly from biomass precursors [z], our study utilizes spent wastewater nano-adsorbents —specifically RM or biochar-based RM as the precursor material. This valorization route converts exhausted adsorbents, which are usually considered agro-waste, into high-performance electrocatalysts, establishing a closed-loop approach consistent with circular economy principles. To our knowledge, this is the first report to repurpose RM residues for urea oxidation catalysis.
Unlike previously reported biochar-based electrocatalysts for the urea oxidation reaction (UOR), which are generally synthesized from pristine or intentionally prepared biomass, this study introduces a unique valorization pathway by reusing spent biosorbents as precursors for catalyst fabrication [16]. The exhausted adsorbents, previously employed for wastewater treatment, were transformed into functional biochar catalysts through controlled pyrolysis [16], effectively linking pollution remediation with electrocatalytic valorization. This dual-purpose approach not only minimizes solid waste generation but also advances the concept of a circular economy by converting waste-derived materials into high-performance UOR catalysts. This is the first report to exploit spent biosorbents of Biochar/RM/PG as feedstock for biochar from RM source based electrocatalysts in urea oxidation applications.
To our knowledge, this is the first report to repurpose spent Biochar/RM/PG adsorbents for urea oxidation catalysis. The resulting composite demonstrated exceptional electrocatalytic activity and stability, rivaling many engineered non-precious catalysts. This work provides a sustainable blueprint for next-generation energy materials by transforming waste streams into high-performance anodes for direct urea fuel cells.

2. Results and Discussion

2.1. Material Characterization

The physicochemical properties of the synthesized materials were thoroughly characterized to understand their structure-property relationships.
The XRD analysis results of both raw Reishi mushroom (RM) and its biochar composite (Biochar/RM) are presented in Figure 1. To evaluate the crystalline characteristics of the materials, powder X-ray diffraction was performed over a 2θ range of 5–80°. As carbon constitutes a major component of the mixture, the XRD patterns Figure 1 indicate that both materials are predominantly amorphous. The XRD pattern of RM exhibits a broad peak centered around 2θ = 20.73°, corresponding to the amorphous carbon structure, with no additional peaks at higher diffraction angles. In contrast, the Biochar/RM pattern displays two distinct diffraction peaks at 2θ = 24.03° and 42.71°, which correspond to the (002) and (100) crystal planes of graphite, respectively [17]. The (002) plane represents disordered microcrystalline carbon, while the (100) plane reflects a graphitized crystal structure. These results suggest that thermal carbonization led to the formation of a mixed structure comprising both amorphous carbon and semi-graphitized microcrystalline domains [18].
Fourier-transform infrared (FTIR) spectroscopy was employed to identify the functional groups present in raw Reishi mushroom (RM) and its biochar composite (Biochar/RM). The FTIR spectra recorded in the 500–4000 cm−1 range are shown in Figure 1b. In the spectrum of raw RM, a broad band between 3200 and 3600 cm−1 corresponds to the stretching vibrations of hydroxyl (-OH) groups [19]. Peaks observed at 2926 cm−1 and 1399 cm−1 are attributed to the stretching and bending vibrations of saturated C–H bonds, respectively [17]. The absorption band at 1645 cm−1 represents the C=O stretching of amide I, while the band at 1560 cm−1 is associated with N–H deformation of amide II. The amide III band, appearing at 1315 cm−1, corresponds to C–N bond vibrations [17]. Additional bands at 1074 cm−1 and 1036 cm−1 are assigned to C–O stretching vibrations in alcohol hydroxyl groups.
A distinct peak at 894 cm−1 indicates the β-configuration of D-glucose units, signifying the presence of a glucan structure [20]. This band, characteristic of the D-glucopyranose fingerprint region, is consistent with the findings reported by Barar et al. [21]. The band at 569 cm−1 represents the bending vibrations of saccharide groups [20,22].
In the Biochar/RM spectrum, peaks at 3417 cm−1 and 3132 cm−1 correspond to –OH stretching vibrations, while a peak at 1631 cm−1 is attributed to C=O stretching and that at 1399 cm−1 to C–H vibrations. Compared with raw RM, the Biochar/RM spectrum shows significantly attenuated or missing peaks, indicating the decomposition of organic functional groups during pyrolysis. This transformation confirms that the thermal carbonization process converted the Reishi mushroom matrix into carbon-rich material.
Field Emission Scanning Electron Microscopy (FE-SEM) was employed to examine the surface configuration of the synthesized materials. As shown in Figure 2a,b, the raw Reishi mushroom (RM) before hormone loading exhibits a compact, homogeneous layered structure resembling a stomach lining, with features like octopus sucker-like formations (Figure 2b). In contrast, after progesterone loading (RM/PG), as shown in Figure 2c,d, the surface morphology changes markedly, revealing a porous, fibrous, and rod-like structure. This transformation indicates that progesterone incorporation disrupted and broke down the compact layers of the original matrix.
Surface texture and morphology were further analyzed using ImageJ software to quantify parameters related to roughness, waviness, and 3D surface topology, as illustrated in Figure S1. Roughness refers to fine, small-scale surface irregularities occurring over short wavelengths (Figure 3). It is typically measured with a stylus profilometer, which records vertical deviations from the mean line. Common roughness parameters include Ra, Rz, and Rq, which describe overall surface texture [23]. Waviness refers to larger-scale, lower-frequency surface deviations arising from machining, casting, or material distortion. It is usually measured with a form tester that scans a wider area using a high-resolution sensor [24].
A comparative analysis of surface parameters, including Rq (Rrms), Ra, Rsk, Rku, Rv, Rp, Rt, FPO, MFOV, FAD, and SA, revealed notable differences between RM before and after hormone loading (RM/PG). All parameters showed an increase after progesterone loading, except Rv and Rsk, which exhibited a decrease (Table S1).
Field Emission Scanning Electron Microscopy (FE-SEM) was used to examine the surface configuration of the prepared materials. As shown in Figure 4a,b, the biochar/Reishi mushroom (Biochar/RM) before hormone loading exhibits compact, homogeneous layers arranged in an overlapping manner with fibrous and crutch-like structures. In contrast, after progesterone loading (Biochar/RM/PG), as shown in Figure 4c,d, the surface morphology changes noticeably, displaying a more porous, fibrous, and rod-like appearance. This alteration suggests that progesterone incorporation disrupted the compact layers, leading to increased pore formation.
Surface texture and morphology were further analyzed and shown in Figure S2a–l. Roughness and wavelengths and representation at Figure 4. It is typically quantified using a profilometer that measures vertical deviations from the mean surface line [25]. Waviness, on the other hand, represents larger-scale surface deviations occurring across longer wavelengths and at a lower frequency than roughness. It may result from factors such as machining, casting, or material deformation. Waviness is typically measured using a form tester that scans a broader surface area with a precision sensor [26]. A comparative evaluation of roughness parameters including Rq (Rrms), Ra, Rsk, Rku, Rv, Rp, Rt, FPO, MFOV, FAD, and SA revealed a general decrease in all parameters after hormone loading, except for Rku, Rp, Rt, and FAD, which showed an increase Figure 5 & Table S2.
To quantitatively assess the textural properties underpinning the electrocatalytic performance, nitrogen adsorption–desorption analysis was performed on the spent adsorbents RM/PG and Biochar/RM/PG. As shown in Figure 6a,b both materials exhibit Type IV isotherms with a distinct H3-type hysteresis loop, which is characteristic of mesoporous materials with slit-shaped pores [X]. The Biochar/RM/PG composite displayed a significantly higher nitrogen uptake across the entire relative pressure (P/P0) range compared to RM/PG, indicating a more developed and extensive porous network.
The corresponding pore size distributions (PSD), derived from the adsorption branch using the Barrett-Joyner-Halenda (BJH) method, are presented in Figure 6c,d. The PSD for RM/PG shows a primary peak centered at approximately 5 nm, while Biochar/RM/PG exhibits a more intense and broader peak shifted to around 7 nm. This confirms that the pyrolysis process not only increases porosity but also generates larger mesopores.
The quantitative textural parameters are summarized in Table 1 The BET specific surface area (SBET) of Biochar/RM/PG was calculated to be 246 m2/g, which is substantially larger than that of RM/PG (151 m2/g). Consistent with this, the total pore volume of Biochar/RM/PG (0.40 cm3/g) was more than double that of RM/PG (0.16 cm3/g). This remarkable enhancement in surface area and porosity for Biochar/RM/PG is attributed to the synergistic effect of thermal decomposition during pyrolysis, which creates a conductive carbon framework with intrinsic porosity, and the subsequent intercalation and surface modification by progesterone molecules. This highly porous and accessible structure is pivotal for electrocatalysis, as it provides abundant active sites and facilitates the diffusion of urea molecules and reaction products, thereby directly contributing to the superior UOR performance observed.

2.2. Electrocatalytic Urea Oxidation Performance

2.2.1. Optimization of Catalyst Loading

To evaluate the influence of catalyst mass and potential mass transport effects, the UOR performance of the optimal Biochar/RM/PG catalyst was systematically assessed at loadings of 0.35, 0.71, and 1.07 mg cm−2 (Figure S3). This pronounced plateau in performance indicates that mass transport limitations such as the diffusion of urea to active sites and the removal of gaseous products (e.g., N2 and CO2) begin to govern the reaction kinetics at higher loadings, preventing a proportional gain in activity with increased catalyst mass [23,27]. This volume is the bare minimum required to ensure stable film formation on the GCE. Lower volumes utilized in early experiments resulted in poorly adhering films and insufficient surface coverage, which led to unstable electrochemical reactions. As a result, we chose the smallest ink volume that resulted in a catalyst layer that was consistent, repeatable, and mechanically stable [22,24].

2.2.2. Catalytic Activity of Different Materials

The electrocatalytic performance of the prepared catalysts toward the urea oxidation reaction (UOR) was evaluated using cyclic voltammetry (CV) in 1.0 M KOH containing 1.0 M urea (Figure 7). The CV data were fitted and analyzed using Origin 8.1 software to determine the relationship between progesterone (PG) loading and oxidation peak current density. The current densities followed the order: Biochar/RM/PG (223 mA cm−2) > RM/PG (172 mA cm−2) > Biochar/RM (140 mA cm−2) > RM (124 mA cm−2).
This trend demonstrates that both calcination and progesterone incorporation significantly enhance catalytic performance. Converting RM to Biochar/RM improves conductivity and porosity, facilitating charge transfer [28]. PG adsorption further increases the density of active sites and modifies the electronic structure of the biochar, resulting in lower onset potential and higher oxidation current density [29].
The 79% enhancement in current density for Biochar/RM/PG over pristine RM highlights a strong synergistic effect, likely due to nitrogen- and carbon-rich residues from PG decomposition forming additional catalytic sites. Therefore, Biochar/RM/PG exhibits the most efficient electron transfer and is identified as the optimal anode material for urea electrolysis [30].

2.2.3. Influence of Urea Concentration and Scan Rate

The influence of urea concentration on the electrocatalytic performance was systematically investigated. Figure 8a–d presents the cyclic voltammetry (CV) responses of the prepared electrodes in 1.0 M KOH with urea concentrations varying from 0.0 to 1.0 M at a scan rate of 100 mV/s. The electrochemical response in the urea-free solution (0.0 M) established a baseline, showing negligible current density. A significant increment in anodic current density was observed for all catalysts upon the addition of urea, confirming their intrinsic catalytic activity towards the urea oxidation reaction (UOR) [31]. The maximum current densities recorded at 1.0 M urea were 125.55 mA cm−2 for RM, 141.25 mA cm−2 for Biochar/RM, 172.03 mA cm−2 for RM/PG, and 225.52 mA cm−2 for Biochar/RM/PG. The Biochar/RM/PG composite consistently delivered the highest current density across all concentrations, demonstrating its superior catalytic efficiency. This enhancement is attributed to PG acting as a surface structure modifier, which enhances interfacial conductivity and creates a highly porous, well-ordered architecture that facilitates electron transfer [32].
To investigate the reaction kinetics and mass transfer effects, CV measurements were performed in 1.0 M KOH + 1.0 M urea at scan rates ranging from 5 to 60 mV/s (Figure 9). The generated current density was significantly affected by the scan rate for all prepared samples. The anodic peak current increased substantially with increasing scan rate, while a corresponding cathodic peak reduction was visible at the lower scan rate of 5 mV/s. This behavior indicates contributions from double-layer capacitance, faradaic reaction processes, and charge transfer kinetics. At lower scan rates, the extended contact time between electrolyte ions and catalytic active sites allows for more complete reactions and higher faradaic efficiency. In contrast, the shorter dwell time at higher scan rates restricts these interactions, leading to an increased charging current. The linear relationship between peak current density and the square root of the scan rate confirmed a diffusion-controlled UOR process for all catalysts. The Biochar/RM/PG and RM/PG composites exhibited the steepest slopes in this relationship, indicating their superior mass transfer characteristics and larger electroactive surface areas. Furthermore, the scan rate-independent current densities observed for these composites, particularly evident in Figure 9c,d, demonstrate better accessibility of electrolyte ions to the active sites [33,34,35]. The enhanced electrochemical characteristics of Biochar/RM/PG can be attributed to the formation of additional electroactive sites facilitated by the integration of organic progesterone residues within the biochar matrix, which collectively provide more active sites for the urea oxidation reaction.

2.2.4. Catalyst Durability

The operational durability and structural stability of the synthesized catalysts, critical parameters for practical application, were rigorously evaluated using combined chronoamperometry (CA) and cyclic voltammetry (CV). Stability was first assessed via CA at a constant potential of 0.6 V in 1.0 M KOH with 1.0 M urea for 3600 s (Figure 10). All catalysts exhibited an initial current stabilization period before reaching quasi-steady state performance. Notably, the Biochar/RM/PG composite demonstrated a characteristic current increase during the initial phase, consistent with an electrochemical conditioning process. This behavior can be attributed to progressive electrolyte wetting of the hierarchical mesoporous structure (Table 1) coupled with potential-induced optimization of nitrogen functional groups derived from progesterone decomposition. Similar electrochemical conditioning has been reported for N-doped carbon catalysts, where applied potentials enhance the formation of active pyridinic-N sites that facilitate urea adsorption and oxidation [36,37,38].
Following this conditioning phase, the Biochar/RM/PG composite maintained superior stability with the highest current retention among all samples [39], while other catalysts exhibited more pronounced current decay due to progressive blockage by reaction intermediates [40].
Post-stability UOR performance was further evaluated by CV (Figure 11). The Biochar/RM/PG catalyst retained 96% of its initial peak current density (163.550 vs. 225.52 mA cm−2) and no significant potential shifts, confirming exceptional robustness of the active sites. To further challenge the catalyst under dynamic operating conditions relevant to fuel cell cycling, we performed an accelerated stress test involving 2000 continuous CV cycles. As shown in Figure 11, the Biochar/RM/PG composite exhibited exceptional stability, with the 1st and 3600 s showing minimal divergence. The catalyst retained 93% of its initial peak current density after 2000 cycles (Figure 11a), providing strong evidence of its resilience against potential-induced degradation and dissolution.
The outstanding stability of Biochar/RM/PG originates from synergistic effects: the resilient graphitic biochar framework provides mechanical stability, while the electrochemically conditioned nitrogen species from progesterone create stable, active centers resistant to deactivation [40]. This combined analysis validates the exceptional operational and structural stability of the Biochar/RM/PG composite, establishing its promise as an advanced anode material for direct urea fuel cells.
The Biochar/RM/PG composite was subjected to post-chronoamperometry (CA) FESEM study to determine whether it retained its structure after extended operation (Figure 11b). The image verifies that there is no obvious collapse, aggregation, or breakdown and that the catalyst maintains its general architecture, including its porous carbon framework. This suggests that under the evaluated UOR circumstances, the material demonstrates outstanding structural stability.

2.2.5. Performance Benchmarking and Synergistic Mechanism

The performance of our best catalyst, Biochar/RM/PG, is competitive with many intentionally synthesized non-precious catalysts reported in the literature, such as Ni-Co oxide nano-grass; and other carbon-supported systems. To clearly demonstrate the superior catalytic activity of our Biochar/RM/PG catalyst compared to typical non-noble metal catalysts reported in recent literature, we present a comparative summary table (Table 2). This table compiles key performance metrics including peak current density, onset potential, and stability, highlighting that Biochar/RM/PG exhibits notably higher peak current density (225.52 mA/cm2) and enhanced operational stability than traditional Ni-Co based catalysts.
The performance of our best catalyst, Biochar/RM/PG, is competitive with many intentionally synthesized non-precious catalysts reported in the literature, such as Ni-Co oxide nano-grass [41,42] and other carbon-supported systems [43].
The superior performance and stability are attributed to a synergistic mechanism where multiple factors contribute to enhanced UOR activity. The Biochar/RM base provides a highly conductive, porous, and stable 3D network that facilitates efficient electron transport and electrolyte diffusion to active sites. Most significantly, the adsorbed progesterone (PG) acts as an in situ dopant during thermal treatment and electrochemical operation. The nitrogen-rich PG decomposes and incorporates nitrogen atoms into the carbon matrix, creating N-doped carbon active sites. As demonstrated in studies of N-doped carbons for electrocatalysis [44], specific nitrogen functional groups—particularly pyridinic N—significantly enhance UOR activity by optimizing the adsorption of urea molecules and facilitating the desorption of key reaction intermediates [45]. This mechanistic insight aligns with our electrochemical data showing superior charge transfer characteristics and lower onset potentials. Finally, the increased surface roughness and porosity, as confirmed by FESEM, provide abundant accessible active sites while the resilient graphitic structure ensures long-term structural integrity during UOR operation.
Table 2. Comparative Performance of Reported UOR Catalysts.
Table 2. Comparative Performance of Reported UOR Catalysts.
Catalyst/SystemCurrent Density (mA/cm2)Potential (V vs. RHE)Stability/NotesCitation
Ni0.15Co0.85-MOF (BMOF)10 @ 1.33 V1.33 (10 mA/cm2)72 h at 40 mA/cm2; high stability[46]
Co,Ge-Ni Oxyhydroxide448 @ 1.4 V1.4 (448 mA/cm2)High Faradaic efficiency (84.9% NO2−)[47]
NiCo2O4@CoS/Ni-Foam78 @ 0.5 V0.5 (78 mA/cm2)High durability; core–shell architecture[48]
Ni/Co Mixed Oxide/Hydroxide—Low onset; low VEnhanced selectivity and fast kinetics[49]
NiCo LDH Hydroxides——Fast kinetics; low energy barrier[50]
Cu-FMOF-NH2 (MOF)10 @ 1.31 V; 50 @ 1.47 V1.31 (10 mA/cm2)Water-stable; outperforms RuO2[51]
Ni-doped CuO Nanoarrays100 @ 1.366 V1.366 (100 mA/cm2)Robust stability; low Tafel slope[37]
Ni3N Nanosheet Array10 @ 1.35 V1.35 (10 mA/cm2)Durable; bifunctional (HER/UOR)[52]
Cr-Ni(OH)2100 @ 1.38 V1.38 (100 mA/cm2)Stable 200 h @ 10 mA/cm2[53]
Oxyanion-Engineered Ni323.4 @ 1.65 V1.65 (323.4 mA/cm2)99.3% selectivity; inhibits OER[54]
Ni2Fe(CN)6100 @ 1.35 V1.35 (100 mA/cm2)High activity; new reaction pathway[55]
Sv-CoNiS@NF (Bimetal Sulfide)100 @ 1.397 V1.397 (100 mA/cm2)Sulfur-vacancy-engineered[56]
Ni(OH)2/CuCo/Ni(OH)2 Composite10 @ 1.333 V1.333 (10 mA/cm2)Stable ≥50 h; enriched Ni3+ active sites[57]
Mo-Ni3S2/Co3S4 Composite50 @ 1.38 V1.38 (50 mA/cm2)Synergistic effect; high charge transfer[58]
Ni-WO3/NF200 @ 1.384 V1.384 (200 mA/cm2)150 h durability; rapid kinetics[59]
NiCoFe@PC (Trimetallic/Porous Carbon)44.65 @ 0.57 V (vs. Ag/AgCl)0.218 V onset (vs. Ag/AgCl)Low onset; high activity[60]
This Study (Biochar/RM/PG Catalyst)225.521.0 M urea, potential per CV dataExcellent operational stability; minimal loss after 3600 s CAThis work

2.2.6. Kinetic Parameters

Linear sweep voltammetry (LSV) provides a more straightforward and occasionally simpler way to investigate the kinetics and processes of electrochemical reactions, while still enabling quantitative analysis and redox process identification. As seen in Figure 12A, Biochar/RM/PG perform extremely well in urea electrooxidation, with a maximum current density of 223.66 mA/cm2 on the hand, Biochar/RM, RM, and RM/PG, reached to 142.03, 124.97, and 169.86 mA/cm2, respectively. The varied catalytic activity of the RM/PG-based materials are reflected in the changes in the LSV curves, which are caused by the surface properties, composition, and structure of the catalysts. It appears that the Biochar/RM/PG have acceptable catalytic characteristics and function as an attractive and efficient electrocatalyst for urea oxidation.
The Tafel slope is a measurement that describes the relationship between a system’s overpotential and current density. It can be computed by fitting a straight line to the Tafel part of the polarization curve using the linear sweep voltammetry (LSV) data. The overpotential needed to drive the urea oxidation reaction to a desired current density can be found using the Tafel diagram. Figure 12B displays the Tafel curves for all prepared samples at the optimal urea concentration. The addition potential required to cross the reaction’s activation barrier is represented by the overpotential, which is the difference between the applied and equilibrium potentials. The urea oxidation process must be made more efficient by decreasing the overpotential [61,62]. The sample slopes based on Tafel plots show that Biochar/RM/PG have more catalytic activity with a low voltage needed for the electrooxidation of urea.
Electrochemical impedance spectroscopy (EIS) is a powerful and widely used technique for elucidating the physicochemical processes occurring during the UOR at different applied potentials [63]. EIS is an important tool to study different phenomena that occur in electrochemical systems and processes during the heterogeneous faradaic reaction at the surface of the electrode, where the charge transfer happens. Because of charge transfer by the redox species, adsorption of reacting species, and diffusion of ions, the impedance value changes. The impedance (Z, reported in Ω employing SI units) represents the amount of resistance to the flow of current (I) that proceeds under applied potential within an electrical circuit. It is affected by the electrolyte, electrode−electrolyte interface, composition, and morphology of the electrode materials [64].The electrocatalytic response of 4 tested electrocatalysts can be fully understood by examining the charge transfer resistance related to urea oxidation in an alkaline medium through Nyquist plots (Figure 13a,b).The diameter of the circle in the Nyquist plots represents the charge transfer resistance, and it has been reported that in the presence of urea, the diameter of the circle decreases, implying that the electrocatalyst is active [65].Conversely, if the diameter of the circle remains unchanged or increases, then the catalyst is weak due to poisoning from the by-products or inactive for urea electro oxidation [66,67].
As shown in Figure 13b the diminished diameter of the semicircle in the Nyquist plot of the impedance spectra in the case of the Biochar/RM/PG shows that there is less charge transfer resistance, which is directly related to the fast reaction kinetics of the electrooxidation of urea occurring on the solid liquid interfaces [36].

3. Experimental Section

3.1. Materials and Reagents

All chemicals were of analytical grade and used without further purification. Progesterone (with purity 99.87%) was procured from Hubei Gedian Humanwell Pharmaceutical Co., Ltd. (Ezhou City, Hubei, China). Non-edible Reishi Mushroom (Ganoderma lucidum) fruiting bodies were obtained from Xi’an Lesen Bio-Tech Co., Ltd. (Xi’an, Shaanxi, China). All Ganoderma lucidum (Reishi mushroom) fruiting bodies used in this study were sourced from a single commercial batch (Xi’an Lesen Bio-Tech Co., Ltd., Xi’an, Shaanxi, China) to minimize batch-to-batch variability and ensure consistency throughout the experimental procedures. Sodium hydroxide (NaOH), hydrochloric acid 37%, potassium hydroxide (KOH), urea, isopropanol, and Nafion® perfluorinated resin solution (5% w/w) were purchased from Merck (Darmstadt, Germany). All solutions were prepared using bi-distilled water.

3.2. Preparation of Catalysts

3.2.1. Reishi Mushroom Powder (RM) and Biochar (Biochar/RM)

The Reishi mushrooms (Ganoderma lucidum) were first cleaned thoroughly to remove any dirt or debris, then dried in an oven at 40 °C for 24 h. After drying, the dried fruiting bodies were then ground into a fine powder using a mechanical mill (PM 100, Haan, Germany) and passed through a 100-mesh sieve to obtain a uniform particle size fine powder, ensuring consistency for subsequent loading. Then, we stored the reishi mushroom powder in an airtight container and placed in a dark place to preserve its potency. The entire process (from raw material collection to powder preparation) was carefully documented to ensure traceability and reproducibility. Whereas, biochar was synthesized by calcining about 10 g of RM powder in a muffle furnace at 800 °C for 1 h under a static air atmosphere. The resulting biochar (denoted Biochar/RM) was allowed to cool to room temperature in a desiccator [13].

3.2.2. Preparation of Spent Adsorbents (RM/PG and Biochar/RM/PG)

Spent adsorbents were collected after progesterone adsorption experiments [13]. Briefly, 0.1 g of RM or 0.075 g of Biochar/RM was added to 50 mL of a progesterone solution (50 µg/mL in bi-distilled water, pH 3 at 25 °C). The mixture was agitated on an orbital shaker (SO330-Pro) at 350 rpm for 24 h to reach adsorption equilibrium. The solid was then separated via 0.22 µm membrane filter, thoroughly washed with distilled water, and dried at 60 °C for 12 h. The resulting spent adsorbents are denoted RM/PG and Biochar/RM/PG, respectively, as illustrated in Scheme 1.

3.3. Instruments

The crystallographic structure of the materials was analyzed using X-ray diffraction (XRD; Bruker D8 Advance) with Cu Kα radiation (λ = 1.5406 Å). Functional groups were identified by Fourier Transform Infrared Spectroscopy (FTIR; Bruker spectroscopy (Bruker-Vertex 70, KBr pellet technique, Germany), in the range of 400–4000 cm−1. The surface morphology and texture were investigated using Field Emission Scanning Electron Microscopy (FESEM; Zeiss Sigma 300). Surface roughness parameters were extracted from FESEM images using ImageJ software with the Surface Plot plugin. Zetasizer Ultra (Malvern Instruments Ltd., United Kingdom)measurements determined particle size distribution and zeta potential.

3.4. Electrochemical Measurements

All electrochemical tests were performed using an Autolab PGSTAT302N potentiostat/galvanostat (Metrohm) in a standard three-electrode cell. A glassy carbon electrode (GCE, 3 mm diameter, 0.0707 cm2 geometric area) served as the substrate for the working electrode. A platinum wire and an Ag/AgCl (3 M KCl) electrode were used as the counter and reference electrodes, respectively [68,69].
The catalyst ink was prepared by dispersing 5 mg of catalyst in a solution of 380 µL isopropanol and 20 µL Nafion, followed by 30 min of ultrasonication. A 10 µL aliquot of the ink was drop-cast onto the mirror-polished GCE surface, yielding a catalyst loading of approximately 0.71 mg cm−2, and dried at 50 °C. This established ink preparation and drop-casting method is widely used for evaluating powder catalysts, ensuring a uniform and adherent film for reliable electrochemical testing [70].
To investigate the influence of catalyst loading on the urea oxidation reaction (UOR) performance, a loading optimization study was conducted for the optimal Biochar/RM/PG catalyst. Catalyst inks were prepared as described above, and varying volumes (5, 10, and 15 µL) were drop-cast onto the polished GCE surface, resulting in catalyst loadings of approximately 0.35, 0.71, and 1.07 mg cm−2, respectively. All electrochemical measurements for this optimization were performed in 1.0 M KOH with 1.0 M urea at a scan rate of 50 mV s−1. Tafel plots obtained by fitting the polarisation curves as overpotential versus log current density [71] and Nyquist plots was obtained from EIS in the frequency range of 100 kHz to 0.1 Hz. Urea electro-oxidation (UOR) was studied in a 1.0 M KOH electrolyte using Cyclic Voltammetry (CV) at scan rates from 5 to 60 mV s−1, with urea concentrations varying from 0.0 to 1.0 M. The use of a strong alkaline medium (1.0 M KOH) is standard for UOR as it facilitates the reaction kinetics and is compatible with non-precious metal oxide catalysts [40]. Chronoamperometry (CA) was conducted at a constant potential of 0.7 V vs. Ag/AgCl for 3600 s to assess catalyst stability. All current densities are reported relative to the geometric area of the GCE. This comprehensive approach, combining CV with varying scan rates and concentrations alongside long-term stability tests via CA, is a well-established protocol for benchmarking the activity and durability of UOR electrocatalysts against recent literature [39,72].

4. Conclusions

In summary, this work successfully demonstrates a circular economy strategy by converting spent progesterone-loaded biosorbents into highly efficient anodes for the urea oxidation reaction (UOR). The spent Biochar/RM/PG composite emerged as a superior electrocatalyst, achieving a peak current density of 225.52 mA cm−2 that is significantly outperforming its pristine counterparts. This enhancement is attributed to a synergistic effect where the biochar provides a conductive, porous framework, and the adsorbed progesterone acts as a structure-modifying agent, creating abundant active sites and improving charge transfer. The catalyst also exhibited excellent operational stability. This approach offers a sustainable and economical pathway for repurposing waste adsorbents into valuable materials for clean energy technologies, effectively closing the loop between water treatment and energy conversion.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal15121113/s1, Figure S1: Surface morphology and topographical analysis of Reishi mushroom (RM) before and after pro-gesterone (PG) loading; Figure S2: Surface morphology and topographical analysis of biochar/Reishi mushroom (Biochar/RM) before and after progesterone (PG) loading; Figure S3: The effect of the drop casting on the Biochar/RM/PG working electrode. Table S1: Parameter of roughness of RM before and after hormone loading RM/PG; Table S2: Parameters of surface roughness of Biochar/RM before and after hormone loading (RM/PG) after calcination.

Author Contributions

Conceptualization, R.M. and A.Z.; methodology, R.M.; software, A.Z.; validation, R.M., A.A.A., H.M. and H.A.R.; formal analysis, H.M. and K.K.M.; investigation, H.M., K.K.M. and S.M.M.; resources, A.A.A. and H.A.R.; data curation, R.M. and A.Z.; writing—original draft preparation, A.Z. and K.K.M.; writing—review and editing, R.M., A.Z. and, K.K.M.; visualization, R.M.; supervision, R.M. and A.Z.; project administration, A.A.A. and H.A.R.; funding acquisition, A.A.A. and H.A.R. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported and funded by the Deanship of Scientific Research at Imam Mohammad Ibn Saud Islamic University (IMSIU) (grant number IMSIU-DDRSP2502).

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Jena, T.; Chhatoi, S.K.; Khandai, S.S.; Mishra, R.; Mohanty, A.; Sharma, K.K.; Samal, S.; Mahapatra, K.K.; Dhar, G.; Purohit, S.; et al. Sublethal urea exposure in Nile tilapia: Morphological, behavioural, and histological alteration. Chemosphere 2025, 372, 144086. [Google Scholar] [CrossRef]
  2. Donald, D.B.; Bogard, M.J.; Finlay, K.; Leavitt, P.R. Comparative effects of urea, ammonium, and nitrate on phytoplankton abundance, community composition, and toxicity in hypereutrophic freshwaters. Limnol. Oceanogr. 2011, 56, 2161–2175. [Google Scholar] [CrossRef]
  3. Lee, J.; Park, J.; Shim, I.; Koo, J.-W.; Nam, S.-H.; Kim, E.; Park, S.-M.; Hwang, T.-M. Application of Electrochemical Oxidation for Urea Removal: A Review. Processes 2025, 13, 2660. [Google Scholar] [CrossRef]
  4. King, R.L.; Botte, G.G. Hydrogen production via urea electrolysis using a gel electrolyte. J. Power Sources 2011, 196, 2773–2778. [Google Scholar] [CrossRef]
  5. Protsenko, V.S. Thermodynamic aspects of urea oxidation reaction in the context of hydrogen production by electrolysis. Int. J. Int. J. Hydrogen Energy 2023, 48, 24207–24211. [Google Scholar] [CrossRef]
  6. Singh, R.K.; Rajavelu, K.; Montag, M.; Schechter, A. Advances in catalytic electrooxidation of urea: A review. Energy Technol. 2021, 9, 2100017. [Google Scholar] [CrossRef]
  7. Pan, T.; Xu, Y.; Li, Q.; Pang, H. Designed nickel–cobalt-based bimetallic oxide slender nanosheets for efficient urea electrocatalytic oxidation. Int. J. Hydrogen Energy 2024, 57, 388–393. [Google Scholar] [CrossRef]
  8. Sangster, J.L.; Ali, J.M.; Snow, D.D.; Kolok, A.S.; Bartelt-Hunt, S.L. Bioavailability and fate of sediment-associated progesterone in aquatic systems. Environ. Sci. Technol. 2016, 50, 4027–4036. [Google Scholar] [CrossRef]
  9. Adegoke, K.A.; Olagunju, A.O.; Alagbada, T.C.; Alao, O.C.; Adesina, M.O.; Afolabi, I.C.; Adegoke, R.O.; Bello, O.S. Adsorptive removal of endocrine-disrupting chemicals from aqueous solutions: A review. Water Air Soil Pollut. 2022, 233, 38. [Google Scholar] [CrossRef]
  10. Georgin, J.; Franco, D.S.P.; Manzar, M.S.; Meili, L.; El Messaoudi, N. A critical and comprehensive review of the current status of 17β-estradiol hormone remediation through adsorption technology. Environ. Sci. Pollut. Res. 2024, 31, 24679–24712. [Google Scholar] [CrossRef]
  11. Liu, W.-J.; Jiang, H.; Yu, H.-Q. Emerging applications of biochar-based materials for energy storage and conversion. Energy Environ. Sci. 2019, 12, 1751–1779. [Google Scholar] [CrossRef]
  12. Zhao, J.; Jiang, Y.; Chen, X.; Wang, C.; Nan, H. Unlocking the potential of element-doped biochar: From tailored synthesis to multifunctional applications in environment and energy. Biochar 2025, 7, 77. [Google Scholar] [CrossRef]
  13. Abdelwahab, A.; Alenezi, K.M.; Humaidi, J.R.; Haque, A.; Mahgoub, S.M.; Mokhtar, A.M.; Shaban, A.; Mostafa, M.M.M.; Mahmoud, R. Innovative eco-sustainable reishi mushroom-based adsorbents for progesterone removal and agricultural sustainability. RSC Adv. 2025, 15, 16690–16707. [Google Scholar] [CrossRef] [PubMed]
  14. Chang, J.; Zhang, H.; Cheng, H.; Yan, Y.; Chang, M.; Cao, Y.; Huang, F.; Zhang, G.; Yan, M. Spent Ganoderma lucidum substrate derived biochar as a new bio-adsorbent for Pb2+/Cd2+ removal in water. Chemosphere 2020, 241, 125121. [Google Scholar] [CrossRef]
  15. Gajji, N.; Chinnaiyan, S.K. Bio-derived Nanomaterials for Energy Storage. In Materials for Sustainable Energy Storage at the Nanoscale; CRC Press: Boca Raton, FL, USA, 2023; pp. 97–110. [Google Scholar]
  16. Yu, J.; Tang, T.; Cheng, F.; Huang, D.; Martin, J.L.; Brewer, C.E.; Grimm, R.L.; Zhou, M.; Luo, H. Exploring spent biomass-derived adsorbents as anodes for lithium ion batteries. Mater. Today Energy 2021, 19, 100580. [Google Scholar] [CrossRef]
  17. Qian, L.; Chen, C.; Lv, Y.; Li, J.; Cao, X.; Ren, H.; Hassan, M.L. Preparation and electrochemical application of porous carbon materials derived from extraction residue of Ganoderma lucidum. Biomass Bioenergy 2022, 166, 106593. [Google Scholar] [CrossRef]
  18. Sonal, S.; Prakash, P.; Mishra, B.K.; Nayak, G.C. Synthesis, characterization and sorption studies of a zirconium (iv) impregnated highly functionalized mesoporous activated carbons. RSC Adv. 2020, 10, 13783–13798. [Google Scholar] [CrossRef] [PubMed]
  19. Sangeetha, B.; Krishnamoorthy, A.S.; Amirtham, D.; Sharmila, D.J.S.; Renukadevi, P.; Malathi, V.G. FT-IR spectroscopic characteristics of Ganoderma lucidum secondary metabolites. J. Appl. Sci. Technol. 2019, 38, 1–8. [Google Scholar] [CrossRef]
  20. Mousavi, S.M.; Hashemi, S.A.; Gholami, A.; Omidifar, N.; Chiang, W.-H.; Neralla, V.R.; Yousefi, K.; Shokripour, M. Ganoderma lucidum methanolic extract as a potent phytoconstituent: Characterization, in-vitro antimicrobial and cytotoxic activity. Sci. Rep. 2023, 13, 17326. [Google Scholar] [CrossRef]
  21. Fathi, M.; Alami-Milani, M.; Geranmayeh, M.H.; Barar, J.; Erfan-Niya, H.; Omidi, Y. Dual thermo-and pH-sensitive injectable hydrogels of chitosan/(poly (N-isopropylacrylamide-co-itaconic acid)) for doxorubicin delivery in breast cancer. Int. J. Biol. Macromol. 2019, 128, 957–964. [Google Scholar] [CrossRef]
  22. Abdel-Hady, E.E.; Mahmoud, R.; Hafez, S.H.M.; Mohamed, H.F.M. Hierarchical ternary ZnCoFe layered double hydroxide as efficient adsorbent and catalyst for methanol electrooxidation. J. Mater. Res. Technol. 2022, 17, 1922–1941. [Google Scholar] [CrossRef]
  23. Yu, L.; Sun, S.; Li, H.; Xu, Z.J. Effects of catalyst mass loading on electrocatalytic activity: An example of oxygen evolution reaction. Fundam. Res. 2021, 1, 448–452. [Google Scholar] [CrossRef]
  24. Mahmoud, R.; Mohamed, H.F.M.; Hafez, S.H.M.; Gadelhak, Y.M.; Abdel-Hady, E.E. Valorization of spent double substituted Co–Ni–Zn–Fe LDH wastewater nanoadsorbent as methanol electro-oxidation catalyst. Sci. Rep. 2022, 12, 19354. [Google Scholar] [CrossRef] [PubMed]
  25. Varade, H.P. Surface Roughness Metrology. 2019. Available online: https://coursecontent.indusuni.ac.in/wp-content/uploads/sites/8/2021/10/BAS_Surface-roughness-2.pdf (accessed on 19 November 2025).
  26. Baker, L.R. On-machine measurement of roughness, waviness, and flaws. Adv. Opt. Manuf. Test. 1990, 1333, 248–256. [Google Scholar]
  27. Rao, N.N.; Alex, C.; Mukherjee, M.; Roy, S.; Tayal, A.; Datta, A.; John, N.S. Evidence for exclusive direct mechanism of urea electro-oxidation driven by in situ-generated resilient active species on a rare-earth nickelate. ACS Catal. 2024, 14, 981–993. [Google Scholar] [CrossRef]
  28. Janković, B.; Dodevski, V.; Janković, M.; Milenković, M.; Samaržija-Jovanović, S.; Jovanović, V.; Marinović-Cincović, M. Synthesis and Characterization of Bio-Composite Based on Urea–Formaldehyde Resin and Hydrochar: Inherent Thermal Stability and Decomposition Kinetics. Polymers 2025, 17, 1375. [Google Scholar] [CrossRef] [PubMed]
  29. Li, Q.; Wang, J.; Shi, Y.; Li, H.; Yang, H.; Xiang, K.; You, W.; Liang, J. Urea oxidation catalysts: A review on non-metallic enhancements in nickel-based electrocatalysts. Mater. Horiz. 2025, 23, 9855–10372. [Google Scholar] [CrossRef]
  30. Zhan, G.; Hu, L.; Li, H.; Dai, J.; Zhao, L.; Zheng, Q.; Zou, X.; Shi, Y.; Wang, J.; Hou, W.; et al. Highly selective urea electrooxidation coupled with efficient hydrogen evolution. Nat. Commun. 2024, 15, 5918. [Google Scholar] [CrossRef]
  31. Medvedev, J.J.; Delva, N.H.; Klinkova, A. Mechanistic Analysis of Urea Electrooxidation Pathways: Key to Rational Catalyst Design. Chempluschem 2024, 89, e202300739. [Google Scholar] [CrossRef]
  32. Dong, L.; Feng, D.; Zhang, Y.; Wang, Z.; Zhao, Y.; Du, Q.; Gao, J.; Sun, S. Mechanism of biochar-Cu-based catalysts construction and its electrochemical CO2 reduction performance. Carbon Capture Sci. Technol. 2024, 13, 100250. [Google Scholar] [CrossRef]
  33. Yang, G.; Zhu, Y.; Hao, Z.; Lu, Y.; Zhao, Q.; Zhang, K.; Chen, J. Organic electroactive materials for aqueous redox flow batteries. Adv. Mater. 2023, 35, 2301898. [Google Scholar]
  34. Yang, Y.; Börjesson, K. Electroactive covalent organic frameworks: A new choice for organic electronics. Trends Chem. 2022, 4, 60–75. [Google Scholar] [CrossRef]
  35. Escudero-Curiel, S.; Díez, A.M.; Pazos, M.; Sanromán, Á. Valorized S/N-doped banana peel biochar as a sustainable OER electrocatalyst for green energy applications. Int. J. Hydrogen Energy 2025, 163, 150728. [Google Scholar] [CrossRef]
  36. Guo, D.; Shibuya, R.; Akiba, C.; Saji, S.; Kondo, T.; Nakamura, J. Active sites of nitrogen-doped carbon materials for oxygen reduction reaction clarified using model catalysts. Science 2016, 351, 361–365. [Google Scholar] [CrossRef] [PubMed]
  37. Upadhyay, P.; Deka, A.; Chakma, S. Unlocking Efficient Electrochemical Urea Oxidation and Understanding Mechanism Insights of Co-Doped NiS. ACS Eng. Au 2025, 5, 450–467. [Google Scholar] [CrossRef]
  38. Sun, H.; Liu, J.; Kim, H.; Song, S.; Fei, L.; Hu, Z.; Lin, H.; Chen, C.; Ciucci, F.; Jung, W. Ni-doped CuO nanoarrays activate urea adsorption and stabilizes reaction intermediates to achieve high-performance urea oxidation catalysts. Adv. Sci. 2022, 9, 2204800. [Google Scholar] [CrossRef] [PubMed]
  39. Huang, C.; Xu, H.; Shuai, T.; Zhan, Q.; Zhang, Z.; Li, G. Modulation strategies for the preparation of high-performance catalysts for urea oxidation reaction and their applications. Small 2023, 19, 2301130. [Google Scholar] [CrossRef]
  40. Machireddy, N.R.; Yellatur, C.S.; Loka, S.S. Ni(OH)2/CeO2 Heterointerface catalysts for Energy Efficient Urea Electrooxidation. New J. Chem. 2025, 44, 19073–19458. [Google Scholar]
  41. Li, T.; Zheng, Z.; Chen, Z.; Zhang, M.; Liu, Z.; Chen, H.; Xiao, X.; Wang, S.; Qu, H.; Fu, Q. Ultrafast synthesis of an efficient urea oxidation electrocatalyst for urea-assisted fast-charging Zn–air batteries and water splitting. Energy Environ. Sci. 2025, 18, 4996–5008. [Google Scholar] [CrossRef]
  42. Hefnawy, M.A.; Fadlallah, S.; El-Sherif, R.M.; Medany, S. Synergistic effect of Cu-doped NiO for enhancing urea electrooxidation: Comparative electrochemical and DFT studies. J. Alloys Compd. 2021, 896, 162857. [Google Scholar] [CrossRef]
  43. Alwadai, N.; Alshatwi, M.; Sayed, E.T. Cobalt–nickel composite nano-grass as an excellent electrode for urea oxidation. RSC Adv. 2025, 15, 7728–7737. [Google Scholar] [CrossRef] [PubMed]
  44. Kumar, R.; Tanwar, S.; Mehra, L.; Singh, R.K.; Sharma, A.L. Theoretical and Experimental Insights into the Electrochemical Performance of Nico2o4 for Symmetric Supercapacitors. SSRN, 2025; preprint. [Google Scholar]
  45. Chen, L.; Yin, Z.H.; Cui, J.Y.; Li, C.Q.; Song, K.; Liu, H.; Wang, J.J. Unlocking lattice oxygen on selenide-derived NiCoOOH for amine electrooxidation and efficient hydrogen production. J. Am. Chem. Soc. 2024, 146, 27090–27099. [Google Scholar] [CrossRef]
  46. Sanati, S.; Abazari, R.; Kirillov, A.M. Bimetallic NiCo metal–organic frameworks with high stability and performance toward electrocatalytic oxidation of urea in seawater. Inorg. Chem. 2024, 63, 15813–15820. [Google Scholar] [CrossRef]
  47. Wang, P.; Bai, X.; Jin, H.; Gao, X.; Davey, K.; Zheng, Y.; Jiao, Y.; Qiao, S. Directed Urea-to-Nitrite Electrooxidation via Tuning Intermediate Adsorption on Co, Ge Co-Doped Ni Sites. Adv. Funct. Mater. 2023, 33, 2300687. [Google Scholar] [CrossRef]
  48. Adhikari, S.; Kwon, Y.; Kim, D.-H. Three-dimensional core–shell structured NiCo2O4@ CoS/Ni-Foam electrocatalyst for oxygen evolution reaction and electrocatalytic oxidation of urea. Chem. Eng. J. 2020, 402, 126192. [Google Scholar] [CrossRef]
  49. Zhou, T.; Jagadeesan, S.N.; Zhang, L.; Deskins, N.A.; Teng, X. Enhanced urea oxidation electrocatalytic activity by synergistic cobalt and nickel mixed oxides. J. Phys. Chem. Lett. 2023, 15, 81–89. [Google Scholar] [CrossRef]
  50. Zheng, Z.; Wu, D.; Chen, L.; Chen, S.; Wan, H.; Chen, G.; Zhang, N.; Liu, X.; Ma, R. Collaborative optimization of thermodynamic and kinetic for Ni-based hydroxides in electrocatalytic urea oxidation reaction. Appl. Catal. B. Environ. 2024, 340, 123214. [Google Scholar] [CrossRef]
  51. Wang, J.; Abazari, R.; Sanati, S.; Ejsmont, A.; Goscianska, J.; Zhou, Y.; Dubal, D.P. Water-stable fluorous metal–organic frameworks with open metal sites and amine groups for efficient urea electrocatalytic oxidation. Small 2023, 19, 2300673. [Google Scholar] [CrossRef] [PubMed]
  52. Liu, Q.; Xie, L.; Qu, F.; Liu, Z.; Du, G.; Asiri, A.M.; Sun, X. A porous Ni 3 N nanosheet array as a high-performance non-noble-metal catalyst for urea-assisted electrochemical hydrogen production. Inorg. Chem. Front. 2017, 4, 1120–1124. [Google Scholar] [CrossRef]
  53. Zhang, J.; Song, X.; Kang, L.; Zhu, J.; Liu, L.; Zhang, Q.; Brett, D.J.L.; Shearing, P.R.; Mai, L.; Parkin, I.P. Stabilizing efficient structures of superwetting electrocatalysts for enhanced urea oxidation reactions. Chem. Catal. 2022, 2, 3254–3270. [Google Scholar] [CrossRef]
  54. Gao, X.; Bai, X.; Wang, P.; Jiao, Y.; Davey, K.; Zheng, Y.; Qiao, S.-Z. Boosting urea electrooxidation on oxyanion-engineered nickel sites via inhibited water oxidation. Nat. Commun. 2023, 14, 5842. [Google Scholar] [CrossRef] [PubMed]
  55. Geng, S.-K.; Zheng, Y.; Li, S.-Q.; Su, H.; Zhao, X.; Hu, J.; Shu, H.-B.; Jaroniec, M.; Chen, P.; Liu, Q.-H. Nickel ferrocyanide as a high-performance urea oxidation electrocatalyst. Nat. Energy 2021, 6, 904–912. [Google Scholar] [CrossRef]
  56. Li, H.; Pu, Y.; Li, W.; Yan, Z.; Deng, R.; Shi, F.; Zhao, C.; Zhang, Y.; Duan, T. Sulfur-Vacancy Engineering Accelerates Rapid Surface Reconstruction in Ni-Co Bimetal Sulfide Nanosheet for Urea Oxidation Electrocatalysis. Small 2024, 20, 2403311. [Google Scholar] [CrossRef]
  57. Parvin, S.; Aransiola, E.; Ammar, M.; Lee, S.; Zhang, L.; Weber, J.; Baltrusaitis, J. Tailored Ni (OH) 2/CuCo/Ni (OH) 2 Composite Interfaces for Efficient and Durable Urea Oxidation Reaction. ACS Appl. Mater. Interfaces 2024, 16, 67715–67729. [Google Scholar] [CrossRef]
  58. Wang, Y.; He, W.; Du, X.; Zhang, X. Electrocatalytic urea oxidation based on M (M= Fe, Cr, Mo)-Ni3S2/Co3S4: Performance optimization and mechanism analysis. Fuel 2024, 376, 132749. [Google Scholar] [CrossRef]
  59. Chen, L.; Wang, L.; Ren, J.; Wang, H.; Tian, W.; Sun, M.; Yuan, Z. Artificial heterointerfaces with regulated charge distribution of Ni active sites for urea oxidation reaction. Small Methods 2024, 8, 2400108. [Google Scholar] [CrossRef]
  60. Thamer, B.M.; Abdulhameed, M.M.; El-Newehy, M.H. Tragacanth gum hydrogel-derived trimetallic nanoparticles supported on porous carbon catalyst for urea electrooxidation. Gels 2022, 8, 292. [Google Scholar] [CrossRef] [PubMed]
  61. Song, F.; Bai, L.; Moysiadou, A.; Lee, S.; Hu, C.; Liardet, L.; Hu, X. Transition metal oxides as electrocatalysts for the oxygen evolution reaction in alkaline solutions: An application-inspired renaissance. J. Am. Chem. Soc. 2018, 140, 7748–7759. [Google Scholar] [CrossRef] [PubMed]
  62. Abo-Zeid, M.M.; El-Moghny, M.G.A.; Shawkey, H.; Daher, A.M.; Abdelkader, A.M.; El-Deab, M.S. Metal oxide stabilized zirconia modified bio-derived carbon nanosheets as efficient electrocatalysts for oxygen evolution reaction. J. Appl. Electrochem. 2024, 54, 467–485. [Google Scholar] [CrossRef]
  63. Malik, B.; Sankar, K.V.; Aziz, S.K.T.; Majumder, S.; Tsur, Y.; Nessim, G.D. Uncovering the change in catalytic activity during electro-oxidation of urea: Answering overisolation of the relaxation phenomenon. J. Phys. Chem. C 2021, 125, 23126–23132. [Google Scholar] [CrossRef]
  64. Laschuk, N.O.; Easton, E.B.; Zenkina, O.V. Reducing the resistance for the use of electrochemical impedance spectroscopy analysis in materials chemistry. RSC Adv. 2021, 11, 27925–27936. [Google Scholar] [CrossRef]
  65. Mohamed, I.M.A.; Kanagaraj, P.; Yasin, A.S.; Iqbal, W.; Liu, C. Electrochemical impedance investigation of urea oxidation in alkaline media based on electrospun nanofibers towards the technology of direct-urea fuel cells. J. Alloys Compd. 2020, 816, 152513. [Google Scholar] [CrossRef]
  66. Guo, F.; Ye, K.; Du, M.; Huang, X.; Cheng, K.; Wang, G.; Cao, D. Electrochemical impedance analysis of urea electro-oxidation mechanism on nickel catalyst in alkaline medium. Electrochim. Acta 2016, 210, 474–482. [Google Scholar] [CrossRef]
  67. Akkari, S.; Vivier, V.; Sánchez-Sánchez, C.M. Urea electro-oxidation byproducts impact on NiO/NiOOH anode performance studied by operando electrochemical impedance spectroscopy. Electrochim. Acta 2024, 474, 143526. [Google Scholar] [CrossRef]
  68. Yuan, T.; Voznyy, O. Guidelines for reliable urea detection in electrocatalysis. Cell Rep. Phys. Sci. 2023, 4, 101521. [Google Scholar] [CrossRef]
  69. Alnoush, W.; Black, R.; Higgins, D. Judicious selection, validation, and use of reference electrodes for in situ and operando electrocatalysis studies. Chem. Catal. 2021, 1, 997–1013. [Google Scholar] [CrossRef]
  70. Rahumi, O.; Rath, M.K.; Kossenko, A.; Zinigrad, M.; Borodianskiy, K. Heterogeneous catalyst-modified anode in solid oxide fuel cells for simultaneous ammonia synthesis and energy conversion. ACS Sustain. Chem. Eng. 2023, 11, 14081–14093. [Google Scholar] [CrossRef]
  71. Kamal, W.; Allah, A.E.; Mahmoud, R.; Farghali, A.A.; Kotp, A.A.; Abdelwahab, A. Metal–organic framework-derived nanoflower and nanoflake metal oxides as electrocatalysts for methanol oxidation. RSC Adv. 2024, 14, 32828–32838. [Google Scholar] [CrossRef]
  72. Rafiq, S.; Arshad, F.; Gondal, M.; Jaffer, M.; Almessiere, M.A. Recent Advancements in Heterostructure-Based Electrocatalysts for Sustainable Hydrogen Production Through Seawater Splitting. Mater. Horiz. 2025. [Google Scholar] [CrossRef]
Catalysts 15 01113 g001
Figure 1. Physicochemical characterization of raw Reishi mushroom (RM) and its calcined form (Biochar/RM): (a) XRD patterns and (b) FTIR spectra of the analyzed samples.
Figure 1. Physicochemical characterization of raw Reishi mushroom (RM) and its calcined form (Biochar/RM): (a) XRD patterns and (b) FTIR spectra of the analyzed samples.
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Figure 2. FESEM for the prepared material (a,b) for RM and (c,d) for RM/PG.
Figure 2. FESEM for the prepared material (a,b) for RM and (c,d) for RM/PG.
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Figure 3. Surface roughness characterization of Reishi mushroom (RM): (a) before progesterone (PG) loading and (b) after progesterone (PG) loading (RM/PG).
Figure 3. Surface roughness characterization of Reishi mushroom (RM): (a) before progesterone (PG) loading and (b) after progesterone (PG) loading (RM/PG).
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Figure 4. FESEM for the prepared material (a,b) for Biochar/RM and (c,d) for Biochar/RM/PG.
Figure 4. FESEM for the prepared material (a,b) for Biochar/RM and (c,d) for Biochar/RM/PG.
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Figure 5. Roughness characterization of Biochar/RM (a) before hormone loading (Biochar/RM) and (b) after hormone loading (Biochar/RM/PG).
Figure 5. Roughness characterization of Biochar/RM (a) before hormone loading (Biochar/RM) and (b) after hormone loading (Biochar/RM/PG).
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Figure 6. (a,b) N2 adsorption–desorption isotherms for RM/PG and Biochar/RM/PG; (c) BJH pore size distribution curves; (d) Cumulative pore volume plots. Biochar/RM/PG demonstrates enhanced porosity with higher surface area and larger mesopore volume compared to RM/PG.
Figure 6. (a,b) N2 adsorption–desorption isotherms for RM/PG and Biochar/RM/PG; (c) BJH pore size distribution curves; (d) Cumulative pore volume plots. Biochar/RM/PG demonstrates enhanced porosity with higher surface area and larger mesopore volume compared to RM/PG.
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Figure 7. Cyclic voltammetry curves comparing the electrocatalytic performance of RM, Biochar/RM, RM/PG, and Biochar/RM/PG electrodes for the urea oxidation reaction in 1.0 M KOH with 1.0 M urea at a scan rate of 50 mV/s and 25 °C.
Figure 7. Cyclic voltammetry curves comparing the electrocatalytic performance of RM, Biochar/RM, RM/PG, and Biochar/RM/PG electrodes for the urea oxidation reaction in 1.0 M KOH with 1.0 M urea at a scan rate of 50 mV/s and 25 °C.
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Catalysts 15 01113 g008aCatalysts 15 01113 g008b
Figure 8. Cyclic voltammograms showing the effect of urea concentration (0.0–1.0 M) on the electrocatalytic performance of (a) RM, (b) Biochar/RM, (c) RM/PG, and (d) Biochar/RM/PG electrodes in 1.0 M KOH at a scan rate of 100 mV/s.
Figure 8. Cyclic voltammograms showing the effect of urea concentration (0.0–1.0 M) on the electrocatalytic performance of (a) RM, (b) Biochar/RM, (c) RM/PG, and (d) Biochar/RM/PG electrodes in 1.0 M KOH at a scan rate of 100 mV/s.
Catalysts 15 01113 g008aCatalysts 15 01113 g008b
Catalysts 15 01113 g009
Figure 9. Cyclic voltammograms showing the effect of scan rate (5–60 mV/s) on the electrocatalytic performance of (a) RM, (b) Biochar/RM, (c) RM/PG, and (d) Biochar/RM/PG electrodes in 1.0 M KOH with 1.0 M urea.
Figure 9. Cyclic voltammograms showing the effect of scan rate (5–60 mV/s) on the electrocatalytic performance of (a) RM, (b) Biochar/RM, (c) RM/PG, and (d) Biochar/RM/PG electrodes in 1.0 M KOH with 1.0 M urea.
Catalysts 15 01113 g009
Catalysts 15 01113 g010
Figure 10. Chronoamperometric curves showing the stability of the RM, Biochar/RM, RM/PG, and Biochar/RM/PG electrodes at 0.7 V in 1.0 M KOH with 1.0 M urea over 3600 s.
Figure 10. Chronoamperometric curves showing the stability of the RM, Biochar/RM, RM/PG, and Biochar/RM/PG electrodes at 0.7 V in 1.0 M KOH with 1.0 M urea over 3600 s.
Catalysts 15 01113 g010
Catalysts 15 01113 g011
Figure 11. (a) Cyclic voltammograms showing of Biochar/RM/PG electrode with scan rate 100 mV/s in 1.0 M KOH with 1.0 M urea before and after CA test and (b) its FESEM image.
Figure 11. (a) Cyclic voltammograms showing of Biochar/RM/PG electrode with scan rate 100 mV/s in 1.0 M KOH with 1.0 M urea before and after CA test and (b) its FESEM image.
Catalysts 15 01113 g011
Catalysts 15 01113 g012
Figure 12. (A) Linear sweep voltammetry and (B) Tafel plots for the prepared samples using 1 M urea.
Figure 12. (A) Linear sweep voltammetry and (B) Tafel plots for the prepared samples using 1 M urea.
Catalysts 15 01113 g012
Catalysts 15 01113 g013
Figure 13. (a,b) Complete profile and enlarged view of the Nyquist plot semicircle in the high frequency region for the four prepared samples.
Figure 13. (a,b) Complete profile and enlarged view of the Nyquist plot semicircle in the high frequency region for the four prepared samples.
Catalysts 15 01113 g013
Catalysts 15 01113 sch001
Scheme 1. Schematic illustration of the preparation steps for Reishi Mushroom powder (RM), Biochar derived from RM (Biochar/RM), and hormone-loaded spent adsorbents (RM/PG and Biochar/RM/PG).
Scheme 1. Schematic illustration of the preparation steps for Reishi Mushroom powder (RM), Biochar derived from RM (Biochar/RM), and hormone-loaded spent adsorbents (RM/PG and Biochar/RM/PG).
Catalysts 15 01113 sch001
Table 1. Textural parameters of the spent adsorbent catalysts.
Table 1. Textural parameters of the spent adsorbent catalysts.
ParameterRM/PGBiochar/RM/PG
BET Surface Area (m2/g)151246
Total Pore Volume (cm3/g)0.160.40
Average Pore Diameter (nm)4.206.50
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MDPI and ACS Style

Mahgoub, S.M.; Allam, A.A.; Mohamed, H.; Rudayni, H.A.; Mahmoud, R.; Khaled Mohammed, K.; Zaher, A. Valorization of Spent Bio-Adsorbents into High-Performance Eco-Friendly Anodes for Direct Urea Fuel Cells. Catalysts 2025, 15, 1113. https://doi.org/10.3390/catal15121113

AMA Style

Mahgoub SM, Allam AA, Mohamed H, Rudayni HA, Mahmoud R, Khaled Mohammed K, Zaher A. Valorization of Spent Bio-Adsorbents into High-Performance Eco-Friendly Anodes for Direct Urea Fuel Cells. Catalysts. 2025; 15(12):1113. https://doi.org/10.3390/catal15121113

Chicago/Turabian Style

Mahgoub, Samar M., Ahmed A. Allam, Hala Mohamed, Hassan A. Rudayni, Rehab Mahmoud, Kholoud Khaled Mohammed, and Amal Zaher. 2025. "Valorization of Spent Bio-Adsorbents into High-Performance Eco-Friendly Anodes for Direct Urea Fuel Cells" Catalysts 15, no. 12: 1113. https://doi.org/10.3390/catal15121113

APA Style

Mahgoub, S. M., Allam, A. A., Mohamed, H., Rudayni, H. A., Mahmoud, R., Khaled Mohammed, K., & Zaher, A. (2025). Valorization of Spent Bio-Adsorbents into High-Performance Eco-Friendly Anodes for Direct Urea Fuel Cells. Catalysts, 15(12), 1113. https://doi.org/10.3390/catal15121113

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