Municipal Sludge-Derived Activated Carbon as a Highly Efficient and Selective Electrocatalyst for H₂O₂ Synthesis

Abstract

Low-cost, efficient, and durable electrocatalysts for two-electron oxygen reduction (2e⁻ ORR) to synthesize hydrogen peroxide (H₂O₂) are essential for sustainable chemical manufacturing. Herein, we report a sustainable route to prepare activated carbon electrodes (ASC) from municipal sludge via chemical activation and pyrolysis. The electrode activated with 30 wt.% H₃PO₄ and calcined at 600 °C for 2 h exhibited high catalytic activity. This electrode featured a hierarchical micro-/meso-/macroporous structure with a high BET surface area of 806.20 m²·g⁻¹, abundant carboxyl groups (surface O content of 4.0 at.%), and high hydrophobicity (contact angle of 121.6°). The unique hierarchical porosity, high surface area, hydrophobic surface, and carboxyl functionalities synergistically enhanced O₂ mass transfer, exposed abundant accessible active sites, and stabilized the triple-phase interface. Electrochemical evaluation revealed that 30 wt.% H₃PO₄-ASC achieved a high H₂O₂ selectivity of 86–92% with an electron transfer number of 2.2, approaching the ideal 2e⁻ ORR pathway. Under optimized conditions (pH = 3, j = 5.0 mA·cm⁻²), it produced 997.3 mg·L⁻¹ of H₂O₂ in 90 min, with a current efficiency of 68.8% and an energy consumption of 27.6 kWh·kg⁻¹ H₂O₂. Moreover, the electrode retained 81.1% of its initial H₂O₂ production after 15 cycles, demonstrating good reusability and long-term stability. This work offers a sustainable strategy for high-value utilization of municipal sludge and advances the development of efficient electrocatalysts for green H₂O₂ production.

1. Introduction

Hydrogen peroxide (H₂O₂) is recognized as one of the 100 most important basic chemicals [1]. Its decomposition products are only O₂ and H₂O, making it a typical green chemical. With a redox potential of approximately 1.8 V, H₂O₂ exhibits strong oxidizing properties and is therefore widely used in wastewater treatment, chemical synthesis, fabric and pulp bleaching, among other applications. In particular, H₂O₂ can achieve sterilization and disinfection by releasing reactive oxygen species that penetrate cell membranes and disrupt enzyme and protein activities, thus playing a crucial role in the prevention and control of infectious diseases such as COVID-19 [2,3].

The industrial anthraquinone process dominates large-scale H₂O₂ production, but it suffers from complex processes, high safety risks, and the generation of waste streams [4,5]. In contrast, the electrochemical oxygen reduction reaction (ORR) for H₂O₂ production uses only water, air, or oxygen as raw materials, generating no additional pollutants. This method is simple, environmentally benign, and represents a promising decentralized process [6]. The electrode material is a key influencing factor. Currently, the main electrocatalytic materials include noble metal catalysts (e.g., Au, Pt-Hg alloys) [7], transition metal-nitrogen-carbon (M-N-C) catalysts (e.g., Fe-N-C, Co-N-C) [8], carbon nanotubes [9], graphene-based catalysts [10], and carbon black [11].

Although noble metal catalysts exhibit high selectivity for H₂O₂ production, they suffer from drawbacks such as scarcity of raw materials and high costs [12]. While M-N-C catalysts possess excellent catalytic activity, the metal active sites tend to catalyze the decomposition of H₂O₂, and the leaching of metal ions may cause secondary pollution [13]. Carbon nanotube and graphene-based catalysts offer large specific surface areas and superior electrical conductivity, but their preparation is costly, and they show low selectivity for H₂O₂ production [14]. In contrast, commercial carbon black catalysts have relatively low costs; however, they are limited by small specific surface areas, insufficient active sites, and rapid degradation of reusability performance [15]. Therefore, the development of low-cost, non-metallic electrocatalytic materials with high selectivity for H₂O₂ production and strong reusability and stability has become a current research hotspot.

In China, the annual production of municipal sludge reaches 90 million tons. The primary disposal methods are sanitary landfilling and incineration, with a resource utilization rate of less than 25% [16]. These practices not only occupy vast land resources but also increase carbon emissions and cause secondary pollution [17]. The carbon content (on a dry basis) of municipal sludge ranges from 30% to 40%. Through pyrolysis and activation, biomass-derived carbon electrode materials with different catalytic performances can be prepared [18]. For example, Deng et al. [19] used municipal sludge as a raw material to prepare an N, P, S, O co-doped porous carbon catalyst (MSB-P), achieving a specific surface area of 98.96 m²·g⁻¹ and an NH₄⁺ yield of 35.76 μg·h⁻¹·mg⁻¹ for electrocatalytic N₂ reduction. Guo et al. [20] prepared an iron- and manganese-containing sludge-derived carbon catalyst from dewatered sludge, which exhibited a rich porous structure, a high specific surface area, and good electrical conductivity, with an electrochemically active surface area (ECSA) of 86.44 mF·cm⁻². Li et al. [21] prepared sludge biochar (SBC) containing graphitic nitrogen from municipal sludge and loaded it onto nickel foam to construct an SBC@Ni electro-Fenton electrode, achieving a H₂O₂ concentration of 80.3 mg·L⁻¹ from O₂ reduction.

Inspired by these works, we adopt municipal sludge as the precursor and utilize different activators (H₃PO₄, ZnCl₂, KOH, and KHCO₃) to fabricate sludge-derived activated carbon through a facile calcination procedure. The resulting carbon materials are then fabricated into composite electrodes using graphite felt as the substrate and polytetrafluoroethylene (PTFE) as the binder. The surface morphology, pore structure, elemental composition, and surface chemical functionalities of the sludge carbon and its composite electrodes are characterized by scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) surface area analysis, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), and contact angle measurements. The electrochemical performance of the municipal sludge-derived activated carbon composite electrodes is evaluated using cyclic voltammetry (CV), linear sweep voltammetry (LSV), ECSA, electrochemical impedance spectroscopy (EIS), and rotating ring-disk electrode (RRDE) techniques. The effects of the type of inorganic activator, activator concentration, and current density on the H₂O₂ production of the electrodes are systematically investigated, and the reusability and stability of the as-prepared electrodes are examined.

2. Results and Discussion

In this study, a series of municipal sludge-derived carbon composite electrodes were prepared using different activators (H₃PO₄, ZnCl₂, KOH, KHCO₃) followed by calcination and loading onto graphite felt. The obtained electrodes were designated as Blank-ASC (no activator), x wt.% H₃PO₄-ASC (where x represents the mass percentage concentration of the activator H₃PO₄, x = 10,20,30,40,50), ZnCl₂-ASC (40 wt.% ZnCl₂), KOH-ASC (20 wt.% KOH), and KHCO₃-ASC (20 wt.% KHCO₃). An electrode without sludge carbon (only PTFE) was named PTFE.

2.1. Morphological and Surface Chemical Characterization

Figure 1 shows the SEM morphologies and water contact angles of the different sludge carbons. Blank-ASC presented a rough surface with indistinct pores and a low specific surface area, reflecting a basic carbon skeleton with limited porosity. Its contact angle was 110.2°, indicating moderate hydrophobicity. ZnCl₂-ASC featured a denser surface with a contact angle of 118.6°, while KOH-ASC consisted of aggregated particles and delivered a contact angle of 119.3°; KHCO₃-ASC exhibited a loose flake-like structure and a lower contact angle of 113.2°. In contrast, H₃PO₄-ASC displayed a dense yet highly porous architecture with well-developed hierarchical pores in the range of 2.5–12 µm. This unique structure is expected to significantly reduce mass transfer resistance and prevent the encapsulation of active sites [22]. Moreover, H₃PO₄-ASC exhibited the highest contact angle of 121.6°, indicating the strongest surface hydrophobicity. The good hydrophobic interface is beneficial for enriching dissolved oxygen from the electrolyte and stabilizing the gas–liquid-solid triple-phase catalytic boundary, thereby enhancing O₂ interfacial mass transfer and the kinetics of the selective reduction reaction [23].

Figure 1. SEM image of (a) Blank-ASC, (b) H₃PO₄-ASC, (c) ZnCl₂-ASC, (d) KOH-ASC, (e) KHCO₃-ASC; (f) Contact angles of municipal sludge-derived activated carbons with different activators.

To quantitatively corroborate the SEM observations, N₂ adsorption–desorption measurements were performed, and the BET surface area, pore volume, and average pore size of all samples are summarized in Table 1. Blank-ASC exhibited a moderate BET surface area of 155.84 m²·g⁻¹. H₃PO₄-ASC showed a high specific surface area of 806.20 m²·g⁻¹, substantially larger than that of ZnCl₂-ASC (388.70 m²·g⁻¹), KOH-ASC (69.33 m²·g⁻¹), and KHCO₃-ASC (163.26 m²·g⁻¹). The N₂ adsorption–desorption isotherms (Figure 2) of H₃PO₄-ASC exhibited a typical Type III behavior: a steep uptake at low relative pressure (P/P₀ < 0.1) indicated the presence of abundant micropores, while a sharp rise at high relative pressure (P/P₀ close to 1.0) confirmed the existence of macropores, consistent with the SEM observations (Figure 1). Pore-size distribution analysis further confirmed that H₃PO₄-ASC possessed a hierarchical micro-meso-macroporous structure, with a notable contribution from mesopores alongside micropores and macropores. This unique hierarchical scaffold, combined with its high surface area and superior surface hydrophobicity (contact angle of 121.6°), synergistically minimizes O₂ mass-transfer resistance, prevents electrolyte flooding of active sites, and stabilizes the triple-phase reaction interface. As a result, H₃PO₄-ASC provides abundant accessible active sites for the dissolved O₂ and facilitates the rapid diffusion of the produced H₂O₂ away from the electrode surface, which are critical factors for achieving high selectivity and activity in the 2e⁻ ORR [23].

Table 1. BET surface area, pore volume, and average pore size of municipal sludge-derived activated carbons prepared with different activators.

Sample	Specific Surface Area (m2·g−1)	Pore Volume (cm3·g−1)	Average Pore Size (nm)
Blank-ASC	155.84	0.27	4.72
H3PO4-ASC	806.20	3.00	5.41
ZnCl2-ASC	388.70	0.46	1.19
KOH-ASC	69.33	0.31	1.64
KHCO3-ASC	163.26	0.43	1.34

Figure 2. Nitrogen adsorption-desorption isotherms and pore-size distribution curves of (a) Blank-ASC, (b) H₃PO₄-ASC, (c) ZnCl₂-ASC, (d) KOH-ASC, (e) KHCO₃-ASC.

The FTIR spectra (Figure 3) provide insight into the surface chemical functionalities. All sludge carbons showed characteristic FTIR absorption bands at approximately 3400 cm⁻¹ and in the 1000–1100 cm⁻¹ region, assigned to the O-H stretching of hydroxyl/carboxyl groups [24], and the C-O stretching vibration of aliphatic C-OH and C-O-C ether linkages [25], respectively. Blank-ASC showed peaks at 1610 cm⁻¹ (aromatic C=C), 1385 cm⁻¹ and 780 cm⁻¹ (C-H bending) [26,27,28], confirming a basic aromatic carbon framework but with limited functional diversity. ZnCl₂-ASC exhibited a similar pattern with a slight improvement in carbon skeleton integrity (1590 cm⁻¹, 680 cm⁻¹) but no new oxygen groups. KOH-ASC displayed peaks at 1640 cm⁻¹ and 1385 cm⁻¹, suggesting that alkali etching mainly optimizes the local carbon structure without enriching active oxygen functionalities [29]. KHCO₃-ASC showed a C=C peak at 1560 cm⁻¹ but no significant oxygenated features. In contrast, H₃PO₄-ASC exhibited several distinct peaks: a broad band at 3130 cm⁻¹ (carboxylic acid dimer O-H) [30], a strong absorption at 1280 cm⁻¹ which can be attributed to the overlapping contributions of C-O stretching in carboxyl groups [31] and P=O/P-O-C stretching vibrations introduced by H₃PO₄ activation [32]. In addition, the band in the 980–920 cm⁻¹ region is assigned to the asymmetric stretching of P-O-P or P-O-C linkages, as well as C-O-P bending modes [32], indicating the presence of phosphate species grafted onto the carbon framework. The O-H out-of-plane bending at 785 cm⁻¹ [28] is also observed. These phosphorus-containing functionalities, along with carboxyl groups, collectively modulate the surface polarity and may contribute to the enhanced 2e⁻ ORR activity, although the carboxyl groups are considered the primary active sites.

Figure 3. (a) FTIR spectra of municipal sludge-derived activated carbons, and (b) XPS of H₃PO₄-ASC.

The XPS survey spectra (Figure 3b) further corroborated these findings. The H₃PO₄-ASC sample displayed clear P 2p, O 1s and C 1s signals, with a surface oxygen content of 4.71 at.% and a phosphorus content of 0.4 at.%, confirming the successful grafting of both oxygen-containing and phosphorus functionalities onto the carbon framework during H₃PO₄ activation. The C 1s high-resolution spectrum revealed peaks corresponding to C-C (284.6 eV) and O-C=O (288.8 eV), confirming the presence of carboxyl and hydroxyl groups. The O 1s spectrum further showed contributions from C=O (531.5 eV) and C-O (532.8 eV) species [33]. While the high-resolution P 2p spectrum exhibited a characteristic peak at ~134.6 eV, which is typically assigned to P-O-C and phosphate (PO₄³⁻) species [32]. This result is fully consistent with the FTIR assignments. Collectively, these features unequivocally demonstrate that H₃PO₄ activation introduces abundant carboxyl and other oxygen-containing groups onto the carbon surface. Such oxygen functionalities not only serve as highly active sites for the 2e⁻ ORR by effectively modulating the electron distribution at the electrode interface, optimizing the interfacial charge transfer environment, and precisely steering the reduction of O₂ to H₂O₂, but also endow the electrode with excellent catalytic activity at the molecular structural level [34]. Although the phosphorus content is relatively low, its presence may synergistically modulate the surface charge environment in conjunction with the dominant carboxyl active sites.

2.2. Effect of Activator Type on H₂O₂ Electrosynthesis

Figure 4 compares the cumulative H₂O₂ concentration over 90 min for electrodes prepared with different activators. Within 90 min of reaction, Blank-ASC increased from 6.9 to 60.7 mg·L⁻¹, ZnCl₂-ASC from 5.8 to 69.4 mg·L⁻¹, KOH-ASC from 6.2 to 63.2 mg·L⁻¹, and KHCO₃-ASC from 5.5 to 56.8 mg·L⁻¹. In sharp contrast, H₃PO₄-ASC started at a much higher initial concentration (15.6 mg·L⁻¹) and reached 95.1 mg·L⁻¹ after 90 min, maintaining the highest production rate throughout. The superior performance of H₃PO₄-ASC arose from its most abundant pore channels, the strongest hydrophobicity, and the highest content of oxygen-containing functional groups, which were more favorable for O₂ mass transfer and the 2e⁻ ORR, thus resulting in the highest H₂O₂ yield [35].

Figure 4. Effect of activator types on the accumulated concentration of H₂O₂, experimental conditions: Na₂SO₄ solution concentration is 0.05 M, current density is 0.5 mA·cm⁻², pH = 3, O₂ aeration rate is 500 mL·min⁻¹.

2.3. Electrochemical Mechanism

The ORR pathways were further elucidated by rotating ring-disk electrode measurements. As shown in Figure 5a,b, Blank-ASC showed an electron transfer number of approximately 3.2 and an H₂O₂ selectivity of 55–65%, indicating a mixed 2e⁻/4e⁻ pathway. ZnCl₂-ASC showed an electron transfer number of about 2.6 and a H₂O₂ selectivity of 75–80%; KOH-ASC had an electron transfer number of about 3.0 and a H₂O₂ selectivity of 61–64%; KHCO₃-ASC presented an electron transfer number of about 3.8 and a H₂O₂ selectivity of 50–55%. Notably, H₃PO₄-ASC achieved the highest H₂O₂ selectivity of 86–92% and an electron transfer number of about 2.2, which was close to the ideal 2e⁻ pathway for H₂O₂ production. This near-ideal selectivity underscored that the carboxyl-rich surface effectively suppressed the 4e⁻ pathway, likely by destabilizing the O-O bond scission intermediates that led to H₂O formation [34].

Figure 5. Electrochemical characterization of municipal sludge-derived activated carbons activated by different agents: (a) electron transfer number, (b) H₂O₂ selectivity, (c) LSV, (d) CV, (e) ECSA, (f) EIS.

The LSV curves in Figure 5c indicated that H₃PO₄-ASC demonstrated the best ORR catalytic activity among the tested samples, with a more positive onset potential and a larger limiting current density, while the electrocatalytic reduction activities of ZnCl₂-ASC, KOH-ASC, and KHCO₃-ASC decreased successively.

The CV curves and ECSA results shown in Figure 5d,e indicated that H₃PO₄-ASC possessed the largest ECSA of 63.79 µF·cm⁻², followed by ZnCl₂-ASC (55.45 µF·cm⁻²), while KOH-ASC and KHCO₃-ASC showed much smaller values of 23.88 µF·cm⁻² and 11.27 µF·cm⁻², respectively. The high ECSA of H₃PO₄-ASC was a direct consequence of its high specific surface area and hierarchical porous structure, which exposed a greater number of active sites. Furthermore, the rich oxygen functional groups not only contributed to ECSA but also enhanced the catalytic activity, as evidenced by the high selectivity even at the relatively low ECSA compared to some metal-based catalysts [36].

The EIS in Figure 5f shows that H₃PO₄-ASC had the smallest charge transfer resistance (R_ct). indicating the most facile electron transfer across the electrode/electrolyte interface. The carboxyl groups likely lower the interfacial barrier by moderately enhancing the local hydrophilicity of the active sites without flooding the hydrophobic pore channels, a delicate balance uniquely achieved by H₃PO₄ activation [37]. In contrast, ZnCl₂-ASC, KOH-ASC, and KHCO₃-ASC exhibited progressively larger R_ct values, which correlated with their inferior catalytic kinetics.

Taken together, H₃PO₄-ASC simultaneously achieved the highest H₂O₂ selectivity, the lowest electron transfer number, the largest ECSA, and the smallest R_ct, unequivocally establishing it as the optimal catalyst among the studied systems.

2.4. Effect of H₃PO₄ Concentration on Electrosynthesis

The effect of H₃PO₄ concentration (10–50 wt.%) on H₂O₂ accumulation is shown in Figure 6. The cumulative H₂O₂ concentration of all electrodes increased linearly with time. After 90 min, 30 wt.% H₃PO₄-ASC exhibited the highest cumulative H₂O₂ concentration of 95.1 mg·L⁻¹, followed by 20 wt.% H₃PO₄-ASC with 89.6 mg·L⁻¹, 40 wt.% H₃PO₄-ASC with 85.7 mg·L⁻¹, and 10 wt.% H₃PO₄-ASC with 84.8 mg·L⁻¹; the lowest cumulative concentration of 70.8 mg·L⁻¹ was obtained with 50 wt.% H₃PO₄. This volcano-shaped trend indicates that an optimal activator concentration is required: at low concentrations of H₃PO₄, insufficient activation leads to limited porosity and fewer carboxyl groups; at excessively high concentrations, over-etching may collapse the porous structure or introduce excessive phosphorus residues that block active sites [38]. Therefore, 30 wt.% was identified as the optimal concentration.

Figure 6. Effect of activator concentration on the accumulated concentration of H₂O₂, experimental conditions: Na₂SO₄ solution concentration is 0.05 M, current density is 0.5 mA·cm⁻², pH = 3, O₂ aeration rate is 500 mL·min⁻¹.

2.5. Effects of Current Density on H₂O₂ Yield and Current Efficiency

Figure 7 illustrates the impact of current density (0.5–7.5 mA·cm⁻²) on H₂O₂ yield and current efficiency. The cumulative concentration increased with current density, reaching 1062.8 mg·L⁻¹ at 7.5 mA·cm⁻² and 997.3 mg·L⁻¹ at 5.0 mA·cm⁻² after 90 min. However, the current efficiency (CE) decreased from ~75.0% at 0.5 mA·cm⁻² to ~50.0% at 7.5 mA·cm⁻², while energy consumption (EC) increased sharply from 27.6 kWh·kg⁻¹ H₂O₂ (at 5.0 mA·cm⁻²) to 51.5 kWh·kg⁻¹ H₂O₂ (at 7.5 mA·cm⁻²). Moreover, at j = 7.5 mA·cm⁻², the electrolyte became noticeably hot, indicating that excessively high current density induced severe side reactions and Joule heating, which is detrimental to the stable operation of the system [39]. Although a higher current density yields more H₂O₂ in absolute terms, the trade-off in CE and EC, along with thermal instability, makes 5.0 mA·cm⁻² the optimal condition for balancing productivity, energy efficiency, and long-term stability.

Figure 7. (a) Effect of current density on H₂O₂ accumulation, and (b) corresponding current efficiency and energy consumption, experimental conditions: Na₂SO₄ solution concentration is 0.05 M, pH = 3, O₂ aeration rate is 500 mL·min⁻¹.

2.6. Reusability and Stability

The reusability and stability of the 30 wt.% H₃PO₄-ASC electrodes were assessed over 15 consecutive cycles (Figure 8). The cumulative H₂O₂ concentration gradually declined from 783.3 mg·L⁻¹ (1st cycle) to 634.4 mg·L⁻¹ (15th cycle), retaining 81.1% of the initial H₂O₂ yield. The current efficiency remained above 47.0% even after 15 cycles, and energy consumption fluctuated modestly between 33.6 and 39.0 kWh·kg⁻¹ H₂O₂.

Figure 8. (a) Accumulated H₂O₂ concentration from 15 repeated electrocatalytic reactions using 30 wt.% H₃PO₄-ASC composite electrodes, (b) corresponding current efficiency and energy consumption, experimental conditions: Na₂SO₄ solution concentration is 0.05 M, current density is 5.0 mA·cm⁻², pH = 3, O₂ aeration rate is 500 mL·min⁻¹.

To elucidate the chemical origin of this moderate deactivation, XPS and Raman analyses were performed on the electrode before and after 15 cycles. Surprisingly, the high-resolution O 1s spectra (Figure 3b and Figure 9a) revealed that the surface oxygen content increased markedly from 4.71 at.% to 11.90 at.% after cycling, rather than decreasing. In the C 1s spectra, the C-C component slightly increased from 97.14% to 97.95%, while the carboxyl carbon (O=C-O) fraction decreased only marginally from 2.86% to 2.05% (Table 2), indicating that the intrinsic carbon skeleton and the key carboxylic acid groups were largely retained. In the O 1s spectra, however, the proportions of C=O and C-O declined from 65.18% to 52.86% and from 34.82% to 34.37% (Table 2), respectively, while a new O-F_x species (chemisorbed oxygen) emerged after cycling at 12.77% (absent in the fresh electrode). This distinct redistribution of oxygen species—coupled with the increase in total oxygen content—suggests that the prolonged electrolysis induced partial surface oxidation, generating new oxygen-containing functional groups (e.g., carbonyl, hydroxyl, and ether groups) [40], but also converting part of the original C=O/C-O moieties into more strongly chemisorbed oxygen species (O-F_x). Importantly, the carboxyl groups remained essentially intact (only a slight decrease from 2.86% to 2.05%), confirming that these crucial active functionalities were not chemically destroyed [33]. Meanwhile, the phosphorus content remained at a low level (from 0.40 at.% to 0.53 at.%) after cycling.

Figure 9. (a) XPS of 30 wt.% H₃PO₄-ASC composite electrode after 15 reuses, (b) Raman peaks and (c) CV of 30 wt.% H₃PO₄-ASC composite electrode before and after 15 reuses.

Table 2. The proportions of carbon and oxygen with different chemical states in 30 wt.% H₃PO₄-ASC composite electrode before and after 15 recycling cycles.

Sample	C (%)		O (%)			P (%)
	C-C	O=C-O	C=O	C-O	O-Fx	P 2p3/2
H3PO4-ASC	97.14	2.86	65.18	34.82	0	100
After 15 reuses	97.95	2.05	52.86	34.37	12.77	100

Concurrently, Raman spectroscopy (Figure 9b) showed only a modest increase in the I_D/I_G ratio from 1.85 to 2.18. The pristine electrode exhibited a relatively high I_D/I_G ratio of 1.85, indicating a disordered carbon structure with abundant defects and limited graphitization. After 15 cycles, this ratio increased modestly to 2.18, suggesting further disruption of the crystalline structure, increased defect density, and even less pronounced graphitization, which would inevitably compromise the ORR catalytic performance of the electrode [41]. Note that the Raman measurements were performed on carbon/PTFE mixtures coated on graphite felt, not on pristine powder; thus, weak PTFE fluorescence, C-F bands, and felt carbon signals may appear as minor extra peaks alongside the main D/G bands, but they do not compromise the assessment of carbon structural changes [42,43]. These spectroscopic findings are further corroborated by the CV curves before and after cycling (Figure 9c), which exhibited a slight decrease in peak current but no appreciable reduction in the enclosed area. The enclosed CV area, which is proportional to the number of electrochemically accessible active sites, showed no significant reduction, indicating that the active site density remained largely unchanged despite the surface oxygen enrichment.

To complement the intermittent cycling results, a prolonged continuous electrolysis test lasting 10 h was conducted to assess the operational stability under sustained polarization (Figure 10a). The H₂O₂ concentration increased steadily within the initial 4 h and peaked at 1099.0 mg·L⁻¹. Beyond 4 h, the H₂O₂ accumulation rate decreased progressively, which was mainly attributed to Joule heating-induced H₂O₂ decomposition at the relatively high current density (5.0 mA·cm⁻²) and further reduction-decomposition of H₂O₂ at the cathode. Notably, post-test LSV characterization revealed that the used electrode exhibited a certain decrease in the reduction onset potential and limiting current density (Figure 10b), confirming that the decline in H₂O₂ accumulation after 4 h was attributable to a combination of the electrode activity decay and H₂O₂ decomposition.

Figure 10. (a) H₂O₂ accumulation during the electrocatalytic reaction of the 30 wt.% H₃PO₄-ASC electrode for 10 h; (b) LSV of the 30 wt.% H₃PO₄-ASC electrode before and after 10 h of use.

Taken together, the moderate activity decline is associated with surface chemical evolution and thermal effects. The 30 wt.% H₃PO₄-ASC electrode thus exhibits good reusability and structural robustness, positioning it as a durable and sustainable electrocatalyst for practical H₂O₂ electrosynthesis.

In addition, Table 3 compares the performance of 30 wt.% H₃PO₄-ASC with reported sludge-derived catalysts. Our electrode achieves significantly higher H₂O₂ selectivity (90%, n = 2.20) and yield (997.3 mg·L⁻¹ in 90 min) than literature values. The outstanding performance is attributed to the hierarchical micro-/meso-/macroporosity, high surface area, abundant carboxyl groups, and strong hydrophobicity, while the higher EC results from the applied current density and structural resistance. Overall, this sludge-derived carbon is a promising low-cost electrocatalyst for H₂O₂ production.

Table 3. Comprehensive comparison of 30 wt.% H₃PO₄-ASC with representative sludge-derived activated carbon catalysts reported in the literature.

Catalyst	H2O2 Selectivity	n	H2O2 Yield (mg/L)	EC (kWh/kg)	Ref.
Fe/Mn-SBC 500	70%	1.93	5.42 (90 min)	--	[20]
SBC@Ni 1000	--	--	80.3 (60 min)	--	[21]
HTC	--	--	100.0 (120 min)	11.5	[44]
NSBCs@Ni-F	74%	--	196.0 (50 min)	12.0	[45]
30 wt.% H3PO4-ASC	90%	2.20	997.3 (90 min)	27.6	This work

3. Materials and Methods

3.1. Raw Materials and Reagents

Municipal sludge (obtained from Everbright Water (Nanjing) Co., Ltd., Nanjing, China); potassium hydroxide, potassium bicarbonate, phosphoric acid, sulfuric acid, sodium hydroxide (AR, Shanghai Lingfeng Chemical Reagent Co., Ltd., Shanghai, China); zinc chloride (AR, Xilong Scientific Co., Ltd., Shantou, China); nitric acid (65–68%), hydrogen peroxide (30%), graphite felt (100 mm × 50 mm × 2 mm) (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China); anhydrous sodium sulfate (AR, Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China); potassium titanium oxalate (AR), polytetrafluoroethylene (PTFE, 60 wt.%) (Shanghai Macklin Biochemical Technology Co., Ltd., Shanghai, China); anhydrous ethanol (AR, Wuxi Yasheng Chemical Co., Ltd., Wuxi, China); potassium bromide (GR, Shandong Xiya Chemical Co., Ltd., Linyi, China); Nafion solution (5%, Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China); ruthenium-iridium titanium mesh electrode (100 mm × 50 mm × 2 mm, Shaanxi Baoji Titanium Xin Machinery Processing Factory, Baoji, China); high-purity O₂, high-purity N₂ (99.999% by volume, Nanjing Special Gas Factory Co., Ltd., Nanjing, China). Ultrapure water was used throughout the experiments.

3.2. Material Characterization

Electric blast drying oven (DHG-9030A, Shanghai Jinghong Experimental Equipment Co., Ltd., Shanghai, China); tube furnace (TL1200, Nanjing Boyuntong Instrument Technology Co., Ltd., Nanjing, China); muffle furnace (KSL-1100X, Hefei Kejing Materials Technology Co., Ltd., Hefei, China); UV-Vis spectrophotometer (UV-3300, Shanghai Mapada Instruments Co., Ltd., Shanghai, China); electronic balance (SQP, Sartorius Scientific Instruments (Beijing) Co., Ltd., Beijing, China); digital ultrasonic cleaner (KQ3200V, Kunshan Ultrasonic Instruments Co., Ltd., Kunshan, China); circulating water multipurpose vacuum pump (SHZ-D(III), Henan Yuhua Instruments Co., Ltd., Gongyi, China); magnetic stirrer (FJS-4, Jincheng Fuwei Experimental Instrument Factory, Jintan District, Changzhou, China); DC power supply (KXN-3040D, Shenzhen Zhaoxin Electronic Instrument Equipment Co., Ltd., Shenzhen, China).

3.3. Catalyst Synthesis

(1) Pretreatment of municipal sludge: The municipal sludge collected from the secondary sedimentation tank of the wastewater treatment plant was first filtered through a 40-mesh sieve to remove mixed kitchen waste debris. It was then filtered through a 100-mesh filter cloth, washed with ultrapure water under ultrasonication for 1 h, and dried in an oven at 105 °C to constant weight. Finally, the dried sludge was ground into powder particles smaller than 40 mesh.

(2) Pretreatment of graphite felt: The graphite felt was cut into small pieces of 5 × 10 cm, immersed in anhydrous ethanol under ultrasonication for 30 min, then washed with ultrapure water and ultrasonicated for another 30 min, followed by drying in an oven at 105 °C.

(3) Preparation of municipal sludge-derived activated carbon: 15 g of the pretreated sludge was mixed with different activators (10–50 wt.% H₃PO₄, 40 wt.% ZnCl₂, 20 wt.% KOH, and 20 wt.% KHCO₃) and impregnated for 2 h. The mixture was first dried in an oven at 105 °C to constant weight and then placed in a tube furnace under a N₂ atmosphere with a flow rate of 500 mL·min⁻¹. The temperature was ramped to the calcination temperature at a heating rate of 5 °C·min⁻¹. and maintained for the corresponding calcination time. The calcined product was ground into powder in an agate mortar, then mixed with 80 mL of 1 mol·L⁻¹ HNO₃ solution and stirred for 4 h. The suspension was filtered under suction and washed with ultrapure water until the filtrate became neutral. Finally, the product was dried in an oven at 105 °C to constant weight to obtain the municipal sludge-derived activated carbon.

(4) Preparation of composite electrodes: 0.200 g of the above-prepared municipal sludge-derived activated carbon was mixed with 20 mL of 40 wt.% PTFE solution and stirred for 20 min to ensure thorough mixing. The mixture was then uniformly coated onto a 100 mm × 50 mm piece of graphite felt. The coated felt was dried in an oven first at 80 °C for 2 h, then at 100 °C for 1.5 h. After drying, it was placed in a muffle furnace, heated to 300 °C at a rate of 10 °C·min⁻¹, and calcined at 300 °C for 1 h to obtain the sludge carbon composite electrodes.

The as-prepared electrodes were designated as Blank-ASC (sludge carbonized directly without any activator), x wt.% H₃PO₄-ASC (where x represents the mass percentage concentration of the activator H₃PO₄, x = 10,20,30,40,50), ZnCl₂-ASC (activated with 40 wt.% ZnCl₂), KOH-ASC (activated with 20 wt.% KOH), and KHCO₃-ASC (activated with 20 wt.% KHCO₃). An electrode without sludge carbon, coated only with 40 wt.% PTFE solution was named PTFE.

3.4. Material Characterization

The surface morphology and pore structure of the electrodes were observed using a Zeiss Gemini 300 scanning electron microscope (Carl Zeiss AG, Oberkochen, Germany). The contact angle on the electrode surface was measured with a JC2000X contact angle goniometer (Shanghai Zhongchen Digital Technology Apparatus Co., Ltd., Shanghai, China) to determine the hydrophilicity/hydrophobicity of the electrode material. The types of oxygen-containing functional groups on the surface of the biochar treated with different activators were analyzed using a Thermo Fisher Scientific Nicolet iS20 Fourier transform infrared spectrometer (Thermo Fisher Scientific, Waltham, MA, USA), with the wavenumber range of 400–4000 cm⁻¹, a resolution of 4 cm⁻¹, and 32 scans. The nitrogen adsorption/desorption isotherms at 77 K were obtained using an Autosorb-iQ-MP high-vacuum gas sorption analyzer (Quantachrome Instruments, a brand of Anton Paar, Boynton Beach, FL, USA). The samples were degassed at 120 °C for 6 h prior to measurement. The specific surface area (SSA) was calculated according to the BET method with five relative pressure points in the interval of 0.05–0.30. X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific K-alpha, Waltham, MA, USA) was carried out to analyze the surface elemental states and their distribution ratios on the biomass-derived carbon material. The degree of graphitization was characterized using a Raman spectrometer (DXR2xi Dispersive Microscopic Confocal Laser Raman Spectrometer, Thermo Scientific, Waltham, MA, USA).

3.5. H₂O₂ Electrosynthesis Experiments

The electrochemical reaction system consisted of an electrochemical reaction cell (500 mL volume), a DC regulated power supply, and an oxygen cylinder. The cathode was a 100 mm × 50 mm composite electrode, and the anode was a 100 mm × 50 mm stainless steel plate, with a distance of 25 mm between them. A 250 mL of 0.05 mol·L⁻¹ Na₂SO₄ solution was used as the electrolyte, and the initial pH was adjusted to 3. The system was first purged with O₂ to saturate the dissolved oxygen, and then the electrolysis for H₂O₂ production was carried out. Samples were taken at regular intervals, and the H₂O₂ concentration in the solution was determined by the potassium titanium oxalate method.

The current efficiency (CE) of H₂O₂ electrosynthesis was calculated according to Equation (1):

CE = [(n × F × C × V)/(I × t)] × 100%,

(1)

where CE is the current efficiency (%), n is the number of electrons transferred in the ORR (n = 2), F is the Faraday constant (96,485 C·mol⁻¹), C is the concentration of H₂O₂ produced (mol·L⁻¹), V is the volume of the electrolyte (0.250 L), I is the current intensity (A), and t is the electrolysis time (s).

The energy consumption (EC) of H₂O₂ electrosynthesis was calculated according to Equation (2):

EC = (U × I × t)/(3600 × C × M × V),

(2)

where EC is the energy consumption (kWh·kg⁻¹ H₂O₂), U is the applied voltage (V), I is the current intensity (A), t is the electrolysis time (s), C is the concentration of H₂O₂ produced (mol·L⁻¹), M is the molar mass of H₂O₂ (34.01 g·mol⁻¹), and V is the volume of the electrolyte (0.250 L).

3.6. Electrochemical Measurements

All electrochemical tests were performed in a three-electrode system using a DH7003 electrochemical workstation (Donghua Analytical Instruments Co., Ltd., Taizhou, China). The electrochemical cell contained 250 mL of 0.05 mol·L⁻¹ Na₂SO₄ solution. A saturated calomel electrode (SCE) was used as the reference electrode, a platinum plate as the counter electrode, and the sludge carbon composite electrode (10 mm × 20 mm) as the working electrode.

For CV and LSV, the electrolyte was first purged with O₂ at 500 mL·min⁻¹ for 15 min to achieve dissolved oxygen saturation. CV scans were recorded in the potential range of −1.0 to 0.2 V vs. SCE at a scan rate of 50 mV·s⁻¹. LSV measurements were performed in the same potential range at a scan rate of 50 mV·s⁻¹.

For ECSA and EIS measurements, the electrolyte was purged with N₂ at 500 mL·min⁻¹ for 15 min to remove dissolved oxygen. ECSA was determined by measuring the double-layer capacitance in the potential range of −0.2 to 0.2 V vs. SCE at scan rates of 5–50 mV·s⁻¹. The current density difference (Δj) at 0 V vs. SCE was plotted against the scan rate, and the slope of the linear fit gave the double-layer capacitance (C_dl). EIS measurements were performed at the open-circuit potential over a frequency range of 0.01–10,000 Hz with an amplitude of 5 mV and a static time of 2 s. The impedance spectra were analyzed using Zview software (Version 2).

RRDE tests were conducted to calculate the H₂O₂ selectivity and electron transfer number. A catalyst ink was prepared by ultrasonically dispersing 0.1 mg of sludge carbon, 0.25 mL of anhydrous ethanol, and 0.125 mL of Nafion solution for 10 min. Then, 3.0 µL of the suspension was pipetted onto the disk electrode and dried; this procedure was repeated twice. LSV was performed at a rotation speed of 1200 rpm in the potential range of 0.5 to −1.5 V vs. SCE at a scan rate of 10 mV·s⁻¹, with the ring electrode potential set at 1.0 V vs. SCE.

All potentials measured vs. SCE were converted to the reversible hydrogen electrode (RHE) scale using Equation (3):

E (vs. RHE) = E (vs. SCE) + 0.244 + 0.059 × pH,

(3)

The ECSA was calculated according to Equation (4):

ECSA = C_dl/C_s,

(4)

where ECSA is the electrochemical active surface area (μF·cm⁻²), C_dl is the measured double-layer capacitance (μF·cm⁻²), and C_s is the specific capacitance (40 μF·cm⁻²).

The H₂O₂ selectivity and electron transfer number were calculated using Equations (5) and (6):

H₂O₂ = [(I_R/N)/(I_D + I_R/N)] × 200%,

(5)

n = (4 × I_D)/(I_D + I_R/N),

(6)

where H₂O₂ stands for H₂O₂ selectivity (%), I_R is the ring current (mA), I_D is the disk current (mA), N is the collection coefficient (N = 0.4077 in this work), n is the number of transferred electrons.

4. Conclusions

In this study, a series of municipal sludge-derived activated carbon electrodes (ASC) were successfully synthesized via a simple calcination process using different activators (H₃PO₄, ZnCl₂, KOH, and KHCO₃), and their catalytic performance for the 2e⁻ ORR toward H₂O₂ electrosynthesis was systematically investigated. The key findings are summarized as follows:

(1) The optimal conditions for preparing municipal sludge-derived activated carbon were activation with 30 wt.% H₃PO₄ and calcination at 600 °C for 2 h. Under the conditions of an initial electrolyte pH of 3 and a current density of 5.0 mA·cm⁻², the 30 wt.% H₃PO₄-ASC electrodes achieved an H₂O₂ yield of 997.3 mg·L⁻¹ after 90 min of reaction, with a current efficiency of 68.8% and an energy consumption of 27.6 kWh·kg⁻¹ H₂O₂.

(2) The good performance of 30 wt.% H₃PO₄-ASC was attributed to three synergistic factors: (i) a high specific surface area (806.20 m²·g⁻¹) with a hierarchical micro-meso-macroporous structure—micropores provided abundant active sites, while mesopores and macropores (2.5–12 µm) effectively facilitated O₂ diffusion and H₂O₂ detachment; (ii) abundant carboxyl and other oxygen-containing groups that serve as highly active and selective sites for the 2e⁻ ORR, with additional phosphorus functionalities (P-O-C/phosphate species) synergistically modulating the surface charge environment; and (iii) suitable surface hydrophobicity that enriches dissolved O₂ at the catalyst interface. These features together gave an ECSA of 63.79 µF·cm⁻², a near-ideal electron transfer number of 2.2, and a H₂O₂ selectivity of 86–92%.

(3) The electrode exhibited good reusability, retaining 81.1% of its initial H₂O₂ production after 15 cycles. XPS analysis revealed that surface oxygen content increased from 4.71 at.% to 11.90 at.% after cycling, while the phosphorus content changed slightly from 0.40 to 0.53 at.%. Raman spectroscopy confirmed that the disorder and defects of electrode materials increase (I_D/I_G increased slightly from 1.85 to 2.18). All of these may be related to the decrease in electrode catalytic activity.

In summary, this work demonstrates that municipal sludge can be transformed into a high-performance and cost-effective electrocatalyst for green H₂O₂ production, delivering yield and selectivity substantially superior to those of previously reported sludge-derived carbon catalysts. Moreover, the reasons for the excellent H₂O₂ electrosynthesis have been preliminarily explained. Further investigations are still required to optimize large-scale preparation, elucidate the detailed reaction mechanism (e.g., via DFT calculations or in situ spectroscopy), and explore its applications in pollutant degradation and other oxidative processes.