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Article

One-Step Carbonization of Monosaccharide and Dicyandiamide to Oxygen and Nitrogen Co-Doped Carbon Nanosheets for Electrocatalytic O2 Reduction to H2O2

by
Dan Wang
1,2,
Yanan Liu
1,
Kun Wan
1,
Danning Feng
1,
Yan Pei
1,
Minghua Qiao
1,*,
Xiaoxin Zhang
3 and
Baoning Zong
3
1
State Key Laboratory of Porous Materials for Separation and Conversion and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Department of Chemistry, Fudan University, Shanghai 200438, China
2
Changzhou Vocational Institute of Engineering, Changzhou 213164, China
3
State Key Laboratory of Catalytic Materials and Chemical Engineering, Research Institute of Petroleum Processing, SINOPEC, Beijing 100083, China
*
Author to whom correspondence should be addressed.
Catalysts 2025, 15(5), 459; https://doi.org/10.3390/catal15050459
Submission received: 25 March 2025 / Revised: 28 April 2025 / Accepted: 3 May 2025 / Published: 7 May 2025
(This article belongs to the Special Issue MXenes-Based Composites and Their Applications in Photocatalysis, Electrocatalysis and Thermocatalysis)
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Abstract

:
The electrocatalytic reduction of O2 via two-electron reaction (2e-ORR) to H2O2 represents a promising alternative to the current anthraquinone process, since it is advantageous in the sustainable and decentralized production of H2O2. Herein, we report the development of oxygen and nitrogen-rich few-layered graphene-like materials (ms-dcda) by the one-step carbonization of biomass-sourced monosaccharides (D-glucose, D-fructose, D-galactose, D-ribose, D-xylose, L-arabinose, and D-mannose) with the aid of dicyandiamide for electrochemical O2 reduction to H2O2. The ms-dcda materials were porous and possessed wrinkled morphology typical of graphene nanosheets. In H2O2 production via 2e-ORR in an acidic electrolyte, these ms-dcda materials were all active and stable catalysts, among which glu-dcda derived from D-glucose and dicyandiamide displayed the lowest onset potential of 0.553 V and the highest selectivity of up to 91.6%. The catalyst was also highly stable in chronoamperometric tests. Selective chemical titration of the C–OH and C=O groups revealed that the latter is far more active and selective than the former in 2e-ORR. Moreover, a positive correlation between the contents of C=O and pyrrolic N and the H2O2 partial current suggests that the pyrrolic N group also contributes to 2e-ORR. This work affords a facile strategy for the sustainable fabrication of metal-free carbon-based catalysts efficient for H2O2 electrosynthesis.

Graphical Abstract

1. Introduction

Hydrogen peroxide (H2O2) is an important industrial chemical that is widely used as an environmentally friendly oxidant for chemical synthesis, bleaching, sanitization, and environmental remediation, and as a potential energy carrier [1,2,3,4]. Nowadays, the large-scale production of H2O2 is dominated by the multi-step anthraquinone process, which has the shortcomings of expensive feedstock, high pollution, complicated processing procedures, and inadaptability to on-site H2O2 production [4,5]. The development of an economic, sustainable, and decentralized H2O2 production process is highly desired to solve the problems of high expenditure for H2O2 production and high safety risk during the storage and transportation of concentrated H2O2.
Recently, the oxygen electrocatalytic strategy has been proposed as a promising approach for on-site and on-demand H2O2 production, in which O2 undergoes a two-electron reduction reaction (2e-ORR) [6,7,8,9,10]. The two-electron pathway involves ·OOH, while its further reduction produces O· and ·OH (·OOH + e− → O· + OH−), leading to the undesired four-electron reduction reaction (4e-ORR) pathway [11,12]. In principle, in order to suppress the four-electron pathway, the key is to prevent the bond-breaking reaction of ·OOH [11,13]. Since it is challenging to construct active sites specific for either of them, a mixed two-electron and four-electron pathway usually occurs [14,15]. Therefore, it is of great significance to develop a catalyst platform with versatile tunability in structure to regulate the oxygen reduction pathway.
Previous studies have explored various catalysts, including metal-based catalysts [16,17] and metal-free carbon-based materials [18,19,20,21]. While metal-based catalysts exhibit high activity, their limited availability and high cost limit their practicality [22]. Carbon-based catalysts have gained attention due to their earth-abundance, tunable electronic properties, and structural flexibility [23]. To elevate their catalytic performance, doping strategies such as heteroatom incorporation, defect modulation, and charge-transfer engineering have proven instrumental in refining electronic configurations, enhancing active site accessibility and stabilizing reaction intermediates [24,25,26].
The doping of carbon-based materials with heteroatoms such as O and N is regarded as an effective strategy to enhance the catalytic performance in 2e-ORR. First, due to the difference in electronegativity between carbon and heteroatom, the integrity of the π-conjugated system of the carbon materials can be disturbed, which redistributes the charges and hence alters the adsorption behavior towards the reaction intermediate [27]. Second, heteroatom doping can incorporate functional groups onto the carbon substrate, which may serve as the active sites for 2e-ORR. For example, for the O-doped carbon catalysts, C–O–C [28], COOH [29], quinone [30], or C=O groups [29,31] have been suggested to facilitate the two-electron pathway. For the N-doped carbon catalysts, pyrrolic N [32,33], a combination of pyridinic N and pyrrolic N [34], or isolated N in the form of graphitic N [35,36] has been proposed to favor H2O2 production. Moreover, the synergy between O and N dopants has also been reported to enhance the catalytic performance of the carbon catalysts in 2e-ORR [37,38].
In the present work, we synthesized O and N co-doped porous wrinkled few-layered graphene-like materials (ms-dcda) by the one-step carbonation of monosaccharides (ms) in the presence of dicyandiamide (dcda). D-Glucose, D-fructose, D-galactose, D-ribose, D-xylose, L-arabinose, and D-mannose were used, which are monosaccharides that can be massively extracted from biomass [39]. During thermal treatment, dicyandiamide can polymerize into g-C3N4, which serves as an endogenous template to direct the growth of two-dimensional (2D) graphene from monosaccharide such as glucose [40,41]. At 750 °C, g-C3N4 could completely decompose [33,40,42], so post-treatment is not needed to remove the template. The advantages of this strategy lie not only in the confined growth of graphene between the layers of the dicyandiamide-derived g-C3N4, but also in the simultaneous O doping from monosaccharide and N doping from dicyandiamide into the framework of graphene. The effect of the types of monosaccharides on the physicochemical properties of the ms-dcda materials was systematically studied for the first time. In 2e-ORR, the ms-dcda materials displayed good activity, selectivity, and stability in an acidic electrolyte. Among them, glu-dcda prepared from glucose and dicyandiamide exhibited the best catalytic performance, making it a competitive candidate for low-cost but high-efficiency decentralized H2O2 production from O2 electrocatalytic reduction.

2. Results and Discussion

2.1. Basic Physicochemical Properties

The ms-dcda materials were synthesized by the one-step thermal treatment of monosaccharides and dicyandiamide. During thermal treatment, dicyandiamide condenses into the layer-structured g-C3N4 at about 450 °C [41], which acts as the endogenous temporary template to confine the growth of monosaccharide-derived carbon material within the interlayer gaps by binding the aromatic carbon intermediates via donor–acceptor interaction [40]. As confirmed by the thermogravimetric result of a homogeneous mixture of glucose and dicyandiamide (Figure S1), the g-C3N4 template completely decomposed at 750 °C, thus leaving free-standing graphene-like carbon nanosheets [40]. The formation of the 2D carbon nanosheets was directly observed by transmission electron microscopy (TEM). As shown in Figure 1, the ms-dcda materials were composed of nanosheets with some wrinkles, similar to the 2D sp2-hybridized graphene. The high-resolution TEM (HRTEM) images revealed the few-layer structure with the interlayer distances of 4.44 to 4.45 Å at different regions (Figure 1b,d,f), which are larger than that of graphite (3.35 Å) [43]. Indeed, the X-ray diffraction (XRD) patterns of the ms-dcda materials showed the (002) peak at 2θ of 25.1° (Figure 2), which is negatively shifted relative to the corresponding peak of graphite at 2θ of 26.6° (JCPDS 26-1079). This discrepancy can be attributed to the existence of microstructural defects, i.e., non-sp2 carbon atoms, and the incorporation of the O and N heteroatoms from monosaccharides and dicyandiamide, respectively [33,41]. The energy dispersive spectroscopy (EDS) element mapping images (Figure 3) showed the homogeneous distribution of the O and N elements on the ms-dcda materials, which is consistent with the organic elemental analysis (OEA) results compiled in Table 1. It is identified that the O content was in the range of 8.46–18.68 wt %, and the N content was in the range of 17.39–19.44 wt %. The high N doping content is expected to effectively alter the electronic structure and facilitate O2 adsorption [33]. In addition, it is note-worthy that the bulk O/C and N/C ratios of glu-dcda are more than three times those of the NWCN-4 material with the same glucose-to-dicyandiamide ratio but prepared in a two-step carbonation strategy [41], demonstrating that the one-step strategy is especially effective in fabricating carbon materials with high O and N doping levels.
N2 physisorption revealed that the ms-dcda materials all displayed a type IV isotherm with sharp uptake at low relative pressure and the H4 hysteresis loop at a relative pressure of ca. 0.42 (Figure 4a), manifesting the coexistence of micropores and mesopores. The pore size distributions of all the ms-dcda materials illustrated in Figure 4b are also similar. In addition, aside from two distinct peaks for micropores at ca. 0.67 nm and 1.2 nm, there is a broad distribution in the range of 2–20 nm, demonstrating the formation of the micro- and meso-porous structure. As summarized in Table 1, the ms-dcda materials all had large specific surface areas (SBET) of > 300 m2 g−1. In contrast, we found that the thermal treatment of glucose in the absence of dicyandiamide under the same conditions resulted in dense and hard solids with negligible N2 adsorption, highlighting the essential role of dicyandiamide in enhancing the porosity of the ms-dcda materials. It is remarkable that the SBET of glu-dcda is much higher than those of NWCN-4 [41] and the NG materials with even lower glucose-to-dicyandiamide ratios prepared by the two-step carbonation strategy [40], substantiating that the one-step strategy is advantageous for the fabrication of high-surface area O- and N-doped carbon materials.
Raman characterization is commonly used to determine the structure of carbon materials. As shown in Figure 5, there are bands at 1350 and 1580 cm−1 for the ms-dcda materials. The former is associated with the non-sp2 carbon defects at the edge or on the basal plane of graphene and termed as the D band, whereas the latter is related to the sp2 carbon of graphene and termed as the G band [44,45]. The intensity ratio of the two bands (ID/IG) reflects the degree of disorder, which fell in the range of 1.06–1.10 for the ms-dcda materials.
The surface chemical properties of the ms-dcda materials were investigated by X-ray photoelectron spectroscopy (XPS). The C 1s spectra in Figure 6a can be fitted into four peaks at binding energies (BE) of ca. 284.5, 285.4, 286.4, and 287.3 eV, assignable to graphitic C, C−N, C−O, and C=O, respectively [34]. As displayed in Figure 6b, the O 1s spectra were fitted into two peaks at 531.8 eV from C=O and at 533.3 eV from C–O [28]. Table 2 summarizes the total O content and the fraction of each O species on the surface of the ms-dcda materials. The surface O contents were in the range of 2.99 to 4.93 at %. Moreover, the relative content of the C=O and C–O species was greatly influenced by the type of monosaccharide, which awaits further investigation and elucidation.
The N 1s spectra (Figure 6c) were deconvoluted into four peaks, which are assigned to pyridinic N (398.2 eV), pyrrolic N (399.8 eV), graphitic N (401.1 eV), and oxidic N (402.9 eV) [46]. Table 3 summarizes the total N content and the fraction of each N species on the surface of the ms-dcda materials. It shows that the surface N contents of the ms-dcda materials were quite high, between 16.1 and 18.2 at %, which conform to the bulk N contents, manifesting their N-rich characteristics. Moreover, for each surface N species, their contents decreased in the order of pyridinic N > graphitic N > pyrrolic N >> oxidic N, with slightly more pyrrolic N than graphitic N only on glu-dcda.

2.2. 2e-ORR Catalytic Performance

The electrocatalytic performances of the as-fabricated ms-dcda catalysts in 2e-ORR to H2O2 were comparatively studied using LSV at identical catalyst mass under reaction conditions of rotation speed of 1600 rpm and scan rate of 10 mV s−1 in the O2-saturated 0.1 M HClO4 electrolyte. It was found that the type of monosaccharide exerted great influence on the catalytic performance of the resulting ms-dcda catalysts in 2e-ORR. According to the polarization curves in Figure 7a, glu-dcda displayed the most positive onset potential of 0.553 V. As compared in Table S1, the glu-dcda catalyst exhibits a significantly lower overpotential compared to most carbon-based catalysts in 2e-ORR to H2O2 in acidic electrolytes. Moreover, glu-dcda possessed the highest H2O2 partial current of 0.170 mA at 0 V, and its activity is consistently the highest across the whole potential range investigated. In contrast, fru-dcda possessed the lowest H2O2 partial current of 0.123 mA at 0 V and the least positive onset potential of 0.468 V. For gal-dcda, the onset potential (0.496 V) and the H2O2 partial current (0.145 mA) were in-between those of glu-dcda and fru-dcda. Therefore, if unspecified, glu-dcda, fru-dcda, and gal-dcda were chosen as representatives for further electrocatalytic testing and characterization. As plotted in Figure 8, the Tafel slopes of glu-dcda, fru-dcda, and gal-dcda were 146.8, 159.1, and 147.2 mV dec−1, respectively, which complies with their sequences of the onset potential and H2O2 partial current and reflects the fastest 2e-ORR kinetics on glu-dcda.
The H2O2 selectivity and electron transfer number (n) were calculated from the iR and iD measured on the RRDE. As presented in Figure 7b,c, all the ms-dcda catalysts maintained impressively stable H2O2 selectivity and electron transfer number in the range of 0.40–0 V. Among the ms-dcda catalysts, glu-dcda also afforded the highest H2O2 selectivity, which varied in a narrow range of 89.0–91.6% and maximized at 0.40 V. In contrast, ara-dcda afforded the lowest H2O2 selectivity varying in the range of 82.9–86.3%.
As the primary product on the ms-dcda catalysts, H2O2 is a highly reactive oxidizing agent, so the stability is of priority for the catalyst used in 2e-ORR [29]. Hence, accelerated durability testing (ADT) [47,48] was adopted to evaluate the cyclic stability of the ms-dcda catalysts, which involved electrochemically cycling the catalyst at a rapid scan rate of 200 mV s−1 between 1.0 and 0 V for 3000 cycles. Figure 9a,c,e,g–j show that the CV curves of the ms-dcda catalysts after 3000 cycles were similar to those of the first cycles. It should be mentioned that the ms-dcda catalysts are more stable than the N and B co-doped carbon (BN-C1) [47] and mesoporous carbon (MesoC) catalysts [48], manifesting their excellent electrochemical stability in 2e-ORR in acidic electrolyte. To clarify whether or not the material itself had been changed after ADT, we collected the capacitance curves in the absence of O2 by saturating the electrolyte with N2 for glu-dcda, fru-dcda, and gal-dcda after 3000 cycles of ADT. It turns out that the capacitance curves are virtually identical before and after ADT (Figure 9b,d,f), further evidencing the robustness of the ms-dcda catalysts.
Since glu-dcda is superior to other ms-dcda catalysts in both 2e-ORR activity and selectivity, the electrocatalytic H2O2 production and accumulation was tested on this catalyst using the chronoamperometric method. During the test, an aliquot of the electrolyte was sampled at intervals to obtain the real-time H2O2 concentration. As shown in Figure 10, the H2O2 concentration increased linearly with respect to the reaction time, reflecting that glu-dcda can continuously and steadily produce H2O2. Within 8 h, the amount of H2O2 reached 481 mmol gcat−1, corresponding to a H2O2 production rate of 60 mmol gcat−1 h−1. The excellent stability of glu-dcda in continuous H2O2 production was also evidenced by the highly constant cathodic current at around −0.35 mA throughout the reaction.
We further investigated the effects of preparation conditions of glu-dcda on the 2e-ORR catalytic performance. By fixing the mass of dicyandiamide at 5.0 g while changing the mass of glucose, a series of xglu-dcda catalysts were prepared, in which x represents the mass of glucose in grams. As shown in Figure S2, 0.75glu-dcda gave the lowest H2O2 partial current at 0 V. For 0.25glu-dcda and 1.0glu-dcda, the H2O2 partial currents at 0 V increased to 0.131 mA. The H2O2 partial current at 0 V further increased to 0.170 mA for 0.50glu-dcda, 1.25glu-dcda, and 1.50glu-dcda. In the range of 0.4–0 V, the H2O2 partial current of 0.50glu-dcda was always higher than those of the others, but its H2O2 selectivity is much lower than those of 1.25glu-dcda and 1.50glu-dcda. The H2O2 selectivities over 1.25glu-dcda and 1.50glu-dcda were similar and remained at about 90% in the range of 0.4–0 V.
To investigate the effect of thermal treatment temperature, a series of glu-dcda catalysts were prepared at 700, 800, 900, and 1000 °C for a fixed time of 2 h and labeled as glu-dcda-700, glu-dcda-800, glu-dcda-900, and glu-dcda-1000, respectively. As shown in Figure S3, the ORR activity of glu-dcda-700 is especially low, which can be associated with the incomplete decomposition of g-C3N4. According to Figure S1 as well as the findings of Li et al. [40], g-C3N4 does not completely decompose until 750 °C. Thus, the decomposition residue of g-C3N4 may seriously cover the surface and block the pores and consequently reduce the number of active sites exposed. The catalytic activity increased at thermal treatment temperatures of 800–1000 °C and was essentially temperature-independent. However, the selectivity to H2O2 over glu-dcda-800 was higher than those over glu-dcda-900 and glu-dcda-1000, inferring that a higher thermal treatment temperature is adverse to the survival of some active sites specific for 2e-ORR.

2.3. Elucidation of Surface Active Sites

In general, an electrocatalyst with higher SBET can provide more active sites and is thus beneficial to the electrocatalytic reaction [44]. In the present case, since gal-dcda with the highest SBET of 416 m2 g−1 is moderately active and selective, while rib-dcda with the lowest SBET of 303 m2 g−1 is secondary to glu-dcda in activity, the discrepancy in the SBET can be excluded as a main factor that affects the catalytic performance of the ms-dcda catalysts in 2e-ORR. On the other hand, for the MesoC catalyst, Bao and co-workers proposed that the carbon defects are the active sites in 2e-ORR [48]. However, the small difference in the ID/IG ratios for the ms-dcda catalysts suggests that the abundance of the carbon defects or the size of the conjugated graphene units is not responsible for the distinct difference in their catalytic performances.
Therefore, electrochemical capacitance measurement was used to determine the electroactive surface areas of glu-dcda, fru-dcda, and gal-dcda as representatives. Their CV curves and the corresponding plots of the average current density against the scan rate are illustrated in Figure 11. The ECSA values were calculated to be 18.41, 13.29, and 16.12 mF cm−2 for glu-dcda, fru-dcda, and gal-dcda, respectively, which is consistent with the trend of the H2O2 partial current at 0 V. This correlation implies that the difference in the 2e-ORR activity lies in the difference in the abundance of the electroactive sites for the ms-dcda catalysts.
Heteroatom doping has been reported to play an important role in promoting the catalytic performance of the carbon catalysts in 2e-ORR. Using the nitric acid-oxidized carbon nanotubes by O-CNT, Cui and co-workers [28] improved both the ORR activity and selectivity to H2O2 in basic and neutral electrolytes. Characterizations showed that the C–O and C=O groups on O-CNT served as active sites for 2e-ORR, and the activity and selectivity were positively correlated with the oxygen content. Joo and co-workers [29] synthesized graphitic ordered mesoporous carbon (GOMC) by pyrolyzing bitumen using Al-SBA-15 as the hard template and then oxidized the GOMC with nitric acid to introduce oxygen functional groups (O-GOMC). With the aid of chemical titration, the C=O groups were identified to contribute the most to the H2O2 partial current in 2e-ORR. By pyrolyzing acetyl vanillin (VA) using MgO as the hard template, Chen et al. [31] prepared O-doped carbon nanosheet catalysts (O-CNS). Among the C–OH, C=O, and COOH groups, they verified that C=O is the most effective active site for 2e-ORR by chemical titration.
For the ms-dcda catalysts, XPS characterization revealed the presence of the C–O and C=O species (Figure 6). Therefore, according to previous reports [29,31,49,50], benzoic anhydride and phenylhydrazine were used to block the C–OH and C=O groups, respectively. Treatment of glu-dcda with benzoic anhydride and phenylhydrazine resulted in catalysts designated as glu-dcda-BA and glu-dcda-PH, respectively. The electrocatalytic tests on the site-blocked catalysts revealed a considerable decrease in activity as compared to the pristine catalysts, while the extent is dependent on the blocked functional group (Figure 12a). The glu-dcda-PH catalyst showed the more pronounced decrease in catalytic performance, with the H2O2 partial current at 0 V decreasing from 0.169 mA to 0.084 mA and reduction in H2O2 partial current of 50.3% as compared to 33.1% for glu-dcda-BA. Furthermore, its onset potential (0.374 V) is also more negative than that of glu-dcda-BA (0.418 V). These selective site-blocking results verify that both the C–OH and C=O groups are capable of catalyzing H2O2 electrosynthesis, with the C=O group being more active than the C–OH group, which is consistent with the findings of Liu and co-workers [31] and Joo and co-worker [29]. Moreover, the H2O2 selectivity over glu-dcda-PH is also lower than that over glu-dcda-BA (Figure 12b), indicating that the C=O group is also more selective than the C–OH group in 2e-ORR. Liu and co-workers pointed out, by DFT calculations, that the C=O group delivers the ΔGOOH* of 4.24 eV, which approaches the volcano vertex of the Sabatier curve [31].
On the other hand, deconvolution of the N 1s spectra reveals that there are mainly pyridinic N, pyrrolic N, and graphitic N species on the ms-dcda catalysts (Figure 6c and Table 3). It has been reported that a nitrogen atom with higher electronegativity than the carbon atom could activate the π-conjugated system and impart positive charge to adjacent carbon atoms, thus facilitating the adsorption of the ·OOH intermediate [48]. However, the delocalized lone-pair electrons from the pyridinic N could induce excessive charge transfer from the π2p bonding orbitals to the π2p* antibonding orbitals of O2, resulting in significantly weakened O–O bonding and undesired dissociation of the ·OOH intermediate into O· and ·OH [34,51,52]. Therefore, a four-electron ORR pathway was proposed to preferentially occur on carbon atoms adjacent to pyridinic N [33]. Antonietti and co-workers [32] prepared mesoporous nitrogen-doped carbon catalysts by carbonizing 1-butyl-3-methylimidazolyl dicyandiamide (BMP-dca) in the presence of silica nanospheres as the hard template. They found that increasing the carbonization temperature resulted in a higher degree of graphitization, lower pyrrolic N content, and lower H2O2 yield, suggesting that pyrrolic N is more favorable for 2e-ORR. Li et al. [33] observed that the much larger number of pyrrolic N species may alter the electronic structure toward optimized adsorption of the ·OOH intermediate, thus leading to superior selectivity in the 2e-ORR pathway. On the other hand, the graphitic N is more positively charged and the carbon atoms surrounding graphitic N may act as Lewis acids [53], which are not favorable for the adsorption of ·OOH intermediate. Therefore, aside from the C=O group, the pyrrolic N or carbon atoms adjacent to pyrrolic N can also significantly contribute to the 2e-ORR pathway.
In light of the experimental results and viewpoints in the literature, we calculated the surface C=O and pyrrolic N content on the basis of Figure 6b,c; the results are also summarized in Table 2. It turns out that the H2O2 partial current at 0 V correlates linearly with the surface C=O and pyrrolic N content (Figure 13), substantiating that the C atoms in or adjacent to these species are the active sites for 2e-ORR to H2O2. We also analyzed the correlation with the surface contents of C=O and other N species; however, only highly scattered data points could be obtained.

3. Materials and Methods

3.1. Chemicals

D-Glucose, D-fructose, D-galactose, D-xylose, L-arabinose, D-mannose, dicyandiamide, anhydrous ethanol, HCl, and CHCl3 were purchased from Sinopharm Chemical Reagent. D-Ribose, nafion, isopropanol, and HClO4 were purchased from Sigma-Aldrich. Phenylhydrazine (PH) and benzoic anhydride (BA) were purchased from Aladdin. All chemicals were used without further purification. N2 and O2 gases were purchased from Shanghai TOMOE Gases.

3.2. Preparation

In a typical synthesis, monosaccharide (1.25 g) and dicyandiamide (5.0 g) were dissolved in deionized water (200 mL). The mixture was stirred and evaporated at 80 °C until dryness. The solids were ground into fine powders and calcined at 800 °C at a ramping rate of 3 °C min−1 in flowing N2 for 2 h. The obtained black materials were denoted as glu-dcda, fru-dcda, gal-dcda, ara-dcda, rib-dcda, xyl-dcda, and man-dcda.
Chemical titration of the oxygen-containing functional groups on glu-dcda was conducted by referring to previous works [49,50]. For the titration of the phenol group, 50 mg of glu-dcda and 1.0 g of BA were added into 10 mL of CHCl3. After continuous stirring under the blanket of N2 at 60 °C for 24 h, the catalyst was centrifuged and washed with anhydrous ethanol to remove the unreacted BA. Then, the phenyl group-blocked catalyst was dried in N2 atmosphere at 60 °C for 24 h and termed as glu-dcda-BA.
For the titration of the carbonyl group, 200 mg of PH and 10 μL of HCl (38%) were dissolved in 10 mL of CHCl3. Then, 50 mg of glu-dcda was added into the solution. After continuous stirring under the blanket of N2 for 72 h at ambient temperature, the catalyst was centrifuged and washed with anhydrous ethanol to remove the unreacted PH. The carbonyl group-blocked catalyst was dried in N2 atmosphere at 60 °C for 24 h and termed as glu-dcda-PH.
The catalyst ink was prepared by mixing 2 mg of catalyst, 300 μL of deionized water, 50 μL of 5% Nafion, and 150 μL of isopropanol under ultrasonication for at least 1 h. Then, 10 μL of the catalyst ink was dripped onto the glassy carbon disk electrode and dried naturally at room temperature. Prior to catalyst coating, the electrode was polished with 0.3 μm corundum aqueous suspension for 5 min and 0.05 μm corundum aqueous suspension for another 5 min and ultrasonicated in deionized water for 30 s.

3.3. Electrocatalytic Testing

An AFMSRCE rotator (PINE Instruments, Durham, NC, USA) and a CHI760E electrochemical workstation (CH Instruments, Bee Cave, TX, USA) were used for electrocatalytic testing by employing the three-electrode configuration. On the rotating ring disk electrode (RRDE), the glassy carbon disk (0.247 cm2) functioned as the working electrode, while the Pt ring (0.186 cm2) was used to collect the H2O2 partial current. The reference electrode and counter electrode were the reversible hydrogen electrode (RHE) and graphite rod, respectively. Linear sweep voltammetry (LSV) was used to assess the catalytic activity and selectivity to H2O2 from 1.0 to 0 V at a scan rate of 10 mV s−1 and a rotation speed of 1600 rpm in a 0.1 M HClO4 electrolyte. O2 was bubbled throughout the reaction. The total current (iD) was measured on the catalyst-loaded disk electrode. On the other hand, the Pt ring electrode was potentiostated at 1.2 V for the oxidation of H2O2 while avoiding the ORR reaction. Hence, the ring current (iR) represents the H2O2 partial current, which enables the in situ quantification of H2O2. The onset potential was defined as the potential at 0.01 mA cm−2 of the H2O2 partial current density j(H2O2) [54]. Details for the calculation of the H2O2 selectivity and the number of electrons transferred (n) have been described in our previous work [38].
Chronoamperometry stability was tested using the three-electrode configuration by fixing the potential on the rotating disk electrode (RDE, 0.196 cm2) at 0.1 V in the O2-saturated 0.1 M HClO4 electrolyte. The rotation speed was 1600 rpm. During the test, a small aliquot was removed at intervals and then complexed with the TiOSO4/H2SO4 reagent. The concentration of H2O2 was photometrically determined on a Shimadzu UV-1800 UV–Vis spectrophotometer (Kyoto, Japan) [55], and the corresponding calibration curve for the quantification of H2O2 is shown in Figure S4.

4. Conclusions

One-step carbonization of seven monosaccharides in the presence of dicyandiamide produced few-layered carbon nanosheets with high specific surface area and high abundance in oxygen and nitrogen species. These ms-dcda materials are all active and stable catalysts for the electroreduction of O2 to H2O2 via the 2e-ORR pathway, among which glu-dcda exhibited the highest activity and selectivity. The highest ECSA and the highest surface C=O and pyrrolic N content may account for the superior catalytic performance of glu-dcda. The facile and biomass-based fabrication strategy and the insight into the active sites for electrocatalytic reduction of O2 to H2O2 shed light on the design of low-cost but efficient carbon-based electrocatalysts. The catalysts pave the way to on-site H2O2; generation in remote or resource-limited settings, such as wastewater treatment plants, medical facilities, or portable disinfection systems. This work provides a promising pathway for advancing electrochemical technologies in the energy and environmental sectors.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal15050459/s1, 1. Characterization; 2. Cyclic voltammetry studies; Table S1: The electrochemical catalytic performance of some carbon-based catalysts in acidic electrolytes [34,35,38,56,57,58,59,60,61,62,63]; Figure S1: TG-DTG curves of a homogeneous mixture of D-glucose and dicyandiamide; Figure S2: The effect of the mass of D-glucose on the catalytic performance of the glu-dcda catalysts. (a) LSV curves and (b) H2O2 selectivity; Figure S3: The effect of the thermal treatment temperature on the catalytic performance of the glu-dcda catalysts. (a) LSV curves and (b) H2O2 selectivity; Figure S4: Calibration curve for the quantification of H2O2 by the colorimetric method

Author Contributions

D.W., investigation, validation, methodology, writing—original draft, formal analysis; Y.L., investigation; K.W., investigation; D.F., investigation; Y.P., methodology, validation; M.Q., conceptualization, formal analysis, supervision, writing—review and editing, funding acquisition; X.Z., methodology, project administration; B.Z., conceptualization, writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the State Key Research and Development Project of China (2021YFA1501404), the National Natural Science Foundation of China (22272030), and the Science and Technology Commission of Shanghai Municipality (2024ZDSYS02).

Data Availability Statement

Data will be made available on request.

Conflicts of Interest

Authors Xiaoxin Zhang and Baoning Zong were employed by the Research Institute of Petroleum Processing, SINOPEC. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Catalysts 15 00459 g001
Figure 1. TEM images of (a,b) glu-dcda, (c,d) fru-dcda, and (e,f) gal-dcda.
Figure 1. TEM images of (a,b) glu-dcda, (c,d) fru-dcda, and (e,f) gal-dcda.
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Figure 2. XRD patterns of the ms-dcda materials.
Figure 2. XRD patterns of the ms-dcda materials.
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Figure 3. EDS mapping of the C, N, and O elements on (a) glu-dcda, (b) fru-dcda, and (c) gal-dcda.
Figure 3. EDS mapping of the C, N, and O elements on (a) glu-dcda, (b) fru-dcda, and (c) gal-dcda.
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Figure 4. (a) N2 adsorption−desorption isotherms and (b) pore size distributions of the ms-dcda materials.
Figure 4. (a) N2 adsorption−desorption isotherms and (b) pore size distributions of the ms-dcda materials.
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Figure 5. Raman spectra of the ms-dcda materials.
Figure 5. Raman spectra of the ms-dcda materials.
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Figure 6. (a) C 1s, (b) N 1s, and (c) O 1s spectra of the ms-dcda materials.
Figure 6. (a) C 1s, (b) N 1s, and (c) O 1s spectra of the ms-dcda materials.
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Figure 7. RRDE ORR results of the ms-dcda catalysts in the O2-saturated 0.1 M HClO4 electrolyte. (a) Linear sweep voltammograms, (b) H2O2 selectivity calculated from the Pt ring current in (a), and (c) electron transfer number. Reaction conditions: 40 µg of catalyst, scan rate of 10 mV s−1, and rotation speed of 1600 rpm. The Pt ring was potentiostated at 1.2 V vs. RHE for H2O2 detection.
Figure 7. RRDE ORR results of the ms-dcda catalysts in the O2-saturated 0.1 M HClO4 electrolyte. (a) Linear sweep voltammograms, (b) H2O2 selectivity calculated from the Pt ring current in (a), and (c) electron transfer number. Reaction conditions: 40 µg of catalyst, scan rate of 10 mV s−1, and rotation speed of 1600 rpm. The Pt ring was potentiostated at 1.2 V vs. RHE for H2O2 detection.
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Figure 8. Tafel slopes of glu-dcda, fru-dcda, and gal-dcda in the O2-saturated 0.1 M HClO4 electrolyte with a scan rate of 10 mV s−1 and a rotation speed of 1600 rpm.
Figure 8. Tafel slopes of glu-dcda, fru-dcda, and gal-dcda in the O2-saturated 0.1 M HClO4 electrolyte with a scan rate of 10 mV s−1 and a rotation speed of 1600 rpm.
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Catalysts 15 00459 g009aCatalysts 15 00459 g009b
Figure 9. (a,c,e,g–j) Stability performance under accelerated durability testing (ADT) conditions in the O2-saturated 0.1 M HClO4 electrolyte, and (b,d,f) capacitance measurements under a N2-saturated 0.1 M HClO4 electrolyte before and after ADT testing. (a,b) glu-dcda, (c,d) fru-dcda, (e,f) gal-dcda, (g) ara-dcda, (h) rib-dcda, (i) xyl-dcda, and (j) man-dcda.
Figure 9. (a,c,e,g–j) Stability performance under accelerated durability testing (ADT) conditions in the O2-saturated 0.1 M HClO4 electrolyte, and (b,d,f) capacitance measurements under a N2-saturated 0.1 M HClO4 electrolyte before and after ADT testing. (a,b) glu-dcda, (c,d) fru-dcda, (e,f) gal-dcda, (g) ara-dcda, (h) rib-dcda, (i) xyl-dcda, and (j) man-dcda.
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Figure 10. The current (left) and photometrically determined amount of H2O2 normalized by the mass of glu-dcda (right) as a function of the reaction time. Reaction conditions: catalyst loading of 200 μg cmgeo−2, rotation speed of the working electrode of 1600 rpm, and the working voltage of 0.1 V vs. RHE. The 0.1 M HClO4 electrolyte was continuously bubbled with O2.
Figure 10. The current (left) and photometrically determined amount of H2O2 normalized by the mass of glu-dcda (right) as a function of the reaction time. Reaction conditions: catalyst loading of 200 μg cmgeo−2, rotation speed of the working electrode of 1600 rpm, and the working voltage of 0.1 V vs. RHE. The 0.1 M HClO4 electrolyte was continuously bubbled with O2.
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Catalysts 15 00459 g011
Figure 11. Cyclic voltammograms in the non-Faradic potential region at varying scan rates of 5, 10, 15, 20, and 25 mV s−1 in the N2-saturated 0.1 M HClO4 electrolyte for the (a) glu-dcda, (b) fru-dcda, and (c) gal-dcda catalysts. (d) The corresponding plots of average current density vs. scan rate.
Figure 11. Cyclic voltammograms in the non-Faradic potential region at varying scan rates of 5, 10, 15, 20, and 25 mV s−1 in the N2-saturated 0.1 M HClO4 electrolyte for the (a) glu-dcda, (b) fru-dcda, and (c) gal-dcda catalysts. (d) The corresponding plots of average current density vs. scan rate.
Catalysts 15 00459 g011
Catalysts 15 00459 g012
Figure 12. RRDE ORR results of glu-dcda before and after chemical titration in the O2-saturated 0.1 M HClO4 electrolyte. (a) Linear sweep voltammograms (LSV), and (b) H2O2 selectivity.
Figure 12. RRDE ORR results of glu-dcda before and after chemical titration in the O2-saturated 0.1 M HClO4 electrolyte. (a) Linear sweep voltammograms (LSV), and (b) H2O2 selectivity.
Catalysts 15 00459 g012
Catalysts 15 00459 g013
Figure 13. Plot of the H2O2 partial current at 0 V vs. RHE as a function of the surface C=O and pyrrolic N content of the ms-dcda catalysts.
Figure 13. Plot of the H2O2 partial current at 0 V vs. RHE as a function of the surface C=O and pyrrolic N content of the ms-dcda catalysts.
Catalysts 15 00459 g013
Table 1. The bulk compositions and textural properties of the ms-dcda materials.
Table 1. The bulk compositions and textural properties of the ms-dcda materials.
SampleBulk Composition (wt %)SBET
(m2 g−1)		Vpore
(cm3 g−1)		dpore
(nm)
C H O N
glu-dcda68.562.5111.5417.394000.525.2
fru-dcda59.912.1318.6819.283250.384.6
gal-dcda66.662.3912.0418.914160.504.8
ara-dcda70.062.428.4619.063170.374.7
rib-dcda69.312.418.8419.443030.364.8
xyl-dcda68.892.3610.7418.013550.444.9
man-dcda69.002.439.7318.843380.344.1
Table 2. The O 1s fitting results of the ms-dcda materials.
Table 2. The O 1s fitting results of the ms-dcda materials.
SampleSurface O (at %)Percentage in Surface O (%)Surface C=O (at %)Surface Pyrrolic N + C=O
(at %)
C=O C–O
glu-dcda4.9353.746.32.656.43
fru-dcda4.6449.150.92.285.11
gal-dcda4.3955.544.52.445.68
ara-dcda4.2144.955.11.895.22
rib-dcda4.1167.232.82.765.80
xyl-dcda3.3853.546.51.815.12
man-dcda2.9966.533.51.995.48
Table 3. The N 1s fitting results of the ms-dcda materials.
Table 3. The N 1s fitting results of the ms-dcda materials.
SampleSurface N (at %)Percentage in Surface N (%)Surface
Pyrrolic N (at %)
Pyridinic NPyrrolic NGraphitic NOxidic N
glu-dcda16.650.922.822.34.03.78
fru-dcda18.347.515.533.04.02.83
gal-dcda16.146.020.130.03.93.24
ara-dcda18.246.018.332.82.93.33
rib-dcda17.748.617.230.33.93.04
xyl-dcda17.146.919.428.55.23.31
man-dcda16.643.321.127.08.63.49
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Wang, D.; Liu, Y.; Wan, K.; Feng, D.; Pei, Y.; Qiao, M.; Zhang, X.; Zong, B. One-Step Carbonization of Monosaccharide and Dicyandiamide to Oxygen and Nitrogen Co-Doped Carbon Nanosheets for Electrocatalytic O2 Reduction to H2O2. Catalysts 2025, 15, 459. https://doi.org/10.3390/catal15050459

AMA Style

Wang D, Liu Y, Wan K, Feng D, Pei Y, Qiao M, Zhang X, Zong B. One-Step Carbonization of Monosaccharide and Dicyandiamide to Oxygen and Nitrogen Co-Doped Carbon Nanosheets for Electrocatalytic O2 Reduction to H2O2. Catalysts. 2025; 15(5):459. https://doi.org/10.3390/catal15050459

Chicago/Turabian Style

Wang, Dan, Yanan Liu, Kun Wan, Danning Feng, Yan Pei, Minghua Qiao, Xiaoxin Zhang, and Baoning Zong. 2025. "One-Step Carbonization of Monosaccharide and Dicyandiamide to Oxygen and Nitrogen Co-Doped Carbon Nanosheets for Electrocatalytic O2 Reduction to H2O2" Catalysts 15, no. 5: 459. https://doi.org/10.3390/catal15050459

APA Style

Wang, D., Liu, Y., Wan, K., Feng, D., Pei, Y., Qiao, M., Zhang, X., & Zong, B. (2025). One-Step Carbonization of Monosaccharide and Dicyandiamide to Oxygen and Nitrogen Co-Doped Carbon Nanosheets for Electrocatalytic O2 Reduction to H2O2. Catalysts, 15(5), 459. https://doi.org/10.3390/catal15050459

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