Covalent Organic Frameworks with Molecular Electronic Modulation as Metal‐Free Electrocatalysts for Efficient Hydrogen Peroxide Production

Tuning the electronic property of active center to balance the adsorption ability and reactivity of oxygen is essential for achieving 2e− oxygen reduction reaction (ORR) for electrocatalytic synthesis of hydrogen peroxide (H2O2), still represents a grand challenge. Herein, different by‐design building blocks are introduced to regulate the electronic structure of catalytically active centers in covalent organic frameworks (COFs). Theoretical calculation reveals that adsorption ability of oxygen molecule (O2) can be finely tuned by the regulation of electronic structure and the binding strength of O2 is positively correlated with the electron donating ability of active center. As a result, the newly designed TP‐TD‐COF shows higher 2e− ORR activity and selectivity owing to the stronger electron donating ability and the higher adsorption strength of O2 on electron‐rich active center. This study reveals fundamental structure–activity relationship in H2O2 synthesis and offers a strategy for designing metal‐free COF catalysts through rational modulation of their electronic properties at molecular level.


Introduction
As a green valuable chemical oxidant, hydrogen peroxide (H 2 O 2 ) is widely used for chemical synthesis, textile bleaching, wastewater treatment, and antisepsis. [1] Presently, the industrial-scale H 2 O 2 production is dominantly manufactured by the energyintensive anthraquinone process, which involves a series of complicated steps, such as hydrogenation, oxidation, extraction, and purification. Although this approach is favorable for the production of highly concentrated H 2 O 2 at the large-scale level, it requires expensive noble metal catalysts, large-scale infrastructure, and generates substantial waste. [2] Therefore, alternative strategies with low cost and energy consumption are desired for the eco-friendly and in-demand H 2 O 2 production. Recently, electrochemical H 2 O 2 production from oxygen reduction reaction (ORR) has been considered an attractive alternative approach, owing to its mild reaction conditions, environmental friendly process, and high-energy conversion efficiency. [3] Despite the electrochemical synthesis of H 2 O 2 shows many potential advantages, the practical process still suffers from great challenges for the reason that O 2 molecule is preferably reduced to H 2 O via the 4e À pathway due to favored thermodynamics for majority of catalysts. [4] Therefore, it is challenging and highly desired to develop electrocatalysts with high 2e À selectivity to achieve this sustainable route.
The vital factor for realizing efficient 2e À ORR catalysts depends substantially on the proper binding strength between the catalytic active sites and oxygen/oxygen intermediates. Too strong binding would easily lead to the dissociation of O 2 molecules toward 4e À path, while too weak binding may result in higher selectivity to H 2 O 2 but lower reactivity at the same time. [5] It is well known that the adsorption behavior of oxygen greatly relies on the surface electronic structure of catalysts. [6] Therefore, Tuning the electronic property of active center to balance the adsorption ability and reactivity of oxygen is essential for achieving 2e À oxygen reduction reaction (ORR) for electrocatalytic synthesis of hydrogen peroxide (H 2 O 2 ), still represents a grand challenge. Herein, different by-design building blocks are introduced to regulate the electronic structure of catalytically active centers in covalent organic frameworks (COFs). Theoretical calculation reveals that adsorption ability of oxygen molecule (O 2 ) can be finely tuned by the regulation of electronic structure and the binding strength of O 2 is positively correlated with the electron donating ability of active center. As a result, the newly designed TP-TD-COF shows higher 2e À ORR activity and selectivity owing to the stronger electron donating ability and the higher adsorption strength of O 2 on electron-rich active center. This study reveals fundamental structure-activity relationship in H 2 O 2 synthesis and offers a strategy for designing metal-free COF catalysts through rational modulation of their electronic properties at molecular level.
the ideal electrocatalyst with flexible tunability in electronic structure is highly desired for simultaneous control of 2e À ORR pathway and high catalytic activity. Up to now, various materials have been investigated as 2e À ORR electrocatalysts such as noble metal/alloy, single atom catalysts, and functional carbon materials (Figure 1). [3aÀd] For example, optimized adsorption ability of oxygen could be achieved through the alloying of multiple components with different electronegativity, such as Pt-Hg or Au-Ni, to tune their electronic structures, which are proved to be favorable for the 2e À pathway with relatively low potential and high selectivity of H 2 O 2 . [7] However, the scarcity and high cost of noble metals hinder their widespread utilization in the future. Likewise, the single atom catalysts have drawn considerable attention for the electrocatalytic reduction of O 2 to H 2 O 2 owing to their high atomic utilization and tunable electronic structure. [6a,8] Nevertheless, the fundamental understanding of the correlation between their electronic structure and activity remains elusive. Moreover, the metal single sites tend to aggregation and be poisoned during the electrocatalysis. Pure carbon materials represent a type of promising metal-free catalysts. Their electronic structures could be easily modulated through the introduction of heterogeneous atoms, which could facilitate the adsorption of oxygen and significantly improve the electrocatalytic 2e À ORR activity. [9] However, the existing problem for carbon materials is that it remains controversial in identifying their active sites and difficult to illustrate structure-property relationship. Taken together, it is highly desired to construct novel 2e À ORR electrocatalysts with flexible tunability in electronic structures and well-defined structure for establishing reliable structure-activity relationship toward high-efficient electrochemical synthesis of H 2 O 2 .
Covalent organic frameworks (COFs), as an emerging class of porous crystalline materials with carbon network-like structure, are made up of lightweight elements and linked by stable covalent bonds. The striking features of COFs are reflected in their molecularly structural tunability and desired permanent porosity. [10] These features will enable the precise manipulation of building blocks within the well-defined framework architectures, offering powerful means for the electronic modulation of active sites. Therefore, COFs, as a privileged class of reticular materials, can integrate their beneficial features to design advanced electrocatalysts with well-defined structure and active sites for electrochemical applications. [11] Yao and co-workers reported the metal-free thiophene-sulfur COFs could catalyze the ORR via 4e À pathway with enhanced activity, making it promising in Zn-air setup. The theoretical calculations suggest that pentacyclic thiophene-sulfur building blocks are active centers with lower over potential. [12] Generally, regulating the local charge distribution of active centers and enlarging dipole moment is conducive to proceeding 2e À ORR among these conjugated organic molecules. For example, the pyrene-based organic molecules were modified by the electron-rich oxygenated group to regulate their electronic structure, thus affording efficient 2e À ORR activity. [13] Therefore, we envision that taking advantage of rational link units could potentially enhance charge distribution and dipole moments of active units in COFs, resulting in enhanced selectivity and electrochemical activity toward 2e À ORR, instead of promoting ORR through 4e À process.
Herein, inspired by the customizable properties of COFs, we reported the modulation of COF's electronic properties at molecular level to develop metal-free electrocatalysts for efficient H 2 O 2 production via selective 2e À ORR process ( Figure 1d). Different Figure 1. a-c) Illustration of the development made previously in designing the electrocatalysts toward 2e À ORR: Pt group catalysts; single atom catalysts and carbon catalysts. d) Illustration of COF electrocatalysts studied in this work with activity-dependent for the potential-limiting step (*O 2 !*OOH), and adjustment of 2e À ORR activity of different COFs by tailoring the electronic structure of active sites with by-design building blocks.
www.advancedsciencenews.com www.small-structures.com trigonal nodes possessing distinctive electron configuration are selected to construct COFs with the ORR electroactive bithiophene linker. Theoretical calculation reveals that the real active sites are carbon atoms adjacent to thiophene-S and their adsorption ability of O 2 molecule can be precisely tuned by the regulation of active site's charge density difference with O 2 through the selection of different building blocks in COFs. The collaborative experimental and theoretical evidence proved that adsorption energy of O 2 molecule on the real active sites is positively correlated with 2e À ORR performance ( Figure 1d). The best screened TP-TD-COF delivers a H 2 O 2 production rate of 158 mmol g À1 catalyst h À1 with satisfying stability and selectivity. Moreover, it shows an outstanding capability for the dye degradation. This work reveals the structure-activity relationship in the 2e À ORR process. More importantly, it paves a way for the design of efficient metal-free 2e À ORR COF catalyst using by-design functional building blocks.
To characterize the chemical structure and composition information of TB-TD-COF, Fourier transform infrared (FT-IR) spectra, 13 C NMR spectroscopy, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), energy-dispersive X-ray (EDX) element mapping, scanning electron microscopy (SEM), N 2 adsorption-desorption measurement, and UV-vis spectroscopy absorption were performed. As shown in FT-IR spectra ( Figure S2, Supporting Information), the stretching bands located at 1610 cm À1 for TB-TD-COF in FT-IR spectra are ascribed to the C═N bonds. Expectedly, the bands of N─H and C═O disappeared in COFs. The 13 C NMR spectroscopy revealed the typical characteristic peaks at 148.9 ppm, assigned to C atoms in the C═N bond ( Figure S3, Supporting Information). Furthermore, the C, N, and S elements are reflected in the XPS spectra of these COFs. For the C 1s spectra, the TB-TD-COF exhibits three typical satellite peaks appearing at the binding energy range from 282.5 to 290.4 eV, which are assigned to sp 2 C═C, C═N (Imine)/C─N, and C─S bonds ( Figure S4, Supporting Information). The N 1s spectra confirms the appearance of C═N linkage signal at around 398.7 eV ( Figure S5, Supporting Information). The S spectra exhibit the characteristic peaks of S 2p 3/2 and S 2p 1/2 at approximately 163.5 and 164.8 eV, which are ascribed to thiophene units ( Figure S5, Supporting Information). All the characterization data indicate the starting materials have a complete conversion into an imine-linked structure. TGA measurement was used to evaluate the thermal stability of TB-TD-COF under N 2 atmosphere. The result indicates that these COFs have excellent stability with thermal decomposition temperatures above 400°C ( Figure S6, Supporting Information). SEM and TEM image reveal that TB-TD-COF shows the irregular particle ( Figure S7 and S8, Supporting Information). The EDX and element mapping reveal that C, N, and S are uniformly distributed in TB-TD-COF ( Figure S9 and S10, Supporting Information), suggesting the high elemental homogeneity. The N 2 adsorptiondesorption isotherm study shows that TB-TD-COF has a Brunauer-Emmett-Teller (BET) surface area of 630.40 m 2 g À1 ( Figure S11, Supporting Information). The bandgap was calculated to be 2.21 eV for TB-TD-COF according to UV-vis absorption ( Figure S12, Supporting Information). The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) are also calculated ( Figure S13 and S14, Supporting Information). The HUMO positions for TB-TD-COF were À6.13 eV.

Electrocatalytic ORR Study of TB-TD-COF
To evaluate the ORR catalytic performance of the prepared COFs, the electrochemical measurements were performed in 0.1 M KOH electrolyte through a RRDE system. Before the measurement, the collect efficiency (n) for the electrodes were calibrated by the redox reaction of [Fe(CN 6 )] 4À /[Fe(CN 6 )] 3À ( Figure S15, Supporting Information). First, the linear sweep voltammetry (LSV) plots for different COFs were obtained in O 2 saturated solution with a scan rate of 10 mV s À1 at the 1600 rpm. As shown in Figure 2a, TB-TD-COF shows comparable current on the disk and ring electrode, revealing the presence of H 2 O 2 production. Remarkably, the TP-TD COF shows a half-wave potential of 0.55 V and an onset potential of 0.71 V. The selectivity and transfer-electron number were calculated to evaluate the catalytic efficiency (Figure 2b). TB-TD-COF shows H 2 O 2 selectivity of 50.9-67.4% and the average transfer-electron number increased to 2.86, indicating favorable 2e À path on TB-TD-COF during ORR process.
To understand ORR mechanism and catalytic activities on the TB-TD-COF, DFT calculations were performed. The calculated free-energy diagrams of the 2e À and 4e À ORR are presented in Figure 2c,d and S16, Supporting Information. It is well known that the intrinsic activity of the electrocatalysts can be determined by overpotential. The potential-determining step is oxygen protonation to *OOH with the overpotential value of 0.40 V (TB-TD-COF) for H 2 O 2 synthesis, while corresponding overpotential value toward H 2 O formation is 0.93 V. Obviously, the ORR occurring on TB-TD-COF prefers to proceed through the 2e À mechanism compared with 4e À mechanism, which is in line well with the experimental result. It is accepted that the adsorption ability of O 2 molecule in active sites is crucial for the ORR catalytic performance. Therefore, possible binding sites around thiophene-S structure were selected to determine their adsorption ability for O 2 . For TB-TD-COF, the adsorption energy of O 2 molecule on site 1, site 2, and site 3 is 0.31, À0.57, and 0.22 eV ( Figure S17, Supporting Information), respectively. The reported JUC-528 with promising ORR activity was then selected as a comparison because JUC-528 and TB-TD-COF are similar with each other except their different thiophene units. [12] JUC-528 shows the adsorption energy of À0.21, À0.43, À0.31, À0.50, and 0.58 eV on site 1, site 2, site 3, site 4, and site 5 (all possible binding sites around thiophene-S, Figure S18, Supporting Information), respectively. Notably, JUC-528 and TB-TD-COF have distinct adsorption characteristics of O 2 molecule. More importantly, it was found that site 2 has the lowest adsorption energy of O 2 molecule among three sites in Figure 2. a) The LSV plot of TB-TD-COF, and b) corresponding the H 2 O 2 selectivity and electron-transfer number. c) Calculated Gibbs free energy diagram of 2e À ORR and d) 4e À ORR. e) The optimized adsorption sites of O 2 on the site 2 of TB-TD-COF and f ) on the site 4 of JUC-528. g,h) The corresponding charge density difference (CDD). The isosurface value was taken to be 0.005 e Å À3 . The charge depletion and accumulation are displayed in green and purple, respectively. www.advancedsciencenews.com www.small-structures.com TB-TD-COF, which is À0.07 eV lower than that of site 4 with the lowest adsorption energy in JUC-528 (Figure 2e,f ). It suggests that TB-TD-COF has higher activity toward O 2 molecule than JUC-528. As the O 2 absorption behavior mainly depends on the geometric and electronic structure of active sites, the corresponding sites were also chosen as active sites for O 2 molecules to analyze the electronic environment of different COFs. Expectedly, the surface charge density difference of active site with O 2 in TB-TD-COF is 0.90 e À , which is 0.06 e À richer than that of JUC-528 (Figure 2g,h). As previous works indicate, richer charge density difference of active site with O 2 on the active site is conducive to accelerating electron transfer from catalyst to O 2 molecule, which may not only benefit the adsorption of O 2 , but also result in easier subsequent activation of adsorbed O 2 molecules. [16]

Design of New COFs with Distinctive Electronic Structures
Combined with the above analysis of calculation results, it is anticipated that the enrichment of the charge density on the active site would boost adsorption of O 2 , thus enhancing the catalytic performance of catalysts. Therefore, the choice of building blocks together with bithiophene unit is feasible to obtain customizable COFs with variable electronic structures. Typical trigonal nodes (tris(4-aminophenyl)amine (TP) and 4,4 0 ,4 00 -(1,3,5-triazine-2,4,6-triyl)trianiline (TT)) were selected to exert electronic regulation over the active site TD. Their calculated configuration, molecular orbital levels, and electrostatic potential surface (EPS) map are presented in Table 1 and Figure S19 and S20, Supporting Information, showing their distinctive geometric and electronic structures. Moreover, dipole moment, as a quantitative indicator of symmetry and charge distribution, was also calculated for all monomers. The gradient values of their dipole moments, TP (3.12 D), TT (2.03 D), and TB (1.07 D), also indicate that the choice of these building blocks is appropriate to some extent. The theoretical calculations were first performed on the repetitive unit models of different COFs constructed by TD and those selected trigonal nodes to reveal the oxygen adsorption behavior on their possible active sites. The model of TP-TD COF, the adsorption energy of O 2 molecule on site 1, site 2 and site 3 are À0.32, À0.74, and À0.52 eV ( Figure S21, Supporting Information), respectively, and corresponding values in the model of TT-TD-COF are À0.26 eV (site 1), À0.62 eV (site 2), and 0.60 eV (site 3) ( Figure S22, Supporting Information). It was observed that TP-TD COF and TT-TD-COF have a smaller adsorption energy than TB-TD-COF on site 2, indicating the higher activity toward O 2 molecule. Expectedly, the following analysis reveals that the charge density difference with O 2 in TP-TD-COF and TT-TD-COF is 0.98 e À and 0.94 e À on corresponding site 2, which is 0.4 e À and 0.8 e À higher than that of TB-TD-COF. This result is consistent with experimental HOMO/LOMO analysis in Figure S33-S36, Supporting Information, suggesting TP-TD-COF and TT-TD-COF have a stronger electron donating ability toward O 2 . Moreover, this evidence suggests the enhancement of O 2 adsorption energy for TP-TD-COF and TT-TD-COF, thus resulting in the possible enhancement of their 2e À ORR activity.

Synthesis and Electrocatalytic Activity of TP-TD-COF and TT-TD-COF
To experimentally investigate the electrocatalytic 2e À ORR activity of these COFs, TP-TD-COF and TT-TD-COF were synthesized. [17] Various characterizations, including PXRD, TEM and corresponding element mapping, EDX, FT-IR, 13 C NMR spectroscopy, XPS spectra, UV-vis spectroscopy absorption, N 2 adsorption-desorption measurement, TGA, and element analysis were utilized to understand the chemical structures Table 1. The calculated molecular configuration, electrostatic potential surface maps (ESP) and dipole moments, O 2 adsorption energy, and charge density difference with O 2 of TB-TD-COF, TT-TD-COF, and TP-TD-COF. and composition information of TP-TD-COF and TT-TD-COF (see the detailed discussions in Figure S23-S44 and Table S1, Supporting Information). As shown in Figure 3a, TP-TD-COF, TT-TD-COF, and TB-TD-COF show obvious differences in the disk and ring current, indicating the distinct 2e À ORR performance. Remarkably, the TP-TD-COF shows a half-wave potential of 0.62 V and an onset potential of 0.76 V, which are higher than that of TT-TD-COF (0.60 and 0.74 V) and TB-TD-COF (0.55 and 0.71 V) (Figure 3b). Additionally, the highest H 2 O 2 oxidation current on the ring electrode was also observed on the TP-TD-COF. Moreover, it is worth noting that the TP-TD-COF shows the highest production turnover frequency (TOF, 4.0 s À1 ) and kinetic current density ( J K , 5.96 mA cm À2 ) among these COFs at 0.55 V ( Figure S45, Supporting Information). Remarkably, these results indicate that TP-TD-COF has the best activity toward 2e À ORR among them, which is further evidenced by the result of cyclic voltammograms test ( Figure S46, Supporting Information). Furthermore, the selectivity and transfer-electron number were calculated based on the produced ring and disk current (Figure 3c,d). For the TP-TD-COF, the selectivity of H 2 O 2 is determined to be 81.9-86.2% with an average transfer-electron number of 2.24 in a wide potential ranging from 0.2 to 0.6 V, suggesting that it mainly undergoes the 2e À pathway during ORR process. TT-TD-COF shows selectivity of 68.9-81.8% and average transfer-electron number of 2.52, correspondingly. Apparently, there is significantly enhancement to selectivity toward H 2 O 2 for TP-TD-COF and TT-TD-COF compared to the TB-TD-COF (selectivity: 50.9-67.4%; average transferelectron number: 2.86). The results of electrochemical studies clearly revealed that the activity of these COFs is positively correlates well with the charge density of active center. Moreover, electrochemical impedance spectroscopy (EIS) measurement was conducted. As shown in Figure S47, Supporting Information, the TP-TD-COF has a smaller semicircle than that of TT-TD-COF and TB-TD-COF from the Nyquist plots, indicating easier electron transfer from the surface of catalyst to O 2 molecule. The stability test of TP-TD-COF was conducted by chronoamperometry test at 0.55 V for 10 000 s. It can be observed that there is no obvious decay in the ring current and disk current during the whole time ( Figure 3e). Additionally, the selectivity and transfer electron number were determined to be around 85.0% and 2.21 after 10 000 s ( Figure S48, Supporting Information). The ring and disk density of TP-TD-COF underwent only small changes when the time was extended to 18 h ( Figure S49, Supporting Information). Moreover, the current density of TP-TD-COF and chemical structure shows slight changes after the long-term stability test for 20 h ( Figure S50 and S51, Supporting Information). Furthermore, the TP-TD-COF shows only slight degradation of activity after continuous 1000 cycles (Figure 3f and S52, Supporting Information). These results confirm that TP-TD-COF displays enhanced activity, selectivity, and stability toward 2e À ORR.   0.83 V ( Figure S55, Supporting Information). The result well uncovers that the ORR catalyzed by TP-TD-COF and TT-TD-COF prefers to proceed through the 2e À mechanism compared with 4e À mechanism, which is in accordance with the TB-TP-COF (Figure 4c). More importantly, the overpotential values of TP-TD-COF and TT-TD-COF are much lower than that of TB-TD-COF (0.4 V), thus showing their higher 2e À ORR activity. In this step, the optimized *OOH intermediates are   all end-on configurations on surface of three catalysts, which leads to the maintenance of the unbroken O─O bonding in the intermediate species ( Figure S56, Supporting Information). The maintenance of *OOH on the catalyst would be favorable to the formation of H 2 O 2 , thereby resulting in the selectivity toward H 2 O 2 , which is in good agreement with the observed experiment result. The enhanced activity and selectivity toward H 2 O 2 formation on TP-TD-COF can be attributed to its synergistic geometric and higher charge density difference of its active centers with O 2 based on the above studies ( Figure 4d). Therefore, combining with the theoretical and experimental analysis, it can be concluded that by regulating the electronic structures of electroactive unit with suitable building blocks, we can obtain COFs with efficient performance toward 2e À ORR.

Dye Degradation by Electrocatalytic Production of H 2 O 2
In view of the above interesting findings, a series of reactions were performed by combing ORR with dye degradation. As shown in Figure 5a, the prepared TP-TD-COF catalyst was supported on the carbon paper, and then equipped with in an H-cell electrolyzer filled with 0.1 M O 2 saturated KOH. The LSV plot in O 2 saturated 0.1 KOH showed a much larger current density than that of the collected current density in the Ar-saturated 0.1 KOH (Figure 5b), indicating excellent oxygen reduction capability of TP-TD-COF. Subsequently, the potential was applied at 0.55 V for 10 000 s to obtain the H 2 O 2 product (Figure 5c). The concentration of generated H 2 O 2 was 158 mmol g À1 catalyst h À1 based on colorimetric quantification method using cerium sulfate titration ( Figure S57, Supporting Information). It is well known that the Fenton reaction is widely used to treat pollutant, in which hydroxyl radicals would generate by the reaction between Fe 2þ and H 2 O 2 and thus can react with pollutant. Given the excellent 2e À ORR electrocatalytic activity of TP-TD-COF, we further used the generated H 2 O 2 to degrade the organic dyes (methylene blue [MB]), (the detailed experiment was provided in experimental part). From the optical image, we can observe that the MB was fleetly faded from the color to colorless within a few minutes ( Figure 5d). Meanwhile, the UV-vis absorption spectra of MB with a concentration of 10 ppm showed a great degrease in corresponding absorption peaks within 2 min. Until to the 5 min, no obvious adsorption peaks were appeared ( Figure 5d). Likewise, methyl orange (MO) and rhodamine B (RB) could degrade through Fenton reaction ( Figure S58, Supporting Information). Combing with the experiment of degradation of organic dyes, we further confirmed that TP-TD-COF can be used as promising electrocatalyst to produce H 2 O 2 , which could simultaneously be applied to effectively degrade various organic pollutant in waste water through Fenton reaction.

Conclusion
In summary, we have designed and synthesized a serial of metal-free COF electrocatalysts with well-defined structures and controllable active sites toward 2e À ORR. On the basis of DFT calculations and experimental results, the selectivity and activity of these COFs toward ORR are predominately determined by the binding strength of O 2 . The binding strength of O 2 is positively correlated with the electron distribution of active center, which could be modulated by the rational design of trigonal node in engineering COF's structure, Consequently, the by-design TP-TD-COF with the stronger electron donating ability on the electron-rich active center shows the highest adsorption strength of O 2 , thus resulting in more effective 2e À ORR activity and selectivity. Furthermore, the enhanced H 2 O 2 production provided by the TP-TD-COF electrode effectively facilitates the Fenton process and thus rapidly degrades the organic dyes like MB, MO, and RB. This work illustrates fundamental structureactivity relationship during 2e À ORR process in virtue of COF's engineering and paves a strategy for designing metal-free COF catalysts through rational design of building blocks.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.