Marginal defects modied graphene with S-C-N-C groups for highly selective oxygen reduction to H2O2

hydrogen peroxide (H 2 O 2 ) is crucial for substituting traditional energy-intensive 23 anthraquinone process. Heteroatom-doped carbon materials have shown great potential 24 toward 2e-pathway for catalyzing oxygen reduction to hydrogen peroxide (ORHP). 25 However, conventional nanocarbon electrocatalysts show slow kinetics toward ORHP 26 due to the weak binding strength with OOH* intermediate, resulting reduction of O 2 to 27 H 2 O. Here, sulfur and nitrogen dual-doped graphene (SNC) electrocatalyst consisting 28 of S-C-N-C functional group are synthesized through hydrothermal self-assembly and 29 nitridation processes with thiourea as sulfur source. In S-C-N-C functional group, 30 pentagon-S and pyrrolic-N are covalently grafted onto the edge of graphene and 31 produce marginal carbon ring defects, which provide highly active sites for catalyzing 32 ORHP. The obtained SNC catalysts deliver an outstanding ORHP activity and 33 selectivity for H 2 O 2 production, while retaining remarkable stability. The experimental 34 and computational results reveal that marginal S-C-N-C functional groups afford an 35 appropriate adsorption strength with OOH* intermediate and a low reaction barrier as 36 well, which are essential for the activity of ORHP. 37

hydrogen peroxide (H2O2) is crucial for substituting traditional energy-intensive challenging. 89 Herein, sulfur and nitrogen dual-doped graphene (SNC) with unique marginal To get a further cognition of chemical composition and electronic structure of 160 SNC moieties, Raman spectra, electron paramagnetic resonance (EPR), and X-ray 161 photoelectron spectroscopy (XPS) were conducted. As revealed in Figure 2a, the 162 intensity ratio of D and G band (ID/IG) for SNC (1.147) is clearly higher than that of SC 163 (1.098), NC (1.087), and SSC (1.054), suggesting that thiourea as sulfur source 164 introduces more structural defects than that of sulfur powder. Additionally, increasing detected at the region of S 2p spin-orbit, demonstrating that sulfur in SNC is effectively 224 introduced by thiourea. Specifically, compared to SSC, the S 2p in SNC shift toward to 225 higher binding energy, which can be attributed to the formation of thiophene-S, giving 226 rise to these micro-displacements 23 (Supplementary Figure 15). Moreover, as 227 nitridation temperature increasing, the content of O=S=O species obviously increases 228 accompanied by the gradually decrease of -SO3H groups, indicating that the hydroxy 229 bonded coordinated with sulfur atom could be removed with high-temperature CVD 230 process ( Supplementary Figure 16 and 17). 231   290  288  286  284  282  406 404 402 400 398 396 394  536  534  532  530  172  170  168  166  164  162   500  1000  1500  2000  2500  3000   The emergence of D+G band at 2920 cm -1 affirms the increased structure disorder. b, 236 Isothermal curve of N2 adsorption and desorption, inset is pore diameter distribution 237 diagram. c, EPR spectra of graphene matrixes. d, High revolution of C 1s spectra with 238 peaks deconvoluted into five subpeaks, while heteroatom refers to the C-S, C-N, and 239 C-O bonds in graphene matrixes. e, High resolution N 1s spectra. The SC shows 240 unapparent characteristic peak in N 1s spectra. f, The deconvolution of O 1s spectra. g, 241 S 2p spectra.

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Next, to further unravel coordination features of SNC moieties, so ft X-ray 243 absorption near edge structure (XANES) and solid-state nuclear magnetic resonance 244 (ssNMR) were applied. And commercial thiourea was investigated for comparison. As 245 illustrated in Figure 3a, the peak around 285.5 eV is assigned to unoccupied π* states 246 originated from the out-of-plane bonds in the sp 2 bonding configuration; the signals  Furthermore, 13 C ssNMR was utilized to gain insights into chemical bonding of SNC. As revealed in Figure 3d, SNC, SC and SSC have typical signals for sp 2 carbon 284 (124.3 and 125.9 ppm) 33 , whereas the resulting NC feature is not pronounced and only 285 shows a weak signal of C=O species at 184.9 ppm, indicating that S-doping play a vital 286 role in enhancing the graphitization. Compared with SSC, SNC has an additional sharp 287 peak located at 128.6 ppm, which can be indexed to C=S bonds inherited from thiourea.

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Besides, the peak at 137.5 ppm is ascribed to the -SO3H groups from sulfuric acid,  between π* and σ* states. The sharp peak at 288.0 eV can be assigned to the π* 304 excitation of C-N in thiourea. b, The XANES of N K-edge. The peak at 402.2 is 305 assigned to amino groups in thiourea. c, The XANES of S-K edge, inset is the micro-306 shift for SNC and SSC. The thiourea exhibits a feeble peak of sulphide at 2470.8 eV. d, 307 The 13 C NMR spectroscopy. The feature around 164.0 ppm is assigned to S-C-N 308 configurations from thiourea.  388 Surprisingly, SNC yields the H2O2 selectivity of 98.9% with the n value of 2.02 ( Figure   389 5c), indicating a nearly 2e-ORR process for H2O2 production. Additionally, rGO 390 maintains a high H2O2 selectivity of ~70%, which can be attributed to oxygen 391 containing groups retained after annealing, accelerating 2e-ORR process 9,20 . Although    Table 5). 463 Nevertheless, not all models create adsorption sites for OOH*. Accordingly, the higher overpotential (0.17 eV) for OOH* adsorption (Figure 6c).  sites. Thus, the C-SO2 and C-SO3 groups in SNC is more beneficial to 4e-ORR.

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To verify the pyrrolic-N is favorable for H2O2 production in S-C-N-C groups, we 513 transformed pyrrolic-N to pyridinic-N configuration in Edge 1 and investigate the 514 corresponding change of ΔGOOH* (Supplementary Figure 36). Of note, the Edge 1 515 doped with pyridinic-N transfers from the strongly bind side to weakly bind side, while 516 the UL decreases from 0.45 to 0.33 V, generating a relatively unstable system for OOH* 517 adsorption. This is mainly due to the delocalized lone pair electrons carried by   Conclusions.

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In summary, we have adopted SNC catalyst with S-C-N-C functional groups via  where IR is the ring current, ID is the disk current and N is the collection efficiency.

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The ECSA of the electrode was determined by measuring the Cdl of the catalytic 666 surface, which is derived from the CV cycles (Scan rates were 20, 60, 100, 140, 180, 667 220, 260 and 300 mV s -1 ) measured at the range of 0.86 to 1.06 V vs. RHE where no Simulation Package (VASP 6.2) with the projector augmented wave ( P A W ) 672 pseudopotential method 45 . The exchange-correlation potential was described by 673 Perdew-Burke-Ernzerhof (PBE) functional 46 with generalized gradient approximation 674 (GGA). The cutoff energy for the plane-wave was set to 500 eV. In the geometry 675 optimization, the convergence criteria for the energy and force were 10 -4 eV and 10 -2 676 eV Å -1 , respectively. A large vacuum spacing (more than 15 Å) was taken to prevent Since RHE is taken as the reference, the pH was set to be zero in this calculation. The

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UL is defined as the lowest potential at which all the reaction steps are downhill in free 690 energy.

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Data availability. The data that support the plots within these paper and other findings 692 of this study are available from the corresponding author upon reasonable request.