Constructing Interfacial Boron‐Nitrogen Moieties in Turbostratic Carbon for Electrochemical Hydrogen Peroxide Production

Abstract The electrochemical oxygen reduction reaction (ORR) provides a green route for decentralized H2O2 synthesis, where a structure–selectivity relationship is pivotal for the control of a highly selective and active two‐electron pathway. Here, we report the fabrication of a boron and nitrogen co‐doped turbostratic carbon catalyst with tunable B−N−C configurations (CNB‐ZIL) by the assistance of a zwitterionic liquid (ZIL) for electrochemical hydrogen peroxide production. Combined spectroscopic analysis reveals a fine tailored B−N moiety in CNB‐ZIL, where interfacial B−N species in a homogeneous distribution tend to segregate into hexagonal boron nitride domains at higher pyrolysis temperatures. Based on the experimental observations, a correlation between the interfacial B−N moieties and HO2 − selectivity is established. The CNB‐ZIL electrocatalysts with optimal interfacial B−N moieties exhibit a high HO2 − selectivity with small overpotentials in alkaline media, giving a HO2 − yield of ≈1787 mmol gcatalyst −1 h−1 at −1.4 V in a flow‐cell reactor.


Synthesis of 1-Butyl-3-methylimidazol-2-ylidene borane (ZIL):
The synthesis of zwitterionic liquid, 1butyl-methyl-imidazol-2-ylidene borane, was carried out according to the procedure stated in ref [1]. [1] In a typical synthesis, 16 ml 1-methylimidazole (0.2 mol) and 60 ml dichloromethane were placed in 500 ml round bottom flask, and 27.415 ml iodobutane (0.24 mol, 1.2 eq.) was added with a dropping funnel while the mixture was stirred. The mixture was heated to 50 o C and refluxed overnight. On the next day, the reaction mixture was checked by NMR-spectroscopy until there was no imidazole peak, and then the solvent was removed in vacuum. The crude product (56.94 g) of 1-Butyl-3methylimidazolium iodide (IL-H-I) was used for further reactions.
Then 56.94 g (0.214 mol) of IL-H-I was dispersed in 200 ml toluene. 9.71 g (0.257 mol, 1.2 eq.) sodium borohydride was added to the mixture, and the mixture was heated to 105 o C and stirred overnight. On the following day, the hot toluene phase was decanted. The other phase was washed twice with hot toluene (105 o C) and decanted again. The collected toluene phase was retained for one day until some solid precipitation of residual sodium borohydride appeared, and then filtered. The toluene was removed by rotary evaporation, and the crude product was dissolved in dichloromethane to further precipitate sodium borohydride and other solid impurities. After filtration, the dichloromethane was removed in vacuum. 26.74 g of 1-Butyl-3-methylimidazol-2-ylidene borane (ZIL) product was obtained as a transparent liquid.
Synthesis of CNB-ZILs: ZIL and DCDA (in a weight ratio of 1:40) were mixed by ball milling, and the mixture was heated continuously for 4 h to 600 o C in the inert N2 atmosphere for 2 h. Then it was heated for another 2 h to 800 o C (or 3 h to 900 o C, 4 h to 1000 o C) for 1 h. After cooling down to room temperature, the black free-standing turbostratic carbon product was obtained. All samples are referred to as CNB-ZIL X, where X stands for the preparation temperature. For comparison, pure nitrogen-doped carbon (NC) was also synthesized by heating a mixture of D(-)-Fructose and DCDA (in a weight ratio of 1:40) through the same method.
Structural and electronic property characterization: X-ray diffraction (XRD) measurements were carried out on a Bruker D8 Advance instrument with Cu-Kα radiation. Transmission electron microscopy (TEM) was performed on a Phillips/CM 200 microscope operated at an accelerating voltage of 200 kV. Scanning electron microscopy (SEM) was carried out on FEI Nova NanoSEM 450 instrument. High angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) and energy dispersive X-ray (EDX) mapping were obtained on a spherical aberration corrected transmission electron microscope (JEOL JEM-ARM200F STEM system) which was operated at 200 kV. Electron energy loss (EELS) spectra was observed on a double-Cs-corrected JEOL ARM 200F instrument operated at 200 kV. Fourier transformed infrared (FTIR) spectra were recorded by a Varian 600 FTIR spectrometer. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Thermo ESCALAB 250Xi X-ray Photoelectron Spectrometer. Ultraviolet Photoelectron Spectroscopy (UPS) was collected with a He I (21.2 eV) radiation source. The Fermi level of gold reference was used to calibrate the system. CNB-ZIL samples were used as synthesized as described above and pressed into indium foil for the near-edge X-ray absorption fine structure (NEXAFS) experiment. N K-edge, B K-edge and C K-edge NEXAFS spectroscopy measurements were collected at a 55° angle of incidence in a vacuum chamber with a base pressure of 10 -7 mbar on the soft X-ray beamline at the Australian Synchrotron, ANSTO. [2] The signals were acquired in total electron yield (TEY) mode and the data were processed using the program called QANT. [3] Pore structure properties of the materials were measured via nitrogen adsorption and desorption at 77 K using a volumetric technique on a Quantachrome Quadrasorb SI porosimeter. Prior to analysis, the samples were degassed under vacuum at 150 o C for 20 h.
Electrochemical measurements: The electrochemical measurements were performed in 0.1 M KOH aqueous solution using a three-electrode system on a CHI 760E electrochemical workstation (CH Instruments) assisted with rotation equipment (ALS, Japan) at room temperature. Prior to tests, a homogeneous catalyst ink was prepared by dispersing 5 mg of the catalyst and 25 μL of Nafion solution (5%) into 1 mL ethanol solution, followed by a sonication process for 30 min. The asobtained ink was then drop-cast onto the polished working electrode with a fixed mass loading of 0.125 mg cm -2 . The catalyst-mounted rotating ring-disk electrode (RRDE, glassy carbon, diameter: 4 mm with a Pt ring) was directly used as working electrode for ORR evaluation. Graphite rod and saturated calomel electrode (SCE) were employed as counter and reference electrodes, respectively.
All of the obtained potentials were calibrated to a reversible hydrogen electrode (RHE, ERHE = ESCE + 0.2415 + 0.059 × pH) and all current densities were normalized to a geometric surface area. Linear sweep voltammetry for ORR was conducted at a scan rate of 5 mV· s -1 without any iR correction. The Tafel slope was calculated using the Tafel equation: η = b log (j/jo) (η, b, j and jo represent the overpotential, Tafel slope, current density and exchange current density respectively), and a scan rate of 1 mV s -1 was used to obtain the polarization curves for Tafel plots. Long-term ORR stability tests on CNB-ZIL 8 was carried out under an operating potential of 0.3 V vs. RHE. HO2selectivity (or molar selectivity) of the catalysts was calculated from the current of both disc and ring electrodes using the following equation: where Id is disk current, Ir is ring current, and N is current collection efficiency of the Pt ring. N was determined to be 0.32 In a similar manner, the fresh electrolytes were pumped into the liquid chamber (with a flow rate of 90 mL h -1 ) and flow through the catalyst surface (frontside of the electrode).

Reaction Mechanism: ORR:
It is well established that ORR could proceed through a 2-electron-transfer pathway in alkaline media, that is: O2 + H2O + 2e -→ HO2 -+OH -(at pH > 11.6, U0 O2/HO2-= 0.74 V vs. RHE) Notably, in some other cases, the ORR could also proceed through a 4-electron-transfer pathway rather than a direct 2eprocess as mentioned above in alkaline media, producing the hydroxyl anions as the dominate products: O2 + 2H2O + 4e -→ OOH* + H2O + OH -+ 3e - OOH* + H2O + OH -+ 3e -→ O* + H2O + 2OH -+2e - O* + H2O + 2OH -+2e -→ OH* + 3OH -+e - OH* + 3OH -+ e -→ 4OH - where * represents the surface site. However, this process is not preferred in the electrochemical synthesis of hydrogen peroxide, which could lead to a decreased selectivity toward the desirable peroxide species. Additionally, during the 2e -ORR process, another unfavourable side reaction is the further 2ereduction of HO2product generated on the surface of electrode, which is:   Figure S2a and b. Figure S2c shows a porous nanostructure on the carbon basal plane of CNB-ZIL catalysts. Further performing the HR-TEM measurements evidence a highly disordered turbostratic carbon structure on CNB-ZIL 8 in Figure S2d, which might be ascribed to the homogeneous doping of B and N heteroatoms into the carbon framework that heavily disturbs its graphitic structure.        To eliminate the interference of the structural defects in carbon materials, ex situ and in situ Raman analysis were further conducted on CNB-ZIL catalysts. As shown in Figure S11a, no obvious characteristic peaks belonging to highly crystalized graphitic carbon (e.g. G band at 1597 cm -1 ) or structural defects (e.g. D band at 1347 cm -1 ) can be observed in CNB-ZIL 8. This might be due to the highly isolated distribution of B-N pairs in turbostratic carbon that forms a polymer-like B-N-C network without obvious graphitic carbon structure, as also confirmed by XRD measurement ( Figure   S4). In this regard, no correlation between structural defects and the high 2e -ORR performance on CNB-ZIL 8 can be established. To further exclude the influence of the structural defects on ORR activity of CNB-ZIL 8, in situ Raman characterizations were also performed. It is revealed that no significant changes on Raman spectra ( Figure S11b) can be detected under the ORR conditions (0.7 to 0.2 VRHE), indicating that no emergence of structural defects could be observed during the reaction.
This further precludes the possible involvement of any carbon defects in the 2e -ORR process on CNB-ZIL 8. Notably, higher pyrolysis temperatures lead to the appearance of D and G bands in the Raman spectra of CNB-ZIL 9 and 10 ( Figure S11a), suggesting the emergence of a slightly more crystalized graphitic carbon network with structural defects that gradually forms under high temperatures. However, despite the appearance of possible structural defects, no detectable changes on the full width at half peak maximum (FWHM) of D and D' (at ~1620 cm -1 ) bands in Raman spectra can be observed for CNB-ZIL 9 regardless of applied potentials during ORR, indicating the lack of an interaction between carbon defects with ORR intermediates, which also rules out their possible contribution to the catalytic performance. Thus, based on the Raman measurements above, the possible interference of carbon structural defects on ORR activity can be eliminated.     [17] (I-C60)@SWNT ~0.75 ~78%@0.4 VRHE N/A [18] g-N-CNHs ~0.75 ~20%@0.