Electrochemically synthesized H2O2 at industrial-level current densities enabled by in situ fabricated few-layer boron nanosheets

Abstract

Carbon nanomaterials show outstanding promise as electrocatalysts for hydrogen peroxide (H₂O₂) synthesis via the two-electron oxygen reduction reaction. However, carbon-based electrocatalysts that are capable of generating H₂O₂ at industrial-level current densities (>300 mA cm^-2) with high selectivity and long-term stability remain to be discovered. Herein, few-layer boron nanosheets are in-situ introduced into a porous carbon matrix, creating a metal-free electrocatalyst (B_n-C) with H₂O₂ production rates of industrial relevance in neutral or alkaline media. B_n-C maintained > 95% Faradaic efficiency during a 140-hour test at 300 mA cm^-2 and 0.1 V vs. RHE, and delivered a mass activity of 25.1 mol g_catalyst^-1 h^-1 in 1.0 M Na₂SO₄ using a flow cell. Theoretical simulations and experimental studies demonstrate that the superior catalytic performance originates from B atoms with adsorbed O atoms in the boron nanosheets. B_n-C outperforms all metal-based and metal-free carbon catalysts reported to date for H₂O₂ synthesis at industrial-level current densities.

Fig. 1. Synthesis and characterizations of Bn-C.

a Schematic illustration of the fabrication of B_n-C. b The XRD patterns. c Raman spectra. d N₂ adsorption–desorption isotherms (inset, the pore diameter distributions) for B_n-C and control catalysts. e Low-magnification HR-TEM image of B_n-C, scale bar 100 nm. f High-magnification HR-TEM image and the FFT pattern (inset) of B_n-C, scale bar 5 nm. g High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image and boron EDS mapping image (red dots) for B_n-C, scale bar 100 nm.

Fig. 2. Electronic structure and environment of Bn-C.

a–c High-resolution C 1 s, B 1 s and O 1 s XPS spectra. d B K-edge XANES spectra for B_n-C, B₁-C, B powder. e C K-edge XANES spectra for B_n-C, B₁-C and Pure C. f EPR signals of B_n-C and control catalysts.

Fig. 3. Electrocatalytic performance of catalysts: H₂O₂ production in O₂-saturated alkaline and neutral electrolytes.

a–c Electrocatalytic measurements in an O₂-saturated alkaline electrolyte (0.1 M KOH, pH = 13), ORR polarization curves (solid lines) with the corresponding H₂O₂ current on ring electrode (dashed lines) at 1.2 V vs. RHE at 1600 rpm without iR-compensation (the ring current had a collection efficiency of 0.37), the LSV curves were measured three times in Supplementary Fig. 17 (a), H₂O₂ selectivity (b), chronoamperometry stability test for 12 hours at 0.65 V vs. RHE (c). d–e Electrocatalytic measurements in an O₂-saturated neutral electrolyte (0.1 M Na₂SO₄, pH = 7), ORR polarization curves (d), the LSV curves were measured three times in Supplementary Fig. 19. H₂O₂ selectivity (e), chronoamperometry stability test for 12 hours at 0.25 V vs. RHE (the line beak of ring current was due to the repeatedly electrochemical reduction of Pt ring) (f). g Tafel slopes in 0.1 M KOH. h ECSAs normalized against BET SSAs. i Radar plot comparing the 2e^- ORR performance of B_n-C with many previously reported catalysts (Supplementary Table 2).

Fig. 4. H2O2 production tests at high current densities in a flow cell.

a, b LSV curves for 2e^- ORR in 1.0 M KOH and 1.0 M Na₂SO₄ electrolytes with 80% iR-Compensation (0.84 ± 0.09 Ω for 1.0 M KOH and 3.13 ± 0.13 Ω for 1.0 M Na₂SO₄). Corresponding FEs and production rates of H₂O₂ under different current densities are shown. The LSV curves were measured three times in Supplementary Fig. 29. c, d 140-hour stability test for B_n-C at 300 mA cm⁻² in 1.0 M KOH at 0.42 V vs. RHE and in 1.0 M Na₂SO₄ at 0.10 V vs. RHE, respectively. e Performance comparison of catalysts in a flow cell in terms of durability, FE and yield rate (Supplementary Table 4).

Fig. 5. Depolymerization of lignin in black liquid from papermaking.

a Schematic illustration showing depolymerization of lignin in papermaking black liquid using in situ electrochemically synthesized H₂O₂ (inset, the black liquid cesspool, and solution). b The LSV curves with and without black liquid (150 mg L⁻¹) addition in 0.5 M KOH electrolyte with 80% iR-compensation (resistance value is 2.62 ± 0.15 Ω) measured three times (Supplementary Fig. 30). c UV-vis spectra before and after treating black liquid (inset: the photograph of black liquid solution before and after treatment). d Stability test with continuous addition of black liquid.

Fig. 6. Catalytic mechanism for H₂O₂ production process.

a Setup for in situ Raman measurements. b In situ Raman spectra of B_n-C during ORR at the potentials from 0.8 to 0 V vs. RHE. c Fitted Raman spectra of B_n-C with and without a potential of 0 V vs. RHE. d The O₂-TPD profiles for B_n-C, B₁-C and Pure C. e Models of boron nanosheets dispersed on graphene (B_n-C) with one to nine O atoms (gray, pink and red representing C, B and O atoms, respectively). f Free energy diagram of 2e^- ORR (gray, pink and red representing C, B and O atoms, respectively). g ORR simulated activity volcano plot of samples. h Free energy diagram of 2e^-/4e^- ORR on the O₈-B_n-C. i Reaction barrier changes of *OOH intermediate for different number of O atoms. j Catalytic mechanism of O₂ reduction to H₂O₂ on O₈_B_n -C.