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In-plane Carbon Lattice-defect Regulating Electrochemical Oxygen Reduction to Hydrogen Peroxide Production over Nitrogen-doped Graphene Lei Han, Yanyan Sun, Shuang Li, Chong Cheng, Christian E. Halbig, Patrick Feicht, Peter Strasser, Siegfried Eigler, and Jessica Liane Hübner ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03734 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 5, 2019

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ACS Catalysis

In-Plane Carbon Lattice-Defect Regulating Electrochemical Oxygen Reduction to Hydrogen Peroxide Production over Nitrogen-Doped Graphene Lei Han,[a]‡ Yanyan Sun,[b]‡ Shuang Li,[b] Chong Cheng,[a] Christian Halbig,[a] Patrick Feicht,[a] Jessica Liane Hübner,[b] Peter Strasser,[b]* Siegfried Eigler[a]* [a] Institute of Chemistry and Biochemistry, Freie Universität Berlin, Takustrasse 3, 14105 Berlin, Germany [b] Department of Chemistry, Chemical Engineering Division, Technical University of Berlin, 10623 Berlin, Germany ABSTRACT: Carbon-based materials are considered to be active for electrochemical oxygen reduction reaction (ORR) to hydrogen peroxide (H2O2) production. Nevertheless, less attention is paid to the investigation of the influence of in-plane carbon lattice defect on the catalytic activity and selectivity toward ORR. In the present work, graphene precursors were prepared from oxo-functionalized graphene (oxo-G) and graphene oxide (GO) with H2O2 hydrothermal treatment, respectively. Statistical Raman spectroscopy (SRS) analysis demonstrated the increased in-plane carbon lattice defect density in the order of oxo-G, oxo-G/H2O2, GO, GO/H2O2. Furthermore, nitrogen-doped graphene materials were prepared through ammonium hydroxide hydrothermal treatment of those graphene precursors. Rotating ring-disk electrode (RRDE) results indicate that the nitrogen-doped graphene derived from oxo-G with lowest in-plane carbon lattice defects exhibited the highest H2O2 selectivity of >82% in 0.1 M KOH. Moreover, high H2O2 production rate of 224.8 mmol gcatalyst-1 h-1 could be achieved at 0.2 VRHE in H-cell with faradaic efficiency of >43.6%. Our work provides insights for the design and synthesis of carbon-based electrocatalysts for H2O2 production.

KEYWORDS carbon lattice-defect; oxygen reduction; hydrogen peroxide; nitrogen-doped graphene; electrocatalysis; selectivity

The ever-increasing demand of hydrogen peroxide (H2O2) with energy-extensive consumption and adverse environmental impact of the well-developed anthraquinone-based industrial H2O2 production method has stimulated researchers’ interest in the development of facile and environmental friendly route to H2O2 production.1-5 As an alternative method, the direct catalytic H2O2 production from hydrogen and oxygen has been proposed,6-8 but the use of high-cost noble-metal-based catalysts and the potential explosion of the mixture of hydrogen and oxygen made this method unattractive for practical applications. In contrast, electrochemical two-electron oxygen reduction reaction (ORR), which has long been overlooked and is undesirable in fuel cells compared to four-electron ORR, may provide a green and safe route to H2O2 production.2, 918 Nevertheless, the key challenge lies in the development of the low-cost two-electron ORR catalysts with high catalytic activity. Platinum-group-metal (PGM)-based materials are considered to be the best ORR catalysts, but mainly facilitate the four-electron process.19-20 Recent researches demonstrated that the ORR reaction pathway over these PGMbased materials could be tuned from the four-electron to

the two-electron reaction pathway through eliminating accessible PGM ensemble reactive sites using various strategies, including coating amorphous carbon layers on their surfaces,21 alloying with inactive element,2, 22-25 and singleatom dispersion on conductive supports.26-28 Despite some promising results have been achieved, the high-cost, dissatisfactory activity and selectivity of these developed PGM-based materials restricts their practical applications. Unlike these PGM-based materials, carbon materials possess advantages, such as low-cost, facile preparation, and high conductivity. Thus, they have been widely used as conductive supports or additives in various electrochemical energy conversion devices, and were also proposed as promising ORR catalysts.13, 29-34 Moreover, the overpotential for a two-electron reduction of O2 to hydrogen peroxide in alkaline conditions is rather low for most of carbonbased materials. It is necessary to rupture a strong O=O bond for a four-electron reduction of O2, while for twoelectron reduction the O-O bond remains. In particular, graphene as a sheet of a single layer (monolayer) of carbon atoms has received much more attentions due to its unique physicochemical properties since its discovery in 2004.35-39 The typical preparation method of graphene is the chemical reduction of exfoliated graphene oxide (GO) obtained

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by Hummer’s method, which possessed many reactive oxygen-containing groups and abundant in-plane carbon lattice defects due to the over-oxidation of carbon framework during the preparation procedure of GO.40-41 Benefiting from these unique features, distinct graphene-based nanostructured materials, such as heteroatom-doped graphene, have been developed as advanced four-electron ORR catalysts,35, 42-46 probably due to the presence of abundant defects as catalytic reactive sites. Recently, oxo-functionalized graphene (oxo-G) was also creatively prepared by our group through temperature-controlled oxidation of graphite at low temperature,41, 47-50 which was completely different from the previously reported Hummer’s method. During the preparation process, long oxidation time at temperatures below 4 °C was applied to minimize CO2 formation and thus, over-oxidation was avoided. Therefore, this resultant oxo-G bears oxo-addends onto the basal plane, and a preserved hexagonal lattice of carbon atoms. After thermal processing of oxo-G, only in-plane lattice point defects of mainly few-atom vacancies next to an intact hexagonal carbon lattice were identified, and also some pores with few-nm diameter were observed.41, 49 In this context, we investigate the tenability of the ORR reaction pathways with the scope to regulate the selectivity of the graphene-based ORR catalysts toward H2O2 production through tuning the in-plane carbon lattice defect density of the graphene precursor. Accordingly, in the present work, oxo-G and GO were both first mixed with H2O2 followed by hydrothermal treatment (labeled as oxo-G/H2O2 and GO/H2O2), where the addition of the H2O2 was employed to further increase the concentration of in-plane lattice defects within the framework of graphene via an oxidative-etching process,51 as illustrated in Figure 1. Subsequently, starting from the resultant four graphene precursors (oxo-G, oxo-G/H2O2, GO, GO/H2O2) with different inplane lattice defect densities, a series of nitrogen-doped graphene materials were prepared thorough mixing with ammonium hydroxide (25 wt%) followed by hydrothermal treatment. Nitrogen doping has been widely considered to be promising way to improve the ORR performances of carbon-based catalysts because the incorporation of electron-accepting nitrogen atoms induces a relatively high positive charge density on adjacent carbon atoms.52-53 As expected, the distinct selective ORR performances of the resultant nitrogen-doped graphene were observed by means of rotating ring-disk electrode (RRDE) technique. Moreover, H2O2 production was also evaluated in homemade two-chamber H-cell using chronoamperometric method.

Figure 1 Scheme of the preparation procedure of GO precursors with different densities of in-plane lattice defects.

Oxo-G was prepared through the mild oxidation of graphite at low temperature below 4 °C,41, 47 whereas GO

was prepared under the harsh oxidation condition based on the modified Hummer’s method.54 Scanning electron microscopy (SEM) results showed that the resultant oxo-G and GO possessed the same plate structure with similar flake sizes (Figure S1a, c). After the reaction with H2O2 under hydrothermal conditions, wrinkled plate structures were formed for the resultant oxo-G/H2O2 and GO/H2O2 (Figure S1b, d). Statistical Raman spectroscopy (SRS) was further carried out to investigate the influence of H2O2 hydrothermal treatment on the average in-plane carbon lattice defect density of graphene precursors.50, 55-56 Generally, there are two kinds of defects for graphene precursors, including functionalization defects and lattice defects (also called vacancy defects).56 Therefore, prior to the SRS measurement, the samples were prepared through depositing the graphene precursors on a Si/SiO2 wafer with subsequent chemical reduction by the use of hot vapor of hydrogen iodine and trifluoroacetic acid (HI/TFA) in order to exclude the interference from functionalization defects.56 As displayed in Figure 2, the decrease of ID/IG ratio with an increase of the full-width at half-maximum of the 2D band (Γ2D) in the order of oxo-G, oxo-G/H2O2, GO, and GO/H2O2, indicate the successive increased in-plane carbon lattice defect density,50, 56 which may be attributed to the introduction of H2O2 during the hydrothermal process.

Figure 2 Statistical Raman spectroscopy (SRS) of samples deposited on 300 nm SiO2/Si wafer. This plot shows the functionalization of the reduction of oxo-G, oxo-G/H2O2, GO, and GO/H2O2 by hydrogen iodine and trifluoro acetic acid (HI/TFA). г2D: the full-width at half-maximum of the 2D band.

Nitrogen-doped carbon materials have been widely investigated as ORR catalysts in the past decades.9, 12, 31, 38 Here, nitrogen-doped graphene was also prepared by the same ammonium hydroxide hydrothermal treatment of these graphene precursors in order to investigate the influence of different in-plane carbon lattice defects of graphene precursors on the ORR performances of the derived nitrogen-doped graphene. The morphological structures of the resultant N-doped graphene were first characterized by SEM and the results were shown in Figure 3. Wrinkled plate structures were observed for oxo-G/NH3·H2O and oxo-G/H2O2/NH3·H2O, which were different from those of GO/NH3·H2O and GO/H2O2/NH3·H2O with smooth plate

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ACS Catalysis structures. X-ray photoelectron spectroscopy (XPS) analysis (Figure S2) indicates that nitrogen has been successfully introduced into the graphene framework in the form of five different nitrogen doping species including pyridinic-N (398.3 eV), pyrrolic-N (399.9 eV), quaternary-N (400.9 eV), graphitic-N (402.1 eV), and pyridine-N-oxide (404.1 eV).10, 14 In addition, the variation trend in the N content of the resultant catalysts followed the increased order of oxoG/H2O2/NH3·H2O (3.3%), GO/H2O2/NH3·H2O (5.6%), oxoG/NH3·H2O (7.9%), GO/NH3·H2O (8.6%), which was consistent with that from elemental combustions analysis results (Table S1). SRS was also conducted to investigate the change of the density of defects after nitrogen doping including functionalization defects, carbon lattice defects and nitrogen doping defects (Figure S3). The density of defects increased in the order of oxo-G/H2O2/NH3·H2O, oxoG/NH3·H2O, GO/NH3·H2O, and GO/H2O2/NH3·H2O, which may provide the catalytic active sites for the ORR process.

Figure 3 SEM images of oxo-G/NH3·H2O (a), oxoG/H2O2/NH3·H2O (b) and GO/NH3·H2O (c), and GO/H2O2/NH3·H2O (d).

The oxygen reduction activity of the resultant catalysts was first evaluated by cyclic voltammetry (CV) in N2(dashed line) or O2-saturated (solid line) 0.1 M KOH (Figure 4a). As evident, no obvious oxygen reduction peaks were observed in N2-saturated solution, whereas there were well-defined oxygen reduction peaks in O2-saturated solution for all four nitrogen-doped graphene materials, indicating their pronounced catalytic activity toward ORR. Besides, the resultant oxo-G/NH3·H2O exhibited the highest peak current density of 0.403 mA cm-2 at 0.757 VRHE (potential relative to reversible hydrogen electrode (RHE)) whereas the most positive peak potential was achieved over the resultant GO/H2O2/NH3·H2O (0.764 VRHE). The selectivity of the resultant catalysts toward ORR was further examined by RRDE technique using linear sweep voltammetry at the rotating speed of 1600 rpm. It can be observed that the positive shift of the onset potential (defined as the potential at the current density of 0.01 mA cm-2)57 of ORR and the increase of the oxygen reduction current density followed the order of oxo-G/NH3·H2O, oxoG/H2O2/NH3·H2O, GO/H2O2/NH3·H2O, and GO/NH3·H2O. Interestingly, there are two different regions for the ring

current from the oxidation current of H2O2 produced on the disk electrode: the increased trend is observed in the order of oxo-G/NH3·H2O, oxo-G/H2O2/NH3·H2O, GO/NH3·H2O, and GO/H2O2/NH3·H2O above the applied potential of 0.5 VRHE whereas the resultant oxo-G/NH3 exhibited the highest ring current below 0.5 VRHE. However, the resultant oxo-G/NH3·H2O exhibited the highest H2O2 selectivity of more than 82% within the investigated applied potential range, which were comparable to those reported ORR catalysts such as noble-metal-alloy24 and nitrogen-doped or un-doped carbon materials.5, 29, 31, 58

Figure 4 Electrochemical ORR performances of oxoG/NH3·H2O, oxo-G/H2O2/NH3·H2O, GO/NH3·H2O, and GO/H2O2/NH3·H2O: (a) cyclic voltammetries, (b) linear sweep voltammetry performed by a rotating ring-disk electrode (RRDE) technique where the ring current is collected on the Pt ring at a constant potential of 1.2 VRHE, and (c) calculated H2O2 selectivity (%) as a function of electrode potential. Electrochemical measurements were performed in N2-(dashed line) or O2-(solid line) saturated 0.1 M KOH at a scan rate of 5 mV s-1 without (a) and with (b, c) 1600 rpm at room temperature and the catalyst loading amount was set to 0.1 mg cm-2.

In addition, we also treated oxo-G by H2O hydrothermal treatment without H2O2 and ammonium hydroxide (labeled as oxo-G/H2O, Figure S4) as comparison to investigate the influence of carbon lattice defect and nitrogen doping defect on the performance of the catalysts toward H2O2 production. SEM result demonstrated the similar nanoplate structure for all oxo-G-derived materials including oxo-G/H2O, oxo-G/H2O2, oxo-G/NH3·H2O, and oxoG/H2O2/NH3·H2O (Figure 3a-b, Figure S1b and Figure S4). CV results (Figure S6) demonstrated there are the well-defined oxygen reduction peaks for all oxo-G-derived materials, but the peak potential of oxygen reduction shifted positively in the order of oxo-G/H2O (0.655 VRHE), oxo-G/H2O2 (0.659 VRHE), oxo-G/NH3·H2O (0.723 VRHE), and oxoG/H2O2/NH3·H2O (0.762 VRHE). Meanwhile, the same trend of the oxygen reduction current density was observed from RRDE results. Unexpectedly, the other three oxo-G-derived materials exhibited the almost same H2O2 selectivity in the range of 76.3% and 83.6% except for oxoG/H2O2/NH3·H2O with the decrease of H2O2 selectivity. In order to gain further insights into the key factors controlling the ORR activity and selectivity, the electrochemical active surface area (ECSA) was estimated according to

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electrochemical double-layer capacitance (Cdl) based on the positive proportional relationship between ECSA and Cdl.59-60 As presented in Figure S7, the GO/H2O2/NH3·H2O possess larger Cdl of 8.02 mF cm-2 than oxo-G/NH3·H2O (4.45 mF cm-2), oxo-G/H2O2/NH3·H2O (3.54 mF cm-2), and GO/NH3·H2O (7.74 mF cm-2). Interestingly, the observed trend for Cdl was consistent with that of the density of inplane carbon lattice defects of the corresponding graphene precursors without nitrogen doping except for oxo-G/H2O2 (Figure 2). In particular, the resultant GO/H2O2/NH3·H2O with the largest ECSA and highest in-plane carbon lattice defect density did not exhibit the highest ORR activity and H2O2 selectivity, which may be due to the fact that excess in-plane carbon lattice defects lead to reduced conductivity. In contrast, the resultant GO/NH3·H2O with moderate ECSA and in-plane carbon lattice defect density exhibited the highest ORR activity with the lowest H2O2 selectivity, whereas the resultant oxo-G/NH3·H2O with the lowest inplane carbon lattice defect density exhibited the highest H2O2 selectivity. These results implied that the optimization of defect site density of graphene precursors may be vital for regulating the catalytic activity and reaction pathway of the catalysts. In addition, we also have performed inductively coupled plasma optical emission spectrometry (ICP-OES) to detect the content of residual manganese of oxo-G and GO. The results demonstrated that the content of residual manganese of oxo-G and GO are 0.38 wt% and 0.11 wt%, respectively. In the recent reported work,61 the influence of different residual manganese content on the ORR performances was investigated, including 0.056 wt%, 0.036 wt%, 0.0032 wt% and 0.0013 wt%. The reported results showed that the ORR catalytic activity increased with the residual manganese content increased, and the reaction pathway tended to four-electron ORR process, which is different from the present work. In their work, nitrogen doping was achieved through high-temperature annealing treatment whereas in the present work nitrogen doping was achieved through hydrothermal method. This may be why the influence of the residual manganese on the ORR performances is different. Based on the results above, the H2O2 production performance over the optimal oxo-G/NH3·H2O was further evaluated in a two-chamber H-cell using the chronoamperometric method, and the results are shown in Figure 5. As observed, the H2O2 production rate normalized to the catalyst loading amount of 0.1 mg cm-2 varied with the applied potential from 0.3 to 0.1 VRHE. The highest production rate of 224.8 mmol gcatalyst-1 h-1 could be achieved at 0.2 VRHE. Moreover, the H2O2 faradaic efficiency (FE) was also calculated according to the passed charge amount and the charge derived from practical H2O2 production amount, and could exceed 43.6% in the investigated applied potential. These results implied that the resultant oxoG/NH3·H2O is a promising ORR catalyst for H2O2 production in alkaline solution.

Figure 5 (a) H2O2 product amount normalized to catalyst loading amount over the reaction time and (b) H2O2 faradaic efficiency (FE, %) at different applied potentials of 0.1 V, 0.2, and 0.3 VRHE performed in O2-saturated 0.1 M KOH with the catalyst loading amount of 0.1 mg cm-2.

In summary, various graphene precursors with different in-plane carbon lattice defect density have been successfully prepared from oxo-G and GO by H2O2 hydrothermal treatment and analyzed by SRS analysis. As expected, the density of in-plane carbon lattice defects increased in the order of, oxo-G/H2O2, GO, GO/H2O2. After ammonia hydrothermal treatment, the oxo-G-derived nitrogen-doped graphene exhibited the highest H2O2 selectivity of >82%, and high H2O2 production rate of 224.8 mmol gcatalyst-1 h-1 at 0.2 VRHE in a H-cell with faradaic efficiency of >43.6% in 0.1 M KOH. Our work provides new insights for the design and synthesis of carbon-based electrocatalysts for H2O2 production.

ASSOCIATED CONTENT Supporting Information. Additional material synthesis, structural characterization and supporting electrochemical characterization. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected].

Author Contributions ‡These authors contributed equally.

ACKNOWLEDGMENT This work received funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)–392444269. P.S. acknowledges partial financial support by the Bundesministerium für Wirtschaft (BMWi) under grant 0350013A (ChemEFlex).

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ACS Catalysis

Table of Contents

Graphene precursors with the increased in-plane carbon lattice defect density have been prepared from oxo-functionalized graphene (oxo-G) and graphene oxide (GO) with H2O2 hydrothermal treatment, respectively. After ammonium hydroxide hydrothermal treatment, the resultant nitrogen-doped graphene derived from oxo-G with lowest in-plane carbon lattice defects exhibited the highest H2O2 selectivity of >82% in 0.1 M KOH, and high H2O2 production rate of 224.8 mmol gcatalyst-1 h-1 at 0.2 VRHE in H-cell with faradaic efficiency of >43.6%.

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