Efficient Electrochemical Hydrogen Peroxide Production from

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Efficient electrochemical hydrogen peroxide production from molecular oxygen on nitrogen-doped mesoporous carbon catalysts Yanyan Sun, Ilya Sinev, Wen Ju, Arno Bergmann, Soeren Dresp, Stefanie Kühl, Camillo Spoeri, Henrike Schmies, Huan Wang, Denis Bernsmeier, Benjamin Paul, Roman Schmack, Ralph Kraehnert, Beatriz Roldan Cuenya, and Peter Strasser ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03464 • Publication Date (Web): 21 Feb 2018 Downloaded from http://pubs.acs.org on February 22, 2018

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Efficient Electrochemical Hydrogen Peroxide Production from Molecular Oxygen on Nitrogen-Doped Mesoporous Carbon Catalysts

Yanyan Sun1, Ilya Sinev2, Wen Ju1 , Arno Bergmann1, Sören Dresp1, Stefanie Kühl1, Camillo Spöri1, Henrike Schmies1, Huan Wang1, Denis Bernsmeier1, Benjamin Paul1, Roman Schmack1, Ralph Kraehnert1, Beatriz Roldan Cuenya2,3, Peter Strasser1*

1

Department of Chemistry, Chemical Engineering Division, Technical University of Berlin, 10623 Berlin, Germany 2

3

Department of Physics, Ruhr-University Bochum, 44780 Bochum, Germany

Department of Interface Science, Fritz-Haber Institute of the Max-Planck Society, 14195 Berlin, Germany

*Email: [email protected]

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ABSTRACT: Electrochemical hydrogen peroxide (H2O2) production by two-electron oxygen reduction is a promising alternative process to the established industrial anthraquinone

process.

Current

challenges

relate

to finding

cost-effective

electrocatalysts with high electrocatalytic activity, stability and product selectivity. Here, we explore the electrocatalytic activity and selectivity toward H2O2 production of a number of distinct nitrogen-doped mesoporous carbon catalysts and report a previously unachieved H2O2 selectivity of ~95-98 % in acidic solution. To explain our observations, we correlate their structural, compositional and other physico-chemical properties with their electrocatalytic performance and uncover a close correlation between the H2O2 product yield and the surface area and interfacial Zeta potential. Nitrogen doping was found to sharply boost H2O2 activity and selectivity. Chronoamperometric H2O2 electrolysis confirms the exceptionally high H2O2 production rate and large H2O2 faradaic selectivity for the optimal nitrogen-doped CMK-3 sample in acidic, neutral and alkaline solutions. In alkaline solution, the catalytic H2O2 yield increases further, where production rates of the HO2- anion reaches a value as high as 561.7 mmol h-1 g-1catalyst with H2O2 faradaic selectivity above 55%. Our work provides a guide for the design, synthesis, and mechanistic investigation of advanced carbon-based electrocatalysts for H2O2 production.

KEYWORDS: nitrogen-doped carbon, two-electron oxygen reduction, hydrogen peroxide production, selectivity, electrocatalysis


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1. Introduction Hydrogen peroxide (H2O2) is worldwide highly sought as important chemical feedstock for industries and environmental treatments,1-2 as well as possible promising energy carrier and oxidant in renewable energy conversion technologies.3-5 Currently, the industrial synthesis route to H2O2 is based on the anthraquinone process involving the sequential hydrogenation/oxidation of anthraquinone molecules.2 Despite its state-of-the-art large-scale production, there still exist serious practical sustainability challenges such as non-energetic-efficiency, industrial waste handling as well as high-cost associated with storage and transportation. The other alternative method is the direct catalytic H2O2 production from molecular oxygen and hydrogen.6-9 However, during the process, the potential explosion hazard from the mixtures of oxygen and hydrogen and the use of platinum-group noble-metal catalysts make the success of this method doubtful. This is why new facile, efficient and eco-friendly routes to H2O2 production are needed. The four-electron electrochemical reduction of molecular oxygen (oxygen reduction reaction, ORR), proceeding in acidic environments according to equation 1, O 2 + 4H + + 4e - → 2H 2 O

E 0 =+1.229V vs SHE

(1)

has gained tremendous attention due to its key role in various energy conversion/storage systems such as fuel cells and metal-air batteries.10-12 Currently, most of researches are focused on the development of improved catalysts for the four-electron pathway from oxygen to water in order to maximize the energy conversion efficiency of the devices. However, the energetically undesirable


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two-electron oxygen reduction pathway (equation 2) O 2 + 2H + + 2e- → H 2 O 2

E 0 =+0.695V vs SHE

(2)

generates hydrogen peroxide and is an interesting pathway for chemical synthesis purposes. In an aqueous electrolytic device, by combining the anodic half-cell reaction (equation 3), 2H 2 O → O 2 + 4H + + 4e-

E 0 =+1.229V vs SHE

(3)

the overall H2O2 electrolysis cell reaction proceeds according to equation 4

O2 + 2H 2O → 2H 2O2 E 0cell =0.534V

(4)

This reaction makes the in-situ H2O2 production from renewable power sources possible, and the scalability of electrochemical devices enables local, on-demand H2O2 production reducing costs associated with storage and transportation. Besides, if the reaction (equation 2) is performed in a fuel cell, it is possible to recover the energy released accompanied by the production of H2O2. Based on the discussions above, electrochemical two-electron oxygen reduction provides a promising method for H2O2 production, with its key challenge being the design and synthesis of oxygen electrocatalysts displaying high activity and selectivity. Recently, oxygen reduction electrocatalysts based on noble-metal (eg. Pt, Pd and Au) have been explored for H2O2 production.13-16 For example, Rossmeisl’s group employed density functional theory (DFT) calculations to screen a series of oxygen reduction electrocatalysts for H2O2 production based on metal alloys including Pt-Hg, Pd-Hg and Ag-Hg.1, 17-18 In their work, the introduction of catalytically inactive Hg aimed to isolate the active sites of reactive metals like Pt and Pd, thus resulting in the


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optimal binding energy of HOO* (adsorbed intermediate for H2O2 production) and achieving high selectivity towards H2O2 production. Choi et al. proposed that the configuration of O2 adsorption on the surface of catalysts including “side-on” and “end-on” determined the selectivity of the catalysts. From this point, they coated amorphous carbon layers on the surface of Pt to induce selective end-on adsorption of O2 by eliminating accessible Pt ensemble sites to further suppress the four-electron pathway.15 Compared to these noble-metal materials reported above, carbon materials are more promising alternative electrocatalysts for H2O2 production due to its high abundance, low cost and acceptable stability under mass transport limited operating conditions.19-21 Indeed, Berl et al. has first proposed the H2O2 production from oxygen reduction on carbon electrode in 1939.22 Since then, various carbon materials such as carbon fibers and graphite have been reported to be active for H2O2 production.23-24 In particular, mesoporous carbon offers advantages over other carbon materials thanks to high surface area and favorable mass transport.25 The catalytic activity and selectivity of mesoporous carbon with respect to the oxygen reduction were reported to be further improved upon heteroatom doping.25-26 In particular, nitrogen doping was reported as an effective method for improving the electrochemical performance of carbon materials.27-29 However, prior work of ORR on N-doped carbons focused largely on optimizing the catalyst synthesis, whereas reports dedicated to the systematic exploration of the correlations between H2O2 reactivity, selectivity and surface physicochemical properties have been missing. This report attempts to change


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that. Here, we explore the electrocatalytic activity and selectivity toward H2O2 production of a number of distinct nitrogen-free and nitrogen-doped mesoporous carbon catalysts and report a previously unachieved H2O2 selectivity of ~95-98 % in acidic solution. To explain our observations, we correlate their structural, compositional and other physicochemical properties with their electrocatalytic performance and uncover a close correlation between the H2O2 product yield and the surface area and interfacial Zeta potential. Balanced amount of nitrogen doping was found to sharply boost H2O2 activity and selectivity. Chronoamperometric H2O2 electrolysis confirms the exceptionally high H2O2 production rate and large H2O2 faradaic selectivity for the optimal nitrogen-doped CMK-3 sample in acidic, neutral and alkaline solutions. In alkaline solution, the catalytic H2O2 yield increases further, where production rates of the HO2- anion reaches a value as high as 561.7 mmol h-1 g-1catalyst with H2O2 faradaic selectivity above 70%. Our work provides a guide for the design, synthesis, and mechanistic

investigation

of

advanced

carbon-based

electrocatalysts

for

H2O2 production.

2. Experimental 2.1 Chemicals Ketjen black EC-300J and Ketjen black EC-600JD were supplied by AkzoNobel polymer chemistry. Black pearls 2000 and Vulcan XC 72R were obtained from Cabot. Graphene nanoplates (2-10 nm) were purchased from ACS Material. CMK-3 was obtained from East High Tech Limited. 1-ethyl-3-methylimidazolium dicyanamide


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(EMIM-dca, a kind of ionic liquid) was purchased from Sigma-Aldrich. Hydrogen peroxide test kits were acquired from Merck. All chemicals were used without further treatment. 2.2 Synthesis of nitrogen-doped mesoporous carbon Prior to nitrogen doping, CMK-3 was firstly pretreated in 2.4 M HNO3 overnight to modify its surface area and the porosity,30 and then dried at 100 °C. In a typical procedure for nitrogen doping, 30 mg acid-treated CMK-3 were dispersed in 5 mL of deionized water by ultrasound for about 15 min, and then mixed with different amount of ionic liquid (IL) (EMIM-dca, 25 µl, 50 µl, 100 µl) overnight under magnetic stirring for homogenization. Subsequently, the resulting mixture was firstly heated at 180 °C for 2 h and then heated to different annealing temperature (600 °C, 700 °C, 800 °C and 900 °C) for 4 h at the rate of 5 °C/min in a porcelain boat under a continuous flow of nitrogen gas. To clarify, the resultant N-doped CMK-3 samples were labeled as CMK3ILX_YT (with X the IL in µL and Y is annealing temperature in °C). 2.3 Characterization Powder X-ray diffraction (PXRD) patterns were recorded on a Bruker D8 Advance instrument with Cu Kα radiation (λ = 1.54056 Å) in the 2θ range of 10° to 90°. The surface area was determined by nitrogen adsorption measurements (Quantachrome Autosorb-1-C). The morphologies of the catalysts were investigated with a scanning electron microscope (SEM, JEOL 7401F) at an acceleration voltage of 10 kV and transmission electron microscopy (TEM, FEI Tecnai G220 S-TWIN) at an


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accelerating voltage of 200 kV. Cu grids with a lacey carbon layer and 300 mesh were used for preparation. X-ray photoelectron spectroscopy (XPS) were conducted on an ultra-high vacuum (UHV) system, equipped with a monochromatic Al Kα source (hν = 1486.5 eV) operated at 14.5 kV and 300 W, and Phoibos 150 (SPECS GmbH) analyzer. For each sample a survey and high-resolution C 1s, O 1s and N 1s regions were measured. The C 1s signal of graphitic-like carbon was used for binding energy calibration and assigned to 285 eV. The Casa XPS software with pseudo-Voight Gaussian-Lorentzian product functions and Shirley background was used for peak deconvolution. Atomic ratios were calculated from XPS intensities corrected by the corresponding sensitivity factors provided by the manufacturing company (SPECS). Raman spectra measurement was recorded on a Bruker SENTERRA Raman system. Zeta potential was determined by Zetasizer Nano instrument (Malvern, Nano Z). The solutions for Zeta potential measurement were prepared by dissolving catalysts into Milli-Q water. Thermo Gravimetric Analysis (TGA) was used to investigate the thermal behavior of CMK3, IL and NCMK3IL50_800T using a thermal simultaneous thermal analyzer (Perkin Elmer STA 8000). All the samples as well as the empty crucible were measured as the following the subsequent procedure: with the nitrogen gas flow at 100 mL /min, firstly hold for 60 min at 30 °C and then heat from 30 °C to 900 °C at 5 °C/min, at 900 °C hold for 240 min. Finally let it cool down naturally and the weight loss was recorded. 2.4 Electrochemical measurement All electrochemical measurements were conducted in a home-made three


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compartment electrochemical glass cell with a Biologic bipotentiostat and a rotator (PINE Instruments model: AFMSRCE, USA). The three electrode arrangement consisted of a Pt mesh as counter electrode, mercury/mercurous sulfate (AMETEK USA) as reference electrode, and a glassy carbon electrode including rotating disk electrode (RDE, 0.19653 cm2) and rotating ring-disk electrode (RRDE, 0.2475 cm2) measurement with a Pt ring as working electrode. All potentials are referred to the reversible hydrogen electrode (RHE). The oxygen reduction activity was investigated by cyclic voltammetry (CV) in O2-saturated electrolyte at a scan rate of 5 mV s-1. Prior to the measurement, the Pt ring of the RRDE electrode was firstly electrochemically cleaned using cycling voltammetry by sweeping the potential between 0.05 V and 1.2 V at a scan rate of 20 mV/s in N2-saturated electrolytes until steady CVs were obtained. Then the non-faradaic current as the capacitive current could be determined by collecting CV sweeping the potential between 0.05 V and 1.1 V at a scan rate of 5 mV/s in N2-saturated 0.5 M H2SO4. After purging with O2 at least 30 min, the linear sweep voltammetry (LSV) of ORR was collected in O2-saturated 0.5 M H2SO4 at the rotation speed of 1600 rpm. Finally, the faradaic currents were corrected by subtracting the corresponding capacitive current. The ring currents were recorded by fixing the ring potential at 1.2 V in order to detect the H2O2 produced on disk electrode. The catalyst ink was prepared through suspending the catalyst in a mixture containing Milli-Q water, isopropanol and Nafion solution (5 wt%, Sigma-Aldrich). After sonication, the catalyst ink was dropped onto the freshly polished glassy carbon electrode and dried


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in the oven at 60 °C for 10 minutes. The collection efficiency (N, 37%) of the RRDE electrode used (Pine AFE6R2GCPT) was determined by using the reversible [Fe(CN)6]4-/[Fe(CN)6]3- system.28 Selectivity of the catalysts towards H2O2 formation was calculated according to the well-known relation: H 2O2 % =

200 × I ring

N×|I disk | + I ring

where Iring is the ring current and Idisk is the disk current.15 The number of transferred electrons (n) at the disk electrode during oxygen reduction process was calculated as follows:

n=

4×|Idisk | |Idisk | + I ring / N

Bulk H2O2 production was carried out in a custom-made two-compartment cell with Nafion 212 membrane as separator. The Nafion membrane was pretreated with 5% (v/v) H2O2 for 1 h at 80 °C and 10% (v/v) H2SO4 for 1 h at 80 °C. Both the cathode compartment and anode compartment were filled with 145 mL of the same electrolyte including 0.5 M H2SO4 (pH=0.3), 0.1 M K2SO4 (pH=7), and 0.1 M KOH (pH=13), respectively. The loading amount of catalyst on the working electrode was fixed at 0.05 mg cm2. H2O2 production was conducted by chronoamperometry at 0.1 V, 0.2 V and 0.3 V with 85% manual iR compensation. To quantify the amount of H2O2 produced (yield), samples collected at certain time intervals were analyzed using a commercial hydrogen peroxide test (Merck KGaA, Germany). In detail, 2 mL aliquot of the electrolyte was taken from the cathode compartment of the electrochemical cell. Then commercial peroxide test solutions were added rapidly to the solution above


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according to the instruction. It should be here noted that, since the pH of the measurement solution for the commercial peroxide test kit must be within the range 6.6-7.7, the electrolytes from the electrochemical cell need to firstly be neutralized using 0.1 M Na3PO4 for 0.5 M H2SO4 or 0.1 M KH2PO4 for 0.1 M KOH. After 10 min (reaction time) at room temperature, the resultant mixture solution was filled into a standard 70 µL cuvette for UV-Vis absorption spectra measurements. The UV-Vis spectra were recorded on an AvaSpec-2048TEC-2 equipped with a deuterium halogen light source (Avantes, Broomfield, USA).31 During the process, the color of the resultant mixture solution changed from colorless to orange since hydrogen peroxide reduces copper (II) ions to copper (I) ions in the presence of a phenanthroline derivative, which exists in the commercial peroxide test solution. The UV–Vis measurements were then performed at 453 nm. The concentration-absorbance curves were calibrated using standard hydrogen peroxide solutions including 0.05 mg L-1, 0.1 mg L-1, 0.3 mg L-1, 0.5 mg L-1, 1 mg L-1, 2 mg L-1, and 2.5 mg L-1 of hydrogen peroxide. The hydrogen peroxide concentrations were subsequently determined based on the calibration curve. In order to ensure a precise result, the absorbance value should be under 1. If the absorbance value exceeds 1, the solution after neutralization should be further diluted before adding the test kits. The H2O2 faradaic selectivity was calculated from the H2O2 yield against the quantity of charge passed:

H 2 O 2 faradaic selectivity (%)=2CVF/Q where C is the H2O2 concentration (mol L-1), V is the volume of electrolyte (l), F is the Faraday constant (C mol-1), Q is the amount of charge passed (C).21


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3. Results and Discussion

Figure 1: (a) Powder XRD patterns, (b) the BET surface area and (c) the Zeta potential in water (pH=7) of various typical carbon materials including CMK-3, Black pearls 2000, Vulcan XC 72R, Ketjen black EC 300J, Ketjen black EC 600JD, Graphene nanoplates. The Zeta potential measurements were repeated five times and its average value with error bar is displayed here. 3.1 Structure-activity relationship screening of suitable carbon catalysts Our study sets out with a broad exploration of a variety of suitable carbon materials for H2O2 production, followed by a down selection and a more detailed investigation


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of structure-composition-activity relationships of the selected materials after heteroatom-doping and under varying preparation conditions. The initial set of suitable carbons was selected as to span a diverse range of crystallinity, surface areas, and structural morphologies. Our broader screening of six potentially suitable carbon materials involved the correlation of their physicochemical properties and O2-to-H2O2 activity. Properties considered include crystal structure (PXRD), BET surface area, and Zeta potential. Figure 1a reports the diffraction patterns (top to bottom) of CMK-3, Black pearls 2000, Vulcan XC 72R, Ketjen black EC 300J, Ketjen black EC 600JD and graphene nanoplates. As shown, all carbon materials display characteristic peaks of amorphous carbon at 25o and 44o.32-33 Raman spectra measurement is also conducted to judge the degree of graphitization, the defects and disordered structures. As shown in Figure S1a, Raman results demonstrated that all carbon materials exhibit typical D band (1328 cm-1) and G band (1582 cm-1), where the D band is related to disordered graphitic structures and edge defects, the G band reflects the degree of graphitization.21,

28

It can be also observed that CMK-3 possesses characteristic

features of amorphous carbon with moderate intensity ratio of the D band to G band (ID/IG). In addition, given that the apparent electrocatalytic performances may sensitively depend on their true surface areas,21 nitrogen adsorption isotherms were taken to determine BET surface areas (Figure 1b), which varied from 1314.7 m2 g-1 for Black pearls 2000 to 16 m2 g-1 for the graphene nanoplates. Besides, the surface charge of carbon-based catalysts were reported to control their dispersion behavior


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and stability and therefore play an essential role in the electrocatalytic performance,34-35 which is why the Zeta potential was considered as second screening parameter.36 Generally, large absolute values of the experimental Zeta potential suggest that there are large amounts of surface charges, which prevent the aggregation of particles improving the dispersion stability due to the strong repulsive force.35, 37 Of particular importance in the present case is the fact that large surface charge may aid in the catalytic oxidation and reduction process of reactant present in the system.38 As shown in Figure 1c, three carbon materials showed negative Zeta potentials from -0.036 mV to -31.1 mV, whereas the other three ones exhibited positive Zeta potentials from 6.85 mV to 16.9 mV, indicating different surface charges for different carbon materials. Finally, we conducted rotating ring disk electrode (RRDE) measurements to examine the electrocatalytic activity of all carbon materials toward oxygen reduction to H2O2. Figure 2b represents the linear sweep voltammetry curves during the H2O2 generation process. It can be clearly observed that none of the carbon materials reaches a diffusion-limited current plateau highlighting the kinetic hindrance of this surface electrochemical process. Among them, CMK-3 exhibited the highest overall catalytic reactivity for oxygen reduction (disk current density, jdisk, up to -1.06 mA cm-2), and the largest HO2- specific productivity (ring current, iring, up to 0.07 mA) at 0.1 V (Figure 2a and b). Moreover, CMK-3 displayed the most positive onset potential around 0.44 V. The H2O2 selectivity trends and the number of transferred electrons derived from the RRDE voltammograms are displayed in Figure 2c and d.


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For all carbon materials investigated, both H2O2 selectivity trend and the number of transferred electrons depend on the applied potential. Nevertheless, the number of transferred electrons for CMK-3 at 0.1 V is most close to 2, thus indicating that it mainly follows two-electron oxygen reduction pathway.

Figure 2: Linear sweep voltammetry of rotating ring disk electrode (RRDE) with (a) the ring current collected on the Pt ring at a constant potential of 1.2 VRHE, (b) the disk geometric current density, (c) H2O2% selectivity and (d) the number of transferred electrons (n) as a function of electrode potential of various carbon materials including CMK-3, Black pearls 2000, Vulcan XC 72R, Ketjen black EC 300J, Ketjen black EC 600JD, and Graphene nanoplates. In all cases, measurements were performed in O2 saturated 0.5 M H2SO4 at a scan rate of 5 mV/s with 1600 rpm at room temperature and the catalyst loading was set to 0.05 mg cm-2. Three times measurement result of H2O2 % selectivity and the mean number of transferred electrons with error bar for CMK-3 were also shown in Figure 2c and d. To get a deeper understanding of the physico-chemical properties controlling the catalytic H2O2 selectivity, we analyzed the relationship between BET surface areas, Zeta potential, and H2O2 selectivity at 0.1 V in 0.5 M H2SO4 (Figure 3). We can ACS Paragon Plus Environment

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clearly observe that relative large BET surface area and the positive Zeta potential value are beneficial for H2O2 production, like CMK-3, Black pearls 2000, and Ketjen black EC 600JD. These results indicate that BET surface area and Zeta potential are important descriptors for determining electrocatalytic selectivity of the catalysts. On one hand, high surface areas reflect a large number of active sites and are thus beneficial for the overall catalytic performances. On the other hand, recently Terakura’s group reported the reaction mechanism for H2O2 production on carbon materials catalysts (CMCs), where the special hydrogen site is firstly formed by the hydrogenation of the catalytic site on CMCs, then is abstracted by the approached O2 molecules to form an OOH− ion and finally react with H+ to form a H2O2.39 This mechanism proposed is mainly based on which the barriers for O2 molecule abstracting H on CMCs are lower than that for O2 adsorption. Meanwhile, the catalysts with high H2O2 selectivity reported in the present work were found to generally possess the positive Zeta potential, implying that the surface of these catalysts is easily protonated40 and thus may be beneficial for the formation of the special hydrogen site. From this viewpoint, the H2O2 production process over these reported carbon catalysts follows this mechanism, which also well explains why the positive value of Zeta potential is beneficial for H2O2 production. Based on the above considerations, the reaction step scheme of H2O2 production is displayed in Figure S2. Besides, we also draw the relationship between the ID/IG ratio and H2O2 selectivity to uncover the effect of the degree of graphitization and defect sites on H2O2 selectivity (Figure S1b). It can be found that H2O2 selectivity gradually increases as ID/IG ratio


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increases from 0.02 to 1.13, whereas H2O2 selectivity declines slightly by further increasing ID/IG to 1.29, indicating that the defect sites are favorable for H2O2 production.21

Figure 3: The relationship between Zeta potential and BET surface area of the structurally different carbon materials: Graphene nanoplates, Vulcan XC 72R, Ketjen black EC300J, CMK-3, Black pearls 2000 and Ketjen blackEC 600JD. Note: The radius of the circles scales and the numbers in circles denote the H2O2% selectivity at 0.1 V in 0.5 M H2SO4. 3.2 Preparation and characterization of nitrogen-doped mesoporous carbons Based on its excellent faradaic H2O2 selectivity and its balanced BET surface area, CMK-3 was selected as the carbon scaffold to be functionalized with nitrogen atoms using a combination of an acid wash and thermal treatment in the presence of a N-rich ionic liquid (IL) (Figure 4a). In our work, CMK-3 here not only serves as cabon source also use as a template for nitrogen-doped carbon. The PXRD results in Figure S9 shows that the (002) graphite reflections of pristine CMK-3 are downshifted by 4-6°, evidencing that the interlayer distance of graphite (002) is expanded.41-42


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Furthermore, SEM and TEM were conducted to investigate the morphological changes upon N-doping. SEM results before and after functionalization (Figure 4b-c and 4e-f) showed no obvious change in carbon morphology. TEM results (Figure 4d and g) also indicate that ordered structures were maintained after functionalization.

Figure 4 (a) Synthetic schematic of IL-derived N-doped CMK-3 as follows: Step 1, first pretreatment of CMK-3 in 2.4 M HNO3 overnight; Step 2, mixing acid-treated CMK-3 with different amount of IL (25 µL, 50 µL, 100 µL) overnight under magnetic stirring; Step 3, annealing at 180 °C for 2 h to remove H2O and subsequently annealing at different temperature (600 °C, 700 °C, 800 °C and 900 °C) for 4 h under N2 yielding N-doped CMK-3 labeled as CMK3ILX_YT (with X the IL in µL and Y is annealing temperature in °C). (b-c, e-f) SEM images and (d, g) TEM image of (b-d) CMK-3, and (e-g) NCMK3IL50_800T.


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Figure 5 (a, b) Comparison BET surface area, (c, d) Pore size distribution, (e, f) Zeta potential measured in water (pH=7) of CMK-3, CMK-3 after acid treatment, NCMK3IL25_800T, NCMK3IL50_800T, NCMK3IL100_800T, NCMK3IL50_600T, NCMK3IL50_700T and NCMK3IL50_900T. Elemental analysis (Table S1) confirmed that the N-content of the catalyst samples could be tuned by simply changing the mixing ratio of the IL and CMK-3, and annealing temperature. The resulting N-content followed the initial IL amount, whereas the N-content declined with higher annealing temperature. The TGA curves of CMK-3, IL and NCMK3IL50_800T (Figure S10) suggested the onset of IL decomposition at around 300 oC under a N2 atmosphere with weight losses increasing ACS Paragon Plus Environment

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with temperature, which is consistent with the element analysis. We hypothesize that the decomposition of the IL at high temperature actually promotes the formation of micro- and meso-pores. Our hypothesis is supported by the trends in BET surface areas with IL doping and temperature in Figure 5. There exists an optimal level of N doping (Figure 5a and c) and the BET values actually increase with annealing temperature due to formation of micropores (Figure 5b and d). The effect of N doping amount and annealing temperature on the Zeta potential is reported in Figure 5e and f. Consistent with our discussion above, acid treatment introduces negative oxygen-containing functional groups43, which are then offset by increasing amounts of N-doping seen by the Zeta potential variation from -24.4 mV to 11.3 mV. Similarly, heat treatment is detrimental to the negatively charged surface groups,28 which is why the Zeta potential essentially increases from -27 mV to +24.6 mV with annealing temperature.

To gain qualitative and quantitative information on the chemical nature of the N-functionalities embedded into the CMK-3 carbon matrix, high resolution X-ray photoelectron spectroscopy (XPS) spectra were used to identify and distinguish various N-species by means of their core level energies. Figure 6a displays high-resolution XPS spectra of the N 1s core level region for all N-doped carbon materials. It can be observed from Figure 6b and Figure S11 that the N 1s spectra of all N-doped carbon materials except NCMK3IL50–900T can be fit with four distinct peaks, which are assigned to pyridinic-N (398.8 eV), pyrrolic-N (400.4 eV), quaternary-N (401.2 eV), and graphitic-N (402.5 eV), in increasing binding energy


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order, respectively.44 The total content of nitrogen doping and the relative amount of different types of N-species are summarized in Table 1. According to Table 1, several features can be observed: (1) the total content of nitrogen increases with increasing IL concentration, but decreases with increasing the annealing temperature; (2) as the

Figure 6 High-resolution XPS spectra of the N 1s core level region of N-doped carbon: (a) summary of all samples, (b) deconvolution of the N 1s spectrum of NCMK3IL50_600T in different N-components as shown. Table 1 The total content of nitrogen doping and the relative amount of the different nitrogen species detected by XPS (in at%)

Sample

N% (at%)

graphitic

pyridinic

Pyrrolic

Quarternary

NCMK3IL100_800T

0.8

8

50

10

32

NCMK3IL50_900T

0.2

23

31

-

46

NCMK3IL50_800T

0.6

8

53

5

34

NCMK3IL50_700T

1.6

5

54

11

30

NCMK3IL50_600T

2.4

3

53

17

27

NCMK3IL25_800T

0.5

11

49

9

31


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annealing temperature increases, the relative amount of quaternary-N and graphitic-N increases whereas the relative amount of pyridinic-N and pyrrolic-N decreases since the stability of quaternary-N and graphitic-N is superior to that of pyridinic-N and pyrrolic-N at higher annealing temperature.34 3.3 Non-stationary faradaic H2O2 selectivity and the number of transferred electrons The electrocatalytic activities and selectivity of the obtained samples toward ORR were further examined through the RRDE method. We firstly investigated the effect of different loading amount of catalysts on their electrocatalytic activities and selectivity, and the results are shown in Figure S12. It can be seen that as the loading amount decreases, the selectivity of H2O2 production increases, which is accordance with the previous reported literature.45 It is demonstrated that the lower loading amount of the catalysts results in the sparse distribution of active sites and the smaller probability that H2O2 molecules are further reduced to H2O. Thus the optimal loading amount is fixed as 0.05 mg/cm2 thereafter. After the introduction of nitrogen (Figure S13), an increased oxygen reduction current is observed, and the onset potential of oxygen reduction shifts positively from 0.44 to 0.55 V with increasing nitrogen doping amount. Nevertheless, there is no obvious effect of annealing temperature on the electrocatalytic activity except at 900 oC (Figure 7). The H2O2 selectivity and the number of transferred electrons of the oxygen reduction during electrochemical reaction were also determined from RRDE experiment. As shown in Figure S13, nitrogen doping boosted the selectivity of H2O2 and the number of transferred electrons dropped from 2.5 to 2.1 (Figure S13c-d). Also, the annealing temperature


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had a strong effect on catalyst selectivity with an optimum at 800 oC (Figure 7c-d). Finally, the effect of different pH environments on the electrocatalytic activity of the most promising NCMK3IL50_800T catalyst was also addressed (Figure S14). In

Figure 7 Linear sweep voltammetry of RRDE with (a) the ring current, (b) the disk geometric current density, (c) H2O2% selectivity and (d) the number of transferred electrons (n) as a function of electrode potential of NCMK3IL50_YT (Y=600°C, 700°C, 800°C, 900°C). Conditions: the catalyst loading on the disk was set to 0.05 mg cm−2 and measurements were performed in O2 saturated 0.5 M H2SO4 at a scan rate of 5 mV/s with 1600 rpm at room temperature. H2O2 % selectivity and number of electrons were based on three independent repeated measurements. alkaline environment, the resultant NCMK3IL50_800T exhibited the highest catalytic activity, whereas the highest selectivity can be obtained in acid solution. We ascribe this difference to the reactivity of hydroxyl species which were reported to promote an outer-sphere electron transfer process during ORR in alkaline solution, which predominantly leads only to two-electron peroxide intermediate as the final product.46 Meanwhile, peroxide reduction is also reported to be more kinetically favorable in


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alkaline solution than in acidic solution.46 It should be highlighted that the H2O2 selectivity of NCMK3IL50_800T prepared at 800 °C reached 95-98% in the potential range from 0.1 to 0.3 V, which exceeds those reported previously under similar conditions including noble-metal alloys,1, 17 transition-metal-based47 and metal-free catalysts21, 28, 48 (Figure S15 and Table S3).

Figure 8 The relationship between Zeta potential and BET surface area of (a) the resultant CMK-3, CMK-3 after acid treatment, and different IL doping amount (NCMK3IL25_800T, NCMK3IL50_800T, and NCMK3IL100_800T), and (b) different annealing temperatures (NCMK3IL50_600T, NCMK3IL50_700T, NCMK3IL50_800T, and NCMK3IL50_900T). Note: The radius of the circles scales and the numbers in circles denote the H2O2% selectivity at 0.1 V in 0.5 M H2SO4. In order to identify factors controlling the H2O2 selectivity, the relationships of the BET surface area, and Zeta potential with H2O2 selectivity at 0.1 V in 0.5 M H2SO4 were investigated as function of the level of N doping and annealing temperature (Figure 8). It can be seen that after nitrogen doping, the H2O2 selectivity increased with BET surface area increasing up until a value of 1541 m2/g (see catalyst annealed at 800 °C), after which a sharp decline was observed (see catalyst annealed at 900 °C). This may be due to the fact that when the annealing temperature was further increased to 900 °C, the ratio of microporous increased, which is detrimental for H2O2


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production.28 Interestingly, we also observed that the resultant nitrogen-doped CMK-3 catalysts such as NCMK3IL50_600T, NCMK3IL50_700T, and NCMK3IL50_800T, have very negative Zeta potential value, however, performed much better selectivity than pristine carbon materials like graphene nanoplates and Vulcan XC 72R. Further, the relationship between the ID/IG ratio and H2O2 selectivity was also investigated to uncover the effect of the degree of graphitization and defect sites on H2O2 selectivity


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Figure 9 Relationship between the H2O2 % selectivity at 0.1 V and a) total amount of doped nitrogen (N) , b) amount of quaternary nitrogen (N*Nquaternary), c) amount of pyridinic nitrogen (N*Npyridinic), d) amount of graphitic nitrogen (N*Ngraphitic), and e) amount of pyrrolic nitrogen (N*Npyrrolic): (1) CMK-3, (2) CMK-3 after acid treatment, (3) NCMK3IL25_800T, (4) NCMK3IL50_800T, (5) NCMK3IL100_800T, (6) NCMK3IL50_600T, (7) NCMK3IL50_700T and (8) NCMK3IL50_900T. of the nitrogen-doped CMK-3 catalysts (Figure S16-S17). The analysis revealed that for the nitrogen-doped catalysts, the H2O2 selectivity generally increased with decreasing ratio of ID/IG (see Figure S16 at constant annealing temperature and S17 at constant N-precursor amount) whereas the two samples annealed at 700 °C and 800 °C are considered virtually identical. These results illustrated that compared to Zeta potential and defect effects, the nitrogen doping effect may play a more dominant role for the H2O2 selectivity of the nitrogen-doped CMK-3 catalysts during the ORR process. This is why we went a step further and analyzed the correlation between the total amount of nitrogen doping and the amount of different types of N-species, and H2O2 selectivity (Figure 9). This analysis revealed that H2O2 selectivity represents a volcano trend as a function of nitrogen content. The N functionalities do play a beneficial role at low nitrogen content; however, appear to be detrimental for the H2O2 selectivity at high nitrogen content. Further, the relationship between H2O2% and active surface site density (ASSD) was also plotted (Figure S18). It can be observed that the N-functionalities also play a beneficial role at low active surface site density, however, appear to be detrimental for the H2O2 selectivity at high active surface site density. These relations make us believe that the “nitrogen doping” outweighs the other parameters investigated, and that a balanced ASSD of about 0.004 mg m-2 is optimal for the H2O2% yield in the present system. As mentioned by


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Stephens’s group,1 H2O2 can be electrochemically further reduced into H2O at those close N functionality. It is thus reasonable that an excess surface density of N-functional sites would facilitate the decomposition of H2O2, which is detrimental for the free H2O2 yield. Thus, we propose that in order to maximize H2O2 producing on nitrogen-doped carbon catalyst, a balanced active surface site density is required. The notion of a balanced active surface site density can be reformulated that the individual N sites should maintain an optimal distance from each other in order to keep the undesired 4-electron pathway to water to a minimum; this reaction/catalyst couple thus exhibits what is known as “site isolation effect”.49-51 From this viewpoint, the low H2O2 selectivity for NCMK3IL50_700T and NCMK3IL50_600T arises from an insufficient site isolation of neighboring N-sites at high nitrogen doping amounts. 3.4 H2O2 productivities during bulk electrolysis over extended test time H2O2 faradaic selectivity is one of the important parameters for evaluating the performance of catalyst, which requires the real H2O2 production during the ORR process. The amount of H2O2 produced in the bulk electrolyte can be detected through a photometric method reported in our previous work.25,

48

Figure 10a shows the

accumulated amount of H2O2 normalized by catalyst loading amount over the reaction time for NCMK3IL50_800T at the different applied potential from 0.3 V to 0.1 V in O2 saturated 0.5 M H2SO4. It can be observed that the H2O2 production rate increases as the potential is shifted negatively in the investigated potential range. Remarkably, the H2O2 production rate of 159.9 mmol h-1 g-1catalyst at 0.1 V (Figure 10a) and H2O2 faradaic selectivity of more than 70% (Figure 10b) can be achieved in the


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investigated applied potential, which was comparable or even better than that of the reported catalysts such as nitrogen-doped carbon25, catalysts47,

52

(Figure

S19a

and

Table

48

S4).

and transition-metal-based Besides,

the

NCMK3IL50_800T also exhibits excellent stability during the

as-prepared successive

electrochemical H2O2 production within 6 h in acid solution (Figure S20a). We also measure the corresponding H2O2 faradaic selectivity in neutral (0.1 M K2SO4) and

Figure 10 (a, c, e) H2O2 production amount normalized by catalyst loading amount


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over the reaction time, and (b, d, f) H2O2 faradaic selectivity of NCMK3IL50_800T catalysts in O2 saturated 0.5 M H2SO4 (a, b), 0.1 M K2SO4 (c, d), and 0.1 M KOH (e, f). alkaline (0.1 M KOH) solutions. Obviously, pH has a pronounced influence on the H2O2 production rate (Figure 10a, c, e). It can be seen that H2O2 production rate of 547.07 mmol h-1 g-1catalyst can be achieved in neutral solution, which is obviously higher than that reported in the literature (Figure S19b and Table S5). In alkaline solution, the resultant NCMK3IL50_800T exhibits the highest H2O2 production rate at 0.1 V, and can achieve the H2O2 production rate of 561.7 mmol h-1 g-1catalyst (Figure 10e), which is 1.17 and 3.51 times higher than that in 0.1 M K2SO4 and 0.5 M H2SO4, respectively. Generally, concentrated basic solution containing H2O2 are widely used in the pulp and paper industry.47 Thus on-site high H2O2 production rate in alkaline solution is very attractive for this application. However, the H2O2 faradaic selectivity of the resultant NCMK3IL50_800T in neutral and alkaline solutions are both lower than that in acid solution (Figure 10b, d, f), which may be attributed to the different production/decomposition mechanism of peroxide over the catalysts in different electrolytes with different pH value.48 Despite this, the resultant NCMK3IL50_800T also exhibits good stability in both neutral and alkaline solutions (Figure S20b-c). These results indicate that the as-prepared NCMK3IL50_800T is a promising electrocatalyst for H2O2 production. 4. Conclusion We have screened different chemically distinct carbon and N-doped carbon catalyst materials for electrochemical H2O2 production by means of measuring their BET surface areas, Zeta potentials, Raman spectra, and XPS. For pristine carbon materials,


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BET surface areas, Zeta potentials, and defect sites were found to play a key role in determining the catalytic activity and selectivity. In fact, large BET surface area and positive Zeta potential, and high defect sites were beneficial for H2O2 production. Moreover, nitrogen doping further improved the H2O2 selectivity. Different from pristine carbon materials, the H2O2 selectivity of N-doped carbon catalyst materials is more dominated by nitrogen doping effect, not Zeta potentials and defect sites. Besides, the effect of electrolyte’s pH on the selectivity towards H2O2 production and H2O2 production rate was also observed. It should be noted that for CMK-3 with the optimal nitrogen-doping, high selectivity, H2O2 production rate and large H2O2 faradaic selectivity in acidic, neutral and alkaline solution can be achieved. Our work not only describes a facile and low-cost synthetic method, but also provides new insights for the design and mechanistic investigation of metal-free heteroatom-doped carbon materials.


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5. Associated content Supporting Information: Additional structural characterization and supporting electrochemical characterization

6. Author Information Corresponding author: [email protected] 7. Acknowledgements This work received funding by the German Federal Ministry of Education and Research (Bundesministerium für Bildung und Forschung, BMBF) under the grant 03SF0531B “HT-LINKED”. IS and BRC also acknowledge funding from the Cluster of Excellence RESOLV at RUB (EXC 1069). ZELMI of Technical University Berlin is acknowledged for their support with TEM measurements. Yanyan Sun thanks China Scholarship Council (CSC) for the financial support. We also thank Prof. Zhongwei Chen (University of Waterloo, Canada) and Kun Feng (University of Waterloo, Canada) for helping Raman spectra measurement.


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8. References (1) Siahrostami, S.; Verdaguer-Casadevall, A.; Karamad, M.; Deiana, D.; Malacrida, P.; Wickman, B.; Escudero-Escribano, M.; Paoli, E. A.; Frydendal, R.; Hansen, T. W.; Chorkendorff, I.; Stephens, I. E.; Rossmeisl, J., Enabling direct H2O2 production through rational electrocatalyst design. Nat. Mater. 2013, 12, 1137-1143. (2) Campos-Martin, J. M.; Blanco-Brieva, G.; Fierro, J. L., Hydrogen peroxide synthesis: an outlook beyond the anthraquinone process. Angew. Chem. Int. Ed. 2006, 45, 6962-6984. (3) Han, L.; Guo, S.; Wang, P.; Dong, S., Light-driven, membraneless, hydrogen peroxide based fuel cells. Adv. Energy Mater. 2015, 5, 1400424. (4) Mousavi Shaegh, S. A.; Nguyen, N.-T.; Mousavi Ehteshami, S. M.; Chan, S. H., A membraneless hydrogen peroxide fuel cell using Prussian Blue as cathode material. Energy Environ. Sci. 2012, 5, 8225. (5) Mase, K.; Yoneda, M.; Yamada, Y.; Fukuzumi, S., Seawater usable for production and consumption of hydrogen peroxide as a solar fuel. Nat. Commun. 2016, 7, 11470. (6) Arrigo, R.; Schuster, M. E.; Abate, S.; Giorgianni, G.; Centi, G.; Perathoner, S.; Wrabetz, S.; Pfeifer, V.; Antonietti, M.; Schlögl, R., Pd supported on carbon nitride boosts the direct hydrogen peroxide synthesis. ACS Catal. 2016, 6, 6959-6966. (7) Maity, S.; Eswaramoorthy, M., Ni–Pd bimetallic catalysts for the direct synthesis of H2O2– unusual enhancement of Pd activity in the presence of Ni. J. Mater. Chem. A 2016, 4, 3233-3237. (8) Paunovic, V.; Schouten, J. C.; Nijhuis, T. A., Direct synthesis of hydrogen peroxide using concentrated H2 and O2 mixtures in a wall-coated microchannel – kinetic study. Appl. Catal. A: Gen. 2015, 505, 249-259. (9) Wilson, N. M.; Flaherty, D. W., Mechanism for the direct synthesis of H2O2 on Pd clusters: heterolytic reaction pathways at the liquid-solid interface. J. Am. Chem. Soc. 2016, 138, 574-586. (10) Sa, Y. J.; Park, C.; Jeong, H. Y.; Park, S. H.; Lee, Z.; Kim, K. T.; Park, G. G.; Joo, S. H., Carbon nanotubes/heteroatom-doped carbon core-sheath nanostructures as highly active, metal-free oxygen reduction electrocatalysts for alkaline fuel cells. Angew. Chem. Int. Ed. 2014, 53, 4102-4106. (11) Tan, Y.; Xu, C.; Chen, G.; Fang, X.; Zheng, N.; Xie, Q., Facile synthesis of manganese-oxide-containing mesoporous nitrogen-doped carbon for efficient oxygen reduction. Adv. Funct. Mater. 2012, 22, 4584-4591. (12) Yang, W.; Yue, X.; Liu, X.; Zhai, J.; Jia, J., IL-derived N, S co-doped ordered mesoporous carbon for high-performance oxygen reduction. Nanoscale 2015, 7, 11956-11961. (13) Choi, C. H.; Kim, M.; Kwon, H. C.; Cho, S. J.; Yun, S.; Kim, H. T.; Mayrhofer, K. J.; Kim, H.; Choi, M., Tuning selectivity of electrochemical reactions by atomically dispersed platinum catalyst. Nat. Commun. 2016, 7, 10922. (14) Yang, S.; Kim, J.; Tak, Y. J.; Soon, A.; Lee, H., Single-atom catalyst of platinum supported on titanium nitride for selective electrochemical reactions. Angew. Chem. Int. Ed. 2016, 55, 2058-2062. (15) Choi, C. H.; Kwon, H. C.; Yook, S.; Shin, H.; Kim, H.; Choi, M., Hydrogen peroxide synthesis via enhanced two-electron oxygen reduction pathway on carbon-coated Pt surface. J. Phys. Chem. C 2014, 118, 30063-30070. (16) Zheng, Z.; Ng, Y. H.; Wang, D. W.; Amal, R., Epitaxial growth of Au-Pt-Ni nanorods for direct high selectivity H2O2 production. Adv. Mater. 2016, 28, 9949-9955. (17) Jirkovsky, J. S.; Panas, I.; Ahlberg, E.; Halasa, M.; Romani, S.; Schiffrin, D. J., Single atom


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(52) Barros, W. R. P.; Reis, R. M.; Rocha, R. S.; Lanza, M. R. V., Electrogeneration of hydrogen peroxide in acidic medium using gas diffusion electrodes modified with cobalt (II) phthalocyanine. Electrochim. Acta 2013, 104, 12-18.


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Efficient Electrochemical Hydrogen Peroxide Production from

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