Kinetic Understanding of the Reduction of Oxygen to Hydrogen

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C: Physical Processes in Nanomaterials and Nanostructures

Kinetic Understanding of the Reduction of Oxygen to Hydrogen Peroxide over an N-Doped Carbon Electrocatalyst Yun Wu, Azhagumuthu Muthukrishnan, Shinsuke Nagata, and Yuta Nabae J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b12464 • Publication Date (Web): 29 Jan 2019 Downloaded from http://pubs.acs.org on January 31, 2019

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The Journal of Physical Chemistry

Kinetic Understanding of the Reduction of Oxygen to Hydrogen Peroxide over an N-Doped Carbon Electrocatalyst Yun Wua, Azhagumuthu Muthukrishnana, Shinsuke Nagataa, and Yuta Nabaea* aDepartment

of Materials Science and Engineering, Tokyo Institute of Technology, 2-12-1 S8-26,

Ookayama, Meguro-ku, Tokyo 152-8522, Japan S Supporting Information ○

Abstract Nitrogen-doped carbon catalysts for the electrochemical oxygen reduction reaction (ORR) have great potential to substitute precious metal alloy catalysts for the electrochemical synthesis of H2O2 based on a fuel cell setup. Consequently, obtaining a kinetic understanding of nitrogen-doped carbon catalysts in acidic media is essential. In this study, the mathematically modified Damjanovic model and Nabae model were applied to calculate the kinetic constants for an N/C catalyst prepared from polyimide particles. The results show that the N/C catalyst has high selectivity for the two-electron reduction of O2, making it a promising catalyst for the electrochemical production of H2O2. This study gives quantitative insight into the oxygen reduction reaction mechanism over an N/C catalyst prepared by the pyrolysis of polyimide nanoparticles.

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Introduction Hydrogen peroxide is currently one of the most important chemicals for pulp bleaching, environmental protection, and chemical production1. Conventionally, hydrogen peroxide is manufactured on an industrial scale by the anthraquinone oxidation process in a multi-step operation with high energy consumption and significant waste generation, making it an unsustainable process, which negatively affects production1. The serious disadvantages of the anthraquinone process have motivated many researchers towards the development of a one-step synthetic method for H2O2 from its elements: hydrogen and oxygen2, 3. However, the direct handling of hydrogen and oxygen has a major risk: explosion. A polymer electrolyte membrane fuel cell (PEMFC) setup could be used to produce hydrogen peroxide. This setup separates the hydrogen oxidation and oxygen reduction reactions4, 5. The electrochemical synthesis of hydrogen peroxide from hydrogen and oxygen shows significant advantages, that is, a lower chance of explosion of the H2/O2 mixture. In addition, the setup generates electric power. In the synthetic process, hydrogen loses electrons at the anode, yielding electrons and protons. The electrons are transferred through the external circuit, and the protons pass a proton exchange membrane (Nafion), finally being combined with oxygen and producing hydrogen peroxide at the cathode. In the four-electron oxygen electroreduction pathway, the product is water, which suppresses the electrochemical synthesis of hydrogen peroxide6. Therefore, it is essential to explore selective catalysts for O2 reduction to H2O2 via the two-electron reduction process. The existing precious metal alloy catalysts show high activity and selectivity for oxygen reduction to hydrogen peroxide6, 7. However, the high cost and scarcity of precious metals (e.g., platinum and palladium) hinder their application.


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A number of efforts have been made to develop new nonprecious metal catalysts for the electrochemical synthesis of hydrogen peroxide8–10. Recently developed non-precious nitrogendoped carbon catalysts (N/C) are much cheaper and have high selectivity for oxygen reduction to hydrogen peroxide in acidic media. The N/C catalyst is considered to undergo a (2 + 2)-electron reduction pathway for oxygen11. However, the quantitative evaluation of the kinetics of the oxygen reduction reaction (ORR) is required to confirm the real mechanism over the N/C catalyst. Previously, the standard Damjanovic model was proposed to calculate the kinetic rate constants of the ORR, k1 (four-electron reduction pathway), k2 (two-electron reduction pathway), and k3 (further reduction of hydrogen peroxide) over catalysts based on the assumption that k2 is much smaller than k112, 13. This model works well for Pt- and Pt-alloy-based catalysts. However, it results in large errors when it was applied to non-precious-metal catalysts (e.g., the N/C catalyst) because a considerable amount of H2O2 is produced. To avoid such problems, our research group proposed a different approach14. In our method, hydrogen peroxide voltammetry is conducted separately to evaluate the rate constant of the hydrogen peroxide reduction reaction (HPRR) more accurately, and the obtained data are combinatorially analyzed with those from the ORR experiments based on the mathematically modified Damjanovic model (Figure S1a). Furthermore, the overestimation of the four-electron reduction pathway can be successfully corrected by careful analysis of the effect of the catalyst loading density utilizing the proposed calculation model, which, for concision, is denoted as the Nabae model and shown in Figure S1b. In the analysis of the Damjanovic model, the (2 + 2)-electron reduction process is treated as a four-electron reduction process. After the correction of the catalyst loading density of the Nabae model, the (2 + 2)-electron reduction processes can be separated, and the currents and rate constants for the ORR can be correctly evaluated.


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In the present study, the N/C catalyst was prepared by the pyrolysis of polyimide nanoparticles. The above-mentioned mathematically modified Damjanovic model and the Nabae model were used to clarify the inherent kinetics for N/C catalyst to clarify the reduction process of oxygen over this N/C catalyst further. The results show that the N/C catalyst is promising for the selective synthesis of H2O2 (two-electron reduction process) in industry using a fuel cell setup. To decrease or prevent the further reduction of hydrogen peroxide, the amount of catalyst matrix should be minimized.

Experimental Catalyst synthesis: The N/C catalyst was prepared following procedures reported elsewhere after minor modification15, 16. Polyimide nanoparticles were prepared from pyromellitic acid dianhydride (PMDA, TCI, purified by sublimation) and 1,3,5-tris(4-aminophenyl)benzene (TAPB, TCI, used as-received) in the presence of N,N-dimethyldodecylamine (TCI, used asreceived) as a dispersant. A solution of TAPB (1.41 g, 4 mmol) in acetone (50 mL, >99.0%, Wako) was added to a solution of PMDA (1.31 g, 6 mmol) and the dispersant (0.3 wt.%) in acetone (50 mL). The molar ratio of PMDA to TAPB was 1.5:1. The mixture was stirred for 30 min at 0 °C. After the evaporation of the solvent, the curing reaction proceeded by heating the poly(amic acid) at 240 °C under vacuum to obtain polyimide nanoparticles. The prepared polyimide nanoparticles were carbonized by two-step pyrolysis. The polyimide precursor was heated to 900 °C for 5 h in a nitrogen atmosphere, and then heated again to 800 for 1 h in an ammonia atmosphere (50% balanced by nitrogen).


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Characterization: The chemical composition of the resultant carbon was determined using a CHN elemental analyzer (PerkinElmer 2400-II). The Brunauer–Emmett–Teller (BET) surface area was determined to be 992 m2 g-1 (Belsorp Mini II, Bel, Japan). Field-emission-scanning electron microscopy (FE-SEM; Hitachi S-5500) was used to investigate the particle morphology of the catalyst. X-ray photoelectron spectroscopy (XPS) was performed using a spectrometer (JPS9010MC, Jeol) equipped with a monochromator and an Al anode at 12 kV and 25 mA. To eliminate the electrostatic charging effect, the binding energy was corrected with respect to the C 1s signal at 284.6 eV derived from aromatic carbon. Then, the 1s XPS spectrum was carefully deconvoluted into four different nitrogen species: pyridinic nitrogen, pyrrolic nitrogen, graphite-like nitrogen, and nitrogen oxide using XPSPEAK 4.1. The background was determined according to the Shirley algorithm. Gaussian functions (the rest being Lorentzian) and the full width at half-maximum (FWHM) values were fixed at 80% and 1.5 (eV), respectively. The different nitrogen species were qualitatively determined from the corresponding peak area. RRDE voltammetry: The details of the RRDE voltammetry is described elsewhere14. Briefly, the N/C catalyst (2.5 mg) and 50 µL of Nafion solution (DE521) were dispersed in 150 Milli-Q water (Millipore; 18.2 M) and 150 µL of ethanol by sonication in the mixture of ice and water for 30 min to prepare homogeneous ink. 4 µL of ink was cast onto glassy carbon disc of a rotating ring-disk electrode (RRDE) corresponding to 100 µg cm-2. The other loading densities (120 - 160 µg cm-2) for RRDEs were prepared by adding different catalyst weight (3 - 4 mg) in a similar way to that of 100 µg cm-2. The modified RRDE was used throughout the measurements for ORR, HPRR (2.5 mM H2O2), and collection efficiency (2 mM FeCl3) in an electrochemical cell at room temperature using a reversible hydrogen electrode as reference electrode and a carbon rod connected with high surface area carbon cloth as counter electrode. The rotation speed was 1600 rpm. The disc potential


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was swept from 0 to 1.0 V while the ring potential was held at 1.1 V. The exact concentration of H2O2 was evaluated by titration. Based on the ring current and disk current, the obvious electron transfer number (n) and hydrogen peroxide yield were calculated using the following equations:

𝑛=

4𝐼𝐷 𝐼𝐷 +

(1)

𝐼𝑅 𝑁

𝐻2𝑂2(%) =

200 ∗

𝐼𝑅 𝑁

𝐼𝐷 +

𝐼𝑅 𝑁

(2)

Here, ID and IR are the disk current and ring current, respectively. N is the collection efficiency, which was measured in a 0.5 M H2SO4 solution containing 2 mM FeCl3. The kinetic rate constants for the ORR were evaluated using the mathematically modified Damjanovic model and Nabae model, as shown in Figure S1. The individual currents, I1′, I2′, and I3′, were calculated by substituting the ORR data and the evaluated value for k3 in the individual parameters into the following equations: 𝐼′2

𝐼′3

=

(

1 2

=

(

1 𝐼R 2 𝑁

―𝑁+

= 𝐼D ―

𝐼R 2

+

( ) (1 +

𝑁

4𝑘3

𝑁

( ) (1 +

4𝑘3

𝑁

𝐼R 2

𝐼r

𝐼R 2

𝐼′1

( ) (1 +

4𝑘3

))

1

𝑍2𝜔2

)

1

𝑍2𝜔2

))

1

𝑍2𝜔2

(3)

(4)

(5)


6

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where ID and IR are, respectively, the disk and ring currents in the ORR, N is the collection factor, and ω (s-1) is the rotation speed. The Z2 value was referenced from our previous study and was 6.50  10-4 cm s-1/2 14. Then, I1′ and I2′ were converted into k1′ and k2′ using equations (6) and (7). 1 𝑘′1

=

4𝐴𝐹𝑐O2

𝑘′2 =

𝐼′1

1+

―

2𝐼′2 𝐼′1

1 2

𝑍1𝜔

2𝐼′2 𝐼′1

(6)

𝑘′1

(7)

The individual currents, I10, I20, and I30, were calculated using the following equations: 𝐼01 = 𝑎(𝐼′1 +2𝐼′2) 𝐼02 =

𝐼′1 + 2𝐼′2 ― 𝐼01 2

𝐼03 = 𝐼D ― 𝐼01 ― 𝐼02

(8) (9) (10)

The intercept, a, was calculated from the I1′/(I1′ + 2 I2′) vs. γ plot. Finally, k10 and k20 were evaluated using the same treatment as equations (6) and (7). k3′ (equal to k30) was evaluated using HPRR voltammetry.

Results and discussion Catalyst characterization Figure 1 shows FE-SEM images of the N/C catalyst. Fairly uniform catalyst particles approximately 150 nm in diameter were observed. The BET surface area was determined to be 992 m2 g-1 by nitrogen adsorption, and the chemical composition of the resultant carbon was determined by elemental analysis: C 88.8 wt.% and N 5.3 wt.%, indicating that the nitrogen content was high. The relative content of different kinds of nitrogen species was further confirmed by XPS


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measurements, as shown in Figure 2. The N 1s spectra were deconvoluted into four peaks: pyridinic (398.2 eV), pyrrolic (400.3 eV), graphitic (401.4 eV), and oxidized (402.8 eV) nitrogen species 17. As shown in Table 1, the different nitrogen species contents (atomic ratio) were 2.09 at.% (pyridinic), 1.66 at.% (pyrrolic), 0.946 at.% (graphitic-like), and 0.262 at.% (nitrogen oxide). It has been proposed that pyridinic and graphitic nitrogen atoms are active sites for the ORR18–20. Pyridinic nitrogen accounted for 2.09% in the sample, which is much higher than those of other nitrogen species. Pyridinic nitrogen is located on the edge plane of graphite, where it bonds with two carbon atoms. The edge carbon atoms are very active compared with those in the basal plane of graphite. When a nitrogen atom is introduced to the edge site bonded with the carbon atoms, it results in the adjacent carbon next to the pyridinic nitrogen becoming a Lewis base, which could absorb oxygens molecule in the initial step of the ORR21. Graphitic nitrogen is also described as “quaternary nitrogen”, which is anchored in the graphite basal plane and bonded to three carbon atoms22. Both pyridinic and graphitic nitrogen atoms in graphene affect the electronic states of the adjacent carbon atoms, and these carbon atoms are considered to be catalytically active23. In addition, pyrrolic nitrogen has also been assumed to enhance ORR activity because it is similar to pyridinic nitrogen24–26. These nitrogen species have been proposed as catalytically active sites for the ORR27, 28. However, they may be active for two-electron reduction rather than the four-electron reduction in acidic media; this requires further confirmation9.


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Figure 1. FE-SEM image of the catalyst used in this study.

Figure 2. N1s XPS spectrum of the N/C catalyst with deconvoluted curves. The deconvoluted peaks correspond to pyridinic nitrogen (magenta), pyrrolic nitrogen (green), graphite-like nitrogen (navy blue), and nitrogen oxide (violet), respectively. The overall fitting line in the spectrum is red.


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Table 1 Atomic Ratios of Different Nitrogen Species Calculated from XPS Measurements Pyridinic Pyrrolic Graphitic-like Nitrogen N N N oxide N/C

2.09

1.66

0.946

0.262

Electrochemical behavior over the N/C catalyst To characterize the electrocatalytic behavior of O2 reduction over the as-prepared N/C catalyst, RRDE measurements were performed in O2-saturated 0.5 M H2SO4 solution at a loading density of 120 µg cm-2. As shown in Figure 3(a), the N/C catalyst exhibited a large disk current and larger ring current compared with other reported N/C catalysts for H2O2 synthesis9,10. The electron transfer number over the N/C catalysts ranged between 3 and 2, as shown in Figure 3(b), and the percentage of H2O2 produced varied from 50% to 80%. This indicates that O2 reduction over N/C tends toward the two-electron reduction pathway. Figure 3(c) shows the RDE voltammogram for the HPRR over the N/C catalyst. The potential was scanned from 0 to 1.0 V to minimize the influence of O2 by the oxidation of H2O2 at a high potential. The kinetic constant for H2O2 reduction, k3, was calculated using the Koutecky–Levich (K-L) equation: 𝐴 𝑖

= 2𝐹𝐶

1

H2O2𝑘′3

1

1

+ 2𝑍2𝐹𝐶H O 𝜔

―2

(11)

2 2

where i is the reduction current for the HPRR. Z2 was estimated to be 6.50  10-4 cm s-1/2. The twoelectron reduction pathway for oxygen was also determined from the slopes of the Tafel plots in Figure 3(d). The Tafel slope for the reaction over N/C was 149 mV dec-1, indicating that the electron transfer over N/C must overcome a high energy barrier compared with that over a Ptbased catalyst (60 mV dec-1 for the four-electron reduction pathway)29. Based on the above discussion, the N/C catalyst is a promising electrocatalyst for O2 reduction to H2O2.


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Figure 3. (a) RRDE voltammogram for the ORR over the N/C catalyst in O2-saturated 0.5 M H2SO4 solution. (b) Calculated percentage of hydrogen peroxide and the number of electrons transferred over the N/C catalyst. (c) RDE voltammogram for the HPRR over the N/C catalyst. (d) Tafel plots. Rotation: 1600 rpm. Scan rate: 10 mV s-1. Catalyst loading density: 120 µg cm-2.

Kinetic analysis over the N/C catalyst Figure 4(a) shows the RRDE voltammograms for the ORR over the N/C catalyst, indicating that the ORR activity increased with increasing loading density. As the loading density increased, the half-wave potential shifted positively to a higher potential range and the diffusion limiting current


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increased while the ring current density decreased. The ORR data were further analyzed kinetically. Figures 5(a,b) show the individual currents (I1′, I2′, and I3′) and corresponding kinetic rate constants (k1′, k2′, and k3′) evaluated using the mathematically modified Damjanovic model. The I-E and k-E curves are extremely smooth, validating the mathematical modification for the analysis based on the Damjanovic model. The smoothness appears because these parameters are directly calculated from the above equations rather than evaluated from the slope and intercept of the plots against the rotation speed. However, as demonstrated in our previous study, these analytical results still overestimate the rate constant for the direct four-electron reduction pathway, and a correction for a quasi-four-electron or quasi-two-electron pathway is necessary. Consequently, the kinetic parameters were further separated to investigate the inherent kinetic features for this specific N/C catalyst. I1′/(I1′+ 2I2′) vs. γ is plotted in Figure 4(b), and the intercepts were used to calculate I10, I20, and I30, respectively, which are shown in Figure 5(c). In addition, k10, k20, and k30 were calculated, which are displayed in Figure 5(d). This figure shows that, if there was no catalyst loading density correction to the calculation, the current for the four-electron process (I1' and corresponding k1') were overestimated, whereas the current for the two-electron process (I2' and corresponding k2') was underestimated. Figure 5(d) shows that N/C catalyst had a very large k20 compared with the negligible value of k10, indicating that the N/C catalyst had high selectivity for the two-electron ORR process. In addition, k30 was negligible, suggesting that the N/C catalyst exhibited poor catalytic activity for the HPRR. This shows that the N/C catalyst has great potential for H2O2 production. When the N/C catalyst loading density was decreased to 30 µg cm-2, the percentage of H2O2 was much greater than 80% over the whole potential range (Figure S6). This indicates that decreasing the catalyst loading could suppress the further reduction of H2O2 in the catalyst matrix layer. Compared with the values for the N/C catalyst listed in Table 2,


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the Fe/N/C catalyst prepared from the pyrolysis of Fe-containing polyimide particles has higher k10 and k30 values, indicating that Fe plays an important role in enhancing the activity of the fourelectron reduction pathway of oxygen reduction and the further reduction of hydrogen peroxide14. In other words, the N/C catalyst without Fe facilitates the electrochemical synthesis of hydrogen peroxide based on a fuel cell setup.

Figure 4. (a) RRDE voltammograms for the ORR over the N/C catalyst with different loading densities in O2-saturated 0.5 M H2SO4 solution. (b) I1′/(I1′+ 2I2′) vs. γ plot at various potentials. Rotation: 1600 rpm. Scan rate: 10 mV s-1.


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Figure 5. (a) Individual currents calculated based on the mathematically modified Damjanovic model. (b) Kinetic constants calculated based on the mathematically modified Damjanovic model. (c) Individual currents calculated based on the Nabae model. (d) Kinetic rate constants calculated based on the Nabae model. Rotation: 1600 rpm. Scan rate: 10 mV s-1. Loading: 120 µg cm-2. Data for other loading densities are shown in Figures S2–S5. Table 2. Kinetic Rate Constant Comparisons between the N/C and Fe/N/C Catalysts with a Loading of 120 µg cm-2 k10/k20

k30/cm s-1

N/C (0.3V)

0

0.00031

N/C (0.4V)

0

0.00017

Fe/N/C (0.3V)14

1.2

0.0057

Fe/N/C (0.4V)14

0.92

0.0035


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Based on the inherent kinetic rate constant evaluations over the N/C catalyst, only the twoelectron reduction of O2 occurred. Consequently, it was inferred that the electroreduction of O2 to H2O2 involves two coupled electron and proton transfer steps7, 30. ∗ 𝑂2 + 𝐻 + + 𝑒 ― → ∗ OOH ∗ OOH + 𝐻 + + 𝑒 ― → ∗ + H2O2

(12) (13)

In these equations, * denotes the active site, and *OOH denotes the single adsorbed intermediate for H2O2 production. First, an oxygen molecule became adsorbed on the active site, and this absorbed O2 was reduced to its intermediate, *OOH. Probably, the carbon atoms next to nitrogen atoms are responsible for the adsorption of oxygen21. Finally, the intermediate was combined with a proton and an electron to form hydrogen peroxide. During the process, the O-O bond was not broken31.The absence of transition metal centers probably contributes to the retention of the O-O bond. Consequently, the N/C catalyst favored hydrogen peroxide electrosynthesis. This mechanism corresponds to I20 and k20. If there was catalyst matrix layer formed on the electrode, H2O2 was further reduced to H2O, corresponding to the calculated values of I30 and k30.

Conclusions In this study, we evaluated the kinetic rate constants for the ORR over an N/C catalyst using the proposed mathematically modified Damjanovic and Nabae models. The mathematically modified Damjanovic model was first used to calculate the currents (I1', I2', and I3') and (k1', k2', and k3')


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using one rotational (1600 rpm) speed to reduce the calculation noise compared with the conventional Damjanovic model. The obtained current values (I1', I2', and I3') were further applied for the calculation of the inherent currents (I10, I20, and I30) with different loading density corrections, which were finally transformed to the inherent kinetic rate constants (k10, k20, and k30). The calculations show that the N/C catalyst has high selectivity for the two-electron reduction of O2, indicating that the N/C catalyst prepared from polyimide particles is very promising for H2O2 production based on a fuel cell design. Furthermore, to avoid the further reduction of the produced H2O2 in the catalyst matrix layer, the catalyst loading density should be decreased. In summary, this study provides quantitative insight into the oxygen reduction reaction mechanism over an N-doped carbon catalyst. ASSOCIATED CONTENT The Supporting Information is available free of charge. Supporting Information (PDF): Individual currents and corresponding rate constants for various catalyst loading densities calculated based on modified mathematically Damjanovic model and Nabae model. AUTHOR INFORMATION Corresponding Author *Tel: +81-3-5734-2429. Fax: +81-3-5734-2429. E-mail address: [email protected] Notes The authors declare that there are no competing financial interests.


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ACKNOWLEDGMENTS This work was financially supported by the New Energy and Industry Technological Development Organization (NEOD), Japan.

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Kinetic Understanding of the Reduction of Oxygen to Hydrogen

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