Article pubs.acs.org/JPCC
Enhancement of Electrochemical Hydrogen Insertion in N‑Doped Highly Ordered Mesoporous Carbon Dan Liu,† Dong Zheng,‡ Lili Wang,† Deyu Qu,*,† Zhizhong Xie,† Jiaheng Lei,† Liping Guo,† Bohua Deng,† Liang Xiao,† and Deyang Qu*,‡ †
Department of Chemistry, School of Science, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, Hubei, P.R. China ‡ Department of Chemistry, University of Massachusetts Boston, 100 Morrissey Boulevard, Boston, Massachusetts 02482, United States ABSTRACT: An N-doped highly ordered mesoporous carbon material is synthesized. The nitrogen functionalities are found to be chemically bonded on the carbon surface, and the mole ratio of N to C is 1:100. The electrochemical H insertion into the carbon interlayers is enhanced 1.5 times for the N-doped carbon electrode. Such a promotion of H insertion is believed to result from the interference of the Ndopants with the H+H recombination and the carbon surface electronic structure; therefore, the competitive hydrogen evolution path is discriminated against in favor of the H insertion path.
I. INTRODUCTION Hydrogen has been widely considered as a renewable and environmentally benign energy resource. Finding a safe, cheap, and reliable method to store and transport hydrogen remains a daunting challenge. A wide variety of methods attempt to tackle this challenge, for example, hydrogen compression and liquefaction under cryogenic conditions, chemical hydrides, and gas-on-solid adsorption. Another method that can be investigated is electrochemical hydrogen storage in which atomic hydrogen is generated through electrochemical water electrolysis and becomes adsorbed into carbonaceous materials. This technique has been demonstrated as a good method for the storage of hydrogen under ambient conditions.1−6 Recently, this hydrogen insertion carbon material has also been successfully used as the negative electrode in an asymmetric supercapacitor7,8 and as the anode in a fuel cell.9 Although it is very appealing for a hydrogen insertion carbon electrode to be used in an asymmetric supercapacitor, with all of the benefits, for example, performance under ambient conditions, low cost, and safety, the amount of hydrogen stored in the carbon matrix is still below expectation for practical H2 storage applications. Various efforts have been made to increase the H2 storage capacity: use of carbon nanotubes,10 optimization of carbon pore structure,11 and surface structure optimization.12 In our recent report,13 surface catalytic poison was found to substantially enhance the electrochemical H2 storage in the carbon electrode. The electrosorbed H on the surface of the carbon electrode can either insert into the carbon matrix or recombine with the neighboring H forming gaseous H2. The surface catalytic poison enhances H insertion by discriminating against the later process.13 Ordered mesoporous carbon was investigated as hydrogen-storage material.14 It was suggested © 2014 American Chemical Society
that porosity was the dominant factor in determining the electrochemical hydrogen-storage capacity. However, it is possible that irreversible oxidation of the nitrogen functional groups could prevent the hydrogen from interacting with the material and thus reduce this storage capacity. In this report, the enhancement of electrochemical hydrogen storage is also discovered on a nitrogen-doped carbon with highly ordered mesoporous structure (HOMC).
II. EXPERIMENTAL DETAILS All chemicals were of the best quality available commercially and used as received. The preparation of N-doped HOMC has been reported elsewhere.15,16 Resorcinol, hexamethylenetetramine (HMT), triblock poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) copolymers Pluronic F127, 1,3,5trimethylbenzene (TMB), aqueous ammonia (28 wt %), and deionized water were mixed with the molar ratio of 1/0.5/ 0.0159/0.33/2.7/300. The mixture was stirred for 1 h at room temperature and then 24h at 80 °C. Then the product was collected and calcined at 900 °C for 3 h under a nitrogen atmosphere. TEM images were taken with a JEM 2100F electron microscope operating at 200 kV. SEM images were taken using a Hitachi S-4800 field-emission scanning electron microscope. XP spectra were obtained using a VG Multilab 2000 X-ray photoelectron spectrometer with a Mg KR radiation. Narrow-scan spectra of the C 1s and N1s regions were obtained with 15 eV pass energy and 100 W electronReceived: December 10, 2013 Revised: January 14, 2014 Published: January 15, 2014 2370
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in the galvanostatic curves are the potential for hydrogen evolution. Clearly, the overpotential for N-HOMC shown in Figure 1B is larger than that for HOMC. The doping of nitrogen onto the carbon surface could alter the electronic structure on the surface. For example, it could make the carbon atoms electron-deficient by attacking electrons, which would make H more amicable on the carbon surface. High hydrogen reduction overpotential indicates the limitation in H2 evolution, particularly described by Heyrovsky’s reaction:
beam power and are solution of 0.1 eV. Binding energy was calibrated with a C 1s peak at 284.6 eV. Micromeritics ASAP 2020 porosimeter was used for the surface area and porosity measurements. Nitrogen was used as absorbent gas. Density functional theory (DFT) software from Micromeritics was also used. Before electrochemical measurements, all carbon materials were reflux-washed with acetone in a Soxhlet extractor for ∼24 h to remove the physical bonded surface functional groups. The porous carbon powder (85 wt %) was mixed with carbon black (10 wt %) and Teflon suspension (5 wt % of dry material). After being thoroughly mixed, the paste was left to air-dry. The resulting Teflon-bonded carbon was rolled into a thin film; then, the electrode was punched out of film. The disc electrodes were sandwiched between two pieces of nickel foam current collectors. Aqueous KOH solution (30 wt %) was used as an electrolyte in all measurements. A CH Instruments 660C electrochemical workstation was used for electrochemical measurements. An Hg/HgO reference electrode was used in all measurements.
CHad + H 2O + e → H 2 + OH + C
(1)
The nitrogen surface functional groups could also prevent the desorptive recombination (Tafle reaction)6 as a poison catalyst would do:13
2CHad ⇆ H 2
(2)
Both effects can impact the kinetics of the hydrogen evolution reaction and result in high overpotential for the reaction. It is worth mentioning that the voltage difference during the charge and discharge of the N-doped HOMC and HOMC electrodes shown in Figure 1B mainly resulted from the H insertion and extraction kinetics. The open circuit potentials (OCPs) for the N-doped HOMC and HOMC in various stages are tabulated in Table 1. Figure 2 shows the comparison of TEM and SEM images of the HOMC and N-doped HOMC. An ordered pattern on the nanometer scale and worm-like morphology on a micrometer scale are clearly demonstrated for both carbon materials, which are consistent with those previously reported.16 The existence of N on the HOMC is verified by the XPS measurement. Figure 3B shows XP spectrum in N 1s region. An N 1s peak with binding energy around 401 eV is clearly displayed for the N-doped HOMC but not evident for the pristine HOMC. Deconvolution of the peak reveals three peaks that can be assigned to pyrrolic nitrogen (400.7 eV), quaternary nitrogen (401.6 eV), and oxidized pyridinic nitrogen (402.9 eV), respectively. The C 1s peak is shown in Figure 3A. The binding energy of 284.6 eV is in good agreement with that of C 1s reported in the literature.17 The relative molar amount of N and C was determined using the following equation17
III. RESULT AND DISCUSSION Figure 1A,B shows the comparison of cyclic voltammetry and constant current charge/discharge curves for N-doped HOMC
C N/CC = (AN /RN)/(A C /R C)
(3)
where A is the peak area and R is the atom sensitivity factor. With RN = 0.205 and RC = 0.38,17 the ratio of N to C (CN/CC) was found to be 1%. The HOMC without N exhibits similar morphology. Figure 4 shows the comparison of pore distribution of HOMC and N-doped HOMC. The pore distributions in both materials were very similar; the pores in both materials were narrowly distributed around the size of 3.5 nm. The total pore volume of HOMC was higher than that of N-doped HOMC. The capacity of the electrochemical H adsorption has been proven to relate to the pore structure of the host material,5 and both pore size distribution and pore volume play important roles. Comparing the pore distribution shown in Figure 3, one would assume that the capacity of H electrochemical absorption in HOMC would be higher than that in N-HOMC. However, as shown in Figure 1B, almost 1.5 times as much H became electrochemically absorbed in N-HOMC compared with HOMC. Therefore, although porosity may play a role, other factors must be considered. The electrochemical properties of a porous carbon material are closely related to the surface functionalities.18 In general,
Figure 1. Cyclic voltammograms (A) with scan rate of 10 mV s−1 and charge−discharge curves (B) under an applied current of 1 mA of nitrogen-doped HOMC electrode (solid line) and HOMC (dashed line) recorded in a three-electrode cell in 30% KOH solution.
and HOMC, respectively. As shown in Figure 1A, the hydrogen evolution potentials for both electrodes are around −1.2 V versus Hg/HgO. However, the hydrogen evolution overpotential for N-HOMC is larger than that for HOMC, and the reaction kinetics for N-HOMC is also more sluggish compared with that for HOMC. The phenomena are consistent with that demonstrated in Figure 1B; the flat potential regions 2371
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Table 1. Comparison of Open Circuit Potential (OCP) for N-Doped HOMC and HOMC in Various Stages of Charge and Discharge N-doped HOMC OCP (V vs Hg/HgO)
HOMC OCP (V vs Hg/HgO)
−0.23 −1.04 −0.85 −0.25
−0.28 −1.025 −0.81 −0.23
before charge after 10 mA, 4 h charge, 1 min rest after 10 mA, 4 h charge, 48 h rest after 10 mA full discharge, 48 h rest
Figure 4. Comparison of pore size distribution for N-HOMC and HOMC. Figure 2. TEM image (A,B) of nitrogen-doped HOMC and HOMC, respectively; (C,D) high-magnification SEM images of nitrogen-doped HOMC and HOMC, respectively.
an aromatic-to-alkane π bond restructuring.20 The presence of oxygen surface functional groups has a negative effect on the hydrogen insertion, which could result from the saturation of the active sites on the carbon surface with oxygen groups21 or aromatic-olefin restructuring. The nitrogen groups, however, would be associated with resonating electrons of carbon aromatic rings and capability of binding with protons due to their basicity nature, both of which could stabilize the electrosorbed H on the carbon surface. The electrochemical hydrogen insertion is a competitive route against the gaseous evolution of H2 pathway. In an alkaline environment, atomic H first becomes electrosorbed on the carbon surface after H2O molecules are reduced; then, the electrosorbed H could either insert into the carbon host or become gaseous H2, escaping from the electrode surface. Figure 5 shows the comparison of the typical impedance spectra for an N-doped HOMC and a HOMC. In Figure 5A, the semicircles at the high-frequency region resulted from the charge electrochemical reduction reaction and the relative straight line in the low frequency region represents the H diffusion into the carbon matrix due to the fact that H diffusion is the rate-determining step.13 The equivalent circles used to fit the two processes are also illustrated in Figure 5A. Because the graphite microdomain, Lc (which is the diffusion length for H), in the high surface area carbon is so small, the H diffusion path is short enough that H would penetrate the entire thickness during the low-frequency modulation. Thus, a finite Warburg impedance was used, which indeed fit the ac impedance spectra much better than a semi-infinite model, as shown in Figure 5A. A finite Warburg impedance can be expressed as:
Figure 3. Comparison of XP spectrum of the nitrogen-doped HOMC and HOMC in the regions (A) C 1s and (B) N 1s.
Z = (1 − i)σω−1/2 tanh[δ( i/ωD)1/2 ]
oxygen-containing surface groups would make a carbon surface more acidic in character,19,20 while the basic character of a porous carbon is more likely to result from nitrogen functional groups.19 Inductive effects resulting from the electron-withdrawing surface oxygen groups were found to be responsible for
(4)
where δ represents the effective diffusion thickness and D is the effective diffusion coefficient of the particle. Zω = (1 − i)σω−1/2 tanh[δ( iω/D)1/2 ] 2372
(5)
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coefficient could change as well. Because H is appreciably smaller than C atoms in the host material, it could move either between the graphene layers or within the nanopores created by the graphene sheets. However, the high surface area carbon matrix, like the one we used in the study, consists of a large amount of defective graphite microdomains, in which the H is believed to be stored. H may not move within such defective microdomains in the same manner as Li diffuses within a relatively well-crystallized graphite host in a Li-ion battery anode. The H could be trapped on a low-energy defect site forming a metastable state. Such modification would alter the diffusivity of H in the host. Other thermodynamic factors, for example, concentration gradients, electrode potential would have impacts on such a metastable state. Figure 5B shows the comparison of the change of K−2 on the N-doped HOMC and HOMC with reduction potentials. It seems that the change of diffusion coefficient for the two carbon electrodes has the same trend during the reduction. The diffusion coefficients for the both electrodes started at similar low values before the H under-potential deposition occurred at below −0.8 V versus Hg/HgO. The diffusion coefficient for the both electrodes starts to increase at around −0.9 V, which is consistent with what we previously reported.13 Apparently, the increase in diffusion coefficient resulted from the enhancement of H insertion into the carbon matrix by increasing H surface coverage because a substantial amount of H atoms were generated and became electrosorbed on the carbon surface. At high overpotential, for example, less than −1.3 V, the diffusion coefficients for both electrodes went back to their initial values before the reduction. It is believed that the carbon matrixes were fully loaded with H. Almost all electrochemically generated H atoms recombined, forming gaseous H2. At the potential, the H insertion into carbon matrix ceased. Figure 6
Figure 5. (A) AC impedance spectra (circle) and fitting results (line) for N-doped HOMC and HOMC electrodes. Inserts are the equivalent circuits used for the fitting of the ac impedance spectra. The equivalent circuit on the left was used to fit the spectra in the frequency range of 100 K to 1 Hz, and the one on the right was used for the frequency range of 1 to 0.001 Hz. (B) Comparison of K−2, which is proportional to the hydrogen diffusion coefficient inside the carbon materials. K−2 was obtained by fitting the impedance spectra within 1 to 0.001 Hz.
when K=
2δ 2 2δ 2 or D = 2 = 2δ 2K −2 D K
(6)
Then Zω′ (real) =
σ (sinh K ω + sin K ω ) ω (cosh K ω + cos K ω ) σ (sinh K ω − sin K ω ) ω (cosh K ω + cos K ω )
(7)
(8)
Figure 6. Comparison of the discharge curve for N-doped HOMC under various discharge currents.
Both σ and K can be obtained by means of the least-squares fitting of the ac impedance results. As demonstrated in eq 6, the change of K−2 has the same trend as that of the diffusion coefficient (D) assuming that diffusion path did not change during polarization. It is worth emphasizing that unlike the diffusion in a continuous medium, for example, ionic diffusion in a solution, which follows Fick’s laws, the diffusion coefficient inside a solid electrode is closely associated with both the electronic and physical structures of the host material, especially with the defects in the host material. Therefore, as the physical conditions of the host material and external thermodynamic driving forces, for example, overpotential change, the diffusion
shows the discharge curves of the N-doped HOMC electrode discharged at different currents. The discharge capacity is clearly related to the discharge current density, which further demonstrated the kinetics control of the process. The major difference between the H diffusion coefficients for N-doped HOMC and regular HOMC was demonstrated in the potential range between −0.9 and −1.2 V versus Hg/HgO. The H-diffusion rate in the N-doped HOMC was significantly higher than that of regular HOMC, as shown in Figure 5B. The sharp increase in H diffusion coefficient for the N-doped HOMC could result from the higher H coverage, slightly stronger interaction between H and the carbon surface, and
Zω″(imaginary) =
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higher overpotential, which will help to increase the H surface coverage. To force H ad-atoms into hardly accessible narrow slot-shaped nanopores, the H concentration gradient needs be high enough and the process itself becomes effective only at very high H coverage on the outer porous carbon material surface. The higher H-surface coverage would not only help to accelerate the H insertion rate into the carbon matrix but also enable H to access the pores otherwise barely accessible. Therefore, both the diffusion coefficient and H capacity for the N-HOMC were higher than the HOMC.
IV. CONCLUSIONS A nitrogen-doped highly ordered mesoporous carbon has been prepared via a one-pot aqueous route and applied to electrochemical hydrogen insertion. After a small number of nitrogen groups prebonded on the surface of HOMC, the prepared carbon materials show a 100 mV negative shift in hydrogen evolution overpotential compared with a pristine HOMC. The capacitance of the synthesized N-doped HOMC is about three times greater than that of the HOMC. The enhancement of the H electrochemical absorption inside NHOMC is believed to result from the change of the electronic structure of the carbon surface, which could not only hinder the hydrogen evolution (both Heyrovsky and Tafle pathways), thus increasing the H surface coverage and improving the H insertion into the carbon matrix, but also make the H ad-atoms more stable.
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AUTHOR INFORMATION
Corresponding Authors
*Deyu Qu: Tel: +86 27 87756662. E-mail: deyuqu_72@yahoo. com. *Deyang Qu: Tel: +1 617 287 6035. Fax: +1 617 287 6185. Email:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the Fundamental Research Funds for the Central Universities China 2013-aV016 and 2013-Ia-034 and the Natural Science Foundation of China (61274135). The authors thank Dr. Xiao-Qing Liu for his assistance conducting TEM.
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REFERENCES
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