Electrochemical hydrogen storage in ironnitrogen dual-doped ordered mesoporous carbon

Electrochemical hydrogen storage in ironnitrogen dual-doped ordered mesoporous carbon

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Available online at www.sciencedirect.com

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Electrochemical hydrogen storage in ironenitrogen dual-doped ordered mesoporous carbon Deyu Qu a, Xiaoduo Xu a, Lina Zhou a,b, Wenjing Li a, Jorryn Wu a, Dan Liu a,*, Zhi-zhong Xie a, Junsheng Li a, Haolin Tang b,** a

Department of Chemistry, School of Chemistry, Chemical Engineering and Life Science, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, Hubei, PR China b State Key Laboratory of Advanced Technology for Material Synthesis and Processing, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, Hubei, PR China

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abstract

Article history:

Due to the advantage of catalytic activity on the process of hydrogen sorption/desorption

Received 10 October 2018

and high affiliation with hydrogen, iron nanoparticles are a hybrid within an ordered

Received in revised form

mesoporous carbon with nitrogen functional groups on the surface and are used as a host

24 January 2019

for improving the electrochemical hydrogen storage. This iron and nitrogen dual-doped

Accepted 28 January 2019

ordered mesoporous carbon material is synthesized through a nano-casting method

Available online xxx

which uses SBA-15 as a hard template and iron phthalocyanine as the iron, nitrogen and carbon source. After characterization of the synthesized material by SEM, TEM, XRD,

Keywords:

Raman, XPS and Nitrogen sorption/desorption measurements, this hybrid material

Electrochemical hydrogen storage

demonstrated a hydrogen storage capacity of 120 mAh g1. This is associated with the fast

Fe and N dual-doped mesoporous

kinetics of electrochemical hydrogen insertion, low self-discharge and the good cycling

carbon

capability. Furthermore, the hydrogen stored inside the synthesized composited carbon

Sublimation and capillary assisted

material shows excellent rate performance under various states of discharge current

nano-casting

densities. Results suggest that the enhanced electrochemical hydrogen storage perfor-

High hydrogen storage stability

mances may due to the electro-catalytic activity of iron nanoparticle, basicity nature of nitrogen functionality on the surface, mesoporous carbon with large surface area, large porosity and interconnected pore structure. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Using environmental benignity and renewable energy sources to reduce the greenhouse effect is a world-wide challenge. Hydrogen is considered to be a promising clean energy carrier with high energy density per mass. However, transportation and storage of hydrogen in a safe and effective manner is still a crucial factor impeding the wide application of hydrogen

energy. To become a host for high capacity of hydrogen storage, some requirements should be fulfilled, such as fast kinetic of hydrogen sorption/desorption, stable thermodynamic storage, long cycling life, light weight and cost-effectiveness. Electrochemical hydrogen insertion into carbonaceous material, which can be operated in ambient temperatures and mild pressures, is believed to be a promising approach [1e13]. In this way, hydrogen is inserted into the interlayer of the carbon materials in the atomic form, which are generated through

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (D. Liu), [email protected] (H. Tang). https://doi.org/10.1016/j.ijhydene.2019.01.273 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Qu D et al., Electrochemical hydrogen storage in ironenitrogen dual-doped ordered mesoporous carbon, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.01.273

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electrolysis of the aqueous solution upon the cathodic polarization of carbon. Among various other carbonaceous materials, ordered mesoporous carbon, especially heteroatom doped material with hierarchical structure, have already been applied in hydrogen storage as well as other electrochemical energy storage applications [10e37]. Our previous reports exhibited that nitrogen doped ordered mesoporous carbon can improve the hydrogen storage capacity and stability due to altering the electronic structure of the carbons surface and the stabilization of adsorbed hydrogen. Moreover, Platinum (Pt) doped carbon material was also investigated and showed enhancements to the kinetics of the hydrogen storage process through the spillover effect [10,11]. Other than the precious metal Pt, which is too expensive, transition metals were also believed to promote the hydrogen storage performance upon the carbon materials [18e24]. Iron, for example, has been demonstrated to improve the electrochemical hydrogen storage performance resulting from its high binding energy with hydrogen and electro-catalytic activity [19e24,38,39]. Furthermore, heteroatom doping can also generate more defect sites on the carbon surface to lower the surface energy and promote the hydrogen insertion [10,11,21]. Therefore, a hybrid material containing iron nanoparticles, nitrogen functional groups and carbon was developed and introduced in this study as a host for improving electrochemical hydrogen storage performance. This nitrogen and ironnanoparticle dual-doped carbon with highly ordered mesoporous structure (abbreviated as [email protected]) has been developed in our group recently [40]. Here, the electrochemical hydrogen storage capacity and stability, as well as the kinetics of electrochemical hydrogen sorption/desorption process upon this material are investigated. The carbon material without iron doping but carrying the same meso-structure and nitrogen functional group doping (abbreviated as [email protected]) is also synthesized and investigated as comparison.

Experimental section Synthesis of ordered mesoporous silica SBA-15 template The synthesis of the SBA-15 template was reported elsewhere [41]. Pluronics P123 triblock copolymer poly(ethylene oxide)-bpoly(propylene oxide)-b-poly(ethylene oxide) (Mave ¼ ~5800, Sigma-Aldrich) was dissolved in 1.6 M HCl solution and stirred at 35  C overnight. Tetraethyl orthosilicate (TEOS, Sinopharm) was then quickly added into the solution. After vigorous stirring for 5 min, the mixture was stored at 35  C for 20 h. Then the mixture was transferred to a hydrothermal reactor and heat treated at 130  C for 24 h. The solid product was collected and washed several times with ethanol and water. After being air-dried, the as-obtained sample was calcinated in air at 550  C for 6 h.

Synthesis of Fe and N dual-doped mesoporous carbon ([email protected]) The [email protected] material was synthesized through a so called “nanocasting” method using ordered mesoporous silica SBA15 as hard template and iron phthalocyanine (abbreviated as

FePc) as the iron, nitrogen and carbon source. As shown in Fig. S1, the FePc were sublimated first and their molecules entered the SBA-15 in the gas phase, then, they condensate inside the capillary structure of the template. After calcination and removal of the silica template, the [email protected] material with the replication of the negative structure of SBA-15 was achieved. 0.2 g FePc and 0.2 g SBA-15 mixed together and fully grounded. The mixture was treated at 550  C for 1 h followed by calcination at 700  C for 6 h the heating rate of 3  C min1 under argon atmosphere. The as-made carbon-silica composite was collected and immersed in an aqueous solution of HF (10 wt %) for 24 h to produce [email protected] Part of sample was further immersed in a boiled 6 M HCl solution for 7 h to remove Fe from [email protected] composite. In the end, the [email protected] and [email protected] materials were then obtained after filtration, washing with water and air drying.

Material characterizations Scanning electron microscope (SEM) images were taken using a Hitachi S-4800 field emission scanning electron microscope. Transmission electron microscopy (TEM) images were taken with a JEM 2100F electron microscope operating at 200 kV. Xray photoelectron (XP) spectra were obtained using a VG Multilab 2000 X-ray photoelectron spectrometer with Al KR radiation. Narrow-scan spectra of the C 1s, N 1s, and Fe 2p regions were obtained with a 25 eV pass energy, 300 W electron-beam power and resolution of 0.1 eV. Binding energy was calibrated with a C 1s peak at 284.6 eV. Raman spectra were collected on a Renishaw Invia Plus laser Raman spectrometer. The wide-angle X-ray diffraction measurement was performed on a Bruker D8 Advance diffractometer with a Cu Ka radiation (l ¼ 1.5406  A) operating at 40 kV, 40 mA. A Micromeritics ASAP 2020 porosimeter was used for the surface area and porosity measurements. Nitrogen was used as absorbent gas. Density function theory (DFT) method was also used to calculate the fine pores. A Perkin Elmer optima 2100DV ICP-OEX spectrometer was used for ICP measurements. The weight percentage of Fe in [email protected] materials is revealed as 1.3% by the ICP measurement. To make the electrode, [email protected] or [email protected] material (80 wt %) was mixed with conductive graphite (15 wt %) and Teflon suspension (5 wt % of dry material). After being thoroughly mixed, the pastes were left in air to dry. The resulting dough was rolled into a thin film, and then the electrode was punched out of the film with a geometric surface area of 1 cm2. The disc electrode with an overall mass of 9.5 ± 0.5 mg was then sandwiched between two pieces of a nickel foam current collector. An AutoLab electrochemical workstation (PGSTAT100N) was used for electrochemical measurements. The cyclic voltammogram and charge/discharge measurements were performed in a three-compartment electrochemical cell. The reference electrode was a Hg/HgO electrode, and all potentials in this study were referred to this reference electrode. A Pt mesh was used as a counter electrode. Aqueous KOH solution (30 wt%) was used as the electrolyte in all measurements. Electrochemical impedance spectroscopy was carried out at the frequency region from 0.01 Hz to 100 kHz with an amplitude of 5 mV.

Please cite this article as: Qu D et al., Electrochemical hydrogen storage in ironenitrogen dual-doped ordered mesoporous carbon, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.01.273

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Fig. 1 e SEM (A) and high resolution SEM (B) images of the [email protected] material.

Results and discussion Structure and morphology Fig. 1 shows the SEM images of [email protected] material. The short rod-like morphology with a size of around 0.6 mm is observed. The TEM images shown in Fig. 2A and B exhibit the stripe-like image along the (110) direction and a honeycomb-like image along the (001) direction, respectively. This clearly manifests that the synthesized [email protected] carries the highly ordered mesoporous structure indicating the successful replication of the negative structure of SBA-15. The EDS images, shown in Fig. S2, clearly illustrate the existence of iron and nitrogen along with the carbon material. This indicates the successful formation of Fe and N co-doped carbon material. The content of nitrogen shown in EDS image is very limited. This may suggest that the nitrogen functional groups are formed on the carbon surface, resulting in the generation of defect sites on the surface and promotion of the hydrogen insertion. It should be noted that, in Fig. 2A, several filament structures are observed. As indicated in a previous study [42], they are the carbon nanotubes generated from the calcination of the iron phthalocyanine, which adsorb on the surfaces or the pore openings of the SBA-15. The diffraction peaks around 26 and 43 , which represent the existence of small domains of graphite, are observed in

XRD patterns shown Fig. 3A. There are two peaks located around 1325 cm1 and 1592 cm1 which can be assigned to the D and G bands of carbon, respectively, and are detected in the Raman spectrum shown in Fig. 3B. It is widely recognized that the D band represents defects or structurally disordered carbon and the G band is attributed to the ordered sp2 bonded carbon. The ID/IG intensity ratio can be used to tell the graphitic microstructure of carbon materials. The lower the value, the higher the degree of graphitization of the carbon [43e46]. The ID/IG ratio is 1.05 indicating the formation of a carbon with weakly ordered sp2 bonded graphitic carbon domains, which agrees well with the XRD result. Those results indicate the amorphous carbon is formed within the [email protected] material. It should be noted that there are no diffraction peaks observed at 2 theta of 44.673 and 65.021 in the XRD pattern, which can be indexed to iron (JCPDS No. 060696). This may due to the fact that the content of iron in the synthesized [email protected] material is too low (only 1.3% based on ICP measurement) to be detected by XRD investigation. The N2 absorptionedesorption isotherms and the corresponding pore size distribution (PSD) curves of the [email protected] and [email protected] samples are shown in Fig. 4A and B, respectively. Both samples exhibit the standard type-IV isotherm with clear capillary condensation steps, indicative of the existence of mesoporous structure. Type-H2 hysteresis loop indicates the “ink-bottle” type pores with small entrances [13,14,47e49]. The measured BrunauereEmmetteTeller (BET) area of the Fe-

Fig. 2 e (A) TEM and (B) HRTEM image of [email protected] Please cite this article as: Qu D et al., Electrochemical hydrogen storage in ironenitrogen dual-doped ordered mesoporous carbon, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.01.273

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Fig. 3 e (A) XRD patterns and (B) Raman spectrum of the [email protected] material.

Fig. 4 e N2 adsorption/desorption isotherm (A) and pore size distribution (B) of the [email protected] and [email protected]

[email protected] and [email protected] sample was 853 m2 g1 and 710 m2 g1, respectively. The PSD curves for both samples (Fig. 4B) exhibit two peaks centered at 0.8 nm and 3.4 nm, suggesting the micro-meso hierarchical structure within the two carbon samples. Moreover, both samples have the identical N2 adsorption/desorption isotherms and PSD curves reveal that the removal of the Fe don't damage the pore structure of the materials. The synthesized [email protected] material is also investigated by X-ray photoelectron spectroscopy (XPS). Fig. 5A shows the full-range of the XPS survey. The carbon, oxygen, nitrogen, and iron have shown up. Fig. 5BeD shows the high-resolution XP spectra of C 1s, Fe 2p, and N 1s, respectively. The C 1s spectrum shown in Fig. 5B can be fitted to four different types of C functionalities that correspond to C]C (284.1 eV), CeC (284.8 eV), CeN (285.8 eV), and C]O (288.2 eV) bonds, respectively. The N 1s peak, presented in Fig. 5D, can be deconvoluted into three component peaks. They are pyridinic nitrogen (398.1 eV), pyrrolic nitrogen (399.4 eV) and graphitic nitrogen (400.5 eV). The binding energies of 710.7 and 723.7 eV shown in Fig. 5C are assigned to metallic Fe 2p1/2 and Fe 2p3/2 peaks, respectively. These results further prove that N and Fe heteroatoms have been doped into the mesoporous carbon framework.

Electrochemical hydrogen storage The electrochemical hydrogen storage performances of the synthesized [email protected] are investigated. Fig. 6A and B shows the charge/discharge curves and cyclic-voltammograms for [email protected] and [email protected], respectively. As shown in Fig. 6A, in

the charge-discharge profiles at a rate of 100 mA g1 (based on the mass of the whole electrode) in 30 wt% KOH aqueous solution, three regions exist in the charging curves. They are believed to be related to double-layer charging process, electrochemical hydrogen-insertion process and hydrogen evolution reaction, which match well with previous studies [7,8,10,11]. Similar with the discussions in the previous reports, as shown in Fig. 6A, once the charging time is over 1 h and the electrode potential reaches around 1.05 V vs. Hg/ HgO, the potential curves start to flatten out. In this region, the hydrogen evolution reaction takes place and competes with the electrochemical hydrogen-insertion process [8,10,11]. Therefore, as the charging time continuously increases, the coulombic efficiency of the hydrogen storage significantly drops. Due to this effect, no more than 3 h of charging time is tested in this study. The discharge curves clearly show that the hydrogen storage capacity of [email protected] is higher than that of [email protected], which may be due to the fact that certain amounts of hydrogen can be adsorbed on the surface of iron nanoparticles. This can also be revealed from the charge curves in Fig. 6A, while the potential change for the [email protected] electrode in the electrochemical hydrogen-insertion region is more sluggish comparing with the [email protected] electrode. This phenomenon agrees with the cyclic voltammograms in Fig. 6B, while the capacitance of [email protected] is larger than that of [email protected] Other than this, the CVs for both samples are similar. The current peaks at around 1.1 V and 0.25 V relate to the hydrogen and oxygen evolution reactions, respectively, and a pair of cathodic/anodic board peaks in the range of 0.2 V and 0.8 V can be attributed to the surface bonded nitrogen functional groups on the both samples [10,50].

Please cite this article as: Qu D et al., Electrochemical hydrogen storage in ironenitrogen dual-doped ordered mesoporous carbon, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.01.273

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Fig. 5 e XPS survey spectra (A) and high-resolution spectra in the region (B) C 1s, (C) Fe 2p and (D) N 1s of the [email protected] material.

Fig. 6 e (A) Chargeedischarge profile of the [email protected] (solid) and [email protected] (dashed) electrodes under an applied current density of 100 mA g¡1. (B) Cyclic voltammogram of the [email protected] (solid) and [email protected] (dashed) electrodes at a scan rate of 5 mV s¡1 in 30% KOH aqueous solution.

Fig. 7 shows the comparison of the discharge curves of [email protected] and [email protected] electrodes after both the electrodes where charged at varying periods of time. It can be seen from Fig. 7A, after charging both electrodes for 10 min, the H storage capacities in [email protected] and [email protected] electrodes are 24.4 and 16.9 mAh g1(based on the mass of the whole electrode), respectively. As the charging time increase to 20 and 30 min, as shown in Fig. 7B and C, the capacity of hydrogen storage in [email protected] electrode increases to about 45.7 and 66.6 mAh g1, respectively. In the case of the [email protected] electrode, the hydrogen loading is 33.2 and 50.2 mAh g1, respectively. When the charging time increases to 1, 2 and 3 h, as shown in Fig. 7DeF, the hydrogen storage capacity of [email protected] electrode increases to 109.6, 117.5 and 120.0 mAh g1, respectively. They are still higher than the values of 88.6, 92.4 and 92.0 mAh g1 recorded from the

[email protected] electrode, respectively. This result indicates that the Fe nano-particles doped on carbon material may act as a catalyst and promote the kinetic of H insertion process through spillover effect, which agrees with the previous reports [19e24]. As suggested in previous studies, the transition or precious metals doped on carbonaceous materials can adsorb the hydrogen and dissociate the hydrogen molecule into the atomic form, which significantly enhances the hydrogen storage capacity [15,22,51]. Obviously, the presented hydrogen storage capacity of assynthesized [email protected] material, which is only 120 mAh g1, is not attractive especially comparing with those of metal hydride alloys, transition metal nanoparticle composite materials and recently developed ternary oxide composite materials, which demonstrate much higher hydrogen storage capacity [52e82]. But the disadvantages of those metal-based

Please cite this article as: Qu D et al., Electrochemical hydrogen storage in ironenitrogen dual-doped ordered mesoporous carbon, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.01.273

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Fig. 7 e Comparison of the discharge curves for [email protected] (solid) and [email protected] (dashed) electrode after charged for (A) 10 min, (B) 20 min, (C) 30 min, (D) 1 h, (E) 2 h and (F) 3 h.

hydrogen storage materials, such as heavy, expensive and poor cycle life, limit their applications. Carbonaceous-based materials, on the other hand, with the merit of good conductivity, light weight, cost-effectiveness and environmental benignity can be used as a promising candidate for electrochemical hydrogen storage. The self-discharge of the hydrogen storage inside the [email protected] electrode is investigated. After charging the [email protected] and [email protected] electrodes at 100 mA g1 for 2 h, the electrodes remain in open-circuit potential (OCP) for various amounts of resting time followed by the discharging at an applied current density of 100 mA g1. As shown in Fig. 8A, during the initial stage of resting, a quick positive potential shift is observed. This potential change is found to be more significant on [email protected] electrode than of that on the [email protected] electrode. Then the potential positive changing on both electrodes is slowing down but the changing rate on ironfree electrode is still faster than that on the iron doped one. The observed potential positive shift during the resting period can be addressed to the leakage of the stored hydrogen (selfdischarge). Obviously, the hydrogen leakage becomes slower with the Fe doping. As exhibited in Fig. 8A, there is still about 81.5 mAh g1 capacity of hydrogen left in [email protected] electrode

after 24 h resting time. Meanwhile, the hydrogen storage left in [email protected] after resting for the same period of time is only 57.4 mAh g1. It can also be observed in Fig. 8B, after the same charge process and resting time, the capacities of hydrogen remaining in [email protected] electrode are always larger than those in [email protected] electrode. This demonstrates the low selfdischarge of electrochemical hydrogen storage in [email protected] material. The rate performance and the cyclic stability of the electrochemical hydrogen storage in the [email protected] electrode are also examined. Fig. 9A and B shows the rate capability of [email protected] and [email protected] electrodes, respectively. After charging the electrode with 100 mA g1 for 2 h, the discharge capacities of [email protected] electrode are found to be 125.3, 117.4, 99.4 and 85.3 mAh g1 at rates of 10, 100, 1000, and 2000 mA g1, respectively. In the case of the [email protected] electrode, its discharge capacities are 110, 92.4, 65.2 and 28 mAh g1 at rates of 10, 100, 500, and 1000 mA g1, respectively This clearly demonstrates that [email protected] electrode has much better rate performance indicating the iron doping will enhance the kinetic of electrochemical hydrogen desorption process. The cyclic performances for the [email protected] and [email protected] electrodes are investigated and the results are presented in

Please cite this article as: Qu D et al., Electrochemical hydrogen storage in ironenitrogen dual-doped ordered mesoporous carbon, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.01.273

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Fig. 8 e (A) Charging curves, open circuit potential profiles and discharge curves of [email protected] (solid) and [email protected] (dashed) electrodes under applied current density of 100 mA g¡1 in 30 wt% KOH solution. (B) Discharge capacities of [email protected] (circle & solid line) and [email protected] (star & dashed line) electrodes under applied current density of 100 mA g¡1 after charging the electrode with 100 mA g¡1 for 2 h in 30 wt% KOH solution followed by resting at various of time at open circuit potential.

Fig. 9C. One can easily tell that the [email protected] electrode carries good cyclic stability over the [email protected] electrode. When both the electrodes operate between 0 and 1.20 V under an applied current density of 100 mA g1 in 30 wt% KOH solution, the discharge capacities upon the [email protected] electrode slowly

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decreased from 117.4 mAh g1 to around 74.3 mAh g1 with 63.3% of retention after 200 cycles. Meanwhile, the discharge capacities of [email protected] electrode are found to decrease significantly from initial value of 93.2 mAh g1 to 31.2 mAh g1 around 33.5% retention after 200 cycles. The above results illustrate that with only 1.3% (weight percentage) of metallic Fe loaded onto composited carbon, the kinetic of electrochemical hydrogen insertion/desertion as well as the stability of hydrogen storage inside carbon material is significantly enhanced compared with those with iron free carbon material. It should be noted that, as demonstrated in previous studies, the order mesoporous structure and nitrogen doping can also improve the electrochemical hydrogen storage capacity and stability. For example, ordered mesoporous carbon carrying large surface areas, large porosity and interconnected pore structure may result in the fast electrolyte transportation and high adsorption capacity for hydrogen storage. And the basicity nature of nitrogen doping can stabilized the surface-adsorbed hydrogen. Fig. 10 shows the comparison of the impedance spectra for [email protected] and [email protected] electrodes. The Nyquist plot for both electrodes show two depressed semi-circles at the highmedium frequency region, resulting from the H generation from electrochemical reduction reaction followed by the H insertion into the electrode, and a relative straight line in the low frequency region representing the H diffusion in the carbon matrix [7,9e11,14]. The equivalent circuit shown is used to numerically fit those processes. Obviously, this equivalent circuit fit the spectra well. The electrochemical hydrogen generation and insertion into the carbon can be characterized by the charge transfer resistance, Rct,H2O and Rct,Hinsert,

Fig. 9 e Discharge curves of the [email protected] (A) and [email protected] (B) electrodes under various of applied current after charging the electrode with 100 mA g¡1 for 2 h and (C) Cycling performance of the [email protected] (circle) and [email protected] (star) electrodes operated between 0 and ¡1.2 V under applied current density of 100 mA g¡1 in 30 wt% KOH solution. Please cite this article as: Qu D et al., Electrochemical hydrogen storage in ironenitrogen dual-doped ordered mesoporous carbon, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.01.273

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H formation and insertion processes, for the [email protected] electrode, the results are smaller than those for the [email protected] electrode. Meantime, the K2 value for the iron-doped carbon material is over 1 order of magnitude higher than that for the iron-free one, indicating the fast H diffusion in [email protected] material. This further proves that the iron doping can improve the electrochemical hydrogen storage performances in the composited carbon material, which matches well with afore-presented results.

Conclusion Fig. 10 e AC impedance spectra of [email protected] (circle) and [email protected] (star) electrodes and the fitting results (solid line). Inset are the high frequency region of Nyquist plot and the equivalent circuit used for the fitting of the ac impedance spectrum.

respectively. A finite-length Warburg element represents the H diffusion in the high surface area porous carbon since the diffusion path is short enough to let H penetrate the entire medium thickness during the low frequency modulation. The finite-length Warburg element can be expressed as [9e11]: h i 1=2 Z ¼ ð1  iÞsu1=2 tanh dði=uDÞ

(1)

Where d represents the effective diffusion thickness and D is the effective diffusion coefficient of the particle. "  1=2 # Zu ¼ ð1  iÞsu1=2 tanh d iu=D

(2)

when sffiffiffiffiffiffiffi 2d2 2d2 or D ¼ 2 ¼ 2d2 K2 K¼ D K

(3)

In this study, metallic Fe nano-particles and nitrogen functionality are in-situ formed on an ordered mesoporous carbon material through a nanocasting method using SBA-15 as a hard template and iron phthalocyanine as precursors. The formed Fe-N co-doped ordered mesoporous carbon is then used as host for the electrochemical hydrogen storage. An electrochemical hydrogen storage capacity of 120 mAh g1 is demonstrated. It also exhibits low self-discharge (81.5 mAh g1 of hydrogen storage capacity left after 24 h resting), good cyclic stability (63.3% of retention after 200 cycles) and good rate performance (85.3 mAh g1 at 2 A g1). The enhanced electrochemical hydrogen storage performance is suggested to be attributed to the doped iron nanoparticles, which catalytically promotes the kinetics of the electrochemical hydrogen storage process.

Acknowledgment This work was partially supported by the National Natural Science Foundation of China (11474226, 51676143), Fundamental Research Funds for the Central Universities (WUT: 2017-IB-003, 2018-IB-028, 2018-IB-022). The authors thank Dr. Xiao-Qing Liu for his assistance conducting TEM.

then pffiffiffiffi pffiffiffiffi sðsinh K u þ sin K u Þ pffiffiffiffi pffiffiffiffi Z0u ðrealÞ ¼ pffiffiffiffi uðcosh K u þ cos K u Þ

(4)

pffiffiffiffi pffiffiffiffi sðsinh K u  sin K u Þ pffiffiffiffi pffiffiffiffi Z0u ðimaginaryÞ ¼ pffiffiffiffi (5) uðcosh K u þ cos K u Þ qffiffiffiffiffi 2 Here, the parameter K is defined as 2dD . And both s and K can be obtained by means of least square fitting of the ac impedance results. As demonstrated in eq. (3), the value of K2 is proportional to the diffusion coefficient (D). The fitted results are shown in Table 1. The results clearly show that the values of Rct,H2O and Rct,Hinsert, corresponded to

Table 1 e The value of Rsol, Rct,H2O, Rinsert,H and K¡2 obtained through the numerical fitting of the ac impedance spectrum. Rsol (U) [email protected] [email protected]

0.50 ± 0.005 0.48 ± 0.006

Rct,H2O (U) Rinsert,H (U) 0.38 ± 0.02 1.51 ± 0.04

0.14 ± 0.01 1.05 ± 0.06

K2 (s1) 1.7 ± 0.15 0.16 ± 0.005

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.01.273.

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