Influence of phosphoric acid treatment on hydrogen adsorption behaviors of activated carbons

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    The scope of this work investigates the relationship between the amount of oxygen-functional groups and hydrogen adsorption capacity with different concentrations of phosphoric acid. The amount of oxygen-functional groups of activated carbons (ACs) is characterized by X-ray photoelectron spectroscopy. The effects of chemical treatments on the pore structures of ACs are investigated by N2/77 K adsorption isotherms. The hydrogen adsorption capacity is measured by H2 isothermal adsorption at 298 K and 100 bar. In the results, the specific surface area and pore volume slightly decreased with the chemical treatments due to the pore collapsing behaviors, but the hydrogen storage capacity was increased by the oxygen-functional group characteristics of AC surfaces, resulting from enhanced electron acceptor-donor interaction at interfaces.


    hydrogen storage , activated carbon , chemical treatments

  • 1. Introduction

    Hydrogen has attracted a great deal of attention as a clean energy source and alternative fuel. Economical and efficient on-board hydrogen storage systems are critical to the success of the proton exchange membrane fuel cell technology for transportation applications [1-3]. The currently available technologies for hydrogen storage are gas compression, cryogenic liquefaction, intercalation in host metal, metal hydrides, and hydrogen physisorption. Many studies are focusing on improving present technologies and searching for advanced materials such as adsorbents [4]. Among them, carbonaceous materials are being investigated as potential hydrogen storage media because of their high specific surface area and large pore volume [5], and these carbonaceous materials include carbon nanotubes (CNTs) [6], carbon nanofibers (CNFs) [7,8], graphene [9,10], and traditional activated carbons (ACs) [11,12]. In particular, ACs have the advantage of low mass density, availability, and low cost compared with other carbonaceous materials such as CNTs, CNFs, and graphene [3,13,14]. Most of all, ACs have been widely used as adsorbent to remove organic or inorganic pollutants because of their extended specific surface area, high adsorption amount and rate, and specific surface reactivity [15,16].

    Surface complexes onto carbonaceous materials are considered as an important factor in hydrogen adsorption [17]. It has been reported that hydrogen adsorption capacity on CNTs modified by acidic or basic chemical treatments are increased or diminished because of electron-withdrawing or -donating effects, and, as such, are primarily divided into acidic and basic groups, respectively [18].

    In this study, we have investigated chemically treated ACs with H3PO4 concentrations. We have also studied the effects of pore structure and oxygen-functional groups of ACs on hydrogen adsorption behaviors.

    This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License ( which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

    2. Experimental

       2.1 Materials and sample preparation

    ACs were purchased from Tokyo Chemical Industry Co.Ltd. (Tokyo, Japan) Prior to the phosphoric acid treatment, ACs were washed several times with distilled water and dried in a vacuum oven at 90℃ for 24 h. One gram of the ACs was immersed with 50 mL different concentration phosphoric acid solutions for 12 h at 40℃. The H3PO4 treated ACs samples were filtered and washed with distilled water and dried in a vacuum oven for 24 h at 80℃. The H3PO4-treated ACs had as-received H3PO4 concentrations denoted as AC-0.5, AC-1, and AC-2, respectively.

       2.2 Characterization

    The surface analysis of ACs was studied by X-ray photoelectron spectroscopy (XPS). XPS measurements were made with a Thermo Scientific (USA), K-Alpha device with a monochromated Al Kα X-ray source (1486.6 eV). Survey spectra were recorded at O1s and C1s photoelectron peaks. The porous texture of AC was investigated by N2/77 K adsorption/desorption isotherms using a gas adsorption analyzer with a BELSORP (Japan).

       2.3 Hydrogen storage capacity

    The hydrogen uptake experiment was conducted under an ambient condition of 298 K and 100 bar, conditions compatible with future electric-vehicle applications. In each experiment,about 0.2 g of samples were loaded in a stainless chamber. Prior to measurement, the chamber was evacuated at 423 K for 2 h.After the chamber was cooled to room temperature, hydrogen was introduced until a pressure of 100 bar was attained. An ultra-high-purity grade (99.9999%) of hydrogen was used in this study so that the influences of moisture and other impurities could be excluded.

    3. Results and Discussion

    XPS analysis was carried out to analyze the elemental composition of the chemically treated ACs with the H3PO4 concentrations. The results are summarized in Table 1. After the H3PO4 treatments, the oxygen content is increased in the range between 12.1 and 17.5%. The O1s/C1s ratio is also increased with an increase in the H3PO4 concentration.

    Fig. 1 shows the nitrogen adsorption/desorption isotherms for the samples studied. It was found that the all samples are composed of micropores and mesopores, showing from the Type I (relative pressure < 0.05) and Type IV (relative pressure range between 0.5 the 0.99), based on the classification recommended by International Union of Pure and Applied Chemistry [19]. Detailed information on the textural properties of the samples is listed in Table 2, and mesopore volume was determined from the Barret-Joyner-Halenda equation. As shown in Table 2, specific surface area and mesopore volumes gradually decreased with an increase in the H3PO4 concentrations. The porosities of AC-

    0.5, AC-1, and AC-2 were found to be 1,404 m2/g, 1,370 m2/g, and 1,270 m2/g for a specific surface area, respectively. It was found that the specific surface area and mesopore volume of all of the treated samples are slightly decreased by the introduction of oxygen-functional groups in the inner walls of pores, resulting in a decrease of mesopore sizes.

    Fig. 2 shows the hydrogen adsorption behaviors of the sam-

    ples.It was found that all of the chemically treated ACs showed higher hydrogen uptake than did the as-received sample. This result clearly indicates that the presence of oxygen-functional groups influences the hydrogen uptake of AC, meaning that the oxygen-functional group plays the role of more accessible sites on the ACs to hydrogen molecules. Thus, the hydrogen adsorption capacity of as-received and AC-0.5 and AC-1 samples after chemical treatments increased from 0.22 wt% to 0.37 and 0.43 wt%, respectively. From the results, the formation of hydrogenfriendly sites on AC by the H3PO4 treatment is important factor in hydrogen adsorption, resulting in electron acceptor-donor interaction at the interfaces [20,21]. It is revealed that the concentration of H3PO4 is important in enhancing the hydrogen adsorption behaviors. In addition, it is interesting note to that the hydrogen adsorption capacity of the AC-2 sample decreased to 0.30 wt%, which seems to be approaching its upper limit under these conditions.

    4. Conclusions

    In this work, the influence of chemical treatments with the H3PO4 concentrations on AC was investigated for hydrogen adsorption behaviors at 298 K and 100 bar. The hydrogen adsorption capacity was enhanced in the presence of oxygen functional groups for the all chemically treated samples. The chemical treatments can generate a higher amount of oxygen-functional groups and enhanced hydrogen adsorption capacity of ACs. However, in the case of excessively treated AC (AC-2), the hydrogen adsorption capacity begins to decrease. Consequently,the optimization of chemically treated ACs is essential for increasing the hydrogen adsorption capacity.

  • 1. Schlapbach L, Zuttel A 2001 Hydrogen-storage materials for mobile applications. [Nature] Vol.414 P.353 google doi
  • 2. Elam CC, Padro CEG, Sandrock G, Luzzi A, Lindblad P, Hagen EF 2003 Realizing the hydrogen future: the International Energy Agency's efforts to advance hydrogen energy technologies. [Int J Hydrogen Energy] Vol.28 P.601 google doi
  • 3. Dillon AC, Heben MJ 2001 Hydrogen storage using carbon adsorbents:past present and future. [Appl Phys A: Mater Sci Process] Vol.72 P.133 google doi
  • 4. Xu WC, Takahashi K, Matsuo Y, Hattori Y, Kumagai M, Ishiyama S, Kaneko K, Iijima S 2007 Investigation of hydrogen storage capacity of various carbon materials. [Int J Hydrogen Energy] Vol.32 P.2504 google doi
  • 5. Zacharia R, Kim KY, Hwang SW, Nahm KS 2007 Intrinsic linear scaling of hydrogen storage capacity of carbon nanotubes with the specific surface area. [Catal Today] Vol.120 P.426 google doi
  • 6. Park SJ, Lee SY 2010 Hydrogen storage behaviors of platinum-supported multi-walled carbon nanotubes. [Int J Hydrogen Energy] Vol.35 P.13048 google doi
  • 7. Park SJ, Kim BJ, Lee YS 2008 Patent trends of carbonaceous materials for hydrogwn storage (III): major applicants & technology flowchart. [Carbon Lett] Vol.9 P.35 google
  • 8. Park SJ, Kim BJ, Lee YS, Cho MJ 2008 Influence of copper electroplating on high pressure hydrogen-storage behaviors of activated carbon fibers. [Int J Hydrogen Energy] Vol.33 P.1706 google doi
  • 9. Ma LP, Wu ZS, Li J, Wu ED, Ren WC, Cheng HM 2009 Hydrogen adsorption behavior of graphene above critical temperature. [Int J Hydrogen Energy] Vol.34 P.2329 google doi
  • 10. Lopez-Corral I, German E, Volpe MA, Brizuela GP, Juan A 2010 Tightbinding study of hydrogen adsorption on palladium decorated graphene and carbon nanotubes. [Int J Hydrogen Energy] Vol.35 P.2377 google doi
  • 11. Chen GX, Hong MH, Ong TS, Lam HM, Chen WZ, Elim HI, Ji W, Chong TC 2004 Carbon nanoparticles based nonlinear optical liquid. [Carbon] Vol.42 P.2735 google doi
  • 12. Huang CC, Chen HM, Chen CH 2010 Hydrogen adsorption on modified activated carbon. [Int J Hydrogen Energy] Vol.35 P.2777 google doi
  • 13. Panella B, Hirscher M, Roth S 2005 Hydrogen adsorption in different carbon nanostructures. [Carbon] Vol.43 P.2209 google doi
  • 14. Li J, Cheng S, Zhao Q, Long P, Dong J 2009 Synthesis and hydrogenstorage behavior of metal-organic framework MOF-5. [Int J Hydrogen Energy] Vol.34 P.1377 google doi
  • 15. Park SJ, Jang YS 2002 Pore structure and surface properties of chemically modified activated carbons for adsorption mechanism and rate of Cr(VI). [J Colloid Interface Sci] Vol.249 P.458 google doi
  • 16. Park SJ, Jin SY 2005 Effect of ozone treatment on ammonia removal of activated carbons. [J Colloid Interface Sci] Vol.286 P.417 google doi
  • 17. Moreno-Castilla C, Lopez-Ramon MV, Carrasco-Marin F 2000 Changes in surface chemistry of activated carbons by wet oxidation. [Carbon] Vol.38 P.1995 google doi
  • 18. Park SJ, Lee SY 2009 Hydrogen storage behaviors of carbon nanotubes/metal-organic frameworks-5 hybrid composites. [Carbon Lett] Vol.10 P.19 google
  • 19. Rather SU, Zacharia R, Naik M-u-d, Hwang SW, Kim AR, Nahm KS 2008 Surface adsorption and micropore filling of the hydrogen in activated MWCNTs. [Int J Hydrogen Energy] Vol.33 P.6710 google doi
  • 20. Lee SY, Park SJ 2010 Effect of temperature on activated carbon nanotubes for hydrogen storage behaviors. [Int J Hydrogen Energy] Vol.35 P.6757 google doi
  • 21. Lee SY, Park SJ 2010 Effect of chemical treatments on hydrogen storage behaviors of multi-walled carbon nanotubes. [Mater Chem Phys] Vol.124 P.1011 google doi
  • [Table 1.] Elemental composition of the samples studied
    Elemental composition of the samples studied
  • [Fig. 1.] N2 adsorption/desorption isotherms at 77 K porosity parameters of the chemically treated activated carbons (ACs) with the H3PO4 concentrations.
    N2 adsorption/desorption isotherms at 77 K porosity parameters of the chemically treated activated carbons (ACs) with the H3PO4 concentrations.
  • [Table 2.] N2/77K textural properties of the samples studied
    N2/77K textural properties of the samples studied
  • [Fig. 2.] Hydrogen adsorption behaviors of the chemically treated activated carbons (ACs) with the H3PO4 concentrations.
    Hydrogen adsorption behaviors of the chemically treated activated carbons (ACs) with the H3PO4 concentrations.