Cryogenics 101 (2019) 36–42
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Adsorption equilibrium of hydrogen adsorption on activated carbon, multiwalled carbon nanotubes and graphene sheets
Yulong Fenga, Ji Wanga,b, , Yujun Liua, Qingrong Zhengc a
School of Naval Architecture, Dalian University of Technology, Dalian 116024, China Collaborative Innovation Center for Advanced Ship and Deep-Sea Exploration, Dalian 116024, China c Provincial Key Laboratory of Naval Architecture & Ocean Engineering, Jimei University, Xiamen 361021, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Hydrogen storage Carbon material Adsorption Toth equation Isosteric heat of adsorption
For obtaining the technical data to develop storage medium with appropriate structure for hydrogen storage system, behavior of hydrogen adsorption on carbon based adsorbents with diﬀerent micro structures was comparatively studied. Activated carbon (SAC-02), multi-walled carbon nanotubes (MWCNTs) and graphene sheets (GS), which respectively have a speciﬁc surface area about 1916 m2/g, 155 m2/g and 221 m2/g, were selected to test adsorption equilibrium at temperature from 77.15 K to 113.15 K and pressure up to 6 MPa. Toth equation was applied to estimate the absolute adsorption amounts, and isosteric heat and limit isosteric heat of hydrogen adsorption were used to investigate the inﬂuence of structures variation for hydrogen storage. This study shows that the GS exhibits a higher potential in hydrogen storage though it is a mesoporous adsorbent, and the relative errors of predicted results from Toth equation were less than 2.1%, The value of the isosteric heat of hydrogen adsorption on GS is about 4.01–5.88 kJ/mol, which is higher than 3.67–3.96 kJ/mol for hydrogen adsorption on the SAC-02 and 2.47–4.36 kJ/mol for hydrogen adsorption on the MWCNTs, respectively. It also reveals that adsorbents with fold structure would be more beneﬁcial to adsorbing hydrogen than the adsorbents with pore structure or ﬂat plane, and a higher speciﬁc surface area is necessary when the GS was applied to hydrogen storage system.
1. Introduction Due to the easy release of the adsorbed gas and relatively lower cost of the adsorbents as well as the non-toxic gas emission, hydrogen physisorption has been investigated intensively for hydrogen storage since 1980s [1,2]. However, it has been demonstrated from recent experimental results that hydrogen storage capacities using physisorption are still far from the economic targets set out by United States’ Department of Energy (DOE) and International Association of Energy (IAE) under normal conditions [3,4]. Nevertheless, some methods for increasing hydrogen storage capacity, for example by decreasing the temperature and selecting appropriate carbon based adsorbents, can result in signiﬁcantly improved adsorption capacity . Therefore, it is necessary to research the inﬂuence of low temperature conditions on hydrogen storage performance, and explore the adsorption equilibrium model and interaction mechanism between hydrogen molecules and atoms of diﬀerent adsorbents. Theoretically, hydrogen physisorption on a porous adsorbent relies on the strength of Van der Waals’ force (VDW) between hydrogen
molecules and the adsorbent surface. The VDW force can be aﬀected by temperature and intermolecular distance, therefore, a further research should be carried out about hydrogen adsorption on adsorbents with diﬀerent structures, over a large temperature range including cryogenic temperature [6–8]. Moreover, the carbon based materials, which have rich pore structure, extensive industrial application and low-temperature-resistant, have been researched to turn into an appropriate hydrogen storage material since 1960s [9–11]. And there are obvious diﬀerences on its interactive properties between hydrogen molecules and carbon materials surface when the microscopic structure and formation mechanism are diverse [11–14]. Thus, it is signiﬁcant to explore adsorption equilibrium of hydrogen on the carbon materials with typical micro morphology. Furthermore, there is still uncertainty about the physical state of adsorbed hydrogen on carbon, and whether it should be considered a supercritical liquid or subcritical quasi-liquid [5,15–18]. In this paper, in order to avoid the inﬂuence of adsorbed phase, the Toth equation, which can directly calculate the absolute adsorption enthalpy without any assumption on the state of the adsorbed phase, was used for
Corresponding author. E-mail address: [email protected]
https://doi.org/10.1016/j.cryogenics.2019.05.009 Received 3 October 2018; Received in revised form 21 May 2019; Accepted 28 May 2019 Available online 29 May 2019 0011-2275/ © 2019 Elsevier Ltd. All rights reserved.
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scanning electron microscopy (SEM) was employed. Pictures of the three samples are shown in Fig. 1. As seen in these images, massive cylindrical circular holes are distributed on the surface of SAC-02 and hydrogen molecules can be attracted to carbon atoms on the wall of hole. Concerning MWCNTs, there is a smaller surface area for column carbon ﬁber, and the dense stacking structure of the wall of nanotubes restricts the quantity and interaction strength of carbon atoms binding to hydrogen molecules. In Fig. 1(c), it is seen that the GS has a pocket-like morphology and wrinkled paper structure on the surface. Hence, carbon atoms, which distribute in both sides of the folded porous structure, can interact with hydrogen molecules, yielding a greater potential for hydrogen storage. N2 adsorption isotherm at 77.15 K was measured by a Micromeritics ASAP 2010 sorptometer apparatus. Then, the speciﬁc surface area and the pore size distribution (PSD) could be calculated from BET plot and NDFT theory. Results are shown in Fig. 2 and Table 1. Fig. 2 and Table 1 indicate that the speciﬁc surface area and microporous volume of coconut shell SAC-02 are much higher than MWCNTs and GS. But unlike activated carbon SAC-02 and MWCNTs, the mean pore width diameter of GS is in the mesoporous range. Compared with other two samples, the smaller microporous volume and apparent density can be found on GS.
adsorption equilibrium analysis [19,20]. Hence, it is desirable to explore diﬀerence in adsorption capacities for hydrogen adsorption on diﬀerent carbon materials with various microstructures. According to previous research [5,11], carbon-based adsorbents can be divided into three kinds of microporous structure: cylindrical shape, plate type and irregular fold (such as activated carbon, carbon nanotubes and grapheme sheets et al). In this paper, we present equilibrium data of hydrogen adsorption on typical carbon based materials at cryogenic temperature, with a goal of identifying the factors which most strongly aﬀect the capacity of physisorption. After characterization of experimental samples, the isotherms of hydrogen adsorption on activated carbon, multi-walled carbon nanotubes and graphene were measured at a relatively wide range of pressure and temperature. The Toth equation is then used to calculate the absolute adsorption enthalpy. Finally, we determined the isosteric heat and limit isosteric heat of hydrogen adsorption on three carbon based materials.
2. Experiment 2.1. Structural characterization For this study, carbon adsorbents were obtained as follows: 1) activated carbon (SAC-02) synthesized from granular coconut shell (produced by Ningde Xinsen Activated Carbon Co), 2) multi-walled carbon nanotubes (MWCNTs) produced from catalytic decomposition of CH4, and 3) graphene sheets (GS) prepared from thermally expanded graphite oxide [21–23]. All gases used in production of samples were ultra-high-purity grade supplied by Air Liquide Xiamen Co Ltd. The density of bulk gas phase was calculated by MBWR equation, and then experimental adsorption amounts can be computed. In order to assess the structure and morphology of adsorbents,
2.2. Adsorption experiments About 0.6851 g of the activated carbon SAC-02, 0.6134 g of the MWCNTs and 0.5562 g of GS were selected as the adsorbents. Volumetric method was used to measure adsorption equilibrium data on automated Sieverts apparatus PCTPro E&E, an instrument especially designed for measuring gas sorption properties of materials. To ensure consistency of experimental data, all samples were regenerated under
Fig. 1. The SEM images of activated carbon SAC-02 (a), MWCNTs (b) and GS (c). 37
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Fig. 2. PSD of activated carbon SAC-02 (a), MWCNTs (b) and GS (c) determined by analyzing de-sorption isotherm of N2 at 77.15 K. Table 1 Structural parameters of the tested samples. Sample
BET speciﬁc surface area/m2·g−1
Mean pore width or diameter/nm
SAC-02 MWCNTs GS
1916 155 221
1.91 1.7 4.71
0.251 0.166 0.0301
0.48 0.27 0.225
especially in lower temperature region. Theoretically speaking, SAC-02, MWCNTs and GS are all carbon based adsorbents, their interaction mechanisms primarily are intermolecular attraction between carbon atoms and hydrogen molecules, the excess adsorption amounts cannot directly describe the interaction mechanism between hydrogen molecules and carbon atoms of adsorbents, therefore, the relationships between adsorption property and superﬁcial structure of three adsorbents should be further explored.
vacuum at 413 K for at least 4 h before each isotherm measurement, and the equilibrium experiments were measured four times at each temperature. A range of temperatures for adsorption measurements was obtained by using liquid nitrogen (77.15 K), liquid argon (87.15 K) and liqueﬁed natural gas (113.15 K) for cooling. Adsorption isotherms are shown in Fig. 3; additional information about the measurements can be found in [5,11]. The excess adsorption amounts in Fig. 3 were obtained directly from the automated apparatus, and the results illustrate that the experimental isotherms of hydrogen adsorption on carbon materials belong to the Langmuir Type . Compared with SAC-02 and MWCNTs, the maximum hydrogen adsorption on GS appears at lower temperature and lower pressure range, leading us to conclude that GS has a higher potential in hydrogen storage at the lowest pressures. When combining with Table 1, hydrogen adsorption capacity has a linear relationship with increasing surface area and microporous volume. The microporous volume of GS is much smaller, and the mean pore width of GS is about 4.71 nm, as a result that the GS can’t eﬀectively adsorbed hydrogen. However, experimental data indicates that adsorption growth rate of hydrogen on GS is signiﬁcantly higher than hydrogen on MWCNTs,
3. Analysis of adsorption equilibrium 3.1. Absolute adsorption amount For practical applications, it is the absolute adsorption amount that can be more meaningful to evaluate the performance of an adsorbent. Therefore, the excess adsorption amounts should be turned into absolute adsorption amounts. According to the Gibbs’ deﬁnition of adsorption , the relation between absolute amounts nabs and excess amounts nexc can be described as [10,25,26]
nexc = nabs − va ρg = va (ρa − ρg ) = nabs (1 − ρa / ρg ) 38
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Fig. 3. Experimental isotherm of excess adsorption amounts of hydrogen adsorption on SAC-02 (a), MWCNTs (b) and GS (c) at large temperature region.
Here the nexc,i is the excess adsorption amounts from experiments, the mean deviation δ is used to evaluate the accuracy in ﬁtting the Toth equation against experimental data, that's
where ρg is the density of bulk gas phase which can be calculated by 32 term Modiﬁed Benedict-Webb-Rubin equation of state . va is the speciﬁc volume of the adsorbed phase, ρa is the density of adsorbed phase. In order to avoid the error caused by assumption on the state of adsorbed phase, the Toth equation, which can accurately convert the excess adsorption amounts into the absolute adsorption amounts without the assumption of adsorbed state, was used for adsorption equilibrium analysis. The Toth equation can be described as
nabs = n 0
bf [1 + (bf )t ]1/ t
bf ⎛⎜1 − ρg ⎞⎟ [1 + (bf )t ]1/ t ⎝ ρa ⎠
The parameters n 0 , b , t and ρa determine the accuracy of ﬁtting results, which can be obtained by nonlinearly ﬁtting the equations against experimental data by minimizing the residual as follow: 2
Re sidual =
∑ ⎨ ⎡⎢n0 [1 + (bf )t ]1/ t ⎜⎛1 − i
ρg ⎞ ⎤ ⎫ − nexc, i ⎟ ⎬ ρa ⎠ ⎥ ⎦i ⎭
i i − ncal |nexc | × 100% i nexc
i is experimental excess Here N is the equilibrium points of isotherm, nexc i is the excess adsorption adsorption amount at ith equilibrium point, ncal amount predicted by Toth equation. In this paper, the experimental data was ﬁtted by Sftool toolbox of MATLAB software, the relative errors of the predicted results from Toth equation were less than 2.1%, and the results from regression were listed in Table 2, the isotherms of adsorption of absolute amounts are listed in Fig. 4. According to the ﬁtting results, the values of b and t had no regular relationship with the temperature, the saturation adsorption amount n 0 are seen to decrease as the temperature increases, beyond that, a stronger increasing relationship is found between saturation adsorption amount and speciﬁc surface area of adsorbents. Fig. 4 shows that the absolute adsorption amount of hydrogen adsorption on the activated carbon SAC-02 occupies the absolute superiority in three types of absorbents, due to the much larger speciﬁc surface area and microporous volume, the carbon atoms of activated carbon SAC-02 are more convenient to interact with hydrogen molecules. As there are marked diﬀerences in speciﬁc surface area and microporous volume among three adsorbents, in order to explore which micro-shape is more conducive to adsorbing hydrogen, the isosteric heat of hydrogen adsorption on adsorbents is analyzed.
Here n 0 is the saturated adsorption capacity, b and t are parameters which determine the ﬁtting accuracy of Toth equation, the f is the bulk gas fugacity, here in order to obtain accurate fugacity of hydrogen in correspondence with the equilibrium pressure, the fugacity was determine by SRK equation . When combined with Eqs. (1) and Eq. (2) can be transformed as
nexc = n 0
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Table 2 Parameters of Toth equation plotted by non-linear ﬁt of the experimental data of hydrogen adsorption on tested samples. Sample
n0/mmol·g b/mmol·g−1MPa−1 t δ
18.97 1.46 1.23 1.12
16.12 0.61 1.30 0.91
14.27 0.30 1.27 0.88
10.24 0.59 0.59 0.74
8.28 0.29 0.72 0.91
5.32 0.25 0.80 0.85
14.5 9.964 0.31 2.07
11.42 0.756 0.43 0.89
6.35 0.70 0.73 0.95
means the carbon atoms of GS is more prone to interact with hydrogen molecules, and the GS has higher hydrogen storage potential. Furthermore, by increasing the speciﬁc surface area and microporous volume, the GS will be an optimistic hydrogen storage material. In addition, compared with other two adsorbents, the variation trend of SAC02’s qst is smoother, the reason is that the motion of hydrogen molecules is no longer intense in lower temperature, and a larger speciﬁc surface area and microporous volume make SAC-02 complete a large amount of adsorption in a small temperature range, therefore, the diﬀerence value of the isosteric heat of adsorption among a wide scope of absolute adsorption amounts is small, and it is easier to achieve the hydrogen coverage when SAC-02 were chosen as absorbent at lower pressure. According to data of Fig. 5, for the MWCNTs and GS, the absolute adsorption amounts even overstep the limits of experimental measurement data when nabs reached to 1.2 mmol/g, the persuasions of calculated results would become unreliable, then the limit isosteric heat of adsorption was chosen to discuss the interaction between hydrogen and carbon-based adsorbents.
3.2. Isosteric heat of adsorption The isosteric heat of adsorption qst can intuitively express the interaction strength between hydrogen and adsorbents [29,30]. According to the Clausius-Clapeyron equation, qst can be determined as [5,31]
qst = −ΔHads = −R (
d ln f )n d (1/ T )
where the ΔHads is molar enthalpy diﬀerence between bulk gas and absorbed gas, f is the fugacity of diﬀerent absolute adsorption amounts and R is universal gas constant. Combined Toth equation and Eqs. (6), the calculated results of qst are showed in Fig. 5. Fig. 5 displays the isosteric heat of adsorption is gradually reduced as the absolute adsorption amounts increase, and adsorption amount of hydrogen on the GS is 4.01–5.88 kJ/mol, which is higher than 3.67–3.96 kJ/mol of hydrogen adsorption on the SAC-02 and 2.47–4.36 kJ/mol of hydrogen adsorption on the MWCNTs, which
Fig. 4. Isotherm of adsorption of absolute amounts of hydrogen adsorption on SAC-02 (a), MWCNTs (b) and GS (c) at large temperature region. 40
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which means it isn't within the optimal pore size of hydrogen adsorption. However, by increasing the speciﬁc surface area and microporous volume, it is projected that GS could be advantageous as a hydrogen storage material. (2) Toth equation can accurately predict the absolute adsorption amounts of three absorbents within a temperature range from 77.15 K to 113.15 K and pressure up to 6 MPa. The relative errors of the predicted results from Toth equation were less than 2.1%, and a stronger increasing relationship can be found between saturation adsorption amount and speciﬁc surface area of adsorbents. (3) The GS exhibited the best hydrogen adsorption potential among three carbon-based adsorbents. The value of the isosteric heat of hydrogen adsorption on GS is about 4.01–5.88 kJ/mol, which is higher than 3.67–3.96 kJ/mol for hydrogen adsorption on the SAC02 and 2.47–4.36 kJ/mol for hydrogen adsorption on the MWCNTs, respectively. And the limit isosteric heat of hydrogen adsorption on GS is 6.32 kJ/mol, which is higher than 4.15 kJ/mol for hydrogen adsorption on the SAC-02 and 4.32 kJ/mol for hydrogen adsorption on the MWCNTs. The results showed GS has the best property for hydrogen storage, which means that adsorbents with fold structure would more beneﬁcial to hydrogen storage than the adsorbents with pore structure or ﬂat plane.
Fig. 5. Isosteric heat of hydrogen adsorption on adsorbents determined by absolute adsorption amount from Toth model.
Table 3 Limit isosteric heat of hydrogen adsorption on three adsorbents.
Declaration of Competing Interest
Limit isosteric heat of adsorption/kJ·mol−1
The authors declared that there is no conﬂict of interest. Acknowledgments
The limit isosteric heat of adsorption is calculated by temperature dependence of Henry law constants. When the adsorption equilibrium pressure tends to zero, the isotherm of hydrogen adsorption can be expressed as
This research was ﬁnancially supported by the Fundamental Research Funds for the Central Universities. (Grand No. DUT19GF111).
n = k ′·p
Here the n is adsorption capacity, p is the adsorption equilibrium pressure and k′ is Henry law constants. The n is plotted against ln (P/n) by Virial method, when n = 0, the k′ can be estimated. Then the limit isosteric heat of adsorption qst0 can be transformed as following equation 
d ln k ′ ⎞ qst0 = RT + R ⎛ ⎝ d (1/ T ) ⎠ ⎜
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The results are showed in Table 3. According to the analysis of limit isosteric heat of adsorption, the GS has highest hydrogen adsorption capacity, and it illustrates that the GS has strongest interaction with hydrogen though all three carbon-based materials. In this paper, it can be found that the micro-structure of GS become an important factor to aﬀect the ability of storage hydrogen, and the adsorbents with fold structure would more beneﬁcial to adsorbing hydrogen than the adsorbents with pore structure or ﬂat plane. 4. Conclusions For obtaining the technical data needed to develop the hydrogen storage systems based on adsorption on carbon based materials, the isosteric heat of adsorption, which was evaluated based on the hydrogen adsorption data covering a larger temperature range, was used to investigate the inﬂuence of structures variation for hydrogen storage. Toth equation was applied to estimate the absolute adsorption amounts and the limit isosteric heat of adsorption was also employed to explore the hydrogen storage performance of carbon-based materials with different structures. The conclusions are summarized as follows: (1) The relatively small speciﬁc surface area and microporous volume restrict hydrogen storage capacity of GS. Comparing with the SAC02 and MWCNTs, and the mean pore width of GS is about 4.71 nm, 41
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