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graphene complex membrane for polymer electrolyte membrane fuel cell: A molecular dynamics simulation study
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Chitosan/graphene complex membrane for polymer electrolyte membrane fuel cell: A molecular dynamics simulation study Hong-Ping Zhang a,d,*, Neha S. Gandhi b, Yuantong Gu c, Yaping Zhang a, Youhong Tang d,** a
Engineering Research Center of Biomass Materials, Ministry of Education, School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang, Sichuan, 621010, China b School of Mathematical Sciences, Queensland University of Technology, 2 George Street, Brisbane, 4000, Australia c School of Chemistry Physics and Mechanical Engineering, Queensland University of Technology, 2 George Street, Brisbane, 4000, Australia d Centre for NanoScale Science and Technology, College of Science and Engineering, Flinders University, South Australia, 5042, Australia
highlights Molecular model for bio-nanocomposites and their mechanism of proton conductivity in PEMFCs is presented in this work. Diffusion behavior of hydronium ions in chitosan/graphene complex systems was studied using 3 ns long MD simulations. Mechanisms of proton conductivity of chitosan/graphene composite were explored at an atomistic scale. Adding graphene and adjusting pH are efficient ways for improving the proton conductivity of membrane.
article info
abstract
Article history:
Chitosan has been considered attractive in polymer electrolyte membrane fuel cells
Received 25 July 2019
(PEMFCs) due to its excellent film forming and fuel barrier properties. Reflecting the limi-
Received in revised form
tation of its low proton conductivity, various materials were used to improve the proton
12 March 2020
conductivity of chitosan, through combination with inorganic materials like graphene
Accepted 13 March 2020
oxide. We present an ideal molecular model for bio-nanocomposites and their mechanism
Available online xxx
of proton conductivity in PEMFCs. In this study, the diffusion behavior of hydronium ions in chitosan/graphene complex systems at various temperatures, concentrations and pH
Keywords:
values were studied systematically using 3 ns long molecular dynamics (MD) simulations
Chitosan
with an aim to provide the mechanisms of proton conductivity of chitosan/graphene
Graphene
composite at an atomistic scale. Various amounts of water content (10%, 20%, 30% and
Molecular dynamics simulation
40%), pH values (achieved by adjusting the protonation degree of amino groups of chitosan
PEMFC
by 20%, 40%, 60%, 80% and 100%) and numbers of graphene sheets (1, 2, and 3) were
Proton conductivity
considered during MD simulations at 4 temperatures (298 K, 320 K, 340 K and 360 K). Our results indicated that the chitosan system containing 40% water was the most suitable polymer electrolyte membrane and temperature was a key factor affecting diffusion proton. Adding graphene to the chitosan system and adjusting the pH values of chitosan were
* Corresponding author. Engineering Research Center of Biomass Materials, Ministry of Education, School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang, Sichuan, 621010, China. ** Corresponding author. E-mail addresses: [email protected] (H.-P. Zhang), [email protected] (Y. Tang). https://doi.org/10.1016/j.ijhydene.2020.03.124 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Zhang H-P et al., Chitosan/graphene complex membrane for polymer electrolyte membrane fuel cell: A molecular dynamics simulation study, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.03.124
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demonstrated to have a significant effect on improving the proton conductivity of the membrane. © 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Burning petroleum and fossil fuels creates a major global environmental problem, particularly related to pollution and global warming. There is an urgent need to reduce or eliminate dependence on non-renewable sources of energy; seeking new, clean emissions and efficient energy sources to power automobiles, for example, is a significant challenge. Polymer electrolyte membrane fuel cells (PEMFCs) have attracted much research interest due to their high efficiency and potential for use in a relatively low-temperature range, such that they are regarded as an environmentally friendly energy form [1e3]. In PEMFCs, protons travel through a hydrated polymer membrane that acts as an electrolyte layer. The realistic electrochemical performance of PEMFCs is closely related to several factors such as the composition of the electrolytic solution, the chemical and physical properties of catalysts, the proton exchanging efficiency of the membrane, the environmental temperature and so on. Among these factors, the membrane plays a very important role by virtue of its direct effect on proton transfer. Nafion® series membranes (Dupont Co., USA.) have been widely used in PEMFCs due to their excellent proton transfer efficiency and good chemical stability [4]. Many researchers have tried to develop new candidates for Nafion® series membranes with lower costs and better proton transfer efficiency. Chitosan, as a natural polysaccharide, has been explored as one of the candidates for synthetic polymers [5e9]. Chitosan has been extensively studied during past decades, in multiple research fields due to its abundant production, biodegradability and chemical activity. It has been widely studied and applied in biomedical engineering, paper production, healthy food production and water treatment. Due to the free amino groups on the chitosan side chains, it also can be a cationic polyelectrolyte. Reflecting these ionic properties, chitosan has been studied as an ionic membrane in chloride ion conduction and as a proton exchange membrane in PEMFCs. Wan et al. [8] studied the ionic conductivity of chitosan membranes fabricated by casting methods. They found that the intrinsic ionic conductivity was as high as 104 S cm1 after hydration for 1 h, whereas it was 2.6 102 S cm1 for commercial Nafion® N117 film [10]. Those results indicated that chitosan holds promise for application in alkaline polymer electrolyte fuel cells, exceeding that of the Nafion® N117 film. Mukoma et al. [7] studied sulfuric acid cross-linked chitosan as a proton exchange membrane in PEMFCs. These cross-linked chitosan exhibited superior water uptake properties to that of the Nafion® N117 film but had poorer thermal stability and proton conductivity properties. Their experimental studies indicated that chitosan could be a low-cost potential membrane material for PEMFCs provided their thermal stability and ionic
conductivity could be increased. Subsequently, the effects of temperature, water content and deacetylation on the mechanism of proton transfer were investigated by Srinophakun et al. [11] using atomistic molecular dynamics (MD) simulations. The results indicated that the proton conductivity property followed Arrhenius behavior. The degree of deacetylation had little effect on the proton conductivity of chitosan at a 40% water content. The morphology change of the ionexchange membrane (Nafion® 1100) under shearing by the dissipative particle dynamics method has also been studied [12]. It was found that low water content and long chain length led to the formation of water-rich tubes aligned with the shear direction. The formation of such channels could ultimately lead to an increase in proton conductivity at low humidity. The size of these water tubes resulted from a combined influence of shear strain, chain relaxation time, interfacial tension and water content. Although these studies have helped our understanding of the ionic conductivity mechanism in PEMFCs, there is still scoped to find a low-cost ion exchange membrane with high proton conductivity. Graphene has been reported to improve the proton conductivity of the membrane in PEMFCs [13e17]. Zarrin et al. demonstrated that functionalized graphene oxide could significantly improve the proton conductivity properties of Nafion® membrane. Related research into the use of chitosan/graphene composites as ion exchange membranes has seldom been performed, especially studies of the mechanism at the atomistic scale [18]. Although it was indicated by MD study [11] that proton conductivity of chitosan could be highly depended on the water content and temperature, the effect of pH which is directly related to the stability of chitosan was not considered. Thus, we used MD simulations to explore the proton conductivity of chitosan/ graphene composites under different pH conditions and various graphene contents. The effects of graphene content, water content, protonation degree of chitosan and temperature on proton conductivity were systematically investigated.
Computational details In the study, all the MD simulations were performed using the Forcite module in Materials Studio 5.0 (Biovia, San Diego, CA). The model of the study system was built by the amorphous cell module. First, the chitosan monomer, hydronium ion, hydroxyl ion and water molecule were constructed and optimized by the DFT program Dmol3 in Materials Studio. The double numerical basis set with polarization function (DNP), that is comparable to the 6-31G** basis set, was utilized during the simulation [19,20]. The core electrons were treated with DFT semicore pseudopotentials. The exchange-correlation energy was calculated using the Perdew-Burke-Ernzerhof
Please cite this article as: Zhang H-P et al., Chitosan/graphene complex membrane for polymer electrolyte membrane fuel cell: A molecular dynamics simulation study, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.03.124
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generalized gradient approximation [21]. Graphene nanosheet consisting of 104 carbon atoms was constructed with 7 carbon atom rings along the a direction and 7 carbon atom rings along the b direction. The surface area was about 17:9 16:7 ¼ 298:9 A2 . The graphene nanosheet was also optimized by the DFT methods mentioned above. Then, 3D amorphous systems with periodic boundary conditions were built by the amorphous cell module, containing a chitosan polymer chain with 10 repeating units, the hydronium ions and many water molecules. We employed the COMPASS force field [22]. The composition of each cell is summarized in Table 1. All systems contained 8 chains of polymeric chitosan with 10 amino groups; 80 hydronium ions; 80 hydroxyl ions; and 10%, 20%, 30%, or 40% water respectively, as shown in Fig. 1. Water percentage is relative to the mass fraction. The figure illustrates the population of the water surrounding the polymeric chitosan, referring to the practical use of this membrane in a fuel cell. Before the MD simulations, the energy minimization was carried out with the maximum derivative 0.1 kcal/mol. The Ewald summation method is used to calculate the van der Waals and Columbic nonbonding interactions [23]. Further an NVT ensemble based MD was applied on these systems, Andersen thermostat [24] method was used for temperature control. The temperatures during the dynamics simulation were 298, 320, 340 and 360 K. 3000 ps long MD simulations were used to bring these systems to the dynamic equilibrating state with a time steps of 1.0 fs. Then, further 1 ns production runs were carried out for the data analysis. Also, the pH effect on the proton conductivity was considered by setting a series of pH values on the chitosan chains. Diverse pH values were realized by varying the protonation degree of amino groups of chitosan. A range of protonation levels of 20, 40, 60, 80 and 100% were taken into account. Similar procedures have been adopted by Franco et al. [4] and Bahlakeh et al. [5]. Finally, graphene nanosheets were also included in the simulation to study their effects on the proton conductivity of chitosan in the aqueous state. The different graphene contents were achieved by varying the number of graphene nanosheets (consisting of 104 carbon atoms, with the area of 17 A 19 A). Hydronium ions were added in the simulation systems
according to the number of protons and pH values with the aim of neutralizing the system. All the details of the different simulation systems (pristine chitosan, protonated chitosan, and chitosan/graphene) are presented in Table 1 and their snapshots after production runs are shown in Fig. S2. Mean square displacement (MSD) analysis, radial distribution function (RDF) analysis, and coordination number calculations were carried out to investigate the mobility of hydronium ions and proton conductivity. The correlation of MSD with the diffusion behavior of hydronium ions was calculated with the following equations: MSD ¼
D ¼
1 d lim N t / ∞ dt
N X
〈½Ri ðtÞ ½Ri ð0Þ2 〉
(1)
i ¼1
MSD 6
(2)
N is the number of diffusion particles, Ri(t) refers to the position vector of the particle i at a certain time t. D stands for the diffusion constant. The system under consideration in this work is a 3D cell, thus the relationship constant between the D and MSD is chosen to be 6. The D has the direct relationship with proton conductivity, generally, the proton conductivity behaves better with high diffusivity. RDF can be a useful tool for analyzing the interactions between different particles. It depicts the probability of finding particle B at distance r from particle A normalized according to the possibility estimated for the total allocation at an equivalent arbitrary density. It can be defined as: gAB ðrÞ ¼
nB NB 2 4pr Dr V
(3)
where nB is the number of atoms located around A atom inside a spherical shell of thickness Dr, and NB is the total number of B atoms applied for an amorphous cell. The coordination number could be calculated according to the RDF. It could also be used to analyze the interactions between different components of the hydronium ion diffusion systems. The equation for the coordination number is:
Table 1 e Description of different simulation systems with different water content, chitosan protonation levels and graphene content. Systems
Volume ( A3)
Chitosan chains No.
Chitosans þH2O þH3Oþ
22460 25404 29045 33868 44170 45128 45983 46851 47670 47218 50776 54268
8 8 8 8 8 8 8 8 8 8 8 8
Chitosans at different protonation levels þ H2O (40%) þ H3Oþ
Chitosans þ graphene þ H2O (40%)þ H3Oþ
H3Oþ H2O molecule No. No. 92 219 376 584 584 600 616 626 637 632 680 725
80 80 80 80 80 80 80 80 80 80 80 80
-OHNo.
Graphene sheet No.
Comments
80 80 80 80 96 112 128 144 160 80 80 80
e e e e e e e e e 1 2 3
10% water 20% water 30% water 40% water 20% protonation 40% protonation 60% protonation 80% protonation 100% protonation
(First part for the system (chitosans þ H2O þ H3Oþ) is a longer validation run (4 ns) of reference [11]).
Please cite this article as: Zhang H-P et al., Chitosan/graphene complex membrane for polymer electrolyte membrane fuel cell: A molecular dynamics simulation study, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.03.124
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Fig. 1 e Simulated cells containing chitosan chains (10 monomers per chain), hydronium ions, hydroxyl ions, and (a) 10% water, (b) 20% water, (c) 30% water and (d) 40% water. The water molecules are shown as spheres and the chitosan and hydronium ions are shown as sticks.
nB ¼ 4p
NB V
Z r2 gAB ðrÞ dr
(4)
Results and discussion The computational results of the MD simulations are discussed here. The conductivity of hydronium ions was investigated by MSD, RDF and coordination number analysis.
Validation of diffusion behavior of hydronium ions in pristine chitosan In order to validate the MD protocol and reproduce the dynamic properties of hydronium ions as reported by the Srinophakun et al. [11], we performed 3ns simulations of hydronium ions in pristine chitosan systems at four different temperatures and varied water contents. Fig. S1 shows the MSD results of hydronium ions for 1 ns in chitosan with different water contents at 298 K, 320 K, 340 K and 360 K. It was found that the mobility of hydronium ions is directly related to temperature and water content. A notable increase in the mobility of hydronium ions is found with an increase of temperature for a given water content. For 40% water content,
the MSD value rises from 200 A to about 700 A as the temperature changes from 298 K to 360 K, as shown in Figs. S2(a)e S2(d). The importance of temperature on the self-diffusion of substances has been demonstrated previously [25e30]. The diffusion coefficient and structure of LiF-KF mixture were studied with NMR and MD simulation by Rollet et al. [29]. The importance of temperature for the diffusions of F, Li, and K ions has been confirmed. A similar conclusion was obtained by Schmid et al. [25] in the study of benzene diffusion in MOF5 by the MD simulation. Water content was found to be another factor that has a strong relationship with the diffusion of protons in our simulation systems. At a certain temperature level, it can be found that the MSD values of protons clearly increase with an increased water content. This phenomenon is consistent across all tested temperature conditions (298 K, 320 K, 340 K and 360 K). The diffusion coefficients of hydronium ions is enhanced by increasing the water content (See Table S1). This is consistent with the previous reports obtained by MD simulations of PEM and AEM [31]. The relationship between the MSD and the diffusion coefficients is depicted in equation (2) and previous report [32]. Table S1 shows the diffusion coefficient of hydronium ions in the pristine chitosan systems. It is evident that both water content and temperature are beneficial for improving the diffusion coefficient of hydronium ions in the pristine chitosan system. However, the diffusion coefficient of hydronium
Please cite this article as: Zhang H-P et al., Chitosan/graphene complex membrane for polymer electrolyte membrane fuel cell: A molecular dynamics simulation study, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.03.124
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ions lies in the range of 1.2 1010 m2/s to 11.8 1010 m2/s for all the chitosan systems with varied water contents, under the 4 different temperatures. RDF results indicated that the chitosan/hydronium ions/ water system was very stable at 298 K, as shown in Fig. S3. The interaction between hydronium ions and amino groups of chitosan is stronger than the normal van der Waals interaction. The main peak of RDF is located at around 1.7 A, greater than the bond length of NO (1.15 A). A similar phenomenon has been observed on the RDF curves for hydronium ionshydroxyl groups of chitosan and hydronium ions-water molecules. This finding indicates that the distance between hydronium ions and chitosan is similar to that between hydronium ions and water molecules. The distribution of hydronium ions in the chitosan/water systems is relatively uniform. Another phenomenon worth mentioning is that the peak values of RDF for hydronium ions-hydroxyl groups of chitosan, hydronium ions-amino groups of chitosan and hydronium ions-water molecules clearly decreased with an increase in water content (from 10% to 40%) in these systems. From this observation, it can be understood that with an increase in water content, the number of hydronium ions decreases per unit volume, although the general distribution of hydronium ions does not change. For the RDF, each peak is for each atom, the first peak is for H of hydronium, and the second peak for oxygen of hydronium. The position of first peak is preserved in all concentrations studied indicating that the most probable structure of H3Oþ ions is hardly affected by chitosan molecules. There are two hydration shells and hydronium- Chitosan is more stable compared to waterhydronium. It is no need to analyze the third peak, the interactions stand by it is too weak. Our trends of the MSD and RDF results are consistent with previously published results [11], although their results are based on very short MD simulation (only 200 ps). As the diffusion process is time dependent, a simulation that is too short, may result in the main phenomenon being will be missed. The mean square displacement varies apparently after 200 ps, thus, longer MD simulations were needed to equilibrate the system in our current work. Table S2 shows the coordination number (n) of hydronium ion, water and functional groups of chitosan for pristine chitosan. It can be found that the functional groups (amino group and a hydroxyl group) of chitosan have very high hydration: 1H2O/amino group and 2H2O/hydroxyl group. It seems that protons have a preference for functional groups with chitosan, i.e. 2 protons/amino group and 4 protons/hydroxyl group. It is understandable according to the strucutres of the other proton conductive membranes. There are many hydrophilic functional groups on the side chains of the Nafion 117 membranes, also the sulfonic acid groups of the sulfonated polyether ether ketone (SPEEK) are hydrophilic functional groups.
Effect of pH values on proton conductivity As the environments in the regular fuel cell are mainly acidic, the effect of pH value on the stability of the chitosan based membrane in the fuel cell will directly affect the proton conductivity of chitosan [30]. The amino group in chitosan can be readily protonated at an acidic or neutral solution with a
5
charge density depending on pH value. These protonated amino groups affect a whole range of features of chitosan. To our knowledge, the effect of pH on proton conductivity has not yet/seldom been studied. Here, the effect of pH on the diffusion behavior of hydronium ions in the chitosan system was carefully investigated. The pH effect was imposed through the variation of the degree of protonation of amino groups of chitosan chains. The treatment of the protonation of amino groups of chitosan in our study was similar to that reported by Franco et al. [4]. Random amino groups were chosen to be protonated and 5 degrees of protonation were taken into account. The 40% water content chitosan systems were considered to explore the pH effect. It was found that protonation could be an effective way to improve the diffusion behavior of hydronium ions in chitosan systems. This was demonstrated by the analyses of MSD, RDF and diffusion coefficient. Fig. 2 shows the results of MSD of hydronium ions in protonated chitosan systems. These results demonstrate a similar phenomenon to that described with hydronium ions in pristine chitosan, such that with an increase in temperature, the MSD values also clearly increase. Meanwhile, the MSD values of the hydronium ions in protonated chitosan systems are significantly larger than those in the pristine chitosan systems. Table S3 shows the diffusion coefficients of hydronium ions in the protonated chitosan systems. It can be seen that the diffusion coefficient of hydronium ions is improved 10 times through the protonation treatment of chitosan. Nevertheless, the protonation degree has little effect on the diffusion coefficient of the hydronium ions. Compared to the effect of the degree of protonation, the effect of temperature is positive. Figure 3 shows the RDF results of hydronium ionprotonated chitosan systems. From Fig. 3(a) and (b), it is evident that hydronium ions are localized around the amino groups of chitosan, rather than the protonated amino groups of chitosan. It is worth mentioning that after the protonation procedure, the hydronium ions/protonated chitosan/water systems are more relaxed than the non-protonated chitosan systems. The volume change between the hydronium ions/ protonated chitosan/water systems and the non-protonated chitosan systems can be found in Table 1. The first peak of RDF occurs at around 2.5 A for the hydronium ions/protonated chitosan/water systems, as shown in Fig. 3(a), whereas the first peak of RDF occurs at around 2 A for the hydronium ions/ non-protonated chitosan/water systems. Fig. 3(c) shows the RDF between hydronium ions and hydroxyl groups of chitosan. It can be seen across all for temperatures that with an increase in the degree of protonation of the chitosan, the first peak value clearly tends to decrease. This phenomenon is also observed in Fig. 3(a). These figures illustrate that, with an increase in the degree of protonation of chitosan, the volume of the studied systems increased, as could be found from Table 1. Table S4 shows the coordination number (n) results for the protonated chitosan systems. It is evident that, with an increase in the degree of protonation, protons interact with the hydroxyl groups of chitosan and protonated amino groups rather than with the amino groups. However, it is also clearly evident that the entire interaction between protons and chitosan decreases compared to the pristine chitosan system.
Please cite this article as: Zhang H-P et al., Chitosan/graphene complex membrane for polymer electrolyte membrane fuel cell: A molecular dynamics simulation study, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.03.124
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Fig. 2 e Time dependent mean square displacement (MSD) of hydronium ions for chitosan systems with different protonation levels (20, 40, 60, 80 and 100%) at different temperatures (298 K, 320 K, 340 K and 360 K).
This phenomenon might explain the increase in proton conductivity upon protonation of chitosan.
The effect of graphene on proton conductivity It is widely known that graphene can be an effective filler for improving the electrochemical behavior of polymer matrix. It has been reported that graphene can be helpful for improving the proton conductivity of chitosan [18,33e35]. Alireza et al. [33] studied the effect of the addition of graphene oxide, sulfonated graphene oxide and sulfonated chitosan into the chitosan matrix on the proton conductivity of chitosan, in the context of its application in a fuel cell. It was found that the addition of graphene oxide and sulfonated graphene oxide could significantly improve the proton conductivity of chitosan [33]. Zhang et al. [35] also demonstrated that the proton conductivity could be apparently improved by adding graphene based materials into the chitosan matrix. However, to our knowledge, there are very limited theoretical studies reported on composites.
Here, we utilized the MD simulation to investigate the mechanism of the phenomenon at the atomic scale. Three different graphene contents were considered in the study by adding 1 to 3 graphene sheets in the chitosan matrix. The details of the systems aiming to investigate the effect of graphene on the proton diffusivity in chitosan are listed in Table 1. The effect of graphene can be comparable with that of the protonation of chitosan. Adding graphene can also multiply the diffusion coefficient of chitosan by 10, as can be intuitively found in Table S5. Meanwhile, the graphene content (from 1 to 3 graphene sheets) has little effect on the diffusion behavior of hydronium ions. Fig. S2 shows snapshots of the chitosan/ water/hydronium ions/graphene systems after 3000 ps MD equilibrating calculations. It can be seen that graphene tends to be dispersed in the chitosan matrix. Fig. 4 shows the MSD results of the system. Temperature is still a positive factor for the diffusion of hydronium ions. The number of graphene sheets has little influence on the MSD of hydronium ions, a finding that is consistent with the results of the diffusion coefficient. The RDF of the graphene/chitosan systems is shown
Please cite this article as: Zhang H-P et al., Chitosan/graphene complex membrane for polymer electrolyte membrane fuel cell: A molecular dynamics simulation study, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.03.124
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Fig. 3 e RDF of chitosan systems with different protonation levels (20, 40, 60, 80 and 100%) at different temperatures (298 K, þ ¡ þ 320 K, 340 K and 360 K): (a) H3Oþ - N of NH2 on chitosan; (b) H3Oþ - N of NHþ 3 on chitosan; (c) H3O - OH on chitosan; (d) H3O - H2O.
Fig. 4 e Time dependent mean square displacement (MSD) of hydronium ions for graphene/chitosan systems with different graphene contents: containing (a) 1 graphene sheet; (b) 2 graphene sheets; and (c) 3 graphene sheets.
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in Fig. 5. The temperature is found to have little effects on the interactions between hydronium ions and water (Fig. 5(a) to 5(c)). It indicates that the effect of temperature on the interactions between hydronium ions and chitosan (Fig. 5(d) to (i)) is rarely less than that between hydronium ions and graphene. It can be inferred from Fig. 5(j)-(l) that the interactions between hydronium ions and graphene are sensitive to temperature, although the interaction is very weak. This finding might be associated with the excellent thermal conductivity
of graphene. However, in these systems, the effect of temperature on the interactions between hydronium ions and functional groups of chitosan can be ignored, differing from the diffusion systems of hydronium ions without graphene. According to the analyses of MSD, RDF, and the diffusion coefficient of hydronium ions, it can be concluded that graphene can be effective in improving the diffusion of hydronium ions. According to our findings, there are 2 effective ways to improve the diffusion behavior of hydronium ions in chitosan:
Fig. 5 e RDF of graphene/chitosan systems: (a) H3OeH2O for 1 graphene; (b) H3OeH2O for 2 graphene sheets; (c) H3OeH2O for 3 graphene sheets; (d) H3OeN of chitosan for 1 graphene sheet; (e) H3OeN of chitosan for 2 graphene sheets; (f) H3OeN of chitosan for 3 graphene sheets; (g) H3OeOH of chitosan for 1 graphene sheet; (h) H3OeOH of chitosan for 2 graphene sheets; (i) H3OeOH of chitosan for 3 graphene sheets; (j) H3O-graphene of chitosan for 1 graphene sheet; (k) H3O-graphene of chitosan for 2 graphene sheets; (l) H3O-graphene of chitosan for 3 graphene sheets. Please cite this article as: Zhang H-P et al., Chitosan/graphene complex membrane for polymer electrolyte membrane fuel cell: A molecular dynamics simulation study, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.03.124
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Fig. 6 e Time dependent mean square displacement (MSD) of hydronium ions for protonated e chitosan/graphene systems (with 1 graphene) with different protonation levels (20, 40, 60, 80 and 100%) at different temperatures (298 K, 320 K and 360 K): (a) 298 K; (b) 320 K; and (c) 360 K. (1) the protonation process for chitosan and (2) the addition of graphene to the chitosan. These results show that the diffusion coefficient of hydronium ions could be multiplied by 10. Based on the current results, we attempted to address the combined effect of graphene in the presence of protonated chitosan on the diffusion behavior of the hydronium ions in a fuel cell. The MSD results in Fig. 6 for hydronium ions in protonated chitosan/graphene systems are very similar to those of the hydronium ions in protonated chitosan systems. From the diffusion coefficient results, no apparent difference can be found between protonated chitosan/graphene systems and protonated chitosan systems (Table S6). These results demonstrate that combining the protonation of chitosan with the addition of graphene did not further improve the diffusion behavior of the hydronium ions. As mentioned above, the better diffusivity indicates better proton conductivity. Shirdast et al. [33] studied the relationship between proton conductivity of chitosan and the addition of graphene oxide or sulfonated graphene oxide. Two things were demonstrated, one being that with the addition of graphene oxide (5 wt%), the proton conductivity of chitosan is improved from 1.3 to 1.9 mS/cm1 and with the addition of sulfonated graphene oxide, it was improved from 1.3 to 3.3 mS/cm1. Chitosan/sulfonated graphene oxide composites have better proton conductivity than chitosan/graphene oxide, which might be due to the hydrogen bond between chitosan and graphene oxide. There is no apparent difference between the proton conductivity of chitosan with the different added amounts of sulfonated graphene oxide (5 wt % or 10 wt %). Zhang et al. investigated the proton conductivity of chitosan with the addition of sulfonated graphene oxide and observed similar trends. These trends correlate well with our simulation data. For the mechanism of graphene in improving the diffusion behavior of hydronium ions in chitosan, we consider the main reason should be that the inducing of graphene could create some spatial channels on the micro scale which can be helpful for the diffusion of the small molecules including water molecules and hydronium ions.
diffusion of hydronium ions in chitosan, compared with the previously reported experimental and theoretical studies of chitosan or chitosan/graphene composites for fuel cell applications [11,13,15,33e35]. Compared to the reported short duration theoretical study (only 200 ps) [11], similar conclusions were obtained: that the 40% water content was the most suitable condition for the transportation of hydronium ions. Temperature played a more important role in the diffusion behavior of hydronium ions than the water content. The experimental study of chitosan/graphene or chitosan/graphene oxide composites, demonstrated the significant role of graphene in improving proton conductivity of the chitosan matrix. However, no further details and mechanisms have been studied. Furthermore, our study investigated the effect of the protonation of chitosan (pH) and the addition of graphene on the diffusion behavior of hydronium ions in chitosan. MSD, self-diffusion coefficient and RDF analysis indicated that the protonation of chitosan and addition of graphene could weaken the interactions between protons and chitosan, which may explain the improved diffusion behavior of hydronium ions in chitosan. Herein, the diffusion behavior of hydronium ions in chitosan can be improved either through the protonation of chitosan, or the addition of graphene sheets. However, the combined use of protonated chitosan and graphene did not offer further enhance the transport of hydronium ions. An improved understanding of the mechanism of the proton conductivity of chitosan (by chemical modification of the chitosan or graphene) is vital for guiding real experimental analyses of chitosan based composites for PEMFCs. The molecular model for bio-nanocomposites presented in this work, will allow for the mechanism of proton conductivity in other biopolymers, by graphene or other nanomaterials, to be accurately modelled.
Acknowledgements This research was supported by the National Natural Science Foundation of China (NSFC; Grant No. 31300793, 21406182, 41672039).
Conclusions In this study, long time MD simulations (3 ns) were utilized to investigate the effect of the degree of protonation of chitosan (pH), graphene content, water content and temperature on the
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2020.03.124.
Please cite this article as: Zhang H-P et al., Chitosan/graphene complex membrane for polymer electrolyte membrane fuel cell: A molecular dynamics simulation study, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.03.124
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Please cite this article as: Zhang H-P et al., Chitosan/graphene complex membrane for polymer electrolyte membrane fuel cell: A molecular dynamics simulation study, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.03.124
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Chitosan/graphene complex membrane for polymer electrolyte membrane fuel cell: A molecular dynamics simulation study Hong-Ping Zhang a,d,*, Neha S. Gandhi b, Yuantong Gu c, Yaping Zhang a, Youhong Tang d,** a
Engineering Research Center of Biomass Materials, Ministry of Education, School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang, Sichuan, 621010, China b School of Mathematical Sciences, Queensland University of Technology, 2 George Street, Brisbane, 4000, Australia c School of Chemistry Physics and Mechanical Engineering, Queensland University of Technology, 2 George Street, Brisbane, 4000, Australia d Centre for NanoScale Science and Technology, College of Science and Engineering, Flinders University, South Australia, 5042, Australia
highlights Molecular model for bio-nanocomposites and their mechanism of proton conductivity in PEMFCs is presented in this work. Diffusion behavior of hydronium ions in chitosan/graphene complex systems was studied using 3 ns long MD simulations. Mechanisms of proton conductivity of chitosan/graphene composite were explored at an atomistic scale. Adding graphene and adjusting pH are efficient ways for improving the proton conductivity of membrane.
article info
abstract
Article history:
Chitosan has been considered attractive in polymer electrolyte membrane fuel cells
Received 25 July 2019
(PEMFCs) due to its excellent film forming and fuel barrier properties. Reflecting the limi-
Received in revised form
tation of its low proton conductivity, various materials were used to improve the proton
12 March 2020
conductivity of chitosan, through combination with inorganic materials like graphene
Accepted 13 March 2020
oxide. We present an ideal molecular model for bio-nanocomposites and their mechanism
Available online xxx
of proton conductivity in PEMFCs. In this study, the diffusion behavior of hydronium ions in chitosan/graphene complex systems at various temperatures, concentrations and pH
Keywords:
values were studied systematically using 3 ns long molecular dynamics (MD) simulations
Chitosan
with an aim to provide the mechanisms of proton conductivity of chitosan/graphene
Graphene
composite at an atomistic scale. Various amounts of water content (10%, 20%, 30% and
Molecular dynamics simulation
40%), pH values (achieved by adjusting the protonation degree of amino groups of chitosan
PEMFC
by 20%, 40%, 60%, 80% and 100%) and numbers of graphene sheets (1, 2, and 3) were
Proton conductivity
considered during MD simulations at 4 temperatures (298 K, 320 K, 340 K and 360 K). Our results indicated that the chitosan system containing 40% water was the most suitable polymer electrolyte membrane and temperature was a key factor affecting diffusion proton. Adding graphene to the chitosan system and adjusting the pH values of chitosan were
* Corresponding author. Engineering Research Center of Biomass Materials, Ministry of Education, School of Materials Science and Engineering, Southwest University of Science and Technology, Mianyang, Sichuan, 621010, China. ** Corresponding author. E-mail addresses: [email protected] (H.-P. Zhang), [email protected] (Y. Tang). https://doi.org/10.1016/j.ijhydene.2020.03.124 0360-3199/© 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Zhang H-P et al., Chitosan/graphene complex membrane for polymer electrolyte membrane fuel cell: A molecular dynamics simulation study, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.03.124
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demonstrated to have a significant effect on improving the proton conductivity of the membrane. © 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Burning petroleum and fossil fuels creates a major global environmental problem, particularly related to pollution and global warming. There is an urgent need to reduce or eliminate dependence on non-renewable sources of energy; seeking new, clean emissions and efficient energy sources to power automobiles, for example, is a significant challenge. Polymer electrolyte membrane fuel cells (PEMFCs) have attracted much research interest due to their high efficiency and potential for use in a relatively low-temperature range, such that they are regarded as an environmentally friendly energy form [1e3]. In PEMFCs, protons travel through a hydrated polymer membrane that acts as an electrolyte layer. The realistic electrochemical performance of PEMFCs is closely related to several factors such as the composition of the electrolytic solution, the chemical and physical properties of catalysts, the proton exchanging efficiency of the membrane, the environmental temperature and so on. Among these factors, the membrane plays a very important role by virtue of its direct effect on proton transfer. Nafion® series membranes (Dupont Co., USA.) have been widely used in PEMFCs due to their excellent proton transfer efficiency and good chemical stability [4]. Many researchers have tried to develop new candidates for Nafion® series membranes with lower costs and better proton transfer efficiency. Chitosan, as a natural polysaccharide, has been explored as one of the candidates for synthetic polymers [5e9]. Chitosan has been extensively studied during past decades, in multiple research fields due to its abundant production, biodegradability and chemical activity. It has been widely studied and applied in biomedical engineering, paper production, healthy food production and water treatment. Due to the free amino groups on the chitosan side chains, it also can be a cationic polyelectrolyte. Reflecting these ionic properties, chitosan has been studied as an ionic membrane in chloride ion conduction and as a proton exchange membrane in PEMFCs. Wan et al. [8] studied the ionic conductivity of chitosan membranes fabricated by casting methods. They found that the intrinsic ionic conductivity was as high as 104 S cm1 after hydration for 1 h, whereas it was 2.6 102 S cm1 for commercial Nafion® N117 film [10]. Those results indicated that chitosan holds promise for application in alkaline polymer electrolyte fuel cells, exceeding that of the Nafion® N117 film. Mukoma et al. [7] studied sulfuric acid cross-linked chitosan as a proton exchange membrane in PEMFCs. These cross-linked chitosan exhibited superior water uptake properties to that of the Nafion® N117 film but had poorer thermal stability and proton conductivity properties. Their experimental studies indicated that chitosan could be a low-cost potential membrane material for PEMFCs provided their thermal stability and ionic
conductivity could be increased. Subsequently, the effects of temperature, water content and deacetylation on the mechanism of proton transfer were investigated by Srinophakun et al. [11] using atomistic molecular dynamics (MD) simulations. The results indicated that the proton conductivity property followed Arrhenius behavior. The degree of deacetylation had little effect on the proton conductivity of chitosan at a 40% water content. The morphology change of the ionexchange membrane (Nafion® 1100) under shearing by the dissipative particle dynamics method has also been studied [12]. It was found that low water content and long chain length led to the formation of water-rich tubes aligned with the shear direction. The formation of such channels could ultimately lead to an increase in proton conductivity at low humidity. The size of these water tubes resulted from a combined influence of shear strain, chain relaxation time, interfacial tension and water content. Although these studies have helped our understanding of the ionic conductivity mechanism in PEMFCs, there is still scoped to find a low-cost ion exchange membrane with high proton conductivity. Graphene has been reported to improve the proton conductivity of the membrane in PEMFCs [13e17]. Zarrin et al. demonstrated that functionalized graphene oxide could significantly improve the proton conductivity properties of Nafion® membrane. Related research into the use of chitosan/graphene composites as ion exchange membranes has seldom been performed, especially studies of the mechanism at the atomistic scale [18]. Although it was indicated by MD study [11] that proton conductivity of chitosan could be highly depended on the water content and temperature, the effect of pH which is directly related to the stability of chitosan was not considered. Thus, we used MD simulations to explore the proton conductivity of chitosan/ graphene composites under different pH conditions and various graphene contents. The effects of graphene content, water content, protonation degree of chitosan and temperature on proton conductivity were systematically investigated.
Computational details In the study, all the MD simulations were performed using the Forcite module in Materials Studio 5.0 (Biovia, San Diego, CA). The model of the study system was built by the amorphous cell module. First, the chitosan monomer, hydronium ion, hydroxyl ion and water molecule were constructed and optimized by the DFT program Dmol3 in Materials Studio. The double numerical basis set with polarization function (DNP), that is comparable to the 6-31G** basis set, was utilized during the simulation [19,20]. The core electrons were treated with DFT semicore pseudopotentials. The exchange-correlation energy was calculated using the Perdew-Burke-Ernzerhof
Please cite this article as: Zhang H-P et al., Chitosan/graphene complex membrane for polymer electrolyte membrane fuel cell: A molecular dynamics simulation study, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.03.124
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generalized gradient approximation [21]. Graphene nanosheet consisting of 104 carbon atoms was constructed with 7 carbon atom rings along the a direction and 7 carbon atom rings along the b direction. The surface area was about 17:9 16:7 ¼ 298:9 A2 . The graphene nanosheet was also optimized by the DFT methods mentioned above. Then, 3D amorphous systems with periodic boundary conditions were built by the amorphous cell module, containing a chitosan polymer chain with 10 repeating units, the hydronium ions and many water molecules. We employed the COMPASS force field [22]. The composition of each cell is summarized in Table 1. All systems contained 8 chains of polymeric chitosan with 10 amino groups; 80 hydronium ions; 80 hydroxyl ions; and 10%, 20%, 30%, or 40% water respectively, as shown in Fig. 1. Water percentage is relative to the mass fraction. The figure illustrates the population of the water surrounding the polymeric chitosan, referring to the practical use of this membrane in a fuel cell. Before the MD simulations, the energy minimization was carried out with the maximum derivative 0.1 kcal/mol. The Ewald summation method is used to calculate the van der Waals and Columbic nonbonding interactions [23]. Further an NVT ensemble based MD was applied on these systems, Andersen thermostat [24] method was used for temperature control. The temperatures during the dynamics simulation were 298, 320, 340 and 360 K. 3000 ps long MD simulations were used to bring these systems to the dynamic equilibrating state with a time steps of 1.0 fs. Then, further 1 ns production runs were carried out for the data analysis. Also, the pH effect on the proton conductivity was considered by setting a series of pH values on the chitosan chains. Diverse pH values were realized by varying the protonation degree of amino groups of chitosan. A range of protonation levels of 20, 40, 60, 80 and 100% were taken into account. Similar procedures have been adopted by Franco et al. [4] and Bahlakeh et al. [5]. Finally, graphene nanosheets were also included in the simulation to study their effects on the proton conductivity of chitosan in the aqueous state. The different graphene contents were achieved by varying the number of graphene nanosheets (consisting of 104 carbon atoms, with the area of 17 A 19 A). Hydronium ions were added in the simulation systems
according to the number of protons and pH values with the aim of neutralizing the system. All the details of the different simulation systems (pristine chitosan, protonated chitosan, and chitosan/graphene) are presented in Table 1 and their snapshots after production runs are shown in Fig. S2. Mean square displacement (MSD) analysis, radial distribution function (RDF) analysis, and coordination number calculations were carried out to investigate the mobility of hydronium ions and proton conductivity. The correlation of MSD with the diffusion behavior of hydronium ions was calculated with the following equations: MSD ¼
D ¼
1 d lim N t / ∞ dt
N X
〈½Ri ðtÞ ½Ri ð0Þ2 〉
(1)
i ¼1
MSD 6
(2)
N is the number of diffusion particles, Ri(t) refers to the position vector of the particle i at a certain time t. D stands for the diffusion constant. The system under consideration in this work is a 3D cell, thus the relationship constant between the D and MSD is chosen to be 6. The D has the direct relationship with proton conductivity, generally, the proton conductivity behaves better with high diffusivity. RDF can be a useful tool for analyzing the interactions between different particles. It depicts the probability of finding particle B at distance r from particle A normalized according to the possibility estimated for the total allocation at an equivalent arbitrary density. It can be defined as: gAB ðrÞ ¼
nB NB 2 4pr Dr V
(3)
where nB is the number of atoms located around A atom inside a spherical shell of thickness Dr, and NB is the total number of B atoms applied for an amorphous cell. The coordination number could be calculated according to the RDF. It could also be used to analyze the interactions between different components of the hydronium ion diffusion systems. The equation for the coordination number is:
Table 1 e Description of different simulation systems with different water content, chitosan protonation levels and graphene content. Systems
Volume ( A3)
Chitosan chains No.
Chitosans þH2O þH3Oþ
22460 25404 29045 33868 44170 45128 45983 46851 47670 47218 50776 54268
8 8 8 8 8 8 8 8 8 8 8 8
Chitosans at different protonation levels þ H2O (40%) þ H3Oþ
Chitosans þ graphene þ H2O (40%)þ H3Oþ
H3Oþ H2O molecule No. No. 92 219 376 584 584 600 616 626 637 632 680 725
80 80 80 80 80 80 80 80 80 80 80 80
-OHNo.
Graphene sheet No.
Comments
80 80 80 80 96 112 128 144 160 80 80 80
e e e e e e e e e 1 2 3
10% water 20% water 30% water 40% water 20% protonation 40% protonation 60% protonation 80% protonation 100% protonation
(First part for the system (chitosans þ H2O þ H3Oþ) is a longer validation run (4 ns) of reference [11]).
Please cite this article as: Zhang H-P et al., Chitosan/graphene complex membrane for polymer electrolyte membrane fuel cell: A molecular dynamics simulation study, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.03.124
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Fig. 1 e Simulated cells containing chitosan chains (10 monomers per chain), hydronium ions, hydroxyl ions, and (a) 10% water, (b) 20% water, (c) 30% water and (d) 40% water. The water molecules are shown as spheres and the chitosan and hydronium ions are shown as sticks.
nB ¼ 4p
NB V
Z r2 gAB ðrÞ dr
(4)
Results and discussion The computational results of the MD simulations are discussed here. The conductivity of hydronium ions was investigated by MSD, RDF and coordination number analysis.
Validation of diffusion behavior of hydronium ions in pristine chitosan In order to validate the MD protocol and reproduce the dynamic properties of hydronium ions as reported by the Srinophakun et al. [11], we performed 3ns simulations of hydronium ions in pristine chitosan systems at four different temperatures and varied water contents. Fig. S1 shows the MSD results of hydronium ions for 1 ns in chitosan with different water contents at 298 K, 320 K, 340 K and 360 K. It was found that the mobility of hydronium ions is directly related to temperature and water content. A notable increase in the mobility of hydronium ions is found with an increase of temperature for a given water content. For 40% water content,
the MSD value rises from 200 A to about 700 A as the temperature changes from 298 K to 360 K, as shown in Figs. S2(a)e S2(d). The importance of temperature on the self-diffusion of substances has been demonstrated previously [25e30]. The diffusion coefficient and structure of LiF-KF mixture were studied with NMR and MD simulation by Rollet et al. [29]. The importance of temperature for the diffusions of F, Li, and K ions has been confirmed. A similar conclusion was obtained by Schmid et al. [25] in the study of benzene diffusion in MOF5 by the MD simulation. Water content was found to be another factor that has a strong relationship with the diffusion of protons in our simulation systems. At a certain temperature level, it can be found that the MSD values of protons clearly increase with an increased water content. This phenomenon is consistent across all tested temperature conditions (298 K, 320 K, 340 K and 360 K). The diffusion coefficients of hydronium ions is enhanced by increasing the water content (See Table S1). This is consistent with the previous reports obtained by MD simulations of PEM and AEM [31]. The relationship between the MSD and the diffusion coefficients is depicted in equation (2) and previous report [32]. Table S1 shows the diffusion coefficient of hydronium ions in the pristine chitosan systems. It is evident that both water content and temperature are beneficial for improving the diffusion coefficient of hydronium ions in the pristine chitosan system. However, the diffusion coefficient of hydronium
Please cite this article as: Zhang H-P et al., Chitosan/graphene complex membrane for polymer electrolyte membrane fuel cell: A molecular dynamics simulation study, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.03.124
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ions lies in the range of 1.2 1010 m2/s to 11.8 1010 m2/s for all the chitosan systems with varied water contents, under the 4 different temperatures. RDF results indicated that the chitosan/hydronium ions/ water system was very stable at 298 K, as shown in Fig. S3. The interaction between hydronium ions and amino groups of chitosan is stronger than the normal van der Waals interaction. The main peak of RDF is located at around 1.7 A, greater than the bond length of NO (1.15 A). A similar phenomenon has been observed on the RDF curves for hydronium ionshydroxyl groups of chitosan and hydronium ions-water molecules. This finding indicates that the distance between hydronium ions and chitosan is similar to that between hydronium ions and water molecules. The distribution of hydronium ions in the chitosan/water systems is relatively uniform. Another phenomenon worth mentioning is that the peak values of RDF for hydronium ions-hydroxyl groups of chitosan, hydronium ions-amino groups of chitosan and hydronium ions-water molecules clearly decreased with an increase in water content (from 10% to 40%) in these systems. From this observation, it can be understood that with an increase in water content, the number of hydronium ions decreases per unit volume, although the general distribution of hydronium ions does not change. For the RDF, each peak is for each atom, the first peak is for H of hydronium, and the second peak for oxygen of hydronium. The position of first peak is preserved in all concentrations studied indicating that the most probable structure of H3Oþ ions is hardly affected by chitosan molecules. There are two hydration shells and hydronium- Chitosan is more stable compared to waterhydronium. It is no need to analyze the third peak, the interactions stand by it is too weak. Our trends of the MSD and RDF results are consistent with previously published results [11], although their results are based on very short MD simulation (only 200 ps). As the diffusion process is time dependent, a simulation that is too short, may result in the main phenomenon being will be missed. The mean square displacement varies apparently after 200 ps, thus, longer MD simulations were needed to equilibrate the system in our current work. Table S2 shows the coordination number (n) of hydronium ion, water and functional groups of chitosan for pristine chitosan. It can be found that the functional groups (amino group and a hydroxyl group) of chitosan have very high hydration: 1H2O/amino group and 2H2O/hydroxyl group. It seems that protons have a preference for functional groups with chitosan, i.e. 2 protons/amino group and 4 protons/hydroxyl group. It is understandable according to the strucutres of the other proton conductive membranes. There are many hydrophilic functional groups on the side chains of the Nafion 117 membranes, also the sulfonic acid groups of the sulfonated polyether ether ketone (SPEEK) are hydrophilic functional groups.
Effect of pH values on proton conductivity As the environments in the regular fuel cell are mainly acidic, the effect of pH value on the stability of the chitosan based membrane in the fuel cell will directly affect the proton conductivity of chitosan [30]. The amino group in chitosan can be readily protonated at an acidic or neutral solution with a
5
charge density depending on pH value. These protonated amino groups affect a whole range of features of chitosan. To our knowledge, the effect of pH on proton conductivity has not yet/seldom been studied. Here, the effect of pH on the diffusion behavior of hydronium ions in the chitosan system was carefully investigated. The pH effect was imposed through the variation of the degree of protonation of amino groups of chitosan chains. The treatment of the protonation of amino groups of chitosan in our study was similar to that reported by Franco et al. [4]. Random amino groups were chosen to be protonated and 5 degrees of protonation were taken into account. The 40% water content chitosan systems were considered to explore the pH effect. It was found that protonation could be an effective way to improve the diffusion behavior of hydronium ions in chitosan systems. This was demonstrated by the analyses of MSD, RDF and diffusion coefficient. Fig. 2 shows the results of MSD of hydronium ions in protonated chitosan systems. These results demonstrate a similar phenomenon to that described with hydronium ions in pristine chitosan, such that with an increase in temperature, the MSD values also clearly increase. Meanwhile, the MSD values of the hydronium ions in protonated chitosan systems are significantly larger than those in the pristine chitosan systems. Table S3 shows the diffusion coefficients of hydronium ions in the protonated chitosan systems. It can be seen that the diffusion coefficient of hydronium ions is improved 10 times through the protonation treatment of chitosan. Nevertheless, the protonation degree has little effect on the diffusion coefficient of the hydronium ions. Compared to the effect of the degree of protonation, the effect of temperature is positive. Figure 3 shows the RDF results of hydronium ionprotonated chitosan systems. From Fig. 3(a) and (b), it is evident that hydronium ions are localized around the amino groups of chitosan, rather than the protonated amino groups of chitosan. It is worth mentioning that after the protonation procedure, the hydronium ions/protonated chitosan/water systems are more relaxed than the non-protonated chitosan systems. The volume change between the hydronium ions/ protonated chitosan/water systems and the non-protonated chitosan systems can be found in Table 1. The first peak of RDF occurs at around 2.5 A for the hydronium ions/protonated chitosan/water systems, as shown in Fig. 3(a), whereas the first peak of RDF occurs at around 2 A for the hydronium ions/ non-protonated chitosan/water systems. Fig. 3(c) shows the RDF between hydronium ions and hydroxyl groups of chitosan. It can be seen across all for temperatures that with an increase in the degree of protonation of the chitosan, the first peak value clearly tends to decrease. This phenomenon is also observed in Fig. 3(a). These figures illustrate that, with an increase in the degree of protonation of chitosan, the volume of the studied systems increased, as could be found from Table 1. Table S4 shows the coordination number (n) results for the protonated chitosan systems. It is evident that, with an increase in the degree of protonation, protons interact with the hydroxyl groups of chitosan and protonated amino groups rather than with the amino groups. However, it is also clearly evident that the entire interaction between protons and chitosan decreases compared to the pristine chitosan system.
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Fig. 2 e Time dependent mean square displacement (MSD) of hydronium ions for chitosan systems with different protonation levels (20, 40, 60, 80 and 100%) at different temperatures (298 K, 320 K, 340 K and 360 K).
This phenomenon might explain the increase in proton conductivity upon protonation of chitosan.
The effect of graphene on proton conductivity It is widely known that graphene can be an effective filler for improving the electrochemical behavior of polymer matrix. It has been reported that graphene can be helpful for improving the proton conductivity of chitosan [18,33e35]. Alireza et al. [33] studied the effect of the addition of graphene oxide, sulfonated graphene oxide and sulfonated chitosan into the chitosan matrix on the proton conductivity of chitosan, in the context of its application in a fuel cell. It was found that the addition of graphene oxide and sulfonated graphene oxide could significantly improve the proton conductivity of chitosan [33]. Zhang et al. [35] also demonstrated that the proton conductivity could be apparently improved by adding graphene based materials into the chitosan matrix. However, to our knowledge, there are very limited theoretical studies reported on composites.
Here, we utilized the MD simulation to investigate the mechanism of the phenomenon at the atomic scale. Three different graphene contents were considered in the study by adding 1 to 3 graphene sheets in the chitosan matrix. The details of the systems aiming to investigate the effect of graphene on the proton diffusivity in chitosan are listed in Table 1. The effect of graphene can be comparable with that of the protonation of chitosan. Adding graphene can also multiply the diffusion coefficient of chitosan by 10, as can be intuitively found in Table S5. Meanwhile, the graphene content (from 1 to 3 graphene sheets) has little effect on the diffusion behavior of hydronium ions. Fig. S2 shows snapshots of the chitosan/ water/hydronium ions/graphene systems after 3000 ps MD equilibrating calculations. It can be seen that graphene tends to be dispersed in the chitosan matrix. Fig. 4 shows the MSD results of the system. Temperature is still a positive factor for the diffusion of hydronium ions. The number of graphene sheets has little influence on the MSD of hydronium ions, a finding that is consistent with the results of the diffusion coefficient. The RDF of the graphene/chitosan systems is shown
Please cite this article as: Zhang H-P et al., Chitosan/graphene complex membrane for polymer electrolyte membrane fuel cell: A molecular dynamics simulation study, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.03.124
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Fig. 3 e RDF of chitosan systems with different protonation levels (20, 40, 60, 80 and 100%) at different temperatures (298 K, þ ¡ þ 320 K, 340 K and 360 K): (a) H3Oþ - N of NH2 on chitosan; (b) H3Oþ - N of NHþ 3 on chitosan; (c) H3O - OH on chitosan; (d) H3O - H2O.
Fig. 4 e Time dependent mean square displacement (MSD) of hydronium ions for graphene/chitosan systems with different graphene contents: containing (a) 1 graphene sheet; (b) 2 graphene sheets; and (c) 3 graphene sheets.
Please cite this article as: Zhang H-P et al., Chitosan/graphene complex membrane for polymer electrolyte membrane fuel cell: A molecular dynamics simulation study, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.03.124
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in Fig. 5. The temperature is found to have little effects on the interactions between hydronium ions and water (Fig. 5(a) to 5(c)). It indicates that the effect of temperature on the interactions between hydronium ions and chitosan (Fig. 5(d) to (i)) is rarely less than that between hydronium ions and graphene. It can be inferred from Fig. 5(j)-(l) that the interactions between hydronium ions and graphene are sensitive to temperature, although the interaction is very weak. This finding might be associated with the excellent thermal conductivity
of graphene. However, in these systems, the effect of temperature on the interactions between hydronium ions and functional groups of chitosan can be ignored, differing from the diffusion systems of hydronium ions without graphene. According to the analyses of MSD, RDF, and the diffusion coefficient of hydronium ions, it can be concluded that graphene can be effective in improving the diffusion of hydronium ions. According to our findings, there are 2 effective ways to improve the diffusion behavior of hydronium ions in chitosan:
Fig. 5 e RDF of graphene/chitosan systems: (a) H3OeH2O for 1 graphene; (b) H3OeH2O for 2 graphene sheets; (c) H3OeH2O for 3 graphene sheets; (d) H3OeN of chitosan for 1 graphene sheet; (e) H3OeN of chitosan for 2 graphene sheets; (f) H3OeN of chitosan for 3 graphene sheets; (g) H3OeOH of chitosan for 1 graphene sheet; (h) H3OeOH of chitosan for 2 graphene sheets; (i) H3OeOH of chitosan for 3 graphene sheets; (j) H3O-graphene of chitosan for 1 graphene sheet; (k) H3O-graphene of chitosan for 2 graphene sheets; (l) H3O-graphene of chitosan for 3 graphene sheets. Please cite this article as: Zhang H-P et al., Chitosan/graphene complex membrane for polymer electrolyte membrane fuel cell: A molecular dynamics simulation study, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.03.124
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Fig. 6 e Time dependent mean square displacement (MSD) of hydronium ions for protonated e chitosan/graphene systems (with 1 graphene) with different protonation levels (20, 40, 60, 80 and 100%) at different temperatures (298 K, 320 K and 360 K): (a) 298 K; (b) 320 K; and (c) 360 K. (1) the protonation process for chitosan and (2) the addition of graphene to the chitosan. These results show that the diffusion coefficient of hydronium ions could be multiplied by 10. Based on the current results, we attempted to address the combined effect of graphene in the presence of protonated chitosan on the diffusion behavior of the hydronium ions in a fuel cell. The MSD results in Fig. 6 for hydronium ions in protonated chitosan/graphene systems are very similar to those of the hydronium ions in protonated chitosan systems. From the diffusion coefficient results, no apparent difference can be found between protonated chitosan/graphene systems and protonated chitosan systems (Table S6). These results demonstrate that combining the protonation of chitosan with the addition of graphene did not further improve the diffusion behavior of the hydronium ions. As mentioned above, the better diffusivity indicates better proton conductivity. Shirdast et al. [33] studied the relationship between proton conductivity of chitosan and the addition of graphene oxide or sulfonated graphene oxide. Two things were demonstrated, one being that with the addition of graphene oxide (5 wt%), the proton conductivity of chitosan is improved from 1.3 to 1.9 mS/cm1 and with the addition of sulfonated graphene oxide, it was improved from 1.3 to 3.3 mS/cm1. Chitosan/sulfonated graphene oxide composites have better proton conductivity than chitosan/graphene oxide, which might be due to the hydrogen bond between chitosan and graphene oxide. There is no apparent difference between the proton conductivity of chitosan with the different added amounts of sulfonated graphene oxide (5 wt % or 10 wt %). Zhang et al. investigated the proton conductivity of chitosan with the addition of sulfonated graphene oxide and observed similar trends. These trends correlate well with our simulation data. For the mechanism of graphene in improving the diffusion behavior of hydronium ions in chitosan, we consider the main reason should be that the inducing of graphene could create some spatial channels on the micro scale which can be helpful for the diffusion of the small molecules including water molecules and hydronium ions.
diffusion of hydronium ions in chitosan, compared with the previously reported experimental and theoretical studies of chitosan or chitosan/graphene composites for fuel cell applications [11,13,15,33e35]. Compared to the reported short duration theoretical study (only 200 ps) [11], similar conclusions were obtained: that the 40% water content was the most suitable condition for the transportation of hydronium ions. Temperature played a more important role in the diffusion behavior of hydronium ions than the water content. The experimental study of chitosan/graphene or chitosan/graphene oxide composites, demonstrated the significant role of graphene in improving proton conductivity of the chitosan matrix. However, no further details and mechanisms have been studied. Furthermore, our study investigated the effect of the protonation of chitosan (pH) and the addition of graphene on the diffusion behavior of hydronium ions in chitosan. MSD, self-diffusion coefficient and RDF analysis indicated that the protonation of chitosan and addition of graphene could weaken the interactions between protons and chitosan, which may explain the improved diffusion behavior of hydronium ions in chitosan. Herein, the diffusion behavior of hydronium ions in chitosan can be improved either through the protonation of chitosan, or the addition of graphene sheets. However, the combined use of protonated chitosan and graphene did not offer further enhance the transport of hydronium ions. An improved understanding of the mechanism of the proton conductivity of chitosan (by chemical modification of the chitosan or graphene) is vital for guiding real experimental analyses of chitosan based composites for PEMFCs. The molecular model for bio-nanocomposites presented in this work, will allow for the mechanism of proton conductivity in other biopolymers, by graphene or other nanomaterials, to be accurately modelled.
Acknowledgements This research was supported by the National Natural Science Foundation of China (NSFC; Grant No. 31300793, 21406182, 41672039).
Conclusions In this study, long time MD simulations (3 ns) were utilized to investigate the effect of the degree of protonation of chitosan (pH), graphene content, water content and temperature on the
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2020.03.124.
Please cite this article as: Zhang H-P et al., Chitosan/graphene complex membrane for polymer electrolyte membrane fuel cell: A molecular dynamics simulation study, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.03.124
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Please cite this article as: Zhang H-P et al., Chitosan/graphene complex membrane for polymer electrolyte membrane fuel cell: A molecular dynamics simulation study, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2020.03.124