Electronic Structure of a Polybenzimidazole-Wrapped Single-Wall

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Electronic Structure of PolybenzimidazoleWrapped Single-Wall Carbon Nanotube Kulbir Kaur Ghuman, and Tsuyohiko Fujigaya J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00247 • Publication Date (Web): 28 Jun 2018 Downloaded from http://pubs.acs.org on June 29, 2018

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The Journal of Physical Chemistry

Electronic Structure of Polybenzimidazole-Wrapped Single-Wall Carbon Nanotube Kulbir K. Ghuman1* and Tsuyohiko Fujigaya1,2,3* 1

International Institute for Carbon Neutral Energy Research, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka, 819-0395, Japan

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Department of Applied Chemistry, Graduate School of Engineering, Kyushu University, 744 Motooka Nishi-ku, Fukuoka 819-0395, Japan 3

JST-PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan

ABSTRACT: Polymer-wrapping on carbon nanotubes (CNT) surface by Polybenzimidazoles (PBIs) allows us to immobilize platinum nanoparticles on pristine CNTs and to use it as the electrocatalyst of polymer electrolyte membrane fuel cells (PEMFCs), thereby improving the durability of PEMFCs. In this work, for the first time, we present the fundamental insights into the electronic structure and the interaction present in the single wall CNT (SWCNT) and PBI composite through comprehensive theoretical study supported by experiments. Our analyses predict that PBI possesses stable helical wrapping around SWCNT due to the strong non-covalent π-π interaction between them, in which PBI participates more than SWCNTs. It is also found that the functionalization of SWCNTs by PBI is independent of the SWCNT chiralities and that the

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functionalization does not affect the intrinsic properties of SWCNTs making SWCNT/PBI-based membrane electrode assembly a good candidate for high performance PEMFCs.

1. INTRODUCTION Polymer Electrolyte Membrane Fuel Cells (PEMFCs) are efficient, environmentally clean, and a key element of the emerging hydrogen economy. The current challenge in PEMFCs research is that the best performing materials used to make its key components utilize elemental compositions in short supply and are too pricey while worst performers use earth abundant low cost elemental compositions. The membrane electrode assemblies (MEA) of conventional PEMFC consist of electrode catalysts composed of Platinum (Pt)/carbon black/nafion. These PEMFCs possess high costs due to the use of Pt catalysts at the electrodes, and nafion membranes as the electrolyte. Secondly, they are not durable due to the loss in the active surface area of Pt catalysts and breakdown of the electrolyte membrane, with operating time1. In our recent research, we showed that the MEA composed of Pt-deposited polybenzimidazole (PBI)-wrapped carbon nanotubes (CNTs) exhibit a high fuel cell performance with a remarkable durability of 500,000 accelerated potential cycles with only a 5% initial potential loss 2. These CNT/PBI based PEMFCs can be operative even under dry conditions above 100 oC with high durability3,4 which afford many benefits such as decreased carbon monoxide poisoning of the catalyst metal particles, increased catalytic reaction rate, easy removal of generated water5. This enhancement in the durability of CNT/PBI/Pt MEA is mainly due to the high electrical conductivity, large surface area, highly crystalline structure, and high electrochemical stability of CNTs6-11. However, to extract the full potential of CNTs used in making MEA of PEMFCs, one must functionalize CNTs by polymers (such as PBI, which is an excellent CNT dispersants and


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helps in the nanofabrication of CNT/polymer nanocomposites6) which assist in overcoming the major drawback of CNTs, i.e. the lack of binding site for homogeneous metal loading. Further, it is well known that not only the proton conductivity of the membranes, but also that of the electrocatalyst strongly affects FC performance since it is essential to form a triple phase structure. A principle strategy to improve the proton conductivity of the FC electrocatalyst is to cast an ionomer such as perfluorosulfonate onto the surface of the electrocatalyst 12. However, pristine multi-wall CNTs (MWCNTs) are unable to provide good binding sites for sufficient attachment of such an ionomer. Thus, when using such an ionomer, it leaches from the MEA during long-term FC operating conditions, resulting in a lower FC performance13. In order to overcome this barrier, CNTs are wrapped with PBI first and then coated with proton conducting ionomer which results in highly improved proton conductivity and stability of PEMFCs14. That being said, even after using these strategies, current PEMFCs are still far away from meeting the target of $40/KW power system. Thus, we need to further tune the performance of current CNT/PBI based PEMFC system and elaborate better MEA designs and synthesis protocols, for which a consistent understanding of the interaction mechanisms between different materials interfaces of CNT/PBI/Pt based PEMFC is highly desirable. Therefore, as a first step in this work, a comprehensive investigation of electronic structures of the single-wall CNT (SWCNT) and PBI heterostructure was conducted by using density functional theory (DFT) calculations. The objective here is to provide insights into the SWCNT/PBI interface, by analyzing the structure, stability, and the influence of SWCNT chiralities on PBI wrapping, as it may directly influence the performance of Pt NPs and the ionomers present in CNT/PBI/Pt based PEMFC. Further it should be noted that in experiments usually MWCNTs are used, even though SWCNTs are capable of providing better performance, due to the reason that MWCNTs are extremely cheaper than


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SWCNTs. In this work, however, SWCNTs were investigated instead of MWCNTs theoretically (as well as experimentally) due to the typically higher curvature radius and related macromolecular architecture complexity of MWCNTs. However, we expect that the reduced sidewall chemical reactivity of MWCNTs as compared to SWCNTs will not alter the qualitative results obtained in this study and the similar behavior should be expected for MWCNT/PBI system. In the following article, the PBI selectivity for SWCNTs with different chiralities is discussed by investigating single helical polymer wrapping around SWCNTs and the degree of interaction between them. A detailed analysis of binding energies, energy band alignment, and work functions has been conducted to provide the qualitative explanation about the interaction between SWCNT and PBI. Some of the results were further confirmed through experiments.

2. MATERIALS AND METHODS 2.1 Computational Model and Method. In this work we considered three SWCNT/PBI interfaces, first in which PBI wraps around armchair (9, 9) SWCNT of diameter 12.37 Å (denoted AC/PBI, Fig. 1 (a)), second in which PBI wraps around zigzag (16,0) SWCNT of diameter 12.70 Å (denoted ZZ/PBI, Fig. 1 (b)), and third in which PBI wraps around chiral (13,4) SWCNT having diameters of 12.22Å (denoted CH/PBI, Fig. 1 (c)). The distance between PBI and SWCNTs (AC, ZZ, CH) ranges from 3.0 to 3.6Å and the cell size was chosen so that the (x, y) axes perpendicular to the nanotube (z) axis, were apart by the vacuum of about 14Å in order to avoid interaction between neighboring tubes. Further, both SWCNTs and PBI polymers were geometrically periodic along z-axis. The Quantum-ESPRESSO code, PWSCF package15, was used to perform the calculations. All calculations were performed by using the long range nonlocal van der Waals density functional, vdW-DF216. Interaction of electron-core


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was approximated by the Vanderbilt ultrasoft pseudopotentials17 available for Perdew-BurkeErnzerhof (PBE)18 in the Quantum Espresso package. The kinetic energy cut-offs of 30 ad 120 Ry were used for the smooth part of the electronic wave functions and augmented electron density, respectively. In all the calculations Brillouin zone integrations were performed using (a)

(b)

(c)

Figure 1. Optimized structures for (a) Armchair (9,9)/PBI (AC/PBI), (b) Zigzag (16,0)/PBI (ZZ/PBI), and (c) Chiral (13, 4)/PBI (CH/PBI) interfaces. PBI is wrapped helically around the CNT forming a periodic CNT/PBI supercell. The first column is the top view and second column shows the side view. H, C and N atoms are depicted in small cyan colored spheres, large yellow spheres and large light gray spheres, respectively.


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Monkhorst-Pack19 grid of 1× 1× 6 k-points (refer supplementary information, Table S3). All calculations are spin polarized and the structures were relaxed by using a conjugate gradient minimization algorithm until the magnitude of residual Hellman-Feynman force on each atom was less than 10-3 Ry/Bohr. In the electronic density of states (DOS) and projected density of states (PDOS) plots a conventional Gaussian smearing of 0.01 Ry was utilized. 2.2 Experimental Materials and Measurements. PBI was provided from Sato Light Industrial Co., Ltd. All the chemicals were used as received. SWCNTs produced by high-pressure catalytic CO decomposition (HiPco, 0.7-1.2 nm) was purchased from NanoIntegris. The PBI-wrapped SWCNTs (SWCNT/PBI) was prepared as described elsewhere.20 The X-ray photoelectron spectroscopy (XPS) spectra were measured using an AXIS-ULTRADLD (Shimadzu, Co., Japan). Au wire was used as an internal standard.

3. RESULTS AND DISCUSSION First, we optimized the SWCNT/PBI heterostructures and presented their structural properties and energetics. These optimized geometries, corresponding to the equilibrium situation at 0 K, were then used to calculate the electronic properties of the functionalized SWCNTs. 3.1 Stability and Geometry. To investigate the stability of SWCNT/PBI interface, we helically wrapped PBI around SWCNTs and optimized it using ground state DFT. Such complex material interface model where both PBI and SWCNT form a periodic supercell, is considered for the first time for theoretical simulations according to best of authors knowledge. The optimized geometries of AC/PBI, ZZ/PBI, and CH/PBI interfaces, are shown in Fig. 1 (a), (b) and (c), respectively. It can be seen from the Fig. 1 that the van der Waals interactions stabilizes all three SWCNT/PBI interfaces upon relaxation and in all three systems, the PBI interacts non-covalently


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with the SWCNTs. After optimizing these models we calculated the binding energies per atom for each SWCNT/PBI heterostructures of size n using following equation: 𝛿𝐻𝐵𝐸 = (𝐸𝑆𝑊𝐶𝑁𝑇+𝑃𝐵𝐼 − 𝐸𝑆𝑊𝐶𝑁𝑇 − 𝐸𝑃𝐵𝐼 )/𝑛, where ESWCNT+PBI (ESWCNT ) is the energy of the SWCNT with (without) helically wrapped PBI and EPBI is the energy of the isolated PBI polymer calculated in the same supercell. Hence, a negative δHBE indicates stable adsorption whereas a positive value indicates unstable adsorption. Binding energies calculations for AC/PBI (δHBE = -0.016 eV/atom), ZZ/PBI (δHBE = -0.016 eV/atom), and CH/PBI (δHBE = -0.014 eV/atom) indicate no significant difference in binding energy between metallic and semiconducting tubes. We confirmed this result by conducting binding energy calculations for small PBI (formed from few monomers) aligned axially with the (9,9), (16,0) and (10,8) SWCNTs, by using vdW-DF2 functional with ultrasoft pseudopotentials available for Generalized Gradient Approximation (GGA) as well as localdensity approximation (LDA) (refer Fig. S1 and Table S1 in the Supplementary Information). These results are also consistent with previous literature calculations for other SWCNT/polymer interactions, which showed that both axial and helical polymer alignment on the SWCNT surface is a function of polymer backbone stiffness and side group type21-23. However, it should be noted that overall for both (axial and helically wrapped SWCNT/PBI) cases zigzag and armchair nanotubes seems to show slightly better binding with the PBI which might prove beneficial in experiments where lots of SWCNT/PBI assemblies are used. 3.2 Interaction between SWCNT and PBI. At the interface, the work functions of PBI and SWCNTs compete with each other which determines the kind of interaction between PBI and SWCNTs. Therefore, next we calculated the work functions of bulk PBI, pristine SWCNTs and functionalized SWCNTs. The work function of the systems considered here are obtained from the Fermi energy (Ef) as = 𝜙 − 𝐸𝑓 , where ϕ represents the vacuum level calculated from the planar


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(a)

(b)

(c)

Figure 2. Planar average of the electrostatic potential along the y-axis for the (a) Armchair (9,9)/PBI (AC/PBI), (b) Zigzag (16,0)/PBI (ZZ/PBI), and (c) Chiral (13, 4)/PBI (CH/PBI) interfaces. Here, red line represents the Fermi level. For all models PBI is wrapped helically around the SWCNT forming a periodic SWCNT/PBI supercell, shown in the right panel. average of the electrostatic potential in the unit cell along the y-axis. More precisely, ϕ is the


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energy level of an electron positioned at rest at a distance sufficient for the electron to experience the full impact of the surface dipole with zero kinetic energy and is calculated as: Vbare + VH +

Figure 3. The charge density difference for (16,0) Zigzag SWCNT/PBI (ZZ/PBI) composite. The positive and negative isosurfaces are represented in red and blue, indicating regions of VXC, where Vbare is the local part of the ionic pseudopotential, VH is the Hartree potential and VXC is the exchange–correlation potential24. While the Ef of metallic systems is always well defined, we stick to the following definition for the Fermi energy of semiconductor SWCNT and PBI: 𝛿𝐻𝑓𝑆𝐶,𝑃𝐵𝐼 = 𝐸𝐻𝑂𝑀𝑂 + 𝐸𝑔 /2, where EHOMO is the energy of the highest occupied molecular orbital and Eg is the band gap of the system. It has been reported before that this definition of Ef is consistent with the experimental results25. Fig. 2 represents the electrostatic potentials along y-axis for the three PBI functionalized SWCNT systems. Further, we summarized the calculated work functions, in addition to the Fermi levels and vacuum levels for bulk PBI, SWCNTs and SWCNT/PBI systems in Table S2. It should be noted from Table S2 that the work function of PBI is less than SWCNTs in all three cases, which indicates that during the wave function overlap between PBI and SWCNTs, which results in non-covalent π-π interaction between them, the participation of PBI is higher than SWCNTs. The coupling between PBI and SWCNT is further indicated by charge density difference, calculated by subtracting the electronic charge of a SWCNT/PBI composites form the individual PBI and SWCNT species, shown in Fig. 3. All three


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systems showed similar trend, therefore we only represent the charge density difference of ZZ/PBI system in Fig. 3, where positive and negative isosurfaces are represented in red and blue color, indicating regions of electron accumulation and depletion, respectively. Furthermore, it should be noted from Table S2 that the work function of armchair, zigzag and chiral nanotubes get lowered by 0.193, 0.143 and 0.219 eV, respectively after functionalization them with PBI. It has been reported earlier that the nanotubes with greater functionalization usually experiences greater lowering of the work functions from their pristine counterpart26. However, present study indicates that the PBI functionalization of SWCNTs, having diameter of about 1.2 nm, is almost independent of the chiralities of the nanotubes. The slightly smaller lowering of the work function of the zigzag nanotube might be due to a bit larger diameter of zigzag nanotube compared to the other nanotubes considered in this study. To further confirm the work function analyses X-ray photoelectron spectroscopy (XPS) for SWCNT and SWCNT/PBI systems was conducted (Fig. 4(a)). In XPS, the C1s binding energy peak position is referenced to the Fermi-

Figure 4 (a) XPS C 1s narrow scans of SWCNT (black line) and SWCNT/PBI (red line). (b) Schematic energy level diagram illustrating the relationship between shift in C1s core level measured in XPS and calculated work function (W) of SWCNT and SWCNT/PBI.


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level of SWCNT27. Consequently, a shift in the C1s core level represents the shift in Fermi-level which in turn changes the work function (= 𝜙 − 𝐸𝑓 ) of SWCNT and SWCNT/PBI systems, as illustrated in Fig. 4(b). Our XPS measurements show that the C1s peak of SWCNT (284.3 eV) shifts to higher binding energy (284.6 eV) after wrapping it by PBI, thereby reducing the work function of SWCNT/PBI system as compared to pristine SWCNT (Fig. 4(b)). This observation agrees well with our calculated work function analyses. It should be noted that this interpretation of XPS may not be applicable for strongly interacting materials as strong interactions can cause chemical shifts that may dominate the C1s peak position. However, in this work as PBI and SWCNT interact weakly, we can very well reference C1s peak to the Fermi level. 3.3 Electronic Structure. The functionalization of the SWCNTs can lead to changes in the band structure of the pristine SWCNT. Therefore, to present the most characteristic effects of SWCNT functionalization on the electronic structure, we calculated the total DOS for the AC/PBI, ZZ/PBI, and CH/PBI systems, represented in Fig. 5 (a), (b) and (c), respectively. In these figures, the DOS for SWCNT, PBI and SWCNT/PBI are represented by red, blue and black respectively. The Fermi level (represented by black dashed line) is set at zero energy for all the calculations. In order to check the accuracy of our methodology we also calculated the Kohn–Sham (KS) HOMO– LUMO band gap of about 0.54 eV for the pure Zigzag (16,0) SWCNT, which compares very well with the previous works28,29. It should be noted that the excitonic effects, which are considered as important in the description of SWCNT electronic states are ignored in these calculations. As seen in the literature, considering excitonic effects in the calculations usually corrects the band gap which is otherwise underestimated by DFT29. However, since the objective of this work is to find out the relative effect of PBI wrapping on SWCNTs of different chiralities, by comparing their electronic properties, KS level densities should be sufficient. Moreover, considering excitonic


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effects for the large systems considered in this study requires huge cost of theoretical and computational framework, which is currently beyond the scope of this study. The DOS figures in the left panel of Fig. 5 indicate that valence band edge (VBE) and conduction band edge (CBE) are mainly formed by SWCNT energy states. Whereas, DOS beyond VBE and CBE consists of well mixed PBI and SWCNT states, indicating stronger coupling in both VB and CB states. (a)

(b)

(c)

Figure 5. The total density of states (DOS) for the (a) Armchair (9,9)/PBI (AC/PBI), (b) Zigzag

(16,0)/PBI (ZZ/PBI), (c) Chiral (13, 4)/PBI (CH/PBI) composites. Left panel shows DOS of SWCNT/PBI system (in black) with DOS of SWCNT and PBI plotted in red and blue, respectively. Right Panel shows the partial DOS (PDOS) for the p electrons of C and N atoms of PBI, s electrons of N and H atoms of PBI, and p electrons of C atoms of SWCNT. The Fermi level is set to zero and is represented by the vertical dashed line.


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Further, to analyze the chemical origin of the bands, the projected density of states for the p electrons of C and N atoms, and s electrons of N and H atoms of PBI and p electrons of C atoms of SWCNT are plotted in right panel of the Fig. 5. PDOS for AC/PBI (Fig. 5 (a)), ZZ/PBI (Fig. 5

(a) HOMO

(b) LUMO

Figure 6. (a) Highest occupied molecular orbital (HOMO) and (b) lowest occupied molecular orbital (LUMO) densities for Zigzag (16,0)/PBI (ZZ/PBI) composite. Isosurface value used is 3

0.00015 e/Å . (b)), and CH/PBI (Fig. 5 (c)) shows that even after functionalization VBE and CBE are formed due to the 2p electrons of C of the SWCNTs. This is also indicated by the charge densities of frontier orbitals calculated for the three composites. Since all three systems showed similar trend, we represent (Fig. 6) the charge densities of the frontier orbitals of ZZ/PBI system only. Fig. 6 clearly represents that both HOMO and LUMO of the SWCNT/PBI composites are localized onto the SWCNT only. This analyses indicates that the intrinsic properties, such as electrical conductivity, of SWCNT, essential for the high performance of PEMFCs, will remain unaltered even after functionalizing it with PBI.

CONCLUSIONS


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Overall in this study, we modelled periodic supercell having PBI helically wrapped around SWCNTs of different chiralities and investigated their geometric stability, electronic structure, and interaction mechanism using first principles calculations. It was found that all three nanotubes were stable after PBI wrapping due to the non-covalent π-π interaction between SWCNT and PBI, for which PBI is found to be mainly responsible. Further, binding energy and work function analyses indicate that PBI functionalization of SWCNTs, which have the diameter of about 1.2 nm, is independent of the chiralities of the SWCNTs. Furthermore, from density of states analyses it was found that after functionalization, SWCNTs do not loose their intrinsic characteristics, thereby maintain their high electrical conductivity which is one of the most important characteristic required for better performance of CNT/PBI/PT based PEMFCs. The fundamental insights into SWCNT/PBI composite found in this study might help in designing improved MEAs with enhanced performance for next generation PEMFCs.

ASSOCIATED CONTENT Supporting Information. The PDF file with following content is available free of charge: Optimized structure and binding energy calculation results for PBI aligned axially with the SWCNTs of different chiralities. Calculated vacuum levels (ϕ), Fermi Energy (Ef) and work functions (W) for Bulk PBI, Pristine SWCNTs and helically wrapped SWCNT/PBI composites. Total energy/atom of helically wrapped ZZ/PBI system using non-spin polarized calculations at different K-meshes and using spin polarized (SP) calculations at 1X1X6 K-mesh.

AUTHOR INFORMATION


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Corresponding Author *E-mail: Kulbir K. Ghuman ([email protected] ) *E-mail: Tsuyohiko Fujigaya ([email protected]) Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was supported in part by the International Institute for Carbon Neutral Energy Research (WPI-I2CNER) sponsored by the World Premier International Research Center Initiative (WPI) and the Nanotechnology Platform Project (Molecules and Materials Synthesis) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. The computations were performed by using Computational Science Research Center, Okazaki, Japan and the HPC supercomputers at International Institute for Carbon Neutral Energy Research (I2CNER), Kyushu University, Japan. The authors thank Prof. Aleksandar Staykov for fruitful discussions. REFERENCES (1)

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Electronic Structure of a Polybenzimidazole-Wrapped Single-Wall

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