Sandwiching h-BN Monolayer Films between Sulfonated Poly(ether

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Sandwiching h‑BN Monolayer Films between Sulfonated Poly(ether ether ketone) and Nafion for Proton Exchange Membranes with Improved Ion Selectivity Jiaman Liu,†,∥ Liwei Yu,‡,∥ Xingke Cai,† Usman Khan,† Zhengyang Cai,† Jingyu Xi,‡ Bilu Liu,*,† and Feiyu Kang*,†,§,⊥ ACS Nano Downloaded from pubs.acs.org by TULANE UNIV on 02/16/19. For personal use only.

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Shenzhen Environmental Science and New Energy Technology Engineering Laboratory, Shenzhen Geim Graphene Center (SGC), Tsinghua-Berkeley Shenzhen Institute (TBSI), Tsinghua University, Shenzhen 518055, China ‡ Institute of Green Chemistry and Energy, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China § Shenzhen Key Laboratory for Graphene-based Materials and Engineering Laboratory for Functionalized Carbon Materials, Shenzhen Geim Graphene Center (SGC), Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China ⊥ Laboratory of Advanced Materials, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China S Supporting Information *

ABSTRACT: Two-dimensional (2D) hexagonal boron nitride (hBN) has attracted great interest due to its excellent chemical and thermal stability, electrical insulating property, high proton conductivity, and good flexibility. Integration of 2D h-BN into commercial proton exchange membranes (PEMs) has the potential to improve ion selectivity while maintaining the proton conductivity of PEMs simultaneously, which has been a longstanding challenge in membrane separation technology. Until now, such attempts are only limited in mechanically exfoliated small area h-BN and in proof-ofconcept devices, due to the difficulty of growing and transferring large area uniform h-BN monolayers. Here, we develop a space-confined chemical vapor deposition approach and achieve the growth of wafer-scale uniform h-BN monolayer films on Cu rolls. We further develop a Nafion functional layer assisted transfer method which effectively transfers as-grown hBN monolayer films from the Cu roll to sulfonated poly(ether ether ketone) (SPEEK) membrane. The as-fabricated Nafion/h-BN/SPEEK sandwich structure is used as the membrane and compared with the pure SPEEK membrane for flow batteries. Results show that the sandwich membrane exhibits ion selectivity 3-fold greater than that of a pure SPEEK membrane (i.e., 32.1 × 104 vs 9.7 × 104 S min cm−3). In addition, we fabricate vanadium flow batteries using the Nafion/ h-BN/SPEEK sandwich membrane and find that the sandwich structure does not affect the proton transport but inhibits vanadium crossover at low current density (98% is achieved. This result further verifies the barrier property of the monolayer h-BN even with high ionic flux. The VE, which is defined as the ratio of discharge voltage divided by charge voltage, is similar for the three membranes at low current densities but decreases a little at high current densities. In the sandwich membrane, due to the insulating nature of h-BN, more voltage loss is contributed by the internal resistance with increasing current density. Additionally, the effective channel, that is, honeycomb pore within h-BN, for proton permeation is limited. As a result, with increasing current density, transport of additional protons is inhibited. These two factors synergistically contributed to the reduced VE at high current density (>100 mA cm−2) of the sandwich membrane. Overall, the EE, which combines with CE and VE, reaches around 91% in the sandwich membrane at 40 mA cm−2. This value is 4% higher than that of the pristine SPEEK membrane, confirming that higher ion selectivity is achieved with the introduction of h-BN monolayer film.

METHODS CVD Growth of h-BN. Prior to growth, the Cu foil (25 μm, 99.8%, Alfa Aesar, no. 46365) was electrochemically polished using a homemade electrochemical cell to smoothen its surface. The electropolishing solution was composed of orthophosphoric acid (90 mL) and ethylene glycol (160 mL). A DC power supply was used to supply constant voltage of 2 V for 30 min. After electrochemical polishing, the Cu foil was cleaned with deionized water and finally blow-dried with nitrogen. The Cu foil was rolled into a Cu roll and loaded into the 1 in. furnace for subsequent h-BN growth. Prior to heating, the whole system was evacuated and backfilled three times with Ar gas to remove air and moisture. The furnace was heated up to 1025 °C within 40 min and maintained at this temperature for 30 min under 20 sccm H2 and 20 sccm Ar for annealing treatment. After that, the ammonia borane (NH3·BH3, 25 mg, 97%, Aladdin, B131882) loaded in the separated chamber was heated to 70 °C and then carried into the growth chamber by the carrier gas (20 sccm of Ar and 20 sccm of H2). The growth lasted for 10−60 min. At the end of the growth, the heating of the NH3·BH3 was switched off and the furnace was cooled to room temperature under 50 sccm of Ar. Transfer of h-BN by PDMS/PMMA. The as-grown h-BN was transferred onto 300 nm SiO2/Si substrate and holey carbon TEM grids using a PMMA-assisted transfer process. PMMA was spincoated at 4000 rpm for 60 s on top of the CVD-grown h-BN on Cu. The samples were heat-treated at 150 °C for 90 s for the solidification of PMMA. PDMS was then attached on top of PMMA. The other side without PDMS/PMMA was plasma-treated in air to remove hBN. The samples were then rinsed in ammonium persulfate solution (0.1 M) to etch Cu. Afterward, the obtained PDMS/PMMA-coated h-BN was cleaned with deionized water, and PMMA was then released on 300 nm SiO2/Si substrates. The adhesion of the h-BN film to the substrate was strengthened by heat treatment at 100 °C for 30 min. The PMMA layer was then removed using hot acetone. For the preparation of TEM samples, only PMMA was used during the transfer process to transfer h-BN onto a lacey carbon-coated Cu TEM grid. Transfer of h-BN by Nafion and Fabrication of Nafion/h-BN/ SPEEK Membranes. In the NFL-assisted transfer process, the asgrown h-BN film on Cu was first flattened using clean aluminum foil. A solution of Nafion (20 wt % in DMF) was spin-coated on top of the as-synthesized CVD h-BN on Cu three times with a total thickness on the scale of several hundred nanometers. After each spin-coating, the Nafion film was heat-treated at 80 °C for 5 min to evaporate its solvent. Subsequently, the film was further heat-treated at 200 °C for 30 min to increase its mechanical strength. It was then left for 10 h in the ammonium persulfate solution (0.1 M) to etch Cu. The floating Nafion/h-BN film was rinsed three times with deionized water. After that, the Nafion/h-BN film was scooped with the SPEEK membrane. Finally, the Nafion/h-BN/SPEEK sandwich membrane was dried for 12 h in a fume cupboard and hot-pressed at 140 °C for 5 min to further increase the adhesion between each layer. We note that too high hot-pressing temperature will degrade the sulfonation degree of the SPEEK membrane. The as-fabricated sandwich membranes were soaked in H2SO4 (1 mol L−1, 12 h). Subsequently, the membranes were immersed into deionized water for 12 h to remove the excess acid and then stored in deionized water for further use. Characterization. The morphology and the coverage of the h-BN on Cu were determined by optical microscopy (Carl Zeiss Microscopy, USA) and scanning electron microscopy (5 kV, Hitachi SU8010, Japan). Raman spectra were collected using a 532 nm laser excitation (Horiba LabRAB HR800, Japan). X-ray photoelectron spectroscopy (K-Alpha, Thermo Fisher, USA) was performed to investigate chemical composition of h-BN. The UV−visible absorption spectra (Cary 5000 UV−vis−NIR, Agilent, USA) was measured to estimate the optical band gap of monolayer h-BN on the

CONCLUSIONS In summary, we developed a space-confined CVD method to grow continuous monolayer h-BN film in the Knudsen diffusion regime. To improve the mechanical properties of the h-BN, we developed a nondestructive Nafion functional layer assisted transfer method (NFL method) to transfer 2D hBN and sandwich the h-BN between Nafion and SPEEK. We use such a sandwiched Nafion/h-BN/SPEEK membrane for proton transport applications, which showed excellent ion selectivity due to the use of continuous h-BN monolayers. Specifically, the sandwich structure has shown 3-fold improvement of proton/VO2+ selectivity due to high permeability of protons yet impermeability of VO2+ in h-BN monolayer. By using the membrane in the all VFBs, the rate performance improved significantly, reaching a CE of ∼98% at high current density and an EE of ∼91% at low current density. This work not only showed the importance of confined space for the CVD growth of large area uniform 2D h-BN but also G

DOI: 10.1021/acsnano.8b08680 ACS Nano XXXX, XXX, XXX−XXX

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ACS Nano quartz substrate. The optical band gap energy was determined by using the formula of a direct band gap semiconductor, α = C(E − Eg)1/2/E (where α is the absorption coefficient, C is a constant, E is the photon energy, and Eg is the optical band gap energy). The plot of (αE)2 versus E should be a straight line. Therefore, when (αE)2 = 0, E should be equal to Eg. The thickness of h-BN film was estimated using contact mode AFM (Cypher, Asylum Research, USA). TEM, HRTEM, and SAED were performed at an acceleration voltage of 300 kV (FEI Tecnai F30, USA).

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ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b08680. Supplementary methods, figures, tables, and discussions (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Bilu Liu: 0000-0002-7274-5752 Author Contributions ∥

J.L. and L.Y. contributed equally.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Nos. 51722206 and 21576154), the National Key R&D Program (2018YFA0307200), the Economic, Trade and Information Commission of Shenzhen Municipality for the “2017 Graphene Manufacturing Innovation Center Project” (No. 201901171523), the Shenzhen Basic Research Project (Nos. JCYJ20170307140956657, JCYJ20160613160524999, JCYJ20170412152620376, JCYJ20170412170756603, and ZDSYS20170303165926217), and the Development and Reform Commission of Shenzhen Municipality for the development of the “Low-Dimensional Materials and Devices” discipline. The authors thank S.M. Feng, J.Z. Li, W.H. Wang, and Z.Z. Pan for fruitful discussions. The authors acknowledge the Materials and Devices Testing Center of Graduate School at Shenzhen, Tsinghua University for the use of characterization facilities. REFERENCES (1) He, G.; Li, Z.; Zhao, J.; Wang, S.; Wu, H.; Guiver, M. D.; Jiang, Z. Nanostructured Ion-Exchange Membranes for Fuel Cells: Recent Advances and Perspectives. Adv. Mater. 2015, 27, 5280−5295. (2) Ulaganathan, M.; Aravindan, V.; Yan, Q.; Madhavi, S.; SkyllasKazacos, M.; Lim, T. M. Recent Advancements in All-Vanadium Redox Flow Batteries. Adv. Mater. Interfaces 2016, 3, 1500309. (3) Kusoglu, A.; Weber, A. Z. New Insights into Perfluorinated Sulfonic-Acid Ionomers. Chem. Rev. 2017, 117, 987−1104. (4) Xi, J.; Li, Z.; Yu, L.; Yin, B.; Wang, L.; Liu, L.; Qiu, X.; Chen, L. Effect of Degree of Sulfonation and Casting Solvent on Sulfonated Poly(Ether Ether Ketone) Membrane for Vanadium Redox Flow Battery. J. Power Sources 2015, 285, 195−204. (5) Dai, W.; Shen, Y.; Li, Z.; Yu, L.; Xi, J.; Qiu, X. SPEEK/Graphene Oxide Nanocomposite Membranes with Superior Cyclability for Highly Efficient Vanadium Redox Flow Battery. J. Mater. Chem. A 2014, 2, 12423−12432. H

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DOI: 10.1021/acsnano.8b08680 ACS Nano XXXX, XXX, XXX−XXX