Nanofiber-based Proton Exchange Membranes: Development of

Subscriber access provided by University of Virginia Libraries & VIVA (Virtual Library of Virginia)

Perspective

Nanofiber-based Proton Exchange Membranes: Development of Aligned Electrospun Nanofibers for Polymer Electrolyte Fuel Cell Applications Parashuram Kallem, Numan Yanar, and Heechul Choi ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03601 • Publication Date (Web): 06 Nov 2018 Downloaded from http://pubs.acs.org on November 6, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 51 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

Nanofiber-based Proton Exchange Membranes: Development of Aligned Electrospun Nanofibers for Polymer Electrolyte Fuel Cell Applications

Parashuram Kallem1, Numan Yanar1, Heechul Choi1,2*

1School

of Earth Sciences and Environmental Engineering, Gwangju Institute of Science and

Technology (GIST), 261 Cheomdangwagi-ro, Buk-gu, Gwangju, Republic of Korea 2Center

for Membranes, Advanced Materials Division, Korea Research Institute of Chemical

Technology (KRICT), Daejeon 34114, Republic of Korea Corresponding author’s email address: [email protected]; Phone: 82-62-715-2441; Fax: 8262-715-2423.

1 ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 51

Abstract To ensure sustainable energy and environments for the future, nanotechnology is continuing to contribute new solutions and prospects. Materials with nanofibrous structures are attractive when it comes to addressing several energy issues. Carbon-based fuel dependability is an especial concern, since employing this fuel results in the constant discharge of enormous amounts of greenhouse gas emissions into the ecosphere, as well as diminishing fossil fuel reserves. Hence, it is urgent to decrease the reliance on fossil fuels and focus on using renewable sources, such as solar and hydrogen energy. Due to the recent challenges associated with society’s current energy needs and emerging ecological concerns, the pursuit of novel, low-cost, and environmentally friendly energy conversion and storage devices has attracted growing attention. Due to their high efficiency, high power density, and low greenhouse gas emissions, polymer electrolyte membrane fuel cells (PEMFCs) have drawn extensive interest as energy sources for automobiles, portable electronics, and residential power generation. The main challenges associated with PEMFC concern the development of a robust, durable, low-cost proton exchange membrane (PEM). In this regard, electrospinning has generated considerable interest as a promising method for fabricating nanofiber-based PEMs owing to the specific properties associated with its advanced features, including its high surface area, low density, high pore volume, and easy scale up. This review summarizes the recent work on the development of PEMs based on electrospun nanofibers, especially emphasizing aligned electrospun nanofibers and giving a brief overview of the fabrication, properties, and fuel cell application. In addition, this review briefly highlights the strategies utilized for the recent developments of nanofiber-based PEMs for high-temperature PEMFCs, as discussed in the recent literature.


Page 3 of 51 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Keywords: Polymer electrolyte membrane fuel cells, Electrospinning, Aligned nanofibers, Uniaxially aligned nanofibers, Enhanced mechanical properties, Proton exchange membrane, High-temperature PEMs



Page 4 of 51

Introduction As the present era is dominated by hydrocarbon-centered power generators (gas, oil, and coal), greenhouse gas (GHG) emissions are endlessly increasing. Humankind is facing a future of diminishing fossil fuel reserves, growing energy demands, and a greater understanding of the environmental ramifications of fossil fuel use. It is now of global importance for GHG emissions related with energy production to be decreased significantly to control the consequences of climate change and environmental pollution. The European Union (EU) has fixed the following two targets: First, GHGs should be reduced to 20% below their 1990 levels by 2020; second, there should be a 20% penetration of renewable energy by 2020. Recently, in its 2007 Energy White Paper, the United Kingdom (UK) set out that it would commit to an 80% GHG emission reduction compared with 1990 levels by 2050. The UK and EU targets are both remarkable, although there is now a mutual trend among many nations concerning targets for a low carbon future 1-2. In recent years, many researchers have dedicated their work to developing new alternative and renewable energy sources, including solar energy, wind energy, hydro power, biomass energy, geothermal energy, nuclear energy and hydrogen energy; such efforts include fuel cell technology, which can serve to bridge the present and forthcoming energy demand– supply gap in a sustainable way. As a result of their high efficiencies and low emissions, fuel cells have recently been identified as a crucial technological solution in the path to realizing a low carbon built environment. Thus, fuel cell technology is currently a vital research focus throughout the world 3-6. The first “fuel cell” was invented accidentally by Sir William Grove as a “gaseous voltaic battery” in 1839; Grove noticed that electricity could be generated by reversing the electrolysis of water using hydrogen and oxygen 7. The term “fuel cell” was first used by Mond and Langer in 1889 8. The fuel cell created by the General Electric (GE) Company in 4 ACS Paragon Plus Environment

Page 5 of 51 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the 1950s was utilized in the Gemini space mission in 1962. In the 1960s, GE’s fuel cell unit making was finished by completed Teflon in the impetus layer, especially by making the electrolyte contiguous. Huge developments started in the mid-1970s, promoting the reception of the Nafion® layer, which was finally fluorinated9-10. A fuel cell is an electrochemical device that converts a fuel’s chemical energy into electrical energy effectively without combustion. Fuel cells offer an efficient and sustainable mechanism for energy conversion. Furthermore, fuel cells are compatible with renewable energy sources and modern energy carriers (e.g., hydrogen gas, natural gas, biogas etc.) for the development of sustainability and security of energy. In summary, fuel cells provide a cleaner, flexible and more efficient conversion for chemical-to-electrical energy2, 11-12. Thus, they are expected to serve as the future energy conversion devices of the future. The global fuel cell industry market is estimated to reach $19.2 billion by the year 2020

13,

with the United States, Japan, Germany, South Korea, and Canada acting as the frontier countries for the development and commercialization of fuel cells. Research attention to this field has increased drastically, especially because multiple major companies have introduced fuel cell products to the world. Toyota began marketing their hydrogen fuel cell car, the Mirai; other automotive companies are also focusing on making their own fuel cell electric vehicles (e.g., the Mercedes B-Class, Honda Clarity, and Hyundai Tucson) the U.S. Department of Energy (DoE)

15

14.

According to

currently, fuel cells can be built at low volume for

approximately $280/kW. In contrast, if the latest advanced materials and components applied in the laboratory-scale can be applied into industry with high volume, the cost will be approximately $55/kW. This represents a marked improvement, but it is still short of the ultimate DoE target of $30/kW.



Page 6 of 51

Polymer electrolyte membrane fuel cells (PEMFCs) Among the types of fuel cells, PEMFCs are extremely flexible thus, widely used with the largest range of applications. PEMFCs are suitable for use in vehicles, as well as both portable and stationary applications. These PEMFCs are the extremely favorable candidates for transport applications thanks to their high power density, high efficiency, fast startup time, and easy but safe handling. Nevertheless, PEMFCs are quiet expensive to be viable or economically adequate4, 12, 16-17. The electrochemical processes in the fuel cell take place at the catalyst layers. In PEMFCs, the processes at the anode (negative terminal) and cathode (positive terminal) are as follows respectively (equation 1 and 2): Anode: 2H2 → 4H+ +4e-

(1)

Cathode: O2 + +4e- + 4H+ → 2H2O.

(2)

Hydrogen fuel is provided to the anode of the fuel cell, where it is oxidized to form electrons and protons by producing heat. Each takes a separate direction to the cathode. The protons migrate through the polymer electrolyte membrane, and the electrons are transferred through an external circuit to generate electricity. At the cathode, oxygen gets into reaction with the coming protons to form water. Heat and water are the only byproducts of this electrochemical process. A fuel cell usually contains three active components, which is actually an electrolyte sandwich between a fuel electrode (anode) and an oxidant electrode (cathode). Figure 1 is a schematic demonstration of a single PEMFC. The heart of the PEMFC is a polymer electrolyte/proton exchange membrane (PEM), which acts as a proton conductor, fuel barrier, and mechanical separator sandwiched between the anode and cathode18-20. 6 ACS Paragon Plus Environment

Page 7 of 51 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[Figure 1]

Polymer electrolyte/proton exchange membrane (PEM) In the PEMFC scenario, the challenging part is mainly related to the electrolyte membrane’s performance and durability. The electrolyte membrane can be regarded in two parts: First, there is the polymeric membrane material occurring of a backbone, side chains, and any fillers or support materials to increase the desirable material properties. Second, there is the proton carrier (water or an ionic medium, such as phosphoric acid (H3PO4) or an ionic liquid11, 21. Perfluorosulfonic acid (PFSA) polymer and its derivative ionomers (e.g. Nafion) have emerged as “state of the art” materials for fuel cell applications. The Nafion®’s proton conductivity significantly depend on its water content, since water is responsible simultaneously for sulfonic acid protons dissociation, which offer greatly mobile hydrated protons and suitable percolation of these hydrophilic inclusions. Nafion shows high proton conductivity of up to 100 mS cm–1 under fully hydrated conditions 22. Briefly described, the major shortcomings related to PFSA-based PEMs are as follows: i) the very expensive ($350–500/m2), ii) proton conductivity decreasing with decline in humidity level thus, failure to function at elevated temperature, iii) low mechanical properties at 100 °C, iv) cracks and shrinkage at dehydrated environments, accelerating gas crossover, and v) chemical and thermal degradation 23. Polymer-based electrolyte membranes have been actively investigated in recent years for the development of a new generation of proton exchange membranes that are adequate for high7 ACS Paragon Plus Environment


temperature (above 100 °C) applications

24-26.

Page 8 of 51

Membranes under active development have

been actively reviewed in recent years; they can be classified into four following groups: i) modified PFSA membranes 26-27; ii) alternative membranes based on partially fluorinated and aromatic hydrocarbon polymers

11;

iii) inorganic–organic composites

28;

and iv) acid–base

polymer membranes 29, usually a basic polymer doped with a non-volatile inorganic acid or blended with a polymeric acid. In PEMFCs, PEMs demand technical progress regarding their performance, including the PEM resistance reduction by enhancement of the ion conductivity and/or minimize the PEM thickness, controlling sudden fuel crossover, enhancement of longterm durability, and decreasing of the fabrication price 11-12, 30-31. Electrospun nanofiber-based PEMs Nanotechnology is contributing new resolutions and prospects to the future of ensuring sustainable energy and environments for the future. Nano-fibrous structured materials are efficient to answer several fuel concerns. F. Xu et al32. fabricated mineral nanofiber (nonelectrospun) reinforced composite PEMs with increased water retention capability in PEMFCs. This study shows a facile approach to improve performance of Nafion membranes by introducing nanofibers into Nafion matrix. In this study, Palygorskite as a type of natural mineral fiber with nano-size channel structures33 was introduced into Nafion matrix to enhance water uptake (10% more than pure Nafion membrane at 373 K), proton conductivity (75% more than pristine Nafion membrane at 0% RH), and mechanical properties (25% comparing with the pristine Nafion membrane)32. Electrospinning is progressively recognized as a powerful approach for introducing unique phase-separated nanofiber-based architectures into composite PEMs. Since the early 1990s, significant advancements have taken place related to understanding the complex electrospinning techniques and controlling nanofiber formation; this has influenced the 8 ACS Paragon Plus Environment

Page 9 of 51 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


growth of nanotechnology, as well as the inception of innovative analysis approaches34-35. The electrospun three-dimensional (3D) network nanofibers show the distinctive features, such as a high specific surface area, fully interconnected pores, high porosity, and high orientation or alignment of nanofibers; attributed with these noteworthy characteristics, electrospun nanofibers have been detected as feasible candidates for PEMFC applications

20,

35-37.

As Figure 2 illustrates, the number of published research articles on electrospun materials for fuel cells has increased significantly, especially in recent years (2007–2017); consequently, there is a need to monitor the current results and identify future challenges.

[Figure 2]

Two types of electrospun architectures have been utilized for the development of PEMFCs with the aim of modifying PEMs and improving their performance (Figure 3). The first comprises electrospinning a non- or low conductive polymer into a porous matrix; once the pores are immersed with a proton-conducting medium, this act as mechanical reinforcement (Figure 3A). The second architecture comprises electrospinning a proton-conducting polymer incorporated in non-conducting matrix. In this case, the electrospun nanofibers generate the tracks for proton conduction, whereas the inert polymer matrix provides the mechanical reinforce (Figure 3B).

[Figure 3]



Page 10 of 51

Electrospinning of PFSA ionomers, for example Nafion® the most typically applied polymer was primarily reported by Laforgue et al.

38

using poly (vinyl alcohol) (PVA) or

polyethylene oxide (PEO) as the inert carrier polymer. Later various research groups have used poly (acrylic acid) (PAA), PVP, or PEO, with comparable results. Table 1 presents the results of different investigations associated with the PEMs comprising Nafion® as electrospun nanofibers or the carrier matrix.

[Table 1]

In the field of non-PFSA-based PEMs, other sulfonated polymers, such as poly(ether ether ketone) (PEEK) and poly(arylene ether sulfone) (PAES), and other polymers like polyimide, polyacrylonitrile (PAN), and polybenzimidazole (PBI), have also been electrospun into fibers. A summary of the publications in this topic is provided in Table 2.

[Table 2]

A review on electrospun nanofiber composite polymer electrolyte fuel cells and electrolysis membranes was published in 2011 by Cavaliere storage devices by Sood

37

36,

while one on energy conversion and

came out in 2016. However, there has not been a review dealing

with PEMs composed of uniaxially aligned electrospun nanofibers and methods. This review summarizes the work on the development of polymeric nanofiber–based PEMs composed of


Page 11 of 51 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


uniaxially aligned electrospun nanofibers; it also provides a brief overview of their fabrication, properties, and application to PEMs for fuel cell uses. This review also briefly describes the strategies utilized for the most recent developments concerning electrospun conducting nanofibers and the current literature on their application in high-temperature PEMFC applications. PEMs composed of aligned electrospun nanofibers Electrospinning can offer uniaxial alignment of the polymer chains in nanofibers, thereby facilitating greater mechanical properties and promoting the formation of interconnected channels, resulting in enhanced proton conductivity

37.

Generally, the arrangement of

electrospun nanofibers depends on the types of collectors used, such as a rotating drum, metal frame, and rotating mandrel. Figure 4A and 4B displays the two types of electrospun nanofiber arrangements, non-woven (random) and aligned. All the studies reported in Table 1& 2 have utilized non-woven electrospun nanofibers for fabrication of PEMs.

[Figure 4]

There are pioneer works focused on building PEMs composed of uniaxially aligned electrospun nanofibers for generating high proton conductivity via the formation of interconnected channels, where the ion is rapidly transported through the nanofiber. Figure 4C displays the schematic proton conductive process of conducting aligned electrospun nanofiber. Generally, the polymers within the aligned nanofibers are strongly oriented in the axial direction of the nanofiber, may facilitate the rapid proton transport. Furthermore, when the polymer having hydrophilic and hydrophobic domains is electrospun,



Page 12 of 51

its hydrophilic and hydrophobic in the polymer may be separated to the inside of the polymer solution and the outside as the air surface, respectively. As a result, the structure of the proton channel forms within the electrospun nanofibers due to network of the acid groups, nanofiber lead to the rapid proton transport without forming a proton sites. Moreover, as the polymer chains in the uniaxially aligned nanofibers are strongly oriented in the axial direction

39,

which can result in enhanced mechanical properties

40-44.

Figure 5

displays the schematic representation of two types of conductivity testing directions. Recent articles have considered surface-aligned PEMs (i.e., the electrospun fibers are aligned along the surface of the PEMs) and achieved significant enhancement in proton conductivity through in-plane proton conductivity (i.e., conductivity via the surface direction) and the through-plane direction (i.e., protons are transported through the thickness direction of the PEMs)

40, 42, 45-46.

An overview of the researches utilizing this approach is provided in Table

3. [Figure 5]

[Table 3] Tamura et al. initiated the development of PEMs composed of uniaxially aligned electrospun nanofibers for fuel cell applications

30, 40, 42.

Recently, significant approach has been brought

related to the development of promising PEMS based on sulfonated aromatic hydrocarbon polymers, which have been extensively synthesized as alternative materials owing to their outstanding chemical, thermal, and mechanical stabilities 47-50. Composite membranes containing uniaxially aligned sulfonated polyimide (SPI) 1,4,5,8naphthalene tetracarboxylic dianhydride (NTDA)–4,4’-diamino-biphenyl 2,2’-disulfonic acid


Page 13 of 51 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(BDSA)–2,2-bis(4-[4-aminophenoxy)phenyl]-hexafluoropropane) (r-APPF) nanofibers have been successfully fabricated by the solvent casting method using the procedure described in Tamura et al. 40 and shortly as follows: i) First, a polymer solution, NTDA-BDSA- r-APPF in DMF (11 wt%), is filled in a syringe, and then a required high voltage (24 kV) was used to produce an electrically charged jet of polymer solution between the syringe needle and grounded plate collector. The experimental set-up contains of a high-voltage power supplier, syringe with a blunt-tipped metallic needle, and grounded plate collector used for the preparation of the uniaxially aligned nanofibers are shown in the Figure 6. In principle, once the electrical forces overcome the surface tension, as well as the fluid viscosity, a liquid jet is ejected through the needle tip. The Taylor cone shape is altered by the applied voltage. When the sprayed jet is electrically drawn to the collector electrode, it stretches and it may breaks into narrower jets due to the mutual electrostatic contraction and repulsion forces, correspondingly. Most of the solvent evaporates from the polymer jet before reaching the collector electrode, developing in the formation of nanofibers.

In order to fabricate uniaxially-aligned nanofibers, parallel

aluminum electrodes on glass plate (7cm×7cm), were used as a grounded collector. The distance between the spinneret and the grounded plate was 10 cm. ii) fix the obtained nanofibers by a glass ring to maintain the aligned structure and followed by acidification with HCl solution, iii) dry the nanofibers in a vacuum oven at 80 °C for 24 h,

iv) composite

PEMs fabrication via pouring the polymer solution (sulfonated polyimide) to the resulting nanofibers and followed by dry in a oven at 100 °C for 24 h, v) acidification (with 0.1 M HCl solution) of PEMs with uniaxially aligned nanofibers and followed by washing with deionized water and drying in a vacuum oven at 80 °C for 24 h.



Page 14 of 51

[Figure 6]

Figure 7 displays the chemical structure of the SPI, schematic representation of the proton channel structure in the SPI nanofiber, and microscopic investigations of sulfonated random copolyimide nanofibers aligned on the collector after electrospinning.

[Figure 7]

Composite PEMs containing uniaxially aligned SPI nanofibers showed a high in-plane proton conductivity of 0.88 S/cm at 80 °C, which was much higher than that of the membrane without nanofibers (i.e., 0.036 S/cm). Moreover, the stability of the composite membranes, such as oxidative and hydrolytic stability, was highly enhanced with an increase in nanofiber 40.

Although the sulfonated six-membered (naphthalenic) SPI (NTDA-BDSA-r-APPF) has

been employed as a promising material for use as a PEM, it is extremely hard to fabricate ultrafine, uniform non-beaded electrospun nanofibers from this six-membered ring polyimide solution, since the polyimide exhibits inadequate solubility for organic solvents. Thus, the fabrication of the electrospun polyimide nanofibers from the six-membered ring polyimides has a shortcoming. To overcome this, it has been revealed that the electrospun nanofiber fabricated from the five-membered ring polyimides comprising the fluorinated group outcomes in an ultrathin with uniform non-beaded manufacture51-52. Following this approach, Tamura et al. 42 synthesized novel composite PEMs composed of uniaxially aligned ultrathin electrospun nanofibers from a novel fluorinated five-membered ring sulfonated copolyimide, 2,2’-bis(3,4-dicarboxyphenyl)hexafluoro propane dianhydride (6FDA)-BDSA-r-APPF. The 14 ACS Paragon Plus Environment

Page 15 of 51 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


chemical structure of the 6FDA-BDSA-r-APPF polyimide is displayed in Figure 8a. Here, the composite PEM comprising thinner nanofibers and higher nanofiber contents presented remarkable in-plane proton conductivity up to 0.3 S/cm at 90 °C and 98% RH. As shown in Table 4, this in-plane proton conductivity revealed greater values compared with that measured for the composite PEM in the perpendicular direction. Furthermore, these conductivity values were higher (by one order) than that of the PEM without nanofibers prepared by a conventional solution-casting method due to the development of ionic channels in the nanofibers; as a result, the protons were rapidly and efficiently transported in the nanofibers to increase the conductivity value. Furthermore, the composite PEM displayed higher chemical stabilities than that of the PEM without nanofibers 42.

[Table 4]

In 2014, Takemori et al. 46 prepared uniaxially aligned SPI, 6FDA-BDSA-r-APPF, nanofibers (with diameters of ~100 nm) using parallel collector electrodes

46.

The achieved proton

conductivities of the SPI nanofibers (obtained by a V2 voltage of 3.0 kV) were estimated to be 7.1 and 0.082 S/cm at 90 °C, 95% RH and 90 °C, 30% RH, respectively (Figure 8b & 8c).

[Figure 8]

These proton conductivity values of the SPI nanofibers were two times higher than that of corresponding SPI and Nafion PEMs in all the temperature intervals (Figure 8b). Figure 8c shows that the proton conductivities of the SPI nanofibers fell with decreasing relative 15 ACS Paragon Plus Environment


Page 16 of 51

humidity; nevertheless, the SPI nanofibers provided greater proton conductivities than that of SPI PEMs did, and they were similar to the Nafion PEM at still low relative humidities 46. PEEK is classified as another alternative nonfluorinated polymer material. Based on a focused review

53,

high performance in terms of proton conductivity and thermochemical

properties, in addition to lower cost, low fuel crossover, and easier availability, can be observed for sulfonated PEEK (SPEEK)-based polymers. Relatively less studies have been performed on the electrospinning of SPEEK nanofibers for the preparation of composite PEMs

54-58.

Nevertheless, studies on SPEEK nanofibers have mainly focused on composite

PEMs, and there has been little independent consideration of SPEEK nanofibers. Pure SPEEK nanofibers fabricated using the solution-blowing method were combined into the Nafion® matrix

59.

The developed PEM showed higher conductivity and better thermal

stability and methanol selectivity. When the nanofiber content was 10 wt%, the obtained proton conductivity was 90 mS cm–1 (at 20 °C and 100% RH), which was greater than that for Nafion® 117 (83 mS cm–1)

59.

SPEEK was combined with hydrophilic SiO2 and

electrospun into nanofibers, and composite SiO2/SPEEK PEMs were fabricated by impregnation with Nafion® ionomer

54.

The resulting PEMs displayed improved water

retention and swelling resistance, in addition to greater proton conductivity, compared with cast films. The maximum power density achieved was 170 mW/cm2 (at a high temperature [120 °C] and low humidity [40% RH]), which was 2.4 times higher than that of recast Nafion® (71 mW/cm2 under the same conditions) 54. An interesting approach to producing aligned electrospun SPEEK nanofiber-based PEMs for fuel cell applications was recently described

44,

whereby SPEEK nanofibers at different

degrees of sulfonation (DS) were fabricated in the design of random and aligned nanofibers. SEM observations of SPEEK nanofibers (Figure 9) showed the decrease of nanofiber diameter with increasing DS from 74% to 81% for both random and aligned collected 16 ACS Paragon Plus Environment

Page 17 of 51 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


nanofibers, mainly due to the availability of more SO3H groups at higher DS in polymer solution that increases the electrical conductivity. The through-plane proton conductivity values of developed PEMs were improved with DS at room temperature due to the incorporation of more sulfonic acid groups into nanofibers. The proton conductivities of randomly oriented and aligned nanofibers were measured at 25 °C and 100% RH at 0.0098–0.0722 S/cm and 0.0592–0.0907 S/cm, respectively 44. The aligned nanofibers based PEM shown high proton conductivity than the random collected nanofibers did due to the very organized pathway

60

for existing proton transfer, such as in the aligned

ionomers in an electrospun polymer 40, 61-62.

[Figure 9] Recently, mechanically stable composite PEMs based on SPEEK electrospun nanofibers incorporated in a short-side-chain Aquivion® (perfluorosulfonic acid matrix) were developed by Boaretti et al.

58.

It was observed that the composite SPEEK/Aquivion® PEMs showed

higher mechanical tensile strength compared with the pristine Aquivion membrane owing to the high stiffness of the porous SPEEK electrospun mat. The Young’s modulus values of composite PEMs with DS of 62% and 73% were 280 MPa and 268 MPa, respectively, which were higher than those of the non-reinforced membrane (184 MPa) by increases of 52% and 45%, respectively. Especially, a greater enhancement in the mechanical properties was seen for the crosslinked PEM with a DS of 94%: The obtained Young’s modulus value was 440 MPa, 150% higher than that of pristine (non-reinforced) membranes and 60% higher than PEMs with DS of 62% and 73%. However, the PEM with the lower DS (62%) exhibited good dimensional stability, which was comparable to that of a non-reinforced PEM 58.



Page 18 of 51

So far, several studies discussed above have achieved conductivity through the in-plane direction, while few of them have been gained in the through-plane direction. There has been strategic emphasis on constructing through-plane proton conductive channels via adding electric/magnetic fields to casting the dope solution or filling porous supports with polyelectrolytes. In the exposure of phase-separated structures to an electric field, interfacial polarization constructs the dispersed ionic domains to form strings aligning along the direction of the electric field—although it is challenging to control the ionic strings’ alignment at the nanoscale level21,

63-66.

Filling porous supports (such as polyimide and

porous silicon) with polyelectrolytes rapidly enhances the proton conductivity with decreasing pore diameter, although it is still hard to construct open through-plane holes, especially straight ones at the nanoscale level 67-69. For meeting the challenging needs of high through-plane proton conductivity in PEMFC operations, Gong et al. 41 developed a novel approach to fabricating nanoscale through-plane proton conductive channels along the thickness direction of PEMs by means of aligned electrospun sulfonated poly(phthalazinone ether sulfone ketone) (SPPESK) nanofibers. The complete steps of electrospinning and stacking procedure described in Gong et al.

41,

as

illustrated in Figure 10A: a. The Figure 10A: b-f displays the morphologies of the developed aligned electrospun (SPPESK) nanofibers. The average fiber diameter of uniaxial alignment of the electrospun nanofibers was about 157 ± 52 nm). To enhance the connection between the interfibers and decrease the conductivity gap among both directions of the thickness aligned PEMs, a smooth treatment was performed as an important part in the fabrication process (Figure 10A: d). The developed electrospun PEMs contained long-range ionic clusters through the thickness direction of the PEM, illustrated in the higher total throughplane conductivity compared with the cast SPPESK membranes (nearly twice the level). Because of smooth treatment, the thickness aligned electrospun SPPESK PEMs exhibited 18 ACS Paragon Plus Environment

Page 19 of 51 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


greater single-cell power density (nearly 1.2 fold) and lower hydrogen crossover (33%) as compared with Nafion 115. Smooth treatment also improved the interconnection among nanofibers; consequently, the tensile strength of the thickness aligned electrospun PEM increased significantly after smooth treatment, with an obtained value of 19.3 MPa (Figure 10B), which is about 1.5-fold higher than that of Nafion 115 (13.3 MPa) 41.

[Figure 10]

High temperature PEMs composed of electrospun nanofibers Fuel cell functioning at high temperature (high-temperature [HT]-PEMFCs) is required: At temperatures more than 120 °C, most of the operational drawbacks of the low-temperature PEMFCs, such as water management, catalyst CO poisoning, and efficiency (electrochemical reaction rates plus polarization effects) are overcome, while the cogeneration possibilities are greatly improved21,

70-72.

The main tasks when operating at high temperature are mainly

associated with PEM performance and durability. PFSA-based membranes like Nafion® are available for PEMFCs

73.

Nevertheless, its application in high temperature fuel cells is

limited, as Nafion® PEMs perform well at elevated humidity levels only, limiting its use to temperatures below 100 °C. Polymer-based electrolyte membranes have been significantly analyzed in recent times for the advancement of a new generation of PEMs that are suitable for applications at hightemperature22,

24-25, 74.

Among these, polybenzimidazole

(poly[2,2-(m-phenylene)-5,5-

bibenzimidazole]) (PBI) membranes have taken extensive attention by increasing the



Page 20 of 51

temperature tolerance of conventional PEM materials; this is due to PBI’s excellent chemical and thermal stability

75-81.

PBI belongs to a big family of aromatic heterocyclic polymers

having benzimidazole units. The strong hydrogen bonding among the =N and –NH– groups in PBI is the major molecular force, which results in close chain packing. Thus, it also results in good mechanical strength of the PEM82-84. The pKa of the benzimidazole group is about 5.5, which enables its absorption of acid as a plasticizer. In fact, PBI needs to be doped with acid to get sufficient proton conductivity for fuel cell operation (over 0.05 S·cm–1) since its intrinsic conductivity is negligible (i.e., 10–12 S·cm–1)

85.

One of the most commonly used

mineral acids is phosphoric acid (H3PO4). It is thermally stable at high temperatures, even above 100 °C. Especially, acid-doped PBI PEMs have been familiar as capable PEM candidates functioning at high temperatures11, 82, 85. More specifically, phosphoric acid–doped PBI PEMs 83, 86-89 have shown significant proton conductivity and fuel crossover behavior. As can be observed in Figure 11, Nafion® and acid–doped PBI (PBI/H3PO4) polymeric membranes are the most efficient PEMs 90. They vary only in terms of operating temperature. Nevertheless, PBI is promising to show significant performance at high temperatures of up to 200 °C, along with its advantages of superior mechanical properties, less manufacturing price, and thermal durability 14, 91.

[Figure 11]

It has been investigated that acid-doped PBI-blend PEMs could undergo from deterioration due to the slow extraction of water‐soluble phosphoric acid once vapor has been released 92. Furthermore, it has also been described that these films lose their mechanical properties after


Page 21 of 51 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


acid doping. It was found that another macroscopic configuration, that is, nanofibers and PBI fiber–incorporated films, could affect these drawbacks by enhancing the mechanical properties93-95. The fabrication of electrospun PBI nanofibers was first reported by Kim et al. 93,

and it is described that sulfonic acid treatment enhanced these PBI nanofibers’ mechanical

properties. Usually electrospun nanofibers with randomly orientation benefit from their highly oriented chains along the fiber axis; because of this, they have much better tearing and tensile properties than that of casted films do96-97. Li et al. 94 fabricated polybenzoxazine (PBz)-modified PBI nanofibers via an electrospinning process. In this approach, the nanofibers were thermally crosslinked via the ring-opening addition reaction of the PBz’s benzoxazine groups, which permitted successive impregnation of the electrospun mat with PBI. The developed composite PEM showed greater acid-doping levels (13) as well as dimensional stability. Moreover, the composite PEM exhibited high proton conductivity of 0.17 S cm–1 at 160 °C under anhydrous conditions, which is about 2fold greater than that of the neat PBI PEMs. The PEM with 10 wt% of crosslinking agent displayed a 3-fold higher Young’s modulus (6570 ± 660 MPa) compared with the neat PBI membrane. Later, the structural design of PBI nanofiber based PEMs at various phosphoric acid (PA) doping levels was studied by Jahangiri et al. 95. In this work, the authors presented an experimental process that started with investigating the influence of electrospinning parameters, such as polymer concentration, applied voltage, and feeding rate on the pure PBI nanofiber morphology. They found that the PEMs’ tensile strength increased with the doping level, while the strain at break (%) diminished due to the brittle nature of the formed H‐bond network (Figure 12).

[Figure 12] 21 ACS Paragon Plus Environment


Page 22 of 51

The results suggested that PBI PEMs with 72‐hour dope displayed a proton conductivity of 123 mS/cm could be a promising candidate to apply for PEMFCs. In most cases, the main drawback is associated with the failure mechanisms of PBI based PEMS from the oxidative degradation of membranes by means of oxidative radicals originating from the oxygen diffusion across the PEM under operational conditions. The crucial degradation mechanism of membranes is the peroxide radicals attack on the PEMs having hydrogen-containing bonds82, 98-99. It has been reported that the flexible spacer groups in the PBI backbone could assist in increasing oxidative stability, such as in PBI having ether linkages 104,

100,

azole-containing groups

101,

sulfone linkages

102,

fluoro-containing groups

103-

and pyridine-containing groups 105, and. In contrast, Weber et al. 106 reported that a single

spacer group in a polymer chain cannot be resistant in long-term oxidative stability tests. These attached non-aromatic groups may create weak locations for the polymer backbone due to the polarization of chemical bond, enabling a center scission via a radical attack. To overcome these drawbacks, recently, Muthuraja et al.

107

synthesized novel electrospun

nanofibers by means of introducing double spacer groups (ether and sulfone) in the backbone of polymer, such as poly(aryl sulfone ether benzimidazole) (SO2-OPBI),via a condensation reaction for increased the proton conductivity and chemical durability properties. Figure 13A displays the poly(aryl sulfone ether benzimidazole) synthetic pathway and fabrication process of electrospun membrane, together with the casting procedure. The obtained proton conductivity of the prepared nanofiber SO2-OPBI membranes was 0.0667 S cm–1 at 160 °C (acid doped level of 338%), which was higher than that of the dense membrane (0.033 S cm–1 at 160 °C and 221% acid doping). As displayed in Figure 13B, under the prepared Fenton reagent, the nanofibers and dense membranes exhibited better


Page 23 of 51 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


oxidative stability than the reported PBI-based membranes did

102, 108-113.

Moreover, it was

observed that the oxidative stabilities of OPBI/silica, PBI/ZC-SiO2, silane crosslinked PBI and PDADMA-TFMS/PBI membranes were poorer than that of m-PBI. However, the major shortcoming of the m-PBI membranes utilized in PEMFCs was their low oxidative chemical stability compared with PFSA and crosslinked modified PBI membranes. Moreover, these developed SO2-OPBI membranes with the sulfone and ether linked in the polymeric chain showed greater resistance and flexibility compared with m-PBI 107.

[Figure 13]

Conclusions and future perspectives Thanks to their enhanced efficiencies and low emissions, fuel cells have been regarded as a crucial technological alternative on the path to realizing a low-carbon built environment. However, in commercializing the PEMFC technology targeted by the DoE, it is necessary to develop new functional materials with focused nanomaterial architectures that can address the shortcomings related to reliability, cost, durability, and infrastructure. There are ongoing pioneer research efforts for achieving a low-cost material with higher durability. In addition to several paths under investigation, one-dimensional (1D) nanomaterials are expected to have favorable effects in the context of specific challenges because the structural control of nanomaterials can be as significant as their composition. The electrospun network nanofibers show specific benefits, such as a high specific surface area, fully interconnected pores, high porosity, and high alignment of nanofibers. This review has shown that the composite PEMs containing aligned nanofibers can be strongly recommended due to their ability to achieve high proton conductivity, low gas permeability 23 ACS Paragon Plus Environment


Page 24 of 51

in the fuel, and good chemical, thermal, and mechanical stabilities. Many research attempts are still being carried out with a focus on constructing uniaxially aligned nanofiber-based PEMs to achieve high proton conductivity through the building of interconnected networks. In this work, the polymer chains in the PEM are effectively ordered in the axial direction, leading to improved durability. Such materials may turn out to be promising PEMs, and they can potentially be useful for fuel cell applications. Regardless of these accomplishments, there are challenges that remain to be overcome before the large-scale usage of electrospun nanofiber-based PEMs becomes possible in fuel cell applications, and this is because the process parameters that influence the electrospinning technique are not yet completely understood. Some other limitations include the feasibility of electrospinning specific polymer materials; need to achieve uniform, low-diameter nanofibers without beads; and limited understanding of the correlation between the processing parameters and morphology. Another aspect that requires significant attention is the design technique, which ought to offer ideal process parameters for cost-effective power generation and sustaining the system’s lifespan to prevent rapid degradation. HT-PEMFCs are efficient, clean systems for stationary and automotive applications compared with conventional PEMs, as no humidifier is required; in addition, the heat delivered is useful for further applications. PBIs/functionalized PBIbased PEMs have been employed for HT-PEMFCs due to their high proton conductivity and thermomechanical stability. More research efforts must still be carried out to resolve the shortcomings related to durability, catalyst cost, agglomeration, material corrosion, and acid leaching in PEMFCs. Moreover, tremendous research efforts are ongoing to elucidate the material aspects and increase the startup time to establish efficient heating strategies and enhance durability. These issues require further investigation through extensive experiments, as they are vital factors in securing the ideal performance before marketing this technology. 24 ACS Paragon Plus Environment

Page 25 of 51 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Acknowledgements The authors gratefully acknowledge the Gwangju Institute of Science and Technology (GIST) for financial support through the Research Grant BK21 Plus project.

References 1. Böhringer, C.; Rutherford, T. F.; Tol, R. S. J. THE EU 20/20/2020 targets: An overview of the EMF22 assessment. Energy Economics 2009, 31, Supplement 2, S268-S273, DO: 10.1016/j.eneco.2009.10.010 2. Elmer, T.; Worall, M.; Wu, S.; Riffat, S. B. Fuel cell technology for domestic built environment applications: State of-the-art review. Renewable Sustainable Energy Rev. 2015, 42, 913931, DOI: 10.1016/j.rser.2014.10.080 3. Thanapalan, K. K. T.; Williams, J. G.; Liu, G. P.; Rees, D. MODELLING OF A PEM FUEL CELL SYSTEM. IFAC Proceedings Volumes 2008, 41 (2), 4636-4641, DOI: 20080706-5-KR-1001.00780 4. Daud, W. R. W.; Rosli, R. E.; Majlan, E. H.; Hamid, S. A. A.; Mohamed, R.; Husaini, T. PEM fuel cell system control: A review. Renewable Energy 2017, 113, 620-638, DOI: 10.1016/j.renene.2017.06.027 5. Sopian, K.; Wan Daud, W. R. Challenges and future developments in proton exchange membrane fuel cells. Renewable Energy 2006, 31 (5), 719-727, DOI: 10.1016/j.renene.2005.09.003 6. Shen, M.; Meuleman, W.; Scott, K. The characteristics of power generation of static state fuel cells. J. Power Sources 2003, 115 (2), 203-209, DOI: 10.1016/S0378-7753(02)00731-0 7. Grove, W. R. LXXII. On a gaseous voltaic battery. The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science 1842, 21 (140), 417-420, DOI: 10.1080/14786444208621600 8. Mond, L.; Langer, C. A New Form of Gas Battery. Proc. R. Soc. London 1889, 46, 296-304. 9. Wang, Y.; Chen, K. S.; Mishler, J.; Cho, S. C.; Adroher, X. C. A review of polymer electrolyte membrane fuel cells: Technology, applications, and needs on fundamental research. Appl. Energy 2011, 88 (4), 981-1007, DOI: 10.1016/j.apenergy.2010.09.030



Page 26 of 51

10. Das, V.; Padmanaban, S.; Venkitusamy, K.; Selvamuthukumaran, R.; Blaabjerg, F.; Siano, P. Recent advances and challenges of fuel cell based power system architectures and control – A review. Renewable Sustainable Energy Rev. 2017, 73, 10-18, DOI: 10.1016/j.rser.2017.01.148 11. Chandan, A.; Hattenberger, M.; El-kharouf, A.; Du, S.; Dhir, A.; Self, V.; Pollet, B. G.; Ingram, A.; Bujalski, W. High temperature (HT) polymer electrolyte membrane fuel cells (PEMFC) – A review. J. Power Sources 2013, 231, 264-278, DOI: 10.1016/j.jpowsour.2012.11.126 12. Sharaf, O. Z.; Orhan, M. F. An overview of fuel cell technology: Fundamentals and applications. Renewable Sustainable Energy Rev. 2014, 32, 810-853, DOI: 10.1016/j.rser.2014.01.012 13. Tollefson, J., Tollefson J. US Congress revives hydrogen vehicle research. Nature 2009, 460, 442-443, DOI: 10.1038/460442b 14. Rosli, R. E.; Sulong, A. B.; Daud, W. R. W.; Zulkifley, M. A.; Husaini, T.; Rosli, M. I.; Majlan, E. H.; Haque, M. A. A review of high-temperature proton exchange membrane fuel cell (HT-PEMFC) system. Int. J. Hydrogen Energy 2017, 42 (14), 9293-9314, DOI: 10.1016/j.ijhydene.2016.06.211 15. DOE TECHNICAL TARGETS FOR POLYMER ELECTROLYTE MEMBRANE FUEL CELL COMPONENTS.https://energy.gov/eere/fuelcells/doe-technical-targets-polymer-electrolytemembrane-fuel-cell-components (accessed October 20, 2016). 16. Chen, H.; Song, Z.; Zhao, X.; Zhang, T.; Pei, P.; Liang, C. A review of durability test protocols of the proton exchange membrane fuel cells for vehicle. Appl. Energy 2018, 224, 289-299, DOI: 10.1016/j.apenergy.2018.04.050 17. Zhang, T.; Wang, P.; Chen, H.; Pei, P. A review of automotive proton exchange membrane fuel cell degradation under start-stop operating condition. Appl. Energy 2018, 223, 249-262, DOI: 10.1016/j.apenergy.2018.04.049 18. Raza, R.; Akram, N.; Javed, M. S.; Rafique, A.; Ullah, K.; Ali, A.; Saleem, M.; Ahmed, R. Fuel cell technology for sustainable development in Pakistan – An over-view. Renewable Sustainable Energy Rev.2016, 53, 450-461, DOI: 10.1016/j.rser.2015.08.049 19. Rikukawa, M.; Sanui, K. Proton-conducting polymer electrolyte membranes based on hydrocarbon polymers. Prog. Polym. Sci. 2000, 25 (10), 1463-1502, DOI: 10.1016/S00796700(00)00032-0 20. Bakangura, E.; Wu, L.; Ge, L.; Yang, Z.; Xu, T. Mixed matrix proton exchange membranes for fuel cells: State of the art and perspectives. Prog. Polym. Sci. 2016, 57, 103-152, 10.1016/j.progpolymsci.2015.11.004 21. Kallem, P.; Eguizabal, A.; Mallada, R.; Pina, M. P. Constructing Straight Polyionic Liquid Microchannels for Fast Anhydrous Proton Transport. ACS Appl. Mater. Interfaces 2016, 8 (51), 35377-35389, DOI: 10.1021/acsami.6b13315 22. Li, Q.; He, R.; Jensen, J. O.; Bjerrum, N. J. Approaches and Recent Development of Polymer Electrolyte Membranes for Fuel Cells Operating above 100 °C. Chem. Mater. 2003, 15 (26), 48964915, DOI: 10.1021/cm0310519 23. Mark F. Mathias, R. M., Hubert A. Gasteiger, Jason J. Conley,; Timothy J. Fuller, C. J. G., Shyam S. Kocha, Daniel P. Miller,; Corky K. Mittelsteadt, T. X., Susan G. Yan, Paul T. Yu. Two Fuel Cell Cars In Every Garage? The Electrochemical Society Interface 2005, 24-35. 24. Bose, S.; Kuila, T.; Nguyen, T. X. H.; Kim, N. H.; Lau, K.-t.; Lee, J. H. Polymer membranes for high temperature proton exchange membrane fuel cell: Recent advances and challenges. Prog. Polym. Sci. 2011, 36 (6), 813-843, DOI: 10.1016/j.progpolymsci.2011.01.003 25. Hogarth, W. H. J.; Diniz da Costa, J. C.; Lu, G. Q. Solid acid membranes for high temperature (¿140° C) proton exchange membrane fuel cells. J. Power Sources 2005, 142 (1-2), 223-237, DOI: 10.1016/j.jpowsour.2004.11.020 26. Li, H.-Y.; Liu, Y.-L. Nafion-functionalized electrospun poly(vinylidene fluoride) (PVDF) nanofibers for high performance proton exchange membranes in fuel cells. J. Mater. Chem. A 2014, 2 (11), 3783-3793, DOI: 10.1039/c3ta14264g


Page 27 of 51 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


27. Dupuis, A.-C. Proton exchange membranes for fuel cells operated at medium temperatures: Materials and experimental techniques. Prog. Mater. Sci. 2011, 56 (3), 289-327, DOI: 10.1016/j.pmatsci.2010.11.001 28. Eguizábal, A.; Lemus, J.; Urbiztondo, M.; Garrido, O.; Soler, J.; Blazquez, J. A.; Pina, M. P. Novel hybrid membranes based on polybenzimidazole and ETS-10 titanosilicate type material for high temperature proton exchange membrane fuel cells: A comprehensive study on dense and porous systems. J. Power Sources 2011, 196 (21), 8994-9007. DOI: 10.1016/j.jpowsour.2011.03.006 29. Ngamsantivongsa, P.; Lin, H.-L.; Yu, T. L. Crosslinked ethyl phosphoric acid grafted polybenzimidazole and polybenzimidazole blend membranes for high-temperature proton exchange membrane fuel cells. J. Polym. Res. 2016, 23 (2), 1-11, DOI: 10.1007/s10965-015-0911-3 30. Tanaka, M. Development of ion conductive nanofibers for polymer electrolyte fuel cells. Polymer Journal 2015, 48 (1), 51-58, DOI: 10.1038/pj.2015.76 31. Kallem, P.; Drobek, M.; Julbe, A.; Vriezekolk, E. J.; Mallada, R.; Pina, M. P. Hierarchical Porous Polybenzimidazole Microsieves: An Efficient Architecture for Anhydrous Proton Transport via Polyionic Liquids. ACS Appl. Mater. Interfaces 2017, 9 (17), 14844-14857, DOI: 10.1021/acsami.7b01916 32. Xu, F.; Mu, S.; Pan, M. Mineral nanofibre reinforced composite polymer electrolyte membranes with enhanced water retention capability in PEM fuel cells. J. Membr. Sci. 2011, 377 (1), 134-140, DOI: 10.1016/j.memsci.2011.04.027 33. Xu, F.; Xu, R.; Mu, S. Nano mineral fiber enhanced catalyst coated membranes for improving polymer electrolyte membrane fuel cell durability. J. Power Sources 2011, 196 (24), 10563-10569, DOI: 10.1016/j.jpowsour.2011.08.098 34. Xue, J.; Xie, J.; Liu, W.; Xia, Y. Electrospun Nanofibers: New Concepts, Materials, and Applications. Acc. Chem. Res. 2017, 50 (8), 1976-1987, DOI: 10.1021/acs.accounts.7b00218 35. Liao, Y.; Loh, C.-H.; Tian, M.; Wang, R.; Fane, A. G. Progress in electrospun polymeric nanofibrous membranes for water treatment: Fabrication, modification and applications. Prog. Polym. Sci. 2018, 77, 69-94, DOI: j.progpolymsci.2017.10.003 36. Cavaliere, S.; Subianto, S.; Savych, I.; Jones, D. J.; Rozière, J. Electrospinning: designed architectures for energy conversion and storage devices. Energy Environ. Sci. 2011, 4 (12), 4761, DOI: 10.1039/c1ee02201f 37. Sood, R.; Cavaliere, S.; Jones, D. J.; Rozière, J. Electrospun nanofibre composite polymer electrolyte fuel cell and electrolysis membranes. Nano Energy 2016, 26, 729-745, DOI: 10.1016/j.nanoen.2016.06.027 38. Laforgue, A.; Robitaille, L.; Mokrini, A.; Ajji, A. Fabrication and Characterization of Ionic Conducting Nanofibers. Macromol. Mater. Eng. 2007, 292 (12), 1229-1236, DOI: 10.1002/mame.200700200 39. Kakade, M. V.; Givens, S.; Gardner, K.; Lee, K. H.; Chase, D. B.; Rabolt, J. F. Electric Field Induced Orientation of Polymer Chains in Macroscopically Aligned Electrospun Polymer Nanofibers. J. Am. Chem. Soc. 2007, 129 (10), 2777-2782, DOI: 10.1021/ja065043f 40. Tamura, T.; Kawakami, H. Aligned electrospun nanofiber composite membranes for fuel cell electrolytes. Nano Lett. 2010, 10 (4), 1324-8, DOI: 10.1021/nl1007079 41. Gong, X.; He, G.; Wu, Y.; Zhang, S.; Chen, B.; Dai, Y.; Wu, X. Aligned electrospun nanofibers as proton conductive channels through thickness of sulfonated poly (phthalazinone ether sulfone ketone) proton exchange membranes. J. Power Sources 2017, 358, 134-141, DOI: 10.1016/j.jpowsour.2017.05.022 42. Tamura, T.; Takemori, R.; Kawakami, H. Proton conductive properties of composite membranes containing uniaxially aligned ultrafine electrospun polyimide nanofiber. J. Power Sources 2012, 217, 135-141, DOI: 10.1016/j.jpowsour.2012.05.118 43. Fennessey, S. F.; Farris, R. J. Fabrication of aligned and molecularly oriented electrospun polyacrylonitrile nanofibers and the mechanical behavior of their twisted yarns. Polymer 2004, 45 (12), 4217-4225, DOI: 10.1016/j.polymer.2004.04.001 27 ACS Paragon Plus Environment


Page 28 of 51

44. Sadrjahani, M.; Gharehaghaji, A. A.; Javanbakht, M. Aligned electrospun sulfonated polyetheretherketone nanofiber based proton exchange membranes for fuel cell applications. Polym. Eng. Sci. 2017, 57 (8), 789-796, DOI: 10.1002/pen.24453 45. Wu, B.; Pan, J.; Ge, L.; Wu, L.; Wang, H.; Xu, T. Oriented MOF-polymer composite nanofiber membranes for high proton conductivity at high temperature and anhydrous condition. Sci. Rep. 2014, 4, 4334, DOI: 10.1038/srep04334 46. Takemori, R.; Ito, G.; Tanaka, M.; Kawakami, H. Ultra-high proton conduction in electrospun sulfonated polyimide nanofibers. RSC Adv. 2014, 4 (38), 20005-20009, DOI: 10.1039/C4RA02155J 47. Campagne, B.; David, G.; Améduri, B.; Jones, D. J.; Rozière, J.; Roche, I. Novel Blend Membranes of Partially Fluorinated Copolymers Bearing Azole Functions with Sulfonated PEEK for PEMFC Operating at Low Relative Humidity: Influence of the Nature of the N-Heterocycle. Macromolecules 2013, 46 (8), 3046-3057, DOI: 10.1021/ma400239f 48. Yamazaki, K.; Wang, G.; Tanaka, M.; Kawakami, H. Sulfonated block-graft copolyimide for high proton conductive and low gas permeable polymer electrolyte membrane. J. Power Sources 2012, 216, 387-394, DOI: 10.1016/j.jpowsour.2012.05.097 49. Matsushita, S.; Kim, J.-D. Organic solvent-free preparation of electrolyte membranes with high proton conductivity using aromatic hydrocarbon polymers and small cross-linker molecules. Solid State Ionics 2018, 316, 102-109, DOI: 10.1016/j.ssi.2017.12.033 50. Miyake, J.; Miyatake, K. Fluorine-free sulfonated aromatic polymers as proton exchange membranes. Polymer Journal 2017, 49, 487, DOI: 10.1038/pj.2017.11 51. Karube, Y.; Kawakami, H. Fabrication of well-aligned electrospun nanofibrous membrane based on fluorinated polyimide. Polym. Adv. Technol. 2010, 21 (12), 861-866, DOI: 10.1002/pat.1511 52. Fukushima, S.; Karube, Y.; Kawakami, H. Preparation of ultrafine uniform electrospun polyimide nanofiber. Polymer Journal 2010, 42, 514, DOI: 10.1038/pj.2010.33 53. Iulianelli, A.; Basile, A. Sulfonated PEEK-based polymers in PEMFC and DMFC applications: A review. Int. J. Hydrogen Energy 2012, 37 (20), 15241-15255, DOI: 10.1016/j.ijhydene.2012.07.063 54. Lee, C.; Jo, S. M.; Choi, J.; Baek, K.-Y.; Truong, Y. B.; Kyratzis, I. L.; Shul, Y.-G. SiO2/sulfonated poly ether ether ketone (SPEEK) composite nanofiber mat supported proton exchange membranes for fuel cells. J. Mater. Sci. 2013, 48 (10), 3665-3671, DOI: 10.1007/s10853-013-7162-7 55. Wang, J.; He, Y.; Zhao, L.; Li, Y.; Cao, S.; Zhang, B.; Zhang, H. Enhanced proton conductivities of nanofibrous composite membranes enabled by acid–base pairs under hydrated and anhydrous conditions. J. Membr. Sci. 2015, 482, 1-12, DOI: 10.1016/j.memsci.2015.02.015 56. Liu, X.; Yang, Z.; Zhang, Y.; Li, C.; Dong, J.; Liu, Y.; Cheng, H. Electrospun multifunctional sulfonated carbon nanofibers for design and fabrication of SPEEK composite proton exchange membranes for direct methanol fuel cell application. Int. J. Hydrogen Energy 2017, 42 (15), 1027510284, DOI: 10.1016/j.ijhydene.2017.02.128 57. Dong, C.; Hao, Z.; Wang, Q.; Zhu, B.; Cong, C.; Meng, X.; Zhou, Q. Facile synthesis of metal oxide nanofibers and construction of continuous proton-conducting pathways in SPEEK composite membranes. Int. J. Hydrogen Energy 2017, 42 (40), 25388-25400, DOI: 10.1016/j.ijhydene.2017.08.136 58. Boaretti, C.; Pasquini, L.; Sood, R.; Giancola, S.; Donnadio, A.; Roso, M.; Modesti, M.; Cavaliere, S. Mechanically stable nanofibrous sPEEK/Aquivion® composite membranes for fuel cell applications. J. Membr. Sci.2018, 545, 66-74, DOI: 10.1016/j.memsci.2017.09.055 59. Xu, X.; Li, L.; Wang, H.; Li, X.; Zhuang, X. Solution blown sulfonated poly(ether ether ketone) nanofiber-Nafion composite membranes for proton exchange membrane fuel cells. RSC Adv. 2015, 5 (7), 4934-4940, DOI: 10.1039/C4RA10898A 60. Sato, T.; Hayasaka, Y.; Mitsuishi, M.; Miyashita, T.; Nagano, S.; Matsui, J. High Proton Conductivity in the Molecular Interlayer of a Polymer Nanosheet Multilayer Film. Langmuir 2015, 31 (18), 5174-5180, DOI: 10.1021/acs.langmuir.5b00036 61. Dong, B.; Gwee, L.; Salas-de la Cruz, D.; Winey, K. I.; Elabd, Y. A. Super Proton Conductive High-Purity Nafion Nanofibers. Nano letters 2010, 10 (9), 3785-3790, DOI: 10.1021/nl102581w 28 ACS Paragon Plus Environment

Page 29 of 51 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


62. Beydaghi, H.; Javanbakht, M. Aligned Nanocomposite Membranes Containing Sulfonated Graphene Oxide with Superior Ionic Conductivity for Direct Methanol Fuel Cell Application. Ind. Eng. Chem. Res. 2015, 54 (28), 7028-7037, DOI: 10.1021/acs.iecr.5b01450 63. Lee, J.-Y.; Lee, J.-H.; Ryu, S.; Yun, S.-H.; Moon, S.-H. Electrically aligned ion channels in cation exchange membranes and their polarized conductivity. J. Membr. Sci. 2015, 478, 19-24, DOI: 10.1016/j.memsci.2014.12.049 64. Park, M. J.; Balsara, N. P. Anisotropic Proton Conduction in Aligned Block Copolymer Electrolyte Membranes at Equilibrium with Humid Air. Macromolecules 2010, 43 (1), 292-298, DOI: 10.1021/ma901980b 65. Liu, D.; Yates, M. Z. Electric field processing to control the structure of poly(vinylidene fluoride) composite proton conducting membranes. J. Membr. Sci. 2009, 326 (2), 539-548, DOI: 10.1016/j.memsci.2008.10.031 66. Isaacs Sodeye, A. I.; Huang, T.; Gido, S. P.; Mays, J. W. Polymer electrolyte membranes from fluorinated polyisoprene-block-sulfonated polystyrene: Microdomain orientation by external field. Polymer 2011, 52 (24), 5393-5396, DOI: 10.1016/j.polymer.2011.10.005 67. Yamaguchi, T.; Zhou, H.; Nakazawa, S.; Hara, N. An Extremely Low Methanol Crossover and Highly Durable Aromatic Pore-Filling Electrolyte Membrane for Direct Methanol Fuel Cells. Adv. Mater. 2007, 19 (4), 592-596, DOI: 10.1002/adma.200601086 68. Chen, H.; Palmese, G. R.; Elabd, Y. A. Membranes with Oriented Polyelectrolyte Nanodomains. Chem. Mater. 2006, 18 (20), 4875-4881, DOI: 10.1021/cm061422w 69. Bureekaew, S.; Horike, S.; Higuchi, M.; Mizuno, M.; Kawamura, T.; Tanaka, D.; Yanai, N.; Kitagawa, S. One-dimensional imidazole aggregate in aluminium porous coordination polymers with high proton conductivity. Nat. Mater. 2009, 8, 831, DOI: 10.1038/nmat2526 70. Lemus, J.; Eguizábal, A.; Pina, M. P. Endurance strategies for the preparation of high temperature polymer electrolyte membranes by UV polymerization of 1-H-3-vinylimidazolium bis(trifluoromethanesulfonyl)imide for fuel cell applications. Int. J. Hydrogen Energy 2016, 41 (6), 3981-3993, DOI: 10.1016/j.ijhydene.2015.11.006 71. van de Ven, E.; Chairuna, A.; Merle, G.; Benito, S. P.; Borneman, Z.; Nijmeijer, K. Ionic liquid doped polybenzimidazole membranes for high temperature Proton Exchange Membrane fuel cell applications. J. Power Sources 2013, 222, 202-209, DOI: 10.1016/j.jpowsour.2012.07.112 72. Quartarone, E.; Angioni, S.; Mustarelli, P. Polymer and Composite Membranes for ProtonConducting, High-Temperature Fuel Cells: A Critical Review. Materials 2017, 10 (7), DOI: 10.3390/ma10070687 73. Peighambardoust, S. J.; Rowshanzamir, S.; Amjadi, M. Review of the proton exchange membranes for fuel cell applications. Int. J. Hydrogen Energy 2010, 35 (17), 9349-9384, DOI: 10.1016/j.ijhydene.2010.05.017 74. Shao, Y.; Yin, G.; Wang, Z.; Gao, Y. Proton exchange membrane fuel cell from low temperature to high temperature: Material challenges. J. Power Sources 2007, 167 (2), 235-242, DOI: 10.1016/j.jpowsour.2007.02.065 75. He, G.; Li, Z.; Zhao, J.; Wang, S.; Wu, H.; Guiver Michael, D.; Jiang, Z. Nanostructured Ion-Exchange Membranes for Fuel Cells: Recent Advances and Perspectives. Adv. Mater. 2015, 27 (36), 5280-5295, DOI: 10.1002/adma.201501406 76. Fujigaya, T.; Nakashima, N. Fuel Cell Electrocatalyst Using Polybenzimidazole-Modified Carbon Nanotubes As Support Materials. Advanced Materials 2013, 25 (12), 1666-1681, DOI: 10.1002/adma.201204461 77. Xiao, L.; Zhang, H.; Scanlon, E.; Ramanathan, L. S.; Choe, E.-W.; Rogers, D.; Apple, T.; Benicewicz, B. C. High-Temperature Polybenzimidazole Fuel Cell Membranes via a Sol−Gel Process. Chem. Mater. 2005, 17 (21), 5328-5333, DOI: 10.1021/cm050831+ 78. Eguizábal, A.; Sgroi, M.; Pullini, D.; Ferain, E.; Pina, M. P. Nanoporous PBI membranes by track etching for high temperature PEMs. J. Membr. Sci. 2014, 454, 243-252, 10.1016/j.memsci.2013.12.006 29 ACS Paragon Plus Environment


Page 30 of 51

79. Eguizábal, A.; Lemus, J.; Pina, M. P., On the incorporation of protic ionic liquids imbibed in large pore zeolites to polybenzimidazole membranes for high temperature proton exchange membrane fuel cells. J. Power Sources 2013, 222, 483-492, DOI: 10.1016/j.jpowsour.2012.07.094 80. Eguizábal, A.; J.Lemus; Roda, V.; Urbiztondo, M.; Barreras, F.; Pina, M. P. Nanostructured electrolyte membranes based on zeotypes, protic ionic liquids and porous PBI membranes: Preparation, characterization and MEA testing. Int. J. Hydrogen Energy 2012, 37 (8), 7221-7234, DOI: 10.1016/j.ijhydene.2011.11.074 81. Araya, S. S.; Zhou, F.; Liso, V.; Sahlin, S. L.; Vang, J. R.; Thomas, S.; Gao, X.; Jeppesen, C.; Kær, S. K. A comprehensive review of PBI-based high temperature PEM fuel cells. Int. J. Hydrogen Energy 2016, 41 (46), 21310-21344, DOI: 10.1016/j.ijhydene.2016.09.024 82. Li, Q.; Jensen, J. O.; Savinell, R. F.; Bjerrum, N. J. High temperature proton exchange membranes based on polybenzimidazoles for fuel cells. Prog. Polym. Sci. 2009, 34 (5), 449-477 DOI: 10.1016/j.progpolymsci.2008.12.003 83. Li, Q.; He, R.; Jensen, J. O.; Bjerrum, N. J. PBI-Based Polymer Membranes for High Temperature Fuel Cells – Preparation, Characterization and Fuel Cell Demonstration. Fuel Cells 2004, 4 (3), 147-159, DOI: 10.1002/fuce.200400020 84. Zuo, Z.; Fu, Y.; Manthiram, A. Novel Blend Membranes Based on Acid-Base Interactions for Fuel Cells. Polymers 2012, 4 (4), 1627-1644, DOI: 10.3390/polym4041627 85. Asensio, J. A.; Sanchez, E. M.; Gomez-Romero, P. Proton-conducting membranes based on benzimidazole polymers for high-temperature PEM fuel cells. A chemical quest. Chem. Soc. Rev. 2010, 39 (8), 3210-39, DOI: 10.1039/b922650h 86. Li, Q.; He, R.; Berg, R. W.; Hjuler, H. A.; Bjerrum, N. J. Water uptake and acid doping of polybenzimidazoles as electrolyte membranes for fuel cells. Solid State Ionics 2004, 168 (1–2), 177185, DOI: 10.1016/j.ssi.2004.02.013 87. Ma, Y. L.; Wainright, J. S.; Litt, M. H.; Savinell, R. F. Conductivity of PBI Membranes for HighTemperature Polymer Electrolyte Fuel Cells. J. Electrochem. Soc. 2004, 151 (1), A8, DOI: 10.1149/1.1630037 88. He, R.; Li, Q.; Bach, A.; Jensen, J.; Bjerrum, N., Physicochemical properties of phosphoric acid doped polybenzimidazole membranes for fuel cells. J. Membr. Sci. 2006, 277 (1-2), 38-45, DOI: 10.1016/j.memsci.2005.10.005 89. Lobato, J.; Cañizares, P.; Rodrigo, M. A.; Linares, J. J.; Aguilar, J. A. Improved polybenzimidazole films for H3PO4-doped PBI-based high temperature PEMFC. J. Membr. Sci. 2007, 306 (1-2), 47-55, DOI: 10.1016/j.memsci.2007.08.028 90. Wieser, C. Novel Polymer Electrolyte Membranes for Automotive Applications – Requirements and Benefits. Fuel Cells 2004, 4 (4), 245-250, DOI: 10.1002/fuce.200400038 91. Zhang, J.; Xie, Z.; Zhang, J.; Tang, Y.; Song, C.; Navessin, T.; Shi, Z.; Song, D.; Wang, H.; Wilkinson, D. P.; Liu, Z.-S.; Holdcroft, S. High temperature PEM fuel cells. J. Power Sources 2006, 160 (2), 872-891, DOI: 10.1016/j.jpowsour.2006.05.034 92. Guan, Y. S.; Pu, H. T.; Jin, M.; Chang, Z. H.; Wan, D. C. Preparation and Characterisation of Proton Exchange Membranes Based on Crosslinked Polybenzimidazole and Phosphoric Acid. Fuel Cells 2010, 10 (6), 973-982, DOI: 10.1002/fuce.201000071 93. Kim, J. S.; Reneker Darrell, H. Polybenzimidazole nanofiber produced by electrospinning. Polym. Eng. Sci. 2004, 39 (5), 849-854, DOI: 10.1002/pen.11473 94. Li, H.-Y.; Liu, Y.-L. Polyelectrolyte composite membranes of polybenzimidazole and crosslinked polybenzimidazole-polybenzoxazine electrospun nanofibers for proton exchange membrane fuel cells. J. Mater. Chem. A 2013, 1 (4), 1171-1178, DOI: 10.1039/C2TA00270A 95. Jahangiri, S.; Aravi, İ.; Işıkel Şanlı, L.; Menceloğlu, Y. Z.; Özden-Yenigün, E. Fabrication and optimization of proton conductive polybenzimidazole electrospun nanofiber membranes. Polym. Adv. Technol. 2018, 29 (1), 594-602, DOI: 10.1002/pat.4169 96. Bazbouz Mohamed, B.; Stylios George, K. The tensile properties of electrospun nylon 6 single nanofibers. J. Polym. Sci., Part B: Polym. Phys. 2010, 48 (15), 1719-1731, DOI: 10.1002/polb.21993 30 ACS Paragon Plus Environment

Page 31 of 51 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


97. Yao, J.; Bastiaansen, W. C.; Peijs, T. High Strength and High Modulus Electrospun Nanofibers. Fibers 2014, 2 (2), DOI: 10.3390/fib2020158 98. Prakash, S.; Rajesh, A. M.; Shahi, V. K. Chlorine-tolerant poly electrolyte membrane for electrochemical dye degradation. Chem.Eng.J. 2011, 168 (1), 108-114, DOI: 10.1016/j.cej.2010.12.047 99. Liao, J. H.; Li, Q. F.; Rudbeck, H. C.; Jensen, J. O.; Chromik, A.; Bjerrum, N. J.; Kerres, J.; Xing, W. Oxidative Degradation of Polybenzimidazole Membranes as Electrolytes for High Temperature Proton Exchange Membrane Fuel Cells. Fuel Cells 2011, 11 (6), 745-755, DOI: 10.1002/fuce.201000146 100. Kim, T.-H.; Kim, S.-K.; Lim, T.-W.; Lee, J.-C. Synthesis and properties of poly(aryl ether benzimidazole) copolymers for high-temperature fuel cell membranes. J. Membr. Sci. 2008, 323 (2), 362-370, DOI: 10.1016/j.memsci.2008.06.040 101. Kim, S.-K.; Kim, T.-H.; Jung, J.-W.; Lee, J.-C. Polybenzimidazole containing benzimidazole side groups for high-temperature fuel cell applications. Polymer 2009, 50 (15), 3495-3502, DOI: 10.1016/j.polymer.2009.06.018 102. Yang, J.; Li, Q.; Cleemann, L. N.; Xu, C.; Jensen, J. O.; Pan, C.; Bjerrum, N. J.; He, R. Synthesis and properties of poly(aryl sulfone benzimidazole) and its copolymers for high temperature membrane electrolytes for fuel cells. J. Mater. Chem. 2012, 22 (22), 11185-11195, DOI: 10.1039/C2JM30217A 103. Li, Q. F.; Rudbeck, H. C.; Chromik, A.; Jensen, J. O.; Pan, C.; Steenberg, T.; Calverley, M.; Bjerrum, N. J.; Kerres, J. Properties, degradation and high temperature fuel cell test of different types of PBI and PBI blend membranes. J. Membr. Sci. 2010, 347 (1), 260-270, DOI: 10.1016/j.memsci.2009.10.032 104. Kumbharkar, S. C.; Islam, M. N.; Potrekar, R. A.; Kharul, U. K. Variation in acid moiety of polybenzimidazoles: Investigation of physico-chemical properties towards their applicability as proton exchange and gas separation membrane materials. Polymer 2009, 50 (6), 1403-1413, DOI: 10.1016/j.polymer.2009.01.043 105. Quartarone, E.; Magistris, A.; Mustarelli, P.; Grandi, S.; Carollo, A.; Zukowska, G. Z.; Garbarczyk, J. E.; Nowinski, J. L.; Gerbaldi, C.; Bodoardo, S. Pyridine-based PBI Composite Membranes for PEMFCs. Fuel Cells 2009, 9 (4), 349-355, DOI: 10.1002/fuce.200800149 106. Weber, J.; Kreuer, K. D.; Maier, J.; Thomas, A. Proton Conductivity Enhancement by Nanostructural Control of Poly(benzimidazole)-Phosphoric Acid Adducts. Adv. Mater. 2008, 20 (13), 2595-2598, DOI: 10.1002/adma.200703159 107. Muthuraja, P.; Prakash, S.; Shanmugam, V. M.; Manisankar, P. Stable nanofibrous poly(aryl sulfone ether benzimidazole) membrane with high conductivity for high temperature PEM fuel cells. Solid State Ionics 2018, 317, 201-209, DOI: 10.1016/j.ssi.2018.01.012 108. Jheng, L.-C.; Hsu, S. L.-C.; Tsai, T.-Y.; Chang, W. J.-Y. A novel asymmetric polybenzimidazole membrane for high temperature proton exchange membrane fuel cells. J. Mater. Chem. A 2014, 2 (12), 4225, DOI: 10.1039/c3ta14631f 109. Wang, S.; Zhao, C.; Ma, W.; Zhang, N.; Zhang, Y.; Zhang, G.; Liu, Z.; Na, H. Silane-cross-linked polybenzimidazole with improved conductivity for high temperature proton exchange membrane fuel cells. J. Mater. Chem. A 2013, 1 (3), 621-629, DOI: 10.1039/C2TA00216G 110. Han, M.; Zhang, G.; Liu, Z.; Wang, S.; Li, M.; Zhu, J.; Li, H.; Zhang, Y.; Lew, C. M.; Na, H. Crosslinked polybenzimidazole with enhanced stability for high temperature proton exchange membrane fuel cells. J. Mater. Chem. 2011, 21 (7), 2187-2193, DOI: 10.1039/C0JM02443K 111. Chu, F.; Lin, B.; Qiu, B.; Si, Z.; Qiu, L.; Gu, Z.; Ding, J.; Yan, F.; Lu, J. Polybenzimidazole/zwitterion-coated silica nanoparticle hybrid proton conducting membranes for anhydrous proton exchange membrane application. J. Mater. Chem. 2012, 22 (35), 18411-18417, DOI: 10.1039/C2JM32787B



Page 32 of 51

112. Singha, S.; Jana, T. Structure and Properties of Polybenzimidazole/Silica Nanocomposite Electrolyte Membrane: Influence of Organic/Inorganic Interface. ACS Appl. Mater. Interfaces 2014, 6 (23), 21286-21296, DOI: 10.1021/am506260j 113. Rewar, A. S.; Chaudhari, H. D.; Illathvalappil, R.; Sreekumar, K.; Kharul, U. K. New approach of blending polymeric ionic liquid with polybenzimidazole (PBI) for enhancing physical and electrochemical properties. J. Mater. Chem. A 2014, 2 (35), 14449-14458, DOI: 10.1039/C4TA02184C 114. Subianto, S.; Cavaliere, S.; Jones, D. J.; Rozière, J. Effect of side-chain length on the electrospinning of perfluorosulfonic acid ionomers. J. Polym. Sci., Part A: Polym. Chem. 2013, 51 (1), 118-128, DOI: 10.1002/pola.26286 115. Choi, S. W.; Fu, Y. Z.; Ahn, Y. R.; Jo, S. M.; Manthiram, A. Nafion-impregnated electrospun polyvinylidene fluoride composite membranes for direct methanol fuel cells. J. Power Sources 2008, 180 (1), 167-171, DOI: 10.1016/j.jpowsour.2008.02.042 116. Li, H.-Y.; Lee, Y.-Y.; Lai, J.-Y.; Liu, Y.-L. Composite membranes of Nafion and poly(styrene sulfonic acid)-grafted poly(vinylidene fluoride) electrospun nanofiber mats for fuel cells. J. Membr. Sci. 2014, 466, 238-245, DOI: 10.1016/j.memsci.2014.04.057 117. Chen, H.; Elabd, Y. A., Polymerized Ionic Liquids: Solution Properties and Electrospinning. Macromolecules 2009, 42 (9), 3368-3373, DOI: 10.1021/ma802347t 118. Yu, D. M.; Yoon, S.; Kim, T.-H.; Lee, J. Y.; Lee, J.; Hong, Y. T. Properties of sulfonated poly(arylene ether sulfone)/electrospun nonwoven polyacrylonitrile composite membrane for proton exchange membrane fuel cells. J. Membr.Sci. 2013, 446, 212-219, DOI: 10.1016/j.memsci.2013.06.028 119. Lim, J.-M.; Won, J.-H.; Lee, H.-J.; Hong, Y. T.; Lee, M.-S.; Ko, C. H.; Lee, S.-Y. Polyimide nonwoven fabric-reinforced, flexible phosphosilicate glass composite membranes for hightemperature/low-humidity proton exchange membrane fuel cells. J. Mater. Chem. 2012, 22 (35), 18550-18557, DOI: 10.1039/C2JM33406B 120. Lee, H.-J.; Kim, J.-H.; Won, J.-H.; Lim, J.-M.; Hong, Y. T.; Lee, S.-Y. Highly Flexible, ProtonConductive Silicate Glass Electrolytes for Medium-Temperature/Low-Humidity Proton Exchange Membrane Fuel Cells. ACS Appl. Mater. Interfaces 2013, 5 (11), 5034-5043, DOI: 10.1021/am400836h 121. Choi, J.; Lee, K. M.; Wycisk, R.; Pintauro, P. N.; Mather, P. T. Nanofiber Network IonExchange Membranes. Macromolecules 2008, 41 (13), 4569-4572, DOI: 10.1021/ma800551w 122. Won, J.-H.; Lee, H.-J.; Lim, J.-M.; Kim, J.-H.; Hong, Y. T.; Lee, S.-Y. Anomalous behavior of proton transport and dimensional stability of sulfonated poly(arylene ether sulfone) nonwoven/silicate composite proton exchange membrane with dual phase co-continuous morphology. J. Membr.Sci. 2014, 450, 235-241, DOI: 10.1016/j.memsci.2013.09.019 123. Subramanian, C.; Weiss, R. A.; Shaw, M. T. Fabrication and Characterization of Conductive Nanofiber-Based Composite Membranes. Ind. Eng. Chem. Res. 2013, 52 (43), 15088-15093, DOI: 10.1021/ie402072e 124. Gong, C.; Liu, H.; Zhang, B.; Wang, G.; Cheng, F.; Zheng, G.; Wen, S.; Xue, Z.; Xie, X., High level of solid superacid coated poly(vinylidene fluoride) electrospun nanofiber composite polymer electrolyte membranes. J. Membr.Sci. 2017, 535, 113-121, DOI: 10.1016/j.memsci.2017.04.037 125. Dos Santos, L.; Rose, S.; Sel, O.; Maréchal, M.; Perrot, H.; Laberty-Robert, C. Electrospinning a versatile tool for designing hybrid proton conductive membrane. J. Membr.Sci. 2016, 513, 12-19, DOI: 10.1016/j.memsci.2016.04.002 126. Hickner, M. A.; Ghassemi, H.; Kim, Y. S.; Einsla, B. R.; McGrath, J. E. Alternative Polymer Systems for Proton Exchange Membranes (PEMs). Chemical Reviews 2004, 104 (10), 4587-4612, DOI: 10.1021/cr020711a


Page 33 of 51 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Biographies Biography

Parashuram Kallem is currently a postdoctoral researcher at Gwangju Institute of Science and Technology (GIST), Republic of Korea where he is working on surface modification of electrospun nanofiber & hollow fiber based membranes for osmotic power generation applications. He obtained his Ph.D. in the framework of Erasmus Mundus Doctorate in Membrane Engineering program in 2017 from University Zaragoza, Spain and University of Montpellier, France. He finished his B.Sc. at the Osmania University, Hyderabad, India in 2006 and then obtained his M.Sc. in Chemistry from Banaras Hindu University, Varanasi, India in 2008. In 2008, he moved to CSIR-CSMCRI (Central Salt and Marine Chemicals Research Institute) Bhavnagar, India as a project research scholar. He is interested in



Page 34 of 51

development of novel materials for the applications of energy storage and conversion, water purification, and power generation. Biography

Numan Yanar is a graduate student and a research assistant at Gwangju Institute of Science and Technology (GIST), Republic of Korea, and an affiliated researcher of Korea Research Institute of Chemical Technology (KRICT). He obtained his Bachelor’s Degree in the Department of Civil Engineering at Middle East Technical University, Turkey and Master’s Degree in the School of Earth Science and Environmental Engineering at GIST. He focuses on the development of high performance desalination membranes, sustainable energy harvesting and the application of 3D printing technologies to enhance the performances of membranes for water-energy nexus.


Page 35 of 51 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Biography

Heechul Choi is a Professor in the School of Earth Science and Environmental Engineering at Gwangju Institute of Science and Technology, Republic of Korea. Currently, he is also the President of Korean Society of Environmental Engineers (KSEE), an Adjunct Professor at Korea Research Institute of Chemical Technology (KRICT) and a fellow of the Korean Academy of Science and Technology (KAST). His research group focuses on the novel strategies for nanofiber and electrospinning technologies, the development of energy harvesting membranes for a green environment and 3D printing technologies for environmental applications. Prof. Choi obtained his Ph.D. Degree in the Department of Civil Engineering at Texas A&M University, USA. Until now, he has over a hundred publications in well-known international journals.



Page 36 of 51

Figures:

Figure 1: Schematic representation of a single fuel cell (PEMFC).

Figure 2: Research articles published in the period of 2007–2017 related the following topics: (A) Electrospun nanofiber based membranes for fuel cells (Scopus data: keywords: electrospinning + fuel cell, source: https://www.scopus.com), (B) Polymeric electrospun 36 ACS Paragon Plus Environment

Page 37 of 51 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


based PEMS membranes for PEMFc (Graphs are drawn by the author by taking Scopus data: keywords: electrospinning + fuel cell filtered with sub-keywords /proton exchange membrane/ polymer electrolyte, source: https://www.scopus.com).

Figure 3: Two types of PEMs based on electrospun nanofibers: (A) proton-conducting polymer support comprising an inert electrospun nanofiber mat, (B) inert polymer support surrounding by an interconnected proton conducting electrospun nanofibers.



Page 38 of 51

Figure 4: (A) and (B) Schematic illustration of non-woven (random) and aligned electrospun nanofiber arrangements respectively. (C) Schematic proton conductive process of conducting aligned electrospun nanofiber.

Figure 5: Two types of conductivity testing directions.


Page 39 of 51 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6: Schematic illustration of the preparation of aligned electrospun nanofibers. (Reprinted from ref

40

with

permission from the American Chemical Society.

Copyright 2010 American Chemical Society.)



Page 40 of 51

Figure 7: (a) NTDA-BDSA-r-APPF chemical structure. (b) Schematic illustration of sulfonated copolyimide nanofiber. (c, d) TEM photographs of aligned nanofiber’s crosssections in the radial direction and in the axial direction respectively. (e) SEM photograph of aligned electrospun nanofiber fabricated on a special designed collector. (Reprinted from ref 40

with permission from the American Chemical Society.

Copyright 2010 American

Chemical Society.)


Page 41 of 51 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8: (a) Chemical structure of 6FDA-BDSA-r-APPF, (b, c) SPI nanofibers prepared from different electrospinning conditions (V2 = 0.5, 1.0, and 3.0 kV), the SPI cast membrane, and the Nafion membrane: (b) temperature dependence (at 95% RH) proton conductivity, and (c) relative humidity dependence (at 90 °C) proton conductivity. (Reprinted from ref

46

with



permission from the Royal Society of Chemistry.

Page 42 of 51

Copyright 2014 Royal Society of

Chemistry.)

Figure 9: SEM photographs of SPEEK nanofibers fabricated at various DSs and structures with consequent diameter distribution: randomly collected nanofibers at (a) DS = 74% and 42 ACS Paragon Plus Environment

Page 43 of 51 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(b) DS = 81%, aligned nanofibers at (c) DS = 74% and (d) DS = 81%. Electrospun nanofiber were fabricated from concentration of 22 wt%, distance of 15 cm, flow rate of 0.1 mL/h, and applied voltage of 15 kV. (Reprinted from ref

44

with permission from the Wiley Online

Library. Copyright 2014 Wiley Online Library.)

Figure 10: (A) Morphologies of both the aligned and thickness aligned electrospun nanofiber based PEMs: a) Schematic illustration of the prepared thickness aligned nanofiber based PEM, b) aligned electrospun nanofiber’s SEM image, c) optical image of the thickness aligned electrospun PEM (left) and the cast PEM (right). SEM photographs of the thickness aligned electrospun composite PEM: d) smooth treated surface, e) without any smooth treated surface, f) cross-section. (B) Tensile strength of the hydrated SPPESK series membranes as a 43 ACS Paragon Plus Environment


Page 44 of 51

function of strain. (Reprinted from ref 41 with permission from Elsevier B.V. Copyright 2017 Elsevier B.V.)

Figure 11: Conductivity window of Nafion and PBI membranes at 80 °C–130 °C. (Reprinted from ref

90

with permission from the Wiley Online Library. Copyright 2004 Wiley Online

Library.)


Page 45 of 51 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 12: (A) Stress‐strain curves of undoped and phosphoric acid–doped at various hours electrospun PEMs; (B) the application of nanofiber mats for mechanical testing are illustrated as an inset. (Reprinted from ref 95 with permission from the Wiley Online Library. Copyright 2018 Wiley Online Library.)



Page 46 of 51

Figure 13: (A) Synthetic route of poly(aryl sulfone ether benzimidazole) (SO2-OPBI) and membrane fabrication. (B) Several HTPEMs oxidative stabilities comparison. Oxidative stability of dense SO2-OPBI, nanofiber SO2-OPBI, and synthetic m-PBI and others (Asym PBI-150

108,

OPBI/silica

SCPBI 112,

109,

Co–80%SO2–PBI

and PBI-TMBP20%

110.

102,

PDADMA-TFMS/PBI

(Reprinted from ref

107

113,

PBI/ZC-SiO2

111,

with permission from the

Elsevier B.V. Copyright 2018 Elsevier B.V.)


Page 47 of 51 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Tables: Table 1. Overview of PFSA based PEMs comprising electrospun nanofibers and polymer matrix. Electrospun

Solvent

material Nafion®

IPA/water

Carrier

Details

polymer/additive

conductivity

s

(mS/cm)

5.0–28.0

wt%

PVA Mw: 2 × Nafion®

IPA/water

Fumion®

DMAc

Aquivion®

DMAC/wat er

Proton

105

Defect free fibers achieved at Fine nanofibers achieved at

105)

16.7 wt% PEO wt%

PEO

Short

chain

DMAC/

fluoride

Acetone

Polyvinylidene

DMF

38

3.5–5.9

38

114

of

PFSA

58 (80 °C; 95% RH)

exhibit

greater

54 (120 °C; 95%

Mw: 6000 to 1 ×

ionomers

106 Da

crystallinity

RH)

Fibers with 300 nm diameter

97 (80 °C; 95% RH)

obtained

66 (120 °C; 95%

1 wt% PEO Mw: 6000 to 1 ×

106

Da Polyvinylidene

8.7–16

28.6 wt% PVA

PEO Mw: 1.1 × 1.5

Ref.

114

RH)

Nafion®

Electrospun PVDF mat pores

2.25 (65 °C)

115

82 (95 °C; 95% RH)

26

106 (95 °C; 95%

116

impregnated with Nafion® Nafion®

PVDF

nonwoven

functionalization with Nafion

fluoride

chains

through

a

3-step

reaction path then doped in Nafion® Polyvinylidene

DMF

Nafion®

PVDF

fluoride

nonwoven

functionalization with Nafion chains, grafting

then with

followed

RH)

by

poly(styrene

sulfonic acid) then doped in Nafion® Sulfonated poly(ether

DMF

Nafion®

ether

ketone) and silica

Composite PEM of blend

77 (90 °C; 100%

nanofiber mat impregnated

RH)

54

with Nafion

Polymerized Ionic

MeCN/DM

Nafion/

liquid

F

poly(acrylic acid) Mw: 4.5 1 × Da

Polymerized 105

ionic

poly(MEBIm-BF4)

liquid, was

7.1 × 10-4 (30 °C;

117

10% RH)

mixed with Nafion/PAA for spinning



Page 48 of 51

Table 2. Overview of non-PFSA-based PEMs comprising electrospun nanofibers and polymer matrix. Electrospun

Solvent

Carrier polymer

Details

Proton conductivity

material

Ref.

(mS/cm)

Polyacrylonitrile

DMSO

Sulfonated poly(arylene

Methyl methacrylate and

164 (80 °C; 100%

ether sulfone)

acrylonitrile

monomers

RH)

synthesized

then

118

stabilized in an oxidizer at 240 °C Polybenzimidazol

DMAc

Polybenzoxazine

PBI nanofibers thermally

e

170 (160 °C)

94

In situ technique used to

25 (30 °C; 60% RH)

119-120

synthesize Silicate glass

for phosphoric acid

electrolyte

166 (80 °C; 80% RH)

crosslinked via the ringopening addition reaction of the benzoxazine groups

Polyimide

-

Sulfonic acid/phosphoric functionalized

acid– silicate

glass electrolyte Sulfonated

DMAc

(NOA) 63

for sulfonic acid Electrospun mats imbibed

86 (70% fiber volume

poly(arylene

with

fraction, 25 °C, 100%

ether sulfone)

Adhesive

Norland 63

Optical adhesive

121

RH)

then UV crosslinking Sulfonated

DMAc

Silicate

PEM fabricated via in-situ

poly(arylene

sol–gel

synthesis

ether sulfone)

silicate directly inside the

60 (30 °C; 100% RH)

122

100 (25 °C; 98% RH)

123

36.6 (80 °C; 100%

41

of

electrospun mat Sulfonated

-

polystyrene-PEO Sulfonated

poly

DMF

(phthalazinone ether

Vinyl

terminated

Crosslinking of nanofiber

poly(dimethylsiloxane)

mat with matrix

SPPESK/glycol/H2O

PEMs

solution

ordered/aligned through-

sulfone

with

RH)

plane nanofibers

ketone) Sulfonated poly(ether

DMAc

-

ether

Nanofibers

at

various

degrees of sulfonation in

ketone)

90.7 (25 °C; 100%

44

RH)

the forms of random and aligned nanofibers

Phosphotungstic acid

coated

DMAc

Crosslinked chitosan

High

level

uniformly

acid

was

coated

on

poly(vinylidene

nanofibers using a facile

fluoride)

polydopamine

23 (RT, 100% RH)

124

assisted


Page 49 of 51 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


coating Poly(vinylidene

DMF

2-

Ratio

of

2-

fluoride-

4(Chlorosulfonylphenyl

4(chlorosulfonylphenyl)et

hexafluoropropyl

)ethyltrichlorosilane)/

hyltrichlorosilane)/tetraet

ene)/SO3H silica

tetraethyl orthosilicate)

hyl orthosilicate) for the

130 (80 °C; 100%

125

RH)

studied conductivity Poly(trimethylhex

DMF

Poly(diallyldimethylam

Spray coating applied for

amethylenetereph

moniumchloride)-

LbL deposition of poly

thalamide)

sulfonated

electrolytes on the nano

poly(phenyleneoxide);

fiber surface

7 (RT; 100 RH)

126

Table 3. Overview PEMs comprising aligned electrospun nanofibers and polymer matrix. Electrospun material

Solvent

Carrier

Details

Proton conductivity

polymer

1,4,5,8-Naphthalene

DMF

tetracarboxylic dianhydride (NTDA)–4,4’-diamino-

Ref.

(mS/cm)

Sulfonated

The stability of the

880

polyimide

composite membranes,

(80 °C)

solution

such as oxidative and

biphenyl 2,2’-disulfonic acid)

hydrolytic

(BDSA)–2,2-bis(4-(4-

was

aminophenoxy)phenyl (r-

improved

40

stabilities, significantly

APPF)–hexafluoropropane 2,2’-Bis(3,4-

DMF

dicarboxyphenyl)hexafluoro propane dianhydride)

Sulfonated

Ultrathin and uniform

polyimide

non beaded structure.

solution

42

(90 °C; 98% RH)

proton

conductivity

(6FDA)–BDSA–r APPF Sulfonated poly(ether ether

in-plane

300

DMAc

-

ketone)

Randomly and aligned

90.7

nanofibers at different

(25 °C; 100% RH)

44

degrees of sulfonation 6FDA-BDSA-r-APPF

DMF

nanofibers

APPF nanofibers (with

82

diameters of ~100 nm)

(at 90 °C, 30% RH)

46

using parallel collector electrodes Sulfonated

DMF

SPPESK/glyc

PEMs

with

poly(phthalazinone ether

ol/H2O

through-plane

sulfone ketone)

solution

nanofibers

aligned

36.6

41

(80 °C; 100% RH)



Page 50 of 51

Table 4. Overview of conductivity values of PEMs with and without nanofibers in parallel (In-plane) and perpendicular (Through-plane) direction ( Table is prepared by using the data from the article of Tamura et al. 42 ) Proton conductivity (S cm-1) at 90 °C, 98%RH PEM

Parallel (In-plane)

Perpendicular (Through-plane)

PEM without nanofibers

0.0836

0.0823

Composite PEM with nanofibers ( Fibre diameter:

0.212

0.0805

0.154

0.0808

0.122

0.0837

77 nm) Composite PEM with nanofibers ( Fiber diameter: 102 nm) Composite PEM with nanofibers ( Fiber diameter: 157 nm)


Page 51 ACS of Sustainable 51 Chemistry & Engineering 1 2 3 4 5 6 7

ACS Paragon Plus Environment

Nanofiber-based Proton Exchange Membranes: Development of

Recommend Documents