Immobilized phosphotungstic acid for the construction of proton exchange nanocomposite membranes with excellent stability and fuel cell performance

https://doi.org/10.1016/j.ijhydene.2020.04.159Get rights and content

Highlights

  • Mesoporous g-C3N4 (mg-C3N4) nanosheets were one-step synthesized.

  • Phosphotungstic acid (HPW) was immobilized onto mg-C3N4 nanosheets surface.

  • SPAES/HPW/mg-C3N4 membranes were uniform, ductile and physicochemical stable.

  • SPAES/HPW/mg–C3N4–1.5 reached power output of 582 mW/cm2 at 80 °C.

  • The HPW leakage was largely suppressed by immobilization.

Abstract

Phosphotungstic acid (HPW) has a good potential as nanofillers in nanocomposite proton exchange membrane with the prerequisite of solving the leakage issue. It is immobilized onto mesoporous graphitic carbon nitride (mg-C3N4) nanosheets surface, and then incorporated into sulfonated poly (aryl ether sulfone) (SPAES) membrane. Structures of the HPW/mg-C3N4 nanocomposites and corresponding SPAES/HPW/mg-C3N4 membranes are characterized by spectroscopic techniques. Fundamental properties and fuel cell performance of the fabricated nanocomposite membranes, and the leakage of HPW are investigated. Along with the highly suppressed HPW leakage, the SPAES/HPW/mg-C3N4 membranes show improved dimensional stability, water affinity and physicochemical stability, as well as better proton conductivity and fuel cell performance. At 80 °C and 60–100% RH, the SPAES/HPW/mg–C3N4–1.5 membrane exhibits 2–3.6 times peak power densities (354.9–584.2 mW/cm2) of the pristine SPAES membrane, and proton conductivity of 203 mS/cm, dimensional change less than 7.5% and weight loss of 1.4% in Fenton oxidation test at 80 °C.

Introduction

Proton exchange membrane fuel cells (PEMFCs) have been extensively studied due to the overwhelming characteristics, such as high power density, low environmental impact and diverse applications [1,2]. As a core part of a PEMFC, proton exchange membranes (PEM) has significant influence on cell performance and durability [2]. So far, the most popular researched PEMs are classified into two types: one type is the perfluorosulfonic acid-based membranes like Nafion, which was used under limited conditions due to the poor thermal resistance, high humidity-dependent proton conductivity and serious fuel/gas permeability; the other type is the sulfonated hydrocarbon polymer-based membranes, such as sulfonated poly (aryl ether sulfone) (SPAES), sulfonated poly (aryl ether ketone) (SPAEK) and so on [3], which have been considered as quite promising candidates for their good thermal and mechanical properties, as well as fine film-forming capability and low cost [4]. However, critical property of a PEM, proton conductivity, highly depends on the ion exchange capacity (IEC). In most cases, high IEC always accompanies excessive water absorbing-swelling and poor membrane stability for the sulfonated hydrocarbon polymer-based PEMs [5,6].

Incorporating hydrophilic inorganic nanoparticles has been generally accepted as an effective approach to improving the proton conductivity and stability of a PEM at the same time [7,8]. Zero-dimension (0D) nanomaterials (e.g., titanium oxide [9], zirconium oxide [10], cerium oxide nanoparticles [11]), one-dimension (1D) nanomaterials (e.g., single-wall/multi-wall carbon nanotubes [12,13], titanium dioxide nanotubes [14]) and two-dimension (2D) nanomaterials (e.g., graphene oxide [15,16] and graphitic carbon nitride (g-C3N4) nanosheets [17,18]) have been used as nanofillers. Scholars have reported encouraging membrane performance enhancements, especially in the aspects of mechanical strength and fuel/gas barrier property [9,17,19,20]. Comparing to 0D and 1D nanomaterials, 2D nanomaterials are considered as the most ideal materials owing to their larger specific surface area and higher aspect ratio [18].

Among the 2D nanomaterials, g-C3N4 nanosheets is a layer stacked conjugated polymer consisting of s-triazine repeating units, it has the advantages of low cost, simple synthesis, good thermal and physicochemical stability, and “earth-plentiful” nature [21,22]. Rich amino groups of g-C3N4 nanosheet make it easy to be modified [23]. Jiang and co-workers [17] used g-C3N4 nanosheets as nanofillers to fabricate SPEEK nanocomposite membranes for direct methanol fuel cells, the prepared SPEEK/g-C3N4 nanocomposite membranes showed a 68% increase in mechanical strength and a 39% increase in maximum power density. They contributed these enhancements to the acid-base interaction between g-C3N4 nanosheets and SPEEK. Incorporating oxidized g-C3N4 nanosheets into SPEEK membranes was also useful for applications of vanadium radium redox flow battery [24]. However, direct incorporation of hydrophobic g-C3N4 nanosheets into the hydrophilic PEMs always faces the incompatibility problem. Hydrophilic functionalization of the g-C3N4 nanosheets will be favorable to enhance the compatibility with PEM materials and to improve the PEM performance [25,26].

Phosphotungstic acid (HPW) has served as nano filler for PEM modifications [19,27]. Encouraging results have been reported because of two features of HPW: one feature is the hydronium ion forms (H3O+, H5O2+ and H9O4+) of protons in HPW, which allows it to act as a conductor for proton transport; the other feature is its special Keggin-type structure, which builds available channels for proton transport [28,29]. However, a nonnegligible drawback of HPW is that it is highly soluble in water [30], appropriate measures should be taken to prevent it from leaking from the membrane matrix. For example, HPW can be effectively immobilized on amino groups by hydrogen bonding [31]. For this reason, immobilizing HPW onto g-C3N4 nanosheet surfaces not only can combine the advantages of HPW and 2D g-C3N4 nanosheets to improve the membrane performance, but also can effectively inhibit the leakage of HPW. Meng and co-workers have successfully anchored HPW onto g-C3N4 nanosheets and prepared SPEEK nanocomposite membranes [19]. The obtained SPEEK/HPW/g–C3N4–1.0 membrane exhibited excellent mechanical strength (tensile strength of 51.1 MPa, elongation at break of 223.3%), and maximum powder density of 65 mW/cm2 in a direct methanol fuel cell at 60 °C. However, the excessive membrane swelling above 60 °C needed to be resolved for applications at elevated temperatures.

Compared with g-C3N4 nanosheets, the mesoporous g-C3N4 (mg-C3N4) nanosheets own larger specific surface area and richer amino/imino groups on the surface and could provide more sites for HPW immobilization. Hence, we attempted to immobilize HPW onto its surface and then fabricate nanocomposite membranes with SPAES. In the SPAES/HPW/mg-C3N4 nanocomposite system, the mg-C3N4 nanosheets interacted with the SPAES copolymers through an acid-base interaction to form an inorganic-organic system, while the HPW immobilized on mg-C3N4 nanosheets provides extra hopping sites for proton transport. We anticipated that the double compositing system could enhance the interactions between inorganic nanofillers and SPAES copolymers, and relieve the HPW leakage at the same time.

Section snippets

Materials

4,4′-Difluorodiphenyl sulfone (DFDPS) and 4-4′-biphenol (BP) were recrystallized by toluene and acetone, respectively. N, N′-dimethylacetamide (DMAc) and toluene were distilled prior to use. Fuming sulfuric acid (SO3, 50%), melamine, ammonium chloride, potassium carbonate, sodium hydroxide, and dimethyl sulfoxide (DMSO) were used directly. Sulfonated poly (aryl ether sulfone) copolymers (SPAES) were lab-synthesized (Fig. S1).

Mesoporous g-C3N4 (mg-C3N4) nanosheets preparation

Mesoporous g-C3N4 nanosheets were synthesized according to a published

Characterizations of mg-C3N4 nanosheets and mg-C3N4/HPW nanocomposites

The preparation procedure of the mg-C3N4 nanosheets and mg-C3N4/HPW nanocomposites was displayed in Fig. 1. The usual preparation procedure for g-C3N4 nanosheets is like that for graphene by the aid of chemical exfoliation in concentrated acids. Here, we utilized a simple one-step method that ammonium chloride was used as a blowing agent during melamine polymerization, which decomposed into hydrogen chloride and ammonia gases. Except for the help of simultaneous exfoliation, it also assisted

Conclusion

A series of SPAES/HPW/mg-C3N4 nanocomposite membranes were fabricated and applied in a H2/O2 fuel cell. Mesoporous graphitic carbon nitride (mg-C3N4) nanosheets were synthesized by a one-step calcination bubble template method, then phosphotungstic acid (HPW) was immobilized onto the surface of mg-C3N4 nanosheets. The gained HPW/mg-C3N4 nanocomposites (HPW: 10% of SPAES, mg-C3N4 nanosheets: 0–2.0% of SPAES) were simultaneously incorporated into the SPAES matrix to fabricate nanocomposite

Acknowledgements

We gratefully thank the financial support from National Natural Science Foundation of China (No. 51708295 & 21276128).

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