Influence of alkaline 2D carbon nitride nanosheets as fillers for anchoring HPW and improving conductivity of SPEEK nanocomposite membranes

https://doi.org/10.1016/j.ijhydene.2017.02.050Get rights and content

Highlights

  • The g-C3N4 nanosheets behave like double-sided adhesive to form hydrogen bonds with HPW and acid–base pairs with SPEEK.

  • PEM maintains stable working performance, because HPW is effectively anchored.

  • PEM has a good methanol-blocking property due to having torturous methanol diffusion channel.

Abstract

In the present study, novel composite membranes were prepared based on sulfonated poly (ether ether ketone)/phosphotungstic acid/carbon nitride nanosheets (SPEEK/HPW/g-C3N4). The alkaline ultrathin g-C3N4 nanosheets in the membranes behaved like “double-sided adhesive”, forming hydrogen bonds with the HPW molecules to anchor hydrophilic HPW without leaking. Moreover, the amine groups of nanosheets formed acid–base pairs with –SO3H of the SPEEK polymer matrix, facilitating the Grotthuss-type transport of proton to improve conductivity. The g-C3N4 inorganic particles provided tortuous pathways for methanol transport to suppress the methanol permeability coefficient. The selectivity of the SPEEK/HPW/g-C3N4-1.0 was 2.3 times higher than that of SPEEK/HPW and 1.5 times higher than that of pristine SPEEK. SPEEK/HPW/g-C3N4 hybrid membranes exhibited stable and durable operation for 240 h under 100% RH at 60 °C. Moreover, membranes exhibited superior mechanical property, with maximum elongation at break of 223.3%.

Introduction

Direct methanol fuel cell (DMFC) is one of the most ideal energy sources in the 21st century because of its renewability, rapid start-up, low environmental pollution, and high energy density compared with fossil energy [1], [2], [3]. The research regarding DMFC has attracted increasing attention worldwide [4], [5], [6], [7]. As the “heart” of DMFC, the property of proton exchange membrane (PEM) directly determines the performance of fuel cell [8]. Desirable PEM should exhibit high proton conductivity, low methanol permeability, excellent mechanical properties and thermodynamic stability. However, the Nafion series membranes (DuPont), which are the most established and primary type of commercialized membrane materials, could hardly sustain satisfactory performance when applied in DMFC. These membranes possess high fuel permeability, and their mechanical strength drops sharply with increasing temperature. Moreover, the high cost of Nafion prohibits its widespread use. Numerous studies have attempted to develop new high-performance and low-cost membrane materials as substitutes to Nafion [9], [10], [11], [12], [13], [14]. Among these materials, sulfonated poly (ether ether ketone) (SPEEK), a low-cost polymer material with good mechanical stability and low methanol permeability, has exhibited the most promising potential as a material for PEM. However, SPEEK has narrower and less separated water-filled proton-conducting channels and more dead-end branches compared with Nafion [15]. These characteristics limit the proton conductivity of SPEEK affecting the overall fuel cell performance. Many inorganic proton conducting materials have been introduced into the SPEEK matrix to improve its performance [16], [17], [18].

Heteropoly acids, such as phosphotungstic acid (HPW), are regarded as efficient proton conductors because of their strong acidity and unique structure [19]. HPW has been widely used to enhance proton conductivities of Nafion [20] and aromatic polymers [21], [22]. Aílton S. Gomes et al. prepared HPW-modified-SPSF composite membrane. The conductivity of membrane significantly increased from 13.2 mS/cm to 39.5 mS/cm at 30 °C when HPW was incorporated [23]. Zhe Wang et al. found that proton conductivity of composite membrane increased with increasing content of HPW. The proton conductivity of SPAEKS/PVDF–HPW composite membrane increased from 62 mS/cm to 98 mS/cm as the content of HPW increased from 10% to 50% at 80 °C [24]. Studies in the previous decade have shown that PEMs with HPW have decay stability because of the leakage of hydrophilic HPW in water. Meanwhile, the methanol permeability coefficients of PEMs increase after incorporation of HPW [25], [26]. Thus, a technique that cannot only trap HPW within the membrane but also suppress methanol permeability should be established.

Two-dimensional (2D) inorganic nanomaterials have excellent chemical and physical properties [27]. Nanosheet surfaces functionalized using different groups (e.g., acid, imidazole, and zwitterionic groups) exhibit high proton conductivities and uniform dispersion in the matrix [28], [29], [30], [31]. Moreover, inorganic nanosheets can suppress methanol permeability. Thus, PEMs incorporated with inorganic nanosheets show good performance in fuel cell [32]. Traditional experiments have proved that amino groups can effectively anchor HPW within membranes through hydrogen bond [33] and improve conductivity by providing more proton sites and interaction of acid–base pairs [34], [35].

In this study, 2D ultrathin graphitic carbon nitride (g-C3N4) nanosheets (Fig. 1) with –NH2 and –NH basic functional groups were prepared and dispersed firmly into the SPEEK/HPW acid system. The alkaline g-C3N4 nanosheets behaved like “double-sided adhesive” to form hydrogen bonds with HPW molecules and acid–base pairs with the SPEEK matrix. We investigated the water uptake (WU), dimensional stability, proton conductivity, and methanol permeability of composite membranes to evaluate their potential for practical applications in DMFCs.

Section snippets

Materials and reagents

Poly (ether ether ketone) (VICTREX® PEEK 450PF) was purchased from Victrex Company and used after drying for 12 h at 100 °C. HPW was provided by Sinopharm Chemical Reagent Co., Ltd. Melamine was obtained from Tianjin Guangfu Chemical Research Institute. Dimethyl sulfoxide (DMSO) and H2SO4 (95–98 wt%) were purchased from Beijing Chemical Reagent Factory. All solvents were of reagent grade and used without any treatment.

Synthesis of SPEEK

Typically, 20 g of PEEK powder was added into 250 ml of concentrated H2SO4,

Characterization of ultrathin g-C3N4 nanosheets

The TEM image of g-C3N4 in Fig. 2a is nearly transparent, indicating the successful fabrication of ultrathin morphology of g-C3N4 nanosheets. The selected area electron diffraction displays regular hexagon diffraction spot, suggesting that the g-C3N4 nanosheets are crystalline material.

The XRD pattern (Fig. 2b) of g-C3N4 matches the standard spectrum of g-C3N4 (JCPDS 87-1526). The absorption peaks at 27.65° and 12.93° correspond to the (002) crystal surface with d-spacing of 0.357 nm and (100)

Conclusions

We propose an effective method that can simultaneously anchor HPW, promote proton conductivity, reduce methanol permeability, and improve mechanical performance. The results of UV–vis spectroscopy and conductivity stability test indicate that HPW was successfully anchored in SPEEK/HPW/g-C3N4 composite membrane. The DMFC equipped with the SPEEK/HPW/g-C3N4-1.0 membrane yielded a higher power density than that offered by the DMFCs with pristine SPEEK and SPEEK/HPW membranes under the same

Acknowledgements

We thank the North China Electric Power University for the support in the methanol permeability measurements. This study was supported financially by the National Key Research and Development Plan (Grant no: 2016YFC0303700) and National Science and Technology Major Project (Research Index: 201505017-002).

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