The demand for energy is increasing with the development of the society. Fuel cells and lithium-ion batteries (LIBs) are typical representatives in the field of new energy sources. The membrane is one of the key components in fuel cell and lithium battery. In the field of fuel cells, Nafion proton exchange membranes (PEMs) has been applied widely which have excellent mechanical properties, good chemical stability and relatively high proton conductivity. However, its application is limited due to its complex synthesis process, high cost, high methanol permeability and easy swelling in high temperature working environment. In the field of LIBs, polyolefin membranes have the advantages of high mechanical strength, good chemical stability and low cost. However, they also have some disadvantages, such as low polarity, poor affinity with electrolytes and poor thermal stability. The renewable biomaterial nanocellulose (NCC) has many advantages, such as the renewability, low density, high specific strength, high modulus and high aspect ratio, and also can be modified through grafting specific groups on it. In this work, NCC and high performance polymers poly (aryl ether ketone) (PAEK) with excellent mechanical properties and good chemical stability were applied in PEMs and separators to solve the problems.
Firstly, sulfophenylated poly(ether ether ketone ketone) (SPEEKK) was prepared through the polymerization between phenylhydroquinone and 1,4-bis(4-fluorobenzoyl)benzene with post-sulfonation. NCC was grafted with carboxyl group by the modification of maleic anhydride to obtain MN. The SPEEKK was mixed with different content of MN to obtain electrospinning solution. The PEMs was obtained through the combination of electrospinning technology and solution casting. At 100 °C, MN2 (with 2% MN) showed better water absorption than MN0 (with 0% MN content), which was 28%. And at 90 °C, MN2 also showed a good proton conductivity of 0.09 S cm^-1. Its tensile strength was as high as 48.7 MPa, and Young’s modulus and elongation at break was1.3 GPa and 34%, respectively. This illustrated the combination of electrospinning and solution casting to be a promising choice to improve the proton conductivity of nanocomposite PEMs, which was promoted by the three-dimensional hydrogen bond proton transport network formed by the well oriented MN in the SP-PEEKK resulted by the electrospinning.
The composite solution of PEEK and carbon nanoparticles (C), which was obtained by the carbonization of nanocellulose (NCC) in the sulfuric acid solution, were coated on the PE separator to prepare Janus composite separator (PE@PC). The C nanoparticles had good compatibility with PEEK. PE@P7C composite membrane had higher porosity and loading rate than PE. There was strong interaction between C=O, C-O-C and-SO₃H groups in PEEK and -OH in C nanoparticles, which improved the wettability of electrolyte solution in the membranes. The wettability of PE@P7C2 was the best. Its contact angle was 34.3^o in the test. Compared with PE membranes, the thermal stability and dimensional stability of PE@P7C composite membranes at high temperature were improved obviously. The thermal shrinkage resistance of PE@P7C2 film was improved 43% compared to PE membranes when kept at 150 ^oC for 30 min. Its discharge capacity could be 130 mAh/g when the current density was 2 C. The good wettability between PEEK and electrolyte ensured the stable transmission of Li⁺. C nanoparticles had the function of electron collector like other carbon materials which improved the efficiency of electronic transmission. Therefore, the PE@PC composite membrane can be a promising suitable material for separators in LIB applications.

Nanocellulose; membranes; Fuel cells; Lithium-ion batteries