Highly sulfonated poly ether ether ketone chelated with Cu²⁺ as a proton exchange membrane at sub-zero temperatures

Improving the proton conductivity (σ) of proton exchange membranes at low temperatures is very important for expanding their application areas. Here, sulfonated poly ether ether ketone (SPEEK) membranes were prepared with different sulfonation degrees, and its maximum ion exchange capacity is 3.15 mmol/g for 10 h at 60 °C. Highly sulfonated SPEEK membrane exhibits ultra-high water uptake and excellent proton conductivity of 0.074 S/cm at -25 °C due to its abundant -SO₃H. Nevertheless, its high swelling ratio and low mechanical strength are not conducive to the practical application of the membrane. Luckily, by employing the chelation of Cu²⁺ with -SO₃^- on the SPEEK chain, Cu²⁺-coordinated SPEEK membranes were prepared, and they not only retain high -SO₃H content but also possess robust mechanical properties and good dimensional stability compared to pristine SPEEK membrane. Meanwhile, the σ of the SPEEK-Cu membrane reaches 0.054 S/cm at -25 °C, and its fuel cell maximum power (W_max) reaches 0.42 W/cm² at -10 °C, demonstrating superior low-temperature performance in comparison to other reported materials. Particularly, water states in the prepared membranes are quantified by low-temperature differential scanning calorimetry. Because much more water bound to the plentiful -SO₃H and Cu²⁺ inside the membrane endows it with anti-freezing performance, the decay of the σ and the W_max for the SPEEK-Cu membrane is retarded at sub-zero temperatures. It is envisioned that composite membranes comprising metal ions such as Cu²⁺-SPEEK have a high potential for sub-zero fuel cell applications.