Sulfonated poly(ether ether ketone) (SPEEK) is a proton-conducting polymer with certain proton conductivity and thermodynamic properties. However, it still has the disadvantage of low proton conductivity and needs to be modified to improve its performance. Co-MOF-74/phosphate-4-phenylimidazole/SPEEK (Co-MOF-74/[IM2][H2PO4]/SPEEK) ternary composite membranes were prepared using the solution casting method. A small amount of agglomeration on the surface of the Co-MOF-74/[IM2][H2PO4]/SPEEK ternary composite membrane with an increase in Co-MOF-74 content and a gear-like structure were observed in the cross section of the prepared composite membrane through SEM. The proton conductivity of the Co-MOF-74/[IM2][H2PO4]/SPEEK ternary composite membrane increased. The reason is that the metal–organic framework (MOF) encapsulates the ionic liquid (IL) through hydrogen bonding, reducing the loss of IL, thereby improving the proton conductivity of the ternary composite membrane. At 120 °C, the doping amount of Co-MOF-74 was 2.5 wt % within the Co-MOF-74/[IM2][H2PO4]/SPEEK membrane, the proton conductivity of the Co-MOF-74/[IM2][H2PO4]/SPEEK reached 25.96 mS·cm–1, and the IL loss rate in the Co-MOF-74/[IM2][H2PO4]/SPEEK ternary composite membrane ranged from 25 to 30%. The Co-MOF-74/[IM2][H2PO4]/SPEEK ternary composite membrane can be applied within 320 °C. These indicate that encapsulating IL in the MOF not only increases the proton conductivity of the SPEEK membrane but also improves the thermal stability of the SPEEK membrane.
