SPEEK

SPEEK stands for sulfonated poly(ether ether ketone), a sulfonated derivative of the high-performance thermoplastic poly(ether ether ketone) (PEEK). By introducing sulfonic acid groups onto the aromatic backbone, SPEEK becomes a proton-conducting polymer used primarily as a proton exchange membrane in fuel cells and other electrochemical devices. The sulfonation increases hydrophilicity and enables proton transport when hydrated, while retaining much of the mechanical rigidity of the PEEK backbone.

Synthesis and degree of sulfonation: SPEEK is typically produced by sulfonating PEEK with concentrated sulfuric acid or similar reagents under controlled conditions. The degree of sulfonation (DS) — the fraction of repeat units bearing sulfonic acid groups — is tunable, commonly ranging from about 30% to 60% depending on reaction time, temperature, and acid strength. A higher DS improves proton conductivity and water uptake but can reduce thermal/mechanical stability and increase swelling.

Properties: SPEEK membranes retain much of PEEK’s thermal and chemical robustness, with high glass-transition temperatures and good chemical resistance. Proton conductivity in hydrated SPEEK depends on DS and humidity; typical values span from 0.01 to 0.1 S/cm at elevated temperatures under humid conditions. Water uptake and swelling generally rise with DS, which can compromise mechanical integrity. SPEEK is often more affordable than Nafion and can be processed by solution casting or extrusion; performance is frequently enhanced by crosslinking or incorporating inorganic or organic fillers to improve stability and reduce methanol crossover.

Applications: primary use as a PEM in polymer electrolyte membrane fuel cells and direct methanol fuel cells, as well as in other electrochemical devices where proton conduction is required.