Effects of the clay loading on conductivity behavior of sulfonated polyetheretherketone (s-PEEK) electrolyte films

In this work we investigated the effect of clay addition on various properties of proton conducting membranes of sulfonated polyether ether ketone (s-PEEK) for fuel cell application. We prepared membranes by Doctor-blade casting technique and we used two types of clay, a commercial montmorillonite and a home purified one, in the range of 5-20 wt%. We performed XRD, ion exchange capacity (IEC), water uptake (WU) and ionic conductivity measurements at various temperatures. The XRD patterns showed typical peaks of the clay confirming the effective introduction and the stability of the clay even after the thermal and the acid treatment of the membranes. The IEC measurements showed increased values by introducing the clay at around 5 wt% and a progressive reduction occurs with the increase of the clay amount. The WU reported at different temperatures showed different behaviours depending on the clay used. At 30° C and for the home clay, we observed a slight decrease in the WU value around 5Wt% and the values remain as a constant for all the superior amounts. At 80°C lower values than sPEEK sample are visible with the 5 and 15wt% while a slight increase occurs with the 10wt%. At 95°C the composite membrane having 5wt% of clay has the highest WU but a reduction is recorded by increasing the amount up to 15wt% and a further increase is visible with the 20wt%. For the commercial clay the behaviour at 30°C is the same but by increasing the temperature at 80°C we observe an increase in the WU with using 5 and 10 wt% and a successive reduction by increasing the loading up to 20wt%. At 95°C the trend is the same for both clays with the highest WU for 20wt%. the proton conductivity of the membranes carried out at 100%RH as a function of temperature showed increased values with the increase of temperature up to 95°C and a drop at 120°C occurs due to the swelling effect that degrades the polymeric matrix and produces the drop of mechanical properties. The proton conductivity of the sPEEK is the highest in the whole range of investigated temperature and the introduction of clay reduces the conductivity. By comparing the evolution of proton conductivity of composite membranes containing various amounts of clay we observe a reduction at 60°C for 5, 10 and 20wt% only for the commercial clay composite and this trend is not observed for the home purified clay composite for which the conductivity increases until 95°C. As a conclusion of this work we found that the 15wt% loading permits the best compromise between chemical-physical and electrochemical data and for this it will be considered for our future fuel cell tests