Sulfonated lignin intercalated graphene oxide membranes for efficient proton conduction

Graphene oxide (GO) membrane has triggered great attention in proton conduction, benefitting from the well-aligned interlayer nanochannels and oxygen-containing groups. However, GO membrane exhibited a relatively low proton conductivity because of the deficiency of proton-conducting groups on basal plane. Herein, sulfonated lignin (SL) was explored as a multifunctional intercalator to prepare highly conductive GO/SL membranes. The resultant GO/SL membrane with the SL mass fraction of 100% exhibits the highest proton conductivity of 346 mS cm−1 at 80 °C, 100% RH and 7.5 mS cm−1 at 80 °C, 40% RH, which are 3 times and 13 times higher than that of pristine GO membrane, respectively. Bearing abundant sulfonic acid groups and ether bonds, SL acts as both proton donors and acceptors, affording additional proton hopping sites and forming continuous hydrogen bond network for proton transfer. The continuous interlayer nanochannels along with the low-energy-barrier proton hopping pathways render GO/SL membranes highly elevated proton conductivity. Moreover, the GO/SL membrane with the SL mass fraction of 100% achieves a significantly increased maximum power density by 162.7% up to 169.2 mW cm−2 at 60 °C, 100% RH.