Effect of the incorporation of sulfonated chitosan/sulfonated graphene oxide on the proton conductivity of chitosan membranes

• Sulfonated chitosan (SCS) and SGO are added to CS to prepare nanocomposite membranes.
• Nanocomposite membranes show better thermal/mechanical properties than pure CS.
• SCS and SGO enhance proton conductivity of CS in a synergistic manner.
• Adding 5 wt% SGO to CS/SCS causes about 6-fold gain in conductivity and selectivity.
• Experimental proton conductivity data are predicted by a Nernst–Planck based model.

Chitosan biopolymer (CS) has been attracting considerable interest as polymer electrolyte in fuel cells. However, proton conductivity of chitosan is low and it is necessary to enhance its conductivity. In this work, 10 wt% sulfonated chitosan (SCS) and different amounts of sulfonated graphene oxide (SGO) nanosheets are incorporated into a chitosan membrane to investigate their effects on the electrochemical properties of the membrane. The proton conductivity and methanol permeability tests conducted on the CS/SCS/SGO membranes show that the conductivity is increased by 454%, the permeability is reduced by 23% and hence the selectivity is increased by 650%, relative to the neat chitosan, at SGO content of 5 wt%. Furthermore, combined addition of SCS and SGO to chitosan causes much more proton conductivity enhancement than the individual additives due to the synergistic effect of SCS and SGO. The observed synergistic effect reveals the importance of the chemical functionality of chitosan and nanofillers in the formation of ionic cluster domains with enhanced size within the membranes for proton transport. Finally, a Nernst–Planck based model is applied to the experimental proton conductivity data in order to shed more light on the role of GOs in the proton conductivity mechanism of chitosan.

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