First-principles study on proton dissociation properties of fluorocarbon-and hydrocarbon-based membranes in low humidity conditions

We present a theoretical study on the proton dissociation properties of the membranes for polymer electrolyte fuel cells. A density functional theory method is used to study the influence of fluorocarbon and hydrocarbon backbones on proton dissociation, the interaction of water molecules with the sulfonic acid group, and the energy barriers for proton dissociation. Better proton dissociation properties of CH₃SO₃H compared to CF ₃-SO₃H are observed from statistical analyses of the optimized structures for both systems. However, the calculated energy barriers for proton dissociation are lower for CF₃SO₃H than for the CH₃SO₃H system. At the same time, the interaction of water molecules is stronger for CH₃SO₃H than for CF ₃SO₃H. Also, the analysis of the hydrogen-bonding network in both systems shows that the number of hydrogen bonds formed around the sulfonic acid group in CH₃SO₃H is larger than that in CF₃SO₃H. Therefore, the decrease of the energy barrier with increasing number of coordinating water molecules, pronounced in the case of CF₃SO₃H, may lower the barrier, which enhances good proton conductivity of a hydrocarbon-based polymer in low humidity conditions. Thus the hydration ability of a sulfonic acid group is an important factor for realizing better proton dissociation in low humidity conditions.