RT cD s , where F is the Faraday constant, R is the gas constant, T is the temperature at which D s is calculated, and c is the concentration of protons in the system. We estimated c to be 0.0028 mol/cm 3 using the c24C system, which corresponds to the number of moles of protons per volume of the membrane (discussed in the Supporting Information). We estimated the proton conductivity of graphanol over the temperature range T = 300-800 K ( to 527 • C), to vary between 313-1567 mS/cm. Graphanol's high proton conductivity at low temperature is a major advantage for PEM-FCs because most high temperature PEM-FCs suffer from delayed startup at low temperatures.