Proton conduction and gas permeation through polymer electrolyte membranes during water electrolysis
Jülich / Forschungszentrum Jülich GmbH, Zentralbibliothek, Verlag (2018) [Book, Dissertation / PhD Thesis]
Page(s): 1 Online-Ressource (VI, 212 Seiten) : Illustrationen
Water electrolysis is an effective, durable and emission free technique to convert electrical energy by the electrochemical decomposition of water into chemical energy. During water electrolysis in an acidic electrolyte, water is oxidized to oxygen and protons at the anode. The protons permeate through the electrolyte to the cathode where they are reduced to hydrogen. Acidic polymer electrolyte membranes (PEMs) are typically used to provide the proton conductivity between the electrodes and to separate the evolved gases. The proton conduction in PEMs takes place in an aqueous phase in the form of water channels that are separated from the solid polymeric phase. This thesis is dedicated to a multiscale description of the permeation of protons and gases through PEMs during water electrolysis, characterizing the physical processes within the microscopic structure of PEMs (in the order of 10 nm and the resulting macroscopic efficiency loss. In the literature, the best characterized PEM is Nafion, which serves as an exemplary material for the physical transport processes in PEMs. Using impedance spectroscopy, the proton conductivity of Nafion membranes was measured to be constant in a frequency range of three orders of magnitude and below amplitudes of 1 V. By relating the length scale of the proton movement in Nafion to the applied frequencies and voltages, the conduction was found to be independent of scattering processes between protons and pore walls of the water channels. Moreover, the conductivity of an aqueous solution with similar proton concentration as the water channels in Nafion was found to be 6 times higher than that of Nafion membranes. The origins for this reduced macroscopic conductivity were ascribed to microscopic geometric restrictions for the proton permeation through the morphology of the aqueous phase. By using the electrochemical monitoring technique, the gas permeability of Nafion membranes was measured to be independent of pressure, which was explained by a solely diffusive permeation process. The alternating permeation through the aqueous phase, solid phase and their intermediate phase was modelled with a resistor network that represents the microscopic structure of Nafion. By comparing measured and modelled hydrogen permeabilities of Nafion, water was estimated to act as a plasticizer that increases the permeability of the solid polymeric phase. The conductivities and permeabilities of six different PEMs were examined and compared. A model to describe the hydrogen and oxygen cross-permeation through PEMs during water electrolysis was developed, in which the influence of the proton flux on the gas diffusion was physically described. This model was evaluated with measured anodic hydrogen contents of an operating electrolysis cell. Based on this model and the measured proton conductivity and gas permeability of Nafion, the efficiency loss caused by the proton conduction and gas cross-permeation during water electrolysis was modelled and used to computationally optimize the membrane thickness. To reduce the efficiency loss that is caused by the hydrogen cross-permeation through the PEM during water electrolysis, a novel technique with an additional electrode in the electrolyte was introduced, where hydrogen in the PEM is electrochemically oxidized and sent back to the cathode. In the focus of efficiency, pressurized operation and atmospheric pressure operation in combination with subsequent compression were compared.