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In the first part of this study, the hydrogen and oxygen permeabilities of Nafion were measured. The aim of the second part of this study presented here is to physically characterize the influence of the aqueous phase, the solid phase, and the intermediate phase in Nafion on the macroscopic hydrogen and oxygen permeabilities. Hereto, a resistor network model morphologically representative for Nafion based on structural investigations reported in the literature is presented in which the different phases are described by individual permeabilities. As a result of the simulations, an enlarged permeability of the solid phase in comparison to that of dry Nafion had to be assumed in order to reproduce the measured influence of temperature and relative humidity on the permeability. This increase of the permeability of the solid phase toward greater water uptake was explained by the effect of water as a plasticizer and the resulting softening of the polymeric matrix. On the basis of the identified mechanisms, approaches to reduce the gas permeability of polymer electrolyte membranes are identified and discussed

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A pore network model has been applied to a both sides of a fuel cell membrane electrode assembly. The model includes gas transport in the gas diffusion layers and catalyst layers, proton transport in the catalyst layers and membrane, and percolation of liquid water. This paper presents an iterative algorithm to simulate a steady state isothermal cell with a 3D pore network model for constant voltage boundary condition. The proposed algorithm provides a simple method to couple the results of the anode and the cathode sides by iteratively solving the uncoupled equations of the transport processes. It was found that local water blockages at the GDL/CL interface not only affect concentration polarization, but also might change ohmic polarization of the cell. Depending on the liquid water configuration in the porous electrodes, the protons generated in the anode need to travel longer paths to reach the active sites of the cathode; consequently, the IR loss will be increased in the presence of liquid water. This finding highlights the strength of pore network models which resolve discrete water blockages in the electrodes. Polymer electrolyte membrane fuel cells are one of the key technologies required to realize a sustainable energy economy because they provide energy storage. A typical PEMFC is a stack of electrochemical cells, and the heart of each is a sandwich of several porous layers around a thin polymer electrolyte membrane, referred to as a membrane-electrode assembly (MEA). In the typical arrangement each side consists of a gas diffusion layer (GDL), and a catalyst layer (CL). The GDL is usually a carbon-fiber based paper and acts as a spacer to allow gaseous reactants to reach regions of the catalyst layer under the flow field ribs, and as a bridge to allow electron access to catalyst sites over the flow field channels. The CL is composed of a mixture of ionomer such as Nafion and carbon-supported platinum catalyst particles, and is adhered to the surface of the membrane as a porous coating around 10-20 μm thick. The ionomer phase in the CL allows protons to reach the catalyst sites, while the carbon particles provide pathways for electrons, and the porosity allows transport of gaseous reactants (oxygen and hydrogen) and product (water). Under some conditions the cathode produces liquid water, which can accumulate in the pore spaces, blocking the access to the reaction sites. Liquid water can also be found on the anode side, for instance if temperature fluctuations occur since the hydrogen is humidified. Understanding the role of liquid water and its impact on fuel cell operation has been a longstanding challenge for the industry.1-3 Complete water removal from the cell is not an option because the currently used membrane materials must be hydrated to function.When electrical current is drawn, several sources of voltage loss are incurred due to the inefficiencies of current generation and transport processes. Voltage losses can be broken into three categories: activation polarization η act , ohmic polarizat...

Polymer electrolyte membrane water electrolysis cells were studied using in-operando neutron radiography, revealing insights into the gas-water distribution inside the cells. Cells were operated at current densities up to 2 A/cm² and for water flow rates ranging from 0.5 ml/(min cm²) up to 5 ml/(min cm²). The averaged gas amount in the flow channels was quantified, revealing that the ratio of gas to water inside the channels decreases for an increasing water flow rate on the anode side. This examination also demonstrates that neutron radiography is very suitable to study gas/water two-phase flow phenomena in running electrolysis cells.