OPTIMIZATION OF TITANIUM LIQUID/GAS DIFFUSION LAYERS IN PROTON EXCHANGE MEMBRANE ELECTROLYZER CELLS

Polymer electrolyte membrane electrolyzer cells (PEMECs), which are reverse PEM fuel cells (PEMFCs), are effective energy storage medium by producing hydrogen/oxygen from water using electricity from renewable energy sources. This is due in part because of its efficiency, high energy density, compact design, and large capacity. In a PEMEC, a liquid/gas diffusion layer (LGDL) is located between the catalyst layer and the current distributing flow field. The LGDL is expected to transport electrons, heat, and reactants/products to and from the catalyst layer with minimum voltage, current, thermal, interfacial, and fluidic losses. Carbon materials (carbon paper or carbon cloth), typically used in PEMFCs, are unsuitable in the anode of PEMECs due to the high ohmic potential and highly oxidative environment of the oxygen electrode. The carbon corrosion and consumption will result in poor interfacial contacts that will degrade performance and efficiency. Advanced and multifunctional LGDLs with desired properties and high durability in corrosive environments are critical for improving efficiency and performance in electrochemical devices. This thesis highlights recent efforts to optimize anode LGDL properties for high-efficiency PEMECs.

By controlling the parameters of the LGDLs, a greater understanding of the physical interactions and multi-scale interfacial effects that occur in the anode is reached. The main objectives of this thesis are as follows: (1) In-situ investigations of the effect of previous objectives on the performance and efficiency of a PEMEC, and provide extensive analysis on testing results; (2) analysis of the corrosion that can occur in the anode during electrolysis operation; (3) development of a standard set of procedures and metrics for designing and fabricating metallic thin film LGDLs.

In this work, a set of metallic LGDLs having different thicknesses and porosities are designed and examined. It is found that the performance of the PEMEC will decrease along with an increase of LGDL thickness, but increase with a decrease of pore size. The porosity of the titanium LGDLs has less impact on PEMEC performance than in PEMFCs. The ohmic resistance plays a dominant role in electrolyzer performance, and improved performance can be obtained even at a lower porosity by reducing ohmic losses.