Investigations of catalyst interfacial phenomena for enhanced fuel cell and water electrolyzer performance

Abstract

Climate crisis and abnormal climate phenomena are appearing much faster and more frequently than expected, and it is necessary to develop an eco-friendly energy society to replace the current carbon energy system. Because of its gravimetric energy density and abundance advantages, hydrogen is being recognized as the ultimate energy carrier for storing and distributing renewable energy sources (solar, wind or geothermal energy) to societies. In an ideal hydrogen energy society, hydrogen gas is extracted from water by direct solar energy splitting or electrolysis and then distributed to mobile or power plants to produce electricity or applied to industries for ammonia synthesis or steelmaking processes. Hydrogen energy society thus requires devices for converting the electricity into hydrogen gas and vice versa, which are water electrolyzer and fuel cell, respectively. However, they yet need further developments for commercialization, especially with respect to stack cost reduction. Basically, it comes from the expensive and scarce materials applied in the membrane-electrode assemblies, which contains noble metal catalysts and fluorinated ionomers, and therefore industry is trying to reduce the content or develop cheaper materials. This thesis begins with the general background of hydrogen energy society and the basic principles of the hydrogen devices. It then addresses the requirements of membrane-electrode assemblies for efficient and durable fuels cell and water electrolyzers. Specifically, this work is about the catalyst interfacial phenomena in the fuel cell and water electrolyzer using the proton exchanging polymer electrolyte membrane (PEM). Chapter 2 deals with a strategy to enhance performance of the polymer electrolyte membrane fuel cell (PEMFC) by addressing the interfacial property of catalyst and ionomer. By avoiding the ionomer film on the Pt surface, the transport resistance of oxygen gas was successfully reduced, which greatly improved the performance of the low Pt loading catalyst layer at high current density. Chapter 3 presents deep analysis to elucidate the interfacial properties on Ir catalyst and Ti porous transport layer. It claims that Schottky contact between Ir catalyst and native oxide layer on Ti transport layer result in severe conductivity problems. And it was figured out that the interfacial properties vary with catalyst layers due to the differences in the materials or structural differences

for instance, the high ionomer contents in catalyst layers lead to severe band bending and low work function catalyst brings out negligible band bending.