Impact of Surface Treatment on Processing of Iridium Oxide Catalysts for Electrolyzers

Hydrogen is an important component of many industries, including agriculture, where hydrogen serves as a reactant in the production of ammonia for fertilizer. Hydrogen is also frequently discussed as a part of green energy. In addition to being used in fuel cells, hydrogen has great potential as an energy storage medium for renewable energy sources. Using hydrogen as storage could relieve over-burdened grid systems when more power is being produced than can be used or stored by the means currently available. However, most hydrogen produced is currently made through the steam reformation of methane, a process that produces a great deal of greenhouse gas emissions of both CO2 and methane. To truly be a green energy source, alternative means of hydrogen production must be found that can meet demands and remain economically viable. One of the most promising methods to accomplish the goal of making hydrogen production environmentally friendly is water electrolysis. Water electrolysis requires only water and an energy input, usually electricity, to produce oxygen and hydrogen. When integrated with renewable energy sources to provide the energy required for this process to run, water electrolysis has no greenhouse gas emissions. Out of the three systems currently being studied, proton exchange membrane (PEM) technology is the closest to being economically viable on a large scale. One of the barriers to large scale use of PEM electrolyzers is catalyst cost. The two most commonly used PEM catalysts are platinum and iridium oxide. Much of the research done with the PEM system focuses on how to improve catalyst activity without sacrificing stability. This includes researching new catalysts or combinations of catalysts, and trying to find ways to use the catalyst more efficiently when assembling electrolyzers. To improve catalyst function, understanding the properties of the catalysts is crucial. This thesis is the first step in fully characterizing the iridium oxide component of the PEM electrolyzer. As part of processing, the catalyst is combined with tetrafluoroethylene (TFE), such as Teflon® by The Chemours Company. The interaction between the catalyst and the TFE has not been extensively studied, and the lack of knowledge surrounding the interaction between the TFE and the catalysts can cause problems in the catalyst processing steps that precede electrolyzer assembly. However, it is known that a surface with higher surface energy will wet less easily.1 The TFE-binding process at Proton Onsite uses a dispersion of TFE with a surfactant, making the wettability of the catalyst important. A catalyst with lower wettability will decrease the contact the catalyst surface’s has with the TFE dispersion and lower the TFE content of the finished catalyst. One way to affect the surface energy of the catalyst is to change the crystal arrangement present in the sample, which can be accomplished by changing the composition through oxidation and reduction of the catalyst.2,3 Therefore, the underlying hypothesis for this thesis is that reducing the surface oxidation of the iridium oxide will reduce the amount of Teflon that can be incorporated into the catalyst during processing. To test this hypothesis, characterization was performed on as-received iridium oxide catalyst, oxidized catalyst, and reduced catalyst. The catalyst’s surface was characterized using x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). Following the initial characterization, all three catalysts were TFE-bound and more characterization was performed, using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and SEM. With the information gathered from the characterization performed for this thesis, we will gain a better understanding of the properties of the catalysts they work with, and will be able to optimize the processing steps involved in making electrolyzers to lower costs, improve efficiencies, and prevent future processing problems