Improving performance of catalysts for water electrolysis: The MnOx case

Research output: ResearchPh.D. thesis – Annual report year: 2015

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This Ph.D. thesis presents work on non-noble metal oxide catalysts for the oxygen evolution reaction, OER. This reaction is currently a bottleneck in electrolyzer technologies, which are promising for energy storage purposes. In particular, Polymer Electrolyte Membrane, PEM, cells are attractive for decentralised hydrogen stations. PEM electrolyzers rely on scarce noble metals to achieve high effciency and durability, which limits the scalability of the technology. Finding new catalysts for OER is therefore a thriving research field with new materials being reported frequently. However, many of these new reports include little information about stability, which is evaluated solely from short term electrochemical testing. The first part of this project was therefore dedicated to designing a meaningful stability protocol. Manganese oxide thin films were prepared with sputter deposition and the stability was evaluated with Electrochemical Quartz Crystal Microbalance measurements combined with Inductively Coupled Plasma - Mass Spectrometry. The results showed that a stable electrochemical performance can be achieved, while a constant mass loss is occurring. The proposed protocol can guide future research efforts in evaluating novel materials for the OER.
Unfortunately, most non-noble metal based OER catalysts reported to this date work in alkaline solutions, where cheap NiFe electrodes are already utilized in commercial systems. For acidic media, relevant for the acidic membrane in PEM electrolyzers, there is a lack of strategies aimed at designing catalysts without noble metals. It turns out that MnO2 is a stable material in the OER relevant potential range in acid. In this project, MnO2 thin films were therefore prepared to evaluate their usefulness for PEM electrolyzers. Anodic dissolution of MnO2 was found to be an issue and a strategy is presented for stabilizing the surface. From density functional theory calculations it was found that titanium could segregate to surface sites prone to dissolution. Thus, MnO2 thin films were modied with titanium using a reactive co-sputtering method and tested in acid. The results indicate that the stability could be improved with more than 40 %, while the activity decreased with 10 %.
Finally, for MnO2 to be useful as an OER catalyst in PEM cells, the activity should be improved. Mixtures of manganese oxide and gold have been reported to exhibit activity enhancements and, hence, a nal part of the thesis focus on this system. Mixed thin lms were prepared, which exhibited ve times higher current density compared to pure Mn oxides. X-ray Diraction measurements indicated that Au domains of approximately 3 nm were important for this enhancement. Furthermore, from an in-situ X-ray Absorption Spectroscopy study it was found that Mn oxidises at a more cathodic potential when Au is nearby. This experimental study serves as a starting point for understanding the beneficial interaction between gold and manganese oxides.
Original languageEnglish
PublisherDepartment of Physics, Technical University of Denmark
Number of pages233
StatePublished - 2015
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