(349c) On-Line Flow Cell Trace Elemental Analysis Elucidates Degradation Mechanisms in Transition Metal Oxygen Electroreduction Catalysts

Commercial proton exchange membrane hydrogen fuel cell cathodes are presently composed of platinum-based materials that exhibit exceptional oxygen reduction reaction (ORR) activity and stability. However, platinum electrodes remain prohibitively expensive for transitioning to a more sustainable economy dependent on hydrogen fuel cells. Less expensive transition metal ORR catalysts have been thoroughly investigated for identifying high activity alternatives with established methods though the stability of alternative materials is not as well-understood. Methods for assessing catalyst degradation typically involve surface characterization, electrolyte aliquot collection for metal quantification, and activity loss measurements, all performed after catalysis for extended periods of time. Experiments of these types can provide information about how catalyst materials degrade over time but usually cannot resolve mechanistic information or phenomena that occur on shorter time scales. More intensive experiments using synchrotron radiation have been utilized to probe degradation mechanisms in platinum-based fuel cell catalysts but investigations on alternative materials are limited due to the accessibility of synchrotron facilities and high time requirements for such experiments. In our work, we utilize an in-house technique consisting of an on-line electrochemical flow cell linked to an inductively coupled plasma-mass spectrometer (ICP-MS) for evaluation of fuel cell catalyst degradation in acidic media. Gas-saturated electrolytes of varying chemical identity and a constant pH of 1 flow through the cell and across a working electrode composed of platinum group-free metals. Dissolved metal ions are carried downstream to the ICP-MS where they can be measured with part per trillion resolution and minimal signal broadening, enabling millisecond resolution of degradation processes occurring at the catalyst surface. We evaluate the oxygen mass transport characteristics of a commercial flow cell against a custom-machined flow cell and optimize experimental conditions for comparability to the established rotating disk electrode (RDE) technique. Measurements on metal foils with applied cycled potentials in oxygen-saturated electrolyte display potential dependent corrosion similar to what we would expect from a typical Pourbaix (potential-pH) diagram of stability. However, cycling these metals to ORR-relevant potentials demonstrates degradation during ORR that increases with ORR current to varying degrees depending on the metal in a way that has not been previously characterized. Much of the degradation in the ORR potential range is not present when saturating the electrolyte with nitrogen, implying that the presence of oxygen can accelerate degradation. Probing the phenomenon of degradation during ORR further with differing scan rates and directions for the applied potential sweep reveals more information about the timescale of surface corrosion processes and informs the development of protocols that can enhance the lifetime of these catalyst materials under operation conditions. In combination with on-line testing in various pH 1 electrolytes, we make specific recommendations for optimal potential ranges and electrolyte compositions that optimize activity and stability to advise the fuel cell community in developing platinum group metal-free fuel cell cathode materials. We are also developing new flow cell designs to enable the testing of other non-precious ORR catalyst morphologies such as nanoparticle catalysts. The on-line ICP-MS flow cell platform demonstrates promise for accelerating materials stability analysis for next generation platinum-free ORR catalysts through practical experiments and direct comparability to established electrochemical techniques.