Durability Assessment of Transition Metal Oxygen Electroreduction Catalysts with on-Line Flow Cell Trace Elemental Analysis

Abstract

Proton exchange membrane hydrogen fuel cell (PEMFC) cathodes for commercial applications are typically based on platinum group materials that exhibit remarkable oxygen reduction reaction (ORR) activity and stability. Despite this performance, the high cost of such platinum electrodes prevents a more rapid transition to a more sustainable hydrogen economy involving PEMFCs. Platinum-free transition metal ORR catalysts have been identified and tested as high activity, low-cost alternatives with well-established protocols, though understanding the durability of these materials would require more experimental efforts. Techniques for measuring catalyst degradation typically involve activity loss measurements, surface-sensitive microscopy and spectroscopy, and liquid aliquot collection for metal loss quantification that are all performed post-catalysis for extended periods of time. Experiments using this approach provide valuable information about overall catalyst material degradation but usually cannot elucidate mechanistic or potential-dependent phenomena that may occur on shorter time scales. More rigorous experiments with synchrotron radiation have been utilized to investigate the degradation mechanisms of platinum-based fuel cell catalysts but studies on other transition metal materials are less widespread due to the high time requirements and limited accessibility of synchrotron beamlines for such experiments. In our work, we utilize an on-line electrochemical flow cell coupled with an in-house inductively coupled plasma-mass spectrometer (ICP-MS) for evaluation of fuel cell catalyst degradation in acidic electrolyte environments. Oxygen and nitrogen gas saturated electrolytes with a constant pH of 1 but differing chemical identity are flowed through the cell and across a working electrode composed of non-platinum transition metals. Dissolved metal ions are convectively transported to the ICP-MS where they are measured with up to part per trillion resolution with a time delay of ~4 seconds, enabling millisecond resolution of transient catalyst surface degradation. We evaluate the oxygen mass transport capabilities of our custom-machined flow cell against a commercial flow cell and compare to the much more established rotating disk electrode (RDE) technique for distinguishing ORR activity. Cyclic voltammetry measurements in oxygen-saturated electrolyte on various metal foils indicate potential dependent corrosion in agreement with what is expected from a Pourbaix (potential-pH) diagram of stability. In contrast, potential cycling these materials in the ORR-relevant range reveals degradation during ORR electrocatalysis that increases with ORR current to differing amounts as a function of the metal in a way that has not been previously characterized. Notably, degradation appears to be suppressed in the ORR potential range when saturating the electrolyte with nitrogen compared to oxygen, suggesting that surface-bound oxygen and ORR intermediates have a potential-dependent role in enabling degradation. Probing degradation during ORR conditions further with varied cyclic voltammetry scan rates and directions reveals more fundamental information about the timescales of catalytic corrosion processes and may inform the development of activation protocols that enhance the lifetime of ORR catalyst materials during operation. Through electrochemical flow cell testing in various pH 1 electrolytes, we make suggestions for electrolyte compositions and catalyst operation potential ranges that maintain activity and stability to advise fuel cell catalyst researchers in developing platinum-free PEMFC cathode materials. We are also developing additional flow cell configurations to allow testing of other non-precious ORR catalyst morphologies such as nanoparticle catalysts on carbon supports. The on-line ICP-MS flow cell architecture accelerates materials stability and degradation experiments for rapid assessment of platinum-free ORR electrocatalysts through in-house experiments and direct comparability to established electrochemical techniques.