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X-rays reveal intimate life of Pt catalyst in a hydrogen fuel cell
Large energy losses during the transformation of hydrogen to electrical energy, together with the relatively short lifetime of key components, makes the implementation of hydrogen energy schemes challenging. X-ray techniques were used to study the degradation mechanisms of Pt catalyst in an operational hydrogen fuel cell, clearly identifying the dynamics and interplay.
Hydrogen appears to be a viable energy vector and the hydrogen economy is emerging as an important part of our energy future. However, the commercialisation of polymer electrolyte membrane fuel cells (PEMFCs) requires further optimisation at the device level to improve durability in automotive applications. A greater understanding of the degradation mechanisms inside membrane electrode assemblies (MEAs) during cell operation is necessary to define the optimal performance envelope, and to identify conditions that lead to shortened lifetimes and performance loss. Directly linking the device operating parameters (e.g., cell temperatures, flow rates, power output) to the evolution of catalyst nanostructure is increasingly desired to accelerate the development of high- performance systems. The majority of electrochemical degradation can be attributed to catalyst particle growth. Quantifying specific processes such as aggregation, coalescence and ripening that lead to this growth has remained challenging, due to the difficulty of measuring catalyst nanostructure in situ. The vast majority of what is known regarding catalyst degradation has been learned comparing catalysts at beginning and end of life.
In this work, the MEA nanostructure inside an operational PEMFC was followed with a high-speed, high-resolution synchrotron X-ray probe at beamline ID31. Depth profiles through the plane of the MEA can be obtained by illuminating the device in the plane of the membrane. With this
approach, the nanomorphology of each individual layer in a stack of films can be selectively probed free of overlap or interference (cathode, anode, membrane, diffusion layers, etc.). This technique was used to study the aggregation state of the Pt electrocatalyst nanoparticles, which can be semi-quantitatively estimated using a combination of small-angle X-ray absorption spectroscopy (SAXS) and X-ray diffraction (XRD) (Figure 126). Aggregation is a difficult property to determine using other techniques, especially for commercially relevant, high Pt-loading industrial catalysts.
Measurements inside the catalyst layer revealed that the Pt nanoparticles evolved through three distinct phases of aggregation, coalescence and ripening during an accelerated stress test. Particle migration and aggregation occurring during the initial stages of ageing appeared to control the later phases of degradation (Figure 127).
At the start of the accelerated ageing test, the SAXS/ XRD ratio of the Pt cathode rapidly increased, reaching a maximum after 400 500 cycles. This initial potential cycling increased the mobility of small Pt nanoparticles, which were weakly attached, to move along the carbon support and rapidly aggregate. Particles trapped in aggregates are comparatively stable and less mobile, causing a switch in the dominant degradation mechanism. Ongoing Pt corrosion/redeposition resulted in the coalescence and Ostwald ripening of the aggregated Pt particles into larger crystalline grains (cycles 500 1500). After this point, the population of original small particles was mostly consumed and large particles remained, which were presumably more firmly bound to their supports. Continued cycling resulted in a gradually increasing particle size with a relatively constant, lower level of aggregation. While all of these mechanisms are certainly active over the entire cycling period, it appears that the relative importance of each mechanism changes over the course of the catalyst life cycle, as the nanomorphology of the Pt evolves.
Fig. 126: a) Schematic showing the geometry of SAXS/XRD line scans through the cross-section of the fuel cell membrane electrode assembly. b) SAXS curves collected in the plane of the membrane, cathode catalyst layer and carbon fibre paper
gas diffusion layer before ageing. c) XRD pattern of the cathode catalyst layer showing reflections from metallic Pt.