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Fig. 136: The hotter, more acidic conditions inside fuel cell devices accelerate nanoparticle catalyst

ageing phenomena (corrosion, aggregation, ripening) versus conventional electrochemical

testing environments. The Pt degradation is approximately five-fold faster inside a real device,

as seen by XRD and electron microscopy (top). The XRD also quantitatively tracks the corrosion over

time, allowing for a detailed understanding of the catalyst life cycle (bottom).

Mapping heterogeneous ageing inside operational hydrogen fuel cells

Imaging catalyst degradation with diffraction tomography inside large, functional fuel cells reveals high spatial heterogeneity linked to chemical gradients in pH, hydration and temperature inside the cell. While absent in model systems, these effects are critical for understanding device performance.

Hydrogen fuel cells, devices which convert the energy stored in hydrogen gas to electricity, require large quantities of expensive platinum nanoparticle catalysts to electrochemically reduce O2 to water. The durability of this catalyst is currently a limiting factor in consumer automotive applications and the emerging hydrogen economy.

Recent work from beamline ID31 has contributed to a rising consensus that heterogeneous chemical microenvironments are both ubiquitous inside electrochemical systems, and play a significant role in determining the performance and durability of devices [1]. X-ray diffraction and small-angle scattering computed tomography (XRD-CT/SAXS-CT) inside a full-size, operational fuel cell show that electrocatalyst degradation phenomena such as corrosion, Ostwald ripening and nanoparticle aggregation are tightly correlated with the distribution of liquid water inside the device. This water distribution depends on the operating conditions of the cell and the larger-scale engineering geometry of the device, which define the gradients of chemical and physical quantities affecting the performance and degradation far more than previously anticipated. This heterogeneous environment leads to large differences in ageing throughout the electrode, seen in Figure 135, which also correlate to the nanoscale swelling of the ionomer membrane visualised by SAXS-CT.

Furthermore, it was demonstrated that Pt catalyst degradation inside a full hydrogen fuel cell is massively accelerated versus the conventional liquid cells typically used to evaluate catalyst performance/stability in electrochemical laboratories. X-ray diffraction was able to determine both the crystallite size and quantity of Pt catalyst remaining over an accelerated ageing test of 10 000 voltage cycles, simulating the lifespan of a fuel-cell vehicle (Figure 136). The size of the Pt nanoparticles aged under conventional lab conditions (room temperature, ultrahigh purity water, etc.) increased from an initial 4.5 nm diameter to 6 nm. The same catalyst particles inside the electrode of the working device (gas-phase, 80°C, 10x lower pH) grew to ~11 nm, yielding dramatically lower active surface areas.

Fig. 135: XRD-CT maps of Pt nanoparticle electrocatalyst in a fuel-cell cathode showing low particle size heterogeneity before ageing (a), which becomes much more polydisperse

after ageing (b). c) Higher-resolution maps of the aged sample show lined patterns produced by chemical gradients inside the cell. Bright-field transmission electron microscopy

of the Pt catalyst before ageing (d) and after ageing (e) confirm the quantitative analysis via XRD-CT.