The study begins with the observation that in the experimental setup for inductively coupled plasma mass spectrometry (ICP-MS) measurements, which necessitates a small electrolyte volume, the stability number comes out significantly higher. The authors argue that the small volume results in a high concentration of metal ions for a given amount of dissolved catalyst, which subsequently limits further dissolution due to the shift in equilibrium (as defined by the Nernst equation). Using RuO2 and IrO2 as benchmark noble-metal OER catalysts, a number of other experimental factors are then investigated. The length of experiment (on short timescales, <1 hour), applied current density, electrode rotation rate and amount of Nafion binder applied all show negligible impact on the measured stability number of RuO2. The adhesion properties of the electrode substrate also do not impact the stability number, though are associated with electrode failure due to mechanical breakdown. These findings do not hold for all materials, however, and in particular, for non-noble metal catalysts the dissolution rate of the metal is not constant with time or applied current density. The overarching factor that connects these disparate trends is whether the concentration of dissolved catalyst is affected by any given parameter for that system.
An important outcome of this work is that measurements between cell architectures with radically different electrolyte volumes, such as membrane electrode assembly, H-cells and flow cells are likely to give different stability data, which may not reflect intrinsic stability of the catalyst. Indeed, the authors’ analysis of published data suggests that no non-noble catalysts to date can compete with IrO2 when it comes to activity or stability (pictured) and that in many cases the stability of non-noble catalysts have been overestimated due to experimental conditions. On the other hand, RuO2 is significantly more active than IrO2 but pays for that with a decreased stability. A suggested focus of future work is therefore to design strategies to augment the intrinsic stability of RuO2. Meanwhile, the goal of finding non-noble catalysts to replace IrO2 in practical-scale water electrolysis perhaps remains much more distant than we had hoped.
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