Mass activity of supported Ir electrocatalysts for acidic OER: How variation in Ir loading during thin-film RDE investigations can lead to inaccurate activity enhancements

As state-of-the-art OER electrocatalysts for acidic water electrolysis are based on scarce metals Ir, strategies such as supporting, alloying, and/or compounding Ir with other materials/elements are frequently adopted to increase the Ir loading normalized OER activity, mass activity ( I O E R m I r ). Here, we explore the effect of the Ir loading on RDE ( m I r ) on the obtained I O E R m I r values and demonstrate that with increasing m I r , I O E R m I r decreases sharply while the electrode area normalized OER activity ( J O E R ) increases, reaching to relatively stable values for m I r > 15 μg for the RDE tip diameter of 5 mm. Hence, when the RDE measurements are performed by keeping the ‘electrocatalyst’ loading constant, the mass activities of the supported electrocatalysts having lower Ir content are enhanced artificially as compared to those of the unsupported reference having relatively higher Ir contents. Therefore, for fair OER activity comparisons between the supported and the unsupported electrocatalysts, RDE measurements should be performed either (i) by using similar m I r values or (ii) by varying the m I r values to obtain a I O E R m I r vs. m I r plot to determine the mass activity by interpolation/extrapolation of the I O E R m I r vs. m I r data. Finally, the claims of high mass activity should be further supported by single cell studies. • Ir mass specific OER activity ( I O E R m I r ) depends strongly on Ir mass on RDE ( m I r ). • For same electrocatalyst loading, supported catalysts exhibit higher I O E R m I r • OER activity measurement using same ‘ m I r ’ is recommended for a fair comparison. • Intrinsic activity independent of m I r may be obtained by fitting I O E R m I r vs. m I r • Stable I O E R m I r and the RDE area-specific OER current are observed m I r > 15 μg.