Additionally, for a wholistic characterization of the catalytic performance it is instructive to calculate the mass activity, i.e. the current at the overpotential normalized by the mass loading . Interestingly for Cr2O3/rGO we find a high mass activity of 100 A/g. At a mass loading of 0.1 mg and an overpotential of 370 mV, this mass activity is among the high performing OER catalysts (Table ) . Furthermore, this performance is stable for at least 200 cycles lasting 24 h as shown in Figure -f. The reproducibility of the long-term experiment has been checked. It is noticed that the slight change of cell voltage in the first run is not reoccurring in the second run and can thus be considered as a measurement artefact.