for Pt-poly with RF of 90±20. Our Pt disk has η 10 of -110 mV, which closely agrees with the results of McCrory et al. . Given that the RF values of Ni-poly do not change for more than 10% upon the exchange with Rh (Figures and, Supplementary Information), and that the Rh-exchanged electrodes have measured HER current densities (normalized per geometric surface area) close to that of Pt (Figure ), surface-specific activities of Rh-exchanged Ni-poly actually exceed the activities of Pt (Figure ). A direct comparison with the results of Nguyen et al. , who investigated HER activity of NiRh 3 alloy nanosponges in alkaline media, can be made. With the catalyst loading of 169 μg cm -2 , the authors reported η 10 of -107 mV for NiRh 3 alloy and -119 mV for commercial Pt/C catalyst .