Pt-Ru alloy is known as a benchmark HOR catalyst that outperform Pt in alkaline media . Given the promotion of both Ru and Pt by pN-C as described above, we then set out to develop PtRu/pN-C catalysts. We found adding a small amount of Pt (25%) in Ru/pN-C (denoted as Pt0.25Ru0.75/pN-C) results in a highly active catalyst. Shown in Figure , with an ultralow loading of 2.3 μgPGM/cm 2 , the HOR activity of Pt0.25Ru0.75/pN-C outperformed its monometallic counterparts. The half-wave potential of this catalyst is merely 16 mV. Increasing the Pt content beyond 25% didn't have remarkable improvements on apparent HOR activity (Supplementary figure ). Therefore, we chose Pt0.25Ru0.75/pN-C for further studies as this sample contained the least amount of Pt. A control experiment performed in N2-saturated KOH suggested the anodic current originated exclusively from hydrogen oxidation (Supplementary Figure ). After calculating the ECSA, the Pt0.25Ru0.75/pN-C catalyst has a j0,mass of 2608.7 mA/mgPGM and a j0,ECSA of 2.12 mA/cm 2 PGM, which are 3.4 and 2.5 times higher than the benchmark Pt0.25Ru0.75/C, respectively. Both Pt0.25Ru0.75/pN-C and Pt0.25Ru0.75/C can be held at diffusion limiting current to a potential higher than 1 V vs RHE without deactivation (Supplementary Figure ). Overall, the activity of the PtRu/pN-C samples are higher than all other reported PGM HOR catalysts (Supplementary Table ). Accelerated durability test (ADT) showed the half-wave potential for HOR only increased 7 mV for Pt0.25Ru0.75/pN-C after 2500 CV scans from -0.1 V to 0.35 V vs RHE with a scan rate of 150 mV/s, indicating the robustness of the catalyst (Supplementary Figure ). XRD suggested Pt0.25Ru0.75 had less distinguished diffraction peaks, possibly because it is in a transition from the fcc (Pt) to hcp (Ru) structure (Supplementary Figure ). Microscopic images showed ultrasmall nanoparticles evenly distributed on the pN-C (Supplementary Figures Although still a matter of debate , the higher HOR activity of PtRu over Pt is proposed to originate from a decreased HBE of Pt sites , due to a Pt to Ru charge transfer that lowers the d band center of Pt. Our XPS data support this proposal (Supplementary Figure ). In addition, our in-situ ATR-SEIRAS data indicate stronger water binding (Supplementary Figure ), and our CO stripping data indicate higher OHBE upon alloying of Pt with the more oxophilic Ru (Supplementary Figure ). These factors might also contribute to the high activity of PtRu according to the apparent HBE theory and the bifunctional mechanism , respectively. The superior activity of Pt0.25Ru0.75/pN-C, then, is a result of both the modulation of the electronic structure of Pt nanoparticles by alloying with Ru, and more optimal interfacial water structure and water binding strength due to Pt SAs. Note that our data suggest the Ru-containing catalysts do not follow the OHBE theory proposed by McCrum and Koper for decorated Pt single crystals . Adding considerable Ru into Pt is known to switch the reaction mechanism from Volmer-Tafel to Volmer-Heyrovsky , evidenced by a change of the Tafel slope from 135 mV/dec to 43 mV/dec . The change of mechanism likely leads to a change of relevant descriptors. The anode and cathode catalyst loadings were 0.18 mg/cm 2 and 0.55 mg/cm 2 , respectively. Test conditions: cell temperature at 80.0 °C, optimized dew points for anode/cathode were 71℃/73℃, H2 and O2/air flow rate were all 1 L/min with no back pressure. (C) Fuel cell performance test for a HEMFC with a Pt0.25Ru0.75/pN-C anode catalyst and a commercial Fe-N-C cathode catalyst. The anode loadings were either 0.16 mgPGM/cm 2 or 0.11 mgPGM/cm 2 . Cathode catalyst loading was 1.34 mg/cm 2 . Test conditions: cell temperature at 80.0 °C, optimized dew points for anode/cathode were 73℃/75℃, H2 and O2 flow rate were all 1 L/min with no back pressure. The orange star represents the U.S. DOE 2022 target for HEMFC . (D) Comparison of the PGM utilization (H2-O2 as feeding gas) of HEMFCs.