Fig. 1. Schematic diagram of a proposed catalyst design strategy for high activity HOR.

Fig. 2. (a) Schematic illustrating the synthesis of Ni/Zn1-NC. The black, blue, green and pink small balls represent C, N, Ni and Zn atoms respectively. (b) XRD images of Ni/Zn1-NC and Ni/rGO. (c) TEM images of Ni/Zn1-NC. (d) The particle size distribution of nanoparticles in (c). (e) HAADF-STEM images of Zn1-NC in Ni/Zn1-NC. Some Zn-SAs were delineated with yellow circles. (f) HAADF-STEM images of Ni/Zn1-NC. Some Zn-SAs were delineated with yellow circles. (g) HAADF-STEM and EDS mapping of Ni/Zn1-NC.

Fig. 3. (a) HOR polarization curves for Ni/Zn1-NC, Ni/rGO and 20? wt% commercial Pt/C for an electrode rotating speed of 1600?rpm. The loading of all Ni-based catalysts was 0.612 mgcatalyst cm-2. The loading of commercial Pt/C was 20.4 µgPGM cm-2. The scan rate of Ni-based catalysts was 0.5 mV s-1. The scan rate of commercial Pt/C was 1 mV s-1. (b) Butler-Volmer plots of the HOR current densities (jk) for Ni/Zn1-NC, Ni/rGO and commercial Pt/C. (c) Comparison of normalized mass activity (jk,m) at an overpotential of 50?mV and specific activity (j0,s). (d) Polarization and power density curves of AEMFCs fed with H2-O2 and equipped with with Ni/Zn1-NC, Ni/rGO and Pt/C (0.1 mgPGM cm-2) as anodes and 40 wt% commercial Pt/C (0.22 mgPGM cm-2) as cathodes. Test conditions: the cell temperature was 95 °C, the anode dew point temperature was 86 °C, and the cathode dew point temperature was 89 °C, respectively. Back pressure of 250 kPag was applied to the gases at both the anode and cathode, and the supply flow rates of H2 and O2 were both 1.2 L?min-1. (e) Comparison of the reported PPD of PGM-free anode AEMFCs. (f) Comparison of the total PGM utilization (H2/O2 as the feeding gas) of AEMFCs. (g) Polarization and power density curves of AEMFCs fed with H2/CO2-free air and equipped with Ni/Zn1-NC and Pt/C (0.1 mgPGM cm-2) as the anode and 40 wt% commercial Pt/C (0.22 mgPGM cm-2) as the cathode. Test conditions: the cell temperature was 95 °C, the anode dew point temperature was 86 °C, and the cathode dew point temperature was 89 °C, respectively. Back pressure of 250 kPag was applied to the gases at both the anode and cathode, and the supply flow rates of H2 and CO2-free air were 0.2 and 1 L min-1, respectively. (h) H2/CO2-free air AEMFC stability test curves of Ni/Zn1-NC at 200 mA cm-2. The test conditions were as follows: the cell temperature was 80 °C; the optimized dew points for the anode and cathode were 80 and 80 °C, respectively. The back pressure was 250 kPag, and the flow rates of H2 and CO2-free air were 0.8 and 0.6 L min-1, respectively.

Fig. 4. XANES (a) and EXAFS (b) spectra at the Zn K-edge of Zn1-NC, indicating the formation of the Zn-SA structure. (c) XPS spectra of Ni 2p3/2. (d) Work functions of Ni, rGO and Zn1-NC. (e) Work function difference between rGO, Zn1-NC and Ni. (f) Schematic diagram of the Mott-Schottky junction in Ni/Zn1-NC and Ni/rGO. (g) DFT calculations of the optimized structures of Ni/rGO and Ni/Zn1-NC (brown, blue, pink, and gray balls represent C, N, Zn, and Ni atoms, respectively). (h) Positive charge density on the Ni sites of Ni/Zn1-NC and Ni/rGO. (i) Binding energies of Ni nanoparticles on Zn1-NC and rGO.

Fig. 5. (a) In-situ SEIRAS spectra of Ni/Zn1-NC were recorded in 0.1 mol L-1 KOH saturated with H2 at 0 to 0.2 V versus RHE, and the OH stretching vibration peaks in the range of approximately 3000 to 3600 cm-1 were deconvolution. (b) Stark tuning rate of Ni/rGO and Ni/Zn1-NC according to in-situ SEIRAS. (c) Deconvolution of the OH stretching vibration peaks (approximately 3000 to 3600 cm-1) at 0.1 V versus RHE was performed for Ni/Zn1-NC and Ni/rGO. (d) The relative fraction of the peak at ~3240 cm-1 of Ni/Zn1-NC and Ni/rGO with the ratio of applied potential (from 0 to 0.2 V versus RHE). (e) EPZC values measured for Ni/Zn1-NC and Ni/rGO. (f) Nyquist plots with Ni/Zn1-NC at different current densities. (g) Nyquist plots with Ni/Zn1-NC and Ni/rGO at 50 mA cm-2. (h) Corresponding DRT results of Ni/Zn1-NC and Ni/rGO. (i) Schematic of fast OH- transfer kinetics.