Highly Durable Fuel Cell Electrocatalyst with Low-Loading Pt-Co Nanoparticles Dispersed Over Single-Atom Pt-Co-N-Graphene Nanofiber

The limited durability of Pt electrocatalysis toward cathodic oxygen reduction reaction is a remaining challenge, yet crucial for the development of Proton Exchange Membrane Fuel Cell. Here, we present a robust catalyst consisted of ~ 3 nm PtCo core-shell nanoparticles supported by porous Graphene-Nitrogen nanofiber decorated with single-atomic Pt and Co, termed PtCo@Pt-Co-GNF. The approach uses polymer-fiber containing Polyacrylonitrile and Cobalt-Metal-Organic-Framework as precursors to the Graphene-Nitrogen template where Pt clusters are in-situ reduced for high temperature synthesis. In a fuel cell with a total Pt loading (anode + cathode) of 0.083 mg cm -2 , the new catalyst delivers unprecedented mass activity of 2.48 A mg Pt-1 at 0.9 V iR-free , and a high Pt utilization of 11.9 kW g Pt-1 at 150 kPa abs initially. Operando X-ray absorption spectroscopies (XAS) of PtCo@Pt-Co-GNF show that the structures of Co moieties under working and ex-situ condition are different, and Co moieties are modified during oxygen reduction reaction. Density strengthened Co Pt sites, the catalytic activity. Theoretical simulation further manifests enhanced the carbon substrate lowers the reaction thermodynamic favoring the H 2 O rather than H 2 O 2 over Co-N 4 . result along the underwent 80 o C and 100 % RH and the back pressures of both cathode and anode were set at ambient (101 KPa). After completion of the voltage cycles, a voltage recovery (VR) process was performed: the cell was held at 0.1 V under H 2 -Air for 1 h at 40 o C, 150 % RH, and 150 kPa abs , followed by application of various voltages between 0.1 V and 0.85 V for different time periods 47 . Current-voltage polarization was then recorded to measure changes in catalyst activity and durability. The H 2 -Air cell and H 2 -O 2 cell durability tests were performed on the same MEA with PtCo@Pt-Co-GNF as cathodic catalyst. For Pt mass activity evaluation, the H 2 -O 2 cell was held at 0.9 V for 15 min, and the Pt mass activity was recorded by averaging the last-minute value after applying hydrogen cross-over correction and a short VR process for cathode at 0.9 V with flowing N 2 for 5 min 5 . Each durability evaluation of PtCo@Pt-Co-GNF was conducted 3 times to ensure reproducibility. EIS was recorded at various voltages of 0.9 V, 0.8 V, 0.7 V, 0.6 V, 0.5 V, 0.4 V for both H 2 -O 2 cell and H 2 - Air cell at 1000 Hz. Prior to the experiments, a “break - in” step was applied 47 : first, the cells were heated to 80 o C and held at open circuit voltage (OCV) in H 2 -Air. After this step, a series of voltage cycles between 0.6 and 0.9 V with different RH were conducted. was monochromatic Al Kα radiation (hυ = 1486.6 eV) which was set at Ar+ -ion and electron beam sample neutralization, fixed analyzer transmission mode. The spot size for X-ray beam was set to 100 μm. The spectra of all samples were collected at pass energy of 23.50 eV and electron escape angle of 45 o to the sample plane. The catalyst morphologies and elemental compositions were characterized by high-resolution transmission electron microscopy (HR-TEM) and Electron energy- loss spectroscopy (EELS) using Argonne’s Chromatic Aberration-Corrected TEM (ACAT) operated at 80 KV. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images and energy-dispersive X-ray spectroscopy (EDS) were taken on a FEI Talos operating at 200 KV. Scanning electron microscopy (SEM) images were acquired with a Quanta 400F ESEM microscope. The specific surface area and the micro/meso/macro-porosity of the catalyst were evaluated by the Brunauer-Emmett-Teller (BET) method at liquid nitrogen temperature in a Micromeritics ASAP2020 surface area analyzer. The degree of graphitization of the Co-N-GF nanofiber was determined with an Invia Renishaw