Bifunctional activity of NiCo on carbon nanotube catalyst for oxygen reduction and evolution on anion exchange membrane-unitized regenerative fuel cells (AEM-URFCs)

Cobalt or nickel/cobalt on carbon nanotube (Co@CNT, NiCo@CNT) catalysts were synthesized by heat treatment of the metal-organic framework, ZIF-67. The X-ray patterns of the synthesized ZIF-67 and the catalysts confirmed their crystalline structures. The field-emission scanning electron microscopy (FESEM) images of ZIF-67 after pyrolysis showed that the dodecahedral crystalline structure of ZIF-67 was retained, and the organic ligands were converted into CNTs and graphitic carbon in an H2 gas environment. The transmission electron microscopy images of the synthesized catalysts confirm the presence of the Ni or Co atoms at the tips of the carbon nanotubes. The NiCo@CNT catalyst exhibited a higher surface area and larger pore volume than those of Co@CNT. X-ray photoelectron spectroscopy confirmed the presence of pyridinic nitrogen, which forms Co-Nx and Ni-Nx, active groups. In anion exchange membrane-unitized regenerative fuel cells (AEM-URFC), NiCo@CNT as a bifunctional oxygen electrode (BOE) catalyst showed a higher current density and lower Tafel slope compared to that of Co@CNT. The cell with the NiCo@CNT catalyst in BOE exhibits the highest current density of 130.0 mA cm−2 at 0.3 V (fuel cell mode) and 32.5 mA cm−2 at 2.0 V (water electrolyzer mode) among all catalysts. This cell exhibited a maximum 38.90 % round trip efficiency, which was comparable to that of the benchmark noble metal (Ir-black) catalyst (38.40 %).