ZIF-67-derived Co nanoparticles embedded in N-doped porous carbon composite interconnected by MWCNTs as highly efficient ORR electrocatalysts for a flexible direct formate fuel cell

Non-precious metal materials are considered as the most promising catalysts for oxygen reduction reaction (ORR) due to their high catalytic activity, low cost, superior stability as well as good electrocatalytic selectivity. Here, in situ encapsulation of Co nanoparticles into N-doped porous carbon (Co@NPC) nanocages interconnected by carbonized multi-walled carbon nanotubes (C-MWCNTs) are prepared through direct carbonization of the MWCNTs-based ZIF-67 precursor. The obtained hybrid material (denoted as Co@NPC/C-MWCNTs) displays an excellent ORR performance with a half-wave potential of 0.79 V, effective four-electron transfer and superior formate-tolerance ability. The superior ORR activity of the Co@NPC/C-MWCNTs hybrids is mainly ascribed to the abundant catalytic active sites contributed by the Co@NPC nanocages, the high electrical conductivity and large specific surface area rooting in C-MWCNTs, and synergetic effects between Co@NPC nanocages and CMWCNTs. The membraneless flexible direct formate fuel cell (DFFC) using Co@NPC/C-MWCNTs as the cathode catalyst delivers a peak power density of 4.32 mW cm-2 stably. Notably, the performance of the flexible DFFC can still maintain 90.9% of its initial value after repeated bending for 1000 times. Moreover, two flexible DFFCs connected in series mode can easily power some low-power electronics. The report not only opens a new door to design high-performance non-precious metal ORR catalysts for DFFCs, but also greatly boosts the development of flexible energy conversion and storage devices.