Unique Graphitization of Ultra-Small Nano ZIF-8 and Its Application in High-Performance Fe–N–C Fuel Cell Catalyst

Zeolitic imidazolate framework-8 (ZIF-8) is widely used as precursor for carbon-supported single-metal-atom catalysts (M–N–C), but typically yields amorphous carbon with poor stability in proton-exchange-membrane fuel cells (PEMFCs). Traditional methods would lose active site density while increasing the graphitization degree. Here, this study reports that ultra-small nano ZIF-8 can produce graphene-nanostacks-based Fe–N–C that has improvements on both activity and stability. First, anti-sintering nanoporous ZIF-8 (nano-ZIF8) particles are synthesized with unit size of 8 nm (the smallest so far) using a destruction-reconstruction strategy. In situ microscopy reveals that the volatilization of Zn has a decisive effect on the graphitization degree of ZIF-8 and such process is size-dependent. Nano-ZIF8 particles have completed Zn volatilization before graphitization process beginning, leading to graphene nanostacks. These nanostacks preferentially host edge-type FeN₄ sites, which have doubled intrinsic activity and much improved stability compared with conventional Fe−N−C. In PEMFC, the catalyst achieves an activity of 48.5 mA cm⁻²@0.9V_iR-free, meeting the US Department of Energy 2025 activity target. In addition, quantitative analysis of active site density is conducted in PEMFC for the first time using nitrite stripping voltammetry. The unique graphitization behavior of nano-ZIF8 paves a new avenue for advancing M–N–C catalysts.