# Contribution of Mg-templated porosity to activity and durability in Fe–N–C O2 reduction catalysts

## TL;DR

This paper shows how adding MgCl2·6H2O to Fe–N–C catalysts improves their performance for oxygen reduction in fuel cells, but also makes them degrade faster.

## Contribution

The study introduces a new structure–activity design principle for Fe–N–C catalysts based on Mg-templated hierarchical porosity.

## Key findings

- MgCl2·6H2O induces large micropores and small mesopores, enhancing O2 reduction activity.
- Increased porosity correlates strongly with activity (R2 = 0.98).
- Enhanced porosity leads to higher degradation rates due to carbon oxidation and Fe loss.

## Abstract

Atomically dispersed Fe in N-doped carbon (Fe–N–C) catalysts are leading platinum-group-metal-free candidates for the O2 reduction reaction in proton exchange membrane fuel cells (PEMFCs). Zeolitic imidazolate framework (ZIF-8) derived Fe–N–C present the most promising performance; however, they possess a narrow distribution of small micropores, which limits active site accessibility. Here, to induce hierarchical porosity in Fe–N–C, we report a systematic study on MgCl2·6H2O-templated ZIF-8-derived Fe–N–C catalysts for the O2 reduction reaction. MgCl2·6H2O addition induced complete Zn removal, collapse of the ZIF-8 framework, and formation of large micro- and mesopores, with graphene-like structures. N content was markedly reduced, with conversion from pyridinic to pyrrolic N species. Rotating disc electrode tests showed a progressive increase in O2 reduction activity with MgCl2·6H2O, which is strongly correlated (R2 = 0.98) to the formation of large micropores and small mesopores (1–4 nm). This introduces an indirect structure–activity design principle for Fe–N–Cs. The enhanced Fe–N–C porosity also leads to increased degradation rates under accelerated stress test conditions, which we attributed to the oxidation of disordered carbon domains and active Fe loss. This study highlights a key trade-off between porosity-driven O2 reduction activity and durability in Fe–N–C catalysts.

The formation of Mg-templated pores within 1–4 nm strongly correlates with activity and degradation of Fe–N–C catalysts for the acidic O2 reduction reaction.