Lattice-strained metal–organic-framework arrays for bifunctional oxygen electrocatalysis

Oxygen electrocatalysis is central to technologies such as fuel cells and electrolysers, but challenges remain due to the lack of effective earth-abundant electrocatalysts and insufficient understanding of catalytic mechanisms. Here we demonstrate that robust bifunctional oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activity can be achieved by inducing lattice strain in noble-metal-free metal–organic frameworks (MOFs). Lattice-strained NiFe MOFs exhibit mass activities of 500 A gmetal−1 at a half-wave potential of 0.83 V for the ORR and 2,000 A gmetal−1 at an overpotential of 0.30 V for the OER, which are 50–100 times that of pristine NiFe metal–organic frameworks. The catalyst maintains ~97% of its initial activity after 200 h of continuous ORR/OER reaction at a high current density of 100–200 mA cm−2. Using operando synchrotron spectroscopies, we observed a key superoxide *OOH intermediate emerging on Ni4+ sites during both the ORR and OER processes, which suggests a four-electron mechanistic pathway.