# Stacking Order-Mediated Spin-State Modulation in Iron Phthalocyanine Covalent Organic Frameworks Enables Efficient Oxygen Reduction Reaction

## TL;DR

This paper shows how changing the stacking order in iron phthalocyanine COFs can improve their performance in oxygen reduction reactions.

## Contribution

The study introduces a novel method to modulate the spin state of FePc COFs via stacking order to enhance ORR activity.

## Key findings

- AA-stacked FePc COF shows a half-wave potential of 0.856 V vs RHE for ORR.
- Zinc-air batteries with AA-stacked FePc COF cathodes achieve 1.64 V cell voltage and high specific capacity.
- High-spin state of Fe in AA-stacked COF promotes efficient ORR kinetics via spin-selective charge transport.

## Abstract

Covalent organic
frameworks (COFs) such as iron phthalocyanine
(FePc) have been considered as potential electrocatalysts. Herein,
we provide important insights into modulating the intrinsic activity
of FePc COFs for the oxygen reduction reaction (ORR) by adjusting
their stacking configuration. The eclipsed, AA-stacked and the staggered,
AB-stacked FePc COF configurations were obtained via adjusting the
interlayer interaction forces. Electrochemical studies reveal that
the AA-stacked FePc COF exhibits a half-wave potential of 0.856 V
vs RHE, which is 0.195 V higher than that of the AB-stacked FePc COF.
The assembled zinc-air battery, using AA-stacked FePc COF as the cathode,
demonstrates a high cell voltage of 1.64 V vs Zn2+/Zn alongside
with a superior specific capacity of 935.79 mA h–1gZn
–1. The upshift in the valence band center and the high effective magnetic
moment in the eclipsed, AA-stacked FePc COF suggest that the states
are occupied at high energy levels, indicating a high-spin state of
Fe. Density functional theory calculations suggest that the long-range
spin channels aligned with iron columns in the eclipsed, AA-stacked
FePc COF facilitate the spin-selective charge transport through interlayer
band dispersion. The mechanism associated with the high-spin state
of Fe promotes the cleavage of the *OO and *OOH intermediates, accelerating
the ORR kinetics. Our study reveals that the stacking order of FePc
COFs is important for modulation of the charge transfer and electron
spin states, showing how to control the spin electronic characteristics
of COFs through the stacking configuration-dependent interlayer interactions.