Direct work function tuning via boron-acceptor substitution on an iron phthalocyanine ligand for a boosted oxygen reduction reaction in brine-seawater batteries

Abstract

Highly conductive concentrated brine seawater can be reused as an electrolyte in aluminium–air seawater batteries used in on-board marine applications; however, the severe chloride corrosion in brine seawater often causes Pt-based oxygen reduction reaction (ORR) electrocatalysts at the cathode to degrade rapidly. Fe macrocyclic molecules, such as those in iron phthalocyanine (FePc), are reported to exhibit low affinity to chloride adsorption. On the other hand, the strongly bound O* and OOH* intermediates in the FeN₄ active sites and the localized electron orbitals are well-known to restrict their ORR performance. In this study, by combining a room-temperature plasma-assisted material modification strategy with density functional theory (DFT) calculations and thermodynamic modelling, we successfully substituted boron as an acceptor on the FePc ligand to induce significant electron delocalization in the macrocyclic FePc structure, thereby reducing the ORR energy barrier of FePc. In an alkaline saline environment (0.1 M KOH + 1 M NaCl), B-FePc displays superior catalytic activity (0.932 V vs. RHE) at the half-wave potential with moderate stability, which surpassed the performance of a commercial 20 wt% Pt/Vulcan electrocatalyst and most of the recently reported electrocatalysts. When used as an air cathode catalyst in a brine seawater-based Al–air battery (1 M KOH + 1 M NaCl + seawater), B-FePc as a cathode catalyst exhibited a peak power density of 71.0 mW cm⁻² and an exceptional stability following its discharging for 60 h at 20 mA cm⁻² through a mechanical recharging process.