Coupling scrap iron valorization with hydrogen evolution via a novel iron oxidation-to-ferrate electrochemistry

Abstract

The exponential growth of global scrap iron generation will present significant environmental challenges without sustainability management. Current recycling relies heavily on energy-intensive metallurgical processes that incur substantial carbon emissions and air pollution. Here, zero-carbon conversion of scrap iron into high-value potassium ferrate (K₂FeO₄) by an electrochemical pathway is reported. The core of the transformation lies in an electrochemical oxidation process which we have termed iron oxidation-to-ferrate (IOF). The IOF mechanism may be complicated, but a volcano-shaped relationship between the efficiency of the IOF process and the oxygen evolution reaction (OER) activity is revealed. The performance of the IOF, including faradaic efficiency and yield, is governed by the optimal binding strength of the critical Fe–OO* intermediate, in which the key intermediates (Fe–O, Fe–OO) and the formation of Fe(VI)O₄²⁻ are involved. The composition of the scrap iron significantly impacts the performance; γ-FeO(OH)-rich (lepidocrocite) with higher O-loss energy exhibits superior FE and K₂FeO₄ yield compared to α-FeO(OH)-rich forms (goethite). Furthermore, IOF can be integrated with the hydrogen evolution (HER) in membrane alkaline water electrolysis, which achieves the efficient co-production of green H₂ (requiring only 2.12 V at 0.5 A cm⁻², FE for H₂ ∼100%) and K₂FeO₄ with remarkable stability (<0.5 mV h⁻¹ decay over 100 h), enabling a 1.3-fold increase in total product value compared to conventional alkaline water electrolysis. This electrochemical strategy offers a sustainable, decentralized solution for valorising widely distributed scrap iron, mitigating environmental risks while advancing decarbonization goals.