Reductive Leaching of Iron and Magnesium out of Magnesioferrite from Victorian Brown Coal Fly Ash

This paper for the first time reports the reductive leaching of an iron-rich brown coal fly ash composed principally of a chemically resilient magnesioferrite (MgFe₂O₄) matrix. The simultaneous mobilization of Fe and Mg out of magnesiorferrite here aims to produce abundant Mg²⁺ that can convert into high-purity MgCO₃, through a mineral carbonation process for CO₂ capture, and abundant Fe²⁺/Fe³⁺ that can convert into value-added high-purity Fe-rich compounds such as FeOOH. Sulfur-bearing compounds, including Na₂S, Na₂S₂O₄, and FeS₂, were used as reductants, on the basis of the fact that S is one of the inherent elements in fly ash that has a Fe-reductive capability. Synchrotron-based X-ray absorption near-edge spectroscopy was used to quantitatively determine the speciation of Fe (Fe²⁺ or Fe³⁺) and S (SO₄^2– or S^2–) in the leachate produced. Leaching with Na₂S₂O₄ and FeS₂ was found to produce the most Fe²⁺ (more than 70% of total eluted Fe) in the leachate at 200 °C. Increasing the leaching temperature is beneficial in increasing the reactivity of FeS₂, leading to a greater amount of Fe²⁺ produced at 200 °C, whereas Na₂S₂O₄ reached its best performance at 100 °C. This is due to a quicker dissolution of Na₂S₂O₄ into the leachate to promote the reduction of inherent Fe³⁺-bearing ash matrix in the liquid phase, whereas FeS₂ mainly remains as a solid, which is less reactive. None of the mechanisms involved affected the total Mg²⁺ cations eluted. Increasing the molar ratio of S to Fe from 0.125 to 0.5 completely reduced all aqueous Fe³⁺ present to Fe²⁺ for both reductants. Concurrent with this was an incremental change in total aqueous Fe amount when Na₂S₂O₄ was used. No significant increase in total Fe eluted was observed when FeS₂ was used. The fate of S differs for both cases, with S mostly mobilized in the leachate when Na₂S₂O₄ was used while predominantly being in the solid leaching residue in the case of FeS₂. In light of this, the use of FeS₂ is more promising on a large scale, although it is less active

Polypyridyl Iron Complex as a Hole-Transporting Material for Formamidinium Lead Bromide Perovskite Solar Cells

An efficient hole-transporting material (HTM) is indispensable for high-performing perovskite solar cells (PSCs), which have recently emerged as a breakthrough photovoltaic technology. Here, we demonstrate the capacity of the transition metal complex (6,6′-bis­(1,1-di­(pyridin-2-yl)­ethyl)-2,2′-bipyridine)-iron­(II/III) trifluoromethanesulfonate ([Fe­(bpyPY4)]­(OTf)_2+<i>x</i>) to act as an additive-free, solution-processable HTM in PSCs based on the formamidinium lead bromide absorber. State-of-the-art physical methods have been employed to characterize [Fe­(bpyPY4)]­(OTf)_2+<i>x</i> and, in particular, to demonstrate its significantly higher conductivity compared to that of the conventional HTM spiro-OMeTAD. A maximum power conversion efficiency of 2.2% was obtained for a device employing [Fe­(bpyPY4)]­(OTf)_2+<i>x</i>, which is the first evidence of the applicability as a HTM in a PSC of a solid material in which conductivity is provided by a redox transformation of a transition metal