Bis(formazanate) iron(ii) complexes as cathode materials for one-compartment H2O2 fuel cells

In this paper, we describe the synthesis of two six-coordinate, pseudo(octahedral) bis(formazanate) Fe(ii) complexes based on newly developed redox-active benzothiazole-substituted formazanate ligands. Complexes [FeII(L1)<inf>2</inf>], 1, and [FeII(L2)<inf>2</inf>], 2, were synthesized by reacting 1-(benzothiazol-2-yl)-5-phenyl-3-(pyren-1-yl)formazan (L1H) and 1-(benzothiazol-2-yl)-5-(2-benzoyl-4-chlorophenyl)-3-phenylformazan (L2H), respectively, with appropriate Fe(ii) precursors at room temperature. The molecular structures of both bis(formazanate) iron complexes were established using single-crystal XRD, and other characterization methods were utilized to further characterize these complexes, as well as the newly synthesized ligands. Furthermore, the cyclic voltammetry studies of these compounds are documented, revealing that both complexes can undergo electrochemical reductions to create anionic and dianionic species. These complexes were further employed as cathodes in one-compartment membrane-less H<inf>2</inf>O<inf>2</inf> fuel cells, operating in 0.5 M H<inf>2</inf>O<inf>2</inf>, with nickel foam serving as the anode. The maximum power densities achieved by the designed H<inf>2</inf>O<inf>2</inf> fuel cells for complexes 1 and 2 were 1.88 mW cm−2 and 3.08 mW cm−2, respectively. This study demonstrates the significant potential of formazanate-based compounds in the development of cathode materials for H<inf>2</inf>O<inf>2</inf> fuel cells. © 2025 The Royal Society of Chemistry.