Tuning Redox Transitions via the Inductive Effect in LaNi_1–xFe_xO_3−δ Perovskites for High-Power Asymmetric and Symmetric Pseudocapacitors

We have synthesized a series of LaNi1–xFexO3−δ (LNF; x = 0, 0.15, and 0.55) perovskites along with brownmillerite–SrFeO2.5 (BM-SFO) to utilize Ni’s high electronegativity combined with the inductive effect via iron substitution to make higher voltage anion-based symmetric and asymmetric pseudocapacitors. The materials’ structures were well characterized, and their electrochemical performance was evaluated using cyclic voltammetry (CV) and galvanostatic charging and discharging (GCD) tests in 1 M KOH. We found that substituting Ni for Fe in the B site increased the Ni2+/3+ redox to higher potentials while the Fe3+/4+ redox peaks shifted to lower potentials due to the inductive effect. The LNF materials displayed a surface anion-redox pseudocapacitance mechanism as opposed to a pseudocapacitive intercalation mechanism that enabled a LaNi0.45Fe0.55O3−δ (LNF55) symmetric pseudocapacitor to discharge with high power (250 000 W kg–1) at an energy density of 4.3 Wh kg–1. It had the largest discharge peak (1 V) for any symmetric anion-based pseudocapacitor reported to date. The BM-SFO//LNF55 asymmetric pseudocapacitor had a 1.8 V potential window and the largest discharge voltage at 1.1 V of any anion-based pseudocapacitor reported to our knowledge, yielding an energy density of 19.5 Wh kg–1 at a power density of 394 W kg–1.