The design and fabrication of high volumetric energy density supercapacitors lead to high density materials to be considered as electrode material [1]. So far, most of the studies have been carried out on mono-cationic oxides such as RuO₂, MnO₂ or Fe₃O₄, while polycationic oxides, for which more than a single redox center are potentially involved, have been mostly neglected [2]. As such, we have investigated Ba_0.5Sr_0.5Co_xFe_1-xO_3-δ phases, with 0.75 ≤ x ≤ 0.90, so-called BSCFs, as pseudocapacitive electrode materials. Our motivations towards these perovskite phases are on their high density (close to 6 g.cm^-3) and on both Co and Fe as potentially active redox centers.

BSCFs phases have been prepared by a glycine-nitrate process at 850°C and resulting powders show the same structural arrangement and identical morphology characteristics in the whole series of compositions. Electrochemical performances have been evaluated by cyclic voltamperometry at room temperature, in various aqueous electrolytes: 5M and 1M LiNO₃, 0.5M Li₂SO₄, Na₂SO₄ and K₂SO₄. Prepared BSCFs showed promising pseudocapacitive behavior as electrode material and performances obtained are dependent on the Co/Fe ratio but also on the electrolyte nature. A maximum capacitance of 87 F.g^-1 at 5 mV.s^-1 was obtained for the electrode based on the BSCF material with x = 0.80 in 5M LiNO₃. This specific capacitance corresponds to a volumetric capacitance of about 500 F.cm^-3 (as shown on the figure below) i.e. five times greater than that of activated carbon based electrode. This demonstrates the potential of high density oxides as electrode materials for improving the volumetric energy density of supercapacitors. Cycling ability of this electrode was also evaluated upon 2000 cycles as well as its self-discharge.

Figure: Specific and volumetric capacitance of BSCFs based electrode vs. Co content in 5M LiNO₃ at 5 mV.s^-1.

Charge storage mechanism was investigated by in situ X-Ray Absorption Spectroscopy, performed on the ROCK beamline at SOLEIL Synchrotron (France). Experiments were carried out at both Fe and Co K-edges in the aim to determine the Co and Fe valency at various states of charge. Data analysis demonstrates that both Co and Fe are electrochemically involved in the charge storage mechanism. Their respective contribution to the capacitance depends on BSCF composition and on the scan rate, in accordance with the redox kinetics at Fe vs. Co centers. Thanks to these in situ measurements, an advanced description of the involved charge storage mechanism is provided.

[1] N. Goubard-Bretesché et al: Improving the volumetric energy density of supercapacitors. Electrochim Acta, 206, 458 (2016).

[2] O. Crosnier et al: Polycationic oxides as potential electrode materials for aqueous-based electrochemical capacitors. Submitted to Current Opinion in Electrochemistry, February 2018.