Air stable photo-redox active elusive Cr(iii)-tri-dithiolene-radical complexes as a new class of pseudo-supercapacitors with high capacitance: enhancement of energy storage with heavier chalcogenide

Abstract

Chromium, an earth-abundant metal with an accessible redox potential of −0.74 V vs. RHE, emerges as a promising element for the design of high-energy-density supercapacitors. Beyond their intriguing chemistry, energy storage devices employing stable redox-active radicals of main-group elements gain increasing attention for sustainable applications. Herein, the pseudo-supercapacitive properties of two unprecedented, photo-redox active, air-stable Cr(III)-triradical complexes with the general formula [Cr(III)(SS-NHC = E˙⁻)₃] (1, E = S; 2, E = Se; (SS-NHC = E) : S₂-donor dithiolene ligand) are reported. Electrochemical anodes fabricated by coating these complexes onto stainless-steel substrates exhibited excellent supercapacitance performance. Complex 1 delivered a specific capacitance of 145.2 F g⁻¹, while complex 2 showed a markedly higher value of 340.9 F g⁻¹ at 5 mV s⁻¹, centred at E_1/2 = −0.72 V. Cycling stability reached 81.2% for complex 1 and 87.3% for complex 2 after 3000 cycles. The superior activity of complex 2 arises from the substitution of sulfur with selenium in the SS-NHC = E˙⁻ ligand, which enhances redox activity, conductivity, and ion transport. Ragone plot analysis confirmed a higher energy density (14.63 Wh kg⁻¹ at 252 W kg⁻¹) for complex 2 compared to complex 1 (8.76 Wh kg⁻¹ at 211 W kg⁻¹). Mechanistic studies revealed that complex 1 operated via mixed surface- and diffusion-controlled processes, whereas complex 2 predominantly followed diffusion control, enabling efficient charge transport and improved pseudocapacitive behaviour.