Polyvinyl alcohol and hydroquinone-based electrolyte coupled with mesoporous carbon nitride for high energy density supercapacitors

Abstract

The pursuit of next-generation supercapacitors with high energy density requires not only the design of robust electrode material, but also the engineering of the electrolyte. Herein, we explore a redox-mediated gel polymer electrolyte (GPE) using polyvinyl alcohol (PVA) and hydroquinone (HQ)/benzoquinone (BQ) couple to enhance the supercapacitive performance of mesoporous graphitic carbon nitride (mg-CN). The mg-CN nanosheet was synthesised via a carboxymethyl cellulose-assisted templating approach. The structural, spectroscopic and textural properties studies revealed the formation of mg-CN with abundant defect sites and sp² carbon domains as well as a hierarchical porous structure with a surface area of 139 m² g⁻¹. Owing to the reversible redox reaction of the HQ/BQ couple leading to pseudocapacitance, mg-CN delivered a specific capacitance of 481 F g⁻¹ at a scan rate of 5 mV s⁻¹ in the 1 M H₂SO₄ + 0.01 HQ-based electrolyte, which is more than twice that of pristine H₂SO₄ (198 F g⁻¹). Evidently, the capacitive-diffusive current contribution study indicated that 42.71% of the specific capacitance obtained in the HQ-based electrolyte is contributed by a diffusion-controlled process, as compared to the 19.81% obtained in the 1 M H₂SO₄ electrolyte. A symmetrical supercapacitor device fabricated using the redox-mediated GPE (1 M H₂SO₄ + PVA + 0.01 M HQ, PVA/HQ) exhibited a remarkable energy density of 47.42 Wh kg⁻¹ at a power density of 6500 W kg⁻¹, along with superior cycling stability. Owing to the improvement in the ionic conductivity of the gel offered by the charge carriers originating from the HQ (1 M H₂SO₄ + PVA: 371.44 vs. PVA/HQ: 401.81 mS cm⁻¹), the device delivers an energy density of 47.42 Wh kg⁻¹ when the power density is increased to 6500 W kg⁻¹, an indication that the PVA/HQ electrolyte can be employed for the fabrication of a high-rate supercapacitor.