Enhanced electrochemical performance of ammonium phosphomolybdate for symmetric and asymmetric supercapacitors in an H ₂ SO ₄ /KI redox additive electrolyte

Ammonium phosphomolybdate (APOMO) was successfully synthesized via a microwave-assisted method. To optimize morphology and performance, two capping agents, oxalic acid (OA) and citric acid (CA), were used, resulting in OA-APOMO and CA-APOMO variants, respectively. Among these, CA-APOMO exhibited a uniform cubic-like structure, as confirmed by SEM, contributing to improved charge storage through enhanced faradaic and non-faradaic interactions at the electrode-electrolyte interface. Structural and chemical characterizations (XRD, Raman spectroscopy, FTIR spectroscopy, and XPS) confirmed the purity, molecular vibrations, and oxidation states of CA-APOMO. Electrochemical evaluations were conducted in both 2 M H₂SO₄ and redox-enhanced 2 M H₂SO₄ + 0.025 M KI electrolytes. The CA-APOMO electrode exhibited specific capacities of 399 mAh g^-1 (in 2 M H₂SO₄) and 500 mAh g^-1 (in 2 M H₂SO₄ + 0.025 M KI) at 3 A g^-1 in a half-cell setup. The full-cell symmetric and asymmetric devices fabricated, the asymmetric device was constructed using CA-APOMO as the positive electrode and MWCNT as the negative electrode. In the symmetric device, CA-APOMO//CA-APOMO achieved a specific capacity, energy density, and power density of 113.11 mAh g^-1, 43.82 Wh kg^-1, and 385.70 W kg^-1 in 2 M H₂SO₄, which improved to 167.77 mAh g^-1, 55.78 Wh kg¹, and 322.46 W kg^-1 with the redox additive and increased the specific capacity of 27.33% compared to without additive electrolyte. For the asymmetric device CA-APOMO//MWCNT at a higher current density (3 A g^-1), the device delivered 84.16 mAh g^-1 and 59.65 Wh kg^-1 in 2 M H₂SO₄ and 121.66 mAh g^-1 and 74.09 Wh kg^-1 with the redox additive, indicating good rate capability, with corresponding power densities of 2126.13 W kg^-1 and 1826.87 W kg^-1, respectively. Density functional theory (DFT) analysis showed that APOMO possesses a 2.11 eV direct band gap at the Γ point, with an electron effective mass of 0.16823m_o and hole effective mass of -0.15951m_o, supporting its electronic conductivity and electrochemical activity. This work highlights the design of effective electrodes through morphology tuning and the use of redox-active electrolytes for advanced supercapacitor applications.