Sodium molybdate – an additive of choice for enhancing the performance of AC/AC electrochemical capacitors in a salt aqueous electrolyte

Abstract

Sodium molybdate (Na₂MoO₄) has been used as an additive to 1 mol L⁻¹ lithium sulfate electrolyte for electrochemical capacitors based on activated carbon (AC) electrodes, in order to reduce the corrosion of stainless steel current collectors. We demonstrate that the MoO₄²⁻ anions improve the overall capacitance owing to pseudofaradaic processes. In a two-electrode cell, capacitance values of 121 F g⁻¹ have been achieved up to 1.6 V using 1 mol L⁻¹ Li₂SO₄ + 0.1 mol L⁻¹ Na₂MoO₄, as compared to 103 F g⁻¹ when 1 mol L⁻¹ Li₂SO₄ is used. Further, by using a two-electrode setup equipped with a reference electrode, we could demonstrate that, at 1.6 V, the positive electrode potential reaches a value of 0.96 V vs. NHE in 1 mol L⁻¹ Li₂SO₄, crossing the thermodynamic potential limit of oxygen evolution (E_ox = 0.846 V vs. NHE), and the pitting potential, E_pit = 0.95 V vs. NHE. By contrast, in 1 mol L⁻¹ Li₂SO₄ + 0.1 mol L⁻¹ Na₂MoO₄, the pseudofaradaic contribution occurring at −0.05 V vs. NHE due to MoO₄²⁻ anions drives the positive electrode to reach only 0.798 V vs. NHE. Hence, the oxidation of the AC and corrosion of the stainless steel current collector at the positive electrode are unlikely in Li₂SO₄ + Na₂MoO₄ when the capacitor operates at 1.6 V. During potentiostatic floating of the capacitor at 1.6 V for 120 hours in Li₂SO₄ + Na₂MoO₄, the capacitance and resistance remain constant at 125 F g⁻¹ and ∼1.0 Ω, respectively, while the resistance increases from 1.4 Ω to 3.1 Ω in Li₂SO₄. Overall, the addition of MoO₄²⁻ anions to Li₂SO₄ aqueous electrolyte allows the capacitance to be enhanced, corrosion of the positive stainless steel current collector to be inhibited and the AC/AC electrochemical capacitor to demonstrate stable performance up to 1.6 V.