The Role of Sodium Sulfate Supporting Electrolyte in Ammonium Transport and Reduction at Interface Between Platinum Cathode and Solution

Ammonium is a potential hydrogen fuel and can be recovered from high ammonium wastewater via electrodeionization (EDI) process. Since NH₄⁺ is a weak acid ion, sodium sulfate (Na₂SO₄) is used as a supporting electrolyte to improve electrolyte’s conductivity. This manuscript investigated the NH₄⁺ behaviors in high Na₂SO₄ solution through electrochemical analysis methods and molecular dynamics (MD) simulations. Ionic strength increased, leading to a decreased ionic activity, and thus negatively influenced NH₄⁺ transportation with the increasing concentration of Na2_SO₄ solution. Na⁺ competitively occupied the place of electric double layer (EDL) and impeded NH₄⁺ to get closer to the electrode surface. Besides, water molecules played a critical role in determining the net charge density and the potential drop. The experimental tests and theoretical simulation demonstrated that NH₄⁺ reduction (NH_4(aq)⁺ + e^- → 0.5 H_2(g) + NH_3(g)) in the cathode was strengthened in the low concentration range (0-0.25 M Na₂SO₄) but inhibited in the concentration range of 0.5-1.5 mol L^-1.