Fig. 2: Characteristics and in-situ monitoring of 3Na-PW₁₂ anolyte in charge and discharge process.

a Redox reactions of H-PW₁₂. b Galvanostatic charge and discharge (GCD) curves of H, 1Na, 2Na and 3Na-PW₁₂ as anolyte (15 ml, 0.1 mol l⁻¹) with NaI (15 ml, 1 mol l⁻¹) as catholyte at 25 mA cm⁻². c Raman spectra of H, 1Na, 2Na and 3Na-PW₁₂ (0.1 mol l⁻¹). The unit ‘a. u.’ stands for ‘arbitrary unit’. d CV in-situ monitoring of 3Na-PW₁₂ (0.1 mol l⁻¹) during GCD process at 25 mA cm⁻² with a saturated Ag/AgCl electrode and the scan rate of 100 mV s⁻¹ at room temperature without iR-correction. e In-situ potential and pH monitoring of 3Na-PW₁₂ (0.1 mol l⁻¹) during the GCD process: (i) GCD curves at 25 mA cm⁻², (ii) potentials of 3Na-PW₁₂ anolyte, (iii) pHs and (iv) theoretical HER potentials. Conditions for GCD curves measurements: electrolyte solution flow rate: 90 ml min⁻¹, at room temperature, without iR-correction.