Efficient Production of Hypochlorite in Water Using Boron-Doped Diamond Electrodes
Authors: Tsuji Yuuri , Fiorani Andrea , Einaga Yasuaki 
Published: December 2024

Hypochlorite (OCl-) has various uses such as wastewater treatment, sterilization, and bleaching. Generally, OCl- (in basic condition, NaOCl) is produced by dissolving Cl2 gas into aqueous NaOH. Cl2 gas has intrinsic problems of carry, storage, and safety use because of its toxicity, and it is desirable to avoid using Cl2 gas. Electrolysis of a NaCl aqueous solution is one of the methods to produce OCl- from the oxidation of the chloride ion without using Cl2 gas. However, there is a problem of selectivity because the reaction potentials between O2 evolution and OCl- production are close. Also, Cl- can cause electrodes corrosion. To overcome these problems encountered in the ClO- production, we focus our attention on boron-doped diamond (BDD) electrodes which have low electrocatalytic activity for water oxidation to O2, combined with high physical and chemical stability.

BDD electrodes are prepared by microwave plasma-assisted chemical vapor deposition (MPCVD), introducing H2 gas for the plasma, in combination with CH3COCH3 and B(OCH3)3 as source of carbon and boron, respectively.

The electrolytic solutions were 0.5 M NaCl at pH 11 (in mixed 0.1 M Na3PO4 and Na2HPO4) and at pH 14 (in 1 M NaOH). Electrolysis was performed for 1 hour by chronoamperometry (CA) using 1% BDD as the working electrode, Pt as the counter electrode, and Ag/AgCl (sat. KCl) as the reference electrode.

To optimize the conditions, electrolysis of NaCl solution was performed in various conditions, such as pH, applied potentials, and electrolyte salts. The solutions at each pH were prepared with phosphate buffer and NaOH.

To detect and quantify the produced OCl-, UV-vis spectra were measured, which show a peak at 292 nm for OCl-. The amount of OCl- produced after the electrolysis was quantified according to the calibration curve of absorption vs OCl- concentration, furthermore the faradaic efficiency was calculated.

To compare the efficiency, selectivity, and stability with other standard electrodes, grassy carbon electrode was also used for the electrolysis.

After 1 hour electrolysis of NaCl solution at pH 14, no absorbance peak was observed at 292 nm and OCl- was not detected. On the other hand, after 1 hour electrolysis of NaCl solution at pH 11, OCl- was detected within all the potential range. Also, after the electrolysis of the solution at pH 9, OCl- was detected. These results showed that NaCl solutions at lower pH were beneficial to produce OCl-.

Fig.1 shows the OCl- production and faradaic efficiency for each electrolysis potential at pH11. As the applied potential was increased, the OCl- production increased accordingly. The faradaic efficiency also increased with the potential reaching around 70% with the electrolysis at 6.0 V for 1 hour.

Reasonably, the remaining 30% of the current is used in the competing reaction, the oxygen evolution reaction. In line with this hypothesis, at pH 14 only oxygen evolution reaction occurred without OCl- production, because water oxidation is favored at high pH.

Though OCl- production occurred with the glassy carbon electrodes, the faradaic efficiency was lower than that with BDD electrodes. When the applied potential was the same 3.0 V, the faradaic efficiency with BDD was higher than that with GC (BDD:28.9%, GC:18.9%), though OCl- production was similar (BDD: 130μmol cm-2 , GC: 134μmol cm-2).

The surface condition of GC changed from smooth to rough, while BDD showed negligible modifications.

Therefore, BDD electrode is better for OCl- production in selectivity and stability than glassy carbon electrode.

Figure 1