High Production of Hydrogen Peroxide from Carbonated Water Through Electrochemical Synthesis by Boron-Doped Diamond

<b>Introduction </b>H₂O₂ is recognized as a powerful oxidizing, bleaching and water treatment agent. It attracts great interest as a green oxidant because it decomposes to water and oxygen. Most of H₂O₂ is nowadays produced by large-scale anthraquinone auto-oxidation (AO) process, but recently efforts have been made to establish alternative synthetic strategies, because AO process employs anthraquinone, known as a carcinogen, in addition to organic solvents. Electrochemical methods offer some advantage, such as direct synthesis in aqueous electrolytes, and oxygen or water as starting reagents. The boron-doped diamond (BDD) electrode is physically and chemically stable, and it is known for its specific electrochemical property to produce OH radical easily¹. For the purpose of analytical applications, we previously demonstrated that BDD electrode can produce H₂O₂ from water oxidation. Here, we focused our attention on the optimization of H₂O₂ production on BDD.<br/><br/><b>Experimental </b>Electrochemical experiments were conducted in a two-chamber cell separated by a Nafion membrane with a three-electrode setup (working electrode and counter electrode: BDD with 1% B/C ratio, reference electrode: Ag/AgCl). H₂O₂ was produced by electrochemical oxidation in chronoamperometry (CA) at 2.5 V. We investigated the effect of electrolyte solution on the H₂O₂ production by comparing the electrolyte salt (phosphate and carbonate), and by pH variation. H₂O₂ was detected by UV-visible spectrophotometry after the reaction with KMnO₄.<br/><br/><b>Results & Discussion </b>CA was conducted for 2 hours in 1 M KHCO₃ and 1 M K₂CO₃aqueous solution respectively as the anolyte, and time production was compared. The maximum production was 16.8 μmol cm^-², whereas it was 118 μmol cm^-² with K₂CO₃. This suggests that CO₃²^- anion was superior to HCO₃^- anion for H₂O₂ formation. Due to their equilibrium between CO₃²^- and HCO₃^-, the ratio of their presence changes with pH. To verify the contribution of CO₃²^- to increased H₂O₂ production, 1 M KHCO₃ solution and 1 M K₂CO₃ solution were mixed to reach the desired pH. CA performed from pH 8 to 12 demonstrated that the amount of H₂O₂ production increased concurrent to the CO₃²^- fraction.<br/>To confirm whether this was due to the effect of the basic solution or carbonate ions, hydrogen peroxide production was conducted either by using 1 M K₂CO₃ or 1 M K₃PO₄ solutions (pH 11.9 and 12.9, respectively), and it was found that only the K₂CO₃ solution produced hydrogen peroxide.<br/>Concerning the mechanism, our laboratory suggested that OH radicals generated by electrochemical oxidation of OH^- on BDD oxidized CO₃²^- to C₂O₆²^-, that reacts with water finally producing hydrogen peroxide by hydrolysis.²^,³ Hydrogen peroxide formation in this work can be considered to have proceeded by a similar reaction pathway.<br/>In addition to the BDD electrode, glassy carbon (GC), fluorinated tin oxide (FTO), and Ni electrodes were used in a 2-hour electrolysis to compare the time dependence of hydrogen peroxide production. The results showed that BDD &gt;&gt; GC &gt; FTO &gt; Ni. Characterization of GC and BDD by SEM after electrolysis demonstrated that the surface of GC electrode was clearly damaged and corroded, while BDD did not reveal any apparent change confirming the high stability of BDD surface. From these results, we demonstrated that BDD can be used profitably to synthetize hydrogen peroxide with a high production rate from aqueous solutions containing carbonate.<br/><br/><b>References</b><br/>P-A. Michaud, et. al., <i>J. Appl. Electrochem.,</i> <b>2003</b>, <i>33</i>, 397.<br/>Irkham, et. al., <i>J. Am. Chem. Soc., </i><b>2020</b>, <i>142</i>, 1518.<br/>Irkham, et. al., <i>Anal. Chem.,</i> <b>2021</b>, <i>93</i>, 2336.