Pulsed‐Current Operation Enhances H ₂ O ₂ Production on a Boron‐Doped Diamond Mesh Anode in a Zero‐Gap PEM Electrolyzer

A niobium (Nb) mesh electrode was coated with boron-doped diamond (BDD) using chemical vapor deposition in a custom-built hot-filament reactor. The BDD-functionalized mesh was tested in a zero-gap electrolysis configuration and evaluated for the anodic formation of H₂O₂ by selective oxidation of water, including the analysis of the effects on Faradaic efficiency towards H₂O₂ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:mfenced><mml:msub><mml:mi>FE</mml:mi> <mml:mrow><mml:msub><mml:mi>H</mml:mi> <mml:mn>2</mml:mn></mml:msub> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> </mml:msub> </mml:mfenced> <mml:annotation>${\left( {{\rm{FE}}_{{\rm{H}}_2 {\rm{O}}_2 } } \right)}$</mml:annotation> </mml:semantics> </mml:math> induced by pulsed electrolysis. A low electrolyte flow rate ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:msub><mml:mover><mml:mi>V</mml:mi> <mml:mo>˙</mml:mo></mml:mover> <mml:mi>anolyte</mml:mi></mml:msub> <mml:annotation>${{\dot {\rm V}}_{{\rm{anolyte}}} }$</mml:annotation> </mml:semantics> </mml:math> ) was found to result in a relatively high concentration of H₂O₂ in single-pass electrolysis experiments. Regarding pulsed electrolysis, we show an optimal ratio of on-time to off-time to obtain the highest concentration of H₂O₂. Off-times that are "too short" result in decreased <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>FE</mml:mi> <mml:mrow><mml:msub><mml:mi>H</mml:mi> <mml:mn>2</mml:mn></mml:msub> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> </mml:msub> <mml:annotation>${{\rm{FE}}_{{\rm{H}}_2 {\rm{O}}_2 } }$</mml:annotation> </mml:semantics> </mml:math> , whereas "too long" off-times dilute the product in the electrolyte stream. Using our electrolyzer setup with an anodic pulse of 2 s with 4 s intervals, and a <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics> <mml:msub><mml:mover><mml:mi>V</mml:mi> <mml:mo>˙</mml:mo></mml:mover> <mml:mi>anolyte</mml:mi></mml:msub> <mml:annotation>${{\dot {\rm V}}_{{\rm{anolyte}}} }$</mml:annotation> </mml:semantics> </mml:math> of 0.75 cm³ min^-1, resulted in the best performance. This adjustment increased the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msub><mml:mi>FE</mml:mi> <mml:mrow><mml:msub><mml:mi>H</mml:mi> <mml:mn>2</mml:mn></mml:msub> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> </mml:msub> <mml:annotation>${{\rm{FE}}_{{\rm{H}}_2 {\rm{O}}_2 } }$</mml:annotation> </mml:semantics> </mml:math> by 70 % compared to constant current electrolysis, at industrially relevant current densities (150 mA cm^-2). Fine tuning of BDD morphology, flow patterns, and anolyte composition might further increase the performance of zero-gap electrolyzers in pulsed operation modes.