Enhanced Electrochemical Ozone Production via sp2 Carbon Content Optimization in Boron Doped Diamond Electrodes using Laser Micromachining

The role and bonding arrangement of deliberately added sp2 carbon in maximising the current efficiency, output and longevity of boron doped diamond (BDD) electrodes for electrochemical dissolved ozone generation is elucidated. We show, using a zero-gap cell (ZGC) arrangement, how systematically increasing sp2 carbon results in increased ozone concentration and current efficiency. sp2 carbon addition is made using nanosecond pulse laser micromachining which converts BDD to sp2 carbon. Two ZGC geometries are investigated which both incorporate a Nafion membrane sandwiched between two BDD electrodes. Through-holes (perforations) are integrated into either the membrane or the BDD, the latter using laser micromachining which also converts the hole walls to sp2 carbon. Increasing the number of through-holes (or changing hole geometry), in the perforated BDD, increases the sp2 carbon content of the electrode (from 5 to 100 %). For the perforated Nafion membrane in contact with a planar BDD electrode, patterned laser micromachining of the BDD surface is used to control the sp2 carbon content of the electrode (from 4 to 100 %). For both electrodes, sp2 carbon content is significantly higher than is possible using diamond growth. sp2 carbon contents >40 % and >60 % for the planar and perforated BDD electrodes, respectively, are found to be particularly effective, allowing electrode designs to be proposed for optimised ZGC ozone generation. Notably, the sp2 carbon introduced via laser micromachining is shown to be extremely stable over 20 h (anode potential ∼ 10 V) in contrast to glassy carbon, which corrodes within 10 min. Whilst both are 100 % sp2 carbon, the laser-machined surface (after ozonolysis) is amorphous whereas the glassy carbon contains disorganized graphitic layers. This work also highlights the intriguing stability of amorphous sp2 carbon towards high oxidative potentials.

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MiniLab S060

2.4. Electrode processing

After laser processing, all electrodes were immersed in concentrated sulfuric acid (H2SO4) with potassium nitrate (KNO3) at 0.75 g/mL for 30 min at ∼200 °C, followed by rinsing with ultrapure water (Milli-Q, resistivity <18.2 MΩ cm−1) before repeating the first heating step in just H2SO4 for 30 min. This process removes any weakly-attached sp2 carbon introduced during laser machining and leaves behind a very robust form of sp2 carbon [25]. Electrical contacts were added by sputtering a thin layer of Ti (10 nm) followed by a second layer of Au (400 nm) (Moorfield MiniLab 060 Platform Sputter system) onto either the contact tabs of the perforated electrodes or the back, lapped face of the patterned planar electrodes. The contact was annealed at 400 °C for 5 h in order to create an ohmic electrical connection.

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