Design of Electrochemical Ozone Production Zero Gap Cells Based on Optimising sp2 Carbon Content in Boron Doped Diamond Electrodes

In this work we elucidate 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. In particular we show, using a zero-gap cell (ZGC) arrangement, how systematically increasing sp2 carbon results in an increase in ozone concentration and current efficiency. sp2 carbon addition is made using nanosecond pulse laser micromachining which converts the BDD to sp2 carbon. Two ZGC geometries are investigated which incorporate a Nafion membrane sandwiched between two BDD electrodes, with through-holes integrated into either the membrane or the BDD. Holes in the BDD are generated using laser micromachining which also converts the hole walls to sp2 carbon. Increasing the number of through-holes (or changing hole geometry) increases the sp2 carbon content of the electrode (from 5-100%). For the planar electrode, the proportion of the surface which is laser micromachined controls the sp2 carbon content (from 4-100%). This approach enables significantly higher sp2 carbon contents 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. The sp2 carbon introduced during laser micromachining is shown to be extremely stable over 20 hr (anode potential ~ 10 V) in contrast to glassy carbon, which corrodes within 10 mins. Whilst both are 100% sp2 bonded carbon, the sp2 carbon in the laser-machined surface is fully amorphous whereas in glassy carbon it contains disorganised graphitic layers. This work also highlights the intriguing stability of amorphous sp2 carbon towards high oxidative potentials.