Comparison of microcrystalline and ultrananocrystalline boron doped diamond anodes: Influence on perfluorooctanoic acid electrolysis

Abstract

• Microcrystalline (MCD) and ultrananocrystalline (UNCD) BDD anodes were compared. • MCD exhibited 50-fold higher PFOA removal rate than UNCD. • PFOA (0.24 mmol L −1 ) removal using MCD was achieved at only 1.4 kWh m −3 . • MCD contained more carbon sp 3 , less H-terminated carbon and less boron than UNCD. • Electrochemical function relates to crystal & chemical properties of diamond films. This work aims to study the effect of the distinctive chemical and structural surface features of boron doped diamond (BDD) anodes on their electrochemical performance for perfluorooctanoic acid (PFOA) degradation. Commercial BDD anodes were compared: (i) a microcrystalline (MCD) coating on silicon; and (ii) an ultrananocrystalline (UNCD) coating on niobium. MCD gave rise to the complete PFOA (0.24 mmol L −1 ) degradation in 4 h, at any applied current density in the range 1–5 mA cm −2 . On the contrary, only 21% PFOA removal was achieved when using UNCD at 5 mA cm −2 under comparable experimental conditions. Similarly, the total organic carbon (TOC) was reduced by 89% using MCD, whereas only 13% TOC decrease was obtained by UNCD. In order to explain the dissimilar electrochemical activities, the morphological and chemical characterization of the electrode materials was developed by means of FESEM microscopy, XPS and Raman spectroscopy. The UNCD anode surface showed characteristic ultrananocrystalline grain size (2–25 nm), higher boron doping and greater content of H-terminated carbon, whereas the MCD anode was less conductive but contained higher sp 3 carbon on the anode surface. Overall, the MCD electrode features allowed more efficient PFOA electrolysis than the UNCD anode. As a result of their distinctive performance, the energy needed for the maximum PFOA degradation (after 4 h) using MCD anode was only 1.4 kWh m −3 , while the estimated energy consumption for the UNCD anode would be 37-fold higher. It is concluded that the use of the MCD anode involves considerable energy costs savings.