(Invited) Electrically Conductive Diamond Membrane for Electrochemical Separation Processes

Abstract

Boron-doped diamond has been applied as an outstanding electrode material for electrochemical applications. Using templated growth techniques, a variety of diamond nanostructures like diamond wires and diamond foams can be fabricated [1, 2]. Using quartz or glass fiber paper as growth templates, porous diamond membranes are fabricated [3]. Compared to polymer membranes or membranes based on sp2 carbons, diamond membranes can survive in aggressive media and high temperature conditions. Also, the ion selectivity can be controlled by surface terminations as well as the bias potential of the membrane. Therefore, this kind of membranes is very promising for the electrochemical separation processes such as desalination, pollutants concentration and protein separation. However, both the fabrication and application of diamond membranes are so far reported only in very limited cases. In this paper, we report our recent progress with respect to the templated diamond growth using quartz fiber paper as the growth template [3]. With optimized microwave CVD growth, nanocrystalline diamond film is fully coated onto the quartz fiber as has been confirmed via SEM. The growth conditions in terms of temperature, methane concentration and boron/carbon ratio are investigated in detail. The diamond quality is evaluated via Raman spectroscopy. The potential window of the diamond membrane is determined via cyclic voltammetry in an aqueous NaClO4 solution to be over 3 V. By applying potential to the membrane, reactive ions will accumulate inside the membrane and change its porosity. As a result, the ion flux through the membrane can be well tuned. This ion selectivity can be used to separate charged molecules. As a proof of concept, Rhodamine B, Ni2+ and Cu2+ are separated by the diamond membrane under different biases. Finally, due to the robustness of diamond, the membrane can be cleaned via a high potential (+2 V vs Ag/AgCl) oxidation process in situ. Metal deposits and organic contaminants are proved to be removable by this method. Therefore, this membrane is also suitable for applications where only simple in-situ maintenances are possible. This project has received funding from the European Union's Horizon 2020 Programme under Grant Agreement no. 665085 (DIACAT). [1] Gao F, Wolfer MT, Nebel CE. Highly porous diamond foam as a thin-film micro-supercapacitor material. Carbon. 2014;80(0):833-40. [2] Gao F, Lewes-Malandrakis G, Wolfer MT, Muller-Sebert W, Gentile P, Aradilla D, et al. Diamond-coated silicon wires for supercapacitor applications in ionic liquids. Diamond Relat Mater. 2015;51:1-6. [3] Gao F, Nebel CE. Diamond-Based Supercapacitors: Realization and Properties. ACS Appl Mater Interfaces. 2015.