High Pressure High Temperature Synthesis of Highly Boron Doped Diamond Microparticles and Porous Electrodes for Electrochemical Applications

Abstract High pressure high temperature (HPHT) synthesis of crystallographically well-defined boron doped diamond (BDD) microparticles, suitable for electrochemical applications and using the lowest P and T (5.5 GPa and 1200°C) growth conditions to date, is reported. This is aided through the use of a metal (Fe-Ni) carbide forming catalyst and an aluminum diboride (AlB2) boron source. The latter also acts as a nitrogen sequester, to reduce boron-nitrogen charge compensation effects. Raman microscopy and electrochemical measurements on individual microparticles reveal they are doped to metal-like levels, contain negligible sp2 bonded carbon and display a large aqueous solvent window. A HPHT compaction process is used to create macroscopic porous electrodes from the BDD microparticles. Voltammetric analysis of the one-electron reduction of Ru(NH3)63+ is used to identify the fundamental electrochemical response, revealing large capacitive and resistive components to the current-voltage curves, originating from solution trapped within the porous material. Scanning electrochemical cell microscopy is employed to map the local electrochemical activity and porosity at the micron scale. Such electrodes are of interest for applications which require the electrochemical and mechanical robustness properties of BDD, e.g. when operating under high applied potentials/currents, but with the additional benefits of a large, electrochemically accessible, surface area.