Modification of boron-doped diamond electrodes for application in electrochemistry

Nanoparticles have gained extensive research interests since they were successfully synthesized in the last century. Their unique physical and chemical properties that are distinct from the bulk forms are versatile and have been widely applied in various fields. Electrochemistry is a common technique that yields both high accuracy and efficiency in chemical analysis (electroanalysis).
This thesis investigates nanoparticles, both carbon-containing and metal, both pure metal and alloys, as electrode modifiers for boron-doped diamond electrodes (BDDEs) in electrocatalysis and electroanalysis. Specifically, we focused on how the surface elemental components and morphologies can affect the performance of BDDEs in electrocatalysis and electroanalysis. The experiments were carried out using a range of electrochemical voltammetry methods to understand the kinetics of different reaction systems, accompanied with physical instrumental analysis, including electron microscopy, UV-Visible spectroscopy, and X-ray photoelectron spectroscopy to analyse the composition and structure of nanoparticle modifiers.
The use of nanoparticle modified BDDEs in catalysing the hydrogen evolution reaction (HER), which is one of the most critical reactions in fuel chemistry to produce clean and sustainable hydrogen gas as a renewable energy source was explored and used as the most important example reaction throughout this whole thesis to judge the quality of an electrode modifier as a heterogeneous catalyst. I explored the electrodeposition of three morphologies (polyhedral, dendritic, and spherical) of Ag nanoparticles and two morphologies (dendritic and spherical) of AgAu nanoalloys using various electrochemical conditions spanning deposition potential, deposition method, and deposition time. The reaction kinetics were analysed, and the best candidates were selected based on Tafel analysis and the current densities in voltametric measurements. Notably, spherical Ag and spherical AgAu modified BDDEs showed the best performance in the HER electrocatalysis, which are potentially less expensive yet efficient alternatives for Pt/Pd containing catalysts.
Colloidal Ag and Au nanoparticles and AgAu nanoalloys were also synthesised using the in-situ chemical reduction method. To avoid natural aggregation, carboxylated graphene nanoflakes (cx-GNF) were used as stabilisers. cx-GNF was able to prevent the aggregation of nanoparticles through a non-covalent interaction between the metal nanoparticles and the graphenic surface. Compared with electrodeposited, unstabilised colloidal, and sodium citrate (a widely used stabiliser for nanoparticle synthesis) stabilized colloidal nano-species, cx-GNF stabilized colloidal nanoparticles showed outstanding structural stability of over 14 days and enhanced performance in electrocatalysis of the HER.
The high surface-to-volume ratio of metal nanoparticles makes them naturally suitable for aiding electrochemical sensing. The sensing performance of AuNPs of different synthesis methods including electrodeposition, unstabilised in-situ reduction, and cx-GNF stabilized in-situ reduction were examined and compared. The effect of pH was also investigated for selected systems since the solvation of slightly acidic cx-GNF in alkaline solutions will cause the loss of metal nanoparticles. The optimal pH values were selected for each individual system based on the current signals. AuGNF showed the highest detection signal for various sensing systems, and we were able to detect As (III) ions with a LOD of 0.1 nM, 4-nonylphenol with a LOD of 0.3 μM, glucose with a LOD of 1 mM, and hydroquinone with a LOD of 0.5 nM. Notably, simultaneous detection of hydroquinone and catechol was achieved successfully with LOD of catechol 50 times lower than hydroquinone.
The effect of surface composition, morphology, and stabilizers of nanoparticle modifiers were further investigated in more complex tri-metal alloy systems. PdAuCu tri-metal nanoalloys were synthesized using electrochemical deposition of AuCu nanoalloy, followed by galvanic replacement reaction between Pd and Cu, and in-situ chemical reduction with cx-GNF stabilisation. The electrocatalysis of various fuel cell reactions, including the HER, oxygen reduction reaction (ORR), and ethanol oxidation reaction (EOR) were studied. As a result, the galvanic replacement reaction of electrodeposited AuCu was able to fully covert Cu to Pd, and gives a similar performance in every reaction compared with colloidal tri-metal PdAuCu stabilized by cx-GNF, but with minimized usage of expensive Pd.

Keywords: metal nanoparticles
carbon nanoparticles
electrochemistry
fuel cell chemistry
boron doped diamond electrode
Advisor: Foord, JS