Assessment of Boron Doped Diamond Electrode Quality and Application to In Situ Modification of Local pH by Water Electrolysis

Summary

A protocol is described for the characterization of the key electrochemical parameters of a boron doped diamond (BDD) electrode and subsequent application for in situ pH generation experiments.

Abstract

Boron doped diamond (BDD) electrodes have shown considerable promise as an electrode material where many of their reported properties such as extended solvent window, low background currents, corrosion resistance, etc., arise from the catalytically inert nature of the surface. However, if during the growth process, non-diamond-carbon (NDC) becomes incorporated into the electrode matrix, the electrochemical properties will change as the surface becomes more catalytically active. As such it is important that the electrochemist is aware of the quality and resulting key electrochemical properties of the BDD electrode prior to use. This paper describes a series of characterization steps, including Raman microscopy, capacitance, solvent window and redox electrochemistry, to ascertain whether the BDD electrode contains negligible NDC i.e. negligible sp² carbon. One application is highlighted which takes advantage of the catalytically inert and corrosion resistant nature of an NDC-free surface i.e. stable and quantifiable local proton and hydroxide production due to water electrolysis at a BDD electrode. An approach to measuring the local pH change induced by water electrolysis using iridium oxide coated BDD electrodes is also described in detail.

Introduction

Choice of electrode material is of great importance when conducting any electroanalytical study. In recent years, sp³ carbon (diamond) doped with sufficient boron to render the material "metal-like" has become a popular choice for a wide range of electroanalytical applications due to its excellent electrochemical (and thermal and mechanical) properties^1,2,3. These include corrosion resistance under extreme solution, temperature and pressure conditions⁴ ultra-wide solvent windows, low background currents, and reduced fouling, in comparison to other commonly used electrode materials^5-7,3. However, increasing non-diamon....

Protocol

NOTE: BDD electrodes are most commonly grown using chemical vapor deposition techniques, attached to a growth substrate. They leave the growth chamber H-terminated (hydrophobic). If grown thick enough the BDD can be removed from the substrate and is termed freestanding. The freestanding BDD growth surface is often polished to significantly reduce surface roughness. Cleaning the BDD in acid results in an oxygen (O)-terminated surface.

1. Acid Cleaning BDD

1. Place a beaker of concentrated sulfuric acid (H₂SO₄; ~ 2 ml or deep enough to cover the diamond) on a hot plate at RT and insert the BDD.
2. Add potassium nitrate (KNO

Results

Raman spectra and electrochemical characteristics were obtained for representative BDD macrodisc electrodes with different dopant densities, and both significant and negligible levels of NDC, Figures 1 and 2. Figures 1A and B show typical Raman data for NDC-containing thin film microcrystalline BDD and larger grain freestanding BDD, doped above the metallic threshold, respectively. The presence of NDC is identifiable by t.......

Discussion

Starting with an O-terminated surface is advocated because the H-terminated surface is electrochemically unstable, especially at high anodic potentials^7,40,41. Changing surface termination can affect the electron transfer kinetics of inner sphere couples, such as water electrolysis (used herein to change the local solution pH). Furthermore, if the BDD contains significant NDC at grain boundaries it is also possible that upon application of the extreme anodic/cathodic potentials advocated in this article for pH.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Pt Wire			Counter Electrode
Saturated Calomel Electrode	IJ Cambria Scientific Ltd.	2056	Reference Electrode (alternatively use Ag|AgCl)
BDD Electrode			Working Electrode
Iridium Tetrachloride	VWR International Ltd	12184.01
Hydrogen Peroxide	Sigma-Aldrich	H1009	(30% w/w) Corrosive
Oxalic Acid	Sigma-Aldrich	241172	Harmful, Irritant
Anhydrous Potassium Chloride	Sigma-Aldrich	451029
Sulphuric Acid	VWR International Ltd	102765G	(98%) Corrosive
Potassium Nitrate	Sigma-Aldrich	221295
Hexaamine Ruthenium Chloride	Strem Chemicals Inc.	44-0620	Irritant
Perchloric Acid	Sigma-Aldrich	311421	Oxidising, Corrosive
2-Propanol	Sigma-Aldrich	24137	Flammable
Nitric Acid	Sigma-Aldrich	695033	Oxidising, Corrosive
Sputter/ Evapourator			With Ti & Au targets
Raman			514.5 nm laser
Annealing Oven			Capable of 400 °C
Ag paste	Sigma-Aldrich	735825	or other conductive paint
Potentiostat
pH Buffer solutions	Sigma-Aldrich	38740-38752	Fixanal buffer concentrates
Phenolphthalein Indicator	VWR International Ltd	210893Q
Methyl Red Indicator	Sigma-Aldrich	32654