Electrochemistry in Lead Dioxide Electrodeposition

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Experimental Design and Procedures

This chapter presents the research design and experimental procedures in our research project. Raw materials and preparation procedures of Ti/SnO2-Sb/PbO2 samples are described in detail. Characterisation techniques such as X-ray diffraction (XRD) and scanning electron microscope (SEM) have been used to determine the structural and morphologic properties of the prepared samples. High-performance liquid chromatography (HPLC) combined with total oxygen carbon (TOC) have been employed to perform degradation ability analysis.

Chemicals and Materials

Raw titanium (Ti) sheets were cut from a high-purity titanium plate at a thickness of 1 mm. The size and working dimension of the Ti sheets will be further illustrated in subsequent chapters. The chemicals used for substrate preparation, electrodeposition, and degradation tests were of analytical grade, which are listed below: tin chloride (SnCl4, Sigma-Aldrich), polyethylene glycol (PEG, Sigma-Aldrich), antimony oxide (Sb2O3, Sigma-Aldrich), isopropanol (C3H8O, Sigma-Aldrich), methanesulfonic acid (MSA, CH3SO3H, Sigma-Aldrich), lead methanesulfonate (Pb(CH3SO3)2, Sigma-Aldrich), hydrochloric acid (HCl, Sigma-Aldrich), sulphuric acid (H2SO4, J.T. Baker), oxalic acid (H2C2O4, J.T. Baker), acetone (CH3COCH3, J.T. Baker), lead nitrate (Pb(NO3)2, Sigma-Aldrich), nitric acid (HNO3, J.T. Baker), acetonitrile (C2H3N, Sigma-Aldrich) and trifluoroacetic acid (TFA, Sigma-Aldrich), acetonitrile (C2H3N, Sigma-Aldrich), maleic acid (C4H4O4, Sigma-Aldrich), catechol (C6H6O2, ≥99%), 4-hydroxybenzoic (C7H6O3, Sigma-Aldrich), fumaric acid (C4H4O4, Sigma-Aldrich), hydroquinone (C6H6O2, Sigma-Aldrich) and benzoquinone (C6H4O2, Sigma-Aldrich). All solutions were prepared using ultrapure water acquired from a Milli-Q (Merck Millipore, USA) water purification unit. The electrical resistivity of the ultrapure water was ≥18 MΩ cm.
Also, deionised water (DI water) obtained from an Elix (Merck Millipore, USA) purification unit was used for sample washing.

Preparation of Ti/ SnO2-Sb/PbO2 Electrodes

Pre-treatments of titanium sheets

The fresh Ti sheets were initially treated by grinding and polishing with sandpapers down to 2000 grit. Surface oxides were removed in this process, giving rise to a smooth and shiny surface free of scratches. The polished Ti sheets were then carefully washed with ultrapure water and acetone respectively to clean the residual surface organics and debris. Afterwards, these Ti sheets were soaked in boiling oxalic acid solution (10 wt. %) for one hour. This chemical etching process completely removed the surface oxides and resulted in a rough and matte surface. After being washed with acetone several times, the pre-treatment of Ti sheets was completed, and the as-prepared Ti sheets were referred to as Ti substrates hereafter.

Thermal deposition of SnO2-Sb interlayer

In our sample preparation, the thermally deposited SnO2-Sb layer serves as an interlayer between the Ti substrate and the outside PbO2 layer. The precursor solution for interlayer preparation was made up of 14 mL C3H8O, 10 mL concentrated HCl, 0.12 g Sb2O3, and 1.6 mL SnCl4. Firstly, the Ti substrate was soaked into the precursor solution for 10 s. Then, the soaked Ti substrate was heated at 100 oC for 10 min in a muffle furnace, followed by heating at 500 oC for another 10 min. The same dipping and heating processes were repeated six times. Then, the treated Ti sheets were annealed at 500 oC for one hour. After being cooled in the air, a SnO2Sb interlayer was successfully prepared on the Ti substrate, and hereafter the thermally treated sheets were referred to as Ti/SnO2-Sb substrate. As proposed by previous reports, the presence of the SnO2Sb interlayer can give rise to prolonged service life and increased adhesion to titanium-based PbO2 electrodes [1, 127].
The crystal structure information of the Ti/SnO2-Sb substrate is presented in Fig. 3.1, which reveals multiple broad peaks that originated from the deposited SnO2-Sb interlayer. It should be noted that the X-ray scans failed to find the presence of antinomy, probably due to its limited amount in the interlayer. Moreover, the surface microstructure of Ti/SnO2-Sb substrate was recorded. As depicted in Fig. 3.2, the Ti substrate was covered by the thermally deposited SnO2-Sb interlayer. Fig. 3.2 (a) shows the deposited SnO2-Sb interlayer is at a mean thickness of ~2 um. Besides, Fig. 3.2 (b) clearly shows a cracked-mud like surface morphology of the Ti/SnO2-Sb substrate. We should note this observation matches the description of the SnO2-Sb layer obtained by thermal decomposition in previous reports [9, 48].

Electrodeposition of lead dioxide

PbO2 was anodically deposited on the Ti/SnO2-Sb substrate. In most cases, galvanostatic deposition was carried out in the preparation of PbO2. Apart from that, pulse reverse current (PRC) deposition was also employed as a novel PbO2 electrodeposition technique, which will be described in detail in Chapter 7.
In the PbO2 deposition, a typical bipolar electrolytic beaker cell was employed with two paralleled electrodes immersing in electrolyte solutions. The Ti/Sb-SnO2 substrate was used as the anode, while a copper sheet with the double working dimension served as the cathode. The working dimension of the electrodes will be described in detail in subsequent chapters. The distance between the two electrodes was kept constant at 15 mm. A PTFE magnetic stirrer was placed in electrolyte solutions to trigger the bath agitation at 300 rpm. This electrolytic beaker cell was placed in a thermostatic bath at a designated temperature. The electrodeposition of PbO2 coatings was carried out in a fume hood to avoid the potential hazard of vapour leakage. The detailed electrodeposition parameters will be illustrated in individual chapters, including deposition time, current density, acid strength, and Pb2+ concentration. To better clarify the sample preparation procedures, a schematic of the preparation pathway of Ti/SnO2-Sb/ PbO2 samples is presented in Fig. 3.3.
After the electrodeposition process, the as-prepared Ti/SnO2-Sb/PbO2 sample was immediately ultrasonically rinsed with ethanol several times. Then the prepared samples were dried and placed in a desiccator pumped to a vacuum level of ≤ 1 KPa using a rotary vane pump. This is to prevent the PbO2 coating from reacting in a redox reaction with water or the remaining acidic solution on the sample surface. As mentioned in section 2.3.1, lead is thermodynamically metastable in its high oxidation state. Without a positively charged potential, PbO2 could easily transfer into Pb(II) in acidic or even neutral solutions [2].
Fig. 3.4 records the microstructure information deriving from one of the prepared Ti/SnO2-Sb/PbO2 samples. In both Figs. 3.4 (a) and 3.4 (b), we can see a homogenous PbO2 layer was uniformly deposited on the substrate without pores and cracks. This is in a good agreement with extensive reports which electrodeposited uniform PbO2 coatings on the Ti/SnO2-Sb substrate [1, 9, 74]. It should be noted that the electrodeposition process is controlled by a number of parameters. The structural and morphologic properties of the PbO2 coatings can be markedly different case to case, depending on the particular deposition parameters/conditions.

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Electrodeposition temperature

In order to investigate the appropriate electrodeposition temperature, a series of experiments have been performed at different deposition temperatures, as shown in Fig. 3.5. The temperature range was chosen between 35 oC and 65 oC based on the previous works [23, 127]. With other parameters remaining identical, the increasing deposition temperature from 35 oC to 65 oC evidently improved the coating quality. At 35 oC, PbO2 failed to adequately cover the substrate, revealing a poorly deposited coating made up of a number of isolated “columns” (Fig. 3.5 (a)). High magnification reveals the “columns” were composed of numerous rather fine crystals, Fig. 3.5 (b). As the deposition temperature increased to 50 oC, the obtained PbO2 coating almost covered substrate, while large holes and cracks were observed (Fig.3.5 (c)). However, at an elevated temperature of 65 oC, the substrate was entirely covered by the PbO2 coating with significantly enhanced uniformity, Fig. 3.5 (d). The finding in Fig. 3.5 proves that the deposition process is facilitated by increasing temperature (≤65oC), giving rise to improved deposition quality of prepared PbO2 coatings.
In reference to previous research in the literature [23, 128], an increased PbO2 deposition temperature leads to improved coating uniformity because of the enhanced nucleation and grain growth processes [23]. At a high temperature, the nucleation process benefits from the faster Pb2+ diffusion and the less positive onset potential of PbO2 deposition. Meanwhile, the grain growth process is facilitated due to the accelerated solid state diffusion [129, 130]. Therefore, the increased temperature prefers a more uniform PbO2 coating as revealed in Fig. 3.5 (d). Nevertheless, an excessively high deposition temperature can damage PbO2 coatings [127]. When referring to the study by Zhao et al. [127], the PbO2 electrodeposition at 85 oC gave rise to non-uniform PbO2 coatings with a number of cracks and pits, causing a deteriorated long-term stability of PbO2 electrodes. A feasible explanation is that water solution evaporates considerably at an extremely high temperature, which severely inhibits the nucleation and 51 growth process of PbO2 and damages the coating quality. From the above discussion, our research thesis chooses a relatively high electrodeposition temperature of 65 oC. At this temperature, the prepared PbO2 coatings are presumed to have better deposition quality (Fig. 3.4), while avoiding the possible detrimental effects brought by a rather high deposition temperature as stated in Ref. [127].

Physicochemical Characterisation

Characterisation of microstructure and crystal structure

The microstructure information (e.g., surface and cross-section morphologies) of the prepared samples was characterised using a scanning electron microscope (SEM, Quanta 200F, FEI, USA). The crystal size observed in the coating surface was compared using the evaluation method based on the ASTM Standards [131, 132]. The thickness of PbO2 coatings was estimated from the cross-sectional images using Image J.
The crystal structure of these samples was characterised utilising an X-ray diffractometer (XRD, D2 Phaser, Bruker AXS, Germany) with Cu Kα radiation (λ=0.15406 nm). The scanning rate in XRD tests was 0.02° per second in the 2θ range from 20° to 80°. The grain size of PbO2 samples was estimated using the Scherrer formula from the peak broadening at half maximum intensity (FWHM) of the strongest diffraction peak, which is proposed in previous literature [1, 9, 28].
Cyclic voltammetry (CV) tests were carried out to investigate the electrochemical behaviours of the prepared samples. A CHI604D (Chenhua, China) electrochemical workstation was employed as the power supply. A typical three-electrode beak cell configuration was used in the CV measurements. A platinum sheet was used as the counter electrode, and a saturated calomel electrode (SCE) was used as the reference electrode. Different working electrodes were used in different chapters with the same working dimension of 10 mm×10 mm. Further details of the used working electrodes will be illustrated in related chapters.
In most cases, the sulphuric acid solution (0.5 M) was used as the electrolyte in CV measurements. Also, different electrolyte types (e.g., methanesulfonate electrolyte, nitric electrolyte) were used to study the initial electrodeposition process of PbO2, which will be described in detail later. The CV measurements were conducted at room temperature. Specific technique parameters (e.g., scan rate, electrolyte, and scan range) of the CV measurements will be illustrated in related chapters. It should be noted that all the given potentials in the thesis are referred to SCE.

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Stability test

Chemical stability is a core concern in PbO2 preparation and application. Excellent long-term stability of PbO2 electrodes prevents the potential hazard from the dissolution of toxic lead in the electrolysis. It should be noted that the estimated service life of a PbO2 electrode is usually far beyond practical industry requirements when referring to previous literature [2]. Although possible chemical corrosion still threatens the practical uses of PbO2 electrodes, previous works have suggested that only several millimetres of thickness of PbO2 anodes can support a continuous operation for years in industry plants [133].
In general, the stability of electrodes can be estimated based on a simulation test performed in rather extreme conditions (e.g., strong acidic medium, huge applied current density) – the accelerated life test [134]. Referring to the empirical equation proposed by Hine et al. [134], the estimated service life of electrodes can be calculated by:
SL ~
Where SL stands for the estimated service life, j stands for the applied current density, and n is a given constant in the range of 1.4~2.0. Compared with the extreme condition applied in accelerated service life tests, the practical working circumstance is considerably milder.
In our cases, the accelerated service life of the Ti/SnO2-Sb/PbO2 samples was investigated using a typical bipolar electrolytic beaker cell. A titanium sheet (working dimension 30 mm× 20 mm) was used as the cathode, and the sample electrode (working dimension 30 mm × 10 54 served as the anode. The applied current density was at 500 mA/cm2. The test was conducted in 1.0 M H2SO4 solutions at room temperature. Bath agitation was triggered by a PTFE magnetic stirrer at 400 rpm, and the inter-electrode distance was kept constant at 15 mm. Applied cell potential was recorded periodically throughout the test. The time point was recorded when the cell potential surged up and surpassed 10.0 V, which is referred to as the accelerated serve lifetime [135].

Electrochemical Degradation of Organics

Electrochemical oxidation of benzoic acid

The electrochemical oxidation of benzoic acid (BA) was performed in a bipolar beaker cell. The prepared Ti/SnO2-Sb/PbO2 electrode with a working area of 30 mm×10 mm was used as the working anode, and a Ti sheet with a working area of 30 mm×20 mm was used as the cathode. The distance between the two electrodes was constant at 15 mm. A magnetic PTFE stirrer in the electrolyte was used to agitate aqueous solutions at 400 rpm. The galvanotactic electrolysis of BA was conducted with a current density of 150 mA/cm2. The electrolysis temperature was kept constant at 25 oC. Further details of the electrolyte composition and applied current density will be given in related chapters.

HPLC analysis

High-performance liquid chromatography (HPLC) is an advanced technique in analytical chemistry. This column chromatographic technique has been extensively accepted for qualitative and quantitative examination of each component dissolved in the aqueous solution, achieving a rather high accuracy at the part-per-million (ppm) level. In the HPLC measurement, the pressurised flow (known as the “mobile phase”) carrying sample solution is pumped through a column containing the adsorbent material (referred to as the “stationary phase”). During this process, each individual component in the sample mixture is separated corresponding to its different retention time in the column, which is a result of its different reaction rate with the adsorbent material.
A typical HPLC system contains three essential parts: a sampler, pumps, and a detector. The sampler transports the sample mixture into the mobile phase stream. A different composition gradient can be generated in such process. The pumps provide the driving force to transfer the flow throughout the system. The detector produces a digital signal deriving from each component passing through the column. Fig. 3.6 displays a flow scheme of a typical HPLC system with a brief description of each unit.

Table of Contents
Abstract
Acknowledgements 
Table of Contents
List of Figures
List of tables 
1. Introduction 
1.1 Research Background and Motivation
1.2 Objectives
1.3 Thesis Framework
2. Literature review 
2.1 Background
2.2 Basics of Lead Dioxide
2.3 Electrochemistry in Lead Dioxide Electrodeposition
2.4 Electrodeposition of Lead Dioxide
2.5 Modifications of Lead Dioxide
2.6 Applications in Wastewater Treatment
2.7 Other Applications
2.8 Motivations of research
3. Experimental Design and Procedures 
3.1 Chemicals and Materials
3.2 Preparation of Ti/ SnO2-Sb/PbO2 Electrodes
3.3 Physicochemical Characterisation
3.4 Electrochemical Degradation of Organics
4. Effects of Electrolyte Composition on the PbO2 Electrosynthesis Process from a Methanesulfonate Electrolytic Bath 
4.1 Introduction
4.3. Results and Discussion
4.4 Conclusion
5. Effects of Deposition Time and Current Density on the PbO2 Electrosynthesis Process a Methanesulfonate Bath 
5.1 Introduction
5.2 Experimental Procedures
5.3 Results and Discussion
5.4. Conclusion
6. Structural and Electrochemical Characterisation of PEG-modified PbO2 Electrodes Synthesised from a Methanesulfonate Bath 
6.1 Introduction
6.2 Experimental Procedures
6.3 Results and Discussion
6.4 Conclusion
7. Characterisation of PbO2 Electrode and its Electrochemical Degradation against Benzoic Acid 
7.1 Introduction
7.2 Experimental Procedures
7.3 Results and Discussion
7.4. Conclusion .
8. Summary and Recommendations 
8.1 Summary of Conclusions
8.2 Recommendations for Future Work
References
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Characterisation and Application of Lead Dioxide Electrodeposited from Methanesulfonate Electrolytes

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