Photoelectrochemical water splitting
Photoelectrochemical water splitting (PECWS) employing solar light is the final target of this work aimed at investigating photoelectrocatalysts for water oxidation. Solar light is absorbed by water oxidation photoelectrocatalysts to lower the potential demand that is required to allow water oxidation. Also, material properties may change due to the solar energy absorption. For example, their electronic conductivity may become higher under light (photoconductivity) thus facilitating the charge transfer and electron transport efficiencies. In addition, the power and optical spectrum of the absorbed light may have a great influence on the PECWS activity, as higher efficiency provides more photon energy that could be used for water splitting.
In order to carry out PECWS experiments, a special electrochemical reactor was designed and fabricated in LISE (see Figure II.10). According to literature, the absorbance of the MnO2 is expected to occur within the 300 – 600 nm wavelength range[102,103]. Thus, the reactor contains a quartz crystal window allowing the light to enter the reactor and shine the sample without energy loss in the desired wavelength range. As far as the electrodeposition procedure is concerned, the other parameters kept the same values.
Basic mechanism of water oxidation on a clean Pt electrode: preliminary checking experiments.
MnO2 electrodeposition occurs on many different types of substrates through a 2-electron oxidation reaction from Mn2+ cations used as precursors. The mechanism of the formation of MnO2 thin films on a working electrode has been widely discussed, several mechanisms have been proposed, but it is difficult to define one single route because it is influenced by a large variety of factors such as the ion concentration, pH and deposition techniques[104-107]. As a consequence, a deep understanding of the electrodeposition mechanism is important to obtain a highly active and stable MnO2 thin film possessing a very good photoelectrocatalytic activity. For this purpose, a bulk Pt electrode was employed as a model electrode material in the first part of our investigations. It offers a smooth surface, a simple chemical composition, and an excellent electronic conductivity, and more importantly, it can be reused easily after polishing, careful cleaning and rinsing.
Electrochemical cleaning of bare platinum electrodes
The surface conditioning of the substrate has a great influence on the properties of the electrodeposited films, as it involves the mastering (selection or suppression) of adsorbed chemicals, oxidized superficial layer and roughness. Thus, it is necessary to treat and clean the working electrode surface before applying the electrodeposition procedure in order to produce homogeneous films and make sure the obtained results are reproducible.
For an ideally cleaned Pt electrode, the observation of the electroactivity of hydrogen in the negative potential range of a cyclic voltammogram (CV) obtained in a sulfuric acid aqueous solution provides a very convincing test, as shown in Figure III.1. For potential values lower than 0.5 V vs. Ag/Ag2SO4, the cathodic peaks reveal the reduction of adsorbed protons whereas the anodic peaks correspond to the oxidation of adsorbed H2 molecules and the subsequent production of freely diffusing protons. These adsorption phenomena are known to occur only on perfectly clean platinum substrates. The anodic wave appearing in the positive potential region is related to the oxidation of Pt into platinum oxy-hydroxide(s) (Pt(O)x(OH)y) whereas the cathodic peak situated at 0 V vs. Ag/Ag2SO4 reveals the reduction of the abovementioned oxidation product(s) . Moreover, for a perfectly clean platinum electrode, this cyclic voltammogram remains unchanged during consecutive potential cycles. On the other hand, corresponding peak and wave currents may increase in the case of an ongoing electrochemical cleaning.
Water oxidation on bare and clean platinum electrodes : influencing parameters
Water oxidation is a complicate multi-step electrochemical reaction; it is sensitive to many experimental parameters. In particular, those defining the composition of the aqueous electrolytic solution, including pH, identity and concentration of ionic species, are always important factors that affect the performance of the electrode material, as shown with the following experiments.
Influence of the ionic concentration of the aqueous electrolyte.
Figure III.4 shows the linear sweep voltammetry curves of water oxidation on a Pt electrode in an acidic electrolytic solution for different ion concentrations. These experiments were carried out in a home-made electrochemical cell allowing the relative positioning of the three electrodes to be easily reproduced (see section II.7). In these conditions, one can expect ohmic drop effects that do not depend on the electrode positioning. For NaNO3/HNO3 solutions (see Figure III-4a), as the salt concentration increases , the anodic current related to water oxidation and measured at a given potential (1.5 V vs. Ag for example) increases too, but when the salt concentration is high enough, for example when [NaNO3]/[HNO3] ratios achieve 7 and 15, this anodic current is lower. However, for the Na2SO4/H2SO4 solution, this activity increases continuously with the concentration.
Electrodeposition and electrochemical reduction of MnO2 films
In order to get more familiar with the electrodeposition and electrochemical reduction mechanisms of manganese dioxide in our experimental conditions, a Pt electrode was used as a substrate for MnO2 electrodeposition. Figure III.8 is the CV curve of Pt electrode in a MnSO4/H2SO4 aqueous electrolytic solution (pH 2), which has been employed previously in our group. The anodic peak located in the positive potential range located between 0.5 V — 1.0 V vs. MSE is ascribed to the oxidation of Mn2+ into Mn4+, according to the following redox reaction: (III.1).
The following anodic wave appearing after this peak at potentials higher than 1.0 V is the start of the water anodic oxidation wave. In the backward scan, a large cathodic wave extending from about 0.6 V vs. MSE to about -0.7 V vs. MSE reveals the complex reduction mechanism of the electrodeposited MnO2 film. This wave is followed by the electrochemical reduction of protons.
Table of contents :
Chapter I: Introduction
I.2 Oxygen evolving complex (OEC)
I.3 Artificial photosynthesis
I.4 Water splitting
I.5 Oxygen evolution reaction (OER) catalyst
I.6 Manganese dioxides
I.7 Material preparation methods
I.8 Methods for OER evaluation
I.9 Motivation of this thesis
Chapter II: Experimental Methods
II.2 Adopted chemicals
II.3 Preparation of MnO2 thin films
II.3.2 Deposition electrolytes
II.3.3 Electrodeposition techniques
II.3.4 Post heat treatment
II.4 Characterization of structural, morphological and optical properties.
II.4.1 X-ray diffraction spectroscopy
II.4.2 Field emission gun scanning electron microscopy
II.4.3 Atomic force microscopy
II.4.4 Ultra-violet visible spectroscopy
II.4.5 X-ray photoelectron spectroscopy
II.5 Characterisation of electrochemical properties
II.5.1 Cyclic and linear sweep voltammetries
II.5.3 Electrochemical impedance spectroscopy
II.6 Photoelectrochemical water splitting
Chapter III: Basic mechanism of electrodeposition of stable MnO2 thin films on different substrates
III.2 Basic mechanism of water oxidation on a clean Pt electrode: preliminary checking experiments.
III.2.1 Electrochemical cleaning of bare platinum electrodes
III.2.2 Water oxidation on bare and clean platinum electrodes : influencing parameters
III.3 Electrodeposition of MnO2 films on Pt electrode
III.3.1 Electrodeposition and electrochemical reduction of MnO2 films
III.3.2 Influence of electrolyte pH on MnO2 electrodeposition
III.3.3 AFM images of MnO2 films electrodeposited on Pt electrode.
III.3.4 Optimisation of the electrodeposition conditions of MnO2 films on a Pt electrode
III.4 MnO2 electrodeposition on ITO
III.4.1 ITO electrode cleaning
III.4.2 Electrodeposition of MnO2 on ITO electrode
III.4.3 I-V curves of electrodeposited ITO/MnO2 films
III.4.4 Characterization of electrocatalytic activity of ITO/MnO2 films towards OER
III.5 MnO2 electrodeposition on FTO
III.5.1 FTO electrode cleaning
III.5.2 Electrodeposition of MnO2 films on FTO electrode
III.5.3 Nucleation and growth mechanism of MnO2 at different potentials
III.6 Comparison of glass/ITO and glass/FTO electrodes
III.7 MnO2 electrodeposition on a-CNx electrodes
Chapter IV: Heat treatment effect on the OER electrocatalysis of FTO/MnO2 electrodes .
IV.2 Effect of heating at low temperature
IV.2.1 On MnO2 films electrodeposited on FTO at 1.0 V vs. MSE
IV.2.2 On MnO2 films electrodeposited on FTO at 0.6 and 1.2 V vs. MSE
IV.3 Effect of heating at moderate temperature
IV.3.1 On MnO2 films electrodeposited on FTO at 0.6 V vs. MSE
IV.3.2 On MnO2 films electrodeposited on FTO at 1.2 V vs. MSE
IV.4 Effect of heating high temperature on FTO/MnO2 films
IV.5 Solar water splitting on heated FTO/MnO2 films
Chapter V: Influence of cations insertion on the electrocatalytic activity of MnO2 films towards OER
V.2 Preparation of FTO/C-MnO2 films containing different cations (C)
V.3 OER tests on electrodeposited FTO/C-MnO2 films
V.4 Structural and morphological analysis of electrodeposited FTO/C-MnO2 films
V.5 Element analysis of electrodeposited FTO/C-MnO2 films
A.1 Calibration of reference electrode
A.2 Calibration of solar simulator