DESIGN AND CHARACTERIZATION OF CU2O PHOTOCATHODES FOR PHOTOELECTROCHEMICAL WATER SPLITTING.
Tesi di Dottorato
Data di Pubblicazione:
2018
Citazione:
DESIGN AND CHARACTERIZATION OF CU2O PHOTOCATHODES FOR PHOTOELECTROCHEMICAL WATER SPLITTING / A. Visibile ; tutor: S. Rondinini ; co-tutor: A. Vertova, A. Minguzzi. DIPARTIMENTO DI CHIMICA, 2018 Feb 12. 30. ciclo, Anno Accademico 2017. [10.13130/a-visibile_phd2018-02-12].
Abstract:
The exploitation of renewable energy sources is one of the most addressed aspects for the sustainable development of human activities. Molecular hydrogen can be considered one of the most interesting energy vectors to fulfill humankind’s needs. In this context, photoelectrochemical water splitting (PEC-WS) using solar energy to produce H2 at semiconducting materials is considered one of the most interesting technologies. In the present study, Cu2O was selected as cathode material for its correct band position for hydrogen evolution, together with non–toxic and low-cost starting material.
The research activity was devoted to the development of photocathodes for the PEC-WS and their characterization under working conditions with different techniques. The work was divided in:
I) The role of the underlayer in Cu2O photocathodes for PEC-WS;
II) Characterization of copper oxide based materials under PEC-WS by X-Ray Absorption Spectroscopy (XAS)
III) Characterization of Photoactive Semiconductor Materials by Cavity Micro-Electrodes (C_ME) & Scanning ElectroChemical Microscopy (SECM)
IV) Development of new protective layers for the PEC_WS systems: FeOOH and CuO/Cu2O core-shell systems
V) Application of the Density Function Theory (DFT) to study doping materials and vacancy formation in Cu2O
1) The complete electrode scheme is constituted by (i) a transparent conductive support (usually FTO), (ii) a conductive underlayer of electrodeposited Cu, that is assumed to enhance the electron-hole separation (iii) the semiconductor, (iv) a thin transparent protective overlayer of about 80 nanometers. In the present study, the underlayer of Cu replaces the expensive and toxic Cr/Au underlayer. Therefore, a specific protocol for the preparation of the Cu2O photoconverter was developed and validated, obtaining good results in terms of generated photocurrent (Figure 1). Appropriate modifications of underlayer deposition conditions also lead to large increase in the Cu2O photocurrents, thus denoting that the underlayer has a marked influence on the system performances.
In the final deposition protocol, several parameters were defined and controlled: deposition potential and the relevant current intensity; temperature, pH and stirring conditions; loading (C/cm2) of Cu(0) underlayer and Cu2O active layer. In particular, the thickness of the Cu underlayer was controlled in order to obtain a transparent layer while maintaining high electric conductivity. Transparency of the entire support is an important feature, allowing the use of the electrode in both front and back illumination configurations. Moreover, a complete study of the copper lactate bath was performed with electrochemical methods and X-ray Absorption Spectroscopy (XAS).
The experiments show that the best fit is obtained by a model where four lactate ions act as monodentate ligands, 1:4 (Cu:L), in a distorted tetrahedral geometry.
Notwithstanding the good results in photocurrent, the actual Cu2O photoconverter is able to work at the highest potential only for a few minutes and for this reason the development of a protective overlayer was mandatory as explained later.
2) The short life of Cu2O photoconverter is due to photodegradation. In order to investigate this phenomenon, XAS measurements at Cu-K edge were performed to define photodegradation products, individuate stability potential window and elucidate the possible combined effects of light and potential.
In-situ and operando techniques like X-Ray Absorption Near Edge Structure (XANES), Extended X-Ray Absorption Fine Structure (EXAFS) and Fixed Energy X-Ray Absorption Voltammetry (FEXRAV) allow us to better understand material behavior. Changes in copper oxidation states upon light and/or applied potential were obse
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A. Visibile
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