Rosestolato, Davide

Institutional Research Information System University of Ferrara

Chemical and Physical Characterization of Electrocatalysts based on Iridium Oxide and prepared by Sol-Gel and Physical Vapor Deposition

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 Abstract 
 
The request for electrode materials that exhibit satisfactory catalytic activity and high 
stability under particular conditions of potential and current density, has stimulated the 
synthesis of new devices, characterized by improved performances. Among these, electrodes 
based on RuO2 and IrO2 have found wide application in the field of DSA anodes® 
(Dimensionally Stable Anodes), thanks to their features of good electrocatalytic activity and 
high resistance to corrosion. An increase of the stability of materials can be obtained through 
the utilization of mixtures of oxides: along with the electrocatalytic oxide, other ones (TiO2, 
SnO2, etc.) are added, which have the double purpose of increasing the corrosion resistance of 
the electrode, while diluting the main oxide to minimize the production costs of the device. 
As described in Chapters 1 & 2, the present research aims to the improvement of 
iridium-based electrodes, to be used for chlorine and oxygen evolution reactions, and to the 
development of “better” materials, which should be characterized by reduced costs of 
production and/or higher catalytic activity and stability. Their main uses concerns the chlor-
alkali industry, where they are exploited as anodes for the chlorine production, and several 
electro-metallurgy processes, where the anodic process is represented by the oxygen 
evolution; a short review on these and others uses has been presented in Chapter 3. A not 
conventional preparative was chosen: the reactive sputtering, which belongs to the family of 
Physical Vapor Depositions (PVDs). The industrial preparative of DSA anodes® typically 
exploits the most common sol-gel technique, for reasons related to costs and operational 
simplicity; however, the sputtering synthesis may offer advantages, in terms of activity and 
reactivity of the final device, and, therefore, a specific investigation has been planned. Both 
preparation methods have been deeply discussed in Chapter 4. 
Chapter 5 deals with the theoretical details on which nuclear techniques are based, 
because of the fact that their utilization for the characterization of this kind of materials is 
uncommon; a quite detailed review of theoretical notions about RBS and ERDA has been 
reported. 
This work of thesis has been focused on films constituted by mixed oxides of iridium 
and titanium, prepared through the two different methods mentioned above (sol-gel and 
reactive sputtering), at 350 and 450 °C. Details on the preparation have been resumed in 
Chapter 6. 
Structural and morphological analyses (AFM, RBS, ERDA, XRD, SEM-EDX, XPS) have 
been coupled to electrochemical studies (CV, PC, EIS), aiming to obtain a thorough 
description of the studied materials. All experimental details are included in Chapter 7. The 
 relationship between the structural and catalytic properties represents a key-step in the 
development of a new catalyst, and the information available in the literature is often unclear 
or contradictory, as in the case of the reaction mechanisms proposed for the chlorine 
evolution at noble-metal-oxide-based materials. 
The ex-situ and on-site characterizations via XRD, RBS, AFM, CV and ERDA are 
discussed in Chapter 8. Preliminary studies on the behavior of the mixture, carried out via 
XRD, showed that for quantities exceeding 20 %mol in iridium, the crystallization of both 
components occurs in the rutile form (with reference to TiO2, this phase is not kinetically 
favored under these experimental conditions) with partial formation of a solid solution, and 
the growth preferentially follows specific crystallographic directions (e.g., the (110)). RBS, 
AFM, ERDA and CV analyses, carried out on samples prepared by sputtering, evidenced that 
films are more compact than those obtained by sol-gel, and constituted by columnar structures 
that are the source of a “meso/nano”-porosity; the latter can strongly affect the mass transport 
of reagents in the innermost portions of the electrode. Samples prepared by sol-gel, 
conversely, are characterized by a strongly wrinkled surface, which leads to an increase of the 
real active area of two orders of magnitude. The different structure of the materials greatly 
influences the stability of the devices: one of the causes of deactivation of DSA electrodes® 
consists on the passivation of the titanium metal at the interlayer: this event is plausibly more 
probable when the material exhibits cracks and macro-pores, which allow the permeation of 
the electrolyte solution, leading to device decommissioning. Moreover, a compactness of the 
deposit can also increase the resistance to dissolution of the electrocatalytic material, 
prolonging greatly the service life. 
The mechanism of chlorine evolution reaction (ChlER) has been also investigated, and 
a detailed discussion can be found in Chapter 9. The investigation required a careful analysis 
using impedance spectroscopy and also the application of a mathematical model proposed by 
Conway in the early 90s. This approach has led to identify a sequence of stages in agreement 
with the so-called Volmer-Heyrovsky mechanism. The whole picture of experimental data 
becomes interpretable by assuming the involvement of a rather high coverage by reaction 
intermediates, which allows explaining the abnormalities occurred in terms of Tafel slopes 
and reaction orders. The impedance spectroscopy analysis pointed out an important 
contribution associated to the porosity for the samples prepared at 350 °C. Polarization 
experiments carried out at different temperatures provided information on the reactivity of the 
active site and on how this reactivity depends on both temperature and method of preparation. 
The high values of coverage involved in the process (around 0.7 - 0.8) have been confirmed 
by ex-situ measurements carried out on ad hoc samples, after heavy treatments of polarization 
 under ChlER conditions: RBS and XPS confirmed the presence of chlorine (radical anion) on 
sample surfaces (Chapter 10), and even in the bulk in the case of films fired at the lowest 
temperature. Moreover, Chapter 10 included XRD tests on sample after strong polarizations 
under chlorine evolution and accelerated service life tests. Finally, Chapter 11 collects the 
conclusions drawn on the basis of the whole picture of experimental results collected. 


ROSESTOLATO, Davide

Archivio istituzionale della ricerca - Università di Ferrara

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Chemical and Physical Characterization of Electrocatalysts based on Iridium Oxide and prepared by Sol-Gel and Physical Vapor Deposition

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