Selected silicone membranes are investigated in order to find attractive multifunctional materials as liquid confining flexible agents with good heat resistance as well as low affinity towards the chemical specimens commonly used in dye sensitized solar cells (DSSC). In fact, the leakage and/or volatilization of liquid components inside DSSC remains one of the most critical obstacles in the progress of this technology from laboratory scale to large area applications. Dynamic mechanical spectroscopic, differential scanning calorimetric, and thermogravimetric analyses on dry membranes were performed in order to investigate their mechanical properties and their thermal stability. By a further comparative study between the equilibrium uptake and the adsorption-desorption process of a nitrile based solution, useful indications about the ability of these networks to encapsulate the liquid mixture were inferred. Moreover, a rough evaluation of porosity was also carried out, giving results which are in agreement with surface morphology observed by scanning electron microscopy and atomic force microscopy. In the light of the results obtained by the different experimental techniques the confinement capability of these membranes towards the liquid components inside a DSSC is discussed.
In the last few decades, DSSCs have attracted the interest of many academic and industrial communities as powerful and promising devices for the conversion of solar energy in electricity at low cost and low environmental impact [
Among elastomeric materials currently available, silicone networks represent a special class of polymers which have been widely applied to various industrial and consumer products. Their unique characteristics such as high thermal stability, excellent mechanical and chemical resistance, water-proof, low toxicity, and sealing capability derive from the presence of silicon oxide fragments in the silicone network [
This work focuses on the mechanical properties, thermal stability, and surface morphology of three membranes obtained by commercial room temperature vulcanizing (RTV) silicone rubbers which are expected to display most of the desirable solvent resistance required in the manufacture of these devices. Dynamic mechanical spectroscopy (DMS) measurements were carried out in dry and wet membranes in order to investigate the influence of the liquid components in the temperature region around the glass-rubber transition. The thermal behaviour of dry silicone membranes (SMs) was investigated by performing thermogravimetric (TG) and differential scanning calorimetric (DSC) experiments. The TG results on solvent saturated samples were compared with liquid uptake data and a relationship between the solvent desorption and the confinement characteristics of the selected membranes towards the liquid components was found.
Finally, SMs were analyzed by scanning electron microscopy (SEM) and atomic force microscopy (AFM) in order to investigate their surface morphology which strongly influences the absorption-desorption properties of the membranes. The microscopic analysis revealed different surface morphologies which were in agreement with both the experimental density and porosity values.
The encouraging information gained from the present experimental results and theoretical discussion could be of practical interest in view of the large implications in the development of polymeric materials for highly add-value applications.
Unsupported silicone specimens were supplied by Bluestar Silicones (France) and used as received for the preparation of the silicone rubber membranes. SMs for physical characterization were obtained by mixing the specific RTV-resin silicone casting rubber with the associated catalyst in accordance with the weight ratio specified in Table
Mass density (
Samples (abbreviated names) | RTV silicone resin/catalyst |
|
Sample weight (mgr) | Liquid uptake (%) | Porosity (%) |
|
|
---|---|---|---|---|---|---|---|
Dry | Wet | ||||||
SM689 | 96/4 | 1.48 | 359.4 | 360.3 | 0.25 | 0.45 | 148 |
SMTixo | 95/5 | 1.21 | 220.4 | 222.3 | 0.86 | 1.23 | 147 |
SM400 | 95/5 | 1.19 | 302.1 | 324.8 | 7.50 | 10.32 | 143 |
bDetermined on the dry samples at a driving frequency of 3 Hz.
Wet specimens were prepared by dipping the dry membranes in acetonitrile/methoxypropionitrile solution (AN/MPN; molar ratio 1 : 1) in stopped bottles at room temperature. At regular intervals, the samples were removed from the solvent and repeatedly weighed (electronic balance with an accuracy of ±0.01 mg) until their weight kept constant. A period of
The AN/MPN solution was chosen since nitrile based solvents are commonly used as liquid components in DSSCs due to their high solvation activity for the redox couple and rather high boiling temperature. In the sequel of this work, the acronyms
The AN/MPN uptake of the selected membranes was measured as a function of dipping time in the nitrile based solution and calculated as follows:
Furthermore, a rough evaluation of porosity,
The temperature behaviour of the mechanical loss modulus
Films with a thickness of about 50
A DSC1-
SEM was carried out by a field emission scanning electron microscope Phillips XL 30, with an accelerating voltage of 20.0 kV. The image obtained allowed to investigate the surface morphology of SMs down to 20
The AFM analysis was performed with a Veeco-Innova microscope operating in high amplitude mode. Ultra sharpened Si tips were used (MSNL-10 from Veeco Instruments, with anisotropic geometry, radius of curvature
To illustrate the influence of the AN/MPN mixture on the relaxation dynamics of the studied SMs by crossing the glass transition temperature region, the experimental results for the loss modulus
Temperature behaviours of the modulus
With regard to the lower temperature loss-peak, there is a wide agreement that it arises from the primary
According to recent mechanical and quasielastic neutron scattering (QENS) measurements [
Differently, the almost total invariance observed in the relaxation feature of
Finally, the frequency independence of the third contribution to
The study of thermal stability details the temperature range where the silicone membrane keeps unchanged its specific characteristics without undergoing any degradation processes. This represents a very important piece of information which has to be considered for the use of these materials as liquid confining agents in DSSCs. With this aim, the thermal degradation behaviour of the selected dry membranes was investigated by thermal gravimetric analysis and in Figure
TG (a) and DTG (b) curves of the dry
These features can be seen more clearly in the derivative TG curves (DTG) where the weight loss which is observed as a decrement in TG traces is strongly enhanced and revealed as a large peak. More precisely, DTG curves (see Figure
The liquid uptake data, obtained by (
Thermogravimetric measurements were performed over the temperature interval between 293 K and 473 K, because no degradation processes were observed in this range. Figure
TG (a) and DTG (b) curves in the wet silicone membranes:
In order to get a more quantitative comparison between the desorption behaviours in the different membranes, the more convincing derivative thermogravimetric curves were also analyzed (see Figure
Interestingly, the peaks of the DTG traces are particularly broad and characterized by an asymmetric shape in the low temperature side. This peculiarity suggests that this feature could derive from the overlapping of two contributions arising from the desorption processes at temperature below 403 K. To separate the two contributions and better quantify the weight loss percentage, a fit of the DTG curves was performed by considering two Gaussian functions. The validity of this approach can be assessed from Figure
Table
Values of Gaussian fit parameters of the studied wet membranes. The surface roughness and average pore depth obtained by AFM analysis on dry membranes are also reported.
Samples |
|
FWHH (K) |
|
Surface roughness (nm) | Average depth (nm) | |||
---|---|---|---|---|---|---|---|---|
|
|
|
|
|
| |||
SM689 | 308 | 341 | 313 | 311 | 7.5 | 10.7 | 0.4 | 4.5 |
SMTixo | 308 | 352 | 309 | 325 | 11.4 | 36.3 | 0.5 | 2.9 |
SM 400 | 308 | 355 | 307 | 308 | 14.1 | 59.0 | 1.3 | 3.6 |
The peak temperature and
In order to provide detailed information about the microscopic surface morphology, which affects the membranes performances, SEM and AFM comparative analysis has been carried out. SEM measurements were collected over a selected area of the membrane surface and the two-dimensional images for
SEM images of the studied
With the aim to better understand the membrane surface morphology, an AFM analysis on all the selected specimens was performed and information about the roughness as well as pore depth distribution of membranes was inferred. Figure
AFM images of the studied membranes: (a) 2D images in contrasting shadows and heights; (b) 3D images and depth histograms.
By comparing the AFM images of the three membranes, some considerations can be put forward. The The The
According to Hirose et al. [
It is interesting to observe that all these AFM results well parallel the thermogravimetric data. In fact, with a greater roughness a decreased
The mechanical, calorimetric, thermogravimetric, SEMs and AFM experimental results show that these membranes are characterized by a low level of liquid uptake and desorption, putting forward their promising employment as sealant for the liquid solution inside DSSC. In this study, the
A study of the absorption-desorption process in a set of RTV-silicone membranes has been performed to explore their possible application as sealing materials for liquid solutions which are essential for high electrochemical performances of a DSSC. Thermogravimetric analysis has been applied to evaluate the liquid confining properties of membranes fully saturated by solvent, revealing that the absorption-desorption process is mainly driven by the pore distribution at the surface and inside the bulk membrane. SEM and AFM analysis has disclosed superficial pore distributions which are in agreement with porosity and uptake data, justifying the observed absorption-desorption behaviour. The totality of the experimental results indicate the membrane
This work, combining thermogravimetric measurements, liquid uptake data, and microscopic surface morphology analysis, represents an initial framework to find appropriate materials which could lead to overcome the problem regarding the leakage of volatile liquid components in DSSCs.
None of the authors have a direct financial relation with the trademarks mentioned in this paper that might lead to a conflict of interests for any of them.
The authors are grateful to Dr. R. Pedicini (ITAE-CNR, Messina) and to Dr. F. Ruffino (Physical Department, University of Catania) for their collaboration concerning the SEM and AFM measurements, respectively. This research was supported by “EFOR” Project (Energia da FOnti Rinnovabili, Iniziativa CNR per il Mezzogiorno L. 191/2009 art. 2 comma 44) and by “SAGRO” Project financed by Regione Siciliana (Assessorato Regionale Attività Produttive progetti PO FESR 2007/2013 linea d’intervento 4.1.1.1). Furthermore, the authors acknowledge Programma Operativo Nazionale Ricerca e Competitività 2007–2013—project PON 01_02257 FOTORIDUCO2.