1. UNIVERSITÀ DEGLI STUDI DI PADOVA
DIPARTIMENTO DI INGEGNERIA INDUSTRIALE
CORSO DI LAUREA IN INGEGNERIA CHIMICA E DEI MATERIALI
Tesi di Laurea in
Ingegneria Chimica e dei Materiali
(Laurea triennale DM 270/04 – indirizzo Chimica)
SYNTHESIS AND CHARACTERIZATION OF
MESOPOROUS CARBON
AS SUPPORT FOR
METALLIC NANOPARTICLE CATALYSTS
Relatore: Prof. Armando Gennaro
Laureanda: Gisela Auxiliadora Cepeda Arque
ANNO ACCADEMICO 2014 – 2015
3. 2
Riassunto
La necessità di trovare fonti alternative di energia è uno dei problemi più grandi che
affronta la società odierna. Tra le proposte alternative per la produzione d’energia
notevole importanza rivestono le celle a combustibile. Tuttavia, le tecnologie
sviluppate fino ad ora non sono abbastanza mature per poter soddisfare i fabbisogni
della società.
Lo scopo di questo studio è la sintesi di carboni mesoporosi dopati con eteroatomi, in
particolare N, e con nanoparticelle di Pt, per poi analizzare il prodotto finale sia
morfologicamente che elettrochimicamente nella reazione di riduzione dell’ossigeno
(ORR), tutto questo con il fine di considerare il possibile uso di questi elettrodi nelle
celle a combustibile.
La sintesi è stata condotta per pirolisi, in atmosfera inerte, di un precursore organico
contenente anche l’eteroatomo oggetto di interesse. In particolare si è scelto il
carbazolo, che contiene, appunto, anche N, oltre a C e H. Per ottenere un prodotto
mesoporoso, si è utilizzata silice mesoporosa come materiale templante, ottenendo,
dopo pirolisi e rimozione della silice con trattamento con NaOH, un carbone
mesoporoso (MC) di elevata area superficiale e con un elevato volume dei pori.
Tali MC sono stati utilizzati anche come supporto per nanoparticelle di Pt, depositate
per riduzione chimica di un composto di Pt, al fine di verificare l’attività catalitica
dello stesso MC e del MC contenente Pt.
I MC così preparati sono stati testati nella ORR in ambiente acido, osservando che
nel CV del MC senza Pt si possono rilevare due segnali: uno legato alla presenza di
gruppi chinone/idrochinone, il secondo legato proprio al processo di ORR.
Invece per il MC contenente anche Pt, la presenza del segnale dovuto al gruppo
chinone/idrochinone non c’è. Non solo, si può anche osservare che la presenza del Pt
sposta il valore del potenziale di picco di riduzione di O2 a valori più positivi.
Inoltre, si è visto che il meccanismo del processo di riduzione di O2 è diverso. Nel
MC senza Pt la riduzione avviene con il trasferimento di due elettroni, mentre nel
MC con Pt vengono coinvolti 4 elettroni.
4. 3
Abstract
The need of finding alternative energy sources is one of the biggest problems of our
society nowadays. Among the alternatives proposed for the production of alternative
energy, Fuel Cell (FC) are very important. Yet, technologies developed so far are not
enough to satisfy the demands of society.
The objective of this study is the synthesis of a mesoporous carbon (MC) doped with
heteroatoms, in particular with N, and with nanoparticles of Pt, in order to analyze
the final product both morphologically and electrochemically in the oxygen
reduction reaction (ORR). All this with the intention of considering the utilization of
such electrodes in the FCs.
The synthesis was carried out by pyrolysis, in an inert atmosphere, using an organic
precursor containing the heteroatom of interest. The choice for the synthesis was
carbazole, an organic compound that not only contains C and H atoms but also N. In
order to obtain a meosporous product a mesoporous silica template was used and,
after pyrolysis and removal of the silica with NaOH treatment, an MC of high
surface area and a high pore volume was obtained.
Such MCs were also used as support for Pt nanoparticles, deposited by chemical
reduction of a Pt compound, in order to analyze the catalytic activity of the MC and
the MC containing Pt.
The MCs synthesized in this way were tested for the ORR in acid ambient, observing
from the CV of the MC without Pt that two main signals can be retrieved: one due to
the presence of quinone/hydroquinone functional groups and the other due to the
oxygen reduction.
On the contrary, the CV for the MC containing Pt showed no peak related to the
quinone/hydroquinone group. It can also be observed that the presence of Pt shifts
the peak potential towards more positive values.
In particular, it can be remarked that electron transfer mechanism is different. In the
MC without Pt, reduction takes place through the 2 electron step pathway, while in
the MC containing Pt, 4 electrons are involved.
5. 4
Index
CHAPTER 1 – Introduction 6
1.1 Fuel Cells.. 6
1.1.1 Fuel Cells Overview ... 6
1.1.2 Thermodynamics Consideration for a Fuel Cell... 8
1.2 Anodic and Cathodic Reactions . 10
1.3 Mesoporous Carbon.... 12
1.3.1 Mesoporous Carbon 12
1.3.2 Mesoporous Carbon Doped with Hetroatoms... 13
CHAPTER 2 – Techniques and Instruments. 16
2.1 Linear Sweep and Cyclic Voltammetry 16
2.2 Rotating Disk Electrode Method 17
2.3 Scanning Electron Microscopy 19
2.4 Transmission Electron Microscopy 20
2.5 X-ray Photoemission Spectroscopy 21
CHAPTER 3 – Experimental … 22
3.1 Chemicals… 22
3.2 Preparation of the compound… 22
3.3 Electrochemical tests… 24
3.4 Characterization tests… .24
CHAPTER 4- Results and Discussion 26
4.1 Chemical and Morphological characterization 26
4.2 Analysis of the Capacitance of the Compound 30
4.3 Study of the Oxygen Reduction Reaction 31
CONCLUSSIONS . 35
ABREVIATIONS 36
NOMENCLATURE 37
BIBLIOGRAPHY 38
7. 6
CHAPTER 1
INTRODUCTION
1.1 Fuel Cells
1.1.1 Fuell cells overview
In the last few years, scientific community has been trying to develop alternative
methods for energy production. The motives for which research has been focused in
this area have diverse origins. However, those that can be considered as the most
important are undoubtedly the finite resources of fossil fuels and their constant
raising prices and environmental pollution. Combustion of fossil fuel releases
dangerous gases for the environment, that not only are harmful for human health, but
also contribute to the Greenhouse Effect.
Among the several substitutes proposed for production of alternative energy, fuel
cells can be found. They are essentially a type of battery, which main purpose is to
convert chemical energy into electrical energy. Theoretically, fuel cells are capable
of distributing energy as long as the reactants involved in the chemical process are
available, which means that ideally they can operate indefinitely if the reactants
feeding is continuous. However, in reality, degradation and malfunctioning of
components may limit the working life of these batteries.(1)
The possibility of converting chemical energy into electrical energy was first
demonstrated at the beginning of the 19th century by Humphry Davy. Subsequently,
research related to what would be the first fuel cell proceeded with the studies of
scientist Christian Friedrich Schönbein in 1838. Notwithstanding, the creation of the
first fuel cell is attributed to Sir William Groove in 1839.
Groove conducted a series of experiments with what he defined a gas voltaic battery,
which allowed him to prove that it was possible to produce electric current from an
electrochemical reaction involving hydrogen and oxygen in presence of a platinum
8. 7
catalyst. Yet, the term fuel cell was not used until 1889, when scientists Charles
Langer and Ludwig Mond started their research of using coal emissions as fuel for
the fuel cells.
In 1932, the engineering Professor Francis Bacon of Cambridge University modified
the device created by Langer and Mond in order to design the first Alkalyne Fuel
Cell (AFC), but it wasn’t until 1959 that Bacon demonstrated a practical 5 kW fuel
cell system.(2)
Since then many types of Fuel Cells have been developed(3)
:
Figure 1.1 Classification of the Fuel Cells
There are several kinds of fuel cells, and each operates a bit differently. But in
general terms, hydrogen molecules enter a fuel cell at the anode where an
electrochemical reaction strips them of their electrons, producing protons that
dissolve in the electrolyte. The negatively charged electrons provide the current
through wires to do work.
9. 8
Oxygen enters the fuel cell at the cathode and, in some cell types (like the one
illustrated above), it there combines with electrons returning from the electrical
circuit and hydrogen ions that have traveled through the electrolyte from the anode.
In other cell types the oxygen picks up electrons and then travels through the
electrolyte to the anode, where it combines with hydrogen ions.
The electrolyte plays a key role. It must permit only the appropriate ions to pass
between the anode and cathode or vice versa. If free electrons or other substances
could travel through the electrolyte, they would disrupt the electrochemical reaction.
Whether they combine at anode or cathode, together hydrogen and oxygen form
water, which drains from the cell. As long as a fuel cell is supplied with hydrogen
and oxygen, it will generate electricity.(4)
Figure 1.2 Structure of a FC(5)
1.1.2 Thermodynamic considerations in a fuel cell(6)
The overall reaction that takes place is:
The total voltage of the cell, ΔE, is given by the difference of the two semi-reactions
while the equilibrium potential of each semi-reaction is given by Nernst equation.
10. 9
[ ]
[ ]
The term , for each semi-reaction, is the standard electrode potential and [red]
([ox]) is the activity of reducing (oxidizing) species. Consequently, the voltage of the
cell in absence of current is given by the equation:
[ ] [ ]
[ ] [ ]
Variation of Gibbs free energy, ΔG, can be defined for the same reaction as:
[ ] [ ]
[ ] [ ]
,
where is the standard Gibbs free energy change.
The cell voltage and the Gibbs free energy can be related by means of the equation
.
As said before, each electrochemical reaction is characterized by the Nernst equation.
Nevertheless, all the previous thermodynamics considerations were under the
hypothesis of no current flow. When a current flows, that is, electrical work is
performed, a deviation of the potential from the equilibrium occurs. The deviation
from the equilibrium value is the overpotential η.
For a redox reaction at one electrode, the current density j is given- in the simplest
case- by the Butler-Vollmer equation:
[ ( ) ( )]
where jo is the exchange current density and α is the electron transfer coefficient.
This equation holds when the charge transfer dominates the reaction at small values
of j and η.
11. 10
Other limiting factors, like mass transport hindrance, are present in real systems and
are described in terms of potential losses.
1.2 Anodic and cathodic reactions
The two semi-reaction aforementioned are the anodic reaction of hydrogen oxidation,
while the cathodic reaction is usually oxygen reduction.
Hydrogen oxidation occurs readily on platinum-based catalysts. The kinetics of this
reaction are very fast and, in a fuel cell, hydrogen oxidation is usually controlled by
mass transfer limitations. Hydrogen oxidation also involves the adsorption of the gas
onto the catalyst surface followed by dissociation of the molecule and an
electrochemical reaction to form two protons in presence of an acid electrolyte.
Pt(s) represents a free surface site and Pt-H is an adsorbed hydrogen atom on the Pt
active site. The overall reaction of hydrogen oxidation is
,
where .
Although this process is a fast electrochemical reaction, with rate constants of about
10-5
mol sec-1
cm-2
, some problems may arise in a fuel cell when impure hydrogen is
used. Operating a fuel cell with pure hydrogen gives the best performance but pure
hydrogen can be expensive and difficult to store.
Alternatives to pure hydrogen include natural gas, propane, or alcohols. These
substances have to be reformed, however, into hydrogen and, even after gas clean-up,
some contaminants, such as CO species, can persist in the fuel feed. CO poisons the
catalyst by blocking active sites. Consequently, sites are no longer available for
hydrogen adsorption and subsequent oxidation.
12. 11
When it comes to the reduction of oxygen, the reaction can proceed by two overall
pathways in aqueous electrolytes:
1) Direct four-electron pathway
a) alkaline electrolyte:
b) acidic electrolyte:
2) Peroxide pathway
a) alkaline electrolyte:
followed by:
or:
b) acidic electrolyte:
followed by:
or:
The direct four electron pathway is preferable as it does not involve peroxide species
in solution and the charge efficiency (Faradic efficiency) of the reaction is greater.
This pathway, however, consists of a number of steps in which molecular oxygen
dissociates at the surface and combines with hydrogen ions. The adsorption of an
oxygen species to the surface of the metal particles is essential for electron transfer.(6)
It is desirable to have the O2 reduction reaction occurring at potentials as close as
possible to the reversible electrode potential (thermodynamic electrode potential)
with a satisfactory reaction rate. Exchange current density is an important kinetic
parameter representing the electrochemical reaction rate at equilibrium. For an
electrochemical reaction, both forward and backward reactions can occur. At
equilibrium, the net current density of the reaction is zero. The current density of the
forward reaction equals that of the backward reaction. This current density is called
exchange current density. The magnitude of the exchange current density determines
how rapidly the electrochemical reaction can occur. The exchange current density of
13. 12
an electrochemical reaction depends on the reaction and on the electrode surface on
which the electrochemical reaction occurs.(7)
Developing electrocatalysts with both high selectivity and efficiency for the oxygen
reduction reaction (ORR) is critical for several applications, including fuel cells and
metal-air batteries. Carbonaceous materials play critical roles over the course of the
developments of ORR catalysts and their physicochemical properties have significant
influences on their overall activity and durability.(8)
1.3 Mesoporous Carbon
1.3.1 Mesoporous Carbon
Recent developments of carbon electrodes that possess graphitic structure, corrosion
resistance, and remarkable electrical conductivity bring tremendous opportunities
toward developing advanced catalysts.(8)
Carbon materials have been used in a wide field of applications like adsorption,
catalysis, water and air purification or energy storage. These materials are inert,
exhibit a high specific surface area, a large pore volume and possess a high
mechanical stability. However, since they have been originally prepared by
carbonization of natural precursors like wood or coal, their chemical and textural
properties are not well defined.
Furthermore, commonly used charcoals are predominantly microporous resulting in
diffusion limitations in catalytic and adsorption processes. Therefore many efforts
have been taken to develop carbon materials with tailored properties.(9)
The development of new porous carbons has become possible in these last years by
using a templating approach. The method is based on the filling of the organized
porous structure of silica host by a carbon precursor in gaseous or liquid phase
followed, if necessary, by carbonization.
14. 13
The carbon, which is recovered after silica removal, corresponds to a negative replica
of the pristine silica template. It displays an organized and interconnected porous
volume including micropores and mesopores associated with high values of specific
surface area and pore volume. The specificity of the template approach is that the
carbon is formed in a confined porous network (pore size < 10 nm), which confers to
it structural characteristics and textural properties that cannot be obtained by
conventional preparation methods.(10)
However, the synthesis of ordered materials is relatively cost-intensive due to the
expensive templates (i.e. MCM-48 or SBA-15). Therefore the field of applications
of such materials is restricted.
Carbon precursor molecules, which are not deposited inside the template pores, will
lead to non-porous carbon. Since the pore filling is mainly determined by the
mobility of the carbon precursor, carbon loss can be caused by several processes: (i)
leaching of precursor by the used solvent; (ii) removal of carbon species by water
which is formed by condensation stages during the carbonization; (iii) calcination,
depending on temperature and heating range.(9)
Mesoporous carbons have shown promising applications in energy storage systems,
catalysts, electrodes and separation processes. It has been reported that the
introduction of heteroatoms, e.g. nitrogen in a carbon support favored the catalytic
activity towards oxygen reduction.(11)
1.3.2 Mesoporous Carbons Doped with Heteroatoms
Nitrogen containing carbons have attracted increasing interest among the scientific
community over the past few years, as they improve the properties of bulk carbon for
use in various applications. The incorporated nitrogen functional groups have a deep
effect on the carbon properties. In particular, electrical conductivity, basicity,
oxidation stability, and catalytic activity are directly affected and often enhanced
when nitrogen is introduced in the carbon structure.
15. 14
The performance of these materials is strongly dependent on the amount of nitrogen
in the carbon host as well as on its local structure. Nitrogen doping in carbon
materials can be performed either directly during synthesis or by post-synthetic
treatment. However, post-treatment methodologies often yield only surface
functionalization. The bulk material properties are not affected. In contrast, “in situ”
doping during solid synthesis using nitrogen containing precursors ensures the
homogeneous incorporation of nitrogen throughout the entire carbon material.
Bulk nitrogen-doped carbons have been commonly investigated as metal-free
catalysts for the ORR. There is a great interest in replacing the expensive fuel cells
electrodes containing supported noble metals such as Pt and Ru.
The catalytic mechanism of ORR is a matter of discussion especially regarding the
role of the metal nanoparticles and the nitrogen within the structures. The main
hypotheses concerning the mode of operation are briefly summarized as follows: (i)
the metal nanoparticles are the catalytically active sites, which through thermal
treatment become well dispersed, so enhancing their catalytic activity; (ii) the
catalytically active centers are the metal nanoparticles, which are influenced by the
surrounding carbon and nitrogen atoms; (iii) the metal nanoparticles just promote the
formation of the catalytically active center in the C/N-system, but have no catalytic
function during ORR.
To date, none of these hypotheses have been proved as exclusively valid. It is
speculated that the catalytic mechanism of ORR may involve elements from all three
hypotheses, which would explain the controversial analysis of similar experimental
findings. However, it is generally accepted that the nitrogen incorporated into the
structures plays a key role in the catalytic function.
The role of nitrogen in the ORR reaction is not fully understood but a major
contribution is attributed to pyridinic nitrogen atoms on the edges of the graphitic
planes. On these sites, oxygen activation can be expected to occur at C-sites in the
proximity of nitrogen atoms. Furthermore, it is unclear why some N-doped carbons
catalyze the two-electron mechanism and others the four electron mechanism. On the
16. 15
other hand, theoretical studies show that the polarization of the C-N bond of
quaternary nitrogen leads to increased interaction between the adjacent carbon atoms
and the oxygen radicals, which induces enhanced catalytic activity. Depending on the
surface nitrogen density, other electron mechanisms may be favored.(12)
17. 16
CHAPTER 2
TECHNIQUES and INSTRUMENTS
2.1 Linear Sweep and Cyclic Voltammetry
Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) are the most widely
used voltammetric techniques for studying redox reactions of both organic and
inorganic compounds, because they are unmatched in their ability to provide
information on the steps involved in electrochemical processes with only a modest
expenditure of time and effort in the acquisition and interpretation of data. These
electroanalytical methods require rather simple and inexpensive instrumentation and
provide information not only on the electrochemical quantities typical of a redox
process, but also on possible chemical reactions coupled with charge transfer steps.
In both LSV and CV, a small (<0.1 cm2
), stationary working electrode is dipped in
an unstirred solution containing an excess of supporting electrolyte to repress
migration of charged reactants and products, so that any transfer of electroactive
species to and from the electrode surface can occur only through diffusion. In LSV
the potential of the working electrode changes linearly with time, as shown in the
upper plot of Figure 2.1(a), starting from a potential Ei, where no electrode reaction
occurs, and moving to the potential where reduction (more negative values) or
oxidation (more positive values) of the investigated analyte takes place; the
instantaneous potential Et applied at time t is given by equation
where v is the scan (or sweep) rate, that is, the absolute value of the rate of the
potential change, |dE/dt|, which is constant within any sweep. The sign depends on
the potential scan direction (positive for anodic sweeps and negative for cathodic
sweeps).
18. 17
Figure 2.1 Potential waveforms for linear sweep (a) and cyclic (b) voltammetry and
the resulting voltammograms.
The current is measured throughout the experiment and the resulting current–
potential curve (voltammogram) displays the typical shape shown in the lower plot in
Figure 2.1(a), which also reports the main parameters. They are peak current (ip), that
is, the maximum current value; peak potential (Ep), that is, the potential
corresponding to ip; and half-peak potential (Ep/2), that is, the potential at which i =
ip/2.
Peak position on the potential scale (Ep) is related to the formal potential of the redox
process and to its reversibility degree (conditioning also the peak shape Ep−Ep/2),
thus providing information on the analyte involved. Instead, peak height (ip) gives
information on the analyte concentration, the number of electrons involved in the
electrochemical process, and the possible presence of coupled chemical reactions.(13)
2.2 Rotating Disk Electrode Method
For a research on the electrode reaction mechanism and kinetics, particularly those of
ORR, it is necessary to design some tools that could control and determine the
reactant transportation near the electrode surface and its effects on the electron
transfer kinetics. A popular method, called the rotating disk electrode (RDE) has
been widely used for this purpose, particularly for the ORR.
19. 18
For electrolytic solution containing an excess of supporting electrolyte, the ionic
migration term can be neglected, suggesting that there are only two major process
left for material transport: diffusion and convection. If there is no solution
convection, the thickness of diffusion layer near the electrode surface will become
larger and larger with prolonging the reaction time, resulting in non-steady-state
current density. However, if there is a vigorous solution convection such as stirring
and electrode rotation, the thickness of diffusion layer will be fixed, leading to a
steady-state current density.
Therefore, the convection controls the thickness of the diffusion layer and the
diffusion controls the transport rate of the reactant through the diffusion layer. Using
RDE apparatus to precisely control the electrode rotating rate, the quantitatively
control of diffusion layer thickness can be realized, resulting in feasible quantitative
analysis of electrode reaction kinetics.
The central part of the RDE theory and technique is the convection of electrolyte
solution. Due to the solution convection, the reactant in the solution will move
together with the convection at the same transport rate. Let’s first consider the
situation where the flow of electrolyte solution from the bottom of the electrode edge
upward with a direction parallel to the electrode surface to see how the diffusion-
convection layer can be formed and what is its mathematical expression.(14)
Equations used for RDEs are as follows:
(the Koutecky-Levich equation) where j is the disk current density, jk is the kinetic
current density, and jLev is the Levich current density. jk can be expressed as
20. 19
where n is the overall electron transfer number, A is the electrode area, is the
kinetic constant of electron transfer, CO2 is the concentration of dissolved O2, and
Scatalyst is the surface concentration of the catalyst, or the catalyst loading.
jLev can be expressed as
where DO2 is the diffusion coefficient of O2, is the kinematic viscosity of the
electrolyte solution, and ω is the rotation rate represented by rpm.
For RDE data analysis, three non-electrochemical kinetic parameters, such as the
diffusion coefficient of O2, the kinematic viscosity of the electrolyte solution, and the
solubility of O2 must be known accurately. These parameters are all temperature
dependent. Their values are also slightly dependent on the electrolyte used. (7)
2.3 Scanning Electron Microscopy (SEM)
The scanning electron microscope is one of the most versatile instruments available
for the examination and analysis of the microstructural characteristics of solid
objects. During the scanning, the area to be examined, or the microvolume to be
analyzed, is irradiated with a finely focused electron beam, which may be static or
swept in a raster across the surface of the specimen.
The types of signal produced when the electron beam impinges on a specimen
surface include secondary electrons, backscattered electrons, Auger electrons,
characteristic x-rays, and photons of various energies. These signals are obtained
from specific emission volumes within the sample and can be used to examine many
characteristics of the sample (composition, surface topography, crystallography, etc.)
In the SEM, the signals of greatest interest are the secondary and backscattered
electrons, since these vary as a result of differences in surface topography as the
21. 20
electron beam is swept across the specimen. The secondary electron emission is
confined to a volume near the beam impact area, permitting images to be obtained at
relatively high resolution.
The three dimensional appearance of the images is due to the large depth of field of
the scanning electron microscope, as well as to the shadow relief effect of the
secondary electron contrast. Other signal are available which prove to be similarly
useful in many cases.(15)
2.4 Transmission Electron Microscopy (TEM)
An optical microscope, used in transmission mode, typically consists of five parts:
the light source; the condensor lens, which focuses the light beam onto the sample; a
transparent sample, usually a thin section of rock or tissue; the objective lens,
sometimes in contact with the sample through a contact medium (oil); and the
magnifying lens(es), often combined with the ocular or eyepiece. The number of
lenses may vary but most optical microscopes have all five components.
A TEM can be divided into similar sections. The five sections of a TEM are: the
electron gun, the illumination stage, the objective lens, with an electron transparent
sample immersed into a magnetic field, the magnification and projection system,
often with three or more lenses, and the detector (a viewing screen, photographic
camera, etc).
The TEM mainly provides information about the internal structure of the analyzed
sample. The sample has to be thin enough to allow the electron to pass through it.
During the passing some of the electrons are absorbed into the structure while other,
accordingly to the atomic placement disuniformity of the crystal, are deviated
irregularly.(16)
22. 21
2.5 X-Ray Photoemission Spectroscopy (XPS)
The study of surfaces can be done using electronic spectroscopies, speccially XPS
and AES. In the XPS (X-ray Photoelectron Spectroscopy), the image obtained is the
result of the collection of the electrons emitted by the sample after being eccited with
an x-ray energy.
The electronic spectroscopy analyzes the electrons that are expelled by
photoelectric effect from the material for a qualitative or for a semi-
quantitative analysis. Three different sources that trigger the photoemission process
can be distiguished:
1. X-ray photoemission spectroscopy, that draws the electrons close to the atomic
nucleus,
2. Auger electron spectroscopy, that draws the electrons far from the nucleus by
Auger effect,
3. UV photoemission spectroscopy, that uses a less energetic radiation and that
allows the ionization of the more external electrons.
While using XPS, the sample is irradiated with a monocromatic X-ray source (the
most diffuse sources are Al K (h = 14.86 keV, ) and Mg K (h = 12.53 keV), but
for more profound cores Cu, Ti and W are used.
The emitted electrons are captured by a system of electromagnetic lenses and then
sent to a kinetic energy analyzer, therefore registered by a multichannel revealer
where an spectrum I vs BE (Binding Energy) can be obtained.
At the bottom of the spectrum, the peaks can be identified from the atomic orbital
source based on the BE (the presence of Auger peaks could be present too
though).(17)
23. 22
CHAPTER 3
EXPERIMENTAL
3.1 Chemicals
Mesoporous Silica (200 nm particle size, 4 nm pore size, Sigma-Aldrich), carbazole
(Sigma-Aldrich, >95%), ethanol (Fluka HPLC, >99,8%), Nafion (5 ww% in lower
aliphatic alcohols and water, contains 15/20% in H2O), acetone (Sigma Aldrich,
>99,5%), NaOH (VWR, >99%)
3.2 Preparation of the mesoporous carbon (MC)
The procedure consists in the dissolution of 1 g of silica and 1 g of carbazole in 20
mL of acetone. The silica purpose in the solution is to act as a template matrix and to
guarantee the mesoporosity of the carbon.
The solution is dried in oven for over an hour at 60 °C, in order to remove all traces
of acetone (as a result, a purplish powder was obtained; furthermore, the presence of
a brown crust could be observed at the borders of the powder, demonstrating signs of
oxidation).
Once the desiccation process has ended, the compound is taken into a quartz oven
where the pyrolysis of the carbon takes place.
Before the quartz oven can be used, a flux of nitrogen at room temperature is sent
into the quartz tube for about one hour, in order to eliminate all traces of oxygen. The
presence of oxygen could lead to combustion instead of pyrolysis.
The oven is programmed in such a way as to start raising its temperature at a rate of
5 °C//min, after the first hour, until it reaches 750 °C. This process takes
approximately 2.5 h. Once the setup temperature is reached, the compound stays
inside the tube for 5 h. Then, the system cools down through heat exchange with the
room, for which it takes one entire night to bring the compound to room temperature.
24. 23
The carbon powder that is obtained from the pyrolysis is treated with a liquid
solution made up by 20 mL of NaOH and 20 mL of ethanol. The NaOH reacts with
the silica and dissolves into the mix, while the carbon precipitates.
The carbon is separated by vacuum filtration with the aid of a nylon nanometric filter
(GVS, nylon 0,2 µm, 47 mm membrane diameter). The filtration is done under
vacuum for two main reasons:
1) vacuum accelerates the process of filtration
2) vacuum prevents the filter from detaching itself from the bottom surface of
the Buchner, avoiding in this way the precipitation of carbon particles into the
filtered solution.
Once the carbon is filtered, the solution that has to be deposited on the electrodes is
prepared. Firstly, the MC is grounded with a mortar in order to obtain an even finer
powder. 2.5 mg of this powder is added to 2.35 mL of water and 150 μL of nafion.
Nafion has the only purpose of keeping the carbon in suspension. The solution is
then sonicated for 3 min.
The deposition of the Pt nanoparticles into the mesoporous carbon was done as an
ulterior doping. This procedure consists in dissolving 20 mg of the MC in 16 mL of
water and then adding this to a mixture made by 0,0107 g of PtCl2 dissolved in 2 mL
of H2O and 2 mL of C2H5OH. The resulting solution was then sonicated for about 1
h.
To this mixture, 0.032 g of NaBH4 dissolved in 4 mL of water was added and then it
was sonicated a second time for 2 h. The resulting solution was then filtered, washed
with water and dried at 80 ºC
25. 24
3.3 Electrochemical tests
The electrochemical activity measurements were carried out by a cyclic voltammetry
(CV) and rotating disk electrode (RDE) voltammetry, using an AUTOLAB 100
potentiostat. A conventional three electrode configuration, consisting of a glassy
carbon (GC, Tokai) with an area of 0.071 cm2
, as the working electrode, a Pt as the
counter electrode and a saturated calomel (SCE) as reference electrode, was used.
The GC was polished to a mirror finish with diamond paste (3-,1-,0.25-µm particle
size).
Then 10 µL of suspension was careful pipetted onto the clean GC electrode. After
that, it was left to dry at air room temperature overnight.
The LSV/CV and RDE study of oxygen reduction was carried out in 0.1 M H2SO4
and the solutions were purged with Ar before each measurement, whereas for the
ORR test, the electrolyte was bubbled with high purity O2 gas for at least 30 min to
ensure O2 saturation.
3.4 Characterization tests
X-ray photoemission spectroscopy (XPS) measurements were performed in a UHV
chamber base pressure <5 × 10−9
mbar), equipped with a double anode X-ray source
(PSP), a hemispherical electronanalyzer (VG Scienta) at r.t., using non-
monochromatized Mg-Kαradiation (hν = 1253.6 eV) and a pass energy of 50 eV
and 20 eV for the survey and the single spectral windows, respectively. The
calibration of the Binding Energy (BE) scale was carried out using Au 4f as
reference (BE Au 4f = 84.0 eV).
For the characterization of the NMCs, the nitrogen amount was determined by
normalizing the intensity of the N 1s XPS peak for the integrated area of the C 1s
photoemission peak (both 20 corrected for the differential cross section and anelastic
mean free path of photoelectrons), obtaining a nitrogen concentration of 5.7 at% for
the best sample (see below). The XPS peaks of carbon and nitrogen were separated
into single chemical shifted components (after Shirley background removal), using
26. 25
symmetrical Voigt functions; the χ2
was minimized by the use of nonlinear least
squares routine.
To perform XPS measurements, 2.5 mg of the MC10 or Pt@MC10 powder were
dispersed in 50 μL of toluene and then gently sonicated (for 5 min) in order to
efficiently disperse the powders; the solutions were then drop-casted on
electropolished polycrystalline copper or GC substrates (with a surface area of 1
cm2
). Thus, the samples were first dried overnight under nitrogen flux to obtain
homogeneous films; then they were vacuum-dried for 2 h at about 10-6
mbar.
Scanning electron microscopy (SEM) and transmission electron microscopy (TEM)
images were obtain using a Zeiss Supra 35VP Gemini scanning electron microscope
operating at 5 kV and FEI Tecnai G2 transmission electron microscope operating at
100 kV.
27. 26
CHAPTER 4
Chemical and electrochemical characterization
The mesoporous carbon was fully characterized by a whole series of chemical and
electrochemical techniques in order to understand the electrochemical behavior and
the catalytic activity as well as the surface chemistry and morphology. The
electrochemical analysis was carried out by different techniques, such as cyclic
voltammetry (CV) and rotating disk electrode (RDE), while the morphological and
chemical analysis were acquired by SEM,TEM and XPS investigations.
4.1 Chemical and Morphological characterization
Both figures 4.1a and b show the SEM images of the MC10 (name given to the
mesoporous carbon that was synthesized from carbazole) at a 50 and 200 nm scale.
For the most part orderly, well-defined spheres can be seen which implies that the
carbon has assumed perfectly the structure of the silica template used to synthesize
the MC. However, some of the spheres have not a perfectly round surface, but appear
rough. Although the synthesis started with silica grains of 200 nm, it can be observed
from figures 4.1 that the size of the carbon particles varies within the structure. The
histogram in Figure 4.1c shows the carbon particles size distribution centered around
a diameter of 320 nm. From a 100 specimens sample, it was determined the diameter
of particles varying between 100 and 550 nm; almost 25% of the measures fall
within the 350/400 nm diameter range and most of the particles show a diameter
between 250 and 500 nm. Therefore, most of the carbon particles are bigger than the
starting silica. Some exceptions can be observed as expected from the normal
distribution curve.
28. 27
Figure 4.1 SEM images of MC10 at different amplification: (a) 1 µm, (b) 200 nm.
(c) distribution of particle size.
Figure 4.2 SEM images of Pt@MC10 at different amplification: (a) 200 nm,
(b) 100 nm.
Diameter (nm)
RelativeFrequency
50 100 150 200 250 300 350 400 450 500 550 600
0
4
8
12
16
20
24
a
a b
c
b
29. 28
Figures 4.2a and b show the SEM images relative to the Pt@MC10, scaled at 200 nm
and 100 nm. The carbon structure proves to be unaltered after the deposition of the
platinum nanoparticles .
Figure 4.3 TEM images of the Pt@MC10.
Figure 4.3a and b show the TEM images of the Pt@MC10, which serves the purpose
of showing the regular porous structure of the compound. In this case Pt NPs are not
easily visible probably because confined inside the pore structure.
X-ray photoemission spectroscopy (XPS) was used to study the atomic percentage
of each element in MC10. In figure 4.4a it’s reported the survey for MC10, in which
it is possible to observe signals of photoelectrons emitted from orbital 1s of C, 1s of
N and 1s of O.
ba b
30. 29
Figure 4.4 a) Suvey for MC10; b) high resolution spectrum of photoemission from
orbital 1s of C; c) high resolution spectrum of photoemission from orbital 1s of N.
In Figure 4.4b and c there are high resolution signals of photoemission from C1s and
N1s, respectively. In this case, with a deconvolution of signal done with the program
‘XPS fit’ is possible to detect the state of bonding of each element in the sample. The
deconvolution of signal of carbon shows three main components: graphitic carbon,
carbon linked to nitrogen and oxidized carbon. The deconvolution of signal of
nitrogen has four main components: pyridinic nitrogen, pyrrolic nitrogen, graphitic
nitrogen and oxidized nitrogen. In table 4.1 the BE (eV) and full width at half
maximum (FWHM) of each peak are reported.
Table 4.1 Binding energy (BE) and full width at half maximum (FWHM) of
different XPS signals.
BE (eV) FWHM (eV) BE (eV) FWHM (eV)
Components N1s peak Components C1s peak
Pyridinic N 398.3 1.80 C-C 284.6 1.34
Pyrrolic N 399.3 1.72 C-N 285.4 1.74
Graphitic N 400.6 1.42 C-O 287.3 1.80
Oxidized N 401.6 1.30
ba
c
31. 30
4.2 Analysis of the Capacitance of the Compound
The behavior of MC10 was examined by cyclic voltammetry (CV) in Ar saturated
0.5 M H2SO4 solution, at different scan rates in order to determinate the material
capacitance. Figure 4.5 reports the electrochemical behavior at different scan rates.
-0,6 -0,4 -0,2 0,0 0,2 0,4 0,6 0,8 1,0
-1,5
-1,0
-0,5
0,0
0,5
1,0
1,5
I/mA
E/V vs SCE
Velocity Scan (Vs)
002
005
01
02
03
04
05
08
1
2
Figure 4.5 Cyclic voltammetries of MC10 at different scan rates.
It is commonly known that nitrogen doping improves the capacitive behavior of
materials and this is generally attributed to the electron donor capability of nitrogen
atoms. Specifically, the strong electron donor nature of nitrogen atoms promotes
reinforcement in π bonding, enhancing wettability of the material at the
electrode/electrolyte interface.(18).
Therefore, the presence of nitrogen functional
groups induces a pseudocapacitive effect, caused by the increased adsorption of
proton end electrolyte ions in the double layer.(19)
A well defined quasi-reversible redox peak couple was observed in the potential
range between 0.2 and 0.4 V vs. SCE. The anodic-cathodic peak separation increases
with the increasing scan rate and peak currents are linearly proportional to the scan
rate ranging from 0.02 to 2 Vs-1
, as typical for a non-diffusive, surface-controlled
process. The presence of these peaks may be ascribed to redox active surface oxygen
32. 31
functional groups such as hydroquinone/quinone couple, which usually undergo
proton coupled electron transfer.(20)
Figures 4.6a and b show the trend of the specific capacitance for the MC10 obtained
as mean value for a set of experiments. The weight considered for the specific
capacitance is that of the mass deposited on the electrode, which is equivalent to 0.01
mg, whereas the geometric surface area (0.0706 cm2
) was considered for the specific
area capacitance calculation.
In both cases a the maximum value of the capacitance coincides with peak present at
0.3 V, which is due to the presence of the aforementioned functional groups, while
the two minima correspond to the potentials relative to the maximum values of the
anodic and cathodic current, respectively.
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8
0.00
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.10
C/QV-1mg-1
E/ V vs SCE
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8
0.004
0.005
0.006
0.007
0.008
0.009
0.010
C/QV-1cm-2
E/ V vs SCE
Figure 4.6 Specific capacitance for: (a) weight and (b) area of MC.
4.3 Study of the Oxygen Reduction Reaction
The study of the ORR was conducted on both MC10 and its platinum modified
version, Pt@MC10. The O2 reduction peak potential and peak current were
determined by CV while the limiting current, the onset potential and the half-wave
potential were determined by linear sweep voltammetry (LSV) at rotating disk
electrode (RDE).
In Fig. 4.7a and b the cyclic voltammetry behavior of both MC10 and Pt@MC10,
respectively, are reported, in a O2 saturated 0.5 M H2SO4 solution at different scan
rates (0.02, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.8, 1 V/s).
ba
33. 32
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
-1.0
-0.5
0.0
0.5
1.0
I(mA)
E/V vs SCE
Velocity Scan (Vs-1
)
0.1
0.2
0.5
0.8
-0.4 -0.2 0.0 0.2 0.4 0.6 0.8
-1.0
-0.5
0.0
0.5
1.0
I(mA)
E/V vs SCE
Scan Velocity (Vs-1
)
0.1
0.2
0.5
0.8
Figure 4.7 Cyclic Voltammetry of a O2 saturated 0.5 M H2SO4 solution on:
a) MC10; b) on Pt@MC10
In the case of MC10 two signals are present: the first one is a quasi-reversible peak
couple associated to the above mentioned quinone/hydroquinone redox couple; the
second, at more negative potential (-0.367 V vs. SCE), is associated to oxygen
reduction. In the case of Pt@MC10 the peak at more positive potential is associated
to oxygen reduction (0.09 V vs. SCE), while the peaks between 0 and -0.2 V vs. SCE
are due to underpotential reduction and oxidation of adsorbed hydrogen on Pt. In this
case the peak of quinone/hydroquinone couple is absent.
Figure 4.8a and b show the LSV at RDE, undertaken at different rotating rates, from
800 to 3500 rpm, in a O2 saturated 0.5 M H2SO4 solution and at a scan rate of 5 mVs-
1
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
-4
-3
-2
-1
0
1
I/mAcm-2
E/V vs SCE
Rotation Rate (rpm)
800
1100
1400
1700
2000
2300
2600
2900
3200
3500
0,0 0,2 0,4 0,6 0,8
-5
-4
-3
-2
-1
0
I/mAcm-2
E / V vs. SCE
rotation rate (rpm)
800
1100
1400
1700
2000
2300
2600
2900
3200
3500
Figure 4.8- RDE linear sweep voltammetry for ORR on: (a) MC10 and (b)
Pt@MC10.
In the case of the MC10, the current hardly reaches a well-defined diffusion limiting
plateau, the effect was observed to be intrinsic of this system since other types of
doped mesoporous carbon give better defined plateau currents. However, it can be
a b
ba
34. 33
asserted that the current reaches a plateau-like behavior at potential below 0.0 V vs.
SCE, while in the case of the Pt@MC10 it can be seen that the plateau is anticipated
at a potential above 0.0 V vs. SCE.
Table 4.2 Electrochemical parameters for ORR in a O2 saturated 0.5 M H2SO4
solution.
Electrode
Ep
a
(V vs. SCE)
Eonset
b
(V vs.
SCE)
E1/2
b
(V vs. SCE)
104
Ip
a
(A)
104
Ilim
b
(A)
MC-10 -0.367±0.04 0.036±0.04 -0.092±0.06 -1.7 ± 0,03 -1.7±0.03
Pt@MC-10 0.09±0.07 0.57±0.08 0.46±0.09 -2.72 ±0.01 -2.79±0.01
a
Values obtained from the CV. b
Values obtained from the RDE at 2000 rpm.
Table 4.2 summarizes the main electrochemical data for both electrodes. The onset
potential in the platinum doped carbon is ca. 0.5 V more positive with respect to the
material without platinum. It can be observed that generally, all potentials are shifted
to more positive values in the platinum doped version and the limit current and peak
current values are almost doubled in their values.
The anticipation of the potential peak and onset values means that the ORR takes
place more easily at Pt@MC10, due to the presence of platinum.
To investigate the mechanism of the electron transfer process the Koutecky-Levich
equation was used. The overall reaction of the ORR can take place in two different
ways. The first one is the two-electron transfer, which consists in the formation of
peroxide species during the reaction (eq. 4.1 and 4.2):
(4.1)
(4.2)
the second mechanism is the four electron pathway which leads to the formation of
water during the reaction (eq. 4.3):
(4.3)
35. 34
By means of the slope obtained from the Koutecky-Levich(7)
equation (eq.4.4):
4.4
the number of electrons involved in the ORR can be determined. Figure 4.4 shows
the variation of the number of electron transferred with the potential for both cases.
For the MC10, the average number of electrons is 2.68, which means that the two
electron pathway is the prevalent mechanism. However, the number of electrons for
the Pt@MC10 is 4.6 which is a clear indication that in this case the 4-electron
transfer pathway is preferred.
-0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2
0
1
2
3
4
5
6
7
8
n°e-
E/ V vs SCE
MC10
Pt@MC10
Figure 4.9 Number of electrons for the ORR obtained by RDE linear voltammetries
at different electrode potentials in a O2 saturated 0.5 M H2SO4 solution.
The potentials reported in figure 4.9 are associated to the diffusive regime in which
the electron transfer takes place, which are closely linked to the ORR. Once more it
can be seen that the reduction process for the platinum starts at a more positive
potential with respect to the metal free carbon.
36. 35
CONCLUSSIONS
In the present thesis, a structural and electrochemical study on MC, doped with N
heteroatoms and containing Pt nanoparticles, has been carried out. In particular, the
attention was focused on the electrochemical characterization of the ORR. The most
relevant results will be briefly described.
From the structural evaluation, it can be seen that the MC carbon synthesized retains
the original ordered and spherical structure possessed by the silica template. The
presence of Pt nanoparticles cannot be observed from the images but it can be
inferred from the electrochemical behavior of the doped compound. The nitrogen
bonds were determined through the XPS.
The electrochemical analysis shows that the MC shows two main signal for the CV,
one due to the presence of quinone/hydroquinone functional groups and the second
one due to the actual oxygen reduction reaction. In the Pt@MC only the signal
relative to the ORR is present and the potential at which the reaction takes place
shifts towards more positives value with respect to MC. The difference in peak
potential between the two MCs is ca. 0.45 V, while the difference in onset potential
is ca. 0.5 V.
Another main difference between MC and Pt@MC is the electron transfer
mechanism. With the use of RDE technique it was determined that the at the MC the
ORR takes place through the two electron transfer mechanism, which consists in the
formation of peroxides species. On the other hand, at the Pt@MC, ORR follows the
four electron pathway, which leads to the formation of water as reduction product.
37. 36
ABBREVIATIONS
AFC Alkaline Fuel Cell
BE Binding Energy
CV Cyclic Voltammetry
FC Fuel Cell
GC Glassy Carbon
LSV Linear Sweep Voltammetry
MC Mesoporous Carbon
ORR Oxygen Reduction Reaction
RDE Rotating Disk Electrode
SCE Saturated Calomel Electrode
SEM Scanning Electron Micrsocopy
TEM Transmission Electron Microscopy
XPS X-ray Photoemission Spectroscopy
38. 37
NOMENCLATURE
α Electron Transfer Coefficient
A Electrode Surface Area
C Concentration
D Diffusion Coefficient
Eº Standard Electrode Potential
E Electrode Potential
F Faraday Constant
G Gibbs Free Energy
Gº Standard Gibbs Free Energy
j Current Density
jo Exchange Current Density
jLev Levich Current Density
jk Kinetic Current Density
k Kinetic Constant for Electron Transfer
n Number of electrons transferred
[Ox] Concentration of the Oxidized Species
[R] Concentration of the Reduced Species
R Gas Constant
S Surface
v Scan Rate
ν Kinematic Viscosity
ω Rotation Rate
39. 38
BIBLIOGRAPHY
1. Gennaro, A. Lectures Notes
2. FuelCellToday. Johnson Matthey Plc 2014 (www.fuelcelltoday.com)
3. Gross, D. CleanTech 2010, (www.cleantechvistor.com)
4. Woodrow W.C.; Grant, C. Global Energy Innovation: Why America Must
Lead 2011 Praeger, Santa Barbara (CA).
5. www.robotplatform.com/knowledge/energy/fuel_cells.html
6. Stimming, U; Carrette, L.; Friedrich, A. K.; ChemPhysChem 2000, 1, 162-
193.
7. Song, C.; Zhang, J. PEM Fuel Cells Electrocatalysts and Catalyst Layers,
2008 Springer, London.
8. Cheng, Y.; Zhang, H.; Varanasi; C.V.; Liu, J. Sci. Rep. 2013, 3, 3195.
9. Böhme, K.; Einicke, W.D.; Klepel, O. Carbon 2005, 43, 1918-1925.
10. Gadiou, R.; Didion, A.; Gearba, R.I.; Ivanov, D.A.; Czekaj, I. ; Kötz, R.; Vix-
Guterl, C. J. Phys. Chem. Solid 2008, 69, 1808-1814.
11. Huang, Y.; Yang, F.; Xu, Z.; Shen, J. J. Colloid Interf. Sci. 2011, 363, 193-
198.
12. Sevilla, M.; Yu, L.; Fellinger, T.P.; Fuertes, A.B.; Titirici, M.-M. RSC Adv.
2013, 3, 9904-9910.
13. Bontempelli, G.; Toniolo, R. Encyclopedia of Electrochemical Power
Sources 1st
Edition, Elsevier, 2009.
40. 39
14. Xing, W.; Yin, G.; Zhang, J.; Rotating Electrode Methods and Oxygen
Reduction Electrocatalysts, 1st
Edition, Elsevier, 2014.
15. Goldstein, J.; Newbury, D.; Echlin, P.; Joy, D.C.; Fiori, C.; Lifshin, E.;
Scanning Electron Microscopy and X-ray Microanalysis, Springer, London,
1981.
16. De Graef, M. Introduction to Conventional Transmission Electron
Microscopy Cambridge University Press, 2003.
17. Calliari, I. Lecture Notes
18. Kim, N.D.; Kim, W.; Joo, J.B.; Oh, S.; Kim, P.; Kim, Y.; Yi,. J. Power
Sources 2008, 180, 671-675.
19. Wang, Q.; Yan, J.; Fan, Z. Electrochim. Acta 2014, 146, 548-555.
20. Yuan, X.; Yuan, D.; Zeng, F.; Zou, W.; Tzorbatzoglou, F.; Tsiakaras, P.;
Wang, Y.; Appl. Catal. B: Environ. 2012, 129, 367-374
