electrochemical characterization of k3mn(cn)6 and related metallates
TRANSCRIPT
1
ELECTROCHEMICAL CHARACTERIZATION OF K3MN(CN)6 AND
RELATED METALLATES
Marco Renzi
Instituto Superior Técnico, Universidade Técnica de Lisboa
January 2013
Abstract. This work has been focalized on the preparation and electrochemical characterization of
mixed hexacyanomanganates analogues of Prussian Blue. The preliminary approach regarded the
synthesis and electrochemical and spectroscopic characterization of K3Mn(CN)6 compound, that to
the best of our knowledge, has never been studied before. The resulting electrochemistry is very
complex because of the numerous oxidation states of Manganese. The goal of this work is to verify
the possibility to obtain mixed hexacyanometallates containing Mn3+
taking into account the general
formula of hexacyanometallates AMAMB(CN)6, we are trying to use Manganese either in position
MA or in position MB having Iron in the other position.
The further research has been dedicated to the chemical and electrochemical synthesis of
Fe3[Mn(CN)6]2 (FeHCMn) for which we couldn’t find references in the literature. Starting from the
informations found in the few papers dealing with the compound Mn3[Fe(CN)6]2 (MnHCFe), in
which it’s hypothesized the possibility of isomerization between Fe and Mn presents in the
molecule, we studied the electrochemical and spectroscopic behavior of the compound FeHCMn.
Keywords. Prussian Blue, Potassium hexacyanomanganate, Iron hexacyanomanganate, modified
electrode, film growth mechanism, electrocatalysis, isomerization.
Introduction
The hexacyanometallates are a class of amazing and evergreen compounds that include Prussian
Blue (PB) and its analogues (PBA). Recent investigation on their chemical-physical properties has
stimulated an interest in particular towards magnetism, electrochromism, thermochromism, cation
exchange, etc. of complex solids. The magnetic properties of PBA’s were summarized in 1997 by
Dunbar in a review on modern perspectives of the chemistry of metal cyanide compounds1. In 1980,
Trageser and Eysel oxidized Na3[MnIII
(CN)6] with perchloric acid and obtained a brown-purple
material formulated as MnII[Mn
IV(CN)6]•1.14H2O
2. By the late 1980s, no molecule-based magnet
had been reported with an ordering temperature higher than 90 K. The exchange reactions may be
divided into three categories: those involving exchange of central metal atoms with metal ion; those
2
involving electron transfer between complex cyanide, differing only in the oxidation state of central
metal atom; and those of Adamson, et al., involving exchange of the cyanide ligand with cyanide
ion or HCN3. The studies in this field concerns in particular compound like iron-
hexacyanoruthenate(II) or Ruthenium Purple (RP), which thin films deposited on Glassy Carbon
electrode were found to be either ionic size discriminatory or very sensitive to the level of aqueous
electrolyte doping with some organic solvents. Acetonitrile as a solvent and/or Li+ have a great
distorting effect on RP redox waves4. There are a variety of studies about dehydrated solid Prussian
Blue analogues that have been used for hydrogen storage, in particular we have Ga[Co(CN)6] and
Co3[Co(CN)5]2 compared to those of M3[Co(CN)6]2 (M = Mn, Fe, Co, Ni, Cu, Zn); due to its clean
combustion and high heating value, hydrogen is currently being considered as a replacement for
fossil fuels in mobile applications5. The kinetic studies towards thin films deposited on Glassy
Carbon electrode of iron-hexacyanoruthenate(II) (Ruthenium Purple RP), corresponding to the iron
and ruthenium centers, obtained results that indicated evidence for catalytic behavior towards water
decomposition4. PB and its analogues are well-known compounds, which are very useful in
preparation and fabrication procedure of commercial cells6.
Thus, taking in mind the above considerations, we focused this work on the following aims:
We report the preparation of K3Mn(CN)6 crystals and we made on this molecule also
electrochemical and spectroscopical experiments. Mixed hexacyanometallates such as FeHCMn,
CoHCMn NiHCMn and MnHCFe, were synthesized in chemical and electrochemical way; we
made on these molecules also electrochemical and spectroscopical experiments
Experimental Section
Potassium cyanide KCN (Carlo Erba ≥ 98%), Manganese chloride tetrahydrate MnCl2•4H2O (Carlo
Erba ≥ 99%), Iron sulphate heptahydrate FeSO4.7H2O (Aldrich ≥ 99%), Nickel nitrate hexahydrate
Ni(NO3)2•6H2O (Carlo Erba ≥ 99%), Cobalt nitrate hexahydrate Co(NO3)2•6H2O (Sigma-Aldrich ≥
98%), concentrate Phosphoric acid H3PO4 (SAFC 85 %), concentrated Nitric acid (Baker 65%) and
Potassium hexacyanomanganate (III) (Sigma-Aldrich 99,97 %) were used as received. Nafion (Ion
Power, Inc, LIQUION Solution LQ-1005, 1000EW 5% wt) was from Ion power. All solutions were
prepared with house-distilled water that was further purified with Millipore Milli-Q nanopure water
of resistivity ≈ 17 M Ω cm.
Electrochemical experiments were carried out with the CH Instruments (Austin, TX) model 630C,
in a standard three-electrode cell. The working electrode was Glassy carbon (GC); Pt wire was used
as the counter electrode. The GC (0.7 cm diameter) working electrode was polished with 0.05 µm
3
alumina on a micro-cloth polishing pad before each experiment and carefully rinsed with distilled
water. All potentials were expressed versus Calomel saturated (KCl) reference electrode.
The IR spectra were recorded in the ATR mode (Attenuated Total Reflectance), with PerkinElmer
Spectrum 100 Series FT-IR spectrometer.
SEM experiments were performed with JEOL Model JSM-5400 scanning electron microscope
equipped with a Shimadzu 800HS EDX.
The UV-Vis spectra were recorded using a Hewlett-Packard 8452A diode array spectrophotometer.
The colloidal dispersion of MHCMn was transferred onto the surface of a conventional microscopic
glass slide and spread (painted) with another glass slide to produce a uniform layer.
X-ray diffraction (XRD) data were recorded using an automated PHILIPS diffractometer equipped
with Soller slits, 1° divergence-slits, 0.1 mm receiving slits a graphite diffracted-beam
monochromator.
Synthesis of K3Mn(CN)6. Standard literature preparation was employed for the synthesis following
the method of Lower and Fernelius7. Managanese (III) orthophosphate: To a warm solution of 32.4
g of manganese (II) chloride, MnCl2∙4H2O, or an equivalent amount of nitrate or acetate, in 50 ml
of water, is added 30 g of syrupy phosphoric acid and 10 g of concentrated nitric acid. On
concentrating to near dryness, this mixture first turns an amethyst color and gradually deposits a
green-gray precipitate, which, if the mixture is not well stirred or if is evaporated too far, is apt to
stick fast to the bottom of the beaker. After cooling, water (about 50 ml) is added, and the
precipitate is collected on Büchner funnel, washed well with water and dried. Potassium
hexacyanomanganate (III): Thirty grams of potassium cyanide is dissolved in 80 ml of water and
the solution heated to 80°. After removing the source of heat, 8 g of manganese (III) orthophosphate
is added slowly with continuous stirring. Caution: if the solution is overheated at this point,
manganese (III) oxide is precipitated, which must be removed by filtration before continuing the
operations. After a short time, the mixture becomes deep red, and all of phosphate dissolves. The
solution is allowed to cool and is air-evaporated. If a crust forms and no more evaporation takes
place, enough water (5 to 10 ml) is added to dissolve the crust, leaving the red crystalline needles of
potassium hexacyanomanganate (III). These are collected on a Büchner funnel and sucked dry. The
filtrate can be evaporated again or treated whit alcohol and a small additional crop crystals
collected. For purification, the salt is recrystallized from water. Potassium hexacyanomanganate
(III) forms dark-red prisms. When boiled with water, this salt is decomposed and all the manganese
is precipitated as hydrated manganese (III) oxide. When a solution of the hexacyanomanganate (III)
is treated whit potassium amalgam, it is reduced to the dark-blue hexacyanomanganate (II).
4
Synthesis of M3[Mn(CN)6]2 powders. The three species FeHCMn, NiHCMn and CoHCMn were
prepared, respectively, by mixing aqueous solution of FeSO4, Ni(NO3)2 and Co(NO3)2 at
concentration 0.1 M, with hexacyanomanganate (III) anion at the same concentration and quantity.
The additions were made in ice bath at 0°C to avoid, or at least slow down, the fast degradation at
room temperature of K3Mn(CN)6. The precipitates were filtered, washed several times with distilled
water and dried in a desiccator over silica gel; the vacuum was applied to accelerate the drying8,9
.
Synthesis of Mn3[Fe(CN)6]2 powders. The MnHCFe was prepared by dropwise mixing 50 mL
solution (0.1 M) of MnCl2 with 50 mL solution (0.1 M) of potassium ferricyanide. The mixture was
shaken thoroughly, centrifuged and washed several times with distilled water to eliminate excess of
metal ion. The precipitates were filtered, and dried in a desiccator over silica gel; the vacuum was
applied to accelerate the drying10
.
Electrode preparation by physical deposition. A sample volume of 10 µL of MHCMn dispersion
in water was dropped on the surface of GC electrode and left to dry. To prevent leaching of the
compound from the electrode surface, few drops of 5% w/v solution of Nafion were added and left
to dry. Using the FeHCMn (III) dispersion, we obtained a smooth and uniform surface of the GC,
while using the NiHCMn (III) and CoHCMn (III) dispersion we couldn’t obtain a uniform surface.
Preparation of MHCMn and MnHCFe films by cyclic voltammetry. The film of MHCMn was
grown on the surface of the GC electrode by potential cycling at scan rate of 50 mV s−1
in the 1 M
KNO3 or 1 M KCl electrolyte solutions containing 1·10-3
M of FeSO4, Ni(NO3)2 and Co(NO3)2
respectively, and 1·10-3
M K3[Mn(CN)6]. The MHCMn electrodeposition was taken at 0°C to
prevent the degradation of solution. Typically, 50 full potential cycles, starting from 1 V and ending
at -0.1 V, were applied. Care was not taken to degas the deposition solutions.
The same method was used to modify the GC electrode with MnHCFe film by electrochemical
deposition; this test was used to check the similarity with MHCMn11,12
.
Results and Discussion
Preliminary electrochemical study of K3Mn(CN)6. A solution of 10-3
M K3Mn(CN)6 in 1 M KCl
as supporting electrolyte was tested in a conventional three electrodes electrochemical cells using a
GC working electrode at room temperature (≈ 25°C) and at 0°C. As reference compound was added
10-3
M K3Fe(CN)6 , that shows a reversible monoelectronic voltammetric wave at ≈ 0.2 V.
5
1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.8 -1.0
-7
-6
-5
-4
-3
-2
-1
0
1
2
B
Background KCl 1M
HCMn 10-3M + HCFe 10
-3M 25°C
HCMn 10-3M + HCFe 10
-3M 0°C
Cu
rre
nt/A
Potential/V
A
AI
BI
CI
C
Figure 1: Cyclic Voltammograms of K3Mn(CN)6 10-3
M + K3Fe(CN)6 10-3
M solution in KCl 1 M on a Glassy Carbon
electrode; Scan rate: 100 mV/s.
The Figure 1 shows a quasi-reversible couple C/CI at reducing potential while there is an
irreversible peak BI at oxidizing potential at room temperature, that become reversible at 0°C.
To assign the couple we made a potential scan inversion experiment; the data extracted from the
graphics are shown in Table 1.
Table 1: Dependence on temperature and E1/2 of electrochemical peaks of K3Mn(CN)6.
K3Mn(CN)6 25 °C E1/2 0 °C E1/2
A/AI Rev 0.26 V Rev 0.29 V
B/BI No-Rev 0.85 V Rev 0.88 V
C/CI Rev 0.50 V Rev 0.45 V
D/DI No-Rev 0.14 V - -
K3Fe(CN)6 free solution. We eliminated the reference Ferricyanide redox couple, to see if there
could be some effect on the other redox couples (Figure 2).
6
1.0 0.5 0.0 -0.5 -1.0
-3
-2
-1
0
1
2
Background KCl 1 M
K3Mn(CN)
6 10
-3 M
Cu
rre
nt/A
Potential/V
C
CI
B
BI
BI
a
DI
D
EI
Figure 2: Cyclic Voltammogram of K3Mn(CN)6 10-3
M in KCl 1 M on a Glassy Carbon electrode; Temperature: 0°C;
Scan rate: 100 mV/s.
There are several processes:
- The redox couple B/BI, due to K3Mn(CN)6, decreases becoming less reversible; at the same
time appears a single peak BIa that seems to be directly correlated to D/D
I redox couple
degradation and formation of MnO213,14,15
;
- The peack of the redox couple C/CI, due to K3Mn(CN)6, increases becoming less reversible;
- There is a new formation of D/DI non-reversible redox couple at the same potential of A/A
I,
due to degradation of K3Mn(CN)6, and subsequent formation of: Mn(OH)2, Mn(OH)3,
MnO213,14,15
; probably even EI is due to the same reason.
The next step involved successive addition of K3Fe(CN)6 to see the change that it made on the
redox couple. The redox couple D/DI disappears with the increase of K3Fe(CN)6 concentration,
while the redox couple A/AI, due to K3Fe(CN)6, becomes visible.
The redox couple D/DI disappears because the increased concentration of K3Fe(CN)6 gives rise to
higher peaks that are in the same area; instead the other redox couple B/BI and C/C
I are not
influenced; the actual shift is due to degradation.
Scan rate study of K3Mn(CN)6.
7
1,0 0,8 0,6 0,4 0,2 0,0
-2,0
-1,5
-1,0
-0,5
0,0
0,5
1,0
0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8
-1,0
-0,8
-0,6
-0,4
-0,2
0,0
0,2
0,4
0,6
0,8
1,0
1,2
1,4
y = -8.379 10-7 + 1.738 10
-5 x
slope = 1.738 10-5
y = 2.72 10-7 - 9.905 10
-6 x
slope = 9.905 10-6
Ip/
A
Sqrt of Scan rate
Ipc
Ipa
500 mV/s
200 mV/s
100 mV/s
50 mV/s
20 mV/s
10 mV/s
5 mV/s
2 mV/s
Cu
rre
nt/A
Potential/V
a
0,0 -0,2 -0,4 -0,6 -0,8 -1,0
-2,0
-1,5
-1,0
-0,5
0,0
0,5
1,0
1,5
2,0
0,0 0,5
-1,6
-1,4
-1,2
-1,0
-0,8
-0,6
-0,4
-0,2
0,0
0,2
0,4
0,6
0,8
1,0
1,2
1,4
1,6
y = -9.35 10-7 - 2.212 10
-5 x
slope = 2.212 10-5
y = 1.342 10-6 + 2.092 10
-5 x
slope = 2.092 10-5
Ip/
A
Sqrt of Scan rate
Ipc
Ipa 500 mV/s
200 mV/s
100 mV/s
50 mV/s
20 mV/s
10 mV/s
5 mV/s
2 mV/s
1 mV/s
Cu
rre
nt/A
Potential/V
b
Figure 3: Cyclic Voltammograms recorded in solution of K3Mn(CN)6 10-3
M in KCl 1 M at different scan rates (500,
200, 100, 50, 20, 10, 5, 2 and 1 mV/s) of a) oxidation span and b) reduction span; Temperature: 0°C.
In Figure 3 we studied the dependence on different scan rate of the two redox couple separately.
In both cases it is possible to observe a linear dependence from the square root of scan rate both at
low scan rates and at high scan rates, as expected for a linear semi-infinite diffusion regime. The
anodic and cathodic potentials don’t shift as typical of reversible species.
Different KCl concentration study of K3Mn(CN)6.
8
1.0 0.5 0.0 -0.5 -1.0
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
KCl 1 M
KCl 0.1 M
KCl 0.01 M
Cu
rre
nt/A
Potential/V
B
BI
C
CI
Figure 4: Cyclic Voltammograms recorded in solution of K3Mn(CN)6 10-3
M at different concentration of KCl (1 M,
0.1 M, 0.01 M); Temperature: 0°C; Scan rate: 100 mV/s.
Table 2: Values of anodic and cathodic peak potential (B/BI and C/CI) of K3Mn(CN)6 in KCl at different
concentration; Scan rate: 100 mV/s.
Log conc. Ecat (V) B/BI Ean (V) B/BI Ecat (V) C/CI Ean (V) C/CI
0 0.857 0.915 -0.545 -0.354
-1 0.826 0.899 -0.717 -0.289
-2 0.805 0.898 -0.885 -0.196
Slope = 0.026 Slope = 0.0056 Slope = 0.17 Slope = 0.079
Cathodic potential B and anodic potential CI show a slope close to 59 mV characteristic of a
monoelectronic process. The different slopes of BI and C can be related to different electrodic
kinetics during the insertion/desertion process of K+.
This feature together with the different morphology of peaks needs a detailed analysis to clarify the
electrochemical process of the redox couples.
Spectroscopical analysis of K3Mn(CN)6. The IR spectrum of anhydrous K3Mn(CN)6 shows C-N
stretching of cyano ligand at 2122 cm-1
and 2113 cm-1
, as reported by other author16,17,18
. The UV-
Vis absorption spectrum of 10-3
M K3Mn(CN)6 solution held at 0°C to prevent degradation show
two absorption bands: a weak shoulder at 270 nm that is due to disproportionation and
decomposition reactions that takes place, as reported by Trageser and Eysel19
, and another band at
326 nm, responsible of the solution’s pale yellow color as reported by other authors20,21
.
Spectroscopical analysis of FeHCMn and MnHCFe. The IR spectrum of anhydrous FeHCMn
shows a shift on C-N stretching at 2068 cm-1
and a weak shoulder adsorption at 2120 cm-1
; as found
9
in literature22
. The IR spectrum of anhydrous MnHCFe shows a shift on C-N stretching at 2147 cm-
1. The UV-Vis absorption spectra of FeHCMn dispersed in water shows a band at 198 nm. The cell
parameter (a = 10.19) of FeHCMn has been calculated from XRD data using the software unit cell,
(©Team Holland and Simon Redfern); the values are very close to the corresponding metal
hexacyanoferrate.
4000 3500 3000 2500 2000 1500 1000
40
50
60
70
80
90
100
2000 1500
40
50
60
70
80
90
100
K3Mn(CN)
6
FeHCMn
MnHCFe
%T
Cm-1
2068
2113
1604
2122
2120
2147
1609
K3Mn(CN)
6
FeHCMn
MnHCFe
%T
Cm-1
2068
2113
16041096
1007
2122
2120
2147
1609
Figure 5: IR spectrum of K3Mn(CN)6, FeHCMn and MnHCFe powders.
Film preparation and characterization of FeHCMn and MnHCFe.
1.0 0.8 0.6 0.4 0.2 0.0
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
Cu
rre
nt/A
Potential/V
a
10
1.0 0.8 0.6 0.4 0.2 0.0
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
Cu
rre
nt/A
Potential/V
b
Figure 6: Electrodeposition of a) FeHCMn and b) MnHCFe film on a Glassy Carbon electrode by consecutive potential
cycling; Scan rate: 50 mV/s.
The electrodeposition process occurs, even if the compound seems not homogeneous, because in
the cathodic potential we see a behavior due to amorphous-like compound. To compare the
results11,12,23
and see if there is any possibility to have an interchange between Iron and
Manganese22,24,25,
we performed the electrochemical deposition of MnHCFe. There is a growth of
peak at 0.5 V and 0.3 V during reduction scan and at 0.4 V, 0.55 V and 0.9 V during oxidation scan,
as expected.
The modified electrode is characterized in 1 M KCl solution and we have an high background
current, with the same shape of the last cycle of the electrodeposition.
Film preparation and characterization at different pH. The electrodeposition of FeHCMn film
on a Glassy Carbon electrode was made with standard procedure: 10 ml KNO3 1 M + K3MnCN6 10-
3 M + FeSO4 10
-3 M; the temperature was of 0°C in refrigerated cell and the scan rate was 50 mV/s.
The GC electrode has been characterized in 1 M KNO3 supporting electrolyte adjusted at different
pH values (1, 3, 5, 7) by addition of HNO3. The more the solution is acid, the more the shape of
cyclic voltammetry has improved. Anyway there is a larger separation of the peaks, which increases
at lower pH. The strange results obtained for low scan rate is due to the instability of the film,
which degradation does not permit the identification of the potential. Also the film seems to be
more stable at pH 1.
Film preparation and characterization in Buffer solution at pH 7.5. In this test we followed the
deposition procedure for MnHCFe reported in literature12
by using sodium phosphate buffer
solution (PBS), because this seems to work better. We tried to use the same conditions also to
deposit FeHCMn, and verify the difference with MnHCFe to compare if there is any possibility to
11
have an interchange between Iron and Manganese. As matter of fact if the experiment is done at
buffered pH, there is a different behavior, with less background current.
Time stability study of FeHCMn and MnHCFe films. This experiment was done to study all of
reactions that take place during repetitive cycling. The current decreases probably because of the
loss of deposited film and after 100 cycles are lost all the features of the compound.
0 200 400 600 800 1000
0
1
2
3
4
5
Coulomb (C) Ox
Coulomb (C) Red
C
Cycle number
a
0 100 200 300 400 500 600
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
b Coulomb (C) Ox
Coulomb (C) Red
C
Cycle number
Figure 7: Plot of Qcat and Qan Vs Cycle number of a) FeHCMn and b) MnHCFe.
Different KCl concentration study of FeHCMn and MnHCFe films. In this experiment we
made the cyclic voltammograms performed at different KCl concentration (1 M, 0.1 M, 0.01 M) on
FeHCMn and MnHCFe films at scan rates 100 mV/s and 10 mV/s, in order to verify the influence
of K+ concentration on the redox process. The relative Nernst equation can be written as
E= K + 0.059Log K+
that in the logaritmic scale gives a variation of peak parameters of 59 mV per decade of K+ ion
concentration. In Table 3 are reported the values of anodic and cathodic peak potentials. The slope
different from 59 mV can be related to different electrodic kinetics during the insertion/deinsertion
process of K+. This feature together with the different morphology of peaks needs a detailed
analysis to clarify the electrochemical process of the redox couple.
Table 3: Values of anodic and cathodic peak potential of a) FeHCMn and b) MnHCFe in KCl at different concentration
(1 M, 0.1 M, 0.01 M); Scan rate: 100 mV/s and 10 mV/s.
a
Log conc. Ecat (V) 100 mV/s Ean (V) 100 mV/s Ecat (V) 10 mV/s Ean (V) 10 mV/s
0 0.362 0.73 0.402 0.667
-1 0.358 0.659 0.654 0.654
-2 0.347 0.632 0.594 0.594
Slope = 0.075 Slope = 0.049 Slope = 0.065 Slope = 0.0365
b
Log conc. Ecat (V) 100 mV/s Ean (V) 100 mV/s Ecat (V) 10 mV/s Ean (V) 10 mV/s
12
0 0.391 0.732 0.387 0.69
-1 0.326 0.714 0.377 0.654
-2 0.322 0.707 0.321 0.594
Slope = 0.0345 Slope = 0.0125 Slope = 0.033 Slope = 0.048
Film preparation and characterization of FeHCMn and MnHCFe under alternative
condition. In this experiment (Figure 8) we made electrodeposition of FeHCMn and MnHCFe
films on a Glassy Carbon electrode. The main difference with the other test is the width of the
potential span; actually we used-1 V as lower potential for the first 10 cycles and 1.2 V as high
potential for the following cycles.
1,0 0,5 0,0 -0,5 -1,0
-3
-2
-1
0
1
2
Cu
rre
nt/A
Potential/V
a
1,0 0,5 0,0 -0,5 -1,0
-1,4
-1,2
-1,0
-0,8
-0,6
-0,4
-0,2
0,0
0,2
0,4
0,6
0,8
1,0
1,2
1,4
1,6
1,8
Cu
rre
nt/A
Potential/V
b
Figure 8: Electrodeposition of a) FeHCMn and b) MnHCFe films on a Glassy Carbon electrode by consecutive
potential cycling; Temperature: 0°C; Scan rate: 50 mV/s.
13
After deposition the electrode is carefully washed with deionized water and left to dry in air. The
modified electrode is characterized in different supporting electrolyte solution.
1.0 0.5 0.0 -0.5 -1.0
-8
-6
-4
-2
0
2
4
FeHCMn in KCl 1 M
MnHCFe in KCl 1 M
Cu
rre
nt/A
Potential/V
a
1.0 0.5 0.0 -0.5 -1.0
-6
-4
-2
0
2
4 b
FeHCMn in K2SO
4 0.5 M
MnHCFe in K2SO
4 0.5 M
Cu
rre
nt/A
Potential/V
Figure 9: Comparison between characterizations of FeHCMn and MnHCFe modified electrode in a) KCl 1 M and b)
K2SO4 0.5 M.
We can notice that in the cathodic scan there are the same peaks for both electrolytes This further
evidence supports the thesis of the isomerization process of Iron and Manganese in the complex.
Conclusions
Preliminary studies performed in this work of thesis gave us the following results:
We studied the electrochemistry of K3Mn(CN)6 that, to the best of our knowledge, never before was
explored. Due to several different oxidation states possible for Manganese, the electrochemistry
resulted rather complex and depending on the exploited potential span. Mixed hexacyanometallates
such as MHCMn and MHCFe, were synthesized in chemical and electrochemical way. Even for
these compounds the electrochemistry resulted very complex. However a comparison of cyclic
voltammograms of both compounds (FeHCMn and MnHCFe) show a similar behavior confirming a
possible isomerization mechanism between Fe and Mn.
Acknowledgments
I would like to express my gratitude to my supervisor, Prof. Silvia Zamponi, whose expertise,
understanding, and patience, added considerably to my graduate experience. I appreciate her vast
knowledge and skill in many areas and her assistance in writing this thesis. I would like to thank my
co-supervisor Prof. Mario Berrettoni from University of Bologna for his supervision, advice, and
guidance as well as giving me extraordinary experiences through out this work. I would like to
thank also my other co-supervisor Prof. Armando Pombeiro from Instituto Superior Técnico of
Lisbon, for the calm and patience shown in the teaching me how to work and live out of Italy.
14
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