A D V A N C E S IN M A T E R I A L S F O R E L f c C T R O C A T A L Y S I S
S. T r a s a t t l "
DeptrUKM "/ Fkv*4uri I'hemixtrvtmJ tilecm/ckemutirv, I nnvr.\it\ ufMilan,
Via Venetian 21. 20133 Milan. Italy
Abstract
Advances in research and d.ielopmcnt of materials for eleclrocala lysis arc renewed
After n scrutmv of the factors go\cming the clcclrocatalylic activity of materials and an
anah si> of ihe emerging trends. ad\ ances are illustrated by presenting recent unpublished
results from Ihe author's laboratory: H ; e\olulion on Ni +RuO composites, and on RhO,
Rut) . anodes and cathodes lor intermittent electroh'SB, ()• cuilulion on IIOJ I SnO •
<)• reduction on Co spinels prepared from different precursors
:
Keywords; Elecirocatalysis. o\ide electrodes, hydrogen evolution. o\ygcn evolution,
oxygen rcduclion. inlermittent electrolysis
Introductory Concepts
Search for new or improved materials is restless in the field of electrochemistry . Materials are
involved in power sources (batteries, fuel cells, capacitors) [1,2], in electrochemical
reactors
(electrodes for electrosynthesis. environmental processes, surface treatments) [3,4], as well as in
various devices (sensors, electrochromic and electronic devices)
Electrical
elecirolyzers
energy
(AJ'<(J = Al'«/*/) is produced in power
sources and consumed in
Materials are needed to optimize the efficiency in the two cases: (a) maximum / with
maximum A C in power sources; (b) maximum / with minimum A C in electrolyzers A C is the output
voltage in (ti) and the input voltage in (b)
Components o f A f a r e :
ar =
where
the (+)
ae
sign is for electrolyzers
±ir\
± m
and the (-)
± AI ,
(i)
-
sign is for power
sources. A£
is the
thermodynamic (minimum) value (at /~0); En, is ihe sum of cathodic and anodic overpotentials.
IR
is the ohmic drop in ihe interelecirode gap, the electrodes and the connections. AT, simulates the
Stability, i.e.. deterioration with lime o f the other terms.
While AE
depends on the nature of the electrode reactions and cannot be changed without
changing them, i) depend
typically on the electrode
materials (assuming no mass transport
limitations arc operative) Therefore, n can be modified by changing electrode materials: this is the
typical case of (heterogeneous) elecirocatalysis IR
is essentially related to cell design: thus, it is
' Tram a Iccuirc gn.cn ai ihe XI Mccling of Ihe Portuguese Electrochemical Sociel>. 1R-21 April 2001. Pono. Portugal.
Portugaliae Etectiocnimica Acle, 1 9 ( 2 0 0 1 ) 1 9 7 - 2 0 3
-
— 198 —
199 —
more relevant lo engineering (although the conductivity o f electrode materials can have an impact!
The e led meat a lytic activity is typicallv assessed on a relative basis Reference electrode materials
Finally, the stability is as a rule a typical problem o f materials
for ihe main electrode reactions are summarized in Table 4 These materials are a reference either
Table I
Technological Demands
•
Improve the electrocatalytic activity for wanted reactions
•
•
Depress (he electrocatalytic activity for unwanted reactions
Stabilize electrode materials toward wear
•
Replace materials containing precious metals with (cheaper)
materials based on non-precious metals
Find substitute for polluting materials
•
becaisc customarily used in technology, or because they exhibit Ideal activity
P i ] Various
materials have already been lested for the various reactions, which can be grouped imo general
classes as tentatively indicated in Table S The largest variety o f materials has been explored for H ;
evolution (d| and O ; reduction (7.8] The most common classes o f materials are melals (prescm in
all cases) and oxides {idem, except II
ionization)
In this paper, attention will be focussed on
oxides, for more lhan three decades investigated in the author's laboratory [•>. 10]
Table -I
Emphasis will be placed in this paper on materials for clcctrocaialysis In this respect, research
Reference Electrode Materials
work is addressed to satisfy technological demands which arc summarized in Table 1 In view o f
•
Hydrogen evolution: N i , t c. steel
eqn (1), electrodes for technological applications should satisfy the requirements listed in Table 2,
•
Oxygen evolution N i . Pb (Ag.Ca.Su
•
Oxygen reduction: Pt
•
Chlorine evolution: D S A (mixed oxides)
«
Hydrogen ionization: Pt
Table 2
Electrodes for Technological Applications
Requirements
)
•
High surface area
•
High electrical conduction
•
•
G o o d electrocatalytic properties
Long-term mechanical and chemical stability
•
•
Minimized gas bubble problems
Enhanced selectivity
electrocatalytic elfecl
•
Availability and low cost
two eHeels is a largei o f R & D to be able to improve or optimize electrode materials The immediate
»
Health safely
The en ha nee ii lent o f the electrode activity as electrode materials are replaced can be related either
to electmnic fadon
(chemical structure and composition o f materials) or to geometric
(surface area o f materials) The former is a true electrocatalytic effect, the latter only an apparent
Nevertheless, in technology both effects are welcome The separation o f (he
target o f technology is to reduce En. in eqn (1). whatever the way
The main reactions for which eiecirocatalysis plavs an essential role in technological applications
are; C l j . 0 ; and H ; evolution, 0 ; and H? ionization, oxidation o f organic molecules, as well as a
number o f environment-related reactions [5] listed in Table 3
Table 3
•
factors
Environmein-Related Items
Water electrolysis (intermittent)
Table 5
Hydrogen
evolution
Oxides
Metals
Alloys
Carbides
Sulphides
Iniermetallics
•
S O ; oxidation
•
O i eleclrogeneralion
•
•
•
C I O ; electro synthesis
C O : reduction
H ? 0 ; direct synthesis
•
Water pollutant degradation
•
NO.,-destruction
•
•
C'hromate replacement in chlorate industry
DesuUtilization o f natural gas
achieved
•
C H conversion
technology o f preparation.
•
•
On-site hypochlorite generation
C N destruction
4
M a t e r i a l s for Electrodes
Hydrogen
Oxygen
ionization
evolution
Oxygen
reduction
Metals
Carbides
Oxides
Melals
OxideMelals
Carbons
M a c recycles
Chlorine
evolution
Oxides
Metals
A survey o f the specific literature reveals that some general trends are the following
synergelic effects and lh) maximizing surface area
The former
fa) Pursuing
consists in using
materials as well as in achieving more intimate mixing in mixed compositions
composite
The latter is
by improving the physical dispersion o f materials as well as by devising
specific
Emerging items in the preparation o f electrocatalytic materials are summarized in Table 6. In
particular high-energy ball milling can produce nanocrystallinc materials [11], the same target o f
(he sol-gel methodology [12] which however gives less striking results. Iniermitlent electrolysis is a
" Fav
1MOV224. E-mail uasani o icilM cilca n
—
-
200 —
201
-
typical condition o f elecirolyzers powered by solar or eolian energy [ 13) Gas diffusion electrodes
are being extended more and more to electrolyzers [14,16]. Finally, the N E M C A (non Faradaic
electrochemical modification o f catalytic activity) is an effect discovered by Vaycnas [17] which
promises to be o f high technological interest [IS]
Table 6
A Few Emerging Iteim
•
•
Composite materials (metal, oxide, polymer matrix)
Intermittent electrolysis
•
Sol-gel methodology (surface area)
•
•
High-energy ball milling (nanocrystalline materials)
Gas diffusion electrodes (anodes and cathodes)
I iiiiuv I - Current dcnsilv for H- evolution on Ni»RuO; composite materials n amount orRuO; particles in ihc Ni
hull Intel Tafcl lines tor II evolution from alkaline solution. RuO? conlcnl in lhe bath Kl g d m '
«
N E M C A effect (electrochemical promotion)
h i w f ' - Poicmial i» lime HI HXi in A cm - forH: evolution from alkaline solution on Ni and NiiRuO_> composite
material. RuO; coiucnl in Ihc Ni bath: HI gdm ,
In the field o f clccnocatalysis the author's laboratory has been active for several decades [19,23]
Targets have recently been: (a) study o f new materials (metals, oxides, conducting polymers); (b)
comparison o f polycrystalline vs monocrystalline materials [24]; (c) synergetic effects in composite
materials; (d) investigation o f structure/activity relationships. In what follows a few specific cases
will be illustrated
amcur* of RuO?, m I g
tfn"
bme. 11 h
1
Fig. 2 shows that while Ni increases its overpotential with time, R u O : remains stable with a lower
ove'potential o f 0 4 V The stability o f the composite material is also evident i f the voltammetric
charge o f used samples is compared with that o f fresh electrodes
A one-to-one correlation (unit
slope) is clear indication o f stability o f the surface stale o f the composite sample.
While electrodes deposited from a Watts bath showed the best performance, lower activity
was exhibited by those prepared from the thiosulphate bath, despite ihe good activity o f sulphides in
Ni + RuOi Co-deposited Layers
H evolution [6)
:
In alkaline solution N i is used as a cathode for H evolution Although chemically stable, its
:
electrocatalytic activity is moderate and decreases progressively with use [5,6] O n the other hand
Ru0
2
is more active but more expensive [25]. Also, the mechanical stability o f a R u O : film is
lower. Thus, a composite material has been prepared comprising N i and R u O ; , obtained by
electrodeposition o f N i from a N i bath containing R u O ; particles in suspension [26]. The target is to
obtain a N i electrode "pigmented" with R u O : particles embedded onto the N i matrix and therefore
Intermittent water electrolysis
Although electrolysis is a clean technology, it cannot be cleaner than ihe energy source used
to power it A truly ecological technology calls for a renewable energy source l o power an
ciectrolyzer Solar and eolic energy conversions are the most advanced for applications A problem
with them is that ihe clecirical energy provided is noi constant but depends on natural events
mechanically stable
(winds, clouds, etc.) Therefore, electrolysis becomes unavoidably intermittent [28] Intcrmittence
N i + R u O ; were deposited from three different baths, identified by the main component, i.e.,
sulphate (Watts), chloride and thiosulphate, to investigate the effect o f the composition o f the bath.
The amount o f R u O : in suspension was varied between 0 and 10 g dm
The occurrence o f co-deposition was verified by S E M and X P S . Cyclic voltammetry showed
that the presence o f R u O : increases the surface charge
Sulphur was also deposited from the
thiosulphate bath [27]
produces more severe conditions for the stability o f materials Thus, special tests are required and
aJ-Moc materials have to be identified
Intermittent electrolysis was simulated by appropriate current-lime sequences
solar energy ihe change in power can be more sudden than in the case of eolic energy. Therefore,
the current-time sequence was simulated by the current function illustrated in Fig 3 where each step
lasted I min and ihe maximum current was 200 m A cm" . F i g 3 shows that N i + R u O
:
Fig. I shows that the activity o f NI cathodes is dramatically enhanced by the presence o f
R u O : particles, so much as to approach reversibility at low current densities The Tafel slope
decreases from ca 100 m V . typical o f N i . to ca 40 mV. typical o f R u O ; . already with 1-2 g dm"'
R u O : in suspension Although R u O ; is expensive, the composite material with Ni contains a very
small amount o f precious compounds
In ihe case o f
:
composite
electrodes are stable under intermittent electrolysis and as active as pure R u O ; .
Fig 4 shows intermittent electrolysis wiih anodes C o spinels are active materials per se [29)
If nixed with FeOi [30] their activity is enhanced. In particular. 10-15% F e O produces the best
r
activation, whereas other compositions are less active
O n the basis o f Figs
3 and 4, water
-
202
-
-
203 —
intermittent elecirolvsis can profitably be carried out with a cathode o f N H R u O i and an anode o f
smallest size The B E T surface area was in fact the highest The lattice constant is almost ihe same
N i C o i O * * IO%FeO».
lor all precursors, but the phase from the citrate contains also some C o O , The different panicle sizes
-1.3
were also confirmed by different voltammctric charges, ihe highest being that o f C o i O j from
-H
ca rbonale
£ -16
v...
111
s(1>
-1.7
-1.8
ffinn
40
BO
time, i / h
40
AC
lime.i/h
3 - PoKmial w lime LOQ mA cm • for H- evolution from nlLaliuc solution on Mi und N i R u O tt»mposile
male-mis in condi lions e f j o M n f M M clcclrohsis IDtii: t2)2 g IB* : (-•) l u g dm RuO.-: (J)RuO.+
flfl
:
Figure 4- Potenli.il m lime at 200 mA cni" for 0 : evolution from alkaline solution on Ni mid NiCo^O.+FcO,
composite materials in conditions of intermittent elecirolvsis. (1) NiCo<0<: (2) 5%; (3) 10%; (4) 15%: (5) 20%:(6)4«%:
(TlKirl-iFcO,,
:
-Q*
-D?
01
D
L<
rime(h)
hif-urc .i - Vollammolric curies in 0 saturated solution of COjQrbased electrodes prepared by thermal decomposition
ol'diffcrciil precursors I mol dm' KOH: Suppon: Ni. Active layer: 80 \v % carbon + 2(1 w % oxide.
:
1
Figure (> - Long-lcnii performance of gas diffusion electrodes loaded with C o i O , prepared by thermal decomposition of
different precursors 7 5 mot dm"' K O H : 6(1 °C: 111 mA cm \
C o j O i - b a s e d electrodes for O ; reduction
Fig 5 shows voltammetric curves for 0 ; reduction The sequence o f activity is carbonate > nitrate >
0> reduction is important for fuel cells, batteries [31,32] as well as for electrolyzers (air
cathodes [14,16], to decrease AA in eqn. (1)). in particular, in metal-air batteries the activation o f
citrate > oxalate
carbon carriers with specific electro catalysts can be different depending on the features o f
conditions (60 °C, 7.5 mol dm" K O H . 30 m A cm"'). While the oxide from nitrate is initially more
Fig 6 shows the performance o f electrodes in a pilot plant under operating
1
the
power source. For mechanically rechargeable baiteries, no bifunctionality is needed for the O ;
active, that from carbonate is more stable and is the only one remaining more active than the pure
electrode (no 0 ; evolution takes ever place): in this case, electrocatalysts need not be resistant to 0 ;
carton carrier
Further experiments have shown that the pyrolysis o f macrocycles still produces more active
evolution and macrocycles can be used. In the case o f electrochemically recharged batteries,
electrocatalysts
Afunctional O i electrodes are necessary, although ihcy may be less active for Ihe specific reaction
This means that synergetic effects are operative between the metal oxide and
nitrogen moieties coming from the ligands. Experiments arc now in progress to explore the
o f 0 ; reduction.
possibility o f synthesizing active catalysts from C o oxide precursors and nitrogen-containing
In the case o f macrocycles o f transition metals, those o f C o [33] are usually the most active.
compounds less expensive than macrocycles [36]
The activity is particularly enhanced if macrocyles are subject to an appropriate p retreat mem based
on pyrolysis [34], Analysis o f the residues o f pyrofysis reveals that Co o\ide is present in the
I r O i - d o p e d S n O ; Anodes
resulting material Since macrocycles are expensive, we have investigated C o spinels as possible
Oxygen evolution in acid environment constitutes a very severe test for electrocatalysts. Only
substitutes In particular, we have scrutinized the effect o f different precursors on the activity o f the
oxide obtained by thermal decomposition at 400 °C. Precursors were: C o nitrate, carbonate, citrate
precious metal oxides are (relatively) stable [37], Among these, I r 0
and oxalate. Previous studies [35] showed that the point o f zero charge of the resulting oxide is the
resistant [38]. A practical problem is the cost o f the precious compound. A possible solution is the
same, although morphological features differ substantially.
use o f composite materials where the precious compound is dispersed in a less active but more
Oxides were prepared as powders,
2
is in principle the most
stab'e matrix.
as films supported on N i or mixed with carbon
Previous studies [39] have shown that IrO; + S n 0
Experiments o f O ; reduction were carried out in K O H solution Materials were also tested with gas-
2
mixed oxides exhibit a strong surface
enrichment w ith IrO? In addition, mixing lrO? and S n O : together the surface charge o f the resulting
diffusion electrodes in a pilot plant
material is higher than for the pure oxides. Thus, I r 0 + S n O ; electrodes were prepared by thermal
X R D and T E M revealed that Co.O.i from carbonate is composed by particles with the
2
5
-
204
-
—
205
-
decomposition of IrtNO.Oi and SnClj at 400 C on T i as a support. O i evolution was studied in
oxide exhibits a step at about -0.25 V ( S C E ) , which is probably related to oxide reduction. In fact,
HCIOj solution
the backward curve remains flat, with a very low Tafel slope which indicates hydrogenalion of the
U
Fig. 7 shows that the addition of IrO? to S n O ; decreases the Tafel slope from 120 m V to ca.
surface. This is clearly evident in Fig. 10 showing the dependence of the Tafel slope on the
60 m V . typical of IrOi. Fig. 8 shows that the activity for O- evolution increases with the IrO;
temperature of calcination The value for the forward and the backward curves converge toward a
content by a factor of almost 500 It is clear that IrOj is the active component and the sharp increase
common value at the highest temperature, while as the temperature decreases the Tafel
in activity is related to the strong surface enrichment with the precious metal oxide Comparison of
diverge with a very low backward value.
the voltammetric charge for fresh electrodes with that for aged electrodes shows that the composite
materials are very stable under O ; evolution from acid solution
slopes
It is interesting that if the activity (/) is normalized to the unit of surface concentration of
active sites ijlq*, where q* is the voltammetric charge), the normalized activity remains low up to
In conclusion. IrO; dominates the properties of the mixed oxides from 10 mol%. The highest
activity is achieved with a IrO; nominal content of > 20 m o l % , which reduces the cost of the anodes
hi
500 °C and then increases dramatically This probably indicates that the activity is dominated by
geometric effects at low temperatures, and by electronic effects at higher temperatures.
substantially.
70
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> -0 2
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LU
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•1
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...
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o 30
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10
20
-1.0
60
DO
350
1.0
450
500
T/*C
bail I m* tm>
figure 9 - Tafel lines for Hj evolution from acid solution on RhO, electrodes prepared by thermal decomposition at
different temperatures. (1)430 °C: (2) 530 °C Open symbols: Forward potential scan Solid symbols: Backward
potential scan.
J
nominal IrO? cnnlnnl I nwl %
Figure 7 - Tafel slope as a function of iiO- content for O: evolution from acid solution on lrO;+SnO: mixed o\idc
electrodes. (0) Forward and (»1 backward potential scan
Figure* - Apparent currcai density at E- 1.5 V (SHEl as a function ofliO: content forO; eiolulion from acid
solution on ItO +SnO mixed oxide electrodes. ( ) Forward and(») backward potential scan.
;
JW
1
m jia
;
RhO.y-based Electrodes
Figure 10 -Tafel slope for H evolution from acid solution on RJtO, elect rodes prepared at different calcination
leinperaiurcs. (1) Forward and (2) backward potential scan.
;
R h O , + R u O j M i i e d O x i d e Electrodes
Precious metal oxides have been shown to be active for H ; evolution [21,25] In particular,
they are. more stable and less sensitive to metallic impurities than metal electrodes While results arc
Fig. 11 shows a comparison among R h O * R u O ; and IrO? for H ; evolution in acid media. The
activity varies in the order R h O , > l r O j > R u 0 . Since both R h O , and IrO; are much more
2
available for R u O ; and IrO; both in alkaline and in acid solutions, no data are available for RhO,,
although occasional results have shown that it is more active than other precious compounds
RhO, electrodes on Ti were prepared by thermal decomposition at 400-600 °C of RhCl.i
hydrate [40],
evolution was studied in H ; S 0 4 solution Voltammetric curves show that R h O , is
probably reduced by H2 evolution, although it retains its stability The voltammetric curve after H?
evolution exhibits a shape which resembles that of the pure metal [41], with a region of hydrogen
adsorption/desorption and unSymmetrical peaks of surface oxidation/reduction. However, after
reduction, the electrode becomes much ntore active for
evolution, which suggests that the
reduction o f the oxide probably produces a finely dispersed form
of metallic Rh particles,
something like a "Raney" [6] R h , even if the entire oxide phase is not reduced.
Fig. 9 shows steady-state potentiostatic curves. The forward curve for the lower temperature
expensive than R u O ; , it is certainly o f interest to try to reduce the amount of RhO,- by mixing it
with RuO-. Therefore, mixed oxides were prepared by thermal decomposition of R h C h and R u C b
hydrates at 400 °C.
X P S analysis has shown that there is some surface enrichment with R u O with a maximum of
r
about 2 0 % at 50 m o l % S E M analysis has shown that the morphology does not change visibly with
composition, although the layer appears without cracks at intermediate Rh contents, whereas it is
cracked near pure RhO.,, Voltammeiric curves indicate that the surface charge is much higher for
pure Rh oxide than R u oxide However, the charge increases up to 2 0 % RhO„ shows a maximum
there, then increases steadily and dramatically for RhO,. > 5 0 % At the same time the voltammetric
curves suggest that R h O , is presumably reduced as the Rh content is > 60%.
— 206
—
14 N Kuruva and H Aikawa. hJectmMm
Ada 45 (2000)4251
15 N F u r u y a a n d N M i n c o . J . Affl. Etectntcliem.
8-0.4
lo
E
20 (1990) 47S
T Morimoto. K Suzuki. T Malsubara and N. Yoshida, Eleclrochim.
Acta 45 (2000) 4257.
17 C G Vaycnas. S Bebclis and S. Ladas. Nature 343 (1990) 625.
IS
•OS
-3.0
-2.0
-J.0
0.0
tO
60
SO
TOmmal RhçonKçnt i mo! %
3
Figure II - Tipicat Tafc! lines for H; cvoluuon from acid solulion on IK);. RuO: and RhO, prepared b> [hernial
decomposiiion al 40(1 °C
Figure 12- IVieiiiul al 3 mA cm' as a function of lhe RhO, conicni orRuO:*RhO, mivcd o\idc clccirodcs for H
evolution from acid solulion.
Fig
Bokeloh. J Nicole and C h
Comninellis. Electrochem.
19 S Trasatti../ Electr«anat
Chem. I l l (1980) 125.
20. S
O'Grady
In particular, the potential decreases by ca. 0 15 V as only 20 moI% RhO, is
Trasatti and
EllgjllttliMg,
:
W E
Soild-Siale
Lett. 2
in Advances
and
Electrochemical
Hydrogen
H
Technologies.
Wendt E d , Elsevier. Amsterdam
(1990), pp I, 104
22 S Trasatti in The I Lctrochemts'n
of Novel Materials.
J Lipkowski and P N Ross Eds
VCH
Publishers Inc., 1994, p. 207
added Thus, the maximum activity is achieved with only 1/5 of Rh, with a substantial decrease in
23. S Trasatti tn Iitlerfacutl I lean/chemistry
the cost of the electrode
in Electrochemistry
H Gerischer and P Oelahay Eds . V o l 13. Inierscience, New York, 1980, p. 177.
21. S. Trasatti in Electrochemical
12 shows that lhe potential ai constant current decreases (lower overpotential) as the RhO,
content increases
Wodiunig. F
(1999(281
20
10
i mA crn" J
S
Theory Tiactic:
Applications.
A Wieckowski E d . ,
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Acknowledgements The author thanks the following co-HOriVcrs for their conlnbudon A C Ta\ares. D
Milani. C. Mocchi. M . Brcgolato. M Campari. C P. Dc Pauh Financial support from M U R S T . C N R.
and E C.C. isgralcfijlh acknowledged
24
B. E Conway. EJectrochemicalSiipercapacitors,
3
K Rajeshwar. J G Jbanezand G M . Swain,./ AppL Edectrochem. 24 (1994) 1077
4.
A . M Couper. D PletchcrandF C. Walsh, Chem. Rev. 90(1990)837
5
S Trasatti, hit. J. Hydrogen
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T
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Wellington E d . . Elsevier Applied
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Acta, 45 (2000) 4195.
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in Electrochemical
Technology, V o l . 7, S. Sealey E d . , The
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7.
E Yeager. Eleclrochim.
8
C. Fischer, N. Alonso-Vante. S. Fiechter and H Tributsch. J. AppL Electrucltem.
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31
R W. Reeve. P A Christensen, A Hamnett. S A Haydock and S C Roy. J.
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1004.
S Trasatti and G. Buzzanca../ Electroanal.
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Chem. 29(1971) App. I
10 S. Trasatti and G. Lodi in Electrodes of Conductive Metallic
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V . Gorodetskii, V
CHEMICAL AND E L E C T R O C H E M I C A L CHARACTERIZATION O F C O B A L T
19 1198") 19
BEARING FERRITES ORIGINATING FROM T H E PURIFICATION O F C O B A L T
21 (1991) 335
A Neberchilov and M . M . Pecherskii, Russ. .1. Ekarovhem.
CONTAINING W A S T E W A T E R
30 (1994)
916.
39. C. P. De Pauli and S. Trasatti,./. Electroanal.
40
1
2
1
3
1
Y u . E Roginskaya, O. V . Morozova, G. I. Kaplan, R R. Shifrina, M . Snilmov and S Trasatti.
Ekctrochim.
41
E. Barrado , F. Prieto , F . J . Garay , J . Medina and M. Vega
Chem. 396 (1995) 161.
Aciatt
(1993) 2435.
M . l . Florit. A. E Bolzàn and A . J . Arvia,./. Electroanal.
1
Chem. 3 9 4 ( 1 9 9 5 ) 2 5 3 .
Departamento de Química Analítica. Departamento de Física de la Materia Condensada,
5
Crista log rati a y Mineralogia; Facultad de Ciências, Universidad de Valladolid. 47005 Valladolid
(SPAIN). E-mail: ebarrado@qa tiva.es
;
Centro de Investigaciones Químicas, Universidad Autônoma dei Estado de Hidalgo, Crta. Pachuca-
Tulancingo, Km 4.5, 42076 Pachuca, Hidalgo (MEXICO).
Abstract
In this research, lhe efficiency of the "ferrite process" for the purification of wastewater
heavily contaminated with Cobalt (99.99% in the optimal conditions) was verified, and the
three cobalt-bearing ferrites produced using three different F e ' / C o
í
íf
molar ratios (15/1, 7/1
and 3/1] were characterized by chemical analysis (ICP-AES and potentiometric titration),
X R F . X R D and D S C , indicating C o . F e V . F e V ) , (x = 0.16, 0.35, and 0.65 respectively) as
the most probable structure of the solids Electrochemical analysis of the solid cobalt ferrites
was performed using a carbon paste electrode in H C I d and HCI media. In each case, the
first cyclic-voltammogram showed the participation of solid species in the elect roc fie mi cal
transformation process. In second and successive scans, the voitammograms indicated the
redox couples fe *„d + 1e" <=> F e '
5
S
?
M S
(E = 0.525 V vs AgCI/Ag) and Co ' + 2e <=> Co(s) (E =
2
-0.230 V) in HCIO<. and Feci;'™ + 1e •=> F e C I " ^ + C f ( E = 0.475 V) and C o C k " * + 2 e
o Co(s) + x C r ( E »-0.320 V) in HCI.
Keywords:
Cobalt, Wastewater treatment; Metal removal. Cobalt-ferrites, Voltammetry,
Carbon paste electrode.
INTRODUCTION
Uncontrolled industrial and agricultural practices, combustion processes and mining
have led to the accumulation of toxic heavy elements info the environment [1.2J. The
growing concern of increasing environmental levels has prompted rigorous restriction
Poifuga/iae Eleclrochimica
Ada. 19 (2001) 209-313