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Investigation of the carbon-oxygen (air) electrode
J- ~Ie&d. Ckem.. 180 (1984) 619-637 Elsevier ‘Sequoia S.A., Laubfie - Piinted in me Netherlands
I
619
..
.’ I~sm.~&-moy
-.
..
OF -TgfE &B~N-~~GE~
(ATFt)ELELCT.I~&E *
..
:
6-V. SHTEINBERG, A.JJ. DRIBINSKY, I.A. KUKUSHKINA, L.N. MOKOROUSOV and
VS. EAGORKY A’.N. Ahhkin Instittite of Electrochemistry, Academy of Sciences of rke U.S.S.R. Leninsky prospect 31. 11~071.Moxow (LJS.S.R.) (Received 15th May.1984)
..
ABSTXACT
The microkinetics of oxygen reduction on various types of active carbon, and the capillary properties of active carbon and carbon electrodes have been investigated. The reaction rate on active carbon catalysts depends on the ratio of basic and acid surface groups. but more significantly so on the microporosity of carbon. It has b?zn demonstrated that the macropores in carbon are Iyophobic in alkaline electrolytes, contributing essentially to the gas porosity and to oxygen mass transfer processes in the air electrode. Ion transfer occu;; through minute micropores of active carbon and lyophiiic pores of the secondary struct,ure of the carbnn electrode. The optimal active carbon-to-hydrophobic component ratio depends on the capillary propenies of carbon.
(I) INTRODUCTION
.,
The utilizatioli of carbon materials as a base for oxygen &d air electrodes was suggested about 100 years ago; c&nmercial prdduction of carbon-air electrodes for some type+ of chemical power sources was started-more than 50 years ago. However, the problem of, m&~fact&ing cheap highly-active oxygen and air electrodes based on carbon materials still remains urgent. The investigations of carbon electrodes were significantly extended and intensified in the sixties in connection with the development of oxygen electrodes for fuel cells. At that time, moderr, types of caibon electiodk were develo$ed-the two- and mdty-layer electrodes [1-31-w&h allow&d. the electrode thickn% to be considerablydecreased and the electrode characteristics improved. The investigations of Schwabe and co-workers showed the &tract&&tics of oxygen electrodes to be essentially dependent on the type of carbon ,material [4], its -pr&minary treatment and the subsequent gas activation of tht? &ctrodes is]; .as well ,& -on the& porous structtire. Schwabe [4] inferred that the electrode activity deijends.only on p&r& df, JO,30 nm size; more minute pores do riot tffect the‘:l&rode ~&iv&. .The’ dGpend&ce of the rate of cathodic oxygen * Dedicated to the memory of Professor Dr. Dr. h. c.
Kurt
Schwabe.
620 reduction on the pH of the solution for carbon electrodes with platinum on carbon as catalyst was also investigated [6]. These investigations were basically concerned with carbon electrodes containing a more active catalyst-silver, platinum or. metal oxides; The characteristicsof pure. carbon electrodes were studied mainly with a view to estimate the activating effect of the catalyst added. It was shown later that oxygen electrodes with pure carbon catalysts (active carbon) can be used in power sources with ‘alkaline electrolytes [7-111, these electrodes being considerably cheaper. Comprehensive investigations of the physico-chemical properties of carbon materials, particularly their electrocatalytic activity in the oxygen reduction reaction, and of the current genefation process in the carbon gas-diffusion electrode are being carried out in the A.N. Frumkin
Institute of Electrochemistry of the U.S.S.R. Academy of Sciences with a view to elucidating the means of improving carbon-oxygen and carbon-air electrodes. Parts of these studies were carried out in collaboration with the Central Laboratory of Electrochemical Power Sources (CLEPS) of the Bulgarian Academy of Sciences and the I. Heyrovskjl Institute of Physical Chemistry and -Electrochemistry (HIPhChE) of the Czechoslovak Academy of Sciences. Some of the results of these investigations are presented in this paper. (11) EXPERIMENTAL
Active carbon catalysts have an intricate porous structure, the effective pore radii differing by more than four orders of magnitude (0.5 nm-10 pm). The structure of a carbon electrode is characterized not only by the primary structure of the active carbon grains, but also by the secondary structure of the pore space between these grains. An essential characteristic of the waterproofed carbon electrode, besides its porous structure, is the hydrophilic-hydrophobic (capillary) properties of the pores. The porous structure and the capillary properties were investigated by conventional methods: mercury penetration, measurement of benzene and water vapour sorption and weighing the specimens in air, wetting and non-wetting liquids. A number of new methods were also developed to estimate the capillary properties of certain pore classes. The double porosimetry method involves comparison of the mercury and electrolyte or water intrusion curves in, identical specimens.‘ In the case of a purely hydrophobic medium, the capillary equilibrium in the pore space obeys. the equation: p=
-2acosB/r
0)
where r is the pore radius, 0 is the surface tension of thr liquid and 0 is the contact angle. For two different non-wetting liquids, for example, mercury and electrolyte solution, with the same sequence of filling, the ratio of the penetration pressures for the electrolyte and mercury in the same pores is given by the equation: (0, cosel/P1)B=(~~~sez/P2)P
'(5)
621
or A In Pp = In{ c/c,)
i- ln(cos
B,/cos
6,)
(3)
where A In Pp is the distance between the penetration curves (F-In P) for the electrolyte and mercury and v is the volume of pores without liquid. In the case of hydrophilic-hydrophobic materials, such as carbon, the capillary properties of pores with 6 > 90 O_ sts well as the volume of pores with fl< 90 O, can be determined by the above,method [12]. Similarly to the mercury penetration method. the double po.rosimetry method provides a means for estimating the structure and capillary properties of carbon pores in the radii range from 103 to 3.5 nm. The structures of hydrophilic and hydrophobic pores in a specimen can also be determined separately by the “freezing” method This method is based on comparison of the mercury penetration curve for an initial specimen and for a specimen with the hydrophilic pores preliminarily fibed up with water or electrolyte solution, the liquid then being frozen. Mercury penetration is performed at a temperature below the freezing point of the solution. The mercury penetration curve for a frozen specimen correspljnds to the hydrophobic pore radii distribution; the difference between the penetration curves for the initial and frozen specimens corresponds to the radii distribution of hydrophilic pores [13]. To estimate the wetting of minute pores by the electrolyte, the charging curue method was used [14]. The value of the apparent doubie-layer capacity CX determined from the charging curves in an inert gas atmosphere depends on the size of. the wetted surface for a particular material. The change of C” under different water repellency treatments of carbon in the process of electrode formation, or under long-term storage or operation indicates a change in the wetting by the electrolyte of micro- and mesopores malting the main contribution to the double-layer capacity of carbon. The characteristics of carbon-oxygen and carbon-air electrodes were investigated using two types of two-layer gas-diffusion electrodes. In the first type of electrodes the active layer consisted of a mixture of active carbon and polytetratluoroethylene (PTFE) particles; the hydrophobic layer was a thin plate or film of porous PTFE. In -the second type of electrodes the active layer was a mixture of active carbon and a hydrophobic component prepared by depositing PTFE on carbon black: the hydrophobic layer consisted entirely of waterproofed carbon black. The second type of electrode, developed at CLEPS [9,15], were used in the studies performed in collaboration with CLEPS and HIPhChE [16-181. The true catalyst activity in the working carbon electrode may be overshadowed by mass transfer processes. In order to determine the true ele-:trocatalyiic activity of carbon catalysts, a model floating gas-diffusion electrode MS developed [15]. This consist of a very thin layer of carbon particles (approxinlately a monolayer of particles of lo-40 I.crn diameter, total mass less than 20 g .m-*) on a porous hydrophobic conducting substrate. Carbon is deposited or the porous substrate without a binder, and is not subjected to any heat treatment, being retained on the hydrophobic surface of the substrate by adhesion alone. The electrode is placed into
.’
:
622 ,,.
a hermetic glass cell so that the hydrophobic substrate is located. in the gas phase and the catalyst is immersed in the electrolyte (Fig. J),.The amount of carbon .in contact with electrolyte and substrate is ,determined directly m the cell from comparison of the electrode .double-layer capacity with. the capacity of. .1 g of the given carbon powder 1201. Such an arrangement of the .elec;rode -eliminates mass transfer limitations over a wide range of current values (of 2-3. orhers-of magnitude), i.e. the inner surface of ‘the carbon. ,grains is equtilly accessible in the. oxygen reduction reaction_ This permits the oxygen reduction- microkinetics to be-studed directly on active carbons and makes.it possible tocompare the-true electrocatalytic activity of carbon catalysts with various structural and surface properties-The direct contact between the carbon grains and the conducting surface in the model electrode also allows the .oxygen reduction process to .be investigated in low conductivity : .~ carbons, for instance, in fossil coals [21].
Gas
n
-L
-Hydrophobic substrate
=A
== -
‘\
-
-
-
--
-u L_
Fig. 1. Mcdel floating gas-diffusion
electrode.
Corbon particles
623 III. RESULTS
A&D
DISCUSSION
Active carbons
(1lI.I)
(III.
1.1) .Capil/ary properties of active carbon
The- porosity, hydrophilic-hydrophobic, properties and lyophobic behaviour in alkaline ele&rolyte (7 M KOHj of commercial active carbon AG-3, other types of active carbon based on fossil coals (bituminous coal, lignite, peat carbon) and active carbon HS-4 (Czechoslovakia), Nor-it NK,(The Netherlands) have been investigated. To eliminate the influence of mineral impurities often present in commercial active carbons, the carbons were Subjected to preliminary treatment in concentrated hydrofluoric and hydrochloric acids with subsequent removal of halide traces by washing, drying and heating in a hydrogen flow. Some of the carbons were subjected to further activation in a vapour gas mixture p20 + CO,). The investigations were carried out by means of the combined mercury, water and alkali penetration technique, as well as by the charging curves method. The capillary .properties of minute-pores (< 3.5 nm) not accessible for,mercury &.$ration were investigated in more detail by comparing the sorption of benzene and Gater vapours. The micropores of. AG-3 carbon were found to be lyophihc in 5 M KOH soluticn, being wetted by electrolyte after contact of short .duration_ The macropores are lyophobic and are filled with alkaline electrolyte only under pressure. The contact angle for alkali solution calculated by eqn. (2) from the shift of the alkali penetration
1.2
Active
carbon
AS-3
1.8
CI_
$nP/Torr)
-. Fig. 2. Mercury, alkali
a ad
wate; penetration c~xves for AG-3 carbon.
_’
624 TABLE
._~
1
Basic structure characteristics
of carbons a
; , ”
($1
Powder weight/g dme3
AC-3 K-1
13
596
K-2 K-3 K-4
37 53 87
510 420 375
Carbon
type
Burning out
,
T-1
-
545
T-2
-
370
KM-2 Norit-NK+
-
10% PTFE HS-4
-
_..
,- .
: Vma
,1.17 O-62
0.7 0.38
0.83 1.03
: O-45 0.53. 0.74
1.44
Vmi.
.Vphoh
. 0,28-~ -0.74 0.20.
0.42
0.32 0.35 .0.42.
O.&l 0.52 0.87..
Vphi,
/_
:m.! i-’
. 0.43. 0.2
97 ‘23.
-. 0.39
: 48 _
.0.28 ,_ 0.58
.:
....,-I s,,
Pore,volume/cm3 g-’ Vx
--:. .-
95 .. ,170
OSl0.57 0.41
85.
.. 8/O‘. .. ‘.
-. 105 ., ‘-:
95
101
0.99
0.58.
1.71
0.92
0.34
1.1
0.61
235
105
540
0.83
0.4
0.38
b.46
0.37’
-29
106
-
1.08. 0.55
0.81 0.34
.-.
0.91 022
0.17 0.33
:
-’ -7 -
‘.
..- .95: 1G4 ..lO$
96.
: 7
a Vr = total pore volume; V,. = volume of macropores: Vmi = volume of micropores: V&. Vphi, = volumes of Iyophobic and iyophilic pores, respectively; S,,,, = surface of mesopores; 0 = contact angle in 7 M KOH solution. curve
with
pores
are not
respect
to that
wetted
mercury (Fig. 2) is B,,, 7 M KOH solution, &en
of
by the
=
95-100”
after
contact
[22].
T&
macro-
df Jong duration
(Fig. 3). As the surface tension of water is lower, carbon is &ad_uafly wetted by it. Therefore, stable hydrophobic properties of carbon can be achieved only by waterrepellency treatment (Fig. 3). Similar results were obtained with other types of active carbon, which allows the lyophobic properties of _macropori-s in alkaline electrolyte, '1.0 t
Fig_
3.
(0)
Active carbon AG-3;
Variation
in
time
of
the
gas
porosity
of AG-3
carbon.
(8) active carbon AG-3 +20%
mFE
( X) Active
-.
carbon
AG-3+10%
Paraffin;
as we1l.s. the Iydphihc~properties of micropores, _to be.regarded as a general property of active carbons. This inference. is essential for understanding- the nature of current -gene&&n in the carbon-_gas&iff~sion electrode. The ~volumes ,of the macro- and _micropdres; and Accordingly of the lykphilic and lyophobic pores, in various types of
active carbon, are-compared@ Table 1. ..- The volumes of all the .ciasses of ports increase -after additional. activation of carbons,in-a vapour gas.&xture (H,O + CO,) at.a temperature of 90C°C; because p&of the carbon .burns out according to .the reaction: .. _ -. ., It is of interest that long-term activation (high degree of burning OUL)also affects
c+co,.;2co
significantly the lyophobic properties of the pores: the contact angle of the macro/ cm3 g” *-\
ec
LnLn(P,.P-‘)y,
-Fig. 4. &other&of benzene and .yaier sorption on. active. carbons with various activation degrees. (@) active carbon KM-3; (0) active carbon KM-2.
,.
626
-:
. .
pores increases and slightly lyophobic. pores with:’ effective .radii less thati’ 5 ‘r& emerge, Enhanced hydrophobic properties .of a~carbqn el&t&dk ,after a&$Vation by gas was pointed out in ref. 5:The capillary ekope&es of‘tiinute poieS were. stud&d in more detail .by sorption methods. .: :. : . ‘,’ - -’ .. ..-: I.‘, ’ Comparison of the sorption isotherms of betienk and w&r vapouis-shows that ; with low degrees of burning out the volume of sorbed benzene VC+; is fieai’ly, equal to that of sorbe’d water VH;o (Fig. 4), i.e. all the rnicropbres ark hydrophilk. After long-term activation, the volume -of sorbed. benzene increases‘ more th& that or water (VC+, > G20 ), which indicates waterproofing of part of the.minute pqres. At the same time, the shift of the sorption isotherm of water with respect to that of benzene increases, which the authors consider as more -.eviden& for the -es&aked
-2
0 LnLn(P,
2
4
6
_P-‘1
Fig. 5. Isotherms of benzene and water sorption on KM-2 carbon with various surface treatments. (a, A) Initial: (1;:) treated in 0, atmosphere;’ (v,v) treated in CO, atmosphere; (CI,E!I)treated in (3% atmosphere; (C. @) anodic oxidatkn.
.;. 627
hyclroehobic propertiesof minute pores. Thus, the-activntion processcauses porosity and lyophobic properties to increase simultaneously, .the latter increasing first in large pores.and then in more and more minute ones. Subjecting active carbons to treatment- in an oxidizing atmosphere, for instance, in air at 400 o C, or to anodic oxidation in an alkaline electrolyte results in the pores becoming gradually hydrcphilic, starting from minute pores and extending to large ones (Fig. 5). Chemical analysis of the surface functional groups on carbon specimens subjected to various treatments, and with various degrees of acti\.ation showed that the total content of oxygen-containing groups, as well as the ratio of alkaline and acid groups, depends .on the kind of treatment applied. Comparison of the data of chemical analyses with the capillary properties of carbon shows ihe enhanced lyophobic properties to be due to a decrease in the amount of acid (phenolic. carboxylic) groups and an increase in basic groups, while lyophilic properties result from an increase in the amount of acid groups on the carbon surface. (III.l.2) Microkinetics of oxygen reduction on active carbon Oxygen reduction kinetics were studied under conditions of equal accessibility of the inner carbon surface by means of a model floating; electrode in sulphate. phosphate, carbonate and KOH solutions, the range of pH 0.3-14 [23]. The steady--state polarization curves in these solutions for AG-3 and K-4 carbons are presented in Figs. 6a and 6b. Two Tafel slopes are characteristic for these curves, - b, = 50-55 mV and b2 = loo-150 mV. In solutions of pH 14, the first Tafel slope has a constant value over a range of almost three orders of magnitude of the current. With decreasing pH, the E,/log I curves (where E, is the potential referred to the rhe in the same solution) shift towards the negative side, the, first slope becoming less marked. With the more active K-4 carbon, the 6, slope is clearly marked in acid as well. At a constant current value, the shift of the curves with changing pH is aE/apH = -40 mV. In the first and second slope regions, the current depends linearly on partial oxygen pressure, i.e. the reaction is first order for oxygen. Thus from the data obtained, it can be seen that there are, two kinds of liinetic dependence for active carbon: j = /po, [ H’Ioe9 e_EFlRT
(4)
and
which turn one into the other with changing potential. The slope b, = 50-55 mV in the potential region E, 3 0.8 V is recorded for all the active carbon types investigated_ The investigation of oxygen reduction kinetics on smooth carbon materials, for example on pyrolytic graphite, showed,- as a rule, that there is only one slope, 2RT/F, and that the reaction rate does not depend on the pH of solution [ll]. This
,628 .
..
can be -reasonably understood. oxygen molecule to be slow: ~2~.~wxis
the trknsition
assuming
I.
~- ‘,
of_ Ihe first electkon to- thk .,_.._. .’ : ‘. ;.. .‘_ _, ;:;-.. I’ I_ ‘_.. c .,_._
(02 Lds + =- ++ to,_ Lhs a-
Ld* + HOH --j HO, + OH-
HO,.+
e- + HOH + H,O,
+ OH-
I. ‘.. d
*
14.0
,.
.-.
.:
e’ 0’ o/’ 4/
03
7.6
.’ /
*/’
irn Hd
u
/.
<
da
z E J
d 2-
0
0.6
0.6
PH
Fig. 6. Polarization cw-ves of oxygen red&ion
AG-3 carbon; (b) K-4 carbon.
on a model electrode in
sob&& of different PH. (6
629
.-
The same.reaction scheme fits eqn. (5) satisfactot’.ly. To explain the-first slope 6,, the . surface oxides formed on carbon at potentials E, > 0.8 1: were.assumed to exert a retarding action similar to that assumed for- platinum and palladium at potentials .more positive.than 0.8 V [24]. (-rir.1_3) Electrocatalytic aaivity of c&on catalysts of d$ferent m-m&-es Comparison of the electrocatalytic activity of carbon catalysts of different structures was carried out-with the model gas-diffusion electrode; for which the carbon catalyst is not affected by a:ay treatment of the electrode during its preparation (mixing with a binder, heatin g, etc). From the preliminary results of the study on the capillary properties of carbon, as well-as from refs. 4 and.25, the authors assumed that the strictly electrochemical step of the oxygen reducticn process proceeds in mesopores, whose properties tie ‘intermediate between those of the “dry” macropores and the electrolyte-filled micropores. Therefore the investigation was performed on carbon types with significantly varying mesoporosity. The capillary properties of these carbons have been described in Section (111.1.1). The polarization curves for oxygen reduction in 1 M KOH solution are in all the cases similar to the curves pH - 14 in Figs. 6a and 6b, differing only in the current values. The activities (I (mA g-r) at constant E, = 0.9 V) of the carbons arc compared in Table 2. It can be seen that the activities of carbons from various sources differ by more than one order of magnitude. For each carbon series the activity increases slightly with increasing activation degree. The authors failed to detect any correlati.on between the activity and the mesopore surface of different carbons: in some carbon series the activity increased with increasing mesopore surface, in other series a contrary trend was observed, and in some series there was no dependence on the mesopore surface. The activity in different- carbon series with equal mesopore surface differs by a factor of .lO-30. At the same time, a correlation between the activity and the TABLE 2 Activities of various carbons in the oxygen reduction reaction on a model electrode in 1 M KOH sohtion
CapacityC” at
Activity I at E 0.9V/
Activity I at E = 0.9V/
E, = 0.7-0.8 V/F g-’
mAg-’
t&F-’
75 76 75 80 70.
90 65 85 135 55
1.2 0.9
K-2
80
440
5.5
K-3 K-4 KM-2 KM-3
90 90 70 69
650 650 1015 580
7.2 7.2 14.5 8.4
Norit-NK x-IS-4
70 90.
67 30
1.0 0.3
Carbon type AG-3 B-2 T-l T-5 K-l
1.1 1.7 0.8
. ..‘. 6% microporosity of carbon was .detected (Fig. 7); though no quantitative-cdrrelation ,.:_< .:. between these parameters was established. The results are in good agreement with. the data of cap&y ‘mvestigation, &rich show that in strongly activated carbons, for example in KM-3 carbon (Table l), the micropores alone are wetted by the electrolyti. The high electrochemical activity.of KM-3 carbon in the model electrode (Table 2) points to the electrochemical reaction proceeding in micropores or on the boundary between micro- and mesopores, Thus, when mass transfer limitations were eliminated, the oxygen reduction kinetics were found to depend significantly on the microporosity of carbon. (11.1;4) Effect of carbon surface treatment Active carbon KM-2, found to have the- highest activity in the .model electrode, was chosen as the object. It was subjected to various treatments which did not affect its porous structure but led to a change in the composition-of the surface groups (heating in NH,, Cl,, air, CO, flows, anodic oxidation in 7 M KOH solution with
I
0
0.4
0.2 Vmi /
Clll 3
g-l,
Fig. 7. Dependence of the carbon activity on the micropore voiume.
1
631
.stzbsequent wash&g t? remoye alkali, and ,drying). Changing the composition of the surface ,&oups allowed the activity of carbon?n the model electrode with 1 M KOH +- el&Qo!yte to-be changed tw0 to three-fold, with a natural electrolyte by a factor of_ 5-8. In an alkaline‘electrolyte *he greatest activity was found with carbon having b&c _anc a sm&. Flaunt of acid surface grq.~ps. (II7.2)
Capillag
prOperties
ofcarhn
electrodes
:The ca&llary Froperties .of &bon electrodes were investigated by the same methods as those of active carbor;;s, Gith the exception of the sorption methods. The effective co.nductivity of the electrolyte in the active layer pores was also measured. Comparison of the 7 M KOH intrusion curves for active carbon AG-3 with similar
tyophobic
pores Lyoptiilic
Active
c&-bon partictf5
Fig. 8. Model structureof the carbon-FIFE
PTFE active layer.
pores
..
632
-:
:
curves for an electrode prepared froili a .mixturk of-_AG-3 karb& pSr!icle~~~an~da. PTFE suspension, and calculation of thk -tie&i r&KS of ..the gas pores .(Tgasj’ .in carbon and in the electrode active layer showed the main Vo!uk~e-of g&$&s in :ti& electrode to be contained in the carbon &ai,tis. .Thti high ‘de&e bf dispersion‘of the an effetidtiv++ sUp& ix-A0 gaseous phase in carbon (& = 0.3-O-35 pm)‘-provides the carbon grains and, consequently, participation of. the inner carbon surface in current generation. PTFE mahes the- external surface of carbon graitis, ik the secondary structure pores, water-repellent. thus providing a contact bti:t\veen’the carbon grains and the- gaseous phase [26]. The strkture of- the carba. :l?TFE electrode is shown schematically in Fig:& The relationship between the capillary properties of the second type of -ca:.bon Hydrophobic
.--r-~~,,
.‘. Active
component
(h.c.)
/
_
ccrhon
\
*y
h.C. 0.E
L i
0.f
‘ST ol
9 >
03
‘0,
0,
0.;
Active
carbon
1
ri S_,
\ c
3 I
6.
I. :\
‘0.
4
I
l\ \
0
I
10
-12.
Ln (Pfl0I-r) Fig. 9. Alkali penetration curves in active layers of different composition.
-
633
electrodes.and.
the, carbon catalyst-to-hydrophobic
component
ratio.was
investigated
in. collaboration_ with: CLEPS &rd the. HlPhChE. The waterproofed carbon black gas+pplying layer of these electrodes was found (by means of the double porosimetry~technique) to .contain.pores of less than 2 &-I size, more than 90% of the pore volume being’lyophobic in alkaline electroiyte., The contact angle of these pores in alkali &as 6 = 113-i15 P. The degree of ~lyophobic. porosity of the active layer consisting of a mixture of HS-4 and carbon black depends chiefly on .the content of the latter (Fig. 9).‘Figure 9 shows that. the lyophobic pores of the secondary structure formed. during electrode .preparation (hatched area) can also. be revealed by the technique of alkali intrusion into the active layer and its individual components. The& pores appear .to be gaps. between the carbon grains and hydrophobic component. From the change in electrolyte content in the active layer, with varying ratio of active carbon and waterproofed carbon black, it was inferred that the secondary structure pores are filled with electrolyte [17]. (111.3) Effect electrodes
of structure
on the characteristics
of carbon-oxygen
and
carbon-air
(1113.1) Electrodes with carSon-PT_FE actioe layer The relationship between the activity of these electrodes and the ratio of active carbon AG-3 and PTFE masses was studied. Figure 10 represents this relationship
E/RlV
-300
I-
_--o--
-----_o_
-L -\* -.
<--.
t
-250
‘.
I.
-200
--
50-
,_e_---'----.-,_
-_
e, ,A---
--.
_------___------_
e -150
* -1co
"__--._.
." --Q
0 20
1 30
Fig. 10:Dependence of the curxmt density on the air-electrode electrode; (---) after 500 h loa&g.
I 50
on the PTFE
content.
( -.)
initial
: 634
;.
,.
--
i-.
.:
.-
for air electrodes after%peiation:for‘1 and :l%‘h,at-25 %i&‘cm’~~It &n bk.sekri that at low polai-izati& va.lueS( EHg;HgO=- +:lOE) kv) the &i&y decrt+ari;essiiglitl+ witi increasing FIFE content; at -higher -pola&ation +%&es the -plot of: +i&y. tigaikst -. FTFE content has a maximum; .though“a, very flat’ one-Such a~~rel&on&ip_.differS significantly from that of the &r eIect@de -witli, 'platinu&y ‘iatalykt. [27]::- After operation for 150 h the activity maxikntiti shifts towards greater PTFE conte+td;‘the -. activity increasing in ‘time at low polaikqti’o~ k&es, aild decrksing- at ‘hi& &es: Comparison of these data with the -data. on the capillary.-prope&es,- ‘And iin the effective electrolyte conductivity in the active laytr.sh&s that in the&r {electrodes studied, at. cur&t densities .less thzin 50-70 mA;/crii’, the activity is limited by ohmic resistaiices in the liquid phase. Increase in acti,vity,with-operation tim’e is di.xe
20(
151
-250 N
/
.’
‘5
/
e,Of
_---a
’ . .
1’ +-----_,_ -.
\
DL 0.21
I
\
\\ 8 \
‘\ \ \ /-
1
-200 -250
:200
-150
‘.
-
-150
----o__
-0. -1-00
.c’--0.4
-300
\
‘.
5,
a-
\\
\
‘.
%
0.5
t
3.6
I
0.7
t
0.8
S-8
Fig. 11. Dependence of the current density on the air electrode (A) active carbon AG-3: (a) activecubor; KM-2.
oti th.5 hydrophobic
&mponent
content.
63f;
:
_So&i@g of., the act&layer and increase in the. effective.conductivity in the electrode pores:_At.&ti& de&es great& than 100 _mA/cm*~ &ffusion,resistances in. .the oxygen. :supply from .the ,gaseotis phase.-into .the reacition zone become app.arent, l&d the- soaking.of the active Jtiyer causes ‘activity.to decrease in time (Fig. .lO).,-Aftei-ciperation for 300.6, the capillary properties andPopertition.characteristics :do not change.; The electrodes 6peiate:steadily &r 7000 -h at j.= 25 mA/cm’. ,‘_ !, .(III.3.2) Ekrodes of carbon and wtzterproofed carbon black : ‘.In klectrodes of.the s&ofid type the effect of the carbon catalyst-to-waterproofed carbon black ratio -on .activity was studied, .th& carbon catalysts chosen varying in their eltitrocatalytic-activity in the model electrode,- and in. their capillary properties [17]. The polarization of these electrodes was studied both in air and pure oxygen, which allowed the diffusion resistance in oxygen supply in the air electrode to be characterized quantitatively.in terms of the AEi parameter [28]. (A& = ( Eo, - E,,,)i -ml the_
0
2:3 t
oActive
\,
\
carbon
Active
i-
iiS_&
carbon h.c.
1
o-
O-
Active,cbrbon
A&,
0, 0. QuOH g active
: . .
..’
layer
-”
Fig. 12. D&c cdencx of AEi -dn the electr&te content in the electrode..
636
-is the shift of the electrode potential at constant, current density due to replacement of pure oxygen by air + oxygen supply. -Compar&n of the carbon &Aivi~y in the. working electrode with that in th& model electrode with ‘an equally accessible surface allowed the catalyst efficiency in the working electrode. to-be e&mated. .From’ t-he data obtained, it follows that the optimal ratio of carbon ‘catalyst-and hydrophobic component in the active layer, and- ‘accordingly the optimai -electrolyte content; depends on the capillary properties of the,carbon catalyst (Figs. 11 and 12):Analysis, of all the experimenta data showed the gas pores in the carbon.catalysf to play an essential role in the oxygen mass transfer processes, which becomes particularly apparent at high current densities (more than 100 mA/c&). :Thercfore, ‘the’maximum activity at high current densities is achieved in air electrodes tith the carbon catalyst having a large volume of lyophobic poresAt low current densities (less, than 50 mA/cm*), the most active air electrodes are prepared from the carbon catalyst of maximum activity in the model electrode_ Thus, -the choice of carbon- catalyst providing maximum activity in air electrodes depends on the particutar operating regimes of the electrodles. (IV) CONCLUSION
The investigations show the porous structures of active carbons to inchrde lyophobic macropores and lyophilic micropores. The lyophilic-lyophobic properties of carbons depend on the composition of the surface groups which can be changed by various treatments with gas. When the mass transfer limitations are eliminated, !he strictly electrochemical step of the oxygen reduction reaction occurs in micropores or on the boundary of micro- and mesopores. The electrocatalytic activity of a carbon catalyst depends essentially on microporosity. The lyophobic macropores in carbon are gas-supplying channels playing an essential role in the oxygen mass transfer in the carbon electrode. The concepts developed give a full interpretation of the experimental relationships between the air electrode activity and the composition of the active layer, and allow the carbon catalyst and the structure of the active layer to be chosen so as to achieve maximum operating characteristics in air electrodes under given operating conditions. REFERENCES 1 K-V. Kordeschin C. Berger. Handbook of Fuel Cell Technotogy, Prenrice-Hall, Engiewood Cliffs, 1968, p. 361. 2 K. Schwabe and E. Hallax, Pat. DDR, Cl 21B, 14/01, 21B, 7/01. (HOlm) No. 56837, 15.07.67. 3 R.H. Burshtein and D.L. Kondrasbov, J. Appl. Chem. 29 (1956) 1691. 4 K. Schwabe, J. Electrochem. Techn. 3 (1965) 189. 5 K. Schwabe, Rev. Energ. Primaire, 2 (1966) 81, Discuss. 120. 6 K. Schwabe, R. Kopsel. K. Wisener and E. WinkIerEIectrochim. Acta, 9 (1964) 413. 7 J. Mrha, M. Musilova and J. Jindra, Collect. Czech. Chem. Commun., 36 (1971) 638. 8 P. ZoItowski. D-M. Dra%? and L. VorkapiC J. Appl. EIectrochem.. 5 (1975) 79. 9 1. IIiev, S. Gamburtzev, A. Kaisheva; E. Vakanova, J. Muchovski and EBudevski, Commun. Dept.
’
Chem.. Bulg. Acad. Sci.; 7 (1974) 223. 10 W. Kuczinsky, J. Janiak, T. Kuczinsky and J. Olstinska, Chem. Stosow., 19 (1975) 211. 11 M.R. Tarasevich and GIV. Shteinberg. Izv. Vyssh. Uchebn. Z-tvcd., Chem. Chem. Techn., (in Russian)_. 12 LG. Abidor, Ya.B. Shimshelevich and VS. Bagotzky, BIektr&hirniya, 9 (1973) 186.
26 (1983) 40
13 A.V: Dribinsky, M.R Tarasevich and RH. Burshtein, Eiektrokhimiya, 7 (1971) 1144. 14 L.N. Mokrousov, N.A. Urisson and G.V. Shteinberg, Eiektrokhimiya, 9 (1973) 683. 15 I. Iliev, S. Gamburtzev, A. Kaisheva and E. Budevski, Commun. Dep. &em., Buig. Acad. Sci.. 8 (1975) 359. 16 S. Gamburtzev, I. Iliev, A. Kaisheva, G.v. Shteinberg and L.N. Mokrottsov, 28th ISE Meeting, Yarna. 1977, Extended Abstracts, Vol. 2. International Society of Electrochemistry. Vama-Druzhba, Bulgaria, 1977, p_ 303. 17 G.V. Shteinberg. A.V. Dribinsky, 1.4. Kukushkina. M. Musilova and J. Mrha J. Power Sources. 8 (1982) 17. 18 S. Gamburtzev, I. Iliev, A. Kaisheva, G-V. Shteinberg and L.N. Mokrousov, Elektrokhimiya, 16 (198C!) 1069. 19 G.V. Shteinberg, LA. Kukuahkina and V.S. Bagotzky, Pat. USSR N 746272 (1980) 14.03. 20 G.V. Shteinberg, LA. Kukushkina, V.S. Bagotzky and M.R. Tarasevich, Electrokhimiya. 15 (1979) 527. 21 N.G. Spitsina, G.v. Shteinberg, K.N. Nikitin and LA. Kukushkina, Izv. Vyssh. Uchebn. Zaved., Chem. Chem. Techn., 25 (1982) 319. 22 A.V. Dribinsky, L.N. Mokrotucov, LG. Abidor, G.V. Shteinberg, M.R. Tarasevich and VS. Bagotzky, Elektrokhimiya, 13 (1977) 284. 23 IA. Kukushkina, G.V. Shteinberg, M.R. Tarasevich and VS. Bagotzky, Electrokhimiya, 17 (1981) 234. 24 M. Appel and A J. Appleby, C.R. Acad. Sci., 280 (1975) 551. 25 A.J. Appleby and J. Marie, Electrochim. Acta, 24 (1979) 195. 26 G-V. Shteinberg, A-V. Dribinsky, L.N. Mokrousov and LA. Kukushkina, in ref. 16, p_ 268. 27 A.P. Baranov, G.V. Shteinberg and VS. Bagotzky, Elektrokhimiya, 7 (1971) 387. 28 L.Y. Johansson, J. Mrha and R. Larsson, Electrochim. Acta 18 (1973) 255.
I
619
..
.’ I~sm.~&-moy
-.
..
OF -TgfE &B~N-~~GE~
(ATFt)ELELCT.I~&E *
..
:
6-V. SHTEINBERG, A.JJ. DRIBINSKY, I.A. KUKUSHKINA, L.N. MOKOROUSOV and
VS. EAGORKY A’.N. Ahhkin Instittite of Electrochemistry, Academy of Sciences of rke U.S.S.R. Leninsky prospect 31. 11~071.Moxow (LJS.S.R.) (Received 15th May.1984)
..
ABSTXACT
The microkinetics of oxygen reduction on various types of active carbon, and the capillary properties of active carbon and carbon electrodes have been investigated. The reaction rate on active carbon catalysts depends on the ratio of basic and acid surface groups. but more significantly so on the microporosity of carbon. It has b?zn demonstrated that the macropores in carbon are Iyophobic in alkaline electrolytes, contributing essentially to the gas porosity and to oxygen mass transfer processes in the air electrode. Ion transfer occu;; through minute micropores of active carbon and lyophiiic pores of the secondary struct,ure of the carbnn electrode. The optimal active carbon-to-hydrophobic component ratio depends on the capillary propenies of carbon.
(I) INTRODUCTION
.,
The utilizatioli of carbon materials as a base for oxygen &d air electrodes was suggested about 100 years ago; c&nmercial prdduction of carbon-air electrodes for some type+ of chemical power sources was started-more than 50 years ago. However, the problem of, m&~fact&ing cheap highly-active oxygen and air electrodes based on carbon materials still remains urgent. The investigations of carbon electrodes were significantly extended and intensified in the sixties in connection with the development of oxygen electrodes for fuel cells. At that time, moderr, types of caibon electiodk were develo$ed-the two- and mdty-layer electrodes [1-31-w&h allow&d. the electrode thickn% to be considerablydecreased and the electrode characteristics improved. The investigations of Schwabe and co-workers showed the &tract&&tics of oxygen electrodes to be essentially dependent on the type of carbon ,material [4], its -pr&minary treatment and the subsequent gas activation of tht? &ctrodes is]; .as well ,& -on the& porous structtire. Schwabe [4] inferred that the electrode activity deijends.only on p&r& df, JO,30 nm size; more minute pores do riot tffect the‘:l&rode ~&iv&. .The’ dGpend&ce of the rate of cathodic oxygen * Dedicated to the memory of Professor Dr. Dr. h. c.
Kurt
Schwabe.
620 reduction on the pH of the solution for carbon electrodes with platinum on carbon as catalyst was also investigated [6]. These investigations were basically concerned with carbon electrodes containing a more active catalyst-silver, platinum or. metal oxides; The characteristicsof pure. carbon electrodes were studied mainly with a view to estimate the activating effect of the catalyst added. It was shown later that oxygen electrodes with pure carbon catalysts (active carbon) can be used in power sources with ‘alkaline electrolytes [7-111, these electrodes being considerably cheaper. Comprehensive investigations of the physico-chemical properties of carbon materials, particularly their electrocatalytic activity in the oxygen reduction reaction, and of the current genefation process in the carbon gas-diffusion electrode are being carried out in the A.N. Frumkin
Institute of Electrochemistry of the U.S.S.R. Academy of Sciences with a view to elucidating the means of improving carbon-oxygen and carbon-air electrodes. Parts of these studies were carried out in collaboration with the Central Laboratory of Electrochemical Power Sources (CLEPS) of the Bulgarian Academy of Sciences and the I. Heyrovskjl Institute of Physical Chemistry and -Electrochemistry (HIPhChE) of the Czechoslovak Academy of Sciences. Some of the results of these investigations are presented in this paper. (11) EXPERIMENTAL
Active carbon catalysts have an intricate porous structure, the effective pore radii differing by more than four orders of magnitude (0.5 nm-10 pm). The structure of a carbon electrode is characterized not only by the primary structure of the active carbon grains, but also by the secondary structure of the pore space between these grains. An essential characteristic of the waterproofed carbon electrode, besides its porous structure, is the hydrophilic-hydrophobic (capillary) properties of the pores. The porous structure and the capillary properties were investigated by conventional methods: mercury penetration, measurement of benzene and water vapour sorption and weighing the specimens in air, wetting and non-wetting liquids. A number of new methods were also developed to estimate the capillary properties of certain pore classes. The double porosimetry method involves comparison of the mercury and electrolyte or water intrusion curves in, identical specimens.‘ In the case of a purely hydrophobic medium, the capillary equilibrium in the pore space obeys. the equation: p=
-2acosB/r
0)
where r is the pore radius, 0 is the surface tension of thr liquid and 0 is the contact angle. For two different non-wetting liquids, for example, mercury and electrolyte solution, with the same sequence of filling, the ratio of the penetration pressures for the electrolyte and mercury in the same pores is given by the equation: (0, cosel/P1)B=(~~~sez/P2)P
'(5)
621
or A In Pp = In{ c/c,)
i- ln(cos
B,/cos
6,)
(3)
where A In Pp is the distance between the penetration curves (F-In P) for the electrolyte and mercury and v is the volume of pores without liquid. In the case of hydrophilic-hydrophobic materials, such as carbon, the capillary properties of pores with 6 > 90 O_ sts well as the volume of pores with fl< 90 O, can be determined by the above,method [12]. Similarly to the mercury penetration method. the double po.rosimetry method provides a means for estimating the structure and capillary properties of carbon pores in the radii range from 103 to 3.5 nm. The structures of hydrophilic and hydrophobic pores in a specimen can also be determined separately by the “freezing” method This method is based on comparison of the mercury penetration curve for an initial specimen and for a specimen with the hydrophilic pores preliminarily fibed up with water or electrolyte solution, the liquid then being frozen. Mercury penetration is performed at a temperature below the freezing point of the solution. The mercury penetration curve for a frozen specimen correspljnds to the hydrophobic pore radii distribution; the difference between the penetration curves for the initial and frozen specimens corresponds to the radii distribution of hydrophilic pores [13]. To estimate the wetting of minute pores by the electrolyte, the charging curue method was used [14]. The value of the apparent doubie-layer capacity CX determined from the charging curves in an inert gas atmosphere depends on the size of. the wetted surface for a particular material. The change of C” under different water repellency treatments of carbon in the process of electrode formation, or under long-term storage or operation indicates a change in the wetting by the electrolyte of micro- and mesopores malting the main contribution to the double-layer capacity of carbon. The characteristics of carbon-oxygen and carbon-air electrodes were investigated using two types of two-layer gas-diffusion electrodes. In the first type of electrodes the active layer consisted of a mixture of active carbon and polytetratluoroethylene (PTFE) particles; the hydrophobic layer was a thin plate or film of porous PTFE. In -the second type of electrodes the active layer was a mixture of active carbon and a hydrophobic component prepared by depositing PTFE on carbon black: the hydrophobic layer consisted entirely of waterproofed carbon black. The second type of electrode, developed at CLEPS [9,15], were used in the studies performed in collaboration with CLEPS and HIPhChE [16-181. The true catalyst activity in the working carbon electrode may be overshadowed by mass transfer processes. In order to determine the true ele-:trocatalyiic activity of carbon catalysts, a model floating gas-diffusion electrode MS developed [15]. This consist of a very thin layer of carbon particles (approxinlately a monolayer of particles of lo-40 I.crn diameter, total mass less than 20 g .m-*) on a porous hydrophobic conducting substrate. Carbon is deposited or the porous substrate without a binder, and is not subjected to any heat treatment, being retained on the hydrophobic surface of the substrate by adhesion alone. The electrode is placed into
.’
:
622 ,,.
a hermetic glass cell so that the hydrophobic substrate is located. in the gas phase and the catalyst is immersed in the electrolyte (Fig. J),.The amount of carbon .in contact with electrolyte and substrate is ,determined directly m the cell from comparison of the electrode .double-layer capacity with. the capacity of. .1 g of the given carbon powder 1201. Such an arrangement of the .elec;rode -eliminates mass transfer limitations over a wide range of current values (of 2-3. orhers-of magnitude), i.e. the inner surface of ‘the carbon. ,grains is equtilly accessible in the. oxygen reduction reaction_ This permits the oxygen reduction- microkinetics to be-studed directly on active carbons and makes.it possible tocompare the-true electrocatalytic activity of carbon catalysts with various structural and surface properties-The direct contact between the carbon grains and the conducting surface in the model electrode also allows the .oxygen reduction process to .be investigated in low conductivity : .~ carbons, for instance, in fossil coals [21].
Gas
n
-L
-Hydrophobic substrate
=A
== -
‘\
-
-
-
--
-u L_
Fig. 1. Mcdel floating gas-diffusion
electrode.
Corbon particles
623 III. RESULTS
A&D
DISCUSSION
Active carbons
(1lI.I)
(III.
1.1) .Capil/ary properties of active carbon
The- porosity, hydrophilic-hydrophobic, properties and lyophobic behaviour in alkaline ele&rolyte (7 M KOHj of commercial active carbon AG-3, other types of active carbon based on fossil coals (bituminous coal, lignite, peat carbon) and active carbon HS-4 (Czechoslovakia), Nor-it NK,(The Netherlands) have been investigated. To eliminate the influence of mineral impurities often present in commercial active carbons, the carbons were Subjected to preliminary treatment in concentrated hydrofluoric and hydrochloric acids with subsequent removal of halide traces by washing, drying and heating in a hydrogen flow. Some of the carbons were subjected to further activation in a vapour gas mixture p20 + CO,). The investigations were carried out by means of the combined mercury, water and alkali penetration technique, as well as by the charging curves method. The capillary .properties of minute-pores (< 3.5 nm) not accessible for,mercury &.$ration were investigated in more detail by comparing the sorption of benzene and Gater vapours. The micropores of. AG-3 carbon were found to be lyophihc in 5 M KOH soluticn, being wetted by electrolyte after contact of short .duration_ The macropores are lyophobic and are filled with alkaline electrolyte only under pressure. The contact angle for alkali solution calculated by eqn. (2) from the shift of the alkali penetration
1.2
Active
carbon
AS-3
1.8
CI_
$nP/Torr)
-. Fig. 2. Mercury, alkali
a ad
wate; penetration c~xves for AG-3 carbon.
_’
624 TABLE
._~
1
Basic structure characteristics
of carbons a
; , ”
($1
Powder weight/g dme3
AC-3 K-1
13
596
K-2 K-3 K-4
37 53 87
510 420 375
Carbon
type
Burning out
,
T-1
-
545
T-2
-
370
KM-2 Norit-NK+
-
10% PTFE HS-4
-
_..
,- .
: Vma
,1.17 O-62
0.7 0.38
0.83 1.03
: O-45 0.53. 0.74
1.44
Vmi.
.Vphoh
. 0,28-~ -0.74 0.20.
0.42
0.32 0.35 .0.42.
O.&l 0.52 0.87..
Vphi,
/_
:m.! i-’
. 0.43. 0.2
97 ‘23.
-. 0.39
: 48 _
.0.28 ,_ 0.58
.:
....,-I s,,
Pore,volume/cm3 g-’ Vx
--:. .-
95 .. ,170
OSl0.57 0.41
85.
.. 8/O‘. .. ‘.
-. 105 ., ‘-:
95
101
0.99
0.58.
1.71
0.92
0.34
1.1
0.61
235
105
540
0.83
0.4
0.38
b.46
0.37’
-29
106
-
1.08. 0.55
0.81 0.34
.-.
0.91 022
0.17 0.33
:
-’ -7 -
‘.
..- .95: 1G4 ..lO$
96.
: 7
a Vr = total pore volume; V,. = volume of macropores: Vmi = volume of micropores: V&. Vphi, = volumes of Iyophobic and iyophilic pores, respectively; S,,,, = surface of mesopores; 0 = contact angle in 7 M KOH solution. curve
with
pores
are not
respect
to that
wetted
mercury (Fig. 2) is B,,, 7 M KOH solution, &en
of
by the
=
95-100”
after
contact
[22].
T&
macro-
df Jong duration
(Fig. 3). As the surface tension of water is lower, carbon is &ad_uafly wetted by it. Therefore, stable hydrophobic properties of carbon can be achieved only by waterrepellency treatment (Fig. 3). Similar results were obtained with other types of active carbon, which allows the lyophobic properties of _macropori-s in alkaline electrolyte, '1.0 t
Fig_
3.
(0)
Active carbon AG-3;
Variation
in
time
of
the
gas
porosity
of AG-3
carbon.
(8) active carbon AG-3 +20%
mFE
( X) Active
-.
carbon
AG-3+10%
Paraffin;
as we1l.s. the Iydphihc~properties of micropores, _to be.regarded as a general property of active carbons. This inference. is essential for understanding- the nature of current -gene&&n in the carbon-_gas&iff~sion electrode. The ~volumes ,of the macro- and _micropdres; and Accordingly of the lykphilic and lyophobic pores, in various types of
active carbon, are-compared@ Table 1. ..- The volumes of all the .ciasses of ports increase -after additional. activation of carbons,in-a vapour gas.&xture (H,O + CO,) at.a temperature of 90C°C; because p&of the carbon .burns out according to .the reaction: .. _ -. ., It is of interest that long-term activation (high degree of burning OUL)also affects
c+co,.;2co
significantly the lyophobic properties of the pores: the contact angle of the macro/ cm3 g” *-\
ec
LnLn(P,.P-‘)y,
-Fig. 4. &other&of benzene and .yaier sorption on. active. carbons with various activation degrees. (@) active carbon KM-3; (0) active carbon KM-2.
,.
626
-:
. .
pores increases and slightly lyophobic. pores with:’ effective .radii less thati’ 5 ‘r& emerge, Enhanced hydrophobic properties .of a~carbqn el&t&dk ,after a&$Vation by gas was pointed out in ref. 5:The capillary ekope&es of‘tiinute poieS were. stud&d in more detail .by sorption methods. .: :. : . ‘,’ - -’ .. ..-: I.‘, ’ Comparison of the sorption isotherms of betienk and w&r vapouis-shows that ; with low degrees of burning out the volume of sorbed benzene VC+; is fieai’ly, equal to that of sorbe’d water VH;o (Fig. 4), i.e. all the rnicropbres ark hydrophilk. After long-term activation, the volume -of sorbed. benzene increases‘ more th& that or water (VC+, > G20 ), which indicates waterproofing of part of the.minute pqres. At the same time, the shift of the sorption isotherm of water with respect to that of benzene increases, which the authors consider as more -.eviden& for the -es&aked
-2
0 LnLn(P,
2
4
6
_P-‘1
Fig. 5. Isotherms of benzene and water sorption on KM-2 carbon with various surface treatments. (a, A) Initial: (1;:) treated in 0, atmosphere;’ (v,v) treated in CO, atmosphere; (CI,E!I)treated in (3% atmosphere; (C. @) anodic oxidatkn.
.;. 627
hyclroehobic propertiesof minute pores. Thus, the-activntion processcauses porosity and lyophobic properties to increase simultaneously, .the latter increasing first in large pores.and then in more and more minute ones. Subjecting active carbons to treatment- in an oxidizing atmosphere, for instance, in air at 400 o C, or to anodic oxidation in an alkaline electrolyte results in the pores becoming gradually hydrcphilic, starting from minute pores and extending to large ones (Fig. 5). Chemical analysis of the surface functional groups on carbon specimens subjected to various treatments, and with various degrees of acti\.ation showed that the total content of oxygen-containing groups, as well as the ratio of alkaline and acid groups, depends .on the kind of treatment applied. Comparison of the data of chemical analyses with the capillary properties of carbon shows ihe enhanced lyophobic properties to be due to a decrease in the amount of acid (phenolic. carboxylic) groups and an increase in basic groups, while lyophilic properties result from an increase in the amount of acid groups on the carbon surface. (III.l.2) Microkinetics of oxygen reduction on active carbon Oxygen reduction kinetics were studied under conditions of equal accessibility of the inner carbon surface by means of a model floating; electrode in sulphate. phosphate, carbonate and KOH solutions, the range of pH 0.3-14 [23]. The steady--state polarization curves in these solutions for AG-3 and K-4 carbons are presented in Figs. 6a and 6b. Two Tafel slopes are characteristic for these curves, - b, = 50-55 mV and b2 = loo-150 mV. In solutions of pH 14, the first Tafel slope has a constant value over a range of almost three orders of magnitude of the current. With decreasing pH, the E,/log I curves (where E, is the potential referred to the rhe in the same solution) shift towards the negative side, the, first slope becoming less marked. With the more active K-4 carbon, the 6, slope is clearly marked in acid as well. At a constant current value, the shift of the curves with changing pH is aE/apH = -40 mV. In the first and second slope regions, the current depends linearly on partial oxygen pressure, i.e. the reaction is first order for oxygen. Thus from the data obtained, it can be seen that there are, two kinds of liinetic dependence for active carbon: j = /po, [ H’Ioe9 e_EFlRT
(4)
and
which turn one into the other with changing potential. The slope b, = 50-55 mV in the potential region E, 3 0.8 V is recorded for all the active carbon types investigated_ The investigation of oxygen reduction kinetics on smooth carbon materials, for example on pyrolytic graphite, showed,- as a rule, that there is only one slope, 2RT/F, and that the reaction rate does not depend on the pH of solution [ll]. This
,628 .
..
can be -reasonably understood. oxygen molecule to be slow: ~2~.~wxis
the trknsition
assuming
I.
~- ‘,
of_ Ihe first electkon to- thk .,_.._. .’ : ‘. ;.. .‘_ _, ;:;-.. I’ I_ ‘_.. c .,_._
(02 Lds + =- ++ to,_ Lhs a-
Ld* + HOH --j HO, + OH-
HO,.+
e- + HOH + H,O,
+ OH-
I. ‘.. d
*
14.0
,.
.-.
.:
e’ 0’ o/’ 4/
03
7.6
.’ /
*/’
irn Hd
u
/.
<
da
z E J
d 2-
0
0.6
0.6
PH
Fig. 6. Polarization cw-ves of oxygen red&ion
AG-3 carbon; (b) K-4 carbon.
on a model electrode in
sob&& of different PH. (6
629
.-
The same.reaction scheme fits eqn. (5) satisfactot’.ly. To explain the-first slope 6,, the . surface oxides formed on carbon at potentials E, > 0.8 1: were.assumed to exert a retarding action similar to that assumed for- platinum and palladium at potentials .more positive.than 0.8 V [24]. (-rir.1_3) Electrocatalytic aaivity of c&on catalysts of d$ferent m-m&-es Comparison of the electrocatalytic activity of carbon catalysts of different structures was carried out-with the model gas-diffusion electrode; for which the carbon catalyst is not affected by a:ay treatment of the electrode during its preparation (mixing with a binder, heatin g, etc). From the preliminary results of the study on the capillary properties of carbon, as well-as from refs. 4 and.25, the authors assumed that the strictly electrochemical step of the oxygen reducticn process proceeds in mesopores, whose properties tie ‘intermediate between those of the “dry” macropores and the electrolyte-filled micropores. Therefore the investigation was performed on carbon types with significantly varying mesoporosity. The capillary properties of these carbons have been described in Section (111.1.1). The polarization curves for oxygen reduction in 1 M KOH solution are in all the cases similar to the curves pH - 14 in Figs. 6a and 6b, differing only in the current values. The activities (I (mA g-r) at constant E, = 0.9 V) of the carbons arc compared in Table 2. It can be seen that the activities of carbons from various sources differ by more than one order of magnitude. For each carbon series the activity increases slightly with increasing activation degree. The authors failed to detect any correlati.on between the activity and the mesopore surface of different carbons: in some carbon series the activity increased with increasing mesopore surface, in other series a contrary trend was observed, and in some series there was no dependence on the mesopore surface. The activity in different- carbon series with equal mesopore surface differs by a factor of .lO-30. At the same time, a correlation between the activity and the TABLE 2 Activities of various carbons in the oxygen reduction reaction on a model electrode in 1 M KOH sohtion
CapacityC” at
Activity I at E 0.9V/
Activity I at E = 0.9V/
E, = 0.7-0.8 V/F g-’
mAg-’
t&F-’
75 76 75 80 70.
90 65 85 135 55
1.2 0.9
K-2
80
440
5.5
K-3 K-4 KM-2 KM-3
90 90 70 69
650 650 1015 580
7.2 7.2 14.5 8.4
Norit-NK x-IS-4
70 90.
67 30
1.0 0.3
Carbon type AG-3 B-2 T-l T-5 K-l
1.1 1.7 0.8
. ..‘. 6% microporosity of carbon was .detected (Fig. 7); though no quantitative-cdrrelation ,.:_< .:. between these parameters was established. The results are in good agreement with. the data of cap&y ‘mvestigation, &rich show that in strongly activated carbons, for example in KM-3 carbon (Table l), the micropores alone are wetted by the electrolyti. The high electrochemical activity.of KM-3 carbon in the model electrode (Table 2) points to the electrochemical reaction proceeding in micropores or on the boundary between micro- and mesopores, Thus, when mass transfer limitations were eliminated, the oxygen reduction kinetics were found to depend significantly on the microporosity of carbon. (11.1;4) Effect of carbon surface treatment Active carbon KM-2, found to have the- highest activity in the .model electrode, was chosen as the object. It was subjected to various treatments which did not affect its porous structure but led to a change in the composition-of the surface groups (heating in NH,, Cl,, air, CO, flows, anodic oxidation in 7 M KOH solution with
I
0
0.4
0.2 Vmi /
Clll 3
g-l,
Fig. 7. Dependence of the carbon activity on the micropore voiume.
1
631
.stzbsequent wash&g t? remoye alkali, and ,drying). Changing the composition of the surface ,&oups allowed the activity of carbon?n the model electrode with 1 M KOH +- el&Qo!yte to-be changed tw0 to three-fold, with a natural electrolyte by a factor of_ 5-8. In an alkaline‘electrolyte *he greatest activity was found with carbon having b&c _anc a sm&. Flaunt of acid surface grq.~ps. (II7.2)
Capillag
prOperties
ofcarhn
electrodes
:The ca&llary Froperties .of &bon electrodes were investigated by the same methods as those of active carbor;;s, Gith the exception of the sorption methods. The effective co.nductivity of the electrolyte in the active layer pores was also measured. Comparison of the 7 M KOH intrusion curves for active carbon AG-3 with similar
tyophobic
pores Lyoptiilic
Active
c&-bon partictf5
Fig. 8. Model structureof the carbon-FIFE
PTFE active layer.
pores
..
632
-:
:
curves for an electrode prepared froili a .mixturk of-_AG-3 karb& pSr!icle~~~an~da. PTFE suspension, and calculation of thk -tie&i r&KS of ..the gas pores .(Tgasj’ .in carbon and in the electrode active layer showed the main Vo!uk~e-of g&$&s in :ti& electrode to be contained in the carbon &ai,tis. .Thti high ‘de&e bf dispersion‘of the an effetidtiv++ sUp& ix-A0 gaseous phase in carbon (& = 0.3-O-35 pm)‘-provides the carbon grains and, consequently, participation of. the inner carbon surface in current generation. PTFE mahes the- external surface of carbon graitis, ik the secondary structure pores, water-repellent. thus providing a contact bti:t\veen’the carbon grains and the- gaseous phase [26]. The strkture of- the carba. :l?TFE electrode is shown schematically in Fig:& The relationship between the capillary properties of the second type of -ca:.bon Hydrophobic
.--r-~~,,
.‘. Active
component
(h.c.)
/
_
ccrhon
\
*y
h.C. 0.E
L i
0.f
‘ST ol
9 >
03
‘0,
0,
0.;
Active
carbon
1
ri S_,
\ c
3 I
6.
I. :\
‘0.
4
I
l\ \
0
I
10
-12.
Ln (Pfl0I-r) Fig. 9. Alkali penetration curves in active layers of different composition.
-
633
electrodes.and.
the, carbon catalyst-to-hydrophobic
component
ratio.was
investigated
in. collaboration_ with: CLEPS &rd the. HlPhChE. The waterproofed carbon black gas+pplying layer of these electrodes was found (by means of the double porosimetry~technique) to .contain.pores of less than 2 &-I size, more than 90% of the pore volume being’lyophobic in alkaline electroiyte., The contact angle of these pores in alkali &as 6 = 113-i15 P. The degree of ~lyophobic. porosity of the active layer consisting of a mixture of HS-4 and carbon black depends chiefly on .the content of the latter (Fig. 9).‘Figure 9 shows that. the lyophobic pores of the secondary structure formed. during electrode .preparation (hatched area) can also. be revealed by the technique of alkali intrusion into the active layer and its individual components. The& pores appear .to be gaps. between the carbon grains and hydrophobic component. From the change in electrolyte content in the active layer, with varying ratio of active carbon and waterproofed carbon black, it was inferred that the secondary structure pores are filled with electrolyte [17]. (111.3) Effect electrodes
of structure
on the characteristics
of carbon-oxygen
and
carbon-air
(1113.1) Electrodes with carSon-PT_FE actioe layer The relationship between the activity of these electrodes and the ratio of active carbon AG-3 and PTFE masses was studied. Figure 10 represents this relationship
E/RlV
-300
I-
_--o--
-----_o_
-L -\* -.
<--.
t
-250
‘.
I.
-200
--
50-
,_e_---'----.-,_
-_
e, ,A---
--.
_------___------_
e -150
* -1co
"__--._.
." --Q
0 20
1 30
Fig. 10:Dependence of the curxmt density on the air-electrode electrode; (---) after 500 h loa&g.
I 50
on the PTFE
content.
( -.)
initial
: 634
;.
,.
--
i-.
.:
.-
for air electrodes after%peiation:for‘1 and :l%‘h,at-25 %i&‘cm’~~It &n bk.sekri that at low polai-izati& va.lueS( EHg;HgO=- +:lOE) kv) the &i&y decrt+ari;essiiglitl+ witi increasing FIFE content; at -higher -pola&ation +%&es the -plot of: +i&y. tigaikst -. FTFE content has a maximum; .though“a, very flat’ one-Such a~~rel&on&ip_.differS significantly from that of the &r eIect@de -witli, 'platinu&y ‘iatalykt. [27]::- After operation for 150 h the activity maxikntiti shifts towards greater PTFE conte+td;‘the -. activity increasing in ‘time at low polaikqti’o~ k&es, aild decrksing- at ‘hi& &es: Comparison of these data with the -data. on the capillary.-prope&es,- ‘And iin the effective electrolyte conductivity in the active laytr.sh&s that in the&r {electrodes studied, at. cur&t densities .less thzin 50-70 mA;/crii’, the activity is limited by ohmic resistaiices in the liquid phase. Increase in acti,vity,with-operation tim’e is di.xe
20(
151
-250 N
/
.’
‘5
/
e,Of
_---a
’ . .
1’ +-----_,_ -.
\
DL 0.21
I
\
\\ 8 \
‘\ \ \ /-
1
-200 -250
:200
-150
‘.
-
-150
----o__
-0. -1-00
.c’--0.4
-300
\
‘.
5,
a-
\\
\
‘.
%
0.5
t
3.6
I
0.7
t
0.8
S-8
Fig. 11. Dependence of the current density on the air electrode (A) active carbon AG-3: (a) activecubor; KM-2.
oti th.5 hydrophobic
&mponent
content.
63f;
:
_So&i@g of., the act&layer and increase in the. effective.conductivity in the electrode pores:_At.&ti& de&es great& than 100 _mA/cm*~ &ffusion,resistances in. .the oxygen. :supply from .the ,gaseotis phase.-into .the reacition zone become app.arent, l&d the- soaking.of the active Jtiyer causes ‘activity.to decrease in time (Fig. .lO).,-Aftei-ciperation for 300.6, the capillary properties andPopertition.characteristics :do not change.; The electrodes 6peiate:steadily &r 7000 -h at j.= 25 mA/cm’. ,‘_ !, .(III.3.2) Ekrodes of carbon and wtzterproofed carbon black : ‘.In klectrodes of.the s&ofid type the effect of the carbon catalyst-to-waterproofed carbon black ratio -on .activity was studied, .th& carbon catalysts chosen varying in their eltitrocatalytic-activity in the model electrode,- and in. their capillary properties [17]. The polarization of these electrodes was studied both in air and pure oxygen, which allowed the diffusion resistance in oxygen supply in the air electrode to be characterized quantitatively.in terms of the AEi parameter [28]. (A& = ( Eo, - E,,,)i -ml the_
0
2:3 t
oActive
\,
\
carbon
Active
i-
iiS_&
carbon h.c.
1
o-
O-
Active,cbrbon
A&,
0, 0. QuOH g active
: . .
..’
layer
-”
Fig. 12. D&c cdencx of AEi -dn the electr&te content in the electrode..
636
-is the shift of the electrode potential at constant, current density due to replacement of pure oxygen by air + oxygen supply. -Compar&n of the carbon &Aivi~y in the. working electrode with that in th& model electrode with ‘an equally accessible surface allowed the catalyst efficiency in the working electrode. to-be e&mated. .From’ t-he data obtained, it follows that the optimal ratio of carbon ‘catalyst-and hydrophobic component in the active layer, and- ‘accordingly the optimai -electrolyte content; depends on the capillary properties of the,carbon catalyst (Figs. 11 and 12):Analysis, of all the experimenta data showed the gas pores in the carbon.catalysf to play an essential role in the oxygen mass transfer processes, which becomes particularly apparent at high current densities (more than 100 mA/c&). :Thercfore, ‘the’maximum activity at high current densities is achieved in air electrodes tith the carbon catalyst having a large volume of lyophobic poresAt low current densities (less, than 50 mA/cm*), the most active air electrodes are prepared from the carbon catalyst of maximum activity in the model electrode_ Thus, -the choice of carbon- catalyst providing maximum activity in air electrodes depends on the particutar operating regimes of the electrodles. (IV) CONCLUSION
The investigations show the porous structures of active carbons to inchrde lyophobic macropores and lyophilic micropores. The lyophilic-lyophobic properties of carbons depend on the composition of the surface groups which can be changed by various treatments with gas. When the mass transfer limitations are eliminated, !he strictly electrochemical step of the oxygen reduction reaction occurs in micropores or on the boundary of micro- and mesopores. The electrocatalytic activity of a carbon catalyst depends essentially on microporosity. The lyophobic macropores in carbon are gas-supplying channels playing an essential role in the oxygen mass transfer in the carbon electrode. The concepts developed give a full interpretation of the experimental relationships between the air electrode activity and the composition of the active layer, and allow the carbon catalyst and the structure of the active layer to be chosen so as to achieve maximum operating characteristics in air electrodes under given operating conditions. REFERENCES 1 K-V. Kordeschin C. Berger. Handbook of Fuel Cell Technotogy, Prenrice-Hall, Engiewood Cliffs, 1968, p. 361. 2 K. Schwabe and E. Hallax, Pat. DDR, Cl 21B, 14/01, 21B, 7/01. (HOlm) No. 56837, 15.07.67. 3 R.H. Burshtein and D.L. Kondrasbov, J. Appl. Chem. 29 (1956) 1691. 4 K. Schwabe, J. Electrochem. Techn. 3 (1965) 189. 5 K. Schwabe, Rev. Energ. Primaire, 2 (1966) 81, Discuss. 120. 6 K. Schwabe, R. Kopsel. K. Wisener and E. WinkIerEIectrochim. Acta, 9 (1964) 413. 7 J. Mrha, M. Musilova and J. Jindra, Collect. Czech. Chem. Commun., 36 (1971) 638. 8 P. ZoItowski. D-M. Dra%? and L. VorkapiC J. Appl. EIectrochem.. 5 (1975) 79. 9 1. IIiev, S. Gamburtzev, A. Kaisheva; E. Vakanova, J. Muchovski and EBudevski, Commun. Dept.
’
Chem.. Bulg. Acad. Sci.; 7 (1974) 223. 10 W. Kuczinsky, J. Janiak, T. Kuczinsky and J. Olstinska, Chem. Stosow., 19 (1975) 211. 11 M.R. Tarasevich and GIV. Shteinberg. Izv. Vyssh. Uchebn. Z-tvcd., Chem. Chem. Techn., (in Russian)_. 12 LG. Abidor, Ya.B. Shimshelevich and VS. Bagotzky, BIektr&hirniya, 9 (1973) 186.
26 (1983) 40
13 A.V: Dribinsky, M.R Tarasevich and RH. Burshtein, Eiektrokhimiya, 7 (1971) 1144. 14 L.N. Mokrousov, N.A. Urisson and G.V. Shteinberg, Eiektrokhimiya, 9 (1973) 683. 15 I. Iliev, S. Gamburtzev, A. Kaisheva and E. Budevski, Commun. Dep. &em., Buig. Acad. Sci.. 8 (1975) 359. 16 S. Gamburtzev, I. Iliev, A. Kaisheva, G.v. Shteinberg and L.N. Mokrottsov, 28th ISE Meeting, Yarna. 1977, Extended Abstracts, Vol. 2. International Society of Electrochemistry. Vama-Druzhba, Bulgaria, 1977, p_ 303. 17 G.V. Shteinberg. A.V. Dribinsky, 1.4. Kukushkina. M. Musilova and J. Mrha J. Power Sources. 8 (1982) 17. 18 S. Gamburtzev, I. Iliev, A. Kaisheva, G-V. Shteinberg and L.N. Mokrousov, Elektrokhimiya, 16 (198C!) 1069. 19 G.V. Shteinberg, LA. Kukuahkina and V.S. Bagotzky, Pat. USSR N 746272 (1980) 14.03. 20 G.V. Shteinberg, LA. Kukushkina, V.S. Bagotzky and M.R. Tarasevich, Electrokhimiya. 15 (1979) 527. 21 N.G. Spitsina, G.v. Shteinberg, K.N. Nikitin and LA. Kukushkina, Izv. Vyssh. Uchebn. Zaved., Chem. Chem. Techn., 25 (1982) 319. 22 A.V. Dribinsky, L.N. Mokrotucov, LG. Abidor, G.V. Shteinberg, M.R. Tarasevich and VS. Bagotzky, Elektrokhimiya, 13 (1977) 284. 23 IA. Kukushkina, G.V. Shteinberg, M.R. Tarasevich and VS. Bagotzky, Electrokhimiya, 17 (1981) 234. 24 M. Appel and A J. Appleby, C.R. Acad. Sci., 280 (1975) 551. 25 A.J. Appleby and J. Marie, Electrochim. Acta, 24 (1979) 195. 26 G-V. Shteinberg, A-V. Dribinsky, L.N. Mokrousov and LA. Kukushkina, in ref. 16, p_ 268. 27 A.P. Baranov, G.V. Shteinberg and VS. Bagotzky, Elektrokhimiya, 7 (1971) 387. 28 L.Y. Johansson, J. Mrha and R. Larsson, Electrochim. Acta 18 (1973) 255.