- Email: [email protected]
Electrochemical and electrocatalytic behaviour of iron—base amorphous alloys in 1 M KOH at 25°C
Ekcwachim&a Acta. Vol. 38. No. 6 pp. 821-825.1993 RinIcd in GUI
0
Britain
0013~4686/‘93 $6.00 + 0.00 1993. Pergamoll PIWS Ltd.
ELECTROCHEMICAL AND ELECTROCATALYTIC BEHAVIOUR OF IRON-BASE AMORPHOUS ALLOYS 1 M KOH AT 25°C J. C~ousm~,* J. P. CROUWR* and F.
IN
Bmnmxt
*Laboratoire de Physico-Chiic de8 Mattriaux, kquipe corrosion et kctrochimie, Universite de Rovence, 13331 Marseille cedex 3, France TDepartment of Materials and production Engineering, the University of Naples, Piaxxale Tecchio 80125, Naples, Italy (Received 5 October 1992) Aba&ac-The electrochemical and the electrocatalytic properties of Fe,,Co,,Si,,B,, (Vitrovac 76C0, (BDI) iron Gl4X Fe,Ni,,p,,B, (AC), Fe6s,,,Nii0,,s Cr,,,,Zr,B,, (BCC) and of Fe60Ni,,Cr,JrsB,, base amorphous alloys in deaerated 1 M KOH aqueou8 solution and at 25°C was investigated. Poor electrocatalytic activity was exhibited by the amorphous alloys in the as-quenched state, while enhanced electrocatalytic activity was observed after anodic oxidation in situ at constant current density. The more active the amorphous alloy is in the alkaline solution, the better is its catalytic activity. The catalytic activity generally increases by incressing the oxidation current. This result has been attributed to an increase in the active surface of the catalyst after the anodic treatment rather than to an enhancement of the electronic properties of the surface. The G14 alloy exhibited the best catalytic properties among the alloys investigated. After an anodic treatment with a charging current of 0.5mAcmW2 for 2 mitt, this alloy shows catalytic activity comparable to that of pure polycrystaIline Ft in the same environment. Key words: electrocatalyse, amorphous alloy, hydrogen evolution, electrochemistry, alkaline solution.
INTRODUCTION One of the most investigated electrochemical reactions is the hydrogen evolution reaction (HER) from both acid and alkaline solutions. Several methods have appeared in the literature dealing with suitable cathodes for this reaction as pointed out in a recent review by Trasatti[l]. In spite of the effort put in in this field, there is a need for new materials and/or for new physical and/or mechanical modifications of cathodes to enhance their catalytic activity. Among others, new types of materials showing interesting catalytic properties have a structure intermediate between amorphous and crystalline materials. These nanocrystalline alloys can be prepared from powder crystalline metals, this process giving a bulk microcrystallite alloy. For instance, the NiMo alloy[2] is a tee. nickel crystal supersaturated with molybdenum whose dimension is in the order of 5OA.Heat treatments at low temperature have been reported as alternative suitable techniques to modify the structure of the surface of amorphous alloys[3]. A continuous change has been observed, of the electrochemical properties of these heat treated alloys, depending on the temperature at which the alloys were treauXl[3]. Recently, it has been reported that the crystalline surface of a metal can be modified by electrochemical oxidation. The catalytic activity of bulk nickel was observed to increase after anodic oxidation. In addition, the overpotential for hydrogen evolution was observed to decrease by increasing the oxidation potential up to a maximum value after which the overpotential was observed to increase again[4].
Cathodic currents ten times higher than those observed on freshly polished electrodes were reported in the literature after anodic oxidation[5], but no explanation has been so far reported. In the last decade, amorphous alloys gained interest mainly for their corrosion resistance properties. In the past few years, however, attention has been payed to these materials as cathodes for the HER[b 91. In addition, it has been shown that these alloys need to be activated by using HF or via an electrochemical treatment (anodic oxidation) to exhibit good catalytic properties. After acid or electrochemical activation, however, an increase of the exchange current density, i,, was observed, while the Tafel slope remains constant and equal to that of the untreated alloy. These observations led to the conclusion that the activation process affects the surface of the catalyst rather than the electronic state of the surface. Excellent cathodic properties were observed using the Fe&o,,Si,,B,, amorphous alloy after an anodic treatment at 70°C in 30 weight per cent KOH[8]. This alloy did not exhibit the same high catalytic properties when the anodic treatment was carried out at 25”C[8]. Good catalytic properties have been claimed using this alloy in the as-quenched state[7, IO]. The aim of this paper is to further support the view that in situ anodic treatments enhance the catalytic activity of iron base amorphous alloys. In addition, an attempt will be made to relate the catalytic activity of the anodically activated alloys to the electrochemical properties of the amorphous alloys in the same environment as determined by the potentiodynamic curves. 821
J. CROUSIER et al.
822 EXPERIMENTAL
A classic three-electrochemical cell was used to carry out anodic and cathodic polarization curve measurements and anodic potentiostatic transients. The counter electrode was a platinum sheet, placed in a separated compartment, and all the potentials were recorded with respect to a saturated sulfate electrode (sse). The equipment consisted of a computer controlled (IBM-AT) PAR model 273 potentiostat, capable of controlling the experiments, collecting and plotting the data. Four amorphous alloys were investigated in this paper, namely Fe,&o,,Si,,B,, (Vitrovac, G14), Fe,eNi,oP1,B, (AC), Fe,s,,,Ni,,,,,Cr,,,,Zr,B,, (BCC) and Fe,,Ni,,Cr,,Zr,B,, (BDI). The G14 alloy was kindly furnished by Vakuumschmelze GmbH, Hanau. The amorphous ribbons were sealed in glass supports and were used as-quenched, ie without any mechanical polishing. Before each experiment, the electrodes were degreased with alcohol and rinsed several times with bi-distilled water and finally cleaned in an ultrasonic bath. The bright face of the alloy was used as the electrode, the dull face being masked by a high water resistant lacquer. The electrodes were immersed in the test solution without drying. The electrolyte solution was 1 M KOH solution prepared from grade reagent and high purity bidistilled water obtained with a millipore system. The solution was deaerated by bubbling pure Argon gas for two hours before the experiment. An argon blanket was maintained over the surface of the quiescent solution throughout the experiments to avoid oxygen during the test. Anodic and cathodic polarization experiments were carried out at a scanning rate of 0.5 mVs_‘. Cathodic polarization curves on the as-quenched amorphous alloys were performed after the samples were polarized at -2000mV for 10min to reduce any air-formed oxide. The same procedure was adopted for the alloys after an anodic galvanostatic experiments under different anodic charging currents, i,, , for 2 min. RESULTS Electrochemical behaviour
The electrochemical behaviour of the amorphous alloys investigated in this paper is illustrated in Fig. 1. All amorphous alloys exhibit an active to passive transition with a potential-current shape curve depending on the alloy. The potentiodynamic curve of the BDI alloy shows several anodic peaks that can be related to the formation or to the restructuration of the passive layer. Anodic peaks were observed at - 1400, - 1200 and -600mV vs. sse. In addition, this alloy does not exhibit a constant passive current. This result suggests that the structure and the properties of the passive layer may change by changing the applied potential. For potential values above -400 mV, an increase of the passive current is observed up to + 1OOmV at which oxygen evolution was observed. The behaviour of the AC alloy is also reported in Fig. 1. Only a clear anodic peak at
Oti
vi vi L
‘E d
-1000
-
i,
VAcm
-2
Fig. 1. Anodic polarization curves of iron base amorphous alloys in 1 M KOH at the temperature of 25°C.
- 1300mV was observed, followed by a constant passive current in the potential range -800 to 0. Similar behaviour is exhibited by the BCC alloy, with a passivity potential E, = - 12OOmV, followed by a constant passive current in the range llOOOmV. The electrochemical behaviour of the G14 alloy exhibits two very close anodic peaks at about - 1200 mV followed by a further anodic peak at high potential (- 200 mV). Even if the shape of the anodic curve shows a passive behaviour, the passive current is of the same order of magnitude as the critical current for passivity. At high values of the anodic potential, however, a further decrease of the current is observed. This decrease of the passive current can be associated to both a restructuration of the passive layer as well as to the formation of a new phase or a new oxide. Finally, transpassivity is observed to occur at potential values close to 0 V vs. sse. Results reported in Fig. 1 show that all amorphous alloys exhibit a passivity potential at about - 1200mV vs. sse, followed by an anodic behaviour more or less complex depending of the specific alloy. Clear passivity for the amorphous alloys investigated in this paper were observed only for potential values higher than - 1200mV. The passive currents, ip, order : -300mV are in the up to BDI < AC < BCC < G14, while for potential values higher than - 300 mV, the BDI alloy is more active than the AC alloy. Oxygen evolution was observed to occur for potential values close to 0 V vs. sse. The passivity of amorphous alloys has been the subject of numerous investigations. The high corrosion resistance exhibited by amorphous alloys compared to the same alloys in the crystalline state has been ascribed to the selective dissolution of the less noble metal in the active range leading to an enrichment of the outer of the passive layer[ll]. An increase of chromium content in the passive layer of iron-chromium amorphous alloys has been reported in the literature, while the passivity of iron-nickelphosphorus alloys has been attributed to an enrichment in phosphorous of the passive layer[ll]. Since the chromium content of the BCC alloy is lower than that of the BDI alloy, the lower passive current observed in the latter case is consistent with this finding. The absence of Ni and Cr in the G14
823
Behaviour of iron-base amorphous alloys
amorphous alloy, modified profoundly the anodic behaviour of this alloy in the test solution. Since the passivity is related to the formation of a stable oxidehydroxide at the metal solution interface, it is worthwhile to observe that the critical current for passivity of the G14 alloy is comparable with the passivity current, thus suggesting that the G14 alloy cannot be considered as a high corrosion resistant alloy in 1 M KOH. Anodic oxidation The anodic dissolution of the iron-base amorphous alloys at 25°C in 1 M KOH was carried out at different current densities for 2min and are characterized by an E-t curve with different plateaus depending on the alloy and on the values of i., . Figure 2 shows the results for the BDI alloy for different values of i., . At i,, equal to 10 ~Acm-‘, a sharp rise in the potential is observed for a short time followed by a smooth increase with time. The final potential was - 1330mV. For i,, equal to 100PA cm-*, the E-t curve appears more complex. Three plateaus were clearly observed at potential values equal to - 1200, -900 and -700mV vs. sse. Since the number of plateaus indicates the number of steps involved in the dissolution mechanism of the alloy, the BDI alloy seems to dissolve via a complex mechanism or via the formation of different oxide-s. Only two clear plateaus were observed for i., equal to 5OOpAcm-*, while for ia, equal to 1 mAcm-*, the potential reach the oxygen evolution potential in less than 15s. It is worthwhile to observe that the two plateaus at 5OOficm-* occurs at potential values very close to those at which anodic peaks were observed in the anodic potential curve as can be seen from Fig. 1. Results for the AC alloy are shown in Fig. 3 for i., in the range 0.1-l mA cm-*. Only one plateau at E = - 1200mV was observed. This value of the potential is very close to the passivity potential E, as can be seen from the anodic curve shown in Fig. 1. For i., values greater than 1 mAcrn-*, the potential reaches the oxygen evolution potential in a few seconds. These results were omitted in this figure for the sake of simplicity.
400
.I’..1’I’I..
I mAcm
BDI
-2
,.,./ r;
,;
.j
d -1200
,,;
_C”
/*
i’
.
-2
0.1 mAcm
/.d------c--c--
,/-----
-1600’
0
’ 20
.
’ 40
’ 60
t ’ 80
time,
.
’ 100
.
’ 120
.
’ 140
’
s
Fig, 3. Potential vs. time curves for the AC amorphous alloy after different anodic treatment in 1 M KOH at the temperature of 25°C.
Figure 4 represents the results for the BCC alloy. For values of i., in the range 10-1OO/.~Acm-*, a smooth increase of the potential with time was observed, while for i., values in the range 0.31 mAcm-*, a clear plateau at - 12OOmV was observed. For anodic oxidation current equal to lOmAcm_*, the potential increased sharply reaching the oxygen evolution potential in a few seconds. Also in this case, it is worthwhile to observe the close analogy between the potential value at which passivity is observed as shown in Fig. 1 with the value of the potential at which the plateau is observed as can be seen from the data represented in Fig. 4. It can, therefore, be concluded that the plateau observed in this figure is related to the onset of passive film formation at the metal-solution interface. Finally, some results for the G14 alloy are shown in Fig. 5 for i., values equal to 0.3, 2 and 3 mA cm- *. Also in this case, only a clear plateau at E = - 1200mV was observed. For i., equal to 3 mA cm-*, a sharp variation of the curvature in the E-t plot can be seen for E = - 300 mV. These values of the potential are very close to the E, values as shown in the anodic polarization curve of this alloy (see Fig. 1). It must be mentioned that the results shown in Fig. 5 are not fully consistent with those reported in the literature[8]. It has been
I-
4 v! ln
-400
t g
d
-2 IOpAcm
0
20
40
60
80 time,
100
120
-800
-
140
5
Fig. 2. Potential vs. time curves for the BDI amorphous alloy after different anodic treatment in 1 M KOH at the temperature of 25°C.
0. I mAcm -1200
-1600’
0
’ 20
.
’ 40
.
’ 60
’ 80 time,
’ 100
’ 120
’ 140
’
5
Fig. 4. Potential vs. time curves for the BCC amorphous alloy after different anodic treatment in 1 M KOH at the temperature of 25°C.
824
J. CROUSIER et al. 400
-“I
614 ,r-,’
.’
3mAcm
Pi : . . . . . . . ..___...........
-2
-5
. .._._._._._._._._._.~.~.~.~.~.~.~ Nl .
,.
B .A -6
0
’
20
’
40
.
n.
60
n
’
80
100
n
120
’
140
-
‘-.-.
0
-7
10-l
. 0
100
IO’
time, 5
.
102
IO’
IO4
i ac , Wcm
Fig. 5. Potential vs. time curves for the G14 amorphous alloy after different anodic treatment in 1 M KOH at the temperature of 25°C.
t
0
.
0
’
. -.-._.-.-.-.-.-.-.-.-.-.-
_ y’
.........................
:.J cc ._._._._._ 4..._._._._._......‘._._*_~_._._._........r......_._._._._._
-?
-1600’
_. . . . . . . . . li...m
Fe
-1
Fig. 6. Exchange current densities, i,, in pAcme2 vs. the anodic charging current densities, i,,, in pAcme2 in 1 M KOH at the temperature of 25°C.
reported the absence of any plateau in the E-t curve
for an anodic charging current of 2mAcmm2 at 25°C. There is no simple explanation for such a different result even if it should be mentioned that Huot et nl. adopted a different experimental procedure (mechanical polishing) before carrying out the anodic oxidation of the amorphous alloy.
Cathodic polarization Cathodic polarization curves were obtained under potentiostatic conditions at low scan rate on the asquenched and anodically oxidized amorphous alloys. The curves follow a typical Tafel behaviour from which the Tafel slope, b, and the exchange current density, i, , were evaluated. These values are reported in Table 1 together with the Tafel slope and the exchange current density of polycrystalline Pt, Fe, Ni and Co in 1 M KOH at 25°C as taken from the literature[7,9].
The values of i, are also presented in Fig. 6 as a function of i., . The values of i, for the amorphous alloys in the as-quenched state are indicated with an arrow on the left hand scale. The values of i, and of b for the G14 alloys are in good agreement with those reported in the literatureC7-J As can be seen from this figure, all alloys exhibit an increase of i, by increasing the anodic oxidation current and reach a plateau at high values of i,,. The AC and the G14 alloys, however, show a decrease of i, for high values of i,,, thus the best performance with these alloys were achieved after an anodic treatment with an oxidation current density of 0.3 and 2mAcme2, respectively. A similar behaviour has been reported by Huot et al.[8] for the G14 alloy in 30% by weight of KOH at 70°C. The maximum value of i, was obtained after an anodic oxidation treatment at 1 mAcm-‘[8].
Table 1. Log of the exchange current densities i,, in ~Acm-‘, and Tafel slope, 6, in mV for HER on amorphous iron base alloys after anodic activation at different current densities for 2min. in 1 M KOH at 25°C. Values of log i, for polycrystalline Pt, Fe, Ni and Co in 1 M KOH at 25°C as taken from the literaturc[7,9] are also reported Anodic treatment @Acm-2) as-quenched 10 50 100 300 500 700 loo0 zoo0 3ooo 104 10’
BDI -logic
-b
AC -log i,
-b
BCC -log i,
-6
6.74 6.55
105 109
6.92 6.38
109 133
6.31 6.06
105 109
6.49
119
127 109
105 105 108
108 133 118
5.72 5.63
6.05 5.70 5.62
6.48 5.54 5.68 5.80
112
5.56
109
5.45 5.45
105 109
Polycrystalline materials
-log i,
-b
Platinum Iron Nickel Cobalt
4.62 5.00 8.29 8.80
120 135 62 65
G14 -log i,
-b
5.86 5.70 5.80 5.32 5.20 4.55
97 133 118 112 120 135
4.56 4.96
137 111
825
Bchaviour of iron-base amorphous alloys
DISCUSSION As already pointed out, interest in amorphous alloys as good catalysts for the HER reaction is base4l on the assumption that the electronic properties of a surface depends on its structure and, therefore, the amorphous state is expected to result in some modification of electronic properties. In addition, due to their high corrosion resistance, corrosion problems arising at the cathode when shutdown operation are required, should be an issue of less concern. On the basis of the data presented in the literature as recently reviewed by TrasattiCl], it is not well assessed yet that the amorphous state is really the origin of enhanced catalytic activity. Recent results obtained with the G14 alloy at high temperature[S], suggest that the high catalytic activity exhibited by this alloy after anodic oxidation, is due to the formation of crystalline Fe,O* on the surface whose cathodic reduction leaves an active porous Fe layer on which the HER occurs. No changes in the Tafel slope were reported suggesting that the enhanced catalytic activity exhibited by this alloy after anodic treatment, is due to a surface rather than to an electronic effect. A similar conclusion can be drawn also from the data of Alemu and Jiittner[7], even if the low value of the Tafel slope (95mV) exhibited by this alloy in the as-quenched state at 25°C (about - 96 mV), would suggest a combined catalytic and surface effect. Results obtained in this paper (see Fig. 6) suggest that the in situ anodic oxidation can be considered as a general electrochemical technique capable of leading to an increase in the activity of the amorphous alloy as compared to that in the asquenched state. The effect of the anodic charging current on the catalytic activity, depends on the alloy even if some features appear to belong to all the amorphous alloys investigated in this paper. As can be seen from Fig. 6, the BDI, the AC, and the G14 alloys, show a sharp rise of i, at specific values of i,. These values are 10, 100 and 300~Acm-2, for the BDI, AC and G14 alloys, respectively. These “critical charging current density” are very close to the critical current for passivation as can be seen from the data represented in Fig. 1 and the values reported in Table 1. The BCC alloy exhibits a smooth variation of i, with in, even if a significant improvement of the catalytic activity was observed at i., = 100~cm-2. This evidence further supports the view that the enhanced catalytic activity exhibited by these alloys after anodic treatment is due to the formation of an oxide layer on the top of the amorphous surface. Changes of the amorphous sample colours during the anodic treatment were clearly observed only after the anodic treatment at the “critical charging current density” at which the samples were covered by a dark brown oxide. This oxide was not fully reduced pretreatment subsequent cathodic the ~2OOOmV for 10min) before the cathodic polarization curve was carried out. Catalytic activity of amorphous alloys as observed in this paper, can, thus, be attributed to the surface extent and to the electronic properties of the oxide-hydroxide layer
rather than to the properties of an iron active surface. The values of the Tafel slope as can be seen from Table 1 are in the range of - 110 to 135mV that is very close to the value of 120mV observed for pure Fe and mild steel[l]. It can, therefore, be concluded that the enhanced catalytic activity exhibited by the iron base amorphous alloys investigated in this paper after anodic treatment, is due to a surface rather than electronic effect. Finally, the decrease of the catalytic activity observed with the G14 alloy after an anodic treatment with 3mAcm-‘, is similar to that reported in the literature[8]. This result can be explained on the basis of the following consideration. After an anodic treatment with 3 mAcm_‘, a final potential equal to + 98 mV was reached. At the end of the anodic treatment with 1 mA cm- ‘, the final potential was equal to - 830mV. As can be seen from the potentiodynamic curve reported in Fig. 1, at high values of the anodic potential a new oxide is formed (Co(OH), ?), or a restructuration of the passive layer occurs leading to the formation of a compact rather than porous layer. This result leads, in turn, to a reduction of the catalytic surface, and thus of the values of iv,
CONCLUSIONS Results obtained in this paper indicate that the catalytic activity vs. the HER in an alkaline medium can be enhanced after an anodic treatment. Less corrosion resistant is the alloy in the alkaline solution, higher is its catalytic activity. The increased catalytic activity alter anodic treatment has been attributed to the formation of a porous layer on the top of the amorphous surface, rather than to an enhancement of the electronic properties of the amorphous alloy.
REFERENCES 1. S. Trasatti, in Advances in Electrochemical Science und En@eering, Vol. 2 (E&cd by H. Gerisher and C. W. Tobias) pp. l-86. VCH Publishers, New York (1992). i. J. Y. Huot, M. L. Trudeau and R. Schulz, J. electrothem. Sot. 138, 1316 (1991). 3. Y. Massiani, P. Gas, J. Crousier and J. Bemardini, Appl. Sur&Sci. 20, 132 (1984). 4. J. P. Diard, B. Le Gorrcc and S. Maximovitch, Electrochim. Acta 35, 1099 (1980). A. C. Mackrides, J. electrochem. Sot. 109,977 (1962). :: M. Enyo, T. Yamazaki, K. Kai and K. Suzuki, Electrochim. Acta 28, 1573 (1983). 7. H. Alemu and K. Jiittner, Electrochim. Acta 33, 1101 (1988). 8. J. Y. Huot, M. L. Trudeau, L. Brossard and R. Schulz, J. electrmhem. Sot. KU&2224 (1989). 9. K. Lian, D. W. Kirk and S. J. Thorpe, Electrochim. Acta 36,537 (1991). 10. G. Kreysa and B. Hakansson, J. electroanal. Chem. u)l, 61(1986). 11. P. C. Scarson, P. V. Nagarkar and R. M. Latanision, in Modem Aspects of Electrochemistry Vol. 21, (Edited by R. E. White, J. O’M. Bockris and B. E. Conway), p. 121. Plenum, New York (1990).
0
Britain
0013~4686/‘93 $6.00 + 0.00 1993. Pergamoll PIWS Ltd.
ELECTROCHEMICAL AND ELECTROCATALYTIC BEHAVIOUR OF IRON-BASE AMORPHOUS ALLOYS 1 M KOH AT 25°C J. C~ousm~,* J. P. CROUWR* and F.
IN
Bmnmxt
*Laboratoire de Physico-Chiic de8 Mattriaux, kquipe corrosion et kctrochimie, Universite de Rovence, 13331 Marseille cedex 3, France TDepartment of Materials and production Engineering, the University of Naples, Piaxxale Tecchio 80125, Naples, Italy (Received 5 October 1992) Aba&ac-The electrochemical and the electrocatalytic properties of Fe,,Co,,Si,,B,, (Vitrovac 76C0, (BDI) iron Gl4X Fe,Ni,,p,,B, (AC), Fe6s,,,Nii0,,s Cr,,,,Zr,B,, (BCC) and of Fe60Ni,,Cr,JrsB,, base amorphous alloys in deaerated 1 M KOH aqueou8 solution and at 25°C was investigated. Poor electrocatalytic activity was exhibited by the amorphous alloys in the as-quenched state, while enhanced electrocatalytic activity was observed after anodic oxidation in situ at constant current density. The more active the amorphous alloy is in the alkaline solution, the better is its catalytic activity. The catalytic activity generally increases by incressing the oxidation current. This result has been attributed to an increase in the active surface of the catalyst after the anodic treatment rather than to an enhancement of the electronic properties of the surface. The G14 alloy exhibited the best catalytic properties among the alloys investigated. After an anodic treatment with a charging current of 0.5mAcmW2 for 2 mitt, this alloy shows catalytic activity comparable to that of pure polycrystaIline Ft in the same environment. Key words: electrocatalyse, amorphous alloy, hydrogen evolution, electrochemistry, alkaline solution.
INTRODUCTION One of the most investigated electrochemical reactions is the hydrogen evolution reaction (HER) from both acid and alkaline solutions. Several methods have appeared in the literature dealing with suitable cathodes for this reaction as pointed out in a recent review by Trasatti[l]. In spite of the effort put in in this field, there is a need for new materials and/or for new physical and/or mechanical modifications of cathodes to enhance their catalytic activity. Among others, new types of materials showing interesting catalytic properties have a structure intermediate between amorphous and crystalline materials. These nanocrystalline alloys can be prepared from powder crystalline metals, this process giving a bulk microcrystallite alloy. For instance, the NiMo alloy[2] is a tee. nickel crystal supersaturated with molybdenum whose dimension is in the order of 5OA.Heat treatments at low temperature have been reported as alternative suitable techniques to modify the structure of the surface of amorphous alloys[3]. A continuous change has been observed, of the electrochemical properties of these heat treated alloys, depending on the temperature at which the alloys were treauXl[3]. Recently, it has been reported that the crystalline surface of a metal can be modified by electrochemical oxidation. The catalytic activity of bulk nickel was observed to increase after anodic oxidation. In addition, the overpotential for hydrogen evolution was observed to decrease by increasing the oxidation potential up to a maximum value after which the overpotential was observed to increase again[4].
Cathodic currents ten times higher than those observed on freshly polished electrodes were reported in the literature after anodic oxidation[5], but no explanation has been so far reported. In the last decade, amorphous alloys gained interest mainly for their corrosion resistance properties. In the past few years, however, attention has been payed to these materials as cathodes for the HER[b 91. In addition, it has been shown that these alloys need to be activated by using HF or via an electrochemical treatment (anodic oxidation) to exhibit good catalytic properties. After acid or electrochemical activation, however, an increase of the exchange current density, i,, was observed, while the Tafel slope remains constant and equal to that of the untreated alloy. These observations led to the conclusion that the activation process affects the surface of the catalyst rather than the electronic state of the surface. Excellent cathodic properties were observed using the Fe&o,,Si,,B,, amorphous alloy after an anodic treatment at 70°C in 30 weight per cent KOH[8]. This alloy did not exhibit the same high catalytic properties when the anodic treatment was carried out at 25”C[8]. Good catalytic properties have been claimed using this alloy in the as-quenched state[7, IO]. The aim of this paper is to further support the view that in situ anodic treatments enhance the catalytic activity of iron base amorphous alloys. In addition, an attempt will be made to relate the catalytic activity of the anodically activated alloys to the electrochemical properties of the amorphous alloys in the same environment as determined by the potentiodynamic curves. 821
J. CROUSIER et al.
822 EXPERIMENTAL
A classic three-electrochemical cell was used to carry out anodic and cathodic polarization curve measurements and anodic potentiostatic transients. The counter electrode was a platinum sheet, placed in a separated compartment, and all the potentials were recorded with respect to a saturated sulfate electrode (sse). The equipment consisted of a computer controlled (IBM-AT) PAR model 273 potentiostat, capable of controlling the experiments, collecting and plotting the data. Four amorphous alloys were investigated in this paper, namely Fe,&o,,Si,,B,, (Vitrovac, G14), Fe,eNi,oP1,B, (AC), Fe,s,,,Ni,,,,,Cr,,,,Zr,B,, (BCC) and Fe,,Ni,,Cr,,Zr,B,, (BDI). The G14 alloy was kindly furnished by Vakuumschmelze GmbH, Hanau. The amorphous ribbons were sealed in glass supports and were used as-quenched, ie without any mechanical polishing. Before each experiment, the electrodes were degreased with alcohol and rinsed several times with bi-distilled water and finally cleaned in an ultrasonic bath. The bright face of the alloy was used as the electrode, the dull face being masked by a high water resistant lacquer. The electrodes were immersed in the test solution without drying. The electrolyte solution was 1 M KOH solution prepared from grade reagent and high purity bidistilled water obtained with a millipore system. The solution was deaerated by bubbling pure Argon gas for two hours before the experiment. An argon blanket was maintained over the surface of the quiescent solution throughout the experiments to avoid oxygen during the test. Anodic and cathodic polarization experiments were carried out at a scanning rate of 0.5 mVs_‘. Cathodic polarization curves on the as-quenched amorphous alloys were performed after the samples were polarized at -2000mV for 10min to reduce any air-formed oxide. The same procedure was adopted for the alloys after an anodic galvanostatic experiments under different anodic charging currents, i,, , for 2 min. RESULTS Electrochemical behaviour
The electrochemical behaviour of the amorphous alloys investigated in this paper is illustrated in Fig. 1. All amorphous alloys exhibit an active to passive transition with a potential-current shape curve depending on the alloy. The potentiodynamic curve of the BDI alloy shows several anodic peaks that can be related to the formation or to the restructuration of the passive layer. Anodic peaks were observed at - 1400, - 1200 and -600mV vs. sse. In addition, this alloy does not exhibit a constant passive current. This result suggests that the structure and the properties of the passive layer may change by changing the applied potential. For potential values above -400 mV, an increase of the passive current is observed up to + 1OOmV at which oxygen evolution was observed. The behaviour of the AC alloy is also reported in Fig. 1. Only a clear anodic peak at
Oti
vi vi L
‘E d
-1000
-
i,
VAcm
-2
Fig. 1. Anodic polarization curves of iron base amorphous alloys in 1 M KOH at the temperature of 25°C.
- 1300mV was observed, followed by a constant passive current in the potential range -800 to 0. Similar behaviour is exhibited by the BCC alloy, with a passivity potential E, = - 12OOmV, followed by a constant passive current in the range llOOOmV. The electrochemical behaviour of the G14 alloy exhibits two very close anodic peaks at about - 1200 mV followed by a further anodic peak at high potential (- 200 mV). Even if the shape of the anodic curve shows a passive behaviour, the passive current is of the same order of magnitude as the critical current for passivity. At high values of the anodic potential, however, a further decrease of the current is observed. This decrease of the passive current can be associated to both a restructuration of the passive layer as well as to the formation of a new phase or a new oxide. Finally, transpassivity is observed to occur at potential values close to 0 V vs. sse. Results reported in Fig. 1 show that all amorphous alloys exhibit a passivity potential at about - 1200mV vs. sse, followed by an anodic behaviour more or less complex depending of the specific alloy. Clear passivity for the amorphous alloys investigated in this paper were observed only for potential values higher than - 1200mV. The passive currents, ip, order : -300mV are in the up to BDI < AC < BCC < G14, while for potential values higher than - 300 mV, the BDI alloy is more active than the AC alloy. Oxygen evolution was observed to occur for potential values close to 0 V vs. sse. The passivity of amorphous alloys has been the subject of numerous investigations. The high corrosion resistance exhibited by amorphous alloys compared to the same alloys in the crystalline state has been ascribed to the selective dissolution of the less noble metal in the active range leading to an enrichment of the outer of the passive layer[ll]. An increase of chromium content in the passive layer of iron-chromium amorphous alloys has been reported in the literature, while the passivity of iron-nickelphosphorus alloys has been attributed to an enrichment in phosphorous of the passive layer[ll]. Since the chromium content of the BCC alloy is lower than that of the BDI alloy, the lower passive current observed in the latter case is consistent with this finding. The absence of Ni and Cr in the G14
823
Behaviour of iron-base amorphous alloys
amorphous alloy, modified profoundly the anodic behaviour of this alloy in the test solution. Since the passivity is related to the formation of a stable oxidehydroxide at the metal solution interface, it is worthwhile to observe that the critical current for passivity of the G14 alloy is comparable with the passivity current, thus suggesting that the G14 alloy cannot be considered as a high corrosion resistant alloy in 1 M KOH. Anodic oxidation The anodic dissolution of the iron-base amorphous alloys at 25°C in 1 M KOH was carried out at different current densities for 2min and are characterized by an E-t curve with different plateaus depending on the alloy and on the values of i., . Figure 2 shows the results for the BDI alloy for different values of i., . At i,, equal to 10 ~Acm-‘, a sharp rise in the potential is observed for a short time followed by a smooth increase with time. The final potential was - 1330mV. For i,, equal to 100PA cm-*, the E-t curve appears more complex. Three plateaus were clearly observed at potential values equal to - 1200, -900 and -700mV vs. sse. Since the number of plateaus indicates the number of steps involved in the dissolution mechanism of the alloy, the BDI alloy seems to dissolve via a complex mechanism or via the formation of different oxide-s. Only two clear plateaus were observed for i., equal to 5OOpAcm-*, while for ia, equal to 1 mAcm-*, the potential reach the oxygen evolution potential in less than 15s. It is worthwhile to observe that the two plateaus at 5OOficm-* occurs at potential values very close to those at which anodic peaks were observed in the anodic potential curve as can be seen from Fig. 1. Results for the AC alloy are shown in Fig. 3 for i., in the range 0.1-l mA cm-*. Only one plateau at E = - 1200mV was observed. This value of the potential is very close to the passivity potential E, as can be seen from the anodic curve shown in Fig. 1. For i., values greater than 1 mAcrn-*, the potential reaches the oxygen evolution potential in a few seconds. These results were omitted in this figure for the sake of simplicity.
400
.I’..1’I’I..
I mAcm
BDI
-2
,.,./ r;
,;
.j
d -1200
,,;
_C”
/*
i’
.
-2
0.1 mAcm
/.d------c--c--
,/-----
-1600’
0
’ 20
.
’ 40
’ 60
t ’ 80
time,
.
’ 100
.
’ 120
.
’ 140
’
s
Fig, 3. Potential vs. time curves for the AC amorphous alloy after different anodic treatment in 1 M KOH at the temperature of 25°C.
Figure 4 represents the results for the BCC alloy. For values of i., in the range 10-1OO/.~Acm-*, a smooth increase of the potential with time was observed, while for i., values in the range 0.31 mAcm-*, a clear plateau at - 12OOmV was observed. For anodic oxidation current equal to lOmAcm_*, the potential increased sharply reaching the oxygen evolution potential in a few seconds. Also in this case, it is worthwhile to observe the close analogy between the potential value at which passivity is observed as shown in Fig. 1 with the value of the potential at which the plateau is observed as can be seen from the data represented in Fig. 4. It can, therefore, be concluded that the plateau observed in this figure is related to the onset of passive film formation at the metal-solution interface. Finally, some results for the G14 alloy are shown in Fig. 5 for i., values equal to 0.3, 2 and 3 mA cm- *. Also in this case, only a clear plateau at E = - 1200mV was observed. For i., equal to 3 mA cm-*, a sharp variation of the curvature in the E-t plot can be seen for E = - 300 mV. These values of the potential are very close to the E, values as shown in the anodic polarization curve of this alloy (see Fig. 1). It must be mentioned that the results shown in Fig. 5 are not fully consistent with those reported in the literature[8]. It has been
I-
4 v! ln
-400
t g
d
-2 IOpAcm
0
20
40
60
80 time,
100
120
-800
-
140
5
Fig. 2. Potential vs. time curves for the BDI amorphous alloy after different anodic treatment in 1 M KOH at the temperature of 25°C.
0. I mAcm -1200
-1600’
0
’ 20
.
’ 40
.
’ 60
’ 80 time,
’ 100
’ 120
’ 140
’
5
Fig. 4. Potential vs. time curves for the BCC amorphous alloy after different anodic treatment in 1 M KOH at the temperature of 25°C.
824
J. CROUSIER et al. 400
-“I
614 ,r-,’
.’
3mAcm
Pi : . . . . . . . ..___...........
-2
-5
. .._._._._._._._._._.~.~.~.~.~.~.~ Nl .
,.
B .A -6
0
’
20
’
40
.
n.
60
n
’
80
100
n
120
’
140
-
‘-.-.
0
-7
10-l
. 0
100
IO’
time, 5
.
102
IO’
IO4
i ac , Wcm
Fig. 5. Potential vs. time curves for the G14 amorphous alloy after different anodic treatment in 1 M KOH at the temperature of 25°C.
t
0
.
0
’
. -.-._.-.-.-.-.-.-.-.-.-.-
_ y’
.........................
:.J cc ._._._._._ 4..._._._._._......‘._._*_~_._._._........r......_._._._._._
-?
-1600’
_. . . . . . . . . li...m
Fe
-1
Fig. 6. Exchange current densities, i,, in pAcme2 vs. the anodic charging current densities, i,,, in pAcme2 in 1 M KOH at the temperature of 25°C.
reported the absence of any plateau in the E-t curve
for an anodic charging current of 2mAcmm2 at 25°C. There is no simple explanation for such a different result even if it should be mentioned that Huot et nl. adopted a different experimental procedure (mechanical polishing) before carrying out the anodic oxidation of the amorphous alloy.
Cathodic polarization Cathodic polarization curves were obtained under potentiostatic conditions at low scan rate on the asquenched and anodically oxidized amorphous alloys. The curves follow a typical Tafel behaviour from which the Tafel slope, b, and the exchange current density, i, , were evaluated. These values are reported in Table 1 together with the Tafel slope and the exchange current density of polycrystalline Pt, Fe, Ni and Co in 1 M KOH at 25°C as taken from the literature[7,9].
The values of i, are also presented in Fig. 6 as a function of i., . The values of i, for the amorphous alloys in the as-quenched state are indicated with an arrow on the left hand scale. The values of i, and of b for the G14 alloys are in good agreement with those reported in the literatureC7-J As can be seen from this figure, all alloys exhibit an increase of i, by increasing the anodic oxidation current and reach a plateau at high values of i,,. The AC and the G14 alloys, however, show a decrease of i, for high values of i,,, thus the best performance with these alloys were achieved after an anodic treatment with an oxidation current density of 0.3 and 2mAcme2, respectively. A similar behaviour has been reported by Huot et al.[8] for the G14 alloy in 30% by weight of KOH at 70°C. The maximum value of i, was obtained after an anodic oxidation treatment at 1 mAcm-‘[8].
Table 1. Log of the exchange current densities i,, in ~Acm-‘, and Tafel slope, 6, in mV for HER on amorphous iron base alloys after anodic activation at different current densities for 2min. in 1 M KOH at 25°C. Values of log i, for polycrystalline Pt, Fe, Ni and Co in 1 M KOH at 25°C as taken from the literaturc[7,9] are also reported Anodic treatment @Acm-2) as-quenched 10 50 100 300 500 700 loo0 zoo0 3ooo 104 10’
BDI -logic
-b
AC -log i,
-b
BCC -log i,
-6
6.74 6.55
105 109
6.92 6.38
109 133
6.31 6.06
105 109
6.49
119
127 109
105 105 108
108 133 118
5.72 5.63
6.05 5.70 5.62
6.48 5.54 5.68 5.80
112
5.56
109
5.45 5.45
105 109
Polycrystalline materials
-log i,
-b
Platinum Iron Nickel Cobalt
4.62 5.00 8.29 8.80
120 135 62 65
G14 -log i,
-b
5.86 5.70 5.80 5.32 5.20 4.55
97 133 118 112 120 135
4.56 4.96
137 111
825
Bchaviour of iron-base amorphous alloys
DISCUSSION As already pointed out, interest in amorphous alloys as good catalysts for the HER reaction is base4l on the assumption that the electronic properties of a surface depends on its structure and, therefore, the amorphous state is expected to result in some modification of electronic properties. In addition, due to their high corrosion resistance, corrosion problems arising at the cathode when shutdown operation are required, should be an issue of less concern. On the basis of the data presented in the literature as recently reviewed by TrasattiCl], it is not well assessed yet that the amorphous state is really the origin of enhanced catalytic activity. Recent results obtained with the G14 alloy at high temperature[S], suggest that the high catalytic activity exhibited by this alloy after anodic oxidation, is due to the formation of crystalline Fe,O* on the surface whose cathodic reduction leaves an active porous Fe layer on which the HER occurs. No changes in the Tafel slope were reported suggesting that the enhanced catalytic activity exhibited by this alloy after anodic treatment, is due to a surface rather than to an electronic effect. A similar conclusion can be drawn also from the data of Alemu and Jiittner[7], even if the low value of the Tafel slope (95mV) exhibited by this alloy in the as-quenched state at 25°C (about - 96 mV), would suggest a combined catalytic and surface effect. Results obtained in this paper (see Fig. 6) suggest that the in situ anodic oxidation can be considered as a general electrochemical technique capable of leading to an increase in the activity of the amorphous alloy as compared to that in the asquenched state. The effect of the anodic charging current on the catalytic activity, depends on the alloy even if some features appear to belong to all the amorphous alloys investigated in this paper. As can be seen from Fig. 6, the BDI, the AC, and the G14 alloys, show a sharp rise of i, at specific values of i,. These values are 10, 100 and 300~Acm-2, for the BDI, AC and G14 alloys, respectively. These “critical charging current density” are very close to the critical current for passivation as can be seen from the data represented in Fig. 1 and the values reported in Table 1. The BCC alloy exhibits a smooth variation of i, with in, even if a significant improvement of the catalytic activity was observed at i., = 100~cm-2. This evidence further supports the view that the enhanced catalytic activity exhibited by these alloys after anodic treatment is due to the formation of an oxide layer on the top of the amorphous surface. Changes of the amorphous sample colours during the anodic treatment were clearly observed only after the anodic treatment at the “critical charging current density” at which the samples were covered by a dark brown oxide. This oxide was not fully reduced pretreatment subsequent cathodic the ~2OOOmV for 10min) before the cathodic polarization curve was carried out. Catalytic activity of amorphous alloys as observed in this paper, can, thus, be attributed to the surface extent and to the electronic properties of the oxide-hydroxide layer
rather than to the properties of an iron active surface. The values of the Tafel slope as can be seen from Table 1 are in the range of - 110 to 135mV that is very close to the value of 120mV observed for pure Fe and mild steel[l]. It can, therefore, be concluded that the enhanced catalytic activity exhibited by the iron base amorphous alloys investigated in this paper after anodic treatment, is due to a surface rather than electronic effect. Finally, the decrease of the catalytic activity observed with the G14 alloy after an anodic treatment with 3mAcm-‘, is similar to that reported in the literature[8]. This result can be explained on the basis of the following consideration. After an anodic treatment with 3 mAcm_‘, a final potential equal to + 98 mV was reached. At the end of the anodic treatment with 1 mA cm- ‘, the final potential was equal to - 830mV. As can be seen from the potentiodynamic curve reported in Fig. 1, at high values of the anodic potential a new oxide is formed (Co(OH), ?), or a restructuration of the passive layer occurs leading to the formation of a compact rather than porous layer. This result leads, in turn, to a reduction of the catalytic surface, and thus of the values of iv,
CONCLUSIONS Results obtained in this paper indicate that the catalytic activity vs. the HER in an alkaline medium can be enhanced after an anodic treatment. Less corrosion resistant is the alloy in the alkaline solution, higher is its catalytic activity. The increased catalytic activity alter anodic treatment has been attributed to the formation of a porous layer on the top of the amorphous surface, rather than to an enhancement of the electronic properties of the amorphous alloy.
REFERENCES 1. S. Trasatti, in Advances in Electrochemical Science und En@eering, Vol. 2 (E&cd by H. Gerisher and C. W. Tobias) pp. l-86. VCH Publishers, New York (1992). i. J. Y. Huot, M. L. Trudeau and R. Schulz, J. electrothem. Sot. 138, 1316 (1991). 3. Y. Massiani, P. Gas, J. Crousier and J. Bemardini, Appl. Sur&Sci. 20, 132 (1984). 4. J. P. Diard, B. Le Gorrcc and S. Maximovitch, Electrochim. Acta 35, 1099 (1980). A. C. Mackrides, J. electrochem. Sot. 109,977 (1962). :: M. Enyo, T. Yamazaki, K. Kai and K. Suzuki, Electrochim. Acta 28, 1573 (1983). 7. H. Alemu and K. Jiittner, Electrochim. Acta 33, 1101 (1988). 8. J. Y. Huot, M. L. Trudeau, L. Brossard and R. Schulz, J. electrmhem. Sot. KU&2224 (1989). 9. K. Lian, D. W. Kirk and S. J. Thorpe, Electrochim. Acta 36,537 (1991). 10. G. Kreysa and B. Hakansson, J. electroanal. Chem. u)l, 61(1986). 11. P. C. Scarson, P. V. Nagarkar and R. M. Latanision, in Modem Aspects of Electrochemistry Vol. 21, (Edited by R. E. White, J. O’M. Bockris and B. E. Conway), p. 121. Plenum, New York (1990).