Electroreduction of molecular oxygen on preferentially oriented platinum electrodes in acid solution

The oxygen electroreduction reaction was studied on two different preferentially oriented ((111)-type and (100)-type and on a conventional polycrystalline (PC) platinum rotating disc electrodes in acid solutions at 30 °C. At low overpotentials, Tafel lines of −0.060 V decade−1 were obtained on the three electrodes in oxygen-saturated 1.0m H2SO4 and 1.0m H2SO4 + ym K2SO4 (0 ⩽y ⩽ 1). At high over-potentials the usual Tafel slope of -0.120V decade−1 was observed on both (111)-type and PC platinum electrodes in 1.0m H2SO4, whereas a slope of −0.165V decade−1 was found on (100)-type platinum. In oxygen-saturated 1.0m H2SO4 the surface coverage by O-containing adsorbates on (100)-type platinum was greater than on both (111)-type and PC platinum. Rotating ring-disc electrode data showed that a higher amount of H2O2 was produced on (100)-type platinum than on the other platinum surfaces. The overpotential against current density plots are influenced by the anion concentration depending on the type of preferentially oriented platinum.


Introduction
The oxygen-electroreduction reaction deserves special attention because of its importance for metal corrosion in oxygen-containing solutions, air-metal batteries and fuel cells [1][2][3][4][5][6].In the area of electrochemical energy conversion technology, platinum electrocatalysts are particularly vital for this reaction.Tafel plots for oxygen-electroreduction on smooth polycrystalline (PC) platinum in both diluted H 2SO 4 and HC 104 exhibit a -0.120V decade 1 slope at high current densities (c.d.), and a -0.060V decade-1 slope at low c.d.The kinetics of the reaction has been explained in terms of the initial charge transfer step being rate-determining, assuming for the intermediates Temkin adsorption conditions at low c.d. and Langmuir adsorption conditions at high c.d. [7,8].On the other hand, the -0.060V decade -1 slope at low c.d. has also been associated with a rate determining chemical step following the initial electron transfer [7].Another possible reaction pathway comprising a set of parallel reactions involving two types of active sites with different adsorption energies, has been proposed [9].Accordingly, adsorbed O-containing species could be desorbed easier from a certain type of sites by a direct electroreduction to H20 through a 4-electron transfer reaction, or alternatively forming H202, via a 2-electron transfer reaction, the latter further decomposing to H20.Otherwise, the kinetics of oxygen-electroreduction on PC platinum in H3PO4 solutions at different concentrations [10,11], and in the presence of specifically adsorbed anions [12,13], depends strongly on the anion concentration, particularly at high c.d.These effects were attributed to the blockage of electroactive platinum sites by anion adsorption.
Studies of oxygen-electroreduction on both wellordered low Miller index and stepped platinum surfaces in acid solutions [7,14] suggested that there is no large influence of platinum surface structure on the kinetics of the reaction.However, it was recently [15] claimed that oxygen-electroreduction on Pt(lll), Pt(100) and Pt(ll0) in 1.0M H2SO4, 1.0M H3PO 4 and 0.1 M HC1 + 1.0M HzSO 4 becomes a structure-sensitive reaction.Such results justify further work on this subject by using platinum electrodes whose surfaces were preferentially oriented either as (1 1 1)-type Pt or (1 0 0)-type Pt.The conclusions from this work can be correlated to those derived previously for platinum single crystal (SC) electrodes.The easy preparation and handling of these preferentially oriented surfaces, in addition to their stability during operation, are promising approaches for their applications in electrochemical devices [16].

Experimental details
Platinum discs (99.999% purity, 3 mm dia.) were used as working electrodes.The preparation of either (1 1 1)-type or (1 0 0)-type preferentially oriented platinum electrodes was made as described elsewhere [16].For comparative purposes smooth PC platinum discs were occasionally employed.A large platinized-Pt plate (10cm 2 of geometric area) counter electrode, facing the working electrode for minimizing ohmic drop effects, was used.A RHE in the same solution, connected to the rest of the cell through a Luggin-Haber capillary tip, was employed as reference elec-0021-891X © 1994 Chapman & Hall 1 19 trode.All potentials in the text are referred to the RHE scale.The real surface area of the working electrodes was estimated by comparing the H-adatom voltammetric charge recorded at 0.1 V s -1, to the Hatom monolayer charge density on the different oriented platinum surfaces [17].
Prior to the preparation of the preferentially oriented surfaces, the PC platinum discs were mechanically polished with different grades of alumina down to 0.05 #m grit, repeatedly rinsed with triplydistilled water, and finally, immersed for 1 h in the working electrolyte solution.The electrochemical behaviour of the preferentially oriented platinum electrodes was followed through the H and O-atom electrosorption voltammograms at 0.1Vs -1 in 1.0m H2SO 4. SEM micrographs of the platinum surfaces were also made.
All experiments were conducted at 30°C.Previously to each run a potential routine was applied to the working electrode to assure cleanness and reproducibility of the surface.It comprised a potential step at 1.4V for 5 s to oxidize adsorbed impurities, followed by a potential step to 0.1 V for another 5 s to electroreduce the O-containing surface layer.The resulting prereduced platinum surface was employed to obtain the steady state potentiostatic polarization curves at 2000r.p.m. by recording the current for various preset potentials in the 0.1-1.0Vrange.Under these conditions the steady currents were attained after about 5 min.
The values of Ik, the net kinetic current, were derived from I, the experimental current, through the following equation: Ilim --I where Ilim/(lli m --I) is the diffusion contribution correction and Ilim is the observed limiting diffusion current.
A Tacussel rotating ring-disc electrode (RRDE) device was also used.The characteristics of the rotating electrode were rt = 0.20cm, r2 = 0.22cm and r 3 = 0.24 cm, where r 1 is the radius of the disc, and r2 and r 3 are the radii of the inner and the outer circles of the platinum ring electrode.The collection efficiency of the RRDE was 0.22.
The steady state coverage of the platinum electrode by O-containing surface species at different potentials was determined from q0, the charge density derived from the current transients recorded after applying a potential-step from the preset potential down to 0.3 V, i.e. a potential where the O-containing surface species are no longer preset.The fractional surface coverages by O-adatoms, 0o, were calculated from the qo/2qH ratio, where qH denotes the H-atom monolayer charge density on each preferentially oriented surface [17].

General behaviour of PC and preferentially oriented platinum surfaces
There are multiple states of adsorbed hydrogen at the PC platinum surface (starting material) as observed in the corresponding cyclic voltammograms of Fig. 1.The two principal current peaks are associated with H-adsorption on Pt (1 1 1) and Pt (1 1 0) facets (weakly adsorbed hydrogen), and Pt (1 0 0) facets (strongly adsorbed hydrogen).The distribution of different crystallographic facets is strongly dependent on the pretreatment of the PC platinum electrode.The percentage (in average) of distribution of strongly (s) and weakly (w) adsorbed-H at the starting PC plati- num follows the same trend than that previously reported for a high temperature treated PC platinum electrode [20,21], that is, about 31% (s) and 53 % (w).
The PC platinum electrode was subjected to a symmetric square wave periodic potential (SSWPP) between 0.60 and 1.45V at 1.0kHz for 40min.The voltammetric response of the resulting new surface (Fig. l(a)), hereafter denoted as (1 1 1)-type Pt, is similar to that reported previously for a platinum surface with a high proportion of reaction sites for weakly adsorbed H-atoms, and platinum single crystal stepped surfaces with (1 1 1) narrow terraces [22 24].
On the other hand, after the SSWPP treatment between 0.05 and 1.70 V at 1.0 kHz during 20 min., the voltammetric profile (Fig. l(b)) of the treated surface, hereafter denoted as (1 0 0)-type Pt, approaches that reported for platinum surfaces with a high proportion of reacting sites for strongly adsorbed hydrogen atoms, and platinum single crystal stepped surfaces with (1 0 0) narrow terraces [22,23,25].

Tafel behaviour
The Tafel plots for oxygen-electroreduction on (1 1 1)type Pt in oxygen saturated (Po2 = 1 atm) 1.0M H2SO 4 at 30 °C show two straight line portions (Fig. 2).In the low overpotential region (1.o.r.) the value of the Tafel slope is bw = -0.067V decade -I and the value of the corresponding exchange current density is J0 = 3 x 10 -6 #A real cm -2, whereas in the high overpotential region (h.o.r.), bm= -0.121 V decade -1, and j0 = 6.0 x 10 .3#A real cm -2 (Table 1).The same qualitative behaviour can be observed for (100)-type Pt (Fig. 2), but in this case, bT =-0.064Vdecade -1 and J0 = 6 x 10-6 #A cm -2 in the 1.o.r., and bT=-0.165Vdecade-1 and J0 = 6.0 x 10 -2#A cm 2 in the h.o.r.It should be noted that the behaviour of PC platinum is similar to that of (1 1 1)-type Pt (Fig. 2).The Tafel lines for the three types of platinum exhibit a crossover point at about 0.92V, i.e. in the potential region corresponding to the onset of OH irreversible adsorption [26], indicating a change in the kinetics of the electrode reaction.

The degree of surface coverage by O-containing species
The dependence of 0o on the applied potential was o.
-H o. - determined in both oxygen-saturated and oxygen-free 1.0 M H2SO 4 covering the 0.60-1.00Vrange.For the three types of platinum the plots show almost linear 0 o against E dependences from 0.75V up to 1.0V (Fig. 3).For the (1 1 1)-type and PC platinum.In oxygen-saturated 1.0 M H2SO 4 the slopes of the 0 o against E plots, are the same and equal to 0.85 V -1, but their extrapolation to 0o = 0 tyields E0o_ 0 = 0.67 V for the (1 1 1)-type and E0o= 0 = 0.65V for PC platinum.In this respect, the behaviour of (1 0 0)-type Pt is somewhat different, since all surface coverage values are much greater than those found on other platinum surfaces.In this case, the slope of the 0o against E straight line is 1.13V 1 and E0o=0 = 0.65V.E0o= 0 can be considered as the potential related to the hypothetical condition corresponding to 0o = 0. Values of 0 o for (1 0 0)-type Pt in oxygen-free 1.0 M H2SO 4 are at each potential much lower than the corresponding ones found in the oxygen-saturated solution.Besides that, 0o values for both (1 1 1)-type and PC platinum surfaces in the oxygen-free solution, are only slightly lower than those found for (1 0 0)-type Pt under the same experimental conditions.

Rotating ring-disc data
RRDE-voltammetry data obtained with preferentially oriented and PC platinum electrodes in oxygen-saturated 1.0 M H2SO 4 are depicted in Fig. 4. In these runs the disc potential was scanned at 0.010Vs -1 and the potential at the ring was held at 1.2 V, i.e. in the potential region where the diffusion controlled electrooxidation of H202 to 02 takes place.Hence, the H202 produced during the oxygen-electroreduction at the disc electrode could be collected and detected at the ring electrode.
The ring current, IR, against the disc potential, ED, plots at a; = 2000 r.p.m, for the three types of platinum electrodes show principally two peaked values of IR, namely at about 0.40 V (peak I) and at about 0.18V (peak II).A control experiment, run under nitrogen saturation, revealed that peaks I and II are exclusively recorded when the oxygen-electroreduction takes place.For (1 0 0)-type Pt there is a relatively large contribution of peak I which is shifted positively to about 0.44 V, whereas peak II is considerably diminished.On the other hand, peak I for both (1 1 1)-type and PC platinum is lower and shows a hump near 0.50V.

Influence of the solution composition
The addition of K2SO 4 to 1.0M H2SO 4 solution, changes the kinetic parameters resulting from the Tafel plots for the three types of platinum as shown in Table 1.Higher K2SO 4 concentration increases the value of ba-at the h.o.r, for (1 1 1)-type and PC platinum in contrast to the invariant values of ba-found for (1 0 0)-type Pt which remain higher than the usual Tafel slope of 0.120 V decade -1.Furthermore, the presence of K2SO 4 in the electrolyte decreases the corresponding j0 values for the three types of platinum in both Tafel regions.Since the changes in pH and (HSO4)/(S42) equilibrium after addition of KzSO 4 are not significant, the increase in the bT value at the h.o.r, for both (1 1 1)-type and PC platinum must be principally attributed to the increase of the specific adsorption of anions.On the other hand, the (1 0 0)-type Pt is covered by a large amount of O-containing species, so there is practically no effect of the anionic adsorption on bT in contrast to the decreasing value of J0 as the K2SO4 concentration is increased.
Tafel plots were also obtained for the preferentially oriented platinum surfaces from current-potential curves run at 30°C in weakly adsorbing media, namely, dilute HzSO 4 and HC104 solutions.The corresponding kinetic parameters confirm the above mentioned trend.Thus, Tafel slopes of -0.120 V decade -1 and -0.124 V decade -l were observed at the h.o.r, for (1 1 1)-type Pt in oxygen saturated 0.1 M HzSO 4 and 0.1 g HC104 respectively, whereas a bx value of-0.136Vdecade-1, that is, a Tafel slope higher than 2.3(2RT/F), was found for (1 0 0)-type Pt in both electrolytes.

Discussion
The preceding results show that comparable values of bT and J0 are derived from the steady state oxygenelectroreduction Tafel plots on both PC and (1 1 1)type Pt electrodes in 1.0 M H2SO 4. The corresponding kinetic parameters (Table 1) are in part coincident with data reported earlier [8,9,27,28].The value bx = -0-120Vdecade-1, was interpreted through a mechanism involving the first single electron transfer as r.d.s, and Langmuir adsorption conditions for the reaction intermediates [7,28].Otherwise, the value bT =-0.060Vdecade -1 was associated with either a rate determining chemical step following the primary electron transfer [7] or the first electron transfer becoming r.d.s, under Temkin adsorption conditions for the O-containing surface intermediates [7,8].On the other hand, the results obtained on (100)-type Pt electrodes in 1.0g HzSO 4 show bx = -0.165V decade -1 in the h.o.r., i.e. a Tafel slope higher than 2.3(2RT/F), and the same bx = -0.060V decade -1 in the 1.o.r., this behaviour being maintained in more dilute acid solutions, such as 0.1M HzSO 4 and 0.1M HC104.Furthermore, Tafel slopes higher than 2.3(2RT/F) were also found at the h.o.r, on both (1 1 1)-type and PC platinum after addition of K2SO 4 to the electrolyte solution.
Values of bx higher than 2.3(2RT/F) for the oxygen-electroreduction on PC platinum in the h.o.r. were reported by Yeager et al. [10] and Glass et al. [12] in H3PO 4 and in HCl-containing solutions [13,15].These values of bx were explained through a blockage of platinum electroactive sites for the reaction by specifically adsorbed anions.As a first approach this explanation could be applied to the kinetics of the reaction in K2SO 4 + H2SO 4 solutions.On the other hand, interference by a differential anion adsorption in the electrosorption of hydrogen and oxygen adatoms on the platinum single crystal has also been reported [29,30].Tafel slopes for the oxygen-electroreduction on PC platinum higher than 0.120 V decade -1 were also found in the h.o.r, in alkaline solutions [31], and in this case H202 electrogeneration was demonstrated.It should also be noted that in H3PO4 solutions [10] about 10% of the overall oxygen electroreduction proceeeds via H202.The RRDE data obtained in the present work on (100)type Pt (Fig. 4) reveal, at the ring electrode, a current contribution due to the HzOz-electrooxidation higher than that found on either (1 1 1)-type or PC platinum.Accordingly, it appears that for (1 0 0)-type Pt the Tafel slope exceeding 2.3(2RT/F) is related to the fact that a part of the oxygen-electroreduction reaction is via H202, as it is detected at the ring electrode.Therefore, there are two reasonable explanations for the values of bx exceeding -0.120 V decade -1 based on a blockage effect, namely, the possible adsorption of either anions or peroxide species produced in the proper reaction.
The present results show that 0o at constant potential, for (1 0 0)-type Pt in oxygen-free 1.0 M H2804, is slightly higher than that found on either (1 1 1)-type or PC platinum.The values of 0 o for the latter coincide with those previously reported by different authors [7,28,32,33].This fact agrees with the higher voltammetric charges found for the O-electrosorption on Pt (1 0 0) as compared to Pt (1 1 1) and Pt (1 1 0) single crystals [15].On the other hand, the present results indicate that 0o values for (1 00)-type Pt in oxygen-containing solution are much greater than in oxygen-free solution.Thus, at 0.97V for (100)-type Pt in oxygen-saturated 1.0g H2SO4, 00 = 0.78, whereas in the oxygen-free 1.0M H2SO 4 00 = 0.28.It should be noted that according to the preceding analysis 0 o may actually involve two O-adsorbed species, one resulting from the upd discharge of water, the other produced from the oxygen-electroreduction intermediates.It is likely that larger contributions of O-adsorbed electroactive intermediates, such as peroxide species, are particularly involved on (100)-type Pt in the oxygen-saturated acid solution.
The values of j0 resulting from the extrapolation of the h.o.r, linear Tafel region to the Er(Oz/H20)= 1.23V for (100)-type Pt, are one order of magnitude higher than those resulting for other platinum electrodes.On the other hand, the values of j0 obtained for PC and (1 1 1)-type Pt are in good agreement with those already reported for PC platinum by Damjanovic et al. in H2SO 4 solution at pH 1.9 [8], and by Yeager et al. [10] in 85.5% H3PO 4 at 25 °C.
The presence of a high concentration of adsorbable anions, such as SO]-and HSO4 (Table 1), tends to hinder the adsorption of O-containing species produced in the oxygen-electroreduction reaction through a competitive adsorption at platinum electroactive sites.In this respect, the tetrahedral structures of SO 2-and HSO4 favour their adsorption on surfaces with the (1 1 1) trigonal symmetry [34].This fact correlates with the increase of bx with the anion concentration at the h.o.r, on (1 1 1)-type Pt (Table 1).In contrast, when the structural coupling is not facilitated weaker adsorption effects on the kinetics of the reaction are expected.This is apparently the case for SO42-and HSO4 on (1 0 0)-type Pt.The mismatch of the corresponding geometries helps to produce a cathodic shift of the O-electrosorption potential range as compared to (1 1 1)-type Pt.This is confirmed by the O-electrosorption voltammetric profiles of the different preferently oriented surfaces (Fig. l).
The preceding explanation is consistent with the conclusions of Angerstein-Kozlowska et al. [35] that the presence of specifically adsorbed anions produces a dual effect on the O-electrosorption, namely, a potential dependent blockage of surface sites and a shift of the O-electroadsorption potential range [36] due to the changes on the local field by adsorbed anions.When the oxygen-electroreduction is carried out on an oxidized platinum electrode, oxygen molecules cannot bond strongly to the surface, because a large number of the platinum sites are covered by O-adsorbed atoms.In this case, the free energy of adsorption of the oxygen molecules is not large enough to break the O-O bond and, consequently, the most probable mechanism for the reaction is a 2electron path yielding H20 2 [37].On the other hand, it is known that the amount of H20 2 detected at the ring electrode in 0.1 M NaOH increases with the degree of oxidation of the platinum disc surface, as described by Luk'yanycheva et al. [38].The O-electroadsorption region appearing at lower potentials on the (1 0 0)-type Pt electrode and the higher amounts of H20 2 generated from this type of surface, are in accordance with the above considerations.
At potentials more positive than 0.9 V (vs RHE), anions tend to desorb as a result of their competition with O-containing species [39,40].Accordingly, the values of bT tend to coincide for the different platinum electrodes.This is shown through the steady state Tafel plots on the preferentially oriented'Pt surfaces in H2SO 4 solutions containing different amounts of K2SO4, whose kinetic parameters are given in Table 1.

Conclusions
(i) The oxygen-electroreduction kinetics on (1 1 1)type Pt electrodes in acid media is found to be similar to that reported by many authors for smooth PC platinum.
(ii) Through the specific blockage of the electrode surface, the oxygen-electroreduction behaves as a surface structure-sensitive reaction in the h.o.r.(iii) The blockage effect can be attributed to the presence of adsorbable anions and to the formation of peroxide species in the oxygen-electroreduction.
(iv) Results suggest that the surface blockage by SO 2-and HSO4 anions is favoured on (1 1 1)-type Pt, whereas peroxide species are mainly responsible for the (1 0 0)-type Pt blockage.