Temperature dependence of kinetic parameters related to oxygen electroreduction in acid solutions on platinum electrodes

The oxygen electroreduction reaction (OERR) kinetics at (111)- and (100)-type Pt electrodes has been studied in aqueous 1.0 M H2SO4 solutions in the 8–62°C range. On both Pt electrodes and at all temperatures, two Tafel regions have been observed. The Tafel slope in the low current density range is −0.053 ± 0.005 V decade−1, irrespective of temperature and the surface texture of the electrode. For (111)-type Pt, the Tafel slope in the high current density range increases with temperature, approaching the −2.303(2RT/F) ratio, whereas for (100)-type Pt, it reaches a temperature-independent value equal to −0.165 ± 0.003 V decade−1. Temperature dependences of OERR Tafel slopes on both types of Pt electrodes offer the possibility of further insights into the mechanism of the reaction, through the interpretation of both the adsorption enthalpy and entropy of intermediates produced along the OERR. Compensation effects, involving activation enthalpy and entropy variations with the applied potential, are also considered to account for experimental data.


INTRODUCTION
It is well known that the efficiency of Pt-acid electrolyte fuel cells is limited by the OERR overpotential [1][2][3].The rate of this reaction at oxide-free Pt surfaces, which is the first single electron transfer to molecular 02 [4,5], appears to be determined by the adsorption of products formed at the rate determining step (rds).The OERR station* ary kinetics on smooth polycrystalline (PC) and facetted Pt electrodes fit Tafel lines with two distinct Tafel regions, namely, a low current density region, LCD, with a slope (bT)LCD g --0.060V decade-1 in both acid and alkaline solutions, and a high current density region, HCD, with Tafel slopes, (bT)nCD, ranging from -0.120 to -0.165Vdecade-l, depending on both the electrode topology and the degree of surface coverage by adsorbable anions and OERR intermediates [6,7].
The influence of temperature on the OERR kinetics deserves further work, because unlike simple processes, the interpretation of the electrochemical activation parameters is still rather unsatisfactory.
Generally, the adsorption of intermediates in electrocatalytic reactions influences the rate of the electron transfer step.For the OERR, Huang et al. [8] and Appleby [9] have found no significant changes of (bx)aci) in concentrated aqueous H3PO 4 in the 25-250°C range.It has been recently found, however, that these kinetic results are influenced by the presence of a Pt anodic film on the electrode, formed at high potentials, and by the Pt surface restructuring upon potential cycling [10].Conversely, Damjanovic and co-workers [11][12][13] have reported values of (bT)ncn increasing with temperature in diluted aqueous HCIO 4 and H2SO 4 below 70°C, as predicted by classical theories of electron transfer reactions at electrified interfaces.Reports by Conway and McKinnon on the hydrogen evolution reaction (HER) in ethanolic and methanolic acid solutions [14], and by Conway et al. on the bromine evolution reaction (BER) on graphite [15], have shown that the temperature dependence of (bT)nC D is related to the nature of the electrode and the operating temperature range [14,16,17].It is clear that, despite the number of works about OERR on PC Pt and well-defined Pt electrodes [17][18][19][20][21], there are some fundamental kinetic aspects which are still far from being well established.
This work deals with the kinetics of the OERR in diluted acid media, at different temperatures, followed through the rotating disc electrode (rde) technique on two differently facetted Pt surfaces.New results offer the possibility of obtaining thermodynamic data related to the activated process involved in the OERR, and of advancing an interpretation about the temperature dependence of Tafel slopes, under several operating conditions.
The working electrode real surface area (Av) was estimated from the H-adatom charge density reading from voltammograms run at 0.1Vs -1, after the double layer correction, considering as a reference the H-adatom monolayer charge density at each Pt single crystal electrode surface [23].A 10cm 2 geometric area platinized-Pt counter electrode facing the working electrode, and a reversible hydrogen electrode (the), connected to the rest of the cell through a capillary tip, were employed.A conventional two compartment cell of ca.350cm 3 capacity was used, and the arm of the reference electrode compartment was separated from the working cell by means of a Teflon stopcock.The reference electrode compartment was held at 25°C to perform the experiments at constant potential (non-isothermal conditions) [24, 251.Potentials in the text are referred to the the scale.Oxygen (99.99% purity) and nitrogen (99.998% purity) were treated as indicated elsewherel-26], and then bubbled through the solution until saturation was reached.
The kinetics of the OERR at different temperatures was followed by using the rde technique.Prior to each run, disc electrodes were cleaned employing a potential routine already described in the literaturel '7].Stationary polarization curves were obtained under potentiostatic conditions, by holding the disc potential (ED) at a preset value in the 0.05 V ~< E D ~< E,.,t range, Er, t being the open circuit potential of Pt in the O2-saturated (Po2 = 1 atm) acid solution, and by reading the stationary current after no less than a 4min stability.Polarization curves were independent of whether E D was changed upwards or downwards within the 0.05 V to E,=,, potential range.The stationary polarization curves were displayed as E v vs. log j plots (Tafel plots), where j is the cathodic current density at the disc electrode referred to A D. Tafel plots were properly corrected for mass transport contribution, assuming a first order kinetics with respect to O21271.

Electrochemical behaviour of Pt electrodes
Cyclic voltammetry was used to test both the stability of facetted Pt electrodes and the absence of impurities in the solution, which could interfere with the kinetics of the OERR under different experimental conditions[ '28].
Cyclic voltammograms of (100)-and (11 l)-type Pt electrodes were run at 0.10Vs -1, between 0.5 and 1.5V, in O2-free 1.0M aqueous H2SO4, in the 8-62°C range.The voltammetric response of (Ill)type Pt is similar to that previously reported for a Pt surface with a large fraction of reaction sites for weakly adsorbed H-atoms, whereas for the (100)type Pt it approaches that reported for Pt surfaces with a large fraction of reacting sites for strongly adsorbed H-atoms[ '7].Cyclic voltammograms for both Pt facetted surfaces showed no appreciable changes in the H-and O-adatom contours after 1 h temperature holdings in the 8-62°C range.This was taken as an indication that Pt electrodes were stable during the kinetic experiments.

Stationary polarization curves
Tafel plots for the OERR on (111)-and (100)-type Pt in O2-saturated 1.0 M aqueous H2SO 4, at different temperatures (Fig. 1), show, in all cases, a low current density and a high current density linear region.The value of (bT)LCD is slightly lower than the --2.3(RT/F) ratio, and tends to decrease as the temperature is raised, irrespective of the electrode texture.
Otherwise, (bT)HC D increases with temperature and fits the -2.3(2RT/F) ratio for (11 D-type Pt, in agreement with the results reported by Sepa et al. for   OERR on PC Pt electrodes in dilute aqueous H2SO 4 and HCIO4[I 1 , 12].For (100)-type Pt, an almost temperature invariant (bx)HcD value is obtained, as earlier reported by Huang et al. [8] for the OERR on PC Pt in concentrated aqueous HaPO4 solutions.Temperature invariant Tafel slopes were also reported by Zabusova et al. [29] for the HER on PC Pt in H2SO4 and HCI ethanolic solutions.
Values of (Jo)LCD and (Jo)MCD were also obtained by extrapolation of Tal'el lines to the reversible 02 electrode potential.OERR kinetic parameters, derived under different experimental conditions, are assembled in Table 1.

Arrhenius plots
From Tafel plots obtained at different temperatures, electrochemical Arrhenius curves, in j vs. 1/T, were made in the HCD range (Fig. 2), providing relationships between j and temperature, at constant Ev [9].The effective electrochemical enthalpy of activation, AJ~,~f, was obtained from the slope of these lines, and the logarithm of the pro-exponential factor in the rate equation, which contains the effective electrochemical entropy of activation, Ag~f, was also derived from the intersection of Arrhenius straight lines with (l/T) = 0.
OERR Arrhenius behaviour considerably depends on the Pt surface texture.Thus, at the 02 electrode reversible potential, AJ~f = 48.8kJ mol-i for (111)type Pt, whereas AJ~re~ f ----10.6kJmol-lfor(100)type Pt.It should be pointed out that the A~r~f value for (111)-type Pt is similar to that reported by Sepa et aL for the OERR on PC Pt in dilute aqueous H2SO, , and HCIO4 [ll].
The potential dependence of A/-~e~ f is also different for both types of Pt electrodes (Fig. 3).For (Ill)type Pt, A~e~ f increases as Eo approaches the 02 electrode reversible potential value, whereas for (100)-type Pt, A/'~e~ f appears to be practically constant at least within the experimental error (Fig. 3).
The pro-exponential factor, (ln j)l/r= o, obtained from Arrhenius plots also shows a different potential behaviour for each type of facetted Pt electrodes (Fig. 4).For (100)-type Pt, (ln jh/r= o changes with the applied potential, whereas this dependence becomes negligible for (lll)-type Pt.For the latter, the assumption that potential effects on the activation entropy term are negligible could be justified.

DISCUSSION
4.1.Some aspects of the 0 2 electrocatalysis on Pt For the electrocatalytic reaction, such as the OERR, the height of the activation barrier depends on the extent of reactants and/or the adsorption of products, which changes their ground-states without any appreciable modification in the shape of the energy vs. reaction coordinate plot.Figure 5 shows the influence of the adsorption of OERR reactants and products through Morse curves.Considering the case of O2-reactant adsorbates without product adsorption, the activation barrier height, A(~*, is defined by the intersection of curves ( 1) and (2).Conversely, when the reactant adsorption energy is more important than that of the product, A~f increases with respect to A~*.
Since the OERR implies an initial O2-Pt interaction in the electrochemical interface, the knowledge of the 02 adsorbate configuration is required.According to gas-phase studies under ultra high vacuum (uhv) conditions [30,31], the molecular or dissociative 02 adsorption on Pt depends on the electronic characteristics of the surfacer30 and refs therein].Accordingly, the O2-adsorbate structure at the electrochemical interface in the HCD region would also depend on the surface texture of the electrode, leading to either side-on peroxide (-02-) or OH-and O-type adsorbates, whether molecular or dissociative 02 adsorption takes place.From recent semi-empirical quantum chemistry calculations of 02 adsorption on Pt clusters, peroxo adsorbates were found to be the most likely intermediates on Pt(100) and Pt(111), this intermediate being less stable on the latter [32,33].Otherwise, in the LCD region, ie at more positive potentials, end-on peroxide (O2H-) adsorbates are more likely, due to the decrease in d-orbital Pt vacancies at the onset of the Pt oxide layer formation.
An alternative three-step pathway for the OERR mechanism in aqueous acids, consistent with our experimental results for Pt facetted electrodes, is presented.The mechanism formally comprises an O2 adsorption equilibrium process, step (I), followed by a single electron transfer to the O2-adsorbate, step (II), which becomes the RDS in the HCD region, and the subsequent electrodecomposition of the peroxoadsorbate into several products, all of them collected in step (III).Then, the overall rate of the OERR in the HCD region can be expressed in terms of the current density of step (II), in, as:

(I)
where r is the nuclear transmission coefficient for reactants, ca+ is the H + ion concentration in the bulk of the solution, 0o~ is the surface coverage by # the O2-reactant adsorbates, A(~ is the change in the electrochemical activation free energy, AA~d stands for the difference in the electrochemical adsorption free energy between reactants and products, and gives the fractional contribution of AA{~ao to the activation energy assisting the process in the forward direction.
The net change of the activation barrier height due to the adsorption of reactants and products can be considered in the expression of the current density through ),AA(~ad.In fact, ~ represents the fraction of the electrical potential drop at the electrode/solution interface, which affects the adsorbed species [34].Furthermore, potential energy curves can be drawn as straight lines near the crossing point, so that ~ is nearly constant over the entire reaction coordinate.Changes in the activation barrier heights as a result of adsorption were firstly considered by BrSnsted [35], later extended to electrochemical systems by Horiuti and Polanyi [36] and reexamined by Appleby [37].From the above considerations, the effective activation energy for the OERR, A(~r, will include two different contributions, ie A(~ # and AA~ad. Since adsorption changes the electrochemical activation energy, the knowledge of the degree of surface coverage by OERR intermediates as a function of potential is required for each facetted Pt surface.In this respect, 0T, the stationary degree of surface coverage by the total O-containing adsorbates, was determined in O2-saturated 1.0M aqueous H2SO4 on (111)-and (100)-type Pt electrodes (Fig. 6).Thus, gr in O2-saturated sulfuric solution on (100)-type Pt was always greater than that on (lll)-type Pt, ie gr = 0.52 for (100)-type Pt and ,9 T = 0.17 for (111)type Pt at 0.85V.However, the situation changes when the value of ~ is determined in O2-free aqueous H2SO4 solutions, since it becomes almost the same, ie OT = 0.16 at 0.85 V, for both Pt facetted electrodes (Fig. 6).Since gr in O2-saturated 1.0M aqueous H2SO, can reach relatively high values, AA(~,d must be considered as dependent on gr AA~ad = AA~ad.st=o +f('gT)" (2) In our case, a Temkin-type isotherm can be considered for f(,gT) , as a linear function of ~T.Accordingly, the expression for f(ST) becomes: f(~qT) = r'-gT, (3) where r = (c~AAt~ad/c~gr) at constant temperature.
Since it was assumed that AA~d for the OERR intermediates depends on ~gT, AA~d results potential-dependent, and the following relationship can be established: AA(~.d = AAGa~,st= o + r,gE= o + rKE. ( On the other hand, from the proposed reaction scheme 0o~, the degree of surface coverage exclusively by O2-reactant adsorbed intermediates can be obtained, and substituting it in equation ( 1), the OERR current density results
Equation ( 6) allows us to evaluate OERR kinetic parameters at different potentials and temperatures.

The influence of the electrode potential on the OERR kinetics
Let us consider the influence of the potential on the kinetics of the OERR through equation (6).A potential applied to the Pt/aqueous electrolyte interface implies a change in the Fermi level of PT electrons, reflected as a shift in AG* through atFE, a being the electrochemical transfer coefficient assisting the OERR in the forward direction.Earlier works [19 and refs therein] have shown that the electrode potential influence on the reaction rate does not always correspond to a temperature-independent a value.
The temperature dependence of a can be better considered as that derived from the potential dependences of both An* and Ag*, since the OERR occurs at an electrified metal/electrolyte interface.Then, considering that A~ = An -TAg , a can be expressed as a s --a s T [18,19], where a n and a s are defined as: Equations (7a) and (7b) take into account the internal influence of the interfacial field on the metal and the external effect on the double-layer caused by the adsorbed species (compensation effect) [16].
On the other hand, it should be noted, that the physical origin of the temperature dependence of a is still far from being completely understood.According to Conway [19], it could be related to potential and temperature effects on solvent dipole orientations in the electrode interphase, although Bockris and Gocbev [21] considered that the solvent orientation effect is too small to account for the observed experimental temperature dependence of ~c Since the potential dependence of Ag* may be either positive or negative [15,38], another theoretical explanation of the temperature dependence of a, based upon nuclear tunnelling to the classical transfer [39,40] was advanced, but these considerations only lead to small effects on the electrode reaction rate.
Furthermore, the possibility that Ag* could reach a maximum, as the reaction coordinate goes from reactants to products, has already been considered [20].In principle, a linear decrease in Ag* with the reaction coordinate should be expected as both vibrational and translational energies of adsorbed products are probably lower than those of reactants, although obviously this situation largely depends on both the type of the adsorption isotherm and the extent of the own adsorption process.
In addition, it could be argued that as OX increases, the values of AAg, d, the change of the electrochemical adsorption entropy between products and reactants, will also vary.This is only possible when the surface concentration of OERR intermediates is great, since lateral interactions, which are responsible for the existence of rs, are important only when gr =~ 1.In this case, it can be considered that AAgad could vary according to: AAg, d = AASad.st=0+ rs~z=o + rsKE (8) where r s = (aAAgaa,/cq,gT) E and AASad st = 0 stands for the change of AASad between reactants and products at ~1 T = 0.Under this assumption, and considering that AAt~,a = AA/Z/,d -TAAgad and A~* = An* -TAg*, their substitution in equation ( 6) leads to the following current density expression for the HCD region: The electrochemical activation parameters derived from Arrhenius plots correspond to the effective enthalpic and entropic barriers for the electron transfer at a given potential.Then the formal expressions for AA]~eff and AAgeff can be derived from equation ( 9 9) can be neglected.Moreover, AAff, d can be considered to vary little with ~r, since in the HCD region it is lower than 0.6.Then, a simpler equation describing the potential dependence of in can be obtained x exp[-aHFE/RT1 exp[asFE/RT] (12) where K contains all the electrode potentialindependent terms.

The temperature-dependence of OERR Tafel slopes
The influence of the surface texture on the kinetics and the mechanism of the OERR at Pt electrodes has been emphasized [6,7,.However, this influence does not reflect throughout (bT)LCD except for (bT)ncD.Values of (bT)nCD close to -2.3(2RT/F) were found (lll)-type Pt and are often explained through a reaction scheme involving the first electron transfer as RDS, under Langmuirian adsorption conditions, for the intermediates [44, 451.However, this mechanism cannot be applied to OERR results for (100)-type, since Tafel slopes of ca.2.3(2.8RT/F)were found in the HCD region.Rotating ring-disc electrode (rrde) data [7,27] showed that an appreciable amount of H202 is formed on (100)-type Pt during the OERR [7,27] whereas the formation of H202 on (111)-type Pt becomes negligible.Such high Tafel slopes were attributed to the influence of surface coverage by peroxide-adsorbates on the current density through AA~,d [7,37].4.3.1.The (lO0)-type Pt electrodes.Let us examine our results for the OERR on (100)-type Pt in aqueous acid solutions.From Arrhenius plots, the potential dependence of AJ~ef f leads to (OAI'7i.n/OE)=-5.1kJmo1-1(Fig. 3).Taking into account equation (10), it follows that On the other hand, since the electric dependence of AAn,d can be exl~ressed through d(AAn,d)= FdE, and since AAH,d also fits the expression d(AAtTl~d)=rsKdE, it can be concluded that r s K = F. Hence, considering equation (13), it results [aa --(1 + y)] = --0.052.
Furthermore, the slope of the On J)t/r=o vs. E plot (Fig. 4) is as F, so, from the numerical values derived from Fig. 4, the value as is obtained, namely, as = -1.2x 10 -3.

2.3RT (bT).CD
Hence, for the OERR on (100)-type Pt in an aqueous environment, (bT)~CD becomes temperature independent and equal to -0.163Vdecade -1 value, ie a Tafel slope which agrees with experimental figures at all temperatures (Table 1).
4.3.2.The (111)-type Pt electrodes.For (111)-type Pt, 0T < 0.2 in the HCD region, so that AAC~,d can be considered almost constant and equal to AAGad.0T= o" Accordingly, the potential contribution of AAC~,d to the overall current density (equation 12) becomes negligible as it should be expected from a Langmuirian behaviour.In this case, the potentialdependence of A~t yields a slope equal to a s F.
Taking into account the experimental (~A/-~eff/~E) value (50 kJ mol-1), the conventional a H = 0.5 value is obtained.Otherwise, (ln J) l/r = o is almost potential-independent, so, it will lead to an almost negligible a s value.In fact, from the slope of the (lnj)l/r= o vs. E plot (Fig. 4) it results in a s ~10 -6.
Since all=0.5 and (~(lnj)l/r=o/aE)~O, the (bT)nCV for the OERR on (111)-type Pt can be written as follows (bT)aCD = --2.3RT/a s F. (15) This expression of (bT)HC v becomes temperature dependent and it is consistent with (bT)acD= --0.118V decade-1 at T = 298 K, found experimentally.In this case, simplified classical electrode kinetics is applicable to the OERR on this type of Pt electrode.
The preceding discussion was mainly based on the assumptions that the temperature dependence of OERR kinetics is influenced by adsorption properties of intermediates and compensation effects.However, it should be expected that other effects, such as differences in the degree of anion adsorption, temperature dependences of interfacial solvent structure, coadsorption processes, non-adiabatic electron transitions, and proton tunnelling, could also be involved.Some of these questions were previously considered by Appleby[201 and Bockris and Gochev [21], but their potential dependence still remains rather ambiguous.Certainly, some extra considerations concerning the transmission coefficients of atoms and electrons could be made, ie the adiabaticity of the process.Actually, a general approach to the role of the electron in the transitionstate complex in electrocatalysis has still not been considered.Simplification of the classical electron transfer theory would, then, lead to erroneous results.Perhaps, further advances in the OERR mechanism could be made through the knowledge of the configuration of the activated complex formed on both Pt electrode surfaces.This goal can be approached by applying quantum chemistry calculations in relation to the OERR, as will be reported in a forthcoming publication[461.(iii) The influence of the electrode potential on the OERR was considered in both A/~ ~ and Ag * leading to ct = ~t H -at s T. A temperature-dependent ~t value is responsible for the major potential dependence of Ag #, for (100)-type Pt.
(iv) Tafel lines at HCDs on (11 D-type Pt follow a conventional activated-controlled process with (bT)nCD dependent on temperature, whereas Tafel lines on (100)-type Pt at HCDs exhibit a temperature independent (br)ncn value as a consequence of the greater potential dependence of both Ag # and AACad •

Fig. 6 .
Fig.6.Influence of adsorbed products involved in the OERR on the potential energy (Morse) curves.Curve (1) stands for the O2-reactant adsorbed intermediate.Curve(2) stands for non-adsorbed intermediates.Curve (3) stands for adsorbed intermediates.A(~# is defined at the intersection of curves (I) and (2); A(~.d stands for the decrease in the minimum energy ordinate from that of curve(2) to that of curve (3).AG,~f is defined at the intersection of curves(1) and (3).