Accessing the Inaccessible: Analyzing the Oxygen Reduction

Letter Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX-XXX

www.acsami.org

Accessing the Inaccessible: Analyzing the Oxygen Reduction Reaction in the Diffusion Limit Alessandro Zana,†,‡,§ Gustav K. H. Wiberg,†,‡,§ Yu-Jia Deng,‡,⊥ Thomas Østergaard,‡ Jan Rossmeisl,‡ and Matthias Arenz*,‡,§ ‡

Nano-Science Center, Department of Chemistry, University of Copenhagen, Copenhagen 1165, Denmark Department of Chemistry and Biochemistry, University of Bern, Bern 3012, Switzerland ⊥ School of Chemistry and Chemical Engineering, Qingdao University, Qingdao 266000, China §

S Supporting Information *

ABSTRACT: The oxygen reduction reaction (ORR) is one of the key processes in electrocatalysis. In this communication, the ORR is studied using a rotating disk electrode (RDE). In conventional work, this method limits the potential region where kinetic (mass transport free) reaction rates can be determined to a narrow range. Here, we applied a new approach, which allows us to analyze the ORR rates in the diffusion-limited potential region of high mass transport. Thus, for the first time, the effect of anion adsorption on the ORR can be studied at such potentials.

KEYWORDS: oxygen reduction reaction, fuel cells, anion adsorption, rotating disk electrode, diffusion limited current, adsorption isotherm, computational modeling

T

studied at such potentials using RDE. To test our approach, we compared the extracted MT free ORR rates to computational modeling of the adsorption isotherms of ions in solution. We start our discussion considering the behavior of polycrystalline Pt in a O2 saturated 0.1 M HClO4 electrolyte. In Figure 1a, ORR polarization curves of polycrystalline Pt recorded at different rotation rates are divided into three different potential regions:1 (i) the region of low overpotential (ca. 1.0−0.9 VRHE), where the reaction is under kinetic control; a change in rotation rate does not result in a change in reduction current. Thus, the reaction is not affected by oxygen MT to the Pt surface. (ii) At higher overpotentials (ca. 0.9−0.6 VRHE), the reaction rate becomes rotation rate dependent and proceeds under mixed kinetic-diffusion control; i.e. the reduction current is controlled by kinetics as well as oxygen MT. (iii) At electrode potentials between 0.6 and 0.05 VRHE, the ORR is under “pure” oxygen MT control. In this potential region, a change in rotation rate results in a change of current density described by the Levich equation, eq 1:

he oxygen reduction reaction (ORR) is one of the key processes in electrocatalysis because of its importance for the development of fuel cell (FC) technology. In FCs, the ORR is the cathode process, whereas at the anode hydrogen is oxidized to protons. Both reactions must proceed at the same rate, which in a device is influenced by kinetics, but also mass transport, membrane conductivity, etc. In research, the ORR is therefore studied in electrochemical half-cells using rotating disk electrodes (RDE). Thus, the reaction can be investigated independent of the anode process under well-defined mass transport and solution resistance.1,2 The method became very popular with the invention of the thin film (TF) method for ORR catalyst testing.3−6 A major drawback, however, is that the reactant (oxygen) needs to be dissolved in the electrolyte solution and therefore only very low mass transport (MT) is achieved. Because of these MT limitations, catalysts can only be evaluated in a narrow potential window outside the typical operation conditions of FC systems for which high mass transport (high power density) is particular interesting.7 Therefore, several research groups tried to extend MT with varying approaches.8−11 In the work, for the first time the ORR has been analyzed under conditions of high oxygen MT using RDE. Conversely to previous efforts to increase MT or standard RDE studies that cannot analyze the diffusion limited (DL) potential region, we applied a new approach, which allows us to analyze the ORR rates in the DL region. Thus, for the first time, the effect of anion adsorption (site blocking model) on the ORR can be © XXXX American Chemical Society

Jdl = 0.62nFA−1D0.67ν−0.166Cω1/2

(1)

According to eq 1, Jdl at any potential in the oxygen MT limited potential region should change proportionally to the square root of the rotation rate ω1/2. Turbulent flow1 at the solid/ Received: September 13, 2017 Accepted: October 24, 2017 Published: October 24, 2017 A

DOI: 10.1021/acsami.7b13902 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces

Figure 1. (a) ORR polarization curves for different rotation rates. The potential of a PC Pt disk in O2 saturated 0.1 M HClO4 electrolyte was swept at scan rate was 100 mVs−1. (b) Corresponding Tafel curves; the measurements are recorded at room temperature.

characterizing the rate of change of the apparent standard Gibbs energy of adsorption with the surface coverage by OH species. As discussed by Rossmeisl et al., this equation generally holds for reactions; however, it does not explain trends in catalytic activity as seen in the well-established volcano curves of the ORR.13 Furthermore, the fact that ORR analysis is only possible around 0.9 VRHE implies that the ORR is always dominated by the exponential, but not the (1 − θad)x term. To allow studying the influence of anion adsorption, we need the ORR analysis to be extended to high overpotentials. This is the aim of the presented work. To test the anion site blocking model on the MT free ORR rates, we (i) studied the ORR in 0.1 M HCl electrolyte and (ii) applied a modified analysis according to the K-L equation. Studying the ORR in 0.1 M HCl electrolyte instead of 0.1 M HClO4 pronounces the site blocking effect and simplifies kinetic modeling of the anion adsorption isotherms. In the Supporting Information, it is demonstrated that the analysis is also applicable to 0.1 M HClO4; however, in weakly adsorbing electrolytes, the uncertainty in the measurements is increased. In the analysis, instead of adopting a diffusion correction approach to extrapolate Jk, we use the K-L eq (eq 2). Plotting 1/J vs 1/ ω1/2, the intercept of 1/J at 1/ω1/2 = 0 represents the inverse of the current extrapolated at infinite rotation rate. In Figure 2, we report a K-L plot with equidistant 1/ω1/2 values. By fitting a straight line to the measured values of 1/J as a function of 1/ ω1/2, we extract the value of 1/J at infinitive rotation rate (see Methods section for a more detailed discussion). In contrast to previous work, we are thus able to extract oxygen MT-free ORR rates (Jkl) even in the apparent diffusion limited potential range. Reiterating the analysis every 10 mV for the whole potential range, we obtain a potential dependent behavior Jkl as demonstrated in Figure 3. Comparing Jkl for three different chloride concentrations, i.e. 0.01 M, 0.1 and 1 M it becomes obvious that the ORR reaction rates are critically inhibited by increasing the Cl− concentration. In addition, a clear maximum in Jkl (negative current as reduction process) is observed. Interestingly, the potential of this maximum, Emax, falls into a potential region of the potential of zero total charge (PZTC) of Pt.14,15 A related property is the potential where ion coverage exhibits a minimum. Thus, the results indeed indicate a correlation between the adsorbate coverage on the Pt surface and the ORR rate. Looking closely at the results, it is furthermore seen that Emax slightly shifts to more negative

liquid interface may result in currents deviating from eq 1, therefore we experimentally confirmed that a well-defined laminar flow is achieved for all adopted rotation rates. A deviation from a straight line intersecting at origin, when plotting J as a function of ω1/2 implies that kinetic limitations affect the electron transfer process.1 In this case the current density can be described by the Koutecký−Levich (K-L) equation, eq 2: 1 1 1 1 1 = + = + −1 0.67 −0.166 J Jk Jdl Jk 0.62nFA D ν Cω1/2

(2)

Rewriting 2 to express Jk, we obtain the Tafel equation, eq 3: Jk =

Jdl J Jdl − J

(3)

As displayed by eq 3, also referred to in literature as diffusion correction, the applicability of the analysis is limited to a narrow potential window: i.e., when J ≪ Jdl.4 For example, in 0.1 M HClO4 electrolyte the potential window where diffusion correction can be applied is ca. 0.90 ± 0.05 VRHE. In this potential range, Jk calculated according to eq 3 is plotted in a logarithmic plot as a function of the applied electrode potential E, see Figure 1b. Extrapolation of the kinetic currents to potentials more relevant for FC systems is extremely inaccurate, because of bent curves (potential-dependent Tafel slopes). As mentioned, the outlined Tafel analysis is in general used to analyze catalysts, both with respect to performance (trying to identify improved catalysts), but also for fundamental studies. In the latter category, fall investigations of the influence of anion adsorption on the ORR. In literature, electrolytedependent ORR activities are usually assigned to a site blocking mechanism.12 The model of anions as spectator species during the ORR is widely accepted. Markovic and co-workers proposed a relation between the rate and the amount of free Pt sites according to eq 4: j = nFkCO2(1 − θad)x e−βFE / RT e−γrθOHad / RT

(4)

Where n is the number of electrons, k is the rate constant, CO2 is the oxygen concertation, θad is the total surface coverage with ions and OHad, x is either 1 or 2 depending on the site requirements of the adsorbates, j is the measured current density, E is the applied potential, γ and β are the symmetry factors (assumed to be 1/2), and rθOHad is a parameter B


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determining step and that the ORR rate depends only on the amount of available free Pt sites. Further it is assumed that for the O−O cleavage two adjacent free sites are required: O2 + 4(H+(aq) + e−) + 2*→

2O* + 4(H+(aq) + e−) ⇋ 2H 2O(l) + 2*

Where * is a free site and O* is an adsorbed oxygen atom. The ORR current density in eq 4 therefore can be simplified according eq 5 to

j = j0 θ*2

(5)

Where θ* is the coverage of active free sites, j0 is a proportional factor depending both on the reaction kinetics and macroscopic experimental set up. Figure 4 compares the measured and modeled ORR currents, plotted in a Tafel fashion. It is important to emphasize that the Figure 2. K-L plots for different potentials of ORR RDE polarization curves recorded in 0.1 M HCl electrolyte. To obtain equidistant points, the adopted rpm rates are 813, 1111, 1600, 2500, and 4444 rpms, respectively.

Figure 4. Modeled and measured MT free current density as a function of HCl concentration: 1 M (blue), 0.1 M (black), and 0.01 M (red). The modeled currents j are plotted in dashed lines and the measured in solid. Figure 3. Plot of the MT free Jkl curve in 0.01, 0.1, and 1 M HCl electrolyte solution. The inset is a magnification of the measurements at higher electrolyte concentrations. The shaded area represents the uncertainty in the measurements.

modeling parameters are determined by comparison to only one electrolyte concentration (1 M) and the remaining two curves (0.1 and 0.01 M) are obtained with the thus determined parameter set (i.e., j0, ΔG0H, ΔG0Cl, εH−H, εCl−Cl; see Table 1

potentials on the RHE-scale with increasing the HCl concentration. To test the site blocking model, we now compare Jkl to a kinetic model of the potential dependence of the number of free Pt surface sites. The computational model takes the potential dependence of proton (H+) and anion (Cl−) adsorption into account (a detailed discussion of the model can be found in SI) and can be applied to any other electrolyte as well. The adsorption energy of these ions depends on the potential of the Pt working electrode and thus their coverage on the Pt-electrode will change with varying potential. The coverage of Cl− and H+ on the Pt surface is modeled by a Frumkin isotherm,16 which also considers Cl−Cl and H−H repulsion energy. The basic assumption in the model is a proportionality between the ORR and the number of free sites (see the Supporting Information). We assume that at very high overpotential (E < 0.6 VRHE) the O−O cleavage is the rate-

Table 1. Comparison between Parameters Fitted According to Site Blocking Model and Literature Values parameter

fit

literature value

ΔG0H (eV) εH−H (eV) ΔG0Cl (eV) εCl−Cl (eV) j0 (mA cm−2)

−0.18 0.12 −1.42 0.18 240

−0.2,17 −0.118 0.1517 −1.3119 0.719 9620 (Value for Pt(111))

and the Supporting Information). The site blocking model captures the experimental trends around Emax extremely well. The anion concentration dependence as well as shift in Emax with concentration is reproduced. Moreover, the resulting adsorption energies (ΔG0H, ΔG0Cl) are comparable with those determined from density functional theory calculations, see Table 1. The same is the case for the H−H repulsion energy C



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εH−H. Only the value for εCl−Cl deviates, which is expected as mean field model does not capture repulsion energies very well. εCl−Cl is in reality a function of coverage, but here it is kept constant and fitted to match the current profile in the low coverage region. A higher slope in the modeled current is therefore expected beyond Emax. The site blocking model also offers a straightforward explanation for the observed shift in Emax: that a change in the electrochemical potential of Cl− with its concentration leads to a shift in the adsorption isotherm. By comparison, the corresponding (the concentration of H+(aq) and Cl−(aq) in the electrolyte must be equal) adsorption isotherm of H+ is relative insensitive to its concentration on a RHE scale (see the Supporting Information). According to the modeling Emax thus displays the potential of zero ion (H+ and Cl−) coverage on the Pt surface. This implies that the adsorption potential of Cl− is shifted by 120 mV on an RHE scale when its concentration changes by 1 order of magnitude. The model also reveals that the dissociation of oxygen must be the rate limiting step at high overpotentials, since the difference in the magnitude of the currents could not be reproduced, when a direct proportionality between j and θ* was assumed (i.e., assuming x = 1 in eq 4). In conclusion, our results demonstrate a feasible approach to extract MT free ORR rates using RDE in a potential range that is usually considered diffusion limited. Although the effective mass transport to the catalyst is not enhanced, the method allows to study important phenomena such as the influence of anion adsorption on the effective reaction rate. So far, the longdebated site blocking model could only be investigated in a potential region that is dominated by the adsorption of oxygenated species, but not anions. Comparing experiment and modeling, it is shown that anion adsorption starts already at very low potentials and competes with proton adsorption. Interestingly, the difference between the concentration dependence of the adsorption isotherms of H+ and Cl− leads to a shift of the potential of minimum ion coverage with electrolyte concentration.

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ACKNOWLEDGMENTS This work was supported by the Danish DFF through Grant 4184-00332, the Danish Council for Strategic Research (4M Centre) and the Villum center for the science of sustainable fuels and chemicals.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b13902. Cyclic voltammetry of Pt in different electrolytes (0.1 M HClO4, 0.01 M HCl, 0.1 M HCl, 1 M HCl); description of the methodology used for the extrapolation of the Jkl; Jkl measurements in 0.1 M HClO4; modeling details and Frumkin adsorption isotherms for hydrogen and chlorine (PDF)

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Letter

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Matthias Arenz: 0000-0001-9765-4315 Author Contributions †

A.Z. and G.K.H.W. contributed equally

Notes

The authors declare no competing financial interest. D


Letter

ACS Applied Materials & Interfaces (17) Karlberg, G. S.; Jaramillo, T. F.; Skulason, E.; Rossmeisl, J.; Bligaard, T.; Norskov, J. K. Cyclic voltammograms for H on Pt(111) and Pt(100) from first principles. Phys. Rev. Lett. 2007, 99 (12), 126101. (18) Norskov, J. K.; Bligaard, T.; Logadottir, A.; Kitchin, J. R.; Chen, J. G.; Pandelov, S.; Stimming, U. Trends in the exchange current for hydrogen evolution. J. Electrochem. Soc. 2005, 152 (3), J23−J26. (19) Gossenberger, F.; Roman, T.; Gross, A. Equilibrium coverage of halides on metal electrodes. Surf. Sci. 2015, 631, 17−22. (20) Rossmeisl, J.; Karlberg, G. S.; Jaramillo, T.; Norskov, J. K. Steady state oxygen reduction and cyclic voltammetry. Faraday Discuss. 2009, 140, 337−346.

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Accessing the Inaccessible: Analyzing the Oxygen Reduction

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