Article pubs.acs.org/JPCC
Oxygen Reduction Reaction in Ionic Liquids: The Addition of Protic Species Elise E. Switzer,†,§ Robert Zeller,†,§ Qing Chen,† Karl Sieradzki,† Daniel A. Buttry,‡ and Cody Friesen*,† †
School of Energy, Matter and Transport Engineering, Arizona State University, PO Box 871606, Tempe, Arizona 85287-1606, United States ‡ Department of Chemistry & Biochemistry, Arizona State University, PO Box 871604, Tempe, Arizona 85287-1604, United States S Supporting Information *
ABSTRACT: The effect of proton donors on the mechanism of the electrochemical oxygen reduction reaction (ORR) is examined in the supporting ionic liquid 1-butyl2,3-dimethylimidazolium trifluoromethanesulfonate (C4dMImTf). ORR in aqueous media is contrasted with that in aprotic media and in aprotic ionic liquid (IL) systems with the addition of protic species. This study elucidates the effect of proton activity encompassing almost thirty orders of magnitude for both platinum (Pt) and glassy carbon (GC) electrodes. In neat aprotic C4dMImTf for both platinum and glassy carbon electrodes, ORR proceeds entirely through a one electron process as expected. In ILs with protic additives, ORR approaches a four-electron pathway regardless of the identity of the protic additive on Pt, whereas ORR on GC is limited to a two-electron process due to a lack of Hads and Oads species.
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INTRODUCTION ORR as a Function of Proton Activity. The oxygen reduction reaction (ORR) in aqueous systems has been intensively studied for decades, and many high quality reviews exist.1−4 These tend to focus on the mechanisms of ORR as a function of pH4−6 or on the selection and design of catalysts.7−9 In aqueous solutions, platinum and its alloys are the best ORR catalysts across most pH ranges,4,10,11 effectively completing the full four-electron reduction to water or hydroxide in aqueous acid or aqueous alkali, respectively. The series mechanism of oxygen reduction initially involves the single electron transfer to the adsorbed dioxygen molecule (O2) to produce the superoxide radical (O2●−) O2,ads + e− → O•− 2,ads
(1)
Figure 1. Pourbaix diagram of selected oxygen species in aqueous solutions showing equilibrium species as a function of pH and electrochemical potential (V vs NHE) at standard state. Adapted from ref 13.
regardless of solution pH.4 Superoxide is then irreversibly protonated by the hydronium ion H3O+ in aqueous acid + • O•− 2,ads + H → HO2,ads
(2)
or H2O in aqueous alkali O•− 2,ads
+ H 2O →
HO•2,ads
+ OH
−
limit of water, hydrogen peroxide is stable. The pH independent formation of the superoxide species O2●− is shown as the bold horizontal line at ca. −0.3 V. Above the vertical dotted line at pH 11.7, the dominant species is the hydroperoxyl anion (HO2−). By expressing the thermodynamics of the series 2e− oxygen reduction reaction
(3)
At this point, a cascade of intermediate reactions occurs with the final products consisting of peroxide, water, and/or hydroxide depending on the particulars of the substrate and electrolyte.4,12 Figure 1 is a Pourbaix diagram for some selected oxygen species in aqueous solutions. In Figure 1, lines (a) and (b) define the stability limits of water. At potentials below line (a), water is reduced to form hydrogen gas at a pressure of 1 atm, while at potentials above line (b) water is oxidized to form oxygen. Within the stability © 2013 American Chemical Society
O2 + 2H+ + 2e− → H 2O2
(4)
Received: January 24, 2013 Revised: April 9, 2013 Published: April 11, 2013 8683
dx.doi.org/10.1021/jp400845u | J. Phys. Chem. C 2013, 117, 8683−8690
The Journal of Physical Chemistry C
Article
in terms of the electrochemical potential E, the relationship between oxygen partial pressure PO2, H2O2 concentration, and pH is E = E H0 2O2 −
PO2 ⎤ 2.303RT ⎡ ⎥ ⎢pH + log F [H 2O2 ] ⎦ ⎣
hydrophobicity/hygrophobicity) were held essentially constant. The approach then was not to examine protic ionic liquids directly, as properties vary widely, but rather to work in a supporting aprotic ionic liquid into which small quantities of protic species were titrated.
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(5)
EXPERIMENTAL SECTION Materials. A number of considerations were made in selecting the supporting IL for these protic additive studies. Foremost among these was the pKa of the conjugate acid of the anion, which effectively places an upper bound on the additive acid strength that can be added to the supporting IL as protons from stronger acids would be captured by the anion, thereby leveling the solvent acidity. The bis(trifluoromethylsulfonyl)imide (Tf2N) anion is slightly preferable to triflate (Tf) because its produces hydrophobic ILs with lower viscosities and melting points; however, its classification as a superacid with a pKa lower than HTf is disputed.18−20 The cation of the ionic liquid can have a strong effect on electrochemical reactions via complexation.21,22 Cations must be noninteracting with superoxide and the electrode surface. Good first approximations for cations that do not react with superoxide are those which are relatively stable to alkaline conditions such as the tetraalkylammoniums. Disubstituted imidazoliums are not suitable because the proton on the C2 carbon is rapidly removed to form a carbene in the presence of superoxide or other strong bases.21 By replacing the C2-proton with a methyl, this path was eliminated. Based on these considerations, 1-butyl-2,3dimethylimidazolium trifluoromethanesulfonate (C4dMImTf) was chosen as the supporting IL for these studies. The melting point of this system (ca. 40 °C) required our experiments be carried out at 50 °C. The selection of protic species for addition to the supporting IL was based on tabulated pKa values to effectively establish a range of proton activities encompassing almost thirty orders of magnitude for both platinum (Pt) and glassy carbon (GC) electrodes. These protic species and their pKa values in both water and aprotic media are listed in Table 1.
It is this variation in potential with respect to pH that is plotted as the sloping bold line in Figure 1 for log(PO2/[H2O2]) = 0 and with the pKa of H2O2 equal to ca. 11.6. Equation 5 expresses the linear relationship between oxygen reduction potential and pH for reaction 4. Through the Henderson− Hasselbach equation, the pH is related to the strength of a general acid, HA, through its pKa. Accordingly, for oxygen reduction in the presence of a general acid O2 + 2HA + 2e− ↔ H2O2 + 2A−
(6)
the expression becomes E = E0 +
[H 2O2 ][A−]2 RT ln 2F [HA]2 PO2
(7)
which represents the Nernst equation for oxygen reduction using a general acid as the proton donor. The expression may take the form of E = E0 +
[H 2O2 ] 2.3RT 2.3RT 1.15RT pH − pK a + ln F F F PO2 (8)
which explicitly separates the contribution of proton binding energy (pKa). Accordingly, the oxygen reduction potential should shift by the pKa of the proton donor. ORR in Aprotic Media. With the mechanism of ORR so closely tied to the availability of protons, either from an acid source or the deprotonation of water, some investigations have pursued oxygen reduction in aprotic solvents such as DMF and DMSO.14−17 The findings of these studies have revealed some interesting complexities in the ORR mechanism. Across a broad range of solvents and catalysts, the rate limiting step in ORR is usually the injection of the first electron to form superoxide.4 In aprotic solvents this is a readily reversible process, but it becomes irreversible with the addition of weakly acidic species such as phenol (pKa = 10).17 The proton transfer from phenol to superoxide is surprising given the superoxide radical’s relatively low pKa of ca. 4.7 in aqueous media.16 However, the conjugate base of superoxide is able to drive the protonation reaction
Table 1. List of Proton Sources Based on pKa Values proton source
−14
triflic acid (HTf) acetophenone:HTf methanesulfonic acid (HMeS) pyridinium triflate (PyrTf) N,N-diethyl-N-methylammonium triflate (DEMATf) dimethyl malonate (dMM) 2-butyl-1,1,3,3-tetramethylguanidinium triflate (TTMGTf) water (H2O)
− − 9 2O•− 2 + H 2O → O2 + HO2 + OH K = 0.91 × 10
(9)
so completely that it behaves as though it has a proton affinity similar to pKa = 24.16 Equation 9 expresses the result of superoxide protonation in aqueous media and the rapid followup reactions that produce the hydroperoxyl anion HO2−. Also, it has been estimated that the pKa of HO2 in DMF is ca. 12 due to the much weaker solvation in nonaqueous solvents.16 Ionic liquids provide a rare opportunity to examine oxygen reduction in aprotic media and in protic ionic liquids with tuned pKa’s ranging from species much more acidic than hydronium to those with acidities comparable to water. We set out to explore oxygen reduction across a ∼30 unit pKa range in protic additives but needed to develop an experiment wherein the physical properties (e.g., viscosity, oxygen solubility,
pKa (water)
pKa (aprotic)
ref
1.6 3.4 11
18−20, 23 24 24 24 24
13
15.7 23.6
24 25
15.7
32
23
−6.2 −2.6 5.2 10.6
0.3
Methods. Oxygen solubility was determined gravimetrically by measuring the mass change of a degassed sample upon saturation with oxygen at 0.96 atm (ambient pressure in Tempe). A 100 mL vial of C4dMImTf was degassed under vacuum for 5 days before recording the initial mass. Immediately following, dry oxygen was purged through the solution for two days to obtain the oxygen saturated mass. The mass of oxygen was confirmed by degassing the solution for one day to regain the initial mass. 8684
dx.doi.org/10.1021/jp400845u | J. Phys. Chem. C 2013, 117, 8683−8690
The Journal of Physical Chemistry C
Article
Figure 2. Cyclic voltammograms of oxygen-saturated aprotic BdMImTf (
