Molecular Oxygen Activation at a Conducting Polymer: Electrochemical Oxygen Reduction Reaction at PEDOT Revisited, a Theoretical Study

Molecular Oxygen Activation at a Conducting Polymer:

Electrochemical Oxygen Reduction Reaction at PEDOT Revisited, a

Theoretical Study

Viktor Gueskine,

*

Amritpal Singh, Mikhail Vagin, Xavier Crispin, and Igor Zozoulenko

*

Cite This:J. Phys. Chem. C 2020, 124, 13263−13272 Read Online

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ABSTRACT: Molecular oxygen requires activation in order to be reduced, which

prompts extensive searching for efficient and sustainable electrode materials to drive electrochemical oxygen reduction reaction (ORR), of primary importance for energy production and storage. A conjugated polymer PEDOT is a metal-free material for which promising ORR experimental results have been obtained. However, sound theoretical understanding of this reaction at an organic electrode is insufficient, as the concepts inherited from electrocatalysis at transition metals are not necessarily relevant for a molecular organic material. In this work, we critically analyze the basics of electrochemical ORR and build a model for our DFT calculations of the reaction

thermodynamics based on this analysis. Altogether, this work leads to a conclusion that outer sphere electron transfer that currently attracts increasing attention in the context of ORR is a viable mechanism at a conducting polymer electrode.

1. INTRODUCTION

The electrochemistry of molecular oxygen is attracting considerable attention in our age of transition to greener fossil-free economy. Electrical energy storage devices such as fuel cells and batteries run the oxygen reduction reaction (ORR) at their cathode that transforms molecular oxygen into water in a 4e overall process, which, for acidic media, can be written as

O₂+4H++4e→2H O₂

The renewal of interest in ORR is also due to the research into new ways for hydrogen peroxide electrolytic production in the framework of green chemistry. In this case, molecular oxygen undergoes a 2e reduction:

O₂+2H++2e→H O_{2 2}

ORR is usually characterized with high overpotentials and is thus notoriously sluggish, calling for judicious choice of the electrode material to facilitate this multielectron proton-coupled process. While platinum and other platinum-group metals (PGM) are nowadays the most efficient electrocatalysts of 4e reduction, in particular, in acidic media,1 2e reduction rather requires different electrode materials, such as mercury or gold containing alloys.2 This very fact reflects clearly that the mechanism of this complex multistage process cannot be the same in different conditions and calls for its better under-standing. While actually employed ORR electrodes are based on PGM or other rare and expensive elements, intensive searching for efficient electrode materials free from these shortcomings is also underway. Recently exploited ORR

electrodes thus also include graphene-based nanostructures3,4 and conducting polymers.5 The latter constitute a class of materials that can be synthesized and processed at low temperatures, thus ensuring a fast payback time when integrated into energy devices, and provide a great degree of freedom with respect to designing theirπ-electronic structure. The performance of conducting polymer modified electro-des in electrochemical reactions has been studied practically since the very inception of the field and continues to be an expanding topic.6,7However, the approach is mostly empirical, and the efforts are mostly directed at the global, macroscopic characterization, such as surface or bulk location of the reaction, rather than at discovering the mechanism at the molecular level. Sometimes it is even unclear if the electro-chemical reaction takes place at all at a CP modified electrode, unless clear inhibition was demonstrated. This is, for example, the case of hydrogen evolution reaction (HER) through the poly(N-methylpyrrole) (PMPy)film that becomes electrically insulating in the cathodic region, so that the charge transfer location is limited to the metal surface.8In this notable case of HER, the very possibility for a CP to drive this reaction was questioned,9 and though answers were brought since then,10 the role of a CP remains controversial, unless it is used as an

Received: April 20, 2020

Revised: May 21, 2020

Published: May 21, 2020

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electrically and ionically conducting matrix for a composite with an established electrocatalyst as the main active component.11 It is incontestable that kinetics of a redox reaction on a CP can be very fast, which can lead to interesting applications.12,13 We believe that an effort to clarify the chemical role of a CP, essentially a molecular material, in an electrochemical reaction at a molecular scale by quantum chemical modeling is necessary to complement electrochemical studies and will lead to a more rational choice of CP electrodes for specific electrochemical transformations.

Here, we focus on the ORR at a p-type polymer poly(3,4-ethylenedioxythiophene) [PEDOT]. PEDOT has emerged as novel and reliable material for recycling power and energy storage devices due to its stability and well-established manufacturing technology.14−17 Scientific community has been attracted toward the investigation of PEDOT in the electrodes for ORR18−27since Winther-Jensen et al. demon-strated that PEDOT electrode catalyzes the ORR with efficiency of the same order of magnitude as that of platinum.5 While the role of PEDOT in composite electrodes can be limited to that of conducting binder, and metal impurities can be responsible for electrocatalysis in some cases, the activity of pure PEDOT free of any metal impurities was unambiguously demonstrated in the works from this laboratory,28,29 and relevant experimental observations were made therein. However, the mechanism of action of the conducting polymer in ORR remains unclear. Earlier theoretical treatments29,30did not sufficiently take into account the complexity of the system. Therefore, we feel it necessary to revisit the issue.

2. PRELIMINARY CONSIDERATIONS

2.1. ORR on PEDOT: What Is Known from Experi-ments. From previous experimental studies of ORR on PEDOT, we have important observations relative to the role of the conducting polymer in this process. First of all, experimental evidence suggests that the reduction reaction follows a 2-electron transfer pathway that produces hydrogen peroxide.29,27

PEDOT is prepared by chemical or electrochemical oxidation of the monomer and is therefore in its oxidized, p-doped, electronically conducting state. When such PEDOT is electrochemically reduced at different potentials, it is partially dedoped, so that its electrical resistance increases to different extents, and then disconnected from electrochemical polar-ization and exposed to air in the wet state, the following changes take place: (i) its resistance gradually decreases, (ii) the optical spectrum shows an increase of the absorption characteristic of the doped form along with a decrease of the absorption of the undoped form, and (iii) the ratio of the vibrational intensity of the quinoid (doped) to benzenoid (undoped) group increases.28 All this data, in mutual agreement, attest to chemical oxidative doping of PEDOT with molecular oxygen. It was logically suggested that thefirst product of such a reduction of TDO should be the superoxide radical anion 2O2−•, though it was not identified. In another recent study,29in situ Fourier-transform infrared spectroscopy (FTIR) reveals that, under polarization of PEDOT at the cathodic potentials where ORR starts, a new band appears, and it is assigned to OOH covalently bound to theβ-carbon of the thiophene ring of PEDOT. Furthermore, this band of an apparent ORR intermediate disappears upon more negative polarization, when hydrogen peroxide, thefinal product of 2e reduction, is detected. In addition to the reversible processes

leading to ORR, other changes in the XPS spectra of reduced PEDOT can be attributed to the formation of carbonyl and/or sulfon group, and thus to oxygen attachment to the backbone. Hydrogen peroxide production from molecular oxygen with the help of PEDOT may thus pass through the following three intermediates. One of them is the superoxide radical anion 2_O

2−• or, depending on pH, its conjugated Brønsted acid, hydroperoxyl radical 2_HO

2•, formed when reduced (neutral) PEDOT is oxidized (p-doped) by O2. The third one is a peroxo-adduct of PEDOT.

2.2. Thermodynamic and Mechanistic Factors in Reactions of Molecular Oxygen, a Triplet Diradical. The possibility of superoxide formation in the course of ORR, which is not limited to PEDOT but can be detected even with Pt electrodes,31 underscores the multistage nature of this multielectron reaction. The formal thermodynamic description of the complete 4e ORR-oxygen evolution reaction (OER) states that it is at equilibrium at 1.23 V (vs standard hydrogen electrode (SHE)) at standard conditions. If such an equilibrium is effective, it is expected that a deviation from this reversible potential in the cathodic or anodic direction would lead to the onset of ORR or OER, respectively. In practice, this is far from being the case, and the deviations of the onset potentials from the thermodynamic value called overpotentials are usually attributed primarily to kinetic hindrance. Note, however, that thermodynamic equilibrium at 1.23 V (SHE) is the equilibrium between the initial andfinal states differing by 4 electrons, which cannot be transferred simultaneously. In reality, as any long journey starts from the first step, many-electron reduction of molecular oxygen starts from thefirst electron transfer (ET), and its thermodynamics and kinetics should not be obscured by the overall thermodynamics. As written step by step for acidic media, the standard potentials32are

This sequence yields, of course, the well-known standard potential for the 4e reaction: (E1+ E2+ E3+ E4)/4 = +1.23 V, as the reversible potential of the multistep reaction is nothing but the average of the potentials of its steps (weighted by the number of electrons exchanged in each step).

Note that the most difficult (most cathodic) reduction step is thefirst one. We conclude immediately that the deviation of ORR onset from the 4e theoretical value (to be derived from 1.23 V by Nernst equation, taking into account the actual activities of the reactants and products) is primarily of thermodynamic and not kinetic origin. A thoughtful review33 clearly pointed to the notion of thermodynamic overpotentials in multistep electrochemical reactions. Furthermore, a huge difference between the onsets (overpotentials) of ORR and OER also has primarily a thermodynamic origin: ORR starts with the very cathodic first step, while OER with the very anodic fourth. This problem was recognized long ago in the context of homogeneous catalysis and biochemistry.34 The second reduction leading from a radical to a stable product, hydrogen peroxide, and terminating the 2e pathway is thermodynamically facile, as follows from the standard potentials above. The reduction of hydrogen peroxide to water starts with the energetically demanding third step

requiring O−O bond cleavage and ends up with the very easy fourth leading to water. A catalyst ideal from the thermody-namic viewpoint would ensure such intermediate energies as to provide equal equilibrium potentials for every step,35 only then, e.g., can complete 4e ORR to water take place at +1.23 V (in standard conditions). It remains that the first ET merits particular attention.

The difficulty of the first reduction step finds its explanation in the electronic structure of molecular oxygen: in its ground state, it is a triplet diradical, 3O2, whose peculiar electronic structure is still the subject of active research in theoretical chemistry.36,37 Notably, it is the only truly stable and ubiquitous diradical known, and such stability may seem quite puzzling. As a matter of fact, the unpaired electrons of this diradical occupy two degenerate antibonding molecular orbitals, so the reduction of3O2involves further population of these, which is rather unfavorable; it was recognized rather early.34Thus, in spite of the diradical character of the molecule and high electronegativity of the oxygen atom, dioxygen is a rather weak oxidant from the thermodynamic point of view, as far as thefirst ET is concerned.

Considering the possibility of direct addition of molecular oxygen to an organic molecule, the triplet nature (S = 1) of the former is again the key, as a regular reaction pathway conserves the total spin. As all other stable molecules are closed-shell, that is, spin-singlet (S = 0), the total spin of the products should be triplet (S = 0 + 1 = 1). Triplet adducts usually have high energy, so the direct addition of dioxygen even to unsaturated singlet organics is prohibited by spin. On the other hand, formation of a pair of spin-doublet radicals (S = 1/2 + 1/ 2 = 1) is allowed, which explains the propensity of radical mechanisms of such oxidation as combustion.

Recent highly correlated computational studies of simplest gas-phase reactions with the participation of dioxygen illustrate these general considerations. The reaction of molecular hydrogen, 1H2, and 3O2 cannot lead directly to H2O2 but rather proceeds via hydrogen abstraction and formation of two radicals, 2HO2• and 2H•.

38

The reactions of triplet dioxygen not only with a saturated hydrocarbon ethane39but also with unsaturated ethene40,41 pass through radical mechanisms. Though spin selection rules are not absolute, the forbidden character of the so-called intersystem triplet-to-singlet crossing makes it very improbable; that is, the formation of a stable singlet adduct is kinetically hindered.42

However, living organisms manage to reduce molecular oxygen smoothly and controllably with the help of enzymes. Most enzyme cofactors contain transition metals (e.g., iron in heme), whose various spin states open the way to complex-ation with molecular oxygen. Still, a particular class stands apart among the oxygen reducing enzymes (oxidases) as the only one not containing transition metals, namely, flavin dependent enzymes (see, e.g., ref43and references therein). In this case, molecular oxygen activation is just ET from a reduced form of flavin, a closed-shell anion, to 3_O

2 with the formation of superoxide radical anion 2O2−• and flavin semiquinone radical. Then, following an easy spin flip in a pair of two separate radicals, these can recombine, yielding a spin-singlet adduct. Note that such a two-step mechanism conserves spin and allows the system to circumvent direct attachment of dioxygen to the singlet organic cofactor that could only lead to a high-lying triplet product. Otherwise, superoxide is further reduced without attachment.

2.3. Electrochemical ORR: The Role of Electrode Material. We are in a position to consider the role of electrode material in electrochemical ORR, namely, in its difficult first ET.

The pattern for the mechanistic understanding of ORR has been dominated for a long time by the way the reaction takes place at platinum in an acidic medium. The foundation for this has been laid by the seminal computational work by Nørskov et al.44 and subsequent publications from the same group. Activation of dioxygen in this case is due to the chemisorption of the molecule at the metal surface, and the famous volcano plots reflect relative bonding energies of dioxygen itself and its reduction products. It is generally accepted that the chemisorption of dioxygen precedes ET.44 Naturally, the chemisorption alters the thermodynamics of the subsequent inner-sphere ETs from the metal to the adsorbates and constitutes the basis of the electrocatalytic effect.

It is noteworthy that while the attachment of triplet dioxygen to closed-shell organics is prohibited, as discussed above, this restriction does not generally apply to a metal surface, which is rather chemically unsaturated, so that spin states are quasi-degenerate and ill-defined. Further reaction mechanism depends on whether two oxygen atoms in the chemisorbed dioxygen molecule remain bonded. If this is the case, a so-called associative pathway may favor H2O2 formation, that is, 2e reduction. On the other hand, a dissociative pathway is realized when no bond between the two oxygen atoms survives chemisorption. It leads directly to water, that is, a 4e reduction.1

Carbon-based graphene and graphite materials have been demonstrated to show considerable ORR activity. Note that they usually contain high density of defects; furthermore, these can be specially created.3,4 Numerous theoretical studies of ORR on graphene materials were aimed atfinding conditions to allow dioxygen chemisorption. Not surprisingly, the presence of defects creating well-defined radical sites was found essential,45−49though spin restrictions as the reason for this are usually not clearly formulated. The chemisorption of dioxygen prior to ET makes ORR follow the inner-sphere mechanism similar to that operating at active metals.

In parallel, the evidence for a different ORR mechanism, not based on dioxygen chemisorption as a prerequisite, started emerging. Oxygen activation by chemisorption becomes impossible when the electrode surface is inactive toward oxygen intrinsically50 and also due to the double layer structure, as in the case of gold or mercury51,52or intentionally passivated.53 Then, the reduction of molecular oxygen takes place prior to chemisorption, by an outer-sphere ET mechanism. In general, breaking of the bond between the two oxygen atoms of the molecule is not favored in this case, which leads to H2O2 as the primary product (not taking its possible disproportionation into account), that is, a 2e mechanism. We note that this electrochemical mechanism is reminiscent of chemical ORR in biological systems mentioned above. A bridge between these two can be found in electrochemical ORR at the electrode surface functionalized with quinones54or even including riboflavin.55

2.4. OOH+_{Intermediate in the Theoretical Studies of} ORR. We feel that it necessary to devote a special section to discard an unfounded and erroneous ORR mechanism dangerously spreading in a number of theoretical works, where OOH+ _{was considered as the starting active species of} ORR in organic systems such as graphene. To the best of our

knowledge, OOH+ _{intermediate was first mentioned in the} context of ORR modeling in Wang and Balbuena’s work in 2005.56 By performing Car−Parrinello molecular dynamics simulations, the authors noticed that if molecular oxygen was deliberately initially placed quite close to the proton of H3O+(H2O)2 in the vicinity of negatively charged Pt surface, then at some moment of time, interatomic distances can discern proton transfer to dioxygen and thus transient formation of OOH+ _{which was already chemisorbed to} platinum 0.07 ps later. At no point in this work was OOH+ referred to as a viable species that could be found in any discernible concentration. Nevertheless, a few authors uncriti-cally picked up OOH+_{, allegedly formed by protonation of} oxygen molecule in aqueous acidic media, as the starting active species of ORR in their simulations justifying it by Wang and Balbuena’s work. As it happens, the statement started gradually gaining credibility by citing the precursors. A previous work from this group30also fell into this trap.

The temptation to use OOH+ _{as a reactive particle is} apparently due to the low reactivity of triplet dioxygen itself. However, one more observation is worth making explicitly in relation to OOH+: its spin state. With molecular oxygen being in the triplet ground state, its reaction with a proton containing no electrons can only lead to a triplet,3OOH+. However, in the previous work from this group,30 as well as probably in the other work, singlet 1OOH+ is somehow the reactive intermediate. Note that3_OOH+ _{would not help ORR much,} given the same spin restrictions as for3O2.

OOH+_{as an isolated particle is indeed a stable species in any} spin state. However, as it is supposed to be formed in aqueous solution, it is sufficient to add a single explicit water molecule to the simulation tofind out, expectedly, that H2O has a much higher proton affinity than O2. According to our calculation, taking into account implicit water, the proton exchange reaction

H O₂ +OOH+→H O₃ + +O₂

favors the products by 2.65 eV with3_OOH+_{as reactant (and} by 3.24 eV with1OOH+, admitting that intersystem crossing to triplet product happens somehow) thus demonstrating that OOH+ is not a viable species in water.

Of course, water is an incomparably stronger base than molecular oxygen and wins the competition for proton easily. By having no explicit water in simulation, this observation can be overlooked. The mistake is the result of confusion between the absolute stability of OOH+ _{as such, that is, energy gain} upon its formation, and its relative stability in the presence of other relevant molecules and alternative pathways.

3. APPROACH TO MODELING ORR AT PEDOT Conducting polymers are, from a chemical viewpoint, molecular materials. Indeed, molecular dynamics simulations performed recently in this laboratory57,58picture PEDOT as an ensemble of partially aggregated solvated chains. Water uptake in PEDOT films and its swelling are common experimental observations.59 Therefore, an electrochemical reaction on a conducting polymer electrode, be it accompanied or not by chemical bond making and breaking, is more similar to ET in molecular systems than to electrocatalysis at bulk electrodes. We assume the possibility of electron exchange between the PEDOT oligomer and the outer reservoir (current source/ electrodes), as determined by the applied potential. The effect

of the applied potential can be, in the first approximation, limited to changing the ratio between the reduced (PEDOT0) and oxidized (PEDOT+) forms of the polymer, according to the Nernst equation, assuming the equilibrium is attained. Though ORR takes place at cathodic potentials, where PEDOT0_{dominates, at any applied potential the presence of} PEDOT+_{cannot be ignored. Note that at the potentials where} the insulating reduced form dominates, a recent model60 of electrochemical transformation of structurally similar material shows that near insulator-to-conductor transition such Nernstian compositional variation precedes the change of the Galvani potential inside the clusters.

We are interested,first of all, in molecular oxygen activation, that is, the transformations dioxygen can undergo in its first contact with the PEDOT electrode. In this context, we examine the thermodynamics of two distinct types of primary reaction of molecular oxygen with PEDOT oligomer: (i) attachment of triplet dioxygen, obviously conserving total spin, and (ii) outer sphere ET to molecular oxygen. One-electron reduction of3O2would produce active intermediates,2O2−•or 2_HO

2•, which, according to thermodynamics, can be easily reduced further to hydrogen peroxide, and we do not study this second reduction here. However, we explore if these intermediates can react with1PEDOT0or2PEDOT+•forms of the oligomer. If the medium is sufficiently acidic, the superoxide radical anion 2O2−• is rather protonated to hydroperoxyl radical 2_HO

2• (pKa is 4.8

61

). As for the peroxo-adducts to PEDOT, they are expected to be protonated already at higher pH values, as hydrogen peroxide has the pKa of 11.7, and that of an organic peroxide is probably comparable. All this is summarized inScheme 1.

As active superoxide/hydroperoxyl intermediates are formed in the outer-shell ET, new possibilities of secondary reactions with their participation are open, be they stages in ORR or PEDOT degradation pathways. These reactions, taking protonation also into account, are summarized in Scheme 2. Note that for the adducts, we consider three regio-isomers, namely, with O−O bonded to the α or β carbons of a middle ring, or to theα carbon of the terminal ring.

The way to the molecular origin of electrochemical potentials is through thermodynamics, and the link is pedagogically demonstrated in the IUPAC explanatory note on the absolute electrode potential.62 Indeed, equilibrium potentials can be readily obtained from the reaction free energies available from experiments or theoretical calculations. One can rely on the thermodynamic treatment of a half-cell Scheme 1. Reaction Pathway for Primary Addition (PA) of the Oxygen, Corresponding to the First ET, When the Molecular Oxygen3O2Is Directly Attached to Oxidized and Reduced Forms of PEDOTa

a_{Pathway PAXa corresponds to the primary addition in the case of} acidic media, i.e., in the presence of H3O+.

reaction, that is, one involving electrons, as essentially identical to the treatment one would apply to an “ordinary” chemical reaction63and on the status of electrochemical potential as the Fermi energy of electrons.64The electron in a half-cell reaction thus becomes merely a reactant but with particular properties: unlike other chemicals, its free energy is variable as it is determined by the electrode potential. A chemical reaction is at equilibrium when its free energy change is zero. By adjusting the free energy of electrons, any half-cell reaction can be put to equilibrium, and this determines its standard potential.

To make it clear, the absolute electrode potential of the standard hydrogen electrode (SHE) is deduced by replacing its half-cell reaction with the following thermodynamic cycle:62

where (g) and (s) stand for the gas and solution phases, respectively. Note that as the thermodynamic potentials do not depend on path, any path leading from the reactants to the products can be chosen, which allows us to bypass the subtle problems of actual distribution of electrostatic potentials in an electrochemical cell. It is important to stress that the hydrogen ionization step implies that free electrons are at infinity at rest, designated e(inf), where their energy is considered zero. The following interpretation can be attached to the thermodynamic cycle. Clearly, with e(inf)the half-reaction is not at equilibrium as its free energy is positive. To adjust it to zero, the electron should be placed not at infinity, e(inf), but rather at the Fermi energy of−4.45 eV, denoted e(SHE). This is nothing else than the absolute potential of the standard hydrogen electrode, and the same value is obtained from interface potential differences involving mercury electrode.62 This means that in order to construct free energy diagrams at 0 V vs SHE,−4.45 eV per electron in an electrochemical half-reaction should be added to the free energies of the molecular species. We would also like to stress on this occasion, if need be, the primordial importance of taking into account solvation energies, in particular, for ions, bringing a major contribution to

electro-chemical potentials: with gas phase energies, nothing even remotely qualitative can be deduced.

4. COMPUTATIONAL DETAILS

The current DFT calculations were accomplished with the Gaussian 09 package.65By using the thermochemistry block of this package, we obtain the gas phase Gibbs free energies for all the species. We chose the range separated hybrid functional wB97XD accounts for 22% Hartree−Fock (HF) exact exchange at short-range and 100% HF exact exchange at long-range, with Grimme’s D2 dispersion effects.66,67 Dis-persion effects are used to account for van der Waals interactions in a particular calculation with DFT for wide variety of molecular complexes. The van der Waals interactions between atoms and molecules play an important role in many chemical systems as these interactions control the structures of DNA and proteins, the packing of crystals, the formation of aggregates, and orientation of molecules on surfaces or in molecularfilms. The basis set used for studying the reactions involving anions, which is essential for ORR, was 6-31+G(d), that is including diffuse functions. Note that diffuse functions are essential: without these, calculated gas phase electron affinities are severely underestimated, while at the 6-31+G(d) level, the calculated values are in good accord with the experimental ones: e.g., for 3O2+ e → 2O2−• our calculated free energy difference is 0.46 eV and the experimental electron affinity is 0.45 eV.68 The gas phase energetics of reactions involving3_O

2are very accurate; we do not need to refrain from explicit DFT calculations involving these species, as done by Nørskov et al. 15 years ago, who ingeniously used thermodynamic equivalents instead.44As a matter of fact, we do not have such equivalents.

The geometry optimizations were performed without imposing any constraints on initial structure. Octamer of EDOT monomer was chosen as the oligomer model of the polymer, as in a previous theoretical study from this laboratory,69 in which the structure and energetics of the neutral and charged forms were studied in detail at the same level of theory. In particular, based on the results obtained in that study, we will consider a single polaron of the spin S = 1/2 per oligomer, denoted2PEDOT+•, where 2 stands for the spin Scheme 2. Reaction Pathways Corresponding to the First Outer-Shell ET, Followed by the Secondary Addition (SA) of Molecular Oxygen Reduction Products to Oxidized and Reduced Forms of PEDOTa

a_{Pathways ETa and SAXa correspond to the electron transfer and secondary addition in the case of acidic media, i.e., in the presence of H}

multiplicity, M = 2S + 1 = 2. Note that unconstrained geometry optimization places this polaron at the middle of the chain. The reduced form here is obviously neutral spin singlet (S = 0; M = 1) 1_{PEDOT. Frequency calculations were also} carried out to verify stationary points as minima (zero imaginary frequency) and to provide zero-point vibrational energy corrections and thermal corrections producing gas phase free energies at room temperature.

The solvent effects were calculated in water (dielectric constant = 80) using a self-consistent reactionfield based on the SMD model.70Special attention was given to the accuracy of solvation energy provided by the explicit solvent model. The role of environment is indeed crucial.

In particular, biological mechanisms are based on charge transfer and thus involve charged intermediates stabilized by surrounding species, while in the gas phase only highly reactive neutral radicals could be formed. Our own preliminary calculations confirm this. The reduction of molecular oxygen by reduced protonated flavin in gas phase is predicted to be hugely unfavorable (by 1.71 eV), though the calculated gas phase electron affinity of molecular oxygen is excellent (0.46 eV) as mentioned above, and the calculated gas phase ionization potential of lumiflavin is reasonable71 (2.17 eV). Still, in aqueous solution or in a protein environment the reaction does take place. This observation reminds us of the necessity to take solvent effects into account in the calculations aimed at meaningful thermodynamic estimates for reactions involving ions in solution.

Comparison to the available solvation energies of ions reveals that for 2_O

2−•, that is, in the absence of hydrogen bonding, the calculated solvation energy of 3.49 eV is underestimated only by 0.11 eV,72 while for H3O+, when hydrogen bonds are essential, our calculated solvation energy of 4.08 eV is more severely underestimated by 0.41 eV.73We will introduce these empirical corrections specifically mention-ing it where necessary. We estimate incomplete account of medium effects as a major source of errors in this sort of calculation. All the computed reaction energies given in this work are therefore free energies in aqueous medium, which allows us to relate them to equilibrium electrode potentials. 5. COMPUTATIONAL RESULTS

5.1. PA1 (Scheme 1). Primary addition of 3_O

2to neutral 1_PEDOT0 _{is found to be thermodynamically unfavorable by} 1.56 and 1.58 eV forβ- and α- thiophene ring substitutions, respectively, in a middle EDOT moiety and 1.19 eV at the terminalα-positions, denoted as 1, 2, and 3 (seeChart 1). It can be understood as such an addition of a triplet species to a singlet molecule leads to triplet products, which are usually high-lying in energy. Note that the terminal substitution (position 3) is the least energetically demanding, apparently because it does not break the entire conjugation. These are gas

phase free energies, but the solvent effect for these neutral species is negligible, as checked for the terminal position 3.

5.2. PA2 (Scheme 1). Primary addition of3_O

2to oxidized 2_PEDOT+•_{: the calculated free energies showed that such} addition leading to spin-doublet products is again energetically unfavorable, by 1.51, 1.40, and 1.13 eV for positions 1, 2, and 3, respectively, in gas phase. Note that the energetics is almost the same for positive and neutral PEDOT. For the least energetically demanding terminal position 3, the solvent effect reduces the energy increase to 0.83 eV, still insufficient for stability reversal. The radical cationic (polaron) state of PEDOT is thus also not chemically reactive toward molecular oxygen.

5.3. PA1a and PA2a (Scheme 1). Primary addition of3_O 2 to neutral1_PEDOT0 _{with protonation stabilizes the product.} This is what a chemist would expect: H3O+ is a very strong acid, much stronger than hydrogen peroxide and most likely organic peroxides. Therefore, protons would rather be bound to a peroxo-group than to water. Numerically, wefind that the proton binding free energy to a water molecule, for an artificial reaction H₂O(aq) + H+_{(g) → H3O}+_{(aq), is −10.7 eV. For} various peroxo-adducts of PEDOT, we designate here generically PEDOT-OOx_{; we find the proton binding free} energy, for the similar artificial reaction PEDOT-OOx(aq) + H+_{(g) → PEDOT-OOH}x+1_{(aq), quite insensitive to the spin} and charge (x) of the peroxide, about−12.5 eV. Both binding energies are huge, as the reactions involve bare proton and as such are irrelevant for any experiment. However, from these two artificial reactions, the free energy can be predicted of the generic reaction of interest: for PEDOT-OOx_{(aq) + H}

3O+(aq) → PEDOT-OOHx+1_{(aq) + H}

2O(aq), it is about −1.8 eV, highly favorable, as expected. We do not strive to a higher precision here, as we are aware of underestimating the hydration energy of H3O+by ca. 0.4 eV, which will bring the increase due to the protonation of the peroxo-adduct down to −1.4 eV. Turning back to the primary addition energies, we see that such stabilization would reverse the trend for many of them and make them favorable, in particular, at the most reactive terminal position. It appears, therefore, that increasing the medium acidity (lowering pH) thermodynamically facilitates molecular oxygen attachment to PEDOT and can thus open the way to it.

5.4. ET and ETa (Scheme 2). Outer-shell ET with molecular oxygen reduction is meant to take place without any chemical attachment. It is therefore sufficient to compare the free energies of reduction (electron attachment) to molecular oxygen and PEDOT+, taking into account hydration. From these reduction free energies, the standard redox potentials of the half-reactions can be estimated by using the relationship introduced above, E0SHE[V] = −4.45 − ΔG [eV].

For 1e oxygen reduction, 3_O

2+ e →2O2−•, the calculated ΔGcalc= −3.94 eV, which, taking into account the empirical correction of−0.11 eV for superoxide anion hydration, gives ΔGcorr=−4.05 eV. This corresponds to E0SHE=−0.40 VSHE, in a very good agreement with the experimental value of−0.33 VSHE.

For oxygen reduction with protonation,3_O

2+ H3O++ e−→ 2_HO

2•+ H2O, we obtainΔGcalc=−5.22 eV. Lowering of the reaction free energy in the case is due to much higher basicity of 2_O

2−• compared to water: the calculated free energies of bare proton addition to them are−12.00 eV and −10.72 eV, respectively, which precisely explains the difference of −1.28 eV betweenΔGcalcwith and without protonation. Correcting Chart 1. PEDOT Octamer

for the underestimated H₃O+_{solvation energy by +0.41 eV, we} arrive atΔGcorr=−4.81 eV. This corresponds to E0

SHE= +0.36 VSHE, and the agreement with the tabulated standard potential (at pH 0) of−0.07 VSHEis significantly worse: we apparently predict the reaction free energy too negative by ca. 0.3 eV, most likely because of hydration energy errors for hydrogen bonding species other than H₃O+_{. The trend that a proton} coupled reduction of molecular oxygen is thermodynamically facilitated at low pH is reproduced in our calculations, but the oxidative power of dioxygen in acidic medium is overestimated. The calculated free energy of reduction of positive to neutral PEDOT, 2PEDOT+• + e− → 1PEDOT, is −4.44 eV. It corresponds to the standard potential of 0.0 VSHE.

It should be noted that we adopt here a very simple approach to free energies and standard potentials, while even much more advanced modeling of species relevant to ORR71is not free of discrepancies.

Finally, our direct free-energy theoretical estimates for the ET from neutral (reduced) PEDOT to molecular oxygen are

While predicted unfavorable in neutral medium, the outer-sphere reduction of molecular oxygen by reduced PEDOT is apparently made feasible, in standard conditions, by the presence of acid.

5.5. Secondary Additions of Molecular Oxygen Reduction Products to Oxidized and Reduced Forms of PEDOT (Scheme 2). Superoxide and hydroperoxyl, unlike molecular oxygen, are highly reactive radicals, so if they are formed in solution as products of outer sphere ET, it is reasonable to inquire if these can attach to the reduced and oxidized PEDOT, possibly with the help of protons.

The reaction of neutral 1PEDOT with superoxide 2O2−• (SA1) is still unfavorable, as well as with molecular oxygen: its free energy is +0.50 eV. On the other hand, the recombination of two radicals, 2_PEDOT+• _{and superoxide (SA2), is slightly} favorable, by −0.04 eV. The effect of protonation can be represented in two ways: either adding H3O+ to the above reaction (SA1a and SA2a), or via the presence of 2HO2• reactant (SA1a′ and SA2a′), which itself is the product of protonation of 2_O

2−•. As already discussed above, the calculated gain for the first case, due to the protonation of the products of SA1 and SA2, is about −1.8 eV, passing to −1.4 eV taking into account the correction for H3O+ hydration: this is largely sufficient to render both SA1a and SA2a quite favorable. Now, comparing the parent reactions SA1 and SA2 with their primed counterparts SA1a′ and SA2a′, they differ by the protonation of the reactant, 2_O

2−•(aq) + H+_(g)→2_HO

2•(aq)°, with the free energy gain of −12.0 eV, and of the product, PEDOT-OOx(aq) + H+(g) → PEDOT-OOHx+1(aq) with the gain of−12.5 eV. The total gain in the reaction free energy of SA1a′ and SA2a′ vs, respectively, SA1a and SA2a, is thus −0.5 eV calculated and just −0.1 eV corrected. This is hardly sufficient to make1_{PEDOT reactive} toward 2_HO

2• (SA1a′) but reinforces the reactivity of the radical cation2PEDOT+•.

6. DISCUSSION

We are now in a position to discuss how our computational results accord with the relevant experimental observations and what implications can we infer.

The oxidized form of the polymer,2PEDOT+•, is unreactive toward molecular oxygen, in spite of the radical character of both species. Indeed, it is well-known that the doped form of PEDOT is stable in air infilms and in aqueous solution, which facilitates the use and commercialization of the latter.

The neutral1_PEDOT0_{, on the contrary, is predicted to react} with molecular oxygen in two ways: both chemical bonding that leads to a peroxo-adduct and outer-sphere ET without adduct formation are favorable, in acidic media. Both these reactionsfind supporting experimental evidence;28,29however, in experiment acidity is apparently not required.

Our theoretical estimate of the redox potential of PEDOT obtained for single-molecule oligomer, E0_(PEDOT+_/ PEDOT0) = 0.0 VSHE, calls for an assessment. Wide plateaus, rather than well-defined peaks, encountered in the cyclic voltammograms of PEDOT complicate direct comparison to experiment. On the other hand, the onset of the resistance increase upon potentiodynamic (5 mV/s) reduction of PEDOT is observed around −0.45 VSHE.28 This threshold likely corresponds to a significant decrease of the fraction of the conducting oxidized form. As the equimolar ratio between the oxidized and reduced forms corresponds to the standard potential E0_{and changes by an order of magnitude per 59 mV} of overpotential according to the Nernst equation, the resistance increase threshold may be situated roughly 0.1 V more cathodic than E0. Here, we suppose that equilibrium is attained in the potentiodynamic experiments (which is far from guaranteed). Altogether, this estimate places E0(PEDOT+/PEDOT0) no more negative than −0.35 VSHE. Comparing our theoretical value of 0.0 VSHEto this estimate, we rather overestimate the free energy of reduction by a few hundred meV. Such an overestimation is indeed possible, as the hydration energy of the charged reactant can indeed be undervalued. In reality, neutral PEDOT is thus probably a stronger reductant than we can predict; therefore, its possibility to reduce oxygen may not be limited to acidic media.

Furthermore, the feasibility of an electrochemical reaction depends on the ordering of the redox potentials of the reactants taking into account the actual concentrations, according to Nernst equation, and not just the standard potentials E0. For oxygen gas (oxidized form), its partial pressure is normally 1/5 of its standard value of 1 atm. The equilibrium concentration of its chemically active reduced forms,2O2−•or2HO2•, can be safely assumed to be orders of magnitude lower than the standard concentration (activity) of 1 M. One order of magnitude decrease in these concentrations leads to the anodic (positive) shift of the reduction potential by 59 mV. Therefore, the equilibrium potential of thefirst ET to molecular oxygen in realistic conditions would be considerably more anodic (positive) than its standard potential. In other words, its reduction will be easier.

The secondary reactions involve reactive intermediates, namely, 2_O

2−• and 2HO2•, quite prone to form adducts with PEDOT, according to our modeling here and in agreement with experimental observations.28,29 Still, such secondary reactivity of both forms of PEDOT is quite different. The neutral PEDOT is relatively inert, probably attaching only superoxide 2O2−• if protonation is coupled. The oxidized

radical cation form 2_PEDOT+• _{is predicted to be reactive} toward both these radicals derived from molecular oxygen. The experimental observations concerning the disappearance of peroxo-adducts, along with hydrogen peroxide production, upon further cathodic polarization probably finds its explanation in the drastic decrease of the reactive oxidized form.

For the outer-sphere reduction of molecular oxygen, it is, on the contrary, the neutral, reduced form of PEDOT that is obviously responsible. Further reduction to hydrogen peroxide of the 1e intermediates,2_O

2−•and its protonated form2HO2•, in solution is quite likely, given much more positive potentials of the second ET, as discussed above.

In the case of outer-sphere mechanism, the role played by the conducting polymer cannot be called electrocatalysis, strictly speaking, as no chemically bound reactive intermediate is formed at this point, but rather redox catalysis or outer-sphere redox mediation. Among its practical advantages is avoiding electrode material damage or passivation.

From the technical point of view, among the factors limiting the precision of this study is the accuracy of the hydration energies. While we were able to confirm the excellent quality of calculated gas-phase energies, implicit solvent modeling underestimates hydration energies, in particular in the case of charged hydrogen bonding species. Such errors are beyond the desired electrochemical precision of 100 meV. Never-theless, we believe that the main conclusions of this work are sufficiently founded.

■

Corresponding Authors

Viktor Gueskine − Laboratory of Organic Electronics, ITN, Linköping University, Norrköping SE-60174, Sweden;

orcid.org/0000-0002-7926-1283; Email:viktor.gueskine@ liu.se

Igor Zozoulenko − Laboratory of Organic Electronics, ITN and Wallenberg Wood Science Center, ITN, Linköping University, Norrköping SE-60174, Sweden; orcid.org/0000-0002-6078-3006; Email:igor.zozoulenko@liu.se

Authors

Amritpal Singh − Laboratory of Organic Electronics, ITN, Linköping University, Norrköping SE-60174, Sweden Mikhail Vagin − Laboratory of Organic Electronics, ITN,

Linköping University, Norrköping SE-60174, Sweden Xavier Crispin − Laboratory of Organic Electronics, ITN and

Wallenberg Wood Science Center, ITN, Linköping University, Norrköping SE-60174, Sweden

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.jpcc.0c03508

Notes

The authors declare no competingfinancial interest.

■

We thank the Knut and Alice Wallenberg Foundation through the project “H2O2”, the Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO Mat LiU No. 200900971), and the Swedish Research Council (VR 2019-05577 and forskningsmiljö).

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