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This computational investigation explored the mechanism of oxygen reduction reaction (ORR) in PEDOT at the DFT level of theory. Free energy profiles were calculated to find the energetically favourable pathway out of four possible pathways i.e. 1. Mechanism involving OOH+, 2. ORR via chemical absorption of oxygen, 3. Possible mechanism for ORR in acidic medium and 4. Outer-Sphere electron transfer mechanism for ORR.
The free energy profiles along the reaction route provided enough evidence to conclude that ORR follows outer-sphere electron transfer mechanism for ORR. We suggested that OOH+ is an unjustified particle in real reaction conditions and hence the all ORR mechanisms considering OOH+ need to be reconsidered.
The power storage devices such as fuel cells and batteries are needed in development of electronic devices and power generation technologies to realise the global aim towards sustainable power. The cathode electrode of the fuel cell is the main component of fuel cells responsible for the oxygen reduction reaction (ORR), which is the rate limiting step resulting in the combination of protons with oxygen and electrons to produce water.
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[1-3] There are two reaction pathways for ORR to proceed: 1. Four electron- four step process involving reduction of oxygen with protons to produce water. 2. Two-electron two step process resulting in the formation of H2O2 as an intermediate.
The conventional fuel cells containing proton exchange membrane and metallic electrodes require platinum (Pt) as catalyst. [4] In addition to the high cost of platinum, platinum electrode suffers CO poisoning and drift phenomenon resulting in widespread utilization of fuel cell technology difficult.
[5-7] There have been great interests toward the development of new materials which can provide efficiency in comparison to platinum for ORR such as graphite, graphene, carbon nanotubes, fullerenes and related nano-materials. [8-12] Enamours theoretical studies have been carried out within the framework of density functional theory [13-17] and molecular dynamics approaches [18-22] towards mechanism of ORR to understand the role dopants and nature of electrocatalytic activity.
Poly(3,4-ethylenedioxythiophene) [PEDOT] has been emerged as novel and reliable material for recycling power and energy storage devices due to its stability, well-established manufacturing technology and excellent optical and electronic properties. [23-26] Scientific community has been attracted towards the investigation of ORR on PEDOT [27—36] since Winther-Jensen etal. demonstrated that PEDOT electrode catalyse the ORR with efficiency similar to Platinum. [37]
Mechanistic studies for this immense important reaction, ORR in PEDOT, are still limited [36, 38-39, 40]. The detailed investigation of various factors of ORR in PEDOT may help in the development of PEDOT-based fuel cells having better performance. The most crucial step in mechanism of ORR is the activation of oxygen molecule via binding to the PEDOT or through direct electron transfer to oxygen. PEDOT can activate oxygen towards ORR through the four possible routes 1. Mechanism involving OOH+, 2. ORR via chemical Absorption of Oxygen, 3. Possible mechanism for ORR in acidic medium and 4. Outer-Sphere electron transfer mechanism for ORR. (Fig 1).
In the previous study from this group [Sandeep 2017] in the initial, that is, crucial as thermodynamically most demanding, step of ORR molecular oxygen was replaced by another particle, namely OOH+, whose interaction with PEDOT, the electrode material and potential catalyst, was found favorable. By supposing that (i) OOH+ can be formed by attachment of the proton to molecular oxygen and (ii) OOH+ can subsequently react with an organic molecule, [Sandeep 2017] followed a certain trend that can be found in a number of theoretical studies. We find it necessary to criticize both these assumptions in detail here in order stop further spreading of these errors in future.
OOH+ was first mentioned, to the best of our knowledge, in the context of ORR modeling in 2005 [Yixuan Wang and Perla B. Balbuena J. Phys. Chem. B 2005, 109, 14896-14907]. By performing Car-Parrinello molecular dynamics simulations, the authors noticed that if molecular oxygen was initially placed quite close to to the proton of H3O+(H2O)2 in the vicinity of negatively charged Pt surface, then at some moment of time interatomic distances allowed to 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 OOH+ was referred to as a viable species that could be found in in any discernable concentration. Nevertheless, a few authors uncritically picked up OOH+ formation in aqueous acidic solution by proton exchange justifying it by the Wang and Balbuena's work. By doing this, they showed a grave confusion between the absolute stability of OOH+ as an isolated particle, that is energy gain upon its formation in the absence of any competing pathways, and its relative stability in the presence of water. Of course, a molecule of water is incomparably more basic than molecular oxygen and wins the competition for proton easily, as it will be demonstrated in more detail below.
Furthermore, dealing with molecular materials such as organic semiconductors, it is imperative to take care of the spin of the reactants and products, as any reaction pathway conserves the total spin, and total spin changes, being forbidden, can only proceed with a very low probability via intersystem crossing of the pathways with different spin multiplicities (e.g. singlet and triplet) at the points of degenerate energy. Note that spin restrictions are apparently unimportantin the case of inorganic atomic crystals, such as metals, whose spin is not well defined. Consequently, modeling ORR on metals or oxides do not discuss spin, which is not the approach we can afford to adopt here.
Molecular oxygen being in the triplet ground state, its reaction with proton containing no electrons can only lead to a triplet, 3OOH+. Even if this particle were available, its reaction with neutral, that is singlet PEDOT could proceed only via a triplet pathway, which is rarely favorable. Note that the previous work from this group [Sandeep 2017], as well as probably the others, considered OOH+ being spin singlet.
Second possible mechanism for the activation of oxygen molecule is through chemical binding of oxygen with PEDOT (mechanism 2) (Fig 1.2). Many theoretical mechanistic studies of ORR have proposed his route for the oxygen activation in different carbon based materials such as, Carbon nanotubes (CNTs), Graphene etc. [ 15, 41-42] However, in these studied the chemical binding of oxygen to the catalyst is energetically unfavourable as free energy change is positive [15, 41-42]. This finding raises question about the high efficiency of the catalytic materials towards ORR.
Due to these controversial steps in the mechanism involving OOH+ and ORR via chemical absorption of oxygen lead us to revisit the ORR mechanism. We have proposed two new possible mechanism for ORR in PEDOT, mechanism for ORR in acidic medium (mechanism 3) and Outer-Sphere electron transfer mechanism for ORR (mechanism 4). (Figs 1.3 and 1.4) In mechanism 3, PEDOT reacts with H3O+ and oxygen simultaneously to give intermediate X. (Fig 1.3)
In Outer-Sphere electron transfer mechanism for ORR (mechanism 4), electron is transferred from PEDOT to oxygen without any real chemical bond between PEDOT and oxygen (Fig 1.4).
To get detailed insight into the reaction mechanism of the ORR catalysed by PEDOT we studied the energetics of these four routes within framework of DFT calculations. In present study we investigate the detailed mechanism of ORR in PEDOT to find the best possible pathway.
The current calculations were accomplished at wB97XD/6-31G(d) level of DFT as implemented in Gaussian 09 package [43]. Diffuse functions were included for studying reactions involving anions. The geometry optimizations for reaction pathway of oxygen reduction reaction were performed without imposing any constraints on initial structure. Range separated hybrid functional wB97XD accounts for 22% HartreeFock (HF) exact exchange at a short range and 100% HF exact exchange at long range, with Grimme’s D2 dispersion effects [44, 45].
Dispersion effects are used to account for van derWaals interactions in a particular calculation with DFT for wide variety of molecular complexes. The van derWaals 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, orientation of molecules on surface or in molecular films. Frequency calculations were also carried out to verify stationary points as minima (zero imaginary frequency) and to provide zero-point vibrational energy (ZPE) corrections and thermal corrections. The solvent effects were calculated in water (dielectric constant = 80) using a self-consistent reaction field (SCRF) based on SMD model, a continuum solvation model developed by Truhlar and co-workers. [46] SMD model calculates the solvation energy through the bulk electrostatic contribution arising from self-consistent reaction field treatment and short-range interactions between solute and solvent molecules in the first solvent shell. [46]
Mechanism 1: Mechanism involving OOH+
As discussed above in the introduction, a large number of mechanistic studies of ORR in different materials have considered the formation unrealistic particle OOH+ by ignoring solvent effects and just considering H+ instead of H3O+. We investigated the formation of OOH+ in water in two possible spin states singlet and triplet. (Fig 2.1) The formation of OOH+ was found to highly energetically unfavourable by 3.24 eV (singlet) and 2.65 eV (triplet). (Fig 3.1). So it is quite reasonable to discard the mechanisms involving OOH+ and investigate other possibilities.
Mechanism 2: ORR via chemical absorption of O2
The second possible mechanism for the ORR in PEDOT is chemical absorption of O2. We considered the chemical absorption of O2 at different positions in middle and end of PEDOT (1, 2 and 3) as well as different oxidation level of PEDOT (0, +1, +2) as shown in the Fig 2.2. Absorption of O2 in PEDOT is found to be energetically unfavourable by 1.56 eV, 1.58 eV and 1.19 eV at positions 1,2 and 3, respectively (Fig 2.2, 3.2). As PEDOT is the p-doped conducting polymer, so we investigated O2 reaction with PEDOT in p-doped state that is PEDOT(+1). DFT calculated free energies showed that absorption of O2 in PEDOT (+1) is energetically unfavourable by 1.51 eV, 1.40 eV and 1.13 eV at positions 1, 2 and 3, respectively in gas phase (Fig 2.2, 3.2). With these calculations we were able to conclude that O2 absorption is unfavourable in PEDOT gas phase and the least energetically demanding position is the end of PEDOT and PEDOT (+1). Next, we considered the solvent effects of water for the O2 absorption at end of the PEDOT (0, +1 and +2) using SMD model of solvation. Free energy changes in water depicted that absorption of the O2 is unfavourable for PEDOT (0), PEDOT (+1), and PEDOT (+2) by 1.19 eV, 0.83 eV and 0.63 eV, respectively (Fig 2.2, 3.2).
Theoretical study of the above two routes Mechanism involving OOH+ and ORR via chemical absorption of O2 shows that ORR in PEDOT cannot be explained by either of them.
Mechanism 3: Possible mechanism for ORR in acidic medium
We investigated another possible route for the ORR in acidic medium as mechanisms 1 and 2 could not explain the activation of O2 in PEDOT (Fig 2.3). In this mechanism, PEDOT react with H3O+ and O2 to give intermediate H and I, as shown in Fig 2.3. Intermediate H and I are same chemical structures having different spin, H is triplet and I is singlet. Free energy calculations revealed that formation of H is energetically favourable by 0.29 eV and I is energetically favourable by 0.81 eV (Figs 2.3, 3.3). Formation of I leads to spin violation in reaction as spin on the reactant side is triplet while I is singlet. Therefore, energetically favourable formation of spin allowed intermediate H could explain ORR in acidic medium. However, this mechanism is limited to ORR in acidic case only and to explain ORR in neutral and basic medium we cannot track this mechanism. To fully explain the ORR in all reaction conditions (neutral, basic and acidic) there should be more general mechanism.
Mechanism 4: Outer-Sphere electron transfer mechanism for ORR
We investigated the fourth mechanism for the activation of O2 in PEDOT through outer-sphere electron transfer from PEDOT to O2 (Fig 2.4) and this transfer of electron happens without any chemical attachment between PEDOT and O2. In acidic medium the transfer of electron from PEDOT to O2 result in the formation OOH. and the reaction is energetically favourable by 0.23 eV (Fig 3.4). In neutral and basic medium, the transfer of electron from PEDOT to O2 results in the formation of O2- with the slight unfavourable free energy change of 0.20 eV (Fig 3.4). It is quite reasonable to consider this mechanism as 0.20 eV is small number and could we arising from implicit model of solvation that ignore the real interactions of water molecules with reaction species.
Fig 3 Relative free energy profiles of different possible mechanism of oxygen reduction reactions in conducting polymer PEDOT. Fig 3.1 Free energy profile for the formation of OOH+. Fig 3.2 Free energy profile for the chemical absorption of O2 at different position in PEDOT in gas phase. Values in bracket are for the free energy changes in water. Fig 3.3 Free energy profile for the ORR in acidic medium. Fig 3.4 Free energy profile for the outer-sphere electron transfer mechanism.
Theoretical studies presented herein make significant contributions in mechanistic understanding of oxygen reduction reactions in PEDOT. The propensity of the reaction towards outer-sphere electron transfer has been established by investigating the energetics of four possible mechanism 1. Mechanism involving OOH+, 2. ORR via chemical Absorption of Oxygen, 3. Possible mechanism for ORR in acidic medium and 4. Outer-Sphere electron transfer mechanism for ORR by adopting a reliable computational approach. The conclusions drawn in this work provide detailed mechanistic insights into the oxygen reduction reaction and may give assistance to experimentalists in the development of the better, more efficient fuel cells in future.
Insights into Oxygen Reduction Reaction Mechanisms in PEDOT: A DFT Study. (2024, Feb 22). Retrieved from https://studymoose.com/document/insights-into-oxygen-reduction-reaction-mechanisms-in-pedot-a-dft-study
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