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C: Energy Conversion and Storage; Energy and Charge Transport

Exploring Hydrogen Storage in PEDOT: A Computational Study Nitin Shriram Wadnerkar, Magnus Berggren, and Igor V. Zozoulenko J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b10812 • Publication Date (Web): 11 Jan 2019 Downloaded from http://pubs.acs.org on January 20, 2019

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The Journal of Physical Chemistry

Exploring Hydrogen Storage in PEDOT: A Computational Study Nitin S. Wadnerkar, Magnus Berggren, and Igor Zozoulenko* Laboratory of Organic Electronics Department of Science and Technology, Linköping University, 60174 Norrköping, Sweden

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Abstract A reliable hydrogen-based energy technology requires promising materials for safe storage and transport of hydrogen. Here, the storage of hydrogen in the organic polymer poly(3,4-ethylenedioxythiophene) (PEDOT) is explored using density functional theory calculations. It is demonstrated that hydrogen chemisorption on PEDOT is feasible with the maximum gravimetric uptake of ~2.8 wt% in ambient condition, whereas physisorption is possible only at very low temperatures or at high pressure. The Gibbs absorption energies, electronic structure and absorption spectra are calculated for the cases of chemisorption of a single hydrogen atom, a hydrogen pair, and hydrogen-saturated chain for both neutral and oxidized PEDOT. Various experimental routes for PEDOT hydrogenations are discussed.


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I. Introduction One of the first solid-state materials ever considered for the reversible storage of hydrogen were metal hydrides, which are the compounds where the hydrogen is bounded to a metal.1 In these compounds the metals (such as aluminum, magnesium, lithium etc. or their alloys) consume a significant gravimetric portion of the respective material, which makes them rather impractical. Also, disposing or recycling of the metal hydrides represents a major environmental concern. Because of these technological and environmental issues a major interest has recently been focused on the carbon materials, in particular graphene and nanotubes.2-9 The sp2 covalent-bonding arrangement of carbon atoms in the honeycomb geometry allows efficient binding of hydrogen atoms providing a large gravimetric hydrogen storage capacity of ~7.7 wt%. The high surface-volume ratio makes these materials efficient for the hydrogen storage, but hydrogen desorption certainly requires high temperatures.3 Recently, much attention has been devoted to organic polymers as hydrogen storing and -carrying materials because they are inherently flexible, chemically inert, easy to synthesize, light-weight, and easy to dispose or recycle. These polymers include proton conductive polymers,10-12 porous organic polymers,13-14 organic polymers capable of chemical storage,15 and hydrogen perm-selective polymer membranes.16 A related study of regiospecific protonation was reported for a conjugated organic polymer such as cyclopentadithiophene and azulene,17 where the absorbed protons gain electrons from the polymer and turn into adsorbed hydrogens. One of the most studied conducting polymers, poly(3,4-ethylenedioxythiophene) (best known as PEDOT), is widely used in variety of applications owing to its stability, excellent electronic and optical properties and well-developed synthesis and processing techniques.18-19 It is explored and utilized as the electrode in photovoltaics, sensors, supercapacitors, batteries, and fuel cells, as the electro-active material in electrochemical transistors, electrochromic displays, bioelectronics, and also as the areal coating in antistatic applications, and more (For a recent review on PEDOT, see 20 and references therein). In addition, PEDOT has been reported to demonstrate high electrocatalytic activities for the oxygen reduction reaction.21-23 Recently, it was shown that PEDOT can serve as an efficient catalyst in the generation of hydrogen.24 This conclusion was subsequently questioned in Ref. 25 where the observed catalysis was attributed to unintentional iron oxide residues present on a conducting substrate beneath the actual polymer film. However, to our knowledge, no studies have been reported so far exploring PEDOT for hydrogen storage applications. With its stability characteristics, wide usage and welldocumented processing protocols, PEDOT is an option and of potential great interest for several largescale energy conversion and storage applications, hydrogen storage is yet an additional one so far not considered. Currently, a fundamental theoretical understanding of the hydrogen storage mechanisms on PEDOT is not available. At the same time, a detailed knowledge of the thermodynamical aspects, energetics, reaction pathways and corresponding reaction barriers is thus in the strong demand. Without proper knowledge any attempts to explore PEDOT-based hydrogen storage systems is difficult. Hence, in the present paper we utilize quantum-mechanical density functional theory (DFT) and provide a detailed atomistic insight into hydrogen storage on PEDOT chains. In particular, we explore both physisorption and chemisorption of hydrogen and demonstrate that the latter is energetically possible at the ambient conditions, whereas the former can take place only at very low temperatures or high pressure. We hope that the results of our computational study would motivate and guide corresponding experimental works investigating the hydrogen storage in PEDOT.



II. Computational details All the calculations were made using Gaussian 09 suit of programs. 26 The geometrical optimizations were done using the range separated hybrid functional ωB97XD 27 which includes 100% long-range Hatree-Fock (HF) exact exchange, a small fraction (about 22%) of short-range HF exact exchange, a modified B97 exchange density functional for short-range interaction, the B97 correlation density functional, and a model of Grimme's D2 dispersion corrections.28 For the case of even number of electrons in the system the restricted calculations were performed with the total spin S=0, whereas for the case of an odd numbers of electrons, the unrestricted spin calculations were performed for S=1/2, see for detailed discussion Ref. 29. The smaller 6-31G* basis set was used along. Transition states are located using the Berny algorithm.30-31 The density of states and orbital occupancies were evaluated. The excited state energy calculations were calculated using TD-DFT 32-35 for accurate description of band gaps through excited-state absorption spectra. The optical band gap energy Eg is determined using the Tauc relation,36 where for the case of direct optical transitions the quantity (αhν)2 is linearized with respect to hν, with α being the absorption coefficient. The dependence (αhν)2 typically exhibits a linear behaviour as a function of hν in the range hν > Eg, Thus, the intersection of the linear fit of (αhν)2 with the abscissa (hν) gives the value of the optical band gap energy Eg.37 The length of PEDOT chains is not known exactly, but they are estimated to consist of 5-20 monomer units 19, 38-39. In our calculations, we consider PEDOT chains composed of seven EDOT (3,4ethylenedioxythiophene) monomer units, see Fig. 1a. The obtained results were also cross-verified for longer PEDOT chains where no substantial difference was found, see Supplementary Information for details. This is expected because a hydrogen atom (or molecule) is bounded to (or interact with) only neighboring carbon atoms on the PEDOT chain. It is important to stress that the pristine (i.e. as polymerized) PEDOT is oxidized and it can be subsequently reduced by electrochemical means. Therefore, in our calculations we consider both neutral and positively charged PEDOT chains. The electronic and the Gibbs free adsorption energies are respectively defined as, ΔE = {E[X-nH]- E[X]} - n E[H], ΔG = {G[X-nH]- G[X]} - n G[H], where X and X-nH stand respectively for PEDOT chain and a PEDOT chain with n absorbed H-atoms, and E[…] and G[…] are the corresponding energies. The Gibbs free energy is given by G[X]=E[X] + Gcorr[X], where Gcorr[X] = Ezpe[X] + Ethm[X] +kBT – T S[X]. Here, Ezpe[X] is the zero-point energy, Ethm[X] is the energy due to sum of translational, rotational, and vibrational motion of X, kB is the Boltzmann’s constant, and S[X] is the sum of entropy for translational, rotational, and vibrational motion of X.

III. Results and discussion In the following sections we investigate a physisorption of molecular hydrogen on PEDOT chains due to the Van der Waals forces (Section IIIA), and a chemisorption of hydrogen atoms by means of formation of chemical bond with carbon atoms in the PEDOT chain (Section IIIB).

III.A. Physisorption of molecular hydrogen on PEDOT chains To model the hydrogen physisorption, we use the molecular hydrogen and allow it to interact with an isolated neutral PEDOT chain via the weak van der Waals interaction. We have initiated physisorption process by placing a H2 molecule in the vicinity of a PEDOT chain at the distance of around 4 Å with Page 4 of 16 ACS Paragon Plus Environment

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its bond axis parallel to the chain plane and allowed the geometry to fully relax. After optimization, the H2 molecule is found to be physiosorbed by the PEDOT chain at a distance of around 3 Å, directly above the middle of the corresponding aromatic ring with its bond-length axis tilted to the PEDOT polymer plane (see Fig. 1b). The Gibbs adsorption energy of the hydrogen molecule was evaluated to be ΔG = 0.24 eV, which means that the physisorption of H2 is not energetically favorable at the ambient conditions but instead requires an additional energy. (Note that ΔG is not sensitive to the position of the ring where a H2 molecule is absorbed to. For example, for the terminal rings, ΔG = 0.23 eV). For the case of a charged chain (PEDOT+), the Gibbs adsorption energy of the hydrogen molecule, ΔG = 0.28 eV, is slightly higher than for the above discussed case of a neutral PEDOT (see Fig. 1b).

Figure 1. (a) Chemical structure of PEDOT oligomer consisting of 7 monomer units, and a definition of α and βcarbons. (b) Relative Gibbs free energy profile along the reaction coordinate for physisorption of a single H2 molecule on PEDOT (blue color) and PEDOT+ (red color). Insets show the reactants and products. The corresponding distances from the H2 molecule (from H1 and H2 atoms) to the closest carbon (C1, C2, C3, and C4) and sulfur (S1) atoms of the aromatic ring unit on which it is adsorbed are also shown. Temperature T~300K. (c) The averaged Gibbs free adsorption energy for H2 as function of the temperature. Here, backbone- and ethoxy groups are not shown for the sake of clarity. Inset shows PEDOT polymer with seven H2 molecules adsorbed (one on each aromatic ring unit) and their relative distances from respective closest carbon atoms. (d) The averaged Gibbs free adsorption energy for H2 as function of the pressure.

We also performed geometry relaxations with successive addition of H2 molecules on PEDOT chain and found that H2 molecules are adsorbed on aromatic ring unit of PEDOT via van der Waals interactions, forming a layer of H2 on the PEDOT chain. Calculations reveal a H2 molecule on each EDOT monomer viz. seven H2 molecules on seven aromatic ring units of considered PEDOT polymer as shown in Fig. 2b. We have made attempts to add one more H2 molecule to this H2 adsorbed geometry but, this additional H2 molecule is found to reside at a longer distance (≈ 6 Å) from PEDOT chain than other adsorbed H2 molecules. The calculated average Gibbs adsorption energy was of 0.23 eV/H2. This obtained adsorption energy was practically the same for the π-π stacked-PEDOT chains (0.20 eV/H2). Page 5 of 16 ACS Paragon Plus Environment


Thus, obtained positive adsorption energies signify that H2 physisorption on PEDOT is not energetically favorable at ambient conditions. It is noteworthy that H2 physisorption on graphene, in contrast to the case of PEDOT, is energetically favorable with the absorption energy in the range of ΔG ≈ - 0.04-0.05 eV/H2.40-41 Our results show that the physisorption of H2 might require either low temperatures or high pressures, or both, of these conditions. To verify this, we evaluated Gibbs free adsorption energies for molecular hydrogen absorption at different temperatures (and by keeping pressure constant viz. 1 bar) for the structure with seven H2 molecules adsorbed on a single PEDOT polymer chain (see Fig. 2c). The trend in Fig. 2c reveals that H2 adsorption is energetically favorable only below the temperature of around 25 K, which could provide its theoretical gravimetric uptake of 1.9 wt%. However, this temperature range is apparently too low to be useful in practical applications. We also estimated adsorption energies for molecular hydrogen at different pressures (at the constant temperature T~300 K) for the same structure. Figure 1d reveals that H2 physisorption is energetically favorable at very high pressure ~ 6 kbar and above, which is apparently too high for practical applications. Finally, we note that the results reported in this section correspond to the case where H2-molecules are physiosorbed on one side of the chain. We also did calculation for the case where H2-molecules were physiosorbed on different sides of the chain, where we obtained practically the same results for the absorption energies ΔG.

III.B. Chemisorption of hydrogen atoms on PEDOT chains III.B.1. Chemisorption of a single hydrogen atom and a hydrogen pair on a PEDOT chain Chemisorption of atomistic hydrogen on a PEDOT chains is possible because of the sp2-hybridization of the carbon atoms and the conjugated character of the carbon backbone in PEDOT, where the unsaturated p-orbitals can accommodate an external hydrogen atom. Figure 2 shows the calculated change of the Gibbs free absorption energy for a single H atom on a PEDOT chain, which demonstrates that the chemisorption of atomic hydrogen is energetically favorable, ΔG = -1.83 eV and -2.26 eV for neutral and oxidized (PEDOT+) chains, respectively. A transformation from sp2 to sp3 hybridization for the carbon atoms in the PEDOT backbone leading to breaking of the π-bond between C-atoms and producing additional σ-bond between C and H atoms represents the basic mechanism of the hydrogen chemisorption on PEDOT. To verify this, we have measured the bond lengths and found that they are in the range of 1.49 - 1.50 Å for C atoms that are next to the absorbed hydrogen, and ≈ 1.10 Å for C-H bond, whereas bond angles are 116.6○ and in the range of 107.6-110.8○ for C-C-C and C-C-H, respectively. These values are close to the standard ones for the carbon sp3 hybridization (1.54 Å and 109.5 for all angles), and for the C-H σ-bond (1.09 Å). Note that only α- and β-carbon atoms on the PEDOT backbone can change their hybridization to sp3 and thus chemisorb the hydrogen atoms. The calculations show that chemisorption of the atomic hydrogen on α-carbon is energetically more favorable (by 0.22 eV) than that on β-carbon. This holds true for the case of PEDOT+ as well. It is noteworthy that an extra negative charge (~ -0.2e) is transferred from the absorbed hydrogen to the carbon atom where it is absorbed (see Fig. 2b). Also, optimized geometries show that chemical adsorption of the H-atom on α-carbon induces a large torsional angle (non-coplanar) to accommodate the change from sp2 to sp3 hybridization, see Fig. 2b. It is noteworthy that the calculated hydrogen adsorption energy for the case of PEDOT is larger than the one for the case of graphene, where ΔG ≈ 0.75-0.84 eV.42 For a π-π stacked-PEDOT the calculated Gibbs free adsorption energy for a single H atom is close to the one calculated for a single chain (e.g. ΔGπ-π = -1.50 eV for a α-carbon and neutral chains). However, for the π-π stack, the torsional distortions induced by the adsorbed hydrogen is much smaller as


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Figure 2. (a) Gibbs absorption energy along the reaction coordinate for hydrogen chemisorption on PEDOT (blue color) and PEDOT+ (red color). The reactants and products in the inset represent geometries before and after DFToptimization of H-atom adsorbed on alpha-carbon of PEDOT/PEDOT+ chain, respectively. (b) Mulliken charge distribution before and after absorption of the hydrogen atom at α-carbon (H-atom is indicated by an arrow).

compared to a single chain because of the interchain van der Waals interaction that keeps the chains together and tends to make the π-π stack flatter, see Fig. S1a. Note that the α-carbon, to which H-atom is chemically adsorbed, is found to be slightly popped out from the chain plane producing small distortion around it.

Figure 3. (a) Illustration of the “flipped” and “frustrated” positions of hydrogen atoms. (b) Gibbs free absorption energies per H-atom, ΔG/H, for configurations depicted in (c). (c) Geometries of two chemisorbed hydrogen atoms on different carbon sites (shown by arrows) in the neutral PEDOT polymer chain before and after DFToptimizations (red color boxes). Here, two hydrogen atoms are absorbed at the nearest carbon atoms belonging to the same or neighboring monomer units (configurations (1-6) and (7-8), respectively). (Note that only the middle part of the PEDOT chain is showed, and some of EDOT monomers at both ends of the chain and backbone-ethoxy groups are not displayed for the sake of clarity).

Let us now investigate properties of the system in which a hydrogen pair binds to a PEDOT chain in different ways. We have initiated our calculations for the systems in which two hydrogen atoms are placed in the middle of the chain near the neighboring carbon atoms belonging to the same or neighboring monomer units. Chemisorbed hydrogen atoms on the adjacent carbon atoms on the same side are called here as “H-frustrated” whereas inverted to each other around the adjacent carbon atoms are called as “H-flipped” as shown in Fig. 3a.5 There are eight possible configurations to place two atoms either on one side or on both sides of a single PEDOT polymer chain as shown in Fig. 3c. (Note that we also investigated configurations where hydrogen atoms are placed on the carbon atoms separated by larger distances, and we found that these configurations are less or equal energetically favorable than Page 7 of 16 ACS Paragon Plus Environment


closely adsorbed ones. This is expected, because well-separated H atoms affect each other to much lesser extent, and hence the system can be considered as the one with two non-interacting H-atoms). Figure 3b shows the Gibbs free adsorption energies for different configurations as represented by “1 to 7” in Fig. 3c. The most energetically preferential configurations are 3-6, whereas the least preferential ones are 1,2,7. For the case of configuration 8, the PEDOT chain breaks during the hydrogen absorption. This behavior can be related to the energetics of the transformation from sp2 to sp3 hybridization for the carbon atoms during hydrogen absorption. Indeed, for the case of configuration 1 and 2, both H-atoms are absorbed on β-carbons belonging to the same chain. All neighboring atoms of these carbons belong to one monomer units, and therefore the sp2 to sp3 transformation requires significant distortion of the same monomer unit. In contrast, for the cases 3-6, one of the carbon atoms always has a neighbor on a different monomer unit. As a result, the sp2 to sp3 transformation requires a distortion including a bond between two neighboring chains, which is energetically more preferable than the distortion within the same chain. For the case of configurations 7 and 8, the sp2 to sp3 transformation requires a significand bending at the position of the bond connecting two neighboring monomers. For the configuration 7 the bending is the most pronounced ones and, as a result, this configuration is the least thermodynamically preferential. For the case of configuration 8, the bond is broken during the bending such that the chain breaks into two parts. For the former configuration we also examined the case of two π−π stacked PEDOT chains and also found the chain with the absorbed hydrogen breaks despite the van der Waals forces that keep two chains in the stack together, see Fig. S1b. However, for the case of a charged PEDOT, we found that the chain remains intact after the hydrogen absorption. This due to the fact that PEDOT+ chain adopts a quinoid form (due to a polaron formation) in which the bridging bonds between two monomers convert from single to double bonds (C=C). These bonds are apparently much stronger than the single bonds for the case of a neutral chain, and, as a result, the chain does not break. Figure 4 shows the electronic structure and molecular orbital diagrams for PEDOT and PEDOT+ chains before (Fig. 4a and Fig. 4b, respectively) and after chemical H-adsorption (Fig. 4c, Fig. 4e and Fig. 4d, Fig. 4f respectively). (Note that a detailed analysis of the electronic structure, spin states and optical absorption spectra of PEDOT for different oxidation levels has been recently reported in Ref. 29). For the case of PEDOT, PEDOT+-H, and PEDOT-2H the number of electrons is even, and therefore we consider the spinless ground state, S=0. As a result, the electronic structure is spin-degenerate, see Fig. 4a,d,e. For the cases of PEDOT+, PEDOT-H, and PEDOT+-2H, the number of electrons is odd. As a result, the ground state has a spin S=1/2, and its degeneracy is lifted, see Fig. 4b, Fig. 4c, Fig. 4f. The electronic structure of PEDOT shows a HOMO-LUMO gap of ≈ 5.73 eV. (Fig. 4a). PEDOT+ correspond to the case when one electron is removed from the chain. This leads to the formation of an unoccupied polaronic state in the gap (Fig. 4b). For the case of PEDOT-H one of the states (HOMO↑) is somehow shifted into the gap and can be considered as a “hydrogen” state related to chemisorption of a hydrogen atom (Fig. 4c). Electronic structure of PEDOT+-H exhibits both polaronic state and the “hydrogen” (HOMO) state (Fig. 4d). The electronic structure of a PEDOT chains with a pair of absorbed H-atoms is showed in Fig. 4d – Fig. 4f. For the case of PEDOT-2H two occupied “hydrogen” states shifted into the gap can be recognized (states HOMO and HOMO-1 in Fig. 4e), and the electronic structure for the case of PEDOT+-2H exhibits an empty polaronic state, and two occupied “hydrogen” states, see Fig. 4f.


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Figure 4. Electronic structure and representative orbitals for (a) PEDOT, (b) PEDOT+, (c) PEDOT-H, (d) PEDOT+-H, (e) PEDOT-2H, (f) PEDOT+-2H. (A pair of absorbed H-atoms corresponds to configuration 5 from Fig. 3). Solid lines show the occupied electron orbitals, whereas dotted lines show unoccupied ones. Arrows ↑↓ correspond to spin-up and spin-down electrons.

Figure 5 shows the absorption spectra of PEDOT and PEDOT+ chains with a single absorbed H-atom and a pair of H-atoms. Here, the dominant transitions contributing to the absorption spectra corresponding to the largest configuration interaction (CI) coefficients as given by the TD-DFT calculations are indicated by arrows. For the main peaks in neutral PEDOT and its hydrogen adsorbed cases, major contribution is coming mostly from the valence to conduction band transition (Fig. 4a, Fig. 4c and Fig. 4e). Because of the formation of a polaronic state, the main peaks at higher wavelengths for the case of PEDOT+-H and PEDOT+-2H are due to the transition from the states in the valence band (HOMO and HOMO-1) into the corresponding polaronic state (Fig 4d and Fig. 4f). The overall optical spectra progressively shift to the shorter wavelengths as the PEDOT and PEDOT+ chains acquire more hydrogen, which can be used for experimental analysis of the steps in hydrogenation of PEDOT. These Page 9 of 16 ACS Paragon Plus Environment


theoretical predictions for changes of the absorption spectra upon hydrogenation could be helpful to experimentalists to analyze and identify the hydrogen storage in PEDOT.

Figure 5. Absorption spectra for (a) PEDOT, (b) PEDOT+, (c) PEDOT-H, (d) PEDOT+-H, (e) PEDOT-2H, (f) PEDOT+-2H and corresponding orbital transitions contributing to the main peaks. (The electronic structures shown in the insets correspond to Fig. 4). Solid lines in the representative orbitals show the occupied electron levels, whereas dotted lines show unoccupied ones. Arrows ↑↓ correspond to spin-up and spin-down electrons.

III.B.2. Hydrogen saturated PEDOT polymer chain We model the H-saturated PEDOT chain with four different possible H-atoms conformations I-IV as sketched in Fig. 6a. Among these conformations, a “flipped” geometry III is the most energetically stable, see Fig. 6a. For this case, C-C bond lengths are 1.52-1.53 Å, and C-H bond lengths are 1.09-1.10 Å, whereas bond angles are 107.5○ and 108.6-109.8○ for C-C-C and C-C-H, respectively. These values are close to the standard ones for the carbon sp3 hybridization (1.54 and 1.09 Å for C-C and C-H bonds and 109.5○ for all angles). Thus, all the carbon atoms are connected via sp3 hybridization in the tetrahedral arrangement. Because of this arrangement, all sulfur atoms are situated on the same side of the chain as shown in inset to Fig. 6b. For the fully hydrogenated PEDOT, the estimated gravimetric hydrogen uptake is ~2.8 wt%. Page 10 of 16 ACS Paragon Plus Environment

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Figure 6. (a) Gibbs free absorption energies per H-atom, ΔG/H, of four different conformations (I-IV) of the hydrogen saturated PEDOT polymer chain shown in the upper panel (backbone and ethoxy groups are not shown for the sake of clarity). (b) The density of states and the band energy diagram of most energetically stable Hsaturated single PEDOT chain corresponding to the configuration III. The optimized chain geometry is shown in the inset. (c) The absorption spectra of a bare PEDOT chain (black) and fully H-saturated PEDOT chain in the configuration III (red) and corresponding transitions contributing to the main peaks. The inset shows the calculated direct optical band gap values estimated using Tauc method.

Figure 6b shows the electronic structure and the density of states of a fully saturated PEDOT for configuration III. Its HOMO-LUMO gap of Eg ≈ 9.95 eV, is much larger that the corresponding gap for the bare PEDOT, Eg ≈ 5.73 eV (see Sec. III.B.1). For estimation of the optical band gaps, we performed excited state energy TDDFT calculations. In Fig. 6c the absorption spectra of the H-saturated PEDOT chain is compared to the case of a bare PEDOT chain, where the dominant transitions contributing to the absorption peaks corresponding to the largest configuration interaction (CI) coefficients as given by the TD-DFT calculations are indicated by arrows. As expected, the absorption peak is due to transition between states in the valence and the conduction bands. The calculated direct optical band gap values estimated using Tauc method (as described in the Method section) is shown in inset of Fig. 6c. The calculated absorption spectra and the optical band gaps show that the absorption spectrum of fully H-saturated PEDOT is highly blue shifted (from visible to ultraviolet region) compared to that of bare PEDOT as the optical band gap changes from 2.3 eV to 7 eV upon hydrogenation.

III.C. Experimental routes towards PEDOT hydrogenation While in this study we theoretically demonstrate that the hydrogenation of PEDOT is thermodynamically feasible, the experimentally relevant question is how this hydrogenation can be achieved in practice. As mentioned in Introduction, the hydrogenation of graphene has lately become an area of significant research interest, and various experimental techniques have been developed to prepare hydrogenated graphene. Because both graphene and PEDOT have the same sp2 bonding between carbons in their backbones, many experimental methods developed for graphene can be used for PEDOT as well. Below, we briefly describe these techniques and we hope that they might be instrumental in practical realization of PEDOT hydrogenation. (For a detailed account of these and other hydrogenation techniques in graphene see a recent review 43). (a) Hydrogen plasma treatment and hydrogen thermal cracking. The hydrogen plasma treatment is one of the first experimental techniques used for reversible hydrogenation.44 Within this technique a sample is exposed to a cold hydrogen plasma which is subsequently absorbed at the surface. It has been argued that H+, H2+, and H3+ can possess enough energy to penetrate the graphene membrane which effectively becomes transparent to both protons and electrons.45 The thermal cracking method is conceptually similar to the plasma treatment. In this method the hydrogen gas is first converted into atomic hydrogen using hot filaments, and subsequently absorbed by a sample.46 (b) Birch reduction technique. In the Birch reduction the alkali metal (e.g. Li) is first dissolved in a solvent (e.g. liquid ammonia) to create Li+ ions and solvated electrons. Then the solvated electrons are transferred to graphene, which results in an ionic compound consisting of Page 11 of 16 ACS Paragon Plus Environment


reduced graphene covered with positively charged Li+ ions. On the final step the reduced graphene easily accepts protons from an acidic source such as water or alcohol, hence forming a hydrogenated graphene.47 The advantage of this technique as compared to the plasma treatment is that it fast, efficient and leaves material undamaged. (c) Electrochemical hydrogenation. The electrochemical setup used for hydrogenation includes the graphene as a cathode and an acidic electrolyte providing a source of H+ cations. The graphene is reduced by application of a negative voltage. This makes protonation energetically possible, which leads to the formation of hydrogenated graphene.48 Note that electro-chemical methods potentially represent the most efficient and versatile way of the hydrogenation of PEDOT because PEDOT is one of the best mixed electron-ion conductors ideally suited for electrochemical operation in devices involving polymer-electrolyte interfaces.20 However, for efficient experimental electrochemical hydrogenation, much better understanding of the electrochemistry PEDOT is needed. Currently, many of its aspects are not understood, such as the energetics and kinetics of the hydrogen evolution on PEDOT electrode that can lead to de-hydrogenation and formation of molecular hydrogen, the effect of the morphology on the hydrogen evolution including the role of PEDOT crystallites and “flipped” and “frustrated” geometries of absorbed hydrogen atoms, the effect of the oxidation level and the applied voltage. We plan to address these and related issues in our forthcoming studies.

IV. Conclusion Using the density functional theory, we provided a detailed atomistic insight into the hydrogen storage in organic conducting polymer PEDOT. In particular, we explored both physisorption and chemisorption of hydrogen and demonstrated that the latter was energetically possible at the ambient conditions, whereas the former can take place either only at very low temperatures or high pressure. The Gibbs absorption energies, electronic structure and absorption spectra were calculated for the cases of chemisorption of a single hydrogen atom, a hydrogen pair, and hydrogen-saturated PEDOT chains. The energetics of the hydrogen absorption is considered for both neutral and positively charged PEDOT chains. Also, using the time-dependent DFT we calculated the absorption spectra for hydrogenated neutral and positively charged chains and compared them with those of pristine chains (i.e. without absorbed hydrogen). Our theoretical predictions for changes of the absorption spectra upon hydrogenation can be instrumental to experimentalists to analyze and identify the hydrogen storage in PEDOT. Finally, various experimental routes for PEDOT hydrogenations are discussed. We hope that the results of our computational study would motivate and guide further experimental and theoretical works investigating the hydrogen storage in PEDOT.

■ ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXX. The adsorption energies ΔG for the hydrogen physisorption and chemisorption on PEDOT chains of different lengths Optimized geometry of two π- π stacked-PEDOT chains with a single H-atom and a pair of H-atoms adsorbed on the top chain.

■ AUTHOR INFORMATION Corresponding Author * E-mail: [email protected].


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ORCID Igor Zozoulenko: 0000-0002-6078-3006 Magnus Berggren: 0000-0001-5154-0291

Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS The authors thanks Dr. Viktor Gueskine for many stimulating discussions. This work was supported by Swedish Research Council (2017-04474 and 2016-05990), and Peter Wallenberg foundation (PWS-2016-0010). IZ and MB thank the Advanced Functional Material Center at Linköping University for support. The computations were performed on resources provided by the Swedish National Infrastructure for Computing (SNIC) at NSC and HPC2N.

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