Teppei Ogura

Rate coefficients of H-atom abstraction from ethers and isomerization of alkoxyalkylperoxy radicals

Group rate expressions for the hydrogen(H)-atom abstraction reactions from ethers by hydrogen atoms and hydroxyl(OH) radicals and the intramolecular hydrogen-transfer isomerization reactions of alkoxyalkylperoxy radicals, which result from the H-abstraction from ethers followed by the addition of O(2), have been evaluated based on the quantum chemical calculations and experimental data. With the relative method proposed in the present study, it was shown that the rate coefficients of the reactions, for which only poor experimental information is available, can be reliably evaluated by calculating and extracting the difference from the well-established reactions of alkane hydrocarbons. The major features on the H-abstraction reactions from O-adjacent sites of ethers compared to those from alkanes were the suppression of the activation energy due to the decrease of the C-H bond dissociation energy and non-next neighbor substituent effect from the alkyl group on the counter side of -O-. For the hydrogen transfer isomerization reactions, similar suppression of the activation energy as well as the change in the ring strain energy was found as a major feature.

Chemical Degradation Mechanism of Model Compound, CF3 ( CF2 ) 3O ( CF2 ) 2OCF2SO3H , of PFSA Polymer by Attack of Hydroxyl Radical in PEMFCs

To enhance the durability of perfluorosulfonic acid (PFSA) polymer for proton-exchange membrane fuel cells (PEMFCs), we theoretically analyzed the degradation mechanism of PFSA by the attack of a hydroxyl (OH) radical. We used CF 3 (CF 2 ) 3 O(CF 2 ) 2 OCF 2 SO 3 H as a model compound representing the PFSA side chain because the experimental result suggested that the ether group in the PFSA side chain is vulnerable to the OH radical attack. We performed density functional theory calculation to discuss the degradation reaction mechanism of the ether group in the model compound of the side chain and OH radical. Under high humidity condition, we clearly demonstrated the degradation mechanism and reactivity of C-0 bond cleavage in the ether group by the OH radical. This result shows reasonable agreement with the experimental one. However, the OH radical prefers the reaction of the sulfonic acid group to the ether group under the low humidity condition. We found the different reactivity of the OH radical under the low and high humidity conditions. To improve the durability of PFSA, we proposed four directions: (i) enhancement of deprotonation, (ii) protection of ether group by steric hindrance, (iii) enhancement of C-O bond strength, and (iv) substitution of the ether group by other chemical groups. The latter two directions have been theoretically explored more in detail.

Theoretical Study on Chemical Degradation Mechanism of Nafion Side Chain by the Attack of OH Radical in Polymer Electrolyte Fuel Cell

We theoretically analyzed the chemical degradation mechanism of Nafion side chain by OH radicals on the basis of density functional theory calculations. We found that the cleavage of the C–O bond in ether groups is a main pathway of the degradation from the Nafion side chain by OH radicals under well-hydrated condition. The ether group, which is located near the sulfo group, is vulnerable to the OH radical attack rather than the other ether group connecting the side chain with the polymer main chain.

Theoretical Study on Dissoloved Structure of Pt Complex in Polymer Electrolyte Fuel Cell

We theoretically analyzed the formation energy and solvation free energy of four- and six-coordinated Pt(II) and Pt(IV) complexes with three types of ligands (H2O, OH-, and CF3SO3-) as a model of dissolved Pt species to understand the Pt electrocatalyst degradation and dissolution mechanisms. All calculations were performed under the generalized gradient approximation (GGA) with Becke-88 exchange and Lee-Yang-Parr correlation functionals (BLYP). Solvent effects in water were estimated using the conductor-like-screening model (COSMO). The calculated results clarified that Pt(IV) complexes are more energetically favorable than Pt(II) complexes, indicating that the Pt(IV) complexes are more probable as dissolved species than Pt(II) complexes. We also analyzed the geometrical parameters (Pt...O) and atomic charge of O in ligands. These local relaxations about geometry and atomic charge are one of the factors to determine the stability of dissolved Pt species.