(609f) Ab Initio Calculations of Chemical Degradation of PFSA Membranes

Proton exchange membrane (PEM) based fuel cells are potential power sources usable for vehicles, industrial and household applications with increased efficiency and lower air pollution over currently utilized petroleum based power sources. The PEM acts as an electrolyte and hence the performance index (cell voltage and current density) of the cell is controlled by the membrane. Three types of perfluorosulfonic acid (PFSA) based membranes with polytetrafluoroethylene backbones are commonly used in the fuel cells. These membranes are: Nafion, 3M and SSC. These membranes differ mainly in side chains. Side chains for the membranes are: Nafion (-O-CF₂-CF(CF₃)-O-CF₂-CF₂-SO₃H), 3M (-O-CF₂-CF₂-CF₂-CF₂-SO₃H) and SSC (-O-CF₂-CF₂-SO₃H).  The cost of the fuel cell is greatly affected by the cost of the membrane. Earlier research shows that the PEM often begins to degrade after a short period of time (≈ 5000 h). Hence durability of the membranes is one of the major concerns with these devices.

PFSA membranes can degrade physically and/or chemically. Physical degradation can occur due to membrane creep, microcrack fracture, pin hole formation at high temperature, morphological changes, etc. Chemical degradation occurs due to exposure of the membrane to radicals (hydroxyl and/or peroxyl radicals). PFSA ionomers consist of two parts: the side chain and the backbone. The side chains consist of the terminal sulfonic acid group and CF₂ segments attached to ether oxygen atoms. The backbone generally consists of fluorocarbon. In the present study, membrane degradation was studied independently for both components of the ionomer. Degradation of side chain was assessed by computing the energy of reaction for breaking the bond (C?C, C?O and C?S) by hydroxyl radicals. Hydroxyl radicals are very reactive and are formed by homolytic cleavage of a peroxide molecule. Backbone degradation begins by attack of carboxylic acid end groups which are highly reactive in the hydrogen peroxide (H₂O₂) medium. Experimental studies on the fuel cells confirm the formation of H₂O₂. At the anode side of the cell, hydrogen molecules get adsorbed on the Pt catalyst and exist in atomic form inside the catalyst. Oxygen molecules permeate through the membrane from the cathode side of the cell and are reduced on the Pt catalyst by atomic hydrogen to form H₂O₂.

The backbone degradation mechanism of PFSA membranes was studied through the transition states of the various reactions. Degradation is initiated by the attack of hydroxyl radical on the carboxylic end group of a PFSA membrane. Backbone degradation mechanism is also known as ?unzipping mechanism'.  The reactions have been given in equations 1-4. Reaction starts with the ?COOH end group and at the end of the one series of reactions, membrane ends with another ?COOH group which further takes part in degradation.

                                                                         (1)

                                                                                                     (2)

                                                                                                       (3)

                                                                                            (4)

All calculations were carried out with the Gaussian 03 suite of programs. Optimizations for all species were performed using B3LYP hybrid density functional theory and the 6-311++G** basis set without symmetry constraints. Transition states for the elementary reactions in the unzipping mechanism were determined using the same method and basis set.  Reaction (1) was split into the following two reactions:

                                                                          (1a)

                                                                                               (1b)

For reactions (1a) and (1b) transition states were located by optimizing the structure at the peak of the potential energy scan (PES) curve. For reactions (3) and (4), transition states were located by the synchronous transit-guided quasi-Newton (QSTN) method.  Intrinsic reaction co-ordinate (IRC) calculations were performed with each transition state to confirm the connecting reactant(s) and product(s). Energies at the stationary points were further corrected with CCSD(T) method using the same basis set.

Degradation of side chain was studied for three types of PFSA membranes: Nafion, 3M and SSC. All membranes start degrading with the cleavage of C?S bond and membranes end with OH group. After this reaction, Nafion membrane undergoes the breakage of CF3-pendent. In the case of 3M and Dow membranes, side chain leaves the backbone with the breakage of C?C bond between side chain and backbone. This mechanism occurs in the next step for Nafion membrane. All above reactions are exothermic. Homolytic cleavage of bonds shows that C?F bond is the strongest bond among all bonds (C?F bond: 100-113 kcal/mol, C?C bond: 73-85 kcal/mol, C?O bond: 67-81 kcal/mol and C?S bond: 59-61 kcal/mol). The backbone of the membrane is degraded by unzipping mechanism. Bond lengths and bond angle for reactant(s), product(s) and transition state are given in the Table 1. Table also contains the heat of the reaction for the corresponding reaction. Heats of reaction show that majority of the reactions are highly exothermic.  This means that once the backbone of the membrane ends with the carboxylic acid group, the degradation reaction is almost spontaneous.

In the Table 1, values of variables (distance between two atoms and bond angle) for reactant(s) represent the corresponding values for the reactants complex (same in the case of TS and product(s)). For reaction (1a), value of O?H distance confirms that hydrogen atom is moving from CF₃CF₂COOH to .OH radical to give the products. The combined energy of products increases with increment of distance between them. Thus, the products' energy is going higher than the energy of transition state (see Table 1). Results for reaction (1b) show that the radical reaction is exothermic. Our calculation shows that there exists no real transition state in this case. Reaction (2) is a radical reaction and the heat of reaction confirms that the reaction is highly exothermic. In the 3^rd reaction moving atoms are H and F of CF₃CF₂OH molecule. Increasing values of O?H and C?F distances confirms that H and F atoms are leaving the molecule and the decreasing distance between H-F confirms the formation of HF molecule. Thus it gives the CF₃COF and HF as products. In the last reaction, increasing distance between H and O atoms in H₂O molecules suggests the departure of H atom from H₂O molecule. The decreasing distance between this H atom and F atom of CF₃COF confirms the formation of HF molecule. Further, decreasing distance between O atom of H₂O molecule and C atom of the CF₃COF confirms the formation of C?O bond. This leads the formation of CF₃COOH and HF as products. Energies of transition states for reactions (3) and (4) show that these reactions are relatively slower than other reactions. Final reaction produces another carboxylic end group in the backbone of the membrane which will lead to the same series of the reactions for further degradation of PFSA membranes.

Table 1 bond length and bond angle of reactants, products and transition state and heat of the reactions

The results in this study clearly show that the weak link of the membrane is the C?S bond and degradation of the membrane begins with the breakage of this bond. Side chain separates from the backbone of the membrane with the cleavage of C?C bond. Backbone containing ?COOH end groups initiates the unzipping mechanism which causes the degradation of backbone. The results validate the unzipping mechanism through location of the transition state structures.