Adsorption of Polyaromatic Backbone Impacts the Performance of Anion Exchange Membrane Fuel Cells

Abstract

The performance of anion exchange membrane fuel cells (AEMFCs) employing Pt or PtRu electrocatalyst and ionomers with different polyaromatic backbones is correlated with the density functional theory (DFT)-calculated adsorption energies of the ionomer fragments on the metal surfaces. The performance of the AEMFCs tested in this work significantly changes depending on the backbone structure of polyaromatic ionomer or the type of the catalyst used at the anode. For the same anode catalyst, the performance decreases in the order poly­(fluorene) > poly­(p-biphenyl alkylene) > poly­(terphenyl alkylene)­s, which is in excellent agreement with the decrease in the DFT-calculated interaction energies between the catalyst surface and the corresponding ionomer fragment. Namely, DFT-calculated adsorption energies decrease in the order: p-terphenyl ≥ m-terphenyl > biphenyl > diphenyl ether > benzene ≥ o-terphenyl > 9,9-dimethyl fluorene. The trend in the adsorption energies is explained on the basis of the structural and conformational features of the ionomer fragments. Namely, strong adsorption of the polyaromatic ionomer fragments correlates with the number of benzene rings with a low rotational barrier that can bind parallel to the metal surfaces, leading to strong interaction and hybridization of the aromatic π-orbitals with metal electronic states. The results of this work suggest, therefore, that the interaction between the ionomer and electrocatalyst should be taken into account when designing high-performing ionomers even before considering other factors such as hydroxide conductivity, gas permeability, and water uptake of the ionomeric binder.

Durability challenges of anion exchange membrane fuel cells

“…Although some phenyl groups in the side chain can adsorb on to the ORR catalyst, the side chain phenyl group can be minimized with cationic group substitution. 209 Matanovic et al showed that some phenyl groups, e.g., polyfluorene, have relatively low phenyl adsorption energy, 210 and thus, the AEMFC durability can be improved. 61,211 The second mitigating strategy is to use lessphenyl group adsorbing catalysts.…”

Poly(fluorenyl aryl piperidinium) membranes and ionomers for anion exchange membrane fuel cells

“…Lee et al 35 and Maurya et al 34 reported alkyl ammonium poly(terphenylene) AEMs and alkyl ammonium fluorene ionomers. Alkyl ammonium-type AEPs 34 , 37 exhibited preferable alkaline stability (stable in 1 M NaOH at 80 °C for 720 h) compared to BTMA-type AEPs, while the PPD of these AEMFCs was limited to below 1.6 W cm −2 . Wang et al 17 also reported stable poly(biphenyl piperidinium) (PFBP-0) ionomers and copoly(aryl piperidinium) (c-PAP) AEMs.…”

Polystyrene-Based Hydroxide-Ion-Conducting Ionomer: Binder Characteristics and Performance in Anion-Exchange Membrane Fuel Cells

“…The space between the main chain leading to the aryl-ether-free C–C bond and the functional group must be examined to resolve this phenyl adsorption problem. Therefore, this study employs a different approach from that in previously reported studies on the phenyl adsorption of polyaromatic-based ionomers [ 23 , 24 , 43 ]. We demonstrated that the alkyl side chains directly affected the AEMFC performance while maintaining the backbone of the aryl-ether-free PS ionomer without changing the polymer backbone structure.…”