Tetrazole substituted polymers for high temperature polymer electrolyte fuel cells

Abstract

While tetrazole (TZ) has much lower basicity than imidazole and may not be fully protonated in the presence of phosphoric acid (PA), DFT calculations suggest that the basicity of TZ groups can be increased by the introduction of a 2,6-dioxy-phenyl-group in position 5 of TZ. This structure allows hydrogen bonds between TZ protons and ether oxygen atoms, and thereby establishes a resonance stabilised, co-planar structure for tetrazolium ions. Molecular electrostatic potential (MEP) calculations also indicate that tetrazolium ions possess two sites for proton hopping. This makes such materials interesting for use in a high temperature fuel cell (HT PEMFC). Based on these findings, two polymers incorporating the proposed TZ groups were synthesised, formed into membranes, doped with PA and tested for fuel cell relevant properties. At room temperature, TZ-PEEN and commercial meta-PBI showed an equilibrium uptake of 0.5 and 4.7 mol PA per mol heterocycle, respectively, indicating that PBI has higher affinity for PA than TZ-PEEN. The highest achieved PA uptake was ca. 110 wt%, resulting in a proton conductivity of 25 mS cm À1 at 160 C with a low activation energy of about 35 kJ mol À1 . In a first HT PEMFC test at 160 C, a peak power density of 287 mW cm À2 was achieved.

Breaking through the Cracks: On the Mechanism of Phosphoric Acid Migration in High Temperature Polymer Electrolyte Fuel Cells

“…This redistribution process does not depend only on MEA material parameters, such as the membrane doping level, 10,11 but is also a function of operation parameters, especially of the current density. Eberhardt et al 12 showed the invasion of the anode gas diffusion layer (GDL) and the anode flow field after increasing the current density for the first time and were confirmed by similar findings by Becker et al, 13,14 and Henkensmeier et al 15 This process of acid redistribution into the anode GDL and flow field channel leads to a loss of phosphoric acid over time. Loss of electrolyte is a major limitation for the lifetime of a HT-PEFC, especially when factoring in the DOE target for combined heat and power applications of 60'000 hours of operation for 2020.…”

Finely Adjusted Intrinsically Microporous Copolyimide Electrolyte Membranes for Fuel Cells Operating in a Wide Temperature Range

“…In Figure S6, the as-prepared 65 wt % PA-doped co-PI membranes showed higher power density than the conventional sPBI membrane operated at different temperatures [5.5 (30 °C), 6 (80 °C), and 22 mW cm –2 (160 °C)]. Especially, the peak power density of PI-TB-N30C at 160 °C was superior to those of some reported polymers, such as ABPBI (300 mW cm –2 , H 2 /air), OPBI (320 mW cm –2 , H 2 /air), and Tz-poly­(arylene ether) (287 mW cm –2 , H 2 /air), due to the improved proton transmission channel from the high crown ether contents.…”

Three New Difunctional Electrocatalysts Built from Polyoxometalates and Cu–Tpy Units: Experimental and Theoretical Study

“…The molecular electrostatic potential (MEP) can provide the reactivity sites of electrophilic and nucleophilic attacks. 67,68 The electron-rich and electron-defect regions are labeled by red and blue of compounds 1−3 (see Figure 8). Frontier Molecular Orbital Analysis.…”