Ai Lien Ong

Anion-exchange membranes for alkaline polymer electrolyte fuel cells: comparison of pendent benzyltrimethylammonium- and benzylmethylimidazolium-head-groups

Radiation-grafted alkaline anion-exchange membranes (AAEM) containing pendent groups with either benzyltrimethylammonium (BTM) or benzylmethylimidazolium (BMI) functionality were successfully synthesised from the same base membrane and with identical ion-exchange capacities. The conductivity of the new BMI-AAEM is comparable to the BTM-benchmark AAEM. The fuel cell performance obtained with the BMI-AAEM was, however, significantly poorer due to in situ AAEM degradation. FT-Raman spectroscopic studies on the stability of the two head-groups at 60 °C in aqueous potassium hydroxide (1 mol dm−3) indicates that the BMI-group is intrinsically less chemically stable in strongly alkaline conditions compared to the BTM-benchmark head-group. However, the stabilities of both head-groups are improved when treated at 60 °C in lower pH aqueous carbonate and bicarbonate solutions (1 mol dm−3). Contrary to a portion of the prior literature, there appears to be no real advantage in using anion-exchange polymer electrolytes containing pendent imidazolium groups in highly alkaline systems.

Alkaline polymer electrolytes containing pendant dimethylimidazolium groups for alkaline membrane fuel cells

Novel anion exchange membranes (AEMs), based on poly(phenylene oxide) (PPO) chains linked to pendant 1,2-dimethylimidazolium (DIm) functional groups, have been prepared for evaluation in alkaline polymer electrolyte membrane fuel cells (APEFCs). Successful functionalisation of the PPO chains was confirmed using 1H-NMR and FT-IR spectroscopies. The ionic conductivities of the resulting DIm–PPO AEMs at 30 °C are in the ranges of 10–40 mS cm−1 and 18–75 mS cm−1 at 60 °C. The high ionic conductivities are attributed to the highly developed microstructures of the membranes, which feature well-defined and interconnected ionic channels (confirmed by atomic force microscopy, AFM, measurements). Promisingly, the ion-exchange capacities (IECs) of the DIm–PPO AEM are maintained after immersion in an aqueous KOH solution (2 mol dm−3) for 219 h at 25 °C; a previously developed monomethyl imidazolium PPO analogue AEM (Im–PPO) showed a significant decline in IEC on similar treatment. This reduction in undesirable attack by the OH− conducting anions is ascribed to an increase in steric interference and removal of the acidic C2 proton [in the monomethyl Im-groups] by the methyl group in the DIm cationic ring. Moreover, the maximum power densities produced in simple beginning-of-life single cell H2/O2 fuel cell tests increased from 30 mW cm−2 to 56 mW cm−2 when switching from the Im–PPO AEM (fuel cell temperature = 50 °C) to the DIm–PPO-0.54 AEM (fuel cell temperature = 35 °C) respectively (even with the use of lower temperatures).

The alkali stability of radiation-grafted anion-exchange membranes containing pendent 1-benzyl-2,3-dimethylimidazolium head-groups

Radiation-grafted anion-exchange membranes (AEM) containing pendent benzyltrimethylammonium, 1-benzyl-3-methylimidazolium and 1-benzyl-2,3-dimethylimidazolium functional head-groups were synthesised with ion-exchange capacities in the range 1.7–1.9 meq g−1. The ionic conductivities of the AEMs were also comparable (24.5 ± 1.8 mS cm−1 at 50 °C). The alkali stability (in aqueous potassium hydroxide (1 mol dm−3) at 60 °C) of the 1-benzyl-2,3-dimethylimidazolium head-groups was superior to the 1-benzyl-3-methylimidazolium but inferior to the benzyltrimethylammonium benchmark head-groups. Radiation-grafted AEMs containing pendent 1-benzyl-2,3-dimethylimidazolium head-groups are not suitable for application in electrochemical devices containing highly alkaline environments.

Aromatic polyelectrolytes via polyacylation of pre-quaternized monomers for alkaline fuel cells

To overcome alkali-resistant and synthetic hurdles to alkaline anion-exchange membranes (AAEMs) for alkaline fuel cells, the polyacylation of pre-quaternized monomers as a straightforward and versatile approach has been proposed for the first time. Via this approach, novel aromatic anion-exchange polyelectrolytes featuring a long pendent spacer (i.e., –O–(CH2)4–) instead of a conventional benzyl-type spacer (i.e., –CH2–) are successfully synthesized, and exhibit not only high OH− and CO32− conductivity (91 mS cm−1 and 51 mS cm−1 at 60 °C, respectively) but also outstanding alkaline stability (e.g., no degradation of ammonium groups after aging in 6 mol dm−3 NaOH at 60 °C for 40 days). Using this kind of AAEM, a promising peak power density of 120 mW cm−2 is obtained on a preliminary H2/O2 single cell at 50 °C. This powerful synthetic approach together with exceptional membrane properties should pave the way to the practical application of this kind of AAEMs in alkaline fuel cells.

Development of CaMn1−xRuxO3−y (x = 0 and 0.15) oxygen reduction catalysts for use in low temperature electrochemical devices containing alkaline electrolytes: ex situ testing using the rotating ring-disk electrode voltammetry method

Various inorganic solid state catalysts are of interest for use as cathode catalysts in low temperature alkaline fuel cells including alkaline polymer electrolyte fuel cells. This ex situ study compares the oxygen reduction reaction in aqueous KOH (1 mol dm−3) electrolyte on solid state and sol–gel synthesised CaMn1−xRuxO3 (x = 0 and 0.15) catalysts along with fuel-cell-grade Pt-based benchmark catalysts. The inclusion of Ru (e.g. in the CaMn0.85Ru0.15O3 examples) led to enhanced electronic conductivities compared to the Ru free exemplars. Rotating ring disk electrode hydrodynamic voltammetry was successfully used to determine the electron transfer numbers and hydrogen peroxide production yield for each catalyst. The electron transfer numbers of a number of the catalysts promisingly approach n = 4 (the same as for the platinum benchmarks). However, the on-set potentials of the CaMn1−xRuxO3 (x = 0 and 0.15) catalysts were less than that of the Pt-based benchmarks and they also degraded in the alkaline conditions used (with a further decrease in onset potentials on degradation): the results from the study lead to the hypothesis that the degradation is related to the electrochemically generated peroxide.

The reaction between Nafion sulfonyl fluoride precursor membrane and 1,4-dimethylpiperazine does not yield reliable anion-exchange membranes

Recent reports that the reaction between Nafion sulfonyl fluoride precursor and the cyclic diamine 1,4-dimethylpiperazine yields stable anion-exchange membranes appear to be premature. On aqueous work up, membranes with high cation-exchange capacities and zero anion-exchange capacities are produced.

Effect of carbonate anions on Bi-doped Ca2Ru2O7 pyrochlores that are potential cathode catalysts for low temperature carbonate fuel cells

This ex situ electrochemical study investigates how the oxygen reduction reaction (ORR) on Bi-doped Ca2Ru2O7 pyrochlore catalysts is affected by the addition of carbonate to the aqueous KOH (1 mol dm−3) electrolyte. The parent Ca2Ru2O7 catalyst has been previously reported to be selective towards the generation of CO32− on the reaction of O2 with CO2 (in the presence of H2O) at the cathode of low temperature alkaline polymer electrolyte fuel cells containing alkaline anion-exchange membranes (AAEM): a target is to develop low temperature carbonate fuel cells involving CO32− conduction through the AAEM (for potential CO2 utilisation). Rotating ring disk electrode (RRDE) voltammetry was used to probe the ORR behaviours of Ca2Ru2−xBixO7−y catalysts with x = 0.25, 0.5, 0.75, and 1. The results show that as more Bi was doped into the pyrochlore catalysts, the poorer the on-set potentials compared to the parent Ca2Ru2O7 (which itself yielded a poorer on-set potential to a benchmark Pt black catalyst). Higher levels of Bi-doping tended to reduce n values with higher levels of peroxide generated: all of the pyrochlore catalysts tested gave higher peroxide yields compared to the Pt black benchmark. However, the presence of CO32− in the O2-saturated KOH (1 mol dm−3) electrolyte appeared to improve kinetic performance of the Bi-doped pyrochlore catalysts (the effect being greatest with the x = 0.75 catalyst).