No abstract

The formal potential of the Fe͑III͒/Fe͑II͒ couple shifts markedly in the negative direction by complexation with ethylenediamine tetraacetate ͑EDTA͒, oxalate, and citrate. The potentials of the complexes with EDTA and oxalate are less pH-dependent than with citrate. But, the relatively high pH of around 6.0 is favorable electrochemically due to high corresponding currents. Complexation of Fe͑III͒/Fe͑II͒ couple can provide fast electrode kinetics except for the complex with citrate. But, the solubility of the complex with citrate is up to 0.8 M. Charge-discharge measurements were conducted with the iron-complex/Br 2 redox cells. The results show that performance of the cells with 0.1 M Fe͑III͒/Fe͑II͒-oxalate or Fe͑III͒/Fe͑II͒-citrate is relatively poor due to slow kinetics for the Fe͑III͒/Fe͑II͒-citrate and the unstability of the ferric form for the Fe͑III͒/Fe͑II͒-oxalate, whereas performance of the iron-citrate/Br 2 cell is improved considerably by increasing concentration of the Fe͑III͒-citrate complex. Also, energy efficiencies of up to approximately 80 and 70% could be obtained for the cell with 0.1 M Fe͑III͒/Fe͑II͒-EDTA and 0.8 M Fe͑III͒/Fe͑II͒-citrate, respectively. The preliminary study shows that novel Br 2 /iron-complex cells are technically feasible in redox flow batteries but need further investigation.

The sulfonated poly(ether ether ketone) (SPEEK) / polytetrafluoroethylene (PTFE) composite membranes were produced to aid the swelling properties. The PTFE porous films were used as reinforcing material for the SPEEK/PTFE composite membranes. Scanning electron micrographs showed that SPEEK resin was distributed uniformly in the composite membrane and completely plugged the micropores; there is a continuous thin SPEEK film present on the PTFE surface. The dimensional stability and mechanical strength of the composite membranes were apparently improved. The composite membrane had a smaller oxygen permeability than the Nafion® membrane despite it being larger than that of the SPEEK homogenous membrane. The performance and stability of the PEMFC with SPEEK/PTFE composite membranes were also tested. Results showed that the fuel cell had a larger resistance than the SPEEK homogenous membrane or the Nafion® membrane, but these disadvantages are compensated for with the thinner composite membrane, because of its low gas permeability and high mechanical strength. Stability testing showed that the cell performance could be kept steady for more than 150 hours. The structure of the composite membrane didn't undergo any change and a tight bond remained between the composite membrane and the catalyst layers of electrode after more than 300 hours of intermittent operation.

Solution-cast membranes from sulfonated polyimide (SPI) and its blend were prepared from polyethersulfone (PES) and SPI. The water uptake and swelling were tested and compared between the SPI membrane and the four kinds of blend membranes. Through comparison of the stability of the membranes, we concluded that the PES could greatly increase the stability of the whole membrane and restrict the swelling. However, the PES did not decrease the water uptake very much. We also compared the fuel cell performance with different membranes. The performance was decreased when the content of the PES in the blend membrane increased. The loss of the fuel cell performance with the blend membranes did not decrease very much before the content of the PES was exceeded 20%. It was prospected that the blend membrane could increase the stability of the SPI and, more importantly, even replace the commercial Nafion membranes.