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Freezing of water in a polymer electrolyte membrane fuel cell ͑PEMFC͒ may cause severe problems in driving a fuel cell vehicle during the winter time. Characteristics of PEMFC which suffered low temperatures below zero celsius were examined with thermal cycles during which the temperature of the environment chamber was cycled from 80 to Ϫ10°C. With the thermal cycles, the cell performance was degraded due to the phase transformation and volume changes of water. Effects of freezing of water in a PEMFC on the electrode structure and polarization resistance were examined by Brunauer, Emmett, Teller analysis, cyclic voltammetry, and ac impedance spectroscopy.

c Hyundai Motor Company, Fuel Cell Vehicle Team, Korea For the mobile application, performance of polymer electrolyte membrane fuel cells ͑PEMFCs͒ should be maintained with being exposed to subzero temperatures in the winter time. To simulate the situation, a PEMFC was operated at 80°C, stopped, cooled to and kept at Ϫ10°C for 1 h, and heated to 80°C for the next operation. With the thermal cycle, cell performance was measured and found to degrade at a degradation rate of 2.3% based on current density at a cell voltage of 0.6 V. The degradation was attributed to freezing of water that was produced during operation and remained in the PEMFC after the operation. To prevent the performance degradation, water was removed from the PEMFC by supplying dry gases or an antifreeze solution to the PEMFC before the cell temperature fell to below 0°C. By using the gas-purging and the solution-purging method designed in this work, the performance degradation rate was successfully reduced to 0.06 and Ϫ0.47%, respectively.The polymer electrolyte membrane fuel cell ͑PEMFC͒ is a promising alternative power source for electric vehicles in the near future. One of the challenges that the PEMFC faces is to maintain its integrity when outdoor temperatures are below 0°C in the winter. During operation of a PEMFC, water is produced at the cathode side by the electrochemical reaction and, in most cases, is supplied through the reactant gases to keep the membrane hydrated. 1,2 If operation of the PEMFC is stopped and outdoor temperature is below 0°C, the water remaining in the PEMFC may freeze with expanding its volume by about 9%, because the density of water and ice at 0°C is 0.9998 and 0.9168 g/cm 3 , respectively. 3 Then when the PEMFC is restarted, ice melts into water again, reducing its volume. Such a repetitive volume change of water occurring inside the PEMFC may degrade cell performance.Recently, effects of water freezing on the performance of PEMFCs have been studied. 4,5 Kagami et al. 4 started a PEMFC at temperatures below 0°C with feeding dry reactant gases and measured cell voltage at various constant current densities. Except at very low current densities, the cell voltage decreased quickly. The higher the current density was, the steeper the decrease in the cell voltage was due to increase in water and ice formation rate. Based on the results, they suggested that for self-starting of PEMFCs without external heating, the start-up temperature of a PEMFC should be above Ϫ5°C. Cho et al. 5 investigated effects of repetitive freezing and melting of water in PEMFCs on their characteristics. To simulate the situation of the mobile application, they operated a PEMFC at an operating temperature of 80°C, stopped, cooled, and kept it at Ϫ10°C for 1 h, and heated it to 80°C again for the next operation. They reported that with repetition of the thermal cycle, performance of the PEMFC degraded due to deformation of the catalytic layers accompanied by a decrease in electrochemical active surface area and an increase in charge-transfe...

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of platinum-based catalysts at the PEFC cathode to accelerate the otherwise sluggish kinetics of the oxygen reduction reaction (ORR). Thus, the development of inexpensive noble metal-free catalysts for the ORR is one of the key research topics for future fuel cell designs. Among various types of non-noble metal catalysts reported so far, one promising class is metal-nitrogen-carbon (Me-N x -C y ) type materials in which transition metals, particularly Fe and Co, are bound to nitrogen doped carbon (NDC). The early reports on the application of such materials date back to the sixties of the last century when Jasinski demonstrated that N-coordinated transition metals can be active sites for the ORR. [1] Similarly, the ORR-activity of metal-free NDCs was shown based on experimental and theoretical results. [2] The moderate acidic performance of NDCs can be enhanced by introducing non-noble metal ions such as iron or cobalt. [3] The research on such non-noble metal catalysts was greatly promoted by the groundbreaking results of competitive activities compared to standard Pt-based catalysts obtained by Lefèvre et al. in 2009. [4] A porous activated carbon was used as a conductive backbone and mixed with phenanthroline as nitrogen and iron(II) acetate as iron source. After heat treatment, first in inert and second in NH 3 -atmosphere, a very active ORR-catalyst in acidic medium was obtained. Later, Wu et al. reported highly active and stable ORR electrocatalysts Iron-or cobalt-coordinated heteroatom doped carbons are promising alternatives for Pt-based cathode catalysts in polymer-electrolyte fuel cells. Currently, these catalysts are obtained at high temperatures. The reaction conditions complicate the selective and concentrated formation of metal-nitrogen active sites. Herein a mild procedure is introduced, which is conservative toward the carbon support and leads to active-site formation at low temperatures in a wet-chemical metal-coordination step. Active-site imprinted nitrogen doped carbons are synthesized via ionothermal carbonization employing Lewis-acidic Mg 2+ salt. The obtained carbons with large tubular porosity and imprinted N 4 sites lead to very active catalysts with a half-wave potential (E 1/2 ) of up to 0.76 V versus RHE in acidic electrolyte after coordination with iron. The catalyst shows 4e − selectivity and exceptional stability with a half-wave potential shift of only 5 mV after 1000 cycles. The X-ray absorption fine structure as well as the X-ray absorption near edge structure profiles of the most active catalyst closely match that of iron(II)phthalocyanine, proving the formation of active and stable FeN 4 sites at 80 °C. Metal-coordination with other transition metals reveals that Zn-N x sites are inactive, while cobalt gives rise to a strong performance increase even at very low concentrations.

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The structure of proton transport channels is controlled and fixed in sulfonated poly(styrene-b-butadiene-b-styrene) (SBS) triblock copolymer membranes. Microphase-separated SBS membranes were cross-linked by UV irradiation in the presence of a photoinitiator and then sulfonated by acetyl sulfate. Sulfonated cross-linked SBS (scSBS) membranes with fixed nanosized proton transport channels were then prepared. The scSBS membranes show good proton conductivity, comparable to that of Nafion, and a low methanol permeability, more than 1 order of magnitude smaller than that of Nafion. Sulfonated non-cross-linkable poly(styrene-b-(ethylene-r-butylene)-b-styrene) membranes and sulfonated cross-linked poly(styrene-r-butadiene) membranes, which have less developed proton transport channels, were also prepared for comparison. The effects of the presence and size of the proton transport channels on the proton conductivity and methanol permeability were investigated. It is concluded that both the structure and the fixation of the proton transport channels are crucial to the functioning of proton exchange membranes of direct methanol fuel cells.

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