Freeze-start cycles

Scanning electron microscopy (SEM) MEA cross-section showing an example of catalyst fragmentation after 350 freeze-start cycles from -15 °C, humidified gas purge on shut-down. 

Chiem et al. (2008) reported freeze-start from -20 °C to 50% power in 21 seconds, and 50 freeze- start cycles from -30 with an average start-up time to 50% power... 

Storing and titrating antibodies correctly are often easy solutions when no staining is observed. Some antibodies are extremely sensitive to repeat freeze-thaw cycles and others have a limited shelf life, as evidenced by a short expiry dates, ft is important to check the compatibility of the primary and secondary antibodies before starting IHC, as using the wrong secondary IgG is a common but easily corrected problem. As mentioned, antigen retrieval can be a problem when tissue has been overfixed, so special attention should be paid to fixation time and, once optimized, should be held constant for all subsequent runs. 

In another, similar study, Mukundan et al. [260] performed 100 freeze-thaw cycles (from -40 to 80°C) with different types of CFPs and CCs. After 100 cycles, no obvious degradation was observed in the carbon cloth DL in fact, the performance of the fuel cell slightly improved. On the other hand, after 45 cycles, the CFPs showed significant breakage of the carbon fibers at the edges between the flow channels and the landing widths (or ribs). Thus, it was concluded that this breakage could potentially become a serious failure mechanism in PEM fuel cells when the system was started at subzero temperatures. 

The vector for protein expression in the periplasmic space of E. coli (3) (Fig. 1) contains a lac promotor, which can be induced with IPTG (Isopropyl-D-thiogalacto-pyranoside). Translation starts with the OmpA signal sequence of the outer membrane protein A of E. coli, followed by the chemokine cDNA. The signal sequence leads to protein secretion into the periplasm, where it is cleaved off by bacterial enzymes. The periplasmic space has an oxidizing milieu, which enables disulfide bond formation and contains molecular chaperones, which inhibit aggregation and support correct folding of proteins (8). The lysis of the periplasmic space is performed by four freeze/thaw cycles,... 

Bacteria that remained viable after 48 freeze-thaw cycles were used to initiate new cultures and these were subjected to additional freeze-thaw cycles as described. Individual isolates, which had been previously identified, as well as the control cultures, were also subjected to freeze-thaw cycles. Occasionally, single isolates in 10% TSB would supercool rather than freeze at temperatures close to 0°C, and to ensure that all samples froze at the same temperature, a few sterilized Agl crystals were added to the vials at the start of the experiments. For some experiments, cells were harvested by centrifugation (6,000 xg for 10 min) and the cell pellet washed with 10% TSB and kept at 0°C until analysis. Spent media was obtained by centrifuging, as above, and filtering (0.45 pm). 

Similar to the process described in Option 1 above, the antibiotic is first dissolved in a solvent mixture of acetone/water and frozen at a temperature of — 40 C. The freeze crystallization cycle is then started by raising the temperature to between — IO C and — 30°C and holding the system for... 

DMSO is a very good solvent. Around 90 % of the compounds that end up in a library will dissolve in DMSO. In chloroform or acetone, the figure is only 70 %. Although it is easy to understand the choice for DMSO, not until quite recently was there any concern as to what DMSO might actually do to the compounds. It is not an inert solvent. DMSO is not only mildly reactive but absorbs large amounts of water when exposed to normal air. This became a problem when aware of the problems associated with stability in DMSO solution, companies started to store daughter plates at low temperatures (down to -20°C) only to find large increases in the water content from repeated freeze-thaw cycles. 

Resistance thermometers are inserted in one to six of the ampoules/ vials (depending on the specified need) and the location of each is recorded on the Product Record. Once the temperature of the thermometers in the ampoules/vials reaches the shelf temperature, the freeze-drying cycle is started. The information gained from these thermometers is used in comparisons between batches and sometimes as an indicator as to the speed of freeze-drying, in the knowledge that the containers with the probes are not typical of the batch. 

Fast prediction of emulsion stability is important for product development. As shown by this work, measurement of microstructure parameters such as droplet size distribution over storage time gives very precise information on starting and kinetics of instability and the mechanisms behind it. However, it takes time. Easy viscosity measurements (analysis of flow curves) were not sensitive enough to detect flie first changes in the emulsion structure. Freeze-thaw cycles gave results that did not always correspond to shelf-life under normal conditions. 

The number of freeze-thaw or freeze-start-up cycles that the fuel cell mirst survive is of particirlar importance for stack dirrability. The US Department of Energy has set PEM fuel cell stack transportation technical targets of freeze start-up time to 50% of rated power in 30 s from -20 °C, and unassisted start from -40 °C for 2010 and 2015, with 5000 h dirrability. For stationary fuel cell stack systems operating on reformate, a freeze-start-up time of <30 s to rated power at -20 °C, and survivability from -35 °C to +40 °C, with a 40 000 h lifetime was targeted in 2011 (US Department of Energy, 2007). 

Performance at checkpoints between freeze-starts vs cycle number (full load). (Source Haas and Davis, 2009, reproduced by permission of The Electrochemical Society.)... 

Even though there is not an official freeze/thaw cycling protocol proposed by US DoE, the effect of subzero temperatures on fuel cell performance and membrane properties was investigated by various research groups (Cappadonia et al., 1994 Cha etal., 2001 Datta eta/., 2002 Cho etal., 2003,2004 McDonald eta/., 2004, Yan et al., 2006). Cell temperature is cycled from operational ( 80 °C) to cold-start (-5 °C, -10 °C, -15 °C, -40 °C) values and polarization curves ate measmed. 

Oszcipok et al. found similar results with Cho s in their investigation of starting up the MEA from -10°C (Oszcipok et al, 2005). They observed 5.4% loss in current at 450 mV per freeze-thaw cycle, and consecutive losses in ECSA after each start-up from -10°C. However, contradictory results were obtained by Knights et al. who experienced little performance loss for a fuel cell subjected to 55 freeze-thaw cycles (Knights et al, 2004). 

Regardless, structural alterations and water trapped in CCL described in Section 9.2.2 about effects of freezing-thaw cycles are also causes of performance degradations during cold start. However, they are emphasized during cold start as ice is produced in the core of the CCL and near the catalytic sites. Although an active area decrease is systematically observed by CV at the CCL, preponderance between ohmic, diffusion, and activation limitations is not clearly identified. 

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