Sub-freezing Temperatures Research Articles

Cryoelectrolysis-electrolytic processes in a frozen physiological saline medium.

BackgroundCryoelectrolysis is a new minimally invasive tissue ablation surgical technique that combines the ablation techniques of electrolytic ablation with cryosurgery. The goal of this study is to examine the hypothesis that electrolysis can take place in a frozen aqueous saline solution.MethodTo examine the hypothesis we performed a cryoelectrolytic ablation protocol in which electrolysis and cryosurgery are delivered simultaneously in a tissue simulant made of physiological saline gel with a pH dye. We measured current flow, voltage and extents of freezing and pH dye staining.ResultsUsing optical measurements and measurements of currents, we have shown that electrolysis can occur in frozen physiological saline, at high subzero freezing temperatures, above the eutectic temperature of the frozen salt solution. It was observed that electrolysis occurs when the tissue resides at high subzero temperatures during the freezing stage and essentially throughout the entire thawing stage. We also found that during thawing, the frozen lesion temperature raises rapidly to high subfreezing values and remains at those values throughout the thawing stage. Substantial electrolysis occurs during the thawing stage. Another interesting finding is that electro-osmotic flows affect the process of cryoelectrolysis at the anode and cathode, in different ways.DiscussionThe results showing that electrical current flow and electrolysis occur in frozen saline solutions imply a mechanism involving ionic movement in the fluid concentrated saline solution channels between ice crystals, at high subfreezing temperatures. Temperatures higher than the eutectic are required for the brine to be fluid. The particular pattern of temperature and electrical currents during the thawing stage of frozen tissue, can be explained by the large amounts of energy that must be removed at the outer edge of the frozen lesion because of the solid/liquid phase transformation on that interface.ConclusionElectrolysis can occur in a frozen domain at high subfreezing temperature, probably above the eutectic. It appears that the most effective period for delivering electrolytic currents in cryoelectrolysis is during the high subzero temperatures stage while freezing and immediately after cooling has stopped, throughout the thawing stage.

Radiative cooling to deep sub-freezing temperatures through a 24-h day-night cycle.

Radiative cooling technology utilizes the atmospheric transparency window (8–13 μm) to passively dissipate heat from Earth into outer space (3 K). This technology has attracted broad interests from both fundamental sciences and real world applications, ranging from passive building cooling, renewable energy harvesting and passive refrigeration in arid regions. However, the temperature reduction experimentally demonstrated, thus far, has been relatively modest. Here we theoretically show that ultra-large temperature reduction for as much as 60 °C from ambient is achievable by using a selective thermal emitter and by eliminating parasitic thermal load, and experimentally demonstrate a temperature reduction that far exceeds previous works. In a populous area at sea level, we have achieved an average temperature reduction of 37 °C from the ambient air temperature through a 24-h day–night cycle, with a maximal reduction of 42 °C that occurs when the experimental set-up enclosing the emitter is exposed to peak solar irradiance.

Is the Geographic Range of Mangrove Forests in the Conterminous United States Really Expanding?

Changes in the distribution and abundance of mangrove species within and outside of their historic geographic range can have profound consequences in the provision of ecosystem goods and services they provide. Mangroves in the conterminous United States (CONUS) are believed to be expanding poleward (north) due to decreases in the frequency and severity of extreme cold events, while sea level rise is a factor often implicated in the landward expansion of mangroves locally. We used ~35 years of satellite imagery and in situ observations for CONUS and report that: (i) poleward expansion of mangrove forest is inconclusive, and may have stalled for now, and (ii) landward expansion is actively occurring within the historical northernmost limit. We revealed that the northernmost latitudinal limit of mangrove forests along the east and west coasts of Florida, in addition to Louisiana and Texas has not systematically expanded toward the pole. Mangrove area, however, expanded by 4.3% from 1980 to 2015 within the historic northernmost boundary, with the highest percentage of change in Texas and southern Florida. Several confounding factors such as sea level rise, absence or presence of sub-freezing temperatures, land use change, impoundment/dredging, changing hydrology, fire, storm, sedimentation and erosion, and mangrove planting are responsible for the change. Besides, sea level rise, relatively milder winters and the absence of sub-freezing temperatures in recent decades may be enabling the expansion locally. The results highlight the complex set of forcings acting on the northerly extent of mangroves and emphasize the need for long-term monitoring as this system increases in importance as a means to adapt to rising oceans and mitigate the effects of increased atmospheric CO2.

Does the emulsification procedure influence freezing and thawing of aqueous droplets?

Here we investigate the freezing and thawing properties of aqueous solutions in oil emulsions, with a particular focus on investigating the influence of the oil and surfactant and the stirring time of the emulsion. Specifically, we employ optical cryomicroscopy in combination with differential scanning calorimetry to study the phase behavior of emulsified 25 wt. % ammonium sulfate droplets in the temperature range down to 93 K. We conclude that the nucleation temperature does not vary with oil-surfactant combination, that is, homogeneous nucleation is probed. However, incomplete emulsification and non-unimodal size distribution of dispersed droplets very often result in heterogeneous nucleation. This in turn affects the distribution of freeze-concentrated solution and the concentration of the solid ice/ammonium sulfate mixture and, thus, the phase behavior at sub-freezing temperatures. For instance, the formation of letovicite at 183 K critically depends on whether the droplets have frozen heterogeneously or homogeneously. Hence, the emulsification technique can be a very strong technique, but it must be ensured that emulsification is complete, i.e., a unimodal size distribution of droplets near 15 μm has been reached. Furthermore, phase separation within the matrix itself or uptake of water from the air may impede the experiments.

(Invited) Imaging Fuel Cell Components: From Flow Field Channels to Catalyst Layers

Commercially viable polymer electrolyte membrane fuel cell (PEMFC) systems for transportation and stationary power applications require optimal combination of performance and durability at a competitive cost. Optimized water management across a fuel cell is key to improving performance and maintaining high conductivity of the ionomer in PEM and catalyst layers (CLs), while preventing excessive flooding in CLs, gas diffusion layers (GDLs), channels, and manifolds. Durability is also greatly affected by water transport as certain degradation mechanisms are accelerated depending on the humidity levels. As well, catalyst layers that suffered carbon corrosion due to long operation and/or high number of starts/stops are more susceptible to flooding. Imaging in situ and ex situ enables crucial insight into the microstructure of the fuel cell components and materials, associated water transport, and their effect on cell performance, dynamic response, and degradation mechanisms. Several case studies are presented to highlight the use of optical imaging, neutron imaging, and micro X-ray computed tomography (microXCT) to investigate fuel cell components across a range of length scales. Laboratory-scale microXCT instruments are used ex situ to resolve the three-dimensional structure of the GDL and electrodes, before and after various accelerated stress tests. Optical imaging in operating fuel cells offers a cost-effective combination of high temporal resolution (tens of frames per second) and high spatial resolution (several µm). The imaging requires special cell design with visual access to the imaged area (GDL surface and channels [1] or catalyst layer surface [2]). We employed optical visualization in a variety of studies, such as to visualize liquid water dynamics in the cathode and anode flow fields, or at the interface between the CL and the GDL. Besides water transport studies, optical imaging can quantify in-plane gas distribution in the flow fields during startups and shutdowns [3]. Neutron imaging is a powerful tool to visualize and quantify water transport in operating fuel cells. It has a high sensitivity to small amounts of water inside a cell, while having high transmission through the common fuel cell hardware. At a lower spatial resolution of 250 µm, neutron imaging is used to measure in-plane water distribution with high temporal resolution and large field of view of up to 20 cm by 20 cm. We studied the performance with novel NSTF (nano structured thin film) electrodes and GDL materials [4], and also visualized water and ice formation when cell was subjected to repeated starts at sub-freezing temperatures [5]. Since neutron imaging provides water content integrated along the neutron beam, we combined neutron imaging with the optical visualization to study water transport in different flow field geometries [6] as well as in an operating electrolyzer [7]. Such approach with simultaneous imaging provides additional information about the water location, and in certain situations allows distinguishing between channel and GDL water, or between anode and cathode channels and manifolds. High-resolution neutron imaging is able to measure water distribution across the cell thickness at spatial resolution as high as 10 µm. Image processing procedure is developed to measure the water uptake and observe Schroeder’s paradox in situ in PEMs [8,9]. Further, properties of the microporous layer (MPL) of the GDL were manipulated to prevent excessive flooding in cathode catalyst layers. Anode flooding is evidenced by optical visualization, simultaneous imaging, and high-resolution neutron imaging. Increased water transport across the membrane, from cathode to anode, can be detrimental as liquid water on the anode side may cause localized fuel starvation and cause irrecoverable degradation, similar to accelerated carbon corrosion during unassisted startups and shutdowns. However, for certain novel cathodes, which are prone to flooding, e.g. nano-structured thin film (NSTF) electrodes and non-precious group metal (non-PGM) catalysts, water removal through the anode has proven to be a viable option to improve the performance by reducing cathode water content. The authors acknowledge support from US Department of Energy EERE FCTO, Los Alamos National Laboratory LDRD (Laboratory Directed Research and Development) program, Federal Transit Administration, US Department of Commerce, and NIST Center for Neutron Research.

Neutron Imaging Detector with Scintillation Light Magnification and Amplification

Neutron imaging is an ideal method to study water transport phenomena in proton exchange membrane fuel cells (PEMFCs). Neutrons readily penetrate aluminum, carbon and platinum, yet have a high sensitivity to hydrogenous materials, including water. In addition, unlike x-rays, neutrons are a non-destructive probe and do not alter the performance of PEMFCs due to irradiation. As such, there have been numerous studies that revealed the heat and mass transport phenomena in the flow fields and gas diffusion layers [1], freeze operation [2], impacts of carbon corrosion [3], and the impact of porosity in thick non-precious metal catalyst layers [4]. The primary limitation of neutron imaging is the achievable spatial and temporal resolutions. Current state of the art spatial resolution is about 20 µm, with a 20 minute image acquisition time. The spatial resolution is primarily limited by the range of the charged particles that are the result of the neutron capture reaction and are used to detect the neutron. This range is of order 5 µm for many common detector schemes. The temporal resolution is limited by the brightness of neutron sources, which are in general many orders of magnitude less bright than synchrotron x-ray sources; thus to have reasonable time resolution conventional neutron imaging requires the use of beam defining apertures that are of order 1 mm for high resolution imaging of PEMFCs, so that one cannot make use of geometric image magnification. We are pursuing three efforts to improve the achievable image spatial resolution and time resolution. The simplest approach to improving the spatial resolution is to place a narrow slit in front of the test section and scan it across the active area. The challenge of this method is to create a narrow, well-defined, completely opaque slit with opening area ~1 µm. KAERI and Pusan National University have devised a Gadox powder filling method that enables creating such slits, enabling imaging with 1 µm resolution [5]. By amplifying the scintillation light and using a high speed camera, it is possible to record individual neutron scintillation events and using a centroid algorithm determine the position of the neutron with high precision (less than 10 µm). The measured through-plane water content of a PEMFC is shown in figure 1 for the present state of both methods. The final effort is the development of a neutron microscope objective based on reflective neutron optics [6]. With a lens, one no longer requires the use of small beam defining apertures and thus the image temporal resolution can increase by at least a factor of 100. We will report on the progress of the first optics to reach 20 µm resolution and discuss the possibility of achieving 1 µm spatial resolution through neutron image magnification. References T.A Trabold et al, “Use of neutron imaging for proton exchange membrane fuel cell (PEMFC) performance analysis and design”, Handbook of fuel cells v. 5, 2009. R Mukundan et al, “Performance of PEM fuel cells at sub-freezing temperatures”, ECS Transactions 11 (1), 543-552, 2007.JD Fairweather et al, “Effects of cathode corrosion on through-plane water transport in proton exchange membrane fuel cells”, JECS 160 (9), F980-F993, 2013.D Spernjak et al, “Water Management in PEM Fuel Cells with Non-Precious Metal Catalyst Electrodes”, 228th ECS Meeting abstracts, p. 1540, 2015.J. Kim et al, “Fabrication and characterization of the source grating for visibility improvement of neutron phase imaging with gratings”, RSI 84(6):063705, 2013.D. Liu et al “Demonstration of achromatic cold-neutron microscope utilizing axisymmetric focusing mirrors”, Applied Physics Letters 102 (18), 183508, 2013. Figure 1 Comparison of the image quality of the current state of the art (a), the slit imaging method with 4 µm resolution (b), and the reconstructed image from centroiding (c). Figure 1

Numerical study of gas purge in polymer electrolyte membrane fuel cell

Gas purge for eliminating water from gas diffusion layer (GDL) and membrane is of great importance for polymer electrolyte membrane fuel cell (PEMFC) start-up at subfreezing temperature. A gas purge model was developed to investigate the water transport phenomenon in GDL and membrane. In simulations, hydrogen and air were used as purge streams in anode and cathode, respectively. Effects of purge conditions on gas purge were numerically studied. In addition to relative humidity, flow rate and temperature, the flow configuration of purge gases was studied for the first time. In order to improve purge performance, several purge protocols were proposed and discussed in detail with respect to purge effectiveness and energy saving.

Supercooling Points of Murgantia histrionica (Hemiptera: Pentatomidae) and Field Mortality in the Mid-Atlantic United States Following Lethal Low Temperatures.

The harlequin bug, Murgantia histrionica (Hahn), is a serious pest of brassicaceous vegetables in southern North America. While this insect is limited in its northern range of North America, presumably by severe cold winter temperatures, specific information on its cold hardiness remains unknown. We determined the supercooling points (SCPs) for Maryland and Virginia adult populations and found no significant difference among these populations. SCPs were similar for adults ([Formula: see text] = -10.35 °C; [Formula: see text] 2.54) and early and late instar ([Formula: see text] = -11.00 °C; [Formula: see text] 4.92) and between adult males and females. However, SCPs for first instars ([Formula: see text] = -21.56 °C; [Formula: see text]1.47) and eggs ([Formula: see text] = -23.24 °C; [Formula: see text]1.00) were significantly lower. We also evaluated field survival of overwintering harlequin bug adults during extreme cold episodes of January 2014 and January 2015, which produced widespread air temperatures lower than -15 °C and subfreezing soil temperatures in the Mid-Atlantic Region. After the 48-h episode in 2014, bug mortality in exposed field sites averaged 88%, compared to <5% mortality of bugs sheltered in an unheated greenhouse (recorded minimum temperature 4.4 °C). After the 2015 episode, ∼80% of adults that were established in the field the previous November and then sheltered in an unheated garage during the episode, died, in contrast to 96% mortality in exposed field sites. Our results provide new information on M. histrionica overwintering biology, and thermal limitations to its distribution, which leads to improved predictive capabilities to forecast pest severity.

The nematode Caenorhabditis elegans survives subfreezing temperatures in an isochoric system

This study is the first experimental evidence showing that a living multicellular organism, the nematode Caenorhabditis elegans, can survive subfreezing temperatures in an isochoric (constant volume) thermodynamic system, while immersed in a simple isotonic solution, without the addition of cryoprotectants. Some of the test conditions were more extreme than those found at the ice/water interface of the Antarctic subglacial Vostok lake. On earth, life takes place in an isobaric (constant pressure) environment. In isobaric systems, subfreezing temperature survival of organisms in nature and subfreezing temperature preservation of living material for biotechnology and medicine, is made possible by use of cryoprotective chemicals additives. Our theoretical thermodynamic studies suggested that in an isochoric system, living biological material could survive subfreezing temperatures, without any cryoprotective chemicals. By confirming the theoretical predictions, this paper suggests a new technology for subfreezing preservation of cells, organs and organisms of possible value for biotechnology and medicine as well as new possible mechanisms of living organism survival in nature.

Last Glacial Maximum and deglacial abyssal seawater oxygen isotopic ratios

Abstract. An earlier analysis of pore-water salinity (chlorinity) in two deep-sea cores, using terminal constraint methods of control theory, concluded that although a salinity amplification in the abyss was possible during the LGM, it was not required by the data. Here the same methodology is applied to δ18Ow in the upper 100 m of four deep-sea cores. An ice volume amplification to the isotopic ratio is, again, consistent with the data but not required by it. In particular, results are very sensitive, with conventional diffusion values, to the assumed initial conditions at −100 ky and a long list of noise (uncertainty) assumptions. If the calcite values of δ18O are fully reliable, then published enriched values of the ratio in seawater are necessary to preclude sub-freezing temperatures, but the seawater δ18O in pore fluids does not independently require the conclusion.

Performance analysis of liquid flooded compression with regeneration for cold climate heat pumps

In regions with sub-freezing winter temperatures, the performance of conventional air-source vapor compression heat pumps drop drastically during extended periods of low outdoor ambient temperatures. Modifying the basic cycle configuration with liquid flooded compression and regeneration reduces the thermodynamic inefficiencies associated with the heat pump equipment and increases energy efficiency with less degradation in heating capacity especially at low ambient temperatures. Flooding non-volatile liquid along with refrigerant vapor stream into the compressor leads to a near isothermal compression process and thereby reduces desuperheating losses in the condenser. Regeneration improves the system performance by reducing throttling losses in the expansion device. The performance of a heat pump operating with flooded compression and regeneration has been analyzed for three sub-freezing outdoor ambient climatic regions: Boston, Indianapolis and Minneapolis. The results indicate that at −10 °C ambient temperature, COP (heating) of the flooded compression with regeneration cycle was approximately 13% higher than the standard vapor compression cycle. The Heating Seasonal Performance Factor (HSPF) of a flooded compression with regeneration cycle for Boston, Indianapolis and Minneapolis was approximately estimated to be 9%, 10% and 13% higher than the standard vapor compression cycle respectively.

The effect of undissolved air on isochoric freezing

This study evaluates the effect of undissolved air on isochoric freezing of aqueous solutions. Isochoric freezing is concerned with freezing in a constant volume thermodynamic system. A possible advantage of the process is that it substantially reduces the percentage of ice in the system at every subzero temperature, relative to atmospheric freezing. At the pressures generated by isochoric freezing, or high pressure isobaric freezing, air cannot be considered an incompressible substance and the presence of undissolved air substantially increases the amount of ice that forms at any subfreezing temperature. This effect is measurable at air volumes as low as 1%. Therefore eliminating the undissolved air, or any separate gaseous phase, from the system is essential for retaining the properties of isochoric freezing.

Durability of Polymer Electrolyte Membrane Fuel Cells Operated at Subfreezing Temperatures

The structure, composition, and interfaces of membrane electrode assemblies (MEA) and gas-diffusion layers (GDLs) have a significant effect on the performance of single-proton-exchange-membrane (PEM) fuel cells operated isothermally at subfreezing temperatures. During isothermal constant-current operation at subfreezing temperatures, water forming at the cathode initially hydrates the membrane, then forms ice in the catalyst layer and/or GDL. This ice formation results in a gradual decay in voltage. High-frequency resistance initially decreases due to an increase in membrane water content and then increases over time as the contact resistance increases. The water/ice holding capacity of a fuel cell decreases with decreasing subfreezing temperature (−10°C vs. −20°C vs. −30°C) and increasing current density (0.02 A cm−2 vs. 0.04 A cm−2). Ice formation monitored using in-situ high-resolution neutron radiography indicated that the ice was concentrated near the cathode catalyst layer at low operating temperatures (≈−20°C) and high current densities (0.04 A cm−2). Significant ice formation was also observed in the GDLs at higher subfreezing temperatures (≈−10°C) and lower current densities (0.02 A cm−2). These results are in good agreement with the long-term durability observations that show more severe degradation at lower temperatures (−20°C and −30°C).

Comparison of cloud properties between cloudsat retrievals and airplane measurements in mixed-phase cloud layers of weak convective and stratus clouds

Cloud microphysical properties including liquid and ice particle number concentration (NC), liquid water content (LWC), ice water content (IWC) and effective radius (RE) were retrieved from CloudSat data for a weakly convective and a widespread stratus cloud. Within the mixed-phase cloud layers, liquid-phase fractions needed to be assumed in the data retrieval process, and one existing linear (p 1) and two exponential (p 2 and p 3) functions, which estimate the liquid-phase fraction as a function of subfreezing temperature (from −20°C to 0°C), were tested. The retrieved NC, LWC, IWC and RE using p 1 were on average larger than airplane measurements in the same cloud layer. Function p 2 performed better than p 1 or p 3 in retrieving the NCs of cloud droplets in the convective cloud, while function p 1 performed better in the stratus cloud. Function p 3 performed better in LWC estimation in both convective and stratus clouds. The REs of cloud droplets calculated using the retrieved cloud droplet NC and LWC were closer to the values of in situ observations than those retrieved directly using the p 1 function. The retrieved NCs of ice particles in both convective and stratus clouds, on the assumption of liquid-phase fraction during the retrieval of liquid droplet NCs, were closer to those of airplane observations than on the assumption of function p 1.

Digging through the past: the evolutionary history of burrowing and underground feeding in rhinophrynid anurans

The Rhinophrynidae, endemic to North America, are a highly specialised group of fossorial anurans that, despite a long geologic history, has low diversity. Today, rhinophrynids are represented by one taxon, Rhinophrynus dorsalis, which occurs in extreme southern Texas, USA, and southwards through Mexico and into Central America, in dry tropical to subtropical forests along coastal lowlands. The skeleton of R. dorsalis is highly specialised for burrowing and subterranean feeding, with individuals spending most of their time underground feeding on ants and termites and emerging only after periods of heavy rain to breed. In the fossil record, rhinophrynids with skeletons specialised for burrowing first appear in the late Paleocene (Tiffanian) of Wyoming, USA, and are subsequently known from the Western Interior of North America until the late Eocene (Chadronian), with the exception of one late Pleistocene occurrence in Mexico. From the late Paleocene (Tiffanian) until the present, rhinophrynids became more specialised for burrowing. The tibiale and fibulare became relatively shorter and stouter, the distal condyle of the femur became expanded and divided into two condyles, and the distal prehallux bone and distal phalanx of the first metatarsal were modified as spades by at least the middle Eocene (Uintan). Adaptations for feeding underground might have occurred as early as the middle Eocene (Bridgerian), as suggested by the elongate neural arch of Eorhinophrynus septentrionalis. The middle Eocene (Uintan) Chelomophrynus bayi shares several specialisations with Rhinophrynus dorsalis for feeding underground, so it is presumed that it too was a subterranean feeder. These specialisations include a somewhat protracted snout, lack of teeth on the jaws and vomers, forward shift of the jaw suspensorium and most likely the shoulder girdle, and possibly the structure of the hyoid apparatus. As the climate became more seasonal and subfreezing temperatures developed after the Eocene–Oligocene transition, rhinophrynids apparently never developed the ability to hibernate to avoid freezing and instead shifted their range south.

Coarse grained simulation reveals antifreeze properties of hyperactive antifreeze protein from Antarctic bacterium Colwellia sp.

The novel hyperactive antifreeze protein (AFP) of Antarctic sea ice bacterium Colwellia sp. provides a target for studying the protection of psychrophilic microgoranisms against freezing environment. Interestingly, the Colwellia sp. hyperactive antifreeze protein (ColAFP) was crystallized without the structural dynamic characteristics. Here, the result indicated, through coarse grained simulation of ColAFP under various subfreezing temperature, that ColAFP remains active at temperature of equal and greater than 275K (∼2°C). Extensive simulation analyses also revealed the adaptive mechanism of ColAFP in subfreezing environment. Our result provides a structural dynamic understanding of the ColAFP.

Dynamic Coupling of Iron, Manganese, and Phosphorus Behavior in Water and Sediment of Shallow Ice-Covered Eutrophic Lakes.

Decreasing duration and occurrence of northern hemisphere ice cover due to recent climate warming is well-documented; however, biogeochemical dynamics underneath the ice are poorly understood. We couple time-series analyses of water column and sediment water interface (SWI) geochemistry with hydrodynamic data to develop a holistic model of iron (Fe), manganese (Mn), and phosphorus (P) behavior underneath the ice of a shallow eutrophic freshwater bay. During periods of persistent subfreezing temperatures, a highly reactive pool of dissolved and colloidal Fe, Mn, and P develops over time in surface sediments and bottom waters due to reductive dissolution of Fe/Mn(oxy)hydroxides below the SWI. Redox dynamics are driven by benthic O2 consumption, limited air-water exchange of oxygen due to ice cover, and minimal circulation. During thaw events, the concentration, distribution and size partitioning of all species changes, with the highest concentrations of P and "truly dissolved" Fe near the water column surface, and a relatively well-mixed "truly dissolved" Mn and "colloidal" Fe profile due to the influx of geochemically distinct river water and increased circulation. The partitioning and flux of trace metals and phosphorus beneath the ice is dynamic, and heavily influenced by climate-dependent physical processes that vary in both time and space.

Start-up Effects in Alkaline Fuel Cell Stacks

Any fuel cell operation relies on successful start. In particular, successful and rapid startup of proton exchange membrane fuel cell (PEMFC) from cold start is of great importance for their commercialization. Unlike PEMFCs, liquid alkaline fuel cells (AFCs) are less prone to freezing problems due to the use of aqueous potassium hydroxide solution as electrolyte. The aqueous potassium hydroxide solution has low freezing temperature which prevents the blocking of pores at sub-freezing temperatures to a certain extent. However, the successful start-up in liquid alkaline fuel cells is subjected to other issues which are not common in solid membrane type fuel cells. In liquid electrolyte AFCs, the only barrier between fuel and oxidant is the liquid electrolyte. If during startup the electrolyte is not fully filled within the electrolyte chamber, the reactant gases can enter into the said chamber and creates gas pockets. This phenomenon is highly non-desirable as combustible gas mixture can be trapped and the useful voltage from the cell declines. Few experiments were designed to understand this startup effect and to prevent the failure. The results show that filling-up all the electrolyte chambers prior to reactant gases entry prevents this phenomenon. Furthermore, a relatively high pressure electrolyte chamber filling at startup evacuates the static air from the chamber.

Genes Upregulated in Winter Wheat (Triticum aestivum L.) during Mild Freezing and Subsequent Thawing Suggest Sequential Activation of Multiple Response Mechanisms.

Exposing fully cold-acclimated wheat plants to a mild freeze-thaw cycle of −3°C for 24h followed by +3°C for 24 or 48h results in dramatically improved tolerance of subsequent exposure to sub-freezing temperatures. Gene enrichment analysis of crown tissue from plants collected before or after the −3°C freeze or after thawing at +3°C for 24 or 48h revealed that many biological processes and molecular functions were activated during the freeze-thaw cycle in an increasing cascade of responses such that over 150 processes or functions were significantly enhanced by the end of the 48 h, post-freeze thaw. Nearly 2,000 individual genes were upregulated more than 2-fold over the 72 h course of freezing and thawing, but more than 70% of these genes were upregulated during only one of the time periods examined, suggesting a series of genes and gene functions were involved in activation of the processes that led to enhanced freezing tolerance. This series of functions appeared to include extensive cell signaling, activation of stress response mechanisms and the phenylpropanoid biosynthetic pathway, extensive modification of secondary metabolites, and physical restructuring of cell membranes. By identifying plant lines that are especially able to activate these multiple mechanisms it may be possible to develop lines with enhanced winterhardiness.

Catalytic hydrogen–oxygen reaction in anode and cathode for cold start of proton exchange membrane fuel cell

Fuel cell vehicles (FCVs) have shown the potential of commercialization in recent years. The concerns on the startup ability of proton exchange membrane (PEM) fuel cell stack from subfreezing temperature have risen. The hydrogen–oxygen catalytic reactions assisted cold start method is developed and analyzed in this study. It utilizes a small amount of hydrogen/air mixture to react at low temperature in the catalyst layers (CLs) through platinum catalyst. The interactions between this assisted method and various startup modes are the major issue to be discussed. Anode catalytic reaction with air mole fraction higher than 16% is effective to assist a 30-cell stack starting from −25 °C within 13 s in maximum power mode. However, cathode catalytic reaction cannot sustain a successful startup. The anode humidification effect plays an important role to reduce the stack resistance, and to increase the inherent heat generation rate. In maximum power mode and high current density constant power mode, anode catalytic reaction assisted cold start can be achieved within 10–20 s from −40 °C. Anode air mole fraction must be higher than 18% to ensure the successful cold start in these two modes. For constant power mode, the operating power must be lower than 12 W per cell. In constant current mode, when the current density is low, there would be less demand for anode catalytic reaction to achieve successful startup from −40 °C, indicating that lower current density operations have better survivability in low temperature. Nevertheless, much longer start duration is required for lower operating current. Generally, high current density operating mode with high air mole fraction is a more practical and energy efficient cold start strategy, as the startup time can be reduced significantly. Cold start from about −20 °C without ice accumulation is feasible using this method, which may have reduced concern about degradation. Increasing the volume of CL (porosity and thickness) also helps reduce the ice formation.