How will you identified an active layer?
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Answer:
Active Layer
The active layer, in general terms, represents the upper layer of soil or rock that is subject to annual freezing and thawing in areas underlain by permafrost (Fig. 5). Pragmatically, however, the active layer is thermally defined, such that its base occurs at the depth of maximum seasonal penetration of the 0°C isotherm into the ground (Burn, 2004). Active-layer thickness varies substantially. The thinnest active layers may be as little as a few centimeters in the coldest regions of continuous permafrost at high elevation near the polar plateau in Antarctica. The thickest are 10 m or more in relatively warm regions of discontinuous permafrost, for example in the mountains of northern Norway, where the mean annual ground temperature at 10 m depth is close to 0°C and the bedrock is relatively dry.
Fig. 5
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Fig. 5. Modern active layer truncates the top of the ice wedge at ~ 0.5 m depth, Summer Island, Tuktoyaktuk Coastlands, NWT, Canada. The base of a paleo-active layer is marked by a thaw unconformity at ~ 2 m depth, which truncates massive icy sediments and a composite sand-ice wedge. The deep paleo-active layer formed ~ 10,000 years ago, when the climate was warmer than present. © 1991, photograph, J.B. Murton.
Fuel Cells: Materials
N.P. Brandon, in Encyclopedia of Materials: Science and Technology, 2004
3.2 Electrodes
A catalytically active layer sits adjacent to the electrolyte membrane within both the anode and cathode of a PEMFC. The layer is supported on a PTFE-treated carbon paper which acts as a current collector and gas-diffusion layer, thus enabling the transport of gas, water, and electrons. For operation with pure hydrogen and air, platinum is the most active electrocatalyst. To reduce cost, nano-particles of platinum on a carbon support have been developed, and on-going development of stable nano-structures continues to be an important aspect of fuel cell development.
However, a key aspect of electrocatalyst development relates to the development of anode catalysts for operation on reformed hydrogen from methanol, natural gas, or gasoline. This is because, while gas-diffusion electrodes with a loading of 0.1–0.2 mg cm−2 of dispersed platinum on carbon show very small polarization losses when operating on pure H2, these losses are raised to unacceptable values when even small amounts of CO are present (10–100 ppm). This is always the case in fuel mixtures originating from reformed natural gas or alcohols, unless stringent measures are taken to reduce the CO content to low levels during fuel processing. Many attempts have been made to understand the mechanism of CO poisoning using a large variety of experimental approaches. It is generally proposed that CO poisoning occurs because of a strong adsorption of CO on the catalyst surface, which blocks the hydrogen adsorption step.
There are two main strategies to overcome the CO poisoning problem in PEMFCs. The first approach is to operate the PEMFC at higher temperatures, which helps suppress the adverse impact of CO. However, this requires the development of new high-temperature membranes, in itself a significant materials challenge, and one discussed by Kreuer (2001).
The second approach requires the development of new CO tolerant electrocatalysts. Most work in this area has concentrated on Pt–M, where M is usually a transition metal, e.g., Mo, Fe, Ni, Co, and Mo.
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