Current distribution temperature, effects

Two combinatorial studies also utilized thermocouple arrays to measure temperature distribution. Yan et al.133 used an electrically segmented plate with three thermocouples to investigate the effects of a dynamic load on current and temperature distribution. The results are described further in Section 3.2. Maranzana et al.134 developed a transparent cell with 20 thermocouples and 20 electrically isolated segments of gold wire to monitor performance... 

For the scale up of a chemical reactor, inadequate mixing may result in spatial variations in, for example, reactant composition or temperature. An electrochemical reactor (cell) is a chemical reactor where the reduction and oxidation reactions are spatially separated on cathodes and anodes. The flow of ionic current through the electrolyte results in an electric field through the electrolyte. Since charged species move in response to an electrical field [1-3] and since the potential difference across the double layer impacts reaction rate, electrical field effects can significantly impact current distribution. Thus, in contrast to a chemical reactor, perfect mixing to eliminate all concentration fields does not necessarily result in uniform reaction rates. 

The model is used to study the effect of a resistive spot on temperature and current distribution in a 15-cell fragment of a planar SOFC stack (Figure 5.21). Two variants have been calculated. In the base case, the stack contained no spots this variant provides the reference undisturbed distribution of stack temperature Tsase-... 

In the absence of flooding, current distributions are usually comparable to temperature distributions, as observed by Hakenjos et al. (2004) or Maranzana et al. (2008), regardless of the hypothesis made above complexity of fuel cell operation means that some approxiniate methods used in engineering processes give poor reliability. In cases of flooding, current density is lower in the flooded area the effect on local temperature can hardly be predicted since Gaghardo et al. 

Natarajan, D. and Van Nguyen, T. 2005b. Current distribution in PEM fuel cells. Part 2 Air operation and temperature effect. AIChE Journal 51 2599-2608. 

The preceding discussion illustrates the effects of overall compression and temperature. Additional effects have also been attributed to the inhomogeneous (and usually periodic) compression exerted by the bipolar plate topologies. In a recent effort, Chi and coworkers reported a numerical model that considers the nonuniform compression and the resulting variations in species concentration, temperature, and current distributions (Chi et al., 2010). Such models can become effective design tools for new PTL materials, but they require careful validation against experimental data. 

When temperatures are varied, in addition to the surface-morphology differences, current distribution over the electrode surface also seems to be affected. By comparison, surface coating is more uniform when plated at a lower temperature, while surface plated at room temperature shows clear edge effects, as shown in Fig. 8. This indicates a better current distribution is achieved when polymerization is carried out at a lower temperature. It is reported that more uniform surface coating could give higher conductivity [59]. 

Since the temperature in the PEFC typically varies by at most 15°C, the effect of temperature distribution on the reaction kinetics will be relatively small compared to the temperature interaction with the liquid water distribution and relative humidity that controls membrane ionic conductivity. The local relative humidity in the anode phase typically controls the local current density of an underhumidified fuel cell because electro-osmotic drag exacerbates anode dryout, while water generation at the cathode diminishes any electrolyte dryout in the cathode catalyst layer [11]. That is, if the other parameters are constant, the local current distribution can be predicted with a knowledge of the anode in an underhumidified cell [11]. Anode dryout can be the result of the loss of only a few hundredths of a milligram of water per square centimeter active area in the catalyst layer... 

Attempts have been made to develop methods for the production of aromatic isocyanates without the use of phosgene. None of these processes is currently in commercial use. Processes based on the reaction of carbon monoxide with aromatic nitro compounds have been examined extensively (23,27,76). The reductive carbonylation of 2,4-dinitrotoluene [121 -14-2] to toluene 2,4-diaLkylcarbamates is reported to occur in high yield at reaction temperatures of 140—180°C under 6900 kPa (1000 psi) of carbon monoxide. The resultant carbamate product distribution is noted to be a strong function of the alcohol used. Mitsui-Toatsu and Arco have disclosed a two-step reductive carbonylation process based on a cost effective selenium catalyst (22,23). 

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