Degradation current density distribution

The main drawback to electrochemical machining lies in the need to design a tool (cathode) for each new job. Moreover, the design process to select a suitable electrolyte and obtain the correct current density distribution remains a skilled art rather than a science. It is often necessary to test the tool and to modify it by trial and error. In addition, the need to use large volumes of electrolyte solutions does not fit in well to a mechanical workshop because, however carefully they are handled, corrosion and chemical degradation in the environment are potential hazards. 

The second example of field emission of Si-based nanowires is that of B-doped Si nanochains [80]. The SiNCs were attached onto a Mo substrate by a conductive carbon film. The anode-sample separation ranges from 120 to 220 pm. The turn-on field was 6 V pm and smaller than that (15 V pm ) for the SiNWs. The field-emission characteristics of the SiNCs were analyzed according to the FN theory [81]. All the FN curves with different anode-sample separations fall in nearly the same region and have similar Y intercepts, showing that the SiNCs are uniformly distributed. A stability test showed no obvious degradation of current density and the fluctuation was within +15%, indicating that the B-doped SiNCs are a promising material for field emission applications. 

It has to be kept in mind that usually the temperature influence is less important, since large temperature gradients inside the fuel cell are normally avoided, for several reasons. First, a large temperature gradient imposes thermal stress on the respective materials and is a source of accelerated degradation or failure. Second, the electrochemical reaction is sensitive to temperature. An increase in temperature leads to an increase in current density. This in turn would amplify a nonideal current distribution, leading to an increase in losses connected with cross-currents. 

As for other electrochemical processes, current distribution can be linked to flow rate distribution in the cell stmcture (Kandlikar et al. 2009a), as it can be easily imagined that areas with poor circulation of reacting gas are of restricted electrochemical activity locally low current densities correspond to high cathode potentials, which accelerate degradation phenomena of the MEA, at both the electrode structure and the membrane, as explained in other chapters. 

With distributed current collection hardware, current distribution during cell operation can be evaluated (Yoshioka et al. 2005). It is found that when the inlet RH is low, the highest current density is observed near the gas outlet, where the humidity is relatively high. Thus, the membrane chemical degradation can be accelerated in the gas inlet region since lower RH usually accelerates degradation. 

Ethylene glycol (EG, C2H6O2) is ubiquitously used in the automotive industry as an engine coolant, and hence a distribution infrastructure already exists. Also, EG has a crossover current density roughly half that of methanol [69]. However, PEFC performance with EG is still relatively low, with a fuel cell specific energy density about 20-40% less than that of the same fuel cell utilizing methanol. Additionally, EG has been shown to rapidly degrade PEFC elecfiolyte material, which obviously limits its potential PEFC applications. 

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