Mass Transport Region

The stability and durability of Pt alloys, especially those involving a >d transition metal, are the major hurdles preventing them from commercial fuel cell applications. "" The transition metals in these alloys are not thermodynamically stable and may leach out in the acidic PEM fuel cell environment. Transition metal atoms at the surface of the alloy particles leach out faster than those under the surface of Pt atom layers." The metal cations of the leaching products can replace the protons of ionomers in the membrane and lead to reduced ionic conductivity, which in turn increases the resistance loss and activation overpotential loss. Gasteiger et al. showed that preleached Pt alloys displayed improved chemical stability and reduced ORR overpotential loss (in the mass transport region), but their long-term stability has not been demonstrated. " These alloys experienced rapid activity loss after a few hundred hours of fuel cell tests, which was attributed to changes in their surface composition and structure." ... 

The conversion of CO was measured with fresh catalyst, over 6, 12 and 24 pieces of nets fitted into the cassette, in a temperature range of 600-700°C, with an average flow of 100 Nm /h. At those temperatures, the catalyst is in the outer mass transport region, the activity depends only on the rate of transport of reactants from the gas bulk to the surface of catalyst. The conversion is therefore only slightly dependent on variations in temperature. In Figure 2 the conversion of CO is plotted versus the number of nets, together with simulated data, and Ga, = 100 Nm /h, according to the model for net-based catalysts developed by Fredrik Silversand 6]. 

Fig. 48. Schematic diagram delineating the three mass-transport regions for turbulent flow as a function of electrode length.

Although the voltage loss in the kinetic region is dominated by slow reaction kinetics, there are also some minor losses due to ohmic resistance and mass transport. Similarly, in the resistance (mass transport) region, most of the voltage loss arises from the ohmic resistance (mass transport), but losses due to slow kinetics and mass transport (ohmic resistance) also exist. 

Referring between Figures 2.8a and 2.8b, the reactant concentration c in the activation region is very close to Cbuik°, in the mass transport region c, is near 0, and in the iR loss region Cr is between and Cm-According to the Nemst equations shown in Equations 2.15 and 2.19, for H2, the voltage loss at current density j due to mass transport is... 

Figure 2. Middle (charge transfer-region)- and low-frequency-range (mass transport-region) of the impedance spectra of sensor 1 (left) and sensor 2 (right).

Sketch a typical high-temperature fuel cell (SOFC, MCFC) polarization curve operating at temperature A. Then, sketch a polarization curve B with reduced temperature conditions. Be sure to think about all of the effects of this change. (Ignore the effects on mass transport region until Chapter 5.)... 

A number of different types of experiment can be designed, in which disc and ring can either be swept to investigate the potential region at which the electron transfer reactions occur, or held at constant potential (under mass-transport control), depending on the infomiation sought. 

Ohmic losses AEohmic originate from (i) membrane resistance, (ii) resistance of CLs and diffusion layers, and (iii) contact resistance between the flow field plates. Although it is common practice to split current-voltage characteristics of an MEA into three regions— kinetic (low currents), ohmic (intermediate currents), and mass transport (high currents) [Winter and Brodd, 2004]—implicit separation of vt Afiohmic is not always straightforward, and thus studies of size and... 

The nature of mass transport in MEMED has been confirmed with both ampero-metric and potentiometric studies of bromine transfer from an aqueous phase to DCE [79]. Figure 17 shows typical amperometric data for this case, in which the DCE phase acts as a sink for Br2, and a depleted region of Br2 is measured adjacent to the droplet in the aqueous phase. Video images are also provided, which correspond to particular times during the amperometric transient at position (3) the edge of the developing concentration boundary layer, around the drop, reaches the electrode the concentration profile is then mapped out between points (3) and (4). The measured current, i, can be related to the local concentration, c, via... 

Obviously, there will be a range of pressures or molecular concentrations over which the transition from ordinary molecular diffusion to Knudsen diffusion takes place. Within this region both processes contribute to the mass transport, and it is appropriate to utilize a combined diffusivity (Q)c). For species A the correct form for the combined diffusivity is the following. 

Normally, clay in soil is not present as individual particles but is clustered to aggregates that consist wholly of clay or of a mixture of clay and other mineral and/or organic soil material. Mass transport of soil material along cracks and pores, common in cracking soils in regions with alternating wet and dry periods, does not necessarily enrich the subsoil horizons with clay. 

Having defined our near electrode region, we turn now to consider the various techniques that can be employed in the in situ investigation of the reactions that occur within it. The various methods that can be employed will each provide different types of information on the processes occurring there. As has already been discussed, cyclic voltammetry is the most common technique first employed in the investigation of a new electrochemical system. However, in contrast to the LSV and CV of adsorbed species, the voltammetry of electroactivc species in solution is complicated by the presence of an additional factor in the rate, the mass transport of species to the electrode. Thus, it may be more useful to consider first the conceptually more simple chronoamperometry and chronocoulometry techniques, in order to gain an initial picture of the role of mass transport. 

If kc is very large such that we are in the mass transport-limited region of the current and the second term in equation (2.162) — 0, then the observed current is its limiting value ... 

---