Electrolytes mass transport

Inside the bulk of the electrolyte, mass transport is mainly because of migration, a mechanism of ionic motion caused by the presence of an applied electric field. In the electrochemical cell the potential drop creates an electric field that is much more intense in the regions near the surface of the electrodes, but is sufficiently intense in the bulk of the electrolyte to promote the migration of the ions to the border of the diffusion layers. 

The practical problem encountered in PEMFC is keeping the membrane wet under operating conditions. The balance of water with its transport determines the water content in the cell during the reactive mode of operation. Water-transport processes are a function of the current and the characteristics of both the membrane and the electrodes (Mosdale, 1996 Motupally, 2000). The temperature of operation ofthe cell has a significant influence on its performance. The performance of the PEMFC increases with temperature until 90°C, which reflects a lowering of the internal resistance of the cell. This decrease is due largely to the decrease in ohmic resistance of the electrolyte. Mass transport, which is limited (i) by diffusion of reactants through the PEMFC assembly to the active Pt sites. 

If the concentration of hydrogen ions is small, Eq. IV, 38 has to be applied in the unabridged form.. Computed and experimental i—U curves agree well on platinized platinum [22] and smooth electrodes of Pt, Ir, and Rh [23] in acid electrolytes. Mass transport of OH" ions... 

An important assumption was that the solution was dilute (in this case natural water of approximately lOOp.p.m. total dissolved solids) since there are difficulties in applying mass transport equations for certain situations in concentrated electrolyte solution, where a knowledge of activities is uncertain and this can lead to large errors. 

Both share more or less the same merits but also the same disadvantages. The beneficial properties are high OCV (2.12 and 1.85 V respectively) flexibility in design (because the active chemicals are mainly stored in tanks outside the (usually bipolar) cell stack) no problems with zinc deposition in the charging cycle because it works under nearly ideal conditions (perfect mass transport by electrolyte convection, carbon substrates [52]) self-discharge by chemical attack of the acid on the deposited zinc may be ignored because the stack runs dry in the standby mode and use of relatively cheap construction materials (polymers) and reactants. 

In the first papers dealing with SEI electrodes it was suggested that the passivating layer consists of one or two layers [1, 2], The first one (the SEI) is thin and compact the second (if it exists), on top of the SEI, is a more porous, or structurally open, layer that suppresses the mass transport of ions in the electrolyte filling the pores of this layer. 

Inside a pit in electrolytic solution, anodic dissolution (the critical dissolution current density, and diffusion of dissolved metal hydrates to the bulk solution outside the pit take place simultaneously, so that the mass transfer is kept in a steady state. According to the theory of mass transport at an electrode surface for anodic dissolution of a metal electrode,32 the total increase of the hydrates inside a pit, AC(0) = AZC,<0),is given by the following equation33,34 ... 

De Marco R, Mackey DJ, Zirino A (1997) Response of the jalpaite membrane copper(lI) ion-selective electrode in marine waters. Electroanalysis 9 330-334 Kozicki MN, Mitkova M (2006) Mass transport in chalcogenide electrolyte films - materials and applications. J Non-Cryst Solids 352 567-577... 

Under realistic conditions a balance is secured during current flow because of additional mechanisms of mass transport in the electrolyte diffusion and convection. The initial inbalance between the rates of migration and reaction brings about a change in component concentrations next to the electrode surfaces, and thus gives rise to concentration gradients. As a result, a diffusion flux develops for each component. Moreover, in liquid electrolytes, hydrodynamic flows bringing about convective fluxes Ji j of the dissolved reaction components will almost always arise. 

It was shown in Section 1.8 that in addition to ion migration, diffusion and convection fluxes are a substantial part of mass transport during current flow through electrolyte solutions, securing a mass balance in the system. In the present chapter these processes are discussed in more detail. 

The above brief analysis underlines that the porous structure of the carbon substrate and the presence of an ionomer impose limitations on the application of porous and thin-layer RDEs to studies of the size effect. Unless measurements are carried out at very low currents, corrections for mass transport and ohmic limitations within the CL [Gloaguen et ah, 1998 Antoine et ah, 1998] must be performed, otherwise evaluation of kinetic parameters may be erroneous. This is relevant for the ORR, and even more so for the much faster HOR, especially if the measurements are performed at high overpotentials and with relatively thick CLs. Impurities, which are often present in technical carbons, must also be considered, given the high purity requirements in electrocatalytic measurements in aqueous electrolytes at room temperature and for samples with small surface area. 

Summing up this section, we would like to note that understanding size effects in electrocatalysis requires the application of appropriate model systems that on the one hand represent the intrinsic properties of supported metal nanoparticles, such as small size and interaction with their support, and on the other allow straightforward separation between kinetic, ohmic, and mass transport (internal and external) losses and control of readsorption effects. This requirement is met, for example, by metal particles and nanoparticle arrays on flat nonporous supports. Their investigation allows unambiguous access to reaction kinetics and control of catalyst structure. However, in order to understand how catalysts will behave in the fuel cell environment, these studies must be complemented with GDE and MEA tests to account for the presence of aqueous electrolyte in model experiments. 

Oxidation of Adsorbed CO The electro-oxidation of CO has been extensively studied given its importance as a model electrochemical reaction and its relevance to the development of CO-tolerant anodes for PEMFCs and efficient anodes for DMFCs. In this section, we focus on the oxidation of a COads monolayer and do not cover continuous oxidation of CO dissolved in electrolyte. An invaluable advantage of COads electro-oxidation as a model reaction is that it does not involve diffusion in the electrolyte bulk, and thus is not subject to the problems associated with mass transport corrections and desorption/readsorption processes. 

In addition to the transport of charge, the current flow in an electrolyte is also accompanied by mass transport. The migration flux of species / is given by the equation... 

Mass transport can be by migration, convection or diffusion. As discussed in chapter 1, in the presence of strong electrolyte migration can be neglected, as can convection if the solution is unstirred, at a uniform temperature and the timescale of the experiment is short (i.e. a few seconds). Thus, we can make the first distinction between electrode reactions that are dominated by step 1, diffusion-controlled, and those for which steps 1 and 2 contribute to the overall observed rate. 

For the investigation of charge tranfer processes, one has the whole arsenal of techniques commonly used at one s disposal. As long as transport limitations do not play a role, cyclic voltammetry or potentiodynamic sweeps can be used. Otherwise, impedance techniques or pulse measurements can be employed. For a mass transport limitation of the reacting species from the electrolyte, the diffusion is usually not uniform and does not follow the common assumptions made in the analysis of current or potential transients. Experimental results referring to charge distribution and charge transfer reactions at the electrode-electrolyte interface will be discussed later. 

---