Activation loss

Activation losses are due to reaction kinetics and are associated with the initial dramatic voltage losses of a fuel cell j-V curve. Next, we will discuss the origin of activation losses and ways to improve kinetic performances. 

It is obvious that the rate of an electrochemical reaction is finite because an energy barrier impedes the conversion of reactants into products. Therefore the current produced by an electrochemical reaction is limited. The relation between the energy barrier and the current density is as follows  

At thermodynamic equilibrium, the forward and reverse current densities are equal, which is shown as below  

We can manipulate the size of the activation barrier by varying the cell voltage, which is a distinguishing feature of the electrochemical reaction. The forward activation barrier is decreased by a F, while the reverse activation barrier is increased by (1 — a)nF,. Here r] is the voltage 

Equation (11.10) is known as the Butler-Vohner equation. Here t] is labeled as the activation loss and given the subscript act, as in which represents the voltage that is sacrificed to overcome the activation barrier and forces the reaction to completion. From the Butler-Volmer equation we know that current density increases exponentially with activation loss. 

Activation losses. These are caused by the slowness of the reactions taking place on the surface of the electrodes. A proportion of the voltage generated is lost in driving the chemical reaction that transfers the electrons to or from the electrode. As we shall see in Section 3.4 below, this voltage drop is highly non-linear. 

Ohmic losses. This voltage drop is the sbaightforward resistance to the flow of electrons through the material of the electrodes and the various interconnections, as well as the resistance to the flow of ions through the electrolyte. This voltage drop is essentially proportional to current density, linear, and so is called ohmic losses, or sometimes as resistive losses. 

Mass transport or concentration losses. These result from the change in concentration of the reactants at the surface of the electrodes as the fuel is used. We have seen in Chapter 2 that concentration affects voltage, and so this type of irreversibility is sometimes called concentration loss. Because the reduction in concentration is the result of a failure to transport sufficient reactant to the electrode surface, this type of loss is also often called mass transport loss. This type of loss has a third name - Nemstian . This is because of its connections with concentration, and the effects of concentration are modelled by the Nemst equation. 

These four categories of irreversibility are considered one by one in the sections 

The activation losses are nonlinear with current as seen from earlier discussions. Typically, the activation losses introduce a sharp initial drop in the cell open circuit EMF with current load. Losses are different at each electrode, cathode, or anode as the double layer configuration is different. The activation loss is directly related to the energy barrier (resistances) for oxidation and reduction at the electrodes. This energy barrier depends on several parameters as seen from the BV equation. 

The activation losses for anode and cathode are given by Equations 5.113 and 5.114 as 

From the previous section, it was seen that the activation losses depend primarily on the exchange current density, jo, and the charge transfer coefficient a. For a reaction with larger exchange current density, the required overpotential is smaller and hence there are lower electrode losses. Thus, increasing jo enhances the electrode kinetic performance. In order to understand how jo can be increased, we look at the definition of exchange current density. It is given by 

From Equation 5.118, we see that an increase in temperature increases Iq. The reactant has higher thermal activity— higher intensity thermal vibrations with increase in temperature. The increased thermal activity enhances the possibility of reactant energy to reach the activation energy level and 

Reaction plane with catalysts for hydrogen oxidation. 

The fuel crossover losses are due to the wastage of fuel passing through the electrolyte. Fuel crossover losses are prominent in the fuel cell operation at low temperatures. The activation losses are due to the slow reaction kinetics on the surface of the electrodes. Ohmic losses are due to the resistance offered to the flow of electrons and ions through electrodes and electrolytes, respectively. The concentration losses are due to the change in concentration of the reactants at the surface of electrodes. 

Activation losses are the main cause of voltage drop (mainly at cathode) for the low- and medium-temperature fuel cells. In 1905, Tafel obtained a pattern for the verification of overvoltage against current density as shown in Fig. 2.2. 

The voltage drop due to activation polarisation is represented by the Tafel equation as follows  

Equation (2.34) is the Butler-Volmer equation and can be used as an alternative for the Tafel equation. It should be noted that the Tafel equation holds good only for i io-Basically, activation polarisation is dominant at low ciment density because the electronic barriers are to be overcome prior to ion and current flow. Activation polarisation is directly influenced by the rate of electrochemical reaction and is attributed to the sluggish electrode kinetics. When the exchange current density is 

Equation (2.35) is the same as Eq. (2.30). Hence, the activation polarisation may be due to one electrode or both electrodes. If the value of io is increased then the activation polarisation is reduced, hence the focus should be on increasing the exchange current. The exchange current density can be increased by using catalysts. In addition to this, increasing the pressure and temperature (Fig. 2.1), would elfectively increase the current density. Increasing the reactant concentration as well as electrode roughness would probably lead to an increase in current density due to occupancy of catalyst sites by the reactants. 

---

For fixed-bed reactors containing rapidly deactivating catalysts, the scheduled changes ia operating variables to accommodate activity loss can have a marked effect on mn length. This is exemplified by acetylene hydrochiorination to produce vinyl chloride ia tubular fixed-bed reactors. Steel reactors,... 

The predominate role of the 2inc and aluminum oxides in the ICI catalyst is to reduce the rate of sintering and loss of metallic copper surface area, which, in addition to poisoning, is one of the modes of activity loss with time for this catalyst. 

A rapid activity loss (a few days) was observed with whole cells. Immobilisation increased the stability and continuous production of L-phenylalanine was possible using alginate bead immobilised cells of P. fluorescerts for 60 days. However, to achieve this the cofactor pyridoxalphosphate had to be continuously added to the beads to correct for the dissociation of the cofactor and loss from the cells. 

With a well constituted catalyst of this type, sintering is not an important cause of activity loss at normal operating temperatures even if the... 

During the third operating period (1230-1380 hrs), a breakthrough of 4 mg H2S/m3 synthesis gas was simulated this caused an enormous activity loss. The point in the catalyst bed where adiabatic end temperature was reached dropped from 22 to 44% of bed depth while conversion in the first 23.8% of the bed decreased from 100 to 78%. 

In addition to actual synthesis tests, fresh and used catalysts were investigated extensively in order to determine the effect of steam on catalyst activity and catalyst stability. This was done by measurement of surface areas. Whereas the Brunauer-Emmett-Teller (BET) area (4) is a measure of the total surface area, the volume of chemisorbed hydrogen is a measure only of the exposed metallic nickel area and therefore should be a truer measure of the catalytically active area. The H2 chemisorption measurement data are summarized in Table III. For fresh reduced catalyst, activity was equivalent to 11.2 ml/g. When this reduced catalyst was treated with a mixture of hydrogen and steam, it lost 27% of its activity. This activity loss is definitely caused by steam since a... 

The activity loss measured here is caused by recrystallizations. This was demonstrated by using scanning electron microscopy to determine nickel crystallite size in the same catalyst samples. These tests revealed that the catalyst used in demonstration plants has only a slight tendency to recrystallize or sinter after steam formation and loss of starting activity. 

In contrast to this, the enzyme resin is stressed less by gas sparging than by stirring (see Fig. 18 and 20). The same activity losses were observed first with 1 to 8 times greater specific adiabatic compression power Pj/ V than the maximum power density necessary for stirring. As in the case of the smooth disc, the effects of power input are only weak. The type of gas sparger and therefore the gas exit velocity are of no recognisable importance. The behaviour of the enzyme resin particles is thus completely different from that of the clay min-eral/polymer floes and the oil/water/surfactant droplet system, which are particularly intensively stressed by gas sparging. 

The experimental results in Fig. 27 show the influence of the reactor system (see Fig. 28) on the disintegration of enzyme activity. It was found that the low-stress bladed impeller results in less activity loss than the propeller stirrer which causes much higher maximum energy dissipation ,. The gentle motion the blade impeller produces means that stress is so low that its disadvantage of worse micro mixing in NaOH (in comparison with the propeller) is more than compensated. 

Fig. 27. Activity loss a/a of Acylase enzym resign with the reaction time t of the enzymatic deaccylation of Penicillin G to 6-Aminopenicillanic acid (reactor design see Fig. 28)...

The simple catalyst embedding technique has been applied to ethylene polymcaization in slurry In this technique, active catalytic components are embedded into styrene polymer matrix. The resulting polyethylene shows better morphology and higher bulk density than those produced by homogeneous catalyst. No activity loss was also observed with the... 

A similar catalyst to which 3.9% arsenic had been added in the laboratory was tested and its activity (Figure 3) compared to the activity of a fresh catalyst and also to that of the used catalyst. The activity loss of the used catalyst containing 3.6% As corresponds closely with that for the prepared sample, indicating that arsenic added by impregnation acts like that deposited under actual operating conditions. When the used catalysts were regenerated in air at 482°C, the arsenic was not removed. 

Let us now use the sequence of elementary steps to explain the activity loss for some of the catalysts The combination of hydrogen chemisorption and catalytic measurements indicate that blocking of Pt by coke rather than sintering causes the severe deactivation observed in the case of Pt/y-AljOj The loss in hydrogen chemisorption capacity of the catalysts after use (Table 2) is attributed mainly to carbon formed by methane decomposition on Pt and impeding further access. Since this coke on Pt is a reactive intermediate, Pt/Zr02 continues to maintain its stable activity with time on stream. 

GL 18] ]R 1] ]P 19a-d] High-molecular-weight deposits are assumed to form on the catalyst and can be removed by heating to 130 °C [60, 62], The activity loss is fast, but recoverable. In contrast, palladium loss, another cause of deactivation, is not as fast, but is irrecoverable. This loss decreases gradually in the course of processing, in some cases reaching near-constant behavior. 

The N4 complexes are rather stable in acidic solutions. However, sometimes the stability is not high enough, particularly so at higher temperatures. It was quite unexpected, therefore, to hud that after pyrolysis at temperatures of 600 to 800°C the catalytic activity of these compounds not only failed to decrease but in some cases even increased. The major result of pyrolysis is a drastic increase in catalyst stability. Tests have been reported where after pyrolysis such catalysts have worked for 4000 to 8000 h without activity loss. The reasons for the conservation of high activity after pyrolysis are not entirely clear. The activity evidently is associated with the central ion that has attained a favorable enviromnent of pyrolysis products. 

This test was successfully applied for the hydrosilylat-ion of trimethyl(vinyl)silane by triethoxysilane catalysed by Rh or Pt colloids. The addition of mercury to catalytic mixture led to catalytic activity loss, consistent with a heterogeneous catalyst [12,14]. 

Recent work has identified a critical role for a conserved arginine Arg-34 in the N-terminus for agonist binding and receptor activation, loss of which leads to a dysfunctional receptor [134]. 

Stability tests of the FePcClie-S catalyst confirms that this material is stable under the reaction conditions, as it was previously reported (17). Neither leaching of the complex nor significant changes in the UV-vis spectra of the catalyst after reaction (Figure 49.3) were detected. The catalyst can be reused at least three times without a significant activity loss. 

The ruthenium-catalyzed isotope exchange of boron atoms in decaborane is remarkable because several bonds are selectively broken and formed with a nanoscale catalyst without altering the structure of the decaborane. Highly enriched [10B] decaborane can be obtained by repeated treatment (six times) of decaborane with 10B2H6 in presence of Ru(0) NPs in ILs (entry 3, Table 1.5 Scheme 1.5), where the catalyst was recycled three times in batch experiments without significant activity loss [107]. 

The first-stage effluent temperature has been limited to 560 °C in order to prevent excessive catalyst activity losses. The heat of reaction data is slightly inconsistent with the reported activation energies, but use of this expression demonstrates the ease with which temperature dependent properties may be incorporated in the one-dimensional model. 

The multilayer shells can also provide a protective barrier for the loaded enzyme in environments where enzyme-degrading substrates such as proteases may be present [67]. Dissolved catalase was inactivated immediately by protease, losing its entire activity within 60 min in solution. For catalase loaded in BMS spheres, inactivation is slower, with an activity loss of about 20 % in 60 min. Notably, a negligible decrease in... 

Sodium periodate also may affect tryptophan residues in some proteins. The oxidation of tryptophan can result in activity losses if the amino acid is an essential component of the active site. For instance, avidin and streptavidin may be severely inactivated by treatment with periodate, since tryptophan is important in forming the biotin-binding pocket. In addition, many other amino acid residues are susceptible to oxidation by periodate (Chapter 1, Section 1.1). Limiting the time of oxidation is important to restricting oxidation to diol groups while not affecting other protein structures. 

Purified preparations of calf intestinal AP maintained in solution are usually stored in the presence of a stabilizer, which is typically 3 M NaCl. The enzyme also may be lyophilized, but it may experience activity loss with each freeze-thaw cycle. AP is not stable under acidic conditions. Lowering the pH of an AP solution to 4.5 reversibly inhibits the enzyme. It is recommended that all handling, storage, and use of AP be done under conditions >pH 7.0 to maintain the highest possible catalytic activity. 

---