Kinetic loss

Besides the mentioned thermodynamic losses, there always exist kinetic losses arising from the competitive non-radiative quenching of the excited state. For instance in photovoltaic devices, the undesired thermal recom-... 

Fig. 5.65 Dependence of the solar conversion efficiency (CE) on the threshold wavelength (Ag) for a quantum converter at AM 1.2. Curve 1 Fraction of the total solar power convertible by an ideal equilibrium converter with no thermodynamic and kinetic losses. Curve 2 As 1 but the inherent thermodynamic losses (detailed balance and entropy production) are considered. Continuous line Efficiency of a regenerative photovoltaic cell, where the thermodynamic and kinetic losses are considered. The values of Ag for some semiconductors are also shown (according to J. R. Bolton et al.)... 

The kinetics for the oxidation of hydrogen are fast and contribute minimally (< 5% of the kinetic losses).6 In the case of the ORR, the kinetics are exponentially dependant on the voltage in the cathode, as is the case for electron transfer reactions.6 As the voltage drops from the equilibrium value, the kinetic current increases exponentially. However, since the kinetics for the ORR are slow over all... 

All fuel cells exhibit kinetic losses that cause the electrode reactions to deviate from their theoretical ideal. This is particularly true for a direct methanol PEFC. Eliminating the need for a fuel reformer, however, makes methanol and air PEFCs an attractive alternative to PEFCs that require pure hydrogen as a fuel. The minimum performance goal for direct methanol PEFC commercialization is approximately 200 mW/cm at 0.5 to 0.6 V. 

The performance of a fuel cell is most often reported in the form of a polarization curve. Such a curve is shown in Figure 3. Roughly speaking, the polarization curve can be broken down into three main regions. At low currents, the behavior of a fuel cell is dominated by kinetic losses. These losses... 

Most of the models show that fuel-cell performance is a balance among the various losses shown in Figure 3, in particular, ohmic losses and mass-transport limitations, which both increase with current. The reason for this is that the kinetic losses are hard to mitigate without significantly changing op-... 

Wang, C.H. and Broadbent, F.E. Kinetic losses of PCNB and DCNA in three California soils. Soil Sci Soc. Am. Proc., 36 742-745, 1972. 

Outlet pressure drops were measured based on the frictional and kinetic losses from the outlet of the relief device to the discharge point, typically atmosphere or the entrance to a main relief header. 

Eioss is the energy loss per water molecule in the photowater-splitting process. Eioss involves fundamental losses imposed by thermod)mamics (entropy change associated with the creation of excited states) as well as losses due to nonidealities in the conversion process (transport of electron/holes, electron/holes recombination, kinetic losses, etc.). Eioss takes a minimum value of around 0.3-0.4eV in ideal photocatalysts due to thermod)mamic losses, and this value rises to 0.8eV in practical photocatalysts (Bolton, 1996). 

Optimum thickness. At fixed composition a phase diagram of the catalyst layer can be generated, which establishes a relation between the optimum thickness interval of the catalyst layer and the target current density jo (or jo interval) of fuel cell operation. The optimum compromise between kinetic losses and mass transport losses is realized in the intermediate regime. The existence of an upper limit on the thickness beyond which the performance would start to deteriorate is due to the concerted impact of oxygen and proton transport limitations, whereas considered separately each of the effects would only serve to define a minimal thickness. 

The highest electronic resistances were observed at low Nafion loadings, indicating that the ionomer played a significant role as a binder [211], Meanwhile, kinetic losses pass throngh a minimnm correlated with the electrochemically active snrface area of the catalyst estimated from cyclic voltammograms [209] The higher the electrochemically active surface area, the lower the kinetic losses. This volcano type of cnrve reflects the optimnm in the metal utilization factor u. Below, we try to nnderstand how carbon properties may influence these characteristics. 

As discussed above, the electrochemical oxidation of a fuel can theoretically be accomplished at very high efficiencies (e.g. 96% for gas-phase product water or 83% for liquid product water for the H2/O2 reaction at 25 °C, see Fig. 8.3) as compared to heat engines utilizing the combustion of a fuel. However, in practice, fuel cells experience irreversible losses due to resistive and reaction kinetic losses (see Fig. 8.4), and efficiencies of fuel cell stacks rarely exceed 60% at rated load. The irreversible losses appear as heat and, for example, a 1 kW fuel cell operating... 

For a fuel cell operating at 0.7 V, a power output of 0.5 W cm requires a current density of at least 0.7Acm . If the open circuit voltage (OCV) is IV, then the allowed voltage loss of 0.3 V can be achieved only if the area specific resistance of combined ohmic and kinetic losses does not exceed 0.45 12 cm. ... 

The kinetic losses are a strictly increasing function of current density I e [0, Co/5], where the maximum current is dependent upon oxygen concentration and mass transport limitations. Since is invertible, we formally solve the membrane voltage balance, eqn. (131), obtaining the local current I y),... 

The combined diffusion and kinetics losses at an electrode p (Thampan et al., 2001 Vilekar Datta, 2010) is ... 

Considering (Mily the thermodynamics of the DMFC (used here as a representative of direct alcohol fuel cells), methanol should be oxidized spraitaneously when the potential of the anode is above 0.05 V/SHE. Similarly, oxygen should be reduced spontaneously when the cathode potential is below 1.23 V/SHE, identical to a H2-O2 fuel cell. However, kinetic losses due to side reactions cause a deviation of ideal thermodynamic values and decrease the efficiency of the DMFC. This is presented in Fig. lb, which includes various limiting effects as kinetics, ohmic resistance, alcohol crossover, and mass transport. The anode and cathode overpotentials for alcohol oxidation and oxygen reduction reduce the cell potential and together are responsible for the decay in efficiency of approximately 50 % in DMFCs [13, 27]. 

These kinetic losses at the anode lead to a high anode overvoltage and, therefore, the cell voltage in the DMFC is lower at a respective value of current density as compared to a H2-fed cell, as is the achievable power density. 

RDE analysis assumes perfect first-order kinetics, which is not strictly true, and errors fi om this imperfect correction could become large at low potentials. The authors also smdied the relative humidity (RH) effects on ORR kinetics in PEMFCs [32], They found that when RH is above 50-60%, the kinetics are independent of the RH, but they observed significant ORR kinetic losses at lower RH. The reduction of ORR kinetics at low RH was interpreted as most likely due to a lowering of the proton activity (therefore only indirectly related to the lowering of the water activity) via hydration of acidic groups and the sequestering of protons at low RH. 

---