Electrochemical high theoretical efficiency

The bracketed difference is the Gibbs energy of the reaction that would occur on direct contact. As the Gibbs energy differs from the reaction enthalpy by TAS (T thermodynamic temperature, AS reaction entropy), a positive AS would result in theoretical efficiencies greater than 100% (then the environment would cool). Usually, those efficiencies are approximately 100%, as AS is negligible. This high theoretical efficiency is another asset of electrochemical conversion devices. 

The main attractiveness of fuel cells follows from the definitions given above. It comprises the high theoretical efficiency associated with direct conversion of chemical energy into electrical energy by means of galvanic cells [10] the selectivity of the electrochemical process and the advantage of a continuous metabolism by using the ambient air to oxidize the steadily supplied fuel. 

As it has been described in various other review articles before, the conversion efficiencies of photovoltaic cells depend on the band gap of the semiconductor used in these systems The maximum efficiency is expected for a bandgap around Eg = 1.3eV. Theoretically, efficiencies up to 30% seem to be possible . Experimental values of 20% as obtained with single crystal solid state devices have been reported " . Since the basic properties are identical for solid/solid junctions and for solid/liquid junctions the same conditions for high efficiencies are valid. Before discussing special problems of electrochemical solar cells the limiting factors in solid photovoltaic cells will be described first. 

Porous metallic structures have been used for electrocatalysis (Chen and Lasia, 1991 Kallenberg et al., 2007). Porous electrodes are made with conductive materials that can degrade under high temperatures at high anodic potential conditions. This last problem is of less importance for fuel cell anodes, which operate at relatively low potentials, but it can be of importance for electrochemical reactors. Porous column electrodes prepared by packing a conductive material (carbon fiber, metal shot) forming a bar are frequently used. Continuous-flow column electrolytic procedures can provide high efficiencies for electrosynthesis or removal of pollutants in industrial situations. Theoretical analysis for the electrodeposition of metals on porous solids has been provided by Masliy et al. (2008). 

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... 

The anodic polarization behavior of graphite is shown in Fig. 8 along with the dependence of chlorine current efficiency on current density (c.d.). Thus, at current densities beyond 10 A/dm2, the potential-log c.d. variation is nonlinear, and the chlorine current efficiency decreases with increase in current density. The deviation of the Tafel behavior at high current densities is attributed to the oxide layer on the anode and to the simultaneous discharge of oxygen.39 Comparison of the kinetic parameters evaluated from the experimental data (see Table 1 A) with the theoretical values presented in Table IB for various reaction pathways suggests the slow-step to be electrochemical desorption in the low current density region ... 

---