The Influence of Cell Design

The optimal design of liquid-junction photovoltaic cells shares constraints with solid-state photovoltaic cells.25 209 Current collectors cast shadows and can reduce the amount of sunlight absorbed in the semiconductor. A constraint unique to the liquid-junction cell is the placement of the counterelectrode relative to the semiconductor-electrolyte interface. Shadows, which reduce efficiency and cause local currents in solid-state photovoltaic cells, may lead to localized corrosion in photoelectrochemical cells. Mass-transfer and kinetic limitations at the counterelectrode and resistance of the electrolyte can play important roles in the optimal design of the liquid-junction photovoltaic cell. These considerations are treated qualitatively by Parkinson.210 

The design of a photoelectrochemical configuration is illustrated here for the slotted-semiconductor electrode presented in 

The primary resistance for this system is presented in Fig. 8 as a function of L/D with h/G as a parameter. The maximum power density is presented in Fig. 9 as a function of L/h with h/G as a parameter. The maximum power density for this system is obtained with a small gap. For h/G = 0.5 (G = 1 cm), the maximum power density was 47.8 W/m2, and the maximum power efficiency was 5.4%. The current density under maximum power conditions was 15mA/cm2 delivered at 477.6mV. For h/G = 10 (G = 0.05 cm), the maximum power density was 67.7 W/m2, and the maximum power efficiency was 7.7%. At maximum power the current density was 15.2mA/cm2 delivered at 534.6 mV. 

The hierarchy of photovoltaic cell efficiencies is presented in Table 1. Semiconductor effects, such as recombination, reduce the power efficiency of a GaAs-based device from a value of 37%, based solely upon band gap, to 15.3%. Reflection losses, with an 

The maximum power efficiency is presented as a function of illumination intensity in Fig. 10 for the slotted-electrode cell. The cell was designed with the design parameters calculated to be optimal under peak AM-2 illumination. The power efficiency decreases with increasing illumination due to the influence of electrolyte resistance and kinetic and mass-transfer limitations at the counterelectrode. These phenomena become increasingly important as current densities increase, and mass-transfer limitations at the counterelectrode can result in an upper limit for cell currents. 

---

The influence of cell design on overcharge prec-sure characteristics, that is. potential improvement in oxygen recombination characteristics, is illustrated in Figure 19.14. 

Electrodes. At least three factors need to be considered ia electrode selection as the technical development of an electroorganic reaction moves from the laboratory cell to the commercial system. First is the selection of the lowest cost form of the conductive material that both produces the desired electrode reactions and possesses stmctural iategrity. Second is the preservation of the active life of the electrodes. The final factor is the conductivity of the electrode material within the context of cell design. An ia-depth discussion of electrode materials for electroorganic synthesis as well as a detailed discussion of the influence of electrode materials on reaction path (electrocatalysis) are available (25,26). A general account of electrodes for iadustrial processes is also available (27). 

Knowing that a given combination of anode and cathode half-cell reactions will proceed sponta-neonsly does not ensure that the electrode reaction rates will be sufficiently high for practical applications. Reaction kinetics at the anode and cathode and mass transfer of reactants/products to/from the electrodes may play important roles in an electrochemical cell and may influence the choice of cell design and operating conditions. These important points will be addressed later in this chapter. 

Adoption of this approach to microbial process development cannot occur until methods exist for determining the influence of reactor design and operating parameters on single-cell metabolic control actions and reaction rates. If this Information is available, population balance equations and associated medium conservation equations provide the required bases for reactor analysis (1, ). For example, for a well-mixed, continuous-flow Isothermal mTcroblal reactor at steady-state, the population balance equation may be written ... 

The experiment based on the two-level factorial design as described by Box and Wilson (19) was carried out in order to check the influence of the individual glucanases (g ) of Streptomyces sp. 1228 lytic enzymes system on the degree of yeast cell lysis (y). 

A laboratory-made electrolytic cell was designed as an electrolytic generator of the molecular hydride of Cd. The influence of several parameters on the recorded signal was evaluated by the experimental design and subsequently optimised univariately... 

---