Open-circuit reduction

Desilvestro, J., Neumannspallart, M. 1985. Photoredox reactions on semiconductors at open circuit reduction of Fe on WO, electrodes and particle suspensions. J Phys Chem 89 3684-3689. 

In open-circuit reduction the material passes only once through the machine, whereas in closed-circuit reduction the material discharged from the machine is separated by screening or classifying (the latter usually by air separation) into fine and coarse particles, the latter being returned to the machine for further reduction (Fig. 1 a). 

The three basic types of size reduction circuits used to produce a fine product ate shown in Eigure 1. The final stages of the grinding circuit are typically operated in closed circuit, at comparatively high circulating loads, so that the material has tittle chance of being broken a second time before it is removed from the circuit by a classifier. Rod mills are operated normally in an open circuit. 

Under both short-circuit and open-circuit conditions, a solar cell produces no electric power, the power is consumed internally in the cell and is dissipated as heat. When a resistive load is connected to a cell in sunlight, a photogenerated voltage, F, is induced across the load and a current flows through it. The existence of requites that the flow of majority carriers be reduced from that in the open-circuit condition there must be a higher battier potential than in the open-circuit case (Fig. 2d). This higher barrier potential (V6 — ) indicates a smaller reduction from Since the photogenerated... 

The term electrochromism was apparently coined to describe absorption line shifts induced in dyes by strong electric fields (1). This definition of electrocbromism does not, however, fit within the modem sense of the word. Electrochromism is a reversible and visible change in transmittance and/or reflectance that is associated with an electrochemicaHy induced oxidation—reduction reaction. This optical change is effected by a small electric current at low d-c potential. The potential is usually on the order of 1 V, and the electrochromic material sometimes exhibits good open-circuit memory. Unlike the well-known electrolytic coloration in alkaU haUde crystals, the electrochromic optical density change is often appreciable at ordinary temperatures. 

Cone crushers can be operated in open circuit with capacities shown in Table 20-12, or in closed circuit in parallel with scalping screens for multistage size reduction, as Table 20-13 shows (the product sizes are not comparable). The feed should not contain more than... 

Legault, Mori and Leckie have used open-circuit potential vs. time measurements and cathodic reduction of rust patinas for the rapid laboratory evaluation of the performance of low-alloy weathering steels. The steel specimens are first exposed for 48 h to the vapour of an 0-(X)l mol dm sodium bisulphite solution maintained at 54°C (humid SOj-containing atmosphere) to stimulate corrosion under atmospheric conditions. They are then subjected to two types of test (tt) open-circuit potential-time tests for periods up to 3 000 s in either distilled water or 0 -1 mol dm Na2S04 and... 

Figure 2.3. Catalysis (0), classical promotion ( ), electrochemical promotion ( , ) and electrochemical promotion of a classically promoted (sodium doped) ( , ) Rh catalyst deposited on YSZ during NO reduction by CO in presence of gaseous 02.14 The Figure shows the temperature dependence of the catalytic rates and turnover frequencies of C02 (a) and N2 (b) formation under open-circuit (o.c.) conditions and upon application (via a potentiostat) of catalyst potential values, UWr, of+1 and -IV. Reprinted with permission from Elsevier Science.

Table 4.2 lists the same catalytic systems but now grouped in terms of different reaction types (oxidations, hydrogenations, reductions and others). In this table and in subsequent chapters the subscript D denotes and electron donor reactant while the subscript A denotes an electron acceptor reactant. The table also lists the temperature and gas composition range of each investigation in terms of the parameter Pa/Pd which as subsequently shown plays an important role on the observed r vs O global behaviour. Table 4.3 is the same as Table 4.2 but also provides additional information regarding the open-circuit catalytic kinetics, whenever available. Table 4.3 is useful for extracting the promotional rules discussed Chapter 6. 

Illumination of solar cells causes a reduction of efficiency and fill factor, as a result of light-induced creation of defects (Staebler-Wronski effect. Section 1.1.2.5). This reduction is halted after several hundred hours of illumination. The reduction is correlated with solar cell thickness. A large intrinsic layer thickness leads to a large reduction of efficiency and fill factor compared to a small intrinsic layer thickness. The solar cell properties can be completely recovered by annealing at about 150°C. The open circuit voltage and short circuit current decrease only slightly. 

Fig. 42. Single positively and negatively going scans through the vanadia redox region. The electrolyte is K2S207 with 4.0% K2S04,1.0% V2Os, 370 °C, lOOmV/s. The open circuit potentials are denoted by X, symbols are as follows (A) positive, (C) negaitve, (1) air environment, (2) 1.0% S02 in N2, (3) 1.0% S02, 10.0% 02 in N2, and IV and V represent the vanadia complex oxidation and reduction as before.

Fig. 2. Current-potential curves in Evans diagram [29] format for reduction of Cu2+ ions and oxidation of H2CO. and are the equilibrium, or open circuit, potentials for the Cu2+ reduction and H2CO oxidation reactions, respectively. Assuming negligible interfering reactions, the vertical dashed lines indicate the exchange current densities for the two half reactions, and the deposition current for the complete electroless solution. Adapted from ref. 23.

The O2 reduction reaction affects not only the steady-state deposition kinetics, but also the initiation of deposition, the so-called induction time [126, 127], At the beginning of the deposition process, the open circuit potential (Eoc) of either a uniformly catalytically active substrate, or a catalyst particle on an insulator, will be higher than that required for electroless deposition to occur. This is a consequence of the surface of the catalyst being covered with O or OH species which mask the catalytic activity of the surface the value of would be expected to be in the range of... 

FIGURE 3.5 (a) Impedance and (b) polarization behavior of a freshly prepared La072Sr018MnO3 electrode for 02 reduction as a function of cathodic polarization time at 200 mAcm 2 and 900°C in air. The impedance was measured at open circuit and the numbers are frequencies in hertz. (From Jiang, S.P. and Love, J.G., Solid State Ionics, 138 183-190, 2001. With permission.)... 

Figure 3.5 [36], For the 02 reduction reaction on freshly prepared LSM electrodes, the initial polarization losses are very high and decrease significantly with the cathodic polarization/current passage (see Figure 3.5b). Consistent with the polarization potential, the impedance responses at open circuit decrease rapidly with the application of the cathodic current passage. For example, the initial electrode polarization resistance, RE, is 6.2 Qcm2 and after cathodic current treatment for 15 min RK is reduced to 0.7 Qcm2 see Figure 3.5 (a). The reduction in the electrode polarization resistance is substantial. The analysis of the impedance responses as a function of the cathodic current passage indicates that the effect of the cathodic polarization is primarily on the reduction in the low-frequency impedance [10]. Such activation effect of cathodic polarization/current on the electrochemical activity of the cathodes was also reported on LSM/YSZ composite electrodes [56-58], Nevertheless, the magnitude of the activation effect on the composite electrodes is relatively small.

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