Yield value depletion potential

Data from electrochemical impedance diagrams yield a simplified quantitative analysis for an appropriate interpretation of the linear sweep voltammetry (LSV) experiments. In fact, the Si electrode potential measured with respect to the reference electrode represents the value within the bulk of the material. The direct current flow for the electrochemical reaction has to overcome the resistance of the space charge layer, which can reach extremely high values when a depletion layer is formed. For p-type Si in the potential range for the HER onset, this excess surface resistance is over 10 f2 cm. Thus, even with a bias of —1 V, the DC... 

The experiments were performed with single crystal (111) p-Si electrodes with a resistivity of about 5.5 ohm cm non-aqueous electrolytes were used consisting of absolute methanol containing tetramethylammonium chloride (TMAC) or acetonitrile containing tetraethyl ammonium perchlorate (TEAP). The flat-band potentials or p-Si in the two electrolytes were determined from Mott-Schottky plots (in the dark) in the depletion range of the p-Si electrode, from open-circuit photopotential measurements, and from the values of electrode potential at which anodic photocurrent is first observed in n-type Si electrodes. These three methods all yielded consistent flat-band potential values for p-Si of + 0.05V (vs SCE)... 

In the fully developed H2 starvation region, where H2 is completely depleted, the cathode electrode potential reaches its maximum value as determined by the mixed potential of all electrochemical reactions involved at the cathode, namely, COR and OER. The reaction current therein is determined by 02 crossover (ixthrough membrane to anode, where only ORR takes place in the absence of H2. Charge balance yields... 

Suppose we now consider a series of step experiments in the anthracene solution discussed earlier. Between each experiment the solution is stirred, so that the initial conditions are always the same. Similarly, the initial potential (before the step) is chosen to be at a constant value where no faradaic processes occur. The change from experiment to experiment is in the step potential, as depicted in Figure 5.1.3a. Suppose, further, that experiment 1 involves a step to a potential at which anthracene is not yet electroactive that experiments 2 and 3 involve potentials where anthracene is reduced, but not so effectively that its surface concentration is zero and that experiments 4 and 5 have step potentials in the mass-transfer-limited region. Obviously experiment 1 yields no faradaic current, and experiments 4 and 5 yield the same current obtained in the chronoamperometric case above. In both 4 and 5, the surface concentration is zero hence anthracene arrives as fast as diffusion can bring it, and the current is limited by this factor. Once the electrode potential becomes so extreme that this condition applies, the potential no longer influences the electrolytic current. In experiments 2 and 3 the story is different because the reduction process is not so dominant that some anthracene cannot coexist with the electrode. Still, its concentration is less than the bulk value, so anthracene does diffuse to the surface where it must be eliminated by reduction. Since the difference between the bulk and surface concentrations is smaller than in the mass-transfer-limited case, less material arrives at the surface per unit time, and the currents for corresponding times are smaller than in experiments 4 and 5. Nonetheless, the depletion effect still applies, which means that the current still decays with time. 

Low carrier concentrations - In some cases, the background (or bulk) concentration of free carriers is so low that the entire film is depleted at aU values of the applied potential. This means that the semiconductor film effectively behaves as an insulator with a constant (potential-independent) capacitance. This is not often observed for metal oxide photoelectrodes, which usually have free carrier concentrations between 10 and 10 cm due to either impurities, intrinsic defects, or intentionally introduced dopants. However, there are exceptions. Unpublished results from the author s laboratory show that Ti02 films deposited by spray deposition of high-purity TTIP (99.99%) dissolved in ethanol yield insulating films. A different situation... 

In principle, two fundamentally different methods can be applied to solve this task. The first one is determination of the residual concentrations of the fissile nuclides after irradiation and calculation of the burnup from the difference between final and initial values. For this purpose, the uranium and plutonium fraction has to be separated from the fission and activation products and from each other (e. g. by extraction chromatography) subsequently, the concentrations of the individual isotopes, in particular of the fissile isotopes, are analyzed by mass spectrometry. Well-established analytical techniques for performing such analyses are available, so that only small error margins are to be expected in the determination of the concentrations of the isotopes under consideration. However, there are two problems that can potentially cause systematic errors. The first one is the well-known question of the accuracy of results which have been obtained as a difference between two numbers, which limits the accuracy at lower burnup values in particular. The second problem is that the fissile nuclides are not only consumed by nuclear fission but by neutron capture as well in order to avoid systematic errors here, the capture-to-fission ratio valid for the particular irradiation conditions has to be taken into account in the calculation of depletion during irradiation. If one recalls the complicated buildup and decay mechanisms of actinide nuclides during reactor irradiation (see Fig. 3.5.), it is obvious that such correction requires complex calculations. On the other hand, the direct determination of the residual concentration of fissile nuclides is not influenced by errors due to inaccuracies in the fission yields of fission products to be measured nor by migration-induced inho-mogenities in the fuel. 

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