Electrode j-V Measurement

Enhancing the catalysis at the surface of PEC electrodes results in a lower kinetic overpotential and an increase in photocurrent. The effectiveness of the catalysts after surface treatment can be determined by utilizing three-electrode j-V measurements (see Section Three-Electrode j-V and Photocurrent Onset ) as well as IPCE measurements (see Chapter Incident Photon-to-Current Efficiency and Photocurrent Spectroscopy ). It may also useful to perform Mott-Schottky (see Section Mott-Schottky ) to determine any impacts these catalysts may have on the band structure (e.g., due to Eermi level pinning). 

It is important to determine the conductivity and flat-band potential ( ft) of a photoelectrode before carrying out any photoelectrochemical experiments. These properties help to elucidate the band structure of a semiconductor which ultimately determines its ability to drive efficient water splitting. Photoanodes (n-type conductivity) drive the oxygen evolution reaction (OER) at the electrode-electrolyte interface, while photocathodes (p-type conductivity) drive the hydrogen evolution reaction (HER). The conductivity type is determined from the direction of the shift in the open circuit potential upon illumination. Illuminating the electrode surface will shift the Fermi level of the bulk (measured potential) towards more anodic potentials for a p-type material and towards more cathodic potentials for a n-type material. The conductivity type is also used to determine the potential ranges for three-electrode j-V measurements (see section Three-Electrode J-V and Photocurrent Onset ) and type of suitable electrolyte solutions (see section Cell Setup and Connections for Three- and Two-Electrode Configurations ) used for the electrochemical analyses. 

An example of a 2-electrode j-V measurement and the corresponding ABPE plot is shown in Eig. 8.3 for several nanostructured Ti02 photoanodes. 

Fig. 6.10 Three-electrode j-V plot of a W03-based PEC electrode in dark black curve) and under AM 1.5 G illumination red curve). Measurement was performed in a 0.33 M H3PO4 electrolyte using a scan rate of 25 mV/s...

Day, T.M., Wilson, N.R., and Macpherson, J.V. (2004) Electrochemical and conductivity measurements of single-wall carbon nanotube network electrodes. Journal of the American Chemical Society, 126, 16724-16725. 

It is possible that the steep, steady-state current-voltage (/-F) curves frequently observed with PbNe and other azides prior to detonation are reverse currents at Schottky barriers, dominated by interface states. They do not appear to be single-injection currents because the same electrode that causes the steep, steady-state j-V dependences fails to produce the characteristic transient /-F-r dependences expected from the single-injection mechanism [21,22]. The reverse-bias current mechanism is also consistent with temperature-dependence measurements reported on AgNa [17] and with electrode material-dependence measurements reported on TIN3 and CuN [18]. It is essential to settle this point before one can use such measurements to establish initiation mechanisms. 

The integrated planar silver chloride elecfiode uses a thin layer of 150 jm polymer that consists of a heat curing epoxy resin poly-hydroxy-ethylmethacrylate (PHEMA) to immobilize the KCl elecfiolyte. The potential drift of the reference elecfiode reduced to 59 j,V/h after a conditioning phase of several hours. However, this reference electrode was only used for PO2 measurement, while an external reference elecfiode was used for pH measurement. 

The result of this analysis is a plot of photocurrent density as a function of the potential measured versus a reference electrode. An example j—V curve for a WO3 film is shown in Fig. 6.10. In this case, the photocurrent onset occurs at approximately 0.27 V versus SCE (0.54 V vs. RHE) which corresponds to a VonsecoER of 0.69 V (1.23-0.54 V). Since the onset of photocurrent does not occur cathodic of the reversible potential for HER ( er = —0.27 V versus SCE in this electrolyte), this electrode is unable to split water without an additional bias. At potentials more anodic than 1.2 V versus SCE, the photocurrent density saturates at 3.5 mA/cm. In addition to photocurrent, reverse bias dark current onsets at 1.65 V versus SCE due to shunting or breakdown as mentioned previously. 

The standard procedure for stability testing utilizes the 2-electrode short-circuit measurement detailed in Chapter 2-Electrode Short Circuit and j-v . See Section Cell Setup and Connections for 3- and 2- Electrode Configurations for a discussion on basic cell setup and electrolyte selection. 

J In each case, it is necessary to work above certain minimum values. When the trapping potential is less than 0.4 V, the measured rate constant is dependent upon the value of the potential and is too low. When the drift field is less than 0.35 V cm , calculated residence times in the resonance region disagree with those measured from linewidths. An important feature of the flat cell used in these measurements (1,27 cm spacing between drift electrodes) is that measured total ion currents are independent of magnetic field strength. 

Unlike photovoltaic cells, most photo-electrochemical systems do not operate without an external source. In a more general approach, the potential of the working electrode can be varied with respect to the equilibrium potential Veq (or to the potential of a RE) by means of an external voltage source (e.g. a potentiostat) connected between WE and CE [21, 24]. The current density (indicated here by j) is measured both in the dark and under illumination as a function of the applied potential V. This is described in more detail in Sect. 2.1.2.2. 

A bipolar PU electrode pair lead measures the local potential difference and is therefore actually also an electric field strength [V/m] probe. If the conductivity is known, it is also a current density probe since J = aE. Because the electric field is a vector field and the bipolar lead has an orientation being the length Lpu between the PU electrodes, the measured voltage is the dot product v = E-Lp = J-Lpu/ct- The larger the electrode area,... 

Figure 12.3 displays the model predicted j-V characteristic curve compared with the measured one, for an AT400 aluminum-air cell (Chan and Savinell, 1991). The AT400 design of the aluminum-air cell was built by the Eltech Systems Corporation, Chardin, Ohio decades ago (Chan and Savinell, 1991), with the cell gap (5 ) of 0.0014 m and electrode height (L) of 0.13 m. An observed deviation of less than 12% of the experimental data in Figure 12.3 falls within the error limits, and thus validates the present model. 

A mixed ionic electronic current 1 is sent through the cell which includes the MIEC and two reversible electrodes. The change in electrode mass is measured, which reflects the ionic charge transported, and this yields /j. The electronic component of the current is then obtained from 1 = 1- /j. From /j, 4, the Nemst voltage, V, and the apphed voltage V one obtains ... 

Source Values are compiled from the following sources Bard, A. J. Parsons, R. Jordon, J., eds. Standard Potentials in Aqueous Solutions. Dekker New York, 1985 Milazzo, G. Carol , S. Sharma, V. K. Tables of Standard Electrode Potentials. Wiley London, 1978 Swift, E. H. Butler, E. A. Quantitative Measurements and Chemical Equilibria. Freeman New York, 1972. 

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