Potentiostatic Pulse

PULSE TECHNIQUES WITHOUT TRAPPING 1. Potentiostatic Pulse 

Pulse methods may be applied to both foil and bulk specimens provided that a suitable mathematical model is used in the data analysis. However, most applications of the potentiostatic pulse technique have involved just one surface of a metal specimen. The technique is suitable for bulk specimens since only a single surface need be exposed to the electrolyte, so it offers a practical advantage over permeation methods in that pinholes and sealing problems associated with thin membranes are avoided. Also, diffusion in such specimens can be treated in terms of a semi-infinite boundary condition, which is mathematically appealing. 

The principle of the technique is shown schematically in Fig. 6. The metal is charged with hydrogen at a constant potential Ec for a time after which the potential is then stepped to a more positive value As a result, H atoms diffuse back to the same surface and are reoxidized, resulting in an anodic current transient. The pulse technique was used by Bockris and co-workers to 

Solving Pick s second law subject to these conditions yields 

At the end of the charging pulse, the hydrogen coverage and C(0, t) are assumed to drop instantaneously to zero and the concentration profile exhibits a maximum as shown in Fig. 7. Although the hydrogen atoms eventually diffuse back to the input surface, some will temporarily continue to diffuse into the metal 

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The electrical double-layer structure of a Pt/DMSO interface has been investigated using the potentiostatic pulse method.805 The value of C at E = const, as well as the potential of the diffuse layer minimum, have been found to depend on time, and this has been explained by the chemisorption of DMSO dipoles on the Pt surface, whose strength depends on time. Eg=Q has been found11 at E = -0.64 V (SCE in H2O). 

In this method, which was proposed in 1957 by Geoffrey C. Barker, a series of potentiostatic pulses of increasing amplitude (Fig. 23.6a) are applied to the electrode. Between pulses the electrode is at a potential where there is no reaction during... 

FIGURE 23.6 (a) Sequence of potentiostatic pulses of increasing amplitude (b) linear... 

The method of potentiostatic pulses is sometimes combined with the DME (called pulse polarography). hi this case the pulse frequency should match the drop frequency, where each pulse is used at a definite time in the drop life, hi Barker s method, large pulse amphrndes are used. Other versions of the potentiostatic pulse technique are square-wave and staircase voltammetry here smaU-amphtude pulses are used. 

In the following, a general model [3.94] including formation of an expanded 2D Meads phase on a homogeneous substrate as well as a first order phase transition leading to a condensed 2D Meads phase is discussed for potentiostatic pulse polarization experiments. In this treatment, surface diffusion of Meads is neglected. 

First, Harrison et al. [3.36] studied Me UPD in the systems Ag(lll)/Pb, Cf, Ag(polycrystalline)/Tl, Cf, and Ag (polycrystalline)/Pb, acetate by cyclic voltammetry and potentiostatic pulse measurements. The authors claimed that a non-monotonous current transient represents a necessary criterion for 2D nucleation and growth involved in the 2D Meads overlayer formation. However, the experimental results presented did not give evidence for a first order phase transition. 

Bewick and Thomas [3.110-3.114, 3.270] measured electrochemically and by optical means different Me UPD systems Ag(A 0/Pb, H, ClOd", acetate and citrate, CnQikt)/ h H C104, acetate, and AgQikt)m SOd with Qikt) = (111), (100), and (110). Potentiostatic pulse measurements showed non-monotonous current transients for Ag(lll) substrates which are attributed to a first order phase transition. As an example, a current transient in the system Pig hkt)/Vf, H, SOd is shown in Fig. 3.46. In the case of Ag(lOO) and Ag(llO) substrates, higher order phase transitions were supposed. Clear evidence of a participation of 2D nucleation and growth steps in the 2D Meads phase formation process was found in the system Cu(lll)/Pb H", ClOd", acetate [3.270]. Non-monotonous current transients and a discontinuity in the q(lsE,fi) isotherm were observed (Fig. 3.13). 

Figure 3.46 Current density transient from a potentiostatic pulse experiment in the system Ag(lll)/ 5 X 10-3 M TI2SO4 + 5 X lO l M Na2S04 + lO M HCIO4 at T = 298 K [3.110], Initial and final underpotentials AJEi = 550 mV, A f = 10 mV.

Figure 3.49 Current density transients from potentiostatic pulse experiments in the system Ag(lll)/3 X 10 3 M Pb(CH3COO)2 + 5 x Ifrl M NaC104 + lO M Na2H-citrate at 7= 298 K [3.94]. Initial and final underpotentials AEj/mV = 180, AEf/mV = 124 (1) 122 (2) 120 (3).

Information about the influence of 2D UPD phases on thermodynamics and kinetics of subsequent 3D Me nucleation and growth can be obtained by UPD-OPD transition experiments. In general, the experiment has two stages. In the initial stage i, a 2D Me UPD phase is formed and eventually equilibrated at a selected underpotential AE. The final stage f of the system is characterized by an external potentiostatic pulse to t]f into the OPD range. There are two possibilities for pulse excitation techniques potentiostatic or galvanostatic conditions. 

Potentiostatic pulse polarization is recommended for UPD-OPD transition experiments since both undersaturation and supersaturation are held constant in the initial and final states ... 

Pulse metal deposition offers the possibility of producing alloy layers as well as multilayered systems. If, e.g., the metal ion discharge of the components of an alloy follows different Tafel characteristics, a defined alloy composition can be achieved by an appropriate potentiostatic pulse routine as shown for the deposition of brass, bronze, and other binary and ternary alloys within a wide compositional range. Furthermore, alloys with changing composition gradients, which are of high technical interest, can be produced. 

Potentiostatic pulses (max, eight pulses) Position, electrochemical technique (pulse), delay time before pulses (s), number of cycles, potential 1 (mV), pulse time 1 (ms),... Optional ..., potential 2 (mV), pulse time 2 (ms),..., potential 8 (mV), pulse time 8 (ms)... 

A potentiostatic pulse profile to generate H+ was applied to two microelectrodes in the electrochemical robotic system causing the precipitation of the polymer on a suitable substrate (Fig. 14.29a). Parallel localized deposition of enzyme spots was achieved (Fig. 14.29b). This approach was a first step toward systematic improvement of biosensor microstructures within the electrochemical robotic system. 

Figure 1.21 Double logarithmic plot of current density and time t for a potentiostatic pulse of from 0 to 4 V (SHE) on four different grains with different orientations. Microelectrode diameter was 100 pm.

Figure 1. Potentiostatic (pulse) current-time curves at hanging Hg-drop electrode. Solid line = 10 M Cd in 1 M KNOj dotted line = charging current from 1 M KNOj electrolyte.

A theoretical analysis has been carried out for galvanostatic and potentiostatic pulse regimes [27]. The idea that developed is a bit the same as backflushing with pressure driven-membrane operation such as microfiltration or ultrafiltration. The time dependencies of the extent of the concentration polarization near the membrane surface during the pulse are described theoretically for both pulse regimes and a qualitative discussion of the pause duration is presented. The main characteristic of the non-stationary process is the transition time between the state without polarization and the state with stationary polarization. 

Fig.7. In-situ STM images of copper potentiostatic pulse plating on gold. Electrolyte 0.001 M CUSO4 and 0.05 M H2SO4 in Millipore water, (a) Clean surface, d = 9 mn. (b) Ten pulses of 0/-100 mV, each 500 ms duration (stripes), d— 44 nm. (c)Copper crystallites created by the process, E=0 mV, Et = 42 mV, d = 29 mn. It = 4.2 nA.

Figure 7. Effect of mercury on load capability of 0.2 mm Zn powder electrode. Potentiostatic pulse, charge of potential versus Hg/HgO from 1.380 V to 1.363 V (10 M KOH/0.1 M ZnO).

The analytical model originally developed for the potentiostatic pulse technique applied only to pure diffusion control and did not take trapping into account [41]. 

Fig. 9 Potential profile and current response for the potentiostatic pulse technique [106]. (Reprinted from Acta Metall., Copyright 1987, with permission from Elsevier Science.)...

A good insight into the anodic oxide formation is gained from potentiostatic pulse measurements. Figure 19 shows current transients i t) of anodic oxide formation on aluminum at pH = 6.0. Various potential steps from 0 V (hess) were chosen to an oxide formation potential between 3.3 and 5.9 V [77]. This corresponds to an increase in field strength from 6.6 to 10.1 MVcm . The initial film thickness of 7.4 nm is given by a prepolarization to 3V (hess). Each experiment must be performed on a different sample with respect to the irreversible... 

Fig. 19 Current transients of potentiostatic pulse experiments on aluminum at pH = 5.0 covered by an initial film thickness of 7.4 nm. The influence of the applied field strength on the time to current maximum is clearly seen. The field strengths ranges from 10.1 MVcm down to 5.5 MVcm . The current density in the overshoot decreases monotonically with decreasing field strength.

Potentiostatic pulse experiments yield a different, good example. Starting from... 

For the study of diffusion phenomena in solids it is also possible to work with potential pulses (potentiostatic pulses) or with constant current pulses (galvanostatic pulses). Examples described in the following paragraphs are based on the coulometric titration method described in Chapter 3. Weppner and Huggins reviewed these methods.In a continuous series of pulses the concentration of lithium in a sheet of aluminum is increased. The diffusion in each pulse is followed by either potential or current measurements. 

Figure 5.17 Potentiostatic pulses of lithium deposition into an aluminum alloy electrode i = (limiting law for t r /D, Cottrell equation, after John Wen et al.), radius of the aluminum wire r = 0.9 mm, charge passed during pulse Q = 1 Al C, 415 °C. (Reproduced with permission from Ref. [13], 1979, The Electrochemical Society.)...

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