Electrochemical shift

This is called electrochemical shift and simply stems from the fact that the Fermi level of the reference electrode is not equal to that of the working electrode and thus to the Fermi level of the detector. Furthermore if one changes UWr via a potentiostat the core level electron binding energies of species associated with the reference electrode will shift according to Eq. (5.66), i.e. the XPS analyzer acts also as a (very expensive) voltmeter. 

The working electrode, assuming it is the electrode under observation, should preferably be grounded. If the reference electrode is grounded instead, one should be constantly aware of the above electrochemical shifts. 

Figure 5.37. Transient effect of constant current application and interruption on the Pt/YSZ catalyst potential UWr and on the XPS signal at Eb = 528.8 eV (location of 8-0 Is peak) and at Eb = 181.7 eV (electrochemically shifted position of the Zr 3d5/2 peak).6 Reprinted with permission from the American Chemical Society.

It is interesting to notice again the merging of the chemical and of the electrochemical shift upon anodic (AUWR=1.1 V) polarization (Fig. 5.39c).67... 

The origin of the electrochemical shift is not well understood yet. It is very likely that the phenomenon can be explained on the basis of known effects contributing to the chemical shift (see Section 3.2.5). It is clear, however, as long as the effect is not understood, interpretation of binding energies of weakly adsorbed electrochemical species will be difficult. 

When a redox-active transition metal is used as the signalling imit of a receptor, anion binding is coupled to electron transfer, i.e. anion binding changes the redox potential (couple) of the transition metal. This electrochemical shift can be represented as A, the difference in redox potentials between the receptor anion complex and the receptor alone. Concomitantly, electron transfer at the redox centre also changes the affinity of the receptor for the guest species. These coupled processes are linked thermodynamically by Eq. 1, where Kred and Kox are the stability constants of the reduced and oxidised forms of the receptor anion complex respectively [7]. 

Fe electrodes with electrochemically polished (cathodically pretreated for 1 hr) and renewed surfaces have been investigated in H20 + KF and H20 + Na2S04 by Rybalka et al.721,m by impedance. A diffuse-layer minimum was observed at E = -0.94 V (SCE) in a dilute solution of Na2S04 (Table 19). In dilute KC1 solutions E,njn was shifted 40 to 60 mV toward more negative potentials. The adsorbability of organic compounds (1-pentanol, 1-hexanol, cyclohexanol, diphenylamine) at the Fe electrode was very small, which has been explained in terms of the higher hydro-philicity of Fe compared with Hg and Hg-like metals. 

Experimental results corroborate that shifts of 1.2 eV are always present if any of the variables acting on the electrochemical process are changed the solvent, the salt, or the temperature of work. We cannot attribute the observed shift to solvatochromic, counter-ion-chromic, or thermochromic effects taking place inside the film during oxidation-reduction processes. So, as predicted, these shifts are a consequence of the way the chains store or relax energy through conformational changes stimulated by electrochemical oxidation or reduction, respectively. 

As with alternating electrical currents, phase-sensitive measurements are also possible with microwave radiation. The easiest method consists of measuring phase-shifted microwave signals via a lock-in technique by modulating the electrode potential. Such a technique, which measures the phase shift between the potential and the microwave signal, will give specific (e.g., kinetic) information on the system (see later discussion). However, it should not be taken as the equivalent of impedance measurements with microwaves. As in electrochemical impedance measurements,... 

Electrochemical impedance spectroscopy leads to information on surface states and representative circuits of electrode/electrolyte interfaces. Here, the measurement technique involves potential modulation and the detection of phase shifts with respect to the generated current. The driving force in a microwave measurement is the microwave power, which is proportional to E2 (E = electrical microwave field). Therefore, for a microwave impedance measurement, the microwave power P has to be modulated to observe a phase shift with respect to the flux, the transmitted or reflected microwave power APIP. Phase-sensitive microwave conductivity (impedance) measurements, again provided that a reliable theory is available for combining them with an electrochemical impedance measurement, should lead to information on the kinetics of surface states and defects and the polarizability of surface states, and may lead to more reliable information on real representative circuits of electrodes. We suspect that representative electrical circuits for electrode/electrolyte interfaces may become directly determinable by combining phase-sensitive electrical and microwave conductivity measurements. However, up to now, in this early stage of development of microwave electrochemistry, only comparatively simple measurements can be evaluated. 

Figure 5.36. Effect of electrochemical O2 pumping on the Zr 3dj XPS spectra of Pt/YSZ at 400°C (a) Zr 3d5/2 spectrum shift from AUWr=0 (solid curve) to AUwr=1. 2 V (dashed curve) (b) effect of overpotential AUv/r on the binding energy, Eb) and kinetic energy, (AEk--AEb) shifts of Zr 3dS/2 (filled circles, working electrode grounded) and Pt 4f7/2 (open circle, reference electrode grounded).6 Reprinted with permission from the American Chemical Society.

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