Solid electrochemical

Figure 7. Comparison of (a, solid) electrochemical and (b, dashed) UHV measurements of the H, coverage/potentiaI differential versus potential on Pt(lll).1.) cathodic sweep (25 mV/s) voltammogram in 0.3 M HF from Ref. 20, constant double layer capacity subtracted, b.) dB/d(A ) versus A plot derived from A versus B plot of Ref. 26. Potential scales aligned at zero coverage. Areas under curves correspond to a.) 0.67 and b.) 0.73 M per surface Pt atom.

Typically, the reference level for the solution redox potential is chosen to be the normal hydrogen electrode (NHE). Some tabnlations nse the saturated calomel electrode (SCE) as the reference level with the difference between these two scales well-known to be NHE = —0.2412 V versus SCE. The fundamental problem lies in the determination of the absolnte energy of the NHE relative to vacuum. Although a method to determine directly the absolute electrochemical potential of an NHE has not yet been described, a recent indirect measnrement has indicated that it is approximately 4.4 eV below the vacnum level. This value is often used to relate the solution electrochemical potential scale to the solid electrochemical potential scale. It provides the best approximation that is presently available to calculate the... 

The present availabihty of numerous types of solid electrolytes permits transport control of various kinds of mobile ionic species through those solid electrolytes in solid electrochemical cells, and permits electrochemical reactions to be carried out with the surrounding vapor phase to form products of interest. This interfacing of modem vapor deposition technology and solid state ionic technology has led to the recent development of polarized electrochemical vapor deposition (PEVD). PEVD has been applied to fabricate two types of solid state ionic devices, i.e., solid state potenfiometric sensors and solid oxide fuel cells. Investigations show that PEVD is the most suitable technique to improve the solid electrolyte/electrode contact and subsequently, the performance of these solid state ionic devices. 

In PEVD, an applied voltage is used to transport (A) through the substrate (E). Usually, (E) is an exclusive ionic conductor for (A ) or (A ). It serves as a solid electrolyte in a closed-circuit solid electrochemical cell, and is coimected to an external electrical circuit with a dc electrical source by two electronic conducting electrodes at the sink and source sides of (E). Consequently, only ionic carriers can be transported through (E) to (D). The electronic... 

A substrate (E) in a solid electrochemical cell with an external electric circuit connected from the source side by a counter electrode (C) and from the sink side by a working electrode (W),... 

The charged reactant for the sink electrochemical reaction is supplied by the solid electrochemical cell of a PEVD system. The solid phase (E) is an exclusive ionic conductor for (A +) or (A ), and serves as the solid electrolyte. (C) and (W) are solid electronic conducting phases, and contact (E) from both sides as counter and working electrodes, respectively. They coimect with the external electric circuit, which consists of a dc source and other possible measurement devices. Because the conductivity changes in nature from ionic to electronic at the electrode/electrolyte interfaces, the solid electrochemical cell in a PEVD system effectively separates the transport paths of ionic and electronic charged carriers... 

The role of the source (O) in a PEVD system is to provide a constant supply of the solid-state transported reactant (A) during a PEVD process. Theoretically, it can be either a solid, liquid or vapor phase, as long as it can supply the ionic reactant (A ) or (A ) to the solid electrolyte (E) and the electronic reactant (e) or (h) to the counter electrode (C) via a source side electrochemical reaction. Therefore, the source must be in intimate contact with both solid electrolyte (E) and counter electrode (C) for mass and charge transfer between the source and solid electrochemical cell at location I of Figure 3. Practically, it is preferable to fix the chemical potential at the source. Any gas or solid mixture which does not react with the cell components and establishes a constant chenfical potential of (A) is a suitable source. For instance, elemental (A) provides (A +) or (A ) according to the following reaction... 

This reaction does not have to be a thermodynamically favorable one, since an external dc electric potential is applied via the solid electrochemical cell in the PEVD system to drive the reaction in the desired direction. Thus, the activity of (A) at the reaction site is controlled by the applied dc electric potential. 

For convenience and simplicity, some assumptions are made. The solid electrolyte (E) is assumed to be an exclusive ionic conductor of mono-valent cation (A+). Two porous electroiuc conducting electrodes (C) and (W) are attached to the solid electrolyte (E) from the source and sink side, respectively. An external electric circuit with a dc source is coimected to the solid electrochemical cell via both electrodes. 

Due to conservation of charges, the total current / in any part of the solid electrochemical cell of the PEVD system is the same. 

Electrochemical Control and Monitoring of a PEVD Process Through the Solid Electrochemical Cell of a PEVD System... 

The most distinguishing feature of PEVD process control is made possible because of the solid electrochemical cell involved in a PEVD system. Compared with other vapor deposition techniques, utilization of solid electrochemical cells is one of the most significant advantages of the PEVD technique, since deposition process control and monitoring are easy to realize. By connecting several measurement devices to the external electrical circuit of a PEVD system, the reaction in a PEVD process is easily monitored and possibly controlled by the electrical current and applied potential, respectively. ... 

The current, I, in a PEVD process can be recorded simultaneously by an ammeter in the external circuit to reveal the kinetics of the PEVD reactions. As discussed in the last section, solid-state reactant (A) needs to be transported as a combination of ionic and electronic species from the source to the sink side through the solid electrochemical cell to participate in a PEVD reaction with vapor phase reactant (B). The PEVD reaction rate, and subsequent product (D) formation rate, v(t), can be expressed as... 

Although it is not as severe in PEVD systems as in aqueous electrochemical systems in which various kinds of mobile ions are present in the electrolytes, it should be pointed out that, in the presence of reactants at the sink electrode surface, other electrochemical reactions might also take place in parallel with the desired one at the sink side. If side reactions exist, usually such parallel reactions contributions to the measured current are not easy to quantify. If it is desired to use current to monitor the reaction and product formation in PEVD, side reactions should be eliminated or at least controlled. Fortunately, only one ionic species is usually mobile in a solid electrochemical cell because of the nature of the solid electrolyte. As long as the vapor phase is properly controlled, usually one electrode reaction is predominant over a wide range of PEVD applied potentials. Virtually 100% current efficiency for product formation can be expected. 

Sodium is selected as the solid state transported reactant in PEVD. This is because not only is Na" a component in the PEVD product phase Na COj, but also the mobile ionic species in the solid electrolyte (Na "-[3"-alumina) and in the auxiliary phase of the sensor. Thus, PEVD can take advantage of the solid electrochemical cell (substrate) of the sensor to transport one reactant (sodium) across the substrate under an electrochemical potential gradient. This gradient... 

This anodic reaction provides sodium ions and electrons to the solid electrolyte and the inert Pt counter electrode, respectively, at the source side. Both the sodium ions and electrons will then travel through the solid electrochemical cell along previously-mentioned ionic and electronic paths to sustain the PEVD cathodic reaction for Na COj product formation at the sink side. Eurthermore, based on anodic reaction 60, the chemical potential of sodium is fixed by the vapor phase at the source side. Under open circuit conditions, this type of source can also serve as the reference electrode for a CO potentiometric sensor. 

The potential profiles in this PEVD system are illustrated in Figure 17. Although there is no driving force due to a difference in the chemical potential of sodium in the current PEVD system, the applied dc potential provides the thermodynamic driving force for the overall cell reaction (62). Consequently, electrical energy is transferred in this particular PEVD system to move Na COj from the anode to the cathode of the solid electrochemical cell by two half-cell electrochemical reactions. In short, this PEVD process can be used to deposit Na CO at the working electrode of a potentiometric CO sensor. 

The PEVD process takes advantage of the solid electrochemical cell of an SOFC. Oxygen is chosen to be the solid state transported reactant. At the source side (the cathode of the SOFC), oxygen in the source gas phase is reduced to oxygen anions (O ) through a cathodic reaction... 

Utilization of solid electrochemical cells is one of the most significant advantages of the PEVD technique, since deposition process control and monitoring are easy to realize. 

The PEVD sample utilized in this investigation is a solid electrochemical cell with a ytterbia and yttria stabilized zirconia pellet (8%Yb303-6%Y303-Zr03) as the solid electrolyte to conduct oxygen anions from the source to the sink side. A commercially available Pt thick film paste was screen printed on the center of both surfaces of the solid electrolyte disk. Two Pt meshes, with spot welded Pt leads,... 

The technique of cyclic voltammetry or, more precisely, linear potential sweep chronoamperometry, is used routinely in aqueous electrochemistry to study the mechanisms of electrochemical reactions. Currently, cyclic voltammetry has become a very popular technique for initial electrochemical studies of new systems and has proven very useful in obtaining information about fairly complicated electrochemical reactions. There have been some reported applications of cyclic voltammetry for solid electrochemical systems. It is worth pointing out that, although the theory of cyclic voltammetry originally developed by Sevick, ° Randles, Delahay, ° and Srinivasan and Gileadi" and lucidly presented by Bard and Faulkner, is very well established and understood in aqueous electrochemistry, one must be cautious when applying this theory to solid electrolyte systems of the type described here, as some non-trivial refinements may be necessary. 

In principle, a whole range of applications can be envisaged using these solid electrochemical cells in situations where cells containing liquids could not function, e.g. some fuel cells. 

Thermodynamic data in the Th-C system were obtained from measurements on solid electrochemical cells of the following types, at 1073 to 1273 K ... 

Solid electrochemical cells are more restricted in the structure because of the solid nature of the ionic conductor. Schematic configurations of solid-state cells are presented in Fig. 2(a-c). Figure 2(a) shows an all-solid cell with the SE, SE(M ),... 

K. Kamada, M. Tokutomi, N. Enomoto, J. Hojo, Electrochemical micromachining using a solid electrochemical reaction at the metal/p "- AI2O3 microcontact, Electrochim. Acta 52 (2007) 3739-3745. 

---