Electrochemical Pumping

Investigations carried out on membrane reactors based on pure oxide ion-conducting membranes have been reviewed [12]. Compared to the M lEC-membrane reactor, SEMRs allow for direct control of the oxygen permeation rate due to the faradaic coupling of oxygen flux and cell current. The SEMR has advantages over conventional catalytic reactors [3, 5], including  

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O.A. Mar ina, V.A. Sobyanin, V.D. Belyaev, and V.N. Parmon, The effect of electrochemical pumping of oxygen on catalytic behaviour of metal electrodes in methane oxidation, in New Aspects of Spillover Effect in Catalysis for Development of Highly Active Catalysts, Stud. Surf. Sci. Catal. 77 (T. Inui, K. Fujimoto, T. Uchijima,... 

Figure 5.39. Characterization of the spillover species by photoelectron spectra of the Ols region taken from a 0.02 pm2 spot on the Pt surface (a) The residual O Is spectrum after the cleaning cycles (b) The Ols spectrum measured in 02 atmosphere (pO2=lxI0 6 mbar) (c) The Ols spectrum obtained during electrochemical pumping in vacuum with UWr = 1.1 V. R1 and R2 are the components which are formed by adsorption from the gas phase and by electrochemical pumping. The fitting components of the residual oxygen are shown with dashed lines. Photon energy = 643.2 eV, T 350-400°C.67 Reprinted with permission from Elsevier Science.

Figure 5.46. Local brightness variations in the three windows marked in Fig. 5.45 during electrochemical pumping at T = 695 K.28 Reprinted with permission from WILEY-VCH.

For both reactions studied, NO+CO and NO+propene, the effect of electrochemically pumped Na in increasing the extent of NO dissociation is large and significant. This is because unpromoted low index planes of Pt, Pt(lll), are relatively inert towards NO dissociation and we adscribe the NO dissociation as the key reaction-initiating step. Such dissociation of diatomic molecules in the field of coadsorbed cations has been discussed in detail by Lang et al [29], The rates of production of CO2. N2 and NjO all depend on... 

There is a possibility, in principle, of developing the inverse process namely, the emission of coherent light stimulated by an electrochemical reaction. This process could form the basis for a new type of laser with electrochemical pumping. 

By far the most important practical use of this sensor is for automotive applications, namely for the control of the air to fuel ratio. It compares favorably with the surface conductivity or high temperature potentiometric sensor (Logothetis, 1987). Other gases could be detected on the same principle provided that the right materials for the electrochemical pump were used. The electrode materials/solid electrolytes used for the construction of potentiometric high temperature sensors (see Table 6.7) could serve as guidance. 

Another type of high temperature solid state O2 sensor that has been developed is based on the principle of electrochemical pumping of oxygen with Zr02 electrolytes. These sensors have higher sensitivity (generally, a first power dependence on Pq) than the Nernst cell and the resistive device and possess a number of other characteristics that make them very promising for many new applications. 

Xie et al. [20] reported the fabrication chip for pumps and an electrospray nozzle. The process used to fabricate the electrochemical pump chips with electrospray nozzle is shown in Fig. 2.11. A 1.5 xm layer of Si02 was grown on the surface of a 4 inch silicon wafer by thermal oxidation. The front side oxide layer was patterned and removed with buffered FIF. XeF2 gaseous etching was used to roughen the silicon surface in order to promote the adhesion between subsequent layers and the substrate. The first 4.5 p,m parylene layer was deposited. 

Figure 2.11 Schematic representation of fabrication process for electrochemical pumps on a silicon substrate [20].

Xie et al. [9] reported on-chip generation of gradient elution by using electrochemical pumping in LC-ESI, which contains two 3 p-L on-chip solvent reservoirs, two electrochemical pumps, and an ESI nozzle. Solvent pumping was controlled galvanostatically at a flow rate of 200 nL/min. The authors reported a higher electrical current was used to increase flow rate or to achieve... 

Bohm, S., Olthuis, W., Bergveld, P., An integrated micromachined electrochemical pump and dosing system. Biomed. Microdevices 1998, 1(2), 121-130. 

We will begin with a description of electrochemical sensors or more specifically composition sensors based on electrochemical principles (i.e., we refer to an electrochemical detection of composition). Another group of applications refers to devices in which the transference of mass and charge is used primarily to change composition or produce chemicals (electrochemical pumps and electrochemical reactors, or electrochemical filters) we will term such devices composition actors. At the end we will discuss energy conversion and storage devices (which we do not subsume under the term composition actors as here the energy aspect is to the fore). 

Electrochemical pumps have also been successfully used to promote reactions catalytically. If oxygen is pumped through a zirconia cell into a reaction chamber which is, e.g., filled with hydrocarbons, not only the oxygen that is transferred, reacts. The anode itself can act catalytically. It seems that this so-called NEMCA effect ( Nonfaradaic Electrochemical Modification of Catalytic Activity 51) relies on a hindered surface reaction as a consequence of which the applied potential is translated into a concentration polarization, as reflected by the enrichment of a not fully oxidized oxygen species (e.g., O or O2") at the interface.51"53 For this species, redistribution equilibrium may be assumed that... 

In any case, the fundamental problem to be solved also for this kind of membrane is to develop technologies capable of producing, with relatively low costs, very thin membranes in line with the work presented in Ref. 27. This would allow reduced power consumption in electrochemical pumping applications, or increased energy conversion efficiency in fuel-cell applications [76,101]. 

Electrochemical pumps and compressors that is, devices for dosage, separation, compression or removal of oxygen (or hydrogen), according to Faraday s law. The set-up devoted to separate oxygen from air has been also referred to as either the ion transport membrane (ITM) or solid electrolyte oxygen separation (SEOS) [6]. 

These devices are widely used for the control and monitoring of oxygen content in gas mixtures or liquid metals. Usually, the electrochemical pump and oxygen sensor are based on YSZ tubes with platinum electrodes. 

Solid-state electrochemistry is an important and rapidly developing scientific field that integrates many aspects of classical electrochemical science and engineering, materials science, solid-state chemistry and physics, heterogeneous catalysis, and other areas of physical chemistry. This field comprises - but is not limited to - the electrochemistry of solid materials, the thermodynamics and kinetics of electrochemical reactions involving at least one solid phase, and also the transport of ions and electrons in solids and interactions between solid, liquid and/or gaseous phases, whenever these processes are essentially determined by the properties of solids and are relevant to the electrochemical reactions. The range of applications includes many types of batteries and fuel cells, a variety of sensors and analytical appliances, electrochemical pumps and compressors, ceramic membranes with ionic or mixed ionic-electronic conductivity, solid-state electrolyzers and electrocatalytic reactors, the synthesis of new materials with improved properties and corrosion protection, supercapacitors, and electrochromic and memory devices. 

Conventional heterogeneous metal catalysts are commonly enhanced by the addition of so-called promoter species that are used to modify intrinsic metal surface chemistry with respect to activity and/or selectivity. Electrochemical promotion (EP) provides an in situ, reversible and efficacious means of catalyst promotion and it allows for a systematic study of the role of promoters in heterogeneous catalysis. EP studies relevant to the three-way catalytic chemistry i.e., control of automotive CO, NO and hydrocarbons emissions, demonstrate that major enhaneements in activity of Pt catalyst supported on a"-Al203 (a sodium ion conductor) are possible when Na is electrochemically pumped to the catalyst surface. In the case of the important reactions involving NO reduction by CO or by hydrocarbons, major enhancements in selectivity towards N2 (from 15 to 70%) have been also achieved. The promotional effect of Na is due to enhanced NO chemisorpion and pronounced NO dissociation on the Pt surface. 

The behaviour of these systems may be quantitatively rationalised in terms of changes in adsorption energies (and therefore reaction activation energies) caused by changes in catalyst work function which result from backspillover of electrochemically pumped ions from the solid electrolyte to the active metal component [8]. The Electrochemical Promotion literature has been renewed recently [8]. 

Figure 4. Na Is XPS. (a) Showing effect of catalyst potential in pumping sodium to/from the Pt film under UHV conditions at 600K. (b) Illustrating electrochemical pumping of vacuum-deposited Na away from the surface under UHV conditions at sufficiently high temperature and positive catalyst potential.

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