Microstructure of polarized electrochemical vapor deposition (PEVD) products

Pergamon PII: S0968-4328(97)00062-0

MicronVol. 29, No. 4, pp. 251--259, 1998 '~ 1998 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0968~1328/98 $19.00+0.00

Microstructure of Polarized Electrochemical Vapor Deposition (PEVD) Products ERIC Z. TANG, DOUGLAS G. IVEY* and THOMAS H. ETSELL Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2G6, Canada (Received 13 June 1997; accepted 24 September 1997)

Abstract--Many types of electrocrystallization microstructures have been studied for depositions from aqueous solutions and molten salt electrolyte systems. However, our knowledge is still limited for solid electrolyte systems. This paper presents initial microstructural results of the product phases produced during polarized electrochemical vapor deposition (PEVD), which takes advantage of a solid state ionic material to transport the reacting species and utilizes electrochemical reaction of the ionic species with the vapor phase for deposition. The microstructure of PEVD products during various deposition conditions has ma!nly been characterized using scanning electron microscopy (SEM). In this study, a PEVD product (Na2CO3) was deposited on the cathode of a CO2 potentiometric sensor. Preferred growth and a faceted structure oriented along the porous Pt electrode surface were found. The unique electrocrystallization microstructure of the PEVD products is due to the availability of the reacting species for electrochemical reaction, electronic or ionic shorting at the substrate surface, and the preferred crystallographic directioni for ionic conduction in the product phase during both crystal nucleation and growth stages. The product microstructural studies indicate that PEVD is an attractive thin film deposition technique to improve the solid electrolyte/electrode contact in a number of solid state ionics devices, such as sensors and fuel cells. Furthermore, the studies in this paper also suggest that the microstructure of the PEVD products is related to the thermodynamic considerations for the electrochemical reaction in PEVD and electrical properties for the PEVD product phase. © 1998 Elsevier Science Ltd. All rights reserved Key words: PEVD, CVD, microstructure, electrocrystallization, facet, Na2CO3, solid state ionics, polarization, electrochemical reaction.

INTRODUCTION There are two types of conductivity in nature: electronic and electrolytic. The first does not involve material transport while the second does (Geller, 1981). In contrast to pure electronic conduction, ionic conduction is always accompanied by chemical redox reactions which occur at the electrode and solid electrolyte interface. Thus, it is a mixed physical and chemical phenomenon instead of a pure physical phenomenon. Around the turn of the century and shortly thereafter, the discovery of solid state ionics materials and certain developments in mathematical physics and physical chemistry led to the recent interest in the field of ionic transport in solids. Contrary to classical solids, an important aspect of solid state ionic (SSI) materials with high ionic mobility is the rapidity with which they can respond, at relatively low temperatures, to externally imposed conditions. Just as the electric potential gradient is the driving force in electronic conduction, the electrochemical potential gradient can be considered as the total driving force for the transport of charged species in SSI materials. The material transport process in SSI materials consists of both diffusion and conduction (Tillement, 1994), and has a range of applications (Rickert, 1982). Recently, controllable materials transport in SSI materials has been applied to modify chemical vapor deposition (CVD). Electrochemical vapor deposition (EVD) (Isenberg, 1981) and polarized electrochemical *Corresponding author.

vapor deposition (PEVD) (Tang et al., 1996a) are examples. Instead of transporting all reacting species through the vapor phase as in CVD processes, solid state ionic technologies are applied in this modification to transport some reacting species in the ionic state through SSI materials. Thus, electrochemical reactions rather than chemical reactions occur for depositing products. A typical PEVD system is schematically shown in Fig. 1. Basically, it takes advantage of a solid state ionic conductor, which can transport at least pne reacting species in the ionic state. Under an electrochemical potential gradient, created mainly by a d.ci bias potential, conducting ions accumulate at the interface between the solid electrolyte A and the blocking ]electrode B. Polarization occurs, and the chemical activities of the mobile ions at the interface increase. After reaching a certain level, an electrochemical reaction with the surrounding gas occurs to form a PEVD prod hact phase C. Utilization of a solid electrochemical cell is one of the most significant advantages of the PEVI) technique, since SSI materials can act as electrochemical transducers. In addition to the same controllabl~ parameters as in a CVD process, such as temperature, and vapor phase partial pressure, by connecting a simple external electrical circuit to a PEVD system, thermodynamic and transport quantities can be converted into easily and precisely measurable electrical quantities, e.g. voltages and currents. Due to the reactants supplied by solids as charged species, and the nature of the electrochemical reaction in 251

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aL

/

soliq

I dcsourceI Fig. 1. Schematicof polarized electrochemicalvapor deposition(PEVD).

PEVD, the nucleation and crystal growth behavior of MATERIALS AND METHODS the PEVD products is unique. Previously, many types of electrocrystallization microstructures have been studied Fig. 2(a,b) schematically show a typical PEVD profor depositions from aqueous solutions and molten salt cess setup for Na2CO 3 deposition at the cathode of a electrolyte systems. However, our knowledge is still CO 2 solid state electrochemical sensor. In this PEVD limited for solid electrolyte systems. The present under- system, a Na+-[]-alumina disc (Ceramatec, Inc.), 16 mm standing of solid state electrochemistry is largely due to in diameter and 5 mm in thickness, was used as the solid Wagner's electrochemical theory of tarnishing, as the electrolyte. Then, a screen printed platinum thick film concepts he initially elucidated have remained intact to (about 7/~m thick) was applied to one side of the disc. the present time (Wagner, 1933). However, the electro- On the other side of the Na+-13-alumina disc, a sodium chemical reactions in PEVD have distinct differences source Na2CO3 disc, with the same dimensions, was compared with gas-solid reactions, and the nucleation attached mechanically. Finally, the source and solid and crystal growth behavior in PEVD will be different electrolyte discs were sandwiched by two Pt meshes with from those predicted by the tarnishing theory for scale Pt leads. The sample system was then put into a furnace growth. In PEVD, materials transport in the electrode, under a constant flow rate of 30 sccm on both sides. Dry electrolyte and product phase affects product formation. air with 293 ppm CO 2 was used at the anode; dry air The conductivity mechanism in these materials imposes with 1% CO 2 was used at the cathode. By selecting NO some restrictions on the otherwise wide variety of poss- or SO2 in place of CO2, the PEVD setup can form ible combinations of defect chemical parameters. Prefer- NaNO 3 and Na2SO 4 phases at the cathode, respectively. ential motion of these defects in a certain direction is The operating temperature was kept in a range which caused either by a concentration gradient of the migrat- was high enough to ensure ionic conduction, but was ing species or by an electric field. On the other hand, the below the melting points of the discs. Two temperatures, solid electrochemical cell in a PEVD system provides a 625 K and 670 K, were selected for this study. A bias more direct way to study its deposition products, and potential of 5 V was applied by a steady d.c. source to the electrochemical reaction for product formation can set the Pt thick film as the cathode of the PEVD system. be monitored and controlled by the electrical current During the process, which lasted from 15 to 240 min per and applied potential, respectively. sample, current through the PEVD system was moniThe aim of this paper is to present initial microstruc- tored by a multimeter. Typically, the current varied from ture results of the products of the PEVD processes with an initial value of several mA's to a final value of tens of well-defined electrochemical reaction thermodynamic IzA's. Integration of the current over the deposition time and kinetic conditions. gives the value of the sodium ionic flux in a PEVD

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253

(a)

)le holder g

Pt mesh

d, PEVD cathode ~r working electrode

Pt lead, PEVD anode sensor reference electrode Gas tight disc

:k film

Na2CO3 source disc

,-alumina disc electrometer

(b)

! !

dc source

A +

-

ii

B ~/o

' .....

4

ammeter thermo(

gas out ' \

..... alumina tube --- cover

/- gas out

'

i

dry air with 293 ppm CO2

sample and

sample

holder

dry air with 1% CO2

Fig. 2. PEVD process setup for Na2CO 3 deposition at the cathode of a CO 2 potentiometric sensor: (a) PEVD sampl~ system; (b) PEVD facility.

process. According to Faraday's law, the ionic flux is related to the amount of products formed in a PEVD process. The details regarding electrochemical measurements in PEVD processes have been reported elsewhere (Tang et al., 1996b). The microstructures and the chemical composition of the product phases at the cathode were studied before and after each PEVD process by scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS) and X-ray diffraction (XRD). The SEM used was a Hitachi H-2700 SEM equipped with a Link eXL X-ray detector operated in windowless mode.

goes to the left at the anode. Consequently, Isodium ions and electrons are given up by the source b a2CO 3 disc. Sodium ions then travel through the Na ~-13-alumina electrolyte to the cathode under the ele,:trochemical gradient, while electrons are supplied by the external circuit to the cathode. Thus, the electroche nical potential of the ionic reacting species at the subs :rate surface is increased by the applied d.c. potentia 1. Recently, researchers also found that applying a d.c.]potential to polarize a solid electrochemical cell alters the work function eq~ of both electrodes at the subst[ate surfaces (Vayenas et al., 1992). As a result, the electrochemical reaction ( equation (1)) proceeds to the right, and the Na2CO 3 phase is formed at the cathode.

RESULTS AND DISCUSSIONS

PEVD product deposition

PEVD product nucleation

Fig. 3(a,b) show SEM secondary electron (SE) plan view images of the cathode before and after PEVD deposition, respectively. Fig. 4 shows an SEM SE crosssection image of the cathode after PEVD deposition. The mechanism of Na2CO 3 formation in the PEVD process is schematically shown in Fig. 5. Before the d.c. bias potential is applied, the PEVD electrochemical reaction +

CO2+~O2+2Na +2e- ¢~Na2CO 3

(1)

equilibrates at both the cathode and anode. Under a forward d.c. bias potential, this electrochemical reaction

As shown in Fig. 5, nucleation can only occur at the solid electrolyte, electrode and gas phase~hree-plaase boundary where all reacting species (CO 2, 02, Na+ and e-) are readily available. Consequently, depositions show selectivity in PEVD as indicated b~¢ Fig. 6, in which the PEVD product (Na2CO3) is only gteposited on a substrate at area A, where the Pt thickl film is connected to the external circuit to supply electrons for the electrochemical reaction. The electrode on the substrate surface in area B of Fig. 6 is discontinuouS, so that no electrons were supplied for electrochemicali reaction (1); hence, nucleation was hindered, and nO deposition Occurs.

E.Z. Tang et al.

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Fig. 3. SEM SE plan view images of the cathode before (a) and after (b) PEVD. Bars=5,um.

dc source

e

source electrolyte Na#O, Nat/~-alumina anode

2

cathode

Na+

2

k ~

/~-~-i~2+o2

CO2+O2

,

th~hase

l n°a

Na +. 1~ a l ~ i n a l /

~

-

Fig. 5. The mechanism of the PEVD process for Na2CO3 deposition at the cathode.

Fig. 4. SEM SE cross-sectionimage of the cathode after PEVD. (A) Na+-~-alumina solid electrolyte;(B) porous Pt thick film electrode; (C) PEVD product Na2CO3. Bar=5/~m.

PEVD product growth Surface shorted preferred growth Deposition in a PEVD process usually has preferred surface growth as indicated by the dotted line in Fig. 7.

The PEVD product, Na2CO 3 in this case, first wraps the electronic conducting surface with a thin layer, then uniformly increases in thickness to decrease the aspect ratio. Fig. 8(a-e) are SEM SE images showing the stepwise coverage of the PEVD product on the substrate. Fig. 8(f) is a cross-section image corresponding to the final coverage shown in the plan view image of Fig. 8(e). In order to give the prerequisites for understanding the mechanism of PEVD product growth, it is necessary to consider the transport processes of the charged reacting species in the PEVD product phase. During the crystal growth stage, the availability of reacting species for the electrochemical reaction also controls the growth of the PEVD products. Ionic reacting species and electrons must diffuse through the PEVD product phase to the surface to react with the reacting species in the vapor phase. However, as indicated in Fig. 7 there exists ionic

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Along the X direction, i.e. the surface of the ionic conducting material, the reaction limiting step is electron transport in the product phase. The growth distance, x, can be expressed as: x = ( VNa2CO3a eA Gt/F 2)1/2

(2)

a e is the average electronic conductivity ir the product phase, AG is the Gibbs free energy of the elc.ctrochemical reaction, Vya2co3 is the molar volume of qa2CO3, and t is time. Along the Y direction, i.e. the arface of the electronic conducting material, the reactior limiting step is sodium ion transport in the product phase. The growth distance, y, can be expressed as: y=(4/~)(VNa2CO3~ionAGt[F2)1/2

(3)

where Oion is the average ionic conductivity in the product phase. The ability of PEVD to deposit thin SSI materials can be expressed by the ratio of the Y to X dirdction growth length, y/x, which can be written as y x

Fig. 6. SEM SE image showing the selectivityof the PEVD process. (A) substrate area with continuous Pt thick film is covered with deposit; (B) substrate area with discontinuous Pt thick film is uncovered. Bar=10/~m.

Fig. 7. PEVD product growth behaviorunder surface shorting conditions.

1.265~/a-~a +

(4)

This ratio is only related to the ionic alad electronic conductivity of the PEVD product (Rapp and Shores, 1970). Thus, overgrowth along the electroni~ conducting surface indicates the ionic conductivity is[ significantly higher than the electronic conductivity ira the PEVD product NazCO 3 phase. Once the entire Pt electrode is covered, no electronic shorted surface exists. Thus, for an ioni~ conducting product phase, further growth in thickness has to involve diffusion of both ionic species and~lectrons to the surface to react with the gas phase, iThe growth rate depends on the thickness of the depc~sition layer. Because the increase in thickness is expected to follow a parabolic behavior, the thicker the produc t , the slower the growth rate. Thus, a dense and uniform coverage is expected to decrease the aspect ratio of toe surface as indicated by the dotted lines in Fig. 7. Thisbehavior has been verified by X-ray mapping of a mopnted crosssection sample in Fig. 9, in which the sodiqm elemental map indicates the NazCO 3 PEVD product ~corresponds to the densely dotted area in the schematiclin Fig. 7). Anisotropic growth

and electronic shorting along the solid electrolyte surface (X) and electrode surface (Y), respectively. Although the growth kinetics along those surface directions will still be parabolic, no perceptible open circuit emf will be expected over the growth distance, and charged species are driven by the concentration gradient (Ilschner-Gensch and Wagner, 1958). This is a unique case compared with common gas-solid reactions.

The crystal growth behavior in PEVD is not as simple as discussed in the previous section. The appearance of whiskers, blades and platelets in the reaction product introduces a further complication, partict~larly in the first period of growth before the shorted surface in the thin layer growth region is covered. Faceting of PEVD products usually occurs in this region as shown in Fig. 8(b). However, the faceted structure in PEVD usually disappears when the thickness of the PEVD product

z

Fig. 8. SEM SE images of the product Na2CO 3 on a cathode of a CO2 potentiometric sensor at various stages during a PEVD process, (a) Plan view image of a porous Pt thick film at the cathode before PEVD; (b) plan view image of the cathode when deposit just covers porous Pt thick film electrode; (c) plan view image showing the deposit step increase in thickness; (d) plan view image showing further increase in thickness to fill out pores; (e) plan view image showing the final coverage of the product at the cathode in this PEVD process; (f) cross-section image of the cathode showing the final coverage of the product. (A) Na+-J3-alumina solid electrolyte; (B) porous Pt thick film electrode; (C) PEVD product Na2CO 3 Bars= 1 ~tm.

Microstructure of PEVD products

Fig. 9. X-ray maps of a cathode cross-section sample after PEVD.

increases as shown in Fig. 8(c,d). Furthermore, the appearance of a faceted structure also depends on the PEVD process conditions, such as deposition temperature and applied d.c. potential. Generally, the higher the applied voltage and temperature, the more pronounced the faceting structure and the thicker the deposition layer before faceting disappears. Fig. 10(a,b) are plan view SEM SE images of two identical substrates which have undergone different deposition conditions. For the same amount of PEVD flux of 1.84 C deposited, the one deposited at a lower temperature (625 K) shows a smooth surface coverage, while the other, deposited at higher temperature (670 K), still exhibits a faceted structure. In gas-solid reactions, preferred orientation of reaction products is a common phenomenon because of the freedom loss due to ionosorption of the reacting species on the layer, surface reactions with the lattice defects connected with nucleation processes, or field transport phenomena within the deposition layer under certain crystal growth conditions (Ogumi et al., 1995). Orientation and faceting of the deposition products have also been found in EVD. For instance, tin-doped In20 3 deposited by EVD exhibited preferred orientation, with {110} planes oriented parallel to the surface (Enloe and Wirtz, 1986). When an yttria-stabilized zirconia (YSZ) film was created by EVD, its surface morphology was reported to be dependent on the fabricating temperature (Carolan and Michaels, 1990). At temperatures below 1348 K, facets with { 100} planes oriented parallel to the surface were formed. However, the reason for this behavior is unclear and still open to investigation. It is believed that the possible mechanisms for faceting in EVD might not arise solely from one kinetic process. Since electronic shorting along the surface of the solid electrode exists in a PEVD system, the mechanisms for faceting in PEVD are probably not as complicated as in

257

EVD and ordinary gas-solid reaction systems. The main reason for anisotropic growth in PEVD, we believe, is because the ionic conductivities of the ionic conducting phase are usually not isotropic, i.e. they vary with crystallographic directions. Generally speaking, the mechanisms for ionic and electronic condudtion in solids I are quite different. Ionic conduction is mainly related to crystal structure; electrical conduction is d~termined by the electronic bandgap, which depends more on the individual properties of the constituent ~ons (Heyne, 1981). Usually, ionic conductivity in solids[ is extremely sensitive to crystallographic considerations. Huggins (1978) pointed out that the dominant factor for the motion of mobile ions in solids relates to ~he geometry of the static part of the crystal structure arid the potential energy profiles along which the mobil~ ions move. Calculations have been made using the minlmum energy path model for the fluorite structure; the presence of a small potential energy barrier tunnel along !<001 > crystallographic directions was found. This crystallographic direction shows a much higher ionic conductivity than others. During initial crystal growth in a PEVD process, one crystallographic direction in the product phase may show a much higher ionic conductivity than others. A product crystal with this direction exactly along the electronic shorted surface would grow mucl~ faster than the others, resulting in evolutionary survivali as shown in Fig. 1l(a). This leads to an anisotropic facetitexture with preferred crystallographic orientation in the PEVD thinlayer growth region. As illustrated in Fig. I l(b), in the case of shorted surfaces along the porous Pt thick film, a crystal with a preferred ionic conducting crystallographic direction oriented exactly along this surface will grow faster than the others. This kind of preferred growth will be stopped when the crystal ~s unable to further accommodate the surface contour with this preferred crystallographic direction. Further growth will be along other less conducting crystallographic directions. Once the entire electronic conducting surface is covered by the PEVD product, further deposition has to increase the thickness of the deposited layer. Electrochemical reaction and growth at this stage are controlled by electrons diffusing through the PEVD product. The electronic conductivity is usually more uniform than the ionic conductivity along each crystallographic direction in PEVD products, making the growth ok the PEVD product more isotropic. The growth rate then depends on the thickness of the deposition layer again, and eventually the faceted structure will disappear and a uniform deposited layer is expected. The deposition temperature and applied dic. potential determine the degree and morphologies of lanisotropic growth in the thin-layer growth region. This indicates that the externally imposed conditions affect the ionic conductivity along the preferred crystallographic directions. For instance, higher temperatures increase the ionic conductivity along the preferred crystallographic direction as well as the ability of the crystal lattice to bend to accommodate the shorted surface with

258

E.Z. Tang et al.

Fig. 10. The effect of deposition temperature on faceted structure: (a) lower temperature (625 K); (b) higher temperature (670 K). A flux of 1.84 C is deposited on both samples. Bars=5/tm.

(a) v & tu l [u l B

0

A

X

(b) PEVD flux

V

1 Solid electrolyte

Solid eleelxode

Fae~ting crystals

Normalin~luets Fig. 11. Anisotropic growth of the PEVD product: (a) crystal (A) exhibits evolutionary survival in PEVD due to its preferred ionic conducting crystallographic direction [uvw] parallel to the electronic shorting surface; (b) the mechanism of forming faceted structure at a porous Pt thick film surface.

this direction. Thus, the faceting structure at higher deposition temperature becomes more prominent. The SEM SE image in Fig. 12 represents an extreme case in which the PEVD product (NaNO3) grows out of the pores of a porous Pt electrode along a preferred

crystallographic direction. In order to accommodate the shorted surface, the crystals bend almost 90 ° to continue to grow along the outer pores of the Pt thick film surface. Finally, induced stresses tear the crystals apart and create a 'flower-like' texture.

Microstructure of PEVD products

259

direction for ionic conduction in the product phase during both crystal nucleation and growth stages. REFERENCES

Fig. 12. SEM SE image of PEVD product (NaNO3) growth a~ the electrode surface under the condition in which ionic conduction along certain crystallographic directions is much higher than the others. Lattice bending along the electronic shorting surface causes the 'flower-like' texture. Bar---15/tm.

The anisotropic growth behavior of ionic conducting PEVD products is more easily studied on a long flat electronic shorted surface. Because of a limitation in the surface area, product crystals with their preferred ionic conducting crystallographic direction parallel to the electronic shorted surface will preferentially grow and squeeze out other crystals. This offers a unique method of studying the ionic conductivity of the product along crystallographic directions under well-defined thermodynamic and kinetic conditions in a PEVD process by the TEM diffraction methods (Tang et al., 1996c).

CONCLUSIONS For the PEVD product, Na2CO3, deposited on the cathode of a C O 2 potentiometric sensor, preferred growth and faceting oriented along the porous Pt electrode surface were found in this study. The unique electrocrystallization microstructure of the PEVD products is due to the availability of the reacting species for electrochemical reaction, electronic or ionic shorting at the substrate surface, and the preferred crystallographic

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