Applied Chemistry / Nanotechnology/ Glow Discharge
My research work is on glow discharge, which can be regarded as simulation of lighting on primitive hydrosphere.
Contact glow discharge electrolysis is a novel electrolysis in which a plasma is sustained by do glow discharges between an electrode and the electrolyte surrounding it. It develops spontaneously during normal electrolysis at sufficiently high voltages and is characterized by a stable gaseous sheathing around the anode or the cathode across which glow discharges occur. The chemical results of CGDE are remarkable: the products are novel for ordinary electrolysis and their yields exceed greatly the Faraday law value.
Indian Journal of Chemistry,Vol.34A, June 1995. pp, 459-461
Chemical effects of anodic contact glow discharge electrolysis in aqueous formic acid solutions: Formation of oxalic acid
Susanta K Sen. Gupta*. Rajeshwar Sing & Ashok K. Srivastava
Department of Chemistry, Faculty of Science, Banaras Hindu
University, Varanasi, 221 005
Received 31 August 1994; revised and accepted 6 January 1995
A quantitative study on the liquid phase chemical effects of anodic contact glow discharge electrolysis (CGDE) in aqueous formic acid solutions shows that oxalic acid as well as hydrogen peroxide are formed in quantities significantly in excess of the Faraday law value. Results are discussed in the light of Hickling’s radiolytic mechanism.
Contact glow discharge electrolysis (CGDE), where plasma is sustained by dc glow discharges between one of the electrodes and the surface of the liquid electrolyte around it, provides a potential reaction medium for obtaining yields several times the Faraday law value1-7. Quantitative studies on chemical effects of CGDE in K2SO4 solutions led to the view that there are two reaction zones, the liquid electrolyte near the plasma and the plasma around the glow discharge electrode, and that the major reaction zone is the former for anodic CGDE, and the latter for cathodic CGDE3. Moreover, the body of evidence obtained for anodic CGDE favours strongly the idea that positive gaseous ions emerging from the plasma are energized enough to fragment the solvent molecules in the liquid near it into radicals which trigger off further reactions in the liquid phase leading to non-faradaic yields1-3, 5,6,8. In fact, the potentiality of anodic CGDE for generating radicals in high local concentrations was explored for synthesis of compounds such as amino acids9 and N, N-dimethylaminoacetonitrile10. Chemical effects of anodic CGDE of aqueous solutions containing simple organic compounds, which could react with H and OH radicals, would obviously be of interest to investigate. This note is a report of the chemical results of anodic CGDE of formic acid solutions.
Chemical effects of anodic CGDE of formic acid solutions were studied in a H-type Corning cell with electrode compartments divided by a G-5 glass sinter and having the arrangement for collecting the electrolyzed solutions and the electrode gases from each compartment as described in a previous publication3, at an applied voltage of 450 V under the following standard conditions : anolyte, varying concentrations (0-0.6 M) of formic acid in 0.05 M K2SO4; catholyte, 0.05 M K2SO4; anode, Pt wire (length 5 mm, dia. 0.35 mm); cathode, Pt foil (1 cm x 1 cm, 0.2 mm thick); ambient temperature, 85 ± 2°C; and pressure, atmospheric. Under these conditions, fully-grown CGDE is instantly produced at the anode after switching on the power. Current was supplied from an Aplab 7322 medium voltage D.C. Power supply. Current through the cell and voltage across it were measured on a Phillips PM 2518X digital multimeter. Quantities of electricity passed during anodic CGDE were measured by a precalibrated ESC 640 digital coulometer.
Anodic CGDE of formic acid solutions was found to produce H2O2 and oxalic acid in the anolyte and O2, H2, CO and CO2 in the anode gas. Cathodic yield, on the other hand, was the Faraday law value of H2. Anolyte solutions (in 20.0 ml aliquots) were estimated for H2O2 plus oxalic acid using a pre-standardized 7.7 X 10 -3 M ceric sulphate solution. And for H2O2 only by iodometry using a standardized 19.7 X 10 -3 M sodium thiosulphate solution. Anode gas mixture was analyzed in a precision-model Haldane-type gas analysis apparatus following the standard procedure.
Results and discussion
Yields (in moles) of hydrogen peroxide and oxalic acid, the solution-phase products of anodic CGDE of aqueous formic acid, increased with increase in quantities of electricity passed. However, the variation is not linear throughout the electrolysis. The differential yield of a product (X), G (X) which is conveniently represented by a dimensionless quantity3 as the ratio of the yield observed (in mol per mol electron) at a particular point of electrolysis to the yield stipulated by Faraday’s law is maximum in the beginning and then continues to fall until the integral yield reaches a stationary value. The pattern was found typical for both H2O2 and H2C2O4 atleast over the concentration range (0-0.6 M) of formic acid solutions employed in the study. The nature of the yield curves suggests strongly that the primary species generated by
Table 1 – Values of G0 (H2O2) and G0 (H2C2O4) of anodic
CGDE in 0.05 M K2SO4 containing varying concentrations of
HCOOH (0.1- 0.6 M)
[HCOOH] G0(H2O2) G0(H2C 2O4)
(M) (± 0.2) (± 0.2)
Nil 3.8 —
0.1 3.0 0.7
0.2 2.4 1.0
0.3 2.2 1.2
0.4 2.0 1.4
0.5 1.8 1.6
0.6 1.6 1.8
CGDE in the anolyte, which gives rise to H2O2 and H2C2O4 would also trigger off their decomposition leading finally to their stationary-state yields. In fact, these yield curves are described satisfactorily by Eq. (1)
(X) = 1 – exp – ——— — …(1)
Where (X) is the integral yield of the product X, n is the faradaic yield, G0 is the initial G, k is the coefficient for decomposition of X, q is the number of moles of electrons passed and V is the volume of the anolyte.
The value of G0 (H2O2) of fully grown anodic CGDE (at 420V, the mid-point voltage1-3,11, the critical voltage for the formation of anodic CGDE; and beyond) in aqueous inert-type electrolytes such as K2SO4 was found invariant to applied voltages and other external variables, and is the limiting yield of reactions originating in the liquid-phase reaction zone near the plasma-anolyte interface3. Values of G0(H2O2) and G0(H2C2O4) obtained (from the tangent at the origin of the respective curves) for a set of formic acid solutions in 0.05 M K2SO4 over the concentration range 0-0.6 M were compiled in Table-1. It is interesting to note that the yields of both the products are generally in significant excess of the Faraday law value (i.e. Go > 1) . Further, as the concentration of HCOOH in the anolyte is increased, there occurs a fall in G0H2O2) simultaneous rise in G0(H2C2O4) by practically the same amount, the sum of the two G0 values remaining almost constant at 3.4. Obviously, HCOOH can act as potent scavenger for the precursor of H2O2, the OH radical, leading to formation of H2C2O4.
The chemical effects observed bear interestingly a distinct similarity to those of radiolysis and anodic GDE (glow discharge electrolysis in which the anode is placed in the gas space above the anolyte) of aqueous formic acid solutions1, 12. As a matter of fact, there exists a strong resemblance in the nature of liquid -phase chemical effects of anodic CGDE, anodic GDE and radiolysis1, 3. According to a radiolytic mechanism originally proposed by Hickling et al.1, 8,13 for the chemical effects of anodic GDE of aqueous electrolytes, each positive gaseous ion in the plasma during entry to the anolyte gets accelerated in the steep cathode fall near the plasma-anolyte interface and dissociates several liquid water molecules there into H˙ and OH˙ radials. These species could enter into a variety of reactions depending on the substrates present in the anolyte, in a way similar to that in radiolysis. In the light of Hickling’s mechanism, the elementary reactions in Scheme 1 seem to be of major relevance in the liquid-phase reaction zone of anodic CGDE of aqueous formic acid solutions:
H˙ + H˙ à H2 …(2)
H˙ + OH˙ à H2O …(3)
OH˙ + OH˙ à H2O …(4)
OH˙ + H2O2 à HO2 + H2O …(5)
Followed by OH˙ + HO˙2 à O2 + H2O
OH˙ + HCOOH à H2O + COOH˙ …(6)
COOH˙ + COOH˙ à (COOH)2 …(7)
OH˙ + (COOH)2 à H2O + COO – COOH …(8)
OH˙ + COO – COOH˙ à H2O + 2CO2
(The reactions of H˙ with HCOOH and other carboxyl species being slower than the reactions 2 and 3 by several orders of magnitude 12 were not included.)
Thus, according to Scheme 1, the passage of each mol electron of electricity during anodic CGDE would generate several moles of OH radicals which would give rise to a G0 value summed over the products H2O2 and H2C2O4 significantly greater than unity as observed (Table 1). Further, as the concentrations of H2O2 and H2C2O4 build up in the anolyte (reactions 4 and 7), their decompositions proceed (reactions 5 and 8) leading finally to a stationary state in their yields. Thus, general features of solution-phase chemical results of anodic CGDE in aqueous solutions containing a simple organic compound like formic acid can be satisfactorily explained on the basis of Hickling’s radiolytic mechanism. However, the gaseous products evolved at the glow discharge anode were found to contain a significant proportion of CO besides the expected components O2 , H2 and CO2 (reactions 2-8) indicating that formic acid molecules undergo vapor phase decomposition to a substantial degree in electrical discharges within the plasma around the anode.
One of the authors (AKS) expresses his gratitude to the CSIR, New Delhi for financial support in the form of a research fellowship.
- Hickling A, in Modern aspects of electrochemistry, VOl. 6. Edited by J O’M Bockris & B E Conway (Butterworths, London) (1971) p. 329.
- Hickling A & Ingram M.D. Trans Faraday Soc, 60 (1964) 783.
- Sengupta S.K. & Singh O.P. electro anal Chem., 369 (1994) 113.
- Sengupta S.K. & Palit S.R. Indian Chem. Soc. 52 (1975) 91.
- Singh O.P, Ph.D. thesis, Banaras Hindu University (1991).
- Hickling A. & Newns G.R., J chem. Soc, (1961) 5186.
- Sengupta S.K.,Indian J Chem., 25A (986)261.
- Bullock A T, Gavin D L & Ingram M D, J chem. Soc, Faraday Trans 1, 876 (1980) 648.
- Harada K, Suzuki S. & Ishida H, Biosystems, 10 (1978) 247.
- Tezuka M, Yazima T & Tsuchiya A, Proc IntSymp on Plasma Chemistry – 8 (Tokyo) (987) p.1816.
- Sengupta S K & Singh O P, J electro anal Chem., 301 (1991) 189.
- Spinks J W T & Woods R.J., An introduction to radiation chemistry (Wiley, New York) (1976) p. 247-359.
- Hickling A & Ingram M.D., J electro anal Chem., 8 (1964)
Indian Journal of Chemistry
Vol. 36A, November 1997, pp. 945-950
Origin of contact glow discharge electrolysis in aqueous solution:
Effects of electrolyte temperature and surface tension
Susanta K. Sen Gupta*, Ashok K. Srivastava & Rajeshwar Singh
Department of Chemistry, Faculty of Science
Banaras Hindu University, Varanasi 221 005, India
Received 17 December 1996; | revised 14 August 1997
The effects of electrolyte temperature and surface tension on the breakdown of normal electrolysis and its transition to contact glow discharge electrolysis (CGDE) in aqueous solutions have been studied. It is found that local vaporization of the solvent close to an electrode by Joule heating and the breaking up of vapor – liquid boundary surfaces in the contiguous streams of vapor bubbles are the prime causes of gaseous, sheathing of the electrode and the breakdown of normal electrolysis vis-à-vis its transition to CGDE.
Contact glow discharge electrolysis (CGDE) is an unconventional electrolysis where plasma is sustained by DC glow discharges between one of the electrodes and the electrolyte surrounding it. Ordinary electrolysis when carried out at a sufficiently high voltage breaks down and switches spontaneously over to CGDE. The onset of breakdown is marked by a sharp drop in current with simultaneous formation of a gaseous sheath over an electrode across which glow discharges pass resulting in CGDE at the electrode. The electrode where the current density is larger, electrolyte resistivity is higher or electrolyte surface tension is lower, tends to be the location of CDGE1-4. A remarkable feature of CGDE is that its products at the glow discharge electrode are novel for normal electrolysis, such as H2 at the anode. O2 at the cathode, etc., and in yields several times the Faraday law values1,2,5-7. A number of investigations have been carried out on original and chemical effects of the phenomenon1-15 as well as spectroscopy of its light emission16,17. However, there are several important aspects which need systematic investigation. The mechanism of the breakdown of normal electrolysis vis-à-vis transition to CGDE which is associated with a stable gaseous sheathing around an electrode well a stable gaseous sheathing around an electrode well underneath the liquid electrolyte needs systematic probing. It is thus of interest to investigate the role of Joule heating during electrolysis is causing vaporization of the solvent near an electrode and that of electrolyte surface tension in anchoring the bubbles of vapor over the electrode surface in the form of a stable continuous sheath. In fact, both electrolyte temperature and surface tension have been found to influence profoundly the breakdown of normal electrolysis. The results obtained on their effects are discussed here.
Material and Methods
The work was carried out by studying current-time (i-t) and current-voltage (I-V) characteristics of electrolysis using the following standard conditions: cell, symmetrical Corning H-type (height 11 cm; diameter, 30mm) having a 7.5 cm long bridge (diameter, 15 mm) fitted with G-5 sintered glass disc separator at a height of 1.5 cm from the base of the cell; anode 5 mm long platinum wire of diameter 0.35 mm; cathode usually a platinum foil (1 cm x 1 cm) of 0.20 mm thickness, however, a 5 mm long platinum wire of 0.35 diameter (i.e. identical with the anode) was used in the study of I-t characteristics; the electrodes wire dipped to a depth of 3 cm; electrolytes aqueous 0.05 M K2SO4 with or without a surfactant, sodium dodecyl sulphate (SDS), N-cetyl-N, N, N-trimethylammonium bromide (CTAB) or Aerosol O at varying concentrations (0.25 – 10mM) but MCMC; pressure, atmospheric; ambient temperature, 10 to 93 ± 2°C.
Current was supplied from an Aplab-7322 medium voltage DC power supply which provides a maximum current of 1.5 A at voltages upto 600 V through a calibrated multirange milliampere (AE). The voltages across the circuit were measured on a calibrated voltmeter (Alock).
Results and Discussion
The current-time (I-t) characteristics of electrolysis at an appropriately high voltage show typically three successive stages (Fig.1); normal electrolysis (PQ), a transition period (QR) and CGDE (RS). The minimum time (tB) necessary for the normal electrolysis to breakdown and change over to CGDE falls off significantly with the rise in electrolyte temperature (Table 1).
Fig.1 – Current-time (I-t) characteristics of electrolysis of 0.5 M K2SO4 solution at 100 V for transition to anodic CGDE between Pt – wire electrodes (length 5 mm and dia. 0.35 mm) at ambient temperature, 60 ± 2°C
Fig.2 – Current-voltage (I-V) characteristics of electrolysis of 0.05 M K2SO4 solution between Pt-wire anode (length 5mm and dia. 0.35 mm) and a cathode of 1 cm2 Pt-fail of thickness 0.20 mm at two different ambient temperatures (-0-, 32 ± 2°C and ∆, 75 + 2°C)
Table 1 – Effect of temperature on the duration of normal electrolysis and transition period during electrolysis at a fixed voltage [electrolyte, 0.05 M K2SO4; electrodes, 5 mm long Pt wires of diameter 0.35 mm; applied voltage, 100 V at which normal electrolysis and transition region could be distinguished at different temperatures]
Temp. (°C) Duration of normal Duration of transition Maximum current observed
Electrolysis (min) period (min) (mA)
45 ± 2 16 10 580
60 ± 2 6 5 470
75 ± 2 3 5 420
Table 2- Effect of temperature on the voltage limit beyond which normal electrolysis would cease to remain stable and the minimum voltage beyond which the transition region would start immediately after switching on the power
[Electrolyte, 0.05 M K2SO4; electrodes, 5 mm long Pt wires of diameter 0.35 mm]
Temp (°C) Voltage limit for persistence of stable Minimum voltage for the onset of
Normal electrolysis (V) transition period at the anode (V)
45 ± 2 98 232
60 ± 2 84 140
75 ± 2 70 124
85 ± 2 64 70
Effect of temperature on normal electrolysis and transition region
With increasing temperature the magnitude of the maximum voltage up to which normal electrolysis would persist (without any tendency towards breakdown), decreases (Table 2). Further, the minimum voltage for the instantaneous commencement of the transition period decreases considerably with rising temperature. Thus, higher the electrolyte temperature, easier is the breakdown of normal electrolysis. Higher temperature would require less dissipation of heat from Joule heating for local vaporization and vapor sheathing over the electrode and thus facilitate the breakdown of normal electrolysis. Moreover, the duration of normal electrolysis as well as transition period decreases with rising temperature (Table 1).
Effect of temperature on breakdown voltage (VB) and mid-point voltage (VD)
The current-voltage (I-V) characteristic of electrolysis (Fig. 2) shows a current maximum (the point B) at a particular voltage called the breakdown voltage (VB) at which normal electrolysis breaks down at an electrode and a transition region commences; and a current minimum (the point D) at another voltage called the mid-point voltage (VD) at which full glow discharge develops at the electrode4. Results on I-V characteristics of 0.05 M K2SO4 at different temperatures over the range 10-93°C show that with the increase in temperature the breakdown voltage (VB) falls steadily (Table 3). Further, the current at VB(IB) also diminishes considerably with the rise in temperature (Fig.2). This is consistent with the fact that the critical amount of Joule heating required for local vaporization at an electrode would be smaller, higher the electrolyte temperature. The results obtained thus lead to the inference that the primary step in gaseous sheathing of the electrode causing the breakdown of normal electrolysis is solvent vaporization at the electrode by Joule heating.
However, in some of the earlier studies9, 11, the rate of gas evolution was considered to be the determining factor for the breakdown of normal electrolysis.; larger the rate of gas evolution, easier is the breakdown. Following this view point, increase in the electrolyte temperature, which causes lowering of current at VB (as stated earlier) and consequently a lowering in the rate of electrode gas evolution, would hinder the breakdown of normal electrolysis which is just the reverse of what is observed. Further, when temperature is lowered below 20°C, Joule heating effect becomes insufficient to cause solvent evaporation. In fact, no breakdown of normal electrolysis could be observed, although the current passing (1040 mA) vis-à-vis the rate of electrode gas evolution would be several times that which causes the breakdown (IB) at a higher temperature, the possibility of electrode gas evolution rate as the primary cause of breakdown was also ruled out on the ground that the location of CGDE is not necessarily the electrode where gas evolution rate is higher 3,4.
It is further seen from Table 3 that the mid-point voltage (VD) does not change when the temperature is 60°C or above. This indicates that VD is a characteristic of the glow discharge and is unaffected by the conditions of the surrounding electrolyte. Larger values of VD observed for electrolyte temperatures lower than 60°C (Table 3) are due to larger IR drops at the lower temperatures.
Effect of electrolyte surface tension
The results of the study of effect of electrolyte temperature on I-t and I-V characteristics of electrolysis suggest strongly that solvent vaporization near an electrode by Joule heating rather than electrolytic gas evolution is the prime factor for the breakdown of normal electrolysis. He question arises as to how do the bubbles of solvent vapor coalesce to blanket the electrode with a steady continuous gaseous sheath well underneath the liquid electrolyte. The electrode wettablility is an obvious factor in electrode’s gaseous sheathing. It is thus of interest to study the nature of influence that electrolyte surface tension would exert on anchoring of the vapor bubbles to the electrode surface in the form of a continuous sheath vis-a vis the breakdown of normal electrolysis. As a matter of fact, surface tension has a significant effect on the transition of normal electrolysis to CGDE4. Addition of a surfactant to the anolyte lowers the breakdown voltage (V) at the anode as well as IB (Fig. 3) irrespective of whether the surfactant is cationic (CTAB) or anionic (SDS and Aerosol OT)(Table 4).
Among the three surfactants used, Aerosol OT has been found to be the most effective in bringing down the breakdown voltage (VB)(Table5). However, as seen from Table 4, mid-point voltage (VD) does not show any significant change on addition of surfactant (Fig. 3) demonstrating that the onset of full glow discharge is unaffected by the properties of the electrolyte.
It has been further observed that the effect of electrolyte temperature on the breakdown voltage (VB) and the midpoint voltage (VD) is similar whether the anolyte contains a surfactant or not (Table 6).
The observed effect of electrolyte surface tension on the transition of normal electrolysis to CGDE is highly interesting. Lowering of surface tension of the anolyte facilitates vapor sheathing at the anode ad lowers VB; This is just the opposite of what the static theory of contact angle would predict: lowering of surface tension would enhance the anode’s wet ability and hinder its sheathing by the vapor, and raise VB.
An analogous anomalous surface tension effect is known for a phenomenon called ‘boiling crisis’18. Its growth has close resemblances with breakdown of normal electrolysis. It develops during boiling of a liquid over a heated surface when the rate of vaporization goes beyond a critical value and is marked by a sharp drop in the specific heat flow to the liquid and the formation of a steady vapor sheath over the surface.
It appears that there should be a basic cause common to the two phenomena. The occurrence of ‘boiling crisis’ has been explained well in terms of helmholtz-Taylor’s conditions of hydrodynamic instabilities18. According to this theory, when the rate of vaporization becomes sufficiently high, the streams of bubbles passing through the liquid come close enough for their mutual interaction resulting in disruption of the vapor-liquid boundary surfaces, collapse of the net-type structure of vapor streams and gas blanketing of the surface. This hydrodynamic instability would set up more easily, lower the surface tension of the liquid. According to the hydrodynamic approach as applied to solvent vaporization in the vicinity of an electrode, lowering of surface tension acts in the sense of breaking up the vapour-liquid interfaces in the jets of vapor bubbles over the electrode. This facilitates coalescence of vapor bubbles leading to vapour sheathing of the entire electrode and the breakdown of normal electrolysis at a lower current and hence at VB.
Thus the sequence of events leading to fully grown CGDE can be described as normal electrolysis, solvent vaporization by Joule heating ear an electrode, onset of hydrodynamic instabilities leading to complete and steady vapor sheathing over the electrode and glow discharges across the sheath.
This work was supported by a grant from the Council of Scientific and Industrial Research, New Delhi, India. One of the authors (A.K.S.) is grateful to the above agency for a research fellowship.
- Hickling A, in Modern aspects of electrochemistry, 6, edited by J O’M Bockris & B E Conway (Butterworths, London), 1971, 329.
- Hiklong A & Ingram M.D., Trans Faraday Soc., 60 (1964) 783.
- Sengupta S K, J electro anal Chem., 127 (1981) 263.
- Sengupta S K & Singh O P , J electro anal Chem., 301 (1991) 189.
- Sengupta S.K. & Palit R, J Indian chem. Soc., 52 (1975) 91.
- Sengupta S.K.& Palit S R , J Indian Chem. Soc, 53 (1976) 472.
- Sengupta S K & Singh O P, J electro anal Chem., 369 (1994) 113.
- Bullock A T, Gavin D L & Ingram M D, J Chem. Soc, Faraday Trans 1, 76 (1980) 648.
- Olivier J C & Guilpin Ch, J electro anal Chem., 91(1978) 79.
- Mazza B. Pedeferri P & Re G. Metallurgia Ital, 68(1976) 582; Electrochim Acta, 2) 257.
- Harada K. Suzuki S & Ishida H, Experntia, 34 (1978) 17; Biosystems, 19 (1992) 257.
- Kokufuta E, Shibasaki T., Sodeyama T & Harada K, Lett, (1985) 1569.3 (1978) 87.
- Campbell S A , Cunnane V J & Schiffrin D J , J electro anal Chem, 325 (1992
- Kokufuta E, Shibasaki T., Sodeyama T & Harada K, Chem Lett, (1990) 916.
- Thonstad J, Calandra A J & Zavatti J R , Electrochim Acta, 37 (1992) 711.
- Valognes J C , Bardet J P & Mergault P. Spetrochim Acta, 42B (1987) 445.
- Valognes J C, Bardet J P & Mergault P, Spectrochim Acta 43 B (1988) 799.
- Tong L S, Boiling heat transfer and two-phase flow (Wiley, New York), 1965.
Indian Journal of Chemistry
Vol.37A, June 1998, pp. 558-560
A study on non-faradaic yields of anodic contact glow discharge electrolysis using cerous ion as the OH˙ Scavenger: An estimate of the primary yield of OH˙ Radicals
Susanta K. Sengupta*, Rajeshwar Singh & Ashok K Srivastava
Department of Chemistry, Faculty of Science,
Banaras Hindu University, Varanasi 221 005, India
Received 22 July 1997; revised 1 January 1998
Chemical yields of contact glow discharge electrolysis (CGDE) show strong deviations from faradaic behavior. The products are novel for normal electrolysis and yields significantly exceed the Faraday law value. Anodic CGDE gives rise to Ce4+ and H2O2 in the anolyte in yields significantly exceeding the faradaic value besides the faradaic yield of O2. The relative yield of Ce4+ and H2O2 depends on the concentration of Ce3+ and H2O2 depends on the concentration of Ce3+ and the quantity of electricity passed. A kinetic analysis of the variation of the initial differential yield of Ce4+,G0 (Ce4+) with the concentration of Ce3+ after applying certain approximations leads to an estimate of the generation of 12 moles of OH· Radicals in the liquid phase reaction zone of anodic CGDE for the passage of each mol electron of electricity.
In contact glow discharge electrolysis (CGDE) plasma is sustained b dc glow-discharges between an electrode and the electrolyte around it. Conventional electrolysis switches spontaneously over to full CGDE when the voltage applied reaches a threshold value, the mid-point voltage (VD)1-3. Highly remarkable are the chemical effects of CGDE. The products are novel for conventional electrolysis and in yields several times the Faraday law value1,2,4-9. The non-faradaic yields of anodic CGDE of inert-type aqueous electrolytes comprising H2, H2O2 and OE2 (O2 in excess of the faradaic yield) originate in two reaction zones 8: the liquid phase near the plasma-anolyte interface were sufficiently energized gaseous H2O+ ions from the plasma break up the liquid water molecules on collision into H˙ and OH˙ Radicals 10 which trigger off further reactions giving rise to H2O2 and the majority of the yields of H2 and OE2 and the plasma round the anode where a part of H2 ad OE2 are produced by dissociation of water vapor in glow discharges. Anodic CGDE is thus a potential technique for generating radicals in a liquid at a high local concentration. This is substantiated by conversion of aqueous formic acid into oxalic acid9, and of liquid ammonia into hydrazine4 both in yields several times the Faraday law value. I is thus of interest to study systematically the chemical effects from the liquid phase reaction zone of anodic CGDE in the presence of potential OH˙ Radical scavengers. Herein are discussed the results for Ce3+, a scavenger.
Chemical effects of anodic CGDE of Ce3+ solutions were studied using a H-type Corning cell fitted with a G-5 glass sinter dividing the electrode compartments and having facilities for collecting electrolyzed solutions as well as electrode gasses from each compartment as described in an earlier report8. Power was supplied at 450 V (when anodic CGDE generates in full form just switching on the power3, 8,9 and at current densities 900 ± 50 mA cm-2. The quantities of electricity passed were up to 25 C. the following standard conditions were employed for the study : anolyte, 0.5 M H2SO4 plus varying concentrations (0.02) M – 0.12 M) of Ce2(SO4)3; catholyte, 0.02 M H2SO4 plus 0.05 M K2SO4 ; anolyte and catholyte were pre-boiled and degassed; anode, Pt wire of length 5 mm and diameter 0.35 mm; cathode, 1 cm2 Pt foil of thickness 0.20 mm; ambient temperature, 85± 2°C; and pressure, atmospheric. Current was supplied from an Aplab 7322 medium voltage d c power supply which provided a maximum of 1.5 A at voltages up to 600 V. Quantity of electricity passed during anodic CGDE was measured on a precalibrated ESC 640 digital coulometer.
The yields produced in the anolyte comprised H2O2 and Ce4+, their relative amounts depending on the number of coulombs passed and the concentrations of Ce3+. H2O2 in the anolyte was determined by titrating a 20.0 ml aliquot with standardized 4.4 X 10-4 M KMnO4 acidified with 2 M H2SO4. The Ce4+ content was estimated by the addition of a known excess of 1.25 X 10-3 M H2O2 to a 20.0 ml aliquot followed by titration of the remaining H2O2 with acidified 4.4 X 10-4 M KMnO4 solution (no interference from Ce3+ could be detected with standard Ce4+ solution). A satisfactory material balance was observed among Ce4+, H2O2 and OE2, and H2 produced at the glow discharge anode.
Results and discussion
Ce4+ and H2O2 formed in the anolyte during anodic CGDE of Ce3+ solution interact mutually to yield Ce3+ back and O2. The relative yield of the two products was found to depend on both the concentration of Ce3+ and the quantity of electricity (1) passed. Further no net formation of Ce4+ when [Ce3+] < 0.02 M and H2O2 when [Ce3+]≥ 0.04 M could be observed. The yield of H2O2 increased with q approaching a limiting value for [Ce3+]≤ 0.02 M but decreased at higher [Ce3+]. On the other hand the yield of Ce4+ only increased with q approaching a limiting value.
A useful way of expressing the yield of a product (X) of CGDE is by quoting its G0 – value, a dimensionless quantity1.8 defined as:
Initial differential yield of X (mol per mol electron)
G0(X) = ——————————————————————
Faradaic yield of X (mol per mol electron)
In analogy with G-value for the yield (molecules per 100 eV) in radiation chemistry, the initial differential yield being the slope of the tangent at the origin of the yield vs. q curve. The values obtained for G0(Ce4+) and G0 (H2O2) at different [Ce3+] (Table 1) show clearly that anodic CGDE gives rise to several times the faradaic yield for Ce4+ and H2O2. Further, G0(H2O2) fell with [Ce3+] reaching zero at ~0.03 M Ce3+ where G0 (Ce4+) commenced and rose with further increase in [Ce3+] approaching finally a limiting value, indicating the occurrence of other reactions competing kinetically with the oxidation of Ce3+ during anodic CGDE. In the region of low [Ce3+], H2O2 formation is predominant and no Ce4+ could be detected. However, at higher [Ce3+], Ce4+ formation dominate over H2O2 formation showing a more effective kinetic competition of a Ce3+ with an OH˙ For another OH˙, the precursor for H2O2.
During anodic CGDE highly energized gaseous H2O+ ions from the plasma bombard liquid H2O molecules at the plasma-anolyte interface to break them up into H˙ and OH˙ Radicals which mutually interact to yield H2, H2O2 and OE2 in accordance with Hickling’s radiolytic mechanism1 (the rate constant values quoted are from pulse radiolysis data11) :
H˙ + H˙ à H2 (k1 = 1.3 X 1010 M-1 s-1) …(1)
H˙ + OH˙ à H2O (k2 = 2 X 1010 M-1 s-1) …(2)
OH˙ + OH˙. à H2O2 (k3 = 5.3 X 109 M-1 s-1) …(3)
OH˙ + H2O2 à HO˙2 + +H2O(k4=2.7 x 107 M-1 s-1) …(4)
OH˙ + HO˙2 à H2O + O2 (k5 = 9 x 109 M-1 s-1) …(5)
Thus, in the absence of any radical scavenger, G0 (H2O2) would measure the total non-faradaic yield from the liquid phase reaction zone. However in the presence of Ce3+, a potent OH˙ Radical scavenger, the reaction.
Ce3+ + OH˙ à Ce4+ + OH– (k6 = 7.2 X 107 M-1 s-1) …(6)
Would compete kinetically with the reactions (2) and (3). Further Ce4+ would undergo reduction by H2O2 into Ce3+ and O2. Applying the principle of competition kinetics, an expression for G0(Ce4+) is obtained
[Ce3+] – A
G0(Ce4+) = n ——————–
[Ce3+] + A + B
where, n is the number of moles of OH˙ Radicals produced for the passage of one mol electron of electricity, A and B stand for K3/K6 [OH˙] and K2/K6 [H˙] respectively. The expression predicts an upward trend for G0 (Ce4+) with [Ce3+] towards a limiting value as observed (Table 1). The same considerations predict a downward trend (Table 1) in G0(H2O2) with [Ce3+] :
A – [Ce3+]
G0(H2O2) = n ———————
[Ce3+] + A + B
The concentration of the radicals will change as these advances into the bulk anolyte. The kinetic situation in the liquid phase reaction zone of he anodic CGDE is thus highly complex and requires a complete knowledge of concentration profiles of all the reactant species over the reaction zone for its solution. However, the application of a crude approximation where A and B can be considered as constants, justifiably for species like Ce3+, the rate constant for the reaction of which with OH˙ Radicals is low enough (k6 = 7.2 X 10-7 M-1 s-1), leads to the expression1.12 (9)
1 1 1 2A + B
————— = ————- + ————- ———————- …(9)
G0(Ce4+) n n [Ce3+] – A
The plot ————- versus ————- at large enough [Ce3+] was found
Linear as predicated by Eq. (9). From the intercept of the plot, the number of moles of OH˙ Radicals (`n’) generated by 1 mol electron of electricity was estimated to be 12.8. This agrees fairly well with the `n’ value of 8 estimated from anodic glow discharge electrolysis (GDE where the anode is placed in the gas space above the anolyte surface) of Ce3+ solutions1,12 indicating significant similarity between the mechanism operating in the liquid-phase reaction zone of anodic CGDE and the one in anodic GDE. Further, there exists a strong analogy between the chemical effects of anodic CGDE and those of radiolysis of aqueous solutions. It is thus worth comparing the yield of radicals from the two processes. Radiolysis of water (pH = 0.46) by y-rays or fast electrons with energies in the rage 0.1 – 20 MeV produces the following yields in molecules per 100 eV11,13 .
H2O à OH˙ (2.90) + H˙ (3.65) + H2(0.40) + HO˙2 (0.008) + H2O2 (0.78)
This implies a maximum yield 4.5 moles for OH˙ Radicals per mol electron of electricity if the H2O+ gas have average energies of 100 eV as against the yield of more than 12 moles per mol electron of electricity by anodic CGDE. Thus it appears that during anodic CGDE the average energy of the positive gaseous ions entering into the liquid anolyte from the plasma over the anode is appreciably greater than 100 eV.
The work was supported by a grant from the CSIR, New Delhi, India. One of the authors (AKS) is grateful to the CSIR for a research fellowship.
- Hickling A, in Modern aspects of electrochemistry, Vol.6, edited by JO’M Bockris & B E Conway (Butterworths, London) (1971) p.329.
- Hickling A & Ingram M D, Trans, Faraday Soc, 60 (1964) 783.
- Sengupta S K & Singh O P, J electro anal chem., 301 (1991) 189.
- Hickling A & Newns G R, J chem. Soc, (1961) 51186.
- Sengupta S K & Palit S R, J Indian chem. Soc, 52 (1975) 91.
- Sengupta S K & Palit S R, J Indian chem. Soc, 53 (1976) 472.
- Sengupta S K, Indian J Chem, 25 (1986) 261.
- Sengupta S K & Singh O P, J electro anal chem., 369 (1994) 113.
- Sengupta S K, Singh R & Srivastava A K, Indian J Chem, 34A (1995) 459.
- Bullock A T, Gavin D L & Ingram M D, J chem. Soc, Faraday Trans I, 76 (1980) 648.
- Spinks J W T & Woods R J, An introduction to radiation chemistry (Wiley, New York) 2nd (1976) p.247.
- Denaro A R & Hickling A, J electrochemical Soc, 105 (1958) 265.
- Buxton GV, Radiat Res Rev, 1 (1968) 209.
ELECTROCHEMICAL SCIENCE AND TECHNOLOGY
A Study on the Origin of Non-faradaic Behavior of Anodic
Contact Glow Discharge Electrolysis
The Relationship between Power Dissipated in Glow Discharges and Non-faradaic Yields
Susanta K. Sengupta, Rajeshwar Singh, and Ashok K. Srivastava
Department of Chemistry,
Faculty of Science, Banaras Hindu University, Varanasi – 221 005 India
Chemical effects of contact glow discharge electrolysis (CGDE) at an electrode where a plasma is sustained by dc glow discharges between the electrode and the surrounding electrolyte, are remarkably non-faradaic. A critical analysis of the chemical results of anodic CGDE at varying voltages, currents, power supplies, and PHs clearly shows that non-faradaic yields originate in two separate reaction zones: the plasma around the anode and the liquid anolyte near the plasma – anolyte interface. The yield from the former zone appear from 250V onward (the beginning of the onset of partial glow discharge ) and very linearly with the power dissipated in the glow discharge. The yields from the latter zone appear from 410 V onward (the beginning of the full glow discharge) and are independent of the power dissipated in the glow discharge. The relative contribution of the two zones to the total non-faradaic yields is dependent on the voltage applied: the plasma zone having a share of 100% up to 400 V, followed by 20% up to 450 V, and thereafter rising steadily to 57% at 500 V.
Contact glow discharge electrolysis (CGDE) at an electrode, where plasma is sustained by de glow discharges between the electrode and the surrounding liquid electrolyte, develops spontaneously during conventional electrolysis, if the voltage applied goes beyond a threshold value called the breakdown voltage (VB). The glow discharge at the electrode becomes full when the voltage is raised to a critical value called the midpoint voltage (VD), the magnitude of which depends only on the polarity of the electrode (420 V for the anode and 160 V for the cathode in aqueous electrolytes) and is unaffected by the electrolyte’s composition, concentration, temperature , and surface tension1-3. A remarkable feature of CGDE is its highly non-faradaic chemical effects. The yields obtained at the glow discharge electrode (anode or cathode) are several (two to five) times the faradaic value and include products novel for conventional electrolysis such as H2 at the anode, O2 at the cathode, I2 at the cathode.1-4-6 Moreover, this technique allows anodic production of Fe31,1-2 hydrogen peroxide, 1,2,7oxalic acid, 8 hydrazine, 9 and amino acids10-16 from appropriate electrolyte systems in yields several (two to eight) times the corresponding Faraday law value. Such non-faradaic chemical effects are also observed during anodic CGDE in acetonitrile and dinethylformamide media. 17However, it is of interest to point out that no non-faradaic yields are obtained during CGDE of molten electrolytes.18
According to a recent finding7 on chemical yields of CGDE in K2SO4, solutions, its non-faradaic yields: H2, H2O2, and OE2(oxygen in excess of the faradaic yield) at the anode seem to be produced in two separate reaction zones the liquid near the plasma –electrolyte interface and the plasma around the electrode, and this happened through dissociation of water molecules by appropriate energy –transfer processes. To probe further the proposed two – reaction zone model for non faradaic chemical effects of CGDE, a detailed investigation on the formation of all the products of anodic CGDE (i.e.,H2 OE2and H2 O2) in an inert – type aqueous electrolyte over a voltage range from its breakdown voltage (VB) to 500 V was undertaken, and the results abtained are critically analyzed here.
To minimize possible changes in ionic composition and resistivity of the electrolyte near the electrodes in the course of electrolysis due to acid / alkali formation in the anolyte / catholyte, a phosthate buffer: 0.12 M KH2PO4 – 0.07 M KOH of ionic strength 0.43, and pH 7.0 was employed as the electrolyte of choice for the study.
A Corning H-type (height, 11 cm ; diam. 30 mm ) divided cell (Fig.1) having a bridge of length 7.5 cm ( internal dia, 15mm) Fitted with a g-5 sintered glass disk separator at 1.5 cm height from the base of the cell fitted with a platinum wire anode (length, 5 mm, diam. 0.35 mm) and a platinum foil cathode (1 x 1cm, 0.2 mm phick) was used for electrolysis at an ambient temperature of 85± 20c and at atmospheric pressure.
The cell was further equipped with the necessary facilities for collecting both electrode gases and electrolyzed solutions from either electrode chamber.
The setup was found quite convenient for producing CGDE at the anode whose current – potential characteristics in reference to a saturated calomel electrode (SCE) are described in Fig. 2. Chemical results of electrolysis were studied at voltages ranging from 150 V (near the breakdown voltage, VB corresponding to the point B in Fig. 2) to500V. Current was supplied from an Aplab 73 22 medium voltage de regulated power supply. Current and voltages were measured using an OSAW multirange millimeter and a Philips PM 2518X digital multimeter. Quantities of electricity passed during anodic CGDE were measured by a precalibrated ESC 640 digital coulometer.
Fig. 1. A schematic drawing of the cell setup used for studying anodic CGDE.
Anodic CGDE of phosphate buffer solutions was found to produce a liberal mixture of H2 and O2 in the anode gas and H2 O2 in the anolyte. Further, the yields of the products were found to exceed the Faraday law values considerably. However, the cathodic product during anodic CGDE comprised H2 only and in yields as stipulated by faraday’s laws.
The gases produced at both the glow discharge anode and the counter electrode were analyzed quantitatively using a modified form of a precision model Haldane gas analysis apparatus, in which O2 was measured by absorption in alkaline pyrogallol and H2 was measured by combustion in the presence of a given excess of O2 over a heated platinum coil H2O2 produced in the anolyte was measured by permanganometry preceded by the necessary standardizations.
Results and Discussion
Chemical effects of anodic CGDE are partly faradaic and partly non-faradaic. The quantity and composition of non-faradaic yields have a strong dependence on the magnitude of the voltage applied. A plot of the normalized integral yield, y(mol/mol electron of electricity) of the non-faradaic product of anodic CGDE H2 or (H2O2+OE2) (O2 in excess of the faradaic yield in terms of equivalent H2O2) against applied voltages (Fig. 3) show clearly that non-faradaic yields are only abtained in significant quantities at voltages beyond 250 V. The yields, after remaining fairly constant at 0.36 + 0.03 mol / mol electron over 250 to 400 V, undergo a sharp rise between 400 and 420 V. The rise in the yields, however, sloes down drastically between 420 and 450 V. Again, with further increases in voltage, the yield keep on rising steadily at least up to 500 V (beyond which the platinum wire anode tends to melt down).
On the other hand, at any voltage during anodic CGDE no non-faradaic yields could be observed in the cathodic compartment pointing to the operation of only a conventional charge – transfer mechanism at the cathode-producing faradaic yield at H2.
It is interesting to note from Fig.3 that a satisfactory material balance between H2 on one hand and H2O2 + OE2 on the other hand is maintained over the entire range from 150 to 500 V. Evidently, non-faradaic yields of anodic CGDE originate from the breakup of H2O molecules presumably through generation of H˙ and OH˙ Radicals by some energy transfer process. The generation of these radicals in the liquid electrolyte during anodic CGDE was proved earlier by an ESR study using 5,5-dimethylpyrroline-1-oxide (DMPO) as be spin trap.19
Although, the normalized integral yield (y) vs. voltage curves for H2 and H2O2 plus OE2 (Fig.3) are practically super imposable, significant differences are noted when the individual normalized integral yield (y) vs. voltage curves of H2, H2O2, and OE2 are compared. Though the curves for H2 and OE2 resemble each other (Fig.3 and 4), those for H2O2 and OE2 differ markedly (Fig.4). The yield of H2O2 commences at 405 V, undergoes a sharp rise between 410 and 420 V and thereafter attains a stationary value of 0.50 mol/mol electron (y vs. V curve in Fig. 4).
The difference observed between the individual yield (y) vs. Voltage curves of H2O2 and OE2 is further manifested when the yields of H2O2 and OE2 for a given quantity of electricity are compared at different values of current. Although increasing the current results in a larger dissipation of power in the glow discharge, it considerably raises the yield of OE2, but does not affect that of H2O2 to any significant extent (Table1). Thus, the reactions on during in the glow discharge, which contribute to OE2 formation, do not gives rise to H2O2. The result is clearly indicative of the formation of H2O2 from reactions taking place only in the liquid phase near the plasma-anolyte interfacial region and that of OE2 at least in considerable part due to reactions initiated within the plasma around the anode. When H2O2, formed in the liquid anolyte during anodic CGDE, accumulates to a sufficient concentration, it undergoes decomposition to OE2 due to its further reaction with OH˙20and finally approaches a stationary state value (Fig. 5). Thus the total non-faradaic yield from the liquid-phase reaction zone (H2O2 plus OE2 from decomposition of H2O2 ) can be computed from the initial differential yield of H2O2, G0 (H2O2) (the slope of the tangent at the origin of the curve for yield of H2O2 vs. q, the quantity of electricity at a given voltage Fig. 5).
In fact, the initial differential yield of H2O2, G0(H2O2) varies with voltage in the same way as its normalized integral yield, y(H2O2) (Fig. 4) and beyond 420 V it rapidly reaches a stationary value of 1.70 mol/mol electron. The constancy in G0(H2O2) with voltage beyond 440V, in spite of a variation in power dissipation from 17.3 to 29.6 W lends further support to the indication obtained above that go (H2O2) is the measure of the total non-faradaic yield of anodic CGDE from the liquid-phase reaction zone. Further, the sharp rise in go (H2O2) between 400 and 420 V, near the mid point voltage (VD, corresponding to point D in Fig. 2), which is the critical voltage for the onset of full glow discharge in anodic CGDE, 1-3,7 show that the liquid phase reaction zone gets activated only when the glow discharge becomes full.
Table 1. Effect of current on y (H2O2) and y (OE2)
(Electrolyte, 0.12 M KH2PO4 – 0.07 M KOH; buffer of pH 7.0;
Anode Pt wire of length 5mm and diam 0.35 mm; cathode, 1cm2 Pt foil of
Thickness 0.20 mm; anolyte, temperature, 85 ± 2 °C;
Catholyte temperature, 32 ± 2c; quantity of electricity passed
30.00 C (3.11 X10-4 mol electron).
y(H2O2 ) y (OE2)n
Average mol/mol mol/mol
Current Voltage Watts electron electron
(mA) (V) (W) (± 1%) (± 1%)
40.1 450 17.3 0.49 0.82
52.1 480 23.9 0.50 1.26
62.5 500 29.6 0.50 1.70
*(OE2 ), O2 in excess of the Faraday law value.
Fig. 5. Variation of the yield of H2O2 with quantity of electricity (q) during anodic CGDE of KH2PO4 . KOH buffer of pH 7.0 at 430V.
Results of the effect of anolyte pH on the yield of H2O2 and OE2 lead to further interesting conclusions. When the anolyte pH is raised above 11.5, through the normalized integral yield, y (H2O2) drops sharply to a much lower value as one would expect due to base catalysis of H2O2 decomposition.21 The initial differential yield, g0 (H2O2) remains unaltered (Table 2). The constancy in g0 (H2O2) to changes in anolyte pH can be reasonably explained as being due to the initiation of the reaction near the plasma – anolyte interface by species originated in the anodic plasma and not in the liquid anolyte. The species entering the anolyte, presumably H2O+ gas would be sufficiently energized at the steep cathode fall of anodic CGDE near the plasma anolyte interface. It is further seen from Table 2 that g0 (H2O2) is significantly less then y(H2O2+OE2) pointing to the origin of a part of OE2 in a different reaction zone, i.e., presumably the plasma around the anode. This is still further manifested on comparing y(H2O2+OE2) at 12.0 and other pHs (Table 2). The larger yield at pH 12.0 results from the higher average current (55 mA) and the larger power dissipation (54 W) in the glow discharge at this pH as compared to an average current of 40 mA and power dissipation of 39 W at other pHs.
As already inferred, non-faradaic yields of anodic CGDE originate from the breakup of H2O molecules and the yields from the liquid-phase reaction zone comprised g0 (H2O2) each of H2O2 plus OE2, and H2. Thus, the “excess” non-faradic yields over and above the yields from the liquid-phase reaction zone can be computed as the total yield (y) of H2 (or H2O2 plus OE2) minus g0 (H2O2), i.e.,
y(H2) – g0 (H2O2) or y(H2O2 +OE2) – g0(H2O2)
A plot of these excess non faradaic yields against voltage (Fig. 6) shows the excess yields from in significant quantities from 250 V onward. The yields remain fairly constant at 0.36 mol/mol electron up to 420 V and thereafter register an almost linear rise.
It is interesting to note that during anodic CGDE, the glow discharge remains only partial over the Kelloggregion1.22 from 250 till 420V, the midpoint voltage, and becomes full therafter.1-3,7 More interestingly, there exists a close similarity between the voltage variation of the excess non-faradaic yields, y(H2) – g0 (H2O2) and that of the power dissipation in the glow discharge (Pdissp) (Fig. 6). Pdissp after remaining nearly constant (15.7± 0.1W) over the range 250-420V, the kelloggregion, 1,22 records an almost linear rise. In fact, a linear dependence of y (H2) – g0 (H2O2) on Pdissp could be observed (Fig. 7); the least squares line being…
y(H2) – g0(H2O2) = 0.13(Pdissp – 13.3) 
The result is very significant and indicates clearly that the plasma around the anode is the reaction zone for the excess non-faradaic yields. The analysis of the results of the study thus shows clearly that non-faradaic yields of anodic CGDE originate in two different reaction zones. The plasma around the anode and the liquid phase near the plasma – anolyte interface it also proves the soundness of the two-reaction zone model proposed earlier.7
In the reaction zone within the plasma around the anode, H2O vapor molecules dissociate into H2 and OE2 apparently following the mechanism of decomposition of water vapor in electrical discharges23
H2O à H˙ + OH˙ (2)
H˙ + H˙ à H2 (3)
OH˙ + OH˙ à H2O + —- O2 (4)
H˙ + OH˙ à H2O (5)
The mechanism is consistent with the appearance of H lines and OH bands in the emission spectra from anodic CGDE.24
On the other hand, in the liquid-phase reaction zone near the plasma-anolyte interface, several liquid H2O molecules break into H2 and H2O2 plus OE2 on being bombarded by each H2O+gas from the anodic plasma. This, as indicated already, gets sufficiently energized during entry to the anolyte, apparently following Hickling’s radiolytic mechanism1 originally proposed for interpreting chemical effects of anodic glow discharge electrolysis (GDE) where the anode is placed in the gas space above the electrolyte and the glow discharge is struck between the anode and electrolyte surface1
H2Ogas + nH2O ^^^à nOH˙ +nH˙ (6)
H˙ + H˙ à H2 (7)
OH˙ + OH˙ à H2O2 (8)
OH˙ + H2O à HO˙2 +H2O (9)
OH˙+ HO˙2 à H2O2 + O2 (10)
OH˙ + H˙ à H2O (11)
The formation of H˙ and OH˙ during anodic CGDE, as already pointed out, has been confirmed by an ESR study. 19 The H2O˙gas on entry into the anolyte would also give rise to a faradaic yield of O2 by normal electrolytic charge –transfer mechanism1, 7
H2O˙gas + H2O à H2O + H2O˙ à OH˙ + H2O˙ (12)
OH˙ + OH˙ à H2O + —- O2 (13)
Thus the total yield of anodic CGDE for the passage of each mole electron of electricity would consist of
- Faradic yields. 0.25 mol of O2 at the anode and 0.50 mol of H2 at the cathode;
- Nonfaradaic yields from the plasma reaction zone : m mol of H and m/2 mol of OE2 at the anode only at voltages beyond 250 V (the onset of partial glow discharge) where m relates to Pdissp as m = 0.13(Pdissp – 13.3) (Eq.1).m is fairly constant (0.36 ± 0.03) over 250 to 420 V, the Kellogg region where the glow discharge remains partial, and it varies linearly with Pdissp beyond 420 V when the glow discharge becomes full; and
- Non-faradaic yields from the liquid –phase reaction zone near the plasma –anolyte interface g0 (H2O2) mol each of H2O2 + OE2 and H2 at voltages beyond 410 V. The magnitude of g0 (H2O2) in mole/mole electron is 0.50 at 410 V, jumps to 1.60 at 420 V, and becomes stationary at 1.70 beyond 450 V.
It is interesting to note that the percent contribution of the plasma reaction zone to the non-faradaic yield of anodic CGDE is 100% over 250 to 400 V but drops through 41% at 410 V to 21 ± 1% over 415 to 450 V and thereafter increases with voltage and becomes 57% at 500 V (Table III).
It is worth pointing out that not only unconventional electrolysis at a glow discharge electrode but also conventional electrolysis can give rise to non-faradaic chemical effects. Electrochemical oxidation of H2 in the presence of O2 on a catalyst-anode of finely dispersed platinum on a graphite support in an aqueous KOH at 1-2 V was found to raise the rate of open-circuit catalytic oxidation of H2 and O2 to an extent by as large as 2,000% of the faradic efficiency.25 It was inferred that hydroxide ions act as promoters, and each ion supplied to the catalyst-anode surface causes oxidation of up to 20 hydrogen atoms. This non-faradaic phenomenon seems to be due to the effect of the changing catalyst-electrode potential, work function, and the local structure at the interface on the binding strength of the absorbents (Pt = 0 and Pt – H), reaction intermediates, and the transition state for the catalytic reaction. In contrast, non-faradaic yields of CGDE which comprise novel products (like H2 at the anode) also originate from the breakup of H2O molecules both in the plasma around the electrode and near the plasma-electrolyte interface by highly energized ions generated in the plasma.
During anodic CGDE of aqueous solutions, chemical effects at the anode are produced in three distinct modes
- Normal electrolytic charge-transfer processes at the plasma-anolyte interface giving rise to faradaic yield of O2 (0.25 mol/mol electron).
- Energy transfer processes similar to those in radiolysis of aqueous solutions in the liquid phase near the plasma-anolyte interface producing non-faradaic yields of H2 and H2O2 plus OE2. The liquid-phase reaction zone gets activated near or above the midpoint voltage (VD) of anodic CGDE. The yields from this reaction zone are insensitive to the power dissipated in the glow discharge and remain stationary at the g0 (H2O2) value, 1.70 mol/mol electron.
- Energy transfer processes similar to those in electrical discharges through water vapor within the plasma around the anode lead to additional non-faradaic yields of H2 and OE2.
The magnitude of these yields is linearly related to the power dissipated in the glow discharge region of anodic CGDE.
Council of Scientific and Industrial Research, New Delhi supported the work. The authors thank the referee of the original manuscript for drawing their attention to the studies on non-faradaic behavior during electrochemical activation of catalytic reactions and for useful comments.
Manuscript submitted October 15, 1997; revised manuscript received January 12, 1998.
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Contact Glow discharge electrolysis: a study on its origin in the light of the theory of hydrodynamic instabilities in local solvent vaporization by Joule heating during electrolysis.
Susanta K. Sengupta *, Ashok K. Srivastava, Rajeshwar Singh
Department of Chemistry,
Faculty of Science, Banaras Hindu University, Varanasi 221 005, India
Received 23 July 1996; revised 29 November 1996
Normal electrolysis at high voltages switches over spontaneously to contact glow discharge electrolysis. Raising the temperature and/ or lowering the surface tension of the electrolyte profoundly facilitates the transition. Results of a systematic study on the influence of these two factors on the breakdown voltage for normal electrolysis have been critically analyzed. It has been concluded that solvent vaporization near an electrode by Joule heating during electrolysis and the onset of hydrodynamic instabilities in local solvent vaporization are the prime causes for the breakdown of normal electrolysis. © 1997 Elsevier Science S.A.
Keywords: Contact glow discharge electrolysis; Hydrodynamic instabilities.
Contact glow discharge electrolysis (CGDE) is an unconventional electrolysis where electrochemical process occur in glow discharges at a plasma electrolyte interface. The phenomenon develops spontaneously at an electrode during conventional electrolysis whenever the applied voltage is sufficiently high irrespective of the electrolyte being aqueous, non-aqueous or molten. The onset of CGDE is marked by a steep drop in the current with the simultaneous appearance of a luminous sheath of gas at either the cathode or the anode. The electrode where the current density is larger, electrolyte resistivity is higher or electrolyte surface tension is lower tends to be the centre of CGDE [1-4]. A remarkable feature of CGDE is that its chemical yield at the glow discharge electrode is several times the Faraday law value and the products are novel for conventional electrolysis such as H2 at the anode and O2 at the cathode, etc. [1m2m5-7]. A good number of studies have been reported on the origin and chemical effects of the phenomenon  as well as spectroscopy of its light emission [18,19]. However, there several important aspects which need systematic investigation. It has been inferred from some of the evidence that the gaseous sheathing over an electrode is caused by a film of solvent vaporized locally due to Joule heating [1,2]. Although the view has been favoured by subsequent studies [3-7,9], a few of these are inclined to consider the volumetric rate of electrolytic gas evolution as a significant factor and even the determining factor for the transition of normal electrolysis to CGDE [9-12]. The aspect thus needs further probing. The mechanism of the growth of a stable gaseous sheath which is steadily anchored to the contour line of the electrode well underneath the electrolyte solution during normal electrolysis is of intrinsic interest and calls for a systematic investigation. Two variables which have been found to influence profoundly the breakdown of normal electrolysis in the transition to CGDE are electrolyte temperature and surface tension. Herein, the results of a study on the influence of these two factors on the transition from normal electrolysis to CGDE are discussed in the light of the theory of hydrodynamic instabilities in solvent vaporization near an electrode due to Joule heating during electrolysis.
The work was carried out by studying current-time (I-t) as well as current-voltage (I-V) characteristics of electrolysis under the following set of standard conditions : Cell, symmetrical Corning H-type (height 11 cm; diameter, 30 mm) having a 7.5 cm long bridge (diameter, 15 mm) fitted with a G-5 sintered glass disc separator oat a height of 1.5 cm from the base of the cell; anode, platinum wire of length 5mm and diameter 0.35 mm; cathode, usually a platinum foil (1 cm X 1 cm) of 0.20 mm thickness, although a 45 mm long platinum wire of 0.35 mm diameter (i.e. identical with the anode) was used in the study on I-t characteristics : the electrodes were dipped to a depth of 3cm; electrolytes, aqueous 0.05 M K2SO4 ( the reference inert-type electrolyte) with or without a surfactant additive (sodium dodecyl sulphate (SDS), N-cetyl N, N, N-trimethyl ammonium bromide (CTAB or Aerosol OT at varying concentrations). Surface tension was measured by a precalibrated stalagmometer; pressure, atmospheric; ambient temperature, (32 to 82) ± 2°C. Current was supplied from an Aplab-7322 medium voltage d.c. power supply which provided a maximum current of 1.5 A at voltages up to 600 V. The current passing through the cell and voltage across the circuit were measured using a Philips PM2518X digital multimeter.
- Results and discussion
- Current-time characteristics at different temperature
The current-time (I-t) characteristic of electrolysis of 0.05 M K2SO4 between identical platinum wire electrodes at an applied voltage of 100 V at 45 ± 2°C (Fig.1) consists of an almost linear part PQ representing normal electrolysis followed by a transition period QR. At R, where the current drops by a large amount (325 mA), a gaseous mantle of pulsating thickness develops at the anode and the situation stabilizes. The fluctuation in current diminishes with further progress in time up to the point S beyond which a stable gaseous film forms around the anode. Fig. 1 further shows that when the ambient t temperature is raised to 75 ± 2°C, the formation of the gaseous sheath (corresponding to the point R) occurs much earlier. This can be explained as a higher electrolyte temperature would require less Joule heating for local vaporization and thus facilitate the vapour sheathiong on the electrode which results in the breakdown of normal electrolysis. It is worth noting that the maximum current passing through the cell just before gas sheathing the electrode (the point Q, Fig. 1) is considerably reduced (580 mA to 420 Ma) on raising the temperature from 45°C to 75°C.
If the rate of electrolytic gas evolution can be considered as the prime cause for the breakdown of normal electrolysis, raising the temperature, which causes lowering of the current maximum (corresponding to the point Q) and consequently a significant lowering in the volumetric rate of electrode gas evolution, would not have facilitated the electrode gas sheathing and the breakdown of normal electrolysis. Moreover, when the ambient temperature is lowered below 20°C, Joule heating becomes insufficient to cause solvent vaporization and no breakdown of normal electrolysis could be observed even at 600 V, although the current passing (1040 mA) and the rate of gas evolution is several times that which can cause the breakdown at a higher temperature. The same conclusion is also arrived at from the results of the effect of temperature on current-voltage (I-V) characteristics of electrolysis, the breakdown voltage for normal electrolysis VB and the current passing at VB falling from (320V, 670 mA) to (160V, 410 MA) on raising the temperature from 35°C to 75°C. The results of the study thus confirm the soundness of the earlier view point [1,2] that the primary cause for the breakdown of normal electrolysis is vaporization fo the solvent due to Joule heating in the vicinity of an electrode.
- Effect of anolyte surface tension
The present body of evidence is strongly in favour of the view that solvent vaporization and not electrolytic gas evolution at the electrode is the prime factor for the breakdown of normal electrolysis there. The question arises : how do vapour bubbles formed at an electrode coalesce and form a stable sheath of vapour around the electrode well underneath the electrolyte solution ? It is thus of interest to study the influence of the electrolyte surface tension on gaseous sheathing on the electrode resulting in the breakdown of normal electrolysis. It has been shown that surface tension has a profound effect on the transition of normal electrolysis to CGDE . Addition of a surfactant to the anolyte decreases the breakdown voltage VB (for normal electrolysis) at the anode as well as the current at VB (to be called the critical current Ic) to a significant extent whether the surfactant added to the anolyte is cationic (CTAB) or anionic (SDS) (Table 1). Thus, lowering of surface tension promotes considerably the growth of vapour film at the electrode. This aspect is discussed further in Section 3.3.
- Applicability of the theory of hydrodynamic instabilities in solvent vaporization near an electrode by Joule heating during electrolysis
The observed effect of electrolyte surface tension on the transition of normal electrolysis to CGDE is highly interesting. It is well known that according to the static theory of contact angle, lowering of anolyte surface tension would enhance the wettability of the anode and hinder the growth of bubbles on it and thus raise the breakdown voltage VB for normal electrolysis, whereas the observed effect is just the opposite. A similar effect of surface tension is observed on the growth of a phenomenon known as `burn-out’ or `boiling crisis’ . This develops in the course of boiling a liquid in contact with a heating solid surface when the surface becomes blanketed by a continous permanent sheath of vapour resulting in a sharp drop in the specific heat flow from the surface to the liquid. This occurs when the rate of vaporization vis a’ vis the specific heat flow goes beyond some critical value. The onset of `boiling crisis’ is significantly facilitated by lowering of surface tension of the liquid. Furthermore, there is a strong analogy in the morphological events of vapour or gas evolution leading to the onset of both CGDE and `boiling crisis’. The I-V (current-voltage) characteristics of electrolysis changing over to CGDE and ф-▲T (specific heat flow, ф vs. surface temperature relative to the liquid ▲T) characteristics of boiling changing over to `boiling crisis’ are very similar.
It appears that there is a basic cause common to the two phenomena. The onset of `boiling crisis’ is well explained in terms of the conditions of hydrodynamic instabilities described by Helmholtz and by Taylor [20-24]. So, it is worth examining the applicability of the hydrodynamic aapproach in solvent vaporization at an electrode due to Joule heating during electrolysis. Such an approach based on the Helmholtz and Taylor instability conditions in gas evolution at an electrode during molten salt electrolyses has been fairly successful in explaining the growth of anode effect (anodic CGDE) in molten salt electrolyses . A similar line of reasoning was advanced for aqueous salt electrolses [10,11].
However, as inferred from the present study, solvent vaporization and not electrode gas evolution is the prime factor for the breakdown of normal electrolysis. It is thus worth attempting the hydrodynamic approach in solvent evaporation at an electrode during electrolysis to understand the mechanism of breakdown of normal electrolysis. According to this approach, when the rate of vapour evolution at an electrode attains the critical value for the onset of instabilities, streams of vapour bubbles in the electrode region would have their vapour|liquid boundary surface broken and also become close enough to coalesce into a continous gaseous blanket on the electrode. An increase in surface tension would hinder the onset of hydrodynamic instabilities. A proportionality between the critical rate of solvent vaporization Rc and the critical electrolytic current density jc (which corresponds to the current at breakdown voltage VB on the I-V curve) may be postulated. For the case of wire electrodes of radius r, having high enough length to radius ratio, two correlations would follow [10,11,20] :
Jcr ½ = constant for a given electrolyte surface tension;
Jcу – ½ = constant for a given electrode.
from the expression for the critical rate of solvent vaporization Rc at wire electrodes of radius r
PG(PG + PL)
X — (PL –PG) + ————————
y (r + ▲r)2
where p is the density, g is the acceleration due to gravity, ▲r is the thickness of the vapour layer, L (subscript) denotes liquid, G (subscript) denotes gas, neglecting (g/y)(pL –pG) with respect to 1/(r + ▲r)2 = 1/r2, ▲r = 0.01 mm  is much less than r (0.10-0.25 mm) selected in the study.
Obviously, it is of basic interest to examine the applicability of the two correlations for the breakdown of normal electrolysis of an electrolyte containing varying concentrations of different surfactants and for wire anodes of different radii. The results obtained on the correlation between the critical current density jc for a 5 mm long platinum wire anode of 0.175 mm radius and surface tension y of 0.05 M K2SO4 anolyte containing varying concentrations of SDS or CTAB (Table 1) are quite encouraging. In spite of significant variations in surface tension of the anolyte medium, values of jc y – ½ were found to remain fairly constant within ± 6.8%. Further, the nature of charge on the surfactant ion does not influence the results. Thus, according to the hydrodynamic theory, lowering of surface tension acts in the sense of breaking up the gas|liquid interface in the streams of vapour bubbles over the anode and thus facilitates the coalescence of bubbles leading to vapor sheathing of the entire electrode surface and consequently the breakdown of normal electrolysis at a lower jc and hence lower VB. The other coreralation jcr ½ = constant within ± 6.3% has also been found applicable over a significant range of the values of the radii of the anode wire (0.10 to 0.25 mm) whether the anolyte contains any surfactant or not (Table 2). An electrode of lower radius would require a higher jc (jcr ½ = constant) and thus a lower I (I=constant X 2lr ½ , where l is the length of the wire) and a lower breakdown voltage VB.
Thus the sequences of events leading to fully grown CGDE can be described as normal electrolysis, solvent vaporization by Joule heating near an electrode, onset of hydrodynamic instabilities leading to complete and steady vapour sheathing over the electrode and glow discharges across the sheath.
All these results indicate that solvent vaporization close to an electrode due to Joule heating, not electrolytic gas evolution,k and the onset of hydrodynamic instabilities in solvent vaporization at the electrode, are the two prime factors for the transition of normal electrolysis to CGDE. The conditions of higher electrolyte temperature, lower electrolyte surface tension and thinner electrode favour the breakdown of normal electrolysis.
The work was supported by a grant from the Council of Scientific and Industrial Research, New Delhi, India. One of the authors (A.K.S.) is grateful to the above agency for a research fellowship.
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