Electrode processes on graphite electrodes
Citation for published version (APA):
Janssen, L. J. J. (1975). Electrode processes on graphite electrodes. Chemie, Ingenieur, Technik, 47(4), 151-.
https://doi.org/10.1002/cite.330470414
DOI:
10.1002/cite.330470414
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Published: 01/01/1975
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Wissenschaftliche Forschungsarbeit
188/75
-
Synopse
CHEMIE
INGENIEUR
TECHNIK
Electrode processes on graphite
electrodes
*
L.
1.
J. Janssen* *
Since the invention of DSA electrodes for chlorine produc- tion in the chlorine-alkali industry the use of graphite electrodes has declined. At the present time about 70% of the chlorine produced is formed at graphite electrodes [l]. The following factors are of special interest for chlorine evolution a t graphite: the nature and roughness of the graphite electrode surface and the reaction mechanism of chlorine formation.
It is well known
[a]
t h a t the potential of the graphite anode varies during the evolution of chlorine as a function of the time t of anodic polarization. The potential initially rises t o a maximum ; thereafter it decreases rapidly at first and then gradually until a constant value is reached. The maximum value of the potential during anodic polarization depends on the geometric current density and on the composition of the anolyte [3]. The capacitance cp,
determined by the potentio- static potential jump method, and thus the roughness fac- tor [2], is likowiso a function of the time of polarization: the capacitance cp increases slowly with time until the potentialLO
0 L '
600 700 800 900 1000 1100 1200 1300 rnV 1500 E V S S C E
Fig. 1. Voltammograms on Acheson graphite electrodes which had been polarized with a n anodic current density of 30 mA/cmZ
in a 4 M SaC'I c 1 31 HCI solution a t 25 "C for different times. The numbers next to the curves indicate the total quantity of charge per em2 geometric surface area which had been passed through the electrode.
reaches its maximum ; with prolonged polarization the capacitance increases rapidly a t first and then slowly rea- ches a constant value at about the same time as the potential becomes constant. The nature of the surface of the graphite anode also depends on the time of anodic polarization [2]. This is clearly demonstrated by the e/J curves determined after several anodic polarization cycles. These curves are represented in Fig. 1. The pronounced change in the .s/J curve occurs simultaneonsly with a marked increase in surface area of the electrode. This ageing process is due to the oxidation of graphite to CO and COZ and manifests itself
Lecture, presented at the annual meeting of the GDCh-Working Group "Applied Electrochemistry" a t Leverkusen (Germany), October 16-17, 1974.
* * Dr. L . J . J . .Janssen, Laboratory of Electrochemistry, Eind-
hoven Universit,y of Technology, Eindhoven, The Netherlands.
in a change in the nature of the graphite electrode and a n increase in surface area.
From voltammograms recorded in 4 M NaCl
+
1 M HC1 solution for highly orientated pyrolytic graphite electrodes, whose edge plane or basal plane served as the electrode sur- face, and for Acheson graphite electrodes, Janssen [4] has concluded t h a t two different surface compounds, lp oxygen and h p oxygen, are formed on all these graphite electrodes. I n the stationary state lp oxygen is present in the potential range of chlorine evolution below about 1250 mV and hp oxygen a t potentials higher than about 1350 niV vs SCE.Hine
et al. [5] also support this conclusion.The voltammograms [4] on the electrodes investigated for a potential sweep between about 1 100 mV and about 1500 mV are similar t o the voltammogram given in Fig. 1 for a n electrode which had been charged with 21 Ahlcm2. During chlorine evolution a n aged electrode is consequently covered with h p oxygen and/or with chlorine atoms.
Atomic chlorine is adsorbed on the surface of the graphite crystallites [6] during chlorine evolution. The degree of coverage with atomic chlorine is proportional to Q c (0), i.e. t o
the quantity of charge required for liberation of all the chlorine present on a graphite anode during the electrolysis. From the experimental relationship between the current density, the potential, and Q C ( O ) , it clearly appears [7] t h a t for a n aged anode the chlorine evolution occurs according to the Volmer/Heyrovsky mechanism, viz.
C1- + clad
+
e-C1-
+
Clad + Clz+
e- Heyrovsky reactionwhere the Heyrovsky reaction is the rate-determining step. Further, it is established t h a t for a n aged electrode in 4 M NaCl
+
1 M HC1 the ratio between the exchange current density of the Volmer and that of the Heyrovsky reaction is equal to 20, the transfer coefficient of the Heyrovsky reac- tion is 0.5, the activation energy is 8.4 kcal/mol chlorine, and the degree of coverage with atomic chlorine is 0.05 at the reversible chlorine/chloride electrode potential.Chlorine is also formed according to the Volmer/Heyrovsky mechanism at graphite electrodes whose su:.face differs from t h a t of an aged electrode ; however, the kinetic parameters then depend on the nature of the surface of the graphite
anode. Received: November 26. 1974
Volmer reaction
[l] V . de Nora, ISE-Congress, Brighton 1974.
[2] L. J . J . Janssen, J . G. Hoogland, Electrochim. Acta [London]
1 4 , 1097 [1969].
[3] L. J . J . Janssen, to be published.
[4] L. J . J . Janssen, Electrochim. Acta [London] 19, 257 119741.
[5] F . Hine, M . Yasuda, M . Iwata, J. electrochem. Soc. 1 2 1 , 749 [6] L. J . J . Janssen, J . G . Hoogland, Electrochim. *Acta [London]
[7] L . J . J . Janssen, J . G. Hoogland, Electrochim. Act,a [London]
119741.
1 5 , 339 [1970]. 1 5 , 941 [1970].
Keywords: Chlorine-alkali electrolysis, Electrotlc procssses, Gra-
phite electrodes.
The complete manuscript of this article covers 20 pages and in-
cludes 12 figures and 6 references It is available as a photocopy
or microfiche (No. 188175) An order form will be found on the
page facing the back cover of this journal.
Chemie-/ng.-Techn. 47. lahrg. 1975 l Nr. 4 151
