Properties of Slurry Electrodes from Activated Carbon Powder

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Abstract
Slurry electrodes (suspensions of particulate electrode material) are suitable for carrying out electrochemical processes outside an electrochemical cell while removing the required charge from one electrode of such a cell. The suspended material must be stable against electrochemical attack within a certain range of potentials and must exhibit a sufficient capacity. Properly pretreated activated carbon powder can be polarized in acidic aqueous solution within potentials between about −0.2 and +1.0 V (vs. standard hydrogen electrode). In alkaline solutions, the limit towards positive potentials in considerably lower, about +0.3 V. Within these ranges, an almost constant capacity of about 100 … 120 F/g is observed, and there are no primary indications for Faradaic processes. While the capacity can essentially be attributed to a double‐layer behaviour, electrochemical surface reactions without a distinct redox potential may well be present; this presumption is supported by an examination of the ion exchange between the electrolyte within the pores of the particles and the external electrolyte. Frequent contacts with the “feeder” electrode are required for charging the capacity of the particles because, for particles with diameters between 0.2 and 0.5 mm, typically less than 0.01% of the charge necessary for complete adaptation to the feeder electrode potential is transferred during a single contact. This is due to the short duration of a collision (2 to 7 μs at liquid flow rates about 1 m/s towards the electrode) and to the sum of electrolyte and contact resistances. In 4 M H2SO4, the electrolyte resistance ranges from 5 to 2 Ω. The values of the contact resistance (15 to 5 Ω) are compatible with the behaviour expected from the theory of an elastic collision.

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