Ohmic field or gate field?
In bipolar electrochemistry a current through the electrolyte generates a field that induces the potential difference ΔV = E·l across a conductive object of length l. Under which conditions does an external field act on an electrode surface, and what role does the current flow through the electrolyte play?
Two kinds of field have to be distinguished: an ohmic field carried by a current through the electrolyte (E = j/κ), and a gate field applied behind an insulator without direct current through the electrolyte. Geometrically, the generation 2 EFCE cell is a gate arrangement: the field electrode lies behind 6 µm of mica.
- Ohmic field
- directly in front of the 5 µm disc at 10 pA and κ ≈ 2 S/m: ≈ 0.25 V/m
- Control field
- between field electrode and working electrode: order of magnitude 10⁴–10⁵ V/m
- Gate charge
- 20 V across 6 µm of mica (εr ≈ 6): ≈ 0.018 µC/cm², corresponding to about 10⁻⁴ monolayers; it resides at the mica
Gate and ohmic arrangement in the same cell
- Gate arrangement: field electrode behind mica, control curve as reference.
- Ohmic arrangement: field electrode in direct contact with the electrolyte, otherwise identical geometry.
- Frequency and dilution series: variation of the controller frequency (100–1000 Hz) and of the electrolyte concentration to separate charging currents in the gap from Debye screening.
The effect does not depend on the DC path. Possible causes are charging currents in the gap or coupling via leads and reference.
The effect requires a current-carried field. The gate geometry is then ineffective, and the cell has to be rebuilt accordingly.
- Possible coupling via the leads, since the working electrode is grounded and the reference carries current.
- Whether the field reaches the interface has not been demonstrated (Debye length ≈ 1.2 nm).
- The unit of the field axis in the first measurement (×10⁴ or ×10⁵ V/m) is still being clarified.