Isothermal or non isothermal? Direct or indirect heating? Permanently or pulsed?


Isothermal heating
of an electrolysis cell is not a new method, although e.g. a „hot-tub voltammetry“ has been defined. Fast and independet T-variation in isotherm systems is not useful till now.

By non isothermal heating (of the electrode or of a near-electrode solution layer), real thermoelectrochemistry is attainable.

Permanent heating can be done directly or indirectly.
T keeps always below the boiling point.
Constant electrode temperature is achievable.
A micro stirring effect arises.

Pulse heating is done preferably by direct heating, otherwise the T variation is sluggish.
T values far above boiling point are attained.
Solution keeps quiet, nearly without any convection.

History of permanent heating


Directly, electrically
DUCRET, CORNET (1966): „Électrode a convection thermique“

Indirectly, electrically
HARIMA and AOYAGUI (1976): Gold layers heated electrically.

Directly, optically
Laser beams focused on electrode surfaces.

The experiments listed above did not find much attention. The scientific community started to interest when hot-wire electrochemistry became successful due to the trick with the „symmetrical electrode arrangement“. New techniques were proposed:

  • Microwaves
  • High frequency heating
  • resistive heating of a solution layer by alternating current
Principle of Hot-Wire-Electrochemistry

The performance of voltammetric sensors can be improved considerably when operating at increased temperature regime. This way slow reactions proceed with much higher rate, and transport processes become faster.

The new method proposed here can be used to built amperometric microsensors which may find application, e.g., as detectors in flowing solution. These sensors may work at temperature far above the boiling point of water without causing the solution to boil, i.e., to form vapour bubbles.


The arrangement consists of thin wire pairs in a symmetric configuration. Every pair is heated by high frequency (up to 1 MHz) ac current, and the same time polarized potentiostatically. During the heating time, temperature of the wire and of the neighbouring solution layer rises sharply, but the rise ceases more and more then. After longer time (ca. 0.2 s) there starts convection, with the consequence of establishing a stationary state.

The actual temperature of the electrode surface during current measurement can be controlled precisely to a predetermined value by varying the ac heating current amplitude. We know also very well the temperature profile from wire into the solution (for the time before convection has started). The technical problem of recording very low electrolysis current (down to nanoamperes dc current) when simultaneously ac current of very high magnitude (up to 1 ampere ac) is applied to the working electrode was solved adequately. The symmetric electrode pair arrangement permits to compensate for stray current that could be induced from heating circuit to measuring circuitry inside the electrolysis cell.

There are two ways to work with hot wire electrodes:

  • A stationary temperature by continuous heating can be established.
  • Short heating pulses are applied in a special sequence alternating with polarisation cycles. Thus, voltammetry above boiling point comes within reach.

Advantages of electrically heated electrodes

Advantages of electrically heated electrodes in comparison to hydrodynamic electrodes:
  • small dimensions, cheap
  • sigmoidal curve shape like with hydrodynamic electrodes
  • current magnitude like with makroelectrodes
  • sensitivity higher than rotating disk, but lower noise
  • temperature can be varied as an additional parameter
Convection at heated electrodes


Voltammograms at heated electrodes are characterized by sigmoidal shape. The reason for sigmoidal curve shape is a stationary diffusion layer.

It can be assumed, that the diffusion layer is positioned inside a more than 10 times thicker hydrodynamic layer.

In the field of heated electrodes there is an analog for LEVICH´s equation. The structure is as expected: a quiet diffusion layer, surrounded by a „thermal distribution layer“ which takes the place of the „hydrodynamic layer“.

In all experiments with aqueous solutions, and also with some non aqueous solutions, it has been found, that a strict proportionality exists between temperature of wire surface and of the square of heating current amplitude.
It can be concluded that a "thermal distribution layer" of constant thickness must exist.

Comparison of permanently heated microelectrodes with hydrodynamic electrodes

  • heated microelectrodes follow equations with a degree of complexity not higher than that of hydrodynamic electrodes
  • heated electrodes result in noise free voltammograms
  • heated electrodes are small, cheap and easy to maintain
  • heated microelectrodes can be placed at remote places
  • heated microelectrodes can be operated with arbitrarily adjustable temperature as an additional parameter. Their surface temperature can be controlled. Hence, they act like a micro-thermostat.
  • heated microelectrodes display a controlled temperature only at the place where this is desired. Outside a layer of some hundreds of microns, no temperature change is excited.
History of pulsed heating

1975
Barker, Gardner
Laser pulses
Hg el.; fundamentals
1982
Benderskii, Velichko
relaxation of double layer
1983
B. Miller
T modulation voltammetry
1988
Feldberg
kinetics by T jump method
1992
Olivier
Thermo-electrochemical impedance
1983
Gabrielli
Joule heating of wires
kinetics (T jump)
1994
Gründler
sensors, anal. chem.
1998
Compton
microwave heating
anal. chem.
2001
RF heating el.
anal. chem.
2002
Baranski
Joule heating of solution
fundamentals


Theory of pulsed heating - The symmetric electrode arrangement
Heat pulses
 


Temperature rise and decay at heated electrodes: delayed start and subsequent cooling effect.


Actual current-time function for a potential staircase with overlaid heat pulses: redox entropy made visible.

Temperature Puls Voltammetry

 


Temperature Puls Voltammetry, long-time version

Reversible redox couples (ferri-ferrocyanide and iron(II)/iron(III)sulfate:
Each heat pulse yields one point in the diagram. Altogether, they add to give voltammograms.
Top diagram cor-responds to a temperatur of 160°C.
Reaction medium is the metastable, superheated water. We are working in the region of boiling retardation.



Temperature Puls Voltammetry, short-time version
TPVs of K4[Fe(CN)6] in 0,1M K2SO4:
Temperature with all diagrams 80°C (by varied pulse duration).
Top sigmoide curve: Permanent heating.
Lower sigmoidal curve: without heating (room temperature).
.