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.

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
  

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.
  
  

• 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
  

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.

• 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.