Nonequilibrium Thermodynamics for the Stability Study of Contact Glow Discharge Electrolysis

Abstract

Beyond a critical voltage in a two‐electrode configuration, the conventional electrolysis is no longer sustained when a thin gaseous envelope covers the working electrode, and the emission of visible light is observed. The system is examined as a process far from thermodynamic equilibrium, with the identification and computation of conjugated thermodynamic fluxes and forces generating the entropy production. The quadratic form of the entropy variational function is employed for the stability discussion, in the Lyapunov's sense of stability for steady state autonomous systems. Thermodynamic arguments, compared with experimental observations on the gas film dynamics, show that the stability of the phenomenon is mainly governed by thermal and electrochemical contributions of the electrochemical cell.

On the N-shaped Conductance and Hysteresis Behavior of Contact Glow Discharge Electrolysis

“…Thus, CGDE phenomena exhibits two levels of configuration: a global voltage-controlled N-shaped negative differential resistance, with an embedded current-controlled S-shaped negative differential resistance around the critical voltage [28]. This makes the system transition between two distinctly stable states: conventional electrolysis and plasma micro-discharges [10].…”

“…The voltage range containing the hysteresis is highly dependent on the electrochemical configuration of the cell, as this has to do with the critical voltage of the system. The voltage U c is not only a function of the electrolytic medium (chemical composition, conductivity, temperature) but also of the geometric configuration of the cell (electrodes size and shape, interelectrode distance, electrodes' areas ratio), and local fluctuations of other thermodynamic variables [10]. Hysteresis is encountered in a wide range of disordered systems which can be trapped in metastable configurations.…”

“…The system becomes more and more macroscopically complex, and is no longer able to sustain the standard electrolysis state; it becomes inevitably unstable while transitioning into a new state or order following an abrupt change in the current transport mechanism http://dx.doi.org/10.1016/j.electacta.2015.03.154 0013-4686/© 2015 Elsevier Ltd. All rights reserved. [10]. The first point C in this region is characterized by the coordinates critical or threshold voltage, U c , and critical current, I c , which are defined by various parameters, such as the geometric configuration of the electrochemical cell, the working electrode surface state, and the chemical composition and conductivity of the electrolyte [11], whereas at the second point D the current tends to zero confirming that the gas film is electrically isolating the electrode.…”

“…Additionally, the current mathematical models are essentially hydrodynamic [11,8,19] or stochastic [5,4,6], involving analyses based on local dynamic variables most of the time difficult to estimate, such as the size and geometry of gas bubbles, or their fractional electrode coverage [20]. The proper understanding and control of CGDE via global thermodynamic and measurable variables is necessary [10] to detect unwanted behaviors such as in the case of HallHéroult process for aluminum smelting, for example. The anode effects, which are due to brief upsets of the electrochemical conditions during the electrolysis of alumina in molten cryolite, lead to undesirable voltage increases with unstable production rates and extremely harmful environmental impacts [19].…”

Band‐Pass Filter and Relaxation Oscillator using Electric Double‐Layer Capacitor

“…Carbon graphite films were prepared using a two‐electrode CGDE setup as reported is previous papers ,,. A polycrystalline graphite rod of 0.5 cm diameter was slightly immersed in a 20 wt.% sulfuric acid solution and served as the plasma‐emitting cathode, and a large surface area platinum coil was used as the counter electrode .…”