Reports about ozone generation can be traced back to 1785 when van Marum, a Dutch physicist, found electric discharge in the air results in a characteristic ozone odor. In 1801 the same odor was observed during a water electrolysis experiment carried out by Schönbein [5- 7].
Because ozone is a highly reactive gas under ordinary conditions, it has to be generated “on site” [5,20,54]. Nowadays, the “on site” ozone generation is carried out in great scale using the silent electric discharge process via Corona reactors, while in the case of medium and small scale applications ozone can be also generated via water electrolysis using specially designed Electrochemical reactors [5-7,54,57].
The main advantage presented by Electrochemical Ozone Production (EOP) is the high concentration achieved in the gaseous phase (O2 + O3), which can range from 10 up to 35
wt% [63-80]. EOP is gaining popularity due to a couple of features that are not achieved using the conventional Corona process (e.g. production of the ultra pure ozone at very high concentrations) [54]. Besides, specially designed Electrochemical reactors operating in electrolyte-free water permit the direct application of ozone into water streams, thus
eliminating problems concerned with the mass transport from gas to the condensed phase [54,78,79].
Costs associated with ozone production using Corona reactors have dropped by almost 50% in the last two decades and, therefore, a great number of new industrial applications has appeared in recent years [5,54]. Depending on the particular ozone generation system, ozone generation consumes power at a rate of 8 to 17 Wh g-1 O3 [5,9,54].
Ozone generation via Corona process - where a dry gas, either air or pure oxygen, is
subjected to a silent electrical discharge, a reaction between the oxygenated species (O• + O2),
assisted by the free energetic electrons that are created from an electric spark, takes place in the gas phase generating the O3-molecule [7,54,81,82].
Figure 3 presents a scheme describing a Corona reactor and the elementary processes leading to ozone formation, which take place during the silent electric discharge inside the discharge chamber.
The voltage required to produce ozone by corona discharge is proportional to the pressure of the source gas in the generator and the width of the discharge gap. Theoretically, the highest yield (ozone produced per unit area of dielectric) would result from a high voltage, a high frequency, a large dielectric constant, and a thin dielectric [9,20,54].
However, there are practical limitations to these parameters. As the voltage increases, the electrodes and dielectric materials are more subject to failure. Operating at higher frequencies produces higher concentrations of ozone and more heat requiring increased cooling to prevent ozone decomposition. Thin dielectrics are more susceptible to puncturing during maintenance. So, the design of any commercial Corona reactor requires a balance of ozone yield with operational reliability and reduced maintenance.
Two different geometric configurations for the electrodes are used in commercial ozone generators [9,27,81,82]: (i) concentric cylinders and (ii) parallel plates. The parallel plate configuration is commonly used in small generators and can be air cooled. The glass dielectric/high voltage electrode in commercial generators resembles a fluorescent light bulb and is commonly referred to as a “generator tube” [81,82].
Most of the electrical energy input to an ozone generator (about 85 percent) is lost as heat [54]. Because of the adverse impact of temperature on the production of ozone, adequate cooling should be provided to maintain generator efficiency. Excess heat is removed usually by water flowing around the stainless steel ground electrodes. The tubes are arranged in either a horizontal or vertical configuration in a stainless steel shell, with cooling water circulating through the shell.
Ozone generators are classified by the frequency of the power applied to the electrodes. Low frequency (50 or 60 Hz) and medium frequency (60 to 1,000 Hz) generators are the most common found in the water industry, however some high frequency generators are available [54]. Medium frequency generators are efficient and can produce ozone economically at high concentrations, but they generate more heat than low frequency generators and require a more complicated power supply to step up the frequency supplied by utility power. New installations tend to use medium or high frequency generators [82].
Figure 3. Corona reactor and the elementary processes taking place during the silent electric discharge (adapted from ref. [54]).
Although the corona technology requires a lower specific power consumption the concentration of the O3 in the gaseous phase (O2 + O3) presented by conventional devices is
low (~2.5 to 7.5 wt%) [81,82], thus restricting its use in several important applications involving degradation of the recalcitrant pollutants. In very special cases the O3-concentration
furnished by a Corona device can reach a maximum of ∼15 wt% [9,54].
In principle, ozone generation can be also carried out via Photochemical technology, where pure oxygen or air, when irradiated by the UV light (λ = 254 nm) inside a photochemical reactor, produces ozone [7]. However, the photochemical ozone generation presents a very high specific energy demand (~ 1 kWh g-1) due its low efficiency and, therefore, it is very expensive when compared with the Corona process [5,20,54].
Electrochemical Ozone Production (EOP) – EOP presents a couple of features that are
not achieved with the Corona process, thus making it an interesting alternative for several medium and small ozone applications: (i) investment costs (per unit mass of produced O3) are
considerably lower than for the conventional Corona technology and (ii) concentrations of O3
in the product gas that can be achieved are higher [5,7,54].
Several technological advances related to EOP were achieved in recent decades [54,57,67,79]. Electrolytic ozonizers based on Solid Polymer Electrolyte technology (SPE), which operate in electrolyte-free water and under ambient temperature conditions, permit ozone application directly into water streams ready for various oxidizing and/or disinfectant applications. In this case the total energy demand is minimized, since O3-production is carried
out at ambient temperature (refrigeration is not necessary) and the high ozone mass transfer rate obtained in this case avoids the accessories such as gas diffusers and pumping systems [54].
EOP can be also carried out employing electrolytic ozonizers using specially designed electrolytes [67]. This technology furnishes a very high efficiency (≥ 35 wt%), which makes this electrochemical ozonizer rather competitive with the corona technology in several different applications where a higher O3-concentration in the gaseous phase is necessary. In
this case the cathodic process is the reduction of the oxygen present in the air and, as a consequence, the specific energy demand is very close to that presented by a conventional Corona device [54,57,67].
As previously discussed, the reaction mechanism leading to ozone generation during water electrolysis is a rather complex one [6,75]. This electrode process is less favorable from the thermodynamic point of view when compared to the oxygen evolution reaction. Therefore, one has that EOP can only be carried out using specially design electrocatalysts [54]. Fundamental and applied aspects concerned with the EOP process were reported by Da Silva et al. [5-7,54,75-77].
The fundamental process taking place in the membrane electrode assembly (MEA – see Fig. 4), which comprises a “sandwich” of gas diffusion electrodes (anode and cathode) pressed against a solid polymer electrolyte (e.g. Nafion 117 from Dupont®) using porous current collectors is presented below:
ANODIC PROCESSES (+): Oxygen Evolution vs. Ozone Production H2O(aq.) + S → S---HO•(ads.) + H+(aq.)---SPE + e-(current collector)
S---HO•(ads.) → S---O•(ads.) + H+(aq.)---SPE + e-(current collector)
2S---O•(ads.) → 2S---O2(ads.) → O2(g) (oxygen evolution – Eo = 1.23 V)
2S---O2(ads.) + S---O•(ads.) → O3(g) (ozone production – Eo = 2.07 V))
CATHODIC PROCESS (−):
2H+(aq.)---SPE + 2e-(current collector) → H2(g) (hydrogen production),
where: S is an active surface site and SPE is the solid polymer electrolyte.
Figure 4 presents the design of an electrochemical ozonizer based on the SPE technology recently developed by Da Silva and Jardim (Patent – EGT/FAPEMIG, Brazil).
The main difference between the ozone generation process using the electric silent discharge and the electrolysis processes is the fact that ozone are produced in the bulk gas phase and at the solid/aqueous interface, respectively. While in the case of the bulk gas phase reaction the electric spark can dissociate the O3-molecule inside the Corona (ozone
decomposition step), thus limiting the efficiency of this process for ozone production, in the case of the surface reaction taking place at the electrode/solution interface inside the Electrochemical reactor the electric potential gradient does not lead to O3-degradation; the
ozone molecule is constantly removed from the electrode surface by forced convection and then spontaneously dissolved in water.
Therefore, EOP permits production of ozonated water containing a very high ozone concentration [54]. Besides, the electrolysis of water (condensed phase) resulting in formation of the O2-O3 mixture (gaseous phase) eliminates the use of compression systems in order to
Figure 4. Design of an electrochemical ozonizer based on the SPE technology developed by Da Silva and Jardim.
Since pure hydrogen is generated during EOP, one has that this clean fuel can be in principle used in a hydrogen fuel cell in order to reduce the total energy demand dissipated in the whole process [54]. Figure 5 shows a scheme representing a hybrid environmentally friendly electrochemical system proposed for ozone production using the electrochemically generated hydrogen as an auxiliary energy source.
Figure 5. Scheme representing the hybrid environmentally friendly electrochemical system proposed for ozone production using the electrochemically generated hydrogen as an auxiliary energy source (adapted from ref. [54]).