Electrochemical ozone production (EOP) is an attractive technology for disinfection and sterilization purposes. This work reports a study on the EOP performance of the solid polymer electrolyte (SPE) electrolyzer, including the optimization of electrode configuration and operation conditions. It is proven that the EOP performance is highly affected by electrode configuration. Tests using BDDs with different B/C ratios demonstrate that BDD-4.9 provides more reaction sites and faster electron transfer rate, exhibiting a high electrocatalytic activity for EOP. Regarding electrode thickness, 0.54 mm in thickness is the most suitable for the EOP from the perspective of less power consumption. Moreover, operation conditions were evaluated. It was found that increasing water flow rate is an effective strategy for promoting ozone dissolution, and within the present experimental range, the water flow rate of 63 L·h−1 was identified. Meanwhile, through the study of all processes occurring inside the electrolyzer at higher current densities, the optimum current density was determined to be 125 mA·cm−2. Based on these results, ozone water presents excellent performance in the killing of Escherichia coli with high inoculum concentrations, indicating potential application performance in the field of environment.

  • Ozone production by electrochemical technology.

  • The SPE electrolyzer with specially designed clamping system served as the ozone reactor.

  • Using a boron-doped diamond (BDD) as the electrode material.

  • The SPE electrolyzer can continuously and effectively produce ozone by a flow-through system with low energy consumption.

  • Ozone water shows a considerable bactericidal effect on different concentrations of E. coli.

Graphical Abstract

Graphical Abstract
Graphical Abstract

Ozone, with a high oxidation potential of 2.076 V (vs RHE) (Ershov & Morozov 2008), can rapidly and efficiently remove a large variety of contaminants and pathogens (Rosal et al. 2008; García-Espinoza et al. 2021) and is widely used in the fields of sterilization, disinfection, and water treatment(Uhm et al. 2009; Goztas et al. 2014; Lara-Ramos et al. 2020). Meanwhile, ozone is very environmentally friendly compared with many other oxidants such as chlorine because it is unstable and spontaneously decomposes into oxygen, leaving no harmful residuals during its application (Awad et al. 2006a). Due to these properties, intensive interest has been generated in efficient methods of ozone production.

Conventional technologies for generating ozone have been developed including corona discharge and ultraviolet radiation (Azam et al. 2019; Rodríguez-Peña et al. 2021c). However, these two methods cannot satisfy the application requirements in some aspects owing to their inherent limitations. For instance, the corona discharge not only requires very high voltages and additional equipment but also releases harmful nitrogen oxides (Lees et al. 2018). Ultraviolet radiation shows high energy consumption. In addition, both methods can only produce ozone gas, which is limited to space environment disinfection. Electrochemical technology casts a light on ozone production, which can directly produce a high concentration of ozone water at low voltages, thereby skipping the problems associated with dissolving ozone gas into water (Zhang et al. 2017; Sandin et al. 2020). Therefore, this technology is available for substance surface disinfection such as the hands and food as well as surgical equipment washing. The structure of the electrolyzer is crucial to the actual application of this technology.

At present, the solid polymer electrolyte (SPE) electrolyzer is regarded as a more rational design to generate ozone water than with the separated electrolyzer. In a SPE electrolyzer, the following reactions (equations 1–3) occur at the anode and cathode.
(1)
(2)
(3)

It is noteworthy that the formation of O3 by oxidation of water (see equation (1)) is in strong competition with oxygen evolution (see equation (2)), which is thermodynamically preferred at a lower redox potential (Santana et al. 2005). In addition, the electrodes suffer from corrosion and passivation due to the harsh environment that is induced by the formation of hydrogen ions at the anode interface. Thus, electrode materials play a prominent role in ozone production. Firstly, the anode materials need high overvoltage for the oxygen evolution reaction (OER) to encourage the generation of O3 and retard the evolution of O2, resulting in the high current efficiencies. Secondly, the anode materials should have good chemical and thermal stability to resist chemical corrosion.

In previous studies various electrode materials, especially PbO2 (Wang et al. 2008; Wang & Jing 2012), Pt (Awad et al. 2006b), and glassy carbon (Foller & Kelsall 1993), have been thoroughly investigated and show remarkable performance for ozone production. Nevertheless, they still have some drawbacks, such as electrode erosion caused by oxidation reactions and considerably extreme conditions required (Rodríguez-Peña et al. 2021b). As an emerging potential candidate, boron-doped diamond (BDD) meets the abovementioned requirements well, and its inert surface with weak adsorption properties endows the ability to permit the stable presence of OH· and O·, which act as precursors for ozone production. Moreover, it shows excellent electrical conductivity (Mortet & Soltani 2011), a wide electrochemical window (Zhang et al. 2021), and high electrocatalytic activity, which could improve the mass transfer process and reach higher energy efficiencies.

However, several studies revealed that BDD electrode had relatively low electrochemical ozone production (EOP) performance as a result of high cell voltage. The composition and structure of BDD have been demonstrated to be key to solving the problem. Arihara et al. (2007) prepared a BDD electrode by optimizing the number of holes, hole size, and electrode thickness, achieving a maximum current efficiency of 47%. Honda et al. (2013) compared BDD electrodes with various B/C ratios using an electrolyte-free system and it was found that heavily BDD electrodes with sp2 impurities yielded a high concentration of ozone.

In addition, operation conditions are also very important for the EOP performance. Kraft et al. (2006) utilized BDDs as cathode and anode, investigating the effects of current density, flow rate, and electrolyte conductivity on ozone production in a SPE electrolyzer, and it was found that high flow rate and low conductivity of water are beneficial to ozone production. Rodríguez-Peña et al. (2021a) proposed a phenomenological model to explain the influence of the current intensity applied, helping to understand the basis of the electrochemical processes that occur inside the cell and promote the production of ozone.

In summary, the further advance of the EOP performance can be improved by optimizing electrode configuration and operating conditions simultaneously. Herein, based on SPE technology, we conducted the structural design of the ozone reactor, in which BDDs were employed as electrode materials to research the performance of ozone production. The influence of electrode configuration and operation conditions on ozone generation performance were systematically investigated. Finally, the bactericidal activity of ozone water is evaluated. This study provides technical support for the large-scale application of the EOP technology.

Materials and reagents

All of the chemical reagents were directly used without treatment. Indigo carmine (C16H8N2Na2O8S2), potassium dihydrogen phosphate (KH2PO4), dibasic sodium phosphate (Na2HPO4), sulfuric acid (H2SO4, 98%), hydrogen peroxide (H2O2), tryptone, sodium chloride (NaCl), bovine serum albumin (BSA), nutrient broth (NB), agar, and sodium thiosulfate (Na2S2O3) were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd. Escherichia coli (8099) was supplied by the National Bio-Protection Engineering Center of China. Nafion 117 (Dupont, 50 mm × 50 mm) as a SPE membrane was obtained from Shanghai Fanyue Electronic Technology Co., Ltd. BDDs employed as electrodes (40 mm × 40 mm) were bought from by Luoyang Yuxin Diamond Co., Ltd. Deionized water used to prepare all the solutions was produced by the Millipore Milli-Q system (conductivity: 1 μs·cm−1).

Experimental setup

As illustrated in Figure 1, there is a Nafion 117 membrane between two BDD electrodes, gaskets outside the electrodes, titanium current collectors outside the gaskets. The gaskets were adequately compressed through a specially designed clamping system, so that BDD electrodes were firmly attached to both sides of the Nafion 117 membrane. This compact structure helps not only reduce ohmic loss, but also prevent gas-liquid channeling and water leakage. Fluid manifolds that are machined into titanium plates ensure uniform distribution of water in the electrolyzer and provide the necessary conditions for full contact between BDD electrodes and water. Additionally, it can export the gas and heat generated in time to reduce the resistance loss of the flow channels. Titanium as a current collector has the function of providing a conductor for the transfer of charges.

Figure 1

Structure of the SPE electrolyzer.

Figure 1

Structure of the SPE electrolyzer.

Close modal

As described in Figure 2, the electrolyzer was powered by a DC power supply (M8811, Maynuo, China). Deionized water was continuously fed into both anode and cathode compartments using two peristaltic pumps (BT100S-1, LEAD FLUID, China) at different flow rates (9, 18, 27, 36, 45, 54, 63, 72, and 81 L·h−1). Water is oxidized to ozone and oxygen at the anode surface. Hydrogen ions in hydrated form move through the membrane to the cathode. Meanwhile, electrons reach the cathode through the external circuit. Finally, they meet at the cathode surface and the reduction reaction occurs to evolve hydrogen. The experiments were performed under galvanostatic conditions (0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, and 5 A).

Figure 2

Scheme of the experimental setup for electrochemical ozone water production.

Figure 2

Scheme of the experimental setup for electrochemical ozone water production.

Close modal

Analytical techniques

Measurement of ozone concentration

The concentration of ozone water was determined by indigo disulfonate spectrophotometry (IDS). The photometric measurements were performed in a UV-VIS spectrophotometer (Hach, Model: DR5000).

Assessment of ozone production performance

The EOP performance was evaluated by current efficiency and specific power consumption.

The proportion of electric current that is used to produce ozone is defined as current efficiency, and is calculated by Equation (4):
(4)

where n is the stoichiometric number of electrons transferred, F is the Faraday's constant (9.6485 × 104 C·mol−1), is the water flow rate (L·h−1), is the concentration of ozone water (g·L−1), M is the molecular weight of ozone (48 g·mol−1), and I is the current (A).

Specific power consumption is the energy consumed to produce one gram of ozone, and is calculated by Equation (5):
(5)
where is corresponded to cell voltage (V).

Composition and structure characterization

The composition and morphology of the BDD electrodes were characterized by a scanning electron microscope (SEM, Sigma 300, Zeiss, Germany) combined with energy dispersive X-ray spectrometry (EDS, Smart EDX, Zeiss, Germany). The crystal structure of the BDD electrode was analyzed by X-ray diffraction (XRD, Ultima IV, Rigaku, Japan) with Cu-Kα radiation (λ = 1.5418 nm) at a scanning rate of 2° min−1. The crystal quality of the BDD electrode was examined by a Raman spectrometer (LabRAM HR Evolution, HORIBA Scientific, France) with a 532 nm laser.

Evaluation of the bactericidal performance of ozone water

Preparation of E. coli suspension: prior to the experiments, the E. coli seed tube was thawed and sub-cultured for 18–24 h at 37 °C on nutrient agar (NA) (Graça et al. 2011). After NA growth, at least three well-isolated colonies were selected and transferred with an inoculation loop into a tube containing 10 mL of nutrient broth and then was cultivated in a constant temperature incubator (MCo-175, SANYO, Japan) for 18–24 h at 37 °C. The initial prepared E. coli suspension was diluted to the required concentrations and maintained in a refrigerator at 4 °C for later use.

Suspension quantitative sterilization test: a mixture of 0.5 mL of the E. coli suspension and 0.5 mL of organic interference (3% BSA) was exposed to 4.0 mL of ozone water for 2 min, followed by taking 0.5 mL of sample and adding it to 4.5 mL of a neutralizer (PBS, 0.03 mol·L−1, pH 7.2, with 10 g·L−1 sodium thiosulfate) (Santos et al. 2021). After the exposed time of 10 min, the sample was inoculated in triplicate in plate count agar (PCA) for 18–24 h at 37 °C. A positive control repeated the above operation using tryptone saline solution (TPS) instead of ozone water.

The death rate of E. coli is calculated using the mean colony counts of three agar plates according to Equation (6):
(6)
where is the colony count of the positive control (cfu·mL−1), is the colony count of the test (cfu·mL−1).
Additionally, growth reduction of E. coli is calculated according to Equation (7):
(7)

Characterization of the BDD electrodes

As shown in Figure 3(b) and 3(c) elements distribute on the surface of BDD electrodes uniformly, indicating the well incorporation of boron atoms into the diamond structure. The carbon content of three BDD electrodes is determined to be 98.95 wt%, 97.45 wt%, and 95.33 wt%, respectively, and the boron content is determined to be 1.05 wt%, 2.55 wt%, and 4.9 wt%, respectively. Therefore, the weight ratios of boron and carbon (B/C ratios) of the three BDD electrodes are 1.1%, 2.6%, 4.9%, which are denoted as BDD-1.1, BDD-2.6 and BDD-4.9, respectively.

Figure 3

(a–c) EDS results and elemental mapping images of three BDD electrodes.

Figure 3

(a–c) EDS results and elemental mapping images of three BDD electrodes.

Close modal

Observing the morphology of the BDD electrodes by SEM, as shown in Figure 4. The grain size of BDD electrodes is varied from 11 to 87 μm. And the array of BDD electrodes is continuous and uniform, and their grains are smooth and sharp, proving they have good crystallinity as well as no obvious cracks. The tightly arranged structure of the BDD electrodes shows strong resistance to oxidation and corrosion, which is beneficial to prevent the penetration of electrolyte and improve the stability of the electrode.

Figure 4

SEM images of (a) BDD-1.1, (b) BDD-2.6, and (c) BDD-4.9 electrodes.

Figure 4

SEM images of (a) BDD-1.1, (b) BDD-2.6, and (c) BDD-4.9 electrodes.

Close modal

XRD was used to further investigate the crystal structures of BDD electrodes, as shown in Figure 5(a). For three BDD electrodes, the characteristic diffraction peaks of diamond are clearly observed at 43.94°, 75.22°, and 91.42°, which are assigned for the (111), (220), and (311) planes of the diamond. The diamond (220) is the dominant growth direction in the BDD-1.1 electrode and the diamond (111) is the dominant growth direction in both of BDD-2.6 and BDD-4.9 electrodes. And the (111) plane of the BDD-4.9 electrode is more intense than BDD-2.6. This observation confirms that the (111) plane promotes the incorporation of boron atoms the most and has a crucial impact on the electrical and electrochemical properties such as conductivity and active surface area.

Figure 5

(a) XRD patterns and (b) Raman spectra of BDD-1.1, BDD-2.6, and BDD-4.9 electrodes.

Figure 5

(a) XRD patterns and (b) Raman spectra of BDD-1.1, BDD-2.6, and BDD-4.9 electrodes.

Close modal

The Raman spectra of the BDD electrodes were taken to determine their crystal quality (Figure 5(b)). For three BDD electrodes, the characteristic peak at 1,324 cm−1 corresponds to the diamond having a significant negative shift compared to the standard diamond peak (1,324 cm−1), which can be attributed to nonideal crystal form appearing in the diamond. Two typical broadband peaks of BDD-2.6 and BDD-4.9 electrodes can be observed at 501 and 1,209 cm−1, which are attributed to the heavily doping of boron (Okino et al. 2004). Besides, the spectrums of BDD-2.6 and BDD-4.9 electrodes also display a typical broadband peak at 1,538 cm−1, which is induced by the G (graphite carbon) bands (Coffinier et al. 2011). Notably BDD-2.6 and BDD-4.9 electrodes only have a low amount of graphite carbon, confirming almost no lattice defects in BDD electrodes (Zhao et al. 2014).

The changes in EOP performance with B/C ratio

To explore the effect of the boron doping amount on the EOP performance, the three BDD electrodes under different current densities were tested at an electrode thickness of 0.32 mm and a flow rate of 36 L·h−1 (Figure 6).

Figure 6

The dependence of (a) ozone concentration, (b) cell voltage, (c) current efficiency, (d) specific power consumption on current density using BDD-1.1, BDD-2.6 and BDD-4.9 electrodes.

Figure 6

The dependence of (a) ozone concentration, (b) cell voltage, (c) current efficiency, (d) specific power consumption on current density using BDD-1.1, BDD-2.6 and BDD-4.9 electrodes.

Close modal

Figure 6(a) demonstrates that the ozone concentration produced by the three BDD electrodes is virtually identical when the current density is lower than 125 mA·cm−2. On the contrary, accompanied with increasing current density, BDD-4.9 electrode shows higher ozone concentration and it is up to 7.686 mg·L−1 at the current density of 312.5 mA·cm−2. The result verifies that a BDD electrode with a high B/C ratio can promote formation of dissolved O3.

As shown in Figure 6(b), the cell voltage increases with the current density, and at each given current density, the value of BDD-4.9 electrode is significantly lower than those of BDD-1.1 and BDD-2.6 electrodes. Moreover, it can also be seen from the slope of the curve, the higher the B/C ratio, the slower the rising rate of cell voltage. Those results indicate that increasing the boron doping amount is an effective strategy to lower cell voltage under the condition of keeping high BDD quality, which is because that the enhanced boron doping can reduce resistance and yield higher conductivity (Liu et al. 2011).

As observed in Figure 6(c), the initial current efficiency increases with increasing current density regardless of the studied B/C ratio. After reaching a maximum value at 125 mA·cm−2, the current efficiency decreases with increasing current density. This result confirms a medium current density is more conducive to produce O3. At the case of current density below 125 mA·cm−2, the reason for the increase of current efficiency is that the OER process is gradually replaced by the EOP process with increasing current density. At the case of current density over 125 mA·cm−2, a decrease in the current efficiency may be a result of the following factors: firstly, the anodic oxidation of water to produce hydrogen peroxide competes with the production of ozone, as described in Equation (8):
(8)
Secondly, hydrogen peroxide as a predator of ozone can interact with ozone and destroy it, as shown in Equation (9):
(9)

It has to be highlighted the kinetics of Equation (9) depends on the concentration of H2O2. Higher current density leads to an increase in the concentration of H2O2, enhancing the kinetics in Equation (9) and accelerating ozone destruction (Michaud et al. 2003). Meanwhile, increasing current density increases the heat generated on the three-phase interface that is a contact surface formed among the BDD electrodes, Nafion 117, and water. This phenomenon leads to a rise in temperature, which accelerates the chemical decomposition of ozone and reduces the current efficiency. The three factors mentioned above affect negatively the accumulation of ozone, causing a decay in the current efficiency.

It can also be seen from Figure 6(c), the BDD-4.9 electrode has the higher current efficiency than BDD-1.1 and BDD-2.6 electrodes under different current densities, and it reaches 26.6% when the current density is 125 mA·cm-2. This trend can be explained by the mechanism of ozone generation (Equations (10–13)) proposed by Babak et al. (1994) and Da Silva et al. (2003), (Silva et al. 2001)
(10)
(11)
(12)
(13)
According to the mechanism, the terminal O3 generation depends on Equation (13), and increasing the coverage of the key intermediates of O2* and O* on the surface of the active centers increases their encounter chances, thereby accelerating the EOP kinetics. Hydroxyl radical intermediates are the precursor of oxygen intermediates. BDD with a high B/C ratio exhibits a larger active surface area, which can provide more reaction sites to promote the absorption of hydroxyl radical, thus increasing concentrations of oxygen intermediates on electrode surface (Gu et al. 2021). Therefore, a higher current efficiency was achieved when using the BDD-4.9 electrode. Besides, the BDD electrode with a high B/C ratio shows a higher carrier concentration, which accelerates electron transfer rate (Bogdanowicz et al. 2020; Dettlaff et al. 2020), and a faster electron transfer rate can enhance the terminal rate of O3 generation, thus improving current efficiency. Based on the results represented above, we conclude that increasing B/C ratio of BDD electrodes is a feasible strategy to improve ozone production rate, and the BDD electrode with a high B/C ratio shows a high electrocatalytic activity for EOP.

Figure 6(d) displays that regardless of the studied B/C ratio, the specific power consumption with current density first decreases down to a minimum value, and then increases. The specific power consumption decreases depending on the boron doping level in the order of BDD-1.1, BDD-2.6, and BDD-4.9. The minimum value of 67.44 Wh·g−1 is obtained by BDD-4.9 electrode at a current density of 125 mA·cm−2, indicating that the BDD electrode with a high B/C ratio has remarkable EOP performance.

The changes in EOP performance with BDD electrode thickness

To investigate the influence of electrode thickness on the EOP performance, BDD electrodes with different thicknesses in the range of 0.32–0.98 mm were tested at a current density of 125 mA·cm−2, a B/C ratio of 4.9%, and a water flow rate of 36 L·h−1 (Figure 7).

Figure 7

The dependence of (a) ozone concentration and cell voltage, (b) current efficiency and specific power consumption on electrode thickness.

Figure 7

The dependence of (a) ozone concentration and cell voltage, (b) current efficiency and specific power consumption on electrode thickness.

Close modal

As depicted in Figure 7, the ozone concentration and current efficiency increase accordingly as the electrode thickness is from 0.32 mm to 0.54 mm, this might be because that in the case of thinner electrodes, increasing electrode thickness enlarges the reaction interface, which contributes to the synthesis of ozone. However, as the electrode thickness exceeds 0.54 mm, the ozone concentration and current efficiency both begin to decrease. This might be because that a further increase in the electrode thickness increases the reaction resistance, leading to the increase of electric current that is used to overcome ohmic loss and then reducing the current efficiency. The maximum values of ozone concentration and current efficiency can be clearly observed at the BDD electrode with 0.54 mm in thickness and they are 35.46% and 5.88 mg·L−1, respectively.

Another very important observation in Figure 7(b) is that specific power consumption drops with electrode thickness rapidly down to a minimum value, and then increases. The drop of specific power consumption is due to that the increasing reaction interface increases O3 formation rate and its raise is attributed to the elevation of cell voltage from 4.3 V at 0.32 mm in thickness to 7.43 V at 0.98 mm in thickness (shown in Figure 7(a)), which makes much power wasted in the production of heat. Under the trade-off relationship between higher EOP efficiency and higher cell voltage, a minimum specific power consumption of 46.11 Wh·g−1 is found to be around 0.54 mm. Those results demonstrate that a medium electrode thickness presents the superior EOP performance and the optimum electrode thickness is 0.54 mm.

The changes in EOP performance with water flow rate

Many previous studies have demonstrated that EOP performance is highly dependent on the water flow rate. To further elucidate this interesting point, the optimization of water flow rate was performed at a current density of 125 mA·cm−2, a B/C ratio of 4.9%, and a thickness of 0.54 mm (Figure 8).

Figure 8

The dependence of ozone concentration, current efficiency and specific power consumption on the water flow rate.

Figure 8

The dependence of ozone concentration, current efficiency and specific power consumption on the water flow rate.

Close modal

As illustrated in Figure 8, the current efficiency with the water flow rate increases up to a steady state value and then it is maintained. The current efficiency attains a maximum value of 51.29% at a water flow rate of 63 L·h−1. The increase of current efficiency at a water flow rate below 63 L·h−1 is partly attributed to the employment of a flow system which builds a smooth way for the transfer of water to the electrode surface and the release of ozone from the electrode surface. Meanwhile, the higher water flow rate increases the ozone release rate, thus avoiding ozone destruction reactions. Another reason for the trend is that at lower water flow rate, the amount of ozone in the gas phase is promoted over that in the liquid phase, and increasing the flow rate is expected to effectively promote a change in this ratio (Onda et al. 2005). At a water flow rate over 63 L·h−1, a stabilization in the current efficiency can be attributed to an equilibrium rate between the formation and destruction of ozone.

However, specific power consumption with the water flow rate decreases down to a steady state value and then it stays constant. A maximum value of 37.95 Wh·g−1 is achieved at a water flow rate of 63 L·h−1. This is because the higher water flow rate allows rapid dispersion of the heat generated during electrolysis, thus minimizing ohmic loss. It can also be seen from Figure 8, the concentration of ozone water decreases as water flow rate is increased and it attains 4.86 mg·L−1 at the highest current efficiency and the lowest production energy.

Evaluation of the bactericidal effect of ozone water

After determining the optimal electrode configuration and operating conditions, 4.86 mg·L−1 of ozone water was produced by the SPE electrolyzer. To evaluate the bactericidal activity of ozone water, the death rate and growth reduction of E. coli with the inoculum concentrations ranged between 1 × 103 and 3 × 109 cfu·mL-1 were measured (Figure 9).

Figure 9

Action of ozone water on the E. coli with different concentrations. Analysis of (a) the death rate and (b) growth reduction for the microorganism E. coli.

Figure 9

Action of ozone water on the E. coli with different concentrations. Analysis of (a) the death rate and (b) growth reduction for the microorganism E. coli.

Close modal

As the concentration of E. coli increases, the death rate and growth reduction both decrease. When the E. coli concentration is in the range of 1 × 103–3 × 103 cfu·mL-1, the death rate is 100% and the growth reduction is higher than 5 log. Under this condition the ozone water meets the disinfection standard and can be used as a disinfectant. When the E. coli with the inoculum concentrations of 1 × 109 to 3 × 109 cfu·mL-1 was exposed to ozone water for 10 min, the death rate and growth reduction were equal to 96.36% and 1.44 log, respectively. At this time the ozone water reaches the standard of bactericidal effect, demonstrating that the ozone water is also effective in inactivating the E. coli with high inoculum concentration. These results indicated that the ozone water shows a considerable bactericidal effect on different concentrations of E. coli and has great prospects for application in the environment field.

In summary, the SPE electrolyzer equipped with a couple of BDD electrodes was rationally designed and assembled, which can continuously and effectively produce ozone by a flow-through system with low energy consumption.

Electrode configuration studies revealed that the BDD electrode with a high B/C ratio (BDD-4.9) possess higher electrical conductivity and larger active surface area compared with the BDD electrode with a low B/C ratio, resulting in much improved ozone production efficiency, and the BDD electrode with 0.54 mm in thickness shows the superior EOP performance. The study concerning operating conditions indicated that ozone production is favored by the medium current density (125 mA·cm−2) and higher water flow rate (63 L·h−1).

The highest current efficiency of 51.29% and the lowest specific power consumption of 37.95 Wh·g−1 were achieved, which corresponds to an ozone concentration of 4.86 mg·L−1. This concentration of ozone water is effective in inactivating the E. coli with inoculum concentrations in the range of 1 × 103–3 × 109 cfu·mL−1. Application of the SPE electrolyzer for disinfection and sterilization will be expected in the future.

This work was supported by the National Natural Science Foundation of China (No. 51878659), Military Logistics Research (No. AWS18J004) and Youth Program of Military Medical Research Project of PLA General Hospital (QNF19051).

The authors declare no conflict of interest.

All relevant data are included in the paper or its Supplementary Information.

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