Another way to electrochemically produce H₂O₂ is the anodic oxidation of water, which proceeds as follows ：2H₂O→H₂O₂+2H⁺+2e‾. Water oxidation is a solid-liquid two-phase interfacial reaction, which only uses water as raw material without oxygen supply, and has mass transfer advantages over oxygen reduction reaction requiring gas-liquid-solid three-phase interface. The key to increase the concentration of H₂O₂ produced by water oxidation is to inhibit the competitive reaction. The competitive reaction of 2-electron oxidation of water (2e-WOR) to produce H₂O₂ is mainly 4-electron oxidation of water (4e-WOR) to produce oxygen. In order to improve the selectivity of H₂O₂, the 4-electron oxygen production reaction must be inhibited, and the main method is to use semiconductor electrodes with high oxygen production overpotential.Such as BiVO₄, TiO₂, WO₃, SnO₂, CaSnO₃, ZnO, etc.,Commercial carbon felt, carbon cloth, carbon paper and boron-doped diamond as anodes also have high oxygen overpotential, which are often used to oxidize aquatic H₂O₂^{[1,31][32][33]}.

Another way to suppress the 4-electron oxidation reaction is to manipulate the adsorption energy of the intermediate. When the Gibbs adsorption free energy value （ΔG_*OH） of * OH is between 1.6 and 2.4 eV, H₂O₂ is the dominant product, and ideally, 1.76 eV is the optimal thermodynamic value^[34]. Strong adsorption of materials with ΔG_*OH values below 1.6 eV leads to O₂ precipitation, and weak adsorption of materials with ΔG_*OH values above 2.4 eV leads to · OH production^[34]. Therefore, reasonable regulation of OH adsorption is a feasible way to achieve efficient H₂O₂ production. At a sufficiently positive potential, the oxygen bubbles generated by 4 e water oxidation move upward from the electrode and then collapse and disappear. If these oxygen bubbles are confined near the active site, the accumulated O₂ molecules can further interact with the OH on the surface of the catalyst, thus adjusting the binding strength in favor of 2-electron oxidation^[34]. By coating the catalyst surface with a hydrophobic polymer to trap the in situ generated O₂ gas close to the active site, Wang's group observed that once the generated O₂ gas was confined to the surface, its H₂O₂ selectivity increased significantly^[35]. Sun's group controlled the adsorption strength by doping a specific number of strongly OH-adsorbed ruthenium atoms into the lattice of weakly OH-adsorbed TiO₂, and accumulated 20 mL of 422 mg/L H₂O₂ in 10 min under the optimal conditions^[36].

At present, due to poor conductivity (metal oxide) or low selectivity (carbon material), the concentration of H₂O₂ aqueous solution produced by the reported electrode is only several hundred micromoles per liter, which can reach 24 ~ 33 mmol/L under strong alkaline conditions with high concentration of bicarbonate electrolyte, equivalent to 0. 1% of the mass concentration, which is still a certain gap from the actual disinfection requirement of 3%^[32,37][38]. In order to increase the concentration, some reports adopt the idea of cathodic oxygen reduction-anodic water oxidation, and the two poles cooperate to produce H₂O₂. The reactor structure and H₂O₂ generation path are shown in Figure 1. In addition, the long-term stability of the electrode is also a problem to be solved in practical applications.

Similar to the electrochemical process, photocatalysis or photoelectrocatalysis can also generate H₂O₂ through the pathway of water oxidation^[39]. Illuminated semiconductor photocatalytic materials, such as TiO₂, g-C₃N₄, BiVO₄, etc., can generate photogenerated electrons and holes. Photogenerated electrons are similar to the electrons provided by the cathode in the electrochemical process, which can reduce dissolved oxygen in water to produce H₂O₂. Holes are similar to the positive charges of the anode, which can oxidize aquatic H₂O₂. In the photocatalytic process, oxygen reduction and water oxidation, two ways of producing H2O2, can be carried out simultaneously^[40]. The structure of the reactor for photoelectrocatalytic H₂O₂ production and the reaction path of the two-pole synergistic H₂O₂ production are shown in Fig. 2.

Obviously, photocatalysis saves electrical energy relative to electrochemical processes, but requires illumination. In addition, the electrochemical process requires aqueous solution to conduct electricity, that is, there must be electrolyte in water, while photocatalysis can produce H₂O₂ aqueous solution without other impurities using pure water as raw material^[41]. The focus in the field of photocatalytic production of H₂O₂ is mainly on the development of new materials, but the cumulative concentration of H₂O₂ can rarely exceed 0.1%^[42,43]. The main goal of photocatalytic methods is to further improve the concentration and volume of H₂O₂ produced, and the current exploration approach is still to improve the light absorption, promote the separation of photogenerated charges and enhance the efficiency of surface reaction, which is still emphasized by traditional photocatalysis^[44].

In addition, there are also some methods for in-situ preparation of H₂O₂ with water as raw material, such as thermal catalysis method, ultrasonic piezoelectric catalysis method, plasma pyrolysis method, bioelectrochemical method, microdroplet method and so on. Although these methods have not been reported much, they provide new ideas or have application potential, and they are very promising to obtain H₂O₂ aqueous solution that meets the requirements of in-situ disinfection. This paper summarizes the latest reports in recent years and introduces these methods in turn.

The direct synthesis of H₂ and O₂ to produce H₂O₂ is a well-known thermocatalytic reaction^[31,45]. Because of the explosion risk of mixing H₂ and O₂, it is necessary to find safe raw materials. Wate is an ideal hydrogen source, and that direct oxidation of （H₂O+0.5O₂=H₂O₂ΔG=116.7 kJ/mol） with O₂ is a thermodynamically uphill reaction, thus requiring an energy input^[46]. A common form of energy supply is thermal coupling, in which the heat released in an exothermic reaction drives an endothermic reaction, for example, the heat released in the oxidation of carbon monoxide to carbon dioxide provides heat for the gold-catalyzed reaction of water and oxygen to H₂O₂^[47]. The product of liquid phase thermal coupling is a mixture of H₂O₂ and organic matter, which must be separated from the liquid phase. The high cost of the separation process is the disadvantage of this method. In addition, most of the energy of this process is used to heat the bulk of the diluent rather than to drive the desired reaction, which is energy inefficient for dilute solutions. Kung et al. Proposed to conduct the thermal coupling reaction in the vapor phase to produce H₂O₂, which can increase the reactant concentration and greatly improve the energy efficiency^[48]. Their study showed that reactions such as epoxidation of alkenes and selective oxidation of alkanes to alcohols could not provide sufficient thermodynamic driving force, while the heat released from the oxidation of alcohols to aldehydes and acids could drive the oxidative aquatic H₂O₂ reaction. This approach requires consideration of the spontaneous decomposition of the resulting H₂O₂ at high temperatures.

Another form of thermal catalysis is that thermoelectric materials use the separation of positive and negative charges caused by temperature difference to convert heat energy into electrical energy to produce H₂O₂. Thermoelectric voltage is the key factor affecting the concentration and volume of H₂O₂ produced by thermoelectric materials. Bismuth telluride （Bi₂Te₃） has excellent physical properties and unique electronic structure at room temperature, which can produce higher thermoelectric voltage than other common thermoelectric materials such as antimony telluride （Sb₂Te₃） and lead telluride (PbTe). Lin et al. Prepared Bi₂Te₃@CFF materials by coating bismuth telluride nanoplates on carbon fiber fabrics^[49]. Whether the temperature difference is positive or negative, the Bi₂Te₃@CFF shows significant antibacterial activity and good durability. Control experiments confirmed the generation of superoxide radicals during the thermocatalytic reaction, which favors the generation of H₂O₂, and also demonstrated that the disinfection ability comes from the in situ generated H₂O₂. A two-dimensional structure with a high surface area is responsible for that superior thermal catalytic performance of bismuth telluride nanoplate (NPs). The antibacterial filter based on Bi₂Te₃NPs is installed in the air conditioner, and the thermocatalytic reaction is triggered by the temperature difference between the inside of the air conditioner and the outside environment. The experimental results show that 5 mg bismuth telluride NPs can produce 30μM H₂O₂ within 20 min when the temperature difference is 30 K. The temperature difference was generated by a blower, and the sterilization rate of the 1×1 cm Bi₂Te₃@CFF against 1 mL of Escherichia coli with a concentration of 2×10⁶CFU/mL reached 72% within 20 min, and the operation was stable for at least 30 days^[49].

Piezoelectric catalytic effect is a concept proposed in 2010^[50]. Under the action of external force, piezoelectric materials will produce a polarizing electric field, which induces the separation of electron-hole pairs and shows catalytic ability. In recent years, there have been more and more reports on the production of H₂O₂ by piezoelectric catalysis. For example, by vibrating piezoelectric microfibers, water can be split piezocatalytically with an energy conversion efficiency of 18%^[50]. Electron-hole pairs induced by piezoelectricity can react with O₂ or water respectively, both of which can produce H₂O₂^[51].

Perovskite is the main piezoelectric material. In order to enhance piezoelectric polarization, manufacturing defects, doping metal elements and constructing composite materials are effective ways. Perovskites are a class of oxides with the general molecular formula ABO₃. A and B are rare earth (alkaline earth) elements and transition metal elements, respectively, and a and B can be replaced by other metals with similar radii without changing the perovskite crystal structure. In general, the net charge of each unit cell in perovskite crystal is zero, but when the titanium ions in the unit cell are slightly off the center, the electric polarity will be generated, and the unit cell will be converted into an electric dipole. When mechanical stress acts on the crystal, the position of titanium ions changes, which changes the polarization of the crystal and produces piezoelectric effect.

The most widely studied perovskite in the field of H₂O₂ production is lead zirconate titanate, with formula Pb（Ti_x, Zr_1-x）O₃, formed by solid solution of PbTiO₃ and PbZrO₃. The polarization degree of the lead zirconate titanate is adjusted by changing the external force, so that the intensity of a local electric field can be adjusted, and the adsorption capability of the surface of the lead zirconate titanate to OOH groups can be adjusted, and the aim of improving the yield of H₂O₂ is achieved^[52].

To avoid the use of toxic lead, researchers have explored the production and H₂O₂ of perovskite piezoelectric catalytic materials such as barium titanate. In order to improve the piezoelectric properties, ternary perovskite barium zirconate titanate has also been developed. Due to the coexistence of three phases (orthorhombic, tetragonal and rhombic), the polarization energy barrier is reduced, resulting in a significant enhancement of piezoelectricity. Combined with the suitable concentration of oxygen vacancies, the H₂O₂ selectivity of barium zirconate titanate reaches 80% under ultrasonic irradiation. The research of Li's group further reveals that piezoelectric polarization in ternary perovskites promotes the generation, separation and transport of electron-hole pairs, thus promoting the catalytic reaction^[53].

Piezoelectric catalytic performance depends on the intensity of piezoelectric polarization. Fabricating defects, doping metal elements and constructing composite materials are effective methods to enhance piezoelectric polarization. Huang's group fabricated ultrathin Bi₄Ti₃O₁₂ nanosheets with a thickness of only 4.2 nm, which is only equivalent to 1 – 2 Bi₄Ti₃O₁₂ unit cells, and has abundant surface oxygen defects (OVs) due to a large number of exposed surfaces^[54]. Compared with ordinary Bi₄Ti₃O₁₂, this ultrathin Bi₄Ti₃O₁₂ nanosheet rich in OVs is more sensitive to external force, which can generate a stronger piezoelectric polarization field, promote the separation and migration of piezoelectric free charges, prolong the lifetime of piezoelectric free charges, and reduce the adsorption energy of O₂ molecules to promote their activation into superoxide radicals. Benefiting from these advantages, the ultrathin Bi₄Ti₃O₁₂ nanosheets with the optimal concentration of OVs showed excellent piezoelectric catalytic H₂O₂ generation performance, and 0. 05 G of catalyst added to 100 mL of ethanol – water solution could generate 161 μmol of H₂O₂ in 2 H under ultrasound, which was converted to 0. 0054% by weight, significantly higher than 86 μmol (0. 0029%) of ordinary Bi₄Ti₃O₁₂ nanosheets with a thickness of 25. 8 nm^[54].

In addition to construction defects, doping piezoelectric materials with metal elements can improve polarization. For example, doping Nb⁵⁺ into the lattice of BaTiO₃ can introduce charge compensation, increase the carrier concentration and charge migration rate of BaTiO₃, and thus obtain higher polarization electric field intensity under the same mechanical force. Coupling piezoelectric effect with photocatalysis can effectively improve the ability to produce H₂O₂, but perovskite materials generally have the photocatalytic ability of ultraviolet light response, which can not utilize the largest proportion of visible light in the solar spectrum, while loaded carbon quantum dots (CDs) can usually help perovskite to obtain visible light response ability. Zhai's group used niobium-doped barium titanate （BaTiO₃：Nb） to enhance piezoelectric polarization, and then loaded CDs as visible light sensitizer and electron acceptor to increase the concentration of H₂O₂^[55]. Under co-irradiation of visible light and ultrasound, 15 mg of CDs modified Nb-doped BaTiO₃ catalyst was added into 30 mL of ethanol-water mixed solution to produce H₂O₂ at a rate of 1360μmol/(g<sub> catalyst </sub > · H) (0. 0023%), which was 27 times higher than that of BaTiO₃ 48μmol/(g<sub> catalyst </sub > · H)^[55].

The third way to strengthen the polarization electric field is to construct composite materials. A composite constructed with spherical zinc sulfide and nanopiezoelectric barium titanate modified multilayer In₂S₃ nanosheets （ZnS/In₂S₃/BS₃/BaTiO₃） produced about 378 µM of H₂O₂（0.0013%） in 100 min under co-irradiation of visible light and ultrasound (piezo-photocatalysis), and the concentration of H₂O₂ produced by ZnS/In₂S₃/BaTiO₃ could reach 1160 µM (0.0039%) in 600 min of continuous reaction. The enhanced yield of H₂O₂ under piezo-photocatalysis is attributed to the enhanced rate of the surface reaction due to the piezoelectric effect induced polarization electric field which promotes the separation of photogenerated charges^[56].

Curie temperature and piezoelectric coefficient are important parameters reflecting the properties of piezoelectric materials. Once the service temperature exceeds the Curie temperature, the piezoelectric materials will lose their piezoelectric properties; The piezoelectric coefficient reflects the ability of a piezoelectric material to convert mechanical energy into electrical energy or electrical energy into mechanical energy. The higher the piezoelectric coefficient, the higher the energy conversion efficiency. The Curie temperature of perovskite RbBiNb₂O₇ exceeds 1000 ℃, which is higher than that of most piezoelectric materials and can operate in a wider temperature range. The piezoelectric coefficient of organic polytetrafluoroethylene (PTFE) is as high as 600×10^-12C\N, and it is easy to obtain permanent polarization by ultrasonic irradiation. Li's group loaded RbBiNb₂O₇ on PTFE and obtained a synergistic effect^[57]. Firstly, the hydrophobicity of PTFE is beneficial to the adsorption of oxygen, and the piezoelectric polarization decomposes water to produce H₂O₂ and oxygen, which can produce H₂O₂ by oxygen reduction because oxygen is bound on the surface of the material. Secondly, the built-in electric field formed by PTFE and RbBiNb₂O₇ promotes the separation and migration of polarization charges, and improves the energy efficiency of H₂O₂. The addition of 10 mg RbBiNb₂O₇\PTFE to 1 mL of ethanol-water solution produces H₂O₂ at a rate of 219.23μmol/(g<sub> catalyst </sub > · H), which is converted to 0.0074% by weight. This value is 12 and 3 times that of RbBiNb₂O₇ alone and PTFE alone, respectively^[57]. In addition to perovskite, some metals, carbon materials, organics and so on can also be used as piezoelectric materials to produce H₂O₂.

Metal Pt also has the ability of piezoelectric polarization to produce H₂O₂. With the help of ionic liquid, a single atom of Pt was anchored on the surface of silica （Pt1/SiO₂）, and the resultant catalyst could generate 3027.1µmol/（L·h）H₂O₂（ (0.01% by weight) under sonocatalysis. This was the highest concentration of sonocatalytic H₂O₂ production reported in the literature at that time. The high yield of H₂O₂ is attributed to the efficient bidirectional catalysis of Pt₁. ：Pt₁ not only has high water adsorption energy and low water activation energy, but also can selectively activate O₂, so it can simultaneously produce H₂O₂ through two pathways of oxygen reduction and water oxidation^[58].

The metal-free graphite-phase carbon nitride （g-C₃N₄） is a layered two-dimensional structure, and the strain gradient generated between the layers leads to an obvious piezoelectric response when an external force is applied to the layered material^[50]. The piezoelectric properties of two-dimensional g-C₃N₄ are caused by its nanoscale ferroelectric properties and the nanoscale triangular noncentrosymmetric holes in its lattice structure. In addition, g-C₃N₄ has abundant pyridinic nitrogen, which is the active site for adsorption and activation of oxygen^[50]. Therefore, g-C₃N₄ is an ideal material for the production of H₂O₂ by piezocatalytic oxygen reduction. The above analysis was supported by experiments, and the production rate of H₂O₂ under ultrasonic irradiation reached 34 μmol/H when 50 mg of catalyst was put into 100 mL of pure water, which was converted to 0.0011% by weight^[50]. Although this concentration is still far from the requirement of disinfection, this work opens up a new direction for the production of H₂O₂ from pure water and oxygen without the use of metal catalysts. The charge transfer and reaction path of the decomposition of aquatic H₂O₂ by piezoelectric effect under ultrasonic and g-C₃N₄ is shown in Figure 3.

Organic polymers can also produce H₂O₂ as metal-free piezoelectric materials. Zhang's group has developed a new flexible polymer film composed of polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) matrix and silica nanoparticle filler^[59]. PVDF-HFP has weak piezoelectric effect, while SiO₂ has no piezoelectric effect, and the H₂O₂ of PVDF-HFP or silica alone under ultrasonic irradiation is less than 10 μmol/ (L · H). After forming the composite film, silica can not only enhance the polarization electric field of PVDF-HFP, but also adsorb the reaction intermediate, which can produce 246 μmol/ (L · H) of 00083 (0.00083% by weight) under ultrasonic irradiation. Computer simulation and experimental results show that the H₂O₂ generation ability originates from the enhancement effect of the piezoelectric polarization electric field on the gas-liquid-solid interface reaction, the hydrogen bond formed by the oxygen atom of silica and the hydrogen atom of PVDF enhances the piezoelectric response, and the adsorption energy of the surface hydroxyl group of silica to the oxygen reduction intermediate -OOH is moderate, which is beneficial to the selective generation of H₂O₂. The flexible film has the advantages of simple preparation method, environmental friendliness, long-term storage stability, easy recovery after use, and practical application potential^[59].

Plasma is a globally electrically neutral assembly of electrons, ions, excited particles, and neutrals. Plasma contains a large number of highly reactive particles, which can trigger complex chemical reactions, so it is widely used in the fields of environment, energy and materials. There are three possible mechanisms for plasma to produce H₂O₂: positive and negative charges in plasma are transferred into water to produce H₂O₂ by oxidizing water or reducing oxygen, respectively; The gas-phase hydroxyl radical are combined to generate a H₂O₂ which is then dissolve into a liquid phase; H₂O⁺ ions in water are dehydrogenated and oxidized to hydroxyl radicals, which combine to produce H₂O₂^[60].

Plasma can be generated by glow, arc, corona and dielectric barrier discharge. Locke's group has developed a gliding arc plasma reactor based on the principle of arc discharge. The two curved electrodes of the reactor form a gradual gap, which gradually expands from 2 mm to 20 mm^[61]. They apply a high voltage between the two stages to break down the air to form a plasma, and then spray small water droplets into the plasma between the electrodes through a nozzle. The small gap of millimeter level ensures that the water mist is fully contacted with the plasma, which produces H₂O₂ based on the principle mentioned in the previous paragraph. They used a 250 Hz pulse power supply to avoid the temperature rise caused by continuous power supply, prevent the evaporation and gasification of water droplets and inhibit the thermal decomposition of H₂O₂. The final achievement of a 1 mL/min influent flow rate yielded a H₂O₂（ weight percent of 0.034%) of 10 mM. The concentration of H₂O₂ in the liquid phase product decreased with the increase of influent flow rate, and the concentration of H₂O₂ produced was 0. 9 mM when the influent flow rate was 20 mL/min. At this time, the energy produced per kWh was about 80 gH₂O₂^[61]. Ranieri et al. Showed that compressed air was used to carry small droplets with a diameter of about 0. 3 μm into the microsecond pulse dielectric barrier discharge plasma device, which could generate H₂O₂ in the droplets and evenly cover the surface of agricultural products to complete sterilization and preservation^[62]. Using a plasma discharge cross-section of 80 cm², it took only 2 min to complete the disinfection of a 100 L space at a power density of 1 W/cm² and a compressed air flow rate of 50 L/min^[62].

Discharge time, discharge power, liquid phase conductance and other factors can affect the yield of H₂O₂. Takeuchi et al. Systematically studied the process mechanism of H₂O₂ generation by plasma discharge, and found that the H₂O₂ concentration in the liquid phase increased linearly with the plasma treatment time and discharge current, and was independent of the discharge gap distance^[63]. The H₂O₂ is mainly generated at the gas-liquid interface, and the H₂O₂ generated in the gas phase and the H₂O₂ generated in the liquid film near the interface migrate into the solution through diffusion. Under the optimal conditions, about 22 mg/L of H₂O₂ was produced in 25 mL of pure water in 3 min, and the energy efficiency was 3.9 g H₂O₂/kWh^[63]. The generation of H₂O₂ depends on the plasma volume and the interface area between plasma and liquid phase. In general, as the power increases, the plasma volume expands and the interface with the liquid phase increases. This provides more opportunities for water molecules to collide with electrons, producing more • OH and H₂O₂. Liquid conductivity is also a key factor affecting the H₂O₂ produced by plasma. Vor Voráč's group obtained aqueous solutions with conductivities of 0.01 ~ 36 mS/cm by adding KCl to deionized water^[64]. For the microsecond pulse plasma, the conductivity increases from 0.01 mS/cm to 0.3 mS/cm, resulting in a decrease of 62% in the H₂O₂ generation rate from 0.12 μmol/s to 0.045 μmol/s. For nanosecond pulsed plasma in argon plasma, the generation rates of H₂O₂ corresponding to liquid films with conductivity of 0.01 mS/cm and 36 mS/cm are 0.16 μmol/s and 0.14 μmol/s, respectively, which are reduced by about 13%. The results show that the increase of liquid film conductivity is not conducive to the generation of H₂O₂, and the lower the pulse frequency is, the stronger the adverse effect is^[64]. These works confirm that plasma discharge can also produce H₂O₂ in conductive solution, thus indicating that this method can be used for seawater or other electrical water sterilization.

In the background of plasma, the location of H₂O₂ is divided into gas phase and liquid phase. In order to study the contribution of different positions to the generation of H₂O₂, Li's group compared the concentration of H₂O₂ generated by bubble pulse discharge on the water surface and underwater. Regardless of the discharge mode, the yield and production rate of H₂O₂ increased with the increase of voltage, and the rate of H₂O₂ generated by bubble pulse discharge in 200 mL pure water was 0. 0196 mg/min, which was significantly higher than 0. 012 mg/min in water surface pulse discharge^[65]. Considering that the bubbling process forms more gas-liquid interfaces within the liquid film, this result indicates that H₂O₂ is more easily formed in the gas phase^[65]. Tachibana et al. Investigated the H₂O₂ production per unit energy consumption of a similar bubbling discharge system and a water surface discharge system. After 10 min of discharge, the bubbling system produced 270 mg/L of H₂O₂ in 12 mL of water and 1 gH₂O₂ per kWh of electricity, while the water surface discharge system produced about 93 mg/L of H₂O₂ in 15 mL of water and 0.1 gH₂O₂ per kWh of electricity^[66]. This result further demonstrates that the plasma method is more economical to produce H₂O₂ in the gas phase^[66]. Plasma requires a millimeter-scale discharge gap, and how to generate it on a large scale has been a research hotspot of this technology. Vlachos group has developed a new type of plasma reactor for distributed production of H₂O₂, which is based on the principle of dielectric barrier discharge and is characterized by filling a high-density gas-liquid interface^[67]. It can be operated at 4 kV voltage and 0.7 W power to produce 2.2 mM H₂O₂, and the concentration of 33 mM H₂O₂ can be further obtained by increasing the power, which makes the technology easy to expand through the series-parallel connection of unit reactors, and is expected to achieve large-scale production of H₂O₂^[67].

In humid environment, the surface of fine particles in soil and atmosphere will spontaneously produce H₂O₂. Because the micro-droplet with fine particles as the core is a necessary condition for the generation of H₂O₂, this process can be called micro-droplet process^[68]. The microdroplet process produces H₂O₂ with only water and fine particles, without the application of voltage, catalyst, or added chemicals. It is generally believed that the H₂O₂ produced in the microdroplet process is produced by the recombination of two hydroxyl radicals. When micro-droplets and fine particles come into contact, water molecules collide with atoms or molecules on the solid surface, and electron transfer occurs at the solid-liquid interface. The hydroxide anion loses an electron to produce a hydroxyl radical, and the two radicals recombine to produce H₂O₂^[68].

In order to fabricate a solid-liquid interface that can simulate the microdroplet process, Chen et al. Used polydimethylsiloxane to construct a closed straight channel (diameter 20 μm, length 100 μm) on glass, and the H₂O₂ could be detected by introducing deionized water^[68]. In this process, the oxygen atoms required for the formation of H₂O₂ may come from the oxygen-containing groups on the surface of fine particles. To test this idea, silica was used as a substrate, and the substrate was activated by O₂ plasma to increase the density of hydroxyl groups, which showed that the density of hydroxyl groups was positively correlated with the amount of H₂O₂ generated at the solid-liquid interface. Isotope experiments have confirmed that the oxygen atoms in the H₂O₂ come from the hydroxyl groups of the substrate, and it is difficult to produce H₂O₂ by using microdroplets made of water containing hydroxyl radical scavengers. The analysis shows that ion transfer, overlap of electron cloud and change of surface hydroxyl group can cause surface charge transfer when fine particles contact with water, which leads to the recombination of solid surface hydroxyl groups and the production of H₂O₂^[69]. This method of microdroplet generation of H₂O₂ uses only water as raw material and does not require energy input, which is a new method worthy of study.

Will there be H₂O₂ at the gas-liquid interface? The gas-liquid interface of the droplet has a strong electric field, about 10⁹V/m^[70]. This electric field is strong enough to cause the hydroxide at the gas-liquid interface to lose an electron, ionizing the hydroxide ion to form a hydroxyl radical. Because of the higher curvature of the droplets, the charge density is greater and the electric field strength is greater, resulting in an increase in the efficiency of H₂O₂ generation. At droplet diameters below 20 μm, the amount of H₂O₂ production increased significantly. In addition, hydronium and hydroxide ions are separated and unevenly distributed in the droplet, which enhances the electric field strength on the surface of the droplet^[71]. Although that electric field enhance the generation of H₂O₂ inside the droplet, no H₂O₂ was detected at the gas-liquid interface. This approach yielded H₂O₂ concentrations up to 30 μm.

This method, which uses neither chemicals nor added energy to synthesize H₂O₂, requires only water and equipment to produce the spray^[72,73]. Ultrasonic atomization is the direct production of H₂O₂ by microdroplet method. When the ultrasonic energy is high enough, the tiny bubbles (cavitation nuclei) existing in the liquid vibrate, grow and continuously accumulate energy under the action of the ultrasonic field. When the energy reaches a certain threshold, the cavitation bubbles will collapse sharply. This phenomenon is called cavitation. The cavitation phenomenon can accelerate the diffusion of oxygen in water and increase the rate of H₂O₂ production, and the H₂O₂ spray produced can easily reach more than 30 cm^[74].

In order to intensify the cavitation phenomenon and increase the H₂O₂ production, zinc flakes were introduced on the ultrasonic nebulizer^[74]. The presence of surface defects, such as cracks, on the zinc sheet increases the nucleation sites of cavitation bubbles compared to cavitation in a homogeneous liquid phase. In turn, the microjets formed by the collapse of cavitation bubbles continuously impact and grind the zinc sheet, further increasing the number of surface defects (cracks, holes, surface roughness, etc.), thus exposing more active sites. The increase of nucleation sites can improve the cavitation effect. Therefore, the corrosion of zinc flake and the formation of H₂O₂ can promote each other. The reaction equation for the production of H₂O₂ by this method is as follows.

The yield of H₂O₂ was 21 times higher than that without zinc flakes, and 4.75μg/mL Zn²⁺ was produced in the spray droplets^[74]. The yield of H₂O₂ was positively correlated with zinc content and reaction time. 121.25μM H₂O₂ was obtained by treating E. coli contaminated feeding bottles with the produced microdroplets for 30 min, and the sterilization rate reached 93. 53%. This ultrasonic atomization using a functional zinc coating successfully increased the yield of H₂O₂ while generating Zn²⁺, providing a new idea for the development of microdroplet disinfection methods^[74]. Although the concentration of H₂O₂ produced by microdroplet method does not meet the requirement of 3% disinfection, it has application potential and broad application scenarios, and is worthy of further study.