Recent advances towards aqueous hydrogen peroxide formation in a direct current plasma–liquid system

The aqueous phase hydrogen peroxide (H 2 O 2aq ) produced from the plasma–liquid interactions can directly or synergistically (with other substances) affect the liquid chemistry, and therefore it is important to unfold the H 2 O 2aq formation mechanism. However, up to now, a consensus on the H 2 O 2aq formation mechanism is not reached. This review aims to survey the recent advances on the understanding of the H 2 O 2aq formation mechanism in the system of a direct current discharge plasma operated over a liquid electrode. Theoretical and experimental analyses indicate that the recombination of dissolved OH radicals (OH aq ) is the dominant process for the H


| INTRODUCTION
Plasma generated in gas is a state of matter, which is an electrically neutral substance including positively and negatively charged particles, and in most of the cases, the negatively charged particles are electrons.Besides charged particles, a variety of excited neutral molecules, atoms, and radicals are produced in the plasma by energy transfer from the energetic electrons in most cases.Therefore, a highly chemical reactivity is provided by plasma, based on which plasma is widely used in the modern processing technology [1].
When the liquid in the plasma-liquid system is an aqueous solution, unavoidably, the water molecule will be involved in the plasma-liquid interactions.As a result, atomic hydrogen (H) and hydroxyl (OH) radicals are generated in the plasma by electron collisions, etc. Hydrated electrons (e aq ) are also produced from the dissolution of the gaseous electrons into the aqueous solution [43][44][45].If the discharge plasma is operated in ambient air, nitrogen oxides are usually generated in the gas phase and then dissolved in the solution to form nitrate - 405     (NO 3
Many other species might also be produced if other discharge gases are used.Among these reactive species, the reactivity of OH radicals and e aq is very strong, resulting in a short penetration depth of the gaseous OH/electron entering the solution (∼10 2 nm) [44].The short penetration depth of these highly reactive species such as OH radicals and e aq can influence only the very layer of the solution surface, while the relatively long-lived reactive species, such as H 2 O 2 , NO 3 − and NO 2 − will involve in the bulk solution reactions.They can offer multiple chemical advantages for the plasma-treated solutions.Especially, when the aqueous hydrogen peroxide (H 2 O 2aq ) is formed in the solution, it is possible to produce highly reactive OH radicals in the bulk solution through Fenton's reaction [52][53][54][55] or UV irradiation [56][57][58].Moreover, the reactive ONOO − /ONOOH can be formed when nitrite is present [18,50].Compared with the short-lived species, such as OH, ONOO − /ONOOH, which can penetrate the solution with a very thin thickness, the H 2 O 2aq species can exist in the bulk solution and then the chemistry of the bulk solution can be tuned by the plasma-generated H 2 O 2aq species with a long durance.Therefore, to clarify the formation process of the H 2 O 2aq species in the plasma-liquid system is of great importance.
Depending on the application, different power sources have been used to generate discharge plasma in the plasmaliquid system.The power sources range from direct current (DC) to microwave power supplies [59].In some cases, modulated electrical signals are also applied to generate discharge plasma with controllable parameters [60,61].As described in Ref. [62], a DC discharge plasma is macroscopic time independent, which is simpler than radio frequency or other discharges.If the driven power is not a DC source, for the plasma-liquid system with the liquid as an electrode, the processes happening at the plasma-liquid interface are usually complicated compared with a DC plasma-liquid system.The discharge plasma inside a liquid is a transient discharge, and thereby it is difficult to investigate and analyse the processes at the plasma-liquid interface for the plasma operated inside a liquid.Moreover, the DC plasma has a simple potential distribution along the plasma column axis, which makes it easy to give a quantitative analysis of the species and charge transfer at the plasma-liquid interface.Therefore, we focus our investigation and discussion on the plasma-liquid system of a DC discharge plasma operated over a liquid electrode.As mentioned in our previous paper [63], the conclusions drawn from the DC plasma-liquid system are fully or partially applicable to systems supplied with other power sources.For example, the conclusions obtained from the DC plasma-liquid system with the liquid as a cathode or an anode should still hold for the positive or the negative period of the plasma driven by an alternating current power supply.Herein, we choose the DC plasma-liquid system to investigate the H 2 O 2aq formation mechanism.
In a review paper, B. R. Locke et al. [64] surveyed the methods for forming hydrogen peroxide in discharge plasma with liquid water, and they pointed out that the H 2 O 2 species can be produced via various methods as water participates in a plasma.There exist several proposals on the mechanism of the H 2 O 2aq formation in the plasma-liquid system.In the early studies of the plasma-liquid interactions, there was a surmise that the gaseous OH radicals (OH g ) are produced by the water vapour dissociation by Equation ( 2), and then the OH g species dissolves into the solution to form aqueous OH radicals (OH aq ).The H 2 O 2aq species are thus formed in the solution through the recombination of dissolved gaseous OH radicals (Equations ( 2), (3), ( 5)) [65][66][67][68][69][70][71][72].Later, some kind of a modified viewpoint was proposed, that is, the H 2 O 2aq is formed inside the liquid by the recombination of the OH aq generated in situ in the liquid phase by positive ion bombardment [73][74][75][76][77][78] or by ultraviolet (UV) and vacuum ultraviolet photolysis of water [79,80] (Equations ( 4), ( 5)).Several more recent research studies [81][82][83][84] suggested that gaseous H 2 O 2 is formed in the gaseous plasma by the combination of the OH g species and then a gas to liquid transfer of the gaseous H 2 O 2 (H 2 O 2g ) leads to the formation of the H 2 O 2aq (Equations ( 2), ( 6), (7)).Up to date, the exact mechanism of the H 2 O 2aq formation in the DC plasma-liquid system is still in dispute, although the consensus in these proposals is that the OH radicals are the building block of the H 2 O 2aq species.An electrocatalytic production of H 2 O 2aq species recently attracts much attention, which is based on the oxygen reduction reaction at the electrode [85][86][87][88].The precursor of the H 2 O 2aq is molecular oxygen in the electrocatalytic method, while it is water in the plasma-liquid system.
In this review, we will present some of the most recent advances of the H 2 O 2aq formation mechanism in the DC plasma-liquid system with the liquid as one discharge electrode.First, we discuss several fundamental processes in the DC plasma-liquid system with the liquid as a cathode or an anode, including charge and mass transport at the plasmaliquid interface.Second, we provide a general equation for the H 2 O 2aq production rate by considering the possible sources of the H 2 O 2aq species, and we find that the exact H 2 O 2aq formation mechanism is dependent on the discharge type.A series of experiments demonstrated that for a DC glow discharge plasma-liquid system, the H 2 O 2aq formation is mainly through the recombination of dissolved OH radicals.Moreover, we infer that for a corona discharge plasma-liquid system, the H 2 O 2g dissolution will be the major contributor of the H 2 O 2aq formation.Some insights are also provided for tuning the production rate of the H 2 O 2aq species in the DC plasma-liquid system.The main objective of this review is to offer a concise but comprehensive understanding of the H 2 O 2aq formation mechanism in the DC plasma-liquid system with a liquid electrode, which will be helpful for further applications and development of the plasma-liquid system.

| General characteristics in the DC discharge plasma-liquid interface
At low pressures, a glow discharge will be ignited when a certain value of voltage is applied to the parallel metal plate electrodes located at both ends of a gas-filled glass tube.One electrode acts as the cathode and the other as the anode.Typically, to sustain the low pressure glow discharge plasma, a few hundred voltages are needed depending on the discharge distance, media, and pressure.Before the discharge is ignited, the potential from the cathode and to the anode will increase linearly along the tube axis [62].Because of the travelling of positive ions towards the cathode and electrons to the anode, positive and negative space charges are accumulated at both electrodes, respectively.Consequently, most of the potential is dropped near the cathode, and a small potential is dropped at the anode due to the slow mobility of massive positive ions.Figure 1 presents the characteristic potential distribution between the discharge electrodes for a DC glow discharge plasma [89].The regions with a potential fall near the cathode and the anode are called as the cathode sheath and anode sheath, respectively.The potential variation between the cathode sheath and anode sheath is very small.The voltage fall across the cathode sheath is usually called as the cathode fall (V C ).To maintain the continuity of current at the cathode, the charge carriers are positive ions and secondary electrons, while the charge carriers at the anode are dominantly electrons [59].We will show that the DC plasma-liquid system has similar characteristics as those in the glow discharge plasma with two metal electrodes.
The size of atmospheric pressure discharge plasma is usually limited to the order of ∼mm at least in one dimension because of the high collision frequency at atmospheric pressure.It is difficult to investigate the properties of atmospheric pressure plasma using a Langmuir probe.While at a low pressure, the situation is totally different.Using a single Langmuir probe, Kaneko et al. [14] studied the potential distribution along the Ar plasma column at a pressure of 40 Pa, where a kind of ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate, which possesses extremely low vapour pressure, was used as an electrode in the DC plasma-liquid system.The results are shown in Figure 2. Clearly, the potential distribution is similar to that presented in Figure 1.When the liquid is a cathode, a cathode fall of ∼300 V is observed near the liquid cathode surface.There is almost no potential difference between the plasma and the liquid surface when the liquid is an anode.
Although the results shown in Figure 2 were obtained at the low gas pressure, it is safe to assume that they hold for the discharge plasma operated at an atmospheric pressure, since many researchers have already discussed and measured a similar cathode fall of atmospheric pressure plasma [90][91][92][93][94][95][96][97].Therefore, there exists a bombardment of energetic positive ions or a shower of low-energy electrons towards the liquid surface when the liquid is a cathode or an anode.The average energy of electrons in DC atmospheric pressure plasma is in F I G U R E 2 Potential distributions between the discharge electrodes for Ar plasma-liquid systems using ionic liquid as (a) a cathode, and (b) an anode.The Ar pressure is 40 Pa, and the discharge current is 1 mA.Reprinted with permission from Ref. [14].Copyright 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim F I G U R E 1 Qualitative potential distribution for a direct current (DC) glow discharge plasma between two parallel electrodes with a gap of L. VC is known as the cathode fall [89] the range of 1-2 eV [2].Because of the small potential fall near the liquid anode, the plasma electrons will touch the liquid surface with an energy less or a little greater than the energy of the electrons in the plasma (1-2 eV), and then some of the electrons will dissolve into the liquid, forming hydrated electrons if the liquid is an aqueous solution [43][44][45].The positive ions from the plasma will be accelerated by the strong electric field at the cathode region when they travel across the cathode sheath.If there is no energy loss by collisions, the energy increase of the positive ions will be on the order of ∼eV C (hundreds of eV).However, the high collision frequency at atmospheric pressure will lead to some energy loss, and the energy increase of the positive ions can be estimated based on the plasma properties of d C and V C etc. (see Appendix A).For example, in a DC Ar plasma-liquid system used to investigate the H 2 O 2aq formation [98], d C and V C were estimated to be about 70 mm and 590 V, respectively.The kinetic energy of Ar + was estimated to be about 0.2 eV.By the method shown in Appendix A, the E Ar+ is estimated to be 50.31eV.Although this energy is much smaller than that achieved without collisions (590 eV), it is still possible for the positive ions to induce sputtering [99], evaporation, and secondary electron emission at the liquid surface.
The above discussion demonstrates that the main charge transfer is the movement of the accelerated positive ion towards the liquid surface when the liquid is a cathode, while it is the movement of electrons towards the liquid surface when the liquid is an anode.This difference will lead to a different solution chemistry, affecting the H 2 O 2aq yield, which we will discuss in the next section.
From kinetic theory, we can estimate the partial flux (Γ m ) and the power flux (P m ) of the mth type of neutral particle in the plasma towards the liquid surface, and the rate of the mth type of particle impinging on the liquid surface (R m ) [100] by Equations ( 8)- (10).
where M m is the mass, T m is the temperature, n m is the number density of the mth type of particle, and k B is the Boltzmann constant (1.38 � 10 −23 J/K).S is the contact area of the plasma and the liquid surface.Based on the above analysis, the major processes taking place at an Ar plasma-liquid interface for systems using an aqueous solution as a discharge electrode can be schematically depicted in Figure 3.These processes can be simply classified as two types: One is from the gaseous plasma towards the liquid phase, and the other is from the liquid phase towards the gaseous plasma.

| Species transport from the gaseous plasma towards the liquid phase
For the liquid as a cathode, positive Ar + ions are accelerated across the cathode sheath to bombard the liquid surface, by which the liquid surface will be disturbed, and then evaporation, sputtering, secondary electron (e 2nd ) emission are entailed from the liquid.
For the liquid as an anode, electrons are pulled out from the plasma to enter the liquid by the small electric field near the liquid anode.When these electrons enter the aqueous solution, they will be hydrated to form the e aq species in the solution.
In both cases of the liquid being a cathode and an anode, photons emitted from the excited particles in the plasma might enter the liquid, probably inducing some secondary processes in the liquid.Neutral particles in the plasma will also be transported to the liquid surface following the kinetic theory (Equations ( 8)-( 10)).
Because of the strong bombardment of energetic positive ions towards the liquid surface, energy transport from the plasma to the liquid for the liquid being a cathode is much greater than that for the liquid being an anode, if other discharge parameters are the same.This is confirmed by an investigation of the temperature evolution in a DC plasmatreated aqueous solution of NaCl (Figure 4) [5].The temperature of the treated solution for the liquid as a cathode increases much faster than that for the liquid as an anode.

| Species transport from the liquid phase towards the gaseous plasma
Ar ions are accelerated across the cathode sheath to bombard the liquid surface, by which the liquid surface will be disturbed and then evaporation, sputtering, and the e 2nd emission are entailed from the liquid phase.From the aqueous solution to the gaseous plasma, the components of the solution can be transported to the gaseous plasma by these processes.Sputtering and the e 2nd emission are absent when the liquid is an anode since the energy of electrons approaching the liquid surface is relatively low.Here, we are interested in the transport of water molecules into the gaseous plasma.The number density of water molecules transported to the gaseous plasma by these three processes can be expressed as Ref. [5,62,101] where n Spu , n Ele and n Eva are the number densities of water molecules entering the plasma phase due to the sputtering, electric field-induced hydrated positive ion emission, and evaporation, respectively.k spu is the sputtering coefficient, I d is the discharge current, E i is the energy of incident positive ion, C Ion is the hydrated positive ion concentration in the liquid phase, V C is the cathode fall, n 0 is the number density of water molecules in the liquid phase, ΔH vap is the heat of vapourisation, k is the Boltzmann constant, and T is the liquid temperature.The n Ele is an increasing function of the C Ion and E C , while it depends on these parameters in a complicated way [101].Therefore, the number density of water molecules (n H2Og ) transported to the gaseous plasma for the liquid being a cathode is a function of I d , C Ion , and V C (Equation 14, E i is a function of V C , see Appendix A).If the liquid is an anode, the n H2Og is only the final term of Equation (14).
According to Equation (2), the time derivative for the number density of gaseous OH radicals (n OHg ) formed in the plasma should be proportional to the electron density (n e ) and n H2Og , since the OH is mostly generated by the electron-collision-induced dissociation of water vapour in the plasma, where k 2 is the rate constant of Equation ( 2).The production rate of the OH g is related to the electron density and the discharge gas humidity.However, the electron density will decrease when the discharge gas humidity increases as reported in Ref. [102].Therefore, the highest production rate of the OH g species will be reached at a trade-off point between the electron density and the humidity.
In general, the transport of species at the plasma-liquid interface will be very different for systems with a liquid cathode and a liquid anode due to the different voltage drop at the liquid surface as shown in Figure 2.
For the liquid as the cathode, the positively charged species in gaseous plasma will be driven towards, while the negatively charged species will be repelled from the liquid surface.The neutral species in the plasma enter the liquid mainly by diffusion.The liquid components are transferred to the gaseous plasma mainly by sputtering and evaporation caused by the ion bombardment of the liquid phase [103].
For the liquid as the anode, there is no ion bombardment on the liquid surface; the negatively charged species in the gaseous plasma is transferred to the liquid driven by a small voltage drop.The neutral species in the plasma enter the liquid by diffusion.However, the liquid components will be transferred to the gaseous plasma mainly by the evaporation process, which is insignificant compared with that in the case of the liquid cathode.

| PATHWAYS OF THE AQUEOUS H 2 O 2 FORMATION IN THE DC PLASMA-LIQUID SYSTEM
Based on the analysis in Section 2, the H 2 O 2aq might originate from the dissolution of gaseous H 2 O 2 and from in situ solution chemistry.In the plasma and in the solution, the H 2 O 2 species might be eliminated by decomposition or recomposition with some substances.
Time evolution of the solution temperature in both liquid as a cathode and as an anode electrode.The treated liquid was an aqueous NaCl solution (conductivity of 4800 μS cm −1 ).The discharge gap was   4), ( 6), ( 16) and ( 17), respectively.The OH aq species can also be scavenged by some scavengers (S i etc.) existing in the solution.Thus, the change of [OH aq ] with respect to time can be expressed by d where G OHg and G OH (UV) are the OH g dissolution rate and the OH aq generation rate by the plasma-induced UV photolysis of water, respectively.k Si is the rate constant of the reaction between OH aq and S i .
In a pseudo steady state, d[OH aq ]/dt = 0 ([OH aq ] should be a small constant because of its super high reactivity) and then we have As a result, Equation ( 19) turns out to be d For the DC plasma-liquid system with a particular aqueous solution, G OHg and G H 2 O 2g can be assumed to be constants for a fixed discharge current based on Equation (10).G OH (UV) can also be considered to be a constant for a fixed discharge current.
P i k S i ½S i � and P i k C i ½C i � are also constants for a particular solution.Therefore, the production rate of the H 2 O 2aq species is mainly a function of G OHg , G H 2 O 2g and G OH (UV).Each of the potential parameters capable to produce the H 2 O 2aq species is recently explored to clarify the dominant process for the H 2 O 2aq formation in the DC plasma-liquid system.
First, the viewpoint of the H 2 O 2g dissolution being the main contributor of the H 2 O 2aq formation is investigated.J. Liu et al. [5] studied the production of the H 2 O 2aq species in a DC plasma-liquid system with the liquid being a cathode or an anode.They found that the production rate for the system with the liquid as a cathode is much larger than that with the liquid as an anode.The pH value of the solution can strongly influence the H 2 O 2aq yield, even in the case with the liquid as the cathode, and the H 2 O 2aq production rate for a plasma-liquid system with an aqueous solution of the NaCl cathode (pH = 5.89) is 15.6 times greater than that for a system with an aqueous solution of the NaOH cathode (pH = 12.86) as shown in Figure 5.
It is known that hydrogen peroxide in an aqueous solution takes an equilibrium state as When the solution is in the acidic range, the molecular form of hydrogen peroxide (H 2 O 2 ) will be dominant.If the solution moves to the basic range, the ionic form of hydrogen peroxide (HO 2

−
) will be the main species.We should note that Henry's Law is applied to gases that are physically dissolved but do not react in the solution to form a different species [104,105].Then, Henry's Law can be applied to Equation (7), which is a physical dissolution, but not to Equation (23), which is a chemical reaction.If the H 2 O 2aq species is from the dissolution of the H 2 O 2g species, then we have where k H H 2 O 2g is the Henry's law coefficient and p H 2 O 2g is the gas-phase partial pressure of the H 2 O 2g species.Thus, the total concentration of the dissolved H 2 O 2g species is the sum of the physical dissolved one (Equation ( 24)) and the reacted one (Equation ( 23)), and as a result, the effective Henry's law coefficient should be dependent on the pH value as described in Ref. [105].Because Equation ( 23) will proceed to the right side when pH is higher, the effective Henry's law coefficient of hydrogen peroxide should be greater in the solution of a higher pH value.Consequently, the total dissolved H 2 O 2g species should be higher in the solution with a higher pH value, if other conditions are the same.
We will demonstrate the fact that the colourimetric method will not influence the measurement of the H 2 O 2aq concentration in a wide range of pH values.
Using a colourimetric method with a strongly acidified aqueous solution of titanium sulphate, we measured the H 2 O 2aq concentrations for aqueous solutions with identical amounts of H 2 O 2 at the pH range of 2-13 (Figure 6).The result indicates that the colourimetric method for the H 2 O 2aq quantification is independent of the pH values, which has also been reported by Z. Chen et al. [106] for aqueous solutions at a pH range of 4-10.Thus, the pH value will not influence the colourimetric quantification of the total dissolved H 2 O 2g species.Here, it is worth noting that a previous investigation have implied some H 2 O 2aq might be decomposed at the pH value of 12 [98].However, we found that there is no obvious H 2 O 2 decomposition at that pH value as we rechecked the measurement, presumably some mistake occurred during the previous measurement.
If the H 2 O 2aq species originates mainly from the H 2 O 2g dissolution and the colourimetric method can quantify the total dissolved H 2 O 2g species, the H 2 O 2aq production rate should be higher at higher pH values.However, the result is opposite.This indicates that the contribution of the H 2 O 2g dissolution to the H 2 O 2aq formation in the DC plasma-liquid system is insignificant.Thus, a question is raised: What causes the great difference on the H 2 O 2aq production rate at different pH values?
Since the Henry's Law coefficient of H 2 O 2g is relatively high, several orders greater than that of the OH g [107], the dissolution of H 2 O 2g must contribute to the H 2 O 2aq formation according to Equation (22), if the number density of H 2 O 2g is high.While the result is not the case.Therefore, we assume that the number density of H 2 O 2g in the DC plasma-liquid system might be very low.It is well known that the thermal decomposition of H 2 O 2 is a temperature-dependent process, and the decomposition of the H 2 O 2 vapour is approximate to 100% when the temperature is higher than 760 K, as reported by P. A. Giguère et al. [108].We know that the gas temperature of the plasma in the DC plasma-liquid system is in the range of 2000-3000 K [2,98], and therefore the H 2 O 2g formed in the plasma phase might be thermally decomposed.Consequently, the number density of H 2 O 2g is insignificant and that of the G H2O2g as well.From these arguments, we come to the conclusion that the H 2 O 2g dissolution is not the main contributor for the H 2 O 2aq formation in the DC plasma-liquid system.A reasonable interpretation of the big difference in the H 2 O 2aq production rate will be presented later when we discuss the contribution of the OH g dissolution to the H 2 O 2aq formation.
Second, we will check the contribution of photolysis of water to the H 2 O 2aq formation in the DC plasma-liquid system.X.He et al. [98] investigated the production of the H 2 O 2aq species by the plasma-generated irradiations.A quartz plate (1 mm in thickness and 50 cm in diameter) was placed just over the surface of an aqueous NaCl solution, only permitting the transport of the plasma-generated photons.Two graphite rods (3 mm in diameter, 100 mm in length and 2 mm in distance) placed in parallel on the quartz plate surface functioned as the grounded electrode.Ar gas (20 standard cubic centimetres per minute) was supplied to a hollow tungsten steel electrode through a water bottle.This design roughly imitates the discharge condition of humid Ar.The discharge current was 30 mA and the discharge gap was 2 mm.
Figure 7 presents the H 2 O 2aq concentration as a function of the plasma exposure time.Evidently, the yield of the H 2 O 2aq species by the plasma-induced photolysis of water is significantly small.Thereby, Equation ( 22) turns out to be d To explore the influence of dissolved OH g on the H 2 O 2aq formation, a OH scavenger of dimethyl sulfoxide (DMSO, (CH 3 ) 2 SO) was added to the plasma-treated solution, due to the miscibility of the low-volatility DMSO in water and the high reaction rate constant of DMSO with OH radicals (6.6 � 10 9 M −1 s −1 ) [109,110].Moreover, it has been confirmed that the DMSO does not scavenge the H 2 O 2aq species [98].When DMSO is present in the solution, the OH aq species will quickly react with DMSO by Equation (26).
Then Equation ( 25) turns out to be d where DMSO is not included in the representative OH aq scavenger, S i , and P i k S i ½S i � can also be taken as a constant for a particular solution.K DMSO is the rate constant of the reaction between OH aq and DMSO. Figure 8 shows the DMSO effect on the yield of the OH aq in a DC plasma-liquid system with the liquid as a cathode.As demonstrated in Figure 8, the production rate of the H 2 O 2aq species decreases as the DMSO concentration increases, which is consistent with Equation (( 27)).When the DMSO concentration is greater than some level, for the competing reactions of Equations ( 5) and ( 26), the latter one will be dominant.As a result, the yield of the H 2 O 2aq species in the plasma treated solution approaches zero.The critical DMSO concentration is located between 50 millimole per litre (mM) and 100 mM in Figure 8.
Here, we turn to the results described in Figure 5.The reaction rate constant between the hydroxide ion (OH − ) and the OH aq species is very high (Equation ( 28)), k 27 = 1.3 � 10 10 M −1 s −1 ) [109], which is about two times greater than that between the DMSO and the OH aq species, In Figure 5, the OH − concentration for the NaOH solution is about 100 mM, approaching the critical value of the DMSO concentration.Therefore, most of the OH aq species are eliminated by the OH − ions in the solution, leading to an insignificant yield of the H 2 O 2aq species.
It is worth noting that a recent paper [81], suggested that the dissolution of H 2 O 2g is the dominant contributor for the H 2 O 2aq species in the DC plasma-liquid system.They used 4 mM of disodium terephthalate to scavenge the dissolved OH radicals in solutions.Even for the DMSO concentration that is as high as 50 mM, the dissolved OH radicals are not eliminated completely (Figure 8).This amount of disodium terephthalate is insufficient to completely eliminate the dissolved OH radicals, and therefore only a part of the dissolved OH radicals are scavenged.Consequently, the H 2 O 2aq species can still be generated in the solutions by the recombination of the dissolved OH radicals.This might mislead the authors to make that conclusion.
The electrochemical process is also a potential pathway for the H 2 O 2aq formation in the acidic medium.Molecular oxygen (O 2 ) from the water electrolysis or from the ambient air can be reduced to form H 2 O 2 by a two-electron transfer process (Equation ( 29)) [111][112][113].Moreover, the O 2 species can capture an electron to form O 2 − and then the O 2

−
species combines with H + to form a HO 2 species.Therefore, the H 2 O 2aq can be produced by the disproportionation of the HO 2 species (Equations ( 30) & ( 31)) [106,114,115].However, active and stable catalysts are usually required in the electrochemical synthesis of the H 2 O 2aq species [116].In the DC plasma-liquid system, the immersed electrode is usually the substance with a poor or no catalytic property for the H 2 O 2 synthesis.Instead of the production of the H 2 O 2aq species, molecular oxygen (O 2 ) and molecular hydrogen (H 2 ) are usually the products of the electrochemical process at the liquid cathode by reducing the reaction of H + + e → H 2 and at the liquid anode by the oxidising reaction of 2H 2 O → O 2 + 4H + + e, respectively [117].Thus, we consider that the electrochemical pathway for the H 2 O 2 synthesis is insignificant in the DC plasma-liquid system without an active catalytic electrode.
From the above analysis, we can draw a conclusion that the H 2 O 2aq species in the DC plasma-liquid system originates mainly from the recombination of dissolved OH radicals, while the contributions of the dissolution of gaseous H 2 O 2 and plasma-induced photolysis of water are insignificant.
A question is still required to be answered: Why is the production rate of the H 2 O 2aq species in the DC plasma-liquid system with a liquid cathode at a fixed discharge current is much greater than that with a liquid anode?The contribution of the H 2 O 2g dissolution to the H 2 O 2aq formation can also be ruled out by this fact.The liquid acting as the cathode or the anode will not affect the H 2 O 2g dissolution much, and then the production rate of the H 2 O 2aq should not be much different at both cases.However, the production rate of the H 2 O 2aq for the liquid cathode is much greater.The n OHg of the DC plasmaliquid system quantified by several techniques ranges from 10 18 to 10 23 m −3 depending on the discharge type and plasma parameters [80,[118][119][120][121][122].Using laser-induced fluorescence spectroscopy, the n OHg is found to increase almost linearly with the increasing discharge current in the DC air plasmaliquid system with a liquid cathode or a liquid anode [120].The obtained n OHg is 8.95 � 10 21 m −3 for the case of a liquid cathode and 1.78 � 10 22 m −3 for the case of a liquid anode at a fixed discharge current of 28.5 mA [120], and similar results also reported in Ref. [121].These results mean that the n OHg is in a similar magnitude for the liquid being a cathode or being an anode at a fixed discharge current, which might be the result of a trade-off between the electron density and the water vapour content in the gaseous plasma (Equation ( 15)).Thus, for a fixed discharge current, G OHg should be in the same order for the DC air plasma-liquid system with the liquid being a cathode or being an anode since G OHg is proportional to the n OHg (Equation ( 10)).Based on Equation (25), the difference of d[H 2 O 2aq ]/dt between the liquid cathode and the liquid anode is determined by the OH aq and H 2 O 2aq scavengers (S i and C i ).When the liquid acts as an anode, electrons pulled out from plasma are transported to the solution, leading to the formation of e aq , which is almost absent when the liquid acts as a cathode.The e aq species can directly or indirectly involve in the consuming reactions for the OH aq and the H 2 O 2aq species as described in Equations ( 32)- (38).
Compared with the system with the liquid being a cathode, the system with the liquid being an anode will add additional OH aq and H 2 O 2aq scavengers into the solutions such as e aq and OH − .For a fixed discharge current where G OHg is in the same order for the DC plasma-liquid system with the liquid being a cathode or being an anode, these additional OH aq and H 2 O 2aq scavengers will increase the terms of P i k S i ½S i � and (25), and consequently, the production rate of the H 2 O 2aq species, d[H 2 O 2aq ]/dt, in a system with the liquid being an anode will be relatively small.
It is possible to enhance the production rate of the H 2 O 2aq species in a system with a liquid anode by tuning the e aq concentration and the pH value of the solution.Figure 9 gives the influences of the pH values and the e aq species on the production rate of the H 2 O 2aq species in a system with a liquid anode [116,117].
For the effect of the pH value, the results indicate that the H 2 O 2aq species are formed only in solutions with intermediate pH values of 3.34 and 4.00.The insignificant yield of the H 2 O 2aq species at the low pH value might be ascribed to Equation (1), which is effective at pH values below 3 [124].At high pH values, OH − consumes some OH aq species, and Equations ( 32)- (38) as well, inducing very small yield of the H 2 O 2aq species.For the solutions with intermediate pH values, Equation (1) does not take place and the OH − ions concentration is not so high that the H 2 O 2aq yield is relatively enhanced.
For the effect of the e aq species, the H 2 O 2aq production rate is very low for solutions with pH values of 2.00 and 11.50, which is similar to the case by only tuning the pH value.The H 2 O 2aq production rate of solutions with pH values of 7.00 and 8.34 are comparable to that of the solution with a pH value of 4.00 without reducing the e aq concentration.The H 2 O 2aq production rates are insignificant for solutions with pH values of 7.00 and 8.34 without reducing the e aq concentration (Figure 9a).This is attributed to the suppression of the e aq concentration.
From Equation ( 22), the production rate of the H 2 O 2aq species in the DC plasma-liquid system with a liquid anode can be enhanced by improving the in situ OH aq production with some method, although the in situ OH aq production by plasma-induced photolysis of water is ruled out.It may work if the solution is mixed with some substance which could improve the in situ OH aq yield.As an endeavour, ethanol has been used as the additive to improve the production rate of the H 2 O 2aq species in a DC plasma-liquid system with a liquid anode [125].
When atomic oxygen (O) is present, the OH radicals can be rapidly generated by the hydrogen abstraction reactions [126] with a total rate constant of 1.02 � 10 8 M −1 s −1 [127] (Equations ( 39)-( 41) Equation ( 39) is found to be the major hydrogen abstraction at 298 K [127].The product of Equation (39), alphaethanol radical, CH 3 CH(OH), can further react with O to generate OH radicals by Equation ( 42) with a great rate constant of 1.9 � 10 11 M −1 s −1 [127,128], Figure 10 indicates that a certain content of ethanol addition indeed improves the production rate of the H 2 O 2aq species in a DC plasma-liquid system with a liquid anode.If the content of ethanol is higher than some value, the production rate of the H 2 O 2aq species is again suppressed.n OHg was found to be not much deviated when ethanol was added into the solution.Therefore, Equations ( 39)-( 42) should take place in the solution and improve the OH aq species in situ in the solution.The suppression of the production rate of the H 2 O 2aq species at the high content of ethanol might be explained as follows.Although Equations ( 39)-( 42) can in situ improve the OH aq formation, there exist another set of reactions in which ethanol acts as a scavenger of the OH radicals (Equations ( 43)-( 45)).The products of Equations ( 39)-( 42) might also function as the scavengers of the OH radicals.When the content of ethanol is excessive, the role of the OH scavenger of ethanol becomes dominant, resulting in the suppression of the H 2 O 2aq formation.Although the dissolved gaseous OH radicals have been confirmed to be the dominant contributor for the formation of the H 2 O 2aq species in a DC plasma-liquid system with plasma being in contact with the liquid, the dominant contributor might vary with the plasma type as we consider Equation (22).If the plasma is a corona discharge for which the active plasma zone is far from the liquid surface, the liquid functions as a discharge electrode.In this case, the gaseous H 2 O 2 species can be formed in the corona due to the low gas temperature.The short-lived gaseous OH radicals cannot reach the liquid surface because of the long distance between the active plasma and the liquid surface, but the long-lived gaseous H 2 O 2 species can reach the liquid.Therefore, the dominant contributor for the formation of the H 2 O 2aq species in this case should be the dissolution of the gaseous H 2 O 2 species similar to a system using a gliding arc discharge operated in water-sprayed oxygen gas [129].The in situ generation of the OH aq species can also happen if strong UV plasma emission is generated or some substance is added into the solution to enhance the OH aq formation.

| SUMMARY AND OUTLOOK
Recent advances for the H 2 O 2aq formation in a DC plasmaliquid system are surveyed.Although there are several potential sources for the generation of the H 2 O 2aq species, the recombination of dissolved gaseous OH radicals is confirmed to be the dominant contributor for the H 2 O 2aq formation in the DC plasma-liquid system by a theoretical analysis and solid experimental results.In both systems of using the liquid as a cathode or as an anode, there exists a bombardment of particles on the liquid surface, while the particles are high-energy positive ions for the case of a liquid cathode, whereas low-energy electrons for the case of a liquid anode.This fact causes different secondary reactions in the bulk liquid, and consequently, strongly influences the production rate of the H 2 O 2aq species.The theoretical analysis for the production rate of the H 2 O 2aq species has also provided valuable information on understanding the contributors for the H 2 O 2aq species in other kind of plasma-liquid systems as well as on the methods to efficiently improve the production rate of the H 2 O 2aq species in the DC plasma-liquid system.
After elucidating the mechanism for the formation of the H 2 O 2aq species in the DC plasma-liquid system, one of the future challenges will be an efficient improvement for the production rate of the H 2 O 2aq species.As shown in Section 3, to enhance one or all the potential factors might realise this aim.Among factors of H 2 O 2g , OH g and in situ generation of OH aq (by adding enhancing substance), the former two are better parameters to choose than the last one because the last one can add some by-products in the solution besides the H 2 O 2aq species.The number density of the OH g species is proportional to the electron density in plasma and the water vapour content in the environment of the discharge zone.While the water vapour content can also affect the electron density, there exists a trade-off between the electron density and the water vapour content to obtain a maximum number density of the OH g species at a fixed discharge current.To improve the amount of H 2 O 2g , one must decrease the gas temperature in plasma and/or increase the number density of the OH g species.The gas temperature in plasma might be decreased to a reasonable degree by using a pulsed DC power source [130] because of a decrease in the dissipated energy.Because the electron energy achievement and power transfer are more efficient in the pulsed DC plasma [130], the radicals' generation might be more efficient in the pulsed DC plasma.Thereby, the production rate of the H 2 O 2g for the pulsed DC plasma might be higher than that for a DC plasma.
It has been reported that a short-pulsed electrical excitation can enhance the emission vacuum-ultraviolet photons in a dielectric barrier discharge plasma [131].Therefore, a short-pulsed electrical excitation such as the nanosecond pulse DC excitation power source might be possible to enhance the UV-light emission in the plasmaliquid system.The in situ generation of OH aq in the solution can be improved by the enhanced UV photons, and as a result the production rate of H 2 O 2aq species might be enhanced.
To develop and extend the applications of the DC plasma-liquid systems where the H 2 O 2aq is important for the liquid chemistry, it is of great importance to achieve a plasma-liquid system, which can efficiently produce the H 2 O 2aq species under a high rate.A disciplinary knowledge on plasma physics, plasma chemistry, chemistry as well as diagnostics is required to overcome the gap between the present low production rate and the future high yield requirement of the H 2 O 2aq species.There are also other ways to increase the efficiency of the H 2 O 2aq production, such as using water spray instead of bulk water, by which design the contact efficiency between the plasma species and the water is enhanced, and subsequently the energy yield can be increased [132,133].If the H 2 O 2aq species can be efficiently produced at a high rate by the DC plasma-liquid system, we will obtain the green chemical oxidant only from electricity and water.Both industry and our routine life might benefit from this achievement, such as bleaching, propellant, and disinfection [134].In the future, the direct synthesis of H 2 O 2aq from the DC plasma-liquid system might be an alternative for the currently indirect, high-cost anthraquinone process [135].

APPENDIX A
Energ y of positive lons impinging on the liquid surface in a dc atmospheric pressure plasmaliquid system with the liquid being a cathode Assuming a collisional sheath near the liquid surface, the velocity (v iC ), and the energy (E iC ) of the positive ion at the cathode [62] where V C , d C , μ i , and m i are the cathode fall, the cathode sheath thickness, the mobility of the positive ion, and the mass of the positive ion, respectively.The mobility of the positive ion can be given as [136] μ i ¼ 760 p T 273:16 where μ i is the mobility of the positive ion at the background gas pressure of p (in Torr) and gas temperature of T (in K), and μ io+ is the mobility of the positive ion at standard temperature and pressure (at 760 Torr and 273.16K).
In a DC Ar plasma-liquid system, we have estimated the plasma properties where d C is about 70 μm, V C is about 590 V, and the kinetic energy of Ar + is about 0.2 eV.From Ref. [137], we obtain μ Ar+0 of 4 cm 2 V −1 s −1 .Using these results, the estimated E Ar+ is 118.5 eV, which is much smaller than 590 eV due to the energy loss by collisions.Using these data, we achieve CHEN ET AL.
OHg should be proportional to the number density of gaseous OH radicals (Equation (10)) for a fixed discharge current.If our analysis is right, the production rate of the H 2 O 2aq species can be tuned by the OH and H 2 O 2 scavengers (S i and C i ) in the solution.FI G U R E 6 The H 2 O 2aq concentrations measured at different pH values for aqueous solutions with identical amounts of H 2 O 2 .The pH values of solutions were adjusted by adding a diluted H 2 SO 4 or NaOH solution.The pH values of the x axis are referred to the H 2 O 2 solutions before mixing with the strongly acidified aqueous solution of titanium sulphate CHEN ET AL.

F I G U R E 7
Concentrations of the H 2 O 2aq generated in an aqueous solution exposed by a direct current (DC) plasma-generated photons.The initial conductivity of the NaCl solution is 4800 μS cm −1 .In order to treat the solution only with plasma-produced photons, the plasma species except the photons are excluded by separating the plasma and the solution using a quartz plate.Reprinted with permission from Ref. [98].Copyright 2018 IOP Publishing F I G U R E 8 The H 2 O 2aq concentrations as a function of the plasma treatment time for aqueous solutions of NaCl with dimethyl sulfoxide (DMSO) concentrations of 0, 10, 20, 50, 100, and 200 mM.The solution acted as a cathode.The discharge current was 30 mA and the discharge gap was 3 mm.Reprinted with permission from Ref. [98].Copyright 2018 IOP Publishing 412 -CHEN ET AL.
The H 2 O 2aq concentrations as a function of the plasma treatment time for solutions with different pH values, and (b) for solutions of different pH values with the suppression of the e aq species.The liquid acts as an anode and the direct current (DC) discharge current is 20 mA.The [e aq ] reduction is performed by placing a tungsten wire (0.5 mm in diameter), just touching the liquid surface.Reprinted with permission from Ref. [63, 123].Copyright 2020 EDP Sciences F I G U R E 1 0 Curves of the H 2 O 2aq concentration versus plasma treatment time for solutions with different contents of added ethanol.The total volume of the liquid is 200 mL with a pH value of 4.0 (adjusted by H 2 SO 4 ), the discharge current is 20 mA, and the liquid acts as the anode.Reprinted with permission from Ref. [125].Copyright 2019 IOP Publishing where A i and C i are the substances that react with H 2 O 2g and H 2 O 2aq to produce products B i and D i , respectively.The changes of the H 2 O 2g (n H 2 O 2g ) number density and the H 2 O 2aq concentration ([H 2 O 2aq ]) with respect to time can be expressed by dn H 2 O 2g OHg and n Ai are the number densities for the H 2 O 2g and A i , respectively.G H 2 O 2g is the dissolution rate of the H 2 O 2g species.k 4 , k 6 , k Ai , and k Ci are the rate constants of Equations ( where n