Fluidized Bed Process Hydrogen Peroxide

For this hydrogen peroxide production technology , SL TEC cooperates with one of the branch companies of China National Chemical Engineering Co., Ltd. '' SEDIN Ningbo Engineering Co., Ltd. to offer a turnkey hydrogen peroxide plant.

Contact us to discuss your requirements of Hydrogen Peroxide Production Plant. Our experienced sales team can help you identify the options that best suit your needs.

There are several methods for hydrogen peroxide production , among them the AO(auto-oxidation) process is the most prevailing one. In this process, 2-alkyl anthraquinone is mixed with organic solvent to make up the working solution, which is hydrogenated in presence of catalyst, and the resulting is oxidized by air (or oxygen) in counter-current way before being extracted, regenrated, purified and enriched to get commercial hydrogen peroxideproduct. Depending upon the different reactor type in the hydrogenation unit , AO process is further divided into Fixed Bed Reactor Process and Fluidized Bed Reactor Process . SL Tec provides both processes.

' Due to the absence of potassium carbonate drying column and relating control problems, the process flow of Hydrogen Peroxide Manufacturing Plant is shorted, the product quality is better and more favorable for the production of high purity hydrogen peroxide , like food grade and electronic grade.

' The WS capacity of Hydrogen Peroxide Manufacturing Plant is as high as 12g/L; 35-40% hydrogen peroxide can be directly collected from the extraction column; the total yield reaches up to 97%.

Hydrogen Peroxide Manufacturing Plant Process Flow Description

1.Hydrogenation Unit

The working solution (WS) is pressurized and enters into the WS Temperature conditioner before being charged to the bottom of the hydrogenation column with Pd catalyst; fresh hydrogen from the surge tank is filtered and combined with the recycling hydrogen from the hydrogen compressor before entering the bottom of the hydrogenation column. The WS and hydrogen flows up in a co-current way,  part 2-EAQ is hydrogenated to HEAQ, and the non-reacted hydrogen flows from the hydrogenation column to the separation and cooling unit before mixing with the fresh hydrogen and enters into the hydrogenation column again.

The hydrogen and the hydrogenated solution goes through gas-liquid separation, and the resulting hydrogenated solution is filtered, cooled and fed to the hydrogenated solution storage tank. While the filtered Pd catalyst powder is re-flushed by the re-flushing liquid into the bottom of the hydrogenation for re-use.

The hydrogenated solution in the storage tank is pressurized, and goes through two-stage filters to avoid the entering of the catalyst powder into the hydrogen peroxide solution and causing any possible mistake.

2.Oxidation Unit

The oxidation column consist of air distributor, re-distribution plate, distribution cooler and packings. In this column, the hydrogenated solution from the last step is oxidized and the oxidation solution is obtained. The off-gas obtained from gas-liquid separation is cooled and condensed to get the mixture of aromatic and water before entering into the off-gas recovery cooler to have heat exchange with the low temp. oxidation gas from the expander. The heat exchanged off-gas enters into the off-gas recovery tank to separate out the condensed aromatic, which is to be recovered.

3.Extraction and Purification Unit

3.1 Extraction and Purification of Hydrogen Peroxide

The oxidation solution from the oxidation unit overhead tank is sent to the lower part of the extraction column, and dispersed into numberless globule and float to the column top; at the same time, the pure water from the overhead tank is heat exchanged to increase the temp. and charged to the extraction column upper section to have counter-current extraction with the upwards floating oxidation solution. The crude hydrogen peroxide is obtained at the bottom of the extraction column, while the resulting oxidation solution flows out of the extraction column top and enters into the working solution coalesce and the working solution water separator.

The crude hydrogen peroxide is fed to the purification full with aromatic which flows from the purification column overhead tank to have counter-flow contact to remove the organic impurities in the hydrogen peroxide, and the resulting hydrogen peroxide intermediate flows out of the purification column bottom, while the heavy aromatic from the top is charged to the used aromatic storage tank.

3.2H3PO4 Solution Make Up and Adding H3PO4 Solution into Pure Water

3.3Automatic analysis of oxygen content at the extraction column top and the purification column top and heavy aromatic vapor dilution by nitrogen

3.4Working solution vacuum drying

4.Re-generation Unit

During the hydrogenation and the oxidation, small amount of anthraquinone degrading compounds will be generated; to reduce the consumption of the EAQ consumption and keep the cleanness of the working solution, part of the hydrogenation solution or the recycling working solution shall be introduced to the alumina oxide bed for regeration.

This work examines the role of oxygen supply in the improvement of the hydrogen peroxide (H 2 O 2 ) electrochemical production efficiency and the generation of high H 2 O 2 concentrations in electrochemical processes operated in a discontinuous mode. To conduct this study, a highly efficient Printex L6 carbon-based gas diffusion electrode (GDE) as a cathode was employed for the electrogeneration of H 2 O 2 in a flow-by reactor and evaluated the effects of lowering the operation temperature (to increase solubility) and increasing the air supply in the system on H 2 O 2 electrogeneration. The results obtained in this study show that unlike what is expected in flow-through reactors, the efficiency in the H 2 O 2 production is not affected by the solubility of oxygen when GDE is employed in the electrochemical process (using the flow-by reactor); i.e., the efficiency of H 2 O 2 production is not significantly dependent on O 2 solubility, temperature, and pressure. The application of the proposed PL6C-based GDE led to the generation of accumulated H 2 O 2 of over 3 g L '1 at a high current density. It should be noted, however, that the application of the electrocatalyst at lower current densities resulted in higher energy efficiency in terms of H 2 O 2 production. Precisely, a specific production of H 2 O 2 as high as 131 g kWh '1 was obtained at 25 mA cm '2 ; the energy efficiency (in terms of H 2 O 2 production) values obtained in this study based on the application of the proposed GDE in a flow-by reactor at low current densities were found to be within the range of values recorded for H 2 O 2 production techniques that employ flow-through reactors.

So far, when it comes to the development and application of techniques for the production of H 2 O 2 through ORR, most of the effort has been devoted toward the development of new cathodic materials (which use lab-scale cells) or specific applications of H 2 O 2 (such as cells for wastewater treatment). 19 , 22 ' 24 No substantial effort has been devoted toward investigating and developing new efficient techniques that are capable of producing H 2 O 2 at high concentrations. 14 , 16 Thus, the present work aims to develop and optimize the operational parameters of a new carbon-based gas diffusion electrode (GDE) applied in a flow-by electrochemical reactor with a view to obtaining high H 2 O 2 concentrations and high production efficiency. The choice of the electrochemical reactor operating mode to be flow-by is since the use of GDE in flow-by reactors has advantages in relation to the flow-through reactors, as it minimizes the formation of bubbles on the electrode surface, which increases the ohmic drop and also reduces the possibility of the electrolyte salt precipitation inside the GDE, blocking its channel structure and deactivating it over time. 25

Improvement of oxygen supply in the electrochemical cell since the low solubility of oxygen in the cell causes the efficiency of the process to be controlled by diffusion. As reported in the literature, to help tackle this problem, some important progress has been made by

The range of application of H 2 O 2 has progressively increased in recent years, and the annual consumption of this oxidant is estimated to increase to 6 million tons in . 2 , 7 ' 9 Most of the current production of H 2 O 2 occurs through the anthraquinone process. To meet the increasing demand for H 2 O 2 , alternative efficient techniques for the production of the oxidant are currently being studied, and one of the techniques that have been found to be highly promising is the electrochemical production of H 2 O 2 via oxygen reduction reactions (ORR)'see eq 1 . 2 , 4 , 9 ' 11 The ORR technique employs oxygen as the raw material in the electrochemical process. Over the past few years, there has been a huge interest among researchers in the use of ORR via the 2-electron pathway for the electrogeneration of H 2 O 2 ; this technique has become extremely popular because it is an energy-intensive multistep process, which has been proven to have the following advantages: high efficiency, good operational safety, and low environmental impact. 7 , 8 There have been several reports in the literature regarding the mechanism of operation of the ORR process. As demonstrated in the literature, through the application of carbon-based cathode materials, O 2 is easily reduced to H 2 O 2 via the transfer of two electrons at a potential of 0.682 V vs SHE 4 , 11 ' 13

The past few decades have seen a dramatic increase in the search for new technologies that are capable of producing chemical oxidants at substantial concentrations and in a highly efficient manner. Hydrogen peroxide (H 2 O 2 ) is a highly efficient, eco-friendly chemical oxidant, which has a wide range of applications in different sectors. 1 H 2 O 2 has a high reduction potential (E 0 = 1.77 vs standard hydrogen electrode, SHE) and produces nontoxic water when applied; as a result, this oxidant is widely applied in several industrial processes, including the synthesis of chemical products, paper bleaching, and wastewater treatment. 2 ' 6 As part of the efforts to combat the SARS-CoV-2 virus, H 2 O 2 was widely employed as a reagent in the formulation of decontamination and disinfection products and for the cleaning of contaminated respiratory masks for reutilization due to its antimicrobial properties. 3

The lifetime of the electrode was evaluated by applying a current density of 200 mA cm '2 using Arbin Instruments (model FBTS'20 V). Cyclic voltammetry analysis was performed before and after the electrochemical durability tests in a potential window of 0.0 to '0.8 V and at a scan rate of 50 mV s '1 using Autolab PGSTAT302N equipment and Ag/AgCl 3M as a reference electrode.

Hydrogen peroxide was quantified (in mg L '1 ) using titanium(IV) oxysulfate solution as an indicator reagent, and the quantification analysis was performed by UV'vis spectroscopy (at λ = 408 nm) using an Agilent 300 Cary series UV'vis spectrophotometer. The method adopted for the quantification of H 2 O 2 in this study was based on the technique proposed in previous studies reported in the literature. 1 , 6 , 19

The carbon black (CB) material used for the conduct of the experiments was Printex L6 carbon (PL6C)'acquired from Degussa Brazil. Before manufacturing the GDE, the catalytic material'Printex L6 carbon is heat-treated at 120 °C for 24 h to remove water residues and organic interferences. 15 After that, the carbon black material was mixed with 20 or 40% (w/w) of PTFE dispersion in 400 mL ultrapure water for 2 h until the mixture was completely homogenized. The catalytic mass was then filtered to remove excess water. Ten grams of the wet catalytic mass was deposited and spread over the carbon cloth (geometric area of 126 cm 2 ). The electrode was dried at 120 °C for 15 min and was subsequently treated through the application of a pressure of 5 tons and a temperature of 290 °C for 2 h. The electrode was then cut into a circular shape of 20 cm 2 .

The flow-by electrochemical reactor used in this work operates under atmospheric pressure because the reactor is not hermetically closed due to the continuous entry of gas into the cathode compartment. O 2 gas was continuously injected into the cathode compartment, and this was monitored with the aid of a gas flowmeter. An Autolab PGSTAT302N potentiostat coupled with a BOOSTER 10A was used as a power supply. A Pt//Ag/AgCl 3.0 M was employed as a pseudo-reference electrode and was coupled to the electrochemical cell, as described by Beati et al. 26 The platinum wire and the Ag/AgCl reference electrode were added to an external chamber containing the same electrolyte as the electrolyte from the electrochemical reactor (0.1 mol L '1 of Na 2 SO 4 ).

As can be seen in , the experimental system was set up using a flow-by electrochemical reactor with Printex L6 carbon/PTFE deposited on carbon cloth employed as the gas diffusion cathode and a dimensionally stable anode-chlor alkali (DSA-Cl 2 ) used as the anode. The interelectrode gap was 8.0 mm, and both electrodes occupied a geometric area of 20.0 cm 2 . The electrolyte solution, 0.1 mol L '1 Na 2 SO 4 , pH 2.5, was fed to the electrochemical cell from the reservoir tank (with a capacity of 1.0 L) through a peristaltic pump operating at a flow rate of 50.0 L h '1 (under an electrolyte flow of 50 L h '1 , the flow regime in the reactor is laminar, with a Reynolds number of '600 and an internal rate (v 0 ) of '0.190 m s '1 ). 18 A thermostatic bath coupled to the reservoir tank was used to control the temperature.

3.'Results and Discussion

3.1. Effect of Temperature

One of the most efficient electrochemical experimental setups employed in the production of H2O2 via ORR through the 2-electron pathway involves the coupling of electrodes in a flow-through reactor operating at high pressure.1,6,19 The production of hydrogen peroxide is limited by the availability of oxygen on the cathode surface, and the solubility of this gas can be increased significantly when the system is operated under high pressure and at low temperature, as has been previously demonstrated.1 In addition, the low temperature applied in the process can help decrease the rate of H2O2 decomposition. By performing the electrolysis under these optimal conditions using a flow-by reactor in a discontinuously operated bench-scale plant, it was possible to obtain a maximum accumulated H2O2 concentration of approximately 400 mg L'1 at 0.9 Ah L'1 with a temperature of 11.5 °C and a pressure of 2 bar and a specific production of H2O2 of about 110 g kWh'1.1 The production of higher H2O2 concentration was not feasible in this discontinuous process because, under these conditions, there is an equilibrium between the rates of H2O2 production and decomposition, and from this point onwards, the process becomes unproductive. Thus, the only way to obtain higher H2O2 production efficiency is to change the operation mode from discontinuous to continuous mode,1 where the hydrogen peroxide removed is protected against self-decomposition.

It should be noted that the physical mechanisms associated with the reduction of oxygen in GDE are very different from those that occur in electrodes made up of flow-through cells; furthermore, it is interesting to evaluate whether the decrease in temperature also exerts an influence over the physical mechanisms related to oxygen reduction in GDE. Thus, to evaluate the effects of temperature and O2 solubility in flow-by reactors using GDE, electrolysis experiments were carried out at 5, 15, and 25 °C, with a maximum O2 solubility of 14.8'11.2, 10.4, and 8.69 mg L'1, respectively. As can be seen in A, unlike what is observed under the application of flow-through electrodes, the electrolyte temperature and O2 gas solubility do not play an influential role in the efficiency of GDE when applied in a flow-by electrochemical cell.

The electrolysis experiments carried out at 5, 15, and 25 °C, under atmospheric pressure and O2 flow at 50 mL min'1, yielded very similar concentrations of H2O2, with a mean value of 543 mg L'1 and a standard deviation of 5.6 mg L'1 at 1 Ah L'1. The average concentrations of H2O2 electrogenerated under the three temperature levels amounted a specific production of H2O2 of 93.5 g kWh'1'this is slightly lower than the values obtained for flat electrodes equipped in flow-through cells'the values reported for these electrodes ranged between 101 and 135 g kWh'1.1,19

Furthermore, as can be seen in eqs 2 and 3, the decomposition of H2O2 in the bulk solution or on the anode surface did not cause a decline in H2O2 concentration; this behavior was observed by Monteiro et. al in a flow-through cell.1 The main advantage of the flow-through electrochemical reactor is that the solution flow passes through the anode and cathode, which improves the oxidation or reduction rate, as well as the efficiency of the electrochemical process, because it improves the convection of material transfer in the electrode surface. According to Lu and Zhang,27 flow-through reactors have two advantages over the flow-by electrochemical reactors, which are (i) high mass transfer efficiency and (ii) electron-transfer efficiency. In the flow-by reactor, the mass transfer is highly limited by the fact that the electrodes are in parallel with the solution flow. Thus, the rate of decomposition of H2O2 is more pronounced in flow-through cells than in flow-by electrochemical cells; as such, the application of flow-through cells does not allow one to obtain higher electrogenerated H2O2 concentrations. It is worth emphasizing that the GDE works by increasing the efficiency of O2 mass transfer at the cathode, which improves this limitation of mass transfer in the flow-by electrochemical cell; however, the DSA-Cl2 anode, which is parallel to the GDE, is limited to the mass transfer

2

3

It is noteworthy that the ORR process involving the production of H2O2 mostly occurs in the triple phase of the GDE since the O2 that is solubilized in the electrolyte does not practically interact with the electrode surface, and as such, it does not take part in the process. This phenomenon, observed in flow-through cells and which is characterized by the occurrence of higher H2O2 production efficiency at low temperature, would occur if H2O2 production was higher at 5 °C, once the solubility of O2 is higher at this temperature than at other temperature levels.

3.2. Effect of O2 Flow Rate

As previously stated, the main limitation of the electrochemical production of H2O2 is the solubility of O2. High-pressured devices or accessories that promote the drag of bubbles such as the venturi mixer (improving the gas'liquid contact surface area) have been shown to enhance the efficiency of the electrochemical process by effectively supplying the O2 needed as a raw material.6,19 GDE is a suitable alternative device that helps to minimize the mass transfer constraint in the reactor, but the flow rate of O2 gas that passes through the cell may influence the performance of the electrochemical system in very different ways.18,22 In view of that, one needs to evaluate the O2 gas input so that there is no shortage or excess of the reagent, as this will impact the efficiency of H2O2 production. In the present study, the O2 gas injection flow into the cathode compartment was evaluated by varying the O2 reagent input from 0.5 to 15 cm min'1. The experimental conditions were kept at 15 °C. As can be seen in , one can clearly observe that an increase in the gas flow resulted in an increase in the maximum concentration of H2O2 accumulated in the electrochemical device up to 1,159 ± 13.6 mg L'1 at 2.5 cm min'1. With regard to the O2 flow between 0.5 and 1.25 cm min'1, the amount of O2 recorded was lower; in other words, the oxygen did not interact with all of the ORR active sites available on the GDE and because of that the efficiency of H2O2 production and the accumulated concentration of H2O2 produced were found to be lower in the discontinuous process (536.4 and 863.1 mg L'1 for 0.5 and 1.25 cm min'1, respectively).

For more information, please visit formaldehyde plant.

Looking at the images in the inset of , one will observe the formation of large air pockets at conditions above 5 cm min'1; this is attributed to the excess gas that entered the cathode compartment, which increased in diameter as the injected O2 flow increased. At 15 cm min'1, the air pockets covered a large part of the electrode surface, reducing the three-phase contact area of the GDE; this resulted in a decrease in the amount of H2O2 accumulated by 510 mg L'1. This negative effect represented a 2.7-fold decrease in H2O2 production efficiency.

The nondependence of the GDE on O2 solubility and on its 3D-multichannel structure allows the electrode to provide an unlimited supply of oxygen at the electrode/electrolyte interface. The O2 that passes through the GDE can directly interact with the ORR active sites present in the Printex L6 carbon, and this promotes the generation of high concentrations of H2O2 in the system. In addition, the versatility of operation and easy installation of the GDE allow it to be operated in bench cells and in full-scale electrochemical reactors with volumes ranging from milliliters to hundreds of liters.

A point worth mentioning is that the O2 gas humidity does not play an important role in the ORR when using the GDE as a cathode. The catalytic layer of the GDE is almost totally hydrated due to the partial permeation of the electrolyte; in this way, when the O2 gas flow is passing through the channels of the GDE, the gas tends to increase its degree of humidity.28 Thus, regardless of the gas being prehumidified (relative humidity'RH of 50%) or dry (RH < 10%), a self-humidification occurs as the gas passes through multichannels of the GDE, as well as a decrease in proton resistance decreases due to water enrichment along the channel.28 Also, it is worth emphasizing that the oxygen reduction reaction must occur in an aqueous environment so that O2 is reduced to H2O2. Thus, it is essential that the catalytic layer of GDE is partially wet, and this parameter can be controlled by the PTFE content.

Xia et al. used gas diffusion electrodes based on carbon nanotubes with 40% PTFE loading to produce H2O2 in an undivided electrochemical cell with a volume of 0.16 L.29 The authors evaluated the effect of the O2 flow injected directly into the GDE'this was one of the parameters investigated in their study. Based on their results, the increase in O2 flow promoted an increase in O2 mass transfer within the GDE, and this in turn led to an increase in H2O2 production. However, when the O2 flow rate reached 280 mL min'1, there was a decline in the accumulated H2O2 concentration (compared to the O2 flow rate of 210 mL min'1).29 The excess of O2 flow led to the formation of bubbles that covered the surface of the electrode, and this led to a decrease in the production of H2O2. A similar outcome was noted in our present study. Xia et al. obtained the best H2O2 production efficiency at a flow rate of 210 mL min'1, where the accumulated H2O2 concentration was mg L'1 (with a current efficiency of 88.5% in 60 min of electrolysis).29 Remarkably, under the operating conditions employed by Xia et al.,29 the amount of O2 consumed was twice as high as the amount of O2 consumed in the present work even though our proposed system operated for an additional 1 h ( ± 13.6 mg L'1 in 120 min).

Another study that deserves being mentioned is that of Lima et al. where the authors employed a Printex L6 carbon-based GDE (similar to the electrocatalyst employed in our present study) to evaluate H2O2 production in an electrochemical cell. For comparison purposes, Lima et al. employed different amounts of carbon (8 g) and PTFE loading (40%) in their analysis (in the present study, 0.67 g carbon loading and 20% PTFE loading were employed).24 With the GDE exhibiting a relatively larger thickness due to the higher amount of carbon in its composition, the authors had to apply a pressure of 0.2 bar of O2 gas in the cathode compartment for the electrode to work in the best condition.24 Under these conditions, Lima et al. reported having obtained an accumulated H2O2 concentration of '750 mg L'1 after 120 min of electrolysis in an electrochemical cell. Interestingly, despite consuming a higher amount of reagent (O2), the amount of H2O2 concentration obtained in their study24 corresponds to only 65% of H2O2 concentration obtained from the application of the Printex L6-based GDE proposed in our present study. This shows that high amounts of carbon or high PTFE loadings are not required in the composition of the GDE since the ORR process, involving H2O2 production, occurs slightly below the electrode surface (on the catalytic layer of GDE), and thus the use of thinner electrodes can lead to satisfactory results.

3.3. Effect of PTFE (%) Loading and Current Density

The percentage content of PTFE employed in the GDE exerts an influential role on the hydrophobicity of the electrode; in other words, increasing the PTFE content in the GDE composition makes the electrode more hydrophobic and this inhibits the permeability of the aqueous solution through the electrode. With regard to the electrode proposed in the present study, the application of more than 40% PTFE in the GDE was found to render the device excessively hydrophobic, and this made the system behave like a conventional/flat electrode. On the other hand, the application of lower contents of PTFE resulted in higher permeability of the solution in the GDE (higher degree of wettability). With low contents of PTFE, the preliminary assays indicated that at values below 20%, the solution completely permeates the electrode according to the use of the GDE; this fact was observed by an electrolyte soaking in the cathode compartment when a GDE containing 10% PTFE was used. Thus, it was not possible to generate H2O2 electrosynthesis data for GDE containing values below 20%.

It should be noted, however, that there is a minimum PTFE loading value that will prevent the solution from soaking through the electrode. Thus, it is essentially important to find an ideal PTFE loading that helps to prevent high hydrophobicity or high wettability of the electrode. The results obtained from the thorough analysis conducted in the present study helped obtain some useful insights in this regard. Based on our findings, the ideal PTFE loading should be between 20 and 40%; this is because below 20% PTFE loading, flooding occurs on the electrode, while there is high resistance to solution permeability when one applies a PTFE loading above 40% (see ).

A thorough investigation was carried out to evaluate H2O2 generation at different current densities using the GDE with PTFE loadings of 20 and 40% (see for the results obtained). One will observe that the accumulated H2O2 concentrations (obtained in 120 min of electrolysis) for the 20% PTFE'GDE were between 1.1- and 1.4-fold higher than those obtained for the 40% PTFE'GDE at all of the current densities evaluated. The 20% PTFE'GDE contains the equivalent of 80% of Printex L6 carbon by mass; this is 20% more than the amount of PL6C in the 40% PTFE'GDE.

The difference in the carbon content between the two electrodes (20% PTFE'GDE and 40% PTFE'GDE) resulted in an increase of almost 46% in current efficiency for the production of H2O2 at a current density of 100 mA cm'2; this effect can be attributed to a higher amount of ORR active sites available for O2 adsorption on the carbon surface. It is also worth emphasizing that lower contents of PTFE contain a higher amount of carbonaceous matrix (since the catalytic mass of GDE is composed of Printex L6 carbon and PTFE). The carbon matrix is responsible for promoting the electrochemical production of H2O2, and thus, the greater the content of carbonaceous material, the greater the amounts of ORR active site present in the GDE.

In previous works reported in the literature,4 it shows that Printex L6 carbon contains only carboxyl-type oxygenated functional groups (COOH) in its chemical composition (18.6% content'data referring to an analysis by X-ray photoelectron spectroscopy, XPS). As reported in the literature,2,4 the carboxyl group is the oxygenated functional group that has the greatest influence on H2O2 electrosynthesis, followed by the carbonyl functional group (C=O). The oxygenated functional group on the surface of the carbonaceous material is responsible for the displacement of electrons from its adjacent carbon, making it an excellent active site for the adsorption of the O2 molecule, and for tending to the formation of the OOH* intermediate, which is the only intermediate that favors the formation of H2O2 (the * symbolizes that the species is adsorbed at the active site).

Looking at , one will observe that an increase in the current density resulted in an increase in the accumulated H2O2 concentrations obtained for both GDEs, but there was no ideal current density to work with. The accumulated H2O2 concentrations for 20% PTFE'GDE were .3 ± 19.1, .7 ± 40.6, and .0 ± 47.3 mg L'1 at 75, 150, and 200 mA cm'2; for 40% PTFE'GDE were .3 ± 14.3, .1 ± 23.6, and .9 ± 38.5 mg L'1 at 75, 150, and 200 mA cm'2, respectively.

Thus, for a better understanding of the electrochemical production of H2O2, one needs to observe , which shows the relationship between the concentrations of H2O2 generated as a function of time for both GDEs. It can be noted that the application of current densities higher than 150 mA cm'2 led to a decrease in H2O2 concentration after 90 min of electrolysis due to the decomposition of H2O2 within the solution and on the anode surface (as discussed in eqs 2 and 3). The consumption of H2O2 by these parallel reactions causes a decline in the current efficiency; at very high current densities, the current efficiency may also decrease by favoring the ORR via 4-electron transfer on the cathode surface. The 20% PTFE'GDE exhibited a maximum current efficiency of 85.3% at a current density of 50 mA cm'2. From this value onwards (85.3%), the current efficiency only decreased until it reached 73% at 200 mA cm'2.

The same behavior (decline in current efficiency) was observed for the 40% PTFE'GDE; however, this electrode recorded a maximum current efficiency of 75.9% at a current density of 25 mA cm'2. Thus, one can conclude that for an operation aimed at obtaining a higher current efficiency, one needs to employ low current densities. However, when the aim is to obtain high H2O2 concentration in a short period of time, one will need to employ a high current density.

As can be seen in the first 60 min of the graph in , the 20% PTFE'GDE recorded kinetic constant values that were 24, 47, 51, and 34% higher than those of the 40% PTFE'GDE at current densities of 50, 100, 150, and 200 mA cm'2, respectively. Both electrodes exhibited an apparent pseudo-order kinetic constant of zero (i.e., the generation of H2O2 is independent of the concentration of O2 and H+). The 20% PTFE'GDE showed H2O2 production rate values of 10.7, 15.5, 19.9, and 28.5 mg L'1 min'1 at current densities of 50, 100, 150, and 200 mA cm'2, respectively. Valim et al.30 reported a H2O2 production rate of 5.9 mg L'1 min'1 when operated at '1.0 V vs Ag/AgCl, whereas Carneiro et al.15 reported a slightly higher rate of 7.6 mg L'1 min'1 at the same conditions. In both works, the GDE was modified with metallic oxides, whose modification is to improve the selectivity and catalytic activity of the carbonaceous material. Moreira et al.31 reported surprising values of 38 mg L'1 min'1 when operated at 100 mA cm'2 using a Sudan-Red-modified Printex L6 carbon-based GDE.

Finally, an interesting element to consider in our analysis of the efficiency of the electrochemical process is the cell potential (Ecell) and the difference of potential between the cathode and pseudo-reference electrode Pt//Ag/AgCl (Ecat-ref) values; this is because these potentials may indicate changes in the reactor setup or even in the electrode fabrication method. With that in mind, an analysis was conducted to evaluate whether the amount of carbon in the composition of the GDE can affect the cathode potential and cell potential values since it affects the conductivity of the electrode. Interestingly, both the 20% PTFE'GDE and 40% PTFE'GDE recorded very similar Ecat-ref and Ecell values, as seen in A; this shows that both electrodes exhibit similar electrochemical behavior, despite the difference in carbon content.

As expected, an increase in the current density resulted in an increase in the potential values; also, higher Ecell values were recorded at higher current densities. This outcome suggests that, under these working conditions, the anode DSA-Cl2 plays an essential key role in the process, especially in the parallel reactions that lead to the decomposition of H2O2, as can be observed in eq 3 or in the formation of predator species for H2O221 such as ozone, which plays a role in the decomposition of H2O2 once it is formed and disappears immediately afterward. It is worth noting that the use of more active anodes, such as BDD anode, can increase the intensity of parallel reactions and negatively influence the production of H2O2.

Based on the cell potential values, one can estimate the specific production of H2O2 in g kWh'1; this is an important and more realistic parameter that can help evaluate the applicability of the electrodes in real systems. The term specific production of H2O2 represents how much hydrogen peroxide is produced per power generated per time, which can be expressed in grams per kW per hour (g kWh'1). The specific production of H2O2 was calculated using eq 4, where CH2O2 is the concentration of H2O2 (in mg L'1), V is the volume (in L), E is the cell potential (in V), I is the current (in A), and t is the time (in h)

4

B shows the results obtained from the analysis of the specific production of H2O2 as a function of current density. One will observe that an increase in current density (j), which consequently results in an increase in the cell potential value, causes a decrease in the specific production of H2O2; in other words, smaller grams of H2O2 are produced per kWh at high current densities (the application of j > 100 mA cm'2 results in the specific production of H2O2 values less than 37 g kWh'1). On the other hand, at lower current density; i.e., 25 mA cm'2, an extremely high specific production of H2O2 values were recorded'the 20% PTFE'GDE and 40% PTFE'GDE recorded the specific production of H2O2 of 131.5 and 122.6 g kWh'1, respectively.

A comparison of the specific production of H2O2 values obtained for the proposed 20% PTFE'GDE and 40% PTFE'GDE with the values obtained for other gas diffusion electrodes reported in the literature pointed to the superior performance of the electrodes proposed in the present study. For illustration purposes, Lima et al.24 employed 40% PTFE'GDE at a current density of 50 mA cm'2, where they obtained a maximum H2O2 concentration of 750 mg L'1, with a specific production of H2O2 of '23.5 g kWh'1; this is roughly 2.9- and 4-fold smaller compared to the values obtained for the 40% PTFE'GDE and 20% PTFE'GDE proposed in our present work. Moreira et al.31 also employed 20% PTFE'GDE, where they obtained the specific production of H2O2 of '9.5 g kWh'1 at 100 mA cm'2; this is roughly 5 times lower than the value obtained in this work. Barros et al.16 employed an unmodified GDE for the electrochemical generation of H2O2 in a potentiostatic mode at '1.1 V (the cell current value should be approximately 150 mA cm'2), where they obtained a specific production of H2O2 of approximately 28 g kWh'1 and H2O2 accumulated a concentration of 6,400 mg L'1; a comparison of the conditions applied in their work with the GDE developed in our present work showed that our proposed GDE exhibited a slightly higher H2O2 production efficiency of 30 g kWh'1.

The efficiency in the production of H2O2 of the GDE developed in this work is because it is composed of a diffusion layer based on a carbon cloth and a catalytic layer based on Printex L6 carbon and PTFE. Some GDE reported in the literature (e.g., the work by Lima et al.,24 Moreira et al.,31 and Barros et al.16) employ a single layer that acts as both a diffusion and catalytic layer. In those case, up to the point at which the solution permeates the GDE, it is called the catalytic layer, while after this point, where the solution does not permeate, it is called the diffusion layer. The use of carbon cloth as the diffusion layer facilitates the diffusion of O2 gas to the catalytic layer, and therefore, it was possible to achieve a higher production of H2O2.

Flow-by electrochemical cells are characterized by higher cell voltage compared to pressurized nondivided microfluidic electrochemical cells. The main advantage of the pressurized nondivided microfluidic cells lies in the short separation distance between the cathode and the anode; it is this short distance between the cathode and the anode that allows these cells to have lower cell voltage and ohmic resistance compared to flow-by cells. It is worth noting that the lower the cell voltage, the less amount of specific production of H2O2. In previous studies conducted by Moratalla19 and Monteiro,1 the authors obtained the specific production of H2O2 values that ranged between 101 and 135 g kWh'1 at a current density of 5 mA cm'2; the values they obtained are slightly higher than those obtained in our present study at 25 mA cm'2. It should be noted, however, that the aforementioned studies [1, 20] employed different electrode technologies in the electrochemical process, which was more dependent on temperature and pressure.

3.4. Durability Test for GDE

The electrochemical resistance of the 20% PTFE'GDE and 40% PTFE'GDE was evaluated by applying a current density of 200 mA cm'2 and the operation time needed for the Ecell to increase exponentially. As can be seen in A, the electrode containing 20% of PTFE loading maintained the Ecell constant for 36 Ah (this corresponds to 7.5 days), after which the voltage increased significantly. The electrode with 40% PTFE loading exhibited a longer lifetime, reaching an uninterrupted life span of 48 Ah (10 days). After the aforementioned lifetime, both GDEs exhibited a more resistive current profile, as can be seen in the cyclic voltammograms obtained before and after the durability test (see B); this behavior can be attributed to the fact that the electrodes lost a significant part of the catalytic film (the catalytic mass containing Printex L6 carbon and PTFE) deposited under the carbon cloth. In addition, the GDEs were found to have been soaked by the electrolyte.

shows the SEM images related to the removal of the catalytic film from the carbon cloth in the 20% PTFE'GDE. Before the durability test, the catalytic mass was deposited homogeneously and uniformly over the carbon cloth substrate ( , before). During the durability test, the catalytic film (Printex L6 carbon + PTFE) started to exhibit some cracks (like cracked soil). An increase was observed in the thickness of the cracks over time until parts of the catalytic film were removed from the carbon cloth substrate ( , after).

Are you interested in learning more about MEG Plant? Contact us today to secure an expert consultation!