A DEVICE AND METHOD FOR THE PRODUCTION OF HYDROGEN PEROXIDE

Abstract

A method produces hydrogen peroxide in an aqueous solution by electrochemical reduction of oxygen. An oxygen containing gas is supplied to an electrochemically active side of a cathode contained in a cathodic compartment. The cathode contains a porous gas diffusion electrode, one side of which contains a carbon based electrochemically active layer capable of catalyzing the reduction of oxygen to hydrogen peroxide. The cathodic compartment is in fluid communication with an anodic compartment. At least one at least partly water soluble, weak protonic electrolyte is supplied to a catholyte. The weak protonic electrolyte has a pKa which is at least one unit higher than the pH of the catholyte at the onset of the oxygen reduction reaction to hydrogen peroxide. The catholyte is not pH buffered and the pH of the catholyte is let to evolve in course of the reaction.

Claims

1-18. (canceled)

19. A method for producing hydrogen peroxide in an aqueous solution by electrochemical reduction of oxygen, wherein an oxygen containing gas is supplied to an electrochemically active side of a cathode contained in a cathodic compartment, wherein the cathode comprises a porous gas diffusion electrode one side of which comprises a carbon based electrochemically active layer capable of catalyzing the reduction of oxygen to hydrogen peroxide, the cathodic compartment being in fluid communication with an anodic compartment, characterized in that at least one at least partly water soluble, weak protonic electrolyte is supplied to a catholyte, wherein the weak protonic electrolyte has a pKa which is at least one unit higher than pH of the catholyte at an onset of the oxygen reduction reaction to hydrogen peroxide, wherein the catholyte is not pH buffered and the pH of the catholyte is let to evolve in course of the reaction.

20. The method as claimed in claim 19, wherein the pKa of the weak protonic electrolyte is at least 1.25 units higher than the pH of the catholyte at the onset of the oxygen reduction reaction.

21. The method as claimed in claim 19, wherein the weak protonic electrolyte is a weak protonic acid having a pKa in a range of 2.0≦pKa≦8.0 or a weak protonic base having a pKa between 6.0≦pKa≦12.0.

22. The method as claimed in claim 19, wherein the pH of the catholyte at the onset of the oxygen reduction reaction is at least 2.5 and the weak protonic electrolyte has a pKa of at least 3.5.

23. The method as claimed in claim 19, wherein the weak protonic acid is selected from the group consisting of weak organic and weak inorganic acids including acetic acid, citric acid, oxalic acid, lactic acid, gluconic acid, ascorbic acid, formic acid, glycolic acid, potassium monohydrogen phosphate, potassium dihydrogen phosphate, ammonium chloride, boric acid, sodium hydrogen sulphate, sodium hydrogen carbonate, ammonium chloride, and mixtures of two or more thereof.

24. The method as claimed in claim 21, wherein the weak protonic base has a pKa between 7.0 and 11.0.

25. The method as claimed in claim 24, wherein the weak protonic base is selected from the group consisting of ammonia, trimethylammonia, ammoniumhydroxide, pyridine, conjugated bases of acetic acid, citric acid, oxalic acid, lactic acid, gluconic acid, ascorbic acid, formic acid, glycolic acid, potassium monohydrogen phosphate, potassium dihydrogen phosphate, ammonium chloride, boric acid, sodium hydrogen sulphate, sodium hydrogen carbonate, or a mixture of two or more thereof.

26. The method as claimed in claim 19, wherein the oxygen containing gas is selected from the group consisting of air, pure oxygen, a mixture of oxygen with one or more inert gases including N.sub.2, Ar, He, or a mixture of two or more thereof.

27. The method as claimed in claim 19, wherein the electrochemically active layer comprises electrically conductive carbonaceous particles with a catalytically active surface comprising protonic acidic functional groups.

28. The method as claimed in claim 19, wherein the method is operated in a continuous process, and the weak protonic electrolyte is supplied with a constant flow rate or a variable flow rate.

29. The method as claimed in claim 19, wherein a convective mass transfer is created in the catholyte and/or an anolyte.

30. A device for producing hydrogen peroxide in an aqueous solution by electrochemical reduction of oxygen in a cathodic compartment of the device, the cathodic compartment comprising at least one cathode and an aqueous catholyte fluid, the device further comprising at least one anodic compartment in fluid communication with the cathodic compartment, wherein the cathode comprises a porous gas diffusion electrode one side of which comprises a carbon based electrochemically active layer capable of catalyzing the reduction of oxygen to hydrogen peroxide, wherein an oxygen containing gas is supplied to an electrochemically active side of the cathode, characterized in that the aqueous catholyte fluid comprises a co-catalyst for the reduction of oxygen to hydrogen peroxide, wherein the co-catalyst comprises at least one weak protonic electrolyte in a partially dissociated state, wherein the weak protonic electrolyte has a pKa which is at least one unit higher than pH of the catholyte at an onset of the oxygen reduction reaction to hydrogen peroxide.

31. The device as claimed in claim 30, wherein the pKa of the weak protonic electrolyte is at least 1.25 units higher than the pH of the catholyte at the onset of the oxygen reduction reaction.

32. The device as claimed in claim 30, wherein the weak protonic electrolyte is a weak protonic acid having a pKa in a range of 2.0≦pKa≦8.0 or a weak protonic base having a pKa between 6.0≦pKa≦12.0.

33. The device as claimed in claim 30, wherein the electrochemically active layer comprises electrically conductive carbonaceous particles with a catalytically active surface comprising protonic acidic functional groups.

34. The device as claimed claim 30, wherein a side of the electrochemically active layer of the gas diffusion electrode facing a gas phase is coated with a layer of a hydrophobic material which is permeable to oxygen, wherein the hydrophobic material is selected from the group consisting of polyvinyldifluoride (PVDF), polytetrafluoroethylene (PTFE or Teflon), and PSU.

35. The device as claimed in claim 30, wherein the at least one anodic compartment and the at least one cathodic compartment are separated from each other by an ion permeable membrane including a cation permeable membrane.

36. The device as claimed in claim 30, wherein a convective mass transfer is created in the catholyte and/or an anolyte in the device.

37. The method as claimed in claim 19, wherein the pKa of the weak protonic electrolyte is at least 1.50 units higher than the pH of the catholyte at the onset of the oxygen reduction reaction.

38. The method as claimed in claim 19, wherein the pKa of the weak protonic electrolyte is at least 2.0 units higher than the pH of the catholyte at the onset of the oxygen reduction reaction.

39. The device as claimed in claim 30, wherein the pKa of the weak protonic electrolyte is at least 1.50 units higher than the pH of the catholyte at the onset of the oxygen reduction reaction.

40. The device as claimed in claim 30, wherein the pKa of the weak protonic electrolyte is at least 2.0 units higher than the pH of the catholyte at the onset of the oxygen reduction reaction.

Description

[0056] The present invention is further illustrated in the figures and description of the figures below.

[0057] FIG. 1 is a schematic view of the device of the present invention.

[0058] FIG. 2 shows the steady state current density as a function of time for different oxygen-reducing gas-diffusion cathodes, with and without CeO.sub.2 catalyst, after 120 min of electrocatalytic production of hydrogen peroxide. N: Norit, V: Vulcan-Norit, AB: Acetylene Black-Norit. All electrodes were composed of a combination of 80% carbon mixture and 20% polymer (PVDF).

[0059] FIG. 3 shows: Experiments with a cathode based on activated carbon particles in the active layer, using different concentrations of phosphate as electrolyte:

[0060] FIG. 3a) Concentration of H.sub.2O.sub.2, obtained as per CE.

[0061] FIG. 3b) Concentration obtained based on the analytical method.

[0062] FIG. 3c) pH of Anolyte and Catholyte.

[0063] FIG. 4 shows the analytical concentrations of the H.sub.2O.sub.2 obtained in the experiments of Example 2.

[0064] FIG. 5 shows electrokinetic biasing curves plotted varying the potential of the cathode vs Ag/AgCl in the range −0.5 to 0.5 at a scan rate of 1 mV.Math.s.sup.−1, experiments with a cathode based on graphite particles as electrocatalytically active material, using different concentrations of phosphate as electrolyte.

[0065] FIG. 6 shows H.sub.2O.sub.2 production as a function of reaction time and concentration of phosphate added to the catholyte.

[0066] FIG. 7 shows the electrokinetic biasing curves plotted varying the potential of the cathode vs Ag/AgCl in the range −0.5 to 0.5 at a scan rate of 1 mV.s.sup.−1, experiments with a cathode based on graphite particles as electrocatalytically active material, using different concentrations of citrate as electrolyte.

[0067] FIG. 8 shows H.sub.2O.sub.2 production as a function of reaction time and concentration of phosphate added to the catholyte.

[0068] FIG. 9 shows experiments with a cathode based on graphite particles as electrocatalytically active material, using different concentrations of weak acid as electrolyte. a) moles produced after 4 h experiment, using hydrogen phosphate as electrolyte; b) moles produced after 4 h experiment, using citrate as electrolyte.

[0069] FIG. 10 shows the evolution of the pH of the catholyte with time, for varying concentrations of citric acid as weak protonic electrolyte.

[0070] FIG. 11 shows the evolution of the pH of the catholyte with time, for varying concentrations of HKH.sub.2PO.sub.4 as weak protonic electrolyte.

[0071] The device of the present invention for the production of hydrogen peroxide in an aqueous solution or aqueous electrolyte by reduction of oxygen shown in FIG. 1, comprises at least one anodic compartment 5 and at least one cathode compartment 15. If so desired a plurality of anodic and cathode compartments may be present as well. If a plurality of anode and cathode compartments is provided, they are preferably arranged in a unipolar arrangement, with a plurality of alternating positive and negative electrodes forming a stack separated by ion permeable membranes. In a unipolar design, electrochemical cells forming the stack are externally connected, the cathodes are electrically connected in parallel as well as the anodes.

[0072] The anode or anodes 1 are immersed in an anode compartment comprising an aqueous anolyte fluid 2. The cathode or cathodes 10 are immersed in a cathode compartment comprising an aqueous catholyte fluid 12. The anodic compartment and cathodic compartment are in fluid communication to allow transport of cations, in particular transport of protons from the anodic compartment to the catholyte compartment, and transport of anions from the cathodic compartment to the anodic compartment. As anolyte fluid, any anolyte considered suitable by the skilled person may be used. In particular any aqueous electrolyte, conventionally used in electrochemical reduction reactions may be used. The anolyte may for example comprise an aqueous solution of an electrolyte selected from the group of sulphates, phosphates, chlorides and mixtures of two or more of these compounds. The anolyte chamber may comprise a supply member for feeding anolyte fluid. The catholyte chamber may comprise a supply member for feeding catholyte fluid. The catholyte may be different from the anolyte, but anolyte and catholyte may also be the same. Suitable catholyte materials include an aqueous solution of an electrolyte selected from the group of sulphates, phosphates, chlorides and mixtures of two or more of these compounds.

[0073] The device of the present invention may be operated in batch or continuous mode. When operated in continuous mode, electrolyte may continuously be recirculated or fresh electrolyte may be supplied, in particular to the cathode compartment. When operated in batch mode, electrolyte containing the reaction product may be withdrawn in particular from the cathode compartment and replenished by a corresponding batch of fresh electrolyte.

[0074] The anode and cathode compartment 5, 15 may be made of any material considered suitable by the skilled person, but are preferably made of a polymeric material. Suitable materials include polyvinylidene difluoride (PVDF), polytetrafluorethylene (PTFE), ethylene tetrafluoroethylene (EFTE), polyvinylchloride (PVC), chlorinated polyvinyl chloride (CPVC), polyacrylate, polymethylmethacrylate (PMMA), polypropylene (PP), high density polytethylene, polycarbonate and blends or composites of two or more of these compounds.

[0075] The anode and cathode compartment 5, 15 are separated from each other by an ion permeable membrane 11 to control exchange of cations and anions between both compartments, as described above. To improve structural integrity, the ion-permeable membrane 11 separating the anode and cathode compartment 5, 15 may be reinforced with a rigid support, for example a rigid support made of a sheet, a fleece, which may be woven or non-woven or otherwise made of a porous polymer or a web or a mesh of metal fibres or metal fibres arranged in a woven or non-woven structure.

[0076] The cathode 10 used in the device of this invention is preferably a gas diffusion electrode, to ensure a sufficiently high mass transfer of oxygen to the electrochemically active layer present at the cathode, and a sufficiently high reaction yield, taking into account the limited solubility of oxygen in water. The gas diffusion electrode is preferably a multilayered electrode comprising a current density distributor 3 for supplying electric current to an electrochemically active layer 4 deposited on top of the current distributor. The electrochemically active layer 4 is chosen such that it is active in the catalysis of the reduction of oxygen to hydrogen peroxide.

[0077] The electrochemically active material 4 is preferably a material which has a higher electric conductivity than the current density distributor. This permits the electrochemically active material to take away or bring the electron from and to the current density distributor. With “electrochemically active layer” is meant a layer of a material in which the electrochemical reduction of oxygen to hydrogen peroxide takes place, in particular a layer of a material having high electrical conductivity, which is porous to gas, in particular oxygen, and electrolyte. Materials suitable for use as electrochemically active layer have been described above, and comprise electrically conductive carbon particles comprising surface functional group with acidic protons, having a high specific surface area as measured by the BET method described in ASTM D5665, in particular carbon particles selected from the group of graphite, carbon nanotubes, graphene, carbon black, activated carbon or synthetic carbons.

[0078] The gas diffusion electrode that is used as the cathode 10 in the device of this invention preferably comprises a current density distributor 3 or a current distributor, which may be made of any material and form considered suitable by the skilled person. Preferably however, use is made of a mesh type current density distributor, having a porous mesh received in a circumferential electrically conductive frame or an array of several porous meshes. The current density distributor is connected to a source of electric energy along a current feeder, for supplying electrical energy to the current density distributor. The mesh comprises a plurality of electrically conductive paths. Within the scope of the present invention with “mesh” is meant a woven, knitted, braided, welded, expanded mesh, bars or threads of electrically conductive fibers, having holes between the bars, fibers or threads to provide porosity, or a plate, sheet, foil, film made of an electrically conductive material having a plurality of perforations or holes to provide porosity. The wording “mesh” is meant to include a square meshes with a substantially rectangular shape and orientation of the conductive wires and insulating threads, but the mesh may also be tubular, or a coil film, or a otherwise shaped three-dimensional materials. Still other types of meshes suitable for use with this invention include perforated sheets, plates or foils made of a non-conductive material, having a plurality of wires or threads of a conductive material interlaced in the direction parallel to the current flow. A further type of mesh suitable for use with the present invention includes lines/wires of a conductive material, which extend parallel to the current flow direction, printed on a perforated sheet, foil or plate.

[0079] One side of the current density distributor 3 is coated with an electrochemically active layer 4 capable of catalyzing the reduction of oxygen to hydrogen. The layer of electrochemically active material 4, i.e. the layer which is catalytically active in the reduction of oxygen to hydrogen peroxide as described above, is preferably applied to the side of the current density distributor facing the gas phase. The electrochemically active layer usually has an interface with electrolyte on one surface (i.e. the side facing the current distributor) and a water repellant (hydrophobic gas diffusion) layer 13 on the other.

[0080] The electrochemically active layer 4 may be coated on the side facing the gas phase 13, with a water repellant layer 13 or a hydrophobic gas diffusion layer to minimize the risk of water leaking through the electrode into the gas phase. This water repellant layer 13 may also be deposited on top of the electrochemically active layer 4. Suitable materials for use as the water repellant layer include polyvinyldifluoride (PVDF), polytetrafluoroethylene (PTFE or Teflon), PSU, but other materials considered suitable by the skilled person may be used as well.

[0081] The device preferably comprises a supply member for supplying an oxygen containing gas to the side of the cathode comprising the electrochemically active layer. Any oxygen containing gas considered suitable by the skilled person may suitably be used. Examples include pure oxygen, a mixture of oxygen with one or more inert gases for example N.sub.2, Ar, He or a mixture thereof, air, etc. Preferably, in the device of this invention the gas supply rate may be adapted, to permit controlling the hydrogen peroxide production rate and yield.

[0082] The cathodic compartment comprises on a side 6 opposite the side of the cathode comprising the electrochemically active layer 4, an inlet for supplying at least one weak protonic electrolyte, preferably an aqueous electrolyte. Preferably the flow rate with which the weak protonic electrolyte is variable and may be adapted taking into account the oxygen conversion rate to hydrogen peroxide.

[0083] The anode 1 used in the device of this invention may be a conventional electrode, or may be a gas diffusion electrode similar to the cathode.

[0084] The device may further comprises means for creating convective mass transfer in the catholyte. This may improve H.sub.2O.sub.2 reclamation from the electrochemically active layer, by increasing convective mass transfer at the catholyte site. This may also improve replenishment of H.sup.+ at the position of the acidic active sites present on the surface of the electrochemically active layer. Means for creating convective mass transfer may also be provided at the anolyte site, to promote proton transfer towards and through the ion permeable membrane. The means for creating convective mass transfer may comprise those known to the skilled person, for example a stirrer, gas supply, a spacer material capable of creating turbulent flow conditions.

[0085] The device of this invention may also comprises means for optimizing the oxygen residence time at the cathode. This may be achieved by the presence of means for creating convective mass transfer in the gas phase at the cathode.

[0086] The invention is further illustrated by the examples and comparative experiments described below.

[0087] Comparative Experiments.

[0088] Experiments were performed in a half cell electrochemical reactor (FIG. 1), which contained a cathode (working electrode), a reference electrode, a counter electrode, and an ionic separator between working and counter electrode. Ag/AgCl 3 M KCl (+200 mV vs SHE) was used as reference electrode (Koslow Scientific), whereas a Pt disk fixed by laser welding over a titanium (Ti) plate was used as a counter electrode. Cathode and counter electrode were separated by liquid electrolyte and the separating membrane, Zirfon® (AGFA), unless otherwise stated. The electrodes and membrane have a projected electrode surface area of 10 cm.sup.2.

[0089] Sodium chloride solution (0.07 M) was used as both anolyte and catholyte, adjusted at pH 2.7 with 37% HCl. Air was fed to the cathodic air-compartment, at a flow rate of 402 mL min.sup.−1 and an overpressure of 10 mbar. Electrolyte recirculated in batch mode through the cells, with a peristaltic pump (Watson-Marlow), at 20 rpm speed (equivalent to ˜100 mL min.sup.−1). The experiments were carried out at room temperature (18±2° C.).

[0090] A Bio-Logic VMP3 potentiostat/galvanostat and frequency response analyzer was used in order to perform the electrochemical measurements. EC-Lab v.10.23 software was used for data acquisition and analysis.

[0091] Chronoamperometric experiments were carried out at a different set of independent potentials (0.350, 0.250, 0.150, 0.050, −0.050, −0.150, −0.250, and −0.350, V vs Ag/AgCl, respectively) during a period of 120 minutes at each condition. After that time, steady state was achieved in all systems.

[0092] Afterwards, Electrochemical Impedance Spectroscopy (EIS) was recorded at a frequency range from 10 kHz to 10 mHz, with 6 points per logarithmic decade, using an amplitude of 10 mV. Validity of the data was verified by using the Kramers-Kronig transforms. Data consistency was assessed by visual inspection of successful regression to experimental data with several electrical analogues, composed of Voigt elements (Dominguez-Benetton et al., 2012). Subsequently, cyclic voltammetries (CV) were recorded in 3 cycles at 1, 10 and 100 mV s-1, respectively, in a potential range from −0.450 to 0.450 V vs Ag/AgCl.

[0093] A spectrophotometric method was used to determine the concentration of H.sub.2O.sub.2 in solution. Reagent A was prepared by mixing 33 g potassium iodide, 1.0 g sodium hydroxide, and 0.1 g ammonium molybdate tetrahydrate dissolved and diluted into 500 mL deionized water. This solution was kept in dark conditions to inhibit the oxidation of I—. If the solution becomes colored, it should be remade. Reagent B was prepared by 10.0 g of potassium hydrogen phthalate (KHP) dissolved in deionized water and diluted to 500 mL.

[0094] The standard calibration curve was prepared from known concentrations of H.sub.2O.sub.2. 60 μL of 30% H.sub.2O.sub.2 were mixed with 100 mL of deionized water, in a volumetric flask. The concentration of this standard is 200 mg L.sup.−1 H.sub.2O.sub.2. The series of dilutions were done by taking an appropriate amount of H.sub.2O.sub.2 solution for the desired concentrations, as shown in Table 1.

TABLE-US-00001 TABLE 1 Known concentrations of H.sub.2O.sub.2 for calibration curve. Final volume H.sub.2O.sub.2 concentration in mL of 200 mg L−1 with deionized standard samples (mg L.sup.−1) H.sub.2O.sub.2 standard water (mL) 0.0 0.0 100 0.5 0.25 100 1.0 0.50 100 2.0 1.0 100 3.0 1.5 100

[0095] Further analysis was done by pipetting 3.0 mL of “Reagent A”, 3.0 mL of “Reagent B”, and 3.0 mL of standard or sample into a beaker. The contents of the mixture was allowed to react for a minimum of 5 minutes, before reading the absorbance of the solution at 351 nm (reference needed). Besides the known concentrations, problem sample obtained after chronoamperometric experiments were scanned to validate the absorbance wavelength.

[0096] CeO.sub.2 Catalyst Preparation

[0097] CeO.sub.2 at 4% by weight on the carbon support material was prepared by using the PPM (Polymeric Precursor Method). Precursor solution was prepared with 40 g of citric acid (CA) and 320 g of ethylene glycol (EG) at a 50:400 wt % ratio, at 60° C. The catalyst was prepared by adding 0.8 g of metal into precursor solution, to satisfy 1:50:400 (Metal:CA:EG) ratio. 19.2 g of activated carbon were added into the resin. This mixture was homogenized in an ultrasonic bath for 60 min and thermally treated at 400° C. for 2 h, under N2 atmosphere. The PPM is used to produce the catalyst with a high surface area, to effectively mix different metal ions and to produce stable metal-chelate complexes to enhance H.sub.2O.sub.2 production (Assumpçao M H M T et al. 2012).

[0098] Preparation of Gas-Diffusion Electrodes.

[0099] Electrodes with and without catalyst were prepared according to the method described by (Alvarez-Gallego Y et al. 2012). The multilayered electrodes consisted of a current density distributor (metal gauze), with a layer of an electrocatalyst on a carbon support embedded in a porous polymer matrix applied on one side, and a hydrophobic gas-diffusion layer was present on the opposite side.

[0100] A cold-rolling method was utilized for the preparation of the gas diffusion cathodes, from carbonaceous powders and polymer binder. The carbonaceous powders and combinations thereof utilized to fabricate the different types of electrodes object of this study are described in Table 2. Stainless steel 316L gauze (wire diameter μ100 m, mesh 44) was used as the current density distributor. Polyvinilydenefluoride (PVDF) was chosen as polymer binder, for both the active layer and the hydrophobic gas-diffusion layer (HGDL). The hydrophobic particles in the hydrophobic backing were FEP 8000. A typical HGDL is composed of 50% by weight PVDF and 50% by weight of FEP 8000.

TABLE-US-00002 TABLE 2 Carbon material used for cathode Metallic Catalyst production (% wt) PVDF (% wt) CeCl2 (% wt) 1 Norit (80) 20 — 2 Norit (76) 20 4 3 Norit (50), Vulcan 20 — (30) 4 Norit (46), Vulcan 20 4 (30) 5 Norit (50) Carbon 20 — Acetylene Black (30) 6 Norit (46), Carbon 20 4 Acetylene Black (30)

[0101] The percentage of the carbon materials for the fabrication of electrodes was identified on the basis of performance during the cold calendaring of carbon materials. The maximum possible percentage of carbon to incorporate with Norit was found 30% by weight, due to manufacturing restrictions.

[0102] Current density (j, A m.sup.−2) was recorded as a function of time (up to 120 min) for the different oxygen-reducing gas-diffusion cathodes, in chronoamperometric (CA) experiments at different potentials. These results were obtained at 18±1° C., in batch mode with recirculation of electrolyte. As presented in FIG. 2, the activated carbon electrode (Norit), without metallic electrocatalyst incorporated, was found to be the most active for oxygen reduction in terms of current density. Specially, this activity was significantly enhanced at −350 mV vs Ag/AgCl, where 51.75 A m-2 were reached. This was in good agreement with the evolution of H.sub.2O.sub.2 concentrations measured for this material, as presented in FIGS. 3a and 3b. Both the highest current density and the highest concentration of H.sub.2O.sub.2 were found at −350 mV vs Ag/AgCl. Compared to the other electrode materials without metallic catalyst incorporated, at −350 mV vs Ag/AgCl (i.e. Vulcan-Norit (8.42 A m-2) and Vulcan-Acetylene Black (17.9 A m-2)), the electrode made with only Norit as carbon material showed improvement in current density on 6 and 2.9 times, respectively.

Example 1

[0103] Addition of 10 mM of each of the following weak acids: KH.sub.2PO.sub.4, K.sub.2HPO.sub.4, CH.sub.3COONa and a mixture thereof were added in independent electrolytes and experiments were run for 2 h at the polarization potential −350 mV vs Ag/AgCl, as described above. Concentration of hydrogen peroxide, pH and conductivity were measured. The highest concentration of 20.5 mg L−1 H.sub.2O.sub.2 was found at −350 mV vs Ag/AgCl with K.sub.2HPO.sub.4, whereas 19.4 mg L−1 were obtained from sodium acetate and KH.sub.2PO.sub.4. The lowest concentration was found with the mixture of all weak acids even though; the highest concentration of 423 mg L−1 was obtained in this case based on EC. Therefore, it is considered that in the latter case, the low efficiency is due to preferential cathodic deprotonation of the weak acids and not to the formation of H.sub.2O.sub.2. Despite of the concentration of H.sub.2O.sub.2, pH of the electrolyte was found 6.51 with K.sub.2HPO.sub.4, while without the weak acid it was 11.77. Based on the performance in pH and H.sub.2O.sub.2 production, K.sub.2HPO.sub.4 was selected for further optimization, by using 100 mM of K.sub.2HPO.sub.4.

[0104] The results are illustrated in FIG. 3a-3c.

Example 2

[0105] Example 1 was repeated, this time using K.sub.2HPO.sub.4 as a weak acid, in varying concentrations as illustrated in FIG. 4.

Example 3

[0106] Example 2 was repeated, this time using a cathode whose active layer consisted of 80% graphite and 20% polymer binder. KH.sub.2PO.sub.4 was selected as weak acid, in varying concentrations as illustrated in FIGS. 5 and 6 and in table 3 below. From table 3 it may be observed that there is a dependence between the concentration of KH.sub.2PO.sub.4 used and the power administrated to the system for the electrosynthseis reaction to take place, the power density required was up to 40% of the value without KH.sub.2PO.sub.4

[0107] In FIG. 5 it can be appreciated that, at a selected cathode potential the current density increases as phosphate concentration increases in the electrolyte (meaning an increase in reaction rate)

TABLE-US-00003 TABLE 3 Ewe/V vs. Ag/AgCl/KCl (3.5M) j/mA .Math. m−2 P/mWm−.sup.2 0 mM −0.315 −3.995 1.26 10 mM −0.260 −3.965 1.03 100 mM −0.189 −3.953 0.75 500 mM −0.232 −4.05 0.94 1000 mM −0.195 −3.939 0.77 1500 mM −0.250 −4.064 1.02

Example 4

[0108] Example 1 was repeated, this time using citric acid as a weak acid, in varying concentrations as illustrated in FIGS. 7 and 8 and in table 4 below. From table 4 it may be observed that the concentration of citric acid/citrate electrolyte did not affect substantially the power suministrated to the system for the electro synthesis reaction to take place. From FIG. 8 it can be appreciated that, in contrast with experiments using phosphate as electrolyte (example 3), with citrate H.sub.2O.sub.2 mole production did not vary substantially when citrate concentration increases from 100 mM to 500 mM, or even increased (making the process less favourable).

TABLE-US-00004 TABLE 4 Ewe/V vs. Ag/AgCl/KCl (3.5M) j/mA .Math. m−2 P/mWm−.sup.2  0 mM −0.315 −3.995 1.26  10 mM −0.310 −4.070 1.26 100 mM −0.346 −4.089 1.42 500 mM −0.312 −4.022 1.26

[0109] In FIG. 9 the results obtained in terms of moles of H.sub.2O.sub.2 produced after 4 h of experiments in example 3 are compared with those obtained in example 4, making evident the beneficial effect of using HKH.sub.2PO.sub.4 as weak protonic electrolyte.

[0110] FIG. 10 shows the evolution of the pH of the catholyte with time, for varying concentrations of citric acid as weak protonic electrolyte.

[0111] FIG. 11 shows the evolution of the pH of the catholyte with time, for varying concentrations of HKH.sub.2PO.sub.4 as weak protonic electrolyte.

REFERENCES

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