Hydrogen Peroxide Electrosynthesis Using Recycled Graphite Granules as 3D Cathode. Comparison with Other Commercial Materials and Optimization Studies

This study aims to evaluate the ability of the graphite granules (GG) recycled from spent ZnC batteries to act as 3D cathode for hydrogen peroxide (HP) electrosynthesis (HPE) in eco-friendly conditions (unbuffered 0.05 M Na2SO4 solution, air as O2 source). The performances of GG were compared to those of other usual carbonaceous cathode materials for HPE such as graphite bloc, graphite felt and reticulate vitreous carbon (RVC), using a divided filter-press electrochemical reactor. The operational parameters such as the polarization mode, electrolyte and air flow rates, applied potentials or imposed currents, and aeration mode were optimized by 1 h tests of electrosynthesized HP accumulation (EHPA). Considering as optimization criteria the best compromise between the final HP concentration, global current efficiency and electrical energy specific consumption, we find that the most efficient material for EHPA was RVC of 500 ppi, exploited in galvanostatic mode and using an original aeration system. In optimized conditions, for the GG cathode, very promising efficiency indicators were evaluated, suggesting that better results can be obtained by electrode geometry optimization and GG pretreatment.

In this context, we performed comparative HPE optimization tests in eco-friendly conditions using recycled graphite granules (GG) from spent Zn-C batteries and other conventional unmodified carbonaceous materials (GB, GF and RVC).

Experimental part Experimental techniques, setups and equipments
During the present work, for each tested cathodic material and the corresponding filter-press electrochemical reactor (FPER) configuration, two experimental techniques were used. Firstly, in order to establish the starting conditions for the optimization studies, preliminary measurements were performed by hydrodynamic linear voltammetry (HLV) with the simultaneous on-line monitoring of the electrosynthesized HP concentration. Subsequent, 1 h tests of electrosynthesized hydrogen peroxide accumulation (EHPA) were completed in potentiostatic or galvanostatic mode for several combinations of the operational parameters.
The experimental setups used for the present studies, described schematically in Figure 1, included, as main constituent, a modified Microflow Cell ® FPER (ElectoCell A/S, Denmark) equipped with Nafion 117 (DuPont, USA) cation exchanges membranes (C.E.M.) and polytetrafluoroethylene (PTFE) tube (O.D. = 1.6 mm, I.D. = 1 mm) Luggin capillaries for the electrodes potential measurement. In order to prevent the interruption of the references liquid junctions by air bubbles, the PTFE Luggin capillaries were permanently flushed during the all experiments with anolyte and catholyte, at a flow rate of 1 mL/min, using the two sections (P2.1 and P2.2) of the peristaltic pump (PP) P2 of Reglo Digital MS2/8 type (ISMATEC, Switzerland).  For both HLV and EHPA measurements, as indicated in figure 1.A, the anodic section of the experimental setups was quasi-identical, including the anodic part of the FPER equipped with 1 or 2 counter-electrodes (C.E.) of 10 cm 2 active areas, a glass anolyte tank (A.T.) of 200 mL and one section (P1.1) of the P1 PP. The C.E. and P1 PP were of DSA-O2 (Ti/IrO2-Ta2O5 from ElectoCell A/S, Denmark) and Reglo Analog MS2/8 (ISMATEC, Switzerland) type, respectively.
The cathodic section of the experimental setup for HLV measurements has included, as indicated in fig. 1.B, the cathodic part of the FPER equipped with different models of working electrodes (W.E.) made from carbonaceous materials, a polypropylene catholyte tank (C.T.) of 5 L, the second section (P1.2) of the P1 PP and an original flow-through hydrogen peroxide detector (FTHPD) [40] connected at the output of the FPER cathodic compartment (OUT4). Additionally, in order to saturate the catholyte with air, a membrane pump (P3) and a porous PTFE dispenser immersed in the C.T., were used.
For EHPA studies, as indicated in Figure 1.C, the cathodic section of the experimental setups included the cathodic compartment of the FPER equipped with the carbonaceous cathodes, a glass C.T. of 200 mL and the FTHPD inserted between the P1.2 section of the P1 PP and the input of the FPER cathodic compartment (IN4). Concerning the air feed during the EHPA tests, three different solutions were tested. As indicated in the section (C1) of Figure 1.C, in a first embodiment, the air was pumped, at a flow rate controlled by the PP P4 (also of Reglo Digital MS2/8 type), directly in the C.T. In a second embodiment (see section (C2) from Figure 1.C), the air was pumped with P4 and injected in the catholyte flow before the entrance in the FPER cathodic compartment (input IN4). Finally, in a third embodiment, the sections C2 and C3 were used together, the last one acting as a supplementary Air-Water Mixer (A.W.M.). For this purpose, a PTFE body centrifugal pump of MD-10 model (IWAKI CO., LTD, Japan) was used, assuring an intensified air dispersion in the catholyte flow.
Concerning the cathode construction, as described in figure 2, five different carbonaceous materials and corresponding FPER configurations were tested. For the first two embodiments (Figure 2.A and 2.B), commercially available GB and RVC 100 ppi (pores per inch) electrodes, both of 5 mm thickness (ElectoCell A/S, Denmark) were used. For the third and fourth configuration (figure 2.C and 2.D), a rectangular piece of RVC 500 ppi (33x30x8 mm 3 ) and a 6 mm thickness GF layer of 0.082 g/cm 3 (both from Alfa Aesar), respectively, were fixed in corresponding polymethylmethacrylate (PMM) spacers. In the last embodiment, described in Figure 2.E, recycled GG were used as 3D cathode for HPE. In this purpose, the cylindrical graphite current collectors were carefully extracted from 10 Toshiba AA spent Zn-C batteries, washed consecutively with 48% H2SO4, 48% H2SO4 + 5% oxalic acid and doubledistilled water. Subsequently, the recycled graphite rods were air dried, crushed using a pair of pliers and classified by sieving. Finally, the new 3D cathode was made by fixing a 3 mm th home madeickness layer of GG, having the diameter between 0.65 and 2 mm, over a 3 mm thickness layer of GF and a GB electrode. For all tested carbonaceous materials, the W.E. exposed geometrical areas were of 10 cm 2 .

2.Materials and methods
All the measurements were carried out in an unbuffered, low ionic strength solutions (0.05 M Na2SO4) used as anolyte and catholyte solutions, prepared from solid Na2SO4 10 H2O (Sigma-Aldrich, p.a. grade) and double-distilled water, with an intrinsic pH of ~ 4.2. After the EHPA experiments, the final HP concentration ([H2O2]F) was evaluated by titration with 0.1 N KMnO4 standard solution. A 48% H2SO4 solution was used to acidify the catholyte samples before titrations and to clean the recycled graphite rods.

Experimental procedures
Before each set of HLV experiments, the A.T. and C.T. were filled with 100 mL and 5 L of fresh 0.05 M Na2SO4 solutions, respectively. Subsequently, all the setup hydraulic elements were filled with electrolyte and the catholyte was saturated with air for 30 min. Finally, the measurements were performed, for each tested cathodic material, at equal anolyte and catholyte flow rates (Qel) of 4, 8, 20 and 40 mL/min and a scan rate (v) of 5 mV/s.
For the EHPA studies, the A.T. and C.T. were filled with 100 mL samples of fresh catholyte and anolyte, all the setup hydraulic elements were filled with electrolyte and the measurements were carried out for 1 h.
For an increased accuracy of the HP concentration electrochemical monitoring, before each VLH or EHPA experiment, the D.E. from the FTHPD was conditioned (

A. Measurements by HLV
As pointed before, the main goal of the HLV measurements was to identify the starting experimental parameters for the optimization studies. Practically, the influences of the Qel and applied W.E. potential (EW.E.) on the HPE rates were evaluated for all tested cathodic materials. Because the instant HP concentration ([H2O2]t) evaluated with the FTHPD strongly depends on the Qel (due to the dilution factor), for a more relevant comparison, the recorded D.E. currents (ID.E.) were converted to instant generation rates (I.G.R.) values using the calibration data [40] (evaluated at different Qel), the corresponding Qel values and the following equation: where K represents the calibration constant corresponding to the Qel value. Based on this treatment, in Figure 3 is presented an example concerning the influence of the EW.E. on the IW.E. and I.G.R. values recorded by HLV on RVC 100 ppi at different Qel values. https://doi.org/10.37358/Rev. Chim.1949 As it can be seen from Figure  The results presented in Figure 4 also suggest that, for identical W.E. exposed geometrical areas of 10 cm 2 and Qel of 40 mL/min, at the above mentioned optimal EW.E. values, the obtained I.G.R. increased in the next order: GB < RVC 100 ppi < RVC 500 ppi < GG < GF.

B. Measurements by EHPA B.1. Treatment of EHPA data
In a first approach, for all tested materials, the more efficient conditions for EHPA were considered as the optimal compromise between the [H2O2] F, global current efficiency (C.Ef.G) and global electrical energy specific consumption (WS.G).
[H2O2] F was evaluated, for each experiment, by titration with 0.1 N KMnO4, and used to calculate the practical mass (mP) of the accumulated HP. C.Ef.G was evaluated as: where mT, z, F, MHP and tF represent the theoretical mass of the accumulated HP, number of transferred electrons (2 in this case), Faraday constant (96485 As/mol), HP molar mass and final experiment time, respectively, the other terms being defined before.
Considering ET the voltage at the FPER terminals, WS.G was evaluated as: Unfortunately, the ID.E. evolutions during all experiments reveals that the HP accumulation rate do not remains constant, recording a significant decrease toward the experiment ends. Consequently, for a more accurate evaluation of the EHPA process efficiency evolution, the intermediate values for HP concentration ([H2O2]I), current efficiency (C.Ef.I) and electrical energy specific consumption (WS.I) were also evaluated using dedicated LabView applications. Practically, the recorded data were divided, according eq. (4), in equal duration segments, t, of 180 s, and, for each data segment, the intermediate HP mass variation, mI, was calculated using eq. (5): where the [H2O2]t was calculated using the [H2O2]F value and considering that the recorded ID.E. is directly proportional (linear correlation) to the HP concentration.

B.2. Preliminary optimization studies in potentiostatic mode
For the first optimization studies in potentiostatic mode, we decided to use as cathode the commercially available RVC electrode. As presented in Figure 2.B, it consisted in a rectangular piece (33x30x5 mm 3 ) of 100 ppi RVC fixed with graphite doped conductive epoxy in a 5 mm thickness GB, allowing, theoretically, an electrolyte flow parallel with the current lines. Based on the literature suggestions [30][31][32][33], the catholyte oxygenation was made by air sparging in the C.T. according to the experimental setup described in Figure 1.C(C1) (9) several competitive (parasitic) processes can occur, such as total (4 electrons) reduction of dissolved O2, H2 evolution and HP electrochemical reduction at the cathode surface or HP bulk disproportionation, described by equations (10) -(13), respectively [11,16,34]:   (13) Moreover, the processes corresponding to the equations (12) and (13) are favored by the increase of the electrosynthesized HP concentration due to the accumulation process, inducing a permanent degradation of the EIEHPAP [11,16,34,37]. the poor conductivity of the catholyte and the non-uniform distribution of the applied potential in the volume of the 3D electrode [34,41], the low concentration of the dissolved O2 can induce punctual increases of the cathode overpotential, favoring the competitive processes described by eqs. (10)÷ (12) and degrading the EIEHPAP. More interestingly, at Qair = 80 mL/min, even if IW.E.M reached the maximum value (-13 mA), indicating an efficient catholyte oxygenation and an intense HPE process, the intermediate and global EIEHPAP reached the worst values. This apparently abnormal behavior, signaled also in literature [42,43], can be explained by the accelerated HP bulk disproportionation and, probably, by the HP stripping due to the excessive air sparging. The last supposition is sustained by the fact that, during the initial tests, the HP accumulation do not occur at all when the intensive aerating system used for the HLV tests (large PTFE diffuser and membrane air pump) was inserted into the C.T. of reduced volume.
Concerning the influence of EW.E. on the intermediate and global EIEHPAP for the RVC 100 ppi cathode, the obtained results presented in Figure 5.B and the last three rows from table 1, confirmed that the optimal polarization potential corresponds to the value of -1.0 V vs. Ref. 1.
On the other hand, it is important to discuss about the electrolyte velocity, uel, in respect to the electrode, which corresponds to the ratio between Qel and the flow surface, SF: For the electrolyte flow parallel with the current lines, as in the case of the RVC 100 ppi cathode, the SF corresponds to the electrode exposed geometrical area, leading, for Qel = 40 mL/min and SF = 10 cm 2 , to an uel value of 4 cm/min. Unfortunately, for industrial applications, in order to preserve the same uel and EIEHPAP values, Qel and Qair must increase proportionally to the electrode surface (e.g. Qel = Qair = 40 L/min for SF = 1 m 2 ), generating large consumption of electrical energy for pumping. To overcome this drawback, we decided to elaborate and test a new FPER configuration, described in Figure 2.C, that included a new RVC 500 ppi cathode and allowed the electrolyte flow perpendicular to the current lines. For this new arrangement, SF was reduced to 2.4 cm 2 , determining, for Qel = 40 L/min, an uel value of 16.66 cm/min. Moreover, taking into account that the use of three-phase system (solid-liquid-gas) seems to be more efficient for HPE [35-38, 44, 45], we decided, as indicated in fig. 1.C(C2), to inject the air directly into the catholyte flow. In the first step, for the Qair influence study, the EW.E. was fixed (based on the HLV results), at -0. 8    Even if the increased IW.E.M values indicated an intensification of the ORR in the three phase system, the degradation of the EIEHPAP can be explained by the inefficient exploiting of the electrode surface (partial blocking) due to the formation of large air bubbles resulting by the simple air injection in the catholyte flow. Moreover, due to the double-faced configuration of the RVC 500 ppi cathode, the effective Qel for each half of the electrode diminished to 20 mL/min, determining, as exemplified in Figure 3, a 25% decrease in the I.G.R.
From another point of view, it is important to note that the big air bubbles formed inside of the 3D cathode due to the simple air injection in the catholyte flow induces temporally blocking (insulation) of the PTFE reference capillary end, making ineffective the potentiostatic control and determining excessive and uncontrolled polarizations at the W.E. level. This unwanted phenomenon becomes more disturbing in the case of GF and GG tested cathodes, determining the cancelation of the corresponding optimization studies in potentiostatic mode. Only the optimization tests for the GB cathode succeeded, values of 6.

B.3. Optimization studies in galvanostatic mode
In order to overcome the drawbacks signaled during the optimization tests in potentiostatic mode, the next experiments were performed in galvanostatic conditions and using an original and improved catholyte aeration system. Practically, as indicated in Figure 1.C(C3), a Teflon® body centrifugal pump was inserted between the point of air injection in the catholyte flow and the input of the FPER cathodic compartment (IN4). This supplementary pump, working at a minimal pressure difference, acts as a efficient air-water mixer that divides the large air injected bubbles in very fine ones and transforms the electrolyte-air mixture in a relative stable emulsion. Moreover, comparing with other intensive aerating arrangements such as the jet-aerators [35][36][37][38], our original aerating systems assures an efficient oxygenation with minimal electric energy consumption and without losses of the electrolyte pressure.
Using the new aerating systems, in the first instance, the more promising cathodic materials (RVC 500 ppi and GG) were tested in galvanostatic mode at Qel = 40 mL/min and using several combinations of Qair and IW.E. values indicated in table 3. Also, in fig. 7.A and 7   Concerning the optimization studies for the recycled GG cathode exploited in galvanostatic mode and using the original aeration system, by correlating the data from fig. 7.B and table 3 (last 6 rows), we concluded that the best EIEHPAP were obtained at Qair = 10 mL/min and IW.E. = -10 mA. Comparing these results with those obtained in optimized conditions on the RVC 500 ppi cathode exploited in similar conditions, very promising values for the global EIEHPAP were obtained. Practically, using this extremely cheap cathodic material, even if the [H2O2]F and C.EF.I values decreased with 53% and 30%, the WS.I presented an excellent value, of 5.38 kWh/kg of HP.
Finally, using the same experimental setup, optimization studies were performed in galvanostatic mode for the GB and GF cathodes, a comparison of the best intermediate and global EIEHPAP for all tested materials being presented in fig. 8 where SW.E. represent the geometrical W.E. surface exposed towards the anode.   fig. 8 and table 4, we concluded that GB induces the best (lowest) WS.G from all tested materials and experimental parameters, but, unfortunately, the lowest G.G.R. Our results, in accordance with other reported data (see lines 1 and 5 from table 4), indicate that enormous FPER are required for large scale industrial applications. The productivity can be increased up to 4 times (see line 6 from table 4) by using pure oxygen, but this approach raises the production costs.
Concerning the RVC 500 ppi cathode tested in galvanostatic mode, it determines the highest C.Ef.G value and a G.G.R. 10 times bigger comparing to the GB electrode, in accordance with other reported results for air or pure O2 sparging in the cathodic compartment of a divided tank reactor (see lines 2, 9 and 10 from table 4). As indicated in the lines 7 and 8 from table 4, the productivity of RVC cathodes in respect to the EHPA process can be increased up to 10 times by using a divided FPER and pure O2 sparging in the C.T., with the corresponding increase of the production costs. Moreover, the high fragility and price of the RCV materials hinder the industrial development, only laboratory scale experiments being reported.
For optimized conditions and identical SW.E. value (10 cm 2 ), GF cathode assures the highest productivity (maximal [H2O2]F and G.G.R. values), but the worst HPE efficiency (lowest C.Ef.G and highest WS.G values), a similar G.G.R. being reported only for a more complicated setup, based on the air injection through the GF cathode (see line 11 from table 4).
Finally, comparing the best results obtained for the recycled GG cathode with those evaluated for the other tested materials, we conclude that it present satisfactory and very promising performances, assuring a G.G.R. 2 times smaller and 5 times bigger in relation to the RVC and GB electrodes, respectively. Also, the obtained productivity is 4 times larger as that reported for a granular cathode from glassy carbon pellets (see line 12 from table 4). Starting from this encouraging results, further improvements of the HPE efficiency for the recycled GG cathode will be tested based on electrode geometry optimization, surface pre-treatment [7,21,24,27], use of surfactants [31] or implementing programmable pulsed current [40,41].

4.Conclusions
To the best of our knowledge, the recycled GG from spent Zn-C batteries were for the first time proposed and tested as 3D cathode for HPE based on electrochemical ORR, their performances being https://doi.org /10.37358/Rev. Chim.1949 Rev. Chim., 71 (3) compared to those of other commercial carbonaceous materials (GB, GF and RVC). For all tested electrodes, the best results were obtained in galvanostatic mode and using the original aerating system implemented, also, during the present work. In these conditions, based on 1 h optimization tests, we conclude that the most efficient material for EHPA was RVC of 500 ppi, [H2O2]F, C.Ef.G and WS.G values of 71 ppm, 74.1% and 6.1 kWh/kg, respectively, being obtained. Even if the use of the GB electrode determine the minimal WS.G (of 4.94 kWh/kg), unfortunately, it induce the minimal G.G.R., of 2.0810 -5 mg s -1 cm 2 . Similarly, the GF electrode leads to a maximal [H2O2]F values (128.1 ppm), but to a minimal C.Ef.G values of 50%.
For the GG cathode, very promising global efficiency indicators were evaluated ([H2O2]F = 34 ppm, C.Ef.G = 53.3% and WS.G = 5.4 kWh/kg), suggesting that this extremely cheap material can be transformed, after a minimal treatment (washing. grinding and sieving), in a versatile 3D cathode for HPE. Moreover, improved results can be obtained by the optimization of other operational parameters (e.g. GG diameter, current profile, etc.) and GG pretreatment.
From another point of view, our studies revealed that the thickness of the 3D cathodes can plays an essential role when low conductivity electrolytes are used for HPE, too thick electrodes being able to induce highly nonuniform distribution of the potential and favoring the parasitic processes. Also, the aeration mode and air flow rate influence significantly the EHPA process efficiency, excessive sparging being able to reduce the HP accumulation rate.