domain: chim sde

text:
M Muñoz-Morales

A Ramírez

Aurelien Cañizares

Conchi J Llanos

Maria Concepcion

Ovin Ania

Conchi Ania
email: conchi.ania@cnrs-orleans.fr

Evaluating key properties of carbon materials as cathodes for the electrogeneration of hydrogen peroxide

Keywords: carbon cathodes, electrical conductivity, hydrogen peroxide, Toray paper, structural defects

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Hydrogen peroxide is an oxidizing chemical widely used in a large number of strategic productive sectors such as the paper and pulp industry, chemical synthesis, food and beverages, water treatment, textiles and laundry, and electronics among others [1]. Furthermore, hydrogen peroxide is a potential energy carrier and can be used in onecompartment H 2 O 2 fuel cells to generate electricity [2,3]. The significance of the global hydrogen peroxide market is evidenced by the over 5 million tons annual production in 2021 (market dominated by the chemical synthesis and the paper and pulp industries), and an expected compound annual growth rate (CAGR) index over 3% in the next five years [4].

Current industrial production is mostly carried out through the anthraquinone autoxidation process [START_REF] Campos-Martín | Hydrogen peroxide synthesis: an outlook beyond the anthraquinone process[END_REF][START_REF] Yang | Toward the Decentralized Electrochemical Production of H2O2: A Focus on the Catalysis[END_REF][START_REF] Hans-Joaquin | Production of Hydrogen Peroxide[END_REF][START_REF] Rust | Manufacture of Hydrogen Peroxide[END_REF], an energy-intensive and complex one with a wide number of technological drawbacks (i.e., large waste production, low efficiency, mass transfer limitation, excessive solvent use) [START_REF] Campos-Martin | Hydrogen Peroxide Synthesis: An Outlook beyond the Anthraquinone Process[END_REF]. These issues have triggered the research efforts to develop more efficient, safe and green methods for the synthesis of hydrogen peroxide (catalytic, enzymatic, photocatalytic, electrochemical). Among them, the production of H 2 O 2 from the electrochemical reduction of oxygen has become an interesting alternative, as it can be operated at room temperature and atmospheric pressure and allows on-site operation, avoiding costs and risks associated to its transport and storage.

The electrochemical production of H 2 O 2 relies on the 2-electron oxygen reduction reaction (2e-ORR). This reaction is typically limited by low oxygen solubility in aqueous electrolytes at room temperature, and depends to a large extent on the cathode material [START_REF] Petrucci | On the ability to electrogenerate hydrogen peroxide and to regenerate ferrous ions of three selected carbon-based cathodes for electro-Fenton processes[END_REF]. Owing to their unique structural and electronic properties, many carbon materials have been investigated for this application, such as graphite felts [START_REF] Song | Performance of graphite felt as anodes in the electro-Fenton oxidation systems: Changes in catalysis, conductivity and adsorption properties[END_REF][START_REF] Özcan | Carbon sponge as a new cathode material for the electro-Fenton process: Comparison with carbon felt cathode and application to degradation of synthetic dye basic blue 3 in aqueous medium[END_REF], reticulated vitreous carbon [START_REF] Ramírez-Pereda | Kinetics of Acid Orange 7 oxidation by using carbon fiber and reticulated vitreous carbon in an electro-Fenton process[END_REF], activated carbon [START_REF] García-Espinoza | Study of simultaneous electro-Fenton and adsorption processes in a reactor containing porous carbon electrodes and particulate activated carbon[END_REF][START_REF] Phan Quang | Advanced electro-Fenton degradation of a mixture of pharmaceutical and steel industrial wastewater by pallet-activated-carbon using three-dimensional electrode reactor[END_REF], carbon paper [START_REF] Dong | Heterogeneous electro-Fenton catalysis with self-supporting CFP@MnO2-Fe3O4/C cathode for shale gas fracturing flowback wastewater[END_REF], carbon felt [START_REF] Santos | Testing the role of electrode materials on the electro-Fenton and photoelectro-Fenton degradation of clopyralid[END_REF]. Compared to noble-metal-based electrocatalysts, carbon-based materials offer the advantage of their low cost, availability, good conductivity, as well as the possibility to modulate their characteristics (e.g., porosity, surface defects, composition) to favor the selectivity towards the 2e-ORR pathway [START_REF] Wang | Recent advances in electrochemical 2e oxygen reduction reaction for on-site hydrogen peroxide production and beyond[END_REF][START_REF] Kim | Efficient hydrogen peroxide generation using reduced graphene oxide-based oxygen reduction electrocatalysts[END_REF][START_REF] Zhou | Selective H2O2 electrosynthesis by O-doped and transition-metal-O-doped carbon cathodes via O2 electroreduction: A critical review[END_REF]. The most commonly used carbon cathodes for the electrogeneration of H 2 O 2 are graphites, carbon blacks and carbon felts [START_REF] An | Highly efficient electro-generation of H2O2 by adjusting liquid-gas-solid three phase interfaces of porous carbonaceous cathode during oxygen reduction reaction[END_REF][START_REF] Zhang | Carbon Black Oxidized by Air Calcination for Enhanced H2O2 Generation and Effective Organics Degradation[END_REF][START_REF] Pérez | Effect of air pressure on the electro-Fenton process at carbon felt electrodes[END_REF]. The latter has several disadvantages related to the cost and poor performance in pilot scales-up flow reactors due to their flexibility [START_REF] Le | Correlation between degradation pathway and toxicity of acetaminophen and its by-products by using the electro-Fenton process in aqueous media[END_REF][START_REF] Poza-Nogueiras | Towards a more realistic heterogeneous electro-Fenton[END_REF][START_REF] Poza-Nogueiras | Exploring the pressurized heterogeneous electro-Fenton process and modelling the system[END_REF]. In the case of carbon black, low faradaic efficiencies are usually obtained when large electrodes are used due to oxygen transfer limitations related to their solubility in water [START_REF] Ren | A novel vertical-flow electro-Fenton reactor for organic wastewater treatment[END_REF][START_REF] Chu | A dual-cathode electro-Fenton oxidation coupled with anodic oxidation system used for 4-nitrophenol degradation[END_REF], although this is counterbalanced by their low cost and a high electronic conductivity. Nevertheless, other researchers testing different carbon matrices without O 2 diffusion problems indicate that if the ORR onset is shifted towards more positive potential the energy consumption is lower and hence the faradaic efficiency is higher [START_REF] Cordeiro-Junior | Catalysis of oxygen reduction reaction for H2O2 electrogeneration: The impact of different conductive carbon matrices and their physicochemical properties[END_REF][START_REF] Assumpção | A comparative study of the electrogeneration of hydrogen peroxide using Vulcan and Printex carbon supports[END_REF].

Other carbon materials have also been largely investigated as cathodes for the electrogeneration of H 2 O 2 [START_REF] An | Highly efficient electro-generation of H2O2 by adjusting liquid-gas-solid three phase interfaces of porous carbonaceous cathode during oxygen reduction reaction[END_REF][START_REF] Li | Graphene-based synthetic fabric cathodes with specific active oxygen functional groups for efficient hydrogen peroxide generation and homogeneous electro-Fenton processes[END_REF][START_REF] Cornejo | Cathodic generation of hydrogen peroxide sustained by electrolytic O2 in a rotating cylinder electrode (RCE) reactor[END_REF]; however, there is a lack of understanding of the correlation between the physicochemical and structural characteristics that includes a wide spectrum of different types of carbon materials and their electrocatalytic activity for the generation of H 2 O 2 using 4 cm 2 electrodes. For instance, it has been reported that the porosity of the carbon cathodes influences the mass transfer rate of the ORR species (being that of H 2 O 2 larger than that of O 2 ) as well as the strength of the oxygen adsorption sites -i.e., binding energy towards the catalyst's surfacefavoring the 2e-ORR pathway (avoiding further reduction to water) [START_REF] Jung | Recent Advances in Electrochemical Oxygen Reduction to H2O2: Catalyst and Cell Design[END_REF]. Surface functionalization of the carbon cathodes has also been reported to have a large impact on the electrocatalytic activity and selectivity for the 2e-ORR [START_REF] Zhang | Graphitic N in nitrogen-Doped carbon promotes hydrogen peroxide synthesis from electrocatalytic oxygen reduction[END_REF][START_REF] Nair | Carbonaceous cathode materials for electro-Fenton technology: Mechanism, kinetics, recent advances, opportunities and challenges[END_REF][START_REF] Moreira | Electrochemical advanced oxidation processes: A review on their application to synthetic and real wastewaters[END_REF]. In this regard, a large number of studies have reported the beneficial effect of heteroatom doping of the carbon cathodes on their H 2 O 2 production [START_REF] Zhou | Selective H2O2 electrosynthesis by O-doped and transition-metal-O-doped carbon cathodes via O2 electroreduction: A critical review[END_REF][START_REF] Li | Graphene-based synthetic fabric cathodes with specific active oxygen functional groups for efficient hydrogen peroxide generation and homogeneous electro-Fenton processes[END_REF][START_REF] Zhang | Multiple active cobalt species embedded in microporous nitrogen-doped carbon network for the selective production of hydrogen peroxide[END_REF]. However, owing to the complexity of achieving selective surface modification of carbon materials, the role of the individual functional groups and their synergy in the electrocatalytic performance remain rather unknown. On the other hand, the role of the electric conductivity of the carbons is scarcely addressed; carbon materials are typically considered good conductors, although such characteristic varies considerable depending on the type of carbon material (e.g., from highly conductive graphite or graphene to poor conductive amorphous porous carbons).

Although there are other important factors for an efficient electrochemical production of H 2 O 2 [START_REF] Pérez | Effect of air pressure on the electro-Fenton process at carbon felt electrodes[END_REF][START_REF] Murayama | Electrosynthesis of Neutral H2O2 Solution from O2 and Water at a Mixed Carbon Cathode Using an Exposed Solid-Polymer-Electrolyte Electrolysis Cell[END_REF][START_REF] Ma | Reduction of oxygen to H2O2 at carbon felt cathode in undivided cells. Effect of the ratio between the anode and the cathode surfaces and of other operative parameters[END_REF] specially when high surface electrodes are studied, a proper selection of the carbon cathode considering the physicochemical, structural and electrochemical parameters of the carbon material is of great relevance for this application.

Thus, the aim of this work was to perform a screening of carbon materials with varied physicochemical features as cathodes for the electrochemical production of H 2 O 2 . The portfolio of carbon materials ranged from graphite, graphene and derivatives, chars, activated carbons, carbon nanotubes and carbon nanofibers. They have been chosen based on their differences in surface chemistry, structural order, porosity and electrical conductivity; the aim is to clarify the role of these descriptors in the electrochemical production of H 2 O 2 . Furthermore, the electrocatalytic performance was measured on relatively large cathodes (i.e., 2 x 2 cm 2 ), higher than most studies from the literature reporting the adequateness of carbon materials for this application (i.e., a few mm 2 ). This is important as it demonstrates the feasibility of large dimension cathodes for the electrochemical production of H 2 O 2 .

Experimental

Reagents

Sodium sulfate, sulfuric acid, PTFE (60% emulsion), 2-methyl propanol, 5%(v/v) Nafion Ⓡ 117 solution and potassium titanium oxide oxalate dihydrate were purchased to Sigma-Aldrich. Polyvinylidene fluoride (PVDF) and N-Methyl-2-pyrrolidone were purchased to Merck. All aqueous solutions were prepared with Milli-Q water (18.2 MΩ cm).

Carbon materials

A series of carbon materials was used for the fabrication of the cathodes, including graphite, carbon black, graphene and derivatives, carbon nanotubes, petroleum coke, and nanoporous carbons. They were selected based on their different structural, porous and physicochemical properties. Details and nomenclature of the samples are compiled in Table 1. A complete characterization of the materials can be found in the Electronic Supplementary Information (Tables S1,S2, Fig. S1-S3).

Table 1. Nomenclature and selected characteristics of the carbon materials used as anodes in the electrocatalytic assays for the production of hydrogen peroxide.

Nomenclature Characteristics

Gra

Graphite, commercial, supplied from Sigma Aldrich

HSU250

Sample HSU treated at 250 °C under inert atmosphere for 30 min to remove surface functionalities HSU500

Sample HSU treated at 500 °C under inert atmosphere for 30 min to remove surface functionalities HSU1000

Sample HSU treated at 1000 °C under inert atmosphere for 30 min to remove surface functionalities

Electrochemical Characterization

A preliminary electrochemical characterization of the carbon powders was carried out Contact angles were measured to determine wetting behaviour of the electrodes.

Measurements were performed based on the sessile drop method, using 2 μl drops of deionised water deposited on the samples with a micropipette in a Digitrop (GBX) instrument. The contact angle was measured after stabilization of the water droplet in contact with the material surface. Averages of three independent measurements were registered for each sample.

Electrocatalytic H 2 O 2 production

For the electrocatalytic assay for hydrogen peroxide generation, a 2 x 2 cm 2 Toray carbon paper® (TGPH-120, thickness 0.19 mm) was used as electrode substrate. Before applying the catalytic inks, the Toray papers was ultrasonicated for 30 min as reported elsewhere [START_REF] Yu | Cost-effective electro-Fenton using modified graphite felt that dramatically enhanced on H2O2 electro-generation without external aeration[END_REF]. The carbon inks were prepared by suspending ca. 1 mg/mL of carbon powders and 50 mg/mL of PTFE (60% emulsion) in 2-methyl propanol, and sonicated for 15 minutes. The Toray papers were immersed in the ink, and sonicated for 15 min; then, the electrodes were dried in an oven at 80ºC for 24 hours to remove the excess of solvent. After drying, the electrodes were weighted to determine the loading coverage (Γ) in mg of carbon per cm 2 , reaching average values ranging from 0.3-0.5 mg cm 2 for all prepared inks. Commercial carbon felt (with and without PTFE coating) was also tested as cathode material for comparison purposes.

The production of H 2 O 2 was performed in an undivided cell (0.1 L) at room temperature and atmospheric pressure, using 0.05 M Na 2 SO 4 at pH 6.5 (adjusted using 0.05 M H 2 SO 4 ) as electrolyte. The 2 x 2 cm 2 Toray papers with the catalytic inks were used as cathode, and a graphite rod as anode (active surface 4.2 cm 2 ). The electrolyses were done in a potentiostatic mode, with an applied potential of -0.9 V using potentiostat/galvanostat (Biologic). This potential was selected to assure the production of hydrogen peroxide in the electrodes (confirmed by a colorimetric method), while the potentiostatic mode was preferred to prevent high currents through the electrodes (due to their differences in conductivity, Table S2). The distance between the anode and cathode was fixed at 1 cm. A continuous supply of oxygen was applied by bubbling air;

the solution was continuously stirred (300-400 rpm) using a magnetic bar to ensure a homogenous distribution of oxygen as reactant. The control electrolysis under a continuous flow of He to assure the absence of oxygen were also recorded. Prior starting the electrochemical treatment, the electrodes were immersed in the electrolyte (wetting) for 30 min under stirring. At time intervals, aliquots of 1 mL were taken for analyzing the concentration of H 2 O 2 . The concentration of H 2 O 2 was measured by following the concentration of the complex formed between H 2 O 2 and Ti(IV) measuring the absorbance at 408 nm on a UV-Vis spectrophotometer (Shimadzu) [START_REF] Pérez | Effect of pressure on the electrochemical generation of hydrogen peroxide in undivided cells on carbon felt electrodes[END_REF]. The nomenclature of the 2 x 2 cm 2 electrodes was X/TP, where X refers to the carbon materials used and TP stands for the Toray paper support.

Applied electric charge was used as a measure of the quantity of electricity supplied to the electrodes with the time and referred to the volume of solution. This parameter helps us to differentiate performance of electrodes according with the total power consumed.

The applied electric charge (Q) was calculated as follows:

𝑄 = 𝐼 * 𝑡 𝑉 ( 1
)
where I is the external current (A) at the applied voltage, t the time (s) and V the volume of the whole tank (L).

The faradaic efficiency (FE) of H 2 O 2 production was calculated as follow [START_REF] Panizza | Direct And Mediated Anodic Oxidation of Organic Pollutants[END_REF]:

𝐹𝐸 (%) = 𝑛 • 𝐹 • [𝐻 2 𝑂 2 ] • 𝑉 ∫ 𝐼 • 𝑑𝑡 𝑡 0 𝑥 100 ( 2
)
where n is the number of electrons transferred for ORR to H 2 O 2 , F the Faraday Constant

(96486 C/mol), [𝐻 2 𝑂 2 ]
is the concentration of H 2 O 2 (mol/L), V the bulk liquid volume (L) in the reactor, I the external current according with the voltage chosen (A) and t the time (s).

The energy consumption (KWh/m 3 ) was calculated as [START_REF] Brillas | Electro-Fenton Process and Related Electrochemical Technologies Based on Fenton's Reaction Chemistry[END_REF]:

𝐸𝑛𝑒𝑟𝑔𝑦 𝑐𝑜𝑛𝑠𝑢𝑚𝑝𝑡𝑖𝑜𝑛 = 𝑈 • 𝐼 • 𝑡 𝑉 ( 3
)
where U is the cell voltage (V), I is the current (A), V is the volume of the solution (m 3 ) and t is the time (h).

Characterization and Data Analysis

The selected carbon materials were thoroughly characterized using various techniques (e.g., gas adsorption isotherms, elemental analysis, Raman spectroscopy). Details of the experimental protocols are shown in the Supplementary Information File (Tables S1 andS2, Fig. S1-S3). A hierarchical cluster analysis method was used to analyze Raman data, following Ward's clustering algorithm (minimum variance method) and the Euclidian distance measure to generate a dendrogram using a script programmed in Phyton with Scipy library selecting linkage function and choosing the Ward method [START_REF] Jain | Data clustering: a review[END_REF][START_REF] Johnson | Hierarchical clustering schemes[END_REF]. The hierarchical cluster analysis was performed on the full Raman spectrum.

Results & discussion

Portfolio of carbon materials

A series of carbon materials with different physicochemical features were selected for this study, aiming to correlate their electrocatalytic activity towards the generation of H 2 O 2 with the characteristics of the carbon (structural order, porosity, surface defects, composition and electrical conductivity). The carbon materials were characterized using various techniques (see Supplementary Information File); these properties are herein discussed in general terms. S1).

Regarding structural properties, the hierarchical cluster analysis of Raman data shown in Fig. 1.d shows four clusters with statistically large structural dissimilarities. A clear correlation was observed between those clusters and the functionalization and conductivity of the carbons. As an example, the first cluster (highlighted in green) is composed of samples graphite, graphene, carbon black and graphitized carbon, which are characterized by a high structural order but also low functionalization and high conductivity. A second cluster (highlighted in blue) is formed by samples with moderate structural order, along with low O/C and high conductivity. The last two clusters are composed of the carbons with a high functionalization and poor conductivity.

This classification will be used as a guide for the discussion on the electrocatalytic performance of the carbons in the following sections.

Pre-screening of the electrocatalytic activity in a glassy carbon support

An initial pre-screening of the electrocatalytic activity of the selected carbons for the production of hydrogen peroxide was examined by Linear Sweep Voltammetry (LSV) in 0.05 M sodium sulphate on a 5 mm diameter glassy carbon support. The LSV curves were recorded in both O 2 -saturated and He-saturated electrolyte by continuously bubbling air or helium in the electrochemical cell (before and during the analysis). This allowed determining the net current density for each electrode (i.e., iO 2 -iHe). The second wave at higher onset potentials is due to the Hydrogen Evolution Reaction (HER) that occurs in parallel in the cathode (r3):

2 𝐻 2 𝑂 (𝑎𝑞) + 2𝑒 -→ 𝐻 2 (𝑔) + 2 𝑂𝐻 -(𝑎𝑞) 𝐸 0 = 0. 82 (𝑣𝑠 𝑅𝐻𝐸) (𝑟3)
It should be mentioned that carbon materials are typically good electrocatalysts for the HER but the choice of the experimental conditions (e.g., availability of oxygen in solution, electrolyte, applied potential) can contribute to suppress this reaction [START_REF] Zhou | Electrocatalytic hydrogen evolution under neutral pH conditions: current understandings, recent advances, and future prospects[END_REF] and enhance the electrogeneration of H 2 O 2 via 2e -ORR.

Fig. 2b,c show the values of the onset potential and the net electric current density evaluated at -0.9 V vs SCE for all the studied carbons. This potential value corresponds to the plateau of the first cathodic wave, and it was chosen as it allows the electrogeneration of H 2 O 2 while minimizing the HER. As seen, current densities in the O 2 -saturated electrolyte were higher than those in He-saturated environment for all the studied carbons. This confirms the electrocatalytic activity of the electrodes for the reduction of oxygen to H 2 O 2 at this potential. The formation of hydrogen peroxide was confirmed by a colorimetric method upon direct measurement in the electrolyte (see experimental section) for all the studied carbons. A RRDE was also used to confirm the electrocatalytic activity of the carbons towards the preferential reduction of oxygen through the formation of H 2 O 2 . Figure S5 shows the ring and disk currents recorded for selected carbons representative of the four studied groups. Significant oxidation currents were recorded in the Pt-ring electrode at sufficiently negative potentials due to the oxidation of H 2 O 2 to O 2 . Maximum oxidation currents were obtained at potentials above of -0.9V for all the samples, corresponding to a number of electrons between 1.9-2.3

and calculated H 2 O 2 current efficiencies over 70%.

The onset potential values for the ORR varied between -200 and -600 mV vs SCE for all the carbons (most values above -600 mV vs SCE). Only for carbons NPBH and PCKOH an onset value could not be clearly identified in the LSV curves; both carbons displayed certain electrocatalytic activity for the production of H 2 O 2 when the electrolysis was carried out in bigger electrodes as it will be discussed in the next section.

For comparison purpose, the performance of the carbons in terms of current density was compared at an applied potential of -0.9 V vs. SCE. This value was negative enough to assure the production of hydrogen peroxide in all the studied samples (confirmed by a colorimetric method), with rather constant current values. It was also the potential chosen for the electrolysis in 4 cm 2 electrodes (next section). Overall, no clear correlation was observed between the onset potential values and the characteristics of the four groups of carbons. In general terms, low E onset values were obtained for groups I and II (characterized by low H/C and O/C ratios and high electrical conductivity), with the exception of graphite and carbon black that presented values similar to those of carbon in groups III and IV (carbons with much lower oxidation extent, and electrical conductivities of at least an order of magnitude lower).

Carbons from groups I and II presented (on average) among the highest net current densities (ca. 0.25-0.5 mA/cm 2 ), pointing out the important role of the electrical conductivity of the cathodes. Similarly, the graphitization of carbon NPQ from Group IV (to render sample NPQgra)

resulted in an increase in the net current density, due to the enhancement in conductivity. However, the net current density of carbons from Group III was comparable to that of sample CNT and CB (groups II and I, respectively), and significantly higher than that of sample GRA in Group I (all of them showing much higher electrical conductivities). This indicates that the conductivity of the cathodeseven though an important parameter-, is not the only parameter governing the electrocatalytic production of H 2 O 2 .

Regarding surface functionalization, different trends were observed. The presence of Ogroups improved the electrocatalytic activity of samples CNT and rGO (compared to CNTox and GO, respectively), despite the considerable loss in electrical conductivity upon functionalization. This would be in agreement with the abundant literature reporting the beneficial role of heteroatom doping on the electrocatalytic activity of carbon materials in the 2e-ORR [START_REF] Zhou | Selective H2O2 electrosynthesis by O-doped and transition-metal-O-doped carbon cathodes via O2 electroreduction: A critical review[END_REF]. In contrast, a similar trend was not observed for the materials of group IV derived from a hydrochar (samples HSU, HSU250, HSU500 and HSU1000) or for carbons NPB and NPBH. The electrocatalytic of sample HSU (oxygen content ca. 20 wt.%) increased upon thermal annealing of the sample to remove the oxygen functionalities. A similar behavior would explain the low performance of sample NPB compared to NPBH treated at 800ºC. This points out that the gained in conductivity upon the thermal treatment becomes a more critical parameter than the loss of wettability/ hydrophilicity of the electrodes.

The latter was corroborated by the similar wetting behavior of all the studied electrodes, as inferred from the contact angle measurements (Fig. S6,S7). In this regard, it is well known that the surface wettability of electrodes can be tuned upon the incorporation of a binder in the composition of the inks [START_REF] Zhao | Superhydrophobic Air-Breathing Cathode for Efficient Hydrogen Peroxide Generation through Two-Electron Pathway Oxygen Reduction Reaction[END_REF] As a result, all the electrodes presented a marked hydrophobic character, despite the different hydrophobic/hydrophilic character of the pristine carbon materials (Table S2). Such hydrophobicity has been demonstrated

to provide enough oxygen to the electrocatalytic sites to favor the two-electron ORR activity for H 2 O 2 production, through the formation of triple-phase (oxygen gas-electrolyte-electrode) contact points [START_REF] Pérez | Improving the Efficiency of Carbon Cloth for the Electrogeneration of H2O2: Role of Polytetrafluoroethylene and Carbon Black Loading[END_REF].

The presence of structural defects in the carbons seems to play a key role in the electrocatalytic activity determined by LSV, as evidenced in Fig. 3 showing the correlations with the structural parameters obtained from the analysis of Raman data using Sadezky'5 bands mode [START_REF] Sadezky | Raman microspectroscopy of soot and related carbonaceous materials: Spectral analysis and structural information[END_REF]. As seen, higher net current densities where obtained for the carbons with high A D1 /A G ratio and low full-width at medium high of band D;

both parameters are strongly related with the structural order of the carbons and the presence of in-plane defects [START_REF] Beyssac | Raman spectra of carbonaceous material in metasediments: A new geothermometer[END_REF]. With the exception of one sample (graphite), the lowest net current densities were obtained for the carbons from Groups III and IV, characterized by A D1 /A G between 1.5-3. Similarly, both groups of carbons present FWMH values for the D band above 100. The similar correlation of the current density with the electrical conductivity (Fig. 3c) rendered a less conclusive trend, with similar current densities for carbons in Groups III and II with different electrical conductivity (as discussed above). Thus, it appears that threshold values of both structural parameters can be used to describe the electrocatalytic performance of the cathodes in terms of current density (a lower impact was observed for the onset potential, Fig. S5). Further correlations will be made based on the electrocatalytic performance of large electrodes (4 cm 2 ) in the next section.

Electrocatalytic H 2 O 2 production of carbon-based materials supported in a 4 cm 2 carbon paper

The electrochemical production of H 2 O 2 was evaluated in 4 cm 2 electrodes (to disregard limiting factors of low surface area electrodes related to limited quantification of H 2 O 2 ).

Constant air bubbling in the vicinity of cathode was applied to minimize any masstransfer issues of dissolved oxygen at the interface electrode/electrolyte [START_REF] Ren | A novel vertical-flow electro-Fenton reactor for organic wastewater treatment[END_REF][START_REF] Pérez | Effect of pressure on the electrochemical generation of hydrogen peroxide in undivided cells on carbon felt electrodes[END_REF]. For this, porous Toray paper was used as support for the fabrication of the cathodes (with the exception of the carbon felt that was used as-received). As indicated in the experimental section, the electrocatalytic assays were carried out in potentiostatic mode at -0.9 V vs SCE to selectively promote the electrochemical reduction of O 2 to H 2 O 2 .

Fig. 4 With the exception of sample Gra, the carbons from Groups I and II exhibited the highest production of H 2 O 2 (absolute and normalized values). This agrees with the prescreening data from the LSV curves in a glassy carbon support (Fig. 2). The production of H 2 O 2 matched the theoretical production for a 100 % of FE at low total charge values for both groups. At higher total charge values (corresponding to longer electrolysis times), the production rate stabilized reaching a plateau value. A decrease in the FE at long times was rather expected, likely due to the contribution of parasitic reactions in non-separated electrochemical cells, as the oxidation (r4) or the reduction (r5) of hydrogen peroxide [START_REF] Ma | Reduction of oxygen to H2O2 at carbon felt cathode in undivided cells. Effect of the ratio between the anode and the cathode surfaces and of other operative parameters[END_REF][START_REF] Pérez | Electrosynthesis of hydrogen peroxide in a filter-press flow cell using graphite felt as air-diffusion cathode[END_REF][START_REF] Pérez | Electrochemical jet-cell for the in-situ generation of hydrogen peroxide[END_REF].

𝐻 2 𝑂 2 (𝑎𝑞) → 2𝐻 + + 2𝑒 -+ 𝑂 2 (𝑔) 𝐸 0 = 1.20 (𝑣𝑠 𝑅𝐻𝐸) (𝑟4) 𝐻 2 𝑂 2 (𝑎𝑞) + 2𝐻 + (𝑎𝑞) + 2𝑒 -→ 2𝐻 2 𝑂 (𝑎𝑞) 𝐸 0 = 1.78 (𝑣𝑠 𝑅𝐻𝐸) (𝑟5)
These reactions are expected to increase with the concentration of hydrogen peroxide, and may be important since we have used a non-separated cell configuration in this study. They can be minimized in separated and flow-cells, thus increasing the production of hydrogen peroxide. Optimizing the hydrogen peroxide has not been considered in this study, which focuses on comparing the electrocatalytic performance of a pool of carbon cathodes.

With the exception of samples PCC and PCKOH (both obtained from the same precursor), carbons from Groups III and IV showed similar H 2 O 2 production following overlapping profiles. This is interesting since these carbons showed different electrocatalytic activity in the pre-screening LSV curves in small dimension electrodes (Fig. 2). Such differences are no longer observed when the electrochemical production of H 2 O 2 is carried out in larger electrodes and for longer times. In this regard, the surface chemistry became a less important parameter in the electrocatalytic activity of the carbons in 4 cm 2 electrodes, as seen in the similar production profiles of electrodes GO/TP and rGO/TP, as well as the series HSU/TP and its derivatives after thermal annealing at 250, 500 and 1000ºC. Hence, the electron transfer (i.e., electronic conductivity) would seem to be the parameter governing the overall performance in the large electrodes and at long electrolysis times.

Fig. 5 shows the dependence of the FE at an applied electric charge of 0.12 Ah L -1 of the electrochemical production of H 2 O 2 for all the X/TP cathodes, and correlations with various physicochemical parameters of the carbons. The FE was calculated at the highest electric charge value where there is a linear increase with the concentration of H 2 O 2 for all the carbons (for comparison purposes). Interestingly, the highest H 2 O 2 production was obtained for carbons of Group I, whereas the FE was higher for the carbons of Group II (ca. 0.8 º/ 1 for CNF). To understand this behavior, it should be reminded that the FE is a concept related with the selectivity for reducing O 2 to generate H 2 O 2 . Thus, the high production in cathode NPQgra accompanied with a FE of ca. 0.52 º/ 1 , indicates the existence of parallel reactions (typically HER or 4e-ORR) that compete with the electrochemical generation of hydrogen peroxide.

The porosity of the carbon materials does not seem to be a relevant parameter on the electrocatalytic activity, with random correlations in some groups between the surface area and/or microporosity of the carbons and the performance (Fig. 5b). This suggests that high micropore volumes associated to high surface areas that correspond to carbons of group III and IV are neither primary catalytic sites nor relevant gas transfer channels in the transport/diffusion of dissolved oxygen or the as-generated H 2 O 2 [START_REF] Casanova | Carbon black as conductive additive and structural director of porous carbon gels[END_REF][START_REF] Beyssac | Raman spectra of carbonaceous material in metasediments: A new geothermometer[END_REF]. On the contrary, low values of specific surface area of carbons in group I and II show a better performance with faradaic efficiencies higher than 0.5 º/ 1 .

For Groups I and II, the FE at a constant applied electric charge followed a rather linear increasing trend with the electrical conductivity; this was not the case for the other carbons (Fig. 5b,c). The electrical conductivity of the electrodes is related to the charge transfer processes occurring at the electrode interface. Thus, high conductivities are needed to increase the current response and minimize current losses. For Group IV, faradaic efficiencies ranged between 0.3-0.6 º/ 1 , despite all the carbons display similar (very low) electrical conductivity. To explain the behavior of carbons in group IV, the differences in the structural features and/or composition of the carbons must be considered.

The correlation between the FE and some structural parameters of the carbons obtained from the analysis of Raman spectra is shown in Fig. 5c,e. Indeed, faradaic efficiencies higher than 0.5 º/ 1 were obtained for those carbons displaying A D /A G ratios lower than 1.5 (mainly carbons from Groups I and II). According to literature, A D /A G values above 1.5 are characteristic of disordered carbons with small size aromatic domains associated to high density of structural defects (such as sp 3 hybridization, impurity atoms, vacancies, grain boundaries, loops...) [START_REF] Puech | Analyzing the Raman Spectra of Graphenic Carbon Materials from Kerogens to Nanotubes: What Type of Information Can Be Extracted from Defect Bands?[END_REF]. The FE of the X/TP cathodes also followed a clear decreasing correlation with the width of the D band, as also discussed for the net current density (I net O 2 -He) determined in the screening LSV curves recorded with the X/GC electrodes (Fig. 3). In this line, Csp 3 -bonds and structural defects have been reported as the active sites for the 2e-ORR to generate H 2 O 2 [START_REF] Wang | Recent advances in electrochemical 2e oxygen reduction reaction for on-site hydrogen peroxide production and beyond[END_REF]. These are considered active sites for favoring the adsorption of O 2 , since oxygen interactions with carbon surfaces at room temperature are related to unsaturated carbon atoms on the carbon surface (i.e., structural defects and dangling bonds) [START_REF] Pérez | Electrochemical jet-cell for the in-situ generation of hydrogen peroxide[END_REF].

Our data shows that structural defects in carbon-based cathodes play a pivotal role in their electrocatalytic activity for the 2e -ORR [START_REF] Ma | A review of oxygen reduction mechanisms for metal-free carbon-based electrocatalysts[END_REF]. More specifically, an A D /A G threshold value of 1.5 could be used to discriminate the electrocatalytic activity of the carbon cathodes for the 2e -ORR, with high production rates (absolute production) and good faradaic efficiency (selectivity).

The performance of the carbon felt cathode (considered as a benchmark carbon cathode in many electrochemical applications) of similar dimensions was very poor in terms of FE and H 2 O 2 production (Figs. 2 and4). Despite the coating of the carbon felt with PTFE layer improved the electrocatalytic performance, it was still lower that most of the studied cathodes, particularly those from Group II where this material would be classified according to the composition, structure and conductivity (Fig. 1). We attribute this to the thickness of the carbon felt compared to the X/TP electrodes (ca. 12 vs 0.19 mm, respectively), that would affect the diffusion of oxygen through the electrode and would hinder a proper comparison of the performance.

Regarding functionalization, no significant differences were observed in the electrocatalytic activity of the series of hydrochars annealed at different temperatures, with similar FE of ca. 0.4 º/ 1 for the samples treated up to 500ºC. This is attributed to the fact that the structural parameters and conductivity of the carbons remained rather unchanged (e.g. A D /A G above the threshold of 1.5). In contrast, the FE increased for the sample treated at 1000ºC, which displayed lower structural defects (A D /A G of 1.5).

Similarly, the FE of samples NPQ (Group III) and NPQgra (Group I) is similar (ca 0.5 º/ 1) , despite the A D1 /A G ratio decreased from 2.1 to 1.2 after the graphitization. For Group II (samples CNT and CNTox), the drop in FE should be linked to the slightly deterioration of the electrical conductivity upon oxidation, since the A D /A G ratio remained constant upon oxidation (Table S2). Working electrode: Toray Paper (4 cm 2 ); counter electrode: graphite rod; reference electrode: saturated calomel (SCE). Grey circle: Carbon Felt; black circle: Carbon Felt with PTFE).

Energy consumption of the cathodes

Fig. 6 shows the calculations of the energy consumption per gram of H 2 O 2 for the different carbon cathodes, using two different criteria: (1) constant applied electric charge of 0.12 Ah L -1 (Fig. 6a) and ( 2) the production of 20 mg L -1 of H 2 O 2 (Fig. 6b).

These values of applied electric charge and H 2 O 2 production were selected to be able to carry out the comparison of the 22 carbon materials studied in this work (given their large differences in electrocatalytic performance). Data of the benchmark carbon felt electrode in two electrode configurations (with and without a PTFE coating) is included for a fair comparison. Overall, the energy consumption of the carbon cathodes was rather low (especially for those carbons classified in Groups I and II), and in all the cases lower than that of the carbon felt benchmark. In this regard, it should be mentioned that the incorporation of a PTFE coating layer to the carbon felt significantly increased the FE and reduced the energy consumption, as previously stated in literature [START_REF] Pérez | Improving the Efficiency of Carbon Cloth for the Electrogeneration of H2O2: Role of Polytetrafluoroethylene and Carbon Black Loading[END_REF].

For an applied electric charge of 0.12 Ah L -1 , the energy consumption of all carbons is rather similar, despite FE span from 0.3 to 0.8 º/ 1 . This is most likely due to the relatively low applied electric charge, that minimizes the decomposition of H 2 O 2 (this being more important as the concentration of hydrogen peroxide increases, as discussed above) and that corresponds to short electrolysis times (Table S3). The balanced performance of samples from Groups III and IV in terms of electrochemical H 2 O 2 production and energy consumption at short electrolysis times should be highlighted.

Particularly if it is considered that many of these carbons are low cost materials that can be easily prepared from abundant and renewable precursors (including wastes), compared to carbons of Groups I and II. This represents an advantage to push their application in electrochemical processes and it would contribute to develop sustainable and cost-efficient electrochemical technologies based on circular economy processes involving materials' preparation. On the other hand, when the energy consumption was expressed as the energy required to reach 20 mg L -1 H 2 O 2 , important differences were observed among the samples.

Indeed, the energy consumption of carbons from Groups III and IV is significantly higher than that of Groups I and II, being the effect more pronounced for samples PCKOH/TP and PCC/TP (both obtained from the same precursor). This behavior is attributed to the longer times needed for carbons from Groups III and IV to produce 20 mg L -1 H 2 O 2 (Fig. 4), whereas shorter times were involved for the calculations at an applied charge of 0.12 Ah L -1 . This points out that other factors may need to be considered for further optimizing the use of these carbons, such as using divided or flow cells to reduce side reactions that decompose H 2 O 2 [START_REF] Pérez | Electrosynthesis of hydrogen peroxide in a filter-press flow cell using graphite felt as air-diffusion cathode[END_REF] or gas diffusion cathodes to reduce O 2 mass-transfer limitations [START_REF] Liao | Efficient hydrogen peroxide production at high current density by air diffusion cathode based on pristine carbon black[END_REF]. Regarding Group II, the energy consumption of CNT-ox/TP is twice larger than that of CNT/TP, revealing the negative impact of the surface functionalization of the carbon cathode (structural parameters remained similar).

Conclusions

We have investigated the dependence of the electrocatalytic activity of carbon cathodes through 2e-ORR to produce hydrogen peroxide, with the characteristics of the carbon materials, by analyzing a pool of 22 carbons with varied physicochemical and structural properties. Electrocatalytic tests have been performed in small (0.2 cm 2 ) and large (4 cm 2 ) electrodes. Our data has shown that the electrocatalytic activity of carbon cathodes to produce hydrogen peroxide is a complex process that requires balanced characteristics in the electrodes to assure good electron transfer and O 2 mass transfer.

While electrical conductivity of the carbons is important, the structural defects play a key role in their electrocatalytic activity, with less impact on composition and porosity.

This dependence is more pronounced when the electrocatalytic tests are carried out in large electrodes and after various hours (ca. 150 minutes). In contrast, when the electrocatalytic tests are run in small electrodes (assays of 2-3 min, typically LSV data reported in the literature), the different performance due to chemical composition of the electrodes is no longer relevant in the overall production rate and faradaic efficiencies.

Thus, it is of paramount importance to perform screening tests in large electrodes to validate the electrocatalytic performance of the cathodes for further process engineering.

Regarding the structure of the carbon materials, two parameters have been identified as where V is the voltage drop (V), I is the current intensity (A), and t is the thickness of the pellets (m). Linear sweep voltammetry (LSV) at a rotating ring-disk electrode (RRDE-3A, ALS, Japan) containing a 3 mm diameter glassy carbon disk surrounded by a Pt ring, in O 2saturated 0.050 M Na 2 SO 4 electrolyte using a VMP potentiostat. The measurements were made in a three-electrode cell that included the RRDE as working electrode, a graphite rod as counter electrode and an Ag/AgCl reference electrode, at a scan rate of 5 mV/s and rotation speeds (ω) between 500-5000 rpm. 20 μL of the inks of the carbon materials (10 mg/mL of carbon powders in 2-methyl propanol and 5 wt.% nafion) were drop-casted on the glassy carbon disk (loading 0.4 mg/cm 2 ). The number of electrons transferred during ORR (n) and the H 2 O 2 yield were determined from the measured disk and ring currents (i D and i R , respectively), considering a collection efficiency (N) of 0.45 and according to the following equations:

n electrons = 4i D /(i D +i R /N) H 2 O 2 yield = 100 * 2i R /(Ni D +i R )
Contact angles θ (°) were measured to determine the hydrophilic and hydrophobic properties of materials and the electrodes (and investigate their wetting behavior).

Measurements were performed based on the sessile drop method, using 2 μl drops of deionised water deposited on the samples with a micropipette in a Digitrop (GBX) instrument. The contact angle was measured after stabilization of the water droplet in contact with the material surface. Averages of three independent measurements were registered for each sample.

Table S1. Elemental analysis (wt.% dry ash basis), electric conductivity and selected textural parameters of the studied carbon-based materials.

Groups

Fig. 1

 1 Fig.1shows a selection of physicochemical properties of the studied carbon materials; namely composition and electric conductivity and a hierarchical cluster analysis of Raman data. The analysis of these properties (see discussion below) allowed us to classify the studied carbons in four groups, according to similarities in the structure, composition and electrical conductivity. This arbitrary classification will serve as a guide for the discussion on the electrocatalytic performance in the following sections. Group I (graphite, graphene, carbon black and graphitized carbon): low H/C and O/C ratios, high conductivity and high structural order; Group II (nanotubes, nanofibers, felt): low O/C and varied H/C ratio, high conductivity and moderate structural order; Group III (nanoporous carbons): low O/C and varied H/C ratio, moderate conductivity and low structural order;

Fig. 1 .

 1 Fig. 1. Van Krevelen diagram for the selected carbon materials; data is shown in (a) linear and (b) logarithmic scale for clarity of comparison; (c) electrical conductivity of the carbon materials measured by the 4-probe method (inset accounts for a magnification of the scale); (d) Dendrogram representation resulting from the hierarchical cluster analysis of the full Raman spectra using Ward's algorithm.

Fig. 2a shows

  Fig. 2a shows the LSV of electrode CB/GC as a representative example of the studied materials; the LSV curves of the complete series of carbons are shown in Fig. S4. As seen, the curve in O 2 -saturated electrolyte showed a marked cathodic wave with an onset potential (E onset ) at ca. -0.6V vs SCE, and a second change in the slope above -1 V vs SCE. The first wave is attributed to the ORR, following either a 4-or a 2-electron transfer mechanism, as indicated in the following competing reactions (stoichiometry shown for atmospheric pressure and acidic pH conditions):

Fig. 2 .

 2 Fig. 2. a) Example of LSV curve of electrode CB/GC in 0.05 M Na 2 SO 4 in He-saturated (blue) and O 2 -saturated (red) conditions; b) current densities from the LSV curves at -0.9 V vs SCE in He-saturated (filled bars) and O 2 -saturated (pattern bars) conditions; c) net current density (iO 2 -iHe) and onset potential values. Working electrode support: glassy carbon (5 mm); counter electrode: graphite rod; reference electrode: saturated calomel (SCE).

Fig. 3 .

 3 Fig. 3. Correlation between a) the ratio A D1 /A G of Raman spectra, b) the FWHM of D band and c) the electrical conductivity vs the net current density of the studied carbons obtained at -0.9 vs SCE. Working electrode: glassy carbon (5 mm); counter electrode: graphite rod; reference electrode: saturated calomel (SCE).

Figure 4 .

 4 Figure 4. Production of H 2 O 2 per mg of carbon material in the series of X/TP electrodes with the applied electric charge in potentiostatic mode (-0.9 V vs SCE) and in 0.05 M Na 2 SO 4 : a) group I; b) group II, c) group III; d) group IV. Line represents the theoretical amount of H 2 O 2 for a 100% faradaic efficiency. Control assays corresponding to Toray Paper and Carbon Felt are included for comparison. Working electrode support: Toray paper; counter electrode: graphite rod; reference electrode: saturated calomel (SCE).

Fig. 5 .

 5 Fig. 5. (a) Faradaic efficiency of H 2 O 2 production at an applied electric charge of 0.12 Ah L -1 using 4 cm 2 electrodes; (b) Faradaic efficiency of H 2 O 2 production (Z axis) at an applied electric charge of 0.12 Ah L -1 with electrical conductivity (Y axis) and specific BET surface area (x axis); (c) Faradaic efficiency of H 2 O 2 production (Z axis) vs A D /A G ratio (Y axis) and electrical conductivity (X axis); (d) Correlation between the FWMH of D band and the faradaic efficiency; (e) Correlation between the ratio A D1 / A G of

Fig. 6 .

 6 Fig. 6. Energy consumption and faradaic efficiency for the production of hydrogen peroxide calculated for the electrodes casted on TP (4 cm 2 ): (a) at an applied charge of 0.12 Ah L -1 ; (b) for the production of 20 mg L -1 of H 2 O 2 . Values corresponding to carbon felt electrodes with and without PTFE coating have been added for comparison purposes.

Figure S1 .

 S1 Figure S1. Raman spectra and deconvolution signals of the studied carbon materials. Spectra have deconvoluted according to Sadezky's 5-band model [2].

Figure S2 .

 S2 Figure S1. (continued)

Figure S3 .PCCFigure S4 .

 S3S4 Figure S3. Comparison of the A D1 /A G ratio obtained from the deconvolution of the Raman spectra of the studied carbon materials.

2 NPBFigure S5 .Figure S6 .Figure S7 .

 2S5S6S7 Figure S5. Linear sweep voltammograms for selected studied carbons in O 2 -saturated 0.050 M Na 2 SO 4 electrolyte using an RRDE, at v scan of 5 mV/s and various rotation speeds (500-5000 rpm). (a) Current of the Pt ring at a E ring of +1.3 V vs Ag/AgCl; (b) current at a glassy carbon disk electrode between 0.2 and -1.3 V vs Ag/AgCl (sweeping towards cathodic values). The catalysts were drop-casted on a 3 mm glassy carbon disk (loading of 0.4 mg/cm 2 ).

Figure S8 .

 S8 Figure S8. Correlation between the onset potential values and a) the A D1 /A G ratio obtained from the deconvolution of Raman spectra; and b) the electrical conductivity of the studied carbons. Working electrode: glassy carbon (5 mm); counter electrode: graphite rod; reference electrode: saturated calomel (SCE).

  shows the evolution of H 2 O 2 over time and versus the applied electric charge for all the studied carbons. Data is expressed in ppm H 2 O 2 (absolute production) and normalized per gram of carbon material in the cathode to facilitate the comparison of the different cathodes. The theoretical concentration of H 2 O 2 corresponding to 100% current efficiency has also been indicated for clarity, as well as control data for the Toray paper support. It should be mentioned that the electric charge passing through the cells is different for each carbon, as this parameter depends on the current density recorded (i.e., electrolysis were carried out in potentiostatic mode).

Table S3 .

 S3 Electrochemical parameters during H 2 O 2 production using a 2 x 2 cm 2 Toray paper (TP) support and catalytic inks of the different carbon materials in a PTFE dispersion (ratio 1:30 wt.%). Counter Electrode: graphite rod; Reference electrode SCE; 0.05 M Na 2 SO 4 electrolyte. Potentiostatic mode. Applied electric charge 0.12 A h L -1 .

		Carbon Material	C	H	N	O	S	Conductivity (S/m)	S BET (m 2 /g)	Vmicropores (cm 3 /g)
	I I Groups Carbon Material FLG 99.8 0.1 0.0 Gra 99.8 0.1 0.0 Intensity (A) time (h) [H 2 O 2 ] (mg/L) 0.1 0.001 146.7 255 0.1 0.1 104.8 9 Efficiency (%) 0.077 Faradaic 0.002 I CB 99.8 0.1 0.0 0.1 0.0 92.1 43 0.017 Bare TP 0.0036 1.67 5.5 7.2
	I I I	NPQgra FLG/TP Gra/TP	99.7 0.01 0.26 0.02 0.012 0.0029	0 0.5 2.01	56 25.8 33.8	25	0.006 73.6 44.4
	II II I II I II II II II III II III II III II III II	CNT CNT-ox CB/TP CNF NPQgra/TP HSU1000 CNT/TP CFelt CNT-ox/TP NPQ CNF/TP NPBH HSU1000/TP NPBKK CFelt NPPW CFelt (PTFE coated) 98.4 1.3 0.0 92.3 2.5 0.0 0.01 96.8 1.6 0.3 0.03 94.6 2.1 0.1 0.014 95.6 1.6 --0.006 96.6 0.6 0.7 0.01 93.1 0.9 0.1 0.005 94.0 0.3 0.8 0.02 99.2 0.3 0.0 0.018	0.3 5.2 1.3 3.1 2.8 2.1 6.0 4.7 0.6	0.0 0.0 0.60 0.0 0.20 0.1 0.43 --1.00 0.0 0.6 0.0 1.00 0.2 0.3 0.0 0.15	16.2 2.56 50.4 8.9 45.5 4.97 62.2 3.5 43.3 0.53 53.8 0.85 42.3 3.33 3.8 3.80 11.2	292 284 154 393 33 1033 1040 961 1357	0.068 66.2 0.106 59.8 0.057 81.7 0.155 56.9 0.007 70.7 0.316 66.7 0.272 5.3 0.292 0.453 32.7
	III III NPQ/TP NPcomp	97.8 0.8 0.0 0.007	1.4	0.0 0.86	10.43 44.2	330	0.112 58.1
	III III III NPBH/TP PCKOH PCC IV GO III NPBKK/TP	95.0 0.6 0.9 94.5 1.1 2.8 0.0088 51.6 4.2 2.6 40.3 3.3 0.7 0.01	0.1 0.9 0.57 --0.60	0.84 8.33 30.7 b.d.l. 43.3	577 2 9	0.228 48.5 0.001 0.001 56.9
	IV IV III NPPW/TP rGO NPB	85.3 1.3 1.1 12.2 84.7 2.6 0.1 12.7 0.006	--0.0 1.00	1.2 0.005 33.3	29 1280	0.007 0.314 43.8
	IV III NPcomp/TP HSU	70.5 4.6 0.1 24.8 0.007	0.1 0.86	b.d.l. 32.4	5	0.002 42.6
	IV III PCKOH/TP HSU250	77.7 3.6 0.1 18.6 0.045	0.1 1.33	b.d.l. 15.4	16	0.004 43.3
	IV III PCC/TP HSU500	85.5 3.1 0.1 11.2 0.045	0.1 1.33	b.d.l. 23.0	354	0.131 27.0
	b.d.l. below detection limits IV GO/TP		0.004		1.50	34.9		45.9
	IV rGO/TP			0.005		1.20	30.8		40.4
	IV NPB/TP			0.007		0.60	44.0		56.8
	IV HSU/TP			0.0033		1.80	0.028		36.8
	IV HSU250/TP			0.003		2.00	37.4		49.2
	IV HSU500/TP			0.003		2.00	31.3		41.1

Acknowledgements M.Muñoz-Morales and J.Llanos wish to express their gratitude to Junta de Comunidades de Castilla-La Mancha, Diputación de Albacete and UCLM for the funding (grants SBPLY/21/180501/000058 and DIPUAB-2022 MUÑOZMORALES respectively). Conchi Ania thanks the funding from the European Union's Horizon 2020 research and innovation program (grant agreement 776816) and Region Centre Val de Loire (PRESERVE, grant EX010666). The authors acknowledge Dr. Vautrin-Ul and Mme. Jebali from ICMN (Orléans) for the access to contact angle and RRDE measurements.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary data

Supplementary data to this article is available at ..............

References Appendix A. Supplementary data

Evaluating key properties of carbon materials as cathodes for the electro-generation of hydrogen peroxide M. Muñoz-Morales 1,2 *, A. Ramírez 2 , Aurelien Cañizares 1 , J. Llanos 2 , Conchi Ania

Supplementary File

Characterization techniques

The porosity of the materials was evaluated by N 2 adsorption isotherms at -196 °C in automatic volumetric analyzer from Micromeritics. The samples were initially degassed under vacuum at 120 °C for 17 h. The nitrogen adsorption isotherms were used to calculate the specific surface area (S BET ) and micropore volume using the Dubinin-Radushkevich equation [1]. Each isotherm was recorded in duplicate on fresh sample aliquots (error was below 2 %). Elemental analysis of samples dried at 100 °C was measured in a LECO CHNS-932 (C, H, N, S) and a LECO-VTF-900 (O) analyzers. Raman spectra were recorded in ambient conditions in a Renishaw InVia Qontor spectrometer equipped with 514.5 nm laser. The spectra were collected under a Leica DM2500 optical microscope with a x50 long working distance objective (ca. 10 mm). The scattered Raman light was dispersed by a holographic grating of 600 grooves/mm, in order to acquire the whole range of interest for carbons (500-5000 cm -1 ). Each spectrum was recorded with an integration time of 5 s; data presented represents the average of three measurements. Deconvolution of Raman spectra was carried out by a fitting procedure using a linear baseline and Gaussian/Lorentzian functions, and the assignment of the peaks according to the ligature [2]. The electrical conductivity of the samples was measured using a four-point probe method following the general principles of ASTM standard methods D4496-87 [3]. Briefly, disk-shaped pellets of the samples (ca. 90 wt.% of carbon powders, 10 wt.% polyvinylidene fluoride binder) were prepared by compaction of the powders under 5 tons pressure. The diameter of the pellets was ca. 10 mm and their thickness varied between 0.10-0.16 mm (total weights between 8-12 mg). The resistance of the pellets was measured at room temperature and atmospheric pressure using a Keithley 2400 multimeter coupled with a 4 probes Everbeing SR-4 module to make the electrical contact, using the van der Pauw technique. It was followed the general principles of ASTM standard methods D4496-87 [4]. Briefly, a constant current (between 1-10 mA) was applied to the surface of the pellets through the probes, and the voltage drop was

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