An oxygen reduction reaction electrocatalyst tuned for hydrogen peroxide generation based on a pseudo-graphite doped with graphitic nitrogen https://dx.doi.org/10.5599/jese.1407 1009 J. Electrochem. Sci. Eng. 12(5) (2022) 1009-1023; https://dx.doi.org/10.5599/jese.1407 Open Access : : ISSN 1847-9286 www.jESE-online.org Original scientific paper An oxygen reduction reaction electrocatalyst tuned for hydrogen peroxide generation based on a pseudo-graphite doped with graphitic nitrogen Kailash Hamal1, Dipak Koirala1, Jeremy May1, Forrest Dalbec1, Nolan Nicholas2 and I. Francis Cheng1, 1Department of Chemistry, University of Idaho, 875 Perimeter Dr, MS 2343, Moscow, ID 83844, USA 2Nano Lab, 22 Bedford St. Waltham, MA, 02453, USA Corresponding author: ifcheng@uidaho.edu; Tel: (208) 885-6387 Received: June 14, 2022; Accepted: August 26, 2022; Published: September 5, 2022 Abstract The carbon material, GUITAR (pseudo-graphite from the University of Idaho thermolyzed asphalt reaction) can be doped with nitrogen in two prevalent forms. In a previous study N(py)-GUITAR had a predominance of pyridinic and pyrrolic moieties with no graphitic nitrogen. In this study N(g)-GUITAR contains a 9.7 % N atomic abundance, with that fraction consisting of 72.3 % graphitic, 23.7 % pyridinic, and 0 % pyrrolic nitrogen. The two materials allow for the examination of hypotheses regarding the importance of the three different nitrogen moieties in the oxygen reduction reaction (ORR). In the previous investigation, the lack of graphitic nitrogen of N(py)-GUITAR gave a preferred pathway of 4e- ORR to H2O. In this investigation, N(g)-GUITAR gave a 2e- ORR pathway to H2O2. This was elucidated by current efficiency and hydrodynamic voltammetry studies. The high predominance of graphitic nitrogen confirms the hypothesis regarding 2e- vs. 4e- ORR pathways with N-doped carbon materials. N(g)-GUITAR was also evaluated for parasitic pathways for H2O2 production. At -0.95 V vs. Ag/AgCl the combination of current efficiency for H2O2 is 96 % in 0.05 M Na2SO4 with a production rate of 4.9 mg cm-2 h-1, is the highest reported in the literature. This indicates possibilities for water purification and treatment applications, which require 10 to 250 mg L-1, depending on conditions. Keywords Hydrogen peroxide; oxygen reduction reaction; electrocatalysis; N-doped carbon Introduction Hydrogen peroxide (H2O2) is a relatively eco-friendly and widely used oxidant in the water purification, chemical, and medical industries [1,2]. It is a greener oxidizing agent than chlorine in that it leaves no halogenated by-products such as trihalomethanes and haloacetic acids and is https://dx.doi.org/10.5599/jese.1407 https://dx.doi.org/10.5599/jese.1407 http://www.jese-online.org/ mailto:ifcheng@uidaho.edu J. Electrochem. Sci. Eng. 12(5) (2022) 1009-1023 OXYGEN REDUCTION REACTION ELECTROCATALYST 1010 effective over a wider pH range [3,4]. A partial barrier to wider implementation is that the predominant method of H2O2 production, the anthraquinone process, is highly energy-intensive and requires large quantities of extraction solvents [2,3]. Furthermore, transportation issues also hinder its widespread application in water treatment. Onsite generation of H2O2 through the electro- chemical process offers minimization of environmental impact as it can be conducted under aqueous and ambient conditions with no hazardous materials or waste [5-7]. This process proceeds by a 2e- electrochemical process outlined in Equation 1. O2 + 2H+ + 2e- → H2O2 E0 = 0.695 V, E0’ (pH 7) = 0.282 V (1) The challenges in the electrochemical production of H2O2 are improving slow kinetics and minimizing competing reactions. Slow electrode kinetics gives significant overpotentials for Equation 1 that require electrode potentials from -0.5 to -1.5 V vs. Ag/AgCl [8,9]. At these potentials, three other parasitic electrochemical reactions are possible. These include the 4e- oxygen reduction reactions (ORR) to water (Equation 2), the hydrogen evolution reaction (HER), (Equation 3), and the 2e- reduction of H2O2 to H2O (Equation 4) [8-10]. O2 + 4H+ + 4e- → 2H2O E0 = 1.23 V, E0’ (pH 7) = 0.817 V (2) 2H+ + 2e- → H2 E0 = 0.00, E0’ (pH 7) = -0.413 V (3) H2O2 + 2H+ + 2e- → 2H2O E0 = 1.763 V, E0’ (pH 7) = 1.35 V (4) It is therefore incumbent that any electrocatalyst for equation 1 must be selective for this process. This has been noted by several investigators [9,11]. For this contribution, we demonstrate an electrocatalyst that is preferential for the reaction in Equation 1 based on GUITAR (graphite from the University of Idaho thermolyzed asphalt reaction). This carbon material consists of an 85/15 mole ratio of sp2/sp3 carbon with a crystallite grain size of 1.5 nm [12,13]. The interlayer d-spacing of 350 pm is slightly expanded over graphite’s 335 pm. It is a pseudo-graphite in that it has visual and microscopic features that resemble graphite but has differing properties. Distinguishing characteristics over graphite and other carbon allotropes is that GUITAR has fast heterogeneous electron transfer (HET) kinetics with the Fe(CN)64-/3- probe at the basal and edge planes while maintaining excellent corrosion resistance [12-14]. The 3-volt aqueous potential window is among the largest reported. GUITAR is grown using a chemical vapor deposition process, which allows for the fabrication of electrodes ranging from the nano- to macro-sizes, depending on the substrate [12,15,16]. A significant consideration is the tunability of the electrocatalyst for performance and selectively between the reactions of Equations 1-4. Nitrogen doping of carbon materials is associated with the enhancement of electrocatalysis for ORR [17,18]. The C-N bond is believed to facilitate the adsorption of O2 [19,20]. Predominate forms of N in sp2 carbon lattices are pyridinic, pyrrolic, N- oxides, and graphitic moieties shown in Figure 1. Figure 1. Nitrogen moieties in the graphite lattice K. Hamal et al. J. Electrochem. Sci. Eng. 12(5) (2022) 1009-1023 https://dx.doi.org/10.5599/jese.1407 1011 In computational studies, graphitic N is hypothesized to drive the 2e- process of Equation 1 [21]. However, experimental investigations have only been conducted on mixed nitrogen moiety carbon materials, therefore definitive experimental evidence has yet to be gathered [21-23]. Pyridinic and pyrrolic moieties have been associated with the 4e- transfer ORR (Equation 2) useful in fuel cell applications [18]. The N-oxide moiety is not associated with any specific ORR pathway. In a previous investigation, a specific nitrogen doping method led to a GUITAR variant with a 0.9 % total nitrogen atomic abundance giving fractions of 0 % graphitic N, 46.0 % pyridinic, 41.3 % pyrrolic, and 12.4 % N-oxide. This electrocatalyst, N(py)-GUITAR, proved to be durable and efficient for the 4e- transfer ORR to H2O [15]. An alternative doping method produced a nitrogen-doped pseudo-graphite which contains 9.7 % N [13]. This material, called N(g)-GUITAR, contains graphitic N as the predominant nitrogen moiety [13]. It is noteworthy that GUITAR is the only known carbon material that can be selectively synthesized for predominantly pyridinic/pyrrolic or with graphitic nitrogen moieties. This provides an opportunity to examine the role of these functionalities in the pathway outlined in Equations 1 and 2. Experimental Materials and chemicals KFD graphite felt (SLG Carbon Company, St. Marys, PA, USA) was used as received. The reagents in this investigation were compressed Ar(g) and O2(g) (>99.5 %, Oxarc, WA, USA), soybean oil (Walmart), paraffin (Gulf Wax), sulfuric acid (96.3 %, J.T Baker Chemical Co, Phillipsburg, NJ, USA). Potassium chloride was obtained from Fisher Scientific (Waltham, NJ, USA), and potassium ferri- cyanide was obtained from Acros Organics (Morris Plains, NJ, USA). Sodium sulfate (EMD Chemicals, Germany), 30 % H2O2 (Fisher Scientific, Belgium), and ethanol (99.5 %, Pharmaco, CT, USA) were used without further purification. All aqueous solutions were prepared with deionized water passed through an activated carbon purification cartridge (Barnstead, model D8922, Dubuque, IA). Electrode fabrication and electrochemical setup N(g)-GUITAR samples were synthesized using the method described in a previous study [13]. In short, acetonitrile vapors were mixed with N2 gas and directed into a muffled tube furnace (heated to 900 °C) containing the substrate, for 25 minutes. Quartz wafers, KFD graphite felts, and Ketjen black were used as substrates. Working electrode areas were isolated, as described in the previous study [16]. This was conducted by covering the copper alligator clip to the potentiostat with paraffin wax so as to avoid corrosion of this ohmic contact. Complete wetting of the electrode was conducted by washing with isopropanol followed by deionized water prior to use [24]. All electrochemical studies were conducted in a three-electrode setup cell with Ag/AgCl/3M KCl reference electrode using either a Bioanalytical System CV-50 W (West Lafayette, IN, USA) or a Gamry Instruments Interface 1000 potentiostat. The cell design is shown in Figure 2. The cell consisted of anodic and cathodic chambers with a separator membrane composed of cellulose acetate on cellulose paper [25]. This design allows for the formation of H2O2 via Reaction 1 without consumption at the anode through Reaction 4. Hydrodynamic experiments were carried out with a Pine Instrument glassy carbon rotating electrode (RDE) (Grove City, PA, USA). For that study, N(g)-GUITAR was deposited on Ketjen black particles and drop-cast onto the RDE disks. This follows a procedure developed in a previous investigation [15]. https://dx.doi.org/10.5599/jese.1407 J. Electrochem. Sci. Eng. 12(5) (2022) 1009-1023 OXYGEN REDUCTION REACTION ELECTROCATALYST 1012 Figure 2. Electrochemical cell design for H2O2 generation H2O2 analysis This was conducted by a colorimetric method [26]. Potassium titanium oxide oxalate dihydrate, (Sigma-Aldrich) forms an aqueous complex with H2O2, which has an optical absorbance maximum at 400 nm. A Beer’s law-based calibration curve was established with H2O2 (Pharmaco, CT, USA) stan- dardized with KMnO4 (J.T. Baker Chemical Co, Phillipsburg, NJ, USA) and sodium oxalate (Fisher Scientific, Waltham, NJ, USA). Results and discussion Material characterization summary This material was fully characterized in a previous study [13]. In short, the X-ray photoelectron spectrograph (XPS) of N(g)-GUITAR flakes indicated nitrogen, oxygen, and carbon content of 9.7, 3.9 and 86.4 %, respectively (see Table 1). The deconvolved nitrogen XPS peak revealed 23.7 % pyridinic N (398.2 eV), 60.5 % graphitic N-center (400.98 eV), 11.8 % graphitic N-valley (403.0 eV), and 4 % N- oxides (405.3 eV). This material was notably devoid of pyrrolic nitrogen. Raman spectrographic analysis of N(g)-GUITAR exhibited an ID/IG ratio of 1.66, indicating more defects from the inclusion of nitrogen relative to pristine GUITAR (ID/IG of 1.15). The XRD analysis of N(g)-GUITAR indicated a similar degree of nano-crystallinity as pristine GUITAR (3.3 nm vs. 2.9 nm grain size) [13]. The composition of N(py)-GUITAR is also described in Table 1. Most noticeable is the lack of graphitic N in this electrocatalyst. Table 1. Content of total atomic nitrogen and deconvolved nitrogen moiety of N(py)-GUITAR and N(g)-GUITAR, determined via XPS Content of total atomic nitrogen, % Deconvolved nitrogen moiety, % Graphitic Pyrrolic Pyridinic N-oxides Reference N(py)-GUITAR 0.9 0.0 41.3 46.0 12.4 [15] N(g)-GUITAR (Used in this study) 9.7 72.3 0.0 23.7 4.0 [13] Formation of the electrocatalyst and surface area The N(g)-GUITAR electrocatalyst was grown onto a KFD graphite felt substrate using chemical vapor deposition (CVD), as described in a previous study [13]. SEM micrographs of bare KFD fiber and N(g)-GUITAR/KFD are shown in Figure 3. The fiber diameter of KFD (7.4 ± 0.9 µm (n=10)) is within K. Hamal et al. J. Electrochem. Sci. Eng. 12(5) (2022) 1009-1023 https://dx.doi.org/10.5599/jese.1407 1013 the range reported in the literature [16,27]. Deposition of N(g)-GUITAR increases this diameter to 10.04 ± 0.9 µm (n = 10). The surface area of 62.2 cm2 per cm2 of geometric area for N(g)-GUITAR/KFD was calculated assuming the electrode comprises cylindrical fibers [16,24]. This value agrees with cyclic voltammetric (CV) studies of Fe(CN)64-/3- using the Randles-Ševčik equation (Equation 5) below, where ip, n, A, C (10-6 mol cm-3), D (7.26×10−6 cm2 s-1), and v are peak current, number of electrons transferred, area, concentration, diffusion coefficient and potential sweep rate, respectively [16]. ip = 268,600 n3/2AC (Dv)1/2 (5) Figure 4A shows the overlayed CV plots at multiple scan rates. A plot of ip vs. v1/2 in Figure 4B indicates semi-infinite linear diffusion with the slope indicating an electrochemically active surface area (ECSA) of 59.0 cm2 per cm2 of geometric area. ECSA for GUITAR/KFD and bare KFD are 50.6 and 61.5 cm2 per cm2 of geometric area, respectively [16]. Figure 3. A) Scanning electron micrographs (SEM) micrographs of as obtained KFD felt. B) N(g)-GUITAR coated KFD. The average diameter KFD fiber is 7.4± 0.9 µm (n = 10) and for N(g)-GUITAR/KFD is 10.0 ± 0.7 µm (n = 10). These measurements were estimated with ImageJ software Potential, mV vs. Ag/AgCl v1/2 / (V / s)1/2 Figure 4. A) Cyclic voltammograms (CV) obtained at various scan rates of 1 mM Fe(CN)6 3-/4- in 1 M KCl. (B) Cathodic peak current (ip) vs. square root of scan rate for the estimation of the electrochemically active surface area (ECSA) from the Randles-Ševčik equation. The best fit line indicates semi-infinite linear diffusion Cyclic voltammetric studies The electrocatalysts were examined for ORR activity by CV in 0.05 M Na2SO4 at 50 mV s-1 under O2 and Ar purged conditions, as shown in Figure 5. Under O2 purge, GUITAR/KFD exhibits two peak potentials (Ep’s) of -0.65 and -1.10 V. This electrolyte is a wastewater surrogate often used in lite- rature [28,29]. Based on literature, this is an indication of the 2e- process ORR through equation (1) https://dx.doi.org/10.5599/jese.1407 J. Electrochem. Sci. Eng. 12(5) (2022) 1009-1023 OXYGEN REDUCTION REACTION ELECTROCATALYST 1014 and the subsequent consumption of H2O2 through the reaction of Equation 4 [30-32]. In the case of N(g)-GUITAR/KFD, there is only a single Ep at -0.77 V, which qualitatively indicates 2e- transfer giving H2O2 in the ORR of Equation (1), without consumption of H2O2 through Equation 4, This will be examined in detail below. Also notable is the relatively large peak current (ip) of 7.0 mA cm-2at 50 mV s-1, which is the highest reported in the literature (0.7- 4.5 mA cm-2) [30-33]. The ip based on ECSA is 0.12 mA cm-2. The CVs for both electrode materials under Ar purge indicate an overpotential of about 1 V for the HER (Equation 3). Figure 5. Cyclic voltammograms at 50 mV/s in 0.05 M Na2SO4 under O2 and Ar saturated conditions for N(g)- GUITAR/KFD and KFD electrodes. The peak potentials (Ep) for GUITAR/KFD are -0.65 and -1.10 V and for N(g)-GUITAR is -0.77 V. The hydrogen evolution reaction (HER) for the scans is also indicated H2O2 production is the major pathway for the N(g)-GUITAR electrocatalyst The electrocatalyst was evaluated for generation of H2O2 by controlled potential electrolysis (CPE) at potentials ranging from -0.35 to -1.50 V vs. Ag/AgCl for 30 min in O2 saturated 0.05 M Na2SO4(aq). The saturation is ensured by bubbling O2 across the electrode surface, which also aids in mass transport. The N(g)-GUITAR cathode ECSA is 260 cm2, corresponding to a geometric area of 4.4 cm2. Control electrodes of the same geometric areas include GUITAR/KFD (ESCA 223 cm2) and KFD (ECSA 270 cm2). Selected current-time profiles with the N(g)-GUITAR/KFD cathode are shown in Figure 6A. All chronoamperograms remained at a steady state during the electrolysis for each potential. This is an indication of constant mass transport of O2 to the electrode and stable electrocatalyst performance. Figure 6C shows the time profile for H2O2 concentration at the applied potential of -1.50 V for N(g)-GUITAR/KFD. Data was obtained by taking 0.5 ml aliquots every 5 min for H2O2 analysis. This system obeys zero-order kinetics with the observed linear increase in H2O2 concentration in Figure 6C. These results show the typical behavior in comparison to other electrochemical H2O2 generation studies in the literature [34,35]. The performance of the control electrodes, GUITAR/KFD and KFD, are also shown in Figure 6C. As can be observed through the slopes, the N-doped material is a much more efficient electrocatalyst than either of the control electrodes. Figure 6B illustrates the effect of driving potential on the rates of H2O2 production (µmol cm-2 h-1). The N(g)-GUITAR/KFD electrode increased the H2O2 generation rate proportionally with driving potential from the range -0.35 to -1.50 V. Both control electrodes decreased in production rate beyond -0.95 V. It is surprising that GUITAR/KFD is much less efficient than bare KFD. This may be from the consumption of H2O2 K. Hamal et al. J. Electrochem. Sci. Eng. 12(5) (2022) 1009-1023 https://dx.doi.org/10.5599/jese.1407 1015 through Reaction 4 and may be observed in the 2nd CV wave at Ep -1.1 V for GUITAR/KFD in Figure 5. Further evidence is presented in the current efficiency studies below. Figure 6. (A) Current-time curves recorded in O2 saturated 0.05 M Na2SO4 at various constant electrode potentials. (B) Amount of H2O2 generated during constant potential analysis and normalized for geometric area. (C) Concentration of measured H2O2 with time. (D) Plot of current efficiency for H2O2 production at different applied potential Under O2 purge, the controlled potential electrolysis gave pH shifts from pH 7.0 to approximately pH 12. The final pH is expected from the reactions of Equations 1, 2 and 4. A Pourbaix diagram in Figure 7 illustrates that this shift in pH does not affect the spontaneity of reactions electrode potentials from -0.35 V to -1.50 V vs. Ag/AgCl. The current efficiency (CEH₂O₂/O₂) for H2O2 production was examined for each of three electrode systems of this study. This is described in Equation 6 where n is a number of electrons transferred in the balanced half-reaction, F is Faraday’s constant, V is a volume of the cell, and i is current [36]. 2 2 2 2 2 H O H O /O 0 100 d t nFVc CE i t =  (6) Figure 6D demonstrates that N(g)-GUITAR/KFD maintains excellent CE H₂O₂/O₂ at all potentials. At the highest driving potential of -1.5 V, it has a CE H₂O₂/O₂ of 88 %, at -1.2 V it is 95 % (also see Table 2, Column D). The CE H₂O₂/O₂ for both KFD and GUITAR/KFD are notably lower, at 38 % and 29 %, respectively at -1.5 V vs. Ag/AgCl. This shows that both control electrodes increase H2O2 production rates from -0.35 to 0.95 V with noticeable drops at higher driving potentials. This may be from the consumption of H2O2 via Reaction 4 and the parasitic HER. These issues do not affect N(g)-GUITAR/KFD performance, as the H2O2 production rate increases with increasing cathodic potential. https://dx.doi.org/10.5599/jese.1407 J. Electrochem. Sci. Eng. 12(5) (2022) 1009-1023 OXYGEN REDUCTION REACTION ELECTROCATALYST 1016 Figure 7. Pourbaix diagram for the reactions in Equations 1-4. The shaded region are the pH and potential conditions of this study Figure 6D demonstrates that N(g)-GUITAR/KFD maintains excellent CEH₂O₂/O₂ at all potentials. At the highest driving potential of -1.5 V, it has a CEH₂O₂/O₂ of 88 %, at -1.2 V it is 95 % (also see Table 2, Column D). The CEH₂O₂/O₂ for both KFD and GUITAR/KFD are notably lower, at 38 and 29 %, respecttively at - 1.5 V vs. Ag/AgCl. This shows that both control electrodes increase H2O2 production rates from -0.35 to 0.95 V with noticeable drops at higher driving potentials. This may be from the consumption of H2O2 via Reaction 4 and the parasitic HER. These issues do not affect N(g)-GUITAR/KFD performance, as the H2O2 production rate increases with increasing cathodic potential. In general, N(g)-GUITAR operates at a higher CE relative to literature electrocatalysts. Figure 8 illustrates the competitiveness of this electrocatalyst at the various applied potentials relative to the literature on non-metal electrocatalysts. Also included are the control surfaces of GUITAR and bare KFD at their maximum CE and production rate. Figure 8. Plot of H2O2 production rate and current efficiency for the N(g)-GUITAR ORR electrocatalyst. The applied potentials are indicated. The highest rates of H2O2 production for GUITAR and KFD are included along with literature electrocatalysts [34,35,37-49] K. Hamal et al. J. Electrochem. Sci. Eng. 12(5) (2022) 1009-1023 https://dx.doi.org/10.5599/jese.1407 1017 This N-doped pseudo-graphite electrode operates at 95 to 100 % CE H₂O₂/O₂ at potentials ranging from -0.35 to -1.2 V (Figure 6D), while literature electrodes are at 45 to 95 % at similar potentials at lower H2O2 production rates [34,35,37-39]. This gives N(g)-GUITAR the ability to effectively operate at higher driving potentials without a loss in CE. At the highest potential of this study, -1.50 V vs. Ag/AgCl, this electrocatalyst operates at 88 % CEH₂O₂/O₂ with a production rate of 250 µmol h-1 cm-2 (8.5 mg h-1 cm-2). This is the highest CE reported for this relatively high rate. Higher production rates have been reported in the literature, but they typically come at the cost of a lower CEH₂O₂/O₂ [38]. Considering these performance metrics (CEH₂O₂/O₂ and H2O2 production rate), N(g)-GUITAR has the best combination of high current efficiency and H2O2 production rate [34,35,37,38]. The hydrogen evolution reaction is a minor pathway for the electrocatalyst The high CEH₂O₂/O₂ indicates that the major pathway for the electrocatalyst is through Reaction 1. The extent of the HER as a competing pathway is examined under the conditions for the 2e- ORR. Previous studies of GUITAR and its variants showed that these materials are kinetically slow for HER. Generally, the overpotential is 1 to 1.5 V and is among the highest of all electrode materials [12,13]. The N(g)-GUITAR/KFD cathode was examined for the HER by chronoamperometry in Ar-purged 0.05 M Na2SO4 at -0.95, -1.20 and -1.50 V vs. Ag/AgCl. Figure 9 shows the respective chronoampero- grams for this study. In the absence of O2 the charge from those currents is proportional to total H2 production. These charges are summarized in Table 2, Column A. When compared the charge obtained under saturated O2 conditions in Figure 6A and summarized in Table 2 Column B, the HER is a minor component of overall cathodic current ranging from 0.2 % at -0.95 V to 7.8 % at -1.5 V (Table 2 Column C). Figure 9. Chronoamperograms recorded in Ar purged 0.05M Na2SO4. The potentials and steady-state currents for the N(g)-GUITAR/KFD electrodes are described in the diagram Assessment of the contribution of the 4e- ORR pathway to the overall cathodic current: rotating disk electrode studies The contribution of the 4e- ORR (Equation 2) to the overall cathodic current was estimated through the measurement of n, the number of electrons transferred in the half-reaction. Contributions to ORR through the reaction in Equation 2 occur with n > 2. To assess this charac- teristic, rotating disk electrode (RDE) experiments were conducted. The value, n is obtained through the Koutecky-Levich (K-L) relationship in Equation 7. Symbols J, Jk, and ω are the current density, https://dx.doi.org/10.5599/jese.1407 J. Electrochem. Sci. Eng. 12(5) (2022) 1009-1023 OXYGEN REDUCTION REACTION ELECTROCATALYST 1018 kinetic current density, and angular velocity, respectively. In the K-L equation, B is 0.62nFν1/6 cO₂DO₂ 2/3, where F is Faraday’s constant, ν is the kinematic viscosity of the electrolyte (0.01 cm2 s-1), cO₂ is the bulk concentration of oxygen (1.20×10 -6 mol cm-3) and DO₂ is the diffusion coefficient of O2 in 0.050 M Na2SO4 (1.90× 10-5 cm2 s-1) [15,36]. 1/2 k 1 1 1 J J B = + (7) For the RDE study, the graphitic nitrogen-rich N(g)-GUITAR electrocatalysts were deposited on Ketjen black particles as described in the experimental and previous studies [15]. In previous investigations, pristine GUITAR had n = 2.6 to 2.8 and a variant of an N-doped GUITAR (N´-GUITAR) which had a predominance of pyridinic and pyrrolic moieties, was found to have n = 3.6 to 3.7 [15]. The background corrected RDE linear sweep voltammograms (5 mV s-1) from 400 to 2400 rpm are shown in Figure 10A. Between the potential range of -0.7 to -1.5 V vs. Ag/AgCl, this electrocatalyst was found to have n = 1.99 (see Figure 10B). This, along with CE data of Section 3.3, strongly indicate that this electrocatalyst has a strong preference for Reaction in Equation 1, with Reaction 2 being a negligible pathway. Figure 10. A) Rotating disk electrode (RDE) linear sweep voltammograms recorded at different rotating rates in 0.05 M Na2SO4. B) The number of electrons transferred from the Koutecky-Levich equation over the potential range of -0.7 to -1.5 V. The average electrons transferred (n) over the entire potential range with one standard deviation is shown Other pathways that lead to the loss of H2O2 The pathways that lead to the loss of H2O2 produced through the 2e- ORR are the electroreduction of H2O2 giving H2O (Equation 4) and disproportionation through the reaction in Equation 8 [10]. 2H2O2 → O2 + 2H2O (8) The two reactions cannot be distinguished from one another and therefore are treated together. Equation 2, the 4e- ORR, can also be considered in this analysis. The contribution of these three pathways that lead to the loss of H2O2 outlined in Equations 2, 4, and 8 can be inferred from Equation 9, and summarized in Table 2, Column E. Pathways for H2O2 consumption, % = 100 – CEH₂ – CEH₂O₂/O₂ (9) The three values in Column E are not statistically different from each other, it is possible that the disappearance of H2O2 is invariant to the applied potentials. K. Hamal et al. J. Electrochem. Sci. Eng. 12(5) (2022) 1009-1023 https://dx.doi.org/10.5599/jese.1407 1019 Table 2. Breakdown of major pathways during oxygen electrolysis in 0.05 M Na2SO4 (aq.) for 30 minutes with n = 3. One standard deviation interval is included Potential, V vs. Ag/AgCl A B C D E Charge obtained under Ar purge (Fig, 6), C Charge obtained under O2 purge (Fig. 5A), C Current efficiency for H2 production via Equation 3, % Current efficiency for H2O2 production via Equation 6, % Pathway for H2O2 degradation pathways (Equations 2, 4 and 7), % -0.95 0.17 ± 0.04 87.4 ± 5.3 0.2 ± 0.06 96 .0 ± 1.0 3.8 ± 1.0 -1.20 1.42 ± 0.10 100.0 ± 4.5 1.4 ± 0.2 95.0 ± 2.0 3.6 ± 2.0 -1.50 10.5 ± 0.5 134.0 ± 3.6 7.8 ± 0.5 87.8 ± 1.8 4.4 ± 1.8 The N(g)-GUITAR electrocatalyst is stable The electrocatalyst stability is demonstrated in the sequence of experiments outlined in Figure 11. The electrode was subjected to a constant potential of -1.50 V for five, 30-minute cycles under O2 purged conditions. The H2O2 production rate (Figure 11A) and current efficiencies (Figure 11B) indicate no degradation in catalyst performance. Figure 11. Stress tests of the N(g)-GUITAR electrode for H2O2 production. Average values and one standard deviation are indicated on each graph. A) Rates of hydrogen peroxide generation at -1.5 V of applied potential in 0.05 M Na2SO4. B) Current efficiencies as calculated using Equation 6 Summary and conclusions The ability to selectively dope GUITAR with different nitrogen moieties is unique to this pseudo- graphitic material. Our previous form of nitrogen-doped material, N(py)-GUITAR, contains 0.9 % nitrogen by atomic abundance with the division in those species as 0.0 % graphitic-N, 46.0 % pyridinic, 41.6 % pyrrolic, and 12.4 % N-oxide [15]. That electrocatalyst gave an ORR pathway through a 4e- mechanism giving H2O (Equation 2). In this study, another form of nitrogen-doped pseudo-graphite, N(g)-GUITAR, has an atomic nitrogen abundance of 9.7 %. Those N species have a division of 72.3 % graphitic N, 23.7 % pyridinic, 0.0 % pyrrolic, and 4.0 % N-oxide [13]. Based on the rotating disk electrode and chronoamperometric studies, N(g)-GUITAR shows a strong preference for the 2e- route to H2O2 production (Equation 1). This result agrees with the literature and indicates the importance of graphitic N for this pathway [21,23]. While N(g)-GUITAR has a relatively high abundance of pyridinic nitrogen (23.7 %), it was observed that this moiety did not significantly contribute to the 4e- pathway. This runs contra to literature hypotheses [18]. However, this result must be viewed in the context that GUITAR is a pseudo-graphite with a plethora of structural defects. The preference for the 2e- ORR pathway along with significant overpotentials for the HER (Equation 3) and H2O2 consumption (Equation 4) gives an electrocatalyst with the highest combination of H2O2 production and current efficiency in literature. 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