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. 2020 Sep 11;5(37):23578–23587. doi: 10.1021/acsomega.0c01974

Highly Active Wood-Derived Nitrogen-Doped Carbon Catalyst for the Oxygen Reduction Reaction

Kätlin Kaare †,○, Eric Yu ‡, Aleksandrs Volperts §, Galina Dobele §, Aivars Zhurinsh §, Alexander Dyck ∥, Gediminas Niaura ⊥, Loreta Tamasauskaite-Tamasiunaite ⊥, Eugenijus Norkus ⊥, Mindaugas Andrulevičius #, Mati Danilson ∇, Ivar Kruusenberg ○,*
PMCID: PMC7512441  PMID: 32984677

Abstract

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In this recent decade, great interest has risen to develop metal-free and cheap, biomass-derived electrocatalysts for oxygen reduction reaction (ORR). Herein, we report a facile strategy to synthesize an electrochemically active nanocarbon material from the renewable and biological resource, wood biomass. The ORR activity of the catalyst material was investigated in 0.1 M KOH solution by employing the rotating disc electrode method. Scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy were employed to obtain more information about the catalyst material’s morphology and composition. The material exhibits outstanding electrocatalytic activity with low onset potential and high current density, similar to that of a commercial Pt/C catalyst in an alkaline medium. The results clearly ascertain that wooden biomass can be easily transformed into novel carbon nanostructures with superior ORR activity and possibility to be used in fuel cells and metal–air batteries.

Introduction

The number of studies dedicated to renewable energy conversion and storage devices, such as batteries, fuel cells, and photovoltaic systems has gained utmost importance as pollution levels caused by large scale consumption of fossil fuels have continued to rise year by year.1 The fuel cell is considered one of the most significant modern breakthroughs in the energy section because of its high efficiency,2 grid independency, and longer operating times in comparison to the battery technologies.3 Despite the many research efforts made to improve performance, efficiency, and durability, large-scale commercialization of fuel cells still remains problematic.4,5 One major concern is the high price of widely used platinum-based catalysts.6 These catalysts also suffer from poisoning due to carbon monoxide which may be present in the supply gas.7,8 Therefore, it is crucial to develop cheaper, platinum-free materials that exhibit similar electrochemical activity and higher stability than current commonly used noble metal-based nanocatalysts.3 Since the oxygen reduction reaction (ORR) at the fuel cell cathode has very slow kinetics because of strong O=O bonds,9 accelerating this reaction becomes a question of optimizing reaction rates using different metals. One possibility is to use transition metal-based and/or heteroatom-doped catalyst materials.10,11 For example, metallo-macrocycles12−15 and transition metal chlorides16−18 incorporated into modified carbon materials have been intensively studied. The option of using heteroatoms for doping nanostructured sp2 carbons is especially attractive because, for example, if the nitrogen is in pyridinic form, its π electrons can be donated to electron-poor reactions, such as the ORR.19 On the other side, the improved ORR activity of heteroatom-doped carbon materials is also directly linked to the charge redistribution.20 For example, adding nitrogen dopants that have higher electronegativity will generate positive charge density on carbon atoms and the O2 chemisorption will change, which in turn will also effectively weaken the O–O bond and thereby promote the ORR.21 Most commonly used heteroatoms for carbon doping are nitrogen,20,22 boron,23,24 sulfur,25,26 and phosphorus.27,28 Special focus has lately been directed to nitrogen-doped carbon as ORR catalysts, as interest in this topic has grown considerably.29 This is mainly because nitrogen-doped carbon materials have shown good activity towards the ORR in alkaline media compared to commercial Pt/C catalysts.30,31 Nitrogen-doped carbon catalysts are most commonly prepared via pyrolysis method, by mixing nitrogen precursors and carbon, then heating above 700 °C to achieve high electrocatalytic activity.20,32 A N-doped catalyst material may contain different forms of nitrogen: pyridinic-N, pyrrolic-N, graphitic-N (or quaternary-N), and pyridinic-N-oxide.33,34 For pyridinic-N, each N atom that is located at the edge is bonded to two C atoms and is a member of a heterocyclic aromatic ring, donating one p-electron to the π system. Pyrrolic-N refers to N atoms that are incorporated into the five-member heterocyclic ring where each N atom is bonded to two C atoms and donates two p-electrons to the π system. In the case of graphitic-N, the N atoms replace the C atoms in the graphene plane. Pyridine-N-oxide binds to two C atoms and to one O atom, so the binding energy has a +5 eV shift in the positive direction. However, there is still a debate over the active sites on these materials that promote the ORR.20,35,36 Rao et al. report the pyridinic-N to be responsible for enhanced ORR activity,35 while others claim that graphitic-N is the major contributor.34

The most common carbon materials used for N-doping include carbon nanotubes,37,38 carbide-derived carbon,39 carbon blacks,40,41 MOFs,42 and graphene.43−45 Presently, graphene is one of the most popular candidates because of its high conductivity and attractive thermal and mechanical properties.46 Among the previously mentioned support materials, biomass-derived carbon has lately been achieving enormous interest.47−53 Lignin-rich wood mass has been reported to be an extraordinary precursor for high surface area carbon synthesis.54 Lignin plays a crucial role in this high surface area nanocarbon formation as it constitutes up to 35 wt % of the dry mass of wood.55 However, for the preparation of the nitrogen-doped carbon catalyst, additional treatment with nitrogen-containing precursors is needed since lignin naturally lacks nitrogen functionalities.56 Borghei et al. have reported good ORR activity for wood-based nitrogen-doped carbon.57 The main problem with wood-derived doped carbon catalysts is the heterogeneity of the final material.55 To form nitrogen-doped carbon nanomaterials with a desirable structure, optimized activation and pyrolysis procedures as well as a compatible nitrogen source and structure-directing agent are needed.58,59 Different nitrogen sources have been used for synthesizing nitrogen-doped carbons, such as urea or melamine,60,61 but the previous work has shown that dicyandiamide (DCDA) may give better doping effects towards the ORR.60

As the world is still mostly dependent on fossil fuels, the conversion of biomass into carbon has received an enormous interest lately.53,57,62 Most of the by-products of the wood industry are traditionally simply burnt,62 so there is a great need to find more efficient ways for valorizing different by-products from wood. This could also help to substitute most fossil-based carbon nanomaterials that are used as a catalyst carrier in energy conversion systems with greener and cheaper biomass-based materials that also possess good electrocatalytic activity. Besides wood-based precursors, other animal- or plant-based precursors could be used, such as chitosan,30,63 dandelion seeds,64 fresh beancurd,65 etc. Herein, we present an easy procedure to synthesize a highly active catalyst material with the widely available alder wood char as the biological waste and DCDA as the nitrogen precursor. The obtained N-doped carbon is used as an electrocatalyst for the ORR in alkaline media, and the activity of this kind of biological waste-based catalyst is comparable to expensive commercial Pt/C materials.

Results and Discussion

The samples were studied using nitrogen adsorption at 77 K, and the isotherms are presented in Figure 1. It can be seen that both samples are highly microporous since the isotherms belong to type I.66 The BET surface area changed negligibly after the second pyrolysis from 2435 to 2245 m2 g–1, but the total volume of pores (Vtotal) and the average pore diameter value (L) increased (Table 1).

Figure 1.

Figure 1

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Isotherms of nitrogen sorption at 77 K of wood-derived N-doped carbon before (AWC) and after secondary pyrolysis (AWC-1).

Table 1. Porosity of Wood-Derived N-Doped Carbon Before (AWC) and After Secondary Pyrolysis (AWC-1).

samples SBET, m2 g–1 SDFT, m2 Vtotal, cm3 g–1 Vmicro(DR), cm3 g–1 L, nm
AWC 2435 1763 1.27 0.86 1.36
AWC-1 (after second pyrolysis) 2245 1594 1.34 0.82 1.63
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The morphology of the catalyst material AWC-1 was further studied with SEM. AWC-1 exhibits an irregular granular structure: it consists of mostly amorphous carbon with a rough surface structure (Figure 2a,c), but at the same time, some parts of the catalyst show the typical structure of nanoplatelets (Figure 2b).

Figure 2.

Figure 2

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(a–c) SEM images of wood-derived N-doped carbon AWC-1.

The microstructure of the prepared N-doped wood-based catalyst material was further investigated with TEM and is shown in Figure 3a–c. Figure 3b shows the material consists of graphitic lattice fringes (this likely correlates to the platelets seen from the SEM images as well) and areas of amorphous carbon that has a porous structure (Figure 3c,d). Thickness of the 10x layered catalyst particle is 3.55 nm, and the interlayer spacing between two layers is ∼0.35 nm. Likewise, the interlayer spacing of few layered graphene is also close to 0.35 nm, revealing that some parts of the catalyst possess a graphene-like structure.67 It could also be seen from the SEM images that the material contains some nanoplatelets, and this is further confirmed by TEM microphotos as well.

Figure 3.

Figure 3

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(a–d) TEM microphotos of wood-derived nitrogen-doped carbon AWC-1.

Surface composition of the catalyst material AWC-1 was studied using XPS. From the XPS survey spectra, peaks of C 1s, O 1s, N 1s can be detected. The high-resolution XPS spectra of C 1s and N 1s are presented in Figure 4a,b, respectively. The C 1s peak mostly consists of sp2 hybridized carbon atoms.68 The nitrogen content in the catalyst material AWC-1 is quite low (1.56 wt %), and therefore, the intensity of the N 1s peak is also rather low, but it can be deconvoluted into different nitrogen species. It is widely believed that the electrochemical activity is related to different nitrogen surface types rather than the overall nitrogen content;69,70 the N 1s peak was fitted and four peaks can be identified: pyridinic-N (398.3 eV), pyrrolic-N (399.7 eV), graphitic-N (400.8 eV), and N–O (402.3 eV).71,72 Most nitrogen was in the pyridinic form (59%) followed by the graphitic form (53%). The amount of pyrrolic-N and N–O was 21 and 23%, respectively. Lai et al. have shown that pyridinic-N lowers the onset potential of the ORR73 and graphitic-N acts as an active center for the ORR, which in turn helps to achieve higher diffusion limiting current density; meanwhile, other nitrogen species, such as pyrrolic-N and N–O have no significant effect on the activity of the ORR on N-doped carbon materials.73,74

Figure 4.

Figure 4

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(a) Deconvulated C 1s spectra and (b) deconvulated N 1s spectra of the catalyst material AWC-1.

Raman spectroscopy is able to provide rich information on the structure and disorder of carbon networks in carbon-based materials.63,75,76Figure 5 shows a 532 nm excited Raman spectrum of N-doped alder wood char-based catalyst materials before (AWC sample) and after (AWC-1 sample) secondary pyrolysis and ball-milling in the spectral region of 400–3200 cm–1. Two intense bands near 1345 and 1594 cm–1 belong to characteristic D and G modes of the carbon network.75−77 The E2g symmetry G band is associated with the in-plane relative motion of carbon atom pairs in sp2 hybridization.76 This mode is always allowed in carbon-based materials. The A1g symmetry D mode arises from breathing vibrations of aromatic rings; thus, presence of defects is required for this mode activation.63,75−77 Parameters of these bands were estimated by fitting the experimental contour with four Gaussian–Lorentzian form components in the frequency region of 900–1800 cm–1. The widths of D and G bands of the sample AWC determined as full width at half maximum (FWHM) were found to be 204 and 79 cm–1, respectively. Such high width of the bands and high relative integrated intensity ratio of D and G bands (A(D)/A(G) ≈ 3.6) indicate considerable disorder in the carbon network of the studied material. It should be noted that the FWHM value of the G band for undoped pristine graphene was found to be 15 cm–1.78 Secondary pyrolysis (AWC-1) results in a considerable decrease in the ratio of integrated intensities (A(D)/A(G) = 2.2) and a slight decrease in the width of the D band (FWHM = 196 cm–1). Considering the peak intensity ratio for D and G bands, we found that I(D)/I(G) decreases from 1.02 to 0.93 upon the secondary pyrolysis of the sample AWC. The high difference in integrated and peak intensity ratios is related with the difference in width of D and G bands (FWHM of the G band is considerably lower). However, both integrated and peak intensity ratios decrease after the secondary pyrolysis of the AWC sample. These spectral changes indicate a slight decrease in the number of graphene layer defects for the sample treated with additional pyrolysis.69 The I(D)/I(G) ratio probes the density of point-like defects related to modifications in the carbon lattice because of the destruction of sp2 hybridization.75 Formation of oxygen-containing functionalities results in the development of sp3 defect sites.75 The observed slight decrease in the I(D)/I(G) ratio might be related to the high temperature-induced deoxygenation of the carbon lattice and recovery of the graphene sp2 structure.

Figure 5.

Figure 5

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Raman spectra of wood-derived N-doped carbon before (AWC) and after second pyrolysis (AWC-1). The excitation wavelength is 532 nm (0.6 mW).

The ORR activity of the previously prepared N-doped wood-based carbon material was studied on GC electrodes in 0.1 M KOH solution by using the RDE method. The results of the RDE experiment are shown in Figure 6a. To evaluate the ORR activity of the catalyst material, the onset potential (Eonset) is an important criterion; for N-doped wood-based carbon (AWC-1), the Eonset is approximately at 0.92 V vs RHE. One can also see from Figure 6a that despite increasing the rotation rate, the onset potential remains the same, indicating at least the short-term stability of the catalyst. Similar tendencies have been previously observed by Hu et al., who studied nitrogen-doped hydroxypropyl methylcellulose for the ORR in the alkaline media.79 Even though the electrocatalytic activity of their catalyst was similar to the catalyst presented in the current work, it is important to note that the catalyst loading used by Hu et al. was higher than used herein. Han et al. have also synthesized N-doped hollow-core, mesoporous nanospheres with the BET surface area of 770 m2 g–1 and 4.4–6.7% nitrogen content but also with a higher catalyst loading.80 Therefore, it is evident that the electrocatalytic ORR activity presented in the current work is comparable or even higher than the ones reported for similar catalyst materials. The Koutecký-Levich (K-L) plots were also constructed using the O2 reduction reaction polarization data shown in Figure 6a. This was done using the K-L equation:81

graphic file with name ao0c01974_m001.jpg 1

where j is the experimentally measured current density from which the background current has been subtracted, jk and jd are the kinetic and diffusion limited current densities, respectively, k is the electrochemical rate constant for O2 reduction, cO2b is the concentration of oxygen in the bulk (1.2 × 10–6 mol cm–3),82F is the Faraday constant (96,485 C mol–1) DO2 is the diffusion coefficient of O2 in 0.1 M KOH (1.9 × 10–5 cm2 s–1),82 ν is the kinematic viscosity of the solution (0.01 cm2 s–1), and ω is the rotation rate of the electrode (rad s–1).83Figure 6b shows the K-L plots retrieved from the ORR polarization curves at different rotation speeds in 0.1 M KOH. The K-L lines are approximately linear, and the intercepts of the extrapolated lines are close to zero, indicating that the oxygen reduction process happening on the electrode is diffusion limited within the potential range studied. The number of electrons transferred per O2 molecule (n) was also calculated at various potentials using the K-L equation (the inset of Figure 6b). This value of n is close to four in the potential range studied, which indicates that oxygen is directly reduced to water. However, it is impossible to determine via K-L analysis if this is a direct four-electron reduction of oxygen or reduction via the HO2 intermediate (2 e– + 2 e– reduction) pathway.

Figure 6.

Figure 6

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(a) RDE polarization curves for oxygen reduction on GC electrodes modified with a N-doped wood-based catalyst (AWC-1) in O2-saturated 0.1 M KOH. ν = 10 mV s–1. ω = (1) 360, (2) 610, (3) 960, (4) 1900, (5) 3100, and (6) 4600 rpm. (b) K–L plots for oxygen reduction on a N-doped wood-based electrode in 0.1 M KOH at various potentials. The inset figure shows the changes of n values in the studied potential range.

Stability is also an important factor for fuel cell or metal–air battery applications. Stability test results for the N-doped wood-based catalyst during 1000 potential cycles between 1 and −0.2 V are shown in Figure 7a. The onset and half-wave potentials did not change much during the 1000 potential cycles in alkaline media. However, a slight change in diffusion current values can be observed, meaning that the catalyst morphology on the electrode changes with no effect on the active sites. Another potential and more likely cause of this could be that a small amount of the catalyst material detaches from the electrode during stability tests. Methanol tolerance tests comparing as-prepared catalyst AWC-1 and commercial Pt/C were carried out to further study the catalyst durability for potential application in direct methanol fuel cells. As seen from Figure 7b, the addition of methanol at 300 s causes a sharp current loss in the case of commercial Pt/C; the relative current drops to ∼30%. Current drop after injection of methanol for AWC-1 is not as huge; the value of relative current decreases a bit, but stabilizes and shows no significant change.

Figure 7.

Figure 7

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(a) Stability of GC electrodes modified with AWC-1 during 1000 cycles. ω = 960 rpm. (b) Chronoamperometric responses of the catalyst material AWC-1 at 0.6 V with the addition of 3 M methanol.

The comparison of oxygen reduction polarization curves in 0.1 M KOH solution is shown in Figure 8. The RDE polarization curves for AWC, Vulcan XC 72R, and commercial 20% and 40% Pt/C catalyst materials have been added for comparison purposes. The onset potential and half-wave potential for the N-doped wood-based catalyst is much more positive compared to AWC and the most commonly used catalyst support, Vulcan XC 72R. The slight negative shift of the onset potential is observable in comparison with 20% Pt/C but at the same time, the diffusion-limited current density is reaching much higher values compared to the commercial 20% platinum catalyst. Wu et al. have synthesized nitrogen-doped graphitic carbon nanoribbons84 exhibiting higher onset potential than presented hereby but with a 2.3 mg cm–2 catalyst loading, which is five times higher than the loading of current catalyst material AWC-1. Wu et al. have also been using ferrous chloride in their catalyst preparation process, which may have a great impact to the electrocatalytic activity. There are many research articles about the conversion of biomass into carbon,30,53,57,64,65,85 and some of these materials show even better electrochemical activity towards the ORR than the material studied herein, but the goal of this work was to use a precursor that is widely available in Europe.

Figure 8.

Figure 8

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RDE voltammetry curves for oxygen reduction on GC electrodes modified with different catalyst materials in O2-saturated 0.1 M KOH. ν = 10 mV s–1, ω = 1900 rpm.

The above-described electrocatalytic properties of the synthesized N-doped wood-derived carbon catalysts may be due to an extremely high surface area and enlarged pore volume of the catalyst, increasing during the secondary pyrolysis step. Other factors include the high percentage of pyridinic-N in the nitrogen-doped material. A synergy of all these properties should be responsible for the remarkable ORR activity of the novel wood-derived electrocatalyst. Overall, this trailblazing work shows the development of highly effective, cheap, and electrochemically active nanocarbon materials for energy storage and conversion applications using wooden biomass as a renewable and biological resource of carbon.

Tafel plots (from the data shown on Figure 8) comparing 20% Pt/C and AWC-1 are shown in Figure 9. At low current densities the slope value for 20% Pt/C is −72.7 mV dec–1 and for AWC-1 −55.6 mV dec–1. At higher overpotentials the corresponding values are −120.6 mV dec–1 and and – 125.0 mV dec–1, respectively.

Figure 9.

Figure 9

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Tafel plots with the linear fits used for the determination of the Tafel slopes.

Conclusions

The nitrogen-doped wood-derived carbon catalyst was synthesized using alder wood char as the carbon source. Compared to the most commonly used commercial carbon XC 72R, wood-derived N-doped carbon AWC-1 exhibits remarkably improved electrocatalytic ORR activity. AWC-1 shows an onset potential of 0.92 V vs RHE and a half-wave potential of 0.85 V vs RHE in an alkaline medium in addition to improved stability compared to commercial Pt/C. The synthesized wood-derived N-doped carbon is a promising alternative to state-of-the-art precious metal-based catalysts and may be an excellent catalyst carrier for many other applications. The superiority of wood-derived catalysts is demonstrated with ORR activity comparable to a commercial 20% Pt/C catalyst in 0.1 M KOH solution. Such electrocatalytic activity in alkaline media can be explained by the synergistic effect of a high surface area and pore volume and high pyridinic nitrogen content. Thus, this work proposes a facile synthetic strategy to design highly active multifunctional wooden biomass-based materials toward different energy storage and conversion applications.

Experimental Section

Preparation of N-Doped Wood-Based Catalyst

Commercial alder charcoal made by SIA Fille 2000 (Latvia) was the precursor with an 82% nonvolatile carbon content. Charcoal was refined in a planetary mill Pulverizette Classic 5 (Fitsch, Germany) using zirconia mortar and beads to reach a fraction of ∼10 m–6 and then impregnated with a 50% NaOH (Chempur, Piekary Slaskie, Poland) solution. The mass ratio of activator to carbonizate was 3 to 1. The obtained mixture was activated in an Ar flow (100 L h–1) at 700 °C for 120 min (Nabertherm 40 L muffle oven, Lilienthal, Germany). The product was washed with deionized water, 10% HCl (Lachner, Neratovice, Czech Republic), and water again until the filtrate pH reached 5. AC was dried overnight at 105 °C. Ash content was 0.1–0.2%. The process is described in details elsewhere.86

The activated sample was further doped with nitrogen using dicyandiamide (DCDA; Sigma–Aldrich, Germany) solution in dimethylformamide (DMF; Lachner, Czech Republic) with a mass ratio of AC/DCDA = 1:20. DMF was later removed in a rotary evaporator. Doping was carried out at 800 °C for 1 h in an argon atmosphere; the sample obtained is denoted as AWC. The sample was later ball-milled using zirconia beads and secondary pyrolysis was performed in the tube furnace at 800 °C in a flowing nitrogen atmosphere. The resulting final catalyst material is denoted as AWC-1.

Physical Characterization

The porous structure of the AC was evaluated from N2 sorption/desorption isotherms determined in a sorptometer Quantachrome NOVA 4000 (Boynton Beach, FL, U.S.A.) by means of NovaWin 11.03 software. Degassing was performed at 300 °C for 2 h. The results were assessed using the theories of Brunauer–Emmet–Teller (BET, in a P/P0 range of 0.05–0.35), Dubinin–Radushkevich (DR, in a P/P0 range of 10–5–0.35), and quenched solid density functional theory (QSDFT, in a whole P/P0 range).

The morphology of the prepared catalyst was characterized using an SEM-focused ion beam instrument (Helios Nanolab 650). The shape and size of the catalyst particles were further examined using a Tecnai G2 F20 X-TWIN transmission electron microscope equipped with an EDAX spectrometer and an r-TEM detector. For microscopic examinations, 10 mg of the sample was first sonicated in 1 mL of ethanol for 1 h and then deposited on a Cu grid covered with a continuous carbon film.

For performing X-ray photoelectron spectroscopy (XPS), a Kratos Axis Ultra DLD X-ray photoelectron spectrometer with monochromatic Al Kα radiation (hν = 1486.6 eV) was used. The pass energy values used for the survey and high-resolution spectra were 160 and 20 eV, respectively. The energy scale of the system was calibrated with respect to Au 4f7/2, Ag 3d5/2, and Cu 2p3/2 peak positions. Vision 2.2.10 software was used for the peak deconvolution and atomic concentration calculation procedures. All spectra fitting procedures were performed using symmetrical peaks and a 70:30 Gauss–Lorentz function ratio unless stated otherwise in the text. Raman spectra were recorded using an inVia Raman (Renishaw, U.K.) spectrometer equipped with a thermoelectrically cooled (−70 °C) CCD camera and microscope. Raman spectra were excited with 532 nm radiation with diode-pumped solid state (DPSS) laser (Renishaw, UK). The 20x/0.40 NA objective lens and 1800 lines/mm grating were used to record the Raman spectra. The accumulation time was set to 40 s. To avoid damage of the sample, the laser power at the sample was restricted to 0.6 mW. The Raman frequencies were calibrated using the polystyrene standard. Parameters of the bands were determined by fitting the experimental spectra with Gaussian–Lorentzian shape components using GRAMS/A1 8.0 (Thermo Scientific) software.

Electrochemical Characterization

To perform rotating disc electrode (RDE) measurements, glassy carbon discs (GCs) with a geometric area (A) of 0.2 cm2 were used as the substrate material. First, the electrodes were polished using 1 and 0.3 μm aluminium oxide (Al2O3; Buehler) paste. After polishing, the electrodes were sonicated for 5 min in isopropanol (Sigma–Aldrich) and Milli-Q water. The catalyst ink with a concentration of 4 mg mL–1 in isopropanol was prepared using 0.25% of AS-04 OH– ionomer (Tokuyama Corp., Japan). The electrodes were evenly covered with the catalyst material by drop coating 20 μL of previously prepared catalyst ink. The loading of the catalyst material on the GC electrode was 400 μg cm–2. After coating, the electrodes were dried in the oven at 60 °C. The electrochemical measurements were carried out using the RDE method. A Pine AFMSRCE (Pine, USA) rotator and speed controlling unit were used for the RDE measurements. The software used for controlling the experiments was Nova 2.1.2 (Metrohm Autolab P.V., The Netherlands), and the potential was applied with a potentiostat/galvanostat Autolab PGSTAT 128 N (Metrohm Autolab P.V., The Netherlands). All electrochemical tests were carried out in a three-electrode cell using Pt foil as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. As all potentials in this work were measured vs SCE, the potentials were calculated in the reversible hydrogen electrode (RHE) scale using the Nernst equation:

graphic file with name ao0c01974_m002.jpg 2

where ESCE is the experimentally measured potential vs SCE, and ESCE0 = 0.242 V at 25 °C. All the potential values in the text are stated against RHE, unless noted otherwise. Electrochemical measurements were performed in 0.1 M KOH solution at room temperature (23 ± 1 °C). For studying the ORR, the solution was saturated with O2 (6.0), and for measuring the background, the solution was saturated with N2 (5.0). A continuous flow of gases was maintained over the solution during the measurement. The RDE for 1000 potential cycles at a scan rate of 100 mV s–1 was applied to test the stability of the catalyst material. The rotation rate was set to 960 rpm during stability tests. Linear-sweep voltammograms (LSVs) were recorded after every 100 cycles at a scan rate of 10 mV s–1. To further study the stability of the prepared catalyst material, methanol tolerance tests were also conducted at 0.6 V in O2-saturated 0.1 M KOH. To perform the methanol tolerance test, the rotation speed was set to 1900 rpm and 3 M MeOH was added at 300 s.

Acknowledgments

This research was financially supported by the M-ERA.NET project “Wood-based Carbon Catalysts for Low-temperature Fuel Cells (WoBaCat),” ERA.Net RUS Plus project “Novel Heteroatom-doped Nanocarbon Catalyst for Fuel Cell and Metal-air Battery Applications (HeDoCat),” and LZP-2018/1-0194 project “Nanostructured Nitrogenated Carbon Materials as Promoters in Energy Harvesting and Storage Technologies (NN-CARMA).” The funding for the research stays at the DLR-Institute of Networked Energy Systems e.V. (Germany) from the Federal Ministry of Education, and Research in the framework of the project NETonia is greatly appreciated. This work was also supported by the European Union through the European Regional Development Fund, Project TK134 EQUITANT.

The authors declare no competing financial interest.

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