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Article

Cobalt-Containing Nitrogen-Doped Carbon Materials Derived from Saccharides as Efficient Electrocatalysts for Oxygen Reduction Reaction

by
Kaidi Veske
1,
Ave Sarapuu
1,*,
Maike Käärik
1,
Arvo Kikas
2,
Vambola Kisand
2,
Helle-Mai Piirsoo
2,
Alexey Treshchalov
2,
Jaan Leis
1,
Aile Tamm
2 and
Kaido Tammeveski
1
1
Institute of Chemistry, University of Tartu, Ravila 14a, 50411 Tartu, Estonia
2
Institute of Physics, University of Tartu, W. Ostwald Str. 1, 50411 Tartu, Estonia
*
Author to whom correspondence should be addressed.
Catalysts 2022, 12(5), 568; https://0-doi-org.brum.beds.ac.uk/10.3390/catal12050568
Submission received: 29 April 2022 / Revised: 16 May 2022 / Accepted: 18 May 2022 / Published: 20 May 2022
(This article belongs to the Special Issue Heterogeneous Electrocatalysis: Fundamentals and Applications)
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Abstract

:
The development of non-precious metal electrocatalysts towards oxygen reduction reaction (ORR) is crucial for the commercialisation of polymer electrolyte fuel cells. In this work, cobalt-containing nitrogen-doped porous carbon materials were prepared by a pyrolysis of mixtures of saccharides, cobalt nitrate and dicyandiamide, which acts as a precursor for reactive carbon nitride template and a nitrogen source. The rotating disk electrode (RDE) experiments in 0.1 M KOH solution showed that the glucose-derived material with optimised cobalt content had excellent ORR activity, which was comparable to that of 20 wt% Pt/C catalyst. In addition, the catalyst exhibited high tolerance to methanol, good stability in short-time potential cycling test and low peroxide yield. The materials derived from xylan, xylose and cyclodextrin displayed similar activities, indicating that various saccharides can be used as inexpensive and sustainable precursors to synthesise active catalyst materials for anion exchange membrane fuel cells.

Graphical Abstract

1. Introduction

Low-temperature fuel cells are promising devices for converting H2 generated from renewable energy back to electricity. Among these, proton exchange membrane fuel cells (PEMFCs) have already found commercial application in the transport sector; however, they rely on high-cost catalysts containing platinum-group metals (PGM) [1]. Anion exchange membrane fuel cell (AEMFC) technology allows using cheaper materials, including non-precious metal catalysts, and their performance has been significantly improved in recent years [2,3,4,5]. The United States Department of Energy has set a target to reach PGM-free AEMFCs by 2030 [6]. Thus, it is necessary to develop high-performance PGM-free catalysts for both O2 reduction reaction (ORR) occurring in fuel cell cathode and hydrogen oxidation reaction on anode.
The PGM-free catalysts for ORR have been intensely developed in recent decades, and among these, transition metal-containing nitrogen-doped carbon (MNC) catalysts have shown the most promising results [1,4,7,8,9,10,11]. These catalysts can be synthesised from existing carbon precursors such as carbon black [12], carbon nanotubes (CNTs) [13], carbide-derived carbon (CDC) [14], CNT–CDC composites [15,16], mesoporous carbon [17,18], graphene [19], natural shungite [20], etc., by doping the surface of these materials in high-temperature pyrolysis with nitrogen and transition metal sources. Alternatively, it is possible to synthesise the nanocarbons by pyrolysing organic precursors while simultaneously doping these with nitrogen and transition metal [21,22,23,24,25,26,27]. This is a cost-effective approach, as besides simple organic compounds, any organic biomass, such as agricultural waste products, can be used as carbon precursors [28,29]. Glucose as one of the simplest and easily producible organic compounds has been a popular choice for carbon precursor and has often been used in the preparation of ORR catalysts [30,31,32,33], but other carbohydrates have also been employed [34,35,36]. However, when synthesising the carbon catalysts from organic precursors, one of the main challenges is to achieve a porous structure suitable for the fuel cell application, preferentially hierarchical porosity. The most common methods to tailor the porous structure is using hard or soft templates [37]. It has been shown that graphitic carbon nitride (g-C3N4), which forms from some simple N sources, such as dicyandiamide (DCDA), urea or melamine during the pyrolysis at about 550–600 °C, can also serve as a template in the synthesis of N-doped carbon materials [22,23,30,33,35,36,38,39]. It decomposes when the temperature is further increased, and the nitrogen-containing gases are released that act as dopants of the carbon material [33,35].
In addition to favourable morphology, the high-performing carbon-based catalysts should have a high number density of ORR-active sites. In alkaline solution, these can be either various metal-free nitrogen-containing functionalities (pyridinic-N, pyrrolic-N, graphitic-N, etc.), nitrogen-coordinated single-atom transition metal centres (M-Nx), or metal-containing nanoparticles, such as carbides, oxides, nitrides, or metals, which are often encased by carbon layers [10,40]. The catalysts prepared by pyrolysis contain many of these centres and despite numerous efforts, the exact mechanism of the ORR has not been revealed as yet. It is very likely that all such sites are involved in the ORR process, and in some works, synergetic effects between these have been also proposed [41,42,43].
Previously, we have successfully used the carbon nitride template-based approach to synthesise PGM-free ORR catalysts from 5-methylresorcinol [22,23]. In this work, saccharides were employed as cheap and sustainable organic precursors to prepare cobalt-containing N-doped carbon catalysts by simple one-step high-temperature pyrolysis. High electrocatalytic activity for ORR in 0.1 M KOH solution was demonstrated by the rotating disk electrode (RDE) method and the dependence of the catalyst performance on the cobalt content was revealed. The ORR activity of the catalysts did not depend on the nature of the saccharide employed; thus, this work provides a feasible route towards the production of sustainable and cost-effective cathode materials for the AEMFC application.

2. Results and Discussion

2.1. Structural Characterisation

The cobalt-containing N-doped carbon catalysts Glu-NC-Co, Glu-NC-4Co, Glu-NC-8Co and Glu-NC-16Co were prepared by pyrolysis of the mixture of glucose, dicyandiamide and various amounts of Co(NO3)2·6H2O. The scanning electron microscopy (SEM) studies of surface morphology of Glu-NC-8Co reveal that pyrolysis of the mixture of carbon, nitrogen and cobalt precursors results in a porous carbon material with irregular structure and a network of interconnected meso- and macropores (Figure 1). It has been proposed that during the pyrolysis, DCDA forms g-C3N4 nanosheets at ca. 550 °C, which act as an in situ template for the formation of carbon nanosheets from glucose and yields a high mesoporosity of the resulting material [30]. By increasing the temperature of pyrolysis over 600 °C, g-C3N4 starts to decompose to reactive gases, such as NH3 and CO2, which induce additional porosity and introduce N-containing active sites to the catalyst [33,35]. According to energy-dispersive X-ray spectroscopy (EDX) results, the catalyst material contained 24 wt% Co, 61 wt% C, 8 wt% of N and 7 wt% of O. Since the metal content is rather high, it is expected that the material also contains cobalt-rich nanoparticles encapsulated in carbon layers, as shown in the previous reports on the materials prepared by similar approach [22,31,34].
N2 physisorption analysis was performed to examine the specific surface area and porosity of the catalyst material. The N2 adsorption/desorption isotherm of Glu-NC-8Co (Figure 2a inset) corresponds to the type III isotherm with the H4 hysteresis loop according to the IUPAC classification [44], which is characteristic of micro-mesoporous materials with a low content of micropores. The specific surface area (SBET) was 275 m2 g−1 and the total pore volume was 0.39 cm3 g−1, of which 0.07 cm3 g−1 was the micropore volume. In the pore size distribution graph (Figure 2a), the maximum is at about 3.5 nm, and it can be seen that most of the pores are less than 6 nm in diameter; however, there are also some larger mesopores of various size present in the material. Such hierarchical porosity is beneficial for the fuel cell catalysts, as the existence of mesopores is required for enhancing the mass transfer of the reactants and products in the catalyst layer and high microporosity ensures a high number of ORR-active sites [45].
The results of Raman spectroscopy (Figure 2b) provide information on the structure of Glu-NC-8Co carbon material. The D-band (at ca. 1350 cm−1) is associated with the defects and disorder in the carbon structure, while the G-band (at ca. 1580 cm−1) corresponds to the regular crystalline graphitic carbon. The highly broadened and intense D band indicates that the material has a rather defective and irregular structure, which is characteristic for the doped carbon. However, a rather intense G-peak suggests the sufficient graphitisation of the material, which has been shown to be promoted by transition metals [46] and is necessary to provide good electrical conductivity to the material.
X-ray photoelectron spectroscopy (XPS) analysis was performed to examine the elemental content of the catalyst surface (Figure 3). The Glu-NC-8Co material was found to contain 9.1 at % N, 2.5 at % Co, 81.6 at % C and 6.8 at % O. Figure 3b shows the detailed XPS spectrum in the N 1s region, deconvoluted into components corresponding to different functional groups in the material. According to the peak fitting, the material contains the highest amount of pyridinic N (43% of the total nitrogen content, peak at 398.5 eV); the other peaks can be assigned to metal-coordinated N (399.5 eV, 15%), hydrogenated N (including pyrrolic N and hydrogenated pyridine [47], 400.6 eV, 25%), graphitic N (402.0 eV, 10%), N-H (403.7 eV, 4%) and N-O (405.2 eV, 2%). Previous studies have shown that pyridinic and metal-coordinated nitrogen moieties have the highest electrocatalytic activity towards the ORR in alkaline solution [25]. The detailed Co 2p spectrum (Figure 3a, inset) consists of multiple peaks corresponding to oxidised forms of Co, which could be metal oxides but also N-coordinated single-atom centres. The identification of these peaks is not possible, because they consist of multiple overlapping peaks due to multiplet splitting and charge transfer effects [48]. The small peak at the lowest binding energy (778.1 eV) can be assigned to metallic Co, which corresponds to about 2% of total surface cobalt. The deconvolution of detailed O 1s and C 1s photoelectron spectra (Figure 3c,d) reveals that there are various functional groups on the surface. The subpeaks can be attributed to O in carbonyl (531.2 eV); ether/alcohol/-C=O in ester (531.5 eV) and –OH in carboxyl/-C-O-C in ester (532.9 eV) [49]; the subpeak with the lowest binding energy (530.3 eV) can be related to metal oxides. The C 1s peak consists of components corresponding to sp2 and sp3 carbon (284.7 and 285.5 eV, respectively) and several peaks that can be attributed to various functional groups with C-O and C-N bonds [49,50].

2.2. Electrochemical Characterisation

The electrocatalytic activity of carbon-based catalyst materials was studied using the rotating disk electrode method (RDE) in 0.1 M KOH saturated with O2. The metal content in the catalyst was varied to achieve the maximum activity. The comparison of RDE polarisation curves in Figure 4a shows that the electrocatalytic activity of Co-containing materials was considerably higher than that of cobalt-free catalyst (Glu-NC), and the activity increased with increasing the metal content. The most positive value of the half-wave potential (E1/2) of −0.18 V vs. saturated calomel electrode (SCE) was achieved with Glu-NC-8Co and two times higher metal content (Glu-NC-16Co) did not further improve this, although Glu-NC-16Co showed slightly higher onset potential (Eonset, corresponds to the potential at which the ORR current density reaches the value of −0.1 mA cm−2) of −0.04 V (Table 1). It can also be seen that the electrocatalytic activity of these catalysts is equal to that of commercial Pt/C catalyst. The catalyst Glu-NC-8Co was treated in acid to obtain the material Glu-NC-8Co-at, but this did not have a positive effect on the ORR activity of the catalyst, as the values of E1/2 and Eonset were slightly more negative for Glu-NC-8Co-at material as compared to Glu-NC-8Co. Although dissolving the metal-rich nanoparticles is necessary to increase the ORR activity of MNC catalysts in acidic solution [51], the effect of it to the electrocatalytic activity in alkaline media is controversial, as these can contribute to the activity of the materials, and the removal of these can decrease the ORR activity [34,39].
To evaluate the applicability of the synthesis method for other carbohydrates, saccharides with different structures were used as organic precursors instead of glucose: xylose, xylan, and cyclodextrin to obtain the catalyst materials Xylose-NC-8Co, Xylan-NC-8Co and CD-NC-8Co, respectively. It can be seen from Figure 4b and Table 1 that the values of Eonset and E1/2 of the O2 reduction for the materials prepared from different saccharides are rather similar to that of Glu-NC-8Co. Thus, it can be assumed that this synthesis method is universal for the preparation of MNC catalysts from saccharides.
To further investigate the ORR pathway on the catalysts, the RDE polarisation curves were recorded at different electrode rotation rates (Figure 5a), and the number of electrons transferred (n) was found from the Koutecky–Levich equation [52]:
1 j   =   1 j k   +   1 j d   =   − 1 n F k c O 2 b   −   1 0.62 n F D O 2 2 / 3 v − 1 / 6 c O 2 b ω 1 / 2
where j is the experimental current density, jk is the kinetic current density, jd is the diffusion-limited current density, n is the number of electrons transferred per O2 molecule, k is the O2 reduction rate constant (cm s−1), F is the Faraday constant (96,485 C mol−1), DO2 is the diffusion coefficient of oxygen (1.9 × 10−5 cm2 s−1) [53], ν is the kinematic viscosity of the solution (0.01 cm2 s−1) [54], c O 2 b is the concentration of oxygen in the bulk solution (1.2 × 10−6 mol cm−3) [53] and ω is the electrode rotation rate (rad s−1). Figure 5b shows the Koutecky–Levich plots for the Glu-NC-8Co material at different potentials and the value of n calculated from the slopes of these, which is close to four over the whole range of potentials studied. Thus, it can be concluded that the final product of O2 reduction is OH−. Similarly, a 4e− reduction in O2 was confirmed for all catalysts (Table 1), except for the metal-free Glu-NC, on which a lower n value suggests partial 2e− reduction to hydroperoxide anion (HO2−).
A comparison of the catalysts with different metal content (Figure 4a) indicates that the presence of sufficient amount of cobalt salt in the carbonisation process is necessary to obtain active ORR electrocatalysts. However, it is expected that many different cobalt- and nitrogen-containing species form during the pyrolysis, and the exact role of these in ORR electrocatalysis is not clear. The nitrogen-coordinated single-atom transition metal centres (MNx) are shown to be active sites for the ORR [25,43,55,56,57,58,59]; however, in alkaline media, a high activity of metal-containing nanoparticles has also been demonstrated [39,41,43,60,61]. Therefore, increasing the catalysts’ activity with increasing cobalt content is an expected result, as the number of electroactive centres is also expected to increase. On the other hand, cobalt is known to promote graphitisation of the carbon material formed during the pyrolysis [46]; thus, the higher metal content may increase the conductivity of the materials, which is also beneficial to the activity.
In order to indirectly show the existence of M-Nx centres in the catalyst and their role in the ORR process, the effect of cyanide anions to the O2 reduction activity was determined. Cyanide ions are known to bind irreversibly to M-Nx centres and block the access of these sites to O2 molecules, thereby inhibiting the ORR rate on the MNC catalysts [12,62]. It can be seen from Figure 6a that the electrocatalytic activity was significantly reduced in the presence of CN−, with Eonset decreasing by 0.11 V and E1/2 by 0.09 V. Thus, it can be concluded that Glu-NC-8Co catalyst contains M-Nx centres, and these have a large contribution to their ORR activity. The remaining ORR activity is presumably related to O2 reduction on metal-free N centres, such as pyridinic N and graphitic N, and possibly on cobalt-containing nanoparticles surrounded by an N-doped carbon shell.
To assess the methanol tolerance of the catalyst, the electrocatalytic activity of O2 reduction was determined in the presence of 3 M of methanol. It can be seen from Figure 6a that methanol had very little effect on the RDE polarisation curves, so it can be concluded that the material has a high methanol tolerance and therefore could also be a suitable cathode catalyst for direct methanol fuel cells.
The stability of the catalyst materials is an important characteristic for the application in the low-temperature fuel cells. As compared to PGM-based materials, non-PGM catalysts show higher durability in AEMFCs [3]. A short-term stability test was performed with Glu-NC-8Co, in which 20,000 potential cycles were applied and O2 reduction polarisation curves were recorded after a certain number of cycles (Figure 6b). It can be seen that the ORR current gradually decreased in the intermediate potential range from −0.3 to −0.9 V vs. SCE, but it remained rather stable in the kinetic region and at more negative potentials. There was only a negligible decrease in the values of Eonset and E1/2 after the potential cycling. Hence, this catalyst can be considered as rather stable in alkaline environment.

3. Materials and Methods

3.1. Preparation of Catalyst Materials

For the preparation of catalysts, 0.15 g of D-glucose (Sigma-Aldrich, St. Louis, MO, USA), 1.5 g of dicyandiamide (DCDA, Sigma-Aldrich) and 0.01–0.16 g of Co(NO3)2·6H2O (Sigma-Aldrich) were dissolved in the mixture of 10 mL of 2-propanol and 20 mL of Milli-Q water and sonicated for about 30 min. The solution was dried at 35 °C overnight, and the dried precursor mixture was pyrolysed at 800 °C for 2 h in N2 atmosphere, raising the temperature by the ramp rate of 10 °C min−1. The materials prepared using 0.01 g, 0.04 g, 0.08 g and 0.16 g of Co salt are designated as Glu-NC-Co, Glu-NC-4Co, Glu-NC-8Co and Glu-NC-16Co, respectively. For comparison, a metal-free material (Glu-NC) was prepared similarly but without Co salt. The catalyst materials were also prepared by replacing glucose with xylan (Sigma-Aldrich), D-xylose (Sigma-Aldrich) and β-cyclodextrin (Sigma-Aldrich), denoted as Xylan-NC-8Co, Xylose-NC-8Co and CD-NC-8Co, respectively. Glu-NC-8Co material was also acid-treated in the aqueous solution of 0.5 M HNO3 (Sigma-Aldrich) and 0.5 M H2SO4 (Sigma-Aldrich) at 50 °C for 8 h; then, it was filtered, rinsed with water, dried and pyrolysed again at 800 °C for 2 h to obtain the material Glu-NC-8Co-at.

3.2. Electrochemical Measurements

Glassy carbon (GC) electrodes with a geometric area of 0.2 cm2 were used for electrochemical measurements by the rotating disk electrode method. The surface of the GC electrodes was polished with 1 and 0.3 μm particle diameter alumina powder (Buehler, Uzwil, Switzerland), sonicated first in Milli-Q water and then in 2-propanol for about 5 min. Then, 4 mg of catalyst was dispersed in the mixture of 653 µL of 2-propanol, 327 µL of water and 20 µL of a 5% solution of Nafion (Sigma-Aldrich) by sonication for 30 min. The GC electrode was drop-coated with two 5 µL aliquots of the suspension to yield the catalyst loading of 200 μg cm−2. For comparison, a commercial Pt/C catalyst (20 wt%, E-TEK, BASF Fuel Cell, Somerset, NJ, USA) was applied by the same method, resulting in the final Pt loading of 40 μgPt cm−2 on the electrode.
Electrochemical oxygen reduction was studied with the RDE method in 0.1 M KOH aqueous solution prepared from KOH pellets (Sigma-Aldrich), saturated with oxygen (99.999%, Linde, Dublin, Ireland), using an EDI101 rotator (Radiometer, Copenhagen, Denmark), CTV101 speed controller (Radiometer) and an Autolab potentiostat/galvanostat PGSTAT30 (Metrohm-Autolab, Utrecht, The Netherlands). The reference electrode was a saturated calomel electrode (SCE), and the auxiliary electrode was a carbon rod. The background currents recorded in the electrolyte solution saturated with argon (99.999%, Linde) were subtracted from the corresponding RDE polarisation data. The methanol tolerance of the catalysts was studied in 3 M solution of methanol in 0.1 M KOH. To study the effect of cyanide anions, the ORR experiments were conducted in the presence of 10 mM NaCN (Sigma-Aldrich) in 0.1 M KOH solution. The short-time stability tests were performed by cycling the potential between −0.5 and 0 V for 20,000 potential cycles at a scan rate (ν) of 200 mV s−1, and the RDE polarisation curves for oxygen reduction were measured after 2000, 5000, and 20,000 potential cycles.

3.3. Physicochemical Characterisation

SEM with EDX was used to study and characterise the surface morphology and composition of the Glu-NC-8Co material. The GC disk was drop-coated with the catalyst material, and the measurements were performed with a Helios Nanolab 600 (FEI, Hillsboro, OR, USA) electron microscope attached INCA Energy 350 (Oxford Instruments, Abingdon, UK) spectrometer.
The N2 adsorption/desorption isotherms were recorded on a NovaTouch LX2 (Quantachrome Instruments, Boynton Beach, FL, USA) at the boiling point of N2 (77 K). The sample was degassed in vacuum at 300 °C for 12 h before measurements. The SBET was calculated according to the Brunauer–Emmett–Teller (BET) theory. The total pore volume was measured at P/P0 = 0.97. The micropore volume and pore size distribution were calculated using quenched solid density functional theory (QSDFT) equilibria model for slit-type pores. TouchWin 1.11 software (Quantachrome Instruments) was used for calculations.
For the Micro-Raman spectroscopic analysis, the catalyst suspension was pipetted onto a Si plate. The spectrum was recorded on an inVia Raman spectrometer (Renishaw, Wotton-under-Edge, UK) with an Ar ion laser operating at 514.5 nm and a confocal microscope (Leica Microsystems, Mannheim, Germany) with 50X objective.
The XPS analysis was conducted with a SCIENTA SES-100 electron energy analyser (VG Scienta, Uppsala, Sweden) and a non-monochromatic twin anode X-ray source Thermo XR3E2 (Thermo Fisher Scientific, Waltham, MA, USA). The Mg Kα X-rays with photon energy 1253.6 eV were used. The survey spectrum was registered in the range of 1000 to 0 eV with a pass energy of 200 eV, step size of 0.5 eV, step duration of 0.2 s and number of scans: 5. For collecting the detailed spectra, the step size of 0.2 eV, step duration of 0.2 s and the number of scans of at least 25 were used. The data were processed with Casa-XPS software (version 2.3.17, Casa Software, Teignmouth, UK); this included removing X-ray satellites. The peak fitting was completed using a Gauss–Lorentz hybrid function (GL 70, Gauss 30 %, Lorentz 70 %) and a blend of linear and Shirley-type backgrounds.

4. Conclusions

Cobalt-containing nitrogen-doped carbon-based catalyst materials for the ORR were prepared by the pyrolysis of saccharides in the presence of cobalt salt and dicyandiamide as a nitrogen precursor. Graphitic carbon nitride formed in situ during the pyrolysis acted as a reactive template to yield porous materials. In the rotating disk electrode experiments, the material with optimised cobalt content had a remarkably high ORR activity in alkaline solution, which was comparable to that of a 20 wt% Pt/C catalyst. The catalyst materials prepared from glucose, xylose, xylan and cyclodextrin had similar activity; thus, it is expected that this synthesis method can be applied to any saccharides. Acid treatment of the catalyst did not increase its electrocatalytic activity. The number of electrons transferred was found to be around 4 for all Co-containing catalysts. Glu-NC-8Co catalyst was stable during potential cycling for 20,000 cycles and had a high methanol tolerance. In the presence of cyanide anions, the electrocatalytic activity of Glu-NC-8Co decreased significantly, indicating that the catalyst contains M-Nx as active centres.
This work provides a simple procedure to prepare active and stable non-precious metal catalyst materials for AEMFC cathodes, starting from inexpensive and abundant precursors.

Author Contributions

Conceptualisation, A.S. and K.T.; methodology, A.S., A.K., V.K., A.T. (Aile Tamm) and K.T.; validation, K.V., A.S. and A.K.; formal analysis, K.V., M.K., H.-M.P., A.K. and A.T. (Alexey Treshchalov); investigation, K.V., M.K., A.K., H.-M.P. and A.T. (Alexey Treshchalov); resources, J.L., V.K., A.T. (Aile Tamm) and K.T.; data curation, A.S.; writing—original draft preparation, A.S.; writing—review and editing, M.K., A.K., A.T. (Alexey Treshchalov), J.L., A.T. (Aile Tamm) and K.T.; visualisation, K.V., A.S., H.-M.P. and M.K.; supervision, A.S. and A.T. (Aile Tamm); project administration, A.T. (Aile Tamm) and K.T.; funding acquisition, J.L., A.T. (Aile Tamm) and K.T. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Ministry of Education and Research, Estonia, through Estonian Research Council, grant numbers PRG4, PRG723 and PRG1509, and by the EU through the European Regional Development Fund, grant numbers TK141, “Advanced materials and high-technology devices for energy recuperation systems”; TK134, “Emerging orders of quantum and nanomaterials” and TK143, “Centre of Excellence in Molecular Cell Engineering”.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 12 00568 g001 550
Figure 1. Scanning electron microscopy images of the Glu-NC-8Co catalyst material.
Figure 1. Scanning electron microscopy images of the Glu-NC-8Co catalyst material.
Catalysts 12 00568 g001
Catalysts 12 00568 g002 550
Figure 2. (a) Pore size distribution curve and N2 physisorption isotherm (inset) for Glu-NC-8Co material. (b) Raman spectrum of Glu-NC-8Co catalyst.
Figure 2. (a) Pore size distribution curve and N2 physisorption isotherm (inset) for Glu-NC-8Co material. (b) Raman spectrum of Glu-NC-8Co catalyst.
Catalysts 12 00568 g002
Catalysts 12 00568 g003 550
Figure 3. X-ray photoelectron spectra for Glu-NC-8Co catalyst: survey spectrum (a) and deconvoluted detailed spectra in Co 2p region (inset of (a)), N 1s region (b), O 1s region (c) and C 1s region (d).
Figure 3. X-ray photoelectron spectra for Glu-NC-8Co catalyst: survey spectrum (a) and deconvoluted detailed spectra in Co 2p region (inset of (a)), N 1s region (b), O 1s region (c) and C 1s region (d).
Catalysts 12 00568 g003
Catalysts 12 00568 g004 550
Figure 4. RDE voltammetry curves of O2 reduction in O2-saturated 0.1 M KOH on (a) glucose-derived catalyst materials and Pt/C; (b) catalysts derived from various saccharides. ω = 1900 rpm, ν = 10 mV s−1.
Figure 4. RDE voltammetry curves of O2 reduction in O2-saturated 0.1 M KOH on (a) glucose-derived catalyst materials and Pt/C; (b) catalysts derived from various saccharides. ω = 1900 rpm, ν = 10 mV s−1.
Catalysts 12 00568 g004
Catalysts 12 00568 g005 550
Figure 5. (a) RDE voltammetry curves of O2 reduction on Glu-NC-8Co catalyst in O2-saturated 0.1 M KOH, ν = 10 mV s−1; (b) Koutecky–Levich plots derived from data in (a) and number of electrons transferred (inset).
Figure 5. (a) RDE voltammetry curves of O2 reduction on Glu-NC-8Co catalyst in O2-saturated 0.1 M KOH, ν = 10 mV s−1; (b) Koutecky–Levich plots derived from data in (a) and number of electrons transferred (inset).
Catalysts 12 00568 g005
Catalysts 12 00568 g006 550
Figure 6. RDE voltammetry curves for O2 reduction on Glu-NC-8Co catalyst in O2-saturated 0.1 M KOH (a) in the presence of 10 mM NaCN or 3 M methanol, (b) before and after a certain number of potential cycles between 0 and −0.4 V. ω = 1900 rpm, ν = 10 mV s−1.
Figure 6. RDE voltammetry curves for O2 reduction on Glu-NC-8Co catalyst in O2-saturated 0.1 M KOH (a) in the presence of 10 mM NaCN or 3 M methanol, (b) before and after a certain number of potential cycles between 0 and −0.4 V. ω = 1900 rpm, ν = 10 mV s−1.
Catalysts 12 00568 g006
Table
Table 1. ORR parameters of the electrocatalysts in 0.1 M KOH solution.
Table 1. ORR parameters of the electrocatalysts in 0.1 M KOH solution.
CatalystE1/2 (V vs. SCE)Eonset (V vs. SCE)n (at −1.1 V)
Glu-NC−0.40−0.213.2
Glu-NC-Co−0.24−0.144.0
Glu-NC-4Co−0.22−0.114.0
Glu-NC-8Co−0.18−0.064.3
Glu-NC-16Co−0.18−0.044.1
Glu-NC-8Co-at−0.20−0.074.1
Xylan-NC-8Co−0.19−0.074.0
Xylose-NC-8Co−0.20−0.074.1
CD-NC-8Co−0.19−0.074.2
Pt/C−0.18−0.043.9
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Veske, K.; Sarapuu, A.; Käärik, M.; Kikas, A.; Kisand, V.; Piirsoo, H.-M.; Treshchalov, A.; Leis, J.; Tamm, A.; Tammeveski, K. Cobalt-Containing Nitrogen-Doped Carbon Materials Derived from Saccharides as Efficient Electrocatalysts for Oxygen Reduction Reaction. Catalysts 2022, 12, 568. https://0-doi-org.brum.beds.ac.uk/10.3390/catal12050568

AMA Style

Veske K, Sarapuu A, Käärik M, Kikas A, Kisand V, Piirsoo H-M, Treshchalov A, Leis J, Tamm A, Tammeveski K. Cobalt-Containing Nitrogen-Doped Carbon Materials Derived from Saccharides as Efficient Electrocatalysts for Oxygen Reduction Reaction. Catalysts. 2022; 12(5):568. https://0-doi-org.brum.beds.ac.uk/10.3390/catal12050568

Chicago/Turabian Style

Veske, Kaidi, Ave Sarapuu, Maike Käärik, Arvo Kikas, Vambola Kisand, Helle-Mai Piirsoo, Alexey Treshchalov, Jaan Leis, Aile Tamm, and Kaido Tammeveski. 2022. "Cobalt-Containing Nitrogen-Doped Carbon Materials Derived from Saccharides as Efficient Electrocatalysts for Oxygen Reduction Reaction" Catalysts 12, no. 5: 568. https://0-doi-org.brum.beds.ac.uk/10.3390/catal12050568

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