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Article

Manganese- and Nitrogen-Doped Biomass-Based Carbons as Catalysts for the Oxygen Reduction Reaction

by
Ance Plavniece
1,*,
Kätlin Kaare
2,
Dijana Simkunaitė
3,
Aldona Balciunaite
3,
Vitalija Jasulaitiene
3,
Gediminas Niaura
3,
Aleksandrs Volperts
1,
Galina Dobele
1,
Luis César Colmenares-Rausseo
4,
Ivar Kruusenberg
2,
Loreta Tamasauskaite-Tamasiunaite
3 and
Eugenijus Norkus
3,*
1
Latvian State Institute of Wood Chemistry, LV-1006 Riga, Latvia
2
National Institute of Chemical Physics and Biophysics, 12618 Tallinn, Estonia
3
Center for Physical Sciences and Technology (FTMC), LT-02300 Vilnius, Lithuania
4
SINTEF Industry, Batteries and Hydrogen Technologies, NO-7465 Trondheim, Norway
*
Authors to whom correspondence should be addressed.
Catalysts 2024, 14(2), 92; https://doi.org/10.3390/catal14020092
Submission received: 29 December 2023 / Revised: 17 January 2024 / Accepted: 19 January 2024 / Published: 23 January 2024
(This article belongs to the Section Catalysis for Sustainable Energy)
Browse Figures
Versions Notes

Abstract

:
Manganese- and nitrogen-doped carbon materials were produced using NaOH-activated wood char and wood-processing residues such as wood chips and black liquor and evaluated as oxygen reduction catalysts for further application in fuel cells or metal–air batteries. The elemental and chemical composition, with special attention given to types of nitrogen bonds and the structure, morphology, and porosity of the obtained catalyst materials were studied. The catalytic activity was assessed in an alkaline medium using the rotating-disk electrode method. It has been shown that synthesized and doped N-Mn catalytic materials based on biomass precursors with different chemical structures are a promising alternative to modern oxygen reduction catalysts based on precious metals.

Graphical Abstract

1. Introduction

In recent scientific research on electrochemical processes, a lot of attention has been paid to highly efficient energy storage devices [1,2]. Energy storage and conversion technologies, including fuel cells, are of great importance as they offer wide possibilities to meet the growing global demand for clean and sustainable energy [3,4]. However, their performance is mostly influenced by the electrochemical reduction in oxygen [5,6]. The oxygen reduction reaction (ORR) is one of the main electrochemical reactions taking place at the fuel cells’ cathode. The dynamics of this rate-determining reaction and the variety of possible pathways have a significant impact on the fuel cells’ overall energy conversion efficiency. However, the complicated reaction mechanisms and slow kinetics of ORR seriously limit their technological development. Therefore, active and selective electrocatalysts are essential to improve the ORR performance. The best, irreplaceable, and most effective catalysts for promoting the ORR via four-electron transfer are usually Pt and Pt-based materials [7,8]. However, the large-scale commercial application of Pt and Pt-based catalysts is still limited by their high cost, scarcity, low resistance to carbon monoxide (CO) poisoning, and insufficient long-term durability under fuel cell conditions. To address these issues, a large number of noble-metal-free nanomaterials such as transition metals or transition metal oxides [9,10], hydroxides [11], sulfides [12,13], nitrides [14], or metal-free nanomaterials such as N-, S-, P-, B-, and F-heteroatom-doped carbon-based catalyst materials have been developed [15,16]. The latter materials are of special interest as they are supposed to be a viable alternative to Pt-based catalysts due to their cost-effectiveness, capability of directing the ORR via a four-electron pathway, high current density, and low onset potential [17]. In addition, carbon-based catalysts are of great relevance given their high conductivity, chemical stability, high specific surface area, controlled porosity, and high number of electroactive sites available.
In this context, biomass-derived carbons have emerged as promising potential alternatives to conventionally produced carbon-based catalysts and hold great promise as a renewable source of sustainable carbon materials for the next generation of energy storage and conversion systems [18,19,20]. Biomass is a hot carbon precursor in terms of its renewability, earth abundance, environmental friendliness, low cost, non-toxicity, sustainability, ease of production, and multiplicity of heteroatoms (N, P, S, etc.,) in its intrinsic composition [18,21]. The plant biomass consists of three main components: lignin, cellulose, and hemicellulose. The source of raw materials from this biomass can be both wood and wood-processing waste, as well as agricultural crops, which allows the use of a wide spectrum of possible raw materials for the preparation of porous carbon ORR catalysts. Catalysts of this nature are typically rich in micro- and mesopores which are required for a high density of multi-active sites, making them extremely efficient. The desired physicochemical properties of ORR catalysts are largely influenced by the synthesis methods used to convert biomass into porous carbon-based catalysts [18,22]. Pyrolysis [23,24], hydrothermal carbonization [25], a combination of both [26,27,28], microwave-assisted [29,30], ionothermal carbonization [31], etc., have been widely used for this purpose. In addition, the required characteristics, such as high surface area, high porosity, and the surface functionality of biomass-derived carbon-based ORR catalysts can be optimized by activation techniques or via doping [32,33].
Nitrogen (N) doping is one of the most effective and widely used methods for improving the activity of biomass-derived carbon materials for ORR catalysts [34,35]. The incorporation of electronegative N atoms into catalyst compositions leads to charge relocation in the sp2-hybridised carbon structure, which facilitates oxygen adsorption and reduction, resulting in a significant increase in ORR efficiency [31]. Extra enhancement of the ORR efficiency is known to be achieved either by the use of duel-doped catalysts with heteroatoms such as N, S, and P [36,37] or co-doped with transition metals such as Fe, Cu, Co, CoNi, and CoFe [38,39,40]. Due to the synergistic coupling impact, including the availability of N species (pyridinic-N and graphitic-N, pyrrolic-N), a large number of active catalytic sites, and generation of multi-level porous structures with a highly developed specific surface area for more oxygen rapid mass transfer, heterogeneous doping leads to extremely efficient electrocatalytic activity of ORR.
In addition, transition metals or their hybrids, such as Fe-N [41,42], Co-CoS [43], and Ni [44], have been used in order to increase the activity of wood-derived carbons for ORR catalysts. A N, P, and S/Fe-codoped carbon (FBC-Fe) with A hierarchical porous structure was prepared of feculae bombycis (FB) and exhibited excellent long-term durability and a remarkable methanol resistance, as well as high electrocatalytic activity, with an onset potential of 0.92 V vs. RHE, and an electron transfer number of 4.1 for the ORR in alkaline media [45]. A wood-derived reduced graphene oxide-supported Fe-based catalyst showed ORR activity with the highest onset potential of 0.83 V vs. RHE and a limiting current density of 5.33 mA cm−2 [41]. A carbonized lignin-free wood (CLFW)@Co-CoS hybrid material has also been developed, where the porous wood pipe structure not only provides space for CoS and Co nanoparticles to load and transport electrolyte ions but also provides a large number of active sites, which improves the electrocatalytic properties of electrocatalysis [43]. However, wood-derived carbon material activities are still relatively moderate, and there is still a need to find a more efficient and optimized catalyst using a simple and easy synthesis route.
Taking into account the information above in this work, three different biomass residue-based manganese- and nitrogen-doped oxygen reduction catalysts materials were studied as catalysts for the ORR.

2. Results

2.1. Physical Characterization

To characterize the porous structure of the obtained doped carbon materials, the surface area and pore size distribution of materials under study were determined from nitrogen adsorption–desorption isotherms at 77 K (Figure 1a). All obtained manganese and nitrogen co-doped carbon materials had a micro-mesoporous structure with high specific surface areas (SSAs). For activated wood char (AWC-Mn-N), hydrothermally carbonized wood (AHTC-Mn-N) and activated black liquor (ABL-MN-N), with Brunauer–Emmet–Teller (BET) 2127, 2195, and 1808 m2 g−1 SSA values were observed, respectively (Table 1). Pore sizes ranged from 1 to 4 nm when wood char and birch wood were used as precursors (Figure 1b), while slightly wider pores were formed when using black liquor as a precursor (ABL-Mn-N).
SEM was used to study the morphology of the obtained catalyst samples. SEM images of AWC-Mn-N (Figure 2a,b), AHTC-Mn-N (Figure 3a,b), and ABL-Mn-N (Figure 4a,b) showed that the shape and size of the deposited Mn particles depended on the raw material. The Mn particles in the AWC-Mn-N catalyst had rough edges (Figure 2a,b), whereas particles in the ABL-Mn-N were round and fluffy (Figure 4a,b). The particle size ranged from 3 to 6 µm. In the case of the AHTC-Mn-N catalyst, the particles resembled flat flakes with a size of 10–40 µm (Figure 3a,b).
Each element’s presence in the catalysts was validated by EDX elemental mapping analysis. The combined element mapping confirmed the relatively uniform distribution of the Mn, N, and C elements in the AWC-Mn-N (Figure 2c), AHTC-Mn-N (Figure 3c), and ABL-Mn-N (Figure 4c) samples.
Moreover, Mn was uniformly distributed in all samples (Figure 2d, Figure 3d and Figure 4d).
Figure 5a compares 532 nm excited Raman spectra of the AWC-Mn-N, AHTC-Mn-N, and ABL-Mn-N samples. All the spectra exhibited broad D and G bands. In addition, a clearly defined feature at 2689 cm−1 was visible in the case of sample AWC-N. This feature belongs to a prominent 2D band and indicates an increase in the structural ordering of this carbon-based sample [46]. Similar Raman spectra were obtained for samples AWC-N, AHTC-N, and ABL-N (Figure S1).
Important quantitative information about the structure of the carbon material can be extracted from the analysis of full width at half maximum of G band, FWHM(G) [47,48]. Therefore, the Raman spectra in the frequency range from 1000 to 1800 cm−1 were fitted by 4 or 5 Gaussian or Lorentzian form components (Figure S2). Besides the well-known D and G components (Lorentzian form), additional Gaussian form bands located at 1275 cm−1 (D*) and 1520 cm−1 (D″) were introduced [47,49]. The D″ band was previously related to amorphisation of the carbon material [50]. The disorder-induced D′ band near 1621 cm−1 was used only in the case when the presence of this band was visible in the Raman spectra as a shoulder near the G band (AWC-N and AWC-Mn-N). The important structural parameter in the sp2-hybridization layered carbon material was an average in-plane crystallite size La [47,48]. The experimentally obtained FWHM(G) value was used to determine the La [47,51,52]:
L a = l c 2 l n C F W H M G − F W H M ( G 0 )
where the photon coherence length lc = 32 nm, C = 95 cm−1, and FWHM(G) and FWHM(G0) are the widths of the G band of the sample under investigation and undoped pristine graphene (15 cm−1), respectively. The equation is valid for measuring the La between 32 and 2.8 nm [47]. The parameters of the G band and estimated La values are given in Table 2. The La varies from 13.5 to 7.4 nm depending on the sample treatment. The largest in-plane crystallite size values were obtained for samples AWC-N and AWC-Mn-N.
The XRD patterns of AWC-Mn-N, AHTC-Mn-N, and ABL-Mn-N are compiled in Figure 6b. Broad bands at ~15°, which indicate amorphous carbon formation, and low-intensity bands at 43, 44, and 64, with an orientated aromatic structure, were observed for all obtained samples. This is in good compliance with the Raman spectroscopy results where all the obtained carbon materials were found to be mostly amorphous, with some crystalline fragments in the structure. All samples show the characteristic XRD peaks of MnO2, Mn3O4, and MnO, respectively [53,54,55]. It should be noted that the XRD peak intensity decreases in the following order: AWC-Mn-N > ABL-Mn-N > AHTC-Mn-N.
XPS was used to further analyze the N-Mn-doped carbon materials’ surface chemical composition. The survey spectra of all obtained samples in Figure 6a indicate the presence of Mn, O, N, and C elements. The content of Mn on the surface decreases in the following order: AWC-Mn-N (2.8 at.%) > AHTC-Mn-N (1.9 at.%) > ABL-Mn-N (1.4 at.%) (Table 3). The materials with amorphous structures have more N on the surface, e.g., AHTC-Mn-N (7.7 at.%) and ABL-Mn-N (7.9 at.%). In the case of AWC-Mn-N, which has a more ordered structure, the surface N content is lower (5.5 at.%). The high-resolution XPS C 1s spectra of the ABL-Mn-N, AHTC-Mn-N, and ABL-Mn-N samples are divided into four major peaks that arise at 284.2 (Csp2), 284.7 (Csp3), 285.4–285.2 (N-sp2-C), and 286.3–286.2 (N-sp3-C) [56] (Figure 6b).
As the N-Mn-doped carbon material catalytic activity is greatly influenced by the N form, the deconvolution of high-resolution N 1s spectra was performed (Figure 6c, Table 4). It was shown that most of the nitrogen is in the catalytically active pyridinic N (or Mn-Nx [57]) form: ABL-Mn-N—38.7%, AHTC-Mn-N—39.7%, and AWC-Mn-N—41.8%. XPS spectra of Mn 2p with spin-orbital duplet Mn 2p1/2 and Mn 2p3/2 at 653.6 ± 0.1 and 642.0 ± 0.1 eV, respectively, with a spin-orbit splitting of 11.6 ± 0.1 eV are demonstrated in Figure 6d [58]. This value confirms the presence of MnO2 in the prepared samples [59,60,61]. As evident, the Mn 2p3/2 peaks were deconvoluted into four peaks at binding energies of 640.6 ± 0.1, 642.0 ± 0.1, 643.3 ± 0.2, and 645.2 ± 0.6 eV, indicating the mixed-valence of the manganese oxide phases (Figure 6d, Table 4). Following the data reported in Refs. [59,60,61,62,63], the position of deconvoluted Mn 2p3/2 peaks is generally assigned to the Mn (IV) or Mn (II) oxidation state at binding energies ranging between 641.85 and 643.0 eV or 640.10 and 641.12 eV, respectively. Therefore, peaks located at 640.6 ± 0.1 and 642.0 ± 0.1 eV confirm the presence of Mn(II) and Mn(IV) species, respectively, in all samples (Figure 6d, Table 4). Furthermore, the additional peaks at 645.2 ± 0.6 eV could be assigned to a satellite shake-up peak located at higher binding energy values than the main component and is a characteristic feature of the MnO phase Mn 2p core peak maximum at 640.6 ± 0.1 eV [58]. Notably, the dominating fraction in the ABL-Mn-N, AHTC-Mn-N, and AWC-Mn-N samples is the MnO2 phase, equal to 37.1, 37.9, and 38.7%, respectively (Table 4). At the same time, the MnO (Mn (II)) phase remains lower compared to that determined for the MnO2 (Mn (IV)) phase. The O 1s spectra for all samples show peaks with binding energies of 530.0 ± 0.1, 531.0–531.7 ± 0.2, and 533.4 ± 0.1 eV (Figure 6e). Usually, the peaks at lower binding energies of 529.6–530.7 eV show the presence of the lattice oxygen species [64]. The binding energy peaks at 530.0 ± 0.1 eV may correspond to the presence of MnO and MnO2 [64]. In addition, the O1s peaks at 531.0–531.7 ± 0.2, and 533.4 ± 0.1 eV, shown in Figure 6e, may be associated with oxygen in the states of C=O and C–OH/C–O–C, respectively [65].

2.2. Electrochemical Characterization

To evaluate different raw materials used for Mn doping, ORR measurements were carried out in 0.1 M KOH by employing the RDE method. The comparison of ORR polarization curves at 1600 rpm is shown in Figure 7 (47.2% commercial Pt/C catalyst tests are included for comparison). The oxygen electroreduction measurements show that the best activity is achieved when ABL is used as a precursor material for doping with manganese and nitrogen. The onset potential for ABL-Mn-N is 934 mV vs. RHE and the half-wave potential is 829 mV vs. RHE. The overall electrochemical measurement results are shown in Table 5. The Eonset for AWC-Mn-N is 924 mV vs. RHE, and E1/2 is 803 mV vs. RHE. The poorest results were achieved with AHTC-Mn-N; for that material, the Eonset and E1/2 values are 899 and 788 mV vs. RHE, respectively. The polarization curves at various rotating speeds for different catalyst materials can be seen in Figure S3a–c. The electrons transferred per O2 molecule (n) are also shown in Table 5. The average n value for the catalyst materials is around 4, and for AWC-Mn-N, it even negligibly exceeds the maximum. This is mostly explained by the high SSAs of the catalyst material since it has been found that mesoporosity and high SSAs promote the mass transport of oxygen to the active sites [66].
Muhyuddin et al. have used lignin, known to be the main component of black liquor [67], to prepare bimetallic catalysts for the ORR. They have achieved better onset and half-wave potential values for just the Mn-doped catalyst material; however, the loading they have used in their work is higher than the catalyst loading used in this work [68]. The better results achieved in this work may be connected to the higher content of nitrogen. The nitrogen content of the best-performing catalyst (ABL-Mn-N) is extraordinarily high (7.9 at.%); this material also contains 66.8% mesopores from the Vtot.
The deconvolution of high-resolution N 1s XPS spectra (Figure 6b, Table 4) showed that for all the catalyst materials under study, the main N group was found to be pyridinic-N, which has been considered as one of the active sites for the ORR [68,69,70]. The second most abundant N group was pyrrolic-N and after that graphitic-N. The catalytic activity of different N groups decreases in the following order: pyridinic-N > pyrrolic-N > graphitic-N > oxidized-N [70]. The worst-performing catalyst material AHTC-Mn-N has a larger particle size (10–40 µm) than that of AWC-Mn-N and ABL-Mn-N (3–6 µm) and this indicates that better electrocatalytic performance is achieved when the particle sizes are smaller.
Cai et al. have also measured an Mn-N-doped carbon black catalyst, but the half-wave potential reported in their work (760 mV) falls short compared to the best E1/2 (829 mV vs. RHE) we have achieved within this work [71]. But Cai et al. have used carbon black as a precursor, which is usually synthesized via environmentally harmful methods, such as incomplete combustion of carbonaceous fuels [72,73] and their complicated synthesis process also consists of numerous steps. In this work, we have used sustainable biomass-based precursors, and the synthesis process is simple and facile.
In the fuel cell working conditions, the stability of the catalyst materials is also one of the key factors. The chronoamperometry results can be seen in Figure 7b. The chronoamperometric testing at 0.6 V vs. RHE was carried out for 20 h. The results are in good accordance and are quite similar for all of the samples under study: for AWC-Mn-N the stability after 20 h remains around 95%, for ABL-Mn-N and AHTC-Mn-N the values are 92% and 87%, respectively. However, for the commercial Pt/C, the stability drops to about 70% after 20 h.
In our previous studies [67,74], we have already compared the influence of the different precursors on the properties of porous carbon materials doped with N. It should be noted that manganese oxide and M-Mn-pyridinic N moieties significantly improve not only catalytic ORR activity (Table 5) but also the diffusion limiting current, which has a stability plateau.

3. Materials and Methods

3.1. Synthesis of Carbon Material

Three different raw materials, alder charcoal (Ltd. “Fille”, Valmiera district, Latvia), birch wood (genus Betula), and black liquor (Horizon Pulp & Paper Ltd., Kehra, Estonia), were used to synthesize Mn-N-doped activated carbons. At first, activated carbons, based on different raw materials, were synthesized. Activated carbon, based on woodchar (AWC), was obtained by mixing 20 g of char with 60 g of NaOH, following an activation procedure at 800 °C.
Since black liquor (lignin-containing residue of the pulping process) [67] already contains NaOH and other sodium salts, activation was performed using a lower activator dosage. Briefly, activated carbon based on black liquor (ABL) was synthesized by mixing 44.44 g of black liquor (45% dry content) with 40 g of NaOH, followed by processing at 700 °C.
The raw material, birch wood, was first pre-treated using hydrothermal carbonization, which was performed in a PARR-4554 (USA) autoclave at 260 °C for 4 h [75]. Then, 20 g of the resulting carbonizate was mixed with 60 g of NaOH and heated at 700 °C in an inert gas atmosphere to obtain activated carbon based on birch wood after hydrothermal carbonization (AHTC).
Co-doping of activated carbons with nitrogen and manganese was carried out by mixing 2 g of activated carbon (AWC, ABL, or AHTC) with 40 g of dicyandiamide (DCDA) and 0.62 g manganese acetate in 250 mL of dimethylformamide (DMF). Then, the solvent was evaporated using a rotary evaporator, and the solid residue was processed at 800 °C. The obtained catalyst materials were named as follows: AWC-Mn-N (alder charcoal based), ABL-Mn-N (black liquor based), and AHTC-Mn-N (birch wood after hydrothermal carbonization).

3.2. Characterization of Carbon Materials

The “Quantachrome Nova 4200e” instrument (Quantachrome Instruments, Boynton Beach, FL, USA) was used for the determination of the specific surface area (m2 g−1), micropore and total pore volume (m2 g−1), and pore width (nm) of the samples by recording N2 adsorption/desorption isotherms [76].
Raman spectra were recorded using an inVia Raman (Renishaw, Gloucestershire, UK) spectrometer equipped with a thermoelectrically cooled (−70 °C) CCD camera and microscope. Raman spectra were excited with 532 nm radiation from a diode-pumped solid state (DPSS) laser (Renishaw, Gloucestershire, UK). The 20×/0.40 NA objective lens and 1800 lines/mm grating were used to record the Raman spectra. The accumulation time was 160 s. To avoid damage of the sample, the laser power at the sample was restricted to 0.4 mW. The Raman frequencies were calibrated using the polystyrene standard. Parameters of the bands were determined by fitting the experimental spectra with Gaussian and Lorentzian shape components using GRAMS/A1 8.0 (Thermo Scientific, Waltham, MA, USA) software.
X-ray diffraction analysis (XRD) was measured using a D2 PHASER (Bruker, Karlsruhe, Germany) diffractometer and Cu-K-alpha as an X-ray source. The measurements were conducted in the 2θ range of 10°–90°.
The morphology and composition of the prepared materials were investigated by scanning electron microscopy (SEM) using an SEM workstation SEM TM4000Plus with an AZetecOne detector (Hitachi, Tokyo, Japan).
The samples’ surface chemical composition was analyzed using X-ray photoelectron spectroscopy (XPS) utilizing the Kratos AXIS Supra+ spectrometer (Kratos Analytical, Manchester, UK), as described in more detail in Ref. [77].

3.3. Electrochemical Measurements

To carry out the ORR measurements, the rotating disc electrode (RDE) method was employed. A rotator from OrigaTrod (Origalys ElectroChem SAS, Rillieux-la-Pape, France) for altering the rotation speeds (ω) from 3600 to 400 was used. For controlling the potential and experiments, a Gamry 1010E potentiostat and Gamry Framework program were used (both from Gamry Instruments, Warminster, PA, USA). The glassy carbon (GC, Goodwell, OK, USA) electrodes with a d of 5 cm were used and coated with a catalyst dispersion. Before coating, the GC electrodes were polished with Al2O3 slurries—first with 1 μm and then with 0.5 μm. For cleaning the electrodes from polishing residues, ultrasonication in ethanol and Milli-Q water was carried out. To coat the electrodes, catalyst materials’ dispersions were made by mixing 4 mg of Mn-N-doped activated carbon-based powders with ethanol and Nafion (10 μL per mg of catalyst). After sonication, the electrodes were coated by pipetting a total of 20 μL of the ink onto the surface of the GC. The catalyst material loaded on the electrode was 400 μg cm−2.
The RDE measurements were carried out in a classic round 5-neck glass cell; a saturated calomel electrode (SCE, SI Analytics) was a reference electrode, and the results were later calibrated vs. reversible hydrogen electrode (RHE); a graphitic rod was used as an auxiliary electrode. The ORR measurements were then carried out in an oxygen-saturated 0.1 M KOH (VWR) solution using linear sweep voltammetry (LSV) at a scan rate of 2.5 mV s−1 and varied values of ω (400, 800, 1200, 2400, 3600 rpm). The experimental data were then further analyzed by employing the Koutecky–Levich (K-L) equation:
1/j = 1/jk + 1/jd = 1/B ω1/2 + 1/jk; B = 0.62nFC0(D0)2/3ν−1/6; jk = nFkC0
where j is the experimentally measured current density value, jk and jd stand for the kinetic and diffusion-limited current densities, n denotes the number of electrons transferred per O2 molecule, F is the Faraday constant (96,485 C mol−1), k is the heterogenous rate constant for oxygen reduction, ω is the rotating rate of the electrode (rad s−1) [78]), D O 2 is the diffusion coefficient of oxygen (1.9 × 10−5 cm2 s−1 [79]), C O 2 b is the oxygen concentration (1.2 × 10−6 mol cm−3 [79]) in 0.1 M KOH, and ν is the kinematic viscosity of the solution (0.01 cm2 s−1).
Chronoamperometry was carried out for 20 h at 0.6 V vs. RHE to perform the stability testing of the catalyst materials.

4. Conclusions

In order to reduce the use of platinum in fuel cells, NaOH-activated and nitrogen- and manganese-doped alternative electrocatalysts were synthesized using three biomass precursors: biochar, hydrothermally carbonized birch wood, and black liquor, a byproduct from the pulp and paper industry.
The resulting activated and doped N-Mn carbon materials had a micro-mesoporous structure (pore size 1–6 nm) with high specific surface areas of more than 2100 m2 g−1 and a mesopore fraction from the total volume of 41.9 and 47%, respectively, for samples of biochar and hydrothermally carbonized wood, while in the case of black liquor, the proportion of mesopores reached 66.8%.
It has been shown that the morphology of carbon materials, namely, the shape and size of particles, varies depending on the type of precursor. All of the materials obtained showed a homogeneous distribution of manganese.
The results of the Raman spectra and X-ray diffraction patterns for all samples showed characteristic bands of amorphous carbon of the biomass. In the case of the charcoal-based sample, the Raman spectrum shows the largest in-plane crystallite size and a clearly defined second-order 2D band at 2689 cm−1, indicating a higher structural order.
Based on XPS studies, it was found that the majority of the nitrogen in all samples is in the catalytically active pyridine form. Carbon materials based on hydrothermally carbonized wood and black liquor with a more amorphous structure have a two-fold higher nitrogen surface content compared to the biochar sample. The choice of precursor does not have a significant effect on the Mn form after doping.
The resulting N-Mn-doped biomass-based carbon materials showed comparable electrocatalytic activity and ORR durability to 40% Pt/C in an alkaline 0.1 M KOH solution with four-electron transfer. The highest activity was achieved when using black liquor as a precursor material for N-Mn doping, with an onset potential of 934 mV to RHE and a half-wave potential of 829 mV. These advantages are possibly associated with a more developed mesoporosity and a higher content of nitrogen in the pyridinic form.
The results of the work contribute to the understanding of the potential of carbon materials and demonstrate the effectiveness of using various biomass precursors with heterogeneous N-Mn dopants for the development of green energy catalysts, taking into account biorefinery concepts.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal14020092/s1, Table S1. G peak position ν(G) and width FWHM(G) and average crystallite size La of carbon-based samples; Figure S1. Raman spectra of (a) nitrogen-doped carbon-based samples (red line) AWC-N, (green line) AHTC-N, and (black line) ABL-N. Excitation wavelength is 532 nm (0.4 mW); Figure S2. Raman spectra of nitrogen-doped carbon-based AWC-N with fitted Lorentzian (D, G, and D′ bands) or Gaussian (D* and D″ bands) form components. Excitation wavelength is 532 nm (0.4 mW); Figure S3. Polarization curves at various rotating speeds for (a) AWC-Mn-N, (b) AHTC-Mn-N, and (c) ABL-Mn-N.

Author Contributions

Conceptualization A.P., K.K. and G.D.; methodology and validation, A.P., K.K. and G.D.; formal analysis, K.K., A.B., V.J. and G.N.; data curation, K.K., A.B., V.J. and G.N.; writing—original draft preparation, A.P., D.S. and K.K.; writing—review and editing, A.P., A.V., G.D. and L.T.-T.; visualization, A.P., A.B. and G.N.; supervision, I.K., L.T.-T. and E.N.; project administration, L.C.C.-R., I.K. and E.N.; funding acquisition, L.C.C.-R., I.K. and E.N. All authors have read and agreed to the published version of the manuscript.

Funding

“Sustainably Produced Carbon Nanomaterials for Energy Applications (SuNaMa)” benefits from a 988,000 EUR grant from Iceland, Liechtenstein, and Norway through EEA Grants. The aim of the project is to develop innovative, high-performance, highly conductive, electrocatalytically active, durable, cost-effective, and high-surface-area nanocarbon materials. Project contract with the Research Council of Lithuania (LMTLT) No. is S-BMT-21-12 (LT08-2-LMT-K-01-055).

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

The authors would like to thank the Latvian State Institute of Wood Chemistry, Center for Physical Sciences and Technology, and SINTEF Industry, Batteries and Hydrogen Technologies for the support provided.

Conflicts of Interest

The authors declare no conflicts of interest.

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Catalysts 14 00092 g001
Figure 1. N2 adsorption–desorption isotherms at 77 K (a) and pore size distribution curves calculated from the corresponding isotherms by the Density Function Theory (DFT) method (b) of Mn-N-doped carbon catalyst samples.
Figure 1. N2 adsorption–desorption isotherms at 77 K (a) and pore size distribution curves calculated from the corresponding isotherms by the Density Function Theory (DFT) method (b) of Mn-N-doped carbon catalyst samples.
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Figure 2. SEM images (a,b) and EDX elemental mapping of all elements (c) and Mn (d) of AWC-Mn-N.
Figure 2. SEM images (a,b) and EDX elemental mapping of all elements (c) and Mn (d) of AWC-Mn-N.
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Catalysts 14 00092 g003
Figure 3. SEM images (a,b) and EDX elemental mapping of all elements (c) and Mn (d) of AHTC-Mn-N.
Figure 3. SEM images (a,b) and EDX elemental mapping of all elements (c) and Mn (d) of AHTC-Mn-N.
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Catalysts 14 00092 g004
Figure 4. SEM images (a,b) and EDX elemental mapping of all elements (c) and Mn (d) of ABL-Mn-N.
Figure 4. SEM images (a,b) and EDX elemental mapping of all elements (c) and Mn (d) of ABL-Mn-N.
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Catalysts 14 00092 g005
Figure 5. (a) Raman spectra (excitation wavelength is 532 nm (0.4 mW)) and (b) XRD spectra of Mn-N-doped carbon catalyst samples (red line) AWC-Mn-N, (green line) AHTC-Mn-N, and (black line) ABL-Mn-N.
Figure 5. (a) Raman spectra (excitation wavelength is 532 nm (0.4 mW)) and (b) XRD spectra of Mn-N-doped carbon catalyst samples (red line) AWC-Mn-N, (green line) AHTC-Mn-N, and (black line) ABL-Mn-N.
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Figure 6. XPS (a) survey, (b) C 1s, (c) N 1s spectra, (d) Mn 2p, and (e) O 1s spectra of manganese- and nitrogen-doped activated carbon based on wood char (AWC-Mn-N), manganese- and nitrogen-doped activated carbon based on HTC wood (AHTC-Mn-N), and manganese- and nitrogen-doped activated carbon based on black liquor (ABL-Mn-N).
Figure 6. XPS (a) survey, (b) C 1s, (c) N 1s spectra, (d) Mn 2p, and (e) O 1s spectra of manganese- and nitrogen-doped activated carbon based on wood char (AWC-Mn-N), manganese- and nitrogen-doped activated carbon based on HTC wood (AHTC-Mn-N), and manganese- and nitrogen-doped activated carbon based on black liquor (ABL-Mn-N).
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Figure 7. (a) LSV curves recorded in 0.1 M KOH for Mn-N-doped catalysts and the commercial 47.2% Pt/C catalyst. ω = 1600 rpm, ν = 2.5 mV s−1. (b) Chronoamperometric responses for Mn-N-doped catalysts and the commercial 47.2% Pt/C catalyst in 0.1 M KOH solution.
Figure 7. (a) LSV curves recorded in 0.1 M KOH for Mn-N-doped catalysts and the commercial 47.2% Pt/C catalyst. ω = 1600 rpm, ν = 2.5 mV s−1. (b) Chronoamperometric responses for Mn-N-doped catalysts and the commercial 47.2% Pt/C catalyst in 0.1 M KOH solution.
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Table 1. Porosity of Mn-N-doped carbon catalysts.
Table 1. Porosity of Mn-N-doped carbon catalysts.
SamplesSpecific Surface Area, m2 g−1Pore Volume, cm3 g−1Average Pore Width, nmMesopores Volume from Total, %
BETDR *DFTTotalMicroMeso
AWC-Mn-N2127190514261.20.70.52.241.9
AHTC-Mn-N2195193214941.30.70.62.447.0
ABL-Mn-N1808161912761.70.61.13.866.8
* Density Function Theory.
Table 2. G peak position ν(G) and width FWHM(G) and average crystallite size La of carbon-based samples.
Table 2. G peak position ν(G) and width FWHM(G) and average crystallite size La of carbon-based samples.
Sampleν(G) (cm−1)FWHM(G) (cm−1)La (nm)
AWC-Mn-N158857.013.1
AHTC-Mn-N160669.88.8
ABL-Mn-N160074.97.4
Table 3. Surface elemental composition calculated from XPS.
Table 3. Surface elemental composition calculated from XPS.
ElementABL-Mn-NAHTC-Mn-NAWC-Mn-N
at. %at. %at. %
N7.9 ± 0.17.7 ± 0.15.5 ± 0.1
O5.0 ± 0.16.5 ± 0.13.5 ± 0.1
C85.7 ± 0.183.9 ± 0.188.2 ± 0.1
Mn1.4 ± 0.11.9 ± 0.12.8 ± 0.1
Table 4. XPS analysis of the elemental composition of ABL-Mn-N, AHTC-Mn-N, and AWC-Mn-N.
Table 4. XPS analysis of the elemental composition of ABL-Mn-N, AHTC-Mn-N, and AWC-Mn-N.
SampleC 1sN 1sO 1sMn 2p3/2
Eb, eVat. %Eb, eVat. %Eb, eVat. %Eb, eVat. %
ABL-Mn-N284.210.2398.038.7530.137.4640.728.6
284.752.0399.030.8531.743.2642.137.1
285.425.0400.626.2533.319.4643.523.8
286.39.4402.64.2 645.810.5
287.13.4
AHTC-Mn-N284.212.0397.939.7530.036.2640.727.3
284.744.7398.934.9531.744.8642.037.9
285.327.2400.725.4533.419.0643.323.8
286.212.7 645.211.0
287.33.3
AWC-Mn-N284.17.6397.641.8529.928.6640.631.9
284.754.2398.632.4531.024.5642.038.7
285.426.0400.114.3531.931.1643.518.2
286.412.2401.411.5533.515.6645.811.2
Table 5. Electrochemical data for Mn-doped samples in 0.1 M KOH.
Table 5. Electrochemical data for Mn-doped samples in 0.1 M KOH.
SampleEonset, E vs. RHE mVE1/2, E vs. RHE mVjL at 0.25 VAverage nStabilityRef.
AWC-N881761−7.23.9-[61]
ABL-N841701−6.23.5-[61]
AWC-Mn-N924803−5.004.395%
AHTC-Mn-N899788−4.594.087%
ABL-Mn-N934829−4.934.092%
47.2% Pt/C979869−5.434.070%
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Plavniece, A.; Kaare, K.; Simkunaitė, D.; Balciunaite, A.; Jasulaitiene, V.; Niaura, G.; Volperts, A.; Dobele, G.; Colmenares-Rausseo, L.C.; Kruusenberg, I.; et al. Manganese- and Nitrogen-Doped Biomass-Based Carbons as Catalysts for the Oxygen Reduction Reaction. Catalysts 2024, 14, 92. https://doi.org/10.3390/catal14020092

AMA Style

Plavniece A, Kaare K, Simkunaitė D, Balciunaite A, Jasulaitiene V, Niaura G, Volperts A, Dobele G, Colmenares-Rausseo LC, Kruusenberg I, et al. Manganese- and Nitrogen-Doped Biomass-Based Carbons as Catalysts for the Oxygen Reduction Reaction. Catalysts. 2024; 14(2):92. https://doi.org/10.3390/catal14020092

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Plavniece, Ance, Kätlin Kaare, Dijana Simkunaitė, Aldona Balciunaite, Vitalija Jasulaitiene, Gediminas Niaura, Aleksandrs Volperts, Galina Dobele, Luis César Colmenares-Rausseo, Ivar Kruusenberg, and et al. 2024. "Manganese- and Nitrogen-Doped Biomass-Based Carbons as Catalysts for the Oxygen Reduction Reaction" Catalysts 14, no. 2: 92. https://doi.org/10.3390/catal14020092

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