Nitro Lignin-Derived Nitrogen-Doped Carbon as an Efficient and

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Nitro Lignin derived Nitrogen Doped Carbon as Efficient and Sustainable Electrocatalyst for the Oxygen Reduction Micaela Graglia , Jonas Pampel, Tina Hantke, Tim-Patrick Fellinger, and Davide Esposito ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b08040 • Publication Date (Web): 17 Mar 2016 Downloaded from http://pubs.acs.org on March 20, 2016

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Nitro Lignin Derived Nitrogen Doped Carbon as Efficient and Sustainable Electrocatalyst for the Oxygen Reduction a,#

Micaela Graglia, a,#Jonas Pampel, aTina Hantke, aTim-Patrick Fellinger*, and aDavide

Esposito* a

Max Planck Institute of Colloids and Interfaces, 14424 Potsdam, Germany.

#

M. Graglia and J. Pampel contributed equally.

E-Mail: [email protected]; [email protected].

KEYWORDS: Electrocatalysis, Lignin, Nitration, Nitrogen Doped Carbon, Ionothermal Carbonization

Abstract

The use of lignin as a precursor for the synthesis of materials is nowadays considered very interesting from a sustainability standpoint. Here we illustrate the synthesis of a micro-, mesoand macroporous nitrogen doped carbon using lignin extracted from beech wood via alkaline hydrothermal treatment and successively functionalized via aromatic nitration. The so

1


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obtained material is thus carbonized in the eutectic salt melt KCl/ZnCl2. The final N-doped carbon shows an excellent activity as electrocatalyst for the ORR reaction.

Lignin is the most abundant aromatic polymer in nature and can be isolated from plant´s cell wall1 where it is present in up to thirty weight percent. Lignin is mostly composed of three phenylpropanoid monomers, namely conyferyl, sinapyl and p-coumaryl alcohols, resulting in a structure that displays different types of aryl ether bonds.2 Commercial lignin is largely available at low costs as by-product of the paper and pulp industry. Recently, great attention has been devoted to the valorization of lignin that is considered a sustainable and low carbon footprint feedstock. Besides efforts for the depolymerization of lignin aiming at the production of valuable fine chemicals, attempts to convert this polymer into porous materials have been reported. Lignin or its modified forms have been used as efficient adsorbents for dyes, heavy metals and phenols. 3,4 Due to its high carbon content it has also been proposed as a more advantageous precursor for the production of activated carbons compared to cellulose.5,6 Accordingly, activated carbons were obtained using different strategies and successfully employed e.g. as adsorbents for the removal of water pollutants.7,8 The preformed aromaticity in the polyphenolic lignin, as well as the presence of redox active functionalities also suggest its use as precursor for the synthesis of carbon materials for electrochemical applications and in this context, reports on lignin-derived supercapacitor electrodes have appeared.9-11 Activated carbons materials are generally characterized by pronounced microporosity with limited total pore volume. However, the additional introduction of meso- and/or macroporosity is critical to enhance the applicability of carbon materials in the fields of supercapacitors and catalyst supports, where mass transport 2


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phenomena play a key role.9,12 Biomass-derived porous carbon can be readily produced using ionothermal methods. In this regard, we recently introduced an ionothermal carbonization approach that can be regarded as an expansion of the more classical ZnCl2 “activation”. In the latter one ZnCl2 droplets, formed throughout carbonization of impregnated biomass, act as micropore template rather than as a chemical leaching agent, like widely described.13 Ionothermal carbonization rationally expands this process by using eutectic mixtures that facilitates the dissolution or swelling of the carbon precursors leading to increased porosity formation up to the level of carbon aerogels.14-16 Interestingly, mesoporous carbons were recently obtained from Kraft lignin and other biomass by ionothermal carbonization in molten salt and the resulting material was used as catalyst support.17,18 In the context of electrochemical applications, nitrogen doped carbons (NDCs) gained a lot of attention among other carbon materials because of their improved conductivity, stability and the more versatile surface chemistry, that can enable electroactive functions.19 One of the most interesting effects of nitrogen doping is the strongly improved catalytic activity for the electrochemical oxygen reduction reaction (ORR).20,21 Selected NDCs showed very promising performances in this regard, at least in alkaline medium. Since the state-of-the art catalysts are based on the expensive and rare platinum, the preparation of cheaper and more efficient ORR catalysts represents a vibrant research area.22 In particular, the preparation of high end products such as NDC fuel cell catalysts using inexpensive materials like lignin would be very desirable. Unfortunately, lignin naturally lacks nitrogen functionalities, and therefore its application for the preparation of NDCs is not straightforward and can only be achieved so far via combination with a second nitrogen containing precursor.23 One of the drawbacks associated with this approach is the expected poor homogeneity of the final materials due to the absence of molecular mixing and related formation of a material with heterogeneous 3


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morphology. Therefore, the covalent functionalization of lignin with nitrogen containing groups might open new and more efficient avenues for the preparation of cheap and sustainable lignin based NDCs. In this report we introduce a two steps synthesis of sustainable micro-, meso- and macroporous NDCs using lignin from beech wood via covalent introduction of nitrogen containing groups and successive ionothermal carbonization (Fig. 1). Isolation of lignin from beech wood chips is accomplished through an alkaline hydrothermal treatment, followed by a straightforward aromatic nitration in order to bypass the natural lack of nitrogen in the biopolymer. The nitrogen functionalized lignin is thus carbonized in a eutectic KCl/ZnCl2 melt as porogen/swelling agent to obtain mesoporous NDCs characterized by high surface area, porosity, and nitrogen content (5-6 wt %N). The so obtained material showed an excellent catalytic activity towards ORR.

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Figure 1. A Schematic pathway for the synthesis of N-doped mesoporous carbon from beech wood lignin; B Synthetic pathway for the conversion of alkali-lignin (L) into Aminated Lignin (AmL) via aromatic nitration

Results and discussion: Lignin was obtained via alkaline hydrothermal treatment of beech wood chips in the presence of Ba(OH)2 at 220 °C for 15 h. Although many different pulping schemes allow for the preparation of lignin,24 this form of treatment was chosen as it simultaneously affords the important bio-based chemical lactic acid, offering therefore an example of integrated biomass schemes with full biomass utilization.25 As previously reported, the alkaline treatment restituted a lactic acid rich dark liquor as well as lignin, isolated from the solid filtrate. A THF-soluble lignin (L) fraction was thus isolated after an acid washing of the pulp in a 10 wt % yield compared to the starting biomass. The obtained lignin showed the characteristic composition, average molecular weight and dispersity (Mw/Mn) of classical alkali-lignins (Table 1).26,27 The 2D HSQC-NMR (Fig. 2A) of L, acetylated under standard conditions for the purpose of the analysis, displayed the presence of methoxyl, β-aryl ether and aliphatic groups typical of syringil and guaialcyl units. In addition, the FT-IR (Fig. 2C) confirmed the presence of hydroxyl and phenol groups, linked to aromatic and methoxyl functionalities. As mentioned before, L does not contain nitrogen functionalities. Only a limited number of processes has been developed for the introduction of nitrogen in the lignin structure and generally, they result in a reduced incorporation, as in the case of the ammonia percolation pretreatment, that would not fulfill the criteria for the preparation of NDCs.28 We considered the aromatic nitration as the simplest method for the selective covalent introduction of 5


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nitrogen containing functionalities on the lignin carbon backbone. Despite the wide choice of effective nitrating systems in the literature, including AgNO2 in the presence of Pd(OAc)2 29or Cu(NO3)2 supported on a zeolite catalyst,30 the simple and cheaper nitration by nitric acid in acetic anhydride in the presence of catalytic amounts of sulfuric acid

31

(Fig. 1B) was

investigated as a proof of principle. In order to optimize the reaction conditions, we firstly performed the nitration of 2-methoxy-4-propylphenol, which features phenylpropanoic, methoxy, and phenolic functionalities and is considered a good model for the more complex structure of lignin. The successful nitration was accompanied by acetylation due to the employed reaction conditions and afforded the desired compound in 55 wt % yield. Therefore, the reaction was performed on L under the optimized conditions, leading to a final compound with 6.7 wt % nitrogen content, according to elemental analysis (Table 1). The gel permeation chromatography (GPC) data of the nitrated lignin (NL) exhibited a sensible decrease in the average molecular weight and in the dispersity value when compared to the starting L. This advantageous side effect can be explained by hydrolysis of more labile ether linkages upon the introduction of acetyl and nitro groups in the lignin backbone (Table 1). Based on the combined information obtained from the model nitration, and the data obtained for NL (elemental analysis, GPC data), we concluded that ca. 50% of the aromatic rings in the lignin structure are nitro-functionalized, fulfilling the requirements for a good nitrogen-doped carbon precursor. The successful introduction of nitrogen functionalities was additionally confirmed by FT-IR (Fig. 2C) which showed characteristic N-O stretching signals (1365 and 1537 cm-1) as well as C-N stretch signals, (1643-1632 cm-1, 1273-1196 cm-1 and 1045 cm-1).32 Moreover, the decreased relative content of aromatic C-H in the 2D-HSQC NMR spectrum of NL was consistent with the occurrence of nitration (Fig. 2B). As in the case of the monomer,

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the nitration step was accompanied by acetylation of hydroxyl and phenol groups in lignin structure and was confirmed by 1H-NMR (Fig. 2D).

Figure 2. A 2D-HSQC NMR of acetylated-lignin (L); B 2D-HSQC NMR of acetylatednitrated lignin (NL); C FT-IR spectra of L versus NL; D 1H-NMR spectra of L, NL and nitrodeacetylated lignin (NDL).

Although NL already represents a promising candidate for the preparation of NDCs, amino lignin (AmL) was additionally prepared as a control material, in order to evaluate how different nitrogen-functionalities would affect the ionothermal synthesis and therefore the 7


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properties of the resulting carbon. NL was deacetylated using a methanolic solution of KOH affording nitrated-deacetylated lignin (NDL). Deacetylation was carried out to prevent possible acyl transfer reactions upon the following reduction, which will result in the loss of free amino functionalities by formation of acetamides. Thus, subsequent continuous flow reduction of NDL was investigated. The reduction conditions were optimized on the deacetylated nitro-2-methoxy-4-propylphenol model, leading to the quantitative isolation of the corresponding aniline, and were further applied to NDL. The reaction was monitored via colorimetric ninhydrin test (Fig. S2) and afforded the desired AmL compound with a 72 wt % yield. The observed variations in the molecular weight, dispersity and relative elemental compositions for the different lignins are reported in Table 1. Table 1. Mw and dispersity (Mw/Mn) of L, NL, NDL and AmL and their elemental composition before and after carbonization.

GPC analysis

Elemental analysis

Sample

Mw (g/mol)

Mw/Mn

NEA (Wt%)

CEA (Wt%)

H (Wt%)

C/N

L

3416

3.1

0.2

64.5

6.2

318

NL

1275

1.5

6.7

52.5

5.6

7.7

NDL

1082

1.4

6.2

53.5

5.6

8.9

AmL

786

1.5

6.2

54.4

6.4

9.1

L-C

----

----

0.4

79.1

1.7

204.2

NL-C

----

----

6.1 (6.7)a

78.5 (81.8)a

1.7

13.2

NDL-C

----

----

5.3 (4.8)a

80.2 (83.1)a

1.7

15.3

AmL-C

----

----

5.6 (3.5)a

80.7 (82.6)a

1.7

14.2

8


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Mw: Weight-average Molecular weight, Mw/Mn: polydispersity, L: alkali lignin, NL: nitro lignin, NDL: nitro-deacetylated lignin, AmL: amino lignin. C indicates the carbonized samples at 850 °C in the presence of KCl/ZnCl2. a: calculated by X-ray photoelectron spectroscopy. At this stage, NL and AmL were employed as nitrogen/carbon sources for ionothermal carbonization in eutectic KCl/ZnCl2 (KZ) salt melts. L was also carbonized in the same fashion to serve as a reference. Noteworthy, according to TGA analysis, L appeared as a noncarbonizable precursor in the absence of KZ, unlike NL and Am-L (Fig. S3). Additionally, KZ was selected due to the promising behavior shown during preliminary tests, which indicated a superior miscibility with the lignin derivatives leading also to higher yields, compared to other mixtures e.g. NaCl/ZnCl2 as shown in Table S1. All lignin samples were ball milled in order to obtain fine homogenous powders, thus grinded with the salt mixture under Argon atmosphere in a weight ratio of 1:5 (lignin derivative:salt). The samples were pyrolized to a final temperature of 850 °C in a nitrogen atmosphere and finally washed twice with deionized water to remove the salts. The ionothermal carbons were obtained in 21.1, 18.7 and 27.9 wt % yield for NL, NDL and AmL, respectively (yields are reported as wt % of the dry lignin precursor). Interestingly, the nitrogen content of the ionothermal carbons was found to be consistently high compared to the functionalized lignin precursor, indicating an efficient incorporation of nitrogen functionalities into the carbon framework (Table 1). The carbonized samples are indicated by a C (for carbonized) tagged to the corresponding name. All carbonized samples showed a similar elemental composition with nitrogen contents of 6.1, 5.6 and 5.3 % N for NL-C, AmL-C and NDL-C respectively. X-ray photoelectron spectroscopy (XPS) analysis of the samples was performed to identify the nature of the surface N-sites within the materials. In line with the combustion elemental analysis, NL-C features the highest nitrogen content (6.7 wt %) according


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to XPS. However, the values appear relatively lower for NDL-C (4.8 wt % N) and AmL-C (3.5 wt % N). In general, such value resulted different compared to the bulk elemental analysis. High resolution XPS gives insight into the relative abundance of different N sites, of which especially pyridinic and quaternary nitrogen are considered active in the ORR. The results are summarized in Fig. 3 and in Table S2. It can be observed that the absolute abundance of pyridinic and graphitic sites follows the order AmL-C < NDL-C < NL-C, indicating a more efficient enrichment of nitrogen sites on the surface of the material derived from NL.

Figure 3. Absolute abundance of nitrogen sites evaluated by XPS for AmL-C, NDL-C and NLC.

Nitrogen adsorption porosimetry was used to analyze the materials porosity (Fig. 4). The specific surface area was calculated by Brunauer-Emmett-Teller (BET) method for the adsorption data at p/p0 < 0.3 using QuadraWinTM Micropore BET Assistant. The external surface area and the micropore volume were obtained by using the slope and the intercept of the linear region of the tplot after the micropore filling respectively (statistical thickness method). Pore size distribution


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and total pore volume were obtained from the adsorption branch due to capillary effects and by employing a QSDFT model for carbon with slit shaped, cylindrical and spherical pores provided by Quantachrome©. A summary of the porosity data can be found in Table 2. Table 2. Pore properties of L-C, NL-C, NDL-C, AmL-C. Pore Volume (cm3g-1) Sample

SBET

Vtot

Sext

Vmicro

Vmeso

C Yield (%)

(m2 g-1) L-C

258

0.152

64

0.084

0.068

61.1

NL-C

1589

0.826

122

0.643

0.183

38.3

NDL-C

1381

0.704

135

0.524

0.180

28.2

AmL -C

1564

0.744

102

0.595

0.149

41.8

L-C: carbonized alkali-lignin, NL-C: carbonized nitro-lignin, NDL-C: carbonized nitrodeacetylated lignin, AmL-C: carbonized amino-lignin. Carbonization of samples is performed at 850 °C in the presence of KCl/ZnCl2.

All products show type IV isotherms characteristic for mesoporous materials, with an offset due to the presence of micropores. We also observed a steep gas uptake at high relative pressures, which is due to the presence of macroporosity. The obtained materials show hierarchical pore systems, which are typical for ionothermal carbons at medium to high salt-to-precursor ratios and also desirable for efficient electrocatalytic conversions. The steep decrease in gas uptake in the desorption branch points to the presence of “ink-bottle” pores (Fig. 4A). The strong increase in


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gas uptake for the nitrogen doped samples compared to the reference L-C sample suggests that the functionalized lignin is more liable towards swelling or dissolution in the salt melt, allowing for the more efficient porogenesis. It is worth mentioning that this increased solubility might also be additionally favored by a decrease in the molecular mass of the nitrated lignin samples. The pore size distribution of the samples indicates intense contribution of micropores for all ionothermal carbons (Fig. 4B). Additionally a smaller peak in the small mesopore range, herein between 2.0 and 3.5 nm, is obtained for the nitrogen doped carbons, but not for the crude lignin based sample (Fig. 4B). Partly, these mesopores explain the high gas uptake at low relative pressures of NDL-C, NL-C and AmL-C as compared to L-C. As compared to previously reported ionothermal carbons, a noticeable low gas uptake in the medium pressure range (~0.30.6 p/p0) was observed in the case of L-C. The value of the related external surface area of Sext = 64 m2 g-1 is very typical for ZnCl2 activated carbons. Similarly, AmL-C (Sext = 102 m2 g-1), NDL-C (Sext=135 m2 g-1) and NL-C (Sext=122 m2 g-1) show only a moderate but sensibly enhanced external surface areas compared to L-C.

Figure 4. A Isotherms and B pore size distributions, obtained from N2-physisorption analysis of L-C, NL-C, NDL-C and AmL-C.


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Scanning electron microscopy was used to get further insight into the products morphology. Comparing the SEM pictures of the non-carbonized samples and respective carbonized materials (Fig. 5), a developed porosity can be observed after carbonization, whereas the precursors show smooth and dense surfaces as can be expected for thermoplasts like lignin. The carbonized samples show a rough surface, which can be assigned to micropores and small mesopores, but also a broad range of larger meso- and macropores, depending on the actual sample. In general the morphology of the materials as observed by SEM is not very homogeneous. This is to be expected from biomass precursors, however the trends in the morphology reflect the gas sorption data well as shown by the selected representative images. A strong morphological difference between NL-C and AmL-C/NDL-C, which is less obvious from nitrogen adsorption data, can be observed by microscopy imaging. NL-C is composed of many small aggregated particles and has large interstitial meso- and macroporosity. AmL-C and NDL-C, in turn, are mostly composed of large bulky chunks containing micro- and mesopores obviously originating from molten salt droplets, but with less connected macroporosity, which is less desirable in terms of mass transport. The origin of these morphological differences can be speculatively attributed to a different mode of interaction for the lignin precursors with the salt melt. NL undergoes significant dissolution in the salt melt and carbon particles are formed throughout the progress of carbonization (bottom-up). AmL and to a higher extent NDL, seem to be only swollen by the eutectic salt melt on the other hand, which allows for the formation of porous carbon, but the final carbon morphology is a consequence of the particle shape of the precursor (top-down). In this regard, the carbonization of AmL and NDL can be described as a border line mechanism located between classical ZnCl2 activation and the more recent ionothermal carbonization, also helping to differentiate the two. Finally, the different interaction of the lignin matrices with the


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salt melt is specifically influenced by the different chemical reactivity of nitro and amino groups with KCl/ZnCl2.

Figure 5. SEM images of: (A) Lignin L, (B) Carbonized Lignin LC, (C) Nitro-Lignin NL, (D) Carbonized Nitro-Lignin NL-C, (E) Aminated Lignin AmL, (F) Carbonized Aminated Lignin AmL-C, (G) Carbonized Nitro-deacetylated Lignin NDL-C

The XRD patterns of the obtained materials (Fig. S5) show the strongly disordered structure of the ionothermal carbons characterized by broad (002) and (110) reflections with low intensity and strong small angle X-ray scattering caused by the micropores. The diffractogramm shows no other crystallinity, such as residual salt or metals/metal oxides. Simple water washing apparently leads to sufficiently pure NDCs. All together the obtained NDCs show promising nitrogen content and mass transport porosity, which are desired for NDC electrocatalysts. Electrochemical tests were therefore carried out in 0.1 M KOH using a standard Gamry three electrode setup with a glassy carbon rotating disk electrode, a graphite counter electrode and a


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Ag/AgClsat reference electrode (Fig. 6A). The lignin derived sample L-C behaves like a typical nitrogen free carbon showing a low half wave potential (E1/2) of 0.67 V (vs. RHE) and an electron transfer number (n) of 1.9 as revealed by Koutecký-Levich (KL) analysis (Fig. 6B and C). In contrast, all nitrogen doped carbons derived from N-functionalized lignin possess a strongly enhanced electrocatalytic activity. The aminolignin derived AmL-C shows a clearly improved n of 3.6 and a 140 mV positively shifted E1/2 if compared to L-C. The highest activity is obtained with the nitrolignin derived sample NL-C resulting in an upshift of 180 mV (40 mV compared to AmL-C), i.e. an E1/2 of 0.85 V (vs. RHE). KL analysis of NL-C reveals an ideal 4 eselectivity at different potentials in the kinetically limited potential region (Fig. 6B). The high selectivity was further corroborated by rotating ring-disk measurements, where peroxide, indicative of a two electron processes, is detected in-situ. The measurements show low peroxide concentration and a high selectivity throughout the whole potential range (Fig. S6). The remarkable difference of AmL-C and NL-C is highly interesting if we consider that both materials show similar surface area and nitrogen content. The different performances result from the special ability of nitro functionalities to favor the more efficient formation of ORR-active N sites such as pyridinic and graphitic nitrogen on the surface of the carbon particles. This explanation is supported by the fact that NDL-C shows a similar onset potential as NL-C therefore indicating a similar surface chemistry, also originating from the presence of nitro groups in the precursor. Nevertheless, the kinetic performance of NDL-C is decreased causing a 10 mV negatively shifted E1/2 which can be explained by the lower SBET (87 % of NL-C) and thus a lower electrochemical active surface area. A second factor to be considered is the improved accessibility due to the particulate morphology in the case of NL-C which favors the access to the active sites leading to the faster kinetics. Interestingly, AmL-C shows similar


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kinetics as NL-C indicating that the improved supply of oxygen to the active sites is mediated by the additional macroporosity, which is apparent from the higher relative gas uptake in the nitrogen physisorption measurements for these two samples (Fig. 4). In line with other recent results these data suggest the importance of combining the “right” active nitrogen sites with an optimized accessible pore system, in order to achieve highly active ORR-catalysts.33 Chronoamperometric stability tests show 78 % retained current density after 8 h and high stability against the methanol crossover, which causes serious catalysts poisoning e.g. at commercial precious Pt/C electrocatalysts (Fig. S7). All in all, the electrochemical performance of NL-C is outstanding for a lignin derived carbon, as the catalyst performs comparable to advanced non-noble metal catalysts (e.g. Fe-coordinated NDCs) from the recent literature (Fig. 6D).22,34-39


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Figure 6. (A) RDE polarization curves in O2-saturated 0.1 M KOH with a sweep rate of 5 mV s−1, 1800 rpm, (B) Koutecký-Levich analysis of L-C, AmL-C, NDL-C and NL-C at different potentials obtained from the polarization curves at different rotation rates. (C) E1/2 of L-C, AmLC, NDL-C and NL-C.C (D) Comparison of the polarization curve of NL-C with different literature benchmark, non-noble metal catalysts (alkaline solution). On the basis of the reported data, the N-functionalized lignin derivatives NL (nitrated) and AmL (aminated) lead to high surface area nitrogen doped carbons with similar properties in terms of elemental composition as well as their porosity after ionothermal carbonization. In addition, NLC exhibits a slightly better electrocatalytic activity than AmL-C. However, the synthetic pathway


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from NL to AmL increases the cost and the complexity of the process, decreases its value and the possibility of a potential industrial scale up without bringing any substantial advantage. Therefore, the use of NL allows for the preparation of a more active electrocatalyst material and offers the advantage of a cheaper and more sustainable process.

Conclusions In this work we reported the preparation of very efficient nitrogen doped carbons as electrocatalyst using widely available and sustainable lignin as the precursor. A straightforward nitration was used for the introduction of nitrogen functionalities in an alkaline lignin obtained from beech wood. Manipulation of the introduced nitro group via reduction was performed in order to generate different analogues and study the impact of different N-containing groups on the properties of the final carbon. The so obtained precursors were thus employed for the synthesis of nitrogen doped hierarchically porous carbon via ionothermal carbonization in molten eutectic KCl/ZnCl2. The nitrogen doped carbons showed high porosity, high surface area and a relatively high nitrogen content (5-6 wt % N). Products from nitrated lignin show higher absolute abundance of pyridinic and quaternary nitrogen, which leads to increased ORR activity. All NDCs show strongly improved catalytic activity as compared to the unmodified lignin based reference. However, the catalytic activity of nitrated lignin leads to a superior ORR catalyst, which in alkaline medium catalyzes the desired four electron process with a half-wave potential of 0.85 V vs. RHE. This sustainable and cheap material presents a performance comparable to current state-of-the art non-noble metal catalysts and is a highly promising materials system for future investigation. The very different performance of both materials, which were obtained by equivalent methods, indicates the strong dependence of the detailed N doped carbon structure


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and morphology. This work discloses a general strategy for the preparation of high value Ndoped functional materials on the basis of lignin using a simple and inexpensive functionalization procedure in combination with a very convenient ionothermal carbonization method. Future studies will focus on the extension of this approach to the preparation of N-X doped carbons (e.g. Sulfur doped materials using Kraft Lignin) as cheap and sustainable electrode materials, such as ORR-electrocatalysts. General materials and methods All the reagents used for synthetic purposes were reagent grade and were used without further purification. Barium hydroxide octahydrate, hydrochloric acid (1 M), 2-methytetrahydrofuran, tetrahydrofuran, 2-methoxy-4-propylphenol, nitric acid, acetic anhydride, sulfuric acid, potassium hydroxide, diethyl ether, sodium sulfate anhydrous, hydrochloric acid 37%¸ methanol, ninhydrin, pyridine and chloroform were purchased by Sigma Aldrich. Beech wood chips were purchased by GOLDSPAN®. Raney-Nickel packed cartridge was purchased by ThalesNano®. All the solvents used for the analysis were of analytical grade and purchased by Sigma Aldrich. 1

H-NMR monodimensional spectra were recorded on a Bruker Spectrospin 400 MHz Ultrashield

Spectrometer in deuterated solvents, with chemical shifts referenced to the residual solvent signals. Spectra of model compound derived samples were recorded dissolving 20 mg of the sample in 0.6 ml of CD3OD. Bidimensional HSQC-NMR were recorded using an Agilent 400 MHz device dissolving 200 mg of acetylated lignin in 0,6 ml of CDCl3. Elemental analysis was performed as combustion analysis using a Vario Micro device. Gel permeation chromatography (GPC) was carried out on a Thermo Separation Products apparatus equipped with an UV/RI detector. The samples were eluted using N-methyl-2-pyrrolidone as


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solvent at 70 °C through two PSS-GRAM columns in series (300 mm, 8 mm2) with an average particle size of 7 µm and porosity between 100–1000 Å. Polystyrene was used as calibration standard. Fourier transform-infrared spectroscopy (FT-IR) spectra were recorded on a Varian 1000-FT-IR spectrometer. GC-MS analysis was performed using an Agilent Technologies 5975 gas chromatograph equipped with a MS detector and a capillary column (HP-5MS, 30 m, 0.25 mm, 0.25 micron). The analytical gradient method started with a first temperature of 50 °C kept for 2 min, then the temperature was increased to 300 °C (rate of 10 °C/min) and the system maintained at the final temperature for 20 min. The injector and detector temperatures were set to 250 °C and 280 °C respectively. Gas (nitrogen) sorption experiments were performed using a Quantachrome Quadrasorb device processing the data with the QuadraWin software. Scanning electron microscope (SEM) images were recorded using a LEO 1550 Gemini microscope. The samples were loaded on carbon coated aluminium holder and measured without any additional coating. Wide angle diffraction analysis (XRD) were obtained by a Bruker D8 diffractometer equipped with a Cu-Kα source (λ = 0.154 nm) and a scintillation counter (KeveX Detector). The reduction reactions were performed using a H-Cube Pro™ flow reactor. X-ray photon spectroscopy (XPS) was measured with a Thermo-VG Scientific ESCALAB 250 X-ray photoelectron spectrometer (Thermo Electron, U.K.) using Al Kα X-ray source (1486.6 eV). The carbon peak served as internal reference and the high resolution N1 s spectra was used for the quantification of the different N sites. Thermogravimetric Analysis coupled MS (TGA-MS) were performed by a NETZSCH TG 209F1 Libra TGA209F1D-0036-L instrument coupled with a Thermostar Mass spectrometer. Electrochemical measurements were conducted at a rotating disk electrode (RDE) set up (Gamry Instruments) in 0.1 M KOH (Titripur®, Merck) equipped with a Ag/AgClsat electrode and a graphite rod (diameter, 6 mm) as reference and counter electrode,


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respectively. Glassy carbon electrodes (Gamry Instruments; diameter, 5.0 mm) served as working electrodes.

ASSOCIATED CONTENT

Supporting Information. All the synthetic methods and characterization of products are described in details in the supporting information This material is available free of charge via Internet at http://pubs.acs.org.

Acknowledgements We acknowledge the Max Planck Society for financial support. The technical staff at the MPI is thanked for standard analysis. C. Rodríguez (University of Vigo) is acknowledged for XPS measurements. Prof. P. Adelhelm (Friedrich-Schiller University of Jena, Germany) is thanked for suggesting WebPlot software to digitalize literature data.

Corresponding Author *E-Mail: [email protected]; [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. #: M. Graglia and J. Pampel contributed equally. REFERENCES


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