Biomass derived 2D carbons via a hydrothermal carbonization method as efficient bifunctional ORR/HER electrocatalysts

Baobing Huang , Yuchuan Liu and Zailai Xie *
State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou 350116, P. R. China. E-mail: zlxie@fzu.edu.cn

Received 13th September 2017 , Accepted 1st October 2017

First published on 3rd October 2017


Biomass derived carbons via hydrothermal carbonization (HTC) are critically important for different applications due to their low cost, eco-friendly properties, light weight and potential for the catalysis research field. However, standard HTC carbons commonly present amorphous and spherical morphology with low surface areas and large particle size. Here we report a 2D morphology of crystalline carbons via a HTC method by employing the biomolecule guanine and diverse carbohydrates (glucose, fructose and cellulose) as carbon precursors. Importantly, the addition of guanine to the HTC process plays a significant role in producing the 2D-morphology of carbon materials from the diverse carbohydrates. Hierarchical porous nitrogen-doped carbons with high surface areas up to 974 m2 g−1 and nitrogen contents as high as 2.39 at% are made by further high temperature carbonization. The current 2D carbons are found to be highly active toward the oxygen reduction reaction (ORR) with a rather positive half-wave potential of 0.87 V (vs. the RHE) and a large limiting current density of 5.9 mA cm−2. These catalysts outperform most other equivalent benchmarks as well as Pt-based electrocatalysts, meanwhile being efficient for the hydrogen evolution reaction (HER) with a very low overpotential of 0.35 V to achieve 10 mA cm−2 in alkaline medium. It is expected that our synthetic protocol can be extended to various bio-nucleobases, yielding all-sustainable bio-based electrocatalysts with a tunable structure, elemental composition, porosity and electronic properties.


Introduction

The oxygen reduction reaction (ORR) is one of the most pivotal reactions in energy converting system (such as fuel cells).1,2 When combined with its inverse process, the so-called oxygen evolution reaction (OER), rechargeable metal–air batteries can be assembled.3 Besides, the hydrogen evolution reaction (HER), involved in water-splitting devices, is capable of supplying hydrogen gas as a clean energy carrier.4 All of the above three reactions belong to the core processes of clean and renewable energy systems. As known, platinum-based catalysts are rather efficient for both the ORR and HER.5–7 However, these noble metal based catalysts possess numerous fatal drawbacks of limited reserves, poor durability and extreme sensitivity to alcohol.8 Therefore, the pursuits of more suitable alternatives to noble metal-based ORR and HER catalysts, particularly those kinds of metal-free catalysts that show excellent performance, low cost, non-toxicity and multifunctionality, are highly encouraged.

One of the prerequisites for efficient electrocatalysts is their porosity (e.g. specific surface area and micro-, meso-, and macro-porous textures) that provides enough space for ionic mass transport.9–13 Porous carbon materials, owing to their numerous merits of high conductivity, excellent mechanical stability, relatively low cost, and easily surface-functionalization, have been extensively studied as metal-free electrocatalysts.14,15 Particularly, carbon materials with heteroatom doping, as a kind of metal-free catalyst, have recently displayed huge potential in the electro-catalysis field, e.g. acting as the first metal-free catalyst for the ORR in 2009,16 for the OER in 2013[thin space (1/6-em)]17 and for the HER in 2014,18 coupled with the first metal-free bifunctional catalyst for the ORR and OER in 2015[thin space (1/6-em)]19 as well as for the ORR and HER in 2016.20 It is widely believed that nitrogen atoms doped into conjugated carbon skeletons could induce a relatively increased charge density on adjacent carbon atoms, enhancing the adsorption and reactivity of intermediates.17,21,22 Co-doping with various heteroatoms (e.g. N–S, N–B, and N–P) also largely improves the electrocatalytic activities mainly via the synergic effect.23–25

Among various carbons, the appearance of graphene has triggered enormous research interest in 2D carbon electrocatalysts.26–33 Such 2D carbons can be efficiently exposed to the electrolyte and the majority of surface active sites on 2D carbons could take part in the electrocatalytic reactions, which makes 2D carbon highly efficient for an array of electrochemical reactions. Owing to these unique functionalities, intense research has been focused on searching for more facile and green synthetic techniques that allow for the fabrication of porous and N-doped 2D carbons.34–37

In terms of green synthesis of carbons, hydrothermal carbonization (HTC) of biomass has demonstrated its capability of converting biomass into functional carbons, which meets the requirements of green chemistry.38–43 However, the major drawbacks of standard HTC carbons using biomass as precursors are lack of porosity and large size of carbon particles, which are undesirable for electrochemical processes when applied as electrode materials. Thus, the next challenge of HTC is to integrate abundant heteroatoms, developed porosity and 2D morphology into carbon frameworks via a one-pot synthetic protocol. However, to the best of our knowledge, the production of 2D materials via the HTC technique with biomass as precursors still remains to be explored.

Here we present a new family of hydrothermal carbons with the characteristics of 2D-morphology, nitrogen-doping and a hierarchical porous nanoarchitecture by employing the biomolecule guanine and diverse carbohydrates (glucose, fructose and cellulose) as carbon precursors. The carbon materials from the different carbohydrates exhibit unprecedented nanosheet morphology, regardless of different elemental compositions and porosities. SEM and TEM characterizations indicate that the HTC treatment of the combination of reactive guanine and monosaccharides favors the formation of N-doped 2D carbon nanosheets. Such unique functionalities and morphology lend significance to their application as high performance electrode materials. The three presented HTC carbons were tested for their ORR-related activity and HER activity. The current 2D carbon made from monosaccharides is found to be highly active toward the ORR with a super-positive half-wave potential of 0.87 V (vs. the RHE) and a large limiting current density of 5.9 mA cm−2, meanwhile being efficient for the HER with a very low overpotential of 0.35 V to achieve 10 mA cm−2 in alkaline medium. Notably, this is the first report on the in situ nitrogen-doping of 2D carbon via a sustainable biomolecule-based HTC approach achieving highly efficient ORR and HER performance.

Results and discussion

Synthesis and characterizations

As depicted in Scheme 1, the synthesis of nitrogen-doped carbon nanosheets prepared from glucose and guanine was illustrated. First, P123 and SO as double surfactants were dissolved in an aqueous solution to form mixed micelles owing to their hydrophobic nature.44 After that, a certain amount of guanine as a powerful nitrogen source and carbohydrates as carbon sources were added to the above solution; the mixtures were then transferred into an autoclave and kept at 180 °C for 8 h. During the HTC process, glucose first dehydrated to 5-hydroxymethyl furfural.40,45 The guanine then reacted with 5-hydroxymethyl furfural via the Schiff base reaction between the amino appended to guanine and the carbonyl groups in 5-hydroxymethyl furfural. Meanwhile, condensation took place around micelle droplets via the weak interaction, resulting in the formation of a hydrothermal composite. The prepared hydrothermal materials were further subjected to carbonization at 1000 °C for 2 h in a N2 atmosphere to obtain conductive carbons. During the high temperature treatment, the embedded soft template P123 gradually decomposed in the form of CO2 or CO gas to produce considerable micropores inside the carbon materials. During the carbonization process, the relatively thin outermost hydrothermal carbon walls were further shrunk to form 2D carbon nanosheets with curved layers and in situ nitrogen doping.
image file: c7ta08052b-s1.tif
Scheme 1 The synthetic route of nitrogen-doped 2D carbon nanosheets for highly efficient ORR and HER.

Regarding the morphology, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were first performed to disclose the surface morphology of the as-synthesized materials. In Fig. 1(a), (d) and (g), SEM shows the homogeneous and partly curved thin sheet-morphology of carbon for Fru/Gu-HTC-1000 and Glu/Gu-HTC-1000, while the heterogeneous morphology possessing different kinds of textures for Cel/Gu-HTC-1000 is observed. With respect to Cel/Gu-HTC-1000, cellulose is relatively difficult to carbonize at 180 °C; hence it mixes with guanine unevenly, leading to a heterogeneous morphology in the final product with different kinds of carbon textures. Although the current sample morphologies are quite similar for Fru/Gu-HTC-1000 and Glu/Gu-HTC-1000, they are significantly different from other HTC carbons. Standard HTC materials are composed of uniform spherical particles of diameters >0.5 μm. In contrast, the presented carbons are composed of a homogeneous 2D-morphology of nanosheets with a mean size of ∼0.2–0.5 μm. Importantly, the aggregated and interconnected nature of nanosheets in these size domains generates porosity.


image file: c7ta08052b-f1.tif
Fig. 1 S(T)EM images of different samples with increasing magnification: Fru/Gu-HTC-1000 (a–c), Glu/Gu-HTC-1000 (d–f), and Cel/Gu-HTC-1000 (g–i).

Furthermore, a rather thin sheet-like morphology can be clearly observed for Fru/Gu-HTC-1000 and Glu/Gu-HTC-1000 in the TEM images in Fig. 1(b) and (e). A fluffy sponge-like structure can be found in Cel/Gu-HTC-1000 in Fig. 1(h). The graphitic structure, especially for the samples Fru/Gu-HTC-1000 and Glu/Gu-HTC-1000, is clearly observed in the HRTEM images. The formation of the 2D morphology is presumed to be related to the interaction of the forming carbon framework and the surface of micelle droplets, such that the surfactant essentially acts as a sacrificial soft template/surface stabilizer. Alternatively, the layered guanine plays an important role in inducing the ultrathin 2D structure, which is in agreement with results reported elsewhere.46

Heteroatom-doping is crucial to modify the physico-chemical properties of the carbons. Elemental analysis of the presented 2D carbons shows different values of the carbon and nitrogen contents depending on the precursors. The nitrogen content of Glu/Gu-HTC-1000 is slightly higher than that of Fru/Gu-HTC-1000 (Table 1), indicating a smaller proportion of glucose-based substances in the carbonized hydrothermal composite, probably resulting from different carbonization degrees of both precursors. X-ray photoelectron spectroscopy (XPS) was carried out to analyze the status of heteroatom doping. The survey spectra of all three samples reveal the presence of C 1s, N 1s and O 1s characteristic peaks in Fig. 2(a), indicating the successful doping of nitrogen alien atoms into the carbon skeletons. It should be pointed out that the guanine precursor was the N source, and oxygen was from both raw materials and moisture adsorbed from air. The high resolution N 1s spectra in Fig. 2(b)–(d) are mainly deconvoluted into three typical peaks located at 398.4 eV, 401.0 eV and 403.0 eV, corresponding to pyridinic N, quaternary N, and N–O species, respectively. Additionally, the elemental mapping images in Fig. 2(e) exhibit uniform dispersion of different kinds of atoms throughout the Fru/Gu-HTC-1000, which further confirms the homogeneous doping of nitrogen atoms into the entire carbon skeleton.

Table 1 Textural properties and elemental compositions
Samples Porosity data Elemental compositionsc, at%
S BET [m2 g−1] V total [cm3 g−1] V meso [cm3 g−1] V micro [cm3 g−1] C N O
a From total N2 uptake at P/P0 = 0.95. b From the DFT method. c Determined by XPS.
Glu/Gu-HTC-1000 624 0.60 0.36 0.17 94.78 2.39 2.83
Fru/Gu-HTC-1000 974 0.84 0.47 0.27 95.77 1.53 2.71
Cel/Gu-HTC-1000 841 0.52 0.12 0.36 93.52 3.26 3.22



image file: c7ta08052b-f2.tif
Fig. 2 (a) XPS survey spectra; (b), (c) and (d) high resolution N 1s XPS spectra of Fru/Gu-HTC-1000, Glu/Gu-HTC-1000 and Cel/Gu-HTC-1000, respectively; (e) HAADF-STEM mapping of Fru/Gu-HTC-1000.

The Raman spectra of the serial materials in Fig. 3(a) all display typical D bands (centered at 1337 cm−1) and G bands (centered at 1589 cm−1), wherein the intensity ratio of D band to G band (ID/IG) is used to evaluate the amount of defects and disorder. Fru/Gu-HTC-1000 possesses the highest ID/IG values up to 1.15, indicating a defect-rich configuration throughout the flawless carbon skeletons, which have been proved to be highly active sites toward electrocatalysis.29,30,47–49 Besides, X-ray diffraction (XRD) was performed to probe the degree of graphitization. Concerning the carbon structure, as shown in Fig. 3(b), all samples exhibit relatively broad (002) diffraction peaks centered at 25.0°, together with observable characteristic (100) reflections at 43.6°, suggesting rather disordered but mainly sp2 hybridized graphitic structures, which is in good accordance with the Raman results.


image file: c7ta08052b-f3.tif
Fig. 3 (a) Raman spectra; (b) XRD patterns of the different samples; (c) nitrogen adsorption/desorption isotherms; (d) the corresponding pore size distributions (PSDs).

The nitrogen sorption behavior was investigated to clarify the porosity of carbon materials. As can be clearly seen in Fig. 3(c) and (d), two samples, Fru/Gu-HTC-1000 and Glu/Gu-HTC-1000, present very striking hysteresis loops at P/P0 > 0.4, along with obviously increased N2 uptake at P/P0 > 0.9 indicating the existence of considerable mesopores and partial macropores, which will greatly favor mass transfer and diffusion in catalytic reactions. For Fru/Gu-HTC-1000, its mesopores and partial macropores may stem from intersheet voids created by their stacking, while in the case of Glu/Gu-HTC-1000, its mesopores and macropores are mainly contributed from the spaces formed by the highly curved ultrathin sheets and broken holes inside them. Regarding the region of micropores, Fru/Gu-HTC-1000 and Cel/Gu-HTC-1000 exhibit higher gas uptake than Glu/Gu-HTC-1000, which could be explained by the fact that thin sheet-like walls and a fluffy monolith might carry more micropores in them compared to ultrathin graphene in Glu/Gu-HTC-1000. Herein, these kinds of micropores mainly originate from the holes introduced by soft templates, which are first embedded into the hydrothermal composites during the hydrothermal carbonization process and then gradually decompose in the form of CO2 or CO gas when the temperature is over 550 °C.

Electrochemical performance toward the ORR and HER

The electrochemical activity of the three catalysts for the ORR was initially investigated. All tests were carried out in an O2-saturated solution and at room temperature. As shown in Fig. 4(a), the cyclic voltammogram (CV) curves recorded in the O2-saturated 0.1 M KOH solution with a scan rate of 50 mV s−1 present an intense oxygen reduction cathodic peak at ca. 0.8 V (vs. the RHE); however, it completely disappears in the N2-saturated 0.1 M KOH solution, suggesting the outstanding ORR activities of the Fru/Gu-HTC-1000 catalyst. Rotating disk electrode (RDE) voltammetry was performed to distinctly compare the ORR kinetics of the different catalysts. The linear sweep voltammetry (LSV) curves in Fig. 4(b) were recorded at a rotating speed of 1600 rpm with a scan rate of 10 mV s−1 in the O2-saturated 0.1 M KOH solution. Among them, Fru/Gu-HTC-1000 displays the most positive onset potential (at ca. 1.00 V vs. the RHE), comparable to that of a commercial 20 wt% Pt/C catalyst (at ca. 1.03 V vs. the RHE), coupled with a larger diffusion-limited current density (ca. 5.9 mA cm−2) and half-wave potential (ca. 0.87 V vs. the RHE) than the Pt/C catalyst (E1/2 = 0.85 V vs. the RHE, jL = 5.6 mA cm−2) under the same testing conditions, indicating the extraordinary ORR activities. With regard to Glu/Gu-HTC-1000, which possesses an equal onset potential to Fru/Gu-HTC-1000 but a bit lower half-wave potential and diffusion-limited current density (E1/2 = 0.86 V vs. the RHE, jL = 5.6 mA cm−2), its ORR performance is comparable to that of the commercial Pt/C catalyst. In contrast, the ORR activity of Cel/Gu-HTC-1000 is much more inferior to that of Fru/Gu-HTC-1000, reflecting in the unsatisfactory onset potential, half-wave potential and diffusion-limited current density (Eonset = 0.94 V vs. the RHE, E1/2 = 0.79 V vs. the RHE, jL = 4.4 mA cm−2), which is largely consistent with the CV results (Fig. S1). Typically as depicted in Fig. 4(c), the current densities increase with ascending rotating speeds, suggesting the diffusion-controlled nature of the oxygen reduction reaction. The average electron transfer number of Fru/Gu-HTC-1000 which is calculated from Koutecky–Levich (K–L) plots showing good linearity at different voltages is ca. 4.0, convincingly demonstrating that the corresponding catalyzed ORR process proceeds through the favorable oxygen to hydroxyl reduction pathway. Moreover, the stability and tolerance to methanol are quite crucial factors to the practical application in fuel cells, wherein the Pt/C catalyst displays imperfect performances. The stability and tolerance to methanol were tested at 0.8 V (vs. the RHE) in O2-saturated 0.1 M KOH at a rotating rate of 1600 rpm, as shown in Fig. 4(d). The current retention rate of Fru/Gu-HTC-1000 could achieve 88.0% after a 20[thin space (1/6-em)]000 s stability test, obviously superior to that of the Pt/C catalyst (74.6%), indicating a higher ORR durability. Besides, as can be seen from the inset, the current direction of Pt/C immediately changed in reverse when methanol was added at roughly 1000 s (final concentration of 1 mol L−1), corresponding to the occurrence of methanol electrooxidation, while Fru/Gu-HTC-1000 was almost immune to methanol influence.
image file: c7ta08052b-f4.tif
Fig. 4 (a) CVs with 50 mV s−1; (b) comparisons of the RDE polarisation curves in O2-saturated 0.1 M KOH at 1600 rpm with 10 mV s−1; (c) RDE polarisation curves at various rpms; (d) current–time (It) response at 0.8 V (vs. the RHE) at 1600 rpm; the inset corresponds to the testing of tolerance to methanol.

Surprisingly, the ORR performance of Fru/Gu-HTC-1000 in acidic electrolyte is also remarkable. As shown in Fig. 5(a), Fru/Gu-HTC-1000 presents a pronounced oxygen reduction peak in O2-saturated 0.5 M H2SO4 solution instead of N2-saturated acidic solution. There is an 80 mV difference in half-wave potentials compared with the Pt/C catalyst, together with a conspicuous diffusion-limited current platform but a much larger diffusion-limited current density of 6.51 mA cm−2 (at 0.2 V vs. the RHE) than Pt/C (5.22 mA cm−2) (Fig. 5(b)). The average electron transfer number calculated from Koutecky–Levich (K–L) plots in Fig. 5(c) reaches 3.92, indicating a 4e pathway. The durability of Fru/Gu-HTC-1000 in Fig. 5(d) also significantly outperforms that of the Pt/C catalyst after 20[thin space (1/6-em)]000 s of testing.


image file: c7ta08052b-f5.tif
Fig. 5 (a) CVs in 0.5 M H2SO4 solution with 50 mV s−1; (b) LSV curves at 1600 rpm with 10 mV s−1; (c) LSV curves at different rpms; (d) It response at 0.65 V (vs. the RHE) at 1600 rpm.

In order to explore the potential of our materials as bifunctional electrocatalysts, we further conducted hydrogen evolution reaction (HER) testing in 1 M KOH solution. As shown in Fig. 6(a), similar to the ORR activity discussed above, the corresponding HER performance of the serial catalysts decreases as follows: Fru/Gu-HTC-1000 > Glu/Gu-HTC-1000 > Cel/Gu-HTC-1000. The best HER catalyst is Fru/Gu-HTC-1000, owing to its outstanding porosity, 2D morphology, and most defect-rich nature revealed by the highest value of ID/IG. It can be distinctly seen from Fig. 6(b) that the potentials required to achieve 10 mA cm−2 are −0.350 V, −0.440 V and −0.462 V for Fru/Gu-HTC-1000, Glu/Gu-HTC-1000 and Cel/Gu-HTC-1000, respectively. In addition, their corresponding Tafel slopes in Fig. 6(c) are 108 mV per decade (Fru/Gu-HTC-1000), 121 mV per decade (Glu/Gu-HTC-1000) and 117 mV per decade (Cel/Gu-HTC-1000), indicating that Fru/Gu-HTC-1000 possesses the most favorable kinetic processes. In the long-term stability testing of Fru/Gu-HTC-1000 in Fig. 6(d), after more than 10 h of operation at a current density of 10 mA cm−2 and a rotating speed of 1600 rpm, the current density shows a little fluctuation, demonstrating a very promising electrocatalyst for the HER in alkaline electrolyte.


image file: c7ta08052b-f6.tif
Fig. 6 (a) LSV curves for the HER with 10 mV s−1 at 1600 rpm; (b) the required potentials at a current density of 10 mA cm−2 for the different catalysts; (c) Tafel plots from the corresponding LSV curves; (d) It response at −0.35 V (vs. the RHE) without iR correction.

Discussion on electrochemical activity

On the basis of above-mentioned data, the current 2D carbons indeed are highly active toward the ORR with quite positive half-wave potentials and large limiting current densities in both acidic and alkaline media and outperform most of the other equivalent benchmarks and Pt-based electrocatalysts (Table 2), meanwhile being efficient for the HER with low overpotentials and Tafel slopes. Compared to the other catalysts, Fru/Gu-HTC-1000 has a slightly positive E1/2 and larger jL than Glu/Gu-HTC-1000, regardless of the similar Eonset. The more extraordinary activity of Fru/Gu-HTC-1000 can be mainly explained by the optimized porosity, such as larger mesopore volumes and a higher specific surface area (Table 1), which is more beneficial to mass transfer and diffusion of reactants/products in electrocatalyzed reactions. Moreover, the 2D morphology also facilitates the exposure of active sites to the electrolyte and for interaction with reactants. On the other hand, the relative total content of graphitic N and pyridinic N is another key factor to boost ORR performance. For Fru/Gu-HTC-1000, the above N total content is the highest among the three samples (Table S2), suggesting the existence of more active sites in this sample. Additionally, the defect-rich texture which has been demonstrated to produce amazing electrocatalytic activities is more pronounced in Fru/Gu-HTC-1000 than Glu/Gu-HTC-1000, determined by the higher ID/IG value of Fru/Gu-HTC-1000. In the case of Cel/Gu-HTC-1000, the electrocatalyzed performance is unsatisfactory. The foremost factors are less-developed porosity and heterogeneous morphology. On the basis of the relationship between structure and activity, we can conclude that the key factors to enhance electrochemical activities of the HTC carbons are the synergistic effect of 2D morphology, defects, N-doping and the hierarchical porous structure.
Table 2 Comparison of some advanced metal-free ORR catalysts in 0.1 M KOH electrolyte
Catalysts E onset (V vs. the RHE) E 1/2 (V vs. the RHE) |jL|@1600 rpm (mA cm−2) References
N-doped porous carbon sheets 0.90 0.77 5.8 50
N-doped porous carbon fiber 0.97 0.82 4.7 51
N,P-doped CNT/graphene nanospheres 0.94 0.82 5.6 52
N,P,F-doped graphene 0.90 0.71 6.0 24
N,S-doped porous carbon 0.99 0.85 5.8 13
N,S-doped graphitic sheets 1.01 0.87 5.1 32
N-doped Fru/Gu-HTC-1000 1.00 0.87 5.9 This work
N-doped Glu/Gu-HTC-1000 1.00 0.86 5.6 This work


Conclusion

In summary, we have developed a new strategy to fabricate 2D-morphology carbons via a facile one-pot HTC method by using the biomolecule guanine and diverse carbohydrates as precursors. During the HTC process, the formation of the 2D nanostructure is driven by the addition of guanine. The porous carbons obtained by this method exhibit thin layers, higher surface areas and tunable nitrogen doping, which are key parameters that determine the performance of electrocatalysts. The resultant 2D carbon indeed is found to be highly active toward the ORR with a rather positive half-wave potential and large limiting current density in both acidic and alkaline media and outperforms most other equivalent benchmarks and Pt-based electrocatalysts. Further work is underway to elucidate the underlying mechanism behind the formation of the 2D structure and the function of the biomolecule guanine.

Experimental

Material synthesis

Typically, 0.075 g of guanine and 0.075 g of fructose were uniformly dispersed in 24 mL of deionized water, followed by the addition of 8 mL of aqueous solution containing 0.030 g of sodium oleate (SO) and 0.018 g of P123 under stirring. The stirring was continued for 20 min, and the mixture was then sealed in a 50 mL Teflon autoclave and kept at 180 °C for 8 h. After cooling down to room temperature, the product was collected by filtration, washed several times with deionized water, and finally dried at 60 °C under vacuum for 8 h. The dried product was subjected to further carbonization at 1000 °C for 2 h in a N2 atmosphere with a heating rate of 5 K min−1. The final product was denoted as Fru/Gu-HTC-1000. Similarly, when 0.075 g of fructose was replaced by the same amount of glucose (0.075 g) or cellulose (0.075 g), the final product obtained was Glu/Gu-HTC-1000 or Cel/Gu-HTC-1000.

Physical characterizations

Elemental analysis of C, H and N was performed by using a Vario EL III CHNOS elemental analyzer. Scanning electron microscopy (SEM) images were acquired on an FEI Nova NanoSEM 230 with Everhart-Thornley secondary electron and in-lens detectors. Transmission electron microscopy (TEM) was done on an FEI Technai 20 microscope operated at 200 kV with a Gatan energy filter. Nitrogen adsorption–desorption isotherms were measured at 77 K on a Quadrachrome adsorption instrument (Quantachrome Instruments). The sample was dried at 150 °C for 6 h prior to nitrogen sorption analysis. X-ray diffraction (XRD) patterns were recorded in reflection mode (CuKα radiation) on a Bruker D8 diffractometer between 5 and 80 °C. Raman spectra were recorded by using a Thermo Scientific DXR Raman Microscope with a 50× magnification lens and a 532 nm laser. X-ray photoelectron spectroscopy (XPS) was conducted on a Thermo Scientific K-alpha photoelectron spectrometer.

Electrochemical measurements

The electrochemical activities of the as-synthesized materials toward the ORR and HER were measured using an IviumStat multichannel electrochemical analyzer in a three-electrode cell at room temperature, wherein a Ag/AgCl (saturated KCl) electrode was selected as the reference electrode, a Pt foil (1 cm2) as the counter electrode and a rotating disk electrode (RDE) functioning as the working electrode. For the rotating disk electrode (RDE) experiment, a glassy carbon electrode (GCE, d = 4 mm) was pre-polished and rinsed clean. 5 mg of the carbon sample was dispersed in a mixture of 0.35 mL of deionized water, 0.7 mL of ethanol and 0.08 mL of Nafion (5 wt%) and then sonicated for 1 h to obtain a homogeneous catalyst ink. A certain amount of the as-prepared inks was dropped onto the surface of the GCE and dried in air for further testing. ORR measurements were conducted in O2-saturated 0.1 M KOH or 0.5 M H2SO4 solution, and HER measurements were conducted after bubbling with N2 for 20 min in 1 M KOH solution. The loading of metal-free catalysts for both the ORR and HER was 0.45 mg cm−2. The electrochemical activity of the commercial 20 wt% Pt/C with a loading of 0.1 mg cm−2 was also evaluated for comparison. The Eonset for the HER is defined as the critical potential at which the current density reaches 0.5 mA cm−2. The diffusion-limited current density for the ORR is determined at 0.4 V (vs. the RHE). All of the potentials are iR corrected and converted to the reversible hydrogen electrode scale E (RHE) (V) = E (Ag/AgCl) + 0.198 + 0.059 pH.

The calculation of the electron transfer number is based on the Koutecky–Levich equations

image file: c7ta08052b-t1.tif

B = 0.62nFC0(D0)2/3(ν)−1/6
where J is the measured current density; Jk and J0 are the kinetic and diffusion-limiting current density, respectively; ω is the electrode rotation rate (rad s−1); n is the overall electron transfer number per oxygen molecule during the ORR process; F represents the Faraday constant (96[thin space (1/6-em)]485 C mol−1); C0 is the bulk concentration of O2 in the electrolyte (1.2 × 10−3 mol L−1 in 0.1 M KOH, 1.1 × 10−3 mol L−1 in 0.5 M H2SO4); D0 is the diffusion coefficient of O2 (1.9 × 10−5 cm2 s−1 in 0.1 M KOH, 1.4 × 10−5 cm2 s−1 in 0.5 M H2SO4); and ν is the kinematic viscosity of the electrolyte (0.01 cm2 s−1).

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (NSFC grant number 21571035), the Award Program for Fujian Minjiang Scholar Professorship, the Open Project Foundation of the Key Laboratory of Physical Chemistry of Solid Surfaces of Xiamen University (2015-18), and the Open Project Foundation of the Key Laboratory of Structural Chemistry of FJIRSM.

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Footnote

Electronic supplementary information (ESI) available: Fig. S and Table S. See DOI: 10.1039/c7ta08052b

This journal is © The Royal Society of Chemistry 2017