J Mater Sci (2022) 57:13403–13413 Composites C O M P O S I T E S & N& A Nnanocomposites OC O M P O S I T E S Facile preparation of glycine-based mesoporous graphitic carbons with embedded cobalt nanoparticles Wenhai Wang1, Alvaro Y. Tesio2, Mara Olivares-Marı́n3,*, Pedro Gómez Romero4, and Dino Tonti1,* 1 Institut de Ciència de Materials de Barcelona (ICMAB), Consejo Superior de Investigaciones Científicas (CSIC), Campus UAB, 08193 Bellaterra, Barcelona, Spain 2 Centro de Investigación y Desarrollo en Materiales Avanzados y Almacenamiento de Energía de Jujuy CIDMEJu (CONICET, Universidad Nacional de Jujuy), Centro de Desarrollo Tecnológico General Savio, 4612 Palpalá, Jujuy, Argentina 3 Departamento de Ingeniería Mecánica, Energética y de los Materiales, Centro Universitario de Mérida, Universidad de Extremadura, 06800 Mérida, Spain 4 Catalan Institute of Nanoscience and Nanotechnology (ICN2), The Barcelona Institute of Science and Technology (CSIC-BIST), Campus UAB, 08193 Bellaterra, Barcelona, Spain Received: 14 March 2022 ABSTRACT Accepted: 8 June 2022 A simple route has been developed for the preparation of mesoporous graphitic Published online: carbons with embedded cobalt nanoparticles just using glycine as a nitrogen 12 July 2022 source, cobalt nitrate and distilled water. After heating the mixture to 300 °C under magnetic stirring, a dry solid product was obtained, which was then Ó The Author(s) 2022 carbonized at 900 8C under argon atmosphere. Changing the glycine/Co molar ratio allowed controlling the size of the cobalt particles and their dispersion in the carbon matrix, the porosity of the carbon and its graphitic character. The carbon–metal composites obtained were tested as oxygen cathodes in Li–O2 batteries. Cells assembled exhibited a full discharge capacity up to 2.19 mAh cm-2 at a current of 0.05 mA cm-2 and over 39 cycles at a cutoff capacity of 0.5 mAh cm-2. This work provides a green, feasible and simple way to prepare mesoporous graphitic carbons with embedded cobalt nanoparticles without involving templates. Handling Editor: Andrea de Camargo. Address correspondence to E-mail: maraom@unex.es; dino.t@csic.es https://doi.org/10.1007/s10853-022-07421-3 13404 J Mater Sci (2022) 57:13403–13413 GRAPHICAL ABSTRACT Introduction produce parasitic reactions, which decompose the components of the cell [9] such as the cathode [10] and the electrolyte [11]. In the global challenge of developing more efficient Without any doubt, one of the most important and ecofriendly energy systems, lithium-ion batteries aspects to developing a Li–O2 battery is the oxygen have historically attracted the greatest interest, but cathode. For a Li–O2 battery, this must fulfill a series today searching for higher energy density, a series of of requirements, such as high oxygen permeability, batteries known as ‘‘new generation batteries’’ are excellent conductivity and catalysis of the oxygen emerging. Among them lithium–oxygen battery (Li– redox reaction. Several non-carbonaceous materials O2) stands out in terms of its energy density [1, 2]. In are currently being studied to be used as cathodes recent years, the study of various aspects of the non- [12, 13]. Nonetheless, carbon-based materials remain aqueous Li–O2 battery has been intensified, covering the most studied materials for cathode application in the stability of the electrolytes, solvents, cathode Li–O2 batteries, in spite of having many known issues materials used and the problems derived from the [5]. charge/discharge reversibility of the battery (spe- The known catalytic effect of cobalt has been cially the oxidation of ORR products). During the introduced in Li–O2 batteries as redox mediators discharge process, O2 is reduced by a series of [14, 15], carbon-free cathodes [16, 17] and carbon mechanisms strongly dependent on current density cathodes [18, 19]. Among the latter, porous carbons [3], solvent [4] and nature of the cathode [5] until it with embedded cobalt and cobalt oxide nanoparticles forms (mainly) insoluble and insulating Li2O2 [6]. have been widely investigated in the field of energy However, the loss of capacity and the low cyclability conversion and storage: lithium-ion batteries [20, 21] that these batteries present is due to the oxygen lithium–sulfur batteries [22, 23] and Li–O2 batteries species [7, 8] produced in the oxygen reduction and [24, 25]; these last were mainly motivated by their evolution reactions, which are highly reactive and ORR and OER catalytic activity [26]. In fact, it has J Mater Sci (2022) 57:13403–13413 13405 been reported that Co nanoparticles embedded in (CoN, Co(NO3)2 6H2O, ACS reagent, C 98%, CAS carbon nanostructures can reduce the charge and 10026–22-9, Sigma-Aldrich). The schematic illustra- discharge polarization of Li–O2 batteries extending tion of the synthesis is shown in Scheme 1. Briefly, its lifetime [27]. Indeed, it can also reduce the charge glycine powder was dissolved in deionized water overvoltage by forming easily decomposable amor- under stirring using a PYREXÒ beaker. Subse- phous Li2O2 [27]. quently, an amount of CoN was added to the previ- Several methods have been already proposed for ous solution until it was completely dissolved. synthesizing Co/porous carbons composites Glycine/CoN molar ratios employed are shown in employing numerous carbon precursors, cobalt Table S2. Then, the solution was heated up to 300 °C sources and experimental methods (see Table S1 in within the same beaker on a digital hot plate stirrer Supplementary material). Most of these methods until a brown dark residue was developed, and the suffer from need of expensive organic precursors release of vapors ended. This step ensured a good (such as cobalt acetylacetonate and potassium hexa- dispersion of nitrates in the matrix of the carbon cyanocobaltate), complicated processing (such as sil- precursor. The nitrogen-rich, thermally decomposed ica, or carbonates as templates) and careful washing. glycine (N-rich TDG) obtained was cooled at room The table below summarizes the strategies used to temperature and subsequently carbonized in a obtain the Co-carbon composites used for oxygen tubular furnace at 900 °C under Ar atmosphere for redox. 1 h using a heating rate of 10 °C/min. Before char- Taking into account all the above, in search of a acterization, the carbonized products were grinded in simpler and more sustainable route, we report herein an agate mortar and sieved through a 180-mesh. The a synthesis process to prepare mesoporous graphitic average carbon yield was around 15 wt.% MGC-Co carbons with embedded cobalt particles to be used as and was labeled as GX–Co–T, where X represents the cathodes in Li–O2 batteries. The synthesis involved glycine amount in the molar ratio G/CoN. For com- the use of commercial glycine, cobalt nitrate and parison purposes, carbonized glycine (G–T) at 900 8C distilled water. Glycine has been used elsewhere as a was also prepared as a bare sample. An overview of reducing fuel for the synthesis of ultraporous metals the samples prepared is provided in Table S2, which by a simple combustion reaction [28]. We performed also includes the respective Co contents calculated an extensive composition and morphology charac- from the ash residues of thermal gravimetry in air terization of these materials, named GX–Co–T (where (see below for conditions). X represents the glycine amount in the molar ratio G/CoN), and we studied the electrochemical prop- Materials characterization erties. Both the morphological characteristics and the electrochemical behavior of the materials strongly Thermogravimetric analysis (TGA) was carried out varied depending on the preparation conditions and with a simultaneous thermogravimetric analysis the amount of Co added. Cells assembled with G6– (TG)-differential scanning calorimetry/differential Co–T delivered a full discharge capacity up to thermal analysis (heat flow DSC /DTA) system 2.19 mAh cm-2 at a current of 0.05 mAh cm-2 over 39 NETZSCH-STA 449 F1 Jupiter. The amount of cobalt cycles without any capacity loss at a cutoff capacity of incorporated in the carbon materials was analyzed by 0.5 mAh cm-2. thermogravimetry from 25 to 900 °C at a heating rate of 5 8C/min under flowing air (80 mL/min). N2 adsorption/desorption experiments were performed Experimental sections at - 196 °C using a Micromeritics ASAP 2020 equipment. Specific surface areas, SBET, were deter- Synthesis of MGC-Co mined by the Brunauer–Emmett–Teller (BET) method. Micropore volumes (Vmi) and external sur- Mesoporous graphitic carbons with embedded cobalt face areas (Sext) were determined by the t-plot nanoparticles (MGC–Co) were prepared by using a method. Total pore volume (VT) was estimated by the two-step simple method with commercial powdered Gurvitsch’s rule as the amount adsorbed at a relative glycine (glycine for synthesis, CAS number 56-40-6, pressure of P/P0 = 0.995. Pore size distributions Sigma-Aldrich) and cobalt (II) nitrate hexahydrate (PSDs) were estimated from the adsorption branches 13406 J Mater Sci (2022) 57:13403–13413 Scheme 1 Synthesis route scheme of mesoporous graphitic carbons with embedded cobalt nanoparticles (MGC-Co). of the isotherms by the Barrett–Joyner–Halenda (BJH) diameter glass fiber filter (filterLab MFV1, 260 lm model. TEM studies were performed with a JEOL thick) soaked with * 100 ll of electrolyte, the anode (JEM1210) instrument, operating at 100 keV. Before a 11-mm-diameter Li metal foil (Rockwood Lithium, examination, the samples were dispersed in anhy- 0.4 mm thick). All Li–O2 cells were assembled in an drous ethanol and deposited on a holey carbon film argon-filled glove box. Once assembled, the cell was on a copper grid. X-ray diffraction (XRD) patterns at purged with dry pure oxygen for 60 s. Electrochem- small and wide angles were obtained on a Siemens ical tests, such as cyclic voltammetry and galvanos- D5000 instrument operating at 40 kV and 20 mA, tatic charge–discharge measurements, were using Cu–Ka radiation (l = 0.15406 nm). X-ray pho- performed using a MTI BST8-WA battery tester Bio- toelectron spectroscopy (XPS) measurements were logic VMP3. All the tests were done at 25 8C. carried out using a SPECS EA10P hemispherical analyzer using a 300 W non-monochromatic X-ray source (Al Ka line with a photon energy of Results and discussion 1486.6 eV). The literature data [30] indicate that glycine decom- Electrochemical measurements poses at temperatures between 220 and 280 8C into glycylglycine (dipeptide) and 2,5-piperazinedione. As carbon powders, the different MGC-Co samples Among others, NH3, H2O and CO2 gases evolve synthesized here were used after grinding in a mortar during the decomposition [30, 31]. At the same time, and sieving through a 180-mesh. The method used cobalt nitrate can be reduced to CoO, N2O and H2O for the preparation of carbon - binder composite at around 242 °C [30, 32]. According to that, after this electrodes was based on the common procedure in first heating step, the dark product obtained pre- which a carbon powder is mixed with polyvinylidene sumably is composed of a complex mixture of gly- fluoride (PVDF, Sigma-Aldrich) as a binder and car- cylglycine (dipeptide), 2,5-piperazinedione and CoO. bon black (Super P, TIMCAL) in the mass ratio of On the other hand, during the carbonization up to 4:4:2 in N-methyl-2-pyrrolidone (NMP, Sigma- 900 8C, the obtained solid product can decompose Aldrich, C 99.0%). The slurry obtained was used to releasing CO2, HCNO, HCN [30] and NH3 leading impregnate a 10-mm-diameter carbon paper to a N-rich carbonaceous product at the end of the (Freudenberg H2315/H23) and finally dried at 80 °C process. At the same time, the carbothermal reduc- for 25 h. The electrode loading was of the order of tion reaction of CoO to elemental metal occurs at 1 mg (* 1.3 mg cm-2). Li–O2 battery used consisted temperatures higher than 800 °C [33] resulting in of a Teflon homemade cell based on the Giessen particles of cobalt embedded in the N-rich carbon battery design [29]. The electrolyte was prepared in a matrix. Additionally, these metal particles that glovebox without exposure to air: 1 M lithium triflate emerged could act as catalysts for the conversion of (LiTf, Sigma-Aldrich, 99.995%) dissolved in diethy- amorphous carbon into more ordered or graphitic lene glycol dimethyl ether (DEGDME, Sigma- carbon [34]. Aldrich, 99.5%). The separator was a 12-mm- J Mater Sci (2022) 57:13403–13413 13407 Figure 1 TEM images of G– T and GX–Co–T samples. The morphology and structure of samples rigid aggregates of plate-like particles or assemblies observed by TEM analysis are shown in Fig. 1. TEM of slit-shaped mesopores [35]. image of carbonized glycine (G–T) revealed a car- According to textural data calculated from N2 iso- bonaceous material with a dense structure with low therms (see Table S3), a total pore volume (VT) porosity. GX–Co-T samples showed clearly embed- maximum is observed with the G36–Co–T (16 wt.% ded cobalt particles within carbon matrix, the sizes Co), with a VT 2.3 times compared to pristine carbon and distributions of which are function of G/CoN G–T. For higher amounts of the experimentally molar ratio. In addition, all these samples revealed to determined Co (and thus of the porogen nitrate pre- have certain porosity which are also dependent on cursor), VT gradually decreased, possibly associated cobalt nitrate loading employed during synthesis. For with the high density of Co content and structural lower G/CoN molar ratios (samples G144–Co–T and degradation (see Fig. 2c). Samples G6–Co–T and G72–Co–T), cobalt particles showed good dispersion G36–Co–T showed clear mesoporous structures and poor agglomeration in the carbon matrix, being compared to pristine carbon. Ordered mesoporous the mean Co particle diameter in the range of carbons with embedded cobalt nanoparticles made 50–200 nm. On the other hand, TEM images of sam- by a novel one-pot laser-assisted approach were ples G36–Co–T, G16–Co–T and G6–Co–T revealed reported recently by Chimbeau et al. [35]. that an increase in the cobalt loading to values higher The structures of the pristine carbon (G–T) and Co- than 16 wt.% induced the formation of larger cobalt doped carbons were analyzed by XRD. XRD patterns particles (up to about 500 nm) that did not disperse of samples are shown in Fig. 2d. Sample G–T only uniformly on the carbon matrix due to a clear presents a weak and broad reflection at around 24° agglomeration. corresponding to the (0 0 2) plane of carbon [36, 37], Figure 2a–b shows the N2 sorption isotherms and which indicates a still highly amorphous nature after the corresponding BJH pore size distributions (PSDs) the thermal treatment. On the other hand, samples of GX–Co–T samples. Pristine carbon obtained prepared using cobalt nitrate showed a diffraction directly from the carbonization of glycine (G–T) was peak at 2h = 44.28 and 51.58 corresponding to (1 1 1) mainly a microporous carbon that shows a type I and (2 0 0) crystal planes of metallic cubic cobalt, isotherm. Nevertheless, the addition of CoN influ- respectively. On the other hand, traces of CoO can be enced the mesostructuration of final products also observed due to the oxidation of cobalt particles changing their isotherms shape to type II with asso- upon to exposure of air at room temperature [38]. In ciated H3-type hysteresis loop that according to the Fig. 2d, it can be appreciated how the (0 0 2) carbon IUPAC classification indicates the presence of non- peak becomes increasingly sharper even when the proportion of cobalt in the sample is small. These 13408 J Mater Sci (2022) 57:13403–13413 Figure 2 a Nitrogen adsorption/desorption isotherms of G–T and GX– Co–T samples; b pore size distribution curves from the adsorption branches of the isotherms by BJH. c Total pore volume vs. experimentally determined Co content in G–T and GX–Co–T samples; d XRD patterns of samples G– T and Co-based samples. facts corroborate that, as previously reported [34], the corresponding to Co(0), Co(II), Co–Nx and the shake- formation of metal particles (e.g., Fe, Co and Ni) up satellite peaks [44]. In addition, the O 1s spectra during carbonization can act as graphitization cata- also prove the existence of cobalt oxide. The domi- lysts at relatively low temperatures. Note that in an nant existence of surface Co2? oxides in the XPS uncatalyzed reaction the graphitization process analysis can be attributed to the thin CoO layer shell occurs at temperatures above 2000 8C [39]. formed by the exposure of cobalt particles to ambient The XPS spectra in Figure S1 and Fig. 3 show the air [38] given the XPS surface sensitivity. surface composition of representative samples, which Cyclic voltammetry (CV) was applied to investi- can be qualitatively related to their bulk composition. gate the electrocatalytic activities of selected Co- GT pristine carbon presents a set of peaks corre- based carbons and G–T within the voltage range of sponding to C 1s (285 eV), N 1s (401 eV) and O 1s 2.0–4.5 V (Fig. 4a). All cathodes exhibit a reduction (532 eV). Thus, the product obtained directly from and an oxidation peak, respectively, ascribed to the the carbonization of glycine (at 900 8C) appears par- formation of Li2O2 and the decomposition of Li2O2, ticularly N enriched. C 1 s spectrum can be mainly respectively [45]. It is worth noticing that G6–Co–T divided into C–C/C= C (284 eV), C–N (285 eV), C–O shows larger ORR and OER peak current than G–T (286 eV) and O–C =O (289 eV) [40], which confirms and G36–Co–T, implying its superior ORR and OER that the carbon is bonded to nitrogen and some activity. Figure 6b shows discharge curves of Li–O2 oxygen groups. Similarly, the high-resolution N 1 s batteries at 0.05 mA cm-2. The discharge capacity of spectrum can be deconvoluted into pyridinic-N G6–Co–T electrode (2.19 mAh cm-2) is much higher (398 eV), Co–Nx (399 eV), pyrrolic N (400 eV), gra- than that of G–T electrode (1.38 mAh cm-2) and G36– phitic N (401 eV) and oxidized N (402 eV) [41, 42]. It Co–T electrode (1.42 mAh cm-2) consistent with what is known that pyridinic N and Co–Nx species play an is expected from the results of the CV. The excellent important role for ORR and OER [43]. The total of performance of G6–Co–T electrode can be attributed pyridinic N and Co–Nx of G6–Co–T (37%) is higher to its higher pyridinic N and Co–Nx content, pro- than that of G–T (21%) or G36–Co–T (26%) (Table S4). viding more active sites for ORR [43], facilitating the Meanwhile, XPS spectra of samples obtained using formation and better distribution of the discharge cobalt nitrate exhibit in addition new peaks, products [46]. J Mater Sci (2022) 57:13403–13413 13409 Figure 3 Deconvoluted XPS spectra C 1s (a), N 1s (b) Co 2p3/2 (c) O 1s (d) of G–T, G36–Co–T and G6–Co–T. Figure 4 Electrochemical performance of Li –O2 batteries in a 1 M LiTf dissolved in DEGDME electrolyte: a CV curves at a scan rate of 20 mV s-1; b full discharge–charge profiles at 0.05 mA cm-2; c cycling performance with both potential (2.0–4.5 V vs Li?/ Li) and capacity (0.5 mAh cm-2) limitations, showing the terminal voltage and capacity for each cycle. 13410 J Mater Sci (2022) 57:13403–13413 The cycling performance of the cathode with G-6, abundant and inexpensive raw material in the G6–Co–T and G36–Co–T was evaluated with the chemical industry. Carbon obtained directly from the limited capacity of 0.5 mAh cm-2 at the current carbonization of glycine was mainly a microporous density 0.05 mA cm-2 (Fig. 4c). G6–Co–T cathode carbon with low degree of graphitization but signif- presents an improved cycle stability, proving once icant N content. However, the addition of cobalt again the benefit of this over the other tested mate- nitrate influenced significantly the structure of final rials. There is an evident improvement on the dis- products turning it mesoporous. It is important to charge potential between GT and G36–Co–T. Here, mention that even with a low proportion of cobalt a the 2.3 times larger total pore volume may allow high degree of graphitization at low temperature maintaining electrochemically active surface area, could be reached. Both the porosity and the Co con- even though clogging occurs finally after discharging tent could be adjusted with the initial ratio of the a similar capacity. However, when the percentage of reagent concentration. For these reasons, this syn- Co increases to 67%, the increase in discharge thetic route can be considered not only economic but capacity obtained is very evident (more than 50%). also green and eco-friendly. Note that in this case, the pore volume is approxi- mately the same (0.13*0.14 cm3 g-1), which justifies that the catalytic effect of the cobalt, together with Acknowledgements pyridine nitrogen content added to the increase in the carbon graphitic character, has a more positive effect This research was supported by the Spanish Ministry on the battery discharge than the total pore volume. of Science and Innovation, through the ‘‘Severo This catalytic effect could imply a modified discharge Ochoa’’ Programme for Centers of Excellence in R&D mechanism that promotes a more homogeneous dis- (CEX2019-000917-S), the projects MAT2017-91404- tribution of precipitate, which at the same time EXP, RTI2018-096273-B-I00, RTI2018-3097753-B-I00, improves its removal, as confirmed by the enhanced with FEDER co-funding, the CSIC program for the anodic current above 3.2 V vs. Li (Fig. 4a), and by the Spanish Recovery, Transformation and Resilience improved cycle life. The discharge capacity and cycle Plan ‘‘Plataforma Temática Interdisciplinar Tran- life of G6–Co–T are comparable to several recently sición Energética Sostenible? (PTI-TRANSENER ?)’’ reported cathodes for Li–O2 batteries (Table S5). funded by the Recovery and Resilience Facility of the European Union, established by the Regulation (EU) 2020/2094. The authors also acknowledge the Gen- Conclusions eralitat de Catalunya (2017SGR1687). W.W. gratefully acknowledges the support from the China Scholar- A novel and straightforward synthetic procedure is ship Council (CSC No.:201808340076). This work has presented here allowing to produce N-enriched been performed within the framework of the doctoral mesoporous graphitic carbon materials embedded program in materials science of UAB (W. W.). with cobalt nanoparticles. This material makes use of the porous structure of a carbon with a high degree of graphitization and also takes the advantage of both Funding catalytic effects of the N and the Co. Li–O2 cells Open Access funding provided thanks to the CRUE- assembled with G6–Co–T electrode (Co = 67%) CSIC agreement with Springer Nature. exhibited a full discharge capacity up to 2.19 mAh cm-2 at a current of 0.05 mAh cm-2 and cycle sta- Declarations bility for over 39 cycles without capacity loss at a cutoff capacity of 0.5 mAh cm-2. Conflict of interest The authors declared that they The reported route is very simple and avoids the have no conflict of interest. problem of residual templates after the reaction, consequently, eliminates the use of strong acids (i.e., Supplementary Information: The online version HF) to remove the template. For the fact of not using contains supplementary material available at even a wash of the final product, the waste genera- https://doi.org/10.1007/s10853-022-07421-3. tion is almost absent. In addition, glycine is an J Mater Sci (2022) 57:13403–13413 13411 Open Access This article is licensed under a Crea- aprotic metal–O2 batteries. Energy Environ Sci 12:2559. h tive Commons Attribution 4.0 International License, ttps://doi.org/10.1039/c9ee01453e which permits use, sharing, adaptation, distribution [8] Lee HW, Kim H, Jung HG, Sun YK, Kwak WJ (2021) and reproduction in any medium or format, as long Ambilaterality of redox mediators towards 1O2 in Li-O2 as you give appropriate credit to the original batteries: trap and quencher. Adv Funct Mater 31:2002559. h author(s) and the source, provide a link to the Crea- ttps://doi.org/10.1002/adfm.202102442 tive Commons licence, and indicate if changes were [9] Mozhzhukhina N, Marchini F, Torres WR, Tesio AY, Men- made. 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