Carbon Capped Zero-Valent Nickel and Cobalt Nanoparticles as

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Carbon Capped Zero-Valent Nickel and Cobalt Nanoparticles as Multitask Hybrid Electrocatalysts Guilherme M Pereira, Thelma Sley Pacheco Cellet, Adley Forti Rubira, and Rafael Silva ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00955 • Publication Date (Web): 13 Aug 2018 Downloaded from http://pubs.acs.org on August 14, 2018

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ACS Applied Energy Materials

Carbon Capped Zero-Valent Nickel and Cobalt Nanoparticles as Multitask Hybrid Electrocatalysts Guilherme M. Pereira, Thelma S. P. Cellet, Adley F. Rubira and Rafael Silva* Chemistry Department, Universidade Estadual de Maringá (UEM) - Av. Colombo 5790 - CEP 87020-900, Maringá, Paraná, Brazil ABSTRACT: In order to attend the worldwide need for clean energy technologies, multifunctional hybrid catalysts based on zero-valent metal nanoparticles protected by N-doped carbon matrices were developed. A cost-effective and scalable sol-gel process is used, in which metal ions coordinated to melamine-formaldehyde resins are the precursors. The carbon capped zero-valent Ni/Co nanoparticles have metal content ranging from 43 wt% to 63 wt% and Ndoped carbon with the N/C ratio between 3% and 20%. It was found that the as-prepared or postmodified hybrid materials can catalyze the three most important electrochemical processes for hydrogen and fuel cell technology (OER, HER, and ORR). For these three reactions, the carbon capped zero-valent Ni/Co nanoparticles have comparable, or better, activity and stability in comparison to state-of-the-art noble metal catalysts as IrO2, Ir/C, Ru/C, and Pt/C. The most active catalysts presented ƞ10 at 390 mV and 332 mV toward OER and HER, respectively, and ƞ0.5 at 403 mV for ORR. These hybrid catalysts proved to sustain their activity even after 12 h of uninterrupted use.

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KEYWORDS: Hybrid materials, Sol-gel method, melamine-formaldehyde resin, N-doped carbon materials, multifunctional electrocatalysts, water splitting, oxygen reduction reaction.

INTRODUCTION At the Paris climate conference COP21, 195 countries agreed to mitigate their CO2 emissions to prevent global warming. To meet the required reduction in anthropological CO2 emission, science has to overcome some challenges in renewable energy technologies. Among these challenges is the development of inexpensive approaches to scale up renewable fuel production. Renewable fuels are needed to replace fossil fuels, the prime villains of environmental pollution. Molecular hydrogen is a promising alternative to fossil fuels,1,2 when produced by water splitting process in a sustainable way from clean energy sources such as solar or wind energy.3 The chemical energy stored in molecular hydrogen can be efficiently converted to electricity in fuel cells.4 To end fossil fuel age and watch the uprising of the hydrogen era we need to improve three fundamental chemical half-reactions: hydrogen evolution reaction (HER); oxygen evolution reaction (OER); and oxygen reduction reaction (ORR). Nowadays, electrocatalysts based on platinum, for HER and ORR, and iridium or ruthenium, for OER, are used as state-ofthe-art catalysts.3,5 Other materials based on metals from the same group of these noble metals are also used as electrocatalysts. For instance, recently was shown a Rh2P catalyst which has HER performance that exceeds the commercial catalyst of Pt at all pH conditions.6 However, the widespread uses of these noble metal-based catalysts, containing scarce and very expensive elements, make them unsustainable materials to large-scale applications. Indeed, many efforts have been done to replace state-of-the-art Pt-, Ir- and Ru-based electrocatalysts by sustainable catalysts based only on earth-abundant elements. Substantial


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advances on inexpensive transition metals, mainly Fe, Co, Ni, Cu, and Mn, have been reported. These transition metals are used in different phases, such as metal alloys,7 oxides/hydroxides,8-10 phosphides/phosphates,11-13 selenides,14 sulfides15, 16 and nitrides.17 It is evident that not only the chemical composition but also the catalyst architecture has to be optimized to favor the kinetics of these reactions. Fast kinetics can be obtained through high surface area catalysts, which enables the fast diffusion of reactants and products in and out of the electroactive region. For instance, porous Ni-doped Co3O4 nanostructured catalysts proved to be active and stable towards OER.18 Additionally, the intrinsic catalytic activity found for metal-free carbon-based catalysts provides the opportunity to design electrocatalyst based solely on active sites located on carbon backbone. Therefore, the combination of an intrinsically active carbon matrix and active metal transition phases has been shown a straightforward approach to achieving multifunctional catalysts. A possible synergetic effect among the different active sites can lead to utterly efficient electrocatalysts. For instance, Co-embedded in N-doped carbon nanotubes (N-CNT) catalyst has shown activity akin to Pt/C for HER at all pH conditions.19 Fe3C nanoparticles encapsulated by bamboo-like N-CNT had displayed an outstanding electroactivity in ORR compared to commercial Pt/C catalyst at acid conditions.20 On the other hand, free-metal N-doped graphiticcarbon nanomaterials presented superior activity for water oxidation compared to IrO2/C in alkaline media.21 Many examples of robust and active electrocatalysts recently reported are based on abundant and inexpensive chemical elements. Therefore, further development of new catalysts has to focus on systems that can be both versatile and feasible. A versatile catalyst can be applied to several electrochemical reactions with activity comparable or better than benchmark catalysts for each individual reaction; and an economically feasible catalyst is a catalyst produced by simple synthetic procedure, which can be easily scaled up.


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The use of atmosphere sensitive phases or nanomaterials produced at low yield brings extra drawback to hydrogen technology, thus all the issues that make the electrochemical production of hydrogen less competitive in relation to fossil fuels have to be eliminated. Therefore, a feasible catalyst should be produced by simple synthetic strategies to reduce its final cost. In the literature, molecular cobalt complexes are shown as active electrocatalysts for H2 evolution. However, the activity of cobalt complexes toward HER has been recently attributed to the formation of metallic cobalt nanoparticles on the working electrode surface.22-24 For instance, highly active Janus cobalt-based materials, prepared by cathodic deposition of Co2+ species in phosphate media, was found to form metallic cobalt nanoparticles on the working electrode.25 Additionally, Janus cobalt-based catalyst is active for both HER and OER, and the mechanism of the catalytic pathway involves nascent Co phases on the metallic cobalt nanoparticle surface. However, this type of catalysts cannot be stored since they are unstable once the applied potential is removed. Herein, we designed inexpensive, simple and scalable synthetic method to produce zerovalent-state Co or Ni nanoparticles capped by N-doped carbon and describe how these platforms can be applied for the three most important half-reactions in renewable energy technology such as HER, ORR and OER. The synthesis procedure consists of a sol-gel method using melamineformaldehyde resin with metal ions (Ni2+ or Co2+) uniformly coordinated to the polymer chains. This facile procedure leads to the formation of metallic Co or Ni nanoparticles coated by Ndoped carbon. In the system presented in this manuscript, the capped metallic phases are stable in harsh conditions. N-doped carbon coating works as a protective layer for the metallic phase, but also as intrinsically active electrocatalyst. Additionally, we discuss the effect of both, metallic phase and N-doped carbon matrix, to verify the catalytic activity and the synergetic


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effect between the phases for the three reactions. Each reaction has a completely different behavior in relation to the dependence on the effect of the transition metal and carbon matrix. For all three half-reactions, we can achieve catalytic activity that is among the best results reported for noble metal-free catalyst, for each individual reaction with the same catalyst platform.

MATERIALS AND METHODS Materials. Co(NO3)2, Ni(NO3)2, formaldehyde solution (37%), Poly(ethylene glycol)-blockpoly(propylene glycol)-block-poly(ethylene glycol) ((PEG)20(PPG)70(PEG)20, Pluronic® 123, average molecular mass ~5800 Da), isopropanol and Nafion® 117 were obtained from SigmaAldrich. NaOH, KOH, and HNO3 were purchased from Synth. Melamine was donated by GPC QUIMICA S.A. Synthesis of melamine-formaldehyde resin (MFR), Co-MFR, and Ni-MFR. Pristine MFR was synthesized dissolving 2.5 g of P123 and 2.22 g (0.0176 mmol) of melamine in 150 mL of distilled H2O at 60 °C and then 1.6 g (0.0530 mmol) of formaldehyde was added and stirred for 1 h. Then, 150 mg (3.8 mmol) of NaOH was added and the mixture was kept under stirring for 5h. Co-MFR and Ni-MFR were made following the same procedure above and after the 5h of stirring, Co(NO3)2 or Ni(NO3)2 was added in a molar ratio of 2:1 of melamine:metal and stirred for more 30 min. Then, the solution was placed in an oven at 180 °C for 4h to finish the crosslinking process. Synthesis of nitrogen-doped carbon (NC) from pyrolysis of MFR, Co-MFR, and Ni-MFR. The resin was placed in a tubular oven where was kept under a continuous N2 flow of


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20 mL min-1. The precursor was heated from room temperature to the first step at 200 ºC, and them to the second step at 300 ºC, with a heating rate of 1ºC min-1. Each thermal treatment step is kept for 30 min. Then, the temperature was increased to the final condition at 700 °C, 800 ºC or 900 ºC, with a heating rate of 3 ºC min-1 and kept at the final temperature for 30 min. After the final pyrolysis, the samples were washed with water to neutral pH and placed to dry in an oven at 60ºC. The nitrogen doped carbon samples containing Co (Co@NC) and Ni (Ni@NC) were also treated with HNO3 2 mol L-1 at 50 ºC for 24 h and washed with distilled water to neutral pH to leach out the metal phases from the carbon matrices. Materials Characterizations. FTIR spectra were recorded in the solid mixture of 2 mg of the material with 200 mg KBr using a Bruker Vertex 70-RAM II spectrometer, with 2 cm-1 of resolution. Surface area values were calculated from N2 adsorption-desorption isotherms at 77 K using a QuantaChrome Nova 1200 by Brunauer-Emmet-Teller method (BET). The pore size distribution was obtained by Barrett-Joyner-Halenda method (BJH) using the desorption branch. The samples were degassed at 150 ºC for 12 h at 10 µmHg. Powder X-ray diffractograms were carried out on a Shimadzu Lab-X XRD6000 using Cu as X-ray source (λ = 0.154 nm, 40 kV, and 30 mA) at a rate scan of 2° min-1. The amount of each metal in samples was determined by thermogravimetric analysis (TGA) using a TGA-Q50 from TA Instruments under synthetic air flow of 50 mL min-1 and heating rate of 10 ºC min-1. Raman spectra were performed with solid samples using a laser of a wavelength of 532 nm and 5 mW of power on a Bruker Senterra micro-Raman spectrometer. Morphology analysis made by scanning electronic microscopy (SEM) and transmission electronic microscopy (TEM). SEM micrographs were obtained on a Shimadzu SS550 Superscan operating in 15 kV and TEM micrographs were recorded on a JEOL


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JEM-1400 at 120 kV. X-ray photoelectron spectroscopy (XPS) was carried out in a Thermo Scientific K-alpha spectrometer using an Al K-α as the monochromatic X-ray source. Electrochemical Characterization. Electrochemical measurements were carried out by linear sweep voltammetry (LSV) on an Autolab PGSTAT302N. Rotating glassy carbon electrode (GCE), with 5 mm of diameter and spin speed of 1600 rpm, was used as working electrode, unless otherwise specified. Saturated calomel electrode (SCE) was used as the reference electrode and graphite rod as counter-electrode. An N2 saturated solution of KOH 1.0 mol L-1 was used as the electrolyte for Hydrogen Evolution Reaction (HER) and Oxygen Evolution Reaction (OER) and an O2-saturated solution of KOH 0.1 mol L-1 was used for Oxygen Reduction Reaction (ORR). For each measurement, a volume of catalyst ink was dropped on GCE to obtain a surface concentration of the catalyst sample of 100 µg cm-2. The ink was prepared by dispersing an amount of catalyst in 1.5 mL of a solution of isopropanol:water (1:3 v/v) containing 160 µL of a 5% Nafion solution. The sweep rate was 2 mV·s-1, otherwise specified, the current was normalized by GCE area and the potential was converted from SCE to RHE (reversible hydrogen electrode) using Equation (1). () = () + 0.242 + 0.0592 ∙ (. ) Koutecky-Levich plots (K-L) were used to determine the number of electrons transferred (n) in ORR.4 The same three electrode system and working electrode preparation described above were used. The working electrode was scanned cathodically at a rate of 2 mVs-1 with varying rotating speeds ranging from 300 rpm to 1600 rpm. The number of electrons transferred was calculated using the Equation 2 (K-L equation).26 1 1 1 1 1 = + = + (. !)


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Where is 0.62"#$0%02/3&−1/6, j is the measured current density, jk and jL are the kinetic and diffusion-limiting current densities, respectively, ω is the angular rotation (ω = 2πf/60, f is rotation speed), F is the Faraday constant (96,485 C mol-1), n is the number of electrons transferred during ORR, C0 is the saturated O2 concentration in the KOH 0.1 mol L-1 (1.2 × 10-3 mol L-1), D0 is the diffusion coefficient of O2 in the KOH 0.1 mol L-1 (1.9 × 10-5 cm2 s-1), and v is the kinetic viscosity of the KOH 0.1 mol L-1 (0.1 m2 s-1).4

RESULTS AND DISCUSSION The sol-gel method is used to prepare hybrids materials composed by zero-valent Co or Ni nanoparticles embedded in nitrogen-doped carbon. The synthetic method explores the synthesis of melamine-formaldehyde resin as the gel precursor, where the metal ions are coordinated, Scheme 1. The three-dimensional structure of melamine-formaldehyde resin works as the medium to control the distribution of the metal ions,27 once the melamine segments in the polymer have several nitrogen atoms from aromatic and secondary amines that can be coordinated to metal centers. Therefore, a large equivalent of metal ions can be held in the polymer matrix.28, 29 The interaction of the metallic ions with the polymer matrix controls the growth process of the metal phase and prevents them from severe aggregation in the carbonization step. In addition, melamine has many nitrogen atoms in its structure, consequently, the amount of N in the final carbonaceous structure will depend on the pyrolysis condition and can achieve more than 30%. Melamine resins were previously reported as a precursor to graphitic carbon nitride (g-C3N4)30 and to N-doped carbon nanostructures.31 Polymerization of resin was carried out following literature reports.32 The thermal stability of the obtained material depends on monomers ratio, and the most stable resin is


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obtained at 1:3 ratio of melamine:formaldehyde.32 Higher degradation temperature provides a slower carbonization process and results in a higher carbon yield.33,

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NaOH is added in the

polymeric resin synthesis and is kept in the system during the carbonization process. NaOH has two main functions in the process: first, it catalyzes the resin polymerization;35 and second, it works as an activation agent in the carbonization process. Carbon materials activated by NaOH consistently has hydrophilic surface and high surface area, which can surpass 3000 m2/g.36 Asprepared samples will be named in this text as Co@NC-T and Ni@NC-T, which T indicates the maximum pyrolysis temperature.

Scheme 1. Representation of synthesis of formaldehyde-melamine resin and Co@NC and Ni@NC.


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The crystalline phases for the metal in the hybrid materials were characterized by XRD, Figure 1A. In the XRD diffractograms of the Co@NC samples it is possible to identify very intense (111) and (200) planes attributed to the face-centered cubic structure of metallic cobalt at 44.3° and 51.6° (JCPDS 15-0806). In addition, a peak with very low intensity is observed at 37.0°, which can be attributed to (311) plane of Co3O4 phase (JCPDS 74-1656). Therefore, Co is predominantly present in the Co@NC samples as Co0 phase with some trace amount of Co3O4. A similar result is observed for Ni@NC samples as can be seen in Figure 1B. At 44.6° and 51.9° is observed intense peaks attributed to (111) and (200) planes of the face-centered cubic structure of metallic nickel phase (JCPDS 87-0712). In addition, two peaks of lower intensity are also observed for Ni@NC at 37.4° and 43.3°, that is attributed to (101) and (012) planes of NiO phase (JCPDS 87-0712). Therefore, nickel is also present in the final hybrid materials mainly as zerovalent nickel phase along with oxide phase in minor quantities.


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Figure 1. (A) XRD patterns of Co@NC and (B) Ni@NC synthesized at different temperatures. (C) TGA of Co@NC and (D) Ni@NC synthesized at different temperatures in air. The metal content in the as-prepared samples is estimated by using thermogravimetric analysis in a synthetic air atmosphere (20% O2 in N2), Figures 1C and 1D. The metal amount ranges from 44% to 64% for both metals at different temperatures. All the materials prepared with Co have lower metal content than those prepared with Ni. Since Co and Ni have very similar molar weight, 58.9 and 58.6 g/mol, respectively, the difference in the metal content in the final hybrid material is a consequence of the relative amount of carbonaceous materials in the samples. In other words, Co is more effective to avoid the carbon volatilization process during the carbonization step. Additionally, the effect of the metal on the carbonaceous structure formed can be seen in the XRD diffractograms, Figures S1A and S1B. The XRD peak attributed to the graphitic structure is observed for both Co@NC and Ni@NC, however, it is broader for Co and


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narrower for Ni samples. Therefore, these results indicate that the carbonaceous material is obtained in a lower yield but in a more organized form from Ni samples. It means that the graphitization process is more effective for Ni, but it also leads to a larger volatilization of heteroatoms moieties. The observed effect of Co and Ni on the carbon graphitization and yielding is in agreement with C affinity to the two transition metals. The metal affinity to carbonaceous moieties can be assigned to the empty d orbitals of the metals that can interact with the π systems in carbonaceous matrices.37 Ni has most stable electronic distribution than Co which means that Ni (3d8) forms a weaker bond with carbon than Co (3d7). Furthermore, the authors also state that the graphitization ability arises from transition metals which are capable to stabilize carbon species on their surfaces by weak bond and also have low affinity by carbon (i.e. low carbon solubility).37 In the TGA analysis of the samples treated with HNO3 is possible to observe the metal removal, Figures 1C and 1D. It is worth to mention that these samples were initially composed of an amount of at least 44% of metallic nanoparticles. After the leaching process, metallic species are still present in the final materials. The amount of metal that resist to the acid treatment varies among the samples from 0.5 to 4.4%. In general, materials prepared at lower pyrolysis temperature have a larger amount of residual metals species, which might be attributed to a higher amount of metal intercalated in the amorphous carbon structure. For the samples Co@NC-H, the XRD diffractograms do not evidence any Co crystalline phase indicating the residual cobalt is probably present as small clusters intercalated in the amorphous structure of Ndoped carbon, Figure S1A. On the other hand, XRD diffractograms of Ni@NC-H present peaks with very low intensity what were previously attributes to metallic Ni and NiO, Figure S1B.


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In carbon materials, the graphitization degree can be provided by D and G band intensity ratio in Raman spectrum. D band is related to amorphous carbon structures, while the G band is attributed to the presence of graphitic structures.34 As shown in Figures 2 and S3, three peaks are observed at 1340, 1580 and 2670 cm-1 attributed to D, G and 2D bands, respectively. As expected, the graphitization degree increases in function of temperature, which can be verified with the decrease of ID/IG for higher pyrolysis temperature. Samples containing Ni demonstrate lower ID/IG values in comparison to Co samples for pyrolysis temperature higher than 800oC, indicating the higher ability of Ni to induce graphitization at high temperature. For instance, the ID/IG for Ni@NC-900 is around two times lower than that observed for Co@NC-900, i.e. 0.48 and 0.88, respectively. Therefore, it can be stated that the carbonaceous network in Ni@NC samples differs from Co@NC in relation to the degree of organization. More crystalline carbon materials are expected to have higher electric conductivity, but the lower content of surface defects (i.e. edges). However, some authors found that Co is more active in carbon graphitization than Ni nanoparticles in temperatures higher than 800 °C.38 Nevertheless, in such cases, the samples were kept at the final pyrolysis temperature for at least 3h, which is 6-fold the time used for the samples prepared herein this work. Therefore, the difference of Co and Ni to induce graphitization appears to be related to the kinetics of the process, suggesting that the kinetics of graphitization on Co should be slower than on Ni nanoparticles.


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Figure 2. Raman shift spectra of (A) Co@NC and (B) Ni@NC prepared at different pyrolysis temperatures after acid treatment. Additional information about the microstructural characteristics of the samples is provided by the nitrogen sorption measurements, depicted in Figures S4A and S4B. It is worthy to note that the specific surface area of the samples is higher than 100 m2/g for materials with large metal loading. The isotherms can be classified as type IV and present hysteresis loops classified as type H3, indicating the presence of the mesoporous structure.39 The samples present virtually the same pore diameters as can be observed in Figures S4C and S4D. For all the samples prepared with metal, the specific area and total pore volume increased after acid treatment as can be seen in Figures S4E and S4F. The surface area change from 174 m2/g to 666 m2/g for Co@NC-900 and Co@NC-900-H, and from 144 m2/g to 479 m2/g for Ni@NC-700 and Ni@NC-700-H,


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respectively. The larger specific surface area observed for the acid treated samples is due to the leaching of the metal phases, since metal particles were occupying the carbon pores contributing to the reduced specific surface area of the materials. TEM, Figure 3, and SEM, Figure S6, were carried out to evaluate the shape and size of the metal phases and their integration with the carbonaceous matrix. The results demonstrate that the morphology of the materials is quite dependent on the metal used and the pyrolysis temperature. For instance, the micrographs of Ni@NC carbonized at 700°C displayed a porous carbon network studded of Ni nanoparticles (Figures 3A and S6A), whereas Ni@NC prepared at 800°C (Figure 3B) and 900°C (Figure 3C) exhibited a mixture of both Ni-embedded rich N-doped carbon nanotubes and core@shell structures, which agrees with ID/IG and XRD results aforementioned. However, the micrographs of Co@NC showed agglomerates of carbon nanosheets (Figures 3D, 3E, 3F and S6B, S6C, S6D) studded with cobalt nanoparticles for all the pyrolysis temperatures. Curves of size distribution for the metallic nanoparticles were obtained from TEM images, Figure S7, and show the diameters of the nanoparticles increase with the temperature. Furthermore, Co nanoparticles are lower than Ni in all the temperatures, which can be related to the higher relative amount of carbon moieties in Co samples compared to Ni samples, stabilizing the particles and hindering the growth during pyrolysis steps.


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Figure 3. TEM images of (A) Ni@NC-700, (B) Ni@NC-800, (C) Ni@NC-900, (D) Co@NC700, (E) Co@NC-800, and (F) Co@NC-900. The electroactivity of all samples was investigated in basic condition using glassy carbon electrode painted with the catalysts inks. These catalysts were applied for all three important half-reactions in hydrogen technology and the results are presented in Figure 4 and Figure S9. It is evident the chemical characteristic of the metal is capable to affect the features of the carbon matrix. Thus, a set of samples was produced by treating the Co@NC and Ni@NC with 2.0 molar HNO3 at 50 ºC for 24 hours to leach out the metal phase and leave the solid materials primary composed of N-doped carbon. The effect of the metals in the catalytic processes promoted by Ndoped carbon is intriguing. In many cases, results demonstrate that metals added in the carbonization step favor the formation of materials with higher catalytic activity. Therefore, the effect of metal can be justified by two different hypotheses: 1) metal ions participate in the catalytic process and therefore the metal is part of the active site; 2) metal ions work only to


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facilitate the formation of active sites that are located in the carbon backbone. The reconciliation of these two conflicting ideas is the consideration that both mechanisms are possible and mutually important. The capacity of a metal to generate active sites is a way to design better carbon-based electrocatalysts.

Figure 4. Electroactivity comparison of the samples pyrolyzed at different temperatures. LSV for OER: (A) Co@NC; and (B) Ni@NC. LSV for HER: (C) Co@NC; and (D) Ni@NC. LSV for ORR: (E) Co@NC, and (F) Ni@NC. Overpotential for the best catalysts at 10 mA/cm2 for (G) OER and (H) HER, and (I) at 0.5 mA/cm2 for ORR. All the measurements were made at 2 mV/s. The performance of the catalysts toward OER is found to vary according to the pyrolysis temperature, as a higher pyrolysis temperature provides a better catalyst, Figures 4A e 4B. Both Co@NC and Ni@NC proved to be active and have a very similar onset potential for OER, but a slight advantage is observed for Co samples if the overpotential at 10 mA/cm2 is analyzed,


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Fig. 4G. The best catalyst, Co@NC-900, is able to perform the OER at 10 mA/cm2 with overpotential as low as 400 mV, Figure 4A. This activity is comparable to literature reports on the state-of-the-art 20% Ru/C (390 mV),40 20% Ir/C (380 mV)40 and 20% IrO2/C (370 mV),21 Fig. 4G. The ƞ10 of the Co@NC-900 is also very akin to recent hybrid electrocatalysts based on Co and Co oxides incorporated on N-doped carbon nanocages (410 mV)41 or N-S-codoped graphene (460 mV)42, respectively. In addition, a higher slope for the polarization curves of Co@NC samples also confirms the better activity of Co sample for OER. The difference in current density might be attributed to the higher specific surface area of the sample Co@NC-900, which is almost 3-fold higher than the surface area of Ni@NC-900 (for more information please see SI, Fig. S4). The structure of the carbon matrix, which is more amorphous for the Co samples according to Raman and XRD data aforementioned, can be used to understand the large difference in the specific surface area. The metal leached samples proved to be less active toward OER than the corresponding as-prepared samples, as can be seen in the LSV curves in the Figures 4A and 4B. In this experiment, the carbon matrix provided by Co samples is believed to be inactive toward OER, considering that the remaining catalytic activity of metal leached samples seems to be due to the presence of metallic phase impurities after the acid treatment, as can be verified in Table S1 and Figure S10. However, the carbonaceous materials obtained from Ni samples seem to present an intrinsic activity, as can be seen in Figure S10, similar to the result presented by Zhao and co-workers.21 The activity for both metallic Co and Ni is attributed to the in situ transformation into highly active (oxy)hydroxides metastable phases at the surface of metallic nanoparticles.43 The faradaic efficiencies of Ni@NC-900 and Co@NC-900 presented in Figures S11A and S11B show virtually a perfect fit among the amount of O2 calculated and measured, indicating the gas produced by the catalysts is basically only O2. Additionally, the


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samples Ni@NC-900 and Co@NC-900 were submitted to an applied potential of 6.60 V for 1h and after that Raman spectra were collected, Figures S11C and S11D. The spectra show D and G bands of the carbon materials even after the electrical stressing condition, indicating the corrosion resistance of the carbon-based materials herein synthesized in harsh environments. The stability test for OER, with Co@NC-900 and Ni@NC-900, was carried out applying a constant potential of 1.7 V (vs RHE), Figure S13A. Both catalysts display a very stable performance and after 12 h of reaction, the current is 20% and 26% lower than the initial current for Co@NC-900 and Ni@NC-900 catalysts, respectively. These values are very akin to the “golden standard” IrO2/C which loses at least 25% of its activity after 12h of direct using.44 The high stability verified for metallic phases trapped in the carbonaceous matrices is attributed to the N-doped carbon nanostructures protective layer, which prevents the metal degradation by the electrochemical process. Studies have shown that graphitic carbon is stable and chemically inert under harsh oxidative environments.45, 46 Several reports have also supported that the presence of nitrogen in graphitic carbon leads to an increase of the corrosion resistance.46-48 The OER kinetics for the best catalysts was investigated by analyzing Tafel slopes, Figure S13B. The Tafel slopes of Ni@NC-900 (76 mVdec-1) and Co@NC-900 (80 mVdec-1) are found to be smaller than the obtained for IrO2/C (97 mVdec-1).49 The lower values of Tafel slopes for Ni@NC-900 and Co@NC-900 indicate more favorable electron transfer processes, which suggests fast kinetics and feasible OER mechanism for these catalysts in relation to IrO2/C.49 The samples are also active for both ORR and HER processes (reduction processes), but the change in the catalysts’ activity in function of metal removal goes in a completely different direction from OER (an oxidative process). The LSV curves for HER process are presented in Figures 4C and 4D. The best sample for HER, Ni@NC-800, presents overpotentials of ca. 270


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mV and 110 mV at 10 mA/cm2 compared to 20% Pt/C and MoS2/Graphene,50 respectively, Figure 4H. Taking as the references the as-prepared samples (i.e. with trapped metallic phases), Ni samples have higher efficiency as a catalyst than Co samples as can be seen in LSV curves normalized by real metal mass loading in Figures S12A and S12C. However, the changes in activity for HER in function of the pyrolysis temperature and metal leaching are more complex than the behavior observed for OER. The pyrolysis temperature of 800 ºC is found as the optimum condition for both Co@NC and Ni@NC samples. Nevertheless, the most surprising result is the observed effect of the metal leaching process. For the group of samples prepared with Co, the metal leaching leads to the improvement of the catalytic activity, while for the samples prepared with Ni the activity of the catalysts are almost completely lost after the metal leaching, Figures S12B and S12D. These results indicate that HER activity in the Ni samples is metal-dependent, whilst in the case of Co samples the metal phase seems to have a secondary role in the catalytic process, and therefore the catalytic sites in the carbon must be the more relevant. The hydrogen binding energies on Ni and Co are very similar, around 205 kJ/mol, and this value is lower than hydrogen binding energy to Pt, which is around 250 kJ/mol.51 According to the volcano plot,52-54 there is a suitable energy binding, neither too weak nor too strong, which provides an optimal performance between the reactant adsorption step and final product desorption step. For Ni and Co, the HER activity could be improved increasing the metal to hydrogen affinity. The increment of the bond affinity of Ni and Co for hydrogen can result from the higher surface energy of nanometric metal phases due to their higher surface tension. In relation to the intrinsic activity of the carbon, the efficiency of carbon derived from Co samples and inactivity of carbon from Ni samples implies to a strong correlation among the organization the carbon matrix with the activity. It is found that the more amorphous structure of carbon is


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linked to a higher activity for HER. The carbon’s intrinsic HER activity of Co@NC-800-H and Co@NC-900-H may be due to the pyridinic nitrogen species that support the formation of the intermediate state for HER (adsorption step of H) as described by Qiao and co-workers in their metal-free N-doped carbon catalyst.55 Besides, as reported by Zheng and co-workers56, graphitic and pyridinic nitrogen species incorporation decrease free-energy of the hydrogen adsorption (∆GH*) which is favorable to HER process. The electroactivity increasing after leaching process will be further discussed in this manuscript correlating with XPS results. The durability test of the best catalysts toward HER, Co@NC-800-H and Ni@NC-800, was performed by applying a potential of -0.32 V (vs. RHE) for 15 h, Figure S14A. The most interesting pattern is observed for Co sample that holds its activity even after 15 h of straight using, whilst the Ni sample shows a current density decay of almost 37%. It is worth to mention that the commercial Pt/C catalyst (20% wt.) loses almost 45% of its current density only after 1 h of reaction, Figure S14A. Therefore, the catalyst prepared here is an undoubtedly better catalyst for long time use than the commercially Pt/C (20% wt.). In order to investigate the kinetics of the catalysts toward HER, the Tafel slopes are shown in Figure S14B. Both samples present high Tafel slopes, 154 mVdec-1 for Co@NC-800-H and 158 mVdec-1 for Ni@NC-800. The mechanism of hydrogen evolution in basic solution is not fully understood, but it is suggested the HER process could follow the Volmer-Tafel, Volmer-Heyrovsky or may derive from multiple pathways depending on the coverage of surface active sites by adsorbed hydrogen.57, 58 The high values of Tafel slopes obtained suggest that the mechanism may arise from the combination of various reactions pathways. The samples are also active for ORR, Figures 4E and 4F, and such as observed for Co samples in HER, it was also found that the metal leaching process is an important stage to improve the


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catalytic activity of both Co and Ni samples toward ORR. The best sample shows a current density of 0.5 mA/cm2 at overpotentials as low as 58 mV and 82 mV regarding Pt/C (20% wt.) and Pd4S,59 respectively, and very akin to other recent catalyst based on graphene modified with Co-N-O,60 Figure 4I. Comparing the half-wave potential to recent electrocatalysts reported as PdS4 and The higher efficiency of the metal leached samples reveals that the observed catalytic activity for ORR is more carbon-dependent, which is the opposite behavior verified for OER. Therefore, metal species are not needed for ORR during electrochemical reaction, but it is necessary in the synthetic route to form highly active catalysts since the blank samples produced by the pyrolysis of the MFR without any metal presented negligible activities for ORR, Figure S15C. When we take a closer look at the LSV curves in Figs. 4E and 4F it is evident that the catalytic activity of the sample after metal leaching had a better improvement regarding current density than at overpotential. Therefore, the metal leaching provides a higher surface area, once void space is formed by the metal nanoparticles removal, which might be leading to a higher current density. In the ORR experiments, it is also observed that samples prepared with Co have a superior catalytic activity compared to Ni samples. This result can be correlated with HER results that also shown Co as better metal to the synthesis of carbon with higher intrinsic catalytic activity. The difference between Ni and Co can be attributed to the higher Ni ability to induce carbon graphitization than Co, as aforementioned. Figure S16 shows LSV curves normalized by the real carbon weight loaded on electrodes indicating the superior activity of the carbon-based catalysts obtained from samples prepared with Co compared to the Ni. The most active samples toward ORR, Co@NC-800-H, and Ni@NC-800-H were evaluated at the constant applied potential of 0.75 V (vs. RHE), Figure S17A. The current density observed for Pt/C drop to 30% of its initial value in 5h of use. The Ni@NC-800-H showed to be unstable


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with a 50% decrease of activity within 1 h of use. On the other hand, the Co@NC-800-H showed an extraordinary stability with a self-activation process. Co@NC-800-H provide a slightly increasing activity during 12h of use, resulting in a current density 10% higher than initially observed. This increase of the current density in function of the catalyst used may be justified by structural changes in the carbonaceous material which facilitate the diffusion of reactions in and products out of the active site surrounds. An important feature to be determined for active catalyst toward ORR is their ability to generate H2O2, which occurs through the incomplete O2 reduction process.61 The amount of H2O2 can be estimated calculating the number of electrons transferred in ORR.62 Figure S19 shows the total number of electrons transferred determined by Koutecky-Levich (K-L) plots (Figure S18) for Ni@NC-800-H and Co@NC-800-H. The results demonstrate a very stable four electrons transfer for Co sample, which indicates that O2 is strictly reduced to H2O via 4-electron reduction pathway. However, for Ni sample the transference varies from two to three electrons depending on potential, what can be attributed to the competition among 2-electron (that leads to H2O2 production)63 and 4-electron pathways. It suggests that Ni sample produces higher amounts of H2O2 than Co sample in the ORR process, which might be the cause of its low stability observed by the chronoamperometric test. The Tafel plots, Figure S17B, were used to predict the kinetics behavior of the best samples. The Tafel slope is an indication of the reaction pathway on the electrode surface and can be correlated with the nature of the adsorbed O2 species and its coverage variation with respect to potential.45 A Tafel slope of 65 mV dec-1 and 60 mV dec-1 are obtained for Ni sample and the-state-of-the-art Pt/C (20% wt.), respectively, whilst the Co sample displays a slope of 44 mV dec-1, which is characteristic of a fast electron transfer followed by a slow chemical step.64


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The correlation of surface chemical composition with electroactivity is studied with XPS analysis, Figures S20 to S24. The metal content on the surface of the samples in function of pyrolysis temperature does not vary significantly, Figure S25. Since the pyrolysis temperature does not change the metal content neither the metal phase on the catalysts surface, the changes observed in the catalytic activity must be due to changes on the carbon matrices. The XPS result reveals that the metal nature has an effect on the nitrogen species profile. The nitrogen content, as well as the ratio among the different nitrogen moieties, is different for the samples prepared from Co and Ni. In Figure S26A is shown that the N/C atomic ratio is drastically reduced by increasing pyrolysis temperature from ca. 20% at 700ºC to 3% at 900 ºC for Co@NC samples. The XPS result also indicated that the metal leaching treatment causes changes on the surface groups of the samples. In the Figure S26B is observed an increase of ca. 5% in N/C ratio after acid treatment in the Co@NC, whereas in the Ni@NC there was no change at all. In a blank sample produced without any metal added, the N/C ratio reduces from 20% to 18% with the acid treatment. The increase of nitrogen groups in the Co samples can be explained by the amorphous feature of the carbonaceous matrix, which is more susceptible to nitration than more graphitic carbon. Figure S27 displays the different N species revealed by high-resolution N1s peaks on XPS. All catalysts spectra show four types of N species before acid treatment such as Npyridinic (N1, 398.4 eV), N-pyrrolic (N2, 400.1 eV), N-quaternary/graphitic (N3, 401.0 eV) and N+-O--pyridinic (N4, 403.0-403.7 eV) and one more peak of N-nitrate (N5, 405.6 eV) after acid treatment which should be resulted by the nitration reaction occurred during HNO3 washing procedure. Pyridinic nitrogen is the most abundant nitrogen species for all samples with ca. 50%, followed by pyrrolic nitrogen. In the sample produced by Ni, the pyrrolic nitrogen content is slightly higher than the observed in the sample produced with Co. The origin of the


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electroactivity of N-doped carbons toward ORR process is quite controversial in the literature. Some authors suggest that the activity toward ORR is due mainly to pyridinic nitrogen, which favors four electron transference,4 while pyrrolic nitrogen, which is in tautomeric equilibrium with hydroxypyridine, is responsible to adsorb O2 in a similar geometric way observed on Pt surface.4,65 It is also reported a negative influence of pyrrolic groups toward ORR activity.66, 67 Other authors claim that ORR activity is correlated to pyridinic and quaternary nitrogen species. It is described that the N1 and N3, are responsible for changing the O2 adsorption configuration from end-on to side-on weakening the oxygen bonding, benefiting the reduction reaction, and leading to a dissociative mechanism.4, 67-71 Nevertheless, Kim and co-workers72 reported that an adsorption in a side-on mode has a higher barrier than an end-on mode on the graphene surface, which means that the ORR mechanism should take an associative pathway. In other words, the associative mechanism is more energetically favorable than the dissociative mechanism, thus ORR on N-doped carbons should occur only by an associative pathway.73, 74 Thus, there is no consensus about which N species play a key role in ORR. Our results do not present high changes in chemical composition of the nitrogen groups, while the activity varies greatly. Figure S27B depicts the percentage of atomic contents of N in each pyrolysis temperature of the samples of Co@NC as-prepared and it really seems that higher contents of N1 and N3 species along a lower amount of N2, benefits the electroactivity toward both ORR and HER in this work. In addition, for HER and OER take place in alkaline medium, the first step should involve H2O or OH- adsorption on catalyst sites, respectively.75, 76 Since H2O/OH- has a weak interaction with pure carbon materials,77 this first step might be impaired on pure carbon catalysts. Thus, a high N and/or O amount on carbon structure may improve the adsorption step by increasing the number of specific interactions, for instance, by hydrogen bond interaction, between water molecules and


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catalyst surface, yielding OER and HER catalysts with high efficiency on this purpose. As presented in Figure S27B, the N content does not significantly vary among the samples, and therefore, it cannot be taken as a predominant effect on changes in the electroactivity of the samples. Nevertheless, the N/O geometric arrangement on catalyst surface also should affect the adsorption behavior. According to Müller and co-workers and McCallum and co-workers, the water adsorption occurs in a cooperative way on what two or more sites are in a suitable distance, which allows adsorbed water molecules in neighboring sites interact with each other, reducing the activation energy of adsorption step.78, 79 This statement suggests that OH bonds on adsorbed water molecules should be extended and thus weaker than on free-water, what may benefit both water-splitting reactions: OER and HER. The metal dispersion of the polymeric resin prior to the pyrolysis step is believed to contribute to the organizations of the N groups directly bonded to the metal center. Thus, after pyrolysis, most of these N groups should retain its position, which even after metal leaching by acid treatment leads to elevated electroactivity of Co@NC-H and Ni@NC-H related to NC-H toward HER/ORR and OER, respectively.

CONCLUSION Trapped zero-valent Co and Ni nanoparticles in N-doped carbonaceous matrices were prepared by pyrolyzing inexpensive and already commercial melamine-formaldehyde resins with Ni2+ and Co2+ ions coordinated. The nature of the metal ion dictate the general properties of the carbon matrix. We found that as weaker is the interaction of the metal with the carbon phase higher is the crystallinity of the carbon phase. The nitric acid treatment changes the general behavior of the catalysts, once this process removes a great part of the metal content, it can cause the surface nitration of the carbonaceous material and also change the pore structure of the solid material.


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Co@NC samples are the most active toward OER and the difference in catalytic activity in relation to Ni@NC arises mainly from the variance in the specific surface area. The acid posttreated samples showed a reducing in catalytic activity indicating the metallic phases as the active sites for OER. In the best case, ƞ10 was achieved at 390 mV with Co@NC-900 and the stability was similar to the benchmark catalysts. For HER, the as-prepared Co@NC proved to be inferior catalysts than Ni@NC samples, which the lower ƞ10 was provided by Ni@NC-800 at 332 mV. However, Co samples had their catalytic activity improved by the acid treatment, what can be justified by the exposure of active sites in the carbon matrix that were previously suppressed by the metal phase. Therefore, Co nanoparticles are able to create active sites toward HER on the carbonaceous matrix more effectively than Ni. In addition, the stability test showed no activity reduction of Co@NC-800-H even after 15h of reaction, whereas Ni@NC-800 had a 37% reduction in its activity. For ORR, more active samples are also obtained after metal removal. The most active sample, Co@NC-800-H, provided 0.5 mA/cm2 at only 90 mV higher than Pt/C but with superior stability. No current reduction is observed for Co@NC-800-H whilst for Ni@NC-800-H decreases 50% of its initial value. The three more important electrochemical reactions to renewable energy technology can be carried out with a system, which gathers active metal and active carbon phases. In the result, the OER is clearly metal-dependent while the opposite process of ORR is more carbon dependent.

ASSOCIATED CONTENT Supporting Information.


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Additional XRD patterns after HNO3 treatment and Raman Spectra, Nitrogen physisorption isotherms, pore diameter distributions, total pore volume, BET surface areas, SEM and TEM images, comparison of the performance at 10 mAcm-2, ID/IG ratio and metal content, OER, ORR and HER stability test and Tafel slopes, Additional LSV curves and normalized by metal content loading, ORR steady-state LSV curves, Koutecky-Levich plots and number of electrons transferred, XPS spectra and atomic contents. (PDF)

AUTHOR INFORMATION Corresponding Author *Corresponding Author: R. Silva ([email protected]) Notes The authors declare no competing financial interest. ACKNOWLEDGMENT GMP thanks the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES-Brasil) for doctorate fellowships. AFR and RS acknowledge the financial supports given by CNPq, Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES-Brasil) and Fundação Araucária-Brasil. Authors also thanks to the COMCAP and Finep for technical support.

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Carbon Capped Zero-Valent Nickel and Cobalt Nanoparticles as

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