Noble Metal-Free MOF-74 Derived Nanocarbons: Insights on Metal

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Noble Metal-Free MOF-74 Derived Nanocarbons: Insights on Metal Composition and Doping Effects on the Electrocatalytic Activity Towards Oxygen Reactions Víctor Karim Abdelkader Fernandez, Diana Mesquita Fernandes, Salete Silva Balula, Luís Cunha-Silva, Manuel Jose Perez-Mendoza, F. Javier Lopez-Garzon, Manuel Fernando R. Pereira, and Cristina Freire ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b02010 • Publication Date (Web): 11 Feb 2019 Downloaded from http://pubs.acs.org on February 13, 2019

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ACS Applied Energy Materials

Noble Metal-Free MOF-74 Derived Nanocarbons: Insights on Metal Composition and Doping Effects on the Electrocatalytic Activity Towards Oxygen Reactions

Víctor K. Abdelkader-Fernández†*, Diana M. Fernandes†*, Salete S. Balula†, Luís CunhaSilva †*, Manuel José Pérez-Mendoza‡, F. Javier López-Garzón‡, M. Fernando Pereira+ and Cristina Freire†

†REQUIMTE/LAQV, Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade de Porto, Rua do Campo Alegre, s/n, Porto, 4169-007, Portugal. ‡Departamento de Química Inorgánica, Facultad de Ciencias, Universidad de Granada, Granada, 18071, Spain. +

Laboratório de Catálise e Materiais (LCM), Laboratório Associado LSRELCM, Departamento de

Engenharia Química, Faculdade de Engenharia, Universidade do Porto, 4200-465 Porto, Portugal.

*Corresponding author: Dr. Víctor K. Abdelkader-Fernández ([email protected]), Dr. Diana M. Fernandes ([email protected]) and Dr. Luís Cunha-Silva

([email protected])

Keywords: MOF-derived carbons, renewable energy, electrocatalysis, oxygen evolution reaction, oxygen reduction reaction, fuel cells

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ABSTRACT The continuous and dramatic increase of global demand for energy resources make it urgent to develop affordable nanostructured materials to act as efficient electrocatalysts (ECs) on the energy-related reactions. MOF templates pyrolysis for the production of nanostructured carbon-based materials is a very promising methodology to produce carbon-based ECs. Herein, we report the preparation, characterization (XPS, CHNS analysis, ICP-OES, XRD and SEM/EDS) and application as OER and ORR ECs of a plethora of nanostructured carbons materials derived from RT-synthetized MOF74 with different metal compositions - Co, Ni and Co/Ni, and dopant heteroatoms: N-, S- and N/S-dual-doping. This has allowed the study of the aforementioned parameters influence on the OER and ORR electrocatalytic activity in alkaline medium. High synergetic effects have been detected in two cases: (1) when N/S-dual doped carbon is produced from a monometallic Co-MOF-74 template - N,S-Co@C, and (2) when an undoped carbon is derived from bimetallic Co/Ni-MOF-74 - Co/Ni@C. These two samples achieve OER performances with η10 = 0.41 and 0.44 V, respectively, along with Tafel slopes of 101 and 93 mV dec-1, being close to the state-of-art OER catalyst performance. In addition, ORR tests showed that the effect of heteroatom doping on ORR activity is always positive, regardless metal composition.

1. INTRODUCTION The current huge global energy demand looks set to continue to expand, reaching a growth of 30% in 2035, mainly due to the increasing prosperity in fast-growing emerging economies.1 Recently, this fact along with climate change produced a rising awareness of environmental responsibilities, making mandatory to exploit renewable 2 ACS Paragon Plus Environment

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energy more than before to bring the benefits for the society.2 In this context, trying to avoid greenhouse gas emissions, but at the same time meeting the (present and future) worldwide energetic requirements, the hydrogen (water) cycle takes a great importance as the basis for highly promising energy conversion systems.3, 4 Specifically, the efficient electrocatalysis of the oxygen evolution reaction (OER),5,

6

the half-

reaction for the water splitting process, will be crucial for the real implementation of fuel cells, regenerative batteries and electrolyzer systems. Some oxides containing platinum group metals (PGMs), e. g., ruthenium and iridium oxides (RuO2 and IrO2), have been reported as the materials with the best electrocatalytic performances for the OER process.7 However, the extremely elevated costs of PGMs, along with the scarceness and lack of reliable suppliers,8-11 strongly encouraged the research on development of alternative and affordable OER electrocatalysts. In the last years, important efforts have been made to develop novel nanocomposites to be used as electrocatalysts towards oxygen evolution, especially trying to combine transition metal oxides with advanced carbon materials.12-14 In this field, crystalline solids based on extended coordination frameworks formed by metal ions and organic ligands, known as Metal-Organic Frameworks (MOFs), have attracted attention due to their very interesting properties: structural variance, compositional versatility, homogeneous distribution of metal atoms and ordered porosity.15 However, in spite of these desirable features, regarding the application of MOFs to electrocatalysis, these materials present some disadvantages, e.g., limited diffusion capacities of reactants through MOF porosity, low reactivity of metal centers due to their saturation with the coordinated linkers, and poor electron conductivity.16 To overcome these drawbacks, the recent research on MOF-based materials applied to 3 ACS Paragon Plus Environment


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the electrocatalysis of energy-related reactions has been focused on the pyrolysis of MOF sacrificial templates to produce derived nanostructured carbon materials. A plethora of works reported the production of OER ECs generated from diverse MOF templates, specifically involving the carbonization of MOFs containing Co,17-24 Ni25, 26 or Fe,27 as well as combinations between several transition metals.28-30 Additionally, two doping approaches have been explored: i) the direct pyrolysis of a sacrificial MOF including a heteroatom-containing organic linker, or ii) the post-synthesis heteroatom source incorporation to the MOF template before its carbonization. A common example for the former approach is the production of N-doped carbons derived from Zeolitic Imidazolate Frameworks (ZIFs),17,

18, 21, 28, 31,

or the synthesis of a N,S-dual

doped carbon via pyrolysis at 800 °C of a MOF involving thiophene-2,5-dicarboxylate (TDC) and 4,4’-bipyridine (BPY) as organic ligands and S, N precursor, respectively.20 In relation to the latter doping approach, different compounds, e. g., selenium,24 phosphorous,21 thiourea26 and melamine27, have been employed as Se, P, N/S and N heteroatom sources, respectively. Despite the variety of materials reported during the last few years, there is a certain lack of systematic studies on the aggregate influence of the metal/s composition along with the type of doping on the electrocatalytic activity of MOF-derived nanocomposites towards the oxygen-related reactions. In addition, up the best of our knowledge and regarding the potential future scaled production of MOF-based ECs, room temperature (RT)-synthesized MOF has not been employed for this kind of electrocatalytic applications. For this reason, tackling these issues, we have focused on the production of doped carbons derived from MOF-74. This type of MOF (also named as CPO-27) is formed by divalent metal ions and 2,5dihydroxyterephtalic acid, H4DOBDC. The choice of MOF-74 as sacrificial template is 4 ACS Paragon Plus Environment

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based on two important factors: the possibility of production with different metals and even, with metal mixtures, and specially, on its very simple preparation at room temperature. In this work we report (i) a facile synthesis of noble metal-free dual N,S-doped MOF-74 derived nanostructured carbon-based materials by a 3-step strategy, and (ii) the study of their electrocatalytic activity towards the OER and ORR processes in alkaline medium. The 3-step approach consists of MOF preparation, incorporation of thiourea (heteroatom source) and finally, simultaneous carbonization/doping treatment (Figure 1). One of the major advantages of this work relies of the fact that MOF-74 templates have been synthetized at RT, avoiding hard-condition solvothermal processes, and contrary to the more extended pyrolysis treatments, carbonizations were performed under relatively low temperature, 500 °C. Focusing on electrocatalytic studies, dual-, mono- and undoped nanocarbons containing Co, Co/Ni and Ni species have been systematically tested to assess the influence of doping and metal composition on the OER (and ORR) catalytic behavior.

Figure 1. Scheme for the 3-step strategy used to produce electrocatalysts consisting of noble metals-free doped carbons derived from a MOF-74 template. M = Co, Ni or Co/Ni, and dopant = dicyandiamide, sulfur or thiourea. 5 ACS Paragon Plus Environment


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2. EXPERIMENTAL 2.1. MOF-74 AND CARBON PREPARATION 2.1.1. Materials Reagents

used

for

MOF-74

preparations,

cobalt(II)

acetate

tetrahydrate,

Co(AcO)2·4H2O (Analytical Reagent, 99%, Merck), nickel(II) acetate tetrahydrate, Ni(AcO)2·4H2O (Analytical Reagent, 99%, Riedel-de-HaënTM), 2,5-dihydroxyterephtalic acid, H4DOBDC (98%, Sigma-Aldrich), and N,N-dimethylformamide, DMF (Analytical Reagent Grade, Fisher Scientific) were employed as received. Regarding the impregnation

step,

thiourea

(Analytical

Reagent

Grade,

99.98%,

Merck),

dicyandiamide, DCDA (99%, Sigma-Aldrich) and elemental sulfur were used as heteroatom sources, while methanol (Analytical Reagent Grade, 99.99%, Fisher Scientific) played the role of solvent. During carbonization, a nitrogen (High Purity: > 99.998 %) flow was employed.

2.1.2. Room temperature synthesis of MOF-74 Co-MOF-74 precursor was prepared by a facile room temperature method.32 Briefly, two DMF solutions (one with 1.0 mmol of organic linker, H4DOBDC, and the other one with 2.6 mmol of metal salt, Co(AcO)2·4H2O) were mixed and magnetically stirred during 20 h. The solid was separated from solvent via centrifugation at 3500 rpm during 10 min, washed with DMF and twice with methanol. Finally, the as-obtained orange-brown powder was kept immersed in methanol during 6 days, replacing the methanol by fresh methanol for at least three times, to complete the purification process of the isolated material. For the synthesis of Ni-MOF-74, the corresponding


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diacetate salt was employed. In the case of Co/Ni-MOF-74 preparation, an equimolar DMF solution of the two salts was used.33

2.1.3. Impregnation of MOF-74 with the heteroatom source In order to incorporate the N/S heteroatoms, 0.4 g of MOF-74 was added to 40 mL of a thiourea 0.13 M methanol solution (MOF-74:thiourea ratio of 50wt.%/50wt.%). The mixture was kept under stirring during 24 h to ensure maximum loading of thiourea on MOF-74. Then, the solid was recovered by centrifugation and air dried overnight. For the N-doping of Co-MOF-74 the same procedure was followed, but using dicyandiamide as heteroatom source instead of thiourea, while elemental sulfur was employed to carry out the S-doping.

2.1.4. Carbonization of MOF-74 Simultaneous carbonization/doping treatments were performed on a tubular furnace, placing the heteroatom source-loaded MOF templates on a ceramic boat. To produce the final carbon-based electrocatalysts, the sacrificial templates were heated up to 500 °C with a ramp temperature rate of

3 °C·min-1, under N2 flow (80 mL·min-1).

Temperature was hold at 500 °C during 6 h. Before starting the heating, a N2 purge was carried out to avoid the presence of air during the carbonization. To produce the nondoped analogous nanocomposites the heating treatment was performed in a similar manner, but utilizing thiourea-free MOF-74 precursors. Final metal/carbon-based nanocomposites are labelled following the structure: X-M@C, where X stands for the doping element (or elements), M stands for the metal (or metals) and C, for carbon.



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2.2. MOF-74 AND CARBON CHARACTERIZATION 2.2.1. Physicochemical characterization Diverse characterization techniques - elemental (CHNS) analysis, inductively coupled plasma optical emission spectrometry (ICP-OES), X-ray photoelectron spectroscopy (XPS), powder X-ray diffraction (XRD), N2 adsorption measurements, transmission electronic microscopy (TEM) and scanning electronic microscopy with energydispersive X-ray spectroscopy (SEM/EDS) - were employed to the complete compositional/structural/morphological analysis of the MOF-74 templates and their derived nanocarbons. Additional technical details can be consulted in the Experimental Section of Supplementary Information (SI).

2.2.2. Electrochemical studies All electrochemical tests were carried out at RT using an Autolab PGSTAT 302 N potentiostat/galvanostat (EcoChimie B.V.) and an Autolab rotating disk electrode (RDE) system (Metrohm AG, Switzerland). This measurement system was controlled by the NOVA v2.0 software. A standard three-electrode cell arrangement was used to perform the electrochemical measurements, consisting of a glassy carbon (GC) working electrode tip (d = 3 mm, Metrohm) modified by ECs coating, an Ag/AgCl reference electrode (3 mol dm-3 KCl, Metrohm) and a counter electrode - a platinum wire (d = 0.6mm, 0.5 m, 99.99+%, Goodfellow) for OER studies or a glassy carbon rod (d = 2 mm, Metrohm) for ORR. The experimentally applied potentials were measured against the Ag/AgCl reference electrode, and then, converted to the reversible hydrogen electrode (RHE) using the pH-depending Nernst formula:


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ERHE = EAg/AgCl + 0.059pH + E°Ag/AgCl

1

where ERHE is the converted potential vs. RHE, E°Ag/AgCl = 0.197 V at 25 °C, and EAg/AgCl is the measured potential vs. Ag/AgCl. Previously to its utilization, the RDE was sequentially polished with 6, 3 and 1 μm particle size diamond polishing compounds (MetaDi®ll, Buehler) and 0.3 μm alumina powder (MicroPolish Alumina, Buehler) until a mirror-like polished surface was obtained, and then rinsed with ultrapure water (electrical resistivity = 18.2 MΩ cm, Millipore). For the electrode modification, 1.0 mg of electrocatalyst and 20 μL of Nafion® 117 solution (Sigma-Aldrich) were dispersed in 250 μL of a 2-propanol (anhydrous 99.5%, Sigma-Aldrich) and ultrapure water solvent mixture (V:V: = 1:1), and then sonicated for at least 30 min to produce an homogeneously dispersed electrocatalyst ink. After that, 7.5 μL of ECs dispersion were dropped onto the GC disk electrode surface and dried under air flux. The electrolyte was 0.1 M KOH solution prepared from ultrapure water (pH = 13.0).

2.2.2.1. OER measurements Oxygen evolution tests were performed in N2-saturated alkaline electrolyte, 0.1 M KOH solution purged with N2 for at least 30 min before the measurement. Linear sweep voltammograms (LSV) were obtained by sweeping the potential from 1.0 to 1.8 V (vs. RHE) with a scan rate of 5 mV s–1 at 1600 rpm of rotation speed. All presented LSV tests were performed with iR-compensation, via previous calculation of the uncompensated resistance (Ru) of the circuit by using i-interrupt approach, and finally, applying an iR-compensation value equal to 0.90×Ru to the LSV measurement. In order



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to study the OER activity of the MOF-derived nanocomposites Tafel slopes were calculated by linear fitting of LSV data to the following equation: η = a + blog|j|

2

where η stands for the overpotential, b corresponds to the Tafel slope, and j is the current density.

2.2.2.2. ORR measurements The cyclic voltammetry (CV) experiments were performed from 1.2 V to 0.25 V (vs. RHE) in both N2- and O2-saturated 0.1 M KOH solution with a sweep rate of 5 mV s-1. LSV experiments were carried out also in N2- an O2-saturated 0.1 M KOH solution, a scan rate of 5 mV s-1 under diverse rising rotation speeds: 400, 800, 1200, 1600, 2000 and 3000 rpm. The numbers of electrons transferred (nO2) per oxygen molecule at different potentials during the reduction of oxygen (ORR) were determined via the Koutecky-Levich (K-L) equation: 1 1 1 1 1 = + = + 𝑗 𝑗𝐿 𝑗𝐾 𝐵𝜔1/2 𝑗𝐾 B = 0.2 nO2FC0(D0)2/3ν-1/6 jK = nO2FkC0

3

where j is the current density (derived from the experimentally measured current intensity), jL and jK stand for the diffusion- and kinetic-limiting current densities, respectively, ω corresponds to the rotation rate, nO2 represents the number of electrons transferred per O2 molecule, F is the Faraday constant (96485 C mol-1), C0 is the O2 concentration in the O2-saturated electrolyte (1.15×10-3 mol cm-3), D0 stands for


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the O2 diffusion coefficient in the electrolyte (1.95×10-5 cm2 s-1), and ν is its kinetic viscosity (8.98x10-3 cm2 s-1).34

2.2.2.3. Assessment of electrochemically active surface areas (ECSA) An ECSA study of the carbon samples has been carried out through comparison of their corresponding double layer capacitance (Cdl) values.17,

23, 35

Experiments and

calculation details are included in the Experimental Section of Supplementary Information (SI).

3. RESULTS AND DISCUSSION 3.1. PHYSICOCHEMICAL CHARACTERIZATION All carbon-based materials were prepared from three different RT-synthesized MOF-74 sacrificial templates: Co-MOF-74, bimetallic Co/Ni-MOF-74 and Ni-MOF-74. Metal contents and surface element concentrations of these precursors were analyzed via ICP-OES (Figure 2a) and XPS analysis (Table S1 in the Supporting Information, SI). Additionally, their structures were characterized by the deconvolution of XPS high resolution spectra (Figure S1, SI) and acquisition of the corresponding XRD powder patterns (Figure 2b):



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Figure 2. (a) Metal concentrations for the three different MOF-74 precursors, obtained by ICPOES analysis, and (b) their corresponding XRD powder patterns (focused on the two characteristics diffraction peaks of MOF-74 structures).

The two diffraction peaks at ≈ 7° and ≈ 12° demonstrate the successful preparation of the three MOF-74 structures at RT.32, 33 Additionally, IR spectra of the three different RT-synthesized MOF-74 sacrificial compounds are identical and reveal the main vibrational bands characteristics of these family of MOFs, further supporting the synthesis of the desirable materials (Figure S5, SI).32 Regarding composition, the three MOFs present similar metal contents (27.4 – 30.1 wt.%), and the Co/Ni-MOF-74 material exhibits a Co:Ni atomic ratio of 1.18 (from XPS data) and of 1.20 (from ICPOES data), denoting a highly homogeneous distribution of the Co(II) and Ni(II) ions into the 3D framework. MOF-74 morphologies were further studied by using SEM (Figure S6, SI), exhibiting the same appearance for the three template materials: particles of several micrometers with no detectable high crystalline units/domains, being in good


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agreement with the low crystallinity degree shown up by the low intensity diffraction peaks in Figure 2b. Focusing on carbon materials produced via MOF-74 pyrolysis, the XPS atomic surface % for all these nanocarbon samples are displayed in Table 1. As consequence of the carbonization process, oxygen surface % are noticeably lower than those for the MOF74 precursors (≈ 32 – 35 at.%, see Table S1, SI). The two bimetallic carbon samples (Co/Ni@C and N,S-Co/Ni@C) present Co:Ni atomic ratios of 1.63 and 2.38, respectively. These are much larger than that for their precursor Co/Ni-MOF-74, 1.18, suggesting that carbonization involves a drastic redistribution of metals, in which cobalt is preferably located in the outer areas of bimetallic Co/Ni-containing clusters or nanoparticles. Regarding the two mono-doped samples, the heteroatom (sulfur or nitrogen) concentration in S-Co@C is significantly larger than in N-Co@C. This trend is also observed for the dual-doped samples (N,S-Co@C, N,S-Co/Ni@C and N,S-Ni@C) where S atomic percentages are always higher than the corresponding N concentrations, probably due to the high tendency of S to form metal sulfides (see Tables 3 and S3, SI) under the carbonization conditions. In any case, the relevant presence of these heteroatoms in the doped carbon compositions indicates the efficiency of the doping strategy.



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Table 1. Surface element composition (XPS) for all MOF-74 derived carbons. Sample

C 68.64 70.12 84.16 62.28 63.86 71.92 61.48 66.70

Co@C Co/Ni@C Ni@C N-Co@C S-Co@C N,S-Co@C N,S-Co/Ni@C N,S-Ni@C

O 23.33 25.06 10.80 28.01 22.05 18.01 27.27 18.14

XPS atomic % (at.%)* Co Ni 8.03 2.99 1.83 5.04 8.59 4.35 5.80 5.36 2.25 6.91

N

S 1.12 1.92 1.60 3.61

9.74 2.36 2.05 4.63

* Surface atomic percentages derived from high resolution C1s, O1s, Co2p, Ni2p, N1s and S2p core-level peak areas.

Surface and bulk compositions are compared in Table 2. In relation to the metal contents, bulk values are larger than those in the surface for all carbons, fact that can be attributed to the coating of the metal nanoparticles by the carbon matrix. In contrast, heteroatoms (N and S) are more abundant near to the surface than in the bulk of doped samples. These heteroatom surface/bulk distributions are probably due to the concentration of these elements in the carbon matrix and in the outer regions of the C-embedded metal-containing nanoparticles.

Table 2. Comparison between surface (XPS) and bulk (ICP-OES and CHNS analysis) elemental compositions for all MOF-74 derived carbons. Sample Co@C Co/Ni@C Ni@C N-Co@C S-Co@C N,S-Co@C N,S-Co/Ni@C N,S-Ni@C

Metal contents (mmol·g-1) Co Ni 1 2 Surface Bulk Surface1 Bulk2 4.8 8.5 2.0 7.5 1.2 6.4 3.4 9.2 5.0 7.9 2.6 4.5 3.6 10.1 3.1 5.6 1.3 4.7 4.1 9.5

1 Surface

Dopant atom contents (mmol·g-1) N S 1 3 1 Surface Bulk Surface Bulk3 0.7 0.4 0.1 0.1 5.8 2.3 1.2 0.5 1.5 0.5 0.9 0.3 1.2 0.3 2.1 1.4 2.7 1.9

concentration values derived from XPS high resolution Co2p, Ni2p, N1s and S2p core-level peak areas.


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3 Bulk

2 Bulk

metal concentrations obtained by ICP-OES analysis. heteroatom concentration values obtained by CHNS analysis.

Chemical varieties of the different elements have been elucidated by the deconvolution of the corresponding XPS high resolution spectra for each sample. An example of these deconvolutions is shown in Figure 3 for the N,S-Co/Ni@C, while the deconvolutions for the elements of the rest of carbon-based materials are collected in Figure S2 (SI). In addition, all positions and full width at half-maximum (FWHM) values for the diverse components in each XPS fitting are included in Table S2 (SI). Focusing on the 3/2 region of Co2p peaks, they can be deconvoluted in three main components approximately located at 778.9, 780.3 and 782.0 eV, attributed to the presence of the metallic state (Co0), cobalt(II) oxide (CoO) and Co interacting with C, respectively.29 In case of the S-containing materials (S-Co@C, N,S-Co@C and N,S-Co/Ni@C), S2p deconvolutions demonstrate the formation of cobalt(II) sulphide (CoS), therefore, the component related to Co0 in Co2p peaks is slightly shifted to lower binding energies and can be simultaneously attributed to CoS (≈778.6 eV).36 Similarly, in Ni2p (3/2 region) fittings, the lowest BE component (≈852.8 eV) can be unambiguously attributed to metallic nickel (Ni0) in the cases of Co/Ni@C and Ni@C, and to both Ni0 and nickel(II) sulphide (NiS) in the cases of N,S-Co/Ni@C and N,S-Ni@C, while the following two components, NiO (i) and (ii), are assigned to the local and nonlocal screening of the Ni2p core hole in nickel(II) oxide.29, 37, 38 The positions of these two components, 854.2 and 855.8 eV, are consistent with those reported for nanoscaled NiO crystals.37 N1s deconvolutions involve three components at ≈398.6, ≈400.4 and ≈401.4 eV, corresponding to pyridinic-like, pyrrolic-like and quaternary-like nitrogen atoms,39 while S2p fittings present four different contributions due to MS (≈161.7 eV), 15 ACS Paragon Plus Environment


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with M = Co(II) and/or Ni(II), and S bonded to C atoms forming diverse functional groups: thiophene-like/thiol (C-SH-C/C-SH) groups (≈164.4 eV), and organo–SOx functions (≈167.6 and 168.2 eV with x = 2 and 3, respectively).36, 40

Co2p components:

― Co0 / CoS ― CoO

― Co-C

interaction ― Satellite

Ni2p components

― Ni0 / NiS

― NiO (ii)

― NiO (i)

― Satellite

N1s components:

― Pyridinic ―Quaternary N

N

― Pyrrolic N S2p components:

― CoS / NiS

― C-SO2-C

― C-S-C /

― C-SO3

C-SH

Figure 3. Deconvolution of XPS high resolution spectra: (a) Co2p (3/2 region), (b) Ni2p (3/2 region), (c) N1s and (d) S2p peaks of N,S-Co/Ni@C. In the S2p deconvolution plot, 1/2 components are omitted for clarity.

Abundances derived from component areas in the peak deconvolutions for the metal/s (Co and Ni) and dopant heteroatoms (N and S) are shown in Table 3 and Table S3 (SI), respectively. A common feature for all samples is that carbon-embedded metal atoms are always present in non-negligible proportions of two oxidation states: metallic state (0) and divalent cations (II) forming oxides (and also sulfides in the doped carbons). Paying attention to the undoped carbons and comparing Co2p deconvolutions, an important increase of the satellite component (from 23.3 to 55.9 at.%) is observed 16 ACS Paragon Plus Environment

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when Co@C and Co/Ni@C are compared. The same situation is found by comparing Ni2p satellites corresponding to Ni@C and Co/Ni@C. This behavior suggests nonnegligible changes in the proportions of M2+ and M0 states, since the presence of satellites in Co2p and Ni2p is associated to Co2+ and Ni2+ ions, respectively. Thus, the variations of the M0:M2+ atomic ratios seem to corroborate this statement, while Co0:Co2+ atomic ratio in Co@C resulted to be 0.293, in case of Co/Ni@C decreased to 0.167. In relation to nickel a similar behavior is detected, being Ni0:Ni2+ atomic ratios for Ni@C and Co/Ni@C, 0.620 and 0.054, respectively. Considering the doped samples, unfortunately, the corresponding Co0:Co2+ and Ni0:Ni2+ atomic ratios cannot be calculated with reliability, due to the metal(II) sulfide components overlap with the M0 bands in the Co2p and Ni2p core-level regions.

Table 3. Atomic concentration (relative abundances) for the different chemical environments of Co and Ni atoms of all carbon samples.

Sample Co0 / CoS1 Co@C Co/Ni@C Ni@C N-Co@C S-Co@C N,S-Co@C N,S-Co/Ni@C N,S-Ni@C

17.4 6.3 11.3 15.0 16.0 14.5 -

Abundance (at.%) Co2p core-level region Ni2p core-level region Co-C CoO Satellite Ni0 / NiS1 NiO2 Satellite interaction 41.7 17.6 23.3 17.7 20.1 55.9 1.8 33.3 64.9 23.0 37.1 39.6 43.7 18.0 26.5 21.0 24.6 39.4 28.8 15.2 40.1 28.3 13.8 43.4 8.9 49.0 42.1 22.3 39.1 38.6

In case of undoped carbons, this component can be unambiguously attributed to M0. The abundance of NiO resulted from the sum of the two NiO (i) and NiO (ii) components. 1

2

Table S3 (SI) shows, on one hand, that the totality of N atoms is directly integrated into the C matrixes in all N-containing carbons, i. e., N-Co@C, N,S-Co@C, N,S-Co/Ni@C and N,S-Ni@C. On the other hand, S atoms do not exhibit the same behavior, since a 17 ACS Paragon Plus Environment


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predominant proportion (always above 57 % in the three dual-doped carbons) is not directly doping the carbon, but combined with the metal/s to generate the corresponding metal(II) sulfides. This behavior has been recently reported for similar dual-doped MOF-derived carbon materials.20, 26 XRD powder diffractograms for all carbon samples are shown in Figure 4. As consequence of the carbonization process, the two diffraction peaks at low angles characteristics of MOF-74 structures (see Fig. 2b) disappeared in the diffractograms of the carbons. In addition, no graphitic (002) peak is observed between 24° - 25°, denoting the formation of a disordered C matrix. Thus, the totality of the features is located at angles above 30° and is assignable to several metal-containing crystalline phases. Identification of the different phases was performed by comparison with the diffraction peaks positions indexed in the corresponding ICDD-International Centre for Diffraction Data cards provided by the Joint Committee on Powder Diffraction Standards (JCPDS) (Figure S3, SI). Metallic phases (space group: Fm-3m, with closepacked structure)41, 42 are clearly observed in all samples (red triangles). In the case of undoped carbons, a slight shift of the peaks is detected due to the different sizes of Co and Ni atoms (see inset in Fig. 4a), being their positions consistent with the presence of metallic Co, Co0.5Ni0.5 and Ni nanocrystals in Co@C, Co/Ni@C and Ni@C, respectively. Contrary to metallic phases, peaks related to metal(II) oxides (black squares) are not visible in all the samples, probably due to a lower dimension and concentration of this type of crystalline phase. Similarly to the case of metal(II) oxides, only one peak positioned at ≈31° is assignable to a metal(II) sulfide phase (yellow squares), being more clearly observable in the diffractogram of N,S-Ni@C, probably due to a higher tendency to form sulfides for Ni than Co (see Table 3). In general, 18 ACS Paragon Plus Environment

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structural data provided by XRD are in fair agreement with those derived from XPS analysis. Comparing the patterns for undoped and doped carbons, an important difference in peak intensities is detected. While undoped carbons exhibit intense narrow diffraction peaks, rather wider peaks are observed for doped samples, especially in the cases of the dual-doped carbons. Co@C, Co/Ni@C and Ni@C present average crystal sizes (calculated by using Sherrer equation) of approximately 27, 22 and 30 nm, respectively; and N,S-Co@C, N,S-Co/Ni@C and N,S-Ni@C exhibit smaller crystal sizes, 11, 12 and 18 nm, respectively.

a Co@C Co/Ni@C Ni@C

Intensity (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30

40

50 60 2theta (°)

70

80

Figure 4. Diffractograms for (a) undoped and (b) mono doped and dual-doped carbons. Red triangles, black circles and yellow squares point up diffraction peaks generated by metallic (0), metal(II) oxide and metal(II) sulfides phases, respectively. Taking advantage from the absence of diffraction peaks at low angles, x axes start from 30° for clarity.



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Consequently, it is clear that the dual-doping (via impregnation of MOF-74 templates with thiourea before pyrolysis) produces a clear effect, not only on the composition but also on the size of metal-containing nanoparticles embedded in the carbon matrixes, resulting in higher nanostructuration degrees, since average crystal sizes get approximately halved. The morphology of the carbons was studied by acquiring TEM and SEM images at different magnifications. TEM micrographs are included in Figure 5 and Figure S9 (SI). Despite the impossibility to register high resolution images due to the intense magnetic properties of the samples, two general conclusions can be made: (i) all of them present a ‘low optical contrast’ C matrix embedding nanoparticles showing high contrast as consequence of their metal composition, and (ii) undoped nanocarbons exhibit higher NP sizes, while their N,S-dual-doped analogues involve noticeably smaller nanoparticles. This last conclusion supports the higher nanostructuration degrees found for the dual-doped nanocarbons in comparison with the undoped samples (cf. average crystal sizes calculated from XRD data).


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Figure 5. TEM micrographs for the samples (a,b) Co/Ni@C, (c) N,S-Co@C and (d) N,S-Co/Ni@C.

Focusing on the undoped carbons, in contrast with Co@C and Ni@C, bimetallic Co/Ni@C presents narrow-distributed NP sizes (15 - 20 nm, aprox.) with an incipient core-shell structure, where the difference of contrast between inner and outer areas of the NPs suggests the existence of an oxidized shell coating a metal-state core (see Fig. 5a and b). This geometric arrangement formed in Co/Ni-MOF-74 derived carbons has been recently described as CoxNi1−x/CoyNi1−yO core/shell nanoparticles by Sun et al.29 Regarding SEM analysis, the corresponding micrographs are gathered in Figure S7 (SI). By comparing the morphologies of MOF-74 templates (Fig. S6, SI) and their derived carbons, carbonization effect is evidenced by the formation of granular textures. This significant change of surface morphologies produced by carbonization leads to the increment of surface areas (SBET): from 116 m2·g-1 in Co-MOF-74 template to 176 m2·g-1 in N,S-Co@C (see Figure S10, SI). Focusing on carbons, these show similar nanostructured arrangements consisting of globular grains (with sizes of approximately of some ten nanometers) very close to each other, with the exception of Co/Ni@C, whose morphology at micron scale (Fig. S7c, SI) is noticeably different. This fact suggests a non-negligible influence of the core/shell NPs formation on the final carbonaceous matrix morphology. Therefore, given the small sizes of the nanoparticles 21 ACS Paragon Plus Environment


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and their carbon coating (shown by TEM analysis), the morphology of the nanocarbon samples that can be assessed by SEM seems to be marginally influenced by metal composition or doping, excepting the presence of mixed Co-Ni core/shell nanostructures. In addition, mapping measurements based on EDS microanalysis were performed (Figure S8, SI). Nitrogen could not be detected due to the poor sensibility of EDS towards nitrogen detection along with the low N concentrations of the samples (see Tables 1 and 2). Regarding metals (Co and Ni) and S (for the doped carbons), maps show up a very homogeneous distribution of those elements in all samples, indicating a good dispersion of the metal-containing nanoparticles in the C matrixes. Note that the black regions in S maps are generated by the topographical shadowing effect (SEM/EDS phenomenon more accused for elements with low atomic numbers). Finally, a more intense blue color is noted in the S distribution map for N,S@Ni-C due to the higher S content of this sample in relation to the others (see Tables 1 and 2).

3.2. ELECTROCATALYTIC PERFORMANCE FOR THE OER 3.2.1. Effect of the type of doping The influence of the type of doping (N-doping, S-doping and N,S-dual-doping) on the OER electrocatalytic behavior has been assessed via registering the corresponding LSV curves (see 2.2.2.1. OER measurements) for the different Co-containing carbons: Co@C, N-Co@C, S-Co@C, and N,S-Co@C (Figure 6a). At first sight, polarization curves show a slight positive influence on mono-doped samples (N-Co@C and S-Co@C), exhibiting a low increase of current density comparing with that for Co@C. However, the catalytic activity of N,S-Co@C is triggered, producing a very significant increase of current density achieved with lower potentials. This important enhancement of the 22 ACS Paragon Plus Environment

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catalytic activity towards oxygen evolution is also evidenced in the Tafel plots (Figure 6b), with a remarkable slope decrease in case of the dual-doped carbon, N,S-Co@C, in comparison with those for the mono-doped and undoped samples.

Figure 6. (a) OER polarization curves obtained by LSV (KOH 0.1 M solution, scan rate = 5 mV s-1, rotation speed = 1600 rpm) for Co@C, N-Co@C, S-Co@C, and N,S-Co@C. (b) Tafel plots for the same samples.

Table 4 collects the electrochemical parameters regarding the OER activity shown by the four samples. In comparison with Co@C, the overpotential required to achieve 10 mA cm-2 (η10) exhibited by N,S-Co@C decrease in 0.23 V, from 0.64 to 0.41 V vs. RHE. This fact results in a drastic rise of the current density achieved by N,S-Co@C: j1.75 = 43.8 mA cm-2. The much faster kinetic exhibited by N,S-Co@C results in a TS value of 101 mV dec-1, much lower than that for the undoped carbon (Co@C), 167 mV dec-1. All these results evidence a great magnitude synergetic effect between the doping of the carbon matrix with N (and to a lesser extent with S) and the simultaneous formation of cobalt(II) sulfide (CoS), which strongly boost the capacity of N,S-Co@C to catalyze the



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OER process. Noteworthy, neither the independent generation of N-containing functions bond to C atoms (case of N-Co@C) nor the independent CoS formation (case of S-Co@C) are effective to trigger the OER catalytic activity of this type of nanocomposites.

Table 4. OER activity parameters for undoped, mono-doped and dual-doped Co@C samples. Sample Co@C N-Co@C S-Co@C N,S-Co@C

E10 (V vs. RHE)1 1.87 1.85 1.86 1.64 2

OER parameter η10 (V)2 j1.75 (mA cm-2)3 0.64 4.9 0.62 6.0 0.63 5.9 0.41 43.8

TS (mV dec-1)4 167 171 175 101

1 Required potential to reach j = 10 mA cm-2 Overpotential for achieving j = 10 mA cm-2 (respect to E°O2/H2O = 1.23 V vs. RHE) 3 Current density at potential (E) of 1.75 V vs. RHE 4 Tafel slope (calculated by linear fitting of Tafel plot data)

3.2.2. Effect of metal composition and N,S-dual-doping (aggregate influence). In order to evaluate the influence of metal composition, Co/Ni@C and Ni@C have been tested as OER electrocatalysts. Additionally, to carry out the assessment of the metal/dual-doping aggregate effect on catalytic performance, the analogous dualdoped samples, N,S-Co/Ni@C and N,S-Ni@C, were also tested. Regarding the polarization curves for the undoped samples (Figure 7a), CoxNi1−x/CoyNi1−yO core/shell nanoparticles embedded in the carbon matrix of the Co/Ni@C sample drastically trigger the catalytic activity of this material, in comparison with the monometallic samples (Co@C and Ni@C). The notable synergetic effect shown by these carboncoated mixed-metal core/shell structures has been reported by Sun et al.29 Remarkably, we found a drastic increase of current density values achieved with Co/Ni@C in relation to Co@C, namely, a much more pronounced Co-Ni synergy than 24 ACS Paragon Plus Environment

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that recently reported. Trying to explain this fact, two main differences between the Co/Ni-MOF-74 derived materials (tagged as Co/Ni@C in the present article) produced by our group and by Sun et al.,29 were found. From a compositional point of view, the bimetallic nanoparticles in our sample exhibit a surface enriched in cobalt, since it presents 2.0 and 1.2 at.% of surface atomic contents for Co and Ni, respectively, (see Table 1), while in their material the surface concentrations for Co and Ni are practically the same. From a structural perspective, our core/shell NPs are noticeably larger than theirs (≈18 nm and ≈8 nm, respectively) and the no detection of diffraction peaks assignable to an oxidized phase in our sample (see Fig. 4a) suggests that the mixed Co/Ni oxide presumably adopts an amorphous configuration. Thus, taking in account these differences, it is evidenced the determinant influence of composition and structural layout, especially regarding the metal/oxide nanoparticles, on the OER electrocatalytic properties of these type of carbons. Once proven the remarkable improvement generated by dual-doping on the electrocatalytic performance shown by N,S-Co@C (see Section 3.2.1.), N,S-Co/Ni@C was synthesized in order to try to combine both Co-Ni and dual-doping synergetic effects (Figure 7b). Unfortunately, in this case, the expected accumulative positive effect is not observed: N,S-Co/Ni@C achieved higher current density than Co@C, but is far from the results obtained, on the one side, by N,S-Co@C and on the other, by Co/Ni@C. Concerning the two Nicontaining carbons, Ni@C and N,S-Ni@C, their catalytic activity is not influenced by the doping, thereupon, no synergic effect between NiS and the doped C matrix is found. Kinetic of the electrocatalysed process has been assessed through the Tafel plots derived from LSV data (Figure 7c).



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Figure 7. OER polarization curves obtained by LSV (KOH 0.1 M solution, scan rate = 5 mV s-1, rotation speed = 1600 rpm) for (a) undoped samples, Co@C, Co/Ni@C and Ni@C; for (b) dualdoped samples, N,S-Co@C, N,S-Co/Ni@C and N,S-Ni@C; and (c) their corresponding Tafel plots.

The corresponding Tafel slopes, along other relevant parameters are collected in Table 5, showing up the remarkable difference between the electrocatalytic behavior exhibited by the most active carbons, N,S-Co@C and Co/Ni@C, in relation to the dualdoped and bimetallic one, N,S-Co/Ni@C. On one hand, N,S-Co@C and Co/Ni@C 26 ACS Paragon Plus Environment

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present small overpotentials (η10 resulted to be 0.41 and 0.44 V, respectively) and on the other hand, N,S-Co/Ni@C shows a larger one, 0.51 V. This fact is also evidenced by comparing TS values: 101 and 93 mV dec-1 for the two first electrocatalysts, and 148 mV dec-1 for the last one. Thus, no positive aggregate effect of the above described synergies (bimetallic Co/Ni combination and dual-doping) is observed. A plausible explanation for this fact is that N,S-Co/Ni@C involves CoxNi1-xS sulfide (see Table 3 and S3, SI) instead of the more ‘active’ CoS - as in the case of N,S-Co@C -, and at the same time does not show ‘active’ core/shell nanostructures (see Fig. 5d) - as the bimetallic Co/Ni@C sample -. The electrocatalytic parameters for the standard commercial OER references, namely, ruthenium(IV) and iridium(IV) oxides (RuO2 and IrO2) reported in the literature20 and obtained under similar experimental conditions are: for RuO2, E10 = 1.53 V vs. RHE, η10 = 0.30 V, and TS = 65 mV dec-1, and for IrO2, 1.59 V, 0.36 V and 82 mV dec-1, respectively. Therefore, the electrocatalytic activity exhibited by N,S-Co@C and Co/Ni@C (see Table 5) are fairly close to those shown by the commercial materials, especially in the case of IrO2. Also, the results obtained in this work for N,SCo@C and Co/Ni@C are comparable and in some cases even better than those reported in literature for similar materials (see Table 6).



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Table 5. OER activity parameters for Co@C, Co/Ni@C, Ni@C and their corresponding dualdoped analogous samples. Sample Co@C Co/Ni@C Ni@C N,S-Co@C N,S-Co/Ni@C N,S-Ni@C

E10 (V vs.

2

RHE)1 1.87 1.67 2.035 1.64 1.74 2.015

OER parameter η10 (V)2 j1.75 (mA cm-2)3 0.64 4.9 0.44 30.8 0.80 3.4 0.41 43.8 0.51 10.8 0.78 3.4

TS (mV dec-1)4 167 93 208 101 148 238

1 Required potential to reach j = 10 mA cm-2 Overpotential for achieving j = 10 mA cm-2 (respect to E°O2/H2O = 1.23 V vs. RHE) 3 Current density at potential (E) of 1.75 V vs. RHE 4 Tafel slope (calculated by linear fitting of Tafel plot data) 5 Values calculated by extrapolation.

Table 6. Comparison of the OER parameters exhibited by the most active samples in this work and diverse state of the art noble metal-free electrocatalysts recently reported. Electrocatalyst N,S-Co@C Co/Ni@C Ni-hydr(oxy)oxide* NGO/Ni7S6 Ni3S2/NF* Co9S8@SNCC 50-CoxNi1−x@CoyNi1−yO@C Co@NPC-900 Co-PC BNPC1100 MCF/N-rGO* NiCo-POM/NF* MNF-Co(OH)2@RGONF*

Loading (mg·cm-2) 0.39 0.39 0.21 37 1.13 0.85 0.41 0.40 0.10 0.32

Electrolyte

η10 (V)

0.1 M KOH 0.1 M KOH 0.1 M KOH 0.1 M KOH 0.1 M KOH 0.1 M KOH 0.1 M KOH 0.1 M KOH 0.1 M KOH 0.1 M KOH 0.1 M KOH 0.1 M KOH 1.0 M KOH

0.41 0.44 0.39 0.38 0.19 0.33 ≈0.55 0.37 0.38 ≈0.60 ≈0.47 0.36 ≈0.41

Tafel slope (mV·dec-1) 101 93 44 45 159 ≈80 126 96.9 201 126 63

Ref. this work this work 43 26 44 20 29 19 22 45 46 47 48

*Electrocatalytic material non-produced via MOF carbonization approach. Abbreviations: NGO = nitrogen-doped graphene oxide, NF = nickel foam, SNCC = S,N-doped carbon cuboid, NPC = N-doped porous carbon, PC: porous carbon, BNPC = B,N-doped porous carbon, MCF = Mn/Co/Fe, N-rGO = N-doped reduced graphene oxide, POM = polyoxometalate, MNF = magnetic nanoflake, RGONF = reduced graphene oxide nanoflakes

The stability of the most promising samples as OER electrocatalysts, N,S-Co@C and Co/Ni@C, has been studied by the continuous acquisition of multiple LSV polarization 28 ACS Paragon Plus Environment

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curves (Figure S11, SI). In both cases the electrocatalytic activity is decreased ≈21 % in terms of achieved currents after 500 potential sweeps, resulting in a non-significant increase of overpotentials (η10) from 0.41 to 0.43 V for N,S-Co@C, and from 0.44 to 0.45 V for Co/Ni@C. Likely, durability is favored by a stabilization effect of carbon matrixes on the embedded metal-containing nanoparticles.49,

50

The very similar

stability degrees exhibited by these two carbon samples indicates that neither metal composition nor type of doping are decisive factors regarding their durability.

3.3. ELECTROCATALYTIC PERFORMANCE FOR THE ORR Figure 8 shows ORR polarization curves for the undoped and dual-doped carbons. The corresponding CV plots are included in Figure S12. Regarding the three undoped samples, Co@C, Co/Ni@C and Ni@C (Fig. 8a), all of them exhibit low catalytic activities, with Ni@C presenting the best performance. For the corresponding dualdoped analogous samples (Fig. 8b), a non-negligible improvement for the three metal compositions was observed. The best result was achieved for the N,S-Co/Ni@C.



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Figure 8. ORR LSV plots (KOH 0.1 mol dm-3 solution, scan rate = 5 mV s-1, rotation speed = 1600 rpm) for (a) Co@C, Co/Ni@C and Ni@C, and (b) their dual-doped analogous carbons. (c) Variation of the number of electron transferred per O2 molecule with potential for all samples.

Table 7 presents the main ORR electrocatalytic parameters. As it can be observed, doping leads to a general improvement of ORR activity, regardless of the metal composition, shifting onset potentials to more positive values and increasing current densities. The most visible enhancement is observed in the case of bimetallic Co/Ni carbons, shifting Eonset from 0.67 V vs. RHE (for Co/Ni@C) to 0.92 V vs. RHE (for N,SCo/Ni@C), and at the same time increasing the diffusion-limiting current density in approximately 170 % (from 1.22 to 2.11 mA cm-2). In order to evaluate the efficiency of the oxygen reduction process, the number of electrons transferred per O2 mol was determined by the K-L plot slopes (Figure S13). If nO2 = 4, oxygen reduction pathway only comprises a unique step, while if nO2 =2, a two-step mechanism involving production of hydrogen peroxide as intermediate was followed. Regarding the nO2 values, dual-doping does not show a significant effect on the reduction pathway, being the values for the doped samples similar to those obtained for their undoped analogues. 30 ACS Paragon Plus Environment

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Table 7. ORR activity parameters for Co@C, Co/Ni@C, Ni@C, N,S-Co@C, N,S-Co/Ni@C and N,SNi@C. Sample Co@C Co/Ni@C Ni@C N,S-Co@C N,S-Co/Ni@C N,S-Ni@C

Eonset (V vs.

RHE)1 0.64 0.67 0.75 0.76 0.92 0.80

ORR parameter J0.27 (mA cm-2)2 -0.94 -1.22 -1.44 -1.53 -2.11 -2.00

n (0.27 – 0.55 V)3 2.73 2.18 3.05 2.69 2.42

1 Required potential to reach j = 0.1 mA cm-2 Current density achieved at the maximum potential applied (E): 0.27 V vs. RHE 3 Average transfer electron number (calculated from Koutecky-Levich equation) between 0.27 and 0.55 V vs. RHE. 2

Finally in order to get insight on the electrocatalytic behavior exhibited by the MOF-74 derived nanocarbons, their apparent electrochemically active surface areas (ECSA) expressed as double layer capacitances (Cdl) are compared in Figure 9 (see the corresponding double-layer charging CV plots in Figure S14, SI). In relation to the undoped carbons, an important effect of metal composition on Cdl is observed, while the apparent electrochemical area resulted to be 0.007 mF·cm-2 for Co/Ni@C, the two monometallic samples exhibits much larger values: 0.509 and 0.570 mF·cm-2, for Co@C and Ni@C, respectively. This result shows up again that the presence of CoxNi1−x/CoyNi1−yO core/shell nanoparticles in Co/Ni@C makes this sample intrinsically different to the others. Then, regarding dual-doped carbons, the three samples present similar Cdl values, suggesting that the formation of the corresponding metal sulfides and the doping of carbonaceous matrixes with N and S results in similar distributions of the active sites into the carbon structures.



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Figure 9. Current density versus scan rates plots for (a) the undoped carbons and (b) their corresponding dual-doped analogues. Cdl values are included following the sample names.

The comparison of the behaviors towards OER and ORR exhibited by the undoped and dual-doped carbons, along with ECSA data, provides the following information: 

In the absence of doping, metal composition of the samples presents a highly pronounced influence on OER catalytic activity, with the Co/Ni@C sample showing the best performance. This fact is attributed to the catalytic behavior of the carbon-coated CoxNi1−x/CoyNi1−yO core/shell nanoparticles with high activity towards the oxygen evolution reaction. In contrast, metal composition has a limited effect on ORR catalytic activity, exhibiting a similar behavior with no dependency on metal composition (Co, Ni or bimetallic Co/Ni combination).



Effect of dual-doping (with N and S atoms) is also very different towards OER and ORR. In case of the oxygen reduction doping always results in a limited but significant enhancement of activity, while regarding the OER the situation drastically changes: only N,S-Co@C shows a remarkable improvement of OER


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catalytic activity in relation with Co@C, pointing to the CoS – N,S-doped C matrix combination as a promising catalytic system. In the other two cases: (i) Dualdoping combined with bimetal Co/Ni composition generates an intense drop of OER activity, probably due the formation of metal(II) sulfides avoids the generation of ‘active’ core/shell nanostructures, and (ii) doping combined with Ni results in the maintenance of catalytic behavior, allowing us to discard the NiS N,S-doped C matrix combination as an interesting catalytic system in MOF-74 derived electrocatalysts. 

The fact that the two most active samples towards OER, Co/Ni@C and N,S-Co@C, do not present large apparent electrochemically surface areas (especially in the case of Co/Ni@C), points to that likely the synergetic effects shown by both nanocarbons are not based on the generation of numerous active sites, but on a significant enhancement of the charge transfer process between the metalcontaining NPs and the carbon matrixes.



All these data indicates that the OER catalytic behavior of this type of carbons is predominantly governed by the nature and structural arrangement adopted by the metal species generated during the heating carbonization treatments. However, in an opposite way, the ORR electrocatalysis seems to be more influenced by the carbonaceous matrix features than by metal composition.

4. CONCLUSIONS Production of active noble metal-free nanostructured electrocatalysts based on the pyrolysis of RT-synthesized MOF-74 templates has been successfully explored. The developed simple 3-step approach (MOF RT-preparation, dopant incorporation and 33 ACS Paragon Plus Environment


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simultaneous doping/carbonization) has allowed us to generate a variety of doped and undoped carbons containing Co, Ni or Co/Ni metal combinations. This allowed a systematic study of the ‘aggregate’ effect of the doping and metal composition on the OER electrocatalytic behavior of these nanocomposites. Two samples have reached remarkable OER catalytic activities by two different paths: i) the dual-doped Cocontaining carbon, N,S-Co@C (synthesized from Co-MOF-74 precursor and using thiourea as doping agent), whose OER activity evidenced a clear synergetic effect between the N,S-doped carbon matrix and the in-situ formed embedded CoScontaining nanoparticles; ii) the undoped bimetallic carbon, Co/Ni@C (obtained from Co/Ni-MOF-74 direct carbonization), exhibited high activity, due to the formation of highly-active C-embedded metal/oxide core/shell nanoparticles. The preparation of N,S-Co/Ni@C, sample that gather the two beneficial characteristics, i. e., bimetallic Co/Ni combination and dual-doping, demonstrated that the sum of these specific synergetic effects does not result in an improvement of OER activity; evidencing that not always synergies are complementary. Even so, N,S-Co@C and Co/Ni@C showed low overpotentials (0.41 and 0.44 V, respectively) and low Tafel slopes (101 and 93 mV·dec-1, respectively). These electrocatalytic parameters are fairly close to those exhibited by the expensive IrO2 commercial reference: η10 = 0.36 V and TS = 82 mV·dec1.

Unfortunately, the activity towards the ORR electrocatalysis shown by the prepared

carbon-based materials is low, but the comparison of ORR and OER results allow us to corroborate the higher influence of the metal nanoparticles on oxygen evolution process. Consequently, the present work shows up the promising application of RTprepared MOF templates as versatile precursors of noble metal-free nanocarbonbased OER electrocatalysts, emphasizing the necessity of a systematic evaluation of 34 ACS Paragon Plus Environment

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the compositional/structural features effects on the catalytic performance of this type of novel materials. Looking at the near future, continuation of research on this field will be required in order to get a more fine-control of the carbonization and doping processes leading to enhance and efficiently tune the electrocatalytic activity exhibited by these nanocarbons.

ACKNOWLEDGEMENTS The work was funded by Fundação para a Ciência e a Tecnologia de Portugal (FCT)/MEC under FEDER under Program PT2020 - project UID/QUI/50006/2013POCI/01/0145/FEDER/007265,

UID/EQU/50020/2013-POCI-01-0145-FEDER-006984

and project “UniRCell”, with the reference POCI-01-0145-FEDER-016422. Víctor K. Abdelkader Fernández thanks UniRCell project for the post-doctoral grant.

SUPPORTING INFORMATION The Supporting Information is available free of charge on the ACS Publications website. Detailed information for the physicochemical characterization and ECSA tests are provided. Tables with surface composition for MOF-74 precursors, binding energy values for the components in XPS high resolution spectra, and relative abundances of the different types of N and S (from N1s and S2p deconvolutions, respectively) for the doped nanocarbons. Figures include deconvolutions of XPS high resolution spectra for MOF-74 and the derived nanocarbons, JCPDS cards and XRD identifications, IR spectra for MOF-74 samples, SEM images and EDS-maps of the elements present in the nanocarbons, TEM 35 ACS Paragon Plus Environment


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micrographs and N2 adsorption isotherms for selected materials, OER stability tests, ORR CV plots, Koutecky-Levich (K-L) plots and double-layer discharging tests for the carbons.

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GRAPHICAL ABSTRACT


Noble Metal-Free MOF-74 Derived Nanocarbons: Insights on Metal

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