This is the submitted version of the following article:

Zhang X., Luo J., Lin H.-F., Tang P., Morante J.R., Arbiol J.,
Wan K., Mao B.-W., Liu L.-M., Fransaer J.. Tailor-made
metal-nitrogen-carbon bifunctional electrocatalysts for
rechargeable Zn-air batteries via controllable MOF units.
Energy Storage Materials, (2019). 17. : 46 - .
10.1016/j.ensm.2018.11.034,

which has been published in final form at
https://dx.doi.org/10.1016/j.ensm.2018.11.034 ©
https://dx.doi.org/10.1016/j.ensm.2018.11.034. This
manuscript version is made available under the CC-BY-NC-ND
4.0 license
http://creativecommons.org/licenses/by-nc-nd/4.0/
          Tailor-made metal-nitrogen-carbon catalysis for rechargeable Zn-air batteries via controllable MOF unit design




   Tailor-made metal-nitrogen-carbon bifunctional electrocatalysts for
            rechargeable Zn-air batteries via controllable MOF units


Abstract: The majority of chemical syntheses involve the use of catalysts, which play a crucial role in the yield and
conversion rates of chemical reactions. In view of the increasing demand for chemical commodities and specialties
linked to the growth of the world’s population and the living standards, highly efficient and low-cost catalysts are
urgently required. The metal-nitrogen-carbon (M-N-C) catalysts family is one of the most promising candidates. In
this work, a series of benzene-1,3,5-tricarboxylate linker based metal organic frameworks (MOFs) were used as
self-sacrificial templates and tunable platform for designable preparation of M-N-C catalysts. Changing the pillars
between the 2D layers and the nature of the metal ions in the pristine MOFs significantly influenced the structure,
chemical composition and catalytic activity of the resulting M-N-C catalysts for the oxygen reduction reaction
(ORR). Furthermore, the influence of the MOF units on the catalyst performance, the role of the metals in the
M-N-C catalysts and the primary catalytically active sites for ORR were explored by a combination of density
functional theory (DFT), in-depth structural and chemical/elemental characterizations, and electrochemical studies.
Among the prepared catalysts, Co-BTC-bipy-700 exhibited the highest electrocatalytic activity for oxygen
reduction reaction (ORR), which showed a larger limiting current density and similar half-wave potentials with less
catalyst degradation and much higher methanol tolerance than the commercial Pt/C catalyst. Meanwhile, as a
bifunctional electrocatalyst, Co-BTC-bipy-700 catalyst was also employed for oxygen evolution reaction (OER)
and demonstrated a lower overpotential (lowered by 140 mV at a current density of 10 mA cm−2) and better
durability than IrO2. Furthermore, in terms of device performance, the Zn-air battery enabled by Co-BTC-bipy-700
catalyst reached a maximum specific energy as high as 1009.8 Wh kg−1, which is 76.5% of the theoretical value
(1320 Wh kg−1), and demonstrated higher discharge potential and lower charge potential than that based on the Pt/C
catalyst. Importantly, the presented strategy for tailor-made M-N-C catalysts by controlling the synthesis of the
pristine MOFs could offer a guide map for the future design of M-N-C catalysts family not only for electrochemical
reactions but also beyond electrochemistry.

Keywords: metal-organic frameworks; anodic electrodeposition; Metal-Nitrogen-Carbon catalysis; oxygen
reduction reaction; oxygen evolution reaction; rechargeable Zn-air batteries.

Introduction
  Without catalysts, our lifestyle would be significantly different. More than 80% of all chemicals and

pharmaceuticals are made by catalysts.1 Although tremendous efforts have been made by the chemical industry,

noble metal catalysts still dominate the market of energy-related catalysis and fine chemistry.2 It is urgent to find a

suitable alternative due to the prohibitive cost and scarcity of these noble metals. Different types of transition metal

based catalysts have been widely studied.3 Among various alternatives, a new class of metal (or metal

compound)-nitrogen-carbon (M-N-C) catalysts have emerged as one of the most promising candidates, and they are

                                                                1
            Tailor-made metal-nitrogen-carbon catalysis for rechargeable Zn-air batteries via controllable MOF unit design




used in a wide range of chemical reactions such as transesterification4, hydrogenation5-7, oxidative degradation8,

Fischer–Tropsch synthesis9,      10
                                      and especially for electrochemical reactions (e.g. hydrogen evolution reaction

(HER)11-15, oxygen evolution reaction (OER)16-20, oxygen reduction reaction (ORR)21-28 and carbon dioxide

reduction 29, 30).

     The traditional methods for preparing M-N-C catalysts can roughly be divided into two categories. The first one

is the two-step method in which N-doped carbon materials are subsequently loaded with metals. For example,

Shaabani et al. synthesized three-dimensional nitrogen-doped graphene frameworks by hydrothermal and

freeze-drying process, and then immobilized nickel nanoparticles on the prepared substrate.31 The second method is

the one-step method in which a mixture of carbon, nitrogen and metal precursors is pyrolized. Metal/N4 macrocycle

complexes together with some carbon source are often used for fabricating M-N-C catalysts.26, 32, 33 Common

inorganic salts (sulfates, acetates and chlorides) and organic compounds (including dicyandiamide, melamine and

tripyridyl) have also successfully been employed as M-N-C catalyst precursor materials.34-36 Moreover, in order to

improve the mass transport properties and increase the exposure of the active sites, templates were introduced into

the synthesis process, such as SBA-1537, 38 and SiO2 nanoparticles25, 39.

     In view of the above, metal-organic frameworks (MOFs), constructed from metal ions or clusters bridged by

organic ligands, have been considered as ideal self-sacrificial templates for M-N-C catalysts due to their highly

ordered cavities, open channels and rational composition and structure.2, 40 These unique characteristics of MOFs

allow them to serve as both template and precursor materials (metal, carbon and nitrogen). Furthermore, the highly

ordered structure and chemical composition result in a homogeneous distribution of heteroatom and metal particles,

which could provide more active sites after pyrolysis. Therefore, different kinds of MOFs have attracted attention

as     precursors    for    preparing     M-N-C       catalysts,       such   as   cobalt-4′-(4-pyridyl)-4,2′,6′,4′-terpyridine41,

cobalt-imidazolate42, cobalt-benzimidazole43, 44, ZIF-6745-47, ZIF-848, 49, MIL-10150, MIL-10051, MIL-88B-NH352,

Co-BDC53 CPM-2454 and enantiotopic chiral 3D MOFs (Co-BTC-MIDPPA L and R) 55. However, they are only the
                                                                   2
          Tailor-made metal-nitrogen-carbon catalysis for rechargeable Zn-air batteries via controllable MOF unit design




tip of the iceberg as more than 20,000 different MOFs have been created and the number is still increasing56, 57. The

large quantities of MOFs offer extensive possibilities for the M-N-C catalyst family but at the expense of endless

lab work. The previous studies often suffer from the unclear influence of different metal nodes and linkers on the

resulted catalysts. Similar studies for conventional method (mix metal salts with carbon resource, which could be

easily adjusted) have been well discussed recently. However, it is hard to achieve this goal for MOF-derived

materials due to the complexity of MOF structures. The separated studies and unclear influence of MOF units

hampered scientific and industrial progress of MOF-derived catalysts. Therefore, for the sake of minimizing

unnecessary lab work and to avoid both the consumption of time and resources, the critical problem is to gain

rational and comprehensive insight into the influence of MOF units (i.e. organic ligands, metal units and

interactions of these two parts) on key parameters of catalyst performance via experiments and computational

chemistry. Unfortunately, the related research in this aspect is still very scarce. Due to the complexity of MOF

structures, the role of metal and linkers in M-N-C structures based on MOFs is much more difficult to study than

that of M-N-C structures prepared via common or conventional methods which mix precursors of metals, carbon,

and nitrogen. In order to further improve the performance of MOF-derived materials, it is urgent to find a suitable

platform enabled by rational methodologies.

  In this work, rechargeable metal-air batteries, which are one of the most promising energy storage devices for

practical domestic/industrial applications because of their high theoretical energy density, environmental benignity,

safety and low cost,58-62 were chosen as the targeted application. For rechargeable metal-air batteries, OER and

ORR are involved in the charge-discharge process. Therefore, designing an effective bifunctional catalyst is

essential for rechargeable metal-air batteries. According to previous studies,16, 19, 40, 63-66 the following concerns are

important for ORR and OER of M-N-C catalysts: (1) large effective surface area for catalytic active sites; (2) high

degree of graphitization and conductivity; (3) homogeneous distribution of heteroatoms and metal particles; (4)

high Nitrogen content and suitable forms of N-C structure; (5) active metal species sites; (6) intimate connection
                                                                3
          Tailor-made metal-nitrogen-carbon catalysis for rechargeable Zn-air batteries via controllable MOF unit design




between carbon and metal. Bearing these considerations in mind, a series of benzene-1,3,5-tricarboxylate (BTC)

linker based MOFs, which work as a tunable platform due to the well-defined two-dimensional (2D) plane layers,

have been successfully prepared by electrodeposition. The characteristics of the MOF units are changed via

tailoring the pillars between the 2D layers and the central metal ions. After pyrolysis, significant differences in

catalytic performance can be seen with different pristine MOFs. A combination of density functional theory (DFT)

computation, in-depth structural and chemical/elemental characterizations, and electrochemical studies were used

to explore (i) the influences of MOFs unit changes on the catalyst performance, (ii) the role of metal in M-N-C

catalysts, and (iii) the primary catalytically active site of M-N-C structure for ORR. Among the prepared catalysts,

Co-BTC-bipy-700 exhibited excellent electrocatalytic activity for both ORR and OER. For the rechargeable Zn–air

battery test, this catalyst exhibited higher discharge potential and lower charge potential than Pt/C catalyst.

Moreover, the specific energy of Zn-air battery based on Co-BTC-bipy-700 catalyst reached 1041 Wh kg−1 at 3 mA

cm−2, which is 80% of the theoretical value (1320 Wh kg−1). These results show that Co-BTC-bipy-700 catalysts

are promising catalyst materials for rechargeable Zn-air batteries.


Results and discussion
Tuned properties of M-N-C catalysts via MOFs units design (including thermal stability, carbon structure, N

content, distribution, structure and electrochemical double-layer capacitances)

   A series of benzene-1,3,5-tricarboxylate (BTC) based MOFs, which act as a tunable platform due to the

well-defined two-dimensional (2D) layers, have been successfully prepared by anodic electrodeposition (Figure 1).

Ni(HBTC)(DMF)2 (abbreviated as Ni-BTC-DMF herein) was constructed by offset stacking of 2D layers with

honeycomb pores. In each layer, Ni (II) is bonded to three BTC linkers, with one bidentate carboxylic group and

two monodentate carboxylic groups. The free carbonyls in the monodentate carboxylic groups are further bonded to

a proton through hydrogen boding, forming an overall neutral framework. The axial position of Ni (II) was

occupied by DMF molecules. The sheets are pillared by 4,4’-bipyridine along the c axis to form 3D porous
                                                                4
          Tailor-made metal-nitrogen-carbon catalysis for rechargeable Zn-air batteries via controllable MOF unit design




framework structure (Ni(HBTC)(4,4'-bipy)). Ni(HBTC)(4,4'-bipy) possesses two types of channels: honeycomb

channels along the [001] direction and rectangular channels along the [110] direction. The Ni(HBTC)(4,4'-bipy)

(abbreviated as Ni-BTC-bipy herein) was further tailored by replacement of Ni (II) with Co (II) to obtain

Co(HBTC)(4,4'-bipy) (abbreviated as Co-BTC-bipy herein). As evidenced by the SEM images (Figure 1), all the

MOFs exhibited similar hexagonal morphology. The XRD patterns of different MOFs (Figure S1−S3) are in

agreement with the theoretical XRD patterns. These results demonstrate the successful electrodeposition of the

desired MOFs. In addition, the XRD patterns of Ni-BTC-bipy and Co-BTC-bipy are very similar, indicating that

the crystal structures of these two kinds of MOFs are almost the same. As the coordinating ability of Ni (II) and Co

(II) are different, Co-BTC-bipy was prepared at a lower temperature (30 °C). This phenomenon is consistent with

the previous report by Li et al. using solvothermal methods.67 Compared with solvothermal methods, the visible

and adjustable electrodeposition process is easier to control the reaction process and avoids the influence of anions

from salts.68, 69 SEM images of the as-prepared electrodes by anodic electrodeposition of Co-BTC-bipy at 60 °C

and 120 °C are presented in Figure S4. Additionally, the feasibility of large scale production is important for

practical industrial applications. Herein, when two electrodes connected in series, there is no difference for

electrodeposition of Co-BTC-bipy (Figure S5), which shows that this method could be used for preparing

multi-electrodes simultaneously. The same cobalt plate was used 10 times as the substrate for anodic

electrodeposition of Co-BTC-bipy. After each time, the cobalt plate was dipped into dilute HCl solution and

polished with sand paper. After 10 times, the Co-BTC-bipy covered cobalt plate looks the same as that after the first

time (the inset of Figure S5). This proves that our procedure has the potential for large scale production.

  In order to get M-N-C catalysts, the as-prepared MOFs were pyrolyzed under N2 atmosphere at different

temperatures. Based on the thermogravimetric analysis (TGA) results (Figure S6), it is concluded that the onset

temperature of carbonization of the framework for Ni-BTC-DMF, Ni-BTC-bipy and Co-BTC-bipy are 395 °C,

379 °C and 312 °C, respectively. Because of the decreased coordination ability of Co (II), the decomposition
                                                                5
          Tailor-made metal-nitrogen-carbon catalysis for rechargeable Zn-air batteries via controllable MOF unit design




temperature of Co-BTC-bipy is lower than Ni-BTC-bipy. Before decomposition of the framework, there is one

weight loss event each for Ni-BTC-bipy and Co-BTC-bipy, which corresponds to the release of adsorbed molecules

(around 15% and 10% weight loss, respectively). In addition, when the linker is changed, there is no obvious

decomposition temperature changes are observed between Ni-BTC-DMF and Ni-BTC-bipy. However, multi-steps

weight loss events can be observed for Ni-BTC-DMF, with around 45% weight loss, which is probably caused by

the loss of DMF molecules in the structure of Ni-BTC-DMF.70 Based on the composition of Ni-BTC-DMF, DMF

was considered as N source for M-N-C catalysts. Therefore, the loss of DMF molecules from the structure of

Ni-BTC-DMF could lead to the low N content for Ni-BTC-DMF derived from Ni-BTC-DMF.

  The composition and structure of M-N-C catalysts derived from MOFs were investigated by Raman spectroscopy,

CHN analysis, XPS and TEM. Raman spectroscopy was used to analyze the degree of structural order with respect

to a perfect graphitic carbon structure. Generally speaking, the D-band at ~1350 cm−1 and a G-band at ~1585 cm−1

are reflection of ideal and disordered graphitic lattices, respectively. The intensity ratio of D and G bands (ID/IG)

can be regarded as a measure of the carbon crystalline order of carbon materials.71 A reduction of ID/IG ratio of

carbon materials means an increase of graphitic content, which would benefit the charge transfer for

electrochemical catalysts.72 Based on Raman spectra (Figure S6-S9), the ID/IG ratios of different catalysts are

obtained (Figure 2a). The ID/IG values are 1.13, 0.88, 1.47, 0.92, 0.84, and 0.72 for Ni-BTC-DMF-700,

Ni-BTC-DMF-900, Ni-BTC-bipy-700, Ni-BTC-bipy-900, Co-BTC-bipy-700 and Co-BTC-bipy-900 catalysts,

respectively. With increase of the pyrolysis temperature, the ID/IG ratio decreases a lot for all the MOFs derived

catalysts. Compared with Ni-BTC-DMF based catalysts, Ni-BTC-bipy show higher ID/IG ratio, which indicates that

the 4,4’-bipyridine linker can result in more heteroatoms in the carbon structure after pyrolysis. This result is in

agreement with TGA test. Among these pristine MOFs, Co-BTC-bipy based catalyst shows the lowest ID/IG ratio.

The difference of ID/IG ratio between Co-BTC-bipy and Ni-BTC-bipy based catalysts could be caused by the

different catalytic ability of different metals for carbon graphitization. The detailed mechanism of metal
                                                                6
          Tailor-made metal-nitrogen-carbon catalysis for rechargeable Zn-air batteries via controllable MOF unit design




nanoparticles in catalytic graphitization is still not clear, which are probably related to carbon solubility and carbon

diffusion coefficients in the different metal.73 It is worth mentioning that Co-BTC-bipy-700 (ID/IG = 0.84) and

Co-BTC-bipy-900 (ID/IG = 0.72) show even lower ID/IG ratio than multi-walled carbon nanotubes (MCNTs), whose

ID/IG ratio is 0.92 (Figure S10).

  The bulk N content (at.%) of catalysts were explored by CHN analysis (Figure 2b). With increase of the

pyrolysis temperature, the N content decreases for all the samples. Comparing with Ni-BTC-DMF and

Ni-BTC-bipy, Ni-BTC-bipy derived catalysts (5.9 at.% for 700 °C and 2.8 at.% for 900 °C) show much higher N

content than Ni-BTC-DMF (1.9 at.% for 700 °C and 1.3 at.% for 900 °C), which is related to the loss of DMF

molecules in the structure of Ni-BTC-DMF during pyrolysis, which can be proved by TGA results (Figure S6).

These results reveal that the MOF ligand has a big influence on the N content of the as-prepared catalysts. When

two MOFs which have similar N content in the structural unit are pyrolized, the resulting M-N-C catalyst show

higher N content when the metal atoms are directly coordinated to the N atoms. In addition, the type of metal ion

has some effect on the N content as well, which is probably caused by the difference in thermal stability and the

ability to catalyze carbon growth between Ni-BTC-bipy and Co-BTC-bipy. A similar influence of metal particles on

nitrogen content of N-C materials was observed by Kudashov et al. as well.74 The N contents of Co-BTC-bipy

based catalysts are 5.0 at.% and 2.2 at.% at the pyrolysis temperature of 700 °C and 900 °C, respectively.

Furthermore, the metal content was calculated from the CHN analysis and metal oxide residues (based on phase

diagrams of Ni-O2 and Co-O2 binary systems shown in Figure S11), and the results are displayed in Table S2.

  The electrochemical double-layer capacitance, which is related to the electrochemical surface area of different

catalysts (mass loading: ~0.25 mg cm−2) were measured by cyclic voltammetry (CV) on a rotating disk electrode

(RDE) in a non-Faradaic region (Figure S12−S17). As these catalysts are composite materials, it is difficult to

accurately quantify their electrochemical surface areas. However, for qualitative analysis, the electrochemical

double-layer capacitances are still very useful for comparing similar materials with the same mass loading. At a
                                                                7
          Tailor-made metal-nitrogen-carbon catalysis for rechargeable Zn-air batteries via controllable MOF unit design




mass loading of 0.25 mg cm−2 on the RDE, the electrochemical double-layer capacitance of Ni-BTC-DMF-700,

Ni-BTC-DMF-900, Ni-BTC-bipy-700, Ni-BTC-bipy-900, Co-BTC-bipy-700 and Co-BTC-bipy-900 are 2.06, 1.44,

0.81, 0.57, 2.87, and 1.43 mF cm−2, respectively. Comparing the influence of different pillar linkers, the

Ni-BTC-DMF derived materials exhibit higher capacitance than Ni-BTC-bipy based materials. It is speculated that

the intermolecular short contact interactions between coordinated pillar (organic solvent) and coordinated

carboxylate group in Ni-BTC-DMF is weaker than that in Ni-BTC-bipy, leading to obviously release of DMF

pillars. Therefore, more pores and space will be produced during pyrolysis. Concerning different metals, the

detailed function of metal particles during pyrolysis is still unclear. It is well known that different metals have

different catalytic ability for carbon growth, which could eventually lead to different carbon net structures.

  Furthermore, the near-surface elemental compositions (atomic percentage) based on full scan survey XPS spectra

for different catalysts are shown in Table S3. There is a huge difference between the full scan survey XPS spectra

and CHN analysis for metal contents, because the metal nanoparticles are covered by a carbon layer while for XPS

tests, the information depth is less than 10 nm.19, 75, 76 Furthermore, the high-resolution XPS spectra of the Ni 2p

(Figure S18a) and Co 2p (Figure S18b) were used for determining chemical states of transition metals. Because of

low metal content on the surface of Ni-BTC-DMF-900 and Co-BTC-bipy-700, it is hard to get sharp peaks of Ni 2p

or Co 2p. For the other Ni based catalysts, the XPS spectra of the Ni 2p (Figure S18a) all exhibit two main peaks at

853.3 ±0.3 eV and 870.5 ±0.3 eV without any obvious satellites, corresponding to the metallic dominant structure.

Two weak peaks are assigned to NiNx (855.3 ± 0.3 eV) and NiO (856.8 ± 0.3 eV, oxidized in the air),

respectively.77-80 The similar results for Co based catalysts with different locations of peaks (for Co0, CoNx and

CoO)16, 81-83 are shown in (Figure S18b), which indicates Co0 as the dominant chemical states of Co. These results

are in good agreement with HRTEM observations. Elemental compositions (atomic percentage) based on XPS

spectra (full scan survey) show lower N content but the similar trend that the total N content decreases with

increasing temperature for all catalysts. To further comprehend the chemical states of N-heteroatoms, the
                                                                8
          Tailor-made metal-nitrogen-carbon catalysis for rechargeable Zn-air batteries via controllable MOF unit design




high-resolution XPS spectra of the N 1s (Figure 3) are used to investigate N-heteroatom species. All the samples

show obvious peaks of N 1s. The binding energies of about 398.5 ±0.2 eV, 400.5 ±0.3 eV and 401.3 ±0.3 eV were

assigned to pyridinic N, pyrrolic N and graphitic N, respectively. In addition, transition metals could be coordinated

with different number of pyrrolic or pyridinic N atoms in the conjugate carbon plane, namely M-Nx site. Therefore,

the peak intensity for pyridinic N and pyrrolic N should also include a contribution from M-N bonds.25, 40, 84 Upon

deconvolution of the N 1s, the relative contents (based on N content) of different N-heteroatom species are

presented in Figure 3. The graphitic N is most stable at high temperature. The relative content (based on N content)

of pyridinic N was dramatically decreased with increasing temperature. Interestingly, the relative content of

pyrrolic N is almost the same for all samples, even though pyrrolic N is not stable at high temperatures (above

700 °C).85, 86 This could be caused by the change of the formation energy of pyrrolic N in the M-N-C structure,

which in good agreement with the EELS mapping and DFT results. It is discussed in detail in the EELS mapping

analysis. Combining high-resolution XPS spectra with CHN analysis, the contents of different N-heteroatom

species are shown in Table S4. The different linkers have significant influence on N content, which is confirmed by

CHN analysis. Among these catalysts, Ni-BTC-bipy-700 shows the highest N content (5.9 at.% based on total

amount of C, H and N), including 2.2 at.% pyridinic N, 1.4 at.% pyrrolic N and 2.3 at.% graphitic N. The different

N-heteroatom species can have a significant influence on the catalytic activity. The details of different

N-heteroatom species in potential microstructures for each elementary steps of ORR are discussed in the section of

“DFT calculation of active sites in M-N-C structure”.

  The general view and magnified details of the ADF-STEM images are displayed in Figure 4 and Figure

S19−S22. For all the samples, the metal-containing nanoparticles are covered by a carbon layer or embedded in the

carbon layer. The diameters of these particles are in the range of 10 to 100 nm. Moreover, the relative compositions

of Co, N, O and C elements of these samples have been mapped by means of EELS analysis using the

Hartree-Slater model for signal quantification and power law for background removal.87 In order to extract the
                                                                9
          Tailor-made metal-nitrogen-carbon catalysis for rechargeable Zn-air batteries via controllable MOF unit design




maps and compute the relative composition, we have analyzed the corresponding K-edge for C (284 eV), N (401

eV) and O (532 eV) and the L2,3-edge in the case of Co (779 eV). The relative composition maps of C and N were

explored by EELS analysis as well, except for Ni-BTC-DMF based catalysts due to their low N content. The N

content based on total amount of C and N by EELS analysis are 7.70 ± 2.13 at.%, 3.08 ± 0.82 at.%, 6.1 ± 2.9 at.%

and 3.7 ± 2.3 at.% for Ni-BTC-bipy-700, Ni-BTC-bipy-900, Co-BTC-bipy-700 and Co-BTC-bipy-900 catalysts,

respectively. With increasing temperature, the N content decreases. These results are in good agreement with XPS

and CHN analyses. EELS chemical composition maps clearly show a N content in the carbon layer, which

originates from the C-N structure. In addition, it is worth mentioning that metal particles covered by the carbon

layer are often adjacent or close to nitrogen atoms (the relative composition of N in the metal nanoparticles region

is much higher than that on the region without metal particles), suggesting possible M-N bonding,88 which is in

agreement with the XPS analysis. Therefore, the high N atomic content around the metal nanoparticles should

originate from the M-N-C structure. Furthermore, there are some rich C-N cavities produced by the metal

nanoparticles etched from the C-N shell support (Figure 4). The presence of rich C-N cavities together with the

high relative composition of N near the metal particles indicates that the metal nanoparticles could stabilize N in the

graphite framework. This phenomenon might be caused by the change of formation energy of C-N structure after

formation of the M-N-C structure. Even the influences of metal source on the N content in conventional method

have been reported by previous researchers, a reasonable theoretical explanation is still unseen. Formation energies

(in eV) of potential configurations of M-N-C microstructures presented in this work based on DFT were shown in

Table S5, the formation energies of Ngraphite-G (0.62 eV) structure are smaller than both of Ni-Ngraphite-C (4.20 eV)

and Co-Ngraphite-C (5.34 eV).89 However, the formation energies of Npyrid-G (3.32 eV) and Npyrro-G (10.21 eV) are

dramatically reduced after forming the M-N-C structure. The formation energies of Co-Npyrid3-C, Co-Npyrid4-C,

Co-Npyrro2-C and Co-Npyrro3-C are 2.61 eV, 1.31 eV, 7.70 eV and 6.59 eV, respectively. Therefore, the N would be

enriched around metal particles and more for pyridinic N and pyrrolic N, which is in good agreement with XPS
                                                               10
          Tailor-made metal-nitrogen-carbon catalysis for rechargeable Zn-air batteries via controllable MOF unit design




results as well. In addition, all the Ni-based M-N-C structures show smaller formation energies than corresponding

Co-based materials. This might be the reason why Ni-based materials show higher N content than Co-based

materials even with the same structure of the pristine MOFs and using the same pyrolysis condition. Meanwhile,

based on the STEM-EELS mapping, the O signal seems to be always accompanied by C and N signals rather than

metal signal, meaning that the metal nanoparticles are mainly present in their metallic form and M-Nx compounds.

The HRTEM micrographs of Co-BTC-bipy-700 and Co-BTC-bipy-900 (Figure 5) show the morphology of the

nanoparticles, which are surrounded by a graphitic C-N shell. The HRTEM image and its corresponding colored

power spectrum (FFT) obtained on the red square region indicate that the nanoparticle crystallized in the hexagonal

Co crystal phase, [P63/MMC]-space group 194, with lattice parameters of a = b = 0.24968 nm, c = 0.40308 nm and

                                                  ̅ ] direction. Upon increasing pyrolysis temperature, the crystal
α = β = 90°, γ = 120°, as visualized from the [0001

phase of the Co nanoparticles does not change.

DFT calculation of active sites in M-N-C structure

  Although Co-N-C or Ni-N-C for ORR has been explored by DFT in previous studies, only few potential

configurations of M-N-C microstructures have been considered.90, 91 However, different potential configurations of

M-N-C microstructures may exist within the same material for heteroatom doped carbon based catalysts.92 There

are eight different basic configurations of M-N-C microstructures in the M-N-C structure (Figure 6a). Different

N-heteroatom species and structures influence the catalytic activity of overall ORR steps differently. Therefore, it is

important to know which kind of configurations of M-N-C microstructures are primary catalytically active sites for

ORR.

  In this work, the performances of different potential microstructures as an ORR catalyst were evaluated

theoretically by computing their free energy diagram for ORR with each elementary steps (four-electron way) at a

fixed overpotential (η = 0.4 V), which is based on real conditions. The different N-heteroatom species and

structures have a significant influence on the catalytic activity of overall ORR steps (Figure 6a). Firstly, all reaction
                                                               11
          Tailor-made metal-nitrogen-carbon catalysis for rechargeable Zn-air batteries via controllable MOF unit design




steps on the Ni-Ngraphite-C surfaces are downhill except for the OH* formation, which is 1.44 eV uphill. This step is

the rate-determining step (RDS) of the overall ORR for the Ni-Ngraphite-C catalyst. When Co replaces Ni, the OH*

formation is thermodynamically favorable. However, the last step, OH* desorption, is 1.89 eV uphill. Hence, a

four-electron oxygen reduction mechanism should be thermodynamically unfavorable on both Ni-Ngraphite-C and

Co-Ngraphite-C surfaces. There are four kinds of potential configurations of M-Npyrid-C microstructures (Figure 6b).

The step from O2* to HOO* is the RDS during the overall ORR on the Ni-Npyrid-C surface. The corresponding free

energy differences for RDS of Ni-Npyrid-C structures are 0.31 eV, 0.45 eV, 0.51 eV and 0.74 eV for Ni-Npyrid1-C,

Ni-Npyrid2-C, Ni-Npyrid3-C and Ni-Npyrid4-C, respectively. These Ni-Npyrid-C structures show more favorable for ORR

than Ni-Ngraphite-C structures with free energy difference of the rate-determining-step as high as 1.44 eV for the

OH* formation. But all RDS of Ni-Npyrid-C structures are still uphill. Interestingly, for the Co-Npyrid3-C and

Co-Npyrid4-C structure, all the steps are downhill, indicating a four-electron ORR pathway could be

thermodynamically favorable on these structures. However, with the number of connected N atoms decreases,

Co-Npyrid2-C and Co-Npyrid1-C seem to be unfavorable for the OH* desorption. The last possible N-heteroatom

species is pyrrolic N. All ORR steps on the surface of Ni-Npyrro1-C (Figure 6) show downhill except for the OH*

desorption which is slightly uphill (0.03 eV). When the N content increases in the microstructures, the formation of

HOO* becomes unfavorable. Because Ni-Npyrro1-C shows the lowest free energy difference of RDS (0.03 eV)

among all potential configurations of Ni-N-C microstructures (0.31 eV for Ni-Npyrid1-C and 1.44 eV for

Ni-Ngraphite-C), Ni-Npyrro1-C might be the most promising active site for the four-electron ORR.

     For the different Co-Npyrro-C microstructures, a four-electron ORR can only happen favorably on the surface

of Co-Npyrro2-C with all ORR steps going downhill. Therefore, the active sites of Co-Npyrid3-C, Co-Npyrid4-C and

Co-Npyrro2-C could be the primary catalytically active site for ORR in Co-N-C catalysts. It is worth stressing that

Co-Npyrid4-C is not the only active site in Co-N-C catalyst. In fact, Co-Npyrid3-C and Co-Npyrro2-C can play an

important role as well, which is proven by DFT calculations for the first time. This useful information would
                                                               12
          Tailor-made metal-nitrogen-carbon catalysis for rechargeable Zn-air batteries via controllable MOF unit design




clearly guide the further improvement of Co-N-C catalysts. Based on the DFT calculation results, it is concluded as

follows: (i) the primary catalytically active sites for ORR in Co-N-C catalysts include Co-Npyrid4-C, Co-Npyrid3-C

and Co-Npyrro2-C; (ii) the N content of pyridinic N and pyrrolic N could play an important role in the ORR for both

Co-N-C catalyst and Ni-N-C catalyst; (iii) Co-N-C catalysts could show better ORR catalytic performance than

Ni-N-C catalysts. However, in the real conditions, the situation will be much more complicated. A lot of other

factors would have influenced the ORR performance of the M-N-C catalysts. For example, there is no perfect

graphene framework for loading M-N-C structure. In the following parts, we will discuss the present M-N-C

catalysts for the ORR under practical conditions by electrochemical methods.

Enhancement of catalytic performance based on MOFs units design

     The catalytic performance of as-prepared catalysts was explored by LSV (Figure 7). Compared to the blank

electrode, all catalysts show obvious catalytic performance for ORR. Firstly, the effect of the pyrolysis temperature

is studied (Figure 7a−7c). For the Ni-BTC-DMF based catalyst, Ni-BTC-DMF-900 displays better catalytic

performance than Ni-BTC-DMF-700, even though Ni-BTC-DMF-700 shows higher N content and electrochemical

double-layer capacitance (indicating larger electrochemical surface area).93 However, the N content of both

Ni-BTC-DMF-700 and Ni-BTC-DMF-900 are very low (especially pyridinic N and pyrrolic N, whose total content

is less than 0.5 at.%). Among all M-N-C structures, the Ni-Npyrro-C and Ni-Npyrid-C structures are related to active

sites for the ORR based on the DFT results, the structure of carbon might dominate the catalytic performance of

Ni-BTC-DMF based catalysts. Because of this, we call this C-dominant behavior. After changing the pillars to

4,4’-bipyridine, the N content of MOFs-derived catalysts (i.e. M-BTC-bipy-700 and M-BTC-bipy-900 ) increased a

lot. For Co-BTC-bipy based materials, both of these two catalysts show very low ID/IG ratios, meaning high

graphitization of these materials. Therefore, the real structure might be close to the DFT configurations.

Co-BTC-bipy-700 shows better performance than Co-BTC-bipy-900 because Co-BTC-bipy-700 has a higher N

content (especially, pyridinic N and pyrrolic N are almost 5 times higher) and larger electrochemical surface area
                                                               13
          Tailor-made metal-nitrogen-carbon catalysis for rechargeable Zn-air batteries via controllable MOF unit design




(i.e. higher electrochemical double-layer capacitance). These results are in good agreement with DFT and XPS

results. As in this situation, the number of effective active sites might dominate the catalytic performance, which

can be denoted as “N-dominated behavior”. The Ni-BTC-bipy-700 shows slightly higher limiting current density

than Ni-BTC-bipy-900 (Figure 7b). This is caused by the change of N content, which is similar to the

Co-BTC-bipy based materials. On the other hand, this N-dominated behavior effect is not as strong as for

Co-BTC-bipy based materials. This is possible because of the high ID/IG ratio of Ni-BTC-bipy. When the pyrolysis

temperature increases, the ID/IG ratio decreases significantly, which would benefit the ORR. A similar phenomenon

is also observed in Ni-BTC-DMF based materials. Therefore, for Ni-BTC-bipy based materials, the structure of the

carbon, the electrochemical surface area and the N content show a synergistic effect for the ORR, which can be

classified as “synergistic effect behavior”.

  Compared to Ni-BTC-DMF based catalysts, Ni-BTC-bipy based catalysts show better performance at both

pyrolysis temperatures, although Ni-BTC-DMF based catalysts show smaller ID/IG ratio and higher electrochemical

double-layer capacitance than Ni-BTC-bipy. This is caused by the large difference in the N content. With increasing

pyrolysis temperature, the differences in catalytic ability between Ni-BTC-bipy-900 and Ni-BTC-DMF-900 are

smaller than at low pyrolysis temperature. This is probably due to the decrease of the N content for Ni-BTC-bipy

based catalyst at high pyrolysis temperatures and the lower ID/IG ratio of Ni-BTC-DMF based catalyst. Therefore,

when the metal atoms are directly connected to N atoms in the pristine MOF, the resulting M-N-C material has

more favorable catalytic behavior for the ORR. Co-BTC-bipy-700 exhibits better catalytic activity for ORR than

Ni-BTC-bipy-700 (Figure 7e). This could be attributed to two reasons: (1) The metal ions of MOFs will lead to

different active sites (Co-Nx-C show better catalysts performance than Ni-Nx-C), which is proven by DFT results as

well; (2) Different metals have different ability to catalyze carbon growth, which will have an influence on the

structure of the carbon after pyrolysis and on the electrochemical surface area.

     The commercial Pt/C (10 %) catalyst was used as reference sample at same mass loading as Co-BTC-bipy-700.
                                                               14
          Tailor-made metal-nitrogen-carbon catalysis for rechargeable Zn-air batteries via controllable MOF unit design




At this mass loading of commercial Pt/C (10%), the mass loading of Pt is around 50 μg cm−2 (higher than “standard”

loading of 20 μg cm−2). Interestingly, the onset potential of Co-BTC-bipy-700 is lower than Pt/C catalyst (Figure

8a), but with a larger limiting current density and similar half-wave potentials. While long-term stability is an

important parameter for high-performance catalysts, Co-BTC-bipy-700 catalyst again outperforms the Pt/C catalyst.

After 2000 CV cycles (in the range of 0.5 V−1 V vs. RHE), the LSV curve of Co-BTC-bipy-700 catalyst shows

only minor changes whereas the limiting current density of the Pt/C catalyst decreased a lot.

     Furthermore, another advantage of Co-BTC-bipy-700 catalyst is the excellent tolerance to methanol (Figure

8b). With the addition of 1 M methanol, Pt/C catalyst shows an obvious peak of methanol oxidation and the

cathodic peak corresponding to the ORR disappears. On the contrary, there is no change for the Co-BTC-bipy-700

catalyst, revealing its potential applications in direct methanol fuel cells. Moreover, considering economic and

environmental impact, the recycling of catalysts is a big issue. Magnetic separation was recognized as a gentle and

economical way to solve this issue.94-96 The recycling of Co-BTC-bipy-700 catalyst by magnetic separation in 1 M

KOH solution proves that magnetic separation is an effective way to recycle Co-BTC-bipy-700 catalyst (Figure

S23 a−b). Even after one month, the Co-BTC-bipy-700 catalyst can be easily recycled by magnetic separation

(Figure S23 c-d), which can be attributed to the protection of the cobalt by the carbon layer.

     To know the electron transfer process of the ORR involving the as-prepared samples, LSV on RDE were

recorded at rotation speeds from 400 rpm to 2000 rpm as shown in Figure S24a−S31a. Based on Koutecky-Levich

(K−L) plots (Figure S24b−S31b), the electron transfer numbers (n) per oxygen molecule is calculated in the

potential range 0.45 V−0.6 V. Commercial MCNTs and Pt/C were explored using the same method as control

groups,. The calculated values of n at various potentials are presented in Figure 9a. Among these catalysts, MCNTs

show the smallest n values (~1.7) for ORR. The electron transfer numbers of Pt/C and Co-BTC-bipy-700 catalysts

for ORR are 3.9 and 4.0, respectively, which is typical for the 4-electron ORR. The kinetic differences of catalytic

ORR between different samples are investigated by Tafel plots (Figure 9b). Generally speaking, lower Tafel slopes
                                                               15
          Tailor-made metal-nitrogen-carbon catalysis for rechargeable Zn-air batteries via controllable MOF unit design




imply faster kinetics.97 The Tafel slopes calculated from Figure 9b are given in Table S6. All the MOF derived

catalysts show low Tafel slopes, ranged from 53 mV dec−1 to 64 mV dec−1. Obviously, the Tafel slope of

Co-BTC-bipy-700 catalyst (53 mV dec−1) is the lowest. Compared with Pt/C whose Tafel slope is 74 mV dec−1, the

Co-BTC-bipy-700 catalyst exhibit more desirable ORR kinetics.

     In order to figure out the role of the metal in Co-BTC-bipy-700 catalyst for ORR, cyanide anions (CN−) was

used as a molecular probe to explore active ORR catalytic sites on Co-BTC-bipy-700 catalyst as they readily

coordinate with the Co center.98 In addition, the control electrode of cobalt-covered RDE was prepared by

electrodeposition. The cobalt covered RDE did not show catalytic activity for ORR (Figure S32a), indicating that

metallic cobalt is not the active site for ORR, justifying that the contribution of metallic phase to catalytic

performance can be ignored. After the addition of CN−, the onset potential and the diffusion-limiting current density

of the ORR polarization curve of the Co-BTC-bipy-700 electrocatalyst decrease significantly (Figure S32b),

suggesting the blocking of the cobalt sites in Co-BTC-bipy-700 catalyst by CN− ions. This behavior strongly

suggests that cobalt in the M-N-C structure is directly involved in the ORR process. On the other hand, metals can

catalyze the formation of N-C and improve the degree of sp2-hybridized carbon and stabilized N structure in

graphene framework based on XPS, Raman, CHN analysis, CVs and TEM results. All these results suggest that

cobalt in M-N-C structure plays an important role in the ORR activity in both direct and indirect ways.

  Another key reaction for rechargeable metal-air battery is the OER,47, 99 corresponding to the charging process.

The OER catalytic activity of Co-BTC-bipy-700, Pt/C and IrO2 were explored by LSV on RDE at a rotation speed

of 1600 rpm in 0.1 M KOH. Compared to Pt/C and IrO2 catalysts, Co-BTC-bipy-700 catalyst shows lower onset

potential (1.54V vs. RHE) and lower potential at a current density of 10 mA cm−2 (1.63 V vs. RHE) as shown in

Figure 10a. Based on Tafel plots obtained from Figure 10a, the Tafel slopes of Co-BTC-bipy-700 and Pt/C and

IrO2 are 77, 98 and 298 mV dec−1, respectively, indicating that the OER was kinetically faster on the

Co-BTC-bipy-700 catalyst. To assess the OER stability of catalysts, chronoamperometric test is carried out at 1.7 V
                                                               16
          Tailor-made metal-nitrogen-carbon catalysis for rechargeable Zn-air batteries via controllable MOF unit design




vs. RHE. As shown in Figure 10c, IrO2 catalyst suffers a rapid current loss at the initial stage and the total current

loss was about 66% after 24,000 s. On the contrary, more than 95% current can be remained for Co-BTC-bipy-700

catalyst, indicating a high stability for OER. After stability tests, there is no obviously change on the LSV for

Co-BTC-bipy-700 catalyst (Figure 10d).

Home-made Zn-air battery performance

  A home-made Zn-air battery was used to study the catalyst performance under real conditions. A Zn plate

catalyst covered electrode and 6 M KOH with 0.2 M zinc acetate solution were used as the anode, cathode and

electrolyte, respectively. The open circuit voltage of Zn-air battery based on Co-BTC-bipy-700 catalysts is around

1.49 V (Figure S33). .Figure 11a presents the i−V polarization curves of Zn−air batteries based on different

catalysts. The Pt/C-IrO2 (the mass ratio of the two catalysts is 1:1) mixed catalyst was chosen for comparison.

Clearly, Co-BTC-bipy-700 shows higher polarization current density than Pt/C-IrO2 (Figure 11a), which benefits

from better ORR and OER performance of Co-BTC-bipy-700. Moreover, the corresponding power density and

current density during the discharging is shown in Figure 11b. The maximum power density of the assembled

zinc-air battery based on Co-BTC-bipy-700 is determined to be 336 mW cm−2, which is higher than that of the

zinc-air battery based on Pt/C-IrO2 (270 mW cm−2). In order to compare with previous works, the Tafel slop for

each reaction and the overvoltage between ORR and OER (𝐸OER@10 mA cm−2 − 𝐸ORR@3 mA cm−2 base on the

three-electrode system) is used as a descriptor for evaluating the bifunctional electrocatalytic activity of a catalyst

(Table S8). The Co-BTC-bipy-700 catalyst shows that the value of 𝐸OER@10 mA cm−2 − 𝐸ORR@3 mA cm−2 reaches

0.84 V, which is comparable with the highest level of bifunctional electrocatalysts reported recently (Table S8),

including novel metal based catalysts, transition metal based catalysts, doped carbon based catalysts and

metal-doped carbon based catalysts as well as other MOF-derived catalysts. Moreover, the comparison of the peak

power densities of the Zn-air battery based on the as-prepared 2D-CMO electrode in this work and those based on

other electrodes reported in the literature is shown in Figure S34. Even if Co-BTC-bipy-700 is not the best ORR
                                                               17
          Tailor-made metal-nitrogen-carbon catalysis for rechargeable Zn-air batteries via controllable MOF unit design




electrocatalyst, it surpasses most bifunctional electrocatalysts reported to date considering both ORR and OER,

leading to the high Zn-air battery performance. Compared with Pt/C-IrO2 catalyst, the Co-BTC-bipy-700 catalyst

enables the Zn-air battery a higher voltage platform during the discharge process (benefiting from the high catalytic

activity for ORR) and a lower voltage platform during the charge process (benefiting from the high catalytic

activity for OER) at all the tested current densities (Figure 11c). At a current density of 3 mA cm−2, the specific

energy density of the Zn-air battery based on the Co-BTC-bipy-700 catalyst equals 1009.8 Wh kg−1 (based on the

mass of Zn), which is around 76.5% of the theoretical value (1320 Wh kg−1). Meanwhile, the specific energy

density of the Zn-air battery based on the whole system (including anode, electrolyte, cathode, and all other

components of the device) is 4.37 Wh kg−1. Furthermore, long term charge-discharge tests were carried out with

different catalysts. As shown in Figure S35, after 45 cycles, the degradation of the discharge voltage platform for

Zn-air batteries based on Pt/C-IrO2 and Co-BTC-bipy-700 catalysts are around 8%. The decrease of catalyst

performance is probably caused by the passivation of the zinc plate side and the corrosion of the carbon structure or

the carbonate precipitation on the gas diffusion electrode side.100-103 The Zn-air battery based on Co-BTC-bipy-700

catalyst shows lower charging voltage and higher discharging voltage at all cycles than Pt/C catalyst. In addition,

the design diagram of the cell and the demonstration of the Zn-air battery (based on Co-BTC-bipy-700) with an

OCV of around 1.51 V are given in Figure S36.


Conclusions

  In summary, a series of benzene-1,3,5-tricarboxylate (BTC) linker based MOFs have been successfully prepared

via tailoring the pillars between the 2D layers and the metal ions. Subsequent pyrolysis leads to the formation of

M-N-C composites catalysts. The influence of the MOFs unit, including linker and metal ions, on the resulting

M-N-C catalyst performance for ORR were explored by a combination of DFT calculations, morphology and

composition characterization, and electrochemical studies. It is found that (i) the key parameters (such as N content,

N-heteroatom species, electrochemical surface area, and the structure of carbon) of M-N-C catalyst for ORR are
                                                               18
          Tailor-made metal-nitrogen-carbon catalysis for rechargeable Zn-air batteries via controllable MOF unit design




significantly affected by the MOF units; (ii) Based on DFT computation, the Co-Npyrid3-C, Co-Npyrid4-C and

Co-Npyrro2-C are the primary catalytically active sites for ORR in the Co-N-C structure; (iii) Metal particles in

M-N-C structure play an important role for the ORR activity in both direct and indirect ways. Among the prepared

catalysts, Co-BTC-bipy-700 exhibits high catalytic capacity for both ORR and OER, which is beneficial for

rechargeable Zn-air battery. In battery tests, this catalyst exhibited higher discharge potential and lower charge

potential than Pt/C catalyst. The specific energy density of Zn-air battery based on Co-BTC-bipy-700 catalyst was

1009.8 Wh kg−1 at 3 mA cm−2, which is around 76.5% of the theoretical energy density (1320 Wh kg−1). These

results show that the catalytic capacity of M-N-C catalyst can be easily tailored by designable MOFs unit. It is still

difficult to quantitatively control each key parameter of the M-N-C catalyst for chemical reactions. Fortunately, the

present work could offer new guidelines for the molecular design of M-N-C catalysts, which are promising

candidate catalysts for various reactions.


Methods

Electrodeposited MOFs: All MOFs were synthesized by anodic electrodeposition. Nickel plate (99.8% purity,

Goodfellow, UK). and cobalt plate (99.9% purity, Goodfellow, UK) were carfully polished before used as an anode

electrode. The cathode and anode material were the same. The metal plate was partially immersed in the different

synthesis solutions. By applying a current for 8 h, MOFs were grown on the surface of the metal plate. After

completion of the MOF growth, the MOFs were carefully removed from the electrode. The resulting powder was

washed with DMF and EtOH to remove excess ligand. The solution, temperature and current for the synthesis of

different kinds of MOFs were as follows:

Ni(HBTC)(DMF)2: Ni(HBTC)(DMF)2 covered electrodes were synthesized in a solution containing 3 g

1,3,5-benzenetricarboxylic acid (H3BTC, 98% purity ABCR Germany) per 100 mL solvent, including 67 mL

N,N-dimethylformamide (DMF, ≥ 99% Chem-Lab) and 33 mL ethanol (EtOH, 99.9% purity, VWR France), at a

temperature of 120 °C and a current density of 2 mA cm−2 .
                                                               19
          Tailor-made metal-nitrogen-carbon catalysis for rechargeable Zn-air batteries via controllable MOF unit design




Ni(HBTC)(4,4′-bipy): Ni(HBTC)(4,4′-bipy) covered electrodes were synthesized in a solution containing 3 g

H3BTC and 1 g 4,4-dipyridyl (bipy, 98 % purity ACROS ORGANIC) per 100 mL DMF at a temperature of 120 °C

and a current density of 2 mA cm−2 .

Co(HBTC)(4,4′-bipy): Co(HBTC)(4,4′-bipy) covered electrodes were synthesized in a solution containing 3 g

H3BTC and 1 g 4,4-dipyridyl per 100 mL DMF at three different temperatures (30, 60 and 120 °C ) and the current

density of 2 mA cm−2.

Preparation of MOF derived M-N-C catalyst: The different MOFs were pyrolized at 700 °C and 900 °C for 8 h in

a nitrogen atmosphere with a flow rate of 300 cm3 min−1, respectively. The temperature inside the furnace was

gradually increased from room temperature at a heating rate of 5 °C min−1. After pyrolysis, the sample was cooled

down at a cooling rate of 5 °C min−1 in a flowing nitrogen atmosphere until room temperature. The as-prepared

black powder was first washed with 30% H2SO4 with ultrasound (half hour), then maintained at 60 °C overnight to

remove uncovered (carbon) metal nanoparticles. The resulting samples were washed with DI water and EtOH

repeatedly until pH ≈ 6, and then dried at 60 °C. The preparation details of the different MOFs derived M-N-C

catalysts are shown in Table S1.

Material characterizations: X-ray diffraction (XRD) patterns of the samples were recorded on a Bruker AXS D8

diffractometer using Cu Kα radiation (λ = 0.15405 nm) and Ni filter with 2θ ranging from 5°to 30°and a step size

of 0.02° (2 s per step). Thermogravimetric analysis (TGA) was performed in a nitrogen atmosphere on a

thermogravimetric analyzer (AutoTGA 2950HR V5.4A, TA Instruments) using platinum pans at a heating rate of

5 °C min−1. The onset temperature of the weight loss in the TGA was used as the decomposition temperature (Td).

The morphologies and structures of the samples were observed on a FEI/Philips XL30 FEG microscope (SEM). In

order to improve the electronic conductivity of the MOF, the MOF-covered metal plate was sputtered with a 10 nm

layer of Pt. High resolution transmission electron microscopy (HRTEM) and annular dark field (ADF) scanning

transmission electron microscopy (STEM) images were obtained on an FEI Tecnai F20 field emission gun
                                                               20
          Tailor-made metal-nitrogen-carbon catalysis for rechargeable Zn-air batteries via controllable MOF unit design




microscope operated at 200 kV with a point-to-point resolution of 0.19 nm, which is equipped with an annular dark

field (ADF) and an electron energy loss spectroscopy (EELS) detectors. In addition, the obtained images and

spectra were analyzed by Gatan Digital Micrograph software. X-ray photoelectron spectroscopy (XPS) was

recorded with an ESCALAB250 (monochromatic Al Kα with 1486.6 eV). The binding energy was calibrated by

placing the principal C1speak at 284.6 eV. CHN analyses were carried out on a Thermo Scientific Flash 2000

Organic Elemental Analyzer.

Electrochemical characterizations:

Preparation of working electrodes: All catalyst inks were prepared by dispersing 5 mg of catalyst in 1 mL of

isopropanol solvent containing 40 µL of 5 wt.% Nafion (Alfa Aesar Germany) through sonication for 30 min. Then,

for the ORR test, 20 µL of the catalyst ink was loaded onto a glass-carbon electrode (GCE) of 5 mm in diameter

(loading: 0.50 mg cm−2). In order to reduce the influence of oxygen bubbles on the OER test caused by high mass

loading, 10 µL of the catalyst ink was loaded onto a glassy carbon electrode (GCE) of 5 mm in diameter (loading~

0.25 mg cm−2). For comparison, multi-walled carbon nanotubes (MCNTs, Nanocyl, Belgium), Pt/C catalyst (10%

of platinum, Alfa Aesar Germany) and IrO2 (P40V020, particles from Premetek Co.) were conducted on the same

electrochemical tests with the same mass loading. Here, the mass loading Pt is around 50 μg cm−2, which is higher

than the “standard” loading (20 μg cm−2).36 In addition, the cobalt-covered electrode was electrodeposited at a

current density of -40 mA cm−2 in CoCl2 (0.9 mol L−1) aqueous solution with LiClO4 (0.1 mol L−1) as the

supporting electrolyte at room temperature until a mass loading of 0.50 mg cm−2 (based on Faraday's law) was

obtained on the GCE. The details were shown in previous work.70, 104 As the catalysts in this study are composite

materials, electrochemical surface area (ECSA) measured by electrochemical double-layer capacitance or

Brunauer-Emmett-Teller (BET) might lead to errors for current normalization.105 Therefore, the geometrical surface

area of the GCE and constant catalyst mass loading for all catalysts were used as current normalization method.



                                                               21
            Tailor-made metal-nitrogen-carbon catalysis for rechargeable Zn-air batteries via controllable MOF unit design




Catalytic performance: All the electrocatalytic measurements are carried out in a three-electrode cell using a

rotating disk electrode (RDE, PINE Research Company). Before measuring, the electrolyte was purged with O2 (for

ORR) or N2 (for OER) for at least 45 minutes. During the test, the electrolyte was continuously bubbled with O2 or

N2. The ORR and OER catalytic performance of the samples were characterized by linear sweep voltammetry

(LSV), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques on an Autolab

electrochemical workstation at 25 °C. An automatic thermostat bath was used to control the temperature (Julabo

F25, Germany). The supporting electrolyte in all electrochemical experiments is 0.1 M KOH. The currents of the

LSV and CV are normalized by the mass of the active materials and the geometrical area of the glassy carbon

electrode (GCE). The EIS measurements were recorded in a frequency range from 10−2 to 104 Hz at open circuit

potential with a peak-to-peak amplitude of 5 mV. The catalyst covered GCE, a graphite plate (3.5 cm × 5 cm) and a

Hg/HgO electrode were used as the working electrode, the counter electrode and the reference electrode,

respectively. All potentials are referred to the reversible hydrogen electrode (RHE) potential, which was converted

from the Hg/HgO electrode using the following equation:

𝐸𝑅𝐻𝐸 = 𝐸𝐻𝑔/𝐻𝑔𝑂 + 0.098 + 0.059 × pH


All potentials were iR-corrected to compensate for the influence of solution resistances, using the following

equation72, 106, 107:

𝐸𝑖𝑅−𝑐𝑜𝑟𝑟𝑒𝑐𝑡𝑒𝑑 = 𝐸 − 𝑖𝑅

where i is the current and R is the uncompensated ohmic electrolyte resistance measured by EIS at high frequency.

Electrode kinetic data were calculated based on Koutecky-Levich (K−L) equation:

1 1 1     1        1
 = + =     1/2
               +
𝑗 𝑗𝐿 𝑗𝐾 𝐵𝜔       𝑛F𝑘𝑐∞
                2/3 −1/6
𝐵 = 0.62𝑛F𝐷°        𝑣      𝑐∞

where j is the measured current density (A m−2), jL is the diffusion-limited current density (A m−2), jK is the kinetic

current density (A m−2), 𝜔 is the the angular velocity of the rotating electrode (s−1), F is the Faraday constant
                                                                 22
           Tailor-made metal-nitrogen-carbon catalysis for rechargeable Zn-air batteries via controllable MOF unit design




(96485 C mol−1), c∞ is the bulk concentration of O2 in 0.1 M KOH (1.2 mol m−3 ), n is the number of transferred

electrons, Do is the diffusion coefficient of O2 in 0.1 M KOH (1.9×10−9 m2 s−1), and ʋ is the kinematic viscosity of

the electrolyte (10−6 m2 s−1), respectively.45, 108, 109

Rechargeable Zn-Air battery tests: For the full-cell test of our catalysts, we prepared a zinc plate as an anode,

and 6 M KOH with 0.2 M zinc acetate solution was used for the electrolyte (Figure S36). The different air cathodes

were prepared by a mixture of different catalysts with 5 wt.% Nafion solution with the same ratio for half reaction

test (ORR and OER), and a mass loading of 3.5 mg cm−2 on a gas diffusion electrode (carbon fiber paper). All the

Zn-air batteries were tested under ambient atmosphere without a separator for anode and cathode. An assembled

full-cell was characterized by chronopotentiometry (CP) at several charge and discharge currents.

First-principles calculations: All calculations were performed using plane-wave density functional theory (DFT)

employing periodic boundary conditions as implemented in the VASP code110,                                  111
                                                                                                               . The electronic

                                                                                                                    112
exchange-correlation energy was modeled using the Perdew-Burke-Ernzerhof (PBE) functional                                 within the

generalized gradient approximation (GGA). The projector augmented wave (PAW) method was used to describe

the ionic cores113, 114. For the plane-wave expansion, a 550 eV kinetic energy cut-off was used after testing a series

of different cut-off energies. To model metal-nitrogen-carbon (M-N-C) structures, the (5×5×1) supercell M, N

co-doped graphene with lattice parameters of a = b = 12.28 Å was constructed by a periodic boundary condition,

and the vacuum layers were set to be larger than 20 Å to avoid periodic interaction. A Monkhorst-Pack 3 × 3 × 1

                                                               115
k-point grid was used to sample the Brillouin zone                   . The convergence criterion for the electronic structure

iteration was set to be 10-4 eV, and that for geometry optimizations were set to be 0.01 eV/Å on force. To better

describe the dispersion interaction within water adsorption systems, vdW correction was considered by adopting

the Grimme’s D2 scheme 116.

In order to understand the mechanism and the active site of the oxygen reduction pathway on the M, N co-doped

graphene, we use the free energy change (G) for each ORR step as the criterion for assessing the ORR activity.
                                                                23
          Tailor-made metal-nitrogen-carbon catalysis for rechargeable Zn-air batteries via controllable MOF unit design




Specifically, we considered the four-electron reaction pathway for ORR in alkaline media, as listed in

Supplementary equations (2) to (5), which is the dominated mechanism for doped graphene catalysis117. The

free-energy diagrams in the reaction pathways were calculated on the basis of a standard hydrogen electrode (CHE)

model that was introduced by Nørskov et al118. For each step of ORR, the Gibbs free energy of formation is given

by the following equation:

ΔG = ΔE +ΔZPE –TΔS –eU

where ΔE, ΔZPE, and ΔS are the changes of DFT total energy, zero-point energy, and entropy from the initial state

to the finial state, respectively; T is temperature; U is the electrode potential; e is the charge transfer. ΔZPE, and ΔS

can be obtained by the thermodynamics table for gaseous molecules119 and by calculating the vibrational

frequencies for the oxygenated intermediate (see Table S7), respectively.




                                                               24
        Tailor-made metal-nitrogen-carbon catalysis for rechargeable Zn-air batteries via controllable MOF unit design




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Competing financial interests
The authors declare no competing financial interests.




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          Tailor-made metal-nitrogen-carbon catalysis for rechargeable Zn-air batteries via controllable MOF unit design




Figures & Tables




Figure 1. Schematic illustration of the synthesis of MOFs on substrates, schematic views of the crystal structures

showing the coordinative environments around the center metal atoms in Ni-BTC-DMF, Ni-BTC-bipy and

Co-BTC-bipy (all H atoms are omitted for clarity), and view of supramolecular frameworks from the b axis. SEM

images of (a) Ni-BTC-DMF, (b) Ni-BTC-bipy and (c) Co-BTC-bipy electrodeposited on substrates are also given.




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          Tailor-made metal-nitrogen-carbon catalysis for rechargeable Zn-air batteries via controllable MOF unit design




Figure 2. (a) The ID/IG ratios of different catalysts based on Raman spectra (Figure S7− S9). (b) N content (at.%)

of different MOFs after pyrolysis under different temperatures, determined by CHN analysis based on total amount

of C, H and N.




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         Tailor-made metal-nitrogen-carbon catalysis for rechargeable Zn-air batteries via controllable MOF unit design




Figure 3. High-resolution XPS spectra of different N-heteroatom species and their relative content in (a)

Ni-BTC-DMF-700 and Ni-BTC-DMF-900 catalysts, (b) Ni-BTC-bipy-700 and Ni-BTC-bipy-900 catalysts, and (c)

Co-BTC-bipy-700 and Co-BTC-bipy-900 catalysts.




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          Tailor-made metal-nitrogen-carbon catalysis for rechargeable Zn-air batteries via controllable MOF unit design




Figure 4. EELS chemical composition maps of Co-BTC-bipy-700 (top image) and Co-BTC-bipy-900 (bottom

image) electrocatalysts obtained from the blue rectangular area of the ADF-STEM micrograph. Individual C

K-edge (red), N K-edge (green), O K-edge (blue), Co L2,3-edge (pink) composite maps and the relative composition

maps of C (at 94.0 ±2.9 %) and N (at 6.1 ±2.9%) for Co-BTC-bipy-700 and the relative composition maps of C (at

94.9 ±11.6 %) and N (a: 3.7 ±2.3%) for Co-BTC-bipy-900.




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           Tailor-made metal-nitrogen-carbon catalysis for rechargeable Zn-air batteries via controllable MOF unit design




 (a)                                                               (b)

                                                       (10-10)                                                       (10-10)
                                             (1-100)                                                       (1-100)

                                                         (01-10)
                                                                                                                        (01-10)


                                         Co [000-1]                                                    Co [000-1]



Figure 5. TEM images and corresponding power spectra (FFT) of (a) Co-BTC-bipy-700 and (b) Co-BTC-bipy-900

electrocatalysts.




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          Tailor-made metal-nitrogen-carbon catalysis for rechargeable Zn-air batteries via controllable MOF unit design




Figure 6. (a) Free energy variations of ORR elementary steps and (b) rate-determining steps for different M-N-C

structures. Inset in (b) shows top views of the potential configurations of M-N-C structure surfaces. Gold, white and

blue balls represent C, N and M (M = Ni or Co) atoms, respectively.




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          Tailor-made metal-nitrogen-carbon catalysis for rechargeable Zn-air batteries via controllable MOF unit design




Figure 7. Linear scan voltammograms of different catalysts at a rotating speed of 1600 rpm in O2-saturated 0.1 M

KOH at a scan rate of 10 mV s−1: (a) Ni-BTC-DMF-700 and Ni-BTC-DMF-900 electrocatalysts; (b)

Ni-BTC-bipy-700 and Ni-BTC-bipy-900 electrocatalysts; (c) Co-BTC-bipy-700 and Co-BTC-bipy-900

electrocatalysts;   (d)   Ni-BTC-DMF-700,            Ni-BTC-DMF-900,           Ni-BTC-bipy-700         and     Ni-BTC-bipy-900

electrocatalysts; (e) Ni-BTC-bipy-700 and Co-BTC-bipy-700 electrocatalysts.




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          Tailor-made metal-nitrogen-carbon catalysis for rechargeable Zn-air batteries via controllable MOF unit design




Figure 8. (a) Linear scan voltammograms (LSV) of Pt/C and Co-BTC-bipy-700 electrocatalysts on a RDE at a

rotation speed of 1600 rpm in O2-saturated 0.1 M KOH at a scan rate of 10 mV s−1 before and after cycling tests (in

the range of 0.5 V−1 V vs. RHE). (b) Cyclic voltammograms (CV) of Pt/C and Co-BTC-bipy-700 electrocatalysts

at a scan rate of 50 mV s−1 without the addition of methanol and after the addition of 1 M methanol.




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          Tailor-made metal-nitrogen-carbon catalysis for rechargeable Zn-air batteries via controllable MOF unit design




Figure 9. (a) Corresponding electron transfer number during ORR tests at various potentials based on

Koutecky-Levich plots of different electrocatalysts (Figure S24b−S31b). (b) Tafel plots derived from the

polarization curves (Figure S24a−S31a) at a rotating speed of 1600 rpm in O2-saturated 0.1 M KOH at scan rate of

10 mV s−1.




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           Tailor-made metal-nitrogen-carbon catalysis for rechargeable Zn-air batteries via controllable MOF unit design




Figure 10. (a) OER polarization curves of Pt/C, IrO2 and Co-BTC-bipy-700 catalysts on a RDE rotating at 1600

rpm in 0.1 M KOH; (b) Corresponding Tafel plots obtained from the polarization curves; (c) OER stability test of

IrO2 and Co-BTC-bipy-700 catalysts with a constant applied potential of 1.7 V vs. RHE in 0.1 M KOH; (d) OER

polarization curves of IrO2 and Co-BTC-bipy-700 catalysts at a rotation speed of 1600 rpm in 0.1 M KOH before

and after stability tests.




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          Tailor-made metal-nitrogen-carbon catalysis for rechargeable Zn-air batteries via controllable MOF unit design




Figure 11 The i−V polarization (a) and power density (b) curves of Zn-air batteries based on Pt/C-IrO2 and

Co-BTC-bipy-700 catalysts. (c) Charge-discharge curves of Zn-air batteries based on Pt/C-IrO2 and

Co-BTC-bipy-700 catalysts at different current densities after 30 cycles.




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